Theoretical Study Of The Structure And Thermodynamic Properties Of Cooper Sulfur Compounds

In recent years,copper sulfide materials（Cu_xS_y）have been widely used in many fields such as photocatalysis,solar cells,thermoelectric materials,and biomedicine because of their unique physicochemical properties.The wide variety of phase types and the complexity of phase change reactions in the Cu-S system leave many scientific questions unanswered.From the copper-rich phase Cu₂S to the sulfur-rich phase Cu S₂,copper sulfides exist in a range of stoichiometric compositions,with different crystal structures,sizes,morphologies and physical phases exhibiting different properties.Therefore,the controllable synthesis of Cu_xS_ymaterial phase and morphology is crucial for the application of materials,and it is also a huge experimental challenge.With the development of density functional theory and the rapid advancement of computer hardware performance,theoretical calculations have become a practical tool for studying material properties at the microscopic level.Based on the above study background,the structure and thermodynamic properties of copper-sulfur compounds are systematically investigated from the perspective of theoretical calculations.Cu_xS_y materials have been a challenging class of systems for theoretical calculations due to the strong correlation interactions between Cu 3d and S 3p electrons.Based on this,the structural properties,electronic properties,and thermodynamic formation energy of the copper-sulfur system were first systematically evaluated using different exchange-correlation generalized functions(LDA,PBEsol,PBE,PBE+U(U_eff=3,5,7,9e V),and the results showed that when PBE+U(U_eff=5 e V)was used,the structural properties,electronic properties,and thermodynamic formation energy of the copper-sulfur system could be reasonably predicted.properties of the copper-sulfur system when PBE+U(U_eff=5 e V)is used.For the description of thermodynamic properties,PBE+U(U_eff=7 e V)gives results that are close to the experimental values.The thermodynamic stability of different copper-sulfur phases was investigated based on the work on the evaluation of generalized functions,and the thermodynamic stability of the bulk phase of seven common copper-sulfur phases was first investigated.Subsequently,the surface energies of different phases at different pressures P,temperatures T and sulfurization potentials（μ_S）were calculated using ab initio atomic thermodynamics,and the microcrystalline morphologies of different copper-sulfur phases at different sulfur chemical potentials were further constructed based on Wulff’s principle.The results show that the surface energy of sulfur-rich surface gradually increases with the increase of sulfur chemical potential,while the surface energy of sulfur-poor surface gradually decreases with the increase of sulfur chemical potential,and for the surface with stoichiometric ratio,the magnitude of the surface energy is not affected by the ambient sulfur chemical potential and is always a certain value.Low temperature and low H₂S partial pressure favor the stabilization of sulfur-poor surfaces,while the stabilization trend of sulfur-rich surfaces is the opposite.Finally,surface energy and atomic Wulff theory were used to study the morphological changes of Cu_xS_ynanoparticles at different sizes and temperature and to plot the thermodynamic equilibrium phase diagrams between different particles in different phases.The thermodynamic equilibrium phase diagram shows that for both bulk phase and nanoparticles,the monolithic copper will first sulfide to Cu₂S and then to Cu₇S₄phase,and by adjusting the temperature and gas pressure,Cu₇S₄can further sulfide to form pure Cu S phase,and by continuing to increase the temperature and pressure,Cu S is sulfided to form Cu S₂-PNNM phase.This work provides important theoretical reference for the phase regulation and controllable synthesis of copper sulfur compounds in experiments.