www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 67 (2004) 61–65

Synthesis of DAM-1 molecular sieves containingsingle walled carbon nanotubes

Edgar Mu~nnoz a, Decio Coutinho a, Richard F. Reidy b, Arnvar Zakhidov a,Weilie Zhou c, Kenneth J. Balkus Jr. a,*

a Department of Chemistry and the UTD NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083-0688, USAb Department of Materials Science, University of North Texas, Denton, TX 76203, USA

c Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148, USA

Received 12 June 2003; accepted 24 September 2003

Abstract

The mesoporous silica DAM-1 (Dallas Amorphous Material) has been synthesized in the presence of single walled carbon

nanotubes (SWNT) using vitamin E TPGS as the template. It is proposed that this water soluble version of vitamin E wraps and

partially debundles the SWNTs en route to partial encapsulation within the DAM-1 particles. XRD, adsorption, SEM and TEM

provide evidence for occlusion of the nanotubes.

� 2003 Elsevier Inc. All rights reserved.

Keywords: DAM-1 molecular sieves; Carbon nanotubes; Synthesis

1. Introduction

Mesoporous silica has recently been shown to be ef-fective host materials for carbon nanotubes [1–10]. For

example, MCM-41 and MCM-48 have been used to

grow multi-walled carbon nanotubes (MWNT) [5,6].

Aligned MWNTs have been prepared on mesoporous

silicas templated with polyoxyethylene (10) cetyl ether,

Pluronic 123 (P-123) and cetyltriethylammonium chlo-

ride (CTAC) [7]. In this case and others, it is clear that

the MWNTs grow outside the molecular sieve and oftenthe mesoporous structure collapses during MWNT

synthesis. The versatility of these materials as hosts is

exemplified by a recent report of well aligned MWNTs

in patterned SBA-15 and SBA-16 films [8]. More re-

cently the growth of single walled carbon nanotubes

(SWNT) by chemical vapor deposition has been claimed

for SBA-11, SBA-15 and SBA-16 films [9]. The results

from this work suggest that growth of carbon nanotubeswith fixed diameters could be determined by the mo-

*Corresponding author. Tel.: +1-972-883-2659; fax: +1-972-883-

2925.

E-mail address: balkus@utdallas.edu (K.J. Balkus Jr.).

1387-1811/$ - see front matter � 2003 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2003.09.024

lecular sieve pore size. This is a property that has al-

ready been exploited in microporous molecular sieves

for the synthesis of SWNT [11,12]. An alternativestrategy for the preparation of molecular sieve encap-

sulated carbon nanotubes is to incorporate the nanotu-

bes during synthesis of the mesoporous silica. Recently,

arc discharge generated SWNTs were added to an

MCM-41 synthesis followed by heating in air at 450 �Cto remove the template [10]. There was no evidence that

a well ordered mesoporous molecular sieve was syn-

thesized, however, UV–vis and fluorescence spectro-scopy provided support for embedding of the SWNTs in

the silica. The direct encapsulation within the mesopores

during synthesis is a viable strategy. However, to pre-

pare an ordered structure, the template should solubilize

and preferably debundle the SWNTs. Here we describe a

new approach to the encapsulation of SWNTs in meso-

porous silica by incorporating the nanotubes during the

synthesis of DAM-1. This mesoporous silica, made usingvitamin E TPGS [a-tocopheryl polyethylene glycol 1000succinate, C33O5H54(CH2CH2O)23] as the template is a

well ordered, thermally stable molecular sieve with rel-

atively thick pore walls (3–4 nm), high surface area

(562–1200 m2/g), and hexagonally arranged mesopores

in the 3.0–5 nm range [13]. The DAM-1 pore size might

62 E. Mu~nnoz et al. / Microporous and Mesoporous Materials 67 (2004) 61–65

allow encapsulation of individual SWNTs (�1 nm in

diameter) or small-diameter SWNT bundles within the

micelles. We have found that the vitamin E TPGS mi-

celles form stable SWNT dispersions in water, in a

similar fashion to other amphiphilic species such as so-dium dodecyl sulfate and Triton-X [14,15]. Vitamin E

TPGS disperses SWNTs by non-covalently interacting

its hydrophobic head group with the nanotubes and

exposing its polyethylene glycol tail to the water. Results

for the synthesis of DAM-1 with 0.1% and 0.4% by

weight SWNT, including XRD, SEM, TEM and surface

area will be presented.

2. Experimental

SWNTs synthesized by the HiPco method were pur-

chased from Carbon Nanotechnologies Inc. (www.

cnanotech.com). SWNT containing �25% by weight

iron was dispersed in vitamin E TPGS (Eastman

Chemical). Two dispersions were employed to preparethe mesoporous silica. DAM-1 materials prepared with

nanotubes will be referred to as DAM-1-NT. The first

(A) had a composition of 0.1% SWNT in a 2% vitamin

E aqueous solution and the second (B) had a compo-

sition of 0.4% SWNT in 2% vitamin E aqueous solu-

tion. To prepare DAM-1-NT, 0.35 ml of hydrochloric

acid (37%, EM Science) was added to 6.4 g of the

above SWNT dispersion and stirred at room temper-ature for �30 min, after which 0.35 g of tetramethyl-

orthosilicate (TMOS, 98%, Aldrich) was added

followed by stirring at room temperature for �20 min.

The gel was then aged under static conditions at 45 �Cfor 24 h followed by heating at 90 �C for 48 h. After

heating at 90 �C for 24 h, 20 ml of deionized water was

added to the reaction vessel and heated at 90 �C for an

additional 24 h. SWNT-containing DAM-1-NT wasannealed at 850 �C under argon to remove the vitamin

E TPGS template. A sample of the DAM-1-NT was

also heated in air to 550 �C to remove both the tem-

plate and SWNTs.

Powder X-ray diffraction patterns were obtained

using a Scintag XDS 2000 X-ray diffractometer using

CuKa radiation. Samples for scanning electron mi-

croscopy (SEM) were coated with Pd/Au and micro-graphs obtained using a field emission SEM (LEO 1530

VP). Transmission electron microscopy (TEM) was

performed on a JEOL EM operating at 200 kV from a

sample deposited onto carbon coated TEM grids. Sur-

face area and pore size distributions were measured with

a NOVA isotherm instrument Model 2200. A five point

BET equation was used to calculate the surface area,

and the pore size distribution was calculated from thedesorption data using the BJH (Barret–Joyner–Hal-

enda) method.

3. Results and discussion

DAM-1-NT was prepared by first dispersing SWNT

in aqueous vitamin E TPGS solution. Hydrochloric acid

and TMOS was then added to this dispersion yielding ablack suspension. After aging at 45 �C for a few hours,

the gel became very viscous unlike the synthesis gel of

regular DAM-1 where the silicate starts to precipitate

forming a layer at the bottom of the reaction vessel. This

gel was allowed to age at 45 �C up to 24 h, and then

heated at 90 �C for 24 h to promote condensation. At

this point, excess water was added to the gel, which was

again heated for an additional 24 h. Dispersions con-taining 0.1% and 0.4% SWNT in 2% vitamin E TPGS

were employed in the synthesis of DAM-1-NT. The as-

synthesized DAM-1-NT material synthesized using the

0.1% SWNT dispersion yields an XRD pattern (Fig. 1)

typical of hexagonally arranged mesoporous DAM-1

[13]. Extensive work with vitamin E TPGS under vari-

ous conditions has shown that this template forms a

mesoporous hexagonal phase [13]. The d-spacing of 7.32nm (Fig. 1) is slightly larger than what is normally ob-

served for DAM-1 produced under similar conditions

but without nanotubes [13]. Dispersions containing

higher SWNT content produced DAM-1-NT with a

poor XRD pattern as indicated by the low intensity

broad low angle reflection (not shown). DAM-1-NT was

calcined under argon at 850 �C to decompose the vita-

min E template without destroying the SWNT’s. Afterheating, the black molecular sieve powder did not

change color. For comparison, DAM-1-NT was also

calcined in air at 550 �C. In this case, the molecular sieve

powder becomes white, suggesting that most of the

template and SWNTs were removed. The XRD patterns

for DAM-1 calcined at 850 �C in argon and DAM-1

calcined in air at 550 �C shown in Fig. 1 display higher

intensity than the as-synthesized material. Additionallysome of the higher angle reflections start to appear in the

region of 2–4� 2h. The XRD pattern also shows that

DAM-1-NT is thermally stable up to 850 �C in argon.

The main reflection shifts to 6.60 and 7.29 nm for the

DAM-1-NT calcined under argon and in air respec-

tively. After calcination in air at 550 �C to remove all

organics, the pore size is measured to be 3.4 nm and the

surface area is 538 m2/g, which is well inside the rangefor typical DAM-1 materials. The wall thickness was

calculated to be �5.0 nm which is larger than normally

observed. Although the sample appears white, it may

also be possible that there is residual carbon in the pores

which would lower the observed pore size. For the

sample calcined at 850 �C under argon the pore size was

3.1 nm with a surface area of 303 m2/g. Since there was

only a 0.1% by weight loading of SWNTs in the syn-thesis gel, complete filling of the pores was not expected.

Therefore, the smaller surface area and pore size maybe

consistent with the loading.

2-theta (degrees)

Inte

nsity

(a.u

.)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 20 40 60 80 100

pore diameter (Å)

Volu

me

(cm

3 /g)

0.6 1.6 2.6 3.6 4.6 5.6 6.6 7.6 8.6 9.6

A

B

C

Fig. 1. Powder XRD patterns of (A) as-synthesized DAM-1-NT, (B) DAM-1-NT calcined at 850 �C under argon, and (C) DAM-1-NT calcined at

550 �C in air. Inset shows the pore size distribution for DAM-NT calcined in air.

Fig. 2. SEM images (A) and close up (B) of DAM-1-NT (0.1% SWNT)

calcined at 850 �C under argon.

E. Mu~nnoz et al. / Microporous and Mesoporous Materials 67 (2004) 61–65 63

The addition of SWNT (0.1% and 0.4%) to the syn-

thesis of DAM-1 does not appear to have caused a

tremendous swelling of the vitamin E TPGS micelles

based on the observed pore size. Since the addition ofmesitylene and other hydrophobic molecules to the

synthesis usually increases the pore size of mesoporous

materials such as SBA-15 and MCM-41, one might

conclude that the vitamin E TPGS template partially

debundles the SWNTs. This would mean the vitamin E

TPGS hydrophobic core effectively wraps around the �1

nm carbon nanotubes.

Scanning electron micrographs for DAM-1-NTsamples prepared with 0.1% dispersion after annealing

at 850 �C under argon are shown in Fig. 2. The SEM

images show small tubular particles 1–2 lm in length

and up to 250 nm in diameter. It is clear from the SEM

images that SWNTs protrude from the DAM-1-NT

particles that in many cases connect two or more

mesoporous particles. This morphology is remarkably

different from the material produced without theSWNTs, where typically hexagons, gyroids or even

spheres are observed [13]. Additionally, the DAM-1-NT

particle size is considerably smaller than that normally

observed. Clearly the SWNTs have a dramatic effect on

the DAM-1-NT particle growth. An interesting phe-

nomenon is shown in Fig. 2B, where nanotubes from

different particles combine and form a twisted helix.

This is further testament to the strong self-attractionthe SWNTs have for each other. The SEM images of the

DAM-1-NT calcined in air (not shown) reveal only the

mesoporous silica without the nanotubes. Increasing

Fig. 3. SEM images of as-synthesized DAM-1-NT (0.4% SWNT).

64 E. Mu~nnoz et al. / Microporous and Mesoporous Materials 67 (2004) 61–65

the SWNT concentration from 0.1% to 0.4% did not

alter the DAM-1-NT morphology as shown in Fig. 3Aand B. The nature of the silica pore structure and in-

traparticle nanotubes cannot be discerned from these

SEM images. However, we speculate that the vitamin E

TPGS wraps and partially debundles the SWNTs during

synthesis of DAM-1-NT. Although one might expect

small shifts in the Raman spectra of encapsulated

Fig. 4. TEM micrographs of (A) and close up (B)

(debundled) SWNTs, the Raman is somewhat incon-

clusive because the sample is a composite of encapsu-

lated nanotubes and these twisted or re-bundled

nanotubes outside the molecular sieve (Figs. 2 and 3).

Therefore, we obtained TEM images to better under-stand the nature of these composites.

The transmission electron micrographs shown in Fig.

4A and B reveal the well ordered pores of DAM-1-NT.

There are clearly SWNTs protruding from both ends

of the DAM-1-NT particles. Both TEM and SEM

micrographs indicate that SWNTs, either isolated or

small-diameter bundles, are aligned with respect to the

DAM-1-NT channels. Eventually the SWNTs mergeand/or twist into larger bundles outside the mesoporous

material. In many cases, as shown in Fig. 4A (and Fig.

3) the carbon nanotubes bridge two mesoporous DAM-

1-NT particles. Fig. 5A and B further illustrate the

SWNTs emanating from the DAM-1-NT particles and

recombining into bundles. The mesopores associated

with the silica particles in Fig. 5A are not clear and the

mesopores along the lower edge of the DAM-1-NTparticle in Fig. 5B appear somewhat disordered. It is

possible that there is some inclusion of SWNT bundles

of various diameters which would lead to distortion of

the channel structure and disordered regions. However,

defective structures and disordered particles may be a

consequence of the DAM-1 synthesis variables which

are yet to be fully explored.

4. Conclusions

We have demonstrated that the SWNT inclusion in

DAM-1 mesoporous molecular sieves can be achieved

during synthesis. The resulting composite might com-

bine the properties and potential applications of both

carbon nanotubes and mesoporous molecular sieves.Furthermore, the mesoporous material provides a rigid

of as-synthesized DAM-1-NT (0.1% SWNT).

Fig. 5. TEM micrographs of as-synthesized DAM-1-NT (0.1% SWNT).

E. Mu~nnoz et al. / Microporous and Mesoporous Materials 67 (2004) 61–65 65

coating to the confined nanotubes that might facilitate

the SWNT manipulation as well as protect them from

certain chemical environments. The intrinsic properties

of the synthesized SWNT/mesoporous composites are

currently being investigated, as well as improvements in

their synthesis such as the use of different surfactants or

other methods to increase the SWNT occupancy in theresulting mesoporous materials.

Acknowledgements

We thank the Robert A. Welch Foundation and

DARPA for support of this work. We also thank Alan

Dalton for helpful discussions.

References

[1] W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou,

R.A. Zhao, G. Wang, Science 274 (1996) 1701.

[2] S. Xie, W. Li, Z. Pan, B. Chang, L. Sun, J. Phys. Chem. Solids 61

(2000) 1153.

[3] S. Xie, W. Li, Z. Pan, B. Chang, L. Sun, Mater. Sci. Eng. A 286

(2000) 11.

[4] A.M. Cassell, S. Verma, L. Delzeit, M. Meyyappan, J. Han,

Langmuir 17 (2001) 260.

[5] M. Urban, D. Mehn, Z. Konya, I. Kiricsi, Chem. Phys. Lett. 359

(2002) 95.

[6] M. Urban, Z. Konya, D. Mehn, J. Zhu, I. Kiricsi, J. Nanosci.

Nanotech. 3 (2003) 111.

[7] F. Zheng, L. Liang, Y. Gao, J.H. Sukamto, C.L. Aardahl,

Nanolett. 2 (2002) 729.

[8] G. Zheng, H. Zhu, Q. Luo, Y. Zhou, D. Zhao, Chem. Mater. 13

(2001) 2240.

[9] L. Huang, S.J. Wind, S.P. O’Brien, Nanolett. 3 (2003) 299.

[10] M. Alvaro, P. Atienzar, J.L. Bourdelande, H. Garcia, Chem.

Commun. (2002) 3004.

[11] Z.K. Tang, H.D. Sun, J. Wang, J. Chen, G. Li, Appl. Phys. Lett.

73 (1998) 2287.

[12] Z.K. Tang, L. Zhang, N. Wang, X.X. Zhang, G.H. Wen, G.D. Li,

J.N. Wang, C.T. Chan, P. Sheng, Science 292 (2001) 2462.

[13] D. Coutinho, R.A. Orozio-Tevan, R.F. Reidy, K.J. Balkus Jr.,

Micropor. Mesopor. Mater. 54 (2002) 229.

[14] G.S. Duesberg, M. Burghard, J. Muster, G. Philipp, S. Roth,

Chem. Commun. (1998) 435.

[15] J. Liu, A.G. Rinzler, H. Dai, J.H. Hafner, R. Kelley Bradley, P.J.

Boul, A. Lu, T. Iverson, K. Shelimov, C.B. Huffman, F.

Rodr�ııguez-Mac�ııas, Y.-S. Shon, T. Randall Lee, D.T. Colbert,

R.E. Smalley, Science 280 (1998) 1253.