Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO 3 /BiVO 4 +CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO 4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO 4 with more conductive WO 3 nanorods in a form of core-shell heterojunction, where the BiVO 4 absorber layer is thinner than the carrier diffusion length while it's optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72mAcm -2 under 1 sun illumination at 1.23V RHE that corresponds to ∼90% of the theoretically possible value for BiVO 4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56mAcm -2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.
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