Characterization of Sol-gel Derived Cobalt-doped Willemite via 2D Correlation Spectroscopy

Cobalt blue is one of the world's most famous blue pigments, but it presents a threat to human health since it is toxic when inhaled or ingested. A safer and environmentally preferable way of obtaining cobalt blue-like pigments is doping of various nontoxic compounds with small amounts of cobalt. In this work, doping of zinc silicate (willemite, Zn2SiO4) was carried out with 5, 10, and 15 mol% of Co with the aim of obtaining Zn2-xCoxSiO4 solid solution. Thermal properties of sol-gel derived samples were examined with simultaneous differential thermal and thermogravimetric analysis (DTA/TGA), which showed dehydration, decomposition, and burning effects, accompanied with mass loss and willemite crystallization effect at 800 degrees C. X-ray powder diffraction analysis (XRD) indicated the formation of willemite phase at 800 degrees C, accompanied with ZnO phase and increase in willemite crystallinity with annealing temperature rise from 800 to 1100 degrees C. Fourier transform infrared spectroscopy (FTIR) showed characteristic bands for present oxides and their bonds along with adsorbed water and carbon dioxide. Colouration of prepared samples changed with annealing temperature, whereas higher Co concentrations and temperatures accentuated the intense blue colour. Diffuse reflectance spectroscopy (DRS) revealed an increase in absorbance with annealing temperature and specific bands as a result of different Co coordination present in the samples. The 2D correlation analysis of FTIR and UV-Vis spectra of the samples thermally treated at various temperatures was utilized to investigate and correlate the changes in the structure with the rise of the annealing temperature. The obtained correlation facilitated the finding of optimal process parameters for the desired pigment quality.