ZnFe2O4 Leaves Grown on TiO2 Trees Enhance Photoelectrochemical Water Splitting

TiO 2 has excellent electrochemical properties but limited solar photocatalytic performance in light of its large bandgap. One important class of visible‐wavelength sensitizers of TiO 2 is based on ZnFe 2 O 4 , which has shown fully a doubling in performance relative to pure TiO 2 . Prior efforts on this important front have relied on presynthesized nanoparticles of ZnFe 2 O 4 adsorbed on a TiO 2 support; however, these have not yet achieved the full potential of this system since they do not provide a consistently maximized area of the charge‐separating heterointerface per volume of sensitizing absorber. A novel atomic layer deposition (ALD)‐enhanced synthesis of sensitizing ZnFe 2 O 4 leaves grown on the trunks of TiO 2 trees is reported. These new materials exhibit fully a threefold enhancement in photoelectrochemical performance in water splitting compared to pristine TiO 2 under visible illumination. The new materials synthesis strategy relies first on the selective growth of FeOOH nanosheets, 2D structures that shoot off from the sides of the TiO 2 trees; these templates are then converted to ZnFe 2 O 4 with the aid of a novel ALD step, a strategy that preserves morphology while adding the Zn cation to achieve enhanced optical absorption and optimize the heterointerface band alignment.