Graphite photoelectrochemistry: 3. Photoelectrochemical oxidation of surface-confined hydroquinones at highly oriented pyrolytic graphite basal plane electrodes

Photocurrent measurements at irradiated highly oriented pyrolytic graphite basal plane electrodes with adsorbed quinone/hydroquinone (benzoquinone, 1,4-naphthoquinone, and 9,10-phenantherenequinone) couples are reported. The photocurrent was generated by modulated UV-vis light. The potential dependence of the photocurrent, the photocurrent light action spectra, and photocurrent dependence on light modulation frequency are reported. The peak of the anodic photocurrent curve was found close to the formal potential of the adsorbed redox couple. The peak is ascribed to the action of two opposite processes: (1) oxidation of adsorbed hydroquinones by hot holes that are photogenerated in the graphite; (2) relaxation of the adsorbed redox couple to equilibrium by a dark reduction process. A model describing the dependence of the photocurrent magnitude and phase angle on the light modulation frequency as a function of parameters of the dark reduction process is presented. Rate constants calculated by the proposed model for the dark process are consistent with reported values.