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Temperature and Size Dependent Compositionally Tuned Microstructural Landscape for Ag-46at.%Cu Nanoalloy Prepared by Laser Ablation in Liquid Kirtiman Deo Malviya, and Kamanio Chattopadhyay J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09781 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry

Temperature and Size Dependent Compositionally Tuned Microstructural Landscape for Ag-46at.%Cu Nanoalloy Prepared by Laser Ablation in Liquid Kirtiman Deo Malviya* and K. Chattopadhyay Department of Materials Engineering Indian Institute of Science Bangalore 560012

ABSTRACT: We report a temperature and size dependent compositionally tuned microstructural landscape for Ag-46at.%Cu alloy nanoparticles. The microstructural and morphological changes were established through the technique of in situ transmission electron microscopy. The nanoparticles were synthesized by laser ablation of alloy target in an aqueous medium. The as-synthesized particles predominantly contain nano-sized grains of a single phase solid solution with grains having sizes~3±0.5 nm. For particles with smaller sizes (~20 nm), the solid solution decomposes and grains coarsen on heating to yield predominantly bi-crystals containing two phases of Ag-rich and Cu-rich solid solution. The microstructure of the larger particles (≥ 40 nm) evolves through segregation of Ag and their preferential growth near the surface of the particles. This leads to a core-shell like composition distribution at a certain range of temperatures (≥ 200oC) and sizes (≥32 nm). At higher temperatures, these core-shell particles undergo a morphological transition through grain growth yielding bi-crystals of two phases. We present a size and temperature dependent morphology diagram that captures these changes. 1 ACS Paragon Plus Environment


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1. INTRODUCTION The synthesis of alloy nanoparticles has attracted significant attention in recent years due to technological importance 1–9. The synthesis of a desired alloy composition at small scale is challenging due to the issues related to the stability at small scale 10–16. The structure and composition of the nanoparticles may deviate from that expected from the equilibrium phase diagram due to reduction of size of the particles as well as non-equilibrium processing that they may undergo. It is now well established that different morphologies can evolve at small length scale that can alter the properties of the particles. This can lead to newer applications 17–22. In addition to the constraints imposed by the kinetics of the process of synthesis, the selection of morphologies in nanoalloys are often dictated by nature of the interfaces surrounding the nanoparticles. The surface energies of these interfaces are dependent on composition, size and temperature. Recently, a study of the size and composition dependent morphological evolution in the nanoalloys of Ag-Cu system is reported by the authors 23. The synthesis was carried out by laser ablation in an aqueous medium. This work has also reported an enhancement in solid solubility of Cu in the Ag nanoalloys to an extent of 46at.%Cu. This can be compared with the maximum solid solubility of 0.1at%Cu in Ag at room temperature under equilibrium condition 24

. Such a change can be rationalized by considering the kinetics of the ablation process as well

as by invoking the energies due to the presence of additional surfaces and interfaces compared to the bulk 25–27. Reports exist detailing morphology of such evolving structures as a function of size15,23. However, only a limited literature is available that addresses the temperature dependence of evolution of such morphologies 12. An example is a recent report on the sequential deposition of Ag on Cu at temperatures above room temperature that shows the emergence of a

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core-shell morphology 28. Such an information is crucial in the applications like catalysis that take place over a range of temperatures29. The laser interaction with material in liquid medium yields direct synthesis of nanoparticles with distinct microstructures compared to other conventional synthesis techniques 23,25,30–33. The recent development of controlled laser ablation synthesis and resulting microstructure of pure metals34–36, alloys37–39, composites40–42 and biocompatible43–45 nanoparticles show the diverse potential applications of this method. The present work describes the effect of particle size and temperature on the evolution of the structure and morphology of nano-alloy particles having a composition of Ag-46±8at.%Cu. The nano-particles were prepared by laser ablation of the alloy target in a liquid medium. The evolution of morphology of these particles as a function of temperature was monitored in situ inside an electron microscope with the help of a heating holder.

2. EXPERIMENTAL METHODS The alloy nanoparticles of Ag-46±8at.%Cu was prepared by laser ablation, the details of which is given elsewhere23,46. In brief, the ablating target (Ag-60at.%Cu) was prepared by the induction melting of pure Ag (99.999%) and pure Cu (99.995%) of appropriate atomic percentage under the vacuum level of 10-6 mbar. The microstructure of the target is shown in the back-scattered electron (BSE) image with the Ag rich (light gray) and Cu rich (dark gray) phases in figure S1. The alloy target was fixed on the bottom of the flask filled with an aqueous medium (30 ml) containing Polyvinyl Pyrrolidone (PVP) (0.02 M concentration) below 3 mm of the surface of the liquid. The flask was kept under argon atmosphere. A Nd: YAG laser with wavelength 1064 nm, pulse width 8 ns and frequency 10 Hz (Spectra-Physics make) was used for the ablation 3 ACS Paragon Plus Environment


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experiments. 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 (figure S2). The laser beam with a fluence of 20 J/cm2 and a beam diameter of 0.5 mm was focused on the target rotating at a centrifugal force of 5 x g. The typical ablation time used was 30 minutes for the preparation of the sample. For characterization, the solution containing sample was centrifuged at the rpm of 14,000 for 10 minutes and the concentrated solution was washed several times for removal of excess PVP. Samples were preserved in ethanol for the characterization by different techniques. For XRD analysis the concentrated solution was dried on a glass slide to obtain a layer of the nanoparticles. An X-ray diffractometer (Pan Analytical make X’Pert PRO) was used for the XRD data collection in the angular range of 20°- 90° with a step size of 0.2°. The crystallite size and strain were estimated by the Williamson-Hall method. For further characterization of both composition and structure by TEM, the dilute solution of the sample was sonicated for 15 minutes to ensure that the nanoparticles are well separated in the solution. A drop of the colloidal solution containing nano particles is allowed to dry on a carbon coated nickel grid using a lamp and further desiccated in vacuum before loading in a TEM sample holder. For in-situ heating, a Gatan, USA, make double tilt heating holder with the tantalum ring was used.

3. RESULTS The nature of the nanoparticles prepared through laser ablation of Ag-60at%Cu alloy target under aqueous medium is detailed in figure 1. The XRD pattern from the alloy target is shown in fig. 1a. We have also included the XRD pattern from pure Ag nanoparticles synthesized under identical condition. The pattern from alloy nanoparticles is broad compared to that obtained from the pure silver target. The peak fitting of the XRD data from the alloy nanoparticles indicates coexistence of two solid solutions. Analysis of these peaks indicates crystallite size of the 4 ACS Paragon Plus Environment

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ablated particles to be ~3±0.5 nm. Assuming the validity of the Vegard law in the present case, the compositions of the two co-existing solid solution phases were determined to be ~51±7 and 21±5at. %Cu. Transmission electron microscope was used for further characterization of these nanoparticles. The mean size of the particles using this technique is found to be 22±9 nm (fig 1b (1)). A typical bright field image reflecting the distribution of these particles is shown in fig 1b (2). The diffraction pattern from this group of particles shows rings of uniform intensity (fig 1c). No sharp diffraction spots could be seen consistent with the fine sizes of the particles. Imaging under bright field condition at higher magnification could reveal the presence of distinct diffraction contrast from smaller regions (2-3 nm) inside the nanoparticles (fig 1b (3)). Highresolution lattice imaging of two particles of different sizes (~20 nm and ~40 nm) reveals the presence of very small crystallites (fig 1d and 1e). The observed bending of some lattice fringes suggests that these particles are associated with high strain. This is consistent with the results derived from the X-ray patterns from these samples (rms strain 1.69±0.06 and 1.54±0.05 for two solid solutions, Ag rich and Cu-rich, respectively). The small crystallites inside the nanoparticle may have a distribution of composition that is further characterized by Scanning Transmission Electron Microscopy (STEM). The High Angle Annular Dark Field (HAADF) image reveals contrast difference within the particle and confirms the difference in composition of the two phases (fig 1f) as suggested by the presence of strain in HRTEM images of the particle. The high Cu concentration and the compositional segregation within an individual particle under as ablated condition suggest that this is a metastable phase since the equilibrium phase has negligible solid solubility. The heating of the particle can give information about the stability of these particles and the possible phases that can appear along the pathway leading to the final microstructure. The evolution of the diffraction patterns during heating of the particles from 5 ACS Paragon Plus Environment


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room temperature (RT) to 600 °C is shown in figure 2. The samples at room temperature and at 200 oC yield ring patterns with smooth and uniform intensities of the rings. On further heating, spotty rings begin to appear indicating grain coarsening. The intensities of these rings increase with temperature while that of uniform intensity rings fades. The analysis of the spotty rings at 600 °C establishes that they belong to the phase separated Ag and Cu. The lattice parameter measurements (figure 3) indicate a continuous change in the lattice parameters of both Ag rich and Cu-rich phases. Finally, they converge to that of lattice parameters of pure Ag and Cu phases at higher temperatures. The observation of the individual particles under electron beam at different temperatures can show the possible pathway for the phase separation in these nanoparticles. The time for complete interdiffusion depends on the size of the particle. This can cause a difference in the microstructure for different size particles. We have chosen two different length scales of the nanoparticles, ~20 nm and ~40 nm, for the present analysis. Figure 4 shows the morphology of the alloy nanoparticle of size ~20 nm at different temperatures. At the initial stage (at room temperature) each nanoparticle consists of several small grains of single solid solution phase (A) (fig.4a). At 200 °C, the nucleation of a primary Ag-rich phase occurs at the surface of the nanoparticles as shown by the dashed line (fig.4b). This leads to the evolution of two phase morphology containing the freshly nucleated Ag-rich phase and the initial singlephase solid solution. These are marked as B and A respectively in the figure 4b. We shall term this as bi-phase morphology in the rest of the paper. On further heating at 300 °C, the particle reveals the growth of the initial phase (B) together with nucleation of additional Ag-rich phase at the surface (B’)(fig.4c). The direction of growth of the Ag-rich phase is more along the outer surface of the particle. The growth of these two Ag rich phases at the expense of the original solid solution continues when heated to 500 °C(fig.4d). At 600 °C, the two Ag rich grains 6 ACS Paragon Plus Environment

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merged as a single phase and the nanoparticles exhibit completely phase-separated Ag rich and Cu-rich bi-phasic structure ( in literature, often termed as Janus morphology (fig. 4e) 47). A STEM analysis of the composition profile across the two phases in the particle with Janus morphology is shown in (fig. 4f). Figure 5a (inset) shows a particle of size ~40 nm particle that has been heated from room temperature to 600 °C. The conditions are similar to that of the previous case. The HRTEM of the particle before heating reveals the internal structure of the particle consisting of small grains (fig.5a). Heating from room temperature to 300°C leads to the appearance of Ag-rich phase enveloping the surface (fig 5b) resembling a compositionally core-shell structure. The lattice fringe indicates the shell to be silver rich while the Cu-enriched phase exists at the core of the particle. Further heating to 600 °C leads to a rearrangement and eventual formation of a bi-phasic (Janus-like) structure (fig 5d). The elemental mapping of the particle with Ag-L, Cu-K an O-K shell is shown in figure 5c. The EDS analysis shows the oxygen composition is less than 1 at% at the core as well as at the shell of the particle (figure S3). The mapping establishes the Ag-rich phase at the surface of the particle while Cu-rich phase in the core thus confirming a core-shell composition distribution. The details of the TEM microstructure variation with temperature and size were shown in figure S4. The above observations establish that particles having a size of 20 nm and smaller exhibit multiple grains of single solid solution phase. These particles on heating to 200 °C and above transform to a Janus-like biphase morphology consisting of Ag rich and Cu-rich phases. However, for larger size particles, a core-shell like composition distribution with Ag-rich phase enveloping the particle surface could be observed.



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We have mapped the morphological evolution of these nanoparticles as a function of size at different temperatures using in situ microscopy and have generated a morphology selection diagram for these nanoparticles. This is shown in figure 6. The diagram indicates that core-shell morphology of composition distribution evolves in larger particles and shifts to the higher temperature as the size increases. Quantitatively, increase in temperature leads to only bi-phase morphology below 32 nm of particle size. Above 32 nm, the transformation to the bi-phase Janus morphology occurs through an intermediate core-shell morphology.

4.DISCUSSION There exist extensive literature dealing with phase evolution in Ag-Cu alloys, in particular, the evolution of core-shell and Janus-like morphology

48–61

. We like to emphasize at the very

beginning that the size range of the particles that we are reporting falls outside the scope of these studies and many of our results are very different to those reported for particles with a size of few nanometers containing a small number of atoms. The distribution of particle sizes observed in our laser ablation experiments has already been presented in the result section (fig. 1(b1)). As ablated particles of both 20 and 40 nm size contain multiple grains with average initial sizes of about ~3±0.5 nm. Although the diffraction patterns of as-synthesized particles indicate solid solution phase, there exists considerable broadening of the peaks that may be attributed to small sizes of the grain as well as due to a possible local fluctuation in the composition of the grains. The nature of the lattice fringes also indicates the presence of considerable amount of strain in the solid solution that could be due to the large size differences (Ag:Cu:: 144:128 pm) of silver and copper atoms. These aspects are not further discussed in the current work. As we heat the


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sample, a simultaneous recrystallization and decomposition process sets in resulting in changes in both the morphology and composition leading to, either a, core-shell like or a Janus-like morphology. In order to discuss these results, we first summarize the experimental results. A schematic of the phase evolution established from the present experiments for a 20 nm and a 40 nm particle is shown in figure 7. The experimental observations reveal that the multigrain solid solution phases in alloy nanoparticles of both the sizes (20 and 40 nm) start recrystallization (as well as decomposition) with new Ag-rich phase nucleating at the surface of the particle at intermediate stages of evolution. Concurrent diffusion induced phase separation and growth during recrystallization finally lead to the Janus-like biphasic morphology. For larger particles, Ag preferentially segregates at the surface and growth of the Ag-rich recrystallized grain along the surface can lead to a core-shell composition distribution of Ag and Cu at intermediate temperatures. However, at a higher temperature, all the particles exhibit biphasic morphology of Ag rich and Cu-rich bi-crystal. In terms of the free energy, the Ag-rich phase at the surface can be more stable than the Cu-rich phase due to the higher surface energy of Cu (1.85 J/m2) compared to that of the Ag (1.25 J/m2) 62

. Thus, a transient configuration of core-shell morphology provides stability to the particle at

intermediate temperatures. However, the formation of biphasic Janus morphology on further heating indicates that the intermediate state is a metastable state. Our experiments show that Ag-Cu solid solution in a multi-grained nanoparticle, core-shell composition profile does not evolve at smaller length scale. The thermodynamic mediated phase separation tendency of an initial solid solution and consequent recrystallization of nanograins to relieve the inherent strain due to the large size differences between Ag and Cu atoms can be



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kinetically enhanced by the difference in mobility of the two species

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63–67

. For smaller particles,

the process of diffusional unmixing will be faster since the diffusion distance needed for the process is smaller. Hence the bicrystal of the two phases will evolve rapidly driven by diffusion. The preferential growth of Ag grain along the surface utilizing the advantage of lower surface energy can lead to a core-shell morphology. However, this will be significant only when the surface diffusion controls the growth of silver grain. Our experiments indicate a narrow domain for the occurrence of the core-shell structure that initiates at a size of 32 nm at ~ 200 °C. The temperature domains of its existence increase with increasing size. However, at higher temperature normal diffusivity again dominates leading to the biphasic structure.

5. CONCLUSIONS The microstructure evolution in Ag-Cu alloy nanoparticle indicates that laser ablated particles mostly exhibit a multi-grain (~ 3.5nm) morphology. Structural analysis has established them to be a single phase solid solution at room temperature. For the smaller size of particles (< 32 nm), they transform (phase separate) to a biphasic, Janus-like morphology containing bicrystal of Ag rich and Cu-rich phases. The decomposition of the solid solution at intermediate temperatures (≥200 oC) and sizes ≥32 nm promote an intermediate stage of surface segregation of Ag-rich phase on the surface of the alloy nanoparticle. The lower surface energy of Ag compared to Cu is most likely responsible for such intermediate stage. These particles also transform to a biphasic structure at higher temperature consequent to the grain growth of Ag rich and Cu-rich phases.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India Phone: +918022932262. Fax: +918023600472. Present address: Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa-3200003, Israel Contact: +972-549562128

SUPPORTING INFORMATION The microstructure of target, experimental setup, EDS spectrum of the nanoparticle, and temperature dependent TEM micrograph. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS The authors like to thank the microscopy facility at Advanced Facility of Microscopy and Microanalysis (AFMM), India Institute of Science, Bangalore, India.

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Figures

Figure 1. a) The XRD pattern of as ablated nanoparticles of pure Ag and Ag-46at%Cu. b) 1. The particle size distribution of the ablated nanoparticles with a mod of 22 nm. 2. The bright field micrograph of alloy nanoparticles showing the particles 3. The particles at a higher magnification revealing smaller domains under diffraction contrast inside the particles c) A selected area diffraction pattern from the particles showing rings of uniform intensity. (d &e) The lattice fringe imaging of two particles with size ~20 nm and ~40 nm showing very small crystallites (~3nm) within the particles. f) The HAADF image of the particles showing different contrasts inside the particles suggesting composition variations inside the particles.


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Figure 2. a) The diffraction pattern from the group of alloy nanoparticles as synthesized at room temperature (RT). The diffraction pattern shows the uniform intensity ring. b) At 200 °C few diffraction spots apart from the ring is observable. c, d, & e) As the temperature increases the intensity of ring decreases and the spots are more pronounced. f) At 600 °C, the indexed diffraction peaks show the complete separation of Ag and Cu elements. 17 ACS Paragon Plus Environment


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Figure 3. The lattice parameter of the Ag-rich solid solution and Cu-rich solid solution with respect to the temperature. With the increase in temperature the change in lattice parameter converges to the elemental lattice parameters.


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Figure 4. a) An alloy nanoparticle of size ~20 nm as synthesized at room temperature. b) Heating at 200 °C shows the wetting of an Ag-rich grain B on the surface of the particle. Single phase is shown as a region A. c) At 300 °C another Ag rich grain B’ appeared along the surface of the particle. d&e) Further heating of the particle shows the grain growth and finally at 600 °C complete phase separation in Ag and Cu occurs. f) The line scan of the particle shown in ‘e’ revealing characteristic x-ray intensity of Ag and Cu elements. 19 ACS Paragon Plus Environment


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Figure 5.a &b) The high-resolution micrograph from a ~40 nm alloy nanoparticle at room temperature and after heated to 300 °C inside the microscope. Insets show the particles at lower magnifications. At room temperature, particle shows the multigrain within particle while heated to 300 °C shows the surface segregation of Ag-rich phase. c) The elemental mapping of the particle heated to 300 °C with Ag-L and Cu-K . d) Further heating at 600 °C shows the complete phase separation at 600 °C.


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Figure 6. Represents the transformation of the single phase particles to core-shell and biphase with respect to the increase in temperature. Symbols represents: (s) single phase; ( b) biphase; (c) core-shell; coexistence of (sb) single and biphase; (sc) single phase and core-shell; (bc) coreshell and biphase; (sbc) single, core-shell and biphase. The dashed line shows the interface of the two different phases.



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Figure 7. The schematic diagram of the observed experimental results. The small particle (~20 nm) initially contains nano-grains of single phase Ag-Cu solid solution. Recrystallisation and decomposition occur simultaneously with a new grain of Ag-rich phase nucleating near the surface and grow both along the surface and the interior yielding a biphasic Janus morphology. In contrast, the decomposition of a bigger particle (~40 nm) occurs through the growth of Agrich grains along the surface leading to the formation of the core-shell composition distribution of Ag and Cu. However, heating at still higher temperature finally yields biphasic Janus morphology.


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Table of Content Graphics (TOC)


Temperature- and Size-Dependent Compositionally Tuned

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