Thermal decomposition approach for the formation of α-Fe ₂O₃ mesoporous photoanodes and an α-Fe ₂O₃/CoO hybrid structure for enhanced water oxidation

Hematite (α-Fe₂O₃) is one of most investigated oxides for energy applications and specifically for photocatalysis. Many approaches are used to prepare well-controlled films of hematite with good photocatalytic performance. However, most of these methods suffer from a number of disadvantages, such as the small quantities of the product, and the assembly of the nanostructures is usually a secondary process. Herein, we present a facile and large-scale synthesis of mesoporous hematite structures directly on various substrates at moderate temperature and study their photoelectrochemical (PEC) properties. Our approach is based on thermal decomposition of iron acetate directly on a substrate followed by an annealing process in air to produce a continuous mesoporous film of α-Fe₂O₃, with good control of the size of the pores. Improving the PEC properties of iron oxide was achieved by deposition of CoO domains, which were formed by thermal decomposition of cobalt acetate directly onto the hematite surface to produce α-Fe₂O₃/CoO nanostructures. PEC measurements of the hematite film before and after CoO growth were tested. Two methods were used to deposit the cobalt material: (a) thermal decomposition and (b) the most typically used method, adsorption of cobalt salt. The photocurrent of pure hematite was 0.25 mA/cm² at 1.23 V versus reversible hydrogen electrode (RHE), while modification of the hematite surface using the thermal decomposition method showed 180% improvement (0.7 mA/cm² at 1.23 V vs RHE) and 40% improvement (0.35 mA/cm² at 1.23 V vs RHE) via the adsorption method. Moreover, the onset potential was shifted by 130 and 70 mV when the surface of the hematite was modified by the thermal decomposition and adsorption methods, respectively.