Synthesis and photophysical properties of barium indium oxides
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Z. Zou Photoreaction Control Research Center (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
J. Ye Ecomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan (Received 27 February 2002; accepted 24 June 2002)
Barium indium oxides (BaIn2O4, Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8) were synthesized by the citric process and characterized by powder x-ray diffraction. The optical absorption properties of these compounds were investigated by UV–visible diffuse reflectance spectroscopy. It was found that with the increase of the mole ratio of In2O3 in the formula the optical absorption edges of these oxides shift to the longer wavelength side monotonically. The photocatalytic H2 and O2 evolutions under visible light irradiation ( > 420 nm) from aqueous CH3OH/H2O and AgNO3/H2O solutions were performed. Among these oxides, BaIn2O4 was the most stable compound, and other compounds were not stable chemically in the case of water and visible light irradiation.
Much research has been performed on the BaO–In2O3 oxide system including synthesis, structure, defects, and oxide ion conductivity.1–5 Among the system the well-studied compounds include BaIn2O4, Ba4In6O13, Ba2In2O5, Ba3In2O6, Ba5In2O8, etc. BaIn2O4 is crystallized in monoclinic symmetry (space group C2h5 − P21/a, a ⳱ 1.4432 nm, b ⳱ 0.5833 nm, c ⳱ 2.0792 nm,  ⳱ 110.02°), and other compounds appear to be oxygen deficient “multipled” perovskites.6,7 Kwestroo et al.3 reported that the compounds BaIn2O4, Ba4In6O13, and Ba2In2O5 are stable in moist air, Ba3In2O6 and Ba5In2O8 are sensitive to aqueous vapor, and Ba3In2O6 was converted into hydrogarnet Ba3In2(OH)12 by reaction with water. Recently Hashimoto et al. reported that Ba2In2O5 was subjected to structural transformation after incorporation of water vapor.8 Here we report the photophysical properties and photocatalytic properties of the compounds in the BaO–In2O3 system. Polycrystalline barium indium oxide powders were prepared by the citric process. The starting materials including BaCO 3 , In(NO 3 ) 3 ⭈ 3H 2 O, and citric acid [HOOCCH 2 C(OH)(COOH)CH 2 COOH] in chemical stoichiometry were dissolved in a beaker with dilute nitric acid solution. The mixed solution was heated on a hot plate at a temperature about 100 °C and stirred by a magnetic stirrer with a rotation rate 700 rpm. After the J. Mater. Res., Vol. 17, No. 9, Sep 2002
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water in the solution was evaporated completely, the nitric group began decomposing, the solution became denser, and then the magnetic stirrer was taken out. The process in the beaker went on. When the mixed product began self-igniting, the heating was stopped. The beaker containing the residue was kept in a cabinet at 80 °C for 12 h. The dried residue was heated at 450 °C in a furnace for 2 h; thus, the porous gray noncrystal powder was obtained, in which some fine In2O3 particles with ligh
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