Synthesis of Nanostructured Reduced Titanium Oxide: Crystal Structure Transformation Maintaining Nanomorphology

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There is an increasing need for the synthesis of nanostructured reduced oxides because of their attractive properties; for example, compared with titanium dioxide (TiO2), reduced titanium oxides are attractive for photovoltaics, photocatalysts, and fuel cells owing to their narrow band gap enabling absorption of visible light, chemical stability, and relatively high electrical conductivity, comparable to that of graphite. Generally, reduced titanium oxides have been synthesized by 1) thermal reduction of TiO2 at about 1000 8C with H2 gas or Ti powder, 2) photochemical reduction of TiO2 with UV laser irradiation, or 3) direct synthesis from unique precursors or using laser ablation techniques. These reductive techniques in turn cause particle growth or compositional inhomogeneity, making it difficult to obtain high-quality nanostructures. Ti4O7 and Ti8O15 nanowires were synthesized by annealing H2Ti3O7 nanowires in a hydrogen atmosphere. [2] These nanostructures seem the most sophisticated reported so far; however, particle growth is inevitable and the method seems to be inapplicable to other nanostructures. Since nanostructures are of significant importance for exploiting reduced titanium oxides, development of novel techniques for synthesizing them on the nanoscale is needed. Herein we report a novel method for synthesizing nanostructured reduced titanium oxides by reducing nanostructured TiO2 with a strong reducing agent at much lower temperatures than in conventional techniques. Interestingly, nanostructured reduced titanium oxide with the same morphology as its precursor was obtained, though its crystal structure was transformed from the tetragonal to the hexagonal system. The synthesis is applicable to other nanostructured titanium dioxides, and thus opens up fascinating possibilities for designing nanostructures of reduced titanium oxides for a wide range of applications. We chose TiO2 nanoparticles with rutile structure (tetragonal, P42/mnm), which is a high-temperature phase obtained above about 800 8C and irreversibly stable at room temperature, as precursor. The as-received TiO2 nanoparticles were thoroughly mixed with a fourfold molar excess of CaH2 powder, and then heated at 350 8C for 15 d. Low-temperature reduction with binary metal hydrides was first demonstrated by Hayward et al., who used solid NaH as a reducing agent to topotactically synthesize infinite-layer LaNiO2 from perovskite LaNiO3. [11] Since then, alkali and alkaline earth metal hydrides such as CaH2 and LiH have been employed by Hayward et al. and others to obtain unprecedented reduced phases inaccessible by conventional reduction techniques. The powerful reducing activity even at low temperatures of such binary metal hydrides was expected to enable us to synthesize nanostructured reduced titanium oxides from nanostructured TiO2. The final product was a black powder (see Figure 3), which is typical for reduced titanium oxides. Figure 1 compares the synchrotron powder X-ray diffraction (SXRD) patterns of the rutile TiO2 precursor and the reduced product. After reduction, SXRD patterns significantly changed; all major peaks of the product were readily