Surface-Mediated Visible-Light Photo-oxidation on Pure TiO 2 (001) Hiroko Ariga, † Toshiaki Taniike, † Harumo Morikawa, † Mizuki Tada, † Byoung Koun Min, ‡ Kazuya Watanabe, § Yoshiyasu Matsumoto, § Susumu Ikeda, | Koichiro Saiki, | and Yasuhiro Iwasawa* ,† Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, Hongo, Tokyo 113-0033, Japan, Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012, Department of Chemistry, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan, and Department of Chemistry, Graduate School of Frontier Science, The UniVersity of Tokyo, Kashiwa, Chiba 277-8561, Japan Received August 6, 2009; E-mail: iwasawa@chem.s.u-tokyo.ac.jp Usually only ultraviolet light must be used for photochemical reactions on TiO 2 because of its bulk band gap (3.0-3.2 eV). However, we used scanning tunneling microscopy (STM) to observe visible light photo-oxidation reactions of formic acid on the new ordered lattice-work structure of a TiO 2 (001) surface for the first time. Two photon photoelectron and electron energy loss spec- troscopies and density functional theory calculations revealed that the nanostructured surface makes the band gap significantly smaller than 3.0 eV only at the surface layer and that the surface state of the crystal enables a visible light response. We report the first example of pure TiO 2 that shows visible light photochemical activity at the surface. Photochemical materials have become important from an envi- ronmental viewpoint because they have the ability to decompose toxic substances in air and water, 1 and their ability to self-clean has been used in commercial products. 2 Understanding and control- ling the electronic structure of the material is key to making photochemical materials a reality. 3 TiO 2 is one of the most intensively studied photochemical materials because it is stable, nontoxic, and inexpensive. 4 With a band gap of more than 3.0 eV, its photochemical properties appear in the near-ultraviolet region. However, since the major component of the solar light lies in the visible light region, research has been trying to extend the photoactivity of TiO 2 -based materials from 3.0 eV down to the visible light range to utilize solar light more efficiently. Several methods such as dye sensitization, 5 metal or nonmetal doping, 6 and oxygen defect doping 7 have been used, and recently, powder TiO 2 has been converted into vis light photocatalysts by doping. The basic strategy of these methods is to create impurity/defect states in the bulk band gap to enable low energy photo excitation. Previous photochemical studies have mainly focused on the bulk states, and almost no attention has been paid to the surface state because powder materials with inhomogeneous surfaces, in which the surface states are not controllable, have been used. In the present study, we investigated the effect of the unique “lattice-work structure” of a TiO 2 (001) surface on the photochemistry of the surface, particularly the vis light responsible photo-oxidation by using formic acid as a probe molecule. The reaction using UV irradiation has already been studied on a powder TiO 2 catalyst, on which formic acid decomposes to CO 2 and H 2 O without any long- lived intermediate. 8,9 To our knowledge, however, there are no reports for a single crystal TiO 2 surface to date. We found through scanning tunneling microscopy (STM) that the nanostructured surface was photochemically active toward vis light (∼2.3 eV) in spite of the bulk band gap of 3.0 V. Combined electronic and theoretical studies also revealed that the photochemical reaction proceeds through the surface state created from the ordered lattice- work structured surface. A typical empty-state STM image of the TiO 2 (001) lattice-work structured surface is shown in Figure 1a and b. The details of all the experiments and the lattice-work structure are described in Supporting Information (SI) 1 and 2. 10 The ordered lattice-work structure has been characterized by STM topographic analysis, adsorption of probe molecules, and DFT calculations in the previous study. 10 This surface has cross-linked and stacked rows running in the [110] and [1 j 10] directions. Each row of the hill part (black square in Figure 1c) consists of three lines of bright protrusions as shown in the inset of Figure 1b. There is ambiguity for the valley part (red square in Figure 1c) in the surface structure. However, the electronic and atomic structure of the hill part, where the visible- light oxidation of formic acid was observed with STM, fortunately, † The University of Tokyo, Tokyo. ‡ Texas A&M University. § Kyoto University. | The University of Tokyo, Chiba. Figure 1. STM images (constant current topograph, (a) 200 × 200 nm 2 , (b) 40 × 40 nm 2 , V s : 2.00 V, I t : 0.05 nA) of TiO 2 (001) after Ar + -sputtering followed by annealing at 1050 K in a UHV. (Inset of (b)) Magnified image of the topmost terrace (red circle: 4-fold coordinated Ti row, blue circle: 5-fold coordinated Ti rows). (c) Top and side views of a structural model of the lattice-work structure on TiO 2 (001). Both side views are cut with black line in the top view. The black squares are “hill part”, and the red squares are “valley part”. 10.1021/ja9066805 CCC: $40.75  XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000 9 A Downloaded by DALIAN INST OF CHEM PHYSICS on October 8, 2009 | http://pubs.acs.org Publication Date (Web): September 25, 2009 | doi: 10.1021/ja9066805