where h is the Planck's constant, λ and υ are wavelengths and frequencies corresponding to the optical band gap and c is the velocity of light. The optical reflection spectrum of the sample was recorded in the UV–visible range (300–800 nm). The UV–Vis reflection spectrum of NTO nanocrystals obtained at 323 k (50 °C) has been shown in Fig. 7. The DRS spectrum of the powder sample shows the optical characteristic of nano-pigments in both the UV and visible light ranges. With relation to Agui and Mizumaki [39] who studied the intermetallic charge transfer and band gap of MTiO3 (M = Fe, Mn, Co and Ni), three types of electronic transition exist in NiTiO3: between O: 2p and Ti: 3d, between Ni: 3d to Ti: 3d and between Ni: 3d and O: 2p [1] Two absorption peaks were seen in the DRS spectrum of NTO around 450 and 580 nm, which are due to the crystal field splitting [40]. The 3d8 band associated with Ni2+ ion splits up into two sub-bands called the Ni2+ → Ti4+ charge transfer (CT) bands and a broad absorption edge at smaller wavelength which indicates the optical band gap attributed to the O2− → Ti4+ charge transfer interaction. The value of the wavelength is substituted in the formula (1) and energy gap was calculated. The band gap of the NTO nanoparticles was calculated from the reflection peak (335 nm) and found to be about 3.72 eV which is very higher than the previous studies [1] and [7]. According to Fig. 7, it can be seen that the NTO nanoparticles have a highly reflection peak at ∼580 nm with average reflection of 80%, which is higher than the previous reported value in similar researches. In those, the average value was reported to be about 60–70 % [7] and [25]. It is interesting to note that the reflectance of the obtained nano-pigment is much higher than that of the pigments sample derived from solid-state ceramic route [41]. This enhanced reflectance has great potential applications for cool pigment for building coatings. It seems that the improved optical reflection in this research is due to the small size of the nanoparticles. Moreover, the presence of a strong absorption (see Fig. 8) in the range of 570–585 nm of the spectrum (yellow region of the visible light) indicates the color of the synthesized nano-pigments is yellow.