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Preparation of aligned titania nanowires with an aligned carbon nanotube composite template
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Materials Letters 62 (2008) 1979 – 1982 www.elsevier.com/locate/matlet
Preparation of aligned titania nanowires with an aligned carbon nanotube composite template Lijun Ji ⁎, Zhi Wang, Zhi Li, Ji Liang Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, PR China Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 2 July 2007; accepted 29 October 2007 Available online 1 November 2007
Abstract A new kind of aligned multi-walled carbon nanotube (AMWCNT)/hydrogel composite template was prepared through a simple polymerization process. A shell of hydrogel with a thickness of up to tens of nanometers was uniformly coated on single nanotube of AMWCNT. The hydrogel shell 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 few millimeters were obtained after calcinations. This new composite template also provides an efficient route to modify aligned carbon nanotubes with 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 1D templates to obtain 1D materials through two synthetic routes: encapsulating metal and metal oxide nanowires in open-ended nanotubes by forming the CNTs in an arc discharge [1]; or coating inorganic materials on the surface of CNTs by physical or chemical technologies [2,3]. Only dispersed but not aligned CNTs were used in reports, because the confined space between aligned CNTs made reagents against full diffusion and reaction on the surface of CNTs. However, dispersed CNTs are also inconvenient for the formation of continuous and solid functional shell, and obtaining long inorganic nanowires by removing the CNT template is difficult. In most cases, CNTs were actually used as scaffolds to prepare 1D composite materials. Coating carbon nanotubes with titania to get composite nanostructures has attracted scientists' great interest [4–11], ⁎ Corresponding author. Fax: +86 010 62782413. E-mail address: [email protected] (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 nanostructures in photocatalysis [12], photovoltaics [13], and sensing [14]. Titania nanoparticles were coated directly on the surface of CNTs through sol–gel method [4–6] or hydrothermal deposition process [7]. To improve the coating efficiency, an effective way is adding functional molecules in synthesis process, such as polyethyleneimine (PEI) [8,9], cetyltrimethylammonium bromide (CTAB) [10], and peptide [11]. However, only short titania nanotubes can be successfully obtained after removing CNTs templates [4]. Titania nanowire arrays were successfully prepared with porous film template [15,16], but the titania nanowires inside arrays have a length only up to a few hundred micrometers and it is difficult to be synthesized in large scale. In this work, a new kind of AMWCNT/hydrogel composite template was prepared by simple polymerization of monomer molecules in AMWCNT template. Aligned titania nanowires (ATNWs) with lengths up to a few millimeters were prepared with this new composite template. The process can also be applied to functionalize surface of AMWCNT with inorganic materials.
1980
L. Ji et al. / Materials Letters 62 (2008) 1979–1982
2. Experimental
Fig. 1. a) Formation process of AMWCNT/hydrogel template: bulk hydrogel among AMWCNT shrunk and enwrapped CNTs with the loss of water in ethanol solution. b) AMWCNT/hydrogel template.
AMWCNTs prepared in our laboratory by chemical vapor deposition (CVD) were immersed into an ethanol and water (1:1 wt/wt) solution of acrylic acid (AA), dimethylacrylamide (DMAA), methylenebisacrylamide (MBAA) and ammonium persulfate in a weight ratio of 1:1:0.02:0.01, with a total monomer content of 25wt.%. After polymerization at 60°C, the AMWCNTs were embedded in the copolymer. This copolymer was a hydrophilic hydrogel and could swell a great deal of water or ethanol solution. The AMWCNT/hydrogel templates were rinsed in ethanol for several times to remove water in hydrogel. After being rinsed in a mixture of ethanol/tetrabutyle titanate (TBT) (1:1 vol/vol) for 12h, the template was moved into a mixture of ethanol/water (1:1 vol/vol) for a hydrolysis process. After calcinations at 700°C for 6h in air, ATNWs were obtained. Morphology of the specimens was observed with fieldemission scanning electron microscope (FESEM, LEO-1530) and transmission electron microscope (TEM, JEOL-200CX). The AMWCNT/hydrogel/titania composites were embedded in polystyrene and cut into pieces with a thickness of 60nm for ultramicrotome and TEM observation. The ATNWs were dispersed by ultrasonication for TEM observation. The powder X-ray diffraction (XRD) patterns were recorded with a D/MAX-RB X-ray diffractometer. The specific surface area of
Fig. 2. a) TEM image of ultramicrotomed AMWCNT/hydrogel/titania composite, b) TEM image of ATNWs, c) Magnified FESEM image of ATNWs, d) ATNWs show long-range order structure.
L. Ji et al. / Materials Letters 62 (2008) 1979–1982
ATNWs was measured with the BET method on a SORPTOMATIC 1990 system at 77K. 3. Results and discussion The hydrogel shell enwrapping the AMWCNTs is a copolymer of AA, DMAA and MBAA. Polyacrylic acid (PAA) can swell a great amount of ethanol solution. However, pure PAA is too soft to obtain regular morphology on the surface of CNTs without assistance of DMAA. MBAA is a crosslinker. Because hydrogel bulk can shrink hundreds of times in air and more than ten times in ethanol with loss of water, there could be a shrinkage–avulsion process during the formation of the AMWCNT/hydrogel composite template. As shown in Fig. 1a, the AMWCNT template was embedded in hydrogel after polymerization of the monomers. When the template was peeled off from the bulk hydrogel, the hydrogel among AMWCNTs shrunk and ruptured with loss of water, forming uniform hydrogel shells with a thickness up to tens of nanometers on the nanotubes. It is easier to form a thick and dense shell of hydrogel macromolecule on AMWCNTs by this synthesis process than by self-assembling [17] and chemical graft [18] methods when proceeding chemical reaction in confined spaces among AMWCNTs. Because this process has no correlation with the surface properties of CNTs, the AMWCNT
1981
needs not to be modified with nitric acid. Compared with pure AMWCNT, the hydrogel composite nanotubes show larger diameter about 100–150nm and rough surface (Fig. 1b), and part of hydrogel on neighbor carbon nanotubes was not ruptured completely. The speculation about the formation of composite template was confirmed by increasing the content of the monomers and cross linker, which increase the strength of hydrogel. It can be observed with FESEM that the bulk hydrogel was not ruptured completely with the loss of water and retained among the nanotubes. The hydrogel shell is hydrophilic and its volume can swell tens of times in aqueous solution. Thus, it can absorb a great amount of the precursors for further chemical reactions, modification and decoration on the nanotube surfaces. In this case, because acid group in hydrogel can catalyze sol/gel process of titania [19], TBT, including that absorbed in hydrogel shell and on the surface, can be induced to hydrolysis favorably only in hydrogel shell but not on the surface of 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. As shown in Fig. 2a, titania replicated the morphology of hydrogel and enwrapped on carbon nanotubes. The AMWCNT/hydrogel/titania composite has similar morphology with that of AMWCNT/hydrogel template. After calcinations at 700°C in air, ATNWs were obtained. TEM image shows that the titania enwrapping on nanotubes contracted to be nanowires but not nanotubes (Fig. 2b). And the diameter of the ATNWs becomes smaller than that of the AMWCNT/ hydrogel/titania composite before calcinations. The ATNWs partly replicated the AMWCNT templates, including the branches (Fig. 2c), the waved curve and the aligned structure (Fig. 2d). The length of the titania nanowires was about 4mm, and the nanowires were seldom broken according to the FESEM observation. As shown by XRD pattern (Fig. 3a), the nanowires are polycrystal of anatase and rutile. All CNTs and hydrogel were burn off, which are confirmed by the result of energy diffraction spectrum (EDS) (Fig. 3b). The specific surface area of the ATNWs was 31m2/g, according to nitrogen adsorption/desorption measurement. A contrastive experiment was carried out with AMWCNT templates. It confirmed that bundles of titania fibers felted together were obtained after calcinations without the assistance of hydrogel, with a small quantity of tenuous nanowires that might be formed by sol–gel precursor filled into nanotubes.
4. Conclusion In conclusion, a new AMWCNT/hydrogel composite template was prepared by a simple polymerization process of monomer in AMWCNT template. The hydrogel can form a continuous and thick shell enwrapping AMWCNTs. ATNWs up to a few millimeters have been prepared with this composite template. Because the hydrogel shell can absorb precursors in great amount, this composite template is suitable for preparation of AMWCNT composite structures, and it can also facilitate further modification and decoration of AMWCNT surface. Acknowledgments Fig. 3. a) X-ray diffraction pattern confirms the anatase and rutile polycrystal structure of ATNWs, b) EDS confirms carbon nanotubes have been burned off.
This project was supported by China Postdoctoral Science Foundation No. 20060390429 and National Natural Science Foundation of China under grant No. 10332020.
1982
L. Ji et al. / Materials Letters 62 (2008) 1979–1982
References [1] C. Guerret-Plécourt, Y. Le Bouar, A. Loiseau, H. Pascard, Nature 372 (1994) 761–765. [2] J. Sun, L. Gao, M. Iwasa, Chem. Commun. 7 (2004) 832–833. [3] W.Q. Han, A. Zettl, Nano. Lett. 3 (2003) 681–683. [4] D. Eder, I.A. Kinloch, A.H. Windle, Chem. Commun. 14 (2006) 1448–1450. [5] L. Chen, B.L. Zhang, M.Z. Qu, Z.L. Yu, Powder Technol. 154 (2005) 70–72. [6] K. Hernadi, E. Ljubović, J.W. Seo, L. Forró, Acta Mater. 51 (2003) 1151–1447. [7] A. Jitianu, T. Cacciaguerra, R. Benoit, S. Delpeux, F. Béguin, S. Bonnamy, Carbon 42 (2004) 1147–1151. [8] S.-W. Lee, W.M. Sigmund, Chem. Commun. 6 (2003) 780–781. [9] J. Sun, L. Gao, M. Iwasa, Q. Zhang, Carbon 42 (2004) 895–899.
[10] A. Jitianu, T. Cacciaguerra, M.-H. Berger, R. Benoit, F. Béguin, S. Bonnamy, J. Non-Cryst. Solids 345-346 (2004) 596–600. [11] M.J. Pender, L.A. Sowards, J.D. Hartgerink, M.O. Stone, R.R. Naik, Nano. Lett. 6 (2006) 40–44. [12] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544–2550. [13] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol. Energy Mater. Sol. Cells 90 (2006) 2011–2075. [14] A.S. Zuruzi, A. Kolmakov, N.C. MacDonald, M. Moskovits, Appl. Phys. Lett. 88 (2006) 102904. [15] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 857–862. [16] M.S. Sander, M.J. Côté, W. Gu, B.M. Kile, C.P. Tripp, Adv. Mater. 16 (2004) 2052–2057. [17] Z. Wei, M. Wan, T. Lin, L. Dai, Adv. Mater. 15 (2003) 136–139. [18] D. Baskaran, J.W. Mays, M.S. Bratcher, Angew. Chem. Int. Ed. 43 (2004) 2138–2142. [19] L. Ji, J.H. Rong, Z.Z. Yang, Chem. Commun. 9 (2003) 1080–1081.
Materials Letters 62 (2008) 1979 – 1982 www.elsevier.com/locate/matlet
Preparation of aligned titania nanowires with an aligned carbon nanotube composite template Lijun Ji ⁎, Zhi Wang, Zhi Li, Ji Liang Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, PR China Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 2 July 2007; accepted 29 October 2007 Available online 1 November 2007
Abstract A new kind of aligned multi-walled carbon nanotube (AMWCNT)/hydrogel composite template was prepared through a simple polymerization process. A shell of hydrogel with a thickness of up to tens of nanometers was uniformly coated on single nanotube of AMWCNT. The hydrogel shell 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 few millimeters were obtained after calcinations. This new composite template also provides an efficient route to modify aligned carbon nanotubes with 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 1D templates to obtain 1D materials through two synthetic routes: encapsulating metal and metal oxide nanowires in open-ended nanotubes by forming the CNTs in an arc discharge [1]; or coating inorganic materials on the surface of CNTs by physical or chemical technologies [2,3]. Only dispersed but not aligned CNTs were used in reports, because the confined space between aligned CNTs made reagents against full diffusion and reaction on the surface of CNTs. However, dispersed CNTs are also inconvenient for the formation of continuous and solid functional shell, and obtaining long inorganic nanowires by removing the CNT template is difficult. In most cases, CNTs were actually used as scaffolds to prepare 1D composite materials. Coating carbon nanotubes with titania to get composite nanostructures has attracted scientists' great interest [4–11], ⁎ Corresponding author. Fax: +86 010 62782413. E-mail address: [email protected] (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 nanostructures in photocatalysis [12], photovoltaics [13], and sensing [14]. Titania nanoparticles were coated directly on the surface of CNTs through sol–gel method [4–6] or hydrothermal deposition process [7]. To improve the coating efficiency, an effective way is adding functional molecules in synthesis process, such as polyethyleneimine (PEI) [8,9], cetyltrimethylammonium bromide (CTAB) [10], and peptide [11]. However, only short titania nanotubes can be successfully obtained after removing CNTs templates [4]. Titania nanowire arrays were successfully prepared with porous film template [15,16], but the titania nanowires inside arrays have a length only up to a few hundred micrometers and it is difficult to be synthesized in large scale. In this work, a new kind of AMWCNT/hydrogel composite template was prepared by simple polymerization of monomer molecules in AMWCNT template. Aligned titania nanowires (ATNWs) with lengths up to a few millimeters were prepared with this new composite template. The process can also be applied to functionalize surface of AMWCNT with inorganic materials.
1980
L. Ji et al. / Materials Letters 62 (2008) 1979–1982
2. Experimental
Fig. 1. a) Formation process of AMWCNT/hydrogel template: bulk hydrogel among AMWCNT shrunk and enwrapped CNTs with the loss of water in ethanol solution. b) AMWCNT/hydrogel template.
AMWCNTs prepared in our laboratory by chemical vapor deposition (CVD) were immersed into an ethanol and water (1:1 wt/wt) solution of acrylic acid (AA), dimethylacrylamide (DMAA), methylenebisacrylamide (MBAA) and ammonium persulfate in a weight ratio of 1:1:0.02:0.01, with a total monomer content of 25wt.%. After polymerization at 60°C, the AMWCNTs were embedded in the copolymer. This copolymer was a hydrophilic hydrogel and could swell a great deal of water or ethanol solution. The AMWCNT/hydrogel templates were rinsed in ethanol for several times to remove water in hydrogel. After being rinsed in a mixture of ethanol/tetrabutyle titanate (TBT) (1:1 vol/vol) for 12h, the template was moved into a mixture of ethanol/water (1:1 vol/vol) for a hydrolysis process. After calcinations at 700°C for 6h in air, ATNWs were obtained. Morphology of the specimens was observed with fieldemission scanning electron microscope (FESEM, LEO-1530) and transmission electron microscope (TEM, JEOL-200CX). The AMWCNT/hydrogel/titania composites were embedded in polystyrene and cut into pieces with a thickness of 60nm for ultramicrotome and TEM observation. The ATNWs were dispersed by ultrasonication for TEM observation. The powder X-ray diffraction (XRD) patterns were recorded with a D/MAX-RB X-ray diffractometer. The specific surface area of
Fig. 2. a) TEM image of ultramicrotomed AMWCNT/hydrogel/titania composite, b) TEM image of ATNWs, c) Magnified FESEM image of ATNWs, d) ATNWs show long-range order structure.
L. Ji et al. / Materials Letters 62 (2008) 1979–1982
ATNWs was measured with the BET method on a SORPTOMATIC 1990 system at 77K. 3. Results and discussion The hydrogel shell enwrapping the AMWCNTs is a copolymer of AA, DMAA and MBAA. Polyacrylic acid (PAA) can swell a great amount of ethanol solution. However, pure PAA is too soft to obtain regular morphology on the surface of CNTs without assistance of DMAA. MBAA is a crosslinker. Because hydrogel bulk can shrink hundreds of times in air and more than ten times in ethanol with loss of water, there could be a shrinkage–avulsion process during the formation of the AMWCNT/hydrogel composite template. As shown in Fig. 1a, the AMWCNT template was embedded in hydrogel after polymerization of the monomers. When the template was peeled off from the bulk hydrogel, the hydrogel among AMWCNTs shrunk and ruptured with loss of water, forming uniform hydrogel shells with a thickness up to tens of nanometers on the nanotubes. It is easier to form a thick and dense shell of hydrogel macromolecule on AMWCNTs by this synthesis process than by self-assembling [17] and chemical graft [18] methods when proceeding chemical reaction in confined spaces among AMWCNTs. Because this process has no correlation with the surface properties of CNTs, the AMWCNT
1981
needs not to be modified with nitric acid. Compared with pure AMWCNT, the hydrogel composite nanotubes show larger diameter about 100–150nm and rough surface (Fig. 1b), and part of hydrogel on neighbor carbon nanotubes was not ruptured completely. The speculation about the formation of composite template was confirmed by increasing the content of the monomers and cross linker, which increase the strength of hydrogel. It can be observed with FESEM that the bulk hydrogel was not ruptured completely with the loss of water and retained among the nanotubes. The hydrogel shell is hydrophilic and its volume can swell tens of times in aqueous solution. Thus, it can absorb a great amount of the precursors for further chemical reactions, modification and decoration on the nanotube surfaces. In this case, because acid group in hydrogel can catalyze sol/gel process of titania [19], TBT, including that absorbed in hydrogel shell and on the surface, can be induced to hydrolysis favorably only in hydrogel shell but not on the surface of 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. As shown in Fig. 2a, titania replicated the morphology of hydrogel and enwrapped on carbon nanotubes. The AMWCNT/hydrogel/titania composite has similar morphology with that of AMWCNT/hydrogel template. After calcinations at 700°C in air, ATNWs were obtained. TEM image shows that the titania enwrapping on nanotubes contracted to be nanowires but not nanotubes (Fig. 2b). And the diameter of the ATNWs becomes smaller than that of the AMWCNT/ hydrogel/titania composite before calcinations. The ATNWs partly replicated the AMWCNT templates, including the branches (Fig. 2c), the waved curve and the aligned structure (Fig. 2d). The length of the titania nanowires was about 4mm, and the nanowires were seldom broken according to the FESEM observation. As shown by XRD pattern (Fig. 3a), the nanowires are polycrystal of anatase and rutile. All CNTs and hydrogel were burn off, which are confirmed by the result of energy diffraction spectrum (EDS) (Fig. 3b). The specific surface area of the ATNWs was 31m2/g, according to nitrogen adsorption/desorption measurement. A contrastive experiment was carried out with AMWCNT templates. It confirmed that bundles of titania fibers felted together were obtained after calcinations without the assistance of hydrogel, with a small quantity of tenuous nanowires that might be formed by sol–gel precursor filled into nanotubes.
4. Conclusion In conclusion, a new AMWCNT/hydrogel composite template was prepared by a simple polymerization process of monomer in AMWCNT template. The hydrogel can form a continuous and thick shell enwrapping AMWCNTs. ATNWs up to a few millimeters have been prepared with this composite template. Because the hydrogel shell can absorb precursors in great amount, this composite template is suitable for preparation of AMWCNT composite structures, and it can also facilitate further modification and decoration of AMWCNT surface. Acknowledgments Fig. 3. a) X-ray diffraction pattern confirms the anatase and rutile polycrystal structure of ATNWs, b) EDS confirms carbon nanotubes have been burned off.
This project was supported by China Postdoctoral Science Foundation No. 20060390429 and National Natural Science Foundation of China under grant No. 10332020.
1982
L. Ji et al. / Materials Letters 62 (2008) 1979–1982
References [1] C. Guerret-Plécourt, Y. Le Bouar, A. Loiseau, H. Pascard, Nature 372 (1994) 761–765. [2] J. Sun, L. Gao, M. Iwasa, Chem. Commun. 7 (2004) 832–833. [3] W.Q. Han, A. Zettl, Nano. Lett. 3 (2003) 681–683. [4] D. Eder, I.A. Kinloch, A.H. Windle, Chem. Commun. 14 (2006) 1448–1450. [5] L. Chen, B.L. Zhang, M.Z. Qu, Z.L. Yu, Powder Technol. 154 (2005) 70–72. [6] K. Hernadi, E. Ljubović, J.W. Seo, L. Forró, Acta Mater. 51 (2003) 1151–1447. [7] A. Jitianu, T. Cacciaguerra, R. Benoit, S. Delpeux, F. Béguin, S. Bonnamy, Carbon 42 (2004) 1147–1151. [8] S.-W. Lee, W.M. Sigmund, Chem. Commun. 6 (2003) 780–781. [9] J. Sun, L. Gao, M. Iwasa, Q. Zhang, Carbon 42 (2004) 895–899.
[10] A. Jitianu, T. Cacciaguerra, M.-H. Berger, R. Benoit, F. Béguin, S. Bonnamy, J. Non-Cryst. Solids 345-346 (2004) 596–600. [11] M.J. Pender, L.A. Sowards, J.D. Hartgerink, M.O. Stone, R.R. Naik, Nano. Lett. 6 (2006) 40–44. [12] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544–2550. [13] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol. Energy Mater. Sol. Cells 90 (2006) 2011–2075. [14] A.S. Zuruzi, A. Kolmakov, N.C. MacDonald, M. Moskovits, Appl. Phys. Lett. 88 (2006) 102904. [15] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 857–862. [16] M.S. Sander, M.J. Côté, W. Gu, B.M. Kile, C.P. Tripp, Adv. Mater. 16 (2004) 2052–2057. [17] Z. Wei, M. Wan, T. Lin, L. Dai, Adv. Mater. 15 (2003) 136–139. [18] D. Baskaran, J.W. Mays, M.S. Bratcher, Angew. Chem. Int. Ed. 43 (2004) 2138–2142. [19] L. Ji, J.H. Rong, Z.Z. Yang, Chem. Commun. 9 (2003) 1080–1081.