Preparation And Photoelectric Performance Of ZnO Modified Materials Based On Density Functional Theory

Over exploitation of fossil fuels leads to a large amount of carbon dioxide（CO₂）emissions,causing serious environmental problems such as raising sea levels and greenhouse effect.Energy shortage and environmental problems are two major problems that need to be solved in the current human society.In addition,CO₂ is also an ideal raw material for organic synthesis.If this rich C1 resource can be turned into valuable chemicals such as methane,methanol,dimethyl ether and low-carbon hydrocarbons,it can not only reduce the content of CO₂ in the atmosphere,but also obtain new energy,which has important research value.Photoelectrocatalysis technology has the advantages of both photocatalysis and electrocatalysis.The hydrogen for reducing CO₂ comes from water and the energy is from sunlight.It is a clean and excellent technology for CO₂ reduction.ZnO has become the key material in the field of CO₂ conversion due to its superior electrical and optical properties.Nevertheless,ZnO wide bandgap（3.37 e V）and inability to absorb in the visible region hinder its applications to a certain extent.Therefore,it is necessary to modify ZnO for its practical use as a sustainable photoelectrocatalyst.At present,the main methods to adjust the bandgap of semiconductor oxides are element doping and surface vacancy.Based on the above ideas,we use（Bi,S）co-doping ZnO and introduce oxygen vacancy to adjust the bandgap of ZnO,which can improve its photoelectric properties and broaden its application in the field of CO₂conversion.The main contents of this paper are as following:1.The enhanced photoelectric performance of（Bi,S）co-doping ZnO is explored by combining theoretical calculation with experiment measurements.The crystal structures,formation energies,electronic properties and relative effective masses of photogenerated electrons and holes of pure and（Bi,S）co-doped ZnO are investigated by density functional theory（DFT）calculations.Results indicate that（Bi,S）co-doping can generate impurity states and shift the valence band into the higher energy region,causing the decrease in bandgap.Relative effective masses show that the separation rates of photogenerated carriers are enhanced after modification.Moreover,experimental measurements are carried out on the basis of the above theoretical analysis.Pure and（Bi,S）co-doped ZnO samples with different doping ratios are synthesized and characterized.The results confirm that the presence of Bi and S has major effects on oxygen defects in the crystal structure of ZnO.XPS,Raman,PL and UV-vis studies evidence the roles of oxygen defects on bandgap narrowing for（Bi,S）co-doped ZnO samples.As expected,2%-（Bi,S）co-doping ratio has the highest photocurrent(23.58μA cm^-2,nearly 4.7 times higher than pure ZnO)and the smallest electrochemical impedance,which effectively improve the photoelectric performance of ZnO materials.This work will provide some new insights into the modification of other wide bandgap semiconductor materials.2.Oxygen vacancies are efficient to promote the separation of photogenerated electrons and holes,thereby enhancing the photoelectrocatalytic performance of the catalyst.In this work,we synthesize porous ZnO nanosheets featuring abundant oxygen vacancies successfully.SEM reveals that the resultant ZnO sheets afford an abundant surface porosity,thereby contributing to the rich oxygen vacancies.XPS,Raman,PL and ESR spectra show that there are a lot of oxygen vacancies in the rich oxygen vacancies ZnO nanosheets.The increasing of oxygen vacancies results in a narrowing bandgap and increases the visible light absorption of the ZnO.In addition,the first principle calculations further prove that the high concentration of oxygen vacancy creates an impurity level near the valence band and induces the band gap narrowing.It is consistent with the experimental results.The narrowed bandgap further contributes to extended light absorption range and the increased valence band width leads to efficient charge transfer and separation,hence improves the visible light photoreactivity of ZnO.As a result,the rich oxygen vacancies ZnO nanosheets exhibit a superior photoelectric properties,showing the maximum transient photocurrent(78.9μA cm^-2)and minimum electrochemical impedance.This work both experimentally and theoretically indicates that the abundant oxygen vacancies confined in ZnO nanosheets lead to the optimized electronic structure and,consequently,the remarkable photoelectric activity.It provides a new method on how to design high-performance photoelectrocatalytic materials.3.The adsorption behavior and reduction mechanism of CO₂ on ZnO（0001）surface are studied by first principle calculations.The results show that CO₂ tends to be adsorbed at the top position of O atom parallel to ZnO（0001）surface.The calculated adsorption energy is-0.24 e V,meanwhile,the bond angle decreases from 180°to 178.27°,and the bond length decreases by 0.002 nm.It shows that the adsorption of CO₂ on ZnO（0001）surface is very weak,which belongs to physical adsorption.In the process of competing with hydrogen evolution reaction（HER）,the Gibbs free energy change（ΔG）of CO₂ reduction reaction（CRR）is less than HER,which indicates that the ZnO surface is more conducive to CO₂ reduction reaction.The final product of CO₂ reduction is CO,the intermediate transition state is*COOH instead of*OCHO,and the energy barrier of 1.22 e V needs to be overcome.At last,the overpotential required in the process of CO₂ reduction to CO is estimated to be 1.116 V.This work establishes a theoretical foundation for further study of CO₂ reduction mechanism.