Synthesis and Optical Band Gap Study of NIR-Reflective CoFe2O4 Black Pigments Doped with MgO, CaO and Al2O3

Abstract

Near-infrared (NIR) reflectance of cobalt ferrite (CoFe2O4) black pigment has been significantly enhanced by doping with Al2O3, MgO and CaO&MgO derived from dolomite. The CoFe2O4 pigment was synthesized by a conventional solid-state reaction of Co3O4 and Fe2O3 at 1100 oC. Various amounts of dopants including Al2O3, MgO and dolomite which yielded MgO&CaO upon calcination were added into starting raw materials to investigate doping effect. Substitutions of Al3+ for Fe3+, and Mg2+ and/or Ca2+ for Co2+ in the CoFe2O4 structure were manipulated to find an optimum condition that yielded high NIR reflectivity while its black color hue was maintained. Crystal structure, particle morphology and properties of the synthesized pigments were characterized by an XRD, SEM, UV-Vis-NIR spectrophotometer and CIE 1976 L*a*b* colorimetric method. The pure CoFe2O4 black pigment exhibited color lightness L* and NIR reflectance of 19.59 and 19.5%, respectively. The incorporation of the MgO and Al2O3 dopants resulted in a significant increase of the NIR reflectance to 47.6%. Similarly, a significantly enhanced NIR reflectance of 39.8% was obtained at the optimum doping content of Al2O3 and MgO&CaO derived from dolomite. The synthesized pigments exhibited high NIR reflectance that was comparable to the commercial reflective black pigments. Doping the CoFe2O4 with an inexpensive dolomite mineral not only enhanced its NIR reflectance but would also reduce the synthesis cost.