Functionalizing MXenes by Tailoring Surface Terminations in Different Chemical Environments

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.

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

“…Surface attachment past stoichiometry is computationally predicted by exergonic attachment of Fto an already fully terminated Ti3C2O2 MXene. 30 Although our samples show more bound surface groups than the ideal stoichiometric surface functionalization (Ti3C2Tx, x = 2), the ratio of surface groups are O0.58F0.19(OH)0.21Cl0.02 and O0.42F0.29(OH)0.27Cl0.02 for CoF2-MXene and HF-MXene, respectively. Only O and F functional groups are predicted to exist in an ambient oxygen environment.…”

“…Only O and F functional groups are predicted to exist in an ambient oxygen environment. 30 However, using a multilevel computational strategy, Gibb's free energy of formation was found to be the lowest with a ratio of surface groups for a Ti3C2Tx MXene in a solution with pH = 0 (and an open circuit potential of 0 V) was found to be O0.5F0.25OH0.25. 31 Our MXenes, synthesized in a calculated pH of -0.8, support this prediction with experimental evidence.…”

“…Adsorption of Cl as a surface functional group is unlikely due to its lower energy of adsorption compared with that of O, OH, and F. 30 Generally, it is difficult to intercalate anions, and it has been suggested that large anions (Cl, Br, etc.) may be adsorbed to the edges of MXene during synthesis.…”

“…Likewise, computational studies predicted the role of termination groups to stabilize the surface. [29][30][31][32][33][34] These studies provide crucial energetic insights into the thermodynamic stability of MXenes and the surface chemistry of the stacked layers.…”

Prediction of surface termination preference of out-of-plane ordered double-transition metal MXenes (o-MXenes) from first-principles calculations

“…Up to now, a variety of methods for preparing MXenes have been reported. However, common to the vast majority is that selectively etching the A atomic layer, typically Al, from specific Mn+1AXn (MAX) phases with HF acid [9]. This exploits a feature of MAX phase that the bonding strength of M-X is stronger than that of M-A, but also results in the presence of termination groups (-F, -O, -OH) on both surfaces of the as-synthesized MXene sheets.…”