Nanocrystalline transition metal (CoMoO4) prepared by sol gel method: Correlation between powder colors and α/β phase transformations

Abstract

A simple grinding of the purple β-CoMoO 4 molybdate leads to the formation of the green polymorph α-CoMoO 4 (PDF file: 01–073-1331). The change of color under the effect of grinding or pressure in general is called the piezochromism phenomenon. The β-CoMoO 4 molybdate (PDF File: 00–021-0868) is a piezosensitive compound, sensitive to grinding. On the other hand, heating the green α-CoMoO 4 powder to 500°C at room pressure allows obtaining the purple β-CoMoO 4 variety; this is the thermochromism phenomenon. • Sol–gel route was adopted for the synthesis of the CoMoO 4 molybdates. • TGA-DTA, XRD, Structural refinement, Raman spectroscopy, SEM and TEM analysis were used for characterization… • The average size of the particles of the β-CoMoO 4 powders obtained by sol–gel method was estimated of less than 60 nm. • The heat treatment of the green powder α-CoMoO 4 at 500 °C (ambient pressure) allows obtaining the purple variety β-CoMoO 4 (thermochromism phenomenon). • The analysis of the UV–Visible spectra allows us to affirm that the thermochromism phenomenon is due to the displacement of the O 2– →Mo 6+ charge transfer band towards high energies. The molybdenum-based binary oxides CoMoO 4 have been synthesized using the sol–gel method. Thermal analysis (TGA-DTA), X-ray diffraction (XRD), and Rietveld refinement data were used to characterize the nanocrystalline powders that were produced. A simple grinding of the purple β-CoMoO 4 molybdate leads to the formation of the green polymorph α-CoMoO 4 (piezochromism phenomenon). Heating the green α-CoMoO 4 powder to 500 °C at room pressure allows obtaining the purple β-CoMoO 4 variety (thermochromism phenomenon). The T 1/2 transition temperature which corresponds to the transformation of half of the phase α into β is about 435.5 °C. According to the Rietveld refinement results, the refinement parameters showed that the observed and refined XRD patterns are in good agreement and the CoMoO 4 (α and β) monoclinic molybdate samples are totally pure. Scanning electron microscope (SEM) of β-CoMoO 4 powder obtained from cobalt nitrate (G 1 gel) showed a mixture of porous agglomerates with dispersed nanoparticles. Meanwhile, pre-sintering was observed in the powders obtained from cobalt chloride (G 2 gel). Transmission electron microscopy (TEM) analysis of β-CoMoO 4 molybdate indicated quasi-spherical particles with nanometric sizes ranging from 10 to 60 nm. The analysis of the UV–Visible spectra allows us to affirm that the thermochromism phenomenon is due to the displacement of the O 2– →Mo 6+ charge transfer band towards high energies.