| Photocatalytic water splitting using solar energy has received a great deal of attention since it has been considered as one of the potential ways to provide clean and renewable energy of H2. However, most of photocatalysts are only active under ultraviolet (UV) light, which accounts for only a small fraction (5%) of the solar spectrum at the earth's surface. Many transition metal oxynitrides exhibit strong absorption in the visible region because of their p states mixing with O 2p states, with the potential for overall water splitting into H2 and O2. However, the range of oxynitride photocatalysts synthesized to date remains limited. This thesis focuses on the synthesis of Ta5+- based oxynitrides and the investigation of their photocatalytic properties for water decomposition under visible light irradiation. In addition, a novel route for the synthesis of transition metal nitrides using hydrazine as nitrogen source has been developed.ScTaOxNy, Y2Ta2O5N2, LaTaON2, PrTaON2, NdTaON2, SmTaON2 and Gd2Ta2O5N2 have been successfully prepared from partial replacement of O2- by N3- in the crystalline matrix of LnTaO4 (Ln=Sc,Y, La, Pr, Nd, Sm and Gd) by the high-temperature nitridation technique. The absorption edges of these oxynitrides occur at about 570～650 run in the visible region and are siginificantly shifted to the longer wavelength with increasing the molar ratio of N/O in the lattice of the parent oxides.Y2Ta2O5N2 and ScTaOxNy can decompose water into H2 and O2 under visible light and in the presence of appropriate sacrificial reagents. LaTaON2 is active only for water reduction. PrTaON2, NdTaON2, SmTaON2 and Gd2Ta2O5N2 exhibit relatively lower activity only for water oxidation. Y2Ta2O5N2 catalyst could split water into H2 and O2 simultaneously in the presence of I-. Although platinum is generally considered to be the best metal promoter for H2 production due to its low overpotential, it has been shown to exhibit remarkably enhanced efficiency for H2 evolution when Ru was deposited on Y2Ta2O5N2. A 22-times enhancement in H2 production efficiency is observed when both Pt and Ru are present on Y2Ta2O5N2 compared with Y2Ta2O5N2 modified by Pt alone. The deposition order of Pt and Ru also has a remarkable effect on the H2 evolution efficiency. Deposition of Pt on Ru/Y2Ta2O5N2 (Pt/Ru) results in a quite higher activity than the co-deposition of Pt and Ru and reverse redeposition (deposition of Ru on Pt/Y2Ta2O5N2, Ru/Pt). The order of the H2-evolution rate under visible light on Y2Ta2O5N2 follows: Pt/Ru > Pt-Ru > Ru > Ru/Pt ≈ Pt. This is different from the oxide photocatalysts. The order of the H2-evolution rate under UV light on TiO2 follows: Pt > Ru/Pt > Pt/Ru > Pt-Ru≈ Ru. The simultaneous use of Pt and Ru as promoters of photocatalytic water reduction has yet to be reported.Nanocrystalline TiN with high specific surface area (> 200 m2 g-1) has been prepared by the direct reaction between TiCl4 and N2H4 in solution at low temperatures (150℃).
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