We study the influence of Nb doping on the TiO 2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO 2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO 2 rutile nanoparticles.

The solid solution Nb x Ti 1-x O 2+δ has been synthesized by a sol-gel method with 0.0 e x e 0.1. Structural and microstructural properties have been studied by XPS, XRD, Raman spectroscopy, and TEM as a function of the Nb/Ti atomic ratio and thermal treatment temperature (600-900 °C). XRD analyses showed that a percentage of the nominal added Nb was assimilated in substitutional Ti sites in the bulk of TiO 2 adopting a pentavalent state. On the other hand, XPS detected a high concentration of Nb at the surface. On the basis of the careful analysis of the XRD and XPS spectra it is reasonable to assume that the concentration of Nb at the surface of the nanoparticles is higher than that in the bulk, especially for rutile, giving a U-shaped Nb concentration profile. The niobium incorporation stabilizes the titania obstructing the diffusion of anatase-type surface atoms, which prevents grain coarsening and phase transformation. The solubility limit of niobium into titania is greater for the anatase phase (x > 0.1) than for the rutile phase (x ) 0.06). Once the solubility limit of Nb atoms is surpassed, a ternary phase ascribed to TiNb 2 O 7 has been detected. In addition, the crystallite sizes and the percentage of rutile phase were quite similar for Nb contents above the solubility limit. XRD and XPS measurements suggest that niobium mainly enters into titania phase with valence +5, while Ti maintains its higher oxidation state (4+), the extra charge being thus partially compensated by titanium vacancies. As derived from XPS analyses in the core levels and the valence band regions, the addition of Nb resulted in a displacement of the Fermi level toward the conduction band. In consequence, the Nbaltered samples presented a more marked n-type feature, as compared with that of the bare TiO 2 .

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Hydration and oxidation of gadolinium-doped barium cerate, a system with highly promising properties when used as electrolyte for protonic ceramic fuel cells, are investigated by means of density functional calculations. The energy landscape of oxygen vacancies and interstitial protons in this strongly distorted orthorhombic perovskite is computed. Although the most stable sites for protons are found in the close vicinity of the dopant, the picture of a very complex energy landscape emerges, in which some sites far away from Gd are found more stable than other ones in its close vicinity, due to the highly distorted geometry of the host materials. The fully hydrated phase can be approximated by a structure with 16 local minima. Both hydration (water incorporation) and oxidation (oxygen incorporation) are found to be exothermic processes with reaction enthalpies of −1.34 eV/H 2 O molecule and −0.70 eV/O atom, respectively. The hole polaron resulting from the exothermic incorporation of oxygen is found localized on oxygens around the dopant (small polaron) and carries a spin magnetic moment. Finally, the competition between hydration and oxidation is studied and discussed as a function of oxygen and water partial pressures.