Two-step Synthesis Of Laminar Vanadate Via A Facile Hydrothermal Method And Enhancing The Photocatalytic Reduction Of CO₂ Into Solar Fuel Through Tuning The Oxide Vacancies By Vacuum Illumination Treatment

With the depletion of fossil fuels,the global warming caused by the increasing of carbon dioxide concentration in atmospheric has caused extensive research and attention.One of the best ways to solve this problem is to convert carbon dioxide into renewable fuels through photocatalytic reduction,which can both reduce the carbon dioxide concentration and ease the energy crisis.Many researches focused on exploring efficient photocatalytic reduction of CO₂ systems has been conducted.Due to unique properties and potential applications in catalysis and optoelectronic devices,nanomaterials have attracted people's extensive research interest.The crystal structure and microscopic morphology of nanomaterials have a crucial influence on their physical and chemical properties.Therefore,the controllable synthesis of nanomaterials with specific crystal phase and morphology is the focus of research in preparative chemistry and materials science.Oxygen vacancy is one of the most important crystal defect and is supposed to be the prevalent defect in many metal oxides.been revealed that oxygen vacancies can behave as important adsorption and active sites for heterogeneous catalysis which are able to strongly influence the reactivity of metal oxides.Moreover,it has been shown that the photocatalytic properties including the electronic structure,charge transport,and surface properties of semiconductors are closely related to oxygen vacancies.For these reasons,there is a great interest in the development of controllable synthesis of semiconductors with oxygen vacancies and exploiting their unique properties for photocatalytic applications.We did the following two aspects of work:?1?Monoclinic LaVO₄,tetragonal CeVO₄ and monoclinic Pb₂V₂O₇ with laminar structure composed of nanosheets have been successfully synthesized by a two-step facile hydrothermal method using NH₄V₄O₁₀ nanobelts as intermediate products without adding any additional morphology control agent.Due to its unique structure and dispersibility,the NH₄V₄O₁₀ nanobelt can spatially limit the growth of vanadate during the synthesis process,thus forming the unique laminar morphology.The photocatalytic properties of CO₂ reduction for these vanadates has also been studied for the first time.In the liquid-phase CO₂ reduction reaction,these vanadates can transform the CO₂ into ethanol selectively.In the gas phase CO₂ reduction reaction,LaVO₄ mainly generates CO.Compared with the samples obtained by the traditional one-step hydrothermal method,the photocatalytic reduction efficiency of both the liquid phase and gas phase of LaVO₄ synthesized by the two-step method has been significantly improved.?2?The photocatalytic activity of prepared LaVO₄ can be further enhanced by a vacuum illumination treatment using the gas phase photocatalytic reduction system.After the formation of oxygen vacancies,the yield of CO in the gas-phase C0₂ reduction reaction is increased by nearly one-fold.The formation of oxygen vacancies was confirmed by the characterization of UV-vis,XPS,and EPR.Photoluminescence spectroscopy confirmed that oxygen vacancies could inhibit the recombination of photogenerated electron hole pairs and prolong the lifetime of photogenerated electrons.The in-situ IR results further indicate that oxygen vacancies can convert the physical adsorption of CO₂ on the sample surface into chemical adsorption and reduce the activation energy of the CO2 molecule in the reduction reaction.These two properties enhances the performance of photocatalytic CO₂ reduction for LaVO₄.