Nanostructured Control Of Highly Active TiO₂ Material And Study Of Its Photocatalytic Performance

Titanium dioxide(Ti O2) is one of the most widely studied inorganic semiconductor photocatalyst, owing to its high photocatalytic activity, photo- and chemical stability, non-toxicity and ubiquity. However, due to its large band gap(3.0 e V for rutile and 3.2 e V for anatase), it only responds to UV radiation, which is less than 5% of sunlight. In addition, the fast recombination rate of photo-generation electron-hole pairs and slow hot carrier diffusion hinder the photocatalytic performance of Ti O2 and its practical application as an efficient photocatalytic material. Thus, controlled synthesis of Ti O2 with both exposure of high-energy facets and high surface area is an effective way to promote the charge separation and improve the photocatalytic efficiency. Otherwise, intentional introduction of crystal defects(such as oxygen vacancy or Ti3+) into Ti O2 crystal lattices has proved to be one of the most promising methods to broaden visible light absorption, because defects can impact the functional properties of metal oxides, such as electronic structure, charge transport, and catalytic performance.1. The controlled synthesis of anatase titanium dioxide(Ti O2) with both high surface area and high energy facets is technologically important for its application in photocatalysis, photoelectrochemical cells, and solar cells. Here we report a simple and fluorine free hydrothermal method tosynthesize hierarchically nanostructured mesoporous anatase Ti O2 spheres(MATS), which were covered with {001} facets. Mild H2SO4 was used as both a phase-inducer for the formation of the anatase phase and a capping agent to promote oriented growth and formation of {001} facets. Detailed XRD and SEM studies suggested that formation of MATS follows a typical nucleation and growth process. The refining or reconstruction of Ti O2 crystal structure during growth resulted in a mesoporous crystalline framework that exhibits enhanced adsorption and photocatalytic degradation of rhodamine B in comparison with that of commercial Degussa P25 Ti O2.2. We developed a simple method to synthesize Ti O2 nanowire arrays with nearly 100% exposed {001} facets. The coating exhibits good transparency. The thin films of Ti O2 nanowire arrays display a very good photocatalytic degradation of dye molecules and good durability. Based on the above features, the Ti O2 nanowire array coating is advantageous for self-cleaning coating.3. Stable reduced Ti O2 rutile nanorods with well-defined facets were prepared by a solvothermal route in the presence of Zn powder. The as-prepared reduced Ti O2 exhibits high stability in air and water upon light irradiation. The reduced degree(oxygen vacancy concentration) can be tuned by the amount of Zn powder added. Experimental results show a good conversion efficiency in both the full solar spectrum and visible light(l > 420 nm), which supports that it is the introduced oxygen vacancy that accounts for the extension of the photocatalytic activity from the UV to the visible light region. Excess amount of oxygen vacancy will result in a decrease of photocatalytic performance. The present study demonstrates a simple and economical method for narrowing the band gap and for the development of a highly active photocatalyst under visible light.4. To further understand the effect of structural defect of TiO2 on the electrochemical and photocatalytic properties, two synthetic approaches based on hydrothermal synthesis and post-synthetic chemical reduction to achieve oxygen defect-implantation have been developed here. These lead to the formation of Ti O2 nanorods with uniformly distributed defects in either bulk, on the surface or the combination of both in the formed Ti O2 nanorods. Both the approaches utilize the unique Ti N nanoparticles as the reaction precursor. The electron microscopy and BET results indicate that all the studied samples exhibit similar morphology and close specific surface area. XPS and EPR results confirm the existence of VO(oxygen defects). Photocatalytic H2 production is used to evaluate the photocatalytic properties of Ti O2 with different implanted types of VO. By optimizing the concentration of VO among different treated Ti O2 NRs, the materials exhibit significantly higher photocatalytic activities than that of the stoichiometric Ti O2 NRs. IPCE results indicate that the enhanced photocatalytic activity can be mainly contributed to the defect-assisted charge separation, which implies that photo-generated electrons or holes can be captured by VO and suppress the charge recombination process. The results show that the defective Ti O2 obtained by combining two approaches together can achieve the greatest photocatalytic activity enhancement among all the samples.