First-principles Calculation Of Transition Metal Chalcogenides And Electron-phonon Interaction

With the development of human society,traditional energy sources are facing severe exhaustion crisis.The development of clean energy,especially the hydrogen energy,can achieve energy transformation and realize sustainable development.At present,photo(electro)catalytic water splitting is a potentially high-efficient hydrogen production method.However,for reliable industrialization,it is crucial to develop a low-cost high-performance catalyst.The first-principles calculation fully considers the hydrogen production mechanism and expands the application of the catalysts.As a result,it is able to make accurate and reliable predictions of the efficiency of catalysts and consequently reduces the experimental cost.The first-principles calculation of transition metal chalcogenides accurately predicted the high catalytic performance of the material,leading to the focus of attention on this material in the catalytic chemistry research.The calculation of energy band and hydrogen adsorption Gibbs free energy using density functional theory can predict the hydrogen production performance of materials.If the modulation of the hydrogen production band gap of the material can be realized so that the hydrogen adsorption Gibbs free energy can be tuned to close to 0 e V,the development of the high-performance catalytic material will be achieved.On the other hand,the electron-phonon interaction of the material dramatically influences the carrier transport of the material,and thus affects the separation of empty electron-hole pairs inside the material.In this thesis,electron-phonon interactions in MoS₂,TaSe₂and their composites were studied with first-principles calculations.The density functional theory was applied to reveal the layer thickness dependence of the MoS₂band structure and the effects of substitution doping on the band regulation.Further,the Gibbs free energy of catalytically active sites for the hydrogen adsorption of the amorphous MoS₂and Ni₃S₂composite was calculated,indicating a significant increase of the hydrogen-production active cites.Then,the influence of electron doping on the electron-phonon interaction of2H-TaSe₂and 2H-TaS₂was studied by first-principles calculations and Raman spectroscopy under ion liquid control.In the first part of this thesis,we studied the catalytic performance of the 2H-MoS₂and amorphous MoS₂-Ni₃S₂composites.The MoS₂band structure with various layer thicknesses was calculated from the first principles.The band gap of the single layer MoS₂is a direct band gap with a width of 1.65 e V.While the bulk MoS₂has an indirect band gap of 1.21 e V.The band gap of the single layer of MoS₂can be regulated by doping.The doping of Tc and Re will increase the band gap with an increase of the Fermi level at the same time.Inspired by the edge active sites of single-layer MoS₂,we constructed a composite of amorphous molybdenum sulfide and Ni₃S₂with multiple active sites to enhance the electrocatalytic hydrogen production.The Gibbs free energy of hydrogen adsorption was calculated based on density functional theory.The results clearly indicated that significant more hydrogen-production active sites were obtained.In the second part,we studied the layer thickness dependent Raman spectroscopy of thin layer 2H-TaSe₂and 2H-TaS₂and the effects of different electron doping concentrations on the electron-acoustic phonon interaction of the materials.Revealed by the Raman spectroscopy and the phonon spectrum calculation under ionic liquid control,the out-of-plane vibration modes of 2H-TaSe₂and 2H-TaS₂are affected by the electron doping concentration and the applied electric field.The in-plane vibration mode of 2H-TaSe₂is modulated only by the applied electric field,with a blue shift of 7 cm^-1.It is worth noting that there are LO-TO splits when an electric field is applied.As the electron doping concentration increases,the splitting increases gradually.And the intrinsic carrier density of the material also changes with the phonon vibration mode.In summary,by the study of the band structure and its dependence on the substitution doping of the single layer MoS₂,this thesis demonstrated that the material band gap can be modulated by doping.Further,high efficient catalytic performance of the amorphous MoS₂and Ni₃S₂composite was predicted by the Gibbs free energy of hydrogen adsorption.Finally,the electric field modulation of the electron-phonon interaction in 2H-TaSe₂and 2H-TaS₂indicates that the phonon vibration modes of 2D materials can be influenced by the applied field and electronic doping levels,providing new prospects for photo(electro)catalysis and transport properties.