Thermochromic properties of pure NiTiO3 and its Cu- or Co-doped derivatives

Abstract

Nickel titanate is a yellow-brown pigment which adopts the ilmenite crystal structure as corundum and hematite. Since Cr doped Al2O3 and Fe2O3 present a thermochromic behavior, NiTiO3 was also studied for its potential ability to change colour with temperature. Powders were synthesized via a modified Pechini method followed by a calcination step at 700 °C. The thermochromic properties in the 20 °C–400 °C temperature range were collected via the determination of the colorimetric L*a*b* parameters. Namely, the hue continuously shifts from yellow-brown to red-brown, with L*a*b* parameters moving from 60.4/19.2/58.5 to 42.9/30.5/43.1. This colour change is fully reversible even after several cycling. Temperature dependent UV–vis spectroscopy showed that the transparency window at 550–650 nm at room temperature shifts to 575–650 nm at 400 °C. This fully explains the observed colour change. Finally, the effects of Cu or Co doping on NiTiO3 optical and thermochromic properties were also investigated.