Mesoporous CuFe₂O₄ Photoanodes for Solar Water Splitting Oxidation—Impact of Surface Morphology on the Photoelectrochemical Properties

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While nanostructuring is well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe₂O₄ (CFO) thin film photoanodes were prepared by dip-coating and soft-templating.<br/>Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic F-127), polyisobutylene-block-poly(ethyleneoxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquidTM-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm² at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO present the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate is observed for the ordered KLE-based mesoporous CFO, which retain spherical pore shapes at the surface resulting in fewer surface defects.<br/>Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order. These findings will be of importance for the fabrication of improved metal oxide photoelectrodes and address the issue that nanostructuring of photoanodes with low minority charge carrier diffusion lengths are not generally beneficial for solar water oxidation. [1]<br/><br/>[1] Einert, Marcus, et al. "Mesoporous CuFe₂O₄ photoanodes for solar water oxidation: impact of surface morphology on the photoelectrochemical properties." <i>Chemistry–A European Journal</i> 29.24 (2023): e202300277.