Trimetallic PtPdRu Dendritic Nanocages with Three-Dimensional

Article pubs.acs.org/JPCC

Trimetallic PtPdRu Dendritic Nanocages with Three-Dimensional Electrocatalytic Surfaces Kamel Eid,†,‡ Hongjing Wang,† Victor Malgras,§ Zeid A. Alothman,# Yusuke Yamauchi,§,# and Liang Wang*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P.R. China § World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan # Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia S Supporting Information *

ABSTRACT: Control over composition and structure on the nanoscale level is critical for designing highly active and durable catalyst to implement in electrochemical energy conversion. Herein, we report a facile strategy for an efficient synthesis of trimetallic PtPdRu dendritic nanocages with hollow cavity and porous dendritic shell by eroding the interior of the starting PtPdRu nanodendrites in acidic solution. The newly discovered trimetallic dendritic nanocages with an open-framework surface afford 3D molecular accessibility and can be used as highly active and durable catalysts for oxygen reduction reaction due to the synergetic effect derived from their unique porous structure and multimetallic composition.

1. INTRODUCTION The tailored design of porous metallic materials with both porous and metallic properties has attracted intensive interest because of its potential implementation in important applications (e.g., theranostic and catalysis).1−3 In general, porous metallic materials can be prepared through various approaches such as galvanic replacement, Kirkendall effect, dealloying, and templating.4−9 For instance, gold nanocages have been synthesized by the galvanic replacement reaction between silver nanocubes and chloroauric acid for theranostic applications.10,11 The intrinsic activity of the porous metallic materials can be finely tuned by adjusting their compositions, shapes, and pore structures.12−14 It has been reported that mesospace-stimulated optical property of gold film is highly dependent on its mesoporous structures.15 Therefore, the precise control over composition and structure can effectively enhance surface chemical activity. Porous Pt-based materials have been demonstrated as important catalysts for electrochemical energy conversion. Their porous structure effectively maximizes the accessible surface, thereby achieving a useful high mass activity of the rare and expensive Pt catalysts.16−21 For instance, mesoporous Pt nanorods and PtPd nanoparticles exhibit enhanced performance for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in comparison with nonporous Pt nanorods and Pt black, respectively.22,23 Among various porous © XXXX American Chemical Society

Pt-based materials, hollow and dendritic structures afford promising catalytic activity. Several methods have been developed for preparing such Pt-based materials.24−35 The synthesis of Pt-based dendrite enclosing a hollow cavity (i.e., dendritic Pt-based nanocage) is a promising strategy to create active catalysts where the synergetic effects derived from both the hollow cavity and porous dendritic shell play a critical role.23,25−36 We have previously demonstrated that bimetallic PtPd dendritic nanocages synthesized by selective chemical etching are active catalyst for MOR.25 Following this study, it is excepted that a further control over the composition of the dendritic Pt-based nanocages can further enhance their catalytic performances. Inspired by this idea, our target in this study is to develop an efficient method to easily synthesize hollow trimetallic PtPdRu dendritic nanocages with porous dendritic shell for ORR. The newly discovered trimetallic dendritic nanocages with an openframework surface afford 3D molecular accessibility from exteriors and interiors and can be used as a highly active and durable catalyst for ORR due to the synergetic enhancement effect derived from its porous dendritic structure and multimetallic composition.26−35 Furthermore, it has been Received: June 19, 2015 Revised: July 29, 2015

A

DOI: 10.1021/acs.jpcc.5b05867 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

hydrogen on Pt surface, and QH is the charge for Hupd adsorption. The latter parameter can be determined using

demonstrated that the rich edges and corner atoms derived from dendritic structures are highly active sites due to their abundant unsaturated atomic valencies.36 Simultaneously, the dendritic nanocages composed of three distinct metals (i.e., Pt, Pd, and Ru) afford favorable conditions for efficient catalysts due to their strong electronic and strain coupling derived from the lattice contracts of each metal.10,26−28,37−39 This coupling effect greatly facilitates the desorption of intermediates (e.g., O and OH), which is the major rate-limiting factor during ORR.10,37−39 We strongly believe that the proposed strategy is an important design for future multimetallic catalysts.

Q H = 0.5 × Q

where Q, the charge in the Hupd adsorption/desorption region, is obtained after the double-layer correction. The Koutecky−Levich equation was used to calculate the kinetic current, which was described as follows 1 1 1 = + j jk jd

(3)

where j is the measured current density and jk and jd are the kinetic and diffusion-limited current densities. Then, the kinetic current was calculated based on j × jd jk = jd − j (4)

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. RuCl3, K2PtCl4, Na2PdCl4, L-ascorbic acid (AA), poly(vinylpyrrolidone) (PVP, MW = 40 000), HClO4, and HNO3 were purchased from Beijing Chemical Reagent (Beijing, China). Commercial Pt/C catalyst was ordered from Alfa Aesar. 2.2. Synthesis of PtPdRu nanodendrites. The PtPdRu nanodendrites were prepared according to our previous report with a minor modification by using PVP to replace Pluronic F127.40 In a typical synthesis, 0.9 mL of 20 mM RuCl3, 0.9 mL of 20 mM Na2PdCl4, 1.2 mL of 20 mM K2PtCl4, and 0.01 g PVP were mixed together, followed by the rapid addition of 0.3 mL of 0.4 M AA. The reaction solution was continuously stirred for 3 h at room temperature. The product was obtained after three consecutive washing/centrifugation cycles with water. 2.3. Synthesis of PtPdRu dendritic nanocages. In brief, the PtPdRu nanodendrites were mixed with an excess amount of concentrated nitric acid, followed by continuously stirring for 4 days at room temperature. The product was collected after three consecutive washing/centrifugation cycles with water. 2.4. Characterizations. The particle size and morphology were investigated using a HITACHIH-8100 EM transmission electron microscope (TEM) with an accelerating voltage of 100 kV and a JEM-2010 TEM operating at 200 kV. 2.5. Electrochemical Investigations. Cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) measurements were performed using a CHI 832C electrochemical analyzer (Chenhua, Shanghai, China). A conventional three-electrode cell was used, including a Ag/AgCl (saturated KCl) electrode as reference electrode, a Pt wire as counter electrode, and a modified rotating glassy carbon disk electrode (RDE) as working electrode. The modified RDE was coated with 10 μg of different catalysts and dried at room temperature. Then, 3 μL of Nafion (0.05%) was coated on the surface and dried before the electrochemical experiments. ORR measurements were performed on a RRDE-3A rotation system (ALS, Japan) with modified the RDE in an O2-saturated aqueous solution with 0.1 M HClO4 at a rotation rate of 1600 rpm and a scan rate of 10 mV s−1. For the ORR durability tests, 5000 and 10 000 cycles were performed with the same rates. Current densities were normalized in reference to the geometric area of the working electrode. Specific activities were normalized by the ECSAs, and mass activities were normalized by the loading amount of metal, respectively. The electroactive surface areas (ECSA) were calculated using the following equation ECSA = Q H/m × 210

(2)

3. RESULTS AND DISCUSSION The synthetic strategy employed for synthesizing hollow trimetallic PtPdRu dendritic nanocages with porous dendritic walls was achieved by acidic treatment of the starting PtPdRu nanodendrites. In our previous work, we have prepared trimetallic PtPdRu nanodendrites and explored their catalytic activity for MOR and ORR in which triblock copolymer Pluronic F127 was used as capping agent and structure directing agent to guide the formation of the dendritic nanoparticles. 40 Following this work, we here further demonstrated that PVP was also favorable for the formation of the PtPdRu nanodendrites under the similar synthetic condition. The as-made PtPdRu nanodendrites were used as the starting material to prepare the PtPdRu dendritic nanocages for ORR in the present study. Trimetallic PtPdRu nanodendrites were first synthesized by a one-step reduction of the metallic precursors with AA in the presence of PVP. This mild reduction made the nucleation process slow and provided sufficient reaction time for the particle formation.40 During the particle growth, PVP served as both capping and structure directing agent to guide the formation of the dendritic nanoparticles by using its tertiary amine of the pyrrolidone ring and carbonyl group to adsorb onto the particle surface.21 This is a truly simple process that stands out from the traditional thermal decomposition synthesis of trimetallic nanoparticles in a high boiling organic solvent at a high temperature.41−45 The obtained product was well-dispersed nanodendrites with a narrow particle size distribution centered at 23 ± 5 nm (Figure 1a). The nanoscale elemental mapping distinctly confirmed that the nanodendrites consisted of Pt, Pd, and Ru metals (Figure 2a). From energy-dispersive X-ray spectroscopy (EDS) measurement, the atomic ratio of Pt/Pd/Ru was determined to be 1/0.4/0.27. A highly magnified TEM image of one nanoparticle further clearly showed that the particle was an assembly of 3 nm wide arms as building blocks, resulting in a porous dendritic structure (Figure 3a). The d spacing of 0.23 nm was assigned to the (111) crystal plane of Pt fcc structure. The PtPdRu nanodendrites were used as starting material for the synthesis of the hollow PtPdRu dendritic nanocages by chemical etching. The as-made dendritic nanocages with an inner cavity have a similar size distribution to the nanodendrites (Figure 1b). The architecture of their dendritic shell remains highly branched with a wall thickness of ca. 5 nm. The

(1)

where m is the loading amount of metal on the electrode, 210 μC cm−2 is the charge required for monolayer adsorption of B


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Figure 1. TEM images of PtPdRu nanodendrites before chemical etching (a) and dendritic nanocages after chemical etching (b).

corresponding elemental mapping analysis of the PtPdRu dendritic nanocages confirms a unique structure exhibiting a Ptenriched shell and an overall Pt:Pd:Ru atomic ratio of 1:0.07:0.05, suggesting that Pd and Ru are selectively dissolved in the concentrated nitric acid as the atomic ratio of the initial nanodendrites is 1:0.4:0.27 (Figure 2b). During the erosion, Pd and Ru of the starting material were preferentially dissolved, while Pt located in the shell was mostly maintained. It is noted that the crystallinity and the architecture of the nanocages remained stable despite the highly corrosive acidic solution. This stands out from previously reported alloy or core−shell structures in which the structure was significantly altered by the acidic conditions.16−35,44,45 In the dendritic nanocages, the three metals merged together and did not show any sign of segregation, thereby achieving a stable atomic structure. The porous architecture of the dendritic nanocages with open-framework surface affords 3D molecular accessibility from both exteriors and interiors, which is highly favorable for maximizing the surface area. The N2 adsorption−desorption isotherm analysis of the PtPdRu dendritic nanocages gave a high surface area (179 m2 g−1), which was 1.9 times higher than the PtPdRu nanodendrites (94.5 m2 g−1), implying that the cavity inside the nanoparticles dramatically improves the molecular accessibility (Figure S1). This is critical for achieving a high mass activity during catalysis. The obtained surface area is higher than the highest surface area reported for free-standing

Figure 2. HAADF-STEM-EDS elemental mapping images of PtPdRu nanodendrites before chemical etching (a) and dendritic nanocages after chemical etching (b).

Pt aerogels (168 m2 g−1) as well as other reported Pt-based materials (Table S1).20 In the case of dendritic nanocages, a large hysteresis loop was observed, which is commonly assigned to cage-type mesoporous materials with large pore size. Barrett−Joyner−Halenda (BJH) curves reveal that the dendritic nanocages exhibit a narrow pore size distribution, corresponding to the hollow cavity. In comparison, the nanodendrites prior to chemical etching show a random and broad pore size distribution. This approach rationally utilizes the differences in chemical stability of Pt, Pd, and Ru in the trimetallic nanodendrites to achieve an easy synthesis of hollow nanocages with porous dendritic shell by chemical erosion. Pt is much more stable than Pd and Ru in concentrated nitric acid solution.25 The porous dendritic structure facilitates the access of the etching agent and promotes the interior erosion. The formation of PtPdRu dendritic nanocages is highly dependent on the optimized synthetic condition in the typical synthesis. It is very difficult to control the different components Pd and Ru in the dendritic nanocages, which results in irregular nanostructures. Although the metallic nanocages have been previously prepared by C


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Pt-based catalysts are effective electrode materials for both MOR (anodic reaction) and ORR (cathodic reaction) in direct methanol fuel cell (DMFC). The MOR and ORR are equally important for DMFC. As an initial demonstration, the electrocatalytic performance of the PtPdRu dendritic nanocages is investigated for ORR along with the PtPdRu nanodendrities and a commercially available Pt/C catalyst as reference benchmarks. The electrochemical active surface area (ECSA) of PtPdRu dendritic nanocages (96.8 m2 g−1) is higher than those of PtPdRu nanodendrites (64.4 m2·g−1) and Pt/C (58.2 m2·g−1) because of its unique porous structures (i.e., hollow cavity and porous dendritic shell) that provide sufficient active surfaces from both the inner and outer regions (Figure 4a). The onset potential of PtPdRu dendritic nanocages (0.73 V) is more positive than those of PtPdRu nanodendrites (0.687 V) and Pt/ C (0.68 V). The half-wave potential of PtPdRu dendritic nanocages (0.611 V) was a positive shift of 40 mV compared with that of the PtPdRu nanodendrites (0.57 mV) and a positive shift of 70 mV compared with that of Pt/C catalyst (0.53 V). These observations of an accelerated ORR kinetics suggest that the reaction is performed more easily on the PtPdRu dendritic nanocages (Figure 4b). The catalytic activities of the tested three catalysts are different, resulting in the different diffusion-limiting current values, which is consistent with the previously published papers.12,22,27,28,40 The specific activity of PtPdRu dendritic nanocages (2.9 mA cm−2) was 1.81 and 8.78 times higher than those of PtPdRu nanodendrites (1.6 mA cm−2) and Pt/C (0.33 mA cm−2), respectively. The mass activity of PtPdRu dendritic nanocages (2.61 mA μg−1) is 2.08 times higher than that of PtPdRu nanodendrites (1.25 mA μg−1) and 14.1 times higher than that of Pt/C (0.185 mA μg−1) (Figure 4c). The results clearly demonstrated that the PtPdRu dendritic nanocages showed a superior electrocatalytic performance in terms of both specific and mass activities. The mass activity of the PtPdRu dendritic nanocages for ORR was superior to those of the reported

Figure 3. Highly magnified TEM images of one PtPdRu nanodendrite (a) and one dendritic nanocage (b) as well as the corresponding Fourier-filtered lattice fringe images in the core area and in the shell area, respectively. The insets display the corresponding FFT patterns.

multiple step galvanic replacement process, the structure of both the hollow cavity and the dendritic shell were rarely controlled.23,29−32,35,36 In contrast, the sophisticated chemical etching route used in this study can easily achieve a welldefined trimetallic nanocage architecture.

Figure 4. (a) CVs and (b) ORR polarization curves of the three catalysts measured in a N2- and an O2-saturated 0.1 M HClO4 solution, respectively. (c) Comparisons of the specific and mass activities at 0.6 V. (d) Comparisons of ECSAs. D


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Figure 5. ORR polarization curves of PtPdRu nanocages (a), PtPdRu nanodendrites (b), and Pt/C (c), respectively, before and after the durability tests. (d) Comparisons of half-wave potentials.

PtFeCu nanorods (0.222 mA μg−1), PtPdBi nanowires (1.16 mA μg−1), and PtPdBi nanospheres (0.67 mA μg−1) under similar conditions.42,43 After 10 000 cycle tests, the PtPdRu dendritic nanocages preserved most of their initial ECSA (95.5%) and showed only a 3.7 mV degradation in the halfwave potential (Figures 4d and 5). As a comparison, the PtPdRu nanodendrites and the Pt/C maintained only 87 and 51% in ECSA and a degradation of 7.5 and 65 mV in the halfwave potential, respectively (Figures 4d and 5). This clearly demonstrates the superior stability of the hollow PtPdRu dendritic nanocages during ORR. The higher stability of PtPdRu dendritic nanocages than that of Pt/C is mainly ascribed to its porous open-framework structure. Its favorable structure provides a spatially accessible and stable surface that is not susceptible to particle aggregation, while the Pt/C catalyst with solid structural feature suffers inevitable loss in stability because of its susceptible particle aggregation as well as the susceptible C corrosion in acid solution. The structure and composition play important key roles for enhancing the performances. The hollow PtPdRu dendritic nanocages with porous dendritic wall not only afford 3D molecular accessible active sites from both the inner and outer surfaces but also improve the tolerance to undesirable particle agglomeration, which can lead to drastic deterioration of the active sites. Moreover, the confinement effect derived from both the hollow cavity and the dendritic shell with spatially separated branches highly increases the mass transfer of the reactive species inside the porous nanocages. The electronic and strain effects derived from the trimetallic compositions synergistically promote the catalytic activity. The electronic binding energy of Pt is lowered due to the strong interaction between the three metals, thus increasing the amount of adsorbed O2 on the surface and improving the O−O splitting at low potential.37−48 The compressive strain derived from the trimetallic lattice contract reduces the binding strength of the adsorbed intermediates on the catalyst surface (e.g., O2−, O22−,

H2O2, and so on), thus decreasing the inhibiting effect and activating the catalytic sites for the adsorption of O2.37,41−48 Therefore, the trimetallic dendritic nanocages effectively accelerated the kinetics of oxygen reduction.

4. CONCLUSION In summary, we have successfully developed an efficient route for the synthesis of the novel hollow trimetallic PtPdRu dendritic nanocages with porous dendritic shell, which are a highly active and durable catalyst for ORR due to their shape and composition effect. The developed method is very important for the facile creation of multimetallic dendritic nanocages with desired compositions and properties.

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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.jpcc.5b05867.

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Additional characterization data. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21273218). K.E. greatly appreciates the CAS-TWAS President’s Fellowship. We extend our sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding this Prolific Research Group (PRG-1436-04). E


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