REPORTS A clos C) at the ledge of Water-Assisted Highly Efficient the swnt forest that the nano- ubes are dense Synthesis of Impurity-Free aligned from the Low-resolution ansmission electron microscopy (TEM) Single-Walled Carbon Nanotubes studies (Fig. ID) of the as-grown forest reveal the presence of only thin nanotubes and the absence of metallic particles and Kenji Hata, t Don N. Futaba, Kohei Mizuno, Tatsunori Namai, supporting materials that usually comprise a Motoo Yumura, Sumio iijima major constituent of as-grown material High-resolution TEM studies (Fig. IE and We demonstrate the efficient chemical vapor deposition synthesis of single- fig. S1) show that the nanotubes are clean walled carbon nanotubes where the activity and lifetime of the catalysts are SWNTs free from amorphous carbon and enhanced by water. Water-stimulated enhanced catalytic activity results in metal particles. We have taken hundreds of massive growth of superdense and vertically aligned nanotube forests with high-resolution TEM images, and double-or heights up to 2.5 millimeters that can be easily separated from the catalysts multi-walled carbon nanotubes (MWNTs providing nanotube material with carbon purity above 99.98%.Moreover were rarely found. Raman spectra(fig. S2) terned, highly organized intrinsic nanotube structures were successfully at 514 nm excitation showed clear radial fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis firmed the existence of SWNTs. The sizes of 8 the SWNTs were estimated from the peaks Single-walled carbon nanotubes (SWNTs) and sputtered metal thin films (Fe, Al/Fe, to be in the range of I to 3 nm, in agreement are a key aspect in the emerging field of Al,O/Fe, Al,O/Co) on Si wafers, quartz, with those measured by TEM. nanotechnology; however, large-scale syn- and metal foils, which demonstrates the The SWNT forest structure can be easily o thesis is still limited because of the difficul- generality of our approach removed from the substrate with, for example, ties in synthesizing SWNTs. Current synthesis Water-stimulated catalytic activity results a razor blade (movie S1). After removal, the methods suffer from the production of in the growth of dense and vertically aligned substrate is still catalytically active and can 5 impurities that must be removed through SWNT forests with millimeter-scale height grow SWNT forests again, indicating a root- purifications steps, which can damage the in a 10-min growth time. Our best result to growth mode and the presence of the catalysts tions for further processing also presents In contrast with standard ethylene CVD (TGA)was implemented on 10 mg of the as- g nanotubes. Dispersion of SWNTs in solu- date is 2.5 mm in 10 min(Fig. 1, A and on the substrate. Thermo-gravimetric analysis challenges because the smooth-sided tubes growth, where the catalysts are only active grown material(Fig. 2A). No measurable readily aggregate and form parallel bundles for about 1 min, a height increase of the residue remained after heating above 750C or ropes as a result of van der waals in- forests has been observed after 30 min for indicating very high purity. The combustion 5 and general synthetic approach that concur- weight ratio exceeds 50,000%, more than with the peak weight reduction occurring at rently addresses these problems, in which the 100 times as high as that of the high-pressure esult very similar to that of pu- activity and lifetime of the catalysts are carbon monoxide(HiPco) process(4). Pro- -quality SWNTs synthesized by a E dramatically enhanced by the addition of a vided that the amount of water is well controlled amount of water vapor in the controlled, growths are highly reproducible with x-ray me tative elemen- method (5). Quan rescence spec- growth atmosphe We wanted to find a weak oxidizer that but would not damage the nanotubes at the A would selectively remove amorphous carbon Fig. 1. SWNT forest with water-assisted CVD(A)Picture of a 25-mm-tall SwnT forest on rowth temperature, because coating of the 7-mm by 7-mm silicon wafer catalyst particles by amorphous carbon dur- A matchstick on the left and ing chemical vapor deposition (CVD) ruler with millimeter markings reduces their activity and lifetime(n) on the right is for size reference found that water acts in promoting and (B) electron rosco- Py (SEM)image of the same grown by ethylene cvd by using ar or he b D SwnT forest. Scale bar. 1 with H. that contained a small and controlled forest ledge. Scale bar, 1 um.(D) amount of water vapor (2). Balancing the relative levels of ethylene and water was nanotubes Scale bar, 100 nm.(E) crucial to maximize catalytic lifetime. Water- High-resolution TEM image of the assisted growth was successfully carried out SWNTs Scale bar. 5 nm on various catalysts that generate SWNTs, C including Fe nanoparticles (3) from FeCl3 E f Advanced Industrial Science and Technology(AIST), Tsukuba, 305-8565, Japan. "These authors contributed equally to this work fTo whom correspondence should be addressed E-mail: kenji-hata@aist- go. jp 1362 19NovemBer2004voL306SciEncewww.sciencemag.orgWater-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes Kenji Hata,*. Don N. Futaba,* Kohei Mizuno, Tatsunori Namai, Motoo Yumura, Sumio Iijima We demonstrate the efficient chemical vapor deposition synthesis of single￾walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water. Water-stimulated enhanced catalytic activity results in massive growth of superdense and vertically aligned nanotube forests with heights up to 2.5 millimeters that can be easily separated from the catalysts, providing nanotube material with carbon purity above 99.98%. Moreover, patterned, highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis. Single-walled carbon nanotubes (SWNTs) are a key aspect in the emerging field of nanotechnology; however, large-scale syn￾thesis is still limited because of the difficul￾ties in synthesizing SWNTs. Current synthesis methods suffer from the production of impurities that must be removed through purifications steps, which can damage the nanotubes. Dispersion of SWNTs in solu￾tions for further processing also presents challenges because the smooth-sided tubes readily aggregate and form parallel bundles or ropes as a result of van der Waals in￾teractions. We report a rational yet simple and general synthetic approach that concur￾rently addresses these problems, in which the activity and lifetime of the catalysts are dramatically enhanced by the addition of a controlled amount of water vapor in the growth atmosphere. We wanted to find a weak oxidizer that would selectively remove amorphous carbon but would not damage the nanotubes at the growth temperature, because coating of the catalyst particles by amorphous carbon dur￾ing chemical vapor deposition (CVD) reduces their activity and lifetime (1). We found that water acts in promoting and preserving catalytic activity. SWNTs were grown by ethylene CVD by using Ar or He with H2 that contained a small and controlled amount of water vapor (2). Balancing the relative levels of ethylene and water was crucial to maximize catalytic lifetime. Water￾assisted growth was successfully carried out on various catalysts that generate SWNTs, including Fe nanoparticles (3) from FeCl3 and sputtered metal thin films (Fe, Al/Fe, Al2O3/Fe, Al2O3/Co) on Si wafers, quartz, and metal foils, which demonstrates the generality of our approach. Water-stimulated catalytic activity results in the growth of dense and vertically aligned SWNT forests with millimeter-scale height in a 10-min growth time. Our best result to date is 2.5 mm in 10 min (Fig. 1, A and B). In contrast with standard ethylene CVD growth, where the catalysts are only active for about 1 min, a height increase of the forests has been observed after 30 min for water-assisted growth. The SWNT/catalyst weight ratio exceeds 50,000%, more than 100 times as high as that of the high-pressure carbon monoxide (HiPco) process (4). Pro￾vided that the amount of water is well controlled, growths are highly reproducible. A close examination (Fig. 1C) at the ledge of the SWNT forest illustrates that the nano￾tubes are densely packed and vertically aligned from the substrate. Low-resolution transmission electron microscopy (TEM) studies (Fig. 1D) of the as-grown forest reveal the presence of only thin nanotubes and the absence of metallic particles and supporting materials that usually comprise a major constituent of as-grown material. High-resolution TEM studies (Fig. 1E and fig. S1) show that the nanotubes are clean SWNTs free from amorphous carbon and metal particles. We have taken hundreds of high-resolution TEM images, and double- or multi-walled carbon nanotubes (MWNTs) were rarely found. Raman spectra (fig. S2) at 514 nm excitation showed clear radial breathing mode peaks (RBM), which con￾firmed the existence of SWNTs. The sizes of the SWNTs were estimated from the peaks to be in the range of 1 to 3 nm, in agreement with those measured by TEM. The SWNT forest structure can be easily removed from the substrate with, for example, a razor blade (movie S1). After removal, the substrate is still catalytically active and can grow SWNT forests again, indicating a root￾growth mode and the presence of the catalysts on the substrate. Thermo-gravimetric analysis (TGA) was implemented on 10 mg of the as￾grown material (Fig. 2A). No measurable residue remained after heating above 750-C, indicating very high purity. The combustion range of the SWNTs was 550-C to 750-C, with the peak weight reduction occurring at 700-C, a result very similar to that of pu￾rified, high-quality SWNTs synthesized by a laser-oven method (5). Quantitative elemen￾tal analysis with x-ray fluorescence spec￾R EPORTS Fig. 1. SWNT forest grown with water-assisted CVD. (A) Picture of a 2.5-mm-tall SWNT forest on a 7-mm by 7-mm silicon wafer. A matchstick on the left and ruler with millimeter markings on the right is for size reference. (B) Scanning electron microsco￾py (SEM) image of the same SWNT forest. Scale bar, 1 mm. (C) SEM image of the SWNT forest ledge. Scale bar, 1 mm. (D) Low-resolution TEM image of the nanotubes. Scale bar, 100 nm. (E) High-resolution TEM image of the SWNTs. Scale bar, 5 nm. Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan. *These authors contributed equally to this work .To whom correspondence should be addressed. E-mail: kenji-hata@aist.go.jp 1362 19 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org on January 23, 2008 www.sciencemag.org Downloaded from