Molybdenum Oxide Structures Synthetized by Microwave Technique and Its Phase Transformation by Thermal Treatment

Abstract

The use of microwave radiation for the materials synthesis presents great benefits: energy saving, short processing times, increased performance, economical and environmentally friendly procedures, etc. In a microwave-assisted synthesis, process there is an inverse temperature profile compared to conventional methods: heating occurs by conversion rather than by energy transfer, so the temperature distribution in the synthesis is uniform and highly controlled, which allows rapid synthesis, efficient and with better results than traditional heating [1]. This discovery has allowed the synthesis of new materials in more controlled morphologies [1][2][3], additionally a phase transformation can be achieve by thermal treatment [3], and the morphological changes are due to the crystalline structures obtained.Molybdenum oxide (MoO3) materials were synthesized according to previously method reported by Paraguay-Delgado et al [3], for which an aqueous solution of ammonium heptamolybdate ((NH46Mo7O24 · 4H2O) at concentration of 0.3 M and controlling the pH was treated in microwave assisted hydrothermal method at 200°C for 20 minutes, achieving pressures around 41 bar. The microwave-assisted hydrothermal method allows the synthesis two samples of MoO3 with hexagonal bar morphology structures as showed by scanning electron microscopy (SEM) in image 1 and hexagonal MoO3 phase according to X ray diffraction technique (XRD) in figure 2 whit a clear difference in (100) and (210) patterns intensity directions. A complete statistical study made to both samples with pressure variation during the synthesis show DCLML2 MoO3 sample with hexagonal bars 8% shorter and 7% thinner than DCLM1 MoO3 sample. The pressure was for DCLM1 and DCLM2 41 and 38.5 bar respectively.In order to made a phase transformation the MoO3 samples were treated in a muffle at 450°C for 45 min in static atmosphere, the materials after treatment are characterized by SEM, the results are shown in figure 3 and the XRD results are show in figure 2. X ray diffraction show a complete phase transformation in both materials to orthorhombic MoO3 structures and by SEM and high resolution transmission electron microscopy (HRTEM) the morphological changes in the hexagonal bar structures are studied. Deformation in the morphology hexagonal bars structures of MoO3 samples are observed and transformation in the sheets type within the bars was observed, these sheets are observed both horizontal and vertical with respect to the original bar, the transformation process is studied by means of HRTEM and its morphological changes is due to crystalline phase transformation process.References:[1] J.