Facile synthesis of Ag/AgCl/BiOCl ternary nanocomposites for photocatalytic inactivation of S. aureus under visible light

Abstract

Ag/AgCl/BiOCl nanosheets were synthesized by a situ ion exchange between BiOCl nanosheets and AgNO₃ solution followed by visible light reduction at room temperature. The obtained sample was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Benefitting from the fact that Ag nanoparticles can respond to visible light and the ternary composite can effectively separate the photo-generated electrons and holes, the Ag/AgCl/BiOCl nanocomposite displayed enhanced visible-light-driven (VLD) photocatalytic inactivation of S. aureus, which was superior to BiOCl, Ag/BiOCl, AgCl/BiOCl, Ag/AgCl and Ag/AgCl/TiO₂. Moreover, the mechanism of photocatalytic bacterial inactivation was investigated by using different scavengers and a simple partition system which is able to separate the catalyst and bacteria. It was found that the direct contact between Ag/AgCl/BiOCl nanocomposites and bacterial cells was unnecessary for the photocatalytic disinfection, and the diffusing H₂O₂ generated from holes reduction via a multi-electron pathway plays an important role in the photocatalytic disinfection. Finally, the photocatalytic destruction of the bacterial cells was directly observed by atomic force microscopy (AFM). This work provides a potential effective VLD photocatalyst to disinfect S. aureus cells.

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Introduction

Contaminated water is one of the biggest sources of potentially hazardous microorganisms that can cause many millions of cases of disease and disability.^1,2 Since Matsunaga et al. reported for the first time the photocatalytic bactericidal effect over TiO₂ under ultraviolet (UV) illumination in 1985, semiconductor-based materials have been extensively investigated for photocatalytic disinfection.³ However, most efficient semiconductor-based photocatalysts were responsive to UV light, which can only utilize about 4% of solar energy. Therefore, exploring semiconductor-based photocatalytic systems with a visible light (VL) response is urgent for disinfection of waste water. Recently, it was found that a plasmonic metal modified semiconductor showed enhanced VL catalytic activity.⁴ By selecting suitable VL responsive plasmonic metal nanoparticles (NPs) and semiconductor composites, electrons in metal NPs can be injected into the conduction band of a semiconductor, therefore, inducing plasmonic generated electron and hole separation.^5–7 Those works indicate that employing semiconductor-based photocatalytic systems with plasmonic units can give us insight into promoting visible-light-driven (VLD) disinfection for waste water.

Among the reported literature on plasmonic metals, Ag NPs have been extensively studied for its relatively low cost and excellent response in VL region, while finding a suitable semiconductor for the promotion of plasmonic electron and hole separation is not easy.^8–10 Previous report has found that AgCl is a good candidate for the separation of plasmonic generated electron and hole pairs via hole transfer from Ag NPs to AgCl surface.^11,12 However, the detail process for the production of reactive oxygen species (ROS), which is quite important for bacterial disinfection, is unclear. Furthermore, as ROS can be created by either hot electron reduction process or plasmonic induced hole oxidation, which one dominates in VLD photocatalytic disinfection being also un-clarified.^13–15 From this standpoint, studying the mechanism of the disinfection process for Ag/AgCl photocatalytic systems is still needed.

Very recently, it was found that BiOCl nano-semiconductor possess relatively low conduction band edge, which can easily accept hot electrons produced in surface plasmonic resonance (SPR) units or photo-excited organic molecule.^16–18 By using BiOCl as substrate for Ag/AgCl photocatalytic system, plasmonic induced electrons might be injected into the conduction band of BiOCl, which promoting the electrons and holes separation. Therefore, constructing Ag/AgCl/BiOCl ternary composites can be expected to further enhance the photocatalytic performance of Ag/AgCl, and related works have been reported.^19–21 However, those studies investigated the photocatalytic efficiency of organic pollutants, without focusing on microorganisms. In fact, the degradation of organic compound is much different compared to photocatalytic disinfecting pathway for microorganisms. Cheng et al. synthesized Ag/AgBr/BiOBr hybrid composites which showed good performance for pathogenic organism sterilization, but there is a little information on the mechanisms of the VLD photocatalytic inactivation.²² Therefore, systematic works on photocatalytic disinfection are still needed to understand the Ag/AgCl/BiOCl catalytic systems.

In our work, we facilely synthesized Ag/AgCl/BiOCl nanocomposites by an in situ ion exchange between BiOCl nanosheets and AgNO₃ solution followed under VL irradiation and further investigated the photocatalytic disinfection of S. aureus under VL irradiation. The roles of the major reactive species were discussed systematically based on various control experiments. Moreover, a simple partition setup with a semi-permeable membrane was used to determine whether the direct contact between Ag/AgCl/BiOCl nanocomposites and the bacterial cells was a prerequisite for the photocatalytic disinfection of S. aureus.^14,23 This work provides an insight into plasmonic induced disinfection and further the understanding of photocatalytic process in Ag/AgCl under visible light irradiation.

Experimental

Synthesis of catalysts

For the preparation of BiOCl, 0.97 g Bi(NO₃)₃·5H₂O was first dissolved in 40 mL diethylene glycol (DEG), and 20 mL 0.1 mol L⁻¹ KCl solution was added dropwise into the above solution. After being sonicated for 30 min, the precipitation was washed with deionized water and dried at 80 °C in air (S1).

The Ag/AgCl/BiOCl composites were synthesized via immersing BiOCl into AgNO₃ solution under VL irradiation. In the synthesis, 0.8 mmol as-prepared BiOCl powder was dispersed into 40 mL 25 mmol L⁻¹ AgNO₃ aqueous solution to obtain a suspension, then N₂ was bubbled into the suspension for 30 min to remove the dissolved oxygen molecules. After that, the solution was transfer into a 50 mL quartz tube and exposed under a 500 W Xe lamp (Beijing Changtuo Technology Co. Ltd.) with a 400 nm cut-off filter as light source, and vigorous stirring for 6 h. The final products were collected and washed with deionized water for five times (S3). For comparison, a AgCl/BiOCl sample was prepared in the same way without irradiation (S2), Ag/AgCl was prepared without BiOCl, and Ag/AgCl/TiO₂ sample was prepared by using the same weight of titania P-25 instead of BiOCl. The Ag/BiOCl nanocomposites were synthesized by depositing silver nanoparticles (according to our previous study²⁴) to BiOCl (S1) directly.

Characterization

Powder X-ray diffraction (XRD) was performed on Bruker axs D8 Discover (Cu Kα = 1.5406 Å). Scanning electron microscopy (SEM) images were taken on a Hitachi S4800 microscope and transmission electron microscope (TEM) images were carried out on a Titan G2 60-300. UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrometer (Shimadzu UV-2550) by using BaSO₄ as a reference and were converted from reflection to absorbance by the Kubelka–Munk method. X-ray photoelectron spectroscopy (XPS) was performed on a VG Multilab 2000 spectrometer by using Al Kα (1486.6 eV) radiation as light source. The Brunauer–Emmett–Teller (BET) specific surface area of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus.

Photocatalytic inactivation

The VLD photocatalytic inactivation of S. aureus was conducted using a 500 W Xe lamp (Beijing Changtuo Technology Co. Ltd.) with a 400 nm cut-off filter as light source. All glass apparatuses used in the experiments were washed with deionized water, and then autoclaved at 121 °C for 20 min to ensure sterility. The bacterial cells were incubated in nutrient broth solution at 37 °C for 15 h with shaking, and then washed with sterilized saline solution (0.9% NaCl). The photocatalyst and the suspension of washed cell were then added into a flask with an aluminum cover. The final photocatalyst concentration and cell density were adjusted to 100 mg L⁻¹ and about 1 × 10⁶ cfu mL⁻¹, respectively. The reaction temperature was maintained at 25 °C and the reaction mixture was stirred with a magnetic stirrer throughout the experiment. At different time intervals, aliquots of the sample were collected and serially diluted with sterilized saline solution. Then 0.1 mL of the diluted sample was immediately spread on a Luria-Bertani (LB) agar plates and incubated at 37 °C for 24 h to determine the number of viable cells (in cfu). For comparison, experiments also conducted under VL irradiation without photocatalyst (light-control) or under darkness with the presence of photocatalyst (dark-control). All the treatment and control experiments were performed in triplicates.

The separated experiments were carried out using the partition setup reported with a suspension of bacterial cells in saline inside of the semi-permeable membrane as shown in Fig. 1.^14,23 After the desired time, 1 mL suspension inside of the container was sampled and immediately diluted. The density of living cells was then determined by counting the cfu mL⁻¹. For comparison, the disinfection effect in this partition setup was also investigated when EDTA–Fe(II) was added to the outer system. All the above experiments were conducted in triplicates.

Fig. 1 Schematic illustration of partition setup used in the photocatalytic disinfection by the Ag/AgCl/BiOCl under VL irradiation.

Analysis of hydrogen peroxide (H₂O₂)

The production of hydrogen peroxide (H₂O₂) was analyzed photometrically by the horseradish peroxidase (POD)-catalyzed oxidation product of N,N-diethyl-p-phenylenediamine (DPD).²⁵ In the experiment, 50 mg Ag/AgCl/BiOCl was dispersed into 50 mL deionized water with VL irradiation under vigorous stirring. At different time intervals, aliquots of the sample were collected and analyzed the absorption at 551 nm using a Shimadzu UV2800 spectrophotometer after the catalyst being removed by centrifugation.

Atomic force microscopy (AFM) observation

The mixture of Ag/AgCl/BiOCl nanocomposites and S. aureus before and after VL irradiation was sampled and centrifuged. The harvested cells were washed with PBS and then fixed with 2.5% glutaraldehyde at 4 °C for 2 h. After being washed with PBS, the cells were imaged by a Veeco NanoScope IIIa Multimode atomic force microscopy operating in a tapping mode.

Results and discussion

Catalyst characterization

The as-prepared products characterized by powder XRD are shown in Fig. 2a, all the diffraction peaks of the products fabricated without AgNO₃ solution treatment (S1) could be perfectly indexed to the tetragonal phase of BiOCl (JCPDS no. 06-0249) with no impurity diffraction peaks detected, indicating the high purity of BiOCl products. In additional to BiOCl diffraction peaks, there are some peaks observed in the XRD patterns of the products obtained after AgNO₃ treatment with (S2) or without VL illumination (S3) at 2θ = 27.9°, 32.3°, 46.3°, 54.9° and 57.6°, which are corresponding to the pattern of AgCl (JCPDS no. 01-1013), demonstrating the production of AgCl. As no additional Cl⁻ was added in the systems for the preparation of the hybrid composites, the production of AgCl comes from in situ ion exchange between BiOCl nanosheets and AgNO₃ solution. From the amplified XRD patterns of the products, it is clearly found that the diffraction peak of AgCl at 2θ = 27.9° is dominated in the products of S2 and S3 (Fig. 2b). Compared with S2, S3 exhibited the additional peak at 2θ = 38.1° which should be ascribed to Ag (JCPDS no. 65-2871) (Fig. 2c). It should be noting that even through our synthesis method for production of composites is similar to the traditional photoreduction process, the formation of Ag on the products is actually induced by photo-induced disproportionate of AgCl, which has been confirmed by the XRD patterns of AgCl before and after visible light irradiation (Fig. SI1, in ESI†). The above results indicate that the products are AgCl/BiOCl binary and Ag/AgCl/BiOCl ternary composites for the BiOCl products after treated by AgNO₃ solution without and with VL illumination, respectively.

Fig. 2 (a) XRD patterns of S1, S2 and S3, (b) the amplified XRD patterns in the range of 2θ = 27–29°, (c) the amplified XRD patterns in the range of 2θ = 35–40°.

The morphology and structure of the as-synthesized BiOCl and Ag/AgCl/BiOCl ternary composites were characterized by SEM and TEM images. As shown in Fig. 3a and b, both of the two products exhibits nanosheet-like morphology with average diameter of 50 nm and thickness about 10 nm. A typical high-resolution TEM image of S3 in Fig. 3c shows that the produced composites of BiOCl, AgCl and Ag, which is in accord with XRD observation. The distinct fringes with lattice spacing of 0.263, 0.321 and 0.728 nm are assigned planes of Ag (111), AgCl (111) and BiOCl (001), respectively. The corresponding SAED image in Fig. 3d exhibits various groups diffraction spots or rings, indicating the production of composites. The special surface areas of the samples were measured by nitrogen adsorption–desorption analysis (Fig. SI2, in ESI†). The BET surface areas of BiOCl and Ag/AgCl/BiOCl calculated from the results of N₂ isotherms were 25.4, and 27.6 m² g⁻¹, respectively. Those results show that AgNO₃ treatment would not distinctly influence the physical structures of the products, but induced in situ production of AgCl and Ag nanocrystal.

Fig. 3 SEM imagines of S1 (a) and S3 (b), high-resolution TEM (c) and SAED (d) of S3 products.

To further determine the production of AgCl/BiOCl binary and Ag/AgCl/BiOCl ternary composite, we performed X-ray photoelectron spectroscopy (XPS) analysis for the S2 and S3 shown in Fig. 4. From the survey spectra of S2 and S3 (Fig. 4a), it was found that all the characteristic binding energy peaks of Bi, O, Cl and Ag were present, indicating the coexistent of those elements in the products. C signal comes from the reference sample in the instrument. Fig. 4b shows the high-resolution XPS spectrum for Ag of the product. Distinct two peaks which associated with Ag(I) in AgCl are clearly observed in both spectra.²⁶ However, additional two weak peaks ascribed to Ag(0) signal were observed at 371.0 and 365.2 eV for S3, further confirmed the production of Ag/AgCl/BiOCl ternary composite. Compared with S2, the XPS peaks of S3 associating with Ag(I) showed slightly shift, demonstrating the contact between Ag and AgCl components. Combining with the peak areas for different chemical state of Ag and ICP-MS analysis for the products (Table SI1, in ESI†), the mass percentages of Ag and AgCl in Ag/AgCl/BiOCl are estimated to be 0.33% and 1.5%, respectively. Fig. 4c shows the high-resolution XPS spectrum for Bi 4f orbital. The two strong peaks at 159.1 eV and 164.4 eV, belonging to Bi 4f_7/2 and Bi 4f_5/2 respectively, correspond to the Bi³⁺ in BiOCl.²⁷ However, comparing to S2, both of Bi 4f_7/2 and 4f_5/2 peaks of S3 slightly shifted to the high binding energy, indicating strong interaction between Ag and BiOCl in the composites. Similar situations were also observed in the high-resolution XPS of Cl 2p_3/2 and Cl 2p_1/2, as shown in Fig. 4d. The change in the Bi, Cl and Ag signal associated with Ag(I) spectra indicates the production of Ag on the AgCl/BiOCl, which affected the chemical condition of Bi, Cl and Ag in the AgCl/BiOCl, demonstrating Ag was in situ formed between BiOCl and AgCl.

Fig. 4 XPS spectra of the samples of S2 and S3: (a) wide survey scan, (b) Ag 3d, (c) Cl 2p and (d) Bi 4f.

The optical properties of as-prepared pure BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl are shown in Fig. 5. All of the products exhibit an absorption edge range from 350 to 370 nm, indicating that the intrinsic absorption of BiOCl is mainly UV response. The addition of AgCl was shown to improve light absorption in the VL region slightly, while a strong absorption covering the full range from 370 to 800 nm can be observed with the introduction of Ag/AgCl, demonstrating that the excellent VL response property of Ag/AgCl/BiOCl ternary composites. This absorption, which believed to be corresponding to the SPR of Ag NPs, is consistent with the previous reports.^28–30

Fig. 5 UV-vis diffuse reflectance spectra of BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl.

Photocatalytic inactivation performance

The photocatalytic inactivation efficiencies of Ag/AgCl/BiOCl were evaluated by disinfection of S. aureus under VL irradiation. For comparison, the disinfection efficiency over BiOCl, Ag/BiOCl, AgCl/BiOCl, Ag/AgCl and Ag/AgCl/TiO₂ were also tested, which are shown in Fig. 6. In the dark control, the bacterial population remained constant after 3 h treatment, indicating no toxic effect caused by Ag/AgCl/BiOCl alone. Only VL irradiation also had no bactericidal effect on the bacterial cells. When BiOCl was irradiated by VL, it exhibited very low bactericidal activity and only 0.64 log-reductions in the vial cells count was obtained after 3 h treatment. Although, Ag/BiOCl, AgCl/BiOCl, Ag/AgCl and Ag/AgCl/TiO₂ have higher bactericidal activities under VL, only 1.3, 4.1, 4.2 and 2.5 log-inactivation were achieved within 3 h, respectively. Surprisingly, when employing Ag/AgCl/BiOCl ternary composites as photocatalyst, 1 × 10⁶ cfu mL⁻¹ of S. aureus was completely inactivated within 3 h under VL irradiation, exceeding those of BiOCl, Ag/BiOCl, AgCl/BiOCl, Ag/AgCl and Ag/AgCl/TiO₂. Those results show that BiOCl is an excellent component for construction of Ag/AgCl ternary composites system and the Ag/AgCl/BiOCl nanocomposite exhibits the best photocatalytic activity for the bacterial inactivation. The photo-stability of as-prepared Ag/AgCl/BiOCl nanocomposite was evaluated by cycle inactivation experiments under identical condition. Fig. SI3 (in ESI†) shows the corresponding inactivation tests in the presence of Ag/AgCl/BiOCl under visible light irradiation. It was found that after 4 times of tests, the catalyst still show high active for the disinfection, with 4 log-inactivation can be achieved within 3 h. This result suggests that the as-prepared Ag/AgCl/BiOCl can be used for a long period of time or limited recycles. The morphology and response to visible light of as-prepared products after 5 cycles photo-inactivation tests almost remain unchanged, which were observed by SEM and UV-vis DRS, as shown in Fig. SI4 (in ESI†).

Fig. 6 Photocatalytic inactivation efficiency of S. aureus (1 × 10⁶ cfu mL⁻¹) under VL irradiation using different catalysts (100 mg L⁻¹).

Photocatalytic inactivation mechanism

It is generally accepted that the sliver ion (Ag⁺) at high concentrations exhibits bactericidal activity.³¹ In our case, the Ag⁺ concentration eluted from the system of Ag/AgCl/BiOCl nanocomposite and bacterial cell suspension was determined by ICP-MS. It is found that only 0.275 mg L⁻¹ Ag⁺ was eluted from Ag/AgCl/BiOCl nanocomposite when it immerged in bacterial cells solution, which is almost the same with that of the photocatalyst/bacteria suspension in the absence of VL irradiation (0.273 mg L⁻¹). However, Ag/AgCl/BiOCl nanocomposite shows no deactivation efficiency for S. aureus in darkness (dark control in Fig. 6). These facts suggest that the quick inactivation of S. aureus should result from the photocatalytic performance of Ag/AgCl/BiOCl nanocomposite instead of the eluted Ag⁺.

To get the insight of the photocatalytic disinfection process over Ag/AgCl/BiOCl products under VL irradiation, various scavengers were selected to evaluate the contribution of the photo-induced active intermediates such as reactive oxygen species (˙OH, H₂O₂ or ˙O₂⁻) or photogenerated electrons and holes. The scavengers used in this study were sodium oxalate for h⁺, isopropanol for ˙OH, Cr(VI) for e⁻ and EDTA–Fe(II) for H₂O₂, and the corresponding results are shown in Fig. 7.^32–34 The control experiments show that the addition of each scavenger almost had no toxic effect on S. aureus within 3 h (see Fig. SI5, in ESI†). When isopropanol was added, the inactivation efficiency is slightly decreased, indicating a small amount of ˙OH dissociated in solution is involved in the VLD photocatalytic inactivation process. Meanwhile, except the weak bactericidal activity of Cr(VI) without photocatalyst (Fig. SI5†), no inhibition effect is observed when utilizing Cr(VI) as the electron scavenger to quench the reduction pathway. This result indicates that reactive species generated from the reduction site, such as e⁻ and ˙O₂⁻, are not dominated for the photocatalytic deactivation. However, in the presence of sodium oxalate as a scavenger for h⁺ or EDTA–Fe(II) as a quencher for H₂O₂, the photocatalytic bacterial disinfection is greatly inhibited, indicating that h⁺ and H₂O₂ play key roles in the disinfection. It is widely accepted that the production of H₂O₂ can be either initiated from direct reduction of H₂O via multi-electron process by h⁺ or evolution from ˙OH or ˙O₂⁻.^35–38 However, in our systems, the additions of ˙OH quencher (isopropanol) and electron quencher (Cr(VI)) show limited inhibition for the photocatalytic deactivation, which demonstrating that the production of H₂O₂ may be directly originated from hole reduction via multi-electron pathway instead of radical evolution process.

Fig. 7 Photocatalytic inactivation efficiency of S. aureus (1 × 10⁶ cfu mL⁻¹) by Ag/AgCl/BiOCl (100 mg mL⁻¹) with different scavengers (0.5 mM sodium oxalate, 5 mM isopropanol, 0.05 mM Cr(VI), 0.1 mM EDTA–Fe(II)) under VL irradiation.

To further confirm the role of H₂O₂ in the disinfection process, we also conducted the disinfection experiment using the partition system. In the system, a suspension of S. aureus in saline was contained in the membrane packaged container and the photocatalyst particles dispersed in the saline outside of the container. Here, the semi-permeable membrane with the molecular weight cutoff (MWCO) of 3.5 kDa allowed the free entry of smaller molecules such as H₂O and diffusing H₂O₂, but prevented the passage of larger targets such as Ag/AgCl/BiOCl nanocomposite and S. aureus. As shown in Fig. 8a, there was about 5.6 log-reduction in viable cells count within 3 h when the outer system was VL irradiated Ag/AgCl/BiOCl nanocomposite suspension. Since the VL irradiated Ag/AgCl/BiOCl nanocomposite and S. aureus were separated by the semi-permeable membrane, only the diffusing H₂O₂ generated by hole reduction could pass through the semi-permeable membrane to inactivate the bacterial cells inside of container. When EDTA–Fe(II) (a H₂O₂ scavenger) was added into the outer system, the photocatalytic bacterial disinfection is greatly inhibited, which further confirms that the diffusing H₂O₂ could go through the membrane to inactivate the bacterial cells inside the container. The disinfection efficiency of S. aureus in the partition system is slightly lower than that in the non-separated system (shown in Fig. 6), indicating that the photocatalytic disinfection efficiency by Ag/AgCl/BiOCl nanocomposite is mostly originated from the production of H₂O₂. Therefore, we further detected the production of H₂O₂ by POD/DPD method according to the previous report.²⁵ As shown in Fig. 8b, compared to no irradiation the concentration of H₂O₂ significantly increased after 1–3 h of VL irradiation, which confirmed the production of H₂O₂ by Ag/AgCl/BiOCl composites under VL irradiation.

Fig. 8 (a) Photocatalytic disinfection efficiency of S. aureus (1 × 10⁶ cfu mL⁻¹) inside a semi-permeable membrane packaged container when the outer system is the Ag/AgCl/BiOCl suspension (100 mg L⁻¹) under VL irradiation in the presence of 0.1 mM EDTA–Fe(II) or not, (b) absorption spectra of the DPD/POD reagent after reaction with H₂O₂ produced in the presence of Ag/AgCl/BiOCl (100 mg L⁻¹) under VL irradiation.

To understand the destruction process of bacteria by the diffusing H₂O₂ generated by Ag/AgCl/BiOCl nanocomposite under VL irradiation, the structure and morphology of S. aureus at the different stages of photocatalytic inactivation was examined by atomic force microscopy (AFM) and shown in Fig. 9. Before VL irradiation, S. aureus cells exhibit a sphere-like morphology and a relatively smooth surface (Fig. 9a and d). After 2 h irradiation, the bacteria cells maintain the sphere-like morphology, but the cell surface became a lot of depression (Fig. 9b and e). After 3 h of photocatalytic inactivation treatment, the cells were obviously damaged and became collapsed (Fig. 9c and f). The result illustrates that H₂O₂ produced over Ag/AgCl/BiOCl nanocomposites under VL irradiation could damage the structure of bacteria cells, and therefore inducing destruction of bacterial cells and membranolysis.

Fig. 9 AFM of S. aureus under VL irradiation with Ag/AgCl/BiOCl: 0 h (a and d), 2 h (b and e), 3 h (c and f).

On the basis of above experimental results, the mechanism of photocatalytic disinfection by Ag/AgCl/BiOCl nanocomposite under VL irradiation is preliminarily proposed and illustrated in Fig. 10. Photogenerated electron–hole pairs would be formed on the surface of Ag NPs due to the SPR under VL irradiation. Owing to the SPR of Ag NPs and negative charge surface of AgCl, a polarized electronic magnetic field would induce the electron transfer from the negative surface of AgCl to Ag particles, which results in the photogenerated holes in Ag NPs being transferred to the surface of AgCl, and consequently oxidizing H₂O to H₂O₂ via multi-electron process.^11,35,38 In the meantime, the photogenerated electrons in Ag nanoparticles can be injected into the conduction band of BiOCl.^19–21 The above process could induce the efficient separation of plasmon generated electron and hole pairs. Further, the produced H₂O₂ on AgCl surface can disperse into solution and disinfect the bacteria by rupturing the cell wall and severely destroying the cell structure. Thus, the Ag/AgCl/BiOCl photocatalytic system can effectively separate photo-generated electrons and holes, favorable to the improvement of photocatalytic disinfection activity.

Fig. 10 Proposed bactericidal mechanism of Ag/AgCl/BiOCl photocatalyst under visible light irradiation.

Conclusions

In summary, Ag/AgCl/BiOCl ternary composite was synthesized by a situ ion exchange between BiOCl nanosheets and AgNO₃ solution followed by VL reduction. Excellent VLD photocatalytic disinfection performance of Ag/AgCl/BiOCl for S. aureus was achieved due to the SPR of Ag NPs combined with the efficient separation of photo-generated electrons and holes by the ternary composite. The efficient photocatalytic disinfection performance is mainly attributed to the plasmonic generated holes, which leading to production of H₂O₂ via multi-electron transfer. The produced H₂O₂ could attack bacterial cell and inducing destruction of bacterial cells and membranolysis, which resulting in inactivation of cells. As a highly efficient VLD photocatalyst, the Ag/AgCl/BiOCl nanocomposite can be potentially applied in the related bactericidal fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21371139, 21471121), High-Tech Industry Technology Innovation Team Training Program of Wuhan Science and Technology Bureau (2014070504020243).

Notes and references

1. S. Kim, K. Ghafoor, J. Lee, M. Feng, J. Hong, D. U. Lee and J. Park, Water Res., 2013, 47, 4403–4411.
2. P. G. Wu, R. C. Xie, J. A. Imlay and J. K. Shang, Appl. Catal., B, 2009, 88, 576–581.
3. T. Matsunaga, R. Tomoda, T. Nakajima and H. Wake, FEMS Microbiol. Lett., 1985, 29, 211–214.
4. R. B. Jiang, B. X. Li, C. H. Fang and J. F. Wang, Adv. Mater., 2014, 26, 5274–5309.
5. T. Hirakawa and P. V. Kamat, J. Am. Chem. Soc., 2005, 127, 3928–3934.
6. A. Dawson and P. V. Kamat, J. Phys. Chem. B, 2001, 105, 960–966.
7. V. Subramanian, E. Wolf and P. V. Kamat, J. Phys. Chem. B, 2001, 105, 11439–11446.
8. C. Wen, A. Y. Yin and W. L. Dai, Appl. Catal., B, 2014, 160–161, 730–741.
9. X. Zhong, Z. Dai, F. Qin, J. Li, H. Yang, Z. Lu, Y. Liang and R. Chen, RSC Adv., 2015, 5, 69312–69318.
10. K. Kawahara, K. Suzuki, Y. Ohka and T. Tatsuma, Phys. Chem. Chem. Phys., 2005, 7, 3851–3855.
11. P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei and M. H. Whangbo, Angew. Chem., Int. Ed., 2008, 47, 7931–7933.
12. H. Xu, H. M. Li, J. X. Xia, S. Yin, Z. J. Luo, L. Liu and L. Xu, ACS Appl. Mater. Interfaces, 2011, 3, 22–29.
13. H. X. Shi, G. Y. Li, H. W. Sun, T. C. An, H. J. Zhao and P. K. Wong, Appl. Catal., B, 2014, 158–159, 301–307.
14. L. S. Zhang, K. H. Wong, H. Y. Yip, C. Hu, J. C. Yu, C. Y. Chan and P. K. Wong, Environ. Sci. Technol., 2010, 44, 1392–1398.
15. B. Tian, R. Dong, J. Zhang, S. Bao, F. Yang and J. Zhang, Appl. Catal., B, 2014, 158–159, 76–84.
16. H. P. Zhao, Y. F. Zhang, G. F. Li, F. Tian, H. Tang and R. Chen, RSC Adv., 2016, 6, 7772–7779.
17. L. H. Tian, J. Y. Liu, C. Q. Gong, L. Q. Ye and L. Zan, J. Nanopart. Res., 2013, 15, 11.
18. J. Y. Xiong, G. Cheng, G. F. Li, F. Qin and R. Chen, RSC Adv., 2011, 1, 1542–1553.
19. L. Q. Ye, J. Y. Liu, C. Q. Gong, L. H. Tian, T. Y. Peng and L. Zan, ACS Catal., 2012, 2, 1677–1683.
20. W. Xiong, Q. D. Zhao, X. Y. Li and D. K. Zhang, Catal. Commun., 2011, 16, 229–233.
21. Y. H. Ao, H. Tang, P. F. Wang and C. Wang, Mater. Lett., 2014, 131, 74–77.
22. H. F. Cheng, B. B. Huang, P. Wang, Z. Y. Wang, Z. Z. Lou, J. P. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2011, 47, 7054–7056.
23. L. S. Zhang, K. H. Wong, D. Q. Zhang, C. Hu, J. C. Yu, C. Y. Chan and P. K. Wong, Environ. Sci. Technol., 2009, 43, 7883–7888.
24. Z. Lu, K. F. Rong, J. Li, H. Yang and R. Chen, J. Mater. Sci.: Mater. Med., 2013, 24, 1465–1471.
25. H. Bader, V. Sturzenegger and J. Hoigné, Water Res., 1988, 22, 1109–1115.
26. H. Zhang, G. Wang, D. Chen, X. J. Lv and J. H. Li, Chem. Mater., 2008, 20, 6543–6549.
27. L. Q. Ye, K. J. Deng, F. Xu, L. H. Tian, T. Y. Peng and L. Zan, Phys. Chem. Chem. Phys., 2012, 14, 82–85.
28. J. G. Yu, G. P. Dai and B. B. Huang, J. Phys. Chem. C, 2009, 113, 16394–16401.
29. R. C. Jin, Y. W. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz and J. G. Zheng, Science, 2001, 294, 1901–1903.
30. Y. P. Bi and J. H. Ye, Chem. Commun., 2009, 6551–6553,  10.1039/b913725d.
31. S. Pal, Y. K. Tak and J. M. Song, Appl. Environ. Microbiol., 2007, 73, 1712–1720.
32. R. C. Jin, W. L. Gao, J. X. Chen, H. S. Zeng, F. X. Zhang, Z. G. Liu and N. J. Guan, J. Photochem. Photobiol., A, 2004, 162, 585–590.
33. A. A. Khodja, A. Boulkamh and C. Richard, Appl. Catal., B, 2005, 59, 147–154.
34. Y. M. Chen, A. H. Lu, Y. Li, L. S. Zhang, H. Y. Yip, H. J. Zhao, T. C. An and P. K. Wong, Environ. Sci. Technol., 2011, 45, 5689–5695.
35. K. T. Ranjit, I. Willner, S. H. Bossmann and A. M. Braun, Environ. Sci. Technol., 2001, 35, 1544–1549.
36. Y. Kikuchi, K. Sunada, T. Iyoda, K. Hashimoto and A. Fujishima, J. Photochem. Photobiol., A, 1997, 106, 51–56.
37. S. W. Liu, G. C. Huang, J. G. Yu, T. W. Ng, H. Y. Yip and P. K. Wong, ACS Appl. Mater. Interfaces, 2014, 6, 2407–2414.
38. H. Sakai, R. Baba, K. Hashimoto, A. Fujishima and A. Heller, J. Phys. Chem., 1995, 99, 11896–11900.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05754c

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