Photoelectrochemical oxidation of water using nanostructured BiVO ₄ films

Nanostructured BiVO₄ films were synthesized by coevaporation of bismuth and vanadium in an oxygen ambient, a process referred to as reactive ballistic deposition (RBD). The films were tested in various electrolyte solutions to assess their activity for photoelectrochemical water oxidation. Deposition parameters, including the V/Bi atomic flux ratio and the incident angle of deposition, were adjusted. Films deposited with excess vanadium (V/Bi = 2) and incident angles of deposition at 65° showed the highest initial photocurrents with IPCE values above 21% for light wavelengths of 340-460 nm (in 0.5 M Na₂SO₄ at 1.0 V vs Ag/AgCl). With continued illumination the excess vanadium in these films dissolved into the electrolyte and the photocurrents dropped by 60-75% before reaching steady state. The steady-state photocurrent and IPCE values (above 14% for 340-460 nm light) were higher than the initial values for films synthesized with stoichiometric amounts of vanadium and bismuth (V/Bi = 1) and incident angles of deposition at 65°. Stoichiometric BiVO₄ films remained stable under illumination but their photocurrents were limited by surface reaction kinetics. The addition of cobalt as an electrocatalyst to the surface of these films increased their photocurrent by a factor of 3.