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Robust Sub‐Monolayers of Co3O4 Nano‐Islands: A Highly Transparent Morphology for Efficient Water Oxidation Catalysis

The scalable synthesis of highly transparent and robust sub‐monolayers of Co3O4 nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co3O4 nanoparticles and thermally induced self‐organization lead to an ultra‐fine nano‐island morphology with more than 94% light transmission at a wavelength of 500 nm. These transparent sub‐monolayers demonstrate a remarkable mass‐weighted water oxidation activity of 2070–2350 A gCo3O4−1 and per‐metal turnover frequency of 0.38–0.62 s−1 at an overpotential of 400 mV in 1 m NaOH aqueous solution. This mixed valent cobalt oxide structure exhibits excellent long‐term electrochemical and mechanical stability preserving the initial catalytic activity over more than 12 h of constant current electrolysis and 1000 consecutive voltammetric cycles. The potential of the Co3O4 nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co3O4‐GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. These findings provide a highly performing catalyst structure and a scalable synthesis method for the engineering of efficient photoanodes for integrated solar water‐splitting cells.