abstract
MAX phases are layered materials that have been known for over 60 years and MXenes are their metaphorical children, a family of two-dimensional (2D) materials that appeared in 2011. MXenes are often synthesised by etching out the layers of the A element of their parent MAX phases. The remaining 2D MXene layers are very reactive and spontaneously become covered by a layer called the surface termination. The Mo2C MXene was first synthesised in 2015, by using hydrofluoric acid to remove the Ga layers from the Mo2Ga2C MAX phase, and its surface termination was determined to be mostly composed by O groups. While the layers of Mo and C in Mo2Ga2C are arranged in a face-centred cubic manner, those of Mo2C prefer a hexagonal close-packed stacking. This implies that, at some point during the etching process, a phase transition should occur because of the replacement of Ga by O. Herein, we employ density-functional theory calculations to analyse the thermodynamics of this process by assessing the stability of the two stacking phases of the Mo2Ga2C MAX phase, as the Ga atoms are gradually replaced by O ones. Among other results, we identify the preferred locations of O and F atoms in the Mo2Ga2C lattice, propose a possible explanation for the greater presence of O atoms over F ones on the surface of as-synthesised Mo2C MXenes, evaluate the interaction between O atoms in Mo2Ga2C crystals, estimate the energy of Ga-Ga bonds, and lastly, show that the relative stability of the two stacking phases of Mo2Ga2C is reversed when approximately half of the Ga content is replaced by O.
keywords
2-DIMENSIONAL MOLYBDENUM CARBIDE; 1ST PRINCIPLES
subject category
Chemistry; Physics
authors
Gouveia, JD; Gomes, JRB
our authors
José Daniel Lago da Silva Neves Gouveia
Assistant Researcher
José R. B. Gomes
Principal ResearcherProjects
CICECO - Aveiro Institute of Materials (UIDB/50011/2020)
CICECO - Aveiro Institute of Materials (UIDP/50011/2020)
Associated Laboratory CICECO-Aveiro Institute of Materials (LA/P/0006/2020)
Produção de hidrogénio por decomposição de ácido fórmico usando MXenos (HydroForMX)
Collaboratory for Emerging Technologies, CoLab (EMERGING TECHNOLOGIES)
acknowledgements
This work was developed within the scope of the projects CICECO-Aveiro Institute of Materials, with ref. UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, HydroFormX with ref. 2023.12765.PEX, and CRF - CRG 2022 KAUST with ref. ORFS-2022-CRG11-5075.2. We are also thankful to FCT I. P. for the computational resources granted in the framework of project ref. 2024.13633.CPCA.A3 by the FCT 2024 Call for Advanced Computing Projects. JDG thanks the Portuguese Foundation for Science and Technology (FCT) through the grant with ref. 2023.06511.CEECIND, in the scope of the Individual Call to Scientific Employment Stimulus - 6th Edition.