Detailed Studies on the Synthesis Methods and Characterizations of Nanocrystalline Cr 2 O 3 Phase Materials for NIR Pigmentation Applications

Abstract

To make comparative studies on the different synthesis methods of Cr 2 O 3 nanomaterials, efforts were made to syntheses of Cr 2 O 3 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 Cr 2 O 3 nanomaterials obtained by distinct approaches. When the precursor, Cr(NO 3 ) 3 .9H 2 O heat treated at 200 °C shows amorphous in nature, whereas the pure rhombohedral [R-3c h (167)] Cr 2 O 3 phase forms at >350 °C. Upon heat treating (650 °C) the precipitated and hydrothermally obtained products, the amorphous phase changed to the nanocrystalline Cr 2 O 3 phase. The UV–Vis diffuse reflectance measurements exhibit slight variation in the peak intensity of absorbance and calculated E g 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 Cr 2 O 3 phase nanomaterials. The precursor salt calcined at 900 °C exhibits good green color Cr 2 O 3 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 Cr 2 O 3 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 Cr 2 O 3 phase materials.