Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂Photoanode: A Rate Law Analysis

It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2oxidize water. In this paper, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent and transient absorption spectroscopy, we measure both the kinetics of electron extraction (t50%≈ 200 μs, 1.5VRHE) and the kinetics of hole oxidation of water (t50%≈ 100 ms, 1.5VRHE) as a function of applied potential, demonstrating the water oxidation by TiO2holes is the kinetic bottleneck in this water-splitting system. Photoinduced absorption spectroscopy measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2holes under conditions of photoelectrochemical water oxidation. Under these conditions, we find that the surface density of these holes increases nonlinearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is third order with respect to surface hole density, with a rate constant kWO= 22 ± 2 nm4·s-1. Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions, the rate law was second order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20-25 s-1at ∼1 sun irradiance). This showed that the rate-limiting step does not involve OH-nucleophilic attack and demonstrated the versatility of TiO2as an active water oxidation photocatalyst over a wide range of pH.