Active facets on titanium(III)-doped TiO2: an effective strategy to improve the visible-light photocatalytic activity.

The properties and applications of materials are significantly controlled by their physical characteristics, such as size, shape, and structural state. Many processes are governed by interface reactions by which the surface energy and reactivity depend on the spatial configuration, coordination, and structural state of surface atoms and molecules. For crystals, this dependence is directly related to the expression of specific crystallographic faces, which exhibit different surface structures and atomic configurations. These differences explain why some applications, such as molecular adsorption and desorption, gas sensing, drug molecule delivery and release, and heterogeneous catalysis are highly sensitive to the surface atomic structures. Recent progress in the engineering of crystal morphology has included the synthesis of polyhedral silver nanocrystals by the polyol method, the epitaxially seeded growth of highly faceted Pt-Pd nanocrystals, and the controlled overgrowth of Pd-Au core–shell structures enclosed by {111} facets. Apart from these metallic nanocrystals, binary or ternary compounds with preferentially developed facets have also been reported. The facet effect is an important factor for heterogeneous photocatalysts, because surface atom arrangement and coordination intrinsically determine the adsorption of reactant molecules, surface transfer between photoexcited electrons and reactant molecules, and desorption of product molecules. This phenomenon has been well studied in TiO2 photocatalysts. TiO2 is one of the most extensively studied photocatalysts owing to its abundance, nontoxicity, and stability. However, for practical applications, pure TiO2 is not a good candidate because it is only active under ultraviolet (UV) irradiation owing to the band gap of 3.2 eV for the anatase phase. Therefore, band-gap engineering is required to use TiO2 as a water-splitting catalyst under visible-light irradiation. Reduced TiO2 (TiO2 x), containing Ti or O vacancies, has been reported to show visible-light absorption. Various strategies have been applied to synthesize reduced TiO2, such as heating under vacuum [8] or reducing gas, laser irradiation, and high-energy particle bombardment (electrons or Ar ions). A big challenge for the application of reduced TiO2 is that the surface oxygen defects are highly unstable in air owing to the susceptibility of Ti toward oxidation by O2. [13] Recently, we reported a facile one-step combustion method to synthesize partially reduced TiO2. [14] The presence of Ti in the sample extends the photoresponse of TiO2 from the UV to the visible light region, which leads to high visible-light photocatalytic activity for the generation of hydrogen gas from water. However, in the rapid and harsh combustion process, there is very limited control over the crystallization process, which results in the irregularly shaped products. Herein we report the development of a simple solution method to grow non-stoichiometric rutile TiO2 crystals with desired facets. The incorporation of Ti, which extends the light absorption from the UV into the visible range, along with the development of facets with high reactivity, results in a material exhibiting greatly enhanced photocatalytic H2 production activity relative to the combustion product we reported before. Powder X-ray diffraction analysis (Figure 1a) shows that the sample of as-produced TiO2 (sample S1) has rutile structure. All of the diffraction peaks can be assigned to