Bifunctional Material with Organic Pollutant Removing and

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Bifunctional Material with Organic Pollutant Removing and Antimicrobial Properties: Graphene Aerogel Decorated with Highly Dispersed Ag and CeO2 Nanoparticles Xiafang Tao, Yazhou Zhou, Kai Xu, Yunyan Wu, Jianli Mi, Yi Li, Qinqin Liu, Xiaonong Cheng, Nan Zhao, Haifeng Shi, and Juan Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04251 • Publication Date (Web): 27 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Sustainable Chemistry & Engineering

Bifunctional

Material

with

Organic

Pollutant

Removing and Antimicrobial Properties: Graphene Aerogel Decorated with Highly Dispersed Ag and CeO2 Nanoparticles Xiafang Tao,† Yazhou Zhou,*,† Kai Xu,† Yunyan Wu,† Jianli Mi,† Yi Li,† Qinqin Liu,† Xiaonong Cheng,† Nan Zhao,† Haifeng Shi,‡ and Juan Yang *,† †School

of Materials Science and Engineering, and ‡Institute of life sciences, Jiangsu University,

Xuefu Road 301, 212013, Zhenjiang, People’s Republic of China *Email address of corresponding author

Yazhou Zhou: [email protected]

Juan Yang: [email protected]

KEYWORDS: Graphene aerogel; Cerium dioxide (CeO2); Silver nanoparticles (Ag NPs); Photocatalyst; Antimicrobial material

ACS Paragon Plus Environment

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Abstract: The graphene aerogels supporting Ag and CeO2 nanoparticles (NPs) (Ag/CeO2/GA) with bifunctional performances have been fabricated by a modified gelatinization reaction method. Through the control of GO concentration and ultrasonic-assisted pre-reduction process, the small NPs can be dispersed highly into porous structure of GA. The integration of hierarchical porous structure and a synergistic effect between CeO2 and Ag NPs grant them bifunctional properties of organic pollutant removing (100% of methylene blue and 81.8% of bisphenol A were degraded within 12 min and 4 h, respectively), and antimicrobial activity against Escherichia coli (MIC100%Ag: 7.5 ppm, MBC100%Ag: 11.3 ppm). The possible mechanism is the improved photocatalytic performance of CeO2/GA by unique surface plasmonic effect of Ag NPs due to increasing light absorption and photoconductivity for photoexcited holes and electrons. Therefore, the holes and radicals (especially •OH) can be produced continuously and efficiently from Ag/CeO2/GA, which can remove organic pollution from water efficiently while protecting the water from bacterial pollution. Moreover, a good recycling stability and biocompatibility suggest that such bifunctional material has a great promise in water purification applications.

Introduction Graphene has attracted a great attention as nanoscale building blocks to construct the threedimensional (3D) macroscopic graphene aerogels (GAs).1-3 Previous work reported that GAs not only inherit many properties from two-dimensional (2D) graphene such as large specific surface area, high physical and chemical stability as well as excellent electrical, mechanical, and thermal properties, but also present high porosity, extremely low density, and extraordinary elasticity properties.4-6 The constructing macroscopic graphene structure is crucial for promoting the graphene in many potential applications.7 In particular, GA has been demonstrated as a promising


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candidate for oil spill clean-up,8-11 and water purification applications12-14 on based of several advantages over the traditional sorbents: (1) be able to efficiently adsorb the organic pollutants from water at high speed due to its superhydrophobic and superoleophilic properties; (2) super high adsorption capacity because of its high porosity and large surface area; (3) ease separation of GA from water; (4) be compatible with large-scale applications and easily assembled owing to its macroscopic bulk structure, and (5) facile and large-scale synthesis from graphene oxide (GO) using many methods such as self-assemble, hydrothermal, hard template and freeze-casting methods. Unfortunately, there are still concerning issues for GA in practical water purification applications. For example, the recycling capacity should be further improved because it is not easy to remove the adsorbed organic pollutants from GA. Although, the thermal treatment is a useful way, it may cause collapse of the porous structure. It is difficult to clean out the microorganism such as bacteria out of the polluted water. The adsorbed bacteria on the GA would probably proliferate in high speed, which bring the serious microbial contamination to the water. Therefore, synthesis of bifunctional GA with high visible-light photocatalytic degradation and excellent antimicrobial properties is a great strategy to overcome as-described issues. A number of photocatalyst nanoparticles (NPs) such as TiO2, ZnO, Ag3PO4, Ag/AgX, CeO2, etc. have been supported by 2D graphene sheets to form new types of photocatalysts.15-19 Graphene, a good photogenerated electrons acceptor and transporter, plays an important role in improving the photodegradation performance due to its high specific surface area and excellent electrical conductivity. Thus, integration of high adsorption ability of GA and good photocatalytic activity of NPs may be an ideal for fabrication of the new-type of water purification materials. CeO2, an abundant and inexpensive rare earth material, has attracted tremendous attention from excellent photocatalytic activity under visible-light irradation.20 For instance, Reddy21 and Jha22


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have reported the deposition of CeO2 in GA. Nevertheless, there is still a great challenge for the controllable synthesis of NPs/GAs. One of the most popular approaches for the synthesis of NPs/GAs is the integration of metal precursors or NPs with GO into macroscopic bulk structure.2325

The high concentration of GO is necessary for formation of stable macroscopic graphene.26,27

However, due to the high concentration of GO, it’s difficult for the metal precursors or NPs to uniformly disperse on the GO sheets, which results in the big size and serious aggregation of NPs in GAs.28,29 Some researchers deposited NPs into as-prepared GA by self-assembly method, but it also caused serious NP aggregations.30 Thus, creating new strategies for controllable integration of photocatalytic NPs such as CeO2 NPs with GA is highly desirable for the development of excellent water purification materials. Microbial contamination of water (such as Escherichia coli, E. coli) is still a very severe problem that causes gastrointestinal diseases to human, particularly to children in developing countries.31 Although the gastrointestinal diseases can be easily controlled by chlorination of water, the recontamination is a huge problem in purified water. Moreover, the surviving microbes adsorbed on the purified materials such as activated carbon would rapidly proliferate, which is detrimental to the water quality.32 Ag NPs show a great promise in water treatment application owing to the excellent antibacterial activity, controllable biocompatibility as well as the high capacity of removing certain ions.33 It has been reported that incorporating Ag NPs can improve the photocatalytic activity of CeO2 NPs.34 Thus, integration of Ag NPs with photocatalytic CeO2/GA (Ag/CeO2/GA) may be an ideal candidate in development of water treatment materials that are able to purify the organic pollutants and microbes. Herein, bifunctional Ag/CeO2/GA materials have been successfully synthesized using a modified gelatinization reaction method. As far as we know, this might be the first report


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constructing Ag/CeO2/GA with several achieved benefits. (a) The new and versatile strategy has been demonstrated to fabricate NPs/GA with highly-dispersive of NPs; (b) The ultrasonic-assisted process was introduced to form well-dispersed Ag NPs with small size and narrow size distribution in the Ag/CeO2/GA materials; (c) The Ag/CeO2/GA displayed bifunctional performances of removing the organic pollutants such as colorful dye of methylene blue (MB) and colorless bisphenol A (BPA), and excellent antibacterial activity. (d) The cost is acceptable due to the low Ag NP loading (~7.5 wt%). Such attractive materials hold a great potential in water purification applications, and this method is quite superior for creating the new GA-based materials with attractive properties. Experimental Section Synthesis of CeO2/GA Photocatalyst. The CeO2/GA material was synthesized by a modified gelatinization reaction method. The positively charged CeO2 NP colloid (3.9 mg mL-1, particle size: 11.6 nm) (Figure S1) was firstly prepared according to the reported method (see Supporting Information).35 And then, 5 mL of CeO2 colloid was added in 60 mL of GO aqueous solution (0.33 mg mL-1) to form CeO2 NPs/GO, after 1 h vigorous stirring. Then, CeO2 NPs/GO solution was concentrated by centrifuging, followed by removing a certain volume of supernatant to obtain 10 mL of CeO2 NPs/GO solution. The above solution with 20 mg L-ascorbic acid was placed in oven at 90 oC for 4 h to form hydrogel. After extracting, freeze-drying, the CeO2/GA material can be obtained. Synthesis of Ag/CeO2/GA Photocatalysts. The Ag/CeO2/GA material was synthesized by asdescribed method, but the ultrasonic-assisted pre-reduction was processed before the gelatinization reaction. 5 mL (or 6 mL) CeO2 colloid and 0.35 mL of silver nitrate (AgNO3) solution (0.1 M) were added into 60 mL of GO aqueous solution (0.33 mg mL-1) to form the Ag+/CeO2/GO


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precursors after vigorous stirring. The same concentrated process was carried out to obtain 10 mL of Ag+/CeO2/GO solution. Then Ag+/CeO2/GO precursors were pre-reduced by adding 10 mg sodium borohydride (NaBH4) using 10 min ultrasonic-assisted process to obtain Ag/CeO2/rGO solution. Above solution was then treated as same process as CeO2/GA to form the Ag/CeO2/GA. The Ag/CeO2/GA photocatalysts prepared by adding 5 mL or 6 mL of CeO2 colloid were labeled as Ag/CeO2/GA-5 and Ag/CeO2/GA-6, respectively. Characterization. The morphologies of samples were characterized by scanning electron microscope (SEM) on a JSM-7001F and transmission electron microscope (TEM) on a JEOL 2011, Japan. The scanning TEM (STEM) and energy dispersive X-ray (EDX) analyses were obtained using Titan 80-300 S/TEM equipped with a probe spherical aberration corrector (FEI Company) operated at 300 kV. The X-ray photoelectron spectroscopy (XPS) data was performed using a Kratos AXIS-165 with a monochromatized AlKα X-ray anode (1486.6 eV). The X-ray diffraction (XRD) analyses were performed on a Siemens D5000 powder X-ray diffractometer with Cu Kα radiation at 40 kV and 30 mA. The optical properties were characterized by ultravioletvisible (UV-vis) spectra performed on a Shimadzu UV2450 spectrometer (Japan). The actual metal loading was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Thermo-Fisher iCAP 6300, Thermo Fisher Scientiﬁc Inc.). Photoelectrochemical Experiments. The typical three-electrode system was employed to measure the electrochemical impedance spectroscopy (EIS) and transient photocurrent of samples (CHI 660E, Chenhua Instrument, Shanghai, China). The platinum foil and Ag/AgCl (3M) were used as a counter and reference electrodes, respectively. 0.2 M Na2SO4 aqueous solution was employed as electrolyte. A Xe lamp (300 W, a 420 nm cut-off filter) was used as visible light source. The procedures for preparation of working electrodes are as follow: 5 mg photocatalyst


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was dispersed into mixed solution containing 250 μL ethanol, 250 μL ethylene glycol and 40 μL Nafion; 80 μL above catalytic ink was then dropped onto the fluorine tin oxide (FTO) glass with an exposed area of 1 cm2. EIS spectra were recorded in the range from 0.01 Hz to105 Hz at an ac voltage of 10 mV. The photocurrent responses were obtained with 1.2 V bias voltage under light switching on and off. Purification Performance of Organic Pollutant Removal from Water. The dye of MB and colorless BPA were employed to demonstrate the purification property of samples. A solar simulator with Xe lamp (CHF-XM-500W, λ>420 nm, Changtuo, China) was used as the visible light source. In a typical process of degradation of MB, 30 mg Ag/CeO2/GA-5 (CeO2/GA) were dropped in 50 mL of a dye solution (30 mg mL-1) with magnetic stirring without light irradiation. After 30 min, the light was turned on. During the experiment, 2 mL of dye solution was taken out every few minutes. MB concentration was monitored using a UV-vis spectra. For degradation of BPA, the process was similar with MB. But, 50 mL BPA (150 mg L-1) and 30 mg materials were used. A high-performance liquid chromatography (HPLC, Perkin Elmer) was carried out to analyze BPA concentration according to the previous work.36 The details can be seen in Supporting Information. Recycling Stability Test. The recycling and stability properties of the Ag/CeO2/GA catalyst were investigated by recycling the catalyst towards degradation of MB for 5 times. In detail, the aerogel was taken out from the MB solution, and was washed with DI water for 3 times, followed by freeze-drying. The degradation of MB was repeated. This process was repeated for 5 times, and the change of photocatalyst of sample was recorded. After five-recycle experiment, the stability of photocatalyst was measured by XRD, and compared with that of initial catalyst.


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Antimicrobial Property Evaluation. The in vitro evaluation was used to characterize the antimicrobial property of Ag/CeO2/GA against the Gram-negative bacterium E. coli (ATCC 117). The antimicrobial measurements include inhibition zone testing, bacterial growth inhibition, and bacterial time-kill assay. And then, the minimum inhibitory concentration (MIC100%) and minimum bactericidal concentration (MBC100%) can be obtained according to the data. The biocompatibility and cytotoxicity of Ag/CeO2/GA were characterized using a 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) method on the human hepatic carcinoma (HepG2) cell lines. The detailed experimental procedures were shown in the Supporting Information and our previous work.16 Results and Discussion Synthesis and Characterizations of CeO2/GA and Ag/CeO2/GA Catalysts. The synthesis of CeO2/GA includes three main procedures, and its schematic illustration is shown in Figure S1: (1) Preparation of NPs/GO by electrostatic self-assembly (Figure S1a); (2) gelatinization reaction of NPs/GO to form hybrid hydrogel (Figure S1b), and (3) freeze-drying to obtain aerogel with macroscopic porous structure (Figure S1c). It is noted that GO concentration is crucial for the synthesis of graphene-based aerogels with high quality (> 2mg mL-1, usually). In order to highly disperse the NPs onto the surface of GO sheets, a low concentration of GO solution (0.33 mg mL-1) is initially required in this work because of the fully stripped GO sheets (Figure S1a). After that, the GO solution was then concentrated to 2 mg mL-1 by centrifugation (Figure S1b). In doing so, both highly dispersive of NPs and stable macroscopic bulk structure can be achieved (Figure S1c).


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Figure 1. Characterization of CeO2/GA, (a) SEM image, (b, c) TEM images, and (d) HRTEM image. The porous structure of CeO2/GA catalysts was investigated by SEM (Figure 1a). The welldefined 3D graphene frameworks can be clearly seen in the cross-section of CeO2/GA catalyst. The detailed morphologies of CeO2/GA catalysts were further analyzed by TEM (Figure 1b, c). A few of graphene layers are folded to form the typical wrinkled and fibrous structures, wherein the cubic CeO2 NPs disperse on the graphene sheets. There is no any NP aggregation found in GA, indicating that the high dispersion of NPs in GA was achieved. Figure 1d shows the HRTEM image of CeO2 NPs. The NPs with annotations of planes can be clearly seen, which is consistent


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with fluorite CeO2 crystal structure. For comparisons, two additional CeO2/GA materials were prepared under different experimental conditions. One is the use of high concentration of initial GO solution (2 mg mL-1). However, obvious NP aggregations in GA can be found (Figure S3). The other is the use of low concentration of GO solution (0.33 mg mL-1) without centrifuging process, and the macroscopic hydrogel cannot be formed due to the low GO concentration (Figure S4). The results strongly proved that the high-dispersive of NPs and macroscopic structure can be both achieved through the control of the GO concentration. On the basis of the synthesis of CeO2/GA materials, Ag/CeO2/GA material was further fabricated, and the synthesis procedures is schematically illustrated in Figure 2. The Ag+ and CeO2 were attached on the GO surfaces using self-assembly method. After concentrated process, the Ag+/CeO2/GA precursors were obtained (Figure 2a).

The precursors were then treated by

ultrasonic-assisted pre-reduction process. During this process, the Ag NPs can be attached on the rGO surfaces through an in-situ reduction reaction to obtain the Ag/CeO2/rGO (Figure 2b). Finally, the Ag/CeO2/GA material was obtained by gelatinization reaction process (Figure 2c), followed by freeze-drying (Figure 2d). The interaction between NPs and rGO can be strengthened during gelatinization reaction process. Importantly, the ultrasonic-assisted pre-reduction process is crucial for introducing of small Ag NPs with highly-dispersive in GA, which will be discussed in detail later.


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Figure 2. Schematic illustration of synthesis procedures of Ag/CeO2/GA, (a) preparation of Ag+/CeO2/GO by (1) self-assembly method and (2) concentrated process; (b) Ag/CeO2/rGO prepared by ultrasonic-assisted process; (c) Ag/CeO2/graphene hydrogel prepared by gelatinization process; (c) Ag/CeO2/GA prepared by freeze-drying of Ag/CeO2/graphene hydrogel. The SEM image of cross-sectional Ag/CeO2/GA material is shown in Figure 3a. The Ag/CeO2/GA material shows a similar well-defined 3D porous framework structure with CeO2/GA, indicating that the pre-reduction process does not affect the formation of porous structure. Figure 3b shows the TEM image of Ag/CeO2/GA material. The NPs with an average size of 12. 2 nm are uniformly scattered on the graphene sheets, and no obvious NP aggregation is detected in large-scale graphene sheets, indicating that NPs with highly-dispersive in GA were achieved (Figure 3b). In order to further identify the CeO2 and Ag NPs and their distributions, the HRTEM and elemental mapping were employed. As shown in the HRTEM image (Figure 3c), the


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Figure 3. (a) SEM, (b) TEM, (c) HRTEM, (d) HAADF-STEM and (e,f) STEM mapping images of Ag/CeO2/GA-5, (g) SEM and (h) TEM images of Ag/CeO2/GA-6.


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well-resolved lattice fringes with d-spacing of ~ 0.312 nm are observed for the cube-shaped NPs, corresponding to the {111} interplanar distance of fluorite CeO2 structure. The lattice fringes with d-spacing of ~ 0.232 nm can be clearly seen for some particles, which are in high agreement with the fcc Ag {111} planes. The high-angle annular dark field scanning TEM (HAADF-STEM) image (Figure 3d), corresponding elemental mappings for Ce (Figure 3e) and Ag (Figure 3f) also demonstrate both Ag and Ce element existing in GA, and they have a homogeneous dispersion in GA. The crystal structures of CeO2/GA and Ag/CeO2/GA photocatalysts were further characterized by XRD (Figure 4a). For CeO2/GA photocatalyst, an obvious peak can be found at 2 of 22.5°, which is assigned to the C (002) for GA. The other nine peaks appeared at 2 of 28. 6°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7°, 79.1°, and 88.4° are correspondingly assigned to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) crystal planes of CeO2, which are highly consistent with the fluorite CeO2 structure (JCPDS card no. 34-0394). In contrast, Ag/CeO2/GA photocatalyst presents four additional peaks at 2 of 38.1°, 44.3°, 64.4°, and 77.5°, corresponding to the (111), (200), (220), and (311) crystal planes of fcc Ag structure (JCPDS card no. 04-0783). In addition, the CeO2 diffraction peaks in Ag/CeO2/GA photocatalyst are highly consistent with those of in CeO2/GA, indicating that the pre-reduction process does not affect the crystal structure of CeO2. According to the XRD data, the sizes of crystal Ag and CeO2 NPs were calculated using the Scherrer equation:37 𝐿=

0.9𝐾1 𝐵2 𝐶𝑜𝑠(𝑚𝑎𝑥)

where L is the mean NP size, 𝐾1 is the X-ray wavelength (=0.15418 nm), B2 is the line broadening at half the maximum intensity, and max is the Bragg angle of the peak. The average


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sizes of Ag NPs and CeO2 NPs calculated from that equation are 14.5 nm and 12.4 nm, which is in good agreement with that determined from TEM data. The elemental analysis of photocatalysts were characterized using XPS. From the C 1s XPS spectrum of Ag/CeO2/GA material (Figure 4b), only few oxygen-functional groups remained in GA compared with GO, indicating the high reducing degree of GA is achieved using as-described method. The high-resolution Ce 3d XPS spectrum is shown in Figure 4c. The spin-orbit coupling 3d5/2 and 3d3/2 XPS peaks are labeled as V and U, respectively. Eight fitted peaks can be clearly seen, which are in high agreement with reported spectra of CeO2.38 Specifically, the six peaks labeled as V, V2, V3, U, U2 and U3 are attributed to Ce4+ state. We also found the Ce3+ state with peaks labeled as V1 and U1 in XPS spectrum. The result indicated the cerium in the material is present in both Ce4+ and Ce3+ oxidation. Figure 4d shows the Ag 3d XPS spectrum for Ag/CeO2/GA material. Two individual peaks appear at 368.6 eV and 374.6 eV, corresponding to Ag 3d3/2 and Ag 3d5/2, respectively.39,40 The Ag loading in Ag/CeO2/GA material is calculated by ICP, and Ag loading in photocatalyst is only 7.5 wt%. The XRD, XPS, and TEM results have proven that the highly crystalline Ag NPs and CeO2 NPs have been successfully integrated with GA.


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Figure 4. (a) XRD patterns of Ag/CeO2/GA, CeO2/GA and GO, (b) high-resolution C 1s, (c) Ce 3d, (D) Ag 3d XPS spectra of Ag/CeO2/GA, porous structure characterization (e) N2 adsorption/desorption isotherms and (b) pore size distributions.


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It’s well known that the antimicrobial activity of Ag NPs strongly depends on their particle sizes,41 and smaller Ag NPs has better antimicrobial activity than those of bigger Ag NPs.42 In this work, the Ag NPs in Ag/CeO2/GA photocatalyst have a small size (~14.5 nm), which are attributed to the ultrasonic-assisted pre-reduction process. For comparison, another Ag/CeO2/GA material was also synthesized without the pre-reduction process. Due to the existing Cl- in CeO2 colloid, the AgCl aggregations can be formed by reaction with Ag+ and Cl-, resulting in the serious NP aggregations, big size and the large size distribution of Ag NPs on graphene sheets during the gelatinization reaction by L-ascorbic acid (Figure S5). The ultrasonic-assisted pre-reduction is useful to address this issue with the following two benefits: (1) Ag+ can be reduced by the strong reductant NaBH4 quickly, which tend to form the small Ag NPs; (2) ultrasonic-assisted condition is beneficial for further reducing the Ag NP size and improving the NP dispersion.37 However, we should be cautious about NaBH4 dosage because it heavily impacts the construction of macroscopic structure. The optimal NaBH4 dosage is ~10 mg in this experiment. This amount of NaBH4 is enough for reducing the Ag+, but is insufficient for reducing GO. Thus, the Ag/CeO2/rGO mixed solution still presents the high mono-dispersion after the pre-reduction process owing to the sufficient remaining oxygen-functional groups on rGO sheets (Figure 2c), which can be further assembled into hydrogel by gelatinization reaction. When higher amount of NaBH4 (>15 mg) was used, the powder separated from the solution because a lot of oxygenfunctional groups were removed from rGO sheets by NaBH4, and the above hydrogel cannot be formed (Figure S6). Using this method, the high NP loading in GA, while maintaining their highly dispersive and macroscopic structure can be easily achieved. Figure 3g shows the SEM image of CeO2/GA-6 catalyst with the higher NP loading. The SEM image shows the similar porous structure with


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CeO2/GA-5 catalyst, indicating that the change of NP loading does not significantly affect the porous structure. The NPs are also highly dispersive in GA, and no NP aggregation can be found (Figure 3h). Therefore, the method described in this paper is superior for integration of NPs with controllable composition and loading and highly dispersive in GA. The porous structures of photocatalysts were further analyzed by N2 adsorption/desorption measurements. The typical N2 adsorption/desorption isotherms of CeO2/GA and Ag/CeO2/GA materials are shown in Figure 4e. Clearly, both photocatalysts show the type IV adsorption/desorption isotherm with a clear H1 hysteresis loop. Figure 4f shows the pore size distributions of two photocatalysts obtained using BJH model. Both two photocatalysts have hierarchical structures, including the mesopores and macropores. The specific surface area and pore volume of Ag/CeO2/GA material are 123 m2 g-1 and 0.32 cm3 g-1, respectively, which are similar to CeO2/GA material (112 m2 g-1 and 0.23 cm3 g-1). The high specific surface area and large pore volume make such photocatalysts suitable for water purification. Purification Performance of Organic Pollutant from Water. The purification performance of Ag/CeO2/GA material is investigated by its ability in removing the dye molecule of MB and colorless BPA from water. CeO2/GA and GA are investigated as comparisons. Figure 5a showed the MB (30 mg mL-1, 30 mL) removing performance of Ag/CeO2/GA (30 mg). First of all, the adsorption behaviors of GA, CeO2/GA and Ag/CeO2/GA toward MB were investigated in the darkness. The adsorption equilibriums for all the samples are achieved within 15 min. The CeO2/GA and Ag/CeO2/GA have similar adsorption efficiencies of 48%, which are higher than those of GA (38%). In previous works, graphene derivatives such as GO and rGO were used as adsorbents for removal the contaminations from water, particularly in organic pollutants.43 They have the much higher adsorption capacities than those of graphite, activated carbon, layered double


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hydroxide, boron nitride, metal-organic frameworks, and polymer adsorbents due to their strong - stacking, anion-cation interactions and oxygen-functional groups.44-48 In this work, the 3D hierarchical porous structure and high pore volume also promote Ag/CeO2/GA material highly efficient for organic pollutant adsorption. Under the visible-light irradiation, the MB purification efficiency of Ag/CeO2/GA showed in the Figure 5a, which is obtained according to the real-time absorption spectra of MB over the material (Figure 5b). Almost 100% of MB molecules were degraded within 12 min under visible light irradiation using Ag/CeO2/GA as a photocatalyst, while 87.7% of MB molecules were degraded using CeO2/GA as a photocatalyst. In contrast, negligible MB removing was further found when the pure GA was used as photocatalyst, which proves that MB removing under light is attributed to the photocatalytic effect. Thus, Ag/CeO2/GA material exhibits improved MB removing efficiency compared with CeO2/GA material because of enhanced photocatalytic activity of CeO2 with Ag NPs. The recycling capacity is also demonstrated, which is essential for water purification materials in practical applications. It can be clearly seen that the Ag/CeO2/GA photocatalyst still remains a high efficiency of MB purification after 5 recycles, indicating the excellent recycling capacity (Figure 5c). In addition, the XRD of Ag/CeO2/GA after stability measurement is also obtained and compared with initial Ag/CeO2/GA. Obviously, the crystal structure of Ag/CeO2/GA has no obvious change after stability measurement, further proves its high stability (Figure 5d).


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Figure 5. (a) Adsorption and photodegradation properties of GA, CeO2/GA and Ag/CeO2/GA toward (a) MB, and (e) BPA. (b) Absorption spectra of MB during the purification process. Inset of (b) is digital photograph that shows color change during the purification process. Purification stabilities of photocatalyst toward (c) MB, (f) BPA by five-recycle experiments. (d) The XRD patterns of Ag/CeO2/GA before and after five-recycle measurements.


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BPA is an important synthetic compound for synthesis of plastic products such as epoxy resins, polycarbonate, unsaturated polyester-styrene resins, and so on.36 It is also a typical toxic endocrine-disrupting chemical. Owing to its wide applications in manufacturing industries, the exposure of BPA in water environments has raised great concerns about its harmful impact on the ecosystem. However, it is still a challenge to remove BPA effectively and completely from water. Recently work showed that the visible light-driven photocatalyst can be used for degradation of BPA because of their many advantages including environmentally friendly, economic friendly, and efficiency.36,49 Here, the BPA is used as the second model to investigate the purification ability of Ag/CeO2/GA towards organic pollutants. Figure 5e shows the evolution of the BPA degradation in the presence of CeO2/GA and Ag/CeO2/GA photocatalysts. Clearly, all the materials display the strong absorption of BPA without light irradiation. Under visible light irradiation, 81.8% of BPA molecules were degraded within 4 h when Ag/CeO2/GA was used as a photocatalyst, while 54.2% of BPA molecules were degraded by CeO2/GA photocatalyst. The BPA could be completely degraded by Ag/CeO2/GA within 8 h. Therefore, Ag/CeO2/GA shows significantly improved photocatalytic activity compared with that of CeO2/GA. We also investigate the stability of Ag/CeO2/GA photocatalyst towards BPA by five-recycle experiments and compared with that of CeO2/GA photocatalyst. As shown in Figure 5f, Ag/CeO2/GA exhibits 85.1% degradation efficiency of BPA after stability measurement; while 58.5% efficiency is retained for CeO2/GA. Therefore, Ag/CeO2/GA photocatalyst shows the good purification property and stability towards BPA. The photodegradation of organic dyes by CeO2-based materials such as pure CeO2 NPs,20 Ag/CeO2,34 and CeO2/RGO composites19,21 has been reported, which has an obviously low efficiency compared to our CeO2/GA and Ag/CeO2/GA materials as tabulated in Table 1. Our


20

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materials also have higher or comparable efficiencies compared with the state-of-the-art 3D porous photocatalysts such as CeO2/RGA,50 ZnO/rGO foam,51 and RGO/TiO2 aerogel,52 Bi12O15Cl6 photocatalyst,36 Ag-SCN photocatalyst.49 The good integration of macroscopic graphene with photocatalytic NPs has been proven as a great strategy for fabrication of advanced water treatment materials. Table 1. Comparison of purification efficiencies of photocatalysts toward organic pollutantsa Dye Conc (volume)

Photocatalyst (weight)

Dye degraded

hollow CeO2 (10 mg)

RhB

20 mg L-1 (50 mL)

93

180

Visible (300 W Xe)

20

Ag/CeO2 (50 mg)

RhB MB CV

5 mg L-1 (100 mL)

99 97 99

70, 60, 60

Visible (500 W)

34

CeO2/RGO (15 mg)

MO

1 mM (30 mL)

88.3

60

426 nm (14.5W/m2)

19

rGO-CeO2 (50 mg)

MB

10-5 M (50 mL)

72

50

Sunlight

21

CeO2/RGA

RhB

10 mg L-1 (100 mL)

85

120

Solar light (150 W Xe)

50

ZnO/rGO foam (5 mg)

RhB

5 mg L-1 (25 mL)

>95

150

Solar light (0.1W/cm2)

51

RGO/TiO2 aerogel

CBZ

10 mg L-1 (200 mL)

100

90

UV (~13.5W m-2)

52

Bi12O15Cl6 (30 mg)

BPA

10 mg L-1 (40 mL)

Nearly 100

360

Visible (350 W Xe)

34

Visible (155 W Xe)

49

Ag-SCN (30 mg)

Degradation (%)

Degradation Light time (min) source (power)

10 mg L-1 (50 mL)

Ref.

CeO2/GA (30 mg)

MB BPA

30 g L-1, 150 mg L-1 (50 mL)

Nearly 100, 54.2

18, 240


This work

Ag/CeO2/GA (30 mg)

MB BPA

30 g L-1, 150 mg L-1 (50 mL)

Nearly 100, 81.8

12, 240


This work

aRhB:

rhodamine B; MO: methyl orange; RGA: reduced graphene oxide aerogels; CBZ: carbamazepine; AgSCN: Ag-decorated S-doped g-C3N4.


21


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Antibacterial Property. E. coli is one of the most common fecal-derived pathogenic microorganisms in water. The virulent strains can cause gastroenteritis, urinary tract infections, neonatal meningitis, hemorrhagic colitis, and Crohn's disease.53 Thus, E. coli was used as a model to demonstrate the antimicrobial activity of Ag/CeO2/GA photocatalyst. Figure S7 shows the inhibition zones of CeO2/GA and Ag/CeO2/GA photocatalysts toward E. coli. The obvious zone with a diameter of 11.6 mm can be seen for 500 ppm Ag/CeO2/GA photocatalyst, while only 3.4 mm zone can be found for 500 ppm CeO2/GA photocatalyst. Even though the concentration was lowered to 200 ppm, the Ag/CeO2/GA photocatalyst still shows the larger zone of 8.4 mm than that of 500 ppm CeO2/GA material, indicating significant enhancement in antibacterial activity of Ag/CeO2/GA photocatalyst. The antibacterial activity was further evaluated by online-monitoring of bacterial growth in Luria-Bertani (LB) medium within 20 h. The CeO2/GA and Ag/GA were used as comparisons. As shown in Figure 6a, CeO2/GA with the high concentration of 120 ppm is still inactive for E. coli. In contrast, the E. coli growth exhibited obvious inhibition phenomenon with low concentration of Ag/CeO2/GA photocatalyst (80 ppm). Furthermore, the Ag/CeO2/GA is also more active than that of Ag/GA with the similar Ag loading (50 ppm). The E. coli growth can be completely inhibited by 100 ppm Ag/CeO2/GA photocatalyst (Figure 6b). Thus, the MIC100% for Ag/CeO2/GA photocatalyst against E. coli can be determined as around 100 ppm. The results prove that the Ag/CeO2/GA photocatalyst has significantly improved antibacterial activity by decoration of Ag NPs in CeO2/GA photocatalyst.


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(a)

1h

2h

3h

4h

120 ppm

Water

(c)

(b)

150 ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d)

(e)

Figure 6. Antimicrobial characterization of photocatalysts against E. coli. (a, b) The growth curves of E. coli treated with different materials, (c) inhibition of bacterial colonies and (d) bacteria timekill curves of E. coli treated with Ag/CeO2/GA. (e) Biocompatibility measurements of Ag/CeO2/GA against the HepG2 cell lines. The water, Ag/GA, AgNO3 were used as the control experiments.


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The time-killing studies were used to further demonstrate the antimicrobial activity of photocatalysts. The E. coli colonies were obtained after treatment with Ag/CeO2/GA photocatalyst within 4 h (Figure 6c). The kinetics of killing can be calculated according to the colonies, which are shown in Figure 6d. The results show that Ag/CeO2/GA photocatalyst has the dose/timeresponse for killing efficacy against E. coli. The E. coli can be completely killed by 150 ppm Ag/CeO2/GA material within 4 h. Thus, its MBC100% should be less than 150 ppm. In addition, the MIC100%Ag and MBC100%Ag can be normalized in Ag dosage to 7.5 ppm and 11.3 ppm, respectively, according to the Ag content. The low MIC and MBC indicated that the Ag/CeO2/GA material has a higher or comparable antimicrobial activity compared to many of the reported Ag-based materials.54 Interestingly, we found that the only 41.2% of E. coli were killed by 150 ppm Ag/CeO2/GA within 4 h in the completely dark environment. Therefore, the antibacterial activity of Ag/CeO2/GA is affected by light irradiation significantly. Besides the high antimicrobial activity, the biocompatibility of Ag/CeO2/GA photocatalyst was also demonstrated using MTT method. As shown in Figure 6e, the Ag/CeO2/GA photocatalyst shows the dose-response cytotoxicity on HepG2 cells within 24 h incubation. It exhibits an excellent biocompatibility when its concentration lowers to 100 ppm, wherein the high antibacterial against E. coli. In addition, the antibacterial activity of Ag/CeO2/GA photocatalyst is much higher than CeO2-based materials,38,55 and is also higher or comparable with that of some state-of-the-art photocatalysts such as C3N4-supported Ag NPs.20,56,57. Therefore, the high antimicrobial activity and good biocompatibility make Ag/CeO2/GA potential in elimination of bacterial contamination during the practical water purification.


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Proposed Mechanisms of Bifunctional Properties. On the basis of the above results, Ag/CeO2/GA photocatalyst has the excellent organic pollutant purification performance and antimicrobial activity. The high efficiency of Ag/CeO2/GA photocatalyst towards organic pollutant purification is attributed to its excellent adsorption and photocatalytic activity. Specifically, the residual oxygen-functional groups, the strong - stacking, anion-cation interactions, together with the hierarchical porous structure and high pore volume make Ag/CeO2/GA material highly efficient for organic pollutant adsorption.43,44 At the same time, the organic pollutants can be degraded under the visible light irradiation. Therefore, the organic pollutants can be removed from water completely and efficiently. It is well known that the Ag NPs are the highly efficient antibacterial materials. However, in this Ag/CeO2/GA material, the main supplier of antibacterial activity is not Ag NPs due to its ultra-low Ag content in Ag/CeO2/GA (~7.5 wt%). Therefore, we consider that the improved photocatalytic activity by a synergistic effect between Ag and CeO2 NPs contributes the most on bifunctional properties of Ag/CeO2/GA materials. It is well known that the photoexcited radicals such as •OH, •O2- etc. are main active species for photodegradation property of photocatalyst.57 These radicals are also very toxic to bacteria based on the reactive oxygen species (ROS) theory.58 Thus, the investigation of photoexcited radical efficiency by photocatalysis is useful to understand the possible mechanism. Here, the ESR spin-trap technique (with DMPO) was used to detect the photoexcited radicals. The typical DMPO-•OH and •O2- peaks can be detected in both the mixed aqueous solution (Figure 7a) and methanol solution (Figure 7b) of CeO2/GA and Ag/CeO2/GA under visible light irradiation; and no such signals were detected in the dark. The ESR signals were also strengthened with increasing irradiation time, indicating the increase of generated radicals. Furthermore, the efficiency of


25


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photoexcited radicals by Ag/CeO2/GA is much higher than that of CeO2/GA, implying that improved photocatalytic activity was achieved by a synergistic effect of Ag and CeO2 NPs. The ESR results proved that both •OH and •O2- radicals can be generated from Ag/CeO2/GA, with •OH radicals as the predominant species. (a)

(c)

(e)

(b)

(d)

(f)

Rs

Rt

CPE

Figure 7. DMPO spin-trapping ESR spectra recorded in 2 g L-1 CeO2/GA and Ag/CeO2/GA (a) aqueous dispersions and (b) methanol dispersions at different irradiation times, the effects of reactive species scavengers on (c) MB purification efficiency and (d) antibacterial activity of


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Ag/CeO2/GA. (e) transient photocurrent response and (f) EIS spectra of CeO2/GA and Ag/CeO2/GA. In order to further understand the possible mechanism, the effects of radicals on above bifunctional properties of Ag/CeO2/GA photocatalyst were investigated using the radical trapping experiments. The efficiency of MB photodegradation by Ag/CeO2/GA photocatalyst with/without scavengers can be seen in Figure 7c. The hole scavenger of disodium ethylenediaminetetraacetate (Na2-EDTA), the •OH scavenger of tert-butanol (TBA) and the O2•- scavenger of p-benzoquinone (BZQ) were used in the experiments. The inhibition efficiencies of MB photodegradation are estimated to be 78.5% for Na2-EDTA, 64.8% for TBA, and 35.5% for BZQ, respectively. Obviously, the holes and •OH radicals are dominant active species for photodegradation performance because of their strong oxidized activity. Furthermore, the holes are crucial for formation of •OH radicals. Thus, above results are high consistence with ESR analysis. The radical effect on antimicrobial activity was also investigated using the growth curve measurement of E. coli treated by Ag/CeO2/GA material with/without radical scavenger of N-acetyl-L-cysteine (NAC). As shown in Figure 7d, the antibacterial activity of Ag/CeO2/GA was obviously weakened by adding the NAC or without light irradiation, indicates that photoexcited holes and radicals are also crucial for the antibacterial activity. Therefore, Ag/CeO2/GA with improved photocatalytic activity has the both organic pollutant purification and antibacterial properties through photoexcited holes and •OH radicals efficiently. The synergistic effect of Ag and CeO2 NPs can improve the activity of photocatalyst through modifying electronic structure of photocatalyst. As well as we know, the Ag NPs can be used as electron accepters and transporters because of their excellent electronic conductivity.57 Therefore, the photoexcited electrons can be transported rapidly from CeO2 NPs to Ag NPs, which is useful


27


Page 28 of 40

for suppressing the charge recombination and generating more holes and electrons. In order to prove this synergistic effect, the electronic interaction between Ag NPs and CeO2/GA is studied by transient photocurrent measurements, which is shown in Figure 7e. Both CeO2/GA and Ag/CeO2/GA photocatalysis showed a sensitive photocurrent response during the on/off visible light irradiation. The Ag/CeO2/GA photocatalyst has a 4.7 times higher photocurrent value than that of CeO2/GA, indicates its better visible light response and more efficient photo-existed separation of hole and electron pairs. The electronic structure of Ag/CeO2/GA is further investigated by EIS. The EIS Nyquist plots of Ag/CeO2/GA and CeO2/GA electrodes under visible-light irradiation are shown in Figure 7f. The arc radius for Ag/CeO2/GA photocatalyst is much smaller than that of CeO2/GA photocatalyst, indicating that Ag/CeO2/GA has more effective separation of photoexcited holes and electrons and faster interfacial charge transfer. Figure S8 shows the UV-vis diffuse reflectance spectra of CeO2/GA and Ag/CeO2/GA photocatalysts. Compared with CeO2/GA, the absorption edges of the Ag/CeO2/GA photocatalyst has remarkable red shift and significantly enhanced absorption intensities of light.34 That means the Ag/CeO2/GA photocatalyst has more visible light harvesting, and improved photocatalytic activity compared with that of CeO2/GA because of unique optical property of Ag NPs.


28

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Figure 8. The mechanism of organic pollutant purification and antibacterial properties for Ag/CeO2/GA photocatalyst. Thus, the mechanism of Ag/CeO2/GA in bifunctional properties can be confirmed according to above data and analysis, which can be seen in Figure 8. When the Ag/CeO2/GA material is irradiated with visible light, the separated hole/electron pairs of CeO2 conductor in Ag/CeO2/GA can be generated. These photoexcited electrons can react directly with adsorbed O2 on surface of catalyst to form the •O2-. The •O2- or h+ anions can combine with H2O or H+ to produce the •OH. The h+, •OH, and •O2- can degrade the organic pollutants and kill the bacteria in water.18 Importantly, the excellent conductivity makes GA and Ag NPs efficient as acceptors and transporters of photoexcited electrons.18 Thus, the rapid electron transportation from CeO2 NPs to GA and Ag NPs is beneficial for keeping the electrons away from CeO2, effective suppressing the


29


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charge recombination and generating more holes and electrons. In doing so, the radicals can be produced continuously and efficiently under the visible-light irradiation, and make Ag/CeO2/GA efficient removing organic pollution from water while protecting water from bacterial pollution. Furthermore, its good stability and excellent biocompatibility also demonstrate that such Ag/CeO2/GA material holds a great potential in water purification applications. Conclusions In summary, we have developed a facile and efficient approach of modified gelatinization reaction method for the synthesis of Ag/CeO2/GA. The two processes, adjustment of GO concentration and ultrasonic-assisted pre-reduction, play the important roles in the formation of macroscopic porous graphene-supported small and highly dispersive NPs (Size: 14.5 nm for Ag NPs, 11.6 nm for CeO2 NPs). This Ag/CeO2/GA material shows the high efficiency for organic pollutant purification including MB dye and colorless BPA due to its strong absorption ability and highly photocatalytic activity. This material also exhibits excellent antimicrobial property with low MIC100%Ag of 7.5 ppm and MBC100%Ag of 11.3 ppm against E. coli. The unique surface plasmonic effect of Ag NPs not only can harvest more visible light, but also can decrease the recombination of photoexcited holes and electrons thought their photoconductivity for charge transportation. Therefore, the radicals can be produced continuously and efficiently under the visible-light irradiation, enabling Ag/CeO2/GA to efficiently remove organic pollution from water and protect water from bacterial pollution. In addition, the good recycling stability and biocompatibility suggest that such bifunctional material has a great promise in water purification applications. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. ACS Publications website DOI:


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The chemicals, the synthesis of CeO2 NPs and GO, the detailed procedures for antimicrobial properties, the procedures of BPA concentration measurement during the photodegradation process. The TEM images of CeO2 NPs, CeO2/GA and Ag/CeO2/GA prepared with other conditions. Author Information Corresponding Author *E-mail: [email protected] (Y Zhou); [email protected] (J Yang) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant 51572114, 51672112 and 51702129). Y. Z wants to thank supporting from Postdoctoral

Science

Foundation

(2018M630527)

and

China

Scholarship

Council

(201708320150). Z. N thanks to the Key University Science Research Project of Jiangsu province (16KJB430009). References (1) Fu, G.; Yan, X.; Chen, Y.; Xu, L.; Sun, D.; Lee, J. M.; Tang, Y. Boosting Bifunctional Oxygen Electrocatalysis with 3D Graphene Aerogel‐Supported Ni/MnO Particles. Adv. Mater. 2018, 30, 1704609.


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(2) Jiang, Y.; Xu, Z.; Huang, T.; Liu, Y.; Guo, F.; Xi, J.; Gao, W.; Gao, C. Direct 3D Printing of Ultralight Graphene Oxide Aerogel Microlattices. Adv. Funct. Mater. 2018, 28, 1707024. (3) Kumar, R.; Singh, R. K.; Singh, D. P.; Joanni, E.; Yadav, R. M.; Moshkalev, S. A. LaserAssisted Synthesis, Reduction and Micro-Patterning of Graphene: Recent Progress and Applications. Coord. Chem. Rev. 2017, 342, 34-79. DOI: org/10.1016/j.ccr.2017.03.021 (4) Zhou, Y.; Yen, C. H.; Fu, S.; Yang, G.; Zhu, C.; Du, D.; Wo, P.C.; Cheng, X.; Yang, J.; Wai, C. M.; Lin, Y. One-Pot Synthesis of B-Doped Three-Dimensional Reduced Graphene Oxide via Supercritical Fluid for Oxygen Reduction Reaction. Green Chem. 2015, 17, 35523560. (5) Lv, L.; Zhang, P.; Cheng, H.; Zhao, Y.; Zhang, Z.; Shi, G.; Qu, L. Solution‐Processed Ultraelastic and Strong Air‐Bubbled Graphene Foams, Small 2016, 12, 3229-3234. (6) Zhou, Y.; Yang, J.; Zhu, C.; Du, D.; Cheng, X.; Yen, C. H.; Wai, C. M.; Lin, Y. Newly Designed Graphene Cellular Monolith Functionalized with Hollow Pt-M (M=Ni, Co) Nanoparticles as the Electrocatalyst for Oxygen Reduction Reaction, ACS Appl. Mater. Interfaces 2016, 8, 25863-25874. (7) Kumara, R.; Joanni, E.; Singh, R. K.; Singh, D. P.; Moshkalev, S. A. Recent Advances in the Synthesis and Modification of Carbon-Based 2D Materials for Application in Energy Conversion and Storage. Prog. Energy Combust. Sci. 2018, 67, 115-157. DOI: dx.doi.org/10.1016/j.pecs.2018.03.001 (8) Ge, J.; Shi, L. A.; Wang, Y. C.; Zhao, H. Y.; Yao, H. B.; Zhu, Y. B.; Zhang, Y.; Zhu, H.W.; Wu, H. A.; Yu, S. H. Joule-Heated Graphene-Wrapped Sponge Enables Fast Clean-up of Viscous Crude-Oil Spill. Nat. Nanotechnol. 2017, 12, 434-442.


32

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Subrati, A.; Mondal, S.; Ali, M.; Alhindi, A.; Ghazi, R.; Abdala, A.; Reinalda, D.; Alhassan, S. Developing Hydrophobic Graphene Foam for Oil Spill Cleanup. Ind. Eng. Chem. Res. 2017, 56, 6945-6951. (10) Huang, J.; Yan, Z. Adsorption Mechanism of Oil by Resilient Graphene Aerogels from Oil-Water Emulsion. Langmuir, 2018, 34, 1890-1898. (11) Luo, Y.; Jiang, S.; Xiao, Q.; Chen, C.; Li, B. Highly Reusable and Superhydrophobic Spongy Graphene Aerogels for Efficient Oil/Water Separation. Sci. Rep. 2017, 7, 7162. (12) Zhang, P.; Liao, Q.; Zhang, T.; Cheng, H.; Huang, Y.; Yang, C.; Li, C.; Jiang, L.; Qu, L. High Throughput of Clean Water Excluding Ions, Organic Media, and Bacteria from DefectAbundant Graphene Aerogel under Sunlight, Nano Energy 2018, 46, 415-422. (13) Chen, Y.; Song, X.; Zhao, T.; Xiao, Y.; Wang, Y.; Chen, X. A PhosphorylethanolamineFunctionalized Super-hydrophilic 3D Graphene-Based Foam Filter for Water Purification, J. Hazard. Mater. 2018, 343, 298-303 (14) Cheng, C.; Deng, J.; Lei, B.; He, A.; Zhang, X.; Ma, L.; Li, S.; Zhao, C. Toward 3D Graphene Oxide Gels Based Adsorbents for High-Efficient Water Treatment via the Promotion of Biopolymers. J. Hazard. Mater. 2013, 263, 467-478. (15) Zhang, X. Y.; Li, H. P.; Cui, X. L.; Lin, Y. Graphene/TiO2 Nanocomposites: Synthesis, Characterization and Application in Hydrogen Evolution from Water Photocatalytic Splitting. J. Mater. Chem. 2010, 20, 2801-2806. (16) Sun, H.; Liu, S.; Liu, S.; Wang, S. A Comparative Study of Reduced Graphene Oxide Modified TiO2, ZnO and Ta2O5 in Visible Light Photocatalytic/Photochemical Oxidation of Methylene Blue. Appl. Catal. B-Environ. 2014, 146, 162-168.


33


Page 34 of 40

(17) Xiang, Q.; Lang, D.; Shen, T.; Liu, F. Graphene-modified Nanosized Ag3PO4 Photocatalysts for Enhanced Visible-Light Photocatalytic Activity and Stability. Appl. Catal. B-Environ. 2015, 162, 196-203. (18) Zhou, Y.; Chen, R.; He, T.; Xu, K.; Du, D.; Zhao, N.; Cheng, X.; Yang, J.; Shi, H.; Lin, Y. Biomedical Potential of Ultrafine Ag/AgCl Nanoparticles Coated on Graphene with Special Reference to Antimicrobial Performances and Burn Wound Healing. ACS Appl. Mater. Interfaces 2016, 8, 15067-15075. (19) Verma, R.; Samdarshi, S. K. In Situ Decorated Optimized CeO2 on Reduced Graphene Oxide with Enhanced Adsorptivity and Visible Light Photocatalytic Stability and Reusability. J. Phys. Chem. C 2016, 120, 22281-22290. (20) Deng, W.; Chena, D.; Hub, J.; Chen, L. A General and Green Approach to Synthesize Monodisperse Ceria Hollow Spheres with Enhanced Photocatalytic Activity. RSC Adv. 2015, 5, 80158-80169. (21) Choi, J.; Reddy, D. A.; Islam, M. J.; Ma, R.; Kim, T. K. Self-Assembly of CeO2 Nanostructures/Reduced Graphene Oxide Composite Aerogels for Efficient Photocatalytic Degradation of Organic Pollutants in Water. J. Alloy Compd. 2016, 688, 527-536. (22) Jha, S. K.; Kumar, C. N.; Raj, R. P.; Jha, N. S.; Mohan, S. Synthesis of 3D Porous CeO2/Reduced Graphene Oxide Xerogel Composite and Low Level Detection of H2O2. Electrochim Acta 2014, 120, 308-313. (23) Nawaz, M.; Miran, W.; Jang, J.; Lee, D. S. One-step Hydrothermal Synthesis of Porous 3D Reduced Graphene Oxide/TiO2 Aerogel for Carbamazepine Photodegradation in Aqueous Solution. Appl. Catal. B-Environ. 2017, 203, 85-95.


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Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Cui, C.; Li, S.; Qiu, Y.; Hu, H.; Li, X.; Li, C.; Gao, J.; Tang, W. Fast Assembly of Ag3PO4 Nanoparticles within Three-Dimensional Graphene Aerogels for Efficient Photocatalytic Oxygen Evolution from Water Splitting under Visible Light. Appl. Catal. B-Environ. 2017, 200, 666-672. (25) Fan, Y.; Ma, W.; Han, D.; Gan, S.; Dong, X.; Niu, L. Convenient Recycling of 3D AgX/graphene Aerogels (X= Br, Cl) for Efficient Photocatalytic Degradation of Water Pollutants. Adv. Mater. 2015, 27, 3767-3773. (26) Adhikari, B.; Biswas, A.; Banerjee, A. Graphene Oxide-Based Hydrogels to Make Metal Nanoparticle-Containing Reduced Graphene Oxide-Based Functional Hybrid Hydrogels. ACS Appl. Mater. Interfaces 2012, 4, 5472-5482. (27) Kumar, R.; Singh, R. K.; Alaferdov, A. V.; Moshkalev, S. A. Rapid and Controllable Synthesis of Fe3O4 Octahedral Nanocrystals Embedded-Reduced Graphene Oxide using Microwave Irradiation for High Performance Lithium-Ion Batteries. Electrochim. Acta 2018, 281, 78-87. DOI: org/10.1016/j.electacta.2018.05.157 (28) Kumar, R.; Singh, R. K.; Dubey, P. K.; Singh, D. P.; Yadav, R. M. Self-Assembled Hierarchical Formation of Conjugated 3D Cobalt Oxide Nanobead-CNT-Graphene Nanostructure Using Microwaves for High-Performance Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2015, 7, 15042−15051. DOI: 10.1021/acsami.5b04336. (29) Kumar, R. Singh, R. K.; Vaz, A. R.; Savu, R.; Moshkalev, S. A. Self-Assembled and OneStep Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2017, 9, 8880-8890. DOI: 10.1021/acsami.6b14704.


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(30) Zhou, Y.; Yen, C. H.; Hu, Y. H.; Wang, C.; Cheng, X.; Wai, C. M.; Yang, J.; Lin, Y. Making Ultrafine and Highly-Dispersive Multimetallic Nanoparticles in Three-Dimensional Graphene with Supercritical Fluid as Excellent Electrocatalyst for Oxygen Reduction Reaction. J. Mater. Chem. A 2016, 4, 18628-18638. (31) Ottoson, J.; Stenström, T. A. Faecal Contamination of Greywater and Associated Microbial Risks. Water Res. 2003, 37, 645-55. (32) Wu, S. E.; Hwang, K. J.; Cheng, T. W.; Lin, Y. C.; Tung, K. L. Dynamic Membranes of Powder-Activated Carbon for Removing Microbes and Organic Matter from Seawater. J. Membrane Sci. 2017, 541, 189-197. (33) Zhou, Y.; Yang, J.; He, T.; Shi, H.; Cheng, X.; Lu, Y. Highly Stable and Dispersive Silver Nanoparticle–Graphene Composites by a Simple and Low‐Energy‐Consuming Approach and Their Antimicrobial Activity. Small 2013, 9, 3445-3454. (34) Saravanakumar, K.; Ramjan, M. M.; Suresh, P. ; Muthuraj, V. Fabrication of Highly Efficient Visible Light Driven Ag/CeO2 Photocatalyst for Degradation of Organic Pollutants. J. Alloy Compd. 2016, 664, 149-160. (35) Yu, T.; Zeng, J.; Lim, B.; Xia, Y. Aqueous-Phase Synthesis of Pt/CeO2 Hybrid Nanostructures and Their Catalytic Properties. Adv. Mater. 2010, 22, 5188-5192. (36) Wang, C. Y.; Zhang, X.; Song, X. N.; Wang, W. K.; Yu, H. Q. Novel Bi12O15Cl6 Photocatalyst for the Degradation of Bisphenol A under Visible-Light Irradiation, ACS Appl. Mater. Interfaces 2016, 8, 5320-5326. (37) Zhou, Y.; Yang, G.; Pan, H. B.; Zhu, C.; Fu, S.; Shi, Q.; Du, D.; Cheng, X.; Yang, J.; Wai, C. M.; Lin, Y. Ultrasonic-Assisted Synthesis of Carbon Nanotube Supported Bimetallic Pt-Ru Nanoparticles for Effective Methanol Oxidation. J. Mater. Chem. A 2015, 3, 8459-8465.


36

Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Wang, L.; He, H.; Yu, Y.; Sun, L.; Liu, S.; Zhang, C.; He, L. Morphology-Dependent Bactericidal Activities of Ag/CeO2 Catalysts Against Escherichia coli. J. Inorg. Biochem. 2014, 135, 45-53. (39) Zhou, Y.; Yang, J.; Cheng, X.; Zhao, N.; Sun, H.; Li, D. Transparent and Conductive Reduced Graphene Oxide/Silver Nanoparticles Multilayer Film Obtained by Electrical SelfAssembly Process with Graphene Oxide Sheets and Silver Colloid. RSC Adv. 2013, 3, 33913398. (40) Zhou, Y.; Cheng, X.; Du, D.; Yang, J.; Zhao, N.; Ma, S.; Zhong, T.; Lin, Y. GrapheneSilver Nanohybrids for Ultrasensitive Surface Enhanced Raman Spectroscopy: Size Dependence of Silver Nanoparticles. J. Mater. Chem. C 2014, 2, 6850-6858. (41) Levard, C.; Hotze, E. M.; Lowry, G. V.; Brown, G. E. Environmental Transformations of Silver Nanoparticles: Impact on Stability and Toxicity. Environ. Sci. Technol. 2012, 46, 69006914. (42) Xiu, Z.; Zhang, Q.; Puppala, H. L.; Colvin, V. L.; Alvarez, P. J. J. Negligible ParticleSpecific Antibacterial Activity of Silver Nanoparticles. Nano Lett. 2012, 12, 4271-4275. (43) Kumar, R.; Singh, R. K.; Kumar, V.; Moshkalev, S. A. (2019) Functionalized Nanosize Graphene and Its Derivatives for Removal of Contaminations and Water Treatment. In: Naushad M. (eds) A New Generation Material Graphene: Applications in Water Technology. Springer, Cham. (44) Kumar, R.; Singh, R. K.; Kumar, V.; Moshkalev, S. A. (2019) Graphene/Graphene Oxide and Carbon Nanotube Based Sensors for the Determination and Removal of Bisphenols. In: Naushad M. (eds) A New Generation Material Graphene: Applications in Water Technology. Springer, Cham.


37


Page 38 of 40

(45) Gu, P.; Zhang, S.; Li, X.; Wang, X.; Wen, T.; Jehan, R.; Alsaedi, A.; Hayat, T.; Wang, X. Recent Advances in Layered Double Hydroxide-Based Nanomaterials for the Removal of Radionuclides from Aqueous Solution. Environ. Pollut. 2018, 240, 493-505. DOI: org/10.1016/j.envpol.2018.04.136. (46) Yu, S.; Wang, X.; Pang, H.; Zhang, R.; Song, W.; Fu, D.; Hayat, T.; Wang, X. Boron Nitride-Based Materials for the Removal of Pollutants from Aqueous Solutions: A Review. Chem. Eng. J. 2018, 333, 343-360. DOI: dx.doi.org/10.1016/j.cej.2017.09.163. (47) Li, J.; Wang, X.; Zhao, G.; Chen, C.; Chai, Z.; Alsaedi, A.; Hayat, T.; Wang, X. MetalOrganic Framework-Based Materials: Superior Adsorbents for the Capture of Toxic and Radioactive Metal Ions. Chem. Soc. Rev. 2018, 47, 2322-2356. DOI: 10.1039/c7cs00543a. (48) Zhao, G.; Huang, X.; Tang, Z.; Huang, Q.; Niu, F.; Wang, X. Polymer-Based Nanocomposites for Heavy Metal Ions Removal from Aqueous Solution: A Review. Polym. Chem. 2018, 9, 3562-3582. DOI: 10.1039/c8py00484f. (49) Oh, W. D.; Lok, L. W.; Veksha, A.; Giannis, A.; Lim, T. T. Enhanced Photocatalytic Degradation of Bisphenol A with Ag-Decorated S-doped g-C3N4 under Solar Irradiation: Performance and Mechanistic Studies, Chem. Eng. J. 2018, 333, 739-749. (50) Kumar, S.; Kumar, A. Enhanced Photocatalytic Activity of rGO-CeO2 Nanocomposites Driven by Sunlight. Mater. Sci. Eng. B 2017, 223, 98-108 (51) Men, X.; Chen, H.; Chang, K.; Fang, X.; Wu, C.; Qin, W.; Yin, S. Three-Dimensional Free-Standing ZnO/Graphene Composite Foam for Photocurrent Generation and Photocatalytic Activity. Appl. Catal. B-Environ. 2016, 187, 367-374.


38

Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(52) Nawaz, M.; Miran, W.; Jang, J.; Lee, D. S. One-Step Hydrothermal Synthesis of Porous 3D Reduced Graphene Oxide/TiO2 Aerogel for Carbamazepine Photodegradation in Aqueous Solution. Appl. Catal. B-Environ. 2017, 203, 85-95. (53) Tenaillon, O.; Skurnik, D.; Picard, B.; Denamur, E. The Population Genetics of Commensal Escherichia coli. Nat. Rev. Microbiol. 2010, 8, 207-217. (54) Chernousova, S.; Epple, M. Silver as Antibacterial Agent: Ion, Nanoparticle, and Metal. Angew. Chem. Int. Ed. 2013, 52, 1636-1653. (55) Moongraksathum, B.; Chen, Y. W. Anatase TiO2 Co-Doped with Silver and Ceria for Antibacterial

Application.

Catal.

Today

2018,

310,

68-74.

DOI:

x.doi.org/10.1016/j.cattod.2017.05.087. (56) Khan, M. E.; Han, T. H.; Khan, M. M.; Karim, M. R.; Cho, M. H. Environmentally Sustainable Fabrication of Ag@g-C3N4 Nanostructures and Their Multifunctional Efficacy as Antibacterial Agents and Photocatalysts. ACS Appl. Nano Mater. 2018, 1, 2912-2922. DOI: 10.1021/acsanm.8b00548. (57) Wu, Y.; Zhou, Y.; Xu, H.; Liu, Q.; Li, Y.; Zhang, L.; Liu, H.; Tu, Z.; Cheng, X.; Yang. J. Highly Active, Superstable, and Biocompatible Ag/Polydopamine/g-C3N4 Bactericidal Photocatalyst: Synthesis, Characterization, and Mechanism, ACS Sustainable Chem. Eng. 2018. DOI: 10.1021/acssuschemeng.8b02620. (58) Hou, Y.; Li, X.; Zhao, Q.; Chen, G.; Raston, C. L. Role of Hydroxyl Radicals and Mechanism of Escherichia coli Inactivation on Ag/AgBr/TiO2 Nanotube Array Electrode under Visible Light Irradiation. Environ. Sci. Technol. 2012, 46, 4042-4050.


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For Table of Contents Use Only A bifunctional material of graphene aerogel supporting highly-dispersed Ag and CeO2 NPs with organic pollutant removing and antimicrobial properties was prepared by a facile method. Such bifunctional properties are attributed to the improved photocatalytic activity by synergistic effect of Ag and CeO2. The good recycling stability and biocompatibility suggest that such material has a great promise in water purification applications. SYNOPSIS


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Bifunctional Material with Organic Pollutant Removing and

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