Strategy of Charge Compensation for High-Performance Ni²⁺-Activated MgAl₂O₄ Spinel Near-Infrared Phosphor Synthesis via the Sol–Gel Combustion Method

Abstract

Near-infrared (NIR) phosphor conversion light-emitting diodes (pc-LEDs) have great application potential as NIR light sources in many fields such as food analysis, night vision illumination, and bioimaging for noninvasive medical diagnosis. In general, phosphors synthesized by a high-temperature solidphase method have large particle sizes and have to be processed to fine powders by a grinding process, which may introduce surface defects and lower the luminous efficiency. Here, we report a sol−gel sintering method with ammonium nitrate and citric acid as the sacrificing agents to synthesize high purity, nanosized (less than 50 nm) Zr 4+ /Ni 2+ codoped MgAl 2 O 4 spinel NIR phosphors, in which MgAl 2 O 4 spinel is the matrix, Ni 2+ is the luminous center, and Zr 4+ acts as the charge compensator. We systematically characterized the crystal structures and NIR luminescence properties of the Ni 2+ -doped MgAl 2 O 4 and the Zr 4+ /Ni 2+ codoped MgAl 2 O 4 . Under 390 nm light excitation, the emission spectrum of the Ni 2+ -doped MgAl 2 O 4 phosphor covers 900−1600 nm, the half-peak width is 251 nm, and the peak position is located at 1230 nm. We demonstrated that by incorporating small amounts of Zr 4+ as the charge compensator, the NIR emission intensity of the Zr 4+ /Ni 2+ codoped MgAl 2 O 4 nanosized phosphor was doubled over that of the Ni 2+ -doped MgAl 2 O 4 phosphor. The optimal content of the charge compensator was 2 mol %. More importantly, the inclusion of Zr 4+ led to a NIR phosphor with improved thermal stability in luminous properties, and the luminous intensity measured at 100 °C was 33.83% of that measured at room temperature (20 °C). This study demonstrates that NIR phosphor nanomaterials with high-purity and enhanced optical properties can be designed and synthesized through the charge compensation strategy by a sol−gel sintering method.