Electrochemical Performance of Diamond Thin-Film Electrodes from Different Commercial Sources

The electrochemical properties of two commercial (Condias, Sumitomo) boron-doped diamond thin-film electrodes were compared with those of two types of boron-doped diamond thin film deposited in our laboratory (microcrystalline, nanocrystalline). Scanning electron microscopy and Raman spectroscopy were used to characterize the electrode morphology and microstructure, respectively. Cyclic voltammetry was used to study the electrochemical response, with five different redox systems serving as probes (Fe(CN)<sub>6</sub><sup>3-/4-</sup>, Ru(NH<sub>3</sub>)<sub>6</sub><sup>3+/2+</sup>, IrCl<sub>6</sub><sup>2-/3-</sup>, 4-methylcatechol, Fe<sup>3+/2+</sup>). The response for the different systems was quite reproducibile from electrode type to type and from film to film for electrodes of the same type. For all five redox systems, the forward reaction peak current varied linearly with the scan rate<sup>1/2</sup> (ν), indicative of electrode reaction kinetics controlled by mass transport (semi-infinite linear diffusion) of the reactant. Apparent heterogeneous electron-transfer rate constants, <i>k</i>°<sub>app</sub>, for all five redox systems were determined from Δ<i>E</i><sub>p</sub>−ν experimental data, according to the method described by Nicholson (Nicholson, R. S. <i>Anal. Chem</i>. <b>1965</b>, <i>37</i>, 1351.). The rate constants were also verified through digital simulation (DigiSim 3.03) of the voltammetric <i>i</i>−<i>E</i> curves at different scan rates. Good fits between the experimental and simulated voltammograms were found for scan rates up to 50 V/s. <i>k</i>°<sub>app</sub> values of 0.05−0.5 cm/s were observed for Fe(CN)<sub>6</sub><sup>3-/4-</sup>, Ru(NH<sub>3</sub>)<sub>6</sub><sup>3+/2+</sup>, and IrCl<sub>6</sub><sup>2-/3-</sup> without any extensive electrode pretreatment (e.g., polishing). Lower <i>k</i>°<sub>app</sub> values of 10<sup>-4</sup>−10<sup>-6</sup> cm/s were found for 4-methylcatechol and Fe<sup>3+/2+</sup>. The voltammetric responses for Fe(CN)<sub>6</sub><sup>3-/4-</sup> and Ru(NH<sub>3</sub>)<sub>6</sub><sup>3+/2+</sup> were also examined at all four electrode types at two different solution pH (1.90, 7.35). Since the hydrogen-terminated diamond surfaces contain few, if any, ionizable carbon−oxygen functionalities (e.g., carboxylic acid, p<i>K</i><sub>a</sub> ∼4.5), the Δ<i>E</i><sub>p</sub>, <i>i</i><sub>p</sub><sup>ox</sup>, and <i>i</i><sub>p</sub><sup>red</sup> values for the two systems were, for the most part, unaffected by the solution pH. This is in contrast to the typical behavior of oxygenated, sp<sup>2</sup> carbon electrodes, such as glassy carbon.