Characterization of highly concentrated nanoparticle dispersions in evaporating droplets by in-situ Small Angle X-ray Scattering
Colloidal stability is crucial when using nanoparticles in applications such as inkjet printing of conductive structures, fabrication of optical nanocomposites, and quantum-dot enhanced sensors and displays. Several fabrication techniques involve the agglomeration or self-assembly of nanoparticles during solvent evaporation, but little is known about the stability of nanoparticles at high concentrations because most colloidal methods are limited to relatively low particle concentrations.
Here, we study the agglomeration of nanoparticles in a wide range of concentrations up to complete drying in an evaporating solvent using time-resolved, in-situ Small Angle X-ray Scattering (SAXS). We monitor a “hanging droplet” of the dispersion (see figure 1) and simultaneously quantify nanoparticle agglomeration and solvent content. Particle agglomeration causes an increase in the structure factor; solvent evaporation is followed by measuring the “solvent peak” originating from the short-range order of the solvent molecules. Both features occur on different length-scales and are thus well-separated in the reciprocal space recorded by SAXS.
We investigated gold nanoparticles stabilised by an octanethiol ligand shell dispersed in linear alkanes (heptane, nonane, decane), cyclohexane, and toluene. The concentration c’ at which particles agglomerated strongly depended on the solvent. We found c’ that differed by two orders of magnitude from the worst solvent, toluene, to the best (cyclohexane). A detailed analysis of the agglomeration fraction as a function of free nanoparticle concentration revealed a constant concentration cs of non-agglomerated particles during the agglomeration process that was close to c’ for all solvents but cyclohexane. The concentration-dependent colloidal stability in certain solvents is thus reminiscent of ideal molecular solubility. Other solvents lead to considerable deviations, and we will discuss hypotheses to explain their origin.
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|Extended Abstract||Figure1||“Hanging droplet” experimental set-up to study in-situ nanoparticle agglomeration during solvent evaporation||114 KB||Download|