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 the merits of nanostructuring are 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. 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(ethylene oxide) (PIB‐PEO) and poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide) (Kraton liquid™‐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 presented 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 was observed for the ordered KLE‐based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. 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.

Lanthanoid oxysulfides are promising materials for technological applications owing to their magnetic, photoluminescent, catalytic, and optoelectronic properties. Herein, we report the solid-state synthesis and structural characterization of Ln 10 S 14 O (Ln = La, Ce, Pr, Nd, Sm) oxysulfides. Then, we present a thorough discussion on their electronic and photophysical properties. Through Tauc plot analysis and the derivation of the absorption spectrum fitting method (DASF), we determine that all oxysulfides have direct band gaps with energies of 2.84 eV (La), 2.02 eV (Ce), 2.56 eV (Pr), 2.64 eV (Nd), and 2.41 eV (Sm). Furthermore, surface photovoltage spectroscopy (SPS) shows photovoltage (ΔCPD) values of −0.4 to −1.1 V for La-, Pr-, Nd-, and Sm-containing compounds when illuminated near the optical band gap, indicating that these oxysulfides are n-type semiconductors, which is consistent with Mott−Schottky analysis. Photovoltages under sub-band gap illumination energy and photovoltage decay data suggest mid-band gap states possibly arising from the lanthanoid 4f orbitals and/or defects within the crystal structure or at the particle surfaces. These photophysical properties suggest possible applications of the oxysulfides in photoelectrochemical and photovoltaic energy conversion.

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.

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