Preparation And Photocatalytic Performance Of Nitrogen-Doped (001) Facets Dominated Anatase Titania Hierarchical Spheres

Studies on materials for photodegradation of organic dyes and photocatalytic hydrogen production under visible-light have attracted much attention due to the deterioration of environment and the exhaustion of resources. In this thesis, (001) facets dominated N-doped anatase TiO2 nanosheets hierarchical sphere catalysts with various morphologies have been synthezed via a facile nonfluorinated one-pot solvothermal route. The relationship between the morphology, structure, optical absorption properties of the catalysts have been systematically characterized by combinational characterization methods. The photocatalytic degradation of organic dyes MB/RhB and splitting water for hydrogen production under visible light are studied to reveal the inherent relationship between the synthesis, physicochemical properties, morphology of the catalysts and photocatalytic properties. The main conclusions are as follows:(1) Four novel kinds of varied morphological N-doped anatase TiO2 nanosheets hierarchical mesoporous spheres (xNT-(001)-t, x=N/Ti molar ratio, t is the reaction time in h) with dominated (001) facets were first prepared by a one-pot solvothermal route with varied N-doped amount and crystallization time using TTIP, diethylenetriamine and IPA, urea:N-doped nanosheets hierarchical solid spheres xNT-(001)-24 (x=0.12,0.3,1.2), yolk-shell hollow pompons (0.12NT-(001)-36), porous solid pompons (0.12NT-(001)-48) and half-moon hollow spheres(ONT-(001)-24) without N-doping. The series hierarchical spheres possess an average size in 600-1500 nm, and high specific surface areas in 102～460 m2/g and optimal pore diameter in 2.8-3.8 nm. The hierarchical nano-microspheres were self-assembled by nanosheets with the size of 250-750 nm and thickness of 7～12 nm composed of the grains of 11～13 nm. The (001) facets exposures reach 89%-99%. The yolk-shell hollow pompons (0.12NT-(001)-36) possesses the highest surface area and (001) facets exposure. The formation mechanism of TiO2 nanosheets hierarchical spheres was tentatively proposed.(2) We systematically study the effects of N-doping amount and solvothermal reaction time on the properties of photocatalytic degradation of organic dyes MB and RhB. The yolk-shell hollow pompons 0.12NT-(001)-36 exhibits the highest photocatalytic activity with 100% degradation rate of MB and RhB at 10 min and 30 min under visible light, respectively. XPS and UV-vis DRS results show that nitrogen atom has been inserted into TiO2 lattice interstitial site in presence of O-Ti-N or Ti-N-0 linkages, and the localized states introduced by mixing the N 2p and the O 2p states above the valence band edge result in the narrowing band gap of anatase TiO2 thus greatly enhanced visible light absorption. Raman data indicate that (001) facets dominated anatase TiO2 nanosheets hierarchical spheres attain increased the oxygen defects with N-doping which is beneficial for improving the photocatalytic activity under visible light. The photodegradation mechanism of catalyst was tentatively proposed.(3) The visible photocatalytic water splitting for hydrogen evolution has been performed over platinum-loaded hierarchical spheres catalysts via a photoreduction deposition method. The yolk-shell hollow mesoporous pompons 0.12NT-001-36 with 0.5 wt% Pt shows the highest H2 evolution efficiency (45269 mol·g-1·h-1) and QE(71%) in 20%(v/v) methanol-water system. The reason may include the greatly enhanced synergetic effect between the Pt nanoclusters acting as electron trapping center and yolk-shell hollow mesoporous pompons support with the highest surface area and (001) surface exposure rate, thus reducing overpotential of H+/H and inhibiting the recombination of electrons and holes. The photocatalytic H2 production mechanism of the Pt loaded hierarchical spheres catalyst was tentatively proposed.