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Sierpinski gasket-like Pt–Ag octahedral alloy nanocrystals with enhanced electrocatalytic activity and stability
Nano Energy 61 (2019) 397–403
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Sierpinski gasket-like Pt–Ag octahedral alloy nanocrystals with enhanced electrocatalytic activity and stability
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Jiawei Zhanga, Huiqi Lia, Jinyu Yea, Zhenming Caoa, Jiayu Chena, Qin Kuanga,∗, Jun Zhengc, Zhaoxiong Xiea,b,∗∗ a State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China b Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, 361005, China c Institute of Physical Science and Information Technology, Anhui University, Hefei, 230601, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Alloys Electrocatalysis Platinum Nanocrystals Fractal
Architectural engineering of noble metal nanocrystals can result in structural diversity and complexity, thereby providing catalysts with multifunctional properties. Herein, a unique Sierpinski gasket-like Pt–Ag octahedral alloy nanostructure with a three-dimensional hyperbranched architecture containing abundant well-defined Ptrich {111} stable facets and dense low-coordinated active sites is reported. Unlike the well-accepted long-range limiting diffusion govened growth model for fractal structures, our success relies on creating local depletion layer near the crystal surface by surfactant. The as-prepared Sierpinski gasket-like Pt–Ag octahedral nanocrystals allows us to achieve three milestones for electrocatalysts, i.e. high catalytic activity, great durability, and stability towards the methanol oxidation reaction in acidic media. Specifically, the catalyst shows a high specific activity (6.61 mA cm−2, is superior than most of reported Pt-based catalysts), outstanding COads-poisoning tolerance and stability (the morphology and composition of the catalyst are preserved after 2000 cycles). This study points to a new approach to develop highly efficient catalysts in the form of fractal nanostructures with structural diversity and complexity to balance the delicate trade-off between activity and stability of catalysts.
1. Introduction Proton-exchange membrane fuel cells (PEMFCs) are promising and environmentally friendly renewable energy conversion devices [1–3]. Currently, platinum (Pt)-based nanomaterials are state-of-art electrocatalysts in both anode (oxidation of fuels) and cathode (reduction of oxygen) [4–7]. However, the abundance in the earth's crust of Pt is quite rare. With efforts to lower economic costs of PEMFCs, much research interest has been attracted to explore high-performance Pt-based electrocatalysts [8–14]. Ideally, a desired electrocatalyst should not only exhibit high catalytic activity, but also show great structural stability. Nonetheless, due to the incompatible structural requirements for activity and stability, how to balance the stability-activity trade-off via crystal engineering remains a challenging undertaking [15–17]. For example, for the anode catalysts of PEMFCs, fabricating high-energy surfaces (e.g., Pt {110} facets) and size shrinking have been demonstrated as efficient
approaches to increase their catalytic activity [18–20]. However, their long-time performances decay significantly due to the serious poisoning of active sites by surface-adsorbed intermediate spices (e.g., COads) during reactions [21–23]. On the other hand, constructing thermodynamically stable surface structures (e.g., {111} facets) could greatly improve the intermediate-poisoning tolerance and structural stability of Pt-based catalysts, but their catalytic activities are usually unsatisfactory [24–26]. While single-function catalysts hardly meet the standards for desired catalyst, alternative promising way out of this predicament is developing multifunctional NCs which integrate different structural advantages into one complex architecture [27–29]. In this context, fractal structures should be ideal catalyst models, and have garnered a great deal of attention due to their well-known inherent structural aesthetics and complexities [30,31]. Specifically, the hyperbranched characteristic of fractal NCs not only could endow the catalysts with large surface-to-volume ratios and high density of easily accessible low-
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Corresponding author. Corresponding author. State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. E-mail addresses: [email protected] (Q. Kuang), [email protected] (Z. Xie). ∗∗
https://doi.org/10.1016/j.nanoen.2019.04.093 Received 1 April 2019; Received in revised form 25 April 2019; Accepted 29 April 2019 Available online 30 April 2019 2211-2855/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) SEM and (b) HAADF-STEM images of Sierpinski gasket-like Pt–Ag octahedral NCs. (c, d, and e) SEM and TEM images corresponding SAED patterns and 3D models of an individual NC recorded along the [001], [111], and [011] directions, respectively. (f) HRTEM image recorded from the white dashed box in (e). (g) Schematic illustration of the shape transformation from an octahedron to a Sierpinski gasket-like octahedral structure.
of the precursor or the viscosity of solution) based on the long-range limiting diffusion path. More importantly, this work demonstrated the great potential of rationally engineering factal architectures to achieve optimal catalysts with superior activity and stability. 2. Experimental section 2.1. Chemicals Pt black was purchased from Alfa Aesar; platinum acetylacetonate (Pt(acac)2) was purchased from Kunming Institute of Precious Metals (Yun-nan, China); dodecyltrimethylammonium chloride (DTAC) (> 98%) was purchased from TCI; N, N-dimethylformamide (DMF, AR), silver nitrate (AgNO3, AR) was purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were used as received. 2.2. Synthesis of Sierpinski gasket-like Pt–Ag octahedral NCs Fig. 2. (a) XRD pattern, (b) HAADF-STEM image of the Sierpinski gasket-like Pt–Ag octahedral NCs and corresponding elemental mappings, (c) cross-sectional compositional line profile (indicated by the yellow arrow in (b)), and (d) Ag 3d and Pt 4f XPS spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
5 mg of Pt(acac)2, 0.74 mg of AgNO3, and 30 mg of DTAC were mixed in 10 mL of DMF were mixed together. Then, the homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave with a capacity of 25 mL at room temperature. Then the sealed vessel was heated to 150 °C for 6 h. After naturally cooled to room temperature, the products were collected by centrifugation, and washed several times by mixture of ethanol and water (volume ratio 1:3) to remove residual DTAC on samples.
coordinated edge/corner sites, but also promote the permeability of reactant to reach the active sites from surfaces to insides [32,33]. In addition, the fractal NCs have well-defined sophisticated shapes, which may provide the catalysts unexpected catalytic stability [34–36]. Nevertheless, the formation of fractal nanostructures is thermodynamically unfavored for crystals, which prefer to take convex polyhedral shapes to minimize the surface free energy during growth [37]. Herein, by taking the synthesis of Pt–Ag alloy NCs as an example, we report a novel fractal Sierpinski gasket-like octahedral architectures. Impressively, our successful synthesis relies on creating barrier layer near crystal surface to provide local diffusion-limited growth condition. Remarkably, this unique three-dimensional (3D) fractal architecture showed excellent catalytic activity, great COads-poisoning tolerance, as well as great structural stability towards methanol electrooxidation. Their specific activity outperform most of reported catalyst, and the morphology and composition preserve very well after 2000 cycles. It is found that a high density of corner/edge atoms resulting from the hyperbranched nature played a key role for the extraordinary catalytic activity of the catalyst, while the excellent anti-poisoning properties and structural stability were attributed to exposed Pt-rich {111} facets. The synthetic strategy presented here is entirely different from general approaches for fabricating fractal NCs (e.g., varying the concentration
2.3. Instrumentation Scanning electron microscopy (SEM) images were taken using Hitachi S4800 with an acceleration voltage of 20 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images and selected-area electron diffraction (SAED) patterns were obtained using JEM 2100 with an acceleration voltage of 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained using FEI TECNAI F30 microscope operated at 300 kV. X-ray powder diffraction (XRD) pattern was obtained using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurement was investigated on Escalab 250Xi XPS system. cyclic voltammetry (CV) was carried out using CHI 760e electrochemical workstation (Shanghai Chenhua, China). Electrochemical in situ ATR-FTIR spectroscopy were performed on a Nexus 870 FTIR spectrometer (Nicolet) equipped with a liquidnitrogen-cooled MCT-A detector, an EverGlo IR source, at a spectral resolution of 8 cm−1. 398
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Fig. 3. SEM and high-magnification TEM images of Pt–Ag NCs synthesized at varying amounts of DTAC: (a) 8, (b) 15, (c) 18, and (d) 30 mg. Insets correspond to structure models.
calculated by integrating the hydrogen desorption charge after doublelayer correction on CV curves (−0.25 V < E < 0.10 V) in N2-saturated 0.5 M HClO4 solution, and q0 is 210 μC cm−2. For in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), on an Au film chemically deposited basal plane of a hemicylindrical Si prism beveled at 60°, 20 μL of catalyst ink (1.0 mg mL−1) was transferred onto an electrochemically polished Au film via a pipette carefully. During the measurement, N2 was continuously bubbled. The electrolyte solution containing 0.1 M HClO4 + 0.5 M methanol solution, and the spectrum was collected at 0.20 V, with the reference spectra acquired at −0.20 V at a spectral resolution of 8 cm−1 (coaddition of 200 interferograms for each spectrum). 3. Results and discussion In a typical synthesis, Pt(acac)2 and AgNO3 were simultaneously reduced by using DMF as both solvent and reductant in the presence of a surfactant (i.e. DTAC). A representative SEM image (Fig. 1a) showed that the products were composed of well-shaped octahedron-like NCs with an average edge length of 85 ± 15 nm and high purity (> 95%) (see Fig. S1 for a large-area SEM image). By carefully surveying their morphologies, we found the surfaces were not smooth. They are totally different from usual octahedral shape with flat surfaces [38]. HAADFSTEM revealed a three-dimensional (3D) hyperbranched architecture for these particles (Fig. 1b). High-magnification SEM and TEM images of an individual NC with different orientations showed the surface was composed of highly ordered triangle facets (Fig. 1c–e). And this 3D hierarchical structure exhibited a single-crystalline feature, as revealed by the corresponding SAED patterns. More information of the exposed facets was provided by a HRTEM image taken from a tip zone of an individual particle along the [011] direction (Fig. 1f). Continuous lattice fringes with an interplanar spacing of 0.23 nm and parallel to the surface were observed and ascribed to {111} planes of face-centeredcubic (fcc) Pt–Ag alloy. Overall, the architecture of the as-prepared product resembled that of the famous 3D fractal Sierpinski gasket octahedron, though they were not be exactly the same. Structurally, this unique architecture can be regarded as a fractal structure evolved from the recursive iteration of smaller octahedra at the six corners of a primary octahedron (Fig. 1g). As such, the architecture maintained the
Scheme 1. Schematic illustration of the synthesis of Sierpinski gasket-like Pt–Ag octahedral NCs via creating barrier layer near crystal surface by DTAC to provide local diffusion-limited growth condition.
2.4. Electrochemical measurement Before every experiment, the glassy carbon electrode (working electrode) with a 5 mm diameter was carefully polished and washed. The sample ink was prepared by dispersing electro-catalysts in ethanol, and final concentration is 1.0 mg mL−1. Then 5.0 μL of the suspensions was deposited and dried on the working electrode. A Pt slice was served as the counter electrode. Before each electrocatalytic experiment, continuous potential cycling between −0.25 and 1.00 V (reference electrode: SCE) at 50 mV s−1 in N2-saturated 0.1 M HClO4 solution was applied to clean the working electrode. The methanol oxidation reaction (MOR) was measured by CV method in a solution containing 0.1 M HClO4 + 0.5 M methanol solution from 0.0 to 0.9 V at a scan rate 50 mV s−1 For specific activity, the peak currents of catalysts were all normalized to the electrochemically active surface area (ECSA). The ECSA was determined according to the equation ECSA = Q/q0, where the Q is the electric charge 399
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Fig. 4. (a) CV curves for MOR at a scan rate of 50 mV s−1. (b) Comparison of the specific (left) and mass (right) activities of different catalysts for MOR. (c) The setup for in situ ATR-FTIR experiments. (d) in situ ATR-FTIR spectra of the formed COads at 0.20 V during the MOR.
octahedral NCs formed when the amount of DTAC was 30 mg (Fig. 3d). It should be noted that, the addition of DTAC should not be unlimited, otherwise, large hyperbranched octahedral aggregates formed (Fig. S10). It is well-accepted that fractal structures form under diffusion-limited conditions in which the solute is depleted on the crystal nucleus surface, and the whole growth process is controlled by the limiting diffusion of the growth units from the bulk solution to the crystal nucleus surface [37,39]. Since the diffusion path is long-range, the fractal nanostructures can be generated by precisely controlling the concentration of the precursors (i.e., the concentration gradient of the growth units) [40,41]. Alternatively, we very recently found it was also effective to prepare fractal Rh nanoplates by varying the viscosity of solution to control the diffusion of the growth units [42]. Surprisingly, in our current study, neither the concentration of precursors nor the viscosity of solution changed. The surfactant DTAC seemed to play a critical role in creating diffusion-limited growth conditions for the growth of the final fractal nanostructures. Actually, it is well-known that cationic surfactants with long n-alkyl chains such as DTAC could form a compact self-assembled bilayer on the crystal surface [43]. Therefore, it is possible to construct a steric barrier layer between the crystal nucleus surface and the bulk solution (Scheme 1). This closely packed interface may limit the diffusion of the growth units locally near the crystal surface. In other words, being different from typical longrange diffusion path, fractal nanostructures can also form via generation of a local depletion layer for the growth units by constructing an interface barrier layer on the crystal surface. This proposed growth mechanism was further verified by reaction temperature-dependent shape evolution. The morphology of products evolved from the Sierpinski gasket-like octahedral nanostructures to almost perfect octahedra as the reaction temperature increased (Fig. S11). Clearly, the amount of adsorbed DTAC greatly decreased with the reaction temperature, which made the system beyond the scope of a diffusion-limited growth [44]. To demonstrate the structural advantages for electrocatalytic applications, we compared the MOR performance of the as-synthesized Sierpinski gasket-like Pt–Ag octahedral NCs with those of normal
outline of a normal octahedron, with the surface being comprised of numerous triangle {111} faces with plenty of edges and corners. The crystal structure of the Sierpinski gasket-like Pt–Ag octahedral NCs was determined by XRD, which revealed the typical fcc diffraction peaks at intermediate angles of the standard diffraction peaks for pure Ag and Pt, confirming the nature of Pt–Ag alloy (Fig. 2a). According to the Vegard's law, the Pt/Ag molar ratio was calculated to be 1.62, which was consistent with the inductively coupled plasma atomic emission spectroscopy (1.65 ± 0.03) and energy-dispersive X-ray spectroscopy (EDS) results (Fig. S2). Elemental mappings revealed a roughly overlapped distribution of Pt and Ag throughout the entire nanostructure (Fig. 2b). Further EDS line scanning profiles revealed that the signal of Pt appeared prior to Ag, suggesting a Pt enriched surface (Fig. 2c). As confirmed by XPS (Fig. 2d), the near surface Pt/Ag atomic ratio was estimated to be 3.7:1. To understand the formation mechanism, time-dependent reaction experiments were carried out. In the initial growth stage, the product was predominantly comprised of concave octahedral Pt–Ag alloy NCs (Fig. S3a). Subsequently, the tips of the concave octahedral NCs were found to grow faster (Fig. S3b and c) and the Pt content increased in the products (Fig. S4). Once the Sierpinski gasket-like octahedral nanostructures eventually formed at 6 h, the composition and morphology of the products remained nearly unchanged (Fig. S4 and S5). These results revealed that the formation of Sierpinski gasket-like octahedral nanostructure resulted from the continuous growth. In addition, we found that DTAC strongly adsorbed on crystal surfaces (Fig. S6), and the amount of DTAC played an important role in the formation of the final fractal nanostructure. Only severely aggregated particles were generated in the absence of DTAC (Fig. S7). When the concentration of DTAC was insufficient (8 and 15 mg), the products were slightly truncated octahedral Pt–Ag NCs with large amounts of exposed {111} facets, or concave octahedral Pt–Ag alloy NCs bounded by high-index {332} facets with a nearly uniform composition distribution (Fig. 3a and b, see Fig. S8 and 9 for structural details). With the increase of the amount of DTAC (18 mg), hyperbranched octahedra began to form due to the fast growth of the tips of the concave octahedral seeds (Fig. 3c). And perfect fractal Sierpinski gasket-like Pt–Ag 400
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Fig. 5. (a) Current–time curves at 0.50 V for 2000 s, and (b) loss of peak current densities as a function of the number of cycles on different electrocatalysts in a 0.1 M HClO4 + 0.5 M methanol solution. (c–e) TEM images of the Sierpinski gasket-like, concave octahedral, and normal octahedral Pt–Ag NCs after 500 cycles of MOR, respectively. (f) HAADF-STEM image with composition information from EDS spectrum, (g) elemental mappings and line scanning profiles of the Sierpinski gasketlike Pt–Ag octahedral NCs after 2000 cycles of MOR.
improving the utilization efficiency of Pt [34,47]. In addition, since the COads is main poisoning intermediate during MOR, monometallic Pt catalyst usually suffers from serious poisoning problem. In our case, the alloying with Ag could modify the electronic structure of Pt, optimize the adsorption/desorption of COads, further improving its catalytic performance [48–52]. Moreover, the presence of {111} facets can prevent poisoning by COads, because of {111} surface is unreactive in methanol decomposition to form COads, further increasing the catalytic performance [53,54]. The great antipoisoning ability can be verified by the high ratio of the forward oxidation peak current density (jf) to the backward peak current density (jb) [22]. The jf/jb of Sierpinski gasketlike Pt–Ag octahedral NCs is 1.10, which is higher than 0.92 for concave octahedral Pt–Ag NCs, 1.00 for octahedral Pt–Ag NCs, and 0.85 for commercial Pt black. It means that methanol molecules can be oxidized more effectively during the forward potential scan on the Sierpinski gasket-like Pt–Ag octahedral NCs, while less poisoning intermediates generated during the backward scan as compared to other studied electrocatalysts [34]. We further used in situ ATR-FTIR to gain more insight into the COads-poisoning tolerance of the electrocatalysts (Fig. 4c). As shown in Fig. 4d, the full width at half maximum (FWHM) of the in situ formed COad bands (ca. 2030 cm−1, is linear bonded CO) for the Sierpinski gasket-like Pt–Ag octahedral NCs (51 cm−1) was
octahedral Pt–Ag NCs (Fig. 3a), concave octahedral Pt–Ag NCs (Fig. 3b), and a commercial Pt black (Fig. S12). Among the studied electrocatalysts, the Sierpinski gasket-like Pt–Ag octahedral NCs showed the highest electrocatalytic activity. Their electrocatalytic specific activity in the positive direction was 6.61 mA cm−2, much higher than 5.81 mA cm−2 for the concave octahedral Pt–Ag NCs, 3.66 mA cm−2 for the octahedral Pt–Ag NCs, and 1.14 mA cm−2 for the commercial Pt black (Fig. 4a and S13). The Sierpinski gasket-like Pt–Ag octahedral NCs showed significantly higher activities than most of reported Pt-based electrocatalysts under similar conditions (Table S1) [11,17,45]. Furthermore, the mass activity of the Sierpinski gasket octahedron-like Pt–Ag NCs was 726.3 mA·mgPt−1, which was 1.2, 1.6, and 4.2 times higher than those of the concave octahedral Pt–Ag NCs (608.3 mA·mgPt−1), the octahedral Pt–Ag NCs (460.9 mA·mgPt−1), and the Pt black (171.4 mA·mgPt−1), respectively (Fig. 4b). This enhanced electrocatalytic performance of the Sierpinski gasketlike Pt–Ag octahedral NCs could be ascribed to their structural complexity. It has been well established that the presence of low-coordinated edge/corner sites on Pt-based electrocatalysts can greatly increase the overall methanol oxidation rate of the catalyst [10,17,45,46]. Simultaneously, the hyperbranched nature favored the accessibility of active sites on the surface or inside the catalysts, greatly
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comparable to that of the octahedral Pt–Ag NCs (47 cm−1) and much broader than those of the concave octahedral Pt–Ag NCs (37 cm−1) and the commercial Pt black (39 cm−1). A broader FWHM implies higher mobility of COad species, which could suppress the COad coverage on the electrocatalyst [54]. In addition, the COad band on the Sierpinski gasket-like Pt–Ag octahedral NCs was found to split into two peaks revealing the presence of two different kinds of active sites. Compared to other electrocatalysts, the main peak of the Sierpinski gasket-like Pt–Ag octahedral NCs blue shifted noticeably, further demonstrating the weaker CO–Pt bonding strength [55,56]. The great COads-poisoning tolerance of the catalyst improved the catalytic activity and provided the electrocatalysts with outstanding operation durability. As shown in Fig. 5a, after 2000 s, the Sierpinski gasket-like Pt–Ag octahedral NCs only lose 55.7% of its initial specific activity, which is far lower than the values for the concave octahedral Pt–Ag NCs (73.2%), the octahedral Pt–Ag NCs (57.6%), and the commercial Pt black (69.4%). These results highlight the importance of the as-prepared Sierpinski gasket-like octahedral structures, which possess rich {111} facets, which is well-known inactive in COads poisoning [53,54]. Of note, the concave octahedral and octahedral Pt–Ag NCs became hollow after the tests, and this was attributed to the electrochemical dissolution of Ag (Fig. S14a and b). In contrast, the morphology of the Sierpinski gasket-like Pt–Ag octahedral NCs remained almost unchanged (Fig. S14c). The great structural stability can be ascribed to the Pt-rich surface, which effectively prevented Ag leaching during MOR. To further evaluate the structural stability, consecutive CV tests were conducted (Fig. S15). After 500 cycles, the Sierpinski gasketlike Pt–Ag octahedral NCs still showed the highest catalytic activity (Fig. 5b) among the catalysts tested. Strikingly, the morphology for the Sierpinski gasket-like Pt–Ag octahedral NCs remained nearly unchanged (Fig. 5c), while serious shape changes were observed for the concave octahedral and normal octahedral Pt–Ag NCs electrocatalysts (Fig. 5d and e). More importantly, the structure and composition of the Sierpinski gasket-like Pt–Ag octahedral NCs remained unchanged even after 2000 sweeping cycles. As confirmed by STEM and TEM images, their shapes kept well (Fig. 5f, Fig. S16a). And from the analyses of EDS (in the inset in Fig. 5f), their compositions of Pt and Ag in the used sample were almost as same as the fresh sample (Fig. S2). Consistent results were also obtained from the unchanged peak positions of XRD patterns before and after 2000 cycles for MOR (Fig. S16b). Again, Ptrich surfaces of Sierpinski gasket-like Pt–Ag octahedral NCs account for their great shape and composition stability (Fig. 5g). Additionally, the Sierpinski gasket-like Pt–Ag octahedral NCs exhibited great thermal stability. Thus, storage under atmospheric environment for ca. 1 year did not result in shape alterations or MOR catalytic activity changes (Fig. S17).
Acknowledgements We thank the National Basic Research Program of China (No. 2015CB932301), the National Key Research and Development Program of China (2017YFA0206500 and 2017YFA0206801), the National Natural Science Foundation of China (No. 21603178, 21671163, 21721001, 21773190 and 21802110) and China Postdoctoral Science Foundation (2016M602066, 2017T100468). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.093. References [1] S. Guo, S. Zhang, S. Sun, Angew. Chem. Int. Ed. 52 (2013) 8526–8544. [2] Y. Wang, W. Long, L. Wang, R. Yuan, A. Ignaszak, B. Fang, D.P. Wilkinson, Energy Environ. Sci. 11 (2018) 258–275. [3] J.N. Tiwari, R.N. Tiwari, G. Singh, K.S. Kim, Nano Energy 2 (2013) 553–578. [4] R. Rizo, R.M. Arán-Ais, E. Padgett, D.A. Muller, M.J. Lázaro, J. Solla-Gullón, J.M. Feliu, E. Pastor, H.D. Abruña, J. Am. Chem. Soc. 140 (2018) 3791–3797. [5] Y. Wei, S. Chen, M. Ye, C. Ren, J. Ma, R. Long, C.M. Wang, J. Yang, L. Song, Y. Xiong, Nano Energy 39 (2018) 532–538. [6] N. Zhang, Y. Feng, X. Zhu, S. Guo, J. Guo, X. Huang, Adv. Mater. 29 (2017) 1603774. [7] L. Bu, N. Zhang, S. Guo, X. Zhang, J. Li, J. Yao, T. Wu, G. Lu, J. Ma, D. Su, X. Huang, Science 354 (2016) 1410–1414. [8] L. Zhang, L.T. Roling, X. Wang, M. Vara, M. Chi, J. Liu, S.I. Choi, J. Park, J.A. Herron, Z. Xie, M. Mavrikakis, Y. Xia, Science 349 (2015) 412–416. [9] H. Rong, J. Mao, P. Xin, D. He, Y. Chen, D. Wang, Z. Niu, Y. Wu, Y. Li, Adv. Mater. 28 (2016) 2540–2546. [10] W. Zhao, B. Ni, Q. Yuan, P. He, Y. Gong, L. Gu, X. Wang, Adv. Energy Mater. 7 (2017) 1601593. [11] H. Li, Q. Fu, L. Xu, S. Ma, Y. Zheng, X. Liu, S. Yu, Energy Environ. Sci. 10 (2017) 1751–1756. [12] H. Huang, K. Li, Z. Chen, L. Luo, Y. Gu, D. Zhang, C. Ma, R. Si, J. Yang, Z. Peng, J. Zeng, J. Am. Chem. Soc. 139 (2017) 8152–8159. [13] C. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Nat. Mater. 12 (2013) 765–771. [14] Q. Shao, P. Wang, X. Huang, Adv. Funct. Mater. 29 (2019) 1806419. [15] K. Li, X. Li, H. Huang, L. Luo, X. Li, X. Yan, C. Ma, R. Si, J. Yang, J. Zeng, J. Am. Chem. Soc. 140 (2018) 16159–16167. [16] S. Chen, Z. Niu, C. Xie, M. Gao, M. Lai, M. Li, P. Yang, ACS Nano 12 (2018) 8697–8705. [17] L. Bu, S. Guo, X. Zhang, X. Shen, D. Su, G. Lu, X. Zhu, J. Yao, J. Guo, X. Huang, Nat. Commun. 7 (2016) 11850. [18] G. Garcia, M.T. Koper, Phys. Chem. Chem. Phys. 10 (2008) 3802–3811. [19] H. Chu, Y. Shen, L. Lin, X. Qin, G. Feng, Z. Lin, J. Wang, H. Liu, Y. Li, Adv. Funct. Mater. 20 (2010) 3747–3752. [20] L. Zhang, D. Chen, Z. Jiang, J. Zhang, S. Xie, Q. Kuang, Z. Xie, L. Zheng, Nano Res 5 (2012) 181–189. [21] Z. Zhang, X. Tian, B. Zhang, L. Huang, F. Zhu, X. Qu, L. Liu, Y. Liu, Y. Jiang, S. Sun, Nano Energy 34 (2017) 224–232. [22] Y. Wang, H. Zhuo, H. Sun, X. Zhang, X. Dai, C. Luan, C. Qin, H. Zhao, J. Li, M. Wang, J. Ye, S. Sun, ACS Catal. 9 (2018) 442–455. [23] J. Zhang, Q. Kuang, Y. Jiang, Z. Xie, Nano Today 11 (2016) 661–677. [24] S.C.S. Lai, N.P. Lebedeva, T.H.M. Housmans, M.T.M. Koper, Top. Catal. 46 (2007) 320–333. [25] Y. Jia, Z. Cao, Q. Chen, Y. Jiang, Z. Xie, L. Zheng, Sci. Bull. 60 (2015) 1002–1008. [26] W. Gao, J.E. Mueller, Q. Jiang, T. Jacob, Angew. Chem. Int. Ed. 51 (2012) 9448–9452. [27] Y. Yu, Q. Zhang, J. Xie, J.Y. Lee, Nat. Commun. 4 (2013) 1454. [28] G. Du, J. Pei, Z. Jiang, Q. Chen, Z. Cao, Q. Kuang, Z. Xie, L. Zheng, Sci. Bull. 63 (2018) 892–899. [29] R.G. Weiner, S.E. Skrabalak, Angew. Chem. Int. Ed. 54 (2015) 1181–1184. [30] L. Wang, Y. Nemoto, Y. Yamauchi, J. Am. Chem. Soc. 133 (2011) 9674–9677. [31] B. Lim, Y. Xia, Angew. Chem. Int. Ed. 50 (2011) 76–85. [32] L. Wang, Y. Yamauchi, J. Am. Chem. Soc. 135 (2013) 16762–16765. [33] M.A. Mahmoud, C.E. Tabor, M.A. El-Sayed, Y. Ding, Z. Wang, J. Am. Chem. Soc. 130 (2008) 4590–4591. [34] S. Chen, H. Su, Y. Wang, W. Wu, J. Zeng, Angew. Chem. Int. Ed. 54 (2015) 108–113. [35] B. Xia, W. Ng, H. Wu, X. Wang, X. Lou, Angew. Chem. Int. Ed. 51 (2012) 7213–7216. [36] J. Lai, W. Niu, S. Li, F. Wu, R. Luque, G. Xu, J. Mater. Chem. 4 (2016) 807–812. [37] J. Zhang, H. Li, Q. Kuang, Z. Xie, Acc. Chem. Res. 51 (2018) 2880–2887. [38] G. Fu, R. Ma, X. Gao, Y. Chen, Y. Tang, T. Lu, J.M. Lee, Nanoscale 6 (2014) 12310–12314. [39] A.P. Monaco, R.L. Neve, C. Colletti-Feener, C.J. Bertelson, D.M. Kurnit, L.M. Kunkel, Nature 320 (1986) 264–265. [40] Y. Zhou, S. Yu, C. Wang, X. Li, Y. Zhu, Z. Chen, Adv. Mater. 11 (1999) 850–852. [41] M. Cao, T. Liu, S. Gao, G. Sun, X. Wu, C. Hu, Z. Wang, Angew. Chem. Int. Ed. 44
4. Conclusion In summary, we firstly demonstrated the synthesis of a novel Pt–Ag NCs with a 3D Sierpinski gasket-like octahedral architecture. Moreover, in addition to the well-known approaches based on providing longrange diffusion paths, we showed that it was also feasible to create local limiting diffusion growth environments near the crystal surface by surfactants for preparing fractal nanostructures. This understanding would stimulate controllable design and synthesis of similar multifunctional fractal nanostructures. The coexistence of Pt-rich {111} facets and abundant low-coordinated edge/corner sites provided the asprepared Sierpinski gasket-like Pt–Ag octahedral alloy NCs with excellent electrocatalytic activity, great operation durability, and structural stability towards the MOR. This work encourages us to think that multifunctional NCs are a feasible solution to simultaneously achieve electrocatalysts with high activity and good stability.
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Zhenming Cao received his B.S degree in chemical engineering and technology from department of chemistry, University of Jinan in 2012. Then he jointed Prof. Zhaoxiong Xie's group to pursue his Ph.D. degree at Xiamen University. His research interest is mainly focused on the regulation of the crystal phase and related electrochemical properties of noble metal nanocrystals.
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Jiayu Chen received his B.S. degree from department of chemistry, Xiamen University in 2014. Then he continued pursuing his Ph.D. degree at Xiamen University under the supervision of Prof. Zhaoxiong Xie. His research is focused on construction of specific structures of noble metal/oxide interface and their application in catalysis.
Jiawei Zhang received his B.S degree from University of Jinan in 2009. Then he joined department of chemistry at Xiamen University as a graduate and obtained his Ph.D. degree under the supervision of Prof. Zhaoxiong Xie in 2015. He is currently working as a postdoctoral research fellow in Professor Zhaoxiong Xie's group. His research focuses on the controlled synthesis of noble metal nanocrystals and their applications in catalysis and fuel cells.
Qin Kuang received his B.S (2001) and Ph.D. (2008) in Chemistry from Xiamen University. He worked as a postdoctoral research fellow at Hong Kong University of Science and Technology from 2011 to 2012 with Prof. Shihe Yang. He was promoted to a professor of Chemistry at Xiamen University. So far, he has published more than 100 peerreviewed research journal publications and H index reached 46. His current research is focused on surface/interface engineering of inorganic functional nanomaterials and their applications in the energy and environmental fields.
Huiqi Li received her B.S degree from department of chemistry, Heilongjiang University in 2015. Then she continued pursuing her PhD degree at Xiamen University under the supervision of Prof. Zhaoxiong Xie. She is interested in wet chemical synthesis of noble metal-based nanocrystals with excavated structures and their applications in catalysis.
Jun Zheng received his Ph.D. in Chemistry from Xiamen University in 2010. After then, he worked in Anhui University. His current research is focused on inorganic functional nanomaterials and their applications in the environmental and antimicrobial fields.
Jinyu Ye received his Ph.D degree from Department of chemistry of Xiamen University in 2016 under the supervision of Prof. Shi-Gang Sun and is currently an engineer of chemistry at Xiamen University. His research interests include surface electrochemistry, electrocatalysis and spectroelectrochemistry.
Zhaoxiong Xie received his B.S., M. S. and Ph.D. degrees from department of chemistry, Xiamen University in 1987, 1990 and 1995, respectively. During 1997–1998, he was a postdoc in Centre d’Etudes de Saclay, France, and Ulm University, Germany. Since 2002, he holds the position of Professor of physical chemistry at Xiamen University. He won the National Distinguished Young Scientist Fund of China in 2007 and Chang Jiang Chair Professorship in 2014. So far, he has published more than 260 peer-reviewed research journal publications. His current research interests are focused on surface/interfacial chemistry of functional inorganic nanomaterials.
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Sierpinski gasket-like Pt–Ag octahedral alloy nanocrystals with enhanced electrocatalytic activity and stability
T
Jiawei Zhanga, Huiqi Lia, Jinyu Yea, Zhenming Caoa, Jiayu Chena, Qin Kuanga,∗, Jun Zhengc, Zhaoxiong Xiea,b,∗∗ a State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China b Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, 361005, China c Institute of Physical Science and Information Technology, Anhui University, Hefei, 230601, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Alloys Electrocatalysis Platinum Nanocrystals Fractal
Architectural engineering of noble metal nanocrystals can result in structural diversity and complexity, thereby providing catalysts with multifunctional properties. Herein, a unique Sierpinski gasket-like Pt–Ag octahedral alloy nanostructure with a three-dimensional hyperbranched architecture containing abundant well-defined Ptrich {111} stable facets and dense low-coordinated active sites is reported. Unlike the well-accepted long-range limiting diffusion govened growth model for fractal structures, our success relies on creating local depletion layer near the crystal surface by surfactant. The as-prepared Sierpinski gasket-like Pt–Ag octahedral nanocrystals allows us to achieve three milestones for electrocatalysts, i.e. high catalytic activity, great durability, and stability towards the methanol oxidation reaction in acidic media. Specifically, the catalyst shows a high specific activity (6.61 mA cm−2, is superior than most of reported Pt-based catalysts), outstanding COads-poisoning tolerance and stability (the morphology and composition of the catalyst are preserved after 2000 cycles). This study points to a new approach to develop highly efficient catalysts in the form of fractal nanostructures with structural diversity and complexity to balance the delicate trade-off between activity and stability of catalysts.
1. Introduction Proton-exchange membrane fuel cells (PEMFCs) are promising and environmentally friendly renewable energy conversion devices [1–3]. Currently, platinum (Pt)-based nanomaterials are state-of-art electrocatalysts in both anode (oxidation of fuels) and cathode (reduction of oxygen) [4–7]. However, the abundance in the earth's crust of Pt is quite rare. With efforts to lower economic costs of PEMFCs, much research interest has been attracted to explore high-performance Pt-based electrocatalysts [8–14]. Ideally, a desired electrocatalyst should not only exhibit high catalytic activity, but also show great structural stability. Nonetheless, due to the incompatible structural requirements for activity and stability, how to balance the stability-activity trade-off via crystal engineering remains a challenging undertaking [15–17]. For example, for the anode catalysts of PEMFCs, fabricating high-energy surfaces (e.g., Pt {110} facets) and size shrinking have been demonstrated as efficient
approaches to increase their catalytic activity [18–20]. However, their long-time performances decay significantly due to the serious poisoning of active sites by surface-adsorbed intermediate spices (e.g., COads) during reactions [21–23]. On the other hand, constructing thermodynamically stable surface structures (e.g., {111} facets) could greatly improve the intermediate-poisoning tolerance and structural stability of Pt-based catalysts, but their catalytic activities are usually unsatisfactory [24–26]. While single-function catalysts hardly meet the standards for desired catalyst, alternative promising way out of this predicament is developing multifunctional NCs which integrate different structural advantages into one complex architecture [27–29]. In this context, fractal structures should be ideal catalyst models, and have garnered a great deal of attention due to their well-known inherent structural aesthetics and complexities [30,31]. Specifically, the hyperbranched characteristic of fractal NCs not only could endow the catalysts with large surface-to-volume ratios and high density of easily accessible low-
∗
Corresponding author. Corresponding author. State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. E-mail addresses: [email protected] (Q. Kuang), [email protected] (Z. Xie). ∗∗
https://doi.org/10.1016/j.nanoen.2019.04.093 Received 1 April 2019; Received in revised form 25 April 2019; Accepted 29 April 2019 Available online 30 April 2019 2211-2855/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) SEM and (b) HAADF-STEM images of Sierpinski gasket-like Pt–Ag octahedral NCs. (c, d, and e) SEM and TEM images corresponding SAED patterns and 3D models of an individual NC recorded along the [001], [111], and [011] directions, respectively. (f) HRTEM image recorded from the white dashed box in (e). (g) Schematic illustration of the shape transformation from an octahedron to a Sierpinski gasket-like octahedral structure.
of the precursor or the viscosity of solution) based on the long-range limiting diffusion path. More importantly, this work demonstrated the great potential of rationally engineering factal architectures to achieve optimal catalysts with superior activity and stability. 2. Experimental section 2.1. Chemicals Pt black was purchased from Alfa Aesar; platinum acetylacetonate (Pt(acac)2) was purchased from Kunming Institute of Precious Metals (Yun-nan, China); dodecyltrimethylammonium chloride (DTAC) (> 98%) was purchased from TCI; N, N-dimethylformamide (DMF, AR), silver nitrate (AgNO3, AR) was purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were used as received. 2.2. Synthesis of Sierpinski gasket-like Pt–Ag octahedral NCs Fig. 2. (a) XRD pattern, (b) HAADF-STEM image of the Sierpinski gasket-like Pt–Ag octahedral NCs and corresponding elemental mappings, (c) cross-sectional compositional line profile (indicated by the yellow arrow in (b)), and (d) Ag 3d and Pt 4f XPS spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
5 mg of Pt(acac)2, 0.74 mg of AgNO3, and 30 mg of DTAC were mixed in 10 mL of DMF were mixed together. Then, the homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave with a capacity of 25 mL at room temperature. Then the sealed vessel was heated to 150 °C for 6 h. After naturally cooled to room temperature, the products were collected by centrifugation, and washed several times by mixture of ethanol and water (volume ratio 1:3) to remove residual DTAC on samples.
coordinated edge/corner sites, but also promote the permeability of reactant to reach the active sites from surfaces to insides [32,33]. In addition, the fractal NCs have well-defined sophisticated shapes, which may provide the catalysts unexpected catalytic stability [34–36]. Nevertheless, the formation of fractal nanostructures is thermodynamically unfavored for crystals, which prefer to take convex polyhedral shapes to minimize the surface free energy during growth [37]. Herein, by taking the synthesis of Pt–Ag alloy NCs as an example, we report a novel fractal Sierpinski gasket-like octahedral architectures. Impressively, our successful synthesis relies on creating barrier layer near crystal surface to provide local diffusion-limited growth condition. Remarkably, this unique three-dimensional (3D) fractal architecture showed excellent catalytic activity, great COads-poisoning tolerance, as well as great structural stability towards methanol electrooxidation. Their specific activity outperform most of reported catalyst, and the morphology and composition preserve very well after 2000 cycles. It is found that a high density of corner/edge atoms resulting from the hyperbranched nature played a key role for the extraordinary catalytic activity of the catalyst, while the excellent anti-poisoning properties and structural stability were attributed to exposed Pt-rich {111} facets. The synthetic strategy presented here is entirely different from general approaches for fabricating fractal NCs (e.g., varying the concentration
2.3. Instrumentation Scanning electron microscopy (SEM) images were taken using Hitachi S4800 with an acceleration voltage of 20 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images and selected-area electron diffraction (SAED) patterns were obtained using JEM 2100 with an acceleration voltage of 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained using FEI TECNAI F30 microscope operated at 300 kV. X-ray powder diffraction (XRD) pattern was obtained using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurement was investigated on Escalab 250Xi XPS system. cyclic voltammetry (CV) was carried out using CHI 760e electrochemical workstation (Shanghai Chenhua, China). Electrochemical in situ ATR-FTIR spectroscopy were performed on a Nexus 870 FTIR spectrometer (Nicolet) equipped with a liquidnitrogen-cooled MCT-A detector, an EverGlo IR source, at a spectral resolution of 8 cm−1. 398
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Fig. 3. SEM and high-magnification TEM images of Pt–Ag NCs synthesized at varying amounts of DTAC: (a) 8, (b) 15, (c) 18, and (d) 30 mg. Insets correspond to structure models.
calculated by integrating the hydrogen desorption charge after doublelayer correction on CV curves (−0.25 V < E < 0.10 V) in N2-saturated 0.5 M HClO4 solution, and q0 is 210 μC cm−2. For in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), on an Au film chemically deposited basal plane of a hemicylindrical Si prism beveled at 60°, 20 μL of catalyst ink (1.0 mg mL−1) was transferred onto an electrochemically polished Au film via a pipette carefully. During the measurement, N2 was continuously bubbled. The electrolyte solution containing 0.1 M HClO4 + 0.5 M methanol solution, and the spectrum was collected at 0.20 V, with the reference spectra acquired at −0.20 V at a spectral resolution of 8 cm−1 (coaddition of 200 interferograms for each spectrum). 3. Results and discussion In a typical synthesis, Pt(acac)2 and AgNO3 were simultaneously reduced by using DMF as both solvent and reductant in the presence of a surfactant (i.e. DTAC). A representative SEM image (Fig. 1a) showed that the products were composed of well-shaped octahedron-like NCs with an average edge length of 85 ± 15 nm and high purity (> 95%) (see Fig. S1 for a large-area SEM image). By carefully surveying their morphologies, we found the surfaces were not smooth. They are totally different from usual octahedral shape with flat surfaces [38]. HAADFSTEM revealed a three-dimensional (3D) hyperbranched architecture for these particles (Fig. 1b). High-magnification SEM and TEM images of an individual NC with different orientations showed the surface was composed of highly ordered triangle facets (Fig. 1c–e). And this 3D hierarchical structure exhibited a single-crystalline feature, as revealed by the corresponding SAED patterns. More information of the exposed facets was provided by a HRTEM image taken from a tip zone of an individual particle along the [011] direction (Fig. 1f). Continuous lattice fringes with an interplanar spacing of 0.23 nm and parallel to the surface were observed and ascribed to {111} planes of face-centeredcubic (fcc) Pt–Ag alloy. Overall, the architecture of the as-prepared product resembled that of the famous 3D fractal Sierpinski gasket octahedron, though they were not be exactly the same. Structurally, this unique architecture can be regarded as a fractal structure evolved from the recursive iteration of smaller octahedra at the six corners of a primary octahedron (Fig. 1g). As such, the architecture maintained the
Scheme 1. Schematic illustration of the synthesis of Sierpinski gasket-like Pt–Ag octahedral NCs via creating barrier layer near crystal surface by DTAC to provide local diffusion-limited growth condition.
2.4. Electrochemical measurement Before every experiment, the glassy carbon electrode (working electrode) with a 5 mm diameter was carefully polished and washed. The sample ink was prepared by dispersing electro-catalysts in ethanol, and final concentration is 1.0 mg mL−1. Then 5.0 μL of the suspensions was deposited and dried on the working electrode. A Pt slice was served as the counter electrode. Before each electrocatalytic experiment, continuous potential cycling between −0.25 and 1.00 V (reference electrode: SCE) at 50 mV s−1 in N2-saturated 0.1 M HClO4 solution was applied to clean the working electrode. The methanol oxidation reaction (MOR) was measured by CV method in a solution containing 0.1 M HClO4 + 0.5 M methanol solution from 0.0 to 0.9 V at a scan rate 50 mV s−1 For specific activity, the peak currents of catalysts were all normalized to the electrochemically active surface area (ECSA). The ECSA was determined according to the equation ECSA = Q/q0, where the Q is the electric charge 399
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Fig. 4. (a) CV curves for MOR at a scan rate of 50 mV s−1. (b) Comparison of the specific (left) and mass (right) activities of different catalysts for MOR. (c) The setup for in situ ATR-FTIR experiments. (d) in situ ATR-FTIR spectra of the formed COads at 0.20 V during the MOR.
octahedral NCs formed when the amount of DTAC was 30 mg (Fig. 3d). It should be noted that, the addition of DTAC should not be unlimited, otherwise, large hyperbranched octahedral aggregates formed (Fig. S10). It is well-accepted that fractal structures form under diffusion-limited conditions in which the solute is depleted on the crystal nucleus surface, and the whole growth process is controlled by the limiting diffusion of the growth units from the bulk solution to the crystal nucleus surface [37,39]. Since the diffusion path is long-range, the fractal nanostructures can be generated by precisely controlling the concentration of the precursors (i.e., the concentration gradient of the growth units) [40,41]. Alternatively, we very recently found it was also effective to prepare fractal Rh nanoplates by varying the viscosity of solution to control the diffusion of the growth units [42]. Surprisingly, in our current study, neither the concentration of precursors nor the viscosity of solution changed. The surfactant DTAC seemed to play a critical role in creating diffusion-limited growth conditions for the growth of the final fractal nanostructures. Actually, it is well-known that cationic surfactants with long n-alkyl chains such as DTAC could form a compact self-assembled bilayer on the crystal surface [43]. Therefore, it is possible to construct a steric barrier layer between the crystal nucleus surface and the bulk solution (Scheme 1). This closely packed interface may limit the diffusion of the growth units locally near the crystal surface. In other words, being different from typical longrange diffusion path, fractal nanostructures can also form via generation of a local depletion layer for the growth units by constructing an interface barrier layer on the crystal surface. This proposed growth mechanism was further verified by reaction temperature-dependent shape evolution. The morphology of products evolved from the Sierpinski gasket-like octahedral nanostructures to almost perfect octahedra as the reaction temperature increased (Fig. S11). Clearly, the amount of adsorbed DTAC greatly decreased with the reaction temperature, which made the system beyond the scope of a diffusion-limited growth [44]. To demonstrate the structural advantages for electrocatalytic applications, we compared the MOR performance of the as-synthesized Sierpinski gasket-like Pt–Ag octahedral NCs with those of normal
outline of a normal octahedron, with the surface being comprised of numerous triangle {111} faces with plenty of edges and corners. The crystal structure of the Sierpinski gasket-like Pt–Ag octahedral NCs was determined by XRD, which revealed the typical fcc diffraction peaks at intermediate angles of the standard diffraction peaks for pure Ag and Pt, confirming the nature of Pt–Ag alloy (Fig. 2a). According to the Vegard's law, the Pt/Ag molar ratio was calculated to be 1.62, which was consistent with the inductively coupled plasma atomic emission spectroscopy (1.65 ± 0.03) and energy-dispersive X-ray spectroscopy (EDS) results (Fig. S2). Elemental mappings revealed a roughly overlapped distribution of Pt and Ag throughout the entire nanostructure (Fig. 2b). Further EDS line scanning profiles revealed that the signal of Pt appeared prior to Ag, suggesting a Pt enriched surface (Fig. 2c). As confirmed by XPS (Fig. 2d), the near surface Pt/Ag atomic ratio was estimated to be 3.7:1. To understand the formation mechanism, time-dependent reaction experiments were carried out. In the initial growth stage, the product was predominantly comprised of concave octahedral Pt–Ag alloy NCs (Fig. S3a). Subsequently, the tips of the concave octahedral NCs were found to grow faster (Fig. S3b and c) and the Pt content increased in the products (Fig. S4). Once the Sierpinski gasket-like octahedral nanostructures eventually formed at 6 h, the composition and morphology of the products remained nearly unchanged (Fig. S4 and S5). These results revealed that the formation of Sierpinski gasket-like octahedral nanostructure resulted from the continuous growth. In addition, we found that DTAC strongly adsorbed on crystal surfaces (Fig. S6), and the amount of DTAC played an important role in the formation of the final fractal nanostructure. Only severely aggregated particles were generated in the absence of DTAC (Fig. S7). When the concentration of DTAC was insufficient (8 and 15 mg), the products were slightly truncated octahedral Pt–Ag NCs with large amounts of exposed {111} facets, or concave octahedral Pt–Ag alloy NCs bounded by high-index {332} facets with a nearly uniform composition distribution (Fig. 3a and b, see Fig. S8 and 9 for structural details). With the increase of the amount of DTAC (18 mg), hyperbranched octahedra began to form due to the fast growth of the tips of the concave octahedral seeds (Fig. 3c). And perfect fractal Sierpinski gasket-like Pt–Ag 400
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Fig. 5. (a) Current–time curves at 0.50 V for 2000 s, and (b) loss of peak current densities as a function of the number of cycles on different electrocatalysts in a 0.1 M HClO4 + 0.5 M methanol solution. (c–e) TEM images of the Sierpinski gasket-like, concave octahedral, and normal octahedral Pt–Ag NCs after 500 cycles of MOR, respectively. (f) HAADF-STEM image with composition information from EDS spectrum, (g) elemental mappings and line scanning profiles of the Sierpinski gasketlike Pt–Ag octahedral NCs after 2000 cycles of MOR.
improving the utilization efficiency of Pt [34,47]. In addition, since the COads is main poisoning intermediate during MOR, monometallic Pt catalyst usually suffers from serious poisoning problem. In our case, the alloying with Ag could modify the electronic structure of Pt, optimize the adsorption/desorption of COads, further improving its catalytic performance [48–52]. Moreover, the presence of {111} facets can prevent poisoning by COads, because of {111} surface is unreactive in methanol decomposition to form COads, further increasing the catalytic performance [53,54]. The great antipoisoning ability can be verified by the high ratio of the forward oxidation peak current density (jf) to the backward peak current density (jb) [22]. The jf/jb of Sierpinski gasketlike Pt–Ag octahedral NCs is 1.10, which is higher than 0.92 for concave octahedral Pt–Ag NCs, 1.00 for octahedral Pt–Ag NCs, and 0.85 for commercial Pt black. It means that methanol molecules can be oxidized more effectively during the forward potential scan on the Sierpinski gasket-like Pt–Ag octahedral NCs, while less poisoning intermediates generated during the backward scan as compared to other studied electrocatalysts [34]. We further used in situ ATR-FTIR to gain more insight into the COads-poisoning tolerance of the electrocatalysts (Fig. 4c). As shown in Fig. 4d, the full width at half maximum (FWHM) of the in situ formed COad bands (ca. 2030 cm−1, is linear bonded CO) for the Sierpinski gasket-like Pt–Ag octahedral NCs (51 cm−1) was
octahedral Pt–Ag NCs (Fig. 3a), concave octahedral Pt–Ag NCs (Fig. 3b), and a commercial Pt black (Fig. S12). Among the studied electrocatalysts, the Sierpinski gasket-like Pt–Ag octahedral NCs showed the highest electrocatalytic activity. Their electrocatalytic specific activity in the positive direction was 6.61 mA cm−2, much higher than 5.81 mA cm−2 for the concave octahedral Pt–Ag NCs, 3.66 mA cm−2 for the octahedral Pt–Ag NCs, and 1.14 mA cm−2 for the commercial Pt black (Fig. 4a and S13). The Sierpinski gasket-like Pt–Ag octahedral NCs showed significantly higher activities than most of reported Pt-based electrocatalysts under similar conditions (Table S1) [11,17,45]. Furthermore, the mass activity of the Sierpinski gasket octahedron-like Pt–Ag NCs was 726.3 mA·mgPt−1, which was 1.2, 1.6, and 4.2 times higher than those of the concave octahedral Pt–Ag NCs (608.3 mA·mgPt−1), the octahedral Pt–Ag NCs (460.9 mA·mgPt−1), and the Pt black (171.4 mA·mgPt−1), respectively (Fig. 4b). This enhanced electrocatalytic performance of the Sierpinski gasketlike Pt–Ag octahedral NCs could be ascribed to their structural complexity. It has been well established that the presence of low-coordinated edge/corner sites on Pt-based electrocatalysts can greatly increase the overall methanol oxidation rate of the catalyst [10,17,45,46]. Simultaneously, the hyperbranched nature favored the accessibility of active sites on the surface or inside the catalysts, greatly
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comparable to that of the octahedral Pt–Ag NCs (47 cm−1) and much broader than those of the concave octahedral Pt–Ag NCs (37 cm−1) and the commercial Pt black (39 cm−1). A broader FWHM implies higher mobility of COad species, which could suppress the COad coverage on the electrocatalyst [54]. In addition, the COad band on the Sierpinski gasket-like Pt–Ag octahedral NCs was found to split into two peaks revealing the presence of two different kinds of active sites. Compared to other electrocatalysts, the main peak of the Sierpinski gasket-like Pt–Ag octahedral NCs blue shifted noticeably, further demonstrating the weaker CO–Pt bonding strength [55,56]. The great COads-poisoning tolerance of the catalyst improved the catalytic activity and provided the electrocatalysts with outstanding operation durability. As shown in Fig. 5a, after 2000 s, the Sierpinski gasket-like Pt–Ag octahedral NCs only lose 55.7% of its initial specific activity, which is far lower than the values for the concave octahedral Pt–Ag NCs (73.2%), the octahedral Pt–Ag NCs (57.6%), and the commercial Pt black (69.4%). These results highlight the importance of the as-prepared Sierpinski gasket-like octahedral structures, which possess rich {111} facets, which is well-known inactive in COads poisoning [53,54]. Of note, the concave octahedral and octahedral Pt–Ag NCs became hollow after the tests, and this was attributed to the electrochemical dissolution of Ag (Fig. S14a and b). In contrast, the morphology of the Sierpinski gasket-like Pt–Ag octahedral NCs remained almost unchanged (Fig. S14c). The great structural stability can be ascribed to the Pt-rich surface, which effectively prevented Ag leaching during MOR. To further evaluate the structural stability, consecutive CV tests were conducted (Fig. S15). After 500 cycles, the Sierpinski gasketlike Pt–Ag octahedral NCs still showed the highest catalytic activity (Fig. 5b) among the catalysts tested. Strikingly, the morphology for the Sierpinski gasket-like Pt–Ag octahedral NCs remained nearly unchanged (Fig. 5c), while serious shape changes were observed for the concave octahedral and normal octahedral Pt–Ag NCs electrocatalysts (Fig. 5d and e). More importantly, the structure and composition of the Sierpinski gasket-like Pt–Ag octahedral NCs remained unchanged even after 2000 sweeping cycles. As confirmed by STEM and TEM images, their shapes kept well (Fig. 5f, Fig. S16a). And from the analyses of EDS (in the inset in Fig. 5f), their compositions of Pt and Ag in the used sample were almost as same as the fresh sample (Fig. S2). Consistent results were also obtained from the unchanged peak positions of XRD patterns before and after 2000 cycles for MOR (Fig. S16b). Again, Ptrich surfaces of Sierpinski gasket-like Pt–Ag octahedral NCs account for their great shape and composition stability (Fig. 5g). Additionally, the Sierpinski gasket-like Pt–Ag octahedral NCs exhibited great thermal stability. Thus, storage under atmospheric environment for ca. 1 year did not result in shape alterations or MOR catalytic activity changes (Fig. S17).
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4. Conclusion In summary, we firstly demonstrated the synthesis of a novel Pt–Ag NCs with a 3D Sierpinski gasket-like octahedral architecture. Moreover, in addition to the well-known approaches based on providing longrange diffusion paths, we showed that it was also feasible to create local limiting diffusion growth environments near the crystal surface by surfactants for preparing fractal nanostructures. This understanding would stimulate controllable design and synthesis of similar multifunctional fractal nanostructures. The coexistence of Pt-rich {111} facets and abundant low-coordinated edge/corner sites provided the asprepared Sierpinski gasket-like Pt–Ag octahedral alloy NCs with excellent electrocatalytic activity, great operation durability, and structural stability towards the MOR. This work encourages us to think that multifunctional NCs are a feasible solution to simultaneously achieve electrocatalysts with high activity and good stability.
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Zhenming Cao received his B.S degree in chemical engineering and technology from department of chemistry, University of Jinan in 2012. Then he jointed Prof. Zhaoxiong Xie's group to pursue his Ph.D. degree at Xiamen University. His research interest is mainly focused on the regulation of the crystal phase and related electrochemical properties of noble metal nanocrystals.
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Jiayu Chen received his B.S. degree from department of chemistry, Xiamen University in 2014. Then he continued pursuing his Ph.D. degree at Xiamen University under the supervision of Prof. Zhaoxiong Xie. His research is focused on construction of specific structures of noble metal/oxide interface and their application in catalysis.
Jiawei Zhang received his B.S degree from University of Jinan in 2009. Then he joined department of chemistry at Xiamen University as a graduate and obtained his Ph.D. degree under the supervision of Prof. Zhaoxiong Xie in 2015. He is currently working as a postdoctoral research fellow in Professor Zhaoxiong Xie's group. His research focuses on the controlled synthesis of noble metal nanocrystals and their applications in catalysis and fuel cells.
Qin Kuang received his B.S (2001) and Ph.D. (2008) in Chemistry from Xiamen University. He worked as a postdoctoral research fellow at Hong Kong University of Science and Technology from 2011 to 2012 with Prof. Shihe Yang. He was promoted to a professor of Chemistry at Xiamen University. So far, he has published more than 100 peerreviewed research journal publications and H index reached 46. His current research is focused on surface/interface engineering of inorganic functional nanomaterials and their applications in the energy and environmental fields.
Huiqi Li received her B.S degree from department of chemistry, Heilongjiang University in 2015. Then she continued pursuing her PhD degree at Xiamen University under the supervision of Prof. Zhaoxiong Xie. She is interested in wet chemical synthesis of noble metal-based nanocrystals with excavated structures and their applications in catalysis.
Jun Zheng received his Ph.D. in Chemistry from Xiamen University in 2010. After then, he worked in Anhui University. His current research is focused on inorganic functional nanomaterials and their applications in the environmental and antimicrobial fields.
Jinyu Ye received his Ph.D degree from Department of chemistry of Xiamen University in 2016 under the supervision of Prof. Shi-Gang Sun and is currently an engineer of chemistry at Xiamen University. His research interests include surface electrochemistry, electrocatalysis and spectroelectrochemistry.
Zhaoxiong Xie received his B.S., M. S. and Ph.D. degrees from department of chemistry, Xiamen University in 1987, 1990 and 1995, respectively. During 1997–1998, he was a postdoc in Centre d’Etudes de Saclay, France, and Ulm University, Germany. Since 2002, he holds the position of Professor of physical chemistry at Xiamen University. He won the National Distinguished Young Scientist Fund of China in 2007 and Chang Jiang Chair Professorship in 2014. So far, he has published more than 260 peer-reviewed research journal publications. His current research interests are focused on surface/interfacial chemistry of functional inorganic nanomaterials.
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