Preparation And Solar-Energy-Fuel Conversion Performance Of Oxygen Vacancies-rich TiO₂ Based Photocatalysts

Excessive consumption of fossil fuels not only aggravates the energy crisis,but also leads excessive levels of harmful gas and greenhouse gas（such as CO₂）in the atmosphere.Using photocatalytic technology to convert H₂O and CO₂ into solar fuels（such as H₂ and CH₄）can effectively solve the above energy and environmental problems.Titanium dioxide（TiO₂）is considered as one of the most promising photocatalysts due to its advantages of low cost,stable properties and easy preparation.However,bare TiO₂ is difficult to be applied on a large scale,which is mainly due to its weak ability to absorb sunlight and the poor ability to separate photogenerated electrons-holes pairs.Recent studies have shown that the introduction of oxygen vacancies in the TiO₂ lattice can effectively expand the range of light absorption and inhibit the recombination of photogenerated carriers.However,TiO₂ containing oxygen vacancies still faces disadvantages such as insufficient exposed active sites,poor selectivity of photocatalytic reduction of CO₂,and limited efficiency of photogenerated charge separation.In order to overcome the above bottleneck,the oxygen vacancies-rich TiO₂ was modified in this paper:1)the novel holey TiO₂ nanosheets with abundant oxygen vacancies,2)the TiO₂ nanosheets simultaneously equipped with AuCu alloy and oxygen vacancies,3)the TiO₂ nanocrystalline simultaneously embedded with abundant oxygen vacancies and Ru species were prepared respectively.The performance of TiO₂ rich in oxygen vacancies for conversion of solar fuel was further improved by increasing oxygen vacancies concentration,constructing synergistic catalytic sites,and promoting the separation efficiency of photogenerated carriers.According to the characterization and analysis of the micro-morphology,crystal structure,charge separation capacity and photocatalytic activity of the prepared photocatalytic materials,the internal mechanism of the enhancement of photocatalytic performance was discussed.The specific research contents are as follows:1.Holey defected TiO₂ nanosheets with abundant oxygen vacancies（HD-TiO₂NSs）was successfully prepared by hydrolysis growth and calcination method using graphene oxide（GO）as sacrificial template.TEM,SEM,XRD and other characterization indicate that the tetrabutyl titanate（TBOT）precursor hydrolyzed slowly on GO surface and formed porous sheets composed of interlinked TiO₂nanoparticles.After removal of GO template by calcination,HD-TiO₂ NSs with abundant oxygen vacancies were obtained by hydrogen reduction.XPS,EPR,Raman,etc.,proved that compared with bulk TiO₂,HD-TiO₂ NSs with a large number of mesoporous and edge sites were more conducive to maximizing oxygen vacancies concentration in hydrogen reduction process.The optimal HD-TiO₂-5h NSs photocatalyst has high hydrogen evolution performance(8.99 mmol·g^-1·h^-1)and cycling stability,which is 2.83 times and 26.8 times of holey TiO₂ nanosheets（H-TiO₂NSs）and defective bulk TiO₂（D-TiO₂）,respectively.This is mainly due to the fact that the porous structure is conducive to the diffusion and mass transfer of the reactants and the oxygen vacancies on the surface can effectively promote the separation of photogenerated electrons and holes.2.AuCu alloy/TiO₂ nanosheets with oxygen vacancies composite photocatalyst was successfully prepared by hydrothermal-liquid phase reduction-hydrogen reduction method.TEM,XRD,ICP,XPS,EPR,Raman and other characterization indicate that AuCu alloy and oxygen vacancies were successfully introduced on the surface of TiO₂nanosheets.The results of photocatalytic reduction of CO₂ showed that AuCu-TiO_2-xNSs exhibite the highest CH₄ yield(22.47μmol·g^-1·h^-1)and selectivity（90.55%）.PL,FL,EIS and photocurrent analysis show that the oxygen vacancies in AuCu-TiO_2-x NSs promoted the separation of carriers,and more importantly,promoted the oxidation of H₂O,thus providing abundant protons to promote the deep reduction of CO₂ to CH₄.AuCu alloy can not only further promote the separation of photogenerated electrons holes pairs,but also serve as the selective reduction site of CO₂ to CH₄.The CO intermediate may be generated on the surface of Au,and then overflow on the surface of Cu for further hydrogenation until the formation of CH₄,thus significantly improving the yield and selectivity of photocatalytic conversion of CO₂ to CH₄.3.TiO₂ nanocrystals with abundant oxygen vacancies(Ru-TiO_2-x)were successfully synthesized through a facile Ru-doping method for the photocatalytic conversion of CO₂ to CH₄.The concentration of oxygen vacancies could be precisely controlled by regulating the doping amount of Ru species.TEM,XRD,XPS,EPR and other characterization were used to characterize the morphology,crystal structure and surface oxygen vacancies of the prepared photocatalytic materials in detail.The synergistic mechanism of Ru and oxygen vacancies to improve the performance of photocatalytic conversion of CO₂ to CH₄ was investigated by PL,FL,photocurrent and EIS.The1 mol%Ru-doped TiO₂(1%Ru-TiO_2-x)showed the highest CH₄ yield(31.63μmol·g^-1·h^-1)and selectivity（90.93%）,which was significantly higher than that of TiO₂samples containing only Ru or oxygen vacancies.This is mainly due to the doped Ru in 1%Ru-TiO_2-x lattice can generate intrinsic strain,thus inducing the formation of oxygen vacancies.The oxygen vacancies not only effectively inhibit the recombination of photogenerated carriers,but also promote the adsorption and activation of CO₂molecules,which makes the intermediate easily converted to CH₄ through protons coupling.More importantly,the synergistic effect of Ru with strong electrons trapping ability and oxygen vacancies can further promote the separation of photogenerated charge.This could provide the necessary electrons and protons for the deep conversion of CO₂ into CH₄.