Photophysical and photocatalytic properties of Li 2 M(WO 4 ) 2 (M = Co and Ni)
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Xianli Huang Ecomaterials and Renewable Energy Research Center, National Laboratory of Solid State Microstructures, Nanjing 210093, China; and Department of Physics, Nanjing University, Nanjing 210093, China
Haifeng Shi, Hanmin Tian, and Zhaosheng Li Ecomaterials and Renewable Energy Research Center, National Laboratory of Solid State Microstructures, Nanjing 210093, China; and Department of Materials Science and Technology, Nanjing University, Nanjing 210093, China
Tao Yu Ecomaterials and Renewable Energy Research Center, National Laboratory of Solid State Microstructures, Nanjing 210093, China; and Department of Physics, Nanjing University, Nanjing 210093, China
Jinhua Ye Photocatalytic Materials Center, National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan
Zhigang Zoua) Ecomaterials and Renewable Energy Research Center, National Laboratory of Solid State Microstructures, Nanjing 210093, China; and Department of Physics, Nanjing University, Nanjing 210093, China (Received 4 February 2008; accepted 8 August 2008)
Li2M(WO4)2 (M ⳱ Co and Ni) were synthesized by a conventional solid-state reaction method and characterized by powder x-ray diffraction, Brunauer-Emmet-Teller (BET) measurement, ultraviolet-visible (UV-vis) diffuse reflectance spectra, Raman spectroscopy, and photocatalytic evaluation measurements. Photocatalytic water splitting results showed that Li2M(WO4)2 (M ⳱ Co and Ni) exhibited abilities for H2 evolution with Pt cocatalyst from an aqueous methanol solution and for O2 evolution from an aqueous AgNO3 solution under UV light irradiation. Theoretical calculation, absorbance analysis, and photocatalytic H2 evolution experiment revealed that the position of W 5d level shifted to the negative side with respect to the reduced potential of H+/H2. The photocatalytic H2 evolution over Li2M(WO4)2 is discussed from the view of crystal and electronic structure point.
I. INTRODUCTION
Recently, more and more attention has been paid to photocatalytic water splitting for solar energy conversion.1 In principle, if the conduction band (CB) bottom of a semiconductor is lower than that of H+/H2 reduction potential and the valence band (VB) top is higher than that of O2/H2O oxidation potential, it is then thermodya)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0407 J. Mater. Res., Vol. 23, No. 12, Dec 2008
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namically possible to decompose water into hydrogen and oxygen.2 Tungsten oxide (WO3) is an excellent photocatalyst for oxygen evolution.3 However, no hydrogen evolution over WO3, both under visible light and ultraviolet (UV) light irradiation, is observed because of its lower CB (mainly composed of W 5d level) with respect to the reduced potential of H+/H2. Therefore, many efforts have been made to achieve photocatalytic hydrogen evolution and simultaneously keep the ability of photocatalytic oxygen evolution via tuning the electronic structure of WO3.4,5 © 2008 Materials Research Society
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