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Morphology and Phase Controlled Construction of Pt−Ni Nanostructures for Efficient Electrocatalysis Jiabao Ding,† Lingzheng Bu,† Shaojun Guo,‡ Zipeng Zhao,§ Enbo Zhu,§ Yu Huang,*,§ and Xiaoqing Huang*,† †

College of Chemistry, Chemical Engineering, and Materials Science, Soochow University, Jiangsu 215123, China Materials Science & Engineering, College of Engineering, Peking University, Beijing 100871, China § Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States ‡

S Supporting Information *

ABSTRACT: Highly open metallic nanoframes represent an emerging class of new nanostructures for advanced catalytic applications due to their fancy outline and largely increased accessible surface area. However, to date, the creation of bimetallic nanoframes with tunable structure remains a challenge. Herein, we develop a simple yet efficient chemical method that allows the preparation of highly composition segregated Pt−Ni nanocrystals with controllable shape and high yield. The selective use of dodecyltrimethylammonium chloride (DTAC) and control of oleylamine (OM)/oleic acid (OA) ratio are critical to the controllable creation of highly composition segregated Pt−Ni nanocrystals. While DTAC mediates the compositional anisotropic growth, the OM/OA ratio controls the shapes of the obtained highly composition segregated Pt−Ni nanocrystals. To the best of our knowledge, this is the first report on composition segregated tetrahexahedral Pt−Ni NCs. Importantly, by simply treating the highly composition segregated Pt−Ni nanocrystals with acetic acid overnight, those solid Pt−Ni nanocrystals can be readily transformed into highly open Pt−Ni nanoframes with hardly changed shape and size. The resulting highly open Pt−Ni nanoframes are high-performance electrocatalysts for both oxygen reduction reaction and alcohol oxidations, which are far better than those of commercial Pt/C catalyst. Our results reported herein suggest that enhanced catalysts can be developed by engineering the structure/composition of the nanocrystals. KEYWORDS: Composition segregation, nanoframes, Pt−Ni nanocrystals, oxygen reduction reaction, alcohol oxidation reactions

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electronic effect from different metals and the largely improved number of available Pt sites.21−23 From the structural and chemical perspective, one obvious advantage of the highly composition segregated Pt-based nanocrystals is that they can be readily sacrificed to create highly open skeletons, nanocages, or nanoframes, a kind of promising yet new class of nanostructures with 3D large accessible surface area and ideal interconnected edges.23−34 This indicates that tuning the morphology and structure of Ptbased nanocrystals is the key to further tuning and maximizing the catalytic reactions. This has stimulated the researchers to develop synthetic methods for obtaining highly composition segregated Pt-based nanomaterials in a rich variety of morphologies.23−34 However, to our knowledge, the majority of previous reported Pt-based nanomaterials with open structure (skeletons, nanocages or nanoframes structure) are cubes, octahedra/truncated octahedra, and rhombic dodecahe-

he design and synthesis of multicomponent Pt-based nanomaterials with highly controlled structures have attracted extensive research attention, mainly due to their unparalleled structural diversity and the accompanying highly active catalytic properties for various electrocatalytic reactions.1−7 Nevertheless, the performance enhancement of these catalytic systems largely requires the precise control over the structure of multicomponent Pt-based nanomaterials with the lowest Pt amount and very high turnover frenquecy,8−15 mainly due to the scarcity of Pt and high cost of Pt-based nanomaterials.16−20 Recently, nanoscale composition segregation, also called compositional anisotropy, is of more interest in designing superior nanocatalysts because such phenomena can lead to the three-dimensional (3D) nanocrystals with very high surface area and good size and facet control.21−23 Inspired by this, the composition segregated Pt-based nanocrystals can work well for enhancing the electrocatalytic reactions because they can not only inhere the properties of the Pt constituent, but also show excellent performance for catalytic reactions compared with the homogeneous alloys due to the strong © 2016 American Chemical Society

Received: February 2, 2016 Revised: February 28, 2016 Published: March 7, 2016 2762

DOI: 10.1021/acs.nanolett.6b00471 Nano Lett. 2016, 16, 2762−2767

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Nano Letters dra; reports on the production of other mophologies with highly composition segregated feature have not been realized.23−34 Furthermore, it should be noted that the majority of the previous reported results for highly composition segregated Pt-based nanomaterials lack structure tunability, which is however important to the rational design of highperformance Pt-based nanoframes with potentially practical applications.23−34 Therefore, the development of robust approach for the creation of highly composition segregated Pt-based nanomaterials with tunable structure is highly desirable but remains a tremendous challenge. Herein, we demonstrate an effective synthetic approach to synthesize a new class of polyhedral Pt−Ni nanocrystals with highly composition segregated feature. Tetrahexahedral Pt−Ni nanocrystals (THH Pt−Ni NCs) and rhombic dodecahedral Pt−Ni nanocrystals (RDH Pt−Ni NCs) have been selectively prepared by simply changing the ratios of oleylamine (OM)/ oleic acid (OA) without changing the other synthesis parameters. To the best of our knowledge, this is the first report on composition segregated THH Pt−Ni NCs. By virtue of the structure and composition anisotropy, these solid Pt−Ni NCs can be transformed into highly open tetrahexahedral Pt− Ni nanoframes (THH Pt−Ni NFs) and rhombic dodecahedral Pt−Ni nanoframes (RDH Pt−Ni NFs) after acetic acid treatment with almost unchanged shape and size. The resulting Pt−Ni NFs are highly active for oxygen reduction reaction (ORR) with their mass and specific activities more than 20times higher than those of commercial Pt/C (E-TEK, 20 wt % Pt). Our tests show that these Pt−Ni NFs are also efficient in their catalysis for MOR and EOR with their mass and specific activities more than 2−4-times higher than those of commercial Pt/C. The THH Pt−Ni NFs with higher Ni composition generally exhibit higher specific activities than RDH Pt−Ni NFs in the ORR, methanol oxidation reaction (MOR), and also ethanol oxidation reaction (EOR). The control of composition segregated Pt−Ni NCs was realized in a very simple organic solution system by using platinum(II) acetylacetonate ([Pt(acac)2]) and nickel(II) acetylacetonate ([Ni(acac)2]) as metal precursors and OM/ OA mixture as solvent and surfactant in the presence of dodecyltrimethylammonium chloride (DTAC). In the typical preparation of highly composition segregated THH Pt−Ni NCs, 10 mg of Pt(acac)2, 6.5 mg of Ni(acac)2, 25.9 mg of DTAC, 2.5 mL of OM, and 2.5 mL of OA were added into a 35 mL glass vial (see Supporting Information for details). After the glass vial had been capped, the mixture was ultrasonicated for around 1 h. The resulting homogeneous mixture was then heated from room temperature to 180 °C and kept at that temperature for 3 h in an oil bath before it was cooled to room temperature. The resulting colloidal products were collected by centrifugation and washed with an ethanol (8 mL)/cyclohexane (1 mL) mixture. The highly composition segregated THH Pt−Ni NCs were initially characterized by transmission electron microscopy (TEM). The product consists of uniform nanostructures with cubic outline at the first glance, as revealed by TEM images (Figures 1a and S1a). The edge lengths of these nanostructures are in the range of 14−20 nm with an average edge length of 16 nm (Table S1). Interestingly, the TEM image (Figure 1a) also reveals that each nanostructure has square diagonals on each facet, indicating the possible presence of compositional segregation feature in the nanostructure. The morphology of THH Pt−Ni NCs with convex facets was vividly presented by

Figure 1. Morphological and structural characterizations for THH Pt− Ni NCs. Representative (a) TEM image, (b) HAADF-STEM image, (c) TEM images and corresponding geometric models of THH Pt−Ni NCs oriented along two typical projections, (d) HAADF-STEM-EDX elemental mappings for individual THH Pt−Ni NC, and (e) PXRD pattern of as prepared THH Pt−Ni NCs. The XRD pattern of THH Pt−Ni NCs can be split into two set of patterns, which are associated with PtNi and PtNi3, respectively.

high-angle annular dark-field scanning TEM (HAADF-STEM) image with sharp contrast (Figures 1b and S1b). To reveal the specific structure, several TEM images of individual nanocrystals viewed from different projections were carefully collected (Figure 1c). As can be seen from the projection along [100], the NC has square diagonals in the middle with convex outline on the side, which matches the geometric model. The projection along the [110] also matches the corresponding geometric model very well. Thus, it collectively turns out that the nanocrystal is essentially a tetrahexahedron consisting of 24 facets and can be identified as a nanocube with each face capped by a square-based pyramid, matching well the geometric models (Figure 1c).35,36 The highly composition segregated feature was confirmed by the HAADF-STEM-EDS (energydispersive X-ray spectroscopy) elemental mapping analysis, where the distributions of Pt and Ni are not uniform through the whole NC. While the distribution of Ni is even through the whole tetrahexahedron, more Pt is located on the edges of the tetrahexahedron (Figure 1d). The obtained products were further characterized by powder X-ray diffraction (PXRD) (Figure 1e). The PXRD pattern of the colloidal products displays two sets of distinct face-centered cubic (fcc) patterns, which may be indexed to the PtNi and PtNi3 phases with the ratio of 1.40 (PtNi3/PtNi) according to the calculation of the (111) peak, confirming the presence of two phases in the THH Pt−Ni NCs (Figure 1e). The Pt/Ni composition is 32 ± 0.6/ 68 ± 0.6, as confirmed by scanning electron microscopy-EDS 2763

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Nano Letters (SEM-EDS, Figure S1d), being consistent with the result of inductively coupled plasma atomic emission spectroscopy (ICP-AES). Although a thorough literature search for Pt-based nanostructures synthesized by wet-chemical method reveals that Pt-based nanostructures with various sizes, compositions, and morphologies have been achieved, the creation of highly composition segregated THH Pt−Ni NCs has not been reported yet.37−40 To understand the control mechanism of the highly composition segregated THH Pt−Ni NCs, we designed sets of experiments to investigate how the different reagents affect the growth of the THH Pt−Ni NCs. Among all the experimental parameters, DTAC in our synthesis appears to play a significant role in controlling THH Pt−Ni NCs. Figure S2 shows that the structures of products changed dramatically when the amount of DTAC used in the reaction system was changed. When the amount of DTAC was reduced from 25.9 mg to 5.2 mg, the morphology of the as-prepared sample changed from THH into irregular nanocrystals (Figure S2). When the amount of DTAC was increased to 130 mg, THH Pt−Ni NCs could not be obtained either (Figure S2). Furthermore, when the DTAC was changed by its homologues (i.e., trimethyloctylammonium chloride, CTAC, and STAC.) (Figure S3), the THH Pt−Ni NCs mixed with other shaped NCs were produced. Therefore, the selective use and specific amount of DTAC was critical in the high-yield production of THH Pt−Ni NCs. The volume ratio of OM to OA in the synthesis was proven to be another critical parameter on the growth of THH Pt−Ni NCs. As observed by the TEM images (Figure S4a,b), only spherical NCs were obtained when the reactions were performed in the presence of OM alone. Further optimized results showed that only a volume ratio of OM/OA at 2.5:2.5 could produce THH Pt−Ni NCs (Figure S4). For example, the composition segregated Pt−Ni NCs with inhomogeneity in size were created when the ratio of OM/ OA was increased to 3:2 (Figure S4c,d), while the porous NCs were dominated when the synthesis was carried out in the presence of 3 mL of OA and 2 mL of OM (Figure S4e,f). With highly composition segregated THH Pt−Ni NCs at hand, we turned our attention to the creation of highly opened NFs such as THH Pt−Ni NFs. By treating the highly composition segregated Pt−Ni NCs with acetic acid at 100 °C overnight, these solid Pt−Ni NCs were readily transformed into highly open Pt−Ni NFs with almost no shape and size change. With the acetic acid treatment to etch away the majority of Ni in the highly composition segregated Pt−Ni NCs, the surfactant coating was also largely washed, both of which help to activate the Pt−Ni catalyst.41,42 Figures 2 and S5 are the detailed characterizations of the THH Pt−Ni NFs by using the highly composition segregated THH Pt−Ni NCs as sacrificing nanostructures, showing that the THH Pt−Ni NCs are all changed into THH Pt−Ni NFs. Both the TEM (Figure 2a) and HAADF-STEM (Figure 2b) images demonstrate that the THH Pt−Ni NFs have the uniform size of 15 nm (Table S1). The well-defined tetrahexahedral contour of NFs was confirmed by the high-magnification TEM images collected from different orientations, matching well the geometric models of THH NFs (Figure 2c). The obtained THH Pt−Ni NFs were further characterized by PXRD (Figure S5). Compared with its parent solid NCs, the XRD pattern of THH Pt−Ni NFs reveals the vanish of acromia and the shift of the diffraction peaks to lower angle region, suggesting that the product changes into pure alloy phase. The PXRD pattern of the THH Pt−Ni NFs

Figure 2. Morphological and structural characterizations for THH Pt− Ni NFs. Representative (a) TEM image, (b) HAADF-STEM image, (c) TEM images and corresponding geometric models of THH Pt−Ni NFs oriented along two typical projections, and (d) HAADF-STEMEDX elemental mapping results for individual THH Pt−Ni NF.

displays the distinct fcc pattern associated with the PtNi. The HAADF-STEM-EDX elemental mapping analysis was applied to further characterize the distributions of Pt and Ni in the nanoframes (Figure 2d), where the Pt and Ni spread over the NF homogeneously, confirming that the formation of pure alloy phase in final THH Pt−Ni NFs. As determined by SEMEDS, the Pt/Ni composition is 54.6:45.4 (Figure S5), indicating the loss of Ni after the etching treatment. Interestingly, by changing the solvent mixture to 4.0 mL of OM + 1.0 mL of OA with other parameters identical to the preparation of THH Pt−Ni NCs, we also got uniform RDH Pt−Ni NCs (Figure 3a,b). TEM image shows that the product consists of uniform NCs with hexagonal outline (Figure S6). The RDH Pt−Ni NCs showed uniform size with their edge length at 12 ± 3 nm (Table S1). The Pt/Ni composition was determined to be 23:77 by ICP-AES, being consistent with the SEM-EDS result (Figure S6). Both the TEM and HAADFSTEM images indicate the possible presence of composition segregated feature in the nanostructures, as these nanostructures possess different regions that contrast clearly with one another (Figures 3a and S6). The detailed high-magnification TEM images of individual NC viewed from several typical projections show that the obtained NCs have rhombic dodecahedral shape, which matches well the geometric models (Figure S7a). The elemental mappings obtained by HAADFSTEM-EDX reveal that the Ni element distributes homogeneously throughout the whole area and the Pt element has higher content in the edges (Figure 3b). In particular, these composition segregated Pt−Ni NCs are essentially composition segregated NCs with the coexist of fcc PtNi2 and PtNi5 with the ratio is 3.28 (PtNi5/PtNi2) according to calculation of the subpeaks in the (111) peak, as revealed by the PXRD (Figure S6c). By virtue of the structure and composition anisotropy, these solid RDH Pt−Ni NCs can also be transformed into highly open RDH Pt−Ni NFs after acetic acid treatment with almost 2764

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Figure 3. Morphological and structural characterizations for RDH Pt− Ni NCs and RDH Pt−Ni NFs. Representative (a) HAADF-STEM image and (b) HAADF-STEM-EDX elemental mapping results for individual RDH Pt−Ni NC. Representative (c) HAADF-STEM image and (d) HAADF-STEM-EDX elemental mapping results for individual RDH Pt−Ni NF.

Figure 4. Pt−Ni NFs/C as highly efficient catalysts for ORR. (a) CVs recorded at room temperature in 0.1 M HClO4 solution with a sweep rate of 50 mV s−1. (b) ORR polarization curves recorded at room temperature in an oxygen-saturated 0.1 M HClO4 aqueous solution with a sweep rate of 10 mV s−1 and a rotation rate of 1600 rpm. (c, d) Specific activity and mass activity at 0.9 V versus RHE for these three catalysts.

unchanged shape and size. Figure 3, panels c and d and Figure S8 show the typical characterizations of the obtained RDH Pt− Ni NFs. We can see that, after the etching treatment, the inner part of the solid RDH Pt−Ni NCs was etched away, which led to the formation of highly open RDH Pt−Ni NFs (Figure 3c,d). Despite etching, the rhombic dodecahedra were largely maintained (Figures 3c and Figure S7b). The disappearance of acromia in the XRD pattern of RDH Pt−Ni NFs also indicates the changing of phase from segregated phase to pure alloyed phase (Figure S8). To elucidate the distribution of Pt and Ni in the RDH Pt−Ni NFs, HAADF-STEM-EDX elemental mapping analysis was conducted to confirm the transformation of segregated phase into alloy phase (Figure 3d). The Pt/Ni composition of RDH Pt−Ni NFs is 70.8:29.2, showing the much loss of Ni content (Figure S8). We deposited these highly open Pt−Ni NFs on a commercial carbon (C) support to evaluate their catalytic performance toward both alcohol electrooxidations and ORR. Figure S9 is the typical TEM image of the Pt−Ni NFs loaded on the C, which shows that mixing the cyclohexane dispersion of Pt−Ni NFs and C powder under sonication led to the uniform deposition of Pt−Ni NFs on C. The final electrocatalysts were named as THH Pt−Ni NFs/C and RDH Pt−Ni NFs/C, respectively. The THH Pt−Ni NFs/C and RDH Pt−Ni NFs/C were dispersed in ethanol for the electrochemical tests and benchmarked against the commercial Pt/C catalyst (Aldrich, 205915−1G, 99.97%, Figure S10). The Pt loadings of THH Pt−Ni NFs/C, RDH Pt−Ni NFs/C, and commercial Pt/C were all fixed at 2.0 μg. As demonstrated in Figure 4, panel a, by calculating the charge formed during the hydrogen adsorption/ desorption process, the two Pt−Ni NFs catalysts exhibit high electrochemically active surface area (ECSA), with 46.3 m2 g−1 of THH Pt−Ni NFs/C and 54.6 m2 g−1 of RDH Pt−Ni NFs/ C, respectively, similar to that of commercial Pt/C (57.4 m2

g−1). Figure 4, panel b shows the positive-going ORR polarization curves of THH Pt−Ni NFs/C, RDH Pt−Ni NFs/C, and commercial Pt/C catalysts in oxygen-saturated 0.1 M HClO4 solution at a sweep rate of 10 mV s−1 and at a rotation rate of 1600 rpm. The calculated kinetic current density at 0.9 V was normalized over the Pt loading weight and ECSA to give the mass activity and specific activity, respectively (Figure 4c,d). The catalytic specific and mass activities follow the order of THH Pt−Ni NFs/C > RDH Pt−Ni NFs/C > commercial Pt/C. The THH Pt−Ni NFs/C exhibits the highest mass activity and specific activity, 20.9-times and 25.9times higher than that of the commercial Pt/C, respectively. The THH Pt−Ni NFs/C show a specific activity of 6.37 mA cm−2, a 130% specific activity enhancement compared with the RDH Pt−Ni NFs/C (4.93 mA cm−2). The enhanced electrocatalytic performance of THH Pt−Ni NFs/C is likely due to their higher Ni composition, and more edges and corners than those of RDH Pt−Ni NFs/C, indicating the feasibility to control the catalytic performance of such nanoframes through further structure tuning. These Pt−Ni NFs/C catalysts were also applied as electrocatalysts for alcohol oxidation reactions such as MOR and EOR. The MOR tests were performed in the electrolytes of 0.1 M HClO4 + 0.2 M CH3OH at room temperature. In general, the two Pt−Ni NFs/C catalysts display notably higher electrocatalytic activity than commercial Pt/C toward the MOR (Figure 5a). To compare the electrocatalytic activity, the currents normalized by both the ECSA and the mass loading of Pt are shown in Figure 5, panel b. The THH Pt−Ni NFs/C and RDH Pt−Ni NFs/C have the mass activity of 0.84 mA μgPt−1 and 1.04 mA μgPt−1, which are 2.63-times and 3.25-times higher than that of the commercial Pt/C (0.32 mA μgPt−1). However, the specific activities of the THH Pt−Ni NFs/C and the RDH 2765

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activities of THH Pt−Ni NFs/C and RDH Pt−Ni NFs/C are 0.17 mA μgPt−1 and 0.24 mA μgPt−1, which are 5.67-times and 8-times higher than the commercial Pt/C (0.03 mA μgPt−1) after the durability tests. These three catalysts after durability tests were carefully collected and scratched off the electrode by sonication for further TEM characterization. After durability test, the Pt NPs on C aggregated into larger NPs after durability test (Figure S10), while both the alloyed structures and morphologies of THH Pt−Ni NFs/C (Figure S11) and RDH Pt−Ni NFs (Figure S12) largely maintained after 600 sweeping circles, as revealed by TEM images, SEM-EDS, HRTEM images, and HAADF-STEM-EDX elemental mappings. To conclude, a robust wet-chemical approach for the creation of well-defined THH Pt−Ni NCs and RDH Pt−Ni NCs with highly composition segregated feature has been developed. To the best of our knowledge, it is the first case that highly composition segregated THH Pt−Ni NCs has been obtained in a very simple organic solution system. The optimization process of the synthesis demonstrates that the selective use of DTAC and the tuning of OM/OA ratio are very critical for the selective production of well-defined THH Pt−Ni NCs and RDH Pt−Ni NCs. Because of the highly composition segregated feature, these solid Pt−Ni NCs can be readily transformed into Pt−Ni NFs with highly open feature. The detailed catalytic results reveal that these resulting Pt−Ni NFs exhibit much higher performance for both the ORR and alcohol oxidations than those of the commercial Pt/C catalyst, showing a novel class of bimetallic Pt-based catalysts with high performance and improved utilization efficiency of Pt for potentially practical applications.

Figure 5. Pt−Ni NFs/C as high-performance electrocatalysts for alcohol oxidation reactions. Alcohol electrooxidation performance of THH Pt−Ni NFs/C, RDH Pt−Ni NFs/C, and commercial Pt/C. (a) MOR curves were recorded at room temperature in a 0.1 M HClO4 + 0.2 M CH3OH aqueous solution at 50 mV s−1. (b) Specific activity and mass activity for MOR of these three catalysts. (c) EOR curves were recorded at room temperature in a 0.1 M HClO4 + 0.2 M CH3CH2OH aqueous solution at 50 mV s−1. (d) Specific activity and mass activity for EOR of these three catalysts. (e) Variation of normalized peak current densities of MOR in the positive-going potential sweep during potential cycling. Potential was continuously scanned at 50 mV s−1 in 0.1 M HClO4 + 0.2 M CH3OH. TEM images and corresponding geometric models for (f) RDH Pt−Ni NFs/C and (g) THH Pt−Ni NFs/C after MOR. −2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00471. Experimental details and data; supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

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Pt−Ni NFs/C are 2.19 mA cm and 1.90 mA cm , respectively, 3.91-times and 3.39-times greater than the commercial Pt/C (0.56 mA cm−2). The EOR was also applied to compare the activities of the above three catalysts. Similarly, the two Pt−Ni NFs/C catalysts also exhibit higher electrocatalytic activity for the EOR than commercial Pt/C (Figure 5c). Figure 5, panel d summarizes the mass activity and specific activity of the three catalysts toward EOR. The mass activities of THH Pt−Ni NFs/C and RDH Pt−Ni NFs/C are 0.77 mA μgPt−1 and 0.98 mA μgPt−1, which are 2.46-times and 3.16-times higher than the commercial Pt/C (0.31 mA μgPt−1). Nevertheless, the THH Pt−Ni NFs/C has a specific activity of 1.99 mA cm−2, which is the highest compared with RDH NFs/C (1.79 mA cm−2) and commercial Pt/C (0.58 mA cm−2) catalysts, respectively. The durability test of the THH Pt−Ni Pt-Ni NFs/C, RDH Pt−Ni NFs/C, and commercial Pt/C toward MOR were conducted by repeating the CV scans for 600 cycles. The activities of the two Pt−Ni NFs/C remain much higher than the commercial Pt/C even after 600 sweeping cycles. The mass

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the start-up funding from Soochow University and Peking University, Young Thousand Talented Program, the National Natural Science Foundation of China (21571135), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Ralph E. Powe Junior Faculty Enhancement Award. Z.Z., E.Z., and Y.H. acknowledge support from the National Science Foundation (NSF) through Award No. DMR1437263.

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DOI: 10.1021/acs.nanolett.6b00471 Nano Lett. 2016, 16, 2762−2767