Applied Surface Science 254 (2008) 5247–5251

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Applied Surface Science

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Effect of heat-pretreatment of the graphite rod on the quality of SWCNTsby arc discharge

Zijiong Li a,b,*, Liangming Wei b, Yafei Zhang b,**a Department of Physics, Zhengzhou University of Light Industry, Zhengzhou 450007, PR Chinab National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education,

Research Institute of Micro/Nanometer Science & Technology, Shanghai Jiao Tong University, Shanghai 200030, PR China

A R T I C L E I N F O

Article history:

Received 7 January 2008

Received in revised form 13 February 2008

Accepted 13 February 2008

Available online 17 February 2008

Keywords:

Single-walled carbon nanotubes

Arc discharge

Heat-pretreatment

Raman spectroscopy

Field emission

A B S T R A C T

Single-walled carbon nanotubes (SWCNTs) have been synthesized in high yield by the dc arc discharge

technique under heat-pretreatment of the graphite rod conditions. Before executing arc discharge, the

graphite rods containing the catalysts were heat treated at 600, 700, 800 and 900 8C for 1–3 h,

respectively. Effects of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge

were investigated. The heat-treatment temperature and time were found to be crucial for a high yield of

high-purity SWCNTs. Optimum parameter was found to be at the heat-treatment temperature of 800 8Cfor 2 h. The SWCNTs synthesized under the optimum condition have better field-emission characteristics.

The turn-on field needed to produce a current density of 10 mA/cm2 is found to be 1.9 V/mm and the

threshold field where current density reaches 10 mA/cm2 is 3.9 V/mm.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The remarkable properties of single-walled carbon nanotubes(SWCNTs) have attracted extensive interest to make them superfor use in a large variety of applications since they were firstreported in 1993. However, the full potential of SWCNTs will berealized once their growth will be optimized and well controlled[1–4]. In this regard, different techniques such as arc discharge,laser ablation and catalytic chemical vapor deposition (CCVD) forthe growth of carbon nanotubes have been actively pursued.Considering its reasonably high efficiency and the bulk productioncapability, the arc discharge method is still one of the best methodsfor producing SWCNTs in large quantity [5–8]. This methodgenerally involves temperatures higher than 6000 K and the

* Corresponding author at: National Key Laboratory of Nano/Micro Fabrication

Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of

Education, Research Institute of Micro/Nanometer Science & Technology, Shanghai

Jiao Tong University, Shanghai 200030, PR China. Tel.: +86 21 6293 3294;

fax: +86 21 6268 3631.

** Corresponding author. Tel.: +86 21 6293 3294; fax: +86 21 6268 3631.

E-mail addresses: lizijiong@sjtu.edu.cn (Z. Li), yfzhang@sjtu.edu.cn

(Y. Zhang).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.02.036

production always comes along with large proportions ofamorphous carbon, fullerenes and catalyst remnants, etc. Becausemost of the impurities are partly or totally built from sp2 carbon,purification and separation steps are inefficient or problematic forthe least. In spite of the efforts that have been devoted to theelectric arc technique to understand the influence of the main andnumerous physical parameters (typically: arc current, pressure,anode composition and mobility and type of catalysts) [3,7–11],there are still many key problems remaining to be resolved.

In this work, the effects of heat-pretreatment of catalystscontaining graphite rods on synthesizing SWCNTs via arc dischargewere investigated. And we found that the heat-treatmenttemperature and time are crucial for synthesis of higher yieldand higher purity SWCNTs. Electron field emission of the as-prepared SWCNTs was also evaluated.

2. Experimental

SWCNT was synthesized by the sublimation of graphite rod asthe anode in a dc electric arc discharge reactor. The anode had adiameter of 6 mm and a hole was drilled along its axis with adiameter of 4 mm and a depth of 60 mm. This hole was filled with apowder mixture of graphite and catalysts (NiO and Y2O3). Theatomic ratio of Ni to Y in the filling powder was 4:1. The graphite

Fig. 1. SEM images of SWCNTs obtained by heat-pretreatment of the graphite rod containing catalysts at different temperature for 2 h: (a) 600 8C, (b) 700 8C, (c) 800 8C and (d)

900 8C.

Fig. 2. TEM images of SWCNTs obtained by heat-pretreatment of the graphite rod containing catalysts at different temperature for 2 h: (a) 600 8C, (b) 700 8C, (c) 800 8C and (d)

900 8C.

Z. Li et al. / Applied Surface Science 254 (2008) 5247–52515248

Fig. 3. Raman spectra of the as-prepared SWCNTs by heat-pretreatment of the

graphite rod containing catalysts at (a) 800 8C for 2 h, the inset shows an

enlargement of the RBM peaks and (b) 600, 700 and 900 8C for 2 h, IG/ID ratio is also

shown.

Z. Li et al. / Applied Surface Science 254 (2008) 5247–5251 5249

rods containing catalysts were heat-pretreated in the furnaceunder Ar/H2 (15% H2) at a flow rate of 600 sccm. The heat-pretreatment temperature was controlled at 600, 700, 800 and900 8C for 1–3 h, respectively. Then arc discharge was performedwith a current of 90 A and a voltage of 60 V under heliumatmosphere pressure of 300 Torr.

The as-prepared SWCNTs were characterized by scanningelectron microscopy (SEM, JSM-7401F), transmission electronmicroscopy (TEM, JSM-2010), Raman spectroscopy (633 nmexcitation) and thermogravimetric analysis (TGA, PE TGA-7instrument). The field-emission measurements, in a diode config-uration, were performed in a vacuum chamber at a base pressure of�10�4 Pa. The uniform SWCNT films were grown by arc dischargeand subsequently deposited by screen-printing on the entire SiO2

surface. An indium–tin–oxide (ITO) coated glass, which served asthe anode, was separated from the SWCNT cathode by a 180-mmthick mica spacer. The field-emission characteristics (current vs.voltage) were measured with a Keithley 237 electrometer.

3. Results and discussion

A series of experimentations have been conducted to achievethe optimum conditions by heat-treatment of catalysts in graphiterods for synthesis of SWCNT. The effect of heat-treatmenttemperature and time on the yield and purity of SWCNT wasinvestigated, respectively. The SEM and TEM images of the CNTsobtained for a series of heat-treatment temperature are shown inFigs. 1 and 2, respectively. The images clearly show bulks ofmicrometer-long CNTs were densely synthesized with a typicalentangled manner at different heat-treatment temperature. Theimages also show some changes in the morphology and purity ofsample at different heat-treatment. At 600 8C, bundled CNTs withbig diameter are randomly distributed on the surface of samples.The as-prepared CNTs are covered by amorphous carbon andmetallic particles. At 800 8C, the diameter of bundled CNTs hasbecome smaller, and the density and purity of CNTs is obviouslyhigher than that of the 600 and 700 8C samples. At 900 8C, the CNTsdensity is the highest but the purity is lower than that of the 800 8Csample.

Typical Raman spectra from 633 nm laser excitation in Fig. 3areveal that the as-prepared nanotubes by heat-treatment ofcatalysts in graphite rods at 800 8C for 2 h were single-walled, ascharacterized by the strong G-band (tangential stretch mode)and the presence of the sharp radial breathing mode (RBM) inthe low frequency region [12]. The diameter dt (in nm) of theSWCNT can be estimated from the frequency vRBM (in cm�1) ofthe RBM with the equation vRBM = 248/dt [13]. In the RBM peaksof the as-prepared SWCNT synthesized by arc discharge by heat-treatment of catalysts in graphite rods at 800 8C, several peaksappear in a wide range of 150–400 cm�1 with the main peaksare 155.3, 219.1 and 335.1 cm�1. The corresponding diametersof the main peaks are 1.59, 1.13 and 0.74 nm. The G-band peakcan usually be used to reflect the density of as-prepared SWCNT.And the intensity of the G-band to D-band ratio (IG/ID) canusually be used to evaluate the quality of the grown nanotubes.The effect of heat-treatment on the density and purity ofSWCNTs from Raman spectroscopy is shown in Fig. 3b. Asobserved from the SEM and TEM images, the G-band peak isweak at 600 8C. However, as the growth temperature increases,the intensity rapidly increases. Fig. 3(b) also shows the IG/ID

variation with the pretreatment temperature. The lowest IG/ID

ratio was obtained at the pretreatment of 600 8C. And the IG/ID

ratio increases with increasing pretreatment temperature. Afterreaching a maximum intensity at 800 8C, the IG/ID ratio rapidlydecreases as the temperature increases. The IG/ID ratio is �2.4

times larger under the pretreatment of 800 8C conditions thanunder the corresponding 600 8C conditions, which is consistentwith the previous SEM/TEM images of the samples, where theamount of amorphous carbon species is much less under 800 8Cconditions. By contrast, the 800 8C was selected as the optimumheat-treatment temperature.

Fig. 4 shows the results of TGA spectra from the as-preparedSWCNTs with different heat-treatment temperature. The TGAwas made with a thermo-analytical balance on samplesweighing 100 mg burnt in the air at a temperature increaserate of 10 8C/min. It further demonstrated that the yield andpurity of the SWCNTs increases with the increase in heat-treatment temperature. The purity of the as-prepared CNTs ishighest at 800 8C which is corresponding to the observationfrom SEM and TEM images.

Fig. 5 shows the TEM images of as-prepared SWCNTs by heat-treatment of graphite rods containing catalysts for 1, 2 and 3 hat 800 8C. It is clearly shown that the purity of the as-preparedSWCNTs by pretreatment for 2 h is higher than that for 1 and 3 hsamples. Further examination of the SWCNTs using IG/ID ratiofrom Raman spectroscopy reveals that the IG/ID varies amongdifferent heat-treatment time at 800 8C samples. The IG/ID ratiois the highest for 2 h among the heat-treatment time for 1, 2 and3 h. It indicates that heat-treatment time for 2 h is the optimumtime.

Fig. 6. Emission current density vs. electric field (J–E) characteristics of the SWCNTs,

which were synthesized by arc discharge by heat-pretreatment of the graphite rod

containing catalysts at 800 8C for 2 h, the inset shows the corresponding Fowler–

Nordheim plot.

Fig. 4. TGA spectra of SWCNTs obtained by heat-pretreatment of the graphite rod

containing catalysts at different temperature: (a) 600 8C, (b) 700 8C, (c) 800 8C and

(d) 900 8C.

Z. Li et al. / Applied Surface Science 254 (2008) 5247–52515250

Fig. 6 demonstrates the emission current density versuselectric field (J–E) characteristics of the SWCNTs, which weresynthesized by arc discharge by graphite rods containingcatalysts heat-treatment at 800 8C for 2 h. The electric contactwas then made directly onto the SWCNT film, which serves asthe cathode. The turn-on field (Eto) needed to produce a currentdensity of 10 mA/cm2 is found to be 1.9 V/mm. The thresholdfield (Eth) where J reaches 10 mA/cm2 is 3.9 V/mm, and a highemission current density of 24.5 mA/cm2 can be achieved at afield of 5 V/mm. The lower Eto and Eth are superior to the SWCNTthat were grown by arc discharge without heat-treatmenthaving even been reported. Among these, Bonard et al. haveobtained excellent field emitters, yielding Eto = 1.5–4.5 V/mmand Eth = 4–7 V/mm [14–17].

Based on all the evidence mentioned above, it can be inferredthat the as-prepared SWCNTs by rods containing catalysts heat-

Fig. 5. TEM images of SWCNTs obtained by heat-pretreatment of the graphite rod

treatment in this work were realized through the reaction:H2 (gas) + NiO (solid)! Ni (solid) + H2O (liquid) and 3H2 (gas)+ Y2O3 (solid)! 2Y (solid) + 3H2O (liquid). On the one hand,composite particles of Ni and Y can serve as active catalystsfor the SWCNTs nucleate and growth in the arc dischargeatmosphere. On the other hand, the heat-treatment changedthe surface feature of catalyst and the dispersion of the activemetal composition in graphite rods on the condition thatthe heat-treatment temperature is feasible. Moreover, a narrowpart of the rods surface containing uniform catalysts isefficiently heated to increase the sublimation rate during thearc discharge, which resulted in the production of nanotubeswith higher density. It is well-known that the formation of

containing catalysts at 800 8C for different time: (a) 1 h, (b) 2 h and (c) 3 h.

Z. Li et al. / Applied Surface Science 254 (2008) 5247–5251 5251

SWNTs is closely proportional to the evaporation rate of thecomposite rod.

4. Conclusion

In summary, high quality SWCNTs were synthesized via dc arcdischarge by heat-pretreatment of the graphite rods containingcatalysts. The effects of catalysts heat-treatment temperature andtime on the SWCNTs growth were investigated. Optimumcondition was found to be at 800 8C for 2 h for graphite rodsheat-treatment temperature and time, respectively. The SWCNTssynthesized under this condition showed excellent field-emissioncharacteristics with lower turn-on and threshold fields of 1.9 and3.9 V/mm, respectively.

Acknowledgements

This work is supported by National Natural Science Foundationof China No. 50730008, Shanghai Science and Technology GrantNo. 0752nm015 and National Basic Research Program of China No.2006CB300406.

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