Unveiling the Interfacial and Structural Heterogeneity of Ti₃C₂T_x MXene Etched with CoF₂/HCl by Integrated in Situ Thermal Analysis

Abstract

Ti 3 C 2 T x MXene is a member of the recently discovered two-dimensional early transition metal carbide and nitride family of MXenes with potential applications in energy storage and heterogeneous catalysis at elevated temperatures. Here, we apply a suite of in situ techniques to probe Ti 3 C 2 T x MXene's thermal evolutions, including in situ X-ray diffraction (XRD), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and integrated thermogravimetry−differential scanning calorimetry− mass spectrometry (TG-DSC-MS). In light of this set of in situ investigations, we find heterogeneity in the layering of Ti 3 C 2 T x MXene revealed only at higher temperatures. Our findings present behavior up to 600 °C, particularly interlayer water and −OH surface end-capping groups. In one group of layers, their interlayer spacing shrinks as water deintercalates, but the other group of layers unexpectedly shows no change in the interlayer spacing. This is strong evidence that intercalants act as guest pillaring agents in the latter layering group, which stabilize these layers at higher temperatures while keeping the interlayer space accessible.

Energetic Stability and Interfacial Complexity of Ti3C2Tx MXenes Synthesized with HF/HCl and CoF2/HCl as Etching Agents

“…CoF2-MXene has a distinct shoulder to the (002) peak between 2θ = 0.5 -1.0°, reflecting the presence of a pillared structure. 27 Using this shoulder at 0.77° to calculate a c lattice parameter of 36 Å reveals layering with ultra-large interlayer spacing for CoF2-MXene. Smaller cations with lower charges, such as Na + or K + , lead to larger interlayer spaces than multivalent cations like Al 3+ due to their smaller hydration shell, allowing two layers of cations and associated water within an interlayer space.…”

“…15,18,25,26 We recently reported the unexpected structural heterogeneity of Ti3C2Tx MXene etched with CoF2/HCl, which has low AlF3•3H2O impurity. 27 We found that CoF2/HCl etching leads to Ti3C2Tx MXene with pillared layers. Removal of interlayer confined molecular water revealed two distinctive layering systems, specifically, (i) typical Ti3C2Tx MXene layers reduced the interlayer spacing upon water removal, and (ii) pillared Ti3C2Tx MXene layers that retained their open and large interlayer spacing even after deintercalation of water.…”

“…We reported intercalated Al 3+ species that act as pillaring agents and are "trapped" in the interlayer space post-synthesis and washing treatments. 27 Both CoF2-MXene and HF-MXene, feature rich interfacial chemistry with complex surface functionalization. This is most likely due to surface attachment past ideal stoichiometry.…”

“…Figure 2a displays a few particles loosely attached to the Ti3C2Tx MXene particles, which are minor etching residuals described in our previous investigations into the morphology of Ti3C2Tx MXene. 18,27 Ti3C2Tx is oxidized in the air over long periods, showing these small popcorn-like particles on the surface. 62 Our samples do not show detectable oxidation products compared to those with significant TiO2 particle growth on their surface.…”

“…Removal of interlayer confined molecular water revealed two distinctive layering systems, specifically, (i) typical Ti3C2Tx MXene layers reduced the interlayer spacing upon water removal, and (ii) pillared Ti3C2Tx MXene layers that retained their open and large interlayer spacing even after deintercalation of water. 27 Such structural heterogeneity is probably due to the presence of trapped/intercalated Al 3+ and/or Co 2+ species in the interlayer space because of the high diffusion barrier from the strong ionic strength of the CoF2/HCl etching environment. However, the energetics of formation and intercalant-layer guest-host interactions, which govern the formation of this unique Ti3C2Tx MXene system featuring heterogeneously pillared layered structures, have not been experimentally investigated.…”