Studies Of Metal Anode/Solution Interface Processes, Nanostructures And In Situ Raman Spectroelectochemistry

There have been abundant information and complex dynamic behaviors present at the metal anode/solution interface,which depend on the anode potentials,such as formation and evolvement of surface film phases,surface adsorption and desorption,complexation,dissolution or re-deposition of metals,gas evolution,electrochemical oscillations, production of surface nanostructures and etc.Profound study on these interfacial characters of pure metal and alloy anodes/solution contributes to reveal of electrode reaction mechanism,to understand the interface process,consequently to direct the electrode reactions and to construct kinds of metal nanomaterials.In situ Raman spectroscopy has been used for inspecting the micro-zone at metal anode/solution interface.The first-hand information of the species present on electrode surface can be obtained by fitted with the time-resolved and potential-dependent spectra, which will be very helpful to deep recognize the interfacial dynamic of anodic processes at the molecular level.In this thesis,several anodic processes of metals and alloys have been investigated by in situ Raman spectroelectrochemistry.From the analysis of electrochemical behaviors and in situ spectral results,direct identification and reasonable explanations were provided for various types of electrooxidation mechanism.Meanwhile,nano-structured materials for surface-enhanced Raman spectroscopy and electrocatalysis have been fabricated by utilizing the anodic processes.The main contents are summarized as follows:1.The in situ Raman spectroelectrochemical technique has been briefly introduced.The recent research progress of the anodic electrooxidation for pure metals/alloys and correlative nanostructures have been reviewed.2.The electrochemical processes in dealloying of Au-Sn alloys with different ratios in an HCl solution have been first investigated in detail by means of in situ potential-dependent and time-resolved Raman spectra.Two dealloying modes were found occurring within different potential regions in the electrooxidation of Au-Sn alloys. One is the mode known as classical dealloying,where Sn is selectively dissolved;and the other a so-called quasi-dealloying mode found here,in which Au re-deposits automatically after simultaneous dissolution with Sn.Meanwhile,nanoporous gold,thin layers of gold nanoparticles stacked on the surface,and colloidal gold in the solution can be prepared from the Au-Sn alloys simply by an electrochemical control of potential.3.Investigation on the potential-dependent anodic oxidation of a Pt-Ni alloy electrode has been performed in an HCl solution.Spectroscopic information of UV-vis absorption and in situ Raman scattering shows that the alloy undergoes selective Ni dissolution and simultaneous Pt dissolution successively with the increase of the applied potential. The measurements of SEM-EDX,AFM and XRD at selected potentials reveal that Pt enrichment occurs in the alloy degradation accompanying diversiform morphological evolution such as cracks from stress corrosion,ultrafine pores by selective dissolution,and flaky nanoporous films involving simultaneous dissolution and potential accelerated replacement reaction between Ni in the alloy and dissolved PtCl₆^2-.Moreover,the nanoporous films display high electrocatalytic activity toward the methanol oxidation.4.The surface oxidation processes of Pd and Au-Pd alloy electrode in chloride containing solutions have been investigated in detail by means of in situ potential-dependent Raman spectroscopy for the first time.In the HCl solution,characteristic Raman bands such as for the oxidative coordination of Pd with Cl^-,the transformation of soluble Pd(â…¡) to Pd(â…£) complexes,the electrooxidation of Cl^- into Cl₂,and the redox between Cl₂ and Pd were all detected unambiguously during the potential ascending.While in the KCl solution,insoluble salt films of K₂PdCl₄ and K₂PdCl₆ were found on the electrode surface due to their poor solubility.A mechanism scheme is given on the basis of spectroelectrochemical results.Moreover,the electrooxidation of Au-Pd alloy in HCl solution has been further studied.Several species from anodic oxidation were detected by in situ Raman spectra,and the morphomogies at typical anodic potentials were also characterized. It can be concluded that the Au-Pd alloy electrooxidation follows simultaneous dissolution procedure in the HCl medium.5.The quasi-dealloying manner has been grafted onto a pure Au electrode to construct the SERS substrate conveniently.Firstly, electrodeposite a tin overlayer on the substrate as Au|Sn layer,and then co-dissolve the Sn overlayer with Au underneath by controlling the potential at the simultaneous dissolving region.A thin layer of gold nanoparticles(GNPs) sticking to the gold electrode surface can be obtained and it performs good surface Raman enhancement. Besides,we described the synthesis of Sn(â…¡) complex stabilized GNPs by Sn(â…¡) reduction of HAuCl₄ in high acid aqueous solution. This gold colloid was stable for months in oxygen-free surrounding. More interestingly,the GNPs in the colloid were futher assembled into uniform supperlattice by slow sedimentation,which was induced by the oxygen bit by bit.The supperlattice can be served as a long-lived acive SERS substrate.6.Nonliner behaviours for anodic electrooxidation of Cu-Zn and Cu-Ni alloy have been studied in different medium.Two different types of potential and current oscillations were found during the anodic electrooxidation of Cu-Zn alloy in NaOH solution,corresponding to the two crossing cycles in the cyclic voltammograms,respectively. With the help of Raman spectra,the oxide species on the alloy electrode were discussed and possible oscillatory mechanism was given.A new electrochemical oscillatory system was found for the electrodissolution of copper-nickel alloy in sulfuric acid in the region of oxygen evolution,which is different from the oscillatory system for pure Ni electrooxidation.7.Oscillations have been first observed during iodide oxidation in an alkaline solution on a static platinum electrode,where phase transitions at the interface of electrode/solution are essential.The film formation of solid iodine acts as a negative nonlinear feedback slowing down the iodide oxidation due to its poor conductivity,and oxygen gas evolution as a positive nonlinear feedback by destroying the iodine film mechanically and by promoting its dissolution through convection mainly via disproportional reaction.Based on the results of electrochemical experiments and in situ Raman spectroscopy,a tentative mechanism is given concerning the interfacial phase transition and the disproportional reaction.