Revealing the Effects of the Non-solvent on the Ligand Shell of Nanoparticles and Their Crystallization

Abstract

When nanoparticles (NPs) are assembled from solution, a common assembly method of choice is either solution destabilization or solvent evaporation technique. The destabilization of the NP solution by non-solvents results in the formation of faceted supercrystals (SCs) while periodic film-like assemblies are typically formed by solvent evaporation. Here, we reveal the effect of non-solvents in washing, dispersing, and crystallizing NPs. Small angle neutron scattering (SANS) is employed for monitoring the ligand shell of NPs in solutions upon introduction of various non-solvents. The SC crystallization process is traced in situ with small-angle X-ray scattering (SAXS), and the structures of the resulting single-crystalline SCs are examined in detail by mapping the reciprocal space using SAXS and wide-angle X-ray scattering. Our study suggests that the relative miscibility of the non-solvent with solvents and ligands determines the solvation and thickness of the ligand shell and thereby the resulting structure of SCs. In the early stage of crystallization, truncated octahedral PbS NPs form SCs with face-centered cubic (fcc) symmetry. In the later stage, the fcc symmetry is preserved in the SC formed by larger (5.60 nm) NPs, but the SC assembled from smaller (4.14 nm) NPs undergoes a phase transition into body-centered cubic (bcc) lattice via Bain transformation, becoming a polycrystalline SC containing three structurally correlated bcc domains and one untransformed fcc domain. Our study provides the detailed understanding of the non-solvent effect that impacts beyond the formation of SCs, enabling the judicious selection of solvent/non-solvent mixtures for NP purification.

Supercrystallography-Based Decoding of Structure and Driving Force of Nanocrystal Assembly

“…However, an OA-triggered screening effect appears to weaken the strength of the direct interactions between inorganic NC cores so that the extent of orientational ordering is largely reduced in the nucleated fcc superlattice (Figure 10d,e) [55]. With a similar experimental control, large fcc supercrystals with multiple groups of orientational ordering were observed by various research groups [51,56], confirming the reliability of the derived guiding rules.…”

“…Among a large variety of NC systems, PbS NCs have been extensively used to study the spontaneous NC self-assembly, and various superlattices have been observed accordingly, including fcc, bcc and a series of intermediate ones, such as the tetragonal superlattice [18,25,41,46,51,52,53,54,55]. Instead of covering many NC systems with too many details to distract attention, only PbS NCs with intermediate ratios of surface truncations are used as an example here to detail how the design and control of typical environments are experimentally made to activate specific NC interactions towards the ultimate harvest of a single superlattice of fcc or bcc at both small and large scales as desired (Figure 10) [46,55].…”

“…Based on the phase diagrams, one could delicately play the defined parameters using various types of carbon-hydrogen ligands and solvents in experiments to harvest single superlattice of either bcc or fcc or others. Witnessing the agreement between computational predictions and experimental discoveries [34,36,46,51,52,53,54,55,56] and the newly emergent approach of machine learning [60], computational design will certainly come to serve as an increasingly important tool in the future design and fabrication of desired NC superlattices with tailored properties for applications.…”

Smectic Liquid Crystalline Titanium Dioxide Nanorods: Reducing Attractions by Optimizing Ligand Density

“…[32] Excess ligands, even if added after completion of the SA, were also found to be able to affect interparticle spacings and the order in colloidal crystals already formed. [36] It is often the case that ligands are not bound covalently to the surface and thus can be replaced or removed by frequent washing steps [35,[42][43][44][45] and conversely that the presence of excess ligands affects SA. [36,46] Additionally, addition of excess ligands above the critical micelle concentration can cause the formation of micelles which can induce depletion attractions between the NPs.…”

“…[28] In recent years many groups working on the self-assembly of NPs have highlighted the importance of ligands on the interactions between NPs and thus on what type of colloidal (liquid) crystal phases, with different symmetries and interparticle spacings they ultimately form. [29][30][31][32][33][34][35][36][37][38][39][40][41] The role of ligands on self-assembly was also recently reviewed. [29] Here we will only mention some typical examples of the role of ligands on SA as discussed in literature and almost all of relevance to the present…”