Carbon Nanotubes
Single wall carbon nanotubes (SWCNT) can be envisioned as a rolled up graphene sheet into a seamless cylinder with fullerene caps. The van der Waals interaction between sidewalls leads to close-packed “bundles,” which are an important physical property and the dimensions can be observed in a scanning electron micrograph. The role-up vectors from a point of origin on a graphene sheet will determine the so-called "chirality" of the SWCNT, which determines whether the structure will be metallic or semiconducting. The optoelectronic properties of a SWCNT will depend directly on the chiral angle and diameter of the nanotube. The optical spectra become unique “signatures” for each SWCNT, and in the case of semiconducting SWCNTs, band gap energies inversely proportional to diameter are observed typically between 0.5-1.0 eV and the use of optical absorption spectroscopy can distinctly monitor these effects. SWCNT spectroscopy is a core strength of the NPRL with capabilities including high resolution SEM, near-IR fluorescence spectroscopy, optical absorption spectroscopy, Raman spectroscopy, and thermogravimetric analysis, among others. The NPRL is investigating the uses of SWCNTs in a number of applications including conductive wires, Li-ion battery electrodes, thermionic emitters, transparent conductive coatings, and additives in bulk-heterojunction organic photovoltaics. The NPRL produces and purifies their own material using laser vaporization synthesis, and chemical refluxing, respectively.
Multi-walled carbon nanotubes (MWCNTs) are carbon nanotubes comprised of multiple “rolled-up” graphene sheets in concentric cylinders. Many of the structural properties are similar to that of SWCNTs, however; due to the many concentric layers, most MWCNTs are metallic-like conductors and do not have the same discrete bound states or characteristic optoelectronic properties. Other techniques, such as Raman spectroscopy, have routinely been used to investigate the properties (e.g., purity) of the MWCNT materials. MWCNTs are a slightly less expensive alternative to high-purity SWNCTs as they can be produced at a greater rate and can perform nearly as well in applications such as free standing electrodes or as conductive additives in polymer composites. The NPRL synthesizes MWCNTs using a CVD reactor developed in-house and subsequently utilize the resulting materials in applications includeing Li-ion batteries, proton exchange membrane fuel cells, and bulk heterojunction organic photovoltaics.
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Swcnt Synthesis
The NPRL has extensive experience on developing control over the synthesis, characterization, purification and separation of SWCNT chiralities for advanced power generation and storage devices. Recent success has enabled the development of synthesis capabilities that produce carbon nanotubes with enhanced material quality and production scalability. This synthesis control can be obtained by understanding the fundamental material properties in conjunction with innovative fabrication and chemical processes. The NPRL is specifically focused on the synthesis and characterization of SWCNTs produced by laser vaporization. The laser synthesis is performed using either an Alexandrite laser (755 nm) or Nd:YAG laser (1064 nm) which rasters over the surface of metal-doped graphite targets at an average power density of 100 W/cm2. The reactor temperature is constant at 1150 °C under flowing Ar(g) and 760 torr. The raw SWCNT soot is purified using a nitric acid reflux followed by controlled thermal oxidation treatment to maximize purification efficiency.
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Mwcnt Synthesis
MWCNTs are synthesized using an injection chemical vapor deposition (CVD) process at ambient pressure using a process developed in-house at the NPRL and modeled after a reactor developed at the NASA Glenn Research Center. A syringe pump is used to inject a metal organic precursor, cyclopentadienyliron dicarbonyl dimer, dissolved in xylene into a tube furnace at elevated temperature with a controlled rate. A unique coaxial tip has been designed for this reactor which consists of a quartz capillary tube jacketed by a stainless steel tube. The precursor solution travels within the quartz capillary tube and the outer tube acts as a pathway for the carrier gas. At the tip, the carrier gas and precursor solution converge which subsequently aerosolizes the solution assisting its movement into the hot zone of the furnace. Upon reaching the hot zone of the furnace, the metal organic precursor is cracked causing iron to be deposited within the reaction vessel. MWCNTs growth is catalyzed by these particles and proceeds as long as the solution is injected into the vessel. Raw soot containing high purity MWCNT is collected from the tube; vertically aligned MWCNTs have also been grown in this CVD reactor using SiO2 on Si as the substrate.
- Recent Publications:
- DiLeo, Roberta; Landi, Brian; Raffaelle, Ryne., Materials Research Society Symposium Proceedings (2007), 1018E, Paper #1018-EE05-11.
Material Characterization
In addition to novel synthesis capabilities, the NPRL is well positioned for extensive characterization of carbon nanotubes involving microscopic (SEM, AFM), spectroscopic (optical absorption, Raman, fluorescence), surface area, and thermal (TGA, DSC) analysis. Recently, the NPRL has advanced the understanding of SWCNT purity assessment using optical absorption spectroscopy (B.J. Landi et al. J. Phys. Chem. B 2004, 108, 17089; and B.J. Landi et al. J. Phys. Chem. B 2005, 109, 9952). This work analyzed stable dispersions of laser-synthesized SWCNTs in N,N-dimethylacetamide (DMA) to determine the mass fraction of SWCNTs in the carbonaceous portion of a sample. This was strongly aided by the development of constructed sample sets which varied the SWCNT mass fraction of purified SWCNTs with respect to a representative carbonaceous by-product. Such SWCNT calibration samples allowed numerous mathematical approaches to be applied in reference to a known metric of comparison. The publication’s review of the linear subtraction method for the second interband electronic transition of the semiconducting SWCNTs (SE22) peak showed how this linear approach overestimates the actual SWCNT content, supported by both experimental data and a mathematical derivation. Instead, the NPRL developed alternative approaches which showed a better correlation to the constructed sample sets. These included a nonlinear regression model and multiple rapid assessment protocols using peak maxima values (absolute absorbance intensity, peak maxima ratio, tie line slope, and a Beer’s law analysis derived from calculated extinction coefficients). This framework allows the NPRL to work with standardized purity SWCNT materials, and assess the purty of materials purchased from commercial vendors.
There is considerable ongoing effort to develop control over individual single wall carbon nanotube (SWCNT) properties (chirality, length, purity, electronic type ratio, etc.) for a variety of applications. This control can either be accomplished during synthesis or in conjunction with subsequent processing steps aimed at purification and separation of the desired products. Although substantial work has been done in the last decade involving synthesis techniques and conditions for varying SWCNT diameter, the ability to manipulate production of specific SWCNT chiralities is still a sought-after research goal. The synthetic tunability of SWCNT chiralities has been a research challenge, in large part, due to the absence of a characterization technique which can rapidly discriminate the (n, m) SWCNT types in a sample. The discovery of SWCNT fluorescence has provided a straightforward approach with the capability to “map” the semiconducting distribution in a sample. This is a major improvement over absorption spectroscopy where multiple SWCNT features can be convolved within a series of peaks that fluorescence mapping can highlight due to selective excitation. The figure illustrates this advantage of fluorescence mapping given that multiple chiralites absorb (emit) over a very close wavelength range. Therefore, such spectroscopy can assist in understanding effects of experimental parameters on SWCNT distributions during synthesis, as well as offering insight during electronic type separations.
The NPRL has developed a carbonaceous purity assessment technique, using Raman spectroscopy, to assess the purity of the CVD grown MWCNTs. Raman spectroscopy has been performed on a reference sample set containing predetermined ratios of MWCNTs and representative synthesis by-products. Changes in the characteristic Raman peak ratios (i.e., ID/ IG, IG’ / IG, and IG’ / ID) as a function of MWCNT content were measured. Calibration curves were generated from the reference samples and used to evaluate MWCNTs synthesized under different conditions with varying purity. The efficacy of using Raman spectroscopy in conjunction with thermogravimetric analysis for quantitative MWCNT purity assessment is discussed.
- Recent Publications:
- Landi, Brian J.; Raffaelle, Ryne P., J. Nanosci. Nanotech., 7(3), 2007, pp. 883-890(8).
- Landi Brian J; Ruf Herbert J; Evans Chris M; Cress Cory D; Raffaelle Ryne P., J. Phys. Chem. B. (2005), 109(20), 9952-65.
- Landi, Brian J.; Ruf, Herbert J.; Worman, James J.; Raffaelle, Ryne P. J. Phys. Chem. B. (2004), 108(44), 17089-17095.
Carbon Nanotube Electrodes for batteries and fuel cells
Carbon nanotubes have attracted considerable attention for basic and applied research based on their extraordinary electrical, thermal, and mechanical properties. The NRPL is investigating the production of carbon nanotube electrodes for chemical energy conversion applications. Specifically targeted technologies are lithium ion batteries and PEM fuel cells. Both of these devices rely on electrodes that can facilitate charge, ion, and gaseous transport. In addition, specific attributes such as large catalytic surface area, good thermal conductivity, and material strength and flexibility are extremely sought after for these applications. Based on previous work in the NPRL, we are well positioned to capitalize on our fundamental understanding of the material properties to better enhance the performance of these devices.
Images of
(a) as-produced MWCNT product harvested from the CVD reactor;
(b) SEM and TEM images representative of the MWCNTs synthesized using xylenes; and
(c) MWCNT paper fabricated using vacuum filtration.
The NPRL has investigated the lithium ion capacity for multi-walled carbon nanotubes (MWCNTs) synthesized by the injection chemical vapor deposition (CVD) process using the cyclopentadienyl iron dicarbonyl dimer catalyst. The high quality of the as-synthesized MWCNTs has enabled free-standing electrodes to be fabricated independent of polymeric binder or copper support. Results of galvanostatic cycling of these electrodes have proved very promising whereby excellent reversibility and coulombic efficiency (>97% after cycle 3) were demonstrated using propylene carbonate based electrolytes, with no evidence for material degradation. A reversible capacity exceeding 225 mAh/g was measured after 20 cycles when using the electrolyte combination of (1:1:1 v/v) ethylene carbonate (EC):propylene carbonate (PC):diethyl carbonate (DEC) at a charge rate of 74 mA/g (equivalent of C/5 for LiC6). With the in-house CVD capabilities, the NPRL has also investigated the effects of modified synthesis parameters (e.g., exchanging xylenes with pyridine as the precursor solvent). These modifications have improved the lithium ion capacity in the resulting MWCNT paper to 340 mAh/g. In addition, this MWCNT paper showed a stable reversible capacity after 10 cycles, exceeding 225 mAh/g when cycled at an equivalent 1C rate. The high production rate and high quality material make MWCNTs a high-capacity alternative for Li-ion battery anodes.
XRD data for the purified SWCNT paper before (blue, bottom) and after lithiation (red, top). The theoretical Bragg reflections (hk) are listed at each peak for a 2-dimensional triangular lattice (see schematic in the center) with a lattice constant of 17 Å. The peaks attributed to the lithiation process in SWCNTs are designated with an asterisk (*).
The NPRL has also investigated the electrochemical cycling performance of high-purity single-walled carbon nanotube (SWCNT) paper electrodes measured vs. Li for a series of electrolyte solvent components. It has been found that the addition of propylene carbonate (PC) into the conventional ethylene carbonate (EC):dimethyl carbonate (DMC) co-solvent mixture enables a reversible Li-ion capacity of 520 mA-h/g for high purity SWCNTs. A free-standing SWCNT electrode (no polymer binder or metal substrate support) with this electrolyte combination had enhanced cycleability, retaining >95% of the initial capacity after 10 cycles. The NPRL is current investigated methods to mitigate the 1st cycle hysteresis which is common in these materials. Results have indicated the electrolyte selection is critical and which emphasizes the importance of fundamental investigations in to the solid-electrolyte interface (SEI) formation on SWCNT materials. Improvements have also been observed in the Li-ion capacity at higher charge rates (e.g., 1C) resulting in a 2× improvement [in capacity per current] over reported values for conventional graphite anode materials. In addition to the many electrochemical results, NPRL’s expertise in materials characterization has been utilized in postmortem analyses of the SWCNT electrodes. Marked changines in SWCNT paper XRD and Raman spectrectra have been observed and understanding these variations are expected to be paramount in optimizing the battery performance.
- Recent Publications:
- Landi, Brian J.; Ganter, Matthew J.; Schauerman, Christopher M.; Cress, Cory D.; Raffaelle, Ryne P., J. Phys. Chem. C, 112(19), 2008.
- Journal of Nanoscience and Nanotechnology Vol.8, 1–5, 2008
* Click the images to enlarge