Synthesis, Characterization, And Properties Of TiO₂-B Nanomaterials

Titanium dioxide（TiO₂） nanomaterials are important wide band gap semiconductors characterized of chemically inert, nontoxic, biocompatible, etc. TiO₂ show a wide range of interesting chemical-physical properties, including a high refractive index, good electric conductivity, excellent UV-absorption capability, and a remarkable（photo）-catalytic activity. Because of its particular band structure and photochemical stability, nanoscale TiO₂ has been demonstrated to be a suitable platform for many solar-energy conversion applications, such as dye-sensitized solar cells, water-splitting photoelectrolytic systems for hydrogen production, the demolition of pollutants, conversion of CO2 into added-valued chemicals, and small hydrocarbon fuels. TiO₂-B was first synthesized by Marchand et al. using solid chemistry methods in 1980. TiO₂-B is composed of edge- and corner-sharing Ti O6 octahedra with Perovskite-like windows between sites. The computational studies indicated the low Li+ migration energies for paths along the b-axis channel and the [001] direction, suggesting favorable Li-ion mobility. TiO₂-B has many promising applications in photovoltaics, photocatalysis, photo-electrochromics, and sensors. Recently, Ti-based materials（including TiO₂, Li4Ti5O12, etc.） have received increasing attentions as promising Li-ion battery anode material due to their low cost, excellent recharge ability, and improved safety over graphite. Among various polymorphs of TiO₂, TiO₂-B shows more open channels in the lattice and characteristic pseudocapacitive behavior, resulting in an easier Li-ion access to the crystal structure and faster charge-discharge capability. Moreover, TiO₂-B has a higher theoretical capacity（335 m Ahg-1） is comparable to that of graphite（372 m Ahg-1）, and about twice the values of commercialized Li4Ti5O12 spinel and anatase TiO₂（≈170 m Ahg-1）.As a most representative photocatalysts, TiO₂-B is also a green material which is expected to resolve the problem of energy shortage and environmental pollution. Great efforts have been devoted to optimize the synthesis conditions of TiO₂-B nanomaterials. Oxygen vacancies within the nanomaterials or composite nanomaterials with other functional semiconductors results in heterostructures, which could effectively improve the performance of materials. In addition to retain the inherent physical and chemical properties of single components, novel performances may be given to the heterostructures. The synergy（ "1+1>2"） could be achieved since various properties could be integrated into one due to the unique heterostructural characteristics. In this paper, we studied the TiO₂-B in detail to solve the problems as mentioned and the main contents of this thesis are given as follows:1. Monodisperse TiO₂-B nanoparticles with an average size of 5 nm were synthesized by solvothermal method. By adjusting the reaction parameters, ultrathin TiO₂-B nanosheets were prepared with high specific surface area. Ultrathin TiO₂-B nanosheets improve the interfacial kinetics and intercalation properties of lithium-ion batteries due to their synergetic superiorities with a pseudocapacitive mechanism and adequate electrode-electrolyte contact. Ultrathin TiO₂-B nanosheets exhibit a discharge capacity of 220 m Ahg-1 after 50 cycles at 0.5 C in a voltage range of 1.0-3.0 V at room temperature. And 90.8 m Ahg-1 after 5 cycles at 10 C in a voltage range of 1.0-3.0 V at room temperature.2. We developed a facile method to prepare black TiO₂-B nanoparticles with an average size of 5.0 nm by UV irradiation of monodispersed TiO₂-B nanoparticles. After heating the light-induced electron doping(Ti³⁺) TiO₂-B nanoparticles in argon, Air-stable black TiO₂-B nanoparticles were obtained by heating 340 °C and further transformed to black anatase at 360 °C. HRTEM images show that both black TiO₂-B and black anatase TiO₂ have TiO₂/TiO₂-x core-shell heterostructures, where the core TiO₂ is crystalline and the shell TiO₂-x is disordered. TiO₂-B absorbs below 400 nm, while black TiO₂ absorbs wavelengths over the entire spectrum. Electron paramagnetic resonance results show a characteristic single electron signal, which indicates oxygen vacancies in black TiO₂. The black TiO₂ nanoparticles were efficient visible light photocatalysts for degradation of MO. The black samples exhibit a photocatalytic degradation efficiency of 43.5% for MO in 3 h. The black TiO₂ show room temperature superparamagnetic behavior which has potential applications in spintronics fields and catalysis fields.3. Mechanism of forming Black TiO₂-B nanoparticles was proposed. Photogenerated electrons and holes produced in TiO₂ under UV irradiation. The photogenerated electrons can be trapped by Ti4+ cations and reduce it to Ti³⁺ state. The holes can be trapped on the negatively charged surface of the TiO₂ nanoparticles. The temporally charge imbalance in the nanoparticles will induce the migration of O2- to the surface and leave an oxygen vacancy. It is presumable that the holes on the surface attract hydrogen from the methanol, which is a subsequently oxidized to methanal, resulting in the formation of Ti-OH group. After annealing the sample in Ar atmosphere, surface oxygen（mainly Ti-OH groups） released via a dehydration process. This mechanism was supported by the XPS O 1s results. The contribution of surface oxygen decreased significantly in the black TiO₂ comparing to that of the as-prepared monodispersed TiO₂-B.4. Porous TiO₂-B/SnO₂ nanomaterials are prepared. The porous TiO₂-B spheres composed of nanowires are obtained by hydrothermal method. The SnO₂ nanoparticles are uniformly coated on the porous TiO₂-B, and the porous TiO₂-B/SnO₂ core-shell nanocpmposites were prepared. As the anode of lithium ion batteries, core-shell SnO₂/TiO₂-B exhibit high lithium storage capacity and superior rate capability. Such a good lithium storage performance can be attributed to the synergistic effect between SnO₂ and TiO₂-B. The Sn/Ti ratios directly affect the electrochemical properties of the porous TiO₂-B/SnO₂ nanocomposites. We explored that how the amounts of SnO₂ influences the performance of the porous TiO₂-B/SnO₂ heterostructures as anode of lithium battery.