First-Principles Studies On The Electronic Structures And Electrocatalytic H₂ Evolution Activity Of MoS₂ Based Surface/Interface Systems

With the rapid development of society,depletion of petroleum supplies and increasing environmental issues become increasingly severe,which lead to wide explorations of new global-scale sustainable energies.Hydrogen,as a clean and renewable energy resource,has been regarded as one of the most promising alternative energy for diminishing fossil fuels.Electrochemical water splitting is one green and sustainable way to generate molecular hydrogen(H2).To accelerate the efficiency of water-splitting,employing an appropriate electrocatalyst is critical.Currently,platinum(Pt)-based materials have been deemed as the best-performing catalyst for water splitting.However,the limited resource and high cost hinder its widespread application.Therefore,it is urgent to seek cost-effective and earth-abundant catalysts.MoS2 has been regarded as one of the most promising alternatives of Pt to be the next-generation catalyst for hydrogen evolution reaction(HER).Currently,researches on MoS2 for HER catalysis mainly focus on maximizing the exposed active sites,improving per-site activity,activating the inert basal plane via doping,introducing defects and inducing phase transition,in order to enhance the intrinsic conductivity and basal plane activity.Despite these studies,however,the overall activity is still order of magnitude smaller than that of Pt.Up to now,there are several grand challenges,which include low electron injection efficiency due to the Schottky barrier existing at the interface of MoS2 and metal current collector,poor per-site activity of edge sites,limited strategies for activating the basal plane,and weak stability of the material after phase transition.In this thesis,by using density functional theory(DFT)based first-principles calculations,we aim to understand the underlying physical origin and propose efficient solutions to these issues,and expect to shed new light on future design of MoS2-based HER electrode with high activity and robust stability.The main contents of this thesis are as follows:? Using Au(111)as a metal current collector,we studied the effect of electric field on the interfacial electron transport barrier.Before exposed to an external electric field,the Schottky barrier at the interface of MoS2/Au(111)is 0.63 eV,which can be fine-tuned when an external electric field was applied.With positive electric field,the barrier increases with the increase of the magnitude of field.On the contrary,an increasing negative electric field leads to lower barrier height.We further explored the physical origin and found that external electric field may cause a strikingly charge density recontribution around the interface,which may change the potential step across the interface and consequently tune the interfacial barrier height.Moreover,we found that the H adsorption on the basal plane of MoS2 can also be tuned by the electric field.? We studied the effect of metal substrate on the HER activity of MoS2 edge sites,the correlation between the electronic structure of edges and the HER activity,and the effect of electric field on the electronic structures as well as HER activity of edge sites.Results show that the underlying substrate can affect the structure reconstruction and H bonding strength at edges,and consequently has a nonnegligible effect on the HER activity of edge sites.Furthermore,we revealed an excellent descriptor for H adsorption,which highlights the critical role of S p-resonance states near the Fermi level in determining ?Eb.The closer the S p-resonance is to Fermi level,the stronger the H-S bonding.We proposed the use of an electric field to fine tune the position of S p-resonance states,and consequently the H bonding strength.Especially for S edges,an optical HER activity can be obtained at a mediate magnitude of electric field.Explorations on S vacancies in the basal plane of MoS2 show an insensitive feature to electric field due to the presence of Mo d states associated with the structure.?We investigated correlation between the electronic structure of MoS2 with basal pane S vacancy and the catalytic activity towards HER.Furthermore,we integrated MoS2 with various two-dimensional nanostructures and studied the interfacial effect on the electronic structure as well HER catalytic activity of the basal plane.Results show that the characteristics of the S vacancy-induced defect states below normal hydrogen electrode potential(VNNE),especially the position and density of the lowest unoccupied and highest occupied states,determine the HER activity.Furthermore,by assembling MoS2 with 16 common 2D structures into 2D van der Waals heterostructures,the characteristics of the Vs-induced defect states as well as the charge distribution around Vs can be fine-tuned by interlayer interaction,which lowers the S vacancy density(Vs%)that the basal plane needs to obtain an optimal HER activity.Remarkably for MoS2/MXene-OH heterostructure,an optimal free energy can be achieved for a low Vs%of?2.50%,much lower than 9%that in freestanding case,which may enhance the utilization of MoS2 basal plane of significantly.?We studied the role of the metal substrate in influencing the stability of the 1T'-MoS2 and the HER activity of 1T'-MoS2 supported on metal surfaces.Phase transition leads to a change in the S configuration of Mo atom,which affects the splitting of Mo d-orbitals and make Mo d-orbitals in 1T'-MoS2 be partially occupied,weakening the structure stability.By adsorbing 1T'-MoS2 on 15 common metal surfaces,it is found that the electrons transferring from metal to MoS2 overlayer reoccupied these partially occupied d orbitals,improving the stability of 1T'-MoS2.Remarkably on the Mo(001),W(001)and Hf(0001)surfaces,the 1T' phase becomes energetically comparable or even favorable to the 2H phase.Furthermore,kinetic studies on the phase transition between 2H and 1T' phase show that the metals surfaces can also increase the transition barrier of 1T'?2Hy preventing the reverse phase transition of 1T'?2H.Finally,we studied HER activity of 1T'-MoS2 adsorbed on the 3 well-performing metal surfaces,Mo(001),W(001)and Hf(0001),and found that the HER activity of 1T'-MoS2 adsorbed on Mo(001)and Hf(0001)were weakened by serious structure reconstruction,while the active sites of 1T'-MoS2 adsorbed on W(001)were doubled.