3D graphene structure for dye removal: Adsorption, photocatalysis and physical separation capabilities

Accepted Manuscript Well-designed Ag/ZnO/3D graphene structure for dye removal: adsorption, photocatalysis and physical separation capabilities Malihe Kheirabadi, Morasae Samadi, Elham Asadian, Yi Zhou, Chunyang Dong, Jinlong Zhang, Alireza Z. Moshfegh PII: DOI: Reference:

S0021-9797(18)31299-2 https://doi.org/10.1016/j.jcis.2018.10.102 YJCIS 24257

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

17 July 2018 1 October 2018 29 October 2018

Please cite this article as: M. Kheirabadi, M. Samadi, E. Asadian, Y. Zhou, C. Dong, J. Zhang, A.Z. Moshfegh, Well-designed Ag/ZnO/3D graphene structure for dye removal: adsorption, photocatalysis and physical separation capabilities, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.10.102

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

Well-designed Ag/ZnO/3D graphene structure for dye removal: adsorption, photocatalysis and physical separation capabilities Malihe Kheirabadia, Morasae Samadi a, Elham Asadianb, Yi Zhouc, Chunyang Dongc, Jinlong Zhangc and Alireza Z. Moshfegha, b,* a

Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran

b

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588-89694, Tehran, Iran c

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China. *Corresponding author: [email protected]

Abstract In this research, adsorption and photocatalytic degradation process were utilized to remove organic dye from wastewater. To accomplish that, a newly-designed ternary nanostructure based on Ag nanoparticles/ZnO nanorods/three-dimensional graphene network (Ag NPs/ZnO NRs/3DG) was prepared using a combined hydrothermal-photodeposition method. The threedimensional structure of graphene hydrogel as a support for growth of ZnO nanorods was characterized using field emission scanning electron microscopy (FESEM). In addition, diameter of silver nanoparticles grown on the ZnO nanorods with the average aspect ratio of 5 was determined in the range of 30-80 nm by using transmission electron microscopy (TEM). The X-ray diffraction (XRD) pattern was revealed hexagonal Wurtzite structure of ZnO nanorods and the (111) lattice plane of the face-centered cubic (FCC) of the silver nanoparticles. The dye adsorption capacity of the synthesized 3DG was evaluated at about 300 mg/g using kinetic study. The photocatalytic dye degradation under both UV and visible light irradiation exhibited an enhanced activity of the prepared ternary Ag/ZnO/3DG sample in comparison to ZnO/3DG and 3DG structures. Different charge-carrier scavengers were utilized to elucidate the synergistic effect of adsorption and visible-light photocatalytic degradation mechanism for dye removal. The facile photocatalyst recovery as well as the high elimination rate of dye is promising for future applications such as efficient removal of organic contaminants from industrial wastewater under solar irradiation.

1

Keywords: three dimensional graphene; ZnO nanorods; visible active photocatalyst; organic adsorption capacity; silver nanoparticle

1 Introduction Today, one of the major environmental problems relating to the water pollution is the residual of highly toxic organic materials produced by industrial activities. Exposure to these hazardous materials can cause immunological, neurological and circulatory disorders in human and living organism [1]. Therefore, the degradation and elimination of these chemical contaminants from the wastewater are of high priority. In particular, among various techniques, photocatalytic degradation [1,2] and adsorption process [3] have indicated a superior performance for the organic pollutions elimination and removal from wastewater. In this context, photocatalytic processes have been attracting considerable attention during the past few years as a cost effective and green method [2,3]. Metal oxide semiconductors are promising candidates as photocatalyst for degradation of organic pollutants and water purification [5,6]. Among various metal oxides, zinc oxide (ZnO) nanomaterials are of great importance as a n-type semiconductor, which have been extensively investigated in recent decade because of its extraordinary properties such as high electron mobility (155 cm 2V-1s-1), chemical stability, biocompatibility, high photosensitivity and nontoxicity [7]. However, ZnO photocatalysts are only capable of absorbing UV light due to their high exciton binding energy (60 meV) as well as wide band gap energy (3.37 eV) [8,9]. It means that the ZnO can absorb ultraviolet which is only ~4% of the solar spectrum. On the other hand, the fast electron-hole recombination rate of ZnO largely restricts its photocatalytic activity [10]. Thus, different approaches have been applied to extend its light absorption in the visible range and inhibit electron-hole recombination

process

[5].

For

ZnO

material,

various

approach, including

nanocomposite fabrication [5,11-13], surface modification [14,15] and doping with metal and non-metal [7] have been utilized by our group to enhance photocatalytic activity under visible light irradiation, which contains about 43% of the solar spectrum. Many research groups have demonstrated that the modification of ZnO nanomaterials with noble metals, such as Ag [10,16-21], Au [10,16,19,22,23], Pt [19,24] and Pd [19] could remarkably enhance the photocatalytic activity. This improvement has been

2

attributed to the fact that noble metals not only act as an electron sink/trap [16] for retardation in electron-hole recombination, but also their nanoparticles show surface plasmon resonance (SPR) property for visible light absorption [10]. In addition, several researchers have reported that the creation of a Schottky barrier at the metal –ZnO interface could result in a better separation of the photogenerated electrons and holes and enhance the photocatalytic activity through metal modification. [10]. Furthermore, various ZnO nanocomposites with different carbon nanostructures (0D, 1D, 2D) have been investigated by some researchers. Some of these carbon based nanostructures are including carbon quantum dots (C-QDs, 0D) [15], carbon nanotubes (CNTs, 1D) [25], carbon nanofibers (CNFs, 1D) [26], graphitic carbon nitride (g-C3N4, 2D) [27], graphene oxide (GO, 2D) [28] and reduced graphene oxide (rGO, 2D) [29,30]. Among different approaches, decorating graphene with ZnO nanostructures has been found to demonstrate a superior photocatalytic activity and enhanced anti photocorrosion towards degradation of organic pollutants. This enhancement in photocatalytic activity is ascribed to the fact that graphene has a suitable energy level for photoelectron transfer and high electron extraction ability from semiconductors [29].

Moreover,

the

improved

photocatalytic

efficiency

of

ZnO/graphene

nanocomposites can be attributed to the high adsorption capacity of graphene and the extended lifetime of photogenerated charge carriers during the electronic conductivity of graphene [30]. Three dimensional graphene network (3DG) (or graphene hydrogel) architecture with high surface area and good porosity, as well as, facile and green synthesis route and simple separation from the reaction solution have been considered as an effective adsorbent in the field of water refining and contamination removal. Besides, their chemically active functional groups, and strong π−π interaction with organic contaminants they have other advantages for water treatment applications [32,33]. Therefore, the 3D graphene network can be utilized as a support for the growt h of semiconductor photocatalysts which makes it possible to separate photocatalyst from solution after water purification, conveniently [32]. Moreover, the composite of graphene hydrogel and photocatalysts exhibits the synergistic effect of adsorption and photocatalytic activity for effective removing of organic contaminants from environment. In this process, first, the pollutant was adsorbed on the surface of 3DG network and then, the photogenerated charge carriers participate in the reaction leading to the decomposition of pollutants. Besides, the network of 3DG could provide the

3

channels for photogenerated electrons and holes transfer and therefore, in the composite of graphene hydrogel and semiconductor the photocatalytic performance can be considerably improved [29,32,33]. So far, only a few researches have been published on 3DG based photocatalysts with sufficient elimination rate of water pollutants, thus, it needs further studies for improving the efficiency. In this context, different composites such as TiO2/3DG [3,32,34,35], Fe3O4/3DG [33], g-C3N4/3DG [36], AgX/3DG (X= Br, Cl) [37,38], Ag 3PO4/3DG [39] and CeVO 4/3DG [40] were reported for dye adsorption and photocatalytic degradation researches. In this work, we utilized a combinational approach to study adsorption and the photocatalytic process for removing organic dye pollutants. For this purpose, a novel ternary nanostructure of visible active photocatalyst based on Ag nanoparticle/ZnO nanorods/3DG network was designed and synthesized. Although, there have been some reports about the photocatalytic performance of ternary structure of Ag/ZnO/reduced graphene oxide with two dimensional structure [41-44], to the best of our knowledge, this three dimensional structure has not been reported so far. In this work, graphene hydrogel was prepared by a facile and green method using ascorbic acid (Vitamin C) as the reducing agent followed by the growth of ZnO nanorods on its surface using the hydrothermal method. Then, Ag nanoparticles were photodeposited on the synthesized ZnO nanorods/3DG structure. In order to investigate the photocatalytic performance of the prepared nanocomposite, methylene blue (MB) dye was selected as a model organic pollutant. The photodegradation rates of the dye on the different samples were measured under both UV and visible light irradiation. Finally, for better understanding the mechanism of the photocatalytic activity, a variety of scavengers were examined and the charge carrier generation and transfer during the dye photodegradation reaction under both UV and visible illumination were investigated and compared.

2 Experimental 2.1 Materials Hydrogen peroxide (H 2O2, 30%), sulfuric acid (H 2SO4, 95–97%), potassium permanganate (KMnO4), sodium nitrate (NaNO 3), sodium hydroxide (NaOH), hydrochloric acid (HCl, 37 wt. %), AgNO3, ascorbic acid, potassium iodide, isopropanol and ethanol from Merck company were used as received. Graphite powder 4

(99.99%, particle diameter 45μm) and Hexamethylenetetramine (HMTA) were purchased from Sigma-Aldrich Co. Zinc acetate dihydrate ((CH 3COO)2 Zn.2H2O) and Zinc nitrate hexahydrate (Zn(NO 3)2.6H2O) were purchased from Titrachem CO.

2.2 Instrumental analysis The crystal structures and phase states of 3DG/ZnO (NR)/Ag (NP) hybrid hydrogels were characterized by X-ray diffraction (XRD) using a diffractometer (Stoe, stadi p) at voltage of 40 kV and current of 30 mA with Cu Kα line (λ = 0.1541 nm) in a scanning range of 5–80° with scan rate of 0.03°/step. The surface chemical composition of the prepared Ag (NPs)/ZnO (NRs)/3DG sample was determined through X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA system) which equipped with an Al Kα (1486.6 eV) X-ray source. The calibration of all peaks was performed by fixing the C(1s) core level binding energy at the 285.0 eV as a reference. The SDP software (v 4.1) was employed for deconvolution of XPS peaks with 90% Gaussian−10% Lorentzian peak fitting. To study surface morphology of samples, the swollen hydrogels were freeze-dried. Then, the lyophilized 3D graphene composite networks were coated with a thin layer of gold (8 nm) by DC sputtering technique. Finally, the morphology of the samples was investigated by field emission scanning electron microscope (FESEM, Zeiss, Sigma/VP 500) at an accelerating voltage of 15 kV. For preparation of the sample in the transmission electron microscope (TEM, JEM-2100, JEOL), the lacy carbon film on 300 mesh copper grid was used as a support. UV-Vis absorption spectra were recorded using a UV-Vis spectrophotometer (JASCO V-530 Instruments). The optical property of the prepared 3D nanostructures was investigated by the diffuse reflectance UV-Vis (DRS) using an Ava Spec- 2048TEC spectrophotometer. The relevant spectra were recorded on a UV–vis spectrometer (UV1901, Puxi) in the range of 300–700 nm with the resolution of 1 nm. In this analysis, the solid BaSO 4 slices were used as the reference and the slit width was set to 2 nm.

2.3 Methods The ternary structure of Ag (NPs)/ZnO (NRs)/3DG were prepared in four steps. Initially, the 3DG was obtained from graphene oxide (GO). Then, the ZnO nanorods were grown on the 3DG surface to prepare ZnO NRs/3DG. Finally, Ag nanoparticles were photodeposited on the coated ZnO NRs/3DG surface. 5

2.3.1 GO preparation Using by natural graphite powder, GO sheets were synthesized by modified Hummer’s method [45]. In brief, the graphite powder (2 g) was stirred with 12 mL H 2SO4 (98%) for 1h at 80 °C. After that, the mixture was placed in an ice-bath followed by the addition of 2 g NaNO 3 and 80 mL H 2SO4 and stirred for 10 min. Then, 5 wt. % KMnO4 was slowly added to the solution with vigorous stirring for additional 15 min. The green paste was then transferred to an oil bath (38°C) and stirred for another 1 h, subsequently, the solution was diluted with 160 mL deionized (DI) water and stirred and heated at 95°C for 30 min. Finally, the oxidation process was stopped by the addition of 400 mL DI water and 16 mL H 2O2 (30%). The orange-golden solution was filtered and washed with HCl and DI water solution in a 1:10 ratio. The filtered brown dough was dispersed in DI water and cleansed by centrifugation (by an Eppendorf 5702 centrifuge, rotor radius = 10 cm) at 2000 rpm for 15 min and 4000 rpm for 30 min to remove any un-exfoliated and tiny graphite plates, respectively. Finally, GO nanosheets were made by sonication (35 min) of filtered graphite oxide suspension by Sonorex Digitec, 100W; Bandelin, Germany-Berlin.

2.3.2 Preparation of three dimensional graphene (3DG) For the preparation of 3DG, ascorbic acid (32 mg) was added into a 4 mL of GO aqueous solution (1.5 mg/mL) as a reducing agent under vigorous stirring for 30 minutes. Then, 0.5 mL of this solution was transferred to a sealed vial and kept without any perturbation in 100 °C for 1 hour to form 3DG. In order to remove the unreacted ascorbic acid inside the resulting graphene hydrogel (3DG), it was washed several times with DI water and stored in DI water. Finally, the sample was dehydrated by freeze-drying for 24 h.

2.3.3 Preparation of ZnO NRs/3DG hybrid hydrogel The general reaction conditions were followed according to the previous reported method [46] based on the seeding process and nanorods growth. Briefly, in the seeding step, the 3DG network was immersed in 1 mM zinc acetate aqueous solution for 3 minutes and then, washed with ethanol. This procedure was repeated for four times. In the growth step, the 3DG bulk containing ZnO seeds was soaked into 25 mM zinc

6

nitrate and 25 mM HMTA aqueous solutions and was kept at 92 °C in an oil bath for two hours. Then, the sample was washed with DI water and ethanol and then, stored in DI water for preparing other steps.

2.3.4 Preparation of Ag NPs/ZnO NRs/3DG The photodeposition method was used for the formation of Ag NPs on the ZnO NR/3DG nanocomposite [47]. In this process, ZnO NRs/3DG hydrogel was immersed in 0.125 mM silver nitrate aqueous solution with N 2 purging for 45 minutes to adsorb Ag+ on the hydrogel surface. Then, for the reduction of Ag + to zerovalent Ag, the solution containing the hydrogel was irradiated by 15W UV-C lamp (Philips) for 10 minutes. The prepared sample (Ag NPs/ZnO NRs/3DG) was washed with DI water to remove the remaining Ag+ ions.

2.4 Dye adsorption experiment The adsorption capacity of synthesized 3DG vs. time was evaluated by the adsorption of MB in aqueous solution. To perform this experiment, prepared 3DG with mass of 0.3 mg was placed in 10 mL of MB solution with concentration of 10 ppm under air pumping in room temperature. At given time intervals, 2.5 mL of MB solution was drawn and the adsorbed concentration of MB was determined by recording the variation of the maximum absorption peak of MB at 664 nm. The mass of MB adsorbed per unit mass of the 3DG was obtained by using the following mass balance equation [48]: (1) where qt (mg/g) is the MB mass adsorbed per gram of adsorbent in the time t, C 0 is the initial concentration of MB in the solution (mg L -1) and Ct are the MB concentration in time t (mg L-1), m (g) is the mass of the absorbent used (i.e. 3DG) and V (L) is the volume of MB solution.

2.5 Photocatalytic activity test The photocatalytic performance of the prepared samples was measured by degradation of MB solution (10 ppm) under UV and visible light irradiation. To conduct this experiment, 0.3 mg of as-prepared photocatalyst was added into the prepared 10 mL MB solution (10 ppm). Due to the high capacity of the 3DG structure in the adsorption

7

of MB, the solution containing photocatalyst was stirred by air pumping in dark for 24 h to establish an adsorption-desorption equilibrium. Then, the samples were exposed to the UV irradiation (15 W UV-C, linear tube lamp) and visible irradiation (8 W, LED lamp) for photocatalytic activity under UV and visible light, respectively. The distance between lamp and samples was fixed at 5 cm for all experiments. At a given time interval, 2.5 mL of MB solution was drawn to analyze by recording the variation of the maximum absorption peak of MB at 664 nm. This wavelength is characteristic of the MB absorption by the π- conjugated system [5].

2.6 Specific surface area measurement MB is a cationic dye which forcefully absorbs in large amounts on the negatively charged materials such as, clays [49], GO and RGO [50,51]. Since the determination of specific surface area (SSA) of photocatalyst is of great importance in any catalytic process, herein, the amount of absorbed MB on the 3DG is used for calculation of SSA. For this purpose, 0.3 mg 3DG as an adsorbent was immersed in 20 mL MB solution (10 ppm) under air pumping in darkness for 27 hours. Then, the remnant concentration of MB in the solution was obtained from the UV-Vis absorption intensity at 664 nm. Finally, the SSA was estimated by using the following equation [5,51]:

(2)

where M MB is the molecular mass of MB (319.87 g), A v is Avogadro’s constant (6.02×1023 mol-1), AMB is the area covered by one MB molecule (estimated to be 130 Å2), C0 and Ce are the initial and equilibrium concentrations of MB, respectively, V is the volume of MB solution, and m s is the mass of the photocatalyst sample used in the measurement.

3 Results and discussion 3.1 The structure and morphology of the 3D network nanocomposite The overall preparation procedure of the ternary nanocomposite is illustrated in Scheme 1. In the first step, the 3DG network was synthesized via in-situ chemical reduction of dispersed GO solution using ascorbic acid as a mild reducing agent. In this

8

reaction, the oxygen functional groups on the surface of GO graphene sheets (such as alkoxy, carbonyl, carboxyl and epoxy) were locally decreased that resulted in the formation of a network-like structure through self-assembly of the graphene sheets based on π−π interaction between the reduced sheets [3]. The formation of graphene hydrogel network was verified by the color change from brown to black in the reaction mixture as well as the fabrication of well-defined and porous three dimensional black color composition as reported by others [52]. In the next step, ZnO nanorods were grown on the surface of the prepared 3DG network. For this purpose, the 3DG was initially immersed in the zinc acetate solution as a seeding source and the Zn +2 ions electrostatically adsorbed on the surface of 3DG due to the negative charge of partially reduced graphene sheets. The Zn 2+ coated on the 3DG network was placed in the growth solution of zinc nitrate and HMTA, as a known organic base which is widely used for the growth of aligned ZnO nanorods. Finally, in order to fabricate a visible active photocatalyst, Ag nanoparticles were formed on the surface of the ZnO (NRs)/3DG nanocomposite through photodeposition route using light -induced electrochemistry. Since the reduction potential of silver (~ 0.8 eV) is more positive than the conduction band of ZnO (-0.1 eV), the Ag photodeposition on the ZnO surface is favorable. Moreover, the photodeposition of Ag nanoparticles can further stabilize ZnO which is very useful in photocatalysis [53].

9

Scheme. 1. Representation of three basic synthesis stages of Ag nanoparticles/ZnO nanorods/3DG nanocomposites.

In order to determine the surface morphology of the deposited layers in the above stages, SEM was utilized for comparison. SEM image in the Fig. 1a shows the highly porous 3D interconnected graphene network of the prepared 3DG. In addition, according to the TEM observation presented in Fig. 2a, it is demonstrated that utilized

10

reduced graphene was formed from a few numbers of layers. To have a better insight, the specific surface area (SSA) of the as-prepared 3DG was evaluated by measuring the absorption capacity of MB on the 3DG. Using this method, the SSA of the prepared 3DG was estimated about 740 m 2/g using equation 2. This calculation is indicated that the formed 3DG network has a super high surface area with a large adsorption capacity for organic pollutants that made it an efficient adsorbent for water purification and photocatalysis. From the SEM images of ZnO nanorods/3DG (Fig 1b and c), it can be clearly seen that ZnO nanorods formed with a hexagonal structure and a “treelike” growth pattern. The hierarchical “treelike” structure of ZnO nanorods is advantageous and favorable for light scattering and reflection [42]. In addition, according to the SEM observations showing the formation of ZnO nanorods on the 3DG edges, it can be concluded that the initial nucleation step was preferentially occurred at the edges of graphene sheets where the charge density is higher. Hence, if higher concentration of zinc acetate was used in the seeding step, no more ZnO nanorods were grown on the surface of 3DG as reported by other researchers [44]. Thus, it might be concluded that the surface density of ZnO nanorods on the 3DG is limited through chemical impregnation and subsequent hydrothermal method. Figure 1d and e depict that after Ag photodeposition, the morphology of ZnO nanorods changed and became uneven, which is an evidence of the formation of silver nanoparticles on the ZnO nanorods. It is noteworthy to mention that with observation of Fig. 1d, the photodeposited Ag can also grow on the surface of ZnO as flake shape which was previously observed by other researchers as well [45,52]. To support the evidence of Ag formation on the ZnO nanorods, the energy dispersive X-ray (EDX) spectrum of the prepared nanocomposite was conducted for determining the elemental composition (Fig. 1f). As can be seen in this figure, only carbon, zinc, oxygen and silver were detected, which verify the high purity of the prepared samples and perfect formation of ternary structure. The signal of gold (Au) in the EDX spectrum comes from the deposited gold layer on the surface of the sample for SEM measurements. Additionally, as shown in Fig 1g, the qualitative EDX map analysis from ternary nanostructure is good evidence for photodeposition of Ag nanoparticles on both ZnO nanorods and 3DG surface.

11

Fig. 1. SEM images of the freeze dried a) 3DG, b, c) ZnO nanorods/3DG, d, e) Ag nanoparticles/ZnO nanorods/3DG nanocomposites, f) EDX spectrum of Ag nanoparticles/ZnO nanorods/3DG nanocomposites, g) one single ZnO nanorod on the 3DG surface in the Ag NPs/ZnO NRs/3DG sample along with the qualitative EDX 2D map analysis of different elements in the ternary nanostructure.

In order to determine the diameter of silver nanoparticles, we utilized TEM technique. As shown in Fig. 2b, ZnO nanorods are well fastened to the surface of graphene sheets. However, it should be noted that since the diameter of ZnO NRs are so high (≈ 400 nm) the electrons cannot pass through them in the TEM technique. Thus, the deposited Ag nanoparticles are only observable on graphene sheets (as also present in the EDX spectrum shown in Fig 1f) and not on the ZnO nanorods (Fig. 2c).

12

From the TEM images, the diameter of the Ag nanoparticles was obtained in the range of 30-80 nm.

Fig. 2. TEM images of a) 3DG, b) Ag nanoparticles/ZnO nanorods/3DG, and c) observed Ag nanoparticles on the 3DG surface.

Fig. 3. XRD patterns of a) 3DG, b) ZnO NRs/3DG and c) Ag NPs/ZnO NRs/3DG. The inset is the XRD spectrum of the prepared graphene oxide.

The XRD patterns of the synthesized samples are presented in Fig. 3. As can be seen, the diffraction peak of GO is appeared at ~11°, which is the characteristic peak of (001) reflection of GO nanosheets [54]. As a result, the interlayer distance of prepared GO was calculated at about 7.96 Å. After the reduction process of the GO and formation of 3D graphene structure, this peak shifts to 2θ ~ 22.40° (Fig 3b) due to the reduction of oxygenic groups on the GO surface and the restacking of graphene sheets through the π−π interaction between the reduced graphene oxide sheets. In further

13

discussion on the XRD pattern of the ZnO (NRs)/3DG, it was found that the characteristic diffraction peaks at 2θ of 31.5, 34.0 and 36.7° are related to (100), (002) and (101) facet of the hexagonal Wurtzite structure of ZnO (JCPDS 36-1451) [43]. In addition, the observed small peaks at 46.99, 56.63, 62.98, 67.91° are attributed to the (102), (110), (103), (112) planes of ZnO with a wurtzite structure [55]. The absence of Zn(OH)2 characteristic peaks is indicative of high purity of synthesized ZnO nanorods. XRD pattern of the Ag (NPs)/ZnO (NRs)/3DG displays a sharp peak at 38.14° which is attributed to the characteristic peak of (111) crystal plane of silver nanoparticles on the 3DG network and ZnO nanorods. The appearance of small peaks at 44.29 and 64.51° are in agreement with (200) and (220) planes of the face-centered cubic (FCC) of metallic silver, respectively (JCPDS 04-0783) [55]. The results showed that the purging by N 2 during the photodeposition process of Ag on ZnO, it inhibited the formation of AgO and/or Ag 2O compounds [53]. In both XRD patterns of the ZnO (NR)/3DG and Ag (NP)/ZnO (NR)/3DG, the characteristic peak of the reduced graphene sheets is clearly seen. Therefore, based on our XRD data analysis, intended 3D network with ternary structure is formed under optimum conditions. XPS measurements were carried out for precise characterization of surface chemical composition of prepared photocatalyst. The Full XPS survey scan of the Ag (NPs)/ZnO (NRs)/3DG sample was displayed in supporting information (Fig S1). From this spectrum, specified peaks attributed to the core level of the C (1s), Ag (3d), O (1s) and Zn (2p) can be clearly seen which confirm the presence of ZnO and Ag metal on the 3DG surface. The high resolution spectra of these elements were also shown in the Fig. 4. The strong peak of carbon (1s orbital) displays three characteristic peaks (Fig 4a). In the C1s spectrum, the peak at 284.8 eV is ascribed to the C-C bond existed in 3DG [56]. Two peaks at 286.7 and 289.5 eV are attributed to the C-O bonds (epoxy and hydroxyl) and (C=O)-O bond at carboxylate groups on the 3DG surface [56] The presence of carboxylate group in the prepared ternary nanostructure indicates that ZnO nanorods was connected to the 3DG surface in the form of Zn +2 ions and well hybridized on the surface. Such a junction efficiently enhances the electron transfer from ZnO nanorods to 3DG, which resulted in an effective separation of electron hole pairs [44]. Furthermore, two characteristic peaks of the Zn (2p) were observed at 1021.5 and 1044.8 eV are corresponding to the electrons in the 2p 3/2 and 2p1/2 orbitals, respectively, due to spin orbit splitting (Fig 4b). As can be shown in Fig 4c, the

14

appeared peaks at 367.5 and 373.5 eV are attributed to the binding energy of Ag metal, in accordance with binding energy of the Ag (3d 5/2) and Ag (3d3/2), respectively [18].

Fig. 4. XPS high resolution spectra of the Ag NPs/ZnO NRs/3DG structure: a) C 1s, b) Zn 2p, c) Ag 3d, and d) O 1s.

The appeared sharp peak of oxygen (1s orbital) displays three characteristic peaks (Fig 4d). In the O1s spectrum, the peak at 530.3 eV is ascribed to the metal oxide bonds existed in sample [56]. Two peaks at 531.7 and 533.2 eV are attributed to the CO-H bonds (hydroxyl) and (C=O) bond at carboxylate groups on the 3DG surface [56]. In addition, with evaluating XPS spectrum, the amount of deposited silver and zinc oxide on the surface of prepared nanocomposite was 1.3% and 6.1%, respectively. The optical properties of the ZnO NRs/3DG and Ag NPs/ZnO NRs/3DG nanostructures were studied by UV-Visible diffuse reflectance spectroscopy (DRS). As shown in the Fig. 5, the absorption region of the 3DG dominantly occurs in the visible spectrum. This absorption might be due to the internal scattering of the porous structure of 3D structure of grapheme which led to increase in the optical length. In the

15

other word, light was trapped in the pores of the 3DG and therefore, resulted in the increase of light absorption [56].

Fig. 5. UV–Vis diffuse reflectance spectra of the prepared samples. The inset is light absorption of the Ag/ZnO/3D (red) and the ZnO/3DG (blue) in a wavelength range from 400 to 500 nm.

In the ZnO NRs/3DG sample, the visible light absorption decreased considerably as compared to the 3DG, which can be due to the limited light-trapping mechanism due to the formation of the ZnO nanorods on the surface of 3DG. (Fig. 5). Furthermore, it was observed that the modified ZnO/3DG with silver nanoparticles (Ag NPs/ZnO NRs/3DG) has a higher absorbance in the visible region than ZnO/3DG. This phenomenon was also seen by the other researchers [18,57]. In the Ag NPs/ZnO NRs/3DG spectrum, the band appeared at about 370 nm is due to the band-edge absorption of ZnO nanorods. It seems that the band edge absorption of the Ag/ZnO/3DG sample in comparison to the ZnO/3DG exhibits a slight shift toward the smaller wavelength. Hence, it is expected that with the addition of Ag nanoparticles, the electron-hole recombination rate in the ternary nanostructure decreases and as a result the photocatalytic degradation efficiency increases. Considering the DRS spectra shown in the inset of Fig. 5, it is revealed that the Ag/ZnO/3DG has an absorption peak at about 450 nm, which be attributed to the

16

surface plasmon resonance (SPR) effect of silver nanoparticles present on the surface of ZnO/3DG nanostructure [18,58]. Moreover, the appeared broad SPR band is related to the wide size distribution of the prepared Ag nanoparticles. The plasmon absorption band of silver nanoparticles is sensitive to the surroundings and can be shifted based on an applied substrate [56]. The other small absorption peaks observed at about 520 and 630 nm was assigned to the 3DG network structure used as a support in our study.

3.2 Study of MB adsorption kinetics on 3DG structure The adsorption capacity of graphene hydrogel was investigated by MB adsorption capacity on 3DG. Methylene blue is a cationic dye which can be strongly absorbed on the negatively charged surface of graphene sheets due to the electrostatic interaction and π−π stacking interactions between them [50]. The adsorption capacity of the synthesized 3DG is relatively high (~310 mg/g) which is ascribed to its large surface area of 740 m 2/g. This large adsorption capacity of the 3DG can efficiently enhance the photodegradation performance of adsorbed pollutants [42]. It is necessary to note that we have measured adsorption capacity of the Ag/ZnO/3DG system in separate experiments and found that the amount of its adsorption capacity of about 300 mg/g which is similar to that of the 3DG structure. The adsorption capacity of 3DG vs. time was displayed in Fig. 6a. The concentration of dye in the solution was calculated by Beer’ law equation based on maximum adsorption peak for MB at 664 nm. The pseudo-second-order (equation 3) and the intraparticle diffusion model (equation 4) were applied to evaluate the adsorption process [48]:

(3) (4) where k2 is the pseudo-second-order rate constant (g mg-1 min-1), qt and qe (mg g-1) are the amount of adsorbed MB per unit mass of 3DG at equilibrium time and later time t (min), respectively. In the equation 4, k i is the intraparticle diffusion rate constant (mg g-1 min-1/2), C is the constant depicting the boundary layer effects.

17

Fig. 6. a) The effect of contact time, b) pseudo-second-order and c) interparticle diffusion models of MB adsorption on the 3DG

By plotting t/qt against MB adsorption time by 3DG, a straight line with good correlation coefficient (R 2) was obtained (Fig. 6b), indicating that the adsorption kinetics agrees well with the pseudo-second-order model. Besides, for detecting the steps involved during adsorption process, the intraparticle diffusion model can be used (equation 4). According to this model, if the obtained plot from adsorption capacity (q t) vs. the square root of time (t 1/2) be a straight line, the interparticle diffusion can be a rate-controlling factor for diffusion [48,59]. As shown in Fig. 6c, the obtained plot from this model is linear, indicating the diffusion is involved step in the adsorption process and diffusion through interparticles can be the rate-controlling factor. The obtained results and parameters from these models and correlation coefficients which were measured by linear regression (Fig. 6b, c) are given in Table 1.

Table 1. Kinetic parameters of the MB adsorption over the 3DG structure. Intraparticle diffusion model

Pseudo-second-order kinetic R2

C0

k2

qe,exp

qe,cal

(mg L-1)

(g mg-1 min-1)

(mg g-1)

(mg g-1)

10

3.5 × 10-5

308.3

333.3

0.9918

ki

C

(mg g-1 min1/2)

(mg g-1)

15.7

8.5

R2

0.9937

3.3 Photocatalytic activity investigation In order to study photocatalytic

activity of different graphene hydrogel

nanocomposites under UV and visible light irradiation, degradation and decolorization of methylene blue (MB), as a standard sample compound, were conducted in a similar experimental condition. To examine this phenomenon, the solution containing 18

photocatalyst was maintained in dark for 24 h with stirring and air pumping to establish an adsorption-desorption equilibrium and then monitoring the MB absorption peak for each sample (Fig. S2). Experimentally, it has been well established that the photodegradation kinetic of organic pollutants such as dyes can be expressed with a first-order Langmuir−Hinshelwood (LH) model as shown in the following equation [15]: (5)

where k is the first-order rate constant of photocatalytic degradation reaction and C 0 and C are the concentration of MB at initial time t 0 and later time t, respectively. The ln(C0/C) vs. the irradiation time of the synthesized samples including the 3DG, ZnO NRs/3DG and Ag NPs/ZnO NRs/3DG were plotted to evaluate the photocatalytic degradation rate constant (slope of the curves) under UV and visible irradiation (Fig. 7a and b). As can be seen, the obtained curves are nearly linear which indicates that the MB photodegradation can be fitted well and explained by the LH kinetic model. Obviously, as shown in Fig 7a, the photodegradation of MB under UV light irradiation without any photocatalysts is insignificant during the photolysis (k = 0.02 × 10 -2 min-1) and MB did not self-decompose under UV illumination. As a result, the degradation of MB solution under UV irradiance can be ascribed to the photocatalytic activity of the prepared photocatalysts. The results revealed that the photocatalytic efficiency of the Ag NPs/ZnO NRs/3DG nanocomposite is higher than the ZnO NRs/3DG and 3DG under UV light. The UV-light driven photodegradation of MB by the Ag NPs/ZnO NRs/3DG nanocomposite was also studied for three runs (Fig S3). It is well established that the stability of samples after various photodegradation runs depends on the nature of photogenerated electron-hole pairs origenated from the photocatalyst (i.e. ZnO nanorods) [60], however, in this study, the ZnO nanorods were coated on the surface of 3DG which has high adsorption capacity (~310 mg/g). As a result, the photodegradation and adsorption were simultaneously took part in elimination of MB from the solution. During photoirradiation, for higher runs, the 3DG adsorption capacity decreased (due to the saturation phenomenon or occupied active sites), and as a result, the photodagardation efficiency reduced. Hence, the stability of the prepared sample couldn’t be determined exactly. In other words, in higher runs, both number of photogenerated electron-hole pairs and adsorption capacity influenced the reduction of MB elimination from the solution under photoirradiation conditions.

19

Therefore, we could not precisely determine the stability of samples due to the nature of ZnO nanorods, which was described above. To obtain a better understanding on this phenomenon, the adsorption and desorption isotherm of MB can be conducted to provide additional information. The adsorption and desorption isotherm of MB is a well-studied concept which have been fully addressed by various research groups [61-64]. But, this investigation is not included in the present work. However, in order to provide further information, the adsorption-desorption isotherm of the photocatalysts (i.e. Ag NPs/ZnO NRs/3DG nanocomposite) should be fully studied in our future work. Figure. 7b shows the MB photodegradation kinetics of different samples under the visible light. The decolorization rates of MB under visible light for the ZnO NRs/3DG and 3DG are in the same order of magnitude for absence of a photocatalyst (photolysis) which is related to the wide band gap energy of ZnO (3.37 eV) that cannot absorb the visible light and generate electron-hole pair. However, the results revealed that the photocatalytic efficiency of the Ag NPs/ZnO NRs/3DG nanocomposite is more than the ZnO NRs/3DG and 3DG under visible illumination. For instance, the measured k value of the ZnO/3DG under visible light was 0.62×10 -2 min-1, which was increased to 0.89×10-2 min-1 in the case of the Ag/ZnO/3DG ternary structure network. According to the above results, the photocatalytic efficiency of the prepared ternary sample displays an enhancement of ca. 43% under visible illumination and also ca. 40% under UV light in comparison to the ZnO NRs/3DG, which is due to the presence and role of the Ag nanoparticles on the network surface. This noticeably discloses that loading Ag nanoparticles as co-catalyst on the surface of ZnO nanorods led to a considerable improvement in the photocatalyst efficiency under the UV light due to effective charge carrier transfer through Ag NPs, which caused retardation in electronhole recombination that leads to enhance photocatalytic activity. The role of Ag NPs as an electron sink or trap to improve the photocatalytic performance was also reported by the other researchers [10,42]. Moreover, the localized surface plasmon resonance (SPR) effect of Ag nanoparticles, which was confirmed by the DRS spectra, indicated the visible light photoactivity of the ternary nanocomposite. The enhancement in the photocatalytic activity under the visible light due to the SPR effect of the Ag nanoparticles was also demonstrated by the other groups [16,44,58].

20

Fig. 7. Variation of ln(C0/C) vs light exposure time under a) UV and b) visible irradiation for the 3DG, ZnO NRs/3DG, Ag NPs/ZnO NRs/3DG as compared to the blank. c) The decolonization of MB by Ag NPs/ZnO NRs/3DG photocatalyst (with a mass of 0.3 mg) under UV or visible light irradiation.

3.4 Proposed mechanism of photodegradation To elucidate the mechanism of photocatalytic reaction on the surface of the Ag NPs/ZnO NRs/3DG network, the influence of various charge carrier scavengers on the photodegradation efficiency of MB was studied under both UV and visible light irradiation. For this purpose, potassium iodide (KI) as both hole (h +) and hydroxyl radical (•OH) scavenger [5,14] as well as isopropyl alcohol (IPA) as an •OH scavenger [5,65] were used. Moreover, to analyze the effect of dissolved oxygen in the photodegradation reaction, the MB decomposition was carried out under pure nitrogen gas purging. Figure 8a and b shows the effect of scavenger presence and N 2 purging on

21

the k values for the MB degradation on the Ag/ZnO/3DG network under UV and visible light, respectively.

Fig 8. Degradation rate constants of MB on the Ag/ZnO/3DG nanocomposite in the presence of different scavengers and N 2 purging under UV (a) and visible (b) irradiation.

Figure. 8a shows that the photodegradation rate of MB under UV light irradiation decreases in the presence of IPA as compared to the scavenger-free condition. Since, IPA has been known as a •OH scavenger, it can be concluded that •OH species produced via hole-induced water oxidation, has a key role in the photocatalytic degradation reaction. Moreover, the rate constant was reduced dramatically by the addition of KI, as both •OH and holes scavengers. Thus, it is revealed that for photodegradation reaction not only hydroxyl radicals are essential, but also the holes play an effective role. In addition, to specify the role of superoxide radical (O 2•-) species produced through the reaction of photogenerated electron and dissolved oxygen, the rate constant of photodegradation was also measured during N 2 purging and absence of dissolved oxygen. It was observed that with elimination of O 2 species,

22

the photocatalytic efficiency reduced indicating the significant role of O 2 in photodegradation reaction of MB. The photodegradation rate constants of the Ag NPs/ZnO NRs/3DG in the presence of various scavengers under visible illumination showed a totally different trend compared to that of UV light irradiation (Fig 8a). As observed in Fig. 8b, by the addition of IPA, the photodecomposition of MB increased. Therefore, it can well deduce that the hydroxyl radicals do not have any significant role in the phot ocatalytic degradation reaction. This result can be further confirmed by studying the photodegradation rate with the addition of KI. It was revealed that in the presence of KI, the rate constant did not show any meaningful change in comparison to scavengerfree condition, which indicated that under visible light irradiation, the photogenerated holes (h+) and hydroxyl radicals do not have a strong effect on the decomposition of MB. Moreover, the photodegradation rate constant in the N 2 gas purging and absence of O2 showed that it decreased as compared to air saturated solution (no scavenger condition). This indicated that the O 2•- species have a decisive role in photodegradation reaction of MB on the Ag/ZnO/3DG under visible light irradiation.

Scheme 2. A proposed mechanism of charge transfer and photodegradation of MB dye on the Ag/ZnO/3DG surface under visible (a) and UV (b) light.

Considering

the

above-mentioned

discussions

and

analysis,

the

overall

photodegradation mechanisms under visible and UV light irradiation are illustrated in the Scheme 2a and b, respectively. As previously reported by some research groups, the difference in the work function of Ag (4.26 eV) and ZnO (5.2 eV) resulted in the formation of a Schottky barrier at the interface of metal (Ag) and semiconductor (ZnO) [16,66,67]. Therefore, an equilibrium Fermi level (E eq) could be created by direct 23

electron transfer from Ag (with the lower work function) to ZnO (with the higher work function) [16,66]. This newly formed Fermi level produced an electric field close to the interface that prevents the recombination of the photogenerated charges [60]. When the Ag/ZnO/3DG sample is exposed to the visible light, the electrons produced due to the SPR effect of the Ag NPs could be rapidly excited from Eeq to the conduction band (CB) of ZnO nanorods. The existing electrons in the CB of ZnO transfer to conductive network of 3DG and can react with dissolved oxygen to generate superoxide radicals (O2•-). Subsequently, these reactive oxygen species (O 2•-) leads to the degradation of MB. The overall photodegradation reactions under visible light are described as following:

Considering the above reactions, production of CO 2 is indicative of degradation of MB over the photocatalyst. It is worth noted that after photoirradiation of MB solution in the presence of the Ag NPs/ZnO NRs/3DG photocatalyst for 210 minutes, we didn’t observe any additional absorption peak and shift (Fig. S4). Moreover, as can be clearly seen in Fig. S5, the full scan absorption (from 200 to 750 nm) from the photodegraded solution of MB only indicates two characteristic peaks of MB at 664 and 295 nm [68]. We can conclude that under the aforementioned conditions and using the Ag NPs/ZnO NRs/3DG photocatalyst material, MB degraded to CO2 and water and no other kind of colorless organic molecules was formed. A similar results was also reported by others recently [16,68,69]. In other word, if another kind of colorless organic molecule was formed, the observed absorption peak must indicate a clear shift [70]. A similar charge generation and transfer were also reported by P. Fageria et al for the ZnO/Au and ZnO/Ag nanoparticle systems [16]. On the other hand, upon photoirradiation with UV light, the valence band (VB) electrons of the ZnO nanorods were excited to conduction band (CB) and thus, the holes (h +) were generated and remained in the VB. Then, the excited electrons in CB band could be transferred to the Ag nanoparticles as an electron sink and/or 3DG as an electron channel. This electron

24

transfer is a reason of the conductivity of 3DG and the electron capturing ability of Ag nanoparticles, which could effectively hinder the electron-hole recombination in the ZnO nanorods and thus improve the photocatalytic performance. In an another scenario, the transferred electrons from the CB of ZnO to Ag nanoparticles and 3DG reacted with dissolved oxygen to generate superoxide radicals (O 2•-) on the surface of 3D network. Similar to the visible light condition, O 2•- species decompose the MB molecules. Furthermore, the holes in the VB of ZnO react with H 2O to form the hydroxide radicals leading to degrade the MB dye. As a result, unlike the visible light, upon the UV irradiation, both holes and electrons have a strong effect on the complete degradation. The overall photodegradation chemical reactions under UV irradiation are summarized below:

Based on the above reactions, the toxic dye methylene blue is converted to safe compounds (CO 2 and H2O) under UV irradiation.

4. Conclusions Ternary nanostructure network based on the Ag NPs/ZnO NRs/3DG was successfully fabricated using facile and green method for removing organic dyes from waste water. The crystal structure of the prepared samples was verified by means of XRD. According to both SEM and TEM observations, it was determined that the prepared 3DG network is composed of few layer graphene sheets on which the ZnO nanorods are well deposited. Moreover, the hydrothermal growth method applied for the ZnO nanorods fabrication on the edge of the 3DG network with a length and an average diameter of 1.6 µm and 300 nm, respectively. Furthermore, it was also measured that the deposited Ag nanoparticles had an average diameter of 60 nm. It was found that the deposition of Ag nanoparticles onto the surface of ZnO/3DG enhanced the photocatalytic efficiency under UV light irradiation due to the electron capturing 25

properties of these nanoparticles. In addition, the SPR effect of the Ag NPs enhanced photocatalytic activity with higher rate in the Ag NPs/ZnO NRs/3DG network under visible light. Apart from the role of Ag NPs in the improved photocatalytic performance under UV and visible light, the 3DG as a support had three impressive properties on the purification of wastewater: I) High electronic conductivity of graphene to transfer the photogenerated electron and retardation in electron-hole recombination, II) High adsorption capacity of organic dyes due to the formation of strong π−π interaction between them, III) Allow easy separation and recovery of the photocatalyst after water treatment. Several charge carrier scavengers were utilized to determine the photodegradation mechanism of MB dye. It was found that upon the visible light irradiation, the superoxide radicals are responsible for photodegradation of MB under UV light irradiation. In addition, it was determined the both superoxide radicals and hydroxyl radicals are the responsible species for photocatalytic dye degradation. Finally, due to the high adsorption capacity of organic materials and visible light photocatalytic activity of the prepared Ag NP/ZnO NR/3DG network, it can be utilized for efficient removal of organic contaminants from industrial wastewater under solar illumination.

Acknowledgements The authors would like to thank Research Council of Sharif University of Technology for financial support and the Iran Science Elites Federation (Grant of the top 100 national science elites). AZM also thanks the Iran National Science Foundation (INSF) for the financial support through a Grant No. 96009153 and Research Chair Award of Surface and Interface Physics (Grant No. 940009).

References 1) Lee, K. M.; Lai, C. W.; Ngai, K. S.; Juan, J. C. Recent Developments of Zinc Oxide Based Photocatalyst in Water Treatment Technology: A Review. Water Res. 2016, 88, 428-448. 2) Dhandole, L. K.; Kim, S. G.; Seo, Y. S.; Mahadik, M. A.; Chung, H. S.; Lee, S. Y.; Choi, S. H.; Cho, M.; Ryu, J.; Jang, J. S. Enhanced Photocatalytic Degradation of Organic Pollutants and Inactivation of Listeria Monocytogenes by Visible Light Active Rh–Sb Codoped TiO2 Nanorods. ACS Sustain. Chem. Eng. 2018, 6, 4302-4315.

26

3) Li, Y.; Cui, W.; Liu, L.; Zong, R.; Yao, W.; Liang, Y.; Zhu, Y. Removal of Cr (VI) by 3D TiO2-Graphene Hydrogel via Adsorption Enriched with Photocatalytic Reduction. Appl. Catal. B Environ. 2016, 199, 412-423. 4) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. 5) Naseri, A.; Samadi, M.; Mahmoodi, N. M.; Pourjavadi, A.; Mehdipour, H.; Moshfegh, A. Z. Tuning Composition of Electrospun ZnO/CuO Nanofibers: Toward Controllable and Efficient Solar Photocatalytic Degradation of Organic Pollutants. J. Phys. Chem. C. 2017, 121, 3327-3338. 6) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. (1995). Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. 7) Samadi, M.; Zirak, M.; Naseri, A.; Khorashadizade, E.; Moshfegh, A. Z. Recent Progress on Doped ZnO Nanostructures for Visible-Light Photocatalysis. Thin Solid Films 2016, 605, 219. 8) Pawar, R. C.; Cho, D.; Lee, C. S. Fabrication of Nanocomposite Photocatalysts from Zinc Oxide Nanostructures and Reduced Graphene Oxide.Curr. Appl. Phys. 2013, 13, 50-57. 9) Cheng, Y. F.; Jiao, W.; Li, Q.; Zhang, Y.; Li, S.; Li, D.; Che, R. Two Hybrid Au-ZnO Aggregates with Different Hierarchical Structures: A Comparable Study in Photocatalysis. J.Colloid Interf. Sci. 2018, 509, 58-67. 10) Kuriakose, S.; Choudhary, V.; Satpati, B.; Mohapatra, S. Facile Synthesis of Ag–ZnO Hybrid Nanospindles for Highly Efficient Photocatalytic Degradation of Methyl Orange. Phys. Chem. Chem. Phys. 2014, 16, 17560-17568. 11) Samadi, M.; Shivaee, H. A.; Zanetti, M.; Pourjavadi, A.; Moshfegh, A. (2012). Visible Light Photocatalytic Activity of Novel MWCNT-Doped ZnO Electrospun Nanofibers. J. Molecul. Catal. A: Chem. 2012, 359, 42-48. 12) Samadi, M.; Pourjavadi, A.; Moshfegh, A. Z. Role of CdO Addition on the Growth and Photocatalytic Activity of Electrospun ZnO Nanofibers: UV vs. Visible Light. Appl. Surf. Sci. 2014, 298, 147-154. 13) Nourmohammadi, A.; Rahighi, R.; Akhavan, O.; Moshfegh, A. (2014). Graphene Oxide Sheets Involved in Vertically Aligned Zinc Oxide Nanowires for Visible Light Photoinactivation of Bacteria. J. Alloys Comp. 2014, 612, 380-385. 14) Samadi, M.; Shivaee, H. A.; Pourjavadi, A.; Moshfegh, A. Z. Synergism of Oxygen Vacancy and Carbonaceous Species on Enhanced Photocatalytic Activity of Electrospun ZnO-Carbon Nanofibers: Charge Carrier Scavengers Mechanism. Appl. Catal. A: Gen. 2013, 466, 153-160. 15) Ebrahimi, M.; Samadi, M.; Yousefzadeh, S.; Soltani, M.; Rahimi, A.; Chou, T. C.; LiChyong, C.; Kuei-Hsien, C.; Moshfegh, A. Z. Improved Solar-Driven Photocatalytic Activity

27

of Hybrid Graphene Quantum Dots/ZnO Nanowires: A Direct Z-Scheme Mechanism. ACS Sustain. Chem. Eng. 2016, 5, 367-375. 16) Fageria, P.; Gangopadhyay, S.; Pande, S. (2014). Synthesis of ZnO/Au and ZnO/Ag Nanoparticles and their Photocatalytic Application Using UV and Visible Light. Rsc Adv. 2014, 4, 24962-24972. 17) Ta, Q. T. H., Park, S., Noh, J. S. Ag Nanowire/ZnO Nanobush Hybrid Structures for Improved Photocatalytic Activity. J. Colloid Interf. Sci., 2017, 505, 437-444. 18) Kandula, S.; Jeevanandam, P. (2015). Sun-Light-Driven Photocatalytic Activity by ZnO/Ag Heteronanostructures Synthesized via A Facile Thermal Decomposition Approach. RSC Adv. 2015, 5, 76150-76159. 19) Gu, H.; Yang, Y.; Tian, J.; Shi, G. Photochemical Synthesis of Noble Metal (Ag, Pd, Au, Pt) on Graphene/ZnO Multihybrid Nanoarchitectures as Electrocatalysis for H 2O 2 Reduction. ACS Appl. Mater. Interf. 2013, 5, 6762-6768. 20) Raji, R.; Sibi, K. S.; Gopchandran, K. G. ZnO: Ag Nanorods as Efficient Photocatalysts: Sunlight Driven Photocatalytic Degradation of Sulforhodamine B. Appl. Surf. Sci . 2018, 427, 863-875. 21) Chen, X.; Li, Y.; Pan, X.; Cortie, D.; Huang, X.; Yi, Z. Photocatalytic Oxidation of Methane over Silver Decorated Zinc Oxide Nanocatalysts. Nature Commun. 2016, 7, 12273. 22) She, P., Xu, K., Yin, S., Shang, Y., He, Q., Zeng, S., Sun; Liu, Z. Bioinspired Self-standing Macroporous Au/ZnO Sponges for Enhanced Photocatalysis. J. Colloid Interf. Sci. 2018, 514, 40-48. 23) He, W.; Kim, H. K.; Wamer, W. G.; Melka, D.; Callahan, J. H.; Yin, J. J. Photogenerated Charge Carriers and Reactive Oxygen Species in ZnO/Au Hybrid Nanostructures with Enhanced Photocatalytic and Antibacterial Activity. J. Am. Chem. Soc., 2013, 136, 750-757. 24) Yu, C.; Yang, K.; Xie, Y.; Fan, Q.; Jimmy, C. Y.; Shu, Q.; Wang, C. Novel Hollow Pt -ZnO Nanocomposite Microspheres with Hierarchical Structure and Enhanced Photocatalytic Activity and Stability. Nanoscale 2013, 5, 2142-2151. 25) Dai, K.; Dawson, G.; Yang, S.; Chen, Z.; Lu, L. Large Scale Preparing Carbon Nanotube/Zinc Oxide Hybrid and Its Application for Highly Reusable Photocatalyst. Chem. Eng. J. 2012, 191, 571-578. 26) Mu, J.; Shao, C.; Guo, Z.; Zhang, Z.; Zhang, M.; Zhang, P.; Chen, B.; Liu, Y. High Photocatalytic Activity of ZnO− Carbon Nanofiber Heteroarchitectures. ACS Appl. Mater. Interf. 2011, 3,590-596. 27) Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Enhancement of Photocurrent and Photocatalytic Activity of ZnO Hybridized with Graphite-Like C 3N4. Energ. Environ. Sci., 2011, 4(8), 2922-2929.

28

28) Zhang, L.; Du, L.; Yu, X.; Tan, S.; Cai, X.; Yang, P.; Gu, Y.; Mai, W. Significantly Enhanced Photocatalytic Activities and Charge Separation Mechanism of Pd-Decorated ZnO–Graphene Oxide Nanocomposites. ACS Appl. Mater. Interf. 2014, 6, 3623-3629. 29) Qiu, B.; Xing, M.; Zhang, J. Recent Advances in Three-Dimensional Graphene Based Materials for Catalysis Applications. Chem Soc. Rev. 2018, 47, 2165-2216. 30) Li, X., Wang, Q., Zhao, Y., Wu, W., Chen, J., Meng, H. Green Synthesis and Photocatalytic Performances for ZnO-reduced Graphene Oxide Nanocomposites. J. Colloid Interf. Sci. 2013, 411, 69-75. 31) Qiu, B.; Xing, M.; Zhang, J. Mesoporous TiO 2 Nanocrystals Grown in Situ on Graphene Aerogels for High Photocatalysis and Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5852-5855. 32) Chen, X.; Chen, Q.; Jiang, W.; Wei, Z.; Zhu, Y. Separation-Free TiO2-Graphene Hydrogel with 3D Network Structure for Efficient Photoelectrocatalytic Mineralization. Appl. Catal. B Environ. 2017, 211, 106-113. 33) Dong, C.; Lu, J.; Qiu, B.; Shen, B.; Xing, M.; Zhang, J. Developing Stretchable and Graphene-Oxide-Based Hydrogel for the Removal of Organic Pollutants and Metal Ions. Appl. Catal. B Environ. 2018, 222, 146-156. 34) Nawaz, M., Miran, W., Jang, J., & Lee, D. S. One-Step Hydrothermal Synthesis of Porous 3D Reduced Graphene Oxide/TiO 2 Aerogel for Carbamazepine Photodegradation in Aqueous Solution. Appl. Catal. B Environ. 2017, 203, 85-95. 35) Zhang, Z.; Xiao, F.; Guo, Y.; Wang, S.; Liu, Y. One-Pot Self-Assembled Three-Dimensional TiO2-Graphene Hydrogel with Improved Adsorption Capacities and Photocatalytic and Electrochemical Activities. ACS Appl. Mater. Interf. 2013, 5, 2227-2233. 36)

Tong, Z.; Yang, D.; Shi, J.; Nan, Y.; Sun, Y.; Jiang, Z. Three-Dimensional Porous

Aerogel Constructed by g-C3 N4 and Graphene Oxide Nanosheets with Excellent VisibleLight Photocatalytic Performance. ACS Appl. Mater. Interf. 2015, 7, 25693-25701. 37)

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. 38) by

Chen, F.; An, W.; Liu, L.; Liang, Y.; Cui, W. Highly Efficient Removal of Bisphenol A A

Three-Dimensional

Graphene

Hydrogel-AgBr@

RGO

Exhibiting

Adsorption/Photocatalysis Synergy. Appl. Catal. B Environ. 2017, 217, 65-80. 39)

Mu, C.; Zhang, Y.; Cui, W.; Liang, Y.; Zhu, Y. Removal of Bisphenol A over A

Separation Free 3D Ag3PO4-Graphene Hydrogel via an Adsorption-Photocatalysis Synergy. Appl. Catal. B Environ. 2017, 212, 41-49.

29

40)

Fan, C.; Liu, Q.; Ma, T.; Shen, J.; Yang, Y.; Tang, H.; Wang, Y.; Yang, J. Fabrication

of 3D CeVO 4/Graphene Aerogels with Efficient Visible-Light Photocatalytic Activity. Ceram. Int. 2016, 42, 10487-10492. 41)

Gao, P.; Liu, Z.; Sun, D. D. The Synergetic Effect of Sulfonated Graphene and Silver

as Co-Catalysts for Highly Efficient Photocatalytic Hydrogen Production of ZnO Nanorods. J. Mater. Chem. A. 2013, 1, 14262-14269. 42)

Gao, P.; Ng, K.; Sun, D. D. Sulfonated Graphene Oxide–ZnO–Ag Photocatalyst for

Fast Photodegradation and Disinfection under Visible Light. J. Hazard Mater. 2013, 262, 826-835. 43)

Yoo, D. H.; Cuong, T. V.; Luan, V. H.; Khoa, N. T.; Kim, E. J.; Hur, S. H.; Hahn, S. H.

(2012). Photocatalytic Performance of a Ag/ZnO/CCG Multidimensional Heterostructure Prepared by a Solution-Based Method. J. Phys. Chem. C, 2012, 116(12), 7180-7184. 44) Wei,

Yang, T.H.; Harn, Y.W.; Huang, L.D.; Pan, M.Y.; Yen, W.C.; Chen, M.C.; Lin, C.C.; P.K.;

Chueh,

Y.L.;

Wu,

J.M.

Fully

Integrated

Ag

Nanoparticles/ZnO

Nanorods/Graphene Heterostructured Photocatalysts for Efficient Conversion of Solar to Chemical Energy. J. Catal. 2015, 329, 167-176. 45)

Hummers Jr, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc.

1985, 80, 1339-1339. 46)

Yu, M.; Ma, Y.; Liu, J.; Li, X.; Li, S.; Liu, S. Sub-Coherent Growth of ZnO Nanorod

Arrays on Three-Dimensional Graphene Framework as One-Bulk High-Performance Photocatalyst. Appl. Surf. Sci. 2016, 390, 266-272. 47)

Liu, Y.; Wei, S.; Gao, W. Ag/ZnO Heterostructures and their Photocatalytic Activity

under Visible Light: Effect of Reducing Medium. J. Hazard. Mater. 2015, 287, 59-68. 48)

Ai, L.; Jiang, J. Removal of Methylene Blue from Aqueous Solution with Self-

Assembled Cylindrical Graphene–Carbon Nanotube Hybrid. Chem. Eng. J. 2012, 192, 156163. 49)

Yukselen, Y.; Kaya, A. (2008). Suitability of the Methylene Blue Test for Surface

Area, Cation Exchange Capacity and Swell Potential Determination of Clayey Soils. Eng. Geo. 2008, 102, 38-45. 50)

Aboutalebi, S.H.; Jalili, R.; Esrafilzadeh, D.; Salari, M.; Gholamvand, Z.; Aminorroaya

Yamini, S.; Konstantinov, K.; Shepherd, R.L.; Chen, J.; Moulton, S.E.; Innis, P.C. HighPerformance Multifunctional Graphene Yarns: Toward Wearable All-Carbon Energy Storage Textiles. ACS nano 2014, 8(3), 2456-2466. 51)

Yang, C.; Shen, J.; Wang, C.; Fei, H.; Bao, H.; Wang, G. All-Solid-State Asymmetric

Supercapacitor Based on Reduced Graphene Oxide/Carbon Nanotube and Carbon Fiber Paper/Polypyrrole Electrodes. J. Mater. Chem. A 2014, 2, 1458-1464.

30

52)

Sui, Z.; Zhang, X.; Lei, Y.; Luo, Y. (2011). Easy and Green Synthesis of Reduced

Graphite Oxide-Based Hydrogels. Carbon 2011, 49, 4314-4321. 53)

Wenderich, K.; Mul, G. Methods, Mechanism, and Applications of Photodeposition in

Photocatalysis: A Review. Chem. Rev. 2016, 116, 14587-14619. 54)

Kheirabadi, M.; Bagheri, R.; Kabiri, K.; Ossipov, D. A.; Jokar, E.; Asadian, E.

Improvement in Mechanical Performance of Anionic Hydrogels Using Full‐Interpenetrating Polymer Network Reinforced with Graphene Oxide Nanosheets. Adv. Polym. Techn. 2016, 35, 386-395. 55)

Li, Z.; Sheng, L.; Meng, A.; Xie, C.; Zhao, K. A Glassy Carbon Electrode Modified

with A Composite Consisting of Reduced Graphene Oxide, Zinc Oxide and Silver Nanoparticles in A Chitosan Matrix for Studying the Direct Electron Transfer of Glucose Oxidase and for Enzymatic Sensing of Glucose. Microchim. Acta 2016, 183, 1625-1632. 56)

Moussa, H.; Girot, E.; Mozet, K.; Alem, H.; Medjahdi, G.; Schneider, R. ZnO

Rods/Reduced Graphene Oxide Composites Prepared via A Solvothermal Reaction for Efficient Sunlight-Driven Photocatalysis. Appl. Catal. B Environ, 2016, 185, 11-21. 57)

Grabowska, E.; Zaleska, A.; Sorgues, S.; Kunst, M.; Etcheberry, A.; Colbeau-Justin, C.;

Remita, H. Modification of Titanium (IV) Dioxide With Small Silver Nanoparticles: Application in Photocatalysis. J. Phys. Chem. C, 2013, 117, 1955-1962. 58)

Wang, K.; Wu, X.; Zhang, G.; Li, J.; Li, Y. Ba 5Ta 4O15 Nanosheet/AgVO 3 Nanoribbon

Heterojunctions with Enhanced Photocatalytic Oxidation Performance: Hole Dominated Charge Transfer Path and Plasmonic Effect Insight. ACS Sustain. Chem. Eng. 2018, 6, 66826692. 59)

Liu, F.; Chung, S.; Oh, G.; Seo, T. S. Three-Dimensional Graphene Oxide

Nanostructure for Fast and Efficient Water-Soluble Dye Removal. ACS appl. Mater. Interf. 2012, 4, 922-927. 60)

Xu, K.; Wu, J.; Tan, C. F.; Ho, G. W.; Wei, A.; Hong, M. Ag–CuO–ZnO Metal–

Semiconductor Multiconcentric Nanotubes for Achieving Superior and Perdurable Photodegradation. Nanoscale, 2017, 9(32), 11574-11583. 61)

He, X.; Male, K. B.; Nesterenko, P. N.; Brabazon, D.; Paull, B.; Luong, J. H.

Adsorption and Desorption of Methylene Blue on Porous Carbon Monoliths and Nanocrystalline Cellulose. ACS Appl. Mater. Interf. 2013, 5(17), 8796-8804. 62)

Ren, F.; Li, Z.; Tan, W. Z.; Liu, X. H.; Sun, Z. F.; Ren, P. G.; Yan, D. X. Facile

Preparation of 3D Regenerated Cellulose/graphene Oxide Composite Aerogel with High-efficiency Adsorption Towards Methylene Blue. J. Colloid Interf. Sci. 2018, 532, 58-67.

31

63)

Chen, L.; Li, Y.; Du, Q.; Wang, Z.; Xia, Y.; Yedinak, E.; Lou, J.; Ci, L. High

Performance Agar/Graphene Oxide Composite Aerogel for Methylene Blue Removal. Carbohydr. Polym. 2017, 155, 345-353. 64)

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. 65)

Wang, D.; Guo, L.; Zhen, Y.; Yue, L.; Xue, G.; Fu, F. AgBr Quantum Dots Decorated

Mesoporous Bi2 WO 6 Architectures with Enhanced Photocatalytic Activities for Methylene Blue. J. Mater. Chem. A 2014, 2, 11716-11727. 66)

Sangpour, P.; Hashemi, F.; Moshfegh, A. Z. Photoenhanced Degradation of Methylene

Blue on Co-sputtered M: TiO 2 (M= Au, Ag, Cu) Nanocomposite Systems: A Comparative Study. J. Phys. Chem. C 2010, 114, 13955-13961. 67)

Jing, L.; Zhou, W.; Tian, G.; Fu, H. Surface Tuning for Oxide-Based Nanomaterials as

Efficient Photocatalysts. Chem. Soc. Rev. 2013, 42, 9509-9549. 68)

Xia, S.; Zhang, L.; Pan, G.; Qian, P.; Ni, Z. Photocatalytic Degradation of

Methylene Blue with a Nanocomposite System: Synthesis, Photocatalysis and Degradation Pathways. Phys. Chem. Chem. Phys. 2015, 17, 5345-5351. 69)

Zhang, X.; Qin, J.; Xue, Y.; Yu, P.; Zhang, B.; Wang, L.; Liu, R. Effect of

Aspect Ratio and Surface Defects on the Photocatalytic Activity of ZnO Nanorods. Sci. Rep. 2014, 4, 4596. 70)

Guo, X.; Zhu, H.; Li, Q. Visible-light-driven Photocatalytic Properties of ZnO/ZnFe2O4

Core/shell Nanocable Arrays. Appl. Catal. B: Environ. 2014, 160, 408-414.

32

List of figure captions: 1 Scheme. 1. Representation of three basic synthesis stages of Ag nanoparticles/ZnO nanorods/3DG nanocomposites. 2 Fig. 1. SEM images of the freeze dried a) 3DG, b, c) ZnO nanorods/3DG, d, e) Ag nanoparticles/ZnO nanorods/3DG nanocomposites, f) EDX spectrum of Ag nanoparticles/ZnO nanorods/3DG nanocomposites, g) one single ZnO nanorod on the 3DG surface in the Ag NPs/ZnO NRs/3DG sample along with the qualitative EDX 2D map analysis of different elements in the ternary nanostructure. 3 Fig. 2. TEM images of a) 3DG, b) Ag nanoparticles/ZnO nanorods/3DG, and c) observed Ag nanoparticles on the 3DG surface. 4 Fig. 3. XRD patterns of a) 3DG, b) ZnO NRs/3DG and c) Ag NPs/ZnO NRs/3DG. The inset is the XRD spectrum of the prepared graphene oxide.

5 Fig. 4. XPS high resolution spectra of the Ag NPs/ZnO NRs/3DG structure: a) C 1s, b) Zn 2p, c) Ag 3d, and d) O 1s. 6 Fig. 5. UV–Vis diffuse reflectance spectra of the prepared samples. The inset is light absorption of the Ag/ZnO/3D (red) and the ZnO/3DG (blue) in a wavelength range from 400 to 500 nm.

7 Fig. 6. a) The effect of contact time, b) pseudo-second-order and c) interparticle diffusion models of MB adsorption on the 3DG.

8 Fig. 7. Variation of ln(C0/C) vs light exposure time under a) UV and b) visible irradiation for the 3DG, ZnO NRs/3DG, Ag NPs/ZnO NRs/3DG as compared to the blank. c) The decolonization of MB by Ag NPs/ZnO NRs/3DG photocatalyst (with a mass of 0.3 mg) under UV or visible light irradiation.

9 Fig 8. Degradation rate constants of MB on the Ag/ZnO/3DG nanocomposite in the presence of different scavengers and N2 purging under UV (a) and visible (b) irradiation.

10 Scheme 2. A proposed mechanism of charge transfer and photodegradation of MB dye on the Ag/ZnO/3DG surface under visible (a) and UV (b) light.

33

Graphical abstract