AFM Force Curve Investigations On Single Crystalline Electrode/ionic Liquid Interfaces And Influence Of Cation Side Alkyl Chain Length And Ferrocene

Ionic liquids(ILs)are a kind of substance,entirely composed of ionic species and presented as liquid at and near roomtenperature.Compared with traditional electrolyte solutions,ILs have many important features,such as wide electrochemical window,low vapor pressure,high boiling point and good electrical conductivity,it have been widely used in various fields.The structure of the electric double layer of "electrode/ILs"interfaces,compared to the aqueous solution,has distinct characteristics and properties.Recent researches show that ionic liquids can form potential-dependent layering structures near the electrode surface,in addition to ordered adsorption at the surface.However,detailed and comparative investigations of layering structure formed in different ionic liquid systems have not been investigated.Also the influence of charge transfer reaction in ILs has not been investigated on well-defined single crystal electrode/ionic liquid interfaces,which hinders the understanding of reaction kinetics from structural point of view.In this work,we systematically investigate the interfacial structure in three ionic liquid with different alkyl chain length and influence of redox species by combined EC-AFM force curve technique and electrochemical methods.Through study of the number,thickness and stability of the layering structures and their potential-dependency,we analyze the influence of alkyl chain length and the redox reaction to the layering structure,thus providing experimental evidence for gaining reasonable "electrode/ionic liquid" models.Results and conclusions are as follows:1.The potential of zero charge(PZC)of electrode-ionic liquid interfaces for three ionic liquid systems(EMITFSA,BMITFSA and OMITFSA)are measured based on charge density vs.potential curves(q-E curves)by immersion method.2.Effects of the alkyl chain length on the interfacial nanostructure of ionic liquid at the Au(111)-ionic liquids interface are investigated.Three ionic liquids are as follows:EMITFSA,BMITFSA and OMITFSA.EC-AFM force curve technique is employed at potentials positive of and negative of PZC to investigate the number,thickness and stability of the layering structure.Force vs.potential curves(F-E curves)are constructed to display the potential-dependency of the layering structure.The results show that:(1)The number of the layering structure decrease as the alkyl chain length increase;(2)The tendency of F-E curves in three ILs behave the same way.At potentials close to PZC,the cations and anions loosely arranged,which lower the stability of the layering structure.While at the potential negative to PZC so that the electrode surface is positively charged,more anions attached to electrode surface which increase the stability of layering structure.However,too much surface charge will weaken the stability of the inner-most layer because of the lattice saturation of ions.In the potential negative to PZC,F-E curves appears similarly.(3)Rupture force of layered structure associated with different ions has the following sequence:EMI+<BMI+<TFSI-<OMI+,OMI+ has stronger structure because the long cation alkyl chain produces a stronger solvophobic force that increases the cohesive forces within layer.3.Effects of redox species on the interfacial nanostructure of ionic liquid at the Au(111)-ILs interfaces.Ferrocene is chosen as the probe molecule to investigate the nanostructure of three ionic liquids of EMITFSA,BMITFSA and OMITFSA.EC-AFM force curves are measured in both regions where redox reaction takes place and where it does not take place.The number,thickness and stability of the layering structures are obtained and compared with corresponding neat ionic liquid systems.Also F-E curves are constructed to display the features of the layering structure.The results show:(1)The presence of redox species decreases the number of layering structure by squeezing cations into the inner-most layer,and the longer the alkyl chain,the greater the impact.(2)The rupture force of layered structure is increased when redox reaction take place,which is caused by the stronger interaction between the pi electron of cyclopentadienyl and the ions of ionic liquid and increased cohesive forces within the layer.(3)Redox species can change the thickness of layering structure.For short alkyl chain length,ferrocene squeeze into the gap of cations and anions.For long alkyl chain length,cations unfold the side alkyl chain to let ferrocene in,which cause the thickness increase of the layering structure.4.Effect of Ag(111)electrode on the interfacial structure of EDL in ionic liquids.The surface etching and reconstruction of gold single crystalline electrodes mostly employed in current studies makes the interfacial process more complex.So we choose Ag(111)as working electrode to analyze the "electrode/ionic liquid" interfacial structure.EC-AFM force curve technique is used to measure the number,thickness and stability of two ionic liquids with different alkyl chain length.The results show that:(1)The tendency of F-E curves in two ionic liquids behave the same way.As the potential turns negative,the force of the inner-most layer firstly increased and then decreased;(2)The stability of layering structure of Ag(111)is greater than Au(111)because of the surface etching and reconstruction of the gold nanocrystalline electrode while the surface of Ag(111)is much more stable.