To overcome this drawback, the protective or blocking layers can be used to avoid direct contact between the electrolyte and the sensible photoabsorber materials 17. The GaAs surface is partially photoelectrochemically dissolved and etched by photo-generating holes rather than oxidation water. However, the rapid surface charge recombination lowers the overall STH efficiency 16. In particular, GaAs (E g: 1.4 eV) has a remarkable e −/h + mobility and requires small positive onset potentials to drive water oxidation, owing to its excellent optical properties and suitable band potentials 14, 15. Specifically, when oxidizing water to O 2, these materials are either anodically photocorroded or photopassivated by native metal oxides in the competitive reaction 5, 13. Although many photoanode materials (e.g., silicon, gallium arsenide, and gallium phosphide ) have valence-band edges at more negative potentials than metal oxides, as well as typically having optimal bandgaps for efficient solar-driven water splitting, these semiconductors are generally unstable when operated under photoanodic conditions in aqueous electrolyte 10, 11, 12. Due to an inadequate band edge position and bandgap for PEC water oxidation, many semiconductors suffer from poor solar light absorption, inefficient charge separation/transfer, and unavoidable e -/h + recombination at the heterojunction interface thus, there is a need to find ideal photoelectrode candidates with minimal limitations under PEC working conditions 8, 9. Solar-to-hydrogen conversion efficiencies (STH, %) have been far from industrial requirements for decades, although remarkable advances have been made in understanding photoactive materials and enhancing their ability to perform the water-splitting reaction 6, 7. The PEC water splitting reactions-including water oxidation and reduction-depend largely on the structure, components, and surface morphology of the photoelectrode. In addition to the well-known production methods, the photoelectrochemical (PEC) technique is of considerable interest because it directly converts solar energy into H 2 from water 4, 5. Around 10 million metric tons of hydrogen are produced per year, reflecting the enormous demand for this chemical in a variety of applications, such as fuels and many other industrial processes 3. Hydrogen (H 2) production is a highly energy-intensive process that consumes 95% of the world’s fossil-derived electrical energy and about 5% of the world’s renewable energy 1, 2. cm −2) and a stable solar-to-hydrogen conversion efficiency of 9.5%.The Et-GaAs/TiO 2/Ni-Pi║Ni-Pi tandem configuration results in the best unassisted bias-free water splitting device with the highest J ph (~7.6 mA In this work, the electrode also has enhanced photostability under 110 h testing for PEC water oxidation at a steady current density J ph > 25 mA The integration of Ni-Pi bifunctional co-catalyst results in a highly efficient GaAs electrode with a ~ 100 mV cathodic shift of the onset potential. Here, we report chemically etched GaAs that is decorated with thin titanium dioxide (~30 nm-thick, crystalline) surface passivation layer along with nickel-phosphate (Ni-Pi) cocatalyst as a surface hole-sink layer. At present, gallium arsenide represents the most efficient photoanode material for PEC water oxidation, but it is known to either be anodically photocorroded or photopassivated by native metal oxides in the competitive reaction, limiting efficiency and stability. Hydrogen is one of the most widely used essential chemicals worldwide, and it is also employed in the production of many other chemicals, especially carbon-free energy fuels produced via photoelectrochemical (PEC) water splitting.
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