Letter pubs.acs.org/NanoLett
Epitaxial Growth of Multimetallic Pd@PtM (M = Ni, Rh, Ru) Core− Shell Nanoplates Realized by in Situ-Produced CO from Interfacial Catalytic Reactions Yucong Yan,† Hao Shan,‡ Ge Li,† Fan Xiao,† Yingying Jiang,† Youyi Yan,§ Chuanhong Jin,† Hui Zhang,*,† Jianbo Wu,*,‡ and Deren Yang*,† †
State Key Laboratory of Silicon Materials, School of Materials Science & Engineering and Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ‡ State Key Laboratory of Metal Matrix Composites, School of Materials Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China § Department of Forensic Analytical Toxicology, West China School of Basic Science and Forensic Medicine, Sichuan University, Chengdu, Sichuan 610041, People’s Republic of China S Supporting Information *
ABSTRACT: Pt-based multimetallic core−shell nanoplates have received great attention as advanced catalysts, but the synthesis is still challenging. Here we report the synthesis of multimetallic Pd@ PtM (M = Ni, Rh, Ru) nanoplates including Pd@Pt nanoplates, in which Pt or Pt alloy shells with controlled thickness epitaxially grow on plate-like Pd seeds. The key to achieve high-quality Pt-based multimetallic nanoplates is in situ generation of CO through interfacial catalytic reactions associated with Pd nanoplates and benzyl alcohol. In addition, the accurate control in a trace amount of CO is also of great importance for conformal growth of multimetallic core−shell nanoplates. The Pd@PtNi nanoplates exhibit substantially improved activity and stability for methanol oxidation reaction (MOR) compared to the Pd@Pt nanoplates and commercial Pt catalysts due to the advantages arising from plate-like, core−shell, and alloy structures. KEYWORDS: Multimetallic core−shell nanocrystals, epitaxial growth, interfacial catalytic reactions, nanoplates, electrocatalysis
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well-defined shapes, which has received unprecedented research interest recently.8 Unfortunately, the deposition of Pt on the surface of as-preformed seeds typically adopts an island growth mode because of the thermodynamic restriction arising from high surface energy of Pt, which leads to the irregular or dendritic structures.9,10 To this end, Xia and co-workers have developed a kinetically controlled strategy to the epitaxial deposition of Pt shells with tunable thickness at atomic level on Pd nanocrystals with different shapes including cube, octahedron, icosahedron, and decahedron, showing the remarkably enhanced catalytic activities for ORR relative to the commercial Pt/C.11−14 In these syntheses, the epitaxial growth of Pt shells on as-preformed Pd nanocrystals was conducted by tuning the ratio of surface diffusion and deposition rates of Pt atoms via slowly injecting the Pt precursor into a reductive solution at an elevated temperature (e.g., 200 °C). This protocol was also extended to produce Pd@Ir and Pd@Ru octahedra and cubes with a face-centered
latinum (Pt) is a key component of the catalysts for oxygen reduction reaction (ORR) at the cathodes and methanol oxidation reaction (MOR) at the anodes in direct methanol fuel cells (DMFCs).1,2 However, the sluggish reaction kinetics of ORR often requires a high Pt loading to achieve a desirable fuel cell performance, which is severely restricted by its scarce abundance and high cost in the practical applications.3 In addition, the catalysts consisting of pure Pt are extremely vulnerable to the poisoning effect of intermediates such as CO produced in MOR.4 As such, tremendous efforts have been devoted to overcome these limitations, among which epitaxial growth of a thin Pt shell on a substitutional core with a less expensive metal to form bimetallic core−shell nanocrystals has been demonstrated as a promising strategy.5 Obviously, such core−shell structure can maximize the utilization efficiency of Pt by tuning its shell thickness, and thus reduce the Pt loading without compromising the catalytic property. Significantly, the incorporation of a core metal (e.g., Pd) in Pt-based core−shell catalysts substantially enhances the catalytic properties toward ORR and MOR due to the synergetic effect between these two distinct metals.6,7 Seed-mediated growth represents a powerful and versatile approach to the synthesis of such core−shell nanocrystals with © XXXX American Chemical Society
Received: October 29, 2016 Revised: November 22, 2016 Published: November 28, 2016 A
DOI: 10.1021/acs.nanolett.6b04524 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Morphological, structural, and compositional characterizations of the Pd@PtNi nanoplates prepared using the standard procedure. (a) TEM, (b) atomic-resolution HAADF-STEM, and (c) EDX mapping images of the planar Pd@PtNi nanoplates. (d) TEM, (e) HRTEM and HAADF-STEM, and (f) EDX mapping images of the vertically upstanding Pd@PtNi nanoplates.
their large specific surface area.30−32 For example, Xia and coworkers reported the synthesis of bimetallic Pd@Pt core−shell nanoplates with Pt shell of ∼2 nm by manipulating the reaction kinetics using citric acid as a weak reducing agent.33 However, it still remains a tremendous challenge to synthesize Pt-based multimetallic core−shell nanoplates through epitaxial growth of Pt-based alloy on a plate-like seed. This synthetic challenge mainly arises from both the difficulty in coreduction and epitaxial growth of two distinct components in the shell due to the large difference in the electronegativity and bonding energy.34,35 Here, we demonstrate a facile and general method for epitaxial growth of multimetallic Pd@PtM (M = Ni, Rh, Ru) nanoplates with tunable thickness including Pd@Pt nanoplates. This synthesis is facilitated by in situ generation of CO in a trace amount from organic catalytic reactions at the interface between the Pd nanoplates and proper solvents. Compared to the Pd@Pt nanoplates as well as the commercial Pt black and Pt/C, the Pd@PtNi nanoplates exhibit substantially enhanced activity and stability for MOR. Pd@PtM (M = Ni, Rh, Ru, or none) multimetallic nanoplates were generated by a simple solvothermal approach in benzyl alcohol (BA) containing the corresponding metal precursors and poly(vinylpyrrolidone) (PVP) in an argon atmosphere with Pd nanoplates as the seeds (see Supporting Information for details). The as-prepared Pd seeds show average edge length of ∼18 nm and thickness of ∼1.1 nm (Figure S1). Figure 1 shows morphological, structural, and compositional characterizations of Pd@PtNi multimetallic nanoplates prepared using the standard procedure and dispersed into oil phase by phase-transfer method for better observation. From the transmission electron microscopy (TEM) image in Figure 1, panel a, most of Pd@PtNi nanoplates share a hexagonal shape similar to the Pd seeds and present a good morphology uniformity after overgrowth. The population of plate-like shape was over 95% calculated from the TEM images at lower magnification in Figure S2a,b. The aberration-corrected high-angle annular dark-field scanning
cubic (fcc) structure, which hold great potential as advanced catalysts for catalytic reactions such as hydrogen generation from hydrazine.15−18 In a recent study, we have demonstrated a thermodynamically controlled approach to epitaxial growth of Pt shells on five-fold twinned Au cores by decreasing the surface energy of Pt through strong adsorption of amine group.19 These huge successes greatly inspire researchers to produce Pt-based multimetallic nanocrystals with further enhanced catalytic properties for a given reaction. Compared to bimetallic catalysts, nanocrystals consisting of multimetallic components provide an extra adjustability of the composition and structure and thus the optimal control over their catalytic properties.20,21 As a typical example, epitaxially depositing Pt-based alloys on as-preformed seeds has been regarded as the fascinating catalysts that combined both the benefits of core−shell and alloy structures.22−24 To this end, Xia and co-workers have reported the synthesis of
[email protected] core−shell octahedra by a seed-mediated coreduction approach, which shows the significantly enhanced ORR properties in terms of activity and durability owing to the optimized electronic structure of Pt arising from the incorporation of Pd as the core and the alloying of Pt with Ni in the shell.25 For MOR, alloying Pt with oxyphile metals like Cu, Ni, Rh, and Ru is a commonly used strategy to improve the catalytic properties by enhancing the tolerance of Pt toward CO poisoning according to the bifunctional mechanism.26−28 Conformal deposition of such Pt-based alloy on the as-preformed seeds to form multimetallic core−shell nanocrystals can further boost the MOR properties due to the electronic coupling between core and shell metals.29 It should be noted that the core in these Pt-based multimetallic nanocrystals is often composed of other noble metals (e.g., Pd). The utilization efficiency of noble metal in the core is usually neglected in previous literature, but it is of great importance for reducing the overall cost of the catalysts. For this purpose, noble-metal nanoplates, especially for those with thin thickness, are supposed as a promising candidate of the seeds to generate core−shell nanoplates with high utilization efficiency of both core and shell metals due to B
DOI: 10.1021/acs.nanolett.6b04524 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. TEM and HAADF-STEM-EDX mapping images for (a, d) Pd@Pt nanoplates, (b, e) Pd@PtRh nanoplates, and (c, f) Pd@PtRu nanoplates, respectively, prepared using the standard procure, except for the different metal precursors. The scale bar in top, left part in panel d is 1 nm.
TEM (HAADF-STEM) image (Figure 1b) of an individual planar Pd@PtNi nanoplate shows a hexagonal pattern with a lattice spacing of 0.134 nm, which can be indexed to {220} planes of Pd or PtNi alloy. The energy-dispersive X-ray spectroscopy (EDX) mapping image (Figure 1c) of a planar Pd@PtNi nanoplate presents a uniform distribution of Pd, Pt, and Ni elements and indicates the formation of Pd@PtNi multimetallic nanoplates. The atomic resolution HAADFSTEM images (Figure S3a,b) of the planar Pd@PtNi nanoplate show that PtNi alloy prefers to be deposited on the basal {111} planes of Pd nanoplates instead of the side faces. Interestingly, some Pd@PtNi nanoplates tend to assemble in a face-to-face way (Figure 1d and Figure S2c), which can provide the cross-sectional information on the nanoplates. The typical HRTEM image (Figure 1e upside) of a vertically upstanding Pd@PtNi nanoplate reveals the continuous lattice fringes from the Pd core to PtNi shell, which indicates the epitaxial growth of the PtNi shell on {111} planes of the Pd seed. This result is confirmed by HAADF-STEM image (Figure 1e downside), which shows a clear contrast difference between the PtNi alloy shell (∼3 atomic layers) and Pd core (∼5 atomic layers). This core−shell structure is further confirmed by EDX mapping analysis (Figure 1f) and line-scanning profile (Figure S3d) on the cross-section of the Pd@PtNi nanoplates (Figure S3c). The thickness of the multimetallic nanoplates was measured to be ∼2.61 nm with a PtNi shell of ∼0.76 nm (∼3 atomic layers), as shown in Figure S2d. The thickness of Pd@PtNi nanoplates can be tuned by simply varying the amounts of the Pt and Ni salt precursors (see Figure S4). In addition, the inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis (see Table S1) reveals that the Pt/Ni atomic ratio in the Pd@PtNi multimetallic nanoplates is 50.2:49.8, which is close to the molar ratio of the metal precursors fed in the synthesis. X-ray diffraction (XRD) pattern of the Pd@PtNi multimetallic nanoplates (Figure S5a) indicates that they are in an fcc structure with the diffraction peaks slightly shifted to higher angles as
compared to the standard data of pure Pt, which further confirms the alloying of Pt atoms with smaller Ni atoms. The method presented here can also be extended to generate other multimetallic 2D nanoplates such as Pd@Pt and Pd@ PtM (M = Rh, Ru) nanoplates by simply varying the type of the metal salt precursors in the synthesis. Typical TEM, HAADFSTEM, and EDX mapping images achieved from the planar and vertically standing nanoplates (Figure 2; Figures S6 and S7) confirm the epitaxial growth of Pt or PtM (M = Rh, Ru) alloy shells on hexagonal Pd seeds with a core−shell structure. The thickness of these three multimetallic nanoplates is measured to be ∼1.95, 2.54, and 2.59 nm for Pd@Pt, Pd@PtRh, and Pd@ PtRu nanoplates with a shell thickness of ∼0.43 (about two atomic layers), 0.72, and 0.75 nm (about three atomic layers for both), respectively. The ICP-AES data indicate that the atomic ratios of Pt/Rh and Pt/Ru in the multimetallic nanoplates are both equal to ∼1 (Table S1), which are close to the feeding molar ratios of the metal precursors. From the XRD patterns of these three multimetallic nanoplates (Figure S5b), all of the diffraction peaks can be indexed to Pd or Pt with an fcc structure. Compared to the diffraction peaks of Pd@Pt nanoplates, there are slight shifts to higher angles for the Pd@PtRh and Pd@PtRu nanoplates, which indicate the alloying of Pt atoms with smaller Rh or Ru atoms. Similarly, the thickness of Pd@PtM (M = Rh, Ru) nanoplates can be controlled by varying the amounts of the metal precursors (Figures S8 and S9). Taken together, all these results confirm the universality of the presented approach for the synthesis Ptbased multimetallic nanoplates with Pd nanoplates as the seeds. In our approach presented here, Pd nanoplates served not only as two-dimensional templates, but also as catalysts for in situ production of CO through a series of catalytic reactions of BA. According to the previous reports,36,37 these reactions catalyzed by Pd-based nanocrystals mainly include (1) dehydrogenation or hydrogenolysis of BA into benzaldehyde or toluene and (2) decarboxylation of benzaldehyde into benzene and CO. These interfacial catalytic processes could be C
DOI: 10.1021/acs.nanolett.6b04524 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a) GC−MS spectra of four residual solutions including pure BA, BA containing Pd nanoplates, BA containing Pt salt precursor, Ni salt precursor and Pd nanoplates treated by the solvothermal process, and the standard sample containing (1) benzene, (2) toluene, (3) benzaldehyde, (4) BA, and (5) benzoic acid. (b) FTIR spectra of the Pd@PtNi nanoplates prepared using the standard procedure in the absence of PVP and Pd nanoplates treated in pure BA by the solvothermal process. (c) Scheme for the epitaxial growth of Pd@PtM (M = Ni, Rh, Ru) nanoplates realized by interface catalytic reactions.
Figure 4. (a, b) Cyclic voltammograms (CV) of the Pd@PtNi nanoplates, Pd@Pt nanoplates, Pt/C, and Pt black in a mixed solution containing 0.5 M H2SO4 and 0.5 M MeOH at a scan rate of 50 mV/s for MOR normalized by surface area (CO stripping) and Pt mass, respectively. (c) Specific and mass activities at the peak position of forward curve. (d) Current−time (I−t) curves for MOR at 0.85 V (vs RHE) for 1500 s.
a typical synthesis of Pd@PtNi nanoplate, which suggests the catalytic reactions of BA facilitated by other noble metals (e.g., Pt). The presence of CO is further confirmed directly by Fourier transform-infrared (FTIR) studies on the Pd and Pd@ PtNi nanoplates treated after the solvothermal process (Figure 3b). The FTIR spectrum of Pd nanoplates shows a main vibrational peak at 1893 cm−1, which matches well with the three-fold bridge adsorption of CO on Pd {111} planes at room temperature,38 while relatively weak peaks (e.g., at 2029 cm−1)
confirmed by analysis of residual solutions in the synthesis using a gas chromatography mass spectrometry (GC−MS) instrument. From the GC−MS data (Figure 3a), three remarkable peaks associated with benzaldehyde, toluene, and benzene were detected in the presence of Pd nanoplates in comparison with only a small peak of benzaldehyde in the absence of Pd seeds, which imply the in situ production of CO by Pd catalyzed reactions. In addition, larger amounts of these three intermediates were detected from the residual solution in D
DOI: 10.1021/acs.nanolett.6b04524 Nano Lett. XXXX, XXX, XXX−XXX
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which is a key parameter to increase the activity of the catalysts. Clearly, the electronic coupling between Pd and Pt weakens the adsorption of the intermediates (e.g., CO) on Pt surface, which is also responsible for the enhancement in activity. In addition, the incorporation of the oxyphile component (e.g., Pd and Ni) into Pt-based catalysts can catalyze water to form the oxygenated species such as OHads, which favors the removal of COads intermediates strongly adsorbed on Pt sites, and thus substantially enhances their activity by inhibiting the CO poisoning effect. Moreover, the stability of these catalysts toward MOR was investigated by chronoamperometric experiments performed at 0.85 V for 1500 s. From the current−time (I−t) curves in Figure 4, panel d, the Pd@PtNi nanoplates show the higher steady current density relative to other three samples for MOR over the entire time range (1500 s), which indicates the superior durability. From TEM images of the Pd@ Pt and Pd@PtNi nanoplates after the electrochemical measurements (Figure S14), some Pd@Pt nanoplates turned into a ring-like structure, while most of Pd@PtNi nanoplates retained their shapes. As such, the enhanced catalytic durability in the Pd@PtNi nanoplates can be attributed to both better CO tolerance and their structural stability. In summary, we have demonstrated a general and powerful approach for the synthesis of Pt-based multimetallic core−shell nanoplates with tunable thickness through epitaxial growth using Pd nanoplates as the seeds. We found that the key to such epitaxial growth is in situ production of a trace amount of CO through interface catalytic reactions catalyzed by Pd nanoplates, which promotes the formation of Pt-based alloy by coreduction and dramatically decreases the surface energy of Pt-based alloy through strong adsorption. The Pd@PtNi multimetallic nanoplates show substantially enhanced catalytic performance in terms of activity and durability toward MOR compared to the benchmark catalysts such as Pd@Pt nanoplates and Pt/C due to the benefits arising from plate-like, core−shell, and alloy structures. With the success of this synthetic strategy, we can expect the appearance of plenty of novel multimetallic core− shell nanoplates with designed compositions and structures, which might offer new opportunity to construct advanced catalysts with enhanced performance.
of absorbed CO appear in the FTIR spectrum of the Pd@PtNi nanoplates.39 We believe that the in situ produced CO plays key roles in the formation of well-defined Pd@PtM (M = Ni, Rh, Ru) nanoplates from several different aspects. On the one hand, the in situ produced CO adsorbed on Pd nanoplates serve as a strong reducing agent to facilitate the coreduction of Pt and other transition metals into alloys. As such, the PtM alloy can be deposited on the surface of Pd seeds due to the consumption of the adsorbed CO during the coreduction. In addition, CO can dramatically decrease the surface energy of Pt-based alloy through strong adsorption,40 thereby promoting the epitaxial growth of Pt-based alloy on seeded Pd nanoplates. Since the PtM alloy can produce CO through the catalytic reactions as well, the layer-by-layer epitaxial growth is expected. The formation of Pd@PtM nanoplates is summarized in Figure 3, panel c. On the other hand, however, an excessive amount of CO severely inhibits the conformal epitaxial growth of PtM alloy in turn since the deposition sites on the {111} planes of Pd nanoplates are always occupied by the sufficient CO. This demonstration is supported by the synthesis of Pd nanoplates with thin thickness (
