Facet-dependent activity of tailored anatase TiO2 crystals in photoanodes for photocatalytic fuel cells

Abstract

A key component of the photocatalytic fuel cell (PFC) is a photoanode, typically based on highly stable semiconductor (e.g., TiO2) that is capable of effective photoelectrocatalytic oxidation of fuels. Notably, the photo(electro)catalytic activity of TiO2 materials can depend heavily on the microstructural morphology, so that tailored crystals with different shapes and exposed facets can show remarkably different, shape-dependent photoactivities. Herein, we study the PFC performance of photoanodes based on anatase crystals of various shapes – tetragonal truncated bipyramids, sheets and belts, which are characterized by different predominantly exposed facets: {101}, {001}, and {100}, respectively. We demonstrate that photoanodes based on belt-shaped crystals dominated by {100} facets, not observed in natural anatase crystals, exhibit superior performance. Analysis of open-circuit potential and impedance data suggests that this activity is related to faster transfer of photogenerated holes to methanol oxidation sites, avoiding thus the undesired surface trapping and Fermi level pinning effects that are the most likely reason of a poorer performance of materials dominated by {101} and {001} facets. This study highlights the importance of advanced crystal engineering and control over the surface catalytic properties of TiO2 materials for PFCs and other light-driven devices in which efficient alcohol photooxidation is crucial.