Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis

Abstract

ABSTRACT. 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 TiO2 oxidize water. In this article, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent (TPC) and 2 transient absorption spectroscopy (TAS), we measure both the kinetics of electron extraction (t50% ~200 µs, 1.5 VRHE) and the kinetics of hole oxidation of water (t50% ~100 ms, 1.5 VRHE) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water splitting system. Photo-induced absorption spectroscopy (PIAS) measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photo-electrochemical water oxidation. Under these conditions we find that the surface density of these holes increases non-linearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is 3 rd order with respect to surface hole density, with a rate constant = 22 ± 2 nm 4 .s -1 . Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions the rate law was 2 nd 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 -1 at ~1 sun irradiance). This showed that the rate limiting step does not involve OH -nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH.TOC graphic

Selecting between two transition states by which water oxidation intermediates decay on an oxide surface

“…The systematic trends with pH demonstrate that the decay is not related to charge transfer to anions in solution, which are varied to keep the ionic strength constant over the pH range ( Figure 3). Past pH 13, both time constants exhibit a marked decrease, reflecting a faster mechanism, in accordance with many previous electrochemical investigations of O2 evolution [37][38][39] . Further, a H/D kinetic isotope effect (KIE) demarcates a separate mechanism for the slower route.…”

High efficiency water splitting photoanodes composed of nano-structured anatase-rutile TiO2 heterojunctions by pulsed-pressure MOCVD

“…The valence band should be sufficiently oxidising for water oxidation and the formation of oxygen to occur (Eq 1b) [8,9]. The bandgap of the material should be wide enough to surpass the thermodynamic potential for water splitting (1.23 V) and additional kinetic barriers [10,11].…”

Systematic Material Study Reveals TiNb2O7 as a Model Wide‐Bandgap Photoanode Material for Solar Water Splitting

“…Moreover, charge‐transfer efficiency appears to be nearly 100 %, as there are no observable spikes in the photocurrent curve, which indicate that hole current and steady‐state currents are the same and, therefore, surface recombination is minimal (i.e., all charges are transferred directly into chemical products and the energy barrier for chemical formation is minimal) . Other photocurrent bottlenecks most likely remain unaffected, such as aqueous diffusion of reactants and products at the solution interface and intrinsic photocharge‐generation limitations . Under sulfite hole‐scavenger conditions, dark reactions increase by a factor of 2.5 at 1.6 V versus RHE, which indicates that sulfite is a functional hole scavenger for this material study, as it is readily decomposed at minimal applied potentials even in the absence of light.…”