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Anatase TiO2 Single Crystals Exposed with HighReactive {111} Facets Towards Efficient H2 Evolution Hua Xu, Pakpoom Reunchan, Shuxin Ouyang, Hua Tong, Naoto Umezawa, Tetsuya Kako, and Jinhua Ye Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm303502b • Publication Date (Web): 04 Jan 2013 Downloaded from http://pubs.acs.org on January 14, 2013
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Chemistry of Materials
Anatase TiO2 Single Crystals Exposed with High‐Reactive {111} Facets Towards Efficient H2 Evolution Hua Xu,†‡§ Pakpoom Reunchan,‡ Shuxin Ouyang,*,‡ Hua Tong,‖ Naoto Umezawa,‡‖ ‖ Tetsuya Kako,†‡§ and Jinhua Ye*,†‡§ †
Graduate School of Chemical Science and Engineering, Hokkaido University, Sapporo 060‐0814, Japan Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), 1‐1 Namiki, Tsukuba 305‐0044, Japan § International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1‐ 1 Namiki, Tsukuba, 305‐0044, Japan ‖ TU‐NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, 92 Wenjin Road, Nankai District, Tianjin 300072, P.R. China ‡
KEYWORDS: Anatase TiO2; {111} facet; Photocatalysis; Surface chemistry. ABSTRACT: In this study, for the first time, {111} facet exposed anatase TiO2 single crystals are prepared via both F‐ and ammonia as the capping reagents. In comparison with the most investigated {001}, {010}, and {101} facets for anatase TiO2, the density functional theory (DFT) calculations predict that {111} facet owns a much higher surface energy of 1.61 J/m2, which is partially attributed to the large percentage of undercoordinated Ti atoms and O atoms existed on the {111} surface. These undercoordinated atoms can act as active sites in the photoreaction. Experimentally, it is revealed that this material exhibits the superior electronic band structure which can produce more reductive electrons in the photocatalytic reaction than those of the TiO2 samples exposed with majority {010}, {101}, and {001} facets. More importantly, we demonstrate that this material is an excellent photocatalyst with much higher photocatalytic activity (405.2 μmol‧h‐1), about 5 times, 9 times, and 13 times of the TiO2 sample exposed with dominant {010}, {101}, and {001} facets, respectively. Both the superior surface atomic structure and electronic band structure significantly contribute to the enhanced photocatalytic activity. This work exemplifies that the surface engineering of semiconductors is one of the most effective strategies to achieve advanced and excellent performance over photofunctional materials for solar energy conversion.
INTRODUCTION Photocatalysis is an environmental‐friendly and promis‐ ing technology to convert solar energy into chemical en‐ ergy.1‐6 The discovery of water photolysis on TiO2 elec‐ trode by Fujishima and Honda in 1972 was recognized as the landmark event in photocatalysis;7 since then, large amounts of research have been carried out on TiO2 pho‐ tocatalysis to achieve a higher solar‐to‐energy conversion efficiency, such as modification of the electronic structure of TiO2 to extend its light‐absorption region via doping,8, 9 and fabrication of TiO2 based heterojunction materials to enhance the charge separation efficiency.10‐13 Recently, special attention has been paid on the surface chemistry of TiO2, since many physical and chemical processes (e.g. adsorption of reactant molecules, surface transfer of pho‐ toexcited electrons to reactant molecules, and desorption of product molecules) happened on the surface during the photocatalytic reaction.14‐16 Peculiarly, for a crystalline TiO2, the surface geometric (atomic arrangement and
coordination) and electronic structures vary with crystal facets in different orientations, leading to various physical and chemical properties.17 Therefore, tailored synthesis of TiO2 single crystals with optimized reactive facets is high‐ ly desirable for promoting the photocatalytic activity. In the case of anatase TiO2, the shape of this material under equilibrium conditions is a slightly truncated te‐ tragonal bipyramid enclosed with 94% of {101} surfaces (the most thermodynamically stable) and minor {001} surfaces based on the Wulff construction and theoretical‐ ly calculated surface energy ({101} (0.44 J/m2)
