Fabrication of novel narrow/wide band gap Bi₄O₅I₂/BiOCl heterojunction with high antibacterial and degradation efficiency under LED and sunlight

Highlights

• Novel narrow/wide band gap Bi₄O₅I₂/BiOCl composite material was synthesized.

• Bi₄O₅I₂/BiOCl can remove E. coli, S. aureus and TC in LED light and sunlight.

• In LED light, BiOCl acted as an electron-receiving platform.

• In sunlight, Bi₄O₅I₂/BiOCl follows a Z-scheme mechanism.

• The performance of Bi₄O₅I₂/BiOCl in sunlight is better than in LED light.

Abstract

It is necessary to construct photocatalytic heterojunctions with high efficiency to remove bacteria and antibiotics from polluted water. To achieve this goal, by selecting Bi₄O₅I₂ with narrow band gap and BiOCl with wide band gap, a novel Bi₄O₅I₂/BiOCl heterojunction was constructed by a simple precipitation method. Various characterizations including XRD, TEM and XPS confirmed that the Bi₄O₅I₂/BiOCl heterojunction was successfully synthesized. The optimum sample 30% Bi₄O₅I₂/BiOCl can inactivate Escherichia coli (E. coli) in 30 min, Staphylococcus aureus (S. aureus) in 75 min and degrade 76% tetracycline (TC) in 60 min in LED light. To further study the performance of the photocatalyst, the photocatalyst was tested in sunlight. It was found 30% Bi₄O₅I₂/BiOCl could inactivate E. coli in 6 min, S. aureus in 15 min and degrade 84% TC in 60 min under sunlight, realizing a shorter reaction time and a better degradation efficiency. Photocurrent and electrochemical impedance tests (EIS) results confirmed higher electron hole separation efficiency in composites. The mechanism of removing pollutants under LED light and sunlight was verified by their respective capture experiments. This experiment provides a new possibility for the application of photocatalysts in a variety of light sources.

Introduction

Water pollution caused by antibiotics and microorganisms is damaging the environment and human health. To explore effective technology for the removal of antibiotics and microorganisms, photocatalytic technology has been widely concerned and applied in wastewater treatment for its environmentally friendly and sustainable performance [1], [2], [3], [4], [5]. In general, under photoirradiation conditions photocatalysts can produce reactive oxygen species, including superoxide (•O₂^–), hydroxyl (•OH) and holes (h⁺), which played important roles in photodegradation and antibacterial activity [6], [7], [8], [9], [10]. Therefore, it is a necessary condition for photocatalytic reaction to design a highly efficient photocatalyst capable of producing these oxidizing active substances.

Nowadays, bismuth based photocatalysts BiOX (X = I, Br, Cl) have become research hotspots due to their unique layered structure and good visible light response characteristics [11], [12], [13]. As a member of BiOX catalysts, BiOCl shows good photocatalytic activity under UV light, for it has a wide band gap (3.2 ~ 3.5 eV) and cannot be excited by visible light [14], [15], [16]. Due to the single BiOCl system generally has poor photocatalytic activity, heterojunction photocatalysts are usually combined with other semiconductors with interlaced band structures [17], [18]. For example, CNFs/BiOCl can degrade 51% TC within 60 min, Ag/BiOCl can remove 100% E. coil within 120 min, Ag/AgCl/BiOCl can remove 100% S. aureus within 180 min, AgCl/AgI/BiOCl can remove 90% S. aureus within 300 min. The photocatalytic activity was effectively promoted by broadening the range of visible light absorption and promoting the separation of electrons and holes under the built-in electric field. In view of above performance of BiOCl, it was found that the advantage of BiOCl was thought of as follows: (1) the recombination of photoexcited electrons and holes in wide band gap is lower than that in narrow band gap semiconductor; (2) a more positive valence band (VB) is easy to produce holes with strong oxidation property, and enhance the oxidation ability of materials, subsequently improve its photocatalytic properties. Therefore, to achieve an efficient photocatalysts that can produce oxidizing active species for antibacterial and degradation, BiOCl is a good candidate catalyst for the construction of composite photocatalyst.

In recent years, a series of bismuth-rich materials, such as Bi₅O₇I [19], Bi₇O₉I₃ [20], Bi₄O₅I₂ [21], have been prepared. These catalysts have narrower band gap than BiOCl and can absorb visible light. Among them, Bi₄O₅I₂ has a loose nanosheet structure obtained by thermal decomposition of BiOI [22]. Due to its small band gap, Bi₄O₅I₂ has great potential in improving visible light utilization (CB = -1 eV, VB = 1.05 eV) [23]. However, the recombination of photoexcited charges reduces the efficiency of photocatalytic reaction. Therefore, combining a semiconductor with Bi₄O₅I₂ to form composite photocatalysts is a relatively simple and an effective measure to broaden absorption in visible range and improve their photocatalytic performance. For example, AgCl/Bi₄O₅I₂ [24], Yb³⁺/Er³⁺/Bi₄O₅I₂ [25], AgI/g-C₃N₄/Bi₄O₅I₂ [21], Fe₃O₄/Bi₄O₅I₂ [26] etc. In above composites materials, the hybrid structure is formed at the interface, and the transfer of photoexcited charge can be improved through the heterojunction band gap engineering. Therefore, the design of Bi₄O₅I₂ related heterojunction as a high efficiency photocatalyst is still worth studying.

Based on the advantage of BiOCl and the suitable band gap between Bi₄O₅I₂ and BiOCl, we combined Bi₄O₅I₂ with BiOCl to prepare a novel Bi₄O₅I₂/BiOCl heterojunction photocatalyst. The strategy has following advantages: (1) under LED light, although BiOCl cannot be excited, its conduction band can obtain the electrons from narrow band gap Bi₄O₅I₂, promote the separation of electrons and holes; (2) under sun light, both Bi₄O₅I₂ and BiOCl can be excited, the low recombination rate of wide band gap BiOCl and its positive valence band position make photoexcited holes of high oxidation property, which is favorable for the photocatalytic reaction; (3) the performance of Bi₄O₅I₂/BiOCl will be improved by broadening absorption ability in visible range and promoting the separation of electron-hole pairs. Up to now, the research of Bi₄O₅I₂/BiOCl composite has not been reported. Therefore, the combination of Bi₄O₅I₂ and BiOCl is anticipated to make up for their respective defects, decrease the recombination of electrons and holes, improve the photocatalytic oxidation ability, and is conducive to the removal of bacterial and organic pollutants under visible light or sunlight.

In order to realize this idea, we designed a novel Bi₄O₅I₂/BiOCl heterojunction photocatalyst with excellent photocatalytic performance by depositing BiOCl nanoflower on Bi₄O₅I₂ nanosheet. Both in visible light and in sun light, the photocatalytic activity of Bi₄O₅I₂/BiOCl heterojunction was estimated by removing TC, E. coli and S. aureus respectively. This result showed it was efficient for us to use less amount of catalyst and shorter reaction time to realize photocatalytic reaction compared with the related BiOCl-based photocatalysts. In addition, various techniques were adopted to verify the existence of Bi₄O₅I₂/BiOCl heterojunction. Based on the capture experiments and photoelectrochemical characterization, the possible mechanism in LED light and sunlight was proposed, respectively. Therefore, this work may provide a new insight for designing efficient photocatalyst under multiple light sources.