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Materials Letters 62 (20

Preparation of aligned titania nanowires with an aligned carbonnanotube composite template

Lijun Ji ⁎, Zhi Wang, Zhi Li, Ji Liang

Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, PR ChinaDepartment of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China

Received 2 July 2007; accepted 29 October 2007Available online 1 November 2007

Abstract

A new kind of aligned multi-walled carbon nanotube (AMWCNT)/hydrogel composite template was prepared through a simple polymerizationprocess. A shell of hydrogel with a thickness of up to tens of nanometers was uniformly coated on single nanotube of AMWCNT. The hydrogelshell can swell a great deal of reagents in aqueous or alcohol solution for further chemical reactions on the surface of the AMWCNTs.The AMWCNTs coated with titania nanoparticles were prepared with this template and aligned titania nanowires with lengths up to a fewmillimeters were obtained after calcinations. This new composite template also provides an efficient route to modify aligned carbon nanotubeswith inorganic species.© 2007 Elsevier B.V. All rights reserved.

Keywords: Nanomaterials; Nanowires; Sol–gel preparation; Template synthesis; Titania

1. Introduction

Carbon nanotubes (CNTs) were extensively used as 1Dtemplates to obtain 1D materials through two synthetic routes:encapsulating metal and metal oxide nanowires in open-endednanotubes by forming the CNTs in an arc discharge [1]; orcoating inorganic materials on the surface of CNTs by physi-cal or chemical technologies [2,3]. Only dispersed but notaligned CNTs were used in reports, because the confined spacebetween aligned CNTs made reagents against full diffusionand reaction on the surface of CNTs. However, dispersed CNTsare also inconvenient for the formation of continuous andsolid functional shell, and obtaining long inorganic nanowiresby removing the CNT template is difficult. In most cases,CNTs were actually used as scaffolds to prepare 1D compositematerials.

Coating carbon nanotubes with titania to get compositenanostructures has attracted scientists' great interest [4–11],

⁎ Corresponding author. Fax: +86 010 62782413.E-mail address: jilijun@tsinghua.edu.cn (L. Ji).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.10.055

because of the great potential applications of titania nanos-tructures in photocatalysis [12], photovoltaics [13], and sensing[14]. Titania nanoparticles were coated directly on the surfaceof CNTs through sol–gel method [4–6] or hydrothermal depo-sition process [7]. To improve the coating efficiency, an effec-tive way is adding functional molecules in synthesis process,such as polyethyleneimine (PEI) [8,9], cetyltrimethylammo-nium bromide (CTAB) [10], and peptide [11]. However, onlyshort titania nanotubes can be successfully obtained after re-moving CNTs templates [4]. Titania nanowire arrays weresuccessfully prepared with porous film template [15,16], butthe titania nanowires inside arrays have a length only up to afew hundred micrometers and it is difficult to be synthesized inlarge scale.

In this work, a new kind of AMWCNT/hydrogel compositetemplate was prepared by simple polymerization of monomermolecules in AMWCNT template. Aligned titania nanowires(ATNWs) with lengths up to a few millimeters were preparedwith this new composite template. The process can also beapplied to functionalize surface of AMWCNT with inorganicmaterials.

Fig. 1. a) Formation process of AMWCNT/hydrogel template: bulk hydrogelamong AMWCNTshrunk and enwrapped CNTs with the loss of water in ethanolsolution. b) AMWCNT/hydrogel template.

Fig. 2. a) TEM image of ultramicrotomed AMWCNT/hydrogel/titania composite, bshow long-range order structure.

1980 L. Ji et al. / Materials Letters 62 (2008) 1979–1982

2. Experimental

AMWCNTs prepared in our laboratory by chemical vapordeposition (CVD) were immersed into an ethanol and water(1:1 wt/wt) solution of acrylic acid (AA), dimethylacrylamide(DMAA), methylenebisacrylamide (MBAA) and ammoniumpersulfate in a weight ratio of 1:1:0.02:0.01, with a totalmonomer content of 25wt.%. After polymerization at 60°C,the AMWCNTs were embedded in the copolymer. This copoly-mer was a hydrophilic hydrogel and could swell a great deal ofwater or ethanol solution. The AMWCNT/hydrogel templateswere rinsed in ethanol for several times to remove water inhydrogel. After being rinsed in a mixture of ethanol/tetrabutyletitanate (TBT) (1:1 vol/vol) for 12h, the template was movedinto a mixture of ethanol/water (1:1 vol/vol) for a hydrolysisprocess. After calcinations at 700°C for 6h in air, ATNWs wereobtained.

Morphology of the specimens was observed with field-emission scanning electron microscope (FESEM, LEO-1530)and transmission electron microscope (TEM, JEOL-200CX).The AMWCNT/hydrogel/titania composites were embeddedin polystyrene and cut into pieces with a thickness of 60nmfor ultramicrotome and TEM observation. The ATNWs weredispersed by ultrasonication for TEM observation. The pow-der X-ray diffraction (XRD) patterns were recorded with aD/MAX-RB X-ray diffractometer. The specific surface area of

) TEM image of ATNWs, c) Magnified FESEM image of ATNWs, d) ATNWs

1981L. Ji et al. / Materials Letters 62 (2008) 1979–1982

ATNWs was measured with the BET method on a SORPTO-MATIC 1990 system at 77K.

3. Results and discussion

The hydrogel shell enwrapping the AMWCNTs is a copolymer ofAA, DMAA and MBAA. Polyacrylic acid (PAA) can swell a greatamount of ethanol solution. However, pure PAA is too soft to obtainregular morphology on the surface of CNTs without assistance ofDMAA. MBAA is a crosslinker. Because hydrogel bulk can shrinkhundreds of times in air and more than ten times in ethanol with loss ofwater, there could be a shrinkage–avulsion process during theformation of the AMWCNT/hydrogel composite template. As shownin Fig. 1a, the AMWCNT template was embedded in hydrogel afterpolymerization of the monomers. When the template was peeled offfrom the bulk hydrogel, the hydrogel among AMWCNTs shrunk andruptured with loss of water, forming uniform hydrogel shells witha thickness up to tens of nanometers on the nanotubes. It is easier toform a thick and dense shell of hydrogel macromolecule onAMWCNTs by this synthesis process than by self-assembling [17]and chemical graft [18] methods when proceeding chemical reactionin confined spaces among AMWCNTs. Because this process hasno correlation with the surface properties of CNTs, the AMWCNT

Fig. 3. a) X-ray diffraction pattern confirms the anatase and rutile poly-crystal structure of ATNWs, b) EDS confirms carbon nanotubes have beenburned off.

needs not to be modified with nitric acid. Compared with pureAMWCNT, the hydrogel composite nanotubes show larger diameterabout 100–150nm and rough surface (Fig. 1b), and part of hydrogelon neighbor carbon nanotubes was not ruptured completely. Thespeculation about the formation of composite template was confirmedby increasing the content of the monomers and cross linker, whichincrease the strength of hydrogel. It can be observed with FESEM thatthe bulk hydrogel was not ruptured completely with the loss of waterand retained among the nanotubes.

The hydrogel shell is hydrophilic and its volume can swell tens oftimes in aqueous solution. Thus, it can absorb a great amount of theprecursors for further chemical reactions, modification and decora-tion on the nanotube surfaces. In this case, because acid group inhydrogel can catalyze sol/gel process of titania [19], TBT, includingthat absorbed in hydrogel shell and on the surface, can be induced tohydrolysis favorably only in hydrogel shell but not on the surfaceof hydrogel shell, and replicate the morphology of hydrogel shell.This process can also confirm the existence of the hydrogel shell,which is the same as the stain technology for TEM observation. Asshown in Fig. 2a, titania replicated the morphology of hydrogel andenwrapped on carbon nanotubes. The AMWCNT/hydrogel/titaniacomposite has similar morphology with that of AMWCNT/hydrogeltemplate. After calcinations at 700°C in air, ATNWs were obtained.TEM image shows that the titania enwrapping on nanotubes con-tracted to be nanowires but not nanotubes (Fig. 2b). And the diam-eter of the ATNWs becomes smaller than that of the AMWCNT/hydrogel/titania composite before calcinations. The ATNWs partlyreplicated the AMWCNT templates, including the branches (Fig. 2c),the waved curve and the aligned structure (Fig. 2d). The length of thetitania nanowires was about 4mm, and the nanowires were seldombroken according to the FESEM observation. As shown by XRDpattern (Fig. 3a), the nanowires are polycrystal of anatase and rutile.All CNTs and hydrogel were burn off, which are confirmed by theresult of energy diffraction spectrum (EDS) (Fig. 3b). The specificsurface area of the ATNWs was 31m2/g, according to nitrogenadsorption/desorption measurement. A contrastive experiment wascarried out with AMWCNT templates. It confirmed that bundles oftitania fibers felted together were obtained after calcinations withoutthe assistance of hydrogel, with a small quantity of tenuousnanowires that might be formed by sol–gel precursor filled intonanotubes.

4. Conclusion

In conclusion, a new AMWCNT/hydrogel compositetemplate was prepared by a simple polymerization process ofmonomer in AMWCNT template. The hydrogel can form acontinuous and thick shell enwrapping AMWCNTs. ATNWsup to a few millimeters have been prepared with this com-posite template. Because the hydrogel shell can absorb pre-cursors in great amount, this composite template is suitablefor preparation of AMWCNT composite structures, and it canalso facilitate further modification and decoration of AMWCNTsurface.

Acknowledgments

This project was supported by China Postdoctoral ScienceFoundation No. 20060390429 and National Natural ScienceFoundation of China under grant No. 10332020.

1982 L. Ji et al. / Materials Letters 62 (2008) 1979–1982

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