Surface states mediated charge transfer in redox behavior of hemin at GaAs(100) electrodes

EIS and XPS investigations on the interaction of hemin with p- and n-doped GaAs(100) electrodes in PBS solution revealed significant differences concerning both the adsorbed species and the mechanism of the redox process caused by dopant nature. XPS data show that hemin is adsorbed on p-GaAs(100) by its carboxyl groups and adopts a vertical position favorable to a polymeric film formation whereas on n-GaAs(100), the adsorbed hemin is monomeric and has a rather planar configuration involving mainly the OH groups of the organic molecule. Hemin gives rise to a reversible redox process at the p-GaAs(100) electrode whereas at n-GaAs(100), there is only one reduction wave of a considerably lower current density appearing at a more negative potential. The effects of the applied potential on the phase angle measured at p-GaAs(100) point out major changes not only in the insulating properties of the adsorbed layer, as found at n-GaAs(100), but also in the electronic properties of the semiconductor triggered by the hemin redox process. Analysis of the experimental data points to a mechanism of charge transfer through surface states, the observed differences being related to the location of the surface states with respect to the formal potential of the hemin redox couple.