Modification And Photocatalytic Performance Of TiO₂ Based Photocatalysts

With the accelerating global industrialization,energy shortage and environmental pollution have become severe issues,which seriously restrict the sustainable development of human beings.Photocatalytic hydrogen production via water splitting and CO₂ reduction into solar fuels can not only convert abundant solar energy into available hydrogen energy and hydrocarbon chemical fuels,but also degrade organic pollutants and decrease the concentration of greenhouse gases such as carbon dioxide,which have broad application prospects in solving energy and environmental problems.TiO₂ is one of the most widely and deeply studied photocatalyst due to its low cost,nontoxicity and high chemical stability.However,the practical application of TiO2 photocatalyst is restricted by two key scientific problems:Firstly,TiO2possesses a wide band gap?3.2 eV?,which can only be excited by ultraviolet light,leading to a narrow light response range;Secondly,the recombination rate of photogenerated charge carriers is much high,which results in a low quantum efficiency and poor photocatalytic activity.Several strategies,such as cocatalyst depositon,direct Z-scheme heterojunction construction and morphological regulation,are proposed to improve the charge separation and migration rate and promote the photocatalytic performance.Additionally,the photocatalytic mechanism of modified composite photocatalysts is explored.The main research contents are as follows:1.Surface heterojunction can be formed between the{001}and{101}facets of TiO₂ due to the slight difference in energy band structures of two facets,which enables photogenerated electrons to migrate from{001}facet to{101}facet and photogenerated holes to{001}facet from{101}facet.The photogenerated electrons and holes are finally accumulated on the{101}and{001}facets,respectively.Based on this feature,photoreduction and photooxidation methods were adopted to reduce or oxidize PtCl₆^2-and Co²⁺precursors into Pt nanoparticles?NPs?and Co₃O₄ NPs,respectively,leading to the selective deposition of Pt and Co3O4 co-catalysts on{101}and{001}facets of TiO₂,respectively.In this ternary TiO₂-Co₃O₄-Pt composite photocatalyst,surface heterojunction was beneficial for separating photogenerated electrons and holes into{101}and{001}crystal facets,respectively,while Pt and Co₃O₄ served as reduction and oxidation cocatalysts,captured electrons and holes and provided reactive sites for redox reactions.Under the synergistic effect of surface heterojunction and selective deposition of cocatalysts,the recombination rate of photogenerated charge carriers of ternary composite photocatalyst was remarkably reduced,leading to the significant enhancement of photocatalytic hydrogen evolution rate.2.Taking advantage of the spatial separated photogenerated electrons and holes by surface heterojunction,the p-type MnO_x nanoflakes and metallic Pt NPs were selectively loaded on the{001}and{101}facets of TiO₂.Herein,three types of heterojunctions were formed:surface heterojunction between{001}and{101}facets of TiO₂,p-n junction between MnO_x and TiO₂{001}facets,and Scottky junction between Pt and TiO₂{101}facets.The formation of multi-heterojunction structure promoted the spatial separation and transfer of photogenerated electrons and holes to the greatest extent.During the process of photocatalytic CO₂ reduction,photogenerated electrons transferred from{001}to{101}facets of TiO₂,and further migrated and accumulated on Pt NPs to participate in multi-electron reduction reactions,which greatly improved the photocatalytic CO₂ reduction activity of the ternary TiO₂-MnO_x-Pt photocatalyst.3.Ni?OH?₂ nanosheets were vertically grown on the electrospinning TiO₂nanofibers using a simple chemical bath deposition method,forming TiO₂/Ni?OH?₂composite fibers.The hierarchical structure can promote the absorption of incident light,accelerate the diffusion of reactant and product molecules,and increase the number of active sites.Photocatalytic CO₂ reduction test results showed that the bare TiO₂ nanofiber can only produce CH₄ and CO.The addition of Ni?OH?₂ can not only improve the conversion efficiency of CO2 into CH4,but also change the photocatalytic reaction path to selectively generate alcohol products?CH₃OH and C2H5OH?.The enhanced photocatalytic performance and selective generation of alcohol products can be attributed to the following reasons:Firstly,Ni?OH?₂ as an alkaline cocatalyst can improve the adsorption ability of acidic CO₂ molecules;Secondly,the photogenerated electrons were likely to transfer from TiO₂ to Ni?OH?₂and form Ni clusters due to the lower reduction potential of Ni²⁺/Ni than the conduction band bottom of TiO₂,thus restraining the charge recombination in bulk TiO₂.4.The direct Z-scheme TiO₂/CdS hierarchical composite photocatalyst was prepared by the successive ion layer adsorption and reaction?SILAR?method.The hierarchical structure is advantageous to the multiple reflections and scattering of incident light,which promotes the light absorption.In the preparation process,the negatively charged TiO₂ firstly adsorbed Cd²⁺ions,then the negatively charged S^2-ions adsorbed and reacted with Cd²⁺ions to form CdS NPs.Finally,CdS NPs were in-situ loaded on the flower-like TiO2 spheres.Due to the effect of electrostatic adsorption,close-contact interface was formed between TiO2 and CdS,which was favorable for the separation and migration of photogenerated charge carriers.The photoluminescence?PL?analysis of hydroxyl radicals showed that TiO2/CdS composite photocatalyst conformed to the direct Z-scheme photocatalytic mechanism:the photogenerated electrons on the conduction band of TiO2 were combined with the holes on the valence band of CdS,which inhibited the charge recombination in the bulk TiO2 and CdS and maintained the strong reduction capacity of electrons on the conduction band of CdS.The photocatalytic H2 generation activity of the TiO2/CdS composite photocatalyst was significantly improved relative to the bare photocatalyst.