The control of oxide nanoparticle size and shape is of great importance to tune their physical and chemical properties. As a consequence, routine methods are required to determine the nanoparticle morphology. Since diffraction patterns strongly depend on both size and shape, X-ray powder diffraction (XRD) is widely used to determine the particle morphology. Herein, an accurate approach is proposed based on the simulation of morphology-dependent diffraction patterns using the Debye formula and on the comparison of these calculated patterns to the experimental ones. The most representative particle morphology is also obtained. This methodology has been first validated on a simple model system of MgO cubic and octahedral nanoparticles. It has been then applied to three different morphologies of boehmite AlOOH samples, obtained by varying the synthesis pH value. The method is characterized in terms of sensitivity, and extensive comparisons with transmission electron microscopy (TEM) results are also performed: the results appear as consistent. The XRD method is especially efficient for the smallest nanoparticles (about 3-7 nm), since the XRD approach leads to a full and accurate determination of the particle morphology, whereas the TEM observations only provide average diameters of the particles. Moreover, oriented aggregation is observed for all the samples. The developed method can be easily extended to a wide range of nanomaterials.

The ability to design nanoparticles size and shape through the addition of simple and commercially available organic molecules is of particular interest in the catalytic domain because huge amounts of very fine powders are needed. The origin of this effect is all the more difficult to elucidate because the involved interactions are weak. In this paper, we have investigated the shaping of boehmite AlO(OH) nanoparticles in the presence of polyols like xylitol (C(5) alditol) by a combined experimental and theoretical approach. Experimental techniques such as XRD, TEM, IEP measurements, adsorption isotherms measurements, and (13)C MAS NMR experiments demonstrate that the effect of xylitol has a thermodynamic origin and suggest weak interactions between xylitol and the surface. Furthermore, the strongest proportion of lateral faces ((100), (001), and (101)) that of basal face would be in agreement with a preferential adsorption upon lateral surfaces. These results were refined by a computational approach. DFT calculations of surface energies (taking into account temperature and solvation effects) and of NMR shielding constants corroborate that molecular adsorption mode is preferred over all adsorption modes involving exchanges with surface OH groups. The preferred adsorption on lateral surfaces is attributed to the nest effect induced by hydroxyl groups localized on the concavities of the (001) and (101) surfaces, able to stabilize the xylitol molecule by hydrogen-bonding, whereas the basal (010) surface is almost flat. This combined experimental and computational approach thus provides interesting rationalization for the morphology effects observed.

TiO2 rutile nanorods of average length L = 160 +/- 40 nm and average diameter D = 15 +/- 5 nm have been synthesized through a seed-mediated growth process by TiCl4 hydrolysis in concentrated acidic solution. These nanorods were dispersed in water to yield stable (aggregation-free) colloidal aqueous suspensions. At volume fractions phi > 3%, the suspensions spontaneously display a phase separation into an isotropic liquid phase and a liquid-crystalline phase identified as nematic by X-ray scattering. At phi > 12%, the suspensions form a nematic single phase, with large order parameter, S = 0.75 +/- 0.05. Very well aligned rutile films on glass substrate were produced by spin-coating, and their photocatalytic properties were examined by monitoring the decomposition of methylene blue under UV light. We found that UV-light polarized along the quadratic axis of the rutile nanorods was most efficient for this photocatalytic reaction.

No abstract

No abstract

Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fischer-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the transformation at the crystal scale, its detailed mechanism and kinetics. A model ZnO material with well-determined characteristics (particle size and shape) has been synthesized to perform this study. Characterizations of sulfided samples (using XRD, TEM and electron diffraction) have shown the formation of oriented polycrystalline ZnS nanoparticles with a predominant hexagonal form (wurtzite phase). TEM observations also have evidenced an outward development of the ZnS phase, showing zinc and oxygen diffusion from the ZnO-ZnS internal interface to the surface of the ZnS particle. The kinetics of ZnO sulfidation by H(2)S has been investigated using isothermal and isobaric thermogravimetry. Kinetic tests have been performed that show that nucleation of ZnS is instantaneous compared to the growth process. A reaction mechanism composed of eight elementary steps has been proposed to account for these results, and various possible rate laws have been determined upon approximation of the rate-determining step. Thermogravimetry experiments performed in a wide range of H(2)S and H(2)O partial pressures have shown that the ZnO sulfidation reaction rate has a nonlinear variation with H(2)S partial pressure at the same time no significant influence of water vapor on reaction kinetics has been observed. From these observations, a mixed kinetics of external interface reaction with water desorption and oxygen diffusion has been determined to control the reaction kinetics and the proposed mechanism has been validated. However, the formation of voids at the ZnO-ZnS internal interface, characterized by TEM and electron tomography, strongly slows down the reaction rate. Therefore, the impact of the decreasing ZnO-ZnS internal interface on reaction kinetics has been taken into account in the reaction rate expression. In this way the void formation at the interface has been modeled considering a random nucleation followed by an isotropic growth of cavities. Very good agreement has been observed between both experimental and calculated rates after taking into account the decrease in the ZnO-ZnS internal interface.

No abstract

No abstract