Synthesis and Mechanism of Composition and Size Dependent

Article pubs.acs.org/JPCC

Synthesis and Mechanism of Composition and Size Dependent Morphology Selection in Nanoparticles of Ag−Cu Alloys Processed by Laser Ablation Under Liquid Medium Kirtiman Deo Malviya and Kamanio Chattopadhyay* Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: We have synthesized Ag−Cu alloy nanoparticles of four different compositions by using the laser ablation technique with the target under aqueous medium. Following this, we report a morphological transition in the nanoparticles from a normal two-phase microstructure to a structure with random segregation and finally a core−shell structure at small sizes as a function of Cu concentration. To illustrate the composition dependence of morphology, we report observations carried out on nanoparticles of two different sizes: ∼5 and ∼20 nm. The results could be rationalized through the thermodynamic modeling of free energy of phase mixing and wettability of the alloying phases.

1. INTRODUCTION The Ag−Cu system is often used as model system for studying the formation of solid solution under nonequilibrium conditions.1−5 In recent time, nanoalloys of Ag and Cu have also emerged as promising materials for applications such as an antibacterial and conducting ink, fiber-optical sensor, catalyst, thermal transport and in applications involving solar cell because of high electron conductivity and better adhesion properties.6−12 The laser ablation method of solid target under liquid medium has been shown to be an efficient method to produce alloy nanoparticles and their applications.13−15 The simplicity of the method is the major advantage of the process. Even if the alloying elements possess large difference in melting temperature, alloy nanoparticles can be synthesized by the process.16 Kazakevich et al. have shown that the properties of the laser generated nanoparticles of brass strongly depend on the microstructure of the initial target. More recently the advantages of the absence of the chemical reagents or ions in the final preparation of the alloy nanoparticles were used to study the biocompatibility of the alloy nanoparticle and the toxicity with the cells.17,18 The alloy nanoparticles can be synthesized by the laser ablation by the direct interaction of the laser beam with the solid target or by the laser irradiation of the colloidal solution containing the mixture of alloying element.19 Hodak et al. have shown that the irradiation of the colloidal core−shell Ag−Au nanoparticle leads to the absorption of the laser energy and results in melting and formation of the solid solution.20 In the present study we have adopted the laser ablation from the alloy target of the desired composition to generate the alloy © 2014 American Chemical Society

nanoparticles. There are reports on the synthesis of Ag nanoparticle and size control by the use of the different surfactant and laser energy.21−23 But the synthesis of Cu and Ag−Cu system is less studied.24−27 Both elements in the Ag−Cu binary system have face centered cubic (fcc) structure and form a eutectic phase diagram.28 The Ag−Cu system under equilibrium conditions is immiscible at ambient temperature in solid state. The maximum solubility of Cu in Ag is 14.1 atom % Cu at eutectic temperature (1052 K).29 The Hume−Rothery rule for the formation of solid solution states that if two elements having same crystal structure (Ag and Cu both have fcc structure) and size mismatch less than 15% (∼13% in the case of Ag−Cu), they should have large mutual solid solubility. The Ag−Cu system is therefore an exception to this rule and has only 0.1% solid solubility of Cu in Ag at room temperature. The system, however, is miscible in liquid state with a small positive heat of mixing (e.g., at equiatomic composition, ΔH = +6 kJ/mol).30 There are limited investigations on free alloy nanoparticles of Ag−Cu binary alloys.31 Because of the difference in redox potentials, it is difficult to carry out chemical synthesis and reduce simultaneously Ag and Cu. Instability of Cu in aqueous medium causes further difficulty in alloying. Recently, Tsuji and co-workers synthesized Ag−Cu alloy nanoparticles by wet chemical method (polyol method) and studied systematically the morphology of the particles.32 The average sizes of these free alloy particles are more than 100 nm and hence are at the Received: March 7, 2014 Revised: May 16, 2014 Published: May 27, 2014 13228

dx.doi.org/10.1021/jp502327c | J. Phys. Chem. C 2014, 118, 13228−13237

The Journal of Physical Chemistry C

Article

nanoparticles were obtained by ultrasonication of nanoparticles for 15 min. A small drop of this solution on a carbon coated nickel grid followed by vacuum drying was used to prepare the TEM sample. A 300 kV field emission transmission electron microscope (FEI make Tecnai F30) equipped with high angle angular dark field (HAADF) detector and energy dispersive Xray (EDX) was used for TEM characterization. The particle size distributions of alloy nanoparticles were determined by measuring more than 500 individual particles using the Sigma-Pro software.

limiting case of nano regime. To overcome the difficulties of synthesis by chemical routes, we have employed laser ablation technique under liquid medium. Since the pulse laser ablation (PLA) technique is a nonequilibrium process with an ability to achieve very high cooling rate (∼1012 K/s), we reasoned that this may yield alloys of Ag−Cu solid solution. The process is relatively cleaner because of the direct formation of nanoparticles without the use of reducing agent or toxic chemicals. In this paper we first report the synthesis of the oxide free nanoparticles of less than 50 nm by laser ablation under aqueous medium of Ag−Cu alloy targets by the optimization of the PVP concentration. Following successful synthesis, we explored the morphology and the phase stability of these alloy nanoparticles as a function of composition. The study unravels a morphological transition from biphasic structure to a dispersed morphology within a nanoparticle to a core−shell structure with increasing copper concentration.

3. RESULTS 3.1. Synthesis through Laser Ablation of Alloy Targets. The targets for the present study were prepared by induction melting of pure elements followed by chilled mold casting in order to have the target with uniform composition and free from foreign impurities. Figure 1a shows the representative back-scattered scanning electron microscope (BSE) image of eutectic target composition to indicate the scale of microstructure. (The scale of other target composition is similar and not shown.) The composition of the two-phase eutectic is Ag−40 atom % Cu. The dark gray phase indicates the β phase (Cu rich solid solution) while the light gray phase represents α phase (Ag rich solid solution). The submicrometer size of the phases is expected to ensure more uniform composition of the laser plume. It is also realized that the synthesis of the nanoparticles by the laser ablation of bulk target shows the maximum purity.18 Figure 1b shows the X-ray diffraction patterns of all the target samples. The systematic variation of peak intensities of Ag and Cu reflections indicates the variation in Ag and Cu contents. It is notable that there is a shift in Ag peaks, whereas there is very little change in the peak positions of Cu. The change in peak position was established by whole pattern fitting routine (Lebail method), and lattice parameters were obtained from Williamson−Hall plot. The solid solution of Ag and Cu is reported to exhibit 1% deviation from Vegard’s law, and a master curve obtained from experimental data is available.28 We have obtained the compositions from the lattice parameter using this curve. The maximum solubility of Ag in β phase is estimated to be 0.11 atom % Ag while for Cu in α phase it is 1.3 atom % Ag. This is consistent with the SEM EDS analysis of the phases in target microstructures. The laser ablation rate of different target composition is determined by weighing the target after ablation at an interval of 10 min starting from 0 to 90 min (total 5.4 × 104 shots). Figure 1c shows the ablation rate of all the targets used in our experiments. The plots indicate that pure Ag has maximum ablation rate while pure Cu has minimum. In the case of Ag the increase of ablation has been attributed mainly to local heating effects and to the cavitation bubble dynamics mechanism.22 Moreover the nanoparticles that stay in the liquid may absorb subsequent laser pulses and contribute to energy loss.35 The ablation rate decreases as the copper content increases in the target. It may be due to the increase in absorption of the laser energy and the fragmentation of the particles by postirradiation of the laser energy.36 This will be more for the case of Cu as absorption plasmon peak is near-infrared wavelength. As the number density of the nanoparticles increases in the liquid medium, the laser beam will be more attenuated by the absorption and scattering of the beam by the particles. Therefore, the actual intensity of the beam reaching the surface of the target will reduce with time causing a further decrease in

2. EXPERIMENTAL METHODS The target compositions for the laser ablation were Ag (99.999% purity), Cu (99.995% purity), and Ag−X atom % Cu (X = 20, 40, 60, 80). These targets were prepared in-house by induction melting under a vacuum level of 10−6 mbar. The laser ablation is carried out with a Nd:YAG laser (wavelength, 1064 nm; repetition rate, 10 Hz; pulse width, 8 ns; make, Spectra-Physics). The targets were immersed in aqueous medium (double-filtered using Millipore system) below 3 mm of the surface of the liquid. The laser beam is bent and focused on the surface of the target by a 90° prism and a lens of focal length 9 cm. The beam diameter on the surface was kept constant at 0.5 mm, and the target is rotated at 5 rotations per minute. A laser energy of 50 mJ/pulse and 1.8 × 104 shots were used in the aqueous solution of 30 mL of PVP for the analysis of the sample. For the analysis of the laser ablation rate we have used total of 5.4 × 104 shots. Aqueous solution of PVP with 0.02 M concentration is used as a capping agent in aqueous medium. Tsuji et al. has shown that the addition of PVP leads to a narrow distribution of the particles.21 The exact amount of PVP was optimized by us using pure Ag, taking into account the distribution and surface state of the particles. A concentration of 0.02 M PVP yields a size distribution of 32 ± 12 nm with very little oxidation of the surface (Figure S1). We have used same concentration for the synthesis of alloy nanoparticles. We believe in situ conjugation of PVP occurs similar to the findings of Petersen et al.33 We rule out postsynthesis equilibrium because of our initial experiments without PVP yield very large sizes that can be controlled only partially by postsynthesis size equilibrium by polymer liquid.34 For X-ray diffraction and transmission electron microscopy (TEM) studies, particles were centrifuged and washed several times to clean the excess PVP. The nanoparticles (NPs) were dried drop by drop on glass slides for the XRD sample preparation. A Panalytical make X’Pert PRO X-ray diffractometer was used for data collection in reflection mode using Cu Kα doublet radiation in 2θ range from 20° to 90° with a step size of 0.02°. A whole powder pattern fitting procedure (Lebail method) using the software TOPAS, version 4 (BRUKER AXS) was used to analyze the data. The crystallite size, rms strain, and precise lattice parameters were calculated from the data on peak broadenings and peak shifts after separation of the contribution from instrumental broadening. For TEM sample preparation, the dilute solutions of dispersed 13229



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3.2. Evolution of Phases and Morphologies. The X-ray diffraction patterns of synthesized nanoparticles from different target compositions are shown in Figure 2a. The Ag peaks are

Figure 2. (a) XRD pattern of the alloy nanoparticles from different target compositions. It shows the deconvoluted peaks in the range 33− 52°. The dot−dash line shows the Ag peak positions and shot dotted line shows Cu peak positions. The extra peaks are represented by the solid line as a solid solution (ss) peaks. (b) Crystallite size of the synthesized nanoparticles from different target compositions.

present in all the samples, whereas Cu peaks can only be observed in the sample with target composition of Ag−80 atom % Cu. Peak broadening increases from pure Ag nanoparticles to sample with target composition of Ag−60 atom % Cu followed by a decrease with further increase in Cu concentration. There is a set of new peaks that emerges in alloys having composition Ag−(20−60) atom % Cu. The peak fitting of X-ray diffraction pattern suggests the formation of two types of solid solutions. One of them is Ag rich solid solution with Cu (Agss) as reflected by the shift in Ag peaks. The second one is characterized by a set of new peaks closer to Cu reflections indicating Cu rich solid solution with Ag (Cuss). The deconvoluted peaks are shown in Figure 2a as dotted lines. Figure 2b shows the crystallite size of the two solid solution phases. The crystallite size decreases from pure Ag to the ablation of Ag−60 atom % Cu and again increased for Ag−80 atom % Cu. The analysis of X-ray diffraction results indicates formation of two types of phases, Ag rich solid solution with Cu (Agss) and Cu rich solid solution with Ag (Cuss). The detailed study of the microstructure of these nanoparticles using transmission electron microscope can yield further insight into the morphology of these phases. Figure 3a−d corresponds to TEM characterization of synthesized alloy nanoparticles from

Figure 1. (a) SEM back-scattered electron (BSE) image of the representative target surface (eutectic composition). Bright contrast in the image represents α phase (Ag), and the dark contrast represents β phase (Cu). (b) XRD pattern of all the target compositions. (c) Ablated mass versus time plot of different target compositions. Inset represents the ablation rate with time.

the laser ablation rate. Therefore, we have used only the 1.8 × 104 shots such that the ablation rate should not be attenuated, and we observe the synthesized nanoparticles directly from the target surface. Also, it was observed that the second harmonic of the laser wavelength is more attenuated because of the absorption of laser energy by the colloidal Ag nanoparticles, and they show the postirradiation effect (laser fragmentation) with a decrease in particle size.36 13230



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Ag−40 atom % Cu the d spacing of the AgSS shows no change, whereas the CuSS shows the decrease in d spacing (d111 = 0.234 nm for AgSS and d111 = 0.228 nm for CuSS). The alloy nanoparticles synthesized from target composition Ag−60 atom % Cu is shown in Figure 3c,c1. In this case most of the particle shows the variation in atomic contrast inside the particle. The high resolution image of an individual particle shows that the lattice fringes are not uniform, suggesting the existence of smaller domains. The dashed line marked on the particle is an attempt to distinguish the different domains or grains. The image shows that a single nanoparticle consists of many subgrains whose size varies between 2 and 3 nm. The FFT as shown in the inset exhibits two concentric diffuse rings with closely spaced interplanar spacings (0.232 and 0.225 nm). The nanoparticles obtained by laser ablation of the target composition Ag−80 atom % Cu show a different morphology (Figure 3d, d1). The HAADF image shows core−shell morphology of most of the particles. The HRTEM image of a nanoparticle shows AgSS and CuSS forming core−shell morphology with the d spacings of 0.234 and 0.216 nm, respectively. Table S1 shows the comparison of the d111 spacings of the two solid solution phases obtained from TEM and XRD. The low magnification bright field micrographs of the synthesized alloy nanoparticles from all four target compositions are shown in Figure S2. The diffraction pattern from the nanoparticles as shown in Figure S2 (a1−d1) shows the faint Cuss diffraction spots in Ag rich samples which are increasing as the Cu content increases. The uniform intensity diffraction ring can be observed in the case of the laser ablation of the Ag−60 atom % Cu. Further increase in Cu content shows the more pronounced diffraction spots of Cuss (Ag−80 atom % Cu). Figure S2 (a2−d2) shows the distribution of the particle size. The plot of volume weighted size distribution is shown in Figure S2 (a3−d3) that resemble the effect of the results of the XRD which is naturally the volume weighted intensity profile. The volume weighted particle size distribution shows that the XRD results are coming mostly from the size range of 15−60 nm. The observation revels that in the Ag rich alloy nanoparticles the biphasic morphology was observed and as the Cu content increases, the biphasic fraction increases and the further increase in Cu concentration changes the morphology from compositional segregation to the core− shell morphology. The UV−vis spectra of all the samples are shown in Figure S3. We found only one plasmon resonance band for each alloy nanoparticle that can be rationalized with the recent results of Menendez et al.19 These authors show that laser ablation from solid target in methyl methacrylate yields a single peak from Ag−Au alloy nanoparticle. We have not found any split in peak. This suggests that most of the particles form alloy. The damping of the plasmon peak in the case of the laser ablation of target Ag−80 atom % Cu may be due to the core− shell structure formation of the Cu rich and Ag rich solid solution. The TEM analyses of the nanoparticles from all the target composition corroborate XRD results. We further analyzed samples using spectroscopic techniques to obtain the compositions of these phases. In order to evaluate composition dependence of the morphology of alloy particles at small length scale, composition mapping of Ag and Cu elements has been carried out using STEM mode in TEM for nanoparticles with size of 20 ± 2 nm. Figure 4 shows the composition maps of alloy nanoparticles with increasing copper content (Figure 4a−d) obtained from targets with composition Ag−X atom % Cu (X = 20, 40, 60,

Figure 3. (a−d) Representative high resolution TEM micrograph of the synthesized alloy nanoparticles from four different target compositions, namely, Ag−X atom % Cu (X = 20, 40, 60, 80). Inset shows the FFT of the particle. (a1−d1) HAADF image of the four respective target compositions.

the target compositions Ag−X atom % Cu (X = 20, 40, 60, 80), respectively. The HRTEM (Figure 3a−d) and the low magnification HAADF images (Figure 3a1−d1) show the morphology of the alloy nanoparticles with the change in Cu concentration. In the case of the nanoparticles synthesized from Ag rich target compositions Ag−X atom % Cu (X = 20, 40, Figure 3a,b), most of the particles show the biphasic nature consists of Ag rich and Cu rich solid solutions. The increase in Cu concentration shows that the fraction of the Cu rich solid solution increases. The inset shows the FFT of the particles, and it indicates that the diffraction spots of two different d spacings correspond to Ag rich and Cu rich solid solution phases. In the case of Ag−20 atom % Cu the d spacings for AgSS and CuSS are d111 = 0.234 and 0.233 nm, respectively. In case of 13231



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Figure 4. (a−d) Compositional variation in nanoparticles corresponding to target composition Ag−X atom Cu (X = 20, 40, 60, 80), respectively. The Ag L and Cu K mapping shows the distribution of Ag and Cu elements in the particle. Red arrow on HAADF image shows the position of line scan. An inset in the HAADF image indicates the overall composition of the particle.

80), respectively. The HAADF image shows the atomic contrast in the particle, and the Ag L and Cu K mapping shows the distribution of Ag and Cu elements in the particle. The variations of composition across the red arrow in the HAADF image are presented in the last column. The overall composition of the whole nanoparticle with respect to target composition is shown in Figure 5a, whereas the plot of plasmon resonant wavelength with respect to the particle composition is shown in Figure 5b. A plot of composition variation of two solid solutions as a function of overall composition of the nanoparticles is shown in Figure 5c. For Ag rich compositions (Figure 4a,b), nanoparticles are composed of two different phases separated by sharp boundaries. Composition imaging reveals formation of complete solid solution in the nanoparticle of composition Ag−(46 ± 8) atom % Cu (Figure 5c). However, one can see evidence of fine scale segregation of Cu in Ag matrix (Figure 4c). The Cu rich alloy particles of composition Ag−(86 ± 3) atom % Cu (Figure 5c) are characterized by a core−shell structure (Figure 4d). The Cu rich phase forms a core, while the Ag rich phase appears as a shell leading to a compositionally separated core−shell structure.

Figure 5. (a) Composition of whole nanoparticle with respect to the target composition. (b) Variation of the UV−vis spectra with respect to the composition of the alloy nanoparticle. (c) Composition of Ag rich and Cu rich solid solution phases with respect to particle composition, respectively.

4. DISCUSSION The experimental results presented above establish the nature of evolution of two-phase microstructure in the nanoparticles of Ag−Cu alloys as a function of copper concentration for a given size of nanoparticle (∼20 nm). With increasing copper concentration a transition from coexistence of two solid solution phase with sharp interface separating them to solid solution with fine scale segregation could be observed. With further increase in copper concentration, a core−shell morphology emerges. The morphology of the two coexisting phases is often determined by their interphase interface

energies and relative volume fractions.37 The minimization of these energies can dictate the morphology of the alloy nanoparticles under equilibrium condition. The diffusivities of Ag in Cu and Cu in Ag are of the order of 10−12 cm2/s at temperature 873 K for bulk sample, while similar values are reported for thin films at room temperature.38,39 Assuming that diffusion in 20 nm particles can be approximated to follow thin film situation, the time to achieve equilibrium will be of the order of 100 μs at room temperature in the solid state. In the case of alloy forms at higher temperature, diffusion and hence equilibrium will be reached even faster. Hence, with a 13232



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Figure 6. (a) Schematic diagram of the two-phase coexistence nanoparticle with the dihedral angle ψ at the tripe point. (b and c) Size dependent surface energy of Ag and Cu. (d) Size and compositional dependent surface energy of alloy nanoparticle. (e) Surface energy with respect to composition at constant size of the particle. (f and g) Measured dihedral angle for two alloy compositions of the particle Ag−12 atom % Cu and Ag− 32 atom % Cu, respectively.

the alloy nanoparticles. We have recently proposed a model for the surface energy based on thermodynamics.41 With adoption of the model, the size and composition dependent surface energies for Ag−Cu alloy system can be estimated. Figure 8b represents the size dependent surface energy of the pure Ag (γα) and Cu (γβ). As can be seen, with an increase in size, the surface energy gets saturated to bulk value. The threedimensional variation of surface energy with the size and composition is shown in Figure 6d. For a constant size of the particle (∼20 nm) the variation of surface energy with composition is shown in Figure 6e. The size of 20 nm is chosen to enable comparison with the experimental results. The interfacial energy (γαβ) between two solid phases α and β can now be calculated by the relation given by Turnbull that consists of a geometrical and a chemical term.42

reasonable assumption of equilibrium, it is possible to calculate changes in the wetting angle between the phases due to the change in composition of the two phases in the particle. The model adopted assumes the solid state wetting of two phases of equal radius of curvature.40 Figure 6a schematically represents the two-phase particle. The three different interfacial energies are γα, γβ, γαβ, corresponding to the interfacial energy per unit area of the Ag rich (α), Cu rich (β), and the interface of the two phase morphology, respectively. There are three possible cases: (1) the two phases are completely separated as, γαβ > γα + γβ; (2) partial wetting when γαβ < γα + γβ; (3) complete wetting in case γβ > γα + γαβ (assuming β phase have higher surface energy than the α phase). The force balance at the junction of the triple point is shown in Figure 9a. Following Wakai et al., the dihedral angle (ψ) can be determined as a function of two parameters x(γα/γβ) and y(γαβ/γβ) by the expression40 y 2 − x 2 − 2x cos ψ − 1 = 0

geo chem γαβ = γαβ + γαβ

(2)

For two solids with incoherent interface, the geometrical term γgeo αβ can be approximated as the average of the large angle grain boundary energies of the two species by following expression,43

(1)

Since the compositions of two phases change in the alloy nanoparticles as the Cu content increases in the alloy, the surface energies will change with the composition and size of

geo γαβ = (0.35/2)(γα + γβ)

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(3)



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The chemical term can be represented as44 chem γαβ = {xAg ΔHAginCu/[C0(VAg)2/3 ] + xCuΔHCuinAg

/[C0(VNi)2/3 ]}

(4)

where ΔHAginCu (or ΔHCuinAg) is the heat of solution of Ag in Cu (or Cu in Ag) at infinite dilution, VAg and VCu are the molar volumes of Ag (VAg = 10.19 cm3) and Cu (VCu = 7.12 cm3), respectively, and C0 is a constant, the value being 4.5 × 108.44 The values of surface energies (γα and γβ) for estimating dihedral angle were obtained from Figure 6e corresponding to the experimental compositions of the particles (Figure 5a) of 20 nm size. By use of eq 1, the dihedral angle (ψ) can now be calculated. The values for the angle for three compositions are presented in Table 1 together with the experimental values Table 1. Dihedral Angle of Different Morphologies of the Particles composition of alloy nanoparticle

calcd dihedral angle (ψcal) (deg)

exptl dihedral angle (ψexp) (deg)

Ag−12 atom % Cu Ag−32 atom % Cu Ag−86 atom % Cu

102.4 112.0 180.0

102 ± 4 114 ± 6 180a

a

All the measured particles have core−shell morphology, and Ag rich solid solution completely covers the Cu rich solid solution core.

obtained from the micrograph as shown in Figure 6f,g. The experimentally obtained values are in close agreement with the calculated values. The variation of the dihedral angle with the composition successfully explains the case of partial wetting in Ag rich phase and complete wetting and formation of core− shell in Cu rich phase. We can now rationalize the stability of experimentally observed morphology by a thermodynamic analysis. The size dependent free energy model of Srivastava et al. can be used to estimate the changes in free energy of the observed morphology of the two-phase nanoparticles.41 The molar free energy (Gss) of solid solution can be expressed as

Figure 7. Calculated free energy variation of two morphologies (core− shell and biphasic) of the experimentally observed composition of (a) 20 nm and (b) 5 nm size particle.

to the core−shell particle for most of the compositions of the nanoparticle. However, the difference in energies for the biphasic and core−shell morphology is very small for Cu rich particles and is within the error expected for our estimation. The two morphologies show a difference of energy of ∼2.5 kJ/ mol. Under such circumstances we propose that the attachment of large Ag atoms at the outer layer forming the shell is more preferred. To validate the model, we have studied statistically the morphological changes in the alloy nanoparticles for two sizes, 5 and 20 nm for four compositions in our investigation. We have selected 25 particles in each case (totaling 200 particles) to observe the changes in morphology by measuring the composition line scan across these particles. The histograms of observed morphologies in different cases are shown in Figure 8a. A typical line profile indicating core−shell structure for 5 nm particle in a Ag−86 atom % Cu alloy is shown in Figure 8b. We note that for Ag rich alloys, solid solution is more abundant for 5 nm particles; however, this needs to be taken with a degree of caution, as it is often difficult to resolve composition segregation in 5 nm particles. In support of the above experimental observations, we have further calculated the size dependent free energy of the alloy nanoparticles for biphase and core−shell morphology in two extreme composition ranges observed (Ag−12 atom % Cu and Ag−86 atom % Cu) as shown in Figure 9a,b. The observed results show that the free energies of both biphase and core− shell particles increase with a decrease in particle size for both

Gss = xAGA + x BG B + ΔGmix + (2γss(D , x)VAB)/(D/2) (5)

Here, xA and xB are the mole fractions of phase A and B, respectively, GA and GB are the molar free energies of A and B, respectively, ΔGmix is the excess free energy of the alloy and γss(D,x) is the size and composition dependent specific surface energy (surface energy per unit area) of the nanoparticle with solid solution between elements Ag and Cu (Figure 6d). The term VAB represents the molar volume of solid solution, and D is the diameter of the particle. In the case of a two-phase particle (biphasic or core−shell morphology) the addition of interface created between two solid solutions will increase the total free energy of the nanoparticle (eq 5). The increase in interfacial energy (Gint(D)) can be written as Gint = A(γgeo + γchem)

(6)

The terms γgeo and γchem are given by eqs 3 and 4. The term A is the surface area occupied by one mole of interfacial atoms. By use of the surface energies as detailed earlier, the composition dependent free energies of nanoparticles (∼20 nm size) with the biphasic and core−shell morphologies observed in the present study can be estimated (Figure 7a). The free energies of the particles with biphasic configuration are higher compared 13234



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Figure 8. (a) Histogram of the different morphology observed in the four different variations in composition for two different particles sizes (5 and 20 nm). (b) STEM−HAADF mapping of a 5 nm particle showing the variation of Ag and Cu elements. The line scan shows the variation across the diameter of the particle.

Figure 9. Size dependent free energy variation of core−shell and biphase particle for two selected compositions: (a) Ag−12 atom % Cu; (b) Ag−86 atom % Cu.

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ACKNOWLEDGMENTS The authors acknowledge the microscopy facilities available at Advanced Facility for Microscopy and Microanalysis (AFMM), The Indian Institute of Science, Bangalore, India.

alloy compositions. However, for the case of Cu rich composition the difference in free energy between the two morphologies is larger. It suggests that in the case of Cu rich composition the core−shell is more preferred. For a constant size of the particle of 5 and 20 nm the increase in free energy for different compositions are shown in Figure 7a,b. The observation shows that the free energy differences between the two morphologies in all the compositions are higher for 5 nm particles. Thus, at small sizes the alloy nanoparticles favor core−shell structure over biphase morphology. The recent in situ small-angle X-ray scattering study of the cavitation bubble dynamics by Wagner et al. has shown that at early stage primary particle mass is most abundant.45,46 The mass abundance of the agglomerates increases during subsequent rebound. Clearly small particles are seen at a very early stage of cavity formation. For metals and alloys we believe the ablation causes a vapor plume that is composed of atomic clusters. Some of these clusters aid rapid nucleation of liquid. However, the probability of nucleation of pure Ag or Cu is always higher, and initially these particles are most likely to form as a liquid melt. Our estimate indicates a time of 100 μs for diffusion to take place for successful alloying in solid at room temperature for 20 nm particle. For liquid and higher temperature this is much smaller, and hence, we believe that the alloying occurred in the expansion phase of the cavity. The capping by the PVP most likely influences and exhibits the possibility of alloying through agglomeration of different particles. The final microstructure that we see evolves most likely during solidification of melt in these particles. In such a situation, the validity of the above arguments, however, needs to be validated in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesized nanoparticles of different composition and the TEM analysis of solid solution phases. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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5. CONCLUSIONS We have synthesized the nanoparticles of Ag−Cu alloy of four different compositions by the laser ablation of the target under liquid medium. Two coexisting solid solution phases rich in Ag and Cu respectively were observed in alloy nanoparticles. The morphology of the particles changes from biphasic to core− shell with increase in Cu concentration. The stability of theses morphologies is explained by the incorporation of the size and composition dependent surface energies in the calculation of the free energy of the particle. Qualitative estimate of the dihedral angles through the model was validated from experimental estimation of the same. This also explains the morphological transition from biphasic to core−shell structure. Our analysis suggests that the core−shell morphology will be preferred in copper rich alloys and as the size decreases, the effect is more pronounced.

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*E - ma i l : k a m a n i o @ m a t e r i a l s . i i s c . e r n e t . i n . Ph o n e : +918022932262. Fax: +918023600472. Notes

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Synthesis and Mechanism of Composition and Size Dependent

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