Detailed Studies on the Synthesis Methods and Characterizations of Nanocrystalline Cr<sub>2</sub>O<sub>3</sub> Phase Materials for NIR Pigmentation Applications
S. A. Ashika, S. Balamurugan, T. K. Sana Fathima, N. Palanisami, T. Viswanathan
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.