On the importance of catalysis in photocatalysis: Triggering of photocatalysis at well-defined anatase TiO2 crystals through facet-specific deposition of oxygen reduction cocatalyst.

Abstract

Well-defined anatase TiO2 crystals with co-exposed {101} and {001} facets represent a promising platform for fundamental studies in photocatalysis and for the development of novel photocatalytic systems exhibiting higher than usual quantum efficiencies. Herein, we present protocols enabling the photoreductive deposition of Pt nanoparticles onto anatase TiO2 micro-sized (1-3 µm) crystals prepared by hydrothermal growth in fluoride-containing solutions to be carried out either facet-selectively (on {101} facets only) or facet-non-selectively (on both {101} and {001} facets). The photocatalytic behavior of the resulting photocatalysts is studied using investigations of oxidative photodegradation of a test pollutant [4-chlorophenol (4-CP)], photocurrent measurements, and kinetic analysis of the open-circuit photopotential decay. We demonstrate that the deposition of Pt nanoparticles effectively triggers the photocatalytic degradation of 4-CP at anatase crystals that are otherwise completely inactive. The role of Pt in triggering the photocatalysis is demonstrated to consist chiefly in the catalytic enhancement of the reaction rate of oxygen reduction by the photogenerated electrons. Only the platinized {101} facets contribute to photocatalysis, whereas the {001} facets, in the literature often referred to as "highly reactive", are even after platinization completely inactive, most likely due to (1 × 4) surface reconstruction upon the heat treatment necessary to decrease the amount of surface fluorides. Based on our results, we highlight the eminent role of efficient surface catalysis for effective charge separation and provide specific design rules for further development of photocatalysts with high quantum efficiencies.