Enhanced disinfection application of Ag-modified g-C3N4 composite under visible light
Graphical abstract
Introduction
Microbial contamination is always harmful to human health since the existence of human society. Many kinds of bacteria can result in serious illness and even death for humans [1]. Therefore, it is significant and necessary to develop effective, low cost and environmentally benign biocides [2], [3], [4]. Since the photon-based disinfection method by platinum-doped TiO2 mediated photocatalysis was first reported in 1985, photocatalytic disinfection has been extensively investigated and considered as one of the most promising disinfection processes [5], [6]. By employing photocatalyst, photo-disinfection technique has been emerged as a preferable candidate compared with traditional bacterial inactivation methods such as chlorination and UV irradiation. However, chlorination is not environmentally benign and sustainable due to the production of carcinogenic disinfection byproducts and the existence of some free chlorine-resistant bacteria such as Mycobacterium avium [7]. In addition, UV light is not effective for some UV-resistant bacteria and the hazards of direct and intensive use of UV radiation restrict its application [8], [9], [10].
Currently, it is still a significant challenge to develop efficient, cost-effective, environmentally benign visible-light-responsive photocatalysts [11]. To reach the desired efficiency, the ideal photocatalyst should possess a high photo- and chemical stability, a wide light adsorption range covering the entire spectrum of sunlight, minimal recombination of photogenerated charge carriers, and facile preparation method [12]. Recently, graphitic carbon nitride (g-C3N4), a novel metal-free polymeric photocatalyst with narrow band gap energy of 2.7 eV, was reported by Wang et al., exhibiting a good performance for hydrogen and oxygen production by water splitting under visible light [13]. Graphitic C3N4 has attracted extensive attentions in water splitting [14] and environment purification [15] due to its high stability, bulk availability, low cost, and outstanding optical property. However, the photocatalytic efficiency of pure g-C3N4 is limited due to its fast recombination of photogenerated electron–hole pairs, low visible-light utilization efficiency, and small surface area [16]. Among the novel photocatalysts under investigation, one can use two different semiconductors [17], semiconductor-metal [18], or semiconductor-nonmetal [19] composite to overcome the above mentioned disadvantages. Therefore, many methods have been investigated to improve the visible light photocatalytic efficiency of g-C3N4 such as combining with semiconductors [20], [21], [22], [23], doping with non-metals [24], [25], [26], and coupling with metals [27], [28], [29].
By modifying a semiconductor with noble metal nanoparticles including silver, gold, and platinum, the photocatalytic activity of the semiconductor can be significantly improved. Therefore, plasmonic nanostructures which combine the advantages of semiconductor photocatalysts and noble metal species have recently attracted a great deal of interest [30]. Among them, the nanostructures of Ag species embedded in a matrix of g-C3N4, can increase the photocatalytic activity of g-C3N4 [18], [31], [32]. It is generally accepted that the surface plasmon resonance (SPR) effect contributes to the observed enhancement of photocatalytic activity after noble metal modification [33]. It had been proposed that the SPR effect could induce the formation of an intensive electromagnetic, which interacted with the semiconductor and further enhanced the formation rates of h+ and e− in the semiconductor. In addition, the noble metal particles could act as electron sinks, trapping free electron and thereby reducing the recombination rates of photogenerated electrons–holes. It was also reported that the plasmonic nanostructures could scatter resonant photons efficiently, leading to longer optical path lengths for photons in the photocatalyst, which caused enhanced evolution rates of photoinduced charge carriers [34].
Although the photocatalytic bacterial inactivation effect of g-C3N4 have been discussed in the early reports [15], [25], the inactivation efficiency still needed to be further improved owing to the low visible-light adsorption and rapid recombination of charge carriers. Herein, for achieving a better inactivation result and utilizing the abundant solar energy, Ag/g-C3N4 heterostructures were synthesized by the method combining thermal polymerization of melamine precursor with photo-reduction approach. The plasmonic composites are expected to possess better disinfection effect than pure g-C3N4 for the SPR effect from Ag nanoparticles and hybrid effect from g-C3N4. On the one hand, the intense local electromagnetic fields caused by the SPR effects of Ag nanoparticles can accelerate the formation rate of charge carriers with g-C3N4 [34], and the Fermi level of silver promotes the separation of holes and electrons [35], further increasing the quantum efficiency of g-C3N4. On the other hand, it is widely accepted that silver itself is a superior antibacterial agent [36]. The photocatalysts are characterized by TEM, XRD, FTIR, XPS, and BET analyses. The bactericidal effects toward Escherichia coli were investigated using standard plate count, laser scanning fluorescence microscopy, and SEM methods. It can be found that Ag/g-C3N4 composites exhibited significantly better photocatalytic disinfection effect than pure g-C3N4. The promotion mechanisms were also systematically investigated by photo-electrochemical methods including photogenerated current densities, electrochemical impedance spectroscopy (EIS) spectra and Mott–Schottky plots. The generation of reactive species were measured using chemical scavengers and ESR technique, indicating the important roles of h+, e− and O2− during photocatalytic disinfection process.
Section snippets
Materials
All chemicals used in this work are analytical grade. All chemicals were used as-received without any further purification. Silver nitrate (AgNO3), melamine (C3H6N6), and anhydrous ethanol (C2H5OH) were purchased from Jiangtian Chemical Technology Co., Ltd. (Tianjin, China).
Synthesis of Ag/g-C3N4 photocatalysts
Bulk g-C3N4 powder was synthesized according to the early report by Wang et al. [37]. Typically, 10 g melamine was put into a crucible with a lid under ambient pressure in air, and then the crucible was heated at 550 °C for 4 h
Characterizations of photocatalysts
The detailed morphology and microstructure of pure and hybrid photocatalysts were imaged using TEM. The typical TEM images of g-C3N4, Ag(1)/g-C3N4, Ag(3)/g-C3N4, and Ag(5)/g-C3N4, are shown in Fig. 1. Fig. 1a exhibits the platelet-like morphology of g-C3N4, which was very similar to that of graphene. The morphology of Ag(1)/g-C3N4 is shown in Fig. 1b, which also displayed a sheet-like structure. However, Ag nanoparticles are not observed on the surface of g-C3N4. It was possibly attributed to
Conclusions
Novel visible-light-driven Ag/g-C3N4 plasmonic photocatalysts were synthesized by the method combining thermal polymerization of melamine precursor with photo-reduction approach. It was found that Ag nanoparticles were well dispersed on the surface of g-C3N4. The doping of Ag species did not change the crystal structure and morphology of g-C3N4. The composite photocatalysts exhibited significantly enhanced photocatalytic disinfection efficiency than pure g-C3N4 powders. The best disinfection
Acknowledgments
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China as general projects (21377061 and 31170473), and a joint Guangdong project (U1133006), and the Key Technologies R&D Program of Tianjin (13ZCZDSF00300).
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