A micro‐Raman spectroscopic study of Cr(OH)₃ and Cr₂O₃ nanoparticles obtained by the hydrothermal method

Abstract

Cr 2 O 3 nanoparticles, widely used in the industry, can be obtained by calcination of the nanoparticles synthesized via the hydrothermal method. The chemical nature and the morphology of as‐prepared and calcined nanoparticles are investigated by scanning electron microscopy, X‐ray diffraction and Raman spectroscopy. Our results indicate that the as‐prepared nanoparticles mainly consist of amorphous and hydrated Cr(OH) 3 , with only minor amounts of Cr 2 O 3 . By contrast, and as already known before, calcined nanoparticles consist of Cr 2 O 3 . We also demonstrate the effect of inappropriately chosen experimental conditions, because the use of laser intensities above 0.7 mW during the Raman experiments causes a local heating and thus induces the transformation of Cr(OH) 3 into Cr 2 O 3 . The correlation between the laser power and a local heating is further corroborated by thermogravimetric analyses, which show that upon increased temperature, Cr(OH) 3 first dehydrates and then partially condensates to the intermediate CrO(OH) form, to finally attain the crystalline form of Cr 2 O 3 at about 409 °C. Copyright © 2017 John Wiley & Sons, Ltd.