Photochemical Formation of V 4+ in a Layered Vanadium Pentoxide Hydrate
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ERIMENTAL Materials V2 0 5 • n H20 gel was prepared by the ion-exchange polymerization method. 1 A portion of wet gel was freeze-dried for H20 adsorption and ESR spectroscopy. For electron microscope observation the wet gel was treated with H2 0 2 hydrothermally at 200 C. UV-Visible light spectroscopy A wet gel was coated onto a silica glass plate and dried in air in the dark. The thickness was adjusted to give a spectroscopic absorbance in the range 0.5--1. Daylight was irradiated indirectly on the film for 10 h. The effective wave length 697 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
for the chemical process was determined by using filters, UV-35 and UV-D33S produced by Toshiba Glass Co., NiSO 4 solution (500 gl-1) and V205 . n H2 0 film. The influence of atmospheric conditions (vacuum, 02, N2, H2 and H2 0 gases) on the photochemical reactions were also studied. A U-321 0 spectrometer manufactured by Hitachi Co. was used. UV-light was supplied from low pressure Hg lamp (30 W) whose main wave length is 253.7 nm. Electron Spin Resonance (ESR) measurements Radical electrons produced by UV-irradiation were studied by ESR to elucidate the coordination environment of V41 ions in the layered structure. A 10 mg xerogel sample was loaded into a silica glass ESR cell, which was connected to the gas dozing system. ESR measurements were conducted in the X-band range by using JEOL JES-FE3XG.
RESULTS AND DISCUSSION 1. Reduction of V-20 -n H20 by UV-irradiation (a) Color changes under daylight and its dependence on the wave length Figure 1 shows UV-Visible light absorbance spectra of V205 • n H2 0 after irradiation by daylight with varying wave length ranges as a function of time. The as-grown material gives two broad peaks in the UV-range at 272 and 380 nm which coincide with those for a wet gel observed by Gharbi et al. 5 These two bands are ascribed to the charge transfer of electrons from p-orbital of oxygen to d-orbital of vanadium ions and change with the extent of polymerization. As seen in Fig. 1a these two bands decrease simultaneously with irradiation time and the base line above 530 nm increases. When the V205 - n H20-coated silica glass filter is applied, through which only visible light above 440 nm can pass as shown by a broken line, no change appeared on irradiation. Figures 1c and 1d show the spectral changes which were observed when the samples were irradiated with varying UV ranges. In spite of different UV ranges spectral changes are quite obvious. (b) Effect of atmosphere on the reduction of V20 5 -n H2 0 by UV-irradiation. Figure 2a shows the spectral change in a vacuum upon UV-irradiation which is more significant than in air. It is noted that isosbestic point was found at 530 nm, suggesting the equivalence of the processes giving spectral changes below and above that point. The latter is ascribed to the light scattering. Similar changes were also observed in the systems including N2 and H2 . The introduction of 1 atm 02 to the sample treated under vacuum restored the spectrum a
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