Thermodynamic stability of borophene, B2O3 and other B1−xOx sheets

Abstract

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−xOx for compositions ranging from x = 0 to x = 0.6, which correspond to borophene and sheets, respectively. Our results indicate that no stable compounds except borophene and sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and . Other hypothetical crystals such as are unfavorable and some of them underwent spontaneous disproportionation into borophene and . It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming 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.