Tuning the Electronic Valence Structure of Amorphous Carbon Surfaces : Effects on the Kinetics of Electrochemical Phenomena at the Carbon/Solution Interface

The understanding of surface behavior of carbon thin films is, at present, attracting great interest due to its multiple applications dealing with electronics, delivery systems and biomedicine. Merging electrochemical and spectroscopic techniques we investigated the kinetics of charge transfer reactions at the carbon/solution interface and the valence electronic properties of bare carbon surfaces. In this work we aimed to understand how the electronic properties of carbon influence the interfacial charge transfer behavior in order to provide a rational approach towards accurate control of surface chemical reactions and interfacial processes that underpin many of the applications of carbon materials. Amorphous carbon thin films have been prepared via DC magnetron sputtering. Surfaces were then annealed at different temperatures in order to modulate their graphitic content. The charge transfer process involving reversible redox couples (Ru(NH3)6+3/+2, IrCl6+4/+3 and Fe(CN)6-3/-4) was studied by electrochemical impedance spectroscopy (EIS). A modified Randles circuit gave us the best fit of our EIS spectra in order to interpret processes occurring at the electrochemical interface. From the fitted spectra, values of charge transfer resistance were obtained and then used to quantify the standard heterogeneous rate constant for each redox couple. Annealed samples show faster charge transfer kinetics than as-deposited samples; moreover we found that the higher the annealing temperature, the faster the kinetics of charge transfer. Ultraviolet Photoelectron Spectroscopy (UPS) was performed in order to measure the carbon work functions and the photoemission intensity near the Fermi energy. We observed a decrease in surface work function and an increase in photoemission intensity upon annealing suggesting that annealed samples behave as better electron donors than as-deposited ones. Finally, we discuss our preliminary attempts at interpreting these results in the context of existing models of charge transfer at the solid/liquid interface.