A facile and eco-friendly method for the preparation of Au-Pd bimetallic nanoparticles (similar to 7 nm) has been developed based on simultaneous bioreduction of Au(III) and Pd(II) precursors with Cacumen Platycladi leaf extract in aqueous environment. The morphology, structure, and size were confirmed with the aid of transmission electron microscopy, selected area electron diffraction. UV-vis spectroscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy. The results from Fourier transform infrared spectroscopy showed that the C=O and C-O groups in the plant extract played a critical role in capping the nanoparticles. Importantly, the process can be described as pure "green chemistry" technique since no additional synthetic reagents are used as reductants or stabilizers. (C) 2011 Elsevier B.V. All rights reserved.National Natural Science Foundation of China[21036004, 20976146]; Natural Science Foundation of Fujian Province of China[2010J05032, 2010J01052]WOS:00029506830004

Guowu Zhan

Ibrahim Abdul-Rauf

Jiale Huang

Mingming Du

Qingbiao Li

Yao Ma

李清彪

黄加乐

Materials Letters

Xiamen University Institutional Repository

Materials Letters 65 (2011) 2989–2991Contents lists available at ScienceDirectMaterials Lettersj ourna l homepage: www.e lsev ie r.com/ locate /mat le tGreen synthesis of Au–Pd bimetallic nanoparticles: Single-step bioreduction methodwith plant extractGuowu Zhan, Jiale Huang, Mingming Du, Ibrahim Abdul-Rauf, Yao Ma, Qingbiao Li ⁎Department of Chemical and Biochemical Engineering, and National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, and Key Lab for Chemical Biology of Fujian Province,Xiamen University, Xiamen, 361005, PR China⁎ Corresponding author. Tel.: +86 592 2189595; fax:E-mail address: kelqb@xmu.edu.cn (Q. Li).0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.06.079a b s t r a c ta r t i c l e i n f oArticle history:Received 16 March 2011Accepted 21 June 2011Available online 28 June 2011Keywords:BimetalBiosynthesisCacumen PlatycladiElectron microscopyNanoparticlesA facile and eco-friendly method for the preparation of Au–Pd bimetallic nanoparticles (~7 nm) has beendeveloped based on simultaneous bioreduction of Au(III) and Pd(II) precursors with Cacumen Platycladi leafextract in aqueous environment. The morphology, structure, and size were confirmed with the aid oftransmission electron microscopy, selected area electron diffraction, UV–vis spectroscopy, X-ray diffraction,and energy dispersive X-ray spectroscopy. The results from Fourier transform infrared spectroscopy showedthat the C_O and C\O groups in the plant extract played a critical role in capping the nanoparticles.Importantly, the process can be described as pure “green chemistry” technique since no additional syntheticreagents are used as reductants or stabilizers.+86 592 2184822.ll rights reserved.© 2011 Elsevier B.V. All rights reserved.1. IntroductionBimetallic nanoparticles (NPs) with alloy or core-shell structures,are of interest from both technological and scientific point of views,owning to many novel properties on magnetic, optical, electronic, andcatalytic applications [1]. For instance, Au–Pd bimetallic NPs haveproved to be excellent catalysts for a variety of reactions with superioractivity and durability to their monometallic counterparts [2–4].Conventionally, bimetallic NPs are assembled through seed-mediatedgrowth, template synthesis, chemical reduction and laser ablation [5–7].However, in spite of their widespread use, these methods utilize notonly chemical reductants but auxiliary stabilizers, and often requirespecialized and expensive equipment. Currently, increasing environ-mental concerns necessitate the development of new and eco-friendlytechniques for the synthesis of NPs. Accordingly, intense research hasbeen directed to the biosynthesis of NPs utilizing “nature factory” (viz.plants and microorganisms) [8–10], which are intrinsically greener.Regrettably, almost all of theseworks are based solely onmonometallicNPs. The few attempts on biosynthesis of bimetallic NPs were previousreported on Au/Ag [11] and Ti/Ni [12] systems, respectively.In earlier studies, our research groups obtained Au, Pd monome-tallic NPs using different plant extracts in aqueous solutions [13–15].Here, we report for the first time, a facile and green route forpreparing Au–Pd bimetallic NPs using Cacumen Platycladi (CP) leafextract as both a reductant and stabilizer. To confirm the nature of thebimetallic NPs, various characterization techniques such as transmis-sion electron microscopy (TEM), selected area electron diffraction(SAED), UV–vis spectroscopy, X-ray diffraction (XRD), energy disper-sive X-ray (EDX) analysis, and Fourier transform infrared spectroscopy(FTIR) were employed.2. Experimental2.1. MaterialsChloroauric acid (HAuCl4) and palladium chloride (PdCl2), werepurchased from Sinopharm Chemical Reagent Co. Ltd. (China) andused directly without pretreatment. Sundried CP leaves were pur-chased from Xiamen Jiuding Drugstore (China).2.2. Synthesis of NPsTo obtain the CP leaf extract, screened power CP leaf of 1 gdosage was dispersed in 100 mL deionized water by a shaker for 4 h.The extract was then filtrated and used for the synthesis of NPs. Thebimetallic NPs were prepared as follows: a well-mixed aqueoussolution (40 mL) of HAuCl4 (0.25 mM) and PdCl2 (0.25 mM) wasprepared. The CP leaf extract (20 mL) was then added to thissolution with vigorous stirring for 2 h; where spontaneous reduc-tion results in the formation of bimetallic NPs. For comparison, theindividual NPs were prepared in the same manner by substitutingthe aqueous HAuCl4/PdCl2 mixtures by HAuCl4 (0.5 mM) and PdCl2(0.5 mM) solutions, respectively.Fig. 1. TEM image (A), high-resolution TEM image (B), and SAED patterns (C) of the Au–Pd bimetallic NPs. The inset in (A) indicates the size distribution of the NPs.2990 G. Zhan et al. / Materials Letters 65 (2011) 2989–29912.3. Characterization of NPsUV–vis spectra were recorded on a spectrophotometer (TU1900,Pgeneral; China) using a 1 cmquartz cell fromhydrosol samples. TEMobservations, SAED analysis, and EDX analysis were performed on anelectron microscope (Tecnai F30, FEI; Netherlands) with an acceler-ating voltage of 300 kV, where the samples were prepared by dip-ping the copper grid in the NPs hydrosol and allowing water toevaporate. XRD measurements for the dried and powdered sampleswere conducted on an X-ray diffractometer (Phillips; Netherlands)equipped with Cu Kα radiation (40 kV, 30 mA). FTIR experimentswere carried out on a spectrophotometer (Avatar 660, Nicolet; USA),where the samples were ground with KBr and pressed into thinwafer.3. Results and discussion3.1. TEM and SAED analysisTEM image of the Au–Pd bimetallic NPs are shown in the Fig. 1A. Itshowed a well-defined spherical shape with fairly uniform size of 7.4±0.8 nm. The d-spacing of the adjacent (111) lattice of the bimetallicNPs was 2.29 Å (shown in Fig. 1B), while those of the individualAuNPs and PdNPs were 2.36 Å and 2.25 Å respectively. Such a valueconforms to the reported value of Au–Pd alloy structure [16].Moreover, SAED pattern of Au–Pd bimetallic NPs (Fig. 1C) showedfour diffraction rings corresponding to four different crystal planes inthe face-centered cubic structure. They could be identified as poly-Fig. 2. UV–vis spectra (A) and XRD patterns (B) of AuNPs (a), PdNPs (b), phcrystalline bimetallic NPs since two phase-segregated metals produceeight diffraction rings.3.2. UV–Vis and XRD analysisFig. 2A shows UV–vis absorbance behavior of the biosynthesizedAu–Pd bimetallic NPs and those of the individual and the physicalmixture of monometallic ones. The AuNPs showed a characteristicsurface plasmon resonance (SPR) peak centered at 531 nm but nopeak was observed for the PdNPs. The SPR band of the Au wascompletely damped in the bimetallic NPs sample whereas it waspresent in the physical mixture sample. These results apparentlysuggest the generation of Au–Pd bimetallic clusters rather than phase-separate monometallic NPs [4]. Additionally, the bimetallic NPshydrosol showed dark brown in color (inset Fig. 2A), distinctlydifferent from the pure AuNPs (ruby red), pure PdNPs (light brown)and the physical mixture of NPs (ruby red).XRD patterns of the monometallic and bimetallic NPs (Fig. 2B)were compared to verify the formation of Au–Pd bimetallic structure.The physical mixture (AuNPs and PdNPs) showed two groups ofBragg diffraction signals, indicating two separate phases. However,the Au–Pd bimetallic NPs exhibited just a set of Bragg peaks, re-vealing the formation of new bimetallic crystalline species. Further-more, the Au–Pd bimetallic NPs exhibited a broad band at 39.0° (2θ),closer to the mean values of pure Au (111) plane (38.3°; Ref. code:00-004-0784) and pure Pd (40.0°; Ref. code: 00-046-1043); reflect-ing that Au alloyedwith Pd [16]. The bimetallic particle size, based onysical mixture of monometallic NPs (c), and Au–Pd bimetallic NPs (d).Fig. 3. STEM image of the Au–Pd bimetallic NPs (A); EDX elemental maps of Au (B) and Pd (C) concentrations in the NPs; Cross-sectional compositional line profiles of a single Au–PdNP (D).2991G. Zhan et al. / Materials Letters 65 (2011) 2989–2991Scherer formula of the (111) diffraction peak width at half-maximum, was 7.9 nm; consistent with TEM-derived data.3.3. EDX elemental analysisThe composition of the Au–Pd bimetallic NPs was measured byscanning TEM (STEM)-EDX mapping (shown in Fig. 3) to investigatethe element distribution of the NPs. The colored elemental mappingimages show that Au atoms (yellow) and Pd atoms (orange) werehomogeneously distributed over the entire NP structure. Therefore,the Au–Pd bimetallic structure was alloyed in nature. Again, EDXelemental line scanning on a single particle (Fig. 3D) confirmed thatthe bimetallic NP was composed of two alloyed metal elements. EDXdata also showed the mole fraction of Au in the alloy to be 0.51;almost consistent with that of the feed solution (0.50).3.4. FTIR analysisFTIR analysis was conducted to identify the possible biomoleculesresponsible for the reduction of the metal precursors and thestabilization of the NPs [15]. The spectrum of the plant extract(Fig. 4a) showed a number of vibration bands for O\H group(3430 cm−1), C_O group (1730 cm−1), C_C group (1625 cm−1),carboxyl group (1380 cm−1), and C\O group (1290 and 1056 cm−1).Therefore, we conclude that some water-soluble polyhydroxy bio-molecules in the CP leaf extract (e.g. flavonoid and reducing sugar) areresponsible for the bioreduction of the NPs. On the other hand, thebands at 1730 and 1290 (1056) cm−1 corresponding to C_O and C\Ostretching vibrations respectively, were reduced (almost disappeared)Fig. 4. FTIR spectra of CP leaf extract (a) and the Au–Pd bimetallic NPs (b).in the bimetallic NPs (Fig. 4b). The evolution of the peaks is attributedto the interactions between C_O (C\O) and NPs, thus suggesting thatC_O and C\O groups are the capping ligands of the NPs with highaffinity for coordination.4. ConclusionSimultaneous bioreduction of aqueous HAuCl4/PdCl2 mixture withCP leaf extract as both a reductant and stabilizer successfully producedspherical Au–Pd bimetallic nanoparticles with a mean size of 7.4 nm.Characterization results showed that the bimetallic nanoparticleswere alloyed. The method is facile, eco-friendly and offers advantagein synthesizing Au–Pd bimetallic nanoparticles with small diametersand narrow size distribution; a potential for catalysis.AcknowledgmentsThis work was supported by key program (Nos. 21036004) andgeneral program (Nos. 20976146) of the National Natural ScienceFoundation of China, and the Natural Science Foundation of FujianProvince of China (Nos. 2010J05032, 2010J01052).References[1] Toshima N, Yonezawa T. New J Chem 1998;22:1179–201.[2] BonarowskaM, Pielaszek J, Semikolenov VA, Karpinski Z. J Catal 2002;209:528–38.[3] Ksar F, Ramos L, Keita B, Nadjo L, Beaunier P, Remita H. 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Green synthesis of Au-Pd bimetallic nanoparticles: Single-step bioreduction method with plant extract

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Green synthesis of Au-Pd bimetallic nanoparticles: Single-step bioreduction method with plant extract

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