Electron transport determines the electrochemical properties of tetrahedral amorphous carbon (ta-C) thin films

Abstract

Amorphous carbon based electrodes are very promising for electrochemical sensing applications. In order to better understand their structure-function relationship, the effect of film thickness on the electrochemical properties of tetrahedral amorphous carbon (ta-C) electrodes was investigated. ta-C thin films of 7, 15, 30, 50 and 100nm were characterized in detail with Raman spectroscopy, transmission electron microscopy (TEM), conductive atomic force microscopy (c-AFM), scanning tunneling spectroscopy (STS) and X-ray absorption spectroscopy (XAS) to assess (i) the surface properties of the films, (ii) the effect of film thickness on their structure and electrical properties and (iii) the subsequent correlation with their electrochemistry. The electrochemical properties were investigated by cyclic voltammetry (CV) using two different outer-sphere redox probes, Ru(NH3)63+/2+ and FcMeOH, and by electrochemical impedance spectroscopy (EIS). Computational simulations using density functional theory (DFT) were carried out to rationalize the experimental findings. The characterization results showed that the sp2/sp3 ratio increased with decreasing ta-C film thickness. This correlated with a decrease in mobility gap value and an increase in the average current through the films, which was also consistent with the computational results. XAS indicated that the surface of the ta-C films was always identical and composed of a sp2-rich layer. The CV measurements indicated reversible reaction kinetics for both outer-sphere redox probes at 7 and 15nm ta-C films with a change to quasi-reversible behavior at a thickness of around 30nm. The charge transfer resistance, obtained from EIS measurements, decreased with decreasing film thickness in accordance with the CV results. Based on the characterization and electrochemical results, we conclude that the reaction kinetics in the case of outer-sphere redox systems is determined mainly by the electron transport through the films and not the electron transfer between the redox probe and the electrode surface, since the surface region is expected to be identical in all the ta-C films. Finally, it was shown that a titanium interlayer between the Si substrate and ta-C significantly affected the electrical and electrochemical properties of the electrodes, further emphasizing the role of electron transport through the film in determining the electrochemical behavior of ta-C.