Influence of light intensity on the steady-state kinetics in tungsten trioxide particulate photoanode studied by intensity-modulated photocurrent spectroscopy

Abstract

Charge-carrier dynamics during photoelectrochemical (PEC) water splitting over WO3 particulate photoanode deposited on titanium sheet (Ti/WO3) was studied using intensity-modulated photocurrent spectroscopy (IMPS). The WO3 particulate film calcined at 650 °C showed high PEC performance for water oxidation to evolve O2. In linear sweep voltammetry, the photocurrent density proportionally increased with incident light intensity (I0). This means that the incident photon-to-current conversion efficiency (IPCE) of Ti/WO3 was not dependent on I0 in voltammetry. In contrast, there was no proportional relation between I0 and the photocurrent density in chronoamperometry at a constant potential, indicating that the IPCE at steady state decreased with an increase of I0. To elucidate the effect of light intensity on the steady-state IPCE, we investigated the IMPS response of Ti/WO3 photoanodes by applying a simple branching model with respect to the photogenerated holes (h+). It was found that the recombination rate constant (krec) rather than the charge transfer rate constant (kct) significantly depends on I0 at the steady-state condition. The pseudo-first-order krec increased with an increase of I0, suggesting that recombination is increased by the accumulation of photogenerated carriers during continuous PEC water oxidation. This is the reason for the decrease in the steady-state IPCE of Ti/WO3 under high-intensity light irradiation. The effects of applied potentials on the kinetic parameters (kct and krec) were also evaluated for Ti/WO3 under different I0.