We report a synergistic effect involving hydrogenation and nitridation cotreatment of TiO(2) nanowire (NW) arrays that improves the water photo-oxidation performance under visible light illumination. The visible light (>420 nm) photocurrent of the cotreated TiO(2) is 0.16 mA/cm(2) and accounts for 41% of the total photocurrent under simulated AM 1.5 G illumination. Electron paramagnetic resonance (EPR) spectroscopy reveals that the concentration of Ti(3+) species in the bulk of the TiO(2) following hydrogenation and nitridation cotreatment is significantly higher than that of the sample treated solely with ammonia. It is believed that the interaction between the N-dopant and Ti(3+) is the key to the extension of the active spectrum and the superior visible light water photo-oxidation activity of the hydrogenation and nitridation cotreated TiO(2) NW arrays.

CuBi2O4 is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi2O4 have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi2O4 photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi2O4 in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi2O4 as a PEC material. In addition, we report new benchmark photocurrent density and IPCE values for CuBi2O4 photocathodes.

Porous, nanostructured BiVO4 films are incorporated with Mo and W by simultaneous evaporation of Bi, V, Mo, and W in vacuum followed by oxidation in air. Synthesis parameters such as the Bi : V : Mo : W atomic ratio and deposition angle are adjusted to optimize the films for photoelectrochemical (PEC) water oxidation. Films synthesized with a Bi : V : Mo : W atomic ratio of 46 : 46 : 6 : 2 (6% Mo, 2% W) demonstrate the best PEC performance with photocurrent densities 10 times higher than for pure BiVO4 and greater than previously reported for Mo and W containing BiVO4. The films consist of a directional, nanocolumnar layer beneath an irregular surface structure. Backside illumination utilizes light scattering off the irregular surface structure resulting in 30-45% higher photocurrent densities than for frontside illumination. To improve the kinetics for water oxidation Pt is photo-deposited onto the surface of the 6% Mo, 2% W BiVO4 films as an electrocatalyst. These films achieve quantum efficiencies of 37% at 1.1 V vs. RHE and 50% at 1.6 V vs. RHE for 450 nm light.

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.

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBiO photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBiO photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBiO, which can facilitate charge separation. Compared to homogeneous CuBiO photocathodes, CuBiO photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBiO photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBiO photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBiO up to -2.5 mA/cm at 0.6 V vs RHE with HO as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBiO are maintained. Lastly, forward gradient self-doped CuBiO photocathodes are protected with a CdS/TiO heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.