LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Synthesis of Carbon Nanotube-Inorganic Hybrid Nanocomposites: An Instructional Experiment in Nanomaterials Chemistry Miguel de Dios,† Veronica Salgueirino,† Moises Perez-Lorenzo,# and Miguel A. Correa-Duarte*,# †
Department of Applied Physics and #Department of Physical Chemistry, University of Vigo, 36310 Vigo, Spain
bS Supporting Information ABSTRACT: An experiment is described to introduce advanced undergraduate students to an exciting area of nanotechnology that incorporates nanoparticles onto carbon nanotubes to produce systems that have valuable technological applications. The synthesis of such material has been easily achieved through a simple three-step procedure. Students explore cutting-edge research and gain knowledge in nanoscience. KEYWORDS: Graduate Education/Research, Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Colloids, Materials Science, Nanotechnology, Surface Science, UV vis Spectroscopy
anotechnology is a rapidly developing field that seeks to understand, control, and exploit new physical properties that arise from systems at length scales between atoms and bulk materials. In this regard, the experiment here reported is meant to be a resource that allows students from any chemistry background to understand some of the foundations and exciting advances in the area of nanoscience. Applications of nanotechnology, which are already emerging, are highly interdisciplinary and include virtually all fields in engineering and the natural sciences. Therefore, this project also constitutes an effort to stimulate faculty collaboration in program and curriculum development. The work presented here is aimed at the development of an integrated research education activity on carbon nanotubes (CNTs). In addition to their interesting one-dimensional morphology, CNTs exhibit interesting structure-dependent properties in the field of electronics, mechanics, optics, and magnetism.1 Consequently, CNTs have applications as catalysts, sensors, semiconductor devices, data storage and processing equipment, or reinforced nanofiber materials.2 This interdisciplinary relevance confirms CNTs as promising components in nanocomposite architectures. An experiment, based on previous research,3 is described that introduces nanotechnology and materials science to advanced undergraduate students. The CNT-based nanocomposites are prepared through the synthesis and assembly of Pt nanoparticles (NPs) onto surface-primed CNTs.3,4 This method allows the controlled deposition of NPs regardless of their size or shape, keeping the CNT properties unchanged. This is performed by a noncovalent functionalization of the nanotubes consisting of a combination of polymer wrapping and a layerby-layer technique.5 The resulting nanocomposite is characterized by transmission and scanning electron microscopy. Considering both synthesis and characterization protocols, this experiment may be included in advanced chemistry courses. Thus,
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Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
students become familiar with concepts such as self-assembly as well as several synthetic strategies. The goal of this activity is the application of a reliable procedure for preparing CNTs with controlled electronic properties as well as providing training for undergraduate students to explore cutting-edge research and gain advanced knowledge in nanoscience.
’ EXPERIMENT DESCRIPTION CNT Polyelectrolyte Functionalization (CNT PAH)
Multiwalled carbon nanotubes, simplified referred to as CNTs here (Nanolab, 5 15 μm length and 10 15 nm diameter, 95% purity), were dispersed in ultrapure water (18 MΩ cm 1) by following a published procedure.5 CNTs were dispersed in an aqueous solution containing 1 wt % polyallylamine hydrochloride (PAH) and 0.5 M NaCl aqueous solution. The pH of the solution was adjusted to 9.5 to favor the PAH wrapping around the nanotubes (Figure 1B). A combination of rapid stirring (1 h) and sonication (3 h in ultrasonic bath) was used to ensure the dispersion of individual nanotubes in the solution. The excess of PAH was removed by three cycles: centrifugation, removal of the supernatant, and redispersion in ultrapure water, each one spinning at 9000 rpm for 12 h. The CNT PAHs were finally redispersed in pure water by gently stirring and brief sonication. Synthesis of Spherical Platinum Nanoparticles (Pt-NPs)
Spherical platinum nanoparticles (Pt-NPs) were synthesized as follows: sodium borohydride (NaBH4, 2.45 mL, 0.015 M) was added as reducing agent under rapid stirring to a solution containing 43 mL of ultrapure water, 2.5 mL of trisodium citrate dihydrate (Na3C6H5O7 3 2H2O, 0.1 M), and 2.5 mL of chloroplatinic Published: October 31, 2011 280
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Figure 2. UV visible spectra of an aqueous solution of H2PtCl6 (1 10 3 M) and sodium citrate (2 10 3 M) before (presence of [PtCl6]2 ions) and after the addition of NaBH4 (complete reduction of Pt4+ ions).
Given their fiber-like structure, they may even become airborne agents and reach the lungs. Figure 1. Illustration of the synthetic process comprising the polymer (PAH) wrapping of CNTs and the electrostatic self-assembly of the presynthesized Pt-NPs. For clarity, only the outer wall of the multiwalled carbon nanotube is depicted in the figure.
acid hexahydrate (H2PtCl6 3 6H2O, 0.05M) (Na3C6H5O7/H2PtCl4/NaBH4 in a 2:1:0.3 molar ratio). The citrate ions are used as stabilizing agents of the Pt-NPs. The solution was stirred for 10 min (Figure 1A). The formation of metallic Pt nanoparticles with sizes ranging roughly between 2 and 5 nm (characterized by a transparent brown solution) was followed using a UV vis spectrophotometer. The absorption spectrum of the pale yellow H2PtCl6 aqueous solution showed a sharp absorption band that disappeared once all the platinum ions (Pt4+) were reduced to metallic Pt. NPs Deposition onto CNTs (CNT PAH/Pt)
CNT PAH (0.6 mL, 0.5 mg/mL) was added to 50 mL of PtNPs solution (0.5 mM) (Figure 1). After 30 min, the solution was centrifuged (10 min, 8000 rpm) and redispersed in 20 mL of pure water to remove the excess of nonadsorbed nanoparticles.
’ EQUIPMENT The nanostructures were characterized and analyzed using a scanning electron microscope (SEM-FEG; JEOL, JSM-6700F) and a transmission electron microscope (TEM; JEOL, JEM1010). UV vis spectra were performed with an Agilent 8453 UV vis diode array spectrophotometer. ’ HAZARDS Polyallylamine hydrochloride, trisodium citrate dihydrate, chloroplatinic acid hexahydrate, and sodium borohydride may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Fume hoods and appropriate clothing, gloves, and eye protection should be used at all steps of this experiment. Same precautions must be adopted with multiwalled CNTs to prevent potential adverse health effects.
’ RESULTS AND DISCUSSION The experimental procedure is summarized in Figure 1. The CNTs are functionalized by wrapping them with a positively charged polyelectrolyte (PAH) that is used as molecular glue for the electrostatic attachment of the negatively charged Pt-NPs onto the outer surface of the CNTs. The negative charge of the Pt-NPs results from the citrate ions that stabilized the nanoparticles and confer them with a net negative charge. The formation of Pt-NPs consists of different steps that can be followed by UV vis spectroscopy (Figure 2). The H2PtCl6 solution is pale yellow before adding NaBH4, showing a peak located at 260 nm due to the ligand-to-metal charge-transfer transition of the [PtCl6]2‑ ions. After the addition of NaBH4, the color of the solution becomes yellow-brown and then dark gray. The peak at 260 nm in the UV vis spectrum disappears, suggesting that all [PtCl6]2‑ ions are reduced. This hydrosol has absorption in all ranges of the UV vis spectrum. This absorption increases as the wavelength decreases. This is in good agreement with the results of Teranishi et al.6 Such tendency in the UV vis spectra was also reported by Duff et al.7 and points to the formation of the Pt-NPs. The so-called polymer-wrapping technique is a noncovalent functionalization that, in contrast to defect-side and covalentside-wall functionalization, prevents the disruption of the nanotubes keeping intact the characteristic sp2 structure conjugation and, therefore, the CNT electronic structure.8 This polymerwrapping technique relies on the thermodynamic preference of CNT polymer interactions over CNT water interactions, whereas the second stage (attachment of the Pt-NPs) is based on electrostatic and van der Waals attractions between the negatively charged NPs and the positively charged PAH-functionalized surface of the CNTs.9 A simple way to confirm the successful functionalization of CNTs is to ensure that nanotube solutions are visually nonscattering, and no precipitation occurs upon prolonged standing. Given the poor solubility of CNTs in aqueous solvents, an inefficient wrapping of the polyion chains around the nanotube would be manifested as solution turbidity that would be easily 281
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together with their numerous scientific and industrial possibilities, render this procedure reliable and operationally convenient for students to start research activities related to nanoscience and nanotechnology. Thus, the variety of pedagogical outcomes attained by this module can be summarized as • Synthesis of NPs. The simple synthetic procedure reported for the Pt-NPs production is significant because it helps the student to become familiar with common wet-chemistry methods. • Functionalization of CNTs. The polymer-wrapping strategy as a noncovalent approach serves to introduce the challenge of nanotube functionalization in solution. This procedure allows the students to assess the advantages of this technique to achieve well-dispersed CNTs in aqueous solution as well as a high density charge per unit area onto the outer surface of the CNT. It is also critical for the students to be aware of the importance of this functionalization so as to exploit CNTs in technological applications. • Synthesis of CNT/NP nanocomposites. Through this practical example, the students become familiar with the synthesis of one-dimensional carbon nanotube-based inorganic hybrid nanostructures. • Scanning and Transmission Electron Microscopy (SEM and TEM) and Ultraviolet Visible Spectrometry (UV vis). In addition to the theoretical concepts related with these characterization techniques, the students get the opportunity to gain hands-on experience with adequate sample preparation and equipment operation. If suitable electron microscopy facilities are not available, collaboration with other departments with such capabilities or local industries possessing SEM and TEM instrumentation should be considered. Other option may be to send representative samples to laboratories that feature remote characterization. In any case, the students become familiar with the day-today work within a research group.
Figure 3. Digital photographs of the polymer-functionalized CNT in aqueous solution at different concentrations: (a) 0.12 mg/mL; (b) 0.06 mg/mL; (c) 0.03 mg/mL; (d) 0.02 mg/mL; and (e) 0.01 mg/mL.
Figure 4. (a) SEM image of CNT PAH; (b) TEM image of CNT PAH; (c) SEM image of CNT PAH/Pt; and (d) TEM image CNT PAH/Pt.
discerned by the naked eye. At longer times, aggregates formation would be also observed. The deep-black color shown on the initial CNT PAH solutions (Figure 3, sample a) makes difficult to verify the absence or presence of turbidity. Therefore, successive dilutions were made to assess whether these dispersions were optically transparent. As shown in Figure 3 (samples b e), no signs of turbidity or coagulation are observed, and most importantly, no precipitation is detected over months. Accordingly, it can be assumed that the followed experimental procedure yields stable and well-dispersed CNT PAH composites. Platinum-covered CNT PAHs (CNT PAH/Pt) with different Pt loadings can be obtained by controlling both CNT/Pt concentration ratio and deposition time. CNT PAH/Pt nanocomposites were prepared starting with ca. 3 nm spherical Pt-NPs accomplishing homogeneous coatings over the complete surface of the nanotubes (Figure 4). It is interesting to note that the spherical morphology of the metallic NPs appears as preserved onto the CNTs surface once the process is finished (Figure 4D).
’ SUMMARY Among the different synthetic strategies that favor an adequate production of carbon nanotube-based hybrid nanostructures, the experiment reported herein represents a remarkable practical example. In this regard, the recent interest in this novel materials
’ ASSOCIATED CONTENT
bS
Supporting Information Instructions for the students, including postlab questions; notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT V.S. and M.P.-L. acknowledge the financial support from the Ramon y Cajal Program (Ministerio de Ciencia e Innovacion, Spain) and Isidro Parga Pondal Program (Xunta de Galicia, Spain). This work has been also supported by Xunta de Galicia (INCITE09209101PR, INCITE08PXIB209007PR, 2008/077 and 2010/78 (Emerxentes)) and EU (METACHEM, grant number CP-FP 228762-2). ’ REFERENCES (1) (a) Wildoer, J. W. G.; Venema, L. C.; Rinzier, A. G.; Smalley, R. E.; Dekker, C. Nature (London, U.K.) 1998, 391, 59–62. (b) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. Nature (London, U.K.) 1998, 282
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