Infrared Evidence for Hoffman Reaction During Calcination of Surfactant-Templated Silica Films and Analysis of the Resulting Surface Sites

Fourier Transform Infrared transmission spectroscopy is used to characterize surfactant-templated silica films on glass. The calcination process is followed in situ and evidence for the removal of the surfactant along the Hoffman reaction is given. Once calcined, the surface of the pores of the mesoporous films is proven by deuteration experiment to be hydroxylated. Bands are attributed to isolated and hydrogen-bonded silanols. Silylation reaction with hexamethyldisilazane only occurs with isolated silanol groups.     

Quadrupolar transitions evidenced by resonant auger spectroscopy

From absorption spectra, the only way to bring to the fore the occurrence of quadrupolar transitions is to study their angular dependence. Resonant spectroscopies offer a new opportunity to obtain more insight into excited electronic states by studying lineshape and intensity of decay processes. We show here that resonantly excited Ti KL(2,3)L(2,3) Auger spectra of TiO2(110) carry a clear signature of quadrupolar transitions to localized e(g) and t(2g) d-like states, giving access to a direct measurement of crystal field splitting.     

Stability of rocksalt (111) polar surfaces: beyond the octopole

Stable polar oxide surfaces must be simultaneously electrostatically compensated and in thermodynamic equilibrium with the environment. As a paradigm, the MgO(111)-p(2x2) reconstructed surface is shown to involve combinations of Mg-covered terminations with peculiar insulating electronic structure, favored in O-poor conditions, and the O-terminated octopole, stabler in more O-rich environments. Such a picture, which could not have been foreseen by either experiments or simulations separately, goes beyond the Wolf model and reconciles the theory with the experimental data taken in variable thermodynamic conditions.     

Interaction de l'oxygène aux basses pressions avec la solution solide niobium-oxygène à haute température: II. Désorption d'oxydes

Interations between oxygen under low pressure and a niobium-oxygen solid solution at high temperature (2200 >T > 1800 K) have been studied. Evidence was found of the validity of a model where the rates of oxide desorption (Z_NbOx) are related to superficial coverage θ : Z_NbOx = K_NbOxθ^x. It is shown experimentally that it is possible to derive from this model an expression which relates flux to bulk concentration C : Z_NbOx = K_NbOxC^x From the equivalence of these two expressions it results that: (1) the enthalpy of transfer (ΔH_SV between the surface and the bulk of solid solution, and (2) the bond energy (x) of the superficial oxygen atoms, have the same dependence on superficial coverage θ (i.e. influence of the interactions between adsorbed oxygen atoms on both X and ΔH_SV is the same). So, X and Δ_SV are theoretically in the form: ΔH_SV = ΔH_SV(0) ? αθ and X = x(0) ? αθ. In fact, there is a discrepancy beiween experimental values which is discussed.     

Morphology of MgO smoke crystallites upon etching in wet environment

The etching of MgO smoke in wet conditions – an archetype for aqueous dissolution of oxide crystals – is explored on the basis of transmission electron microscope (TEM) analyses. In pure water with neutral pH, in acid solutions and in moist air, the MgO cubic crystallites show first (1 1 0) truncations of the 〈1 1 0〉 edges and then (1 1 1) cuts at the corners. Criteria are given to characterize (1 1 0) and (1 1 1) cuts on the TEM images. It is shown that (1 1 0) truncations appear independently of the formation of the (1 1 1) cuts and are associated with a stationary state. The picture is fully consistent with the previous suggestion of a “constrained Wulff profile” [9] which explains the formation of (1 1 0) cuts by excluding (1 1 1) facets.     

Ionic oxygen species formed in the presence of lithium, potassium and cesium

Various thick oxidized Li, K and Cs layers have been analysed. The O²⁻, O₂²⁻ and O₂⁻ ions have been characterized through the multiplet structures of their valence-band spectra. Furthermore, the energy separations between the O 1s core levels and the O 2p valence levels were seen to take some given values for each oxygen species. This provides a new way to unambiguously identify these species. In addition, the above energy separations compare rather well to the free ion values, thus allowing discussion on the representation of the oxygen ions which are formed in the presence of alkali metals in terms of a point-charge model.     

Interaction de l'oxygène aux basses pressions avec la solution solide niobium-oxygène à haute température: I. Èquilibre de segrégation — chimisorption

Interactions between oxygen under low pressure and a niobium-oxygen solid solution had been studied, in the regime where adsorption is the rate-determining step, from 1000 to 1700 K. It is shown that at saturation of solid solution, there exists a constant limiting value Θ_l of superficial coverage, comparable to a limiting bulk concentration c_l. The ratios θ = Θ/Θ_l and ? = c/c_l are called “relative ratio of occupation” (superficial and bulk). $\text{K}$_SV is the equilibrium constant of segregation between adsorbed and dissolved oxygen atoms: (O_diss^?v) + σ ? (O_chim?σ) + v (σ and v being respectively surface and bulk sites), $\text{K}$_SV = [(1 ? θ)/θ] [?/(1 ? ?)]. The experimentally determined expression: $\text{K}$_SV = 5.7 exp[?(22.1 ? 12.1 θ)/ RT] shows that lateral superficial interactions have a large influence on the enthalpy of transfer between the bulk and the surface of the sample. Adsorption is direct and non activated. At the solubility limit, only a fraction of the superficial sites is occupied. We estimate it to be one half. The sticking probability b of oxygen on a niobium oxygen solid solution is given by b = (1 ? θ/2)², its value at zero coverage being estimated as unity.     

Stoichiometry-dependent chemical activity of supported MgO(100) films

Here, we show that the stoichiometry and, consequently, the chemical activity toward hydroxylation of MgO(100) films grown by reactive deposition on Ag(100) strongly depend on the O(2) partial pressure during film growth. Oxygen-deficient films undergo dramatic relative oxygen uptake either by exposure to a partial pressure of water vapor or by aging in vacuum for a sufficiently long time. Conversely, on stoichiometric monolayer MgO islands, photoemission analysis of the O 1s level and scanning tunneling microscopy images are consistent with the prediction that dissociative adsorption of water occurs only at the borders of the islands.