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Synthesis of Monodispere Au@Co3O4 Core-Shell Nanocrystals and Their Enhanced Catalytic Activity for Oxygen Evolution Reaction Zhongbin Zhuang, Wenchao Sheng, and Yushan Yan* The hydrogen economy can provide an efficient energy system that is free from environmental issues related to the combustion of coal, oil, and natural gas.[1] However, such a system requires a clean and sustainable source of hydrogen, which can be provided by splitting of water either electrochemically or photoelectrochemically.[2] One of the key problems in splitting water is the kinetically sluggish anode reaction, i.e., oxygen evolution reaction (OER, 4OH− → 2H2O + 4e− + O2 in base). An overpotential of several hundred millivolts is often required to achieve a current density of 10 A gcatalyst−1.[3] Recent studies have shown that spinel-type Co3O4 has relatively good OER activities.[2e,3c,4] Hybrid materials have been proposed to further promote the OER activity of Co3O4, such as doping Co3O4 with other metals to make substituted cobaltites[5] or growing Co3O4 on a special substrate.[6] Nanoparticles are usually the preferred form of catalysts because of their high surface areas, and a core–shell structure is one of the typical routes to producing hybrid materials at the nanometer scale. Core–shell nanocrystals (NCs) show remarkable catalytic behavior compared with single-component NCs because of the synergistic effect between the core and the shell.[7] For example, Alayoglu and coworkers have found that Ru@Pt core–shell nanoparticles have the highest activity towards CO oxidation.[8] The CO conversion temperature of this catalyst is 30 °C, which is significantly lower than that of PtRu alloys (85 °C), a mixture of monometallic Pt and Ru nanoparticles (93 °C) and pure Pt particles (170 °C). However, synthesis of high quality core–shell NCs with well-defined structure is still challenging, especially when the core and the shell are of different classes of materials (e.g., elemental metal core and oxide compound shell), so that this type of core–shell NCs has scarcely been reported.[9] In the study reported here, we synthesized monodisperse Au@Co3O4 core–shell NCs, which were converted from monodisperse Au@Co core–shell NCs, with an overall diameter of 8 nm and also examined their catalytic OER activity. Au was chosen as the core because Yeo and Bell reported that cobalt oxide electrochemically grown on a Au electrode has enhanced OER activity, benefiting from the highly electronegative Au
Dr. Z. B. Zhuang, Dr. W. C. Sheng, Prof. Y. S. Yan Department of Chemical and Biomolecular Engineering and Center for Catalytic Science and Technology University of Delaware Newark, DE 19716, USA E-mail: [email protected]
DOI: 10.1002/adma.201400336
Adv. Mater. 2014, DOI: 10.1002/adma.201400336
substrate, which makes the Co3O4 more easily oxidized.[6b] Our electrochemical study shows that our Au@Co3O4 NCs have an OER activity 7 times as high as a mixture of Au and Co3O4 NCs or Co3O4 NCs alone, and 55 times as high as Au NCs, most likely due to a strong synergistic effect between the core and the shell, and this effect does not exist between the physically mixed NCs. Some Au and Co3O4 hybrid NCs have been reported,[10] however, none of them have well-defined core–shell structures and uniform sizes. High-quality Au@Co3O4 core–shell NCs are still desired. In our experiment, a three-step approach (Figure 1a) was adopted to synthesize Au@Co3O4 NCs, comprising synthesis of the Au NC, growth of the Co shell, and oxidation of Co to Co3O4. First, Au NCs were prepared by reducing HAuCl4 with tert-butylamine borane (TBAB) in the presence of oleylamine (OAm) as the ligand, following the procedure described in a previous study by Peng et al.[11] Second, Co shells were grown on the Au NC cores to prepare Au@Co NCs by using Co(acac)2, where acac is acetylacetonate, as the cobalt source and TBAB as the reducing agent. OAm and oleic acid (OA) were introduced to control the shape and uniformity. Third, the Au@Co NCs were loaded on carbon and then the Co shells were oxidized to Co3O4 by calcination in air. The experimental details are described in the Supporting Information. Figure 1b shows the transmission electron microscopy (TEM) image of the as-obtained Au NCs. They have a narrow size distribution with a diameter of 3.6 ± 0.5 nm. Five-fold symmetry is found in the high-resolution TEM (HRTEM) image (Figure S1, Supporting Information), which is in agreement with the literature, indicating the multiple-twinned structure of the Au NCs.[11] Figure 1c shows a TEM image of Au@Co core–shell NCs that were synthesized by growing Co shells on the pre-synthesized Au cores with 0.5 mmol OA. A dark core corresponding to Au can clearly be seen located at the center of the NC, and a uniform lighter Co shell caps around it. The Au@Co NCs are nearly monodisperse with an overall diameter of 8.1 ± 0.7 nm. The thickness of the Co shell is ca. 2 nm. The Au@Co NCs are highly uniform so that they can assemble into an ordered structure (Figure 1d). The energy dispersive spectrometry (EDS) spectra (Figure S2a, Supporting Information) show the signals of Au and Co (atomic ratio 1:4), which confirms the hybrid Au@Co composition. The Co shell seems to be amorphous because no clear lattice fringe can be seen in the HRTEM image (Figure 1e). This may be due to the lattice mismatch between Au (face-centered cubic, fcc) and Co (hexagonal close packed, hcp). The multiple twinned nature of the Au core may also influence the crystallinity of the Co shell. It is noted that Co NCs cannot be synthesized under the same condition
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NCs in air. The Au@Co3O4 NCs are well dispersed on the carbon support. The lattice fringe (Figure 1g) with an interplanar spacing of 0.29 nm is ascribed to the (220) plane of spinel Co3O4. The thickness of the shell remained at ca. 2 nm. The calcination process has also removed the surfactants (i.e., OAm and OA) used in the synthesis to obtain a clean surface. With that, the active sites can be more exposed to the reactant during the catalytic process. The EDS spectra (Figure S2b, Supporting Information) also show that the atomic ratio of Au:Co is maintained at 1:4. The thermogravimetric (TGA) result of Au@Co3O4/C measured in air indicates that it contains 68 wt% carbon support (Figure S3, Supporting Information). Based on the Au/Co ratio obtained from the EDS spectra, it can be calculated that the sample contains 12 wt% Au and 20 wt% Co3O4. X-ray diffraction (XRD) patterns of the as-prepared NCs are shown in Figure 2. Figure 2a illustrates the XRD pattern of Au NCs and it fits the fcc Au standard pattern well (JCPDS No. 65–2870). For the Au@Co NCs (Figure 2b), all the peaks can be indexed to fcc Au and no clear Co reflection peaks can be found. It indicates that the Co shell is amorphous, consistent with the HRTEM result. The XRD pattern of Au@Co3O4/C is shown in Figure 2c. The broad peak at ca. 25° comes from the carbon support. The two strongest peaks from Co3O4, i.e., (311) at 36.8° and (440) at 65.2°, are overlapped with the peaks corresponding to Au(111) at 38.2° and (220) at 64.6°, respectively. However, additional peaks assigned to Co3O4 are clearly present (pointed by arrows), which confirms the shell has been oxidized. Figure S4 (Supporting Information) shows the data plotted with intensity on a logarithmic scale to highlight the Co3O4 signals with lower intensity. The key step to obtain monodisperse Au@Co3O4 NCs is to synthesize high quality Au@Co NCs. In our experiment, both OAm and OA were used as solvents and also ligands. It was found that the amount of OA is critical to the shape and uniformity of the products, as summarized in Figure 3 Figure 1. a) Scheme of the synthetic route to Au@Co3O4 core–shell NCs. b) TEM image of Au (enlarged TEM images and EDS spectra are NCs. Inset: Histogram of the size distribution. c) TEM image of Au@Co NCs. Inset: Histogram also shown in Figure S5 in the Supporting of the size distribution. d) TEM image of a two-layer array of the Au@Co NCs. Inset: Modeled Information). TBAB was used as the reducer projection of the two-layer NC assembly. e) HRTEM image of a single Au@Co NC. f) TEM and its reducibility depends on basicity. When image of Au@Co3O4 NCs supported on carbon. g) HRTEM image of a single Au@Co3O4 NC. only OAm is used, the reducibility of TBAB is the strongest. Co(acac)2 was rapidly reduced to Co monomer without adding Au NCs. The Au NCs not only serve as heterogeneous nuclei but also promote the reduction of CoII.[10h] and the monomer became saturated. Owing to the rapid reduction, the Co monomers not only grow on the Au core, but also Figure 1f shows a TEM image of Au@Co3O4 NCs supported homogeneously form Co nuclei and further grow to Co NCs. on carbon (Au@Co3O4/C) prepared by calcination of Au@Co 2
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Figure 2. a–c) XRD patterns of Au NCs (a), Au@Co NCs (b), and Au@ Co3O4/C sample (c). The standard XRD patterns (Au, JCPDS No.65–2870; Co, JCPDS No. 05–0727; Co3O4, JCPDS No. 43–1003) are shown beneath the plots.
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a)
Thus, a mixture of Au@Co NCs and Co NCs are obtained (identified in Figure S5b). By adding 0.16 mL (0.5 mmol) OA, the reducibility of TBAB was weakened and thus the growth rate of Co was lower. Co growth on the heterogeneous Au core is much easier than homogeneous nucleation, therefore monodisperse Au@Co NCs can be obtained. Thus this condition is optimal for Au@Co NC synthesis. However, if the amount of OA is further increased, the growth rate of Co is too low, so that no Co can be found in the product (indicated by the EDS spectra shown in Figure S5e). At the same time, the Au cores become unstable, probably because OA is a weaker ligand than OAm. When half of the 7.5 mL OAm was replaced by OA, some larger Au particles were observed (Figures 3c and S5d). When all OAm was replaced by OA, the Au NCs completely merged into large irregular pieces (Figures 3d and S5f). As a result of the protection of Co shell, Au@Co NCs are stable and the Au cores are confined in Co shells. The Co shell can also prevent the aggregation of Au during the calcination process and thus uniform Au@Co3O4 core–shell NCs can be obtained. The electrochemical performance of Au@Co3O4/C was investigated using a standard three-electrode system in 0.1 M KOH. The detailed procedures are described in the Supporting Information. The catalyst was uniformly cast onto a 5 mm glassy carbon electrode with a total loading (NCs and carbon supports) of ca. 200 μgAu@Co3O4+C cm−2. The loading of Au@Co3O4 NC catalyst was 64 μgAu@Co3O4 cm−2, in which 24 μgAu cm−2 came from Au and 40 μgCo3O4 cm−2 from Co3O4. Gold NCs supported on carbon (Au/C), Co3O4 NCs supported on carbon (Co3O4/C), and Au and Co3O4 NC mixtures supported on carbon [(Au+Co3O4)/C] with the same Au and/
Figure 3. Scheme of the influence of the amount of oleic acid and oleylamine used in the synthesis on the final product.
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or Co3O4 loading were also studied for comparison. The Co3O4/C catalyst was made by calcination of 7.3 nm Co NCs in air, and the detailed procedures are described in the Supporting Information. The characterization data (TEM, EDS, and TGA) of the Co3O4/C and (Au+Co3O4)/C catalysts are shown in Figures S6, S7 and S8, S9, respectively, in the Supporting Information. Figure 4a shows the polarization curves at a slow scan rate of 5 mV s−1 to minimize the capacitive current. These curves were corrected for solution resistance, which was determined to be ca. 44 Ω by AC-impedance spectroscopy. During the measurements, the working electrode was rotating at 2500 rpm to remove the generated oxygen bubbles. The Au@Co3O4 catalyst shows the lowest onset potential of OER current among these four catalysts, revealing its highest activity. Figure 4b summarizes the current densities at a fixed overpotential of 0.35 V (1.58 V vs. reversible hydrogen electrode (RHE)). The current density is 2.84 mA cmdisk−2 for the reaction catalyzed by Au@Co3O4, which is 7 times as high as that with Co3O4 (0.42 mA cmdisk−2) and 55 times as high as that with Au (0.05 mA cmdisk−2). However, the reaction catalyzed by physically mixed Au and Co3O4 nanocrytsals (Au+Co3O4) has a current
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density (0.40 mA cmdisk−2) similar to that by Co3O4 alone. This indicates that the core–shell structure has a strong synergistic effect between the core and the shell, but this effect does not exist between the physically mixed NCs. Figure 4c shows the Tafel plots of the catalysts. The Tafel slope of Au@Co3O4 is 60 mV dec−1, which is similar to that of Au+Co3O4 (60 mV dec−1) and Co3O4 (59 mV dec−1), but much smaller than that of Au (147 mV dec−1), indicating that the active species are Co3O4. A durability test of Au@Co3O4 was carried out by means of a chronopotentiometry measurement (Figure 4d). An operating potential of 1.54 V (corresponding to an overpotential of 0.31 V) needs to be applied to achieve a current density of 10 A gcatalyst−1. The operating potential was nearly constant and only increased by a few millivolts after 300 min testing, indicating the good durability of Au@Co3O4 in alkaline solution. The-state-of-the-art OER catalyst Ir/C (20 wt% Ir on Vulcan XC-72, Premetek Co., the total loading is 200 μgIr+C cm−2 and 40 μgIr cm−2 for metal catalyst loading) was also studied for comparison. Although Ir has lower overpotential at the beginning of the test, it is not stable in alkaline solution and its overpotential grows quickly. Within an hour Ir has higher overpotential than Au@Co3O4, whose overpotential stays stable
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Figure 4. a) iR-corrected polarization curves of Au@Co3O4/C (64 μgAu@Co3O4 cm−2, in which Au loading is 24 μgAu cm−2 and Co3O4 loading is 40 μgCo3O4 cm−2), (Au+Co3O4)/C (24 μgAu cm−2 and 40 μgCo3O4 cm−2), Co3O4/C (40 μgCo3O4 cm−2), and Au/C (24 μgAu cm−2) catalysts in O2-saturated 0.1 M KOH at a scan rate of 5 mV s−1 and a continuous electrode rotating speed of 2500 rpm. b) Activity of the catalysts at an overpotential of 0.35 V. c) Tafel plots of catalysts. d) Chronopotentiometry curves of Au@Co3O4/C and Ir/C (40 μgIr cm−2) under a current density of 10 A gAu@Co3O4−1 or 10 A gIr−1 in O2-saturated 0.1 M KOH.
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over the testing period. The loss of activity of Ir is probably due to the oxidation of Ir to water-soluble IrO42− or other solvated IrVI ions under the alkaline OER conditions.[3e,g] Co atoms are considered as the active centers for the OER in Au@Co3O4. In spinel Co3O4, there are two types of Co, i.e., CoII and CoIII. According to the Pourbaix diagram, at the OER condition (pH 13, E = 1.23 to 1.7 V vs. RHE), the CoII in Co3O4 would be oxidized to CoIII.[12] Cyclic voltammograms (CV, Figure S10, Supporting Information) were employed to investigate the oxidation of the catalysts at high potential. An anodic current was found above 1.20 V for Au, which is attributed to the surface oxidation.[13] The corresponding reduction peak locates at 1.12 V. For Co3O4, there are two anodic peaks at 1.24 V and 1.47 V respectively and two corresponding cathodic peaks at lower potential. The first peak is assigned to the oxidation of CoII to CoIII and the second peak suggests the further oxidation of CoIII to CoIV.[4b,14] For the Au+Co3O4 catalysts, the CV curve is the combination of those for Au and Co3O4. Both characteristic peaks for Au (reduction peak at 1.12 V) and Co3O4 (anodic peak at 1.47 V and its corresponding reduction peak) can be found. However, the CV curve for Au@Co3O4 resembles that of Co3O4. Two anodic peaks for Co3O4 and their corresponding cathodic peaks can be identified. No Au oxidation peak can be found, which confirms the Au cores have been fully covered by Co3O4. After oxidation of Co3O4, the obtained CoIII compound β-CoOOH are constructed by CoIIIO6 octahedra and the surface is terminated by CoIII-OH species. The further oxidized CoIV = O species are found as the active species for the OER, and they couple with one of the neighboring O atoms, resulting in the formation of an O-O bond belonging to a hydroperoxo CoIV-OOH or to a peroxo CoIV-OO species.[15] Finally the O2 molecule desorbs and the catalyst converts back to CoIII. A scheme of the proposed mechanism is shown in Figure S11 in the Supporting Information. This mechanism suggest that the formation of CoIV is critical in the catalytic process and it has been detected by EPR spectroscopy[16] and in-situ Raman spectroscopy.[6b] Draining electrons from Co increases the CoIV population and enhances the OER activity. Au is the most noble metal with highest electronegativity (2.54 in Pauling scale), which is the most effective in draining electrons from the catalyst. DFT calculations show that the d-band center of Co on Au substrate positively shifts by 0.74 eV relative to that of pure Co (−1.17 eV).[17] The higher d-band center results in a stronger Co-O bond, which make Co easier to be oxidized. From the CV curves shown in Figure S10, the onset of the oxidation from CoII to CoIII in Au@Co3O4 is located at a slight lower potential than that of pure Co3O4, which suggests that indeed the Co3O4 on Au is easier to oxidize. The Hubbard-U corrected DFT calculations also show that both Co3O4 and β-CoOOH are located at the weak binding branch of the OER volcano plot, which means the activity will be enhanced by strengthened binding to oxygen.[18] Benefiting from the Au core, the Co3O4 shell of the Au@Co3O4 NCs have stronger binding to oxygen, and is easier to be oxidized to CoIV so that they have enhanced OER performance. In summary, Au@Co3O4 NC catalysts with uniform overall particle size and shell thickness have been successfully synthesized by carefully adjusting the amount of OAm and OA ligands. Owing to the synergistic effect between the core and
the shell, the Co3O4 shell has stronger binding to oxygen, and thus exhibits enhanced OER activity. At overpotential of 0.35 V, Au@Co3O4 NCs have an OER current density 7 times as high as a Au and Co3O4 NCs mixture or Co3O4 NCs alone and 55 times as high as Au NCs alone. A stable overpotential of only 0.31 V is needed for Au@Co3O4 to achieve a current density of 10 A gcatalyst−1. This work indicates that synergistic effect provided by core-shell structure can significantly enhance the catalytic performance, and high performance catalysts can be produced by rational design of the core–shell type hybrid structure.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We thank Dr. Minrui Gao, Jie Zheng, and Dr. Junhua Wang of University of Delaware for valuable discussions. Received: January 22, 2014 Revised: February 25, 2014 Published online:
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Synthesis of Monodispere Au@Co3O4 Core-Shell Nanocrystals and Their Enhanced Catalytic Activity for Oxygen Evolution Reaction Zhongbin Zhuang, Wenchao Sheng, and Yushan Yan* The hydrogen economy can provide an efficient energy system that is free from environmental issues related to the combustion of coal, oil, and natural gas.[1] However, such a system requires a clean and sustainable source of hydrogen, which can be provided by splitting of water either electrochemically or photoelectrochemically.[2] One of the key problems in splitting water is the kinetically sluggish anode reaction, i.e., oxygen evolution reaction (OER, 4OH− → 2H2O + 4e− + O2 in base). An overpotential of several hundred millivolts is often required to achieve a current density of 10 A gcatalyst−1.[3] Recent studies have shown that spinel-type Co3O4 has relatively good OER activities.[2e,3c,4] Hybrid materials have been proposed to further promote the OER activity of Co3O4, such as doping Co3O4 with other metals to make substituted cobaltites[5] or growing Co3O4 on a special substrate.[6] Nanoparticles are usually the preferred form of catalysts because of their high surface areas, and a core–shell structure is one of the typical routes to producing hybrid materials at the nanometer scale. Core–shell nanocrystals (NCs) show remarkable catalytic behavior compared with single-component NCs because of the synergistic effect between the core and the shell.[7] For example, Alayoglu and coworkers have found that Ru@Pt core–shell nanoparticles have the highest activity towards CO oxidation.[8] The CO conversion temperature of this catalyst is 30 °C, which is significantly lower than that of PtRu alloys (85 °C), a mixture of monometallic Pt and Ru nanoparticles (93 °C) and pure Pt particles (170 °C). However, synthesis of high quality core–shell NCs with well-defined structure is still challenging, especially when the core and the shell are of different classes of materials (e.g., elemental metal core and oxide compound shell), so that this type of core–shell NCs has scarcely been reported.[9] In the study reported here, we synthesized monodisperse Au@Co3O4 core–shell NCs, which were converted from monodisperse Au@Co core–shell NCs, with an overall diameter of 8 nm and also examined their catalytic OER activity. Au was chosen as the core because Yeo and Bell reported that cobalt oxide electrochemically grown on a Au electrode has enhanced OER activity, benefiting from the highly electronegative Au
Dr. Z. B. Zhuang, Dr. W. C. Sheng, Prof. Y. S. Yan Department of Chemical and Biomolecular Engineering and Center for Catalytic Science and Technology University of Delaware Newark, DE 19716, USA E-mail: [email protected]
DOI: 10.1002/adma.201400336
Adv. Mater. 2014, DOI: 10.1002/adma.201400336
substrate, which makes the Co3O4 more easily oxidized.[6b] Our electrochemical study shows that our Au@Co3O4 NCs have an OER activity 7 times as high as a mixture of Au and Co3O4 NCs or Co3O4 NCs alone, and 55 times as high as Au NCs, most likely due to a strong synergistic effect between the core and the shell, and this effect does not exist between the physically mixed NCs. Some Au and Co3O4 hybrid NCs have been reported,[10] however, none of them have well-defined core–shell structures and uniform sizes. High-quality Au@Co3O4 core–shell NCs are still desired. In our experiment, a three-step approach (Figure 1a) was adopted to synthesize Au@Co3O4 NCs, comprising synthesis of the Au NC, growth of the Co shell, and oxidation of Co to Co3O4. First, Au NCs were prepared by reducing HAuCl4 with tert-butylamine borane (TBAB) in the presence of oleylamine (OAm) as the ligand, following the procedure described in a previous study by Peng et al.[11] Second, Co shells were grown on the Au NC cores to prepare Au@Co NCs by using Co(acac)2, where acac is acetylacetonate, as the cobalt source and TBAB as the reducing agent. OAm and oleic acid (OA) were introduced to control the shape and uniformity. Third, the Au@Co NCs were loaded on carbon and then the Co shells were oxidized to Co3O4 by calcination in air. The experimental details are described in the Supporting Information. Figure 1b shows the transmission electron microscopy (TEM) image of the as-obtained Au NCs. They have a narrow size distribution with a diameter of 3.6 ± 0.5 nm. Five-fold symmetry is found in the high-resolution TEM (HRTEM) image (Figure S1, Supporting Information), which is in agreement with the literature, indicating the multiple-twinned structure of the Au NCs.[11] Figure 1c shows a TEM image of Au@Co core–shell NCs that were synthesized by growing Co shells on the pre-synthesized Au cores with 0.5 mmol OA. A dark core corresponding to Au can clearly be seen located at the center of the NC, and a uniform lighter Co shell caps around it. The Au@Co NCs are nearly monodisperse with an overall diameter of 8.1 ± 0.7 nm. The thickness of the Co shell is ca. 2 nm. The Au@Co NCs are highly uniform so that they can assemble into an ordered structure (Figure 1d). The energy dispersive spectrometry (EDS) spectra (Figure S2a, Supporting Information) show the signals of Au and Co (atomic ratio 1:4), which confirms the hybrid Au@Co composition. The Co shell seems to be amorphous because no clear lattice fringe can be seen in the HRTEM image (Figure 1e). This may be due to the lattice mismatch between Au (face-centered cubic, fcc) and Co (hexagonal close packed, hcp). The multiple twinned nature of the Au core may also influence the crystallinity of the Co shell. It is noted that Co NCs cannot be synthesized under the same condition
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NCs in air. The Au@Co3O4 NCs are well dispersed on the carbon support. The lattice fringe (Figure 1g) with an interplanar spacing of 0.29 nm is ascribed to the (220) plane of spinel Co3O4. The thickness of the shell remained at ca. 2 nm. The calcination process has also removed the surfactants (i.e., OAm and OA) used in the synthesis to obtain a clean surface. With that, the active sites can be more exposed to the reactant during the catalytic process. The EDS spectra (Figure S2b, Supporting Information) also show that the atomic ratio of Au:Co is maintained at 1:4. The thermogravimetric (TGA) result of Au@Co3O4/C measured in air indicates that it contains 68 wt% carbon support (Figure S3, Supporting Information). Based on the Au/Co ratio obtained from the EDS spectra, it can be calculated that the sample contains 12 wt% Au and 20 wt% Co3O4. X-ray diffraction (XRD) patterns of the as-prepared NCs are shown in Figure 2. Figure 2a illustrates the XRD pattern of Au NCs and it fits the fcc Au standard pattern well (JCPDS No. 65–2870). For the Au@Co NCs (Figure 2b), all the peaks can be indexed to fcc Au and no clear Co reflection peaks can be found. It indicates that the Co shell is amorphous, consistent with the HRTEM result. The XRD pattern of Au@Co3O4/C is shown in Figure 2c. The broad peak at ca. 25° comes from the carbon support. The two strongest peaks from Co3O4, i.e., (311) at 36.8° and (440) at 65.2°, are overlapped with the peaks corresponding to Au(111) at 38.2° and (220) at 64.6°, respectively. However, additional peaks assigned to Co3O4 are clearly present (pointed by arrows), which confirms the shell has been oxidized. Figure S4 (Supporting Information) shows the data plotted with intensity on a logarithmic scale to highlight the Co3O4 signals with lower intensity. The key step to obtain monodisperse Au@Co3O4 NCs is to synthesize high quality Au@Co NCs. In our experiment, both OAm and OA were used as solvents and also ligands. It was found that the amount of OA is critical to the shape and uniformity of the products, as summarized in Figure 3 Figure 1. a) Scheme of the synthetic route to Au@Co3O4 core–shell NCs. b) TEM image of Au (enlarged TEM images and EDS spectra are NCs. Inset: Histogram of the size distribution. c) TEM image of Au@Co NCs. Inset: Histogram also shown in Figure S5 in the Supporting of the size distribution. d) TEM image of a two-layer array of the Au@Co NCs. Inset: Modeled Information). TBAB was used as the reducer projection of the two-layer NC assembly. e) HRTEM image of a single Au@Co NC. f) TEM and its reducibility depends on basicity. When image of Au@Co3O4 NCs supported on carbon. g) HRTEM image of a single Au@Co3O4 NC. only OAm is used, the reducibility of TBAB is the strongest. Co(acac)2 was rapidly reduced to Co monomer without adding Au NCs. The Au NCs not only serve as heterogeneous nuclei but also promote the reduction of CoII.[10h] and the monomer became saturated. Owing to the rapid reduction, the Co monomers not only grow on the Au core, but also Figure 1f shows a TEM image of Au@Co3O4 NCs supported homogeneously form Co nuclei and further grow to Co NCs. on carbon (Au@Co3O4/C) prepared by calcination of Au@Co 2
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b)
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Figure 2. a–c) XRD patterns of Au NCs (a), Au@Co NCs (b), and Au@ Co3O4/C sample (c). The standard XRD patterns (Au, JCPDS No.65–2870; Co, JCPDS No. 05–0727; Co3O4, JCPDS No. 43–1003) are shown beneath the plots.
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a)
Thus, a mixture of Au@Co NCs and Co NCs are obtained (identified in Figure S5b). By adding 0.16 mL (0.5 mmol) OA, the reducibility of TBAB was weakened and thus the growth rate of Co was lower. Co growth on the heterogeneous Au core is much easier than homogeneous nucleation, therefore monodisperse Au@Co NCs can be obtained. Thus this condition is optimal for Au@Co NC synthesis. However, if the amount of OA is further increased, the growth rate of Co is too low, so that no Co can be found in the product (indicated by the EDS spectra shown in Figure S5e). At the same time, the Au cores become unstable, probably because OA is a weaker ligand than OAm. When half of the 7.5 mL OAm was replaced by OA, some larger Au particles were observed (Figures 3c and S5d). When all OAm was replaced by OA, the Au NCs completely merged into large irregular pieces (Figures 3d and S5f). As a result of the protection of Co shell, Au@Co NCs are stable and the Au cores are confined in Co shells. The Co shell can also prevent the aggregation of Au during the calcination process and thus uniform Au@Co3O4 core–shell NCs can be obtained. The electrochemical performance of Au@Co3O4/C was investigated using a standard three-electrode system in 0.1 M KOH. The detailed procedures are described in the Supporting Information. The catalyst was uniformly cast onto a 5 mm glassy carbon electrode with a total loading (NCs and carbon supports) of ca. 200 μgAu@Co3O4+C cm−2. The loading of Au@Co3O4 NC catalyst was 64 μgAu@Co3O4 cm−2, in which 24 μgAu cm−2 came from Au and 40 μgCo3O4 cm−2 from Co3O4. Gold NCs supported on carbon (Au/C), Co3O4 NCs supported on carbon (Co3O4/C), and Au and Co3O4 NC mixtures supported on carbon [(Au+Co3O4)/C] with the same Au and/
Figure 3. Scheme of the influence of the amount of oleic acid and oleylamine used in the synthesis on the final product.
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or Co3O4 loading were also studied for comparison. The Co3O4/C catalyst was made by calcination of 7.3 nm Co NCs in air, and the detailed procedures are described in the Supporting Information. The characterization data (TEM, EDS, and TGA) of the Co3O4/C and (Au+Co3O4)/C catalysts are shown in Figures S6, S7 and S8, S9, respectively, in the Supporting Information. Figure 4a shows the polarization curves at a slow scan rate of 5 mV s−1 to minimize the capacitive current. These curves were corrected for solution resistance, which was determined to be ca. 44 Ω by AC-impedance spectroscopy. During the measurements, the working electrode was rotating at 2500 rpm to remove the generated oxygen bubbles. The Au@Co3O4 catalyst shows the lowest onset potential of OER current among these four catalysts, revealing its highest activity. Figure 4b summarizes the current densities at a fixed overpotential of 0.35 V (1.58 V vs. reversible hydrogen electrode (RHE)). The current density is 2.84 mA cmdisk−2 for the reaction catalyzed by Au@Co3O4, which is 7 times as high as that with Co3O4 (0.42 mA cmdisk−2) and 55 times as high as that with Au (0.05 mA cmdisk−2). However, the reaction catalyzed by physically mixed Au and Co3O4 nanocrytsals (Au+Co3O4) has a current
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density (0.40 mA cmdisk−2) similar to that by Co3O4 alone. This indicates that the core–shell structure has a strong synergistic effect between the core and the shell, but this effect does not exist between the physically mixed NCs. Figure 4c shows the Tafel plots of the catalysts. The Tafel slope of Au@Co3O4 is 60 mV dec−1, which is similar to that of Au+Co3O4 (60 mV dec−1) and Co3O4 (59 mV dec−1), but much smaller than that of Au (147 mV dec−1), indicating that the active species are Co3O4. A durability test of Au@Co3O4 was carried out by means of a chronopotentiometry measurement (Figure 4d). An operating potential of 1.54 V (corresponding to an overpotential of 0.31 V) needs to be applied to achieve a current density of 10 A gcatalyst−1. The operating potential was nearly constant and only increased by a few millivolts after 300 min testing, indicating the good durability of Au@Co3O4 in alkaline solution. The-state-of-the-art OER catalyst Ir/C (20 wt% Ir on Vulcan XC-72, Premetek Co., the total loading is 200 μgIr+C cm−2 and 40 μgIr cm−2 for metal catalyst loading) was also studied for comparison. Although Ir has lower overpotential at the beginning of the test, it is not stable in alkaline solution and its overpotential grows quickly. Within an hour Ir has higher overpotential than Au@Co3O4, whose overpotential stays stable
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Figure 4. a) iR-corrected polarization curves of Au@Co3O4/C (64 μgAu@Co3O4 cm−2, in which Au loading is 24 μgAu cm−2 and Co3O4 loading is 40 μgCo3O4 cm−2), (Au+Co3O4)/C (24 μgAu cm−2 and 40 μgCo3O4 cm−2), Co3O4/C (40 μgCo3O4 cm−2), and Au/C (24 μgAu cm−2) catalysts in O2-saturated 0.1 M KOH at a scan rate of 5 mV s−1 and a continuous electrode rotating speed of 2500 rpm. b) Activity of the catalysts at an overpotential of 0.35 V. c) Tafel plots of catalysts. d) Chronopotentiometry curves of Au@Co3O4/C and Ir/C (40 μgIr cm−2) under a current density of 10 A gAu@Co3O4−1 or 10 A gIr−1 in O2-saturated 0.1 M KOH.
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over the testing period. The loss of activity of Ir is probably due to the oxidation of Ir to water-soluble IrO42− or other solvated IrVI ions under the alkaline OER conditions.[3e,g] Co atoms are considered as the active centers for the OER in Au@Co3O4. In spinel Co3O4, there are two types of Co, i.e., CoII and CoIII. According to the Pourbaix diagram, at the OER condition (pH 13, E = 1.23 to 1.7 V vs. RHE), the CoII in Co3O4 would be oxidized to CoIII.[12] Cyclic voltammograms (CV, Figure S10, Supporting Information) were employed to investigate the oxidation of the catalysts at high potential. An anodic current was found above 1.20 V for Au, which is attributed to the surface oxidation.[13] The corresponding reduction peak locates at 1.12 V. For Co3O4, there are two anodic peaks at 1.24 V and 1.47 V respectively and two corresponding cathodic peaks at lower potential. The first peak is assigned to the oxidation of CoII to CoIII and the second peak suggests the further oxidation of CoIII to CoIV.[4b,14] For the Au+Co3O4 catalysts, the CV curve is the combination of those for Au and Co3O4. Both characteristic peaks for Au (reduction peak at 1.12 V) and Co3O4 (anodic peak at 1.47 V and its corresponding reduction peak) can be found. However, the CV curve for Au@Co3O4 resembles that of Co3O4. Two anodic peaks for Co3O4 and their corresponding cathodic peaks can be identified. No Au oxidation peak can be found, which confirms the Au cores have been fully covered by Co3O4. After oxidation of Co3O4, the obtained CoIII compound β-CoOOH are constructed by CoIIIO6 octahedra and the surface is terminated by CoIII-OH species. The further oxidized CoIV = O species are found as the active species for the OER, and they couple with one of the neighboring O atoms, resulting in the formation of an O-O bond belonging to a hydroperoxo CoIV-OOH or to a peroxo CoIV-OO species.[15] Finally the O2 molecule desorbs and the catalyst converts back to CoIII. A scheme of the proposed mechanism is shown in Figure S11 in the Supporting Information. This mechanism suggest that the formation of CoIV is critical in the catalytic process and it has been detected by EPR spectroscopy[16] and in-situ Raman spectroscopy.[6b] Draining electrons from Co increases the CoIV population and enhances the OER activity. Au is the most noble metal with highest electronegativity (2.54 in Pauling scale), which is the most effective in draining electrons from the catalyst. DFT calculations show that the d-band center of Co on Au substrate positively shifts by 0.74 eV relative to that of pure Co (−1.17 eV).[17] The higher d-band center results in a stronger Co-O bond, which make Co easier to be oxidized. From the CV curves shown in Figure S10, the onset of the oxidation from CoII to CoIII in Au@Co3O4 is located at a slight lower potential than that of pure Co3O4, which suggests that indeed the Co3O4 on Au is easier to oxidize. The Hubbard-U corrected DFT calculations also show that both Co3O4 and β-CoOOH are located at the weak binding branch of the OER volcano plot, which means the activity will be enhanced by strengthened binding to oxygen.[18] Benefiting from the Au core, the Co3O4 shell of the Au@Co3O4 NCs have stronger binding to oxygen, and is easier to be oxidized to CoIV so that they have enhanced OER performance. In summary, Au@Co3O4 NC catalysts with uniform overall particle size and shell thickness have been successfully synthesized by carefully adjusting the amount of OAm and OA ligands. Owing to the synergistic effect between the core and
the shell, the Co3O4 shell has stronger binding to oxygen, and thus exhibits enhanced OER activity. At overpotential of 0.35 V, Au@Co3O4 NCs have an OER current density 7 times as high as a Au and Co3O4 NCs mixture or Co3O4 NCs alone and 55 times as high as Au NCs alone. A stable overpotential of only 0.31 V is needed for Au@Co3O4 to achieve a current density of 10 A gcatalyst−1. This work indicates that synergistic effect provided by core-shell structure can significantly enhance the catalytic performance, and high performance catalysts can be produced by rational design of the core–shell type hybrid structure.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We thank Dr. Minrui Gao, Jie Zheng, and Dr. Junhua Wang of University of Delaware for valuable discussions. Received: January 22, 2014 Revised: February 25, 2014 Published online:
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