Chemical Physics Letters 514 (2011) 307–310

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Chemical Physics Letters

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Ultra fine scale phase separated microstructure for Ag–Fe nanoparticle

Chandan Srivastava ⇑, Shyam Kanta SinhaDepartment of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 June 2011In final form 19 August 2011Available online 27 August 2011

0009-2614/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.cplett.2011.08.052

⇑ Corresponding author. Fax: +91 080 2360 0472.E-mail address: csrivastava@materials.iisc.ernet.in

We report the formation of Ag–Fe nanoparticles with an ultrafine scale phase separated microstructureconsisting of Ag and Fe3O4 phases. Ag–Fe particles were synthesised by the co-reduction of Ag and Fesalts in water medium. The co-existing Ag and Fe3O4 phase volumes were around �1 nm in one of thedimensions.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Fundamental requirements for the formation of miscibility gapare a positive enthalpy of mixing and a large difference in the sizes(greater than 15%) between the participating component elementatoms [1]. Microstructure of a bulk solid with composition layingin the miscibility gap region of the phase diagram, for a particularbinary system, is composed of volumes of two kinetically and ther-modynamically stable phases [1]. Each of these two phases is a so-lid solution is which interchangeably one of the components is thesolute and other is the solvent [1].

Alterations in bulk phase stability in nano-sized systems havebeen well established [2–4]. One such size induced alteration is mis-cibility between bulk immiscible atoms co-existing in an isolatednano-sized particle [4–8]. Miscibility refers to the co-existence ofcomponent atoms in a single phase solid solution structure [1].Nano-size induced increase in miscibility can be used to manufac-ture novel alloy materials with unique properties that cannot berealized in bulk. With respect to the investigations on the phenom-enon of diminishing elemental immiscibility in nano-sized parti-cles, one scientifically interesting and technologically relevantaspect that remains relatively less explored is the structural natureof the alloy formed between bulk immiscible atoms. By the ‘struc-tural nature of alloy’ we mean what exactly is the nature of thethree-dimensional geometrical packing of bulk immiscible atomsconfined in a nano-metric sized volume. Future studies can focuson the identification and rationalization of the nature of the particlemicrostructure that develops when the co-existing componentatoms making up the volume of the nano-sized particle have a po-sitive heat of mixing and a wide difference in the atomic radii.

This Letter investigates different particle microstructures thatdevelop in chemically synthesised Ag–Fe nanoparticles. At the bulkscale, Ag–Fe system shows miscibility gap both in the solid and li-

ll rights reserved.

(C. Srivastava).

quid phases. High immiscibility in Ag–Fe system results from thelarge difference in sizes (�14%) between Ag and Fe atoms and alarge positive heat of mixing (DH). For equi-atomic Ag–Fe solidsolution the DH value is about +28 KJ/mol [9].

2. Experiment

In the present work, to synthesise the nanoparticles, 0.061 g ofAgNO3, 0.031 g of FeCl3 and 0.1698 g of polyvinylpyrrolidone (PVP)were first dissolved in 80 mL of distilled water. A 4 mL solution ofNaBH4 in water with 0.3 M concentration was separately prepared.The solution containing AgNO3, FeCl3 and PVP was then transferredto a three neck round bottom flask fitted with a reflux condenserand a magnetic stirrer. During the synthesis reaction an inert argonatmosphere was maintained inside the three neck flask. The threeneck flask containing the reaction mixture was then heated. Whenthe temperature of the reaction mixture reached 70 �C, the NaBH4

solution was drop wise added into the reaction mixture using asyringe. After addition of the reducing agent, temperature of thereaction mixture was raised to 100 �C and at this temperaturethe reaction mixture was refluxed for 30 min. At the end of30 min, heating was stopped and the reaction mixture was allowedto cool down to the room temperature under the inert atmosphere.At the room temperature, the reaction mixture was poured into abeaker containing 100 mL ethanol and was then left for about3 h. A black dispersion of nanoparticles collected at the bottomof the beaker was then centrifuged. Particles isolated at the bottomof the centrifuge tube were then dispersed in hexane for furtheranalysis.

Phases of the nanoparticles were determined by the X-ray dif-fraction (XRD) technique using the X-Pert PRO, PANalytical ma-chine using the Cuka radiation source. A 300 keV field emissionFEI Tecnai F-30 transmission electron microscope (TEM) was usedfor obtaining bright field images and composition of individualnanoparticles. Highly dilute nanoparticle dispersion was dropdried onto a carbon coated Cu grid for the TEM based analysis.

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After the acquisition of the bright field image, the C2 aperturesetting in the microscope was changed to smallest (50 l diameter)aperture size and the beam spot size was reduced to about 3 nm.With the new aperture and spot size settings the electron beamwas converged over a nanoparticle and energy dispersive spectros-copy (EDS) signal was acquired from that particular nanoparticle.Background subtracted integrated intensity of peaks correspond-ing to AgK and FeK lines in the EDS spectrum were used for the ele-mental quantification. Digital micrograph software from GATANCompany, USA was used to obtain the fast Fourier transform(FFT) diffraction pattern from the high magnification TEM brightfield image of a nanoparticle. To obtain the inverse FFT image, aparticular FFT diffraction ring was selected using a masking toolprovided in the software. The making tool is an annular circle thatcan select a particular diffraction ring. After applying the mask aseparate function in the software that is identified as ‘inverseFFT’ was employed. The ‘inverse FFT’ function re-created only thatpart of the original high magnification bright field image that cor-responds to the masked diffraction ring.

3. Results and discussion

XRD profile obtained from an assembly of nanoparticles formedby drop drying the nanoparticle dispersion onto a glass slide isshown in Figure 1a. As indicated in Figure 1a, the XRD profile con-tained peaks corresponding only to pure Ag and Fe3O4 phases. A

Figure 1. (a) XRD profile for as-synthesised Ag–Fe nanoparticles. The XRD profilereveals peaks corresponding only to pure Ag and Fe3O4 phases and (b) Lowmagnification TEM bright field images of as-synthesised nanoparticles.

low magnification TEM bright field image of the as-synthesisednanoparticles is provided in Figure 1b. HRTEM imaging and singleparticle compositional analysis revealed compositionally andmicrostructurally three different kinds of nanoparticles in the as-synthesised nanoparticle dispersion. They were;

(i) Single phase nanoparticles that did not have any detectableFe content in them and were composed of only Ag (type Aparticles). A representative high magnification image of ananoparticle belonging to this particle group is shown inFigure 2a.

(ii) Single phase nanoparticles that did not have any detectableAg content in them and were pure Fe3O4 (type B particles). Arepresentative high magnification image of a nanoparticlebelonging to this particle group is shown in Figure 2b. Itshould be noted that the synthesis procedure was carriedout under an argon atmosphere so the magnetite nanoparti-cles formed from the oxidation of Fe nanoparticles once thereaction mixture was exposed to the ambient atmosphere.

(iii) Nanoparticles with a two phase microstructure (type C par-ticles). High magnification images of two such type C nano-particles are shown in Figure 3a and b. Composition ofnanoparticles shown in Figure 3a and b are Ag 78 at.% – Fe22 at.% and Ag 64 at.% – Fe 36 at.% respectively. A represen-tative EDS profile showing Ag and Fe peaks obtained from atype C nanoparticle is shown in Figure 3c. A Fast FourierTransform (FFT) of the high magnification image of a typeC particle is shown in Figure 4b. FFT in Figure 4b is obtainedfrom the region of the nanoparticle’s high magnification

Figure 2. (a) A single phase Ag nanoparticle (type A particle) (b) a single phaseFe3O4 nanoparticle (type B nanoparticle).

Figure 3. (a) A two phase type C nanoparticle having a composition of Ag �78 at.% and Fe �22 at.% and (b) a two phase type C nanoparticle having a composition of Ag�64 at.% and Fe �36 at.% and (c) a representative EDS profile showing Ag and Fe peaks obtained from a type C nanoparticle (Cu peaks in the EDS profile is from the carboncoated Cu TEM grid).

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image lying within the dashed rectangle in Figure 4a. Interms of the interplanar spacing, the FFT in Figure 4brevealed the presence of two different kinds of lattice planesand thus a poly-crystalline microstructure for the type Cparticle. Measurement of the interplanar spacings of theselattice planes revealed that the type C particle containsregions of pure Ag and Fe3O4 phases. The location of thesephases in the particle microstructure was revealed in theinverse FFT images obtained using the Ag and Fe3O4 reflec-tions in FFT diffraction pattern in Figure 4(b). Figures 4(c)and 4(d) respectively are the inverse FFT images obtainedby employing Fe3O4 and Ag reflection in the FFT diffractionpattern shown in Figure 4b. From the inverse FFT images itis apparent that the type C nanoparticles are made up ofextremely fine Ag and Fe3O4 phase volumes which are�1 nm in size in atleast one dimension (in the plane of thepaper). Note that the composition values for the particlesshown in Figure 3a and b only provide the relative amountsof Ag and Fe atoms and do not show any oxygen amount.Reporting of the oxygen content was intentionally ignoredas the oxygen signal in the EDS spectrum is due to the oxy-gen present both in the nanoparticle and in the PVP surfac-tant coating around the particle. The composition valuesonly reflect the fact that inside a single particle both Agand Fe atoms were present. Again, as for type B particles, itis emphasized that the synthesis procedure was carriedout under an argon atmosphere so the Fe3O4 phase presentin the type C particles is a result of oxidation of pure Fephase once the reaction mixture was exposed to the ambientatmosphere.

A similar type of ultrafine scale phase separated microstructureas seen in case of type C particles has been reported earlier forAg–Ni system by He et al. [10]. In their work on understanding the

unexpectedly low heat of crystallization in ‘amorphous’ Ag–Ni filmsmade by dc sputtering, He et al. [10] have shown using extended X-ray absorption fine structures in combination with reverse MonteCarlo and molecular dynamics simulations that the amorphous likeAg–Ni films actually contains nonuniform, spinodal-like structuresrich in Ag and Ni atoms. In the present case, during the synthesisreaction when the reducing agent is injected into the reaction mix-ture some of the Ag and Fe atoms that are simultaneously populatingthe reaction mixture due to the co-reduction of Ag and Fe precursorsare forced to freeze into an atomic agglomerate inside a threedimensional confinement made by the long chain organic PVP sur-factant molecules. Inside this surfactant confinement the amor-phous like arrangement of Ag and Fe atoms then evolves into theobserved ultrafine scale phase separated microstructure similar tothe one seen in case of the Ag–Ni system by He et al. [10]. A very highpositive enthalpy change (DH) involved in the formation of a solidsolution structure containing Ag and Fe atoms provides the requireddriving force to overcome the interfacial energy and coherencestrain energy effects between the �1 nm sized (at least in onedimension) co-existing pure Ag and Fe phase volumes. Furthermore,preference for the ultrafine scale phase separated microstructurewith a spinodal decomposition characteristic over a fully phase-sep-arated state is primarily due to the co-existence of two intermixedspecies (Ag and Fe) of widely different sizes which restricts theatomic diffusivity over larger distances. Furthermore, it should beenergetically favorable to form the ultra fine scale phase separatedmicrostructure with phases resembling equilibrium solids as withsuch atomic configuration some degree of energy minimizationcan be realized even though long-range chemical partitioning is sup-pressed due to restricted atomic diffusion.

In conclusion, in the present work Ag–Fe nanoparticles weresynthesised using the chemical reduction technique. In the as-syn-thesised nanoparticle dispersion three kinds of nanoparticles werepresent. They were single phase nanoparticles containing pure Ag

Figure 4. (a) A two phase type C nanoparticle having a composition of Ag �64 at.% and Fe �34 at.% (b) FFT of the high magnification image of the particle shown in Figure 3aand c. Inverse FFT image constructed using the Fe3O4 (311) rings in the FFT diffraction pattern in Figure 3b and d. Inverse FFT image constructed using the Ag (111) rings inthe FFT diffraction pattern shown in Figure 3b.

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phase, single phase nanoparticle containing pure Fe3O4 phase andnanoparticles containing Ag and Fe atoms with an ultrafine scalephase separated microstructure made up of Ag and Fe3O4 phases.

Acknowledgment

The authors acknowledge the electron microscopy facilitiesavailable at the Advanced Centre for Microscopy and Microanalysis,Indian Institute of Science, Bangalore, India. One of the authors(S.K. Sinha) acknowledges the funding from the Council of Scientificand Industrial Research (CSIR), Government of India.

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