Dynamical scaling properties of nanoporous undoped and Sb-dopedSnO2supported thin films during tri- and bidimensional structure coarsening

Abstract

The coarsening of the nanoporous structure developed in undoped and 3% Sb-doped SnO 2 sol-gel dip-coated films deposited on a mica substrate was studied by time-resolved small-angle x-ray scattering ͑SAXS͒ during in situ isothermal treatments at 450 and 650°C. The time dependence of the structure function derived from the experimental SAXS data is in reasonable agreement with the predictions of the statistical theory of dynamical scaling, thus suggesting that the coarsening process in the studied nanoporous structures exhibits dynamical self-similar properties. The kinetic exponents of the power time dependence of the characteristic scaling length of undoped SnO 2 and 3% Sb-doped SnO 2 films are similar ͑␣ Ϸ 0.09͒, this value being invariant with respect to the firing temperature. In the case of undoped SnO 2 films, another kinetic exponent, ␣Ј, corresponding to the maximum of the structure function was determined to be approximately equal to three times the value of the exponent ␣, as expected for the random tridimensional coarsening process in the dynamical scaling regime. Instead, for 3% Sb-doped SnO 2 films fired at 650°C, we have determined that ␣Ј Ϸ 2␣, thus suggesting a bidimensional coarsening of the porous structure. The analyses of the dynamical scaling functions and their asymptotic behavior at high q ͑q being the modulus of the scattering vector͒ provided additional evidence for the two-dimensional features of the pore structure of 3% Sb-doped SnO 2 films. The presented experimental results support the hypotheses of the validity of the dynamic scaling concept to describe the coarsening process in anisotropic nanoporous systems.

Structural Characterization of Nanocrystalline Sb-Doped SnO2 Xerogels by Multiedge X-ray Absorption Spectroscopy

“…The former favors the incorporation of antimony into the SnO 2 lattice, while the latter favors antimony demixing with a segregation of doping near to the surface region of SnO 2 nanoparticles. Furthermore, we have also evidenced that the incorporation of antimony in substitution for Sn in the SnO 2 lattice is concomitant with a change in the Sb oxidation state from III to V. Indeed, the pentavalent Sb state presents a smaller ionic radius for a 6-fold coordination ( R Sb V = 0.60 Å) than the trivalent Sb state (R Sb III = 0.76 Å), allowing an easier incorporation into the SnO 2 lattice (R Sn IV = 0.69 Å) . In this context, and according to the Sb L 1 edge results (Table ), we propose that xerogels Sb doped with 3 atom % are archetype samples in which Sb atoms are substituted for Sn in the cassiterite SnO 2 crystallites.…”

“…Furthermore, we have also evidenced that the incorporation of antimony in substitution for Sn in the SnO 2 lattice is concomitant with a change in the Sb oxidation state from III to V. Indeed, the pentavalent Sb state presents a smaller ionic radius for a 6-fold coordination (R Sb V ) 0.60 Å) than the trivalent Sb state (R Sb III ) 0.76 Å), allowing an easier incorporation into the SnO 2 lattice (R Sn IV ) 0.69 Å). 24 In this context, and according to the Sb L 1 edge results (Table 1), we propose that xerogels Sb doped with 3 atom % are archetype samples in which Sb atoms are substituted for Sn in the cassiterite SnO 2 crystallites. Although there is a lack of L 1 Sb results for 1.5 atom % antimony-doped SnO 2 xerogels, we assume that for this doping level antimony is as well in the substitutional solid solution in the cassiterite structure.…”

“…20 This feature could be responsible for the best sintering properties of Cu IIdoped ceramics 20 compared to those of undoped and Sb-doped SnO 2 in which grain coarsening is the main phenomenon occurring during firing. 24,27…”

SAXS study of monodispersed silica nanospheres obtained by an amino acid route

“…These features combined with the monotone increase of the nanoparticles volume fraction at increasing periods of reaction time (as shown in Fig. 4), proves that the particles growth process is not governed by a coarsening or a coalescence process [19]. Moreover, it also demonstrates that the nucleation of new particles do not occur at the advanced early time period (N280 min).…”

“…This behavior indicates that silica particles present a smooth and sharp interface. In addition, the validity of Porod's law implies that the colloidal suspension is a "two -electron density" system, i.e., the silica nanoparticles do not contain a significant amount of small pores [19].…”