Photoactivated Electrolysis on Nonporous Chlorogallium Phthalocyanine Thin Film Electrodes

Various metal and semiconductor substrates demonstrated greatly enhanced activity toward the electrolysis of the benzoquinone (Q) hydroquinone (H₂Q) redox couple when coated with multimolecular layers of chlorogallium phthalocyanine (GaPc-Cl) and irradiated with visible wavelength light. These electrodes were nearly reversible toward H₂Q in the light, but were quite inert in the dark, even suppressing corrosion of the metal when silver substrates were used. Exchange current densities on GaPc-Cl/Au electrodes irradiated with approximately 100 mW/cm² polychromatic light were ca. 10³ times greater than on plain Au. The improved voltammetric response of these electrodes over other Pc film electrodes are attributable in part to the sublimation conditions employed, distinguished primarily by a deposition rate of tens of angstroms per minute or less. Varying degrees of film porosity were obtained as evidenced by cyclic voltammetry and x-ray photoelectron spectroscopy. The role of GaPc-Cl in activating the substrate surface is briefly discussed, contrasting the behavior of GaPc-Cl/Au electrodes to plain Au, which shows two distinct rates of Q/H₂Q electrolysis, depending on electrochemical pretreatment.