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 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 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.