Selective aggregation of single-walled carbon nanotubes via salt addition.

Single-walled carbon nanotubes (SWNTs) can be dispersed as individuals in H2O using sodium dodecylsulfate (SDS), typically at 1% (35 mM) concentration. At and above the critical micellar concentration (cmc 8 mM at 25 °C), the degree of ionization of SDS is 0.27.1 Once intertube van der Waals (VdW) attraction is overcome by intense sonication, free SDS adsorbs to SWNT surfaces and creates a net surface density of negative charge, which prevents SWNT reaggregation. In such solutions, the excitonic absorption and emission of light by the various SWNT chiralities can be observed.2-4 Any change in the surface charge density of the surfactant, or its solubility, that eliminates the electrostatic repulsion between the nanotubes will cause SWNTs to aggregate and coagulate.4 By controlling the intermolecular and surface forces of SDS in water, we show here that it is possible to engineer the resultant VdW attraction between SWNTs. Titrating SWNT-SDS dispersions with salt solutions leads to diameter-dependent modulation of the electrostatic repulsive forces; changes in the absorbance and emission spectra of SWNTs are used to study these effects. We ascribe the observed spectroscopic changes to selective aggregation of SWNT chiralities. Our experiments clearly demonstrate that certain SWNT chiralities deviate from a simple diameter-dependent stability trend. A 20.0 mg quantity of as-prepared HiPco SWNTs was dispersed in 200 mL of 35 mM SDS (Fisher, 98%) in D2O (4.4 × cmc), using the standard procedure of shear mixing and sonication followed by ultracentrifugation.2 Microliter volumes of salt solutions (∼1 M in D2O) were added to the SWNT dispersions at room temperature. The experiments were reproduced in up to 10 mL of the SWNT dispersions. Aliquots were taken out to record the spectra. After salt addition, the solutions were either stirred or allowed to stand overnight to attain equilibrium. Absorbance spectra were recorded in a Varian Cary 6000i instrument. Excitation maps of the SWNT photoluminescence were recorded using a homebuilt FT-photoluminescence excitation (PLE) spectrometer with a Xe arc lamp source and a liquid N2-cooled Ge detector from a modified Nicolet NXR-9600 FT-IR spectrometer.5 Excitation power at the sample was typically 1.3 mW, and the emission spectra were integrated over 128 scans. The single line (λEX ) 780 nm) emission spectra (Figure 1) were recorded using the same Nicolet spectrometer with diode laser excitation. Systematic changes in the emission features are observed as NaCl is added to the SWNT dispersion (Figure 1). With respect to the starting SWNT dispersion (Figure 1A), in 0.14 M NaCl, the background is suppressed and the emission intensity is enhanced, particularly for the larger-diameter SWNTs (Figure 1B). As the concentration of NaCl is increased, the emission from certain SWNT chiralities is no longer observed. It is particularly interesting to note that in the band corresponding to the (10,5) and (8,7) emissions, the (10,5) emission is no longer visible in 0.43 M NaCl (Figure 1C); similarly, (11,3) and (9,4) signals are absent. The emission intensity of (7,5) and (6,5) chiralities increase, under the same conditions. In 0.57 M NaCl, the (9,7) emission is also completely lost (Figure 1D). The cmc of pure SDS is reduced to 1 mM in 0.16 M NaCl.6 The addition of Na+ decreases the electrostatic repulsion between SDS molecules and results in an increase in their aggregation number. Since we also observe that the completely aggregated dispersions show the background-only emission spectrum, the increase in the emission intensity and loss of the background emission (Figure 1B) may be due to improved isolation of the SWNTs, related to the ionization degree and aggregation number of SDS under these conditions (see also Supporting Information, Figure S2).6,7 Above 0.5 M NaCl, pure SDS is no longer soluble in water at 25 °C;8 under these conditions, the Kraft point of pure SDS is raised above 25 °C.9 The Kraft point is the melting point of a hydrated surfactant; it is raised by the addition of salt in water. We expect the concentration of NaCl necessary to affect the stability of SWNT/ SDS dispersions to be different from values previously obtained for pure SDS solutions. However, lowering the dispersion temperature should yield an effect similar to elevating the Kraft point. We observe a diameter-dependent change in the absorbance spectrum on centrifuging the SWNT dispersion at 70000g for 3 h, at 10 °C (Figure 1F), similar to the effect of adding NaCl (Figure 1E). Thus, manipulation of SDS equilibria results in loss of free SDS in solution, through precipitation or further micellization. SWNT† Los Alamos National Laboratory. ‡ Center for Basic Sciences, National Renewable Energy Laboratory. Figure 1. Spectroscopy of SWNT-SDS dispersions as a function of salt concentration and temperature. (A) emission spectrum of the starting SWNT-SDS dispersion; (B) same solution in 0.14 M NaCl; (C) in 0.43 M NaCl, and (D) in 0.57 M NaCl; (E) absorption spectrum with and without NaCl (0.17 M); (F) change in the absorption spectrum at temperatures below the critical micellar temperature. Published on Web 01/25/2007