Core â shell novel composite metal nanoparticles for hydrogenation

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Kinetics, Catalysis, and Reaction Engineering

Core – shell novel composite metal nanoparticles for hydrogenation and dye degradation applications Sameer Kulkarni, Mangesh Jadhav, Prasad Raikar, Shantanu Raikar, and Uday S Raikar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06094 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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Core – shell novel composite metal nanoparticles for hydrogenation and dye degradation applications Sameer Kulkarni,a, b Mangesh Jadhav,a Prasad Raikar,c Shantanu Raikar,d and Uday Raikar*,a a

Department of Physics, Karnatak University Dharwad – 580003, Karnataka, India b

c

SKE Society GSS College, Belgaum, Karnataka, India

Dept. of CAE, Centre for Post graduate studies, Visvesvaraya Technological University, Muddenahalli, Chikkaballapur – 562101, Karnataka, India

d

Department of Microbiology, Karnatak University Dharwad – 580003, Karnataka, India *

Corresponding Author: [email protected]

Abstract Biofriendly green sustainable nanocatalyst Au@NiAg with high catalytic activity prepared for hydrogenation and degradation of aquatic contaminants like 4 – nitrophenol, Methyl Orange and Orange – G. Synthesized Au@Ni nanoparticles are having triangular shape while the Au@NiAg nanoparticles are of spherical shape and of smaller size is attributed to the digestive ripening of the Au@Ni where silver ion deposited on the host Au@Ni nanoparticles surface. The Au@NiAg nanoparticles catalyzed hydrogenation of 4 – nitrophenol and reduction of organic dyes follow Langmuir – Hinshelwood kinetics. The Au@NiAg nanoparticles have shown excellent catalytic activity with activity factor of 3167s-1 g-1, 5476 s-1 g-1 and 3810 s-1 g-1 for 4 – NP, MO and OG respectively. Ag nanoparticles act as co catalyst for overall improving the performance of the Au@NiAg. Above all the involvement of leaf extract mediated synthesis will open an area to the production of sustainable, ecofriendly and nontoxic core shell nanocatalysts with exemplary catalytic activity.

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1. Introduction: Today as the population of world increasing use of harmful chemicals in textile industry and other industries is now an inevitable commodity which has become global concern. Due to industrialization the industrial waste water management is a major ecological problem, posing threat to aquatic life and society at large. Some of the organic pollutants soluble in water, which are toxic in nature. For instance compounds like 4 – nitrophenol, organic dyes like methyl orange, orange – G etc. are not biodegradable. 4 – nitrophenol interaction with blood forms methaemoglobin which is responsible for methemoglobinemia, causing cyanosis, confusion, and unconsciousness1. Plenty of research work is happening in recent years to have 4 – aminophenol from hydrogenation of 4 - nitrophenol by using sodium borohydride as reducing agent with the help of nanocatalysts such as core shell noble metal nanoparticles which is chemically viable2,3,4,5. Noble metal nanoparticles like Au and Ag show higher catalytic activity but tend to agglomerate due to their high surface energy by virtue of which, eventually large reduction in the catalytic performance of the metal nanoparticles. To circumvent this problem surface functionalization by organic molecules or by other inorganic chemically active metal or metal oxides is necessary, resulting in the formation of noble metal core and shell formation6. The core shell nanoparticles based on noble metals have gained considerable importance because the knowledge of core shell thickness, shape, size play a crucial role in the catalytic7, SERS8, anti microbial9 applications. The metallic core shell alloy nanoparticles are garnering significant interest from the scientific fraternity because of their unique optical, chemical and physical properties. Various core shell nanoparticles have been proposed as catalysts for reaction such as Ni@PtNi nanoparticles supported on rGO10, Ag@CeO2 nanoparticles for the reduction of 4 – 2 ACS Paragon Plus Environment


Nitrophenol11, Pd @Pt for olefin reduction12, Pd@Ag7, Au@Ni nanoparticles13 for partial hydrogenation of Phenyl acetylene. Shao’s group has reported Pt –Ni nanoparticles with higher catalytic activity than the available catalyst for oxygen reduction 14. Au @ Ni nanoparticles have been synthesized for various potential applications like dehydrogenation of Ammonia Borane15, nitrophenol reduction16 and surface enhanced Raman spectroscopy (SERS)8. However there is a need to develop a simple cost effective, environmentally benign method to synthesize core shell with noble metal alloy nanoparticles having potential applications in the fields like catalysis, medical, energy harvesting, efficient conversion, supercapacitor and sensors. In literature there are numerous papers on synthesis of various nanoparticles but there is a limited number of publications reported on leaf extract mediated synthesis on ternary core shell nanoparticles. We report synthesis of novel Au@Ni and Au@NiAg core shell nanoparticles by employing green leaf extract mediated synthesis. The synthesized core shell nanoparticles were subjected to the structural, physical, elemental and morphological studies by employing the techniques like, UV – VIS absorption spectrum, DSC, AFM, XRD, HAADF – STEM and TEM. Furthermore the catalytic activity of synthesized Au@Ni, Au@NiAg core shell nanoparticles is assessed by reduction of 4 – nitrophenol to 4 – aminophenol, Methyl Orange and Orange - G. Synthesized Au@NiAg nanoparticles have shown excellent catalytic activity towards reduction of 4 – nitrophenol, Methyl Orange and Orange – G organic contaminants. We have showed that how Ag nanoparticles will act as co catalyst by virtue of which the enhancement in catalytic activity of Au@NiAg nanoparticles as compared to other reported catalysts. The relay of electrons from Ag to Au@Ni nanoparticles will enhance the catalytic ability of nanocatalyst as shown by the results obtained indicating the excellent catalytic activity than the previously 3 ACS Paragon Plus Environment

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reported nanocatalysts. Above all the involvement of leaf extract mediated synthesis will open new window of knowledge for the production of sustainable, ecofriendly and nontoxic core shell nanocatalysts with high catalytic activity. 2. Materials and Methods: 2.1 Chemicals: Nickel chloride, Gold chloride, Silver nitrate, 4 – nitrophenol, Orange G, Methyl Orange, Ethanol (EtOH) and Sodium borohydride were purchased from Hi- media. All chemicals were of analytical grade and used without further purification. The Millipore milli – Q grade deionized water is used as solvent. 2.2 Preparation of Adathoda Vasica Leaf Extarct: The Adatoda Vasica leaves from botanical garden in Karnatak University (at Dharwad, Karnataka state, India) were collected. The collected leaves were washed with deionized (DI) water to remove dust and other impurities and dried. 15gm of fine chopped leaves were added to the 100ml of deionized water in a beaker and heated in 900C for about 30min. on a heater with magnetic stirrer followed by the filtration to separate the broth. The leaf extract is stored in freezer for further synthesis of nanoparticles. The pH of the leaf extract is 5.5. 2.3 Green synthesis of Au@NiAg core shell nanoparticles: A two step method is used to synthesize Au@NiAg core shell nanoparticles. Firstly the Au@Ni core shell nanoparticles is prepared by taking aqueous solutions of 1:1 molar ratio of Gold chloride and Nickel chloride. To the above aqueous solution 5ml of Adathoda vasica leaf extract is added and refluxed in the 800C for 10minutes. The color of the solution changes colorless to blue indicating the formation of Au@Ni core shell nanoparticles. The formed Au@Ni nanoparticles were washed with DI water to free from impurities. 4 ACS Paragon Plus Environment


The above said Au@Ni core shell nanoparticles are dispersed in 1mol aqueous solution of Silver nitrate solution and 2.5ml of Adathoda vasica leaf extract is added to the solution. The reaction is carried out in 800C for 10mins & with constant stirring till the color of solution changes to red brown indicating the formation of Au@NiAg core shell nanoalloy. The reaction is cooled to room temperature, centrifuged and washed several times from deionized water and dried to get Au@NiAg nanoparticles for further studies. 2.4 Characterization: The optical properties of the synthesized Au@Ni and Au@NiAg core shell nanoparticles were studied using UV – Vis spectra (UV – VIS spectrophotometer, Ocean Optics, HR4000). The structural and crystal analysis is carried out using X – ray diffractometer (Rigaku) with Cu Kα = 1.54178Å over the 2ϴ angles ranging from 200 – 1000. FTIR – spectra of core shell nanoparticles are recorded in the range 400 – 4000cm-1 using NICOLET 6700, USA instrument. Shape, size of the core shell nanoparticles were carried out using transmission electron microscope (TEM, Tecnai, FEI Company, USA). HAADF-STEM and EDS analysis of Au@Ni and Au@NiAg core–shell nanoparticles were done (TEM- FEI, TITAN, Themis 60-300kV). Morphological studies were carried out using Atomic force microscopy (AFM, Nanosurf, USA). For TEM studies the aqueous solution of synthesized Au@Ni and Au@NiAg core shell nanoparticles were drop casted on copper grid coated with carbon and allowed to dry overnight in open air. The oxidation property of the core shell

nanoparticles was characterized by DSC (2960

Simultaneous instrument) with a heating rate of 10 °C/min from 50 °C to 400 °C under air atmosphere. Elemental composition is studied by X-ray photoelectron spectroscopy (PHI 5000 Versa prob II, FEI Inc).


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2.5 Evaluation of Catalytic activity: The hydrogenation of 4-NP by NaBH4 was chosen as a model reaction for testing the efficiency of the Au@Ni and Au@NiAg core shell nanoparticles. Aqueous solutions of 4-nitrophenol (2.0 mL, 0.125 mM) and NaBH4 (0.1 mL, 0.1 M), were added to a cuvette, stirred the solution at room temperature for 5 min. After adding the catalyst (0.06 mL for Au@NiAg, 0.250mL for Au@Ni, 0.14 mg/mL), the bright yellow solution gradually faded as the reaction progressed. UV-VIS spectra of the reaction mixture were recorded during the course of the catalytic hydrogenation process. For the recycling experiment, the catalysts were separated from solution by centrifugation (5000 rpm, 10 min), and re dispersed into DI water for the next cycle of catalysis. For the degradation of Methyl Orange & Orange G organic dyes, the following typical procedure was used: 2.0 mL aqueous solutions of the dye [MO (1 X 10-4M) & OG (1 X 10-4M)] and freshly prepared NaBH4 (0.1 M, 50µL) were added into a standard quartz cell (path length 1 cm), respectively and then, 60µL of freshly prepared aqueous solution of the Au@NiAg catalyst (0.14mg/mL) was added into the above solution. The advancement in the catalytic reaction was monitored by tracing the changes in the absorption spectra of the mixture at small intervals using an UV-vis spectrophotometer. For comparison, the catalytic activity of Au@Ni catalyst (250µL, 0.14mg/mL) is also studied. 3. Results and Discussion: Green synthesis of Au@Ni and ternary Au@NiAg core shell nanoparticles is carried out. The prepared core shell nanoparticles were subjected to morphological, optical, structural and elemental studies. Furthermore the synthesized ternary Au@NiAg core shell nanoparticles were



studied for catalytic hydrogenation of 4 – nitrophenol to 4 – aminophenol and degradation of Methyl orange and Orange G dyes in room temperature. 3.1 FTIR Analysis: FTIR spectra of Au@Ni and Au@NiAg core shell nanoparticles are presented in figure 1 (a). The vibrational peak at 619cm-1 in both the figure represents Ni-O-H vibrations. Whereas the analysis of other peaks is given in the table.1 confirms the presence of quinazoline or vasicine alkaloids present in the adathoda vasica leaf extract indicating the alkaloids are responsible for the formation of stable Au@NiAg and Au@Ni nanoparticles. 3.2 Optical studies: The metal nanoparticles show the SPR peak in the visible region can be used as the preliminary tool for the identification of formation of metal nanoparticles. In figure1 (b) we observed that the dampened SPR peak at 580nm which is due to Au core. The SPR peak is visible because of very thin Ni shell as discussed by L. Huang et.al 17 for Au@Ni nanotriangles where as for Au@NiAg nanoparticles we observed that there are two plasmonic peaks at 455nm and at 526nm corresponding to Ag and Au respectively. 3.3 Thermal Studies: The DSC data of both Au@Ni and Au@NiAg show similar nature, whereas in Au@Ni nanoparticles two exothermic peaks appeared in the DSC curve (figure1 (c)). The peak located in the range 250 – 3470C peaked at 3200C was attributed to the oxidation of Nickel 18 as well as the peak located in the range 339 – 3590C peaked at 3550C can also be attributed to the Nickel shell nanoparticles19. Similarly in DSC graph of the Au@NiAg core shell nanoparticles the peak located in the range 250 – 3470C peaked at 3200C can be attributed to oxidation of nickel nanoshell 18, whereas the peak at 3880C can be due to the oxidation of Ag nanoparticles 20.


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3.4 XRD Analysis: X –ray diffraction is carried out to study the crystal structure of the synthesized Au@Ni and Au@NiAg core shell nanoparticles. XRD spectrum of Au@Ni and Au@NiAg core shell nanoparticles is shown in the figure 2 (a & b). XRD pattern of Au@Ni shows the peak of Au at the angles 2ϴ = 38.10, 44.30, 64.50, 77.50 & 81.70 with the arrangement of the atoms to the planes (111), (200), (220), (311) & (222) respectively showing face centered cubic structure system with JCPDS No. 04 – 0784. Some of the Ni peaks are also present in the XRD pattern of Au@Ni core shell nanoparticles, viz 2ϴ = 76.30 and 98.40 with face centered cubic lattice structure (JCPDS No.04 - 0850). While in the XRD spectrum of Au@NiAg core shell nanoparticles the Au peaks at the angles 2ϴ = 38.10, 44.30, 64.50, 77.50 & 81.70 with the arrangement of the atoms to the planes (111), (200), (220), (311) & (222) respectively remains same but in addition to that Ag (JCPDS No. 04-0783) peaks at 2ϴ = 38.10, 44.30, 64.50, 77.50 & 81.70 corresponding to the planes (111), (200), (220), (311) & (222) respectively with face centered cubic lattice structure are also appears can be assigned by the change in the intensity of peaks. Also some of the peaks of Ni are observed at 2ϴ = 450, 76.30 and 98.40 corresponding to the planes (111), (220) & (222) with FCC structure (JCPDS No.04 - 0850). 3.5 TEM Analysis: The TEM image of Au @ Ni core shell nanoparticles is shown in the figure3 (a). The average size of the Au@ Ni nanoparticles is mostly of about 115nm having triangular shapes as shown in TEM (figure 3 (a)) as well as in AFM image (supplementary figureS1.(a)). The formation of Au@Ni nanotriangles is governed by the addition of plant extract as discussed by the Pratap Chandran et.al.21. The amount of the extract added to the reaction mixture decides the shape of the nanoparticles. Small amount of extract will lead to the formation of Au@Ni nanotriangles.



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The SAED pattern is shown in the figure3 (b). From the SAED pattern we can see that number of circular concentric rings with bright spots showing the different planes of the lattice structure. The ring patterns in the SAED image corresponds to the (111), (200), (220) and (311) planes of Au core nanoparticles (JCPDS No. 04 – 0784) whereas (200), (311), (222) & (400) corresponds to the Ni shell nanoparticles (JCPDS No.04 - 0850). The HAADF – STEM images of Au@Ni core shell nanoparticles are shown in the figure 3 (c -e). The HAADF – STEM image (figure 3 (d)) clearly shows the Au is mostly located in inside the nanotriangle while the Ni is distributed not only inside but also grown outside the nanotriangle indicating the formation of Au@Ni core shell nanoparticles. From the TEM images (figure 4 (a & c)) of Au@NiAg nanoparticles it is clear that the synthesized core shell alloy nanoparticles are uniform in size about 10nm. The size change from Au@Ni nanotriangles to Au @ Ni Ag core shell alloy nanoparticles as indicated by the AFM images (S1, S2, S3) of reaction mixture at different temperatures due to the digestive ripening of the nanoparticles. The AFM image S2 (supplementary information) of the reaction mixture of Silver ions with Au@Ni nanoparticles and plant extract in aqueous medium at room temperature there is no sign of size reduction while in the AFM image S3 (supplementary information) there is clear indication of size reduction indicating formation of Au@NiAg nanoparticles due to digestive ripening of the Au@Ni nanoparticles

22, 23, 24

. Although the

digestive ripening may be partially responsible for the reduction in size of the nanoparticles because the fact that Au will not be oxidized and further reduced in the seeded-growth step. Since the digestive ripening could not explain the significant particle size decrease from Au@Ni to Au@NiAg, the particle size decrease is possibly due to the Adathoda vasica leaf extract. The TEM image shown in the figure4 (c) clearly shows the core shell formation and also from the TEM image it is found that there is an attachment between Au@Ni core shell with Ag


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nanoparticles having interpalanar spacing 0.235nm of Ag (111) plane. The interesting fact is that the change in the shape of the core shell nanoparticles is due to the underpotential deposition of Silver ions on the Au@Ni nanoparticles. It is well known fact that underpotential shift for the deposition of metal ions on the surface of other metal is proportional to the difference in the work function of the metals 25. The work function for Ag (111) face is 4.74eV 26 while for Ni is 5.04 – 5.35eV. This difference in the work function leads to the evolution of spherical (cubo octahedral) structure of the Au@NiAg core shell nanoparticles

27

. The concentric circular ring

having bright spots SAED pattern is shown in figure 4 (b). The concentric rings in the SAED pattern corresponds to the (111), (200), (220) and (311) planes of Au core nanoparticles (JCPDS No. 04 – 0784) whereas (200), (311), (222) & (400) corresponds to the Ni shell nanoparticles (JCPDS No.04 - 0850) as same as that of Au@Ni nanoparticles. The HAADF – STEM images of Au@NiAg core shell nanoparticles are shown in figure 5 (a - d). The figure 5 (b) clearly shows that Au element is concentrated only in the core of the nanoparticles while the figure 5 (c) shows the distribution of Ni element is not only at the centre but also outside the core indicating the formation of core shell structure with Au as the core and Ni as the shell. HAADF – STEM image (figure 5 (d)) clearly shows the distribution of Ag element which is also present in the nanoparticles. The Ag element distribution pattern shows not only shell formation but also the presence of Ag nanoparticles attached with Au@NiAg nanoparticles. 3.6 XPS Analysis: The wide X- ray photoelectron spectra for both Au@Ni and Au@NiAg nanoparticles are shown in figure6 (a) & (b) respectively. The spectra showed the presence of C1s, Au4f and Ni2p peaks in both figure6 (a) & (b) in addition to that the peak of Ag3d is also prominent in case of Au@NiAg nanoparticles. The peak at 284.8eV is due to C-C bond can be attributed to the



phytochemicals i.e. quinazoline or vascisine alkaloids present in the adathoda vasica leaf extract responsible for the reducing as well as capping of the nanoparticles as supported by FTIR analysis discussed in section 3.1. The inset of both the figures 6 a & 6b show Ni2p3/2 at 854.7eV with satellite peak at 858.9eV and Ni2p1/2 at 871.6eV with satellite peak at 877.9eV. The doublet peaks of Au4f (4f7/2 and 4f5/2) are shown in figureS4 (supplementary information) at 82.2eV & 85.9eV respectively. The doublet peaks of Ag3d (3d5/2 and 3d3/2) are shown in figure S5 (supplementary information) at binding energy of 366.7eV & 372.7eV respectively. 3.7 Catalytic Performance of Au@NiAg core shell nanoparticles: 4 – nitrophenol and other organic dyes such as Methyl orange (MO) and Orange G (OG) are the some of the typical examples of organic pollutants posing threat to the aquatic life and humans at large which are not biodegradable. These organic pollutants are common industrial wastes whose uses are regulated by the U. S. Environmental Protection Agency (EPA). Therefore, the reduction of 4 – nitrophenol to 4 - aminophenol and other organic dyes such as Methyl orange (MO) and Orange G (OG) were chosen for the demonstration of catalytic hydrogenation and reduction studies in the present work. Commonly the 4 – nitrophenol aqueous solution has bright yellow color, upon the addition of NaBH4 solution it turns to be green yellow. The formation of 4 – nitrophenolate ions can be observed by the red shift of absorption peak from 317 – 400 under the alkaline conditions invoked by the NaBH4 solution. As shown in the figureS6 (supporting information) there is no change in the intensity of the peak at 400nm for a long time suggesting the reducing agent NaBH4 is not sufficient for the reaction to precede until the introduction of catalyst even with the large excess of NaBH4 (C(NaBH4)/C(4-NP) = 40:1). Upon the addition of the Au@NiAg nanocatalyst (60µl, 0.14mg/ml) the intensity of the absorption peak gradually decreases and complete reduction takes place within 2 minutes and a new peak at 300nm arises


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suggesting the conversion of 4 – NP to 4 – AP as shown in the figure 7 (a), implying the reduction started immediately and no induction time was observed indicating the Au@NiAg has below critical value of dissolved oxygen35 where as for the Au@Ni nanotriangles we can see that it has some induction time (figure 7 (b)) and once the level of dissolved oxygen falls below the critical value the induction period ends35. While the discoloration of the dyes observed within 90 seconds for Au@NiAg nanocatalyst (figure8). These observations indicate that the synthesized nanocatalyst exhibits excellent catalytic activity towards reduction of organic pollutants. The kinetic equation for the degradation reaction can be expressed as follows: ln (Ct / C0) = ln (At / A0) = k t Where Ct is concentration of the organic pollutant at time‘t’, C0 is the initial concentration at time t = 0. The ratio Ct / C0 is given by the intensity of the absorption at corresponding time‘t’ to the initial absorption intensity. k is the apparent rate constant. For our catalyst Au@NiAg we have got k for 4 –NP is 0.0266s-1, MO is 0.046s-1 and for OG is 0.0324s-1(figure 8) which are excellent as compared to earlier reported values (Table.2). To compare our results with the other reported values we calculated the ratio of k to the amount of catalyst used ‘m’. K = k / m and known as the activity factor. The activity factor for the Au@NiAg for 4 – NP is 3167s-1 g-1, while for MO is 5476 s-1 g-1 and for OG is 3810 s-1 g-1. We have compared our obtained value with earlier reported values and found that our catalyst is far more excellent than the reported values (Table. 2). The reusability of the Au@NiAg nanocatalyst was also evaluated in the conversion of 4 – NP to 4 - AP. As shown in figure9 (a), the catalyst can be recycled and reused for at least six successive cycles, maintaining almost the same level of conversion for six consecutive cycles. These experiments prove a good stability of the Au@NiAg nanocatalyst in the reaction medium.



Scheme1.Mechanism of Conversion of 4 – NP to 4 – AP The schematic representation of the tenable mechanism of the hydrogenation of 4 – NP to 4 – AP in presence of Au@NiAg nanocatalyst is shown in scheme 1. It has been observed that the catalytic reaction on the surface of the Au@NiAg nanocatalyst follows Langmuir - Hinshelwood mechanism, in which both BH4− ions and 4 – NP will adsorbed at the beginning on the surface of the Au@NiAg nanoparticles. The BH4− adsorption on the surface of the nanocatalyst leads to the formation of active hydrogen species which activates the hydrogenation of the adsorbed 4 – NP leads to the formation of 4 –AP. In fact Au@NiAg nanoparticles show an excellent catalytic activity towards the 4 – NP as well as other two dye molecules with excellent K values of about 3167s-1 g-1, 5476 s-1 g-1 and 3810 s-1 g-1 for 4 – NP, MO and OG respectively because of the following possible reasons. Firstly the inorganic core shell formation Au@Ni prevents the agglomeration of Au nanoparticles during the catalytic process by virtue of which retaining the catalytic efficiency

11

. Secondly the size of the catalysts will play an important role in the

catalytic efficiency of the catalysts which can be given by the following formula 1

1

1

r

= (4πr2 ) [(k ) + D] -------------(1) k et

Where r is the radius of the nanoparticles, D is the diffusion co-efficient, ket is the electron transfer rate, kapp is the rate constant. From the above equation smaller the size of the


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nanoparticles higher is the rate of electron transfer which is playing a major role in case of Au@NiAg core shell nanoparticles of about 10nm takes 2 minutes in the catalytic conversion of 4 –NP to 4 –AP. While Au@NiAg core shell nanoparticles prepared at room temperature of about 90nm takes 5minutes in the conversion of 4-NP to 4 –AP (FigureS9). The Ag nanoparticles along with Au@Ni nanoparticles act as host active catalytic sites improving the catalytic performance due to synergistic effect. We have performed the catalytic hydrogenation of 4 – NP in presence of Ag nanoparticles (figure S13 (a)) and the physical mixture of Au@Ni with Ag (figure S13 (b)) nanoparticles and we found that the time for the complete hydrogenation of 4 – NP is 10minutes and 7minutes respectively. Indicating Ag nanoparticles act as co catalysts and synergistic effect is responsible for the efficient reduction reaction as compared to Au@Ni nanoparticles alone. The figure9 (b) shows that in presence of hydroxyl scavengers like EtOH (500mM) the conversion of 4 – NP to 4 - AP slows down. The super oxide radicals formed on surface of the Au@NiAg nanoparticles are scavenged by EtOH hindering the conversion process. Pointing the super oxide radicals like hydroxyl radicals are having a crucial role in the catalytic performance of the Au@NiAg nanoparticles as suggested by Shen et al 36. 4. Conclusion: In present work we have synthesized and characterized a new ecofriendly environmentally benign, highly efficient, recyclable and nontoxic nanocatalyst Au@NiAg core shell nanoparticles. By employing green synthesis assisted by Adathoda vasica leaf extract we have synthesized Au@NiAg nanoparticles. The synthesized Au@Ni nanoparticles are having triangular shape but the Au@NiAg nanoparticles are of spherical shape and of smaller size be attributed to the digestive ripening and silver ion deposition on the host Au@Ni nanoparticles surface. The synthesized nanoparticles are of uniform size and exhibit core shell structure with 14 ACS Paragon Plus Environment


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alloy formation as indicated by the TEM monographs. The HAADF – STEM images clearly indicate the core shell formation in case of Au@NiAg nanocatalyst. This nanocatalyst is very efficient in reduction of organic pollutants. In fact we have got very high activity factor of 3167s-1 g-1, 5476 s-1 g-1 and 3810 s-1 g-1 for 4 – NP, MO and OG respectively these values are superior as compared to other previously reported catalysts for the such processes. The Ni shell acts as a shield protecting the Au nanoparticles in the core avoiding the agglomeration

40, 41

is

one of the main reason for the excellent sustainable reduction of 4 – nitrophenol as well as the decoloration of dyes. The Ag nanoparticles acts as co catalysts further enhancing the catalytic activity of the synthesized core shell nanoparticles. Nanocatalyst has shown excellent stability in recycled reactions. References: (1) Agency for Toxic Substances and Disease Registry, U.S. Public Health Service. July 1992. (2) Guo, M; He, J; Li, Y; Ma, S; Sun, X. “One-step synthesis of hollow porous gold nanoparticles with tunable particle size for the reduction of 4-nitrophenol”,

J.

Hazard. Mater.,2016, 310, 89. (3) Wu, X.Q.; Wu, X.W.; Huang, Q.; Shen, J.S.; Zhang, H.W. “In situ synthesized gold nanoparticles in hydrogels for catalytic reduction of nitroaromatic compounds”, Appl. Surf. Sci., 2015, 331, 210. (4) Shen, W.; Qu, Y.; Pei, X.; Li, S.; You, S.; Wang, J.; Zhang, Z.; Zhou, J. “Catalytic reduction of 4-nitrophenol using gold nanoparticles biosynthesized by cell-free extracts of Aspergillus sp. WL-Au”, J. Hazard. Mater., 2017, 321, 299.


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Tables: Table 1: FTIR functional group analysis Figure1(a) 3440cm-1 1640cm-1

Figure1(b) 3440cm-1 1640cm-1

Functional Group N-H stretching (amides) Amino acids containing NH2 group

1380cm-1

1380cm-1

C-H deformation

1120cm-1

1120cm-1

C-N bond

Table 2: Comparison of apparent rate constant (k) and activity factor (K) of different catalyst for the reduction of 4-NP.

Catalyst

Size (nm)

k (s-1)

K (s-1 g-1)

Reference

Au@NiAg Au@Ni Hollow porous Cu particles

10 115 500

0.0266 0.0105 0.0093

3167 300 186

This work This work

Au/graphene hydrogel Ag/SNTs-4 Pd/SPB-PS

14.6 7.68 3.8

0.0032 0.0384 0.0044

31.7 142 116

29

MgAl-LDH@Au MgAlCe-LDH@Au AgNi HPGNPs Au Fe3O4@CTS-Au NPs(A) Fe3O4@C16@CTS-Au NPs(G) Ni@PtNi NCs-rGO Cu2O-Cu-CuO Co@SiO2@C/Ni-700 Au@Fe3O4 yolk shell

20 20 14 80 17 300/5 300/2

0.013 0.041 0.031 0.0074 0.0026 0.00864 0.0306

650 2050 156 742 1083 432 1530

32

72 50 103 15

0.0045 0.0104 0.0097 0.0155

257 20.7 129 783

10


28

30 31

32 33 2 3 34 34

37 38 39


Captions and Figures: Figure. 1 a) FTIR spectrum of Au@Ni and Au@NiAg coreshell nanoparticles b) UV VIS absorption spectrum of Au@Ni coreshell nanoparticles, Au@NiAg oreshell nanoparticles c) DSC curves of Au@Ni, Au@NiAg coreshell nanoparticles in air atmosphere Figure.2 XRD patterns of (a) Au@Ni, (b) Au@NiAg coreshell nanoparticles Figure.3 HRTEM images (a) of Au@Ni coreshell nanoparticles, (b) SAED pattern. (c)HAADF – STEM image of Au@Ni core shell nanoparticles and corresponding STEM – EDX elemental mapping of Au & Ni (d & e) Figure.4 HRTEM images (a& c) of Au@NiAg coreshell nanoparticles, (b) SAED pattern of Au@NiAg coreshell nanoparticles. Figure.5 HAADF – STEM image of (a) Au@NiAg core shell nanoparticles and corresponding STEM – EDX elemental mapping of Au, Ni & Ag (b-d) Figure.6 XPS spectra of (a) Au@Ni core shell nanotriangles. Inset shows the detailed spectra of Ni2p. (b) Au@NiAg nanoparticles with inset showing detailed spectra of Ni2p.

Figure.7 a) UV-vis absorption spectra of the reaction mixture at 0, 30, 60, 90 and 120s in the reduction of 4- NP to 4-AP with NaBH4 catalyzed by Au@NiAg coreshell nanoparticles.The inset show the plot of ln(Ct/C0) vs. reaction time. Reaction conditions: Aqueous solutions of 4- NP (2.0 mL, 0.125 mM), NaBH4(0.1 mL, 0.1 M) and Au@NiAg(0.06mL of 0.14mg / mL). b) UV-vis absorption spectra of the reaction mixture at 0, 60, 120, 180, 240, 270s in the reduction of 4-NP to 4-AP with NaBH4 catalyzed by Au@Ni coreshell nanoparticles. The inset show the plot of ln(Ct/C0) vs. reaction time. Reaction conditions: Aqueous solutions of 4-NP (2.0 mL, 0.125 mM), NaBH4 (0.1 mL, 0.1 M) and Au@Ni (0.25mL of 0.14mg / mL).

Figure.8 Calibration curves as a function of ln(Ct/C0) vs. reaction time for the reaction mixtures containing aqueous solutions of 4 – NP, MO, OG with NaBH4 and Au@NiAg nanocatalyst Figure.9 a) Reusability of the Au@NiAg nanocatalyst in the conversion of 4 –NP to 4 –AP in six successive cycles. b) The effect of scavenger EtOH on the degradation of 4 - NP in presence of Au@NiAg nanoparticles 23 ACS Paragon Plus Environment

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a

b

c

Figure.1 a) FTIR spectrum of Au@Ni and Au@NiAg coreshell nanoparticles b) UV VIS absorption spectrum of Au@Ni coreshell nanoparticles, Au@NiAg coreshell nanoparticles c) DSC curves of Au@Ni, Au@NiAg coreshell nanoparticles in air atmosphere



a

b

Figure.2 XRD patterns of (a) Au@Ni, (b) Au@NiAg coreshell nanoparticles


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a

b

c

d

e

Figure.3 HRTEM images (a) of Au@Ni coreshell nanoparticles, (b) SAED pattern. (c)HAADF – STEM image of Au@Ni core shell nanoparticles and corresponding STEM – EDX elemental mapping of Au & Ni (d & e)



a

b

Figure.4 HRTEM images (a& c) of Au@NiAg coreshell nanoparticles, (b) SAED pattern of Au@NiAg coreshell nanoparticles.

a

b

c

d

Figure.5 HAADF – STEM image of (a) Au@NiAg core shell nanoparticles and corresponding STEM – EDX elemental mapping of Au, Ni & Ag (b-d) 27 ACS Paragon Plus Environment

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a

b

Figure.6 XPS spectra of (a) Au@Ni core shell nanotriangles. Inset shows the detailed spectra of Ni2p. (b) Au@NiAg nanoparticles with inset showing detailed spectra of Ni2p.



Figure.7 a) UV-vis absorption spectra of the reaction mixture at 0, 30, 60, 90 and 120s in the reductionof 4- NP to 4-AP with NaBH4 catalyzed by Au@NiAg coreshell nanoparticles.The inset show the plot of ln(Ct/C0) vs. reaction time. Reaction conditions: Aqueous solutions of 4- NP (2.0 mL, 0.125 mM), NaBH4(0.1 mL, 0.1 M) and Au@NiAg(0.06mL of 0.14mg / mL). b) UV-vis absorption spectra of the reaction mixture at 0, 60, 120, 180, 240, 270s in the reduction of 4-NP to 4-AP with NaBH4 catalyzed by Au@Ni coreshell nanoparticles. The inset show the plot of ln(Ct/C0) vs. reaction time. Reaction conditions: Aqueous solutions of 4-NP (2.0 mL, 0.125 mM), NaBH4 (0.1 mL, 0.1 M) and Au@Ni (0.25mL of 0.14mg / mL).


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Figure.8 Calibration curves as a function of ln(Ct/C0) vs. reaction time for the reaction mixtures containing aqueous solutions of 4 – NP, MO, OG with NaBH4 and Au@NiAg nanocatalyst

a

b

Figure.9 a) Reusability of the Au@NiAg nanocatalyst in the conversion of 4 –NP to 4 –AP in six successive cycles. b) The effect of scavenger EtOH on the degradation of 4 - NP in presence of Au@NiAg nanoparticles



Supporting Information: AFM images of Au@Ni and Au@NiAg nanoparticles at different temperatures HAADF – STEM image of Au@Ni nanoparticles XPS spectra of Au4f and Ag3d spectra of Au@NiAg nanoparticles Time dependant UV VIS spectra of 4 – NP, MO, OG in presence of Sodium borohydride with and without Au@NiAg, and Au@Ni nanoparticles Time dependant UV VIS spectra of 4 – NP in presence of Sodium borohydride with Ag and Au@Ni/Ag nanoparticles


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