“…Owing to the direct band transition in ZnO, the direct optical band gap energy of each sample is calculated from their reflectance spectra by the extrapolation of the linear region of (F(R)h υ ) 2 plot vs E(h υ ) to the energy axis when (F(R)h υ ) 2 = 0 , where R is percentage reflection and F(R) is the Kubelka‐Munk function, which is shown in Figure . It is interesting to note that when the Co concentration is increased for the Co x Zn 1‐x O nanocomposite (x = 0, 0.02, 0.04, 0.06, and 0.10), the band gap energy is decreased to 3.15, 2.88, 2.75, 2.73, and 2.36 eV, respectively, which is in agreement with the previous investigations for the impurity of 2%, and it is improved for the impurities (>2%) compared to the other studies . This means that the value of the band gap decreases with Co impurity, which is evidenced by a red‐shift in the band gap energy .…”