ZnFe2O4 Leaves Grown on TiO2 Trees Enhance Photoelectrochemical Water Splitting

TiO2 has excellent electrochemical properties but limited solar photocatalytic performance in light of its large bandgap. One important class of visible-wavelength sensitizers of TiO2 is based on ZnFe2O4, which has shown fully a doubling in performance relative to pure TiO2. Prior efforts on this important front have relied on presynthesized nanoparticles of ZnFe2O4 adsorbed on a TiO2 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 ZnFe2O4 leaves grown on the trunks of TiO2 trees is reported. These new materials exhibit fully a threefold enhancement in photoelectrochemical performance in water splitting compared to pristine TiO2 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 TiO2 trees; these templates are then converted to ZnFe2O4 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.