Porous hollow Ag/Ag₂S/Ag₃PO₄ nanocomposites as highly efficient heterojunction photocatalysts for the removal of antibiotics under simulated sunlight irradiation

Highlights

• The Ag/Ag₂S/Ag₃PO₄ heterojunction nanocomposite was fabricated by coprecipitation-refluxing.

• The Ag/Ag₂S/Ag₃PO₄ as an efficient visible-light driven photocatalyst for the antibiotic removal application.

• Morphology engineering of the hierarchical microspheres was performed by controlling the reaction conditions.

• Excellent photocatalytic degradation of tetracycline as model antibiotic was achieved under simulated sunlight irradiation.

• A dependable degradation mechanism for the superior photocatalytic performance was explained.

Abstract

Antibiotic pollutants are a serious and growing threat to human health and the environment that efficient measures must be taken to eliminate them. Here, we report the facile fabrication of porous hollow Ag/Ag₂S/Ag₃PO₄ heterostrucutres for efficient photocatalytic degradation of tetracycline under simulated sunlight irradiation. The morphology manipulation and hetero-nanocomposites construction through a coprecipitation-refluxing approach were applied to enhance the photocatalytic performance of the Ag/Ag₂S/Ag₃PO₄ products. The photodegradation outcomes indicated that the heterojunction Ag/Ag₂S/Ag₃PO₄ photocatalyst with a suitable band gap energy of 2.17 eV, has better degradation performance (∼95%) than individual Ag₂S and Ag₃PO₄ structures after 120 min of simulated sunlight irradiation, even after five recycles. The good photocatalytic activity of Ag/Ag₂S/Ag₃PO₄ nanocomposites could be mainly attributed to the unique hierarchical architectures, promoted visible-light harvesting, reduced a recombination and boosted separation of electron-hole pairs originated from the as-formed heterojunctions. Moreover, we proposed a photocatalytic degradation mechanism based on the radical scavenging results, which disclosed that the ^•O₂⁻ and ^•OH species perform essential tasks for the photodegradation of antibiotics by Ag/Ag₂S/Ag₃PO₄ nanocomposites.

Introduction

In the last few decades, water contaminants caused by medicinal effluents such as antibiotics have become a critical environmental issue that threatens humans, animals, and plants (Tang et al., 2016; Wang et al., 2019a). Wastewaters from pharmaceutical factories, hospitals, and industries possess large amounts of antibiotic compounds that could be released into the aquatic environment (Kümmerer, 2009; Lu et al., 2019). Unfortunately, most of the antibiotics are not destroyable by common treatment methods due to their complicated structures and intrinsic resistance (Khodadoost et al., 2017; Malakootian et al., 2019). Among the various antibiotics, tetracycline is widely applying to treat both human and animal infectious diseases (Lu et al., 2019; Xu et al., 2019). So far, various treatment technologies including coagulation, absorption, filtration, reverse osmosis, biological and advanced chemical treatments have reported for the elimination of antibiotic pollutants from aqueous media (Dimitrakopoulou et al., 2012; Malakootian et al., 2019; Ruan et al., 2019; Xu et al., 2019). The antibiotic compounds do not degrade completely by biological and physical routes, hence, advanced chemical processes are required (Fallahi Motlagh et al., 2019). Among the above-mentioned approaches, the photocatalytic oxidation process demonstrates highly efficient performance for the decomposition of antibiotics and organic pollutants from environmental wastewaters (Chen et al., 2010; Pan and Zhu, 2015; Valian et al., 2017a; Najafian et al., 2018; Ruan et al., 2019). Previously, there have been reports of the excellent performance of the photocatalytic process for degradation of environmental contaminants, which indicate the efficient potential of this technique for the treatment and removal of harmful wastewater in large-scale applications (Namvar et al., 2017; Ghanbari and Salavati-Niasari, 2018; Orooji et al., 2020). This procedure has advantages such as solar-light-driven, high-efficiency, recyclable catalyst, eco-friendly, cost-effective, repeatable, and non-toxicity (Duo et al., 2015; Chen et al., 2017a; Valian et al., 2017b).

In recent years, silver-based photocatalysts have found numerous applications in photocatalytic technologies due to their excellent response to visible-light irradiation. As a novel member, silver orthophosphate (Ag₃PO₄) as an n-type semiconductor with a bandgap of 2.35 eV, has attained special attention owing to its non-toxicity, unique optical absorption, excellent oxidation activity, superior quantum efficiency, and higher visible-light photocatalytic performance than TiO₂, BiVO₄, and BiOCl materials (Yi et al., 2010; Bi et al., 2011a; Hua et al., 2015; Martin et al., 2015; Teng et al., 2015; Cheng et al., 2016; Patil et al., 2016). However, applying bare Ag₃PO₄ in the photocatalytic process is limited due to its photochemical instability, photo-corrosion property, and rapid recombination of photogenerated charge carriers (Chen et al., 2015; Tang et al., 2020). So, the stability and photocatalytic performance of Ag₃PO₄ structures need to be further improved.

On the other hand, silver sulfide (Ag₂S) as a p-type compound with a narrow band gap of about 1.0 eV, has a high absorption coefficient in the whole solar spectrum. This property makes it a promising photosensitizer for photocatalysts under visible and near-infrared light illumination (Bose et al., 2014; Khanchandani et al., 2014). Moreover, silver (Ag) has excellent electrical conductivity and surface plasmon resonance effects, which is a promising candidate for high-intensity electron transferring in the photocatalysts (Yu et al., 2013; Wang et al., 2019b, 2020). Therefore, adding Ag and Ag₂S to Ag₃PO₄ compounds can remarkably augment the stability and photocatalytic performance of the Ag₃PO₄ photocatalyst.

So far, many strategies have been reported to improve the visible-light photocatalytic activity of Ag₃PO₄ materials. One of the effective approaches is construction of heterojunction nanocomposites by coupling Ag₃PO₄ with other appropriate semiconductors, which can reduce the recombination rate of electron-hole pairs and strength the chemical stability of the Ag₃PO₄ structure. Formerly, various Ag₃PO₄-based composites including Ag₃PO₄/TiO₂, Ag₃PO₄/In(OH)₃, Ag₃PO₄/BiVO₄, Ag₃PO₄/BiOI, Ag₃PO₄/SnO₂, CdSe/Ag₃PO₄, Ag₃PO₄/g-C₃N₄, Ag₃PO₄/graphene oxide, Ag₃PO₄/SiO₂, AgX/Ag3PO4 (X = Cl, Br, I), Ag₃PO₄/CeO₂, Ag₃PO₄/Nb₂O₅, Ag/Ag₃PO₄/WO₃, Ag/Ag₃PO₄ and Ag₂S/Ag₃PO₄ have been synthesized (Bi et al., 2011b; Yao et al., 2012; Zhang et al., 2012; Chen et al., 2013; Guo et al., 2013; Hu et al., 2013; Li et al., 2013, 2017; He et al., 2014; Yan et al., 2014; Yang et al., 2014; Shao et al., 2015; Wang et al., 2015; Ma et al., 2016; Kim and Jo, 2019). In the Ag/Ag₂S/Ag₃PO₄ ternary systems, the heterojunction nanocomposites exhibit higher photochemical stability and photocatalytic performance compared to the pure Ag₂S or Ag₃PO₄ due to the matched bandgap structures, rapid transfer of electrons and holes, the promoted optical absorption, and the enhanced separation rate of photo-induced charge carriers (Tan et al., 2017). Subsequently, the Ag/Ag₂S/Ag₃PO₄ heterostructures can be employed as superior photocatalyst for the decomposition of antibiotics pollutants under sunlight irradiation.

Herein, we report the synthesis of hollow hierarchical microspheres of the Ag/Ag₂S/Ag₃PO₄ heterojunction by a simple coprecipitation-refluxing route. The prepared Ag/Ag₂S/Ag₃PO₄ nanocomposites were used as potential photocatalysts for treat tetracycline under simulated sunlight radiation. The Ag/Ag₂S/Ag₃PO₄ nanocomposites show much higher photocatalytic activities (∼95%) than pure Ag₂S and Ag₃PO₄ structures. Moreover, the effect of photocatalyst dosage, antibiotic concentration, morphology, type of antibiotic, and reusability of the photocatalyst on the degradation efficiency of pollutants were studied. Furthermore, the photodegradation mechanism elucidated that the superoxide (^•O₂⁻) and hydroxyl (^•OH) radicals had main role in the degradation process of tetracycline by Ag/Ag₂S/Ag₃PO₄ nanocomposites.