Au Decorated Graphene

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Solar-Light Harvesting Bimetallic Ag/Au Decorated Graphene Plasmonic System with Efficient Photoelectrochemical Performance for the Enhanced Water Reduction Process Saikumar Manchala,†,‡ Lakshmana Reddy Nagappagari,§ Shankar Muthukonda Venkatakrishnan,§ and Vishnu Shanker*,†,‡ †

Department of Chemistry, National Institute of Technology, Warangal, Telangana 506004, India Centre for Advanced Materials, National Institute of Technology, Warangal, Telangana 506004, India § Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science and Nanotechnology, Yogi Vemana University, Kadapa, Andhra Pradesh 516005, India

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S Supporting Information *

ABSTRACT: Nowadays, surface plasmon resonance (SPR) induced hot-electron transfer from noble metals to host materials has been a widely used concept in solar energy conversion. On the other hand, the development of graphene-based photocatalytic systems for photocatalytic water reduction has attracted tremendous interest. In the present article, we develop a novel efficient photocatalytic system for clean energy production, i.e., semiconductor-free, solar-light harvesting graphene sensitized Ag/Au-bimetallic plasmonic system, by cost-effective and eco-friendly one-pot in situ green reduction process. Here, graphene oxide (GO) reduced well under the reflux conditions approximately at 90 °C with vitamin C, and simultaneously the Ag and Au nanoparticles (NPs) deposited on the graphene sheet. The reduction of GO to graphene and deposition of an alloy of Ag and Au nanoparticles on graphene sheet have been analyzed by UV−vis and PXRD. These results revealed that bimetallic Ag/Au-graphene has all the characteristic peaks of graphene and Ag and Au NPs. Morphological studies (SEM, TEM) clearly show that the alloys of Ag and Au NPs are well orderly deposited on the graphene sheet. The synthesized bimetallic Ag/Au-graphene plasmonic system was applied for photocatalytic water reduction process for the first time, and it affords a superior photocatalytic performance for H2 evolution from water reduction than Ag-graphene and Au-graphene plasmonic systems under sunlight illumination and further supported by photocurrent experiments. The rate of H2 evolution of bimetallic Ag/Au-graphene plasmonic system is 1.4- and 2-fold more than that of Au-graphene and Ag-graphene plasmonic systems. For enhanced photocatalytic H2 evolution, a mechanism has been proposed. Here graphene serves as an electron mediator/acceptor and light absorber, and the Ag and Au NPs alloy serves as reaction center for H2 evolution. KEYWORDS: bimetallic Ag/Au-graphene, plasmonic system, water reduction, green synthesis, H2 evolution, photoelectrochemical performance

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INTRODUCTION

bimetallic NPs of noble metals, constituted of two different metals with core−shell structure and alloy nanostructures, have captured enormous attention because of their unique physical properties like optical, magnetic, electronic, and particularly chemical, catalytic properties,5−7 along with their diverse and wide range of applications in catalysis, photonics, surface enhanced Raman scattering (SERS), chemical and biological sensing,8−14 and electrochemical water splitting.15 These properties are distinctly superior to their homogeneous

Energy crisis and environmental consequences such as global warming and pollution have been ringing the alarm bell to the upcoming generation. In the past 10 years, the large growth in the world population led to a significant rise in the emission of greenhouse gases from conventional fossil fuels. Finding ways to solve this problem, clean, sustainable, and renewable energy from especially the conversion of sunlight energy to hydrogen (H2) energy through photocatalytic water splitting is the most pivotal approach.1−3 Over a century, noble-metal nanostructures have attracted the scientific community because of their SPR with respect to visible light, since the discovery of interaction of light with spherical particles present in the air by Mie in 1908.4 However, © XXXX American Chemical Society

Received: April 13, 2019 Accepted: July 15, 2019

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DOI: 10.1021/acsanm.9b00684 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Pictorial Illustration for the Synthesis of Ag-Graphene, Au-Graphene, and Bimetallic Ag/Au-Graphene Plasmonic Systems

via intercalation can directly influence the photocatalytic and electrochemical properties.35 Recently, some of the researchers have reported photocatalytic H2 evolution from Pt-decorated graphene nanocomposites.36,37 However, the high cost and limited sources of Pt greatly hindered its large-scale commercial application. As a member of noble metals, Au is more frequently used in photocatalytic water splitting as a cocatalyst in order to improve the performance of photocatalyst, mainly because of its high work function as well as the lowest activation energy. In the present work, bimetallic Ag/Au decorated graphene nanosheets are designed for photocatalytic water reduction from glycerol−water solution. As for our literature knowledge, the present work is the first report on the photocatalytic water reduction over the bimetallic Ag/Au-graphene plasmonic system without any semiconductor and dye sensitizer. This makes the approach cost-effective and shows high H 2 production compared to most of the reported nanocarbonbased semiconductor materials. Herein, we have looked at using this strategy in synergy to improve photocatalytic H2 evolution via water reduction. Furthermore, the plausible mechanism for enhanced photocatalytic reaction in the bimetallic Ag/Au-graphene plasmonic system has been proposed.

monometallic counterparts (constituted of single metal NPs), resulting from the synergistic effects of both metal atoms. On another hand, from the past 3 decades, carbon nanostructures have played a dominant role in the photocatalysis. They can be used either as supports for arresting/ immobilizing bimetallic nanoparticles from conglutination or as metal-free cocatalysts.16−19 Over the reported carbon nanostructures, graphene became the prominent material for the scientific community since the single layer of graphene was successfully made in 2004 via micromechanical cleavage by Novoselov. The interest in graphene originates from its unique properties such as high electrical and thermal conductivity, large specific surface area, and chemical stability.20,21 Innumerable studies have been investigated on graphene supported monometallic nanoparticles such as Ag,22,23 Au,24,25 Pt,26 Pd27 and bimetallic nanoparticles including Ag/Au,28 Au/ Pt,29 Pd/Pt,30 Ag/Pd,31 Au/Pd,32 Au/Ag33 for sensing, antimicrobial activity, fuel cells, hydrogen-energy storage, and organic transformation applications. The making of graphenebased bimetallic NPs composite is an effectual strategy to develop highly efficient composite hybrids. The smart intercalation of mono- and bimetallic nanoparticles into sheets of graphene can not only prevent the coalescence of both graphene sheets and metal nanoparticles but also offer the superior hydrophilicity by improving the aqueous dispersibility, which increases mass transit in the catalytic reaction.34 The feasibility of tuning the specific bimetal−graphene interactions and synergistic effects in the nanospace of the graphene sheets

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EXPERIMENTAL DETAILS

Materials. Nanographite powder (Sisco Research Laboratories Pvt Ltd., 99 wt %), potassium permanganate (KMnO4, Sisco Research B



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Laboratories Pvt Ltd., 98 wt %), hydrochloric acid (HCl, SDFCL, 37 wt %), sulfuric acid (H2SO4, SDFCL, 98 wt %), hydrogen peroxide (H2O2, SDFCL, 30 wt %), silver nitrate (AgNO3, Merck, 99 wt %), and tetrachloroauric acid (HAuCl4, Sigma-Aldrich, 99.9 wt %) were purchased. Double distilled (DD) water was used in all experiments. Synthesis of Bimetallic Ag/Au-Graphene. Initially, GO was prepared by using commercially available nanographite powder through a modified Hummers method.38 Eventually, bimetallic Ag/ Au decorated graphene nanocomposite was synthesized by using the following method. In brief, the GO solution (1 mg/mL) was prepared by dispersing 0.1 g of GO in 100 mL of DD water. Then add 20 mM vitamin C and warm the reaction flask up to 90 °C. To this warmed solution is sequential addition of 1:1 ratio of AgNO3 and HAuCl4 with the interval of 30 min, and the stirring was continued for 24 h under reflux conditions. For comparative study, Ag-graphene and Augraphene were also synthesized through a similar procedure in the absence of HAuCl4 and AgNO3. Pure graphene and bimetallic Ag/Au nanoparticles were also synthesized under similar conditions in the absence of both the metal salts and graphene. The total synthesis procedure was represented in Scheme 1. Characterization. The as-prepared samples were characterized with powder X-ray diffraction (PXRD) studies by using PANalytical Advance X-ray diffractometer using Ni-filtered Cu Kα (λ = 0.154 06 nm) radiation. The surface morphology and chemical composition were investigated by using scanning electron microscope (SEM), VEGA3, Tescon, USA, integrated with an energy dispersive X-ray spectrometer (EDS). Further, it has been analyzed under a highresolution transmission electron microscope (HR-TEM), JEOL, JEM2100F operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed using Al Kα radiation, on a PHI 5000 Versaprobe instrument, ULVAC-PHI, Chigasaki, Japan. FT-IR spectra were recorded from 4000 to 400 cm−1 in transmission mode on a PerkinElmer Spectrum 100 FT-IR spectrometer using KBr pellet. Raman spectra were noted on a Horiba Scientific confocal Raman microscope with the laser power of 2−5 mW at 532 nm laser excitation. The optical properties were analyzed by using Analytikjena SPECORD 205 UV−visible (UV−vis) spectrophotometer. Photoelectrochemical Experiments. Photoelectrochemical experiments were performed on IVIUM Technologies electrochemical workstation to measure photocurrent response in a conventional three-electrode photoelectrochemical cell, and Na2SO4 (0.1 M) was employed as the electrolyte solution. The working electrode was made up of photocatalyst which was deposited on ITO (indium tin oxide) substrate, while the “Pt” wire and Ag/AgCl (in saturated KCl) served as a counter and reference electrodes. To evaluate the performance of synthesized photocatalysts, a Xe-lamp (300 W) was utilized as the light source under several on−off cycles of photocurrent experiments. Photocatalytic Activity. The photocatalytic water reduction experiments were carried out in a 180 mL of airtight sealed quartz reactor with silicone rubber septum under natural solar-light irradiation at ambient conditions in the Kadapa city. The intensity of light was measured with a digital lux meter (LT Lutron LX-101A) for every hour between 11:00 a.m. and 3:00 p.m. The average light intensity was approximately 9.5 × 104 lx during the photocatalytic experiments. In an ideal experiment, photocatalyst powder was placed into 5 vol % aqueous glycerol (used as a hole scavenger) mixtures at a concentration of 5 mg/100 mL. Dark experiments were carried out for at least 30 min to acquire adsorption equilibrium and ensure equal dispersion. The reactor containing solution mixture was evacuated by using high pure nitrogen (N2) gas for at least 30 min to remove dissolved oxygen (O2). Subsequently, the evacuated reactor was exposed to natural solar light, and magnetic stirring was continued. Finally, the evolved gaseous sample was collected at regular time intervals with an airtight syringe. The evolved H2 gas was quantified with Shimadzu GC-2014 offline gas chromatograph equipped with a thermal conductivity detector (TCD) by using N2 as a carrier gas at 70 °C.

RESULTS AND DISCUSSION Characterization of As-Synthesized Photocatalysts. To monitor the reduction of GO, HAuCl4, and AgNO3 by vitamin C, UV−visible spectroscopy was performed and the results were presented in Figure 1. As presented in Figure. 1,

Figure 1. UV−visible spectra of as-synthesized GO, graphene, Aggraphene, Au-graphene, and bimetallic Ag/Au-graphene.

the UV−visible spectra of all nanocomposites showed a peak at 270 nm assigned to the π−π* transition that designates that the conjugation remains stored and the majority of oxygen moieties are eliminated from the GO.39 In the spectra of Aggraphene and Au-graphene, the peaks at 420 and 585 nm are due to the formation of Ag and Au NPs on the graphene surface. Furthermore, the spectra of bimetallic Ag/Augraphene showed the small and broad peak at 550 nm which was between the 420 and 585 nm due to the formation of bimetallic Ag/Au-alloy NPs on the graphene surface.40 PXRD was carried out to know the structures of the GO, graphene, Ag-graphene, Au-graphene, and bimetallic Ag/Augraphene as shown in Figure 2a,b. And the PXRD pattern of bimetallic Ag/Au nanoparticles is displayed in Figure S1. The nanographite powder displays a sharp and strong peak at 26.6° (dspacing = 0.335 nm) with a basal reflection (002). In the case of GO, the diffraction peak moves to a lower angle of 12.2° (dspacing = 0.725 nm), due to the oxidation of graphite. And in the case of graphene, the diffraction peak at 12.2° vanished while a broad peak centering at 25° (dspacing = 0.356 nm) was noticed. However, the larger dspacing of GO than that of pristine graphite after oxidation is corresponding to the intercalation of water molecules and the formation of oxygen-containing functional groups between the layers of graphite, and the lower dspacing of graphene than that of GO after reduction is due to the removal of oxygen-containing groups from the GO.41 PXRD patterns of the black composite powders clearly show the presence of silver and gold NPs in the products, and the four strongest diffraction peaks can be indexed as (111), (200), (220), and (311) of the cubic silver structure (JCPDS 04-0783) and cubic gold structure (JCPDS 04-0784). Additionally, the average crystallite size can be calculated by using the Scherrer from the PXRD pattern,

D= C

kλ β cos θ DOI: 10.1021/acsanm.9b00684 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 2. (a) PXRD patterns of graphite and as-synthesized GO, graphene and (b) Ag-graphene, Au-graphene, and bimetallic Ag/Au-graphene plasmonic systems. (c) Raman spectra of as-synthesized GO and graphene and (d) Ag-graphene, Au-graphene, and bimetallic Ag/Au-graphene plasmonic systems.

where D = crystallite size (nm), β = fwhm (radians), k = 0.9 (Scherrer constant), λ = wavelength of X-rays (0.154 06 nm), and θ = Bragg’s angle/peak position (radians). The measured “D” values for nanoparticles in Ag-graphene, Au-graphene, and bimetallic Ag/Au-graphene are 34, 42, and 32 nm. Raman spectroscopy is a robust nondestructive technique and significantly differentiates the nature (ordered and disordered) of nanocarbon materials. As represented in Figure 2c, the Raman spectra of GO displayed two major characteristic peaks. The first one is at 1330 cm−1 for the D band corresponding to A1g symmetry breathing mode, and the intensity of the D band depends upon the staging disorder and population of defects. The second one is at 1582 cm−1 for the G band corresponding to E2g symmetry phonon mode of inplane vibrations of all sp2 carbon atoms.42 After reduction, the intensity ratios of both D and G bands obviously increased from 0.98 (GO) to 1.12 (graphene), and one more peak appeared at 2662 cm−1 for 2D band corresponding to the twophonon mode with opposite wave vectors that confirms the formation of graphene.43,44 Furthermore, the Raman spectra of synthesized monometallic (Ag and Au) decorated graphene and bimetallic Ag/ Au decorated graphene plasmonic systems (Figure 2d) showed an increase in the D and G band intensities compared to GO. This might be arising from the surface enhanced Raman scattering of noble metal NPs (Ag, Au) present in the composites45 and further confirms the reduction of GO to graphene. After reduction, as represented in SEM images in Figure 3a− c, the silver and gold NPs are nicely separated from each other

and equally distributed on the graphene sheet. In the case of Ag/Au-graphene composite system shown in SEM image Figure 3c, the bimetallic silver and gold alloy NPs are mixed uniformly and well orderly dispersed on the surface of the graphene sheet. The elemental dispersive X-ray spectroscopy (EDS) discloses the presence of silver, gold, carbon, and oxygen elements in the respective synthesized plasmonic systems. As presented in HR-TEM images Figure 4a−f, the bimetallic Ag/Au NPs in the Ag/Au-graphene plasmonic system are well decorated on the surface of a graphene sheet and appeared as a spherical shape and morphologically look like core−shell. In Figure 4c, the size of Ag core is 22 nm and thickness of the shell is 1.5 nm. It means the size of bimetallic Ag/Au core− shell is 23.5 nm, which is approximately close to that of average size calculated from the Scherrer equation. As exhibited in Figure 4f, the lattice fringes and dspacing = 0.24 nm are in good agreement with the (211) plane of cubic phase of Ag core of bimetallic Ag/Au-graphene plasmonic system in the PXRD pattern.46 XPS analysis was carried out in order to confirm the surface elemental composition and elemental chemical status of assynthesized bimetallic Ag/Au-graphene plasmonic system. As represented in Figure 5a, the survey XPS spectrum divulges the existence of C, O, Ag, and Au elements. In the C 1s highresolution spectrum (Figure 5d) situated at the binding energy (BE) of 284.7 eV was the characteristic peak of the sp2 C C bonds of graphene. In addition, the peaks situated at BEs of 83.9 and 87.6 eV were the characteristic peaks of Au NPs present at the surface of graphene which are attributed to the D


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Figure 3. SEM images and corresponding EDS of as-synthesized (a) Ag-graphene, (b) Au-graphene, and (c) bimetallic Ag/Au-graphene plasmonic systems.

2853 cm−1 correspond to the symmetric and asymmetric stretching frequencies of CH. And another major band noticed at 1720 cm−1 corresponds to the stretching frequency of CO of carboxylic acid and carbonyl groups present at the edges of GO. Finally, the bands at 1248 and 1021 cm−1 are attributed to the stretching frequencies of phenolic C−O and epoxy C−O− C groups, respectively. However, after the reduction of GO to graphene there is an apparent decrease in the intensity of the absorption bands of the oxygen-containing functional groups. Furthermore, the FT-IR spectra of synthesized monometallic (Ag and Au) deposited and bimetallic Ag/Au-graphene plasmonic systems reveal that the decrease in the intensity of the absorption bands can be ascribed to both reductions of GO to graphene by vitamin C and deposition of noble metal nanoparticles on the GO surface. Also, the decrease in the intensity of OH stretching frequency in nanocomposites can be attributed to the interactions between noble metal NPs (Ag and Au) and OH groups of GO. The shape and appearance of all other peaks of plasmonic systems disclose the interaction between noble metal NPs (Ag and Au) and the surface of graphene.49 From FT-IR spectra of plasmonic systems, it can

Au 4f7/2 and Au 4f5/2 peaks (Figure 5b), and the peaks situated at BEs of 368.4 and 374.4 eV were the characteristic peaks of Ag NPs present at the surface of graphene which are attributed to the Ag 3d5/2 and Ag 3d3/2 peaks (Figure 5c). The splitting energy of the Ag 3d doublet (6.0 eV) and Au 4f doublet (3.7 eV) was also confirming the metallic nature of Ag and Au.47 In contrast, the Au 4f7/2, Au 4f5/2, Ag 3d5/2, Ag 3d3/2, and C 1s peaks for the bimetallic Ag/Au graphene plasmonic system shifted to the lower binding energy values compared with the standard BE peaks for pure Au0 (Au 4f7/2 84.0 eV and Au 4f5/2 87.7 eV), Ag0 (Ag 3d5/2368.2 eV, Ag 3d3/2 374.2 eV), which clearly confirms the successful anchoring/deposition of Ag and Au NPs onto the surface of the graphene sheet and also confirms the effective electron transfer between the graphene and metal NPs.48 There were no other peaks noticed, which indicates the high purity of the Ag/Au-graphene plasmonic system. As presented in Figure 6 the FT-IR spectra of GO reveal the characteristic broad band between 3000 and 3700 cm−1 and simultaneously a sharp band at 1628 cm−1 that are correlated to the stretching and bending frequencies of OH groups present on the GO surface. The bands observed at 2922 and E


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Figure 4. HR-TEM images of as-synthesized (a−f) bimetallic Ag/Au-graphene plasmonic system.

Figure 5. (a) XPS survey spectrum of Ag/Au-graphene and (b) high-resolution spectra of Ag 3d, (c) Au 4f, and (d) C 1s.

photocatalyst, and it was observed that there is no H2 production. The H2 production rates for all photocatalysts were displayed in Figure 7a. As shown in Figure 7a, for bare graphene and bimetallic Ag/Au system, no H2 production was detected. The rates of H2 evolution for the Ag-graphene, Augraphene, and bimetallic Ag/Au-graphene plasmonic systems

be concluded that there are strong interactions between noble metal NPs and graphene. Photocatalytic Activity. The photocatalytic capabilities of the synthesized plasmonic systems were carried out under solar light illumination for H2 evolution by using glycerol as a hole scavenger. The control experiments were performed without F


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electron capturing and transporting properties of graphene, and (4) the homogeneous distribution of noble metal nanoparticles over graphene surface. Additionally, the bimetallic features of core−shell Ag/Au nanoparticles have triggered the rate of H2 evolution of bimetallic Ag/Au-graphene plasmonic system. In the case of Ag-graphene and Augraphene, the enhanced photocatalytic performance was explained by the work function of metal atoms, i.e., for Ag 4.5 eV 50 and for Au 5.3 eV.51 The enhanced photocatalytic H2 evolution of bimetallic Ag/Au-graphene plasmonic system compared to Ag-graphene and Au-graphene plasmonic systems was further explained by photocurrent studies. The effective charge transfer in the bimetallic Ag/Augraphene plasmonic system was detected by the photoelectrochemical experiments, as shown in Figure 7d. As noticed, the bimetallic Ag/Au-graphene plasmonic system has shown the high amount of photocurrent response compared to Ag-graphene and Au-graphene plasmonic systems. This fact explains the formation of an active electron transfer interface between the graphene, Ag and Au NPs, which further leads to the enhanced photocatalytic performance of the bimetallic Ag/Au-graphene plasmonic system.52 As mention in Table 1, it can be seen that the bimetallic Ag/Au graphene showed a high rate of H2 production than most of the reported nanocarbon based semiconductor materials.53−60 Tentative Photocatalytic Mechanism. On the basis of our experimental investigation, we propose a suitable photocatalytic mechanism for improved H2 evolution over bimetallic Ag/Au-graphene and monometallic (Ag-graphene or Au-

Figure 6. FT-IR spectra of as-synthesized GO, graphene, Aggraphene, Au-graphene, and bimetallic Ag/Au-graphene.

were found to be 20, 28, and 40 μmol h−1 g−1. The typical time courses of H2 evolution for Ag-graphene, Au-graphene, and Ag/Au-graphene plasmonic systems were shown in Figure 7b. As presented in Figure 7a,b, it is noticed that the introduction of these plasmonic nanoparticles (Ag, Au, and Ag/Au) on the surface of the graphene sheet is crucial for the H2 production. The obtained H2 production for Ag-graphene and Augraphene plasmonic systems compared to graphene and bimetallic Ag/Au system is due to combined effects of (1) enhanced visible-light harvesting property, (2) the SPR property of decorated noble metal nanoparticles, (3) the

Figure 7. (a) H2 production rates of as-synthesized (A) graphene, (B) Ag/Au-bimetallic system, (C) Ag-graphene, (D) Au-graphene, and (E) Ag/ Au-graphene plasmonic photocatalytic systems. (b) Analogical studies of photocatalytic H2 evolution during 4 h under solar light illumination for synthesized Ag-graphene, Au-graphene, and bimetallic Ag/Au-graphene plasmonic photocatalytic systems. (c) H2 production of Ag-graphene, Augraphene, and bimetallic Ag/Au-graphene plasmonic photocatalytic systems under solar light illumination. (d) Transient photocurrent experiments of the as-synthesized Ag-graphene, Au-graphene, and bimetallic Ag/Au-graphene plasmonic systems. G


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ACS Applied Nano Materials Table 1. H2 Evolution Rates over Various Nanocarbon Based Semiconductor Photocatalysts sample no.

photocatalyst

amount (mg)

rate of H2 evolution (μmol h−1 g−1)

light source

ref

1 2 3 4 5 6 7 8 9

Ag/Au-graphene carbonnano fibers@ZnIn2S4 RGO/ZnIn2S4 MWCNT/g-C3N4 RGO/ZnIn2S4-hydrazine C-PDA/g-C3N4 g-C3N4-nanocarbon-ZnIn2S4 CdS/graphene NH2-MIL-125(Ti)/reduced graphene oxide

5 30 50 100 50 100 50 100 30

40 95 40.85 7.58 (Pt as cocatalyst) 27.8 81.1 (Pt as cocatalyst) 50.32 70 91

solar light 300 W Xe-lamp (λ > 420 nm) 300 W Xe-lamp (λ > 420 nm) 300 W Xe-lamp (λ > 400 nm) 300 W Xe-lamp (λ > 420 nm) 300 W Xe-lamp (λ > 400 nm) 12 W UV-LED (λ = 420 nm) 200 W Xe-lamp (λ ≥ 420 nm) 300 W Xe-lamp

present work 53 54 55 56 57 58 59 60

Figure 8. Solar-light responsive proposed mechanisms regarding enhanced H2 evolution from water splitting over the (a) monometallic Aggraphene or Au-graphene and (b) bimetallic Ag/Au-graphene plasmonic systems.

graphene) plasmonic systems, shown in Figure 8a,b. It is very well established that the capturing of visible light by plasmon nanoparticles (Ag and Au) involves generation of the hot electron−hole pair within the noble metal through Landau damping/SPR excitation,52,61−63 according to the conventional plasmon induced direct hot-electron transfer (PDHET) mechanism. As displayed in Figure 8a, under sunlight illumination the generated hot electrons from Ag or Au transferred to the next Ag or Au nanoparticles through the graphene and involved H+ reduction to H2 in the Ag-graphene or Au-graphene plasmonic systems. But in the case of bimetallic Ag/Au-graphene plasmonic system as shown in Figure 8b, under sunlight illumination both Ag and Au in core−shell nanoparticles produce hot electrons and the generated hot electrons in d-bands of Ag core transfer to the Au shell because the electronegativity of Au (2.54) is more than that of the Ag (1.93). Subsequently, the solid electron transporter such as graphene transfers those hot electrons to the next bimetallic Ag/Au-nanoparticles through its extended π electron skeleton64 and increases their availability for the H+

reduction to H2. The enhancement in H2 evolution rate in bimetallic Ag/Au-graphene plasmonic system is attributed to both metal nanoparticles (Ag and Au) in core−shell structure supply electrons continuously, those directly involved in the reduction of 2H+ to H2.

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CONCLUSIONS In summary, the bimetallic Ag/Au graphene plasmonic system was successfully fabricated by one-pot in situ green reduction method for enhanced H2 production from solar-light responsive water reduction. SEM and TEM results suggest that the Ag and Au NPs were well deposited on the graphene surface. XPS and EDS results confirm the formation of bimetallic Ag/Au-graphene. The bimetallic Ag/Au-graphene plasmonic system exhibits the highest photocatalytic H2 production, i.e., 40 μmol h−1 g−1, which is approximately 2 times greater than that of H2 produced by Ag-graphene plasmonic system and 1.4 times greater than that of H2 produced by Au-graphene plasmonic system. The bimetallic Ag/Au-graphene plasmonic system can effectively promote the H


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charge transfer, which results in the highest photocatalytic H2 production that was explained by photocurrent studies. It is mentioned that the present work provides new insights into the expansion and design of semiconductor-free, cost-effective, and highly efficient nanostructured materials for multiple photocatalytic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00684. Synthesis of graphene oxide and Figure S1 showing PXRD pattern of bimetallic Ag/Au nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lakshmana Reddy Nagappagari: 0000-0002-3327-0184 Shankar Muthukonda Venkatakrishnan: 0000-0002-5284-1480 Vishnu Shanker: 0000-0002-1341-2448 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Saikumar Manchala thanks the MHRD, Government of India, for providing a fellowship. REFERENCES

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