Thickness-dependent photoelectrochemical properties of a semitransparent CoO photocathode.

CoO has been widely studied as a catalyst when coupled with a photoactive material during hydrogen production using water splitting. Here, we demonstrate a photoactive spinel CoO electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of CoO samples are comprehensively studied. Our analysis shows that two bandgaps of 1.5 eV and 2.1 eV coexist with p-type conductivity in porous and semitransparent CoO samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of CoO related to its use as a working electrode in PEC cells are extensively studied and show potential for the application in water oxidation and reduction processes. To demonstrate the stability, an alkaline cell was composed for the water splitting system by using two CoO photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h cm. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h·cm indicating the stoichiometric ratio of 1:2. We propose that a semitransparent CoO photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.