José Miguel Calatayud

Improvement of boehmite nanoparticles' aqueous dispersability by controlling their size, shape and crystallinity

In this work, results on the control of size, shape, crystallinity and aqueous dispersability of boehmite prepared by a hydrothermal process are reported. The two step synthetic procedure entailed the precipitation of a xerogel by adding NaOH until pH 10 to a solution of aluminum nitrate at 100 °C without or with additives such as tartaric acid and maltitol, and a subsequent hydrothermal treatment at 150 and 200 °C for different periods of time. The final materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and elemental analysis. The stability and other characteristics of the boehmite nanoparticle aqueous dispersions were determined by measuring the zeta potential (ζ), and the particle size distribution (PSD) by dynamic light scattering (DLS). Additive-free boehmite hydrothermally processed at 150 °C for 24 h displayed average lengths of around 20 nm. On increasing the temperature to 200 °C and the holding time up to 168 h, both the particle length increased (ca. 95 nm) and the particle size distribution widened. Comparatively, the growth of the boehmite nanoparticles in the samples prepared with additives was severely restricted up to 20% of the length of additive-free obtained particles, leading to narrower particle size distributions. Modifications in their shape were also allowed. Interestingly, improved aqueous dispersion stabilities were observed for samples prepared in the presence of an additive. A higher dispersability over a wide pH window was found in maltitol-grafted boehmite nanoparticle dispersions with hydrodynamic sizes closer to the single-particle sizes, as observed by TEM. The different formation and stabilization mechanisms of the boehmite nanoparticles obtained in the presence of additives through the synthetic procedure are also discussed.

V-containing ZrO2 inorganic yellow nano-pigments

In this work we report new results on the preparation, characterization and color properties of the inorganic yellow nano-pigmenting system based on monoclinic V–ZrO2 solid solution nanoparticles. The series of solid solution nanopowders were obtained by a polyol technique where the precipitates, obtained after heating at 180 °C ethylene glycol solutions of vanadyl acetonate and zirconium n-propoxide, were annealed at different temperatures up to 1300 °C for a short duration, in order to improve their crystallinity and control the crystalline form of the final nanoparticles. On annealing at around 450 °C highly crystalline tetragonal V-containing zirconia particles were developed, which transform into the monoclinic form of the V–ZrO2 solid solution after subsequent annealing at 800 °C. Interestingly, the final non-aggregated, well-shaped monoclinic zirconia solid solution particles heated even at as high temperatures as 1000 °C, were sized in the nanometric range displaying well-defined faces and edges. The chromatic coordinates of the prepared nano-pigments after annealing at different temperatures between 800 and 1300 °C indicated yellowness values comparable to conventionally prepared micrometric yellow pigments. The stability of the nano-pigments in aqueous dispersions estimated by zeta potential measurements was good with a low degree of particle aggregation. The used polyol-mediated synthesis can be up-scaled on an industrial level in the preparation of zirconia-based nano-pigmenting systems.

Crystalline microstructure of boehmites studied by multi-peak analysis of powder X-ray diffraction patterns

Nanocrystalline boehmite (gamma-aluminium-oxyhydroxide) is a material of industrial importance, the functionality of which follows from its crystalline microstructure. A procedure for preparing boehmite nanoparticles, comprising the formation of a precipitate by the alkalization of an aqueous solution of aluminium nitrate and subsequent hydrothermal aging, was previously elaborated. The application of an additive (maltitol or tartaric acid) to control the sizes and shapes of crystallites in the produced polycrystalline powder of boehmite was developed. The aim of this work is a study of the effect of the hydrothermal treatment time on nanocrystalline characteristics of boehmite, both in absence and in presence of the additive. The obtained materials were investigated by using X-ray diffraction (XRD) as principal technique and additionally by scanning and transmission electron microscopy. The multi-peak analysis of powder XRD patterns was applied to determine the prevalent crystallite shape, volume-weighted crystallite size distribution, and second-order crystalline lattice strain distribution being principal quantitative characteristics of the crystalline microstructure. Based on these characteristics, three types of the microstructure correlated with the production procedures were observed and discussed in detail. The nanoparticles of boehmites were found to be monocrystalline grains with characteristic habits and sizes of order of ten nanometers weakly dependent on the hydrothermal treatment time.