Anion Adsorption, Ti3C2Tz MXene Multilayers, and Their Effect on Claylike Swelling

MXenes are a relatively new and large family of two-dimensional early transition-metal carbides derived typically by etching the MAX phases in fluoride-containing solutions. Although numerous studies have investigated the role of cations in the interlayer distance (d(0002)) between MXene multilayers (MLs), little is known about the role of anions. Herein, using mainly X-ray diffraction, the effect of anions on d(0002) of Ti3C2TZ MLs, where T-z represents various terminations, was systematically studied. The MLs were produced by etching Ti3AlC2 powders in hydrofluoric (HF) acid alone or in mixtures of HF and mineral acids with anions larger than F, viz., hydrochloric, hydrobromic, hydroiodic, sulfuric, or phosphoric acids. The nature of the cations present in the postetching washing solutions was also varied. The results show that the presence of the larger anions demonstrably facilitates water intercalation and deintercalation. The fact that, for the most part, d(0002) in fully dried MLs is not a function of mineral acid, together with postetching chemical analysis via energy dispersive spectroscopy and X-ray photoelectron spectroscopy showing that the amount of residual anions after etching was quite small, in some cases vanishingly so, leads to the conclusion that the larger anions do not enter the interlayer space but most probably preferentially adsorb onto the positively charged ML edges, keeping the interlayer space open, which, in turn, eases water intercalation and deintercalation.