Article pubs.acs.org/cm
Ordered Intermetallic Pt−Sn Nanoparticles: Exploring Ordering Behavior across the Bulk Phase Diagram Douglas Y. DeSario and Francis J. DiSalvo* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States S Supporting Information *
ABSTRACT: Because the bulk phase diagram of Pt−Sn contains five different compounds (Pt3Sn, PtSn, Pt2Sn3, PtSn2, and PtSn4) with very different congruent or incongruent melting points, we chose this system to investigate factors that influence the homogeneity and structure of intermetallic nanoparticles with different compositions prepared by the same solution phase co-reduction reaction. Pt and Sn chloride precursors were reduced by alkali metal borohydrides in tetrahydrofuran. Of the five compositions, only PtSn forms ordered intermetallic nanoparticles (average domain size of 4.3 nm) at room temperature. As-prepared Pt3Sn nanoparticles adopt an alloy FCC structure and ordered into the cubic Cu3Au structure at the unexpectedly low annealing temperature of 200 °C. Despite their low bulk incongruent melting temperatures, the tin-rich compositions (PtSnx, where x = 3/2, 2, or 4), by contrast, form highly disordered products at room temperature and require annealing at ≥200 °C for crystallization and ordering to be observed. Phases prepared from tin(II) chloride exhibit co-reduction behavior different from that of tin(IV) chloride, the latter requiring overall higher temperatures to produce similarly ordered particles. Spectroscopic studies indicate that this behavior may be due to the formation of Pt−Sn complexes in solution prior to the reduction when tin(II) is used as a precursor, but not for tin(IV), resulting in more consistent nucleation of the target stoichiometric phases. The process of crystallization by annealing is discussed.
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INTRODUCTION Platinum and platinum−metal alloys (Pt−M) have been the subject of extensive research into their use as fuel cell anode and cathode electrocatalysts.1−6 The activity, cost, stability, and poison resistance all play a role in the potential utility of such catalysts, and each of these parameters is itself dependent on multiple properties of the catalyst and on the processing conditions. Pt−M alloys and ordered intermetallics of the same species have proven to be more active than pure Pt nanoparticles for both the anodic and cathodic reactions in polymer electrolyte membrane fuel cells (PEMFCs).5,6 They also generally possess greater resistance to common catalyst poisons, such as CO, and better durability upon electrochemical cycling. Recent work suggests that ordered phases show less leaching of M than alloys of the same composition under oxidizing conditions and on electrochemical cycling up to 1.2 V versus the standard hydrogen electrode (SHE).7 Even binary phases that do not show dramatically improved catalytic activity for fuel cell catalysts have promise as potential synthetic routes to more useful ternary structures, through cation redox substitution reactions.8,9 Ordered intermetallic nanoparticles and alloys with the same composition exhibit a number of differences in their properties. The surface sites of a Pt−M alloy nanoparticle are expected to be randomly occupied by Pt or M, perhaps with probabilities and stoichiometries different from those in the interior of the particle. In an ordered intermetallic compound, every occupied crystallographic site in the interior of the particle is occupied by © 2014 American Chemical Society
only one type of atom. In some materials, alloys and intermetallics can be interconverted by thermal treatment. In others, the alloy phase is metastable; the ordered phase can be obtained by annealing, but the transformation is generally irreversible. The mechanics of these transformations have been the subject of many studies; for example, the work of Sun et al. on Pt−Fe demonstrated how the chemically disordered FCC phase could order into the tetragonal intermetallic phase, utilizing either a MgO coating or organic surfactants to prevent sintering upon annealing to 400−800 °C.10,11 Korgel et al. found that high-temperature annealing under forming gas can be used to interconvert Pt−Fe ordered phases of different stoichiometries.12 In the case of ordered intermetallic nanoparticles, it is interesting to ask if the atomic order extends to the surface, but the tools for studying such surfaces on nanoparticles are still under development. We expect that, if the ordering enthalpy is sufficiently large, the order would be maintained on exposed crystallographic faces of a single-domain nanoparticle. Note, however, that exposure to different environments (air, water vapor, hydrogen, carbon monoxide, etc.) can also modify the surface structure and stoichiometry of either alloys or ordered intermetallic compounds.13,14 It is important to remember that dependence when interpreting the results of catalytic reactions Received: February 27, 2014 Revised: March 19, 2014 Published: March 20, 2014 2750
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are quite different. These temperatures might be expected to provide a rough guide to the annealing temperature needed to achieve ordering in nanoparticles. Few ordered intermetallic phases have been reported to be synthesized as nanoparticles with ordered structures at room temperature; mostly disordered (or amorphous) alloys are more common. Exceptions include the synthesis of Pt−Pb and Pt−Bi under specific conditions.23 However, even in those two cases, the outcomes of room-temperature reactions are dependent on the solvent, precursors, and/or the reducing agent used. Other conditions often result in particles that must be annealed to produce order. For example, in diglyme, when specific Pt and Pb precursors are reduced by sodium naphthalide, they do not form crystalline Pt−Pb nanoparticles until they are heated above 110 °C in the mother liquor.24 In this article, we investigate co-reduction of Pt and Sn chloride salts in tetrahydrofuran (THF) to explore the preparation of nanoparticles of all five Pt−Sn line phases, as well as of the end members Pt and Sn. In particular, we are interested in delineating and exploring the factors that control the ordering behavior of these phases under various synthetic and annealing conditions.
in fuel cells or in a wide variety of practical heterogeneous chemical transformations. In any case, this suggests that the preparation strategies and catalytic behavior of alloys and their ordered intermetallic phases might be quite different. For now, we will ignore the details of the surface chemistry. We focus on the relation between bulk phase diagrams that contain ordered intermetallic compounds and the structural characteristics of nanoparticles that can be synthesized by the solution phase reduction of metal precursors. Generally, roomtemperature preparation leads to alloyed or even amorphous products, whereas annealing at elevated temperatures can lead to ordered structures. For these systems, the enthalpy of formation of the ordered phases is more negative than that of an alloy with the same composition, which could be a factor in increasing the durability and lower rates of leaching of the ordered intermetallics. However, the factors that result in ordering behavior in nanoparticles are not fully understood. Although ordered Pt−M intermetallics have favorable enthalpies of formation in bulk,15−17 competition with surface energies may enhance or suppress ordering in nanoparticles. Even in bulk systems, high-temperature annealing is sometimes necessary for most systems to overcome kinetic barriers to ordering in alloys that can be formed in low-temperature processes or by quenching from high temperatures.18,19 Diffusion rates in a given family of materials with the same structure tend to scale with the bulk melting point.20 Thus, we might generalize by supposing that alloy phase nanoparticles prepared at room temperature could order at annealing temperatures that scale with the melting point of the ordered bulk solid. At the same time, the melting point of nanoparticles of elemental metals is reduced from those of the bulk, again because of positive surface energies.21 Similar melting point reduction should also occur for ordered intermetallic nanoparticles, but perhaps with a different dependence on particle size. To explore the factors that control ordering, we chose to study the Pt−Sn system, because there are five known compounds with narrow phase widths [known as “line compounds” (see Figure 1)], and Pt−Sn is a known catalyst for many types of reactions.5,22 The compositions and melting and decomposition temperatures of the five binary compounds
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EXPERIMENTAL SECTION
Pt−Sn Nanoparticles. All reactions were performed in a glovebox under an inert argon atmosphere. All solvents were purchased from Fisher and dispensed dry from a custom system (Seca solvent system by Glass Contour). Platinum(IV) chloride (Alfa Aesar, 99.9%) and tin(II) chloride (STREM Chemicals, anhydrous, 98%) or tin(IV) chloride (Aldrich, 1 M in heptane) precursors were dissolved in 30 mL of THF at the appropriate molar ratio to prepare the target composition, represented in mole fractions (Pt1−xSnx). Typically, 30 mg of PtCl4 was dissolved in 30 mL of THF, and a stoichiometric amount of tin chloride was dissolved in the same flask. The resulting clear yellow-orange solutions were stirred for 15 min. A borohydride reducing agent, LiBHEt3 (LBH) or KBHEt3 (KBH) (Aldrich, 1 M in THF), was injected quickly, generally in a 20% excess. The excess is added to reduce any acidic hydrogen of likely impurities, such as water, in the THF. The rapidly stirred solution turned black and opaque within ∼1 s of injection. The solution was then stirred for an additional 15 min, after which the resulting suspension and any precipitate were transferred to a centrifuge tube, which was then sealed with a rubber septum and removed from the glovebox. The tube was centrifuged (9000 rpm for 10 min), and the clear supernatant was removed under argon; 30 mL of THF was added to the black precipitate, and the contents of the tube were stirred, sonicated for 1 min, and centrifuged again. This washing procedure was repeated once more with THF and finally with hexane. In the process described above, either soluble LiCl or insoluble KCl is produced as a byproduct. LiCl and organic species are completely removed by the washing process described above, leaving only the Pt1−xMx product as agglomerated nanoparticles. KCl precipitates and encases the Pt1−xMx nanoparticles, preventing the formation of direct metal nanoparticle agglomerates and preventing sintering of those nanoparticles upon annealing. If it is desired, the KCl can be removed by washing with ethylene glycol and/or water.6 As we will see, these two different metal nanoparticle products are not the same at room temperature and display different annealing behaviors. In either case, the resulting black product was dried under vacuum. Some products were studied directly without annealing. In that case, the centrifuge tubes were backfilled with argon after vacuum drying on a Schlenk line and then opened in air. Samples that were to be annealed at a higher temperature were, without being exposed to air, transferred inside the glovebox under argon to silica annealing tubes. These tubes were then removed from the glovebox without being exposed to air and sealed under vacuum using a hand-held torch. Finally, they were heated to the desired temperature for 24 h.
Figure 1. Binary phase diagram of the Pt−Sn system. Dark areas represent single-phase regions. The five vertical line compounds adopt ordered, crystallographically distinct structures when they are prepared by standard solid state, high-temperature techniques. Taken with permission from the ASM Alloy phase Diagram Database.25 2751
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Supported Nanoparticles. Supported Pt−Sn nanoparticles were synthesized using carbon black (Cabot, Vulcan XC-72). Carbon black was heated to 100 °C in air to remove adsorbates such as water, transferred to the glovebox, and added to the stirring yellow-orange solution of metal chloride precursors. After this addition, LiBR3H in a 50% excess was added, and workup of the reaction proceeded exactly as described above. We empirically found that this 50% excess reduced all metals and accounted for possible competing reactions between the reducing agent and probable functional groups that contain acidic hydrogen (OH, COOH, etc.) on the carbon surface, with other species adsorbed on the carbon black, or on the container walls. Previous work in this group6 also showed that nanoparticles can be released from a KCl matrix and dispersed onto carbon black supports without much agglomeration, but that method was not specifically explored in this work. Instrumentation. All powder X-ray diffraction (pXRD) scans were performed on a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation. Scans were performed at a rate of 4°/min, and three scans were averaged for all samples to produce the final diffractograms. Analysis of the diffractograms, including Rietveld refinement, was performed with the PDXL software suite (Rigaku). Thermogravimetric analysis (TGA) was performed on a TA Instruments TGA Q50 apparatus. For all TGA analyses, roughly 6 mg of sample was heated at a rate of 10 °C/min to a maximal temperature of 550 °C, where it was held for an additional 30 min. In all cases, the mass remained constant during the hold period. All scanning electron microscopy (SEM) images and energy dispersive X-ray analysis (EDX) data were acquired on a LEO-1550 field emission scanning electron microscope. SEM specimens were prepared by attaching a piece of sticky carbon paper to an aluminum stub and gently dispersing 1/2.27 Such bulk leaching will lead to large surface areas and perhaps unusual particle
morphologies.28 Nonetheless, attempts to synthesize singlephase products were undertaken with the intent of exploring the nanocrystallization behavior of the complete Pt−Sn system. Pt2Sn3 formed the ordered phase only after being heated to 400 °C; at 200 °C, the product pXRD pattern shown in Figure 6 is close to that expected for PtSn. In this case, no other Pt−
Figure 6. pXRD of Pt2Sn3 nanoparticles. The peaks in the roomtemperature sample are very broad, indicating a high degree of disorder and very small domain sizes. Note the presence of PtSn peaks in the 200 °C pattern. At 400 °C, the transition to Pt2Sn3 (black drop lines) is almost complete (PDF Card 04-007-4094). Purple drop lines and asterisks mark the impurity SnO2 in the sample annealed at 400 °C.
Sn structures are seen. Because the room-temperature phase is nearly amorphous, the data suggest that the first phase to nucleate on heating has a stoichiometry different from that of the amorphous 2:3 composition. Indeed, such behavior has been seen in the nucleation of crystalline domains when amorphous thin films are annealed at low temperatures.11 X = 2/3 (PtSn2). Reduction of compounds with a 1:2 Pt:Sn ratio never resulted in a single-phase product, independent of the annealing temperature. The product that contained the most PtSn2 was obtained at 400 °C (60 wt % PtSn2), and the remainder consisted of PtSn and some SnO2, with other Pt−Sn phases such as Pt2Sn3 present in small quantities. The pXRD is shown in Figure S8 of the Supporting Information. X = 4/5 (PtSn4). Interestingly, pXRD patterns of PtSn4 annealed to 200 °C in Figure S9 of the Supporting Information show the expected closely spaced XRD peaks that identify it as 2753
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a pure ordered phase. Small domain sizes at low temperatures make phase identification of impurity phases difficult, but annealed nanoparticles closely match the database pattern for the target phase. It should be noted, however, that the synthesis of this phase is extremely sensitive to air and water contamination of the glovebox atmosphere. In general, a greater tin concentration in a target phase correlates with an increased level of difficulty in synthesizing that phase as pure nanoparticles of the target composition. In all of these tin-heavy systems, the bulk peritectic decomposition temperatures of the tin-heavy phases are much lower than the congruent bulk melting temperature of either PtSn or Pt3Sn, but the tin-heavy phases order at temperatures on the order of or higher than those found for the two Pt-heavy phases. X = 1 (Sn). Pure tin nanoparticles can be readily prepared via reduction by LBH at room temperature. These particles will oxidize quickly in air, forming a shell of tin oxide on the surface of the nanoparticles. To avoid this, the pXRD was taken under air-free conditions by preparing the sample in the glovebox and covering it with a Mylar film sealed to the sample holder with silicone grease. The Mylar film has broad diffraction peaks that were subtracted from the pXRD in Figure S10 of the Supporting Information. The remaining sharper peaks match the pattern for Sn, with a domain size of 21 nm. Table S11 of the Supporting Information summarizes the temperatures at which the five Pt−Sn line phases first show ordering behavior, the domain sizes resulting from annealing the samples to those temperatures, and evidence of whether the sample was phase pure. Further Characterization and Discussion. Room-temperature ordering behavior in binary intermetallic nanoparticles is an unusual occurrence, given that synthesis or annealing at elevated temperatures is often essential for overcoming the nucleation and diffusional barriers to ordering. Determining the cause of this ordering behavior is more difficult, as it appears to not be an intrinsic property of the co-reduction method. Of the Pt−M systems that have been investigated in the past, only PtBi and PtPb prepared by specific synthetic pathways showed ordering behavior at room temperature, while other pathways did not.13 Other metals, including main group metals such as Sb and all 3d transition metals combined with Pt, require annealing temperatures of at least 400 °C before ordered phases can be observed via XRD after similar co-reduction syntheses. Platinum(IV) and tin(II) chloride are known to make a variety of complexes together in aqueous solutions.28 The formation of such complexes in THF should lead to more compositionally homogeneous metal nanoparticle seeds after co-reduction, and perhaps then to as-made nanoparticles that are mostly or wholly ordered. UV−vis absorbance measurements (see Figure 7) of the pre-reduction chloride-containing solution were taken to determine if there is a noticeable amount of complexation occurring in the solution. The platinum-rich Pt3Sn solution has very broad peaks in the region of 350 nm, whereas more tin-rich solutions (PtSn, Pt2Sn3, and PtSn2) have peaks at 390 and 450 nm. The pure tin chloride and platinum chloride solutions showed no sign of any absorbance peaks in these regions. Studies of the UV−vis spectra of Pt−Sn complexes in THF could not be located; however, spectroscopic measurements of Pt−Sn chloride solutions under aqueous conditions confirm that multiple complexes exist, and that they have distinguishable spectra with
Figure 7. Comparison of UV−vis absorbance spectra of a range of PtCl4 and SnCl2 pre-reduction solutions. The ratios of the chloride precursors in each solution are equal to the ratios present in the target Pt−Sn line phase. Solutions include pure Sn (i) and Pt (ii) chlorides, as well as solutions with 3:1 (iii), 1:1 (iv), 2:3 (v), and 1:2 (vi) PtCl4:SnCl2 ratios. Absorbencies are scaled relative to the concentration of platinum chloride in each solution.
peaks occurring in the region of 300−550 nm.26,29 A qualitative comparison of peak positions suggests that in each case, there is one major Pt−Sn complex that is limited by either the platinum(IV) or tin(II) concentration, and the identity of this complex changes depending on the ratio of chloride precursors. This proves that complexation does occur prior to the reduction, and that the complexes that form are dependent on the ratio of platinum to tin in the solution. The next step was to determine if the complexation affects the reduction reaction and subsequent nanoparticle formation. To test this, SnCl4 was used as the tin precursor in place of SnCl2. There are no peaks to suggest that SnCl4 forms any complexes with platinum chloride, as shown by UV−vis spectrophotometric measurements (Figure 8) that show the
Figure 8. UV−vis absorbance of pure SnCl2 (i), SnCl4 (ii), and PtCl4 (iii) in THF. When mixed with PtCl4 in a 1:1 ratio, SnCl4 shows very little change in its absorbance spectrum compared to that of pure PtCl4 (iv), while SnCl2 displays prominent peaks due to complexation (v).
PtIV−SnIV absorbance is roughly the same as the pure PtIV absorbance, in contrast to that of the PtIV−SnII solution. The pXRDs comparing the products of these reductions are shown in Figure 9; as previously seen, the reduction with SnCl2 gives ordered PtSn nanoparticles at both room temperature and upon annealing to 200 °C, whereas the reduction with SnCl4 is amorphous at room temperature. Annealing the nanoparticles 2754
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dispersed supported nanoparticles by a facile and direct synthetic route. More relevant to this study, however, is the fact that dispersing the nanoparticles on a carbon support prevents agglomeration, which allows the particles to be studied via microscopy to further probe their ordered structures. The process of reducing the metal chloride precursors directly onto the carbon black support has the effect of making the as-made nanoparticles smaller and less well-ordered. For example, the peak at a 2θ value of 42° in PtSn/C prepared at room temperature in the presence of carbon black shows a broader shoulder at smaller angles versus that observed without carbon black (see Figure 10), possibly indicating the presence
Figure 9. pXRD comparison of 1:1 products using SnII and SnIV chloride and PtCl4 precursors reduced with LiBHEt3. Drop lines correspond to the PtSn standard.
to 200 °C results in impure Pt−Sn phases, and at 400 °C, the pXRD pattern matches the reference pattern for Pt2Sn3. Reactions with SnCl4 often resulted in multiphase products, suggesting that the local concentrations of tin and platinum had greater variances than in the samples prepared with SnCl2. Pt2Sn3 showed such behavior, as seen in Figure S12 of the Supporting Information. It seems likely, therefore, that the complexation behavior of PtCl4 and SnCl2 plays an integral role in the formation of the ordered phase at room temperature. Of note is the contrast between these results and those of PtSn4, where both the SnII and SnIV chlorides produced ordered PtSn4 (see Figure S13 of the Supporting Information). The major difference between the two products was the presence of SnO2 in the SnIV sample, but no other Pt−Sn impurity phases could be detected. This is an indication that the specific identities of the pre-reduction Pt−Sn complexes (which depend on the relative concentrations of PtCl4 and SnCl2 in solution) are important factors in whether the co-reduction produces seed nanoparticles with ordered (or close to ordered) structures. It is also possible that the ordered phase formation is hampered by a thin, amorphous surface layer of oxide blocking the diffusion of metal atoms. This surface layer could form because of traces of water or oxygen that are entrapped during the handling of the products in nominally air-free environments. Reductions with KBH were also performed on several Pt−Sn phases to determine if limiting the size of the nanoparticle seeds would have an effect on the identity and ordering behavior of the synthesized phases. As shown and discussed in Figure S14 of the Supporting Information, the overall effect was that ordering behavior was suppressed and required higher annealing temperatures for the formation of ordered intermetallics, but domain sizes were kept smaller by the KCl matrix when compared to the size of nanoparticles reduced by LBH and annealed to similar temperatures, which decreased by 20% upon annealing to only 200 °C. Supported Nanoparticles. Supported, well-dispersed nanoparticles are integral in catalysis to best utilize the large surface area of nanoparticles. The low-temperature ordering behavior of the Pt-heavy phases can be leveraged to make well-
Figure 10. pXRD shows that the reduction of PtCl4 and SnCl2 in 1:1 ratios in the presence of carbon black (Vulcan XC-72) produces smaller domain sizes than equivalent reductions of unsupported nanoparticles. The particle domain sizes increase from 4.3 to 5.5 nm on annealing the carbon−nanoparticle composite to 200 °C.
of some amount of Pt-heavy alloyed phase partially obscured by the larger PtSn (102) peak. PtSn/C was therefore annealed at 200 °C to obtain well-ordered nanoparticles with ordering character equivalent to that of the unsupported as-made nanoparticles. The metal:carbon weight ratio in the reaction mixture was 1:5; however, TGA under flowing air oxidizes both Sn and C, which allows one to calculate the mass of PtSn successfully dispersed onto the carbon support. The TGA profile of PtSn/C (Figures S15 and S16 of the Supporting Information) shows that 71% of the mass is lost over the course of heating to 550 °C, which gives a weight percent of 26% metal for the PtSn/C sample. The small difference from 20% may be due to the retention of C in the sample. For Pt3Sn/C, a similar TGA run and calculation gives a weight percent of 18% metal. In both cases, the difference between the theoretical loading of metal on carbon and the experimental value is within the expected error for the technique. Figures 11 and 12 show TEM and SEM images of the PtSn/C and Pt3Sn/C nanoparticles. PtSn particle sizes were measured via TEM to be 5.5 ± 1.7 nm and Pt3Sn particle sizes to be 4.6 ± 1.4 nm, which is in rough agreement with domain sizes measured by pXRD. Finally, HAADF-STEM images and electron diffraction patterns of the PtSn/C particles were acquired to further test the presence of nanoscale ordering. As seen in Figure S17 of the Supporting Information, diffraction patterns and Miller planes of two example as-made PtSn nanoparticles match expected values for the 1:1 ordered phase. 2755
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to the formation of the Pt−Sn complex in THF prior to the reduction, which has the effect of reducing the extent of randomness of the mixing of the two metals in solution. We hypothesize that this lowers the activation barrier for nucleation of the ordered structures produced by rapid reduction. The formation of the complex appears to affect the preferential nucleation of some phases more than others, depending on the identities of the complexes in solution and the relative ratios of Pt and Sn in the pre-reduction solution, as the synthesis of PtSn4 showed little difference between solutions with complexation and those without. Additionally, Pt3Sn and PtSn were prepared as single-phase carbon-supported nanoparticles through direct reduction with LBH onto carbon black. Reduction in the presence of KCl was shown to suppress ordering behavior at room temperature. PtSn nanoparticles reduced with KBH required annealing to at least 400 °C. At the same time, nanoparticles reduced with KBH were protected from sintering at higher annealing temperatures when compared to nanoparticles reduced with LBH. This indicates that when insoluble salt precipitates out of the THF upon reduction of the metal chloride precursors, the KCl matrix traps small nanoseeds of metals that do not necessarily have the proper stoichiometry to form ordered nanoparticles without the growth of nanoparticles through the diffusion of metal atoms or nanoparticles themselves through the matrix, which can occur only at higher annealing temperatures. Simultaneously, this matrix protects those nanoparticle domains from growing too large and from oxidation upon annealing. Balancing the benefits of the salt matrix’s protection capabilities against the larger barrier to ordering can be a way to tune these reactions to produce ordered nanoparticles with the desired domain size. The three Sn-heavy line phases could be prepared as predominantly single-phase products upon annealing; the lowest percentage of the desired product was obtained for PtSn2 (60%). These phases all required annealing temperatures as high as or higher than the annealing temperatures required for the Pt-heavy line phases, even though the Sn-heavy phases have bulk melting points much lower than those of either of the two Pt-heavy phases. Therefore, both the presence of prereduction complexation and congruent melting behavior are likely beneficial to the formation of small, ordered PtM nanoparticles. The suppression of crystallization of the bulk Snrich structures may be due to the suppression of nucleation in small nanoparticles, so that the expected structures are not formed until higher-temperature annealing results in sufficiently large domains. This behavior may be related to the fact that the room-temperature products are nearly amorphous. Clearly, this is not a problem in Pt3Sn or PtSn prepared in either LBH or KBH, although products from KBH require annealing at temperatures higher than the temperatures of those from LBH. In either case, the domain sizes remain relatively small. These observations suggest that the study of nanoparticle synthesis in different binary intermetallic systems that contain more than one line phase should be undertaken to shed light upon the role of particle size and matrix stabilization in influencing the kinetics of ordering in intermetallic nanoparticles. At the same time, one could explore the possible correlation between low-temperature ordering and congruent melting behavior at specific compositions that is suggested in this study of the Pt−Sn system.
Figure 11. TEM and SEM images of PtSn ordered intermetallic nanoparticles made by reduction of a 1:1 solution of PtCl4 and SnCl2 in the presence of a carbon black support, followed by annealing to 200 °C. Bright spots in the SEM indicate metal nanoparticles on the surface of the support.
Figure 12. TEM image of Pt3Sn intermetallic nanoparticles supported on carbon black, annealed at 300 °C.
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CONCLUSIONS Pt3Sn and PtSn nanoparticles with domains sizes of
