Fabrication Composites By Coupling Mesoporous Titanium Dioxide And Its Derivatives With Copper-containing Species And Synergistic Mechanism In CO₂ Photoreduction

Photocatalytic CO₂reduction into clean energy is of great significance to the environmental problems and energy crisis caused by the excessive consumption of fossil fuel.It is necessary to design and synthesize photocatalytic materials with efficient light harvesting,abundant active sites and high separation efficiency of photogenerated carriers.Titanium dioxide（Ti O₂）and its derivatives(Ba Ti O₃,Sr Ti O₃,and K₂Ti₈O₁₇)have attracted much attention due to their suitable band structure,nontoxicity,stability,and easy preparation.However,pristine materials usually suffer from the drawbacks of insufficient active sites and easy recombination of photogenerated carriers,which limits their application in the field of photocatalytic CO₂reduction.In fact,the fabrication of composite materials can effectively improve the separation efficiency of photogenerated carriers and increase the reactive sites,thereby improving the CO₂conversion efficiency.Copper-based materials（Cu,Cu₂O,and Cu₇S₄）have been widely used to fabricate heterojunctions or cocatalyst systems to enhance the activity of CO₂photoreduction due to their abundant sources and suitable band structure.Based on the above background,in this work,mesoporous Ti O₂and its derivatives(Ba Ti O₃,Sr Ti O₃and K₂Ti₈O₁₇)will be used as photocatalysts to supports with Cu-based materials（Cu,Cu₂O and Cu₇S₄）for fabrication of composite materials.The CO₂photoreduction performance as well as the synergic mechanism will be studied.The main research contents are as follows:1.Mesoporous rod-like Ti O₂is firstly prepared from titanium glycolate precursor by a reflux method,and then Cu₂O nanoparticles are deposited on its surface by a simple chemical reduction method,and finally the Ti O₂/Cu₂O composites have been successfully prepared.The powder X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and ultraviolet-visible diffuse reflectance spectroscopy（UV-vis）are employed to determine the composition and structure.The photocatalytic CO₂reduction test shows that the introduction of Cu₂O particles can significantly enhance the activity of CO₂photoreduction to CH₄,and the yield reaches 1.35μmol g^-1h^-1,which is 3.1 times that of pristine Ti O₂material.Based on the band structure analyzed from Tauc plots and XPS valence band,a p-n heterojunction could be formed between Ti O₂and Cu₂O.The photo/electrochemical test show that the built-in electric field formed between Ti O₂and Cu₂O can promote the effective separation of photogenerated charges,which results in more photogenerated electrons participating in the photocatalytic CO₂reduction process,and accordingly enhances the performance of photocatalytic CO₂reduction.2.The titanium-based perovskite oxides（Ba Ti O₃and Sr Ti O₃）are synthesized from mesoporous Ti O₂by a hydrothermal method,and then simultaneously implanted the rich oxygen vacancies and metallic copper to the materials by ethylene glycol/isopropanol-mediated solvothermal method.The structure and composition have been characterized by XRD,XPS,SEM,TEM,UV-vis and electron paramagnetic resonance（EPR）.The results suggest that the introduction of metallic copper and rich oxygen vacancies can enhance the visible light absorption range of Ti O₂,Ba Ti O₃and Sr Ti O₃.The photocatalytic CO₂reduction test show that the CH₄and CO yields enhance upon the oxygen vacancies-enriched Ti O₂,Ba Ti O₃and Sr Ti O₃samples after solvothermal treatment.Take the Ti O₂as an example,the introduction of different contents of Cu can greatly enhance the CO₂photoreduction to CH₄and CO products.Among them,the Ti O₂/Cu-0.1 sample has the best activity,and its CH₄yield is as high as 14.51μmol g^-1h^-1,which is 28 times that of pristine Ti O₂material.The CO yield increased to 8.97μmol g^-1h^-1.Under the same conditions,the Ba Ti O₃/Cu-0.1 and Sr Ti O₃/Cu-0.1 composites also exhibit significantly enhanced CH₄and CO yields compare to the pristine material before and after solvothermal.Through Tauc plots and XPS valence band analysis,the valence band position of a pristine sample after solvothermal shifts to a more negative position,which correspond to the reduction potentials of CO₂/CH₄and CO₂/CO.It is beneficial to the adsorption of CO₂molecules for the increase of oxygen vacancies caused by solvothermal treatment and the introduction of metallic Cu based on the results of CO₂temperature programmed desorption（CO₂-TPD）test.And photo/electrochemical tests show that the introduction of oxygen vacancies and metallic Cu cocatalysts can accelerate the separation and transfer of photogenerated charges.Based on the above results,the band structure of photocatalyst can be regulated by surface oxygen vacancies.The Cu cocatalyst and the surface oxygen vacancy can promote CO₂adsorption and photogenerated charge separation,which contributes to the enhanced photocatalytic CO₂conversion of Ti O₂-Cu,Ba Ti O₃-Cu and Sr Ti O₃-Cu composites.3.The potassium titanate(K₂Ti₈O₁₇)nanowires are firstly synthesized from the Ti O₂by the hydrothermal method,and then K₂Ti₈O₁₇-Cu₇S₄composites have been prepared by ion exchange method from the Cd S.The XRD,SEM and XPS are used to analyze the composition and structure of the as-prepared samples.UV-vis analysis shows that the composite material has an enhanced light absorption range compared to the pristine K₂Ti₈O₁₇material.The photocatalytic CO₂reduction test shows that the introduction of Cu₇S₄can enhance the photocatalytic activity of K₂Ti₈O₁₇,in which the CH₄yield of the K₂Ti₈O₁₇-Cu₇S₄-0.2 composite is as high as 6.02μmol g^-1h^-1,which is 3.8 times that of pristine K₂Ti₈O₁₇nanowire,and the yield of CO is 0.89μmol g^-1h^-1.Through Tauc plots and XPS valence band analysis,combined with photo/electrochemical tests,it is found that Cu₇S₄in the composites mainly play the role of cocatalyst.The introduction of Cu₇S₄promotes photogenerated charge separation,contributing to the enhanced photocatalytic CO₂reduction performance.