- Email: [email protected]
Sonication assisted gold deposition on multiwall carbon nanotubes
Chemical Physics Letters 372 (2003) 848–852 www.elsevier.com/locate/cplett
Sonication assisted gold deposition on multiwall carbon nanotubes A. F asi a, I. P alink o a
b,*
, J.W. Seo c, Z. K onya a, K. Hernadi a, I. Kiricsi
a
Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. t er 1, Szeged, H-6720, Hungary b Department of Organic Chemistry, University of Szeged, D om t er 8, Szeged, H-6720, Hungary c Institute of Physics of Complex Matter, Ec ole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland Received 3 January 2003; in final form 26 March 2003
Abstract Gold was successfully deposited on multiwall carbon nanotubes (MWNTs). It was achieved by applying a gold colloid stabilised by [tetrakis(hydroxymethyl)phosphonium chloride] (THPC) surfactant and ultrasonic irradiation during preparation. By controlling the quantity of gold and optimising sonication time up to 50 wt% gold could be deposited on MWNTs. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotubes are of current interest to scholars of various backgrounds, because these novel materials offer potential applications ranging from applying them as STM tips [1] via hydrogen storage [2] to using them as construction material for microelectronic devices [3]. Despite uncovering many interesting electric, magnetic, chemical and structural properties true applications are yet to be realized. Close to them comes the fabrication of nanotube field effect transistors, which could be used in nanoscale memory cells [4,5]. For including these electric circuit elements in real-world devices electrically conducting contacts are needed [6].
*
Corresponding author. Fax: +36-62-544-200. E-mail address: [email protected] (I. Palink o).
Gold may be this contact material if it can be deposited onto the outer surface of carbon nanotubes. However, due to the highly hydrophobic nature and very regular structure of carbon nanotubes it may not be an easy exercise. Nevertheless, there are some examples in the literature of depositing metals onto carbon fibers and carbon nanotubes by simple impregnation [7]. In this contribution a method is shown, which allows the deposition of large amounts of gold onto the outer surface of carbon nanotubes upon sonication.
2. Experimental Multiwall nanotube was prepared by the catalytic chemical vapour deposition (CCVD) method. The starting catalyst was Fe,Co=AlðOHÞ3 . The
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00527-X
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
catalyst contained 2.5 wt% of each metal and was prepared by ion adsorption and precipitation. The starting salts were acetates and they were deposited from their slightly basic aqueous solution (pH ¼ 7.3). After 24 h the catalyst was filtered, washed and dried at 400 K overnight. Carbon nanotube was grown at 973 K by the decomposition of acetylene. The product was then purified. The metallic particles were dissolved by immersion into nitric acid solution followed by filtration, the support was separated by repeated sonication in an organic solvent mixture and decantation, amorphous carbon was removed by HF treatment. Gold colloid was prepared from HAuCl4 solution, which was decomposed and stabilised with NaOH and [tetrakis(hydroxymethyl)phosphonium chloride] (THPC) solutions, respectively. Gold deposition occurred by adding the gold colloid to carbon nanotube suspension during sonication. Duration of sonication was varied. After finishing this treatment the resulting substance was filtered, washed three times with ethanol and dried for 5 h at 393 K. The composite material was characterized by transmission electron microscopy (TEM, Philips CM-300 FEG instrument). The reactions of methyloxirane (product of Fluka–Aldrich, racemate was used) were studied in a pulse reactor system applying hydrogen as carrier (45 cm3 = min gas flow). The reaction temperature ranged from 363 to 473 K. The size of the pulse was 1 ll and 10 mg of catalyst was used. The catalysts were reduced under hydrogen flow at 623 K for 1 h. The following methyloxirane pulse sequence was applied for both catalysts: first (I): 363 K, second (II): 393 K, third (III): 423 K, fourth (IV): 473 K; 60 min was allowed to elapse between the pulses. Blank experiment (experiment with bare nanotube) has been performed at the highest temperature applied in catalytic experiments and no transformations of methyloxirane have been found. Analysis of the product mixture was done by a GC-MS system (Hewlett Packard (HP) 5890 gas chromatograph equipped with a HP 5970 quadrupole mass selective detector). Good separation was achieved on a 50-m long CPWAX 52CB coated CHROMPACK WCOT fused silica capillary column by applying a temperature program
849
(303 K for 15 min, 323 K for 10 min and 473 K for 10 min). Product identification was based on clean samples.
3. Results and discussion As it was mentioned previously, gold deposition onto carbon nanotube is not an easy exercise. The well-graphitised carbon nanotube has very regular structure not allowing the attachment of gold particles. If the regular structure is hurt, the chances of successful deposition increases. A very efficient way breaking the regularity of the structure is ultrasonic cavitation. Gold deposition was attempted from gold colloid. Since it was coloured, the success of preparation could be verified even visually: the colloid has lost its colour. This indirect indication could be made direct: electron microscopic images provided with direct proof of the successful synthesis. The choice of stabiliser made a difference. Applying citric acid no deposition occurred (Fig. 1) in spite of sonicating the slurry for 3 h. Using another stabiliser, THPC that is, even 1 h of sonication lead to complete decolourisation of the colloid, which meant the preparation of material with as high as 50 wt% gold loading (Fig. 2).
Fig. 1. TEM image of gold-free carbon nanotubes when gold deposition was attempted – without success – from a 10 wt% gold colloid stabilised with citric acid (3 h sonication; bar length: 200 nm).
850
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
Fig. 2. TEM image of carbon nanotube supported gold with 50 wt% gold loading prepared from THPC stabilised gold colloid (60 min sonication during loading; bar length: 200 nm).
A closer inspection of the nanotube reveals that ultrasonic treatment caused the breaking of the smooth carbon nanotube wall, providing a more irregular structure, giving sites for gold deposition (Fig. 3).
Fig. 3. High-resolution TEM image of carbon nanotube supported gold with 10 wt% gold loading prepared from THPC stabilised gold colloid with 60 min sonication. Small arrows indicate the different facets of gold particles. The particle on the left-hand side is exactly located on a deformed site of the MWNT.
Sonication time influences gold deposition. We have found that 10 min sonication is enough for complete gold deposition when the preparation of a 3 wt% Au/carbon nanotube was aimed (Fig. 4). As a possible application this substance was tested as catalyst. Gold, hardly considered to be a potential catalyst of any useful reaction, entered the catalytic stage when it was found that deposited on supports it was very efficient in the oxidation reaction of CO even at subzero temperatures [8]. It is active in other reactions as well, like in the transformations of the epoxide ring [9]. The chosen reactant, methyloxirane that is, can react in more than one direction. Reaction routes found experimentally is depicted in Scheme 1. Thus, it has a versatile functionality, which is one of the favourite of synthetic chemists when they target the preparation of more complicated molecules from relatively simple synthons [10]. The composition of the mixture on leaving the reactor is summarized in Table 1. The table contains data for Au powder for comparison. The reactant was the same and the applied pulse sequence was also the same. The 3 wt% Au/carbon nanotube composite was active in transforming methyloxirane. Deoxygenation (double ring opening) was found to be the major reaction route throughout the 363–473 K
Fig. 4. TEM image of gold impregnated carbon nanotubes with 3 wt% gold loading prepared from THPC stabilised gold colloid (10 min sonication during loading; bar length: 100 nm).
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
851
probably oxygen from the metal. Possibly, the metal–support interface plays crucial role in this reaction. Crystal defects may be formed even on the seemingly perfect graphite sheet during sonication, hydrogen reduction, and upon methyloxirane transformations. Defect sites, especially at the metal–support interface may be promoters of the reaction allowing partial spillover of the oxygen atoms onto the support following deoxygenation.
4. Conclusions
Scheme 1. Transformation pathways of methyloxirane.
temperature range. Nevertheless, isomerisation (already at 363 K) and from 393 K hydrogenation (products of single ring opening) also took place. Hydrogenation became important at higher temperatures. However, unsupported gold was only slightly active even at 473 K and deoxygenation was inferior to isomerisation. The transformation spectrum was also much narrower. It seems to be clear that the support plays important role not only by dispersing the metal, but also allowing traffic of certain species, most
Large amounts of gold could be deposited onto carbon nanotubes. Sonication upon gold deposition and the stabiliser of the gold colloid [tetrakis(hydroxymethyl)phosphonium chloride] were equally important in the success of preparation. The gold–carbon nanotube composite was an active catalyst in the transformations of methyloxirane.
Acknowledgements Research leading to this contribution was supported by the National Science Fund of Hungary via Grant (T034184). The financial support is highly appreciated. One of us A.F. is a Bekesy postdoctoral fellow. The fellowship is gratefully acknowledged.
Table 1 Product distribution in the ring-opening reactions of methyloxirane (MOX) over 10 mg of Au/carbon nanotube composite (3 wt% Au loading) or gold powder in pulse microreactor applying a pulse sequence (I: 363 K, II: 393 K, III: 423 K, IV: 473 K, pulse size: 1 ll) under hydrogen flow (45 cm3 = min) No.
Composition (mol%) MOX (1)
Propene (2)
Au/carbon nanotube (pretreatment: H2 : I 98.6 1.1 II 96.1 2.2 III 92.2 3.6 IV 82.4 8.7 Au powder I II III IV
Acetone (3)
Propionaldehyde (4)
2-Propanol (5)
n-Propanol (6)
H2 O (7)
0 0.7 1.8 3.4
0 0.1 0.6 2.4
0 0.3 0.7 1.2
0 0 0 0
0 0 0 0
0 0 0 0
3
623 K, 45 cm = min, 1 h) 0.3 0 0.5 0.1 0.9 0.2 1.1 0.8
(pretreatment: H2 : 623 K, 45 cm3 = min, 1 h) 100 0 0 0 100 0 0 0 99.8 0 0.2 0 99.3 0.1 0.6 0
852
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
References [1] G. Nagy, M. Levy, R. Scarmozzino, R.M. Osgood, H. Dai, R.E. Smalley, C.A. Michaels, G.W. Flynn, G.F. McLane, Appl. Phys. Lett. 73 (1998) 529. [2] F.L. Darkrim, P. Malbrunot, G.P. Tartaglia, Int. J. Hydr. Energ. 27 (2002) 193. [3] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [4] M.S. Fuhrer, B.M. Kim, T. Durkop, T. Brintlinger, Nano Lett. 2 (2002) 755. [5] M. Radosavljevic, M. Freitag, K.V. Thadani, A.T. Johnson, Nano Lett. 2 (2002) 761.
[6] J. Liu, M.J. Casavant, M. Cox, D.A. Walters, P. Boul, W. Lu, A.J. Rimberg, K.A. Smith, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 303 (1999) 125. [7] C. Pham-Huu, N. Keller, V.V. Roddatis, R. Schl€ ogl, M.J. Ledoux, Phys. Chem. Chem. Phys. 4 (2002) 514. [8] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [9] A. Fasi, I. Palink o, K. Hernadi, I. Kiricsi, Catal. Lett. 81 (2002) 237. [10] I.P. Smoliakova, Curr. Org. Chem. 4 (2000) 589.
Sonication assisted gold deposition on multiwall carbon nanotubes A. F asi a, I. P alink o a
b,*
, J.W. Seo c, Z. K onya a, K. Hernadi a, I. Kiricsi
a
Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. t er 1, Szeged, H-6720, Hungary b Department of Organic Chemistry, University of Szeged, D om t er 8, Szeged, H-6720, Hungary c Institute of Physics of Complex Matter, Ec ole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland Received 3 January 2003; in final form 26 March 2003
Abstract Gold was successfully deposited on multiwall carbon nanotubes (MWNTs). It was achieved by applying a gold colloid stabilised by [tetrakis(hydroxymethyl)phosphonium chloride] (THPC) surfactant and ultrasonic irradiation during preparation. By controlling the quantity of gold and optimising sonication time up to 50 wt% gold could be deposited on MWNTs. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotubes are of current interest to scholars of various backgrounds, because these novel materials offer potential applications ranging from applying them as STM tips [1] via hydrogen storage [2] to using them as construction material for microelectronic devices [3]. Despite uncovering many interesting electric, magnetic, chemical and structural properties true applications are yet to be realized. Close to them comes the fabrication of nanotube field effect transistors, which could be used in nanoscale memory cells [4,5]. For including these electric circuit elements in real-world devices electrically conducting contacts are needed [6].
*
Corresponding author. Fax: +36-62-544-200. E-mail address: [email protected] (I. Palink o).
Gold may be this contact material if it can be deposited onto the outer surface of carbon nanotubes. However, due to the highly hydrophobic nature and very regular structure of carbon nanotubes it may not be an easy exercise. Nevertheless, there are some examples in the literature of depositing metals onto carbon fibers and carbon nanotubes by simple impregnation [7]. In this contribution a method is shown, which allows the deposition of large amounts of gold onto the outer surface of carbon nanotubes upon sonication.
2. Experimental Multiwall nanotube was prepared by the catalytic chemical vapour deposition (CCVD) method. The starting catalyst was Fe,Co=AlðOHÞ3 . The
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00527-X
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
catalyst contained 2.5 wt% of each metal and was prepared by ion adsorption and precipitation. The starting salts were acetates and they were deposited from their slightly basic aqueous solution (pH ¼ 7.3). After 24 h the catalyst was filtered, washed and dried at 400 K overnight. Carbon nanotube was grown at 973 K by the decomposition of acetylene. The product was then purified. The metallic particles were dissolved by immersion into nitric acid solution followed by filtration, the support was separated by repeated sonication in an organic solvent mixture and decantation, amorphous carbon was removed by HF treatment. Gold colloid was prepared from HAuCl4 solution, which was decomposed and stabilised with NaOH and [tetrakis(hydroxymethyl)phosphonium chloride] (THPC) solutions, respectively. Gold deposition occurred by adding the gold colloid to carbon nanotube suspension during sonication. Duration of sonication was varied. After finishing this treatment the resulting substance was filtered, washed three times with ethanol and dried for 5 h at 393 K. The composite material was characterized by transmission electron microscopy (TEM, Philips CM-300 FEG instrument). The reactions of methyloxirane (product of Fluka–Aldrich, racemate was used) were studied in a pulse reactor system applying hydrogen as carrier (45 cm3 = min gas flow). The reaction temperature ranged from 363 to 473 K. The size of the pulse was 1 ll and 10 mg of catalyst was used. The catalysts were reduced under hydrogen flow at 623 K for 1 h. The following methyloxirane pulse sequence was applied for both catalysts: first (I): 363 K, second (II): 393 K, third (III): 423 K, fourth (IV): 473 K; 60 min was allowed to elapse between the pulses. Blank experiment (experiment with bare nanotube) has been performed at the highest temperature applied in catalytic experiments and no transformations of methyloxirane have been found. Analysis of the product mixture was done by a GC-MS system (Hewlett Packard (HP) 5890 gas chromatograph equipped with a HP 5970 quadrupole mass selective detector). Good separation was achieved on a 50-m long CPWAX 52CB coated CHROMPACK WCOT fused silica capillary column by applying a temperature program
849
(303 K for 15 min, 323 K for 10 min and 473 K for 10 min). Product identification was based on clean samples.
3. Results and discussion As it was mentioned previously, gold deposition onto carbon nanotube is not an easy exercise. The well-graphitised carbon nanotube has very regular structure not allowing the attachment of gold particles. If the regular structure is hurt, the chances of successful deposition increases. A very efficient way breaking the regularity of the structure is ultrasonic cavitation. Gold deposition was attempted from gold colloid. Since it was coloured, the success of preparation could be verified even visually: the colloid has lost its colour. This indirect indication could be made direct: electron microscopic images provided with direct proof of the successful synthesis. The choice of stabiliser made a difference. Applying citric acid no deposition occurred (Fig. 1) in spite of sonicating the slurry for 3 h. Using another stabiliser, THPC that is, even 1 h of sonication lead to complete decolourisation of the colloid, which meant the preparation of material with as high as 50 wt% gold loading (Fig. 2).
Fig. 1. TEM image of gold-free carbon nanotubes when gold deposition was attempted – without success – from a 10 wt% gold colloid stabilised with citric acid (3 h sonication; bar length: 200 nm).
850
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
Fig. 2. TEM image of carbon nanotube supported gold with 50 wt% gold loading prepared from THPC stabilised gold colloid (60 min sonication during loading; bar length: 200 nm).
A closer inspection of the nanotube reveals that ultrasonic treatment caused the breaking of the smooth carbon nanotube wall, providing a more irregular structure, giving sites for gold deposition (Fig. 3).
Fig. 3. High-resolution TEM image of carbon nanotube supported gold with 10 wt% gold loading prepared from THPC stabilised gold colloid with 60 min sonication. Small arrows indicate the different facets of gold particles. The particle on the left-hand side is exactly located on a deformed site of the MWNT.
Sonication time influences gold deposition. We have found that 10 min sonication is enough for complete gold deposition when the preparation of a 3 wt% Au/carbon nanotube was aimed (Fig. 4). As a possible application this substance was tested as catalyst. Gold, hardly considered to be a potential catalyst of any useful reaction, entered the catalytic stage when it was found that deposited on supports it was very efficient in the oxidation reaction of CO even at subzero temperatures [8]. It is active in other reactions as well, like in the transformations of the epoxide ring [9]. The chosen reactant, methyloxirane that is, can react in more than one direction. Reaction routes found experimentally is depicted in Scheme 1. Thus, it has a versatile functionality, which is one of the favourite of synthetic chemists when they target the preparation of more complicated molecules from relatively simple synthons [10]. The composition of the mixture on leaving the reactor is summarized in Table 1. The table contains data for Au powder for comparison. The reactant was the same and the applied pulse sequence was also the same. The 3 wt% Au/carbon nanotube composite was active in transforming methyloxirane. Deoxygenation (double ring opening) was found to be the major reaction route throughout the 363–473 K
Fig. 4. TEM image of gold impregnated carbon nanotubes with 3 wt% gold loading prepared from THPC stabilised gold colloid (10 min sonication during loading; bar length: 100 nm).
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
851
probably oxygen from the metal. Possibly, the metal–support interface plays crucial role in this reaction. Crystal defects may be formed even on the seemingly perfect graphite sheet during sonication, hydrogen reduction, and upon methyloxirane transformations. Defect sites, especially at the metal–support interface may be promoters of the reaction allowing partial spillover of the oxygen atoms onto the support following deoxygenation.
4. Conclusions
Scheme 1. Transformation pathways of methyloxirane.
temperature range. Nevertheless, isomerisation (already at 363 K) and from 393 K hydrogenation (products of single ring opening) also took place. Hydrogenation became important at higher temperatures. However, unsupported gold was only slightly active even at 473 K and deoxygenation was inferior to isomerisation. The transformation spectrum was also much narrower. It seems to be clear that the support plays important role not only by dispersing the metal, but also allowing traffic of certain species, most
Large amounts of gold could be deposited onto carbon nanotubes. Sonication upon gold deposition and the stabiliser of the gold colloid [tetrakis(hydroxymethyl)phosphonium chloride] were equally important in the success of preparation. The gold–carbon nanotube composite was an active catalyst in the transformations of methyloxirane.
Acknowledgements Research leading to this contribution was supported by the National Science Fund of Hungary via Grant (T034184). The financial support is highly appreciated. One of us A.F. is a Bekesy postdoctoral fellow. The fellowship is gratefully acknowledged.
Table 1 Product distribution in the ring-opening reactions of methyloxirane (MOX) over 10 mg of Au/carbon nanotube composite (3 wt% Au loading) or gold powder in pulse microreactor applying a pulse sequence (I: 363 K, II: 393 K, III: 423 K, IV: 473 K, pulse size: 1 ll) under hydrogen flow (45 cm3 = min) No.
Composition (mol%) MOX (1)
Propene (2)
Au/carbon nanotube (pretreatment: H2 : I 98.6 1.1 II 96.1 2.2 III 92.2 3.6 IV 82.4 8.7 Au powder I II III IV
Acetone (3)
Propionaldehyde (4)
2-Propanol (5)
n-Propanol (6)
H2 O (7)
0 0.7 1.8 3.4
0 0.1 0.6 2.4
0 0.3 0.7 1.2
0 0 0 0
0 0 0 0
0 0 0 0
3
623 K, 45 cm = min, 1 h) 0.3 0 0.5 0.1 0.9 0.2 1.1 0.8
(pretreatment: H2 : 623 K, 45 cm3 = min, 1 h) 100 0 0 0 100 0 0 0 99.8 0 0.2 0 99.3 0.1 0.6 0
852
A. Fasi et al. / Chemical Physics Letters 372 (2003) 848–852
References [1] G. Nagy, M. Levy, R. Scarmozzino, R.M. Osgood, H. Dai, R.E. Smalley, C.A. Michaels, G.W. Flynn, G.F. McLane, Appl. Phys. Lett. 73 (1998) 529. [2] F.L. Darkrim, P. Malbrunot, G.P. Tartaglia, Int. J. Hydr. Energ. 27 (2002) 193. [3] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [4] M.S. Fuhrer, B.M. Kim, T. Durkop, T. Brintlinger, Nano Lett. 2 (2002) 755. [5] M. Radosavljevic, M. Freitag, K.V. Thadani, A.T. Johnson, Nano Lett. 2 (2002) 761.
[6] J. Liu, M.J. Casavant, M. Cox, D.A. Walters, P. Boul, W. Lu, A.J. Rimberg, K.A. Smith, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 303 (1999) 125. [7] C. Pham-Huu, N. Keller, V.V. Roddatis, R. Schl€ ogl, M.J. Ledoux, Phys. Chem. Chem. Phys. 4 (2002) 514. [8] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [9] A. Fasi, I. Palink o, K. Hernadi, I. Kiricsi, Catal. Lett. 81 (2002) 237. [10] I.P. Smoliakova, Curr. Org. Chem. 4 (2000) 589.