Generation of mixed metallic nanoparticles from immiscible metals by spark discharge
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- Tabrizi, N.S., Xu, Q., van der Pers, N.M. et al. J Nanopart Res (2010) 12: 247. doi:10.1007/s11051-009-9603-4
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Using a spark discharge system, we synthesized Ag-Cu, Pt–Au and Cu-W mixed particles a few nanometers in size. These combinations have miscibility gaps in the bulk form. The microsecond sparks between electrodes consisting of the respective materials, form a vapour cloud. Very fast quenching of the mixed vapour results in the formation of nanoparticles. To investigate the morphology, size, composition and structure of the particles, TEM, XRD analyses and EDS elemental mapping were performed on the samples. The average compositions were measured by ICP and the specific surface areas were determined by the BET. Our method produces Ag-Cu and Au–Pt mixed crystalline phases that do not exist in macroscopic samples. For Cu-W, alloying is not observed, and the metals are mixed on a scale of about 1 nm.
KeywordsNanoparticlesImmiscible metalsSpark dischargeSynthesis method
Metallic nanoparticles are interesting among others because their electronic structure may undergo a major deviation from that of the bulk and they possess the ability to store excess electrons (Lahiri et al. 2005). Bimetallic nanoparticles are of even greater interest, since they generally show different physicochemical properties as compared to their individual constituents (Wu and Lai 2004). Flexible mixing of materials on a nano scale, down to the atomic scale, would give access to an enormous variety of new material properties tunable not only through size but also through mixing ratios and the scale of mixing. This should lead to myriad applications in areas such as optoelectronics, catalysis, batteries, solar cells, fuel cells, hydrogen storage, magnetic materials and sensors (Wu and Lai 2004). In principle, nanomixing offers additional degrees of freedom for tailoring properties to match the application, and pure materials or stoichiometric mixing ratios are likely to become exceptions in functional materials of the future, if suitable methods of mixing are found.
In the present study, we produced Ag-Cu, Au–Pt and Cu-W nanoparticles. Ag-Cu has many interesting applications (Cagran et al. 2006). For instance, the high electrical conductivity of Ag and low electrical migration of Cu make Ag-Cu nanoparticles suitable as conductive fillers in electrically conductive adhesives (Jiang et al. 2005). Au–Pt nanoparticles can be used in various catalytic reactions (Zeng et al. 2006; Devarajan et al. 2005) and exhibit specific catalytic activity and selectivity in hydrogenation (Patel et al. 2005). The high thermal and electrical conductivity of Cu and the low thermal expansion coefficient of W make Cu/W composites attractive as heat sinks in electronic packages (Kang and Bong Kang 2003).
There have been many attempts to generate mixed nanoparticles through different methods such as vapour quenching, co-deposition sputtering, mechanical alloying and ion-beam mixing or irradiation (Almtoft Pagh et al. 2007; Radic and Stubicar 1998). In the present study, we used spark discharge ablation of electrodes in an inert gas, which was introduced by Schwyn et al. in 1988. This method falls under the category of vapour quenching. Short sparks locally evaporate material, which leads to small vapour clouds that are very rapidly cooled. In comparison to other methods used, it is extremely simple, cost-effective and flexible. Moreover, it has the potential of being scaled up. The size and concentration of particles can be controlled via the energy and repetition rate of the spark. While a recent study has focused on the potential of the method in mixing metals that easily form alloys in macroscopic systems (Tabrizi et al. 2008), the objective of the present study is to investigate the feasibility of generating internally mixed nanoparticles from constituents that are immiscible in the macroscopic case.
Since a large fraction of atoms composing a nanoparticle are situated at the surface and contribute to the excess Gibbs free energy, the phase diagram of the particulate binary system may be modified with respect to the macroscopic systems (Lahiri et al. 2005). In addition, reduction in the melting point of the nanoparticles (Buffat and Borel 1976; Ding et al. 2004) and the presence of defects at the interface in bimetallic nanoparticles enhance inter-diffusion of the metals and modify the alloying characteristics (Lahiri et al. 2005; Birringer 1989). Using Monte Carlo simulations Christensen et al. (1995) investigated the size dependence of phase separation in small bimetallic solid clusters. For the Ag-Cu system, which shows immiscibility in the bulk for a broad range of compositions up to the melting temperature, they found that the maximum temperature where phase separation can occur is strongly size dependent. They also found the absence of phase separation for clusters smaller than a critical size of about 270 atoms. By taking into consideration that electric charge can energetically affect the free energy of the formation of clusters, Ouyang et al. (2006a, b) proposed a charge-dependent thermodynamic model to address the phase transformation between miscible and immiscible for nano-sized alloying particles. By applying a thermodynamic model and an analytic embedded atom method, Xiao et al. (2006) recently showed that the heat of formation of alloy nanoparticles is not only composition but also size dependent and for bulk immiscible systems with a positive heat of formation, a negative heat of formation may be found for alloy nanoparticles of small size and in particular for a dilute solute component.
For production of the Ag-Cu and Cu-W particles, two pairs of sintered electrodes of (Ag72/Cu28) and (W72/Cu28) were utilized while for the Au–Pt system, a pure Au electrode was combined with a pure electrode of Pt (99+% purity). Moreover, to produce monometallic particles a pair of identical electrodes of the respective material was used. The inert gas carrying the particles was Ar (99.999% purity).
High resolution electron microscopy was performed on a Philips CM30-UT-FEG and a FEI Tecnai-200FEG. EDS data were acquired on the FEI Tecnai-200FEG using the STEM mode with a spot size of ~0.3 nm, which guaranteed that the measured spectra only came from the local area. The spectra acquisition, drift correction and data analysis were all processed using the software TIA (Tecnai Imaging & Analysis). The TEM grid was a Ni 200 mesh coated with a carbon film.
The average compositions of the samples were measured by ICP-OES on a PerkinElmer Optima 5300. The X-ray diffraction (XRD) measurements were performed on a Bruker–AXS D5005 diffractometer, equipped with a Huber CuKalpha-1 Ge monochromator in the incident beam and a Braun Position Sensitive Detector PSD-50 M in the diffracted beam. N2 adsorption isotherms were measured on a Quantachrome Autosorb-6B for the determination of the specific surface areas of the particles. A home-built scanning mobility particle sizer (SMPS) measured the particle size distribution on-line.
Results and discussion
Very few particles that are much larger (up to 200 nm) are present. The large size difference with respect to the small ones implies a different formation mechanism (Gray and Pharney 1974). We assume that these particles are sputtered from local patches of molten metal at the electrode surface, as we have previously reported for the case of pure metals (Tabrizi et al. 2009). EDS analysis reveals that the large particles are almost pure Ag. This points to phase separation in microscopic molten patches on the electrode surface. The average composition of the nanoparticles measured at different regions of the sample is about 30 wt% Cu and 70 wt% Ag. This agrees with the composition of the electrodes, which is 28 wt% Cu and 72 wt% Ag.
Compositional analysis of the Ag-Cu sample measured by ICP
Cu/Ag wt ratio in the electrode
Cu/Ag wt ratio in the sample
For the above mentioned compositions of the Ag-Cu alloys, the heat of formation at 700 K is positive around 4.1 and 2.8 kJ/mol, respectively (Najafabadi and Srolovitz 1993). The sharp silver reflections are explained by the big particles mentioned above, one of which is shown in the TEM micrograph of Fig. 2. The reflection of Ag and Ag-Cu clearly show the difference in broadness, indicating that the former peak is mainly due to large Ag particles and the latter is due to nanoparticles.
We do not see characteristic peaks corresponding to crystalline oxide phases. This is probably due to the fact that the oxide phase, covering the particle surfaces as observed in the electron micrographs is amorphous. The undefined reflections are attributed to unknown impurities.
Specific surface area of the Ag-Cu particles
BET specific surface area (m2 g−1)
Calculated BET diameter (nm)
EDS compositional analysis of single particles [Au(+)Pt(−)]
Au/Pt wt ratio
Although the results of Xiao et al. (2006) indicate mixing in full range of concentrations for particle sizes up to 6 nm, our results indicate mixing also for the larger particle in Fig. 7. We assume that this is because the particle is twinned consisting of smaller units. As the whole sample consists of primary particles hardly larger than 6 nm, we do not expect any de-mixed cases either for the larger aggregates.
Compositional analysis of Au–Pt samples measured by ICP
Au/Pt wt. ratio in the sample
Au (+)Pt (−)
Au (−)Pt (+)
The broadness of the XRD peaks is determined by the particle size and lattice imperfections. In the case of mixed phases, the XRD peaks should additionally be broadened, if there is a distribution of compositions rather than one fixed composition. Slight amounts of pure metals were also detected in the samples (see inset). Here the peak sharpness indicates a large particle size, probably due to solidified droplets ejected from the electrode surface, as these have been seen before in the case of Ag above.
Specific surface area of the Au–Pt particles
BET specific surface area (m2 g−1)
Calculated BET diameter (nm)
Typical atomic compositions of the Cu-W particles
Compositional analysis of Cu-W sample measured by XPS
Cu/W wt ratio in the electrode
Cu/W wt ratio on the surface of the sample
Cu-W sintered electrodes
With a large positive heat of mixing (ΔHf = +36 kJ/mol), Cu and W are immiscible even in the liquid state (Gladyszewski et al. 1993; Ouyang et al. 2006a; Haubold and Gertsman 1992). Cu and W have fcc and bcc (body-centred cubic) structures with atomic radii of 0.1278 and 0.137 nm, respectively. If W dissolves in the Cu lattice, the lattice parameter of Cu should increase and similarly a decrease in lattice parameter is expected for W if Cu dissolves in the W lattice (Raghu et al. 2001). Equilibrium phases in the phase diagram of bulk Cu-W calculated from a thermodynamic model include the fcc terminal solid solution (Cu) with extremely limited solid solubility of W, and the bcc terminal solid solution (W) with extremely limited solid solubility of Cu (Massalski et al. 1990). On the basis of first principles calculations, Shu et al. (2003) suggested that the noble metal atoms (Ag, Au and Cu) would like to occupy the vacancy sites of the W(001) surface to form the substitutional surface alloys despite the fact that they hardly form an alloy in the bulk. Dirks and van den Broek (1985) produced Cu-W alloy films by simultaneous vapour deposition of the elements on unheated substrates. They found that at least 10 at.% W may be dissolved in fcc Cu films and 40 at.% Cu may be accommodated in the bcc W alloy films. They concluded that the vapour quenching technique could lead to the formation of homogeneous one-phase alloys over a wide range of compositions. They estimated that the metastable two-phase (fcc + bcc) coexistence region should lie between 40 and 60 at.% tungsten. This is almost in the compositional range of the measured particles in Table 6. Metastable mutual solid solubility in the ball milled Cu-W nanocrystallites was also reported by Raghu et al. (2001). According to Xiao et al. (2006), there is a competition between size effect and compositional effect on the heat of formation of immiscible system. When the formation enthalpy reduces to a smaller value than the interface energy of the system, because of the size effects, interface alloying can occur (Liang et al. 2005).
The electron diffraction pattern in the inset of Fig. 13 was obtained from an area, where only nanoparticles were present. It exhibits some small diffraction spots arranged along the rings, indicating very small crystallites to be present. The dominance of smeared continuous rings gives further support to an amorphous or quasi-amorphous phase.
Together with the EDS line scans of Fig. 11, we conclude that we have mixed Cu and W on a scale larger than the lattice constants but significantly smaller than the particle diameters. The EDS line scan on the upper left side of Fig. 11 would be consistent with a homogeneous mixing, but the line scan on the lower left side shows that very significant changes in composition occur on the scale around one nanometer.
Specific surface area of the Cu-W particles
BET specific surface area (m2 g−1)
Calculated BET diameter (nm)
Using sintered electrodes of Ag/Cu in the spark discharge generator, we produced nanoparticles which exhibited enhanced solid solubility of Cu in Ag and Ag in Cu with respect to the macroscopic case. Effective mixing is also possible by using two electrodes of different compositions, as the circuit applied leads to field reversal during the discharge process. Au–Pt particles produced this way show intermediate phases with compositions which lie in miscibility gap in bulk phase diagram at ambient temperatures. In agreement with this, EDS elemental mapping of a 6 nm particle reveals good mixing. This is in agreement with computation by Xiao et al. (2006). Since gold and platinum are both noble metals with low chemical reactivity, cleanness of the particle surfaces leads to cold sintering and low specific surface areas for pure particles. Admixing of Au and Pt results in an increase in specific surface area of the mixed particles. Applying sintered electrodes of Cu/W, we produced mixed particles a few nanometers in size containing both Cu and W. EDS line scans together with TEM, XRD and electron diffraction give evidence of mixing on a subnanometer scale but above the scale of atomic mixing. In conclusion, spark discharge ablation is a powerful technique for producing new mixed nanoparticulate phases that do not exist in macroscopic systems. Beside using the method for basic studies, the possibility of scaling it up by using multiple discharges bears the potential of producing new materials on a larger scale.
The authors would like to express their gratitude to Miren Echave Elustondo for carrying out particle size distribution measurements and Sander Brouwer for his assistance in BET measurements. The Project is partially funded by the Delft Center of Sustainable Energy (DISE).