Iridium-Based Multimetallic Nanoframe@Nanoframe Structure: An

Jun 9, 2017 - ... B. T.; Brodsky , C. N.; Kuo , C.-H.; Lamontagne , L. K.; Jiang , Y.; Wang , Y.; Tao , F. F.; Huang , W.; Tsung , C.-K. Nanoscale-Sha...
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Iridium-Based Multimetallic Nanoframe@Nanoframe Structure: An Efficient and Robust Electrocatalyst toward Oxygen Evolution Reaction Jongsik Park,†,‡,¶ Young Jin Sa,§,¶ Hionsuck Baik,⊥ Taehyun Kwon,†,‡ Sang Hoon Joo,*,§,∥ and Kwangyeol Lee*,†,‡ †

Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Korea Department of Chemistry and Research Institute for Natural Sciences, Korea University, Seoul 02841, Korea § Department of Chemistry and ∥School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea ⊥ Korea Basic Science Institute (KBSI), Seoul 02841, Korea ‡

S Supporting Information *

ABSTRACT: Nanoframe electrocatalysts have attracted great interest due to their inherently high active surface area per a given mass. Although recent progress has enabled the preparation of single nanoframe structures with a variety of morphologies, more complex nanoframe structures such as a double-layered nanoframe have not yet been realized. Herein, we report a rational synthetic strategy for a structurally robust Ir-based multimetallic double-layered nanoframe (DNF) structure, nanoframe@ nanoframe. By leveraging the differing kinetics of dual Ir precursors and dual transition metal (Ni and Cu) precursors, a core−shell-type alloy@alloy structure could be generated in a simple one-step synthesis, which was subsequently transformed into a multimetallic IrNiCu DNF with a rhombic dodecahedral morphology via selective etching. The use of single Ir precursor yielded single nanoframe structures, highlighting the importance of employing dual Ir precursors. In addition, the structure of Ir-based nanocrystals could be further controlled to DNF with octahedral morphology and CuNi@Ir core−shell structures via a simple tuning of experimental factors. The IrNiCu DNF exhibited high electrocatalytic activity for oxygen evolution reaction (OER) in acidic media, which is better than Ir/C catalyst. Furthermore, IrNiCu DNF demonstrated excellent durability for OER, which could be attributed to the frame structure that prevents the growth and agglomeration of particles as well as in situ formation of robust rutile IrO2 phase during prolonged operation. KEYWORDS: nanoframe, kinetic control, iridium-based nanocrystal, ternary alloy, electrocatalysis, oxygen evolution reaction ollow nanoparticles such as nanocages,1−9 nanoshells,10 nanoboxes,11−13 and nanoframes14−29 have received increasing attention, due to their unusual properties that are distinguishable from solid analogues. They have found extensive utility in areas including catalysis, drug delivery, and optical sensing. Among the classes of hollow nanoparticles, nanoframe or nanoskeletal structures are particularly intriguing with their most open structures.14−29 For catalytic applications, nanoframe structures with thin walls can serve as precious-metal-frugal catalysts, exposing a high percentage of catalytically active surfaces per a given mass. Furthermore, the confined nanospace within nanoframe structures enables facile collisions of reactants on the catalytic

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surfaces, leading to enhanced reactivity. Thus far, nanoframebased catalysts have been focused predominantly on Pt-based catalysts for oxygen reduction reaction in polymer electrolyte fuel cells, with a PtNi alloy as the most notable example.17−19,27 Relatively much less effort has been made with other metalbased nanoframe structures such as Ir or Ru, which are expected to exhibit high catalytic activity for oxygen evolution reaction (OER) in water electrolyzers.24,25,28 Regarding the structural aspect of nanoframes, nanoframes with a variety of Received: January 11, 2017 Accepted: June 9, 2017 Published: June 9, 2017 5500

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Figure 1. Characterization of unetched and etched IrNiCu DNF with rhombic dodecahedral morphology. (a) TEM image of unetched DNF. Inset: Enlarged TEM image. (b) HAADF-STEM image of DNF. Inset: Enlarged HAADF-STEM image. (c) HAADF-STEM image and corresponding elemental mapping analysis images of unetched DNF. Each color indicates Ir (green), Ni (red), and Cu (blue). (d) Combined elemental mapping analysis and (e) line profile data of unetched DNF. (f) HAADF-STEM image and corresponding elemental mapping analysis of DNF. Each color indicates Ir (green), Ni (red), and Cu (blue). (g) Combined elemental mapping analysis and (h) line profile data of DNF. The orange dotted line in HAADF-STEM images (insets of panels e and h) indicates the line scan direction for the corresponding nanocrystal.

morphologies such as triangle,16,20 octahedron,17−19,21,22,27 dodecahedron,23,29 and icosahedron26 are currently available; however, such a morphological diversity is limited to the single nanoframe structures. Although the nanoframes with multiple shells may expand the scope of nanoframe structures as well as enhance structural integrity in catalytic applications, such nanoframe structures have not yet been realized. Herein, we report the synthetic strategy for a double-layered nanoframe structure, namely, nanoframe@nanoframe. Nanoframes can be, in principle, prepared by the selective etching of the leachable component from phase-segregated solid alloy nanoparticles. We envisioned that double-layered nanoframes of a certain metal might be prepared by forming core−shelltype alloy@alloy nanoparticles that would exhibit phase segregations at both core and shell regions and by subsequently leaching out the removable components via etching. We and other groups have recently shown that Ni or Cu can be utilized as the removable component in the phase-segregated alloy nanoparticles.9,17−19 We posited that two different alloy phases of a noble metal, namely, one rich in Cu and the other rich in Ni, can be sequentially formed to give a core−shell component because the decomposition kinetics of disparate Cu and Ni precursors would be inherently different. In order to ensure the continuous supply of noble metal species during the formation of alloy@alloy core−shell structure, it might be necessary to use two or more different noble metal precursors of the same noble metal with different decomposition kinetics. To realize the above-described structure, another important prerequisite is the judicious choice of a precious metal. Thermodynamic

calculations suggest that iridium does not favor the formation of binary alloy phase with Ni or Cu due to the negative segregation energies of IrNi and IrCu phases.30 These data predict that Ir-based multimetallic nanocrystals can serve as desired model systems for core−shell structures, where alloys at both core and shell regions can show phase segregation phenomena. Following the above line of reasoning, in this work, we demonstrate the formation of Ir-based double-layered nanoframe (DNF) structures. We co-decomposed four metal precursors, namely, Ir(acac)3, IrCl3, Ni(acac)2, and Cu(acac)2, and found that the desired IrNiCu DNF structure with a rhombic dodecahedral morphology can be obtained after etching of the multicomponent reaction product. The critical role of employing dual Ir precursors for generating DNF structure is demonstrated, with the use of a single Ir precursor producing Ir-based single nanoframe structures. By simply tuning experimental factors, Ir-based nanocrystals with diverse morphologies and structures could be obtained, including DNF with octahedral morphology and CuNi@Ir core−shell structures. IrNiCu DNF showed high activity for the OER in acidic media, which is better than commercial Ir/C catalyst. Importantly, IrNiCu DNF demonstrated remarkable structural robustness after a long-term durability test, whereas Ir/C underwent significant deactivation with agglomeration of particles. The high durability of IrNiCu DNF can be attributed to the frame structure that minimizes particle coarsening and in situ formation of rutile IrO2 phase during the prolonged operation of OER. 5501

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Figure 2. Temporal evolution of unetched DNF structures. (a−d) TEM images, (e−h) HAADF-STEM images and structural models, and (i−l) line profile images of intermediates at reaction times of (a,e,i) 5 min, (b,f,j) 10 min, (c,g,k) 20 min, and (d,h,l) 40 min. Green, red, and blue colors denote Ir, Ni, and Cu, respectively.

RESULTS AND DISCUSSION In a typical synthesis of IrNiCu DNF, a slurry of Ir(acac)3 (0.01 mmol), IrCl3 (0.01 mmol), Ni(acac)2 (0.04 mmol), Cu(acac)2 (0.02 mmol), CTAC (cetyltrimethylammonium chloride, 0.10 mmol), 1,2-HDD (1,2-hexadecanediol, 0.04 mmol), and oleylamine (15 mmol) was prepared in a 100 mL Schlenk tube with magnetic stirring. Hereafter, this reaction composition will be referred to as the standard composition. After placing the solution under vacuum at 60 °C for 5 min, the Schlenk tube was directly placed in a hot oil bath, which was preheated to 260 °C. After being heated at the same temperature for 40 min, the reaction mixture was cooled to room temperature with magnetic stirring. The reaction mixture, after addition of 15 mL of toluene and 20 mL of ethanol, was centrifuged at 4000 rpm for 5 min. IrNiCu DNF was prepared via selective removal of Ni and Cu components from the resulting nanocrystals by using HCl etchant at 60 °C for 1 h (see experimental details in Materials and Methods). A representative transmission electron microscopy (TEM) image of unetched DNF is shown in Figure 1a. TEM images and ideal models viewed from different zone axes (Figure S1a) reveal the formation of a rhombic dodecahedral morphology. The high-angle annular dark-field scanning TEM (HAADFSTEM) image of unetched DNF and corresponding elemental mapping images for Ir (green), Ni (red), and Cu (blue) in Figure 1c,d, respectively, clearly show the formation of doublelayered structures. To further elucidate the composition of double-layered structural feature, we analyzed line profile data of unetched DNF for Ir (green line), Ni (red line), and Cu (blue line) contents (Figure 1e), which also reveals the formation of double-layered structure. Additional elemental mapping data and line profile analysis are shown in Figure S2. Detailed atomic structure and alloy formation in the unetched DNF were investigated by high-resolution TEM (HRTEM) (Figure S3a). In the core section of the HRTEM image (Figure

S3b), the measured lattice distances are 0.210 and 0.181 nm, which could be indexed to {111} and {200} facets of facecentered cubic (fcc) Cu phase. The lattice distances in this section are close to pure {111}Cu (d = 0.209 nm) and {200} Cu (d = 0.181 nm). However, in the outer periphery of the HRTEM image (Figure S3c), the measured lattice distances are 0.213 and 0.184 nm and could be indexed to {111} and {200} facets, respectively, of the IrNi-based alloy phase. The powder X-ray diffraction pattern (PXRD) of unetched DNF (Figure S4a) shows that the peaks are shifted to higher angle, compared to that of pure Ir, revealing the formation of an Ir-based alloy. The energy-dispersive X-ray spectroscopy (EDS) data of unetched DNF (Figure S4) show the atomic composition of 37.7% Ir, 28.5% Ni, and 33.8% Cu, consistent with the presence of a Cu-enriched phase in the core. The mild acetic acid etching was previously employed to remove template materials such as Ni or Cu.19,31,32 However, the contents of Cu and Ni could not be easily etched out by acetic acid, as shown in Figure S5. The etching behavior of the Ir-based core−shell system can be different from those of Ptbased elements, and it appears that the impermeable Ir shell formation is responsible for the enhanced resistance to acetic acid etching.33 Therefore, in order to completely remove the Cu and Ni phases, we instead employed a stronger acid (HCl) for the formation of DNF. The structure of DNF was analyzed by HAADF-STEM, TEM, and HRTEM images. The HAADF-STEM image and TEM image of IrNiCu DNF (Figures 1b and S1b) clearly show that nanoframes preserve a rhombic dodecahedral morphology after the etching and have the structural feature of nanoframe@ nanoframe. The combined elemental mapping analysis of DNF (Figure 1f,g) shows that, in contrast to unetched DNF, all the components are mixed thoroughly in the entire structure of DNF. The line profile data of DNF for Ir (green line), Ni (red line), and Cu (blue line) contents in Figure 1h demonstrate 5502

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15.6% Ni, and 40.6% Cu by EDS (Figure S11b). The rhombic dodecahedral structural feature could be also seen at 10 min. Further heating of the reaction mixture leads to the formation of a well-defined rhombic dodecahedral structure, observable at 20 min (Figures 2c,g,k and S10c). At this stage, most of the Ni precursor seems to be consumed completely. Therefore, in this step, we could find that the decomposition of the Ni precursor outcompetes the decomposition of both Ir and Cu precursors. The atomic percentages of the intermediate were found to be 28.7% Ir, 41.2% Ni, and 30.1% Cu by EDS (Figure S11c). At 40 min, the Ir component from the IrCl3 precursor starts to grow on the Ni-rich surface (Figures 2d,h,l and S10d). The unreacted Ir precursors further deposited on the nanoparticle surface to result in surface alloying with Ni. The atomic distribution and PXRD data of the intermediates for unetched DNF are shown in Figure S12. The shift of PXRD peaks to lower angle from 5 to 10 min is induced by deposition of Ir and ensuing formation of the Ir-rich phase. In contrast, the shift to higher angle during 10−20 min reaction mainly originates from the growth of Ni on the Ir-rich nanocrystal intermediate. Finally, the shift to lower angles during 20−40 min reaction was observed, indicating the decomposition and growth of Ir. Therefore, the PXRD results are in accordance with the proposed nanocrystal growth mechanism, involving the sequential decomposition of precursors. In summary, the inner nanoframe and outer nanoframe seem to originate from the IrCu alloy and the IrNi alloy, respectively. However, the final DNF structure shows little difference in the compositions of the inner nanoframe and outer nanoframe. Therefore, it appears that interlayer diffusion might have occurred during the chemical etching process to give a thoroughly mixed and stable Ir71Ni24Cu5 phase in the nanoframe of DNF, determined by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis, as shown in Table S1. To identify the role of dual Ir precursors in generating DNF structures, control experiments were performed employing only a single Ir precursor. An interesting morphological development was observed when only one type of Ir precursor was used. With only the Ir(acac)3 precursor, an IrNiCu singlelayered nanoframe (SNF type 1), instead of double-layered nanoframe, was obtained after the etching, as shown in Figure 3a. The HRTEM image of SNF type 1 (Figure 3c) reveals that the lattice spacing of the {111} plane is measured to be 0.213 nm, which is similar to that of DNF {111} plane, indicating the formation of an alloy structure. HAADF-STEM image and corresponding elemental mapping images of SNF type 1 (Figure 3e) also confirm homogeneous distribution of constituting elements throughout the SNF type 1 structure. The overall alloy composition of SNF type 1 is comparable to that of DNF, as confirmed by EDS analysis (Figure S13). Although the decomposition of Ni(acac)2 and Ir(acac)3 precursors is not synchronized, the migration of Ir toward the surface occurred while maintaining the overall morphology.34,35 The similar surface migration phenomenon has been previously documented.27 Etching of the resulting nanocrystals leads to the removal of leachable contents in the nanocrystal center to give Ir-based single-layered frame structures. On the other hand, the use of only IrCl3 as the Ir precursor also affords IrNiCu SNF type 2 with rhombic dodecahedral morphology (Figure 3b). The HRTEM image (Figure 3d) and HAADFSTEM and elemental mapping images (Figure 3f) of the resulting SNF type 2 consistently indicate the formation of an IrNiCu alloy phase. Interestingly, the size of this IrNiCu SNF

that DNF is mainly composed of an Ir-rich ternary alloy phase. The HRTEM image and fast Fourier transformation (FFT) pattern of DNF with a zone axis [110] are shown in Figure S6. An enlarged HRTEM image of DNF (Figure S6b) shows that the nanoframe is composed of an Ir-rich alloy phase; the lattice spacing of {111} plane, {200} plane, and {220} plane are measured to be 0.215, 0.186, and 0.132 nm, respectively, which are shorter than pure {111}Ir (d = 0.221 nm), {200}Ir (d = 0.192 nm), and {220}Ir (d = 0.135 nm). The incorporation of both Cu phase ({111}Cu (d = 0.208 nm), {200}Cu (d = 0.181 nm)) and Ni phase ({111}Ni (d = 0.203 nm), {200}Ni (d = 0.176 nm)) would induce the lattice contraction in an Ir-based alloy phase of DNF. The selected area electron diffraction (SAED) pattern of DNF (Figure S6c) is consistent with HRTEM analysis results. The PXRD pattern of DNF (Figure S7a) shows the shift of diffraction peaks to a lower angle, compared to those of unetched DNF, indicating that the formation of an Ir-based alloy phase is consistent with HRTEM analysis. The EDS data reveal that the atomic composition of DNF is Ir 61.7%, Ni 23.6%, and Cu 14.7% (Figure S7b). The average sizes of unetched DNF and DNF are 18.4 ± 0.8 and 18.3 ± 0.9 nm, respectively (Figure S8). It is clearly shown that the size of nanocrystals remains unchanged after chemically etching the core. To monitor the formation of ternary nanoframes from double-layered nanocrystals in more detail, we obtained the PXRD patterns and EDS-based elemental analysis data of intermediates with varying etching times, as shown in Figure S9. Major peaks of the PXRD patterns are gradually shifted to lower angles with increased etching time from 0 to 60 min, indicating that the composition of formed nanocrystals is gradually changed to an Ir-based phase due to the leaching of Cu and Ni. Moreover, the width of PXRD peaks becomes broader with time, suggesting the thinning of frames. The elemental analysis reveals that the leaching rate of Cu is faster than that of Ni; the atomic percentages of the 15 min intermediate were found to be 52.4% Ir, 24.4% Ni, and 23.1% Cu. The formation of the IrNi alloy phase is more facile than the IrCu phase,30 and the unmixed Cu phase should be more prone to leach out than Ni during the etching process. In order to understand the formation mechanism of DNF, we examined temporal structural evolution of the reaction intermediates for unetched DNF, as shown in Figure 2. At the reaction time of 5 min, the Cu precursor was decomposed first to form small, geometrically poorly defined nanocrystal seeds (Figure 2a,e). At the same time, the small amount of Ir and Ni precursors was also deposited on the Cu seeds through underpotential deposition process. The elemental mapping analysis (Figure S10a) and line profile analysis (Figure 2i) of the intermediate at 5 min clearly show that the nanocrystals of the initial stage mainly consist of IrNiCu ternary alloy phase with a Cu-rich core. Although the reduction potential of Cu is lower than that of Ir, the formation of a Cu-rich nanocrystal is facilitated because the decomposition temperature of the Cu precursor is lower than that of Ir precursors.32,33 The atomic percentages of seeds at 5 min were found to be 37.4% Ir, 21.6% Ni, and 41.0% Cu by EDS (Figure S11a). The structural analysis of an intermediate nanoparticle at 10 min (Figures 2b,f,j and S10b) reveals an interesting structural feature. The outermost part of the nanoparticle is covered by mostly Ir, which leads to a core−shell structure. Therefore, it appears that (1) a Cu-rich seed is initially formed and (2) an Ir-rich shell is formed on the Cu-rich core. At this stage, the atomic percentages of intermediates were found to be 43.8% Ir, 5503

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type 2 is larger than that synthesized from Ir(acac)3, suggesting slower decomposition rate of the IrCl3. Hence, as long as decomposition of IrCl3 occurs, the nanoframe size can increase. The overall schematic illustration for precursor-type-dependent formation of IrNiCu DNF and SNF type 1 and type 2 is shown in Figure 4. We also investigated the impact of concentration of Ni and Cu precursors on the morphology of resulting nanocrystals, with TEM images in Figure S14 showing the unetched nanocrystals. When no Ni precursor was used, porous dendritic IrCu nanostructures are formed (Figure S14a). The porous dendritic morphology is preserved until the amount of Ni(acac)2 was increased up to 0.02 mmol (Figure S14b). Because of the low miscibility between Ir and Cu and fast reduction conditions, the island growth of Ir on the irregular shaped Cu seeds under air conditions could induce dendritic morphology. When the amount of Ni(acac)2 was further increased to 0.04 mmol (standard conditions) and 0.08 mmol, rhombic dodecahedral nanoparticles are obtained (Figure S14c,d). Therefore, the presence of critical amounts of Ni precursor is important in the formation of rhombic dodecahedral morphology. When the concentration of Cu precursor was changed from 0 to 0.02 mmol (standard conditions), 0.04 mmol, and 0.06 mmol, the size of resulting nanocrystals gradually decreased with increased Cu(acac)2 concentration (Figure S14e−h). Notably, the nanocrystals synthesized with higher concentration of Cu(acac)2 yielded hollow nanoframework morphology even without the etching process. We suppose that increased concentration of Cu precursor might generate excessive amounts of Cu seeds at the initial stage, after which the remaining Ir and Ni precursors deposited on Cu seeds through both epitaxial growth and galvanic replacement reaction. The observation of in situ hollowing phenomena could be driven by the instability of the Cu phase.36−40 We found that the concentration of 1,2-HDD, as the reductant, is also critical in the formation of DNF (Figure S15). Below 4 equiv of 1,2-HDD (standard conditions), there is no significant difference in the nanocrystal morphology except the difference in the average size. However, at 8 equiv of 1,2-HDD, rhombic dodecahedral morphology disappeared with the advent of octahedral morphology. We speculate that the reduction of Ir is too facilitated at 8 equiv of 1,2-HDD to

Figure 3. Structural and compositional analyses of IrNiCu nanoframes prepared using a single type of Ir precursor. (a,b) TEM images; (c,d) high-magnification TEM images, FFT patterns, and HRTEM images; and (e,f) HAADF-STEM images and corresponding elemental mapping images of IrNiCu SNF synthesized using (a,c,e) only Ir(acac)3 precursor (SNF type 1) and (b,d,f) only IrCl3 precursor (SNF type 2). Insets in panels a and b show TEM images of unetched IrNiCu SNF. Each color in the elemental mapping images indicates Ir (green), Ni (red), and Cu (blue).

Figure 4. Schematic illustration of precursor-type-dependent formation of IrNiCu DNF and SNF structures. The precursors for each synthetic protocol include (a) Ir(acac)3, IrCl3, Ni(acac)2, and Cu(acac)2; (b) Ir(acac)3, Ni(acac)2, and Cu(acac)2; and (c) IrCl3, Ni(acac)2, and Cu(acac)2. 5504

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Figure 5. Electrocatalytic activity of IrNiCu DNF/C, IrNiCu SNF/C, and commercial Ir/C (20 wt % Ir) catalysts. (a) OER polarization curves of IrNiCu DNF/C, IrNiCu SNF/C, and commercial Ir/C measured in 0.1 M HClO4 before and after durability test of 2500 potential cycles between 1.2 and 1.7 V. (b) Bar graph showing overpotential (η) to drive 10 mA cm−2 and Ir mass activity (jm) at 1.53 V (vs RHE) of the catalysts. Patterned bar indicates the activity parameters after the durability test. (c) HAADF-STEM image, (d) high-resolution HAADFSTEM image, and (e) FFT pattern of IrNiCu DNF/C after the durability test. Green circles in the high-resolution HAADF-STEM image indicate Ir atoms. (f) Iridium oxide unit cell (rutile type) and its orthogonal projection image to the [100] zone axis.

profile data in Figure S17c also indicate the formation of a CuNi@Ir core−shell structure. Finally, we investigated the effect of type of anion and cation on the structure of synthesized nanocrystals. When cetyltrimethylammonium bromide was used instead of CTAC (Figure S18a), little difference in the product morphology was observed, but some very small nanoparticles as impurity were also observed. When dimethyldioctadecylammonium chloride was tested (Figure S18b), random shaped nanocrystals, however, were obtained. Therefore, both CTA+ and Cl− are found to be critical in maintaining the rhombic dodecahedral morphology. Ir-based catalysts have demonstrated excellent electrocatalytic activities for the OER, particularly in acidic media.42−44 We suppose that IrNiCu DNF can serve as a highly promising OER catalyst due to multiple advantages: (i) thin skeletal structure can increase surface area to mass ratio, enabling cost-effective electrocatalysis; (ii) ternary alloy composition can boost OER activity by electronic structure modification; and (iii) double framework can enhance structural robustness. Prior to electrochemical measurements, IrNiCu DNF was supported on carbon black (Vulcan XC-72, Cabot) to afford IrNiCu DNF/C catalyst. Electrocatalytic activities of IrNiCu SNF/C (SNF type 1) and a commercial Ir/ C (Premetek, supported on Vulcan XC-72) were also examined for comparison. We first measured electrochemically active surface area (ECSA) of the catalysts by a CO-stripping method

maintain the {110} facet because the growth rate of Ir is faster than the diffusion rate of Ir. With 0 equiv of 1,2-HDD, HCletched nanocrystals exhibited SNF structure with a rhombic dodecahedral morphology. On the other hand, when the nanocrystals synthesized using 8 equiv of 1,2-HDD were etched with HCl, a DNF structure with octahedral morphology was observed. In summary, the concentration of 1,2-HDD controlled the appropriate reduction rate to divide the metal precursor decomposition steps precisely, thereby inducing a double-layered structure feature. We also carried out a set of experiments by controlling the amount of CTAC, as shown in Figure S16. With lower concentration of CTAC (2.5 equiv) than standard conditions (5 equiv), irregular shaped nanocrystals are observed, owing to insufficient binding of Cl − to the surface. At higher concentrations of CTAC (7.5 and 10 equiv), hierarchical CuNi@Ir core−shell nanostructures are obtained. High concentration of CTAC seems to slow down the reduction of Ir, potentially by forming stable complexes such as [IrCl4]−. More detailed analysis of nanocrystals synthesized using the condition of 10 equiv of CTAC after etching is presented in Figure S17. Although a slight porosity could be observed, the inner CuNi alloy phase is largely unaffected by etching due to the resistance of CuNi alloy phase in acidic conditions (Figure S17a).41 The SAED pattern in Figure S16b suggests that the composition of the hierarchical CuNi@Ir core−shell nanostructure mainly consisted of a NiCu alloy phase. The line 5505

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change could be related to chemical transformation of the catalyst surface from metallic Ir to IrOx.45−47 Second, the common Tafel slope indicates that those catalysts have similar reaction kinetics. Therefore, the higher OER activity of IrNiCu DNF/C can be attributed predominantly to its larger active surface area than those of IrNiCu SNF/C and Ir/C. Dissolution of the Ir species during the durability test is a critical process that significantly contributes to the deactivation of Ir-based OER catalysts.47,48 We conducted inductively coupled plasma mass spectrometry (ICP-MS) of the electrolyte after the durability test (Table S4). IrNiCu DNF/C exhibits the smallest amount of dissolved Ir per electrochemically active surface area of Ir (0.12 μgIr cm−2Ir), which is followed by IrNiCu SNF/C (0.18 μgIr cm−2Ir) and Ir/C (0.24 μgIr cm−2Ir). The dissolved amounts correspond to the loss percentages of 16, 19, and 12% for IrNiCu DNF/C, IrNiCu SNF/C, and Ir/C, respectively. This indicates that IrNiCu nanoframe catalysts possess chemically resistant surfaces to Ir dissolution during the OER compared to Ir/C. Elemental mapping images of IrNiCu DNF/C after the durability test reveal that Ni and Cu atoms are well maintained in the frame (Figure S22). We examined the changes in the chemical states of each species in the catalysts before and after the durability tests by XPS to gain insight into the active phase for the OER. Ir 4f XPS spectra (Figure S23) and deconvolution peaks indicate that most surface Ir atoms existed in metallic state before the OER for IrNiCu DNF/C, IrNiCu SNF/C, and Ir/C.49−52 For IrNiCu DNF/C, a significant portion of Ir was transformed into higher valence Ir (Ir4+ and Ir>4+) after the durability test. IrNiCu SNF/C underwent similar changes in the chemical states of surface Ir to IrNiCu DNF/C. However, the XPS spectra of Ir/C show only a marginal increase in the peak corresponding to oxidized Ir. Mayrhofer and co-workers reported that, during the OER over metallic Ir film, IrOx overlayers were generated in situ, and the resulting catalyst showed a reaction kinetics similar to that of IrO2, which has been known as the active phase for the OER.52 Surface phase transformation from metallic Ir into active IrO2 was much more prominent for IrNiCu DNF/C and IrNiCu SNF/C than for Ir/ C, which may contribute to enhanced dissolution resistance of nanoframe catalysts, compared to that of Ir/C.53 Ni 2p XPS spectra show that the initial chemical state of Ni in IrNiCu DNF/C and IrNiCu SNF/C catalysts is metallic Ni(0) (Figure S24a). After the durability tests, XPS signal intensities for Ni species are significantly decreased, due to the dissolution of Ni species. XPS signal for Cu was almost undetectable due to very low Cu content in IrNiCu nanoframe catalysts (Figure S24b and Table S1). We further scrutinized the structure of IrNiCu DNF/C catalyst after the durability with HAADF-STEM analysis, which reveals that DNF/C is composed of highly crystalline nanoparticles of ca. 2 nm in size (Figure 5c,d). The nanocrystals appear to be Cu- and Ni-doped IrO2 as indicated by elemental mapping analysis (Figure S22). The FFT pattern (Figure 5e) of IrNiCu DNF/C after the durability test clearly shows that the zone axis of Figure 5d is the [100] direction. Notably, the atomic arrangement of Ir atoms in Figure 5d (shown in green circles) can be well matched with Ir atoms in the model structure for rutile phase IrO2 (Figure 5f), suggesting in situ transformation of fcc metallic Ir structure into IrO2-based rutile structure during the long-term durability test. On a final note, we suggest that the OER over IrO2 nanoparticles partially doped with Cu and Ni species would

(Figure S19; see experimental details in Materials and Methods). IrNiCu DNF/C, IrNiCu SNF/C, and Ir/C possess ECSAs of 135 ± 7, 102 ± 3, and 49 ± 2 m2 g−1Ir, respectively. A larger ECSA of IrNiCu DNF/C compared to that of IrNiCu SNF/C and Ir/C can be attributed to highly surface exposed double-frame structure. The OER activity of the three catalysts was measured in 0.1 M HClO4. The OER polarization curves of the catalysts (Figure 5a) and the comparison of overpotentials to drive 10 mA cm−2 and Ir mass activities at 1.53 V (vs RHE) (Figure 5b and Table S2) reveal that IrNiCu DNF/C shows the highest initial activity among the compared catalysts. Large ECSA with highly skeletal structure of IrNiCu DNF/C could contribute to its higher activity compared to that of IrNiCu SNF/C and Ir/C catalysts. The comparison of OER activity with reported Ir-based OER nanocatalysts reveals that IrNiCu DNF/C is a highly promising OER catalyst (Table S3). We assessed long-term durability of the catalysts, which was performed by cycling the potential between 1.2 and 1.7 V (vs RHE) for 2500 cycles. The OER polarization curves after the durability test are displayed in Figure 5a as dotted lines. IrNiCu DNF/C shows nearly identical OER polarization curves before and after 2500 potential cycling, indicating excellent durability. In contrast, the OER activity of IrNiCu SNF/C and commercial Ir/C declined significantly with the increase of overpotential at 10 mA cm−2 by 12 and 14 mV, respectively. Mass activity of IrNiCu DNF/C was well preserved, whereas that of IrNiCu SNF/C and Ir/C decreased by 30% (Figure 5b and Table S2). Excellent structural integrity of IrNiCu DNF/C is confirmed by TEM images taken after the durability test, which show preserved double frame structure (Figures 5c and S20a,d). For IrNiCu SNF/C, some nanoparticles were aggregated after the test (Figure S20b,e). Ir nanoparticles in Ir/C underwent the most severe aggregation, resulting in large particles (Figure S20c,f). We next analyzed the kinetics of the catalysts based on their Tafel slopes for the OER. OER is initiated by water adsorption and the formation of adsorbed OH* intermediate, which is converted to another OH species that is chemically same but energetically different than OH* species (steps 1 and 2, where S indicates the active site). A second proton and electron transfer step yields the oxide intermediate (step 3). Recombination of two oxide intermediates completes one reaction turnover (step 4).45 S + H 2O → S−OH*ads + H+ + e−

(1)

S−OH*ads → S−OHads

(2)

S−OHads → S−Oads + H+ + e−

(3)

S−Oads + S−Oads → 2S + O2

(4)

−1

The Tafel slope of 60 mV dec , which is typically obtained for IrOx catalysts, indicates that steps 1 and 2 are the ratedetermining steps. When step 1 is the sole rate-determining step, the Tafel slope is measured to be 120 mV dec−1.45 Steps 3 and 4 result in Tafel slopes of 40 and 15 mV dec−1, respectively, where the former is observed for metallic Ir surfaces.46 In our case, IrNiCu DNF/C, IrNiCu SNF/C, and Ir/C similarly exhibit Tafel slopes of 48 and 55 mV dec−1 before and after the durability test, respectively (Figure S21). First, the change in the Tafel slope from 48 to 55 mV dec−1 after the durability test suggests that the reaction rate was initially limited by steps 1−3, and step 3 became facile during the durability test. Such a 5506

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reaction mixture was cooled to room temperature with magnetic stirring. The reaction mixture, after being cooled to room temperature and adding 15 mL of toluene and 25 mL of methanol, was centrifuged at 4000 rpm for 5 min. The resulting precipitates were further purified two times by being washed with ethanol/toluene (v/v = 10 mL/5 mL). The resulting precipitates were dispersed in 2 mL of toluene and 2 mL of ethanol and then mixed with 3 mL of 3 M HCl solution. The mixture was placed at 60 °C for 1 h. Finally, the precipitated DNF was centrifuged and washed with ethanol (10 mL) two times and then dried under vacuum. Preparation of IrNiCu Rhombic Dodecahedral SingleLayered Nanoframe. A slurry of Ir(acac)3 (0.02 mmol), Ni(acac)2 (0.04 mmol), Cu(acac)2 (0.02 mmol), CTAC (0.10 mmol), 1,2-HDD (0.04 mmol), and oleylamine (15 mmol) was prepared in a 100 mL Schlenk tube with magnetic stirring. After the solution was placed under vacuum at 60 °C for 5 min, the Schlenk tube was directly placed in a hot oil bath, which was preheated to 260 °C. After being heated at the same temperature for 40 min, the reaction mixture was cooled to room temperature with magnetic stirring. The reaction mixture, after being cooled to room temperature and adding 15 mL of toluene and 25 mL of ethanol, was centrifuged at 4000 rpm for 5 min. The resulting precipitates were further purified two times by washing with ethanol/toluene (v/v = 10 mL/5 mL). The resulting precipitates were dispersed in 2 mL of toluene and 2 mL ethanol and then mixed with 3 mL of 3 M HCl solution. The mixture was placed at 60 °C for 1 h. Finally, the precipitated SNF was centrifuged and washed with ethanol (10 mL) two times and then dried under vacuum. Electrochemical Characterization. Electrochemical measurements were performed using an electrochemical workstation (CHI760E, CH Instruments) with an electrode rotator (AFMSRCE, Pine Research Instrumentation). Rotating ring disk electrode (AFE7R9GCPT), a graphite rod, and Ag/AgCl (saturated KCl) were used as working, counter, and reference electrodes, respectively. Before every measurement, the working electrode was polished on a microcloth with 1.0 μm and subsequently 0.3 μm alumina suspension. Catalyst ink was composed of catalyst and a mixed solution of H2O/ EtOH in the ratio of around 1:10 (v/v). Eight microliters of Nafion solution (D521, Du Pont) was added to the catalyst ink per milligram of catalyst. Ir concentration in the ink was 0.7 μgIr μL−1. Seven microliters of the ink was dropped onto the glassy carbon disk of the working electrode and dried at room temperature, resulting in the Ir loading of 20 μgIr cm−2. The working electrode was equipped to the electrode rotator and soaked in N2-saturated 0.1 M HClO4 (70% Veritas double distilled, GFS Chemicals). All electrochemical data in this work were measured three times, and averaged data were presented. Cyclic voltammetry (CV) was then performed from 0.05 to 1.05 V (vs RHE) at a scan rate of 500 mV s−1 for 50 cycles to clean the surface. Electrochemical impedance spectroscopy (EIS) was conducted from 100 kHz to 1 Hz with AC amplitude of 10 mV at 1.4 V (vs RHE). The x-intercept at a high-frequency region in the Nyquist plot was determined as series resistance for iR compensation. OER activity was measured by CV from 1.2 to 1.8 V (vs RHE) at a scan rate of 20 mV s−1 with the electrode rotation speed of 1600 rpm. Cathodic response of second CV was post-iR-corrected (100%) and used for further analysis and activity comparison (Figure S26). After OER activity measurement, the durability test was performed by CV between 1.2 and 1.7 V (vs RHE) at a scan rate of 50 mV s−1 for 2500 potential cycles. After the cycling, the electrolyte was replaced by a fresh one, and cleaning, EIS, and the activity measurements were repeated. The electrolyte after the durability test was subjected to ICP-MS analysis for determination of the amount of leached Ir. Before the test, electrochemical cell and graphite counter electrode were washed with aqua regia and subsequently rinsed with boiling water several times to remove metal impurities. For CO stripping,55 CO molecules were adsorbed on the catalyst surface under a constant potential of 0.10 V (vs RHE) for 5 min with 30% CO (Ar-balanced) bubbling, and dissolved CO in the electrolyte was flushed by N2 bubbling for the next 20 min. Three cycles of CV at a scan rate of 50 mV s−1 were applied from 0.05 to 1.15 V (vs RHE). The current peak arising in the potential range from 0.7 to 1.15 V

be more facile than that over undoped IrO2. In undoped rutile IrO2, the IrO6 structural unit possesses electronic configuration of t2g5eg0 under an octahedral crystal field. The bonding strength of the adsorbate with empty antibonding states would be strong. When Cu is doped in IrO2, the electronic structure of IrO2 is modified via Jahn−Teller distortion.53 Such a distortion results in partial occupation in eg orbital and upshift in the d-band position. The antibonding states of the resulting adsorbates are then partially occupied, which weakens the interaction between the catalyst and O to an optimal strength and consequently boosts the OER activity (Figure S25). The doping of Ni species is also reported to promote the OER activity; Ni in IrNiOx catalysts has been suggested to generate the active surface Ir−OH species while being dissolved.54 In addition, it is worthwhile to note that there are multiple grain boundaries between the doped IrO2 nanocrystals, which may help relieve the structural deformation taking place under high potential conditions. Such a structural feature could contribute in part to the excellent durability of IrNiCu DNF/C catalyst.

CONCLUSIONS In summary, we have developed a rational synthetic strategy toward Ir-based multimetallic double nanoframe structure, which showed excellent activity and durability for the OER in acidic media. The monodisperse Ir-based DNF could be conveniently obtained by exploiting the differences in decomposition kinetics of different metal precursors. We believe that the simple synthetic strategy of this study can be, in principle, further extended to other catalytically active frame nanostructures. Synthesis of other complex nanoframe structures is currently underway. MATERIALS AND METHODS Reagents. Ir(acac)3 (97%), IrCl3 (99.99%), Ni(acac)2 (95%), Cu(acac)2 (99.9%), CTAC, 1,2-HDD (technical grade, 90%), and oleylamine (98%) were purchased from Sigma-Aldrich. All reagents were used as received without further purification. Material Characterization. TEM and HRTEM studies were carried out in a Tecnai G2 F30ST microscope and a Tecnai G2 20 Stwin microscope, respectively. Aberration-corrected imaging and high spatial resolution EDS were performed at FEI Nanoport in Eindhoven using a Titan Probe Cs TEM 300 kV with Chemi-STEM technology. EDS elemental mapping data were collected using a higher efficiency detection system (Super-X detector with XFEG); it integrates four FEI-designed silicon drift detectors very close to the sample area. Compared to a conventional EDX detector with Schottky FEG systems, Chemi-STEM produces up to 5 times the X-ray generation with the X-FEG and up to 10 times the X-ray collection with the Super-X detector. All scanning transmission electron microscopy images and compositional maps were acquired with the use of HAADF-STEM. PXRD patterns were collected to understand the crystal structures of IrNiCu nanocrystals with a Rigaku Ultima III diffractometer system using a graphite-monochromatized Cu Kα radiation at 40 kV and 40 mA. Metal contents in IrNiCu/C catalysts were determined by an ICP-OES analyzer (700 ES, Varian). Ir concentration after the OER durability test was analyzed by ICP-MS (ELAN DRC-II, PerkinElmer). Preparation of IrNiCu Rhombic Dodecahedral DoubleLayered Nanoframe. A slurry of Ir(acac)3 (0.01 mmol), IrCl3 (0.01 mmol), Ni(acac)2 (0.04 mmol), Cu(acac)2 (0.02 mmol), CTAC (0.10 mmol), 1,2-HDD (0.04 mmol), and oleylamine (15 mmol) was prepared in a 100 mL Schlenk tube with magnetic stirring. After the solution was placed under vacuum at 60 °C for 5 min, the Schlenk tube was directly placed in a hot oil bath, which was preheated to 260 °C. After being heated at the same temperature for 40 min, the 5507

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originates from the oxidative desorption of the monolayer CO on the catalyst surface. The peak area corresponds to the CO stripping charge and the normalization of the value using the known specific charge 420 μC per 1 cm2 of Ir surface to give the ECSA. For XPS analysis of the catalysts after the durability test, 55 μL of the above-stated catalyst ink was dropped onto 0.8 cm × 0.8 cm carbon paper (60 μgIr cm−2) and dried at room temperature. Cleaning, EIS, and the activity measurement were performed as described above. XPS was conducted with the catalyst-loaded carbon paper before and after the durability test using X-ray photoelectron spectrometer (KAlpha, Thermo Scientific) with X-ray source of a monochromatic Al Kα.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00233. Figures S1−S26 and Tables S1−S4 give more details on characterization of our synthesized materials and their electrocatalytic performance data; additional TEM, line profile analysis, HRTEM, XRD, elemental mapping analysis, EDS, ICP-OES, and electrocatalytic performance data (PDF)

AUTHOR INFORMATION Corresponding Authors

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

Sang Hoon Joo: 0000-0002-8941-9662 Kwangyeol Lee: 0000-0003-0575-7216 Author Contributions ¶

J.P. and Y.J.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by IBS-R023-D1, NRF2017R1A2B3005682, Korea University Future Research Grant, and NRF- 2017R1A2B2008464. Y.J.S. acknowledges the Global Ph.D. Fellowship (NRF-2013H1A2A1032644). The authors thank Korea Basic Science Institute (KBSI) for the usage of their HRTEM instrument. REFERENCES (1) Sun, Y.; Mayers, B.; Xia, Y. Metal Nanostructures with Hollow Interiors. Adv. Mater. 2003, 15, 641−646. (2) Skrabalak, S. E.; Chen, J.; Sun, Y.; Lu, X.; Au, L.; Cobley, C. M.; Xia, Y. Gold Nanocages: Synthesis, Properties, and Applications. Acc. Chem. Res. 2008, 41, 1587−1595. (3) Mahmoud, M. A.; Narayanan, R.; El-Sayed, M. A. Enhancing Colloidal Metallic Nanocatalysis: Sharp Edges and Corners for Solid Nanoparticles and Cage Effect for Hollow Ones. Acc. Chem. Res. 2013, 46, 1795−1805. (4) Mahmoud, M. A.; Saira, F.; El-Sayed, M. A. Experimental Evidence for the Nanocage Effect in Catalysis with Hollow Nanoparticles. Nano Lett. 2010, 10, 3764−3769. (5) Zhang, L.; Roling, L. T.; Wang, X.; Vara, M.; Chi, M.; Liu, J.; Choi, S.-I.; Park, J.; Herron, J. A.; Xie, Z.; Mavrikakis, M.; Xia, Y. Platinum-Based Nanocages with Subnanometer-Thick Walls and WellDefined, Controllable Facets. Science 2015, 349, 412−416. (6) Wu, H.-L.; Sato, R.; Yamaguchi, A.; Kimura, M.; Haruta, M.; Kurata, H.; Teranishi, T. Formation of Pseudomorphic Nanocages 5508

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DOI: 10.1021/acsnano.7b00233 ACS Nano 2017, 11, 5500−5509