Influence of temperature on the structural and color properties of nickel molybdates

Abstract In this work, nickel molybdates were synthesized by co-precipitation method, at 70 °C, and were thermally treated at temperatures ranging from 300 to 1000 °C. Firstly, the hydrated precursor (NiMoO4.0.65H2O) was obtained, followed by calcination at 300–600 °C to produce a mixture of β- and α-NiMoO4 phases. The pure α phase could only be obtained above 700 °C. The relationships among the crystal structure, colorimetric parameters, and spectroscopic properties (Raman and UV–vis diffuse reflectance) of the NiMoO4 were investigated. Although the thermochromic β phase was not obtained in the pure form, the heat treatments enabled tuning of the band gaps of the materials, with the color changing from yellow to greenish-yellow and the band gap changing from 3.23 to 2.33 eV. The colorimetric parameters (L*a*b*) were determined and the coloring performances of the NiMoO4 materials were evaluated by incorporating them into a clear polymer substrate material.