Facile and efficient synthesis of carboxylic terminated Ti3C2Tx nanosheets using citric acid

For the last decade, researchers have been reporting various approaches for the synthesis and surface modification of Ti₃C₂T_x MXenes nanosheets. However, the use of concentrated acids and alkalis, elevated reaction temperature, and the subsequent degradation of MXenes nanosheets are considered sore points of the trade. In this study, we formulated an original, efficient, and one-pot protocol for the synthesis of carboxylic terminated Ti₃C₂T_x nanosheets incorporating non-hazardous citric acid with NH₄HF₂. Three different etchants: (i) NH₄HF₂ (ii) NH₄HF₂ and citric acid (iii) NH₄F and citric acid, were used to etch the Ti₃AlC₂ MAX phase. Thus, three types of Ti₃C₂T_x nanosheets were synthesized and comparatively analyzed using XRD, SEM, TEM, EDX, AFM, FTIR, XPS, NMR, BET, TGA and AAS techniques. Firstly, it was established that among Ti₃C₂T_x samples, the best-exfoliated few-layered carboxylic terminated nanosheets of 1.36 nm thickness were achieved using NH₄HF₂ and citric acid. Secondly, the nanosheets displayed extraordinary features owing to their superior morphology, microporous structure, improved surface chemistry, better thermal stability, prolonged dispersity and enhanced adsorption performance in an aqueous media. For instance, the Ti₃C₂T_x nanosheets showed a d-spacing of 1.25 nm and a specific surface area of 42.63 m²/g with numerous active sites in terms of (–COOH) terminations alongside (=O), (-F) and (–OH) terminal groups. In addition, the carboxylic terminated MXene exhibited excellent dispersity and stability in water for 40 days. The nanosheets also displayed adsorption capacities of 338 mg/g and 349 mg/g for Cr(VI) and Pb(II), respectively. The adsorption performance of the MXenes nanosheets was quite encouraging compared to previous studies. Moreover, the carboxylic terminated nanosheets were regenerated over six cycles of adsorption–desorption. Hence, these high-quality Ti₃C₂T_x nanosheets are a promising candidate for environmental application in general and removal of toxic heavy metals from wastewater in specific.