The Electronic Valence Structure Of Amorphous Carbon Thin Films And Its Effects On The Kinetics Of Interfacial Charge Transfer

Much interest is nowadays focused on the development of carbon thin films, which are currently under study for many applications, such as biomedical coatings, delivery systems and electronic components. In this work we investigated the valence electronic properties of bare carbon surfaces and the kinetics of charge transfer processes at the carbon/solution interface using a combination of spectroscopic and electrochemical methods. The aim of this study was to understand how the carbon electronic properties affect the interfacial charge transfer behavior in order to provide a rational approach to the accurate control of surface chemical reactions and interfacial processes that underpin many of the applications of carbon materials. Amorphous carbons were prepared as thin films via DC magnetron sputtering. In order to modulate their graphitic content, surfaces were annealed at different temperatures. Electrochemical impedance spectroscopy (EIS) was performed in solutions containing outer-sphere, reversible redox couples (e.g. Ru(NH3)6+3/+2, IrCl6+4/+3). Processes occurring at the electrochemical interface were interpreted by means of a modified Randles circuit, which was found to give the best fit of our EIS spectra. The standard heterogeneous rate constant was calculated for each redox couple from values of charge transfer resistance obtained from fitted spectra. Annealed samples show faster charge transfer kinetics than as-deposited samples: the heterogeneous rate constant of as deposited carbons was found to be several orders of magnitude smaller than that of annealed carbons, whereas comparable values were obtained for samples annealed at various temperatures. Ultraviolet Photoelectron Spectroscopy (UPS) was carried out 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.