Abstract

The growth mechanisms occurring during the synthesis of nanoparticles has been previously investigated by varying reaction conditions, such as reaction time, temperature, ligand and pH. The growth mechanisms of nanoparticles are reported only for high-temperature synthesis methods, by mechanisms such as Ostwald ripening and oriented attachment. Ostwald ripening involves the dissolution of small nanoparticles into monomers, with successive growth by ripening of the surface of bigger particles with these monomers. Oriented attachment occurs by the alignment of more particles following a preferential crystal orientation, and their successive attachment at the interface between particles by so-called necking. Among the previous studies, the mechanisms behind the growth of lanthanide-doped nanoparticles have been often reported, considering the interesting properties of these nanomaterials. Despite the thorough investigations reported in the literature, investigations into the potential for post-synthesis growth mechanisms in lanthanide-doped nanoparticles at room temperature has never been explored.

The presented research project aims to verify the possibility for tailoring the size and shape of lanthanide-doped nanoparticles at room temperature after their initial bottom-up synthesis at high temperature, in response to the present lack of knowledge in this field. The study of post-synthesis surface modification and growth at room temperature has to be carried out taking into consideration the same parameters that are usually varied in the reported investigations for synthesis at high temperatures, such as reaction time, ligand, pH, and temperature. For this project, the temperature is fixed at room temperature, and all the other parameters were varied. Oleate-capped nanoparticles were obtained by a co-precipitation reaction method. The potential for post-synthesis growth of two nanoparticle compositions were investigated (NaREF4 with RE = Y and Gd) to unveil the mechanism of the occurring surface phenomena. As a general protocol, the synthesized particles were subjected to ligand exchange from the as-synthesized oleate capping ligand to phosphonate ligands by first protonating the oleate ion by acidic treatment, followed by deprotonation of the phosphonate ligand and substitution on the nanoparticle surface in basic conditions. The pH was controlled by using two different bases (NaOH and KOH), and the occurrence of the ligand exchange was assessed by FT-IR spectroscopy. Modifications of nanoparticle size and shape during the post-synthesis growth were assessed by collecting aliquots at established time intervals and characterizing them via transmission electron microscopy and inductively coupled plasma mass spectrometry.

The results demonstrated that the ligand can facilitate changes in morphology and growth at room temperature in presence of different bases. NaREY4 nanoparticles do not show significant growth, but a necking behaviour was observed when NaOH was used as a base in the presence of the ligand, in which the nanoparticles orient themselves and connect within each other by forming “necks” between particles (growth by oriented attachment). On the other hand, NaREF4 nanoparticles showed a significant change in size distribution when KOH was used as a base in the presence of the ligand, by forming smaller particles compared to their initial size, which then condense onto the surface of bigger nanoparticles, proving that growth occurs by Ostwald ripening. These results show that it is quite possible to induce room-temperature growth of lanthanide-doped nanoparticles post synthesis by varying the cation of the base. The use of the ligand is crucial for preventing aggregation and inducing the coalescence of smaller nanoparticles into larger ones. To our knowledge, these observations in post-synthesis growth have been reported for the first time.