Detailed Studies on the Synthesis Methods and Characterizations of Nanocrystalline Cr2O3 Phase Materials for NIR Pigmentation Applications

Abstract

To make comparative studies on the different synthesis methods of Cr2O3 nanomaterials, efforts were made to syntheses of Cr2O3 nanomaterials by precipitation method, hydrothermal method, ball milling method, and thermal treatment method. Different characterization techniques were used to examine the structural, microstructural, thermal, and optical properties of Cr2O3 nanomaterials obtained by distinct approaches. When the precursor, Cr(NO3)3.9H2O heat treated at 200 °C shows amorphous in nature, whereas the pure rhombohedral [R-3c h (167)] Cr2O3 phase forms at >350 °C. Upon heat treating (650 °C) the precipitated and hydrothermally obtained products, the amorphous phase changed to the nanocrystalline Cr2O3 phase. The UV–Vis diffuse reflectance measurements exhibit slight variation in the peak intensity of absorbance and calculated Eg values ranging from 1.82 to 2.3 eV. Significant changes in the NIR reflectance (<5 to ∼50%) values are seen in the 750–2500 nm regions for the Cr2O3 phase nanomaterials. The precursor salt calcined at 900 °C exhibits good green color Cr2O3 phase materials with higher NIR reflectance of ∼50% at ∼2370 nm than the other samples, which is promising for potential NIR reflecting green pigment application. While the calcination of the precursor at 650 °C shows the spherical-shaped nanoparticles, the precipitated product heat treated at 650 °C reveals spherical-shaped nanoparticles in the agglomerated form for the nanocrystalline Cr2O3 phase materials. The Raman bands observed in the wavenumber region, 80–1000 cm−1 with significant differences in the shape of peaks are characteristic of the nanocrystalline Cr2O3 phase materials.