Quantification of photoelectrogenerated hydroxyl radical on TiO2 by surface interrogation scanning electrochemical microscopy

Abstract

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical ˙OH_(ads) generated photoelectrochemically at the surface of a nanostructured TiO₂ substrate electrode. In this transient technique, a SECM tip is used to generate in situ a titrant from a reversible redox pair that reacts with the adsorbed species at the substrate. This reaction produces an SECM feedback response from which the amount of adsorbate and its decay kinetics can be obtained. The redox pair IrCl₆^2−/3− offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical ˙OH_(ads) saturation coverage of 338 μC cm⁻² was found in our nanostructured samples by its reduction with the electrogenerated IrCl₆³⁻. The decay kinetics of ˙OH_(ads) by dimerization to produce H₂O₂ were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ∼k_OH = 2.2 × 10³ mol⁻¹ m² s⁻¹. A radical scavenger, such as methanol, competitively consumes ˙OH_(ads) and yields a shorter SI-SECM transient, where a pseudo-first order rate analysis at 2 M methanol yields a decay constant of k′_MeOH ∼ 1 s⁻¹.