Bi2Sn2O7 nanojunction system

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Construction of direct Z-scheme AgI/Bi2Sn2O7 nanojunction system with enhanced photocatalytic activity: Accelerated interfacial charge transfer induced efficient Cr(VI) reduction, tetracycline degradation and Escherichia coli inactivation Hai Guo, Cheng-Gang Niu, Lei Zhang, Xiao-Ju Wen, Chao Liang, Xue-Gang Zhang, Dan-Lin Guan, Ning Tang, and Guangming Zeng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01448 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Sustainable Chemistry & Engineering

Construction of direct Z-scheme AgI/Bi2Sn2O7 nanojunction system with enhanced photocatalytic activity: Accelerated interfacial charge transfer induced efficient Cr(VI) reduction, tetracycline degradation and Escherichia coli inactivation Hai Guo, Cheng-Gang Niu*, Lei Zhang, Xiao-Ju Wen, Chao Liang, Xue-Gang Zhang, Dan-Lin Guan, Ning Tang, Guang-Ming Zeng* College of Environmental Science Engineering, Key Laboratory of Environmental Biology Pollution Control, Ministry of Education, Hunan University, 2 South Lushan Road, Yuelu District, Changsha 410082, China *Corresponding Author: Cheng-Gang Niu, E-mail address: [email protected], [email protected]; Tel: +86-731-88823820; Guang-Ming Zeng, E-mail address: [email protected] Abstract: The exploration of highly efficient visible light driven photocatalysts for diverse pollutants removal has received great concerns in wastewater treatment. Here, a series of tightly connected AgI/Bi2Sn2O7 nanocomposites were fabricated through an in-situ deposition-precipitation route. The resulting AgI/Bi2Sn2O7 photocatalysts exhibited superior photocatalytic performance for Cr(VI) reduction, tetracycline (TC) degradation as well as Escherichia coli (E. coli) inactivation. It was found that AB-31.97 nanocomposite displayed optimal photocatalytic performance under visible light irradiation, i.e. nearly 100% reduction of Cr(VI) and 7.48-log inactivation of E. coli cells. As for TC, the overall degradation process was reflected by 1

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three-dimensional excitation-emission matrix fluorescence spectra (3D EEMs), and the detailed degradation pathways were proposed through LC-MS system. The promoted photocatalytic performance of the obtained AgI/Bi2Sn2O7 nanocomposites can be attributed to the formed nanojunction structure between AgI and Bi2Sn2O7, which not only accelerates the interfacial charge transfer efficiency, but also preserves the strong redox ability of the photogenerated electrons and holes. Meanwhile, cycling experiments and inductively coupled plasma mass spectroscopy (ICP-MS) measurement manifested that the AB-31.97 nanocomposite also presented outstanding photostability. According to the results of radical trapping experiments and electron spin resonance (ESR) detection, h+, e-, •O2- and •OH all involved in photocatalytic reaction. Based on the experimental results, a plausible Z-scheme charge transfer mechanism was put forward. This work provides a deep insight into the multi-application of nanojunction photocatalysts and highlights a facile way to construct highly efficient photocatalysts for wastewater treatment. Keywords: AgI/Bi2Sn2O7; Photocatalysis; Z-scheme; Wastewater treatment Introduction Over the last few decades, the environmental problems caused by the ever-increasing population have brought tremendous pressure to the scientific community. Among them, the issues related to the wastewater treatment are regarded as one of the most challenging tasks

1-2

. Despite numerous methods, such as

adsorption, flocculation and membrane filtration, have been employed to control the water pollution, owing to the diversity of wastewater, it is quite tough for a single 2


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method to acquire satisfactory results in dealing with different types of wastewater 3. Fortunately, a sign of relief is perceived when the researchers come up with an 4-5

emerging solution known as semiconductor-based photocatalysis

. Up to now, this

technology has been well applied in wastewater treatment, especially for water pollution caused by antibiotics, heavy metal ions or pathogenic microorganisms

6-9

.

But considering that most of the researched photocatalysts (e.g., TiO2, ZnO) are only UV-excited, their practical application still faces a lot of limitations. Therefore, to expand the applicability of the photocatalysis and maximize the utilization of solar energy, it is highly imperative to explore novel and efficient visible light driven photocatalysts. In recent years, Bi-based photocatalysts have aroused widespread concerns due to their exceptional visible light response ability, nontoxicity and chemical stability. So far, numerous bismuth materials including BiVO4, BiO2-x, Bi2WO6, Bi2Sn2O7 and BiOX (X= Cl, Br, I) have achieved great success in wastewater treatment 10-12. Among these materials, benefited from its unique pyrochlore structure and appropriate band gap (~2.7 eV), Bi2Sn2O7 is considered as an ideal photocatalyst and has been widely applied in the remediation of various environmental pollution

13

. For instance, Tian

and co-workers reported a facile hydrothermal procedure to fabricate nanocrystalline Bi2Sn2O7, and the obtained samples exhibited extraordinary photocatalytic performance towards the elimination of As(III) from aqueous solution

14

. Similarly,

another successful case was accomplished by Lu et al, in their experiment, the removal of NO was selected as target reaction to investigate the availability of 3



nanocrystalline Bi2Sn2O7 in the field of air purification

15

. But owing to the rapid

charge recombination and low quantum yield, the further photocatalytic application of pure Bi2Sn2O7 is severely hampered. Herein, several methods, such as heteroatom substitution 16 and heterojunction construction 17-19, have been employed to overcome these problems. Among them, coupling Bi2Sn2O7 with another suitable semiconductor to form heterostructural photocatalytic system has been verified as a feasible strategy, and according to this concept, several efficient Bi2Sn2O7-based photocatalysts are obtained 17-20. In spite of this, it has to be pointed out that there is an inevitable defect existed in traditional heterojunction system, namely, the redox activity of the photogenerated charge carriers is ultimately weakened to some extent through the band to band transfer manner 21. So when it comes to practical wastewater treatment, the charge carriers in such a heterojunction system may not provide sufficient redox activity to meet the reaction requirement. Thus, in order to solve this task and further improve the reactivity of photocatalysts, Z-scheme photocatalytic system has been proposed and thoroughly researched

22-23

. In a typical Z-scheme system, the unique

charge separation behavior will not only boost the transfer of the photogenerated electrons and holes, but also preserve the strong redox activity of the photocatalyst 24-26

. Therefore, constructing a Bi2Sn2O7-based Z-scheme photocatalyst with higher

photocatalytic performance will be the most promising choice for treating wastewater, or even, different types of wastewater. Currently, silver iodide (AgI), another potential photocatalyst with a proper band gap (~2.8 eV), has received great attention in the fields of organic pollution 4


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degradation 27, Cr(VI) reduction 28 and pathogenic microorganisms inactivation 9. But owing to the photo-sensitization effect, the structure of pure AgI is not stable and will be gradually destroyed in light driven reaction. According to previous research, heterojunction construction has been confirmed as an efficient strategy in solving the above problems 29-30. And very recently, we also found that coupling AgI with CeO2 to form a heterojunction can greatly promote both the photoactivity and the photostability of pure AgI

27

. Nonetheless, in most cases the freshly deposited AgI

components are micro-sized agglomerates, which make it hard for them to establish intimate contact with the original supports, thus resulting in a lower separation efficiency of the photogenerated charge carriers 31. Therefore, choosing an appropriate substrate that can well disperse AgI nanoparticle will ultimately bring about a more efficient and stable photocatalytst. In view of the above analysis, the combination of AgI and Bi2Sn2O7 might be a good choice since the nano-sized Bi2Sn2O7 will not only contribute to homogeneously disperse AgI but also can effectively restrain the aggregation of AgI. Meanwhile, once the heterojunction between Bi2Sn2O7 and AgI was formed in nanoscale, the migration distance of the photogenerated charge carriers will be greatly shortened

31-32

. In addition, considering the similar band gaps and the

matched band structure of Bi2Sn2O7 and AgI, the photogenerated charge carriers might prefer to experience a Z-scheme transfer process

33-34

. Accordingly, it is

expected that the final AgI/Bi2Sn2O7 nanocomposites will present excellent photocatalytic activity.

5



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In this work, the novel Z-scheme AgI/Bi2Sn2O7 photocatalysts with nanojunction structure were well-designed through a facile deposition-precipitation process. Physicochemical properties of the obtained AgI/Bi2Sn2O7 nanocomposites, such as chemical composition, morphology structure and optical characteristic, were systematically investigated. Three typical probe reactions (Cr (VI) reduction, TC degradation, E. coli inactivation) were utilized to investigated the availability of the as-prepared samples in the treatment of simulated wastewater that were contaminated by heavy metal ions, antibiotics or pathogenic microorganisms. Each probe reaction was fully explored, and in particular, the degradation process of TC was deeply researched through 3D EEMs and LC-MS technology. Finally, based on the results of active species trapping experiments and electron spin resonance (ESR) measurements, a

plausible

Z-scheme

charge

transfer

mechanism

for

AgI/Bi2Sn2O7

nanocomposite-based reaction system was put forward. Experimental section This part is provided in supporting information. Results and discussion Characterization of the obtained materials Powder XRD analysis was applied for examining the crystallinity, phase and purity of as-prepared samples. As presented in Fig. 1, for pristine Bi2Sn2O7, the distinct diffraction peaks located at 2θ values of 28.89°, 33.46°, 48.01°, 57.02°, 59.71° and 77.60° are detected, which can be assigned to the (222), (400), (440), (622), (444) and (662) crystal planes of the cubic phase Bi2Sn2O7 (JCPDS No. 87-0284) 6


19

,

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respectively. These well-defined diffraction peaks indicate that the formed Bi2Sn2O7 possesses superior crystallinity. With regard to the XRD patterns of AgI, four main diffraction peaks at 22.30°, 23.72°, 39.18° and 46.30° are indexed to (100), (002), (110) and (112) crystal planes perfectly (JCPDS No. 09-0374). From the diffraction patterns of AgI/Bi2Sn2O7 nanocomposites, two sets of diffraction peaks belonging to Bi2Sn2O7 and AgI can be clearly observed, confirming the co-existence of these two phases. In addition, with the increase of AgI content, the peaks intensity of AgI enhances gradually while the intensity of Bi2Sn2O7 peaks decreases slightly. This increase or decrease in the peaks parameters can be credited to the synergistic effect between these two phases, further demonstrating a strong interaction between AgI and Bi2Sn2O7

35

. Besides, no additional diffraction peaks are discovered, indicating that

coupling AgI do not affect the phase of Bi2Sn2O7 obviously.

Fig. 1. XRD patterns of pure Bi2Sn2O7, pure AgI, and AgI/Bi2Sn2O7 nanocomposites. The structural and morphological details of pure Bi2Sn2O7 and AB-31.97 nanocomposite were investigated by TEM analysis (Fig. 2). As delineated in Fig. 2a, 7



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b, most Bi2Sn2O7 nanocrystalline presents an irregular cubic structure. To show more information on the size distribution of the obtained sample, the diameter and length of the nanocrystalline is measured by Nano Measurer software

36

, and the result shows

that their sizes are mainly distributed between 10 nm to 30 nm (Fig. S1). After coupling with AgI (Fig. 2c, d), it can be observed that the structure of nanocomposite become more close-knit, simultaneously, numerous dark nanoparticles appear in the nanocomposite, which are uniformly dispersed and tightly connected with Bi2Sn2O7 nanocrystalline. These results provide solid evidence for the formation of nanojunction between Bi2Sn2O7 and AgI

32

, meanwhile, the small size of these two

phases ensures their sufficient contact, which will shorten the migration distance of charge carriers and further promote the charge separation efficiency 37.

Fig. 2. TEM images of (a, b) pure Bi2Sn2O7, (c, d) AB-31.97 nanocomposite. Since the size of AgI is similar to that of Bi2Sn2O7 nanocrystalline, making it hard 8


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to be distinguished from Bi2Sn2O7. Herein, in order to elucidate the existence of AgI in the nanocomposite definitely, HRTEM measurement was carried out to reveal the structural information from a more micro perspective. As displayed in Fig. 3a, two well-defined lattice fringes with different directions were estimated to be 0.311 nm and 0.266 nm, which can be separately indexed to the (222) and (400) planes of Bi2Sn2O7. Analogously, another two crystals with the lattice spacing of 0.229 nm and 0.374 nm are matching well with the (110) and (002) planes of AgI. More importantly, a distinguished interface between AgI and Bi2Sn2O7 nanoparticles can be clearly observed, revealing that the nanojunction is indeed formed in AB-31.97 nanocomposite. Further, from the results illustrated in EDS mapping images (Fig. 3b-g), it is apparent that Bi, Sn, O, Ag and I elements distribute homogeneously in specific areas of nanocomposite, indicating that the phases of Bi2Sn2O7 and AgI are firmly coupled with each other in nanoscale rather than simply mixed 37. Herein, these results well support the conclusion drawn from TEM images.

9



Fig. 3. (a) HRTEM image and (b-g) TEM-EDS elemental mapping images of AB-31.97 nanocomposite. Additionally, XPS measurement was carried out to investigate the chemical composition and electronic state of the as-prepared AB-31.97 nanocomposite. Fig. 4a displays the fully scanned spectrum, in which the Ag, I, Bi, Sn and O elements can be evidently observed, and all peak positions of constituted elements are accurately monitored by referencing the adventitious hydrocarbon at the binding energy of 284.6 eV 10. As depicted in Fig. 4b, two characteristic peaks belonging to Ag 3d5/2 and Ag 3d3/2 orbitals are detected at 368.17 eV and 374.17 eV, revealing the existence of Ag+ 38

. Meanwhile, the binding energy of I 3d illustrated in Fig. 4c are discovered at

619.15 eV and 630.75 eV, which can be attributed to I 3d5/2 and I 3d3/2, in the form of I- 39. In the case of high-resolution Bi 4f spectrum (Fig. 4d), the observed two peaks at 10


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159.21 eV and 164.51 eV are separately assigned to Bi 4f7/2 and Bi 4f5/2, demonstrating that the chemical state of Bi is +3

40

. Notably, the spectrum of O 1s

presented in Fig. 4e can be deconvoluted into two peaks at the binding energy of 529.71 eV and 531.60 eV, wherein the former refers to Bi-O bond and the latter represents Sn-O bond in the nanocomposite 41. From the Sn 3d spectrum (Fig. 4f), it can be perceived that two featured peaks appear at the binding energy of 486.55 eV and 495.21 eV, corresponding to the Sn 3d5/2 and Sn 3d3/2 of Sn4+ 17. Thereby, these spectral data further verify that the AgI/Bi2Sn2O7 nanocomposite is synthesized successfully.

11



Fig. 4. XPS spectra of AB-31.97 composite: (a) survey spectrum, (b) Ag 3d, (c) I 3d, (d) Bi 4f, (e) O 1s, (f) Sn 3d. Considering that the optical property plays an overwhelming role in determining the performance of photocatalysts, the UV-vis absorption spectra of Bi2Sn2O7, AgI and AB-31.97 nanocomposite were therefore measured and displayed in Fig. 5. Pure Bi2Sn2O7 and AgI exhibit an absorption edge at around 475 nm and 455 nm, respectively, where the steep shape of spectra reveals that the light absorption is 12


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resulted from the band-gap transition. After coupling Bi2Sn2O7 with AgI, it is worth noting that the absorption region of AB-31.97 is almost between these of AgI and Bi2Sn2O7. Nevertheless, the absorption edge of AB-31.97 nanocomposite still shows a slight red-shift, which can be ascribed to the strong interaction between Bi2Sn2O7 and AgI

18

. Generally, the band gap (Eg) of semiconductor-based photocatalyst can be

calculated through the equation

42-43

: Eg = 1240/λ, where λ is the absorption edge.

Accordingly, the band gaps of individual Bi2Sn2O7 and AgI are separately assessed to be about 2.61 eV and 2.73 eV, which are in close agreement with previous reports 13, 27

. Furthermore, the band edge positions of Bi2Sn2O7 and AgI can be acquired through

the empirical formula: ECB =X - Ee - 0.5Eg

(1)

EVB =Eg + ECB

(2)

where X represents the absolute electronegativity of the semiconductor, and Ee is a definite value (~ 4.5 eV) that stands for the energy of free electrons on the hydrogen scale. Thus, the CB and VB potentials of Bi2Sn2O7 are determined to be 0.45 eV and 3.06 eV, respectively. Analogously, as for AgI, the CB and VB edges are separately calculated as -0.38 eV and 2.35 eV.

13



Fig. 5. UV-vis diffuse reflectance spectra of Bi2Sn2O7, AgI and AB-31.97 nanocomposite. Evaluation of photocatalytic activity The photocatalytic activity of the as-synthesized samples was firstly estimated by the reduction of Cr(VI) under visible light irradiation (Fig. 6). Initially, separate experiments were conducted without using photocatalyst or illumination to test the stability of Cr(VI), and the results manifest that the self-reduction of Cr(VI) can be neglected under the lamp irradiation. Instead, after the introduction of photocatalysts, the concentration of Cr(VI) decreases obviously with the extension of irradiation time (Fig. 6a), while no significant change in Cr(VI) concentration is observed in the absence of light, indicating that the photoreduction process is triggered by light-stimulated catalysts. For pristine Bi2Sn2O7 and AgI, merely 31.3% and 51.1% of Cr(VI) are photoreduced after a 40 min irradiation, implying their poor photocatalytic activity. But once AgI nanoparticles are coupled with Bi2Sn2O7, the resulting AgI/Bi2Sn2O7 nanocomposites achieve more superior performance for Cr(VI) 14


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reduction. Furthermore, it is found that the photoreduction activity of AgI/Bi2Sn2O7 nanocomposites enhances steadily until the amounts of AgI reaches 31.97%, and the obtained AB-31.97 nanocomposite realizes optimal removal rate, in which almost all Cr(VI) is photoreduced within 40 min. Notably, by further increasing the amounts of AgI, the photocatalytic performance of AgI/Bi2Sn2O7 nanocomposites do not enhance any longer, but start to decline. This can be ascribed to the fact that despite the introduction of AgI accelerates the interfacial charge transfer, excess AgI will block the light from entering the Bi2Sn2O7 surface and reduce the number of available reaction sites, resulting in the insufficient contacts between active species and target pollutants

44

. Fig. 6b provides the temporal absorption spectra of Cr(VI) (after the

chromogenic reaction) in the presence of AB-31.97. As the irradiation proceeds, the characteristic absorption peak of Cr(VI) declines rapidly and even disappears at last, suggesting that the Cr(VI) can be efficiently removed. Since the existence forms of Cr(VI) species (HCrO4-, Cr2O72-,CrO42-) are closely related to the solution pH and will greatly affect the reduction of Cr(VI), a series of experiments were therefore operated at different pH values (adjusted by 0.1 M H2SO4 or 0.1 M NaOH). As outlined in Fig. 6c, it is apparent that, under the acidic condition, the reduction process of Cr(VI) is significantly accelerated, and the highest removal ratio is realized at pH 2.0. However, the reduction efficiency of Cr(VI) declines sharply with the increase of pH value, especially for alkaline condition, about 17.9% of Cr(VI) is reduced at pH 8.0. The possible reasons underlying this negative correlation between pH value and removal ratio are summarized as below 15


28, 45-48

.


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First, at low pH, the dominant Cr(VI) species is HCrO4- or Cr2O72-, and the reduction process of Cr(VI) can be revealed by Eq. (3). Obviously, the reduction potential (Eθ(Cr2O72-/Cr3+)) is positive enough, which makes it more liable to be reduced. Meanwhile, sufficient H+ in the acidic solution also favors the reaction. Second, under alkaline condition, CrO42- is the main species of Cr(VI)，and the Cr(VI) reduction proceeds according to Eq. (4). Compared with the case in acidic condition, it is more difficult to achieve this process since the reduction potential (Eθ(CrO42-/Cr(OH)3)) is relatively negative. Besides, due to the formation of Cr(OH)3 at higher pH, Cr(III) is prone to precipitate on the surface of photocatalysts. Third, from the perspective of thermodynamics, when the pH increases from 0 to 7, the band edge positions of semiconductors shift negatively, about 59 mV per pH unit. But for the reduction potential of Cr2O72-/Cr(III) pairs, it is about 138 mV. Ultimately, the thermodynamic driving force for the reduction of Cr(VI) decreases by about 79 mV with the increase of each pH unit. Thus, a lower pH is favorable for the removal Cr(VI). 2-

Cr2 O7 +14H+ +6e- →2Cr3+ +7H2 O;

2-

Eθ (Cr2 O7 /Cr3+ ) = 1.33 V vs. NHE

(3)

CrO4 + 4H2 O + 3e- →CrOH3 +5OH- ; Eθ CrO4 /Cr(OH)3 = -0.13 V vs. NHE (4) 2-

2-

16


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Fig. 6. (a) Photocatalytic reduction curves of Cr(VI); (b) Temporal absorption spectra of Cr(VI) with AB-31.97 nanocomposite under visible light irradiation; (c) The effect of initial pH for Cr(VI) reduction in the presence of AB-31.97 nanocomposite. Given the poor biodegradability of most antibiotics, they may easily accumulate in the aquatic system and thus induce a series of adverse effects on human health. Therefore, TC, a typical antibiotic that has been frequently detected in surface water, is selected to investigate the photocatalytic performance of the as-prepared samples. As shown in Fig. 7a, blank experiments validate the stability of TC molecules in the absence of photocatalyst or light illumination. Among these samples, the degradation efficiency of pristine Bi2Sn2O7 and AgI are found to be only 43.2% and 63.4%, which is not satisfactory and requires further improvement. Similar to the Cr(VI) reduction, 17



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after the hybridation of Bi2Sn2O7 and AgI, all AgI/Bi2Sn2O7 nanocomposites display distinct improvements in the degradation of TC, and the degradation efficiencies can be ranked as follow: AB-31.97 > AB-26.06 > AB-37.01 > AB-19.03 > AgI > Bi2Sn2O7. Apparently, AB-31.97 nanocomposite achieves the highest removal ratio, where 83.0% of TC is degraded in 50 min. In addition, the first-order kinetic model was applied to reveal the photodegradation process in a more quantitative manner. The reaction rate constant (K, min-1) can be estimated through the following formula 49

:

lnCt ⁄C0 = -Kt

(5)

where Ct and C0 represent TC concentration at the time t and 0, respectively. As depicted in Fig.7b, the good linear relationship between –ln(Ct/C0) and irradiation time ascertains the applicability of this model. Among, Bi2Sn2O7 and AgI possess comparatively small reaction rate constants, with 0.0110 min-1 and 0.0207 min-1, respectively. In stark contrast, AgI/Bi2Sn2O7 nanocomposites exhibit higher K values under the identical condition, and the biggest one was obtained by AB-31.97 (0.0361 min-1), which is 3.28 or 1.74 fold higher than that of Bi2Sn2O7 or AgI. The formed nanojunction between Bi2Sn2O7 and AgI can be the reasonable explanation for this enhancement in TC degradation, since the intimate interface will provide sufficient electronic mobile channel for charge transfer and shorten their migration distance 50.

18


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Fig. 7. (a) Photocatalytic degradation curves of TC and (b) kinetic curves of TC oxidation over the obtained samples under visible light irradiation; Inset: corresponding reaction rate constants. In order to obtain a deep insight into the photodegradation process of TC, 3D EEMs technology and LC-MS measurement were employed to fully monitor the detailed behaviors of TC molecules from different perspectives. At first, 3D EEMs were operated to grasp the overall change of TC in the reaction system, and the corresponding mapping results are presented in Fig. 8. Before irradiation, the fluorescence signal of initial TC solution is pretty weak (Fig. 8a), which is in accordance with the Chen’s results 10. Considering that the molecular fluorescence of substance originates from its chemical structure, the low production of TC fluorescence can be credited to the existence of several electron-withdrawing groups (e.g., carbonyl group) in the TC molecules, they can severely lower the fluorescence efficiency of TC

51

. As the reaction carried on (Fig. 8b-f), it can be found that the

location of fluorescence center is progressively blue-shifted along the emission axis and the excitation axis. Previous investigation have clarified that this blue-shift might 19



be related to the destruction of the condensed aromatic groups, the dissociation of the large molecules into comparatively small moieties and the cleavage of some specific functional groups during the reaction

52-53

. In addition, more details on the TC

degradation can be acquired from the variation of fluorescence intensity. As shown in Fig. 8b, a new fluorescence center (peak A) appears at Ex/Em=340-355/490-520 nm after 10 min of irradiation. Meanwhile, an additional fluorescence center (peak B), located at Ex/Em=285-330/380-460 nm, can also be observed at 20 min (Fig. 8c). According to the literature 54, both of these two fluorescence centers are resulted from the generation of humic acid-like organic matter during the TC degradation. Besides, the intensity of these fluorescence centers increases slightly with the prolongation of reaction time (Fig. 8d), revealing the accumulation of humic-like organic matter in the residual solution. After reaction for 40 min (Fig. 8e), it is worth noting that the peak A is almost disappeared while the peak B becomes stronger, which might be caused by the mutual transformation of these intermediates 55. But as displayed in Fig. 8f, when the irradiation time is prolonged to 50 min, the intensity of peak B weakens to a certain degree, confirming that these formed intermediates can be further decomposed.

20


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Fig. 8. 3D EEMs of the residual TC solution after visible light irradiation time of (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 40 min and (f) 50 min. Then, LC-MS technique was performed to reveal more details about the behaviors of TC molecules during the AB-31.97 treatment process. Several major intermediates were identified after 50 min of reaction, and their corresponding MS spectra are presented in Fig. S2. Apparently, only one dominant peak with a mass-to-charge ratio (m/z) of 445 is detected at the beginning of reaction, indicating 21



that no other chemicals are existed in the original TC solution. Further combining the results of MS spectra and previous reports

56-59

, the best choice of the formed

intermediates and their possible structural information are summarized in Table S1. Herein, the decomposition of TC molecules and the generation of intermediates are mainly conducted via two routes: the cleavage of specific functional groups and the ring-opening reactions. As presented in Fig. 9, the production of TC 1 (m/z 461) can be ascribed to the hydroxylation reaction, because the high reactivity of •OH enables it to attack the aromatic ring of TC molecules. Afterwards, at the ketone position of TC 1, the addition of electron will induce a deamidation process, and then TC 2 (m/z 418) is observed. Similarly, it has been reported that original TC molecules can also undergo this deamidation process 58, thus leading to the formation of TC 3 (m/z 402). And when the produced TC 3 is further oxidized by •OH, the hydroxylated product (TC 4, m/z 418) can be obtained. On the other hand, owing to the low bond energy of N-C, the N-demthylation process is prone to occur and then the product TC 5 (m/z 417) appears. This process can be further confirmed from the blue-shift of fluorescence center in 3D EEMs (Fig. 8), because the dimethylamino group is a specific auxochromophore, the cleavage of the N-dealkylation will cause a hypsochromic shift in the characteristic peak of the parent chemical. By referring to previous investigation

56, 58

, the generation of TC 6 (m/z 374) can be ascribe to the

deamidation of TC 5 or the loss of N-methyl group in TC 3. Subsequently, accompanied by the continuous effect of active species, a ring-opening reaction will occur on these formed TC 6, and ultimately, TC 7 (m/z 319) and TC 8 (m/z 277) are 22


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yielded.

Fig. 9. Suggested pathways for the photodegradation of TC. In order to support the above viewpoints and further evaluate the performance of the prepared samples, the mineralization degree towards the TC degradation was therefore investigated, and the corresponding total organic carbon (TOC) removal curves were displayed in Fig. S3. Compared to the mineralization efficiency of AgI (15.1%) and Bi2Sn2O7 (9.7%), AB-31.97 exhibits a much higher TOC removal ratio, which reaches about 37.5%. The obtained data mean that, under visible light irradiation, the AB-31.97 nanocomposite owns ability to mineralize TC molecules into CO2 or other intermediate products. Thus, combined with the results of 3D EEMs and LC-MS analysis, it can be inferred that the structure of TC molecules can be destroyed to some extent and the formed intermediates will be gradually decomposed during the photocatalytic process. 23



To further expand the applicability of the as-prepared photocatalysts, another probe reaction, E. coli inactivation, was selected for investigation. The corresponding results are plotted in Fig. 10a, at first, light control was operated without adding any photocatalysts. As the viable cell populations keep almost unchanged, thus, the inactivation effect induced by visible light can be neglected. In contrast, after the introduction of photocatalysts, different inactivation efficiencies are observed. Despite pure Bi2Sn2O7 and AgI exhibit some antibacterial effect, about 0.74-log and 1.09-log of viable E. coli cells is inactivated within 40 min irradiation, it is still difficult to meet the criterion of surface water treatment. As the United States Environmental Protection Agency suggested, 99.9% (3-log) bacterial inactivation and 99.99% (4-log) virus inactivation should be achieved in the treatment of surface water 2. Compared to Bi2Sn2O7 and AgI, AgI/Bi2Sn2O7 nanocomposites display more superb inactivation efficiencies and the amounts of viable cells after 40 min irradiation is about 105.73, 103.73, 0, and 104.81 for AB-19.03, AB-26.06, AB-31.97 and AB-37.01, respectively. Obviously, both AB-26.06 and AB-31.97 nanocomposites meet the requirement of surface water treatment (as the initial amounts of E. coli cells is 7.48-log in this experiment), while the highest inactivation efficiencies is realized by AB-31.97 nanocomposite. Therefore, AB-31.97 nanocomposite holds huge potential for the purification of surface water. Owing to the inherent antibacterial characteristic, Ag+, especially at high concentration, has been considered as an underlying disinfectant 60. In order to clarify the effect of Ag+ on E. coli inactivation, ICP-MS measurement was firstly carried out 24


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to detect the concentration of Ag+ released from AB-31.97 during the photocatalytic disinfection experiment. As outlined in Fig. 10b, the concentration of released Ag+ rises progressively with the prolongation of irradiation time, and reaches a maximum (34.1 µg/L) at the end of reaction. Subsequently, under the identical experiment condition, the antibacterial effect of Ag+ towards E. coli cells was examined by varying the concentration of Ag+ (Fig. 10c). It is found that Ag+ does not have evident toxicity towards E. coli cells during the reaction process and only 0.13-log of viable E.

coli cells is inactivated at the concentration of 100 µg/L, which is in good agreement with these reports

61-62

. In view of the above analysis, the released Ag+ from

AB-31.97 nanocomposite cannot be the leading cause for E. coli inactivation, while the superior inactivation performance of AB-31.97 nanocomposite should originate from radical reaction under visible light irradiation.

25



Fig. 10. (a) Inactivation efficiencies of the as-prepared samples against E. coli; (b) Ag+ released from AB-31.97 during the inactivation of E. coli under visible light irradiation; (c) The toxicity of Ag+ towards E. coli cells. Photostability and recyclability Given that the stability and recyclability of photocatalysts are significant prerequisite for their practical application, herein, the stability of as-obtained AB-31.97 nanocomposite was investigated by all three probe reactions under the same condition for four times. As illustrated in Fig. 11a and Fig. S4, after the accomplishment of four consecutive cycles, the photocatalytic activity of the recovered AB-31.97 nanocomposite does not exhibit evident deactivation, verifying the high stability of the obtained photocatalyst. To show more structural details, the 26


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XRD patterns of AB-31.97 nanocomposite after TC degradation are supplied in Fig. 11b, and results show that the phase structure of the used AB-31.97 is nearly the same as that of the fresh one. Meanwhile, combined with the fact that the concentration of released Ag+ after disinfection experiment is only 34.1 µg/L (Fig. 10b). It can be revealed that both the photocatalytic performance and phase structure of AB-31.97 sample are stable enough whether in the process of Cr(VI) reduction, TC degradation or E. coli inactivation. Accordingly, under the visible light irradiation, AgI/Bi2Sn2O7 nanocomposites can be regarded as potential candidates for practical application.

Fig. 11. (a) Cycling experiments towards the photodegradation of TC over AB-31.97 nanocomposite; (b) XRD patterns of AB-31.97 composite before and after TC degradation. Possible reaction mechanism on the enhancement of photocatalytic activity In order to fully grasp the photocatalytic reaction mechanism, the main active species produced in the reaction system were explored. Herein, KBrO3, IPA, TEMPOL and sodium oxalate Na2C2O4 were selected to capture electrons (e-), hydroxyl radical (•OH), superoxide radical (•O2-) and holes (h+), respectively

12

.

Besides, to obtain more reliable results, both TC degradation and E. coli inactivation 27



were employed as trapping experiments. As depicted in Fig. 12a, after the addition of TEMPOL (2 mM), the degradation efficiency of TC declines from 83.0% to 54.7%, revealing that •O2- plays a conspicuous role in the degradation of TC. Meanwhile, significant inhibitory action can be observed when KBrO3 (2 mM) or Na2C2O4 (10 mM) was added into the reaction system, and only 37.0% or 28.4% of TC is degraded by AB-31.97 nanocomposite after 50 min irradiation. These findings mean that both eand h+ are predominant active species in the process of TC degradation. As for IPA (10 mM), despite the inhibitory degree of TC removal is slight, about 7.1% loss, the contribution of •OH to the TC removal should not be overlooked, and this can be further confirmed from the experiment of E. coli inactivation (Fig. 12b). After the addition of different scavengers, the general trend in the inhibitory effect of E. coli inactivation is similar to that of TC removal, but it is clearly that •OH plays an assisted role in the inactivation of E. coli cells. In other words, •OH is yielded in the AB-31.97-based photocatalytic system. Thus, based on the results of trapping experiments, it can be rationally concluded that h+, e- and •O2- worked as major active species in the photocatalytic process, and the impact of •OH on the reaction should not be neglected either. The ESR spin-trap technology was performed to further validate the active species participated in the photocatalysis. As illustrated in Fig. 12c, when AB-31.97 nanocomposite is exposed to visible light, four typical peaks of DMPO- •O2- adducts are detected in methanol dispersion, and the peak intensity enhances with the prolongation of irradiation time. In contrast, under the same condition, no signal is 28


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emerged in darkness. Analogously, a typical signal with peak intensity of 1:2:2:1 is observed when the light turns on (Fig. 12d), suggesting the generation of •OH during the photocatalytic process 63. Therefore, the ESR results reveal that both •O2- and •OH can be yielded by AB-31.97 nanocomposite under visible light irradiation, which is in good consistency with the results of trapping experiments.

Fig. 12. Influence of different scavengers on the degradation of TC (a) and inactivation of E. coli (b) by AB-31.97 composite; (c) DMPO spin-trapping ESR spectra of AB-31.97 in methanol dispersion for DMPO-•O2- and (d) aqueous dispersion for DMPO- •OH. To analyze the origin of enhanced photocatalytic activity, the PL measurement was firstly conducted to assess the charge transfer and separation behaviors. In 29



Page 30 of 49

general, a strong fluorescence peak denotes an intense charge recombination that occurs in semiconductor photocatalysts

64

. As presented in Fig. 13a, pure Bi2Sn2O7

exhibits the highest fluorescence signal, but once AgI is coupled with Bi2Sn2O7 to form intimately contacted AB-31.97 nanocomposite, the peak intensity declines sharply, implying that the recombination possibility of photogenerated electron-hole pairs is effectively reduced. In addition, it can be also observed that the fluorescence signal of AgI is relatively high, verifying that the photogenerated holes and electrons are inclined to recombine in single AgI, which is consistent with their photocatalytic performance. Additionally, to reveal the recombination process intuitively, TRPL decay spectra are employed to research the charge carrier lifetime of the samples (Fig. 13b). And the detailed analysis of their decay dynamics is performed through fitting the obtained data into a biexponential function: It=A1 exp (-t/τ1 ) +A2 exp (-t/τ2 )

(6)

considering that the average charge carrier lifetime (τave) is usually recognized as an indicator to evaluate the charge carrier separation, herein, it is also calculated through the following equation: τave = ∑ Ai τ2i / ∑ Ai τi

(7)

where A1 and A2 represent the PL amplitudes, and correspondingly, τ1 and τ2 denote the lifetime. Specifically, τ1 originates from the direct recombination between the free electrons and holes, while τ2 belongs to the non-radiative recombination of the trapped charge carriers. As listed in Table S2, the τave of pristine Bi2Sn2O7 is calculated to be 0.36 ns, which is apparently shorter than that of AB-31.97 30


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nanocomposite (0.57 ns), revealing a slower decay dynamics is occurred in AB-31.97 nanocomposite. So these results provide persuasive evidence that the electron-hole pairs are effectively separated by the formed nanojunction between AgI and Bi2Sn2O7. Moreover, in order to acquire more information on the kinetic process of the photogenerated charge carriers in AB-31.97 nanocomposite, photoelectronchemical tests, including photocurrent responses (PC) and electronchemical impedance spectra (EIS) are further implemented

65

. Fig. 13c illustrates the transient photocurrent

responses of the samples under visible light irradiation, all samples exhibit repeatable and relatively stable photocurrent profiles during the successive on/off irradiation cycles, demonstrating their excellent photostability. Noticeably, under the identical condition, it is widely accepted that a larger photocurrent density represents a higher transfer efficiency of photogenerated charge carriers. Thus, the result of photocurrent measurement evidences that AB-31.97 nanocomposite possesses stronger charge separation ability than that of pristine Bi2Sn2O7 or AgI. Besides, the EIS Nyquist plots can also supply additional details for investigating the interfacial charge transfer property. In general, the smaller semicircle diameter of EIS Nyquist plot becomes, the lower the charge transfer resistance is. As displayed in Fig. 13d, the relative arc sizes of the samples can be arranged as follow: Bi2Sn2O7 > AgI > AB-31.97, suggesting that a rapid and effective interfacial charge separation process will be achieved through the heterostructured interaction between Bi2Sn2O7 and AgI. Based on the above analysis, it can be deduced that a greater number of electrons and holes can be utilized to initiate photocatalytic reaction in AB-31.97 nanocomposite-based reaction 31



system, which are advantageous for the subsequent removal of contaminant.

Fig. 13. (a) Photoluminescence spectra, (b) TRPL decay spectra, (c) transient photocurrent responses and (d) EIS Nynquist plots of as-prepared samples. Combined with the above analysis, a possible mechanism for AgI/Bi2Sn2O7 nanocomposite-based reaction system is proposed and presented in Scheme 1. It should be noted that there are two typical charge transfer processes (traditional band to band transfer and Z-scheme mechanism) that may exist in most semiconductor heterojunction photocatalysts, and they will compete with each other. In this regard, if AgI/Bi2Sn2O7 nanocomposite belongs to a traditional type II heterojunction (Scheme 1a), under visible light irradiation, the photogenerated electrons on the CB of AgI will migrate to that of Bi2Sn2O7, and the holes will finally accumulate on the VB of AgI. Despite this transfer manner makes some contribution to the spatial isolation of the 32


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photogenerated electron-hole pairs, the VB potential of the AgI (+2.35 eV) is more negative than that of H2O/•OH (+2.40 eV) 66, so the holes accumulated on the AgI can hardly react with H2O to form •OH. Meanwhile, similar case will occur on the generation of •O2-, because compared with the potential of O2/•O2- (-0.046 eV) 66, the CB edge of Bi2Sn2O7 (+0.45 eV) is more positive. However, according to the results of trapping experiments and ESR analysis, both •OH and •O2- have been verified to be truly generated in the photocatalytic reaction process. Therefore, it can be inferred that the charge separation process of AgI/Bi2Sn2O7 nanocomposite does not follow the traditional heterojunction model, and the Z-scheme mechanism might be more appropriate to interpret the transfer behaviors of photgenerated electron-hole pairs during the reaction (Scheme 1b). In a typical Z-scheme system, the photogenerated electrons on the Bi2Sn2O7 will direct migrate to the VB of AgI and recombine with the photogenerated holes of AgI, while the intimate interface between AgI and Bi2Sn2O7 provides the guarantee for realizing this process

1, 42

. As a result, the

photogenerated electrons and holes will remain on the CB of AgI (-0.38 eV) and VB of Bi2Sn2O7 (+3.06 eV), where the electrons own enough reducibility to meet the requirement of •O2- generation, and the holes with strong oxidizability can react with H2O to produce •OH. Subsequently, these formed active species, •O2-, •OH and holes, will participate in the reaction to degrade the TC molecules or inactivate the E. coli cells. And the main contribution of electrons to these two probe reactions is through the generation of •O2- rather than directly involving in the reaction, but as for Cr(VI) reduction, photogenerated electrons will directly reduce Cr(VI) to Cr(III). Altogether, 33



above analysis suggests that the reaction process over AgI/Bi2Sn2O7 photocatalysts belong to a direct Z-scheme mechanism, which will efficiently promote the charge separation efficiency, and therefore, leading to an excellent photocatalytic activity.

Scheme 1. Schematic diagram of the proposed reaction mechanism in the AgI/Bi2Sn2O7 nanocomposites based photocatalytic system. Conclusion In summary, the novel AgI/Bi2Sn2O7 Z-scheme photocatalysts were successfully fabricated through an in situ deposition-precipitation route. The obtained AB-31.97 nanocomposite exhibits optimal photocatalytic performance towards the removal of diverse contaminants, which can reduce almost all Cr(VI) in 40 min and efficiently degrade 83.0% of TC within 50 min. Besides, AB-31.97 nanocomposite displays superior inactivation efficiency under visible light irradiation, and about 3.0 × 107 CFU/mL of E. coli is completely inactivated at the end of reaction. The enhanced photocatalytic performance of these AgI/Bi2Sn2O7 nanocomposites is primarily 34


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ascribed to the accelerated interfacial charge transfer through the role of the formed nanojunction. Analysis of 3D EEMs and LC-MS measurement reveals that the chemical structure of TC molecules can be effectively destroyed and the formed intermediates are progressively decomposed during the reaction process. The radical trapping experiments and ESR detection confirmed that the •O2-, •OH, h+ and e- are all worked in reaction system. It is anticipated that AgI/Bi2Sn2O7 nanocomposites can be served as promising photocatalysts in the treatment of diverse wastewater, and this work might provide a new direction for constructing highly efficient nanojunction photocatalysts to deal with complicated environmental pollution issues. ASSOCIATED CONTENT Supporting Information Experimental section, size distribution of the synthesized sample, TOC removal efficiency and HP-LC analysis of TC, cycling experiments, and TRPL fitted parameters. AUTHOR INFORMATION Corresponding authors *E-mail: [email protected], [email protected] (Cheng-Gang Niu). *E-mail: [email protected] (Guang-Ming Zeng). ORCID Cheng-Gang Niu: 0000-0002-2530-4858 Notes The authors declare no competing financial interest. 35



Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) (51541801, 51521006).

36


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Table of Content

Synopsis: Nano-sized AgI/Bi2Sn2O7 heterojunction exhibits superior photocatalytic activity for Cr(VI) reduction, tetracycline degradation and Escherichia coli inactivation

49


Bi2Sn2O7 nanojunction system

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