Metal oxides are promising for photoelectrochemical (PEC) water splitting due to their robustness and low cost. However, poor charge carrier transport impedes their activity, particularly at low-bias voltage. Here we demonstrate the unusual effectiveness of phosphorus doping into bismuth vanadate (BiVO4) photoanode for efficient low-bias PEC water splitting. The resulting BiVO4 photoanode shows a separation efficiency of 80% and 99% at potentials as low as 0.6 and 1.0 VRHE, respectively. Theoretical simulation and experimental analysis collectively verify that the record performance originates from the unique phosphorus-doped BiVO4 configuration with concurrently mediated carrier density, trap states, and small polaron hopping. With NiFeOx cocatalyst, the BiVO4 photoanode achieves an applied bias photon-to-current efficiency of 2.21% at 0.6 VRHE. The mechanistic understanding of the enhancement of BiVO4 properties provides key insights in trap state passivation and polaron hopping for most photoactive metal oxides.

Improved photocatalytic activities in highly ordered porous photocatalysts are often attributed to the larger surface area, higher light absorption, and suppressed charge recombination. Other underlying reasons for the improved charge transport, however, remain elusive at this stage. Herein, 3DOM BiVO 4 photocatalysts are examined to understand the carrier dynamics and their effects in photocatalytic water splitting. Quantum confinement arising from the ultrathin and crystalline wall upshifted its conduction band, enabling photocatalytic proton reduction to hydrogen gas under visible-light illumination. Time-resolved microwave conductivity spectroscopy reveals its ∼6 times higher charge mobility and longer charge diffusion length relative to the bulk counterpart. The long lifetime (∼360 ns) of 3DOM BiVO 4 with a power-law decay suggests the improved charge separation and the formation of shallow trapping states. Further investigation by Kelvin probe force microscope discloses a built-in electric field with upward band bending from the internal wall to the interconnection part of 3DOM BiVO 4 .

α‐SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar‐to‐hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α‐SnWO4/NiOx interface is performed by synchrotron‐based hard X‐ray photoelectron spectroscopy (HAXPES). NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2+ to Sn4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo‐based photoemission spectra simulation, the presence of a thin oxide layer at the α‐SnWO4/NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new‐found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid‐state interfaces in multi‐layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.

fuel can be used on-site and on-demand but can also be stored and transported for off-site use. [5] However, practical PEC applications put stringent demands on photoabsorber materials in terms of efficiency, cost, and stability. Significant trade-offs have to be made, and this has thus far impeded the commercialization of PEC technology. [3,4] High solar to hydrogen conversion efficiencies approaching 20% have been achieved with photoelectrodes based on high-quality III-V semiconductors, such as GaInP 2 and GaAs. [6] However, their cost is likely to be prohibitive, and many of them suffer from instability under PEC operating conditions. The primary materials criteria are suitable bandgap energy to absorb a large fraction of solar photons with sufficient energies to enable water splitting, good electrical conductivity to enable photogenerated charge carrier extraction, favorable energy-band positions to enable carrier injection, and long-term stability in an aqueous environment. [4] Additionally, the material should be abundant and inexpensive in order to make PEC technology competitive with the chemical production of hydrogen from coal or natural gas. Almost all possible elemental and binary semiconductors have been investigated as photoelectrodes for water splitting, but none fulfill all the requirements. Therefore, the search will have to be expanded to ternary or even more complex materials. Metal-oxides offer many unique advantages as photoabsorber materials for PEC water splitting. [7-9] They have a variety of The widespread application of solar-water-splitting for energy conversion depends on the progress of photoelectrodes that uphold stringent criteria from photoabsorber materials. After investigating almost all possible elemental and binary semiconductors, the search must be expanded to complex materials. Yet, high structural control of these materials will become more challenging with an increasing number of elements. Complex metal-oxides offer unique advantages as photoabsorbers. However, practical fabrication conditions when using glass-based transparent conductive-substrates with low thermal-stability impedes the use of common synthesis routes of high-quality metal-oxide thin-film photoelectrodes. Nevertheless, rapid thermal processing (RTP) enables heating at higher temperatures than the thermal stabilities of the substrates, circumventing this bottleneck. Reported here is an approach to overcome phasepurity challenges in complex metal-oxides, showing the importance of attaining a single-phase multinary compound by exploring large growth parameter spaces, achieved by employing a combinatorial approach to study CuBi 2 O 4 , a prime candidate photoabsorber. Pure CuBi 2 O 4 photoelectrodes are synthesized after studying the relationship between the crystal-structures, synthesis conditions, RTP, and properties over a range of thicknesses. Single-phase photoelectrodes exhibit higher fill-factors, photoconversion efficiencies, longer carrier lifetimes, and increased stability than nonpure photoelectrodes...

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