Effect of <i>pH</i> and Yb³⁺ doping concentration on the structure and upconversion luminescence properties of GdPO₄:Er³⁺,Yb³⁺

Abstract Upconversion luminescence materials hold significant promise for various applications, including lasers, solar cells, anti-counterfeiting technology, biological labeling, and sensor technology. GdPO 4 stands out as a highly promising upconversion host material thanks to its exceptional chemical and thermal stability. While the upconversion luminescence properties of Er 3+ and Yb 3+ co-doped materials are indeed influenced by the crystal environment of rare earth ions within the matrix, the fundamental understanding of how pH and the high concentration of Yb 3+ can ultimately affect these luminescence properties is still lacking. In this paper, we synthesized a series of GdPO 4 :Er 3+ , Yb 3+ upconversion phosphors by a hydrothermal process followed by heat treatments at 900 °C for 2 h. The results of X-ray diffraction and scanning electron microscopy indicate that pH of the initial solution and doping concentration of Yb 3+ can indeed affect crystal growth of samples. Spectroscopic studies reveal that the structure, size and morphology can influence the properties of upconversion emission in GdPO 4 :Er 3+ , Yb 3+ by affecting the local environment of rare earth ions, the surface of sample and the relaxation processes in the samples. The color coordinates calculation illustrates that these implications can ultimately alter the emission color of the samples. This research enhances our comprehension of the role played by the pH of initial solution and the concentration of Yb 3+ ions in the synthesis of Er 3+ , Yb 3+ ions co-doped upconversion materials, specifically in fine-tuning the emission color for the final upconversion materials.