Cause and Consequence of Carbon Nanotube Doping in Water and Aqueous Media

To utilize carbon nanotubes in real-world applications, we have to master their chemistry. At present there is a lack of understanding regarding what happens during basic manipulations, such as doping with acids, forming suspensions by sonication in water with surfactants, or detecting peroxides. We show that sonication of nanotubes in water leads to the in situ formation of molecular oxygen, causing doping, which can be quenched with ethanol. In the presence of the anionic surfactant sodium dodecyl sulfate, oxygen doping is overshadowed by doping due to the sulfate group. Stable suspensions of undoped nanotubes can be created with Triton-X spiked with ethanol. Hydrogen peroxide does not dope, but in high concentrations or in the presence of catalytic iron nanoparticles it decomposes to yield oxygen, which may dope. Hydrochloric acid does not dope, unlike sulfuric acid. Our results clarify the origins of doping while processing carbon nanotubes in water

To Dope or Not To Dope: The Effect of Sonicating Single-Wall Carbon Nanotubes in Common Laboratory Solvents on Their Electronic Structure

Single-wall carbon nanotubes (SWCNTs) are commonly dispersed via sonication in a solvent prior to functionalization. We show that solvents such as dichloromethane, chloroform, 1,2-dichloroethane, and o-dichlorobenzene lead to an upward shift in the Raman response of the SWCNTs. We have used o-dichlorobenzene as a model molecule to explain this effect, and an upward shift of 9 cm−1 is observed in the D* band. This blue shift is associated with p-type doping and is triggered only when the nanotubes are sonicated in the solvent. Sonication decomposes the chlorinated solvents, and new species (Cl2 and HCl(g)) are formed. The catalytic Fe nanoparticles inherently present in the nanotubes are etched by chlorine and hydrogen chloride to form iron chlorides during sonication in the solvent. The dopant was identified by X-ray photoelectron spectroscopy. With such knowledge of doping, the choice of solvent becomes crucial for any chemical reaction and can be intentionally tuned to produce SWCNTs films for electronics applications

Ambiguity in the Characterization of Chemically Modified Single-Walled Carbon Nanotubes: A Raman and Ultraviolet−Visible−Near-Infrared Study

Single-walled carbon nanotubes (SWCNTs) sonicated in o-dichlorobenzene and benzyl chloride show anomalous behavior when characterized with a Raman microscope and ultraviolet−visible−near-infrared spectroscopy. SWCNTs treated with the aforementioned solvents lead to a small but distinct increase in the Raman D peak, when irradiated with laser power higher than 0.12 mW/μm2. This can be mistakenly interpreted as covalent functionalization, but we have correlated this increase in the D peak to the charring of polymeric material, which is formed during sonication of the aforementioned solvents. At a temperature estimated to be 280 °C, corresponding to a laser power of 0.31 mW/μm2, the polymers are charred, resulting in an increase in amorphous material. This behavior is in contrast to that of the covalently functionalized SWCNTs, which show a decrease in the D peak as the laser power is increased. These samples also show a depletion in the spectral intensity of the optical absorption spectra of the SWCNTs, which is again a result commonly associated with covalent functionalization. However, by using a washing protocol, we find the Raman and optical spectra of the resulting SWCNTs no longer show features associated with functionalization. Species formed during sonication can drastically affect data interpretation. Our results provide an unambiguous assessment of the cause and effect of wet chemical processing and its impact on characterization

Long-Range Periodicity in Carbon Nanotube Sidewall Functionalization

Using the Bingel reaction as a model for side-wall functionalization of single-walled carbon nanotubes, we report the discovery of highly regular, long-distance (several nanometer) patterns and examine the conditions for the occurrence of such patterns, possibly due to long-range induced reactivity. Varying periodicities of the patterns have been observed via scanning tunneling microscopy and are attributed to nanotube geometry. Patterns are most prominent on medium heavy functionalized nanotubes and likely tied to a nucleophilic reaction mechanism