Engineering the Inhomogeneity of Metal-Insulator-Semiconductor Junctions for Photoelectrochemical Methanol Oxidation

Si-based inhomogeneous metal-insulator-semiconductor (MIS) junctions with a discontinuous metal nanostructure on the Si/insulator layer are expected to be efficient photoelectrodes for solar energy conversion. However, the formation of a metal nanostructure with an optimized arrangement on semiconductors for efficient charge carrier collection is still a big challenge. Herein, we report a method for the in situ formation of an n-Si inhomogeneous MIS junction with well-dispersed metal nanocontacts through a self-assembly process during photoelectrochemical (PEC) methanol oxidation. The photovoltage shows a strong dependence on the inhomogeneity of the n-Si MIS junction, which can be precisely tuned by the applied electrode potential and operation time. The appropriate inhomogeneity of the Schottky junction as well as the high barrier regions induced by the metal oxide/(oxy)hydroxide layer synergistically produces a large photovoltage of 500 mV for the n-Si inhomogeneous MIS junction. Finally, the n-Si-based photoanode is coupled with a CO₂-to-formate reaction to realize the production of formate at both electrodes, resulting in a high faradic efficiency (FE) of 86 and 93% for anode and cathode reactions at an operational current of 30 mA/cm², respectively. These findings provide important insights into the design of highly efficient inhomogeneous MIS junctions through an in situ self-assembly route for solar energy conversion and storage.