Diamond and Related Materials 12(2003) 878–883

0925-9635/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.PII: S0925-9635Ž03.00029-3

Growth characteristics of carbon nanotubes using platinum catalyst byplasma enhanced chemical vapor deposition

Jae-Hee Han , Sun Hong Choi , Tae Young Lee , Ji-Beom Yoo *, Chong-Yun Park , Taewon Jung ,a a a a, a b

SeGi Yu , Whikun Yi , In Taek Han , Jong Min Kimb c b b

Center for Nanotubes and Nanostructured Composites, SungKyunKwan University, 300, Chunchun-Dong, Jangan-Gu, Suwon 440-746,a

South KoreaNCRI Center for Electron Emission Source, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Koreab

Department of Chemistry, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul, 133–791, South Koreac

Abstract

In growth of carbon nanotubes(CNTs) using Pt catalyst by plasma enhanced chemical vapor deposition, effects of experimentalparameters, such as NH plasma pre-treatment, NHyC H ratio and growth temperature, on the growth characteristics of CNTs3 3 2 2

were studied in details. It is noteworthy that fine dispersion of numerous nano-sized Pt particles was supported on the wall of aCNT without agglomeration. Application of CNTs with nano-sized Pt particles to the electrodes of the fuel cell can be possible.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Carbon nanotubes; Platinum; NH plasma pre-treatment; Plasma enhanced chemical vapor deposition3

1. Introduction

The remarkable structural, electrical and mechanicalproperties of carbon nanotubes(CNTs) have developedconsiderable fundamental studies and applications, suchas field emission sourcesw1x, nanoscale electronicdevicesw2x, battery electrodesw3x and scanning probesw4x for a decade. Up to now, transition metal(Ni, Coand Fe) and their alloys have been mainly used ascatalyst metal for CNTs growth. The peculiar ability ofthe mentioned metals to form graphitic carbon layers isthought to related to the combination of different factors.These include their catalytic activity towards the decom-position of carbon compounds, the possibility to formunstable carbides and finally, the rapid carbon diffusionin the metal particles. Besides those transition metals,when other elements(e.g. Cuw5x, S w6x, or Pb w7x) areused in the arc-discharge method, only a negligible orno amount of CNTs is formed.In this work, we used platinum(Pt) as the catalyst

*Corresponding author. Tel.:q82-31-290-7413; fax:q82-31-290-7410.

E-mail address: jbyoo@yurim.skku.ac.kr(J.-B. Yoo).

for the growth of CNTs. We studied the growth charac-teristics of CNTs by changing NH pre-treatment of3

substrate, NH flow rate and growth temperature.3

2. Experiments

In this work, the growth of vertically aligned CNTswas carried out on Pt coated Si(1 0 0) substrate withand without titanium(Ti) buffer layer using mixedgases of C H and NH . Pt and Ti layers were deposited2 2 3

by electron beam evaporation. Thickness of Pt and Tilayer was 20 and 100 A, respectively. Direct current-˚plasma enhanced chemical vapor deposition(DC-PECVD) system was used for the CNTs growth. C H2 2

was used as a carbon source and NH was used as a3

catalyst gas, respectively. Detailed processes for thegrowth of CNTs were described in our previous reportw8x. Field emission scanning electron microscopy(FESEM, XL 30 ESEMyFEG) was used to analyze thesurface morphology. For an identification of nanostruc-ture of CNTs, high-resolution transmission electronmicroscopy (HRTEM, JEM 3010, 300 kV) was alsoemployed. X-ray photoelectron spectroscopy(XPS,ESCA 2000) was used to investigate the relative amountof surface atoms and their chemical bonding state.

879J.-H. Han et al. / Diamond and Related Materials 12 (2003) 878–883

Fig. 1. AFM images of(a) as-deposited Pt(20 A)yTi(100 A)ySi substrate and(b) as-deposited Pt(20 A)ySi substrate. AFM images of(c) PtyTiySi˚ ˚ ˚substrate and(d) PtySi substrate after NH plasma pre-treatment of 1 min.3

Optical emission spectroscopy(OES, SPEX 270N) wasused to examine the species in the plasma during theCNTs growth process.

3. Results and discussion

Fig. 1a and b show the images of surface morphologyof as-prepared PtyTiySi and PtySi substrates by atomicforce microscopy(AFM) measurement, respectively.The scan area was 1=1 mm . A uniformly deposited Pt2

layer can be seen for both samples over the wholesubstrate region. The root mean square(RMS) rough-ness values of each are 3.84 and 1.02 A, respectively.˚In case of the PtyTiySi substrate, due to presence of Tibuffer (for adhesion) layer, rugged sites can be easilyseen. In order to control the initial surface morphologyof Pt layer, we performed pre-treatment on the substrateusing NH plasma for 1 min before the CNTs growth.3

The AFM images were depicted in Fig. 1c and d. Itseems that the NH plasma pre-treatment had an effect3

on the surface morphology of film even for 1 min atrelatively small NH plasma intensity of 22 W at3

temperature of approximately 4008C. The surface mor-

phology of the PtySi substrate(Fig. 1d) more signifi-cantly changed with rough feature than that of the PtyTiySi substrate. It seems that the NH plasma3

pre-treatment created many nucleation sites for the CNTsgrowth. After the pre-treatment, the RMS roughnessvalues increased to 5.32 and 7.96 A for the PtyTiySi˚and the PtySi substrate, respectively. In both cases, somepinholes were found. According to the heterogeneousnucleation theoryw9x, if g(surface-vapor)-g(film-sur-face)qg(vapor-film), the nucleation of film wouldoccur, whereg(AyB) is the interfacial energy betweenA and B. In NH pre-treatment process, assuming that3

the interfacial energy between vapor(i.e. NH gas) and3

surface, i.e. Ti(in case of the PtyTiySi substrate) or Si(in case of the PtySi substrate), is equivalent, theinterfacial energy between film(i.e. Pt) and surface ofTi layer is smaller than surface of Si substrate becausethe bonding enthalpy of Pt–Ti(397"13 kJ mol ) isy1

smaller than that of Pt–Si(501.2"18 kJ mol ) w11x.y1

Therefore, the nucleation reaction is easier in the PtySisubstrate than in the PtyTiySi substrate.Lee et al. reported that NH pre-treatment is crucial3

step to control the CNTs growth in their thermal CVD

880 J.-H. Han et al. / Diamond and Related Materials 12 (2003) 878–883

Fig. 2. SEM images of CNTs grown on(a) PtyTiySi substrate and(b) PtySi substrate after NH pre-treatment. SEM images of CNTs grown3

under(c) the standard process and(d) the NH yC H ratio of 6. All scale bars in the inset of Fig. 2 are of 300 nm.3 2 2

system, where the formation of nano-sized catalyticparticles was decisively affected by NH pre-treatment3

w10x. In our case, it is conjectured that there are twopossibilities in the role of NH pre-treatment on the3

morphology of Pt layer. First, physical absorption orincorporation of atomic nitrogen or NH radicals into Ptx

layer may occur. These nitrogen atoms may cause theadditional strain energy between Pt–Pt bonding, resultingin the change in Pt layer into Pt particles as the strainrelaxation of Pt layer. Second, physical etching, i.e.sputtering, of atomic nitrogen or NH radicals on Ptx

layer may develop.After NH pre-treatment, we performed the growth of3

CNTs using plasma using a gas mixture of C H(602 2

sccm) and NH (240 sccm), where voltage and current3

were 620 V and 0.12 A, respectively, at growth temper-ature of 5208C for 15 min. Fig. 2a shows that the CNTs(diameters of 50–200 nm and lengths of 0.5–3mm)were grown on the PtyTiySi substrate with interspacingof 1 mm between them. Pt particles were presented atthe tip of every CNTs. The surface of substrate(the

inset of Fig. 2a) has a rugged morphology, which wouldbe resulted from the quaternary reaction of the C–Pt–Ti–Si system. The CNTs with diameters of 40–170 nmwere vertically, sparsely grown on the PtySi substratewith interspacing of approximately 2–5mm (shown inFig. 2b) although there had been much nucleation-siteslike rugged features on the surface of Pt layer after theNH plasma pre-treatment of 1 min as shown in Fig.3

1d. However, unlike the sample grown on the PtyTiySisubstrate(the inset of Fig. 2a), the inset of Fig. 2bshows that there were plenty of Pt particles with diam-eters of approximately 10–30 nm on the surface ofsubstrate. Fig. 2c shows the CNTs grown under thesame growth condition, as mentioned above(in Fig.2b), only without the NH plasma pre-treatment of 13

min. Hereafter we call the process without such pre-treatment asthe standard process in this report. Onecan see that CNTs(;2.1=10 cm ) were apparently,9 y2

densely grown more than those(;1.3=10 cm ) pre-8 y2

treated with NH plasma on the substrate before the3

CNTs growth(Fig. 2b).

881J.-H. Han et al. / Diamond and Related Materials 12 (2003) 878–883

Fig. 3. (a) XPS wide scanning plots of(i) the as-prepared PtySi substrate;(ii) the NH plasma pre-treated CNTs sample and(iii ) the untreated3

CNTs sample(i.e. the CNTs sample grown underthe standard process). The corresponding relative concentration is showed by the percentagein parentheses.(b) OES result showing relative intensities of various chemical species such as CN, CH, C and H as a function of the NH in2 a 3

the C H –NH plasma.2 2 3

For detailed study on the pre-treatment of NH plasma3

on the sample, XPS measurement was performed withthe Mg K a X-ray (1253.6 eV). Fig. 3a indicates XPSwide scanning plot results of different samples. Therelative concentrations of N 1s(calculated by the areainside the corresponding peak and displayed by thepercentage in parentheses, respectively) on the surface(-50 A deep) in the CNTs samples with and without˚the NH plasma pre-treatment shown in Fig. 2b and c3

were increased by approximately 3.50 and 2.93 times,respectively, compared with the as-prepared PtySi sub-strate depicted in Fig. 1b. The relative concentrations ofC 1s levels were increased by 1.66 and 1.83 times,respectively. On the other hand, the relative concentra-tions of Pt 4f7y2 level were decreased by factors of 76and 80, respectively. The detection limit for nitrogen inthis instrument is approximately 1 at.%. X-ray diffrac-tometry (XRD) analysis was also employed(data notshown). However, no nitrogen-related compounds werefound in the sample after both the initial NH plasma3

pre-treatment. From the XPS and XRD analyses, wesuggest two possibilities. First, the NH plasma pre-3

treatment even only for 1 min could make a largecontribution to the increase in the nitrogen concentrationon the surface region of the substrate, even if thenitrogen concentration in the untreated one might bealso increased during the CNTs growth stage using themixture of C H and NH . Second, atomic nitrogen or2 2 3

NH radicals dissociated in the NH plasma pre-treat-x 3

ment process was physically absorbed or incorporatedinto catalytic sites of Pt layer, not being a physicaletching on the initial Pt layer, so that in growth process,a diffusion of carbon through Pt catalyst was readilyprevented, resulting in remaining higher amount of Pt

on the NH plasma pre-treatment CNTs sample than on3

the untreated one, and decrease in the number densityof CNTs under our experimental condition.Furthermore, to clarify the role of NH in the growth3

process, we performed the growth of CNTs with increasein the NH flow rate from 240 to 300 sccm at the3

constant C H flow rate of 60 sccm(i.e. increase in2 2

NH yC H ratio from 4 to 6) under otherwise identical3 2 2

the standard process. As shown in Fig. 2d, CNTs weregrown more crowdedly than those grown underthestandard process, particularly; Pt particles on the samplesurface were not shown unlike the sample shown in Fig.2c. Presumably, most remained numerous Pt particlesmight be catalyzed, thus numerous CNTs were grownfrom the surface of sample. This growth characteristicwas different from the sample pre-treated by the NH3

plasma before the CNTs growth shown in Fig. 2b. Toinvestigate the role of NH in growth process, we used3

OES measurement in growth process. As can be seen inFig. 3b, C (as a carbon source, Swan system,A P y3

2 g

X P band; 436 nm) radicals increased but CN(as an3u

etching source,B SyX S band; 388.3 nm) decreased2 2

with increase in NH flow rate ranging from 240 to 3003

sccm. From this result, it is considered that increase inNH flow rate in the growth stage would increase carbon3

sources and promote the CNTs growth, leading toincrease in the number density of CNTs, in this experi-mental condition. Chhowalla et al. reported that nano-tube length had a maximum value at C H concentration2 2

of 30% with increment in C H concentration at constant2 2

NH flow rate of 100 sccm, because a higher concentra-3

tion of feed gas in the plasma compensates for theetching by NH but with further increase of feed gas3

concentration, the vertical nanotube growth cannot keep

882 J.-H. Han et al. / Diamond and Related Materials 12 (2003) 878–883

Fig. 4. SEM images of CNTs grown at the growth temperature of 6508C. (a) tilt view at low magnification and(b) cross-section view of regionof bottom of CNTs.

pace with the amount of carbon extruded through themetal catalystw13x. In our case, as mentioned earlier,the concentration of C radical which is the carbon2

source enhancing the vertical growth of CNTs wasincreased with increase in NH flow rate ranging from3

240 to 300 sccm at constant C H flow rate of 60 sccm.2 2

Interestingly, we recently obtained an opposite resultthat the concentration of C radical and the growth rate2

of CNTs were decreased with increase in NH flow rate3

ranging from 90 to 150 sccm at constant C H flow rate2 2

of 30 sccmw14x. This result might be considered to bethe change of the range of the critical concentration ofNH (or C H ) at which the growth rate of CNTs is3 2 2

whether increased or not with increase in NH(or3

C H ) flow rate, depending on the total flow rate of2 2

mixed gases. In other words, if the total flow rate werevaried, the range where further growth of CNTs occurswould be also changed even though the fraction ofNH (or C H ) would be identical.3 2 2

Fig. 4a is a SEM image showing that CNTs withdiameters of 70–190 nm were grown underthe standardprocess except for the increase in growth temperatureof 650 8C. The density of CNTs was nearly similar tothat grown underthe standard process (Fig. 2c), butthe length of nanotubes was increased up to 10mm. Asshown in Fig. 4b, there are many short CNTs withlengths of approximately 300–400 nm at the interfacebetween CNTs and Si substrate were grown, with havingPt particles with diameters of approximately 10–30 nmat each tip.Fig. 5 shows TEM images of a CNT which grown

under the standard process except for the increase ingrowth temperature of 6508C (in Fig. 4). We observedthat nano-sized Pt particles were uniformly and inesti-mably distributed in every CNT wall as shown in Fig.

5a. From Fig. 5b, one can see in details that nano-sizedPt particles are scattered without agglomeration. Thelattice fringes of two nano-sized Pt particles are clearlyseen, and the size of Pt particles is from 2 to 5 nm. Itcan be seen that nano-sized Pt particles are present bothinside and outside the CNT wall. In Fig. 5c, anotherpart of CNT wall is showed, where nano-sized Ptparticles are also distributed and the defect-like layerfringes of CNT are indicated by arrows. There havebeen a few reports on dispersion of nano-sized Ptparticles on carbon materials, such as single-wall carbonnanohorn w11x and nanoporous carbonw12x for theapplication to the fuel cell in recent years. However, inall dispersion method, the impregnation of carbon mate-rials with aqueous solution containing H PtCl or PtCl ,2 6 2

which needs relatively particular preparation step, wasalways performed. In our work, however, we success-fully obtained the CNTs with numerous nano-sized Ptparticles(2–5 nm) using simple electron beam deposi-tion of Pt layer and PECVD system.

4. Conclusion

Even by the NH plasma pre-treatment of 1 min,3

atomic nitrogen or NH radicals could be physicallyx

absorbed or incorporated into the surface of Pt layer, soprevent a diffusion of carbon through Pt catalyst andeventually decrease in the number density of CNTsgrown. When the NHyC H ratio increased, most3 2 2

remained numerous Pt particles might be catalyzed, thusnumerous CNTs were grown from the surface of samplein this experiment. In this report, we successfullyachieved the dispersion of nano-sized Pt particles(2–5nm) onto the wall of CNTs without any agglomerationsby conventional electron beam deposition of Pt layer

883J.-H. Han et al. / Diamond and Related Materials 12 (2003) 878–883

Fig. 5. TEM images of(a) a CNT with nano-sized Pt particles on its wall;(b) nano-sized Pt particles at high magnification and(c) high dispersionof many Pt particles(layer fringes of graphite indicated by arrows).

and growth of CNTs using PECVD system. Using thissimple method, it can be possible for these CNTs withnumerous nano-sized Pt particles to be used as theelectrode of the fuel cell.

Acknowledgments

This work is supported by Korea Science and Engi-neering Foundation(KOSEF) through Center for Nan-otubes and Nanostructured Composites(CNNC).

References

w1x J.-M. Bonard, J.P. Salvetat, T. Stockli, W.A. de Heer, L. Forro,A. Chaterlain, Appl. Phys. Lett. 73(1999) 918.

w2x R. Martel, T. Schmidt, H.R. Shea, T. Hertel, Ph. Avouris, Appl.Phys. Lett. 73(1998) 2447.

w3x A.C. Dillon, K.M. Jone, T.A. Bekedahl, C.H. Kang, D.S.Bethune, M.J. Heben, Nature(Lond.) 386 (1997) 377.

w4x H. Dai, N. Franklin, J. Han, Appl. Phys. Lett. 73(1998) 1508.

w5x X. Lin, X.K. Wang, V.P. Dravid, R.P.H. Chang, J.B. Ketterson,Appl. Phys. Lett. 64(1994) 181.

w6x C.-H. Kiang, W.A. Goddard III, R. Geyers, J.R. Salem, D.S.Bethune, J. Phys. Chem. 98(1994) 6612.

w7x C.-H. Kiang, P.H.M. van Loosdrecht, R. Beyers, et al., Pro-ceedings of the Seventh International Symposium on SmallParticles and Inorganic Clusters, Kobe, Japan.

w8x J.H. Han, J.B. Yoo, C.Y. Park, et al., J. Appl. Phys. 91(2002)483.

w9x M. Ohring, The Materials Science of Thin Films, AcademicPress Limited, London, 1992, p. 198.

w10x C.J. Lee, D.W. Kim, T.J. Lee, et al., Appl. Phys. Lett. 75(1999) 1721.

w11x S. Iijima, Proceedings of Tsukuba Symposium on CarbonNanotube in Commemoration of the Tenth Anniversary of itsDiscovery, Tsukuba, Japan, 2001.

w12x S.H. Joo, S.J. Choi, H. Oh, et al., Nature(Lond.) 412 (2001)169.

w13x M. Chhowalla, K.B.K. Teo, C. Ducati, et al., J. Appl. Phys. 90(2001) 5308.

w14x T.Y. Lee, J.H. Han, S.H. Choi, et al., Diamond Relat. Mater.12 (2003).