ARTICLE pubs.acs.org/JPCC
Single-Walled Aluminosilicate Nanotubes with Organic-Modified Interiors Dun-Yen Kang, Ji Zang, Christopher W. Jones,* and Sankar Nair* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, United States
bS Supporting Information ABSTRACT: A methodology for modifying the interior of single-walled metal oxide (aluminosilicate) nanotubes by covalently immobilizing organic functional entities on the interior surface of the nanotube structure is reported. Characterization of the modified nanotubes by a range of solid-state characterization techniques—including nitrogen physisorption, thermogravimetric analysis, transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and solid-state NMR— strongly indicates that the organic entities are immobilized on the inner surface of the nanotubes by reaction with the silanol groups on the interior wall. The resulting organic-modified single-walled nanotubes (SWNTs) show higher hydrophobicity than bare nanotubes based upon water adsorption measurements. Furthermore, a mechanistic understanding of water adsorption in the modified SWNTs is developed, by interpretation of the water adsorption data with a multilayer adsorption model. The degree of interior surface silanol substitution is estimated, with up to 35% of the silanols being substituted through the present modification chemistry. This methodology of immobilizing various functional entities at the inner wall of aluminosilicate nanotubes opens up a range of previously inaccessible “molecular recognition”-based applications for nanotube materials in areas such as catalysis, molecular encapsulation, sensing, and separation.
1. INTRODUCTION Single-walled nanotubes (SWNTs) have been considered important “building blocks” in the development of nanotechnology for more than a decade. Synthetic carbon SWNTs have been investigated extensively for potential applications18 based upon their unique dimensions and structure. The necessity of developing processing routes to carbon-based SWNT nanostructures and devices has led to intensive study of surface functionalization/modification of both the outer and inner surfaces of carbon SWNTs. While outer-surface modification is usually intended to increase the compatibility of the nanotube with other solidor liquid-phase materials,914 interior modification—especially immobilization of functional groups inside the nanotube by covalent bonds—could open up an array of new applications based upon molecular recognition, such as molecular separation, molecular storage, catalysis, and drug delivery. The infiltration of lipids,15 metals,16,17 and C60 beads18,19 into carbon SWNT channels has been reported. Functional entities have also been grafted at the tips of carbon SWNTs.10,20,21 However, the covalent functionalization of the inner surfaces of SWNTs has remained a long-standing challenge. The inner wall of carbon SWNTs has relatively low reactivity and also suffers from steric and transport limitations in delivering potentially reactive functional entities to the desired sites in the carbon SWNTs.21,22 On the other hand, synthetic metal oxide/hydroxide SWNTs2327 can be expected to possess properties quite different from carbon r 2011 American Chemical Society
nanotubes. Such SWNTs can be used to address the problem of interior functionalization because the metal oxide/hydroxide inner surfaces are more reactive and thus amenable to surface modification than the graphitic sheets of carbon nanotubes. More specifically, synthetic aluminosilicate SWNTs, which were first synthesized in 197728 and thereafter well characterized regarding their dimensions,2835 structure,3640 surface composition,4043 bundling characteristics,29,31,40,44,45 and formation mechanisms,31,4650, have attracted substantial interest in recent years. This SWNT consists of an aluminum(III) hydroxide sheet on the outer surface and is lined with pendant silanol groups on the inner surface (Figure 1). These silanols can potentially be functionalized in a manner analogous to the well-known techniques for functionalization of porous silicas.5158 The capability to control the chemistry of the inner surface of the aluminosilicate SWNTs via introduction of desired functional groups could thus have significant implications for nanotube science and engineering. There have been several reports of the outer surface modification of single-walled aluminosilicate nanotubes.5962 However —as in the case of carbon nanotubes—the inner wall modification has proven to be much more difficult. The direct synthesis of aluminosilicate SWNTs containing covalently attached organic Received: February 1, 2011 Revised: March 16, 2011 Published: March 28, 2011 7676
dx.doi.org/10.1021/jp2010919 | J. Phys. Chem. C 2011, 115, 7676–7685
The Journal of Physical Chemistry C
Figure 1. Structure of the as-synthesized single-walled aluminosilicate nanotube.
groups is one potential approach; however, it is only recently that Bottero et al.43 have reported a successful synthesis of SWNTs containing methyl groups on their inner walls via introduction of methylsiloxane precursors in the reactant solution. The efficacy of this method for introduction of more complex organic functional groups is a subject of considerable future interest. Ackerman et al.63 reported that a silane reagent was used to modify the inner surface of the aluminosilicate SWNT, but no detailed characterization of the resulting material was presented. Modification of the aluminosilicate SWNT interior is impeded by its high hydrophilicity at ambient conditions,40 due to its high inner surface silanol density (9.1 OH/nm2).43 Therefore, a successful interior modification may not be achieved without a high degree of dehydration of the SWNT samples. Our previous SWNT dehydration study showed that a heat treatment at 250300 °C under vacuum, which removes the physisorbed water inside the SWNT channels while preserving the nanotube structure, may be an optimal pretreatment allowing for SWNT interior modification.40 In this report, we describe a general strategy for the interior modification of aluminosilicate SWNTs, as illustrated by three reagents: acetyl chloride, methyltrimethoxysilane, and trichlorosilane. The modified SWNTs are assessed by a combination of solid-state techniques including nitrogen physisorption, thermogravimetric analysis (TGA), powder X-ray diffracton (XRD), transmission electron microscopy (TEM), solid-state NMR, and water adsorption. On the basis of these results, we demonstrate that the organic functional groups are immobilized on the inner wall of the aluminosilicate SWNT by condensation with the inner-surface silanols. Furthermore, a detailed study of water adsorption yields an understanding of adsorption mechanisms in the bare and modified SWNT samples and demonstrates the variation of SWNT surface hydrophilicity resulting from interior modification. The reported methodology for modifying SWNTs with various organic groups can be broadly applied to make the SWNTs functional for a range of applications involving selective interactions of the nanotube with molecules based on their shape, size, and chemical properties.
2. EXPERIMENTAL SECTION 2.1. Interior Modification of Aluminosilicate SWNT. The synthesis and purification of the as-synthesized SWNT sample is reported in our previous work.40 For SWNT interior modification, 500 mg of as-synthesized SWNT powder was first placed in a flask connected to a 15 mTorr vacuum line and heat treated at 250 °C for 24 h, after which it is considered fully dehydrated based upon our previous study. The heat-treated SWNT sample was then
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transferred to a nitrogen glovebox, and ca. 5 mL of hexane solvent was added into the flask. The functionalizing reagent (acetyl chloride, trimethylmethoxysilane, or trichlorosilane) was then transferred into the flask, with the reagent to SWNT hydroxyl group molar ratio being ∼2. The mixture was allowed to stir under nitrogen for 24 h. The flask was then connected to the vacuum line and treated at 180 °C for 24 h to remove the solvent and unreacted reagent. The resulting powder samples were used for characterization studies. The label “NT” denotes the bare SWNT, whereas “NT-A”, “NT-M”, and “NT-T” denote the SWNT treated by acetyl chloride, methyltrimethoxysilane, and trichlorosilane, respectively. 2.2. Solid-State NMR. The SWNT sample was first packed into a 7 mm rotor. 13C, 27Al, and 29Si MAS NMR experiments were carried out on a Bruker DSX 300 spectrometer at frequencies of 75.5, 78.1, and 59.6 MHz. For 13C cross-polarization (CP) MAS NMR studies, the sample was spun at 5 kHz, and a single pulse of π/2 and repetition time of 4 s was used. The sample was spun at 56 kHz for 27Al MAS NMR tests, for which a single pulse of π/6 and a repetition time of 0.1 s was used. For 29Si MAS NMR, directpolarization (DP) and cross-polarization (CP) tests were performed with repetition times of 10 and 5 s, respectively, at π/2 single pulse and 5 kHz spinning rate. The chemical shifts of 13C, 27Al, and 29 Si were referenced to adamantane, aluminum trichloride, and 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt, respectively. 2.3. X-ray Diffraction (XRD). Powder X-ray diffraction (XRD) was performed on a PAnalytical X’pert Pro diffractometer operating with a Cu KR source. The high-resolution diffraction data were collected with a diffracted-beam collimator and a proportional detector, scanning from 2 to 30° two theta with a step size of 0.05°. 2.4. Transmission Electron Microscopy (TEM). Approximately 5 mg of SWNT sample was first dispersed in 10 mL of deionized water. The resulting dispersion was sonicated for 10 min. Around 5 drops of the sonicated SWNT dispersion were added on 300-mesh copper grids coated with Formvar layers. Transmission electron microscopy (TEM) images were recorded on a Hitachi HF2000 field emission gun TEM operated at 200 kV. 2.5. Thermogravimetric Analysis (TGA). The experiment was performed with a Netzsch STA409 instrument. Approximately 20 mg of powder sample was heated under nitrogendiluted air from 25 to 900 °C with a ramp rate of 10 °C/min. 2.6. Nitrogen Physisorption. Nitrogen physisorption measurements were carried out on a Micromeritics Tristar II at 77 K. The sample was placed in an analysis tube and degassed under 15 mTorr at 200 °C for 12 h before the physisorption measurement. 2.7. Water Adsorption. Water adsorption measurements were performed on IGAsorp (Hiden Analytical, Warrington, U.K.) at 25 °C. The sample was outgassed at 200 °C for 8 h prior to recording the isotherm.
3. RESULTS AND DISCUSSION 3.1. Porosity, Structure, and Organic Loading. The nitrogen physisorption isotherms (Figure 2) of the as-made and the three modified SWNT samples all show the characteristics of IUPAC type I isotherms,64 suggesting that the pore channels of the modified SWNT samples are microporous, as expected. More detailed information can then be extracted by employing the BET model65 and t-plot method66 to these isotherm data. The BET model yields the total surface area (SBET), contributed by both interior and outer surfaces of the SWNT.67 (The BET model for interpreting nitrogen physisorption isotherms from 7677
dx.doi.org/10.1021/jp2010919 |J. Phys. Chem. C 2011, 115, 7676–7685
The Journal of Physical Chemistry C
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Figure 2. Nitrogen physisorption isotherms of as-synthesized and modified SWNTs, where NT denotes the bare SWNT, NT-A denotes SWNT treated by acetyl chloride, NT-M denotes SWNT treated by methyltrimethoxysilane, and NT-T denotes SWNT treated by trichlorosilane.
Table 1. SWNT Sample Porosity Derived from Nitrogen Physisorption Data BET method
sample
t-plot method
SBET
Vmp
Sext
(m2/g-Al2O3SiO2)
(cm3/g-Al2O3SiO2)
(m2/g-Al2O3SiO2)
NT
418
0.17
10.1
NT-A
256
0.11
15.9
NT-M
153
0.06
11.3
NT-T
260
0.11
14.0
microporous materials (pore size
