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Engineering of Hollow PdPt Nanocrystals via Reduction Kinetic Control for Their Superior Electrocatalytic Performances Caihong Fang, Jun Zhao, Ruibin Jiang, Jing Wang, Guili Zhao, and Baoyou Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08657 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Applied Materials & Interfaces

Engineering of Hollow PdPt Nanocrystals via Reduction Kinetic Control for Their Superior Electrocatalytic Performances Caihong Fanga*, Jun Zhaoa, Ruibin Jiangb, Jing Wangb, Guili Zhaoa, Baoyou Genga*

a

College of Chemistry and Materials Science, The Key Laboratory of Functional Molecular

Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Center for Nano Science and Technology, Anhui Normal University, Wuhu, 241000, China b

Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education,

Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China. *Corresponding author. E-mail: [email protected]; [email protected].

KEYWORDS: PdPt nanocrystals, hollow structures, controlled reaction kinetics, electrocatalysis, methanol oxidation reaction.

ABSTRACT

Synthesis of hollow metal nanocrystals is greatly attractive for their high active surface areas, which gives rise to excellent catalytic activity. Taking PdPt alloy nanostructure as an example,

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we designed a synthetic tactic for the preparation of hollow metal nanostructures by delicate control over the difference in the reduction kinetic of metal precursors. At a high reduction rate difference, the Pd layer forms from the H2PdCl4 and subsequently is etched, leading to the formation of a hollow space. A solid PdPt structure is achieved when reduction rate of Pd and Pt precursor is comparable. Obviously, the hollow space and composition are tunable as well by adjusting the reduction rate difference. More importantly, the prepared hollow PdPt nanostructures exhibit a branched outer, porous wall, and rough hollow interior. The branched outer and rough hollow interior provide the higher density of unsaturated atoms, whereas the porous wall serves as channels connecting the inner, outer, and the reactive agents. Moreover, the periodic self-consistent density function theory (DFT) suggests that the d-band theory density of state (DOS) of the PdPt nanoalloys is upshifted in comparison to the monometallic component, which will benefit for the improvement in their catalytic performances. Electrocatalytic tests reveal that the PdPt bimetallic NCs, especially for Pt32Pd68 nanostructures, show an excellent catalytic activity and stability toward methanol oxidation reaction in owing to their special structures as well as compositions.

INTRODUCTION The metal nanocatalysts with hollow interiors are of great interest and importance as they can offer much higher specific surface areas and thus enhance the corresponding activities in comparison to their solid counterparts1−4. To date, hollow metal NCs were mainly obtained by the galvanic replacement reaction, in which a sacrificial metallic template was employed5. This method strictly requires perfect redox match between the metal core and the subsequently deposited metal ions in solution. Template methodology is another popular way to grow hollow


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structures, in which the morphology often inherit the employed template nanoparticles exhibit6,7. However, the pre-growth of templating nanostructures is process tedious and reagent waste. Moreover, the porous walls in hollow metal nanostructures are pivotal as well. The existence of porosity through the wall in hollow NCs offers a more opportunity in exploring both the outer and inner surface and engineering the active sites for catalysis. Additionally, past decades have witnessed that edges and kinks are considered to be high responsible to provide more atomic steps and thus catalytic sites, which will significantly enhance the catalytic activity8,9. To this end, there is a strong motivation to develop a simple method to synthesize hollow metal NCs with porous walls, edges and kinks for their enhanced catalytic applications. However, a major challenge during preparation process is designing a proper and easy-operated synthetic routine. In addition to shape control, incorporation of other components into the metal NCs to form bi- or multi-metallic nanostructures10−12, in particular alloy NCs, represents another attractive methodology that can further promote and optimize their catalytic activities and stabilities for various reactions13,14. For example, by incorporating Ni into metallic Pt, the hydrogen evolution reaction activity of PtNi3 nanoframes was enhanced by almost one order of magnitude relative to Pt/C15. As a key progress, Pt3Ni alloy NCs exhibited exceptional electrocatalytic activities and stabilities over their Pt counterpart16,17. To date, PdPt alloy nanostructures are considered to be highly promising candidates for the efficient utilization of Pt and the enhancement of their catalytic properties18,19. Especially, the structure with a hollow feature is more attractive20−23, Taking PdPt alloyed NCs as a typical example, we synthesized hollow and porous NCs following the routines realized by controlling the reduction kinetic of the metal precursors. The prepared NCs exhibit a branched outer and porous structure with a rough hollow interior. The key to achieve this special structure is the delicate control over the reduction kinetic of metal


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precursors. More importantly, the novelty is that the structure is first reported, in which the hollow interior, branched outer, and porous wall coexist in a single nanoparticle. By a kinetic control, the hollow space as well as the composition can be adjusted. Such structures are more interesting and important for the following attractive features: (i) The branched outer and hollow rough interior offer much higher density of unsaturated atoms. Moreover, the porous wall provides cavities between the hollow interior, the branched surface, and the reaction system. They will thus not only increase the specific surface area but also allow for the regent transport. (ii) The hollow interior can reduce Pt and Pd loading, which will undoubtedly reduce the cost. (iii) The stability can be further improved by employing PdPt NCs with a proper composition and shape. RESULTS AND DISCUSSION The synthesis of metal NCs typically involves the reduction of metal precursor to metal atoms, which will nuclear and grow into NCs24−26. As exemplified by Pd@Pt core/shell and PdPt alloy nanostructures, Skrabalak reported a systematic investigation on the synthesis of PdPt bimetallic NCs using the co-reduction of Pd and Pt precursors27. Yamauchi and co-workers also presented the preparation of Au@Pd@Pt core/shell NCs by employing HAuCl4, Na2PdCl6, and K2PtCl4 as the metal precursors and ascorbic acid (AA) as the reductant28. However, most of the nanostructures in literatures are solid. It is still hard to obtain PdPt bimetallic with a hollow structure through only mixing the Pd and Pt precursors together. Furthermore, PtCl62− have higher oxidative ability that can be utilized to etch the preformed Pd NCs to achieve Pt nanostructure with a hollow feature20,29,30. Based on the aforementioned theory, hollow PdPt nanostructures can be realized by rationally designing their synthetic routines, in which monometallic Pd can be reduced from PdCl42− and subsequently play a temporary "template" for


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the subsequent deposition of Pt by controlling the reduction kinetic. This will result in the Ptbased hollow nanostructures. Xia’s group has quantitatively confirmed that the initial reduction rate of metal precursors serves as a knob for the final structure31. Octahedral Pd@Pt core/shell and cubic PdPt alloy nanostructures were synthesized by carefully controlling the ratio of their reduction rates. However, it is still extremely difficult to grow PdPt alloy NCs with hollow and special architects. Moreover, it is known that the atomic diffusion occurs though vacancy exchange and not by the direct interchange of atoms32. The vacancies can condense into porous at dislocations. Difference on the diffusivity of atoms (Kirkendall effect) has been employed to tailor nanoparticles with hollow or porous structures33. In this work, we therefore proposed a synthetic pathway to obtain PdPt bimetallic nanostructures by employing reduction rate control and Kirkendall effect. This synthetic process provides a general route in rationally constructing NCs with hollow architecture and porous wall. Figure 1 shows our designed tactic. Pd cannot serves as seeds because metallic Pd with a small size can be rapidly etched by PtCl62− in the following grown progress. The key knob for this synthesis relies on the formation of metallic Pd, which is subsequently oxidized, by fine controlling the reduction rate difference between Pd and Pt precursor (∆R=RPd−RPt). At a right ∆R value, monometallic Pd atoms first deposit on Pt seeds. In this stage, we proposed that metallic Pd formed a shell-like morphology on the surface of Pt seeds due to its rapid reduction rate, followed by the co-reduction of Pd and Pt metal precursor. Subsequently, this shell-like Pd transfers as the hollow space through Kirkendall effect, leading to the formation of alloyed NC with a hollow feature (Routine 1, Figure 1). On this basis, the hollow space can even be tuned by just changing the ∆R value. At high ∆R, the nanostructure with a large hollow interior can be grown (Routine 2, Figure 1). However, if ∆R is reduced, realized by suppressing RPd or increasing RPt, the reduction rate of Pd and Pt precursor is


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comparable. The distinct monometallic Pd layer cannot be formed, which undoubtedly leads to the fabrication of solid nanostructures (Routine 3, Figure 1).

Figure 1 Schematic describing our synthetic tactics to producing a hollow or solid PdPt alloy nanostructure. In a typical preparation, Pt seeds with a size of 3.6 ± 0.7 nm were utilized (Figure S1). These seeds were then dropped into a growth solution formed by a mixture of H2PtCl4 (0.01 M, 660 µL), H2PdCl4 (0.01 M, 240 µL), ascorbic acid (AA, 0.1 M, 320 µL), and cetyltrimethyammonium chloride (CTAC, 0.1 M, 40 mL). The presence of Cl−, the concentration of Pd2+, the pH value (adjusted by addition of HCl aqueous solution), and the amount of the reductant agent provide a proper reduction rate. Scanning electron microscopy (SEM) images show a spherical shape with good monodisperse (Figure S2a). As expected, the prepared nanostructures possess a hollow interior and a rough surface from the transmission electron microscopy (TEM) images (Figure 2a). The hollow feature is clearly verified by the high-angle annular dark-field TEM (HAADF-STEM) imaging (Figure 2b). A distinct hollow interior can be detected in each nanostructure. We further characterized the as-prepared NCs with highresolution TEM (HRTEM) to get more detail information on their surface features. As shown in


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Figure 2c, the NC clearly exhibits numerous dendritic branches on their surface. The average length of the dendritic branches is around 5.0 nm. Moreover, the wall is porous and a clear hole is clearly distinguished in the center. The overall size was determined to be 42.1 ± 3.5 nm with an inner diameter of 11.2 ± 2.7 nm. The thickness is therefore 30.9 ± 3.5 nm. Furthermore, the surface of the hole is also not smooth as well. The lattice fringe spacing of the wall is 0.243 nm, corresponding to that of the {111} facets of the face-centered cubic (fcc) PdPt bimetallic. X-ray diffraction (XRD) patterns exhibit five evident diffraction peaks assigned to {111}, {200}, {210}, {311}, and {222} planes of fcc Pd and Pt (JCPDS No. 46−1043 and JCPDS No. 04−0802), identifying the crystalline nature of the prepared nanostructures (Figure S2b). However, it is difficult to identify the coexistence of Pt and Pd from the XRD characterization due to the very close diffraction peaks in the standard JCPDS cards of metallic Pt and Pd. X-ray photoelectron spectroscopy (XPS) spectra indicate that the Pt 4f displays two peaks at 74.3 (Pt 4f5/2) and 70.9 eV (Pt 4f7/2), demonstrating the existence of metallic Pt0 (Figure 2d). The characteristic Pd 3d has two peaks at 335.25 and 340.55 eV, corresponding to Pd0 state.

Figure 2 TEM (a), HAADF-STEM (b), HRTEM (c), XPS spectra of Pd and Pt (d) for the typical hollow PdPt nanostructures. Inset in (a) is a magnified PdPt hollow nanostructure.


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In order to further substantiate our designed tactics, we monitored the time-dependent shape and composition evolutions (Figure 3). Samples collected at different reaction times were analyzed by TEM imaging. The NCs present a dense porous structure when the growth was stopped before 24 h (Figure S3 and Figure 3a). As the reaction prolonged to 36 h, the appearance of the hollow characteristic is evidenced in TEM images (Figure 3b). Further increasing the reaction time to 48 h, the void space in each porous NC becomes more distinct (Figure 3c). The yield of nanostructures with a clear hollow interior reaches up to almost 100% when the growth was lasted for 72 h (Figure 3d). To precisely tract the structural changes, we further analyzed the elemental distributions of Pd and Pt for the as-synthesized NCs collected at different growth stages through both energy-dispersive X-ray spectroscopy (EDX) mapping and line profiling. Figure 3e is the elemental mapping results taken from the product collected at 24 h. More Pd atoms were monitored at the center of the nanoparticle, whereas just trace of Pt signal was detected. Line-scan analysis clearly reveals that Pt and Pd have completely different profiles (Figure 3f). The content of Pt across an individual NC is almost identical, where there are two apparent increases in the content of Pd. This distinctive change confirms that much more Pd ions were reduced and deposited, which result in formation of Pd−PdPt composite. Cross-sectional compositional line profiles of both Pd and Pt elemental traces of the NCs, achieved at 72 h, have two identical peaks and one valley (Figure 3h). Undoubtedly, the distribution of the Pd matches well with that of the Pt, which is also been detected in EDX mapping results, confirming an alloy composition (Figure 3g). All of the above positional transformations validate the formation of Pd−PdPt intermediate and the alloy composition of our products. Combined with the results involved in the NCs obtained at 24 h, it can be concluded that the structure would involve a transition from solid Pd−PdPt NCs with excess monometallic Pd to PdPt hollow alloy structure.


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These results agree well with our proposed synthetic routine. Pd is first reduced and subsequently deposited on the seed nanoparticles due to its higher reduction rate34. This preferentially reduced Pd formed an almost monometallic Pd shell on the seeds. As the reaction carried on, the reduction rate of Pd decreases because of its decreasing concentration. In this case, Pt will be reduced in accompany with Pd, which will thus lead to the construction of Pd−PdPt nanostructures. Simultaneously, PtCl62− will oxidize newly formed Pd(0) owing to its positive reduction potential compared to PdCl42−/Pd pairs, resulting in the dissolution of metallic Pd and deposition of Pt atoms following the reaction below35: 2Pd(s) + PtCl62−(aq) + Cl−(aq) → Pt(s) + PdCl42−(aq) The dissolution of Pd in center leads to the hollow space inside. Together with the co-reduction of PtCl62− and PdCl42− during the growth procedure, the alloyed PdPt hollow nanostructures are successfully prepared. Additionally, the atomic ratio of the Pd and Pt decreased from 5:1 to 4:1, measured from EDS during line scanning process, as the product evolved from Pd−PdPt nanostructures to hollow alloy NCs.


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Figure 3 TEM images of the NCs collected at different growth time of 24 h (a), 36 h (b), 48 h (c), and 72 h (d), respectively. (e1−e3) Elemental mapping of a single Pd−PdPt nanostructure collected at 24 h. (f) Line-scanning profiles of the NC grown at 24 h. (g1−g3) Elemental mapping of the PdPt hollow nanostructure prepared at 72 h. (h) Line-scanning profiles of Pd and Pt through a single PdPt hollow NC collected at 72 h. To thoroughly understand the effect of reduction reaction kinetics on the growth of hollow nanostructures, we first increased ∆R value by increasing the reduction rate of PdCl42−. It has been reported that the reduction reaction of metal precursor generally follows a second-order rate law. However, a pseudo-first-order rate law can be employed by supplying the reductant (AA in the present work) in a great excess and keeping the same concentration29. At the initial stage, ∆R can be expressed as following: ∆R = RPd − RPt = k1[PdCl42−][AA] − k2[PtCl62−][AA] = k1'[PdCl42−] − k2'[PtCl62−]


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where k is the rate constant; [PdCl42−] and [PtCl42−] correspond the instantaneous concentration of the metal precursors. At the initial stage, the [PtCl62−] was kept at a constant. In this case, the reduction rate is correlated with the concentration of PdCl42−. We thus progressively altered the amount of PdCl42− to accelerate its reduction rate. ∆R value can thus be increased, leading to a structure with much hollow areas inside the resulted NCs in theory. This predication was corroborated by TEM images of the samples that were synthesized by the addition of 120, 500, and 1000 µL of H2PdCl4 (0.01 M) when H2PtCl6 was kept at 660 µL (0.01 M). The TEM and HRTEM images in Figure 4a and b indicate that these NCs are dense rather than hollow structure by dropping just 120 µL H2PdCl4 (Figure S4). The size was measured to be 29.6 ± 5.1 nm from the TEM images. The obtained NCs have hollow interior if we gradually boosted the addition of H2PdCl4 (Figure 1 and Figure 4c−f). When 240 µL of H2PdCl4 was employed, the void space presents in their central part (Figure 1). Moreover, much more H2PdCl4 can be reduced and formed a pre-growth metallic Pd with a large thickness when more H2PdCl4 was employed (1000 µL). This will lead to the growth of nanostructure with a much larger void space. The thickness of the product becomes much thinner or even changes into fragments to form spherical nanocages or nanorings with some holes distributed on their walls (Figure 4e, f and Figure S4c). In addition, the diameter of the outer varied from 42.1 ± 3.5 to 55.5 ± 6.3 nm (Table S1). Furthermore, the diameter of the void space in the center varied from 11.2 ± 2.7 to 33.9 ± 6.7 nm. These results indicate that the thicknesses of the porous walls were also decreased from 15.5 ± 3.5 to 10.8 ± 2.8 nm with the overall size changing from 42.1 ± 3.5 to 55.5 ± 36.3 nm. Moreover, we also determined the ratios between the diameter of the inner and outer of the nanostructures, which can give an intuitive idea about the hollow degree (Table S1). The ratio


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gradually varies from 0.266 to 0.611 as the increase of the increase of H2PdCl4, which undoubtedly indicates the hollow space turns larger and larger.

Figure 4 TEM and HRTEM images of the PdPt hollow NCs grown from the varying amount of H2PdCl4. (a,b) 120 µL. (c,d) 500 µL. (e,f) 1000 µL. On the other hand, the gap between the reduction rate of the PdCl42− and PtCl62− can be narrowed by increasing or/and decreasing the reduction rate of the PdCl42− and PtCl62−, respectively. Introduction of extra ions is one of the most effective manners. Xia and co-workers have demonstrated that the reduction ratio between the PdCl42− and PtCl62− can be declined from 10:1 to 2.4:1 by the introduction of Br−, because Br− can influence the redox potentials of metal ions due to ligand exchange[18]. This result definitely reveals that Br− will decrease the ∆R value. Therefore, the solid nanostructures are formed on the basis of our aforementioned theory. TEM imaging conducted on the products that were prepared in cetyltrimethyammonium bromide (CTAB) instead of CTAC clearly shows that all of the nanostructures are solid (Figure S5a). On the other hand, we retarded the reduction kinetics of Pd precursor, while the reduction rate of Pt precursor was accelerated by additionally dropping Ag+ into the growth solution[34]. As the increase of Ag+ ions, ∆R value will decrease, leading to the growth of dense nanostructures.


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Expectably, there is a poor purity of hollow nanostructures when we just added a small amount of Ag+ ions (100 µL, 0.1 mM, Figure S5b). Further expanding the adding amount of Ag+ (400 µL, 0.1 M), solid nanoparticles were achieved (Figure S5c). As we all know that the reduction ability of a reductant can considerably affect its reduction kinetics. In our methodology, the reduction property of the reductant (AA) depends on the pH values. However, the pH value has nearly the same effect on the reduction rate of the PdCl42− and PtCl62−.That is the ∆R value has almost no change, which will still make hollow nanostructures. To manifest this hypothesis, we altered the pH values of the growth solution in the range of 3 to 9. TEM images of the products imply that all of the final nanostructures own nearly the same architect with empty space in their centers (Figure S6a−d). In addition, we also investigated the effect of the reaction temperature on the final shape. The temperature always accelerates the reaction rate for both metal precursors. The morphologies will thus exhibit a void feature, which was evidenced by the TEM imaging of the NCs grown at 50 and 70 °C. The PdPt NCs have obvious empty centers when the growth were carried out for only 18 h (Figure S6e,f). However, the size distribution is wide at a higher temperature.


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Figure 5 (a) CV curves for the prepared (PdPt bimetallic NCs)/C and the commercial Pt/C catalyst in a N2-saturated solution of H2SO4 (0.5 M) (b) Mass activity normalized by Pt mass in electrocatalyst in a N2-saturated solution of H2SO4 (0.5 M) and methanol (1.0 M). The inserted annotations are also applied to (a). (c) Mass and specific activities of the bimetallic/C and Pt/C nanocatalyst.

The catalytic performances of various PdPt bimetallic NCs were investigated using electrocatalytic MOR as a model reaction and the results were benchmarked with commercial Pt/C nanocatalyst (20 wt% loading). Cyclic voltammetry (CV) curves obtained in deoxygenated H2SO4 (0.5 M) realized by purged N2 to remove O2 dissolved in an aqueous solution, at a sweep


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rate of 50 mV·s−1. The electrochemical active surface area (ECSA) for each nanostructure was determined to be 65.79, 88.38, 114.19, 35.60, 112.77, 97.70, and 74.67 m2 g−1 for the Pd−PtPd (Pt29Pd71, shown in Figure 3e), Pt32Pd68 (shown in Figure 1e and 3f), Pt26Pd74 (shown in Figure 4a and b), Pt40Pd60 (shown in Figure 4c and d), Pt15Pd85 (shown in Figure 4e and bf), Pt23Pd73Ag4 (shown in Figure S5b), and Pt64Pd17Ag19 (shown in Figure S5c) NCs, respectively (Figure 5a). The ECSA of commercial Pt/C was calculated to be 44.83 m2 g−1. The CV curves obtained for MOR with different catalysts show characteristic methanol oxidation peaks at forward and backward scans. The electrocatalytic activities toward MOR were evaluated in a N2satuated mixture of methanol (1 M) and H2SO4 (0.5 M). Figure 5b displays the mass activity curves obtained from the measured curves normalized to the Pt loading mass. The Pt40Pd60, Pd−PtPd, Pt32Pd68, Pt26Pd74, and Pt15Pd85 catalyst is 695, 1540, 1360, 1410, and 1440 mA mgPt−1, which is 3.14, 6.97, 6.15, 6.38, and 6.52 times of commercial Pt/C (221 mA mgPt−1). For the specific activity calculated by normalized the measured activity to ESCA, the Pt40Pd60, Pd−PtPd, Pt32Pd68, Pt26Pd74, and Pt15Pd85 catalyst is 1.95, 2.34, 1.54, 1.22, and 1.28 mA cm−2, respectively. The specific activity of commercial Pt/C is 0.49 mA·cm−2. In addition, the commercial Pd/C exhibited no electrocatalytic activity toward MOR is acidic electrolytes as the previous works and our measurements (Figure S7)36,37. The specific activity of our catalysts was thus improved to be 3.14, 6.97, 6.15, 6.38, and 6.52 times that of commercial Pt/C. These findings unambiguously reveal that the Pt-based alloyed NCs exhibit remarkable enhancements in their electrocatalytic MOR activity compared to the commercial Pt/C catalyst for both specific and mass activity (Figure 5c, Figure S8), signifying the positive synergy of the incorporation of metal atoms into Pt matrix for electrocatalytic MOR. Besides, the Pt-based bimetallic nanocrystals also exhibit a higher electrocatalytic activity compared to the representative PdPt bimetallic


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electrocatalysts that were previously reported (Table S2). Herein, we ascribed those enhancements to the following reasons: (i) The prepared PdPt alloy NCs offer three morphological features, including the branched outers, hollow structures, and porous walls. The branched outers can provide abundant edges, corners, and step atoms, which highly increase the active area in electrocatalytic tests38−41. The hollow structures can further increase the active sites, especially in the presence of porous walls, which can serve as channels connecting the inner, outer, and the reactive agents. Besides, such hollow shape also decreases the amount of metals, which in turn decrease its cost. (2) The synergistic effect on the basis of d-band theory, brought by the electronic coupling, between Pt and the incorporated Pd atoms42. To further elucidate this coupling effect, we also simulated the density of state (DOS) by employing the periodic self-consistent density function theory (DFT) on the basis of d-band theory (Figure 6). The DOS of metallic PdPt alloys with various Pd contents were obtained and compared to that of pure metallic Pt. It is undoubtedly that the energy of DOS shows a substantially upshift after the corporation of Pd atoms. Moreover, we also calculated the energy of d-band center followed the definition of the d-band center with the following calculation43 ா೑

ா೑

ିஶ

ିஶ

ߝௗି௖௘௡௧௘௥ = ൦ න ‫ߩܧ‬ௗ ሺ‫ܧ‬ሻ ݀‫ ܧ‬൘ න ߩௗ ሺ‫ܧ‬ሻ݀‫ ܧ‬൪ the energy of d-band center in DOS gradually upshifts from −2.78 eV of metallic Pt to −2.12 eV when the Pd atom percentage was set to be 75%, which also exhibited a upshift compared to pure metallic Pd (−2.17 eV). (3) The appearance of the poisoning intermediates produced during electrocatalytic MOR in acid mediate, such as CO, can diminish the electrocatalytic activity. This intermediate can be eliminated as the following reaction:


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Pt +H2O →Pt−OH + Pt−H

(1)

2Pt−OH + Pt−(CO) → 3Pt + CO2 + H2O

(2)

Pt + CH3OH → 3Pt−(COH) + 3Pt−H

(3)

which contains the formation of Pt−H44,45. The adsorption of H thus plays a vital role in the catalytic activity. It has been reported that hydrogen adsorption is controllable by altering the compositions of the PdPt alloys. And the optimized Pt content is 8−21 atom % for hydrogen adsorption46,47. Taken together, nanostructured PdPt alloys with a proper content of Pt show a higher catalytic activity. That is Pd−PtPd (Pt21Pd79), Pt26Pd74 and Pt15Pd85 have a higher activity in our measurements compared to that of Pt40Pd60. These predict can also be confirmed by the introduction of Ag into PdPt alloy NCs, in which the extra Ag atoms will impede hydrogen adsorption. The electrocatalytic activity can be weakly decrease after just a very small percentage of Ag atoms (4% for Pt23Pd73Ag4) were incorporated. However, the activity suffers a substantial decay by 45.9 % when the percentage of Ag atom was increased to 19% (Pt64Pd17Ag19). Furthermore, the CO stripping voltammograms of the bimetallic electrocatalysts, especially for the Pd−PtPd (Pt29Pd71), Pt32Pd68, and Pt26Pd74, clearly indicate that the onset and peak potential of the CO oxidation show a obviously negative shift compared to the commercial Pt/C (Figure S9). This result demonstrates an improved CO tolerance of our PdPt electrocatalysts.


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Figure 6 (a) Top- and side-views of Pt, PdPt, and Pd surface from left to right. Pt and Pd atom was indicated in brown and blue, respectively. PdPt alloy with Pd an atom percentage of 50% was exhibited. (b) Calculated DOS of metallic Pt, PdPt alloy, and Pd. The Pd atom percentage was varied from 0% to 100% in a step of 25% during the simulation. (c) The energy of d-band center calculated from (b) for each metal. In addition to the considerable improvement in both specific and mass activities, the catalysts we grown also exhibited a remarkable durability. The stability was evaluated by both accelerated CV curves and chronoamperometry (CA) technique. After 1000 cycles of accelerated durability at room temperature, the mass activity of Pd−PtPd, Pt26Pd74, and Pt15Pd85 dropped even to 19.2%, 29.4%, and 23.4% of their initial intensity, respectively (Figure 7a). The mass activity of the commercial Pt/C also decreases to 75.1% (166 mA mgPt−1). Specifically, the mass activity of Pt32Pd68 only dropped to 1315 mA mgPt−1 after 1000 cycles, which corresponds to a loss of 3.3% relative to the pristine activity. This value is still more than 6 times of the initial mass activity of the commercial Pt/C catalyst. TEM images of the PdPt NCs after 1000 cycles test demonstrate


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that the deterioration o f the electrocatalytic activities of Pt26Pd74, and Pt15Pd85 nanocatalysts toward MOR can be ascribed to their structural deformation. Their hollow interior were altered or even broken into fragments (Pt15Pd85) (Figure S10). We speculate that at a high concentration of Pd in PtPd alloy NCs the electron injection during electrocatalytic tests would oxidize metal atoms in NCs into ions, leading to collapsing of the nanostructures. On the other hand, extensive aggregation was observed for Pd−PtPd, Pt26Pd74, and Pt15Pd85 NCs. Pt32Pd68 nanostructures exhibit no morphology change and almost no aggregation after MOR electrocatalytic test. The long-term durability was tested at 0.5 V for 3000 s (Figure 7b). All the catalysts suffer an unavoidable and rapid decay before reaching a pseudo state due to the concentration deviation brought by methanol diffusion on the surface of the anode and the formation of Pt oxidation48. The Pt32Pd68 nanostructures show the highest oxidation current after durative test for 3000 s.


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Figure 7(a) Mass activities of the bimetallic and Pt/C nanocatalysts before and after 1000 cycles in a N2-saturated mixture of H2SO4 (0.5 M) and methanol (1.0 M). (b) Chronoamperometry curves for all of the (PdPt bimetallic nanostructures)/C and Pt/C in a N2-saturated mixture of H2SO4 (0.5 M) and methanol (1.0 M) for 3000 s. CONCLUSION In conclusion, we proposed a synthetic tactics to fabricate hollow metal NCs using a templatefree methodology. The success of this preparation mainly relies on the control over the reduction rate difference between metal precursors. At a high ∆R, PdPt alloy nanostructures with a hollow feature were grown, while solid NCs can be obtained with a comparable reduction rate. The concentration of metal precursors, addition of extra ions, and reduction ability were employed to control the reduction rate that can thus verify our designed approach and also synthesize PdPt bimetallic NCs with various structures. Moreover, the PdPt bimetallic NCs, especially for Pt32Pd68 nanostructures, show an excellent electrocatalytic activity and stability toward MOR owing to their special structures as well as compositions. We envision that the approach we proposed here provides a simple and effective way to manipulate the morphology and composition in bimetallic nanostructures even multicomponent metal nanomaterials and therefore will be beneficial to their catalytic applications. EXPRIMENTAL SECTION

Synthesis of porous PdPt alloy NCs The PdPt alloy NCs were grown using a seed-mediated method by employing Pt nanoparticles as the seeds. The seeds were prepared through the addition of a freshly prepared, ice-cold NaBH4


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(0.01 M, 600 µL) into a mixture of H2PtCl6 (0.01 M, 80 µL) and CTAC (0.1 M, 10 mL). The resultant solution was then kept at room temperature for at least 2 h. Typically, we obtained the growth solution by sequentially dropping H2PtCl6 (0.01 M, 660 µL), H2PdCl4 (0.01 M, 240 µL), and ascorbic acid (0.1 M, 320 µL) into aqueous CTAC (0.1 M, 40 mL). pH value was measured to be 3.15. The seed solution (200 µL) was then rapidly injected into the growth solution after it turned from brown to colourless. After aged at 30 ºC for 3 days, hollow and porous Pt32Pd68 NCs were collected by centrifugation before characterizations and further use. In our work, Pt40Pd60, Pt26Pd74, and Pt15Pd85 can be obtained by varying the adding amount of H2PdCl4 in the growth solution to be 120, 500, and 1000 µL following the same growth procedure. In addition, the growth of Pt23Pd73Ag4 and Pt64Pd17Ag19 NCs was realized by just adding extra aqueous AgNO3 (0. 1 mM, 100 µL and 0.01 M, 400 µL). To more clearly show the synthetic parameters, we summarized the agents and the resultant nanostructures as a table (Table S3). Electrochemical measurements towards MOR The glassy carbon electrodes with a diameter of 3 mm (Pine Instrument) were sequentially polished with Al2O3 paste (0.05 mm in diameter, mixed Al2O3 powder with DI water) before they were cleaned by ultrasonication. The as-prepared alloy NCs were first dispersed onto XC-72 carbon. The ink solution was fabricated by adding our NC samples into the mixture of N,N−dimethylmethanamide (DMF) and Nafion (0.5 wt.%) under ultrasonication for about 10 min. The as-prepared ink suspension was then dropped onto the surface of the clean glassy carbon electrode and dried naturally in air at room temperature (for details of the exact mass of each component, see Table S4 in Supporting Information). We employed a three electrodes system, in which an Ag/AgCl electrode and a Pt mesh was employed as a reference and counter electrode, respectively. The potential values in this work are


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therefore referenced to Ag/AgCl saturated electrode. The solution in electrocatalytic test was purged N2 for about 30 min to remove oxygen and achieve a N2-saturated solution before each electrochemical measurement. Chronoamperometry curves were tested by fixed the potential at 0.5. The CO-stripping experiments were performed in a deoxygenated H2SO4 (0.5 M) at a sweep rate of 50 mV·s−1. To realize completely adsorption of CO, the electrolyte was prebubbled with CO for at least 30 min. Characterizations SEM images were acquired on an FESEM Hitachi S4800 microscope. TEM imaging was obtained on an FEI Tecnai G2 20 microscope operating at 200 kV. XRD patterns were carried out on Philips X’ Pert system equipped with Cu Kα radiation (λ= 1.5419 Å, scanning rate = 1.0°/min). The HRTEM were taken on Tecnai G2 20 S-TWIN operated at 200 kV accelerating voltage. XPS spectra were performed on a Thermo ESCALAB 250 system. Elemental mapping and HAADF-STEM imaging were achieved on a JEOL−2010 microscope equipped with an energy-dispersive X-ray analysis system. All of the CV and chronoamperometry curves were conducted on an electrochemical workstation (Zennium, ZAHNER). ICP-AES was acquired by Optima 5300DV (Perkin Elmer). The pH values of the growth solutions were measured on PHSJ-4F (Shanghai INESA&Scientific Instrument Co. LTD).


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FIGURES

Figure 1 Schematic describing our synthetic tactics to producing a hollow or solid PdPt alloy nanostructure.

Figure 2 TEM (a), HAADF-STEM (b), HRTEM (c), XPS spectra of Pd and Pt (d) for the typical hollow PdPt nanostructures. Inset in (a) is a magnified PdPt hollow nanostructure.


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. Figure 3 TEM images of the NCs collected at different growth time of 24 h (a), 36 h (b), 48 h (c), and 72 h (d), respectively. (e1−e3) Elemental mapping of a single Pd−PdPt nanostructure collected at 24 h. (f) Line-scanning profiles of the NC grown at 24 h. (g1−g3) Elemental mapping of the PdPt hollow nanostructure prepared at 72 h. (h) Line-scanning profiles of Pd and Pt through a single PdPt hollow NC collected at 72 h.

Figure 4 TEM and HRTEM images of the PdPt hollow NCs grown from the varying amount of H2PdCl4. (a,b) 120 µL. (c,d) 500 µL. (e,f) 1000 µL.


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s Figure 5 (a) CV curves for the prepared (PdPt bimetallic NCs)/C and the commercial Pt/C catalyst in a N2-saturated solution of H2SO4 (0.5 M) (b) Mass activity normalized by Pt mass in electrocatalyst in a N2-saturated solution of H2SO4 (0.5 M) and methanol (1.0 M). The inserted annotations is also applied to (a). (c) Mass and specific activities of the bimetallic/C and Pt/C nanocatalyst.


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Figure 6 (a) Top- and side-views of Pt, PdPt, and Pd surface from left to right. Pt and Pd atom was indicated in brown and blue, respectively. PdPt alloy with Pd an atom percentage of 50% was exhibited. (b) Calculated DOS of metallic Pt, PdPt alloy, and Pd. The Pd atom percentage was varied from 0% to 100% in a step of 25% during the simulation. (c) The energy of d-band center calculated from (b) for each metal.


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Figure 7(a) Mass activities of the bimetallic and Pt/C nanocatalysts before and after 1000 cycles in a N2-saturated mixture of H2SO4 (0.5 M) and methanol (1.0 M). (b) Chronoamperometry curves for all of the (PdPt bimetallic nanostructures)/C and Pt/C in a N2-saturated mixture of H2SO4 (0.5 M) and methanol (1.0 M) for 3000 s.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM image of the seeds, Additional SEM/TEM images of the PdPt NCs, the specific activity of electrocatalysts, TEM images of the bimetallic NCs before and after electrocatalytic tests, and


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the tables of size information, composition of bimetallic NCs, constituent of the ink suspension were presented at supporting information. AUTHOR INFORMATION Corresponding Author *Corresponding author. E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21501005, 21471006), the Programs for Science and Technology Development of Anhui Province (1501021019), the National Science Foundation of Anhui Provincial Education Department (KJ2015A029), the Recruitment Program for Leading Talent Team of Anhui Province, the Program for Innovative Research Team of Anhui Education Committee, and the Research Foundation for Science and Technology Leaders and Candidates of Anhui Province. REFERENCES (1)

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