Biogenic synthesis of shape-tunable Au-Pd alloy nanoparticles with

Journal of Alloys and Compounds 763 (2018) 399e408

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Biogenic synthesis of shape-tunable Au-Pd alloy nanoparticles with enhanced catalytic activities Rakesh Chowdhury a, Md. Masud Rahaman Mollick b, Yajnaseni Biswas c, Dipankar Chattopadhyay b, Md. Harunar Rashid a, * a b c

Department of Chemistry, Rajiv Gandhi University, Rono Hills, Doimukh 791 112, Arunachal Pradesh, India Department of Polymer Science and Technology, University of Calcutta, 92, A. P. C. Road, Kolkata 700 009, West Bengal, India Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 March 2018 Accepted 29 May 2018 Available online 30 May 2018

Herein, we report the synthesis of bimetallic Au-Pd and monometallic Au and Pd nanoparticles (NPs) by successive reduction of the respective metal ions using phytochemicals in the form of plant extract under ambient conditions. Phytochemicals act both as reducing and shape directing agent for metal and alloy NPs. The synthesized nanostructures were characterized by different microscopic, spectroscopic and diffractometric techniques. Microscopic results confirmed that the shape and size of the alloy nanostructures can be tuned by varying the concentration ratio of metal ions in the reaction medium. Energy dispersive X-ray and X-ray photoelectron spectroscopic analysis confirmed the formation of bimetallic Au-Pd nanostructures. The purity and crystalline properties were studied by X-ray diffraction technique. UVevis spectra were recorded to study the optical properties of the synthesized nanostructures. Further, we tested the catalytic activity of both the monometallic and bimetallic alloy nanostructures in borohydride reduction of hazardous organic dye molecules in aqueous medium. The bimetallic alloy NPs exhibit better catalytic activities compared to their respective monometallic nanoparticles. © 2018 Elsevier B.V. All rights reserved.

Keywords: Biogenic synthesis Au-Pd alloy Nanoparticles Dye reduction Catalysis

1. Introduction In recent years, bimetallic alloy nanoparticles (NPs) have attracted intensive research interest because of their interesting physicochemical and optoelectronic properties. These properties are size and shape-dependent and can be optimized by changing the composition of the constituent atoms. As a result, bimetallic alloy NPs possesses unique catalytic properties which are different from the respective single-phase monometallic NPs [1]. It is reported that bimetallic NPs offer superior catalytic activity compared to their individual monometallic NPs, because of their physical and chemical stability as well as selectivity of catalytic reactions [1,2]. Among the various bimetallic alloy NPs investigated, Au-Pd NPs are particularly interesting because of their high catalytic activities and selectivity of catalytic reactions [3e5]. Also, the combination of Au and Pd offers a highly useful alloy NPs, which is miscible at any ratio as reported in the literature [6]. Besides, alloy

* Corresponding author. E-mail address: [email protected] (Md.H. Rashid). https://doi.org/10.1016/j.jallcom.2018.05.343 0925-8388/© 2018 Elsevier B.V. All rights reserved.

NPs composed of Au and Pd possesses unique optical properties arising due to surface plasmon resonance (SPR) of Au. Consequently, bimetallic Au-Pd NPs finds wide applications in different fields. Among, different potential applications, catalysis of organic reactions by Au-Pd NPs are well established. Therefore, Au-Pd alloy NPs are extensively used by researchers as catalyst in the production of some industrially important chemicals like vinyl acetate [7], H2O2 from H2 and O2 [8], aldehydes from primary alcohols by oxidation [9,10] etc. The effect of Au-Pd alloy catalyst in the oxidation of primary alcohol is remarkable as the reaction rate is accelerated by 10e100 times through the use of such alloy NPs in place of individual Pd or Au NPs respectively [10]. Because of its versatile applications in catalysis, researchers are paying more attention on the synthesis of such bimetallic NPs of different morphology and compositions. In general, synthesis of bimetallic Au-Pd NPs are carried out by both physical and chemical approaches [11e17]. But the disadvantages associated with physical approaches such as use of specific sophisticated equipment, high vacuum operation and high electrical energy limit their applicability. Whereas chemical approaches use reducing agent and in some cases additional stabilizer along with the corresponding

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metal salts in a suitable solvent. Recognizing the need for better control and ease of synthesis, researchers are emphasizing on the use of chemical synthesis to control the size, shape and composition of bimetallic alloy NPs. The chemical syntheses of alloy NPs are accomplished via two categories: (i) co-reduction of metal ions where the simultaneous reduction of the ions of both metals occurred and (ii) seeded synthesis, in which one starts with a seed nuclei or NP of one composition and grows a shell around it [14,17e19]. Among these two, co-reduction method is mostly adopted in materials science research, in which a reducing agent along with a stabilizer is used. In most of the cases, either toxic reducing agent or stabilizing agent are used along with solvents which may be either water or oil [20]. However, the use of toxic chemicals and solvents limit the versatility of chemical approaches in practical applications due to their possible environmental hazards. These disadvantages pushed the researchers to adopt a new economical and green synthetic approach where biomolecules are used as reducing-cum-stabilizing agents for the synthesis of alloy NPs under ambient reaction conditions [19]. Such a technique has been considered to be possible green alternative to traditional chemical method as they do not require additional stabilizing agent and the reactions are usually performed in aqueous medium. Use of phytochemicals present in plant extract is one of such emerging green chemical approaches adopted recently for synthesis of monometallic noble metal and bimetallic alloy NPs. A handful of research papers are published recently on the use different parts of plants consisting medicinal, edible and other valuable plants for the synthesis of metal and bimetallic alloy NPs. The polyphenolic molecules present in the plant extract serves the role of reducing agent and the bye-product formed in the reaction medium stabilizes the formed metal NPs [19,21]. This approach is not only benign and cost effective but also very effective in controlling the size and shape of metallic NPs when the reaction conditions such as temperature, pH of the reaction medium and the concentration ratio of the metal ions to reducing agent are systematically controlled [14,22]. Considering the ease of synthesis, low toxicity and versatility of this approach, several research groups have recently reported the use of extract of different plants to synthesize bimetallic alloy NPs. However, to the best of our knowledge, only the use of leaf extract of cacumen platycladi has been reported for the synthesis of bimetallic Au-Pd NPs [14,19,22]. So, any improvement and selection of different kind of plant extracts for the synthesis of size and shape selected bimetallic Au-Pd NPs is highly desirable and promising. Arunachal Pradesh belongs to North-East part of India which is a hotspot of biodiversity having plenty of valuable and non-valuable plants. Most of the non-valuable plants are invasive in nature and are remained unattended by the researchers. Lantana camara belongs to such group with high invasive nature. Although the population of such plants is very high in this part of India, however, no research group has focused on their useful applications. Very recently, different parts of this plant have been used for the synthesis of metal such as Au and Ag and metal oxide say ZnO NPs [23e26]. Considering the advantages of plants extract based synthesis, in this report, we chosen the flower extract of invasive Lantana camara plant to synthesize bimetallic Au-Pd NPs under ambient reaction conditions without using any additional stabilizer. Heterogeneous catalysis is one of the most important applications of bimetallic alloy NPs. As the catalytic reactions take place only on the surface of the NPs, slight changes in the structure, size, or chemical composition of bimetallic alloy NPs can influence their catalytic properties. It is believed that bimetallic alloy in nanometer dimension should possess enhanced catalytic activities due to the enormous increase in the surface area and the number of edges and

corner atoms which are responsible for catalytic properties. Among different catalytic reactions, reduction of organic dyes has received much attention from researchers due to their harmful effects on the environment. Now a day's aquatic living organisms and the human body of industrial, urban areas facing lots of serious hazards from the unwanted and untreated release of dye containing wastewater and waste materials into our environment by industries [27]. Therefore, it is necessary to remove such organic dyes from wastewater to control the hazardous effect of those dyes. But the removal has become very tough and challenging for their characteristics properties such as resistant to aerobic digestion, stable to heat, light and oxidizing agents and recalcitrant nature [28]. A number of methods such as chemical oxidation [29], degradation [26,30], membrane separation [31], adsorption etc. has been reported for the removal of such dyes [32e35]. Recent advancement in catalytic applications of bimetallic alloy NPs and the ease of removal of dye by reducing them into non-hazardous and nonpolluting components prompted us to choose the reduction of such dyes to examine the catalytic properties of the as-prepared bimetallic Au-Pd NPs. So, herein we also report the reduction of three commonly used dyes viz. methylene blue (MB), methyl orange (MO) and rhodamine B (RhB) by NaBH4 in presence of the asprepared colloidal Au-Pd alloy NPs. 2. Experimental section 2.1. Materials and method Flowers of Lantana camara plant were collected from the Rajiv Gandhi University campus, Arunachal Pradesh, India. Hydrogen tetrachloroaurate trihydrate (HAuCl4$3H2O), palladium chloride (PdCl2) and rhodamine B were purchased from Sigma-Aldrich. Methyl orange, methylene blue and sodium borohydride were purchased from Himedia and Merck, India. All the glassware's were cleaned using freshly prepared aqua-regia (1: 3; HNO3: HCl) and then rinse thoroughly with double distilled water. All the reagents were used without further purification. The solutions were prepared in double distilled water. 2.2. Isolation of plant extract The extract of flowers was isolated following our previously reported method using methanol as extracting solvent [26]. In the extraction process, methanol was preferred over other solvents as low molecular weight phenolics which are responsible for reduction and stabilization of metal NPs are obtained in high yield if methanol is used as extracting agent [36]. For the synthesis purpose, a 4.0 gL1 solution of the extract was prepared in distilled water and filter through muslin cloth to remove any suspended substance. 2.3. Synthesis of bimetallic Au-Pd NPs The synthesis of bimetallic Au-Pd NPs involved the co-reduction of respective metal ions in aqueous medium by flower extract of Lantana camara at 50 C. In a typical synthesis, a mixture of 1.5 mL of 0.4 wt% aqueous extract and 2.25 mL water was taken in cleaned glass vial and placed on a hotplate-cum-magnetic stirrer preheated to 50 C. To the stirring reaction mixture, was added 0.05 mL of PdCl2 (0.01 M) solution drop-wise and kept at the same temperature for 10 min under constant magnetic stirring. After that, 0.2 mL HAuCl4 (0.01 M) solution was added at a time and the glass vial containing the reaction mixture was kept at the same temperature for 1 h under constant magnetic stirring. After 1 h, the glass vial containing the reaction mixture was removed from


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hotplate and allowed to cool down to room temperature normally. The product was purified from any soluble unreacted species by dialysis in a cellulose membrane (Himedia, India; MW cut off in between 12000 and 14000) against water. The purified colloidal suspension was used for characterization. This sample was designated as sample Au5Pd1.25 (Table 1). Similar set of reactions were prepared by varying either the concentration of HAuCl4 or PdCl2 in the reaction medium keeping the concentration of another reactant constant to study the effect of their concentration on the size or shape of alloy NPs. The details of the reaction parameters are provided in Table 1. For instance, to synthesize monometallic Au and Pd NPs; the final concentration of HAuCl4 and PdCl2 was maintained at 5.0 104 and 7.5 104 M (Table 1) respectively in the reaction medium.

(XRD) studies of the formed NPs deposited on microscopic glass slide was carried out in a Phillips X'pert Pro multipurpose diffractometer in thin film mode at an accelerating voltage of 40 kV using Cu ka (l ¼ 1.54 Å) as Xeray source. For UVevis spectroscopic analysis, the requisite volume of the colloidal suspension of NPs was transferred into a quartz cuvette of path length 1.0 cm and the spectrum was recorded in an Agilent Cary60 spectrophotometer in the region of 300e800 nm against water as blank. The FTIR spectra of the crude extract and alloy NPs were acquired in a Thermo Scientific Nicolet iS5 spectrometer in ATR mode.

2.4. Catalytic dye reduction

A series of reactions were carried out between plant extract, PdCl2 and HAuCl4 at varying ratio of Au/Pd to synthesize bimetallic Au-Pd NPs of different composition (Table 1). The transmission electron microscopy (TEM) images of the purified colloidal alloy NPs were recorded to investigate the effect of reaction parameters on the size or shape of the formed Au-Pd alloy NPs. Fig. 1 shows the TEM images of sample Au5Pd1.25 prepared at a molar ratio of Au/Pd of 4: 1. The micrograph shows the formation of particles of mixed morphology consisting spherical, nearly spherical and polygonal (triangular and hexagonal) shaped Au-Pd alloy NPs. However, the populations of spherical alloy NPs are high compared to other morphologies. The sizes of spherical alloy NPs are in the ranges of 10e15 nm. The HRTEM image of such spherical NPs (Fig. 1B) shows the presence of multidirectional lattice fringes with interplanar distance of 0.228 nm which is mean value of the (111) planes of face-centered cubic (fcc) Au (0.236 nm) and Pd (0.225 nm). This value is also conformed to the reported value of Au-Pd alloy NPs [14]. This result indicated the formation of polycrystalline spherical Au-Pd alloy NPs. This was further confirmed from the selected area electron diffraction (SAED) recorded from such spherical Au-Pd NPs (Fig. 1C) where diffraction spots are superimposed with the circle.

To investigate the catalytic activities of the as-prepared bimetallic Au-Pd NPs, we chosen the reduction of three commercially available non-degradable toxic dye pollutants viz. MB, MO and RhB. A typical catalytic MB dye reduction involves the addition of 0.3 mL of MB dye solution to 2.5 mL water taken in a 3 mL volume quartz cuvette followed by addition of 0.05 mL purified suspension of colloidal NPs. The reaction mixture was mixed properly and added to this 0.15 mL of 0.01 M NaBH4 solution. The cuvette containing the reaction mixture was placed in the cell holder of the UVevis spectrophotometer and the absorption spectra were recorded at a time interval of 6 s to monitor the progress of the catalytic dye reduction. Similarly, to study the effect of size or shape of the synthesized NPs, we repeated the catalytic reduction of dyes with remaining samples maintaining identical reaction conditions. Further, to compare the catalytic activity of bimetallic Au-Pd NPs with monometallic Au and Pd NPs, we repeated the catalysis reaction with individual metal NPs also. Following the similar procedure, borohydride reduction of MO and RhB were also carried out in presence of alloy NPs. 2.5. Characterization For transmission electron microscopic (TEM) studies, a drop of aqueous suspension of colloidal NPs was cast on a carbon coated copper grid. The excess solutions were soaked with a tissue paper followed by drying in air. The micrographs were then recorded in a higheresolution JEOL electron microscope (JEM 2100EM) at an accelerating voltage of 200 kV. ImageJ (National Institute of Health) software was used to measure the size and to analyze the HRTEM images. X-ray photoelectron spectroscopic (XPS) analysis of bimetallic Au-Pd NPs deposited on glass slide was carried on an XPS instrument (Omicron: Serial no. 0571) with an Al Ka radiation source under 15 kV voltages and 5 mA current. Xeray diffraction

3. Results and discussion 3.1. Microscopic study of bimetallic Au-Pd NPs

3.1.1. Variation of concentration of PdCl2 When the concentration ratio of Au/Pd was kept at 2: 1 by increasing the concentration of Pd salt, the TEM image of sample Au5Pd2.5 (Fig. S1 in Electronic Supplementary Information, ESI) again shows the formation of particle of mixed morphology. However, the populations of spherical NPs are decreased whereas the population of polygonal alloy NPs is increased significantly compared to the previous sample as clearly noticed in the high magnification TEM images of the sample (Fig. S1B in ESI). The sizes of spherical alloy NPs are in the ranges of 5e10 nm. On the other hand, polygonal NPs are larger in sizes and are varied from 8 to 22 nm. The HRTEM images of a spherical alloy NPs (Fig. S1C in ESI) shows the presence of distinct lattice fringes which are not

Table 1 Details of reaction parameters for the synthesis of bimetallic Au-Pd NPs. Sample IDa Au5Pd1.25 Au5Pd2.5 Au5Pd5 Au5Pd7.5 Au2.5Pd7.5 Au1.25Pd7.5 Au2.5Pd5 Au5Pd0 Au0Pd7.5 a

[HAuCl4] (M) 4

5.0 10 5.0 104 5.0 104 5.0 104 2.5 104 1.25 104 2.5 104 5.0 104 0

[PdCl2] (M) 4

1.25 10 2.5 104 5.0 104 7.5 104 7.5 104 7.5 104 5.0 104 0 7.5 104

[HAuCl4]/[PdCl2]

Morphology (highest population)

4:1 2:1 1:1 1:1.5 1:3 1:6 1:2 e e

Spherical Mixed Polygonal Polygonal Hexagonal Hexagonal Hexagonal Spherical Spherical

The numerical values in subscript in the samples names indicate the concentration of the components in the order of 104 M.

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Fig. 1. (A) TEM image of Au-Pd alloy NPs recorded from sample Au5Pd1.25, (B) HRTEM image and (C) SAED pattern recorded from an alloy NPs.

perfectly aligned. This might indicate polycrystalline nature of such spherical Au-Pd alloy NPs. This was further proved from the SAED pattern shown in Fig. S1D. The interplane spacing measured from the HRTEM image was found to be 0.233 nm. Whereas, the HRTEM image of individual pentagonal and triangular alloy NPs shows the presence of perfectly aligned lattice fringes with lattice spacing of 0.234 and 0.21 nm respectively for triangular and pentagonal alloy NPs (Figs. S1E and S1F in ESI). Further when the concentration ratio of Au/Pd was maintained at 1: 1 again by increasing the concentration of Pd salts to 5.0 104 M, the TEM images of sample Au5Pd5 confirmed the significant decrease in the population of spherical alloy NPs (Fig. 2A). At the same time, the populations of pentagonal and hexagonal NPs are increased vastly as can be seen from the magnified image shown in Fig. 2B and overtook the population of other morphologies. The sizes of such polygonal NPs are in between 14 and 35 nm. The HRTEM image recorded from such polygonal Au-Pd alloy NPs indicated the presence of perfectly aligned lattice fringes with interplane spacing of 0.236 nm (Fig. 2C). This result is a clear indication that highly crystalline alloy NPs are formed in this sample. At a concentration ratio of 1:1.5 (sample Au5Pd7.5), preferentially polygonal alloy NPs are formed (Fig. S2A in ESI) which comprises of triangular, pentagonal and hexagonal shaped alloy NPs. In this case, the spherical NPs are almost absent as seen in the magnified TEM image (Fig. S2B in ESI). However, some elongated hexagonal shaped alloy particles were observed in the TEM image of the sample. The sizes of polygonal alloy NPs are varied from 10 to 40 nm. The HRTEM image of a polygonal shaped alloy NPs as usual show the presence of aligned lattice fringes with interplane spacing of 0.231 nm indicating the formation well crystalline alloy NPs.

3.1.2. Variation of concentration of HAuCl4 Additionally, a set of reactions were also carried by varying the concentration of HAuCl4 keeping the concentration of PdCl2 constant at 7.5 104 M. So, when the concentration ratio of Au/Pd was maintained at 1:3 in sample Au2.5Pd7.5 by lowering the concentration of Au salt to 2.5 104 M, preferentially hexagonal shaped alloy NPs were formed (Fig. 3A). As observed through the microscope, the spherical NPs are completely absent in this sample. Also the populations of other shaped particles are decreased significantly. The end to end sizes of such hexagonal particles are measured to be varied from 11 to 30 nm. The magnified TEM image (Fig. 3B) recorded from a portion of the micrograph indicated that these particles are very thin and the edges of these particles are not very smooth. The HRTEM image of a hexagonal shaped alloy NPs (Fig. 3C) shows the presence of well resolved lattice fringes with interplane spacing of 0.236 nm. As the concentration ratio of Au/Pd was further increased to 1:6 by decreasing the concentration of HAuCl4 solution in sample Au1.25Pd7.5, preferentially nearly uniform multifaceted hexagonal alloy NPs are formed (Fig. 4). The average end to end length of these NPs is 16 nm. Magnified view of such micrograph (Fig. 4B) shows the presence of either very dark or weakly intense spots within the particles. These spots might appear due to defects created in the particles. The HRTEM image of hexagonal shaped Au-Pd alloy NP (Fig. 4C) shows the presence of perfectly aligned lattice fringes with interplane spacing of 0.237 nm indicating crystalline nature of the alloy NPs. Similar results were observed when the ration of Au/Pd was fixed at 1: 2 keeping the concentration of HAuCl4 and PdCl2 at 2.5 104 and 5.0 104 M respectively in sample Au2.5Pd5 (Fig. S3 in ESI). In this case the average end-to-end size of the particles is also 16 nm. To ascertain

Fig. 2. (A) TEM image of Au-Pd alloy NPs recorded from sample Au5Pd5 and (B) high-magnification image of micrograph shown in (A). (C) HRTEM image recorded from an alloy NPs.


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Fig. 3. (A) TEM image of Au-Pd alloy NPs recorded from sample Au2.5Pd7.5, (B) high-magnification image of micrograph shown in (A) and (C) HRTEM image recorded from an alloy NPs.

Fig. 4. (A) TEM image of Au-Pd alloy NPs recorded from sample Au1.25Pd7.5, (B) high-magnification image of micrograph shown in (A) and (C) HRTEM image recorded from an alloy NPs.

the composition of formed alloy NPs, we recorded EDX spectrum from sample Au5Pd1.25 during microscopic analysis. The spectrum shown in Fig. S4 in ESI confirmed the presence of peaks due to elemental Au and Pd with atomic percentage ration of 2.06: 0.39 which is quite satisfactory with the expected ratio of 4: 1. Besides, we observed the presence of peaks due to C and O which are assigned to the extract expected to be present on the surface of alloy NPs. This result confirmed that the formed particles are composed of Au and Pd.

3.2. XPS analysis of bimetallic Au-Pd NPs X-ray photoelectron spectroscopy (XPS) technique was employed further to elucidate the composition of formed alloy NPs. Fig. 5a shows the typical survey scan XPS spectrum of bimetallic Au-Pd NPs (sample Au5Pd5). The presence of signals due to C and O confirmed that the phytochemicals are anchored on the surface of alloy NPs consist of C, H and O only. Besides, various core levels binding energy peaks of Au have also been observed. The high resolution XPS spectra for various elements of bimetallic alloy are presented in Fig. 5b and c. The XPS spectra of alloy NPs in the Au region exhibited two peaks at 83.3 and 86.8 eV which correspond to Au4f7/2 and Au4f5/2 respectively of elemental Au. Fig. 5c shows the presence of doublets at 336.3 and 342.1 eV corresponding to 3d5/2 and 3d3/2 of metallic Pd. The XPS results support the EDX data confirming the formation of bimetallic Au-Pd composition on the surface of formed NPs.

3.3. Microscopic study of monometallic Au and Pd NPs In order to understand the size or shape of individual Au NPs, we repeated the synthesis for monometallic Au and Pd NPs following the same reaction procedure. When the reaction was performed between aqueous HAuCl4 and plant extract only (sample Au5Pd0), the TEM image shows the presence of spherical Au NPs of different two kinds (Fig. 6A). The smaller sized spherical Au NPs are within the size limit of 5 nm whereas the larger spherical Au NPs are within 15 nm. The HRTEM image as recorded from a spherical Au NP (Fig. 6B) exhibits multidirectional lattice fringes confirming the formation of polycrystalline Au NPs. The interplane spacing value as measured from the HRTEM image was found to be 0.233 nm which corresponds to (111) lattice plane of fcc Au. When the reaction was repeated with aqueous PdCl2 solution only; the TEM image of sample Au0Pd7.5 as showed in Fig. 6C exhibit the presence of highly populated nearly spherical Pd NPs. The size of such spherical Pd NPs ranges from 5 to 9 nm. The HRTEM image recorded from such spherical Pd NPs (Fig. 6C) confirmed the presence of perfectly aligned lattice fringes with interplanar distance of 0.232 nm. This indicated the formation of highly single crystalline Pd NPs with preferential (111) lattice plane. From the above microscopic results, it is clear that reduction of individual metal ions by extract results in the formation of spherical monometallic NPs. As the mixed metal ions are reduced, at very low concentration of PdCl2 again we observed the formation of predominantly spherical bimetallic Au-Pd NPs. When the concentration of PdCl2 is increased in the reaction medium keeping the concentration of HAuCl4 constant at 5.0 104 M, the population of

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Fig. 5. XPS spectra of bimetallic alloy NPs recorded from sample Au5Pd5.

(XRD) study was carried out. The XRD patterns of all the samples are shown in Fig. 7. Fig. 7a shows the XRD pattern of monometallic Au NPs only which exhibit diffraction peaks at 2q ¼ 38.6, 44.2, 64.6 and 78.5 corresponding to (111), (220), (222) and (311) planes of fcc Au. Besides, another strong peak was observed at 2q ¼ 58 which could not be identified. However, this peak might appear due to the presence of possible impurities in the plant extract. On the other hand the XRD pattern of monometallic Pd NPs (Fig. 7h) is dominated by diffraction peaks at 2q ¼ 35.8, 46.2, 51.7, 53.3 (very weak) and 66 (very weak). The diffraction peaks at 2q ¼ 46.2, 53.3

Fig. 6. TEM images of Au NPs recorded from sample (A) Au5Pd0 and (C) Au0Pd7.5. HRTEM images recorded from an (B) Au NPs in sample Au5Pd0 and (D) Pd NPs in sample Au0Pd7.5.

polygonal Au-Pd alloy NPs increased. At the same time, the size of polygonal alloy NPs is also increased. Whereas formation of smaller sized alloy NPs with low population was observed under such circumstances. However at a particular concentration of PdCl2, when the concentration of HAuCl4 was varied hexagonal bimetallic Au-Pd NPs are formed predominantly. Also a trend of decreasing particles size was observed with decrease of HAuCl4 concentration. So, it can be inferred that due to high positive reduction potential of Pd2þ/Pd0 0 2þ compared to AuCl took place first forming 4 /Au , reduction of Pd smaller sized nuclei. Subsequently these nuclei act as seed to help in forming anisotropic bimetallic Au-Pd NPs by secondary growth. As during secondary growth supply of HAuCl4 increased, size of the formed alloy NPs are increased.

3.4. X-ray diffraction study To confirm the crystalline properties and phase structures of the formed monometallic and bimetallic alloy NPs, X-ray diffraction

Fig. 7. XRD patterns of different samples of monometallic Au, Pd and bimetallic Au-Pd NPs: (a) Au5Pd0, (b) Au5Pd1.25, (c) Au5Pd2.5, (d) Au5Pd5, (e) Au5Pd7.5, (f) Au2.5Pd7.5, (g) Au1.25Pd7.5, and (h) Au0Pd7.5.


and 66 are assigned to crystalline Pd in the sample as reported earlier [37,38]. The additional diffraction peaks at 35.8 and 51.7 might arise again due to impurity of the phytochemicals. Similar undesirable diffraction peaks were also observed earlier by other researchers [37]. The alloy samples synthesized at an Au/Pd concentration ratio of 4: 1 again shows the diffraction peaks (Fig. 7b) those matches well with the peaks observed in case of Au NPs. Besides, two additional peaks were observed at 2q ¼ 51 and 58 (weak) which matches well with the undesirable peaks observed in monometallic Pd NPs as discussed earlier. As the concentration ratio of Au/Pd was decreased to 2: 1 in sample Au5Pd2.5, the diffraction peak at 58 were diminishing in intensity (Fig. 7c). But for sample with 1: 1 ratio of Au/Pd (sample Au5Pd5), the peak at 51 becomes dominant compared to other peaks (Fig. 7d). But samples where the Au/Pd ratio exceeds 1 (Au5Pd7.5), the peak at 36 become dominant (Fig. 7e). This trend is continued upto sample Au2.5Pd7.5 with Au/Pd ratio of 1:3 (Fig. 7f). But when the concentration of Pd was increased significantly, the peak at 51 was raised along with the peak at 36 (Fig. 7g). The XRD results indicated the formation well crystalline phases of monometallic Au, Pd and bimetallic AuPd NPs. Depending on the composition; there are changes in the intensity of the different diffraction peaks confirming the preferential growth in different crystal planes.

3.5. UVevis absorption study The UVevis spectra of all the samples were recorded and are presented in Fig. 8. The spectrum of monometallic Au NPs is dominated by a sharp absorption peak centered at 534 nm. This is characteristic peak of spherical Au NPs commonly called surface plasmon resonance (SPR) band. Sample Au5Pd1.25 prepared at Au/ Pd concentration ratio of 4: 1 exhibited broad absorption band at 540 nm which is red shifted by 6 nm compared to that exhibited by monometallic Au NPs. Similar absorption band was observed in the spectrum of sample Au5Pd2.5 prepared with Au/Pd concentration ratio of 2: 1. However, the absorption band was further red shifted to 550 and 575 in samples Au5Pd5 and Au5Pd7.5 prepared with concentration ratio of Au/Pd of 1: 1 and 1: 1.5 respectively. On the other hand samples Au2.5Pd7.5 and Au1.25Pd7.5 prepared with Au/Pd

Fig. 8. UVevis spectra of monometallic Au, Pd and bimetallic Au-Pd NPs.

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concentration ratio of 1: 3, 1: 6 and monometallic Pd NPs did not show significant absorption band in the UVevis spectra. The results clearly indicate that the optical properties are tunable by varying the composition of Au or Pd in alloy NPs. 3.6. FTIR spectroscopic study The FTIR spectral analysis was carried out to identify the possible biomolecules responsible for the reduction of metal ions and capping of the bioreduced monometallic or bimetallic alloy NPs synthesized by the flower extract. The FTIR spectra of the crude extract (Fig. 9) some peaks at 3312, 2924, 2853, 1607, 1375, 1260, 1025 cm1. The IR bands observed at 3312 and 1607 cm1 correspond to eOH and eCO stretching modes, respectively. The bands at 1375 and 1025 cm1 correspond to eCeO and eCeOeC stretching modes, respectively. The bands at 2924 and 2853 represent asymmetric and symmetric stretching CeH vibrations. These peaks suggested the presence of phenolics in the plant flower extract. The FTIR spectra of alloy NPs (sample Au5Pd5) (Fig. 9) showed IR bands at 3396, 2921, 2850, 1715, 1466, 1374, 1251, 1165, 1052 and 724 cm1 respectively. Comparison of the spectra of flower extract and alloy NPs indicated that most of the absorption bands of the flower extract also appear in the FTIR spectrum of AuPd alloy NPs with slight shift in the peak position. The shifting of the band due to eOH stretching modes of vibration from 3312 cm1 in extract to 3396 cm1 in alloy NPs suggests that the phenolic compounds present in the extract are responsible for the reduction of metal ions. The results also confirmed that some of the phytochemicals are anchored on the surface of monometallic or

Fig. 9. FTIR spectra of crude extract and extract reduced Au-Pd alloy NPs (sample Au5Pd5).

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bimetallic alloy NPs providing stability to the NPs. 3.7. Borohydride reduction of organic dyes In order to examine the catalytic activities of the synthesized monometallic Au, Pd and bimetallic Au-Pd NPs, we have chosen the catalytic reduction of organic dyes from their aqueous solution. First we tested the catalytic efficiency of the synthesized NPs in the reduction of MB. To start with, we have chosen the sample Au5Pd5 having concentration ratio of Au and Pd ¼ 1. The time-evolution UV spectra of the catalytic reduction of MB by NaBH4 are shown in Fig. 10a. The spectra show the gradual decrease of absorbance of the peak at 665 nm with time. This indicates that the Au-Pd alloy can catalyze the borohydride reduction of MB in quick succession. The reaction nearly completes within 48 s. Further we have plotted the %reduction of dye with time as shown in Fig. 10b to check the extent of reduction in presence of alloy NPs. It can be seen that MB undergoes reduction at a faster rate initially and then slowed down as the reaction proceeds. It is evident from the curve that nearly 50% reduction of dye completed within 6 s and nearly 96% dye reduction was achieved within 42 s. This result indicates that the bimetallic Au-Pd NPs are highly efficient as catalyst for the borohydride reduction of MB from aqueous solution. Further to compare the catalytic efficiency of other catalysts, we repeated the same catalysis reaction with the remaining samples. The % reductions of MB with time for all these catalysts are also shown in Fig. 10b. Comparisons of the results confirm that monometallic Au NPs (sample Au5Pd0) are not much effective as catalyst for the borohydride reduction of MB. On the other hand, monometallic Pd NPs (sample Au0Pd7.5) exhibited relatively better catalytic activity in comparison to that of monometallic Au NPs. With Au NPs, only 8% dye was reduced within 6 s after which the reaction slowed down significantly and a maximum of 12% dye reduction was recorded after 42 s. Whereas, with Pd NPs a total of 86% dye can be reduced within 42 s. When, we introduced Pd on Au to achieve a concentration ratio of Au/Pd ¼ 4, the catalytic activity was increased significantly in comparison to monometallic Au NPs. In this case, 48% dye was undergone reduction within 12 s after which it slowed down and

the curve of %reduction versus time becomes flattened (Fig. 10b). Further increase in Pd loading in the bimetallic alloy NPs (sample Au5Pd2.5 with Au/Pd ¼ 2) significantly enhanced the catalytic activity. The maximum reduction of 98% dye was achieved in this case within 42 s of time. Similar observations were noticed with other samples and the comparative results are presented in Table 2. From Table 2, it is evident that alloy NPs with Au/Pd ¼ 0.5 ratio (sample Au2.5Pd5) catalyzes the reduction of MB at a very faster rate compared to other samples and nearly 99.5% reduction was achieved within 18 s of reaction. In order to determine the apparent rate constant (kapp), the curve between ln A versus time was plotted (Fig. S5 in ESI) taking into account the reaction time of 24 s neglecting the other absorbance values those are flattened. The values of the apparent rate constants as measured from the linear fitted curve between ln A and time are also presented in Table 2. A maximum apparent rate constant of 23.0 102 s1 was obtained with sample Au2.5Pd5. On the other hand kapp value for reduction with monometallic Au NPs is the lowest one and as the Pd content in the alloy NPs was increased; the kapp values increases gradually. Comparison of the results led us to conclude that alloy NPs exhibit excellent catalytic activities compared to the individual monometallic NPs and the catalytic activity is dependent on the size, shape and composition

Table 2 Data of borohydride reduction of MB in presence of different catalyst. The reaction parameters are: [MB] ¼ 1.0 105 M; [NaBH4] ¼ 0.01 M and volume of alloy NPs ¼ 50 mL. Catalyst used (sample id)

Amount of MB reduction (%)

Time (s)

kapp (s1)

Au5Pd1.25 Au5Pd2.5 Au5Pd5 Au5Pd7.5 Au2.5Pd7.5 Au1.25Pd7.5 Au2.5Pd5 Au5Pd0 Au0Pd7.5

50.3 96.3 81.5 78.9 81.5 92.5 99.5 12.0 82.3

24 24 24 24 24 24 24 24 24

2.8 102 14.0 102 6.6 102 6.1 102 6.8 102 9.1 102 23.0 102 0.94 102 7.2 102

Fig. 10. (a) Time evolution UVevis spectra of aqueous MB solution in presence of Au-Pd NPs (sample Au5Pd5) and (b) plots showing the variation of %reduction of MB with time for different samples.


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Fig. 11. Plots showing the variation of %reduction of (a) MO and (b) RhB with time for different samples.

of Au-Pd alloy NPs. Further to check whether the alloy NPs can be used as catalyst for other dye reduction, we carried out borohydride reduction of two other dyes viz. methyl orange (MO) and rhodamine B (RhB). The gradual decrease in the absorbance of MO solution at 465 nm with time signified the catalytic reduction of MO in presence alloy NPs. The plot showing the %reduction of MO with time for all the samples are shown in Fig. 11A which confirm that the alloy or individual Au and Pd NPs are suitable to catalyze MO. But the reduction of MO is significantly slower than MB as discussed in the previous section. In this case, maximum of 96.5% of MB dye was reduced within 3 min with sample Au5Pd5 having Au/Pd ratio of 1. As observed earlier in this case also monometallic Au NPs exhibited the lowest catalytic activity compared to monometallic Pd and bimetallic Au-Pd NPs. When we performed the reduction of RhB with the as-prepared samples, we observed in this case also the decrease of peak due to RhB at 554 nm which indicated the effectiveness of alloy NPs in catalyzing the RhB reduction. The plot between %reductions of dye against time (Fig. 11B) clearly indicates that alloy NPs can also be as catalyst for the reduction of RhB dye. In this case also monometallic Au NPs exhibited the lowest catalytic activity as it can only reduce 11% of RhB dye within 30 min. Whereas, sample Au2.5Pd5 exhibited the highest catalytic activity with a total 95% of RhB dye reduction within 10 min. Comparison of the catalysis results, it can be inferred that Au-Pd alloy NPs shows excellent catalytic activity towards the borohydride reduction of MB where the reduction almost complete within a minute. On the other hand individual Au or Pd NPs are weakly active in comparison to the alloy NPs. Also the composition of alloy NPs plays an important role in the catalytic activity. Similar observations were noticed with MO and RhB dye but the time required in both the cases are much higher than that required for MB reduction.

the colour of MB solution re-appeared within a short period of time. This re-appearance time is different for different samples. It is known that on reduction, MB undergoes conversion to colourless hydrogenated molecule, leucomethylene blue (LMB), which can in turn, be oxidized back to its original form. During the study of the catalytic reduction in the present case, we also noticed that in presence of catalyst, the blue colour of MB faded away in quick succession as discussed in the previous section due to the formation of LMB. The colourless reaction mixture on mild shaking in air leads to regeneration of blue colour of the reaction mixture. Addition of fresh sodium borohydride solution in the mixture resulted again the fading of blue colour. This observation supported the ‘clock reaction’. This process of disappearance and appearance of blue colour in presence of catalyst is continued for several times without much losing the catalytic activity of alloy NPs. However, the time of appearance or disappearance of blue colour increases as the repetition process is continued. This might indicate the deactivation of catalyst surface due to the successive catalytic reduction.

4. Conclusion We successfully synthesized monometallic and bimetallic Au-Pd NPs using flower extract of Lantana camara, a wild invasive weed plant. The shape and optical properties of these alloy NPs can be modulated by varying the concentration ratio of gold and palladium salts in aqueous medium. The use of invasive weed offers advantages over the use of other conventional medicinal and edible plants besides their environmental friendly and cost effectiveness. The nanomaterials were well characterized by different instrumental techniques. The Au-Pd NPs exhibited remarkably high catalytic activities towards the borohydride reduction of MB and other dyes and are reusable. The composition of Au-Pd alloy NPs played significant role in the catalytic activities.

3.8. Reusability of alloy NPs During the borohydride reduction of MB by monometallic or bimetallic alloy NPs, we noticed that after the completion of reduction, if the MB solution was exposed to air with mild shaking,

Conflict of interest The authors declares no conflict of interest.

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Acknowledgement MHR acknowledges Council of Scientific and Industrial Research (Grant No. 01(2910)/17/EMR-II), India for supporting the research. RC acknowledges Science and Engineering Research Board, India for providing the research fellowship (Reg. No. IF150609). We also thank SAIF GU for extending their instrumental facilities to us. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.05.343. References [1] X. Jin, K. Taniguchi, K. Yamaguchi, N. Mizuno, Au-Pd alloy nanoparticles supported on layered double hydroxide for heterogeneously catalyzed aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones to phenols, Chem. Sci. 7 (2016) 5371e5383. [2] J.H. Sinfelt, Structure of bimetallic clusters, Acc. Chem. Res. 20 (1987) 134e139. [3] M. Chen, D. Kumar, C.-W. Yi, D.W. Goodman, The promotional effect of gold in catalysis by palladium-gold, Science 310 (2005) 291e293. [4] G.J. Hutchings, Nanocrystalline gold and gold palladium alloy catalysts for chemical synthesis, Chem. Commun. (2008) 1148e1164. [5] H. Zhang, T. Watanabe, M. Okumura, M. Haruta, N. Toshima, Catalytically highly active top gold atom on palladium nanocluster, Nat. Mater. 11 (2012) 49e52. [6] second ed., in: H.S, R.P. Okamoto, L. Kacpraz (Eds.), Binary Alloy Phase Diagrams, vol. 1, ASM International, 1992. [7] D.T. Thompson, Catalysis by gold/platinum group metals, Platin. Met. Rev. 48 (2004) 169e172. [8] J.K. Edwards, S.J. Freakley, R.J. Lewis, J.C. Pritchard, G.J. Hutchings, Advances in the direct synthesis of hydrogen peroxide from hydrogen and oxygen, Catal. Today 248 (2015) 3e9. [9] J. Long, H. Liu, S. Wu, S. Liao, Y. Li, Selective oxidation of saturated hydrocarbons using AuePd alloy nanoparticles supported on metaleorganic frameworks, ACS Catal. 3 (2013) 647e654. [10] S. Marx, A. Baiker, Beneficial interaction of gold and palladium in bimetallic catalysts for the selective oxidation of benzyl alcohol, J. Phys. Chem. C 113 (2009) 6191e6201. [11] Y. Ohkubo, M. Shibata, S. Kageyama, S. Seino, T. Nakagawa, J. Kugai, H. Nitani, T.A. Yamamoto, Carbon-supported AuPd bimetallic nanoparticles synthesized by high-energy electron beam irradiation for direct formic acid fuel cell, J. Mater. Sci. 48 (2013) 2142e2150. [12] M. Yang, Z. Wang, W. Wang, C.-j. Liu, Synthesis of AuPd alloyed nanoparticles via room-temperature electron reduction with argon glow discharge as electron source, Nanoscale Res. Lett. 9 (2014) 405. [13] L. Shi, A. Wang, T. Zhang, B. Zhang, D. Su, H. Li, Y. Song, One-Step synthesis of AuePd alloy nanodendrites and their catalytic activity, J. Phys. Chem. C 117 (2013) 12526e12536. [14] D. Sun, G. Zhang, X. Jiang, J. Huang, X. Jing, Y. Zheng, J. He, Q. Li, Biogenic flower-shaped Au-Pd nanoparticles: synthesis, SERS detection and catalysis towards benzyl alcohol oxidation, J. Mater. Chem. 2 (2014) 1767e1773. [15] R.G. Weiner, M.R. Kunz, S.E. Skrabalak, Seeding a new kind of garden: synthesis of architecturally defined multimetallic nanostructures by seedmediated Co-Reduction, Acc. Chem. Res. 48 (2015) 2688e2695. [16] J. Xu, A.R. Wilson, A.R. Rathmell, J. Howe, M. Chi, B.J. Wiley, Synthesis and catalytic properties of AuePd nanoflowers, ACS Nano 5 (2011) 6119e6127. [17] M. Yang, K.D. Gilroy, Y. Xia, A general approach to the synthesis of M@Au/Ag

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27] [28] [29] [30]

[31] [32]

[33]

[34]

[35]

[36] [37]

[38]

(M ¼ Au, Pd, and Pt) nanorattles with ultrathin shells less than 2.5 nm thick, Part. Part. Syst. Char. 34 (2017), 1600279-n/a. S.-T. Wong, M. Shamsuddin, A. Alizadeh, Y.-S. Yun, In-situ generated palladium seeds lead to single-step bioinspired growth of AuPd bimetallic nanoparticles with catalytic performance, Mater. Chem. Phys. 183 (2016) 356e365. G. Zhan, J. Huang, M. Du, I. Abdul-Rauf, Y. Ma, Q. Li, Green synthesis of AuePd bimetallic nanoparticles: single-step bioreduction method with plant extract, Mater. Lett. 65 (2011). P.C. Pandey, R. Singh, Controlled synthesis of Pd and Pd-Au nanoparticles: effect of organic amine and silanol groups on morphology and polycrystallinity of nanomaterials, RSC Adv. 5 (2015) 10964e10973. S. Phukan, P. Bharali, A.K. Das, M.H. Rashid, Phytochemical assisted synthesis of size and shape tunable gold nanoparticles and assessment of their catalytic activities, RSC Adv. 6 (2016) 49307e49316. Y. Hong, X. Jing, J. Huang, D. Sun, T. Odoom-Wubah, F. Yang, M. Du, Q. Li, Biosynthesized bimetallic AuePd nanoparticles supported on TiO2 for solvent-free oxidation of benzyl alcohol, ACS Sustain. Chem. Eng. 2 (2014) 1752e1759. B. Ajitha, Y. Ashok Kumar Reddy, S. Shameer, K.M. Rajesh, Y. Suneetha, P. Sreedhara Reddy, Lantana camara leaf extract mediated silver nanoparticles: antibacterial, green catalyst, J. Photochem. Photobiol. B 149 (2015) 84e92. S.S. Dash, B.G. Bag, P. Hota, Lantana camara Linn leaf extract mediated green synthesis of gold nanoparticles and study of its catalytic activity, Appl. Nanosci. 5 (2015) 343e350. B. Kumar, K. Smita, L. Cumbal, Biosynthesis of silver nanoparticles using Lantana camara flower extract and its application, J. Sol. Gel Sci. Technol. 78 (2016) 285e292. S. Phukan, P. Bomjen, T. Shripathi, Rashid, Green route biosynthesis of shapetunable ZnO nanostructures and their photocatalytic applications, ChemistrySelect 2 (2017) 11137e11147. E. Forgacs, T. Cserh ati, G. Oros, Removal of synthetic dyes from wastewaters: a review, Environ. Int. 30 (2004) 953e971. G. Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresour. Technol. 97 (2006) 1061e1085. P.K. Malik, S.K. Saha, Oxidation of direct dyes with hydrogen peroxide using ferrous ion as catalyst, Separ. Purif. Technol. 31 (2003) 241e250. F. Davar, A. Majedi, A. Mirzaei, Green synthesis of ZnO nanoparticles and its application in the degradation of some dyes, J. Am. Ceram. Soc. 98 (2015) 1739e1746. G. Ciardelli, L. Corsi, M. Marcucci, Membrane separation for wastewater reuse in the textile industry, Resour. Conserv. Recycl. 31 (2001) 189e197. A. Afkhami, R. Moosavi, Adsorptive removal of Congo red, a carcinogenic textile dye, from aqueous solutions by maghemite nanoparticles, J. Hazard. Mater. 174 (2010) 398e403. R. Chowdhury, N. Barah, M.H. Rashid, Facile biopolymer assisted synthesis of hollow SnO2 nanostructures and their application in dye removal, ChemistrySelect 1 (2016) 4682e4689. A. Mittal, D. Jhare, J. Mittal, Adsorption of hazardous dye Eosin Yellow from aqueous solution onto waste material De-oiled Soya: isotherm, kinetics and bulk removal, J. Mol. Liq. 179 (2013) 133e140. V. Vadivelan, K.V. Kumar, Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk, J. Colloid Interface Sci. 286 (2005) 90e100. J. Dai, R.J. Mumper, Plant phenolics: extraction, analysis and their antioxidant and anticancer properties, Molecules 15 (2010) 7313e7352. P. Dauthal, M. Mukhopadhyay, Biosynthesis of palladium nanoparticles using delonix regia leaf extract and its catalytic activity for nitro-aromatics hydrogenation, Ind. Eng. Chem. Res. 52 (2013) 18131e18139. K. Mallikarjuna, N. John Sushma, B.V. Subba Reddy, G. Narasimha, B. Deva Prasad Raju, Palladium nanoparticles: single-step plant-mediated green chemical procedure using Piper betle leaves broth and their anti-fungal studies, Int. J. Chem. Anal. Sci. 4 (2013) 14e18.