Anatase TiO₂ With Exposed {001} Facets As Photocatalyst: Controllable Synthesis And Performance

Energy and environmental issues are the two of the biggest challenges in the new 21 century. The semiconductor photocatalyst materials exhibit great potentials for solar energy conversion and environmental protection. TiO2 nano-material is one of the most used semiconductors, which is due to its advantages such as non-toxicity, high chemical stability, super photocorrosion resistance, excellent photocatalytic activity and low-cost et al. However, several problems such as its low quantum efficiency and relatively wide band gap, which can only absob the UV light, greatly restricts its practical applications and commercial benefit. As for anatase TiO2, both theoretical and experimental studies indicated that the{001} facets are much more reactive than the{101} facets. As a result, in this thesis, the photo-response range and photo-generated charge (electrons and holes) separation efficiency of{001}-facet exposed anatase TiO2 were developed by means of microstructure regulation (including hierarchical structure construction and crystal facet control), surface coating and modification. There are three main parts in this thesis:1. Mesoporous TiO2 microspheres composed of carbon-coated anatase nanocrystals with exposed{001} facets (TiO2(001)/C) have been successfully synthesized by a one-pot hydrothermal strategy in the presence of glucose and hydrofluoric acid (HF). It is demonstrated that the HF promotes the growth of anatase nanocrystal with{001} facet exposure, while glucose induces TiO2{001} nanocrystals to assembly into mesoporous TiO2 microspheres, as well as acts as carbon source in the formation of carbonaceous layer deposited on the TiO2 grains via C-O-Ti bond. The TiO2(001)/C microspheres show an excellent visible-light driven photocatalytic performance for the degradation of methyl blue. These results provide insight into the preparation, assembly and modification of TiO2 nanocrystals with active facets for potential application in environment protection.2. Surface hydrogenated TiO2 has triggered intense research interests in photocatalysis due to its substantially improved solar absorption and superior photocatalytic activity. However, most of the hydrogenation methods are always time, energy consuming and annealing in H2 under high temperature often resulted in significant decrease of the percentage of{001} facets exposing. In order to resolve this issue, TiO2 nanosheets with esposed 70%{001} facets are rapidly modified through H2-DBD plasma treatment in several minutes, and their photocatalytic activities are evaluated by methylene blue (MB) degradation. The obtained H2-plasma modified TiO2 nanosheets possess a unique crystalline core/amorphous shell structure (TiO2@TiO2-x). Electron paramagnetic resonance (EPR) spectra confirm the presence of the oxygen vacancy in the as-prepared TiO2 nanosheets. Further analysis based on X-ray photoelectron spectroscopy (XPS) spectra of Ti2p indicates that only the Ti3+ is localized in the surface amorphous shell, which exhibits the improved visible and near-infrared light absorption. Further investigations reveal that the lower recombination rate of photo-induced carriers induced by the Ti34 and oxygen vacancies might be the critical factors for the high activity of H2-DBD plasma modified TiO2 nanosheets.3. Anatase TiO2 nanosheets with exposed{001} facets have been controllably modified under non-thermal dielectric barrier discharge (DBD) plasma with various working gas, including Ar, H2, and NH3. The obtained TiO2 nanosheets possess a unique crystalline core/amorphous shell structure (TiO2@TiO2-x), which exhibits the improved visible and near-infrared light absorption. The types of dopants (oxygen vacancy/surface Ti3+/substituted N) in oxygen-deficient TiO2 can be tuned by controlling the working gases during plasma discharge. Both surface Ti3+ and substituted N were doped into the lattice of TiO2 through NH3 plasma discharge, whereas the oxygen vacancy or Ti3+(along with the oxygen vacancy) was obtained after Ar or H2 plasma treatment. The TiO2@TiO2-x from NH3 plasma with a green color shows the highest photocatalytic activity under visible light irradiation compared with the products from Ar plasma or H2 plasma due to the synergistic effect of reduction and simultaneous nitridation in the NH3 plasma.