Shape-Controlled Synthesis of PlatinumâCopper Nanocrystals for

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Shape-Controlled Synthesis of Platinum-Copper Nanocrystals for Efficient Liquid Fuel Electrocatalysis Hui Xu, Pingping Song, Jin Wang, and Yukou Du Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01729 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Shape-Controlled Synthesis of Platinum-Copper Nanocrystals for Efficient Liquid Fuel Electrocatalysis Hui Xu†, Pingping Song†, Jin Wang†, and Yukou Du*† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China *E-mail: [email protected] (Y.D.)

ABSTRACT Well-defined noble metal nanomaterials are attractive as catalysts for various applications due to abundant surface active sites. However, the shape-controlled synthesis of high-performance Pt-based nanocatalysts remains a forbidden challenge. We herein demonstrate a versatile approach for realizing the systemically controlled syntheses of bimetallic Pt-Cu nanocrystals from concave nanocubes (CNCs), to excavated nanocubes (ENCs), to tripods (TRPs) via simply switching the amount of glycine (reducing agent). These Pt-Cu nanostructures supply a desirable platform for carrying out the structure-dependent electrocatalytic studies in the liquid fuel electrooxidation. Impressively, all the Pt-Cu nanocrystals (NCs) show high activity and outstanding durability for alcohol oxidation. In particular, the Pt-Cu CNCs display unprecedently high activity toward MOR and EOR found to be 2041.1 and 5760.9 mA mg-1 in mass activity, which are 7.9 and 11.5 folds greater than the commercial Pt/C catalysts, respectively, showing a promising class of electrocatalysts for fuel cells. This work sheds great promise for optimizing the electrochemical 1 ACS Paragon Plus Environment

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catalysis by precisely modulating the structure of catalysts. KEYWORDS: Shape-controlled synthesis; PtCu NCs; Structure-property correlation; Electrocatalysts; Fuel cells 1. INTRODUCTION

In the pursuit of high-performance electrocatalysts towards fuel cells reaction, Pt has been popularly considered as the most outstanding and commonly used materials because of its excellent performances.1-5 Alloying Pt with the typical transition metals to generate stable Pt-M alloy (M = Ag, Cu, Fe) has been a feasible strategy to gain the high utilization of scare Pt.6-11 Generally speaking, the d-band center of Pt could be efficiently downshifted after incorporating Pt with the 3d-transition metals, leading to the

substantial improvement of catalytic performances.12-17 Moreover,

the

construction and modification of Pt-based NCs with controlled shapes have also been regarded as a renewed scientific topic for enhancing various electrocatalytic applications due to its unparalleled electronic structures.18 Great efforts have been devoted to the design of unique Pt-based nanomaterials with controlled size, shape, and composition.19-21 Nonetheless, the performance enhancement of electrocatalytic system is highly dependent on both high turnover frequency and Pt utilization efficiency.22-23 Recently, the structure design that combined the fabrications of surface structure and surface composition has been emerging as the effective approach for engineering outstanding nanomaterials because this strategy can generate the unique nanostructure with controlled facet and more surface active areas.24-26 Bear this in mind, the well-defined Pt-based nanomaterials can work well for promoting 2 ACS Paragon Plus Environment

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electrocataltyic reactions because of the coupling of its strong electronic structures from different metals and more available Pt active sites.27-28 Considering the crucial role of structure effects for the enhancements of electrocatalytic performances, it is imperative to design the advanced Pt-based nanocatalysts enclosed with controlled facet and high surface active areas.29-32 Over last decades, great efforts have been focused on regulating and designing the morphology of the nanomaterials and many nanocatalysts with specific shapes such as nanowire,33 nanosheet,34 and nanodendrite35-37 have been developed. Among them, the CNCs, ENCs, and TRPs have attracted extensive attention for the electrocatalysis oxidation of liquids fuels because of their high-index facets, abundant surface active sites, as well as better modulation over electronic effects.38-40 At present, intensive efforts have been devoted to designing Pt-based nanomaterials and tailoring their electrocatalytic properties. Accordingly, various sorts of reductants, surfactants, and additives are employed to serve as the shape-controlled reagents. Among them, glycine has been generally regarded as the excellent structure-directing agents due to the fact that glycine can not only alter the sequential reduction kinetics of metal precursors but also selectively adsorp on the surface of Pt-Cu NCs during growth, and thus control the growth of Pt-Cu NCs.41-43 Furthermore, the strong coordination interactions between glycine and the metal ions resulted in the formation of the Pt–Cu alloy with different morphologies.44 We herein develop a simple approach for the syntheses of a series of well-defined Pt-Cu NCs with controllable structures by varying the amount of glycine and 3 ACS Paragon Plus Environment

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exploring their structure-dependent electrocatalytic properties towards MOR and EOR in the alkaline media. Impressively, by virtue of the synergistic and electronic effects, the as-obtained Pt-Cu NCs can deliver excellent electrocatalytic performances toward MOR and EOR, with both enhanced catalytic activity and durability than Pt/C catalysts. More interestingly, owing to the exposed high-index facets, unique concave nanocube structure, as well as the modified electronic structure, the Pt-Cu CNCs display the greatest electrocatalytic activity and the best durability because of exposed high-index facets. 2. RESULTS AND DISCUSSION The syntheses of Pt-Cu nanocatalysts with tunable shapes, namely, Pt-Cu CNCs, Pt-Cu ENCs, and Pt-Cu TRPs, can be realized through the Scheme 1 (details in the Supporting Information). For instance, the addition of 15 mg glycine can lead to Pt-Cu CNCs, 30 mg glycine can lead to Pt-Cu ENCs, and increasing the amount of 60 mg glycine can lead to the successful preparations of Pt-Cu TRPs. Scheme 1. The synthetic route for achieving Pt-Cu nanostructures with tunable shapes from CNCs and ENCs to TRPs.

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The morphological and structural features of the Pt-Cu NCs were characterized in detail. The representative transmission electron microscope (TEM) images demonstrate that the products are mainly unique nanocubic structure with highly concave features and the synthetic yield approaching 100% (Figure 1a, d, and g). The CNCs are highly monodispersed with the uniform size of 20.9 nm (Figure S1a). According to the high-resolution TEM (HR-TEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of an individual PtCu CNC (Figure 2a), we can find that the surfaces of nanocubes were remarkably concave, strongly confirming the high-yield syntheses of PtCu CNCs. Figure 2b and Figure S2 suggest the nanocrystals possess good crystallinity and the majority of lattice fringes with a lattice space measured to be 0.213 nm, being consistent with the (111) facet of the PtCu nanoalloys. Moreover, the high-index facet such as (220), 5 ACS Paragon Plus Environment

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(222), (221), and (311) could also be observed frequently, which are the essential parameters for further boosting the electrocatalytic performances.45-46 Although Pt-Cu ENCs and Pt-Cu TRPs also have some high-index facets, the amounts of more active {110}, {221}, and {331} are less than those of Pt-Cu CNCs, which might be a crucial parameter for the superior electrocatalytic performances of Pt-Cu CNCs (Figure S2b, c).47-48

Figure 1. TEM images of (a, d, and g) Pt-Cu CNCs, (b, e, and h) Pt-Cu ENCs and (c, f, and i) Pt-Cu TRPs with different magnifications.


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For realizing the controlled syntheses of well-defined Pt-Cu NCs, we made a thorough study on the reaction parameters for the growth of Pt-Cu NCs by tuning the amount of glycine. The results demonstrated that the addition of glycine was crucial for the production of Pt-Cu NCs. Interestingly, some deeply excavated Pt-Cu nanocubes (Figure 1b, e, and h) were obtained when adding 30 mg glycine into the reaction system. The representative TEM images indicated that the resulted Pt-Cu ENCs are highly dispersed with the average diameter around 20.0 nm (Figure S1b). After a detailed observation in the Figure 1h, we can find that the diagonal density shows a linear trend to decrease from the top corner to the center of the square, indicating the six facets of the Pt-Cu nanocubes were deeply excavated.49 Furthermore, when the amount of glycine further raised to 60 mg, the uniform Pt-Cu TRPs consisted of many small branches were obtained (Figure 1c, f, and i). And the average branch length and diameter were measured to be ca. 41.1 and 4.0 nm (Figure S1c). The morphology and structure were investigated via the TEM. As seen in the Figure 2c, three main branches grew in three different directions with an angle of 120°, illustrating the creation of Pt-Cu TRPs from threefold-symmetric growth. Besides, the Pt-Cu TRPs are also composed of many surface protuberances, which is one of the key factors for promoting the electrocatalytic performances.50 The HAADF-STEM and HR-TEM images also confirm the generation of planar tripod structure. From the Figure 2d, we can see that the brightness in the branches is same as that in the tripod center, suggesting that the Pt-Cu TRPs are 2D tripods but not 3D tetrapods. And the HR-TEM image shows the perfect lattice fringes with the 7 ACS Paragon Plus Environment

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interplanar spacing of 0.211 nm, indicating the formation of Pt-Cu alloys.51-52 To further investigate the crystalline features of these Pt-Cu nanocatalysts, the powder X-ray diffraction (PXRD) measurements have also been performed. As seen, in comparison with the standard Pt, the PXRD patterns of all the Pt-Cu nanostructures displayed a slight shift to the higher degree, demonstrating the generation of alloy phases.11 The XPS spectra in the Figure 2f, g also illustrated that both the metallic state Pt0 and Cu0 are absolutely predominant in the composites. More importantly, the XPS spectra of Pt 4f positively shifted to the higher binding energy (BE), while Cu 2p downshifted to a lower BE when compared to the standard Pt and Cu, respectively, indicating the occurrence of charge transfer from Cu to Pt.53-54 The distinct humps, atomic steps, as well as the variation in the electronic structures are also significant for promoting their electrocatalytic properties. As seen in the Figure S3, the Pt/Cu ratios of Pt-Cu NCs were 51.2/48.8, 53.6/46.4, and 52.5/47.5 for Pt-Cu CNCs, Pt-Cu ENCs, and Pt-Cu TRPs, respectively, all of which are close to 1/1, indicating the complete reduction of precursors.55-57


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Figure 2. The HR-TEM images and 3D models of (a and b) Pt-Cu CNCs and (c and d) Pt-Cu TRPs. (e) XRD patterns, XPS spectra of (f) Pt 4f and (g) Cu 2p of three types of Pt-Cu nanostructures To decode the formation mechanism of the Pt-Cu NCs, the morphologies of the products acquired in different reaction conditions were also investigated. As illustrated in Figure S4, when removed the PVP and kept the other conditions unchanged, the concave Pt-Cu nanocubes, excavated Pt-Cu nanocubes, and Pt-Cu


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tripods could also be obtained with the addition of glycine for 15, 30, and 60 mg, respectively. However, the products are prone to stacking together to form the bulk counterparts. Additionally, under the same conditions, the reaction without KI only yielded some irregular nanomaterials (Figure S5), suggesting the critical role of KI for the successful preparation of desirable Pt-Cu CNCs. As a matter of fact, the shape-tunable Pt-Cu nnaostructures were synthesized via co-depositing the Pt0 and Cu0, because I− can strongly interact with Pt4+ , therefore, replacing Cl− to form the [PtCl6−xIx]2− during the reaction, resulting in a close reduction potential of [PtI6]2−/Pt0 (0.40 V vs. RHE) and Cu2+/Cu0 (0.34 V vs. RHE).

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Therefore, the

controllable preparations of well-defined Pt-Cu nanostructures enclosed with high surface areas turn out to provide meaningful model electrocatalysts.


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Figure 3. MOR performances of three types of Pt-Cu nanocatalysts with different shapes and commercial Pt/C catalysts. CV curves of different electrocatalysts conducted in the solutions of (a) 1.0 M KOH and (c) 1.0 M KOH + 1.0 M CH3OH at the scanning rate of 50 mV-1. Histogram of (b) ECSAs and (d) electrocatalytic activities of different electrocatalysts. (e and f) Durability comparison of the different electrocatalysts towards MOR for 1000 cycles. The Pt-Cu NCs can display high activities for the fuel cells reactions, while a


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systematic investigation on the structure-dependent electrocatalytic properties of Pt-Cu NCs remain faint. Accordingly, we herein performed the MOR and EOR tests to deeply understand the influences of Pt-Cu nanostructures on the electrocatalytic properties for alcohol oxidation. According to the cyclic voltammetry (CV) curves, the electrochemically surface areas (ECSAs) were measured to be 49.3, 48.2, 47.4, and 56.8 m2 g-1 for the Pt-Cu CNCs, Pt-Cu ENCs, Pt-Cu TRPs, and Pt/C, respectively (Figure 3a, b). Figure 3c shows the CV curves of various electrocatalysts towards MOR, the mass and specific activities for these four electrocatalysts, based on Figure 3c, are presented in Figure 3d. In general, the Pt-Cu CNCs shows the highest activity of 2041.1 mA mg-1 among these four electrocatalysts, which is 1.2, 1.5, and 7.9 times greater than those of Pt-Cu TRPs (1657.2 mA mg-1), Pt-Cu ENCs (1380.1 mA mg-1), and Pt/C (258.2 mA mg-1), respectively. The MOR specific activity of Pt-Cu CNCs reaches to 4.1 mA cm-2, which is 1.2, 1.4, and 8.2-fold enhancements than those of Pt-Cu TRPs (3.4 mA cm-2), Pt-Cu ENCs (2.9 mA cm-2), and Pt/C (0.5 mA cm-2), respectively. These data have revealed that the Pt-Cu CNCs is one of the best MOR electrocatalysts reported to date (Tables S1). From the Fourier transform infrared spectroscopy (FTIR), we can find that the transmittance of CO2 is remarkably higher than that of formaldehyde, indicating the excellent methanol oxidation capability of Pt-Cu CNCs (Figure S8a).59 The durability of the electrocatalysts are evaluated via the chronoamperometry (CA) measurement conducted at the potential of -0.25 V for 3 h (Figure S6). After 3 h, the normalized current (The current density after 3 h normalized with respect to the initial value) of Pt-Cu CNCs is 13.9%, which is much 12 ACS Paragon Plus Environment

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larger than those of Pt-Cu TRPs (12.1%), Pt-Cu ENCs (11.5%), and Pt/C (6.7%). Moreover, the successive 1000 cycles CVs were also carried out to further study their properties (Figure 3e, f, and Figure S7a-d). After 1000 cycles, the Pt-Cu CNCs lost the 46.3% of the initial activity, while the activity losses of Pt-Cu TRPs and Pt-Cu ENCs are 52.8% and 59.5%, respectively, all of them are significantly less than commercial Pt/C (76.7%), suggesting the outstanding stability of Pt-Cu CNs.

Figure 4. EOR performances of three types of Pt-Cu nanocatalysts with different


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shapes, and commercial Pt/C catalysts. CV curves of different electrocatalysts conducted in the solution of (a) 1.0 M KOH + 1.0 M CH3CH2OH at the scanning rate of 50 mV -1. (b) Histogram of mass and specific activities of different electrocatalysts. (c) CA curves of these electrocatalysts conducted at the potential of -0.25 V. (d) The recorded normalized current and retained mass activity of different electrocatalysts after CA measurements for 3 h. (e and f) Durability comparison of different electrocatalysts towards EOR for 1000 cycles. With regard to the EOR, the activity variations among these four electrocatalysts are similar with the MOR. Still, the Pt-Cu CNCs exhibits the highest electrocatalytic activities among these four electrocatalysts (Figure 4a). From Figure 4b, it can be seen that the Pt-Cu CNCs has the specific activity of 11.7 mA cm-2, which is 1.3, 1.6, and 13.0 times larger than those of Pt-Cu TRPs (9.1 mA cm-2), Pt-Cu ENCs (7.4 mA cm-2), and Pt/C (0.9 mA cm-2), respectively. Moreover, the Pt-Cu CNCs also has the highest mass activity of 5760.9 mA mg-1, which is 1.3, 1.6, and 11.5 folds greater than those of Pt-Cu TRPs (4401.2 mA mg-1), Pt-Cu ENCs (3496.3 mA mg-1), and Pt/C (500.7 mA mg-1), respectively. The EOR activity of Pt-Cu CNCs is also superior to those previously reported Pt-based nanocatalysts (Tables S2). The FTIR analysis also reveals that the main electrocatalytic oxidation products for EOR is CO2 (Figure S8b), further confirming the superior EOR capability.60-61


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Figure 5. TEM images of (a and d) Pt-Cu CNCs, (b and e) Pt-Cu ENCs and (c and f) Pt-Cu TRPs with different magnifications after durability tests. Moreover, we have also tested the EOR durability by CA measurement. From Figure 4c, d, we can find that the Pt-Cu CNCs possess the best durability with the highest retained mass activity and normalized current of 651.2 mA mg-1 and 11.3% after 3h, which are much larger than those of Pt-Cu TRPs (413.1 mA mg-1 and 9.4%), Pt-Cu ENCs (252.8 mA mg-1 and 7.2%), and Pt/C (27.2 mA mg-1 and 5.3%). The 1000 cycles CVs have also been carried out. Remarkably, the electrocatalytic activities of Pt-Cu CNCs, Pt-Cu ENCs, and Pt-Cu TRPs were maintained for 48.3, 43.7, and 39.0%, respectively. However, after successive CVs of 1000 cycles, the catalytic activity for the Pt/C was lost for 80.5%, highlighting the enhanced EOR durability of Pt-Cu NCs. The Pt-Cu NCs displayed extremely high long-term stability towards EOR with no noticeable composition, size, morphology, and shape variations after the 15 ACS Paragon Plus Environment

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stability test, as demonstrated by SEM-EDS analyses and TEM characterizations (Figure S10 and Figure 5). However, Pt/C catalysts were unstable under the identical stability test, for which the particles are seriously aggregated (Figure S11). To further study the influence of morphology on alcohol electrooxidation kinetics in the alkaline media, the electrochemical impedance spectroscopy (EIS) for these four

electrocatalysts-modified

electrodes

were

also

investigated

from

0.1

Hz ∼ 100 kHz. As is displayed in Figure S12 a and b, the diameter of semicircle arc (DIA) of Pt-Cu CNCs is much smaller than those of other three electrocatalysts, indicating the higher charge transfer efficiency.

62-63

As seen in Table S3, the

charge-transfer resistance of Pt-Cu CNCs for the MOR and EOR are 152 and 168 Ω cm2, respectively, which are much smaller than Pt-Cu TRPs (289 and 337 Ω cm2) and Pt-Cu ENCs (368 and 493 Ω cm2), further demonstrating the more effective interfacial charge transport of Pt-Cu CNCs.64 3. CONCLUSIONS In conclusion, we have proposed a feasible approach for synthesizing Pt-Cu nanocatalysts with tunable shapes. Through regulating the reduction kinetic, three distinct Pt-Cu nanocatalysts (PtCu CNCs, PtCu ENCs, and PtCu TRPs) can be readily achieved, which offers a desirable platform to readily study the correlation between the electrocatalytic properties for fuel cells reactions in diverse Pt-Cu nanocatalysts. Significantly, owing to the electronic effects between Pt and Cu, highly exposed surface active areas, as well as high-index facet, these Pt-Cu NCs show remarkable electrocatalytic activity towards MOR and EOR. More interestingly, the optimum 16 ACS Paragon Plus Environment

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Pt-Cu CNCs display the unprecedentedly high electrocatalytic activities of 2041.1 mA mg-1 (4.1 mA cm-2) and 5760.9 mA mg-1 (11.7 mA cm-2) towards MOR and EOR. In addition, 7.9 and 11.5 times enhancement in catalytic activity were obtained in comparison with the commercial Pt/C, shedding great promise for the application in fuel cells. We anticipate that the present work will inspire the rational investigation and fabrication of well-defined nanocatalysts for the energy-conversion and beyond. ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website. Experimental section, EDX, TEM, and FTIR of the as-prepared samples. CVs, scan cycling experiments and chronoamperometric curves of different catalysts modified electrode on electrocatalytic oxidation of ethylene glycol and glycol.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Y.D.). Tel: +86-512-65880089, Fax: +86-512-65880089;

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 17 ACS Paragon Plus Environment

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This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), the Suzhou Industry (SYG201636), the project of scientific and technologic infrastructure of Suzhou (SZS201708), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References 1.

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For Table of Content Use Only An advanced class of Pt-Cu nanocatalysts with tunable shapes have been synthesized. Through regulating the reduction kinetic, three distinct Pt-Cu NCs (PtCu CNCs, PtCu ENCs, and PtCu TRPs) can be readily achieved, which offers a desirable platform to readily study the correlation between the electrocatalytic properties for fuel cells reactions in diverse Pt-Cu nanocatalysts. Significantly, owing to the synergistic and electronic effects originated from Pt and Cu, highly exposed surface active areas, as well as high-index facet, these Pt-Cu NCs show remarkable electrocatalytic activity towards MOR and EOR.


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