Scanning Probe Electrochemical Multiple-Parameter Analyses And Preliminary Researches At Electrodes Of Quartz Crystal Resonators

As an in-situ electrochemical testing method, the electrochemical quartz crystal impedance (EQCI) system can be used to measure changes in electrode mass, solution viscosity and density, interfacial dielectric properties by monitoring quartz electroacoustic impedance spectroscopy, electrochemical impedance spectroscopy etc. It has advantages of convenient operation, high sensitivity, capability of providing multiple in-situ parameters and facile automatization. Because of its electrochemical sensibility, the scanning electrochemical microscope (SECM) as one member of the scanning probe microscope (SPM) family has been widely used in local electroanalysis and interrelated studies, including imaging (conducting or insulating substrate), local electrodepositing and etching, electrochemical and homogeneous reactions, thin film characterization, liquid/liquid interfaces and so on. At present, however, the SECM substrate is motionless and characterized mainly by the electrochemical methods. If the substrate is replaced with a high-frequency-oscillating quartz crystal wafer and characterized by the electroacoustic impedance of the PQC resonance, the PQC surface and/or its localized active region can thus be simultaneously investigated by the electrochemical quartz crystal impedance system, which will provide a wealth of real-time information simultaneously from the SECM and EQCI methods.The studies in this thesis are summarized as follows.1. The recent researches using piezoelectric quartz crystal microbalance and scanning electrochemical microscopy are briefly reviewed.2. The combination of the scanning electrochemical microscopy (SECM)as a novel multi-parameter method for investigating cyclic voltammetric growth of poly(o-phenylenediamine) (PoPD) thin films at Au electrodes in aqueous solutions of various pH values, and the potentiostatic micro-etching (localized degradation) of these films in 0.10 mol L"1 aqueous H2SO4 for comparative examinations on polymer porosity and stability. Two potential-sweep ranges, -0.4 to 0.9 (I) and 0 to 0.9 (II) V vs SCE, and four solutions, acidic (A: 0.20 mol L"1 H2SO4 + 0.10 mol L'1 Na2SO4; B: 0.10 mol L"1 H2SO4 + 0.20 mol L"1 Na2SO4), neutral (C: 0.10 mol I/1 PBS + 0.20 mol L"1 Na2SO4, pH 7.2) and alkaline (D: 0.20 mol L"1 NaOH + 0.20 mol L"1 Na2SO4) aqueous solutions, were selected for PoPD growth. The pH increase for the polymerization solution increased the molar percentage of polyaniline-like chains in PoPD, as quantified from the current peaks at ca. 0.6 V vs SCE for oxidation of -NH2 groups in and the film mass (EQCM measurements) of as-prepared PoPD (grown from solutions C and D) during their redox switching in 0.10 mol L"1 aqueous H2SO4 for the first time. The unusual PQC impedance responses observed at negative potentials (potential range I) in the first several potential cycles during cyclic voltammetric growth of PoPD in acidic and neutral solutions have been reasonably explained as due to the precipitation/dissolution of the poorly soluble phenazinehydrine charge-transfer complexes developed during redox switching of oligomers for the first time, which brought about much less compact PoPD films and their higher degradability than those grown in the same solution but over potential range II. SECM, scanning electron microscopy (SEM) and PQC frequency were used to estimate the sizesof etched micro-scale spots. In addition, the x-, y- or z-axis movement of a Pt microelectrode of 25-um diameter near the PQC electrode was found to influence negligibly the PQCI responses in 1.0 mol L' aqueous Na2SO4 containing K4Fe(CN)6 up to 0.10 mol I/1, and a new protocol of dynamically electrodepositing silver microwires via chemical-lens method was proposed for examining the local mass-sensitivity distribution on the PQC surface.3. The combination of reflectance UV-Vis spectroelectrochemistry with electrochemical quartz crystal microbalance (EQCM) and separate reflectance FTIR characterization were used to investigate the structural interconversion for poly(o-phenylenediamine) (PoPD) between its ladder structure with phenazine units and polyaniline-like linear chains. The poly(o-phenylenediamine) films were potentiostatically (0.8 V vs SCE) grown on Au electrodes from 0.20 mol L'1 H2SO4 (PoPD,) or 0.40 mol L'1 NaOH (PoPD2) aqueous solution containing 0.20 mol L"1 Na2SO4 + 0.10 mol L"1 ophenylenediamine. By considering the mass of deposited PoPD2 film obtained from the EQCM data and the charge consumed under the current peak at ca. 0.6 V vs SCE for oxidation of -NH2 groups in as-prepared PoPD2 during potential cycling in 0.10 mol L'1 aqueous H2SO4, the molar percentage of the polyaniline-like chains was estimated to be 19% (relative to total phenylenediamine units), being in agreement with the result obtained from a formaldehyde-combination experiment through the aminocarbonyl reaction. After 40-cycle potential sweeps between 0.2 and 0.8 V vs SCE the polyaniline-like chains in PoPD2 could be completely converted via intramolecular cyclization into the ladder structure with phenazine units. However, P0PD1 was found to be perfectly composed of the ladder structure...