Electronic Structure and Phase Stability of PdPt Nanoparticles - The

Letter pubs.acs.org/JPCL

Electronic Structure and Phase Stability of PdPt Nanoparticles Takayoshi Ishimoto*,† and Michihisa Koyama*,†,∥ †

INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

∥

S Supporting Information *

ABSTRACT: To understand the origin of the physicochemical nature of bimetallic PdPt nanoparticles, we theoretically investigated the phase stability and electronic structure employing the PdPt nanoparticles models consisting of 711 atoms (ca. 3 nm). For the Pd−Pt core−shell nanoparticle, the PdPt solid−solution phase was found to be a thermodynamically stable phase in the nanoparticle as the result of difference in surface energy of Pd and Pt nanoparticles and configurational entropy effect, while it is well known that the Pd and Pt are the immiscible combination in the bulk phase. The electronic structure of nanoparticles is conducted to find that the electron transfer occurs locally within surface and subsurface layers. In addition, the electron transfer from Pd to Pt at the interfacial layers in core−shell nanoparticles is observed, which leads to unique geometrical and electronic structure changes. Our results show a clue for the tunability of the electronic structure of nanoparticles by controlling the arrangement in the nanoparticles.

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computation costs and available computational resources into consideration. On the contrary, typical particle sizes experimentally synthesized are ∼2 nm or much larger. Because the properties of nanoparticles can largely depend on the particle size,24 it is mandatory to challenge the realistic particle size if we intend to discuss the intrinsic nature or factors governing the experimentally observed properties of nanoparticles. In this study, we investigated the phase stability of core−shell and solid−solution PdPt nanoparticles consisting of 711 atoms (ca. 3 nm) by using the density functional theory (DFT) method and massively parallel computation. The fundamental physicochemical properties of PdPt nanoparticles are also analyzed based on the geometrical and electronic structures. All DFT calculations were performed using the Vienna ab initio simulation package (VASP)25,26 with the projectoraugmented wave method.27,28 The Perdew−Burke−Ernzerhof with generalized gradient approximations29 was used for the exchange and correlation functional. The cutoff energy was set to be 400 eV. 1 × 1 × 1 k-point was sampled by the Monkhorst−Pack grid method.30 We used Bader approach31 to calculate atomic charges based on the VASP results. We first analyzed the stability of Pd−Pt and Pt−Pd core− shell and PdPt solid−solution nanoparticles. Figure 1 shows the excess energy32 with vibrational and configurational entropy corrections at 373 K (details of these definitions are given in the Supporting Information) of Pd−Pt and Pt−Pd core−shells and PdPt solid−solution structures from monometallic Pd and

etal nanoparticles are widely used in environmental applications and energy-related devices such as fuel-cell electrocatalysts, automobile exhaust gas catalysts, and hydrogen-storage materials.1−3 Alloying is a popular measure to tune the physicochemical properties; therefore, manipulation of the chemical composition and atomistic configuration as well as controlling the particle size and shape are important.4−11 In addition, it is expected that the new physicochemical properties will appear in the bimetallic solid−solution alloy nanoparticles.12 PdPt bimetallic nanoparticles have been investigated intensively.13−17 Here we note that most of the PdPt alloys reported to date form a segregated domain structure because Pd and Pt are well known to be immiscible on the atomic level, as deduced from their bulk phase diagram. Kobayashi et al. found that the Pd−Pt core−shell nanoparticle forms a homogeneously mixed PdPt solid−solution structure after the hydrogen absorption/desorption process.18 The change in the atomic level structure leads to an increase in the hydrogen absorption properties.18 On the contrary, the Pt−Pd core−shell nanoparticle retains its structure even after the same hydrogen absorption/desorption process. Many fundamental yet essential questions now arise, such as “Is the PdPt solid−solution nanoparticle thermodynamically stable or just in semistable states?” and “What is the role of hydrogen in the unique structure change?” In efforts to address these scientific questions, the firstprinciples method is one of the effective measures that can be used. To date, the electronic structure calculations for PdPt clusters have been studied intensively using particle models mostly up to ca. 150 atoms (∼1.6 nm),19−23 taking the high © XXXX American Chemical Society

Received: December 11, 2015 Accepted: February 1, 2016

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DOI: 10.1021/acs.jpclett.5b02753 J. Phys. Chem. Lett. 2016, 7, 736−740

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the Pt−Pd core−shell nanoparticles are stable for all composition studied compared with the excess energy without entropy correction (Figure S3). This result suggests that the formation of the PdPt solid−solution nanoparticles from Pt− Pd core−shell nanoparticles will be also observed by controlling the composition ratio of Pd and Pt at higher temperature condition to gain larger stabilization of configurational entropy for PdPt solid−solution nanoparticles, although it is not observed in the preceding study. In addition, the difference in stability between core−shell and solid-solution phases in PdPt nanoparticles appears to be in good agreement with experimental observations about the Pd segregation on the surface;34,35 that is, nanoparticle becomes stable when the Pd with lower surface energy occupies the surface and vice versa. To understand the structural characteristics and difference of core−shell and solid−solution PdPt nanoparticles, we analyzed the structural properties of nanoparticles after geometry optimization. Table 1 shows the average interatomic distance

Figure 1. Excess energy with entropy correction at 373 K of solid− solution and core−shell nanoparticles of Pd201Pt510, Pt405Pd306, Pd405Pt306, and Pt201Pd510 from Pd711 and Pt711 nanoparticles. Calculated values for bulk PdPt solid−solution systems (Pd0.25Pt0.75, Pd0.5Pt0.5, and Pd0.75Pt0.25) are shown as reference. Slice views of core− shell nanoparticles are shown. Green and gray balls are Pd and Pt atoms, respectively.

Table 1. Average Interatomic Distance (Å) of NearestNeighbor Atoms in Solid−solution and Core−shell Nanoparticles of Pd201Pt510, Pt405Pd306, Pd405Pt306, and Pt201Pd510 Pd201Pt510

Pt nanoparticles. All PdPt solid-solution structures were stable compared with the monometallic Pd and Pt nanoparticles. This result means that the PdPt solid−solution structure can be formed stably in the nanoparticle phase, although Pd and Pt are immiscible in bulk phase. In fact, the PdPt solid−solution system in bulk was calculated to be unstable because they showed positive excess energy in this study. The Pd−Pt core− shell nanoparticles (Pd201Pt510 and Pd405Pt306) were unstable compared with the corresponding PdPt solid-solution systems. This can be explained by the difference of surface energy between Pd and Pt monometallic nanoparticles (83.9 and 96.9 meV/Å2) because the surface energy of PdPt nanoparticles tends to become smaller with increasing the number of Pd atoms on the surface for the same composition. Compared with the excess energy without entropy correction (Figure S3), the energy difference between Pd−Pt core−shell and PdPt solid− solution nanoparticles becomes larger after entropy correction. This result indicates that the PdPt solid−solution is more stable than the Pd−Pt core−shell nanoparticle due to the configurational entropy effect with increasing temperature. The PdPt solid−solution nanoparticle is experimentally synthesized from the Pd−Pt core−shell nanoparticle. Our results indicate that the PdPt solid-−solution nanoparticles are thermodynamically stable compared with the Pd−Pt core−shell nanoparticles. It was experimentally observed that the hydrogen absorption/ desorption process leads to the formation of the PdPt solidsolution nanoparticle from the Pd−Pt core−shell nanoparticle.18 From our results showing the stability of the PdPt solid−solution nanoparticle compared with the Pd−Pt core− shell nanoparticle, it is deduced that the role of the hydrogen absorption/desorption process will not be to change the thermodynamic stability but possibly to change the kinetics of configurational change from the core−shell to solid−solution phase. This is the reason why the PdPt solid−solution nanoparticles are formed from Pd−Pt core−shell nanoparticles. Contrary to the Pd−Pt core−shell nanoparticles, the stability between Pt−Pd core−shell and PdPt solid−solution nanoparticles depends on the composition ratio of Pd and Pt, while

total core part shell part

2.782 2.788 2.777

total core part shell part

2.773 2.772 2.780

Pt405Pd306

Pd405Pt306

Solid−Solution Nanoparticles 2.778 2.778 2.778 2.779 2.746 2.750 Core−Shell Nanoparticles 2.788 2.768 2.794 2.766 2.761 2.742

Pt201Pd510 2.775 2.781 2.771 2.786 2.797 2.774

calculated for all the nearest neighbor combinations in PdPt nanoparticles (Figures S4−S6). The average interatomic distance of the total, core, and shell parts in PdPt solid− solution nanoparticles (Figures S4 and S5) is located between those of the monometallic Pd and Pt nanoparticles (Figures S7 and S8). On the contrary, we observed a superficially incomprehensible geometrical change in core−shell nanoparticles. The average interatomic distances of Pd−Pt and Pt−Pd core−shell nanoparticles in Table 1 are shorter and longer than for monometallic Pd or Pt nanoparticles (Figure S7), respectively. From the analysis of each core and shell part, we clarified that the average interatomic distances of Pd of the core and shell parts in core−shell nanoparticles (Figure S6) are shorter than the corresponding core and shell parts in the monometallic Pd nanoparticle (Figure S8). Furthermore, the average interatomic distance of Pt of the core and the shell parts in core−shell nanoparticles (Figure S6) was found to be longer than that of the corresponding core and shell parts in monometallic Pt nanoparticles (Figure S8). Apparently, the geometrical difference between the Pd and Pt parts in core− shell nanoparticles is not explained by simple shape or structural effect of the nanoparticles. We next analyzed the density of states (DOS) of PdPt nanoparticles. Figure 2 shows the band centers calculated from the total DOS and the partial DOS of Pd and Pt (Figures S9 and S10). When we look at the band centers calculated from the total DOS, the band centers for both core−shell and solid− solution nanoparticles are in an almost linear relationship with the composition; however, when we look at the band centers 737

DOI: 10.1021/acs.jpclett.5b02753 J. Phys. Chem. Lett. 2016, 7, 736−740

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Figure 2. Band center obtained from total and partial density of states in solid-solution and core−shell nanoparticles of Pd201Pt510, Pt405Pd306, Pd405Pt306, and Pt201Pd510. Filled and empty symbols are core−shell and solid−solution nanoparticles, respectively.

calculated from the partial DOS of Pd and Pt in solid−solution nanoparticles, we can see that they lie between the band center calculated from the total DOS of the solid−solution and monometallic Pd or Pt nanoparticles, respectively. Here we note that the band center for Pd and Pt in solid−solution nanoparticles does not depend on the composition. On the contrary, Pd−Pt and Pt−Pd core−shell nanoparticles show an obvious difference in the position of the band center for Pd and Pt. The band center of Pd and Pt in Pt−Pd core−shell nanoparticles (Pt405Pd306 and Pt201Pd510) shifts shallower and deeper than the band center of monometallic Pd and Pt nanoparticles, respectively. To understand further details of the electronic structure change of nanoparticles, we analyzed the atomic charges in core−shell and solid-solution nanoparticles. Figure 3 shows the average atomic charge of each layer of core−shell and solidsolution nanoparticles. Layers 1 and 2 correspond to the surface and subsurface, respectively. The subsequent layer is defined accordingly. The layer 5+ means layer 5 and the inner layer. In the monometallic Pd and Pt nanoparticles, the average atomic charges in layer 3 or the inner parts were almost zero. This result means that the core part in nanoparticles takes the homogeneous electronic structure similar to that of the bulk. The atoms in the surface are negatively charged, while the atoms in the subsurface possess the opposite charge. This result indicates that charge transfer in nanoparticles occurs locally from the subsurface to the surface atoms. The PdPt solid− solution nanoparticles show the same tendency as the monometallic systems. Pd−Pt and Pt−Pd core−shell nanoparticles show a different tendency to monometallic Pd, Pt, and PdPt solid−solution nanoparticles, as shown in Figure 3. The average atomic charge of subsurface and surface atoms in Pd405Pt306 becomes more positive and negative than monometallic Pd, Pt, and PdPt solid−solution nanoparticles. The subsurface and surface in Pd405Pt306 consist only of Pd and Pt atoms, respectively. The charge transfer from the subsurface to surface atoms is more evident at the interface between the subsurface Pd and the surface Pt. On the contrary, the average atomic charge of the subsurface and surface layers in Pt405Pd306

Figure 3. Average atomic charge of each layer in solid−solution and core−shell nanoparticles of Pd201Pt510, Pt405Pd306, Pd405Pt306, and Pt201Pd510. Large atoms in model structure of solid-solution (Pd405Pt306) correspond to the atoms in each layer. Small dots in model are not considered for the calculation of atomic charge for each layer. Slice view of each core−shell nanoparticle is shown in the corresponding result. Green and gray balls are Pd and Pt atoms, respectively.

is neutral. Therefore, we can say that those atomic charge distributions resulted from two drivers of polarization in bimetallic nanoparticles, that is, charge transfer from subsurface to surface to compensate for the lower coordination of surface atoms and from Pd to Pt. The charge-transfer direction from Pd to Pt at the interface is explained by the larger work function of Pt compared with Pd: The work function values for the Pd and Pt are 5.12 and 5.65 eV, respectively.36 As previously shown, the metal having larger work function is expected to accept electrons at the interface. A similar trend was also observed for Pd201Pt510 and Pt201Pd510, which correspond to core−shell particle models with two shell layers. The polarization in core−shell nanoparticles will explain the geometrical changes, which are different from PdPt solid− solution nanoparticles. Thus, the Pd−Pd distance in the core and shell parts of Pd−Pt and Pt−Pd core−shell nanoparticles becomes shorter due to the poorer valence electrons, which results from charge transfer to Pt. On the contrary, the Pt−Pt distance in the shell and core parts of Pd−Pt and Pt−Pd core− shell nanoparticles increases due to the richer valence electrons. Furthermore, because of the charge transfer between Pd and Pt, the surface atoms in Pd−Pt and Pt−Pd core−shell nanoparticles have different atomic charges. For example, the average atomic charge of surface Pt in Pd405Pt306 and surface Pd in Pt405Pd306 is about −0.08 and 0.01, respectively. Wang et al. reported that the monolayer of Pt in a Pd−Pt core−shell 738

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nanoparticle has the best catalytic activity for the ORR.13 The different catalytic activities of Pd−Pt and Pt−Pd core−shell nanoparticles might originate from the opposite polarization of surface atoms observed in the results of our calculations. We finally analyzed the correlation between the electronic structure of nanoparticles and the excess energy. Here the band center and atomic charge differences (details of these definitions are given in the Supporting Information) were selected as an index of the electronic structure of nanoparticles. We observed a good correlation between electronic structure and excess energy of nanoparticles (Figure S12). Thus, the results of our analyses provide a clue for the tunability of the electronic structure of nanoparticles by controlling the arrangement of elements in the nanoparticles. In summary, we theoretically investigated the phase stability of PdPt nanoparticles adopting the metal nanoparticle models of ca. 3 nm. The stability of core−shell and solid−solution PdPt nanoparticles was explained by the difference of surface energy of monometallic Pd and Pt nanoparticles and entropy effect, especially configurational entropy. The formation of PdPt solid−solution nanoparticles from Pd−Pt core−shell nanoparticles was found to be a thermodynamically driven process, not an uphill process that occurs under hydrogen absorption/ desorption under experimental conditions. This result is one example of metal nanoparticle systems having thermodynamic stability significantly different from the bulk. In addition, we clearly observed geometrical and electronic structure inhomogeneity of metal nanoparticles. The superficially incomprehensible geometrical and electronic structure changes in core−shell nanoparticles were caused by the charge transfer from Pd to Pt layers at the interface. The electronic structure calculation of realistic metal nanoparticle models is mandatory in understanding such unique thermodynamic properties and will lead to the discovery of further unique properties of metal nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02753. Technical details of the DFT calculations and modeling of nanoparticles. All data of interatomic distance analysis after geometry optimization and density of states with a complete set of DFT results. (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Authors

*M.K.: E-mail: [email protected]. Tel: +81-92-8016968. Fax: +81-92-801-6968. *T.I.: E-mail: [email protected]: +81-92-8026969. Fax: +81-92-802-6969. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The activities of INAMORI Frontier Research Center are supported by Kyocera Corporation. This work is supported by CREST, JST and “Advanced Computational Scientific Program” of the Research Institute for Information Technology, Kyushu University. 739

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