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

To make photoelectrochemical water splitting with metal oxide absorbers an economically viable technology for the production of green hydrogen, further improvements of solar-to-hydrogen conversion efficiency as well as photoelectrode stability have to be accomplished. One step towards optimized photoelectrodes is nanostructuring of the metal oxide absorbers addressing the generally short minority charge carrier diffusion lengths. In this work, mesoporous CuFe2O4 (CFO) thin film photoanodes were prepared by a sol-gel chemistry-based dip-coating and soft-templating strategy. The mesoporous CFOs were fabricated with distinct pore morphologies in order to study the impact of pore ordering and surface structure on the photoelectrochemical properties. The degree of pore ordering and geometry was varied by using different structure-directing copolymer surfactants, which were 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 liquid-PEO, KLE). The distinctly mesostructured CFO materials were characterised by means of scanning electron (SEM) and transmission electron microscopies (TEM), grazing-incidence X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). The structural and electronic properties were correlated with the photoelectrochemical water and sulfite oxidation scans. The non-ordered, Pluronic F-127 templated 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 (1.5 µA/cm2) for water oxidation. This 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. These results are confirmed by intensity-modulated photocurrent spectroscopy (IMPS). These yield the lowest surface recombination rate for the ordered KLE-based mesoporous CFO thin films, which retain spherical pore shapes at the surface resulting in fewer surface defects and thus in the highest water oxidation activity. Vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS) on CFO films in contact with aqueous NaIO4 solution confirms these results and further reveals the presence of a detrimental Schottky junction at the FTO-CFO interface. Lastly, the electronic band diagram of the CFO photoanodes was investigated via X-ray photoelectron spectroscopy providing electronic structure arguments about the photoelectrochemical reactions the material is capable of driving. Post-use XPS data suggest that the concentration of hydroxyl groups, oxygen vacancies and adsorbed water/oxygen has indeed changed during photoelectrochemical operation, however the chemical state of the surface metal cations remained unaffected.