Theoretical Study On Edge Reconstruction And Edge Properties Of 1T-phase Transition Metal Dichalcogenides

Edge reconfiguration arises from the generation of dangling bonds and symmetry breaking at material edges.In order to reduce the energy of the system,the edge atoms are usually rearranged,called edge reconfiguration.Edge reconfiguration affects the stability,electronic structure as well as physical and chemical properties of the materials.As the size of a material decreases,the edge becomes an important factor in determining the properties of the material due to the increase in the occupancy ratio.Therefore,edge reconstruction and related properties of two-dimensional（2D）materials have been a core scientific issue in condensed matter physics research.Studies on some typical 2D materials such as graphene,h-BN,black phosphorene,etc.show that edge reconstruction is of great theoretical significance for understanding the emergence of some new structures and physical properties.Transition metal dichalcogenides（TMDCs）exhibit various and adjustable properties,especially at their edges,and thus have attracted tremendous interest as a class of functional materials.The confinement effect of the quasi-1D edge of 2D TMDCs can give the materials new properties and functions,for example,the Mo S₂monolayers are direct band gap semiconductors,while the Mo S₂ nanoribbons with zigzag edges exhibit metallic properties.In addition,the edges of Mo S₂have high catalytic activity,while the basal surfaces are chemically inert.It is well known that monolayer TMDCs mainly have two basic crystalline phases,the 1H phase and the 1T phase.At present,the edge reconstruction and the related properties of 1H-TMDCs,represented by Mo S₂,have been extensively studied.However,studies related to the edge of its sister material 1T-TMDCs are scarce.Despite reports of possible edge configurations and properties of 1T-TMDCs in recent years,the highly stable reconstructed edges,especially the mechanism of reconstruction and the edge properties are still unknown to a large extent.It greatly restricts the practical applications of 1T-TMDCs.Accurate knowledge of the edge configurations,formation mechanisms,and related properties of 1T-TMDCs is crucial for the design of novel functional materials.This dissertation carries out a systematic study on the edge reconstruction and edge properties of monolayer 1T-TMDCs.1T-phase Hf S₂,VS₂,and Ti Te₂ are selected for the study.Using the first-principle calculations combined with the edge structure search methods based on the particle swarm optimization algorithm,the bonding characteristics and formation mechanism of the edge reconstruction of 1T-TMDCs were investigated,the universal laws of edge reconstruction were revealed,and the new physical and chemical properties induced by edge reconstruction were explored.The following innovative research results were obtained:（1）A study of a global search for reconstructed edges of 1T-TMDCs.Synthesizable and versatile edges have important application value in condensed matter physics and materials science.The edge reconstructions of 1T-TMDCs system are complicated to resolve.Additionally,the microscopic mechanism of edge reconstruction is unclear.Given this,the reconstructed edges are investigated by a systematic global search study using monolayer 1T-Hf S₂ as a typical representative of1T-TMDCs.The first edge structure database of 1T-TMDCs was constructed,containing S-terminal zigzag edges,Hf-terminal zigzag edges,and armchair edges,with a total of 33 stable edges and 66 metastable edges.It has been shown that the local chemical environment of sulfur and hafnium at the edge is a major determinant of the edge atomic configuration.Moreover,edge reconstruction can induce the transitions from indirect to direct band gap of Hf S₂,which can be maintained over a wide range of strain（-9%～+10%）.The studies reveal that the d-orbital occupation of transition metal is an intrinsic factor that determines the pattern and stability of edge reconstruction.This work opens up a new paradigm for the design of edge structures of 1T-TMDCs and provides an important reference for their“edge engineering”studies.（2）A study of dimer-like metal Klein edges and related properties.Self-passivated Klein edges are classical edges in many 2D materials,which are extremely geometrically robust and frequently captured experimentally.Many previous studies reported the presence of Klein edges in 1H-TMDCs and strongly influenced the fundamental properties of nanoribbon edges.However,whether the Klein edges also exist in 1T-TMDCs remains a mystery at present.In this study,the dimer-like metal Klein（named DM-Klein）edge is firstly predicted based on monolayer 1T-VS₂.DM-Klein edge is self-passivated from a zigzag edge with 25%sulfur saturation and has high thermodynamic stability under metal-rich chemical potential conditions.The V-V dimerization of the DM-Klein edge is mainly caused by the strong coupling between the 3(9(92_-2-3(9(92_-2 orbitals.It is also found that DM-Klein edges are prevalent in group IV and group V 1T-TMDCs.The metal-metal bonding strength at the DM-Klein edges is 2～5 times that of the metal-metal bonding strength at the 2D basal plane.The DM-Klein edges of several 1T-TMDCs are theoretically predicted to have excellent hydrogen evolution reaction catalytic activity(|ΔG_H*|<0.15 e V)similar to that of commercial noble metal Pt（111）surfaces.Strong intermetallic d-d orbital coupling and appropriate edge sulfur saturation（25%）are prerequisites for the high stability and high-efficient hydrogen evolution reaction activity of the DM-Klein edges.This work offers insights into understanding the edge self-passivation behavior of 1T-TMDCs and its application in electrocatalytic hydrogen evolution reaction.（3）A study of trimer-like metal zigzag edges and related properties.Slight bond contraction induced by“under-coordinated”atoms is the conventional edge self-reconstruction mode for 2D materials but usually fails to drive the edge to its ground state.At present,unconventional and highly stable edge reconstruction patterns have been found in a variety of typical 2D materials,but have not been reported in the 1T-TMDCs system.In this study,an unconventional（2×1）trimer-like metal zigzag（named TMZ）edge is predicted based on monolayer 1T-Ti Te₂.The TMZ edge is formed from the as-cut metal zigzag edge by drastic bond rotation and positional exchanges of metal/sulfur atoms.The strong coupling of 3d orbitals at the center of the Ti₃ trimers favors the stability of the TMZ edge.The TMZ edge is universal in groups IV,V,and X 1T-TMDCs and has an energetic advantage far beyond conventional edge of bond contraction.The unique tri-metallic synergistic effect enables the eight TMZ edges to exhibit high-efficient hydrogen evolution reaction activity comparable to that of commercial precious metal Pt-based catalysts.With the excellent characteristics of facile synthesis,high stability,and high catalytic activity,TMZ edge is expected to be a promising electrocatalyst for hydrogen evolution reaction.The TMZ edge proposed in this study provides a cost-effective strategy for maximizing the catalytic efficiency of hydrogen evolution reaction of 1T-TMDCs.