Co²⁺-Doped BiOBr_xCl_1-x hierarchical microspheres display enhanced visible-light photocatalytic performance in the degradation of rhodamine B and antibiotics and the inactivation of E. coli

Highlights

• Co²⁺-doped BiOBr_xCl_1-x has been synthesized by microwave-assisted hydrothermal approach.

• Superior performance of Co²⁺-doped BiOBr_xCl_1-x over the pure BiOBr catalysts.

• The doped BiOBr_xCl_1-x is superior in dye degradation and bacterial inactivation.

• The doped BiOBr_xCl_1-x reveals excellent stability and recyclability.

Abstract

In this article, we have synthesized Co²⁺-doped BiOBr_xCl_1-x hierarchical nanostructured microspheres, featuring different degrees of Co²⁺ doping, displaying excellent photocatalytic performance. X-ray diffraction and Raman spectroscopy indicated that the Co²⁺ ions were successfully doped into the BiOBr_xCl_1-x nanocrystals. The photodegradation rate of rhodamine B mediated by a doped BiOBr_xCl_1-x was 150 % greater than that of the non-doped BiOBr. We ascribe the improved photocatalytic capability of the Co²⁺-doped BiOBr_xCl_1-x to a combination of its superior degree of light absorption, more efficient carrier separation, and faster interfacial charge migration. The major active species involved in the photodegradation of RhB also has been investigated. Moreover, the doped BiOBr_xCl_1-x possessed excellent cellular biocompatibility and displayed remarkable performance in the photocatalytic bacterial inactivation.

Introduction

With greater awareness of the need for environmental protection, the development of methods for environmental remediation, water recycling, and the removal of organic contaminants has become increasingly important. Semiconductor photocatalysts (Wu et al., 2019; Tan et al., 2019; Xie et al., 2019; Toe et al., 2018) have received much attention for their great potential in water splitting (Wang et al., 2019), wastewater purification (Yahya et al., 2018), antibiotic degradation (Kansal et al., 2014; He et al., 2019), and microbial inactivation (Zhu et al., 2018). Bismuth oxyhalides are attractive photocatalysts because of their tunable light responses (Mi et al., 2018; Zhang et al., 2006; Li et al., 2011), ease of preparation, and chemical stability. Among them, BiOBr has a particularly suitable energy band position and remarkable photocatalytic activity and has, therefore, garnered considerable attention. The tetragonal configuration of BiOBr, comprising [Bi₂O₂] slabs interleaved with double halogen slabs, produces a strong self-built internal potential, which can facilitate the separation of the photogenerated charge (Cheng et al., 2011). Nevertheless, the photocatalytic capability of BiOBr remains limited by its wide band gap (ca. 2.9 eV) (Jiang et al., 2010) and high electron/hole recombination rate.

Many efforts have been made previously to improve the photocatalytic performance of BiOBr; they can be categorized into three major approaches. (1) The formation of BiOBr-based solid solutions, including BiOCl_1–xBr_x and BiOI_1–xBr_x, to tailor the optical band gap and, thereby, extend the light response range (Liu et al., 2011; Wang et al., 2008). (2) Combining BiOBr with other semiconductors (Wang et al., 2015a; Shenawi-Khalil et al., 2012; Zhang et al., 2016) or noble metals (Guo et al., 2016; Pálmai et al., 2017) to construct p–n heterojunctions that improve the separation of electron/hole pairs. (3) Doping of heteroatoms, including transition metals (Tu et al., 2015a; Li et al., 2017a; Guo et al., 2019a) and non-metallic dopants (Jiang et al., 2015; Wu et al., 2016), to improve the separation of photoexicited electron/hole pairs and narrow the band gap of semiconductors for stronger absorbance. Moreover, the doping process can increase the carrier density and mobility, thereby prolonging the transfer paths for photoexcited electron/hole pairs (Jiang et al., 2012). Among these methods, several drawbacks remain, particularly for the formation of solid solutions and heterojunctions. For example, the formation of BiOBr-based solid solutions might disrupt the layered structure and weaken the self-built potential within BiOBr. Coupling BiOBr with other semiconductors might make it difficult to form well-distributed heterojunctions, while also increasing production costs. Therefore, doping BiOBr with foreign ions appears to be the best way to improve the photocatalytic performance.

Cobalt (Co) is one of the most effective doping species because of its abundance of different electronic states. Moreover, because the ionic radii of Co²⁺ (0.65 Å) is much smaller than the size of Bi³⁺ (1.03 Å) (Huang et al., 2019), Co²⁺ ions should readily be incorporated into the BiOBr lattice to tailor its electronic structure. In addition, some pioneering photocatalysts, including ZnO (Kuriakose et al., 2014), TiO₂ (Wang et al., 2015b), and Fe₂O₃ (Suresh et al., 2017), have exhibited enhanced photocatalytic activity after doping with Co²⁺ ions. Herein, we have prepared the Co²⁺-doped BiOBr_xCl_1-x microspheres via solvothermal approach. We found that modification with Co²⁺ ions could narrow the band gap of BiOBr_xCl_1-x, extending its absorption range and enhancing its degree of charge separation and, thereby, increasing its photocatalytic activity. The photocatalytic capabilities of our as-prepared BiOBr_xCl_1-x powders were investigated by performing the degradation of an organic dye and antibiotic and the inactivation of E. coli. Herein, we also discuss the mechanism behind the improved photocatalytic activity of the Co²⁺-doped BiOBr_xCl_1-x.