Syntheses and Studies on the Resistance Switching and Electrochemical Properties of Transition Metal Dichalcogenides

Author:Wu Li Qian

Supervisor:zhong wei


Degree Year:2019





Engineering nanocomposites or introduction of multiple metal elements into a compound can synergize different functions and achieve the performance optimized and expanded,which is a very promising research topic.In this paper,on the basis of the idea mentioned above,the two aspects of research works for transition metal dichalcogenides have been carried out:the study of resistance switching effect and electrocatalytic hydrogen evolution of molybdenum disulfide/graphene hybrids,and the research of preparation and supercapacitor properities for nickel cobalt selenides.The details are as follows:1.Resistance switching effect and electrocatalytic hydrogen evolution of molybdenum disulfide/graphene hybridsSince the graphene was discovered in 2004,the researches of two-dimensional materials(2DMs)have been the focus of the science.As a typical semiconducting 2DM,molybdenum disulphide(MoS2)nanosheet has received much attention owing to its excellent performance in different fields,including optics,electrics,energy-storage and conversion,mechanics,sensors,biology.Engineering nanocomposites by hybridization of two or more materials are generally thought to overcome the weaknesses of individual counterparts as well as synergize different functions,achieving the performance optimized and diversified.Therefore.synthesizing nanocomposites of MoS2 nanosheets offers great opportunities for obtain fascinating performance of physics and chemistry.Graphene oxide(GO),as the intermediate product of solution-processed graphene,possesses abundant functional groups in the plane and high surface areas.Thus it can be served as substrates for the nucleation and subsequent growth of other nanomaterials,effectively forming nanocomposites.In this work,we synthesized MoS2 and reduced graphene oxide hybrid(MoS2-rGO)by hydrothermal method,and investigated the resistance switching effect and electrocatalytic hydrogen evolution activity.The main contents are as follows:(1)A flexible resistive memory device based on MoS2-rGO employed as the resistance transition layer material was fabricated with the configuration of Ti/MoS2-rGO/ITO/PET.Different from on pure MoS2 and rGO,this device behaves distinct switching effect,and possesses excellent performance:a low operating voltage(about 0.4 V),high repeatability of 200 cycles,and long retention time of more than 10-4 s;multilevel states by controlling the compliance current;bending endurance of over 300 bending cycles.On the basis of analysis for I-V curve and energy-level structures of various states of the device,the resistive-switching effect can be attributed to the electron trapping and de-trapping in the MoS2-rGO heterojunction.(2)Mn doped MoS2 and reduced graphene oxide hybrid(Mn-MoS2/rGO)was evaluated as an electrocatalyst for hydrogen evolution in acidic medium of pH=0.The results indicate that,the hybrid exhibits better catalytic activity with a low overpotential of 110 mV and a small Tafel slope of 76 mV dec-1,than that of MoS2 and MoS2/rGO.Through the contrast of electrochemical parameters and the analyses of experimental characterization for the serial samples,it can be concluded that the improvement for electrocatalytic hydrogen evolution activity of the hybrid catalyst is mainly ascribed to fundamental catalytic activity enhancement of the S-edge of MoS2 owing to Mn ions doping,and subsequently the high exposure of more active edge sites generated by the composite structure of Mn-MoS2 and rGO.2.Supercapacitor properities of nickel cobalt selenidesSupercapacitor is an important electrical energy storage system.Its energy storage is based on direct charge store.Since possessing higher power density,superior charge-discharge rate,and longer cycling life,supercapacitor is deemed as one of the most promising energy storage systems in the future,and numerous efforts in experiment and application have been made.Owing to the intrinsic metallicity and fast electrochemical faradic reactions of the transition metal selenides,the supercapacitors that use them as electrode materials possess the potential to exhibit excellent electrochemical property.In this work,the supercapacitor properities of a novel material,namely bimetallic nickel cobalt selenides(NixCo1-x)0.85Se,were systematically studied.The main contents are as follows:(NixCo1-x)0.85Se were successfully synthesized via one-pot hydrothermal treatment.The bimetallic nickel cobalt selenides(NixCo1-x)0.85Se,as electrode materials of supercapacitors,show more excellent electrochemical performance than that of the monometallic selenides Ni0.85Se and Co0.85Se.Meanwhile,through varying the Ni to Co ratio,the(Ni0.5Co0.5)0.85Se delivers the highest specific capacity,excellent rate capability,and good cycling stability.Upon analysis,the superior electrochemical performance of bimetallic nickel cobalt selenides,ascribe to the synergistic effect between Ni ions and the Co ions.