WO2008097333A2

WO2008097333A2 - Method and apparatus for low-temperature fabrication of graphitic carbon, graphene and carbon nanotubes

Info

Publication number: WO2008097333A2
Application number: PCT/US2007/072858
Authority: WO
Inventors: Louis Eugene Brus; Michael Louis Steigerwald; Erich Walter
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 2006-07-06
Filing date: 2007-07-05
Publication date: 2008-08-14
Also published as: WO2008097333A3

Abstract

The disclosed subject matter provides techniques for the controllable low temperature fabrication of graphitic carbon. The disclosed subject matter also provides a chemical process using commercially available hydrocarbon for the preparation of graphitic carbon, including carbon nanotubes and sheets.

Description

METHOD AND APPARATUS FOR LOW-TEMPERATURE FABRICATION OF GRAPHITIC CARBON, GRAPHENE AND CARBON NANOTUBES

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application Serial No.

60/818,840, filed on July 6, 2006, which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH The disclosed subject matter described herein was funded in part by a grant from the National Science Foundation, DMR 0213574. The United States Government may have certain rights hereunder.

BACKGROUND Technical Field. The disclosed subject matter relates to techniques for the preparation of graphitic carbon, including carbon nanotubes.

Background Art. The deposition of graphitic carbon through the catalytic decomposition and piecewise reconstitution of hydrocarbons is a very well-studied process which results in a broad range of effects, ranging from deleterious effects in the carbonization of hydrocarbon reactors to the remarkable science of single- walled carbon nanotubes ("SWCNTs"). In general the production of graphitic carbon in this way has required the use of high reaction temperatures of 1000⁰C or more, or otherwise through the forcing chemical reaction conditions. For example, U.S. Patent No. 7,056,479 to Dodelet et al. describes certain prior art techniques for generating SWCNTs using laser ablation, ablation using an electric arc and chemical vapor deposition ("CVD").

Such extreme conditions may be dictated by the need to fragment stable carbon sources and/or by the difficulty intrinsic to ordering the initially-prepared carbon network. A lower-temperature and controllable process for the preparation of graphitic carbon (particularly graphene sheets and SWCNTs) would facilitate progress in these research areas by enabling the rational synthesis of appropriate carbon specimens.

Plasma-enhanced chemical vapor deposition ("PECVD") has been proposed as an alternative technique wherein the carbon source material is subjected to lower reaction chamber temperatures, e.g., 400° C. See, e.g., U.S. patent application publication no. US2005/0233263 Al to Park et al. However, as discussed in U.S. Patent No. 7,052,667 to Loutfy et al., the plasma itself is at several thousand degrees C. A need therefore remains for a controllable low temperature technique for the fabrication of graphitic carbon.

SUMMARY

Techniques for the controllable low temperature fabrication of graphitic carbon are disclosed herein. The disclosed subject matter also provides a chemical process using commercially available hydrocarbon for the preparation of graphitic carbon, including carbon nanotubes and sheets.

In some embodiments, the disclosed subject matter provides methods for preparing graphitic carbon, includes combining at least partially reduced soluble iron, a cyclic, nonconjugated polyolefin, and a solvent to form a solution, heating the solution until reflux, adding an oxidizing agent to the solution to form a second solution, and heating the second solution to form a solid, thereby forming graphitic carbon. The graphitic carbon prepared from this reaction can include carbon nanotubes and/or sheets of graphitic carbon. The solutions can be agitated while being heated using e.g., a magnetic spinbar or a mechanical shaker or other means for agitating the solution. In one embodiment, the cyclic, nonconjugated polyolefin can be cyclooctatetraene (COT). In a related embodiment, COT is in the form of bis(cyclooctatetraene)iron. The solvent can be any suitable solvent, for example, toluene, benzene, heptadecane, diphenyl ether, and/or hexane. The oxidizing agent can be, for example, diphenylsulfoxide, diphenylselenoxide, diphenyltelluroxide, trimethylamine-N-oxide, pyridine-N-oxide, and/or dimethylsulfoxide (DMSO). The solution optionally further includes dimethoxyethane (DME).

In some embodiments, the second solution is heated at a low temperature, for example a temperature below 500°C. In one embodiment, the second solution is heated at a temperature of between about 100⁰C and about 13O⁰C, e.g., about 115°C. The second solution is heated for a time period to allow formation of a solid.

In one embodiment, the second solution is heated for between about 12 hours and about 120 hours. In another embodiment, the method for preparing graphitic carbon further includes washing the solid with concentrated mineral acid, e.g., hydrochloric acid.

The reaction can take place in an inert gas atmosphere, e.g., nitrogen, argon, or helium. In a particular embodiment, the reaction takes place in a vessel such as a Schlenk tube.

In a particular aspect, the disclosed subject matter also provides methods for preparing graphitic carbon by combining bis(cyclooctatetraene)iron (Fe(COT)₂), dimethoxyethane (DME), and a solvent to form a solution, heating the solution until reflux, adding dimethylsulfoxide (DMSO) to the solution to form a second solution, and heating the second solution to form a solid, thereby forming graphitic carbon.

In another particular aspect, the disclosed subject matter provides methods for preparing graphitic carbon comprising combining COT with dimethylsulfoxide

(DMSO) and a solvent to form a solution, heating the solution until reflux, adding iron pentacarbonyl (Fe(CO)₅) to the solution to form a second solution, and heating the second solution to form a solid, thereby forming graphitic carbon.

The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate embodiments of the disclosed subject matter and serve to explain its principles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an apparatus in accordance with the disclosed subject matter.

FIG. 2 is an SEM image of tubular nanocrystals formed in accordance with the disclosed subject matter. In particular, the SEM image depicts Fe(COT)2 + DME + DMSO powder sputter coated with Pd/ Au.

FIGs. 3(a)-(d) are TEM images showing sheets and tubes before and after an acid wash.

FIG 3(e) is a selective area electron diffraction pattern corresponding to Figs. 3(a)-(d). FIGs. 4(a), (b) and (d) are TEM images showing tubes.

FIG 4(c) is a selective area electron diffraction pattern corresponding to Figs. 4(a), (b) and (d).

FIG. 5 is a graph showing a Raman shift. Throughout the drawings, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the disclosed subject matter will now be described in detail with reference to the Figs., it is done so in connection with the illustrative embodiments.

DETAILED DESCRIPTION

In some embodiments, the disclosed subject matter provides a method and apparatus for the preparation of graphitic carbon, sheets as well as tubes, from readily available solvent reagents at temperatures under about 500° C.

Referring to FIG. 1, a block diagram of an apparatus in accordance with one embodiment of the disclosed subject matter will be described. A suitable vessel, e.g., a vacuum vessel such as a Schlenk tube 110, including a stopper 120 and a socket joint 130 at its sidearm, can be utilized. The socket joint 130 is suitable for connection to a vacuum manifold for introducing overpressure via an inert gas, such as argon, nitrogen, or helium at atmospheric or other pressures. In one embodiment, a magnetic spinbar 140 and solution containing the reagents needed to form graphitic carbon 150 are introduced into the Schlenk tube 110. In another embodiment, the Schlenk tube 110 is placed in a suitable oil bath 160 to provide necessary heat to react the reagents within the solution 150.

Although a Schlenk tube is preferred, other containers which can be heated without deformation to the desired temperature may be employed. Likewise, agitators other than a magnetic spinbar, such as mechanical shakers and overhead stirrers, may be utilized. The reaction can also be carried out without agitation. The solution 150 is a mixture of at least partially reduced soluble iron, a cyclic, nonconjugated polyolefin, and a solvent. The solution is heated until reflux, and an oxidizing agent is added to the solution to form a second solution. Any oxidizing agent may be used, for example, dimethylsulfoxide (DMSO). Other examples of suitable oxidizing agents are diphenylsulfoxide, diphenylselenoxide, diphenyltelluroxide, trimethylamine-N-oxide, and pyridine-N-oxide. Other sources of iron and oxygen, such as independently prepared iron oxide nanoparticles, may be utilized. In another embodiment, the solution optionally further comprises dimethoxyethane (DME) or another composition useful in increasing solubility. As used herein, the term "partially reduced soluble iron" includes a chemical compound that is soluble and/or dispersible in an organic solvent and that contains iron in an oxidation state less than 3.

As used herein, the term "cyclic, nonconjugated polyolefin" includes a hydrocarbon having the general chemical formula C_nH_n and containing a ring of alternating C-C single and C-C double bonds. Examples include cyclobutadiene and cyclododecahexaene in addition to cyclooctatetraene. In one embodiment, the cyclic, nonconjugated polyolefin includes cyclooctatetraene (COT). In a related embodiment, the COT is in the form of bis(cyclooctatetraene)iron. In place of a cyclic, nonconjugated polyolefin, simple acetylenes such as C₂H₂ may be utilized.

Any suitable solvent may be utilized. This solvent must be capable of dissolving and/or dispersing the iron-containing compound, and it must be capable of dissolving and/or dispersing the hydrocarbon compound. For example, the solvent can be toluene, benzene, heptadecane, diphenyl ether, and/or hexane. In one embodiment, the solution is a mixture of bis (cyclooctatetraene)iron

(Fe(COT)₂), DME, and toluene. In one preferred arrangement, 0.38 mmol of Fe(COT)₂ , 1.52 mmol of DME and 40 ml of toluene may form the solution 150. The solution is brought to reflux and an oxidizing agent, e.g., dimethylsulfoxide ("DMSO"), is added. In the preferred arrangement, 1.52 mmol of DMSO is added. In another particular embodiment, the solution is a mixture of bis(cyclooctatetraene)iron (Fe(COT)₂), dimethoxyethane (DME), and a solvent. An oxidizing agent, e.g., DMSO is added at reflux to form a second solution.

In still another particular embodiment, COT with an oxidizing agent, e.g., dimethylsulfoxide (DMSO) is mixed with a solvent to form a solution. Iron pentacarbonyl (Fe(C0)5 is added at reflux to form a second solution.

In some embodiments of the disclosed subject matter, the second solution, as described herein, is maintained at an elevated, but low temperature, e.g., a temperature below 500°C, for example approximately 100°C, 125°C, 15O⁰C, 175°C, 200°C, 225°C, 250°C, 275°C, 300⁰C, 325°C, 350⁰C, 375°C, 400⁰C, 425°C, 450⁰C, 475°C or 500° C, and for a sufficient period of time, i.e., approximately 6, 9, 12, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, or 120 hours, or longer to permit the reactants to form graphitic carbon. Although the temperature is elevated, it is relatively cool compared to the temperatures required for CVD, PECVD and other prior art techniques. In a preferred arrangement, the temperature is kept at approximately 115° C, and the solution 150 is maintained at that temperature for approximately five days (120 hours).

By the end of the time period, the solution 150 transforms into a solution over a solid. The solid comprises graphitic carbon, e.g., carbon nanotubes and sheets. In one embodiment, the solution 150 is then cooled to room temperature, and any remaining supernatant solution is removed. The deposited solid material, in one embodiment, is collected and may be rinsed with a solvent, e.g., pentane (3 x 10 ml) and dried, e.g., in a vacuum, resulting in a brown-black powder containing millimeter- sized fragments of a reflective black solid.

Referring next to Fig. 2, a SEM image of the deposited solid material will be described. As illustrated, the material includes large amounts of long, tubular structures. Those structures are made primarily of carbon, with smaller amounts of iron and oxygen. Referring next to Figs. 3(a)-(e), the deposited solid material will now be described with reference to TEM images. As shown in Figs. 3(a) and (b), micron- scale tubes and sheets in addition to nanoparticles of FeOx are formed. The Selective Area Electron Diffraction ("SAED") pattern of Fig 3 (e), shows that the sheet and tube structures are crystalline, and the pattern matches that of crystalline graphite. It has become a common practice in the carbon nanotube art to remove the metal catalyst particles from the nanotube product by washing with concentrated mineral acid. When this black solid is sonicated in 37% HCl, 10-20% of the original mass is recovered. As shown in Figs. 3(c) and (d), the FeOx particles have been removed and that what remains is graphite (both sheets and tubes). Thus, the reaction of Fe(COT)₂ with DMSO produces not only nanocrystals of iron oxides, but also graphitic carbon as shown in Equation 1.

Fe(COT)₂ + DMSO + DME → Fe₃O₄ + graphite (1)

Referring next to Figs. 4(a)-(e), the deposited solid material will now be described with reference to hi-resolution TEM (HRTEM) images. Figs. 4(a) and (b) show representative images of two samples produced in the same five day long reaction, as-synthesized and acid-washed. The grids are dominated by areas of crystalline graphite and MWCNTs. A SAED pattern from a single graphitic tube, shown in Fig. 4 (c), matches with carbon nanotube electron diffraction. Measurement of the lattice spacing in all HRTEM images with crystalline planes visible is 3.3 A and matches the interplanar distance for graphite. Fig. 4 (d) depicts a HRTEM image from an acid-washed sample, and it is not crystalline.

Referring next to Fig. 5, the Raman spectrum of the black solid shown in Figs. 2-4 is presented. As show, both the D (-1350 cm^"1) and G (-1600 cm^"1) modes of graphite are present. The peaks at 130 cm^"1 and 172 cm^"1 may be attributed to radial breathing modes of CNTs. The Raman spectra also shows that treating the product with acid appears to damage the graphitic carbon, as acid-treated samples display only weak, broad peaks at 1350 cm^"1 and around 1590 cm^"1. Nanoparticles have drawn interest for use as potential catalysts due to their high surface to volume ratio and possible chemical selectivity (crystal facet/chirality/molecular recognition). The catalytic activity of metals, notably gold, is strongly size dependant. Synthesizing graphite and carbon nanotubes at lower temperatures and in solution, in accordance with the disclosed subject matter, may lead to more control of the synthesis with regard to regrowth, functionalization, size control, preferential growth of carbon tubes vs. sheets, cleanliness, solublization and phase.

The foregoing merely illustrates the principles of the disclosed subject matter. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous techniques which, although not explicitly described herein, embody the principles of described herein and are thus within the spirit and scope of the disclosed subject matter.

Claims

What is Claimed is:

I . A method for preparing graphitic carbon, comprising: a) combining partially reduced soluble iron, a cyclic nonconjugated polyolefin, and a solvent to form a solution; b) heating the solution until reflux; c) adding an oxidizing agent to the solution to form a second solution; and d) heating the second solution to form a solid, thereby forming graphitic carbon.

2. The method of claim 1, further comprising rinsing the solid with a solvent.

3. The method of claim 2, further comprising drying the solid.

4. The method of claim 1, wherein the cyclic, nonconjugated polyolefin comprises cyclooctatetraene (COT)

5. The method of claim 1, wherein the cyclooctatetraene comprises bis(cyclooctatetraene)iron.

6. The method of claim 5, wherein about .38 mmol of bis(cyclooctatetraene)iron is included in the solution.

7. The method of claim 1, wherein the solvent is selected from the group consisting of: toluene, benzene, heptadecane, diphenyl ether, and hexane.

8. The method of claim 1, wherein the oxidizing agent is selected from the group consisting of: dimethylsulfoxide (DMSO), diphenylsulfoxide, diphenylselenoxide, diphenyltelluroxide, trimethylamine-N-oxide, and pyridine-N-oxide

9. The method of claim 8, wherein about 1.52 mmol of dimethylsulfoxide (DMSO) is included in the second solution.

10. The method of claim 1, wherein the solution further comprises dimethoxyethane (DME).

I I. The method of claim 10, wherein about 1.52 mmol of DME is included in the solution.

12. The method of claim 1, further comprising washing the solid with concentrated mineral acid.

13. The method of claim 1, wherein the graphitic carbon comprises a carbon nanotube.

14. The method of claim 1, wherein the graphitic carbon comprises a sheet of graphitic carbon.

15. The method of claim 1, wherein the solution is formed in an inert gas atmosphere.

16. The method of claim 1, further comprising agitating the sample while heating.

17. The method of claim 1, wherein the heating of the second solution comprises heating using an oil bath.

18. The method of claim 1, wherein the second solution is heated at a temperature below 500° C.

19. The method of claim 18, wherein the second solution is heated at a temperature of between about 100° C and about 130° C.

20. The method of claim 19, wherein the second solution is heated at a temperature of about 1 15° C.

21. The method of claim 1, wherein the second solution is heated for between about 12 hours and about 120 hours.

22. The method of claim 21, wherein the second solution is heated for about 120 hours.

23. A method for preparing graphitic carbon, comprising: a) combining bis(cyclooctatetraene)iron (Fe(COT)₂), dimethoxyethane (DME), and a solvent to form a solution; b) heating the solution until reflux; c) adding dimethylsulfoxide (DMSO) to the solution to form a second solution; and d) heating the second solution to form a solid,

thereby forming graphitic carbon.

24. A method for preparing graphitic carbon, comprising: a) combining COT with d imethylsulfoxide (DMSO) with a solvent to form a solution, b) heating the solution until reflux; c) adding iron pentacarbonyl (Fe(CO)₅) to the solution to form a second solution; and c) heating the second solution to form a solid, thereby forming graphitic carbon.