Strain-induced Phase Transition Of Transition Metal Dichalcogenides And Its Electrocatalytic Hydrogen Evolution Performance

Transition metal dichalcogenides?TMDs?,as a kind of novel materials with unique physical and chemical properties,have great potential applications in catalysis,electronics,optoelectronics,batteries,etc.Intriguingly,there are two typical phases,i.e.thermodynamically stable trigonal prismatic?2H?phase and metastable octahedral?IT?phase.In contrast to semiconducting 2H phase with bandgaps of 1-2 eV,the 1T phase has better conductivity and electrochemical activity.However,in the conventional method of preparation of TMDs,thermodynamics favours the presence of the semiconductor phase which is more stable but has lower catalytic activity.At present,the preparation of 1T TMDs still relies on the post-growth treatments from 2H to 1T transformation.However,although the phase transformation of 2H phase can be achieved by means of alkali metal intercalation,electron beam treatment,surface doping,strain controlling,etc.The operation process is cumbersome,and the purity of 1T phase prepared is limited and mainly metastable,which will rapidly change back to 2H phase after thermal annealing.Therefore,it is of great significance to directly prepare IT TMDs with high purity and high stability in both the application field and the theoretical exploration field.Strain engineering has been proved to be a powerful strategy for modulating the electronic and photonic properties of the TMDs.Excitingly,theoretical studies have shown that strain can induce the generation of 1T phase by changing the Fermi energy level and electronic state density of 2D TMDs.Compared with the chemical treatment,phase transition controlled by strain will not damage the 2D sheet,which can maintain the performance of the materials to a large extent.Although strain tuning of 2D TMDs has shown promise for phase engineering,its application has been limited in both practical and fundamental standpoints.The existing approaches for strain tuning of 2D TMDs often rely on overlaying a 2D TMDs on a flexible substrate.By superimposing TMDs on the flexible substrate and squeezing the substrate,the strain is applied to TMDs.However,the substrates are often insulating polymers,that may limit their performances.In this work,we choose one dimensional conductive carbon nanotubes?CNTs?as a substrate.Through Co doping,it forms a curved coating around the one-dimensional carbon nanotubes.The strain generated by the in-situ growth further induces the formation of the highly active IT phase.At the same time,we also studied the formation mechanism of strain and realized the regulation of 1T phase content.The prepared 1T Co-TMDs/CNTs exhibited excellent electrocatalytic hydrogen evolution performance.The content of thesis mainly contains the following aspects:?1?Firstly,WSe₂ was used as the representative,and multi-walled carbon nanotubes?MWNTs?with an outer diameter of about 10 nm were selected to induce its coating growth after cobalt atom doping.The microstructure and composition of Co-WSe₂/MWNTs heterojunction were studied by SEM,element mapping,TEM,XRD,UV,Raman and XPS.The results showed that the morphology of the nanosheet changed and the size decreased obviously through the cobalt atom doping of 2D WSe₂,and a "scale-like" curved package was formed on the surface of the multi-wall carbon nanotubes.This bending growth not only increases the number of active sites,but also induces the formation of 1T and improves the electrical conductivity of the materials.The Co-WSe₂/MWNTs showed excellent catalytic performance and stability with a overpotential of 174 mV(current density of-10 mA cm^-2)and a Tafel slope of 37 mV dec^-1.Meanwhile,it also showed remarkable catalytic activity under alkaline conditions.?2?In order to further explore the generation mechanism of strain-induced phase transition and reasonably regulate the content of 1T phase of WSe₂,we regulated the size of carbon nanotubes and MWNTs were replaced by single-walled carbon nanotubes?SWNTs,2 nm?with greater curvature and carbon nanofibers?CNFs,150 nm?with less curvature.Meanwhile,the effects of mechanical strain and interfacial chemical synergism caused by CNTs and Co doping on WSe₂ were studied.Subsequently,the structure and composition of the materials were analyzed by SEM,TEM,XRD,PL,UV,Raman,XPS.The results show that the cobalt plays a significant role in the in-situ coating.With the help of Co-C strong interaction,the 2D nanosheet is firmly coated on the surface of three carbon substrates.Compared with the traditional methods of preparing metallic phases,the Co-WSe₂/CNTs prepared by strain showedgood thermodynamic stability.It was calculated that among the heterogeneous structures induced by the three CNTs,the Co-WSe₂/SWNTs received the maximum strain?56.5%?and the IT content reached 82.9%.The Co-WSe₂/SWNTs exhibited the best electrocatalytic hydrogen evolution performance and stability in both acidic and alkaline electrolytes.Under acidic conditions,the overpotential is only 147 mV,and the Tafel slope is 33 mV dec^-1,almost comparable to the precious metal platinum.?3?By extending the same strain strategy to other transition metal sulfide compounds,such as MoS₂.The results showed that the effects of mechanical strain and interfacial chemical synergism caused by CNTs and Co doping were also applicable to MoS₂.The content of 1T phase in Co-MoS₂/SWNTs was up to 75.7%,and its overpotential in acid and alkaline electrolyte was respectively 180 mV and 152 mV,which was greatly improved compared with pure MoS₂.Our work makes use of strain to realize effective regulation of TMDs phase transition and expands its application in the field of catalysis.