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Revealing Surface Elemental Composition and Dynamic Processes Involved in Facet-dependent Oxidation of Pt3Co Nanoparticles via in-situ Transmission Electron Microscopy Sheng Dai, Yusheng Hou, Masatoshi Onoue, Shuyi Zhang, Wenpei Gao, Xingxu Yan, George W. Graham, Ruqian Wu, and Xiaoqing Pan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b01325 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 8, 2017

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Revealing Surface Elemental Composition and Dynamic Processes Involved in Facet-dependent Oxidation of Pt3Co Nanoparticles via in-situ Transmission Electron Microscopy Sheng Dai1, Yusheng Hou2, Masatoshi Onoue2, Shuyi Zhang1,3, Wenpei Gao1, Xingxu Yan1, George W. Graham1,3, Ruqian Wu2, Xiaoqing Pan1, 2, * 1

Department of Chemical Engineering and Materials Science, University of

California Irvine, Irvine, CA, 92697, United States 2

Department of Physics and Astronomy, University of California Irvine, Irvine, CA,

92697, United States 3

Department of Materials Science and Engineering, University of Michigan, Ann

Arbor, MI, 48109, United States

*Corresponding author e-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Since catalytic performance of platinum-metal (Pt-M) nanoparticles is primarily determined by the chemical and structural configurations of the outermost atomic layers, detailed knowledge of the distribution of Pt and M surface atoms is crucial for the design of Pt-M electrocatalysts with optimum activity. Further, an understanding of how the surface composition and structure of electrocatalysts may be controlled by external means is useful for their efficient production. Here, we report our study of surface composition and the dynamics involved in facet-dependent oxidation of equilibrium-shaped Pt3Co nanoparticles in an initially disordered state via in-situ transmission electron microscopy and density functional calculations. In brief, using our advanced in-situ gas cell technique, evolution of the surface of the Pt3Co nanoparticles was monitored at the atomic scale during their exposure to an oxygen atmosphere at elevated temperature, and it was found that Co segregation and oxidation takes place on {111} surfaces but not on {100} surfaces.

Keyword Pt-Co nanoparticles, surface elemental distribution, facet-dependent oxidation, in-situ TEM, gas cell


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Main Text Platinum-metal (Pt-M, M = Fe, Co, Ni, etc.) nanoparticles (NPs) have ignited research attention due to their potential application as cathode electrocatalysts in proton exchange membrane (PEM) type fuel cells1-5. Compared with the traditional pure Pt catalyst, Pt-M bimetallic systems exhibit an enhanced oxygen reduction reaction (ORR) activity, with up to a 80-fold improvement, at a reduced cost of precious metal4-10. Based on theoretical simulations11-13, this increase in activity is attributed to the modification of electronic properties by the formation of surface configurations involving strained Pt shells. However, surface segregation can naturally occur in Pt-M NPs, causing the real surface configuration to deviate from the ideal Pt-shell structure. The Ni-rich {111} surfaces found on Pt-Ni nano-octahedrons14 and the Pt3Ni nanoframes created by erosion from polyhedra10, 15 provide a couple of illustrations of the anisotropic elemental distributions that can occur on Pt-M NP surfaces. Surface segregation can be accelerated under ORR conditions, which typically involve an acidic or oxidizing environment. By using density functional theory (DFT) calculations, Chen et al. predicted the segregation of 3d metals to the top {111} surfaces of Pt-M systems in the presence of oxygen16,17. Obviously, surface segregation on Pt-M catalysts plays a critical role in determining the lifetime of active electrocatalysts18,19. Until recently, detailed understanding of surface segregation in Pt-M NPs remains incomplete because the dynamic evolution is mostly deduced from post-testing or ex-situ characterization. These approaches have the following


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disadvantages: (1) Based on ex-situ characterization, only the original and final states of the Pt-M NPs can be revealed while information about the intermediate stage, concerning surface evolution, is missed. (2) Only limited knowledge can be obtained about the structure of Pt-M NPs after electrochemical testing, since their surface conditions are inevitably changed to some extent due to dissolution and roughening18,19. Therefore, in-situ transmission electron microscopy (TEM), enabling the dynamic atomic-scale observation is a powerful and indispensable tool for the investigation of the Pt-M surface evolution20, 21. Here, we present results of an in-situ TEM study of the actual time-dependent surface evolution of Pt3Co NPs in an oxidizing environment, utilizing a novel gas cell technique22-24. Initial surface elemental distributions and facet-dependent oxidation behaviors were revealed and successfully interpreted by our DFT calculations. These results may prove useful for better understanding of the catalyst durability and possible further attempts at surface engineering of Pt-M fuel cell nanocatalysts. A commercial ORR catalyst, carbon-supported Pt3Co powders (Pt3Co/C), provided by Tanaka Kikinzoku Kogyo (TKK) Co. Ltd., was used for our study. In order to create equilibrium-shaped NPs with a random distribution of Pt and Co, characteristic of the disorder state of bulk Pt3Co, the Pt3Co/C sample was pretreated by first heating to 900 °C under N2 for 1 h, then cooling rapidly. Chemical composition of the Pt-Co sample was confirmed by energy dispersive X-ray spectroscopy (EDS), which demonstrates the atomic ratio of Pt to Co is 3 to 1 (Fig. 1a). In addition, a face centered cubic (fcc) structure of the Pt3Co sample is revealed


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by X-ray diffraction (XRD), as shown in Fig. 1b. The four diffraction peaks corresponding to (111), (200), (220) and (311) planes of the fcc structure indicate the Pt3Co NPs are in a disordered phase at this stage since the pre-heating temperature was over the order-to-disorder transition temperature of Pt3Co NPs25, 26. Meanwhile, the lattice parameter of the Pt3Co sample is calculated to be 3.863Å according to peak positions in the XRD pattern. This value is consistent with the lattice constant of disordered Pt3Co NPs in published literature8,27.

Figure 1. Characterization results of the original Pt3Co nanoparticles. (a) Energy-dispersive X-ray spectrum of the Pt3Co sample. The Cu peaks in the spectrum


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are generated from the copper grid. The inset shows the corresponding elemental quantification of Pt and Co. (b) X-ray diffraction pattern of the Pt3Co/C sample. (c) HAADF-STEM image of a Pt3Co NP along the zone axis. The insets are the corresponding FFT pattern and the projection of a truncated octahedron model along the same direction. (d) Average electron energy loss spectra 1~4 collected from the black, blue, green, and red points in (c), respectively. (e) Schematic showing the Co-rich distribution on {111} surfaces on the truncated octahedral Pt3Co NP. Blue and yellow spheres represent Co and Pt atoms, respectively. Black and blue arrows illustrate the EELS set-up of the points acquired from {100} surface and {111} surface, respectively.

Aberration-corrected scanning transmission electron microscopy (AC-STEM) was utilized for detailed characterization of the Pt3Co sample. Figure 1c is one of the typical high-angle annular dark field (HAADF) images of the Pt3Co NPs. Along the zone axis, {111} and {200} planes are directly revealed in the atomic-scale STEM image. Meanwhile, the corresponding fast Fourier transform (FFT) reflects the fcc structure, in which the absence of (100) superlattice spots confirms the disordered Pt3Co phase. It is clear that a well-defined truncated octahedron shape, enclosed by the {111} and {100} facets, appears in the STEM image. The geometry of the NP is in accordance with the projection of a truncated octahedron model along the same direction (see the lower inset in Fig. 1c). To investigate the surface elemental composition, electron energy loss


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spectroscopy (EELS) was applied to the truncated octahedral Pt3Co NPs. All the NPs we selected were orientating along the zone axis, and accordingly four {111} and two {100} surface facets can be imaged edge-on. In this case, EELS signals collected from the outermost plane of the NP can truly reflect the elemental composition of the corresponding {111} or {100} surfaces, which are parallel to the incident electron beam, as illustrated in the schematic of Fig. 1e. Taking the Pt3Co NP in Fig. 1c, for example, EELS signals were collected from a series of points along the NP surface. Among these points, the black ones and red ones were from the {100} surface while the green ones and blue ones were from the {111} surface. All acquisitions were done under the same condition, and the spectra were normalized at the local thickness, which was determined by the ratio of the intensity of energy losses (up to 250 eV) to the zero-loss peak to allow mutual comparison of their intensities28, 29. As a result, the spectra in Fig. 1d illustrate an apparent difference between these surfaces. For the {111} surface, strong intensities of Co-L3,2 edges were detected in the average EELS spectra 2 (from blue points) and 3 (from green points). However, in contrast, the intensities of these two Co-L edges decrease significantly in the average spectra 1 (from black points) and 4 (from red points) from the {100} surface. This suggests that the {111} surface of Pt3Co NPs has a higher content of Co than the {100} surface. The same trend was confirmed by EDS line scan analysis at the Pt3Co NP surfaces (see Fig. S1 in the supporting information), further supporting the suggestion that Co atoms preferentially segregate to the topmost layer on the {111} surface of Pt3Co NPs, as schematically rendered in Fig. 1e.


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In order to examine surface dynamics, an in-situ experiment was performed using a ProtochipsTM Atmosphere gas cell system. The Pt3Co/C sample was heated from room temperature to 350 ˚C at a heating rate of 5 ˚C/s, in 760 Torr of pure oxygen. Figure 2a shows an in-situ HAADF-STEM image of a Pt3Co NP annealed in oxygen at 350 ˚C for only 60 seconds. It is clear that an additional atomic layer with a lower contrast has formed on the {111} surfaces while the {100} surfaces remain unchanged. To be quantitative, intensity profiles were taken along the lines marked by the blue and red arrows, respectively, corresponding to different {111} planes across the outermost {100} and {111} surfaces. Between the two intensity profiles in Fig. 2b, an obvious difference is the presence of an extra peak with a low Z contrast in profile 2. Since HAADF-STEM images reflect the atomic number Z of elements30, the newly-formed layer with lower contrast is tentatively determined to have a Co-rich composition.


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Figure 2. In-situ observation of Pt3Co nanoparticles in an oxidizing environment. (a) High resolution HAADF image revealing the formation of a Co-rich layer only on {111} surfaces of a Pt3Co NP. (b) Intensity profiles taken along the lines indicated by the blue (profile 1) and red (profile 2) arrows in (a), respectively. The intensity of the Co column is indicated by the arrow in profile 2. (c-d) False colored BF-STEM images illustrating the periodical unit cell of the oxide layer on {111} Pt3Co surfaces. (e) Projection of a CoO model along the zone axis. Blue and red spheres represent Co and O atoms, respectively.


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Figure 2c shows another twinned Pt3Co NP after oxygen annealing for more than 10 minutes. Here, the bright-field (BF) STEM image with a false color is presented to better visualize the lighter metal atoms (Co). Similar to the result in Fig. 2a, low-contrast layers are found only on the {111} surfaces of this NP. By comparing the marked green and black distance in Fig. 2c, it is obvious that the segregated layers have a larger lattice spacing than the inner part, consistent with oxygen incorporation into the Co lattice. The periodic unit cell is defined as a blue parallelogram (see Fig. 2c) according to the bi-layer Co on the {111} surface. Quantitative indexing is presented in the enlarged panel Fig. 2d, showing that the dimensions of two sides of the parallelogram are a=2.68 Å and b=3.06 Å, with an intersection angle γ=54.0˚. These parameters are in excellent agreement with those of the Co sublattice unit cell (as indicated in Fig. 2e) in the structure of CoO (along the zone axis); small deviations may arise from the lattice mismatch29 between CoO (4.27 Å) and Pt3Co (3.86 Å). The +2 valence state of Co in the oxide layers is further confirmed by EELS measurement on the oxidized Pt3Co NPs (see Fig. S2 and corresponding analysis in the supporting information). By checking more than twenty Pt3Co NPs, outermost CoO layers are always found on {111}, but not on {100} surfaces, without exception, clearly revealing that facet-dependent oxidation is taking place on the Pt3Co NPs.


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Figure 3. In-situ dynamic surface evolution on the {100} surface of Pt3Co nanoparticles in an oxidizing environment. (a-d) Sequential STEM images showing the additional Pt layer growth on the {100} surface of the oxidized Pt3Co nanoparticle. (e-f) BF-STEM images showing another example of the {100} surface evolution of the oxidized Pt3Co nanoparticle.

As we continued the in-situ oxygen annealing experiment, dynamic surface evolution was observed on the {100} surfaces of the Pt3Co NPs. One example is presented in the sequential in-situ STEM images shown in Figs. 3a-3d, which were taken under 760 Torr of oxygen at 350˚C. At the beginning time (t=0 s), the BF (Fig. 3a) and the corresponding HAADF (Fig. 3b) images demonstrate the oxidized {111} surfaces (indicated by the black arrows) and the clean {100} surface. Here, we mark out #1~#13 {200} planes across the right {111} surface of this NP for the following comparison. As the oxygen annealing went on, it is found that additional Pt atoms were attached on the top {100} surface, presumably through surface atom (or possibly


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gas-phase molecular) diffusion (as indicated by the yellow arrow in Fig. 3c). Then, a new layer (#14) gradually formed by attracting additional atoms at the time t=32 s. In addition, another example is also presented in Figs. 3e-3f. Here, sequential BF images clearly show two new {200} planes (#16 and #17) grew on the upper {100} surface in Fig. 3f after an elapsed time of additional 32 seconds, inducing small steps on {100} surface (as indicated by the white arrow in Fig. 3f). Meanwhile, no obvious change was found on the oxidized {111} surface. It should be noted that the contrast of the newly-grown {200} planes is different from that of the CoO layers on {111} surfaces. The contrast of the atomic columns in layers #16 and #17 is similar to that of the layer #15, which was the previous outermost {100} surface. This indicates the atoms of the newly-grown layers are not the same as the Co layers. Instead, the composition of the layers # 16 and #17 are closer to pure Pt. We believe the observed {100} surface evolution is a result of Pt (or gas-phase PtO2) diffusion, which is accelerated in an oxygen environment32. In previous studies, it has been suggested that Pt atoms possess a higher mobility in an oxygen environment due to the formation of volatile Pt-oxygen species32, 33. Moreover, diffusion of Pt atoms has also been observed in our in-situ measurements of Pt3Co NPs (see Fig. S3 in supporting information) and previous studies34. Therefore, such behavior can also take place on the {100} Pt3Co surfaces, which exhibit a composition close to pure Pt, resulting in structural fluctuations on the {100} surfaces, as shown in Fig. 3. In contrast, the CoO layers on the {111} surfaces block exposure of underlying Pt to the oxygen environment, and any diffusion or further


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reconstruction is essentially stopped. In order to support our experimental observations and gain more fundamental insight, first principles calculations based on the DFT were performed35, 36. For simplicity, yet without loss of generality, a special quasi-random structure (SQS)37 was used to represent the disordered Pt3Co phase and to construct the possible {100} surfaces (e.g. {100} Pt7-Co1, {100} Pt6-Co2, etc.) and {111} surfaces (e.g. {111} Pt15-Co1, {111} Pt13-Co3, etc.), as shown in Fig. S5. In these constructions, our supercells contain 8 atoms per layer for the {100} surface and 16 atoms per layer for the {111} surface (more details of the DFT calculation are provided in the supporting information). The stability of each surface can be quantitatively described by its surface energy (E) that is defined as E=

1 [ Eslab − ( nPt − 3nCo ) µ Pt − nCo EPt3Co ] 2A

(1)

where A is the area of surface, Eslab is the total energy of the slab, and nPt and nCo represent the numbers of Pt and Co atoms at each surface, respectively. µPt is the chemical potential of a Pt atom and is used as the variable. EPt3Co is the total energy of one Pt3Co unit in the disordered alloy. The calculated energies of all possible clean surfaces are plotted as a function of the chemical potential µPt, as presented by the colored solid lines in Fig. 4a. In our experiment, the chemical potential µPt should be in the range from -5.5 eV (Pt atoms from fcc bulk form) to -4.5 eV (Pt atoms from {100} surfaces). Clearly, the pure Pt {100} surface (noted as {100} Pt8-Co0, the black solid line in Fig. 4a) shows the


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lowest surface energy among all the possible {100} surface configurations in the given range of µPt. However, for the {111} case, the energy of the {111} Pt15-Co1 surface is almost the same as that of the pure Pt {111} surface (noted as {111} Pt16-Co0), as indicated by the close blue and green solid lines in Fig. 4a. It is also noticeable that either the {111} Pt-15-Co1 surface or pure Pt {111} surface possesses a much lower surface energy than the pure Pt {100} surface, indicating that {111} facets with and without Co are more preferential than the {100} facets.

Figure 4. DFT calculated surface energies of possible {100} and {111} surfaces of disordered Pt3Co alloy. (a) Dependence of the surface energy on the chemical potential of Pt (µPt). (b) Dependence of surface energy of the clean {100} and {111} surfaces on the Co atom number when the chemical potential µPt is fixed at -5.0 eV. Configuration entropy is taken into account at the temperature of 620 K.


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Moreover, the contribution of configuration entropy to Helmholtz free energy, Eentropy = k BT ln W , where kB, T and W are Boltzmann constant, temperature and

configuration number, respectively, was taken into account for a further comparison of the stability of {100} and {111} surfaces. Fig. 4b presents the free energy of {100} and {111} surfaces with a varying Co atom number in the topmost layer when µPt is fixed at -5.0 eV. Importantly, the free energy of {111} Pt15-Co1 surface becomes noticeably lower than that of the pure Pt {111} surface at 620 K, indicating that the {111} Pt15-Co1 surface is energetically favorable at this temperature, or in a wide temperature range. In contrast, the free energy of {100} surface decreases monotonically as the number of surface Co atom drops to zero. Therefore, the DFT result is consistent with our experimental finding that Co atoms prefer to segregate on the {111} surfaces but not on the {100} surfaces of Pt3Co NPs. To simulate the effect of the oxygen environment, a single oxygen atom was placed on the most stable {111} Pt15-Co1 surface configuration, as well as the {100} Pt7-Co1 surface, for comparison. The surface energy of an oxygen-covered (O-covered) surface can be determined according to E=

1 [ Eslab − ( nPt − 3nCo ) µ Pt − nCo EPt 3Co − EO 2 ] 2A

(2)

where EO2 is the total energy of a free oxygen molecule. While O2 may easily dissociate on Pt under our conditions, with and without Co in the surface layer, O atoms should bind tightly with the surface Co atoms12. As a result, the surface energy drops drastically for both orientations in Fig. 4a, where dashed lines showing the results of O-covered surfaces are noticeably below the corresponding (same-colored)


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solid lines in the given range of µPt. The energy decrease of the {111} Pt15-Co1 surface is more than that of the {100} Pt7-Co1 surface, and hence a Co atom on the {111} surface is more attractive for oxygen atoms than one on the {100} surface. This, together with the fact that segregation of Co on the {111} surface is preferred over segregation on the {100} surface, suggests that selective oxidation of Co should occur only on the {111} surface of Pt3Co NPs, as we observed in our in-situ TEM experiment. To summarize, a tendency for surface segregation on {111} surfaces and facet-dependent oxidation phenomena in equilibrium-shaped Pt3Co nanoparticles with a random distribution of Pt and Co atoms, characteristic of the disordered state of bulk Pt3Co, were observed via in-situ TEM experiment and explained by DFT calculation. It was found that Co shows a tendency for segregating to the {111} surfaces but not the {100} surfaces. Upon exposure to an oxygen atmosphere at 350 ˚C, a few CoO layers formed on the outermost {111} surfaces while the {100} surfaces resisted oxidation. These results suggest the {100} facets may play an important role in maintaining the activity of disordered Pt3Co ORR catalysts. Our findings about the real time evolution of surface composition and structure of this important example of Pt-M NPs may shed new light on matters relating to durability and surface engineering of new Pt-M fuel cell nanocatalysts.


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Acknowledgements The experimental work was supported by the National Science Foundation (grant numbers DMR-1506535 and CBET-1159240). The theoretical work was supported by the National Science Foundation (grant number DMR-1310494). Additional support was provided by the Irvine Materials Research Institute (IMRI).

Competing financial interests The authors declare no competing financial interests.

Associated Content Supporting Information Supporting Information is available free of charge on the ACS Publications website. See supporting information for more EDS and EELS characterization results (Fig S1 and Fig. S2), high mobility of Pt atoms/clusters in oxygen (Fig. S3), discussion of the electron beam effect (Fig. S4), and details of DFT calculations (Fig. S5).

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