Room-temperature morphology engineering of K3ZrF7:Yb/Er nanocrystals for thermal enhancement of upconversion

Potassium heptafluozirconate (K 3 ZrF 7 ) has emerged as a promising host for lanthanide-doped upconversion nanocrystals owing to its unique “soft” crystal structure, which endows it with inherent biodegradability and excellent biocompatibility. Although the ultrafast room-temperature synthesis of K 3 ZrF 7 :Yb/Er nanocrystals has been reported, morphology-controlled growth and its influence on optical properties remain insufficiently understood. In this study, we present a systematic study on the morphology-controlled synthesis of K 3 ZrF 7 :Yb/Er nanocrystals via a facile room-temperature coprecipitation method. By modulating the concentration of potassium acetate and the content of ethylene glycol, a well-defined morphological evolution from cubes to cuboctahedra and ultimately to regular octahedra was achieved, accompanied by a size reduction from the micrometer scale to sub-100 nm. The optimized octahedral nanocrystals exhibited pronounced thermally enhanced upconversion luminescence, with a 5.6-fold increase in emission intensity at 423 K compared to that at 303 K. Furthermore, based on the luminescence intensity ratio between the non-thermally coupled levels of Er 3+ ( 4 F 9/2 and 2 H 11/2 ), a high absolute sensitivity of 301.3% K −1 and the maximum relative sensitivity of 1.27% K −1 were achieved at room temperature. These findings demonstrate that morphology engineering is an effective strategy for tuning the optical and thermal properties of K 3 ZrF 7 -based nanocrystals, highlighting their great potential for high-performance optical thermometry.