Apparent Enhanced Solubility of Single-Wall Carbon Nanotubes in a Deuterated Acid Mixture
An apparent enhanced solubility of single-wall carbon nanotubes (SWNTs) in the deuterated form of the standard 3 : 1 sulfuric () to nitric () acid mixture treatment is reported and attributed to the stronger interaction of deuterium bonds with the single-wall carbon nanotube surface. UV-Visible spectroscopy was used to characterize the apparent enhanced solubility of the SWNTs treated in deuterated forms of the acid mixture in comparison to the standard acid mix, while FTIR was used to analyze the nature of the functional groups generated on the SWNTs as a result of the different acid treatments. The apparent enhanced solubility reported here is consistent with the limited number of computational and experimental results published in the literature regarding the interaction of carbon nanotubes with deuterated solvents; however, a detailed understanding of the underlying mechanism responsible for this observation is currently lacking. The apparent increased solubility observed here could potentially be utilized in many applications where carbon nanotube dispersion is required.
Owing to their excellent electrical and mechanical properties, single-wall carbon nanotubes (SWNTs) have been an area of intense research since their discovery in 1991  and a variety of potential applications have been proposed [2, 3]. Many of these applications will likely require chemicalmodification (functionalization) of the nanotube surface tofacilitate their integration into more complex assemblies, and thus solubilization of SWNTs in a variety of solvent systems has been a considerable research pursuit . One common functionalization technique is treatment of the SWNTs in a 3 : 1 acid mixture of concentrated sulfuric and nitric acid (which we will refer to as the H-acid mixture), which creates defects on the nanotube surface from which carboxylic acid groups are attached [5, 6]. The solubility of thenanotubes after this standard acid treatment is typically much greater than that for the pristine nanotubes. However, we have recently observed that a similar treatment with the deuterated forms of this acid mixture (3 : 1 ratio of D2SO4 : DNO3, which we will refer to as the D-acid mixture) results in an ap-parent significant enhancement in the solubility of the SWNTs. This solubility enhancement is attributed to the strong affinity for deuterium interactions with the nanotube surface.
The purified (>90%) SWNTs used for this study wereBuckyPearls (Carbon Nanotechnologies, Inc., Houston, TX). As previously reported in the literature, the diameters of these SWNTs are in the range of 0.8–1.3 nm . The sur-face properties of the as-received SWNTs have been studied using X-ray photoelectron spectroscopy (XPS) and the re-sults were discussed elsewhere . Deuterated forms (99% of isotopes) of sulfuric acid (D2SO4: catalog number DLM-33-50 with concentration of 98% D2SO4 in D2O) and nitric acid (DNO3: catalog number DLM-33-50 with concentration of 65% DNO3 in D2O) were used in thisstudy. Additional chemicals used in the experiments were heavy water (D2O) from Cambridge Isotope Laboratories (Andover, MA); H2SO4 (SA 123 with concentration of 98% H2SO4 in H2O); and HNO3 (SA 95 with concentration of 65% HNO3 in H2O) from Fisher Scientific (Hanover Park, IL). In thisstudy the acids were used as received without furthermodification. The D-acid mixture (4 ml) was prepared in a vial with a 3 : 1 ratio of D2SO4 to DNO3 to which 2 mg SWNTs (sample I) was added. A similar solution was prepared using H2SO4 and HNO3 (sample II) to which 2 mg of SWNTs were added and treated as a control. In each case the vial was capped and sonicated in a bath sonicator at 350 W for 4 hours at 40°C. As described below, the apparent enhanced solubility of SWNTs in the D-acid mixture was highly re-producible and confirmed by repeating the experiment three times on three separate occasions using the procedure de-scribed above.
3. Results and Discussion
After completing the sonication process, the solubility of the SWNTs was found to be significantly enhanced in the D-acid mixture compared to the solubility obtained from the standard (undeuterated acid) H-acid mixture. This observed apparent enhancement of the solubility of SWNTs in the D-acid mixture was highly reproducible and confirmed by repeating the experiment three times. For further characterization of the enhanced solubility of the D-acid mixture, the concentrated sample I was diluted to 10% solution using heavy water and the D-acid mixture, while sample II was diluted in a similar manner using both distilled water and the H-acid mixture. A photograph of the resulting solutions is shown in Figure 1, where vials A and D contain the D-acid mixture treated SWNTs (sample I) in heavy water (vial A) and in the D-acid mixture (vial D); vials B and C (sample II) contain the H-acid mixture-treated nanotubes in distilled water (vial B) and in the H-acid mixture (vial C). It is clear that vials A and D (sample I) are more transparent when compared to the corresponding vials containing sample II. Also noticeable is the lack of discernible aggregates in the vials prepared using sample I.
Each of the 10% solutions shown in Figure 1 was then centrifuged (IEC Medispin, Needham Heights, MA) at 12000 rpm for 30 minutes. Significant SWNT precipitation collected at the bottom of the centrifuge microtubes (0.6 ml, SciMart, St. Louis, MO) for the H-acid mixture-treatedsolutions (Vials B and C) as shown in Figure 2. However, for the case of SWNTs treated with the D-acid mixture significantly less precipitate was found for the solution diluted in D2O (Vial A), and no precipitate is observed for the solution diluted in the deuterated acid mixture (Vial D). Figures 1 and 2 demonstrate the apparent increased solubilization of SWNTs treated with the D-acid mixture.
Further confirmation of this apparent increased solubility was sought via analysis of the UV-Visible spectra for sample I (Figure 3(a)) and sample II (Figure 3(b)) using a double-beam UV-Vis-NIR spectrophotometer (Cary 500, Varian, Palo Alto, CA). Figure 3(a) shows the UV-Vis spectra obtained for the 10% solution of sample I in the D-acid mixture (Figure 1, Vial D), the supernatant after the centrifuge step (Figure 2, Vial D), and the pure D-acid mixture; corresponding data is shown for the H-acid mixture-treated SWNTs (sample II) in Figure 3(b). Figure 3(a) shows strong absorption between 325 and 400 nm for both the 10% solution and supernatant for the case of the D-acid mixture treatment, with no noticeable absorption for the pure D-acid mixture. However, for the case of sample II (Figure 3(b)), only a weak peak is shown within this wavelength range for the 10% solution in the H-acid mixture, with no detectable absorption observed for the supernatant. This demonstrates that after sonication, the D-acid mixture-treated SWNTs are very finely dispersed . For comparison, the dispersion of sample II is not as fine, such that the conglomeration of the SWNTs within the H-acid solution is reflected by the poor absorption of these samples.
Finally, FTIR spectra (BIO-RAD FTIR FTS 60, Hercules, CA) were collected for the sample I precipitate diluted in D2O and the sample II precipitate diluted in distilled water after each sample was rinsed well with D2O and distilled water, respectively, and centrifuged at 12000 rpm for 30 minutes. The precipitates were collected and dried at 80°C under vacuum overnight prior to the FTIR analysis. For FTIR, the SWNT samples were grounded well with KBr and the sample was made into a pellet form using a hydraulic press. The FTIR spectrum was recorded using these pellets. The FTIR spectrum for the D-acid mixture-treated sample I precipitate (Figure 4(a)) displays bands at ~1732 cm−1 that may be due to the presence of C=O from carboxylic acid groups, various bands between 1000 and 1185 cm−1 consistent with C–O–C bonds and C–O stretching frequencies, and bands between 400 and 900 cm−1 which likely represent aromatic rings . The intermediate bands between 1000 and 1185 cm−1 are also in the range of the C–D bending mode . For the H-acid samples shown in Figure 4(b), the band at 3430 cm−1 represents the stretching frequency of –O–H groups, while the bands at 1732 and 1650 cm−1 are attributed to the C=O bonds in saturated and aromatic carboxylic acid. As expected, when sample I was diluted with D2O, the –O–H stretching was not present at ~3400 cm−1.
The strong presence of C–O, C–O–C and aromatic carbon in the sample I spectra may be caused by a larger number of defects on the nanotube sidewall resulting from the stronger interaction of heavy water (D2O) in the D-acid mixture with the SWNTs. Such a hypothesis is consistent with ab initio results discussed in the literature which suggest that a D-bond is stronger than an H-bond based on their respective binding energies . In addition, a recent computational study of water and single-layer graphite found sufficiently large binding energies that are believed to be important in the interaction of water with carbon nanotubes [13, 14]. We hypothesize that water (D2O and H2O, resp.) in the D-acid and H-acid mixtures interacts with the carbon nanotube walls. In the case of the D-acid mixture, a combination of the higher binding energy of (i) DOD in D-acid mixture and (ii) further dilution of the concentrated solution into heavy water and D-acid mixture (also containing D2O) results in a stronger interaction with the SWNTs, resulting in the apparent increased solubility for the D-acid treatment observed here. Other researchers have also reported that strong C–D interactions are responsible for deuterium attachment to carbon nanotubes . Thus our preliminary conclusion is that the apparent enhanced solubility of the SWNTs subjected to the D-acid treatment is due to stronger interaction of D2O with the SWNTs in comparison to H2O available in the H-acid treatment.
In summary, we have found that the treatment of as-received single-wall carbon nanotubes with the deuterated form of the 3 : 1 sulfuric to nitric acid treatment results in an apparent solubility enhancement of the nanotubes. This observation is consistent with the limited number of computational and experimental results published in the literature regarding the interaction of carbon nanotubes with deuterated solvents; however, a detailed understanding of the underlying mechanism responsible for this observation is currently lacking. This reported experimental observation regarding the interaction of carbon nanotubes with different forms of chemical treatments should be further investigated both experimentally and theoretically. In particular, at the moment one cannot rule out the possibility that the D-acid treatment results in significantly greater levels of damage to the structure of the SWNT, which in effect could lead to fragments of oxygenated polyaromatic hydrocarbons having similar UV-Vis and IR spectra as the oxidized SWNTs. In any case, the apparent increased solubility of the D-acid treated SWNTs (or, alternatively, the large-scale structural damage resulting from the D-acid treatment) is of both scientific and technological interest. The increased solubility observed here could be utilized in many applications where carbon nanotube dispersion is required.
We gratefully acknowledge comments by Sasha Stankovich and the grant support from the NASA University Research, Engineering and Technology Institute on Bio Inspired Mate-rials (BIMat) under Award no. NCC-1-02037. The useful comments from an anonymous reviewer on an earlier version of this manuscript are also appreciated.
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