Theoretical Aspects of Light-Activated Microelectrodes in Redox Electrolytes

Abstract

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. © 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3257616͔ All rights reserved.Manuscript submitted August 19, 2009; revised manuscript received October 12, 2009. Published November 16, 2009 Miller and Rosamilia 1 described the use of a focused laser impinging at normal incidence on the surface of a semiconductor electrode immersed in an electrolyte containing a redox species to generate spatially localized currents with several possible applications. 2The implementation of this novel concept employing n-InP and n-GaAs disks in solutions of ferrocene ͑Fc͒ in well-supported acetonitrile electrolytes at high positive potentials, bias , yielded a behavior closely resembling that of the conventional metallic microelectrodes. 2,3 This article seeks to theoretically examine certain aspects of such light-activated microelectrodes. A particular concern is to establish, under steady-state conditions and large bias , the influence of hole transport within the semiconductor beyond the illuminated area on the measured current, explicitly accounting for interfacial redox processes and mass transport of redox species in the solution phase. Theoretical AspectsThe system to be analyzed in this work involves an n-type InP semiconductor disk immersed in a Fc solution in a well-supported ͑1 M tetrabutylammonium tetrafluoroborate͒ acetonitrile electrolyte under conditions similar to those reported by Miller and Rosamilia. 1More specifically, we seek steady-state solutions for the transport equations for electrons ͑n͒ and holes ͑p͒, including both generation and recombination of n-p pairs, 5,6 self-consistently coupled to Poisson's equation for the electrostatic potential within the semiconductor, , and to the transport equations for the donor ͓red ͑reduction͔͒ and the acceptor ͓ox ͑oxidation͔͒ species in a quiescent solution phase subject to the appropriate boundary conditions. The latter includes second-order kinetics 7,8 to account for the interfacial reactions between p ͑in the valence band͒ and red and between n ͑in the conduction band͒ and ox, which, for the parameters selected for this study, may be regarded as the dominant contributions ...

Theoretical Aspects of Light Activated Semiconductor Microelectrodes: A Generalized Analysis

“…Journal of The Electrochemical Society, 157 ͑10͒ F137-F143 ͑2010͒ F140 stant͒, the ability of holes to escape from the illuminated region also increases. As found in a previous work, 9 the total dimensionless current I dim from these high bias conditions is proportional to or, equivalently, to the photon flux.…”

“…As noted in our previous paper, 9 a measure of the contribution the illuminated region makes to the total current may be shown to be given in dimensionless form by…”

“…The results obtained in experiments in which the laser beam was scanned from the center toward the edge of the SC disk electrode were consistent with those expected for a rotating ring-ring electrode configuration. Very recently, Zhu et al 9 examined certain aspects of a closely related system in which the focused beam was aimed at normal incidence toward the surface of a larger static SC disk surface immersed in a quiescent solution containing a redox couple under steady-state conditions. It was concluded from such studies that, depending on the voltage bias, holes created in the illuminated area can escape beyond the edge of that region and react at the surface with solution phase species present therein and, thereby, increase the effective area of the illuminated region.…”

“…It was concluded from such studies that, depending on the voltage bias, holes created in the illuminated area can escape beyond the edge of that region and react at the surface with solution phase species present therein and, thereby, increase the effective area of the illuminated region. 9 This contribution provides a more detailed theoretical analysis of this system in terms of dimensionless variables and characteristic parameters from which insight has been gained into the way in which experimentally controlled variables affect the local photoinduced currents. A similar theoretical approach was employed by Mannheim et al 10 for the treatment of certain aspects of photoetching in a 2D geometry.…”

Light-activated electrochemistry on alkyne-terminated Si(100) surfaces towards solution-based redox probes

“…This increase in electron transfer kinetics was attributed to a greater number of carriers reaching the depletion layer [16]. Furthermore, Scherson and co-workers [30] have shown that with greater light intensity, when irradiated from the backside, the larger the apparent electroactive area on the electrolyte side of the semi-conductor electrode. The increase in current with light intensity as depicted in Figure 4b is consistent with work by Scherson and co-workers [30].…”