Synergistic Effects in CNTs-PdAu/Pt Trimetallic Nanoparticles with High Electrocatalytic Activity and Stability
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We present a straightforward physical approach for synthesizing multiwalled carbon nanotubes (CNTs)-PdAu/Pt trimetallic nanoparticles (NPs), which allows predesign and control of the metal compositional ratio by simply adjusting the sputtering targets and conditions. The small-sized CNTs-PdAu/Pt NPs (~3 nm, Pd/Au/Pt ratio of 3:1:2) act as nanocatalysts for the methanol oxidation reaction (MOR), showing excellent performance with electrocatalytic peak current of 4.4 A mg Pt −1 and high stability over 7000 s. The electrocatalytic activity and stability of the PdAu/Pt trimetallic NPs are much superior to those of the corresponding Pd/Pt and Au/Pt bimetallic NPs, as well as a commercial Pt/C catalyst. Systematic investigation of the microscopic, crystalline, and electronic structure of the PdAu/Pt NPs reveals alloying and charge redistribution in the PdAu/Pt NPs, which are responsible for the promotion of the electrocatalytic performance.
KeywordsCNTs PdAu/Pt Trimetallic nanoparticles Methanol oxidation reaction Electrocatalytic activity Synergistic effects
CNTs-PdAu/Pt trimetallic nanoparticles (NPs, ~3 nm) were synthesized using a straightforward physical approach of RTILs-assisted sputtering deposition.
As a high-performance nanocatalyst for the methanol oxidation reaction (MOR), CNTs-PdAu/Pt NPs show an electrocatalytic peak current of up to 4.4 A mg Pt −1 and high stability over 7000 s, which is much superior to those of Pt-based bimetallic NPs and a commercial Pt/C catalyst. The optimal atomic ratio of Pd/Au/Pt, which has the best catalytic performance, was found to be 3:1:2.
Synergistic effects arose from charge redistribution among Pd, Au, and Pt in CNTs-PdAu/Pt NPs may be responsible for the promotion of the electrocatalytic activity.
Target-oriented design and controlled synthesis of noble-metal nanoparticles (NPs) have aroused extensive attention because of important applications in diverse fields, such as nanocatalysis [1, 2, 3], chemical sensing [4, 5], and drug delivery [6, 7]. In particular, Pt-based NPs are still the most efficient catalytic materials in clean-energy technologies such as fuel cells [8, 9, 10]. For minimizing Pt consumption and optimizing catalytic performance of Pt-based NPs, tremendous efforts have been devoted to synthesize Pt-based multi-metallic NPs because of their superior selectivity, activity, and/or stability in comparison with their monometallic counterparts [11, 12, 13, 14, 15, 16, 17, 18, 19, 20].
Synergistic effects of Pt-based bimetallic NPs (such as AuPt [15, 16, 17] and PdPt [18, 19, 20] NPs) have been well documented. However, Pt-based trimetallic NPs have not been sufficiently explored, as the presence of multiple components increases the complexity of the controlled NP synthesis and thorough characterization. In-depth probing into Pt-based trimetallic NPs could provide new insights into the correlation between the composition, structure, and catalytic properties of noble-metal nanocatalysts.
Kotaro et al. reported an electrocatalyst comprising Pt monolayers on PdAu alloy NPs, which exhibited highly durable and active catalytic performance toward the oxygen reduction reaction (ORR) . Shin et al. synthesized Au@PdPt core–shell NPs and observed better catalytic activity than bimetallic core–shell NPs toward the methanol oxidation reaction (MOR) . Zhang et al. proposed a theoretical model in which the catalytic activity of alloy-core@Pt NPs varies linearly with the alloy–core composition . Nevertheless, two great challenges remain in the experimental study of Pt-based trimetallic NPs. One is precise control of the compositional ratio of metals by chemical approaches that involve the different reduction kinetics of metallic precursors. The other is elucidation of the dominant synergistic effects in the complex ternary nanostructures [24, 25].
Room temperature ionic liquids (RTILs)-assisted sputtering is a straightforward physical approach to prepare monometallic and bimetallic NPs in an environmental-friendly and by-product-free manner [26, 27]. Various bimetallic NPs with different composition can be synthesized by varying the composition of metal targets without any chemical additives (such as NaBH4 and citric acid). For example, Au@Ag and Pd@Ag core–shell NPs, PtNi and AuPd alloy NPs have been successfully prepared by RTILs-assisted sputtering on various nanosupports, such as graphene, carbon nanotubes (CNTs), and TiO2 NPs [14, 28, 29, 30]. However, it is more challenging to prepare trimetallic NPs using sputtering due to the increased difficulty in controlling their morphology and composition.
In this report, we prepared uniform PdAu/Pt trimetallic NPs decorated on CNTs using a RTILs-assisted sputtering method. CNTs are herein used as the nanosupport, since it has been reported that the high conductivity and huge surface area of CNTs are beneficial for electron transfer and mass transport involved in the MOR [10, 31, 32]. The composition and the catalytic behavior of CNTs-PdAu/Pt NPs were controlled by simply varying the sputtering conditions. The electrocatalytic activity and stability of CNTs-PdAu/Pt NPs toward the MOR were systematically investigated and compared with corresponding Pd/Pt and Au/Pt bimetallic NPs and a commercial Pt/C catalyst. Synergistic effects in the CNTs-PdAu/Pt NPs were also discussed.
3.1 Chemicals and Materials
All chemicals were analytical. Commercial Pt/C (20 wt%) catalyst, KOH, and methanol were purchased from Alfa Aesar and used as received. The RTIL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4], purity > 99%), was purchased from Shanghai Cheng Jie Chemical and purified under vacuum for 24 h before use. CNTs with diameters of 30–50 nm were purchased from Nanjing XFNANO Materials Tech.
3.2 Preparation of CNTs-Supported NPs
3.3 Characterization Techniques
The compositional ratio of the metal components was measured using inductively coupled plasma atom emission spectroscopy (ICP-AES, Vista-MPX). The microscopic structure of the CNTs-supported NPs was characterized using high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2) and high-angle annular dark-field scanning TEM (HAADF-STEM). The crystalline structure of the CNTs and the CNTs-supported NPs was analyzed by X-ray diffraction (XRD, PANalytical Empyrean) with Cu Kα radiation (λ = 1.5418 Å). The electronic structure of the CNTs-supported NPs was probed by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) under ultra-high vacuum and X-ray absorption near-edge spectroscopy (XANES) at the Taiwan Light Source (TLS).
3.4 Electrochemical Measurements
Cyclic voltammetry (CV) and chronoamperometry (CA) measurements were taken using a CHI660E electrochemical workstation with glassy carbon (GC), Ag/AgCl, and a Pt wire as the working, reference, and counter electrodes, respectively. Prior to being coated by the nanocatalysts, the GC electrode was polished using alumina slurry, washed ultrasonically in ethanol and water, and then dried at room temperature (27 °C). The as-prepared nanocatalysts (1 mg) were dispersed in a mixture of 500 μL ethanol and 5 μL Nafion solution (5 wt%) under ultrasonication for 30 min. Afterwards, 10 μL of the suspension was coated onto the GC electrode surface. Prior to electrochemical measurements, the nanocatalysts were activated in 1 M KOH to remove any dissolved oxygen and release active sites. CV tests for the MOR were performed between –0.7 and 0.3 V (vs. Ag/AgCl) at room temperature (27 °C) in an electrolyte containing 1 M KOH and 1 M CH3OH.
4 Results and Discussion
4.1 Morphology, Composition, and Structure of CNTs-PdAu/Pt NPs
Elemental content of Pd, Au, and Pt in CNTs-supported nanocatalysts as determined by ICP-AES measurements
Pd (mg L−1)
Au (mg L−1)
Pt (mg L−1)
To further probe the electronic structure of Au, Fig. 4b shows the Au L 3-edge XANES spectra of the samples, with an inset of a magnified view of the white line region around 11,925 eV (2p-to-5d transition) [28, 37, 40, 41]. The normalized white line intensity reflects the density of Au d-band holes, and a higher intensity corresponds to more d-band holes . The XANES spectra indicate that Au–Pt interactions increase the Au white line intensity (Au d-band holes increase due to electron transfer from Au to Pt), while Au–Pd interactions do the contrary (Au d-hole depletion due to electron transfer from Pd to Au). For the CNTs-supported trimetallic NPs, the Au white line intensity decreases with increase in the Pd content. The PdAu/Pt (3:1:2) NPs show the lowest white line intensity, which results from that, at this Pd/Au/Pt ratio, Au is surrounded by Pd and thus gains a number of electrons from Pd. The trend revealed by the XANES results is well consistent with the XPS analysis: electron transfer occurs from Au to Pt and from Pd to Au, and this electron redistribution is dependent on compositional ratio of the CNTs-PdAu/Pt NPs.
4.2 Catalytic Performance of CNTs-PdAu/Pt Trimetallic NPs
By combining the comprehensive characterization and catalytic testing results, we can understand the synergistic effects in the CNTs-PdAu/Pt NPs. On the one hand, alloying in the CNTs-PdAu/Pt NPs may create a number of tiny Pt ensembles, whose surfaces will act as catalytically active sites for the MOR . On the other hand, adding Au stabilizes Pt against surface oxidation, and adding Pd induces significant electron transfer from Pd to Pt. The latter effect is critical to the modification of electronic structure of Pt, where gaining electrons from Pd is expected to cause a downshift in Pt d-band center relative to the Fermi level [22, 45]. As a consequence, the reduced density of empty states in Pt d-band will weaken the interaction between Pt and the MOR intermediates (such as CO), thus suppressing the CO poisoning of the trimetallic nanocatalysts [13, 28]. This combination of the effects explains well the observation that the CNTs-supported PdAu/Pt (3:1:2) NPs, with the highest Pd content (corresponding to the most electron gain for Pt), possess the highest catalytic activity and stability toward the MOR.
A successive RTILs-assisted sputtering technique was utilized to synthesize CNTs-supported PdAu/Pt trimetallic NPs with a small size and tunable composition. With an optimal Pd/Au/Pt ratio of 3:1:2, the PdAu/Pt NPs achieve an electrocatalytic peak current of up to 4.4 A mg Pt −1 and high stability over 7000 s toward the MOR, which is much superior to those of bimetallic control samples and a commercial Pt/C catalyst. The excellent electrocatalytic performance of this ternary nanocatalyst is ascribed to the synergistic effects arising from favorable charge redistribution among the Pd, Au, and Pt ensembles. Adding Au improves the stability of the catalytically active Pt surface, and adding Pd enhances its resistance to the CO poisoning. The approach presented here offers a simple strategy to predesign and tailor the composition of CNTs-supported trimetallic NPs for catalysis and energy applications.
The authors thank the Taiwan Light Source (TLS) for providing XANES beam time. This work was supported by the National Natural Science Foundation of China (Nos. 61675143, 11661131002), the Natural Science Foundation of Jiangsu Province (No. BK20160277), the Soochow University-Western University Joint Centre for Synchrotron Radiation Research, the Collaborative Innovation Center of Suzhou Nano Science & Technology, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
- 5.T. Zheng, S. Bott, Q. Huo, Techniques for accurate sizing of gold nanoparticles using dynamic light scattering with particular application to chemical and biological sensing based on aggregate formation. ACS Appl. Mater. Interfaces 8(33), 21585–21594 (2016). doi: 10.1021/acsami.6b06903 CrossRefGoogle Scholar
- 10.B. Wu, D. Hu, Y. Kuang, B. Liu, X. Zhang, J. Chen, Functionalization of carbon nanotubes by an ionic-liquid polymer: dispersion of Pt and PtRu nanoparticles on carbon nanotubes and their electrocatalytic oxidation of methanol. Angew. Chem. Int. Ed. 48(26), 4751–4754 (2009). doi: 10.1002/anie.200900899 CrossRefGoogle Scholar
- 14.J.-B. Chang, C.-H. Liu, J. Liu, Y.-Y. Zhou, X. Gao, S.-D. Wang, Green-chemistry compatible approach to TiO2-supported PdAu bimetallic nanoparticles for solvent-free 1-phenylethanol oxidation under mild conditions. Nano-Micro Lett. 7(3), 307–315 (2015). doi: 10.1007/s40820-015-0044-6 CrossRefGoogle Scholar
- 26.H. Wender, L.F. de Oliveira, P. Migowski, A.F. Feil, E. Lissner, M.H. Prechtl, S.R. Teixeira, J. Dupont, Ionic liquid surface composition controls the size of gold nanoparticles prepared by sputtering deposition. J. Phys. Chem. C 114(27), 11764–11768 (2010). doi: 10.1021/jp102231x CrossRefGoogle Scholar
- 27.C.-H. Liu, B.-H. Mao, J. Gao, S. Zhang, X. Gao, Z. Liu, S.-T. Lee, X.-H. Sun, S.-D. Wang, Size-controllable self-assembly of metal nanoparticles on carbon nanostructures in room-temperature ionic liquids by simple sputtering deposition. Carbon 50(8), 3008–3014 (2012). doi: 10.1016/j.carbon.2012.02.086 CrossRefGoogle Scholar
- 28.C.-H. Liu, R.-H. Liu, Q.-J. Sun, J.-B. Chang, X. Gao, Y. Liu, S.-T. Lee, Z.-H. Kang, S.-D. Wang, Controlled synthesis and synergistic effects of graphene-supported PdAu bimetallic nanoparticles with tunable catalytic properties. Nanoscale 7(14), 6356–6362 (2015). doi: 10.1039/C4NR06855F CrossRefGoogle Scholar
- 37.C.-H. Liu, X.-Q. Chen, Y.-F. Hu, T.-K. Sham, Q.-J. Sun, J.-B. Chang, X. Gao, X.-H. Sun, S.-D. Wang, One-pot environmentally friendly approach toward highly catalytically active bimetal-nanoparticle-graphene hybrids. ACS Appl. Mater. Interfaces 5(11), 5072–5079 (2013). doi: 10.1021/am4008853 CrossRefGoogle Scholar
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