Low Temperature Synthesis And Photocatalytic Activity Of Nanostructured TiO₂

In 1972, Fujishima discovered the phenomenon of photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light. Since then, enormous efforts have been devoted to the research of TiO2 material, which has led to many promising applications in areas photovoltaics and photocatalysis. Among the unique properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement, and the transport properties related to phonons and photons are largely affected by the size of the materials. The high surface area brought about by small particle size is beneficial to many TiO2-based photocatalytic materials. Therefore, it is facilitate the photocatalytic activity of TiO2 and further improvement of current and practical TiO2 nanotechnology by preparation of homogeneous TiO2 nanoparticles and modification of TiO2, such as doping with nonmetal or metal atoms, coupling with other narrow band-gap semiconductor and surface dye-sensization.This dissertation developed the low temperature preparation (not exceeding 180℃) method for TiO2 and further synthesized doped TiO2 to obtain excellent photocatalytic TiO2 and other special propertity such as water-solubility. The dissertation reviewed the phase structure, photocatalytic mechanism, synthesis method, modified method and application of TiO2, and then systematically introduced low temperature methods to prepare porous crystalline TiO2, rutile TiO2 nanorod superstructures with excellent photocatalytic activity, C-N-S-tridoped TiO2 nanocrystalline photocatalysts, ultrafine TiO2 nanocrystals by Ni doping and watersolubleⅠ-doped TiO2 with excellent visible light photocatalytic activity. These resulting materials were characterized by XRD,XPS,SEM,TEM and photocatalytic activity test. The detailed works were shown as the following:1. Monodispersed hierarchical porous crystalline TiO2 spheres were produced through a one-step hydrothermal process from amorphous TiO2 spheres. This synthesis method can also be used to product porous SrTiO3 and BaTiO3 nanomaterials. Based on the characterization results, we proposed a formation process of these porous spheres according to a mechanism analogous to the Kirkendall Effect. That is, the porous was formed by outward transport of fast-moving Ti-OH2+or HTiO3-through the oxide layer and a balancing inward flow of vacancies to the vicinity of the TiO2 interface. This study provides a general way to synthesize porous titania-based spheres under low temperature. This study provides a general way to synthesize porous titania-based spheres. These resulting porous spheres may find applications in the fields of catalysis, ferroelectrics, photoelectrics and so forth.2. Rutile is the most stable phase of TiO2. It can often be obtained via high-temperature calcinations (over 700℃) of anatase nanopartieles.However,calcination unavoldably leads to agglomeration and growth of the nanocrystalline particles. In this work, rutile TiO2 nanorods were synthesized by hydrolysis of TiCl4 ethanolic solution in water at 50℃. The assembly of rutiie nanorods could be controlled through simply changing the molar ratios of TiCl4, ethanol and water, resulting in different superstructures with flower or urchin-like morphologies. A possible mechanism for the growth and assembly of rutile nanorods superstructures was proposed on the basis of characterization results. More importantly, we found that those low temperature synthesized superstructures showed significantly higher photocatalytic activities than commercial photocatalyst P25 on degradation of rhodamine B (RhB) in water under artificial solar light. This study provides a simple and inexpensive way to prepare high active rutile nanorods superstructures photocatalysts on a large scale.3. TiO2 can only absorb UV light with wavelengths less than 388 nm, which is about 4% of the solar spectrum. This means most of solar energy in form of visible light cannot be utilized. Doping with nonmetals such as C, N and S can extend the spectral response of TiO2 into the visible region and thus enhances its photocatalytic activity. Thiourea and urea have often been used as nonmetal ion sources because they can supply sulfur, nitrogen, and carbon. In this study, C-N-S-tridoped TiO2 nanocrystals were synthesized by using a facile hydrothermal method at 180℃in the presence of a biomolecule L-cysteine. This biomolecule could not only serve as the common source for the carbon, sulfur and nitrogen tridoping, but also could control the final crystal phases and morphology. XPS analysis revealed that S was incorporated into the lattice of TiO2 through substituting oxygen atoms, N might coexist in the forms of N-Ti-O and Ti-O-N in tridoped TiO2 and most C could form a mixed layer of carbonate species deposited on the surface of TiO2 nanoparticles. The photocatalytic activities of the samples were tested on the degradation of NO at typical indoor air level in a flow system under simulated solar light irradiation. The tridoped TiO2 samples showed much higher removal efficiency than commercial P25 and the undoped counterpart photocatalyst. This study provides a new method to prepare nonmetal doped TiO2 photocatalyst.4. In the traditional liquid method such as sol-gel and precipitation method, the fast rate of hydrolysis is unfavorable for the formation of homogeneous TiO2 parities, and the subsequent calcinations process may result in uncontrollable crystal growth. In this work, we developed a polyol-mediated synthetic method to prepare ultrafine anatase TiO2 nanocrystals of about 2-5 nanometers in size. The advantage of this thethod is that the alcohol itself acts as a stabilizer, which can limit the particle growth and prohibit agglomeration. Due to the high temperatures which can be applied (>150℃) for these highboiling alcohols, highly crystalline oxides are often formed.The physiochemical properties of these nanocrystals are tuned by doping with Ni. High resolution XPS analysis revealed that Ni incorporates into the TiO2 framework to form Ti-O-Ni chain. Nitrogen adsorption measurements further showed that Ni doping can greatly enhance the surface area of these anatase TiO2 nanocrystals from 143 to 266 m2/g. Using UV-vis diffuse reflectance spectroscopy analysis, we found that the Ni doping reduced the band gap from 3.08 to 2.73 eV and permitted these TiO2 nanocrystals to successfully absorb light in the visible region. First principles band structure calculations were carried out to study the electronic origin of the nickel induced optical absorption. The photocatalytic activities of the samples were tested through degradation of NO under typical indoor air flow and simulated solar light environment. Ni-doped TiO2 was found to exhibit much higher photocatalytic activity than its undoped counterpart and P25. The enhancement of photocatalytic activity of Ni-doped TiO2 is attributed to the larger surface area and the band gap narrowing tuned by nickel doping. This study provides a new low temperature method to prepare untral fine TiO2 nanoparticles.5. TiO2 has been used as photodynamic therapy (PDT) agents for cancer treatment recently. However, the insolubility of the TiO2 nanocrystals in water greatly limits their applications in PDT and water-soluble TiO2 nanocrystals are desired. Moreover, all those water-dispersed TiO2 have effective photocatalytic activity only under UV irradiation owing to the large bandgap (3.2 eV) of TiO2, and it is all known that UV radiation may cause direct cellular injury. Thus it is desirable to synthesis of water-dispersed TiO2 with visible light response for photodynamic therapy (PDT). In this study, water-soluble iodine doped TiO2 nanoparticles were synthesized by solvothermal method under 180℃. Interestingly, the obtained nanoparticles can dispersed well in water. The high water solubility mainly comes from the terminated-COOH on the surface of the TiO2 nanoparticals. The characterization results indicate the obtained I-doped TiO2 was about 2-5 nm. The doping form of I is I. The iodine atom may substitute the O atom in the TiO2 lattice to form O-Ti-I band. Doping with iodine can shift the absorption edge of TiO2 to the visible light range and narrow the band gap. We found that those water-soluble I-doped TiO2 showed significantly higher photocatalytic activities than commercial photocatalyst P25 on degradation of rhodamine B in water under visible light. Owing to the excellent water-solubility and visible light photocatalytic ability, the obtained water-soluble I-doped TiO2 have potential to be used in photodynamic therapy (PDT).