Solid Intercalation and Exfoliation of Cl-Terminated Multilayered MXenes toward O-Functionalized Single Layers

Although ion intercalation is becoming a powerful strategy to produce expanded layered materials and atomic layers in aqueous or organic systems, it usually suffers from sluggish kinetics with a long intercalating time of several days. Here, we present a facile approach to produce O-functionalized single-layer MXenes by solid intercalation of Cl-terminated accordion-like MXenes in molten salts and subsequent exfoliation. The process involves the intercalation of metal cations (Li⁺, Na⁺ and K⁺) and anions (CO₃^2-) in molten salts, resulting in substitution with -O surface groups and formation of gases (CO₂). Such unique solid intercalation significantly expands multilayered MXenes in 30 min with an enhancement of interlayer spacing from 11.2 to 12.7 Å, facilitating their easy exfoliation to single layers. The resultant O-functionalized MXenes have a highly expanded structure and elevated permittivity, achieving a reflection loss value of -50.5 dB at a thickness of 1.35 mm for electromagnetic wave absorption.