High Near-Infrared Reflective Zn1–xAxWO4 Pigments with Various Hues Facilely Fabricated by Tuning Doped Transition Metal Ions (A = Co, Mn, and Fe)

A series of novel transition metal ion-substituted Zn_1-xA_xWO₄ (A = Co, Mn, and Fe, 0 < x ≤ 0.1) inorganic pigments with blue, yellow, brown, and pale green colors have been prepared by a solution combustion method and exhibit extremely high near-infrared reflectance (R > 85%). X-ray energy-dispersive spectroscopy analysis makes it clear that transition metal ions have already been incorporated into the host ZnWO₄ lattice and do not change the lattice's initial wolframite structure. The optical absorption spectrum in the UV region of the ZnWO₄ pigment calcined at 800 °C for 3 h is a ligand-to-metal charge transfer from O 2p nonbonding orbits to antibonding W 5d orbits. On account of the doping Co²⁺ (3d⁷), Mn²⁺ (3d⁵), and Fe³⁺ (3d⁵) transition metal ions, these chromophore ions have occupied the distorted octahedral site of Zn²⁺, leading to d-d transition and metal-to-metal charge transfer from the occupied 3d orbits of A²⁺ to unoccupied W 5d orbits in UV and visible ranges and generating some bright colors. Significantly, these inorganic pigments are also endowed with excellent thermal and chemical stability and are conducive to harsh working conditions. All of the analysis results have offered some design strategies for various colorful inorganic pigments with high near-infrared reflectance.