Thermodynamic stability of borophene, B 2 O 3 and other B 1−x O x sheets

• Published in: Journal of physics communications 2020-03, Vol.4 (3), p.31001
• Main Authors: Arnold, Florian M, Seifert, Gotthard, Kunstmann, Jens
• Format: Article
• Language: English
• ISSN: 2399-6528; 2399-6528
• DOI: 10.1088/2399-6528/ab7a76

The recent discovery of borophene, a two-dimensional allotrope of boron, raises many questions about its structure and its chemical and physical properties. Boron has a high chemical affinity to oxygen but little is known about the oxidation behaviour of borophene. Here we use first principles calculations to study the phase diagram of free-standing, two-dimensional B 1− x O x for compositions ranging from x  = 0 to x  = 0.6, which correspond to borophene and B 2 O 3 sheets, respectively. Our results indicate that no stable compounds except borophene and B 2 O 3 sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and B 2 O 3 . Other hypothetical crystals such as B 2 O are unfavorable and some of them underwent spontaneous disproportionation into borophene and B 2 O 3 . It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming B 2 O 3 at the edges. All findings can be rationalized by oxygen’s preference of two-fold coordination which is incompatible with higher in-plane coordination numbers preferred by boron. These results agree well with recent experiments and pave the way to better understand the process of oxidation of borophene and other two-dimensional materials.