The present report illustrates the phenomenon of phase separation leading to the splitting of solid solutionstructured Ag–Co nanoparticles into pure Ag and pure Co nanoparticles upon isothermal annealing inside atransmission electron microscope. In bulk, Ag–Co system shows negligible mutual solubility into a singlephase solid solution structure upto a very high temperature. The Ag–Co nanoparticle splitting revealed thatroom temperature, solid solution atomic configuration, between bulk immiscible Ag and Co atoms co-existing in a nano-sized particle, is a kinetically frozen atomic arrangement and not a thermodynamically sta-ble structure. The observed phase separation behavior resulting in particle splitting at high temperatures canbe used to produce devices for sensor applications.
The literature contains several reports on the phenomenon ofdiminishing bulk elemental immiscibility in the nano-sized particles[1–4]. Unlike, the immiscibility and phase separation phenomenonobserved in the bulk case, component atoms with a large differencein atomic sizes (>14%) and a positive enthalpy of mixing when con-fined in a nano-sized volume tend to form solid solution structuredalloys [5]. Most of published studies on the phenomenon of diminish-ing immiscibility with size do not elaborate on the phase stabilityaspect of the solid solution alloys formed. For nano-sized particlescomposed of bulk immiscible atoms in a solid solution arrangement,the possibility and the mechanism of decomposition of the solid solu-tion structure into bulk equilibrium phases when the required energyneeded for atomic diffusion is imparted remains relatively less ex-plored. For possible future technological applications and for assuringthe structural and functional reliability of these novel alloys formedfrom bulk immiscible atoms it is necessary to design studies focusedon determining whether the solid solution structures, made up ofbulk immiscible atoms confined in a nano-sized volume, is a kineti-cally frozen atomic arrangement or a thermodynamically stablestructure. Furthermore, if upon an input of energy a bulk type phaseseparation occurs relaxing the kinetically frozen structure then theother interesting question is in what geometrical arrangement thenewly formed equilibrium phases would co-exist, that is, what willbe the new microstructure for the nanoparticle? The present studyaddresses the aforementioned issues. Structural stability of a solidsolution alloy, made up of bulk immiscible atoms, under isothermalannealing conditions is investigated. The candidate system chosen is
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nanoparticles containing Ag and Co atoms in a solid solution atomicconfiguration. In bulk, Ag–Co system shows no mutual solubilityinto a single phase solid solution structure upto a very hightemperature [6].
2. Experiment
In the present work Ag–Co nanoparticles were synthesized by thechemical reduction technique. To synthesize the nanoparticles, silveracetate and cobalt acetate salts were dissolved in 25 mL diphenylether solvent. This solution was then transferred to a three neckround bottom flask fitted with a magnetic stirrer, a thermometerand a reflux condenser. An inert argon atmosphere was maintainedinside the three neck flask during the particle synthesis reaction.The three neck flask containing the reaction mixture was heated.When the temperature of the reaction mixture reached 100 °C,0.2 mL of oleic acid and 0.2 mL of oleylamine were injected into thereaction mixture through a syringe. The reactants were then heatedto 200 °C and at this temperature 2 mL of superhydride (LiBEt3H)was injected into the reaction mixture to reduce the metal precursorsalts. The reaction mixture was then heated to the boiling tempera-ture of the phenyl ether solvent (~260 °C) and refluxed for 30 minafter which heating was stopped and the reaction mixture was cooledto room temperature under the inert atmosphere. The as-synthesizednanoparticles were then isolated by centrifuging the reactionmixture.
A 300 keV field emission FEI Tecnai F-30 transmission electron mi-croscope (TEM) was used for obtaining bright field images, electrondiffraction patterns and composition of the nanoparticles. A highly di-lute dispersion of as-synthesized nanoparticles was drop dried onto acarbon coated Cu grid for the TEM based analysis. A standard goldpolycrystalline thin film sample was used for calibrating the camera
constant value, used for determining the d-spacings from the selectedarea electron diffraction patterns.
3. Results and discussion
TEM bright field imaging revealed non-monodisperse nanoparti-cles. Transmission electron microscopy–energy dispersive spectros-copy (TEM–EDS) compositional analysis of a large group ofnanoparticles revealed the presence of both Ag and Co atoms inthem. A representative TEM–EDS curve from a group of nanoparticlesshowing peaks corresponding to the Ag and Co elements is shown inFig. 1(a). Average composition value derived from the analysis of theEDS curve obtained from a large group of nanoparticles was Ag74Co26.Selected area electron diffraction (SAED) pattern obtained from thesame group of nanoparticles from which TEM–EDS curve wasobtained is shown in Fig. 1(b). Interplanar spacing values for theplanes corresponding to ring 1 (r1), ring 2 (r2), ring 3 (r3) and ring4 (r4) (see Fig. 1(b)) in the electron diffraction pattern respectively
Fig. 1. (a) A representative EDS curve obtained from a group of nanoparticles (b) selected(c) TEM bright field image of a region containing a group of nanoparticles (ROI) at roomobtained from the newly formed particles lying within the dashed circles in Fig. 1(d) (Cu sthe ROI for the post-annealed conditions.
were 2.41 A, 2.09 A, 1.47 A and 1.262 A. The ratio of the interplanarspacing values (r1/r2=1.15; r1/r3=1.63; r1/r4=1.91; r2/r3=1.42; r2/r4=1.56; r3/r4=1.165) exactly matched with theratio of the interplanar spacing values for a standard fcc crystal. De-termined from the ratios of the interplanar spacing values, the crys-tallographic planes corresponding to the electron diffraction rings 1to 4 respectively are {111}, {200}, (220} and {311}. Three importantobservations that were made from the SAED pattern are (a) structureof the crystal, as revealed by the SAED pattern, is fcc, (a) the SAEDpattern did not reveal any diffraction signature corresponding to thepure Ag or Co phases or their oxides and (b) a positive deviationfrom the Vagard's law. The lattice parameter for the solid solutioncrystal calculated from the interplanar spacing values is 4.17 A. Thisvalue is larger than the lattice parameter for pure Ag by 0.09A.According to the Vagard's law the lattice parameter for an Ag rich,Ag–Co fcc ideal solid solution crystal should be smaller that the latticeparameter value for pure Ag as Co atoms are smaller than the Agatoms. The observed positive deviation from the Vagard's law has
area electron diffraction pattern from a group of as-synthesized Ag–Co nanoparticles,temperature and (d) after 30 min of heating at 400 °C, (e) EDS compositional profileignal is from the TEM support grid) and (f) electron diffraction pattern obtained from
124 C. Srivastava / Materials Letters 70 (2012) 122–124
been reported to happen in case of solid solution crystals made up ofbulk immiscible atoms with a positive enthalpy of mixing [7]. There-fore, the presence of both Ag and Co atoms, absence of diffraction sig-natures corresponding to the pure Ag and Co phases or their oxides,an fcc structure and a positive deviation from the Vagard's law collec-tively confirm that the nanoparticles in the as-synthesized particlesdispersion were made up of Ag and Co atoms arranged in a solid so-lution atomic configuration.
To investigate the thermodynamic stability of the Ag–Co solid so-lution phase, Ag–Co nanoparticles were isothermally annealed insidethe TEM. Samples for the isothermal annealing experiment were pre-pared by drop drying a highly dilute as-synthesized Ag–Co nanopar-ticle dispersion onto a carbon coated Cu TEM grid. The TEM gridwas then loaded on a hot stage TEM sample holder. The TEM sampleholder was connected to a power supply that provided the requiredcurrent for resistive heating of the sample. During the isothermalannealing experiment, the nanoparticles on the sample grid were ob-served in the low magnification bright field imaging mode and anykind of change was closely monitored and recorded. A TEM brightfield image of a region containing a group of nanoparticles (henceforth called the ROI) at room temperature is shown in Fig. 1(c).Note that the regions within the dashed circles in Fig. 1(c) do not con-tain any nanoparticles. It should be noted that the TEM–EDS curveand the SAED pattern presented respectively in Fig. 1(a) and (b)were obtained from the ROI shown in Fig. 1(c). Keeping the imageof the ROI on the viewing screen the temperature of the sample wasraised to 400 °C at a very fast heating rate (~100 °C/min). At 400 °Cthe isothermal annealing experiment was started. During the isother-mal annealing new nanoparticles gradually started to appear in theROI. TEM bright field image of the region shown in Fig. 1(c) isshown again in Fig. 1(d) the only difference being that the image inFig. 1(d) was acquired after 30 min of heating at 400 °C. Note thepresence of new nanoparticles within the regions of the ROI enclosedby the dashed circles in Fig. 1(d). TEM–EDS compositional analysis ofa group of newly formed particles within the dashed circle revealedtheir identity as pure Co particles. A representative TEM–EDS spec-trum obtained from the newly formed particles is shown inFig. 1(e). SAED pattern shown in Fig. 1(f) was obtained from theROI after 30 min of isothermal annealing treatment. The SAED patternin Fig. 1(f) revealed diffraction signatures corresponding to the pureAg phase and pure Co phases. Comparison of the electron diffractionpatterns provided in Figs. 1(b) and (f) reveals that the diffractionspots corresponding to the pure Ag and pure Co phases appear onlyafter the isothermal annealing operation. Compositional analysisand SAED results thus clearly illustrate the occurrence of phase sepa-ration leading to the splitting of solid solution structured Ag–Co
nanoparticles into pure Ag and Co nanoparticles upon an input ofenergy.
This kind of phase separation phenomenon resulting in nanoparti-cle splitting has not been reported earlier. The appearance of pure Coand pure Ag nanoparticles during the isothermal annealing treatmentrevealed that when sufficient energy is imparted to a nano-sized par-ticle containing Ag and Co atoms in a solid solution atomic configura-tion a bulk type phase separation occurs which decomposes the solidsolution structure leading to a phase separation. The interesting as-pect however is the fact that the phase separation occurs in such away that the solid solution Ag–Co nanoparticles prefer to eventuallysplit into isolated nanoparticles with composition and structure cor-responding to the bulk equilibrium phases rather that adopting bulktype two phase microstructure in which the two equilibrium phaseswould co-exist separated by a heterophase interface inside a nano-particle. Particle splitting is favored as a phase separation without itwould result in co-existing equilibrium phases with a high energystrained coherent heterophase interface [8]. This kind of phase sepa-ration behavior resulting in particle splitting at high temperatures canbe used to produce devices for sensor applications.
4. Conclusion
In summary, it is observed that Ag–Co solid solution nanoparticlesgradually split into pure Ag and pure Co nanoparticles upon isother-mal annealing at 400 °C in a vacuum atmosphere. This kind of phaseseparation behavior illustrates that the Ag–Co solid solution structureformed between bulk immiscible Ag and Co atoms is a kinetically fro-zen atomic arrangement and not a thermodynamically stablestructure.
Acknowledgment
The author acknowledges the electron microscopy facilities avail-able at the AFMM Center, Indian Institute of Science, Bangalore, India.
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