Photochemical Hydroboration− Oxidation of Single-Walled Carbon

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Photochemical Hydroboration-Oxidation of Single-Walled Carbon Nanotubes Mark D. Ellison,* Lisa K. Buckley, Greg G. Lewis, Claire E. Smith, Ewa M. Siedlecka, Catherine V. Palchak, and Jaime M. Malarchik Department of Chemistry, Ursinus College, CollegeVille, PennsylVania 19426 ReceiVed: June 19, 2009; ReVised Manuscript ReceiVed: August 27, 2009

The products of photochemical hydroboration-oxidation of single-walled carbon nanotubes (SWCNTs) with a variety of borane complexes have been analyzed using FTIR, UV/vis/NIR, and Raman spectroscopies. Analysis of reaction intermediates suggests that the reaction proceeds by attaching H and BH2 groups, similar to the hydroboration of an alkene. Attempts to hydroborate SWCNTs at temperatures from 0 to 100 °C yielded no functionalization, suggesting that photochemical activation is necessary. The use of optical filters indicates that light in the wavelength range of 250-300 nm is necessary to hydroborate SWCNTs. Treatment with hydrogen peroxide in basic solution resulted in the attachment of OH groups to the hydroborated SWCNTs. UV/vis/NIR spectra of the resulting hydroxylated SWCNTs are similar to those of pristine nanotubes, indicating that the functionalization is not extensive and that the SWCNTs remain largely intact after the functionalization. This reaction process could lead to new pathways of functionalizing SWCNTs. Introduction Single-walled carbon nanotubes (SWCNTs) show great promise for a wide variety of applications, owing to their high mechanical strength and unique electrical properties.1,2 Many applications will require functionalization of SWCNTs for solution processing or orderly arrangement. Numerous covalent functionalization strategies have been reported, including carboxylation,3-5 fluorination,6-10 diazonium reactions,11-17 a free-radical reaction,18 a dichlorocarbene reaction,19 1,3-dipolar additions,20-24 a Birch reduction,25 a nitrile imine reaction,26 and a nitrene reaction.27 In general, these reactions enhance the solubility of SWCNTs, and some attached functional groups28-33 can even alter the electrical properties of the nanotubes. Indeed, covalent functionalization shows strong promise for altering the electrical properties of nanotubes.30,31,33-39 A common feature of these functionalization methods is that, because of the aromatic stability of SWCNTs, they all utilize highly reactive chemicals. One set of highly reactive compounds whose chemistry with SWCNTs has not been studied is boranes. Diborane gas can be used in the production of SWCNTs to yield boron-doped SWCNTs,40 but to our knowledge, no reports of the reactions of boranes with SWCNTs have been published. Boranes react readily with organic compounds, leading to a variety of products depending on the organic compound.41 Borane itself (BH3) is such a strongly reactive molecule that it must be complexed with a Lewis base such as tetrahydrofuran (THF) or an amine to be used as a laboratory chemical. Borane complexes are used for hydroboration-oxidation of CdC double bonds, and they are strong reducing agents, reducing carboxylic acids, ketones, and aldehydes to hydroxyl groups.42 This rich chemistry between boranes and organic chemicals suggests that borane complexes might offer a means of chemically functionalizing SWCNTs. For these reasons, we decided to undertake the study of the chemical reactions of borane complexes with SWCNTs. A number of borane complexes were studied, including BH3-THF, BH3-triethylamine, BH3-tert-butylamine, and * Corresponding author. E-mail: [email protected].

BH3-dimethyl sulfide. As noted earlier, we are not aware of any published reports of reactions between borane complexes and SWCNTs. However, one theoretical study of hydroboration of SWCNTs has been performed. Using density functional theory (DFT) calculations, Long et al. calculated that hydroboration of an SWCNT would be “thermoneutral” with an activation barrier of about 10 kcal/mol.43 This suggests that such a reaction would be reversible and the intermediate would be unstable. However, as will be discussed later, our own DFT calculations yielded somewhat different results. Our initial experimental results suggested that, under thermal conditions (0-100 °C) reaction did not take place, so we utilized a mercury lamp to provide energy in an attempt to drive the reaction. Few studies have been performed on the photochemistry of SWCNTs. Approaches for photochemically functionalizing nanotubes include the use of azide photochemistry to lead to DNA attachment to multiwalled carbon nanotubes44 and the photoinduced attachment of benzhydrol to SWCNTs.45 More recently, Tour et al. have demonstrated the photochemical functionalization of surfactant-dispersed SWCNTs with water.46 In that study, the selective functionalization of semiconductor SWCNTs with OH groups initiated by irradiation with 254-nm light was reported. SWCNTs have a strong absorbance in the UV region near 260 nm, characteristic of π f π* transitions of extended aromatic systems.47 We postulated that exciting this transition could promote electrons to π* orbitals in the SWCNTs, making the SWCNTs more reactive toward the borane complexes. For these reasons, we decided to study the photochemical reactions of borane complexes with SWCNTs. Experimental Section SWCNTs produced by the HiPco method (Carbon Nanotechnologies, Inc.) were purified by heating in water-saturated air followed by sonication in 6 M HCl, washing with deionized (DI) water, and drying.3,4 The manufacturer specified a purity of about 80% by mass, with the major impurity being iron. Spectroscopic analysis for iron in the filtrate indicated that the purification process removed about 15% of the prepurification mass as iron, which would leave about 5% iron by mass

10.1021/jp905805k CCC: $40.75  2009 American Chemical Society Published on Web 10/05/2009

Photochemical Hydroboration-Oxidation remaining in the samples. The Raman spectrum of purified SWCNTs (see Figure 6a below) shows a radial breathing mode at 260 cm-1 and G band at 1582 cm-1, both characteristic of nanotubes,48 and does not show a strong D band, which would be indicative of functionalization of the SWCNTs or nanoscale carbonaceous impurities in the sample. Finally, the treatment with HCl can result in hole carrier injection, which could result in altered reactivity of the SWCNTs. A second consequence of hole carrier injection is a strong reduction in the Raman scattering in the purified SWCNTs.3 Figure 6 shows that the Raman scattering of the purified SWCNTs is comparable in intensity to that of the functionalized SWCNTs, indicating that hole carrier injection from the HCl treatment does not occur to a significant extent. UV/vis/NIR spectra (see Figure 7 below) of the purified SWCNTs show peaks from the van Hove transitions, in agreement with the literature,3 indicating that the purification did not destroy the nanotubes. BH3-THF (1.0 M BH3 in THF), BH3-triethylamine (97%), BH3-tert-butylamine (97%), BH3-dimethyl sulfide (10.0 M BH3 in dimethyl sulfide), NaOH (97%), and 30% H2O2 solution (ACS Reagent) were purchased from Sigma-Aldrich and used as received. All reactions were performed in oven-dried glassware under a nitrogen atmosphere. B2H6 (Voltaix, Inc., 5% in nitrogen) was used as received. In this work, reactions that did not involve a mercury lamp are termed thermal reactions, and those that involved a mercury lamp are called photochemical reactions. The thermal reactions involved only attempts at a hydroboration reaction. The photochemical reactions involved a hydroboration reaction, sometimes followed by an oxidation reaction. As the initial group of experiments, the thermal experiments were performed in a nitrogen glovebox. The mercury lamp apparatus would not fit in the glovebox, so the photochemical experiments were performed in a reaction vessel under a continuous nitrogen gas purge. For typical reaction conditions, about 10 mg of purified SWCNTs (0.8 mmol C) were placed in an oven-dried roundbottom flask or quartz or Pyrex photochemical reaction vessel. The vessel was purged with nitrogen, and sufficient borane complex was added so that the mole ratio of BH3 to C was 2:1. Because BH3-tert-butylamine is a solid, it was dissolved in a minimal amount of triethylamine before being added to the reaction vessel. The reactants were mixed and stirred for times ranging from 3 to 120 h. As noted earlier, the first attempts at reaction were at temperatures between 0 and 100 °C. All borane complexes were reacted with SWCNTs at room temperature, and some were reacted at lowered or elevated temperatures. Specifically, BH3-THF was reacted with SWCNTs at room temperature and at 0 °C. The 0 °C reaction was carried out on a thermoelectric cooler to avoid the presence of water vapor from an ice bath. BH3-THF was not heated because, at 55 °C, diborane gas will evolve from the complex. BH3-triethylamine was reacted with SWCNTs at room temperature. It was also refluxed neat and in toluene at 95 °C. BH3-tert-butylamine was reacted at room temperature and refluxed in triethylamine at 95 °C. Finally, BH3-dimethyl sulfide was reacted at room temperature. The photochemical reactions were carried out in a mercury lamp photochemical reaction system (ACE Glass). The entire apparatus was housed in a ventilated, light-tight box. A 450-W mercury lamp was housed in a water-cooled quartz shroud, which kept the temperature in the reaction vessel at room temperature, thereby ensuring that any change in reactivity was caused by the light emitted from the lamp and not from any

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18537 temperature changes. The reaction vessels were either quartz, which transmits all radiation above ∼200 nm, or Pyrex, which transmits all radiation above ∼300 nm. (See Results.) The reaction vessels were dried in an oven prior to use, reactants were added and stirred with a stir bar, and the vessel was kept under a constant nitrogen purge. Additionally, several photochemical experiments were carried out to determine whether a wavelength dependence for the functionalization exists. In these experiments, the mercury lamp was wrapped in black paperboard and aluminum foil having a hole that snugly fit a 2.54-cm diameter, 2.50-mm-thick filter. Three filters (Edmund Optics) were purchased for these experiments. The first, U330, transmits light from the UV to the edge of the visible range, or from about 250 to 400 nm. The second, U360, transmits light from the mid-UV to the visible range, or from about 300 to 400 nm. The third, B440, transmits light from the near edge of the visible range to the middle of that range, or from about 380 to 500 nm. These three filters cover the UV to the visible in a nearly mutually exclusive manner, allowing us to ascertain if one particular wavelength region is responsible for exciting the reactants and leading to reaction. For the hydroboration reactions, after the designated reaction time, the reaction mixture was centrifuged at 4000 rpm, and the remaining borane complex was decanted. The nanotubes were washed with methanol and centrifuged, and the methanol was decanted. This methanol washing was performed twice. Then, the nanotubes were washed with diethyl ether and centrifuged, and the diethyl ether was decanted. This diethyl ether washing was performed twice. Finally, the nanotubes were dried in an oven at 60 °C for 2 h. For the hydroboration-oxidation reaction, after the reaction mixture had been centrifuged and the supernatant had been decanted, 1.0 M NaOH and 30% H2O2 were added to the nanotubes and allowed to react for 8-10 h. The amounts of NaOH and H2O2 were such that the mole ratio of NaOH to H2O2 to B was 0.33:1.2:1, based on the previously reported mole ratios for organic hydroboration-oxidation.49 Functionalization was checked using FTIR, Raman, and UV/ vis/NIR spectroscopies. Infrared spectra were obtained by placing the sample on a Ge crystal in a single-bounce attenuated total reflection (ATR) accessory (Pike Technologies MIRacle) of a ThermoNicolet 6700 FTIR spectrometer with a liquidnitrogen-cooled HgCdTe detector. Background scans were obtained with just the Ge crystal, and the spectrometer computed the ratio of the sample scan to the background scan. Typically, 1000 scans were averaged, and the resolution was 8 cm-1. UV/ vis/NIR spectra were collected using a Perkin-Elmer Lambda 35 UV/vis/NIR spectrometer and dispersing the SWCNTs in dimethyl formamide (DMF). Raman spectra were obtained using an Ocean Optics R-3000 Raman spectrometer with an excitation wavelength of 785 nm. Results Figure 1a shows a typical FTIR spectrum of purified SWCNTs. No peaks are evident, indicating that any functional groups present are at levels below the detection threshold of the FTIR spectrometer. Figure 1b shows a typical FTIR spectrum of purified SWCNTs mixed with BH3-THF at room temperature for 24 h. The IR spectrum of the SWCNTs that “reacted” with BH3-THF is not noticeably different from that of the purified nanotubes, indicating that mixing SWCNTs with BH3-THF does not lead to reaction. Extending the reaction time to 120 h did not result in any observable peaks in the IR spectrum. The FTIR spectra of SWCNTs mixed individually

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Figure 1. FTIR spectra of (a) purified SWCNTs and (b) SWCNTs reacted with BH3-THF at room temperature for 24 h.

with BH3-triethylamine, BH3-tert-butylamine, and BH3-dimethyl sulfide, for reaction times up to 120 h, all appeared quite similar to Figure 1b, indicating that no appreciable reaction takes place between these complexes and SWCNTs. Borane complexes, particularly BH3-THF, react with water to form boric acid, which would reduce the amount of BH3-THF available for reaction, if water were present in our reaction flask. Although the reaction was carried out under a nitrogen atmosphere in a glovebox, it was important to eliminate this possibility. To slow down this possible side reaction, we mixed BH3-THF and SWCNTs at 0 °C. The FTIR spectrum of the SWCNTs after 120 h had elapsed was quite similar to Figure 1b, indicating that reaction between these species does not take place at 0 °C. Finally, SWCNTs were refluxed in BH3-triethylamine at 95 °C, in BH3-triethylamine in toluene at 95 °C, and in BH3-tert-butylamine in triethylamine at 95 °C. In all of these experiments, the FTIR spectra of the SWCNTs were similar to Figure 1b. These experiments fairly conclusively demonstrate that SWCNTs do not react with these borane complexes even at elevated temperatures. Taken as a whole, the thermal reaction experiments suggest that borane complexes do not react with SWCNTs in the temperature range 0-95 °C. Further evidence for the infeasibility of the thermal reactions is provided by ab initio calculations. We performed DFT energy calculations for the hydroboration of an SWCNT segment by BH3. The calculations used a B3LYP density functional and the 6-311+G* basis set. The full results are the subject of a forthcoming article,50 but in general, they indicate that the hydroboration reaction is not energetically favorable, being endoergic with ∆Erxn ≈ 100 kJ/mol. One additional thermal reaction was investigated. Purified SWCNTs were placed in a glass tube in a gas line evacuated to about 100 mTorr and then exposed to B2H6 gas for 1 to 6 h. FTIR spectra did not show evidence for any reaction, suggesting that SWCNTs do not react with B2H6 at room temperature. The results of the thermal experiments and the computational chemistry suggest that the reaction might need to be driven by an energy source. Therefore, we utilized a mercury lamp to provide energy to spur the reaction. The FTIR spectrum of SWCNTs that had been mixed with BH3-THF in a quartz reaction vessel and irradiated with light from a mercury lamp for 3 h is shown in Figure 2a. Peaks just below 3000 cm-1 are consistent with C-H stretching motions, and peaks between 2200 and 2400 cm-1 are indicative of B-H stretches.51 These peaks suggest that a hydroboration reaction occurred, generating

Ellison et al.

Figure 2. FTIR spectrum of SWCNTs reacted with BH3-THF under mercury lamp irradiation for 3 h (a) in a quartz vessel and (b) in a Pyrex vessel.

Figure 3. Nanotube section with attached BH2 and H groups, showing a structure consistent with vibrations observed in the FTIR spectrum in Figure 2a.

a product consistent with the IR spectrum shown in Figure 3. In this reaction, BH2 and H groups added to adjacent carbon atoms. These groups would disrupt the aromaticity of the ring to which they added, making the carbon atoms to which they are bonded sp3-hybridized. This is consistent with the sp3hybridized C-H stretches observed below 3000 cm-1 in Figure 2a. Because these results suggested that a photochemical reaction was taking place, we decided to investigate the wavelength dependence of the reaction. Next, the SWCNTs and BH3-THF were mixed in a Pyrex reaction vessel and irradiated with light from the mercury lamp for 3 h. Figure 2b shows the FTIR spectrum of the SWCNTs after this process. The spectrum does not indicate that any reaction has occurred. The Pyrex reaction vessel absorbs light below about 300 nm (see Supporting Information), so this result strongly indicates that light below 300 nm is necessary for the reaction to occur. SWCNTs exhibit a well-known maximum absorbance near 260 nm. Together, these results suggest that light below 300 nm excites the SWCNTs, leading to reaction. The Supporting Information includes the spectrum of the mercury lamp, as measured from inside the quartz reaction vessel in the photochemical apparatus using a fiber-optic spectrometer (Ocean Optics USB4000). Shown are the unfiltered light and light filtered by the U330, U360, and B440 filters.

Photochemical Hydroboration-Oxidation

Figure 4. FTIR spectra of SWCNTs reacted with BH3-THF for 6 h under mercury lamp irradiation with light passing through (a) filter U330 (b) filter U360, and (c) filter B440.

These measurements were not meant to quantitatively establish the light intensity at the reactants’ location but rather to demonstrate the wavelengths of light that reach the reactants under various conditions. Figure 4 shows the FTIR spectra of SWCNTs stirred in BH3-THF and irradiated by the mercury lamp through the optical filters for 6 h. Figure 4a, of SWCNTs from the reaction mixture exposed to light filtered by the U330 filter, is similar to Figure 2a, showing evidence of a hydroboration reaction. Spectra b and c of Figure 4, of SWCNTs from the reaction mixture exposed to light filtered by the U360 and B440 filters, respectively, show no peaks, indicating no reaction. Taken with the results shown in Figure 2, these data strongly indicate that UV light in the range of 250-300 nm is responsible for driving the hydroboration reaction. BH3-THF was the only borane complex to appear to undergo this reaction. All of the other borane complexes investigated did not undergo a photochemical hydroboration reaction. To further establish that a hydroboration reaction had occurred, the hydroborated SWCNTs were treated with hydrogen peroxide in a basic solution to undergo an oxidation step. In organic chemistry, the hydroboration of a double bond followed by treatment with hydrogen peroxide results in H and OH groups being added across the double bond.42 The photochemical hydroboration reaction on SWCNTs suggested that the BH2 group could be displaced by an OH group. Therefore, SWCNTs were mixed with BH3-THF in the quartz reaction vessel and irradiated with light from the mercury lamp for 6 h. Afterward, the excess BH3-THF was decanted, and the SWCNTs were reacted with a basic hydrogen peroxide solution for 8-10 h. A FTIR spectrum of these SWCNTs is shown in Figure 5. This spectrum exhibits a broad O-H stretch peak centered at 3360 cm-1, suggesting that OH groups were added to the SWCNTs. C-H stretch peaks for sp3-hybridized carbon are present below 3000 cm-1, indicating that the H groups from the hydroboration reaction remain after the oxidation reaction. Other peaks in the spectrum are also consistent with the presence of H and OH groups. Peaks at 1090 and 1260 cm-1 are C-O stretches,52 and the broad peak at 1400 cm-1 indicates a C-H bend.53,54 The small peak at 1725 cm-1 indicates a CdO stretch from a carbonyl group, suggesting that the hydrogen peroxide likely also produced a small number of carboxylic acid groups on the SWCNTs. Finally, a small peak remains at 2370 cm-1, suggesting that most, but not all, of the BH2 groups were replaced with OH groups. A Raman spectrum of the hydroborated-oxidized SWCNTs is shown in Figure 6b. The presence of the radial breathing mode

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Figure 5. FTIR spectrum of hydroborated-oxidized SWCNTs.

Figure 6. Raman spectrum of (a) purified SWCNTs and (b) hydroborated-oxidized SWCNTs.

peak at 257 cm-1 and the G band at 1581 cm-1 confirms that the SWCNTs have not been destroyed during the reaction. The peak above 800 cm-1 is from the glass substrate used to support the SWCNTs in the Raman experiments. The RBM mode frequencies of the purified SWCNTs and the hydroboratedoxidized SWCNTs agree within the resolution of the instrument, 4 cm-1, so it is not possible to use a change in RBM frequency to suggest functionalization. The presence of the D band at 1284 cm-1 indicates some functionalization of the SWCNTs.13,15,48 The ratio of the peak areas of the D band to the G band can provide a rough representation of the degree of functionalization of the SWCNTs. For the purified SWCNTs, the peak area ratio is 0.026. For the hydroborated-oxidized SWCNTs, this ratio is 0.12, indicating a small to moderate amount of functionalization, but much less than for other highly functionalized nanotubes.15 Finally, a UV/vis/NIR spectrum of the hydroborated-oxidized SWCNTs is shown in Figure 7b. Van Hove peaks are evident in this spectrum, again confirming that the SWCNTs are present and not destroyed in the reaction process. Discussion The FTIR, Raman, and UV/vis/NIR spectra strongly suggest that a photochemical hydroboration reaction takes place when SWCNTs are mixed with BH3-THF and irradiated with light from a mercury lamp. Treatment of these hydoborated SWCNTs with a basic hydrogen peroxide solution results in SWCNTs that are functionalized with H and OH groups. The overall reaction process is illustrated in Scheme 1. The vibrations of the H and OH groups are consistent with the FTIR spectrum in Figure 5.

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Figure 7. UV/vis/NIR spectrum of (a) purified SWCNTs and (b) hydroborated-oxidized SWCNTs.

SCHEME 1: Overall Process of Photochemical Hydroboration-Oxidation of SWCNTs

The IR absorptions of the B-H stretches in the hydroborated SWCNTs fall in a region that overlaps the vibrations of CO2. Therefore, it is important to consider whether these peaks could be caused by CO2. The vibrational frequencies of CO2 molecules physisorbed on and trapped in interstitial spaces in SWCNT bundles have been extensively studied.55-57 In those studies, the CO2 peaks range in frequency from 2320 to 2360 cm-1. In our FTIR spectra, such as that in Figure 2a, the peaks are as high as 2390 cm-1 and as low as 2265 cm-1 in frequency. Thus, although the peaks observed in the spectra of the hydroborated SWCNTs span the range of previously observed CO2 peaks, because we observe peaks at frequencies well above and below any that have ever been observed for CO2 in SWCNTs, we attribute the peaks to B-H stretches. Of course, because of the overlap, we cannot rule out the possibility that the reaction conditions cause some CO2 to be trapped in the SWCNT bundles. Nevertheless, the oxidation that follows hydroboration of the SWCNTs provides strong evidence that the peaks in the range of 2200-2400 cm-1 are primarily B-H stretches. Treatment of SWCNTs with a basic hydrogen peroxide solution could itself oxidize the SWCNTs. Kataura et al. achieved the selective oxidation of semiconducting SWCNTs by refluxing them in 30% H2O2 at 90 °C,58 and Eklund et al. found evidence of oxidation of SWCNTs that had been refluxed in 30% H2O2.59 Control experiments that we performed by taking purified SWCNTs that had not been hydroborated and mixing them with the basic solution of hydrogen peroxide led to FTIR spectra similar to Figure 2b, indicating that elevated temperatures are necessary for hydrogen peroxide to measurably oxidize SWCNTs on the time scale of several hours. The experiments performed here do not directly elucidate the mechanism by which the photochemical hydroboration reaction proceeds. Suzuki et al. have demonstrated that electron-beam irradiation or ultraviolet light irradiation produces defects in single-walled carbon nanotubes.60-62 They put an upper limit of about 20 eV on the activation energy of formation of these defects, which they postulated to be adatom-vacancy pairs.61 Additionally, the activation energy for reversing the defect formation process was found to be about 1 eV, low enough that it can proceed at a moderate rate at room temperature. Finally, they determined that the reverse activation energy was

Ellison et al. significantly greater than 1 eV when the nanotubes were exposed to air, which they attributed to stabilization of the defects by adsorption of gas molecules.62 It is possible that the UV light used in our experiments produced adatom-vacancy defects similar to those described by Suzuki et al., although it is unlikely. The experiments of Suzuki et al. used synchrotron radiation in the vacuum UV and soft X-ray regions, so the photons used in their experiments were at least more than twice as energetic as the photons used in our experiments. DFT calculations indicate that the activation energy for producing an adatom-vacancy defect is about 10 eV.63 Because the highest-energy light used in our experiments is 254 nm, with an energy of about 4.9 eV, the photons in our photochemical experiments have much less energy than is required for producing such defects. Consequently, we consider it possible but unlikely that the reactions studied in this work proceed at an adatom-vacancy defect. The experiments performed with quartz and Pyrex reaction vessels and with the optical filters strongly suggest that light in the wavelength range of 250-300 nm is responsible for initiating the hydroboration reaction. SWCNTs exhibit a λmax of about 260 nm, indicating that absorption of UV light by SWCNTs is responsible for the reaction. Tour et al. attributed the photohydroxylation of SWCNTs by water to absorption of UV light by the SWCNTs, raising the SWCNTs to an excited state that is more reactive toward nearby molecules.46 This mechanism is also a reasonable explanation for the photochemical reactivity of SWCNTs with BH3. SWCNTs functionalized with OH groups could be useful in light-harvesting applications. A very recent report demonstrates the attachment of fluorescent dyes to functional groups on SWCNTs.64 The Borguet group at Temple University intends to test dye-functionalized SWCNTs for their effectiveness as active elements in solar cells.65 This method of attaching OH groups on SWCNTs could allow for more dye molecules to be linked to the nanotubes, potentially increasing their lightcapturing ability in solar cells. Finally, the precedent of organoborane chemistry suggests that the photochemical hydroboration of SWCNTs could lead to new pathways for functionalizing carbon nanotubes. Conclusions The photochemical hydroboration-oxidation of single-walled carbon nanotubes has been demonstrated. Experiments show that UV light in the range of 250-300 nm is absorbed by the SWCNTs, leading to reaction with borane to yield BH2 and H groups. The BH2 groups can be oxidized by a basic hydrogen peroxide solution to OH groups. This reaction could be used to increase the number of functional groups on SWCNTs, and it opens up new means of functionalizing SWCNTs. Acknowledgment. The authors thank Erica Ellison for a critical proofreading and editing of the manuscript. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. This work was also supported by funds from Staiger and Van Sant grants from Ursinus College. The authors gratefully acknowledge Michael Prushan of LaSalle University for the use of the Raman spectrometer. Supporting Information Available: Emission spectra of Hg lamp viewed from quartz and Pyrex reaction vessels; emission spectra of Hg lamp viewed through the filters used in this work. This material is available free of charge via the Internet at http:// pubs.acs.org.

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