that hydrogen has the abilities to maintain a reasonablyclean metal surface by gasification residual carbon speciesand mutually enhance the adsorption of hydrocarbons atthe exposed metal faces and therefore influence the carbonnanostructures. Yagasaki and Masel [17] have proposedthat clean active metal surfaces can undergo reconstructionupon heating and that the presence of hydrogen can have asignificant effect on this process, deciding on the final car-bon nanostrutures. Nevertheless, since the complexity ofthe chemical effects induced by hydrogen, and the influencesof some of CNT growth parameters such as the gas compo-sition or flow rate, the temperature in the furnace and thecatalyst on the carbon nanostructure, in depth systematicstudy is needed.

In summary, large amounts of ACNTs were preparedwith Ni/Al2O3 as catalyst in hydrogen gas atmosphere byCVD at 480 �C. Ni particles are generally known as a cat-alyst for the growth of graphitic carbon nanotubes, here wespeculate that amorphous nanotubes may result from thecooperative function of a low temperature and hydrogencarrier gas.

Acknowledgements

The authors acknowledge the financial support by Tian-jin Municipal Science and Technology Commission (No.05YFJZJC01900) and Doctor Fund from the EducationalMinistry of China (No. 20050056062).

References

[1] Iijima S. Helical microtubules of graphitic carbon. Nature1991;354:56–8.

[2] Dai H. Nanotube growth and characterization. In: Dresselhaus MS,Dresselhaus G, Avouris P, editors. Carbon nanotubes: syn-

thesis, structure, properties and applications. New York: Springer;2000. p. 29.

[3] Helveg S, Carlos LC, Jens S, Hansen PL, Clausen BS, Rostrup NJR.Atomic-scale imaging of carbon nanofibre growth. Nature 2004;427:426–9.

[4] Hofmann S, Ducati C, Kleinsorge B, Robertson J. Direct growth ofaligned carbon nanotube field emitter arrays onto plastic substrates.Appl Phys Lett 2003;83:4661–3.

[5] Minea TM, Point S, Granier A, Touzeau M. Room temperaturesynthesis of carbon nanofibers containing nitrogen by plasma-enhanced chemical vapor deposition. Appl Phys Lett 2004;85:1244–6.

[6] Boskovic BO, Stolojan V, Khan RUA, Haq S, Silva SR. Large-areasynthesis of carbon nanofibres at room temperature. Nat Mater2002;1:165–8.

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[9] Ci LJ, Wei BQ, Xu CL, Liang J, Wu DH, Xie SS, et al.Crystallization behavior of the amorphous carbon nanotubesprepared by the CVD method. J Cryst Growth 2001;233:823–8.

[10] Yang Y, Hu Z, Wu Q, Lu YN, Wang XZ, Chen Y. Templateconfinedgrowth and structural characterization of amorphous carbon nano-tubes. Chem Phys Lett 2003;373(5–6):580–5.

[11] Zhao TK, Liu YN, Zhu JW. Temperature and catalyst effects on theproduction of amorphous carbon nanotubes by a modified arcdischarge. Carbon 2005;43:2907–12.

[12] He CN, Zhao NQ, Du XW, Ding J, Shi CS, Li JJ. Low-temperaturesynthesis of carbon onions by CVD. Scripta Mater 2006;54:689–93.

[13] Rohsenow M, Hartnett JP, Ganic EN. Handbook of heat transferfundamentals. 2nd ed. McGraw-Hill: New York; 1985.

[14] Owens WT, Rodriguez NM, Baker RTK. Carbon filament growth onplatinum catalysts. J Phys Chem 1992;96:5048–53.

[15] Rodriguez NM, Kim MS, Baker RTK. Promotion effect of carbon-monoxide on the decomposition of ethylene over an iron catalyst.J Catal 1993;144:93–108.

[16] Baker RTK, Chludzinski JJ, Sherwood RD. A comparison of thecatalytic influence of nickel, iron and nickel–iron on the gasificationof graphite in various gaseous environments. Carbon 1985;23:245–54.

[17] Yagasaki E, Masel RI. Coadsorption of ethylene and hydrogen on(2X � 1) Pt(110)-observation of a weakly bound form of ethylene.Surf Sci 1990;226:51–60.

Fabrication of patterned catalyst films for carbon nanotubeby selective electroless deposition

Jin Young Kim a, Chang-Ho Noh a, Ki Yong Song a,*, Hae Jung Son a,Eok Chai Hwang a, Sung Hen Cho a, Tamara V. Byk a, Ha Jin Kim a,

Jong Min Kim a, Byung Yun Kong b, Nae Sung Lee b, Yun Sung Woo c

a Materials Laboratory, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Republic of Koreab Nano Science and Technology, Sejong University, Seoul 143-747, Republic of Korea

c Max-Planck-Institut fur Festkorperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany

Received 21 August 2005; accepted 13 February 2006Available online 18 April 2006

0008-6223/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2006.02.019

* Corresponding author. Tel.: +82 31 280 6727; fax: +82 31 280 9349.E-mail address: songk@samsung.com (K.Y. Song).

1862 Letters to the Editor / Carbon 44 (2006) 1845–1869

Keywords: Chemical treatment; Surface treatment; Carbon nanotubes

Carbon nanotubes (CNTs) are currently one of the mostintensively studied research subjects, because their intrinsicproperties such as nanometric dimensions, high aspect ra-tio, good thermal and electrical conductivity, and highYoung’s modulus, may open up many application oppor-tunities in advanced industries and the nanosciences [1–3]. However, the future integration of CNTs with conven-tional microelectronics, and their use in the developmentof novel devices, require that CNTs be grown in highly or-dered arrays, or be located at specifically defined positions,such as on pre-patterned catalysts.

Several approaches to fabricating patterned catalystfilms have been reported to date, including conventional re-sist-based photolithography [4], selective modification ofcatalyst-binding monolayers [5], microcontact printing ofcatalysts [6], ink-jet printing of nanoparticle catalysts [7],and selective electroless deposition (ELD) [8]. Amongthem, selective ELD, a method in which metal is electro-lessly-plated from a predetermined pattern, has recentlyshown great promise for the micro-/nanostructuring of awide range of materials. In particular, selective ELD isproving increasingly useful in semiconductor [9,10], andpackaging technologies [11,12]. A major requirement priorto performing selective ELD is the catalytic Pd seed (theplacement and activation of the seed on the surface) andits selectivity: the seed must be confined to specified regionsof the substrate and be completely absent from other re-gions. These processes can be achieved either by the selec-tive deactivation of a catalytically-active substrate or by theselective activation of a non-reactive surface.

Recently, we succeeded in performing the controlleddeposition of Pd metal nanoparticles using the photocata-lytic properties of materials such as TiO2 or SnO2. Our re-sults showed that the Pd nanoparticles selectively adheredto the photocatalytically-activated surfaces on the photo-catalysts. Substrates patterned with such Pd nanoparticlesin turn are able to induce the selective ELD of metals thatare inhibited at their bare surfaces. Based on these results,it would appear that selective ELD presents a cheap, solu-tion-based metal deposition approach for the fabrication ofpatterned catalyst films to initiate CNT growth. In this arti-cle, we describe a simple but powerful means to fabricatepatterned films of CNTs using the selective ELD of a Nicatalyst.

Fig. 1 schematically outlines the selective ELD processused for the patterned growth of CNTs on glass or Si wafersubstrates. The substrates were cleaned for 10 min byimmersion in Piranha solution [a 7:3 (v/v) mixture of 98%H2SO4 and 30% H2O2], followed by rinsing in deionizedwater and drying on a 120 �C hot plate (Caution: piranhasolution reacts violently with most organic materials andmust be handled with extreme care). To obtain a uniform

photocatalytic layer of TiO2 over a large area, n-butylpolytitanate (Tyzor BTP, DuPont, 5% in 2-propanol) wasfirst deposited on the substrate by spin coating and thenbaked at 100 �C in air. The as-produced films were trans-parent and amorphous, with a uniform thickness of 0.03–0.1 lm using surface profiler (TENCOR P-10). Irradiationof TiO2 photocatalysts with photons having energiesgreater than the TiO2 band gap gives rise to excited-stateelectrons in the conduction band and holes in the valenceband [13]. The UV–Visible spectra of the as-producedamorphous TiO2 film showed that the main absorptionedge was situated at �320 nm, corresponding to an opticalband gap of 3.95 eV, whereas the absorption edge for ana-tase TiO2 occurs at �380 nm (3.2 eV) [14]. As a result, theUV irradiation of TiO2 using a wavelength of less than320 nm will generate excited electron/hole pairs, enablingthe use of its photocatalytic properties. Subsequently, a

Fig. 1. Schematics of the selective ELD processes used for patternedgrowth of CNTs onto substrates.

Letters to the Editor / Carbon 44 (2006) 1845–1869 1863

water-soluble polyvinyl alcohol (PVA) layer was spin-coated on top of the TiO2 film from a PVA solution [(Poly-sciences, 5%, MW: 6000), oxalic acid (0.05 M) and trietha-nolamine (0.01 M)] and then dried for several minutes at100 �C. The thickness (0.04 lm) of the PVA layer was esti-mated using the surface profiler.. Here, the PVA layer wasused as a sacrificial hole-scavenging layer to suppress therecombination of the excited electrons and holes formedduring the irradiation of TiO2 [15–19]. In this respect, thehole scavenging layers prolong the lifetime of the excitedelectron, enabling them to induce the reduction of the me-tal cations. In this work, we used the broad wavelengths ofa 1 kW high-pressure Hg lamp, where the irradiance at thesample position was about 10 mW/cm2.

After UV irradiation of the TiO2 surface through achrome-on-glass photomask for 1.5 min, the patternedsubstrates were immersed in an aqueous solution of a Pdsalt (PdCl2 0.3 g L�1, KCl 10 g L�1, HCl 10 mL L�1) for1.5 min. The photoactivated TiO2 possesses reducingpower for Pd2+ ions and Pd metal layer simultaneouslydeposited on the irradiated area by the photocatalytic reac-tion. A Pd(0) layer pattern accurately matching the photo-mask was obtained by light irradiation between 1.5 and2 min. When the irradiation time was shorter than1.5 min, however, the Pd(0) pattern was incomplete, be-cause the extent of Pd(II) reduction in the irradiated area

was insufficient. In addition, when the irradiation timewas longer than 2 min, the Pd(0) patterns appearedblurred. In this case, it is likely that not only the Pd(II) ionson the irradiated regions are completely reduced to Pd(0),but also the Pd(II) ions near the photomask are reducedvia the lateral diffusion of excited electrons in the TiO2 thinfilm through the already reduced Pd(0) layer.

Both a scanning electron microscopy (SEM) image(Fig. 2(a)) and high-resolution transmission electronmicroscopy (TEM) data (Fig. 2(b)–(d)) confirm the selec-tive deposition of the catalytic Pd seeds on the amorphousTiO2 film. As shown in Fig. 2(a), the major requirement ofselectivity for the deposition step is fulfilled: the Pd seedsare confined to the orthogonal lines and completely absentfrom the inter-line areas. The cross-sectional magnifiedimage of the Pd pattern in Fig. 2(b) shows that the Pd nano-particles were uniformly deposited on the pre-patterned sur-face. The particle diameter of the Pd nanoparticles wasestimated to be about 5 nm, and the observed lattice spac-ings of 3.8902 A agreed well with JCPDS data. Also, fromthe selected area diffraction pattern of the particles, thecrystal structure of the Pd nanoparticles showed a singlecrystalline fcc structure with no other phase. The activityof the TiO2 photocatalyst will be lost during the processof Pd deposition. The reason for this is that once Pd isdeposited on the TiO2 top layer, the photocatalytic reac-

Fig. 2. SEM image (a), high-resolution TEM images (b, c) and selected area diffraction pattern (d) of the line-patterned Pd film deposited by thephotocatalytic reaction.

1864 Letters to the Editor / Carbon 44 (2006) 1845–1869

tion no longer proceeds, regardless of whether irradiationis continued or not. Therefore, it is understood that anultrathin Pd metallic layer may be formed on the surfaceof the TiO2. It was confirmed from inductively coupledplasma (ICP) atomic emission spectroscopic measurementsthat ca. 1.5 · 10�7 g/cm2 Pd was adsorbed onto the TiO2

thin film during this process. This amount of Pd was suffi-cient to initiate ELD.

For the patterned growth of CNTs, electroless Ni depo-sition was carried out on the patterned Pd film using aNi electroless solution (NiCl2 Æ 6H2O 12 g L�1, H3BO3

28 g L�1, H2NCH2CO2H 37.5 g L�1, and H2NNH2 Æ xH2O25 g L�1). The Ni pattern with a line thickness of 0.1–0.5 lm and line width of 10 lm was produced only onthe Pd pattern using selective ELD (Fig. 3(a)). However,because the Ni film prepared by ELD shows only a sparsestructure, its electrical conductivity was lower than that ofa typical sputtered Ni film. The ‘‘Scotch tape test’’ demon-strated that the Ni film adhered well to the substrate. Inter-estingly, it also is noted that the Ni film deposited by ELDshows a surface morphology of nanosized clusters, whichare expected to provide the nucleation sites for CNTs with-out any pretreatment prior to CNT growth (Fig. 3(b)) [20].

The patterned Ni catalyst sample prepared by selectiveELD was subjected to a rapid thermal CVD, where theCNTs were grown in the presence of a mixture of argon(500 sccm) and acetylene (200 sccm) for 10 min at 1.6 Torr.SEM images of the patterned CNTs grown on the Ni cat-alyst film are shown in Fig. 4(a)–(d). It is seen that theCNTs grow selectively to form periodic arrays of patternedorthogonal lines. Even though the CNT growth tempera-ture (550 �C) is quite low, highly dense and long CNTsare produced. It is thought that such a low temperaturegrowth of CNTs is attributed to the nanosized Ni clusters(Fig. 3(b)) preformed by ELD prior to the CVD reaction.Carbon atoms may saturate and precipitate more easilyon the nanosized Ni clusters.

High-resolution transmission electron microscopy(TEM) images (Fig. 4(e) and (f)) again reveal the high yieldand the structure of the multi-walled carbon nanotubes:well-graphitized walls and some extended defects, whichare typical for catalytic deposition. The average outer

diameter varies between 15 and 30 nm, and the walls arepartially covered with amorphous carbon. Catalytic parti-cles are sometimes found inside and at the ends of theCNTs, resulting from the catalytic decomposition of car-bon-containing gases.

In conclusion, we fabricated patterned Ni catalyst filmsfor the formation of CNTs at low temperature by combin-ing the selective ELD and photocatalytic properties ofTiO2. The Ni catalyst was electrolessly-deposited on selec-tive Pd seeded areas deposited on the photocatalytically-activated TiO2 surface. Although further refinement isneeded, the results presented here show that selective

Fig. 3. Titled (a) and magnified (b) SEM images of the patterned Ni film prepared by selective ELD.

Fig. 4. SEM images (a–d) of patterned CNTs grown on different scalestogether with their high-resolution TEM images (e and f).

Letters to the Editor / Carbon 44 (2006) 1845–1869 1865

ELD is a viable technique for CNT manufacturingapplications.

References

[1] De Heer WA, Chatelain A, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270:1179–80.

[2] Wong SS, Joselevich EJ, Woolley AT, Cheung CL, Lieber CM.Covalently functionalized nanotubes as nanometre-sized probes inchemistry and biology. Nature 1998;394:52–5.

[3] Ren ZF, Huang ZP, Xu JW, Wang JH, Bush P, Siegal MP, et al.Synthesis of large arrays of well-aligned carbon nanotubes on glass.Science 1998;282:1105–7.

[4] Huang S, Dai L, Mau AWH. Controlled fabrication of large-scalealigned carbon nanofiber/nanotube patterns by photolithography.Adv Mater 2002;14:1140–3.

[5] Dressick WJ, Dulcey CS, Georger JH, Calvert JM. Photopatterningand selective electroless metallization of surface-attached ligands.Chem Mater 1993;5:148–50.

[6] Kind H, Bonard JM, Forro L, Kern K, Hernadi K, Nilsson LO,et al. Printing gel-like catalysts for the directed growth of multiwallcarbon nanotubes. Langmuir 2000;16:6877–83.

[7] Ago H, Murata K, Yumura M, Yotani J, Uemura S. Ink-jet printingof nanoparticle catalyst for site-selective carbon nanotube growth.Appl Phys Lett 2003;82:811–3.

[8] Emmanuel D, Matthias G, James V, William SG, Paul AA, John CF,et al. Electroless deposition of NiB on 15 inch glass substrates for thefabrication of transistor gates for liquid crystal displays. Langmuir2003;19:5923–35.

[9] Emmanuel D, Matthias G, Roy HM, Heinz S, Bruno M. PatterningNiB electroless deposited on glass using an electroplated Cu mask,microcontact printing, and wet etching. Langmuir 2003;19:5892–7.

[10] Howard EK. Recent advances in semiconductor performance andprinting processes for organic transistor-based electronics. ChemMater 2004;16:4748–56.

[11] Hiroshi Y, Shigehito D, Kensuke A, Kazuo G. Selective electrolesscopper deposition on aluminum nitride substrate with patternedcopper seed layer. Thin Solid Films 2005;491:18–22.

[12] Li Yi, Chen Dongsheng, Lu Qinghua, Qian Xuefeng, Zhu Zikang,Yin Jie. Selective electroless deposition of copper on polyimidesurface by microcontact printing. Appl Surf Sci 2005;241:471–6.

[13] Fujishima A, Honda K. Electrochemical photolysis of water at asemiconductor electrode. Nature 1972;238:37–8.

[14] Takikawa H, Matsui T, Sakakibara T, Bendavid A, Martin PJ.Properties of titanium oxide film prepared by reactive cathodicvacuum arc deposition. Thin Solid Films 1999;348:145–51.

[15] Ramanathan K, Avnir D, Modestov A, Lev O. Sol-gel derivedormosil-exfoliated graphite-TiO2 composite floating catalyst: photo-deposition of copper. Chem Mater 1997;9:2533–40.

[16] Morishita S. Photoelectrochemical deposition of copper on TiO2

particles. Generation of copper patterns without photoresists. ChemLett 1992:1979–82.

[17] Wu L-Q, Xu N, Shi J. Novel method for preparing palladiummembranes by photocatalytic deposition. AIChE J 2000;46:1075–83.

[18] Colon G, Hidalgo MC, Navio JA. Influence of carboxylic acid on thephotocatalytic reduction of Cr (VI) using commercial TiO2. Lang-muir 2001;17:7174–7.

[19] Yamazaki S, Iwai S, Yano J, Taniguchi H. Kinetic studies ofreductive deposition of copper (II) ions photoassisted by titaniumdioxide. J Phys Chem A 2001;105:11285–90.

[20] Tsai TK, Chuang CC, Chao CG, Liu WL. Growth and field emissionof carbon nanofibers on electroless Ni–P alloy catalyst. DiamondRelat Mater 2003;12:1453–9.

SiC whisker-toughened MoSi2–SiC–Si coating to protectcarbon/carbon composites against oxidation

Fu Qian-Gang, Li He-Jun *, Li Ke-Zhi, Shi Xiao-Hong, Hu Zhi-Biao, Huang Min

C/C Composites Research Center, Laboratory of Ultrahigh Temperature Composites, Northwestern Polytechnical University, Xi’an 710072, PR China

Available online 18 April 2006

Keywords: Carbon/carbon composites; Oxidation; Coating

The ultrahigh strength of carbon/carbon (C/C) compos-ites at temperatures above 2273 K make them promisingcandidate materials for high temperature structural applica-tions [1,2]. However, they are oxidized when used in anoxidizing environment at high temperature, leading to a

decrease in mechanical properties [3]. A multi-layer protec-tive coating, using SiC as an internal buffer layer, has beenproposed [4–6] and MoSi2, which has a high melting pointand excellent oxidation resistance, could be a useful outer-layer material. However, the mismatch of thermal expan-sion coefficient between the MoSi2 outer-layer and the SiCbonding layer usually leads to cracking of the MoSi2coating, which produces channels for oxygen attack. There-fore, pure MoSi2 cannot be used as the outer-layer, and a

0008-6223/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2006.03.002

* Corresponding author. Fax: +86 29 88492642.E-mail addresses: fqg007@126.com (F. Qian-Gang), lihejun@nwpu.

edu.cn (L. He-Jun).

1866 Letters to the Editor / Carbon 44 (2006) 1845–1869