Oxynitride materials for solar water splitting
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oduction Photocatalytic reactions have been studied extensively in recent years from the viewpoint of environmental accountability and energy conversion. For environmental purposes, TiO2 with anatase and rutile crystal structures has been predominantly used because of its superior material and economic properties (e.g., chemical stability, non-toxicity, cost-effectiveness).1,2 In recent research, modified TiO2 such as N-doped TiO2 has been examined in order to extend the absorption edge into the visible light region.3 In application to energy conversion, where research was initially triggered by the potential of TiO2-based photoelectrochemical (PEC) reactions for the decomposition of water into H2 and O2,4 a range of materials has been examined as powder photocatalysts. These materials include transition metal oxides containing metal ions of Ti4+, Zr4+, Nb5+, Ta5+, or W6+ with d0 electronic configuration and typical metal oxides having metal ions of Ga3+, In3+, Ge4+, Sn4+, or Sb5+ with d10 electronic configuration.5–9 Thus, the group of successful photocatalysts for overall water splitting is comprised solely of metal oxide-based systems. Although some non-oxide materials such as CdS and CdSe have been examined, particularly for visible-light catalysis, successful photocatalytic systems have yet to be achieved, primarily due to the lack of oxygen productivity and inherent instability of the materials, which are derived from oxidative self-decomposition of the materials.10,11 The two major obstacles to the development of powder photocatalysts are the discovery of new stable photocatalytic
materials and the construction of a suitable visible light-driven photocatalytic system. The application of metal oxides for photocatalysis in the visible-light region is complicated by the deep valence band positions (O 2p orbitals) of these materials, resulting in a bandgap that is too large to harvest visible light.12 Since the N 2p orbital has a higher potential energy than the O 2p orbital, using a metal nitride or metal oxynitride as a photocatalyst has promise. Figure 1 shows the schematic band structures of a metal oxide (NaTaO3) and oxynitride (BaTaO2N), both of which have the same perovskite structure. The top of the valence band (i.e., highest occupied molecular orbital, HOMO) for the metal oxide consists of O 2p orbitals. When N atoms are partially or fully substituted for O atoms in a metal oxide, the HOMO of the material is expected to be shifted higher compared to the corresponding metal oxide without affecting the level of the bottom of the conduction band (i.e., lowest unoccupied molecular orbital, LUMO). As expected, density functional theory (DFT) calculations for BaTaO2N indicated that the HOMO consists of hybridized N 2p and O 2p orbitals, whereas the LUMO is mainly composed of empty Ta 5d orbitals.6 The potential of the HOMO for the oxynitride is located at higher potential energy than that for the corresponding oxide due to the contribution of N 2p orbitals, making the bandgap energy sufficiently small to respond
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