Thermodynamic stability of borophene, B2O3 and other B1−x O x sheets

• Published in: Journal of physics communications 2020-03, Vol.4 (3)
• Main Authors: Arnold, Florian M, Seifert, Gotthard, Kunstmann, Jens
• Format: Article
• Language: English
• Subjects: Boron
• ISSN: 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 B1−x O x for compositions ranging from x = 0 to x = 0.6, which correspond to borophene and \({{\rm{B}}}_{2}{{\rm{O}}}_{3}\) sheets, respectively. Our results indicate that no stable compounds except borophene and \({{\rm{B}}}_{2}{{\rm{O}}}_{3}\) sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and \({{\rm{B}}}_{2}{{\rm{O}}}_{3}\). Other hypothetical crystals such as \({{\rm{B}}}_{2}{\rm{O}}\) are unfavorable and some of them underwent spontaneous disproportionation into borophene and \({{\rm{B}}}_{2}{{\rm{O}}}_{3}\). It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming \({{\rm{B}}}_{2}{{\rm{O}}}_{3}\) 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.