Linking the Ligand Shell Structure of Nonpolar Nanoparticles to their Colloidal Stability

by Dr Bart-Jan Niebuur (INM – Leibniz Institute for New Material)

Monday 10 Jun 2024, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

Ligand-stabilized gold nanoparticles (Au-NPs) are promising building blocks for materials with well-defined structures and properties. Their colloidal stability is essential for their combination with other components and the formation of superstructures that define functionality. The stability of Au-NPs coated with short, linear hexadecanethiol ligands is governed by a disorder-to-order transition of the ligand shell [1]. In this presentation, I address three strategies to improve the colloidal stability of Au-NPs by stabilizing the disordered state of the ligand shell, namely by the use of solvent mixtures, molecular additives, and alternative ligand designs. Small-angle X-ray scattering (SAXS) was used to quantify the Au-NP colloidal stability, which is correlated with the ligand shell structure, studied using small-angle neutron scattering (SANS) in combination with molecular dynamics (MD) simulations.

Firstly, it is found that, in mixtures of cyclohexane and hexadecane, the agglomeration temperature (T_agglo) depends non-linearly on solvent composition, and is lower than expected for ideal mixing. SANS showed that, at intermediate solvent compositions, the ligands are preferentially solvated by cyclohexane. This stabilizes their disordered state and, therefore, improves the colloidal stability of the Au-NPs [2].

Inspired by these results, cyclic amine and sulfide additives at low concentrations were used to test whether they enrich around Au-NPs. While the amines lowerd T_agglo by not more than 2 K, sulfides lowered T_agglo by up to 28 K at a concentration of 64 mM, but T_agglo increases again above. SANS pointed to the replacement of hexadecanethiol by additive molecules on the Au-NP surface, which strongly hinders ligand ordering, thereby reducing T_agglo. Furthermore, MD simulations showed, that clustering of additive molecules chemisorbed on the gold surface reduces the stabilizing effect, thereby increasing T_agglo at high additive concentrations [3].

Lastly, the ligand design is tested as a means to stabilize the disordered state of the ligands. Coating Au-NPs with kinked and branched ligands reduced T_agglo by ~100 K as compared to Au-NPs coated with their linear counterparts. MD simulations showed, that this is due to a complete absence of ligand ordering [4].

References

[1] D. Monego, T. Kraus, A. Widmer-Cooper et al., ACS Nano 14, 5278–5287 (2020).
[2] M. R. Hasan, B.-J. Niebuur, A. Widmer-Cooper, T. Kraus et al., ACS Nano 17, 9302–9312 (2023).
[3] T. V. Knapp, B.-J. Niebuur, A. Widmer-Cooper, T. Kraus et al., submitted.
[4] T. V. Knapp, D. Monego, B.-J. Niebuur, T. Kraus, A. Widmer-Cooper et al., in preparation.