To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphene sheets that comprise CNT surfaces are often modified to tailor properties, such as dispersion. In this article, we provide a critical review of the techniques used to explore the chemical and structural characteristics of CNTs modified by covalent surface modification strategies that involve the direct incorporation of specific elements and inorganic or organic functional groups into the graphene sidewalls. Using examples from the literature, we discuss not only the popular techniques such as TEM, XPS, IR, and Raman spectroscopy but also more specialized techniques such as chemical derivatization, Boehm titrations, EELS, NEXAFS, TPD, and TGA. The chemical or structural information provided by each technique discussed, as well as their strengths and limitations. Particular emphasis is placed on XPS and the application of chemical derivatization in conjunction with XPS to quantify functional groups on CNT surfaces in situations where spectral deconvolution of XPS lineshapes is ambiguous.

The sorption properties of natural organic matter (NOM) with oxidized multiwalled carbon nanotubes (O-MWCNTs) in simple electrolytes has been studied, as well as the effect that NOM concentration, pH, and O-MWCNT surface chemistry have on CNT stability under environmentally relevant conditions. As O-MWCNT oxygen content increased, NOM sorption decreased in simple electrolytes for a common set of solution conditions. For each O-MWCNT, NOM sorption increased with increasing ionic strength and decreasing pH, although the sensitivity of NOM sorption to these water quality parameters increased as the O-MWCNT oxygen content increased. Collectively, these observations indicate that NOM sorption by O-MWCNTs is determined by favorable hydrophobic π-π interactions that are moderated by repulsive electrostatic forces between negatively charged carboxylic acid functional groups on the O-MWCNTs and NOM. Stability studies conducted in artificial groundwater revealed that CNT stability is influenced by both the NOM concentration and pH, but stability was largely independent of the O-MWCNT oxygen concentration. These findings contrast with the marked effect that surface oxygen has on CNT stability in simple electrolytes. Electrophoretic mobility measurements revealed that the stabilizing effects of adsorbed NOM are due to the introduction of steric repulsion between NOM-coated CNTs, rather than from changes to surface charge.

Total internal reflection microscopy (TIRM) is used to measure SiO2 colloid ensembles over a glass microscope slide to simultaneously obtain interactions and stability as a function of pH (4-10) and NaCl concentration (0.1-100 mM). Analysis of SiO2 colloid Brownian height excursions yields kT-scale potential energy vs separation profiles, U(h), and diffusivity vs separation profiles, D(h), and determines whether particles are levitated or irreversibly deposited (i.e., stable). By including an impermeable SiO2 "gel layer" when fitting van der Waals, electrostatic, and steric potentials to measured net potentials, gel layers are estimated to be ~10 nm thick and display an ionic strength collapse. The D(h) results indicate consistent surface separation scales for potential energy profiles and hydrodynamic interactions. Our measurements and model indicate how SiO2 gel layers influence van der Waals (e.g., dielectric properties), electrostatics (e.g., shear plane), and steric (e.g., layer thickness) potentials to understand the anomalous high ionic strength and high pH stability of SiO2 colloids.

Motivated by the ability of UVC radiation to destroy harmful pathogens in drinking and waste water treatment plants, we have investigated the effect of 254 nm (UVC) radiation on the physical and chemical properties of oxidized multiwalled carbon nanotube (O-MWCNT) suspensions.