Purple nanometrics pigments based on cobalt-doped manganese molybdate: Synthesis, characterization, structural, thermal, optical, colorimetric and chemical properties

Abstract

This work is related to the characterization of nanometrics materials of the βMnMoO4-βCoMoO4 system, prepared by co-precipitation and sol-gel routes from the corresponding metal salts. The structural, morphological and colorimetric characteristics of the obtained powders were studied using thermogravimetric and differential thermal analysis (TGA-DTA), X-ray diffraction (XRD), infrared and Raman spectroscopy, scanning electron microscopy (SEM) and colorimetry (CIE-L*a*b* system). The surface specific area was calculated using the Brunauer–Emmett–Teller analysis (BET) in the adsorption/desorption isotherm. The X-ray diffraction patterns highlighted the formation of a continuous solid solution Mn1-xCoxMoO4 (0 ≤ x ≤ 1) of monoclinic symmetry (C2/m) isotype to β-MnMoO4 and this regardless of the synthetic route adopted. FTIR and Raman spectroscopy analysis showed that the chemical bonds’ nature in the Mn1-xCoxMoO4 compounds corresponded to the strong Mo–O–Mo, Mo–O and Co–O–Mo vibrational modes. The micrographs (SEM) highlighted the form of nanorods with sizes ranging from 10 to 200 nm for compositions x = 0.3 and x = 0.6, prepared by co-precipitation. The specific surface area measurements indicated that the powders obtained by the sol-gel process present lower values than those prepared by co-precipitation, regardless the cobalt substitution rate. The specific surface area of the purple nanoscale powder (x = 0.6) obtained by co-precipitation was about 7.52 m2/g. Colorimetric parameters analysis of the powders heat treated at 1100 °C showed that the degree of the component (-b*) was significant for compositions x = 0.6, regardless of the synthesis method adopted.