Solution processing route to multifunctional titania thin films: Highly conductive and photcatalytically active Nb:TiO2

This paper reports the synthesis of highly conductive niobium doped titanium dioxide (Nb:TiO2) films from the decomposition of Ti(OEt)4 with dopant quantities of Nb(OEt)5 by aerosol-assisted chemical vapor deposition (AACVD). Doping Nb into the Ti sites results in n-type conductivity, as determined by Hall effect measurements. The doped films display significantly improved electrical properties compared to pristine TiO2 films. For 5 at.% Nb in the films, the charge carrier concentration was 2 × 1021 cm-3 with a mobility of 2 cm2 V-1 s-1. The corresponding sheet resistance is as low as 6.5 Ω sq-1 making the films suitable candidates for transparent conducting oxide (TCO) materials. This is, to the best of our knowledge, the lowest reported sheet resistance for Nb:TiO 2 films synthesized by vapour deposition. The doped films are also blue in colour, with the intensity dependent on the Nb concentration in the films. A combination of synchrotron, laboratory and theoretical techniques confirmed niobium doping into the anatase TiO2 lattice. Computational methods also confirmed experimental results of both delocalized (Ti 4+) and localized polaronic states (Ti3+) states. Additionally, the doped films also functioned as photocatalysts. Thus, Nb:TiO2 combines four functional properties (photocatalysis, electrical conductivity, optical transparency and blue colouration) within the same layer, making it a promising alternative to conventional TCO materials. Solution processing of Nb:TiO2 has presented many challenges to the materials community. Nb:TiO2 made by solution processing has been consigned to sensor/catalysis applications. Here, a solution route is presented to highly conductive and photocatalytically active Nb:TiO2 films. A blue color is observed for the niobium doped films with XPS and computational methods showing a stable localized Ti3+ state at the anatase surface compared to the bulk. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.