In-situ Electrochemical Scanning Tunneling Microscopy Study At "Electrode/Ionic Liquid" Interface

The "electrode/electrolyte" interface is the location of electrochemical reactions,and the fundamental research on its structure and property plays an important role in electrochemical field.Room-temperature ionic liquid,totally composed of asymmetric cations and anions,has advantages in various fields because of wide electrochemical windows,low vapor pressure,good conductivity and other designable properties.Though ionic liquids have been extensively used in electrodeposition,capacitor and electrocatalysis,the studies on "electrode/ionic liquids" interfaces are still on their initial stage.Moreover,most of the studies employed polycrystalline electrode without well-defined structure,which limits the correlation between electric double layer structure and kinetics of charge transfer.Well-defined single-crystal electrode not only benefits understanding surface electrochemical process,but also favors the correlation of experimental and theoretical results.By employing in-situ ECSTM,AFM force curve technique,and electrochemical methods,various imidazole ionic liquids adsorption behaviors on different lattice-arrangement crystal and their potential dependency have been systematically studied.The results are outlined as follows:1.In the system of "Pt(100)/OMIPF6",investigation on how electric double layer changes with potential has been carried out from the aspects of ionic liquid adsorption.While comparing with the result in ionic liquid BMIPF6,it is found:(1)The OMI+forms ordered structure on the surface,which exists steady within potential range of about 1.2 V.The OMI+ adsorption structure on Pt(100)surface is given.(2)0.6 V can be considered as critical potential at which cations are displaced by anions.This demonstrates that chemical interaction between OMIPF6 and Pt(100)surface can only be overcome at much positive potential to make OMI+ desorb from surface.(3)The BMI+ forms worm-like structure on the surface.Owing to the long alkyl chain length,OMI+ can interact strongly with Pt to form ordered structure.Because the ordered structure covers the surface,the corrosion from ionic liquid can be suppressed.Therefore,it can be inferred that the chemical interaction is an important factor that influences the structure of Pt(100)/OMIPF6.2.A comparative study on the electric double layer and the potential dependence between Au(lll)/EMMITFSI and Au(111)/EMITFSI has been carried out.The following conclusions can be drawn:(1)In EMMITFSI,two pairs of sharp peaks appear which remain steady within specific potential region.The peaks were not observed in EMITFSI.(2)STM experiments indicate that the electrode surface experience the process that from the initial flat clean without adsorption to the appearance of adsorption structure,eventually full of single-layer adsorption.While in EMITFSI,the worm-like structure exists with the whole potential region more positive than PZC.(3)AFM force curve reveals layering structure in EMMITFSI can exist within potential range more positive than that in EMITFSI,indicating the special interaction between cations and anions in EMMITFSI.Besides,the thickness of layer structure presents various change,and feature peak and valley coincide with those of CV.The above infers the peaks relate to the layer structures to some degree.Hence,it can be concluded that the special EMMI+ structure leads to strong interaction between ions and electrode which makes the peaks appear.3.The structure of the electric double layer relates to both of electrode and solution.Investigation on "Pt(100)/EMMITFSI" system contributes to understand the structure deeply by comparing with the interface investigated above.The following conclusions can be drawn:(1)The characteristic peaks cannot be observed since the interaction between ions and Pt are not as strong as that of Au(111),(2)The ordered structure is composed of anions,which is different from that of OMI+,concerning the smaller cation and special structure.(3)Severe etching doesn't occur on Pt,which can be attributed to the stronger interaction between Au and ionic liquid to a certain extent.