The Study Of Photocatalytic Reduction Of CO₂ Over Ultrathin Porous TiO₂ Nanosheets

The massive combustion of fossil fuels has aggravated the greenhouse effect.Photocatalytic reduction of CO₂ takes advantage of solar energy as an energy source to reduce CO₂ to hydrocarbon fuel for resource recycling.The realization of this technology can alleviate the greenhouse effect effectively as well as help solving the energy crisis.In this paper,the physicochemical properties,photocatalytic reduction CO₂ characteristics and mechanism of two ultrathin TiO₂ nanosheets with different crystal compositions were systematically analyzed and studied,aiming at promoting the design of new catalysts and the industrial application realize of photocatalytic reduction CO₂ technology.Two kinds of ultrathin TiO₂ nanosheets with different crystal forms were prepared by solvothermal and wet chemical stripping method respectively.Post-calcination in a tube furnace at 350°C with air/oxygen atmosphere for 2h/4h was proceeded with the intension of removing the organic residues in the sample while maintaining the flake morphology,the relationship between the photocatalytic properties of the samples and the physicochemical properties before and after calcination was analyzed and revealed systematically.The results show that the post-calcination treatment can effectively inhibit the recombination of photogenerated electron-hole pairs and extended its photoresponse range.Composed of TiO₂-B and anatase crystal forms,TiO₂?D?-2 nanosheet with layered mesoporous structure,and the TiO₂-B{010}facet exposing properties are more efficiency than TiO₂?T?-2,which consists of three crystal forms of TiO₂-B,anatase and rutile.In order to reveal the transformation mechanism of TiO₂ crystal face and morphology with the calcination process and improve its photocatalytic activity,two kinds of original ultra-thin TiO₂ nanosheets were calcined in a muffle furnace at 350°C with air atmosphere for 10h.The relationship between the photocatalytic activity and crystal form and morphology change was analyzed.The results indicate that part of TiO₂-B was transformed to anatase during calcination,the light response range is broadened further,and the specific surface area is reduced.TiO₂?D?-10 is mesoporous and TiO₂?T?-10 is porous structure.The heterojunction between the different crystal forms and faces can separate the photo-generated charges effectively,thus the photocatalytic activity per unit area of TiO₂-10 is twice than that of the TiO₂-2 nanosheets.For the purpose of investigating the mechanism difference of photocatalytic reduction of CO₂ over different TiO₂ with morphology,crystal form and surface exposure properties distinction,the in-situ Fourier transform infrared spectroscopy was adopted to monitor the intermediate products of photocatalytic reduction of CO₂ over ultra-thin porous TiO₂nanosheets,reveal the reaction mechanism and speculate on possible reaction pathways.The results elucidate that numerous crystal defects of TiO₂?T?-based nanosheets enhanced their ability of CO₂ adsorption.TiO₂-10 has the strongest CO₂ adsorption capacity and the most active intermediates due to their porosity structure and the change of crystal form.The proportion of TiO₂-B in TiO₂?T?-10 is extremely low,and no HCOO^-on its surface was detected,which reveals different photocatalytic pathways with TiO₂?D?-10.