Nano-sized NaF inspired intrinsic solvothermal growth mechanism of rare-earth nanocrystals for facile control synthesis of high-quality and small-sized hexagonal NaYbF4:Er

In an effort to address the solvothermal control synthesis issues of hexagonal upconversion nanocrystals (UCNCs) without additional co-doping, especially for NaYbF4:Er, we find for the first time that the mostly used sodium sources of NaOH and sodium oleate function through a totally unexpected NaF–HF pathway rather than the commonly accepted pathway of each of them. Specifically, NaOH reacts with NH4F and converts to nano-sized NaF completely in methanol, leading to reduced NH4F decomposition/HF generation in NaOH based synthesis. By contrast, sodium oleate-oleic acid hybrid suppresses NaF generation, leading to enhanced NH4F decomposition/HF generation in sodium oleate based synthesis. HF-flask SiO2 reaction, HF-to-NaF etching and HF tail gas were also detected for the first time. Results show the nano-sized NaF and in situ generated HF play key roles in the formation of cubic monomers and the cubic-to-hexagonal transition of UCNCs, respectively. Inspired by these results, a facile binary sodium strategy was proposed to achieve control synthesis of high-quality and small-sized hexagonal UCNCs via simultaneous regulation of nano-sized NaF and HF levels. HF was also verified as an origin of Ostwald ripening for UCNCs. The impacts of Ostwald ripening on the evolution of UCNCs were thus investigated. Our findings not only offer an advanced growth mechanism, but also present an effective control synthesis tool for UCNCs.