Combined Optoelectronic and Electrochemical Study of Nitrogenated Carbon Electrodes

Modification of carbon materials via incorporation of nitrogen has received much attention in recent years due to their performance as electrodes in applications ranging from electroanalysis to electrocatalysis for energy storage technologies. In this work we synthesized nitrogen-incorporated amorphous carbon thin film electrodes (a-C:N) with different degrees of nitrogenation via magnetron sputtering. Electrodes were characterized using a combination of spectroscopic and electrochemical methods, including X-ray photoelectron spectroscopy, ellipsometry, voltammetry, and impedance spectroscopy. Results indicate that low levels of nitrogenation yield carbon materials with narrow optical gaps and semimetallic character. These materials displayed fast electron-transfer kinetics to hexammine ruthenium(II)/(III), an outer-sphere redox couple that is sensitive to electronic properties near the Fermi level in the electrode material. Increasing levels of nitrogenation first decrease the metallic character of the electrodes, increase the impedance to charge transfer and, ultimately, yield materials with optical and electrochemical properties consistent with disordered cluster aggregates rather than amorphous solids. A positive correlation was found between the resistance to charge transfer and the optical gap when using the outer-sphere redox couple. Interestingly, the use of ferrocyanide as a surface-sensitive redox probe resulted in a monotonic increase of the impedance to charge transfer vs nitrogen content. This result suggests that surface chemical effects can dominate the electrochemical response, even when nitrogenation results in enhanced metallic character in carbon electrodes.