Controllable Preparation Of Two-Dimensional Metal Sulfide/Oxide And Properties Of CO₂ Photoreduction

Environmental and energy issues,which need to be urgently solved,have become the worldwide problems.The excessive emission of greenhouse gases carbon dioxide?CO₂?seriously hinders the sustainable development of human beings.Therefore,conversion of the CO₂ into available carbon forms has particularly caught the world's attention.Many researchers have focused on electroreduction of CO₂ into hydrocarbon fuels,such as methane?CH₄?,ethylene?C₂H₄?,methanol?CH₃OH?and formic acid?HCOOH?,which could help to simultaneously solve the energy issues and environmental problems.Compared with electroreduction of CO₂,solar-driven CO₂ reduction into hydrocarbon fuel has been generally considered to be an appealing solution because solar is clean and inexhaustible.Unfortunately,these photocatalysts usually suffer from very low CO₂ photoconversion efficiency,far below the requirements of practical implementation,mainly due to the fast recombination rate of photoexcited electron-hole pairs during the photocatalytic process.Therefore,based on two-dimensional metal sulfide/oxide materials,this dissertation establishes an ideal model to gain in-depth understanding on the property of CO₂ photoreduction and disclose the intrinsic mechanism of the photocatalysis process by virtue of in situ characterization techniques and density functional theory calculation.The main research contents of this paper are as follows:1.Defective one-unit-cell ZnIn₂S₄ layers boosting solar-driven CO₂ reduction:we initially construct ideal models of one-unit-cell ZnIn₂S₄ atomic layers with tunable defect concentrations.Aberration-corrected scanning transmission electron microscopy directly verifies the distinct zinc vacancy concentrations confined in one-unit-cell ZnIn₂S₄ atomic layers,further attested by positron annihilation spectrometry and electron spin resonance analysis.Density functional calculations unveil that the existence of zinc vacancies ensures efficient carrier transport and higher charge density.Ultrafast transient absorption spectroscopy,surface photovoltage spectroscopy and photoluminescence spectroscopy manifest the higher zinc vacancy concentration is favor of accelerating carrier separation rates.Therefore,one-unit-cell Zn₂S₄ layers with abundant zinc vacancies exhibit a carbon monoxide formation rate of 33.2 ?mol g^-1 h^-1,approximately 3.6 times higher than that of the one-unit-cell ZnIn₂S₄ layers with poor zinc vacancies.2.Partially oxidized SnS₂ atomic layers achieving efficient CO₂ photoreduction:we initially built SnS₂ atomic layers and then deliberately create domains with different oxidation degrees on their surfaces.High resolution transmission electron microscopes,Raman spectroscopy and X-ray photoelectron spectra demonstrate the presence of Sn oxide domains confined in the SnS₂ atomic layers.Surface photovoltage spectroscopy and photoluminescence spectroscopy certify that Sn oxide domains are beneficial to charge-carrier separation kinetics.In situ Fourier transform infrared spectroscopy spectra unveiled COOH*radical is the main intermediate;and meanwhile,density functional theory calculations disclose the COOH*formation is the rate-limiting step.In addition,the locally oxidized domains result in electron localization on Sn atoms near the O atoms,thus lowering the activation energy barrier through stabilizing the COOH*intermediates.As a consequence,the mildly oxidized SnS₂ atomic layers exhibit the carbon monoxide?CO?formation rate of ca.12.28?mol g^-1 h^-1,approximately 2.6 and 2.3 times higher than those of the SnS₂ atomic layers and the poorly oxidized SnS₂ atomic layers.3.Defective Nb₂O₅ atomic layers regulating product selectivity of solar-driven CO₂ reduction:we initially construct metal oxide atomic layers and then introduce oxygen defects on their surfaces.As an example,defective one-unit-cell Nb₂O₅ atomic layers are successfully synthesized.X-ray photoelectron spectra reveal Nb⁵⁺ is transformed into Nb⁴⁺in the Nb₂O₅ atomic layers,which is indeed induced by oxygen defects.Photoluminescence spectra indicate that the existance of oxygen defects favores the separation of photogenerated electron-hole pairs.In situ Fourier transform infrared spectra disclose the COOH*and CH₃O*radical are the main intermediates;while density functional theory calculations reveal the introduction of oxygen defects enable the formation of CH₃O*to be a spontaneous process,which effectively regulates the reaction pathway,thus facilitating the generation of methane.As a result,the Nb⁴⁺doped Nb₂O₅ atomic layers exhibit the selectivity of methane ca.81.7%,markedly higher than that of the Nb₂O₅ atomic layers?43.4%?.