Construction Of Two-dimensional Molybdenum-based Model Electrocatalysts For In Situ Electrochemical Scanning Tunneling Microscopy Characterization

Currently,the design of hydrogen precipitation reaction（HER）electrocatalysts is becoming increasingly diverse,and the catalytic mechanism is subsequently complicated.The construction of realistic model catalysts with well-defined components and structures for in situ characterization studies has become an important way to reveal catalytic active sites and reaction mechanisms.Among them,two-dimensional（2D）model materials,represented by molybdenum disulfide（Mo S₂）,are particularly suitable as in situ surface analysis platforms,but it is still challenging to modulate the species and distribution of surface active sites at nanoscale and to realize in situ visual characterization.Electrochemical scanning tunneling microscopy（EC-STM）,which can track the dynamics of catalyst surfaces during the electrocatalytic process in real-time and in real space,is a powerful tool for identifying active sites and understanding reaction mechanisms.Based on this,we have developed servel unconventional strategies to construct interfacial,strained,amorphous,and edge sites of 2D molybdenum-based materials at nanoscale,which can be used as platforms for EC-STM combined with“tunneling current noise analysis”for in situ visualization characterization of various active sites at nanoscale.It may provide useful new perspectives for subsequent studies.（1）Strain-enhanced hydrogen evolution activity of Mo O₂was successfully visually presented at nanoscale using EC-STM.Horizontal and vertical configurations of Mo O₂single-crystal sheets were prepared on the Au（111）surface,and compressive strain sites were generated by extrusion deformation of adjacent horizontal Mo O₂sheets.Extensive in situ EC-STM results showed no regional noise differences between the plane and edge sites of the Mo O₂sheets,indicating HER activity between the plane and edge sites is comparable.Two different levels of intensity-enhanced noise were observed at the strained regions on both sides of the bounday of the extruded Mo O₂sheets,while no intensity-enhanced noise was observed at the unstrained regions,suggesting a positive strain effect that enhances the HER activity of Mo O₂was formed in the strained regions.This work extends the visual characterization of strain engineering in electrocatalysis and related fields.（2）An unconventional moiré-guided etching strategy is proposed to construct locally amorphous Mo S_xnanodomains.In Mo S₂/Au（111）,triangular or hexagonal Mo S_xnanodomains were constructed,while in Mo S₂/Au（100）,striped Mo S_xnanodomains were obtained.It can be found that the shape of the amorphous nanodomains follows the moirépattern feature,resulting in an“outside-in”etching mechanism.This strategy can also be developed as a new tool for the preparation of amorphous and crystalline Mo S₂nanoribbons.Thus,a 2D nanomodel platform that can expose both amorphous Mo S_xand crystalline Mo S₂phases is constructed and applicable for in situ EC-STM studies.Through EC-STM results,the amorphous Mo S_xsurface is visually presented to have a higher HER activity than the crystalline Mo S₂surface at nanoscale.This is a new moirésuperstructure application scheme that can be extended to other 2D materials.（3）A strategy for the rapid synthesis of thin 2D Mo S₂nanobelts with horizontal and vertical layered structures was developed based on a self-designed vertical induction heating device.It was shown that hydrogen and heating rate are important factors for the formation of Mo S₂nanobelts with both configurations.This strategy can be extended not only to a variety of conductive substrates such as Pt（111）and graphene,but also to the preparation of nanobelts of metal compounds such as tungsten,vanadium and nickel.As a result,a two-dimensional striped platform suitable for EC-STM has been constructed,which exposes both vertical layered edge surfaces and horizontal layered planes and their edges with several nanometers in height,allowing visually presenting the difference in HER activity between vertical and horizontal exposed edges and planes of Mo S₂layers at nanoscale.This work advances the in situ analysis of vertically exposed edge sites of 2D materials.