Surface Engineering of Nanoparticles by Plasma Enhanced Chemical Vapor Deposition
Open Access
- Author:
- Shahravan, Anaram
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 01, 2012
- Committee Members:
- Themis Matsoukas, Dissertation Advisor/Co-Advisor
Themis Matsoukas, Committee Chair/Co-Chair
Darrell Velegol, Committee Member
Robert Martin Rioux Jr., Committee Member
Joan Marie Redwing, Committee Member - Keywords:
- Plasma enhanced chemical vapor deposition
Nanoparticle coating
Nanoparticle synthesis
Hydrophobic coating
Encapsulation - Abstract:
- Altering near-surface properties of nanoparticles or nanopowders without affecting their desirable bulk characteristics has attracted attention in many industries. Among different approaches that have been used to modify surface properties, plasma enhanced chemical vapor deposition is discussed in this dissertation. This work focuses on adapting plasma polymer coating on the surface of micro- and nano-sized particles in a dry environment. Plasma deposited polymer on the surface of nanoparticles improves their physical and chemical properties, and also provides a barrier for encapsulation, protection, and also a source to attach new organic functional groups to change the chemistry of the surface. Another important achievement of this work is introducing a very novel condition to produce responsive particles in the plasma. Deposition process in plasma can be controlled by different parameters such as reactor pressure, hydrocarbon flow rate, and plasma generator power. Here, conditions to produce thin-films or synthesis of responsive nanoparticles are discussed. Core-shell nanostructures were synthesized by plasma deposition in a radio-frequency plasma reactor. Silica and KCl nanoparticles are encapsulated by deposition of isopropanol-based films of amorphous hydrogenated carbon. By controlling the deposition time, under constant deposition rate of 1 nm/min, particles are encapsulated in a layer of plasma polymer with thickness between 15 and 100 nm. Films are robust, chemically inert, and thermally stable up to 250 C. The permeability of the shells is determined by depositing films of various thickness onto KCl nanoparticles and monitoring the dissolution of the core in aqueous solution. The dissolution profile is characterized by an initial rapid release, followed by a slow release that lasts up to 30 days for the thickest films. The profile is analyzed by Fickian diffusion through a spherical matrix. We find that this model captures very accurately the entire release profile except for the first 12 hours during which, the dissolution rate is higher than that predicted by the model. The overall diffusion coefficient for the dissolution of KCl is 3*10^-21. Wetting characteristics of nanoparticles are also tuned using plasma enhanced chemical vapor deposition technique. The plasma polymer coating impart the properties of the precursors used in the plasma technique, without the requirement for nanoparticle surface preparation. For a range of chosen precursors, the water contact angle of a sessile droplet on coated copper oxide nanoparticles is shown to vary from 54 to 76, 92, and 108 degrees. Stable suspensions of coated nanoparticles are fabricated and are studied to be used as nanofluids. These fluids that contain fine nanoparticles are widely used as heat transfer liquid because of their thermal conductivity enhancement due to the existence of the particles. Thermal conductivity of stable suspensions of silica and copper oxide is studied for different nanofluids with polar and non-polar solvents, oils and refrigerants, as their base fluid. Hydrophobic coatings are tailored as a passivating thin-film for aluminum nanoparticles as well. Since they repel water, no thick oxide layer can form on their surfaces. Coated particles are kept in a harsh environment with high humidity and their aluminum content is measured using thermogravimetric analysis. Experimental data reveals that particles are protected against contamination and oxidation. Aluminum particles are the prime candidate as an additive to propellent, combustibles, and explosives due to their high enthalpy of combustion. Deposition of plasma polymer coating also improves their combustion by releasing high energy at elevated temperatures. Hydrocarbon plasmas are known to produce particles and films consisting of cross-linked amorphous hydrogenated carbon. Here, we document the formation of liquid-like particles which subsequently solidify via a process that releases hydrogen and produces a solid microbubble with micron-size diameter, nanometer-size shell thickness, and high volume fraction, in excess of 90%. These materials are produced in a toluene plasma under conditions that promote low degree of cross-linking (low power, high pressure). Liquid-like droplets produced under these conditions are seen to blow up in TEM under irradiation by the electron beam and to produce solid bubbles with diameter of about 3 micrometers. The solid-to-liquid transformation is also observed under laser irradiation of sufficient power, as well as under heating. We present evidence that the formation of these microbubbles is due to solidification of the liquid-like precursor that is accompanied by release of hydrogen. This mechanism is confirmed by a simple model that provides quantitative description of the particle size before and after solidification. These unique stimuli-responsive particles exhibit the potential of using temperature, electron beam, or laser as a tool to change their size and structure which may find application in thermal insulators, lightweight materials and light scattering agents.