Room-Temperature Synthesis of Ag−Ni and Pd−Ni Alloy

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Room-Temperature Synthesis of Ag-Ni and Pd-Ni Alloy Nanoparticles Zhenyuan Zhang,† Tina M. Nenoff,*,† Kevin Leung,† Summer R. Ferreira,† Jian Yu Huang,‡ Donald T. Berry,§ Paula P. Provencio,| and Roland Stumpf† Department of Surface & Interface Sciences, MS-1415, Center for Integrated Nanotechnologies, MS-1314, Department of Hot Cells and Gamma Facilities, MS-1143, Department of Radiation-Solid Interactions, MS-1415, Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185 ReceiVed: December 17, 2009; ReVised Manuscript ReceiVed: July 26, 2010

Ni-based alloy nanoparticles (NPs) are synthesized using room-temperature radiolysis. Density functional theory (DFT) and various nanoscale characterization methods are used to provide a strong basis for understanding and describing metastable phase regimes of alloy NPs whose reaction formation is determined by kinetic rather than thermodynamic reaction processes. Two series of nickel alloyed NPs, Ag-Ni and Pd-Ni, are analyzed and characterized via various analytical characterization techniques. Different ratios of Agx-Ni1-x alloy NPs and Pd0.5-Ni0.5 alloy NPs are prepared using a high γ irradiation dose rate. Images from high-angle annular dark-field show that the Ag-Ni NPs are not in a core-shell configuration but, rather, a homogeneous alloy structure. Energy filtered transmission electron microscopy maps further elucidate the homogeneity of the metals in each alloy NP. Of particular interest are the normally immiscible Ag-Ni NPs that have been shown to form core-shell structures in thermodynamically driven reactions. All evidence supports that homogeneous Ag-Ni and Pd-Ni alloy NPs are successfully synthesized by a high dose rate radiolytic methodology. DFT modeling is used to support that nanoparticle alloying proceeds through the less energetically favorable path of formation, achievable via high dose rate radiolysis. Introduction Ni-based bimetallic bulk alloys are of great interest for a diverse range of applications, including superalloys for aircraft and engine components,1 shape memory materials for civil structures,2 biomedical devices,3 and hydrogen dissociative membranes.4 However, there is interest in lower-temperature synthetic routes as a means of making bulk alloys with fewer defects. One method to achieve fewer defects is the process of making high surface area nanoparticles (NPs). An additional benefit of such high surface area nanoparticles is a much lower predicted sintering temperature to bulk than is required from the melt method.5 Generally, bimetallic NPs can form either core-shell or alloy NPs. Currently, Ni-based nanoparticles (NPs) are being studied for their technologically important catalytic and magnetic properties.6-10 Most methods of synthesizing Ni-based bimetallic NPs are based on elevated temperatures, such as thermal decomposition of transition-metal complexes,11-14 reverse micelles,15,16 chemical reduction,17,18 electrochemical formation,19,20 molecular beams,21 ion implantation,22 and sonochemical synthesis.23 For example, Pd and Ni can be synthesized into Pd-Ni alloy NPs,10 Pd core-Ni shell NPs,24 and Ni core-Pd shell25 NPs as determined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). However, it is very difficult to synthesize Ag-Ni alloy NPs because of the pronounced lattice mismatch, lower surface energy of Ag, and the complete immiscibility between Ag and Ni.26 It has been confirmed that * To whom correspondence should be addressed. E-mail: tmnenof@ sandia.gov. † Department of Surface & Interface Sciences, MS-1415. ‡ Center for Integrated Nanotechnologies, MS-1314. § Department of Hot Cells and Gamma Facilities, MS-1143. | Department of Radiation-Solid Interactions, MS-1415.

the phase-segregated core-shell structure is the most thermodynamically stable structure for Ag-Ni NPs with Ag surface segregation occurring from the Ni core.26-28 Metastable Ag-Ni alloy NPs have been synthesized via a laser-liquid-solid interaction technique,29,30 and 25 nm crystalline Ag-Ni alloy NPs have been formed by stepwise reduction of Ag+ and Ni2+.31 However, these methods create either Ag-rich and Ni-NiOrich phases29,30 or a mixture of Ag-Ni, Ag, and Ni NPs,31 and a dealloying process of the Ag-Ni NPs may occur.32 The difficulty associated with alloy Ag-Ni NP formation is also supported by simulations and calculation from theory. DFT, global optimization, molecular dynamics, and Monte Carlo simulations have shown that Ni core-Ag shell structures are thermodynamically favored.32-37 In an effort to employ low-temperature formation of Ni-Ag alloys using Ni-based NP chemistry, we turned to the early pioneering work by Belloni38-40 and Henglein.41-43 Their methods use γ radiation as a unique technique to prepare metallic NPs. This method has synthetic advantages over conventional chemical and photochemical methods44 that include (1) a controlled reduction of metal ions without using an excess of reducing agent or producing undesired oxidation products from the reductant, (2) a well-known rate of reduction to reduce metal cations in solution because the number of reducing equivalents generated by radiation is well-defined, (3) radiation is absorbed without interference from light-absorbing solutes and products, and (4) the reducing agent is uniformly formed in the solution.44 Another significant advantage in bi- and multimetallic systems is that the rate of metal reduction may be very high by using powerful gamma sources or beams of electron accelerators (which induce the same reducing radicals as the γ irradiation) and hence can block the intermetallic electron transfer.45 Careful tuning of this procedure through dose time and dose rate allows for the controlled formation of either

10.1021/jp911947v  2010 American Chemical Society Published on Web 08/10/2010

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TABLE 1: Different Stoichiometries of Ag+ and Ni2+ Used To Prepare Ag-Ni Alloy NPs +

-4

[Ag ], × 10 M [Ni2+], × 10-4 M [Ag+] + [Ni2+], × 10-4 M [Ag+]/[Ni2+]

Ag

Ag0.9-Ni0.1

Ag0.7-Ni0.3

Ag0.5-Ni0.5

Ag0.3-Ni0.7

Ni

2 0 2 pure Ag NPs

1.8 0.2 2 9:1

1.4 0.6 2 7:3

1 1 2 1:1

0.6 1.4 2 3:7

0 2 2 pure Ni NPs

core-shell or alloy NPs. We have successfully used radiolysis to form stable AgNi NPs and previously provided a mechanism to explain homogeneous NP alloy formation.46 In this report, we demonstrate that different stoichiometries of Ag-Ni alloy NPs and Pd0.5-Ni0.5 alloy NPs have been prepared via room-temperature radiolysis reactions. The successful formation of alloys is confirmed by a variety of nanoscale characterizations, including UV-vis, TEM/HRTEM, HAADFSTEM, and EFTEM mapping. Utilizing density functional theory (DFT), based on first principles, we show that Ni core-Ag or Pd shell NPs are substantially more stable than model random alloy NPs even for a cluster size of only 240 atoms. This is consistent with our assertion that the NP products are determined by reaction kinetics rather than thermodynamics. These calculations are critical for quantifying the strong preference of core-shell particles over random alloys, rather than relying on the relative energetics based solely on the observed trends in macroscopic systems. Experimental Section Synthesis. Different stoichiometries of Ag-Ni NPs are prepared using the following radiolytic methodology. A 50 mL aqueous solution containing 0 to 2 × 10-4 M AgClO4, 2 to 0 × 10-4 M NiSO4, 3 × 10-4 M sodium citrate, 0.5 M methanol, and 1.5 × 10-2 M (0.66 g/L) poly(vinyl alcohol) (PVA, Mw ) 88 000) is deaerated by bubbling Ar for 12 min and then irradiated in a 60Co-γ source (Sandia National Laboratories Gamma Irradiation Facility (GIF)) at a dose rate of 300 rad/s for 18 min. This corresponds to approximately 4-7 times the dose required for total reduction of Ag+ and Ni2+, depending on the total concentration of metal ions from Ag+ and Ni2+. The total concentration Ag+ and Ni2+ is kept at 2 × 10-4 M (see Table 1) for all alloy ratios. Similarly, for the Pd0.5-Ni0.5 (50% Pd and 50% Ni) alloy NP preparation, a 50 mL aqueous solution containing 1 × 10-4 M Pd(NH3)4Cl2, 1 × 10-4 M NiSO4, 3 × 10-4 M sodium citrate, 0.5 M methanol, and 1.5 × 10-2 M (0.66 g/L) poly(vinyl alcohol) (PVA, Mw ) 88 000) is irradiated at a dose rate of 300 rad/s for 36 min. The reaction is carried out in a 100 mL vessel that is equipped with a side arm containing a 0.5 cm optical path and is sealed with two septa. This allows for the collection of UV-vis spectra without exposing the solution to air. Full reduction of the metal ions is determined when there is no change in UV-vis spectra upon additional irradiation. Specimens for TEM are prepared in a glovebox by dropping the solution on a titanium-carbon grid and subsequent drying under N2 gas in a glovebox. To examine whether Ag-Ni NPs are stable to dealloying, TEM grids with NPs are heated at 125 °C for 6 h and at 100 °C for 9 h in a vacuum oven. Characterization. UV-vis absorption spectra are taken on a Varian Cary 300 Scan UV-visible spectrophotometer. Mean particle diameters, particle size distribution, and morphology are determined by using a JEOL 1200EX (120 kV) bright-field transmission electron microscope (TEM) with Gatan digital imaging. High-resolution TEM and scanning TEM (STEM) images are acquired using an FEI Tecnai G(2) F30 S-Twin (300 kV) TEM at the Center for Integrated Nanotechnologies at

Sandia National Laboratories. This instrument is equipped with Z-contrast capability (to differentiate between elements) with a resolution of 0.14 nm in high-angle annular dark-field (HAADF) mode. The unit is also equipped with energy-dispersive X-ray (EDS) analysis for detection of characteristic X-rays for elemental analysis and with an electron energy-loss spectrometer (EELS) for characterizing composition and energy-filtered imaging. DFT Modeling. Spin-polarized DFT calculations have been performed using the VASP code,47,48 the Perdew-BurkeErnzerhof (PBE) exchange-correlation functional,49 Γ-point Brillouin sampling, a 400 eV plane-wave energy cutoff, and a 0.1 meV energy convergence criterion at each configuration. For Ag-Ni, the face-centered cubic (fcc) simulation cell with a lattice constant of 16 or 18 Å contained a single NP model of 240 atomss120 Ag/Pd and Ni each. The NP structure originated from an unrelaxed bulk fcc Ag metal lattice. A spherical nanoparticle with a radius of 10.8 Å centered in a tetrahedral void site is carved out, and the surface atoms are trimmed to create a core-single shell NP with a C3V symmetry and all 120 Ag on the surface, where a “surface atom” is defined as having at most nine nearest neighbors. The starting configuration of the Ag core-Ni shell NP is obtained by switching Ag and Ni atoms (analog for Pd-Ni). Two alloy NPs have been created using random numbers to assign atom identities in the NP. This 50/50 composition allows a direct comparison of the energetics of the two core-shell configurations as well as the random alloys, while a 240-atom NP is large enough that at least half the atoms can be considered residing in the interior. Conjugate gradient techniques are applied to relax the NP to local minima on the potential energy surfaces. It is observed empirically that, due to its massive reconstruction, the Ni core-Ag shell NP took more than 1000 steps to relax, whereas for other NPs, at most a few hundred steps is sufficient. The root-mean-squared remaining forces were 5.5, 14, 11, and 7.8 meV/Å, respectively, for the four clusters. As will be discussed, there are substantial energetic variations between what appear similar, randomly generated initial structures. This emphasizes the importance of using multiple random initial configurations. For Pd-Ni, the 240-atom fcc simulation cell consists of 120 Ni atoms and 120 Pd atoms. The atoms have been allowed to relax to their local minimum energy geometry until all atomic force components were below 0.015 eV/Å. Eight different orderings of clusters have been tested, including Ni core-Pd shell, Pd core-Ni shell, four different random alloys, and two different ordered alloys. The reference energy for the alloy cluster is determined by the sum of 120 Ni atoms in the pure 240 Ni atom cluster and 120 Pd atomic energies in the pure 240 Pd atom cluster. The reference energy for core-shell configurations is the energy per bulk Ni and Pd atom, corrected by the surface energy.50 Note that the accuracy of PBE-based DFT calculations with respect to metal lattice constants and cohesive energies has been well established in the literature.51 Results and Discussion The series of Ag-Ni NPs and Pd0.5-Ni0.5 NPs are analyzed by a variety of methods, including UV-vis, TEM/HRTEM,

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Figure 1. UV-vis spectra of Ag-Ni alloy NPs as-synthesized after 18 min of irradiation (full reduction). The spectra are from Ag-Ni alloy NPs of 90% Ag, 10% Ni (Ag0.9-Ni0.1); 70% Ag, 30% Ni (Ag0.7-Ni0.3); 50% Ag, 50% Ni (Ag0.5-Ni0.5); 30% Ag, 70% Ni (Ag0.3-Ni0.7); and 0% Ag, 100% Ni (pure Ni).

HAADF-STEM, and EFTEM mapping. The characterization methods allows us to fully explore a wide variety of NP compositions and stoichiometries and correlate our findings to DFT modeling. Ag-Ni Nanoparticles. Agx-Ni1-x alloy NPs with varying ratios of x (Ag) to 1-x (Ni) (see Table 1) are prepared under similar reaction conditions (e.g., the same total metal concentrations, stabilizer concentrations, and irradiation time), and their UV-vis spectra are shown in Figure 1. The plasmon peak position of Ag0.9-Ni0.1 NPs (90% Ag and 10% Ni) is around 391 nm, similar to that of pure Ag NPs synthesized with the same conditions. Alloy NPs with lower Ag concentrations of Ag0.7-Ni0.3, Ag0.5-Ni0.5, and Ag0.3-Ni0.7 NPs all have plasmon peak positions of around 383 nm. The relative intensity of the plasmon peak is decreased as the Ni concentration is increased across all curves. The absorption spectrum for pure Ni NPs has a very weak peak around 330 nm, according to Mie theoretic calculations,52 and most literature reports show no absorption band for Ni NPs. However, the presence of Ni can dampen the Ag plasmon band and blue shift the peak position.26,27 Therefore, alloy NPs with the lowest concentration of Ni, Ag0.9-Ni0.1, have the highest intensity due to the Ag plasmon band, whereas those with the highest concentration of Ni, Ag0.3-Ni0.7, have the lowest intensity (see Figure 1). Nanoparticle sizes and shapes are consistent with the reaction conditions and are determined by a number of reaction factors. The general particle size seems to be determined by the percentage of Ni within the NPs; the particle size decreases as the Ni percentage increases (see Figure 2 and Table 2). Furthermore, stabilizing agents, such as PVA and citrate ions, used in the reaction, may also have an effect on particle size and shape, as they offer good steric isolation and stability between individual NPs. In particular, citrate is known for its capping agent properties,53 strong particle size and shape effects,54 and good solution buffering abilities that prevent particle coarsening or ripening.55 As anticipated, the reaction solution containing both stabilizers, produced spherical NPs (Figure 2). It is important to note that citrate ions may be less reducing than radicals and electrons, possibly contributing to radiation-induced germ formation56 during radiolysis (though we see no evidence of this effect). Aging of the NPs consistently has an effect on the NPs’ size.46 This is evident when comparing TEM images for “assynthesized” to “6 days aged” Ni NPs with a variety NP stoichiometries. In the Ag-Ni series, this is supported by a damping and a 16-19 nm red shift of the band in UV-vis spectra (see Figure S1 in the Supporting Information as

Figure 2. TEM image of Ag, Ag-Ni, and Ni NPs as-synthesized: (A) pure Ag NPs, (B) Ag0.9-Ni0.1, (C) Ag0.7-Ni0.3, (D) Ag0.5-Ni0.5, (E) Ag0.3-Ni0.7, (F) pure Ni NPs. All scale bars are 50 nm. Sample (D) is from ref 46.

compared with Table 1). After 6 days, there is no change in the spectra, indicating no change in particle size. The damping and shift may be caused by a ripening process, as there is a slight increase in particle size (compare Figure S2 and Table S1 in the Supporting Information with Figure 2 and Table 1). Low dose rate radiolysis experiments have been completed in which the evolution of NP formation is studied with respect to dose absorbed, in the UV-vis spectra.39 The alloys studied (e.g., Au and Ag) both have large extinction coefficients in which plasmon position shifts can be monitored. Contrary to those studies, our high dose rate experiments are on systems in which metal (Ni and Pd) NPs do not show plasmon bands. Therefore, to fully characterize our NPs, we use a combination of microscopy techniques. TEM and HRTEM microscopy are used to determine the structure of our bimetallic NPs. Several groups have reported TEM images that clearly show core-shell structures of various bimetallic NPs.57-59 For example, Ferrer et al., have demonstrated obvious Au core-Pd shell NPs in HRTEM.57 TEM has also been used to identify either core-shell57-59 or monodispersed NPs.10,12,13,17,22 In contrast, HRTEM results of Ag0.5-Ni0.546 and the other Ag-Ni NPs presented here show metal alloy lattice patterns (with no evidence of Ag and Ni contrast in the images). In this current work, we note that a few of the pure Ag and Ag-Ni NPs have some shading on the NPs as viewed by TEM (see Figures 2 and S2 (Supporting

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TABLE 2: Particle Size in Diameter and Size Distribution of Ag, Ag-Ni, and Ni NPs size in diameter, nm size distribution

Ag

Ag0.9-Ni0.1

Ag0.7-Ni0.3

Ag0.5-Ni0.5

Ag0.3-Ni0.7

Ni

8.5 24%

7.4 24%

5.7 20%

5.4 15%

4.0 18%

3.4 19%

Information)). This is due to the different particle surface morphologies and orientations rather than a core-shell structure (as is further confirmed with pure Pd NPs in Figure S3 in the Supporting Information). Neither our TEM nor our HRTEM images show core-shell structures in the Ag-Ni NPs. This is confirmed by our subsequent detailed analyses. Using HRTEM, we previously reported that the (111) lattice spacings of Ag0.5-Ni0.5 are 0.223 and 0.226 nm, slightly larger than the prediction (using the dihedral angle of one pair of {111} with reflections showing 69.8°).46 This is very near the lattice spacing prediction of 50% Ag and 50% Ni of 0.220 nm (see Figure 3a). Similarly, we have studied the Pd0.5-Ni0.5 alloy nanoparticle by HRTEM, in which a ) 0.214 nm, b ) 0.218 nm, and the dihedral angle between a and b is 69.7°. By

Figure 3. (a) HRTEM image of Ag-Ni nanoalloys. In this image, a ) 0.223 nm, b ) 0.226 nm, and the dihedral angle between a and b is 69.8°. For lattice spacing, Ni(111) ) 0.203 nm, Ag (111) ) 0.236 nm, and the prediction from Vegard’s law for 50% Ag and 50% Ni is 0.220 nm. (Reprinted with permission from ref 46.) (b) HRTEM image of a Pd0.5-Ni0.5 alloy nanoparticle. In this image, a ) 0.214 nm, b ) 0.218 nm, and the dihedral angle between a and b is 69.7°. For lattice spacing, Ni(111) ) 0.203 nm, Pd(111) ) 0.225 nm, and the prediction from Vegard’s law for 50% Pd and 50% Ni is 0.214 nm. The dihedral angle between {111} reflections is 70.5° in theory. This HRTEM image demonstrates that the lattice spacing of 50% Pd and 50% Ni may be present in our NP alloys.

comparison, the lattice spacing for pure Ni(111) ) 0.203 nm and pure Pd(111) ) 0.225 nm; from Vegard’s law, the prediction for a 50% Pd and 50% Ni alloy NP is 0.214 nm. The dihedral angle between {111} reflections is 70.5° in theory. The HRTEM data indicate alloying in this NP. HRTEM data were also collected for all AgxNi1-x samples. However, the data are inconclusive with respect to any changes in lattice spacing from stoichiometric variations between NP compositions. It is important to note that the change in lattice spacing of those samples is predicted to be on the order of 0.006 nm (0.06 Å) from Ag0.5-Ni0.5. This resolution is below the detection limit of our HRTEM, and therefore, this methodology is insufficient to allow for a direct comparison between the alloy stoichiometries and their respective lattice spacings for this system. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is used as a powerful technique to visualize structural and chemical information of the nanoscale NPs. The intensity of HAADF images is approximately proportional to Z1.7 (Z, atomic number).57,60 Because of the intensity difference from atomic numbers (Z), bimetallic Ag-Ni and Pd-Ni NPs can give a clear elemental contrast in HAADF images, such that the presence or absence of a core-shell structure of bimetallic NPs can be confirmed from the Z-contrast imaging. For example, Ferrer et al. showed that Pd core-Au shell NPs gave a clear contrast in the HAADF image,57 thereby complementing earlier published results that have not used HAADF analysis.61 HAADF-STEM images from Ag0.7-Ni0.3, Ag0.3-Ni0.7, and Ni NPs show no evidence of core-shell structures (see Figure 4). Both sets of images are very similar to the image of pure Ni NPs in Figure 4E. This evidence indicates that Ag and Ni are homogeneously distributed throughout each Ag0.7-Ni0.3 and Ag0.3-Ni0.7 NP, as we presented earlier for the Ag0.5-Ni0.5.46 A single Ag0.7-Ni0.3 NP (see Figure 4A), has been analyzed by EDS. The analysis identifies strong peaks from both Ag and Ni. A decrease in the signal-to-noise ratio compared with other compositions is probably due to the high percentage of Ni in Ag0.3-Ni0.7 NPs. In fact, the intensity of the Ni peak in Figure 4C is very similar to that of a pure single Ni NP from Figure 4E. Our attempts to detect Ni in Ag0.9-Ni0.1 NPs from singleparticle EDS have not been successful. The low Ni percentage in Ag0.9-Ni0.1 NP is probably below the detection limit of our instrument. However, the presence of Ni is confirmed in each nanoparticle by the AgNi NP EFTEM mapping studies that clearly show that both Ni and Ag are present within one single particle and that both elements are homogeneously distributed in each NP as described below. As shown in Figure 1, the consistent trends in the UV-vis results indicate that the Ag percentage decreases from Ag0.9-Ni0.1 to Ag0.3-Ni0.7, as shown by the intensity decrease from the Ag plasmon band. The exact stoichiometry of each individual NP in single-particle EDS in Figure 4 has been analyzed by the EDS analysis software (Table 3). Though there is substantial noise in each data set, due to the small size of the NPs, an approximate value can be determined per NP. The largest error in EDS is for Ag0.7-Ni0.3 in Figure 4B due to a

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Figure 4. HAADF-STEM image of (A) Ag0.7-Ni0.3, (C) Ag0.3-Ni0.7, and (E) pure Ni NPs. Scale bars: 100 nm. Single-particle EDX of (B) Ag0.7-Ni0.3, (D) Ag0.3-Ni0.7, and (F) pure Ni NPs. The single-particle EDX analysis is from a particle as indicated by a red circle in (A), (C), and (E). The EDX analysis shows the peaks from both Ag and Ni for (B) Ag0.7-Ni0.3 and (D) Ag0.3-Ni0.7 NPs and thus indicates the presence of both elements in one particle for these NPs.

TABLE 3: EDS Quantitative Analysis of Single Agx-Ni1-x NPs ratios from preparation ratios from EDS analysis a

Ag0.7-Ni0.3

Ag0.5-Ni0.5a

Ag0.3-Ni0.7

70% Ag, 30% Ni 91% Ag, 8.6% Ni

50% Ag, 50% Ni 44% Ag, 56% Nia

30% Ag, 70% Ni 43% Ag, 57% Ni

Component analysis is from Figure 3b in ref 24.

large signal-to-noise ratio for this sample. The analysis shows that there is 56% Ni and 44% Ag in one single Ag0.5-Ni0.5 NP and 57% Ni and 43% Ag in one Ag0.3-Ni0.7 NP (Table 3). Elemental mapping by energy filtered transmission electron microscopy (EFTEM) provides information on particle chemical uniformity and topochemistry and distribution of chemical components with a high spatial resolution.62 This elemental map can be characteristic for particular elements, and therefore, images of the elemental distribution can be recorded with either atomic or nanometer resolution.62,63 EFTEM maps of carbon, Ni, and Ag and the zero loss image of our Ag0.9-Ni0.1 NPs are shown in Figure 5. The spatial distribution of all the bright spots from the elemental maps of Ni and Ag is consistent with that in the zero loss image. The carbon map shows that the background of the NPs is bright where the amorphous carbon is present on the TEM grid, whereas the NPs are dark. The spatial distribution of NPs in the carbon map is also consistent with that found in the Ni and Ag maps. The Ni and Ag maps indicate that both Ag and Ni are present in the Ag0.9-Ni0.1 alloy

NPs. Importantly, the shape of bright spots from both Ni and Ag maps, as well as that from the zero loss image, is the same. This implies that Ni and Ag are uniformly distributed in each individual nanoparticle. The EFTEM maps for the Ag0.7-Ni0.3 and Ag0.3-Ni0.7 NPs samples are shown in Figure 6; the data demonstrate that both Ni and Ag are present in each NP. Furthermore, the spatial distributions in both Ag0.7-Ni0.3 and Ag0.3-Ni0.7 NPs images are consistent with the zero loss image. Therefore, the EFTEM map results from both Ag0.7-Ni0.3 and Ag0.3-Ni0.7 show that Ni and Ag are uniformly distributed throughout each entire particle and further confirmed the homogeneity of the alloy structure in these NPs. Together, our HRTEM, HAADF-STEM, and EFTEM map results all show the homogeneous distribution of Ni and Ag in our Ag0.9-Ni0.1, Ag0.7-Ni0.3, and Ag0.3-Ni0.7 bimetallic NPs and confirm NP alloy formation. In combination with our previous results from Ag0.5-Ni0.5 NPs,46 we believe that Ag-Ni

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Figure 5. EFTEM maps of Ag0.9-Ni0.1 NPs: (A) zero loss image, (B) carbon map, (C) Ni map, and (D) Ag map. All scale bars are 50 nm.

alloy NPs can be formed at any ratio of Ni and Ag even though they are immiscible in bulk. Heating experiments have been carried out to test the stability of these Ag-Ni NPs due to possible dealloying effects as shown by Sastry et al.31 Similar to the case of Ag0.5-Ni0.5,46 results from both HAADF-STEM and EFTEM maps confirmed that the homogeneity of the alloy structure of Ag-Ni NPs is retained after mild heating to 100 °C for 12 h. Pd-Ni Nanoparticles. For Pd-Ni, we were successful in synthesizing and confirming the alloy structure of the Pd0.5-Ni0.5 NPs. As in the AgxNi1-x system, a combination of UV-vis, TEM-HRTEM, HAADF-STEM, and EFTEM mapping methods were used to confirm a homogeneous distribution of Ni and Pd in alloyed Pd0.5-Ni0.5 NPs.

Zhang et al. The UV-vis spectrum is shown for as-synthesized Pd-Ni alloy NPs after 36 min of irradiation (curve a), and it indicates no absorption band present (Figure 7). Although Mie theoretic calculations indicate that Pd has an absorption band around 225 nm,52 our spectrum of pure Pd NPs does not show any absorption peak near 225 nm (see Figure S3 in the Supporting Information). This may be due to interference from the presence of postirradiated PVA in solution. Furthermore, TEM results of our Pd NPs (see Figure S3 in the Supporting Information) are similar to those reported by Henglein.64 The as-synthesized Pd0.5-Ni0.5 NPs are not spherical but elliptical in shape with an aspect ratio of 1.33 (5.1 nm in transverse length and 6.8 nm in longitudinal length on average). There is a damping in intensity in the UV-vis (see Figure 7A, curve b) with no change in spectra after 6 days. A ripening process may cause the intensity decrease in UV-vis, but the particle size change is very small, with a 0.1 nm increase for both transverse and longitudinal lengths and no change for the aspect ratio (see Figure 7C). In addition, the lattice spacing derived from the HRTEM image (see Figure 3b) shows that 50% Ni and 50% Pd may be present in the alloy NPs. Therefore, both TEM and HRTEM confirm that there is no core-shell structure in our Pd0.5-Ni0.5 NPs. HAADF-STEM and EFTEM mapping results for Pd0.5-Ni0.5 NPs agree that Ni and Pd are homogeneously distributed within the whole particle (see Figure 8) with no core-shell structures present. Single-particle EDS of one Pd-Ni NP also shows that both Ni and Pd are present. The Ni and Pd EFTEM maps (see Figure 9) demonstrate that both Ni and Pd are present in Pd0.5-Ni0.5 NPs and the spatial distribution of Pd0.5-Ni0.5 NPs in both images is consistent. The spatial distribution of Pd0.5-Ni0.5 NPs is also consistent with the corresponding zero loss image in Figure 9A. DFT Modeling of Alloy versus Core-Shell, Ag-Ni NPs. To support the experimental findings that the NP creation is kinetically and not thermodynamically determined, we used DFT to consider the energies of representative NPs. The initial and final configurations of a Ni core-Ag shell particle are seen in Figure 10. As Ni is more reactive and has a much higher surface energy than Ag, this configuration should be favored, as is

Figure 6. EFTEM maps of Ag0.7-Ni0.3 NPs (panels A-C) and Ag0.3-Ni0.7 NPs (panels D-F). For Ag0.7-Ni0.3 NPs, (A) zero loss image, (B) Ni map, and (C) Ag map. For Ag0.3-Ni0.7 NPs, (D) zero loss image, (E) Ni map, and (F) Ag map. All scale bars are 50 nm.

RT Synthesis of Ag-Ni and Pd-Ni Alloy NPs

Figure 7. (A) UV-vis spectra of Pd0.5-Ni0.5 NPs as synthesized after 36 min of irradiation (curve a) and aged after 6 days (curve b). Absorbance is normalized to a 1 cm optical path. TEM images of Pd0.5-Ni0.5 NPs (B) as-synthesized and (C) after 6 days of aging. Scale bars for both (B) and (C) are 50 nm.

indeed found to be the case. The final configuration is found to retain a C3V-like geometry (which is not imposed in the calculation) and exhibits a 0.55 µB magnetization per Ni atom. Two randomly generated Ag-Ni alloy NPs are depicted in Figure 10C, D. Their magnetizations are 0.53 and 0.56 µB per Ni atom. They exhibit energies 49.5 and 43.7 eV higher than the Ni core NP, respectively. On a per atom basis, this amounts to 0.18 eV (7.2 kBT at room temperature (where kB is Boltzmann’s constant)) and 0.21 eV (8.2 kBT) higher energies than compared with the Ni core/Ag shell NP. The fact that the energy difference per atom in alloys versus core-shell NP far exceeds kBT indicates that entropic factors would not change our conclusions. The entropic terms are necessarily on the order of kBT per atom. For example, whereas entropy favored alloy formation, the classic formula for configuration entropy gain in a 50/50 random alloy relative to the phase-segregated solid components is (-ln 0.5)kBT, or 0.018 eV, per atom.65 This amounts to 4.3 eV for the 240-atom cluster. The difference in the vibrational (i.e., phonon) entropies should have the same order of magnitude. Such entropic contributions are dwarfed by the large energy preference for the Ni core/Ag shell NP.

J. Phys. Chem. C, Vol. 114, No. 34, 2010 14315 Thus, even though our conjugate gradient procedure can only search for local minima in the potential energy surface, the Ni core structure is found to be significantly more favorable than random alloy structures that expose Ni atoms at their surfaces. We stress that, despite their thermodynamic stability, Ni core structures are not observed in our γ-radiolysis synthesis experiments. Our DFT calculations are, therefore, definitively consistent with kinetically controlled AgNi NP alloy. For completeness, the initial and final configurations of the Ag core-Ni shell NP are depicted in Figure 10E,F. Because this core-shell NP exposes a maximum number of Ni atoms at the surface, it is initially unstable and experienced large structural changes during structural relaxation from its initial fcc lattice structure. The final local minimum configuration exhibits some Ni4 tetrahedral motifs, not just a single layer of Ni at the surface, to alleviate surface energy costs. Furthermore, some Ag atoms are ejected to the surface. Despite all these structural changes, the energy of this Agcore-Nishell NP remains the highest among all examined, 57.2 eV above the Ni core NP. As discussed above, on a per-atom basis, this energy is much higher than kBT. The magnetization is also the largest, at 0.59 µB per Ni atom, reflecting the high Ni surface fraction. We have not investigated the mechanisms and free energy barriers involved in converting the metastable alloy NPs into thermodynamically stable core-shell NPs. For metastable Au core-Ag shell particles and using approximate empirical force fields, not first-principles methods, this process has been estimated to take 107 years in the absence of vacancies.66 We anticipate this to be an important direction for our Ni-based NP studies in the future. DFT Modeling of Alloy versus Core-Shell, Pd-Ni NPs. The Ni-Pd total energy calculations show a clear preference for a Ni core-Pd shell structure (Figure 11A). The least stable arrangement is the reverse, that is, Pd core-Ni shell, which is 40.6 eV higher in energy (Figure 11B). The six different alloy configurations are intermediate to the core-shell structures in stability. The four random alloy NPs’ (not shown) energies range from 16.6 to 20.0 eV above the Ni core-Pd shell NP, whereas the two layered structures (one of which is depicted in Figure 11C) are 16.3-20.7 eV higher in energy. The magnetization of the NiPd NPs is almost twice that of the AgNi NPs. Total magnetizations are higher for those particles that have a larger fraction of Ni atoms at the surface. These DFT calculations show that the Ni core/Pd shell NP is energetically more stable than random or layered alloy NPs by at least 0.068 eV (2.6 kBT per atom). The magnitude of this energy preference in the core-shell NP exceeds the configurational entropy per atom of a random alloy NP by a factor of 3.8 at room temperature. We stress that the relative

Figure 8. (A) HAADF-STEM image of Pd0.5-Ni0.5 NPs. Scale bar ) 20 nm. (B) Single-particle EDX. The particle for EDX analysis is from (A), as indicated by the red circle. The EDX analysis shows the peaks from both Ni and Pd and thus indicates the presence of both elements in one particle.

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Figure 9. EFTEM maps of Pd0.5-Ni0.5 NPs: (A) zero loss image, (B) Ni map, and (C) Pd map. All scale bars are 50 nm.

Figure 10. NP models containing 120 Ag and Ni atoms each. (A, B): initial and final Ni core-Ag shell configurations. (C, D): final configurations of two random alloy NPs. (E, F): initial and final Ag core-Ni shell configurations.

Figure 11. NP models containing 120 Pd and Ni atoms each: (A) Ni core-Pd shell configuration, (B) Pd core-Ni shell configuration, (C) layered Pd-Ni structure.

thermodynamic stability of two NPs depends on their total free energy difference ∆G via the Boltzmann factor exp(-∆G/ kBT), not the free energy difference per atom. Indeed, in macroscopic systems, any finite free energy advantage of one phase over another induces a phase transition.65 Thus, a 16.3 eV energy difference between core-shell and alloy NPs, even

when attenuated with a 240 × 0.018 eV (or 4.3 eV) favorable configuration entropy for the alloy, would be thermodynamically insurmountable. These results are consistent with our assertion that the experimentally observed Pd0.5-Ni0.5 alloy NPs are not determined by thermodynamics but are kinetically controlled. Other reports have also shown that Pd-Ni

RT Synthesis of Ag-Ni and Pd-Ni Alloy NPs alloy NPs,10 Ni core-Pd shell NPs,25 and Pd core-Ni shell24 NPs are synthesized by either a one-step coreduction (to form an alloy) or a stepwise (to form a core-shell) approach. The above considerations emphasize the importance of using accurate DFT methods to ascertain the relative energies of different NPs and not rely on assumptions based on macroscopic alloy stability. Conclusion Room-temperature radiolysis is used to synthesize Ni-based alloy NPs. Various nanoscale characterization methods are employed to analyze the structure of these Ni-based alloy NPs, and density functional theory modeling is used to understand their kinetic growth mechanisms. We have shown that radiolysis techniques can access novel metastable phase regimes not previously formed by traditional alloying techniques. The various characterization techniques together provide strong evidence of an alloyed NP structure whose reaction formation is via kinetic processes, rather than thermodynamic processes. We have demonstrated that different stoichiometries of Ag-Ni alloy NPs and Pd0.5-Ni0.5 alloy NPs can be synthesized at room temperature by this high dose rate radiolytic method. The Agx-Ni1-x and Pd0.5-Ni0.5 NPs are well characterized by UV-vis, TEM/HRTEM, HAADF-STEM, and EFTEM mapping. The analysis results clearly show that homogeneous alloy NPs are made, as opposed to the thermodynamically favored core-shell NPs. Single-particle EDS shows that both Ag/Ni and Pd/Ni, respectively, are present within each particle. The consistency from both the shape and the spatial distribution of NPs in EFTEM maps indicate that both Ag (or Pd) and Ni are present and that both components are homogeneously distributed throughout the whole particle. In addition, our results from EFTEM maps show that Ni and Ag are present in a wide variety of ratios for Ag-Ni NPs (Ag0.9-Ni0.1, Ag0.7-Ni0.3, Ag0.5-Ni0.5, and Ag0.3-Ni0.7) according to the initial metal ion ratios, and the homogeneity is retained. Radiolysis provides us an important (and possibly universal) method of synthesizing uniform alloy NPs, in metastable phase regimes not accessible by traditional methods. Alloy NPs of other immiscible metal pairs, which do not react with water, may also be prepared by a radiolytic method or other methods that can reduce metallic precursors very quickly. The various TEM characterization techniques employed in this study will be used in the future to distinguish between the type of core-shell NPs formed (e.g., Nicore-Agshell vs Agcore-Nishell) or if a mixture of core-shell and alloy NPs is produced, as indicated in the case of Au-Pd NPs by Ferrer et al. through slow reduction of reactants.57 Our ongoing research is focused on the low dose rate synthesis of Ag-Ni NPs (e.g., 400 rad/min or 4 Gy/min) to study both the reducibility and the subsequent NP formation of other transition metals into a Ni-based system. We are also examining the low-temperature sintering of all of our synthesized NPs into bulk phases, for use in future mechanical testing. Low dose rate synthesis (i.e., slow reduction) of Ag-Ni NPs will allow us to correlate the dose rate with the NPs’ structure. We also plan to continue using ab initio molecular dynamics theoretical techniques to investigate redox potentials for small homonuclear (e.g., mono- and diatomic) and bimetallic clusters. Acknowledgment. This work was supported, in part, by the Laboratory Directed Research and Development (LDRD) program of Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory operated by Sandia

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