Oxygen vacancies and p-n heterojunction modified BiOBr for enhancing donor density and separation efficiency under visible-light irradiation

Highlights

• Abundant oxygen vacancies were produced in BiOBr through a facile hydrothermal method and acid treatment.

• Oxygen vacancies induce a new electron donor level into BiOBr which can extend visible-light absorption region.

• BiOBr with rich oxygen vacancies has increased carrier concentration and exposure of high activity {001} facets.

• p-type BiOI was deposited on n-type BiOBr by SILAR method to form 2D p-n heterojunction.

• BiOBr-O_v/BiOI photoanode presents 4.58 folds of photocurrent density than bare BiOBr nanosheet.

Abstract

It remains a great challenge to overcome the problems of poor conductivity and rapid charge recombination in bismuth oxybromide (BiOBr). Herein, for the first time we report the synergistic effect of oxygen vacancies (O_v) and p-n heterojunction to improve photoelectrochemical performance of BiOBr. Benefitting from the new electron donor level caused by oxygen vacancies, the visible-light absorption region, carrier concentration and exposure of {001} facets of BiOBr are all increased. In the meantime, the 2D p-n heterojunction between BiOBr and BiOI leads to a built-in electric field and thus significantly accelerating the charge separation and transfer. Upon visible light irradiation, the resulted BiOBr-O_v/BiOI photoanode presents robust photoelectrochemical performance, achieving a photocurrent density of 1.33 mA/cm² at 1.23 V vs. RHE. This work not only provides new insight for designing efficient photoelectrodes but also highlights the importance of oxygen defect-engineering strategy in photoelectrochemical water splitting.

Introduction

Photoelectrochemical (PEC) water splitting based on semiconductor materials is an ideal technology to produce oxygen and hydrogen [1,2]. Since TiO₂ had been used as PEC material to split water and produce H₂ by Fujishima and Honda in 1972, a mass of materials (such as WO₃, Fe₂O₃, and BiVO₄) have been demonstrated promising photoelectrodes for PEC water splitting [[3], [4], [5], [6]]. For decades, it has been being the core of the research for seeking and developing suitable photoelectrodes with high PEC performance. Recently, bismuth oxyhalides (BiOX, X = Cl, Br, I) which belong to the category of Bi-based semiconductors have attracted more and more attention due to their unique two-dimensional (2D) layered structure and high photocatalytic activity [7,8]. All the bismuth oxyhalides are crystallized in a typical tetragonal crystal structure with the alternating arrangement of [Bi₂O₂]²⁺ slabs and X⁻ atoms layer. The atoms in the BiOX layered structure are connected by strong covalent bonds, while the BiOX atomic layers are connected by van der Waals force which is weak and leading to the peeling of the layer structure in the direction of [001] [9,10]. This kind of open-type 2D layered structure provides sufficient space for the polarization of atoms and orbits, which induces the formation of a built-in electric field. The electric field is perpendicular to the bismuth oxygen layer and the halogen layer and is beneficial to the separation of electrons and holes [11,12]. On the other hand, the indirect band gap of BiOX could suppress charge recombination to some extent, thus further enhancing their PEC performance [13,14]. These characteristics all prove the excellent potential of BiOX semiconductors for applying in PEC water splitting.

Nevertheless, the problems of poor conductivity and rapid charge recombination still exist in BiOX, which hinders their practical application. To overcome these challenges, a variety of methods have been developed, including doping, building heterojunction, depositing noble metal, controlling crystal facets and so on [[15], [16], [17], [18], [19]]. Specially, raising the exposed degree of {001} facets of BiOX could improve the photocatalysis property under visible light irradiation. For instance, Li et al. have discussed the transfer and catalytic behavior of photo-induced electrons at BiOX (001) surfaces and interfaces based on density functional theory, and the result indicates that BiOX (X = F, Cl, Br, I) {001} surfaces display an important influence on photocatalytic performance [20]. Recently, people find that the surface oxygen defect which is similar to heteroatoms doping is a promising way to improve the PEC performance of photoelectrodes by affecting their physical-chemical properties and electronic structure [[20], [21], [22]]. Wu et al. have introduced abundant oxygen vacancies and V⁴⁺ species in multi-layer BiVO₄ and the 3-layer BiVO₄ shows highest PEC performances in sulfite oxidation and stability owing to the modulated band structure by the oxygen vacancies and V⁴⁺ species [23]. In terms of BiOX, because of the low bond energy and long bond length of Bi–O bond, oxygen vacancies are easily introduced into BiOX [24,25]. More importantly, there are reports suggest that the oxygen vacancies are also beneficial for enlarging the exposure surface area of {001} facets in BiOX [26]. However, the extensive and deep understanding between the interaction of oxygen vacancy and PEC performance in BiOX is still desired.

Among bismuth oxyhalides, BiOBr owns the most suitable band gap energy (∼2.7 eV) for PEC water splitting, while BiOCl (∼3.2 eV) only responses to ultraviolet light and BiOI (∼1.7 eV) is limited in separation of photogenerated electron-hole pairs. Therefore, we choose BiOBr as research object in our work. Through a simple, facile hydrothermal method and acid treatment, BiOBr nanosheets with abundant oxygen vacancies (BOB-O_v) was successfully prepared on fluorine-doped tin oxide (FTO) transparent conductive glass, and then BiOI (BOI) was fabricated on the BiOBr substrate to construct 2D p-n heterojunction. Upon the introduction of oxygen vacancy (O_v), the exposure of {001} facets have increased and a new doping energy has been created in the band gap, thus increasing donor density and accelerating charge separation. The resulted BOB-O_v/BOI photoanode presents an excellent PEC performance, which highlights the oxygen vacancy engineering for rationally fabricating efficient photoelectrodes in PEC water splitting.