Synthesis of Zn²⁺-Doped Strontium Manganese Vanadate Black Powders with High Near-Infrared Reflectance

Zn2+-doped strontium manganese vanadate, SrMn1−xZnxV2O7 (0 ≤ x ≤ 0.15), was synthesized as near-infrared (NIR) reflective black powder by a citrate sol−gel method and its crystal structure, particle size, optical properties, and color were investigated. Single-phase SrMn1−xZnxV2O7 (0 ≤ x ≤ 0.15) samples were successfully obtained, and the lattice volume decreased with an increasing Zn2+ concentration. All of the samples exhibited optical absorption below 650 nm, which corresponds to the bandgap transition between the valence band (VB) consisting of the Mn3d and the O2p orbitals and the conduction band (CB) consisting of the V3d orbital. The absorption spectrum of SrMn1−xZnxV2O7 shifted to longer wavelengths as the Zn2+ content increased because the lowest potential level of the conduction band was lowered by the introduction of the Zn4s orbital. Consequently, the color of the sample changed step by step from bark to black with an increasing Zn2+ concentration. The darkest SrMn0.93Zn0.07V2O7 absorbed visible light at wavelengths of less than 680 nm. In addition, this powder sample exhibited an excellent NIR reflectance value (R = 67.2%), which was much larger than that of the commercially available pigments (R < 50%).