Electrochemistry Reaction And In-situ Raman Investigations Of Ionic Liquids At Very Negative Potentials

Metals are often described by Jellium mode which have uniform continuous positive background and free electrons that uniformly distribute in bulk and spill to out of metals at surface and interface,and it plays important role in researching and theoretical calculation of basic issues of double electrode layer and so on.Based on the relevant study,the spillover tail of interfacial electrons is relevant to electrode materials,nanometer scale surface structure and electrode potential.However,the effect of electron spillover on specific electrochemistry reaction have not been thoroughly studied.For instance,electron spillover can increase surface electron density,the relationship between electron spillover and electrochemical reaction which requires the participation of electrons have very few research;the surface plasmon resonance(SPR)generated under laser illumination at nanostructures is based on collective electron oscillation,and the researches on the synergetic influence of SPR and electron spillover on electrochemistry reaction and improvement of resolution at z-direction of SERS measurement are rarely reported.Ionic liquids as a new kind solvent have very negative cathodic electrochemical window which can meet the requirement for studying electron spillover and synergetic effects with SPR on the electrochemistry reaction at very negative potentials.Meanwhile,the stability issues and cathodic decomposition mechanism of ionic liquids involved in the study would provide guidance for their practical application.In this thesis,we first perform traditional electrochemical measurement to systematically study the cathodic reduction process of different imidazolium ionic liquids on different electrode materials with different surface topography under the illumination of different lasers and power densities.By combining with SERS technique,we further investigate the influence of electron spillover and SPR to electrochemical reduction reaction.In addition,normal Raman spectroscopy(NR)measurement and density functional theory(DFT)calculation are combined to affirm reaction products and speculate reasonable reaction mechanism.The main content and results are outlined as follow:1.Influence of electron spillover and SPR in electrochemical reactions The metal electrodes will have obvious electron spillover at relative negative potentials,so the system being studied should include redox species that can react at negative potentials in electrolyte whose cathodic reaction potential is more negative than the reaction potential of redox species.Various redox species and electrolytes have been studied and compared,and finally the cathodic reduction of imidazolium ionic liquids are chosen as system for detailed investigations.Bases on the electrochemistry results of different ionic liquids and experiment condition,the cathodic potential of reaction on electrochemically roughen Ag electrodes is more positive than that on smooth Ag electrodes,i.e.rough surface is beneficial to spilling electron and promoting reaction.However,the reaction at Au electrodes is not affected by surface topography,probably due to the weak electron spillover at Au surface.Furthermore,for Ag electrodes,electron spillover and SPR can further promote electrochemistry reduction reaction,and 532 nm laser have stronger influence on the promotion of reaction,as Ag nanoparticles have stronger SPR under the illumination of 532 nm laser.2.The influence of electron spillover in electrochemical surface-enhanced Raman spectroscopy(EC-SERS)of ionic liquids In order to research the effect of electron spillover on SERS,the electrochemistry SERS measurements of EMITFSI ionic liquids on Ag electrodes are carried out.With negative shifting of potential,a new peak appears in electric double layer region,and is discerned to arise from the vibration of intermediate product carbene,i.e.reduction product can be detected at a potential positive than the potential where massive bulky reduction occurs.By combining NR and DFT calculation,the main reaction products are determined,which contain carbene,single-bond dimer and double-bond dimer,and reaction mechanisms are proposed.However,even though the applied potential goes to very negative,the intensity of SERS does not get further enhanced after the reaction occurs,inconsistent with the expectation of electron spillover enhanced Raman scattering.It is proposed that a film-like layer of reaction products may have been formed on the electrode surface and thus inhibited electron spillover to enhance SERS intensity.On contrary,when the system contains certain amount of water,which may prevent the formation of the film-like structure of products,the SERS intensity increases along with negative shifting of potential.3.Study of Ag nanoparticles size on cathodic reaction Surface curvature of metals can also affect electron spillover,further affect electrochemical reaction,so we investigate the influence of different silver nanoparticles size on cathodic reaction of EMI-TFSI.We tried various methods to obtain different curvature electrode surface with uniformly distributed Ag nanoparticles(50-400 nm),Ssuch as electrodeposition and electrochemical grafting.The electrochemistry results of EMITFSI on Ag nanoparticle electrodes show that the Ag nanoparticles size have very little influence on the reaction.So further detailed investigations are necessary.