Barry Miller

Electrochemical Behavior of Boron‐Doped Diamond Electrodes

Boron-doped diamond electrodes have many possibilities for electroanalytical and electrosynthetic application because of their stability and range. However, their combination of semiconductive origin, nearly metallic resistive characteristics when heavily doped, and surface inertness make the understanding of electron transfer at these materials complex. The transition from reversible behavior, with a wide range of outer sphere redox couples with potentials positive of about -0.5 V vs. SCE, to extremely slow kinetics, with systems such as the halogen/halide couples, requires placing all the above properties in context. Experimental data pertaining to these issues and speculation concerning the controlling factors are discussed.

Dopamine and Ascorbate Analysis at Hydrodynamic Electrodes of Boron Doped Diamond and Nitrogen Incorporated Tetrahedral Amorphous Carbon

Rotating and hydrodynamically modulated rotating disk electrodes of boron-doped diamond (BDD) and tetrahedral amorphous nitrogen incorporated carbon (taC : N) were employed to extend analytical sensitivity and linear current response of these materials for dopamine analysis at physiological pH to 200 nM. Hydrodynamically modulated rotating disk electrode (HMRDE) techniques discriminate against nonmass transfer limited components, allowing more complete implementation of the wider electrochemical window of these materials. Experiments performed in the presence of ascorbate showed typical catalytic enhancement of the dopamine oxidation current and a corresponding loss of response to dopamine quinone at the ring in a rotating ring-disk electrode (RRDE) configuration. Utilizing these phenomena, a RRDE method for simultaneous determination of both dopamine and ascorbate concentrations in solutions containing comparable levels of both species is presented.

Rotating ring–disk electrode studies of the oxidation of p-methoxyphenol and hydroquinone at boron-doped diamond electrodes

The stepwise electrochemical oxidation of p-methoxyphenol and hydroquinone at boron-doped diamond (BDD) electrodes was investigated in perchloric acid media using rotating ring–disk electrode (RRDE) and hydrodynamic modulation techniques. Initial oxidation of these compounds to p-benzoquinone at the disk may be confirmed by reduction of the quinone at the ring electrode. Further oxidation, ultimately to CO2, can be followed by the loss of redox active products at the ring. Comparison of BDD anodic activity to those of graphite, gold, and platinum electrodes, utilizing RRDE configurations, shows that the high oxygen evolution overpotential exhibited by BDD allows for more efficient anodic conversion of p-methoxyphenol and hydroquinone to products beyond the quinone stage. BDD, unlike the other electrode materials, showed no corrosion or fouling when used under these conditions. These results complement product analysis studies by other groups on BDD for organic contaminant remediation by exhaustive electrolysis.

Electrodeposition and Nucleation of Copper at Nitrogen-Incorporated Tetrahedral Amorphous Carbon Electrodes in Basic Ambient Temperature Chloroaluminate Melts

The electrodeposition of copper on the atomically smooth nitrogen-incorporated tetrahedral amorphous carbon (taC:N) electrode has been studied in basic ambient temperature AlCl 3 /1-ethyl-3-methylimidazolium chloroaluminate melts. A high overpotential for nucleation of copper on taC:N and no underpotential deposition features are observed, comparable to the behavior of boron-doped diamond electrodes. Electrochemical deposition and stripping of copper on taC:N show that most of the deposit is anodically dissolved only when the potential reaches that of Cu(I) oxidation in a system in which Cu(I) and Cu(II) are both stable. The low density of intrinsic active sites for nucleation and its early saturation with increasing overpotential are responsible for the slight deviation from a model of the ideal progressive type of nucleation at high overpotentials.

Cathodic and Anodic Deposition of Mercury and Silver at Boron‐Doped Diamond Electrodes

The mechanisms of silver and mercury metal deposition on boron-doped diamond electrodes were investigated using a prior model of three-dimensional diffusion controlled nucleation/growth to interpret the data. The mechanism of nucleation depends upon the nucleation overpotential and metal ion concentration. At low overpotentials, instantaneous nucleation can be observed and, at high overpotentials or concentrations, the process becomes progressive. No underpotential phenomena are observed, consistent with exceptionally weak interaction between the deposited metal and the diamond surface. Active sites corresponding to a low density of surface sites on the electrode which act as critical nuclei during the electrodeposition. Anodic reactions of silver were also investigated. It was found that the resulting product depends on the silver concentration in the solution. Soluble Ag(II) forms only at low concentration (<0.5 mM Ag + ). Increasing concentration leads to formation of sparingly soluble oxysalts of formula Ag 7 O 8 X, where X is NO - 3 or CIO - 4 , with voltammetric characteristics and adhesion similar to these found for cathodic metal deposits.

Copper Electrodeposition and Dissolution on Tetrahedral Amorphous Carbon Incorporating Nitrogen

Metal deposition and dissolution on diamond and diamond-like materials have different characteristics than on metallic electrodes and typical graphitic carbons. The behavior of copper on the recently developed tetrahedral amorphous carbon incorporating nitrogen (taC:N) film materials is addressed with quantitative speciation and voltammetry through rotating ring-disk electrode techniques. The nearly atomically smooth and widely stable taC:N film on polished Si wafers shows high nucleation overpotentials for copper deposition followed by anodic dissolution requiring participation of both the stable Cu(I) and Cu(II) states present in chloride media. The adhesion of copper films plated from conventional sulfate baths is strongly dependent on transport conditions. The anodic characteristics of the deposits reflect both the difficult nucleation process and the relative inertness of the taC:N interface.

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.