Robust Sub‐Monolayers of Co 3 O 4 Nano‐Islands: A Highly Transparent Morphology for Efficient Water Oxidation Catalysis

The scalable synthesis of highly transparent and robust sub‐monolayers of Co 3 O 4 nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co 3 O 4 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 g Co3O4 −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 Co 3 O 4 nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co 3 O 4 ‐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.