Effect of Ni doping and synthesis temperature on the properties of NIR-reflective ZnFe2O4 black pigments

Abstract

NIR-reflective Ni-doped ZnFe2O4 black pigments with the general formula of Zn(1-x)NixFe2O4 have been synthesized by a conventional solid-state reaction of ZnO, Fe2O3 and NiO. The effect of Ni doping was studied by adding various amounts of NiO, with x ranging from 0 to 1. Proportional amounts of ZnO, Fe2O3 and NiO were homogeneously mixed together by means of wet milling, followed by filtration and drying. Then, the mixtures were calcined at 1000 and 1100 °C for 3 h. XRD analysis of the undoped pigments revealed the presence of pure ZnFe2O4 in all synthesis conditions. The addition of NiO dopant into the ZnFe2O4 caused a gradual shift of the diffraction peaks toward higher angles due to the substitution of Zn2+ by the smaller Ni2+ to form the Zn(1-x)NixFe2O4. Upon the increase of NiO doping content and calcination temperature, the color hue of the pigments changed from light brown to brownish black, and a gradual reduction of the NIR reflectance was observed. For the black pigments which were synthesized at 1100 °C, the band-gap energy changed from 2.05 eV for ZnFe2O4 (x = 0) to 1.65 eV for NiFe2O4 (x = 1). At the optimum synthesis condition, resulting in black color hue and high NIR reflectance, the Zn0.5Ni0.5Fe2O4 pigment synthesized at 1100 °C had high NIR reflectance of 55.3%.