![]() m-Planes exhibit negatively charged surface states within the energy bandgap, causing near-surface upward energy band bending, which is most commonly encountered in the n-type semiconductors. The underlying difference between these two approaches is the type of surface states for the m-plane InGaN NW sidewalls and the c-plane surface of the InGaN layers and InN QDs. Nanostructured n-type InGaN nanowires (NWs) and InGaN layers, activated with InN quantum dots (QDs), employed as photoanodes, showed high efficiencies in the oxygen evolution reaction (OER) in solar water splitting. Solar hydrogen produced by this method of photoelectrochemical water splitting is the ideal green fuel to replace fossil fuels in a sustainable hydrogen energy economy 1. ![]() In addition, for one of the most important reactions, the photoelectrochemical splitting of water into hydrogen and oxygen, InGaN possesses the required straddling alignment of the conduction band and valence band with the water H +/H 2 reduction potential and O 2/H 2O oxidation potential for up to ~50% of the In content and shows the best bandgap energy for solar light absorption for 30–40% In. To begin with the right material, InGaN is one of the best choices due to the wide tunability of the direct optical bandgap over the whole visible range by varying the In content, the high absorption coefficient, high carrier mobility, and chemical and mechanical resilience. The key strategies to boost the performance of photoelectrodes are nanostructuring, surface catalyst coupling, light management, heterostructuring, and doping to optimize the crucial processes of light absorption, photocarrier separation and transfer, and surface reaction. ![]() Final deposition of the earth-abundant NiOOH co-catalyst boosts the photocurrent of the InGaN/Cu 2O/NiOOH complete NW photoanode into the competitive mA/cm 2 range. The functional InGaN/Cu 2O heterostructure core-shell NW photoanode is chemically self-stabilized at positive applied voltage by a thin CuO surface layer. Thick Cu 2O layers on top of the InGaN NWs act as common photocathodes. For sufficiently thin Cu 2O layers, the upward energy band bending in the depletion zone extends up to the surface for optimized hole transport and surface reaction. The large photocurrent is due to the maximized photocarrier separation and hole transfer to the surface in the depletion zone of the p–n heterojunction established by the p-Cu 2O layer, forming a thin, uniform shell-layer around the n-InGaN NW core by electrodeposition. The heterostructuring and doping concepts have proved to obtain a novel n-InGaN/p-Cu 2O nanowire (NW) photoanode by strong enhancement of the photocurrent compared to a bare InGaN NW photoanode in solar water splitting. ![]()
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