RESUMO
MoO3 and MoO3-x nanostructures were grown in a simple one-step process by direct evaporation of MoO3 pellets from a tungsten resistive source in presence of helium or hydrogen at pressures from 100 to 1200 Pa. This method uses no templates, catalysts or oxidizing agents. It leads to one dimensional (1-D) crystalline nanostructures mixed with amorphous material in variable ratios. Amorphous structures grew preferentially when hydrogen was used as carrier gas while crystalline material predominated when helium was used. In fact, only crystalline structures were found when the evaporation was carried out under a helium pressure of 600 Pa with source temperatures between 763 and 910 degrees C. Hydrated MoO3 phases with different water concentrations were preferentially formed using hydrogen. X-ray photoelectron spectroscopy detected only molybdenum in its +6 oxidation state in the samples grown under helium, exhibiting the same chemical composition of the source material. Molybdenum in its +6 as well as its +5 oxidation states was detected in the samples obtained under hydrogen at 600 Pa. Hydroxyl groups were identified in samples grown using both gases. The effect of the helium pressure on the growth kinetics and crystallinity of the samples is discussed according to the kinetics conditions (supersaturation, evaporation, cooling and convection rates) driving to the formation of nanostructures in the inert-gas condensation. Finally, the effect of hydrogen on the growth of MoO3 is discussed.
RESUMO
Molybdenum trioxide nanostructures were grown by direct evaporation of MoO3 from a tungsten boat resistively heated in the presence of hydrogen or helium as carrier gas at pressures from 100 to 600 Pa. Crystalline structures such as, nanoribbons, nanofibers, nanoneedles and nanoparticles were obtained at source temperatures below 900 degrees C. On the other hand, at source temperatures above 1000 degrees C, nanoporous structures were obtained. The latter were found more often when hydrogen was used as carrier gas.
