Influence of micro-structural features on the colour of nanocrystallised powders of hematite and visible-NIR reflectance spectra simulations

Abstract

Pigments are quite complex materials whose appearance involves many optical phenomena. Here, we focused on hematite as it is a traditional pigment, whose origin of coloration has been well discussed in the literature. Pure nanocrystallised α-Fe2O3 hematite powders have been synthesised using different synthesis routes. These powders have been characterised by X-ray powder diffraction and scanning electronic microscopy. The colour of the samples has been studied by visible-NIR spectrophotometry. We obtained hematite with both various grain morphologies and noticeably different shades going from orange-red to purple. Colorimetric parameters in CIE L*a*b* colour space and diffuse reflectance spectra properties were studied against the structural parameters. For small nanocrystals, hue is increasing with the grain size until a critical diameter of about 80 nm, where the trend is reversed. Since classical scattering theories are not able to model this trend, we believe a combination of multiple physical phenomena occurring at this scale may explain it.