Polymer Stabilized Bimetallic Alloy Nanoparticles: Synthesis and Catalytic Application

Polymer Stabilized Bimetallic Alloy Nanoparticles: Synthesis and Catalytic Application

Colloid and Interface Science Communications 24 (2018) 62–67 Contents lists available at ScienceDirect Colloid and Interface Science Communications ...

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Colloid and Interface Science Communications 24 (2018) 62–67

Contents lists available at ScienceDirect

Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom

Polymer Stabilized Bimetallic Alloy Nanoparticles: Synthesis and Catalytic Application ⁎

Charu Dwivedia, , Abhishek Chaudharyb, Srija Srinivasanc, Chayan K. Nandic,

T



a

Department of Chemistry, School of Physical Sciences, Doon University, Dehradun, Uttarakhand, India Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India c School of Basic Sciences, Indian Institute of Technology, Mandi, Himachal Pradesh, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Bimetallic nanoparticles Alloy nanoparticles 4-Nitrophenol 4-Aminophenol

A simple, single pot synthesis method has been described for the synthesis of AueAg alloy nanoparticles using cationic long chain polymer. Unlike other methods for alloy NP synthesis, our method is unique because of its fast synthesis rate and robust nature. The high stability of the synthesized monometallic and bimetallic nanoparticles is attributed to the combined effect of the steric hindrance and electrostatic repulsion provided by polymeric stabilizer. Efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of alloy nanoparticles and NaBH4 is investigated and is found to depend upon the gold content in the nanoparticles.

1. Introduction Bimetallic nanoparticles (BNPs) are emerging as a new class of nanomaterial due to their unique electronic, catalytic and optical properties [1]. Such fascinating characteristic properties of BNP are different from the single component metallic nanoparticles. Moreover, these properties of the BNP can be tailored according to the specific application by controlling the size, shape and metallic composition of the particles [2]. BNP containing gold as one of the elements have begun to show opportunities for developing novel catalytic systems. Several research groups have already demonstrated diverse production techniques of bimetallic nanoparticles of this type [3]. A large segment of the studies focus on noble metal Au core-shell nanoparticles. Metallic core and gold shell combination is in particular seems very appealing as gold provides an established platform for surface functionalization, and being a noble metal gold shell provides protection against oxidation and helps to maintain long-term stability of the particles [4]. Despite all these advantages, the core-shell particles have their own challenges. For example, it is difficult to control the uniformity and thickness of the metal shell and additionally the high density of the grain boundaries at the gold surface offers poor diffusion barrier [5]. These shortcomings of the core shell BNP can be minimized by forming alloy nanoparticles. Gold containing alloy nanoparticles are solid solutions where other metal atoms substitute gold sites in the face center cubic lattice [6]. The incorporation of even a little content of the other metal changes the catalytic activity significantly. Haruta and Hutchings, in their groundbreaking work around 1987, introduced Au based catalysts [7]. Since



then, nano-particulate gold catalysts have been intensively investigated, to be used as catalyst in oxidation and reduction reactions. The aerobic oxidation of alcohols the aldol reaction, addition of alcohols in alkynes, hydrogenation of alkenes and reduction of nitrocompounds, etc., are just to name a few examples [8]. AuePd, AuePt, and AueCu have already been investigated as well as AueAg as bimetallic gold catalysts [9–11]. In case of AgeAu, the difference in the work functions of the two metals leads to electron enrichment in the gold near the interface which facilitates the reduction of the reactant on the gold surface as compared to monometallic nanoparticles [12]. There are many reports available on core-shell AgeAu BNP but literature concerning the synthesis of AgeAu bimetallic alloy nanoparticles is scarce. Very recently, Zhang et al. have reported synthesis of poly(Nvinyl-2-pyrrolidone) (PVP)-protected Ag/Au BNPs by physical mixtures using colloidal dispersions of Ag and Au NPs [13]. In another work, authors have described a method for the synthesis of AgeAu alloy BNPs by exposing maltose coated AgNP seeds to Au3+ ions in EO100PO65EO100 (Pluronic F127) aqueous solutions [14]. The subsequent Au3+ reduction on the surface of the silver NPs leads to alloy BNP formation. Generally AgeAu alloy BNP are synthesized by reacting preformed Ag and Au nanoparticles as co-reduction of silver and gold precursors. However, this method does not yield alloy BNP with uniform size and composition. Herein, we have reported a simple, one pot synthesis method for AgeAu BNP, using a bio-compatible polymer, poly diallyldimethyl ammonium chloride (PDADMAC), as stabilizing agent. The synthesized alloy BNPs have been analyzed thoroughly using various techniques. The catalytic activities of as prepared alloy BNPs with

Corresponding authors. E-mail addresses: [email protected] (C. Dwivedi), [email protected] (C.K. Nandi).

https://doi.org/10.1016/j.colcom.2018.04.001 Received 30 October 2017; Received in revised form 31 March 2018; Accepted 3 April 2018 2215-0382/ © 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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2.5. Catalytic Studies

various compositions have also been investigated for reduction of 4nitrophenol.

The catalytic reduction of 4-nitrophenol is chosen as model reaction to study the catalytic efficiency of both monometallic and bimetallic nanoparticles. This reduction reaction is known to follow pseudo-first order decay kinetics. All reactions were performed in a standard 3 mL quartz cuvette with a 1 cm path length. Stock solutions of the 4-nitrophenol was prepared first and an appropriate amount of the same solution was added into the reaction cell containing predetermined amount of double distilled water. To the 4-NP solution, freshly prepared NaBH4 solution (final concentration in the reaction mixture1.6 mM) was added just before the start of measurement followed by addition of monometallic or bimetallic nanoparticles. To study the effect of the concentration of 4-NP on the rate of reaction, the concentration was varied from 33 to 100 μM keeping concentration of nanoparticles and NaBH4 constant at 100 μL and 1.6 mM, respectively. In another experiments, the effect of the concentrations of the synthesized nanoparticles on the catalytic reduction of 4-NP was investigated by varying the concentrations from 50 to 100 μL at fixed concentration of 4-NP (60 μM) and NaBH4 (1.6 mM). The concentration of 4-nitrophenolate was determined from the absorbance at 400 nm (ε = 19,200 M−1 cm−1).

2. Material and Methods 2.1. Materials All glassware were washed with aqua regia (3 HCl: 1 HNO3), followed by rinsing with double distilled water for several times. All the chemicals gold (III) chloride hydrate (HAuCl4.3H2O 99.99%), sodium borohydride (NaBH4, 99%), silver nitrate (AgNO3, 99.5%), 4-nitrophenol, poly diallydimethylammonium chloride (PDADMAC, 20% aq. solution) were purchased from Sigma Aldrich. Double distilled deionized water (18.3 mΩ, Elga Pure Lab Ultra) was used throughout the preparation of solutions. 2.2. Synthesis of Monometallic Gold and Silver Nanoparticles Monometallic silver and gold nanoparticles were prepared by reduction of AgNO3 and HAuCl4 in the presence of PDADMAC by NaBH4 in aqueous medium. A solution of 100 mM AgNO3 was prepared by dissolving 169.87 mg of AgNO3 in 10 mL water. A stock solution containing 0.1 wt% NaBH4 was prepared by adding 10 mg of NaBH4 in 10 mL of water. 2% PDADMAC aqueous solution was used as stock solution. To 4.15 mL of double distilled water, 100 μL of 2% PADADMAC solution was added followed by addition of 25 μL of 100 mM AgNO3. The solution was stirred well and then NaBH4 solution (250 μL of 0.1 wt%) was added into it with vigorous stirring. The solution turned light yellow instantaneously indicating formation of AgNP. The AgNP colloid solution was left overnight and was used for further studies. The monometallic gold nanoparticles were prepared by similar procedure using HAuCl4 as gold precursor. An instantaneous color change from light yellow to bright ruby red on addition of NaBH4 indicated the formation of GNP.

3. Results and Discussion Fig. 1 shows the UV–vis spectra for gold, silver monometallic nanoparticles and all of the AueAg nanoparticles with varying gold concentration. Au and Ag nanoparticles show their characteristics absorption peaks at 520 and 410 nm, respectively. On simultaneous reduction of gold and silver ions by sodium borohydride in the same solution, gold–silver alloy particles are formed. The alloy formation is concluded from the fact that the optical absorption spectrum shows only one plasmon band and the wavelength at which maximum absorbance occurs in a linear fashion. In general, the core-shell type nanoparticles show two plasmon bands in the absorption reason corresponding to the constituent core and shell metal [15]. In another scenario, if the metal of shell layer forms a thin uniform film (3–4 nm) on the core particles, the surface plasmon absorption band depicts only one peak resulting from the metal of shell layer. Such possibility of core-shell formation is ruled out by the TEM and HRTEM images of the synthesized AueAg nanoparticles [15]. The average size of the monometallic Ag and AuNP is determined to be 11 nm and 6 nm, respectively (Fig. 2a-b, 2e-f and Fig. S1, Supporting information). Although, the polydispersity in the size of the monometallic particles is observed, still the large fraction of nanoparticles has size < 10 nm. The polydispersity in the shape as well as size of the alloy nanoparticles is comparably less than that of monometallic nanoparticles and their average size is found to be ~4 nm. As it is evident from the Fig. 2k, the alloy nanoparticles does not show any difference in the contrast on the surface and core of the nanoparticles, therefore, it can be concluded that the nanoparticles are nano alloy of Ag and Au. Moreover, the distortion observed in the fringe pattern of the AgeAu nanoparticles in Fig. 2l also indicates towards the defects created during alloying of Ag with Au. The co-reduction of the metal salts results in formation of Au0 and 0 Ag in the reaction mixture which coalesce to form nanoparticles. Since, melting point of metal decrease with the decrease in their size, the diffusion coefficient of the metals increases and alloying of Ag and Au takes place in the nanodomain [16]. Moreover, the similarity in the atomic size of Au and Ag also makes the interdiffusion between Au atoms and Ag atoms easy. During the synthesis process the initial nucleation of both Au and Ag nanoparticles may start simultaneously and the growth process completes in several seconds evident by the change in color of the reaction mixture (Fig. 1d). The lattice parameters of silver and gold are almost identical and both crystallize in an fcc lattice an and can form solid solutions (alloy) [17]. This is also evident from the HRTEM image of the AgeAu alloy nanoparticles in which no

2.3. Synthesis of Bimetallic Gold –Silver Alloy Nanoparticles AgeAu alloy nanoparticles were prepared by NaBH4 induced coreduction of silver and gold salts in presence of PDADMAC. The concentration of AgNO3 was kept constant at 0.5 mM and concentration of gold salt was varied from 0.05–2.5 mM. The final concentration of the stabilizing polymer, e.g. PDADMAC was maintained at 0.04% by wt. The concentration of reducing agent was varied from 0.001%-0.01% in different solutions. In all the cases the reaction was instantaneous and accompanied with the visible color change in the solution. 2.4. Characterization of the Synthesized Monometallic and Bimetallic Nanoparticles Particle size and dispersity of the synthesized nanoparticles were characterized by using a TECNAI 200 kV TEM (FEI, Electron Optics). Fast Fourier transform (FFT) analysis was done from selected area of the HRTEM image of the edge of the synthesized nanoparticles, which has been enclosed by the white box in the image. The lattice images were constructed by the selective masking of the spots obtained from the FFT analysis of the area enclosed in the white box. The UV–vis absorption spectra were recorded on a Shimadzu 2450 UV–vis Spectrophotometer. The zeta potential of the nanoparticles were measured by using Zeta Sizer Nano, equipped with a HeeNe laser illumination at 633 nm in a single photon counting mode using avalanche photodiode for signal detection (Malvern Instrument). XRD diffraction patterns were recorded on Rigaku SmartLab X-Ray diffractometer using Cu Kα radiation as X-ray source (λ = 1.5418 Å) at room temperature. The voltage and current for the measurement were kept 45 kV and 100 mA respectively. 63

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Fig. 1. UV–visible spectra of (a) monometallic AuNP and Ag NP, (b) bimetallic AueAg Alloy NP, (c) digital image of the synthesized monometallic and bimetallic NP colloid solutions, (in left to right order, AgNP, Ag:Au::5:1, 2:1, 1:1, 0.5:1, 0.04:1 and AuNP) and (d) plot of maximum absorption of the AueAg NP with increase in Au content.

continuous red shift of surface plasmon band with a significant decrease in the adsorption intensity. The electronegativity difference between Au and Ag is quite considerable and charge transfer occurs between Ag and Au. But the change in the electron density is not responsible for red shift as the bulk plasma frequencies of Au and Ag are identical, however, it does result in damping of SPR band and the absorption intensity decrease [19]. These results are in agreement with the findings of Mulvaney et al. that the increase in Au content causes nonlinear decrease in the absorption intensity [20]. The linear red shift in the SPR with increase in the Au content could be due to the perturbation in the d-band energy levels of the metal to prevent the change in free electron concentration [19]. The application of the synthesized bimetallic NP as catalyst was investigated by carrying out the NaBH4 induced reduction of 4-NP to 4AP. The catalytic efficiency of these bimetallic NP was further compared with that of the monometallic gold and silver nanoparticles. The reduction of 4-NP to 4-AP has been established as a model reaction to evaluate the catalytic activity as this reaction is quite rapid, yield single and stable reduction product and can be monitored spectrophotometrically [21]. The reduction of 4-NP to 4-AP proceeds by an intermediate step involving formation of nitrophenolate ion under alkaline condition (Scheme 1). The electron rich AgeAu alloy nanoparticles surface acts as electron reservoir and facilitates the reduction of 4-NP into 4-AP. The positive surface charge on the synthesized nanoparticles attracts the negatively charged nitrophenolate ions as well as BH4− towards the metallic nanoparticles surface and hence facilitates the electron transfer from BH4− (donor) ion to the nitrophenolate (acceptor) ion through metallic nanoparticle surface [22]. This electrostatic pull offered by the positively charged nanoparticles decrease the induction time as observed by many researches and as soon as the NaBH4, is added into the reaction mixture the catalytic reduction of 4-

recognizable segregation of Ag or Au nanoparticles can be visualized (Fig. 2i). The multi-ring pattern in the SAED of the AgeAu composite NP also confirms the alloying of the two metals (Fig. S2, Supporting information) [18]. The XRD patterns and HRTEM image of the synthesized monometallic and bimetallic nanoparticles (Ag:Au::1:1) are depicted in Fig. S3, Supporting information. It is observed that the XRD patterns for AueAg bimetallic systems exhibit broader characteristic peaks and the characteristic peaks for Au and Ag cannot be distinguished. The XRD data is in accord with the SAED pattern of the bimetallic nanoparticles and suggests the formation of poorly crystalline and less ordered structures as usually observed for nanoparticles. The synthesized nanoparticles are found to be quite stable over the time. The stability of nanoparticles can be explained by the combined effect of steric hindrance and the electrostatic repulsion provided by the stabilizing PDADMAC chains. The zeta potential of the monometallic and the bimetallic nanoparticles is determined to be in the range of +34 to +40 mV for different nanoparticles (Table S1, Supporting information). The effect of polymer concentration, metal precursors and reducing agents was also investigated by synthesizing the monometallic and bimetallic nanoparticles at different reaction conditions. With the increase in NaBH4 concentration a slight blue shift in the SPR band of nanoparticles is observed indicating towards smaller sized particles (Fig. S4, Supporting information). However, the concentration of metallic precursors cannot be varied in a beyond a certain range as the positively charged PDADMAC tends to flocculate in presence of concentrations of high metal ions. The effect of concentration each metal ion Ag+ and AuCl4−on the formation of bimetallic nanoparticles was also studied. As it can be seen from Fig. 1c, with the increase in the Au content in the bimetallic nanoparticles, the SPR band shifts towards red side and color of the solution also changes from yellow to reddish brown (Fig. 1d). The increase in the gold content brings about the 64

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Fig. 2. HRTEM image and corresponding particle size distribution histogram of (a–d) monometallic GNP, (e–h) AgNP and (i–l) AgeAu bimetallic alloy nanoparticles (Ag:Au::1:1). The distortion in the lattice fringes in image l indicates alloy formation. 65

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synthesized monometallic and bimetallic catalyst systems are given in Table1. As evident from Fig. 2 the difference in size of monometallic and bimetallic nanoparticles is not very large and for comparing the rate of reaction for 4-NP reduction, all the nanoparticles were synthesized by near uniform synthesis procedure. The rate of catalytic reduction of 4NP to 4-AP is quite fast with monometallic AuNP and the bimetallic alloy nanoparticles with higher gold content and is slowest for monometallic AgNP (Fig. S5 and S6, Supporting information). However, the fastest reduction is observed in case of bimetallic alloy nanoparticles with Ag:Au of 0.2:1 (Fig. S7, Supporting information). For subsequent compositions with the higher silver content, a decrease in the catalytic efficiency is observed (Fig. S8–S10 Supporting information). Higher silver content has changed the electronic property of gold and the catalytic activity is decreased still the catalytic efficiency of the synthesized alloy nanoparticles is quite higher than the reported alloy nanoparticles. The kapp values for the reduction reaction are determined at different concentrations of nanoparticle catalyst and 4-NP and furnished in Table 1. The effect of concentration of catalyst and 4-NP is also studied. It is observed that the rate of reaction increases with increase in the catalyst concentration or decrease in the concentration of 4-NP (Fig. S11–16, Supporting information). The variation in time taken for the completion of reaction is linear with concentrations of the catalyst and the reactant as reported in the literature. 4-NP reduction to 4-AP is a multistep process and progress of reaction is promoted by metal catalyst. During reaction, metallic catalyst binds with the 4-Nitrophenolate ion through two oxygens of nitro groups. In case of excessively strong binding of adsorbate with metal, the reaction will not occur and if this binding is very weak then the reactant/adsorbate will not have any interaction with the catalyst at all [24]. These metal-reactant interactions or in other words the strength of chemisorption depends significantly on the electronic properties and hence d-band center of metal catalyst. The electronic states of the incoming adsorbate overlap with d band of metal and split it into bonding and antibonding states [25]. This phenomenon pushes down the d-band further below the Fermi level and the population in the antibonding states increases which in turn weakens the interaction of the adsorbate with metal. Therefore, by optimizing the position of d band centre, chemical reactivity of the catalyst can be improved. This could be the reason for the highest catalytic efficiency of the Ag:Au:: 0.2:1 and significantly higher catalytic efficiency of the all the alloy nanoparticles composition as compared to monometallic AgNP. The additional reason for the lowest catalytic efficiency of AgNP could be due to its relatively larger size as the position of d band center from Fermi level also varies with the size of metal catalyst. For PtNP and AuNP is observed that the d band center shifts towards Fermi level with the decrease in the size of NP [26]. It indicates that, on interaction with adsorbate's sp. and metal d band, the antibonding states will be pushed upwards and will be higher in the energy from Fermi level. The population in bonding state will be high and the interaction will be stronger. The band center of Ag and Au are much below the Fermi level and the same of Ag is even

Scheme 1. Schematic representation of reduction of 4-NP to 4-AP in presence of AgeAu alloy nanoparticles.

NP starts. The excess NaBH4 used in the reactions shifts the pH of the reaction to alkaline side which not only converts the 4Nitrophenol to 4Nitrophenolate ion but also prevents BH4− ion from rapid hydrolysis [23]. The nitrophenolate ion has strong absorption at 400 nm and hence the catalytic activity of the nanoparticles can be monitored by tracking the changes in the absorption spectra of 4-nitrophenolate ion at 400 nm [21]. Fig. S5–S11 (Supporting information), show the absorbance spectra of nitrophenolate ion, at different time intervals, in the absence and presence of the synthesized monometallic and bimetallic nanoparticles. With the progress of reaction a gradual decrease in the absorption peak at 400 nm and concomitant appearance of a new peak at 298 nm corresponding to 4-AP is observed. From the absorption data it can be seen that the reduction reaction completes within a time 4.5 min, 33 min and 2.5-to-26 min in the presence of AuNP, AgNP and different compositions of AueAg alloy NP, respectively, when concentration of 4-NP is 53 μM and the volume of nanoparticle sol is 50 μL in 3 mL. The reaction was found to be faster for AuNP catalyzed system than all the AgeAu NP composition except Ag:Au::0.2:1. Since the concentration of NaBH4 used in the reaction is quite higher than that of 4-NP or the metal catalysts, the reaction can be considered to follow pseudo-first order rate kinetics. Therefore, the apparent rate constants (kapp) for the metal NP catalyzed reaction were calculated from the slope of linear sections of plot of At/Ao vs time, i.e., the ratio of absorbance At of 4-NP at time, t, to its value A0 measured at t = 0, (Fig. S5, Supporting information). The apparent rate constant kapp can be determined from Eq. (1) [21].

dCt = kapp Ct dt

(1)

where Ct is concentration of 4-NP at time t. The values of kapp, for the

Table 1 Pseudo first order apparent reaction rate constants determined for reduction of 4-NP to 4-AP using synthesized monometallic and bimetallic nanoparticles. Sample Ag:Au

kapp(s−1) [4-NP] (μM)

33 AgNP AuNP 0.2:1 0.5:1 1:1 2:1

Volume of nanoparticle solution (μL / 3 mL) 53

−4

2.81 × 10 2.12 × 10−2 – 8.91 × 10−3 5.41 × 10−3 1.27 × 10−3

100 −4

2.79 × 10 2.01 × 10−2 – 6.17 × 10−4 5.10 × 10−3 1.02 × 10−3

50 −4

2.65 × 10 1.95 × 10−2 – 5.56 × 10−3 4.47 × 10−3 9.56 × 10−4

66

80 −4

5.21 × 10 1.11 × 10−2 1.63 × 10−2 5.31 × 10−2 2.41 × 10−2 1.45 × 10−3

100 −4

5.96 × 10 1.41 × 10−2 1.89 × 10−2 5.93 × 10−3 5.52 × 10−3 1.61 × 10−3

6.13 × 10−4 1.83 × 10−2 1.91 × 10−2 6.31 × 10−3 6.91 × 10−3 1.82 × 10−3

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References

below Au [27]. Therefore, the extent of attraction between adsorbate and the AgNP is comparatively less and the catalytic efficiency is less than that of AuNP. Further, the possible oxidation of AgNP during progress of reaction also results in the decrease in the catalytic activity. It is reported that the strong nucleophile like BH4− can bring down the reduction potential of AgNP and can make the surface of AgNP more susceptible towards oxidation [28]. The poisoning of AgNP with the oxide layer can hamper the catalytic activity of AgNP. In case of monometallic AuNP, the rate of reaction is quite fast but the comparatively similar reaction rate has been observed for the AueAg alloy nanoparticles. This could be due to the alteration in the d band center of both the metals on alloying, as during alloying of two disparate metals there is a rearrangement of charge to equilibrate Fermi levels. And also the strain induced during alloying brings d band center closer to Fermi level which results in the increase in the energy of antibonding sates [29]. Therefore, during interaction of adsorbates with the nanoalloy surface, the antibonding states may remain depopulated and the stronger interaction will bring down the activation barrier. Therefore, the rate of reaction is found to increase with the increase in the Au content in the bimetallic alloy nanoparticles.

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4. Conclusion We have developed a single pot facile and versatile method for synthesis of bimetallic AgeAu alloy nanoparticles (size < 10 nm) using cationic polymer (PDADMAC). The synthesis procedure demonstrated here is quite robust and can be adopted for synthesis of monometallic and bimetallic nanoparticles with different chemical compositions. The physical characterization of the synthesized nanoparticles confirms the alloying of Au and Ag in the nanoparticles without phase segregation and the particles. The application of the alloy nanoparticles as a catalyst has been established by investigating the 4-NP reduction in aqueous medium. The bimetallic alloy nanoparticles exhibit superior catalytic activity towards reduction of 4-NP and the efficiency of the nanoparticles increase with the increase in the gold content. 5. Associated Content UV–vis absorption spectra of the synthesized nanoaprticles; HRTEM images of the nanoparticles; SAED patterns and XRD data of the nanoparticles; Zeta potential values of the nanoparticles; nanoparticle catalyzed of reduction of 4-NP to 4-AP; kinetics of catalytic reduction of 4-NP to 4-AP in presence of different concentrations of 4-NP as well as monometallic AgNP and AuNP, and, different compositions of bimetallic alloy nanoparticles. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgment The author C. Dwivedi acknowledges Doon University Dehradun and all the author acknowledge Indian Institute of Technology Mandi (IIT Mandi) and Department of science & Technology India for all the financial support. Advanced Material Research Centre Facilities of IIT Mandi is also being acknowledged where all the experimental work were carried out. Appendix A. Supplementary Data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2018.04.001.

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