Stable colloids in molten inorganic salts

Abstract

A colloidal solution is a homogeneous dispersion of particles or droplets of one phase (solute) in a second, typically liquid, phase (solvent). Colloids are ubiquitous in biological, chemical and technological processes1,2, homogenizing highly dissimilar constituents. To stabilize a colloidal system against coalescence and aggregation, the surface of each solute particle is engineered to impose repulsive forces strong enough to overpower van der Waals attraction and keep the particles separated from each other2. Electrostatic stabilization3,4 of charged solutes works well in solvents with high dielectric constants, such as water (dielectric constant of 80). In contrast, colloidal stabilization in solvents with low polarity, such as hexane (dielectric constant of about 2), can be achieved by decorating the surface of each particle of the solute with molecules (surfactants) containing flexible, brush-like chains2,5. Here we report a class of colloidal systems in which solute particles (including metals, semiconductors and magnetic materials) form stable colloids in various molten inorganic salts. The stability of such colloids cannot be explained by traditional electrostatic and steric mechanisms. Screening of many solute–solvent combinations shows that colloidal stability can be traced to the strength of chemical bonding at the solute–solvent interface. Theoretical analysis and molecular dynamics modelling suggest that a layer of surface-bound solvent ions produces long-ranged charge-density oscillations in the molten salt around solute particles, preventing their aggregation. Colloids composed of inorganic particles in inorganic melts offer opportunities for introducing colloidal techniques to solid-state science and engineering applications.

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Figure 1: Nanocrystal colloids in molten inorganic salts.
Figure 2: The nanocrystals stay intact in molten inorganic salts.
Figure 3: The origin of colloidal stability in molten salts.

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Acknowledgements

We thank P. Guyot-Sionnest for discussions, L. Wang for help with taking photographs, V. Srivastava for providing data on GaAs NCs in molten salts, A. Filatov for discussions and help with XPS measurements, F. Zhai and T.-Y. Zheng for help with NMR measurements, Z. Li and M.H. Hudson for providing UCNPs and Fe3O4 NCs, and T. Shpigel for reading the manuscript. This work was supported by the National Science Foundation (NSF; DMR-1611371), Air Force Office of Scientific Research (AFOSR; FA9550-14-1-0367), Department of Defense (DOD) Office of Naval Research (grant number N00014-13-1-0490), and by the II-VI Foundation. G.H. acknowledges support from the National Institutes of Health (NIH; R01 MH103133) and the Human Frontier Science Program (RGY-0090/2014). N.B.L acknowledges support from the University of Chicago Research Computing Center. The work used facilities supported by the NSF MRSEC (DMR-14-20703), and resources of the Center for Nanoscale Materials and Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (DE-AC02-06CH11357).

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Contributions

H.Z. performed and designed the experiments, analysed data, and co-wrote the paper. G.H. provided UCNPs. B.L. set up SAXS measurements and analysed data. K.D., N.B.L. and S.V. designed and carried out theoretical analysis and MD simulations. D.V.T. conceived and designed experiments and simulations, analysed data, co-wrote the paper, and supervised the project. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Dmitri V. Talapin.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Photographs illustrating the generality of NC colloids in molten inorganic salts.

a, The eutectic AlCl3/NaCl/KCl molten salt. b, A stable colloid (~5 mg ml−1) of Pt NCs in molten AlCl3/NaCl/KCl (AlCl3:NaCl:KCl = 63.5:20:16.5 mol%, also referred to as AlCl3/AlCl4). The three photographs show the same colloid at different angles. c, Solidified samples of various NCs (labelled) in AlCl3/NaCl/KCl (~5 mg ml−1 for all NCs). d, Additional examples of various combinations of NCs and molten salts, including Pt NCs in molten AlBr3, NaYF4:Yb,Er/CaF2 upconverting nanoparticles (UCNPs) in molten LiCl/LiI/KI, InP NCs in molten CsBr/LiBr/KBr, InP NCs in molten ZnCl2/NaCl/KCl, and UCNPs in molten ZnCl2/NaCl/KCl. The concentrations of NC dispersions shown in d are 5–10 mg ml−1.

Extended Data Figure 2 Integrity and surface chemistry of NCs in molten salts.

a, b, Histograms of the size distributions of Pt NCs: a, with native OA and OLA ligands, and b, NCs recovered from AlCl3/NaCl/KCl and refunctionalized with OA and OLA ligands. c, Photographs of Pt, Pd and Fe3O4 NCs in toluene. These NCs were recovered from AlCl3/AlCl4, redispersed in toluene with additional organic ligands, and purified with toluene/ethanol. d, Fourier transform infrared (FTIR) spectra of Pt NCs with organic ligands (black) and recovered from AlCl3/AlCl4 (red). The absence of vibrational peaks of original organic ligands (for example, C–H stretching at 2,800–3,000 cm−1) suggests that NCs dispersed in AlCl3/AlCl4 are free of organic ligands at the surface6. We speculate that [Al2Cl7] (and the minor Al2Cl6 and AlCl3), a very strong Lewis acid, plays a similar role to Me3O+ in Me3OBF4 as the ligand-stripping agent7. As a consequence, NCs with both organic-ligand-capped surfaces (using a phase transfer method) and bare surfaces could be well stabilized in AlCl3/AlCl4. e, FTIR measurements of CdSe/CdZnS core/shell QDs recovered from the dispersion in NaSCN/KSCN show a shift of the SCN peak to higher frequencies (from 2,060 to ~2,090 cm−1) compared to pure NaSCN/KSCN. According to ref. 16, Cd(SCN)2 powders and SCN-capped CdSe NC solids show characteristic SCN stretching peaks at 2,100–2,150 cm−1, which are at higher frequencies compared to NaSCN and KSCN. Thus, the observed shift was probably due to the formation of Cd– or Zn–SCN complexes at the QD surface.

Extended Data Figure 3 Stability and photoluminescence (PL) of nanocrystals in molten salts.

a, b, Stable colloids of UCNPs in toluene (with OA ligands) and in DMF (bare NCs treated by NOBF4) under ambient light (a) and illuminated by a 980 nm near-infrared (IR) laser beam (b). An orange emission was observed under IR illumination for both solutions. c, Photographs of a solidified UCNP/NaSCN/KSCN composite under ambient light (left panel and top right panel) and illuminated by a 980 nm near-IR laser (middle panel and bottom right panel). The orange emission from UCNPs was preserved. The dark object at the bottom of the vial (shown in the right panels) is the stir bar. d, Photograph of UCNPs recovered from NaSCN/KSCN and redispersed in toluene with additional OA, showing an orange emission under IR illumination. e, PL spectra of UCNPs in toluene and in NaSCN/KSCN (SCN) in both solid and molten states. f, The X-ray diffraction (XRD) patterns of original UCNPs and UCNPs recovered from NaSCN/KSCN. No change in the XRD pattern is observed. g, PL spectra of CdSe/CdZnS QDs in toluene and NaSCN/KSCN (SCN) in both solid and molten states. h, i, The temperature dependence of PL spectra of QDs in heptadecane (C17H36; h) and SCN (i). In both cases, the samples were preheated at 150 °C and allowed to cool down. The red-shifted PL peak at high temperatures was observed in both C17H36 and SCN because of the reduced bandgap. j, l, Photographs of original and recovered/refunctionalized QDs in toluene solution under ambient (j) and UV (l) light. k, Absorption (solid lines) and emission (dashed lines) spectra of QDs with original organic ligands (black) and the ones recovered from NaSCN/KSCN and re-functionalized by OA (red). Minimal changes in the absorption spectra and a slight decrease of PL efficiency were observed after dispersing QDs in molten NaSCN/KSCN for several days.

Extended Data Figure 4 Unstable NC suspension in Lewis neutral salts.

ad, Photographs of unstable, inhomogeneous mixtures of Pt NCs in a, AlCl4, b, AlCl4/AlBr4, c, NO3 and d, NO3/NO2. Pt NCs undergo severe agglomeration owing to the lack of repulsive force. See Extended Data Table 1 for a full list of the components of the salts.

Extended Data Figure 5 Stable NC colloids in ionic liquids.

a, Photographs of stable NC colloids in P+P (Methods). bd, Small-angle X-ray scattering (SAXS) patterns of CdSe (b), QDs (c) and Pt NCs (d) in toluene and P+P. The SAXS patterns of NCs in P+P resemble those of NCs in toluene, indicating high NC colloidal stability. e, Photographs showing the phase transfer of CdSe NCs from octane to [BMIM]+Cl without additional ligands. f, Photographs of stable CdSe NC colloids in [BMIM]+Cl. g, SAXS patterns of CdSe NCs in toluene (with OA/TOP/TOPO) and in [BMIM]+Cl. All samples were gently heated to keep [BMIM]+Cl in the molten state.

Extended Data Figure 6 CdSe NC colloids in imidazolium-based ionic liquids.

Figure shows the effect of anions in ionic liquids on colloidal stability and the absorption features of nanocrystal colloids. a, Photographs showing the phase transfer of CdSe NCs. Top, pure ILs; middle, biphasic systems with CdSe NCs in octane as the top layer and the ILs as the bottom layer; bottom, CdSe NCs transferred from octane to Lewis basic [BMIM]+Cl and [BMIM]+I. In comparison, no phase transfer was observed in the Lewis neutral [EMIM]+[EtSO4] or [BMIM]+BF4. Photographs of b, stable colloids of CdSe NCs in (from left to right) [BMIM]+Cl, [EMIM]+[EtSO4] with added [BMIM]+Cl, and [EMIM]+[EtSO4] with added [BMIM]+I and c, a stable colloid of CdSe NCs in [BMIM]+BF4 with added [BMIM]+Cl. Adding a small amount of surface-binding chloride or iodide ions to [BMIM]+BF4 or [EMIM]+[EtSO4] resulted in a stable CdSe NC colloid (b, c) free of the original organic ligands. dg, UV–visible absorption spectra of CdSe NCs in various ILs and their mixtures with organic solvents. Shown for NCs in d, P+P, e, [BMIM]+I, f, [EMIM]+[EtSO4] with a small amount of [BMIM]+Cl, and g, ILs diluted with toluene or NMF. The absorption spectra of pure ILs and CdSe NCs in toluene are given for reference.

Extended Data Figure 7 Simulations of CdSe surface in molten KCl.

a, Simulated charge density profile as a function of the distance away from the [001] surface of a CdSe nanocube (2.45 nm × 1.85 nm × 1.85 nm) in molten KCl (black dots) and the fit with Ginzburg–Landau theory (solid line). The charge density is defined as [n+(D) − n(D)]/ns, where n(D) and n+(D) are the densities of Cl and K+ at the plane of D while ns is the total ion density. b, Simulated charge density profiles around a CdSe nanocube sized as in a in molten KCl, and the corresponding fits at various temperatures above the melting point of KCl. c, Simulated free energy of layering between two quasi-spherical CdSe NCs (10 nm in diameter) in molten KCl. The quasi-spheres are formed by one parallel facet and 4, 6 or 8 tilted facets (see Supplementary Discussion for more details). d, Simulated free energy of layering between two structureless (or chemical inert) walls in molten KCl (blue dots) and the fit by Ginzburg–Landau theory (black curve). Error bars in d are related to state-state covariance as discussed elsewhere35 (see Supplementary Discussion for additional details).

Extended Data Figure 8 Nanocrystal colloids in liquid metals.

a, Photograph of a dispersion of ligand-stripped Pt NCs in liquid Ga–In eutectic (Tm ≈ 15.5 °C). Pt NCs were homogeneously distributed in the metal matrix with a concentration of ~4 mg ml−1, as indicated by ICP–OES analysis. b, Photograph of two metal wires soldered by a piece of Wood’s metal (50% Bi, 26.7% Pb, 13.3% Sn and 10% Cd by weight, Tm ≈ 70 °C) containing Pt NCs. c, d, TEM (c) and high-resolution TEM (d) images of Pt NCs in a thin layer of Ga–In eutectic, showing that the NCs retain their integrity and are uniformly distributed throughout the metal matrix. e, Powder XRD pattern (traces) of original Pt NCs and NCs recovered from Ga–In. The line pattern (bottom) shows the diffraction peaks for the bulk fcc Pt phase.

Extended Data Figure 9 Colloidal synthesis and chemistry in molten inorganic salts.

a, Photographs of PbX2 (X = Cl, Br, I) in molten CsBr/LiBr/KBr. In each salt mixture, PbX2:CsBr = 0.03 mol%. b, Solidified PbX2 in CsBr/LiBr/KBr under ambient (top) and UV light (bottom). Adding PbX2 (X = Cl, Br, I) to CsBr/LiBr/KBr resulted in the formation of colourless (mixed) compounds in the molten state (a). On solidification, the (mixed) compounds showed vivid colours and strong PL under UV light. This indicates the formation of emissive, ternary or quaternary alkali lead halides in molten salt. c, TEM image of GaAs NCs and d, a photograph of GaAs colloidal solution. e, Raman spectra of bulk GaAs (black), as-synthesized (275 °C) GaAs NCs (red) and GaAs NCs annealed at 500 °C using a CsBr/LiBr/KBr molten salt as the solvent (blue). f, XRD patterns of as-synthesized and annealed GaAs NCs prepared as in e. GaAs NCs formed a stable colloid in molten CsBr/LiBr/KBr when using a ‘solvent-free’ method. After annealing in the inorganic salt solvent at 500 °C, the quality of the GaAs NCs was significantly improved, as indicated by the well-defined TO and LO modes in e, while the NC size was preserved (f).

Extended Data Table 1 Experimentally tested combinations of NCs and molten inorganic salts or ILs forming homogeneous colloids

Supplementary information

Supplementary Information

This file contains Supplementary Methods, a Supplementary Discussion, Supplementary Figures 1-13, Supplementary Tables 1-2 and Supplementary References. (PDF 2502 kb)

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Zhang, H., Dasbiswas, K., Ludwig, N. et al. Stable colloids in molten inorganic salts. Nature 542, 328–331 (2017). https://doi.org/10.1038/nature21041

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