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Fig. 1. Mechanisms of downshifting and down-conversion in Tb 3+ , Yb 3+ and crossrelaxation mechanisms within Tb 3+ iob and Tb 3+ , Yb 3+ ions pair Y 4 Al 2 O 9 is one of the four crystalline phases in the Y 2 O 3 –Al 2 O 3 system with the formation of monoclinic, maintaining Y 2 O 3 :Al 2 O 3 ratio of 2:1. Other phases are cubic garnet with ratio of 3:5 (Y 3 Al 5 O 12 , abbreviated YAG), orthorhombic perovskite with ratio of 1:1 (YAlO 3 , abbreviated YAP) and a metastable hexagonal structure phase YAH, with the same stoichiometry as YAM, observed during the synthesis by soft chemistry methods. Crystals grown by standard Czochralski method crack during cooling because of phase transition in YAM. In our investigations the modified solgel method was used to obtain YAM:Tb 3+ +Yb 3+ samples, in form of nanopowders. The samples were obtained in the Institute of Electronic Materials Technology in Warsaw. The measurements were performed using Photon Technology International spectrophotometer. The properties of Tb 3+ activated YAM hosts were subject of few investigations. In case of YAM: Tb 3+ +Yb 3+ system the main routes for down-conversion and downshifting are presented in the Fig. 1. the 5 D 4 Tb level plays critical role in down-conversion process. The energy of the Tb 3+ 5 D 4 → 7 F 6 transition is twice larger than the energy of the Yb 3+ 2 F 5/2 → 2 F 7/2 transition. This suggests a possibility of efficient energy transfer between 5 D 4 level of single Tb 3+ ion and 2 F 5/2 levels of two Yb 3+ ions. The emission spectrum of Yb 3+ ion possesses two distinct peaks at 976 nm and 1026 nm, which suit transitions from Stark sub-levels of the 2 F 5/2 level. This emission may be excited by direct excitation of Yb 3+ ion at 908 nm or via down-conversion of excitation of Tb 3+ ion at about 274 nm. Such wavelength corresponds to spin-allowed transition of electron from 4f 8 to 4f 7 5d 1 configuration [4]. This down-conversion mechanism is the most desirable due to emission from Yb 3+ coinciding with the range of high spectral response of the standard crystalline silicon cells. The down-conversion is not the only mechanism occurring in Tb 3+ +Yb 3+ doped system. Another way of relaxation of exited 4f 7 5d 1 configuration is by emission o light at wavelengths corresponding to transitions from 5 D 3 → 7 F J and longer wavelength emissions from 5 D 4 → 7 F J (J=6,5,4,3,2). The strongest of those emissions occurs in case of 5 D 4 → 7 F 5 transition, which gives UV irradiated terbium materials a distinct green glow. Spectrum of above mentioned emissions are presented in Fig. 3, along with excitation spectrum of the 5 D 4 → 7 F 5 emission. Shape of the spectrum, again with dominant feature at around 274 nm confirms that both Yb 3+ NIR and Tb 3+ VIS emissions primarily result from the same UV excitation and are thus competitive processes. From point of view of obtaining maximally efficient down-conversion process emissions from higher energy 5 D 3 level to 7 F J multiplets are particularly unfavorable, since they provide pathway of depopulation of this level before excited electrons non-radiatively decay to the lower 5 D 4 level, where they have chance to take part in the cooperative energy transfer Fig. 2. Excitation spectrum of Yb 3+ IR emission at 1026 nm (black) and emission spectrum from excitation of Tb 3+ ion at 274 nm indicates energy transfer from Tb 3+ to Yb 3+ ions 114 Fig. 3. Emission spectrum resulting from excitation of Tb 3+ ion at 274 nm and excitation spectrum of green emission at 544 nm Elektronika 6/2012
chanism, as it provides alternative explanation for Yb 3+ emission under Tb 3+ UV excitation. Due to required multiphonon relaxation intensity of this pathway may be reduced in lower phonon energy hosts, which will be subject of further investigations. Conclusions Emission of NIR light from Yb 3+ ions after excitation of the Tb 3+ was observed indicating existence of transfer mechanisms. Relative intensities of these down-conversion and down-shifting via single Yb 3+ ion mechanisms is yet to be established. Efficient down-shifting resulting in visible light emission was observed. Relatively narrow absorption spectrum with peak at 274 nm limits potential for application of this system in PV systems, due to little sunlight energy contained within that part of the solar spectrum. Fig. 4. Influence of terbium ions quenching effects on shape of the Tb 3+ VIS emission spectrum with Yb 3+ ions. There is however mechanism of cross relaxations within Tb 3+ ion that at high terbium concentration quenches emissions from the 5 D 3 level and excites 5 D 4 . This mechanism reveals itself at Tb 3+ concentrations larger than 5% at., as presented in Fig. 4 (results in YAM: Tb 3+ not containing Yb 3+ ). The third possible mechanism of depopulation of the 5 D 4 level, also shown at Fig. 1, is Tb 3+ Yb 3+ non-resonant cross relaxation resulting in excitation of single Yb 3+ ion and multi-phonon decay to the Tb 3+ 7 F 0 level [5]. Existence of this pathway complicates calculation of energy transfer efficiency of the down-conversion me- References [1] Macdonald D. H.: Recombination and Trapping in Multicrystalline Silicon Solar Cells. PhD. Thesis, The Australian National University, May 2001. [2] Giebink N. C., G. P. Wiederrecht, M. R. Wasielewski: Resonanceshifting to circumvent reabsorption loss in luminescent solar concentrators. Nature Photonics 5, 694–701(2011). [3] Shalav A., B.S. Richards, M.A. Green: Luminescent layers for enhanced silicon solar cell performance: Up-conversion. Solar Energy Materials & Solar Cells 91 (2007) 829–842. [4] Li Li, Wei Xiantao, Chen Yonghu, Guo Changxin, Yin Min, “Energy transfer in Tb3+,Yb3+ codoped Lu 2 O 3 near-infrared down-conversion nanophosphors”, Journal Of Rare Earths, Vol. 30, No. 3, Mar. 2012, P. 197. [5] Terra I.A.A. et. al., “Down-conversion process in Tb 3+ –Yb 3+ co-doped Calibo glasses”, Journal of Luminescence 132 (2012) 1678–1682. Photocatalytic degradation of the organic compounds enhanced by chemical oxidants Justyna Dziedzic, Paweł Nowak, Piotr Warszyński, Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, Krakow Water for drinking and domestic purposes must not contain harmful substances. It should be transparent, colorless, odorless, have a pleasant and refreshing taste and cannot contain pathogenic bacteria [1]. One of the basic problems of water treatment is the removal of organic species, especially humic substances (HS) – compounds, which have not as yet been properly chemically defined. HS represent a major fraction of natural organic matter (NOM) in ground and surface waters. Their presence causes growth of microorganisms resulting in undesirable odor and change of color and turbidity of water [2]. NOM occurrence in water, especially that part which cannot be removed by coagulation, determines demand for chlorine in chlorination process. However, toxicological studies indicate that after chlorination carcinogenic byproducts, such as trihalomethanes, can be found. Therefore, the efficient removal of NOM leads to reduction of the required amount of disinfectant, decreases the risk of formation trihalomethanes and also prevents the formation of biofilm [3]. For these reasons there exists a strong demand for new methods for NOM removal. As photocatalysis, is a promising method for removing organic compounds from water, in recent years, TiO 2 based photocatalysis of humic acids (HAs) has been extensively investigated [4–7]. In this work, the photocatalytic removal of humic acid (HA) under artificial sun light (ASL) and UV irradiation was examined by monitoring changes in the UV absorbance at 254 nm (UV 254 ). That absorbance is the widely accepted measure for determination of the degradation rate of humic acid and is used as the surrogate parameter for the total organic carbon (TOC) – usually applied to determine degree of HA photodegradation [8]. As the alternative method, the measurements of the chemical oxygen demand (COD) were also performed. We considered the effectiveness of TiO 2 photocatalytic oxidation for HS removal from water with the use of titanium dioxide powder (Degussa P – 25) with the addition of oxidants – sodium persulphate. and hydrogen peroxide. The main novelty was observed synergistic effect occurring when TiO 2 and Na 2 S 2 O 8 were used simultaneously, leading to significant increase of efficiency of HA removal. Experiment Materials Degussa P – 25 Aeroxide TiO 2 powder (surface area – 50 m 2 g -1 ) was kindly supplied by Evonic – Degussa and used as a photocatalyst without any modification. Humic acid sodium salt (technical grade) was supplied by Sigma – Aldrich. HA working solution of 40 mg l -1 for photocatalytic degradation experiments was always freshly prepared by dilution of the stock solution (500 mg l −1 ) with water. More details concerning preparation of HA stock solution were given in our previous work [9]. The oxidants – sodium persulphate and hydrogen peroxide were provided by Sigma – Aldrich. Photodegradation experiments The photodegradation experiments were performed in two quartz flat bottomed batch reactors of the volume 40 ml as shown in Fig. 1. In one of the reactors halogen lamp (150 W, Philips) was used as the artificial sunlight (ASL) source, in the other one, dedicated for ultraviolet (UV) region, the high pressure xenon arc lamp (250 W, Optel) was applied. Both lamps were used without any cut-off filters, except the layer of cooling water flowing below the bottom of the cell. The cell illumination was monitored by the radiometer Radiometer RD 0.2/2/100 (Optel). The irradiation intensity was 68.8 mW cm -2 for ASL and 48.8 mW cm -2 for UV source. The photocatalyst suspension was stirred during the experiments with a mechanical stirrer at a constant rate of 300 rpm. The tempera- Elektronika 6/2012 115
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