Research On Electrosynthesis And Electrocatalysis Based On Single Particle Collision Electrochemistry

Two-dimensional transition metal chalcogenides(2D-TMDs),as a two-dimensional layered material with a graphene-like structure,have unique electronic structure and surface chemical properties,showing great research significance and application prospects in optoelectronic devices,biosensing,energy conversion and other fields.Some studies have shown that the physical and chemical properties of 2D-TMDs,such as band gap,nonlinear optics,and specific surface area,are directly related to the number of layers of the materials.Accordingly,the establishment of an efficient method to prepare TMDs with thin-layer or even single-layer structure is prerequisite for studying the intrinsic properties and applications of such materials.Among many reported fabrication methods,the electrochemical exfoliation method has the advantages of mild operating conditions,low cost,and fast response.Therefore,it is expected to be applied to the large-scale preparation of two-dimensional nanosheets,and have attracted creasing attention from researchers.However,the existing electrochemical exfoliation method still faces two major challenges.Firstly,during the expansion progress resulted from the ion intercalation,the bulk material directly used as the working electrode is easily disintegrated,causing incontiguous exfoliation and lower yields.Secondly,such approach is limited to large-size bulk crystals,which is difficult deal with powdered materials.With the development of single-particle impact electrochemistry,the study of the collision behavior between nanoparticles and ultramicroelectrodes in solution can provide important characteristic signals at a single particle level.In addition,we were illuminated by the fluidized electrocatalysis strategy recently rep orted by our research group.In this strategy,by fluidizing the catalytic particles in the electrolyte,the catalysis is activated only when single particles collide with the electrode.Therefore,different catalyst fatigue mechanisms can be significantly illuminated due to the drastically reduced reaction time scale,providing a more stable and efficient current output.This strategy is also expected to be applied to high-yield exfoliation of particle layered materials.In this thesis,a liquid phase exfo liation method based on single particle impact electrochemistry was developed.To be in detail,a cylindrical reaction cell made of glassy carbon was designed and used both as a working electrode and an electrolytic container,which can provide sufficient collision chance so as to efficiently realize the exfoliation of the powdered material.Taking MoS₂ as a model material,the mechanism of the intercalation and exfoliation based on single particle impact electrochemistry was deeply explored.This method wa s further applied to other 2D-TMDs,such as WS₂ and WSe₂,and a series of characterization proved that high-quality2D-TMDs with large lateral size and thin layer can be successfully prepared.Due to the fact that TMDs often show high electroactivity towar d hydrogen evolution reaction(HER),the as resulted three few-layer nanosheets(MoS₂,WS₂ and WSe₂)were also put into study of HER,showing good HER performance.The key points of this thesis are summarized below:(1)Taking MoS₂ powder material as an example,the exfoliation mechanism of the new strategy was discussed in depth.During the preparation process,when the MoS₂ powder particles collide with the electrode,ammonium ions and a large number of free radicals are quickly inserted between the layer s,weakening the van der Waals force between the layers.MoS₂ powder particles are then gently sonicated in a DMF solvent that matches their specific surface energy,so that the swollen MoS ₂ powders can be peeled off into a uniform and stable dispersion of few-layer MoS₂ nanosheets.The single particle impact electrochemistry-based exfoliation method can expand the electrochemical intercalation assisted exfoliation to powdered materials.(2)Morphological characterizations showed that we have successfully prepared few-layer MoS₂,WS₂ and WSe₂ nanosheets with a lateral size distribution of 3-12?m and a thickness distribution of 3.2-3.8 nm.The successful exfoliation of three different powdered materials demonstrated the generality of the curren t exfoliation strategy.The structure and composition of the three prepared nanosheets were systematically analyzed by XRD,Raman,HR-TEM,XPS and other characterization methods.The results proved that the few-layer nanosheets prepared by this method are highly crystalline 2H-TMDs,indicating that this method will not bring additional changes of structure and composition to the material.(3)By performing HER,the few-layer MoS₂,WS₂ and WSe₂nanosheets showed higher catalytic activity and faster kinetics compared to their corresponding bulk powders.At the same time,by examining the single MoS ₂ and WS₂ nanosheets catalyzed HER at different potentials,we found that single MoS ₂ nanosheets showed faster kinetics than WS₂.