Photoelectrochemical Oxidation of Aromatic Hydrocarbons and Decamethyl Ferrocene at the n ‐ GaAs / Room Temperature Molten Salt Electrolyte Interface

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Photoelectrochemical (PEC) oxidation of decamethyl ferrocene , benzo (a) pyrene (BP), 9, 10‐diphenylanthracene (DPA), anthracene (A), and hexamethyl benzene (HMB) on electrodes in contact with the aluminum chloride‐n‐butyl pyridinium chloride molten salt electrolyte, was studied by cyclic and linear sweep voltammetry. The electrochemical behavior of the above compounds was first characterized by cyclic voltametry on carbon. DPA and exhibited quasi‐reversible charge transfer behavior, the other compounds exhibited varying degrees of departure from reversible behavior. Based on the variation of peak potentials and peak separation with scan rate, the charge transfer kinetics were ordered thus: . Slight changes in the composition and an examination of the corresponding variations in the current‐voltage characteristics on illuminated electrodes, permitted mapping of the bandgap energy levels responsible for mediating charge transfer. One set of such states seemed to be located at an energy ∼0.6 eV below the conduction bandedge in . These states were interpreted to arise from specific adsorption of Cl− ions from the electrolyte. Photooxidations of , BP, DPA, A, and HMB were all sustained on at potentials well below the equilibrium (thermodynamic) values on carbon. In the case of DPA and A, evidence from cyclic voltammetry indicated photooxidation to the dication, probably caused by injection of an electron from the cation radical into the conduction band in . The location of the reduction waves on in the cyclic voltammograms for BP, DPA, and A further indicated a second set of surface states situated very close to the valence bandedge. Linear sweep voltammograms on in the dark and under illumination showed the usual rectification behavior associated with n‐type semiconductor/electrolyte junctions. A two‐ to three‐fold increase in the photocurrent was observed when the electrolyte was mixed with toluene (50 v/v). Output parameters obtained from linear sweep voltammetry are presented for PEC cells utilizing BP, DPA, A, and HMB as redox species.