Zinc Ferrite Photoanode Nanomorphologies with Favorable Kinetics for Water‐Splitting

The n‐type semiconducting spinel zinc ferrite (ZnFe 2 O 4 ) is used as a photoabsorber material for light‐driven water‐splitting. It is prepared for the first time by atomic layer deposition. Using the resulting well‐defined thin films as a model system, the performance of ZnFe 2 O 4 in photoelectrochemical water oxidation is characterized. Compared to benchmark α‐Fe 2 O 3 (hematite) films, ZnFe 2 O 4 thin films achieve a lower photocurrent at the reversible potential. However, the oxidation onset potential of ZnFe 2 O 4 is 200 mV more cathodic, allowing the water‐splitting reaction to proceed at a lower external bias and resulting in a maximum applied‐bias power efficiency (ABPE) similar to that of Fe 2 O 3 . The kinetics of the water oxidation reaction are examined by intensity‐modulated photocurrent spectroscopy. The data indicate a considerably higher charge transfer efficiency of ZnFe 2 O 4 at potentials between 0.8 and 1.3 V versus the reversible hydrogen electrode, attributable to significantly slower surface charge recombination. Finally, nanostructured ZnFe 2 O 4 photoanodes employing a macroporous antimony‐doped tin oxide current collector reach a five times higher photocurrent than the flat films. The maximum ABPE of these host–guest photoanodes is similarly increased.