High photocatalytic degradation of tetracycline under visible light with Ag/AgCl/activated carbon composite plasmonic photocatalyst
Graphical abstract
Introduction
Recently, antibiotics are widely used for human and veterinary medicine to treat and prevent disease of human and animals. The effluents of medical industry disposed a large amount of antibiotics content, which mixed with water body will cause severe problems, including generated drug resistant and decreased resistance of human bodies, polluted environment, and even affected on ecosystem [1], [2], [3]. Thus, it is imperative to treat antibiotics before discharging into the environment and reduce the influence of antibiotics in the water. As a result, various physical, chemical, biological methods have been employed to treat the antibiotic contaminated water, such as electrochemical method [4], [5], physical adsorption [6], [7], [8], [9], degradation [10], [11], and photocatalysis [12], [13], [14], [15], [16], [17], etc. Among these methods, photocatalytic degradation is one of the most available techniques to remove antibiotics in the water from an economic and environmental point of view. It is well known that the photocatalysts play important role in the photocatalytic degradation processes. As one kind of popular semiconductors, TiO2 is limited using as one economic photocatalyst because of its low quantum efficiency which owed to wide band gap of TiO2 and led inability to effectively utilize solar irradiation which only contains a small proportion (∼4%) of high energy ultraviolet (UV) [18], [19].
In order to overcome the low quantum efficiency, many photocatalysts have been designed through impurity doping, metals deposition [16], [17], [20], nonmetal elements doping [18] and composite technology [21], [22], [23], [24], etc. which could effectively enhance visible light response and reduce electron–hole recombination during photocatalytic degradation processes. Among these photocatalysts, silver halide is widely interested by the surface plasmon resonance (SPR) which is mainly caused by nanoparticles of metallic Ag under light irradiation and widely used for enhancing the performance of materials. Therefore, SPR could effectively enhance the photocatalytic degradation rate of organic pollutants through interaction between free electrons of metal surface and photons [25], [26], [27]. Therefore, the physical optical phenomenon is used to prepare photocatalytic materials which enlarged greatly their visible light response range and increased quantum efficiency, as well as improving photoelectric conversion efficiency used in the solar cell. Due to this special phenomenon, AgX (X = Cl, Br and I) is widely considered as the visible light photocatalyst. However, the nanoparticles of AgX are easy to agglomeration which always caused low photocatalytic efficiency. Therefore, many supports are used to load AgX nanoparticles, carbon materials, as one kind of supports, which could effectively enhanced the distribution of nanoparticles, electrons transfer, and stability, such as activated carbon (AC) [28], graphene oxides (GO) [16], carbon nanotube (CNT) [29], [30] etc. Among the carbon materials, activated carbon (AC) is greatly used because it is suitable to practical application to treat waste water and unique advantages [31]: AC possesses much microporous could suit to adsorb many pollutants; AC is very available for low prepared price; AC with large size is easy to separate from solution and better for recycling.
Available methods of silver chloride have been reported, such as directly adding an AgNO3 solution into the surfactant solution [32], mixing of AgNO3 and HCl with poly (vinyl alcohol) (PVA) as dispersing agent [33], etc. Following the part of AgCl on prepared materials was reduced by photoreduction or using reductive reagent. As a photocatalyst, photodegradation of organic pollutants were well documented in the literature [34]. In this work, we designed a plasmonic photocatalyst of Ag/AgCl/AC, which combines the surface plasmon resonance of nanoscale metallic Ag and enhanced visible light response, sodium citric as complexing reagent to control the size AgCl by forming complex to retard the release speed of silver ions. The photocatalytic activity was evaluated by degradation of tetracycline under visible light. The properties of Ag/AgCl/AC photocatalysts were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM), Brunauer Emmett Teller (BET) surface area analyzer, UV–vis diffuse reflectance spectra (UV–vis DRS), Photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and the mechanism of improved photocatalytic activity of Ag/AgCl/AC composite plasmonic photocatalyst was also proposed.
Section snippets
Materials and agents
Silver nitrate (AgNO3), sodium chloride (NaCl), and sodium citrate were purchased from Sigma Aldrich. Tetracycline was obtained from Shanghai Shunbo Biological Engineering Co. Ltd. Isopropanol (IPA), ethylenediaminetetraacetate disodium salt dihydrate (EDTA) (99%), benzoquinone (BQ) and anhydrous ethanol were purchased by Shanghai Chemical Reagent Co. Ltd. All chemicals and reagents were used as received without further purification except activated carbon (AC, offered by Beijing Wuyuan
XRD
The phase structure and crystalline of as-prepared composite photocatalysts were investigated by XRD at a 2θ of 5–80° and spectra for composites and pure AgCl and AC were shown in Fig. 3(a) and (b). The pure AC exhibits amorphous phase. Peaks of pure AgCl are sharp intensity and high, indicating as-prepared AgCl possesses excellent phase of crystalline and relatively high content. From Fig. 3(c) and (d), the photocatalysts of AgCl/AC and Ag/AgCl/AC obviously exhibit peaks of silver chloride,
Conclusions
In summary, AgCl/AC photocatalysts were prepared by a simple precipitated method under sodium citrate auxiliary. Ag/AgCl/AC composite plasmonic photocatalysts were prepared by photoreduction under UV-irradiation. The plasmonic photocatalysts exhibit high photocatalytic activity for degradation of TC aqueous solution which mainly due to the LSPR of metallic Ag. The LSPR of metallic Ag plays a key role in enhancing the photocatalytic activity and stability of the photocatalytic degradation
Acknowledgments
We gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 21576125, 21407064), the Natural Science Foundation of Jiangsu Province (BK20131259, BK20130489, BK20140532), the program for Postgraduate Research Innovation in University of Jiangsu Province (no. KYLX1028).
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