Synthesis of well-aligned carbon nanotubes on MCM-41

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1237

Synthesis of w e l l - a l i g n e d carbon nanotubes on M C M - 4 1 Wei Chen, Ai Min Zhang*, Xuewu Yan, Dongcheng Han Department of chemistry, Nanjing University, Nanjing, 210093, P.R. China Fax: +86-25-3317761, E-mail: [email protected] Well-Aligned carbon nanotubes (CNTs) have been fabricated on mesoporous molecular sieves (MCM-41) embedded with iron oxide nanoparticles by chemical vapor deposition (CVD). Benzene with 1% thiophene was used as the carbon source. And large pore size MCM-41 was obtained by using 1,3,5-trimethyl benzene (TMB) as swelling agent. It has been found the mesoporous MCM-41 is an ideal substrate for growing well-aligned carbon nanotubes.

1. I N T R O D U C T I O N Since the discovery of carbon nanotubs, both theoretical models and experimental measurement have demonstrated their remarkable mechanical as well as novel electrical and magnetic properties. Growing Well-Aligned CNTs is important for obtaining functional devices for use as scanning probes [1] and sensors, as new field emitters in panel displays [2], and single-molecular transistors in microelectronics [3]. Aligned carbon nanotubes have been prepared either by postsynthesis fabrication [4] or by synthesis-induced alighment [5]. Recently, Jung Sang Suh [6] fabricated highly ordered two-dimensional CNTs on porous anodic alumina templates; Ren [7] used plasma-enhanced CVD and synthesized self-aligned CNTs on glass substrates. Previous studies show that the template plays an important role in the procedure of CNTs growth. Since the appearance of mesoporous molecular sieves [8,9], such as MCM-41, it has been found that mesoporous molecular sieve (MCM-41) is an ideal substrate for encapsulating catalyst [10,11]. Here we report the well-aligned CNTs have been obtained by using CVD over iron oxide nanoparticles embedded in MCM-41. It is known that the size of formed micelles determines the pore size of final mesoporous materials [12]. Some researchers have already used post-synthesis treatments [13,14], surfactants of different chain lengths [15] and polymers such as triblock-copolymers [16] as templates or incorporation of swelling agent to form large pore mesoporous materials. In the previous studies, 1,3,5-trimethylbenzene (TMB)[17,18] and decane [19] have been used as expanders, and materials with pore size superior to 80 A were obtained. In order to synthesize

* Corresponding author.

1238 CNTs with uniform diameters through controlling the size distribution of active iron particles, we want to synthesis MCM-41 with large pore diameter. Here, we used TMB as swelling agent to expand the pore diameter of mesoporous materials.

2. E X P E R I M E N T A L 2.1. Preparation of catalyst Cetyltrimethylammonium bromide (CTAB) was first dissolved in water with stirring at room temperature to obtain a clean colloidal solution. 1,3,5-trimetyl benzene (TMB), tetraethyl orthosilicate (TEOS) and NaOH were then separately added drop by drop to the solution. After being stirred at room temperature for 1 hour, the homogenous gel with the molar composition of 1.0 cetyltrimethylammonium bromide (CTAB)" x TMB' 20.0 tetraethyl orthosilicate (TEOS): 10.0 NaOH" 1500.0 H20 (0~< x ~< 2.5) was sealed in Teflon autoclaves and statically heated at 373K for 72 hours. Resultant white product was filtered and washed several times with hot deionized water. After drying it was calcined at 773 K in air for 6 hours. The loading of iron oxides onto MCM-41 was carried out by the wet impregnation technique with a 1.6 M aqueous solution of Fe(NO3)3 9H20 for certain time. Then the resulting product was washed with deionized water and dried at room temperature under vacuum for several hours. Afterward the material was calcined at 673K under N2 atmosphere for 6 hours, which led to a transformation of iron nitrate to ferric oxide indicated by the disappearance of the IR band of the NO3-at 1380 cm -~.

2.2. Growth of carbon nanotubes CNTs were prepared in a conventional CVD equipment consisted of a horizontal tubular furnace and gas flow controlling units. A typical growth experiment, about 50 mg catalysts was put into ceramic boat inside a quartz tube. The catalysts were first actived at 500~

for

1.5 h in N2 with flow rate of 60 ml/min, and subsequently reduced by H2 (60 ml/min) at 500~ for l h, then rise to reaction temperature, at 8 0 0 - 9 5 0 ~ maintained for 1 h with the N2 flowing rate of 60 ml/min. Finally the benzene vapor with 1% thiophene was draw into the reaction system by hydrogen gas at certain flowing rate for 30 rain. Carbon nanotubes formed over catalyst were weighed at room temperature.

2.3. Characterization of the carbon nanotubes and catalysts The morphology and diameter of carbon nanotubes were observed by the JEM-200CX type transmission electron microscope (TEM). The crystallogram was determined with Japan

1239 D/max-Y RA X-ray diffractometer using CuK~, radiation (X = 1.54178 ). Pore diameter distribution and specific surface area were performed on an ASAP 2000 adsorption apparatus made by Micromeritics Corporation. The chemical compositions of catalysts were analysed with atom scan 2500 ICP emission spectrometer.

3. R E S U L T S A N D D I S C U S S I O N

3.1. Synthesis of carbon nanotubes The key result we reported in this research work is the synthesis of Well-Aligned CNTs using the new catalyst, MCM-41 embedded with iron nanoparticles. The TEM image of the as-synthesized material (see Figure 1) shows the well-aligned carbon nanotubes with diameter from 10 to 15 nanometers. For a typical 30 rain growth experiment at 900~

the average

weight increase percent using the catalyst contained 2.0 wt.% of ferric oxide is about 22 wt.%, which is relative to the total weight of the catalyst. It is obvious from Fig.1 that nanotubes self-assemble into aligned structures. We have predicted the possible aligned mechanism in our current work. As the nanotubes growth, their outmost walls interact with those of neighboring nanotubes via van der Waals force to form a large bundle with sufficient rigidity. This rigidity enables nanotubes to keep growing along the original direction. Even the outmost nanotubes are held by the inner nanotubes without branching away.

-~., ,~

,

~..,.~ "

~,,

, ~,

]

! Figure 1. Well-aligned carbon nanotubes on MCM-41 embedded with iron oxide nanoparticles We have found that the catalyst preparing process is a crucial step in obtaining the high performance carbon nanotubes. Impregnation with aqueous solution of ferric nitrate for more

1240 than one hour will cause the collapse of the mesoporous structures due to the poor hydrothermal stability of MCM-41 in acidic solutions (pH < 1.0 ), which was indicated by the disappearance of the typical XRD reflection peaks of MCM-41 after impregnation. This collapse would significantly reduce the total surface areas and pore volume. As a result, iron oxide nanoparticles, could not be well dispersed in such template. But, the well-dispersed nanoparticles are very essential to CNTs growth as indicated in other people's work [20-22]. So, in order to avoid this limitation, we adopted different ways: 1. the impregnation time was reduced; 2. impregnation carried out in methanol solution of ferric nitrate; and 3. ultrasonic disperse was adopted. In our experiment, all the typical hkl reflections of the MCM-41 XRD pattern were well maintained after impregnation or ultrasonic loading compared with the assynthesis materials, showing that the loading process in such condition has little influence on the mesoporous phase of MCM-41. The details of the XRD results are shown in figure 2 and figure 3 respectively. Besides the typical MCM-41 reflections, no additional peaks are observed, indicating that no crystalline iron oxide phase has been formed outside the pore structure.

Figure 2. XRD pattern after impregnation with ferric nitrate aqueous solution for l0 rain

Figure 3. XRD pattern atker impregnation with ferric nitrate methanol solution for 1 hour

1241 We also found that the pore diameter, surface area and pore volume of MCM-41 were changed little after impregnation, which was indicated by BET experiment. The results of BET experiment are shown in Table 1. So, with an average MCM-41 pore diameter of 2.9nm, the iron oxide nanoparticles should be dispersed well, which is the vital factor to synthesis Well-Aligned CNTs. Table 1 BET results of as synthesised materials and the materials after impregnation. BJH surface area (mZigi................i;0reV0iume .. ....................P0re::~ciiameter .. ................. (cc/g) (nm) As-synthesised materials

1313.35

0.83

3.40

After impregnation

1297.32

0.78

2.75

3.2. Synthesis of MCM-41 with large pore diameter by using TMB as the swelling agent The pore structure of the mesoporous MCM-41, as the substrate of catalyst, influences immediately on the states of loading iron nanoparticles. Beck et al. [8,9] have demonstrated that the pore size of MCM-41 can be varied as a function of the concentration of expander molecules such as TMB. According to the methodology introduced by Beck et al, we obtained enlarged pore size materials only at the molar ratio of 1.0 CTAB: 2.5 TMB. The experiment results also indicated that the quantity of smeller (TMB) is an important factor on the phase and pore diameter of final mesoporous materials. In our experiment, the molar composition of mixture may be described as: 1.0 CTAB: x TMB: 20.0 TEOS : 10.0 NaOH: 1500.0 H20 ( 0 ~ x ~ 2.5) As a result, the mixture of MCM-41 and MCM-50 or pure MCM-50 were obtained when the CTAB / TMB molar ratio is between the range of 1.0 to 2.0. The lamellar MCM-50 occurred when the molar ratio of TMB/CTAB reached 1.5. However. However when the molar ratio further increasing, the hexagonal MCM-41 was restored again and the pore diameter was enlarged. The chemical composition of mixture and products of synthesis materials for expanding procedure are presented in table 2. We predicted the possible phase transformation mechanism of MCM-41 pore size expanding procedure by using TMB as the swelling agent. As Kunieda et a1.[23] said in his paper, the penetrate tendency was very large for alcohol and aromatic hydrocarbons such as m-xylene. In this case, there will be no significant change in the micelle size by using 1,3,5trimethyl benzene (TMB) as the swelling agent at the lower TMB/CTAB ratio (less than 1.5). But this penetration would destroy the structure of hexagonal MCM-41, and result in the

1242 formation of lamellar MCM-50. While increasing the amount of TMB, TMB molecules would congregate to form "big oil particles", and " dissolve" in the organic hydrophobic tail of the surfactant (CTAB). The hydrophobic solvate interaction of the aromatic molecule with the hydrocarbon tails is analogous to the hydrophilic solvate interaction of water with the charged head groups of surfactants (CTAB). In this sense the inorganic/organic molecular ion pair species are organized with the organic TMB molecules as a co-solvent for the hydrophobic portion of the bi-phase synthesis mixture. As a result, the pore diameter of MCM-41 would be enlarged, which was checked by the increased dl00 value of MCM-41 reflection peaks. Table 2 Chemical compositions of mixture and products of synthesis mesoporous material CTAM

TMB

H20

0.05

0.0

75.0

0

MCM-41 (fine)

0.5

0.05

0.05

75.0

1.0

MCM-41 and MCM-50

0.5

0.05

0.075

75.0

1.5

Disordered MCM-50

1.0

0.5

0.05

0.100

75.0

2.0

MCM-50 and MCM-41

1.0

0.5

0.05

0.125

75.0

2.5

MCM-41

TEOS

NaOH

1.0

0.5

1.0 1.0

TMB :CTAB

Product

Above procedure was indicated by the XRD patterns (see figure 4-8). When the molar ratio of TMB/CTAB was 1.0, the reflection peaks of lamellar MCM-50 were occurred around 2 0 = 3.4 (figure 5), and the dl00Value (44.125 A ) of the 100 reflection peaks of MCM-41 phase was changed little compared with the pure MCM-41 (41.925 A) (figure 4). When the ratio reached 1.5, no reflection peaks of MCM-41 phase were detected and only MCM-50 reflection peaks could be observed (figure 6), which means that MCM-41 phase was completely transformed to MCM-50 phase. At the molar ratio of 2.0, the reflection peaks of MCM-41 phase occurred again, and the dl00 value was increased to 56.718 A (figure 7), indicated that the pore of MCM-41 was enlarged by the expander molecule (TMB). But the MCM-50 phase still existed at this condition. Finally, when the ratio reached 2.5, the reflection peaks of MCM-50 phase were disappeared in the XRD patterns, only the pure MCM-41 with enlarged pore diameter was found, and value of dl00 was 69.531 A (figure 8). But the 110 and 200 reflection peaks of hexagonal MCM-41 couldn't be observed due to the broadening the 100 reflection peak. We also found that the 100 reflection peak shifted toward smaller angel region when the pore was enlarged, which was as the same as the previous work [8,9,19].

1243

2-11o

~

t

20o

1

L,

I

I

-

Figure

4.

TMB

9 CTAB

,~

Figure

= 0.0

~,

5.

r

o,

TMB

,=

r

9 CTAB

e

,,,

o

= 1.0

\ I;

Figure

6.

TMB

9 CTAB

Figure

= 1.5

Figure

8.

TMB

"CTAB

7.

TMB

' CTAB

= 2.0

= 2.5

4. C O N C L U S I O N We have synthesized Well-Aligned CNTS on MCM-41 with diameter from 10 to 15 nanometers. Our synthetic approach involves prepare of mesoporous molecular sieves (MCM-41), impregnation with ferric nitrate aqueous solution, and chemical vapor deposition. All of these allow the production of the Well-Aligned carbon nanotubes. And we have synthesized the MCM-41 with large pore diameter, which will be used to fabricate different

1244 carbon nanotubes grown from the pores of the template in our future work.

REFERENCE 1. J.H.Hanfer, C.L.Cheung, A.T.Woolley, C.M.Lieber, Progress in Biophysics & Molecular Biology, 77 (2001) 73. 2. De Heer, W.A. Bonard, J.M. Fauth, et al., Adv. Mater., 9 (1997 ) 87. 3. S.Frank, P.Poncharal, Z.L Wang, W.A.De Heer, Science, 280 (1998) 1744. 4. W.A. De Heer, W.S.Bacsa, C.A. Telain, T. Gerfin, R.Humphreybaker, L.Forro, D. Ugarte, Science, 268 (1995) 845. 5. S.Huang, L. Dai, A.W.H Mau, J. Mater. Chem, 9 (1999) 1221. 6. J.S. Sub and J.S. Lee, Applied Physics Letters, 75 (1999) 2047. 7. Z. F. Ren, Z. P Huang, et.al science, 282 (1998) 1105. 8. C.T. Kresge, M.E. Leonowicz, W.J.Roth, J.C.Vartuli, J.S.Beck, Nature, 359 (1992) 710. 9. J. S. Beck, et al., J. Am. Chem. Soc., 114 (1992) 10834. 10. F.Michael, et al., Chem. Mater., 274 (1999) 1701. 11. T.Abe, Y.Tachibana, T.Uemastsu, M.Iwamoto, J. Chem. Soc. Chem.Commun. (1995) 1617. 12. A.Corma, Chem. Rev., 97 (1997) 2373. 13. Q.Huo, D.I. Margolez, G.D. Stucky, Chem. Mater., 8 (1996) 1147. 14. A. Sayari, P. Liu, M.Kruk, M. Jaroniec, Chem. Mater., 9 (1997) 2499. 15. A. Sayari,V.R. Karra, R.J. Sudhakar, Presented at the Symposium on Synthesis of Zeolites, Layered compounds and other Microporous Solids, 209th National Meeting of the American Chemical Society, Anaheim, CA, 1995. 16. D. Zhao, J.Feng, Q. Huo, N.Melosh, G.H.Fredrickson, B.F.Chmelka, G.D.Stucky, Science 279 (1995) 548. 17. J. S. Beck, U.S. Patent, 5,057,57296 (1991). 18 P.J. Branton, J.Dougherty, G.Lockhart, J.W.White, Charact. Porous Solids IV(1997) 668. 19 J.L. Blin, C. Otjacques, G. Herrier, and Bao-Lian Su. Langmuir, 16 (2000) 4229. 20 W.W. Li, S.S. Xie, et al., Science, 274 (1996) 1701. 21 J. Kong, A. Cassell, H.Dai, Chem.Phys.Lett., 292 (1998) 4. 22 E. Flahaut et al., Chem.Phys.Lett., 300 (1999) 236. 23 H. Kunieda, K. Ozawa, K.L.Huang, J. Phys. Chem. B, 102 (1998) 831.