First Principles study of Photocatalytic Water Splitting in BO Monolayer: Effect of Strain and Surface Functionalization

TL;DR

This study demonstrates that a boron oxide (BO) monolayer, while having a large $E_g$ of $E_g^{HSE06} \approx 3.8$ eV and UV-only absorption, can be engineered for visible-light photocatalytic water splitting through two strategies: (1) applying mechanical strain to modestly tune the band gap and band-edge positions, and (2) surface functionalization with dual-atom decorations (C, N, Si, Ge, P, As). Dual-atom decoration, particularly with two Si or two Ge atoms, reduces the band gap and shifts absorption into the visible range around $1.6$ eV while maintaining favorable band-edge alignment relative to the water redox potentials; single-atom decorations tend to induce metallic or high-gap insulating states. Stability analyses (negative formation energies and AIMD up to 9–10 ps) confirm that decorated BO surfaces are thermodynamically and dynamically viable. Overall, functionalized BO monolayers emerge as promising candidates for efficient, solar-driven hydrogen production, offering tunable optical and electronic properties suitable for visible-light photocatalysis.

Abstract

Light element based two dimensional (2D) materials are promising photocatalysts for hydrogen production via water splitting. Boron oxide (BO) is a recently synthesized 2D monolayer which has yet to be thoroughly explored for its potential applications. In this article, using first principles calculations, we report, for the first time, the visible-light photocatalytic activity of a BO monolayer for water splitting under mechanical strain and surface modification with single- and double-atom decorations (C, N, Si, Ge, P, As). The pristine BO monolayer exhibits an indirect band gap of 3.8 eV with band edges spanning the water redox potentials, but its optical absorption lies in the UV region (~ 4.5 eV). Strain engineering tunes the band gap and band alignment with a minimal shifting in the optical absorption (~0.5 eV). Single atom decoration produces a metallic state for elements like N, P, As, and an insulating state for single C, Si, Ge with a partial shifting in optical absorption. In contrast, double atom decoration produces substantial band gap reduction, improved band alignment, a pronounced red-shift in optical absorption into the visible range (1.6 to 3.2 eV) thus satisfying the criteria for water splitting. The stability of all the adsorbed configurations was confirmed by negative formation energy and ab-initio molecular dynamics simulations. These findings suggest BO monolayer functionalization can improve photocatalytic efficiency, providing hydrogen generation insights.

Figures (6)

• Figure 1: (a) Schematic of the crystal structure (top view), (b) side view, (c) Electronic localization function (ELF), Electronic band structure using (d) PBE-GGA, and (e) HSE-06 hybrid functional, (f) projected density of states of pristine BO monolayer. The red and green spheres in (a,b) indicate oxygen and boron atoms, respectively.
• Figure 2: (a) The calculated band structure of BO monolayer adsorbed with (a) two C, (b) two N, (c) two Si, (d) two Ge, (e) two P, (f) two As, (g) one Si, (h) one Ge atoms. The dotted horizontal line indicates the Fermi level.
• Figure 3: The band edge positions of pristine and adsorbed BO monolayer with respect to vacuum potential calculated using (a) PBE and (b) HSE-06 functional.
• Figure 4: The calculated projected density of states of BO monolayer after decorating with (a) two C, (b) two N, (c) two Si, (d) two Ge, (e) two P, (f) two As, (g) one Si and (h) one Ge atoms. The dotted vertical line represents the Fermi level.
• Figure 5: The optical absorption spectrum of pristine and adsorbed BO monolayer. The colored strip represents the visible region.