Hydrogen-bond acidic functionalized carbon nanotubes (CNTs) with covalently-bound hexafluoroisopropanol groups

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Hydrogen-bond acidic functionalized carbon nanotubes (CNTs) with covalently-bound hexafluoroisopropanol groups Leonard S. Fifield *, Jay W. Grate Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA

A R T I C L E I N F O

A B S T R A C T

Article history:

Hydrogen-bond acidic fluoroalcohol groups are directly attached to the backbone of

Received 9 September 2009

single walled carbon nanotubes (SWCNTs) via carbon–carbon bonds without the

Accepted 11 February 2010

introduction of intermediate heteroatoms. Hexafluoroisopropanol functional groups are

Available online 15 February 2010

exceptionally strong hydrogen-bond acids, and are added to the nanotube surface (via the substituted benzene para position) as 2-phenyl-1,1,1,3,3,3-hexafluoro-2-propanol (i.e., –(p-C6H4)C(CF3)2OH) using the aryl diazonium approach to create hydrogen-bond acidic carbon nanotube (CNT) surfaces. These groups can promote strong hydrogen-bonding interactions with matrix materials in composites or with molecular species to be adsorptively concentrated and sensed. In the latter case, this newly developed material could potentially find useful application in chemical sensors and in CNT-based preconcentrator devices for the detection of hydrogen-bond basic analytes such as chemical warfare agents and explosives, as has been demonstrated for fluoroalcohol-containing polymers. Ó 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) have attracted great interest for their unique properties and the vast number of potential CNT applications [1]. The use of CNTs in polymeric composites is of considerable interest [2], where the nanotubes must mix with and interact with the composite matrix material. In the field of chemical detection, CNTs may function as selective sorbents, nanoscale architectures providing support for selective functionality, or as components of transducers [3–6]. The hexafluoroisopropanol group (–C(CF3)2OH) has previously been incorporated as a substituent on polystyrene, polyacrylate, polysiloxane, and polycarbosilane polymers, as well as on various dendrimers and hyperbranched polymers [7]. In early studies, it was shown that these groups promoted compatibility of polymer blends by setting up interactions between the hydrogen-bond acidic polymer and other polymers such as polycarbonates and polymethacrylates [8]. Specifically, the hydrogen atom of the hydrogen-bond acidic hydro-

xyl groups of the hexafluoroisopropanol-modified polymer would interact by hydrogen-bonding to hydrogen-bond basic heteroatoms (typically oxygen atoms as carbonyl groups), acting as Lewis bases, on the other polymer in the blend. In this example and throughout this paper, acidity and basicity will always refer to hydrogen-bonding interactions, and not to proton transfer (Bronsted) reactions. The hexafluoroisopropanol group (–C(CF3)2OH) was subsequently incorporated into polymer materials for chemical detection applications, which have recently been comprehensively reviewed [7]. Fluorinated alcohols and fluorinated phenols were identified as favorable hydrogen-bond acidic functional groups to promote sensing polymer interactions with hydrogen-bond basic analytes (explosives, chemical warfare agents, and toxic industrial solvents that are Lewis bases) via hydrogen-bonding interactions, leading to applications in detecting chemical warfare agents and explosives. Such materials have been used as selective layers on transducers including surface acoustic wave (SAW), chemiresistor,

* Corresponding author: Tel.: +1 509 375 6424; fax: +1 509 375 2186. E-mail address: [email protected] (L.S. Fifield). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.02.019

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chemicapacitor, and optical devices. They have also been useful for adsorptive sampling and preconcentration applications. Thus far there are limited reports on the use of fluorinated hydrogen-bond acidic groups in conjunction with CNTs as sensing materials. In one such report, Snow and coworkers impart chemoselectivity to a CNT network chemicapacitor by coating the network with a layer of fluoroalcohol-containing organosilyl polymer (termed ‘‘HC’’) and, alternatively, by functionalizing the silica surface that supports the nanotube network [9]. In another report, the Swager group used noncovalent modification of single walled carbon nanotubes (SWCNTs) with hexafluoroisopropanol-substituted polythiophene to achieve chemoselectivity in a CNT chemiresistor [10]. Ideally, the functional groups in such applications should be at the nanotube surface, and should only be found at the nanotube surface, so that analyte molecules are selectively concentrated at the sensitive surface and not wasted by sorption at other locations or far from the nanotube. For example, the fringing fields of a nanotube network chemicapacitor are strongest closest to the nanotube [9]. We are particularly interested in approaches for the incorporation of functional groups into materials where the chemistry used to bond the group to the material is ‘‘selectivity-neutral’’. By this, we mean that the chemistry does not introduce additional heteroatoms into the material other than those that are part of the selective functional group. Thus, neither the bond-forming reaction that incorporates the functional group, nor the reactions that form the material (e.g. polymerization bond-forming reactions) should add heteroatoms that impart a significant potential for interactions that may be different from the interactions being established by the incorporated functional group itself. This would reduce the selectivity of the material [7]. When fluorinated hydrogenbond acidic functional groups are part of a material, it is desirable to avoid hydrogen-bond basic groups in the same material, because they can have two negative effects: first, they set up interactions with hydrogen-bond acidic adsorbates, reducing the intended selectivity for hydrogen-bond basic adsorbates (where the adsorbate is an analyte in a chemical sensing application). Second, the presence of both hydrogen-bond basic and hydrogen-bond acidic groups in the material may lead to self association, reducing the availability of free hydrogen-bond acidic functional groups for interaction with adsorbates [7]. The influence of hydrogen-bond basic heteroatoms that arise from bond-forming reactions used to functionalize carbon-based materials has been seen previously with fullerenes. These materials will react readily with organic amines to add organic groups via a nitrogen linker atom [11]. Li and Swanson produced such materials for use on SAW sensors and determined vapor uptake for a range of organic vapors [12]. When Grate and coworkers compared these data with vapor uptake by a variety of nonpolar carbon-based materials lacking hydrogen-bond basic heteroatoms, such as nonpolar polymers and graphite, a general correlation was observed between adsorption on the fullerene layer on the SAW device (as indicated by sensor response), and adsorption of the same set of vapors on graphite [13]. However, the results for meth-

anol were an outlier: it was noted that the fullerene was much more sensitive to the hydrogen-bond acidic methanol vapor than would be expected compared to these other nonpolar materials. This was interpreted as evidence for a hydrogenbonding interaction between the nitrogen linker atoms of the fullerene layer and the hydrogen atoms of the methanol vapor hydroxyl groups, leading to an altered pattern of selectivity for the nitrogen modified fullerene material compared to an unmodified nonpolar carbon material. Accordingly, if hydrogen-bond acidic groups were to be attached to the surfaces of CNT materials for sorptive applications, a ‘‘selectivity-neutral’’ approach would dictate that the bonding chemistry should not introduce hydrogen-bond basic heteroatoms. In this regard, the oxidation of CNTs to produce carboxylate groups as an preliminary step to attach selective functional groups, a frequently used pathway to CNT covalent modification [14], will introduce hydrogen-bond basic groups to confound the selectivity [7] of the material. While carboxylic acids are typically thought of as acids in the sense of Bronsted proton transfer reactions, the oxygen atoms of this functional group are (Lewis) hydrogen-bond bases; organic carboxylic acids are well-known to self associate by hydrogen-bonding. Hence, using reactions with surface-bound carboxylic acids to functionalize a CNT surface will necessarily leave a hydrogen-bond basic surface. Similarly, a proximal silica layer [9], even when functionalized, should not be considered selectivity-neutral, since the oxygen atoms of silica surfaces and interfaces are hydrogen-bond basic. In this paper, we describe the synthesis of a CNT material with a fluorinated alcohol hydrogen-bond acidic functionality (–(p-C6H4)C(CF3)2OH) that does not involve an intermediate polymer layer, silica layer, carboxylate groups or other heteroatoms. For this purpose we selected the in situ generated diazonium-based CNT chemistry developed by the Tour group of Rice University [15]. This chemistry reacts an aniline derivative in the presence of isoamyl nitrite with CNT surfaces, releasing the nitrogen from the aniline as part of a nitrogen gas molecule, and linking the phenyl group directly to the CNT surface via a covalent carbon–carbon bond (Fig. 1). Non-fluorinated phenols [16] and benzyl alcohols [17] have previously been added to CNTs using this method. Here we extend this chemistry to include hydrogen-bond acidic functionality by describing the synthesis and characterization of fluoroalcohol-modified SWCNTs via direct covalent bonding of fluoroalcohol-substituted phenyl groups onto the CNT surface.

Fig. 1 – Schematic of in situ generated diazonium CNT modification route.

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Experimental

To synthesize hexafluoroisopropanol containing CNTs, 0.181 mol of 2-(4-aminophenyl)-1,1,1,3,3,3-hexafluoro-2-propanol) (722-92-9) (Acros Organics) was dissolved into 40 ml N,N-dimethylacetamide (DMAc) (127-19-5) (Sigma–Aldrich). Ultrasonication (Branson 250 Sonifier) was used to disperse 0.8 g (0.06 mol C) of purified (9 wt.% residual iron catalyst) HiPco SWCNTs (Carbon Nanotechnologies Inc.) in 60 ml DMAc. The solutions were combined and diluted with 70 ml additional solvent. Following addition of 0.293 mol of isoamyl nitrite (110-46-3) (TCI America), the mixture was heated in an oil bath at 60 °C for 24 h. The functionalized CNTs were recovered by multiple centrifuge/rinse steps using water and 2-propanol. To confirm covalent (chemical) functionalization of the CNTs [18] the visible absorption spectra of CNTs before and after reaction were measured using an Ultraviolet–visible (UV–vis) Recording Spectrophotometer (Shimadzu UV2401PC). Spectra were taken within 1 h following dispersion of 1 mg of dry CNTs (or modified CNTs) in 10 ml DMAc through ultrasonication, as in the reaction procedure above. Concentrations of CNTs (or modified CNTs) in DMAc were adjusted to form suspensions with absorption on scale for comparison of the spectral feature shapes of modified and unmodified CNTs. Thermogravimetric analysis (Netzsch TG 449 C Thermomicrobalance) of the sample was performed following an established protocol for determination of CNT functional group loading [19]. Briefly, 10 mg of sample was heated in argon according to the following temperature profile: the sample was ramped at 5 °C/min to 140 °C, held for 1 h, ramped at 5 °C/min to 470 °C and held for 2.5 h. Functional content of the sample was established by elemental analysis (by Desert Analytics, Tucson, AZ) (C 64.94 wt.%, N 1.64 wt.%, and F 13.50 wt.%); the fluorine content was determined using ion chromatography.

Fig. 2 – UV–vis spectra of functionalized (solid) and unmodified (dashed) SWCNTs showing decrease in structure of the absorption curve. Inset: HBA–CNTs are much more easily dispersed (right) than unmodified SWCNTs (left) in N,N-dimethylacetamide. Photograph shows dispersion contrast after overnight settling.

3.

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Results and discussion

The hydrogen-bond acidic carbon nanotubes, dubbed HBA–CNTs, were prepared according to the method in Fig. 1 as described in the Section 2. A stark contrast in solubility (dispersibility) between the unmodified SWCNTs and the HBA–CNTs is observed when DMAc solutions are allowed to settle overnight after sonication (Fig. 2 inset). The difference in absorbance between solutions (suspensions) of unmodified SWCNT and HBA–CNT in DMAc is shown in Fig. 2. The significant loss of van Hove singularities in the spectrum of the modified nanotubes confirms that the functional groups have been covalently attached to the nanotube surface [18]. Fluoride content in the HBA–CNT was found to be 13.50 wt.%. This corresponds to approximately 29 wt.% functional groups (–(p-C6H4)C(CF3)2OH) added to the SWCNT, or one for every 50 carbon atoms. Correcting for the 9 wt.% of residual iron catalyst in the SWCNT, a functional group loading of 32 wt.% versus C is obtained, or 1 functional group for every 44 carbon atoms. Thermal gravimetric analysis (TGA), using a previously established protocol for determination of functional group loading on SWCNTs [19], revealed a 27% observed mass loss with heating corresponding to one functional group for every 55 carbon atoms in the material (Fig. 3). Correction for residual iron catalyst leads to a calculated loading of approximately one functional group for every 48 carbon atoms, in good agreement with the estimate calculated from elemental analysis. In conclusion, a fluoroalcohol functionality (–(p-C6H4)C(CF3)2OH) was bonded directly to the surface of CNTs without intermediate heteroatoms or polymers. The values for the degree of functionalization, as determined by two independent methods, were in good agreement. Previously, the functionalization of materials with the hexafluoroisopropanol group has been primarily demonstrated with polymeric materials. As the aryl diazonium reaction has been demonstrated on carbon, metallic and semiconducting materials [20,21], this paper illustrates a general way to obtain hexafluoroisopropanol functionality on any of these materi-

Fig. 3 – Mass loss of unmodified (dashed) SWCNTs and modified (solid) SWCNTs with heating in argon. Temperature profile: ramp to 140 °C (5 °C/min), hold 1 h, ramp to 470 °C (5 °C/min), hold 2.5 h.

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als, in a hydrogen-bonding selectivity-neutral way, using SWCNTs as the prototypical example. Since polymers modified with hydrogen-bond acidic groups have been found to be useful in adsorptive sensing and preconcentration applications [7], and in polymer blends [8], it is anticipated that CNTs directly modified with hydrogen-bond acidic groups could be similarly useful.

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Acknowledgements A portion of this research was carried out in the William R. Wiley Environmental Molecular Sciences Laboratory, a US DOE scientific user facility operated for the DOE by the Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the US Department of Energy by Battelle Memorial Institute. The Laboratory Directed Research and Development Program supported this research.

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