Epitaxial Growth of Twinned Au–Pt Core–Shell Star-Shaped

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Epitaxial Growth of Twinned Au-Pt Core-Shell StarShaped Decahedra as Highly Durable Electrocatalysts Ting Bian, Hui Zhang, Yingying Jiang, Chuanhong Jin, Jianbo wu, Hong Yang, and Deren Yang Nano Lett., Just Accepted Manuscript • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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Figure 1. Morphological, structural, and compositional characterizations of Au−Pt star-shaped decahedra prepared using the standard procedure: (a) TEM image, (b) HAADF-STEM image, (c) EDX mapping and (d) line-scan analysis, (e) HRTEM image, and (f) HAADF-STEM image at a higher magnification. The insets in a and b show the magnified TEM and HAADF-STEM images of a single star-shaped decahedron, respectively. The red and green colors in c correspond to Au and Pt elements, respectively. 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

tuning the ratio of deposition and diffusion rates of Pt atoms under kinetic control, showing the substantially enhanced ORR properties in terms of activity and durability relative to Pt/ C.23,24 Although significant advances have been achieved in the synthesis of Pt ultrathin shell catalysts with highly active and durable ORR properties, their ORR properties, especially for durability, still have a large room for further improvement. In addition to composition control and structure design, strain engineering provides another promising approach to tailoring the catalytic properties, and has received abundant research interests lately. In general, nanocrystals with twinned structures can exhibit substantially enhanced catalytic properties for a given reaction relative to their single-crystal counterparts due to the strain.25−27 For example, we recently demonstrated that the area-specific activity of icosahedral Pt3Ni catalysts for ORR was about 50% higher than that of the octahedral Pt3Ni catalysts, even though both shapes are enclosed by the {111} facets.28 Similar result was also reported in icosahedral Pd@Pt core−shell nanocrystals with ultrathin Pt shells.29 This enhancement was attributed to strain-induced electronic effects. However, these twinned nanocrystals were vulnerable to highly corrosive medium for ORR because of the highly active sites at the twin boundaries, leading to the decrease of their durability.

Compared to Pd as the substrate, incorporation of Au into Pt-based catalysts was recently shown to help preventing the electrocatalysts from degradation by up-shifting the dissolution potential of Pt, thereby assuring excellent long-term stability.30,31 Au is also more abundant than Pt in annual production (over ten times production of Au relative to Pt in 2014). Recent studies indicated that Au nanocrystal generally tends to take a multiply twinned shape in small sizes (e.g., < 10 nm) with the most stable forms being the most packaged icosahedral and decahedral structures.32,33 Thus, this multiply twinned structure of Au is a promising candidate to serve as a seed in generating the twinned Pt-based bimetal via seeded growth. More significantly, a combination of Pt with Au impacts greatly the electronic structures due to strong coupling between these two metals, making Au−Pt bimetallic nanocrystals attractive catalysts for ORR.34−36 Among them, generation of Au core completely encased by Pt shell is therefore highly desirable for maximizing the electronic coupling between Pt and Au.20 Such Au@Pt core−shell nanoparticles with surface-engineered twinning can be a unique structure that fully utilizes all the key parameters including facet, interface and internal structure. However, the deposition of Pt on the surface of an Au seed typically adopted an island growth mode because of the restriction from their surface energy and lattice mismatch B

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Figure 2. TEM and HAADF-STEM-EDX mapping images of the Au−Pt nanocrystals that prepared using the standard procedure, except for the different molar ratios of Au to Pt salt precursors: (a) 1:1, (b) 1:0.9, (c) 1:0.8, (d) 1:0.7, (e) 1:0.6, and (f) 1:0.5. 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137

between Pt and Au,37 leading to the formation of Au−Pt nanodendrites.38−41 Despite several recent attempts on the synthesis of Au−Pt conformal core−shell nanoparticles,42,43 the epitaxial growth between Au and Pt has not yet been demonstrated. In addition, the well-known core−shell nanostructures contained a lot of defects in the interface, which are detrimental to the ORR catalytic properties, in particular stability. Therefore, it still remains a challenge to epitaxially grow Pt shell on twinned Au cores with less defects at the interface, especially those with well-defined shape. Here we report a facile approach that allows one-pot epitaxial growth of uniform Pt shell on Au decahedral cores with starlike shape and perfect interface in high yield (>90%). The thickness of Pt epitaxial shell was accurately controlled by varying the amount of the Pt precursor. Compared to carbon supported Au−Pt core−shell spherical nanoparticles and reference Pt/C, Au−Pt core−shell star-shaped decahedra supported on carbon exhibited substantially enhanced catalytic properties in terms of activity and durability for ORR. In a typical synthesis, Au−Pt core−shell star-shaped decahedra were generated by simultaneously injecting HAuCl4 and H2PtCl6 into a mixture of oleylamine (OAm), hexadecyltrimethylammonium bromide (CTAB), and trioctylphosphine oxide (TOPO) at 180 °C with a pipet (see Supporting Information for the experimental details). The color

of the solution turned from yellow to pink immediately upon addition of both the metal precursors and then evolved into dark brown, suggesting the reduction of Au and Pt salt precursors, respectively, and the formation of phase-separated structures. Figure 1 shows morphological, structural, and compositional characterizations of the Au−Pt nanocrystals that were obtained by the aforementioned approach for 3 h with the molar ratio of Au and Pt salt precursors being 1:1 (defined as the standard procedure). From transmission electron microscopy (TEM) image in Figure 1A, most of the nanocrystals were observed to have a star-shaped profile with an average size of 16 nm. The size was defined as the diameter of a circumscribed circle of a pentacle projection, as shown in Figure S1. The population of star-like shape was about 95% based on the analysis of TEM micrographs at low magnifications (Figure S2). The magnified TEM image (inset of Figure S2) clearly shows the presence of 5-fold twins from the center of a nanocrystal, implying the formation of starshaped decahedron. The TEM images obtained from a starshaped decahedron at different tilting angles match well with the corresponding three-dimensional models viewed along different axes (Figure S3). The star-shaped decahedron was also supported by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image in Figure 1B. The elemental distribution of Au and Pt in the star-shaped C

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Figure 3. TEM images of the products prepared using the standard procedure at different reaction times: (a) 1, (b) 20, and (c) 60 min. The upper and lower insets show the corresponding HRTEM and EDX mapping images of a single nanocrystal with 5-fold twins, respectively. The scale bars in the insets are 2 nm. (d) Change in mol % of Pt (ΔPt) after the reaction for 1, 5, 20, 30, 45, 60, 180, and 240 min, respectively. 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197

elevated-temperature (e.g., 180 °C). This demonstration was also supported by the characteristic localized surface plasmon resonance (LSPR) peak of Au nanocrystals centered at 520 nm in UV−vis adsorption spectrum. As the reaction proceeded, the LSPR peak of Au nanocrystals exhibited a slight blue shift, and eventually disappeared, which was due to the formation of Pt shell on the surface of Au nanocrystals. This observation clearly indicated a seeded growth, in which Au salt precursors were preferentially reduced to Au nanocrystals by OAm in the initial stage and then served as seeds for subsequent growth of Pt layer, owing to the relatively larger redox potential of AuCl4−/ Au relative to PtCl62−/Pt (e.g., 0.915 V for AuCl4−/Au and 0.8 V for PtCl62−/Pt versus RHE). This seeded growth mechanism was confirmed by the result of our two-step synthesis in which Au seeds were first produced and then used as seeds to epitaxial growth of Pt shells separately in two steps. This procedure also yielded Au−Pt star-shaped decahedra but with poor size distributions (Figure S5). Based on the separated reduction behavior of Au and Pt salt precursors, we were able to readily tune the thickness of Pt shell on Au core by varying the amount of H2PtCl6 while keeping the concentration of HAuCl4 constant. Figure 2 shows TEM and HAADF-STEM-EDX mapping images of six samples prepared with the different molar ratios of Au to Pt salt precursors varied from 1:1 to 1:0.9, 1:0.8, 1:0.7, 1:0.6, and 1:0.5. As can be seen, all of these six samples were dominated by the Au−Pt starshaped decahedra with a core−shell structure. It is clear that the Pt shell thickness of the star-shaped decahedra decreased with the amount of Pt salt precursor fed in the synthesis. Figure S6 compares the average Pt shell thickness of these six samples that prepared with different molar ratios of Au to Pt salt precursors. The thickness of Pt shell was defined as the distance from the concave vertex of a star to the surface of a sphere along {111} twin plane (inset of Figure S6). The data were obtained from the nanocrystals randomly selected from high-

decahedron was determined by both energy dispersive X-ray (EDX) mapping and line-scan analysis. From the EDX mapping image (Figure 1C), Au could be observed only in the interior (red color), whereas Pt was distributed throughout the entire nanocrystal including the shell (green color), suggesting a core−shell structure. The Au−Pt core−shell structure was also supported by the data of EDX line-scan recorded through the center of a single star-shaped decahedron (see Figure 1D). As observed, the Au trace has one peak in the center, while the Pt trace has two peaks at two ends, further confirming a core−shell structure. The typical high-resolution TEM (HRTEM) image (Figure 1E) of an individual starshaped decahedron clearly shows that the nanocrystal consisted of five single-crystalline domains with a twin-based adjoining plane between two neighboring ones. A star-shaped decahedron can also be described as a truncated decahedron in which the truncation reaches the maximum. In addition, the HRTEM image reveals the continuous lattice fringes from the Au core to the Pt shell, indicating that the Pt shell were grown epitaxially on the Au seed. This result was further confirmed by HAADFSTEM in Figure 1F. The fringes with a lattice spacing of 2.30 Å can be indexed to {111} planes of face-centered cubic (fcc) Au and Pt, indicating that the exposed facets were dominated by {111} planes. The atomic ratio of Au and Pt in this core−shell structure was about 1:1 as quantitatively determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Table S1), which was consistent with the molar ratio of the Au to Pt salt precursors fed into the synthesis. During the synthesis of the Au−Pt star-shaped decahedra, we could monitor the progress of the synthesis through ultraviolet−visible (UV−vis) adsorption analysis, together with the evolution of the solution color (Figure S4). The color of the solution turned from yellow to pink after immediately upon addition of the precursors for 1 min, indicating the formation of Au nanocrystals by fast reduction of HAuCl4 with OAm at D

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resolution EDX mapping images. The average thickness of Pt shell was about 2.2, 2.0, 1.8, 1.6, 1.4, and 1.2 nm when the Au/ Pt molar ratios varied from 1:1 to 1:0.9, 1:0.8, 1:0.7, 1:0.6, and 1:0.5, respectively. On the basis of ICP-AES analysis (Table S1), the Au/Pt atomic ratio of these samples is close to the feeding ratio of Au to Pt salt precursors. In order to clarify the formation mechanism of the Au−Pt star-shaped decahedra, a series of samples were taken from the reacting solution at different reaction times for TEM characterization, as shown in Figure 3. In the initial stage of the reaction (Figure 3a, t = 1 min), a large number of spherical nanocrystals with an average size of 8 nm were obtained. The HRTEM image (inset of Figure 3a) clearly reveals the presence of 5-fold twins from the center of the rounded decahedron. The high-resolution EDX mapping (inset of Figure 3a) shows that very few Pt atoms started to deposit on the surface of the rounded Au decahedra. No Pt was detectable in the product at this time by the ICP-AES analysis (Figure 3d). High reaction temperature (180 °C) was likely responsible for the formation of decahedral nanocrystals, because this structure was considered to be one of the most stable forms under thermodynamic control.44 When the reaction was extended to t = 20 min (Figure 3b), the 5-fold twinned nanocrystals increased to 10 nm in size. A large percentage of Pt (ca. 0.2 molar ratio of Pt/Au) was detected in these 5-fold twinned nanocrystals due to the reduction of Pt salt precursor assisted by Au seeds (see Figure 3d). HRTEM and high-resolution EDX mapping analyses (insets of Figure 3b) indicate that most of the newly formed Pt atoms were preferentially added to five singlecrystalline domains of Au decahedral seeds with only a few Pt atoms being deposited at five twin boundaries to generate the decahedra with five tiny bumps. This preferential epitaxial growth behavior was dominated by thermodynamic control since the decahedron with a truncation at twin boundaries (i.e., Marks decahedron) was more thermodynamically stable than the regular one.45 In addition, the epitaxial growth of Pt shell on Au seeds can also enhance total surface energy because Pt has a higher surface energy than Au.46 The extra surface energy introduced by Pt atoms induces a stress-release response through a localized epitaxial growth of Pt on each singlecrystalline domains of Au decahedral seeds.47 When the reaction proceeded to 60 min (Figure 3c), facets developed to minimize both the strain and total surface energies of a nanocrystal, thus promoting the growth into star-shaped decahedra. In this case, the decahedral Au seeds were completely encased by Pt shell with five protruded tips (insets of Figure 3c). The atomic ratio of Pt to Au in the nanocrystals is close to the ratio supplied in the reaction (see Figure 3d). After this stage, the atomic ratio of Pt to Au was kept largely unchanged in the nanocrystals due to the depletion of precursors in the solution. High-quality star-shaped Au−Pt decahedra of 16 nm in size were obtained when the reaction was conducted for 3 h (the standard procedure). Further extension to 8 h (Figure S7) did not result in any obvious morphological change, indicating that the star-shaped decahedra were highly stable at this elevated temperature. In general, the island growth mode, also known as the Volmer−Weber mode, is preferred for the growth of Pt on Au seeds due to the higher surface energy of Pt than that of Au, resulting in the formation of Au−Pt branched structures.41 The key to layer-by-layer epitaxial growth (Frank−van der Merwe mode) of Au−Pt core−shell nanocrystals was the use of OAm in the solution. According to the previous studies,48 the

incorporation of amine species can dramatically decrease the surface energy of Pt due to the strong adsorption, and thus meet the criteria associated with the surface energy in the epitaxial growth of Pt shell on Au core, resulting in epitaxial growth between Pt shell and Au seeds. This argument was confirmed by the control experiments, as shown in Figure S8. When OAm was replaced by other solvent without amine group such as tetrahydronaphthalene, Au−Pt nanodendrites were obtainable through the island growth (Figure S8a). Au−Pt dendrites evolved into star-shaped decahedra in a core−shell structure, as the amount of OAm was increased in the reaction (Figure S8b). In addition, Au−Pt star-shaped decahedra were also obtained when the reaction was conducted in other amine such as octadecylamine (Figure S8c), further confirming that amine played an important role in facilitating the epitaxial growth of Pt on Au seeds. Besides the solvent, reaction temperature has a great impact on the morphology of the Au−Pt star-shaped decahedra, as shown in Figure S9. Only small nanoparticles with sizes less than 10 nm were observed when the temperature was set to 140 °C (Figure S9a). In contrast, increasing the temperature to 160 °C led to the formation of the star-shaped decahedra (Figure S9b). Significantly, the amount of the star-shaped decahedra was increased as the reaction temperature was further increased to 190 and 200 °C, as shown in Figure S9, c and d. These temperature-dependent experiments further confirmed that the growth of the star-shaped Au−Pt decahedra was a result of thermodynamically controlled reduction. We also systematically investigated the effects of other important experimental parameters on the morphology of the Au−Pt starshaped decahedra. In the standard procedure except for the absence of TOPO, a large number of star-shaped decahedra with nonuniform sizes were obtained (Figure S10a), indicating that TOPO acted as coordinating ligands to ensure such nanocrystals uniform. In addition, we found that the amount of CTAB also had a great influence on formation of the Au−Pt star-shaped decahedra. In the absence of CTAB, spherical nanoparticles with size of about 5 nm were generated (Figure S10b). The HAADF-STEM, EDX mapping and line-scan and HRTEM analysis shows that the spherical nanoparticles were Au−Pt core−shell structures with 5-fold twins (Figure S11). Such 5-fold core−shell nanoparticles were used as a benchmark for ORR in the following section. As the amount of CTAB were increased (e.g., 30 and 100 mg, see Figure S10c and d), the size of the nanoparticles were also enlarged since CTAB has a strong effect on their growth rate by strong complexion with PtCl62− and AuCl4− ions.49,50 Only when the amount of CTAB exceeded 300 mg, high-quality star-shaped decahedra exposed with {111} facets were successfully obtained, indicating that selective adsorption of CTAB on the {111} facets of Pt. The effectiveness of Br− ions from CTAB can be further confirmed by replacing CTAB with the same amount of cetyltrimethylammonium chloride (CTAC, without Br− ions, Figure S10e) and didecyldimethylammonium bromide (DDAB, containing Br− ions, Figure S10f), respectively. The electrocatalytic property of carbon-supported Au−Pt core−shell star-shaped decahedra (Au−Pt/C) catalysts was measured on the three electrode setup. The cyclic voltammetry (CV) curves of these carbon-supported catalysts were recorded at room temperature in Ar-purged 0.1-M HClO4 solutions at a sweep rate of 50 mV/s between 0.05 and 0.1 V. The electrochemically active surface area (ECSA) was calculated by measuring the charge collected in the hydrogen adsorption E

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Figure 4. (a) Polarization curves of AuPt star catalysts with various composition, AuPt NC, and Pt/C. (b) area-specific (is, mA/cm2Pt) and (c) mass (im, mA/μgPt) ORR activities for AuPt star catalysts, AuPt NC, and Pt/C reference catalyst. (d) is and im at 0.9 V (vs RHE).

Figure 5. Polarization curves of (a) AuPt1.03 star, (b) AuPt0.4 star, (c) AuPt NC, and (d) Pt/C before and after ADT. The ADT was carried out between 0.6 and 1.0 V at a scan rate of 100 mV/s−1 for 30 000 cycles in an oxygen saturated 0.1-M HClO4 solution. 359 360 361 362 363 364 365 366

region assuming an adsorbed hydrogen monolayer on metal surface, which is 210 μC/cm2.2 The Au−Pt star-shaped nanocrystals had a similar specific ECSA, which are 46.2 m2/ gmetal for AuPt0.4, 48.8 m2/gmetal for AuPt0.62, 48.1 m2/gmetal for AuPt0.68, 46.6 m2/gmetal for AuPt0.69, 48.4 m2/gmetal for AuPt0.79, and 43.5 m2/gmetal for AuPt1.03, respectively (see Table S2 and Figure S12). This result indicates that with the similar size, the entire surface of Au−Pt star-shaped nanocrystals were covered

by Pt shell in the compositions of Pt between 0.4 and 1. This ECSA measurement is consistent with the EDX mapping (Figure 2). The electrocatalytic property of Au−Pt star-shaped nanocrystals with different Au/Pt atomic ratios in ORR was tested in 0.1 M HClO4 solution and compared with that of AuPt spherical nanocrystals and reference Pt/C (Figure 4). The ORR onset potentials were increased with the increased amount of Pt F

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in the Au−Pt star-shaped nanocrystals and became leveled when the composition became AuPt0.62 or higher in Pt. These data indicate oxygen reduction occurred at high potentials (i.e., low overpotentials) when the layer thickness of Pt increased, indicating increasing activity with average Pt layer thickness, up to a limit of six Pt atomic layers estimated from AuPt0.62. Other Au−Pt star-shaped nanocrystals followed the same trend, that is, to a limited thickness the thicker the Pt layers were, the higher the ORR activity. This phenomenon can be interpreted by lattice mismatch-induced strain effect.51 Since Au has larger lattice and its atomic size is bigger than Pt, expansive strain is generated in the Pt epitaxial layer of Au−Pt core−shell nanocrystals. As previously reported by Nørskov et al.52 the substrate-induced expansive strain tends to cause an upshift of the d-band center, thereby leading to the decreased ORR activity. With the increased layer thickness of Pt, such substrateinduced expansive strain at the outmost Pt layer was gradually alleviated, because the prior formed Pt atoms served as the buffer layer, resulting in the enhanced ORR activity. Among the series of star-shaped, decahedral Au−Pt/C catalysts, the AuPt1.03 has one of the highest ORR mass activity of 0.94 mA/μgPt based on unit mass of Pt, and the highest area activity of 1.09 mA/cm2 (see Table S2), which is over 6.7 and 5 times, respectively, as high as the mass and area activity of Pt/C.2 The AuPt1.03 star-shaped decahedron also shows a five-time enhancement over AuPt NC in both mass and area activities. This improvement is attributed to the synergetic effect of the ORR preferred {111} facets and twin structure, which has also been reported to improve the ORR properties in the cases of Pt icosahedron and Pd@Pt icosahedron.29,53 Specifically, the 5fold twined structure in the Au−Pt star-shaped decahedra played a key role in improving their ORR activity due to the strain-induced electronic effects, which was also demonstrated in our previous report.28 Compared to the AuPt1.08 spherical nanocrystals (i.e., AuPt NC), the AuPt1.03 star-shaped decahedra had similar compositions but exhibited higher mass and area activity, indicating the effect of {111} facets on the enhanced ORR activity. The ORR stability of the star-shaped AuPt1.03 and AuPt0.4 catalysts was further studied by accelerated stability test (ADT) for 30 000 cycles in 0.1 M HClO4 solution. Both catalysts show high stability with a half-wave potential loss of 17 mV for AuPt1.03/C and 22 mV for AuPt0.4/C, respectively. In comparison, the spherical AuPt1.08/C lose 29 mV and the Pt/ C lose 87 mV (Figure 5). The area activity of AuPt0.4/C remained to be 0.20 mA/cm2 with a loss of 10% (Figure 5a) and the AuPt1.03/C remained to be 1.09 mA/cm2 with a loss of ∼10%, which is over 6 times higher than the Pt/C in area activity (see Table S3). On stability, the Pt/C catalyst loses over 50% in mass activity under the same ADT conditions, and AuPt1.03/C kept ∼80% of its mass activity. To exclude the size effect on the ORR stability, the 5-fold twinned Pt icosahedra with the similar size as the Au−Pt star-shaped decahedra were synthesized according to our previous report53 and then evaluated as the catalysts for ORR. From Figure S13, such Pt icosahedra showed a loss of ∼44% in area activity (Figure S13) after 30 000 cycles. The convergent evidence shows that the Au core in the Au−Pt star-shaped, core−shell structure helps enhance the stability in ORR, while {111} structure with proper Pt layer is attributed to the high activity. Figure S14 shows TEM and HAADF-STEM-EDX mapping of the star-shaped AuPt1.03 and AuPt0.4 catalysts after ADT for 30 000 cycles in 0.1 M HClO4 solution. From TEM images in Figure S14a and c,

the AuPt nanocrystals were still well-dispersed on the carbon support after the tests. The HAADF-STEM-EDX mapping images (Figure S14b and d) shows that the Pt atoms were dissolved only at five corners of the star-shaped decahedra, while the {111} twin structures and Pt terrace layers were largely intact after the tests. As a result, the star-shaped AuPt catalysts showed high stability toward ORR, and their activity only dropped slightly after ADT tests. In summary, we developed a facile, one-pot approach to the synthesis of uniform Au−Pt core−shell star-shaped decahedra with different Pt shell thicknesses in high yield. The Pt shell was epitaxially grown on Au core, leading to lattice-matched interface between Au and Pt. A key to the epitaxial growth of Pt on Au was the use of amine, which dramatically decreased the surface energy of Pt by the strong adsorption. Reduced growth rate due to strong complexion between Br− ion and the salt precursor and the selective adsorption on {111} facets of Pt facilitate the formation of the star-shaped decahedra. The Au− Pt core−shell star-shaped decahedra with the thick Pt shell exhibited the enhanced activity toward ORR relative to the Au−Pt spherical nanocrystals and the Pt/C, with the AuPt1.03 star-shaped decahedra being the best catalysts. Significantly, the Au in such star-shaped decahedron can substantially enhance the durability toward ORR relative to the Pt/C due to the likely strong electron interaction arising from lattice-matched interface between Au and Pt. This work provided a new strategy to design advanced Pt-based ORR catalysts with enhanced the ORR catalytic performance through synergistically controlling a range of key structural parameters including facet, interface, internal structure, composition, and size.

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* Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02960. Experimental procedure (sample synthesis, characterization, and electrocatalysis) and figures (Figures S1− S15) (PDF)

AUTHOR INFORMATION

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Corresponding Authors

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

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work on electron microscopy was carried out in the Center for Electron Microscopy of Zhejiang University. We would like to thank Prof. Yong Wang and Ms. Ying Jiang from Zhejiang University for the part of TEM analysis. We acknowledged financial support by the National Science Foundation of China (51372222, 51222202, 51472215, 51522103, and 91333115), the National Basic Research Program of China (2014CB932500 and 2015CB921000), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037 and IRT13R54), the Fundamental Research Funds for the Central Universities (2014FZA4007), the US NSF (CHE-1213926), and start-up funds from Shanghai Jiao Tong University. G

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DOI: 10.1021/acs.nanolett.5b02960 Nano Lett. XXXX, XXX, XXX−XXX

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