Theoretical Aspects of Light Activated Semiconductor Microelectrodes: A Generalized Analysis

Abstract

Theoretical aspects of light activated semiconductor ͑SC͒ microdisk electrodes in redox electrolytes have been examined as a function of the dimensionless photon flux and dimensionless bias potential ⌽ 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 and small ⌽ bias , the local dimensionless flux at the interface normal to the SC surface, J Z ‫ء‬ ͑R,0͒, where R is the dimensionless radius normal to the axis of symmetry, is dominated by the oxidation process in the illuminated region ͑R Յ 1͒ and by the reduction process in the dark area near the edge of the illuminated region. For large ⌽ bias , however, the oxidation process dominates J Z ‫ء‬ ͑R,0͒ 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 ⌽ bias and , a total current flowing through the dark area that can exceed that collected within the illuminated disk.Although the vast majority of studies involving semiconductor ͑SC͒ electrochemistry involve homogeneous illumination of electrode surfaces, 1,2 a few techniques have been described in the literature in which a focused light beam is used to irradiate either statically or dynamically a small section of the SC surface while monitoring, for example, the photogenerated current. 3-6 This tactic makes it possible to create two-dimensional ͑2D͒ images that afford information regarding the nature and spatial distribution of materials, such as films on a given substrate, 7 or induce local reactions, 5 as in photoetching, to produce specific patterns. 8 A unique illustration of this principle has been described by Miller and Rosamilia, 4 who employed a focused laser to illuminate at normal incidence a circular area of a rotating n-type SC disk electrode polarized at a high positive bias in an electrolyte solution containing the reduced form of redox couple ͑red͒. Using these conditions, the holes generated would migrate to the SC surface and react with red to produce a corresponding solution phase oxidized species ͑ox͒, which could then be detected by a concentric metallic ring electrode polarized at a judiciously set potential. 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 beyo...

Metal‐Insulator‐Semiconductor Anodes for Ultrastable and Site‐Selective Upconversion Photoinduced Electrochemiluminescence

“…Due to its indirect band gap and its high charge transport capabilities, photogenerated holes have a long lifetime and a diffusion length. These results are in good agreement with the decay of the PECL profile (Figure S10), the calculation of hole diffusion length (Section 4 in Supporting Information), and previous literature on light‐addressable electrochemistry [4, 61–63] …”

“…These results are in good agreement with the decay of the PECL profile (Figure S10), the calculation of hole diffusion length (Section 4 in Supporting Information), and previous literature on light-addressable electrochemistry. [4,[61][62][63]…”