| Research Abstract |
TiO_2 is an efficient photocatalyst and its application areas have been expanded to self-cleaning building materials and environment preservation technologies, among others. However, it is activated only by ultraviolet (UV) light irradiation, so the modification of its sensitivity to visible light has been required. Hence, doping a foreign element into TiO_2 has been performed and nitrogen -doped TiO_2 has attracted much attention. We successfully fabricated nitrogen, carlbn, and sulfur doped TiO_2 with various degrees of these dopants and evaluated their visible-light induced activities. However, those anion-doped TiO_2 had low activities irradiated with visible light. It was plausible considering the density of states (DOSs) of anion-doped TiO_2 that the introduced anion-p or bit als formed the localized electronic states in the forbidden band. We designed using DOS calculation that the N 2p level shifts to ward the valence band (positive potential) in the Ta, N-codoped TiO_2 system,
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resulting in a delocalization of N 2p level by mixing its valence band. Thus, we could expect that the Ta, N-codoped TiO_2 had high activities, due to increasing oxidation power and hole mobility under visible light irradiation. In fact, the Ta, N-codoped TiO_2 showed higher activity than solely N-doped TiO_2.
With the exception of TiO_2-based materials, only several reports have examined oxide or oxynitride materials that are sensitive to visible light for the oxidative decomposition of organic compounds. We have investigated various such novel photocatalysts, controlling electronic band structures and crystalline structures. For example, we have already reported Ag′-inserted NbO_2F with a ReO_3 structure and α-AgGaO_2 with a delafossite structure utilizing Ag′. Ag′ cation in a 4d^<10> configuration mixes the O2p orbitals, forming the highly dispersed upper valence band. This can be favorable for visible light sensitivity and high photocatalytic activity, because mixing Ag4d with O2p decreases in the band-gap energy and the dispersed valence band leads to high mobility of photo-generated holes. In addition to Ag′ cations and anions in d^0, d^<10>, d^<10>s^2 configurations, such as Cu′, Pb^2 etc., can generate photocatalysts that are sensitive to visible light. The key factors in realizing high-efficiency visible light sensitive photocatalysts are modulation of the closed shell with ligand O2p orbitals and finding crystal structure favorable for holes' mobility. Less
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