Use of Gas-eXpanded Liquids as Tunable Solvents for the Preparation of Well-Defined Nanomaterials
Saunders, Steven R.
Type of Degreedissertation
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This dissertation presents the research performed to study the precipitation and fractionation of noble metal nanoparticles using gas-expanded liquids. The unique and novel properties of nanoparticles are often found to be highly size-dependent, as such, methods of fine tuning the size distribution of synthesized nanoparticles is crucial in order to make use of these properties in fundamental studies and applications. Traditional methods of nanoparticle processing are typically qualitative, time intensive, and produce large quantities of organic solvent waste. Previously, a technique has been developed that uses mixtures of pressurized CO2 and an organic solvent to size-selectively precipitate and fractionate nanoparticles. However, this technique was limited to extremely small processing scales (<200 μL volumes) and required relatively large operating pressures (upwards of 35 bars of applied CO2 pressure). This work presents methods of size-selectively precipitating and fractionating large quantities of nanoparticle dispersions utilizing techniques that make use to the tunable solvent properties of CO2-expanded liquids. The methods presented here demonstrate that processing large volumes of nanoparticle dispersions is possible while producing zero waste. A new apparatus is utilized that is simple and cost-effective to build while being scalable to handle larger volumes and modular. This apparatus, at current scales, is capable of size-selectively precipitating and fractionating a polydisperse nanoparticle sample (up to 20 mL) into monodisperse samples. This apparatus can process volumes two orders of magnitude greater than the previous state of the art. Using this technique, it is demonstrated that by judiciously selecting the operating conditions, the recovered nanoparticle sample can be tailored to a specific average size and size-distribution. Also presented are techniques for tuning the range over which nanoparticles precipitate in a gas-expanded liquid by varying the stabilizing ligand and the composition of solvent media in which the nanoparticles are initially dispersed. A thermodynamically stable dispersion of nanoparticles can be brought to the threshold of stability by adding a secondary poor solvent such that any additional reduction in overall solvent strength of the system would induce nanoparticle precipitation. These methods demonstrate that nanoparticle precipitation is possible at an applied gas pressure 85% lower than what was previously reported. Modeling these precipitation and fractionation processes is crucial in order to predict the average sizes and size-distributions of recovered nanoparticle fractions. Three models are presented which capture the dynamics of the system in different ways. First, an empirical model which relates simple experimental measurements of the precipitation process into average sizes of the nanoparticles that could be dispersed at a given set of conditions. Secondly, a fundamental thermodynamic model which balances the inherent van der Waals attractive forces with an osmotic repulsive force (due to solvent-ligand interactions) and an elastic repulsive force (due to the compression of the ligand tails between two nanoparticles) has provided an understanding the physical phenomenon that causes the precipitation of nanoparticles in a gas-expanded liquid. Finally, a rigorous fundamental thermodynamic and statistical model which makes use of the known physical nature of the stabilizing ligand shell in order to predict the size-distributions of recovered nanoparticle samples from the gas-expanded liquid fractionation process.