The role of molecular shell structure and core size in the colloidal stability of non-polar metal nanoparticles
Non-polar metal nanoparticles are important components in sensing, and photovoltaics, for example. Size, shape and colloidal stability at the relevant processing temperatures are crucial for their successful application. Stability is intimately linked to organic molecules on the metal surfaces that direct size and shape during growth and dominate the interactions between the particles. Non-polar particles are not stabilised by Coulomb repulsion and more prone to agglomeration than electrocratic colloids; classical DLVO theory does not explain their colloidal dynamics. Recent results indicate that certain combinations of metal core, ligand, and solvent lead to surprisingly good colloidal stability [1,2] that depends on the exact molecular structure of the shell, quite in contrast to aqueous charge-stabilized particles.
We prepared systematic size series (3 to 12 nm) of non-polar metal nanoparticles with different organic shells and studied their agglomeration as a function of temperature, shell structure, and solvent. Particles with shells composed of organic thiols with different chain lengths were dispersed in linear alkanes and toluene. We compared linear, branched, and unsaturated ligands. Small Angle X-ray scattering (SAXS) provides a quantitative agglomeration fraction and the structure of the forming agglomerates.
Well-dispersed samples at elevated temperatures are first characterized in detail by SAXS. This provides the particles' form factor. Upon stepwise cooling, the structure factor begins to deviate from unity as particle agglomeration sets in. We believe that at any given temperature, an equilibrium is reached between dispersed and agglomerated particles. Their number ratio is related to the structure factor and can be precisely calculated from SAXS. We then define agglomeration temperatures as the temperatures where 20% of the particles are part of larger structures. The agglomeration temperature for a given particle core diameter changed by several tens of kelvin depending on the structure of the shell and the solvent. The large differences cannot be explained by steric repulsion between long ligand molecules alone: disorder of the ligand shell and its entropic contributions strongly affect stability, too. The stability is so sensitive to the degree of molecular order in the shell that “switching” agglomeration is possibly when using ligands shells that undergo order-disorder transitions and solvents that strongly react to the difference.
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|Extended Abstract||Figure1||Temperature dependent agglomeration of non-polar alkane-thiol-coated gold nanoparticles (core diameter d = 8.3 nm) in decane as a function of alkyl chain length.||112 KB||Download|