Scalable Binder-Free Supersonic Cold Spraying of Nanotextured Cupric Oxide (CuO) Films as Efficient Photocathodes

We demonstrate production of nanotextured ptype cupric oxide (CuO) films via a low-cost scalable supersonic cold spray method in open air conditions. Simply sweeping the spray nozzle across a substrate produced a largescale CuO film. When used as hydrogen evolution photocathodes, these films produced photocurrent densities (PCD) of up to 3.1 mA/cm2 under AM1.5 illumination, without the use of a cocatalyst or any additional heterojunction layers. Cu2O particles were supersonically sprayed onto an indium tin oxide (ITO) coated soda lime glass (SLG) substrate, without any solvent or binder. Annealing in air converted the Cu2O films to CuO, with a corresponding decrease in the bandgap and increase in the fraction of the solar spectrum absorbed. Annealing at 600 °C maximized the PCD. Increasing the supersonic gas velocity from ∼450 to ∼700 m/s produced denser films with greater surface roughness, in turn producing higher PCD. The nanoscale texture of the films, which resembles the skin of a dinosaur, enhanced their performance, leading to one of the highest PCD values in the literature. We characterized the films by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy to elucidate the origins of their outstanding performance. This supersonic cold spraying deposition has the potential to be used on a commercial scale for low cost mass production.