Synthesis And Photocatalytic Performance Of ZnO Nanofiber-based Photocatalytic Materials

Photocatalytic reduction of CO₂ into valuable hydrocarbon fuel via solar energy is a promising strategy for carbon utilization.ZnO has high exciton binding energy,rapid generation of electron(e _CB-)-hole(h _VB+)pairs and strong electron-migration ability,so it is recognized as an ideal photocatalytic material.However,ZnO also has disadvantages of fast recombination of photogenerated charge carriers,poor CO₂adsorption capacity and poor visible light absorption.Therefore,developing efficient ZnO-based photocatalysts and optimizing their preparation or surface modification techniques has become important issues toward ZnO as photocatalyst.In this dissertation,ZnO-based hybrid nanofibers were prepared by loading appropriate cocatalysts on ZnO electrospun nanofibers and constructing step-scheme（S-scheme）heterojunction to achieve effective separation of photogenerated charges and significant enhancement of photocatalytic activity.The reasons for the improvement of photocatalytic performance and the photocatalytic mechanism of ZnO hybrid nanofibers were clarified.The main research contents are shown as follows:1.The Ni-Ni S/C/ZnO heterojunctions for efficient photocatalytic CO₂reduction.Ni S is a two-dimensional layered nanomaterial that shows good performance in super capacitors,sensors and photoelectrocatalytic water splitting.However,there are few reports on using Ni S as a cocatalyst for CO₂ photoreduction.A kind of Ni-Ni S/C/ZnO photocatalyst was synthesized by electrospinning method as well as photodeposition method,and Ni-Ni S nanosheets were aligned and uniformly deposited on the surface of C/ZnO nanofibers.XPS analysis confirmed the existence of metallic Ni and Ni S.Raman and photoluminescence showed that there was chemical interaction after recombination and cocatalyst Ni-Ni S promoted the separation of photogenerated electrons and holes of ZnO.The prepared Ni-Ni S/C/ZnO photocatalyst exhibits enhanced light absorption and CO₂ adsorption,which makes the photocatalytic CO₂ reduction activity significantly enhanced.The yield of CO(5.86 umol g^-1 h^-1)and CH₄(1.14 umol g^-1 h^-1)are greatly improved compared with ZnO,at the same time,the photogenerated holes were successfully consumed by H₂O.The ¹³C isotope tracer experiments confirmed that the reduction products originate from CO₂ instead of other environmental pollutants.This work provides an effective method for designing and synthesizing ZnO-based photocatalysts to achieve efficient photocatalytic CO₂ reduction.2.ZnO/Zn Mn₂O₄ S-scheme heterogeneous synthesis and photocatalytic CO₂reduction performance.Zn Mn₂O₄ is a spinel material,which has a large specific surface area,good electrical conductivity and enough reactive sites.Exploring efficient spinel material to boost photocatalytic CO₂ reduction is important for solar-to-fuel conversion.Herein,ZnO/Zn Mn₂O₄ hybrid photocatalysts were synthesized by high voltage electrostatic spinning and calcination.Density functional theory（DFT）calculations showed that the e _CB-of Zn Mn₂O₄ will spontaneously transfer to ZnO due to the latter had a lower Fermi level（E_F）when compound.An internal electric field pointing from Zn Mn₂O₄ to ZnO was formed at the interfaces.The internal electric field would accelerate the recombination of the e _CB-（ZnO conduction band）as well as h _VB+（Zn Mn₂O₄ valence band）and leave the e _CB-on Zn Mn₂O₄ conduction band as well as the h _VB+on ZnO valence band,which successfully suppressed the recombination of photogenerated charges in ZnO and Zn Mn₂O₄.ZnO/Zn Mn₂O₄ possessed enhanced photocatalytic CO₂ reduction performance and the yield of CO and CH₄ were increased by 3 times and 2 times compared with ZnO nanofibers,respectively.At the same time,H₂O was oxidized to O₂ by h _VB+.The enhancement of activity can be attributed to the enhanced light absorption and CO₂ adsorption,the weaker interface charge transport resistance and the construction of S-scheme heterojunction.This work shows that the new photocatalyst Zn Mn₂O₄ can effectively improve the utilization of light and realize the spatial separation of photogenerated charges,which provides a new idea for the design of ZnO-based S-scheme heterojunction photocatalytic materials.