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 Ti₃C₂T_x 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 Ti₃C₂T_x 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 Ti₃C₂T_x MXene colloidal suspensions over the experimental timeframe as assessed by UV–visible absorption spectroscopy. In contrast, exposure to HOCl/OCl^– induced aggregation of Ti₃C₂T_x MXenes. At low chlorine concentrations, solutions with dark green color were observed, indicating good colloidal stability of Ti₃C₂T_x MXenes. When the chlorine concentration exceeded a threshold, Ti₃C₂T_x suspensions became unstable, forming visible agglomerates. For all conditions evaluated, Ti₃C₂T_x 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 Ti₃C₂T_x nanosheets will be discussed.

Joel Pedersen

Univ of Wisconsin

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