Two-dimensional (2D) transition metal carbides/nitrides (MXenes) possess outstanding properties such as large specific surface area, high metallic conductivity, and hydrophilicity which make them of interest for a variety of applications, including environmental remediation, water treatment, and catalysis. Given the relative newness of this class of materials, many uncertainties exist about their environmental behavior, fate, and transport; and their stability under conditions relevant to natural and engineered environments has not been explored. This study investigates the colloidal and chemical stabilities of Ti3C2Tx MXenes in aqueous suspensions in the presence of O2, HOCl, and/or UV light as a function of pH (5-9) and chlorine concentration (20-200 mg L-1). Modified MAX phase (Al-Ti3AlC2), produced in the presence of excess aluminum, was utilized to synthesize Ti3C2Tx MXene with improved oxidative stability in aqueous suspension. We find that pH in the range of 5-9 and UV irradiation (up to 240 min) do not affect the stabilities of aqueous Ti3C2Tx MXene colloidal suspensions over the experimental timeframe as assessed by UV–visible absorption spectroscopy. In contrast, exposure to HOCl/OCl– induced aggregation of Ti3C2Tx MXenes. At low chlorine concentrations, solutions with dark green color were observed, indicating good colloidal stability of Ti3C2Tx MXenes. When the chlorine concentration exceeded a threshold, Ti3C2Tx suspensions became unstable, forming visible agglomerates. For all conditions evaluated, Ti3C2Tx reacted with free chlorine. The rates of reaction with free chlorine were enhanced under UV irradiation and were highest at pH 9, where OCl– was the dominant species in solution. Chemical changes to the Ti3C2Tx nanosheets will be discussed.

Chemical and colloidal stability of Ti3C2Tx MXenes under environmentally relevant conditions

ACS Fall 2021

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