Photochromism in composite and hybrid materials based on ...

<strong>Photochromism</strong> <strong>in</strong> <strong>composite</strong> <strong>and</strong> <strong>hybrid</strong> <strong>materials</strong> 

<strong>based</strong> on transition-metal oxides 

<strong>and</strong> polyoxometalates 

Abstract 

Progress <strong>in</strong> Materials Science 51 (2006) 810–879 

Tao He, Jiannian Yao * 

CAS Key Laboratory of Photochemistry, Center for Molecular Sciences, Institute of Chemistry, 

Ch<strong>in</strong>ese Academy of Sciences, Beij<strong>in</strong>g 100080, PR Ch<strong>in</strong>a 

Received 25 April 2005; accepted 5 December 2005 

Photochromic <strong>materials</strong> are attractive <strong>and</strong> promis<strong>in</strong>g for applications <strong>in</strong> many fields. One subject 

<strong>in</strong> this area is to prepare <strong>and</strong> study the photochromism <strong>in</strong> <strong>composite</strong> or <strong>hybrid</strong> <strong>materials</strong> <strong>based</strong> on 

transition-metal oxides or polyoxometalates. Their properties not depend only on the chemical 

nature of each component, but also on the <strong>in</strong>terface <strong>and</strong> synergy between them. S<strong>in</strong>ce the charge 

transfer plays a key role <strong>in</strong> the photochromism of these <strong>materials</strong>, it is very important to <strong>in</strong>crease 

the charge (electrons, holes, <strong>and</strong> protons) <strong>in</strong>teractions between the two components <strong>in</strong> either <strong>composite</strong>s 

or <strong>hybrid</strong>s. To realize this, one big challenge is to optimize the two components on a molecular 

or nanometer scale, which is closely relevant to the constituents, sample history (pre-treatment, 

preparation, <strong>and</strong> post-treatment), environment (humidity, presence of reducible or oxidizible matters, 

light-irradiation wavelength, <strong>in</strong>tensity, time, etc.). Based on these, many novel <strong>composite</strong> or 

<strong>hybrid</strong> <strong>materials</strong> with improved photochromism, visible-light coloration, reversible photochromism, 

multicolor photochromism or, possibly, fast photoresponse, have been prepared dur<strong>in</strong>g the last two 

decades or three. This may underscore the opportunity of us<strong>in</strong>g these <strong>composite</strong> <strong>and</strong> <strong>hybrid</strong> <strong>materials</strong> 

as the photonic applications. In present paper, we summarize thoroughly all the recent progress 

<strong>in</strong> these subjects. 

Ó 2005 Elsevier Ltd. All rights reserved. 

* Correspond<strong>in</strong>g author. Tel./fax: +86 10 82616517. 

E-mail address: jnyao@iccas.ac.cn (J. Yao). 

0079-6425/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. 

doi:10.1016/j.pmatsci.2005.12.001 

www.elsevier.com/locate/pmatsci

Contents 

T. He, J. Yao / Progress <strong>in</strong> Materials Science 51 (2006) 810–879 811 

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812 

2. Inorganic/<strong>in</strong>organic <strong>composite</strong> <strong>materials</strong> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 

2.1. Semiconductor/metal <strong>composite</strong> <strong>materials</strong> . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814 

2.1.1. Improved photochromism of WO 3 <strong>and</strong> MoO 3 ................... 814 

2.1.2. Multicolor photochromism ................................. 817 

2.2. Semiconductor/semiconductor <strong>composite</strong> <strong>materials</strong> ...................... 818 

2.2.1. Improved photochromism <strong>in</strong> WO 3/MoO 3 system . . . . . . . . . . . . . . . . . 818 

2.2.2. Improved photochromism <strong>in</strong> other <strong>composite</strong>s conta<strong>in</strong><strong>in</strong>g WO3 ...... 822 

2.2.3. Improved photochromism <strong>in</strong> MoO3/TiO2 system ................. 825 

2.2.4. Visible-light coloration <strong>in</strong> MoO 3/TiO 2 system . . . . . . . . . . . . . . . . . . . 827 

2.2.5. Visible-light coloration <strong>in</strong>duced by CdS . . . . . . . . . . . . . . . . . . . . . . . 828 

2.3. Photochromic <strong>materials</strong> with dopants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830 

2.3.1. Doped TiO 2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830 

2.3.2. Transition-metal-doped titanates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833 

2.3.3. Doped molybdates <strong>and</strong> tungstates . . . . . . . . . . . . . . . . . . . . . . . . . . . 835 

2.3.4. Field-assisted photochromism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836 

2.4. Miscellaneous <strong>composite</strong> <strong>materials</strong> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837 

2.4.1. a-WO3/Si heterostructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837 

2.4.2. H3PW12O40/TiO2 system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839 

2.4.3. Other <strong>composite</strong> systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839 

3. Inorganic/organic <strong>hybrid</strong> <strong>materials</strong> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 840 

3.1. Model molecules, photochromic mechanism, <strong>and</strong> preparation methods . . . . . . . 840 

3.1.1. Model molecules <strong>and</strong> photochromic mechanism . . . . . . . . . . . . . . . . . 840 

3.1.2. Preparation methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 842 

3.1.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 

3.2. Hybrids at molecular level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 

3.2.1. Donor–acceptor systems prepared from POMs <strong>and</strong> aromatic organic 

molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 

3.2.2. Alkylammonium POMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 845 

3.2.3. Hybrids prepared from POMs <strong>and</strong> small biological molecules . . . ..... 847 

3.2.4. Visible-light coloration <strong>in</strong> CT complexes. . . . . . . . . . . . . . . . . . . . . . . 847 

3.3. Hybrids at nanometer level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 848 

3.3.1. Multilayer th<strong>in</strong> films prepared by electrostatic layer-by-layer method . . 848 

3.3.2. POMs embedded <strong>in</strong> polymeric matrices . . . . . . . . . . . . . . . . . . . . . . . 856 

3.3.3. POMs embedded <strong>in</strong> organic/silica matrices . . . . . . . . . . . . . . . . . . . . . 862 

3.3.4. POMs anchored to organic polymeric backbone . . . . . . . . . . . . . . . . . 863 

3.3.5. Self-organized <strong>hybrid</strong>s <strong>based</strong> on POMs <strong>and</strong> DODA . . . . . . . . . . . . . . . 864 

3.4. Miscellaneous <strong>hybrid</strong> <strong>materials</strong> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 867 

3.4.1. TMOs/DMF systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 867 

3.4.2. Molybdenum-oxide cluster/citric acid complexes . . . . . . . . . . . . . . . . . 868 

3.4.3. Molybdenum (VI) oxalate complexes . . . . . . . . . . . . . . . . . . . . . . . . . 870 

4. Conclud<strong>in</strong>g remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 870 

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872

812 T. He, J. Yao / Progress <strong>in</strong> Materials Science 51 (2006) 810–879 

1. Introduction 

The word ‘‘photochromism’’ derives from two Greek words mean<strong>in</strong>g light <strong>and</strong> color, 

which refers to such a phenomenon that the material can change color <strong>in</strong> a reversible 

way by electromagnetic radiation (UV, visible, <strong>and</strong> IR illum<strong>in</strong>ation) [1–6]. The reverse 

process can take place by exposure to the light at a different frequency [1–3], by heat<strong>in</strong>g 

<strong>in</strong> the dark [1–3], by electrochemical polarization [7], or by chemical oxidation [8]. Photochromic 

<strong>materials</strong> exhibit a wide range of optical properties, which makes them attractive 

<strong>and</strong> promis<strong>in</strong>g for a variety of applications. In the book edited by Brown [1], Bertelson has 

given an excellent review about these applications. The sensitivity of the <strong>materials</strong> to light 

radiation makes them useful for self-develop<strong>in</strong>g photography, protective <strong>materials</strong>, dosimetry 

<strong>and</strong> act<strong>in</strong>ometry. The <strong>materials</strong> with good photochromic reversibility have potential 

applications <strong>in</strong> reusable <strong>in</strong>formation storage media, data display, optical signal process<strong>in</strong>g, 

chemical switch for computer, smart w<strong>in</strong>dow (control of radiation <strong>in</strong>tensity), <strong>and</strong> the like. 

These <strong>materials</strong> can also be used as the reagents for photomask<strong>in</strong>g, photoresist, camouflage, 

<strong>and</strong> so on. 

The photochromism was first phenomenologically observed <strong>in</strong> both <strong>in</strong>organic <strong>and</strong> 

organic <strong>materials</strong>, which dates back to the 19th century [2,9–14]. However, subsequent 

developmental work has proliferated the number of organic <strong>materials</strong> considerably. 

Although so far most photochromic <strong>materials</strong> are organic, <strong>in</strong>organic <strong>materials</strong> have some 

advantages over the organic counterparts. They have better thermal stability, strength, 

chemical resistance, <strong>and</strong> macroscopic shape mold<strong>in</strong>g (can be easily shaped as th<strong>in</strong> films, 

coat<strong>in</strong>gs, monoliths, or other suitable forms). In the first half of 20th century, the research 

on <strong>in</strong>organic photochromic <strong>materials</strong> was ma<strong>in</strong>ly focused on alkali halides, alkal<strong>in</strong>e earth 

halides, alkali metal azides, TiO2, titanates, complex m<strong>in</strong>erals, <strong>and</strong> complex mercury salts, 

<strong>and</strong> so on [1,2,10,15–18]. After Deb’s pioneer<strong>in</strong>g work [19–22], a widespread <strong>in</strong>terest 

[7–9,23–38] has been motivated <strong>in</strong> the photochromism of transition-metal oxides 

(TMOs) <strong>and</strong> polyoxometalates (POMs), specifically <strong>in</strong> MoO3 <strong>and</strong> WO3. The photochromic 

response of MoO3 [7] <strong>and</strong> WO3 [36] th<strong>in</strong> film was extended from ultraviolet (UV) light 

to visible light after the cathodic polarization pre-treatment. The photochromic activity of 

TMO films can be improved when irradiated <strong>in</strong> reducible atmosphere [30,31], though from 

which only a few applications can be benefited. For s<strong>in</strong>gle <strong>in</strong>organic photochromic species, 

they usually exhibit poor reversibility (e.g., thermal bleached WO3 or MoO3 cannot be 

photocolored aga<strong>in</strong>), small change <strong>in</strong> optical density after coloration (low photochromic 

activity), narrow response <strong>in</strong> the spectrum (all the TMOs except V2O5 response only to 

the blue or UV light), low fatigability (cannot be cycled many times while ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g performance), 

monotonic coloration (usually blue color for most TMOs), low thermal stability 

of virg<strong>in</strong> or colored states, slow response time, <strong>and</strong> sometimes a high-cost preparation 

with difficulty <strong>in</strong> tailor-make, <strong>and</strong> so forth. 

Dur<strong>in</strong>g the last two decades or three, specifically due to the development of nanotechnology 

<strong>and</strong> nanoscience, a considerable promis<strong>in</strong>g potential of molecular <strong>materials</strong> has 

been ly<strong>in</strong>g on the possibility to create <strong>composite</strong> <strong>and</strong> <strong>hybrid</strong> <strong>materials</strong>. The technological 

drive <strong>in</strong> the quest for this novel class of multifunctional <strong>materials</strong> is the desire to secure a 

property or a comb<strong>in</strong>ation of properties not available <strong>in</strong> any of the <strong>in</strong>dividual component 

of the <strong>composite</strong>s or <strong>hybrid</strong>s <strong>and</strong>/or at least to improve some properties of the active component 

[39–42]. Accord<strong>in</strong>gly, the trend <strong>in</strong> photochromism beg<strong>in</strong>s to concern the <strong>materials</strong> 

comb<strong>in</strong><strong>in</strong>g several properties <strong>in</strong> a synergistic way. In most cases, it is to develop


photochromic systems of <strong>in</strong>organic/<strong>in</strong>organic <strong>composite</strong>s [23,28,29,38] <strong>and</strong> <strong>in</strong>organic/ 

organic <strong>hybrid</strong>s [43–47] <strong>based</strong> on TMOs or POMs. These systems are very <strong>in</strong>terest<strong>in</strong>g from 

the perspectives of basic science <strong>and</strong> technology. The properties expected <strong>in</strong> them do not 

depend only on the chemical nature of each component, but also on the <strong>in</strong>terface <strong>and</strong> synergy 

between them. The latter usually plays a key role <strong>in</strong> tun<strong>in</strong>g the photochromic behavior. 

Thus, a critical po<strong>in</strong>t for the design of these <strong>materials</strong> is the tun<strong>in</strong>g of nature, extension 

<strong>and</strong> accessibility of the <strong>in</strong>ner <strong>in</strong>terfaces [48]. The general tendency <strong>in</strong> research is to create 

<strong>in</strong>timate mix<strong>in</strong>g <strong>and</strong>/or <strong>in</strong>terpenetration at molecular or nanometer level between the two 

components <strong>in</strong> order to obta<strong>in</strong> strong <strong>in</strong>terfacial <strong>in</strong>teractions, specifically a strong charge 

(electron, hole, proton) communication. Accord<strong>in</strong>gly, the preparation methods transfer 

from pretty high-cost physical techniques (such as thermal evaporation or sputter<strong>in</strong>g 

deposition) to readily <strong>and</strong> relatively low-cost ‘‘soft’’ chemical (chimie douce) ones. Hopefully 

most of the aforementioned issues for s<strong>in</strong>gle species, though not all, may be surmounted 

or at least ameliorated <strong>in</strong> these systems. In the <strong>composite</strong> systems, the 

photogenerated electrons <strong>and</strong> holes can transfer between the two <strong>in</strong>organic constituents 

due to the different energy levels, lead<strong>in</strong>g to an improved photochromism [28,29,38,49– 

52], visible-light coloration [23,53,54], or multicolor photochromism [55–57]. Similarly, 

the transfer of photogenerated charge carriers <strong>and</strong>, possibly, protons between <strong>in</strong>organic 

<strong>and</strong> organic moieties <strong>in</strong> <strong>hybrid</strong> <strong>materials</strong> usually results <strong>in</strong> improved photochromic activity, 

reversibility <strong>and</strong> response time [45,58–62]. Another advantage for the <strong>in</strong>organic/ 

organic <strong>hybrid</strong>s results from their high versatility <strong>in</strong> offer<strong>in</strong>g a wide range of possibilities 

to fabricate tailor-make <strong>materials</strong> <strong>in</strong> terms of their extremely versatile chemical <strong>and</strong> physical 

properties, compositions, <strong>and</strong> process<strong>in</strong>g techniques [63–68]. 

In present review, the recent advances of photochromism <strong>in</strong> <strong>in</strong>organic/<strong>in</strong>organic <strong>composite</strong> 

<strong>materials</strong> <strong>based</strong> on TMOs <strong>and</strong> POMs will be considered first, 1 ma<strong>in</strong>ly about the 

doped semiconductors <strong>and</strong> coupl<strong>in</strong>g of a semiconductor with a noble metal or another 

semiconductor on the surface. The progress <strong>in</strong> <strong>in</strong>organic/organic <strong>hybrid</strong> systems <strong>based</strong> 

on TMOs <strong>and</strong> POMs will be discussed <strong>in</strong> next section. The attention will be paid only 

to the systems <strong>in</strong> which photochromism is caused by the <strong>in</strong>organic moiety, not by the 

organic. The conclusions together with a brief outlook will be given at last. In order to 

keep the unity <strong>and</strong> <strong>in</strong>tegrity of the review, it is necessary to <strong>in</strong>clude some papers published 

before 1970s. 

2. Inorganic/<strong>in</strong>organic <strong>composite</strong> <strong>materials</strong> 

Electron–hole pairs can be produced <strong>in</strong> TMOs upon b<strong>and</strong> gap (Eg) irradiation, lead<strong>in</strong>g 

to an obvious change <strong>in</strong> the optical density <strong>and</strong> consequently to a change <strong>in</strong> the color of 

TMOs [30,31]. The resultant absorption has been <strong>in</strong>terpreted by several theoretical models. 

For amorphous <strong>materials</strong>, the models are <strong>based</strong> on the formation of color center or 

hydrogen bronze under illum<strong>in</strong>ation [31], <strong>in</strong> which the absorption is thought to be caused 

by the color center [20,22], <strong>in</strong>tervalence-charge transfer (IVCT) [69,70], or small-polaron 

transition [71]. While for the systems conta<strong>in</strong><strong>in</strong>g (nano)crystals, the absorption arises from 

1 Although some <strong>in</strong>organic <strong>materials</strong> pert<strong>in</strong>ent to TMOs or POMs (such as LiNbO3, Bi4Ge3O12, Bi12MeO20 

(where Me = Ge, Si, Ti, etc.), <strong>and</strong> the like) <strong>and</strong> their <strong>composite</strong>s can exhibit photochromism too, they are usually 

regarded as the photorefractive <strong>materials</strong> rather than the photochromic <strong>materials</strong>. So these <strong>materials</strong> will not be 

discussed <strong>in</strong> the current review.


the free or trapped charge carriers [31]. Although so far discrepancy still exists among different 

models s<strong>in</strong>ce none of them can successfully expla<strong>in</strong> all the experimental results, it is 

sure that the photochromic performance of TMOs is determ<strong>in</strong>ed by the behavior of optically 

excited electron–hole pairs. The coloration performance can thus be tuned readily by 

controll<strong>in</strong>g the behavior of these charge carriers. Possibilities that immediately spr<strong>in</strong>g to 

m<strong>in</strong>d are studies of sensitization. Usually a noble metal with a high work function or a 

(narrow-b<strong>and</strong> gap) semiconductor with more negative or less positive energy levels (vs. 

NHE) is used as dopant or surface compound to modify the TMOs. In former case, a 

Schottky barrier is formed at the metal/semiconductor <strong>in</strong>terface [72,73], which facilitates 

the separation of photogenerated electron–hole pairs <strong>and</strong> leads to an improved photochromism 

of TMOs. In latter case, the effective charge carriers not come only from the photochromic 

TMOs but also from the comb<strong>in</strong>ed semiconductor. An improved photochromism 

<strong>and</strong>, sometimes, visible-light coloration are thus obta<strong>in</strong>ed. 

2.1. Semiconductor/metal <strong>composite</strong> <strong>materials</strong> 

2.1.1. Improved photochromism of WO3 <strong>and</strong> MoO3 

Yao et al. [38,74–76] have deposited a th<strong>in</strong> layer of Au or Pt ( 20 nm) onto the film 

surface of MoO3 by thermal vacuum evaporation technique. Color change due to photochromism 

is usually measured as a change <strong>in</strong> absorbance (optical density) after <strong>and</strong> before 

coloration, 2 DOD, simply the change <strong>in</strong> the optical absorption coefficient at a given wavelength. 

In these systems, DOD at the absorption peak (900 nm) for a UV-colored MoO 3/ 

Au <strong>and</strong> MoO 3 th<strong>in</strong> film is 0.56 <strong>and</strong> 0.21, respectively (Fig. 1) [38]. That is, MoO 3 th<strong>in</strong> films 

modified with Au overlayer exhibit an enhanced UV-light photochromism over the 

prist<strong>in</strong>e films. It has been reported that Au-nanoparticle overlayer prepared by a sp<strong>in</strong>coat<strong>in</strong>g 

method can also improve the photochromism of MoO3 [28,77] <strong>and</strong> WO3 [78] th<strong>in</strong> 

films. 

S<strong>in</strong>ce the Fermi level (EF) ofMoO3 <strong>and</strong> WO3 (4.3–4.9 eV) [28,78–83] is lower than the 

work function of Au (5.1 eV) or Pt (5.64 eV) [73], the contact of Au or Pt with MoO3 or 

WO 3 th<strong>in</strong> film results <strong>in</strong> the formation of a Schottky barrier at the <strong>in</strong>terface (Fig. 2). Under 

this built-<strong>in</strong> electric field photogenerated electrons upon UV-light excitation are drifted 

<strong>in</strong>to the bulk TMOs along the conduction b<strong>and</strong>, while holes to the <strong>in</strong>terface via the valence 

b<strong>and</strong>. So the separation of photogenerated electron–hole pairs <strong>in</strong> the <strong>composite</strong> film is 

more efficient than that <strong>in</strong> the prist<strong>in</strong>e film. Consequently, recomb<strong>in</strong>ation of the photogenerated 

charge carriers is suppressed more efficiently. Another advantage of surface modification 

with Au or Pt is that Schottky barrier is <strong>in</strong> favor of <strong>in</strong>hibit<strong>in</strong>g the surface 

photocorrosion of semiconductor [38]. The use of Au nanoparticles can afford the third 

enhancement mechanism. Much more water is adsorbed at the <strong>in</strong>terface or film surface 

of TMO/metal than that of TMO due to the surface effect of nanoparticles, which is favorable 

to the utilization of photogenerated holes [28,78,84]. This has been confirmed by the 

surface photovoltage spectra, FT-IR <strong>and</strong> XPS results [28,78]. So much more photogenerated 

charge carriers can contribute to the coloration process, result<strong>in</strong>g <strong>in</strong> an improved 

photochromism. In addition, it is argued [80] that the improved photochromism of 

2 In literatures, different authors have used optical density, OD, ABS, or absorptance to represent absorbance. 

In present review we just keep the orig<strong>in</strong>al form <strong>in</strong> the figures that the authors had used <strong>in</strong> their publications 

<strong>in</strong>stead of try<strong>in</strong>g to unify them by one denotation.


Fig. 1. Absorption spectra of (A) MoO 3/Au <strong>and</strong> (B) MoO 3 th<strong>in</strong> film. (a) Prist<strong>in</strong>e film; (b) spectrum taken after 

the film (a) was irradiated with UV light for 3 m<strong>in</strong> <strong>in</strong> air [38]. 

WO 3 by Pt <strong>in</strong> HCO 2H or EtOH is due to the ‘spillover’ effect (high catalysis of noble metals 

on the evolution of hydrogen), which can lead to more hydrogen to penetrate <strong>in</strong>to 

WO3. 

It is reported [38,74,76,77,85] that visible-light coloration of MoO3 th<strong>in</strong> films <strong>in</strong>duced 

by cathodic polarization can also be improved by the deposition of Au or Pt onto the film 

surface (Fig. 3) [38]. This enhancement effect is still attributed to the formation of a Schottky 

barrier at the <strong>in</strong>terface upon the surface modification with a noble metal. Pt produces 

a stronger improvement effect than Au because a stronger built-<strong>in</strong> electric field is formed at 

the <strong>in</strong>terface due to its higher work function.


Fig. 2. Schematic diagram for Schottky barrier <strong>and</strong> charge-transfer process at TMO/Au (here TMO refers to 

MoO3 or WO3) <strong>in</strong>terface. 

Fig. 3. Comparison of visible-light photochromic responses at 900 nm of MoO3, MoO3/Au, <strong>and</strong> MoO3/Pt th<strong>in</strong>film 

samples. DABS1 refers to the change <strong>in</strong> absorbance after polarization <strong>and</strong> DABS2 referes to the one after 

visible-light illum<strong>in</strong>ation [38]. 

Although the addition of Pt, as a surface microcrystall<strong>in</strong>e deposit, to f<strong>in</strong>ely powdered 

(nanosized) TiO2 can promote the production of hydrogen <strong>and</strong> suppress the recomb<strong>in</strong>ation 

of photogenerated charge carriers, it causes a progressive decrease <strong>in</strong> the reversible 

photochromic response with <strong>in</strong>creas<strong>in</strong>g Pt content [86]. In the meantime, optical absorption 

of trapped holes might be observed [87,88]. This is because <strong>in</strong> this case Pt deposited on 

TiO2 particles acts as a scavenger for electrons [86–91]. This is aga<strong>in</strong>st the results for MoO3 

<strong>and</strong> WO3 th<strong>in</strong> films. With larger pieces of semiconductor there is a depletion layer at the 

surface, across which there exists a potential gradient that allows the separation of charge 

carriers; while a potential gradient of this k<strong>in</strong>d does not exist when the diameter of nano-


sized particle is smaller than the thickness of such a space-charge layer <strong>and</strong> <strong>in</strong> such case 

the details of charge separation may not be the same as those <strong>in</strong> bulk semiconductor 

[88,92–94]. That is, electrons transfer efficiently from TiO2 to Pt <strong>and</strong> there are no evidences 

for the back-flow of electrons from the metal <strong>in</strong> Pt/TiO2 to form Ti 3+ [89]. Similar 

phenomenon has also been found <strong>in</strong> Ag/TiO2 nano<strong>composite</strong> system upon irradiation 

(cf. Section 2.1.2). 

2.1.2. Multicolor photochromism 

Conventional photochromic <strong>materials</strong> usually respond <strong>in</strong> a monochromatic way, so 

that multicolor photochromism requires several different <strong>materials</strong> or filters comb<strong>in</strong>ed 

appropriately. Recently a reversible multicolor photochromism has been reported <strong>in</strong> 

TiO2 films loaded with silver nanoparticles by photocatalytic means [55–57]. This multicolor 

photochromism achieved with a simple material is of great significance for the applications 

<strong>in</strong> a rewritable color-copy paper, a high-density multiwavelength optical memory, 

a multicolor-smart glass, a color-changeable pa<strong>in</strong>t, <strong>and</strong> the like. 

After Ag + is embedded <strong>in</strong>to TiO 2 th<strong>in</strong> film, Ag nanoparticles are obta<strong>in</strong>ed by irradiat<strong>in</strong>g 

the resultant film with UV light due to the reduction of Ag + by the excited electrons 

from TiO2 [95–97]. S<strong>in</strong>ce Ag nanoparticles absorb visible light of various wavelengths due 

to surface plasmon resonance <strong>and</strong> that the wavelength depends on local refractive <strong>in</strong>dex 

<strong>and</strong> particle size <strong>and</strong> shape [97–101], brownish-gray color of the as-prepared Ag/TiO2 film 

is ascribed to Ag nanoparticles with various sizes <strong>and</strong> shapes deposited <strong>in</strong> the nanopores of 

TiO 2 film [55–57]. The orig<strong>in</strong>al color can be substantially bleached <strong>in</strong> air upon visible-light 

irradiation due to the plasmon resonance transfer of electrons to oxygen either directly or 

via TiO 2 [56,102] <strong>and</strong> be restored aga<strong>in</strong> on UV-light illum<strong>in</strong>ation. This coloration–decoloration 

process is repeatable. Similar phenomenon has been observed <strong>in</strong> Pt/TiO2 system, 

<strong>in</strong> which color changes from pale blue-gray to pale brown with the <strong>in</strong>creas<strong>in</strong>g amount of 

Pt [86]. 

Interest<strong>in</strong>gly, the <strong>in</strong>itial brownish-gray color of the film changes under a colored visiblelight 

illum<strong>in</strong>ation to almost the same color as that of <strong>in</strong>cident light, which turns brownishgray 

aga<strong>in</strong> by irradiation with UV light (Fig. 4) [57]. Under monochromatic visible-light 

irradiation, the correspond<strong>in</strong>g Ag nanoparticles absorb light, <strong>and</strong> the electrons thus 

excited are accepted by O 2, result<strong>in</strong>g <strong>in</strong> oxidation of the Ag nanoparticles to Ag + ions 

<strong>and</strong> a decrease <strong>in</strong> absorbance at the correspond<strong>in</strong>g wavelength. As a result, only the light 

of the excitation wavelength is reflected or transmitted, while the rema<strong>in</strong><strong>in</strong>g particles 

absorb lights of all the other wavelengths [57]. Thus, the <strong>in</strong>itial color (brownish-gray) 

can change to almost the same color as that of the excitation light. If the generated 

Ag + ions are removed from the pores, the film reta<strong>in</strong>s its color even under UV light irradiation 

[55]. So the apparently uniform Ag/TiO 2 film can be almost any color <strong>and</strong> the role 

of TiO 2 is a repeatable generation of Ag nanoparticles with different absorption wavelengths. 

3 This multicolor photochromic behavior <strong>and</strong> chromogenic property can be 

controlled by regulat<strong>in</strong>g irradiation conditions as well as geometry <strong>and</strong> matrix <strong>materials</strong> 

of nanopores [55]. The formation of anisotropic Ag particles can be suppressed by a 

3 It should be noted that the chromogenic mechanism (photooxidation of Ag particles to Ag + ) of such k<strong>in</strong>d of 

multicolor—exhibit<strong>in</strong>g almost the same color as that of <strong>in</strong>cident monochromatic light—is different from that of 

conventional silver glass [103] or silver halide photography [104], <strong>in</strong> which silver halide is photoreduced to Ag 

particles.


Fig. 4. Multicolored Ag/TiO2 film. Photograph of multicolored spots (7 mm diameter) on the Ag/TiO2 film on a 

glass substrate irradiated successively with monochromatic lights (5 m<strong>in</strong> each). A xenon lamp <strong>and</strong> an UV-cut 

filter (block<strong>in</strong>g light below 400 nm) was used with a 450 nm (blue), 530 nm (green), 560 nm (yellowish-green), 

600 nm (orange) or 650 nm (red) b<strong>and</strong>pass filter (FWHM, 10 nm), or without any b<strong>and</strong>pass filter (white). Light 

<strong>in</strong>tensity, 10 mW cm 2 except for white light (50 mW cm 2 ) [57]. (For <strong>in</strong>terpretation of the reference <strong>in</strong> color <strong>in</strong> 

this figure legend, the reader is refered to the web version of this article.). 

dual-light irradiation (310 <strong>and</strong> 420 nm) for Ag deposition, result<strong>in</strong>g <strong>in</strong> the improvement of 

chromogenic properties [55]. It is claimed that the nanopores with various sizes <strong>and</strong> shapes 

present <strong>in</strong> TiO 2 films act as molds for Ag nanoparticles with various sizes <strong>and</strong> shapes, 

which is the pre-requisite to display various color [55]. S<strong>in</strong>ce the refractive <strong>in</strong>dex of surround<strong>in</strong>g 

<strong>materials</strong> can <strong>in</strong>fluence the surface plasmon resonance wavelength of Ag nanoparticles 

[105] <strong>and</strong> Ag nanoparticles are <strong>in</strong> contact not only with TiO2 but also with air 

[55], the ratio of TiO2 to air as well as the matrix <strong>materials</strong> can be used to tune this chromogenic 

property. For <strong>in</strong>stance, coat<strong>in</strong>g the Ag/TiO2 film with a silica or nitrocellulose 

film can change its spectrum [56]. 

It should be noted that it is <strong>in</strong>evitable that the color images displayed on the film can be 

bleached gradually <strong>in</strong> air by ambient white light due to photochromism. This photochromism 

as well as the rewritability of Ag/TiO 2 film can be temporarily deactivated by modification 

with octadecanethiol or fluorodecanethiol, which can be fully reactivated by 

sufficiently irradiat<strong>in</strong>g the film with UV light [56]. The possible reasons why the photooxidation 

of Ag nanoparticles was <strong>in</strong>hibited by thiols <strong>in</strong>clude the block of electron transfer to 

oxygen <strong>and</strong> water repulsion. The block<strong>in</strong>g effect might be more effective for the suppression 

of bleach<strong>in</strong>g. The reactivation is due to the decomposition of thiols by TiO2 

photocatalysis. 

2.2. Semiconductor/semiconductor <strong>composite</strong> <strong>materials</strong> 

2.2.1. Improved photochromism <strong>in</strong> WO3/MoO3 system 

In the <strong>composite</strong> semiconductor systems, WO3/MoO3 might be the first that one should 

consider s<strong>in</strong>ce both of them exhibit pronounced photochromic response. Furthermore, 

one disadvantage of us<strong>in</strong>g s<strong>in</strong>gle MoO3 or WO3 for display devices is that the absorption 

peak (MoO3, 1.56 eV; WO3, 1.4 eV) [31,106] does not match the peak of eye-sensitivity 

curve (at 2.25 eV). It is reported that the maximum optical absorption of WO 3/MoO 3 films 

can occur at 2.15 eV [106], which is significant because it is very close to the response of 

eyes. The <strong>composite</strong> films can be made by co-evaporation [52,106,107], chemical vapor


Fig. 5. The photochromic responses of (a) 92%WO3–8%MoO3, (b) WO3, <strong>and</strong> (c) MoO3 th<strong>in</strong> films <strong>in</strong> C2H5OH 

(vol%) <strong>in</strong> N2 [52]. 

deposition (CVD) [108] or sol–gel [109] methods. However, so far most researches are 

focused primarily on characterization or electrochromism of WO3/MoO3 <strong>composite</strong> films 

[71,106,108–114] <strong>and</strong> only few are pert<strong>in</strong>ent to the photochromism [52,75,115]. 

The photochromic responses of WO3/MoO3 th<strong>in</strong> films subject<strong>in</strong>g to UV irradiation are 

<strong>in</strong>vestigated <strong>in</strong> reduc<strong>in</strong>g environments, of which the spectra are shown <strong>in</strong> Fig. 5 [52]. WO3/ 

MoO3 <strong>composite</strong> films exhibit an enhanced photochromic absorption <strong>in</strong> ethanol vapor 

when compared to those of pure MoO 3 or WO 3 films. The associated change <strong>in</strong> absorbance 

of the <strong>composite</strong> film is ca. 1.5 times that of WO 3 film, <strong>and</strong> is ca. 2.7 times that 

of MoO3 film. The photochromic performance of the <strong>composite</strong> oxide is dependent on 

the constituent composition of the film. The largest photochromic response takes place 

<strong>in</strong> the <strong>composite</strong> WO3/MoO3 film that conta<strong>in</strong>s about 8 wt% MoO3 (Fig. 6) [52]. Except 

the enhanced absorption, a blue-shift (ca. 0.2 eV) <strong>in</strong> the photochromic absorption for the 

<strong>composite</strong> film is observed, which is similar to the blue-shift observed <strong>in</strong> the electrochromic 

experiments [71,106,108]. All these observations can be rationalized by the IVCT [106] 

or small-polaron absorption [116] <strong>in</strong> the <strong>composite</strong> oxide. 

When WO 3 or MoO 3 film is subjected to light irradiation, it is colored by a simultaneous 

<strong>in</strong>jection of protons <strong>and</strong> electrons to form bronze. The optical absorption is 

due to IVCT or small-polaron absorption arose from the transition of W 5+ to W 6+ 

(W ! W) or Mo 5+ to Mo 6+ (Mo ! Mo) between adjacent ions. In the <strong>composite</strong> oxide, 

another transition, Mo 5+ to W 6+ (Mo ! W), is believed to take place [106,107]. Itis 

assumed that the Mo levels are lower <strong>in</strong> energy than the correspond<strong>in</strong>g W levels. If the 

colored product of the <strong>composite</strong> oxide is schematically represented as HxW1 yMoyO3, 

electrons will be trapped at Mo sites <strong>and</strong> Mo ! Mo <strong>and</strong> Mo ! W transitions will occur 

when x < y; whereas when x > y, the Mo sites are saturated <strong>and</strong> Mo ! W <strong>and</strong> W ! W 

transitions can take place [117]. The energies required for the W ! W, Mo ! Mo <strong>and</strong> 

Mo ! W transitions have been calculated as 1.4, 1.54, <strong>and</strong> 2.13 eV, respectively [106]. 

So the Mo ! W transition corresponds to the highest energy <strong>and</strong> matches the peak of 

eye-sensitivity curve. This expla<strong>in</strong>s the observed blue-shift <strong>in</strong> the photochromic absorption 

of <strong>composite</strong> films, though the shift amplitude <strong>in</strong> the photochromism is smaller (ca. 0.2 eV) 

than that <strong>in</strong> the electrochromism (0.6 or 0.7 eV), which might due to the weak coloration 

<strong>in</strong> photochromism. Such a blue-shift may also exist <strong>in</strong> other mixed-metal oxides. Monk


Fig. 6. The photochromic responses of mixed WO3/MoO3 films of different compositions which were irradiated 

with UV light for 5 m<strong>in</strong> <strong>in</strong> 1.0 vol% C2H5OH <strong>in</strong> N2 [52]. 

et al. [118] have given a semi-empirical approach to correlate an optical-shift parameter S 

(via the frequency maximum of an optical b<strong>and</strong>) with the composition of the mixed-metal 

oxides (Eq. (1)), which might allow the colors of coloration-oxide mixtures to be tailored 

S ¼ mpure mmixture 

mpure 

In addition, Donnadieu et al. [108] have used the Hubbard–Mott model [119] to expla<strong>in</strong> 

this blue-shift. Accord<strong>in</strong>g to their <strong>in</strong>terpretation, the coloration is caused by the electron 

transition between the two Hubbard b<strong>and</strong>s separated by a pseudo-gap. The <strong>in</strong>troduction 

of Mo <strong>in</strong> the WO3 matrix can decrease the value of the <strong>in</strong>terelectron distance (the atomic 

radius of Mo, on which the electrons are preferentially trapped, be<strong>in</strong>g smaller than that of 

W). At the same time, the r<strong>and</strong>om distribution of Mo <strong>in</strong> WO3 matrix <strong>in</strong>creases the localization 

of electrons <strong>and</strong> the repulsion among them. Thus the <strong>in</strong>terb<strong>and</strong> separation 

<strong>in</strong>creases <strong>and</strong> the maximum of the absorption is shifted towards higher energies. The 

<strong>in</strong>creased disorder <strong>in</strong> the film due to the r<strong>and</strong>om distribution of Mo can also expla<strong>in</strong> 

the improved photochromic effect as <strong>in</strong> the case of electrochromism [108,120]. 

ð1Þ


Fig. 7. Photochromic responses of a 92%WO3–8%MoO3 th<strong>in</strong> film <strong>in</strong> four different alcohol vapors (vol%) 

measured at 800 nm: (a) ethanol; (b) methanol; (c) n-propanol; (d) iso-propanol [52]. 

The photochromic response of <strong>composite</strong> WO3/MoO3 films has also been tested <strong>in</strong> different 

alcohol vapors (Fig. 7) [52]. The largest photochromic response of the <strong>composite</strong> 

films is observed <strong>in</strong> ethanol, followed by methanol, n-propanol, <strong>and</strong> iso-propanol vapors, 

which is as the same order as that for MoO3 film [30,121]. In HCOOH vapor, WO3 exhibits 

the largest photochromic response, the <strong>composite</strong> oxide follows <strong>and</strong> MoO3 the least 

(Fig. 8) [52]. It is suggested [121] that the Mo sites <strong>in</strong> the <strong>composite</strong> film are responsible 

for the photochromic response to these alcohols <strong>and</strong> the W sites are responsible for the 

observed photochromic responses when exposed to HCOOH. So the photochromism of 

<strong>composite</strong> WO3/MoO3 th<strong>in</strong> films <strong>in</strong> alcohol vapors can potentially be used for chemical 

sens<strong>in</strong>g purpose. 

Fig. 8. Photochromic responses of (a) WO 3, (b) 92%WO 3–8%MoO 3, (c) MoO 3 th<strong>in</strong> films <strong>in</strong> HCOOH (vol%) <strong>in</strong> 

N2 [52].


2.2.2. Improved photochromism <strong>in</strong> other <strong>composite</strong>s conta<strong>in</strong><strong>in</strong>g WO3 

Apart from the enhanced photochromism of WO3/MoO3 <strong>composite</strong>, the photochromism 

of WO3 can be improved by the oxides of Ti [29,49,83,122–128], Nb[49], Ta[49], 

Zn [51], <strong>and</strong> Zr [49]. Here the photochromic performance <strong>and</strong> enhancement mechanism 

are demonstrated by us<strong>in</strong>g WO3/TiO2 system as an example. 

Fig. 9 [29] shows the typical UV–Vis absorption spectra of WO 3 <strong>and</strong> WO 3/TiO 2 colloids. 

Strong absorption appears at short wavelengths, correspond<strong>in</strong>g to the fundamental 

semiconductor b<strong>and</strong> gap. Upon irradiation with UV light, a typical absorption peak 

centered around 900 nm appears due to the absorption of electrons trapped at the energy 

levels with<strong>in</strong> the forbidden gap of WO3 [29]. After a 3-m<strong>in</strong> UV-light irradiation, WO3/ 

TiO2 colloids turn deep blue <strong>in</strong> color, while WO3 colloids turn very light bluish which 

can hardly be perceived by naked eyes. The experimental results (<strong>in</strong>sets <strong>in</strong> Fig. 9) [29] <strong>in</strong>dicate 

that DOD at 900 nm <strong>in</strong>creases with the <strong>in</strong>creased concentration of TiO2 no matter 

there is a hole scavenger (H 2C 2O 4) present or not <strong>and</strong> the maximum enhancement amplitude 

can reach more than 100-fold. Similar results have also been observed <strong>in</strong> WO 3/TiO 2 

<strong>composite</strong> films [126] <strong>and</strong> <strong>in</strong> WO 3/SrTiO 3 system [129]. Among the oxides of Nb, Ta, Ti, 

<strong>and</strong> Zr, it is claimed [49] that Zr has the strongest enhancement effect on the photochromic 

properties of WO3 th<strong>in</strong> films, which is attributed to the Zr effect on the WO3 structure. 

Although now it is widely accepted that the photochromic performance of WO3 can be 

improved by TiO2, a controversy still exists about the mechanism among different authors. 

S<strong>in</strong>ce the optical coloration <strong>in</strong> WO3 is electronic <strong>in</strong> nature [22], all the authors agree that 

this enhancement should be <strong>in</strong>terpreted <strong>in</strong> terms of the electron energy levels <strong>in</strong> these 

<strong>materials</strong>. The process might <strong>in</strong>volve the transfer of electrons at the <strong>in</strong>terface if the work 

functions are comparable. This is <strong>in</strong>deed the case as confirmed by some authors 

[29,83,123,124]. 

Fig. 9. UV–Vis spectra of (A) WO3/TiO2 <strong>and</strong> (B) WO3 colloids. (—) Virg<strong>in</strong> state; (---) after UV-light irradiation 

for 3 m<strong>in</strong>. The <strong>in</strong>sets show the dependence of DOD at 900 nm on the concentration of TiO 2 for the systems with 

(<strong>in</strong>set <strong>in</strong> A) <strong>and</strong> without (<strong>in</strong>set <strong>in</strong> B) oxalic acid [29].


Fig. 10. Schematic diagram represent<strong>in</strong>g the energy levels of WO 3 <strong>and</strong> TiO 2, <strong>and</strong> charge (photogenerated 

electrons <strong>and</strong> holes) <strong>in</strong>teractions between them. 

S<strong>in</strong>ce the relative positions of the valence b<strong>and</strong> <strong>and</strong> conduction b<strong>and</strong> of WO 3 are lower 

than those of TiO2 [29,83,130,131], when they comb<strong>in</strong>ed together a heterojunction is 

formed at the <strong>in</strong>terface of WO3 <strong>and</strong> TiO2 (Fig. 10). Electrons <strong>and</strong> holes can be generated 

<strong>in</strong> both WO3 <strong>and</strong> TiO2 under UV-light irradiation s<strong>in</strong>ce both of them are good photoresponsive 

<strong>materials</strong>. The photogenerated electrons aris<strong>in</strong>g from TiO2 are <strong>in</strong>jected <strong>in</strong>to the 

conduction b<strong>and</strong> of WO3, whereas the holes orig<strong>in</strong>at<strong>in</strong>g from WO3 move to the valence 

b<strong>and</strong> of TiO2. The holes can oxidize the adsorbed species, such as water or organic residue, 

form<strong>in</strong>g protons that then migrate <strong>in</strong>to WO 3 gra<strong>in</strong>s through diffusion or Coulombic 

attraction. Blue-colored tungsten bronze is produced by the reaction of WO 3 with the 

protons <strong>and</strong> conduction-b<strong>and</strong> electrons. In the meantime, electrons <strong>in</strong> the valence b<strong>and</strong> 

of WO3 can also be excited to the conduction b<strong>and</strong> of WO3 <strong>and</strong> contribute, possibly 

the ma<strong>in</strong> contribution, to the photochromism. However, Chopoorian et al. [123] have suggested 

that UV light is absorbed by TiO2, which <strong>in</strong> turn, photoactivates WO3. This difference 

might be due to that the oxides used by them have different b<strong>and</strong> gap energy <strong>and</strong> 

energy levels, which is a normal case for the same samples with different preparation 

<strong>and</strong> (pre)treatment methods [30,31,117]. In addition, the samples used by Chopoorian 

et al. are sodium (poly)tungstates, whereas the one used by others is WO 3. 

Another enhancement mechanism has been put forward for the photochromism of 

WO3 colloids comb<strong>in</strong>ed with TiO2 nanoparticles [29,51], <strong>in</strong> which no hydrogen bronze 

is formed after coloration <strong>based</strong> on the follow<strong>in</strong>g two arguments. (i) The st<strong>and</strong>ard redox 

potential of hydrogen tungsten bronze (ca. 0.29 V vs. NHE) [132] is <strong>in</strong>sufficiently negative 

for the photogenerated electrons to reduce WO3 <strong>in</strong>to the bronze [133]. (ii) Raman 

spectra are almost identical before <strong>and</strong> after UV-light coloration for both WO3 <strong>and</strong> 

WO 3/TiO 2 colloids [29]. It is suggested that [29,94,131,133,134] the coloration of WO 3 colloids 

are caused by electrons trapped at energy levels with<strong>in</strong> the forbidden gap of WO 3. 

These trapped electrons are metastable <strong>in</strong> air <strong>and</strong> are rather stable <strong>in</strong> an <strong>in</strong>ert atmosphere 

[29,133], which can be optically excited <strong>in</strong>to higher energy levels, lead<strong>in</strong>g to a broad 

absorption <strong>in</strong> red-IR region (Fig. 9) [29]. Thus the colloids turn blue under UV-light irradiation. 

When WO3 <strong>and</strong> TiO2 comb<strong>in</strong>e together, similarly, a heterojunction is also formed 

at the <strong>in</strong>terface (Fig. 10). The positive holes created <strong>in</strong> WO3 can migrate to the valence 

b<strong>and</strong> of TiO2 particles due to a lower valence b<strong>and</strong> of WO3 particles, <strong>and</strong> may be trapped


at the surface thereafter. At the same time, most photogenerated electrons (orig<strong>in</strong>ated 

from both WO3 <strong>and</strong> TiO2) will transfer to <strong>and</strong> be trapped at the energy levels with<strong>in</strong> 

the forbidden gap of WO3. The results of surface photovoltage have confirmed that electrons 

can really transfer from TiO2 to WO3 <strong>in</strong> the <strong>composite</strong> system [29]. So the electrons 

<strong>and</strong> holes orig<strong>in</strong>ally generated <strong>in</strong> both WO3 <strong>and</strong> TiO2 particles are separated more efficiently, 

result<strong>in</strong>g <strong>in</strong> suppress<strong>in</strong>g the recomb<strong>in</strong>ation. This suppression has been proved 

by the results of fluorescence spectra [29,124]. Consequently, <strong>in</strong> the <strong>composite</strong> system 

much more electrons (not only from WO 3 but also from TiO 2) will be trapped <strong>in</strong> the b<strong>and</strong> 

gap of WO3 <strong>and</strong> contribute to the coloration process, lead<strong>in</strong>g to an improved photochromism 

of WO3 colloids. In addition, it has been reported [124] that TiO2 prepared by a 

photoassisted method shows stronger enhancement effects than that prepared by conventional 

sol–gel technique due to the promoted suppression of recomb<strong>in</strong>ation, a narrowed 

b<strong>and</strong> gap for the former, <strong>and</strong> so on. The enhancement mechanism of TiO2 is further supported 

by the comb<strong>in</strong>ation of WO 3 with SnO 2 or ZnO. Similar to TiO 2, ZnO improves the 

photochromism of WO 3 s<strong>in</strong>ce its conduction <strong>and</strong> valence b<strong>and</strong>s are higher than those of 

WO 3 so that electrons can transfer from ZnO to WO 3 [51]. However, SnO 2 cannot 

improve the photochromism of WO3 due to its lower conduction <strong>and</strong> valence b<strong>and</strong>s than 

WO3, <strong>in</strong> which electrons transfer from WO3 to SnO2 [29]. 

In addition, it is noted that [127], if the th<strong>in</strong> films are prepared by aerosol-assisted 

chemical vapor deposition (AACVD), the layered WO3/TiO2 films show significant photochromism, 

while comparable thickness titanium-doped WO3 films show reduced photochromism, 

proportional to the level of Ti dop<strong>in</strong>g (Fig. 11) [127]. Compared with the 

aforementioned WO 3/TiO 2 systems, the <strong>in</strong>hibited effects of Ti dopant <strong>in</strong> AACVD films is 

due to Ti <strong>in</strong>corporation <strong>in</strong>to the WO 3 lattice (the substitution of Ti for W) dur<strong>in</strong>g the preparation, 

i.e., a solid solution of titanium <strong>and</strong> tungsten oxides, rather than the presence of 

<strong>in</strong>terstitial Ti. In fact, no TiO2 peaks appear <strong>in</strong> XRD patterns for titanium-doped WO3 

Fig. 11. Change <strong>in</strong> optical density aga<strong>in</strong>st irradiation time for 1: a doped metal oxide film with metal atom% ratio 

of W:Ti 82:18, 2: a doped metal oxide film with metal atom% ratio of W:Ti 92:8, 3: a TiO 2 underlayer with a WO 3 

overlayer, 4: an undoped WO3 film, all produced by aerosol-assisted CVD, under 254 nm irradiation [127].


films [127]. This <strong>in</strong>dicates that the synthesis method is very important for prepar<strong>in</strong>g such 

k<strong>in</strong>d of <strong>composite</strong> <strong>materials</strong>. 

WO3 <strong>and</strong> TiO2 can also be comb<strong>in</strong>ed together to fabricate a photoelectrochromic system, 

<strong>in</strong> which WO3 film <strong>and</strong> dye-sensitized TiO2 film form the two electrodes of an electrochemical 

cell [135]. The resultant structure exhibits coloration under photoirradiation. Unlike 

conventional photochromic films, the light-absorption process, performed by a ruthenium 

polypyrid<strong>in</strong>e-sensitized nanocrystall<strong>in</strong>e TiO 2 electrode, is separate from the coloration process 

(<strong>in</strong> WO 3 film). Light absorption by the sensitiz<strong>in</strong>g dye leads to electron <strong>in</strong>jection <strong>in</strong>to 

the TiO2 film. When the electrodes are short-circuited, the electrons move from TiO2 

through external circuit to the WO3 film where, <strong>in</strong> the presence of cations small enough 

to <strong>in</strong>tercalate <strong>in</strong>to the WO3 lattice, a bluish-colored tungsten bronze is formed. This can 

be used as the so-called self-powered smart w<strong>in</strong>dow. As a matter of fact, it is the electrochromism 

of WO3 driven by a dye-sensitized solar cell. 

2.2.3. Improved photochromism <strong>in</strong> MoO 3/TiO 2 system 

MoO 3/TiO 2 is another promis<strong>in</strong>g <strong>composite</strong>, which can be prepared by reactive DC 

magnetron sputter<strong>in</strong>g [136] <strong>and</strong> sol–gel related methods [50,123,137]. The effects of TiO2 

on the photochromic behavior of MoO3 are similar to those on WO3 [50,123,136–138]. 

It is said [137] this colored <strong>composite</strong> may exhibit a more neutral color than the pure constituents. 

Kullman et al. [136] have reported that two peaks appear <strong>in</strong> the absorption spectra 

for the photochromatically <strong>in</strong>duced MoO3/TiO2 samples, which shift <strong>in</strong> <strong>in</strong>tensity, 

position, <strong>and</strong> width with changes <strong>in</strong> the composition (Fig. 12) [136]. The absorption is 

strongest <strong>in</strong> the most Mo-rich film; the pure Ti oxide film shows no photochromism at 

all under these conditions. At wavelengths exceed<strong>in</strong>g ca. 500 nm there is an absorption 

maximum, which is split <strong>in</strong>to two components for <strong>in</strong>termediate Mo/Ti ratios. One absorption 

b<strong>and</strong> lies at the red region (ca. 750 nm), which is <strong>in</strong> good agreement with the data on 

pure molybdenum oxide [30,31], whereas another is at ca. 570 nm. The former b<strong>and</strong> has 

been expla<strong>in</strong>ed by the small-polaron absorption due to the electron hopp<strong>in</strong>g between similar 

or different transition-metal sites [136]. But no explanation has been given for the latter 

absorption. Here we suggest that, similar to the WO 3/MoO 3 system (cf. Section 2.2.1), the 

former absorption b<strong>and</strong> might be due to the Mo ! Mo or Ti ! Ti transition <strong>and</strong> the latter 

due to the Mo ! Ti transition. However, this hypothesis needs further experimental 

evidences. In addition, It is reported that [123] the samples conta<strong>in</strong><strong>in</strong>g TiO2 undergo a considerably 

more rapid coloration than the blank, <strong>and</strong> a range of 30–35% TiO2 is the most 

effective <strong>in</strong> the coloration rate. 

Similar to the case <strong>in</strong> WO3/TiO2 system, a heterojunction is formed at the <strong>in</strong>terface of 

MoO3 <strong>and</strong> TiO2 when they are comb<strong>in</strong>ed together s<strong>in</strong>ce the relative positions of the 

valence b<strong>and</strong> <strong>and</strong> conduction b<strong>and</strong> of MoO 3 are more positive or less negative (vs. 

NHE) than those of TiO 2 [50,137]. Efficient electrons <strong>in</strong>jected <strong>in</strong>to the conduction b<strong>and</strong> 

of MoO 3 orig<strong>in</strong>ate not only from MoO 3 but also from TiO 2 under UV-light irradiation. 

As <strong>in</strong> the case of WO3/TiO2, however, Chopoorian et al. [123] have suggested that UV 

light is absorbed by TiO2, which <strong>in</strong> turn, photoactivates the MoO3. This difference might 

be caused by the different samples used <strong>in</strong> different groups, sodium (poly)molybdates for 

Chopoorian [123] <strong>and</strong> MoO3 for others [50,137]. 

It has been po<strong>in</strong>ted out [138] that the transfer of hydrogen atoms between two mechanically 

mixed particulate TiO 2 <strong>and</strong> MoO 3 can take place <strong>in</strong> the suspension system, from 

which the enhancement effect might also be benefited. In addition, s<strong>in</strong>ce the adsorbed


Fig. 12. Spectral absorptance for Mo–Ti oxide films with different compositions measured <strong>in</strong> as-deposited state 

(triangles) <strong>and</strong> after photochromic coloration by UV irradiation <strong>in</strong> air with ethanol vapor for 18 h (squares) <strong>and</strong> 

24 h (circles). The films were deposited onto uncoated glass substrates [136]. 

water plays an important role <strong>in</strong> the photochromism of TMOs films [30,31], it might be 

mean<strong>in</strong>gful to <strong>in</strong>vestigate the changes <strong>in</strong> the amount of adsorbed water after the comb<strong>in</strong>ation. 

Although surface hydroxyl groups or surface acidity have been reported to change 

greatly <strong>in</strong> WO3/TiO2 <strong>and</strong> MoO3/TiO2 systems [139–142], there are no reports on the correlation 

between the photochromism <strong>and</strong> changes <strong>in</strong> surface acidity.


2.2.4. Visible-light coloration <strong>in</strong> MoO3/TiO2 system 

Visible-light coloration is of great importance for efficient utilization of solar energy 

<strong>and</strong> laser sources that cover a broad wavelength from mid-IR to blue. For all TMOs 

except V2O5, however, the energy of the electromagnetic irradiation hm, which <strong>in</strong>duces 

the formation of the colored (activated) species, is usually situated <strong>in</strong> the near-UV range 

of spectrum (correspond<strong>in</strong>g to its optical b<strong>and</strong> gap) [31]. In order to extend the coloration 

photoresponse to visible-light region, the spectral sensitization for these wide b<strong>and</strong> gap 

TMOs, similar to the dye-sensitized solar cell [130,143], becomes a crucial practical 

problem [42]. For MoO3 <strong>and</strong> WO3 th<strong>in</strong> films, visible-light coloration can be <strong>in</strong>duced 

by a slightly cathodic polarization <strong>in</strong> non-aqueous solution [7,36], by a comb<strong>in</strong>ation with 

CdS [23,53,54,144,145] or with TiO2 [50,136]. The first case is out of the scope of present 

review <strong>and</strong> the second will be discussed <strong>in</strong> Section 2.2.5. 

It is reported that the photochromically colored MoO3/TiO2 <strong>composite</strong>s show an onset 

of absorption at a lower energy (<strong>in</strong> the visible-light range) than the prist<strong>in</strong>e films [50,136]. 

Kullman et al. [136] have ascribed it to the modifications of electronic density of states, 

either by charge <strong>in</strong>corporation or by changes <strong>in</strong> the crystall<strong>in</strong>e order. A more detailed 

mechanism has been put forward by Elder et al. [50] to elucidate the visible-light coloration 

<strong>in</strong> nanocrystall<strong>in</strong>e TiO2–(MoO3) core–shell <strong>materials</strong>. 

A series of nanocrystall<strong>in</strong>e TiO2–(MoO3) core–shell <strong>materials</strong> have been synthesized by 

a co-nucleation of metal-oxide clusters at the surface of surfactant micelles [50,146]. The 

nanocrystall<strong>in</strong>e TiO2 (anatase) phase is chemically bonded to MoO3 phase through a heterojunction 

formed at an <strong>in</strong>terface between them. The calc<strong>in</strong>ed TiO 2–(MoO 3) x powders 

display a variety of colors rang<strong>in</strong>g from gray-green to green as a function of MoO 3 content. 

A significant <strong>and</strong> <strong>in</strong>terest<strong>in</strong>g th<strong>in</strong>g is that all samples turn blue or black when excited 

with visible light (ca. 420–460 nm, i.e., 2.88–2.60 eV). This energy is lower than the b<strong>and</strong> 

gap of both MoO3 (2.9 eV) <strong>and</strong> TiO2 (3.2 eV), correspond<strong>in</strong>g to the energy required to 

excite TiO2-core valence b<strong>and</strong> electrons to MoO3-shell conduction b<strong>and</strong> states. This 

energy is correlated with both the nanoparticle size <strong>and</strong> the degree of chemical <strong>in</strong>teraction 

between the TiO2 core <strong>and</strong> the MoO3 shell. When the size of TiO2–(MoO3) nanoparticles 

changes from 8 to 4 nm, it decreases from 2.88 to 2.60 eV with decreas<strong>in</strong>g particle size. 

This is contrast to the most core–shell semiconductor nanoparticles, for which the b<strong>and</strong> 

gap (which is a function of both size quantization effects <strong>and</strong> the relative composition 

of the core–shell particle, i.e., relative thickness of the core <strong>and</strong> shell) <strong>in</strong> the limit<strong>in</strong>g case 

is greater than or equal to the smallest b<strong>and</strong> gap material compris<strong>in</strong>g the core–shell system 

<strong>and</strong> a photoabsorption-energy blue-shift (relative to the b<strong>and</strong> gap energies of the bulk 

<strong>materials</strong>) takes place when the core–shell particle size is <strong>in</strong> the quantum regime (i.e., core 

diameter or shell thickness equal to or smaller than the Bohr radius of a valence/conduction 

b<strong>and</strong> electron) [147,148]. 

The systematic red-shift exhibited by the TiO 2–(MoO 3) x core–shell <strong>materials</strong> is ascribed 

to the change <strong>in</strong> the relative position of the MoO 3-shell conduction b<strong>and</strong> as it evolves from 

less than a monolayer to a two monolayer shell. The optical absorption properties exhibit<strong>in</strong>g 

by the TiO2–(MoO3)x <strong>materials</strong> are due to the charge-transfer (CT) processes at semiconductor 

heterojunction. S<strong>in</strong>ce the chemical bond<strong>in</strong>g between the TiO2 core <strong>and</strong> the 

MoO3 shell is present <strong>in</strong> the system [149], the core–shell wave functions are allowed to 

overlap at the <strong>in</strong>terface, giv<strong>in</strong>g rise to a heterojunction b<strong>and</strong> structure (Fig. 13) [50]. 

The lowest energy excitation is from the TiO 2 valence b<strong>and</strong> to the MoO 3 conduction b<strong>and</strong>, 

acore! shell charge transfer, <strong>and</strong> this energy closely matches the experimental result.


Fig. 13. Arrangement of the TiO2 core <strong>and</strong> MoO3 shell valence b<strong>and</strong>s (VB) <strong>and</strong> conduction b<strong>and</strong>s (CB) for TiO2– 

(MoO 3) 1.8 after heterojunction formation [50]. 

This electronic transition is allowed because of the reduced symmetry at the core–shell 

<strong>in</strong>terface. So the obta<strong>in</strong>ed series of TiO2–(MoO3)x compounds are not a simple l<strong>in</strong>ear comb<strong>in</strong>ation 

of those of the nanocrystall<strong>in</strong>e TiO2 core <strong>and</strong> MoO3 shell, but <strong>in</strong>stead entirely 

new photophysical properties are observed as a result of the core–shell nanoarchitecture 

<strong>and</strong> the electronic transitions this structure supports. 

2.2.5. Visible-light coloration <strong>in</strong>duced by CdS 

Two methods have been reported about the construction of a double-layer CdS/WO3 or 

CdS/MoO3 structure. One is that a polycrystall<strong>in</strong>e CdS th<strong>in</strong> layer is synthesized us<strong>in</strong>g the 

chemical bath deposition technique <strong>and</strong> deposited onto the substrate surface, followed by 

the deposition of an amorphous th<strong>in</strong> film of WO3 or MoO3 by thermal evaporation technique 

[23,53,54]. Another is prepared by sequential evaporation of CdS <strong>and</strong> nanostructured 

MoO 3 films us<strong>in</strong>g thermal <strong>and</strong> activated-reactive evaporation, respectively [145]. 

Fig. 14 [145] presents the optical absorption spectra recorded for MoO 3 <strong>and</strong> CdS/MoO 3 

samples illum<strong>in</strong>ated with a tungsten lamp at different durations. The absorption spectra 

<strong>in</strong>dicate the presence of an absorption b<strong>and</strong> from 500 to 1100 nm centered ca. 850 nm 

due to the formation of color centers. The absorption edge at ca. 500 nm for CdS/ 

MoO3 is due to the CdS ma<strong>in</strong> absorption b<strong>and</strong>, whereas the one at ca. 400 nm for 

MoO3 is due to the ma<strong>in</strong> absorption b<strong>and</strong> of MoO3. Although the shape of this absorption 

b<strong>and</strong> can be expected to be Gaussian from the IVCT model [145], the asymmetric shape is 

<strong>in</strong>terpreted as <strong>in</strong>dicat<strong>in</strong>g the presence of potential fluctuations <strong>in</strong> amorphous films [107]. 

For MoO 3 sample, the color center b<strong>and</strong> appears only <strong>in</strong> the spectrum of the sample irradiated 

for ca. 40 m<strong>in</strong>, <strong>and</strong> the longer irradiation time notably decreases the color center 

concentration [53,54,145]. For CdS/MoO3, however, longer illum<strong>in</strong>ation times produce 

stronger b<strong>and</strong> <strong>in</strong>tensities due to the semiconductor characteristics <strong>and</strong> photosensitivity 

of CdS [150]. Obviously, the <strong>in</strong>tensity of the broad absorption b<strong>and</strong> for CdS/MoO3 is


Fig. 14. The optical absorption spectra of as-deposited <strong>and</strong> irradiated (A) MoO3, (B) MoO3/CdS, <strong>and</strong> (C) MoO3/ 

In:CdS films at different times. Dashed l<strong>in</strong>es <strong>in</strong> (A) represent the Gaussian approximation used to evaluate color 

center concentration [145]. 

more pronounced than that for MoO3, <strong>in</strong>dicat<strong>in</strong>g the enhancement of the color-center 

concentration by us<strong>in</strong>g CdS <strong>in</strong>terlayer. In addition, the photochromic response is more


effective if In-doped CdS is used as the <strong>in</strong>terlayer to modify MoO3 (Fig. 14C) [145] s<strong>in</strong>ce In 

acts as a donor provid<strong>in</strong>g more free electrons to the conduction b<strong>and</strong> by photoexcitation 

[145,151]. 

The mechanism of this visible-light coloration phenomenon, which is <strong>in</strong>terpreted <strong>in</strong> 

terms of charge carrier <strong>in</strong>jection from CdS <strong>in</strong>to TMO film, is different from the one 

<strong>in</strong>duced by cathodic polarization [7,36,85]. Under visible-light irradiation, electron–hole 

pairs will be produced <strong>in</strong> CdS layer s<strong>in</strong>ce it is a narrow-b<strong>and</strong> gap semiconductor 

(Eg 6 2.6 eV) (Fig. 15) [23]. Because CdS <strong>and</strong> TMOs have different energy levels, the 

photogenerated electrons transfer to the conduction b<strong>and</strong> of TMOs, whereas the holes 

to the <strong>in</strong>terface [23,53,54,152]. S<strong>in</strong>ce water undergoes decomposition by holes at the <strong>in</strong>terface 

of CdS/TMO <strong>in</strong> order to generate protons <strong>in</strong>to the TMO layer [23], the blue-colored 

bronze is formed by the reaction of TMO with these electrons <strong>and</strong> protons. Due to the 

presence of O 2 

<strong>in</strong> the S 2 vacancy sites <strong>in</strong> CdS layers prepared by chemical bath deposi- 

tion [53,54], oxygen atoms migrate to the <strong>in</strong>terface of CdS/TMO under the photoirradiation, 

leav<strong>in</strong>g two electrons bounded to the sulfur vacancy. These oxygen atoms can 

immediately react with each other or with the oxygen liberated by light-<strong>in</strong>duced decomposition 

of water at that <strong>in</strong>terface to produce the more stable O2 molecules. This process 

guarantees a more efficient diffusion of protons <strong>in</strong>to the TMO volume. 

2.3. Photochromic <strong>materials</strong> with dopants 

2.3.1. Doped TiO 2 systems 

Although TiO 2 can be colored by b<strong>and</strong> gap excitation due to the trapp<strong>in</strong>g of electrons 

as Ti 3+ species (bulk <strong>and</strong>/or surface Ti 3+ ) [31,32], photochromism is hardly observed <strong>in</strong> the 

absence of a hole scavenger <strong>in</strong> s<strong>in</strong>gle crystals at room temperature [31], <strong>in</strong> bulk TiO2 

Fig. 15. Coloration rate g as a function of the light exposure wavelength for a bare WO 3 film (open symbols) <strong>and</strong> 

a CdS-WO 3 bilayer (closed symbols). The coloration rate is normalized to the <strong>in</strong>cident light <strong>in</strong>tensity as a function 

of the <strong>in</strong>cident wavelength <strong>and</strong> is determ<strong>in</strong>ed as the slope of the obta<strong>in</strong>ed curves of optical density vs. illum<strong>in</strong>ation 

time. For the bare WO3 film, no photochromism is detected when the <strong>in</strong>cident wavelength is greater than 380 nm; 

whereas there is a strong <strong>in</strong>crease of the coloration rate for the CdS/WO 3 bilayer at an onset of 525 nm, which 

roughly corresponds to the b<strong>and</strong> gap energy of CdS [23].


particles [18,153], or <strong>in</strong> a dried gel [154]. Moreover, the photoformed Ti 3+ species are not 

stable <strong>and</strong> are easily quenched <strong>in</strong> the presence of O2. In order to improve the photochromism 

of TiO2, some elements or their oxides are usually used as dopants. 

Fe is the most frequently <strong>in</strong>vestigated dopant for TiO2 <strong>and</strong> photochromism has been 

widely observed <strong>in</strong> Fe-doped rutile [18,153,155–163]. Many authors [153,160–163] agree 

that Fe-doped rutile has a pale yellow color <strong>in</strong> the powdered form, <strong>and</strong> that irradiation 

with UV light superimposes a p<strong>in</strong>kish-brown t<strong>in</strong>ge upon its orig<strong>in</strong>al color, that is reversible, 

<strong>and</strong> bleaches <strong>in</strong> the dark. However, it is noted that a blue photochromic color has also 

been reported [158]. In addition, a (dark subdued) flesh color, depend<strong>in</strong>g on the light 

<strong>in</strong>tensity, has been obta<strong>in</strong>ed <strong>in</strong> Fe-doped anatase type TiO2 [164]. TiO2 also shows 

photochromism when small amounts of some other elements (Co, Cr, Cu, Mn, Mo, 

Nb, Ni, P, Si, V, W, Zn, <strong>and</strong> certa<strong>in</strong> rare earth metals) or their oxides are added to it 

[153,156,157,159,165,166]. Weyl <strong>and</strong> Förl<strong>and</strong> [160] have reported that strongly photochromic 

TiO 2 has been obta<strong>in</strong>ed by <strong>in</strong>corporat<strong>in</strong>g <strong>in</strong>to the rutile a comb<strong>in</strong>ation of the oxides of 

Fe, Nb <strong>and</strong> Ta, <strong>in</strong> which TiO 2 darkens when exposed to light <strong>and</strong> the tan-to-brown color 

fades <strong>in</strong> the dark. McTaggart <strong>and</strong> Bear [153] have claimed that Ni, Cr, <strong>and</strong> Cu are shown 

to give rise to marked effects, while slight effects are caused by Co, Mn, <strong>and</strong> certa<strong>in</strong> rare 

earths (Nd, Pr, Sm, etc.). Karv<strong>in</strong>en [157] has studied the effects of trace element (Cr, Fe, K, 

Nb, P, Si, V, <strong>and</strong> Zn) dop<strong>in</strong>g on the optical properties of nanostructured TiO2. Fe-, V-, or 

Si-doped samples show better photochromism than the undoped ones, whereas Cr-, P-, 

<strong>and</strong> Nb-doped samples exhibit the comparable photochromism to the undoped ones. V 

is the most effective dop<strong>in</strong>g element <strong>in</strong> mak<strong>in</strong>g anatase photochromic, followed by Si, further 

followed by Fe <strong>and</strong> Nb. Most authors have reported optimum impurity levels for the 

photochromic effect at about 0.2% by weight. Remy [159] has claimed that the best result 

about the photochromism of doped-TiO2 has been obta<strong>in</strong>ed with a 0.57 mol% Fe-doped 

sample calc<strong>in</strong>ed at 873 K among the dop<strong>in</strong>g elements of Cr, Co, Cu, Fe, Mn, Mo, <strong>and</strong> Ni. 

Some authors [160,163] have regarded the photochromism of doped-TiO2 as a bulk 

effect due to the impurities <strong>in</strong> lattice. Iron is an effective impurity <strong>in</strong> TiO2 s<strong>in</strong>ce Fe 3+ 

has an ionic radius close to that of Ti 4+ <strong>and</strong> distortion of the lattice occurs when the impurity 

ions fit <strong>in</strong>to a Ti vacancy [17]. When light strikes an impurity ion, for <strong>in</strong>stance, Fe 3+ <strong>in</strong> 

a TiO 2 lattice, an electron is excited from the foreign ion <strong>and</strong> moves either <strong>in</strong>to an oxygen 

vacancy of the defective rutile structure, thus produc<strong>in</strong>g a chromophoric Fe 4+ ion, or the 

electron attaches itself to a Ti 4+ ion to give a colored Ti 3+ [160]. 

Makovskii [158] has also considered the photochromism of doped-TiO2 as a bulk effect, 

but proposed a different mechanism. The change of color from light yellow to blue of rutile 

conta<strong>in</strong><strong>in</strong>g Fe under illum<strong>in</strong>ation is due to the transition of Ti <strong>and</strong> Fe from the tetravalent 

to the trivalent state (Eq. (2), VO represents an oxygen vacancy) [158]. Oxygen vacancies 

are formed <strong>in</strong> TiO 2 with Fe, <strong>and</strong> the conduction electrons, which associated to form the F 

centers, are responsible for the blue color [158]. 

Coloration 

Ti 4þ –VO–ð2eÞ–Fe 4þ ƒƒƒƒ! ƒƒƒƒ Ti 3þ –VO–Fe 3þ 

Bleach<strong>in</strong>g 

S<strong>in</strong>ce most work has been carried out on small TiO 2 particles, McTaggart <strong>and</strong> Bear 

[153] have recognized the <strong>in</strong>fluence of high specific area of this material on its optical 

properties <strong>and</strong> put forward that the photochromic effect is ma<strong>in</strong>ly due to a surface photoreaction 

(i.e., impurity is adsorbed at the surface or <strong>in</strong>terface) with an optimum sample 

treatment temperature of about 300 °C. It is noted that the impurities <strong>in</strong>vestigated have 

ð2Þ


one property <strong>in</strong> common, i.e., two or more valency states. Moreover, a fully oxidized form 

is usually characteristically darker <strong>in</strong> color than a lower form. Upon irradiation, a stable 

low-valence impurity ion on TiO2 surface is oxidized to a colored high-valence state [153]. 

A high-valence form of the ion reverts to a low-valence form with<strong>in</strong> m<strong>in</strong>utes to weeks upon 

cessation of irradiation [153]. 

Clark <strong>and</strong> Broadhead [155] have suggested, s<strong>in</strong>ce TiO 2 is known to show strong surface 

photoreactions, photochromism <strong>in</strong> powdered samples may be due to a comb<strong>in</strong>ation of 

bulk <strong>and</strong> surface mechanism. In fact, the possible photochromic effects occurr<strong>in</strong>g <strong>in</strong> 

doped-TiO2 can be caused by a variety of different mechanisms depend<strong>in</strong>g on sample history 

<strong>and</strong> environment [155]. In Fe-doped rutile particles, the p<strong>in</strong>kish-brown coloration is 

not due to Ti 3+ but due to Fe 2+ –VO (VO signifies a Fe-adjacent anion vacancy) color center 

[155]. Irradiation of the Fe-doped crystal with light of photon energy greater than that 

required to lift the electron from the valence b<strong>and</strong> to Fe 3+ –VO centers will generate Fe 2+ – 

V O centers <strong>and</strong> the majority of the resultant holes are trapped by Fe 3+ ions on sites without 

an adjacent vacancy. It is proposed [155] that the optical absorption of Fe 2+ –V O centers 

is stronger than that caused by Fe 3+ –V O centers, so that irradiation causes an <strong>in</strong>crease 

<strong>in</strong> absorption. The optical absorption of Fe 2+ –VO system will be a comb<strong>in</strong>ation of crystal 

field absorption with<strong>in</strong> the deformed ion <strong>and</strong> CT processes <strong>in</strong>volv<strong>in</strong>g optical excitation of 

an electron from the center to the conduction b<strong>and</strong>. Bleach<strong>in</strong>g is a comb<strong>in</strong>ation of optical 

<strong>and</strong> thermal release of electrons from Fe 2+ –VO centers <strong>in</strong>to conduction b<strong>and</strong> followed by 

recomb<strong>in</strong>ation with the trapped holes, <strong>and</strong> direct recomb<strong>in</strong>ation between the Fe 2+ –VO 

centers <strong>and</strong> non-trapped holes [155]. 

Metal ions doped <strong>in</strong>to TiO 2 <strong>in</strong>fluence the charge-carrier recomb<strong>in</strong>ation <strong>and</strong> <strong>in</strong>terfacial 

CT rates of photogenerated carriers [32,93,167]. The relative efficiency of a metal ion dopant 

depends on whether it serves as a mediator of <strong>in</strong>terfacial charge transfer, or as a 

recomb<strong>in</strong>ation center [93,157]. Enhanced <strong>in</strong>terfacial charge transfer <strong>in</strong> the presence of 

effective dopants appears to be the most important factor <strong>in</strong> the enhancement of photoreactivity 

of doped TiO2, which is relevant to the dopant concentration, the energy level of 

dopant with<strong>in</strong> TiO2, its d electronic configuration, the distribution of dopant with<strong>in</strong> the 

particles, the electron donor concentration, <strong>and</strong> the <strong>in</strong>cident light <strong>in</strong>tensity [93]. Itis 

claimed [86] that the photoresponse is completely absent <strong>in</strong> the presence of 0.85 ion% 

Cr 3+ homogeneously distributed because the homogeneous dop<strong>in</strong>g with Cr 3+ can <strong>in</strong>troduce 

centers which facilitate electron–hole recomb<strong>in</strong>ation. It is reported that Fe 

[156,168], Mo[156], <strong>and</strong> V [156] dopant <strong>in</strong> TiO2 may cause an <strong>in</strong>hibition of hole–electron 

recomb<strong>in</strong>ation. However, Mart<strong>in</strong> et al. [167] have argued that V dopant may promote 

charge-carrier recomb<strong>in</strong>ation with electron trapp<strong>in</strong>g at VO þ 

2 <strong>in</strong> TiO2-25 <strong>and</strong> V 4+ impurities 

as hole trap <strong>in</strong> TiO2-200/400. 

In certa<strong>in</strong> conditions <strong>in</strong>troduction of a metal dopant (such as Fe 3+ ,V 4+ ,Rh 3+ , <strong>and</strong> 

Mn 3+ ) <strong>in</strong>to TiO 2 <strong>in</strong>duces a red-shift <strong>in</strong> b<strong>and</strong> gap transition, sometimes extend<strong>in</strong>g <strong>in</strong>to 

the visible-light range. This is attributed to the CT transitions between the metal ion d electrons 

<strong>and</strong> the TiO2 conduction or valence b<strong>and</strong> or a d–d transition <strong>in</strong> the crystal field [93]. 

For example, it is reported [155] that photochromism has been observed <strong>in</strong> Fe-doped rutile 

s<strong>in</strong>gle-crystal upon irradiation with light of photon energy greater than 2.5 eV. In addition, 

the presence of water <strong>in</strong> some form at the surface or <strong>in</strong>terface appears to be essential 

to the photochromic reaction, which might be due to its reaction with hole or oxygen liberated 

from TiO 2 upon irradiation, or to its enter<strong>in</strong>g <strong>in</strong>to the bond<strong>in</strong>g between impurity 

<strong>and</strong> host oxide [31,153].


2.3.2. Transition-metal-doped titanates 

Strontium titanate (SrTiO3) is a crystal with a cubic perovskite structure at room temperature. 

It is transparent <strong>in</strong> visible range with a b<strong>and</strong> gap of ca. 3.2 eV separat<strong>in</strong>g the oxygen 

2p valence b<strong>and</strong> from the empty titanium 3d conduction b<strong>and</strong> [2,169–171]. When 

s<strong>in</strong>gly doped with Fe [2,169–179], Ni[2,170,176,180], Co[2,170], Cr[2], V[2], or double 

doped with Fe–Mo [153,160,170,173], Ni–Mo [2,170,173], V–Fe [177], <strong>and</strong> Al–Fe [181], 

it is capable of photochromism <strong>and</strong> sometimes a small amount of visible absorption 

appears before coloration. Mn-doped SrTiO 3 does not change at all under illum<strong>in</strong>ation 

[2]. Dop<strong>in</strong>g <strong>in</strong>fluences the charge compensation, which plays an important role <strong>in</strong> photochromic 

SrTiO3 s<strong>in</strong>ce it permits the existence of many different valence states of impurity 

ions <strong>in</strong> the crystal [2,170]. The dop<strong>in</strong>g may also improve the electrical conductivity. Apart 

from the thermal decay, the <strong>in</strong>duced coloration can be bleached with visible light 

[2,170,174,175]. 

Iron has probably been studied more thoroughly as an impurity <strong>in</strong> SrTiO 3 than any 

other transition metals. It is claimed that Fe-doped SrTiO 3 crystal exhibits photochromic 

properties only at low temperature (


Fig. 16. Schematic representation of (a) Ni 3+ donor charge transfer, (b) Ni 3+ –VO–(e) acceptor, <strong>and</strong> (c) Ni 3+ –VO– 

(2e) donor-transfer primary processes. Dashed l<strong>in</strong>es: secondary electron or hole-trapp<strong>in</strong>g processes [176]. 

to Ni 3+ –VO–(e) (Fig. 16) [176]. Acceptor-type charge transfer at near-b<strong>and</strong> gap energy 

k < 500 nm generates Ni 3+ –VO–(2e) centers from Ni 3+ –VO–(e). The holes liberated are 

trapped at cubic Ni 2+ sites which convert back to Ni 3+ . The photochromic b<strong>and</strong> at 

525 nm, characteristic of Ni 3+ , is due to a donor charge transfer of cubic Ni 3+ to become 

Ni 4+ . The b<strong>and</strong> at 575 nm is a Ni 3+ –VO–(2e) donor b<strong>and</strong> which changes this center back 

to the Ni 3+ –VO–(e) state. The strong b<strong>and</strong> at 480 nm is assumed to be caused by an excitation 

of Ni 3+ –VO–(2e) <strong>in</strong>to an unstable configuration or charge state which relaxes back 

<strong>in</strong>to the <strong>in</strong>itial state before the carriers liberated can move away. 

When Fe–Mo double doped SrTiO3 is irradiated with light <strong>in</strong> the 390–430 nm region, 

broad visible absorption b<strong>and</strong>s appear due to the formation of Fe 4+ <strong>and</strong> Mo 5+ via electron 

transfer from Fe 3+ to Mo 6+ [153,160,170,172,173]. This is also observed <strong>in</strong> Fe–Mo 

doped TiO2 [170,173]. This process does not depend on the surface effect. The role of 

Mo is that of an electron trap. When Mo is not present, another transition ion can act 

as both an electron donor <strong>and</strong> an electron trap [170,173,182,183]. In this case, the thermal 

decay rate is faster. For <strong>in</strong>stance, at 300 K thermal bleach<strong>in</strong>g of the photo<strong>in</strong>duced colored 

state occurs with<strong>in</strong> several m<strong>in</strong>utes for the Fe–Mo doped SrTiO3 <strong>and</strong> with<strong>in</strong> less than a 

second for s<strong>in</strong>gly Fe-doped SrTiO3 [170,173]. 

It is reported [184,185] that a light red color is developed from white on exposure of 

calcium titanate (CaTiO 3, E g 

3.4 eV) to light. On the contrary to the report of Tanaka 

[185], MacNev<strong>in</strong> <strong>and</strong> Ogle [184] have claimed that oxygen <strong>and</strong> moisture have no <strong>in</strong>fluence 

on the color response of CaTiO 3. BaTiO 3 also exhibits photochromism, though <strong>in</strong> general 

less <strong>in</strong>tense compared with CaTiO3 due to slight reduction of Ti 4+ to colored Ti 3+ dur<strong>in</strong>g 

the preparation [184]. An impurity is usually present <strong>in</strong> the two titanates, the effect of 

which tends to parallel <strong>in</strong> them <strong>and</strong> <strong>in</strong>creases with its amount [184]. It is also <strong>in</strong>dicated 

[2,170] that CaTiO3 doped with transition metals has photochromic properties very similar 

to SrTiO3. It is claimed [184] that <strong>in</strong> order for an impurity ion to <strong>in</strong>duce photochromism, it 

must have a radius near that of Ti 4+ but not exactly the same, otherwise no distortion 

would occur. Moreover, an effective impurity ion must have a valence other than 4 so that 

electron transfer is possible [184]. Therefore, Fe 3+ ,Zn 2+ ,V 5+ <strong>and</strong> Sb 5+ give the most

pronounced effects (chang<strong>in</strong>g <strong>in</strong>to (dark) violet) whereas Ag + ,Cu 2+ ,Sb 3+ ,Sn 4+ ,Zr 4+ <strong>and</strong> 

Co 2+ produced no detectable color <strong>in</strong> BaTiO3 <strong>and</strong> no detectable <strong>in</strong>crease <strong>in</strong> the slight coloration 

of pure CaTiO3. 

<strong>Photochromism</strong> cannot occur <strong>in</strong> MgTiO3, which is basically dependent upon its different 

crystal structure from that of CaTiO3 [184]. CaTiO3 has a structure <strong>in</strong> which Ca 2+ <strong>and</strong> 

O 2 

ions together form a close-packed lattice <strong>and</strong> small Ti 4+ ion is surrounded octahe- 

drally by six O 2 


ions. In MgTiO 3, however, only O 2 

ions form a close-packed configu- 

ration <strong>and</strong> Mg 2+ <strong>and</strong> Ti 4+ ions exist <strong>in</strong> the <strong>in</strong>terstices. An iron impurity added to MgTiO3 can replace the Mg 2+ <strong>and</strong>/or Ti 4+ ion without produc<strong>in</strong>g an asymmetrical field. Zn 2+ , similar 

<strong>in</strong> size to Mg 2+ <strong>and</strong> Ti 4+ , may replace one or both ions without stra<strong>in</strong> on the closepacked 

O 2 

structure. 

2.3.3. Doped molybdates <strong>and</strong> tungstates 

The photochromic effect <strong>in</strong> PbMoO 4 crystals (with Scheelite structure) is due to the 

charge exchange Mo 6+ +Pb 2+ M Mo 5+ +Pb 3+ [186], which accounts for the appearance 

of all three absorption b<strong>and</strong>s after UV-light illum<strong>in</strong>ation. The one at ca. 435 nm is due to 

Pb 3+ ions <strong>and</strong> the ones at ca. 390 <strong>and</strong> 575 nm are assigned to Mo 5+ ions [186–189]. The 

deviation from compositional stoichiometry toward an excess of MoO3 can also cause 

the appearance of these absorption b<strong>and</strong>s [186]. For PbMoO4 crystals with BaO <strong>and</strong> 

Bi2O3 additions, Ba 2+ <strong>and</strong> Bi 3+ substitute isomorphically for Pb 2+ ions. Consequently, 

the <strong>in</strong>troduction of Ba 2+ reduces photo<strong>in</strong>duced coloration due to the exerted stabiliz<strong>in</strong>g 

effect on the lead sublatttice, <strong>and</strong> Bi-doped crystals exhibit a strong absorption at 

435 nm but no photochromism [186]. 

Another crystal with the same Scheelite structure as PbMoO 4 is PbWO 4, for which the 

most prom<strong>in</strong>ent absorption <strong>in</strong> the range from 350 to 370 nm is usually attributed to the 

existence of Pb 3+ centers [190] <strong>and</strong> the peak at 420 nm seems to be caused by O centers 

[191]. Burachas et al. [192] have po<strong>in</strong>ted out that the radiation-<strong>in</strong>duced photochromic 

effects of PbWO4 are caused by phase transitions <strong>in</strong> the <strong>in</strong>clusions of tungsten oxide 

(WO3 x for the undoped one, <strong>and</strong> W1 yLyO3 x (0 < x < 0.3) for the L-doped one, 

L = Y, La, Gd), of which a valency change is <strong>in</strong>itiated <strong>and</strong> results <strong>in</strong> <strong>in</strong>duced absorption 

<strong>and</strong>, consequently, <strong>in</strong> crystal coloration. S<strong>in</strong>ce PbWO 4 crystals are usually used as sc<strong>in</strong>tillation 

detector <strong>in</strong> high energy physics <strong>and</strong> modern medical imag<strong>in</strong>g, almost all <strong>in</strong>vestigations 

hitherto are focused on how to improve the radiation hardness [190–193], one of the 

most important properties for sc<strong>in</strong>tillation <strong>materials</strong>. By <strong>in</strong>troduction of dop<strong>in</strong>g with La 

[190,192,194,195], Gd [192,195,196], Nb [197], Y [192,198], Sb [195,198], <strong>and</strong> certa<strong>in</strong> 

group-IV impurities (Th) [198], the radiation hardness of PbWO4 is <strong>in</strong>creased. Dop<strong>in</strong>g 

with Zr 4+ ,Si 4+ ,Sc 3+ , <strong>and</strong> Ti 3+ does not improve while dop<strong>in</strong>g with Sn 4+ <strong>and</strong> Yt 3+ even 

considerably decreases the transparency <strong>and</strong> radiation hardness of PbWO 4 [195,198]. 

Upon exposure to UV light, Bi-doped calcium, strontium, <strong>and</strong> barium tungstates <strong>and</strong> 

their solid solutions can change from white to purple, green or p<strong>in</strong>k color, respectively 

[35,199]. Under the same irradiation conditions the <strong>in</strong>tensity of UV-<strong>in</strong>duced coloration 

depends on the Bi content <strong>and</strong> preparation temperature of the tungstates. Bi contents 

higher than 0.1 mol% <strong>and</strong> preparation temperatures higher than 1250 °C give strong coloration. 

In Bi-doped tungstates photogenerated holes can be trapped by cation vacancies 

created by the <strong>in</strong>troduction of Bi 3+ ions or by Bi 3+ themselves [199]. Then electrons, without 

be<strong>in</strong>g recomb<strong>in</strong>ed, can be trapped by oxygen ion vacancies, lead<strong>in</strong>g to coloration of 

the crystals. The similar photochromic mechanism has been put forward by Cronemeyer


<strong>and</strong> Beaubien [200]. The UV-<strong>in</strong>duced color fades gradually at room temperature <strong>in</strong> darkness, 

by thermal fad<strong>in</strong>g, or by optical bleach<strong>in</strong>g [199]. The fad<strong>in</strong>g rate <strong>in</strong>creases with 

<strong>in</strong>creas<strong>in</strong>g Bi content <strong>and</strong> is promoted by visible-light exposure. 

2.3.4. Field-assisted photochromism 

In the application of photochromism to an optical memory, there is a substantial problem 

that the change <strong>in</strong> optical absorption <strong>in</strong>duced by light irradiation degrades with time. 

Faughnan [170] has suggested this degradation results from the thermal reverse reaction. 

For the photochromic TMOs doped by transition-metal ions, Kobayashi et al. [201–204] 

have suggested that the presence of a strong electric field can suppress this thermal reverse 

reaction, <strong>in</strong> which electrons (or holes) emitted from impurity levels to the conduction 

(valence) b<strong>and</strong> drift toward an electrode along the electric filed. This is the so-called 

field-assisted photochromism. A photochromic material is <strong>in</strong>terposed between two th<strong>in</strong> 

electrodes so that a strong electric field can be applied to it (Fig. 17) [201]. It is assumed 

that the charge of doped transition-metal ions, M n+ , is identical with the charge of metal 

ions of a host metal oxide, <strong>and</strong> M n+ ions form an impurity level with the state density of 

Nd at Ed below the bottom of the conduction b<strong>and</strong> (the depth energy Ed is less than onehalf 

of the b<strong>and</strong> gap energy Eg). In the presence of a strong electric field, electrons <strong>in</strong> a 

shallow impurity level of M n+ ions can be thermally emitted <strong>in</strong>to the conduction b<strong>and</strong>. 

The thermal emission of electrons results <strong>in</strong> the variation of the concentration of M n+ ions 

with time. Consequently, the formation of a deep impurity level of M n+ ions is an important 

condition required to realize an optical memory. If the conditions are satisfied, the 

positive space charges due to M n+1 ions are not formed <strong>in</strong> the photochromic material, 

<strong>and</strong> thus the b<strong>and</strong> of the photochromic material is straight as shown <strong>in</strong> Fig. 17A [201]. 

Fig. 17. Schematic illustration of field-assisted photochromism: the b<strong>and</strong> diagrams of a film consist<strong>in</strong>g of ohmic 

electrode, photochromic material, <strong>and</strong> block<strong>in</strong>g electrode (A) before <strong>and</strong> (B) after light irradiation [201].


Under the light irradiation with photon energy hm, such as Ed < hm < Eg Ed, electrons <strong>in</strong> 

M n+ ions can be optically transferred to the conduction b<strong>and</strong>, <strong>and</strong> subsequently electrons 

<strong>in</strong> the conduction b<strong>and</strong> are drifted toward the left electrode along the applied electric field. 

This process is called photoionization of doped transition-metal ions: M n+ ! M n+1 +e. 

Even if light irradiation is turned off, M n+1 ions ly<strong>in</strong>g above the Fermi level of the ohmic 

electrode rema<strong>in</strong> unchanged. The presence of the electric field is effective for the charge 

separation of electron–hole pairs generated by light irradiation <strong>and</strong> for suppress<strong>in</strong>g the 

thermal reverse reaction from the ohmic electrode. In the application of an optical memory, 

M n+1 ions should be reduced optically to M n+ ions. This requirement can be fulfilled 

by the photon-<strong>in</strong>duced charge transfer from the valence b<strong>and</strong> to vacant impurity level 

orig<strong>in</strong>at<strong>in</strong>g from M n+1 ions as shown <strong>in</strong> Fig. 17B [201]. Under the light irradiation with 

a photon energy larger than Eg Ed, M n+ ions are generated <strong>and</strong> successively the photoionization 

of M n+ ions take place. 

These authors have proposed a selection rule for the photoionization <strong>and</strong> charge transfer 

at an arbitrary k vector <strong>in</strong> the Brillou<strong>in</strong> zone by consider<strong>in</strong>g time <strong>in</strong>version [201]. Nonzero 

matrix element is obta<strong>in</strong>ed for the <strong>in</strong>itial state of a M n+ ion conta<strong>in</strong><strong>in</strong>g odd number 

of d electrons <strong>in</strong> the photoionization, <strong>and</strong> for the f<strong>in</strong>al state of a M n+ ion conta<strong>in</strong><strong>in</strong>g 

even number of d electrons <strong>in</strong> the charge transfer: d 2m+1 ! d 2m + e(c.b), <strong>and</strong> 

d 2m + e(v.b) ! d 2m+1 . Us<strong>in</strong>g this selection rule, the authors have argued one can choose 

transition-metal ions to be employed as a photochromic material. This has been demonstrated 

<strong>in</strong> the systems of Co-doped ZnO [202] <strong>and</strong> Cu-doped ZnO [203]. A broad peak 

around 640 nm <strong>in</strong> the photocurrent spectrum is assigned to photothermal ionization of 

Co 2+ ions ðCo 2þ ! Co 3þ þ e CB Þ [202,205], which can be enhanced <strong>in</strong> a strong electric field 

because the barrier height for thermal emission is reduced by the electric field as <strong>in</strong> the 

Pool-Frenkel model [206]. The concentration of Co 2+ ions is decreased by the irradiation 

of 500 nm <strong>and</strong> is recovered to the <strong>in</strong>itial value by turn<strong>in</strong>g off the bias voltage. 

2.4. Miscellaneous <strong>composite</strong> <strong>materials</strong> 

2.4.1. a-WO 3/Si heterostructure 

Tutov et al. [207,208] have prepared an amorphous WO 3/Si (a-WO 3/Si) heterostructure 

by vacuum condensation of thermally evaporated tungsten trioxide powder on n-Si substrate 

under conditions lead<strong>in</strong>g to formation of a transparent WO3 film (stoichiometric) 

<strong>and</strong> the one with color centers (partially reduced). The latter is analogous to a thermochromic 

process without the concomitant structural order<strong>in</strong>g of the film. The tendencies of a 

relative <strong>in</strong>crease of the negative charge <strong>and</strong> the surface states density <strong>in</strong> the a-WO3/Si heterostructure 

dur<strong>in</strong>g photo- <strong>and</strong> electrochromism are the same as that for thermochromic 

process. The concentration of color centers <strong>in</strong>creases monotonically with UV-irradiation 

time <strong>and</strong> reaches a saturated value of 2.4 · 10 20 cm 3 <strong>in</strong> a time of 100 m<strong>in</strong>. The formation 

of color centers <strong>in</strong> photochromic process leads to an expected <strong>in</strong>crease of the surface 

charge, with the additional surface charge located at the <strong>in</strong>terface <strong>in</strong> the a-WO3/Si structure 

be<strong>in</strong>g negative, which correlates with the occupation of the electron states <strong>in</strong> the b<strong>and</strong> 

gap of colored a-WO3 film as determ<strong>in</strong>ed by XPS [209]. 

High-frequency (HF) C–V characteristics have been <strong>in</strong>vestigated as well as an <strong>in</strong>fluence 

of color center formation <strong>in</strong> a-WO3 on charge parameters of heterojunction with UV irradiation 

for different exposure times (Fig. 18) [207]. Obviously, the C–V characteristics of 

the a-WO 3/Si films with an oxide film obta<strong>in</strong>ed under reduc<strong>in</strong>g conditions of deposition


Fig. 18. High-frequency C–V characteristics for (A) WO3 x/n-Si <strong>and</strong> (B) WO3/n-Si heterojunction dur<strong>in</strong>g 

photochromic process. 20 0 ,40 0 ,80 0 —times of UV-irradiation. (*) Marks curves for the structures with virg<strong>in</strong> 

oxide film [207]. 

<strong>and</strong> hav<strong>in</strong>g at the outset a pale-blue color def<strong>in</strong>itely differ from the structures of the uncolored 

film. In the structure with WO 3 x film a s<strong>in</strong>gle energy level of fast surface states is 

observed, located 0.06 eV below the Fermi level <strong>in</strong> Si. It is found that a fast smooth<strong>in</strong>g 

of the peak <strong>in</strong> C–V characteristics occurs with the <strong>in</strong>crease of the irradiation dose, <strong>in</strong>dicat<strong>in</strong>g 

that the <strong>in</strong>itial type of defects (color centers) is ‘‘healed’’, replaced or suppressed by 

other centers characteristic of the photochromic process. However, density of states at this


level <strong>in</strong>creases <strong>in</strong> electrochromic process [207], while this level for the structure with the 

a-WO3 x thermochromic film does not appear [210] s<strong>in</strong>ce the process of thermovacuum 

reduction is concomitant by the order<strong>in</strong>g of the atomic structure. So the nature <strong>and</strong> mechanism 

of photochromism for tungsten trioxide might be different, at least by its electron 

processes observed <strong>in</strong> HF C–V characteristics of a-WO3/Si structure, from those of the 

electrochromic <strong>and</strong> thermochromic ones. This might be expla<strong>in</strong>ed by the model of hydrogen 

photo<strong>in</strong>jection for the formation of color centers proposed by Gavrilyuk <strong>and</strong> co-workers 

(cf. Section 3.4.1) [211]. In this model, some sequential reactions are assumed to result 

<strong>in</strong> the formation of color center. Probably, some of the processes occurr<strong>in</strong>g <strong>in</strong> these reactions 

lead to a suppression of alternative mechanisms of color center appearance <strong>in</strong> a-WO3 

found <strong>in</strong> C–V characteristics. This can expla<strong>in</strong> a reduced concentration of color centers <strong>in</strong> 

photochromic process. Thus the C–V characteristics allow one to observe electron processes 

<strong>in</strong> the color centers pert<strong>in</strong>ent only to the reduction of a-WO3 film. 

2.4.2. H 3PW 12O 40/TiO 2 system 

12-Phosphotungstic acid (H 3PW 12O 40ÆnH 2O, PW 12) can be <strong>in</strong>corporated <strong>in</strong>to M–O 

matrix (like silica gel <strong>and</strong> TiO2) derived <strong>composite</strong>s [212–215]. With regard to the fact that 

TMO gels exhibit redox <strong>and</strong> conduct<strong>in</strong>g properties while silica gels not, titania gel is used 

as a host matrix to prepare the <strong>composite</strong> photochromic PW12/TiO2 gels <strong>and</strong> films via a 

sol–gel route by a dip-coat<strong>in</strong>g technique [214]. Similarity of the characteristic b<strong>and</strong> frequencies 

(FT-IR <strong>and</strong> Raman spectra) to pure PW12 confirms that entrapped Kegg<strong>in</strong> salt 

is well preserved <strong>in</strong>side the gel [213,214]. Dark blue reversible coloration of PW 12/TiO 2 

is obta<strong>in</strong>ed under daylight or UV radiation (k = 366 nm), which is relevant to the reduction 

of PW 12 <strong>and</strong> simultaneous oxidation of organic solvent left <strong>in</strong> the gel lead<strong>in</strong>g to the 

formation of aldehyde <strong>and</strong> arises from the IVCT between metal ions <strong>in</strong> two different oxidation 

states (W 5+ –W 6+ ) [214]. S<strong>in</strong>ce there is a good electrical communication between 

PW12 <strong>and</strong> nanoporous TiO2 [212], a good synergetic effect between the two components 

should be obta<strong>in</strong>ed on the photochromism. However, so far there is no such k<strong>in</strong>d of discussion 

<strong>in</strong> the literatures. 

2.4.3. Other <strong>composite</strong> systems 

Kostecki et al. [216] have prepared a layered TiO2/Ni(OH) 2 <strong>composite</strong> film by electrochemical 

deposition method, which exhibits strong, reversible photochromic properties on 

UV-light irradiation. Vectorial transport of the photogenerated holes <strong>in</strong> TiO2 toward the 

TiO2 surface <strong>and</strong> direct electron transfer from the adjacent Ni(OH)2 phase to the TiO2 

layer result <strong>in</strong> the oxidation of Ni(OH)2 to NiOOH [217], lead<strong>in</strong>g to the darken<strong>in</strong>g of 

the film from nearly transparent to gray or even black, depend<strong>in</strong>g on the light <strong>in</strong>tensity 

<strong>and</strong> exposure time. It is noted that Ni(OH) 2–NiOOH itself is not photoresponsive under 

natural illum<strong>in</strong>ation [216]. The formation of stable Ti 3+ sites can result from the <strong>in</strong>jection 

of photogenerated electrons <strong>in</strong>to TiO2 conduction b<strong>and</strong>, which may contribute to the coloration 

of the <strong>composite</strong> upon illum<strong>in</strong>ation, correspond<strong>in</strong>g to a long tail that extends to 

1000 nm <strong>in</strong> the electronic spectrum. The presence of trapped Ti 3+ species decreases 

the efficiency of further photo<strong>in</strong>duced charge separation <strong>in</strong> TiO2 [218], <strong>and</strong> thereby 

decreases the photochromic efficiency. When the illum<strong>in</strong>ation of <strong>composite</strong> is blocked, 

the opposite process takes place. Electrons accumulated <strong>in</strong> the TiO2 conduction b<strong>and</strong> 

<strong>and</strong>/or trapped <strong>in</strong> Ti 3+ sites become available for a direct <strong>in</strong>terfacial charge transfer (or 

through the surface states) to the NiOOH–Ni(OH) 2 layer <strong>and</strong> thus reduce NiOOH to


Ni(OH)2, result<strong>in</strong>g <strong>in</strong> appearance from gray-black to transparent. The photochromic effect 

<strong>in</strong> the TiO2/Ni(OH)2 film can be significantly enhanced by anodic polarization [216]. 

Similar phenomenon has been observed <strong>in</strong> Prussian blue modified TiO2 [219–221] <strong>and</strong> 

n-SrTiO3 [222,223] electrodes. Illum<strong>in</strong>ation of the <strong>composite</strong> electrode leads to the oxidation 

of colorless Prussian white film to Prussian blue at potentials negative of that measured 

at noble metals. 

3. Inorganic/organic <strong>hybrid</strong> <strong>materials</strong> 

The ability to comb<strong>in</strong>e, <strong>in</strong> a s<strong>in</strong>gle material, <strong>in</strong>organic <strong>and</strong> organic components at 

molecular or nanometer level represents an excit<strong>in</strong>g direction <strong>in</strong> material science with 

extraord<strong>in</strong>ary implications for develop<strong>in</strong>g novel multifunctional <strong>materials</strong>. These <strong>in</strong>organic/organic 

<strong>hybrid</strong>s have the possibility to possess the advantages of both <strong>in</strong>organic 

(high strength, thermal stability, chemical resistance, etc.) <strong>and</strong> organic <strong>materials</strong> (light 

weight, flexibility, versatility, etc.) [224]. The uses of such <strong>hybrid</strong> approach to obta<strong>in</strong> 

greatly improved photochromic properties with opportunities for applications are now 

be<strong>in</strong>g widely studied <strong>in</strong> many groups. In Section 3.1.1., we will see that such (improved) 

photochromism is closely relevant to the charge transfer (CT) between <strong>in</strong>organic <strong>and</strong> 

organic moieties. So for these <strong>hybrid</strong> systems, it is of great importance to construct a 

bridge, through which the charges, specifically the electrons, <strong>and</strong>/or protons can reversibly 

transfer between the two constituents upon photoirradiation. To realize this, first, one 

should choose the proper <strong>in</strong>organic <strong>and</strong> organic molecules, <strong>and</strong>, then, the suitable preparation 

methods. 

3.1. Model molecules, photochromic mechanism, <strong>and</strong> preparation methods 

3.1.1. Model molecules <strong>and</strong> photochromic mechanism 

POMs with well-def<strong>in</strong>ed primary structures, <strong>and</strong> sometimes the TMOs, are usually chosen 

as build<strong>in</strong>g units for such k<strong>in</strong>d of novel <strong>hybrid</strong>s. POMs can be seen as metal-oxide clusters 

due to the similar composition <strong>and</strong> topology between POMs <strong>and</strong> oxides [225]. Similar 

to the behavior of semiconduct<strong>in</strong>g TMOs [226], POMs can be photochemically reduced 

[227] to form colored mixed-valence species while reta<strong>in</strong><strong>in</strong>g the structural <strong>in</strong>tegrity because 

of their well-known special structures, reversible redox activities, high electronic densities 

<strong>and</strong> good proton conductors [35,228–231]. Non-reduced POMs are characterized by oxygen-to-metal 

(O ! M) charge transfer (CT) b<strong>and</strong>s appear<strong>in</strong>g <strong>in</strong> UV region. Reduction of 

POMs to heteropoly blues or heteropoly browns results <strong>in</strong> decrease of the O ! MCT 

b<strong>and</strong>s <strong>and</strong> formation of d–d transition <strong>and</strong>/or IVCT b<strong>and</strong> located <strong>in</strong> the visible <strong>and</strong> near 

<strong>in</strong>frared region. The <strong>in</strong>tensity <strong>and</strong> position of these absorption b<strong>and</strong>s are approximately 

proportional to the number of trapped electrons [232]. Although it is said [27] these 

reduced species are chemically <strong>and</strong> spectroscopically equivalent to tungsten or molybdenum 

bronzes <strong>in</strong> the form of colloidal semiconduct<strong>in</strong>g quantum dots, the photochromic 

mechanism of POMs-<strong>based</strong> <strong>hybrid</strong>s is different from that of TMOs. 

The only absorption b<strong>and</strong> of POMs <strong>in</strong> the UV–Vis range is due to <strong>in</strong>tramolecular 

O ! M lig<strong>and</strong>-to-metal charge transfer (LMCT) transition s<strong>in</strong>ce the metal ions <strong>in</strong> oxidized 

POMs have d 0 electronic configurations [35]. When <strong>hybrid</strong> <strong>materials</strong> <strong>based</strong> on POMs are 

irradiated with UV light, electrons are excited from the low-energy electronic states, which 

are ma<strong>in</strong>ly comprised of oxygen 2p orbitals <strong>in</strong> POM, to the high-energy states, which are


ma<strong>in</strong>ly comprised of metal d orbitals. Consequently, the metal ions have d 1 <strong>in</strong>stead of d 0 

electronic configurations <strong>and</strong> the electrons added to polyanions have a thermally activated 

delocalization with<strong>in</strong> the molecule [226]. For H3PMo12O40, it has been suggested that 

[233,234] the bridg<strong>in</strong>g oxygen (Ob) has exclusive reactivity <strong>and</strong> serves as adsorption sites 

<strong>in</strong> the redox process. So the protons can be attracted to Ob atoms [235] to compensate 

the negative charges of the reduction electrons <strong>in</strong>troduced <strong>in</strong>to the LUMO level which 

has an anti-bond<strong>in</strong>g nature [234]. In addition, it is said that the term<strong>in</strong>al oxygen (O t)is 

apparently the most active atom <strong>in</strong> the redox process s<strong>in</strong>ce O t is located at a protrud<strong>in</strong>g 

position of the Kegg<strong>in</strong> unit <strong>and</strong> the Ot–Mo bond is a double bond [234,236]. Thus electrons 

can be photoexcited from Ot without much difficulty dur<strong>in</strong>g the photochemical reaction 

[236]. 

S<strong>in</strong>ce heteroatoms or counter cations correspond to impurities or lattice defects <strong>in</strong> the 

b<strong>and</strong> model, the photogenerated charge carriers may be trapped <strong>in</strong> electron traps <strong>and</strong> hole 

traps [35]. These traps provide states of localized energy <strong>in</strong> the O ! M LMCT energy gap. 

The trapped carriers can be released by thermal or optical stimulation. If the trap depth is 

larger than kT, the probability for thermal escape from the trap will be negligibly small, 

result<strong>in</strong>g <strong>in</strong> a metastable situation. The d 1 electrons <strong>in</strong> O ! M LMCT states facilitate 

the absorption of visible light via IVCT <strong>and</strong>/or d–d transitions between the neighbored 

metal centers with different valence states [35]. The same type of transition may also be 

possible for the d 1 electrons captured by electron traps. 

The photochromic products may be different when the POMs are comb<strong>in</strong>ed with different 

k<strong>in</strong>ds of organic molecules. The organic selected is usually <strong>based</strong> on organic am<strong>in</strong>es, 

the molecules with p-electrons, or some polymers. If there is hydrogen bond present 

between the oxygen of POM <strong>and</strong> the hydrogen of organic (such as <strong>in</strong> the case of alkylammonium 

<strong>based</strong> <strong>hybrid</strong>s), the photoexcitation of O ! M LMCT b<strong>and</strong>s <strong>in</strong>duces transfer of a 

proton from a hydrogen bonded atom <strong>in</strong> organic (such as alkylammonium nitrogen) to a 

bridg<strong>in</strong>g oxygen atom at the photoreducible site <strong>in</strong> edge-shar<strong>in</strong>g MO6 octahedral lattice 

(Eq. (3)) [35]. The proton transferred to the oxygen atom <strong>in</strong>teracts with the d 1 electron 

of metal atom. Meanwhile, the hole left at the oxygen atom as a result of the O ! M 

LMCT transition <strong>in</strong>teracts with non-bond<strong>in</strong>g electrons on the nitrogen atom to form a 

CT complex, result<strong>in</strong>g <strong>in</strong> separation of electrons <strong>and</strong> holes produced by O ! M LMCT 

transition <strong>in</strong> the POM lattice <strong>and</strong> thus lead<strong>in</strong>g to the stabilization of the colored state 

[35]. In such a case, the hydrogen bond <strong>and</strong> proton transfer are crucial for the photochromic 

process. The organic molecules not serve only as the proton donor for the coloration 

<strong>and</strong> stabilization of <strong>hybrid</strong>s, but also as the electron donor for the formation of coord<strong>in</strong>ation 

bond <strong>in</strong> the photo<strong>in</strong>duced CT complex. If the organic moiety <strong>in</strong> <strong>hybrid</strong> is reducible, it 

may be oxidized by holes. In this case, the colored products are reduced polyanion <strong>and</strong> 

oxidized organic cation or radical rather than the CT complex. This is similar to the photooxidation 

of alkanes catalyzed by polyanions [237–243]. In addition, like the case <strong>in</strong> 

WO 3/TiO 2 system, if the molecule selected is organic semiconductor (such as 2,2 0 -biqu<strong>in</strong>ol<strong>in</strong>e), 

upon photoexcitation electrons can also come from the organic moiety to reduce 

the central metal <strong>in</strong> POM. 4 

4 In some cases, electrons can transfer between organic electron donors <strong>and</strong> POM anions even without light 

illum<strong>in</strong>ation, especially when the addendum atoms—usually Mo VI ,W VI ,orV V —of POM have been substituted 

by d-electron-conta<strong>in</strong><strong>in</strong>g first- or second-row transition-metal cations [242]. However, this is outside the scope of 

the present review. Moreover, no photochromism has been reported <strong>in</strong> these systems.


R 

N 

H 2 

+ 

H 

Mo VI 

O 

O O 

O O 

O 

hv 

O2, Heat<strong>in</strong>g 

R 

N 

H 2 

H + 

O 

O 

Mo V 

O 

O 

The relaxation processes of O ! M LMCT excitation energy <strong>in</strong>clude both the non-radiative 

recomb<strong>in</strong>ation of electrons <strong>and</strong> holes with<strong>in</strong> the energy gap <strong>and</strong> the <strong>in</strong>tramolecular 

energy transfer lead<strong>in</strong>g to a CT emission [35]. The latter corresponds to the O ! M 

LMCT energy gap <strong>and</strong> takes place via radiative recomb<strong>in</strong>ation <strong>and</strong> sensitized emission 

from the heteroatoms or cations. When the photocolored samples are placed <strong>in</strong> air <strong>and</strong> 

kept away from light, <strong>in</strong> most cases, they change back to the orig<strong>in</strong>al color, <strong>and</strong> can be 

recolored aga<strong>in</strong> on exposure to UV light. Heat<strong>in</strong>g can promote this bleach<strong>in</strong>g process. 

If the colored species are stored <strong>in</strong> an <strong>in</strong>ert atmosphere (nitrogen, argon, helium, vacuum 

conditions, <strong>and</strong> the like), however, they can reta<strong>in</strong> blue color for a quite long period. So 

oxygen molecules can <strong>in</strong>itiate the bleach<strong>in</strong>g of the colored samples, which may occur 

through electron transfer from the metal ion with lower oxidation state to the oxygen molecule 

(Eq. (4)), lead<strong>in</strong>g to the back reaction of Eq. (3). 

O2 þ M 5þ ! O2 þ M 6þ 

ð4Þ 

3.1.2. Preparation methods 

To move towards realistic technological implementations of photochromic devices, 

from eng<strong>in</strong>eer<strong>in</strong>g considerations, the coloration <strong>materials</strong> selected should be easily shaped 

as th<strong>in</strong> films, coat<strong>in</strong>gs, monoliths, or other suitable forms. However, it seems difficult to 

prepare the complex molecular structure <strong>in</strong> certa<strong>in</strong> films by the st<strong>and</strong>ard methods of vacuum 

evaporation or by sputter<strong>in</strong>g methods because most of the photochromic compounds 

have the tendency to decompose when heated [9]. S<strong>in</strong>ce most POMs are readily dissolved 

<strong>in</strong> water <strong>and</strong>/or many organic solvents, they might not be used directly <strong>in</strong> the device construction. 

On the other h<strong>and</strong>, however, the high solubility <strong>in</strong> many solvents as well as the 

quite small size of POM anions or clusters (ca. 1–2 nm <strong>in</strong> diameter, with<strong>in</strong> the framework 

of molecular chemistry) makes them promis<strong>in</strong>g as <strong>in</strong>organic components <strong>in</strong> <strong>hybrid</strong> molecular 

<strong>materials</strong>. So far various strategies, especially through ‘‘soft’’ chemistry (chimie douce) 

process that affords versatility <strong>and</strong> relative ease of experimentation, have been adopted to 

prepare th<strong>in</strong> films of <strong>hybrid</strong> <strong>materials</strong>. With different preparation methods, different 

<strong>hybrid</strong> <strong>materials</strong> are formed <strong>and</strong> thus the <strong>in</strong>termolecular or <strong>in</strong>tramolecular charge transfer 

may take place. 

The most commonly used method is the sol–gel derived coat<strong>in</strong>g technique, ma<strong>in</strong>ly 

<strong>in</strong>clud<strong>in</strong>g sp<strong>in</strong>-, dip- <strong>and</strong> cast-coat<strong>in</strong>g methods. The sol–gel process is broadly def<strong>in</strong>ed as 

one <strong>in</strong> which a useful solid product is prepared from a solution or suspension of precursor 

<strong>materials</strong> via hydrolysis <strong>and</strong> polycondensation [244]. In sol–gel process, the organic <strong>and</strong> 

<strong>in</strong>organic parts can be connected by the formation of chemical bonds between the organic 

<strong>and</strong> <strong>in</strong>organic molecules. Its advantage of low-temperature process allows synthesiz<strong>in</strong>g 

<strong>materials</strong> that cannot be prepared by conventional methods. It is convenient to fabricate 

<strong>and</strong> tailor the molecular properties of this new k<strong>in</strong>d of <strong>materials</strong> s<strong>in</strong>ce variable compositions, 

microstructures <strong>and</strong> thickness of th<strong>in</strong> films on different substrates as well as monoliths 

of various shapes can be easily prepared by simply adjust<strong>in</strong>g reaction parameters (for 

<strong>in</strong>stance, the ratio of precursors to water, the nature of solvents, <strong>and</strong> reaction temperature, 

etc.) [244–247]. The f<strong>in</strong>al products can exhibit good optical quality (high transmission <strong>in</strong> 

O 

O 

ð3Þ

visible region) <strong>and</strong> mechanical strength (easy process<strong>in</strong>g) required for application 

[244,247]. Moreover, the porous structure of these <strong>materials</strong> offers free spaces for photochromic 

components undergo<strong>in</strong>g reversible structural changes dur<strong>in</strong>g photochromic transformations. 

In the early research stage for <strong>hybrid</strong> photochromic <strong>materials</strong>, the attention 

has been focused on embedd<strong>in</strong>g organic or organo-metallic chromophores <strong>in</strong> transparent 

matrices or networks (SiO 2,Al 2O 3, ormocers (organically modified ceramics) <strong>and</strong> ormosils 

(organically modified silicas), etc.) made by the sol–gel method [66,248]. In recent years, 

the study of <strong>hybrid</strong>s conta<strong>in</strong><strong>in</strong>g <strong>in</strong>organic photochromic <strong>materials</strong> has become an exp<strong>and</strong><strong>in</strong>g 

research field. To realize the m<strong>in</strong>iaturization of the devices, <strong>in</strong> addition, electrostatic 

(layer-by-layer) self-assembly method is used to fabricate ultrath<strong>in</strong> films <strong>and</strong> sometimes 

even a monolayer of photochromic <strong>in</strong>organic material. Langmuir–Blodgett (LB) technique 

[249–251] may also be used to prepare ultrath<strong>in</strong> films. However, no <strong>in</strong>vestigations about 

the photochromism have been carried out on the LB films, though some of them should 

have it. 

3.1.3. Summary 

Generally speak<strong>in</strong>g, constituent elements, total charges, shapes, <strong>and</strong> sizes of the polyanions, 

as well as the versatility <strong>and</strong> donation ability of electrons <strong>and</strong>/or protons of organic 

molecules, can be readily altered or changed <strong>in</strong> order to tune the photochromic response <strong>in</strong> 

<strong>hybrid</strong>s. The challenge for these new <strong>materials</strong> is to optimize both the <strong>in</strong>organic <strong>and</strong> 

organic components on a molecular or nanometer scale [252]. The nanometer level <strong>in</strong> present 

review refers to the nanoparticles or clusters <strong>in</strong> nanosize rather than the molecules with 

the nanosize. For the <strong>hybrid</strong>s comb<strong>in</strong><strong>in</strong>g the two components at molecular level [46,253– 

263], one should prepare the complex with a CT bridge between <strong>in</strong>organic <strong>and</strong> organic 

moieties <strong>in</strong> the same molecule, specifically the donor–acceptor system. For the <strong>hybrid</strong>s 

comb<strong>in</strong><strong>in</strong>g the two constituents at nanometer level [43,45,62,264–268], one had better prepare 

the <strong>hybrid</strong> <strong>materials</strong> with a CT bridge between POM nanoparticles or clusters <strong>and</strong> 

organic molecules or matrices. In pr<strong>in</strong>ciple, the <strong>hybrid</strong> <strong>materials</strong> with good <strong>and</strong> reversible 

photochromic response may hopefully be obta<strong>in</strong>ed <strong>in</strong> these <strong>hybrid</strong>s. 

3.2. Hybrids at molecular level 


3.2.1. Donor–acceptor systems prepared from POMs <strong>and</strong> aromatic organic molecules 

The first <strong>hybrid</strong>s considered are POMs comb<strong>in</strong>ed with electron-rich aromatic organic 

molecules. These CT complexes <strong>in</strong>clude [H2qu<strong>in</strong>]3[PW12O40] Æ 4EtOH Æ 2H2O (qu<strong>in</strong> = 

qu<strong>in</strong>ol<strong>in</strong>-8-ol) [253], b-[Htmpd]x[tmpd]4 x[Mo8O26] (tmpd = N,N,N 0 ,N 0 -tetramethylp-phenylenediam<strong>in</strong>e) 

[254], b-[Htmpd]x[tmpd]3 x[HMo8O26] [254], (Et4N)5[(ZnTPP)2- 

(XM 12O 40)(Z) n] (TPP = meso-tetraphenylporphyr<strong>in</strong>; X = P, Si; M = Mo, W; Z = Cl, 

Br, I; n = 1 or 2 depend<strong>in</strong>g on the presence of a tri- or tetranegative Kegg<strong>in</strong> anion) 

[255], (QLH) 6[As 2Mo 18O 62] Æ 3QL Æ 1.5EtOH (QL = qu<strong>in</strong>ol<strong>in</strong>e) [261], 10QL Æ H 6P 2- 

W18O62 Æ 11H2O [269], <strong>and</strong> so on. Another more example is the comb<strong>in</strong>ation of phosphomolybdic 

acid (H3PMo12O40) witha-phridylazo-b-naphthol [259], <strong>in</strong> which it is said 

that the organic lig<strong>and</strong>s (oxygen <strong>in</strong> C–O bond of a-phridylazo-b-naphthol) coord<strong>in</strong>ate 

with protons <strong>in</strong> heteropoly acid through coord<strong>in</strong>ation bond. More details about the 

photochromism <strong>in</strong> such k<strong>in</strong>d of CT complexes are discussed <strong>in</strong> those prepared with protonated 

2,2 0 -biqu<strong>in</strong>ol<strong>in</strong>e (biQL) <strong>and</strong> Kegg<strong>in</strong> polyanions (a-isomers, simplified as XM 12) by 

co-crystallization method, hav<strong>in</strong>g the general formula (HbiQL) m[XM 12O 40] Æ nsolv


(X = P (m = 3), Si (m = 4); M = Mo, W; n = 0, 3; solv = H2O, DMF) [258]. The b<strong>and</strong> at 

ca. 410–420 nm <strong>in</strong> electronic spectroscopy (Fig. 19) [258] is characteristic of the synthesized 

compounds <strong>and</strong> is responsible for their yellow color, which is assigned to charge 

transfer between organic base <strong>and</strong> Kegg<strong>in</strong> anions <strong>and</strong> thus gives a direct evidence for 

the <strong>in</strong>termolecular electronic <strong>in</strong>teractions between <strong>in</strong>organic <strong>and</strong> organic moieties <strong>in</strong> 

Fig. 19. (A) Diffuse reflectance spectra of (a) 2,2 0 -biqu<strong>in</strong>ol<strong>in</strong>e, (b) H3[PW12O40] Æ nH2O, (c) (Hbiqui)3[PW12O40]. 

(B) Diffuse reflectance spectra of (a) 2,2 0 -biqu<strong>in</strong>ol<strong>in</strong>e, (b) H 4[SiW 12O 40] Æ nH 2O, (c) (Hbiqui) 4[SiW 12O 40] Æ 4H 2O 

[258].


the solid state. Evidence for the presence of such <strong>in</strong>teractions is also found for the compounds 

with [SiW12O40] 4 

anion <strong>in</strong> dimethylsulfoxide (DMSO) solution, which is 

assumed to result from <strong>in</strong>duced-dipole effects <strong>and</strong> is quite unusual for CT compounds 

<strong>based</strong> on Kegg<strong>in</strong> anions [258]. Under the irradiation of sunlight or a tungsten lamp, 

the color of compounds with XMo12 anions changed from yellow or yellow-green 

to green because the exposure to light-<strong>in</strong>duced reduction of [XMo 12O 40] n 

VI V 

to [XMo11 Mo O40] (n+1) 

. However, all these CT compounds do not exhibit good 

photochromic reversibility [258]. S<strong>in</strong>ce the extent of anion reduction followed the order 

(HbiQL)3[PMo12O40] Æ 3DMF (DMF = N,N-dimethylformamide) > (HbiQL)4[ SiMo12O40] Æ 

3DMF > (HbiQL)3[PMo12O40] > (HbiQL)4[SiMo12O40] Æ 4H2O, DMF molecules may 

play a role <strong>in</strong> the charge transfer that leads to the reduction of anions [258]. Although 

[(DMF)2H]3[PMo12O40] is observed to be photosensitive [258], no evidence has been 

found that <strong>in</strong> these <strong>hybrid</strong>s DMF is directly <strong>in</strong>volved as an electron donor. The solvate–POM 

<strong>in</strong>teractions have also been observed <strong>in</strong> H4SiW12O40 Æ 4HMPA Æ 2H2O (HMPA = hexamethylphosphoramide) CT complex, <strong>in</strong> which there is no formation of 

hydrogen bond [270]. Thus the role of solvents needs further <strong>in</strong>vestigation. 

For most aforementioned CT complexes, however, quite poor or even no photochromism 

[253–255], let alone the notorious irreversibility [258,261], has been observed. Some 

authors have argued that it is the quite weak <strong>in</strong>organic/organic electronic <strong>in</strong>teractions 

allowed <strong>in</strong> the solid state by the CT salts that have prevented the occurrence of an effective 

electron transfer between the two components. But <strong>in</strong> the case that donor <strong>and</strong> acceptor 

coexist <strong>in</strong> the same molecule, if the donor is electron-rich aromatic molecule, specifically 

the organic conductors, it will be not difficult for charge transfer to occur <strong>in</strong> the light of 

‘‘charge transfer theory’’ [271,272]. Thus extensive polarization of charge can take place 

over the whole <strong>hybrid</strong> molecule. Consequently, the electrons photoexcited to the d orbitals 

may transfer to the organic molecule, lead<strong>in</strong>g to a rather weak photochromism. S<strong>in</strong>ce usually 

one should build strong electronic <strong>in</strong>teractions between the <strong>in</strong>organic <strong>and</strong> organic 

moieties, it might be not wise to choose a molecule, especially the group connected to 

POM, with a large p-bond for the fabrication of <strong>hybrid</strong> <strong>materials</strong> with good 

photochromism. 

3.2.2. Alkylammonium POMs 

Another strategy for the synthesis of <strong>hybrid</strong>s at molecular level is to prepare alkylammonium 

POMs by us<strong>in</strong>g POMs <strong>and</strong> organic am<strong>in</strong>e. In fact, almost all the CT complexes 

with truly reversible photochromism are a few alkylammonium salts of polymolybdates 

[35,256,273], though not all of them exhibit reversible coloration [35] <strong>and</strong> some of them 

even show quite poor photoresponse [35,46,262,274]. Alkylammonium POMs are discrete 

molecules which can be differentiated from <strong>in</strong>f<strong>in</strong>ite TMOs <strong>and</strong> are photosensitive 

both as solids <strong>and</strong> <strong>in</strong> solution [35]. When the primary, secondary, <strong>and</strong> tertiary ammonium 

POMs are irradiated with UV light with wavelength correspond<strong>in</strong>g to the 

O ! M LMCT transition (k < 400 nm), the color of the crystal changes from white 

to violet, p<strong>in</strong>k, or reddish brown due to the formation of mixed valence compound 

M VI M V O5(OH) [35]. The first photochromic alkylammonium POMs, [(CH3)2NH2]2O Æ 

3MoO3 Æ H2O, has been reported <strong>in</strong> 1973 [275], which is prepared by a reaction with 

dimethylam<strong>in</strong>e <strong>and</strong> molybdic acid or molybdenum trioxide <strong>in</strong> water, or by the thermal 

decomposition of molybdenum dimethyldithiocarbamate <strong>in</strong> chloroform or methylene 

dichloride solvent. Hydrothermal synthesis method has also been used to prepare


[(C10H20)(NH2)2]2 Æ H3PMo12O40 Æ (H2O)7.5 [46,274], [(C10H20)(NH2)2]2 Æ H3PW12O40 Æ 

(H2O)2.4 [46,274], <strong>and</strong> [Cu(C12H8N2)2]1.5PW12O40 Æ 1.5H2O [260]. 

The alkylammonium POMs prepared by the aforementioned methods are usually <strong>in</strong> 

crystal state, which cannot be used to prepare th<strong>in</strong> films. Moreover, the solutions of alkylammonium 

POMs are turbid, <strong>and</strong> cast<strong>in</strong>g them <strong>in</strong>to th<strong>in</strong> films is not without difficulty. 

This limited processability may be ruled out by a so-called supramolecular template preparation 

method, by which a gelat<strong>in</strong>ous solution of alkylammonium POMs may be 

obta<strong>in</strong>ed so that an ultrath<strong>in</strong> film can be fabricated via coat<strong>in</strong>g technique. For a typical 

synthesis [276,277], the ammonium heptamolybdate ((NH4)6Mo7O24) is first added to a 

transparent alkylammonium solution prepared from alkylam<strong>in</strong>e <strong>and</strong> hydrochloric acid. 

Then the pH of the mixed solution is adjusted to 1–3. A white gel is obta<strong>in</strong>ed after heat<strong>in</strong>g 

the solution at 80 °C for about 8 h with stirr<strong>in</strong>g. The gel, generally stable <strong>in</strong> the dark for 1– 

2 months, is then sp<strong>in</strong>-coated onto the solid supports. The transparent th<strong>in</strong> films can be 

prepared from the long cha<strong>in</strong> alkylam<strong>in</strong>es (C16 <strong>and</strong> C18), but not from the short ones 

(C8, C10 <strong>and</strong> C12) [276,277]. Apart from these resultant (C 16H 33NH 3) 4Mo 8O 26 

[276,277] <strong>and</strong> (C 18H 37NH 3) 5HMo 7O 24 [245], NH 3CH 2CH 2NH 3Mo 3O 10 Æ 4.4H 2O [262], 

[(CH2OH)3CNH3]3PMo12O40 Æ 5H2O [256], [(CH2OH)3CNH3]3PW12O40 Æ 11H2O [256], 

[(CH2OH)3CNH2]2 Æ H4SiW12O40 Æ 10H2O [278], [(CH2OH)3CNH3]2H2SiW12O40 Æ 10H2O 

[256], (C6H18N2)3Mo7O24 Æ 4H2O [279], (C10H18N)6As2Mo18O62 Æ 6CH3CN Æ 8H2O [280], 

(H3dien)2Mo7O24 Æ 4H2O [281] (dien = diethylenetriam<strong>in</strong>e), (H2dien)2Mo8O26 Æ 6H2O 

[281], <strong>and</strong> MoO3(dien) [281] have also been synthesized by similar method. Most of them 

exhibit reversible photochromism. Upon irradiation with UV light, the color of 

(C 16H 33NH 3) 4Mo 8O 26 <strong>composite</strong> film reversibly changes from colorless to violet 

(Fig. 20) [277]. This violet color is quite stable (more than one year) <strong>in</strong> the dark under 

ambient conditions <strong>and</strong> can be bleached by gentle heat<strong>in</strong>g (70 °C) <strong>in</strong> the presence of 

Fig. 20. Absorption spectra of a (C16H33NH3)4Mo8O26 <strong>composite</strong> film (curve A) before <strong>and</strong> (curve B) after 

irradiation with UV light for 10 m<strong>in</strong>. The largest change <strong>in</strong> the absorbance is at 472 nm. The <strong>in</strong>set shows the 

reversibility of the coloration–decoloration cycles [277].


oxygen. The hydrogen bond <strong>in</strong> the <strong>hybrid</strong> is crucial for the photochromism. It is said [281] 

the bond length of Mo(5)–O(17) also plays an important role <strong>in</strong> the photochemical properties 

of (H3dien)2[Mo7O24] Æ 4H2O (such as photochromic <strong>and</strong> anti-cancer activity). 

3.2.3. Hybrids prepared from POMs <strong>and</strong> small biological molecules 

S<strong>in</strong>ce POMs are generally non-toxic to normal cells, as well as due to their rational <strong>and</strong> 

reproducible synthesis <strong>and</strong> flexibility <strong>in</strong> chang<strong>in</strong>g properties, they are attractive for applications 

<strong>in</strong> medic<strong>in</strong>e, such as anti-viral, anti-tumoral, anti-neoplastic <strong>and</strong> anti-carc<strong>in</strong>ogens 

[281,282]. These applications are usually associated with the comb<strong>in</strong>ation of biological 

molecules, specifically of prote<strong>in</strong>. Some of these systems will be discussed briefly <strong>in</strong> present 

review s<strong>in</strong>ce they exhibit photochromic behavior too. Am<strong>in</strong>o acid, as the basic unit of 

prote<strong>in</strong>, has attracted broad attention to its comb<strong>in</strong>ation with POMs [283–289]. Itis 

reported [290] that glyc<strong>in</strong>e (Gly) salt of 12-silicotungstate has activity <strong>in</strong> prevent<strong>in</strong>g plant 

virus, cucumber mosaic virus, the ma<strong>in</strong> harm to green pepper. The solid samples of 

(Gly) 2H 4SiW 12O 40 Æ 5.5H 2O [290] or (HGly–Gly) 3PMo 12O 40 Æ 4H 2O [291] can reversibly 

change color from white to blue under the sunlight irradiation. Fig. 21 [290] shows the 

solid reflectance electronic spectra. For the O ! W CT transition b<strong>and</strong>s, no obvious 

changes take place <strong>in</strong> peak position, but the <strong>in</strong>tensity decreases slightly. In the meantime, 

a new absorption b<strong>and</strong> appears <strong>in</strong> visible region, which is characteristic b<strong>and</strong> of heteropoly 

blue <strong>and</strong> is assigned to the IVCT (W 5+ ! W 6+ ) transition <strong>in</strong> heteropoly anion [230]. Itis 

claimed [290] that the oxidation of Gly occurs simultaneously with the reduction of 

heteropoly anion (W VI O 6 or Mo VI O 6) to heteropoly blue (W V O 5(OH) or Mo V O 5(OH)). 

In addition, 12-molybdenum phosphoric acid can react with dextran ((C 6H 10O 5) n)to 

form an <strong>in</strong>clusion compound [292]. The yellow <strong>in</strong>clusion compound irreversibly changes 

to blue-violet due to the formation of multivalent molybdenum (VI, V, IV) complex upon 

UV-light irradiation. The photochromic product is also some k<strong>in</strong>d of <strong>in</strong>clusion compound. 

The stability of photochromism can be improved by the formation of such <strong>in</strong>clusion compounds 

[292]. 

3.2.4. Visible-light coloration <strong>in</strong> CT complexes 

It is worth not<strong>in</strong>g that the absorption edge of CT complex might shift to the visible region, 

lead<strong>in</strong>g to a visible-light coloration. The complex charge-transfer electron absorption 

Fig. 21. Solid electronic spectra of (Gly)2H4SiW12O40 Æ 5.5H2O. (---) before irradiation; (—) after irradiation. All 

spectra were obta<strong>in</strong>ed from a pellet which consisted of 40% sample <strong>and</strong> 60% MgO by weight. The base of the 

pellet is MgO [290].


Fig. 22. Spectra of a-H3PMoO12O40 <strong>in</strong> TMU (solid l<strong>in</strong>e), <strong>and</strong> <strong>in</strong> H2O (dash l<strong>in</strong>e), both 0.24 mM <strong>in</strong> 10 mm cell at 

25 °C (right y-axis) <strong>and</strong> pseudo-first-order rate constants (circles) for production of PMo12O 4 

40 plotted as function 

of cut-off of <strong>in</strong>cident light (left y-axis) [239]. 

manifold of some soluble metal oxide species <strong>in</strong>clud<strong>in</strong>g heteropoly acids of Mo <strong>and</strong> W is 

known to shift substantially to lower energy on go<strong>in</strong>g from water to organic solvents 

[230,239]. These medium-<strong>in</strong>duced bathochromic spectral shifts render some POM species 

highly photosensitive [239] <strong>and</strong> are directly relevant to the use of POM as photochromic 

<strong>materials</strong>. Prosser-McCartha et al. [239] have found that, although the wavelength maximum 

for H3PMo12O40 is little perturbed on go<strong>in</strong>g from water (kmax. = 310 nm) to TMU (1,1,3,3tetramethylurea) 

(kmax. = 308 nm), the low energy absorption tail is quite shifted to the visible 

region <strong>in</strong> TMU (Fig. 22) [239]. This is ascribed to the electronic <strong>in</strong>teractions between 

POM <strong>and</strong> TMU. The <strong>in</strong>termolecular CT complex of H3PMo12O40 Æ 6DMA Æ CH3CN Æ 

0.5H 2O (DMA = N,N-dimethylacetamide) is also highly photosensitive <strong>in</strong> the near-UV or 

visible region where it absorbs either <strong>in</strong> solution or <strong>in</strong> crystall<strong>in</strong>e form [293]. Similar results 

have been reported [238,294] for H3PW12O40 <strong>in</strong> water <strong>and</strong> different organic solvents, which is 

expla<strong>in</strong>ed by that <strong>in</strong>tramolecular O ! M LMCT transitions predom<strong>in</strong>ate <strong>in</strong> the high-energy 

region of the UV–Visible spectral range near the absorption maximum <strong>and</strong> <strong>in</strong>termolecular 

CT transitions predom<strong>in</strong>ate <strong>in</strong> the low-energy region. The latter <strong>in</strong>termolecular transitions 

are largely solvent-to-POM charge transfer <strong>in</strong> orig<strong>in</strong> [238]. Similar mechanism has been 

put forward [295] to expla<strong>in</strong> the photochromism of [Ln(NMP)4(H2O)4][HxGe- 

Mo 12O 40] Æ 2NMP Æ 3H 2O (Ln = Ce IV ,Pr IV , x =0;Ln=Nd III , x = 1; NMP = N-methyl- 

2-pyrrolidone), <strong>in</strong> which the electronic <strong>in</strong>teractions between POM <strong>and</strong> organic substrate 

result from both secondary Coulombic <strong>and</strong> <strong>in</strong>duced-dipole effects [293]. It is said that the 

<strong>in</strong>termolecular CT complex of H4SiW12O40 Æ 4HMPA Æ 2H2O <strong>in</strong> solid state also has a quite 

large red-shift for the low-energy tail of the CT absorption b<strong>and</strong> <strong>in</strong> the electronic spectra 

[270] <strong>and</strong> the photochromism can be <strong>in</strong>itiated by blue-light irradiation. 

3.3. Hybrids at nanometer level 

3.3.1. Multilayer th<strong>in</strong> films prepared by electrostatic layer-by-layer method 

3.3.1.1. Preparation, structural characterization, <strong>and</strong> photochromic phenomenon. Selfassembly 

(SA) is a phenomenon <strong>in</strong> which a supramolecular hierarchical organization or


order is spontaneously established <strong>in</strong> complex systems without external <strong>in</strong>tervention. 

Attraction of SA is that any number of layers of nanoparticles or platelets of any composition 

can be adsorbed onto a large variety of structurally different substrates (size <strong>and</strong> 

topology) <strong>in</strong> any desired order [47]. Sequential adsorption of oppositely charged polyelectrolytes 

or diam<strong>in</strong>es <strong>and</strong> nanoparticles from solution (Fig. 23) [47], i.e., electrostatic 

layer-by-layer (LBL) self-assembly, is a simple yet powerful approach to construct 

supramolecular architectures with functional <strong>in</strong>organic <strong>and</strong> organic components on solid 

surfaces [296–298]. S<strong>in</strong>ce POM anions (polyanions) are negatively charged, some photochromible 

POMs have been chosen as the <strong>in</strong>organic moiety <strong>in</strong> such <strong>hybrid</strong> system; while 

positively charged polyelectrolytes or small molecules of organic diam<strong>in</strong>es are chosen as 

the counterpart. By this method the value of n, the number of POM/organic bilayer, 

can reach 40 or even more. Apart from electrostatic force, the <strong>in</strong>teractions between polyanions 

<strong>and</strong> their counterparts <strong>in</strong> these <strong>hybrid</strong>s may <strong>in</strong>clude hydrogen bond<strong>in</strong>g, van der 

Waals force, <strong>and</strong> so on [43,58]. Organic molecules can affect the assembled structure of 

<strong>in</strong>organic precursor <strong>and</strong> the stability of the resultant colored complexes, <strong>and</strong> thus tailor 

the structure <strong>and</strong> photochromic properties of the SA th<strong>in</strong> film [299]. Such k<strong>in</strong>d of photochromic 

<strong>hybrid</strong> system was first reported with the ultrath<strong>in</strong> film of P/W (P = poly-(diallyldimethylammonium) 

chloride; W = sodium decatungstate) [47], though without 

detailed characterization <strong>and</strong> discussion about photochromism. Soon after that, many 

other <strong>hybrid</strong> systems with photochromic performance have been reported, for <strong>in</strong>stance, 

WO3/4,4 0 -BAMBp (4,4 0 -bis(am<strong>in</strong>omethyl)-biphenyl) [43,300], WO3/4,4 0 -BPPOBp (4,4 0 - 

bis-(5-pyridium-pentyloxy)-biphenyl) [58,59], WO 3/1,10-DAD (1,10-diam<strong>in</strong>odecane) 

[299,301], PM 12/1,10-DAD (PM 12 =H 3PM 12O 40, M=Mo[62], W[62,302,303]), SiM 12/ 

1,10-DAD (SiM 12 =H 4SiM 12O 40, M=Mo[62], W[62,304]), K 12.5Na 1.5[NaP 5W 30O 110]/ 

poly(ethylenim<strong>in</strong>e) [305,306], [W10O32] 4 /poly(ethylenim<strong>in</strong>e) [306], <strong>and</strong> so on. 

Fig. 23. Schematics of layer-by-layer self-assembly of negatively charged POM <strong>and</strong> positively charged organic. 

Sodium decatungstate (W) <strong>and</strong> poly(diallyldimethylammonium chloride) (P) are used as examples <strong>in</strong> the diagram 

[47].


Fig. 24. AFM images of one layer of (A) P <strong>and</strong> of self-assembled S-(P/W)n films with (B) n = 1 <strong>and</strong> (D) n = 2 <strong>and</strong> 

(C) of a self-assembled S-(P/W) 1/P film on mica substrates. Here S refers to the mica substrate [47]. 

The topography of such <strong>hybrid</strong> films can be obta<strong>in</strong>ed from AFM images (Fig. 24) [47]. 

Accord<strong>in</strong>g to Fig. 24A, the first layer of P shows a featureless AFM image with a height


variation of ca. 2 nm. Adsorption of W onto the P film manifests itself <strong>in</strong> the formation of 

numerous doma<strong>in</strong>s with a height variation of ca. 4 nm (Fig. 24B). Self-assembly of the 

second layer of P to form the P/W/P film decreases the height variation to below 3 nm 

(Fig. 24C), <strong>and</strong> adsorption a layer of W to produce (P/W)2 results <strong>in</strong> the same type of 

doma<strong>in</strong>s (Fig. 24D) as imaged <strong>in</strong> Fig. 24B. The thicknesses of the P/W unit <strong>and</strong> of the first 

P layer are ca. 6.2 <strong>and</strong> 2 nm, respectively. 

UV–Vis absorption spectra (Fig. 25) [43] are usually used to track the SA process of a 

self-assembly multilayer (SAM) th<strong>in</strong> film, from which the lamellar growth process of the 

alternat<strong>in</strong>g <strong>in</strong>organic <strong>and</strong> organic layers can be confirmed. Absorbance correspond<strong>in</strong>g to 

the one from organic molecule <strong>and</strong> from O ! M LMCT <strong>in</strong>creases proportionally with 

<strong>in</strong>creas<strong>in</strong>g n, <strong>in</strong>dicat<strong>in</strong>g a quantitative <strong>and</strong> reproducible SA process. 

Another technique used to characterize the layered structure of SAM film is the smallangle 

X-ray diffraction (Fig. 26) [43], from which the d-space can be determ<strong>in</strong>ed. In these 

<strong>hybrid</strong> SAM systems, the alignment of organic molecules may be <strong>in</strong>cl<strong>in</strong>ed [43,58,59,300] or 

perpendicular [299,301–303] to the <strong>in</strong>organic counterparts. More important is that there is 

no doubt that the <strong>in</strong>organic moiety <strong>in</strong>corporates <strong>in</strong>to the <strong>composite</strong> film without structural 

alteration, <strong>and</strong> the polyanions penetrate <strong>in</strong>to <strong>and</strong> adsorb onto an underly<strong>in</strong>g organic layer 

to form an <strong>in</strong>organic/organic ion complex [47]. 

For the aforementioned <strong>hybrid</strong> systems, the absorbance <strong>in</strong> visible spectral region 

<strong>in</strong>creases under UV-light irradiation (Fig. 27) [301], <strong>in</strong>dicat<strong>in</strong>g the photochromic behavior 

of these SAM films. The changes <strong>in</strong> the absorbance after coloration for some of these systems 

are displayed <strong>in</strong> Table 1. The absorbance returns to its <strong>in</strong>itial state after the colored 

film is heated (such as to 70 °C) <strong>in</strong> the presence of O 2 for a certa<strong>in</strong> duration (such as 1 h 

or 2). 

3.3.1.2. High photochromic reversibility. Although many photochromic POM salts have 

been reported <strong>in</strong> the literature [35,307], it is noted that hitherto only few POMs have been 

reported to exhibit good reversible coloration [35]. For MoO3 <strong>and</strong> WO3, the best photochromic 

TMOs, once the colored samples are bleached by heat<strong>in</strong>g <strong>in</strong> the presence of O2 

they cannot be photocolored aga<strong>in</strong> [19,20,30,31]. However, it is reported that most <strong>hybrid</strong> 

systems prepared by electrostatic LBL SA method exhibit good reversible photocoloration–decoloration 

cycle even <strong>in</strong> the case they are thermally bleached <strong>in</strong> the presence of 

O2 (e.g., more than 20 times for WO3/4,4 0 -BAMBp <strong>and</strong> more than 50 for WO3/4,4 0 - 

BPPOBp <strong>and</strong> PM12/1,10-DAD). This feature thus shows considerable significance <strong>and</strong> 

promise, especially for the devices of reusable <strong>in</strong>formation storage media, display, signal 

process<strong>in</strong>g <strong>and</strong> chemical switch. 

The reason account<strong>in</strong>g for such difference <strong>in</strong> reversibility is that they possess different 

photochromic mechanisms. For TMOs, the presence of oxygen vacancies or defects is critical 

for the photochromism [19,20,30,31,84,308]. If coloration is caused by the formation 

of F-like centers by the capture of one or two photogenerated electrons excited from 

valence b<strong>and</strong> to the defect b<strong>and</strong> formed by oxygen vacancies or defects [19,20,30,31], these 

oxygen vacancies or defects can be destroyed when the colored TMOs are bleached by 

heat<strong>in</strong>g <strong>in</strong> the presence of O2, result<strong>in</strong>g <strong>in</strong> an irreversible coloration–decoloration process. 

If the coloration is due to the formation of hydrogen bronze, the light-<strong>in</strong>duced decomposition 

of water is necessary for the coloration. Upon irradiation, the water adsorbed at surface 

or <strong>in</strong>terface is decomposed <strong>in</strong>to protons <strong>and</strong> highly reactive oxygen atoms [84]. The 

oxygen vacancies or defects <strong>in</strong>side sample can trap the nascent oxygen radicals so that


Fig. 25. Absorption spectra of WO3/4,4 0 -BAMBp SAM films with different number of layers. The <strong>in</strong>set is the 

dependence of optical absorbance of these films on the number of layers at different wavelength [43]. 

Fig. 26. X-ray diffraction pattern of a SAM film composed of 40 alternat<strong>in</strong>g layers of WO3 <strong>and</strong> 4,4 0 -BAMBp [43]. 

the back-reaction of water decomposition is efficiently suppressed. Thus, if the oxygen 

vacancies or defects are destroyed <strong>in</strong> the bleach<strong>in</strong>g process, the sample cannot show photochromic 

effect aga<strong>in</strong> due to the fast back reaction. As for the pure POMs, the photochromic 

irreversibility results from as yet uncharacterized side reactions dur<strong>in</strong>g both the 

coloration <strong>and</strong> decoloration [35]. 

In the case of the SAM <strong>hybrid</strong> systems, especially for those conta<strong>in</strong><strong>in</strong>g am<strong>in</strong>o-end 

groups, as discussed <strong>in</strong> Section 3.1.1, upon photoexcitation a colored CT complex is 

formed. When the colored species are heated <strong>in</strong> the presence of oxygen molecules, the

Fig. 27. Absorption spectra of a 28-layer WO 3/1,10-DAD SAM film (A, A 0 ) <strong>and</strong> a 40-layer WO 3/4,4 0 -BAMBp 

SAM film (B, B 0 ): A <strong>and</strong> B: spectra of freshly prepared or bleached films; A 0 <strong>and</strong> B 0 : spectra of the films irradiated 

with UV light <strong>in</strong> air for 3 m<strong>in</strong> [301]. 

Table 1 

Changes <strong>in</strong> the absorbance of some SAM th<strong>in</strong> films after <strong>and</strong> before the photocoloration 

WO 3/4,4 0 - 

BAMBp 


WO 3/4,4 0 - 

BPPOBp 

WO 3/1,10- 

DAD 

PMo 12/1,10- 

DAD 

SiMo 12/1,10- 

DAD 

PW 12/1,10- 

DAD 

SiW 12/1,10- 

DAD 

References [43] [59] [301] [62] [62] [62] [62,304] 

n 40 40 28 40 40 40 40 a or 17 

Thickness 

(nm) 

27 28 78.7 70 68 64 65 a 

DOD at 0.0055 0.0084 0.045 0.168 0.151 0.030 Not obvious 

1050 nm 

a 

or 0.003 

(at 800 nm) 

a 

From Ref. [62]. 

reverse reaction takes place. The bleach<strong>in</strong>g reaction is <strong>in</strong>itiated by an electron transfer 

from the metal ion with lower oxidation state to the oxygen molecule (Eq. (4)) <strong>and</strong>, consequently, 

the orig<strong>in</strong>al state is restored (Eq. (3)). So the heat<strong>in</strong>g does not destroy anyth<strong>in</strong>g 

pert<strong>in</strong>ent to the coloration process <strong>and</strong> thus the SAM <strong>hybrid</strong> films exhibit good reversible 

photochromism. 

3.3.1.3. High photochromic response. It is noted from Table 1 that the <strong>hybrid</strong>s <strong>based</strong> on 

molybdenum POMs usually show a high photochromic response, while the counterparts 

<strong>based</strong> on tungsten not. The former also exhibits a much slower bleach<strong>in</strong>g speed than 

the latter. Similar phenomena have been observed <strong>in</strong> the systems of CT complexes 

[258,274], POMs embedded <strong>in</strong> polymeric matrices [264,309] or organic/silica matrices 

[267], <strong>and</strong> POMs anchored to organic polymeric backbone [60]. 

It has been po<strong>in</strong>ted out that the photochromic properties of <strong>materials</strong> comb<strong>in</strong><strong>in</strong>g 

POMs with organic moieties are closely relevant to the reduction potentials of polyanions 

<strong>and</strong> to the donor properties of organic molecules [258]. Bear<strong>in</strong>g the aforementioned


photochromic mechanism <strong>in</strong> m<strong>in</strong>d, an electron will transfer from O 2p (valence b<strong>and</strong>-like) 

to the d orbital of central metal (conduction b<strong>and</strong>-like) upon photoexcitation, lead<strong>in</strong>g 

to its reduction. Obviously, the easier the valence-electron transition is <strong>and</strong>/or the more 

difficult the oxidation of the reduced central metal is; the better the photochromic performance 

is. So the POM with a more positive redox potential should give a better photochromic 

response. It is known that the energy level of Mo 6+ ions lies lower than that of 

W 6+ ions [106], which agrees with the fact that the redox potential of polyoxomolybdate 

is more positive than its tungsten-<strong>based</strong> counterpart. Moreover, the b<strong>in</strong>d<strong>in</strong>g energy of O 

2p electrons <strong>in</strong> POMs decreases (i.e., the peak value of absorption b<strong>and</strong> correspond<strong>in</strong>g 

to O ! M LMCT <strong>in</strong> UV–Vis spectra occurs a red-shift) <strong>in</strong> the same order as the redox 

potential of POMs shifts to a more positive value (Table 2). This is reasonable s<strong>in</strong>ce both 

the redox potential <strong>and</strong> b<strong>in</strong>d<strong>in</strong>g energy of valence electrons are characteristics of the ability 

for donat<strong>in</strong>g-withdraw<strong>in</strong>g electrons. Therefore, one can readily choose a proper POM 

for the <strong>hybrid</strong> system possibly with a high photochromic response accord<strong>in</strong>g to its redox 

potential or the b<strong>in</strong>d<strong>in</strong>g energy of valence electrons. This correlation has been confirmed 

by the fact (Table 2) that the photochromic response (DOD or the ratio of reduced central 

metal), when comb<strong>in</strong>ed with the same organic molecules (1,10-DAD), decreases <strong>in</strong> the 

order of H3PMo12O40 >H4SiMo12O40 >H3PW12O40 >H4SiW12O40, which agrees with 

the order of the negative shift of their redox potential. In addition, the molybdenum-<strong>based</strong> 

POMs have higher proton aff<strong>in</strong>ities than their tungsten-<strong>based</strong> counterparts [310,311], <strong>in</strong>dicat<strong>in</strong>g 

the process of proton transfer from am<strong>in</strong>e to the former is easier than to the latter. 

This may also account for that the system conta<strong>in</strong><strong>in</strong>g molybdenum has a better photochromic 

response. 

It is noteworthy here that some <strong>hybrid</strong> systems (PMo 12 or SiMo 12 comb<strong>in</strong>ed with 1,10- 

DAD) have a very high photochromic response (Tables 1 <strong>and</strong> 2). A considerably th<strong>in</strong> film 

(such as around 70 nm) can show an equivalent photochromic response (<strong>in</strong> DOD) as a 

MoO3 th<strong>in</strong> film with the thickness of ca. 1 lm [62]. However, the photochromic performance 

is quite poor for a MoO3 film with the thickness of dozens of nanometers. Furthermore, 

even one PMo12 monolayer deposited onto an am<strong>in</strong>osilane premodified solid 

support exhibits an observable photochromic response, which is doubled after modified 

by an am<strong>in</strong>e monolayer (Fig. 28) [61], confirm<strong>in</strong>g the role of organic moiety for the photochromism. 

This strik<strong>in</strong>g feature is of great significance for the m<strong>in</strong>iature of devices, specifically 

for the 3-D ultrahigh-density optical memory devices. 

It is known that the protons <strong>and</strong> photogenerated electrons <strong>and</strong> holes play a key role <strong>in</strong> 

the photochromism for both SA <strong>and</strong> TMO th<strong>in</strong> films. The superior photochromic 

response of SAM th<strong>in</strong> film to its TMO counterpart may be relevant to the behavior of 

Table 2 

Key parameters for some SAM th<strong>in</strong> films [62] 

POM Redox 

potential 

(V) 

B<strong>in</strong>d<strong>in</strong>g energy 

for O 2p 

electrons (eV) 

Absorption b<strong>and</strong> 

correspond<strong>in</strong>g to 

O ! M LMCT (nm) 

D(OD)max after 

a 30-m<strong>in</strong> 

UV-irradiation 

Ratio of 

reduced Mo or 

W <strong>in</strong> films (%) 

PMo12 0.27 6.45 310 0.168 32 70 

SiMo 12 0.23 6.50 306 0.151 28 68 

PW 12 0.02 7.01 269 0.03 5 64 

SiW12 0.24 7.22 260 Not obvious Not obvious 65 

Thickness of 

a 40-layer 

film (nm)


Fig. 28. UV–Visible spectrum of the PMo12 monolayer. Inset (a) is the UV–Visible spectra of the monolayer 

before <strong>and</strong> after 5 m<strong>in</strong> UV-irradiation, the dashed l<strong>in</strong>e is the photochromic response after the assembly of another 

organic am<strong>in</strong>e layer on the monolayer film. Insert (b) is the photographs of the monolayer before <strong>and</strong> after UVirradiation 

captured by a microscope equipped with a color CCD camera [61]. 

these charges as well as, possibly, to the efficient utilization of excitation light. For TMOs 

film, the positive holes generated <strong>in</strong>side the film transfer to the <strong>in</strong>terface or surface to oxidize 

adsorbed water or small organic molecules <strong>and</strong> the resultant protons are <strong>in</strong>jected 

<strong>in</strong>side the film by coulomb attraction to form hydrogen bronze or to neutralize the system. 

It is clear that there is a mass-transfer step for both holes <strong>and</strong> protons dur<strong>in</strong>g this process, 

which may limit the photochromic activity. For MoO3 th<strong>in</strong> film, <strong>in</strong> addition, the photochromic 

sensitivity <strong>in</strong>creases with the <strong>in</strong>creas<strong>in</strong>g film thickness [312] <strong>and</strong> reaches the 

maximum when the film thickness equals to the light-penetration depth (1/a, ca.1lm, 

a is the absorption coefficient). This may correlate film thickness to the efficient utilization 

of excitation light. Last, for large b<strong>and</strong> gap oxide semiconductors (such as MoO3), it is 

ma<strong>in</strong>ly the carriers generated with<strong>in</strong> the depletion (space charge) layer that will contribute 

to the photochemical reaction because these semiconductors have small mobilities <strong>and</strong> 

short hole (m<strong>in</strong>ority carrier) diffusion lengths [34]. Therefore, only a very th<strong>in</strong> layer of 

MoO3 close to the surface or <strong>in</strong>terface can take part <strong>in</strong> the coloration process. 

For the SA th<strong>in</strong> films, the situations are quite different. First, there is no need for holes 

<strong>and</strong> protons to undergo a long-distance transfer to form the colored species. Second, it 

seems that, unlike the case for MoO3, all the photogenerated carriers may contribute to 

the photochemical reaction s<strong>in</strong>ce there is no depletion layer present <strong>in</strong> the POM clusters 

or nanoparticles due to their small size. More important is that, s<strong>in</strong>ce no obvious photochromic 

response is observed for the pure PMO12 molecules deposited onto an unmodified 

substrate, it is assumed only the molybdenum atoms that <strong>in</strong>teract with am<strong>in</strong>o headgroups 

are photoreducible [61]. The protons not come only from the organic am<strong>in</strong>e (for each 

POM layer) but also from the premodified am<strong>in</strong>osilane (for the first PMo 12 monolayer).


Third, it has been claimed that almost all the polyanions embedded <strong>in</strong> the ultrath<strong>in</strong> SAM 

film is electrochemically active <strong>and</strong> thereby contribute to the electrochromism [47]. Consider<strong>in</strong>g 

the similarity between photochromism <strong>and</strong> electrochromism of POMs [35], itis 

reasonable to deduce that almost all the polyanions self-assembled <strong>in</strong> the SAM film is photochemically 

active <strong>and</strong> can contribute to the photochromism. The last reason might be 

the multireflection of light <strong>in</strong> the SAM th<strong>in</strong> film so that the efficient utilization of light 

can be improved. However, further experimental evidences should be sought to confirm 

these po<strong>in</strong>ts. 

In addition, one big argument for the <strong>hybrid</strong> <strong>materials</strong> is the fast response time for both 

coloration <strong>and</strong> decoloration. Some authors [60] have claimed that the <strong>hybrid</strong> films conta<strong>in</strong><strong>in</strong>g 

tungsten heteropolyoxometallates show faster coloration <strong>and</strong> bleach<strong>in</strong>g than pure 

tungsten oxide films. However, up to now no detailed reports are available. For the <strong>hybrid</strong> 

system, at least <strong>in</strong> pr<strong>in</strong>ciple, one may expect that the response time should be faster than 

the pure <strong>in</strong>organic <strong>materials</strong> s<strong>in</strong>ce, unlike the case <strong>in</strong> TMO films, only the <strong>in</strong>tramolecular 

O ! M LMCT transitions <strong>and</strong>/or the <strong>in</strong>termolecular CT transitions between the neighbored 

<strong>in</strong>organic <strong>and</strong> organic moieties take place <strong>in</strong> the <strong>hybrid</strong> systems so that there is 

no need for the protons <strong>and</strong> holes to travel through a long distance <strong>in</strong> the <strong>hybrid</strong>s dur<strong>in</strong>g 

the coloration <strong>and</strong> decoloration process (Eq. (3)). 

3.3.2. POMs embedded <strong>in</strong> polymeric matrices 

By sol–gel method (cast-, sp<strong>in</strong>- or dip-coat<strong>in</strong>g), th<strong>in</strong> films can be prepared from the 

<strong>hybrid</strong> <strong>materials</strong> synthesized by the methods mentioned <strong>in</strong> Section 3.2. However, the crack 

formation is usually <strong>in</strong>evitable due to the shr<strong>in</strong>kage dur<strong>in</strong>g the dry<strong>in</strong>g process. In order to 

get a better film, the <strong>hybrid</strong>s can be fabricated at nanometer level by a so-called nano<strong>composite</strong> 

method, which can provide a new tool to improve the processability <strong>and</strong> stability of 

nanocrystals with <strong>in</strong>trigu<strong>in</strong>g novel optical, electronic, <strong>and</strong> magnetic capacities [229]. The 

general pr<strong>in</strong>ciples <strong>in</strong> the construction of such nano<strong>composite</strong>s <strong>in</strong>volve the <strong>in</strong>timate mix<strong>in</strong>g 

of constituents with processable matrices (polymers, glasses, <strong>and</strong> ceramics). Polymers are 

<strong>in</strong>troduced <strong>in</strong>to the <strong>hybrid</strong> systems thanks to their versatile structural property <strong>and</strong> functionality. 

A ma<strong>in</strong> advantage of <strong>in</strong>organic/polymeric <strong>hybrid</strong>s is that the polymeric matrix 

can improve the stability <strong>and</strong> make nanometer scale POM clusters disperse very well <strong>in</strong> 

the <strong>composite</strong>s [313]. Entrapment is relevant to cha<strong>in</strong> orientation, chemical reaction <strong>and</strong> 

structural relaxation, as well as the <strong>in</strong>teractions between POM <strong>and</strong> polymeric matrix. Different 

f<strong>in</strong>al film structures may result <strong>in</strong> significantly different potential properties. The 

polymer used as a matrix <strong>in</strong> the <strong>hybrid</strong> <strong>materials</strong> prepared by sol–gel technique should 

meet the follow<strong>in</strong>g requirements: (1) good film-form<strong>in</strong>g <strong>and</strong> adhesive behaviors on many 

solid supports; (2) highly soluble <strong>in</strong> water or other proper solvents <strong>in</strong> order to prevent the 

phase separation dur<strong>in</strong>g the reaction; (3) conta<strong>in</strong><strong>in</strong>g group that prefers to complex with 

many <strong>in</strong>organic salts result<strong>in</strong>g <strong>in</strong> the good dispersion of <strong>in</strong>organic counterparts; (4) the 

resultant film should exhibit good optical quality (high transmission <strong>in</strong> visible range <strong>and</strong> 

low scatter<strong>in</strong>g loss) <strong>and</strong> mechanical strength (easy process<strong>in</strong>g). The polymers used <strong>in</strong> the 

photochromic <strong>hybrid</strong>s <strong>in</strong>clude polyacrylamide (PAM), polyv<strong>in</strong>yl alcohol (PVA), polyethylene 

glycol (PEG), polyv<strong>in</strong>yl pyrrolidone (PVP), <strong>and</strong> the like. 

3.3.2.1. POMs embedded <strong>in</strong> PAM. Kegg<strong>in</strong> type tungsten (PW12) [44,264,265,313–318] or 

molybdenum (PMo12) [264] heteropolyoxometallate acid <strong>and</strong> Dawson type tungsten 

Þ heteropolyoxometallate [309] have been used 

ðP2W18O 6 

62 

Þ or molybdenum ðP2Mo18O 6 

62


as the photoactive constituents <strong>in</strong> <strong>hybrid</strong> films <strong>based</strong> on PAM matrix. Here PW12 is used 

as an example to elucidate the photochromism <strong>and</strong> related phenomena. The geometry of 

PW12 is preserved <strong>in</strong>side the matrix, <strong>and</strong> a CT bridge is built between PW12 <strong>and</strong> PAM via 

hydrogen bond [44,264,265,313–318]. These N–H O hydrogen bonds are considered as 

normal <strong>and</strong> weak ones, of which the bond energy is ca. 20 kJ mol 1 [319]. The PW12 clusters 

or nanoparticles are well dispersed <strong>in</strong> <strong>hybrid</strong> film <strong>and</strong> are surrounded by PAM. The 

average diameter for PW 12 particles at a low concentration (9 wt% PW 12) is ca. 50 nm, 

while ca. 65 nm with simultaneous evolution of agglomeration at a high concentration 

(38 wt% PW12) [313]. Upon UV-light irradiation, the transparent PW12/PAM films are 

photochemically reduced, which can be tailored by tun<strong>in</strong>g PW12 content [313,318]. The 

one with a low concentration of PW12 (9–28 wt%) can be reversibly reduced to heteropoly 

blues <strong>and</strong> that with a high concentration (38–44 wt%) irreversibly reduced to heteropoly 

browns (highly-reduced products) [318]. The brown films might be used as permanent 

optical storage <strong>materials</strong> <strong>and</strong> the blue ones as re-usable <strong>in</strong>formation-storage <strong>materials</strong>. 

Moreover, the IVCT b<strong>and</strong> gap (peak position <strong>in</strong> the absorption spectra) for the films with 

the concentration of PW 12 from 9 to 28 wt% is ca. 2.0 eV (620 nm), whereas that for 

38 wt% <strong>and</strong> 44 wt% is ca. 2.3 eV (540 nm) <strong>and</strong> 2.7 eV (460 nm), respectively. 

No absorption appears <strong>in</strong> visible region before photoirradiation. For the <strong>hybrid</strong> film 

with a low concentration of PW12[44,314,315], after exposure to the UV light two broad 

absorption b<strong>and</strong>s appear respectively at 600–700 <strong>and</strong> 490 nm <strong>in</strong> the UV–Vis spectra of the 

colored film (Fig. 29) [315]. They are assigned to IVCT <strong>and</strong> <strong>in</strong>tensity-enhanced d–d transitions, 

respectively [230,320]. In addition, it is said [44,314,315,318] that the organic radicals 

are formed dur<strong>in</strong>g the coloration process. The colored film can be bleached by 

heat<strong>in</strong>g <strong>in</strong> the presence of oxygen [44,314]. However, it cannot be bleached by oxygen 

at ambient atmosphere, while colored solution of PW12/PAM can fade <strong>in</strong> air at room temperature 

[44,314]. This is relevant to the diffusion velocity of oxygen <strong>in</strong> polymeric network 

[44,314,318]. In addition, before illum<strong>in</strong>ation the uncolored film is soluble <strong>in</strong> water, while 

the photocolored one is stable <strong>in</strong> water due to the formation of cross-l<strong>in</strong>ked networks by 

the radical polymerization between the polymeric radicals. 

Fig. 29. UV–Vis spectra of the <strong>hybrid</strong> film with 9 wt% PW12 irradiated with different durations [315].


The wavelength of d–d transition is unaffected by the extent of anion reduction <strong>in</strong> UV– 

Vis spectra s<strong>in</strong>ce it is not an <strong>in</strong>tervalence transition [230,320], while the wavelength <strong>and</strong> 

density of IVCT b<strong>and</strong> change with <strong>in</strong>creas<strong>in</strong>g irradiation time (Fig. 29). It is claimed 

[315] that this is because the photoreduction proceeds from one-electron to two-electron 

blue stage with <strong>in</strong>creas<strong>in</strong>g irradiation time. After the film is irradiated with UV light for 

2 m<strong>in</strong>, the k max of IVCT b<strong>and</strong> is at ca. 720 nm (1.7 eV), which is assigned to a one-electron 

reduction of POMs [227,320]. As irradiation time <strong>in</strong>creases, the wavelength of IVCT b<strong>and</strong> 

exhibits a blue-shift until a m<strong>in</strong>imum appears. When the irradiation time reaches 10 m<strong>in</strong>, 

kmax of IVCT is at ca. 620 nm (2.0 eV), which is ascribed to a two-electron reduction of 

POMS [227,320]. The relative <strong>in</strong>tensity of IVCT for two-electron reduction is almost twice 

as that for one-electron reduction. 

It has been reported [265,316] that the ultrasound irradiation dur<strong>in</strong>g the preparation 

has a remarkable effect on the microstructure <strong>and</strong> photochromic properties of the <strong>hybrid</strong> 

films. Sonochemistry aris<strong>in</strong>g from acoustic cavitation phenomena—the formation, 

growth, <strong>and</strong> implosive collapse of bubbles <strong>in</strong> a liquid medium—is a useful technique for 

generat<strong>in</strong>g f<strong>in</strong>ely dispersed nanoparticles <strong>in</strong> <strong>composite</strong> <strong>materials</strong> [321–323]. Although the 

stability of <strong>hybrid</strong> film decreases slightly after the sonication [316], for the <strong>hybrid</strong> film with 

a high concentration of PW12, PW12 nanoparticles are well separated from each other <strong>and</strong> 

homogeneously dispersed <strong>in</strong> the polymeric matrix after sonication (Fig. 30B) [265], 

whereas the nanoparticles without sonication agglomerate together (Fig. 30A) [265]. Furthermore, 

compar<strong>in</strong>g with the samples without sonication [264,315,318], one strik<strong>in</strong>g feature 

for the system with sonication is that, after a long time (such as 40 m<strong>in</strong>) irradiation 

with UV light, the colored species of <strong>hybrid</strong> film changes from irreversible heteropoly 

browns to reversible heteropoly blues (Fig. 31) [265]. This is ascribed to the sonochemical 

controll<strong>in</strong>g over <strong>in</strong>terfacial <strong>in</strong>teractions <strong>in</strong> the <strong>hybrid</strong> system [265,316]. Two different <strong>in</strong>terfacial 

<strong>in</strong>teractions result<strong>in</strong>g from non-covalent <strong>in</strong>teractions exist <strong>in</strong> the <strong>hybrid</strong> system, <strong>in</strong> 

which hydrogen bonds <strong>and</strong> electrostatic forces play a key role <strong>in</strong> the <strong>hybrid</strong> films. One 

is between the polyanions <strong>and</strong> polymeric matrix; another is between polymer cha<strong>in</strong>s. Upon 

sonication the latter is weakened while the former is enhanced <strong>in</strong> the meantime, which 

leads to the changes <strong>in</strong> microstructure <strong>and</strong> photochromism of the <strong>hybrid</strong> <strong>materials</strong>. 

3.3.2.2. POMs embedded <strong>in</strong> PVA. Polyv<strong>in</strong>yl alcohol (PVA) is another important polymeric 

matrix used <strong>in</strong> the photochromic <strong>hybrid</strong> <strong>materials</strong> [266,324–326]. The photoactive 

<strong>in</strong>organic <strong>in</strong>cludes WO3 [266,327–329], H4W10O32 [330], PW12 [325], SiW12 [325,331], 

PMo12 [236], H4GeW12O40 [326], H3PW11MoO40 [332], H6P2W18O62 [324], <strong>and</strong> TiO2 

[327,333,334]. Thanks to its spiral structure, PVA with partial carbonyl groups can form 

<strong>in</strong>clusion complex with the multivalent POM complex <strong>and</strong> thus <strong>in</strong>crease the photochromic 

stability [236]. The <strong>in</strong>termolecular hydrogen bond can be formed <strong>in</strong> these <strong>hybrid</strong> <strong>materials</strong> 

[331]. The <strong>hybrid</strong> th<strong>in</strong> films exhibit considerably high proton conductivity, which <strong>in</strong>creases 

with the <strong>in</strong>creas<strong>in</strong>g relative humidity <strong>and</strong> decreases as photochromism occurs <strong>in</strong> film [325]. 

It is reported that both the proton [326] <strong>and</strong> electron [331] conductivity of such <strong>hybrid</strong> 

films <strong>in</strong>crease with the <strong>in</strong>creased content of heteropolyacid (HPA) s<strong>in</strong>ce the HPA exhibits 

relatively good proton <strong>and</strong> electron conductivity. 

PVA/WO3 <strong>hybrid</strong>s can be produced by us<strong>in</strong>g a sol–gel process <strong>in</strong>volv<strong>in</strong>g ion exchange 

of sodium tungstate dihydrate (Na2WO4 Æ 2H2O) [266,328,329]. WO3-rich doma<strong>in</strong>s hav<strong>in</strong>g 

radius of gyration of about 5.7 nm disperse evenly <strong>in</strong> PVA matrix with the correlation 

length of 20–30 nm. In the vic<strong>in</strong>ity of the <strong>in</strong>terface of doma<strong>in</strong>s composed of WO3


Fig. 30. TEM images <strong>and</strong> histogram of the particle-size distribution for <strong>hybrid</strong> films (A) before <strong>and</strong> (B) after 

ultrasound treatment [265]. 

networks, some PVA cha<strong>in</strong>s are partly <strong>in</strong>corporated. The physical properties of the 

<strong>hybrid</strong>s are affected by the morphology. For example, the mechanical properties of the 

<strong>hybrid</strong> are improved markedly with the <strong>in</strong>creased amount of WO3. This is caused by 

strong <strong>in</strong>teractions between PVA cha<strong>in</strong>s <strong>and</strong> WO3 doma<strong>in</strong>s, which are formed by <strong>in</strong>corporation 

of PVA cha<strong>in</strong>s <strong>in</strong>to WO3 doma<strong>in</strong>s hav<strong>in</strong>g hydrogen bond<strong>in</strong>g. The transparent 

<strong>hybrid</strong> changes from (pale) yellow to blue under UV-light irradiation, especially at low 

concentration of WO 3, <strong>and</strong> is bleached with elapsed time <strong>in</strong> the dark after photoirradiation. 

The reversibility of this system is relevant to the humidity s<strong>in</strong>ce the adsorbed water 

<strong>in</strong> the <strong>hybrid</strong>s plays an important role on photochromism [266,335]. By us<strong>in</strong>g a picosecond 

pump–probe technique [328,329], the IVCT b<strong>and</strong>s <strong>in</strong> red-near IR region (1.34, 2.0, <strong>and</strong>


Fig. 31. UV–Vis spectra of <strong>hybrid</strong> films (A) without <strong>and</strong> (B) with ultrasonic pre-treatment: (a) before UV 

irradiation; (b) UV irradiation of (A) for 25 m<strong>in</strong>; (c) after two months <strong>in</strong> air of (b); (d) UV irradiation of (B) for 

40 m<strong>in</strong>; (e) after 7 h <strong>in</strong> air of (d); (f) after 15 h <strong>in</strong> air of (d). The <strong>in</strong>set <strong>in</strong> (B) shows the coloration–decoloration 

cycles of the film at 700 nm [265]. 

2.4 eV) are found to exhibit transient bleach<strong>in</strong>g that decays monoexponentially with the 

characteristic time of similar to 300 ps. These b<strong>and</strong>s are assigned to the IVCT transition 

between the ground state energy level of W 5+ ion <strong>and</strong> energy levels of W 6+ ion split <strong>in</strong> 

rhombically distorted octahedron lig<strong>and</strong> field. Absorption b<strong>and</strong>s at 0.98 <strong>and</strong> near


1.6 eV are ascribed to IVCT transition W 5+ ! W 6+ <strong>in</strong> triangular group of three-cornershared 

WO6 octahedra, reduced by one electron [328,329]. 

H4SiW12O40/PVA [331] <strong>and</strong> H3PW11MoO40/PVA [332] ultraf<strong>in</strong>e fiber aggregates or 

mats have been prepared by an electrosp<strong>in</strong>n<strong>in</strong>g technique. The color of these fibers 

changes from white to blue under UV-light irradiation. In the meantime, PVA is oxidized 

to unsaturated ketone or aldehyde. The blue color decays to white aga<strong>in</strong> under ambient 

conditions. Two broad b<strong>and</strong>s (centered at 500 <strong>and</strong> 650 or 730 nm), correspond<strong>in</strong>g respectively 

to the d–d <strong>and</strong> IVCT transition, appear <strong>in</strong> the spectra of the irradiated fibers. 

H4SiW12O40 is photoreduced via one-electron reaction [331], while for H3PW11MoO40/ 

PVA it is Mo 6+ [332] that is reduced to Mo 5+ upon UV-light illum<strong>in</strong>ation s<strong>in</strong>ce no 

ESR signals from tungsten are observed. This is reasonable s<strong>in</strong>ce the energy level of 

Mo 6+ ions lies lower than that of W 6+ ions [106]. The photoreduction process goes further 

with <strong>in</strong>creased irradiation time. It is noted that for H4W10O32/PVA <strong>hybrid</strong> system, formed 

by the reaction of white powdery tungstic acid <strong>in</strong> aqueous solution conta<strong>in</strong><strong>in</strong>g PVA, the 

reduction degree of decatungstate anion <strong>in</strong> solution is relevant to the acidity [330]. Oneelectron 

reduced form is produced ma<strong>in</strong>ly at high pH value, while two-electron reduced 

form ma<strong>in</strong>ly at low pH value. 

Unlike the PW12/PAM systems [313,315,318], the color of photochemically reduced 

H6P2W18O62/PVA <strong>hybrid</strong> film changes from light blue to deep blue with <strong>in</strong>creas<strong>in</strong>g irradiation 

time <strong>and</strong> HPA content [324]. However, the decay of the absorbance under ambient 

conditions speeds up with the <strong>in</strong>creas<strong>in</strong>g HPA content [324]. This is because the residual 

hydroxy of PVA, which decreases with the <strong>in</strong>creas<strong>in</strong>g content of HPA, can affect the stability 

of the oxidic state of the organic matrix. In addition, it is said [236] that PMo 12/PVA 

solid solution exhibits an irreversible photochromism. 

When TiO2 is irradiated <strong>in</strong> the presence of PVA [327,333,334], a blue color may appear 

due to the absorption of trapped electrons <strong>and</strong>, occasionally, due to the free electrons [31]. 

PVA can function as a hole scavenger <strong>and</strong> be oxidized by positive holes to polymer ketone. 

In the absence of PVA, coloration is hardly observed <strong>in</strong> TiO2 upon UV-light irradiation at 

room temperature. Due to a much smaller permeability of O2 <strong>in</strong>to the PVA matrix, the 

fad<strong>in</strong>g of the resultant blue color of TiO 2/PVA films <strong>in</strong>itiated by the oxidation of O 2 is 

quite slow. 

3.3.2.3. POMs embedded <strong>in</strong> other polymeric matrices. WO3 can also be <strong>in</strong>corporated <strong>in</strong> 

polyethylene glycol (PEG) [336–338] <strong>and</strong> silica end-capped PEG [335]. WO3/PEG <strong>hybrid</strong> 

films show good reversible photochromism. WO3 <strong>in</strong>corporated <strong>in</strong> silica end-capped polytetramethylene 

oxide (PTMO) exhibits the similar photochromism [335]. The mix<strong>in</strong>g of 

PEG with WO3 <strong>in</strong>duces drastic enhancement of photochromism as well as a better film 

formation [337]. The photochromic sensitivity to UV light <strong>in</strong>creases with the amount of 

PEG added [337]. S<strong>in</strong>ce this enhancement effect is <strong>in</strong>dependent of the polymerization 

degree of the additive PEG [338], it is suggested that it is the O–C–H structure, not 

O–H group, that plays a key role <strong>in</strong> the photochromic process. The bond strength of 

C–H <strong>in</strong> O–C–H is much weaker than that of O–C <strong>in</strong> O–C–H or O–H. Thus, the hydrogen 

may be relatively easily released from C–H with oxidation by photogenerated holes. This 

can expla<strong>in</strong> why ketones <strong>and</strong> some carboxylic acids do not show any enhancement effect. 

So the efficient photochromism-enhanc<strong>in</strong>g additives should have the O–C–H structure. 

Polyv<strong>in</strong>yl pyrrolidone (PVP) has also been used as the matrix <strong>in</strong> <strong>hybrid</strong> <strong>materials</strong>. 

PW 12/PVP [339], <strong>and</strong> H 12[EuP 5W 30O 110]/PVP [340], K 12[EuP 5W 30O 110]/PVP [341] <strong>hybrid</strong>


films show similar photochromism to PW12/PAM. The photochromism is due to the 

reduction of POM upon UV-light irradiation <strong>and</strong> bleach<strong>in</strong>g process can be <strong>in</strong>itiated by 

oxygen. Some authors have claimed [339,341] that PVP is oxidized after coloration due 

to the electron transfer between organic matrix <strong>and</strong> heteropolyanion. However, others 

[340] have argued that the photochromic mechanism is similar to that proposed for 

SAM th<strong>in</strong> films, i.e., formation of a CT complex after coloration (Eq. (5)), rather than 

the oxidization of PVP. It is noted that H 12[EuP 5W 30O 110]/PVP film can exhibit photolum<strong>in</strong>escent 

properties apart from the photochromism. After UV-light irradiation, the 

<strong>hybrid</strong> film shows weaker emission than that without illum<strong>in</strong>ation. 

In addition, PMo12 has been embedded <strong>in</strong> polycarbonate (PC) [342] to prepare PMo12/ 

PC complex, <strong>in</strong> which the C@O of PC <strong>in</strong>teracts with the hydrogen of molybdophosphoric 

acid to form hydrogen bond. For this complex, a new absorption peak appears at ca. 

550 nm under irradiation of sunlight due to the one-electron photoreduction of Mo 6+ . 

3.3.3. POMs embedded <strong>in</strong> organic/silica matrices 

A series of nano<strong>composite</strong> films with reversible photochromism have been prepared 

through entrapp<strong>in</strong>g PMo12 [267], PW12 [267,343,344], or SiW12 [267,345] <strong>in</strong>to an 

organic/<strong>in</strong>organic matrix co-hydrolyzed from 3-am<strong>in</strong>opropyltriethoxysilane <strong>and</strong> tetraethylorthosilicate. 

PW12 has also been embedded <strong>in</strong> the matrix prepared from tetraethylorthosilicate 

<strong>and</strong> tetraethyleneglycol [346]. After SiW12 is entrapped <strong>in</strong>to the polymer prepared 

from methacrylamide <strong>and</strong> v<strong>in</strong>yltriethoxysilane, similarly, the resultant system can be 

reacted with tetraethylorthosilicate to form the nano<strong>composite</strong> [347]. It is claimed that 

<strong>in</strong> such a case the SiW12 is embedded <strong>and</strong> dispersed quite well <strong>in</strong> the organic/<strong>in</strong>organic 

matrices. 

The gel films prepared by such method are <strong>in</strong> amorphous state [267,343,344,348]. UV– 

Vis <strong>and</strong> IR spectra <strong>in</strong>dicate that the Kegg<strong>in</strong> type heteropolyanions disperse uniformly <strong>in</strong> 

the am<strong>in</strong>e-functionalized silica gel skeleton <strong>and</strong> there is a strong <strong>in</strong>teraction (hydrogen 

bond<strong>in</strong>g, electrostatic <strong>in</strong>teraction, <strong>and</strong> other non-covalent forces) between the anion <strong>and</strong> 

organic cation ðR–NH þ 

3 Þ [267,343–345,348]. Jude<strong>in</strong>ste<strong>in</strong> et al. [346] have claimed that 

chemical bonds may also be formed <strong>in</strong> such <strong>hybrid</strong>s. For all samples, there is no absorption 

<strong>in</strong> visible region before photoirradiation. After exposure to the UV light, the color of 

<strong>hybrid</strong> film changes from colorless to blue due to the reduction of anion via a one-electron 

step. Accord<strong>in</strong>gly, two broad absorption b<strong>and</strong>s appear <strong>in</strong> the UV–Vis spectra at 486 <strong>and</strong> 

659–745 (for PW12 <strong>hybrid</strong>s) or 689–720 nm (for SiW12 <strong>hybrid</strong>s), respectively [267]. Like <strong>in</strong> 

the case of PMo12/PAM <strong>hybrid</strong>s [264], they are respectively assigned to <strong>in</strong>tensity-enhanced 

d–d transitions <strong>and</strong> IVCT [230,320,349]. The former does not change with the irradiation 

time, while the latter undergoes a blue-shift with <strong>in</strong>creased irradiation time as a result of 

the heteropolyanion reduced beyond the one-electron stage [227]. However, the PMo12 

<strong>hybrid</strong> films show only one broad b<strong>and</strong> at ca. 720 nm <strong>in</strong> the absorption spectra after 

UV irradiation s<strong>in</strong>ce the d–d b<strong>and</strong> is sometimes obscured by IVCT [230]. The photocolored 

samples can be bleached <strong>in</strong> the presence of oxygen <strong>and</strong> be recolored aga<strong>in</strong> on exposure 

to UV light. For PMo12 <strong>hybrid</strong> films, a new absorption peak at ca. 872 nm appears 

ð5Þ


dur<strong>in</strong>g the bleach<strong>in</strong>g process <strong>and</strong> its cont<strong>in</strong>uous decrease <strong>in</strong> bleach<strong>in</strong>g speed is slower than 

that of the absorption peak at ca. 720 nm. This peak is ascribed to a second IVCT whose 

energy is rather low for a d–d transition <strong>and</strong> is expected for the ‘‘extra-group’’ process 

[267,350,351]. It is noted that the reversibility of PW12 <strong>hybrid</strong> films decreases with the coloration–decoloration 

cycles due to the matrix-constra<strong>in</strong><strong>in</strong>g effect of the rigid environment 

[343]. In addition, the organoam<strong>in</strong>o-modified silica <strong>hybrid</strong> films conta<strong>in</strong><strong>in</strong>g PMo 12 also 

exhibits thermochromic properties [348]. 

3.3.4. POMs anchored to organic polymeric backbone 

To allow a strong electronic communication between the two units, the synthesis of 

related compounds <strong>in</strong> which the organic molecule is covalently l<strong>in</strong>ked to the polyanion 

is thus pursued, i.e., <strong>in</strong> such <strong>materials</strong> a reversible photo<strong>in</strong>duced electron-transfer process 

that does not <strong>in</strong>volve major structural changes or further chemical evolution must occur 

between an organic molecule <strong>and</strong> POM. Jude<strong>in</strong>ste<strong>in</strong> [252,352] has reported the synthesis 

<strong>and</strong> characterization of negatively charged macromolecules <strong>based</strong> on organically functionalized 

POMs through W–O–Si–C l<strong>in</strong>ks <strong>and</strong> anchored to an organic polymeric backbone. 

First, the lacunar polyoxotungstate SiW11O 8 

39 , which formally derives from Kegg<strong>in</strong>’s 

structure by the remov<strong>in</strong>g of a ‘‘WO’’ fragment, reacts with trichloro or trialkoxysilanes 

to yield the modified polyoxotungstates, [SiW11O40(SiR)2] 4 

(R = v<strong>in</strong>yl, allyl, methacry, 

styryl), which can be isolated by crystallization. [SiW11O40(SiR)2] 4 units carry<strong>in</strong>g two 

reactive organic groups can be further polymerized <strong>in</strong> the presence of a radical <strong>in</strong>itiator 

<strong>and</strong> yields <strong>hybrid</strong> polymers <strong>in</strong> which POM are l<strong>in</strong>ked by polymethacrylate or polystyrene 

cha<strong>in</strong>s [252,353]. The <strong>in</strong>organic/organic polymers with different spatial repetitions <strong>and</strong> 

structures (l<strong>in</strong>ear or branched <strong>and</strong> compact) are probably obta<strong>in</strong>ed (Fig. 32) [252] depend<strong>in</strong>g 

on the polymerization conditions. Upon UV irradiation, both the solution <strong>and</strong> films 

can turn blue. Their visible spectrum is characterized by a wide absorption b<strong>and</strong> <strong>in</strong> the visible-near-IR 

range centered on 1000 nm, which is characteristic of an <strong>in</strong>tervalence transition 

W 5+ ! W 6+ <strong>in</strong> mixed valence POMs [354]. 

Photochromic <strong>hybrid</strong> <strong>in</strong>organic/organic nano<strong>composite</strong>s <strong>based</strong> on polyether cha<strong>in</strong>s 

with entrapped POMs <strong>and</strong> silica clusters have also been prepared from heteropolyoxometallates 

<strong>and</strong> (3-glycidyloxypropyl)trimethoxysilane [45,60,346,353]. The high solubility of 

POM molecules <strong>in</strong>side polyethylene oxide cha<strong>in</strong>s comes from the oxygen lone pairs, which 

enables the nanoscale dispersion of the components—polyether cha<strong>in</strong>s, silica clusters, <strong>and</strong> 

POM species—lead<strong>in</strong>g to a transparent material [45]. The matrix is made of organic <strong>and</strong> 

<strong>in</strong>organic parts strongly connected through stable carbon–silicon bonds. The graft<strong>in</strong>g of 

the two phases leads to a strong stability of these <strong>materials</strong> aga<strong>in</strong>st heat<strong>in</strong>g (up to 

150 °C) or humid atmosphere. Photocorrelation spectroscopy is used to probe the formation 

of gel framework <strong>and</strong> macrostructure. Forced Rayleigh scatter<strong>in</strong>g is used to study the 

dynamics of POM clusters <strong>and</strong> thereafter the microviscosity <strong>and</strong> microstructure <strong>in</strong>side the 

blends via the measurement of translational diffusion coefficient of entrapped photoreactive 

targets, which do not experience the sol–gel transition but the system rigidification 

with the loss of solvent [346]. In some mixture, the k<strong>in</strong>etics of the chemistry is fast <strong>in</strong> 

regard to gelation. In other mixture, the gel formation is rapid due to the formation of silica 

clusters <strong>and</strong> polyether cha<strong>in</strong>s. For these <strong>hybrid</strong> films, strong blue coloration is obta<strong>in</strong>ed 

under UV light, which is associated with <strong>in</strong>tramolecular electron transfer <strong>and</strong> can be 

described as a polaronic electron-hopp<strong>in</strong>g process. The photochromism is the result of oxidation 

of the end group of polyether molecules by POMs to yield aldehydes RCHO (Eqs.


Fig. 32. Proposed structures for SiW11SiR polymers (each POM is represented by one black disk): (a) l<strong>in</strong>ear 

structure; (b) branched, compact structure [252]. 

(6)–(11)) [45]. For the colored films Mo et al. [60] have suggested to use Lorentz <strong>and</strong> 

Gaussian oscillators to describe the optical behavior <strong>in</strong> the 400–1700 nm wavelength range 

<strong>and</strong> 700–4000 cm 1 wavenumber range, respectively. The behavior for amplitude grat<strong>in</strong>g 

record<strong>in</strong>g demonstrates their potentiabilities as permanent or temporary holographic storage 

media depend<strong>in</strong>g on the atmosphere they are <strong>in</strong> [45]. 

POM n ! hm POM n 

POM n ! k 1 

POM n 

POM n þ RCH2OH ! kQ 

POM ðnþ1Þ þ R _ CHOH þ H þ 

POM n þ R _ CHOH ! kR 

POM ðnþ1Þ þ RCHO þ H þ 

POM ðnþ1Þ þ R _ CHOH ! kR 

POM ðnþ2Þ þ RCHO þ H þ 

POM ðnþ2Þ þ POM n ! 2POM ðnþ1Þ 

(POM (n+2) + POM n 1 ! 2POM (n+1) 

for Eq. (11) <strong>in</strong> the orig<strong>in</strong>al paper [45].) 

3.3.5. Self-organized <strong>hybrid</strong>s <strong>based</strong> on POMs <strong>and</strong> DODA 

A layered nano<strong>composite</strong> film with a f<strong>in</strong>e superlattice structure has been prepared by 

cast<strong>in</strong>g the solution of an ionic complex formed by supramolecular self-organization 

between a dimethyldioctadecylammonium (DODA) <strong>and</strong> a Kegg<strong>in</strong> type phosphomolybdate 

anion ðPMo12O 3 

40 ; PMo12Þ [268,355] or a Dawson type 18-molybdophosphate anion 

ð6Þ 

ð7Þ 

ð8Þ 

ð9Þ 

ð10Þ 

ð11Þ


Fig. 33. Schematic diagram of a possible structure of PMo 12/DODA <strong>composite</strong> film [268]. 

ðP2Mo18O 6 

62 ; P2Mo18Þ [356,357] on a freshly cleaned substrate. It is claimed that the structure 

of polyanion is preserved <strong>in</strong> the <strong>composite</strong> film with a particular orientation <strong>in</strong> the 

multilayer. Each <strong>in</strong>organic layer consists of one polyanion monolayer <strong>in</strong>corporated <strong>in</strong> 

the hydrophilic <strong>in</strong>terlayer with a d spac<strong>in</strong>g of 2.945 nm for PMo 12/DODA <strong>hybrid</strong> 

(Fig. 33) [268] <strong>and</strong> of 3.591 nm for P2Mo18/DODA system (Fig. 34) [357]. It is said 

[268,355–357] that such an ordered <strong>hybrid</strong> film may show a better photochromic response 

Fig. 34. Schematic diagram of a possible structure of P2Mo18/DODA <strong>composite</strong> film [357].


Fig. 35. Spectral changes at 400–1100 nm upon UV irradiation of (A) PMo12-DODA <strong>and</strong> (B) P2Mo18-DODA 

<strong>composite</strong> film. Times (m<strong>in</strong>) are shown <strong>in</strong> the figures. Insets <strong>in</strong> (A): Spectral changes at 190–400 nm <strong>and</strong> EPR 

spectrum of the <strong>composite</strong> film (irradiation time 40 m<strong>in</strong>) at 84 K. Inset <strong>in</strong> (B): unirradiated P2Mo18-DODA 

<strong>composite</strong> film at 190–400 nm [268,357]. 

than that with a r<strong>and</strong>om orientation of POMs (such as the film <strong>based</strong> on polymeric 

matrix). After UV-light irradiation, the film is colored with blue <strong>and</strong> a new broad absorption 

b<strong>and</strong> with a maximum at ca. 770 nm for PMo 12/DODA (Fig. 35A) [268] <strong>and</strong> at ca. 

780–710 nm for P 2Mo 18/DODA <strong>hybrid</strong> (Fig. 35B) [357]. In the meantime a shoulder at 

ca. 520 nm for PMo12/DODA or at ca. 580 nm for P2Mo18/DODA <strong>hybrid</strong> is clearly 

observed. Those b<strong>and</strong>s are characteristic of reduced POM molecular species with d–d

<strong>and</strong>s at ca. 555 nm <strong>and</strong> IVCT (Mo 5+ ! Mo 6+ ) b<strong>and</strong>s at ca. 625–770 nm [230]. Apart 

from the reduction of heteropolyanions to heteropoly blues, DODA is oxidized <strong>in</strong>to 

one or more compounds with carbonyl bonds (C=O) <strong>and</strong> nitrogen–hydrogen bonds 

(N–H). Bleach<strong>in</strong>g takes place when the film is <strong>in</strong> contact with ambient air or O2 <strong>in</strong> the dark 

<strong>and</strong> can be promoted by heat<strong>in</strong>g. If the UV irradiated films are stored under <strong>in</strong>ert or vacuum 

atmosphere, it can reta<strong>in</strong> blue coloration for a quite long time. 

3.4. Miscellaneous <strong>hybrid</strong> <strong>materials</strong> 


3.4.1. TMOs/DMF systems 

The photochromic behavior of TMO th<strong>in</strong> film is very sensitive to the presence of hydrogen 

compounds dur<strong>in</strong>g the illum<strong>in</strong>ation s<strong>in</strong>ce the coloration process is relevant to the photo<strong>in</strong>jection 

of hydrogen [30,31]. Efficient photo<strong>in</strong>jection of hydrogen can be realized <strong>in</strong> 

some saturated TMOs with vacant d orbitals, such as WO 3, MoO 3, V 2O 5, Ta 2O 5, 

Nb 2O 5, etc. [30,31,358]. This may cause a photochromic response. An improved photochromism 

has been observed <strong>in</strong> MoO 3 [121,138,359,360] <strong>and</strong> WO 3 [361–363] when they 

are illum<strong>in</strong>ated <strong>in</strong> the presence of alcohol. This is relevant to the formation of organic radicals 

due to the oxidation of adsorbed molecules by photogenerated holes. Another k<strong>in</strong>d of 

enhanced photochromism of MoO3 [364,365], WO3 [366,367] <strong>and</strong> V2O5 [365] films have 

been reported when these films are prepared by evaporat<strong>in</strong>g TMOs powder <strong>in</strong> a controlled 

DMF atmosphere, for which the molecules are adsorbed on film surface. The adsorption 

of molecules on the oxide surface (without illum<strong>in</strong>ation) is accompanied by the formation 

of a donor–acceptor bond<strong>in</strong>g between DMF molecules <strong>and</strong> coord<strong>in</strong>ationally unsaturated 

surface metal cations [365]. An unshared electron pair of an oxygen atom of molecules is 

pulled <strong>in</strong>to a vacant d orbital of the surface cation, which can be facilitated by the localization 

of the photogenerated hole near the adsorption center. The <strong>in</strong>tramolecular bonds 

of the sorbate molecules are weakened <strong>in</strong> the process, lead<strong>in</strong>g to a catalytic decomposition 

of the adsorbed molecules to form atomic hydrogen. Eventually, the photogenerated hole 

is captured by an electron of the unshared pair form<strong>in</strong>g a positive charged molecule, which 

pre-determ<strong>in</strong>es <strong>in</strong> turn the possible splitt<strong>in</strong>g off of protons. Such detached hydrogen atoms 

can afterwards migrate <strong>in</strong>to the oxide lattice, giv<strong>in</strong>g rise to the formation of local colored 

species. The photo<strong>in</strong>jection of hydrogen leads to the exchange of a hole with a proton, prevent<strong>in</strong>g 

the recomb<strong>in</strong>ation of the photogenerated carriers. For MoO3/DMF system, the 

amorphous film possesses significant photochromic sensitivity at low temperatures (35, 

90, 200, <strong>and</strong> 300 K) <strong>in</strong> an <strong>in</strong>ert atmosphere (such as <strong>in</strong> N2 gas), for which the photochromic 

process is ascribed to the population of different protonic states (bulky <strong>and</strong> surface 

states), which are relevant to the electronic color centers [364]. In addition, the photochromism 

of hexatungstic acid (H 2W 6O 19) [368] <strong>and</strong> 11-tungstophosphate [369] <strong>in</strong> DMF 

solution have also been reported. It is said that [W 6O 19] 2 undergoes a two-electron 

photoreduction <strong>in</strong> DMF upon irradiation to generate the blue species [W 6O 19] 4 . 

Although hydrogen can be directly photo<strong>in</strong>jected <strong>in</strong>to amorphous WO3 film, this process 

is <strong>in</strong>efficient <strong>in</strong> polycrystall<strong>in</strong>e WO3 films due to the small specific surface area <strong>and</strong> 

weak adsorptivity [366]. In addition, the direct photo<strong>in</strong>jection of hydrogen is impossible 

for vanadium dioxide (VO2) because its d orbitals are not vacant, which makes donor– 

acceptor bond formation impossible between the oxide surface <strong>and</strong> organic molecules 

[358]. A double-layer heterostructure has been constructed to realize the photo<strong>in</strong>jection 

of hydrogen <strong>in</strong>to VO 2 [358] or polycrystall<strong>in</strong>e WO 3 [366,367] films (Fig. 36) [367]. To carry


Fig. 36. Structure for carry<strong>in</strong>g out of photo<strong>in</strong>jection of hydrogen with the help of a hydrogen <strong>in</strong>jector. Hydrogen 

<strong>in</strong>jector refers to amorphous WO 3, <strong>and</strong> hydrogen acceptor refers to VO 2 or polycrystall<strong>in</strong>e WO 3 [367]. 

out the photo<strong>in</strong>jection of hydrogen, organic molecules conta<strong>in</strong><strong>in</strong>g oxygen (such as aldehydes, 

alcohols, organic acids, etc.), 5 an element with an unshared electron pair <strong>in</strong> a p 

orbital, are first adsorbed on the heterostructure surface (i.e., the surface of the amorphous 

layer). The hydrogen photodetached from the adsorbed molecules is first <strong>in</strong>jected <strong>in</strong>to the 

amorphous WO 3 layer, <strong>and</strong> then migrates <strong>in</strong>to the VO 2 or polycrystall<strong>in</strong>e WO 3 layer by 

the heterojunction field. When hydrogen is <strong>in</strong>jected, changes <strong>in</strong> the optical, electrical, 

<strong>and</strong> structural properties of oxides [367] <strong>and</strong>, sometimes, even a semiconductor–metal 

transition, are expected to occur at high <strong>in</strong>jection rates [370,371]. This might be of great 

significance for practical applications. 

3.4.2. Molybdenum-oxide cluster/citric acid complexes 

UV-light-<strong>in</strong>duced coloration has been observed <strong>in</strong> cluster systems of molybdenum 

oxide ([Mo7O24] 6 ) <strong>in</strong> aqueous solutions of citric acid [372,373]. Cluster is formed <strong>in</strong> solu- 

tion due to the polymerization of [Mo7O24] 6 

ions, which occurs after the proton concen- 

tration has reached a certa<strong>in</strong> level (for a given pH of the solution) [230]. Interest<strong>in</strong>gly, two 

different photochromism for the same system have been observed under illum<strong>in</strong>ation with 

365 <strong>and</strong> 380–420 nm, respectively. The former illum<strong>in</strong>ation creates an absorption b<strong>and</strong> <strong>in</strong> 

the region of 750 nm, whereas for the latter irradiation absorption peak at 750 nm does 

not appear <strong>and</strong> the absorption-edge tail shifts toward longer wavelengths (Fig. 37) 

[373]. These phenomena have been expla<strong>in</strong>ed <strong>based</strong> on the fact that citric acid can act 

as a lig<strong>and</strong> to attach this complex of molybdenum oxide cluster. In addition, it is noted 

that no changes <strong>in</strong> the optical spectra of the suspensions conta<strong>in</strong><strong>in</strong>g <strong>in</strong>organic acids, such 

as HCl <strong>and</strong> HNO3, take place under UV-light irradiation s<strong>in</strong>ce no lig<strong>and</strong>s can be formed 

for them. 

Clusters may demonstrate a unique feature that differs substantially from those of the 

bulk <strong>materials</strong> <strong>and</strong> molecules. Cluster is small enough to feel the changes occurr<strong>in</strong>g on its 

surface when a lig<strong>and</strong> becomes attached to it. Furthermore, the number of atoms compris<strong>in</strong>g 

it is large enough to create a potential barrier that <strong>in</strong>hibits cluster transfer back to the 

ground state. This barrier appears <strong>in</strong> the atomic rearrangement as the transfer of photoexcited 

electron changes the electronic structure of the complex [373]. Illum<strong>in</strong>ation with 

the light of 365 nm transfers organic acid <strong>in</strong> such a complex to an excited state. Then 

5 Kuboyama et al. [362] have reported that no photochromism had been observed for the tungstic acid gels <strong>in</strong> 

the presence of aldehyde, carboxylic acid <strong>and</strong> ketone. However, no explanation is available about this.


Fig. 37. Transmission spectra of solutions conta<strong>in</strong><strong>in</strong>g molybdenum oxide cluster <strong>and</strong> citric acid. (A) Irradiation 

with a 365-nm light, <strong>and</strong> (B) irradiation with a light of 380–420 nm. (1) start<strong>in</strong>g spectrum; (2) spectrum after 

irradiation for 85 m<strong>in</strong>; (3) spectrum after completion of irradiation for 30 h. The arrow identifies the absorption 

edge [373]. 

the electron is transferred from the organic compound to the metal atom, lead<strong>in</strong>g to a 

change <strong>in</strong> the oxidation state of Mo atom <strong>in</strong> the cluster from Mo 6+ to Mo 5+ <strong>and</strong>, consequently, 

to the formation of an absorption b<strong>and</strong> appear<strong>in</strong>g around 750 nm [373]. In the 

meantime the proton is elim<strong>in</strong>ated from the cluster or from the acid attached to it. However, 

the photoexcitation with the light of 380–420 nm changes the cluster conformation 

<strong>and</strong> shifts the absorption edge caused by excitation from the oxygen orbitals [373]. The 

different behavior of clusters after photoexcitation for these two k<strong>in</strong>ds of photochromism 

agrees with that the electron transfer responsible for the photochromic effect is a process 

sensitive to the differences <strong>in</strong> the complex structure <strong>in</strong>itiated by a relatively small change <strong>in</strong> 

the concentration of the start<strong>in</strong>g chemical components [373].


3.4.3. Molybdenum (VI) oxalate complexes 

Molybdenum (VI) oxalate complexes <strong>in</strong> the solid state can exhibit photochromic behavior 

on exposure to UV irradiation, which was first observed by Rosenheim [374] about 110 

years ago <strong>and</strong> recently confirmed by Mentzen <strong>and</strong> Sauterean [375].K2[MoO3(C2O4)] Æ H2O 

can change from colorless to p<strong>in</strong>k after successive UV irradiation for 30 m<strong>in</strong>, <strong>and</strong> to 

brown for longer times [375]. At the same time, the absorption b<strong>and</strong> gradually shifts 

towards shorter wavelengths with <strong>in</strong>creas<strong>in</strong>g irradiation time <strong>and</strong> stabilizes at 460 nm after 

6 h of irradiation [375]. C<strong>in</strong>drić et al. [376] have reported that the reaction of molybdenum 

(VI) oxide with oxalic acid or with alkali oxalate <strong>and</strong> alkali halides results <strong>in</strong> the formation 

of two series of molybdenum (VI) oxalate complexes: one of the general formula 

M2[Mo2O5(C2O4)2(H2O)2] with a Mo2O5 core <strong>and</strong> the dimeric structure (M = Na, K, 

Rb, Cs), <strong>and</strong> another of the formula M2[MoO3(C2O4)] with a MoO3 core <strong>and</strong> an <strong>in</strong>f<strong>in</strong>ite 

polymeric structure. In both types of structures molybdenum ions are six-coord<strong>in</strong>ated 

be<strong>in</strong>g surrounded by term<strong>in</strong>al oxo-oxygens, bridg<strong>in</strong>g oxygens <strong>and</strong> bidentate bonded oxalate 

lig<strong>and</strong>s. When exposed to UV light, the former <strong>in</strong> the solid state exhibits photochromic 

behavior, chang<strong>in</strong>g color from colorless to green–brown as the consequence of partial 

reduction of Mo 6+ to Mo 5+ only at the crystal surfaces. The latter exhibits remarkably less 

pronounced photochromism <strong>and</strong> changes to a very pale p<strong>in</strong>k, which might be caused by 

the polymeric nature of the complex anion <strong>and</strong> stabilization effect due to the <strong>in</strong>teractions 

between oxalate lig<strong>and</strong>s as well as between oxo-oxygen atoms [376]. In addition, a class of 

compounds prepared from molybdenum (VI) oxide with oxalic acid <strong>and</strong> RCl (R = pyH 

<strong>and</strong> c-picH) or with tetramethylammonium oxalate complex—R 2[Mo 2O 5(C 2O 4) 2(H 2O) 2], 

[(CH 3) 4N] 2[MoO 3(C 2O 4)] <strong>and</strong> [(CH 3) 4N] 2[Mo 2O 5(C 2O 4) 2(H 2O) 2]—are also observed to 

show considerable photochromic effect due to the photoreduction of Mo 6+ to Mo 5+ [377]. 

4. Conclud<strong>in</strong>g remarks 

The design <strong>and</strong> synthesis of <strong>composite</strong>s <strong>and</strong> <strong>hybrid</strong>s <strong>based</strong> on TMOs or POMs are at 

the forefront of the <strong>materials</strong> chemistry research. Many such k<strong>in</strong>d of <strong>composite</strong> <strong>and</strong> <strong>hybrid</strong> 

photochromic <strong>materials</strong> have been developed <strong>and</strong> some significant progresses have been 

made dur<strong>in</strong>g the past several decades. For the <strong>composite</strong> <strong>materials</strong> <strong>based</strong> on TMOs, an 

improved photochromism has been achieved <strong>in</strong> the systems of WO 3 or MoO 3 modified 

by Au, Pt, or a second TMO (Nb, Ta, Ti, Zn, Zr, etc.). Visible-light coloration has been 

obta<strong>in</strong>ed <strong>in</strong> the systems of CdS/WO3, CdS/MoO3, <strong>and</strong> TiO2/MoO3. The field-assisted 

photochromism has been put forward to suppress the thermal degradation of photochromism 

for doped TMOs. Compared with the poor photochromic behavior of s<strong>in</strong>gle POMs, 

the <strong>hybrid</strong> <strong>materials</strong> <strong>based</strong> on POMs may exhibit better photochromic response, especially 

for the molybdenum-<strong>based</strong> ones. An ultrath<strong>in</strong> (such as 70 nm) <strong>hybrid</strong> multilayer film of 

H 3PMo 12O 40 or H 4SiMo 12O 40 coupled with 1,10-diam<strong>in</strong>odecane can exhibit photochromic 

characteristics comparable to that of a 1-lm MoO 3 film. Moreover, even an 

H3PMo12O40 monolayer coupled with an am<strong>in</strong>e monolayer (only about 2-nm thick) show 

an observable photochromic response. Another strik<strong>in</strong>g achievement <strong>in</strong> the <strong>hybrid</strong> <strong>materials</strong> 

is that, unlike MoO3 <strong>and</strong> WO3, many of them show a good reversible photochromism 

even bleached by heat<strong>in</strong>g <strong>in</strong> the presence of oxygen. In addition, the photochromic activity 

of a <strong>hybrid</strong> <strong>in</strong>creases with the positive-shift<strong>in</strong>g redox potential of POM or the decreas<strong>in</strong>g 

b<strong>in</strong>d<strong>in</strong>g energy of O 2p <strong>in</strong> POM, which is helpful <strong>in</strong> the design of novel <strong>hybrid</strong>s with good 

photochromic response. F<strong>in</strong>ally but not the last, except the frequently-observed blue color

<strong>in</strong> TMOs <strong>and</strong> POMs, <strong>composite</strong>s <strong>and</strong> <strong>hybrid</strong>s that can be photocolored to other color 

(brown, purple, green, p<strong>in</strong>k, etc.) have been developed so that the multicolor might be realized. 

Specifically, upon irradiation Ag/TiO2 can change from the <strong>in</strong>itial brownish-gray to 

almost the same color as that of the <strong>in</strong>cident monochromatic visible light. The knowledge 

acquired through these studies will aid <strong>in</strong> the development of new <strong>materials</strong> <strong>and</strong> the 

improvement of present ones. This might underscore the opportunity of us<strong>in</strong>g <strong>composite</strong> 

<strong>and</strong> <strong>hybrid</strong> <strong>materials</strong> as the photonic applications as well as the m<strong>in</strong>iaturization of related 

devices. Specifically, the <strong>hybrid</strong> photochromic <strong>materials</strong> have shown great promise <strong>in</strong> optical 

memory devices us<strong>in</strong>g blue light (such as 405 nm), which makes them the c<strong>and</strong>idates 

for the next generation <strong>materials</strong> as <strong>in</strong>formation storage media. 6 By select<strong>in</strong>g different 

<strong>in</strong>organic photochromic <strong>materials</strong> <strong>and</strong> their <strong>in</strong>organic or organic counterparts, choos<strong>in</strong>g 

different preparation methods, <strong>and</strong> adjust<strong>in</strong>g the irradiation time, <strong>in</strong>tensity <strong>and</strong> wavelength, 

one can optimize the desired photochromic behavior, <strong>in</strong>clud<strong>in</strong>g the different photo<strong>in</strong>duced 

color, of the <strong>composite</strong> or <strong>hybrid</strong> <strong>materials</strong>. However, the design <strong>and</strong> synthesis 

of <strong>composite</strong>s <strong>and</strong> <strong>hybrid</strong>s <strong>based</strong> on TMOs or POMs are still <strong>in</strong> its <strong>in</strong>fancy <strong>and</strong> there are 

still many problems left for the ongo<strong>in</strong>g research. 

So far only a general pattern of color<strong>in</strong>g process has been reliably established, while for 

the specific feature <strong>and</strong> mechanism of the photochromic process, even for s<strong>in</strong>gle species of 

TMOs <strong>and</strong> POMs, discrepancy still exists among different authors. This needs the help 

from the researchers <strong>in</strong> physics, especially <strong>in</strong> solid state physics, photophysics, <strong>and</strong> experimental 

physics. The energy transfer between <strong>in</strong>organic chromophores <strong>and</strong> organic dyes, a 

related topic, should receive much more attention. The <strong>hybrid</strong> approach offers the opportunity 

to br<strong>in</strong>g these energy transfer systems <strong>in</strong>to solid state <strong>and</strong> might lead to the evolution 

of novel photochromic <strong>materials</strong>. It is necessary to develop (more) new systems, for 

which the colored state can be bleached by optical excitation <strong>in</strong>stead of thermal bleach<strong>in</strong>g. 

This is very important for the optical memory <strong>materials</strong>. It is also mean<strong>in</strong>gful to prepare 

(more) new <strong>materials</strong> that can exhibit multicolor photochromism. S<strong>in</strong>ce the optoelectronic 

characteristics of semiconductor particles are drastically modified <strong>in</strong> the nanometer- <strong>and</strong> 

subnanometer-size regime, more attention might be paid to the semiconductor-<strong>based</strong> 

nano<strong>composite</strong>s, <strong>in</strong> which the <strong>in</strong>teractions at the <strong>in</strong>terface may be tuned <strong>and</strong>, consequently, 

the photochromic performance. Last but not the least, the photochromic response 

time is also worth study<strong>in</strong>g s<strong>in</strong>ce, at least <strong>in</strong> pr<strong>in</strong>ciple, the <strong>hybrid</strong>s should have a quite fast 

photoresponse speed. This is crucial for many practical applications, such as <strong>in</strong>formation 

storage, data display, optical signal process<strong>in</strong>g, chemical switch, <strong>and</strong> the like. 

Acknowledgements 


We are deeply <strong>in</strong>debted to all the colleagues <strong>and</strong> students <strong>in</strong> our group, <strong>in</strong>volved <strong>in</strong> the 

field of photochromism: Y<strong>in</strong>g Ma, Ya-An Cao, Wen-Sheng Yang, Yong-An Yang, 

Zhao-Hui Chen, Guang-J<strong>in</strong> Zhang, Hao-Hao Ke, Zi-Sheng Guan, <strong>and</strong> Ke Shao, for 

fruitful collaboration. F<strong>in</strong>ancial supports came from National Science Foundation of 

Ch<strong>in</strong>a, National Research Fund for Fundamental Key Projects No. 973 (G19990330), 

<strong>and</strong> Ch<strong>in</strong>ese Academy of Sciences. 

6 At the moment the memory density of CD disk is 650 MB with 780-nm laser as light source <strong>and</strong> that of DVD 

is 4.7 GB with 650-nm laser. If the light source moves to blue (405 nm), it can reach 40 GB for s<strong>in</strong>gle layer <strong>in</strong> 12cm 

optical disk.


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