In situ liquid‐cell transmission electron microscopy of
Y
‐based precursor growth dynamics at elevated temperatures
Authors: Ambrožič Bojan , Kostevšek Nina , Rozman Kristina Žužek , Bele Marjan , Šturm Sašo 
Published: December 2016

Yttrium oxide (Y2O3) nanoparticles (NPs) as a host for heavy rare earth elements (Yb3+, Eu3+) have shown to be an efficient up‐conversion phosphor material with a great potential ranging from therapy and sensing for drug delivery to photovoltaic applications [1]. In order to achieve desired morphology and size distribution of Y2O3 NPs the nucleation and growth pathways of Y‐based precursors need to be thoroughly understood. Unfortunately, the mechanism controlling the nucleation and growth of NPs are often difficult to assess and are conventionally studied by indirect methods. On the contrary, in‐situ transmission electron microscopy (TEM) combined with the specialized liquid cell offers both, unprecedented experimental and characterization tool for a direct study of nanoparticle's nucelation and growth phenomena from solutions.

To perform in‐situ TEM experiments Jeol JEM 2100 TEM equipped with Protochips Poseidon 300 liquid flow cell with a heating capability was employed. The synthesis of Y‐based precursor NPs was performed from the solution of urea, yttrium acetate and minor amounts of HNO3 to facilitate efficient dissolution of yttrium acetate. The solution was sealed between two specially designed chips forming a close container with the viewing area of 40 × 50 m and the water layer thickness of 150 nm. The urea precipitation method was selected because [2] it can be well controlled by the temperature of the solution, triggering the homogenous decomposition of urea throughout the whole chamber volume and consequently the uniform precipitation of Y‐based precursor nuclei, typically Y(OH)(CO3).

To properly evaluate the electron beam effect during the in‐situ observation the so prepared solution was first observed for 30 minutes at room temperature and at dose rate of 5000 e‐/nm2*s. No evident precipitation occurred during that time. This initial experiment served as a confirmation that additional chemical species that were created during the radiolysis of water (solvated e‐, OH‐, H0, OH0, H2, H2O2, H3O+, HO2, …) under the influence of incoming electron beam did not have significant influence on the nucleation of NPs at the room temperature [3]. The new feature of the in‐situ holder setup, which adds an extremely important thermodynamic variable in the experiment, temperature, allowed us to perform in‐situ heat‐triggered nucleation of Y‐based precursor NPs. Namely, the abrupt nucleation of NPs was observed when the temperatures in the cell was raised above 90 °C. Although different morphologies of nanoparticles could be observed during the nucleation and growth period, in this study we focused only on NP's with clear hexagonally shaped faces (Fig. 1). These particles grew with an average growth rate of a 0.5 nm/s to an average size of 25 nm and remained stable during the whole experimental observation period (Fig. 2). Selected area electron diffraction (SAED) patterns showed that these NPs were crystalline already in the early stage of growth period.

The formation of well crystalline nanoparticles by urea precipitation method is unexpected since the typical products of this reaction result in the formation of Y(OH)(CO3) amorphous precursor. The formation of crystalline NPs can be explained by the fact that radiolytic decomposition of water provides additional reactive species in the final solution [3]. One plausible explanation could be that the increase of [(OH)‐] concentration at elevated temperatures, a combined effect of water and urea decomposition, will promote the precipitation of stable hexagonally shaped Y(OH)3 particles [4].