UV–vis spectroscopy and colorimetric study of the samples
were carried out in a Shimadzu, UV-2450 spectrophotometer in
the 380–780 nm wavelength range using barium sulfate as a
reference. The band gap of the pigment samples was calculated
from the absorption spectra using Shapiro’s method by
extrapolating the onset of absorption to the wavelength axis.
The study of chromatic properties requires the use of an
appropriate system to measure the color objectivity. The CIEL*a*b*
1976 color scales were used for the assessment of the
color properties of the pigment powders. In this system, L* is
the color lightness (L* is 0 for black and L* is 100 for white), a*
is the green ()/red (+) axis, b* is the blue ()/yellow (+) axis.