Insights into the Structural and Chemical Modifications of Nb Additive on TiO₂ Nanoparticles

The solid solution Nb_xTi_1-xO_2+δ has been synthesized by a sol-gel method with 0.0 ≤ x ≤ 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 TiO₂ 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 TiNb₂O₇ 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 Nb-altered samples presented a more marked n-type feature, as compared with that of the bare TiO₂.