KEYWORDStungsten disulfide nanotubes, inorganic nanotube of tungsten disulfide (INT-WS 2 ), Vilsmeier-Haack reagents, polycarboxylation, inorganic nanotube functionalization ABSTRACT Inorganic nanotubes of tungsten disulfide (INTs-WS 2 ) are insoluble in common solvents and practically inert, hindering their usefulness in both research and commercial applications. The covalent attachment of functional species onto the surface of INT-WS 2 is a critical first step in realizing the potential that INT-WS 2 offer for high-performance materials and products. Although a few attempts have been reported regarding preparing modified nanotubes, only a limited range of surface functionalities is possible with these methods. We have developed a versatile method, based on a modified, highly electrophilic acidic VilsmeierHaack reagent, to produce covalently bonded, polycarboxylated functional WS 2 nanotubes that are dispersible in polar liquids, including water. The surface polycarboxylated shell provides a means for additional derivatization, enabling matching compatibility of derivatized nanotubes to both hydrophobic and hydrophilic materials. Nanocomposites incorporating derivatized INT-WS 2 are expected to show improved properties as a result of enhanced interfacial compatibility, made possible by the large number of classes of functionalization available through the initial polycarboxylation step.

Design of experiments (DOE) methodology was used to identify and optimize factors that influence the degree of functionalization (polycarboxylation) of WS 2 INTs via a modified acidic Vilsmeier-Haack reagent. The six factors investigated were reaction time, temperature and the concentrations of 2-bromoacetic acid, WS 2 INTs, silver acetate and DMF. The significance of each factor and the associated interactive effects were evaluated using a two-level factorial statistical design in conjunction with statistical software (MiniTab ® 16) based on quadratic programming. Although statistical analysis indicated that no factors were statistically significant, time, temperature and concentration of silver acetate were found to be the most important contributors to obtaining maximum functionalization/carboxylation. By examining contour plots and interaction plots, it was determined that optimal functionalization is obtained in a temperature range of 115-120 °C with a reaction time of 54 h using a mixture of 6 mL DMF, 200 mg INTs, 800 mg 2-bromoacetic acid and 60 mg silver acetate.