Photoelectrochemically homogeneous nickel oxide photocathode composed of nanocrystals prepared by supercritical hydrothermal synthesis.

This study presents a novel approach for fabricating photoelectrochemically homogeneous photocathodes composed of nickel oxide nanocrystals (NiO NCs) using supercritical hydrothermal synthesis and push-coating (PC) methods. NiO NCs were synthesized in supercritical water using oleic acid (OA) as a surface modifier. The NiO NCs exhibited high crystallinity, small particle size, and narrow size distribution. Compared with the Langmuir-Schaefer method, the PC method provided NiO-NC films with relatively low roughness and high photoelectrochemical performance. In the PC method, NiO-NC films with thicknesses of ∼100-500 nm were obtained by varying the NiO-NC and OA concentrations in the dispersions. The NiO-NC films can be regarded as photoelectrochemically homogeneous photocathodes, as their impedances are proportional to their thicknesses. This homogeneity, in turn, increases the design freedom of NiO photocathodes. In addition, atomic layer deposition allowed the filling of interparticle voids with NiO, improving the hole migration capability of the NiO-NC films. These findings demonstrate the potential of combining supercritical synthesis and PC methods as a green and economically advantageous process for the high-throughput manufacturing of NiO photocathodes, minimizing material loss and limiting the use of hazardous solvents.