PNNL-13501 - Pacific Northwest National Laboratory

Bandgap Energy (eV)
2.9
2.85
2.8
2.75
2.7
2.65
2.6
2.55
TiO2-(MoO3)1.8
TiO2-(MoO3)1.1
TiO 2-(MoO3)0.54
TiO 2-(MoO3)0.18
35 40 45 50 55 60 65 70 75 80 85
TiO2-(MoO3)x core-shell diameter (Å)
Figure 1. An idealized cross-section view of the TiO 2-
(MoO 3) x core-shell particles and variation of band gap
energy with core-shell diameter
Theoretical and experimental work on II-VI and III-V
core-shell nanoparticle systems indicate that band gap Eg
is a function of both size quantization effects and the
relative composition of the core-shell particle
(i.e., relative thickness of the core and shell) (Haus et al.
1993; Kortan et al. 1990). In the limiting case it is logical
to expect the photoabsorption energy of a core-shell
nanoparticle system to be greater than or equal to the
smallest band gap material comprising the core-shell
system.
In addition to this, a photoabsorption energy blue-shift,
relative to the band gap energies of the bulk materials, is
expected when the core-shell particle size is in the
quantum regime (i.e., core diameter or shell thickness
equal to or smaller than the Bohr radius of the
valence/conduction band electron). For these reasons we
expected the photoabsorption energy for the TiO2-
(MoO3)x core-shell materials to be greater than 2.9 eV (Eg
for MoO3), and likely greater than 3.2 eV (Eg for TiO2)
due to the dominant size quantization effects, especially
for TiO2- (MoO3)1.8 where the core-shell size is
approximately 40 Å. For example, a band gap energy
blue-shift is observed for PNNL-1 (Eg 3.32 eV), which
contains nanocrystalline TiO2 with an average crystallite
size of 25 to 30 Å (Elder et al. 1998). In contrast, the
TiO2- (MoO3)x photoabsorption energies range from 2.88
to 2.60 eV, approximately equal to or lower in energy
than bulk MoO3, which places the photoabsorption
energies of TiO2- (MoO3)1.8 in the most intense region of
the solar spectrum. The charge-transfer absorption
328 FY 2000 Laboratory Directed Research and Development Annual Report
properties exhibited by the TiO2- (MoO3)x compounds are
due to charge-transfer processes at the semiconductor
heterojunction that is established as a result of the
chemical bonding between the TiO2 core and the MoO3
shell (Nozik and Memming 1996). This allows the coreshell
wave functions to overlap at the interface giving rise
to a heterojunction band structure. Figure 2 depicts the
valence band/conduction band arrangement in TiO2-
(MoO3)1.8 after heterojunction formation. The lowest
energy excitation is from the TiO2 valence band to the
MoO3 conduction band, a core-shell charge transfer, and
this energy closely matches what we measured for the
photoabsorption energy of TiO2 (MoO3)1.8 (Figure 1).
This electronic transition is allowed due to the reduced
symmetry at the core-shell interface. We attribute the
regular decrease in band gap energy with increasing
MoO3 shell thickness to the reduced confinement of the
electronic states in the shell as it evolves from isolated
MoO3 islands (TiO2- (MoO3)0.18) to nearly two complete
MoO3 mono-layers (TiO2- (MoO3)1.8).
0
+1.0
+2.0
+3.0
E(NHE)
~2.85 eV
Core-Shell Interface
CB
~3.2 eV
MoO3 shell states TiO2 core states
Figure 2. Arrangement of the TiO 2 core and MoO 3 shell
valence bands (VB) and conduction bands (CB) for TiO 2-
(MoO 3) 1.8 after heterojunction formation
Reaction Mechanism and Thermodynamic Properties
VB
The TiO2- (MoO3)x core-shell powders are synthesized by
co-nucleation of metal oxide clusters at the surface of
surfactant micelles. Specifically, after
(NH4)2Ti(OH)2(C3H4O3)2 (Tyzor®, Dupont) was
combined with cetyltrimethylammonium chloride
(spherical micelles) and then diluted with water,
nucleation of TiO2 nanocrystallites occurred at the surface
of flexible rod-like micelles⎯a heterogeneous nucleaton
of nanocrystals from a homogeneous solution. The crosssection
view shown in Figure 3 demonstrates the
nucleation of TiO2 nanocrystallites at the surface of
cetyltrimethylammonium chloride micelles.
CB
~2.65 eV
VB