Energy-Level Matching of Fe(III) Ions Grafted at Surface and Doped in Bulk for Efficient Visible-Light Photocatalysts Min Liu, ‡ Xiaoqing Qiu, ‡ Masahiro Miyauchi,* ,†,∥ and Kazuhito Hashimoto* ,‡,§ † Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan § Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ∥ Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan * S Supporting Information ABSTRACT: Photocatalytic reaction rate (R) is determined by the multiplication of light absorption capability (α) and quantum efficiency (QE); however, these two parameters generally have trade-off relations. Thus, increasing α without decreasing QE remains a challenging issue for developing efficient photocatalysts with high R. Herein, using Fe(III) ions grafted Fe(III) doped TiO 2 as a model system, we present a novel method for developing visible-light photocatalysts with efficient R, utilizing the concept of energy level matching between surface-grafted Fe(III) ions as co-catalysts and bulk-doped Fe(III) ions as visible-light absorbers. Photogenerated electrons in the doped Fe(III) states under visible-light efficiently transfer to the surface grafted Fe(III) ions co-catalysts, as the doped Fe(III) ions in bulk produced energy levels below the conduction band of TiO 2 , which match well with the potential of Fe 3+ /Fe 2+ redox couple in the surface grafted Fe(III) ions. Electrons in the surface grafted Fe(III) ions efficiently cause multielectron reduction of adsorbed oxygen molecules to achieve high QE value. Consequently, the present Fe(III)-Fe x Ti 1−x O 2 nanocomposites exhibited the highest visible-light R among the previously reported photocatalysts for decomposition of gaseous organic compounds. The high R can proceed even under commercial white-light emission diode irradiation and is very stable for long-term use, making it practically useful. Further, this efficient method could be applied in other wide-band gap semiconductors, including ZnO or SrTiO 3 , and may be potentially applicable for other photocatalysis systems, such as water splitting, CO 2 reduction, NO x removal, and dye decomposition. Thus, this method represents a strategic approach to develop new visible-light active photocatalysts for practical uses. 1. INTRODUCTION Heterogeneous photocatalysis using semiconductors has great potential for solving current energy and environmental issues. 1−20 Efficient photocatalysts are typically wide-band gap semiconductors, such as TiO 2 , ZnO, and SrTiO 3 , owing to the high redox potential of photogenerated charge carriers. 10 Holes with high oxidation power in the valence band (VB) and electrons with sufficient reduction power in the conduction band (CB) are generally required for efficient photocatalytic reactions. However, wide-band gap semiconductors are only activated under ultraviolet (UV) light irradiation, which limits their practical applications. The doping of various transition-metal cations or anions into wide-band gap semiconductors has been extensively studied to increase the visible-light absorption of these photocatalysts. 1−10 However, despite extensive research efforts, most systems remain unsatisfactory for practical use. In particular, metal-ion dopants introduce deep impurity levels in the forbidden band of semiconductor photocatalysts, where they act as recombi- nation centers and impair photocatalytic activity. 1−4 In the case of anion-doped semiconductors, isolated states are formed above the VB and cause the quantum efficiency (QE) of the semiconductors to deteriorate, as the holes generated in these isolated states have lower oxidation power and mobility than those in the VB. 1,2,6−9 For example, the QE of nitrogen-doped TiO 2 under visible light is markedly lower than that of pure TiO 2 under UV light. 6,9 These previous studies indicate that it is difficult to improve the visible-light absorption of semi- conductors while maintaining a high QE value, because the reactivities of photogenerated charge carriers in doped levels or narrowed bands are much less than those in the VB and CB. Very recently, our group demonstrated that the surface modification of TiO 2 with co-catalysts, such as Cu(II) and Fe(III) ions, 21−30 induces the efficient interfacial charge transfer 31−35 of VB electrons upon visible-light irradiation and multielectron reduction reactions of oxygen, 36−44 during which the excited electrons are consumed. Co-catalysts also improve the visible-light activities of doped semiconductors. 25−30 For example, Ti 3+ self-doped TiO 2 , which is inactive even under UV Received: February 12, 2013 Published: June 17, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 10064 dx.doi.org/10.1021/ja401541k | J. Am. Chem. Soc. 2013, 135, 10064−10072