Ultra-thin Al2O3 coatings on BiVO4 photoanodes: Impact on performance and charge carrier dynamics

Bismuth vanadate (BiVO4) has emerged as one of the most promising photoanode materials for oxidising water due to its visible light activity and low cost. Recent studies have shown that the performance of BiVO4 photoanodes can be remarkably improved when coated with ultra-thin passivation layers. In this article we investigate the use of ultra-thin Al2O3 layers grown using atomic layer deposition (ALD). At an optimum thickness (similar to 0.33 nm, 3 ALD cycles), the Al2O3 layer favourably shifted the onset potential by similar to 200 mV and increased photocatalytic currents for the water oxidation reaction. When held at 1.23 V-RHE, we observe a remarkable increase in the theoretical solar photocurrent; from similar to 0.47 mAcm(-2) in uncoated BiVO4 to similar to 3.0 mAcm(-2) in Al2O3-coated BiVO4. Using transient photocurrent (TPC) and transient absorption spectroscopy (TAS) the charge carrier dynamics in Al2O3-coated BiVO4 photoanodes were examined for the first time. TPC showed that photogenerated electrons in the BiVO4 layer were extracted within similar to 1 ms. TAS showed that the remaining holes oxidised water from similar to 100 ms to 1 s. Ultra-thin Al2O3 coatings did not improve the reaction kinetics towards water oxidation, but rather, suppressed bi-molecular recombination on the mu s-ms timescale in BiVO4, and increased the yield of long-lived holes on the ms-s timescale required to oxidise water. This is attributed to an inhibition of surface recombination on BiVO4 by Al2O3, which inhibited the early timescale recombination of charge carriers formed within the space charge layer.