EFFECT OF Ni DOPING AND SYNTHESIS TEMPERATURE ON THE PROPERTIES OF NIR-REFLECTIVE ZnFe₂O₄ BLACK PIGMENTS

NIR-reflective Ni-doped ZnFe 2 O 4 black pigments with the general formula of Zn (1-x) Ni x Fe 2 O 4 have been synthesized by a conventional solid-state reaction of ZnO, Fe 2 O 3 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, Fe 2 O 3 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 ZnFe 2 O 4 in all synthesis conditions. The addition of NiO dopant into the ZnFe 2 O 4 caused a gradual shift of the diffraction peaks toward higher angles due to the substitution of Zn 2+ by the smaller Ni 2+ to form the Zn (1-x) Ni x Fe 2 O 4 . 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 ZnFe 2 O 4 (x = 0) to 1.65 eV for NiFe 2 O 4 (x = 1). At the optimum synthesis condition, resulting in black color hue and high NIR reflectance, the Zn 0.5 Ni 0.5 Fe 2 O 4 pigment synthesized at 1100 C had high NIR reflectance of 55.3 %.