Green synthesis of glucose-reduced graphene oxide

Composites Part B 138 (2018) 35–44

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Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Green synthesis of glucose-reduced graphene oxide supported Ag-Cu2O nanocomposites for the enhanced visible-light photocatalytic activity

T

Kamaldeep Sharmaa, Kakali Maitia, Nam Hoon Kima,∗, David Huib, Joong Hee Leea,c,∗ a

Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global Program), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea b Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA c Carbon Composite Research Centre, Department of Polymer & Nanoscience and Technology, Chonbuk National University Jeonju, Jeonbuk 54896, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanocomposites Materials development Chemical properties Surface analysis Environmental degradation

Ternary nanocomposites (NCs) comprising Ag-Cu2O supported on glucose-reduced graphene oxide (rGO) with enhanced stability and visible light photocatalytic activity were synthesized via a facile and green approach using Benedict's solution and glucose solution at room temperature without the need of any toxic reagent, surfactant or any special treatment. Besides mild reducing capability to GO, glucose also induces the functionalization of rGO sheets, preventing the aggregation of reduced sheets and providing in situ stabilization to Cu2O. The resulting Ag-Cu2O/rGO NCs showed excellent photocatalytic efficiency for the photodegradation of methyl orange (MO), and the degradation rate was found to be higher than the pristine Cu2O and Cu2O/rGO NCs. Further, for the first time Ag-Cu2O/rGO NCs showed markedly enhanced photocatalytic efficiency for the photodegradation of phenol solution which is mainly attributed to its high electron injection rate and effective separation of electron–hole pairs. Thus, present strategy explores the facile synthesis way of varieties of Cu2Obased NCs materials using harmless reagents and their feasible applications.

1. Introduction Among various semiconductor metal oxide photocatalysts, Cu2O has been widely explored for the photocatalytic degradation of organic contaminants owing to its low cost, benign nature, short bandgap, visible light harvesting ability, and high photocatalytic efficiency [1–4]. Previously, shape controlled synthesis of Cu2O based nanostructured materials into different morphologies has been carried for the efficient degradation of dyes solution [5–8]. However, short hole-diffusion length and photocorrosion of Cu2O during the photocatalytic reaction decreased the photocatalytic efficiency and photochemical stability, limiting their large-scale industrial applications in photocatalysis [9,10]. Therefore, various efforts have been focused on the design and synthesis of Cu2O-based nanocomposite materials to achieve high photochemical stability and improve the charge separation ability for high photocatalytic efficiency [11,12]. In this context, the fabrication of Cu2O materials with noble metal nanoparticles is an efficient approach to improve the photocatalytic efficiency, as the Schottky barrier developed at the heterojunction interface inhibits the recombination of photogenerated electrons and holes in photocatalytic reactions [13,14]. Further, the surface plasmon resonance (SPR) of noble metal

nanoparticles also enhances the photocatalytic efficiency by increasing the absorption of photons on the surface of the catalyst [15]. However, the use of stabilizing/capping agents for the synthesis of noble metalsemiconductor nanocomposite materials decrease the catalytically active surface area by aggregating on its surface and cause lower catalyst contact with the reactant molecules [16]. Further, surfactants affect the shape, size, and growth of the deposited metal nanoparticles and prompt the formation of large sized aggregated nanoparticles with low photochemical stability [17]. In this context, graphene oxide (GO) was used as an efficient support for the formation of stable composite materials, because of its large surface area, high electron conductivity, and high optical transparency [18–26]. Further, the pendent functional groups (−COOH, epoxide, and −OH) present on the surface of GO provide high water dispersibility and a site for the stabilization of Cu2O NPs [27,28]. Considering the extensive properties and potential applications of Cu2O and Ag, it would be more advantageous to carry out a facile synthesis of rGO supported Ag-Cu2O NCs with enhanced photochemical stability and high catalytic efficiency. Previously, the toxic reagents such as hydrazine were used for the facile preparation of AgCu2O/rGO nanocomposite materials [29]. Hydrazine reduces the oxygen substituents of GO and increases the pH of the catalytic system,

∗ Corresponding authors. Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global Program), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea. E-mail addresses: [email protected] (N.H. Kim), [email protected] (J.H. Lee).

https://doi.org/10.1016/j.compositesb.2017.11.021 Received 28 September 2017; Received in revised form 12 November 2017; Accepted 14 November 2017 Available online 21 November 2017 1359-8368/ © 2017 Elsevier Ltd. All rights reserved.


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2.4. Preparation of Cu2O/rGO nanocomposites

which results in increased particle aggregation and decreased electron injection rate from semiconductor to the noble metal NPs [30–32]. Thus, there is still a need to develop a synthesis way to prepare AgCu2O/rGO nanocomposite materials using harmless reagents. Furthermore, the photocatalytic efficiency of previously reported Ag-Cu2O/ rGO nanocomposite materials has not been explored for the photodegradation of phenol. Very recently, a highly efficient cotton flowerlike hierarchically porous boron nitride as an adsorbent material which displayed fast adsorption rate and high adsorption capacities for methylene blue (MB) and rhodamine B (RhB) dyes was developed [33]. In continuation of our attempts in this way, an economical and ecofriendly route for the green and facile preparation of Ag-Cu2O/rGO NCs using glucose as the reducing agent at room temperature without the need of any toxic or stabilizing agent was developed. Thus, the method developed in this study for the preparation of Ag-Cu2O/rGO NCs at room temperature is fast, facile, green, and free from the toxic reagents, making it more convenient than the methods being reported in literature (Table S1). The resulting Ag-Cu2O/rGO NCs showed high photocatalytic efficiency for the degradation of MO under visible light radiations, which was more active than pristine Cu2O and Cu2O/rGO NCs. This research mainly focused on the feasibility to use Ag-Cu2O/rGO NCs as an efficient photocatalyst for the degradation of phenol under visible light radiations. Further, the photocatalytic efficiency of generated AgCu2O/rGO NCs for the degradation of MO was higher than various rGO based binary and ternary photocatalysts reported in literature (Table S2, the ESI). The excellent photocatalytic activity of glucose reduced GO supported Ag-Cu2O NCs is mainly attributed to its high electron injection rate and effective separation of electron hole pairs.

GO was prepared via the oxidation of pristine graphite by improved Hummer's method [34]. Cu2O/rGO NCs were prepared at room temperature through a facile single-step solution-phase reduction process by simple mixing the aqueous dispersion of Benedict's solution and GO with glucose. In a typical synthetic procedure for the preparation of Cu2O/rGO NCs, 50 mg of GO was dispersed in 100 mL of deionized water using an ultrasonicator for 10 min to obtain a uniform dispersion. Then, 50 mL of Benedict's solution (0.04 M) was poured into the aqueous dispersion of GO under continuous stirring. To the above mixture, 15 mL of aqueous glucose solution (0.5 M) was added dropwise, and the resulting mixture was sonicated for 30 min at room temperature. The resulting brown precipitates were washed several times with ethanol and deionized water and then separated by centrifugation. The solid sample was then dried at 60 °C and used for the photocatalytic experiments. 2.5. Preparation of Ag-Cu2O/rGO nanocomposites Ag-Cu2O/rGO NCs were prepared at room temperature via a simple one-pot, two-step reduction process. The first step is similar to that employed for the synthesis of Cu2O/rGO as mentioned above. Second step involves the instant addition of 1 mL of AgNO3 solution (0.1 M) to the aqueous dispersion of Cu2O/rGO. The resulting mixture was sonicated at room temperature for 1 h. Finally, the obtained precipitates were washed several times with ethanol and deionized water to remove residual ions and excess glucose. The solid product was then centrifuged and dried in an oven at 60 °C for use in the photocatalytic experiments.

2. Experimental methods

2.6. Photocatalytic degradation of MO

2.1. Materials

The photocatalytic efficiency of Ag-Cu2O/rGO NCs was investigated for the photodegradation of a solution of MO under visible light irradiation at room temperature. Typically, 10 mg of photocatalyst was dispersed in 50 mL dye solution (40 mg/L), and the dispersion was then stirred in the dark for 30 min to establish the adsorption–desorption equilibrium. A 60 W tungsten filament lamp (500–700 nm, 0.24 W/ cm2) was used as the irradiation source, and the reaction vial was submerged in a water bath to prevent the photo-heating effect. After that, 5 mL of dye solution was sampled out at a given interval and centrifuged to separate it from the catalyst. The change in the concentration of the resulting MO solution with increasing irradiation time was monitored using an UV-vis spectrophotometer. The photo-degradation efficiency of MO was calculated using the equation:

Copper sulfate (CuSO4.5H2O; ≥99%), trisodium citrate, sodium carbonate, and silver nitrate (AgNO3, 99.99%) were purchased from Sigma-Aldrich (USA). Phenol (C6H5OH; ≥99.99%) and K2S2O8 were purchased from samchun co. (Korea). Methyl orange (MO) was purchased from TCI Co. (Korea). HPLC grade methanol and ethanol were used for all the experiments. All the reagents were of analytical grade and used as such without further purification. The deionized water obtained using an EYELA Still Ace SA-2100E1 (Tokyo Rikakikai Co., Japan) filtering system was used as a solvent.

2.2. Preparation of Benedict's reagent

η = (C0 − Ct)/C0 × 100%

The stock solution of Benedict's reagent (1 M) was prepared by dissolving 17.3 g of trisodium citrate (Na3C6H5O7) dihydrate and 10 g of sodium carbonate (Na2CO3) in 85 mL of deionized water. Then, an aqueous solution of 1.73 g copper sulfate pentahydrate (CuSO4.5H2O) in 10 mL of deionized water was added to the above solution and was further diluted to make a total volume of 100 mL. Then, different concentrations of this solution in the range 0.4–0.04 M were used for the preparation of Cu2O/rGO and Ag-Cu2O/rGO NCs.

(1)

Where η is the photocatalytic efficiency, C0 and Ct are the absorption intensity of the phenol solution after the adsorption–desorption equilibrium and irradiation, respectively. For comparison, the degradation of MO was also performed using Cu2O/rGO NCs and Cu2O nanocrystals as the photocatalysts by following similar photocatalytic procedure. 2.7. Photocatalytic degradation of phenol

2.3. Preparation of Cu2O nanocrystals The photocatalytic efficiency of the generated Ag-Cu2O/rGO NCs was also examined for the photo-degradation of the aqueous solution of phenol (20 mg/L). In detail, 50 mg of the generated Ag-Cu2O/rGO NCs was dispersed in 50 mL aqueous solution of phenol (20 mg/L). Then, the aqueous solution of phenol containing photocatalysts was stirred using a magnetic stirrer in the dark for 30 min to attain the adsorption–desorption equilibrium. A 60 W tungsten filament lamp (500–700 nm, 0.24 W/cm2) was used as an irradiation source, and the

Pristine Cu2O nanocrystals were prepared by mixing 50 mL of Benedict's solution (0.04 M) with 10 mL of aqueous glucose solution (0.5 M) under continuous stirring. The resulting mixture was sonicated for 45 min at room temperature. The reddish orange precipitates of Cu2O were washed several times with ethanol and deionized water to remove excess glucose. The final solid product was separated by centrifugation and dried at 60 °C. 36


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Fig. 1. (A) UV-Vis spectra of pristine Cu2O, Cu2O/rGO and Ag-Cu2O/rGO NCs. Inset showing reaction vials with apparent color change (B) XRD patterns of GO, Cu2O/rGO and Ag-Cu2O/ rGO NCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

40 kV and a current of 100 mA. The Raman spectra were recorded using a Nanofinder 30 3D Laser Raman Microspectroscopy System (Tokyo Instruments Co., Osaka, Japan). The morphology and elemental analysis (EDX) of the materials were analyzed by field-emission scanning electron microscopy (FE-SEM) using a JSM-6701F (JEOL, Japan) installed in the Center for University-Wide Research Facilities (CURF) at Chonbuk National University and field-emission transmission electron microscopy (FE-TEM) using a JEM-2200 FS microscope (JEOL Co., Japan) at the Jeonju Center of KBSI, respectively. Thermogravimetric (TGA) analysis was conducted under air atmosphere to investigate the content of the materials using a Q50 TGA instrument (TA Instruments, USA) within the temperature range 40–800 °C. The degree of reduction of materials was analyzed by X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, UK). The XPS spectra were deconvolution by using Gaussian components with Shirley background subtraction. The Brunauer–Emmett–Teller (BET) surface areas of the final materials were calculated by recording N2 adsorption–desorption isotherms at 77 K with a Micromeritics ASAP 2020. The pore size distribution is analyzed by Barret-Joyner-Halenda (BJH) method.

vial containing sample was submerged in a water bath to prevent the photo-heating effect. After that, 5 mL of the given solution was sampled out at a given intervals and centrifuged to remove the catalyst. The progress of the reaction was monitored by measuring UV spectra at 270 nm of the given aliquots of sample. Further, the photocatalytic efficiency (η) of phenol was evaluated by using equation (1). To evaluate the mechanism for the photodegradation of phenol, the reaction was repeated by adding methanol (10 mmol), iso-propanol (10 mmol), K2S2O8 (1 mmol), and p-benzoquinone (1 mmol) as the hydroxyl radical scavenger, hole scavenger, electron scavenger, and oxygen radical anion scavenger, respectively. 2.8. Photoelectrochemical measurements For photoelectrochemical studies, the cleaned indium tin oxide (ITO) glass plates (1 × 1 cm) deposited with the catalytic materials (5 mg of photocatalyst dispersed in 500 μL isopropanol, 500 μL water, and 20 μL nafion mixture) were used as working electrode. The working electrode was irradiated by using a 60 W tungsten filament lamp (500–700 nm) as a visible light source with an incident light intensity of 0.24 W/cm2. All the photoelectrochemical measurements were operated on CHI 660D electrochemical-workstation using Pt wire and Ag/ AgCl saturated with KCl as the counter electrode and reference electrode, respectively. Photocurrents of catalytic materials were determined by chronoamperometry at 0.2 V applied potential in 1 M Na2SO4 electrolyte solution. Electrochemical impedance spectroscopic (EIS) measurements were carried out in the frequency range of 2 × 105 Hz–0.1 Hz with an applied voltage of 1.0 V vs Ag/AgCl reference electrode.

3. Results and discussion The preparation of Cu2O/rGO NCs involved the simple addition of Benedict's solution into the dispersion of GO at room temperature, followed by the dropwise addition of glucose. Cu2O and rGO were obtained via the green reduction of Benedict's solution and GO by glucose. The sole addition of Benedict's solution into the suspension of GO did not lead to an obvious color change; however, the addition of incremental amounts of aqueous glucose solution into the above mixture deposited the Cu2O NPs on the sheets of rGO. The whole process was accompanied by an apparent color change from light brown to dark brown (the inset, Fig. 1A). The change in the color from light brown to dark brown indicates the partial restoration of sp2 carbon network between the reduced GO sheets, because of the loss of oxygen containing functional groups [35]. Further, the subsequent addition of varying amounts of AgNO3 into the dispersion of Cu2O/rGO NCs formed spherical AgNPs, which were subsequently deposited on Cu2O to generate Ag-Cu2O/rGO NCs (Scheme 1). To control the reaction time taken for the formation of Cu2O/rGO NCs, same reaction was performed with different concentrations of

2.9. Characterization Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 6700 spectrometer at room temperature in the frequency range 4000–400 cm−1. The powder samples were mixed with dry KBr powder and pressed into pellets for the FTIR study. UV–vis spectra were monitored by using a UV–Vis spectrophotometer S-3100. X-ray diffraction (XRD) analysis of powder sample was performed using a D/Max 2500V/PC diffractometer (Rigaku Corporation, Japan) equipped with CuKα targets at a scanning rate of 2θ = 2° min−1 under a voltage of 37


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Scheme 1. Schematic illustration for the formation of glucose-reduced GO supported Ag-Cu2O NCs.

Fig. 2. (A) FT-IR spectra of Ag-Cu2O/rGO, Cu2O/rGO and GO (B) Raman spectra of Ag-Cu2O/rGO and GO.

an FCC lattice structure of Cu2O. Further, the presence of diffraction peaks at 38, 44.1, 64.5, and 77.5° corresponding to Ag along with diffraction peaks of Cu2O in the XRD spectrum confirmed the formation of Ag-Cu2O/rGO NCs. Moreover, the weak intensity of these peaks indicates the low loading of Ag in the nanocomposite material. However, no separate peak of rGO was observed, probably because of reduced contents and high shielding of rGO by Ag and Cu2O. The FTIR and Raman spectroscopy studies further confirmed the formation of Cu2O/rGO and Ag-Cu2O/rGO NCs and the in situ reduction of GO to rGO during the reduction process. The FTIR spectrum of GO shows the presence of prominent peaks at 3394, 1731, 1619 1223, and 1050 cm−1, corresponding to the O–H, ‒C]O, ‒C]C‒, ‒C‒C‒, and C‒O stretching vibrations (Fig. 2A). In the case of Cu2O/rGO NCs, two new absorption peaks corresponding to the Cu(I)–O vibrations at 1590 and 625 cm−1 appeared, confirming the presence of Cu2O (Fig. 2A) [40]. Similarly, the FTIR spectrum of Ag-Cu2O/rGO NCs exhibits the peaks corresponding to the Cu2O group along with the decrease in the intensity of −C]C− vibration of skeletal graphene, probably because of the reduced contents of rGO in the composites. The mass content of rGO present in Ag-Cu2O/rGO NCs was determined to be 6.2% by the TGA analysis (Fig. S1). The Raman spectrum of Ag-Cu2O/rGO NCs displays two characteristic bands for graphitized carbon at 1348 cm−1 (D-band) and 1584 cm−1 (G-band) with the ID/IG ratio of 0.93, which was found to be slightly higher than the ID/IG ratio of the pristine GO (0.89) (Fig. 2B). A slight increase in the ID/IG ratio indicates the decrease in the aggregation of the GO sheets reduced by glucose, probably because of the adsorption of the gluconate ions on the surface of reduced sheets [41]. Moreover, the new peaks at 212 and 620 cm−1

Benedict's solution and aqueous glucose solution. A gradual decrease in the reaction time was observed with decreasing concentration of Benedict's solution and increasing amount of glucose (Tables S3 and S4). Further, the role of GO for the preparation of Cu2O/rGO NCs was also investigated. The preparations of pristine Cu2O in the absence of GO completed in 45 min. Under similar conditions, same reaction was completed in 30 min when performed in the presence of GO. This study indicates that the carboxylic acid, hydroxyl, or epoxide groups present on GO surface also served as nucleation sites for the formation of Cu2O/ rGO NCs [36]. The UV-vis spectrum of Benedict's solution in the presence of glucose exhibited a broad absorption peak at ∼750 nm, corresponding to the formation of Cu2O nanocrystals (Fig. 1A) [37]. However, the absorption peak at 720 nm blue shifted for Cu2O in the Cu2O/rGO NCs, which may be because of the presence of rGO [38], whereas the absorption spectrum of glucose-reduced GO supported AgCu2O NCs showed a local plasmonic absorption band corresponding to AgNPs at 420 nm along with a broad absorption peak of Cu2O at 600 nm. The enhancement in the absorption intensity was primarily because of the induced light scattering caused by the micro-meter sized rGO in the aqueous dispersion (Fig. 1A) [39]. Fig. 1B shows the powder XRD pattern of GO, Cu2O/rGO, and Ag-Cu2O/rGO NCs. The XRD spectrum of GO exhibits a sharp diffraction peak at 10.5° indexed to the (001) plane, which disappeared in the diffraction pattern of Cu2O/rGO and Ag-Cu2O/rGO, indicating its reduction during the reduction of metal precursors. In the diffraction pattern of Cu2O/rGO NCs, the sharp diffraction peaks at 29.8, 36.5, 42.2, 61.3, 73.6, and 76.5° indexed to the (110), (111), (200), (220) and (311) planes confirm the presence of 38


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Fig. 3. (A) XPS survey spectrum of Ag-Cu2O/rGO NCs (B) C 1s peaks for GO (C) C 1s peaks of Ag-Cu2O/rGO NCs (D) Cu 2p peaks and (E) Ag 3d peaks for Ag-Cu2O/rGO NCs.

and 374.25 eV corresponding to the Ag 3d5/2 and Ag 3d3/2 can be assigned to the pristine metallic Ag (Fig. 3E) [44]. The C1s spectrum of Ag-Cu2O/rGO NCs is deconvoluted into a set of five sub-peaks centered at 284.5, 285.6, 286.8, 288.2, and 289.8 eV and were assigned to the C−C or C]C, C–OH, C–O−C, –C]O, and –COOH groups of oxygen functionalities, respectively (Fig. 3C) [45]. The presence of the peak corresponding to the epoxide group (C−O−C) in the reduced sample in comparison to the peaks of the as-synthesized GO indicates the functionalization of the rGO sheets through the formation of chemical bond

ascribed to the formation of Cu2O [42]. The valence structure of various elements in the Ag-Cu2O/rGO NCs and the degree of reduction of GO to rGO in the presence of glucose were investigated by XPS measurements. The presence of peaks corresponding to Ag, Cu, O, and C in the XPS survey spectrum of NCs confirms the formation of Ag-Cu2O/rGO NCs (Fig. 3A). Further, the Cu2p XPS spectrum revealed two dominant peaks at 932.8 and 952.7 eV are ascribed to the Cu2p3/2 and Cu2p1/2 peaks for Cu2O, respectively (Fig. 3D) [43]. Moreover, the XPS peaks at 368.25

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Fig. 4. (A)–(B) SEM and TEM images of Cu2O NCs (C)–(D) SEM and TEM images of Cu2O/rGO NCs.

Fig. 5. (A)–(C) SEM and TEM images of Ag-Cu2O/rGO NCs (D) HRTEM image of Ag-Cu2O/rGO NCs showing lattice fringes of Ag and Cu2O. Inset. Image shows SAED pattern of Ag-Cu2O/rGO NCs.

decreases the size of Cu2O. Fig. 5A–C shows the presence of spherical AgNPs of size ∼60 nm deposited on the Cu2O situated on the surface of rGO sheets. However, a few AgNPs directly lying on the rGO sheets were also observed. The HR-TEM image of Ag-Cu2O/rGO NCs exhibits lattice fringes with a dspacing of 0.23 and 0.24 nm, corresponding to the (111) planes of Ag and Cu2O, respectively (Fig. 5D). The composition and distribution of the constituent elements in Ag-Cu2O/rGO NCs was determined by the TEM-EDX elemental mapping (Fig. 6). The resulting elemental mapping analysis showed the sphere like arrangement of Cu and O elements surrounded by the Ag elements, suggesting the deposition of Ag nanoparticles on the surface of Cu2O (Fig. 6B and C). Moreover, a sheet like arrangement of C and O elements covering the whole sample area suggested the presence of rGO sheets (Fig. 6D and E). The catalytic activity of the glucose-reduced GO supported Ag-Cu2O

between the gluconate ions and the carbon of the reduced GO (Fig. 3B and C) [41]. The Raman and XPS studies confirmed the presence of oxygen functionalities (e.g., −COOH and epoxide) on the surface of glucosereduced GO sheets. The glucose-reduced GO having ether like groups (gluconate) on the surface interact strongly with Cu2O, provide in situ stabilization and facilitate the transfer of photogenerated electrons during the photocatalysis [46–49]. The morphology of Ag-Cu2O/rGO and Cu2O/rGO NCs was determined by the SEM and TEM analyses. The SEM and TEM images of the Benedict's solution in the presence of glucose showed the presence of Cu2O nanocubes with an average diameter of 200 nm (Fig. 4A and B). Similarly, the cube like morphology of Cu2O with an average size of 150 nm enwrapped in the rGO sheets were observed in the SEM and TEM images of Cu2O/rGO NCs (Fig. 4C and D). These results suggest that the existence of GO in the system 40


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Fig. 6. (A) HAADF-STEM image of Ag-Cu2O/rGO NCs. HAADF-STEM-EDS mapping images of AgCu2O/rGO NCs in the same area (B) Cu K edge (C) Ag L edge (D) C K edge and (E) O K edge.

Fig. 7. (A) UV-vis spectra show photocatalytic degradation of MO in presence of Ag-Cu2O/rGO NCs at different irradiation time. Inset. Photographs (a) and (b) showing colour change of aqueous MO solution after 60 min under visible light irradiation and (B) photocatalytic efficiencies of Ag-Cu2O/rGO, Cu2O/rGO and Cu2O NCs for MO degeradation under visible light irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

relative absorbance of dye at different time intervals (Figs. S2A–S2C, the ESI) [51]. The kinetic studies showed that the degradation rate constant for Ag-Cu2O/rGO NCs (0.0973 min−1) was higher than that observed for Cu2O/rGO (0.0759 min−1) and Cu2O (0.0705 min−1) (Fig. S2D and Table S5). These studies reveal the high catalytic efficiency of glucose-reduced GO supported Ag-Cu2O NCs for the photodegradation of MO, which was still better than the other photocatalysts reported in literature (Table S2). The Fermi level of Cu2O lies above the Fermi levels of Ag and rGO [52,53]. Under visible light irradiation, the Cu2O produced photogenerated electrons in the conduction band (CB) and are transferred to Ag and rGO. In Ag-Cu2O/rGO NCs, the functionalization of reduced sheets significantly alters the Fermi level of rGO which may leads to the enhanced electron injection rate from Cu2O to rGO. Thus, the high photocatalytic activity of Ag-Cu2O/rGO NCs is attributed to the high transport rate of photogenerated electrons owing to the combined effect of the functionalized rGO and Ag. The BET surface area and pore size distribution of all the catalytic materials by recording N2 adsorption-desorption isotherms were measured (Fig. 8). The pore size distribution analysis reveals the presence of mesopores in all the three samples with decreasing trend of pore

NCs was investigated by measuring the photocatalytic degradation of MO (40 mg L−1) aqueous solution under visible-light radiations. Initially, the reaction was performed in the absence of catalyst; however, no degradation of MO was observed. Then, MO solution containing generated Ag-Cu2O/rGO NCs photocatalyst was magnetically stirred for 30 min in the dark to establish the adsorption/desorption equilibrium. The resulting solution was irradiated externally using a 60 W tungsten filament lamp (500–700 nm, 0.24 W/cm2) [50]. The reaction vial containing MO solution and catalyst was submerged in a water bath to prevent the photo-heating effect. The decrease in the concentration of MO with increasing irradiation time was monitored by measuring UV–vis spectra at 465 nm (Fig. 7A). The observed degradation efficiency of MO for Ag-Cu2O/rGO NCs after 60 min was found to be 90%. Further, the catalytic efficiency of the generated Ag-Cu2O/rGO NCs was evaluated by performing the photocatalytic degradation of MO in presence of Cu2O/rGO and Cu2O photocatalysts under similar conditions. The degradation efficiency of MO for Cu2O/rGO and Cu2O photocatalysts was found to be 75% and 65%, respectively (Fig. 7B). The first order rate constant for the degradation of MO was determined from the linear regression plot of 41


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[54]. The BET studies indicate the significant contribution of graphene in achieving high surface area. Thus, the large surface area and presence of mesopores provide more binding sites for the adsorption of more reactant molecules and are responsible for the enhanced catalytic efficiency of Ag-Cu2O/rGO. To further confirm the enhanced photocatalytic efficiency of AgCu2O/rGO NCs, the photocurrent responses of Ag-Cu2O/rGO/ITO, Cu2O/rGO/ITO and Cu2O/ITO electrodes were recorded with on–off cycles of visible light radiations. Fig. 9 shows reproducible rise and fall of photocurrent responses for all the electrodes with on–off cycles of visible light for a period of 40 s. The sharp increase in photocurrent density of Ag-Cu2O/rGO/ITO (68.50 μA/cm2) compared to Cu2O/rGO/ ITO (7.56 μA/cm2) and Cu2O/ITO (3.09 μA/cm2) with visible light on is mainly due to significant absorption of visible light and effective separation of electron-hole pairs which exhibits the excellent photocatalytic efficiency of Ag-Cu2O/rGO NCs. These studies show that deposited Ag nanoparticles play an important role in effective separation of electron-hole pairs. The transfer and effective separation of photogenerated electron-hole pairs were also analysed by EIS spectroscopy. The arc radius of Ag-Cu2O/rGO/ITO observed in Nyquist plot under visible radiations is smaller than that of Cu2O/rGO/ITO and Cu2O/ITO which is probably due to the high photogenerated electron injection rate from Cu2O to Ag nanoparticles, which induced the effective separation of photogenerated electron-hole pairs (Fig. S3). Further, the recyclability experiment was performed to investigate the sustainability of Ag-Cu2O/rGO NCs as a heterogeneous catalyst by using the same reaction as a model reaction (Fig. S4). After each catalytic cycle, the catalyst was separated by centrifugation and again used for the next catalytic sequence. These results show that the Ag-Cu2O/ rGO NCs remain stable and maintained its catalytic activity even after three catalytic cycles. The pronounced stability of the Ag-Cu2O/rGO NCs is attributed to the glucose-induced functionalization of rGO, preventing the aggregation of reduced sheets and providing a site for the in situ stabilization of Cu2O. Encouraged by the excellent photocatalytic efficiency of the as-prepared Ag-Cu2O/rGO NCs for the degradation of MO, the photocatalytic activity of these NCs were evaluated for the photodegradation of phenol under the visible light irradiation. Phenols are toxic contaminants, released from the pesticides, herbicides, dyes, paints, textiles, oil refining, plastics, chemical, agro-chemical and petrochemical industries, which can be easily absorbed through the skin, mucous membranes and different body organs [55,56]. Traditionally, various toxic and costly bimetallic catalytic systems have been employed for the photodegradation of phenol [57–59]. Further, their high catalyst loading, pH adjustment, and the use of co-oxidant are the main hurdles in the development of efficient catalytic systems for phenol degradation. In this study, for the first time,

Fig. 8. Nitrogen adsorption-desorption isotherms and pore size distribution (inset.) for Ag-Cu2O/rGO, Cu2O/rGO and Cu2O NCs.

Fig. 9. Transient photocurrent responses of as prepared Ag-Cu2O/rGO/ITO, Cu2O/rGO/ ITO and Cu2O/ITO materials for two 40 s on-off cycles in 1 M aqueous Na2SO4 solution under visible light radiations with an applied voltage of 0.2 V.

diameter from pristine Cu2O (32 nm) to Ag-Cu2O/rGO (22 nm), which may be due to the blockage of some mesopores by rGO and Ag nanoparticles (inset Fig. 8). The BET surface areas of Ag-Cu2O/rGO (10.2669 m2/g) and Cu2O/ rGO (9.8168 m2/g) were found to be higher than the surface areas of pristine Cu2O (2.8379 m2/g). The hysteresis loop in BET curve of AgCu2O/rGO reveals the presence of micropores along with mesopores

Fig. 10. (A) UV spectra show phenol degradation in presence of Ag-Cu2O/rGO NCs under visible light radiation (B) Photocatalytic activities of AgCu2O/rGO, Cu2O/rGO and Cu2O NCs for phenol degradation under visible light irradiation.

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Fig. 11. Proposed mechanism for the photocatalytic degradation of phenol over the Ag-Cu2O/rGO NCs under visible light irradiation.

curtail extensive heating, the use of toxic reagent, long reaction time, and exhibited enhanced photocatalytic efficiency. The present catalytic system is inexpensive, easy to prepare, enabling efficient light harvesting under ambient conditions. Further, GO dispersion reduced and functionalized by glucose prevented the aggregation of reduced sheets and significantly alters the Fermi level of rGO, thereby leading to improved photocatalytic efficiency. Therefore, the resulting Ag-Cu2O/rGO NCs showed enhanced stability and superior photocatalytic efficiency for the photodegradation of MO, higher than that of Cu2O/rGO and pristine Cu2O NCs. The Ag-Cu2O/rGO NCs were utilized successfully as an efficient photocatalyst for the photodegradation of phenol under visible light radiations.

glucose-reduced GO supported Ag-Cu2O NCs have been utilized as an efficient photocatalytic system for the degradation of phenol. The blank experiment was performed in the dark in the presence of Ag-Cu2O/rGO NCs; however, no obvious change in the absorption intensity of phenol was observed. Then, the same solution was irradiated with visible light radiations for 2 h, and the progress of the reaction was monitored by measuring UV spectra of the aliquots at different time intervals (Fig. 10A). The decrease in the absorption intensity of phenol at 270 nm as a function of irradiation time was plotted to determine the concentration of phenol. The Ag-Cu2O/rGO NCs showed maximum degradation efficiency for the photodegradation of phenol under visible radiations, and the photodegradation efficiency was almost 2 times higher than that of Cu2O/rGO NCs and 6 times than Cu2O (Fig. 10B). These results highlight the high catalytic efficiency of Ag-Cu2O/rGO NCs for the degradation of phenol. To get an insight into the mechanism for the photodegradation of phenol, control experiment was performed to determine which active species control the degradation of phenol. For this purpose, phenol degradation reaction was carried out with glucose-reduced GO supported Ag-Cu2O NCs under visible light irradiations in the presence of different scavengers such as methanol as hydroxyl radical (OH•) scavenger, iso-propanol as the hole (h+ VB) scavenger, potassium persulfate (K2S2O8) as the electron scavenger and p-benzoquinone (BQ) as the oxygen radical anion scavenger. The photodegradation efficiency significantly decreased in the presence of methanol, and after 2 h only 12% degradation was observed (Fig. S5). However, no noticeable change in the degradation efficiency was observed with other scavengers. These results suggest that the holes generated during the photoexcitation in the valence band (VB) react with OH− or H2O to generate OH• species, which actively participated in the degradation of phenol. Based on the above results, a plausible mechanism is proposed for the Ag-Cu2O/rGO NCs catalyzed photocatalytic degradation of phenol under visible light irradiations (Fig. 11).

Acknowledgements The authors acknowledge support from the Basic Research Laboratory Program (NRF-2014R1A4A1008140), the Nano-Material Technology Development Program (NRF-2016M3A7B4900117) and the Basic Research Program (2017R1A2B3004917) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.compositesb.2017.11.021. References [1] Li B, Liu T, Hu L, Wang Y. A facile one-pot synthesis of Cu2O/RGO nanocomposite for removal of organic pollutant. J Phys Chem Solids 2013;74:635–40. [2] Wang CH, Hu Y, Jiang Y, Qiu L, Wu H, Guo B, et al. Facile synthesis and excellent recyclable photocatalytic activity of pine cone-like Fe3O4@Cu2O/Cu porous nanocomposites. Dalton Trans 2013;42:4915–21. [3] Susman MD, Feldman Y, Vaskevich A, Rubinstein I. Chemical deposition of Cu2O nanocrystals with precise morphology control. ACS Nano 2014;8:162–74. [4] Huang W-C, Lyu L-M, Yang Y-C, Huang MH. Synthesis of Cu2O nanocrystals from cubic to rhombic dodecahedral structures and their comparative photocatalytic activity. J Am Chem Soc 2012:1261–7. [5] Wang Z, Wang H, Wang L, Pan L. One-pot synthesis of single-crystalline Cu2O hollow nanocubes. J Phys Chem Solids 2009;70:719–22. [6] Kuo CH, Chen CH, Huang MH. Seed-mediated synthesis of monodispersed Cu2O

4. Conclusions This study demonstrates a green and facile route for the efficient synthesis of ternary Ag-Cu2O/rGO NCs at room temperature, and can 43


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