Laboratory for Energy Materials and Nano-Biomedicine

   IN DEPARTMENT OF CHEMICAL AND MATERIALS ENGINEERING

Nanostructured Materials Processing and Characterization

Introduction

Nanotubes are used in many applications because of their desirable bulk properties. Unfortunately, the surface of the nanotubes is often not ideal for the particular application. The ability to deposit well-controlled coatings on nanotubes would offer a wide range of technological opportunities based on changes to both the physical and chemical properties of the nanotubes. Atomic layer controlled coatings on nanotubes, for example, would allow nanotubes to retain their bulk properties but yield more desirable surface properties. These ultrathin coatings could act to activate, passivate or functionalize the nanotubes to achieve both desirable bulk and surface properties.   

The current trend of developing nanophase materials has motivated an increasing need for nanometer-scale structures in a variety of applications.  Indeed, it is clear that to achieve unique mechanical, physical, chemical, and biomedical properties, it is necessary to develop novel synthesis routes by which an ultrathin film can be deposited on the nanoparticle surfaces.   One example is useful for illustration. AlN particles have high thermal conductivity and are added to composites for thermal management applications. One difficulty is that H₂O can corrode the AlN particles and generate NH₃. Ultrathin coatings on AlN particles could protect the AlN particle from H₂O corrosion without compromising its high thermal conductivity. Novel methods to deposit atomic layer controlled coatings on particles are desired that can be applied to model systems and systems of both military and commercial uses. 

Our Research Activities

Development of Plasma Coating Systems

We plan to tailor new nanoparticle/nanotube surface structures by a unique plasma treatment for several novel applications.  The unique properties required in these nanomaterials could be best achieved by our plasma method.  The goal of the research is to design and build a versatile system that is capable of coating a variety of nanoparticles/nanotubes with functional thin films.  However, for a focused research project, we will specifically develop new structures on selected nanoparticle surfaces for the following specific applications:

 (1) Plasma Coating of Carbon Nanotubes for Enhanced Dispersion and Interfacial Bonding in Polymer Composites

Recently, it has been shown in laboratory scale tests that the physical properties and performance of composite materials can be significantly improved by the addition of small percentages (~2%) of carbon nanotube particles. However, there have not been many successful large-scale tests using a wide variety of soft phase materials that show the advantage of using nanotubes as fillers over traditional carbon fibers. The main problem is in dispersing the nanotubes and creating a strong interface between the nanaotube and the polymer matrix. This strong interface between the nanaotube and the polymer matrix is essential to transfer the load from the matrix to the nanotubes and thereby to enhance the mechanical properties of the composite. A crucial reason for these difficulties is that the nanotubes are atomically smooth and have nearly the same diameters and aspect ratios as polymer chains. In addition, the as-produced nanotubes usually form as aggregates that behave differently in response to a load as compared to individual nanotubes. To maximize the advantage of nanotubes as reinforcing particles in high strength composites, the aggregates need to be broken up and dispersed or cross-linked to prevent slippage.

In this research we plan to tailor new nanotube surface structures by a unique plasma treatment for the synthesis of polymer composites.  The unique properties required in these composites could be best achieved by our proposed plasma method.  The goal of the research is to enhance nanotube dispersion in the polymer and the interfacial bonding between the carbon nanotubes and matrix by depositing ultrathin films on the surfaces of the nanotubes, and to study the fundamental interfacial structures related to the bonding mechanisms.  These are new experimental approaches and currently being researched in our laboratory.  Very positive results have been obtained (See papers in “The Latest”).

(2) Effects of plasma nanoparticle surface modification on interfacial behaviors and mechanical properties in carbon nanotubes-Al₂O₃ composites

Carbon nanotubes (CNTs) have been widely used as additives to reinforce strengths ofcomposite materials, such as epoxy,¹ petroleum pitch,² PMMA,³ and alumina composites ⁴ due to their extraordinary mechanical properties.^5-7 Exciting progress in using CNTs as reinforced additives in ceramic composites has been recently reported.^8-12 However, a great challenge still remains in using CNTs as reinforced agents in ceramic composites mainly associated with high temperature process, in which the oxidation process occurs and carbon nanotubes are damaged due to high temperature process (>1500°C). A relatively-low density (< 93%) was achieved for CNT-Fe-Al₂O₃  nanocomposite ceramic sintered at 1500 °C,¹² and thus high pressure sintering and special complex processing methods are required to achieve high density for CNT-reinforced ceramic composites.^8-12

Additionally, no significant enhancement in the mechanical properties for CNTs-Al₂O₃ composite prepared by the hot press and extrusion methods was achieved as a result of aggregation (or inhomogeneous dispersion) of CNTs in the composite materials.⁴ The effective utilization of nanotubes in composite applications depends strongly on the ability to disperse CNTs homogeneously throughout the matrix. Furthermore, good interfacial bonding is required to achieve load transfer across the CNTs-matrix interface, a condition necessary for improving the mechanical properties of ceramic composites. Considerable progress on dispersing CNTs in ceramic matrices^11-16 have been reported and toughening alumina has been achieved by addition of nanosize second phases and the spark plasma sintering.^17,18 However, the critical issues on CNTs dispersion and high temperature oxidation are yet to be addressed. Furthermore, it is of fundamental importance to develop novel process that can modify the surfaces of the nanoparticles and nanotubes in high density CNTs-Al₂O₃ composites and understand the effects of interfaces on the mechanical behaviors and physical properties.

Recently, we have developed a novel plasma polymerization method to functionalize the surface of CNTs.^19-23 Significant enhancements in the mechanical properties in CNTs-polystyrene nano-composites have been observed due to uniform dispersion of CNT in polystyrene matrix and a strong interfacial bonding. By employing this unique plasma polymerization method to modify the surfaces of nanoparticles involved in the CNTs-Al₂O₃ composites, we found significant interfacial enhancement in sintered CNTs-Al₂O₃ composites. This nanoparticle surface modification contributed to considerable improvement in bulk mechanical properties. The study on the relationship between the interface and mechanical properties will be fundamentally important in the development of high-quality nanotube composites for a variety of applications.

In this study, we present the experimental results on the plasma surface functionalized carbon nanotubes and alumina nanoparticles and their effects on the structural interface behaviors. The mechanical property data are reported for the CNTs-Al₂O₃ composites with and without surface modifications. We discuss the possible mechanisms on the enhancement of bulk properties in association with the structural interfaces observed by transmission electron microscopy.

(3) Ion Exchange by Nanoparticle Surface Functionalization

In many of the technologies currently used for industrial process and waste stream treatment, physical and chemical processes dominate, for example, granular-media filtration processes (GMF) and membrane separation devices. Most granular-media filters are simply beds charged with sand, if the object is the removal of suspended materials, or with activated carbon or ion exchange resins, if the objectives are the removal of dissolved organic materials or ionic species, respectively. Membrane systems usually involve the removal of colloidal or dissolved contaminants by means of straining in a thin separation layer. In such a process, dissolved solids can be removed from process and waste streams through the use of semipermeable membranes having pore diameters as small as 3 Å. The membrane consists of a thin film having a thickness of about 0.25 mm, supported by a more porous substructure. In membrane process, the typical water fluxes range from 0.35 to 0.7 L/m²min. The disad­van­tages associated with using membrane process are both high cost and low water fluxes. Small pores limit water flux making this approach less effective in treating a large quantities of water.

However, for the recovery and recycle of high value, strategic and critical metals, a more cost-effective method of ion recovery is required. The proposed technology will not only drastically improve the efficiency of the ion exchange process, compared to traditional ion exchange resin, but enhance that effectiveness by use of a charge modulated electric field, and allow in-situ regeneration of the nanostructured clusters, to avoid generation of a secondary waste stream. In ion exchange resins, the spherical shaped particles range on the order of a few millimeters.  This already sets a low limit on the surface area of the particles that can interact with the ions in water. Using nanoclusers in this research, the surface area is expected to increase significantly. A novel approach using nano-clusters has been carried in the research. This involves coating nano ceramic particles with acrylic thin films. For details, please see our papers in “The Latest.”

(4) Development of Highly conductive and Loss Thin Films via Nanoparticle Surface Coatings

The conductivity of metals is, in general, 1 to 2 orders of higher than that of tin doped indium oxide (ITO), but metal is not transparent.  However, when the thickness of the metal is down to nano-meters, the metal/dielectric multilayer coating exhibits metallic conductivity and dielectric transparency. Resonant tunneling through multiple metal films can enhance the transmission by several orders of magnitude.  The periodic nature of the metal/dielectric lattice causes the light to propagate through the metal layers with extremely low loss.  The most unique feature of the metallic optical filter is the ability to have a single pass band and block all other radiation from static fields to soft X-rays. This remarkable property is a result of the highly dispersive nature of metals.

 This proposed work is to develop a nano-engineered particle: surface bilayer [pyrrole (2-5 nm)/metal (10-40 nm)] coated nanoparticles (TiO₂ and TiN, 20 to 100 nm).  This nanoparticle composes three functionalities: highly transparent, highly conductive and a broadband radiation blocking from static fields to soft X-rays.  Development of such unique nanostructures would not only benefit the specific industrial applications, such as panel displace and anti-static/anti-reflection (ASAR) coating for lenses and CRT, but also the electronic industry in general.

 (5) Coating of Magnetic Nanoparticles for Biological Applications

Currently, all of the commercialized magnetic particles are in the size range of 0.5 – 10 µm.  Applications of paramagnetic nanoparticles (<500 nm) for biological separation and detection have been rarely reported. It has been repeatedly demonstrated in other particle-based reporter technologies (e.g. colloidal gold or latex beads) that colloidally stabilized, small diameter particles can provide higher sensitivity and greater surface area/volume ratios of the reporter. Diagnostic sensitivity is increased when particle size is no longer a limiting factor in analyte recognition. High surface area to volume ratios are critical when functionalizing particles for the attachment of biological ligands. The development of biocompatible fluorescent paramagnetic nanoparticles can drive the magnetic bead technology to an unprecedented level and will open up many new opportunities in the diagnostic and medical industries.

(6) Coating of Rare Earth Doped Oxide Thin Films for Nano-Phosphors Study

Y₂O₃:Eu³⁺ and related materials are common phosphors in optical displays and lighting applications. The resolution of images on a cathode-ray tube display is closely related to the particle size of phosphors. Usually, smaller particles are favored for higher resolution. Many methods, such as homogenous precipitation and the sol-gel method, are used extensively to prepare nano-phosphors. Williams prepared nano-crystalline Y₂O₃:Eu³⁺ by a gas phase condensation technique using carbon dioxide laser vaporization of pressed and sintered pallets of 0.1% Y₂O₃:Eu³⁺. In 2003, Chen and his colleagues prepared layers of Y₂O₃:Eu³⁺ coated on alumina nano particles, which shows anomalous optical properties and phase transition behaviors. One of advantages of this work is that the size of the Y₂O₃ particles can be controlled conveniently by selecting the substrate’s diameter to get the uniform products and the sintering temperature can also drop. In their paper, homogeneous precipitation method was taken and the centrifuging method was used. So, added Y³⁺ and Eu³⁺ ions can not take part in the reaction completely. Thus, the amount of the reactants can not be measured accurately, which leads the uncertainty to the Y₂O₃ layer’s thickness and dispersion of it over the alumina particles.

The idea comes from self assembly mechanism of ionic surfactants adsorbed on mineral oxides that is to find a suitable surfactant, which can disperse the nano Al₂O₃ particles into the H₂O phase uniformity. In the core of hemi-micelles or ad-micelles formed by the surfactant, there are both the Al₂O₃ particles and Y³⁺ ions. And by changing the environment, such as the PH value, temperature and the concentration, the diameter of the micelles can be below 100 angstroms. Micelles are separated by the surfactant molecules, which can avoid the aggregation of Al₂O₃ particles in the precipitation process and the early stage of sintering process. When the furnace temperature is enhanced, the surfactant, usually the organic molecules, which consist of carbon and hydrogen atoms, can be removed by the oxidation effect due to the existence of oxygen in the air. Then the Eu³⁺ can enter into the crystal lattice of the Y₂O₃ by sintering reaction and the diffusion.

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