Characterization and Electrochemical Properties of Oxygenated Amorphous Carbon (a-C) Films

Amorphous carbon (a-C) films with varying oxygen content were deposited by closed-field unbalanced magnetron sputtering with the aim to understand the effect of oxygen on the structural and physical properties of the films and subsequently correlate these changes with electrochemical properties. The a-C films were characterized by transmission electron microscopy, helium-ion microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and time-of-flight elastic recoil detection analysis. The electrochemical properties were studied by electrochemical impedance spectroscopy and cyclic voltammetry with several redox systems (Ru(NH₃)₆^2+/3+, Fe(CN)₆^4−/3−, dopamine and ascorbic acid). The results indicated that the carbon films are amorphous with an I_D/I_G ratio near 2.6. The oxygen content of the films seemed to saturate at around 11 at. %, whereas the amount of surface oxygen functional groups increased steadily with increasing oxygen inflow during deposition. O/C ratio increased from 0.09 to 0.19. A significant increase in film resistivity was observed with increasing oxygen content. Lightly oxygenated a-C films showed a low charge transfer resistance (R_ct) and reversible electron transfer for Ru(NH₃)₆^2+/3+ whereas both R_ct and ΔE_p increased considerably for heavily oxygenated films. The inner sphere redox systems were significantly affected by the surface oxygen functional groups with dopamine and ascorbic acid showing a linear increase in ΔE_p and E_pa, respectively, with increasing oxygen content. Fe(CN)₆^4−/3− did not show a clear trend but was still clearly affected by the increase in oxygen content. The double layer capacitance was about 1 μF/cm² for all the oxygenated a-C films.