Issue 23, 2014

Ostwald-ripening and particle size focussing of sub-10 nm NaYF4 upconversion nanocrystals

Abstract

We have studied the growth behaviour of sub-10 nm NaYF4 upconversion nanocrystals of the hexagonal β-phase and the cubic α-phase. Ostwald-ripening of such particles in oleic acid/octadecene solvent results in broadening of the particle size distribution if the colloid contains particles of one crystal phase only. Narrow size distributions are formed only if β-phase particles grow in the presence of an excess of α-phase particles. Such binary mixtures of α-phase and β-phase particles form intrinsically when colloids of α-phase particles are heated for a sufficiently long time, because seeds of the β-phase nucleate in the solution after some time at high temperatures. Since the number of seeds determines the final size of the β-phase product, control of the nucleation is crucial for controlling the final particle size. We show that the number of β-phase seeds strongly depends on the composition of the α-phase known to form solid solutions Na1−xYF4−x in the range from x = 0 to x = 4/9. Sodium-deficient α-phase particles form a negligible number of β-phase seeds whereas α-phase particles with high sodium content yield a very large number of seeds. By taking advantage of this dependence and modifying the synthesis of the α-phase particles accordingly, small phase-pure β-NaYF4:Yb,Er particles with a size smaller than 6 nm can be prepared in oleic acid/octadecene just as well as much larger particles.

Graphical abstract: Ostwald-ripening and particle size focussing of sub-10 nm NaYF4 upconversion nanocrystals

Supplementary files

Article information

Article type
Paper
Submitted
09 Jul 2014
Accepted
29 Sep 2014
First published
30 Sep 2014
This article is Open Access
Creative Commons BY license

Nanoscale, 2014,6, 14523-14530

Author version available

Ostwald-ripening and particle size focussing of sub-10 nm NaYF4 upconversion nanocrystals

T. Rinkel, J. Nordmann, A. N. Raj and M. Haase, Nanoscale, 2014, 6, 14523 DOI: 10.1039/C4NR03833A

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