The Nanoscale Kirkendall Effect in Pd-Based Intermetallic Phases

Article pubs.acs.org/JPCC

The Nanoscale Kirkendall Effect in Pd-Based Intermetallic Phases Thomas Götsch,† Michael Stöger-Pollach,‡ Ramona Thalinger,† and Simon Penner*,† †

Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria University Service Center for Transmission Electron Microscopy (USTEM), Vienna University of Technology, A-1040 Vienna, Austria

‡

S Supporting Information *

ABSTRACT: Hollow particles of Pd−X intermetallic phases (X = Ga, Ge, Sn) are prepared by reduction of the respective oxide-supported Pd/XO2 and Pd/X2O3 thin films via the nanoscale Kirkendall effect. Void formation and stability was investigated by (scanning) high-resolution transmission electron microscopy and spectroscopy. Analysis of the selected area electron diffraction patterns after different reductive steps revealed a direct correlation of void size and stability with the composition of the respective intermetallic compounds. Voids only appear for Pd-rich Pd2X phases, whereas for intermetallic compounds of lower Pd content, PdX, the particles are solid again. Energyfiltered electron microscopic images show that the voids are indeed empty and formed by faster Pd outward diffusion compared to X inward diffusion.

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INTRODUCTION After the discovery of the Kirkendall effect in 1947,1 which describes the consequences of different diffusivities for two solids in contact, much of the early research was conducted in order to find ways to minimize this effect in macroscopic samples as it is disadvantageous in metallurgical applications. However, since the first published synthesis of hollow cobalt sulfide nanoparticles in 2004 by Yin at al.,2 the research regarding this so-called nanoscale Kirkendall effect has gained momentum due to possible tuning of physicochemical properties of the resulting hollow nanocrystals, as outlined in a recent review by Wang et al.3 The reason why this effect is of particular interest is that the voids in the inside alter the electronic structure of the particle. Hence, different catalytic or optical properties can in turn be achieved in a controlled way. The nanoscale Kirkendall effect subsequently allowed for the preparation of hollow nanostructures for a wide variety of chemical systems using a large array of different techniques. A lot of research has been performed regarding the preparation of hollow chalcogenides, such as oxide particles (e.g., by Railsback at al., who analyzed the Kirkendall effect upon the oxidation of Ni nanoparticles and formation of NiO as a function of particle size4) or the aforementioned sulfides, but also selenides5 and tellurides.6 Furthermore, the effect has been shown to also occur for nanoparticulate nitrides7 and phosphides.8 As a consequence, also more complex systems taking advantage of the Kirkendall effect have been prepared. For example, Kim et al. have utilized it to create Y3Al5O12:Ce3+ garnet phosphors,9 while Gao et al. synthesized chains of hollow cobalt selenide nanocrystals.10 The preparations have not been limited to spherical nanoparticles, as Fan et al. © 2014 American Chemical Society

managed to subsequently prepare single-crystalline ZnAl2O4 spinel nanotubes.11 There have even been cases where another nanoparticle has been trapped inside the hollow shell. These socalled “yolk−shell” nanostructures can be used to, for instance, protect catalytically active platinum particles from sintering by encapsulating them within a CoO hollow nanoparticle while still exhibiting catalytic activity in ethene hydrogenation since the educts are able to diffuse through the shell.2 In terms of metals, there have been reports about combinations of galvanic replacement and the Kirkendall effect, such as the studies by Gonzalez et al. and Goodman et al., who prepared gold particles with a multitude of different porous and hollow morphologies by substituting silver atoms.12,13 The appearance of voids in alloys has only been studied on metal bulk and thin film samples, like those presented by Liu et al.14 However, to the best of our knowledge, there have been no reports regarding the application of the nanoscale Kirkendall effect for preparation of nanoparticles of intermetallic compounds. As this would open exciting new possibilities with respect to deliberate tuning of the electronic, optical, structural, and catalytic properties of nanoparticulate intermetallic systems, we investigated the formation of hollow intermetallic nanoparticles starting from metal-oxide thin films after various reduction treatments using high-resolution (scanning) analytical transmission electron microscopy (TEM). For eventually successful preparation of those nanostructured particles, we therefore took advantage of our dedicated model thin film approach, which uses epitaxially grown metal particles Received: June 3, 2014 Revised: July 15, 2014 Published: July 21, 2014 17810

dx.doi.org/10.1021/jp505458v | J. Phys. Chem. C 2014, 118, 17810−17818

The Journal of Physical Chemistry C

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embedded in the supporting oxide matrix. The resulting system exhibits a large metal-oxide interface, which facilitates reduction of the support and the interdiffusion of atoms, especially at high temperatures. This is a prerequisite for entering the state of the Kirkendall effect. To demonstrate the preparation of hollow nanoparticulate intermetallic compounds with potential technological applications, we chose the group of Pd-based intermetallic phases, which have shown recently to be promising candidates for CO2-selective methanol stream reforming and methanol dehydrogenation.15

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EXPERIMENTAL METHODS Preparation of the Thin Films. The thin films were grown epitaxially on freshly cleaved NaCl(001) substrates using a high-vacuum chamber pumped to a base pressure of 1 × 10−4 Pa. First, the palladium particles were deposited using electron beam evaporation at a substrate temperature of 600 K. For this, a Pd wire (with a purity of 99.99% and a thickness of 0.25 mm, from Goodfellow) was wrapped around a tungsten rod acting as the anode, which was bombarded with electrons from a W cathode looped around the wire. All three supports, GeO2 (Alfa Aesar germanium(IV)-oxide 99.9999%), SnO2 (Alfa Aesar tin(IV)-oxide 99.9%), and Ga2O3 (Alfa Aesar gallium(III)-oxide 99.9999%), were evaporated thermally by resistive heating of a tantalum boat (the substrates were kept at room temperature) at oxygen pressures of 2 × 10−2 Pa (GeO2), 1 × 10−1 Pa (SnO2), and 1 × 10−2 Pa (Ga2O3), respectively. The NaCl substrate was then dissolved in distilled water to yield floating thin films with a nominal thickness of 25 nm. These thin films were subsequently placed upon a gold TEM grid and dried in air at room temperature. For the reductions, which were performed in flowing hydrogen, the TEM grids carrying the samples were placed inside a small furnace using glass wool as a spacer, while the target temperature (for Pd/SnO2 between 423 and 673 K, 573 and 873 K in the case of Pd/GeO2, and 523 and 873 K for Pd/ Ga2O3), which was approached at 10 K s−1, was kept constant for 1 h. Characterization. The samples were analyzed using a 100 Zeiss EM 10C transmission electron microscope (TEM), as well as a a 200 kV FEI Tecnai F20 (scanning) transmission electron microscope ((S)TEM). The phases were identified by selected area electron diffraction (SAED, the patterns of which were calibrated using the Pd reflexes of the as-grown samples) and high-resolution TEM (HRTEM). The particles were also analyzed using high-angle annular dark field (HAADF) imaging, and their chemical composition was investigated by electron energy-loss spectroscopy (EELS), energy-filtered TEM (EFTEM), and energy-dispersive X-ray spectroscopy (EDXS).

Figure 1. TEM bright-field overview images of the Pd/GeO2 (left column), Pd/SnO2 (middle column), and Pd/Ga2O3 (right column) samples in the as-prepared states and after medium reduction temperatures up to 573 K. Higher reduction temperatures are shown in Figure 2 and the corresponding selected area electron diffraction patterns (SADPs) for phase analysis in Figure 3.

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RESULTS AND DISCUSSION Structure, Morphology, and Composition. Figures 1 and 2 show bright-field TEM images of the three studied systems (Pd/GeO2, Pd/SnO2, and Pd/Ga2O3) in the as-grown state and after reduction at medium to elevated temperatures up to 873 K (the indicated temperatures were kept constant for 1 h under flowing hydrogen, flow 0.9 mL s−1). In the as-grown state, none of the particles contain any voids, although most of the particles exhibit a pronounced internal contrast. Apart from possible thickness variations, the contrast is mostly due to different orientations of the particle-building crystallites. However, after treatment in hydrogen at 523 K, a bright

Figure 2. TEM bright-field overview images of the Pd/GeO2 (left column), Pd/SnO2 (middle column), and Pd/Ga2O3 (right column) samples after elevated reduction temperatures up to 873 K. The asgrown samples, as well as lower reduction temperatures, are shown in Figure 1 and the corresponding SADPs for phase analysis in Figure 3.

contrast can clearly be seen within the former Pd particles on SnO2 as well as Ga2O3, which will be shown in the rest of this work to be holes. Between 573 and 673 K, all three systems, including Pd on GeO2, exhibit hollow nanoparticles. In terms of 17811



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the nanoscale Kirkendall effect, this indicates a diffusioncontrolled process, where the outward diffusion of Pd is apparently larger than the inward diffusion of the other species. At higher reduction temperatures, the particles are solid again for Pd/SnO2 (773 K) and Pd/GeO2 (873 K), while a vanishing minority of the gallium oxide-supported former Pd particles are still hollow. We note rather pronounced sintering of the former Pd particles, especially at the highest reduction temperatures, with apparently almost-total “consumption” of the oxide matrices (as can be deduced from the vanishing of the light contrast between the particles). All the films are composed of an array of very large particles obviously formed by increased atom diffusion at these temperatures. Analysis of the corresponding selected area electron diffraction patterns (SADPs, Figure 3) reveals a direct

Table 1. Assignment of the SAED Patterns to Different Intermetallic Phases phases reduction temperature/K 423a 473 523 573 673 773 873

Pd/GeO2

Pd2Ge Pd2Ge Pd2Ge + PdGe

Pd/SnO2 Pd + Pd2Sn Pd + Pd2Sn Pd + Pd2Sn Pd2Sn Pd2Sn + PdSn PdSn + PdSn2

Pd/Ga2O3

Pd + Pd5Ga2 Pd + Pd5Ga2 Pd + Pd5Ga2 Pd5Ga2 + Pd7Ga3 Pd7Ga3 + PdGa

a

The diffraction patterns referenced here that are not shown in Figure 3 are available in the Supporting Information.

Pd/X is reached, pronounced voids appear in the particles. These voids progressively shrink and finally disappear again as the intermetallic compound particles are subsequently depleted in Pd. We note that the experimental results can be consistently interpreted in terms of the (nanoscale) Kirkendall effect. On this basis, the formation of the intermetallic compound must be accompanied by faster Pd outward diffusion as compared to the inward diffusion of the second metal atoms. This second metal is the result of a (partial) reduction of the oxide support by hydrogen and the subsequent diffusion of (here) Ge, Ga, or Sn species into the Pd lattice. This formation mechanism of intermetallic compounds via “reactive” metal−support interaction17 is well established and accounts for the formation of an increasing number of intermetallic phases, also by using other oxide supports. Void formation, however, has so far only been observed in the examples presented here. The results also strongly suggest composition-dependent interdiffusion coefficients of Pd and the second metal component. Whereas the outward diffusion of Pd is clearly dominating at low reduction temperatures and Pd-richer phases, the situation reverses upon increasing the reduction temperature and the associated formation of more Pd-depleted phases. To show that the voids visible in the bright-field images were indeed voids and not due to some other form of contrast, highangle annular dark field (HAADF) micrographs were recorded as well, as exemplarily shown in Figure 4 for the case of the Pd/ SnO2 system. In this mode, strongly scattered electrons (both Rutherford and thermal diffuse) are detected. Thus, since heavy atoms scatter the electrons to larger angles, they appear brighter in HAADF images. This means that, besides thickness contrast, chemical contrast based on different atomic numbers (i.e., Zcontrast) is predominant in these micrographs. In (a), the particles are obtained after reduction at 523 K (with some of the hollow species marked by arrows), whereas reduction at 773 K yielded only solid particles (b). The latter particles appear chemically homogeneous, and their average size has increased compared to those in (a) since the support was apparently completely “consumed” by the intermetallic phaseforming reaction at that temperature. This explains why no signal from the areas between the particles is visible in (b); indeed, the particles are only supported by themselves as they formed a network while sintering together. Both in the bright-field as well as in the HAADF images, still particles without any void are encountered, even at lower reduction temperatures. This can be understood in terms of either still unreacted palladium particles (at the lower temperatures) or the reduction temperature was high enough

Figure 3. SAED patterns for as-grown particles (top), hollow (center), and solid species after high-temperature reductions (bottom) for Pd/ GeO2 (left column), Pd/Ga2O3 (middle column), and Pd/SnO2 (right column). The occurrence of voids within the particles correlates with the formation of Pd2X compounds (X = Ga, Ge, Sn), whereas they vanish for Pd-depleted phases (PdX and PdX2). A list of assigned intermetallic phases is given in Table 1.

correlation between composition of the metal particles and the appearance of the voids: initially, only the reflections of the face-centered cubic structure of Pd are present (the oxides are amorphous in the initial state). Accordingly, only solid metal particles are observed. Increasing the reduction temperature to 573 K induces formation of Pd−X intermetallic phases with a stoichiometry close to 2:1.15,16 In particular, Pd2Ge, Pd2Sn, and a mixture of Pd5Ga2 and Pd are obtained (see also Table 1 for a summary of the obtained phases and Tables S1−S5 in the Supporting Information for a complete assignment of all the reflections to the different structures). For the Pd/Ga2O3 system, also Pd7Ga3 is observed at 773 K. Further increase of the reduction temperature induces the formation of Pddepleted intermetallic compounds with a 1:1 Pd/X composition and even 1:2 = Pd/X. Specifically, PdGe, PdGa, and mixtures of PdSn/PdSn2 nanoparticles are observed. In parallel, bright-field TEM images show that, as soon as the intermetallic compound state close to a stoichiometry of 2:1 = 17812



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Figure 4. HAADF images of Pd/SnO2 at (a) 523 K, where hollow Pd2Sn particles, some of which are exemplarily marked by arrows, are present (in addition to solid Pd species), and at (b) 773 K, where the particles appear chemically homogeneous and solid.

for the hollow particles to collapse. Since nanoparticles with voids represent only a metastable species, heating for a longer period of time should cause them to become solid again. This has been theoretically shown for hypothetical hollow Au species, where the vacancy concentration gradient causes an outward flux of vacancies at elevated temperatures, resulting in a limited time during which the hollow structure is stable (also, the time needed to fill the voids is larger for bigger particles, as it is proportional to r3, where r denotes the particle radius).18 Theoretical calculations or estimations as in the paper by Tu and Gösele dealing with the stability of such hollow particles were, however, not possible in the present case due to missing information on the temperature dependence of the interdiffusion coefficients, as well as the need for data on the interfacial tensions between Pd and the Pd2X compound and tensions as well as between this phase and the supporting oxide. A qualitative interpretation is nevertheless still possible: the higher the temperature the hollow particles are exposed to, the shorter the time before they become solid. High-Resolution TEM Analysis. The high-resolution TEM (HRTEM) images in Figure 5 support this theory: at 523 K, the hollow particles (a) are composed of Pd5Ga2; that is, an intermetallic phase with a stoichiometry of around 2:1 is formed. The measured lattice spacing of 0.22 nm was related to the 0.221 nm spacing of the (221) planes of orthorhombic Pd5Ga2.19 The solid nanoparticles (b) consist of face-centered cubic Pd only, as the analysis of the lattice spacings, as well as the cubic FFT (inset), confirm: the spacings of 0.19 nm in two directions agree well with theoretical values for the (200) and (020) reflexes (i.e., 0.195 nm), respectively.20 Also, the angle between them was measured to be 90°, as expected. The FFT also shows a rectangular array of spots with even spacing. At 673 K, some Pd5Ga2 particles can also be encountered without holes (c), which apparently is due to the described metastability. The identity of this species was assessed by the

Figure 5. HRTEM images of particles in the Pd/Ga2O3 system. Upon reduction at 523 K, the Pd5Ga2 particles are hollow (a), whereas the solid ones consist of metallic Pd (b). At 673 K, the metastable hollow Pd5Ga2 particles have closed again, whereas after treatment in hydrogen at 773 K voids form again as Pd7Ga3 species are generated (d), which are also found at 873 K (e), where the newly formed PdGa particles are solid again (f).

measured lattice spacings of 0.23 ((420) spacing with a theoretical value of 0.236), 0.36 ((4̅10) with 0.361 nm in theory), 0.22 ((221) with 0.221 theoretically), as well as the angles between them: the measured angle between (41̅ 0) and (221) is 68.1°, the theoretical one 70.5°, the one between (420) and (221) is 37.0°, compared to 35.4° in theory, and the (420) and (4̅10) planes span an angle of 77.4 in the image and 80.9° theoretically.19 At 773 K, a new intermetallic phase is formed (d) that is base-centered monoclinic Pd7Ga3, again featuring pronounced voids. The measured lattice spacing of 0.22 nm could be related to the Pd7Ga3 (510) plane, which features a theoretical spacing of 0.221 nm. The 0.24 nm value corresponds to the (4̅02) spacing, which, in theory, is 0.238. The 0.21 nm could be ascribed to the (112) planes that are 0.210 nm apart, theoretically. The angles confirm the assignment: the angle between (510) and (4̅02) is measured to be 65.8° (theory: 62.4°), that between (510) and (112) 52.7° (theory: 55.8°), and between (112) and (4̅02), the angle is 60.4° (theory: 61.9°).21 The FFT-inset also shows angles that deviate strongly from 90°, corroborating the assignment to a monoclinic structure. Reduction at 873 K yields the formation of yet another phase, cubic PdGa, with a 1:1 stoichiometry. However, not all Pd7Ga3 is converted yet, and hence, a number 17813



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exists; hence, they only exhibit the Kirkendall effect once. The assignment of the solid particle in (c) was based on the marked lattice spacings: the 0.21 nm originates from the (121) planes, and the (020) planes cause the 0.29 spacings. The angle between the (020) and the lower (121) lattice planes (43.5°) agrees well with the theoretical value of 46.5° (the (121) spacing seen in the upper region stems from a different grain, and hence, the angle to this plane cannot be evaluated).25 Statistical Analysis of the Nanoscale Kirkendall Effect. To further shed light on the Kirkendall effect, we performed rigorous statistical analyses of the bright-field and HAADF images of the systems, yielding particle diameter, hole diameter, minimum shell thickness (i.e., the minimum distance between the void and the surface of the particle), and percentage of hollow particles as a function of temperature. The three size values were determined by measuring around 70−200 particles per reduction temperature and fitting a Gauss curve to the resulting histograms. The data are shown in Figure 7, where the phases present in the samples are coded according to the labels and the corresponding background colors. For the metal particles supported on tin oxide (a), the particle size (measured along the longest axis) increases monotonously with increasing temperature (within the scope of the standard deviation) from around 9.3(4) to 28.8(4) nm at 773 K (due to coalescence). The hole size, on the other hand, reaches a maximum of 4.2(5) at 473 K and still is 3.0(1) at 673 K. At 773 K, where the 1:1 compound is present, as can be seen from the background color of the plot, no particles have holes; hence, the mean hole size cannot be specified. The fraction of hollow particles in the Pd/Sn system increases to a maximum at 473 K, correlating with the largest hole size. At that temperature, 56% of all particles feature voids. It can be seen that the fraction drops significantly at 673 K, even though the hole size has not drastically been reduced. This is due to the start of the formation of the PdSn compound, which, as described earlier, exhibits only solid morphologies. This means that the resulting hole size was measured for Pd2Sn particles, of which some still contain voids after reductive treatment at 673 K. Also at this temperature, the minimum shell thickness is found to be slightly higher than for the previous preceding reductions (2.8(2) nm instead of 1.9(1) nm). This can be explained by the fact that if Pd2Sn particles collapse due to their metastability those that have the void farther to the outside will do so faster than those particles where the hole is more centered. At 773 K, the particle size increases drastically (due to sintering), but the particles are solid and homogeneous, devoid of holes. For Pd/Ga2O3 (b), the plot looks slightly different: while the particle size still increases continuously, the fraction of hollow particles shows a significant dip at 673 K (from 47% down to 25%, followed by a rise to 35.5%). This is in line with the theory derived from the HRTEM images and the SADPs as it confirms the onset of a second Kirkendall effect as the intermetallic phase transforms from Pd5Ga2 to Pd7Ga3 at 773 K. The dip can also be seen for the hole size, albeit less pronounced. At the highest temperature, that is, 873 K, PdGa starts to form, which is why the void fraction and the hole size finally decrease. As already mentioned, there is only one Pd−Ge compound with 2:1 stoichiometry that can be formed upon reduction, which also shows up in the statistical plot (c) as there is only one maximum in the fraction of hollow particles. It should also be noted that the maximum fraction, namely, 79.5% (at 673 K),

of hollow particles can still be found on the specimen (e) that consists of Pd7Ga3. The lattice spacing of 0.31 nm equals the (4̅01) plane (theory: 0.318 nm) and the 0.25 one to the (401) plane (theory: 0.250 nm). The angle between them also suggests this: 63.8° versus 63.1° in theory.21 In the case of this particular particle, the lattice fringes can also be seen within the void, while for the HRTEM images of samples heated to lower temperatures, this was not the case. This is due to the hole not extending through the nanostructure from top to bottom (as in a nanotube). Rather, the particle is closed on at least one side of the hole, thus revealing the Pd7Ga3 spacings in the highresolution image apparently also “within” the void. This can also be observed for some of the systems reported by Yin et al.2 At this reduction temperature, the metastability-induced collapse of the hollow structure has already started, which is the most likely reason for this. When the reaction to PdGa occurs, the particles are solid (f), as the lattice spacings, their angles, as well as the FFT, showing a cubic arrangement of spots, reveal: the two spacings of 0.22 nm and an angle of 90° (as expected for a cubic structure) between them correspond to the (120̅ ) and (210) planes.22 The fact that, at lower temperatures, the Pd5Ga2 particles are hollow then become solid at intermediate temperatures and, when heated to higher temperatures, Pd7Ga3 species are formed hollow as well suggests that the Kirkendall effect is actually occurring twice for the Pd/Ga2O3 system: once for each of the almost-2:1 phases, while in the regime between the formation temperatures of these compounds the Pd5Ga2 nanoparticles collapse. Figure 6 shows representative HRTEM images of the Pd/ SnO2 and Pd/GeO2 systems, where the hollow species are, too,

Figure 6. HRTEM images of hollow Pd2Sn (a) and Pd2Ge (b) particles formed via the Kirkendall effect. PdGe (c), in contrast, is solid.

exclusively observed for the 2:1 compounds, whereas the solid particle is PdGe. The lattice spacing visible in (a) corresponds to the (210) planes of Pd2Sn (0.24 nm compared to 0.236 nm; note that in this image, too, the lattice fringes are visible inside the hole indicating a not thoroughly perforated particle).23 Likewise, the assignment of the Pd2Ge compound to the particle in (b) was performed using the measured 0.23, which relates to the (120) planes (0.221 nm).24 For the tin and germanium variants, only one phase with a composition of 2:1 17814



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Figure 8. HAADF images of Pd/GeO2 (reduction at 873 K) show Zcontrast within the particles (a). (b) The particle along which the EDX line profile in (c) was measured. The plot shows a decrease of Pd intensity along with the HAADF signal, while the Ge signal remains relatively constant.

are present in this sample, this is the most likely reason for this particular contrast. Across a particle featuring a darker streak (see (b)), a line scan was performed using energy-dispersive X-ray spectroscopy (EDXS). The HAADF signal, the Pd L line, as well as the Ge K line are plotted in (c). The palladium signal follows the HAADF intensity rather closely, also showing a drop in intensity at the same spot. The germanium signal does, however, not change significantly across the darker region. Thus, this area corresponds to a Pd-deficient phase, which is in good agreement with the present phases and the miscibility gap in between, implying that the dark region correlates to PdGe and the light one to Pd2Ge. Formation Mechanism. Figure 9 shows the general scheme by which the voids are possibly formed: the palladium particle reacts with the second metal X (X = Ga, Ge, or Sn) that

Figure 7. Statistics for Pd/SnO2 (a), Pd/Ga2O3 (b), and Pd/GeO2 (c). In each plot, the mean values (taken from a Gauss fit of the corresponding histograms except for the fraction of hollow particles) of the particle size, hole size, and minimum shell thickness (distance between the inner and outer surface of the particle) were plotted as a function of temperature. Additionally, the present phases (written on top of the graphs) are color-coded via the background color.

is the highest for any system. The temperature dependence of the hole size correlates nicely with the hole fraction, and at 873 K, no holes are found anymore, despite the diffraction patterns revealing the simultaneous existence of Pd2Ge (besides PdGe). The corresponding bright-field images of Figure 2, however, show that the particles do not appear homogeneous; rather, some form of contrast within them is visible. To investigate this phenomenon more closely, HAADF imaging was additionally performed (see Figure 8(a)). The inhomogeneities are also visible, confirming that some contribution of Z-contrast is present. The brighter areas of the particles hence have to be correspondingly enriched in Pd compared to the darker ones. As the phase diagram of Pd/Ge exhibits a miscibility gap between Pd2Ge and PdGe,26 that is, the two compounds that

Figure 9. Hollow particles are prepared due to the difference in diffusivities of the involved species, leaving vacancies behind. The thickness and length of the arrows corresponds to the relative values of the diffusion coefficients. If the temperature is high enough, the relative diffusivities are reversed (either due to the formation of Pddepleted phases or due to the metastability of hollow nanoparticles), and the void is filled again. 17815



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is produced by reduction of the supporting oxide to form Pd2X,15,16 whereafter an initial, thin shell of the product is formed4the Pd atoms diffuse faster outward than the X species can diffuse inward through the thin shell, thus leaving behind vacancies in the lattice of the particle. There are, in principle, two ways the system can proceed, depending on the magnitude of self-diffusivity in the particle, which is equivalent to the mobility of vacancies: if their mobility is low, the vacancies coalesce to many small voids at the Pd/Pd2X interface, yielding a Pd core that is still connected to the Pd2X shell in some places via a sort of framework that acts as a diffusion pathway. Once the reaction is finished, the core is vanished, and a centered void is left behind. This can, for instance, be seen for the CoSe particles prepared by Yin et al.2 Another possibility is that the vacancies coalesce immediately into a single void, which is the case if they have a high mobility (i.e., the self-diffusion is fast). This void is located at the Pd/ Pd2X interface (since that is where the first vacancies are generated), disrupting the Pd/Pd2X interface. Since diffusion cannot occur through a void that large, this prevents any further reaction from taking place at this location, and the Pd2X shell can only grow on the opposite side of the particle, where there is still a Pd/Pd2X interface present. After the reaction, during which the newly formed vacancies merge with the large void, a decentral hole (or sometimes even multiple ones) is left behind, as, for example, is the case for oxidized Ni particles.4 In our system, the second path seems to be predominant, as no particles with a core inside the void could be observed. Also, the first mechanism would yield centered holes (which are almost nonexistent in our images), while the voids generated by the second one usually are off-center, which is exactly what we observed. On the basis of these data, we also exclude a predominance of surface diffusion, which has also been discussed as a possible mechanism of void formation.27 In this case, characteristic “diffusion bridges”, along which the pore diffusion occurs, would be visible, yielding centered voids. This is clearly not the case in our systemhence, bulk diffusion of vacancies seems to predominantly prevail. The void subsequently grows, and when the temperature is high enough, the relative diffusivities reverse: X now diffuses faster inward than Pd can move outward, and the holes close. This is concurrent with the formation of PdX compounds but, in the case of Pd5Ga2, also happens due to the metastability of the hollow particles, resulting in solid Pd5Ga2 without any reaction. We can, thus, say that in Pd2X compounds Pd diffuses quicker than X does and that for PdX it is the other way around. A similar relation between composition and formation of Kirkendall voids was reported by Liu at al. for metallic thin films, as they investigated thin Cu/Ni(V)/Al layers upon SnPb solder: in the interfacial area, voids appeared as long as the formed intermetallic phase was CuSn3. As soon as that was transformed to Cu6Sn5 (i.e., a tin-richer compound), the voids disappeared.14 This is very similar to what we found for the Pd−X systems. To show that the holes are indeed voids, line scans were performed across two particles in the Pd/SnO2 system (reduction at 673 K) using a palladium map recorded via energy-filtered TEM (EFTEM) using the Pd−M edge, as shown in Figure 10(a). The hollow particle shows a clear dip in Pd intensity, while the solid one does not. The EFTEM image is shown in (b), with the insets representing bright-field images of the particles the line scans were created from. The brightfield image shows that the hollow particle actually has two

Figure 10. EFTEM line scans (a) across a hollow (orange) and a solid particle (blue) show a clear difference in intensity. The particles used for this line profile are marked in the false-color Pd map (using the Pd−M edge), where the insets show bright-field images of the same particles with the lines along which the profiles were extracted.

adjacent voids. The resolution of the EFTEM image does not seem good enough to resolve the two; the line profile, however, shows a slight increase of intensity in the center of the hole, which could be due to the transition from the first to the second void. As mentioned, nearly all holes we observed were found to be decentral, and some particles even contained multiple ones. Figure 11(a) shows a collection of representative particles from the Pd/GeO2 system. The HRTEM image shows a Pd5Ga2 sample obtained by reduction at 673 K, clearly showing two voids.

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CONCLUSION We have shown that the nanoscale Kirkendall effect can successfully be used to prepare hollow intermetallic compounds on a Pd basis. Three systems, Pd on Ga2O3, SnO2, and GeO2, clearly showed signs of this effect. A direct correlation between the composition of the intermetallic compound and the size and extent of the voids could be established: voids were exclusively observed for Pd-enriched intermetallic compounds of nearly 2:1 composition. Compounds of higher Pd content did not exhibit hollow structures. Compared to other systems reported so far, the approach followed in this contribution is a more complex one. It starts with Pd particles embedded in an oxide matrix, which needs to be reduced by hydrogen first to generate a second diffusion species (Ga, Sn, or Ge). The “Kirkendall” state, obtained by different diffusion rates of Pd and Ga, Sn, or Ge, thus is represented by hollow intermetallic compound particles supported on the oxide matrix. This, in principle, would allow for infinite transfer of the second species but opens up exciting possibilities for their potential use as, for example, catalytic materials. In this respect, it has been shown that intermetallic compounds are promising methanol steam reforming catalysts, but only if they form a synergistic 17816



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Figure 11. (a) Particles with off-centered or multiple voids, corroborating that the vacancies have a high mobility within the Pd2X compounds. (b) HRTEM image of a Pd5Ga2 particle (reduction at 673 K) with two voids. Here, again, the lattice can be seen inside the voids, suggesting that the holes do not extend through the particle in total.

intermetallic-oxide interface by contact with the supporting oxide, in turn enabling efficient water activation for high CO2 selectivity.28 It would, therefore, be tempting to use the oxidesupported hollow intermetallic nanostructures as new catalysts and to determine if any substantial enhancement of catalytic activity could be achieved.

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ASSOCIATED CONTENT

S Supporting Information *

Additional SAED patterns and detailed listings of the assigned diffraction spots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +43 (0)512 507 58003. Fax: +43 (0)512 507 58199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank M. Kogler for her assistance in the reduction of the samples. This work was financially supported by the Austrian Science Fund (FWF) through grant F4503-N16 and has been performed within the framework of the Forschungsplattform Materials and Nanoscience.

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REFERENCES

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The Nanoscale Kirkendall Effect in Pd-Based Intermetallic Phases

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