| As is known to all, current energy shortages and environmental issues have become the worldwide focus in recent years. Especially, the photocatlytic reduction of CO2 into hydrocarbon is a potential way to convert waste materials to useful ones. Among many candidates for photocatalysts, Ti O2 is almost the only material suitable for industrial use at present and in the future. However, controlling the higher combination efficiency of the photo-generated electron-holes is still the most important challenge for researchers. Designing TiO2 catalyst with heterostructure is one of the most efficient ways. The {101} and {001} facets of anatase TiO2 exhibit different band structures and band edge positions based on density functional theory(DFT) calculations. An optimal percentage of exposed {001} and {101}facets in anatase TiO2 can form surface heterojunctions, which are favourable for the separation of photo-generated carriers and enhanced photocatalytic activity. And then more and more electrons will participate in the deep photoreduction of CO2 to CH4.Titanic nanotubes/belts with 1D structure were firstly used as titanium source to control the morphology and structure of TiO2 in isopropyl alcohol/tertiary butyl alcohol solvent at the existence of HF. Anatase TiO2 nanocrystals with large specific surface areas and different proportion of {101} and {001} facets exposed were prepared successfully. It can be concluded that the photocatalytic performance is closely related to the morphology, facets, grain size and specific surface of the photocatalyst. Meanwhile, modified with Pt and GO will make prominent contributions to increasing productive rate of CH4. The samples are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscope, ultraviolet-visible diffuse reflectance spectrum, N2 adsorption-desorption isotherms, fluorescence and Raman spectroscopy. We further clarified the mechanism of the activity of photocatalyst. Based on the above understanding, the main researches of the thesis are listed as below:(1) Ultrafine anatase TiO2 nanocrystals with controllable coexposed {101} and {001} facets were prepared by a facile solvothermol strategy. 1D titanic acid nanotube transformed to applanate truncated octahedral bipyramid TiO2 nanocrystals, and the percentage of {001} facets can be tuned from 5 to 51% with 0-1.5mL HF. PL and EIS spectra results proved that the surface heterojunction between {001} and {101} facets is favourable for the separation of photo-generated charges. The sample with 51% of {001} facets and 49% {101} facets exhibited the highest photocatalytic activity in terms of photoreduction of CO2, and the photoreduction rate of CO2 to CH4 reached 1.58 μmol h-1g-1. Meanwhile, the mean diameter of nanoparticles is about 10-18 nm, and the smaller size reduced the recombination probability of photocarriers in the bulk. After loaded Pt nanoparticles, the photoactivity increased remarkably. Futhermore, the efficiency could remain nearly 80% after four cycle reactions, indicating the catalyst has a long-term stability.(2) Through modulating different solvents, reaction system, 1D structure of titanic acid nanobelts and different volume of HF in tertiary butyl solvothermol strategy, a series of TiO2 nanocrystals with different ratio of{101} and {001} were synthesized successfully. PL and EIS spectra results proved that the surface heterojunction between {001} and {101} facets is favourable for the separation of photo-generated charges. Among of them, sample with 41% of {001} facets and 59% {101} facets using of 0.2mL HF exhibited the highest photocatalytic activity, and the photoreduction rate of CO2 to CH4 was as high as 3μmol h-1g-1. What’s more, the photoactivity increased remarkably to 11.3μmol h-1g-1 after loaded Pt. Pt nanoparticles on the surface of the TiO2 surface serve as an electron trap for improving the separation efficiency of the photo-generated electron-hole pairs.(3) Graphene, with a two-dimensional sp2 carbon network, is considered as an ideal support for catalysts owning to its unique electronic and photoelectronic properties. In order to improve the efficiency of CO2 reduction, highly efficient GO and TiO2 with coexposed {101}/{001} facets via a facile solvo-thermalmethod and its surface was decorated by using reduced graphene oxide(rGO) sheets. The morphology and chemical composition of the prepared GO/TiO2 {001} nanocomposites were examined by using suitable characterization techniques. It was found that they exhibit relatively higher CO2 photo-reduction efficiency with selective formation of methane than that of TiO2 with coexposed {101}/{001} facets. Because rGO has a strong adsorption ability of CO2 and initiates the reaction by binding a CO2 molecule, forming a magnesium carbonate species on the surface, which is helpful in the photoreduction of CO2 to CH4. It is particularly worthy mentioning that r GO possess excellent conductivity, which can work as electron trap and thus suppress the recombination of electron–hole pairs. Therefore,rGO in the nanocomposite played an important role in CO2 photocatalytic reduction. Especially, the 1%GO/TiO2 {101}/{001} exhibited the maximum CH4 yield(15.2 μmol h-1g-1) under 200℃ for 24 h.(4) Besides the same-component heterojunction with different crystal facets, other different-components heterojunction were also investigated, including visible-light-responded AgBr/Ag2 O and Fe3O4/AgBr heterostructure photocatalysts, and they were displayed in the appendix part. Both AgBr/Ag2 O and Fe3O4/AgBr composites were prepared successfully by the precipitation method, and they showed superior photocatalytic activity for MO degradation. This section enriches and develops the heterojunction theory, which perhaps has certain reference to some peers. |
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