Effective Fabrication and Electrochemical Oxygen Evolution Reaction

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Effective Fabrication and Electrochemical Oxygen Evolution Reaction Activity of Gold Multipod Nanoparticles Core – Cobalt Sulfide Shell Nanohybrids Hien Duy Mai, Van Cam Thi Le, and Hyojong Yoo ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01689 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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ACS Applied Nano Materials

Effective Fabrication and Electrochemical Oxygen Evolution Reaction Activity of Gold Multipod Nanoparticles Core – Cobalt Sulfide Shell Nanohybrids Hien Duy Mai, Van Cam Thi Le, and Hyojong Yoo* Department of Chemistry, Hallym University, Chuncheon, Gangwon-do, 24252, Republic of Korea

KEYWORDS: Gold multipod nanoparticles (GMN); ZIF-67; transition metal chalcogenides nanocages; Oxygen evolution reaction (OER); Synergistic catalytic activity; Core-shell nanostructure

ABSTRACT: Inorganic hybrid materials with anisotropic noble metal nanoparticles core and cage-like transition metal chalcogenide shell are promising candidates for a wide variety of applications. Herein, we report an effective fabrication method for gold multipod nanoparticles (GMNs) core – cobalt sulfide shell (GMN@CoxSy) nanostructures. The unique cage-like

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morphology is successfully acquired within nanohybrids (GMN@CoxSy nanocages). The cobaltbased metal-organic frameworks can act as versatile sacrificial templates to the desired hybrid nanomaterials through solution-based etching approaches without any undesirable reshaping of GMNs which are embedded within. The examination of electrocatalytic oxygen evolution reaction (OER) of the prepared nanohybrids reveals that a type of GMN@CoxSy nanohybrids shows a substantially lower overpotential () value (345 mV) compared with those of GMNs (617 mV) and CoxSy nanomaterials (418 mV) at the current density of 10 mA cm–2. The enhanced OER performance is mainly attributed to the highly effective core-shell interfaces stemming from the unique multi-branch topologies of GMN cores as well as the optimized cobalt sulfide shells of the nanohybrids.


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1. INTRODUCTION The design of advanced inorganic hybrid nanomaterials with controlled size, morphologies, and compositions can lead to unique combined and tunable properties of the obtained materials.1-3 In particular, nanohybrids constructed from anisotropic nanoparticles and cage-like materials organized in core-shell conformations are considered to be quite expedient for a wide variety of applications, such as catalysis, biomedical,4 optics,5 electronics,6-7 and energy storage/conversion devices.8-12 In such core-shell arrangements, the outer cage functions as a nanoreactor and not only offer excellent selectivity towards targeted reactants, but also exhibit its own distinct properties. Meanwhile, the inner core materials with size- and shape-dependent properties are effectively protected by the shell, resulting in markedly enhanced stability during the catalytic processes. More importantly, the chemical and physical properties of the ensuing nanohybrids can be manipulated by varying the constituting materials or the core-to-shell ratios.11, 13-16 However, for the construction of well-defined nanohybrids with anisotropic nanoparticle cores and nanocage shells, a lot of challenges still remain to be overcome. For instance, the encapsulation of nanoparticles within pre-synthesized cage materials at the nanoscale involves complicated multistep synthesis, which severely hinders their large scale fabrication. The encapsulation of distinctively shaped nanoparticles within cage-like shell is expected to maximize the core-shell interface areas, thus strengthening the interaction and electronic transfer between them. Also, such metallic nanoparticles core – cage-like shell configurations can provide spacious interior areas which allows homogeneous exposure of all the active sites to contact with the reactants during the catalytic process. The integration of anisotropic noble metal nanoparticles and transition metal chalcogenides or oxides into core–shell nanohybrids is at the forefront of nano-engineering due to their unique


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functionalities that are rarely observed in the individual components.4, 11, 17-20 Numerous studies are being conducted on the fabrication of core–shell nanohybrids by the overgrowth of transition metal chalcogenides or oxides onto as-prepared nanoparticles with morphological isotropy.10, 15, 17-18, 21-25

For example, a variety of metal chalcogenides and oxides have been deposited on noble

metal nanospheres to form core–shell nanostructures. Anisotropic noble metal nanomaterials have been also integrated into hybrid systems as cores.18, 21-23, 26-28 Importantly, the preparative processes for the transition metal chalcogenides or oxides often involve relatively high-temperatures and harsh reaction conditions as well as an excessive use of surfactants or polymers,29-33 which can initiate the undesirable reshaping of the anisotropic metal cores and somewhat hinder their electrochemical applications. Recently, our group has developed a facile and high-yielding fabrication approach for star-shaped gold multipod nanoparticles (GMNs) containing multi-branches with sharp edges and tips.34 The inherited anisotropic multipod topologies of GMNs are composed of a plethora of curvatures and defect sites that could be beneficial to increase the catalytically active core-shell interfaces when growing with secondary materials.35-36 Herein, we report an effective encapsulation method of GMNs within a cobalt chalcogenide nanocage. The as-prepared GMNs embedded within cobaltbased metal-organic frameworks (zeolitic imidazolate framework, ZIF-67)36 are used as sacrificial templates for the preparation of GMNs core – cobalt sulfide (CoxSy) nanocage shell nanohybrids (GMN@CoxSy nanocages). This is an unusual example of a well-established and facile synthetic approach for noble metal nanoparticles core – cage-like shell nanostructures featuring preserved anisotropic morphologies of the core and well-defined hollow spaces of the shell. GMN@CoxSy nanohybrids show substantially enhanced catalytic activities compared with those of GMNs and


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CoxSy nanomaterials in electrocatalytic oxygen evolution reaction (OER). The multicomponent synergistic effects present in these hybrids are discussed.


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2. EXPERIMENTAL METHODS 2.1. Reagents A variety of GMN@ZIF-67 with different ZIF-67 shell morphologies are prepared and used as precursors as shown in Supporting Information. Cobalt (II) nitrate hexahydrate (Co(NO3)26H2O, 98 %, Sigma-Aldrich), 2-methylimidazole (C4H6N2, 2-MeIM, 99 %, Sigma-Aldrich), thioacetamide (CH3CSNH2, 98 %, Sigma-Aldrich), ruthenium (IV) oxide hydrate powder (RuO2xH2O, Sigma-Aldrich), and ethyl alcohol (C2H5OH, 99.9+%, Burdick & Jackson) are used as received. All stock solutions are freshly prepared before each reaction. Prior to use, all glassware is washed with aqua regia (3:1 ratio by volume of HCl and HNO3; Caution: Aqua Regia is highly toxic and corrosive and must be handled in a fume hood with proper personal protection equipment) and rinsed thoroughly with nanopure water. Abbreviations used: GMN = gold multipod nanoparticle; ZIF-67 NPs = cobalt-based zeolitic imidazolate frameworks; GMN@ZIF67 = gold multipod nanoparticles (GMNs) embedded within cobalt-based zeolitic-imidazolate frameworks; GMN@CoxSy = GMNs core – cobalt sulfide (CoxSy) shell nanohybrids 2.2. Synthesis of GMNs embedded within cobalt-based zeolitic-imidazolate frameworks (GMN@ZIF-67) The preparative procedure for the nanomaterials with GMNs embedded within ZIF-67 nanostructures (GMN@ZIF-67) comprises two steps, i.e., the synthesis of GMNs through a seedmediated protocol,34 followed by the overgrowth of ZIF-67 shells onto the as-prepared GMN seeds to form the core-shell nanoparticles.36 Detailed experimentations for the each step are demonstrated in the Supporting Information. The size and shape of GMN@ZIF-67 can be manipulated by varying the 2-MeIM/Co2+ molar ratios (i.e., 1:1, 2:1, and 4:1). The obtained


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GMN@ZIF-67 are denoted as GMN@ZIF-67 (A), GMN@ZIF-67 (B), and GMN@ZIF-67 (C), where (A), (B), and (C) represent the respective 2-MeIM/Co2+ molar ratios (1:1, 2:1, and 4:1). 2.3. Synthesis of GMNs core – cobalt sulfide (CoxSy) shell nanohybrids (GMN@CoxSy nanohybrids) The GMNs core – cobalt sulfide (CoxSy) shell (GMN@CoxSy) nanohybrids are fabricated through the solution-based etching of GMN@ZIF-67 with thioacetamide acting as sulfur precursor. For the synthesis of cage-like nanohybrids (GMN@CoxSy nanocages), the as-prepared dried powder of GMN@ZIF-67 (C) are redispersed in ethanol (0.5 mg mL-1). Next, 10 mL of the GMN@ZIF67 (C) suspension are then transferred into a vial containing 3.9 mL of ethanol solution of thioacetamide (172.5 mM). The reaction mixture is then shaken vigorously, sonicated for 4 min, and placed in an oven at 80 °C for 5 h. The mixture turns to black within 40 min. Upon the reaction completion, the ensuing GMN@CoxSy nanocages are collected by centrifugation, washed with ethanol, and dried under vacuum for 24 h. The obtained GMN@CoxSy nanocages are denoted as GMN@CoxSy (C), where (C) represents the 2-MeIM/Co2+ molar ratios (4:1) for the synthesis of GMN@ZIF-67 (C). The GMN@CoxSy nanohybrids, which do not show cage-like morphologies, are also synthesized using GMN@ZIF-67 (A) and GMN@ZIF-67 (B). The obtained products are denoted as GMN@CoxSy (A) and GMN@CoxSy (B), where (A) and (B) represent the respective 2-MeIM/Co2+ molar ratios (1:1 and 2:1) for the synthesis of GMN@ZIF-67 (A) and GMN@ZIF-67 (B). The other reaction conditions remain unchanged. 2.4. Synthesis of CoxSy nanoparticles


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For comparison, cobalt sulfide (CoxSy) nanoparticles are also converted from their corresponding pristine ZIF-67 templates, which are prepared via the protocol similar to that used for the preparation of GMN@ZIF-67 in the absence of GMN seeds. The obtained products are denoted as ZIF-67 (x) NPs, where x (A, B, and C) represents the respective 2-MeIM/Co2+ molar ratios (1:1, 2:1, and 4:1). The as-prepared ZIF-67 are subsequently treated with thioacetamide. The reaction conditions are similar to those used for the etching of GMN@ZIF-67. The CoxSy products are respectively denoted as CoxSy (A), CoxSy (B), and CoxSy (C). 2.5. Electrocatalytic oxygen evolution reaction (OER) All electrochemical measurements are carried out using a COMPACTSTAT.h analyser (Ivium Technologies) in a typical three-electrode setup at ambient temperature (~25 °C). A Pt wire and Ag/AgCl are employed as counter and reference electrodes, respectively. A glassy carbon rotating disk electrode (RDE) with a diameter of 3 mm serves as the substrate for the working electrode. To prepare the working electrode, 1 mg of catalysts (GMN@CoxSy (A), GMN@CoxSy (B), and GMN@CoxSy (C)) are dispersed in a solution containing 0.15 mL of water, 0.075 mL of ethanol, and 0.015 mL of Nafion solution (5 wt%), followed by sonication (10 min) to form a homogeneous ink. Then, 4.5 µL of the ink is pipetted onto the RDE surface to produce a 0.199 mg cm–1 loading for all samples, including Pt/C catalyst (60%ww Pt loading, VINA Tech Co., Ltd.). The electrocatalytic activity is evaluated via linear sweep voltammograms (LSV) obtained at a scan rate of 10 mV s-1. The electrolyte used in all measurements is 0.1 M KOH, which is pre-bubbled with N2 for at least 45 min. Prior to test, the electrolyte is bubbled with N2 for at least 45 min to saturate it with N2. For comparison, the electrocatalytic performances of bare RDE, GMNs, CoxSy (A), CoxSy (B), CoxSy (C), GMNs/CoxSy, GMN@ZIF-67, Pt/C (60%ww Pt loading, VINA Tech Co., Ltd.), and RuO2 catalysts are also examined using the same experimental parameters.


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RuO2 catalyst was obtained by calcinating ruthenium (IV) oxide hydrate powder (RuO2xH2O) at 450 °C for 4.5 h in air. Subsequent fabrication of working electrode using as-prepared RuO2 catalyst and OER tests were similarly carried out as did for GMN@CoxSy (A) mentioned above. Electrochemical impedance spectroscopy (EIS) measurements are performed in N2-saturated 1.0 M KOH solution by applying AC voltage with 5 mV amplitude in a frequency range from 105 to 0.1 Hz at 1.53 V vs. RHE. To examine the chemical states of GMN@CoxSy (A) after long-term OER tests by X-ray photoelectronic spectroscopy (XPS), 200 µL catalyst ink prepared by mixing 1 mg of GMN@CoxSy (A), 0.15 mL of water, 0.075 mL of ethanol, and 0.015 mL of Nafion solution (5 wt%) is deposited onto indium tin oxide electrode (ITO), followed by rinsing with water to remove loosely bonding catalyst and drying at 60 °C for 20 min. The obtained working electrode is then subjected to chronoamperometry (at a constant potential of 1.647 vs. RHE) for 6000 s. Afterward, the catalyst is thoroughly collected, rinsed with ethanol and water, and dried at 60 °C for further investigation with XPS. 2.6. General methods The nanoparticles are imaged using a Hitachi S-4800 scanning electron microscope (SEM) and a JEOL JEM-2010 Luminography (Fuji FDL-5000) Ultramicrotome (CRX) transmission electron microscope (TEM). Energy dispersive X-ray (EDX) analysis are measured using a JEOL JEM2100F microscope. Samples are prepared for TEM by concentrating the nanoparticle mixture using centrifugation (2 times, 10 min, 6,000 rpm), followed by resuspension in ethanol (100 μL) and immobilization of 10 μL portions of the solution on TEM grids (Ted Pella, Inc. Formvar/Carbon 400 mesh, copper coated). Ultraviolet-visible spectra are recorded using UV-1800 (Shimadzu,


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UV-vis spectrophotometer). Powder X-ray diffraction (PXRD) is performed using a RIGAKU Ultima IV diffractometer using focused-beam Cu Kα radiation (Kα = 1.541 Å) at a continuous scan rate of 0.09 °min-1 in the range 5–90°. The simulated PXRD pattern of ZIF-67 are calculated from single crystal X-ray diffraction (XRD) data using the Mercury 3.3 program. X-ray photoelectronic spectroscopy (XPS) measurement is performed on a K-ALPHA spectrometer (Thermo VG, United Kingdom) with monochromated Al Kα X-ray radiation as the X-ray source for excitation. N2 adsorption isotherms are obtained using BELSORP-mini II (BEL Japan, Inc.). The gases used throughout adsorption experiments are highly pure (99.999%). Prior to the adsorption experiments, all the samples are activated by thoroughly rinsing with ethanol (3  1 mL), followed by drying under vacuum for 24 h prior to the gas sorption measurements.


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3. RESULTS AND DISCUSSION Initially, monodisperse GMNs with star-shaped morphologies and multiple branches oriented outwardly (Figure S1) are prepared via a solution-based seed-mediated growth protocol adapted from literature reports.34 The successful overgrowth of ZIF-67 shells over the GMNs to form GMN@ZIF-67 (C) ((C) represents the 2-MeIM/Co2+ molar ratios (4:1) used for the overgrowth of ZIF-67 shell) is confirmed by the corresponding transmission electron microscopy (TEM, Figures 1a, S2a and S2b) and scanning electron microscopy (SEM, Figure S3a) images and elemental mapping (Figures S2c to S2f). The average size of GMN@ZIF-67 (C) is estimated to be 1249 ± 164 nm (n = 127 particles) (Figure S3b). Subsequently, a nanohybrid composed of GMNs encapsulated within cobalt sulfide (CoxSy) nanocages (GMN@CoxSy (C), where (C) represents the corresponding 2-MeIM/Co2+ molar ratios (4:1) for the synthesis of GMN@ZIF-67 (C)) is successfully synthesized via the solution-based sulfidation of the as-prepared GMN@ZIF-67 (C) with thioacetamide (TAA) at 80 °C. Notably, after the sulfidation process, the etched product (GMN@CoxSy (C)) exhibits a cage-like structure. The distinct hollow spaces within the welldefined nanocages can be clearly distinguished by the sharp contrast between the outer frames and hollow interiors (Figures 1b and 1c). The nanocage has a frame thickness of approx. 30 nm (Figures 1b and 1c) and is robust enough to endure the sulfidation reaction. A low-magnification SEM image (Figure S4a) shows that the GMN@CoxSy (C) nanocages have a slightly smaller average size (1003  145 (n = 113 particles)) as compared with that of the GMN@ZIF-67 (C) templates. The transformation of ZIF-67 shells in GMN@ZIF-67 (C) into cobalt sulfide nanocages is unambiguously revealed by a significant presence of S element and the overlapping between Co and S species observed in the corresponding elemental mapping of a single particle of GMN@CoxSy (C) nanocages (Figures 1d to 1f).


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Nanostructured metal-organic frameworks (NMOFs) have emerged as promising sacrificial templates for the preparation of various inorganic nanomaterials37-51 because of their clear-cut advantages, such as structural, compositional, and functional tailorability at the molecular level.52-61 In addition, the structural robustness and well-defined morphologies of the frameworks are important toward the fabrication of anisotropic hybrid inorganic materials.36, 40, 62-66 Herein, the sulfidation of the as-prepared GMN@ZIF-67 (C) is carried out using TAA as a sulfur source:37-38 CH3CSNH2 + C2H5OH → CH3(NH2)C(OC2H5) ─ SH

(1)

CH3(NH2)C(OC2H5) ─ SH + C2H5OH → CH3(NH2)C(OC2H5)2 + H2S

(2)

x Co2+ (from ZIF-67 shells) + y H2S → CoxSy + 2y H+

(3)

Alkyl thiol or sulfide species could adversely lead to the structural and morphological changes of gold nanoparticles with highly active surface area.45,

67

In the current synthesis,

although GMNs have multi branches and rough surfaces, they remain morphologically unchanged and still encapsulated within the hollow cobalt sulfide nanocages to form a well-defined core-shell configuration. This is mainly because of the protection effect offered by the shell materials that prevents the direct exposure of GMNs towards sulfide ions. The preservation of unique multipod topologies of GMN is important for the maintenance of catalytically active sites of the cores and, in particular, the generation of effective core-shell interface of the nanohybrids, which are considered useful for catalytic applications. Powder X-ray diffraction (PXRD) of GMN@ZIF-67 (C) and GMN@CoxSy (C) shows that the patterns in the low-angle range (5–35°) characterized for crystalline ZIF-67 in the templates disappear after sulfidation (Figures 2a and 2b), suggesting the complete reaction of ZIF-67 shells with TAA to form the respective cobalt sulfide. The face-centered cubic (fcc) structure of the


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metallic gold phase (JCPDS card no. 01-089-3697) is still observed in the products before and after etching (Figure 2a). Based on PXRD patterns of the etched product and the aforementioned elemental mapping (Figures 1d to 1f), we consider that the cobalt sulfide nanocages could exist in the amorphous form (CoxSy) in GMN@CoxSy (C). The disappearance of the characteristic optical peak (at ~ 600 nm), which is assigned to the tetrahedral Co2+–2-MeIM coordination bonds within ZIF-67 after etching,68 is in good agreement with the PXRD data. Importantly, using NMOFs as sacrificial templates also allows the derived nanohybrids to have a high degree of porosity and large surface area, which are critical for catalysis. On this basis, GMN@CoxSy (C) is characterized by N2 adsorption-desorption isotherms at 77 K (Figure 2d). The Brunauer-Emmett-Teller (BET) surface area and pore volume of GMN@CoxSy (C) are estimated to be 78.5 m2 g-1 and 0.41 cm3 g–1, respectively (Table S1). The high degree of porosity and surface area of GMN@CoxSy (C) could be mainly attributed to the well-defined and porous structures of GMN@ZIF-67 (C) precursors. To the best of our knowledge, although numerous studies have been conducted on the fabrication of NMOF-derived inorganic materials, there are very few examples of noble metal nanoparticles core–cage-like shell nanostructures featuring preserved anisotropic morphologies of the core and well-defined hollow spaces of the shell.69 The mechanistic evolution of GMN@ZIF-67 (C) precursors into GMN@CoxSy (C) nanocages is monitored by time-resolved TEM (Figure 3). At a given reaction time, the etched products are quickly isolated and characterized by TEM. In the early stages of the sulfidation process, S2- ions are generated from the hydrolysis of TAA and react readily with Co2+ on the ZIF67 surfaces to form relatively thin CoxSy frames (Figures 3a to 3c). The pre-formed frames then act as physical barriers to prevent the direct contact and chemical reaction between outside sulfide ions and the inner metal ions. As the reaction proceeds, the outer CoxSy frames grow thicker and


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can be clearly observed by TEM after ~ 120 min (Figure 3d). To continue the sulfidation, metal and sulfide ions have to diffuse through the formed CoxSy layer. Owing to the smaller ionic radius of metal ions (72–74 pm) as compared with that of S2- ions (184 pm), the outflux of metal ions is much easier than the influx of S2- ions.38-39, 42 Consequently, the inner metal-organic framework is gradually consumed and the released metal ions penetrate the pre-formed shell to react with S2ions on the outer surface, which finally produces hollow spaces inside the nanocages (Figures 3e and 3f). The gradual formation of S element throughout the etching process is also confirmed by the time-resolved elemental mapping of the etched products (Figure S5) and the plot of S content vs. etching time (Figure S6). One of the major advantages of using NMOF as sacrificial templates is the simplicity and versatility in size, morphological, and compositional controllability of the derived products. More importantly, given a strong correlation between the overall catalytic performance and the structural properties of core/shell components (i.e., size, shape, and core-to-shell ratios), our synthetic strategy can be extended to fabricate a variety of nanohybrids of GMNs encapsulated within cobalt sulfide shells with distinct morphologies by using the corresponding NMOF precursors. Specifically, for a given amount of GMNs, ZIF-67 shells are fabricated by varying the 2MeIM/Co2+ molar ratios (i.e., 1:1 and 2:1) to form different GMN@ZIF-67 precursors (denoted as GMN@ZIF-67 (A) and GMN@ZIF-67 (B), respectively). The SEM images (Figure S7) of GMN@ZIF-67 (A) and GMN@ZIF-67 (B) show that both precursors are similarly composed of GMN cores and flower-type ZIF-67 shells. The prepared precursors are then subjected to TAA treatment using similar etching conditions for GMN@ZIF-67 (C). The obtained GMN@CoxSy nanohybrids are denoted as GMN@CoxSy (A) and GMN@CoxSy (B), where (A) and (B) represent the respective 2-MeIM/Co2+ molar ratios (1:1 and 2:1) for the synthesis of GMN@ZIF-67 (A) and


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GMN@ZIF-67 (B). The UV-Vis spectra of the etched products show that ZIF-67 shells within the precursors are completely consumed by TAA (Figures S8 and S9). The corresponding SEM and TEM images reveal that the GMN@CoxSy (A) (Figures 4a and 4d) and GMN@CoxSy (B) (Figures 4b and 4e) possess relatively thinner cobalt sulfide shells, but do not show hollow cage-like morphologies as seen in GMN@CoxSy (C) (Figures 4c and 4f). The average sizes of GMN@CoxSy (A) and GMN@CoxSy (B) are estimated to be 770  117 (n = 105 particles) and 819  214 nm (n = 106 particles), respectively (Figure S10). The BET surface areas of GMN@CoxSy (A) and GMN@CoxSy (B) are respectively estimated to be 14.1 and 27.3 m2 g–1 (Table S1, and Figures S11 and S12, respectively), and the corresponding PXRD patterns also reveal that the cobalt sulfide phase is amorphous (Figure S13). Oxygen evolution reaction (OER) is a major bottleneck of high efficiency electrochemical water splitting for the production of hydrogen because of its sluggish kinetics (high overpotential ()). Much effort has been devoted to the development of nanomaterials which exhibit practically meaningful OER performance.70-78 Among those candidates, nanostructured cobalt chalcogenides become increasingly attractive due to the earth abundance of Co and considerable activity toward water splitting.70, 79-82 In addition, considering the possible synergistic enhancement effects mainly stemming from the gold core - cobalt sulfide shell interface of the nanohybrids, herein, the OER performances of the prepared nanohybrids are investigated using a typical three-electrode system in 0.1 M KOH at room temperature. The data are summarized in Table 1. As expected, without any catalysts, the bare rotating disk electrode (RDE) has negligible OER activity for the whole applied voltage range; however, the current density remarkably increases in the presence of catalysts (Figure 5a). Particularly, the current density of GMN@CoxSy (A) and GMN@CoxSy (B) is considerably higher than that of GMN@CoxSy (C). For example, at a potential of 1.8 V vs. RHE,


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GMN@CoxSy (A) reaches a current density of 32.2 mA cm–2, which is twice that of GMN@CoxSy (C) (17.6 mA cm–2). The higher OER efficiency of GMN@CoxSy (A) is attributed to the thinner and morphologically unique sulfide shells, which advantageously maximize the interfacial contacts with GMN cores and lead to profound synergism between the core and shell. We also attempted to test the OER performance of GMN@ZIF-67; however, the catalysts quickly degraded because of the basic condition of the OER test. The OER activity of GMN@CoxSy (A) is remarkably higher than that of commercial Pt/C (only 4.5 mA cm–2 at a potential of 1.8 V vs RHE) (Figure 5a and Table 1). When comparing with other well-known catalysts for OER (e.g. RuO2), GMN@CoxSy (A) shows a slightly higher overpotential ( = 345 mV) than that of the prepared RuO2 catalyst (310 mV), rendering them highly desirable for practical applications. In order to further investigate the enhanced OER performance of GMN@CoxSy (A), cobalt sulfides (CoxSy (A), CoxSy (B), and CoxSy (C)) derived from the corresponding ZIF-67 precursors (ZIF-67 (A), ZIF-67 (B), and ZIF-67 (C)) using different molar ratios of 2-MeIM/Co2+ (i.e., 1:1, 2:1, and 4:1, respectively) are also prepared and used as OER catalysts. SEM images of ZIF-67 (A) and ZIF-67 (B) (Figures S14a and S14b) reveal that at the ratios of 1:1 and 2:1, the products show flower-like morphologies which are similar to the shells of GMN@ZIF-67 obtained at the same 2MeIM/Co2+ ratios. Upon increasing the ratio to 4:1, ZIF-67 (C) shows typical rhombic dodecahedral morphology (Figure S14c). The obtained ZIF-67 precursors are subsequently treated with TAA to form cobalt sulfides, of which morphologies can be sustained after the etching process (Figure S15). The average sizes of CoxSy (A), CoxSy (B), and CoxSy (C) are found to be 938  132 (n = 72 particles), 999  155 (n = 103 particles), and 1021  114 nm (n = 91 particles), respectively (Figure S16). Figure S17 reveals that CoxSy (A) and CoxSy (B) exhibit lower overpotentials ( = 418 and 422 mV, respectively) than CoxSy (C) ( = 475 mV) at the current density of 10 mA cm–


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2.

This result is consistent with the catalytic trend observed for GMN@CoxSy nanohybrids,

suggesting that the cobalt shell morphologies are important to the overall catalytic activity. The  value (at 10 mA cm–2) of GMNs is found to be higher than 617 mV, which is substantially higher than that of CoxSy (1:1), suggesting that the catalytically active component for OER is cobalt sulfide. However, when compared with GMN@CoxSy (A) ( = 345 mV at 10 mA cm–2), both separate components (i.e., GMNs and CoxSy (A)) show considerably lower OER activities (Figure 5b and Table 1). The linear sweep voltammetry (LSV) curves of GMN@CoxSy (A) (Figure S18) recorded at different scan rates (3, 5, and 10 mV s-1) show negligible changes in activities, suggesting rapid mass transfer in GMN@CoxSy (A). In addition, the mass transfer of ions in electrolyte during the OER process can be considered relatively straightforward as using rotating disk electrode apparatus. Electrochemical impedance spectroscopy (EIS) of GMN@CoxSy (A), CoxSy (A), and GMNs was also performed in N2-saturated 1.0 M KOH solution at 1.53 V vs. RHE. The corresponding Nyquist plots are shown in Figure S19. The data are fitted using an equivalent circuit model. The values for electrolyte resistance (Rs) and the charge-transfer resistance (Rct) can be obtained and summarized in Table S2. The Rct values for GMN@CoxSy (A) and CoxSy (A) are obtained as 10.2 and 11.6 , respectively, which is significantly smaller than that of GMNs (356.5 ). Given that the Rct is directly related to the interfacial charge transfer reaction during OER process,79 these data is in concert with the common sense that cobalt sulfide surface directly involves in the OER reaction. To further elucidate the enhanced performance of the nanohybrids, separately-prepared GMNs and CoxSy (A) are physically mixed to obtain a mixture of GMNs and CoxSy (A) (denoted as GMNs/CoxSy). As can be seen from Figure 5b and Table 1, the  value of GMNs/CoxSy at 10 mA cm–2 is almost similar to that of CoxSy (A), and markedly lower than that of GMN@CoxSy (A).


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Based on these experimental data, we consider that the enhancement of OER activity of the nanohybrids could stem from several fundamental factors. Firstly, the gold core, which is a highly electronegative metal, may serve as an electron acceptor that facilitates electron extraction and thereby stabilizes cobalt species at high oxidation levels, leading to an enhanced electrochemical activity in the nanohybrids. To clarify this point, the surface composition and valence states of GMN@CoxSy (A), GMNs, and CoxSy (A) are analyzed using X-ray photoelectron spectroscopy (XPS). The Co 2p spectrum for GMN@CoxSy (A) is split into 2p1/2 and 2p3/2 spin-orbit coupling (Figure 6a). After deconvolution, two doublets characteristic of CoII (at 781.7 and 797.5 eV) and CoIII (779.1 and 794.2 eV) are observed, indicating the coexistence of CoII and CoIII in GMN@CoxSy (A).83-84 The S 2p spectrum of GMN@CoxSy (A) shows the peaks at 163.1 and 162.2 eV, which can be respectively assigned to S-CoII and S-CoIII bonds (Figure S20).84 Because of the formation of core-shell structure of GMN@CoxSy (A), the doublet indexed to CoII in GMN@CoxSy (A) shifts to higher binding energy in comparison with that of CoII (at 780.7 and 795.5 eV) in CoxSy (A) (Figure 6a). Meanwhile, the binding energy of Au 4f (at 84.2 and 87.8 eV) in GMN@CoxSy (A) collectively decrease relative to that of Au 4f (at 84.7 and 88.4 eV) in GMNs (Figure 6b). These shifts confirm the electronic interaction between gold and cobalt sulfide, resulting in electron density migration from Co to Au. This phenomenon has also been reported previously, especially in the context of OER activity.85-97 Second, the unique multipod topologies of GMNs with highly active surface area and a plethora of curvature and defect sites, coupled with optimized cobalt sulfide shells, are important for the generation of catalytically important core-shell interface in the nanohybrids. In this case, the morphological anisotropy of GMNs would induce the better interfacial interaction between core and shell. The increase in catalytically active sites by growing secondary materials on anisotropic metal cores, which results in enhanced catalytic activities, has


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been also reported previously.35,

98-101

Third, thin cobalt sulfide shells with rough surfaces in

GMN@CoxSy (A) endows nanomaterials with large exposed surface atoms, which can serve as catalytically active sites. In addition, the cage-like conformation is considered not helpful for effective core-shell interfaces, which are essential for enhanced OER activity. This partly accounts for the lower OER activity of GMN@CoxSy (C) in comparison with GMN@CoxSy (A) that does not have hollow spaces in the structure. In a word, the synergistic effect of every component is important to tune the electrocatalytic activity, providing a promising way to achieve enhanced OER kinetics. We also compare the OER performance of GMN@CoxSy nanohybrid with those of selected metal sulfides or metal nanoparticle core – metal chalcogenide/oxide shell nanostructures (Table S3). Although the OER activity of GMN@CoxSy (A) does not rank among the highest values, we consider that the enhancement effect between GMN and cobalt sulfide would be, to some extent, valuably contribute towards the design strategy of more efficient nanohybrids for energy-related applications. Tafel slope of GMN@CoxSy (A) is estimated to be 138 mV dec–1, which is almost similar to that of CoxSy (A) (145 mV dec–1). Likewise, for CoxSy (B) and CoxSy (C) systems, the addition of GMNs insignificantly changes their respective Tafel slopes (Figure S21 and Table 1). This trend suggests that the encapsulation of GMN cores does not change the rate limiting step of the reaction, yet effectively reduces the onset potential of the nanohybrids in OER catalysis. The stability of given catalysts is an important factor in practical usage. Therefore, chronoamperometry (at a constant potential of 1.647 V vs. RHE) and chronopotentiometry (at a constant current density of 10 mA cm–2) of GMN@CoxSy (A) are investigated. As seen in Figures 5d and S22a, the nanohybrids show excellent stability to continuous operation for a period of 1000 and 2500 min, respectively. We also attempted to characterize the nanohybrids by collecting the catalysts


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deposited on RDE after long-term OER tests. As seen in Figures 22b and 22c, although the shell thickness of GMN@CoxSy (A) seems to decrease, presumably due to the partial dissolution of cobalt sulfide shell in basic and oxidative conditions, the core-shell configurations are somewhat retained after continuous operation. It has been also reported that the surface of metal chalcogenides could be, to some extent, converted to oxide or/and hydroxide since these chemical states are more thermodynamically stable than metal sulfides under oxidizing potentials.102-103 Therefore, the XPS of GMN@CoxSy (A) collected from working electrode after OER tests is investigated. As shown in Figure S23, the features of the Co 2p and O 1s regions of the sample suggest a cobalt hydroxide-like surface structure, indicating that the surface of the catalyst could be partially transformed into oxide/hydroxide species after the electrochemical process. Such transformation would be advantageous to the formation of more catalytically active interfacial metal oxides/hydroxides surfaces as reported in recent literatures.104-106

4. CONCLUSIONS A variety of gold multipod nanoparticles (GMNs) core – cobalt sulfide shell (GMN@CoxSy) nanohybrids are successfully synthesized through a solution-based etching approach using asprepared GMNs-embedded ZIF-67 (GMN@ZIF-67) as sacrificial templates. In a specific stoichiometry and reaction condition, the cage-like conformation can be acquired within nanohybrids (GMN@CoxSy nanocages). This is an unusual example of NMOF-derived noble metal nanoparticles core – cobalt sulfide shell nanohybrids featuring preserved anisotropic morphologies of the core and well-defined hollow spaces of the shell. Electrocatalytic oxygen evolution (OER) performance of the prepared nanohybrids reveals that GMN@CoxSy (A) shows a substantially lower overpotential () value (345 mV) compared with those of GMNs (617 mV)


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and CoxSy (A) (418 mV) at the current density of 10 mA cm–2. The enhanced OER performance is mainly attributed to several factors: (i) electron extraction induced by highly electronegative gold core that can stabilize cobalt species at high oxidation levels; (ii) high degree of catalytically active core-shell interfaces stemming from GMNs with rough surface and a plenty of curvature and defect sites; (iii) the rough and thin surfaces of cobalt sulfide shell endows nanomaterials with large exposed surface atoms.

ASSOCIATED CONTENT Supporting Information containing the detailed experimental procedures for the preparation of GMNs and GMN@ZIF-67 and the supporting data. AUTHOR INFORMATION Corresponding Author [email protected] Present Addresses Department of Chemistry, Hallym University, Chuncheon, Gangwon-do, 24252, Republic of Korea ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A4A1041631 and NRF-2016R1A2B4009281). This work was supported by the Hallym University Postdoctoral Fellowship Program of 2017 (HLM-PF-2017-0003, Dr. Hien Duy Mai).


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Table of Contents


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Figure 1. TEM images of (a) GMN@ZIF-67 (C) and (b and c) resultant GMN@CoxSy (C) nanocages. The elemental mapping of a single particle of GMN@CoxSy (C) with elements: (d) gold (yellow), (e) cobalt (purple), and (f) sulfur (green).


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Figure 2. Powder X-ray diffraction (PXRD) (a and b) and UV-Vis spectra (c) of GMN@ZIF-67 (C) templates (black) and GMN@CoxSy (C) nanocages (red). (d) N2 adsorption/desorption isotherm of GMN@CoxSy (C). Diffraction peaks marked by an asterisk (*) in (a) are indexed to the fcc structure of metallic gold (JCPDS card no. 01-089-3697). For the measurement of UV-Vis spectra in (c), all samples were suspended in ethanol.


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Figure 3. Time-resolved TEM images revealing the evolution of the cage-like shells of CoxSy in GMN@CoxSy (C) nanocages via etching with TAA for: (a) 20, (b) 40, (c) 80, (d) 120, (e) 200 and (f) 300 min.


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Figure 4. SEM and TEM images of (a and d) GMN@CoxSy (A), (b and e) GMN@CoxSy (B), and (c and f) GMN@CoxSy (C) obtained by means of templating with the corresponding GMN@ZIF67 precursors.


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Figure 5. Polarization curves of OER performance by a variety of catalysts tested in 0.1 M KOH at a scan rate of 10 mV s–1 and a continuous electrode rotating speed of 1800 rpm. (a) Specific activities of bare RDE, commercial Pt/C (60% w/w Pt loading) and RuO2, GMN@CoxSy (A), GMN@CoxSy (B), and GMN@CoxSy (C). (b) Specific activities of GMNs, CoxSy (A), GMNs/CoxSy, and GMN@CoxSy (A). (c) Tafel plots of GMNs, CoxSy (A), and GMN@CoxSy (A). (d) Chronoamperometry of GMN@CoxSy (A) at a constant potential of 1.647 V vs. RHE, with an activity loss of 5.2% after 6000 s of continuous operation.


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Figure 6. High-resolution XPS spectra for (a) Co 2p and (b) Au 4f of GMN@CoxSy (A), GMNs, and CoxSy (A).


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Table 1. Summary of electrochemical performances for OER. Current density

Overpotential ()

Tafel slope

at 1.8 V (mA cm–2)

at 10 mA cm–2 (mV)

(mV/dec)

Pt/C

4.5

> 620

169

RuO2

29.9

310

115

GMNs

5.8

> 617

182

CoxSy (A)

20.2

418

145

CoxSy (B)

20.5

422

152

CoxSy (C)

16.3

475

160

GMNs/CoxSy

18.4

442

GMN@CoxSy (A)

32.2

345

138

GMN@CoxSy (B)

26.4

376

136

GMN@CoxSy (C)

17.6

455

155

Catalysts


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Effective Fabrication and Electrochemical Oxygen Evolution Reaction

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