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

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical center dot 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 IrCl62-/3- offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical (OH(ads))-O-center dot saturation coverage of 338 mu C cm(-2) was found in our nanostructured samples by its reduction with the electrogenerated IrCl63-. The decay kinetics of (OH(ads))-O-center dot 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 similar to k(OH) = 2.2 x 10(3) mol(-1) m(2) s(-1). A radical scavenger, such as methanol, competitively consumes (OH(ads))-O-center dot 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) similar to 1 s(-1).