Green chemistry synthesis of wolframites: An investigation about stoichiometric ratios, optical, structural, vibrational, and physico-chemical properties

This work aims to use a new route to synthesize nanostructured materials. The method has an eco-frendly perspective, which seeks to produce materials with dual characteristics (fluorescence and/or plasmon band) with various applications in luminescent sensors or particles for spectral optimization. Cobalt tungstate nanoparticles (CoWO4 - wolframite), with non-stoichiometric ratios of 2:1 and 1:2 of the precursor materials (ammonium paratungstate and cobalt nitrate), were prepared by the proteic sol-gel method. For the synthesis of the nanoparticles, two types of polymerizing agents were used: animal gelatin (collagen) and vegetal gelatin extracted from red algae (Rhodophyta). The prepared materials were calcined between 773 and 1073 K for 2 h. XRD results evidenced a variation of the lattice parameters and a monoclinic symmetry of the P21/c type for CoWO4 and of the P21/n type for WO3. Particle sizes, evaluated by the Scherrer equation, ranged from 13 to 30 nm for specimens synthesized at 773 K and from 20 to 51 nm for specimens synthesized at 1073 K. Raman spectroscopy revealed anharmonicity in the vibrational system and its damping according to the presence of distinct symmetries. The experiment was successful in obtaining a reproducible sol-gel procedure for the synthesis of the CoWO4 nanosystem. The procedure allowed the obtaining of plasmonic and photoluminescent properties only by stoichiometric modification of the precursors. Finally, the process was optimized, and the best synthesis conditions were defined. Furthermore, fluorescence and plasmon band investigation revealed that the nanostructures have maximum emission and absorption at 468 and 410 nm, respectively.