Combined Optoelectronic and Electrochemical Study of Nitrogenated Carbon Electrodes

Abstract

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.

Capacitive storage at nitrogen doped amorphous carbon electrodes: structural and chemical effects of nitrogen incorporation

“…In the absence of an experimental determination of the microroughness factor for each electrode, it is not possible to obtain estimates of the density of states from values of C pzc , as previously done by other groups in the case of nanocarbon electrodes. 12,54 However, it is interesting to note that C pzc values are strongly correlated to the resistance to charge transfer (R ct ) obtained in our previous work 14 Ag/Ag + ). As the R ct had been found to be inversely correlated to the density of states near the Fermi energy in these materials, this observation supports that the trends observed in Fig.…”

“…All a-C:N surfaces have been shown to possess a mixture of pyridinic-N, pyrrolic-N and graphitic-N, and they were found to be smooth and conformal to the substrate surface. 14 Raman spectroscopy was used to characterise the structural properties of the amorphous carbon phase. [22][23][24] Fig.…”

“…This appears possible when considering the structurally disruptive effect of nitrogenation at high concentrations on the graphitic matrix. 14,26 Beyond the development of a pore structure, nitrogenation can also introduce local surface states due to structural disorder in the carbon scaffold which are known to result in increased capacitance at low frequency. 46 Finally, the presence of N-containing functional groups at the a-C:N surface in a protic solvent can promote specic adsorption 50 and redox reactions at these sites (e.g.…”

“…Amorphous carbon lms were obtained via DC-magnetron sputtering (Torr International) from a graphite target (99.999%) at base pressures <2 Â 10 À6 mbar, deposition pressures 2-7 Â 10 À3 mbar and total gas ow of 50 mL min À1 , following previously reported protocols. 14,15 Briey, nitrogenated amorphous carbon lms (a-C:N) were obtained by using N 2 /Ar gas at ow ratios of 2%, 5% and 10% while keeping deposition time constant at 40 min, whereas non-nitrogenated amorphous carbon (a-C) was obtained by carrying out the deposition using 100% Ar during deposition. Electrodes thus deposited are topographically smooth; the mean thickness was determined via spectroscopic ellipsometry, following methods detailed in our previous work, 14,16,17 to be 74, 83, 114 and 123 nm for a-C, a-C:N-2%, 5% and 10%, respectively (see ESI †).…”

“…Spectra were collected at either open circuit potential (OCP) or at varying DC offsets in 0.2 V steps as indicated in the text; 300 s equilibration time was allowed between each potential step. The specic capacitance was obtained via normalisation by the geometric area of the electrodes; this was determined in aqueous solution via a Randles-Sevcik plot, 14,20 and in 0.1 M TBAPF 6 /MeCN via capacitance measurements on a reference GC disk of known area to account for any capillary wetting within the Teon shielding in MeCN. 21…”