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 BO2 3 sheets, respectively. Our results indicate that no stable compounds except borophene and BO2 3 sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and BO2 3. Other hypothetical crystals such as BO2 are unfavorable and some of them underwent spontaneous disproportionation into borophene and BO2 3. It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming BO2 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.