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J. Phys. Chem. C 2009, 113, 5444–5449
Solubilization of Carbon Nanohorns by Block Polyelectrolyte Wrapping and Templated Formation of Gold Nanoparticles Grigoris Mountrichas,† Toshinari Ichihashi,‡ Stergios Pispas,† Masako Yudasaka,‡ Sumio Iijima,‡ and Nikos Tagmatarchis*,† Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Nanotube Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Greece, and NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: December 3, 2008; ReVised Manuscript ReceiVed: January 29, 2009
In this paper, we present a facile method for the noncovalent polymer functionalization of carbon nanohorns (CNHs) using a block polyelectrolyte, leading to water soluble nanohybrid colloids stable for several months. The protocol for the noncovalent functionalization makes use of the amphiphilic poly[sodium (2-sulfamate3-carboxylate) isoprene-b-styrene] block polyelectrolyte (CSI). The hydrophobic polystyrene block is anchored on the nanohorn surface through hydrophobic interactions, while the polyelectrolyte block stabilizes the formed hybrid nanoassembly through electrosteric interactions. The soluble CNHs/CSI nanoassembly is used as template for the in situ synthesis of gold nanoparticles (Aunano) by reduction of auric acid. The gold nanoparticles are preferentially localized at the periphery of CNHs, because of electrostatic interactions with the charged polymer chains. A variety of complementary techniques, including ATR-IR, UV-vis and EDX spectroscopies, thermogravimetry, dynamic light scattering, and transmission electron microscopy are applied for the characterization and study, in terms of structure and composition, of the CNHs/CSI and CNHs/CSI/Aunano ensembles. Introduction The solubilization of carbon nanohorns (CNHs), a unique nanosized allotropic form of carbon within the family of carbon nanotubes (CNTs) possessing a secondary spherical superstructure, free of any metal catalyst species,1 consisting of tubular graphene sheets with conical ends covalently interacting with each other,2 has attracted increased interest. Methodologies based on the covalent attachment of organic units, including azomethine ylides dipolar cycloaddition,3 aryl diazonium salts,4 polymers grafting5 and amines6 onto CNHs or noncovalent interactions (such as supramolecular, van der Waals, π-π stacking,7 and/or electrostatic interactions with aromatic planar and/or charged organic moieties8) with CNHs have already been applied in order to enhance solubility of CNHs in a variety of organic solvents or aqueous media. Moreover, functionalization of CNHs has been performed either at the highly strained conical-tips or at the side-walls.9 On the other hand, carbon-based nanostructured hybrid materials with noble metal nanoparticles are of widespread interest.10 Metal nanoparticles, especially gold nanoparticles, may find their way in a plethora of applications due to their high catalytic activity and their unique optical and electronic properties.11 Thus, it is not surprising that combining carbon nanotubes with metal nanoparticles can further improve their properties and/or even extend the range of potential technological applications.12 In parallel, methodologies based on polymers or polymerization approaches are receiving considerable attention for both the modification of carbon nanotube materials13 and the forma* Corresponding author. Phone: +30-210-7273835. Fax: +30 7273794. E-mail:
[email protected]. † National Hellenic Research Foundation. ‡ AIST.
tion of nanoparticles.14 Blending of polymers with carbon nanotubes is expected to substantially increase not only the mechanical strength but also the electrical and thermal conductivity of the polymeric material.15 Recently, living anionic polymerization was utilized by our group5 in order to covalently functionalize the surface of CNHs, with the diblock copolymer of polystyrene-b-polyisoprene, as well as with the polyisoprene homopolymer, following the “grafting-to” approach. However, keeping in mind that such a covalent functionalization approach incorporates a plethora of defects, in the form of sp3 hybridized carbons on the skeleton of CNHs, an alternative strategy, that complements our previously demonstrated noncovalent functionalization techniques,7,8 is the solubilization of CNHs by efficient polymer wrapping. In this context, herein, an amphiphilic block polyelectrolyte consisting of poly[sodium(sulfamate/carboxylate)isoprene and polystyrene was utilized to bring into aqueous solution the CNHs, without forming any bond with the skeleton of the carboneceous material. Moreover, the current block polyelectrolyte was carefully designed16 in such a way that the nanoensemble consisting of nanohorns and the block copolymer will be utilized as a template for the in situ formation of gold nanoparticles. Adsorption/steric interactions between CSI and CNHs may be different compared with the CSI/CNTs pair, because of the different surface topologies of CNHs (conical shaped nanostructures) and of their “dalia flower” aggregates, as compared with the elongated bundled nanotubes. Furthermore the noncovalent functionalization of CNHs by the use of an amphiphilic block copolymer and the creation of a CNH/block copolymer/Au nanoparticle complex hybrid system are reported for the first time. Experimental Section Materials. The CNHs used in this study were produced by CO2 laser ablation of graphite in the absence of any metal
10.1021/jp810640h CCC: $40.75 2009 American Chemical Society Published on Web 03/17/2009
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SCHEME 1: Idealized Graphical Illustration of (a) Noncovalent Functionalization of CNHs with the CSI Block Copolymer and (b) Templated Synthesis of Gold Nanoparticles on the Periphery of CNHs/CSI Nanoensemble
catalyst under an inert Ar atmosphere (760 Torr) at room temperature. All solvents and monomers were purchased from Aldrich and used without further purification unless otherwise stated. The synthesis of the amphiphilic block polyelectrolyte, poly[sodium (sulfamate/carboxylate)isoprene-b-styrene] (abbreviated as CSI) has been performed by the postpolymerization functionalization of a block copolymer, namely, polystyreneb-polyisoprene, PS120-b-PI1065 (degree of functionalization of the PI block is about 85% molar). The functionalization procedure has been previously described in detail.16 Synthesis of CNHs/CSI Ensemble. In a typical experimental procedure, CNHs and CSI were added in a vial with a weight proportion of 1:3, respectively. Afterward, distilled water was added, so that the final polymer concentration reached 0.15 mg/ mL, and ultrasonication was applied for 5 min. Subsequently, the obtained black solution was filtered through a PTFE Teflon membrane filter (0.1 µm pore size) and washed with large amounts of distilled water in order to remove the free polymer chains. The wet muddy material was then removed from the filter by using a spatula and with additional rinsing with water. Afterward, excessive water was removed by drying under reduced pressure and the nanomaterial was isolated as a solid. The CNHs/CSI nanoassemble could be spontaneously redispersed in water. However, a 1 min long sonication step was typically applied in order to ensure a fine and good dispertion.16 Synthesis of CNHs/CSI/Aunano. The synthesis of gold nanoparticles was achieved at the surface of the polymer functionalized nanohorns. In a typical experimental procedure, an amount of bulk gold was dissolved in concentrated solution of HCl/HNO3 (3/1 v/v) leading to an auric acid solution of 0.3 M. Subsequently, a small amount of the above solution (∼100 µL) was added in a dilute solution of CNHs/CSI (∼5 mL) and left to stand overnight for equilibration. Afterward, a catalytic amount of hydrazine was added in order to reduce the gold cations to metal nanoparticles. Instrumentation. HR-TEM measurements were carried out using a Topcon EM 002B operated at an accelerating voltage of 120 kV for imaging. EDX analysis was performed on the scanning transmission electron microscope (HITACHI HD2300) equipped with energy dispersive X-ray spectroscopy measurement system. Steady-state UV-vis electronic absorption spectra were recorded on a Perkin-Elmer (Lambda 19) UV-visNIR spectrophotometer. Mid-infrared spectra in the region 550-4000 cm-1 were obtained on a Fourier transform IR spectrometer (Equinox 55 from Bruker Optics) equipped with
a single reflection diamond ATR accessory (DuraSamp1IR II by SensIR Technologies). A drop of the solution was placed on the diamond surface, followed by evaporation of the solvent, in a stream of nitrogen, before recording the spectrum. Typically, 100 scans were acquired at 4 cm-1 resolution. Thermogravimetric analysis was performed using a TGA Q500 V20.2 Build 27 instrument by TA in an inert atmosphere of nitrogen. In a typical measurement, 1 mg of the material was placed in the sample pan, and the temperature was equilibrated at 60 °C. Subsequently, the temperature was increased to 600 °C with a rate of 10 °C/min, and the weight changes were recorded as a function of temperature. Light scattering experiments were performed on a ALV/CGS-3 Compact Goniometer System (ALV GmbH, Germany), using a JDS Uniphase 22mW He-Ne laser, operating at 632.8 nm, interfaced with a ALV-5000/EPP multitau digital correlator with 288 channels and a ALV/LSE5003 light scattering electronics unit for stepper motor drive and limit switch control. Autocorrelation functions were collected five times, and they were analyzed by the cumulants method and the CONTIN routine using the software provided by the manufacturers. Hydrodynamic radii were calculated through the Stokes-Einstein relationship. Results and Discussion In an attempt to keep intact the continuous π-electronic network of CNHs, a protocol for the noncovalent functionalization of CNHs with an amphiphilic block copolymer was for the first time employed. Briefly, a block copolymer, consisted of a hydrophobic block (polystyrene) and a polyelectrolyte block (poly[sodium (2-sulfamate-3-carboxylate) isoprene-b-styrene]), allowed to interact with CNH through the hydrophobic block, while the polyelectrolyte block is extended into the solution, stabilizing the nanosystem by electrosteric interactions, that is, both electrostatic repulsion and steric hindrance, as it is illustrated in Scheme 1a. In this manner, an ink-like aqueous solution of CNHs/CSI, stable for months, was obtained. The methodology utilized is simple, straightforward, versatile, efficient, and general in use, since it can be extended in other CNHs/block copolymer systems. Moreover, the above technique leads to rather pure materials, that is, without unbounded polymer, since the free polymer chains are removed by rinsing during filtration. The CSI block copolymer is soluble in a wide pH range17,18 which gives, in principle, the possibility to study the system
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Figure 1. Absorption spectra of CNHs/CSI (blue) and CSI polymer (black), obtained in water.
Mountrichas et al.
Figure 3. TGA results of CNHs (red), CNHs/CSI (blue), and CSI (black).
Figure 2. ATR-IR spectra of CNHs/CSI (blue) and CSI (black). Figure 4. Typical HR-TEM image of the CNHs/CSI nanoensemble.
under variable solution pH. However, the functionality of the water soluble block is changing upon changing the solution pH; that is, at low pH, the amino and carboxylate groups are protonated,17,18 and the solubilization power of the copolymer decreases, probably because of a decrease of the overall effective charge of the chains. Therefore, the studies presented here were conducted at neutral pH. The CNHs/CSI nanoensemble is characterized by a variety of complementary techniques: spectroscopic, thermal, and microscopy. The aqueous solubility of CNHs/CSI allowed us to record suitable electronic absorption spectra without any other surfactant additives. In this context, the UV-vis spectrum of CNHs/CSI is compared with the one of the net polymer CSI (i.e., in the absence of CNHs) and presented in Figure 1. Evidently, for CNHs/CSI, the characteristic absorbance of CNHs is observed,5 while CSI shows no characteristic absorptions. The attenuated-total-reflectance infrared (ATR-IR) spectra of the net polymer and of CNHs/CSI are given in Figure 2. In both spectra, the bands at 1040-1080 cm-1, 1400-1500 cm-1, and 2800-2900 cm-1 attributed to the vibration of the sulfonic, carboxylic, and C-H groups of the CSI chains, respectively, are identified. Thus, the presence of polymer was further verified in the CNHs/CSI ensemble. Moreover, the slope of the spectrum
in the CNHs/CSI is attributed to the unique structural characteristics of CNHs, and it is in agreement with what has been reported in previous works.16 Thermogravimetric analysis of the CNHs/CSI material provided additional information on the amount of the polymer adsorbed onto the CNHs. Pristine CNHs show excellent thermal stability up to 600 °C, while 70% of net CSI polymer degrades at temperatures as high as 500 °C. As it is shown in Figure 3, the CNHs/CSI ensemble undergoes a weight loss of about 40 wt % up to 500 °C due to the loss of the CSI polymer. Obviously, a considerable amount of polymer was needed for achieving the aqueous solubilization of CNHs. At this point, it is interesting to note that the degradation profile of the polymer was influenced by the presence or not of CNHs. In the presence of CNHs, the polymer decomposition starts at 170 °C and continues in the whole experimental temperature range with two distinguishable decomposition temperatures at about 265 and 380 °C. In contrast, the decomposition of the net polymer stops at 470 °C and the above-mentioned distinguishable decomposition temperatures are not observed. These differences are most likely attributed to interactions between the CNHs and the physisorbed polymer chains in the hybrid material.
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Figure 5. Light scattering intensity at 90° vs time for a dilute aqueous solution of CNHs/CSI.
Figure 7. UV-vis-NIR absorption spectra of CNHs/CSI (blue) and CNHs/CSI/Aunano (red), obtained in water.
Figure 6. Average hydrodynamic radius vs time for a dilute aqueous solution of CNHs/CSI.
Figure 8. Typical HR-TEM image of CNHs/CSI/Aunano. Gold nanoparticles loaded on the CNHs/CSI are identified as dark spots.
Morphological insight on the structure of CNHs/CSI is obtained by HR-TEM. A representative image of the nanoensemble is given in Figure 4, indicating that the unique morphological characteristics of CNHs, that is, the secondary superstructure and the conical tips, are preserved upon the noncovalent functionalization with CSI. Moreover, the average size of the CNH spherical aggregate is calculated to be about 100 nm, which is in line with that of the pristine CNHs. The aforementioned observations lead to the conclusion that the physisorbed polymer has no influence on the novel structure of CNHs, and it does not change the dimensions of the carbon nanostructure. Further information on the morphology, stability, and size of the CNHs/CSI hybrid derives from dynamic light scattering measurements. In particular, the mean light scattering intensity of CNHs/CSI was recorded with the aid of light scattering experiments as a function of time after the preparation of the sample. The time window of the study is as long as four months, and the results are presented in Figure 5. The recorded light scattering intensity is found stable during this experimental time frame, highlighting the stability of CNHs/CSI in aqueous media. In any other case, for example, if precipitation or agglomeration
had occurred, changes of the light scattering intensity would have been observed. Beyond the CNHs/CSI stability, dynamic light scattering experiments provides also information on the size of the solubilized nanoensemble. In particular, the changes of the average hydrodynamic radius (Rh) of the CNHs/CSI over the period of four months are given in Figure 6. The Rh of the hybrid is obtained by analyzing the data of the dynamic light scattering using the cumulants method. The diagram of Rh versus time shows that initially the size of the CNHs/CSI is rapidly decreased (about 10 nm) reaching a plateau, after about two weeks. The latter can be attributed to a partial desorption of some polymer chains from the surface of the CNHs. However, since individual chains may be difficult to escape from the nanoensemble, due to the hydrophobicity of the PS block, the desorption of polymeric supramolecular structures (like the supramolecular hemimicelles that have been observed in the case of carbon nanotubes16) may take place. Moreover, it is expected that this partial desorption can cause reorganization of the remaining adsorbed polymer chains thus overall decreasing the size of the ensemble. The partial desorption scenario is also supported from a second method, which was used for the
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Figure 9. EDX spectrum of CNHs/CSI/Aunano. The spectrum was obtained from the area in the red circle, presented at the right panel (negative contrast image of CNHs/CSI/Aunano nanoensemble). The elements of Cu, Fe, Al, and Si are detected because of their presence in the microscope equipment, sample holder, and crystal detector.
analysis of the dynamic light scattering data. In this context, the CONTIN analysis indicates that in some measurements at initial times, there are two different populations, one with Rh 40 nm and one with Rh 110 nm (Supporting Information Figure S1). The population with the small Rh is most likely attributed to a supramolecular structure which is formed by the desorbed polymer chains (most probably block copolymer micelles) and which could not be observed by TEM, most likely due to their low contrast. Besides the above desorption scenario, the reorganization of the polymer chains with or without desorption of polymer chains is another possible explanation of the radius decrease. However, at this point, it is highlighted that desorption/ reorganization of some polymer chains does not influence the stability of the CNHs/CSI material. Moreover, it should be emphasized that the size measured using light scattering is almost double of that observed using the HR-TEM (cf. Figure 4). This difference is rationalized by the presence of extended polyelectrolyte blocks on the surface of CNHs as a result of block copolymer adsorption. These blocks are extended into the aqueous media because of electrostatic repulsion, leading to high Rh values for the nanoensemble in solution. Presumably, these polymeric blocks could not be imaged under the HR-TEM observation due to low contrast. Therefore, the results from these two techniques are rather complementary and not contradictory. In having characterized the CNHs/CSI, the templated synthesis of gold nanoparticles at the periphery of the polymer functionalized CNHs was undertaken (Scheme 1b). The synthesis of gold nanoparticles on the surface of carbon nanotubes has been previously reported.12 However, in the current study, the presence of CNHs as a template for the formation of gold nanoparticles contributes not only to an increased purity1 of the hybrid material but also to a spherical hybrid nanostructure. The synthetic route involves, initially, the addition of auric acid in a solution of CNHs/CSI where interactions are developed between gold and protonated amine and/or carboxyl groups of the polymer chains. Subsequently, the gold ions are reduced by addition of a catalytic amount of hydrazine, leading to the formation of gold nanoparticles. At this stage, the black solution turned into red, an immediate indication of the success of the reaction. Experimental evidence for the nanoparticle formation
comes from electronic absorption spectroscopy. The spectra of the sample before and after the addition of hydrazine are given in Figure 7. A peak with maximum at 551 nm, appearing for the sample after the addition of hydrazine, is characteristic and attributed to gold nanoparticles.19 Moreover, an estimation of the nanoparticles size is possible by examining the peak characteristics, namely, position and mean high width, thus, giving rise to the calculated size of 6.6 nm. The red shift observed for the π-π* transitions in CNHs/CSI/Aunano is most likely caused by interactions between the gold nanoparticles and the CNHs. In blank experiments performed with pristine CNHs and auric acid solution, no appreciable shift for the π-π* transitions was observed (Supporting Information Figure S2). Besides absorption spectroscopy, HR-TEM was also applied for the direct imaging of the gold nanoparticles formed on CNHs/CSI. As it is shown in Figure 8, full coverage of the CNHs surface by gold nanoparticles is observed, at positions where negatively charged polymer chains physisorbed onto the CNHs should exist. Consistently with the results derived from the absorption spectroscopy, the size of the nanoparticles is calculated to be around 5 nm. The synthesis of gold nanoparticles at the surface of CNHs/ CSI is also testified by aid of energy dispersion X-ray (EDX) analysis. This is a powerful technique for the elemental investigation of carbon-rich nanoensembles. In this frame, the EDX spectrum of CNHs/CSI/Aunano is shown in Figure 9 and verifies the presence of gold, as the high electron density material. Conclusion The noncovalent functionalization of CNHs with an amphiphilic block polyelectrolyte, leading to water soluble CNHs/ CSI has been achieved. The nanoensemble is characterized by a wide gamut of techniques, spectroscopy, thermal analysis, and microscopy, which indicate not only the coexistence of polymer and carbon nanohorns in the ensemble but also the preservation of CNHs unique and novel structural characteristics. Finally, CNHs/CSI was successfully utilized as a template for the in situ synthesis of gold nanoparticles by in situ reduction of gold
Solubilization of Carbon Nanohorns ions complexed on the polymer chains. The presence of gold nanoparticles, localized at the periphery of the polymer decorated CNHs, was verified by both HR-TEM and EDX. This is the first proof-of-concept study on the use of a block copolymer for the noncovalent functionalization of CNHs and the templated growth of gold nanoparticles. The protocols followed are general and should be easily extended in other block copolymer/CNH/ nanoparticle systems. Acknowledgment. This work, made under the European Heads of Research Councils and European Science Foundation EURYI (European Young Investigator) Awards scheme was partially supported by funds from the Participating Organizations of EURYI and the EC Sixth Framework Program. Partial financial support from the EU FP7, Capacities Program, NANOHOST project (GA 201729) is also acknowledged. Supporting Information Available: Hydrodynamic radii distributions for CNH/CSI nanoensembles at different times from CONTIN analysis and UV-vis absorption spectra from blank experiment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; kokai, F.; Tkahashi, K. Chem. Phys. Lett. 1999, 309, 165. (b) Bandow, S.; Kokai, F.; Takahashi, K.; Yudasaka, M.; Qin, L. C.; Iijima, S. Chem. Phys. Lett. 2000, 321, 514. (2) Utsumi, S.; Honda, H.; Hattori, Y.; Kanoh, H.; Takahashi, K.; Sakai, H.; Abe, M.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. C 2007, 111, 5572. (3) (a) Tagmatarchis, N.; Maigne, A.; Yudasaka, M.; Iijima, S. Small 2006, 2, 494. (b) Cioffi, C.; Campidelli, S.; Brunetti, F. G.; Meneghetti, M.; Prato, M. Chem. Commun. 2006, 20, 2129. (c) Pagona, G.; Rotas, G.; Petsalakis, I. D.; Theodorakopoulos, G.; Fan, J.; Maigne, A.; Yudasaka, M.; Iijima, S; Tagmatarchis, N. J. Nanosci. Nanotechnol. 2007, 7, 3468. (4) Pagona, G.; Karousis, N.; Tagmatarchis, N. Carbon 2008, 46, 604. (5) Mountrichas, G.; Pispas, S.; Tagmatarchis, N. Chem. Eur. J. 2007, 13, 7595. (6) (a) Isobe, H.; Tanaka, T.; Maeda, R.; Noiri, E.; Solin, N.; Yudasaka, M.; Iijima, S.; Nakamura, E. Angew. Chem., Int. Ed. 2006, 45, 6676. (b) Cioffi, C.; Campidelli, S.; Sooambar, C.; Marcaccio, M.; Marcolongo, G.; Meneghetti, M.; Paolucci, D.; Paolucci, F.; Ehli, C.; Rahman, G. M. A.; Sgobba, V.; Guldi, D. M.; Prato, M. J. Am. Chem. Soc. 2007, 129, 3938.
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