silver iodide composite with enhanced photocatalytic antibacterial and pollutant degradation ability

Accepted Manuscript Synthesis of Zinc Ferrite/Silver Iodide Composite with Enhanced Photocatalytic Antibacterial and Pollutant Degradation Ability Yuanguo Xu, Qingqing Liu, Meng Xie, Shuquan Huang, Minqiang He, Liying Huang, Hui Xu, Huaming Li PII: DOI: Reference:

S0021-9797(18)30589-7 https://doi.org/10.1016/j.jcis.2018.05.066 YJCIS 23644

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

5 March 2018 18 May 2018 21 May 2018

Please cite this article as: Y. Xu, Q. Liu, M. Xie, S. Huang, M. He, L. Huang, H. Xu, H. Li, Synthesis of Zinc Ferrite/ Silver Iodide Composite with Enhanced Photocatalytic Antibacterial and Pollutant Degradation Ability, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.05.066

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of Zinc Ferrite/Silver Iodide Composite with Enhanced Photocatalytic Antibacterial and Pollutant Degradation Ability Yuanguo Xu*a, Qingqing Liua, Meng Xiea, Shuquan Huang, a Minqiang He,a Liying Huang a, Hui Xub, Huaming Li*b a School of Chemistry and Chemical Engineering, School of Pharmacy, Jiangsu University, Zhenjiang 212013, PR China. b Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China. *E-mail: [email protected]; [email protected]

Abstract ZnFe2O4/AgI composites were first prepared successfully with a hydrothermal method, and ZnFe2O4 nanoparticles were uniformly decorated on the surface of AgI particles. The photocatalytic activities of the obtained ZnFe2O4/AgI composites were investigated by the degradation of organic pollutants and the inactivation of bacteria under visible light irradiation. The results showed that the introduction of ZnFe2O4 greatly enhanced the light harvesting ability and improved the separation efficiency of the photogenerated charge carriers, which contributed to the enhanced generation of reactive species and thus promoted the photocatalytic performance. The 5% ZnFe2O4/AgI composite exhibited the optimal photocatalytic disinfection of E. coli (100% removal efficiency in 80 min) as well as the photocatalytic degradation of rhodamine B (RhB) (98.5% removal rate in 40 min). Furthermore, four consecutive cycles also demonstrated the stable photocatalytic activity of the as-prepared ZnFe2O4/AgI composites. In addition, H2O2 was identified as the predominant active species in the photocatalytic inactivation of bacteria. This study indicated that 1

ZnFe2O4/AgI composites are a promising candidate for the treatment of wastewater.

Keywords: photocatalytic; AgI; ZnFe2O4 ; antibacterial; degradation

1. Introduction A variety of environmental pollutants, such as pathogenic bacteria and organic dyes, have been spread throughout the world due to increasing industrial development, which is harmful to human health [1, 2]. Photocatalysis has attracted increasing attention due to its large potential applications in bacteria decontamination and dye pollution in wastewater [3-5]. Since the first use of TiO2 for the disinfection of microbial cells, photocatalytic inactivation of bacteria has attracted considerable interest [6-8]. Even though many semiconductors have been studied for the promotion of photocatalytic reactions [9-11], the poor visible light absorption capacity and the lower separation efficiency of photogenerated electron-hole pairs has limited the extensive application of photocatalysts. Therefore, the exploitation of highly efficient and visible-light-active semiconductor photocatalysts is imperative for addressing the problem of water pollution. Currently, as a photosensitive material, AgI has attracted profound attention due to its superior visible-light-driven photocatalytic activities in both organic pollutant degradation and bacterial inactivation [12, 13]. Unfortunately, pure AgI is unstable and can be reduced to metallic Ag under visible light irradiation [14, 15]. Moreover, due to its relatively lower efficiency of electron-hole pair separation, the low 2

photocatalytic activities of bare AgI are still far from satisfactory for wastewater treatment, and it is essential to adopt approaches for optimization of wastewater treatment performance [16, 17]. To date, coupling with other semiconductors to construct composite photocatalysts has been studied and has been shown to tremendously improve the single semiconductor photocatalytic performance; this method has therefore been considered a favorable approach for addressing the abovementioned

drawbacks

[18-20].

For

instance,

Xu

et

al.

developed

Bi2O2CO3/Bi2MoO6 nanoplates, which displayed superior photocatalytic activity for RhB degradation, and showed relevant degradation efficiency of approximately 64-fold higher than that of pure Bi2 MoO6 [21]. Hu et al. reported visible-light-driven Ag/AgBr/TiO2 photocatalysts, which exhibited significantly enhanced photocatalytic disinfection performance due to better photoinduced electron-hole separation efficiency [22]. Certainly, much more effort has been devoted to improving photocatalytic performances of pure AgI through the construction of photocatalysts with other matched semiconductors, such as AgI/BiVO4 [23], Ag3PO4/AgI [16], AgI/BiOI [24], Bi2SiO5/AgI [25], AgI/WO3 [26], and CNT/AgI [14]. However, only few reports have demonstrated effective photocatalytic disinfection, with most studies only reporting on the dye degradation performance. Thus, it is still important and essential to search for more suitable semiconductors to fabricate highly efficient AgI composite photocatalysts for simultaneously photocatalytic biohazard inactivation and dye degradation. With their relatively narrow band gap of 1.9 eV, spinel ZnFe2O4 nanoparticles have 3

been by far the most extensively reported photocatalysts due to their visible-light response, their excellent photochemical stability, their natural abundance and their low toxicity [27-29]. However, the photoinduced electron-hole pairs of bare ZnFe2O4 tend to recombine rapidly, leading to poor photocatalytic activity [30-32]. For example, the ZnFe2O4 nanomaterials prepared by Liu et al. showed only a 15% RhB degradation rate after visible light irradiation for 300 min [33]. The photocatalytic performance of ZnFe2O4 reported by Kong et al. was also poor, with almost no degradation effect on RhB after 60 min of irradiation [34]. Fortunately, to achieve better photocatalytic performance, several researchers have developed ZnFe2O4 composites with other photocatalysts, such as G-ZnFe2O4 [28], ZnFe2O4/Ag3VO4 [35], ZnFe2O4-C3N4 [36], and Ag/ZnO/ZnFe2O4 [29]. These reports have clearly demonstrated that the introduction of ZnFe2O4 to construct the ZnFe2 O4-based composites enhanced the photoinduced carrier charge separation and photocatalytic activity. Recently, several groups have reported that AgI has better photocatalytic performance than ZnFe2O4. For example, AgI prepared by Xu et al. achieved a degradation efficiency of more than 70% for RhB upon exposure to visible light for 48 min [14]. In addition, Chen et al. reported that AgI had a certain degradation effect on antibiotic tetracycline (TC), and its degradation efficiency reached approximately 60% within 60 min of irradiation [23]. Thus, excellent photocatalytic activity was expected if ZnFe2O4 nanoparticles were introduced to the AgI system in an appropriate manner. Furthermore, to the best of our knowledge, the synthesis of the visible-light-driven ZnFe2O4/AgI photocatalyst for both the photocatalytic inactivation of bacteria and 4

pollutant degradation has not yet been reported. In the present work, ZnFe2O4/AgI composites were favorably prepared through a feasible one-step hydrothermal approach. The microorganism E. coli and the dye RhB were selected as the target pollutants. The introduction of ZnFe2O4 nanoparticles remarkably enhanced the photocatalytic performances of AgI under visible light irradiation. The origin of the improved photocatalytic activity of the ZnFe2O4/AgI photocatalysts was explored by the examination of structure, morphology, and optical properties. The major reactive species during the photocatalytic process were investigated using various chemical scavengers and the ESR technique.

2. Experimental 2.1. Materials and samples preparation Zinc

nitrate

hexahydrate

(Zn(NO3)2·6H2O),

ferric

nitrate

nonahydrate

(Fe(NO3)3·9H2O), silver nitrate (AgNO3), potassium iodide (KI), ethylene glycol (EG), rhodamine B (RhB), Citric acid (C6H8O7•H2O) and ammonium hydroxide (NH3•H2O) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the reagents were of analytical grade and were used without any further purification. Escherichia coli (E. coli) was obtained from Shanghai Lu Wei Technology Co., Ltd. ZnFe2O4 was synthesized by a sol-gel method. Briefly, 4.0402 g Fe(NO3)3·9H2O and 1.4875 g Zn(NO3)2·6H2O were well-dispersed in 100 mL deionized water under magnetic stirring. The citric acid solution (0.032 g/mL) was then slowly placed into the mixed solution described above. Then, the pH of the mixture was adjusted to 7 by 5

the addition of an NH3.H2O solution. Subsequently, this solution was heated to 80 °C and was maintained for 1 h under oil bath conditions and then dried at 90 °C. Finally, the formed gel was placed into an alumina crucible and calcined at 600 °C for approximately 2 h. The obtained products were ZnFe2O4 nanoparticles. The ZnFe2O4/AgI photocatalysts were fabricated as follows: the as-synthesized ZnFe2O4 (0.4 g) was dissolved in 100 mL EG with sonication. Then, 0.4 g AgNO3 was dispersed in 25 mL ethylene glycol (EG) and a 5 mL ZnFe2O4 dispersion solution (0.004 g/mL) was added into the above solution with stirring at 15 °C for 30 min. Then, an aqueous solution of KI (2 g in 20 mL of water) was added dropwise to the suspension, which was stirred for another 1 h at 15 °C. Finally, the suspension was transferred to a 25 mL Teflon-lined autoclave with a stainless steel tank and was kept at 140 °C for 20 h. The obtained sample, denoted as 5% ZnFe2O4/AgI, was washed and dried. Samples 3% ZnFe2O4/AgI and 7% ZnFe2O4/AgI were prepared similarly to 5% ZnFe2O4/AgI by changing the initial amount of ZnFe2O4 to 3 mL and 7 mL, respectively. The AgI was prepared as described above but without the addition of ZnFe2O4.

2.2. Characterization X-ray diffraction (XRD) patterns were examined using a Shimadzu XRD-6000 X-ray diffractometer with high-intensity Cu-Kα radiation (λ=1.54 Å). The morphology of the as-prepared photocatalysts was examined by scanning electron microscope (SEM) measurements (JEOL JSM-7001F). X-ray photoelectron 6

spectroscopy (XPS) spectra were obtained with an ESCALab MKII X-ray photoelectron spectrometer. UV-vis diffuse reflection spectroscopy (DRS) was recorded

using

a

UV-vis

spectrophotometer

(Shimadzu

UV-2450,

Japan).

Photoluminescence (PL) spectroscopy was measured using a Varian Cary Eclipse spectrometer. The electron spin resonance (ESR) was measured with a Bruker model ESR JES-FA200 spectrometer using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the radical scavenger (Sigma Chemical Co.). The photocurrent and electrochemical impedance

spectroscopy (EIS)

measurements

were

carried

out

using

an

electrochemical system (CHI 660B, Chenhua Instrument Company, Shanghai, China). 2.3. Photocatalytic disinfection performance E. coli was selected as a target bacterium to assess the antibacterial capacity of the synthesized photocatalyst in this system. E. coli was cultivated in a nutrient broth at 37 °C for 15 h with shaking. The bacterial cells were then washed twice with sterilized 0.85% saline solution by centrifugation for 5 min and then re-suspended in a sterile saline solution. All the glass apparatuses and solutions used in this experiment were autoclaved at 121 °C for 20 min to ensure sterility. Then, 1 mg of photocatalyst was added into 20 mL solution containing 107.5 cfu/mL of bacterial suspension, followed by stirring for 10 min in the dark to maintain the adsorption-desorption equilibrium. A visible light simulator 300 W Xe lamp filtered through a UV cut off (λ < 420 nm) was used as the light source. At different time intervals, aliquots of the samples were gathered and continuously diluted with sterilized saline solution and then spread on nutrient agar and incubated at 37 °C for 24 h. To evaluate the effect of 7

visible light irradiation, the control experiments without light were conducted in the same situation. The light control group measurements were carried out without a photocatalyst. All the inactivation experiments were performed in triplicate. 2.4. Photocatalytic degradation activity The photocatalytic activities of the ZnFe2O4/AgI composites were evaluated toward the degradation of RhB in solution in a photocatalytic reactor. A visible-light source was equipped with a 300 W Xe lamp and a 420 nm cutoff filter. Then, 70 mg of the samples were dispersed into 70 mL of 10 mg/L RhB aqueous solution, and the entire photocatalytic process was carried out at 30 °C. The dispersion liquid was magnetically

stirred

for

30

min

prior

to

irradiation

to

achieve

the

adsorption-desorption equilibrium. After the irradiation, a 4 mL sample of the reaction dispersion liquid was collected at given time intervals and was centrifuged to remove the catalyst. The supernatant was collected and analyzed using a UV-vis spectrophotometer at a wavelength of 553 nm.

3. Results and discussion 3.1. Compositional and structural information The X-ray diffraction (XRD) patterns of AgI, ZnFe2O4 and ZnFe2O4/AgI composites are shown in Fig. 1. The characteristic peaks at 22.32°, 23.71°, 25.35°, 32.77°, 39.20°, 42.63°, 46.31°, 59.30°, 71.04°, 73.41° and 76.08° were observed, which were indexed to the (110), (002), (101), (102), (110), (103), (112), (203), (300), (213) and (302) crystal planes of AgI (JCPDS no. 09-0374), respectively [24]. For the 8

pure ZnFe2O4, the diffraction peaks at 29.9°, 35.3°, 42.8°, 53.1°, 56.6°, 62.2°and 73.5° were assigned to the (220), (311), (400), (422), (511), (440) and (533) planes of ZnFe2O4 (JCPDS No. 22-1012) [28], respectively. However, for all of the ZnFe2O4/AgI composites, the ZnFe2O4 peaks were not observed in the pattern, possibly due to the low amounts of ZnFe2O4 in the composites. Moreover, there were no impurities formed in the samples, indicating that these XRD spectra verified the existence of ZnFe2O4 and AgI in the composites. X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemical composition of the as-prepared photocatalysts. As shown in Fig. 2a, the survey spectrum showed that the AgI, ZnFe2O4 and 5% ZnFe2O4/AgI composite contained Ag, I, Fe, O and Zn elements. It could be obviously seen from the high-resolution XPS spectra of the Ag element shown in Fig. 2b that the binding energies of 368.7 eV and 374.7 eV were attributed to Ag 3d5/2 and Ag 3d3/2, respectively. The Ag 3d peak in the ZnFe2O4/AgI composite displayed a slight shiftcompared to the bare AgI [14]. Fig. 2c shows the high resolution I 3d spectra, and the two peaks centered at 620.0 eV and 631.5 eV were attributed to the binding energies of I 3d5/2 and I 3d3/2, respectively [37]. I 3d peaks for the ZnFe2O4/AgI sample also revealed a shift. All the observed shifts indicated the existence of interactions between the introduced ZnFe2O4 and AgI [38]. Fig. 2d shows the high-resolution XPS spectra of Fe 2p, and the main peaks at 712.7 eV and 726.2 eV in the 5% ZnFe2O4/AgI composite correspond to Fe 2p3/2 and Fe 2p1/2, respectively [39]. As observed in Fig. 2e, the high resolution Zn 2p spectrum of 5% ZnFe2O4/AgI 9

composite had two obvious peaks at 1022.47 eV and 1045.58 eV, which were ascribed to Zn 2p3/2 and Zn 2p1/2, respectively [40]. In addition, as observed in Figs. 2d and 2e, the Zn and Fe signals were zero in the pure AgI compared to the ZnFe2O4/AgI composite. Thus,XPS analysis further demonstrated the successful preparation of the AgI/ZnFe2O4 composites. 3.2. Morphology and microstructure analysis The morphologies and compositions of the as-prepared ZnFe2O4/AgI composites were investigated by SEM. The SEM images shown in Figs. 3a and 3b reveal large irregular particles of the pure AgI, with a relatively smooth surface. After the introduction of ZnFe2O4 to the AgI, the main morphology of AgI did not changed (Fig. S1a, b), and a small number of nanoparticles on the surface of AgI were observed. It can be observed from the SEM images of 5% ZnFe2O4/AgI (Figs. 3c, 3d), that more nanoparticles were highly dispersed on the surface of AgI, resulting in the surface of AgI becoming rough. Similarly, as shown in the SEM images of 7% ZnFe2O4/AgI (Fig. S1c, d), the surface of the samples became more rough with the loading of a large amount ZnFe2O4. The excessive ZnFe2O4 covered on the surface of AgI may have decreased the ability of the AgI to absorb visible light, leading to the poor photocatalytic activity of the ZnFe2O4/AgI composites. In addition, elemental mapping images (Fig. 3e) showed that the O, Ag, I, Fe and Zn elements were evenly distributed among the ZnFe2O4/AgI composites. To further prove the successful preparation of the ZnFe2O4/AgI composites, the elemental mapping and EDS of AgI were investigated. The results showed that AgI was composed of Ag and I elements, 10

while no other elements were found (Fig. S2). The high-resolution transmission electron microscopy (HRTEM) image of 5% ZnFe2O4/AgI was obtained and is shown in Fig. 3f. It can be seen that two obvious lattice spacings of 0.254 nm and 0.229 nm coincided with the values for the (311) and (110) planes of ZnFe2O4 and AgI, respectively. Therefore, the above results proved the successful preparation of ZnFe2O4/AgI. 3.3. Optical properties Fig. 4 shows the optical properties of the ZnFe2O4/AgI composites, as characterized by UV-vis diffuse reflectance spectra (DRS) measurements. It was observed that the pure AgI absorbed light at wavelengths mainly below 450 nm. However, the absorption band edge of the ZnFe2O4/AgI composites extended toward the longer range of wavelengths compared to pure AgI, which is mainly attributed to the introduction of ZnFe2O4. Since the light harvesting ability of ZnFe2O4/AgI composites was enhanced, the introduction of ZnFe2O4 provided the prerequisite for significant enhancement of the photocatalytic performance. The band gap energies for the pure AgI and ZnFe2O4 obtained by the classical Tauc approach were approximately 2.8 eV and 1.92 eV, as seen in Fig. 4b. Measurements of photoluminescence (PL) spectra are an effective approach for the investigation of the surface processes involving charge carriers, and the intensity of PL spectra is directly proportional to the photogenerated electron-hole pair recombination rate [41]. Fig. 5 presents the PL spectra of AgI and the 5% ZnFe2O4/AgI composite at an excitation wavelength of 360 nm. It is obvious that the 11

PL intensity of 5% ZnFe2O4/AgI decreased significantly compared to the pure AgI, implying that ZnFe2O4 introduction may enhance the separation efficiency of the electron-hole pairs through faster electron transfer, thereby contributing to higher photocatalytic ability. 3.4. Electrochemistry analysis To further understand the separation and migration of photoinduced charge carriers in the ZnFe2O4/AgI composite, the photocurrent response under visible light was examined for several on-off cycles. Examination of Fig. 6a shows that the as-prepared sample electrodes generated constant photocurrent responses and were very stable in cycles. Under visible light irradiation, the 5% ZnFe2O4/AgI composite showed the highest photocurrent intensity among all of the samples, which were approximately 3, 1.9 and 1.8 times higher than those of AgI, 3% ZnFe2O4/AgI and 7% ZnFe2O4/AgI, respectively. In addition, the photocurrent signal of ZnFe2O4 was so weak that it is covered in Fig. 6a and is shown in the inset of the figure at the top right corner. This result indicated that the 5% ZnFe2O4/AgI composite possessed the highest separation rate of photogenerated electron-hole pairs, which could be beneficial for better photocatalytic activity. Fig. 6b reveals the electrochemical impedance spectroscopy (EIS) of the pure AgI, ZnFe2O4, 3% ZnFe2O4/AgI, 5% ZnFe2O4/AgI and 7% ZnFe2O4/AgI composites. It can be seen that the 5% ZnFe2O4/AgI composite electrode exhibited the smallest impedance arc radius, which may facilitate the interfacial charge transfer and thus improve the photocatalytic efficiency. The result of the EIS experiment was consistent with the photocurrent results mentioned above. 12

3.5. Photocatalytic disinfection activity and stability The photocatalytic disinfection performance of the as-prepared photocatalysts towards the inactivation of E. coli was investigated under visible light irradiation. As seen in Fig. 7a, the E. coli inactivation efficiencies for the photocatalysts were neglected under dark conditions with the prolongation of irradiation time, suggesting that the ZnFe2O4/AgI composites do not possess antibacterial capacity in the absence of light irradiation. Fig. 7b shows the disinfection results for AgI and the ZnFe2O4/AgI composites for E. coli under visible light. There was almost no bacterial cell that was inactivated within 100 min under light irradiation only. It can also be seen that over a period of 100 min of light irradiation, approximately 100.4 and 103.6 removal of E. coli population in the presence of pure ZnFe2O4 and AgI, respectively, was obtained, displaying the lower disinfection efficiencies. Fortunately, all the ZnFe2O4/AgI composites demonstrated an improved photocatalytic inactivation performance compared to the pure ZnFe2O4 and AgI photocatalysts. For the 3% ZnFe2O4/AgI, 5% ZnFe2O4/AgI, and 7% ZnFe2O4/AgI composites, the quantity of viable bacteria was 104, 100 and 103.4 after 80 min irradiation, respectively. In contrast, the 5% ZnFe2O4/AgI composite exhibited the highest photocatalytic activity for the inactivation of E. coli, and all of the bacteria were completely inactivated within 80 min, which was much faster than either the 3% ZnFe2O4/AgI or the 7% ZnFe2O4/AgI composite. Additionally, it is generally accepted that at high concentrations, the released Ag+ displays bactericidal activity [13]. In our system, however, only a small amount of Ag+ was eluted from the ZnFe2O4/AgI composite, with a concentration of 13

0.31 mg/L at 80 min of photocatalytic reaction. (The details of the inductively coupled plasma spectrometer measurements used for this measurement are provided in the supporting information). Additionally, the release of Ag+ in this work was lower than that of other reported Ag-containing materials, such as AgBr-Ag-Bi2 WO6 (>0.6 mg/L) [42] and Ag-TiO2 (>0.5 mg/L) [43]. This result showed that the excellent inactivation of E. coli resulted from the photocatalytic performance of ZnFe2O4/AgI composites instead of the released Ag+. To investigate the stability of 5% ZnFe2O4/AgI, the recycling of the catalyst for the photocatalytic inactivation of E. coli was examined, and the results are shown in Fig. 7c. Clearly, the 5% ZnFe2O4/AgI composite exhibited nonsignificant reduction in bactericidal efficiencies, even after four consecutive cycles. This result implied that the 5% ZnFe2O4/AgI composite has good potential for repeated use. Based on the above

analysis,

ZnFe2O4/AgI

composites

exhibited

superior

photocatalytic

inactivation of E. coli (100% removal efficiency in 80 min) compared to the Ag/AgBr/ZnFe2O4 composite (100% removal efficiency in 120 min) previously reported by us. Moreover, due to the lower solubility of AgI, the ZnFe2O4/AgI composite is highly stable and is not easily corroded by light during the photocatalytic reaction process, unlike the Ag/AgBr/ZnFe2O4 material. The morphology changes of E. coli cells in the ZnFe2O4/AgI system at different irradiation times were observed by SEM. To compare the changes of morphology of E. coli, Fig. 8 shows the four highest magnification images, while the smallest magnification SEM images of E. coli are displayed in Fig. S3. As shown in Fig. 8a, 14

the untreated bacteria displayed a typical rod-shape with smooth and intact cell walls, while a part of the central cell became clearly wrinkled after a 20 min reaction with 5% ZnFe2O4/AgI under visible light irradiation (Fig. 8b). With the extension of the irradiation time to 60 min, it could be seen that much more severe damage and some holes appeared on the E. coli cell structure (Fig. 8c). Subsequently, after being treated for 80 min, the cell wall was dramatically ruptured, indicating that disinfection was achieved successfully (Fig. 8d). 3.6. Photocatalytic degradation of organic pollutants The photocatalytic activities of the ZnFe2O4/AgI composites were also investigated through the decomposition of RhB under visible light irradiation, and the result is shown in Fig. 9a. Obviously, RhB was only slightly degraded in the absence of a catalyst, implying that the blank photolytic degradation was ignored. As seen from Fig. 9a, the introduction of different contents of ZnFe2O4 enhanced the photocatalytic performance of AgI. Furthermore, 5% ZnFe2O4/AgI exhibited the best photocatalytic activity. The degradation efficiency of RhB reached 98.5% after 40 min of visible light illumination, which was much better than that for pure AgI and the composites with other ZnFe2O4 contents. Meanwhile, Fig. 9b shows the pseudo-first-order kinetic data for the photodegradation of RhB using different photocatalysts. The rate constants for AgI, 3% ZnFe2O4/AgI, 5% ZnFe2 O4/AgI and 7% ZnFe2O4/AgI were 0.03778, 0.05898, 0.08802 and 0.04425 min-1, respectively. Therefore, 5% ZnFe2O4/AgI possessed the maximum rate constant, which was approximately 2.3 times higher than that of the pure AgI. 15

The reusability and stability of the ZnFe2O4/AgI composites was also investigated, and Fig. 9c shows the comparison of the recycling reactions for the photodegradation of RhB over the pure AgI and 5% ZnFe2O4/AgI composites. It can be seen that the degradation efficiency of pure AgI dropped from 76.3% to 58.1% with four consecutive cycles, indicating that pure AgI is not stable under visible light irradiation. In contrast, the 5% ZnFe2O4/AgI composite still displayed a high degradation efficiency, even after four cycles, showing much better stability than pure AgI. This result indicated that the stability of AgI was significantly improved in the ZnFe2O4/AgI composite. To further study the active species in the photocatalytic process, free radical trapping experiments were performed through the addition of various trapping agents, such as triethanolamine (TEOA) for h+, N2 for O2•– and tert-butanol for •OH. As shown in Fig. 9d, when the TEOA was added into the reaction system, the photodegradation rate of RhB by the 5% ZnFe2O4/AgI composite was inhibited dramatically, revealing that photogenerated holes are the major reactive species. In addition, the photocatalytic activity also decreased under N2 atmosphere, suggesting that O2•– plays a key role in the photocatalytic process. Little change was observed after the addition of tert-butanol, indicating that •OH may not the main active species in this system. Based on these results, holes played a vital role in the photocatalytic degradation activities. To summarize, compared to the findings in the previous similar studies, the ZnFe2O4/AgI composites showed excellent photocatalytic activities. For example, Xu 16

et al. prepared the Ag/AgBr/ZnFe2O4 photocatalyst, which exhibited a lower inactivation efficiency for E. coli (completely killed 105.5 cfu/mL E. coli within 120 min) [33]. Li et al. reported an effective photocatalytic bacterial inaction using Ag2WO4/g-C3N4 composite (0.1 mg/mL) and showed that a number of live bacteria were completely inactivated after 90 min [44]. Ng et al. investigated bacterial inactivation by Fe2O3-AgBr and evaluated the effects of different factors on the inactivation of E. coli. The results showed that Fe2O3-AgBr was able to inactivate E. coli within 4 h under the concentration of 50 mg/L [45]. Li et al. synthesized plasmonic Ag/AgBr/ZnFe2O4 nanocomposites, which could photodegrade RhB completely in 180 min [46]. In view of the above, the ZnFe2O4/AgI composites displayed enhanced photocatalytic performance in terms of degradation and inactivation of E. coli. 3.7. Mechanism of photocatalytic inactivation enhancement Generally, bacterial inactivation by photogenerated reactive species (including •OH, O2•–, e-, H2O2 and h+) has been widely considered to be the main mechanism during the photocatalytic disinfection process [47]. Since understanding the exact reactive species that play the predominant role in the photocatalytic system is crucial for improving the disinfection effect, particular scavengers were employed to quench the special active species, and the exact role of these species was investigated. The various scavengers employed in this work were sodium oxalate for h+, isopropanol for •OH, Cr(VI) for e-, 4-hydroxy-2, 2, 6,6-tetramethyl-piperidinyloxy (TEMPOL) for O2•– and catalase for H2O2. As shown in Fig. 10a, the bacterial cells were generally 17

completely inactivated within 80 min in the absence of scavengers. It was found that •OH and h+ played a lower bactericidal role, and the number of viable bacteria was approximately 103.9 and 104.3 in the presence of isopropanol and sodium oxalate within the same experimental period, respectively. When Cr(VI) was added to this system, e- was suggested to have a moderate role in the inactivation of E. coli. With the addition of TEMPOL to remove O2•–, the obtained disinfection percentage decreased obviously, indicating a significant role of O2•–. Notably, in the presence of catalase, a significant decrease in bactericidal efficiency was achieved after 100 min of irradiation, implying that the generated H2O2 was the most important reactive species in the inactivation experiment. To further investigate the effect of e-, E. coli inactivation in the absence of oxygen was also examined. With argon (Ar) purging to eliminate oxygen, the disinfection efficiency decreased substantially, further confirming the role of e- in the bacterial inactivation experiment. These observations indicated that H2O2 and O2•– were the major reactive species generated in the 5% ZnFe2O4/AgI photocatalytic disinfection process. In addition, as shown in Fig. 10b, the accumulated concentration of H2O2 generated by 5% ZnFe2O4/AgI was higher than that of pure AgI and reached almost 62 µmol/L after 80 min irradiation. This result is probably the main reason for the higher photoactivity of 5% ZnFe2O4/AgI. To further study the role of the reactive oxygen radicals in the photocatalytic process, the ESR spin-trap technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used on the AgI, ZnFe2O4 and 5% ZnFe2O4/AgI composites. It can be found in Fig. 11 that no obvious signals were detected in the dark. Noticeable characteristic 18

peaks of DMPO-O2•– (Fig. 11a) and DMPO-•OH (Fig. 11b) adducts were clearly observed for AgI, ZnFe2O4 and 5% ZnFe2O4/AgI under visible light irradiation. Obviously, the O2•– and •OH signals of the 5% ZnFe2O4/AgI composite were stronger than that of pure AgI and ZnFe2O4, suggesting that the amount of O2•– and •OH generated in the 5% ZnFe2O4/AgI system was much higher than those of AgI and ZnFe2O4. This result revealed that the introduction of ZnFe2O4 leads to efficient separation of electron-hole pairs, which may be beneficial for improved photocatalytic activity. Based on the above results and discussion, a possible antibacterial mechanism of the ZnFe2O4/AgI photocatalyst is proposed in Fig. 12. According to the UV-vis spectroscopy analysis, the band gap energies (Eg) of the pure AgI and ZnFe2O4 were 2.8 eV and 1.92 eV, respectively. The effective separation of the photogenerated electron-hole pairs in the composite were ascribed to the suitable matching energy bands of the photocatalysts. As is shown in Fig. 12, the valence band (VB) values of AgI and ZnFe2O4 were 2.38 eV and 0.38 eV, respectively, and the corresponding conduction band (CB) values were -0.41 eV and -1.54 eV, respectively [7, 33]. It was clear that the CB edge potential of ZnFe2O4 was more negative than that of AgI, and the VB edge potential of AgI was more positive than that of ZnFe2O4. When the visible light was irradiated on the surface of the ZnFe2O4/AgI composites, both AgI and ZnFe2O4 were activated, and the electrons were excited to the CB, leaving holes in the VB. Then, the differences in the band potentials between ZnFe2O4 and AgI could gave rise to an inner electric field at their interface. As a result, the 19

photogenerated e- in the CB of ZnFe2O4 was transferred to the CB of the AgI semiconductor and reduced the ambient oxygen to yield O2•– due to the CB of AgI (-0.41 eV) being more negative than the potential of O2/O2•– (-0.046 eV) [26]. Subsequently, the stronger H2O2 oxidizing reactive species was produced through the formation of O2•–, which further attacks bacterial cells [45]. Furthermore, some photogenerated holes on the VB of the AgI migrated to the VB of ZnFe2O4 due to the lower VB of ZnFe2O4 compared to that of AgI. At the same time, the EVB value of the AgI (2.38 eV) was more positive than the standard •OH/OH- redox potential (1.99 eV); thus, the generated holes oxidized OH- to generate •OH [48]. Moreover, photoinduced h+ also decomposed organic pollutants through direct oxidation of compositions on the cell membrane. Thus, in this photocatalytic system, these reactive species were beneficial for giving rise to dramatic photocatalytic performance.

4. Conclusions In summary, ZnFe2O4/AgI composites have been successfully fabricated through a hydrothermal method. The ZnFe2O4 nanoparticles were dispersed evenly on the surface of AgI. After the introduction of ZnFe2O4, the ZnFe2O4/AgI composites were more efficient for killing E. coli and for decomposing RhB compared to pure AgI under visible light irradiation. The 5% ZnFe2O4/AgI composite displayed the best photocatalytic disinfection of E. coli (100% removal efficiency in 80 min) as well as the photocatalytic degradation of RhB (98.5% removal rate in 40 min). The enhanced photocatalytic performance was attributed to the strong coupling between ZnFe2O4 20

and AgI, which enhanced the absorption of visible light, facilitated the interfacial charge transfer and improved the efficiency of electron-hole separation. Furthermore, four consecutive cycles also demonstrated the stable photocatalytic activities of the as-prepared ZnFe2O4/AgI composites. As indicated by radical scavenger examination, the highly efficient disinfection and photodegradation activities were mainly due to H2O2 and h+. This work provides important inspiration for the development of other ZnFe2O4-based photocatalytic materials for use in water purification.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21777063, 21506079, 21407065) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] D.J. Payne, M.N. Gwynn, D.J. Holmes, D.L. Pompliano, Drugs for bad bugs: confronting the challenges of antibacterial discovery, Nat. Rev. Drug Discov., 6 (2006) 29-40. [2] Y. Cai, C. Li, D. Wu, W. Wang, F. Tan, X. Wang, P.K. Wong, X. Qiao, Highly active MgO nanoparticles for simultaneous bacterial inactivation and heavy metal removal from aqueous solution, Chem. Eng. J., 312 (2017) 158-166. [3] J. Xu, Z. Wang, Y. Zhu, Enhanced visible-light-driven photocatalytic disinfection performance and organic pollutant degradation activity of porous g-C3N4 nanosheets, ACS Appl. Mater. Inter., 9 (2017) 27727-27735. [4] C. Guo, J. Xu, S. Wang, Y. Zhang, Y. He, X. Li, Photodegradation of sulfamethazine in an aqueous solution by a bismuth molybdate photocatalyst, Catal. Sci. Technol., 3 (2013) 1603-1611. [5] G. Jiang, R. Wang, X. Wang, X. Xi, R. Hu, Y. Zhou, S. Wang, T. Wang, W. Chen, Novel highly active visible-light-induced photocatalysts based on BiOBr with Ti doping and Ag decorating, ACS Appl. Mater. Inter., 4 (2012) 4440-4444. [6] J. Li, Y. Yin, E. Liu, Y. Ma, J. Wan, J. Fan, X. Hu, In situ growing Bi 2MoO6 on g-C3N4 nanosheets with enhanced photocatalytic hydrogen evolution and disinfection of bacteria under visible light 21

irradiation, J. Hazard. Mater., 321 (2017) 183-192. [7] S.B. Ning, H.X. Lin, Y.C. Tong, X. Zhang, Q. Lin, Y. Zhang, J. Long, X. Wang, Dual couples Bi metal depositing and Ag@AgI islanding on BiOI 3D architectures for synergistic bactericidal mechanism of E. coli under visible light, Appl. Catal. B: Environ., 204 (2017) 1-10. [8] F. Chen, X. Yang, Q. Wu, Photocatalytic oxidation of escherischia coli, aspergillus niger, and formaldehyde under different ultraviolet irradiation conditions, Environ. Sci. Technol., 43 (2009) 4606-4611. [9] K. Maeda, Rhodium-doped barium titanate perovskite as a stable p-type semiconductor photocatalyst for hydrogen evolution under visible light, ACS Appl. Mater.

Inter., 6 (2014)

2167-2173. [10] E. Liu, L. Kang, F. Wu, T. Sun, X. Hu, Y. Yang, H. Liu, J. Fan, Photocatalytic reduction of CO2 into methanol over Ag/TiO2 nanocomposites enhanced by surface plasmon resonance, Plasmonics, 9 (2014) 61-70. [11] S. Oros-Ruiz, R. Zanella, R. López, A. Hernández-Gordillo, R. Gómez, Photocatalytic hydrogen production by water/methanol decomposition using Au/TiO2 prepared by deposition-precipitation with urea, J. Hazard. Mater., 263 (2013) 2-10. [12] C.H. An, J.X. Liu, S.T. Wang, J. Zhang, Z. Wang, R. Long, Y. Sun, Concaving AgI sub-microparticles for enhanced photocatalysis, Nano Energy, 9 (2014) 204-211. [13] H.X. Shi, G.Y. Li, H.W. Sun, T. An, H. Zhao, P.-K. Wong, Visible-light-driven photocatalytic inactivation of E. coli by Ag/AgX-CNTs (X = Cl, Br, I) plasmonic photocatalysts: Bacterial performance and deactivation mechanism, Appl. Catal. B: Environ., 158-159 (2014) 301-307. [14] Y.G. Xu, S.Q. Huang, H.Y. Ji, L. Jing, M. He, H. Xu, Q. Zhang, H. Li, Facile synthesis of CNT/AgI with enhanced photocatalytic degradation and antibacterial ability, RSC Adv., 6 (2016) 6905-6914. [15] P.S. Archana, N. Pachauri, Z. Shan, S. Pan, A. Gupta, Plasmonic enhancement of photoactivity by gold nanoparticles embedded in hematite films, J. Phys. Chem. C, 119 (2015) 15506-15516. [16] Z.H. Chen, W.L. Wang, Z.G. Zhang, X. Fang, High-efficiency visible-light-driven Ag3PO4/AgI photocatalysts: Z-Scheme photocatalytic mechanism for their enhanced photocatalytic activity, J. Phys. Chem. C, 117 (2013) 19346-19352. [17] Y. Hou, F. Zuo, Q. Ma, C. Wang, L. Bartels, P. Feng, Ag3PO4 oxygen evolution photocatalyst employing synergistic action of Ag/AgBr nanoparticles and graphene sheets, J. Phys. Chem. C, 116 (2012) 20132-20139. [18] X. Yan, X. Wang, W. Gu, M. Wu, Y. Yan, B. Hu, G. Che, D. Han, J. Yang, W. Fan, W. Shi, Single-crystalline AgIn(MoO4)2 nanosheets grafted Ag/AgBr composites with enhanced plasmonic photocatalytic activity for degradation of tetracycline under visible light, Appl. Catal. B: Environ., 164 (2015) 297-304. [19] P. Li, X. Zhao, C.-j. Jia, H. Sun, L. Sun, X. Cheng, L. Liu, W. Fan, ZnWO4/BiOI heterostructures with highly efficient visible light photocatalytic activity: the case of interface lattice and energy level match, J. Mater. Chem. A, 1 (2013) 3421-3429. [20] H.F. Cheng, W.J. Wang, B.B. Huang, Z. Wang, J. Zhan, X. Qin, X. Zhang, Y. Dai, Tailoring AgI nanoparticles for the assembly of AgI/BiOI hierarchical hybrids with size-dependent photocatalytic activities, J. Mater. Chem. A, 1 (2013) 7131-7136. [21] Y.-S. Xu, W.-D. Zhang, Anion exchange strategy for construction of sesame-biscuit-like Bi2O2CO3/Bi2MoO6 nanocomposites with enhanced photocatalytic activity, Appl. Catal. B: Environ., 22

140 (2013) 306-316. [22] C. Hu, Y.Q. Lan, J.H. Qu, X. Hu, A. Wang, Ag/AgBr/TiO2 visible light photocatalyst for destruction of azodyes and bacteria, J. Phys. Chem. B, 110 (2006) 4066-4072. [23] F. Chen, Q. Yang, J. Sun, F. Yao, S. Wang, Y. Wang, X. Wang, X. Li, C. Niu, D. Wang, G. Zeng, Enhanced photocatalytic degradation of tetracycline by AgI/BiVO4 heterojunction under visible-light irradiation: Mineralization efficiency and mechanism, ACS Appl. Mater. Inter., 8 (2016) 32887-32900. [24] H. Cheng, W. Wang, B. Huang, Z. Wang, J. Zhan, X. Qin, X. Zhang, Y. Dai, Tailoring AgI nanoparticles for the assembly of AgI/BiOI hierarchical hybrids with size-dependent photocatalytic activities, J. Mater. Chem. A, 1 (2013) 7131-7136. [25] Z. Wan, G. Zhang, Synthesis and facet-dependent enhanced photocatalytic activity of Bi2SiO5/AgI nanoplate photocatalysts, J. Mater. Chem. A, 3 (2015) 16737-16745. [26] T.Y. Wang, W. Quan, D. Jiang, L. Chen, D. Li, S. Meng, M. Chen, Synthesis of redox-mediator-free direct Z-scheme AgI/WO3 nanocomposite photocatalysts for the degradation of tetracycline with enhanced photocatalytic activity, Chem. Eng. J., 300 (2016) 280-290. [27] S.L. Wu, P.P. Wang, Y.Y. Cai, D. Liang, Y. Ye, Z. Tian, J. Liu, C. Liang, Reduced graphene oxide anchored magnetic ZnFe2O4 nanoparticles with enhanced visible-light photocatalytic activity, RSC Adv., 5 (2015) 9069-9074. [28] D.R. Yang, J. Feng, L.L. Jiang, X. Wu, L. Sheng, Y. Jiang, T. Wei, Z. Fan, Photocatalyst interface engineering: spatially confined growth of ZnFe2O4 within graphene networks as excellent visible-light-driven photocatalysts, Adv. Funct. Mater., 25 (2015) 7080-7087. [29] S.K. Wu, X.P. Shen, G.X. Zhu, H. Zhou, Z. Ji, K. Chen, A. Yuan, Synthesis of ternary Ag/ZnO/ZnFe2O4 porous and hollow nanostructures with enhanced photocatalytic activity, Appl. Catal. B: Environ., 184 (2016) 328-336. [30] A.H. Mady, M.L. Baynosa, D. Tuma, J.J. Shim, Facile microwave-assisted green synthesis of Ag-ZnFe2O4@rGO nanocomposites for efficient removal of organic dyes under UV- and visible-light irradiation, Appl. Catal. B: Environ., 203 (2017) 416-427. [31] J.S. Xie, Q.S. Wu, D.F. Zhao, Electrospinning synthesis of ZnFe2O4/Fe3O4/Ag nanoparticle-loaded mesoporous carbon fibers with magnetic and photocatalytic properties, Carbon, 50 (2012) 800-807. [32] A.R. Upreti, Y. Li, N. Khadgi, S. Naraginti, C. Zhang, Efficient visible light photocatalytic degradation of 17α-ethinyl estradiol by a multifunctional Ag-AgCl/ZnFe2O4 magnetic nanocomposite, RSC Adv., 6 (2016) 32761-32769. [33] Y. Xu, Q. Liu, C. Liu, Y. Zhai, M. Xie, L. Huang, H. Xu, H. Li, J. Jing, Visible-light-driven Ag/AgBr/ZnFe2O4 composites with excellent photocatalytic activity for E. coli disinfection and organic pollutant degradation, J. Colloid Interf. Sci., 512 (2017) 555-566. [34] L. Kong, Z. Jiang, T.C. Xiao, L. Lu, M.O. Jones, P.P. Edwards, Exceptional visible-light-driven photocatalytic activity over BiOBr-ZnFe2O4 heterojunctions, Chem. Commun., 47 (2011) 5512-5514. [35] L. Zhang, Y.M. He, P. Ye, Y. Wu, T. Wu, Visible light photocatalytic activities of ZnFe2O4 loaded by Ag3VO4 heterojunction composites, J. Alloy. Compd., 549 (2013) 105-113. [36] Y. Yao, Y. Cai, F. Lu, J. Qin, F. Wei, C. Xu, S. Wang, Magnetic ZnFe 2O4-C3N4 hybrid for photocatalytic degradation of aqueous organic pollutants by visible light, Ind. Eng. Chem. Res., 53 (2014) 17294-17302. [37] Q. Wang, X.D. Shi, E.Q. Liu, J. Xu, J.C. Crittenden, Y. Zhang, Y. Cong, Preparation and photoelectrochemical performance of visible-light active AgI/TiO2-NTs composite with rich β-AgI, Ind. Eng. Chem. Res., 55 (2016) 4897-4904. 23

[38] S.Q. Huang, Y.G. Xu, Z.G. Chen, M. Xie, H. Xu, M. He, H. Li, Q. Zhang, A core-shell structured magnetic Ag/AgBr@Fe2O3composite with enhanced photocatalytic activity for organic pollutant degradation and antibacterium, RSC Adv., 5 (2015) 71035-71045. [39] H. Song, L.P. Zhu, Y.G. Li, Z. Lou, M. Xiao, Z. Ye, Preparation of ZnFe2O4 nanostructures and highly efficient visible-light-driven hydrogen generation with the assistance of nanoheterostructures, J. Mater. Chem. A, 3 (2015) 8353-8360. [40] L.R. Hou, L. Lian, L.H. Zhang, G. Pang, C. Yuan, X. Zhang, Self-sacrifice template fabrication of hierarchical mesoporous Bi-component-active ZnO/ZnFe2O4 sub-microcubes as superior anode towards high-performance lithium-ion battery, Adv. Funct. Mater., 25 (2015) 238-246. [41] W.Y. Zhu, J.C. Liu, S.Y. Yu, Y. Zhou, X. Yan, Ag loaded WO3 nanoplates for efficient photocatalytic degradation of sulfanilamide and their bactericidal effect under visible light irradiation, J. Hazard. Mater., 318 (2016) 407-416. [42] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Effective photocatalytic disinfection of E. coli K-12 using AgBr-Ag-Bi2WO6 Nanojunction system irradiated by visible light:The role of diffusing hydroxyl radicals, Environ. Sci. Technol., (2010) 1392-1398. [43] X.P. Wang, T.T. Lim, Highly efficient and stable Ag-AgBr/TiO2 composites for destruction of Escherichia coli under visible light irradiation, Water Res., 47 (2013) 4148-4158. [44] Y. Li, Y. Li, S. Ma, P. Wang, Q. Hou, J. Han, S. Zhan, Efficient water disinfection with Ag2WO4-doped mesoporous g-C3N4 under visible light, J. Hazard. Mater., 338 (2017) 33-46. [45] T.W. Ng, L.S. Zhang, J.S. Liu, G.C. Huang, W. Wang, P.K. Wong, Visible-light-driven photocatalytic inactivation of Escherichia coli by magnetic Fe2O3-AgBr, Water Res., 90 (2016) 111-118. [46] X.J. Li, D.L. Tang, F. Tang, Y. Zhu, C. He, M. Liu, C. Lin, Y. Liu, Preparation, characterization and photocatalytic activity of visible-light-driven plasmonic Ag/AgBr/ZnFe2O4 nanocomposites, Mater. Res. Bull., 56 (2014) 125-133. [47] Y. Chen, A. Lu, Y. Li, L. Zhang, H.Y. Yip, H. Zhao, T. An, P.K. Wong, Naturally occurring sphalerite as a novel cost-effective photocatalyst for bacterial disinfection under visible light, Environ. Sci. Technol., 45 (2011) 5689-5695. [48] D. Wu, W. Wang, T.W. Ng, G.C. Huang, D. Xia, H.Y. Yip, H.K. Lee, G. Li, T. An, P.K. Wong, Visible-light-driven photocatalytic bacterial inactivation and the mechanism of zinc oxysulfide under LED light irradiation, J. Mater. Chem. A, 4 (2016) 1052-1059.

24

(112)

(110)

(002)

(300) (213) (302)

(203) (211)

(103)

(102)

(100)

(101)

Intensity (a.u.)

AgI

3% ZnFe2O4/AgI 5% ZnFe2O4/AgI

10

20

30

40

50

ZnFe2O4 (533)

(440)

(511)

(422)

(400)

(220)

(311)

7% ZnFe2O4/AgI

60

70

80

2 Theta (degree) Fig. 1. XRD patterns of AgI, ZnFe2O4 and ZnFe2O4/AgI composites.

C 1s

Ag 3p O 1s

Fe 2p

5% ZnFe2O4/AgI

Ag 3d

I 3d

Survey

Zn 2p

Intensity (a.u.)

(a)

AgI

1200 1000

800

600

400

200

0

Binding Energy (eV)

(b)

(c)

Ag 3d

5% ZnFe2O4/AgI 374.7 eV

368.7 eV

I 3d5/2

631.6 eV

368.8 eV

Ag 3d3/2 374.8 eV

Intensity (a.u.)

Intensity (a.u.)

Ag 3d5/2

I 3d3/2

620.1 eV

5% ZnFe2O4/AgI 620.0 eV

631.5 eV

AgI

378

376

374

372

I 3d

AgI

370

368

366 636

632

628

624

620

Binding Energy (eV)

Binding Energy (eV)

25

616

Fe

(d)

Fe

Fe 2p

Zn

Zn 2p3/2Zn 2p

(e)

Fe 2p 1/2

Satellite Fe

3+

1022.47 eV

3/2

712.7 eV

726.2 eV

5% ZnFe2O4/AgI

AgI

740

735

Intensity (a.u.)

Intensity (a.u.)

Fe 2p

Zn 2p1/2 1045.58 eV

5% ZnFe2O4/AgI

AgI

730

725

720

715

710

705

1050

1045

1040

1035

1030

1025

1020

Binding Energy (eV) Binding Energy (eV) Fig. 2. XPS spectra of AgI, ZnFe2O4 and 5% ZnFe2O4/AgI composite: (a) survey of the sample, (b) Ag 3d, (c) I 3d, (d) Fe 2p, (e) Zn 2p.

26

Fig. 3. SEM images of AgI (a and b), 5% ZnFe2O4/AgI (c and d), elemental mapping of 5% ZnFe2O4/AgI (e), and HRTEM images of the 5% ZnFe2O4/AgI composite(f).

(a)

(b)

ZnFe2O4

Absorbance (a.u.)

7% ZnFe2O4/AgI 5% ZnFe2O4/AgI 3% ZnFe2O4/AgI

ZnFe2O4

(ahv)

2

AgI

AgI

2.8 eV

1.92 eV 200

300

400

500

600

700

800

1.6

2.0

2.4

2.8

3.2

hV (eV)

Wavelength (nm)

Relative intensity (a.u.)

Fig. 4. (a) UV-Vis diffuse reflectance spectra of the as-prepared samples, (b) corresponding Tauc’s plots of the pure ZnFe2O4 and AgI.

AgI

5% ZnFe2O4/AgI

400

450

500

550

600

650

Wavelength (nm) Fig. 5. PL spectra of the pure AgI and 5% ZnFe2O4/AgI composite.

27

1.2

200

Current / 

1.0 0.8 on

a b c d e off

ZnFe2O4 AgI 3 % ZnFe2O4/AgI 7 % ZnFe2O4/AgI 5 % ZnFe2O4/AgI

e

0.6 0.4

d c b

0.2

(b) AgI 3% ZnFe2O4/AgI

150

-Z'' / ohm

(a)

5% ZnFe2O4/AgI 7% ZnFe2O4/AgI ZnFe2O4

100

50

a

0.0

0

0

50

100

150

200

250

0

300

50

100

Time / s

150

200

250

300

Z' / ohm

7

7

(a)

(b)

Cell density (log10 cfu/ml)

Cell density (log10 cfu/ml)

Fig. 6. (a) Transient photocurrent responses of the pure AgI, ZnFe2O4 and ZnFe2O4/AgI composites under visible-light irradiation, (b) and the corresponding electrochemical Impedance spectroscopy (EIS) Nyquist plots of the samples.

6

6

5

5

4

4 ZnFe2O4

3

3

AgI 3% ZnFe2O4/AgI

2

7% ZnFe2O4/AgI 5% ZnFe2O4/AgI

1

Light Control ZnFe2O4

2

AgI 3% ZnFe2O4/AgI

1

5% ZnFe2O4/AgI

7% ZnFe2O4/AgI

0

0 0

20

40

60

80

0

100

20

40

(c)

Cell density (log10 cfu/ml)

80

100

Time (min)

Time (min) 8

60

st

1

rd

nd

3

2

th

4

6

4

2

0 0

60

120

180

240

300

Time (min)

Fig. 7. E. coli (107.5 cfu/mL) inactivation by prepared photocatalysts; (a) in the dark and (b) under visible light (≥ 420 nm); (c) Recycling experiments for the photocatalytic disinfection of E. coli by 5% ZnFe2O4/AgI. Error bars represent standard deviations from triplicate experiments (n=3).

28

Fig. 8. The highest magnification SEM images of E. coli (107.5cfu/mL) treated with 5% ZnFe2O4/AgI (50 µg/mL) under visible light irradiation for (a) 0 min, (b) 20 min, (c) 60 min, (d) 80 min. 4

1.0

(b)

(a) RhB AgI 7% ZnFe2O4/AgI 3% ZnFe2O4/AgI

0.6

5% ZnFe2O4/AgI

3

-Ln (Ct / C0)

Ct / C0

0.8

5% ZnFe2O4/AgI

0.4

3% ZnFe2O4/AgI 7% ZnFe2O4/AgI AgI RhB

2

1

0.2 0

0.0 0

10

(c) 1st

1.0

20

30

Time / min nd

2

rd

3

40

4

0

20

Time / min

40

(d) 0.8

0.6

30

1.0

th

0.8

1 mM TEOA puring N2

0.6

AgI

1 mM tBuOH No quencher

Ct/C0

Ct/C0

10

0.4

0.4

0.2

0.2 0.0 5%ZnFe O /AgI 2 4 0

40

80

120

160

Time (min)

0.0 0

10

20

Time (min)

30

40

Fig. 9. (a) Photocatalytic degradation of RhB by the as-prepared samples. (b) Kinetic fit for the degradation of RhB with only light, AgI, ZnFe2O4/AgI composites. (c) Recycling runs of the degradation of RhB over AgI and 5% ZnFe2O4/AgI composite. (d) Trapping experiment of active species during the photocatalytic degradation of RhB.

29

8

Cell density (log10 cfu/ml)

80

(a)

6

(b) AgI 5% ZnFe2O4/AgBr

H2O2 (mol/L)

60

Catalase TEMPOL Ar Cr(VI) Sodium oxalate Isopropanol 5% ZnFe2O4/AgI

4

2

40

20

0

0 0

20

40

60

80

0

100

20

40

60

80

100

Time (min)

Time (min)

Fig. 10. (a) Disinfection efficiencies of E. coli by 5% ZnFe2O4/AgI in the presence of different scavengers (0.5 mM isopropanol, 0.5 mM sodium oxalate, 0.05 mM Cr (VI), 0.05 µmol/L catalase and 2 mM TEMPOL) under visible light irradiation and (b) H2O2 produced under visible light irradiation. (a)

O2

dark ZnFe2O4

(b)

OH

AgI 5% ZnFe2O4/AgI

Intensity (a.u.)

Intensity (a.u.)

AgI 5% ZnFe2O4/AgI

dark ZnFe2O4

318.4

318.5

318.6

318.7

318.8

318.9

Field (mT)

318.2

318.3

318.4

318.5

318.6

Field (mT)

Fig. 11. DMPO spin-trapping ESR spectra recorded at ambient temperature with AgI, ZnFe2O4 and 5% ZnFe2O4/AgI under visible light irradiation for DMPO-O2•– in a methanol dispersion (a) and for DMPO-•OH in an aqueous dispersion (b).

Fig. 12. Possible reaction mechanism of photocatalytic performance treated with ZnFe2O4/AgI composite under visible light irradiation. 30

Graphical abstract

ZnFe2O4/AgI composites with different weight ratios of ZnFe2O4, prepared via facile one-step hydrothermal method. The obtained photocatalysts exhibited excellent photocatalytic disinfection of E. coli and pollutant degradation under visible light irradiation.

31