Moreover, we observed that, for W = 10, large SNB wereable to form a uniform coating around the CNT(Fig. 2(e) and (f)).

In conclusion, we decorated full-length CNT with vari-able amounts of uniform silica gel nanoparticles having dif-ferent sizes. The described nanoassemblies exhibited manyvaluable features, specifically the high versatility of the sil-ica and of the bare graphitic surfaces, and the possibility toeasily tune the amount, the dimension, and the physicaland chemical properties of the SNB. Moreover the non-destructive sidewall functionalization of CNT by pyrene-terminated molecules permits to use full-length CNT,therefore with pristine properties. Both the silylated CNTand the flCNT–SNB nanostructures represent an excellentbasis for higher order assemblies that could be useful for alarge variety of applications, especially for biosensors andelectronics, where maintaining the pristine properties ofthe CNT is particularly important. More detailed analyti-cal and electrical characterizations of these materials willbe reported in future publications.

Acknowledgement

This work was supported by Grant U54 CA119335 fromthe National Institute of Health.

References

[1] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of Fullerenesand Carbon Nanotubes. New York: Academic Press; 1996.

[2] Liu L, Wang T, Li J, Guo ZX, Dai L, Zhang D, et al. Self-assemblyof gold nanoparticles to carbon nanotubes using a thiol-terminatedpyrene as interlinker. Chem Phys Lett 2003;367:747–52.

[3] Ravidran S, Chaudhary S, Colburn B, Ozkan M, Ozkan CS.Covalent coupling of quantum dots to multiwalled carbon nanotubesfor electronic device applications. Nano Lett 2003;3(4):447–53.

[4] Georgakilas V, Tzitzios V, Gournis D, Petridis D. Attachment ofmagnetic nanoparticles on carbon nanotubes and their solublederivatives. Chem Mater 2005;17(7):1613–7.

[5] Liu Y, Tang J, Chen X, Wang R, Pang GKH, Zhang Y, et al.Carbon nanotube seeded sol–gel synthesis of silica nanoparticleassemblies. Carbon 2006;44(1):165–7.

[6] Bottini M, Tautz L, Huynh H, Monosov E, Bottini N, Dawson MI,et al. Covalent decoration of multi-walled carbon nanotubes withsilica nanoparticles. Chem Commun 2005;6:758–60.

[7] Polymeric nanoparticles can be prepared using several techniques,including the widely used Stober technique [8] and the water-in-oilnanoemulsion system [9]. The former is based on the simplehydrolysis of a silica precursor in alcoholic medium in the presenceof ammonia; the latter uses water droplets inside of reverse micelles asnanoreactors. The size of the final nanospheres is mainly regulated bythe dimension of the water droplets and, therefore, by the molar ratioof water to surfactant (w) and by the molar ratio of water to theprecursor (h). Other relevant parameters are the molar ratio of theprecursor to the catalyst (n), the reactivity of the precursor, the totaltime (t) and temperature (T) of the reaction.

[8] Stober W, Fink A, Bohn E. Controlled growth of monodisperse silicaspheres in the micron size range. J Colloid Interface Sci 1968;26:62–9.

[9] Bagwe RP, Yang C, Hilliard LR, Tan W. Optimization of dye-dopedsilica nanoparticles prepared using a reverse microemulsion method.Langmuir 2004;20:8336–42.

[10] Chen RJ, Zhang Y, Wang D, Dai H. Noncovalent sidewallfunctionalization of single-walled carbon nanotubes for proteinimmobilization. J Am Chem Soc 2001;123:3838–9.

Low temperature synthesis of carbon nanospheresby reducing supercritical carbon dioxide

with bimetallic lithium and potassium

Wen Qian a, Lingzhi Wei a, Fangyu Cao a, Qianwang Chen a,*, Wei Qian b

a Hefei National Laboratory for Physical Science at Microscale and Department of Materials Science and Engineering,

University of Science and Technology of China, Hefei 230026, PR Chinab School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA

Received 29 August 2005; accepted 5 January 2006

Keywords: Carbon nanospheres; Heat treatment; Electron microscopy; Microstructure

Among various structure forms of carbon, carbon nan-ospheres [1] are becoming increasingly important due totheir potential applications in a lot of fields. They can be

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

doi:10.1016/j.carbon.2006.01.010

* Corresponding author. Fax: +86 551 3631760.E-mail address: cqw@ustc.edu.cn (Q. Chen).

Table 1Parameters used for the preparation of nanocomposites [7]

Nanoparticle size w h Precursor

Small (14 ± 1 nm) 8 80 TMOSLarge (42 ± 3 nm) 10 61 TEOS

Letters to the Editor / Carbon 44 (2006) 1298–1352 1303

used as photonic crystals, excellent supports for catalysts,and anode in secondary lithium ion batteries, etc. Predom-inant method for the synthesis of solid carbon nanospheresin macroscopic quantities at low cost is the pyrolysis ofhydrocarbon precursors via high temperature with theaid of catalysts and/or reductants [1,2]. For example, Wangand Kang [1] reported the synthesis of �210 nm solid car-bon nanospheres by pyrolysis of methane at 1110 �C inquartz tube, in which catalytic transition and/or rare earthmetal oxides with mixed valences had been placed. Carbonnanospheres with diameter of 100–300 nm were also ob-tained from a mixture of CH4 and H2 at 1110 �C in thepresence of a metallic iron catalyst [2]. Though the pyroly-sis of hydrocarbon precursors for synthesizing carbon nan-ospheres are very useful and are used widely, there aresome disadvantages with these methods. First, the mosthydrocarbon precursors used in these syntheses are hazard-ous chemicals. Second, for most syntheses, temperatures ofpyrolysis are around 1000 �C and are not practical forlarge-scale industrial production. Evidently, a relativelylow temperature process for preparing carbon nanospheresis needed. In this paper, these two problems are solved bydemonstrating that carbon nanospheres could be synthe-sized by reducing non-toxic CO2 molecules in supercriticalphase with bimetallic Li and K at temperature as low as450 �C. The huge decrease of the synthetic temperatureand great improvement of the structures of graphitic layersare explained by the synergism of bimetallic Li and K in theprocesses of both nucleation and growth of carbonnanospheres.

Carbon dioxide is a cheap non-toxic low-energy mole-cule and is very abundant on the earth. Using of CO2 mol-ecules as the carbon source for the preparation of variousmaterials has long been a goal for synthesis chemists [3].When CO2 is heated and pressurized beyond its criticalpoint (31 �C and 73 atm), the gas and liquid phase mergeinto a single supercritical phase. Supercritical CO2 (ScCO2)molecules have many unusual properties, such as high mis-cibility with other liquids, high mixing rates and relativelyweak molecular association as compared to ordinary con-densed phases, and could serve as an excellent reactant.The synthesis of organic compounds by the hydrogenationof ScCO2 has been successfully demonstrated [3–5]. Re-cently, people also found that ScCO2 can serve as carbonsource for the preparation of carbon nanotubes [6] andnanospheres [7]. We report herein that when the ScCO2

molecules are reacted with bimetallic Li and K, due tothe synergism of two metals, carbon nanospheres couldbe produced at lower temperature (450 �C) and with higherquality.

In a typical experiment here, 12.0 g ‘‘dry ice’’ freshlymade from high purity CO2 gas (99.99+%), 1.0 g metallicLi and 1.8 g metallic K were loaded into a stainless-steelautoclave of 15 ml capacity. The amount of ‘‘dry ice’’was in excess for the oxidation of metallic Li and K inorder to ensure CO2 being in a supercritical state. The pres-sure was autogenic depending on the amount of reactants

added and reaction temperature. The vessel was immedi-ately closed tightly and heated to reaction temperaturesranging from 260 �C to 550 �C, for 10 h. Then it was al-lowed to cool down to room temperature naturally. Thedark solid products were collected, treated with 4.0 mol/lHCl aqueous solution. A microfilter was used to collectthe precipitate and washed with absolute ethanol and dis-tilled water for several times.

The structure and morphology of the products werecharacterized by several techniques, such as X-ray diffrac-tion (XRD) pattern, transmission electron microscope im-age (TEM), scanning electron microscope image (SEM),and high-resolution TEM image. The XRD pattern was re-corded in the 2h range of 20–80� by using a Rigaku (Japan)D/max-cA X-ray diffractometer equipped with graphitemonochromatized CuK-alpha radiation (k = 1.54178 A).The SEM and TEM images were recorded using HitachiX-650 SEM and Hitachi H-800 TEM, respectively. High-resolution TEM (HRTEM) studies were performed at200 kV using a JEOL 2010 TEM.

a

20 30 40 50 60 70 80

002

101

2 theta (degree)

inte

nsi

ty

004

b

Fig. 1. (a) TEM image of the as-prepared products synthesized at 450 �C.The average diameter of carbon nanospheres is about 300 nm. (b) TypicalXRD pattern of the carbon nanospheres with three peaks correspondingto the (002), (101), and (004) reflections of graphite. The sharp andstrong peak at 2h = 26.3� indicates higher ordered graphitic structures ofcarbon nanospheres.

1304 Letters to the Editor / Carbon 44 (2006) 1298–1352

TEM image of products synthesized at 450 �C is shownin Fig. 1(a). Carbon nanospheres with average diameterabout 300 nm can be clearly seen. XRD pattern(Fig. 1(b)) of the carbon nanospheres obtained at 450 �Cshows a strong peak at 2h = 26.3�, corresponding to the(002) reflection of hexagonal graphitic (JCPDS Card Files,No. 41-1487), and the other two weak peaks coming from(101) and (004) reflection. Further, we find that the peakat 26.3� in XRD pattern of our sample is much narrowerthan most previous XRD results from carbon spheres pre-pared by pyrolysis of hydrocarbons. This indicates that ourcarbon nanospheres are composed by higher ordered gra-phitic layers, which is proved by the HRTEM studies inthe following. Electron diffraction pattern and HRTEMimage of selected area at the edge of carbon nanospheresare shown in Fig. 2(a) and (b), respectively. The electrondiffraction pattern of carbon nanospheres comprises threediffraction rings corresponding to (002), (101), and(004) reflections of graphite, which is in agreement withthe XRD results. Fig. 2(b) reveals that the carbon nano-spheres are consisted of well-crystallized graphite, up to18 ordered parallel graphitized layers with size larger than10 nm are seen in HRTEM image. The 0.34 nm inter-layer spacing is the (002) lattice distance in hexagonalgraphite.

When the experiment was done using single metal, Louet al. [7] in our group has reported the synthesis of carbonspheres by reducing ScCO2 with Li at reaction temperatureas high as 650 �C. One of the goals of our research reportedin this letter is to further optimize the experimentalparameters so that higher quality carbon nanospherescan be produced at even lower temperature for cost-effec-tive large-scale industrial production. Binary metal mix-tures could promote the formation of carbon nanotubeswas reported for the first time in 1994 by Lambert et al.[8], who have demonstrated that a much better yield ofthe carbon nanotubes can be obtained with a mixture of

Co and Pt as catalyst than using Co alone. This experimen-tal phenomena was confirmed very soon by Smalley’sgroup [9], who found that by using Ni and Co, the yieldof single-walled carbon nanotubes (SWNT) is 10–100 timeshigher than the yield of either Ni or Co alone. Here, liketheir tubular counterparts, the synthesis of carbon nano-spheres is also promoted by using binary metal mixtures.From the description on the above, we have shown thatby using a mixture of Li and K, the carbon nanospheresare produced at lower temperature (450 �C) and with high-er quality. Although all kinds of binary metal mixtureshave been extensively used in the synthesis of carbon nano-tubes [8–10] since the discovery of Lambert et al. [8], themechanism by which the binary metal mixtures could sig-nificantly enhance carbon nanotube yields is still not wellunderstood. This is due to the synergism between the met-als, carbon source, and growth conditions are very compli-cated. Recently, a simple two-stage reaction mechanismhas been come up with by Goddard and co-workers [11]for explaining synergetic effects of binary metal mixtureson catalyzed growth of single-walled carbon nanotubes(SWNTs). By using quantum mechanics combined withmolecular mechanics, Goddard and co-workers [11]showed that one metal is energy more preferable in the pro-cess of the nucleation and the other metal is energy morepreferable in the growth and defect repair. This two-stagereaction mechanism is consistent with the experimentalobservation from Hoddon’s group [10], who has systemat-ically studied the roles of Ni and Y in the growth ofSWNTS by using near-infrared spectroscopy. It is reason-able to apply the two-stage reaction mechanism in thegrowth of carbon nanotubes to explain synergism of twometals observed by us in the formation of carbon nano-spheres, due to the similarity in their graphitic structures.In order to identify the role played by Li and K in the syn-thesis (nucleation and growth) of carbon nanospheres, wehave done another experiment of only using metallic Kto react with ScCO2. We have found that using single Kdoes not affect the yield of the soot production, however,the carbon nanospheres cannot be generated in the sootwith the reaction temperature range from 300 �C to600 �C. This indicates that K plays a significant role inthe generation of the soot, but no role in the growth of car-bon nanospheres. Combined with all the experimentaltruths that (i) at 650 �C carbon spheres are obtained byonly using metallic Li [7]; (ii) carbon nanospheres couldnot be produced by only using K at temperature rangefrom 300 �C to 600 �C; and (iii) the huge decrease of syn-thetic temperature of carbon nanospheres from 650 �C to450 �C and great improvement of the quality of carbonnanosphere with using a mixture of Li and K, we coulddraw the following picture of synergism of Li and K. Atthe reaction temperature of 450 �C, K is mainly responsiblefor reducing carbon dioxide molecules and nucleation andLi is mainly responsible for growth of carbon nanospheres.Furthermore, the synergism observed by us in the synthesisof carbon nanospheres also indicates the alkali metals

Fig. 2. (a) Electron diffraction pattern of selected area at the edge ofcarbon nanospheres. Three diffraction rings correspond to (002), (101),and (004) reflections of graphite, which is in agreement with the XRDresults. (b) HRTEM image of selected area at the edge of carbonnanospheres. Up to 18 ordered parallel graphitized layers with size largerthan 10 nm are observed. The 0.34 nm interlayer spacing is the (002)lattice distance in hexagonal graphite.

Letters to the Editor / Carbon 44 (2006) 1298–1352 1305

not only just act as reduce agents but also play a catalyticrole.

It is well known that the reaction temperature plays acritical role in the formation of carbon nanomaterials. Toinvestigate the effect of different temperatures on the for-mation of carbon nanospheres, a series of experiments werecarried out by changing experimental temperature of theprocess. We found the process proceeded at the tempera-tures lower than 260 �C could not initiate the reaction.

When the reaction was conducted at 300–400 �C, novel car-bon structures, core-shell carbon spheres, are the mainproducts as shown in Fig. 3(a)–(d), which include four-shellcarbon spheres (Fig. 3(a) and (b)), two-shell carbon spheres(Fig. 3(c)) and three-shell carbon spheres (Fig. 3(d)). Toour knowledge, this is the first time that carbon sphereswith core-multishell structure are observed and no similarreport has been published before. In Fig. 3(c), you couldsee the accretion of core-shell carbon spheres. This phe-nomena has been explained by Wang and Kang [12], andis due to the high chemical activity on the surfaces of car-bon spheres. When the reaction temperature was raised to500 �C, other carbon structures such as larger hollowcarbon spheres with sizes of 500 nm to 10 lm (Fig. 4(a)),multi-walled carbon nanotubes bundles growing fromthe center to outside (Fig. 4(b)), and hollow carbon cala-bashes (Fig. 4(c)) were also synthesized. Hollow carbonspheres and carbon calabashes have been reported by othergroups [13,14] and will be useful materials for gas storage[14].

In summary, solid carbon nanospheres with diameterabout 300 nm have been synthesized by reducing non-toxicCO2 molecules in supercritical phase with bimetallic Li andK at temperature as low as 450 �C. The huge decrease ofthe reaction temperature and great improvement in form-ing ordered structures of graphitic layers are explained bythe synergetic effects of bimetallic Li and K in the processesof the nucleation and growth of carbon nanospheres. Atlower temperature (300–400 �C), a novel carbon structure,core-multishell carbon spheres have been synthesized forthe first time. In addition, we demonstrate that the reduc-tion of ScCO2 at moderate temperature (300–500 �C) couldbe a versatile method for obtaining carbon materials withdifferent structures by showing that larger hollow carbonspheres, hollow carbon calabashes, and multi-walled car-bon nanotubes bundles were also found in our products.Due to high surface chemical activity, carbon nanospheresmay have potential applications in gas storage and thestudies of using carbon nanospheres for hydrogen storageis undergoing in our lab.

Fig. 4. (a) SEM image of large hollow carbon spheres with sizes of 500 nm to 10 lm. (b) TEM image of multi-walled carbon nanotubes bundles growingfrom the center to outside. (c) TEM image of hollow carbon calabashes.

Fig. 3. TEM images of novel core-shell carbon nanospheres. (a) TEMimage of four-shell carbon spheres. (b) The magnified image of boxed areashown in (a). (c) TEM image of two-shell carbon spheres. (d) TEM imageof three-shell carbon spheres. The accretion of carbon spheres is due to thehigh chemical activity on the surfaces of carbon spheres.

1306 Letters to the Editor / Carbon 44 (2006) 1298–1352

Acknowledgment

This work was supported by the National Natural Sci-ence Foundation of China (20321101, 20125103, and90206034).

References

[1] Wang ZL, Kang ZC. Pairing of pentagonal and heptagonal carbonrings in the growth of nanosize carbon spheres synthesized by amixed-valent oxide-catalytic carbonization process. J Phys Chem1996;100(45):17725–31.

[2] Serp P, Feurer R, Kalck P, Kihn Y, Faria JL, Figueiredo JL. Achemical vapour deposition process for the production of carbonnanospheres. Carbon 2001;39:621–6.

[3] Jessop PG, Ikariya T, Noyori R. Homogeneous catalytic-hydroge-nation of supercritical carbon-dioxide. Nature 1994;368:231–3.

[4] Jessop PG, Hsiao Y, Ikariya T, Noyori R. Methyl formate synthesisby hydrogenation of supercritical carbon-dioxide in the presence ofmethanol. J Chem Soc Chem Commun 1995;6:707–8.

[5] Chen QW, Bahnemann DW. Reduction of carbon dioxide bymagnetite: implications for the primordial synthesis of organicmolecules. J Am Chem Soc 2000;122:970–1.

[6] Motiei M, Hacohen YR, Calderon-Moreno J, Gedanken A. Prepar-ing carbon nanotubes and nested fullerenes from supercritical CO2 bya chemical reaction. J Am Chem Soc 2001;123:8624–5.

[7] Lou ZS, Chen QW, Gao J, Zhang YF. Preparation of carbon spheresconsisting of amorphous carbon cores and graphene shells. Carbon2004;42:229–32.

[8] Lambert JM, Ajayan PM, Bernier P, Planeix JM, Brotons V, Coq B,et al. Improving conditions towards isolating single-shell carbonnanotubes. Chem Phys Lett 1994;226:366–71.

[9] Guo T, Nikolaev P, Thess A, Colbert DT, Smalley RE. Catalyticgrowth of single-walled nanotubes by laser vaporization. Chem PhysLett 1995;243:49–54.

[10] Itkis ME, Perea DE, Niyogi S, Love J, Tang J, Yu A, et al.Optimization of the Ni–Y catalyst composition in bulk electric arcsynthesis of single-walled carbon nanotubes by use of near-infraredspectroscopy. J Phys chem B 2004;108:12770–5.

[11] Deng WQ, Xu X, Goddard WA. A two-stage mechanism ofbimetallic catalyzed growth of single-walled carbon nanotubes. NanoLett 2004;4:2331–5.

[12] Wang ZL, Kang ZC. On accretion of nanosized carbon spheres.J Phys Chem 1996;100:5163–5.

[13] Xu LQ, Zhang WQ, Yang Q, Ding YW, Yu WC, Qian YT. A novelroute to hollow and solid carbon spheres. Carbon 2005;43:1090–2.

[14] Wang ZL, Yin JS. Graphitic hollow carbon calabashes. Chem PhysLett 1998;289:189–92.

Synthesis of carbon nanotube-supportednickel–phosphorus nanoparticles by an electroless process

Feng Wang a, Susumu Arai a,*, Ki Chul Park b, Kenji Takeuchi b,Yong Jung Kim b, Morinobu Endo b

a Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, Japanb Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, Japan

Received 25 October 2005; accepted 29 December 2005Available online 13 February 2006

Keywords: Carbon nanotubes; Electrochemical treatment; Electron microscopy; X-ray diffraction; Crystal structure

Carbon nanotubes (CNTs) have many practical and po-tential applications due to their outstanding physical andelectrical properties [1,2]. Recently, there has been greatinterest to exploit the application of CNTs in the area ofcatalysts, where nanotubes can function as supports of het-erogeneous catalysts [3]. Various methods such as impreg-nation, sol–gel and hydrogen reduction have beendeveloped to prepare well-dispersed metallic nanoparticlessupported on CNTs that can be used as the heterogeneouscatalyst in the hydrogenation reaction [4,5] and the ad-vanced electro-catalyst in fuel cells [6,7].

Nickel–phosphorus (Ni–P) nanoparticles are consideredto be potentially applicable to hydrogenation catalysts ow-ing to their high catalytic activity, good selectivity and highsulfur resistance in many hydrogen-relating reactions [8–10]. The preparation of highly-dispersed Ni–P nanoparti-cles on carriers such as SiO2 [11], c-Al2O3 [12] and carbonblack [13] has been investigated in recent decades, however,there has no report for CNT as support materials in mak-ing Ni–P catalysts.

Previously, we have reported the preparation and char-acterization of multi-walled CNTs with continual Ni–Players by an electroless deposition process [14]. Comparingwith other methods, the electroless deposition can be re-garded as a special chemical reduction method with severaladvantages such as large-scale production, controllable

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

doi:10.1016/j.carbon.2005.12.049

* Corresponding author. Fax: +81 26 269 5432.E-mail addresses: wangf@gipwc.shinshu-u.ac.jp (F. Wang), araisun@

gipwc.shinshu-u.ac.jp (S. Arai).

Letters to the Editor / Carbon 44 (2006) 1298–1352 1307