Electrochemical Surface-enhanced Infrared Spectra Of The Application And Development

Surface electrochemistry is the frontier inter-discipline concerned with the interrelation of electrochemistry and surface chemistry,and the development of surface electrochemistry has been promoted by extensive applications of in-situ spectroscopies, scanning probe microscopes and electrochemical quartz crystal microbalance with high sensitivity and unique specificity.Many interesting topics in surface electrochemistry include the determination of adsorption configuration of aromatic compounds on coinage metal electrodes and of the mechanisms associated with electro-catalytic oxidization of small organic molecules on Pt electrode.In-situ surface vibrational spectroscopies play an important role in investigating the above topics.Recent development of surface spectroelectrochemistry calls for an increasing interest in adsorption and reaction on nano-structured electrodes.Along with this trend, surface-enhanced IR absorption spectroscopy(SEIRAS),surface enhanced Raman spectroscopy(SERS) and abnormal IR spectroscopy(AIREs) are deemed as important analytical tools to offer structural information at electrode/electrolyte interfaces.SEIRAS with the Kretschmann ATR configuration(ATR-SEIRAS) has the merits of simpler surface selection rules as compared to SERS,and thus is convenient for discussing the adsorption configurations of aromatic compounds.In addition,with the advantages of higher surface signals and free problems otherwise caused by the thin-layer electrolyte structure with AIREs,ATR-SEIRAS is preferable for dynamically identifying and monitoring intermediates of irreversible reactions.Nevertheless,there is still a large space to be filled in terms of research systems and technique improvement before ATR-SEIRAS becomes perfect.In this thesis,firstly traditional ATR-SEIRAS has been applied to reinvestigate the adsorption configuration of typical aromatic molecules at electrodes and the reaction processes of CO as well as small organic molecules on Pd electrodes;Secondly, ATR-SEIRAS with a new optical window is proposed,based on the penetration of evanescent waves and the theory of "prism-film coupler for plane wave" to surmount the frequency detection limit of traditional ATR-SEIRAS.Thirdly,new design in IR optics switchable for internal and external modes on demand is proposed to take full advantage of the two modes.The main results and conclusions are summarized as follows:â… -1.Investigation of adsorption configuration of isonicotinic acid on Au electrode Isonicotinic acid has three possible sites for attaching the electrode surface,viz. -COOH,N atom andÏ€bond,and thus taken as a model molecule for the study of adsorption geometries.ATR-SEIRAS combined with cyclic voltammetry and differential capacitance measurement has been applied to investigate the adsorption configuration of isonicotinate(INA^-) species on Au electrodes in alkalinized 0.1 mol·L^-1 KClO₄ and 0.1 mol·L^-1 KCl solutions(pH10).The results indicate that INA^- is nearly vertically adsorbed on the Au surface through the two oxygen atoms of its carboxylate group between -0.4 and 0.2 V(SCE).The presence of Cl^- in supporting electrolyte dose not changes the adsorption configuration of INA^- on Au electrodes. Further supporting information from surface-enhanced Raman spectroscopy suggests that at negative potentials from -0.8 to -0.5 V,INA^- species adsorbed on Au electrode of a very low coverage is likely to remain the nearly same configuration as that revealed at more positive potentials.â… -2.Investigation for adsorption configuration of aromatic molecules on Ag electrode surfacePyridine(Py) and benzoic acid(BA) molecules are the classic model molecules for studying adsorption configuration on metal surface,especially by SERS.Due to complicated nature of SERS surface selection rules,ATR-SEIRAS combined with cyclic voltammetry and differential capacitance measurement has been applied to investigate the adsorption configuration of Py and BA species on Ag electrodes.In the presence of 10 mM pyridine in 0.1 M NaF,the result of capacitance curve measurement suggests that adsorption of Py starts at the potential of ca.-1.0 V,and increases up to -0.65 V and then decreases at higher potentials due to partial oxidation of Ag electrodes.The ATR-SEIRA spectra are dominated with vibrational modes having A₁ symmetry,while those having B₂(in-plane) symmetry are hardly observed.It is noteworthy that spectral features for the adsorbed and the bulk pyridine are different in terms of relative band intensity and position.The totally symmetric ring breathing mode(ν₁) blue-shifts from 992 to 1004 cm^-1,suggestive of the adsorption via N end. On the basis of the surface selection rule of SEIRAS,the adsorbed pyridine molecules are bound to the Ag electrode with its ring plane perpendicular or slightly tilted to the local surface without significantly rotating its C₂ axis about the surface normal consistent with the conclusion drawn by SERS in the literature.In 0.1 M NaClO₄+10 mM BA,the capacitance curve measurement suggests that BA adsorbs on Ag surface at ca.-0.9 V and undergoes phase transition at ca.-0.4 V.The ATR-SEIRA spectra are dominated by vibrational modes having A₁ symmetry and the intensity ofν_s(OCO) is the most strong,indicating that benzoic acid at positive potentials higher than -0.4 V is bound to the electrode surface through the carboxylate oxygen atoms with a bridging coordination.â… -3.Fabrication and application of SEIRA-active Pd film electrodePd electrode shows a high catalytic activity toward the oxidation of formic acid in direct formic acid fuel acid(DFAFC).In order to clarify the mechanism for HCOOH oxidation on Pd electrodes by ATR-SEIRAS,it is essential to construct SEIRA-active Pd film electrodes on the basal plane of a Si window.Here,an improved protocol has been proposed for electroless plating of Pd on Si,enabling a strong adhesion between the plated Pd film and the Si substrate,even after repetitive electrochemical cycling in an acid solution.Besides,the Pd film electrode exhibit strong SEIRA effect and with unipolar band shape,laying the basis for reliable peak analysis.The electrochemical measurements manifest that Pd film electrode is excellent for the electro-oxidation of formic acid.Furthermore,preliminary study has been carried out on the electro-oxidation of HCOOH on Pd electrode by employing ATR-SEIRAS,1) to clarify whether poisonous CO_ad will be formed for HCOOH oxidation on Pd electrodes; 2) to supplement the so-called "dual pathway" mechanism for HCOOH oxidation on Pd electrode.â…¡.Fabrication and application of practically modified ATR-SEIRAS for high-quality frequency-extended detection of surface species at electrodesIn electrochemical ATR-SEIRAS application,the Si prism is the most frequently used IR window to support the film electrodes owing to its stability in acidic electrolytes,as well as readily available mature and economical wet processes for fabricating various metal electrodes on Si surface.However,a Si prism per se suffers from strong IR absorption in the frequency lower than 1000 cm^-1,preventing the observation of important spectral fingerprints in this region,hence compromises the structural and mechanistic elucidation at electrodes.Nevertheless,metal film electrodes supported on ZnSe and Ge prisms(having much higher IR transmission in the range of 1000-700 cm^-1 than Si prism) suffer frequently from abnormal electrochemical responses in wide potential excursions especially in strong acid solutions,due to the instability and dissolution of Ge and ZnSe substrate as well as peeling off metal films.To address this issue,primarily based on "the penetration of evanescent wave" and "the theory of prism-film coupler for plane wave",we present here a practically modified ATR-SEIRAS configuration by sandwiching an ultra-thin water interlayer(submicron thick) between a Si wafer and a ZnSe prism.It is very important to introduce water interlayer in this new ATR optics,which significantly enhances the throughput of effective IR beam across the ZnSe/gap/Si/metal film.This new ATR-SEIRAS with ZnSe/water/Si IR window at incidence angle of 70°can yield the strongest surface IR signal down to 700 cm^-1.The advantages of this modified ATR-SEIRAS have been initially applied to explore two selected systems: wide-ranged in-situ ATR-SEIRA spectra includingν_s(OCO) andδ(OCO) modes provided strong evidence in support of the formate intermediate pathway for methanol electro-oxidation at Pt electrode in an acid solution.In addition,new spectral fingerprints revealed comprehensive orientational information about of the p-nitrobenzoate species at Pt electrode as a result of the dissociative adsorption of p-nitrobenzoic acid molecules from an acid solution.From the in-plane and out-of-plane vibrations obtained from 700-1700 cm^-1,three parameters associated with the orientation configuration can be evaluated,including the dihedral angle between the molecular plane and local surface plane,the edge-tilted angle of the molecular plane,and the angle between the symmetry axis of the molecular plane and the local surface normal.â…¢.Design and application of an IR spectral cell switchable for internal and external reflection modesATR-SEIRAS alone is unable to identify the products in bulk solution and to evaluate IR enhancement.In this thesis,we design a spectral cell convenient for the switching between internal and external reflection modes,favoring the acquisition of full information regarding both the surface and the solution.This switchable IR spectroscopy can be applied to identify CO adsorption and its subsequent oxidation to CO₂,as well as elucidate the origin of abnormal IR effect(AIRES) experimentally. The results show that the AIRES is not only related to the thickness and structure of the metal overfilm but also to the reflectivity of the substrate. In summary,conventional ATR-SEIRAS has been applied to investigate the adsorption configuration of prototype molecules on Au and Ag electrodes,as well as the electrocatalytic reaction on Pd and Pt electrodes.We also focus on the improvement of ATR-SEIRAS technique per se for extended detections.The thesis reveals that ATR-SEIRAS is an important analytical tool in surface electrochemistry, analytical chemistry and nanotechnology.This work is expected to contribute to the advancement of electrochemical ATR-SEIRAS.