HiPco Process Research Articles

A detailed Raman study on thin single-wall carbon nanotubes prepared by the HiPCO process

The Raman spectrum of single wall carbon nanotubes (SWNTs) prepared by high pressure CO decomposition (HiPCO process) has been recorded at nine excitation laser energies ranging from 1.83 eV to 2.71 eV. The characteristic nanotubes features: G band, D band and radial breathing mode (RBM) have been analyzed and compared to those of an arc discharge SWNT material of similar diameter. A strong Breit-Wigner-Fano type (metallic) contribution to the G band was found in the spectra measured with green lasers, while spectra measured with red lasers indicate resonances of semiconducting SWNTs. Analysis of the energy dependence of the position of the D band revealed sinusoid oscillations superimposed on a linear trend. The validity of full DOS calculations for HiPCO materials has been confirmed by a match found between the estimated spectral contribution of metallic SWNTs as calculated from the components of the measured G band and as predicted by the (n, m) indexes of the major scatterers of DOS simulations. The RBM region of the HiPCO spectrum is more complex than usually observed for SWNTs. The analysis performed with a Gaussian distribution and improved fitting parameters leads to a mean diameter and variance of 1.05 nm and 0.15 nm, respectively. A bimodal Gaussian distribution had little influence on the error sum but reduced the standard error slightly. The major spectral features of the RBM could be interpreted using available resonance Raman theory.

Electrochemical Tuning of Electronic Structure of Single-Walled Carbon Nanotubes: In-situ Raman and Vis-NIR Study

The population of valence-band electronic states of single-walled carbon nanotubes (SWCNTs) was tuned electrochemically in acetonitrile electrolyte solution. In dry and oxygen-free solution, the electrochemistry of SWCNTs is controlled by capacitive charging. Reversible changes of intensity and frequency of the Raman spectra can be monitored during cyclic voltammetry at low scan rates. Electrochemical charging of SWCNTs can be also traced via reversible bleaching of the electronic transitions in the vis-NIR region. An aprotic medium offers a broader electrochemical window for tuning of electronic properties of SWCNTs. Electrochemical charging of SWCNTs in an aprotic electrolyte solution allows easy and precise control of the electronic structure of SWCNTs. In addition to commercial SWCNTs, a material made from gas-phase catalytic decomposition of CO by the HiPco process was also studied. Selective quenching of vis-NIR and Raman spectra is a useful tool to the analysis of tubes of varying diameter and helicity.

Oxygen-containing functional groups on single-wall carbon nanotubes: NEXAFS and vibrational spectroscopic studies.

Single-walled nanotubes (SWNTs) produced by plasma laser vaporization (PLV) and containing oxidized surface functional groups have been studied for the first time with NEXAFS. Comparisons are made to SWNTs made by catalytic synthesis over Fe particles in high-pressure CO, called HiPco material. The results indicate that the acid purification and cutting of single-walled nanotubes with either HNO3/H2SO4 or H2O2/H2SO4 mixtures produces the oxidized groups (O/C = 5.5-6.7%), which exhibit both pi*(CO) and sigma*(CO) C K-edge NEXAFS resonances. This indicates that both carbonyl (C=O) and ether C-O-C functionalities are present. Upon heating in a vacuum to 500-600 K, the pi*(CO) resonances are observed to decrease in intensity; on heating to 1073 K, the sigma*(CO) resonances disappear as the C-O-C functional groups are decomposed. Raman spectral measurements indicate that the basic tubular structure of the SWNTs is not perturbed by heating to 1073 K, based on the invariance of the ring breathing modes upon heating. The NEXAFS studies agree well with infrared studies which show that carboxylic acid groups are thermally destroyed first, followed by the more difficult destruction of ether and quinone groups. Single-walled nanotubes produced by the HiPco process, and not treated with oxidizing acids, exhibit an O/C ratio of 1.9% and do not exhibit either pi*(CO) or sigma*(CO) resonances at the detection limit of NEXAFS. It is shown that heating (to 1073 K) of the PLV-SWNTs containing the functional groups produces C K-edge NEXAFS spectra very similar to those seen for the HiPco material. The NEXAFS spectra are calibrated against spectra measured for a number of fused-ring aromatic hydrocarbon molecules containing various types of oxidized functional groups present on the oxidized SWNTs.

Purification and Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process)

A purification method is given for extracting the Fe metal catalyst and non-SWNT carbon from nanotubes produced by the HiPco process.1,2 A multistage purification method has been investigated. Sample purity is documented by ESEM, TEM, TGA, Raman and UV-vis−near-IR spectroscopy. Metal catalyzed oxidation at low temperature has been shown to selectively remove non-SWNT carbon and permit extraction of iron with concentrated HCl. Prolonged catalyzed oxidation has been found to preferentially remove smaller diameter tubes. The onset of oxidation of purified smaller diameter HiPco SWNTs is also found to be approximately 100 °C lower than for purified larger diameter tubes produced in the laser-oven process.

Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study

We have demonstrated large-scale production (10 g/day) of high-purity carbon single-walled nanotubes (SWNTs) using a gas-phase chemical-vapor-deposition process we call the HiPco process. SWNTs grow in high-pressure (30–50 atm), high-temperature (900–1100 °C) flowing CO on catalytic clusters of iron. The clusters are formed in situ: Fe is added to the gas flow in the form of Fe(CO)5. Upon heating, the Fe(CO)5 decomposes and the iron atoms condense into clusters. These clusters serve as catalytic particles upon which SWNT nucleate and grow (in the gas phase) via CO disproportionation: CO+CO⇒CO2+C(SWNT). SWNT material of up to 97 mol % purity has been produced at rates of up to 450 mg/h. The HiPco process has been studied and optimized with respect to a number of process parameters including temperature, pressure, and catalyst concentration. The behavior of the SWNT yield with respect to various parameters sheds light on the processes that currently limit SWNT production, and suggests ways that the production rate can be increased still further.