Original Research Paper
Enhanced visible-light-induced photocatalytic disinfection of Escherichia coli by ternary Bi2WO6/TiO2/reduced graphene oxide composite materials: Insight into the underlying mechanism

https://doi.org/10.1016/j.apt.2019.10.005Get rights and content

Highlights

  • Bi2WO6/TiO2/rGO composites with enhanced photocatalytic activity were synthesized.

  • Efficient reduction of GO into rGO promoted separation of photogenerated carriers.

  • Introduction of rGO greatly enhanced light absorption and photocatalytic activity.

  • Photoinduced h+, radical dotOH, e and radical dotO2 are responsible for the disinfection of E. coli.

Abstract

Composites coupling different semiconductors have attracted increasing attention for photocatalytic application owing to low visible-light absorption capability and weak photocatalytic activity of the single-component system. In this study, Bi2WO6/TiO2/rGO ternary composites, successfully synthesized by a facile hydrothermal method, were manifested as an outstanding visible-light-response photocatalyst for the disinfection towards E. coli. X-ray diffraction, Fourier-transform infrared, and X-ray photoelectron spectroscopy demonstrated that GO coupled in the composites was efficiently reduced to rGO during the hydrothermal process, which was greatly beneficial for enhancing light harvest, promoting charge separation, and improving photocatalytic disinfection activity. UV–vis diffuse reflectance spectra, photoluminescence and time-resolved photoluminescence, photocurrent measurements, and ultraviolet photoelectron spectroscopy clearly showed that Bi2WO6/TiO2/rGO composites had narrower band gap energy and much better suppression capability of photoinduced electron-hole recombination in comparison to the pure Bi2WO6 and TiO2, and Bi2WO6/TiO2. The radical trapping results revealed that the photogenerated holes (h+) were the leading active species responsible for the effective inactivation of E. coli under visible-light irradiation. Hence, the underlying mechanism for the enhanced photocatalytic disinfection performance of Bi2WO6/TiO2/rGO composites was proposed. This study provides new insight into the design and development of composite materials with enhanced photocatalytic activity, which can be an inspiring alternative for environmental application.

Introduction

Situation of water quality deterioration caused by pathogenic microorganisms has increasingly raised public concern worldwide. As one of ubiquitous bacteria in the environment, Escherichia coli (E. coli) is always of great interest since most E. coli strains are harmless and pathogenic. E. coli can be commonly found in food products and drinking water [1], [2]. Disinfection is undoubtedly regarded as an effective process for diminishing the harmful pathogens and protecting human beings against infectious waterborne diseases. Chemical disinfection, especially chlorination, is the most popular sanitization technique for water purification [3], [4], [5]. However, evidences continue to grow that chlorine residue can produce significant amounts of harmful disinfection by-products, posing a serious risk to public health [6].

Photocatalysis has attracted considerable attention for water and wastewater disinfection in recent years, mainly due to its favorable efficiency, relatively low toxicity, and potential capability of using solar energy directly [7], [8], [9]. Over the last few years, TiO2 was regarded as the most common photocatalytic material, which has been widely studied for the removal of organic pollutants from wastewater and polluted air [10], [11]. TiO2 was also confirmed as a potent technique for water purification with excellent inactivation efficiency towards E. coli under light irradiation [12], [13]. However, the practical application of TiO2 is restricted due to its wide band gap and poor ability to separate photogenerated electron-hole pairs [14], [15]. Moreover, TiO2 can only absorb UV light, which merely accounts for less than 5% of the entire solar spectrum [16]. Hence, researches on modification of TiO2 for improving its photocatalytic activity under visible-light irradiation become hotspots in the photocatalytic field, such as coupling with metals or other semiconductors to expand spectrum response [17], [18], [19].

Recently, Bi-based photocatalysts have been comprehensively researched due to their unique crystal structures and excellent photocatalytic performance [20], [21], [22]. For example, Bi2WO6 had a flexible Aurivillius structure with a suitable band gap of around 2.8 eV, which was reported as an excellent visible-light-driven photocatalyst for O2 evolution and dye degradation [23]. Seemingly, it is suitable and feasible to couple Bi2WO6 with TiO2 to produce a heterojunction composite for enhancing the photocatalytic activity. Xu et al. [19] successfully coupled Bi2WO6 with TiO2 using a one-step hydrothermal method and demonstrated that Bi2WO6/TiO2 composites had an enhanced visible-light-induced activity in photocatalytic degradation of organic pollutants in contrast to single Bi2WO6 or TiO2. Jia et al. [17] reported that both absorption of visible light and separation of photogenerated carriers of TiO2 were markedly promoted as being coupled with Bi2WO6, resulting in an enhanced photocatalytic disinfection of E. coli. Similar results were also found in other studies [24], [25], [26], manifesting the promising photocatalytic activity of the heterogeneous photocatalysis technology.

Graphene oxide (GO) has become an attractive material in the area of catalysis owing to its unique structure and properties. It contains various oxygenated functional groups, which can provide abundant active sites for nucleation and growth of nanoparticles [27]. It was found that GO possessed features of charge transport in multi-component systems [28]. Owing to its promising electron mobility [29], GO was considered as a promising alternative to be introduced into semiconductors such as TiO2 and Bi2WO6, which could efficiently promote the separation of photocarriers and enhance the photocatalytic efficiency [30], [31]. Therefore, we speculate that the introduction of GO can further enhance the photocatalytic performance of Bi2WO6/TiO2 composites.

In this study, therefore, we successfully fabricated Bi2WO6/TiO2/reduced graphene oxide (rGO) composites via a facile hydrothermal method. The properties of the prepared composite photocatalysts were comprehensively characterized by a variety of microscopy and spectroscopy techniques. The photocatalytic disinfection activity was evaluated towards E. coli under visible-light irradiation (λ > 420 nm). Based on the radical trapping results, a possible mechanism for the enhanced visible-light-response photocatalytic disinfection activity of Bi2WO6/TiO2/rGO composites was proposed.

Section snippets

Materials

All chemicals are of analytical grade. Titanium tetraisopropoxide (TTIP, purity 99%) was purchased from Aladdin Chemical Reagent Co. Ltd (Shanghai, China). Na2WO4·2H2O (purity 99.5%), Bi2(NO3)2·5H2O (purity 99%), NaOH (purity 96%), and anhydrous ethanol (purity 99.7%) were obtained from Sinopharm Chemical Reagent CO., Ltd (Shanghai, China). GO was purchased from Sigma-Aldrich (USA).

Synthesis of Bi2WO6/TiO2 composites

Bi2WO6/TiO2 composites were synthesized via a facile hydrothermal method. Briefly, 2.425 g of Bi(NO3)3·5H2O was

Characterization of the prepared photocatalysts

The morphologies and microstructures of Bi2WO6, Bi2WO6/TiO2, and Bi2WO6/TiO2/rGO composites were characterized by SEM and TEM images (Fig. 1). The bare Bi2WO6 exhibited a typical plate-like structure with an average width of approximately 300 nm excluding the fragmentized pieces (Fig. 1a). As for Bi2WO6/TiO2, it is clear that TiO2 nanoparticles adhered to the surface of Bi2WO6 nanoplates, although particle agglomeration was observed (Fig. 1b). The existence of rGO nanosheets in the composite

Conclusions

In summary, the Bi2WO6/TiO2/rGO photocatalysts were successfully synthesized via a facile hydrothermal method. Compared with the pure Bi2WO6 and TiO2, as well as the Bi2WO6/TiO2 composites, the coupled Bi2WO6/TiO2/rGO exhibited an enhanced visible-light-response activity in photocatalytic disinfection towards E. coli. The introduction of well-deoxygenated rGO could not only enhance the absorption of visible light, but also improve the separation and mobility of photogenerated carriers

Declaration of Competing Interest

None.

Acknowledgements

This work was financially supported by the Graduate Degree Thesis Innovation Foundation of Donghua University (GSIF-DH-M-2019008), the Chinese Universities Scientific Fund (277-10-0001046), the Guangxi Innovation Drive Development Fund (AA17204076), and the Natural Science Foundation of Shanghai (17ZR1400400).

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