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Freeze-Casting of Multifunctional Cellular 3D-Graphene/Ag Nanocomposites: Synergistically Affect Supercapacitor, Catalytic and Antibacterial Properties Prasanta Kumar Sahoo, Niraj Kumar, Shankar Thiyagarajan, Dinbandhu Thakur, and Himanshu Sekhar Panda ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00158 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Sustainable Chemistry & Engineering

Freeze-Casting of Multifunctional Cellular 3D-Graphene/Ag Nanocomposites: Synergistically Affect Supercapacitor, Catalytic and Antibacterial Properties Prasanta Kumar Sahooa,b*, Niraj Kumarc, Shankar Thiyagarajana, Dinbandhu Thakur a and Himanshu Sekhar Pandac* a

Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology

Bombay, Mumbai-76, India. b

Centre for Nano Science and Nano Technology, Siksha ‘O’ Anusandhan University,

Bhubaneswar-30, India c

Department of Materials Engineering, Defence Institute of Advanced Technology, Pune-25,

India.

*Corresponding Authors: 1. Prof.Himanshu Sekhar Panda Department of Materials Engineering, Defence Institute of Advanced Technology, Pune 411025, India Ph:+91-20-24304205 Email:[email protected] 2. Prof. Prasanta Kumar Sahoo Centre for Nano Science and Nano Technology, Siksha ‘O’ Anusandhan University, Bhubaneswar 751030, Odisha, India Ph: 91-67-42350181, Fax: 91-67-42351880 Email: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT Developments of new and highly effective multifunctional materials are shown great interest in recent years. Herein, we report a simple, cost efficient, one-step, surfactant free cellular 3Dgraphene/Ag

nanocomposites

using

freeze-casting

method

and

explore

further

for

supercapacitor, catalytic and antibacterial applications. FE-SEM and HRTEM analysis of nanocomposites were revealed the 3D-cellular network structure having continuous micrometer size open pores with uniformly decorated Ag nanoparticles of average size 25 nm. An electrochemical study was exhibited highest specific capacitance 845 Fg-1 at 5 mVsec-1 and excellent cyclic retention ~97% even after 1000 cycles. Further, 3D-graphene/Ag nanocomposites are applied as catalyst to reduce methylene blue using NaBH4. The rate of reduction was attained above 99% for 3D-graphene/Ag (40%) nanocomposites, which is significantly higher than pristine 3D-graphene. The network like structure of 3D-graphene/Ag nanocomposite filtered out 37% population from total bacterial strains. Also, 3D-graphene/Ag nanocomposite killed almost 100% bacterial strains after three hours of incubation due to merging effect of Ag ions and 3D-graphene. Key words: 3D-graphene, Silver, Micro-pores, Supercapacitor, Catalysis, Antibacteria


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INTRODUCTION Thrust of developing multifunctional materials have drawn considerable interest in the research community because of their unique properties, which is obtained due to combine electrical, mechanical, thermal and biological characteristics. These materials are found potentially applicable in diverse fields, such as energy, catalysis, sensors and medicine. In past few years, graphene, a single-layer of carbon atoms arranged in a perfect hexagonal lattice, has attracted incredible attention for its multifunctional behavior due to extraordinary electrical, thermal and mechanical properties. Amongst them, three-dimensional (3D) graphene macrostructures have attracted great attention due to higher conductivity, surface area and mass transport efficiency, and used in many promising applications, such as super adsorbents,1-3 energy storage and conversion,4-6 sensors,7 catalysis8 and as an antibacterial agent.2,3 Recently, research has focused on improving the properties and the performance of 3Dgraphene by decorating with metals9-13 and metal-oxide14-20 or making use of its composite21, 22 to meet the practical requirements. A synergistic combination of these foreign materials with 3D-graphene enables them to produce materials with superior performances in various applications, such as supercapacitors,14,17 high-performance lithium ion batteries,17-20 catalysis,9 electrocatalysts for fuel cells,10,11 sensors,12 water splitting15 and electromagnetic interference (EMI) shielding.16 Specifically, noble metal decorated 3D-graphene nanocomposite not only avoid aggregation of noble-metal nanoparticles but also maximize the loading of noble-metal nanoparticles, which improved properties.23-25 Among various noble metal nanoparticles (Pt, Pd, Au and Ag), Ag nanoparticles was chosen due to its unique electrical, optical and physicochemical properties.26 Being economical, good conductor and chemically stable, Ag nanoparticles are used in many potential applications, such as catalytic, antibacterial activity,



electrical contact in microelectronics and LED devices.27-32 Numerous methods are employed to fabricate 3D-graphene based noble metal assemblies.9-13 However, the processes are taken up quite expensive and hazardous. Thus a major challenge is to fabricate a well-dispersed 3Dgraphene-based noble metal nanocomposite with improved properties using a cost effective and environmental friendly process. Also, to the best of our knowledge, there is no report available on cellular 3D-graphene/Ag nanocomposite with different percent of Ag nanoparticles. Hence, we hypothesize to prepare cellular 3D-graphene/Ag nanocomposite with different percentage of Ag nanoparticles using a new and cost-effective freeze-casting process and explore the advantages of the 3D-graphene/Ag nanocomposite by implementing in various applications like supercapacitor, catalytic and antibacterial study. In this article, we report a process to synthesize 3D-graphene/Ag nanocomposites with different percentage of Ag using a cost-effective freeze-casting method. The morphology, structure and composition of 3D-graphene/Ag nanocomposites were investigated using a range of characterization techniques. Electrochemical characterization was conducted in the developed nanocomposites and measured highest specific capacitance 845 Fg-1 at 5 mVsec-1 with good cyclic stability. Incorporation of Ag into 3D-graphene acted as conductive additive and enhanced charge transportation and electrode/electrolyte accessibility, which improved electrochemical reaction kinetics. Again, the catalytic activity study of 3D-graphene/Ag nanocomposites was carried out for the reduction of methylene blue (MB) in presence of NaBH4. 3D-graphene/Ag nanocomposites with higher loading of Ag showed improvement in catalytic activity due to large number of active Ag atoms available for the reduction reaction. Again, the detrimental effect of 3D-graphene/Ag nanocomposites on bacterial species was studied. With E.coli, a gram negative strain as model organism, we explored the antibacterial activity using colony counting and


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plating assay method. Antibacterial activity was reached as high as 99.99% in 3D-graphene/Ag nanocomposites with maximum Ag content. EXPERIMENTAL METHODS Materials Graphite flakes of particle size 45 µm (purity 99.99%) and AgNO3 were obtained from SigmaAldrich and used as received. Other chemicals used in the experiments are in analytical grade. They were supplied from Merck Specialties Private Limited, India. Milli-Q water is used as a solvent in all our experiments. Preparation of 3D-graphene/Ag nanocomposites Aqueous dispersion of GO was prepared by following earlier process.33 In a typical synthesis of 3D-graphene/Ag nanocomposites, an aqueous GO solution (1.5 ml, concentration-5 mg/ml) was mixed with AgNO3 (different weight percentage) separately in a cylindrical 4 ml glass vials. Also, ascorbic acid (15 mg) was added to each vial separately and heated to 100 °C in an oven for 30 min. Following this, each vial was frozen for 30 min in a dry ice bath. Subsequently, after being thawed at room temperature, the vial was kept into a heating oven at 100 °C for 4 h, to obtain a 3D-graphene/Ag gel. 3D-graphene/Ag gel was sequentially dialyzed to remove the unreacted species. This was followed by freeze drying and then drying at 50 °C for 24 h. Figure 1 showed a schematic diagram of the synthesis of 3D-graphene/Ag nanocomposites using freezecasting method. The synthesis of 3D-graphene/Ag nanocomposites with loading of 10, 20 and 40 wt % Ag were carried out in presence of 0.625, 1.25, 2.5 mg of AgNO3 salt, respectively. As synthesized 3D-graphene/Ag nanocomposites with loading of 10, 20 and 40 wt % Ag nanoparticles were referred as 3D-graphene/Ag (10%), 3D-graphene/Ag (20%) and 3D-



graphene/Ag (40%) nanocomposites. Also, bare 3D-graphene was synthesized through freezecasting method in the absence of AgNO3 salt.

Figure 1. Schematic representation for the synthesis and formation mechanism of 3Dgraphene/Ag nanocomposites with different loading of Ag by air drying process and their supercapacitor, catalytic and antibacterial applications. Electrochemical study Electrochemical studies are accomplished to study the capacitive behavior of prepared samples. Developed composite electrode material solution (10 mg in 30 ml of DI water) was deposited (30 µl) on graphite electrode surface. IR lamp was used to dry deposited sample and used further as working electrode. All electrochemical studies are carried out in a three electrode electrochemical system (working electrode, reference electrode and counter electrode) to establish their capacitance parameters at ambient temperature. 1 M KOH solution was used as electrolyte for electrochemical reactions. In the three electrode system, a saturated calomel


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electrode (SCE) is used as a reference electrode, platinum wire as a counter electrode, and graphite electrode as a working electrode. Catalytic study The catalytic activities of the 3D-graphene/Ag nanocomposites were determined by reducing MB in presence of NaBH4 as reducing agent. In a typical reduction process, 5 mg of each catalyst was added to the 100 ml aqueous solution of MB (1 mM) in a reaction cell separately and kept under stirring for 10 min. Then, 2 ml freshly prepared NaBH4 (1.5 M) solution was added to the above dye mixture solution. Further, 2 ml solution was taken in a quartz cell from the reaction system at an interval of 3 min. The whole catalytic reduction process of MB was monitored through the UV-Vis spectra using an UV–Vis spectrophotometer in the range of 250– 800 nm at room temperature. Antibacterial activity of 3D-graphene/Ag nanocomposites 3D-graphene/Ag nanocomposites with different concentration of silver (40%, 20% and 10%) are used to study their potential for killing E.coli, a gram negative bacterium by standard plate assay method. Briefly, fresh bacterial solution was prepared by inoculating few bacterial colonies from the standard plates into the freshly prepared media. This media is incubated overnight in the bacterial culture environment (37 °C, 5% CO2 atmosphere). After incubation, the colonies in stock solution were counted and found 6 X 1012 CFUs/ml (CFUs - colony-forming units). Large number of bacterial colonies was taken for mimicking the extreme polluted conditions. 1 ml of this stock solution was transferred to six pairs of test tubes, each containing 200 µg/ml of as synthesized bactericidal agents. The bacterial suspension without the bactericidal 3Dgraphene/Ag nanocomposite is termed as ‘control’. The samples are named as control - S1, GO S2, rGO - S3, 3D-graphene - S4, 3D-graphene/Ag (10%) - S5, 3D-graphene/Ag (20%) - S6, 3D-



graphene/Ag (40%) - S7 for identification. The suspensions are made up to 5 ml with autoclaved milli-Q water. These test tubes were incubated further at bacterial culture environment for 3 h. 100 µl of treated bacterial suspensions were removed from the test tubes and distributed regularly over the nutrient agar plates. The plates were counted for the viable colonies with respect to their dilution factors. The reduction in viable colonies represented the bactericidal property of the as prepared 3D-graphene/Ag nanocomposites. Characterization XRD analyses of the as-prepared samples were performed by using Philips powder diffractometer (PW 3040/60) with Cu Kα radiation (λ = 1.541 Å). A conventional KBr pellet procedure was used for the FTIR study by using a Magna-IR spectrometer-50 (Nicolet) instrument. The Raman spectra of the as-prepared samples were recorded on a Lab RAM HR 800 Micro laser Raman system with an Ar+ laser of 519 nm. The morphology of the as-prepared samples was inspected by HRTEM (Phillips-CM 200 electron microscope, operated at 200 kV) and FEG-SEM (JEOL Model JSM-7600F) instruments. XPS analysis was performed on MULTILAB from Thermo VG Scientific, using Al Kα radiation as monochromator. Deconvolution of the XPS peaks was made by XPSPeak41 software using shirley-type background correction and fitted through Lorentzian-Gaussian functions. The chemical compositions of the as-synthesized 3D-graphene/Ag nanocomposites were determined by an ICP-AES (Prodigy, Teledyne Leeman Labs). Electrochemical measurements were accomplished by electrochemical workstation (Novocontrol Alpha-A analyzer + POT/GAL) in three electrodes electrochemical cell setup. Cyclic voltammetry and galvanostatic discharge data were obtained by using Win Chem software, and the electrochemical impedance spectroscopy (EIS) data was measured by Win Deta software. Catalytic studies of the as-synthesized 3D-graphene/Ag


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nanocomposites were investigated through Ultraviolet-visible (UV-Vis) (model: CE 3021) spectrophotometer. RESULTS AND DISCUSSION The crystalline nature of as synthesized GO, 3D-graphene and 3D-graphene/Ag nanocomposites was examined through XRD and shown in Figure 2(i). The diffraction peaks that correspond to the (002) plane at 10.8º (GO) and the (100) plane at 43º (hcp structure of carbon) are shown in Figure 2(i)a.34 The broad (002) plane in the 3D-graphene represented the stacked graphene sheets with a short range order.35 However, the (002) plane nearly disappeared in the 3D-graphene/Ag nanocomposites. The restacking of reduced GO sheets prevented due to incorporation of Ag nanoparticles into these sheets (Figure 2(i)c-e).36 The strong diffraction peaks appeared at two theta 38.1º, 44.2º, 64.3º, 77.3º and 81.4° due to (111), (200), (220), (311) and (222) planes of face-centered cubic (fcc) Ag crystals, respectively (Figure 2(i)c-e). These results indicated the reduction of GO and Ag ions to graphene and Ag nanoparticles. Again, the loading of Ag nanoparticles in the 3D-graphene/Ag nanocomposites is confirmed from the comparative intensity of (111), (200), (220), (311) and (222) planes in 3D-graphene/Ag (10%), 3Dgraphene/Ag (20%), 3D-graphene/Ag (40%) nanocomposites (Figure 2(i)c-e). Amongst these, 3D-graphene/Ag (40%) nanocomposite showed the highest intensity and suggested the presence of more Ag nanoparticles. Also, Raman spectroscopy is one of the essential nondestructive techniques to differentiate the carboneous materials on the basis of electronic structures. The characteristic Raman spectra of GO, 3D-graphene and 3D-graphene/Ag nanocomposites are shown in Figure 2(ii). GO, 3D-graphene and 3D-graphene/Ag nanocomposites showed two prominent peaks around 1345 and 1580 cm-1 in their Raman spectra which are related to D and G bands, respectively. The D band is assigned to defects in the curved graphene sheets where as G band correspond to the stretching mode of crystal graphite.37 9 ACS Paragon Plus Environment

(ii) Intensity (a.u)

(311) (222)

(220)

(e)

(200)

(002)

(i) (d)

(002)

(c) (b)

10

(100)

(a)

(002)

Intensity (a.u)

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

(111)


20

30

40

50

60

70

80

90

2θ (Degree)

D

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G

(e)

ID / IG= 1.39

(d)

ID / IG= 1.35

(c)

ID / IG= 1.32

(b)

ID / IG= 1.18

(a)

ID / IG= 0.82

1000

1500

2000

2500

Wavenumber (cm-1)

3000

Figure 2. (i) XRD patterns of (a) GO, (b) 3D-graphene, (c) 3D-graphene/Ag (10%), (d) 3D graphene/Ag (20%) and (e) 3D-graphene/Ag (40%) nanocomposites; (ii) Raman spectra of (a) GO, (b) 3D-graphene, (c) 3D-graphene/Ag (10%), (d) 3D-graphene/Ag (20%) and (e) 3Dgraphene/Ag (40%) nanocomposites. The D to G band intensity ratio (ID/IG) is a measure of average size of sp2 domains.38 The D to G band intensity ratio (ID/IG) for GO is calculated to be 0.81. However, ID/IG ratio of 3D-graphene, 3D-graphene/Ag (10%), 3D-graphene/Ag (20%) and 3D-graphene/Ag (40%) nanocomposites are calculated around 1.18, 1.32, 1.35 and 1.39, respectively. The ID/IG ratios of 3D-graphene and 3D-graphene/Ag nanocomposites are higher than GO, which confirmed the successful reduction of GO. Also, presence of Ag ions supported the reduction process of GO. The chemical states of elements present in the GO and 3D-graphene/Ag nanocomposites are investigated through XPS. The XPS spectra of GO and 3D-graphene/Ag (40%) nanocomposites are shown in Figure 3(a). Both the XPS spectra consist of C1s and O1s peaks, but 3D-graphene/Ag (40%) nanocomposite exhibited an additional Ag 3d peak. It suggested the formation and deposition of Ag nanoparticles on the surface of graphene sheets. Also, it is 10 ACS Paragon Plus Environment

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noticed that the C1s peak intensity is lowered than O1s peak intensity in GO, whereas it is opposite in case of 3D-graphene/Ag (40%) nanocomposite. It indicated that GO was successfully deoxygenated and reduced to form graphene. The high-resolution XPS spectra of Ag doublet (3d3/2 and 3d5/2) in the 3D-graphene/Ag nanocomposites are shown in the Figure 3(b). The doublet (3d3/2 and 3d5/2) are centered nearly at 368.1 eV and 374.1 eV in all three 3Dgraphene/Ag nanocomposites. These values are higher than the metallic Ag (0) (3d3/2 = 367.9 eV; 3d5/2= 373.9 eV).39 C1s

400000

3D-graphene/Ag (40 %)

Intensity (a.u)

(a)

600000

Intensity (a.u)

O1s Ag 3d

200000

0 60000

40000

3000 2500 2000 1500 1000 500 0 3000

(b)


Ag3d5/2

Ag3d3/2

2000


1000 30000 2000

3D-graphene

20000


1000 0

0 0

500

Binding Energy (eV) (c)

366

1000

368

370

372

374

376

Binding Energy (eV)

(d)

GO

378

380


C 1s

Intensity (a.u)

C 1s

Intensity (a.u)

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


Raw Fit sp2C sp3C -C-O -C=O -COO

Raw Fit sp2C sp3C -C-O -C=O -COO

282

282

284

286

288

290

292

294

Binding Energy (eV)

284

286

288

290

292

294

Binding Energy (eV)

Figure 3. (a) XPS spectra of GO and 3D-graphene/Ag (40%) nanocomposite, (b) Highresolution Ag 3d core level XPS spectrum of 3D-graphene/Ag (10%), 3D-graphene/Ag (20%) and 3D-graphene/Ag (40%) nanocomposites, (c) High-resolution deconvoluted C 1s core level



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XPS spectra of GO and (d) High-resolution deconvoluted C 1s core level XPS spectra of 3Dgraphene/Ag (40%) nanocomposite. These results suggested that Ag(I) ions were successfully reduced to Ag(0) in all three 3D-graphene/Ag nanocomposites. Also, higher binding energy of Ag 3d appeared due to transfer of electrons from Ag nanoparticles to the graphene sheets.40 In addition, the deconvoluted C1s spectra of GO and 3D-graphene/Ag (40%) nanocomposites are shown in the Figure 3(c,d). The spectrum of GO showed oxygenic functional groups (-COO at 288.4 eV, -C=O at 286.9 eV, and C-O at 286.6 eV) between 286-289 eV, which suggested that the graphite undergoes successful oxidation to GO through the oxidation process. However, there is an effective reduction in the peak intensities of oxygenic functional groups in 3D-graphene/Ag nanocomposites. It supported the Raman data and confirmed the removal of oxygenic functional groups in 3D-graphene/Ag nanocomposites and GO is essentially reduced to graphene through the reduction process.41 Again, it is observed that the shifting of O 1s peak to a lower binding energy in the 3Dgraphene/Ag nanocomposites than the O 1s peak of GO (Figure S1). The shifting might be observed due to the covalent linkage between the sp2 carbon of 3D-graphene sheets and the Ag nanoparticle through the oxygen atom (Ag-O-C).42 The

morphology

of

the

as-synthesized

3D-graphene

and

3D-graphene/Ag

nanocomposites were examined by FE-SEM. Figure 4(a-d) revealed an interconnected and well defined cellular 3D-porous structure of graphene sheets with continuous open pores of micrometer size. On careful examination, it is noticed that the Ag nanoparticles are distributed over the surface of 3D-building block graphene sheets. Figure 4(e) shows the HRTEM image of 3D-graphene/Ag (40%) nanocomposite. Silver nanoparticles having average particle size 36 nm (inset) are distributed uniformly over the surface of graphene sheets without any aggregation.


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Therefore, interconnected porous graphene sheet prevented the aggregation of Ag nanoparticles even at higher concentration of Ag in the 3D-graphene/Ag (40%) nanocomposite. Also, ultrasonication of 3D-graphene/Ag (40%) nanocomposite (10 min) was not succeeded to separate Ag nanoparticles from graphene sheet, which suggested strong interaction between the fillers and matrix.

Figure 4. FEG-SEM images of (a) 3D-graphene, (b) 3D-graphene/Ag (10%), (c) 3Dgraphene/Ag (20%), and (d) 3D-graphene/Ag (40%) nanocomposites, (e) TEM image of 3Dgraphene/Ag (40%) nanocomposite (Insert shows Ag particles distribution curve) and (f) HRTEM image of 3D-graphene/Ag (40%) nanocomposite.



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Again, lattice spacing of Ag nanoparticle calculated ~0.239 nm (Figure 4(f)), which corresponds to the (111) plane of Ag. This is in good agreement with the d-spacing value obtained from the XRD. Further, the distribution of various elements in the 3D-graphene and 3D-graphene/Ag nanocomposites were investigated using elemental mapping through energy dispersive X-ray spectroscopy (EDX) and shown in the Figure S2. The elemental mapping images revealed the uniform distribution of Ag atoms throughout graphene sheets and supported TEM results. The different loading percentage of Ag nanoparticles in the 3D-graphene/Ag nanocomposites estimated through ICP-AES analysis and found around 8, 17 and 38% for 3Dgraphene/Ag (10%), 3D-graphene/Ag (20%) and 3D-graphene/Ag (40%) nanocomposites, respectively. Electro-chemical studies Cyclic

voltammetry,

Galvanostatic

charge-discharge

and

electrochemical

impedance

measurements are employed to estimate the electrochemical performance of the electrode materials (areal mass loading of electrode material on graphite electrode is 0.01 mg/cm2). Significant electrochemical changes are observed in the 3D-graphene and 3D-graphene/Ag nancomposites and shown in Figure 5(a). The enclosed curve area increased with the increase of Ag nanoparticles in the 3D-graphene/Ag nanocomposites. The CV plots suggested two major peaks in 3D-graphene/Ag (40%) nanocomposite, and showed strong redox behavior of metal content (Ag) in 3D-graphene.13,43 The electrochemical redox reactions due to silver can be explained as follows44:

Ag ↔ Ag +

(1)


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Thereafter, cyclic voltammetry measurements have been performed at different scan rate having potential window (-0.2 to 0.4 V) in 1 M KOH electrolyte and the specific capacitance was calculated from the CV studies using following relation45:

dV

(2)

Where ‘m’ is the deposited mass of the material on the graphite electrode, ʋ is the scan rate, I (V) is the response current, Vc_Va is the potential window. Average value of current was obtained from the area under CV curves.

Figure 5. (a) Cyclic voltammetry curves of 3D-graphene and 3D-graphene/Ag nanocomposites at scan rate of 5mVsec-1, (b) Specific capacitance value of 3D-graphene and 3D-graphene/Ag 15 ACS Paragon Plus Environment


nanocomposites at various scan rates, (c) Galvanostatic discharge curves of 3D-graphene/Ag nanocomposites and (d) Specific capacitance value of 3D-graphene and 3D-graphene/Ag nanocomposites at various current densities. Figure 5(b) showed the plot for specific capacitance versus scan rate of 3D-graphene and 3D-graphene/Ag nanocomposites. 3D-graphene/Ag (40%) nanocomposite demonstrated the highest specific capacitance around 845 Fg-1 at 5 mVs-1(876 Fg-1 at 1 Ag-1), which is higher than 3D-graphene nanosheet (366 Fg-1), whereas the measured specific capacitance around 554 Fg-1 and 775 Fg-1 for 3D-graphene/Ag (10%) and 3D-graphene/Ag (20%) nanocomposites, respectively. Fairly high value of specific capacitance might be obtained in 3D-graphene/Ag nanocomposite due to increase in conductivity with sufficient ion transport in 3D-network structure, fast faradic process and high contribution of active sites in material with higher accessible surface area. Furthermore, the cyclic stability of 3D-graphene/Ag (40%) nanocomposite was performed for 1000 cycles at a scan rate of 100 mVsec-1 and shown in Figure 6(a). 3D-graphene/Ag (40%) nanocomposite exhibited excellent retaining capacitance value (~97%) even after 1000 cycles. Such long-term retention value shows the stability of 3Dgraphene/Ag (40%) nanocomposite and an admissible electrode material. The electrochemical impedance spectroscopy (EIS) study was conducted to understand the charge transport mechanism of electrolyte ions between electrode/electrolyte interfaces of different active materials. EIS study was performed for 3D-graphene and 3D-graphene/Ag nanocomposites in the frequency range of 100 kHz to 0.1 Hz with a bias voltage of 0.5 V. Figure S3 showed Nyquist plots of all the samples. Also, the higher frequency region plot is shown in inset of Figure S3. In the low frequency region, the slope of spectra to the real axis suggested the mass transfer limit, electrolytic diffusion and capacitive behavior.46 Nearly vertical line in the


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low frequency region of the Nyquist plot implied good capacitive behavior of the prepared samples. Moreover, semicircle at the higher frequency side suggested faradic charge transfer resistance parameter.47 In the high frequency region, intercept to the real axis offered series ohmic resistance (Rs) by considering ionic resistance of electrolyte, substrate intrinsic resistance and contact resistance.48 The diameter of the semicircle curve suggests the charge transfer resistance at the electrode/electrolyte interfaces.49 Slight increase in diameter of EIS highfrequency spectra in 3D-graphene/Ag nanocomposite indicated increase in contact resistance of the active material. Higher slope gradient in 3D-graphene/Ag (40%) nanocomposite suggested the improvement in capacitive behavior with decrease in the diffusion resistance and ion diffusion length of the electrode/electrolyte interfaces.50 Also, we have fabricated ASC (asymmetric supercapacitor) device by depositing 3D-graphene/Ag (40%) nanocomposite and 3D-graphene on stainless steel (SS) substrates through dip casting. 3D-graphene/Ag (40%) nanocomposite deposited electrode was used as positive electrode and 3D-graphene deposited electrode as negative electrode. These electrodes were first dipped in electrolyte gel (PVA/KOH) for five min and then sandwiched by placing filter paper (Millipore) as a separator between electrodes. Electrolyte gel was prepared by mixing 2 g of KOH and 2 g of PVA powder in 20 ml DI water and stirred at 85 ºC for obtaining clear solution. Thereafter, it was sealed to avoid leakage and any short contact. Two fabricated ASC devices in series are able to light up the commercial red LED (Figure 6(b)), which suggested the suitability of prepared material for supercapacitor application. Table S1 listed the capacitive performance of graphene derivativesAg based nanocomposites.



Figure 6. (a) Capacitive retention values of 3D-graphene/Ag (40%) at 100mV/sec scan rate for 1000 cycle (Inset shows the corresponding cyclic voltammetry curves) and (b) Demonstration of two fabricated ASC devices in series that light up the commercial red LED. Catalytic study Organic dyes are considered to be environmental pollutants that are discharged in to the surface water and ground water through industrial processes.51 These dyes are the main cause for carcinogenic and mutagenic diseases in animals and humans for longer periods of exposure as it hindered the photosynthesis cycle of plant metabolism.52,53 So, it is a demanding task to remove these toxic materials from the wastewater. Numerous techniques, such as photocatalytic degradation, adsorption, catalytic reduction, electro-oxidation and membrane filtration are adopted for the removal of various dyes from the wastewater by using various materials.54-59 Therefore, we used 3D-graphene/Ag nanocomposites with different loading percentage of Ag for the chemical reduction of MB dye in presence of NaBH4. MB is one of the water soluble cationic dyes, which exist as cationic (MB+) form in water. In general, MB shows absorbance maxima at 665 nm (oxidized form) in an aqueous solution, which is ascribed to n-p* transitions in the MB 18 ACS Paragon Plus Environment

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molecule.60 Figure 7(a-c) showed the time-dependent absorption spectra of the MB in the presence of 3D-graphene/Ag nanocomposites and NaBH4 at an interval of 0-15 min. The absorption intensity of MB at 665 nm decreased gradually over time in presence of 3Dgraphene/Ag nanocomposites and NaBH4. It indicated the reduction of MB due to formation of colorless leucomethylene blue (LMB) by NaBH4 in the presence of 3D-graphene/Ag nanocomposite as catalyst. It is noticed that the catalytic reduction of MB is higher in case of 3D-graphene/Ag (40%) nanocomposite (>99%) than other developed nanocomposites. Again, the reduction of MB was also examined in the presence of 3D-graphene and NaBH4 for 12 h. But, no reduction occurred in the presence of 3D-graphene and NaBH4. Figure 7(d) revealed a linear correlation between ln (Ct/C0) and reaction time t (C0: concentration of MB at initial time (t = 0), Ct concentrations of MB at the specific time intervals (t = t)), which suggested the pseudo-first-order catalytic reduction reaction. The pseudo-first-order rate constant (k) was estimated from the slope of the linear plot and shown in the Table S2. The variation in the catalytic reduction rates of 3D-graphene/Ag (40%), 3D-Graphene/Ag (20%) and 3Dgraphene/Ag (10%) nanocomposites might be observed due to different loading percentage of Ag atoms, which causes an increase in available number of active sites and enhances electron transfer process by supplying electrons for the reduction reaction.61 In addition, the adsorption study of MB in presence of 3D-graphene/Ag (40%) nanocomposite without NaBH4 was investigated and shown in Figure S4.



Figure 7. UV-vis absorption spectra of MB reduction by NaBH4 in the presence of (a) 3Dgraphene/Ag (10%), (b) 3D-graphene/Ag (20%) and (c) 3D-graphene/Ag (40%); (d) Plot of ln Ct/C0 versus time for catalytic reduction of MB in the presence of 3D-graphene/Ag (10%), 3Dgraphene/Ag (20%) and 3D-graphene/Ag (40%) nanocomposites. 15% MB was adsorbed on the 3D-graphene/Ag (40%) nanocomposite even after 30 min. However, the catalytic reduction of MB by 3D-graphene/Ag (40%) nanocomposite with NaBH4 was taken only 15 min for complete reduction. Hence, the decoloration of MB was occurred due to the weak adsorption and the catalytic reduction. Adsorption tendency of the catalyst offered more number of dye molecules in the vicinity of the catalysts, leading to more reduction.41 Further, catalytic activity of 3D-graphene/Ag (40%) nanocomposite (Figure 7(c,d)) was 20 ACS Paragon Plus Environment

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compared with RGO/Ag (40%) nanocomposite (Figure S5(a,b)). It is noticed that the 3Dgraphene/Ag (40%) nanocomposite (k = 0.2523 min-1) showed higher reduction rate than RGO/Ag (40%) nanocomposite (k = 0.136 min-1). It is emerged due to the higher surface area and unique 3D-network structure of 3D-graphene than RGO, which enhanced the mass transport efficiency of the reactants.62,63 The rate constant values of 3D-graphene/Ag (40%) nanocomposite are also compared with the other reported Ag and Ag-based materials.64-66 It is noteworthy that the developed 3D-graphene/Ag (40%) nanocomposite shows not only higher efficiency of reduction but also easily separated from the reaction solution through simple filtration process. Antibacterial activity study Carbon based materials can be derived into mesh like structures which could possibly allow the water to flow but retain the bacteria and other pollutants depending on their size.67-71 In this study, the 3D-graphene/Ag nanocomposite was loaded as a membrane into the burette and their ability to filter out bacterial pathogens was studied (Figure 8(a)). The burette is slowly opened to let the bacterial solution flow (through gravity) through the Ag loaded 3D-graphene membrane and filtrate collected in a flask. In this process, bacterial pathogen present in the solution gets filtered (adsorbed) due to their sheet like network. Pores ranging from sub micrometer to several micrometers can capture bacterial strains (pore size dependent). Similar type of graphene based setup was demonstrated by X. Zeng et al.71 The bacterial colonies present prior to filtration and after filtration were studied by measuring the OD600 and shown in Figure 8(b). Membranes used as filter is examined using SEM under higher and lower magnifications. Herein, the bacterial colonies were seen wrapped up under 3D-graphene sheets along with the Ag nanoparticles (Figure 8(c,d)).



Figure 8. (a)Filter setup loaded with 3D-graphene/Ag (40%) nanocomposite, (b) OD600 (expressed in %) measured prior and after filtration, (c) SEM image showing bacterial colonies adsorbed on 3D-graphene/Ag (40%) nanocomposite and (d) Bacteria wrapped over by graphene sheets were observed at higher magnifications. Further, the nanocomposite has to be realized for its bactericidal properties by counting the number of viable colonies in nutrient agar plates after treating the cultured media with 200 µg/ml of as prepared 3D-graphene/Ag nanocomposites for 3 h in a cell culture environment. After treatment, the plates were counted for the number of viable colonies. The reduction in viability percentage and the corresponding values is presented in Table 1 and the same is represented as a bar diagram in Figure 9 (a) (sample marked as S1, S2, S3, S4 and S5 for the pictorial purpose). The number of colonies in control was ~ 4 X 1012 CFUs/ml. It is seen that the 3D-graphene showed a decent activity with viable colonies around 3.80 X 1011 CFUs/ml.


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Table 1 Number of viable colonies counted and its derivation in percentage with respect to counterparts of 3D-graphene loaded with various concentration of Ag. Sample

Viable Colonies (CFUs/ml)

Reduction in viability (%)

Control-S1

4.085 X 1012

___

3D-graphene-S2

3.80 X 1011

74.22

3D-graphene/Ag (10%)-S3

3.66 X 1010

99.10


2.55 X 109

99.93


2.75 X 108

99.99

It is believed that the entrapment and physical wrapping of cells by high surface area graphene sheets, resulted direct interaction between the sharp edges of 3D-graphene and bacterial pathogen, and damaged the integrity of bacterial structure.72 Also, difference in surface electron states between bacterial and graphene sheets (Shottky barrier) partly responsible for restraining the growth. However, bactericidal performance was improved significantly by homogeneously distributing Ag nanoparticles on graphene sheets. Loading 10% Ag nanoparticles enhanced the antibacterial activity close to 99.1%, whereas with 40% Ag nanoparticles in graphene showed as high as 99.99% killing efficiency for the bacterial strains, and illustrated concentration dependent phenomenon. The improvement of antibacterial properties with respect to silver concentration can be realized from recent past reports of Chakraborty et al and Jakka et al.73,74 Clustering of more silver in 3D-graphene possibly releases more silver ions, which induced more oxidative stress in 3D-graphene/Ag nanocomposite, and resulted in disproportion between cells detoxification ability to the reactive intermediates or their repair and production of reactive oxygen.75 Therefore, significant cell death occurs. The above



arguments are supported through CV data (Figure 5a), where prominent oxidation peaks of Ag was observed in 3D-graphene-Ag (40%) nanocomposite.

Figure 9. (a) Plot shows the antibacterial activity of different samples, (b) SEM images showing intact control cells, (c) & (d) show cleaved off death cells after attachment with 3D-graphene/Ag (40%) nanocomposite, Yellow boxes of both (c) & (d) show disintegrated E.coli cells and its debris and Blue box &Inset of (d) shows cells which are cleaved off from centre. Again, the interaction between the bacterial cells and 3D-graphene/Ag (40%) nanocomposites, was investigated by SEM (Figure 9(c,d)). It is observed that the cellular membrane was cleaved off (as shown in yellow boxes) from the center and the contents (cytoplasm) of bacterial cells could be seen around the vicinity. Inset of Figure 9(c) suggested that the bacterial cells are cleaved off from the centre. It is evident from these images that when 24 ACS Paragon Plus Environment

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the bacterial cells come in the proximity of 3D-graphene/Ag (40%) nanocomposite, these get damaged readily by forming cleavage. It leads to the leakage of cellular components and leaves the bacterial cells dead. The main reason behind the cleavage formation is still unknown. Previous reports shown that, after the treatment, the wrinkled/damaged cell wall leads to the leakage of intracellular contents which killed the cells subsequently. The results involving nanocomposites of Ag with graphitic systems are compared with our developed materials and presented in Table S3 with their possible inhibition mechanisms.

100

Reduction (%)

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


80

60

40

20

0

Control 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

Figure 10. Recycling capability of 3D-graphene/Ag (40%) with accountable colonies in each cycle. Recycling ability is one of the most important factors that one has to consider when deriving a water filter membrane. 3D-graphene/Ag (40%) nanocomposite was studied for its recycling abilities and the bactericidal capacity is evaluated for five cycles. Constant incubation period and the stock solution containing the same number of colonies were used throughout this experiment. All the protocols were similar to the previous experiments, except that the same 25 ACS Paragon Plus Environment


sample was extracted after the treatment, washed three times with ethanol-water solution and then re-used for next cycle. The obtained data are represented in Figure 10. The developed 3Dgraphene/Ag nanocomposite showed considerably good recycling capability up to three cycles. At the end of fifth cycle, the reduction in viability has indeed come down to 51.3%. As argued previously, Ag ions concentration is reduced after few cycles and there comes a point where only few active silver are available for the new bacterial pathogens to interact. Also, the above result and its discussion is in line with previous reports.76-78 Nevertheless, the developed 3Dgraphene/Ag nanocomposite has appeared as a potential player for both antibacterial activity with very large killing effect and catalytic reduction of dyes. CONCLUSION A one-step freeze-casting method is used for the large-scale production of 3D-graphene/Ag nanocomposites with varying percentage of Ag ions. These multifunctional 3D-graphene/Ag nanocomposites achieved higher specific capacitance and excellent cyclic stability. The high conductivity and porosity of the 3D- graphene/Ag nanocomposites networks serves as a bridge for charge transfer and easy access for electrolyte ions to the electrode interfaces. Cyclic voltammetry results suggested the redox behavior of metal content (Ag) in 3D-graphene. 3Dgraphene/Ag nanocomposites were used for the reduction of MB in presence of NaBH4. Catalytic activity was improved significantly in 3D-graphene/Ag nanocomposites than 3Dgraphene. Antibacterial activity was investigated with E.coli, a gram negative strain, and showed significant improvement of killing efficiency in silver loaded 3D-graphene nanocomposites. Also, membrane like properties of 3D-graphene/Ag nanocomposites could filter out 37% population from total bacterial strains. ASSOCIATED CONTENT


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Supporting Information: O 1s XPS spectra of GO, 3D-graphene/Ag (10%), 3D-graphene/Ag (20%), and 3D-graphene/Ag (40%) nanocomposites; EDX spectrum and elemental mapping images of 3D-graphene, 3Dgraphene/Ag (10%), 3D-graphene/Ag (20%), and 3D-graphene/Ag (40%) nanocomposites; Electrochemical impedance spectra of 3D-graphene and 3D-graphene/Ag nanocomposites (Inset is showing corresponding high frequency plots); Table contained electrochemical performance of different graphene/silver based electrodes; UV-vis adsorption spectrum of MB in presence of 3D-graphene/Ag (40%) nanocomposite without NaBH4; UV-vis absorption spectra of MB reduction by NaBH4 in the presence of RGO/Ag (40%) nanocomposite and plot of ln Ct/C0 versus time for catalytic reduction of MB in the presence of RGO/Ag (40%) nanocomposite; Table contained the reaction rates and correlation coefficients (from ln Ct/C0–t plots) of MB in the presence of 3D-graphene/Ag nanocomposites. Table contained comparison of present work with others report involving nanocomposites of silver with different graphitic systems and their possible inhibition mechanisms. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors thank the IITB-Monash Research Academy and Defence Institute of Advanced Technology, India for the characterization support.



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Table of Conents (TOC)

Synopsis: Cellular 3D-graphene/Ag nanocomposites emerged as multifunctional nanomaterials and explored for energy storage, catalytic and antibacterial applications.


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Ag

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