| In recent years,researchers are focusing on improving performance of transition metal chalcogenides(TMDCs)in functional devices by means of defect modulation and phase engineering.Owing to its high thermoelectric power factor,high conductivity,rich active sites,high energy density and excellent rate performance,TiS2 has considerable potential application in thermoelectric,catalytic and energy storage devices.However,the origin of the high conductivity of TiS2 is under debate.Besides,the poor structure stability limits its device applications.Study on the modulation of electrical properties and electrocatalytic hydrogen evolution of TiS2 single crystal by defects and structure,and understanding the underlying mechanism,is of great significance for the design of functional TMDCs devices.In this paper,different TiS2 single crystals were synthesized by controlling the experimental conditions of chemical vapor transport method.The crystal structure,electronic structure,elemental composition and chemical environment of TiS2 single crystals were characterized by X-ray diffraction,energy dispersive X-ray spectroscopy,X-ray photoelectron spectroscopy,electron paramagnetic resonance spectroscopy and Raman spectroscopy.Also,the types of defects and the surface structural phase transformation of TiS2 single crystals were identified.The effects of defects on the electrical properties and electrocatalytic hydrogen evolution performance,the defectsdependent surface structural phase transformation and the effects of surface structural phase transformation on the electrocatalytic hydrogen evolution performance of TiS2 single crystals,were investigated by transport measurement and the electrocatalytic hydrogen evolution measurement.The results are as follows:1.The effects of defects on the electrical properties and electrocatalytic hydrogen evolution performance were investigated by introducing S vacancy defects(Sv),Ti interstitial defects(Tii),Ti-O defects,Frenkel defects(TiF)and TiS structural phase defects into TiS2 single crystals.Donor defects Sv and Tii increase the electron concentrations of.TiS2 upon defects ionization mechanism.In particular,the carrier concentration of TiS2-TiS increases by an order of magnitude,which is probably caused by charge transfer from TiS to TiS2.Sv and Tii,especially Tii,can act as effective active sites to reduce the overpotential of hydrogen evolution reaction.However,when the content of Tii reaches a certain level,Tii defects no more exist in single atom form,but form a stable TiS structural phase,which in turn reduces the active sites and has adversely influences on the overpotential of hydrogen evolution reaction.But its high carrier concentration promotes the charge transfer process.2.H2S molecules formed upon the reaction between surface atoms of TiS2 single crystal and adsorbed H2O molecules,result in the removal of surface S atoms and undercoordinated Ti atoms.Surface structural phase transformation from TiS2 to TiS originates from subsequent displacement of under-coordinated Ti atoms to the interlayer octahedral sites.Various types of defects show different effects on phase transformation.Structures with Ti-OH groups at Sv sites and Tii-OH groups at Tii sites are much stable,preventing the bond breaking of Ti(i)O-H and subsequent formation of H2S.As a consequence,higher energies are required for the surface structural phase transformation from TiS2 to TiS.The abundance of Ti-O groups facilitates the formation of H2S and induces the surface structural phase transformation from TiS2 to TiS at room temperature.Besides,the insufficient adsorption of H2O molecules leads to uneven distribution of surface TiS phase.TiS phase formed upon structural phase transformation results in a significant increase of overpotentials of hydrogen evolution reaction.Upon long-time test,the surface exfoliation of TiS2 single crystals with surface structural phase transformation leads to a loss of TiS content,which makes samples show the electrocatalytic hydrogen evolution behavior of the bulk TiS2 gradually.On the one hand,the research results provide a reference for defect and structure engineering of electrical properties and electrocatalytic hydrogen evolution performance of TMDCs materials.On the other hand,study on the surface structural phase transformation paves the way for the design of TMDCs heterostructures with different components and Janus TMDCs materials. |
