Tunable Self-Assembly of YF₃ Nanoparticles by Citrate-Mediated Ionic Bridges

Abstract

Ligand-to-surface interactions are critical factors in surface and interface chemistry to control the mechanisms governing nanostructured colloidal suspensions. In particular, molecules containing carboxylate moieties (such as citrate anions) have been extensively investigated to stabilize metal, metal oxide, and metal fluoride nanoparticles. Using YF3 nanoparticles as a model system, we show here the self-assembly of citrate-stabilized nanostructures (supraparticles) with a size tunable by temperature. Results from several experimental techniques and molecular dynamics simulations show that the self-assembly of nanoparticles into supraparticles is due to ionic bridges between different nanoparticles. These interactions were caused by cations (e.g., ammonium) strongly adsorbed onto the nanoparticle surface that also interact strongly with nonbonded citrate anions, creating ionic bridges in solution between nanoparticles. Experimentally, we observe self-assembly of nanoparticles into supraparticles at 25 and 100 °C. Interestingly, at high temperatures (100 °C), this citrate-bridge self-assembly mechanism is more efficient, giving rise to larger supraparticles. At low temperatures (5 °C), this mechanism is not observed, and nanoparticles remain stable. Molecular dynamics simulations show that the free energy of a single citrate bridge between nanoparticles in solution is much larger than the thermal energy and in fact is much larger than typical adsorption free energies of ions on colloids. Summarizing our experiments and simulations, we identify as key aspects of the self-assembly mechanism the requirement of NPs with a surface able to adsorb anions and cations and the presence of multidentate ions in solution. This indicates that this new ion-mediated self-assembly mechanism is not specific of YF3 and citrate anions, as supported by preliminary experimental results in other systems.

Tailoring the Synthesis of LnF3 (Ln = La–Lu and Y) Nanocrystals via Mechanistic Study of the Coprecipitation Method

“…Considering this effect, we chose a molar ratio metal:citrate of 1:1.5 to use the minimum amount of citrate in which lanthanide (III) citrate precursor was soluble. 43 In addition, our recent reported study 42 (Figure 2).…”

“…A single precipitation method was performed using acetate precursors with tetramethylammonium citrate as stabilizer following a reported method. 42 To achieve a homogeneous nucleation, the complete dissolution of all precursors is needed. This solubility was achieved with a molar ratio Metal:Citrate up to 1:1.5.…”

“…Particles are interacting between them by ionic interactions due to their high fluoride-cation affinity onto the surface. 42 In cubic NCs, high symmetry onto the surface, this interaction is promoted in all directions allowing spherical assembled systems. However, in orthorhombic NCs the interaction by [010] direction is highlighted due to the high fluoride-cation interaction, making a selective organization allowing rod-like supraparticles in the standard synthesis ( + NH 4 promoted interaction) and in acidic method (H 3 O + mediated interaction), following a citrate mediated ionic bridges (standard method) or an ionic condensation (acidic methods).…”

Using evolved gas analysis – mass spectrometry to characterize adsorption on a nanoparticle surface

“…The results agree with the previous set of experimental and computational characterization performed to obtain the real surface image of these systems. 21,22 We show that EGA-MS allows us to identify all the ions attached to the NP surface and to know the coordination of the multidentate ligand (e.g. citrate), disclosing if it is adsorbed by one or two carboxylate moieties.…”

“…We have applied the method to three different systems reported previously: (i) YF 3 nanoparticles of $5 nm, (ii) YF 3 supraparticles of $80 nm and (iii) LaF 3 nanoplatelets of $7 nm size. 21,22 The use of (i) and (ii) allows us to compare compounds of different sizes while maintaining the composition. Group (iii) allows us to observe the effect of different metals in the LnF 3 compounds.…”