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.