Effect of surface composition on the stability of Ti- and V-based oxycarbide and oxynitride MXenes

resumo

Oxycarbide MXenes were recently experimentally identified, opening up the possibility of a vast majority of carbon-based MXenes synthesised to date being in fact oxycarbides, wherein some substitutional oxygen atoms are present amid the layers that were previously thought to contain carbon only. Here, using an approach based on density-functional theory, we study the structural, energetic and electronic properties of subsurface substitutional oxygen atoms on the X layers of MXenes of the form M2XT2, with M = Ti, V; X = C, N; and T = none, O or F. According to the calculations, subsurface O is thermodynamically stable in any concentration on all the analysed MXenes. Subsurface substitutional O atoms in the Ti2C MXene are thermodynamically driven towards the surface, leaving behind vacancies and creating local O surface terminations. Given that this process involves a very low energy barrier, we predict that, under conditions suitable for eliminating the surface termination of MXenes, many vacancies on the layers of X atoms should occur. The O-terminated carbide MXenes, Ti2CO2 and V2CO2, are stable with their hexagonal symmetry up to a threshold amount of subsurface O, around 75 %, which is above the maximum experimentally observed concentration, 65 %. The F surface termination stabilizes the hexagonal structure of MXenes, which no longer distort regardless of the presence and amount of subsurface oxygen. This suggests that MXene synthesis routes based on fluorine-containing compounds are the most indicated for synthesising nitride MXenes. The PBE functional predicts the semiconductor characteristics of the Ti2CO2 MXene to be lost in the presence of subsurface O, even a percentage as low as 6 %, the lowest considered in this work. The hybrid HSE06 functional predicts the oxycarbide material to remain a semiconductor, but with a significantly lowered band gap energy with increasing substitutional O content. Nevertheless, both functionals agree that the mixture of C and O atoms between the Ti layers of the Ti2CO2 MXene increases its conductivity. To the best of our knowledge, this MXene has never been confirmed experimentally to behave as a semiconductor probably because the real samples of this MXene contain subsurface oxygen.

palavras-chave

ELASTIC BAND METHOD; ELECTRONIC-PROPERTIES; OXYGEN; TERMINATIONS; MOLYBDENUM; POINTS

categoria

Materials Science; Physics

autores

Gouveia, JD; Gomes, JRB

Grupos

G6 - Materiais Virtuais e Inteligência Artificial

Projectos

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)

MXenes catalysts for the water gas shift reaction (ForTheShift)

Collaboratory for Emerging Technologies, CoLab (EMERGING TECHNOLOGIES)

agradecimentos

This work was developed within the scope of the projects CICECO- Aveiro Institute of Materials, with refs. UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, and ForTheShift, with ref. 2022.02949.PTDC, financed by national funds through the FCT/MEC (PIDDAC) . We are also thankful to FCT I.P. for the computational resources granted in the framework of project Ref. 2022.15802.CPCA.A2 by the FCT/CPCA/2022/01 Call for Advanced Computing Projects. JDG thanks the Portuguese Foundation for Science and Technology (FCT) through the grants with Refs. 2022.00719.CEECIND and 2023.06511. CEECIND, in the scope of the Individual Calls to Scientific Employment Stimulus - 5th and 6th Editions. The authors thank Dr Pawel Micha- lowski and Prof. Yury Gogotsi for the fruitful discussions.