Development of New Tantalum Base Mixed Oxide for Photocatalytic Decomposition of H_2O into H_2 and O_2

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Effects of electronic state of oxide semiconductor on photocatalysis of H_2O splitting into H_2 and O_2 were investigated on KTaO_3 based oxide. It was found that doping Zr, Hf, La and Ga to Ta site of KTaO_3 is highly effective for increasing the photocatalytic activity to H_2O decomposition. From the hole effects measurement, KTaO_3 is n-type semiconductor and electronic charge density decreases by doping these lower valence cations. Photo-electromotive force measurement also suggests that the life of existed hole and electron is prolonged by doping the lower valence cations. As a result, it becomes clear that the control of electronic charge density in photocatalyst is effective for increasing the photocatalytic activity to H_2O decomposition. Similar dopant effects were also observed on another compounds, i.e., SrTiO_3, Ba_2In_2O_5, and K_6Ta_<10.6>O_<30>. Effects of crystal structure was also investigated. K_2Ta_2O_6 with pyrochlore like structure and KTaO_3 with perovskite structure was successfully prepared by alkoxide method. Although the chemical composition is the same, the activity to photolysis of H2O is greatly different between K_2Ta_2O_6 and KTaO_3. This suggests that the electronic state is varied by the atomic arrangements in crystal structure and the activity to photocatalytic decomposition of water is highly affected by the electronic state of photocatalyst.