Measuring the Size and Stability of Double-Walled Carbon Nanotubes

Carbon nanotubes are cylindrical, quasi-one dimensional structures of one or more rolled-up sheets of grapheme. The inner diameters of this material range from 0.7 to 2 nm, and depending on the preparation technique, can have standard lengths of several microns. Today, carbon nanotubes are increasingly being used in material reinforcement application and hold great potential in electronics, medicine, energy storage and conversion. However, carbon nanotubes are difficult to dissolve in aqueous solutions as individual nanotubes.

SWCNT and DWCNT

Carbon nanotubes, particularly single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) exhibit special optical properties. These optical properties can only be manipulated in solution. In order to understand the aggregation and stability of the constructs, classification of the SWCNTs and DWCNTs and their surfactant coating is important. Some of the characterization techniques used for such measurements comprises analytical ultracentrifugation, zeta potential, and dynamic light scattering. This report describes the application of the DelsaMax PRO to determine the size and stability of SWCNTs and DWCNTs in aqueous solutions as a function of temperature.

Experimental Framework

First, the SWCNTs and DWCNTs were dissolved in 2% Sodium Dodecyl Sulfate (SDS), which is a common surfactant utilized to split individual SWCNTs. With the help of DelsaMax PRO, the hydrodynamic radius and zeta potential of SWCNT and DWCNT solutions were quickly and reliably measured with low volumes. The DelsaMax​ analysis software helped in analyzing the mobility data, regularization data, and correlation data instantly, and this in turn helped in evaluating the quality of the measurement quickly. Moreover, in case of a dust event, the measurement could be halted or run again.

For the experiment, in 20 mL glass scintillation vials, 200 mg of SDS or Sodium Cholate (SC) was mixed with 10 mL of water. Then, 2 mg of SWCNT or DWCNT was added to the vials. In total, four different solutions were prepared i.e. SDS + SWCNT, SDS + DWCNT, SC + SWCNT, and SC + DWCNT (Figure 1). All vials were bath sonicated for a period of 10 minutes to dissolve the nanotube. Then, the solutions were transferred to a conical tube and centrifuged at 3000 rpm. The nanotubes’ concentration in solution was 5.8 mg/L for all measurements. Using UV-Vis-NIR absorption spectroscopy, the mass concentration of the carbon nanotube solutions was measured. Further validation of the mass normalization of the nanotubes was denoted by approximately the same absorption at 400 nm. All spectroscopy measurements were carried out with a 1-cm quartz cuvette utilizing a Beckman Coulter DU 800 Spectrophotometer.

UV-Vis-NIR absorption spectrum of the nanotube solutions.

Figure 1. UV-Vis-NIR absorption spectrum of the nanotube solutions.

Characterization of Carbon Nanotubes

The DelsaMax PRO was employed to assess the stability and size of the carbon nanotubes using Phase Analysis Light Scattering and Dynamic Light Scattering techniques, respectively. Phase Analysis Light Scattering data was reported as zeta potential for each solution. Once the size and zeta potential measurements at 20°C were obtained, the DelsaMax PRO was scaled to 30°C and the size and zeta potential were again collected. For each nanotube-surfactant solution, this process continued at 40°C and 50°C.

Dynamic Light Scattering and Zeta Potential.

Figure 2. Dynamic Light Scattering and Zeta Potential.

Results

At 2% w/v, SDS and SC made stable solutions of SWCNT and DWCNT as specified by zeta potential, shown in figure 2. A zeta potential is believed to be stable at all temperatures when its absolute value is greater than 30 mV. The solutions containing nanotube and surfactant had zeta potentials with absolute magnitude greater than 40 mV. This result implies that probably the hydrophilic part of the surfactant coating expanded out from the nanotube surface into solution. This conclusion was drawn because the size of the nanotubes remained unchanged with respect to temperature and the hydrophobic part of the surfactants was attached to the nanotubes’ surface.

Moreover, in case aggregation occurs for the nanotubes at increased temperatures, it would considerably increase the nanotube size and cause a corresponding decrease in nanotube stability specified by the zeta potential approaching 0 mV, with a sudden rise in polydispersity. However, these indicators were not observed for any of the samples. In all cases, the lengths of the SWCNT and DWCNT should be analogous in SDS and SC after sonication and centrifugation, DWCNT was larger in radius than SWCNT, and in all cases zeta potential measurements displayed a stable solution, thus denoting that the nanotubes were separately dissolved.

Conclusion

The DelsaMax PRO can easily and precisely size a solution of nanoparticles and ascertain their stability. Sometimes, samples tend to contain dust or other contaminants. Using the DelsaMax PRO software, these contaminants are easily detected and removed by deselecting detectors or individual acquisitions for zeta potential and Dynamic Light Scattering, respectively. Also, concurrent measurement increases speed and by gathering both size and zeta potential in a matter of seconds, the possibility of a dust event is considerably reduced when compared to standard size and zeta potential measurements.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.

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