Efficient hematite photoanodes prepared by hydrochloric acid-treated solutions with amphiphilic graft copolymer

Simple and low-cost approaches for the preparation of photoelectrodes are crucial to enable a transition towards a sustainable and circular economy in which sunlight energy is efficiently harnessed and used. Here, a novel and simple process is presented to prepare a sol solution that can be cast by spin coating deposition for mesoporous α-Fe₂O₃ hematite water-splitting photoanodes, reaching 1.05 mA cm⁻² at 1.23 V_RHE under 1 sun illumination. The sol solution is prepared using inexpensive commercial ∼10 nm α-Fe₂O₃ hematite nanoparticles as hematite film precursor, an amphiphilic poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer as a pore template, and HCl acid as an iron oxide phase directing agent. The hydrophilic POEM side chains selectively interact with HCl-treated hematite nanoparticles allowing their dispersion. Moreover, the HCl in the sol solution disperses and dissolves the hematite nanoparticles which re-precipitate as mixed phase γ-FeOOH and β-FeOOH, leading to better performant hematite films due to finer nanostructures, a more pronounced hematite (110) plane, and a more hydroxylated surface. This work demonstrates that synergies between an amphiphilic graft copolymer, hematite nanoparticles and HCl acid can be exploited in the inexpensive spin coating technique to prepare robust, stable and promising hematite photoanodes for energy devices.