Surface Engineering and Umpolung Chemistry of 2D Transition Metal Carbide MXenes Through Electrochemical Reduction

Two-dimensional (2D) materials show unique properties that have led to their extensive study for diverse applications. 2D materials are mainly dielectrics, semiconductors, or semimetals. MXenes are an emerging family of 2D transition metal carbides and nitrides, most of which are metallic conductors. Thus, they are actively studied for their promising performance in energy storage, electromagnetic interference (EMI) shielding, and catalysis. MXenes follow a generalized formula of M<i>_n</i>₊₁X<i>_n</i>T<i>_x</i> (<i>n</i> = 1-4), where M stands for early transition metal (such as Ti, V, or Nb), X is C or N, and T is surface terminations (for example, -OH, -F, and -Cl). Their structures can be described as <i>n</i>+1 hexagonally packed M layers interleaved with <i>n</i> layers of X atoms. The M layers are then capped by the surface termination groups. Examples of MXenes include Ti₂CCl₂ and Ti₃C₂Cl₂.<br/>We show that conductive MXenes can be engineered by controlled electrochemical processes. By tuning the reductive potential applied to a Ti₂CCl₂ MXene electrode soaked in tetrabutylammonium (TBA) chloride acetonitrile electrolyte, MXenes can be intercalated by TBA or get reduced to eliminate terminal Cl atoms. The most interesting observation is the substitution of surface Cl with butyl groups, forming butyl-terminated MXenes. To the best of our knowledge, Alkyl groups have not been covalently grafted to the surface of MXenes. Attempts to substitute Cl with organolithium produced reduced MXene anions, which barely showed affinity towards carbanions. Our reaction was found to be an unconventional Umpolung (polarity inversion) chemistry of MXenes: Ti(δ⁺)-Cl(δ^-) bond in MXenes is considered an analogue of C(δ⁺)-Cl(δ^-) bond. A cathodic current reduces the electrophilic Ti(δ⁺), and its polarity is inverted to nucleophilicity Ti(δ^-). Then the butyl carbocation from TBA can be transferred to MXene surfaces, squeezing out tributylamine.<br/>The alkyl surface groups can be swapped into other organic species such as alkoxides and amides. Alkyl-terminated MXenes also show reactivity for the catalysis of acetylene deletion and hydroamination. The discovery of alkyl-functionalized MXenes is a demonstration that 2D MXenes can behave like organometallics, initiating a bunch of new opportunities at the interface of solid-state chemistry and organic chemistry.