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Selective thiolation of single-walled carbon nanotubes
Synthetic Metals 139 (2003) 521–527
Selective thiolation of single-walled carbon nanotubes Jong Kuk Lim, Wan Soo Yun, Myung-han Yoon, Sun Kyung Lee, Chang Hwan Kim, Kwan Kim1 , Seong Keun Kim∗ School of Chemistry, Seoul National University, Seoul 151-747, South Korea Received 16 August 2002; received in revised form 1 May 2003; accepted 2 May 2003
Abstract Single-walled carbon nanotubes (SWCNTs) were derivatized with thiol groups at the ends of the nanotubes. The carbon nanotubes (CNTs) were treated with acid mixtures and modified through a series of chemical reactions. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy were used to verify the intermediate products of the oxidation and reduction reactions and the final products. The thiolated CNTs were adsorbed on micron-sized silver and gold particles as well as gold surfaces to study the interaction between the thiol groups of the nanotube and the noble metals. The thiol–metal adhesion was studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), wavelength dispersive electron spectroscopy, and Raman spectroscopy. A new type of bonding between the CNT and a noble metal surface was proposed that involves a bow-type single-walled nanotube (SWNT) with its two ends strongly attached to the metal surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube (CNT); Thiolation; Gold and silver colloid; FT-IR and Raman spectroscopy; AFM image
1. Introduction Since its first discovery [1] following the mass production of fullerenes [2], the carbon nanotube (CNT) has been a subject of intense research in chemistry, physics, and materials science [3]. Both single-walled nanotube (SWNT) and multi-walled nanotube (MWNT) come in various sizes and chiralities, thereby exhibiting remarkably diverse physical properties. The CNT is expected to be potentially useful in electronic devices as a quantum wire [4–8], gas sensor industry [9], nanotweezer technology [10], chemical force microscopy [11,12], high resolution atomic force microscopy (AFM), and field emission display [13,14]. A widespread application of the CNT is hindered, however, because there is no proper way to control the production and manipulation of its physical and chemical characteristics. In particular, from a chemical point of view, the CNT is a molecule of little practical interest because of the absence of a functional group, despite the fact that it has a seemingly useful one-dimensional structural motif. Functionalization of the CNT has been pursued in a limited ∗ Corresponding author. E-mail addresses: [email protected] (K. Kim), [email protected] (S.K. Kim). 1 Also a corresponding author.
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00337-0
number of cases [15–17], mostly through linkage bonds. In this paper, we report our success in functionalization of the CNT by directly attaching thiol groups to the CNT through a series of chemical reactions. Such functionalization is selective in the sense that the thiol groups are attached only to the most reactive sites of the CNT, i.e. the ends of the nanotube. Because organic thiol derivatives are generally well known to interact strongly with noble metal surfaces [18], selective thiolation may be used to make a good electrical junction between a CNT and a metal electrode, or to position the CNT relative to a metal surface by taking advantage of the strong thiol–metal interaction. Although the functionalization of the CNT could bring about numerous potential applications, it is difficult to carry out such a reaction in the first place because of the chemical inertness and low solubility of the CNT in any solvent. Despite such difficulties, however, it has been known that chemical reactions can take place at the defect sites of the CNT in a colloidal state [15–17]. Although we used basically the same thiolation schemes employed in these earlier studies, our CNT has notable differences from those of previous studies. First, it does not have a long alkyl chain that can be found in compounds such as CNT–(CH2 )11 –SH synthesized by Smalley and coworkers [15]. Because of the long and flexible alkyl chain, the latter compound does not anchor on metal surface in a specific orientation. Furthermore,
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the long alkyl chain will give rise to a large contact resistance between the metal and the CNT when the CNT is used as a nanowire in an electronic device. To overcome these problems, Liu et al. synthesized another type of compound, CNT–CONH–(CH2 )2 –SH, which has one amide bond and only two methylene groups between the CNT and the thiol group [16]. This compound, however, contains an amide bond that tends to react easily in an acidic or basic environment. In this paper, we report the synthesis of a new form of thiolated CNT, generically represented as CNT–CH2 –SH, which contains thiol groups almost directly linked to the main -conjugated body of the CNT. This compound has a shorter linkage than any other thiolated CNTs reported to date, which means the contact resistance can be minimized for practical applications of the CNT as a nanowire. In addition, this form of thiolated CNT should be chemically much more inert because of the absence of a reactive functional group. Thiol groups were attached, via successive carboxylation, reduction, chlorination and thiolation, to the open ends of the CNT, which were formed by breaking the CNT by sonochemical activation. The intermediate and final products were characterized by various microscopic and spectroscopic methods.
2. Experiment The CNTs we used were SWNTs (purchased from Carbon Nanotechnology, Inc.) in aqueous colloidal suspension with surfactant (Triton X-100) and NaOH. After filtration by using a Teflon membrane filter with a pore size of 200 nm, the CNTs were washed with distilled water and methanol to remove the surfactant and NaOH. The CNTs were treated with a H2 SO4 /HNO3 (3:1) mixture, and sonicated for 24 h at temperatures between 35 and 40 ◦ C. The resulting suspension was filtered with a polycarbonante membrane filter after dilution with distilled water. It was neutralized with NaOH (10 mM) and washed with distilled water to remove the salt produced in the neutralization step. The CNT was suspended in a H2 O2 (30 wt.%, aq.)/H2 SO4 (4:1) mixture, and the suspension was refluxed for 2 h at 70 ◦ C to cut the CNTs in shorter lengths, and thereby produce a larger number of open ends for carboxylation. A typical length of the CNT at this stage as observed by scanning electron microscopy (SEM) and AFM was 200 nm. The remaining mixture was diluted, filtered and neutralized with NaOH (10 mM). The remaining powder was dissolved into absolute ethanol with excess NaBH4 , and this suspension was refluxed at 80 ◦ C for 5 h to reduce the carboxylic acid groups into hydroxyl groups. The suspension was quenched with H2 SO4 (9 M, aq.) until the excess NaBH4 powder disappears. The hydroxyl groups of the CNTs were then substituted with chloride groups by refluxing the suspension in thionyl chloride for 36 h at 80 ◦ C. It was then refluxed in NaOH (5 M, aq.) at 120 ◦ C for 5 h, and the suspension was filtered on the
200 nm pore-sized Teflon membrane filter. The remaining powder was dissolved into absolute ethanol and the ethanol was evaporated. Since the final product was in the form of a basic salt of a thiol, it was dissolved in distilled water and worked up with HCl (1 M, aq.) to be turned into the neutral form.
3. Results and discussion In order to identify the functional groups at the end of the CNT after the acid mixture treatment, we took Fourier transform infrared (FT-IR) spectra. The spectrum (a) in Fig. 1 shows the FT-IR spectrum taken after the acid mixture treatment. The characteristic vibrational modes of carbonyl group (ca. 1700 cm−1 ) and hydroxyl group (ca. 3400 cm−1 ) are apparent. The peaks at ca. 2350 and 2900 cm−1 are due to the ambient CO2 and the parylene coating of the IR optics in the spectrometer, respectively. After converting the carboxylated CNT into a hydroxylated one by reduction with NaBH4 , we obtained an FT-IR spectrum shown in Fig. 1(b). It appears that the carbonyl peak at 1700 cm−1 becomes weaker while the C–O single bond peak at ca. 1100 cm−1 becomes relatively stronger, with the broad hydroxyl peak at ca. 3400 cm−1 still present. These spectra indicate that the reduction has occurred to a significant extent, if not fully. We also tried an alcohol test using CrO3 in aqueous sulfuric acid, which revealed the typical color change from orange to blue–green for a primary or secondary alcohol. Because there are few ways to directly verify the chloride group, we went to the next step to identify the product of the thiolation reaction, without confirming whether or not the chlorination had occurred. During the process of thiolation with thiourea, there is an intermediate step in which the reactant takes the form of a salt, as a result of a reaction with a strong base, NaOH. If we did not work up the final product with an acid, the end product would have been in an anionic form, CNT–CH2 –S− . Before and after the work up of this final product, we took a nuclear magnetic resonance (NMR) spectrum, and compared with each other. Fig. 2(a) is the NMR spectrum taken before the work up, which reveals proton peaks at ca. 2.2 and 3.7 ppm. By comparison, for benzyl thiol, a reference compound, the characteristic NMR peaks are known to occur at 1.7 and 3.7 ppm, respectively for the hydrogen in the thiol group and the hydrogens in the methylene group. Since our final product is in the form of an anion before the work up, the peak for the thiolic hydrogen was not seen at 1.7 ppm in Fig. 2(a). Instead, only the peak for the methylenic hydrogens was observed at 3.7 ppm, as in the case of benzyl thiol. On the other hand, the peak at 2.2 ppm in Fig. 2(a) is for the protons in the methyl group at the end of the CNT, which did not undergo thiolation. After the work up, however, as shown in Fig. 2(b), we observed the peak for the thiolic hydrogen at 1.7 ppm, in addition to the peaks for the methylenic and methylic hydrogens at 3.7 and 2.2 ppm, respectively.
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Fig. 1. IR spectra of the intermediate products, CNT–COOH (a) taken after the successive treatment with acid mixtures, and CNT–CH2 OH (b) after the reduction with NaBH4 . The sample CNT powder was mixed with a small amount of KBr powder to make the IR pellet.
Fig. 2. (a) The proton NMR spectrum before the acid work up of the thiolated CNT suspension. (b) Proton NMR spectrum after the acid work up. A 300 MHz NMR was used for the CNT suspension in CDCl3 (99.8%). The work up generates thiolic protons, whose peak is evident at 1.7 ppm in (b).
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Fig. 3. SEM images of the thiolated CNTs adsorbed on colloidal silver (A) and gold (B–D) particles. Most CNTs on the particle has a dangling end, since only one of the two ends is often attached to the metal particle. The CNTs in the middle of two adjacent silver particles in (A), however, have no dangling ends because now both ends of the CNTs are anchored upon both particles. Inter-particle linkages are formed through the thiolated CNTs. The images in (B) and (C) show that the fiber-like CNTs are extending out from the surface of a single gold particle. When there is an adjacent gold particle, however, the CNT fibers link both particles together on both ends, just as in the case of two adjacent silver particles in (A).
A thiolated CNT is expected to exhibit strong chemisorptive affinity toward a noble metal surface because of its thiol groups at the tube end. In order to confirm such behavior, the thiolated CNT was adsorbed on micron-sized silver and gold particles, whose SEM images are shown in Fig. 3. In Fig. 3(A), the round particles are silver colloidal particles, while the long hairy fibers are temporarily assumed to be the thiolated CNTs. First of all, we note that there are lots of CNTs in the region between the two adjacent silver particles in the right-hand side of the figure. These CNTs appear to adjoin the two particles, probably by their anchorage on each end through the thiol–silver bonds. On the other hand, on a closer examination of the other part of the figure toward the center, we note that there also are many CNTs extending from the surface of the silver particle. This indicates that the thiolated CNT initially attach itself to a silver particle through one end of the tube, with the other end dangling. When another silver particle approaches, with its own CNTs on the surface, the dangling ends of the tubes on both particles find the other particle, and attach themselves to it, thereby adjoining the two particles as shown in Fig. 3(A). The tubes on the opposite side of the other particle remain dangling, probably until they too find another particle within their range. We investigated chemical compositions of different regions of Fig. 3(A) in the wavelength dispersive spectroscopy
mode of electron probe for micro-analysis. It turns out that the region (a) of Fig. 3(A) contained silver (63.0%), carbon (36.9%), and sulfur (0.1%), while the region (b) also contained the same elements but in different ratios (silver 71.1%, carbon 28.7%, and sulfur 0.2%). The large percentage of the carbon content in both regions indicates that there are quite a lot of CNTs on the surface of the silver particle, perhaps much more than we can apparently deduce from the SEM image. On the other hand, the small percentage of the sulfur content merely reflects the relatively small number of the reaction sites to attach the thiol groups at the end of the tube. Similar SEM images were obtained when we switched from a silver to a gold particle. Here too, a large number of hairy fibers are seen extending radially from the surface of the particle, as shown in Fig. 3(B) and (C). If the CNT were not thiolated, and therefore had no anchoring action to the metal surface, it will just lay itself upon the metal particle in a random direction. Therefore, the images of radially extending CNTs in Fig. 3(B) and (C) seem to be in support of the thiolated nature of the CNT in question. When another gold particle is in the neighborhood, the CNT again adjoins the two particles (Fig. 3(D)), as has been observed for the case of two silver particles in Fig. 3(A). The best way to positively identify a thiolated CNT would probably be Raman spectroscopy. Fig. 4(a) is the spectrum
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Fig. 4. Raman spectra of the thiolated CNT in solid (a) and on silver colloidal particles (b). (a) The characteristic CNT peaks at 1350, 1590, and 2680 cm−1 are shown in addition to the two new peaks at 585 and 1095 cm−1 , which represent the S–S and the C=S vibration of the thiolated CNT, respectively. (b) The peak at 585 cm−1 for S–S disappears and a new peak at 689 cm−1 for C–S appears, while the peak at 1095 cm−1 for C=S shifts to 1058 cm−1 .
for the thiolated CNT in solid, which shows the characteristic CNT peaks [19–21] at 1350, 1590, and 2680 cm−1 as well as the two other peaks at 585 and 1095 cm−1 that have not been observed for an untreated CNT. The peak at 585 cm−1 is assigned as the S–S vibration of the disulfide bond between adjacent thiol groups, while that at 1095 cm−1 corresponds to the C=S double bond [22]. By comparison, Fig. 4(b) shows the spectrum of the thiolated CNT adsorbed on silver particle, whose Raman intensities are considerably enhanced. There are some notable changes that come with the adsorption to silver: (1) the peak at 585 cm−1 disappears; (2) a new peak appears at 689 cm−1 ; (3) the peak at 1095 cm−1 shifts to 1058 cm−1 , with a decrease in relative intensity; (4) an intense and broad spectral band appears at 150–250 cm−1 . The disappearance of the S–S vibration at 585 cm−1 is due to the formation of a new bond between sulfur and silver, which drastically weakens or even destroys the disulfide bond between adjacent thiol groups. The newly-formed sulfur–silver bond will also reduce the vibrational frequency of the C=S bond, thereby causing a red shift of its frequency from 1095 to 1058 cm−1 . On the other hand, the emergence of a new Raman peak at 689 cm−1 for the C–S vibration [23] goes parallel with the weakening of the C=S bond, which in some cases will amount to a conversion into a C–S bond. It also explains the reduction in the relative intensity of the C=S vibration that accompanies the red shift in frequency. We suggest that the intense and broad peak at 150–250 cm−1 represents the sulfur–silver bond [23]. In order to investigate the nature of chemical interaction between our thiolated CNT and a noble metal surface, we
adsorbed the CNT on a gold surface and probed the interaction by AFM. Specifically, we wanted to learn the strength of the bond between the thiol groups of the CNT and the gold surface. We adsorbed the CNT by dipping a gold-plated silicon wafer into an ethanol suspension containing the thiolated CNT. During this process, the ethanol suspension was placed upon a magnet and agitated by a sonicator to prevent the CNTs from aggregating. After a series of washing with absolute ethanol and distilled water under the condition of sonication, we obtained a gold surface covered with thiolated CNTs, as shown by the AFM images of Fig. 5. Fig. 5(a) is a top-view image obtained by AFM in the contact mode with a scanning force of 1 nN. The particles shown in the image are 30–40 nm in width and 7–8 nm in height, which is comparable to the size of the sample in the study of Liu et al. [16], although the AFM image appears very different due to the difference in the scales between the lateral and vertical dimensions. In order to see how strongly the CNT is adsorbed on the surface, we increased the scanning force to 2 nN and scanned (or “scratched”) the same area. We then brought the scanning force back down to 1 nN and scanned the same area to obtain the AFM image of Fig. 5(b). This was to see if there occurred any change in the AFM image by the scratching of the surface with a larger force. Fig. 5(b) shows that some dots shown in Fig. 5(a) have disappeared (apparently removed by our scanning with a 2 nN force), while others still remain on the gold surface. We chose another area of the same sample, and obtained its image by a scanning force of 1 nN, which is shown in Fig. 5(c). To see the effect of the scanning force on the
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Fig. 5. The AFM images of thiolated CNTs on a gold surface, obtained by a scanning force of (a) 1 nN and (b) 2 nN in the contact mode. The substrate was a silicon wafer coated by a 9 nm thick gold layer, with a 1 nm thick titanium layer in between. The peak-to-valley roughness of the gold surface was within 1 nm. Reduction in the number of the CNTs in (b) reflects their removal from the viewing area by a larger scanning force. The images (c) and (d) were obtained in another region, with a scanning force of 1 and 5 nN, respectively, in the contact mode. The CNTs are seen to be readily removed in (d) by a scanning force of 5 nN. (e) A schematic representation of the vertical view image of (a). The thiolated CNT has thiol groups at both ends, which bind to the gold surface. The flexible tube body of SWNT allows the CNT to conform its geometry for maximum binding energy at the expense of the bending energy. The result is a “bow-type” bundle of the thiolated CNTs.
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CNT–metal interaction again, we scratched this area with a much larger scanning force of 5 nN this time, and then obtained an AFM image by the typical scanning force of 1 nN. Fig. 5(d) shows the resulting image, and indicates that most particles have been completely removed. In the study of Liu et al. [16], a rod-type model of CNTs on gold surface was proposed based on AFM images such as that of Fig. 5(a). In their rod-type model, the thiolated, single-walled CNTs with an ethylene linkage were regarded as standing upright on the surface. Contrary to their rod-type model, however, we propose another model from our findings. Our model is based on the flexibility of the CNTs, especially in their single-walled form, that allows them to be bent rather easily. When a thiolated CNT is adsorbed on a gold surface, there are two ways the CNT would attach itself to the metal surface: only one end becomes attached while the tube maintains its straight form, or both ends get attached while the tube main body becomes bent. Obviously, bending of the nanotube comes with an increase in energy, but it is compensated by the energy gain through the adsorption of both ends of the CNT to the metal. If the magnitude of the bending energy is larger than that of the adsorption energy, the CNT will attach only one of its ends to the metal and stay straight. Therefore, whether the thiolated CNT stands upright or bends on a metal surface depends entirely on the energetic balance between the mechanical property of the tube body and the chemical strength of the thiol–metal adsorption. A flexible SWNT with many thiol groups at its ends will more likely to bend on metal surface to form the “bow-type” structure, while a more rigid form of SWNT or MWNT with less thiol groups will stand upright on surface with only one end attached, forming a rod-type structure. Fig. 5(e) depicts, in a schematic way, bundles of the thiolated CNTs in the bow-type form on metal surface. Most of these bow-type CNTs are believed to have their middle part up away from the surface as shown in Fig. 5(e), as evidenced by the typical height of 7–8 nm in the AFM image. In conclusion, we synthesized thiolated CNTs with a short alkyl linkage between the CNT and the thiol end through a series of reactions including carboxylation, reduction, chlorination, and thiolation. Various spectroscopic and microscopic methods were employed to identify the intermediate products as well as the final product. The strong interaction between the thiol group of the CNT and a noble metal surface was investigated by AFM. A new model of the adsorption between thiolated CNT and metal was proposed based on the bow-type model that could be preferred over the conventional rod-type model in certain cases.
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Acknowledgements This work was supported by the National R&D Project for Nano Science and Technology for S.K.K. and the Center for Molecular Catalysis Grant for K.K. Personnel support by the BK-21 Program is gratefully acknowledged.
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Selective thiolation of single-walled carbon nanotubes Jong Kuk Lim, Wan Soo Yun, Myung-han Yoon, Sun Kyung Lee, Chang Hwan Kim, Kwan Kim1 , Seong Keun Kim∗ School of Chemistry, Seoul National University, Seoul 151-747, South Korea Received 16 August 2002; received in revised form 1 May 2003; accepted 2 May 2003
Abstract Single-walled carbon nanotubes (SWCNTs) were derivatized with thiol groups at the ends of the nanotubes. The carbon nanotubes (CNTs) were treated with acid mixtures and modified through a series of chemical reactions. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy were used to verify the intermediate products of the oxidation and reduction reactions and the final products. The thiolated CNTs were adsorbed on micron-sized silver and gold particles as well as gold surfaces to study the interaction between the thiol groups of the nanotube and the noble metals. The thiol–metal adhesion was studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), wavelength dispersive electron spectroscopy, and Raman spectroscopy. A new type of bonding between the CNT and a noble metal surface was proposed that involves a bow-type single-walled nanotube (SWNT) with its two ends strongly attached to the metal surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube (CNT); Thiolation; Gold and silver colloid; FT-IR and Raman spectroscopy; AFM image
1. Introduction Since its first discovery [1] following the mass production of fullerenes [2], the carbon nanotube (CNT) has been a subject of intense research in chemistry, physics, and materials science [3]. Both single-walled nanotube (SWNT) and multi-walled nanotube (MWNT) come in various sizes and chiralities, thereby exhibiting remarkably diverse physical properties. The CNT is expected to be potentially useful in electronic devices as a quantum wire [4–8], gas sensor industry [9], nanotweezer technology [10], chemical force microscopy [11,12], high resolution atomic force microscopy (AFM), and field emission display [13,14]. A widespread application of the CNT is hindered, however, because there is no proper way to control the production and manipulation of its physical and chemical characteristics. In particular, from a chemical point of view, the CNT is a molecule of little practical interest because of the absence of a functional group, despite the fact that it has a seemingly useful one-dimensional structural motif. Functionalization of the CNT has been pursued in a limited ∗ Corresponding author. E-mail addresses: [email protected] (K. Kim), [email protected] (S.K. Kim). 1 Also a corresponding author.
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00337-0
number of cases [15–17], mostly through linkage bonds. In this paper, we report our success in functionalization of the CNT by directly attaching thiol groups to the CNT through a series of chemical reactions. Such functionalization is selective in the sense that the thiol groups are attached only to the most reactive sites of the CNT, i.e. the ends of the nanotube. Because organic thiol derivatives are generally well known to interact strongly with noble metal surfaces [18], selective thiolation may be used to make a good electrical junction between a CNT and a metal electrode, or to position the CNT relative to a metal surface by taking advantage of the strong thiol–metal interaction. Although the functionalization of the CNT could bring about numerous potential applications, it is difficult to carry out such a reaction in the first place because of the chemical inertness and low solubility of the CNT in any solvent. Despite such difficulties, however, it has been known that chemical reactions can take place at the defect sites of the CNT in a colloidal state [15–17]. Although we used basically the same thiolation schemes employed in these earlier studies, our CNT has notable differences from those of previous studies. First, it does not have a long alkyl chain that can be found in compounds such as CNT–(CH2 )11 –SH synthesized by Smalley and coworkers [15]. Because of the long and flexible alkyl chain, the latter compound does not anchor on metal surface in a specific orientation. Furthermore,
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the long alkyl chain will give rise to a large contact resistance between the metal and the CNT when the CNT is used as a nanowire in an electronic device. To overcome these problems, Liu et al. synthesized another type of compound, CNT–CONH–(CH2 )2 –SH, which has one amide bond and only two methylene groups between the CNT and the thiol group [16]. This compound, however, contains an amide bond that tends to react easily in an acidic or basic environment. In this paper, we report the synthesis of a new form of thiolated CNT, generically represented as CNT–CH2 –SH, which contains thiol groups almost directly linked to the main -conjugated body of the CNT. This compound has a shorter linkage than any other thiolated CNTs reported to date, which means the contact resistance can be minimized for practical applications of the CNT as a nanowire. In addition, this form of thiolated CNT should be chemically much more inert because of the absence of a reactive functional group. Thiol groups were attached, via successive carboxylation, reduction, chlorination and thiolation, to the open ends of the CNT, which were formed by breaking the CNT by sonochemical activation. The intermediate and final products were characterized by various microscopic and spectroscopic methods.
2. Experiment The CNTs we used were SWNTs (purchased from Carbon Nanotechnology, Inc.) in aqueous colloidal suspension with surfactant (Triton X-100) and NaOH. After filtration by using a Teflon membrane filter with a pore size of 200 nm, the CNTs were washed with distilled water and methanol to remove the surfactant and NaOH. The CNTs were treated with a H2 SO4 /HNO3 (3:1) mixture, and sonicated for 24 h at temperatures between 35 and 40 ◦ C. The resulting suspension was filtered with a polycarbonante membrane filter after dilution with distilled water. It was neutralized with NaOH (10 mM) and washed with distilled water to remove the salt produced in the neutralization step. The CNT was suspended in a H2 O2 (30 wt.%, aq.)/H2 SO4 (4:1) mixture, and the suspension was refluxed for 2 h at 70 ◦ C to cut the CNTs in shorter lengths, and thereby produce a larger number of open ends for carboxylation. A typical length of the CNT at this stage as observed by scanning electron microscopy (SEM) and AFM was 200 nm. The remaining mixture was diluted, filtered and neutralized with NaOH (10 mM). The remaining powder was dissolved into absolute ethanol with excess NaBH4 , and this suspension was refluxed at 80 ◦ C for 5 h to reduce the carboxylic acid groups into hydroxyl groups. The suspension was quenched with H2 SO4 (9 M, aq.) until the excess NaBH4 powder disappears. The hydroxyl groups of the CNTs were then substituted with chloride groups by refluxing the suspension in thionyl chloride for 36 h at 80 ◦ C. It was then refluxed in NaOH (5 M, aq.) at 120 ◦ C for 5 h, and the suspension was filtered on the
200 nm pore-sized Teflon membrane filter. The remaining powder was dissolved into absolute ethanol and the ethanol was evaporated. Since the final product was in the form of a basic salt of a thiol, it was dissolved in distilled water and worked up with HCl (1 M, aq.) to be turned into the neutral form.
3. Results and discussion In order to identify the functional groups at the end of the CNT after the acid mixture treatment, we took Fourier transform infrared (FT-IR) spectra. The spectrum (a) in Fig. 1 shows the FT-IR spectrum taken after the acid mixture treatment. The characteristic vibrational modes of carbonyl group (ca. 1700 cm−1 ) and hydroxyl group (ca. 3400 cm−1 ) are apparent. The peaks at ca. 2350 and 2900 cm−1 are due to the ambient CO2 and the parylene coating of the IR optics in the spectrometer, respectively. After converting the carboxylated CNT into a hydroxylated one by reduction with NaBH4 , we obtained an FT-IR spectrum shown in Fig. 1(b). It appears that the carbonyl peak at 1700 cm−1 becomes weaker while the C–O single bond peak at ca. 1100 cm−1 becomes relatively stronger, with the broad hydroxyl peak at ca. 3400 cm−1 still present. These spectra indicate that the reduction has occurred to a significant extent, if not fully. We also tried an alcohol test using CrO3 in aqueous sulfuric acid, which revealed the typical color change from orange to blue–green for a primary or secondary alcohol. Because there are few ways to directly verify the chloride group, we went to the next step to identify the product of the thiolation reaction, without confirming whether or not the chlorination had occurred. During the process of thiolation with thiourea, there is an intermediate step in which the reactant takes the form of a salt, as a result of a reaction with a strong base, NaOH. If we did not work up the final product with an acid, the end product would have been in an anionic form, CNT–CH2 –S− . Before and after the work up of this final product, we took a nuclear magnetic resonance (NMR) spectrum, and compared with each other. Fig. 2(a) is the NMR spectrum taken before the work up, which reveals proton peaks at ca. 2.2 and 3.7 ppm. By comparison, for benzyl thiol, a reference compound, the characteristic NMR peaks are known to occur at 1.7 and 3.7 ppm, respectively for the hydrogen in the thiol group and the hydrogens in the methylene group. Since our final product is in the form of an anion before the work up, the peak for the thiolic hydrogen was not seen at 1.7 ppm in Fig. 2(a). Instead, only the peak for the methylenic hydrogens was observed at 3.7 ppm, as in the case of benzyl thiol. On the other hand, the peak at 2.2 ppm in Fig. 2(a) is for the protons in the methyl group at the end of the CNT, which did not undergo thiolation. After the work up, however, as shown in Fig. 2(b), we observed the peak for the thiolic hydrogen at 1.7 ppm, in addition to the peaks for the methylenic and methylic hydrogens at 3.7 and 2.2 ppm, respectively.
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Fig. 1. IR spectra of the intermediate products, CNT–COOH (a) taken after the successive treatment with acid mixtures, and CNT–CH2 OH (b) after the reduction with NaBH4 . The sample CNT powder was mixed with a small amount of KBr powder to make the IR pellet.
Fig. 2. (a) The proton NMR spectrum before the acid work up of the thiolated CNT suspension. (b) Proton NMR spectrum after the acid work up. A 300 MHz NMR was used for the CNT suspension in CDCl3 (99.8%). The work up generates thiolic protons, whose peak is evident at 1.7 ppm in (b).
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Fig. 3. SEM images of the thiolated CNTs adsorbed on colloidal silver (A) and gold (B–D) particles. Most CNTs on the particle has a dangling end, since only one of the two ends is often attached to the metal particle. The CNTs in the middle of two adjacent silver particles in (A), however, have no dangling ends because now both ends of the CNTs are anchored upon both particles. Inter-particle linkages are formed through the thiolated CNTs. The images in (B) and (C) show that the fiber-like CNTs are extending out from the surface of a single gold particle. When there is an adjacent gold particle, however, the CNT fibers link both particles together on both ends, just as in the case of two adjacent silver particles in (A).
A thiolated CNT is expected to exhibit strong chemisorptive affinity toward a noble metal surface because of its thiol groups at the tube end. In order to confirm such behavior, the thiolated CNT was adsorbed on micron-sized silver and gold particles, whose SEM images are shown in Fig. 3. In Fig. 3(A), the round particles are silver colloidal particles, while the long hairy fibers are temporarily assumed to be the thiolated CNTs. First of all, we note that there are lots of CNTs in the region between the two adjacent silver particles in the right-hand side of the figure. These CNTs appear to adjoin the two particles, probably by their anchorage on each end through the thiol–silver bonds. On the other hand, on a closer examination of the other part of the figure toward the center, we note that there also are many CNTs extending from the surface of the silver particle. This indicates that the thiolated CNT initially attach itself to a silver particle through one end of the tube, with the other end dangling. When another silver particle approaches, with its own CNTs on the surface, the dangling ends of the tubes on both particles find the other particle, and attach themselves to it, thereby adjoining the two particles as shown in Fig. 3(A). The tubes on the opposite side of the other particle remain dangling, probably until they too find another particle within their range. We investigated chemical compositions of different regions of Fig. 3(A) in the wavelength dispersive spectroscopy
mode of electron probe for micro-analysis. It turns out that the region (a) of Fig. 3(A) contained silver (63.0%), carbon (36.9%), and sulfur (0.1%), while the region (b) also contained the same elements but in different ratios (silver 71.1%, carbon 28.7%, and sulfur 0.2%). The large percentage of the carbon content in both regions indicates that there are quite a lot of CNTs on the surface of the silver particle, perhaps much more than we can apparently deduce from the SEM image. On the other hand, the small percentage of the sulfur content merely reflects the relatively small number of the reaction sites to attach the thiol groups at the end of the tube. Similar SEM images were obtained when we switched from a silver to a gold particle. Here too, a large number of hairy fibers are seen extending radially from the surface of the particle, as shown in Fig. 3(B) and (C). If the CNT were not thiolated, and therefore had no anchoring action to the metal surface, it will just lay itself upon the metal particle in a random direction. Therefore, the images of radially extending CNTs in Fig. 3(B) and (C) seem to be in support of the thiolated nature of the CNT in question. When another gold particle is in the neighborhood, the CNT again adjoins the two particles (Fig. 3(D)), as has been observed for the case of two silver particles in Fig. 3(A). The best way to positively identify a thiolated CNT would probably be Raman spectroscopy. Fig. 4(a) is the spectrum
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Fig. 4. Raman spectra of the thiolated CNT in solid (a) and on silver colloidal particles (b). (a) The characteristic CNT peaks at 1350, 1590, and 2680 cm−1 are shown in addition to the two new peaks at 585 and 1095 cm−1 , which represent the S–S and the C=S vibration of the thiolated CNT, respectively. (b) The peak at 585 cm−1 for S–S disappears and a new peak at 689 cm−1 for C–S appears, while the peak at 1095 cm−1 for C=S shifts to 1058 cm−1 .
for the thiolated CNT in solid, which shows the characteristic CNT peaks [19–21] at 1350, 1590, and 2680 cm−1 as well as the two other peaks at 585 and 1095 cm−1 that have not been observed for an untreated CNT. The peak at 585 cm−1 is assigned as the S–S vibration of the disulfide bond between adjacent thiol groups, while that at 1095 cm−1 corresponds to the C=S double bond [22]. By comparison, Fig. 4(b) shows the spectrum of the thiolated CNT adsorbed on silver particle, whose Raman intensities are considerably enhanced. There are some notable changes that come with the adsorption to silver: (1) the peak at 585 cm−1 disappears; (2) a new peak appears at 689 cm−1 ; (3) the peak at 1095 cm−1 shifts to 1058 cm−1 , with a decrease in relative intensity; (4) an intense and broad spectral band appears at 150–250 cm−1 . The disappearance of the S–S vibration at 585 cm−1 is due to the formation of a new bond between sulfur and silver, which drastically weakens or even destroys the disulfide bond between adjacent thiol groups. The newly-formed sulfur–silver bond will also reduce the vibrational frequency of the C=S bond, thereby causing a red shift of its frequency from 1095 to 1058 cm−1 . On the other hand, the emergence of a new Raman peak at 689 cm−1 for the C–S vibration [23] goes parallel with the weakening of the C=S bond, which in some cases will amount to a conversion into a C–S bond. It also explains the reduction in the relative intensity of the C=S vibration that accompanies the red shift in frequency. We suggest that the intense and broad peak at 150–250 cm−1 represents the sulfur–silver bond [23]. In order to investigate the nature of chemical interaction between our thiolated CNT and a noble metal surface, we
adsorbed the CNT on a gold surface and probed the interaction by AFM. Specifically, we wanted to learn the strength of the bond between the thiol groups of the CNT and the gold surface. We adsorbed the CNT by dipping a gold-plated silicon wafer into an ethanol suspension containing the thiolated CNT. During this process, the ethanol suspension was placed upon a magnet and agitated by a sonicator to prevent the CNTs from aggregating. After a series of washing with absolute ethanol and distilled water under the condition of sonication, we obtained a gold surface covered with thiolated CNTs, as shown by the AFM images of Fig. 5. Fig. 5(a) is a top-view image obtained by AFM in the contact mode with a scanning force of 1 nN. The particles shown in the image are 30–40 nm in width and 7–8 nm in height, which is comparable to the size of the sample in the study of Liu et al. [16], although the AFM image appears very different due to the difference in the scales between the lateral and vertical dimensions. In order to see how strongly the CNT is adsorbed on the surface, we increased the scanning force to 2 nN and scanned (or “scratched”) the same area. We then brought the scanning force back down to 1 nN and scanned the same area to obtain the AFM image of Fig. 5(b). This was to see if there occurred any change in the AFM image by the scratching of the surface with a larger force. Fig. 5(b) shows that some dots shown in Fig. 5(a) have disappeared (apparently removed by our scanning with a 2 nN force), while others still remain on the gold surface. We chose another area of the same sample, and obtained its image by a scanning force of 1 nN, which is shown in Fig. 5(c). To see the effect of the scanning force on the
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Fig. 5. The AFM images of thiolated CNTs on a gold surface, obtained by a scanning force of (a) 1 nN and (b) 2 nN in the contact mode. The substrate was a silicon wafer coated by a 9 nm thick gold layer, with a 1 nm thick titanium layer in between. The peak-to-valley roughness of the gold surface was within 1 nm. Reduction in the number of the CNTs in (b) reflects their removal from the viewing area by a larger scanning force. The images (c) and (d) were obtained in another region, with a scanning force of 1 and 5 nN, respectively, in the contact mode. The CNTs are seen to be readily removed in (d) by a scanning force of 5 nN. (e) A schematic representation of the vertical view image of (a). The thiolated CNT has thiol groups at both ends, which bind to the gold surface. The flexible tube body of SWNT allows the CNT to conform its geometry for maximum binding energy at the expense of the bending energy. The result is a “bow-type” bundle of the thiolated CNTs.
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CNT–metal interaction again, we scratched this area with a much larger scanning force of 5 nN this time, and then obtained an AFM image by the typical scanning force of 1 nN. Fig. 5(d) shows the resulting image, and indicates that most particles have been completely removed. In the study of Liu et al. [16], a rod-type model of CNTs on gold surface was proposed based on AFM images such as that of Fig. 5(a). In their rod-type model, the thiolated, single-walled CNTs with an ethylene linkage were regarded as standing upright on the surface. Contrary to their rod-type model, however, we propose another model from our findings. Our model is based on the flexibility of the CNTs, especially in their single-walled form, that allows them to be bent rather easily. When a thiolated CNT is adsorbed on a gold surface, there are two ways the CNT would attach itself to the metal surface: only one end becomes attached while the tube maintains its straight form, or both ends get attached while the tube main body becomes bent. Obviously, bending of the nanotube comes with an increase in energy, but it is compensated by the energy gain through the adsorption of both ends of the CNT to the metal. If the magnitude of the bending energy is larger than that of the adsorption energy, the CNT will attach only one of its ends to the metal and stay straight. Therefore, whether the thiolated CNT stands upright or bends on a metal surface depends entirely on the energetic balance between the mechanical property of the tube body and the chemical strength of the thiol–metal adsorption. A flexible SWNT with many thiol groups at its ends will more likely to bend on metal surface to form the “bow-type” structure, while a more rigid form of SWNT or MWNT with less thiol groups will stand upright on surface with only one end attached, forming a rod-type structure. Fig. 5(e) depicts, in a schematic way, bundles of the thiolated CNTs in the bow-type form on metal surface. Most of these bow-type CNTs are believed to have their middle part up away from the surface as shown in Fig. 5(e), as evidenced by the typical height of 7–8 nm in the AFM image. In conclusion, we synthesized thiolated CNTs with a short alkyl linkage between the CNT and the thiol end through a series of reactions including carboxylation, reduction, chlorination, and thiolation. Various spectroscopic and microscopic methods were employed to identify the intermediate products as well as the final product. The strong interaction between the thiol group of the CNT and a noble metal surface was investigated by AFM. A new model of the adsorption between thiolated CNT and metal was proposed based on the bow-type model that could be preferred over the conventional rod-type model in certain cases.
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Acknowledgements This work was supported by the National R&D Project for Nano Science and Technology for S.K.K. and the Center for Molecular Catalysis Grant for K.K. Personnel support by the BK-21 Program is gratefully acknowledged.
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