Yttria, a host for heavy rare earth elements, is an important up-conversion material, able to convert lower energy near-infrared light into higher energy visible light, opening the avenue for a wide spectrum of applications from laser technology, photovoltaics to theranostics [1,2]. The efficient use of yttria in the form of nanoparticles (NPs) is related to the understanding of the nucleation and early growth stage kinetics of yttria precursors, formed by the precipitation from the saturated solutions. In contrast to various analytical methods, where the kinetic data are deduced from large sampled volumes, in-situ transmission electron microscopy (TEM) combined with the specialized liquid cell offers the unique possibility to study the spatial and temporal evolution of NPs one-by-one, facilitating a complete reconstruction of early stage events that are vital for the formation of final products [3].

In-situ TEM experiments were performed by utilizing Jeol JEM 2100 LaB₆ TEM operating at 200 kV and liquid cell TEM holder, Protochips Poseidon 300 with a heating capabilities up to 100 °C. The temperature controlled urea precipitation method was used for the synthesis of yttria precursors [4]. Namely, decomposition of urea at elevated temperatures releases precipitating agents (OH^– and CO₃^2-) homogeneously into the reaction system, avoiding localized distribution of the reactants, allowing precise control over the nucleation and growth of yttrium precursor, typically Y(OH)(CO₃). The prepared solution was placed in a liquid sample enclosure contained in the liquid cell TEM holder. Water layer thickness during the observation was between 150 and 300 nm.

The initial solution was observed at a dose rate of 5000 e^–/nm²/s, at room temperature (RT) for 30 minutes. Precipitation was not observed during that period, suggesting that additional chemical species that were created during the radiolysis of water by the incident electron beam did not have significant influence on the nucleation process at RT [5]. The formation rate of NPs increased drastically when the temperatures in the cell were raised above 90 °C. The resulting products were either faceted particles or dendritic nanostructures. While the faceted nanoparticles did not experience significant morphological changes during the observation, this was not true for dendritic nanostructures (Fig. 1). Dendrites first experience rapid growth by developing highly branched, hierarchical structure up to their final size of 50 nm in the first 45 s of the observation. In the second stage, in the period of about 45 s, dendrites undergo rapid fragmentation, resulting in the formation of several spherically shaped particles within the original dendrite volume that were dynamically changing either by the coalescence or Ostwald ripening. Finally, the spherical particles experience a complete dissolution within the observed area, accompanied by the appearance of faceted 150 nm sized NPs in the vicinity of the observation area.

We hypothesize that dendritic structure initially grew by the diffusion limited conditions to the stage when the depletion zone that developed around NPs hindered further growth, followed by coarsening as a result of surface area reduction. The dissolution and formation of NPs with faceted morphology is explained as a combined effect of water and urea decomposition at high temperatures, resulting in increase of [OH^–] concentration, destabilizing initially formed particles and promoting a formation of more stable, plausibly Y(OH)₃ hexagonal particles [6], as shown in Fig. 2.

References:

1 Feldmann, C., et al. (2003). Adv Funct Mater, 13 (7), 511-516.

2 Höppe, H. A. (2009). Angew Chem, Int. Ed., 48, 3572–3582.

3 Ross, F. M. (2015). Science, 350, 350 (6267), aaa9886-9.

4 Qin, H., et al. (2015). Ceramic International, 41, 11598-11604.

5 Schneider, N. M., et al. (2014). J Phys Chem, 118(38), 22373-22382.

6 Huang, S., et al. (2012). Mater. Chem., 22, 16136-16144.

Figures:

Fig. 1 Growth sequence of yttrium based nanoparticles: initial growth of dendrites (a–c), which is followed by defragmentation of dendrites to small spherical nanoparticles (d–f), either with a process of coalescence or Ostwald ripening. In the final stage spherical nanoparticles are fully dissolved (g) and in their vicinity faceted nanoparticles appear (h).

Fig. 2 Schematic diagram of growth process of yttrium based nanoparticles

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