15144
J. Phys. Chem. C 2008, 112, 15144–15150
Complexation between Rhodamine 101 and Single-Walled Carbon Nanotubes Indicative of Solvent-Nanotube Interaction Strength Christopher J. Collison,* Marc J. O’Donnell, and Jessica L. Alexander Department of Chemistry, Rochester Institute of Technology, College of Science, 84 Lomb Memorial DriVe, Rochester, New York 14623-5603 ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: August 6, 2008
Single walled carbon nanotubes (SWNT) have excellent mechanical, electrical, and thermal properties but their manipulation is hampered since they do not readily dissolve in most organic solvents. In particular, the interaction of nanotubes and organics is not universally understood. Using UV-vis absorption and visible fluorescence spectroscopy we show that, in chloroform, the laser dye Rhodamine 101 (R101) forms groundstate complexes with CoMoCAT SWNT, but that in N,N-dimethylacetamide (DMA), no such complexes exist. We suggest a strong interaction between zwitterionic R101 and nanotube in chloroform where the interaction between nanotube and solvent is weak. We describe how this interaction is solvent specific and how R101 and its analogs may be used as an indicator of strength of interaction of solvent. We also suggest that charge transfer/dipole stabilization should be investigated further in the search for more stable and long lasting SWNT dispersions for improved nanotube manipulation. Introduction Single walled carbon nanotubes (SWNT), discovered by Sumio Iijima in 1991,1 have since become materials of high interest because of their potential in a wide variety of applications.2 Nanotubes are akin to rolled up graphene sheets and are often synthesized with a very large aspect ratio of length to diameter. As a result these nanotubes retain fantastic strength along their axes and can be introduced as dopants to increase the strength of a composite.3 The diameter of the tube and the orientation of the carbon-carbon bonds with respect to the lattice (chirality) allow for several different types of tube, each with differing electronic properties, based on new boundary conditions imparted to the rolled up graphene sheet.4,5 Some tubes have metallic electronic properties and some are found to be semiconducting. The different tubes can be identified with spectroscopy only when fully isolated but the ability to isolate these nanotubes6 is not trivial, and is the subject of intense current research. As-produced tubes tend to bundle together because of Van der Waals interactions along the length of the tubes. While these interactions may be low in strength, the effect of them along the entire length of the tube is enough to prevent routine debundling in many simple solvent systems. Unfortunately, this typically means the majority of tubes that are semiconducting cannot be identified since they are often in contact (directly or indirectly) with metallic tubes. Hence the attributes of the semiconducting tubes may not be fully utilized where tubes are bundled together. Certain techniques have been successful in isolating individual tubes. Use of surfactants7 to obtain optical spectra has allowed researchers to characterize samples very well.8 Tubes have also been wrapped with DNA and polymers9 so as to isolate them. However, in these cases, a third system has been introduced and we no longer have pure tubes in a suspension; wrapping tubes prevents chemical or electronic interaction with another * To whom correspondence should be addressed. E-mail: cjcscha@ rit.edu.
target component or system. Tubes have been well separated in purification processes10 using acid intercalation but ultimately this leads to functionalization of the tubes on a large scale. Further heating processes are required to remove the added functional groups so as to reachieve the properties intrinsic to pure unfunctionalized tubes. Before the tubes are heated, however, the tubes must be filtered from their well dispersed state and inevitably this process results in their rebundling. We turn our focus to the isolation of tubes using solvents in two-system mixtures. The dissolving process of any solid11,12 can be understood in terms of the enthalpy of interaction (between solid and solid, solvent and solvent, and solvent and solid) and the entropy of solution of the solid. The entropy of solution for the nanotubes will include the entropy increase of debundling but will also include entropy decreases associated with any ordering of solvent molecules around the nanotubes. It turns out that nanotubes will never truly dissolve because the entropy of mixing is not high and the enthalpy of interaction between solvent and tube13,14 is not usually enough to drive the solution process forward. We therefore tend to talk about nanotubes in terms of their dispersion.15 Giordani et al.16 have discussed the quality of dispersion in terms of the enthalpy of interaction and the entropy of mixing/debundling. They point out that the enthalpy of interaction is dominant. Recent research16-18 has shown the effectiveness of using the solvents 1-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF) to disperse nanotubes. Other solvents, including 1,2-dichlorobenzene are also effective dispersants. Yet there appears to be a trend toward amides being better dispersants. Landi et al.17 suggest the π-stacking in the carbonyl group of the solvent molecule as being important, along with the bond lengths and bond angles. Ausman18 and Giordani16 suggest the importance of electron pair donation. In this paper we investigate the idea that dipole interaction along with π-π interactions may both be vital to a strong interaction with carbon nanotubes. We support the notion that charge transferring functional groups may have a substantial role to play beyond any π-π interactions.
10.1021/jp804359j CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
Complexation between Rhodamine 101 and SWNT
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15145 because of the strength of the solvent interactions between DMA and the tubes. We conclude by suggesting that R101 might be used as a probe for solvent-SWNT interaction strength (further work is underway) and that the dipole of R101 is critical for the strongly interacting complex. Experimental Methods
Figure 1. Molecular structures of lactonic, zwitterionic, and cationic forms of R101.
It is generally assumed that aromatic molecules will participate in π-π stacking with nanotubes.19,20 The carbons found in nanotubes of smaller diameter, however, may lose some sp2 character when compared with interactions of aromatic molecules with the carbons of a truly flat graphene sheet. We believe that the π-π interactions may also be supplemented with electron pair donation and reception through a strong dipole within the adsorbing molecule. Interpretations of our data support the work of Paloniemi et al.21 in this regard. In this work we use a fluorescent laser dye, Rhodamine 101 (R101). The form of R101 in solution varies between the lactonic, zwitterionic, and cationic forms (Figure 1). Only the zwitterionic and cationic forms of R101 will absorb in the visible region of light. In this work we will show that interactions between R101 and the SWNT strongly affect the optical properties of this fluorophore in the aprotic solvent chloroform. We also show that these interactions are solvent dependent; there is no evidence to support any interaction between R101 and the SWNT in DMA. We propose that, in this case, the interaction between solvent and nanotube is substantially stronger than any interaction with the R101. As a result we claim that fluorophores such as R101 can be used to interpret the strength of the solvent-nanotube interaction. The solvent environment plays an important role in the lactonic-zwitterionic-cationic equilibrium. In polar protic solvents such as water and ethanol, the zwitterionic form of R101 is favored whereas in nonpolar (toluene) and polar aprotic (acetone) solvents the lactonic form of R101 is favored.22,23 Upon addition of acid or protic impurities, the cationic form of R101 is favored. In summary, with this work, we show the presence of a strong physical interaction between as-received SWNT and R101. The interaction results in a shift in the R101 absorbance spectrum and a substantial absorbance intensity increase. Temperature dependent fluorescence quenching is also consistent with complexation in the excited state. We propose that the interaction occurs because of a strong dipole interaction. We explain that similar interactions occur with base-washed tubes and with heated purified Arc-discharge nanotubes (although we do not present this data). The same complexes formed in chloroform are not formed in DMA
Fluorescence grade R101, which has the appearance of a purple powder, was used as received from Sigma-Aldrich (Fluka Biochemika), product number 83694. SWNT were used as received from Southwestern Technologies. The SWNT are of the CoMoCAT24-26 variety and contain pristine SWNT, functionalized SWNT, carbonaceous soot, and cobalt-molybdenum catalyst. ACS reagent grade chloroform was used as received from J.T. Baker. The chloroform comes with 1% ethanol as a stabilizer. SWNT stock solutions in a concentration range of 0.050-0.10 mg/mL were made. The SWNT were dispersed in the stock solutions by sonicating for half an hour using a VWR bath sonicator (model 750) at its maximum power setting (90 W average Power, 180 W peak power). Sample vials were cleaned and filled with the proper amount of R101 and solvent such that a target concentration of R101 and nanotubes would be achieved after addition of the appropriate amount of SWNT stock. Once sonication of nanotube stock had finished the appropriate volume was immediately pipetted into the sample vials. R101 concentrations were maintained at concentrations low enough to prevent formation of dimers.27 Typically concentrations of
