Article pubs.acs.org/JPCC
Size and Shape Effects of Pd@Pt Core−Shell Nanoparticles: Unique Role of Surface Contraction and Local Structural Flexibility Wei An and Ping Liu* Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: In this article, we present a density functional theory (DFT) study of nanoparticles (NPs) using a more realistic particle model, which allows us to model Pd@Pt core−shell NPs in size of 1−3 nm (number of atoms: 35−405) and shape [tetrahedron (TH); sphere-like truncated octahedron (SP)] precisely. Our results show that the size and shape have significant effects on the stability and activity of a Pd@Pt NP toward the oxygen reduction reaction (ORR). More importantly it is found for the first time that the variation in activity with particle size is shape-dependent. In addition, under the ORR conditions the adsorbate-driven structural changes on the terraces of nanoparticles can occur, which is relevant for understanding the observed activity and stability. According to our DFT calculations, the catalytic behaviors of Pd@Pt nanoparticles are associated with the surface contraction (compressive strain) and the local structural flexibility, which are strongly size- and shapedependent. Our study demonstrates the importance of modeling more realistic catalysts and in situ study under reaction conditions to draw valid conclusions for nanocatalysts.
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shape. As you will see below, considering more realistic size and shape with no structural constrain is crucial to well describe the stability and activity of NPs for the theoretical modeling. In this article, we report the size and shape effects of a Pd core−1 monolayer Pt shell (Pd@Pt) NP, which has been found as a potential catalyst for the oxygen reduction reaction (ORR).3c,d,8 The stability and ORR activity were estimated using DFT implemented in the Vienna ab initio simulation package (VASP).9 A particle model with size from 1 to 3 nm (number of atoms: 35−405) and shape of tetrahedron (TH) and sphere-like truncated octahedron (SP) was considered (Figure 1), where no geometry constraints were applied. The Pt monolayer on Pd(111) [Pt/Pd(111)] was also included as a reference for big bulk-like particles. See the Supporting Information for details. The aim of the present study is to identify the importance of using a more realistic model to describe the performance of a NP. The first issue to address is stability. What are the effects of size and shape on the stability of a NP? In Figure 2a, our calculations show an increasing stability with the growing particle size of Pd@Pt NPs, no matter what shape the NP adopts (TH or SP). Intuitively, with the increasing size, the Pd atoms in the core behave more and more like bulk Pd on one hand; on the other hand, the ratio of Pt atoms at the steps and kinks over those on the terrace decreases, though the amount of step and kink Pt atoms grows. This helps in lowering the surface energy and increasing the stability of NPs. More
he nanoparticle (NP) catalysts are essential to future sustainable energy technologies, such as fuel cells.1 It has been observed experimentally that the potential of NPs in achieving high activity, selectivity, and stability, strongly depends on local chemical composition,2 size,3 and shape.1b,d,e,4 As size and shape vary, the intrinsic catalytic properties of NPs change associated with the variation in local coordination, surface site distribution, and surface constraint and structural flexibility. However, the current experimental measurements have limited capability in deconvoluting these effects, and so in spite of the significant influence that these changes can have on the catalytic properties, they remain relatively poorly understood. As a result, NP shape and size control remains empirical and challenging. Advances in theoretical methods, in particular density functional theory (DFT), make it possible to describe the structural and catalytic properties of metal/alloy NPs. Extensive efforts have been made recently based on DFT studies for pure metal NPs5 and core−shell NPs.1f,6 However, it should be noted that either certain approximations or oversimplified models were quite often employed for saving computational cost.5d−f,6,7 The validity of such compromises to simulate the experimentally observed nanoparticles can be of debate. For instance, using the slab model to simulate a NP completely excludes the size effect. The model of subnanometer cluster on the other hand may not be able to represent for a NP, which can behave completely differently. In addition, the geometry constraints for the particle model in nanometer size ignore significant interplanar and intraplanar surface constraint as well as local structural flexibility introduced in particular at nanoscale. Finally, little attention has been paid to the particle © 2013 American Chemical Society
Received: June 11, 2013 Published: July 16, 2013 16144
dx.doi.org/10.1021/jp4057785 | J. Phys. Chem. C 2013, 117, 16144−16149
The Journal of Physical Chemistry C
Article
contraction still exists for particle size less than 3 and likely even 4 nm. The previous studies have shown that such contraction greatly affects the stability and activity of metal NPs.3c,d,10 The difference in shape also matters to stability. As shown in Figure 2a, the TH NP is always less stable than that of the SP NP in a comparable size. With the increasing of particle size, the stability of TH NPs increases less significantly than that of SP NPs. That is, SP NPs converge more rapidly than TH NPs toward the bulk surface alloy limit. This agrees well with the experimental observations that SP NPs are more easily synthesized than TH NPs, which requires special kinetically controlled procedures.8 Nevertheless, when particles grow big enough like Pt/Pd(111), both TH and SP particles should behave more like the bulk. Again, the variation in stability is associated with the surface contraction (Figure 2b). At around 1 nm size, the calculated value is −5.9% for the TH NP with respect to Pt/Pd(111) but only −4.7% for the SP NP. Eventually, the surface contraction of both TH and SP NPs decreases to the same degree as Pt/Pd(111) as the particle size increases to bulk-like Pt/Pd(111) (∞). This is aligned well with the trend observed for stability (Figure 2a), suggesting that with the particle growth the SP NPs approach more quickly to the bulk-like geometries than the TH NPs. A similar trend was also found for pure Pd TH and SP NPs,8 which suggests such a trend is not due to Pt atoms introduced in the Pd@Pt system but the distinct shape. In addition, as shown in Figure 2c, TH NPs grow more rapidly than SP NPs in size using the same amount of Pd and Pt atoms. To sustain the same stability, TH NPs require less metal atoms than that of SP NPs. In terms of stability and cost, this seems to give the advantage of TH NPs over SP NPs as electrocatalysts in fuel cells, though as shown in the following TH is not as good as SP. It should be noted that for a Pd@Pt NP the Pd/Pt ratio is not an independent factor, which strongly depends on both shape and size (Figure 2c). Now we move to the activity. How do size and shape affect the activity of a Pd@Pt NP toward the ORR? In our calculations, O-binding energy (BE-O, see Supporting Information) on the (111) terrace of a NP is used as a good “descriptor” for probing the ORR. The complex kinetics for the ORR has been extensively studied.1f,3c,7b,8,10,11 It was found that hydrogenation of O or OH is the rate-determining step on Pt-
Figure 1. Optimized Pd@Pt core−shell NPs in different sizes and shapes (blue, Pt; yellow, Pd). Top: TH. Bottom: SP.
fundamentally, our calculations found that the variation of stability with the particle size can be well explained in terms of surface contraction (Figure 2b). The surface contraction is present on all the terraces of a NP. In the present study, we only consider the (111) terrace, which is the most active facet toward the ORR.3c,4c As shown in Figure 2b, the significant surface contraction in (111) is observed for the NPs in both shapes. When the particle size increases, the contraction decreases toward that of Pt/Pd(111) (∞, Figure 2b). The surface contraction or compressed strain on Pd@Pt NPs is introduced by two factors: one is intrinsic lattice mismatch between Pd and Pt (3.955 Å for Pd and 3.984 Å for Pt in our calculation). For both Pt/Pd(111) and Pd@Pt NPs, in principle it should result in 0.7% intraplanar (Pt−Pt bond) and interplanar (Pt−Pd bond) compression of Pt atoms in the surface layer, while Pd stays intact as the bulk. The other is only associated with NPs due to the finite size effect, where not only the surface-related Pt−Pd and Pt−Pt bonds but also the Pd−Pd bonds in the core prefer to be more compressed than that of Pt/Pd(111). The latter differentiates Pd@Pt NPs from Pt/ Pd(111). When the particle size increases, the surface contraction is decreased (Figure 2b), together with an increased ratio of core Pd atoms over the surface Pt atoms (Pd/Pt ratio) and therefore a lowered surface energy or an enhanced stability (Figure 2c). We also notice that tremendous surface
Figure 2. (a) Relative stability (ES, see Supporting Information) of Pd@Pt core−shell NPs with respect to Pt/Pd(111) as a function of particle size. (b) Relative surface contraction with respect to Pt/Pd(111) (SPt, see Supporting Information) as a function of particle size. (c) Relative stability with respect to Pt/Pd(111) as a function of Pd/Pt atomic ratio, where the corresponding particle size is labled in nm. Pt/Pd(111) was labeled as “∞” to describe the big bulk-like particles and was used as a zero reference. 16145
dx.doi.org/10.1021/jp4057785 | J. Phys. Chem. C 2013, 117, 16144−16149
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the (111) terrace. As shown in Figure 4, we probe several Pt atoms along the lines across the oxygen adsorption site, aiming
based catalysts, and the turnover frequency for the ORR is well correlated with the O- or OH-binding energy. Accordingly, BEO has been used as a descriptor to scale the ORR activity of Ptbased catalysts. A surprising variation in BE-O with the particle size is observed, which is different from that of stability (Figure 3a). Previous studies have shown that the surface contraction
Figure 3. (a) O-binding energy (BE-O, see Supporting Information) at 3-fold hollow sites on the (111) terrace of TH and SP NPs as a function of particle size. (b) Relative variation with respect to Pt/ Pd(111) in (111) surface contraction (ΔSPt, see Supporting Information) before and after O adsorption as a function of particle size. Pt/Pd(111) was labeled as “∞” to describe the big bulk-like particles.
Figure 4. Calculated 5d-PDOS of individual surface Pt atoms (labeled) on the (111) terrace of 2.1 nm TH (left panel) and 1.7 nm SP (right panel) NPs before (blue) and after (red) O-binding.
likely leads a weaker BE-O on the metal surfaces compared to those with no strain or with tensile strain.1f,12 As shown in Figure 3a, it is the case for the TH NPs. Upon going from 1 to 2.6 nm, the surface contraction of the TH NP is smaller (Figure 2b). As a result, the corresponding BE-O is rapidly strengthened by 0.88 eV (Figure 3a). The further decreasing in BE-O from 2.6 nm to Pt/Pd(111) (∞) is relatively slow by only 0.11 eV (Figure 3a). However, the SP NPs behave differently. The BE-O on the (111) terrace displays a volcano-like variation with the particle size (Figure 3a). For the SP NP as small as 1 nm, the corresponding BE-O is as strong as that of Pt/Pd(111) (BE-O = 1.12 eV). This is surprising. It means that the big difference in surface contraction between the NP and the bulk surface (4.7%, Figure 2b) does not weaken the O binding as in the case of TH but stabilizes O as significantly as Pt/Pd(111). In addition, with the particle size increasing from 1 to 2.2 nm, the BE-O of the SP NP is weakened by 0.59 eV (Figure 3a), though the surface contraction is released by 0.7% (Figure 2b). Eventually when approaching to Pt/Pd(111), BE-O should be lowered down back to approximately 1.12 eV (Figure 3a) together with the decreasing surface contraction. Therefore, a volcano-like variation is expected. Due to the high computation cost for modeling the SP NP bigger than 2.2 nm, our calculations cannot locate the peak position of the volcano. The particle size corresponding to this critical point is likely between 2.2 and 5 nm according to our previous studies, showing that the Pd@Pt NPs behave more likely as the bulk with particle size bigger than 5 nm.3c,d All these behaviors for the SP NPs are not reported or expected according to the previous studies1f,3c,7b,8,10,11 and are not observed for the TH case. To understand the different behaviors of the TH and SP NPs, we calculated partial density of states (PDOS) of Pt 5d on
to understand the perturbation of O adsorption on the local structure of the (111) terrace. The big difference seen in Figure 4 is that the electronic structure of Pt atoms in TH and SP responds differently when O is adsorbed on the surface. For TH, the interaction with O does not seem to affect the PtA,D,E,F, which does not bind with O directly. The PDOS of 5d before (blue, left panel in Figure 4) and after (red, left panel in Figure 4) the adsorption is almost the same. The exceptions are Obound PtB,C. The 5d states are downshifted and stabilized, while the unoccupied states are more populated. This suggests that partial electron transfer from the Pt to O atom occurs, and Pt atoms become partially oxidized. Given that, the (111) terrace of TH NPs seems rigid. Such rigidity is also observed at the atomic level. Figure 3b plots the change in the surface contraction (ΔSPt) before and after the O adsorption as a function of particle size. For TH, we observe the release of surface contraction on the (111) terrace after interacting with O, together with the protrusion of Pt out of the surface (ΔSPt−Pd, Figure S1, Supporting Information). The smaller the particle size is, and the bigger ΔSPt and ΔSPt−Pd are. However, one can clearly see that such O-driven surface perturbation is very small for the TH NPs, where the biggest ΔSPt for the 1 nm NP is as low as 0.14%. This again demonstrates the rigidity of the TH NP. That is, for the TH particles (>1 nm) the significant surface contraction on the (111) terrace (Figure 2b) is likely to keep the surface mostly intact under the reaction condition. Accordingly, it is appropriate to interpret the Obinding activity based on the structures of the bare NPs, which have been carried out in the previous studies.1f,2b,12,13 More flexibility in local structures of the (111) terrace is observed for the SP than that of the TH. As shown in the right panel of Figure 4, the 5d of the probed Pt atoms varies after the O 16146
dx.doi.org/10.1021/jp4057785 | J. Phys. Chem. C 2013, 117, 16144−16149
The Journal of Physical Chemistry C
Article
O bindings to catalyze the ORR efficiently but still maintaining reasonably high stability during the reaction. Indeed, the previous experimental study found the superior ORR activity and stability of the sphere-like Pd@Pt NPs with the size 4−5 nm.3d,10 In conclusion, we performed DFT calculations to study the effect of size and shape on the stability and activity of Pd@Pt NPs. According to our study, at the nanoscale (
