Optimization of NIR reflectance of (Zn,Co)AlFeO4 black pigment for LiDAR detection in autonomous vehicles

Abstract

Zn1-xCoxAlFeO4 (0 ≤ x ≤ 0.5) pigments with excellent 905 nm NIR reflectance were synthesized by spray pyrolysis, and the crystal structure and optical properties with changing the change of Co content were investigated. Regardless of the Co2+ content, the crystal structure of (Zn,Co)AlFeO4 produced is in good agreement with that of ZnAlFeO4 in the cubic crystal structure. The lattice constant of (Zn,Co)AlFeO4 gradually decreases and the microstrain increases as the Co2+ content increases, which is attributed to effective substitution of Zn2+ by Co2+. The powder color of (Zn,Co)AlFeO4 was found to be changeable depending on the Co2+ content. ZnAlFeO4 has yellow characteristics, but (Zn,Co)AlFeO4 gradually changes to a dark achromatic color as the Co2+ content increases. This is because (Zn,Co)AlFeO4 effectively absorbs light between 550 nm and 700 nm due to the 4A2 (4F) → 4T1 (4F) transition of Co2+ substituted at tetrahedral sites. The band gap of (Zn,Co)AlFeO4 decreases linearly from 2.43 eV to 1.91 eV with increasing Co2+ content. The minimum Co2+ content to maintain high NIR reflectance while having black characteristics was determined to be 30 % (x = 0.3). Zn0.7Co0.3AlFeO4 black pigment (L* = 25.69, a* = 1.19, b* = 2.41) prepared by spray pyrolysis showed excellent chemical stability, and the reflectance at 905 nm was 32.2 %, which is about 2.7 times larger than that of commercial inorganic black pigment (11.9 %). Therefore, Zn0.7Co0.3AlFeO4 is a good black pigment to improve the LiDAR detection performance in automotive vehicle applications.