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

Electrochemical formation of high-energy species such as hydroxyl radical in aqueous media is inefficient because oxidation of H2 O to form O2 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 O2 production, thus increasing electrochemical efficiency for ∙OH production. Yet, the underlying mechanisms of O2 and ∙OH production at diamond electrodes are not well understood. We demonstrate that boron-doped diamond surfaces functionalized with a hydrophobic, perfluorinated molecular ligand (PF-BDD) have significantly higher electrochemical efficiency for ∙OH production compared with hydrogen-terminated (H-BDD), oxidized (O-BDD) ), or polyethylene 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 H2 O in an outer-sphere electron-transfer process. In contrast, the total electrochemical current, which primarily produces O2 , differs strongly between samples with different surface functionalization, thereby indicating an inner-sphere electron-transfer pathway. XPS shows that while both H-BDD and PF-BDD electrodes are oxidized over time, PF-BDD showed longer stability (≈ 24 hr of use) than H-BDD. This work demonstrates that increasing surface hydrophobicity using perfluorinated ligands selectively inhibits inner-sphere oxidation to O2 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.