(365a) Deposition of Metal Nanoparticles into Wide Area Thin Films and Ordered Arrays Using Co2-Expanded Liquids | AIChE

(365a) Deposition of Metal Nanoparticles into Wide Area Thin Films and Ordered Arrays Using Co2-Expanded Liquids

Authors 

Liu, J. - Presenter, Auburn University
Anand, M. - Presenter, Auburn University


The assembly of supramolecular architectures such as thin films and ordered arrays from stable nanoparticle building blocks is an area of significant interest. Developing strategies that target ordered and wide area thin films provides opportunities for applications in electronic devices, optical materials, sensors, molecular catalysis, and others. Among the most commonly used methods employed to fabricate structurally ordered 2-D superlattice structures involves the simple evaporation of an organic solvent from a nanoparticle dispersion thereby drop casting the nanoparticles on the surface of a substrate. In this case, the organic solvent evaporation results in dewetting effects, capillary forces and surface tensions that are detrimental to the supramolecular assembly of nanoparticles into low-defect and wide area nanoparticle thin films and order arrays.

Compressed CO2 has been used as an alternative solvent to overcome these dewetting instability issues due to its vanishingly low interfacial tension and excellent surface wetting properties. However, much of the focus on CO2 as a solvent for nanoparticle processing has relied heavily on environmentally suspect and expensive fluorinated ligands/surfactants due to the weak solvent strength of CO2 towards conventional hydrocarbon ligands and surfactants. CO2 gas expanded liquid systems provide a tunable alternative to compressed CO2 with a wider range of solvent properties available. These systems allow for the stabilization and controllable deposition of nanoparticles from these CO2 tunable solvent systems utilizing only conventional hydrocarbon stabilizing ligands rather than fluorinated compounds. The addition of CO2 to the organic liquid mixture results in a reduced solvation of the nanoparticle ligand tails thereby inducing nanoparticle precipitation and targeted deposition onto a specified surface. Subsequent addition of CO2 and heating into the supercritical state provides for the removal of the organic solvent thus avoiding the dewetting effects and surface tensions that commonly exist in the evaporating solvent methods. Careful control over the expansion of the liquid solution via CO2 injection allows for precise manipulation of the thermophysical properties that govern this deposition and assembly process. Using this new CO2 expanded liquid particle deposition technique, we have produced low defect, structurally ordered and wide area metallic (Au, Ag, Pt) nanoparticle thin films by advantageously avoiding the detrimental dewetting and surface tension effects in the system. Ligand (e.g. alkane thiols, carboxylic acids) stabilized metallic nanoparticles (Au, Ag, Pt) were precipitated from organic solvent by controllably expanding the solution with CO2.

The availability of stable nanoparticle building blocks with uniform size and shape is a prerequisite to the assembly of the structurally ordered thin films using this CO2-expanded liquid technique. To this end, we have employed two new particle synthesis and separation approaches that yield very monodisperse ligand stabilized nanoparticle dispersions in simple processing steps. The first approach involves a straightforward β-D glucose assisted aqueous phase process for the synthesis of Au nanoparticles followed by extraction of the particles into an organic phase via ligand exchange from the β-D glucose to dodecanethoil. This extraction process results in a Au nanoparticle dispersion with a narrow size distribution (5nm, σ = 1.1 nm) in an organic solvent while leaving the β-D glucose in the aqueous phase and without need for elaborate and time-consuming post synthesis processing to narrow the particle size distribution. In addition, we have developed a rapid and precise nanoparticle size selective fractionation technique that utilizes the pressure tunable solvent properties of CO2-expanded liquids. By pressurizing and expanding a single organic solution with CO2 gas, ligand stabilized metal nanoparticles of a desired mean size were size selectively precipitated and separated for further use in ordered thin film formation. The monodisperse nanoparticles obtained through these two synthesis/separation techniques were successfully employed in the assembly of structurally ordered, low defect and wide-area nanoparticle thin films using this CO2 expanded liquid strategy. UV/Vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), electron diffraction (ED), and energy dispersive spectroscopy (EDS) techniques were used to characterize the nanoparticles and the assembled structures in the study.