Huanfeng Zhu

Electrochemical Oxidation of Hydroxylamine on Gold in Aqueous Acidic Electrolytes: An in Situ SERS Investigation

The electrooxidation of hydroxylamine (HAM) on roughened Au electrodes has been examined in aqueous buffered electrolytes (pH 3) using in situ surface-enhanced Raman scattering (SERS). Two distinct spectral features were observed at potentials, E, within the range in which HAM oxidation was found to ensue, centered at 803 cm(-1) for 0.55 < E < 0.8 V and at 826 cm(-1) for 1.0 < E < 1.40 V versus SCE, attributed, respectively, to adsorbed nitrite and adsorbed NO(2). Similar experiments performed in solutions containing nitrite instead of HAM under otherwise identical conditions displayed only the peak ascribed to adsorbed nitrite over the range of 0.1 < E < 0.8 V versus SCE with no additional features at higher potentials. These observations strongly suggest that under the conditions selected for these studies the oxidation of HAM on Au proceeds at least in part through a pathway that does not involve nitrite as a solution-phase intermediate.

Quantitative Aspects of Ohmic Microscopy

The potential difference between two microreference electrodes, Δφ(sol), immersed in an aqueous sulfuric acid solution was monitored while performing conventional cyclic voltammetric experiments with a Pt disk electrode embedded in an insulating surface in an axisymmetric cell configuration. The resulting Δφ(sol) vs E curves, where E is the potential applied to the Pt disk electrode were remarkably similar to the voltammograms regardless of the position of the microreference probes. Most importantly, the actual values of Δφ(sol) were in very good agreement with those predicted by the primary current distribution using Newmans formalism (Newman, J. J. Electrochem. Soc. 1966, 113, 501-502). These findings afford a solid basis for the development of ohmic microscopy as a quantitative tool for obtaining spatially resolved images of electrodes displaying nonhomogenous surfaces.

Theoretical Aspects of Light-Activated Microelectrodes in Redox Electrolytes

The behavior of semiconductor-based, light-activated microelectrodes in redox electrolytes has been examined theoretically using commercial software to self-consistently solve the transport equations for solid-state and solution-phase species and the electrostatic potential within the semiconductor phase, subject to the appropriate boundary conditions under steady state. The lightlimited currents for such spatially localized microelectrodes, observed for a high voltage bias, bias, under normal irradiation and a strict axisymmetric geometry, were proportional to the photon flux intensity. The results of these simulations afforded strong evidence that under high bias, holes generated by the light on an n-type semiconductor escape beyond the edge of the illuminated disk, leading to a net increase in the predicted current and thus in the effective area of the light-activated microelectrode.

Theoretical Aspects of Light Activated Semiconductor Microelectrodes: A Generalized Analysis

Theoretical aspects of light activated semiconductor (SC) microdisk electrodes in redox electrolytes have been examined as a function of the dimensionless photon flux σ and dimensionless bias potential Φ bias · Dimensionless steady-state profiles for solid-state and solution phase species were obtained by solving self-consistently the transport equations and the electrostatic potential within the SC subject to the appropriate boundary conditions using COMSOL. Analyses of the results obtained revealed that for fixed σ and small Φ bias , the local dimensionless flux at the interface normal to the SC surface, J * Z (R,0), where R is the dimensionless radius normal to the axis of symmetry, is dominated by the oxidation process in the illuminated region (R ≤ 1) and by the reduction process in the dark area near the edge of the illuminated region. For large Φ bias , however, the oxidation process dominates J * Z (R,0) everywhere along the interface. Agreeing with the phenomenon described in our earlier publication, the holes escape beyond R = 1, yielding, for very large values of Φ bias and σ, a total current flowing through the dark area that can exceed that collected within the illuminated disk.