| Due to the relatively high density of unsaturated 5-fold Ti and the unique electronic structure, the (001) surface of anatase TiO2 has been argued theoretically to possess a photocatalytic activity advantageous over the thermodynamic stable (101) one, which is the most exposed facets for common titania crystals. Anatase single crystals with exposed (001) facets thus have been the focus of many researches in recent two years. In this thesis, nanometer-to micrometer-sized anatase single crystals with exposed (001) facets and a novel titania microsphere with nearly all-(001) surface were synthesized by hydrothermal synthesis. The morphology, structure and photon-induced property of the achieved titania were investigated in detail with various techniques of field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM), UV-Vis diffuse reflectance spectra (UV-Vis DRS) and X-ray photoelectron spectra (XPS). The photocatalytic activity of the achieved titania was evaluated by photodegradation of rhodamine B in water.Follows are the main results obtained:1. Hydrothermal Synthesis of Nanometer-to Micrometer-Size Anatase Single Crystals with Exposed (001) Facets.Anatase single crystals with exposed (001) facets were fabricated by the hydrothermal synthesis of Ti plates in aqueous HF solutions with mass concentrations of 0.15～0.80%, in an autoclave at 180℃for 2-12 h. The achieved anatase possessed crystal sizes varying from 0.4 to 13.6μm, and (001) fractions varying from 15 to 49%. The crystal size and the (001) fraction increased with increasing HF concentrations. For a prolonged reaction time, anatase crystals with larger sizes and reduced fractions of (001) facets were achieved. The activity of the anatase crystals to assist photodegradation of rhodamine B in water increased with decreasing crystal sizes and increasing fractions of (001) facets. A decreasing crystal size contributes more to the enhanced photocatalytic activity. Selective erosion of the anatase single crystals along the high energy (001) facets was noted, which resulted in cone-shaped walls with a thickness ranging from several to hundreds of nanometers. The selective erosion contributed to the photocatalytic activity of the (001)-exposed anatase single crystals. After the additive of 100 mg F127 block copolymer in HF solution, the surface morphology of Ti plates changed significantly. The surface is covered with anatase microsphere totally and its photocatalytic activity is higher than that of the anatase layer obtained in absence of the copolymer.2. Hydrothermal Synthesis of N-F-Cr-doped Anatase TiO2 Microspheres with Nearly All-(001) Surface for Enhanced Solar Absorption.A novel titania microsphere with nearly all-(001) surface was synthesized by hydrothermal treatment of a thermal sprayed TiN/Ti coating with HF aqueous solution containing metallic chromium powders. Unlike the conventional (001)-facet exposed anatase crystals, which are highly truncated bipyramids, the crystal achieved in the current investigation is drum-like with a round cross section. The formation of the drum-like crystals was attributed to a balance between the erosion and precipitation of anatase single crystallites. The Cr-doped in anatase was believed to increase the surface deficiency which enhanced the erosion procedure, leading to the drum-like crystals. The XPS analysis confirmed the incorporations of N, F and Cr in the microsized anatase crystals through the one-pot reaction, which led to a significantly enhanced solar absorption. The UV-Vis diffuse reflectance spectrum revealed a band-to-band red-shift of the band gap of the anatase crystals to 1.60 eV, which is contributed mainly to the homogeneous Cr-doping. |
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