HiPco Process Research Articles

High-Quality Graphene Using Boudouard Reaction.

Following the game‐changing high‐pressure CO (HiPco) process that established the first facile route toward large‐scale production of single‐walled carbon nanotubes, CO synthesis of cm‐sized graphene crystals of ultra‐high purity grown during tens of minutes is proposed. The Boudouard reaction serves for the first time to produce individual monolayer structures on the surface of a metal catalyst, thereby providing a chemical vapor deposition technique free from molecular and atomic hydrogen as well as vacuum conditions. This approach facilitates inhibition of the graphene nucleation from the CO/CO2 mixture and maintains a high growth rate of graphene seeds reaching large‐scale monocrystals. Unique features of the Boudouard reaction coupled with CO‐driven catalyst engineering ensure not only suppression of the second layer growth but also provide a simple and reliable technique for surface cleaning. Aside from being a novel carbon source, carbon monoxide ensures peculiar modification of catalyst and in general opens avenues for breakthrough graphene‐catalyst composite production.

Binding Configuration and Surface Coverage of Poly(9,9-dioctylfluorene-2,7-diyl) on Electronic Type-Sorted Carbon Nanotubes

Several schemes have been demonstrated to successfully extract semiconducting (s-) single-walled carbon nanotubes (SWCNTs) from heterogeneous as-grown mixtures containing metallic (m-) SWCNTs. The polyfluorene family of conjugated polymers, specifically poly(9,9-dioctylfluorene-2,7-diyl) (PFO), have been shown to extract s-SWCNTs from mixtures of s- and m-SWCNTs with high selectivity (>99% s-SWCNT) by preferentially wrapping the s-SWCNT components. In addition to electronic type, the selectivity is heavily dependent on the nanotube diameter and chiral angle. To date, there is an inadequate understanding of the polymer-CNT interactions that lead to the high preference for s- over m-SWCNTs in the wrapping process. It is vital to understand these parameters to design separation schemes to reduce the proportion of m-SWCNT impurities to technologically relevant ppm and ppb levels.In this work, we explore the chemistry of non-covalent nanotube-polymer wrapping processes and describe the effects of surface coverage and binding configuration on the separation efficiency. We disperse SWCNTs grown by the HiPCO process, which contain many chiral species of SWCNT with diameters near 1 nm, in toluene solutions of varying PFO concentration. The resulting dispersions are analyzed with optical absorption spectroscopy, excitation-emission photoluminescence spectroscopy, and photoluminescence anisotropy. First, we experimentally quantify the amount of PFO wrapping the SWCNTs by measuring exciton energy transfer from PFO to SWCNTs via photoluminescence spectroscopy. We characterize the dependence of the surface coverage on PFO concentration, finding that the surface coverage increases by a factor of 7 as PFO concentration increases from 0.2 to 2 mg/mL. Second, we demonstrate that the wrapping of nanotubes by PFO is described by a Langmuir adsorption isotherm, in which surface coverage of PFO on the nanotubes follows an S-shaped curve as a function of PFO concentration. The shape of the curve is related to the polymer-nanotube binding energy, which depends on the nanotube electronic type, diameter, chiral angle, solvent, and polymer molecular weight. Third, we determine the binding configuration of the PFO on the nanotube surface as a function of PFO concentration and demonstrate that the PFO becomes more ordered as the surface coverage decreases. In the highly-ordered PFO regime we estimate the wrapping angle of individual PFO strands around nanotubes in solution. Finally, we find that the metal-semiconductor separation efficiency is lowest when the concentration of PFO is high, surface coverage is high, and surface ordering is low.Understanding the PFO wrapping process will be an important step toward attaining high-purity s-SWCNT from this and similar sorting procedures for a range of polymer-CNT systems. These advances will enable further development of technologically relevant high purity s-SWCNTs for next-generation electronic devices.

Separation and Characterization of Double-Wall Carbon Nanotube Subpopulations

Surfactant-encapsulated double-wall carbon nanotubes (DWCNTs) synthesized by the high-pressure carbon monoxide decomposition (HiPco) process were separated by length and electronic characteristics using density gradient ultracentrifugation (DGU). To ensure our study focuses only on the behavior of DWCNTs, dispersed DWCNTs were first isolated following the method of Green et al. [Green, A. A.; Hersam, M. C. Nat. Nanotechnol. 2008, 264, 1], utilizing the differences in buoyant density of DWCNTs from that of impurity single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes contained in the parent soot. By increasing the density difference between the nanotubes and the density gradient medium, we exploited the length-dependent translation of the nanotubes in response to applied centrifugation to isolate narrow length distribution DWCNT fractions. The length-dependent intrinsic optical response of DWCNTs is consistent compared with the previously reported values for SWCNTs. The controlled addition ...

Diameter-Dependent Solubility of Single-Walled Carbon Nanotubes

We study the solubility and dispersibility of as-produced and purified HiPco single-walled carbon nanotubes (SWNTs). Variation in specific operating conditions of the HiPco process are found to lead to significant differences in the respective SWNT solubilities in oleum and surfactant suspensions. The diameter distributions of SWNTs dispersed in surfactant solutions are batch-dependent, as evidenced by luminescence and Raman spectroscopies, but are identical for metallic and semiconducting SWNTs within a batch. We thus find that small diameter SWNTs disperse at higher concentration in aqueous surfactants and dissolve at higher concentration in oleum than do large-diameter SWNTs. These results highlight the importance of controlling SWNT synthesis methods in order to optimize processes dependent on solubility, including macroscopic processing such as fiber spinning, material reinforcement, and films production, as well as for fundamental research in type selective chemistry, optoelectronics, and nanophotonics.

Defect and Nondefect Interstitial Channel Availability in Carbon Nanotube Bundles: Comparison of Modeling with Experiments

There is a controversy in the literature over whether small gases, such as Ne, can adsorb in the interstitial channels (ICs) of carbon nanotube bundles. We distinguish between two types of ICs: nondefect, defined as ICs in perfectly packed arrays of nanotubes having all of the same diameter (homogeneous) defined where three tubes meet, and defect ICs, present in bundles composed of nanotubes having different diameters (heterogeneous) where four or more tubes meet. We have performed grand-canonical Monte Carlo simulations of Ne on various model carbon nanotube bundles in order to explore the role of nondefect and defect ICs in adsorption. We have performed simulations on closed and partially opened homogeneous and heterogeneous bundles. We have computed the specific surface area, the isosteric heats of adsorption, and adsorption isotherms from our simulations, and we have compared these values to experimentally measured quantities for Ne adsorption on carbon nanotubes produced by the HiPco process. Analysi...

Microwave interaction with nonuniform hydrogen gas in carbon nanotubes

In this paper we study the reflection, absorption, and transmission of microwave from nonuniform hydrogen gas in carbon nanotubes, grown by iron-catalyzed high-pressure carbon monoxide disproportionate (HiPco) process. A discussion on the effect of various hydrogen gas parameters on the reflected power, absorbed power, and transmitted power is presented. The nonuniform hydrogen gas slab is modeled by a series of subslabs. The overall number density profile across the whole slab follows a parabolic function. The total reflected, absorbed, and transmitted powers are then deduced and their functional dependence on the number density, collision frequency, and angle of propagation is studied.

In-situ TEM-STM Observations of SWCNT Ropes/tubular Transformations

AbstractSingle-walled carbon nanotubes (SWCNTs) prepared by the HiPco process were purified using a modified gas phase purification technique. A TEM-STM holder was used to study the morphological changes of SWCNT ropes as a function of applied voltage. Kink formation, buckling behavior, tubular transformation and eventual breakdown of the system were observed. The tubular formation was attributed to a transformation from SWCNT ropes to multi-walled carbon nanotube (MWCNT) structures. It is likely mediated by the patching and tearing mechanism which is promoted primarily by the mobile vacancies generated due to current-induced heating and, to some extent, by electron irradiation.

A Model for Nucleation and Growth of Single Wall Carbon Nanotubes via the HiPco Process: A Catalyst Concentration Study

Using the High Pressure carbon monoxide (HiPco) reactor we conducted an experiment on the effects of changing the catalyst concentration. With each catalyst concentration tested the resulting raw HiPco material was characterized for average SWNT lengths, SWNT diameters, residual iron particle size, and large fullerene content. We were able to determine trends in each of these characteristics as the catalyst concentration was changed. As the catalyst concentration was decreased SWNT lengths increased, SWNT diameters increased, the residual iron particle size increased, and the large fullerene content decreased. From these trends we have developed a Competitive Growth model for nucleation and growth of SWNTs via the HiPco process.

Controlled Multistep Purification of Single-Walled Carbon Nanotubes

A controlled and scalable multistep purification method has been developed to remove iron impurity and nonnanotube carbon materials from raw single-walled carbon nanotubes (SWNTs) produced in the HiPco (high-pressure CO) process. In this study, iron nanoparticles, coated by carbon, are exposed and oxidized by multiple step oxidation at increasing temperatures. To avoid catalytic oxidation by iron oxide of carbon nanotubes, the exposed and oxidized iron oxide is deactivated by reaction with C(2)H(2)F(4) or SF(6). The iron fluorides are removed by a Soxhlet extraction with a 6 M HCl solution. The purity and quality of each sample were determined by thermogravimetric analysis (TGA), Raman spectrometry, ultraviolet-visible-near-IR (UV-vis-near-IR) spectrometry, fluorescence spectrometry, and transmission electron microscope (TEM) spectroscopy. The purity and yield of SWNTs are improved due to reduced catalytic activity of the iron oxide. Greater iron oxide removal also resulted from oxidation at higher temperatures.

Gas‐Phase Production of Single‐Walled Carbon Nanotubes from Carbon Monoxide: A Review of the HiPco Process

AbstractFor Abstract see ChemInform Abstract in Full Text.

Permanent Trapping of CO2 in Single-Walled Carbon Nanotubes Synthesized by the HiPco Process

Infrared spectroscopy is used to study trapped and physisorbed CO2 in single-walled carbon nanotube bundles (SWNTs) synthesized by the HiPco process. CO2 is entrapped within the SWNTs by acid oxidation of the unpurified sample followed by vacuum heating to 700 K. The trapped CO2 has a single nu3 mode at 2327 cm-1, is stable during temperature cycling from 77 to 700 K, and remains after venting to room air. CO2 physisorption studies show a nu3 mode at 2330 cm-1 for the as-received HiPco samples, 2340 cm-1 for the acid-oxidized sample, and 2327 and 2340 cm-1 for the oxidized sample after vacuum heating. The sites responsible for the infrared peaks of the physisorbed and trapped species are discussed.

Gas-phase production of single-walled carbon nanotubes from carbon monoxide: a review of the hipco process.

The latest process for producing large quantities of single-walled carbon nanotubes (SWNTs) to emerge from the Rice University, dubbed HiPco, is living up to its promise. The current production rates approach 450 mg/h (or 10 g/day), and nanotubes typically have no more than 7 mol % of iron impurities. Second-generation HiPco apparatus can run continuously for 7-10 days at a time. In the HiPco process nanotubes grow in high-pressure, high-temperature flowing CO on catalytic clusters of iron. Catalyst is formed in situ by thermal decomposition of iron pentacarbonyl, which is delivered intact within a cold CO flow and then rapidly mixed with hot CO in the reaction zone. Upon heating, the Fe(CO)5 decomposes into atoms that condense into larger clusters. SWNTs nucleate and grow on these particles in the gas phase via CO disproportionation: CO + CO --> CO2 + C (SWNT), catalyzed by the Fe surface. The concentration of CO2 produced in this reaction is equal to that of carbon and can therefore serve as a useful real-time feedback parameter. It was used to study and optimize SWNT production as a function of temperature, pressure, and Fe(CO)5 concentration. The results of the parametric study are in agreement with current understanding of the nanotube formation mechanism.

Electronic structure and arrangement of purified HiPco carbon nanotubes

The electronic structure of purified single-wall carbon nanotubes obtained from HiPco process has been probed by X-ray fluorescent spectroscopy. The CKα spectrum of the HiPco nanotubes revealed a significant enhancement of the high-energy maximum intensity compared with the graphite spectrum. Quantum chemical calculations of carbon nanotube models show the spectral variations could be caused by vacancies in the tube walls developed with the thermal and acid treatment of the sample.

Carbon nanotube sheets for the use as artificial muscles

Low voltage artificial muscles are of great importance for an enormous variety of industrial applications. In 1999 a new material, carbon nanotube sheets also called bucky paper, was described with suitable electromechanical properties. We developed an experimental set-up which allows, for the first time, the analysis of actuation forces vertical to the sheet plane. Single walled carbon nanotubes (SWNT) from arc discharge or HiPco process as well as multi walled carbon nanotubes (MWNT) were used as received or after further purification steps to produce carbon nanotube sheets by the filtration technique. The fastest actuation time of three seconds could be, up to now, obtained with the HiPco-material, which is free of amorphous carbon particles. Several parameters influence the electromechanical properties like the sheet thickness, the used electrolyte, the applied voltage and so on. But also the nanotube material itself and the procedure for the bucky paper production must be considered intensively to avoid large differences between different batches and to produce indiscriminate and reproducible carbon nanotube sheets. We characterized the material, among others, by TGA, SEM/EDX, and BET analysis.

Diameter dependent wall deformations during the compression of a carbon nanotube bundle

Raman scattering is used to study the pressure-dependence of the radial (R) bands of bundled single-walled carbon nanotubes (SWNT) produced by the HiPCO process. We find that the normalized pressure derivative of the R-band frequency, $\ensuremath{\Phi}=d(\mathrm{ln}{\ensuremath{\omega}}_{\mathrm{R}})/dP,$ increases with increasing nanotube diameter D as $\ensuremath{\Phi}\ensuremath{\sim}{D}^{2}.$ Using results from elastic theory, we show that the contribution to \ensuremath{\Phi} from isolated SWNT, $(={\ensuremath{\Phi}}_{0}),$ is small, which agrees with previous model calculations. We show here that ${\ensuremath{\Phi}}_{0}$ is independent of D. We conclude that most of the pressure dependence observed for the R bands in SWNT bundles may be identified with tube wall deformations mediated via tube-tube interactions within a bundle and not with the compression of an individual tube.

Nanotubes in Microwave Fields: Light Emission, Intense Heat, Outgassing, and Reconstruction

SWNTs, produced via the HiPco process, display strong microwave absorption (1.01 × 10-5 eV microwave field) with subsequent dramatic light emission, intense heat release (2000 °C), outgassing, mechanical extension, and nanotube reconstruction (welding and loop formation).

Covalent sidewall functionalization of single wall carbon nanotubes.

Alkyllithium reagents may be used to attach alkyl groups to the sidewalls of fluoro nanotubes. Thermal gravimetric analysis combined with UV-vis-Nir spectroscopy has been used to provide a quantitative measure of the degree of functionalization. SWNTs prepared using the HiPco process exhibit a higher degree of alkylation than SWNTs from the laser-oven method, indicating that the smaller diameter fluoro tubes are alkylated more readily. The spectral signature of the pristine SWNTs can be regenerated when the alkylated SWNTs are heated in Ar at 500 degrees C, demonstrating that dealkylation occurs at this temperature. TGA-MS analysis using a sample of n-butylated h-SWNTs showed that 1-butene and n-butane are formed during thermolysis.

Effects of carbonyl bond, metal cluster dissociation, and evaporation rates on predictions of nanotube production in high-pressure carbon monoxide.

The high-pressure carbon monoxide (HiPco) process for producing single-wall carbon nanotubes (SWNTs) uses iron pentacarbonyl as the source of iron for catalyzing the Boudouard reaction. Attempts using nickel tetracarbonyl led to no production of SWNTs. This paper discusses simulations at a constant condition of 1300 K and 30 atm in which the chemical rate equations are solved for different reaction schemes. A lumped cluster model is developed to limit the number of species in the models, yet it includes fairly large clusters. Reaction rate coefficients in these schemes are based on bond energies of iron and nickel species and on estimates of chemical rates for formation of SWNTs. SWNT growth is measured by the conformation of CO2. It is shown that the production of CO2 is significantly greater for FeCO because of its lower bond energy as compared with that of NiCO. It is also shown that the dissociation and evaporation rates of atoms from small metal clusters have a significant effect on CO2 production. A high rate of evaporation leads to a smaller number of metal clusters available to catalyze the Boudouard reaction. This suggests that if CO reacts with metal clusters and removes atoms from them by forming MeCO, this has the effect of enhancing the evaporation rate and reducing SWNT production. The study also investigates some other reactions in the model that have a less dramatic influence.

Identification of Large Fullerenes Formed during the Growth of Single-Walled Carbon Nanotubes in the HiPco Process

Iron-catalyzed, gas-phase disproportionation of carbon monoxide under high pressures is known to produce single-walled carbon nanotubes (SWNT) in high yields. Small amounts of nontubular nanocarbons and iron encapsulated-graphitic shelled nanoparticles are produced concomitantly. Differences in the oxidation kinetics among the SWNT, nontubular carbon, and iron core−graphitic shell nanoparticles have been exploited as a tool for the quantitative determination of high-molecular-weight nontubular, carbon products. The nontubular carbon materials were eliminated by selective oxidation wherein the raw material was subjected to increasing oxidation thresholds followed by acid leaching in an aqueous or gas−solid reaction. Relative concentrations of nontubular nanocarbons in differentially oxidized samples were determined employing laser desorption ionization-mass spectrometry (LDI-MS) with C60 and bismuth triphenyl as internal markers. The identity of the nontubular carbon was examined by nondestructive extracti...

Raman-Active Modes of Single-Walled Carbon Nanotubes Derived from the Gas-Phase Decomposition of CO (HiPco Process)

Raman-Active Modes of Single-Walled Carbon Nanotubes Derived from the Gas-Phase Decomposition of CO (HiPco Process)