The Dynamics of Nanoparticle Growth and Phase Change During Synthesis of β-NaYF₄

Abstract

We implement a model for nanocrystal growth and crystallographic phase transition during the synthesis of β-NaYF4. In this model, the size of the α-phase nanoparticles, formed during the heating of the precursor materials, grows slightly in mean diameter and broadens in distribution width until some particles reach a size at which the β phase is thermodynamically favored. Individual particles crossing this threshold convert to the β phase, and then, being less soluble than the α phase, grow at the expense of dissolving α-phase particles. Implementing a straightforward kinetic formalism for individual particle growth and a variable phase definition depending on particle size, the model reproduces, in a quantitative fashion, the experimentally observed growth dynamics of β- NaYF4:Yb,Er. This work supports a hypothesis that the β-particle seeds arise from a phase transition in individual α-phase particles. The model also suggests that the great variability observed in the duration of the stage during which the α particle ripen, before β particles begin to appear, may be attributed to rather small differences in the size distribution of the α particles formed during the heating of precursor material.

Simple Self-Referenced Luminescent pH Sensors Based on Upconversion Nanocrystals and pH-Sensitive Fluorescent BODIPY Dyes

“…216 Their findings support a thermodynamically driven dissolution/recrystallization process. They even proposed a four stage process for the growth of β-phase UCNPs, which is supported by our results.. 159,217 As the most efficient UCNPs are mainly synthesized in organic solvents, the applicability is limited as the particles are stabilized by hydrophobic ligands. However, much effort was made to overcome solubility issues.…”

Ultrafast Synthesis and Coating of High-Quality β-NaYF4:Yb3+,Ln3+ Short Nanorods

“…42,43 Regarding the less uniform morphology, we can find an explanation on the model of phase transition reported by M. Berry. 38 We hypothesize that the size polydispersity would be related to small differences in the duration of the stage during which the α particle ripens, before β particles begin to appear. Therefore, it would be interesting to explore in the future if mild annealing conditions for long time can be used to refine nanomaterials shape.…”

“…After 5 minutes at 60 ºC, the temperature is rapidly increased to 180 ºC and kept for 10 minutes. In this step, the kinetically controlled nucleation of -NaYF4 seeds and the nanocrystal growth promoting the -to- phase 38 transformation takes place. Further growth of the thermodynamically favored-NaYF4 phase is drastically reduced by a fast cooling of the reactor.…”

Controllable Synthesis of Upconversion Nanophosphors toward Scale‐Up Productions

“…Later in stage three, the α‐phase begins to transform into the β‐phase, followed by ripening and finally particle growth in stage four. [ 59 ] More recently, the high‐temperature co‐precipitation method has been used in the synthesis of a wider range of UCNPs: including NaYF 4 , [ 60 ] NaLuF 4 , [ 61 ] LiYF 4 , [ 62 ] KLu 2 F 7 , [ 63 ] NaGdF 4 , [ 64 ] NaYbF 4 , [ 65 ] LiYbF 4 , [ 66 ] and KSc 2 F 7 . [ 67 ] With proper optimization of the conditions, uniform 4 nm NaREF 4 UCNPs with enhanced upconversion luminescence could be achieved.…”