Study And Preparation Of Ta⁵⁺ And Ti⁴⁺ Based Oxynitrides As Visible-Light-Driven Photocatalysts

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 as found are only responsive under ultraviolet (UV) light, which accounts for only a small fraction (4%) of the solar spectrum on the earth's surface. When N element is introduced into a transition metal oxide, the transition metal oxynitrides exhibit strong absorption in the visible region due to the higher valence band which consisted of hybridized N2p and O2p orbital. That can lead to potential photocatalytic applications under visible-light excitation. In the past several years, there has been a growing concern about the synthesis of new types of high active visible-light-driven oxynitride photocatalyst. In this paper, wet chemical method was used to synthesize transition metal oxide as a highly reactive raw material for the nitridation reaction. Since its high reactive activity, transition metal oxynitrides with large surface area and narrow band gap are expected to be synthesized by ammonolysis of transition metal oxide.The Ta⁵⁺ oxynitrides have been prepared from partial replacement of O^2- by N^3- in the crystalline matrix of Ta₂O₅ by high-temperature nitridation technique using NH₃ as the reactant gas at 800-900℃for 8 h. X-ray powder diffraction, UV-vis absorption spectra, BET specific surface area were adopted for the characterization of the oxynitrides powder. The influence of ammonolysis temperature on the visible-light absorption property and BET surface area of the oxynitrides were investigated.Lanthanum titanate powder was synthesized by high temperature solid-state reaction, then nitrided by NH₃ at 800-900℃for 8-30h. After that, oxynitride powder can be obtained. X-ray powder diffraction, UV-vis absorption spectra, BET specific surface area, element analysis were adopted for the characterization of the oxynitrides powder. It can be seen that both a higher nitriding temperature and a longer holding time can lead to a higher nitrogen content of the oxynitride. As increasing the nitridation temperature, the absorption band of the oxynitride is red-shifted, and the color of the oxynitride varied from yellow to brown.Lanthanum-titanate oxynitrides have been successfully prepared by ammonolysis of lanthanum titanate precursors resulting from wet chemical method such as sol-gel and co-precipitation. The precursors and the resulting lanthanum-titanate oxynitrides were characterized by X-ray powder diffraction, UV-vis absorption spectra, BET specific surface area, element analysis and transmission electron microscopy. Compared to the properties of the oxynitride synthesized by solid-state reaction and sol-gel, it can be demonstrated that the homogenized amorphous nano-sized La₂O₃/TiO₂ with high surface area and reactivity can be obtained by co-precipitation. Nanocrystalline orthorhombic LaTiO₂N (30nm) with a surface area of 27.5 m²/g has been successfully prepared by nitridation at 900℃for 8 h. The absorption edges of lanthanum-titanate oxynitrides occur at about 550-590nm in the visible region and are significantly shifted to the longer, wavelength with increasing the molar ratio of N/O in the lattice of the precursor oxides. All lanthanum-titanate oxynitrides have brilliant colors, varying from yellow to red, depending on the N/O ratio which correlated with nitridation temperature.