Abstract
Two-dimensional metal carbides and nitrides–MXenes─represent a group of materials which have attained growing attention over the last decade due to their chemical versatility, making them highly promising in areas such as energy storage, superconductivity, and heterogenous catalysis. Surface terminations are a natural consequence of the MXene synthesis, conventionally consisting of O, OH, and F. However, recent studies have extended the chemical domain of the surface terminations to other elements, and they should be considered as an additional parameter governing the MXene properties. There is a shortfall in the understanding of how various chemical species could act as terminations on different MXenes. In particular, there is limited comprehension in which chemical environments different terminations are stable. Here, we present an extensive theoretical study of the surface terminations of MXenes in different atmospheres by considering in total six experimentally achieved MXenes (Ti2C, Nb2C, V2C, Mo2C, Ti3C2, and Nb4C3) and twelve surface terminations (O, OH, N, NH, NH2, S, SH, H, F, Cl, Br, and I). We consider fully terminated (single termination) MXenes and also the impact of substituting individual terminations. Our study provides insights into what terminations are stable on which MXenes in different chemical environments, with predictions of how to obtain single-termination MXenes and which MXenes are resilient under ambient conditions. In addition, we propose synthesis protocols of MXenes which have not yet been realized in experiments. It is anticipated that alongside the development of new synthesis routes, our study will provide design rules for how to tailor the surface terminations of MXenes.
Highlights
The top-down route for manufacturing two-dimensional (2D) materials has experienced tremendous progress since the mechanical exfoliation of single layers of graphene from graphite.[1]
The approach was generalized by the removal of the A element, typically Al, from several Mn+1AXn (MAX) phases,[3] where M is a transition metal and X is C and/or N, resulting in the category of 2D materials known as MXenes
Removal of surface terminations improves the electronic conductivity of Ti- and Mo-based MXenes,[10] while it has been predicted that the thermal conductivity is superior for F-terminated compared to that for O-terminated Ti3C2.11 magnetic properties may be controlled as Cr2TiC2 has been foreseen to be nonmagnetic when terminated with O and antiferromagnetic when terminated with OH or F.12
Summary
The top-down route for manufacturing two-dimensional (2D) materials has experienced tremendous progress since the mechanical exfoliation of single layers of graphene from graphite.[1] In 2011, a new approach for creating 2D materials was presented. By the selective etching of aluminum from a layered titanium aluminum carbide, a 2D titanium carbide was obtained.[2] The approach was generalized by the removal of the A element, typically Al, from several Mn+1AXn (MAX) phases,[3] where M is a transition metal and X is C and/or N, resulting in the category of 2D materials known as MXenes. More than 30 MXenes have been synthesized,[4,5] and the list is growing
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