ABSTRACT
TiO2 has been intensively investigated for environmental remediation and energy conversion in the past several decades as a select platform on which an exceptionally wide range of appealing solid-state physical-chemical properties coexist with the potential for low-cost and environmental remediation and energy technologies [1-3]. TiO2 exists in three crystal structures: rutile, anatase, and brookite. Each crystalline structure exhibits speciϐic physical properties, bandgap, surface states, etc. Rutile TiO2 has some advantages over anatase, such as higher chemical stability and higher refractive index. It is of fundamental signiϐicance to explore mild synthetic techniques by which particle shapes, nano-and micro-meter-scale morphologies, and crystallinity are well deϐined and controlled
[3-5]. Moreover, surface chemistry of single crystalline rutile particles has been the subject of intensive studies because their chemical activity depends greatly on surface structures [6]. It has been reported that well-crystallized faceted particles showed enhanced photocatalytic activity compared to particles with poorly crystalline surfaces and that the photocatalytic activity increased with increase in crystallite size, the surface itself being an intrinsic defect [7]. Morphology, exposed crystal face-controlled synthesis of TiO2 nanoparticles, has long been paid attention in order to develop a high-active TiO2 photocatalyst. Among several kinds of synthetic methods for TiO2 nanoparticles, a hydrothermal treatment has been drawing much attention for one of main synthetic techniques of TiO2 nanocrystals because it directly produces well-crystallized nanocrystallities of a wide range of compositions of crystal phases under mild conditions. In addition, controlling exposed crystal faces of rutile nanorod TiO2 by using surface-morphology-controlled reagents or chemical-etching reagents was also important strategy for further improvement of their photocatalytic activities. So many intensive efforts to improve the visible-light responsibility of TiO2 photocatalyst involving impurity doping have been made in the last few decades [8-13]. However, impurity doping sometimes increases defects in TiO2, which also work as recombination center and result in decrease of photocatalytic activity [14, 15]. Recently, some visible-light responsive TiO2 photocatalysts were developed by modiϐication of metal surface complex that works as a sensitizer for a visible light [16-20]. This method has large advantages in simple preparation method and no introduction of defects in TiO2. However, back electron transfer between injected electrons in TiO2 bulk and oxidized metal ions on the surface of TiO2 may easily proceed resulting in signiϐicant decrease in a photocatalytic activity. Therefore, it is necessary for further improvement of photocatalytic activity under visible-light irradiation to modify metal ion site-selectively in the speciϐic site on TiO2 particles. Our previous studies suggest that redox reaction proceeds preferentially on speciϐic exposed crystal faces of TiO2 [21-23]. This kind of preferential reaction was assigned by site-selective deposition of metal or metal oxide on the speciϐic exposed crystal faces under photoexcitation [21-25].
