Such shallow donor levels can form when part of the

Such shallow donor levels can form when part of the C (contamination from the organic electrolyte) is expelled from the anodic film during the thermal treatment [22], although they LDN193189 are also the result of remnant C in the anodic structure. However, their behaviour is distinctly different over the region 380 to 300nn and the IPCE values for the doped NTs are consistently higher than for the undoped NTs: 15% vs 13% at 360 nm, 26% vs 22% at 340 nm, 32% vs 22% at 320 nm and 28% vs 20% at 300 nm. As a guideline, the light penetration depth in TiO2 is ~10 nm at λλ = 300 nm and ~1 μm at 380 nm [39], meaning that under front illumination (as in this study) electrons generated by photons of shorter wavelengths have to travel a longer distance to be collected at the Ti back electrode. Clearly the presence of Nb is beneficial to the electron transport as NTs incorporating Nb shows consistently higher IPCE than undoped NTs over the region 380-300 nm. Similar conclusions have been previously reported by Schmuki's group [18] and [19]. It is well established by EPR studies that in the Nb-TiO2 system, a valence induction effect occurs in the anatase polymorph leading to the formation of Nb5+ and Ti3+ states, meaning that the extra electrons are stabilized on lattice Ti ions, effectively raising the Fermi level at the boundary of the conduction band [40] (in the case of rutile the extra electron remains instead on the dopant as Nb4+[41]). A similar scenario can be expected for TiO2-Nb NTs and therefore the enhanced electron mobility is responsible for the improved photoresponse of the Nb doped TiO2 NTs.