Thermal Facet Healing of Concave Octahedral PtâNi Nanoparticles

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Letter

Thermal facet healing of concave octahedral Pt-Ni Nanoparticles imaged in-situ at the atomic scale: Implications for the rational synthesis of durable high performance ORR electrocatalysts Lin Gan, Marc Heggen, Chunhua Cui, and Peter Strasser ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02620 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 27, 2015

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Thermal facet healing of concave octahedral Pt-Ni Nanoparticles imaged in-situ at the atomic scale: Implications for the rational synthesis of durable high performance ORR electrocatalysts

Lin Gan1,2 , Marc Heggen3, Chunhua Cui1, Peter Strasser1,* 1

The Electrochemical Catalysis, Energy and Materials Science Laboratory, Department of Chemistry, Technical University Berlin, 10623 Berlin, Germany

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Division of Energy and Environment, Graduate School at Shenzhen, Tsinghua University, 518055 Shenzhen, China

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Ernst Ruska Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany

ABSTRACT

We performed in situ transmission electron microscopy of phase-segregated octahedral Pt-Ni alloy fuel cell nanocatalysts under thermal annealing to study their morphological stability and surface compositional evolutions. The pristine octahedral Pt-Ni nanoparticles (NPs) showed Ptrich corners/edges and slightly concave Ni-rich {111} facets. Time-resolved image series unequivocally reveal that upon being annealed up to 500 °C the Pt-rich surface atoms at the corners/edges diffused onto and subsequently covered the concave Ni-rich {111} surfaces, leading to perfectly flat Pt-rich {111} surfaces with Ni-rich subsurface layers. This was further corroborated by in-situ aberration-corrected scanning transmission electron microscopy and

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electron energy loss spectroscopy. Our results propose a feasible approach to construct shaped Pt alloy nanoparticles with Pt-rich {111} surfaces and Ni-rich subsurface layers that are expected to be catalytically active and stable for the oxygen reduction reaction, thus providing important implications for rational synthesis of durably highly active shaped Pt alloy fuel cell electrocatalysts.

KEYWORDS. Octahedral Pt-Ni Nanoparticles; Concave Nanoparticles; Surface Composition; Oxygen Reduction Reaction; Thermal Annealing; In situ TEM


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Owing to the unique catalytic properties of their {111} surfaces,1 shaped octahedral Pt-Ni nanoparticles (NPs) have been recently shown exceptional electrocatalytic activity on the oxygen reduction reaction at fuel cell cathode.2-12 One remained critical challenge is their long-term catalytic durability in fuel cell conditions. To understand the full life cycle of the octahedral Pt bimetallic alloy NPs, recent studies revealed an novel element-specific anisotropic growth, where Pt-rich phase grew first into concave hexapod structure followed by subsequent deposition of Ni-rich phase at the concave {111} surfaces.8,13 As a consequence of this, the Pt-Ni nanooctahedra exhibit unusual site-specific compositional segregations,5,9 e.g., Pt-rich corners/edges and Ni-rich {111} facets,5 which led to complex structural evolutions under the corrosive ORR conditions.5 The corrosive leaching of Pt-Ni nanooctahedra with Ni-rich {111} surfaces caused the loss of the active {111} facets, leading to drastic activity degradation.5 Tailoring the site-specific compositional segregation therefore becomes crucial for developing next generation shaped Pt alloy fuel cell electrocatalysts with durable high activity. It is well known that thermal annealing is an effective approach for tailoring the surface compositions and the associated catalytic properties of bulk metal alloys.1,14-16 Nano segregated Pt-skin surfaces formed by high temperature annealing of Pt-rich bulk alloys particularly {111} single crystal surfaces in vacuum have shown remarkable enhancement in the ORR catalytic activity.1,14,16 For shaped NPs, however, high temperature thermal annealing may induce undesired morphological instability and particle sintering, as exemplified by previous transmission electron microscopy (TEM) studies on thermal annealing of shaped Pt nanoparticles.17 Moreover, a recent environmental TEM study by Pan et al. reports that,18 under a mild thermal annealing at 200 °C in vacuum, a sandwich-like Pt-Ni octahedral NP (Pt-rich shell and core) transformed to an alloy particle with Ni-rich surface instead of nano-segregated Pt-skin


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surface, consistent with previous results based on X-ray photonic spectroscopic analysis.19 To make shaped Pt alloy nanoparticle viable practical catalysts, it is therefore imperative to probe and understand their morphological stability and surface compositional evolutions at atomic scale upon thermal annealing at different temperatures, which has still been poorly understood. In this letter, we report in situ TEM and aberration-corrected scanning TEM (STEM) combined with electron energy loss spectroscopy (EELS) study of the phase-segregated octahedral Pt-Ni NPs under thermal annealing at different temperatures to study their morphological stability and surface compositional evolutions. The pristine octahedral Pt-Ni NPs prepared by DMF-based solvothermal synthesis showed Pt-rich corners/edges and slightly concave Ni-rich {111} facets. Using aberration-corrected STEM-EELS, we further probed the variation of the intraparticle compositional distributions upon the in situ annealing. Our results reveal that the octahedral Pt-Ni NPs can largely maintain their octahedral shape during the annealing up to 500 °C, during which the Pt-rich surface atoms at the corners/edges can diffuse onto and subsequently cover the concave Ni-rich {111} surfaces, thus forming smoothed Pt-rich {111} surface layers with underlying Ni-rich near surface layers. We expect that the previously unexplored surface diffusional healing of phase-segregated octahedral Pt-Ni NPs may constitute an efficient approach for improving the catalytic activity and stability of shaped Pt bimetallic ORR electrocatalysts. The octahedral Pt-Ni NPs were prepared by dimethylformamide (DMF)-based solvothermal reduction at 120 °C and thereafter dispersed on high surface area Vulcan XC carbon support as reported previously.5 To study their morphological and compositional evolutions under thermal annealing, the supported Pt-Ni nanooctahedra were dispersed on the DENSsolutions’ consumable EMheaterchip, which was then loaded onto a DENSsolutions TEM heating holder


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for subsequent in situ thermal annealing studies. A FEI Tecnai 20 STWIN TEM was used to record the in situ TEM images, and in situ STEM-EELS analysis was conducted on a probecorrected Titan 80-300 electron microscope equipped with Gatan image filtering system (GIF, Tridiem 863). Figure 1 shows the TEM image series from an identical space region of the Pt1.5Ni octahedral NPs after annealing at different temperatures for half hour. The as-prepared Pt1.5Ni octahedral NPs (Fig. 1A) at room temperature (RT) exhibit slight particle agglomeration on the carbon support due to the absence of any deliberately added capping agent during the NP synthesis.4 Heating to 300 °C caused no obvious morphological changes (Fig. 1B). Above 400 °C, the agglomerated octahedral NPs started to sinter and gradually lost their morphological shape and rounded (see the enlarged images in Fig. S1). In contrast, separated individual octahedral NPs (highlighted in red circles in Figure 1) still maintained their shape well up to 500 °C and started to become spherical only above 600 °C. These results suggest that the particle agglomeration or coalescence does substantially exacerbate the shape instability of octahedral NPs, likely due to an enhanced surface diffusion to minimize the prominent surface curvature. To gain atomic insights into the morphological stability of individual Pt-Ni octahedral NPs, we recorded time-resolved HRTEM images of a Pt1.5Ni octahedral NP after being in situ annealed at different temperatures for half hour (Fig. 2). Special care was taken to check for and avoid electron beam induced particle changes or damages. With the incident electron beam along its zone axis, the octahedral NP showed slightly concave {111} surfaces (Fig. 2A), consistent with our previous HRTEM observations (the degree of the concavity was quite small and thus cannot be easily resolved at low magnifications).4,5 The slight concavity is likely associated with the previously reported elemental anisotropic NP growth,8 where deposition of


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Ni-rich phase on pre-formed Pt-rich hexapods did not perfectly complete in the current synthesis and thus led to the slightly concave Ni-rich {111} surfaces. Interestingly, upon annealing to 400 and 500 °C (Fig. 2B and C, respectively), the concave surfaces gradually self-healed into perfectly flat {111} facets, indicating that the Pt-rich corner atoms diffused and subsequently filled into concave {111} facets driven by decreased surface steps and thereby decreased surface energies (as schemed in Fig. 2F). As a result, perfectly shaped octahedral NP with flat Pt-rich surfaces formed (Fig. 2C). This thermally induced self-healing of the concave {111} surfaces were also observed in other particles (see more TEM images in Fig. S2) and can have important implications on their surface catalytic properties. The octahedral NPs with flat surfaces turned out to be largely stable as the temperature reached up to 600 °C (Fig. 2D), though their corners started to be round a bit. At 800 °C (Fig. 2E), massive atomic surface diffusion possibly combined with surface melting significantly accelerated and caused the octahedral NPs to spheroidize.


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Figure 1. TEM image series of the Pt1.5Ni octahedral NPs under in situ thermal annealing from RT to 800 °C. Red circles indicate separated octahedral NPs that appear stable up to 500 °C, whereas green circles indicate accelerated morphological changes of agglomerated octahedral Pt1.5Ni nanoparticles.


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Figure 2. HRTEM image series (A-E) of Pt1.5Ni nanooctahedra under in situ annealing from 200 to 800 °C. (F) Illustration of the structural change of the slightly concave Pt1.5Ni nanooctahedra under annealing up to 500 °C. Platinum-rich corner atoms diffused to and subsequently filled on the concave Ni-rich {111} facets, forming perfectly shaped octahedral NPs with flat Pt-rich {111} surfaces.

We further performed in situ aberration-corrected STEM and EELS analysis to study the effect of thermal annealing on the site-specific compositional segregation pattern. Consistent with our previous results, the pristine Pt1.5Ni nanooctahedra at RT featured an unusual Pt-rich corners/edges and Ni-rich {111} facets (Fig. 3A).5 After being annealed at 400 °C for 1h (Fig. 3


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B-D), the {111} surfaces appear Pt-richer, whereas the near surfaces maintain significant enrichment in Ni. This is consistent with our in situ HRTEM results (Fig. 2) that at 400 °C the Pt-rich surface atoms at the corners diffused to and subsequently filled in the concave Ni-rich {111} facets, leading to Pt-rich flat {111} surface and Ni-rich subsurface layers. We are aware that segregation of bulk Pt atoms to the NP surface through bulk diffusion at above 400 °C, as suggested by previous reports,16,19 may also account for the Pt enrichment of the {111} surfaces. However, the energy barrier for Pt surface diffusion is typically much lower compared to the bulk diffusion at the same temperature. As a result, the surface diffusion of the Pt-rich corner atoms onto the concave surfaces appears to be a more plausible mechanism to form smoothed Ptrich {111} surfaces and Ni-rich near surface layers, which are expected to bring in enhanced ORR activity and stability due to favorable d-band center and the associated adsorptive properties of the Pt surface sites.1 The sustained Ni enrichment at subsurface layers towards the bulk after annealing at 400 °C indicates that long-range atomic bulk diffusion at this temperature was still not significant enough to form a uniform alloy composition. This is particularly true for PtNi1.5 octahedral NPs with a higher extent of Ni-enrichment at the {111} facets (Fig. S3). Annealing at higher temperatures or longer time would probably induce more homogenous elemental distribution but may also cause undesired shape transformation, as shown by Fig. 2. Instead, thermal annealing at the intermediate-temperatures around 400-500 °C appears to an efficient approach to improve the catalytic activity and stability by forming Pt-richer {111} surfaces while maintaining the beneficial near surface Ni enrichment. Nevertheless, we are aware that the current as-prepared catalyst suffers from obvious NP agglomeration that would cause significant particle sintering/coalescence and thus lowered activity upon annealing (Fig. 1). Thus the dispersion of


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the concave octahedral NPs on the carbon support must be substantially improved before the revealed thermal facet-healing effect can effectively increase the overall activity and stability.

Figure 3. In situ STEM and EELS spectrum imaging of the element distribution within the octahedral Pt1.5Ni NPs under thermal annealing. (A) A NP oriented along near [001] zone axis before annealing. (B) The same NP after being annealed at 400 °C (B). Note that the NP rotated slightly towards [110] zone axis at 400 °C. (C, D) Selected NPs annealed at 400°C and oriented along [110] zone axis.


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In conclusion, we conducted in situ TEM thermal annealing of the octahedral Pt-Ni NPs prepared by DMF-based solvothermal reduction, which showed Pt-rich corners/edges and slightly concave Ni-rich {111} facets. Time-resolved HRTEM revealed that these Pt-Ni octahedral NPs can largely maintain the octahedral shape when being annealed up to 500 °C in vacuum, during which the Pt-rich corner atoms underwent surface diffusion onto the Ni-rich concave {111} whereas bulk diffusion processes within the NPs appeared insufficient to eliminate the long-range Ni enrichment. Consequently, flat Pt-rich {111} surfaces with Ni-rich subsurface layers formed, which is corroborated by aberration-corrected STEM and EELS elemental mapping. Our results propose a feasible synthetic approach to generate Pt-rich {111} surfaces with Ni-rich subsurface layers on Pt-Ni nanoparticles that are expected to be catalytically more active and stable.1,20 Our conclusions as to the surface diffusional healing of Pt-Ni octahedral NPs also offer a plausible atomic scale mechanism for the rational synthesis of durable shaped octahedral Pt alloy fuel cell electrocatalyst. ASSOCIATED CONTENT Supporting Information. Supplementary figures Figure S1-S3. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGMENT


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We thank the Zentraleinrichtung für Elektronenmikroskopie (Zelmi) at Technical University Berlin for its support of TEM and EDX measurements. This work was supported by Deutsche Forschungsgemeinschaft (DFG) grants STR 596/4-1 (“Pt stability”), STR 596/5-1 (“Shaped Pt bimetallics”) and HE 7192/1-1. L.G. gratefully acknowledges the support by Nature Science Foundation

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(JCYJ20140902110354242). ABBREVIATIONS NPs, nanoparticles; TEM, transmission electron microscopy; STEM, scanning transmission electron microscopy; EELS, electron energy loss spectroscopy. REFERENCES (1) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (2) Zhang, J.; Yang, H. Z.; Fang, J. Y.; Zou, S. Z. Nano Lett. 2010, 10, 638. (3) Wu, J. B.; Gross, A.; Yang, H. Nano Lett. 2011, 11, 798. (4) Cui, C.; Gan, L.; Li, H.-H.; Yu, S.-H.; Heggen, M.; Strasser, P. Nano Lett. 2012, 12, 5885. (5) Cui, C.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Nature Mater. 2013, 12, 765. (6) Choi, S.-I.; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H.-C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X.; Wang, J.; Kim, M. J.; Xia, Y. Nano Lett. 2013, 13, 3420. (7) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Science 2014, 343, 1339. (8) Gan, L.; Cui, C.; Heggen, M.; Dionigi, F.; Rudi, S.; Strasser, P. Science 2014, 346, 1502. (9) Oh, A.; Baik, H.; Choi, D. S.; Cheon, J. Y.; Kim, B.; Kim, H.; Kwon, S. J.; Joo, S. H.; Jung, Y.; Lee, K. ACS Nano 2015, 9, 2856. (10) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. Science 2015, 348, 1230. (11) Strasser, P. Science 2015, 349, 379.


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Thermal Facet Healing of Concave Octahedral PtâNi Nanoparticles

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