The effect of surface oxides on multi-walled carbon nanotube aqueous colloidal properties.

摘要

Carbonaceous nanomaterials are being produced and integrated into consumer products and specialized applications at an accelerating rate. Recently, however, concerns have increased about the environmental, health and safety risks of these nanomaterials, particularly those chemically functionalized to enhance their aqueous colloidal stability and biocompatibility. In this dissertation research, I have investigated the role that surface-oxide concentration plays in the aqueous colloidal stability of multi-walled carbon nanotubes (MWCNTs), a prominent class of engineered nanomaterials. To vary the concentration of surface oxides on the MWCNTs' surface, pristine (unmodified) tubes were treated with a wet-chemical oxidant (e.g., HNO3, H2SO4 /HNO3, KMnO4); the concentration of surface oxides imparted was measured by x-ray photoelectron spectroscopy (XPS). In conjunction with XPS, previously developed chemical derivatization techniques were used to determine the distribution of hydroxyl, carboxyl, and carbonyl functional groups present on the MWCNTs' surface. The length distribution and structural integrity of pristine and oxidized MWCNTs were characterized using atomic force microscopy and transmission electron microscopy, respectively.;To examine the aqueous colloidal stability and aggregation properties of oxidized MWCNTs, sedimentation and time-resolved dynamic light scattering (TR-DLS) experiments were conducted on neat (i.e., ideal) suspensions prepared by prolonged sonication of MWCNTs in Milli-Q water. Over a range of environmentally relevant pH values (4--9) and electrolyte (NaCL, CaCl2) concentrations (0.001--1.000 M), the aggregation and colloidal properties of MWCNTs were found to agree with the basic tenants of DLVO theory, in that ( i) more highly oxidized, negatively charged MWCNTs remained stable over a wider range of solution conditions than lowly oxidized tubes, ( ii) oxidized MWCNTs adhered to the empirical Schulze-Hardy rule, and (iii) in early-time aggregation experiments, MWCNTs exhibited reaction- and diffusion-limited aggregation regimes.;Structure-property relationships developed during this dissertation research showed that linear correlations existed between MWCNT colloidal stability, XPS measured surface oxidation, and the solution pH. Comparisons between surface charge titrations and electrophoretic mobility (EM) measurements showed that MWCNT colloidal stability was more strongly correlated with surface charge than EM. Analysis of chemical derivatization results indicated that carboxylic functionalities on the MWCNT's surface played a dominate role regulating colloidal stability; however, stronger correlations were observed when colloidal stability was compared to the overall level of surface oxidation.;To complement the investigations conducted under ideal solution conditions, the effect that natural organic matter (NOM---an ubiquitous environmental macromolecule) had on the MWCNT's colloidal properties were examined. Before conducting aggregation experiments, a series of NOM adsorption studies were performed. In the first study, NOM adsorption as a function of MWCNT surface oxide concentration was evaluated. Results showed a systematic decrease in NOM adsorption with increasing surface oxidation. In parallel, the sorption of NOM by pristine MWCNTs and a set of highly oxidized tubes was examined under different solution conditions (i.e., pH and ionic strength). NOM adsorption onto pristine MWCNTs was negligibly affected by the solution conditions while NOM sorption by the oxidized MWCNTs increased with increasing ionic strength and decreasing pH. The colloidal stability of MWCNTs was greatly enhanced in the presence of NOM due to steric stabilization, as expected. However, bench top sedimentation and TR-DLS studies indicated that the colloidal stability of lowly oxidized MWCNTs could exceed that of oxidized MWCNTs at environmentally relevant NOM concentrations (∼3 mg/L). Sorption data was used to rationalize these results, in that, because lowly oxidized MWCNTs adsorb more NOM than highly oxidized MWCNTs, they can exert more steric repulsions. The overarching conclusion from this investigation is that the surface chemistry of the underlying particle matters and will likely play an important role in regulating CNT colloidal properties in more realistic environments.