Understanding photoelectrochemical water oxidation reaction on BiVO4-based photoanodes

Photoelectrochemical (PEC) cells are devices that convert the energy of the sun to chemical energy via splitting water to H2 and O2. During solar water splitting, oxygen evolution reaction (OER) takes place on the (photo)anode and hydrogen evolution reaction (HER) takes place on the (photo)cathode. The poor activity of photoanodes in the oxygen evolution reaction is one limiting factor in widespread use of PEC systems. Another factor is the inefficient utilization of the charge carriers. Hence, there is a need for active photo absorber materials to build more functional photoanodes. To match the efficiency of OER to the HER oxide semiconductor photo absorbers with ability to evolve oxygen are often utilized. Semiconductors with suitable light absorption properties in the visible light spectrum, good catalytic properties, adequate electron/ hole mobility and carrier lifetime, high chemical stability and low-cost are required for effective use of PEC devices. Bismuth vanadate (BiVO4), an n-type semiconductor, has gained much attention for its use as a photoanode in water splitting reactions, due to its relatively narrow band gap (Eg = 2.4 eV) among other metal oxide semiconductor photoanodes and suitable band edge positions with respect to OER electrochemical potentials . Higher PEC activities from BiVO4-based photoanodes could be obtained via morphology control, doping and the addition of co-catalyst facilitating the oxygen evolution kinetics. In this study, BiVO4 photoanode films were prepared via hydrothermal synthesis method. Between direct growth and powder deposition onto FTO coated glass substrates methodologies, direct growth was found to be the best approach. Tungsten (W) dopant was incorporated to improve the electronic properties, and subsequently water oxidation kinetics and best activity was obtained from 2.0at% W-doped BiVO4 samples. For further enhancement iridium oxide (IrOx) co-catalyst was loaded on BiVO4 via atomic layer deposition (ALD) method with water and ozone as oxidizers. Best water oxidation activity was obtained from low coverage (30 ALD cycles) water-oxidized IrOx co-catalyst decorated on BiVO4 with 1.73 mA/cm2 current density at 1.23V under simulated solar light illumination. The effect of hydrothermal reaction time and temperature were also investigated, and the sample preparation was optimized to 2h reaction time at 180ᵒC to get the best photocatalytic performances from the BiVO4 photoanodes. It was found that crystallinity and morphology could be modified by using different surfactants. It was shown that increased homogeneity in morphology distribution and crystallinity did not improve the photocurrent densities for the BiVO4 photoanodes.