Towards the understanding of structure-properties relationships in N,Nb doped TiO2 nanopowders: a multidisciplinary experimental and DFT approach

The photocatalytic activity of doped TiO2 nanopowders in the visible region strongly depends on the close, yet not fully understood, interplay among local lattice distortions, nature and concentration of point defects and bulk electronic states. [1] Besides, also the mesoscale morphology is crucial, as pore volume and surface area determine the effective suitability of the material for industrial and environmental applications. [2] We provide here a multidisciplinary experimental (HR-XRPD, EXAFS, EDX, BET, SEM, EPR, DRS) and quantum-mechanical (DFT, DFT+U) exploration of the structural, morphological and electronic properties of differently doped (N, Nb and N/Nb) TiO2 materials. Overall, we disclose rather subtle correlations among (i) the electronic structure and the nature of the point defects and (ii) the chemical nature of the precursor and the dopant-induced lattice distortions. First, we demonstrate that substitutional doping is preferred for both N and Nb guests at low concentrations, whereas higher N concentrations are invariably associated to a significant increase in the number of oxygen vacancies. The latter in turn might act as recombination sites for the photogenerated e-carriers, [3] accounting for the reduction of the photocatalytic efficiency at high dopant loadings. In co-doped N, Nb materials, Nb acts as a bulk reducing center towards nitrogen. This intrinsic charge compensation mechanism implies that shallow mid-gap states due to N atoms near the conduction band are more populated than in single-doped N-TiO2 powders. This provides a possible explanation for the synergistic enhancement of the visible-light absorption showed by N, Nb co-doped nanopowders with respect to their single-doped analogues. We also show that, upon selecting the proper chemical nitrogen precursor and the desired nominal N/Ti concentration, it is possible to tailor crucial microscopic and mesoscopic parameters, such as phase composition, surface area, morphology, and crystallographic cell distortions. Synergism among experimental and theoretical techniques is here decisive to provide a sensible interpretative model for the observed photoelectrochemical activity in titania-based photocatalysts. In this context, access to large-scale X-ray and neutron facilities is foresee to become more and more important in the next future to allow a complete and accurate understanding of the structure-properties relationships in these materials. [1] (a) Lo Presti et al., J. Phys. Chem. C 2014, 118, 4797; (b) Marchiori et al., J. Phys. Chem C 2014, 118, 24152; (c) Spadavecchia et al., Chin. J. Chem. 2014, 32, 1195; (d) Ceotto et al., J. Phys. Chem. C 2012, 116, 1764. [2] P. Rao & M. Dhar, Recent Advances in Basic and Applied Aspects of Industrial Catalysis, 1998, Elsevier; ISBN: 0-444-82920-2 [3] (a) Katoh et al., J. Phys. Chem. Lett. 2010, 1, 3261; (b) Irie et al., J. Phys. Chem. B 2003, 107, 5483.