Enhancing Electrochemical Efficiency of Hydroxyl Radical Formation on Diamond Electrodes by Functionalization with Hydrophobic Monolayers.

Electrochemical formation of high-energy species such as hydroxyl radicals in aqueous media is inefficient because oxidation of HO to form O is a more thermodynamically favorable reaction. Boron-doped diamond (BDD) is widely used as an electrode material for generating OH radicals because it has a very large kinetic overpotential for O production, thus increasing electrochemical efficiency for OH production. Yet, the underlying mechanisms of O and OH production at diamond electrodes are not well understood. We demonstrate that boron-doped diamond surfaces functionalized with hydrophobic, polyfluorinated molecular ligands (PF-BDD) have significantly higher electrochemical efficiency for OH production compared with hydrogen-terminated (H-BDD), oxidized (O-BDD), or poly(ethylene ether)-functionalized (E-BDD) boron-doped diamond samples. Our measurements show that OH production is nearly independent of surface functionalization and pH (pH = 7.4 vs 9.2), indicating that OH is produced by oxidation of HO in an outer-sphere electron-transfer process. In contrast, the total electrochemical current, which primarily produces O, differs strongly between samples with different surface functionalizations, indicating an inner-sphere electron-transfer process. X-ray photoelectron spectroscopy measurements show that although both H-BDD and PF-BDD electrodes are oxidized over time, PF-BDD showed longer stability (≈24 h of use) than H-BDD. This work demonstrates that increasing surface hydrophobicity using perfluorinated ligands selectively inhibits inner-sphere oxidation to O and therefore provides a pathway to increased efficiency for formation of OH via an outer-sphere process. The use of hydrophobic electrodes may be a general approach to increasing selectivity toward outer-sphere electron-transfer processes in aqueous media.