Thickness-Dependent Electrochemical and Spectroelectrochemical Properties of ta-C:N Thin Films on Nonconductive, Flexible, and Transparent Substrates

Abstract This work presents the first comprehensive physical, electrochemical, and spectroelectrochemical characterization of robust and chemically inert nitrogen-doped tetrahedral amorphous carbon (ta-C:N) films deposited atop nonconductive, flexible polymer and transparent glass substrates. Using filtered laser-arc physical vapor deposition, ta-C:N films of varying thickness were deposited onto polyether ether ketone (PEEK), polyester (PE), and glass. To overcome the inherent electrical resistance of the substrate materials, chromium (Cr) and indium tin oxide (ITO) interlayers were introduced at the film-substrate interface. The surface topography and carbon bonding structure of each film were assessed by numerous techniques. Films lacking an interlayer showed significantly reduced conductivity, confirming the necessity of a conductive interlayer to facilitate electron transport on nonconductive substrates. Physical characterization revealed thickness-dependent sp2/sp3 bonding structures and surface uniformity highlighting the tunability of ta-C films. The model redox systems of Ru(NH3)63+/2+ (surface-insensitive) and Fe(CN)63–/4– (surface-sensitive) were employed to provide a holistic view of the electrochemical properties of each film. All films exhibited reversible or quasi-reversible electrochemical behavior with heterogeneous electron transfer rate constants (ko′) comparable to those reported previously on conductive substrates (Ti and Si). Notably, thinner films displayed higher uncompensated resistance (Ru) while maintaining reversible to quasi-reversible kinetics, indicating that conductivity may be governed by bulk film properties rather than the interlayer-film interface. We report the first use of optically transparent ta-C:N electrodes for spectroelectrochemical evaluation. These films deposited atop ITO coated glass displayed comparable redox parameters determined from spectroelectrochemistry to those determined electrochemically. Overall, this work demonstrates that ta-C:N can be adapted for use on nonconductive, flexible, and/or transparent substrates with proper interface engineering, expanding the applications for ta-C electrodes in spectroelectrochemical and electroanalytical chemistry.