NANO
LETTERS

Diameter Enlargement of HiPco
Single-Wall Carbon Nanotubes by Heat
Treatmentxxxx
Vol. 0, No. 0
A-C

M. Yudasaka,*,† H. Kataura,‡ T. Ichihashi,§ L.-C. Qin,† S. Kar,† and S. Iijima†,§,|ICORP-JST, c/o NEC, 34 Miyukigaoka, Tsukuba, Ibaraki, 305-8501, Japan,
Tokyo Metropolitan UniVersity, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan,NEC, 34 Miyukigaoka, Tsukuba, Ibaraki, 305-8501, Japan, and Meijo UniVersity,Tempaku-ku, Nagoya 468-8502, Japan
Received May 30, 2001; Revised Manuscript Received July 4, 2001

ABSTRACTThe diameters of HiPco single-wall carbon nanotubes were increased by heat treatments. The original diameters estimated from Raman spectra
were in the range of 0.79 to 1.2 nm; they increased with the heat-treatment temperature, reaching about 1.47 nm after 1800 °C heat treatment.
Thermogravimetric analyses showed that the burning temperature of the HiPco tubes rose by about 350 °C after 1800 °C heat treatment, andthe metal content, which was about 30% in the original HiPco, decreased to about 2% after heat treatment at 1800 °C.

Introduction. The single-wall carbon nanotube (SWNT)1 hasa cylindrical structure made of a single graphene sheet witha diameter of about 1 nm. Its formation is easy,2-5 and alarge quantity of SWNTs of high purity can be obtained bydegradation of high-pressure CO with Fe(CO)5 at about1000 °C.6
There have been demands for structural control of SWNTs,
and better methods of controlling the diameter and chirality
have been sought. Although chirality control is difficult,partial success in diameter control has been achieved bycontrolling the furnace temperature7,8 or choosing the metal
catalysts9 in the laser ablation process. On the other hand, it
is reported that the diameter can be enlarged locally by
coalescing two or more SWNTs with an electron beam,10
which suggests that diameter enlargement might be possible
by applying heat treatment to SWNTs. This report showsthat the thermal enlargement of SWNT diameters is possible
for the HiPco tubes.
The HiPco tubes were purchased from Carbon Nanotechnologies Inc., and used without any purification. Heattreatment (HT) was carried out in a vacuum of 10-6 Torrfor 5 h in the temperature range from 1000 to 2000 °C. The
structure of the heat-treated HiPco tubes was analyzed by
Raman spectra (excitation wavelength: 488 nm, measuring
area: about 100 µm2), transmission electron microscopy(TEM), and thermogravimetric analysis (TGA). The chemical* Corresponding author.
† ICORP-JST.
‡ Tokyo Metropolitan University.
§ NEC.
| Meijo University.
10.1021/nl010037x CCC: $20.00
Published on Web 00/00/0000© xxxx American Chemical Society

Figure 1. Raman spectra of pristine HiPco sample and samples
heat-treated at various temperatures. Heat-treatment temperatures(°C) and diameters (nm) estimated from the peak position of theradial breathing modes are indicated.

stability of SWNTs and the amount of Fe were analyzed by
TGA performed in oxygen atmosphere (1% oxygen, 99%argon) with a temperature increase rate of 5 °C/min from30 to 1000 °C.
The Raman spectra of the heat-treated HiPco samples areshown in Figure 1. The SWNT diameters estimated fromthe radial breathing modes11 appearing at 100 to 350 nm areindicated at each peak position. The estimated diameters ofthe pristine tubes became larger as the HT temperature
increased. The peaks corresponding to diameters of 1.31,
PAGE EST: 2.2

Figure 2. Transmission electron microscope images of (a) a pristine HiPco sample and (b) one heat treated at 1780 °C taken with the samemagnification. The black bar in (a) corresponds to about 10 nm.

1.47, and 1.75 nm were not obvious in the spectrum of thepristine tubes, indicating that the small-diameter nanotubes
disappeared and changed into larger ones. The tangential
modes of the SWNTs showed peaks at about 1570 and 1592
cm-1.11 The intensity of the Fano line shape originating frommetallic nanotubes at 1500-cm-1 decreased as the HT
temperature increased and was almost insignificant after 1700
°C HT, which is consistent with the disappearance of highfrequency radial breathing modes, since thin metallic nanotubes have optical transition around 2.54 eV (488 nm).12 The
broad peaks centered at about 1350 cm-1 in Figure 1 indicatethat the HiPco sample also contained carbon with disorderedstructures13 or that SWNTs themselves have incomplete
hexagon networks.14 The intensity of the 1350-cm-1 peakrelative to that of the tangential-mode peaks did not show
any systematic changes for heat treatment at various temperatures. This might mean that the1350-cm-1 peak wasprobably due to the carbon with disordered structure that
did not homogeneously distribute in the HiPco sample. Afterthe 1900 and 2000 °C HT, the peaks that were characteristicsof SWNTs became weak.
TEM observation revealed that the diameters of SWNTs
in the pristine HiPco sample (Figure 2a) increased after HT,
for example, 1780 °C HT (Figure 2b). A few fragments ofthe disordered carbon existed in the pristine sample, andpolyhedron-like structures were seen after 1780 °C HT.
Spherical particles of Fe or Fe-C contained in the pristine
sample disappeared, and a small number of large Fe or Fe-C
particles remained after HT. It is difficult to judge whether
the thickness of the nanotube rope was changed by the heat
treatment or not because it ranged widely from that of asingle SWNT to several tens of nanometers.
To study the stability of SWNTs and the quantity ofresidual Fe after HT, we performed TGA in oxygen
atmosphere. The burnoff temperature of the pristine HiPco
sample was 480 °C, which increased to 600 °C after 1400°C HT, followed by 750 °C after 1600 °C HT, 800 °C after
1800 °C HT, and 860 °C after 2000 °C HT. When theBmeasurement finished at 1000 °C, iron oxide remained.
Assuming that the iron oxide was Fe2O3, the quantity of Fe
was estimated, and then the percentage of Fe in the HiPco
sample before the heat treatment was calculated. The pristine
HiPco contained about 30 wt % of Fe, which decreased as
the HT temperature increased: 7 wt % after1400 °C HT, 5wt % after 1600 °C HT, 2 wt % after 1800 and 2000 °C
HT.
Discussion. The diameter of SWNTs in HiPco can beenlarged by heat treatment in a vacuum. The enlarged
diameters do not coincide with any additions of the diametersof thin tubes, so the enlargement is not due to the coalescenceof the thin tubes. It is not clear how the diameter enlargement
occurred. The diameter might increase as a result of by theself-reconstruction accompanying the increase of the chiralindexes. The residual oxygen gas in the vacuum chamberand Fe might catalyze the diameter enlargement.
When we heat treated SWNTs with diameter of about 1.33
and 1.47 nm formed by pulsed Nd:YAG laser ablation(details of the formation conditions are given in ref 13), thepopulation of the former tubes relative to that of the latterdecreased as the HT temperature increased, and when it was
about 1800 and 2000 °C, a few SWNTs with diameters of
about 1.53 and 1.75 nm appeared. The diameter enlargementof the SWNTs formed by laser ablation showed a similartendency to the HiPco tubes.
This study found the phenomenon of diameter enlargement; in other words, it presents a new diameter-control
method for SWNTs. However, the mechanism is unclear.
To clarify the mechanism, precise studies, both experimental
and theoretical, will be necessary.
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