Host–Guest Intercalation Chemistry in MXenes and Its Implications for Practical Applications

Abstract

The ever-increasing demand on developing layered materials for practical applications, such as electrochemical energy storage, responsive materials, nanofluidics, and environmental remediation, requires the profound understanding and artful exploitation of interlayer engineering or intercalation chemistry. The past decade has witnessed the massive exploration of a recently discovered 2D materialtransition metal carbides, carbonitrides, and nitrides (referred to as MXenes), which began to take hold of a myriad of applications owing to the abundant possibilities on their compositions and intercalation states. However, application-targeted manipulation of the material performance of MXenes is constrained by the dearth of deep comprehension on fundamental intercalation chemistry/physics. To this end, the aim of this review is to provide a holistic discussion on the intercalation chemistry in MXenes and the physical properties of MXene intercalation compounds. On the basis of this, potential solutions for the challenges confronted in the synthesis, tuning of material properties, and practical applications are proposed, which are also expected to reinvigorate the exploration of layered materials that are similar to MXenes.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

“…The match of their energy levels is the key for the interaction, which is associated with the chemical structure and surface chemistry of the host. [ 27 ] For graphite‐like materials with non‐polar surface and delocalization π electrons, both electron acceptors and donors are allowed to create interactions by charge transfer effects, and electrons flow away from or into the their conduction band. TMDs‐like materials only accept electrons to fill the d‐orbitals, and thus electron acceptors do not create interactions.…”

“…Apart from the interlayer spacing, the mechanical properties, electrical conductivity, and stability of MXenes are also influenced. [ 27 , 44 , 45 ] The introduction of intercalants (e.g., Li + , dimethylsulfoxide, and tetraalkylammonium hydroxide) is thought to result in a lower conductivity, worse mechanical properties, and instability. [ 27 , 46 ] Removing the intercalants by cation exchange in an acid solution [ 46 ] or annealing [ 47 ] can significantly increase the electrical conductivity.…”

“…[ 55 ] In most cases, the H 2 O intercalation is promoted by the hydration of metal ions due to the strong electrostatic interactions between the negatively charged Ti 3 C 2 T x surface and metal cations, in contrast with the considerably weaker interaction with pure H 2 O. [ 27 ] The zeta potential of the Ti 3 C 2 T x dispersion is lower than −30 mV over a wide pH range, [ 56 , 57 ] which explains why the sluggish exfoliation kinetics is hastened by metal ion intercalation in the HF/Li and LiF/HCl methods.…”

“…[ 25 , 26 ] Interestingly, some metal ions behave differently and may induce a dramatic structural or lattice change of MXenes. [ 27 ] For example, divalent metal ions (e.g., Fe 2+ , Mg 2+ ) can trigger crosslinking to form a 3D network, which is beneficial for ion diffusion. [ 28 , 29 ] Some metal ions with redox activities (e.g., Fe 3+ , Cu 2+ ) may generate atomic defects, pores, or produce MXene degradation.…”

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

“…Nevertheless, the roles of different interlayer species and the guest-host interfacial chemistry play in determining thermodynamic, structures, and interfacial properties of MXene materials have not been systematically explored. [35][36][37][38] There has been progress towards investigating water/ion-layer interactions, yet the energetics of intercalated particle -MXene layer interactions and why they present higher exothermic formation enthalpies from elements than the bulk parent MAX phases after synthesis and high-temperature treatments have not been clearly understood. 11,[39][40][41][42][43] In other words, unlocking MXene's full potential requires an improved understanding of the energetic interplay of intercalants, surface chemistry, and specific layered structures for samples synthesized under different etching conditions and with varied etching agents.…”