Modulating Surface Composition and Oxygen Reduction Reaction

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Modulating Surface Composition and ORR activities of Pt-Ni Octahedral Nanoparticles by Microwave-Enhanced Surface Diffusion During Solvothermal Synthesis Yangbo Ma, Linqin Miao, Weihua Guo, Xiaozhang Yao, Fei Qin, Zhongxiang Wang, Hongda Du, Jia Li, Feiyu Kang, and Lin Gan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01602 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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Chemistry of Materials

Modulating Surface Composition and ORR activities of Pt-Ni Octahedral Nanoparticles by Microwave-Enhanced Surface Diffusion During Solvothermal Synthesis Yangbo Ma, Linqin Miao, Weihua Guo, Xiaozhang Yao, Fei Qin, Zhongxiang Wang, Hongda Du, Jia Li, Feiyu Kang, Lin Gan* Division of Energy and Environment, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China E-mail: [email protected] Abstract: Compositional segregations in shaped alloy nanoparticles can significantly affect their catalytic activity and are largely dependent on their elemental anisotropic growth and diffusion during nanoparticle synthesis. An efficient approach to control the surface segregations while keeping the nanoparticle shape are highly desired for fine-tuning their catalytic properties. Using octahedral Pt-Ni nanoparticles as a typical example, we report a new strategy to modulate the surface composition of shaped bimetallic nanoparticles by microwave-enhanced surface diffusion during solvothermal synthesis. Compared to traditional solvothermal synthesis, the application of microwave significantly promotes atomic diffusion particularly surface diffusion within the Pt-Ni octahedrons, leading to Pt segregation on the {111} facets while largely keeping the octahedral shape. The obtained segregated Pt-Ni octahedral nanoparticles performed excellent activity towards oxygen reduction reaction. The revealed microwave-enhanced surface diffusion in a liquid phase provides a new way to modulate surface compositions of bimetallic alloy nanoparticles at relatively lower temperatures compared to the widely adopted high temperature gas phase thermal annealing.

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Introduction Platinum-Nickel (Pt-Ni) alloy nanooctahedrons with exposed (111) surfaces have attracted wide attentions in recent years due to their high catalytic activity on the cathodic oxygen reduction reaction (ORR) of fuel cells. 1-7 Besides their (111) geometrical surface structure, a Pt-segregated surface and Ni-rich subsurface is also important factor accounting for the high ORR activity.1 The Pt-rich surface constitutes the catalytic active sites, whereas the Ni-rich subsurface is important to enhance the reactivity of Pt surface through a ligand/strain effect.1,6 Thermodynamically, due to different surface energies of alloying elements, Pt surface segregation is favorable in most Pt bulk alloys with transition metals.8-9 However, unusual compositional segregations have been recently reported in several Pt alloy nanopolyhedrons,10-14 showing, for instance, Pt enriched edges/corners and Ni segregated (111) facets in Pt-Ni nanooctahedron prepared by solvothermal synthesis in N, N-dimethyldeformamide (DMF) solvent.10 This peculiar segregation pattern caused the dissolution of Ni-rich (111) surface during acidic ORR electrocatalysis and thereby significant activity drop.10 Tailoring the surface composition of the Pt alloy nanooctahedron is essential for further improving its catalytic activity and stability. Using in situ heating within transmission electron microscope (TEM), it was reported that the ideal Pt-rich {111} surface and Ni-rich subsurface can be formed by the diffusion of surface Pt atoms under heating condition of around 300-400 °C.15-16 However, high-temperature annealing could inevitably cause particle sintering which may also decrease the catalytic activity. Previous work by us and other research groups have shown that it is the different element-specific anisotropic growth plus additional atomic diffusion that leads to the composition segregation in Pt alloy polyhedrons.12,

17-20

In the DMF solvothermal synthesis, Pt grew into

hexapods along firstly and then Ni deposited on the concave surface, resulting in the Pt-rich edges/corners and the Ni-rich (111) surfaces.17-18 Extending reaction time (from 16 h to 42 h) favored Pt-richer surface compositions due to Pt outward diffusion.6 For Pt-Ni nanooctahedron reduced by CO/W(CO)6, truncated octahedral Pt-rich nucleus formed firstly and then transformed to Pt@PtNi core-shell octahedron by a uniform Pt/Ni overgrowth.20 In oleylamine-based solvothermal synthesis, a Pt-rich tetrapod nucleate initially, then Ni-rich phase deposited to form a rhombic dodecahedron, and finally Pt diffuses to the icosahedral edges.19 From these studies, it is expected that composition segregation of shaped Pt alloy nanoparticles could be modulated by 2


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controlling the growth kinetics of different elements. However, direct synthesis of PtNi nanooctahedron with Pt-rich (111) surface have been never reported. In this contribution, we report the direct synthesis of segregated Pt-Ni nanooctahedra with Pt-rich {111} surfaces and Ni-rich subsurface using microwave-assisted solvothermal synthesis. Microwave synthesis has the advantages of direct energy supply, high heating efficiency and fast reaction speed, and therefore has been successfully used for nanoparticle synthesis.21-23 More importantly, we found that coupling microwave heating with solvothermal synthesis can promote enhanced surface diffusion of alloy elements during the growth of Pt-Ni octahedron, leading to Pt-rich surface and Ni-rich subsurface and as a result excellent ORR activity. Contrary to traditional gas-phase thermal annealing at high temperatures, the microwave-enhanced surface diffusion in liquid phase shown here offers a new route to modulate surface compositions of bimetallic nanoparticles at lower temperatures and thereby avoids significant particle sintering. Results and discussion Carbon-supported Pt-Ni octahedral nanoparticles (NPs) were prepared by microwave-assisted solvothermal synthesis in DMF solvent at different temperatures from 120 to 200 °C (schemed as Fig. 1a). Compared to traditional DMF-solvothermal synthesis, the introduction of microwave not only allows a rapid heating of the reaction solution to designated temperatures in 1-2 minutes but also significantly shortens the total reaction time (from 42 h to 6 h). Figure 1b shows the X-ray diffraction (XRD) patterns of the Pt-Ni NPs prepared under different temperatures. Obvious Pt-Ni alloy phase only formed until the temperature exceeds 140°C. With increasing temperature to 180-190 °C, a minor Ni phase also appears, indicating significantly accelerated reduction Ni(acac)2. As the temperature further increased to 200°C, the Ni phase disappeared and the extent of Pt-Ni alloying slightly increased, which is likely ascribed to enhanced bulk diffusion of Pt and Ni within the NPs and thereby a higher alloying capability of Pt-Ni NPs with additional Ni phase. Consistent with the accelerated reduction of Ni at higher temperatures as revealed by XRD, inductively-couple-plasma optical emission spectrometry (ICP-OES) (Fig. 1c) also shows that the Ni content gradually increases as the reaction temperature increases; above 160°C, the Pt:Ni atomic ratio approaches 30:70, which is close to the molar ratio of metal precursors. In addition, the Pt loading in the obtained carbon-supported catalysts became to be stable (ca. 11 wt%) at above 140 °C (Fig. S1). 3



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The surface composition of PtNi octahedral NPs synthesized at different temperatures were further monitored by X-ray photoelectron spectroscopy (XPS, Fig. 1d). Unlike the monotonously decreased bulk Pt atomic ratio with increasing temperature, the surface composition exhibits an abnormal fluctuation at higher temperatures, showing first decrease from 140 to 160 °C but then significant increase from 160 to 190 °C; at 200 °C, the Pt surface composition drops again. The abnormally increased Pt surface composition at 190 °C indicates the likely existence of temperature-dependent Pt surface segregation process.

Figure 1. (a) Illustration of microwave-solvothermal synthesis of octahedral Pt-Ni NPs. (b) XRD patterns of octahedral Pt-Ni NPs synthesized at different temperatures. (c) Bulk composition and (d) surface composition of Pt-Ni NPs synthesized at different temperatures by ICP-OES and XPS analysis, respectively.

To reveal the origin of the abnormal surface composition fluctuation, we further characterized the structure and surface compositions of Pt-Ni nano-octahedrons at three representative temperatures (140, 160, and 190 °C, designated as PtNi-140, PtNi-160 and PtNi-190, respectively) by using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) (Fig. 2). More TEM images are also shown in Fig. S2 and the 4


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particle size distribution analysis suggests a similar average size (ca. 6-8 nm) for all the Pt-Ni NPs synthesized above 140°C. Differently, the PtNi-140 NPs exhibit a distinct concave octahedral morphology (Fig. 2a-b), while electron energy loss spectroscopy (EELS) elemental mapping confirms that their surfaces are Pt rich (Fig. 2c). At 160°C, the NPs grew into a complete octahedron with flat {111} surfaces exposed (Fig. 2d). High-resolution STEM image of the octahedron (Fig. 2e) shows bright edges/corners and dark {111} facets, implying Pt-rich edges/corners and Ni-rich facets, which is also confirmed by EELS elemental mapping (Fig. 2f). It is these Ni-rich facets that may result in the high Ni surface composition of PtNi-160 in the XPS analysis. These results are fully consistent with previously-reported anisotropic growth mechanism and compositional segregation of the PtNi octahedron.10, 17 The concave character of the PtNi-140 NPs indicates an incomplete filling of Ni-rich phase on the pre-formed Pt-rich hexapod NPs within 6 h due to slow Ni reduction rate at the relatively lower temperature. Increasing temperature to 160 °C resulted in significantly accelerated Ni reduction, thereby leading to complete Ni-rich phase filling and accordingly both higher bulk and surface Ni composition. However, when the temperature was further increased to 190°C, STEM images and EELS elemental mapping evidence a subtle surface structure and composition change. The PtNi-190 NPs showed obviously rounded corners (Fig. 1g), in contrary with the sharp corners of the PtNi-160 NPs. Nevertheless, the octahedral shape still largely maintained, although some {100} facets appeared at the expense of a small portion of {111} facets (Fig. 2h). Importantly, the entire {111} surfaces of the octahedron become much brighter in contrast in the high-resolution STEM image (Fig. 2h), indicating significant Pt surface segregation. This is consistent with XPS result and also confirmed by EELS elemental distribution (Fig. 2i). From these results, we conclude that the surface diffusion could be significantly enhanced at 190°C, especially for the Pt atoms enriched at the corners which process lower coordination numbers and thus are more prone to diffuse. As the Pt atoms at the corners diffused onto the entire particle surface, Pt-segregated {111} surfaces and Ni-rich subsurface formed, while the original sharp corners were formed. This phenomenon is quite similar to what we previously observed during in-situ TEM heating of Pt-Ni nanooctahedrons.15 Differently, our results shown here suggest that the surface diffusional process, which usually occurred above 300-400 °C in vacuum/gas atmosphere thermal annealing, can be 5



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also achieved in the microwave-assisted liquid-phase ‘annealing’ and at a lower temperature (190 °C). By further increasing the reaction temperature to 200 °C, both surface diffusion and bulk diffusion were significantly accelerated, accounting for the increased alloying degree (Fig. 1a). As a result, the surface composition approaches to bulk composition (Pt30Ni70), while the NP shape is more close to be spherical (Fig. 2i).

Figure 2. Morphology and elemental distribution of octahedral Pt-Ni NPs synthesized at different reaction temperatures by STEM imaging and EELS elemental mapping. (a-c) PtNi-140 catalyst, showing concave octahedral/hexapod shape; (d-f) PtNi-160 catalyst, with Pt-rich sharp corners and Ni-rich facets; and (g-i) PtNi-190 catalyst showing rounded corners, Pt-rich {111} surfaces and Ni-rich subsurface.

We further tested the ORR catalytic activity of different Pt-Ni NPs synthesized at different 6


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temperatures (Fig. 3a). With increasing synthesis temperature from 120 to 190 °C, the ORR activity gradually increases. The PtNi-190 octahedral catalyst shows both higher mass activity and specific activity (normalized to the electrochemical surface area, Fig. S3) than those synthesized at other temperatures, reaching 1.6 A/mgPt and 4.45 mA/cm2 at 0.9 V/RHE, respectively. For the PtNi-140 catalyst, the lower bulk Ni content and incomplete octahedral morphology (imperfect {111} surfaces) could be the main reasons for its lower catalytic activity; for the PtNi-200 catalyst, its near-spherical NP morphology also leads to lower ORR activity. For the PtNi-160 catalyst, although they have a complete octahedral shape and a high bulk Ni content, their Ni-rich {111} facet lead to drastic dissolution and thus structural instability. After the ORR test, the PtNi-160 NPs transformed into concave octahedron/hexapods (Fig. 3c) with Ni content dropped significantly (Pt80Ni20), thus leading to inferior ORR activity which is also consistent with previous reports.10 In contrast, the PtNi-190 catalyst still largely maintains the octahedral morphology as well as higher Ni content (Pt60Ni40) after ORR test (Fig. 3d), due to the more stable Pt-rich {111} surface in the pristine NPs. Nevertheless, long-term stability test of the PtNi-190 catalyst shows substantial activity drop after 4,000 cycles in the range of 0.6-1.0V (Fig. S4). The imperfect stability could be ascribed to much smaller particle size (5 nm in edge length) of the Pt-Ni octahedron compared to previous reports and may be improved by further optimization of the particle size.24 Our results show that during the microwave solvothermal synthesis the surface diffusion of PtNi nano-octahedron can be enhanced at around 190 °C, leading to the formation of Pt-rich {111} surfaces and Ni-rich subsurface and thereby enhanced ORR activity. This renders it a novel liquid-phase ‘surface annealing’ approach compared to traditional high-temperature gas-phase thermal annealing, during which particle sintering or growth were commonly observed.15-16 We notice that the direct formation of Pt-rich surface was not observed in the Pt-Ni octahedron prepared with oleylamine/oleic acid capping agent,7, 12, 20 despite a similar synthesis temperature range (170-230 °C). On the one hand, this may be due to the strong capping agents which inhibited the diffusion of surface atoms; on the other hand, microwave may also play an indispensable role in promoting an enhanced surface diffusion, considering that microwave can be directly absorbed by surface atoms and thus provides a driving force to overcome the diffusion energy barriers. 7



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Figure 3. Electrochemical performance and morphology changes of PtNi catalyst during electrochemical testing. (a) ORR polarization curves, (b) Mass activity and specific activity. STEM results of Pt-Ni catalyst synthesized at (c) 160 °C and (d) 190°C after ORR test.

To prove the role of microwave in promoting surface diffusion, we further prepared Pt-Ni octahedral NPs by traditional solvothermal synthesis in an autoclave without microwave irradiation in comparison with the PtNi-190 catalyst prepared by microwave solvothermal synthesis under otherwise same conditions (hereafter denoted as AC-190 and MW-190, respectively). We found that, morphologically, the AC-190 catalyst are mostly concave octahedrons (Fig. 4a, b); their edges and corners are also much sharper, and there is Ni enrichment on the concave {111} surfaces, indicating the absence of significant surface diffusion without microwave. Moreover, despite similar bulk compositions, the MW-190 catalyst has a higher degree of Pt-Ni alloying and a higher Pt surface composition (Fig. 4d, e), again indicating that microwave not only increases the surface diffusion but also promotes the bulk atomic diffusion. Due to the higher alloying degree and Pt-richer surface, the MW-190 catalyst performs a higher ORR activity than the AC-190 catalyst (Fig. 4f). Taking all together, our results clearly 8


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demonstrate the beneficial role of microwave in promoting the surface diffusion at 190 °C to form Pt-segregated {111} surfaces.

Figure 4. Morphology, structure and ORR activity of octahedral Pt-Ni catalyst from traditional autoclave solvothermal synthesis (AC-190) in comparison with microwave solvothermal synthesis (MW-190) at 190 °C. (a-b) STEM images and (c) EELS mapping of Pt-Ni AC-190 NPs. (d) XRD patterns of AC-190 and MW-190 NPs. (e) Bulk compositions (from ICP-OES) and surface compositions (from XPS) of AC-190 and MW-190 NPs; (f) Comparison of ORR polarization curves; the inset plot compares the mass activity and specific activity at 0.9 V/RHE.

Conclusion We developed a microwave solvothermal method for the efficient synthesis of Pt-Ni nano-octahedron with Pt-segregated {111} surfaces through microwave-enhanced surface diffusion. Using high-resolution STEM and EELS spectrum imaging, we studied the effects of reaction temperature on the structure, surface composition and ORR catalytic activity of Pt-Ni nano-octahedrons and investigated the effect of microwave on promoting the surface diffusion process. Compared to the traditional solvothermal synthesis, the employment of microwave substantially promoted the atomic diffusion, particularly surface diffusion, between alloying 9



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elements during the solvothermal synthesis at a certain temperature (here 190 °C), enabling the direct synthesis of Pt-Ni octahedrons with Pt-rich {111} surface and Ni-rich subsurface and as a result high ORR activity. Different from the traditional thermal annealing approach, the revealed microwave-enhanced surface diffusion provides a new way to regulate the surface compositions of bimetallic NPs during liquid phase synthesis and can be conducted at relatively lower temperatures (below 200°C), thus would be of great interests for the synthesis of bimetallic NPs with tailored surface compositions.

Experimental section Chemicals. Platinum(II) acetylacetonate [Pt(acac)2] and Nickel(II) acetylacetonate [Ni(acac)2] were purchased from Alfa Aesar. Nafion solution (5 wt.%) was purchased from Sigma-Aldrich. Anhydrous ethanol and N, N-dimethylformamide (DMF) were purchased from Sinopharm. Synthesis of Pt-Ni octahedral NPs. In a typical synthesis, 0.08 mmol Pt(acac)2, 0.2 mmol Ni(acac)2, 120 mg benzoic acid, and 84 mg of Vulcan XC-72 carbon support were added to 20 mL DMF solvent. After stirring for a few minutes, the solution was transferred to a CEM quartz tube with the Activent® vessel cap and then heated in CEM microwave synthesizer (Discover SP) at various temperatures (120°C-200°C) for 6 hours. After the reaction, the mixture was centrifuged, washed by ethanol and ultrapure water for several times and then dried in oven. Traditional solvothermal synthesis of Pt-Ni NPs was conducted in 50 mL autoclave without microwave under otherwise the same condition. Material Characterization. XRD was acquired on a LYNXEYE XE-T diffracometer using Cu Kα source. X-ray photoelectron spectroscopy (XPS) was obtained using a monochromatic Al K (alpha) as an X-ray source in the PHI 5000 VersaProbe II spectrometer. TEM and STEM were conducted on a 300 kV FEI Tecnai F30 field-emission transmission electron microscope equipped with a high angle circular dark field (HAADF) detector. High resolution STEM were acquired using a spot size number of 10. Electron energy loss spectroscopy (EELS) elemental mapping were obtained on a post-installed Gatan image filter system (Quantum 965ER) with dual energy windows under scanning TEM (STEM) imaging mode with a spot size of 7. The energy resolution determined from the full width at half-maximum of zero loss peak was about 1 eV, and Pt M-edge and Ni L2,3-edge were simultaneously collected along a interested area at a dwelling time of 10


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0.1-0.2 s/pixel. Electrochemical measurements. All electrochemical characterizations were performed on an electrochemical workstation manufactured by BioLogic Science Instruments (SP-200). To prepare the working electrode, 8.0 mg carbon-supported Pt-Ni catalysts were ultrasonically dispersed in 5.0 mL mixed solution of isopropanol, ultrapure water (Millipore, 18 MΩ) and 5 w.% Nafion (DuPont) at a volume ratio of 1:4:0.02, respectively. A portion of 10 µL of the ink was piped onto 5 mm-diameter glassy carbon (GC) electrode, then dried in oven at 50 ºC for 20 min. The resulted catalyst film on the GC contains a metal loading of ca. 12 µgPt/cm2. The thin-filmed working electrode was then mounted onto a RDE system (Pine Research Instrument). A Pt wire was used as the counter electrode and a Hg/HgSO4 electrode was used as the reference electrode. To evaluate the electrochemical surface area, cyclic voltammogram (CV) was conducted by 25 cycles in N2-saturated 0.1 M HClO4 solution at 100 mV/s. The ORR activity was evaluated by liner scanning voltammetry (LSV) in O2-saturated 0.1 M HClO4 with a positive scan from 0.05 to 1.0 V/RHE at 5 mV s-1. All currents were normalized by the disk area of 0.196 cm2, and all potentials are referred to reversible hydrogen electrode (RHE) and iR-compensated.

Supporting Information ICP-OES analysis, TEM images, particle size distributions, and CV curves in N2-satruated 0.1 M HClO4 solution of different carbon supported Pt-Ni catalysts, ORR durability test of PtNi-190 catalyst. This material is available free of charge on the ACS Publications website.

Acknowledgements We thank the financial supports by Natural Science Foundation of China (NSFC) (under grant number 21573123 and 51622103), Guangdong Natural Science Foundation for Distinguished Young

Scholars

(2016A030306035),

and

Basic

Research

Program

of

Shenzhen

(JCYJ20160531194754308) in China. This work made use of the TEM facilities at the Electron Microscopy Laboratory, Materials and Devices Testing Center, Graduate School at Shenzhen, Tsinghua University.

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Graphical Abstract

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Modulating Surface Composition and Oxygen Reduction Reaction

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