Design of Highly Efficient Visible Light Sensitive Photocatalysts on the Basis of Interfacial Charge Transfer

  • Dr Kazuhito Hashimoto, Department of Applied Chemistry, The University of Tokyo, Research Center for Advanced Science and Technology, The University, Japan

    • We recently reported a novel visible-light-sensitive photocatalyst by grafting Ti(IV)-O-Ce(III) hetero-bimetallic assemblies onto the inner walls of mesoporous silica and successfully demonstrated that photocatalytic oxidative decomposition can be initiated by a metal-to-metal charge transfer (MMCT) induced by visible light, i.e., electron transfer proceeds from Ti(IV) to Ce(III)(1). Moreover, on the basis of this observed MMCT and the fact that the conduction band (CB) of TiO2 mainly consists of Ti3d orbitals, we expected that photoirradiation can directly induce a charge transfer from atomic metal ions to the CB of TiO2 when they are atomically grafted onto a TiO2 surface. In reality, TiO2 powder with either Ce(III) or Cr(III) ion grafts was successfully demonstrated to show photocatalytic activity under visible light irradiation(1,2). This process corresponds to a type of interfacial charge transfer (IFCT) between discrete energy levels of molecular species and continuous one of solids, which was predicted by Hush(3) and theoretically formulated by Creutz et al.(4). Therefore, we further hypothesized that photoirradiation also induces charge transfer directly from an oxide VB composed of O2p orbitals to atomic metal ions atomically grafted on the oxide surface. In addition, it is reported that a Cu(I) ion occasionally catalyzes multi-electron oxygen reduction. Based on those considerations, we designed novel photocatalysts sensitive to visible light, namely, Cu(II)-grafted TiO2 and WO3 (5). The fabricated Cu(II)/TiO2 and Cu(II)/WO3 photocatalysts decomposed gaseous 2-propanol(IPA) to CO2 via acetone very efficiently under visible light. Compared to N-doped TiO2, which had previously been believed to be the most efficient visible light photocatalyst, the decomposition of IPA under visible light (> 400 nm) by Cu(II)/TiO2 had 5.5 and 2.1 times higher quantum efficiency (QE) and CO2 generation rate, respectively. As for Cu(II)/WO3, it had 11 and 16 times higher QE and CO2 generation rate, respectively.

    Reference
    (1) R. Nakamura, A. Okamoto, H. Irie, K. Hashimoto, J. Am. Chem. Soc. 129 (2007) 9596.
    (2) H. Irie, S. Miura, R. Nakamura, K. Hashimoto, Chem. Lett., 37 (2008) 252.
    (3) N. S. Hush, Electrochim. Acta 13 (1968) 1005.
    (4)C. Creutz, B. S. Brunschwig, N. Sutin, J. Phys. Chem. B, 109 (2005) 10251.
    (5) H.Irie, S. Miura, K. Kamiya, K. Hashimoto, Chem. Phys. Lett., in press.