Thermodynamic stability of Borophene, $\mathrm{B_2O_3}$ and other $\mathrm{B_{1-x}O_x}$ sheets

TL;DR

This study uses first principles calculations to explore the stability and phase diagram of borophene and boron oxide sheets, revealing that only pure borophene and B2O3 are stable, with implications for oxidation behavior.

Contribution

It provides the first comprehensive phase diagram of B–O 2D materials, identifying stable phases and explaining oxidation tendencies of borophene.

Findings

Only borophene and B2O3 sheets are stable phases.

Intermediate compositions are heterogeneous mixtures.

Oxidation inside borophene flakes is thermodynamically unfavorable.

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 behavior of borophene. Here we use first principles calculations to study the phase diagram of free-standing, two-dimensional $\mathrm{B_{1-x}O_x}$ for compositions ranging from $x=0$ to $x=0.6$, which correspond to borophene and $\mathrm{B_2O_3}$ sheets, respectively. Our results indicate that no stable compounds except borophene and $\mathrm{B_2O_3}$ sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and $\mathrm{B_2O_3}$. Other hypothetical crystals such as $\mathrm{B_2O}$ are unstable and some of them were found to undergo spontaneous disproportionation into borophene and $\mathrm{B_2O_3}$. It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming $\mathrm{B_2O_3}$ 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 understand the process of oxidation of borophene and other two-dimensional materials.