Effect of mixed surface terminations on the structural and electrochemical properties of two-dimensional Ti₃C₂T₂ and V₂CT₂ MXenes multilayers

Abstract

MXenes, a family of layered transition metal carbides and nitrides, have shown great promise for use in emerging electrochemical energy storage devices, including batteries and supercapacitors. MXene surfaces are terminated by mixed -O, -F and -OH functional groups as a result of the chemical etching production process. These functional groups are known to be randomly distributed over the surfaces, with limited experimental control over their composition. There is considerable debate regarding the contribution of these functional groups to the properties of the underlying MXene material. For instance, their measured Li or Na capacity is far lower than that predicted by theoretical simulations, which generally assume uniformly terminated surfaces. The extent to which this structural simplification contributes to such discrepancies is unknown. We address this issue by employing first-principles calculations to compare the structural, electronic and electrochemical properties of two common MXenes, namely Ti3C2T2 and V2CT2, with both uniform terminating groups and explicitly mixed terminations. Weighted averages of uniformly-terminated layer properties are found to give excellent approximations to those of more realistic, mixed termination structures. This approximation holds for the lattice parameters, the electronic density of states and the work function. The sodium storage capacity and volume change during sodiation in the interlayer space of these MXenes with mixed surface terminations are also investigated. The redox reaction is shown to be confined to the terminating groups for low concentrations of intercalated Na, with the oxidation state of the metal atoms unaffected until higher concentrations of intercalated Na are achieved. Finally, the average open circuit voltage is shown to be very similar for both Ti3C2T2Na and V2CT2Na with mixed terminations, although it is highly sensitive to the particular composition of the terminating groups.

Surface Termination Dependent Work Function and Electronic Properties of Ti3C2Tx MXene

“…The maximum work function of 4.8 eV is significantly lower than the predicted work function of Ti3C2O2 ( 6 eV), but significantly higher than the predicted work function of bare Ti3C2 (3.9 eV). 41 With desorption of fluorine at higher temperatures, the work function decreases again and saturates at ca. 4.1 eV.…”

“…One possible explanation could be a desorption of residual contaminations, like LiF or Al from the sample preparation process, or atmospheric water, carbon or carbon oxides, which are known to decrease the work function of clean metals via the push-back effect 39,40 . Another reason could be the desorption of OH surface terminations, which are predicted to significantly decrease the work function of Ti3C2 14,41 . Due to the low amount of OH groups as suggested by XPS results, the former is more likely.…”

Are MXenes suitable as cathode materials for rechargeable Mg batteries?

“…Please do not adjust margins could be estimated by weighted averages of the results from uniformly terminated MXenes. 57 From Table S1 we see that Ti3C2Tx has an O/F ratio of 1.36, while V2CTx has an O/F ratio of 0.66.…”

Recent advances in MXenes: From fundamentals to applications

“…It is noted that the above trends in the work functions of functionalzied MXenes are independent of their thickness (n = 1-3) [92]. Interestingly, the recent studies on Ti 3 C 2 O x F y (OH) z with mixtures of surface terminations have shown that the ultralow work function of fully OH terminated MXenes can be attained by 60% OH termination on the surface [12].…”

“…It is difficult to predict the electronic properties of MXenes using a model structure with the mixed adsorption of F, OH, and O on their surfaces. Hence, the majority of theoretical studies have been carried out using models with the uniform adsorption of one of the chemical groups on the surfaces [11] and very few using mixture of these groups [12,13]. Although such ideal structures might be unrealistic at present, they help with the in-depth understanding of the physics and chemistry of MXenes and may guide and motivate ones to perform new experiments.…”