Layered MXenes are normally obtained by chemically etching the A element of MAX phases (M is the early transition element, A is the main group element, and X is nitrogen or carbon) by using fluoride-containing compounds or a Lewis acidic molten salt (LAMS) for example. After removing the A element in the MAX phase, the exposed M atoms can spontaneously coordinate with anions, such as F–, O2–, OH– or Cl–, as the termination (T) species (Mn+1XnTx). In MXenes, cations, cationic surfactants and organic molecules can be intercalated into the van der Waals gaps, facilitating delamination of the layers. MAX phases, MXenes and intercalated MXenes offer interesting platforms for constructing nanomaterials with different chemical building blocks. However, the capability for such structural editing is currently limited to a few types of constituent element and terminating group.
Ding and colleagues propose a structural editing protocol for gap opening and intercalating species, greatly expanding the family of MAX phases and MXenes. The protocol contains four reaction routes: (1) opening non-van der Waals gaps in MAX phases by using LAMS scissors to form atom vacancies; (2) diffusing metal atoms into interlayer vacancies to form MAX phases; (3) removing the surface terminations of multilayer MXenes and opening van der Waals gaps with metal scissors; and (4) terminating MXenes with anions. By following routes 1 and 2, topotactic structural transformation of MAX phases can be achieved with the original A atoms being replaced by new metal atoms A′. For example, the aluminium atoms in Mn+1AlXn are oxidized by the Cu2+ cations in the LAMS scissor CuCl2, to leave atom vacancies. Then, guest metal atoms, such as Ga, In or Sn, in the molten salt diffuse into the interlayers and occupy the vacancies, forming a new Mn+1A′Xn phase. Notably, the A′ component can be different metals and can contain one or more type of metal with varying composition. By following routes 3 and 2, two-dimensional MXenes can be reassembled into three-dimensional MAX phases again: the metal scissor removes the terminations and then the guest atoms stitch the non-terminated MXene layers and reconstruct the MAX phases. With multiple conversions between the MAX phases and the two-dimensional MXenes (route 1–4–3–2), the MAX phase and MXene layer thickness is reduced, forming fully exfoliated lamellae. The spacing between the lamellae becomes sufficiently large for ion intercalation, leading to the formation of metal-intercalated carbides combining the functional features of MXenes with surface terminations and the structural features of the MAX phases.
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