Gas effects on the chemical and structural characeristics of porous MoO3 and MoO3−x grown by vapor condensation in helium and hydrogen

Porous MoO3 and MoO3−x were grown in vapor-phase under He and H2 at pressures between 100 and 1200 Pa. MoO3 was vaporized from a tungsten boat heated between 650 and 1250 °C. Growth of porous material was promoted at 100 and 200 Pa of the carrier gas. At higher pressures and only in hydrogen under particular preparation conditions, samples rich in porous phase were formed. Samples grown either in helium or in hydrogen have similar aspect under SEM, TEM and XRD, but important differences were detected with thermogravimetry and Raman spectroscopy. The thermogravimetric analysis revealed a hydrated compound in the H2-grown samples produced at 100 Pa, whereas a non-hydrated compound grew under helium under the same preparation conditions. The Raman spectra revealed some vibrational modes of β-MoO3 for the He-grown samples at 100 Pa, whereas α-MoO3 was only revealed for the H2-grown samples grown at the same pressure. The effect of the hydrogen pressure on the oxidation state of the porous samples was studied, showing the existence Mo5+ in the porous samples grown at 600 Pa, whereas the samples grown under a lower pressure exhibit the same chemical composition of the source material. The growth kinetics of the porous samples is discussed under different gases as well as the effect of the gas on the chemical and structural characteristics of the samples.