AgVO3 Graphene Aerogels

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In-situ ion exchange synthesis of Ag2S/AgVO3 graphene aerogels for enhancing photocatalytic antifouling efficiency Yuexing Chen, Yong Liang, Maojun Zhao, Ying Wang, Li Zhang, Yuanyuan Jiang, Guangtu Wang, Ping Zou, Jun Zeng, and Yunsong Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05962 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Industrial & Engineering Chemistry Research

In-situ ion exchange synthesis of Ag2S/AgVO3 graphene aerogels for enhancing photocatalytic antifouling efficiency Yuexing Chena, Yong Liangb, Maojun Zhaoa, Ying Wangc, Li Zhanga, Yuanyuan Jianga, Guangtu Wanga, Ping Zoua, Jun Zengd, Yunsong Zhanga,* aCollege

bCollege

of Science, Sichuan Agricultural University, Yaan 625014, China of Pharmacy and Biological Engineering, Chengdu University, Chengdu 610106,

China cCollege

of Water Conservancy and Hydropower Engineering, Sichuan Agricultural University,

Yaan 625014, China dKey

Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, Sichuan

University of Science Engineering, Zigong 643002, China *Corresponding

author: Tel: +86 835 2885782; fax: +86 835 2862227; Email address:

[email protected]

1

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ABSTRACT Efficient charge separation and cycle stability are critical for water purification by semiconductor photocatalyst. Herein, novel Ag2S/AgVO3 graphene aerogels (Ag2S/AgVO3@GAs) were synthesized via an in-situ ion exchange method. The series of characterization results verified that the Ag2S/AgVO3@GAs synergistically integrate the excellent properties of the Ag2S and AgVO3 into the macroscopic porous graphene aerogel. Furthermore, owing to the chelation of chitosan (CS) for AgVO3 and the ion exchange between well-dispersed AgVO3 and Na2S, the Ag2S can in situ grow on AgVO3, which prevents Ag2S and AgVO3 agglomeration/shedding in the photocatalytic reaction and contributes to the enhanced photocatalytic activity and cyclic stability. Benefiting from the unique structure, the Ag2S/AgVO3@GAs with excellent stability displayed the outstanding photodegradation efficiency for methyl orange (97% removal rate in 40 min) and disinfection activity for Escherichia coli (100% antibacterial efficiency in 36 min). Keywords: Three-dimensional graphene aerogel; Heterojunction; Photodegradation; Disinfection; Charge transfer Graphical abstract

The novel Ag2S/AgVO3 graphene aerogels heterojunction, an aerogel filled with 2


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well-dispersed plentiful rod-like catalysts with spherical nanoparticle, was fabricated by an economical and effective method. The composite heterojunction exhibited excellent

photocatalytic

activities

with

high

cyclic

stability

towards

the

photodegradation of organic pollutants and the disinfection of E. coli and S. aureus under visible light. 1. INTRODUCTION Various environmental contaminants, especially pathogenic bacteria and organics, have seriously threatened human health with the evolution of modern industries.

1, 2

Photocatalysis has great potential for applications in bacterial inactivation and dye degradation, which can significantly reduce the outbreak of water-borne diseases and attach increasing attention. 3-5 Although many semiconductors (such as TiO2, 6 g-C3N4, 7

BiOI 8) have been studied for promoting photocatalytic reactions, the high

photoelectron-hole pairs recombination rate, low visible light absorption and recycling difficultly have restricted their wide application. As an important silver compound, AgVO3 has attracted great research interest owing to its relatively high visible-light absorption capacity.

9

However, the fast

recombination of electron-hole significantly constrains the application of AgVO3 in wastewater purification. 10 So far, the construction of heterojunction photocatalysts by coupling with other semiconductors has been studied, which has been regarded as a promising way for promoting the separation efficiency of electron-hole pairs. 11 Wang et al.

12

reported visible-light-driven Ag3PO4/AgVO3 heterostructure, which exhibits

more efficient for decomposing 4-chlorophenol than pure AgVO3 due to the enhanced 3



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separation efficiency of photoelectron-hole and stronger visible-light absorption after the introduction of Ag3PO4. Nevertheless, most studies about AgVO3-based heterojunctions

only

reported

effective

photocatalytic

organics

degradation

performance, with few reporting on photocatalytic disinfection. Therefore, it is still significant to construct a highly efficient bifunctional AgVO3-based heterojunction photocatalyst for the simultaneous use of photocatalytic bacteria inactivation and organics degradation. Recently, Ag2S has been reported as a promising narrow band-gap photocatalyst for pollutant degradation or inactivation of bacteria owing to its excellent optical properties and catalytic activity.

13

As the development of researches for further

enhancing the catalytic performance, several researchers have constructed Ag2S composites with other photocatalysts, such as Bi2WO6/Ag2S, Ag2S/MoS2.

15

13

Ag2S/ZnS,

14

These reports indicated that the introduction of Ag2S reduced the

recombination of photoinduced carrier charge and extended the visible light absorption range, and thus improved the photocatalytic performance of the Ag2S-based composites. Therefore, excellent photocatalytic performances were anticipated if Ag2S was added in the AgVO3 system to construct heterojunction composite with a proper way, which has not yet been reported. Nevertheless, the recovery strategy of composite photocatalysts is also very important, which limits the photocatalytic application of most catalysts including AgVO3 and Ag2S. 16 The latest researches indicate that the graphene aerogels (GAs) with macrostructure and excellent electrical conductivity are not only beneficial to recycling, but effectively 4


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promote the separation of photogenerated charges. 17 Additionally, the agglomeration tendency of AgVO3 and Ag2S also has a seriously impact on their original activity. Considering the extensive background, we prepared a three-dimensional porous Ag2S/AgVO3 graphene aerogels (Ag2S/AgVO3@GAs) by in-situ ion exchange method. In the obtained Ag2S/AgVO3@GAs hybrid, GAs with a 3D porous structure and good electrical conductivity can provide an ideal support for Ag2S and AgVO3 and it can favor recycling and facilitate the charge transfer as well as the mass transport in photocatalytic reaction. Profiting from the anchoring effect of dispersed chitosan (CS) for AgVO3 and the ion exchange between as-prepared AgVO3 and Na2S, the Ag2S can in situ grow on AgVO3, which prevents Ag2S and AgVO3 agglomeration/shedding in the photocatalytic reaction and contributes to the enhanced photocatalytic activity and cyclic stability. Thereby, it is anticipated that the Ag2S/AgVO3@GAs will possess excellent photocatalytic ability for water purification.

Scheme 1 Schematic diagram of the fabrication route for the Ag2S/AgVO3@GAs hybrid. 2. EXPERIMENTAL SECTION 5



2.1. Materials Graphite powders were supported by Huayuan Chemical Co., China. Ammonium vanadate (NH4VO3) and sodium sulfide nonahydrate (Na2S·9H2O) were purchased from Kelong Co., China. Silver nitrate (AgNO3) was obtained from Sinopharm Chemical Reagent Co., China. All reagents were analytical grade and used directly without further purification. 2.2 Synthesis of Ag2S/AgVO3@GAs 3D porous Ag2S/AgVO3@GAs were fabricated via an in-situ ion exchange method as shown in Scheme 1. The preparation processes of graphene oxide (GO) and AgVO3 were described in Supporting Information. Then, the mixture of 30 mg AgVO3 and 2 mL of GO solution (10 mg·mL-1) was sonicated for 30 min. Next, a certain amount of Na2S solution was put into the solution under ultrasound condition. After that, 3 mL above suspension in a 5 mL vial was carried out a hydrothermal reaction at 180 °C for 12 h to form Ag2S/AgVO3 hydrogel. 18 Finally, the as-prepared hydrogel was frozen and dried for 48 h to generate Ag2S/AgVO3 graphene aerogels (Ag2S/AgVO3@GAs). For comparison, Ag2S/AgVO3@GAs were fabricated with different molar ratios of Ag2S and AgVO3, labeled 0.25 Ag2S/AgVO3@GAs, 0.5 Ag2S/AgVO3@GAs, 0.75 Ag2S/AgVO3@GAs, and 1 Ag2S/AgVO3@GAs, where the molar ratio of Ag2S and AgVO3 was 0.25:1, 0.5:1, 0.75:1 and 1:1, respectively. 2.3. Photodegradation experiments In the photodegradation process, the photodegradation performance of as-prepared samples was analyzed through the degradation of methyl-orange (MO, 10 6


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mg·L-1) under 500W Xe lamp with a 420 nm UV cutoff filter, respectively. At first, the sample (ca. 50 mg) was placed into 70 mL MO solution, and then it was exposed to visible-light after maintain adsorption equilibrium for 40 min. Moreover, 1.0 mL MO solution was gathered in every reaction interval and the MO concentration was 18

evaluated using the UV-vis spectra.

All the photocatalytic degradation

measurements were implemented in triplicate. 2.4. Photocatalytic antifouling experiments E. coli and S. aureus were cultured in a nutrient broth at 37 ℃ for 12 h under shaking (200 rpm) condition. Afterwards, bacterial cells were washed and resuspended with 0.9% NaCl. All materials were autoclaved (121 ℃, 30 min) to ensure sterility. In antifouling experiment, 20 mg sample was put into 20 mL suspension consisting 107 cfu/mL of E. coli or S. aureus. Meanwhile, the light source adopted a 500W spherical Xe lamp with a 420 nm UV cutoff filter. At certain time intervals, 1 mL bacterial solution was gathered, and appropriate serial diluted to culture (37 ℃, 24 h) for cell counting. For comparison, light without photocatalysts control and dark control were also conducted. All the antifouling experiments were performed in triplicate. 3. RESULTS AND DISCUSSION 3.1. Morphology and composition of as-prepared materials The XRD pattern (Fig. 1) displays that the pure AgVO3 patterns was consistent with the (JCPDS file: 29-1154) standard data, which reveals that the obtained AgVO3 has a monoclinic structure.

19

The main diffraction peaks (2θ) at 28.4°, 29.8°, 32.8°, 7



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34.1, 33.5°, 39.6, 44.5 and 50.9 are ascribed to (-211), (501), (-411), (-112), (-303), (-611), (710) and (020) crystallographic planes of AgVO3, respectively.

20

The XRD

pattern of the obtained Ag2S coincides with that of the hexagonal Ag2S phase (JCPDS 14-0072).

21

It is clearly observed that all the Ag2S/AgVO3@GAs with different

amounts of Ag2S exhibit a cooccurrence of both Ag2S and AgVO3 phases. Moreover, with the Ag2S amounts increasing, the diffraction peak intensity of Ag2S becomes stronger gradually, meanwhile, the diffraction peak positions of AgVO3 do not shift, indicating that Ag2S does not influence the crystal structure of AgVO3. Notably, Ag2S/AgVO3@GAs is successfully fabricated, and no other peaks appear sufficiently implying that there are no impurities in the samples

22, 23.

Furthermore, the FTIR

spectra (Fig. S1, Supporting Information) show that compared with GO, the peaks of oxygen-containing functional groups mostly diminish for Ag2S/AgVO3@GAs, demonstrating that GOs are reduced in preparation process. 24, 25

Fig. 1 XRD patterns of GAs, Ag2S, AgVO3, 0.25 Ag2S/AgVO3@GAs, 0.5 Ag2S/AgVO3@GAs, 0.75 Ag2S/AgVO3@GAs, and 1 Ag2S/AgVO3@GAs. The SEM image (Fig. 2a) displays that Ag2S/AgVO3@GAs have a macro porous 8


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structure. It can be observed from Fig. S1b that the pure AgVO3 nanoribbons with about 100-150 nm in width and more than 2 μm in length is very smooth and tend to be agglomeration. As a comparison, AgVO3 in 0.5 Ag2S/AgVO3@GAs (Fig. 2b) are much smaller in size and uniformly distribute in GAs without obvious aggregation. The result implies that the addition of chitosan in Ag2S/AgVO3@GAs is beneficial to the dispersion of AgVO3, which can enhance photocatalytic performance. In EDX spectra (Fig. 2c), the element diffraction peaks (C, O, Ag, S, and V) of Ag2S/AgVO3@GAs

were

entirely

observed,

demonstrating

that

the

Ag2S/AgVO3@GAs is successfully obtained. In the element mapping images (Fig. 2d-h), the elements of C, O, Ag, S, and V disperse uniformly, suggesting that Ag2S NPs and AgVO3 uniformly dispersed in GAs.

Fig. 2 SEM images of (a, b) Ag2S/AgVO3@GAs under the different magnifications, Inserted photos: (a) digital image of the as-prepared Ag2S/AgVO3@GAs hybrid; (c) EDX of Ag2S/AgVO3@GAs and (d-h) mapping of Ag2S/AgVO3@GAs. 9



TEM and high-resolution TEM (HRTEM) images of Ag2S/AgVO3@GAs were given in Fig. 3. Pure GAs image exhibits the nanosheet appearance which can be clearly observed in Fig. 3a. Additionally, as shown in Fig. 3b, c, the AgVO3 displays a nano-rod structure with considerable spherical Ag2S nanoparticles on its surface. Meanwhile, Ag2S NPs are well-dispersed on AgVO3 without obvious aggregation, and the nano-rod structure of AgVO3 wasn’t altered after the introduction of Ag2S. Furthermore, small Ag2S nanoparticles (ca. 10-80 nm, Fig. S1) are fixed on the AgVO3 by ion-exchange between AgVO3 and Na2S. However, comparing the Ag2S in 0.5 Ag2S/AgVO3@GAs, the size of pure Ag2S is larger mainly in the 100-380 nm range (Fig. S1b), which indicates that the ion-exchange method can restrain the aggregation of Ag2S and facilitate the formation of homogeneous nanoparticles. The result signified that the ion-exchange method can restrain the aggregation of Ag2S and facilitate the formation of homogeneous nanoparticles, which contributes to promote photocatalytic performance. To further explore the structure of Ag2S/AgVO3@GAs, HRTEM was performed (Fig. 3d). The lattice distance of 0.25 nm attributed to the (112) plane of Ag2S.

21

Meanwhile, the lattice distance of 0.31 and 0.23 nm were in

accordance with the (501) plane of AgVO3 and the (111) plane of Ag0, respectively. 26 Furthermore, the SAED pattern of Ag2S/AgVO3@GAs hybrid implies the polycrystalline nature.

10


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Fig. 3 TEM images of (a) pure GAs and (b, c) Ag2S/AgVO3@GAs under the different magnifications; (d) HRTEM of Ag2S/AgVO3@GAs. I27nset: SAED pattern. The surface chemical states of Ag2S/AgVO3@GAs were measured by X-ray photoelectron spectroscopy (XPS). As depicted in the XPS survey spectrum of the sample (Fig. 4a), it can be observed that the sample contains C, Ag, S, O, and V elements. Notably, the oxygen-containing groups significantly decreased for the C1s XPS spectrum of Ag2S/AgVO3@GAs (Fig. S1a, b, Supporting Information) compared with GO, implying that the GO was successfully reduced in the hydrothermal process.

28

Fig. 4b displayed a typical Ag 3d spectra of

Ag2S/AgVO3@GAs, the characteristic peaks at 367.9 and 373.9 eV attribute to Ag+, while the weak peaks at 368.9 and 374.7 eV are corresponded to Ag0, respectively. 26 Furthermore, the peaks at 161.5 eV and 162.4 belong to S2p3/2 and S2p1/2 binding energy for Ag2S (Fig. 4c), respectively, which is agreement with the previous report. 11



21

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The signals of V2p5/2 and V2p3/2 were observed at 516.8 eV and 524.0 eV,

respectively, corresponding to V5+ in AgVO3, respectively (Fig. 4d).

29

The results

imply the successful fabrication of Ag2S/AgVO3@GAs. The Raman spectra of GO (Fig. 4c) display that the G-band peak appeared at 1615.3 cm-1 is attribute to the sp2 carbon type structure,

30

and the D-band at 1364.4

cm-1 signifies the existence of some defects. 31 The G-band of 0.5 Ag2S/AgVO3@GAs shifts from 1615.3 cm-1 to 1605.4 cm-1 compared with GO, demonstrating the structure of GAs was further perfected via hydrothermal reduction.

32

Also, ID/IG of

GO and Ag2S/AgVO3@GAs are 0.84 and 1.05, respectively, which verifies a lower ordered carbon type for Ag2S/AgVO3@GAs. 33 This may assign to the reduce reaction of GOs in preparation process.

34

As shown in Fig. 4d, the N2 adsorption-desorption

isotherm of Ag2S/AgVO3@GAs exhibits Type IV property and H3-type hysteresis loop, indicating the presence of pores in Ag2S/AgVO3@GAs. The existence of pores structure in Ag2S/AgVO3@GAs is also elucidated by the inset figure in Fig. 2f, which indicates that it is a porous material. Meanwhile, the specific surface areas (SBET) and pore structure information of the obtained samples are presented in Table S1. Compared with other samples, 0.5 Ag2S/AgVO3@GAs show the highest SBET and pore volume, which would help expose more active site and facilitate the mass transport in photocatalytic reaction. 35

12


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Fig. 4 (a) Full-scale XPS survey spectrum and the XPS spectra of (b) Ag 3d, (c) S 2p and V 2p for Ag2S/AgVO3@GAs; (e) Raman spectra of GO and Ag2S/AgVO3@GAs; (f) Nitrogen adsorption-desorption isotherm and the corresponding pore diameter distribution curve of Ag2S/AgVO3@GAs. 3.2. Optical properties The diffuse reflectance spectroscopy (DRS) displays the optical characters of samples. It was clearly found that the absorption band edge of pristine AgVO3 is approximately 680 nm, while the absorption band edge of 0.5 Ag2S/AgVO3@GAs have extended toward the longer wavelengths range. This could be assigned to the adsorption superiority of GAs component and the introduction of Ag2S with narrow 13



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band gap, 36 which is help to enhance the photocatalytic performance. Meanwhile, the band gap energies of pure Ag2S and AgVO3 were 1.1 37 and 1.9 eV 38 according to the typical Kubelka-Munk method39 (Fig. 5b). Additionally, the photoluminescence (PL) spectra of samples were measured (Fig. 5c). The PL intensity of 0.5 Ag2S/AgVO3@GAs decreased significantly compared to pure AgVO3, implying that the charge recombination efficiency is significantly suppressed due to the introduction Ag2S and GAs.

40

It is understandable that the photogenerated charge carriers can

quickly migrate though the graphene network and the construction of heterojunction can further promote charge separation.

41

In addition, the photo-generated carrier

transfer properties were further studied by the EIS Nyquist plots of the obtained materials (Fig. 5d). We know that the smaller arc radius in the EIS Nyquist plot indicates a smaller electrode resistance which will promote the transfer of photogenerated charges

42.

Apparently, 0.5 Ag2S/AgVO3@GAs shows a smaller

impedance arc radius compared to pure AgVO3, indicating that charge transfer efficiency is more excellent

43.

The results are attributed to the rapid and efficient

separation of photogenerated electron-holes Ag2S/AgVO3@GAs

is

significantly

27.

Therefore, the reaction rate of 0.5

increased,

which

photocatalytic degradation and bacteriostatic efficiency.

14


effectively

enhances

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Fig. 5 (a) The UV-visible diffuse reflectance spectra of the Ag2S, AgVO3 and 0.5 Ag2S/AgVO3@GAs samples; (b) Plot of [F(R)hν]2against hν; (c) PL spectra of the AgVO3 and 0.5 Ag2S/AgVO3@GAs; (d) EIS Nyquist plots of the AgVO3 and 0.5 Ag2S/AgVO3@GAs. To further understand the electron-hole transfer circumstances, we determined the transient photocurrent response (Fig. 6a) and Mott-Schottky (Fig. 6b) curves of the samples. In general, the instantaneous photocurrent response is positively correlated with the photo-generated carrier separation efficiency of the sample. 44, 45 As shown in Fig. 6a, the photocurrent of 0.5 Ag2S/AgVO3@GAs is significantly higher than that of pure AgVO3, which indicates the improved electron-hole separation efficiency of 0.5 Ag2S/AgVO3@GAs. Furthermore, according to the Mott-Schottky equation,

27, 46

the flat band potential (Vfb) of 0.5Ag2S/AgVO3@GAs was calculated and showed a significant negative shift in compared to pure AgVO3. The negative shift of Vfb demonstrates an increase in band bending, which helps to improve the separation efficiency of photogenerated carriers, thereby increasing photocatalytic activity. 27 15



Fig. 6 (a) Transient photocurrent responses and (b) Mott-Schottky plots of the AgVO3 and 0.5 Ag2S/AgVO3@GAs. 3.3. Photodegradation performance The photodegradation activities of Ag2S/AgVO3@GAs were measured by degrading MO under visible-light. The dark adsorption efficiency of 0.5 Ag2S/AgVO3@GAs is 26% (Fig. 7a), which is mainly due to the adsorption capacity of 3D porous GAs on MO. Moreover, it can be observed that the introduction of Ag2S with different contents improved the photocatalytic activity compared with the pure AgVO3 under visible light irradiation. Particularly, the 0.5 Ag2S/AgVO3@GAs present the optimal photodegradation efficiency for MO about 97% after 40 min reaction, which was higher than that of pure AgVO3 and the composites with other Ag2S amounts. Simultaneously, the pseudo-first-order kinetic data of MO degradation by the obtained samples was shown in Fig. 7b, c, where can be observed that the 0.5 Ag2S/AgVO3@GAs have maximum rate constant and it is approximately 5.4 times higher than that of AgVO3. Furthermore, the photocatalytic cycle performance (Fig. 7d) of 0.5 Ag2S/AgVO3@GAs was also evaluated. It can be clearly found that the excellent photocatalytic performance of 0.5 Ag2S/AgVO3@GAs has also been well 16


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remained after five cycles, which suggest that the photocatalyst is relatively stable. Meanwhile, the HRTEM, SEM and XRD images of 0.5 Ag2S/AgVO3@GAs before and after reactions (Fig. S5) show that the structure and morphology of the catalyst after reaction remains almost unchanged compared with the fresh catalyst which can be further certificated the excellent stability of 0.5 Ag2S/AgVO3@GAs. The half-life of dye represents half the time of dye degradation, which can be evaluated by intersection of the dye concentration (C/C0) and degradation efficiency (1-C/C0) curves. min.

47, 48

As shown in Fig. 8a, the half-life of

Additionally,

the

photocatalytic

0.5 Ag2S/AgVO3@GAs was 8.27 performance

comparisons

of

Ag2S/AgVO3@GAs with representative Ag-based composites reported elsewhere are summarized in Table S2. Simultaneously, the compression tests in Fig. S6(a-c) and the video imply that Ag2S/AgVO3@GAs have excellent mechanical strength and elasticity, which help recycle easily and improve cyclic stability significantly. It can be seen that the Ag2S/AgVO3@GAs are able to stand on a dog’s tail grass and a feather (Fig. S6d), meanwhile, it can suspend in an aqueous solution due to the density approximately 0.034 g/cm³ below that of water. The results imply that the composite was light, which is beneficial to the contact between the composite and pollutants. Moreover, the mass loss of hybrid before and after use is below 5% (Fig. S6e, f), which may be derived from the strong anchoring effect of chitosan for AgVO3 and thus it can efficiently maintain the cycle stability of the Ag2S/AgVO3@GAs.

17



Fig. 7 (a) Photocatalytic degradation curves of MO, (b) corresponding pseudo-first-order kinetic curves, (c) reaction rate constants over as-prepared materials under visible light irradiation and (d) five successive photodegradation dynamic curves of MO over 0.5 Ag2S/AgVO3@GAs. To further investigate the possible active species in the photodegradation process, the radical scavenging experiment (Fig. 8b) was performed by adding various trapping agents, such as the disodium ethylenediaminetetraacetate (EDTA) for h+, p-benzoquinone (BZQ) for·O2- and tert-butyl alcohol (TBA) for ·OH, respectively. In Fig. 9, it is observed that BZQ and EDTA significantly reduced the photodegradation efficiency, suggesting that the ·O2- and the h+ radicals should play a predominant role in photodegradation reaction. However, little change was observed by introducing TBA, which indicates that the ·OH is not the main active species.

18


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Fig. 8 (a) Half-life calculation for 0.5 Ag2S/AgVO3@GAs catalyst; (b) the trapping experiment of the active species during the photocatalytic reaction of 0.5 Ag2S/AgVO3@GAs. The mineralization ability of the obtained samples for MO was investigated by detecting the concentration change of total organic carbon (TOC). In Fig. S7, the TOC removal rate of 0.5 Ag2S/AgVO3@GAs could reach 73% after 40 min, which is far high than pure AgVO3, suggesting that the 0.5 Ag2S/AgVO3@GAs composite has an improved photocatalytic performance and more effective mineralization capacity than pure AgVO3. In order to avoid the dye sensitization impact, the bisphenol A (BPA) as colorless pollutant was also applied in photocatalytic tests. In Fig. S8a (Supporting Information), it was found that the BPA can hardly be degraded under visible-light irradiation without catalysts. In dark condition, the adsorption efficiency of 0.5 Ag2S/AgVO3@GAs for BPA is 29%. After 60 min irradiation, all the Ag2S/AgVO3@GAs hybrids shown an enhanced degradation activity compared to the pure AgVO3, indicating that the introduction of Ag2S can promote the degradation efficiency, and 0.5 Ag2S/AgVO3@GAs displayed the optimal degradation capacity 19



Page 20 of 29

for BPA approximately 95% after 60 min reaction. The result further signified that the strong coupling between Ag2S and AgVO3 is beneficial to improving photocatalytic performance. Additionally, more than 85% degradation efficiency of the 0.5 Ag2S/AgVO3@GAs maintained after five cycles (Fig. S8b, Supporting Information), implying high cyclic stability of 0.5 Ag2S/AgVO3@GAs. 3.4. Photocatalytic antifouling performance The survival curves of gram-negative E. coli and gram-positive S. aureus are displayed in Fig. 9a, b, where can be observed that the bacterial cells quantity remained nearly unchanged in blank control experiments (without catalysts) within 36 min, suggesting the visible-light photolysis to the bacteria are overlooked. In the dark experiment, the 0.5 Ag2S/AgVO3@GAs showed antibacterial activity against E. coli and S. aureus approximately 0.58, 0.28 log inactivation, respectively, which was ascribed to the bactericidal activity of Ag nanoparticles even without irradiation. 49, 50 Moreover, it can be clearly observed that pure AgVO3 displayed the lower disinfection

activities

for

E.

coli

and

S.

aureus.

Fortunately,

all

the

Ag2S/AgVO3@GAs hybrids shown an enhanced disinfection efficiency compared to the pure AgVO3, indicating that the constructed heterojunction system with the introduction of Ag2S can promote the antibacterial efficiency. For the 0.5 Ag2S/AgVO3@GAs exhibited the optimal disinfection efficiency for E. coli, and nearly no bacteria cells were observed after 35 min visible-light irradiation (Fig. 9a), which was faster than the 0.25 Ag2S/AgVO3@GAs, 0.75 Ag2S/AgVO3@GAs or 1 Ag2S/AgVO3@GAs. Meanwhile, 0.5 Ag2S/AgVO3@GAs could also exhibit highest 20


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inactivation efficiency for S. aureus approximately 2.8 log inactivation within 40 min reaction (Fig. 9b). Apparently, the inactivation efficiency of S. aureus by 0.5 Ag2S/AgVO3@GAs is lower than that of E. coli, which is attributed to the difference of bacterial cells wall composition. The gram-negative bacteria with a relative thinner cell wall will provide less protection to cells. 51 The antibacterial cycle test results for the

E.

coli

suggest

that

the

outstanding

disinfection

activities

of

0.5

Ag2S/AgVO3@GAs against E. coli have also been maintained after five consecutive cycles (Fig. 9c), which implies that the 0.5 Ag2S/AgVO3@GAs photocatalysts have good potential for repeated use.

Fig. 9 (a) Disinfection performance on E. coli and (b) S. aureus, respectively, (c) stability test of 0.5 Ag2S/AgVO3@GAs on E. coli under visible light and five successive applications. Error bars represent standard deviations across triplicate experiments (n = 3). The representative photographs of the petri dish of the antibacterial (E. coli) performance (Fig. S9) further confirmed the high antifouling activity of 0.5 Ag2S/AgVO3@GAs. The live bacteria staining photos show that Live E. coli can be stained blue by methylene blue, whereas inactivated E. coli cannot be stained and it appears colorless. After 36 min of bacteriostasis, almost all E. coli could not be stained, indicating that E. coli was killed by the 0.5 Ag2S/AgVO3@GAs. Additionally, 21



Page 22 of 29

the SEM images (Fig. S10a, b) of bacterial cell display that the bacterial cell surface (E. coli/ S. aureus) is wrinkled and the center is sunken after 36 min of photocatalytic reaction, so that the bacteria are killed. 3.5. Antifouling mechanism To verify the active species in the bacteriostatic experiment, the trapping experiments using various scavengers, such as EDTA for h+, BZQ for·O2- and TBA for ·OH, respectively, were carried out. In Fig. 10, the antibacterial efficiency of E. coli was up to 99.99% after 40 min reaction by 0.5 Ag2S/AgVO3@GAs without any scavenger. Additionally, the antibacterial rate by introducing TBA has little change, demonstrating that the ·OH is not the key active radical in photocatalytic process. Nevertheless, BZQ and EDTA significantly restrained the antibacterial efficiency, approximately 24.23% and 38.42% respectively, suggesting that the ·O2- and the h+ radicals should play a predominant role in photocatalytic process. Furthermore, the controlling experiments displayed that there is no observably toxic effect of the scavengers on E. coli antibacterial reaction.

Fig.

10

Effects

of

scavengers

on

the

bacteriostatic

22


efficiency

of

0.5

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Ag2S/AgVO3@GAs for E. coli under visible-light irradiation. The conduction band (CB) and valence band (VB) edge potentials of AgVO3 are 0.41 eV and 2.31 eV, and those of Ag2S are -0.09 eV and 1.01 eV, respectively (calculated in Supporting Information). According to the band position of the Ag2S and AgVO3, the possible mechanism of the Ag2S/AgVO3@GAs is proposed (Fig. 11). Under visible-light excited, both Ag2S and AgVO3 can generate electrons-holes pairs, and partial electrons could migrate to the rGO surface, facilitating the separation of electrons-holes. 34 Since the CB of Ag2S is more cathodic than AgVO3 and the VB of AgVO3 is more positive than Ag2S, the generated e- from the CB of Ag2S would transfer to the CB of the AgVO3, while the generated h+ from the CB of AgVO3 would migrate to the VB of Ag2S. Moreover, the CB of Ag2S is not more cathodic than O2/·O2- (O2/·O2-, -0.046 eV vs NHE)

52

potential, which cannot reduce O2 to

generate ·O2-. However, the trapping experiments results showed that ·O2- is a main active specie in the photocatalytic process. Accordingly, it can be inferred reasonably that the photoexcited electrons-holes transfer of Ag2S/AgVO3@GAs composites could not via the conventional heterojunction style (Fig. 11a). The Fermi energy of Ag is more positive than the CB level of AgVO3 and more negative than VB of Ag2S. 53, 54

Thereby, a Z-scheme photocatalytic mechanism via Ag generated form the

interface of Ag2S/AgVO3 as a bridge of the photoexcited charge may be more appropriate (Fig. 11b). During the photocatalytic process, the e- formed on the CB of AgVO3 could flow into Ag0 by the Schottky battier, 55 and then recombine with the h+ migrated from the VB of Ag2S.

56

As a consequence, the mechanism keeps efficient 23



Page 24 of 29

separation of the charge. Simultaneously, the e- in the CB of the Ag2S has enough capacity to reduce O2 to yield ·O2-, and the h+ with high oxidation capacity located at the VB of the AgVO3 can oxidize organic pollutants directly.

Fig. 11 (a, b) Possible photocatalytic mechanism scheme of Ag2S/AgVO3@GAs under visible light irradiation. 4. CONCLUSION A three-dimensional porous Ag2S/AgVO3@GAs were successfully synthesized via an in-situ ion exchange method. The Ag2S nanoparticles were introduced AgVO3 system at first and dispersed uniformly on the AgVO3. After introducing Ag2S, the Ag2S/AgVO3@GAs exhibits excellent photodegradation efficiency for organic pollutants

and

antibacterial

ability

with

high

cyclic

stability.

The

0.5

Ag2S/AgVO3@GAs displayed the optimal photodegradation efficiency for MO (97% removal efficiency in 40 min) as well as disinfection activity for E. coli (100% antibacterial rate in 35 min) and the excellent performance have still been remained even after five consecutive cycles. The improved photocatalytic activities should originate from the strong coupling between Ag2S and AgVO3, improving the charge separation rate, promoted the interfacial charge migrate and extend the visible-light response range. As shown in radical scavenger experiments, the excellent 24


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photocatalytic performance was primarily owing to ·O2- and h+. This study will provide an inspiration for the design of high-efficiency and durable photocatalysts to expedite current research on potential applications in water purification. ASSOCIATED CONTENT Supporting information Synthesis method of GO and AgVO3, characterizations, supplementary results and discussion (SEM image, FT-IR, XPS spectra, and TOC of samples, Ag2S particle size distribution histogram), SBET and pore structure information, HRTEM, SEM and XRD before and after reaction, compression test and mass evaluation, photodegradation activities for BPA, SEM images of bacterial cell, optical photographs of the antibacterial (E. coli) performance and photocatalytic performance comparisons). Acknowledgements This research was supported by the National Key R&D Program of China (No. 19GJHZ), the Key Basic Research Program of the Sichuan Provincial Education Commission, P. R. China (No. 10ZB034), the Basic Research Program of the Science & Technology Department of Sichuan Province, P. R. China (No. 2011ZR0067), the National Natural Science Foundation of China (No. 21607109), Sichuan science and technology plan project of International Cooperation, P. R. China (No. 2016HH0081), and the start-up grants of Sichuan Agricultural University for talent, P. R. China (No. 03120313), the Green Catalysis Key Laboratory of Sichuan Provincial Universities, P. R. China (No. LZJ1805). Sincere thanks go to anonymous reviewers for helpful 25



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AgVO3 Graphene Aerogels

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