Synthesis of Zn<sup>2+</sup>-Doped Strontium Manganese\nVanadate Black Powders with High Near-Infrared Reflectance

Zn<sup>2+</sup>-doped strontium manganese vanadate, SrMn<sub>1−<i>x</i></sub>Zn<sub><i>x</i></sub>V<sub>2</sub>O<sub>7</sub> (0 ≤ <i>x</i> ≤ 0.15),\nwas synthesized\nas near-infrared (NIR) reflective black powder by a citrate sol−gel\nmethod and its crystal structure, particle size, optical properties,\nand color were investigated. Single-phase SrMn<sub>1−<i>x</i></sub>Zn<sub><i>x</i></sub>V<sub>2</sub>O<sub>7</sub> (0 ≤ <i>x</i> ≤ 0.15) samples were\nsuccessfully obtained, and the lattice volume decreased with an increasing\nZn<sup>2+</sup> concentration. All of the samples exhibited optical\nabsorption below 650 nm, which corresponds to the bandgap transition\nbetween the valence band (VB) consisting of the Mn<sub>3d</sub> and\nthe O<sub>2p</sub> orbitals and the conduction band (CB) consisting\nof the V<sub>3d</sub> orbital. The absorption spectrum of SrMn<sub>1−<i>x</i></sub>Zn<sub><i>x</i></sub>V<sub>2</sub>O<sub>7</sub> shifted to longer wavelengths as the Zn<sup>2+</sup> content increased because the lowest potential level of\nthe conduction band was lowered by the introduction of the Zn<sub>4s</sub> orbital. Consequently, the color of the sample changed step\nby step from bark to black with an increasing Zn<sup>2+</sup> concentration.\nThe darkest SrMn<sub>0.93</sub>Zn<sub>0.07</sub>V<sub>2</sub>O<sub>7</sub> absorbed visible light at wavelengths of less than 680 nm.\nIn addition, this powder sample exhibited an excellent NIR reflectance\nvalue (<i>R</i> = 67.2%), which was much larger than that\nof the commercially available pigments (<i>R</i> < 50%).