Local Ordering at the Interface of the TiO₂-WO₃ Bi-Layers

Abstract

Reducing the recombination rate of photo-generated electron-hole pairs in the surface of pristine TiO₂ materials can be done, among others, via local spatial charge separation in the photocatalytically active surface region. A local electric field acting for this purpose can be ensured, for instance, in hetero-junction regions formed at the interface between a TiO₂ film and a WO₃ ultra-thin layer. The option for the WO₃–TiO₂ semiconductor pair is related to the suitable fit of their band structures. To model the interface, a more in-depth knowledge of local atomic environment is required. Here, we discuss the local atomic ordering and the related effects in the interface region of TiO₂/WO₃ and WO₃/TiO₂ structures grown on Si(100) substrates. Materials characterization was done by using XRD, XPS and XAS techniques. We demonstrate that tungsten atoms enter as W⁶⁺ or W⁴⁺ cations into the rutile-type TiO₂ lattice, by substituting the Ti⁴⁺ cations. While W⁶⁺-Ti⁴⁺ substitution leaves the surrounding rutile matrix unchanged, the W⁴⁺-Ti⁴⁺ substitution induces a local rutile-to-anatase transition. The current results are relevant in designing new applications structures with enhanced photocatalytic performances.