Tuning the Electronic Properties of the Titanium Carbide Mxene with Variable Sublattice Oxygen Composition

Abstract

MXenes are two-dimensional materials first synthesised circa 12 years ago, with applications in areas such as optoelectronics, supercapacitors, sensors, theranostics or catalysis. Recently, using atomic resolution mass spectrometry, it was demonstrated that most, if not all, carbide MXenes synthesized to date are in fact oxycarbides. Here, using a first-principles approach, based on density functional theory (DFT), we study the substitutional oxygen defect and clusters thereof in the titanium carbide (Ti3C2) MXene surface to form the corresponding oxycarbide. We found that this defect is very stable, at least up to 75 % replacement of the carbon atoms with oxygen. If all the carbon is replaced, the surface transitions to a different geometry, in which its atomic layers are aligned in an ABA fashion. Substitutional O clusters in Ti3C2 impose almost no lattice strain, even at high O concentrations, justifying the delay in the identification of oxycarbide MXenes. We found that the Ti3C2-based oxycarbides most likely display near metallic behaviour regardless of the oxygen concentration on the carbon layer of the MXene.Substitutional oxygen on the Ti3C2 MXene attracts CO and CO2 molecules and allows the dissociation of the latter to occur almost spontaneously, as is known to happen on bare MXenes. The oxycarbide surface considered here with stoichiometry closest to the experimental one, Ti3C1.38O0.62, adsorbs CO and CO2 more strongly than pristine Ti3C2 by 0.45 and 0.25 eV, respectively, which are noticeable differences that could be detectable experimentally.CO and CO2 were also deemed suitable for identifying substitutional oxygen defects in Ti3C2 through vibrational analysis. On the Ti3C1.38O0.62 and Ti3O2 MXene surfaces, the CO stretching mode is downshifted with respect to Ti3C2 by 61 and 87 cm-1, respectively. The presence of oxygen can be even more clearly identified by the downshift of the symmetric stretching mode of adsorbed CO2 by at least 100 cm-1, even at an O/C ratio of 1/31, allowing the experimental detection of this defect.

Keywords: MXenes, density functional simulations, oxycarbides, adsorption