CBET Award Achievements
Notable Accomplishments from CBET Awards

Making Nanotubes and Nanowires with Flames

Stephen Tse, Rutgers University New Brunswick

Background:  Flames are thought of as being destructive, but NSF-sponsored research shows how they can be used to produce nanotubes and nanowires.  Nanostructured materials form a new class of engineering materials with special and useful physical, optical, and electromagnetic properties.  Combustion synthesis of materials has demonstrated a history of scalability and offers the potential for high-volume commercial production for these new materials at reduced costs.

The researcherís findings on new approaches to combustion synthesis of nanomaterials promise to yield practical amounts of tailored carbon nanotubes (CNTs) for the first time.  CNTs have been a focus of interest for some time but the technology has not been available to produce commercial quantities of nanotubes with tailored physical properties with high purity.  This research was the first demonstration of flame-synthesis of CNTs for this purpose on a large scale.

CNTs are among the most favorable building blocks of nanotechnology due to their amazing mechanical, electrical, thermal, and optical properties; and their wide range of potential applications includes high-strength polymers, copolymers, composites, ceramics, moldable forms, molecular electronics and nano-scale machines, superconductors, delivery systems of bio-molecules to cells, fuel storage and batteries, flat-panel displays, and computer memory devices.  In addition, semiconducting oxide nanowires, a unique group of one-dimensional nanomaterials, can be produced by these approaches.  They have well-defined and uniform shapes; user-specified and stable surfaces; and single-crystal volumes that are dislocation-free.  These properties are keys to nanoelectronic applications like use as field effect transistors, ultra-sensitive nano-size gas sensors, nanoresonators, and nanocantilevers.

Results:  Direct synthesis of various oxides of nanowires and nanorods has been demonstrated for tungsten oxide and zinc oxide with diverse morphologies.  The technique developed in this work uses flame-based chemical vapor deposition (CVD) with various catalytic supports, augmented by electrical force fields to improve uniformity and productivity.  Using such controlled combustion provides for:

1)  the high energy to melt or evaporate the solid metal into the liquid/gas phase;

2)  the gas-phase chemical species needed to produce the requisite oxide, for example by oxidation of the metal precursors; and

3)  a temperature gradient favorable for precipitation of the nano-wires and -rods.

The larger objective of this research was to understand the mechanisms involved in CNT formation and growth from methane, ethylene, and acetylene.  To assess these growth mechanisms, spontaneous Raman scattering was used to determine the gas-phase temperature profile and to map CO, C2H2, CH4, H2, and O2 species; and laser-induced fluorescence is used to map OH and C2 distributions.  The flame structure for optimum CNT growth can then be determined for a given flame geometry.  Additionally, the researchers have synthesized oxide materials in nano- wire, rod, and ribbon structures directly from substrates without the help of templates.

Stephen Tse Image A

A.  Well-aligned carbon nanotubes.
Stephen Tse Image B   Stephen Tse Image C

B.  Tungsten-oxide nanowires.
C.  Zinc oxide connected nanorods.
Images Credit:  Stephen Tse, Rutgers University

Scientific Uniqueness:  In flame synthesis of CNTs, combustion of the hydrocarbon fuel intrinsically provides not only the source of process heat to establish the required high-temperature environment but also the basic species that are necessary for growth.  Carbon-based species and moist conditions are often needed to maximize growth of inorganic nanowires.  Combustion readily provides these conditions, as H2O is a natural by-product and carbon species are present from thermal decomposition of the fuel.  Furthermore, they can be locally and specifically concentrated at the substrate by tuning the flow conditions.  Flames are complex, combining various modes of chemistry and transport, but well-defined flame configurations allowed probing of the fundamental controlling mechanisms using laser-based spectroscopy.  For most other current methods of synthesis, conditions are often too chaotic to yield any meaningful relationship with growth models and, thus, with models for the processes.

Work is notable because it is an integrated effort incorporating a novel and robust synthesis technique, in-situ laser diagnostics, and state-of-the-art materials characterization.  Such integration allows not only for detailed fundamental study of the mechanisms involved in CNT and nanowire formation and growth but also for the active control of those basic processes.  Thus, nanomaterials with tailored physical properties can be produced in large quantities with high purity.

Work is broadly multidisciplinary: because it combines combustion science with reaction engineering and materials science.

This project addresses the strategic outcome goals , as described in the NSF Strategic Plan 2006-2011, of:

(1) Discovery:  Because it addresses an area of national importance that encompasses a wide range of technologies and can provide valuable fundamental knowledge that can be immediately used by industry, this project directly addresses the strategic outcome goal of Discovery.

(2) Learning.  Graduate students are involved in conducting the research. Their participation provides substantive learning of research methods, analysis, and communication skills. The project also includes a research internship for a high school teacher, who brings back her experience to the classroom, transferring new knowledge and perspectives to the students.

This Nugget represents transformative research.  The work points science in a new direction by showing that reactivity at the nanoscale can be modified by formation conditions and that it impacts reactivity and morphology at the macroscale.  Furthermore, by providing the supporting science to push through a technological barrier, the work is transformative technologically.

Impact on Industry and/or Society:  The processing of nanomaterials can be complex, involving pretreatment, catalysts, and vacuum systems, yet still yielding low growth rates.  Consequently, studies on various nanoscale materials and their applications are presently limited due to lack of easy processes for high rate, purity, and orientation synthesis of such materials.  The growth of CNTs and semiconducting oxide nanowires over large areas remains especially challenging.  In this work, a flame-synthesis method is employed to grow well-aligned, single-crystal nanotubes and nanowires with small diameters, large coverage densities, and growth rates of microns per minute, without any pretreatment and in open environments.  As such, the technique is promising for large-scale applications due to its simplicity, scalability, and economy.

Program Officer:   Phillip R. Westmoreland
NSF Award Number:   0522556
Award Title:   Catalytic Flame Synthesis of Carbon Nanotubes
PI Name:   Stephen Tse
Institution Name:   Rutgers University New Brunswick
Program Element:   1407
NSF Investments:   National Nanotechnology Initiative (NNI)

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This Nugget was Approved by ENG on 15 April 2008.