Heterojunction electrodes were fabricated by layer-by-layer deposition of WO 3 and BiVO 4 on a conducting glass, and investigated for photoelectrochemical water oxidation under simulated solar light. The electrode with the optimal composition of four layers of WO 3 covered by a single layer of BiVO 4 showed enhanced photoactivity by 74% relative to bare WO 3 and 730% relative to bare BiVO 4 .According to the flat band potential and optical band gap measurements, both semiconductors can absorb visible light and have band edge positions that allow the transfer of photoelectrons from BiVO 4 to WO 3 . The electrochemical impedance spectroscopy revealed poor charge transfer characteristics of BiVO 4 , which accounts for the low photoactivity of bare BiVO 4 . The measurements of the incident photon-to-current conversion efficiency spectra showed that the heterojunction electrode utilized effectively light up to 540 nm covering absorption by both WO 3 and BiVO 4 layers. Thus, in heterojunction electrodes, the photogenerated electrons in BiVO 4 are transferred to WO 3 layers with good charge transport characteristics and contribute to the high photoactivity. They combine merits of the two semiconductors, i.e. excellent charge transport characteristics of WO 3 and good light absorption capability of BiVO 4 for enhanced photoactivity.

A hematite photoanode showing a stable, record-breaking performance of 4.32 mA/cm2 photoelectrochemical water oxidation current at 1.23 V vs. RHE under simulated 1-sun (100 mW/cm2) irradiation is reported. This photocurrent corresponds to ca. 34% of the maximum theoretical limit expected for hematite with a band gap of 2.1 V. The photoanode produced stoichiometric hydrogen and oxygen gases in amounts close to the expected values from the photocurrent. The hematitle has a unique single-crystalline “wormlike” morphology produced by in-situ two-step annealing at 550°C and 800°C of β-FeOOH nanorods grown directly on a transparent conducting oxide glass via an all-solution method. In addition, it is modified by platinum doping to improve the charge transfer characteristics of hematite and an oxygen-evolving co-catalyst on the surface.

Although large research efforts have been devoted to photoelectrochemical (PEC) water splitting in the past several decades, the lack of efficient, stable and Earth-abundant photoelectrodes remains a bottleneck for practical application. Here, we report a photocathode with a coaxial nanowire structure implementing a Cu 2 O/Ga 2 O 3-buried p-n junction that achieves efficient light harvesting across the whole visible region to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%. With a photocurrent onset over + 1 V against the reversible hydrogen electrode and a photocurrent density of ~10 mA cm −2 at 0 V versus the reversible hydrogen electrode, our electrode constitutes the best oxide photocathode for catalytic generation of hydrogen from sunlight known today. Conformal coating via atomic-layer deposition of a TiO 2 protection layer enables stable operation exceeding 100 h. Using NiMo as the hydrogen evolution catalyst, an all Earth-abundant Cu 2 O photocathode was achieved with stable operation in a weak alkaline electrolyte. To show the practical impact of this photocathode, we constructed an all-oxide unassisted solar water splitting tandem device using state-of-the-art BiVO 4 as the photoanode, achieving ~3% solar-to-hydrogen conversion efficiency.

Highly active and stable electrocatalysts for hydrogen evolution have been developed on the basis of molybdenum compounds (Mo2C, Mo2N, and MoS2) on carbon nanotube (CNT)-graphene hybrid support via a modified urea-glass route. By a simple modification of synthetic variables, the final phases are easily controlled from carbide, nitride to sulfide with homogeneous dispersion of nanocrystals on the CNT-graphene support. Among the prepared catalysts, Mo2C/CNT-graphene shows the highest activity for hydrogen evolution reaction with a small onset overpotential of 62 mV and Tafel slope of 58 mV/dec as well as an excellent stability in acid media. Such enhanced catalytic activity may originate from its low hydrogen binding energy and high conductivity. Moreover, the CNT-graphene hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.

A novel photocatalyst, PbBi2Nb2O9 has been discovered that shows high activities for degradation of organic pollutants, generation of photocurrent, and water decomposition into O2 or H2, all under visible right irradiation (λ ≥ 420 nm). This is the first example of an undoped, single-phase oxide photocatalyst that shows such reactivity. Its quantum yields are much higher than those for most of the previously reported materials, especially in water decomposition to generate oxygen (29%). Since it is an oxide, there is much less concern for stability under light irradiation.