Thickness-dependent photoelectrochemical properties of a semitransparent Co₃O₄ photocathode

Co₃O₄ 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 Co₃O₄ electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of Co₃O₄ 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 Co₃O₄ samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of Co₃O₄ 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 Co₃O₄ photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h^-1 cm^-1. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h^-1·cm^-1 indicating the stoichiometric ratio of 1:2. We propose that a semitransparent Co₃O₄ photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.