Single Walled Aluminosilicate Nanotubes With Organic Modified Interiors Kang D. Y. J. Phys. Chem. C...

8/3/2019 Single Walled Aluminosilicate Nanotubes With Organic Modified Interiors Kang D. Y. J. Phys. Chem. C 2011

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Published: March 28, 2011

r 2011 American Chemical Society 7676 dx.doi.org/10.1021/jp2010919| J. Phys. Chem. C 2011, 115, 7676–

7685

ARTICLE

pubs.acs.org/JPCC

Single-Walled Aluminosilicate Nanotubes withOrganic-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

1. INTRODUCTIONSingle-walled nanotubes (SWNTs) have been considered

important “ building blocks” in the development of nanotechnol-ogy for more than a decade. Synthetic carbon SWNTs have beeninvestigated extensively for potential applications1À8 based upontheir unique dimensions and structure. The necessity of devel-oping processing routes to carbon-based SWNT nanostructuresand devices has led to intensive study of surface functionaliza-tion/modification of both the outer and inner surfaces of carbonSWNTs. While outer-surface modification is usually intendedto increase the compatibility of the nanotube with other solid-or liquid-phase materials,9À14 interior modification—especially immobilization of functional groups inside the nanotube by cova-

lent bonds—

could open up an array of new applications basedupon molecular recognition, such as molecular separation, molec-ular 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 tipsof carbon SWNTs.10,20,21 However, the covalent functionalizationof the inner surfaces of SWNTs has remained a long-standingchallenge. The inner wall of carbon SWNTs has relatively low reactivity and also suff ers from steric and transport limitations indelivering potentially reactive functional entities to the desired sitesin the carbon SWNTs.21,22

On theother hand, syntheticmetal oxide/hydroxide SWNTs23À27

can be expected to possess properties quite diff erent from carbon

nanotubes. Such SWNTs can be used to address the problem of interior functionalization because the metal oxide/hydroxide innersurfaces are more reactive and thus amenable to surface modificationthan the graphitic sheets of carbon nanotubes. More specifically,synthetic aluminosilicate SWNTs, which were first synthesizedin 197728 and thereafter well characterized regarding theirdimensions,28À35 structure,36À40 surface composition,40À43 bundlingcharacteristics,29,31,40,44,45 and formation mechanisms,31,46À50 , haveattracted 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). Thesesilanolscanpotentially befunctionalizedin a manneranalogousto the well-known techniques for functionalization of porous silicas.51À58

The capability to control the chemistry of the inner surface of thealuminosilicate SWNTs via introduction of desired functional groupscould thus have significant implications for nanotube science andengineering.

There have been several reports of the outer surface modifica-tion of single-walled aluminosilicate nanotubes.59À62 However—as in the case of carbon nanotubes—the inner wall modifica-tion has proven to be much more difficult. The direct synthesis of aluminosilicate SWNTs containing covalently attached organic

Received: February 1, 2011Revised: March 16, 2011

ABSTRACT: A methodology for modifying the interior of single-walled metal oxide (aluminosilicate) nanotubes by cova-lently immobilizing organic functional entities on the interiorsurface of the nanotube structure is reported. Characterizationof the modified nanotubes by a range of solid-state character-ization techniques—including nitrogen physisorption, thermo-

gravimetric analysis, transmission electron microscopy (TEM),powder X-ray diff raction (XRD), and solid-state NMR —strongly indicates that the organic entities are immobilized onthe inner surface of the nanotubes by reaction with the silanolgroups 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 themodified 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 modificationchemistry. This methodology of immobilizing various functional entities at the inner wall of aluminosilicate nanotubes opens up arange of previously inaccessible “molecular recognition”-based applications for nanotube materials in areas such as catalysis,molecular encapsulation, sensing, and separation.



7677 dx.doi.org/10.1021/jp2010919 | J. Phys. Chem. C 2011, 115, 7676–7685

The Journal of Physical Chemistry C ARTICLE

groups is one potential approach; however, it is only recently thatBottero et al.43 have reported a successful synthesis of SWNTscontaining 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 functionalgroups is a subject of considerable future interest. Ackerman

et al.63

reported that a silane reagent was used to modify the innersurface of the aluminosilicate SWNT, but no detailed character-ization of the resulting material was presented. Modification of the aluminosilicate SWNT interior is impeded by its high hydro-philicity at ambient conditions,40 due to its high inner surfacesilanol density (9.1 ÀOH/nm2).43 Therefore, a successful interiormodification may not be achieved without a high degree of dehydration of the SWNT samples. Our previous SWNT dehy-dration study showed that a heat treatment at 250À300 °C under vacuum, which removes the physisorbed water inside the SWNTchannels while preserving the nanotube structure, may be anoptimal pretreatment allowing for SWNT interior modification.40

In this report, we describe a general strategy for the interiormodification of aluminosilicate SWNTs, as illustrated by three

reagents: acetyl chloride, methyltrimethoxysilane, and trichlor-osilane. The modified SWNTs are assessed by a combination of solid-state techniques including nitrogen physisorption, thermo-gravimetric analysis (TGA), powder X-ray diff racton (XRD),transmission electron microscopy (TEM), solid-state NMR, and water adsorption. On the basis of these results, we demonstratethat the organic functional groups are immobilized on the inner wall of the aluminosilicate SWNT by condensation with theinner-surface silanols. Furthermore, a detailed study of wateradsorption yields an understanding of adsorption mechanisms inthe bare and modified SWNT samples and demonstrates the variation of SWNT surface hydrophilicity resulting from interiormodification. The reported methodology for modifying SWNTs with various organic groups can be broadly applied to make theSWNTs functional for a range of applications involving selectiveinteractionsof the nanotube with moleculesbasedon their shape,size, and chemical properties.

2. EXPERIMENTAL SECTION

2.1. Interior Modification of Aluminosilicate SWNT. Thesynthesis and purification of the as-synthesized SWNT sample isreported in our previous work.40 For SWNT interior modification,500 mg of as-synthesized SWNT powder was first placed in a flask connectedto a 15 mTorr vacuum line andheattreated at 250°Cfor24 h, after which it is considered fully dehydrated based upon ourprevious study. The heat-treated SWNT sample was then

transferred to a nitrogen glovebox, and ca. 5 mL of hexane solvent was added into the flask. The functionalizing reagent (acetylchloride, trimethylmethoxysilane, or trichlorosilane) was thentransferred into the flask, with the reagent to SWNT hydroxylgroup molar ratio being∼2. The mixture was allowed to stir undernitrogen for 24 h. The flask was then connected to the vacuum lineand treated at 180°C for 24 h to remove the solvent and unreacted

reagent. The resulting powder samples were used for characteriza-tion studies. The label “NT” denotes the bare SWNT, whereas“NT-A ” , “NT-M” , and “NT-T” denote the SWNT treated by acetylchloride, methyltrimethoxysilane, and trichlorosilane, respectively.

2.2. Solid-State NMR. The SWNT sample was first packedinto a 7 mm rotor. 13C, 27 Al, and 29Si MAS NMR experiments were carried out on a Bruker DSX300spectrometer at frequencies of 75.5, 78.1, and 59.6 MHz. For 13C cross-polarization (CP) MASNMR studies,the sample was spunat5 kHz,and a singlepulse of π /2and repetitiontimeof4 s was used. The sample was spunat5À6kHzfor 27 Al MAS NMR tests, for which a single pulse of π /6 and arepetition time of 0.1 s was used. For 29Si MAS NMR, direct-polarization (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 13

C,27

Al, and29Si were referenced to adamantane, aluminum trichloride, and3-(trimethylsilyl)-1-propanesulfonic acid sodium salt, respectively.

2.3. X-rayDiffraction (XRD). Powder X-raydiffraction (XRD) was performed on a PAnalytical X ’pert Pro diffractometeroperating with a Cu K R source. The high-resolution diffractiondata were collected with a diffracted-beam collimator and aproportional detector, scanning from 2 to 30° two theta with astep size of 0.05°.

2.4. Transmission Electron Microscopy (TEM). Approxi-mately 5 mg of SWNT sample was first dispersed in 10 mL of deionizedwater.Theresultingdispersion was sonicatedfor10 min. Around 5 drops of the sonicated SWNT dispersion were added on300-mesh copper grids coated with Formvar layers. Transmission

electron microscopy (TEM) images were recorded on a HitachiHF2000 field emission gun TEM operated at 200 kV.

2.5. Thermogravimetric Analysis (TGA). The experiment was performed with a Netzsch STA409 instrument. Approxi-mately 20 mg of powder sample was heated under nitrogen-diluted air from 25 to 900 °C with a ramp rate of 10 °C/min.

2.6. Nitrogen Physisorption. Nitrogen physisorption mea-surements were carried out on a Micromeritics Tristar II at 77 K.The sample was placed in an analysis tube and degassed under 15mTorr at 200 °C for 12 h before the physisorptionmeasurement.

2.7. Water Adsorption. Water adsorption measurements wereperformed on IGAsorp (Hiden Analytical, Warrington, U.K.)at 25 °C. The sample was outgassed at 200 °C for 8 h prior torecording the isotherm.

3. RESULTS AND DISCUSSION

3.1. Porosity, Structure, and Organic Loading. The nitro-gen physisorption isotherms (Figure 2) of the as-made and thethree 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 employingthe 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 BETmodel for interpreting nitrogen physisorption isotherms from

Figure 1. Structure of the as-synthesized single-walled aluminosilicatenanotube.





microporous materials (pore size <2 nm) should be used withcaution, since the concept of “monolayer adsorption” is notdefinitive when the window size is close to the size of adsorbates.Rouquerol et al.50 have assessed the applicability of the BETmodel for microporous materials and have suggested the BETmodel is valid and yields a reliable “monolayer adsorptionquantity ” if two criteria are satisfied: (1) n( P 0 À P ) shouldincrease as P / P 0 in the applicable region, and (2) the fitted linearcorrelation should have a positive intercept, where n is theadsorption quantity, P the pressure of nitrogen, and P 0 thesaturation pressure of nitrogen at 77 K. In our calculations, wechose a pressure range of 0.005 < P / P 0 <0.05 wherein the twocriteria are fulfilled. Finally, the obtained “monolayer adsorptionquantities” were converted into “surface areas” under an assump-

tion that the distances between any two adjacent adsorbednitrogen molecules are identical.) On the other hand, the t-plotmethod is well-known for differentiating mesoporosity frommicroporosity present in the same sample.66,68À70 Specifically,the linear fitting of the isotherms with HÀ J correlations71,72

allows us to estimate the external surface area (Sext) contributed by the outer surface of the SWNT and the micropore volume(V mp) due to the pore volume in the SWNT channels. Thederived t-plots are presented in the Supporting Information, andthe values of SBET , V mp , and Sext are summarized in Table 1. Forthe as-synthesized SWNT, SBET is about 42 times larger than Sext ,suggesting there is significantly larger “accessible” surface areaat the interior of the SWNT in comparison to the outer surface.

The relatively small accessible external surface area of the SWNTis likely due to the packing of SWNTs into bundles. After

treatment with the three different reagents, all the samples show substantial decreases in both V mp and the “internal” surface areaSBET À Sext , thereby providing direct evidence that most of thesurface modification reaction takes place at the interior of theSWNTs and that the introducedorganic entities are immobilizedin the SWNT channels. However, theamount of decrease in SBETÀ Sext and V mp for the three modified SWNT samples is strongly related to the molecular size of the reagent and the fractionalsilanol substitution at the SWNT’s inner surface (which canalso be considered the loading of the reagent). A quantitativeanalysis of the fractional silanol substitution is discussed later inthis report.

A deviation of Sext from bare to modified SWNTs is alsoobserved, due to the variation in SWNT bundling characteristics

between samples. In particular, the Sext for as-synthesized SWNTfromfi ve batches showsan average of 12.2 with a standarddeviationof 4.5 m2/g-Al2O3SiO2 (Supporting Information). On the otherhand, V mp and SBET of the as-made SWNT from fi ve batches show averages of 0.168 cm3/g-Al2O3SiO2 and 417 m2/g-Al2O3SiO2 withrelatively small standard deviations of 0.008 cm3/g-Al2O3SiO2 and18 m2/g-Al2O3SiO2 , respectively. Hence, the diff erences of Sext between bare and modified SWNTs listed in Table 1 are within thestatistical variation, whereas the deviations of V mp and SBET À Sextfrom bare to modified SWNTs are statistically meaningful. As aconsequence, the analysis from nitrogen physisorption measure-ments reveals that the modified SWNT samples possess signifi-cantly lower porevolumesandtotal surfaceareas (dominated by theinner surface area of nanotubular channels) than the bare SWNTs,

whereas no statistically significant deviation in the external surfaceareas is observed, hence clearly suggesting that the surface mod-ification takes place in the interior of the SWNT.

While nitrogen physisorption analysis elucidates the porosity andsurface area of the as-made and modified SWNTs, X-ray diff raction(Figure 3) gives information on the morphology and bundling of the SWNTs. XRD patterns of nanotubular materials have beentheoretically and experimentally studied in detail in previousreports.40,73À76 It is clear that the diff raction patterns of nanotubesforming small bundlesare notdominatedby Bragg diff ractionbutby X-ray scattering, as opposed to ordered porous materials with one-dimensional channels such as 1D-channel zeolites, MCM-41, orSBA-15. Previous XRD studies on single-walled carbon nanotubes

Table 1. SWNT Sample Porosity Derived from NitrogenPhysisorption Data

BET method t-plot method

sample

SBET

(m2/g-Al2O3SiO2)

V mp

(cm3/g-Al2O3SiO2)

Sext

(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

Figure 3. XRD patterns of as-synthesized and modified SWNTs.Figure 2. Nitrogen physisorption isotherms of as-synthesized and mod-ified SWNTs, where NT denotes the bare SWNT, NT-A denotes SWNTtreated by acetyl chloride, NT-M denotes SWNT treated by methyltri-methoxysilane, and NT-T denotes SWNT treated by trichlorosilane.





have elucidated thediff raction pattern for samples containingisolatednanotubes.74À76 The bundling of nanotubes leads to additional

characteristic“shoulder-like

”peaks. Comparisons of experimentaland simulated XRD patterns have provided definitive characteriza-

tions of the bundling of carbon SWNTs. Similarly, ourpreviousXRDsimulation study on the aluminosilicate SWNTs showed that thepatterns of Figure3 unequivocally represent the structure of SWNTsorganized in bundles. The as-synthesized SWNT sample shows highpurity, as reflected in the sharply defined SWNT characteristic peaks(between 3 and 19°). Similarly, the three organic-modified SWNTsamples show well-defined XRD patterns nearly identical to thatof the bare SWNT, with no evidence of structure amorphizationor alteration. The nanotubular structure and the SWNT bundl-ing characteristics therefore remain unchanged after the surfacemodification.

While XRD reveals the nanotubular structure and high bulk

purity of the bare and modified SWNTs, TEM images providelocalizedvisual information on the samples andconfirmtheXRDresults. The TEM image from the as-synthesized SWNT sample(Figure 4a) clearly shows bundles of aligned nanotubes with a ca.2 nm line-to-line distance representing the outer diameter of theSWNT. After treatment with the organic reagents, the channelsof the modified SWNT samples remain intact as shown inFigures 4bÀ4d. Although the SWNTs form dense bundles onthe TEM grid after evaporation of the solvent, one can alsooccasionally observe isolated nanotubes (as seen in Figure 4d).

Thermogravimetric analysis was employed to investigate themass losses associated with heating the SWNTs in diluted air,including losses associated with physisorbed water, surface

hydroxyl groups, and grafted organic groups. The first-derivativeTGA curves are summarized in Figure 5, and the original TGA traces are presented in the Supporting Information. For the bare

Figure 4. TEM images of as-synthesized and modified SWNT bundles. The scale bars (in black or white) represent 20 nm. The pairs of red arrowsrepresent individual SWNTs, with approximately 2 nm diameter.

Figure 5. Diff erential TGA curves of as-synthesized and modifiedSWNTs.





SWNT, two pronounced peaks, one ranging from 25 to 250 °Cand the other from 250 to 450 °C, are observed. They areassigned to the loss of physisorbed water and hydroxyl groups,respectively, following our previous work.40 An additional peak in the 450À600 °C region is observed in all three modifiedSWNTs and is assigned to the loss of the introduced organicentities. Furthermore, considering the low boiling points of thereagents used (52 °C for acetyl chloride, 102 °C for methyl-trimethoxysilane, and 32 °C for trichlorosilane), the relatively high temperature (450À600 °C) at which the organic groups arelost impliesthatthey arecovalently bondedto thesurface andnotphysisorbed on the inner surfaces of the SWNTs.

The mass loadings of physisorbed water, hydroxyl groups, andorganic groups in the bare and modified SWNTs, all normalized by the mass of dry aluminosilicate (Al2O3SiO2) at the end of TGA measurement (900°C), are summarized in Table 2. For thethree modified SWNT samples, decreases of the physisorbed water loading in comparison to the as-made SWNT suggest thatall the surface modifications yield a lower hydrophilicity in theSWNT. A more detailed hydrophilicity analysis, based on water

adsorption measurements, is presented below. Apart from thedecrease of physisorbed water loading, a decrease of the hydroxylgroup loading accompanying the organic group loading is alsoobserved in the three modified samples. This clearly shows thatthe surface silanols in SWNTs are partially substituted by thesurface modification reagents.

3.2. Surface Reaction Schemes. Upon the basis of the resultsreported above, it is likely that the reagents (acetyl chloride,methyltrimethoxysilane, and trichlorosilane) react with surface sila-nols in the SWNT interior and are therefore immobilized on thesurface. Therefore,we propose reaction schemes forthetreatmentof the SWNT with different reagents (Scheme 1) in analogy to thereported surface modifications of silicate materials by acidhalides,77,78 methoxysilanes,52,53,58,79 and chlorosilanes,80À83 respec-

tively. The surface products associated with the proposed reactionschemes are then examined by 29Si and 13C solid-state NMR.

The 29Si CP-MAS NMR spectra (Figure 6a and 6b) provide amolecular-levelcharacterization of the silicon environment in the bare andmodified SWNTs. For as-synthesizedSWNTs, the mostpronounced peak is at À79 ppm and is assigned to the uniqueQ 3(6Al) silicon framework 36,40 which is not commonly found inother aluminosilicate frameworks. The small and relatively broadpeak at À90 ppm is assigned to Q 4(6Al) and considered thecontribution from defect sites involving a small number of condensed silanols present in as-made SWNT samples.36,40

The 29Si CP-MAS spectrum from the as-prepared SWNT sampleimplies the high purity of the sample. For modified SWNT

samples, slight increases of the Q 4(6Al)/Q 3(6Al) ratio are likely due to the heat treatment at 250 °C prior to the modificationreaction40 as well as the immobilization of the introduced silaneson the SWNT inner surface, specifically for NT-M and NT-T.Moreover, the grafted acetyl group is likely to have a negligible

eff ect on Q 4

(6Al)/Q 3

(6Al) ratio since our previous study suggests that the silicon chemical shifts of the SiÀOÀC andSiÀOÀH environments are not significantly diff erent.55 ForNT-M, there are several additional peaks besides the Q 3(6Al)and Q 4(6Al) in the spectrum shown in Figure 6b. These peaksare assigned to T0 (À45 ppm), T1 (À53 ppm), T2 (À59 ppm),and T3 (À63 ppm), respectively, and these assignments are ingood agreement with previous reports.79,84 Furthermore, theagreement of the chemical shifts of the T1 , T2 , and T3 species with those reported for silane modifications on porous silicas alsosuggests that the reaction takes place on the SWNT inner surface(composed of Q 3 silicon). In contrast, a reaction on the SWNToutersurface (composed of octahedrally coordinated aluminum)

Table 2. Normalized Weight of Water/Hydroxyl Groups/Organic Groups in As-Synthesised and Modified SWNTSamples Determined by TGA

normalized mass

sample

physisorbed water

(g-H2O/g-Al2

O3SiO2 %)

hydroxyl group

(g-OH/g-Al2

O3SiO2)

organic group

(g-organic/g-

Al2O3SiO2 %)

NT 30.9 21.3 --

NT-A 15.3 19.4 12.6

NT-M 12.5 14.5 9.2

NT-T 13.3 13.6 8.2

Scheme 1. Reactions at the SWNT Interior by Various Re-agents: Acetyl Chloride, Methyltrimethoxysilane, and Tri-chlorosilane, Denoted by NT-AcCl, NT-MTMS, and NT-TClS, Respectively





would significantly alter the silicon chemical shifts of T1 , T2 , andT3 species due to the formation of SiÀOÀ Al linkages.85À88 Onthe other hand, we also assigned the peaks in the 29Si CP/MASspectrum (Figure 6b) for T0 groups present in the NT-T sample:T03 at À87 ppm, T02 at À73.8 ppm, and T01 at À64.9 ppm basedupon a previous report89 wherein it has been suggested that theT03 chemical shifts arein therangeof À70to À85ppm,T02 in therange of À50 to À68 ppm, and T01 in the range of À30 to À40ppm. However, with regard to the disagreements in the peak assignments, we performed a control experiment by modifying a well-known mesoporous silicate material (SBA-15) with trichlor-osilane (Supporting Information). The 29Si CP-MAS NMR

spectrum for this control sample shows good consistency of thechemical shifts of T0 groups with those seen in the NT-T sample.Finally, it should be cautioned that the relative peak areas in theCP-MAS spectra do not provide reliable quantitative informationon the molar ratio of diff erent T groups. A quantitative analysiscan be performed using 29Si direct polarization (DP) spectra(Supporting Information) and is discussed below.

While 29Si CP/MAS NMR probes the silicon coordination,13C CP-MAS NMR (Figure 7) is an excellent probe of theimmobilizedorganic entities.For theNT-A sample, there are twopeaks clearly assigned to C1 (the methyl group at 22.9 ppm) andC2 (the carbonyl group at 180.8 ppm), thereby strongly support-ing the proposed scheme for acetyl chloride immobilizedon the SWNT inner surface. On the other hand, the two peaks

observed in the spectrum of NT-M are assigned to the C 3 (themethyl group at À4.5 ppm) and C4 (the methoxy group at 48.8ppm),79 in good agreement with the proposed scheme formodification by methyltrimethoxysilane. 27 Al NMR (Figure 8)spectroscopy was then used to investigate the possible alterationof the SWNT outer surface during surface modification. Thepeak at 4 ppm for the as-made SWNT is assigned to octahedralaluminum.38,40,90,91 The 27 Al spectra remain unchanged, and noadditional peaks are observed in the three modified samples. Thisimplies that the nanotube wall remains intact during surfacemodification, since previous studies suggest that any partial de-gradation of the outer wall of the nanotube is accompanied by theappearance of tetrahedral or pentacoordinated aluminum.38,40

No significant shift of the octahedral aluminum peak is seen,suggesting that the silane reagents do not modify the SWNT outersurface.

3.3. Water Adsorption. The water adsorption isothermsmeasured at 25°C are summarized in Figure9, with the adsorptionquantity normalized by the amount of dry aluminosilicate

(Al2O3SiO2) as obtained from the TGA results (Table 2). Aftersurface modification by the three reagents, the water uptakecapacity of the SWNTs decreases substantially to about 60À75%of the bare SWNT capacity, suggesting that the modified samples become more hydrophobic. However, a decrease of water capacity in the modified SWNT can also be rationalized by a lower pore volume (verified by nitrogen physisorption) as well as the variationof surface hydrophilicity after modifications. A mechanistic modelis necessary to gain physical insight into the water adsorptionisotherms in SWNTs. Upon the basis of Grand Canonical MonteCarlo (GCMC) simulation results (Figure 10a) of water adsorp-tion in bare SWNTs (carried out in a manner identical to ourprevious study),40,92,93 it is clear that the water molecules can form

Figure 6. (a) 29Si CP/MAS NMR spectra of SWNT samples. (b) A detailed view of 29Si CP/MAS NMR spectra for NT-M and NT-T.

Figure 7. 13C CP/MAS NMR spectra of NT-A and NT-M.





multiple layers inside the SWNT, beginning from a monolayer atlowchemicalpotential. On thebasis of ourprevious work, it is clearthat the first layer of water molecules is primarily formed by hydrogen bonding between water molecules and surface silanolgroups, whereas the subsequently adsorbed water layers form by hydrogen bonding between water molecules. Therefore, we pro-pose a model for water adsorption in modified SWNTs asillustrated in Figure 10b. The proposed mechanism includes twophenomena: (1) formation of the first adsorbed water layer, withthe water molecules only hydrogen bonding on the sites at which

the surface silanols have not been substituted by organic groups;and (2) a subsequent water layer forms adjacent to the first waterlayer by hydrogen bonding between two adjacent water molecules.The BET model,64,65,94 which captures multilayer adsorptionphenomena, can be used to model the water adsorption mecha-nism in the SWNT

P = P 0

nð1 À P = P 0Þ¼

ðC À 1Þ

nmC ð P = P 0Þ þ

1

nmC ð1Þ

where P is the pressure of water vapor; P 0 is the saturated water vapor pressure at a given temperature; n is the adsorption quantity (g-water/g-Al2O3SiO2); nm is the monolayer coverage (g-water/g- Al2O3SiO2); and C is the ratio of the equilibrium constants for the

monolayer and subsequent multilayer adsorption. Usually, theconstant C is several orders of magnitude larger than unity 64,94

for N2 adsorption at 77 K as well as in our water adsorption fittingresults. Hence eq 1 can be simplified to

P = P 0

nð1 À P = P 0Þ¼

1

nmð P = P 0Þ þ

1

nmC ð2Þ

Basedoneq2,aplotof( P / P 0)/(n(1À( P / P 0)) vs ( P / P 0) is the well-known BET plot for multilayer adsorption phenomena and isapplicable in the moderate relative pressure region. The applicablepressure region for the BET plot is well-defined for nitrogen

physisorptionat 77K (0.05 < P / P 0< 0.35for mesoporous materials64and P / P 0 < 0.05 for microporous materials95). However, somereports have suggested that the BET plot for water adsorption can be applied in therelativepressurerange of0.05 < P / P 0 <0.5.96À99 Wechose data inthe range 0.1 < P / P 0 < 0.35, wherein the four BET plotsshow high linearity, to fit eq 2 (Figure 11). The fitted linearcorrelations all have positive intercepts, implying that it is feasible toapply the BET model in the assumed pressure region.67,68,95 Thefitted slope of the BET equation gives the monolayer water coveragenm , and these values are summarized in Table 3. A decrease of nm between bare and modified SWNTs clearly suggests that a certainfraction of silanols in the SWNT interior are substituted duringsurface modificationand are hence unavailable for monolayer adsorp-tion of water. Furthermore, the introduced reagents create hydro-phobic regions in the SWNT. These two factors are responsible fora lower water uptake capacity of the modified SWNTs in boththe intermediate and high-pressure regions. In contrast, the low-pressure region shows negligible differences since the Henry ’sconstant for initial water adsorption on available silanol sitesremains essentially unaffected.

3.4. Fractional Silanol Substitution. In this section, weestimate the fractional surface silanol substitution after interiormodifications by three introduced reagents, from the results of the different characterization techniques including nitrogenphysisorption, TGA, 29Si NMR, and water adsorption. Thefractional silanol substitution is physically equivalent to thesurface coverage of the introduced organic entities on the inner

Figure 8. 27 Al NMR of as-synthesized and modified SWNT samples.

Figure 9. Water adsorption isotherms at 25 °C of as-synthesized andmodified SWNT samples.

Figure 10. (a) Simulated bare SWNTÀ water models with diff erent water loading obtained by grand canonical Monte Carlo (GCMC)

simulations.(b) Proposed wateradsorption mechanisms in the channelsof bare and modified SWNTs.





wall of the SWNT. The results are summarized in Table 4, andthe methodology is described as follows. The estimate of fractional silanol substitution from the nitrogen physisorptiondata is based upon the assumption that the lower micropore volume of modified SWNTs is due to the introduced organicentities in SWNT channels. Two approaches are then possible to

interpret the micropore volume decreases in terms of the surfacecoverage of organic groups. The first approach assumes that themolar density of the organic groups embedded inside the SWNTchannels is unchanged from its value in a pure liquid phase at25 °C. The mass of the organic groups in the SWNT can then becalculated. This quantity can finally be converted into the molarloading (mol organic group/g-Al2O3SiO2) using an averagemolecular weight for the different immobilized types of groups(for NT-M and NT-T, based upon the molar ratio between Tgroups derived from 29Si DP spectra as presented in theSupporting Information). In the second approach, one canestimate the sizes of the organic groups grafted in the modifiedSWNTs (see Supporting Information) and directly interpret the

pore volume decrease for modified SWNTs in terms of thesurface coverage. The surface coverage can also be derived fromTGA data. We can utilize the organic loading from TGA data(Table 2) to approximate the SWNT inner surface coverage.This approach converts the gravimetric data into a molar loading by using an average molecular weight derived from the NMR results (similar to the approach used for the nitrogen physisorp-tion data in thecase of NT-M and NT-T). Finally, thedecrease of monolayer water coverage (summarized in Table 4) in modifiedSWNTs can be considered equivalent to the decrease of interiorsurface silanol loading due to the modifications and hence allowscalculation of the organic loading. Table 4 shows the results of the estimates from the four different methods. It is clear that25À35% of the interior silanols have been substituted for NT-A,24À38% for NT-M, and 25À30% for NT-T. It is hypothesizedthat the surface reaction in the SWNT interior may ultimately belimited by the slow diffusion of the organic entities due to partial blockage of the SWNT pores by already immobilized organicgroups. This aspect requires further study to reach a conclusivecharacterization. Considering the large stoichiometric excess(8:1) between the surface modification reagents and the silanolgroups on the inner surface of SWNTs, we find that less than 5%of the reagent is successfully immobilized in the NT-A, NT-M,and NT-T materials.

4. CONCLUSIONS We have developed a general method for functionalizing the

inner surface of single-walled aluminosilicate nanotube materials with organic reagents (as illustrated in Figure 12) and presentedthe first unambiguous and comprehensive characterization toreveal the occurrence, extent, and structural details of the inner-surface functionalization in suchmodified SWNTs. Furthermore,a comprehensive investigation of the resulting solids usingnitrogen physisorption, powder X-ray diff raction (XRD), trans-mission electron microscopy (TEM), thermogravimetric analysis(TGA), 29Si and 13C solid-state NMR, and water adsorptionprovides a detailed understanding of the porosity, structure, andsurface chemistry of the functionalized nanotubes. In particular,

Figure 11. BET plots derived from water adsorption isotherms for as-synthesized and modified SWNT samples.

Table 3. Monolayer Coverage Fitted from Water Adsorption

BET Plots

sample nm (g-H2O/g-Al2O3SiO2)

NT 0.174

NT-A 0.118

NT-M 0.107

NT-T 0.131

Figure 12. Illustration of SWNT modified by various reagents.

Table 4. Fractional Silanol Substitution in Bare and ModifiedSWNTs

fractional silanol substitution (# of silanols being substituted in

modified SWNT/ # of silanols in bare SWNT)

sample TGA

N2 physisorption

(liquid density)

N2 physisorption

(molecular size) water adsorption

NT-A 0.35 0.25 0.34 0.32

NT-M 0.33 0.24 0.37 0.38

NT-T 0.26 0.28 0.30 0.25





SWNTs modified with three reagents preserve their nano-tube structure, and the variation in size and type of reagentsallows for the control of the pore volume of the SWNT. We havealso demonstrated that diff erent types of organic groups, includ-ing alkyl, alkoxy, or carbonyl groups, can be immobilized at theSWNT inner surface. Water adsorption in the SWNTs isinterpreted by the BET model to elucidate the adsorption

mechanism in both bare and modifi

ed SWNTs. As an exampleof the potential applications, we show that the functionalizationapproach enables us to control the surface hydrophilicity as wellas the water uptake of the SWNT. Finally, estimates of fractionalsurface silanol substitution for the three modified SWNTs areachieved via diff erent characterization techniques, and consistentresults are obtained. The present study provides a clear basisfor addressing the challenging problem of adding organic func-tionalities to the interiors of SWNT materials and thereby greatly expands their potential applications. For example, by introducing appropriate functional groups, the SWNT can become an excellent candidate for size- and shape-selectivecatalytic reactions, sensing, molecular encapsulation, and molec-ular separations.

’ASSOCIATED CONTENT

bS Supporting Information. Supporting data on the or-ganic-modified nanotube materials, including XRD patterns,TGA traces, NMR spectra, details of nitrogen physisorption dataanalysis, and details regarding the estimates of the size of theimmobilized groups. This material is available free of charge viathe Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]; [email protected].

’ACKNOWLEDGMENT

This work was supported by ConocoPhillips Company. Theauthors also acknowledge Dr. J. Leisen (Georgia Tech), Dr. K. C.McCarley (ConocoPhillips), and Prof. M. Tsapatsis (U. Min-nesota) for useful discussions.

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