Photocatalysis with chromium-doped TiO₂: Bulk and surface doping

The photocatalytic properties of TiO₂ modified by chromium are usually found to depend strongly on the preparation method. To clarify this problem, two series of chromium-doped titania with a chromium content of up to 1.56 wt % have been prepared under hydrothermal conditions: the first series (Cr:TiO₂) is intended to dope the bulk of TiO₂, whereas the second series (Cr/TiO₂) is intended to load the surface of TiO₂ with Cr. The catalytic properties have been compared in the photocatalytic oxidation of formic acid. Characterization data provides evidence that in the Cr/TiO₂ catalysts chromium is located on the surface of TiO₂ as amorphous CrOOH clusters. In contrast, in the Cr:TiO ₂ series, chromium is mostly dissolved in the titania lattice, although a minor part is still present on the surface. Photocatalytic tests show that both series of chromium-doped titania demonstrate visible-light-driven photo-oxidation activity. Surface-doped Cr/TiO₂ solids appear to be more efficient photocatalysts than the bulk-doped Cr:TiO₂ counterparts. It's classified! The photocatalytic properties of TiO₂ modified by chromium depend strongly on the preparation method. To clarify this problem, two types of modified titania are discussed: one with Cr^III doped in the bulk and one with CrOOH clusters on the TiO₂ surface (see picture). Both series show visible-light-driven photo-oxidation activity. However, surface modification appears to be a more efficient strategy.