Au Heterostructure as A

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Energy, Environmental, and Catalysis Applications

Metal Organic Frameworks-Derived Co3O4/Au Heterostructure as A Catalyst for Efficient Oxygen Reduction Lei Liu, Qin Wei, Xuelian Yu, and Yihe Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06292 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Applied Materials & Interfaces

Metal Organic Frameworks-Derived Co3O4/Au Heterostructure as a Catalyst for Efficient Oxygen Reduction Lei Liua,b, Qin Wei*a, Xuelian Yu*b, Yihe Zhang*a,b, a. Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, Jinan, 250022, China b. National Laboratory of Mineral Materials, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, China. ABSTRACT：Porous nanostructures with a yolk-shell complex interior will provide lots of virtues to construct advanced catalysts. In our work, the preparation of novel yolk-shell Au nanocrystals loaded Co3O4 nanocages (Co3O4/Au heterostructure) from a metal organic framework-derived composites was reported. The characterization ways of SEM, EDX TEM, XRD, XPS and BET etc were used to analyze the morphology, structure and composition of the heterostructures. Most importantly, Co3O4/Au heterostructure are a kind of low cost, good performance catalysts for the oxygen reduction reaction to replace the noble-Pt catalysts. The high surface area of the porous structure, as well as the excellent electron transfer properties of well-dispersed Au nanocrystals and also the electronic coupling effect between Co3O4 and Au in the composites are attributed to the good performance. KEYWORDS: heterostructure; Co3O4/Au; electronic coupling; oxygen reduction reaction; metal-organic frameworks.

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INTRODUCTION With the rapid development of economy and science, it is urgent to find a new kind of catalysts with low cost and high efficiency for energy storage and conversion1. Of these methods, oxygen reduction reaction (ORR) catalysts have been paid much attention in zinc-air batteries and fuel cells2-5, because it is known to be the rate-determining step in these devices. To date, noble metal-Pt and its alloys are the main candidates for designing ORR catalyst

6-7

. But, their limited

resources and declining activity during long-term operation have seriously kept them from large-scale commercialization8. Therefore, people make significant efforts to develop platinum-free ORR catalysts, which are mainly focused on carbon-based nanostructures9-11, transition metal oxides12-14 and carbides15-17, etc. Co3O4 is one of the most potential choices owing to its relatively good catalytic properties and low cost. In the past decades, many morphologies of Co3O4 nanomaterials have been prepared, such

as

yolk-shell

nanocages18,

three-dimensionally

ordered

macroporous

supported

gold-palladium alloy19, porous concave nanocubes20, and so on. Nevertheless, due to their low conductivity, it is critical to enhance the electron transport of Co3O4 based electrodes. Combining with conductive phase, such as N-doped graphene21-22 and carbon nanotube23-24, is a valid way to enhance the charge transfer ability and electerocatalytic performance. Besides, metallic nanoparticles, especially gold nanocrystals, which show high electrical conductivity and surface area, have also been widely used as conductive dopants on metal oxides for electrocatalytic application. For example, Jaramillo et al25 prepared a series of Au-core 3d transition metal oxide-shell (Au@MxOy where M = Ni, Co, Fe, and CoFe) morphology catalysts as the oxygen evolution reaction (OER) catalysts. They found that the addition of Au-core makes the catalytic

2


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activity of OER enhance, which may be ascribed to the synergistic reaction between Au-core and MxOy shell. Jiang et al26 synthesized α-Au/CeOx-RGO by coreduction method for the electrochemical reduction of N2. The addition of Au to the CeOx-RGO made much stronger binding and catalytic ability with N2 molecules. In these heterostuctures, the efficiency of electrocatalytic reaction depends both on the properties of support and the size and the dispersion of metal nanoparticles. Therefore, in order to obtain active Co3O4-based catalysts, metal nanoparticles are dispersed on Co3O4-based support usually. As we know, the porous nanostructures have the advantages of high specific surface area and designable channels for ions and molecules27-30, which attract people’s great research interest. Metal-organic frameworks (MOFs) are consisted of metal ions and organic ligands, which are good template to prepare metal oxide and construct special structure, such as porous metal oxide and carbon nanostructure through simple pyrolysis because of their unique thermal behavior. For example, hollow controllable interiors Co3O4 dodecahedrons have been prepared through pyrolysis of ZIF-67- a kind of Co based zeolitic imidazolate framework31. By calcination of Zn-based MOFs (ZIF-8), microporous carbon polyhedrons have been produced32. As a special extension of porous systems, yolk-shell structure demonstrated a different core@blank@shell structure with a porous shell, a blank gap between core and shell, and a core

33

. These unique

structures can not only provide large surface area, but also promote the charge transfer process within electrode. MOFs are also a promising template to construct heterostructures due to their good compatibility with various metal ions. With them as precursor, it is possible to fabricate heterostructures with controllable composition. Taking all of these into account, it is advisable to develop a facile route for preparing noble metal-embedded yolk-shell structures. 3



In our work, a simple methodology for the production of Co3O4/Au heterostructures through pyrolysis of MOF based hybrids with one-step was reported. Au3+ was loaded to the Co-based ZIF-67 to get the precursor (ZIF-67/Au3+) (Scheme 1). Then the ZIF-67/Au3+ polyhedrons were thermally treated at 300 ℃ in air in order to transfer the ZIF-67 into Co3O4 yolk-shell nanocage (YSNCs) and Au3+ ions to Au nanocrystals at the same time. The resulting Au nanocrystals were well dispersed on the porous Co3O4 YSNCs without agglomeration. The products Co3O4/Au heterostructures showd an excellent electrocatalytic activity for ORR with unique structure.

Scheme 1. Schematic preparation process of Co3O4 YSNCs and Co3O4/Au heterostructures.

RESULTS AND DISCUSSION TG analysis (TGA) measurement was carried out to investigate the ZIF-67/Au3+ precursor calcination temperature. As shown in Figure 1a, below 270 ℃, weight loss is about 2.0 wt%. When the temperature ups to about 270 °C, the weight loss drops rapidly and at around 300 °C flattens finally. During the temperature grows from 270 °C to 300 °C, the total weight loss is ∼59.5%, indicating the decomposition of ZIF-67 organic ligands and the pyrolysis of Au3+ in 4


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NaAuCl4 sources. Therefore, the calcination temperature of 300 ℃ is predetermined to obtain the heterostructures. Figure 1b is the XRD curves of ZIF-67/Au3+ precursors, Co3O4, Co3O4/Au heterostructures. The ZIF-67 diffraction peaks are same to the literature report18. After calcination in air at 300 ℃ for 2 h, the characteristic peak changes obviously. With the standard PDF cards of XRD(JCPDS No. 43-1003), we can tell that the diffraction peaks in Co3O4 and Co3O4/Au heterostructures can be ascribed to the cubic phase Co3O4. No characteristic diffraction peaks of Au are observed due to the relatively low loading content. Figure 1c is the FIIR, which is used to characterize the functional groups changes of ZIF-67/Au3+ precursor. For the ZIF-67/Au3+ precursor, the 1575 and 750 cm-1 absorption peaks represent the bending mode and stretching vibration of C=N bond, and the 422 cm-1 absorption peak is the characteristic peak of Co-N stretching vibration. FTIR and XRD of ZIF-67 precursor before and after the treatment with Au3+ ions were also characterized for comparison. As shown in Figure S1, almost no difference exists between ZIF-67 and ZIF-67/Au3+. Noticeably, after calcination in air, at 663 and 570 cm-1, there two new absorption peaks appears. The two new absorption peaks are ascribed to the Co3O4 Co-O vibration. This result indicates that the ZIF-67/Au3+ precursor is complete decomposed. By comparing the spectrogram of Co3O4 to Co3O4/Au heterostructures, there appears a clear absorption at 1610 cm-1 because of the interaction of Co3O4 with the surface of the Au nanocrystals, suggesting the existence of Au nanocrystals in the heterostructures18.

Figure 1. (a) The TG curve of ZIF-67/Au3+ precursors, (b) XRD curves and (c) FTIR spectra of 5



ZIF-67/Au3+ precursors, Co3O4 and Co3O4/Au heterostructures.

Figure 2. ZIF-67(a), Co3O4 (c) and Co3O4/Au heterostructures (e) SEM images. TEM images of ZIF-67 (b), Co3O4 (d) and Co3O4/Au heterostructures (f). HRTEM image of Co3O4/Au heterostructures (g). EDS of Co3O4 (h) and Co3O4/Au heterostructures (i).

SEM and TEM were used to characterize the morphology and structure of the ZIF-67, Co3O4 and Co3O4/Au heterostructures. The ZIF-67 precursor shows regular polyhedron shape with the average size of 500 nm (Figure 2a). After calcination, the nanocages maintain their original shape and size, but with a rougher surface (Figure 2c). TEM images also indicate their inner difference before and after annealing in air. From Figure 2b and d, the inner structure of ZIF-67 is solid. While after calcination, a Co3O4 YSNCs appears. This phenomenon is may be due to the 6


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heterogeneous contraction of non-equilibrium heat treatment 31. Figure 2e and f show the TEM and SEM images of Co3O4/Au heterostructures. It is obvious that the Au nanocrystals with average size of 5 nm are evenly distributed on the Co3O4 surface to form the heterostructures and no change of the size and shape of the Co3O4 cages are observed. The size distribution of supported Au nanocrystals was shown in Figure S2. HRTEM image is shown in Figure 2g. The 10 times lattice spaces in the image of 2.81 nm, 4.65 nm and 2.35 nm correspond to the (220), (111) and (311) crystal planes of Co3O4, respectively. And the 2.35 nm (10 times of characteristic d-spacing) corresponds to the (111) diffraction of Au. EDS analysis shown in Figure 2h and i also confirmed the existence of Au besides the coexistence of carbon, nitrogen, oxygen, and cobalt atoms in Co3O4 and Co3O4/Au samples. The EDX elemental mapping of selected Co3O4/Au heterostructures shows that the Au nanocrystals uniformly distributed on Co3O4 YSNCs (Figure S3).

Figure 3. XPS spectrogram of survey spectra (a), Co 2p (b) and O 1s (c) in the Co3O4 YSNCs and Co3O4/Au heterostructures. XPS spectra of Au 4f (d) in the Co3O4/Au heterostructures and pure 7



Au nanoparticles.

The existence of Co, O, C and Au elements is shown in the full survey spectrum of Co3O4/Au heterostructures XPS (Figure 3a). Figure 3b shows Co 2p XPS spectra of Co3O4 and Co3O4/Au heterostructures. The fitting peaks at 779.4 and 794.6 eV are attributed to Co3+, while the two peaks at 781.5 and 797.2 eV are ascribed to Co2+. O 1s XPS peaks in Co3O4 YSNCs and Co3O4/Au heterostructures are shown in Figure 3c. The broad and asymmetric peaks at 529.8, 531.1, 531.8 eV can be assigned to the surface lattice oxygen (OL), oxygen vacancies (OV) and chemisorbed oxygen (OC), respectively. The existence of Au in Co3O4/Au heterostructures is also confirmed by XPS. As shown in Figure 3d, the peaks at 87.9 eV and 84.2 eV can be attributed to the 4 f5/2 and 4 f7/2 of the zero-valent Au. Therefore, all of the results indicate the formation of Co3O4/Au heterostructures. The electrocatalytic behavior of the Co3O4/Au heterostructures was preparatory tested by the ORR. Figure 4a shows the CV tests of the GCE modified with Co3O4, Co3O4/Au and Pt/C in 0.1 M KOH solution, saturated with Ar or O2. It can be seen clearly that the Co3O4/Au modified electrode displays a much better electrocatalytic activity than the Co3O4 modified electrode by several parameters, containing the positive onset potential (Eonset), the large cathodic peak current density and the positive potential of the cathodic peak current. The impressed electrocatalytic activity of Co3O4/Au is further proved by LSV measurement. The results were shown in Figure 4b and c, and the limiting current density (jd) of Co3O4/Au at 0.17 V vs. RHE is about 5.3 mA cm-2, which is much higher than Co3O4 catalyst and comparable to that of commercial Pt/C (Figure S4). The insets of Figure 4b and c show the corresponding Koutecky-Levich (K-L) plots, which were 8


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used to calculate the ORR electron transfer numbers of Co3O4/Au (4.0) and Co3O4 (3.5) samples respectively. From the K-L plots, we can calculate the kinetic current density (jK) values at 0.6 V. As summarized in Figure 4d, the half-wave potential (E1/2) for the Co3O4/Au catalyst is 60 mV higher than that of the Co3O4 catalyst, and most impressively, Co3O4/Au gains the largest jK value (71.9 mA cm−2) at 0.6V vs. RHE. The value is about 7 times than that of Co3O4 (10.1 mA cm−2), indicating that the catalyst owns fast kinetics for ORR in the alkaline medium. For comparison, we further carried out the ORR catalytic activity of Au nanoparticles and the results were shown in Figure S5. Obviously, the Co3O4/Au catalyst shows the best limiting current density and half-wave potential, which indicates that O2 is easier to be reduced on Co3O4/Au than on the single of Co3O4 or Au nanoparticles. All of these results show the synergistic effect of Co3O4 and Au in the heterostructures.

Figure 4. (a) CVs of Co3O4, Co3O4/Au and Pt/C catalysts modified electrodes in 0.1M O2-saturated (solid line) and Ar-saturated (dash line) KOH solution. LSV curves of Co3O4 (b) and 9



Co3O4/Au (c) modified electrodes at different rotation rates. The insets are the corresponding K-L plots at different potentials. (d) Jk at 0.6 V and E1/2 for different catalysts.

Figure 5. Tafel plots of Co3O4, Co3O4/Au and Pt/C derived from corresponding RDE data.

The mechanistic and kinetic performance of Co3O4 and Co3O4/Au catalysts were further evaluated using Tafel plots (Figure 5). The Tafel slope in the low current density region on Co3O4/Au is 51 mV/dec, which is even smaller than that on Pt/C surface. In the high current density region, the Tafel slop of Co3O4, Co3O4/Au and 20% Pt-C catalysts are 144 mV/dec, 125 mV/dec and 119 mV/dec，respectively. This is attributed to a change in the mechanism of ORR from Temkin to Langmuir adsorption conditions when the current density increases. From the above results, we can conclude that Co3O4/Au catalyst owns high ORR catalytic activities close to that of the commercial Pt/C catalyst in an alkaline medium. These catalytic performances of Co3O4/Au are comparable to most of the advanced catalysts reported in the literature (see Table 1).

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Table 1. Comparison of electrocatalytic performance of Co3O4/Au with various catalysts reported in the literature. E1/2（V vs. RHE） Jd (mA•cm-2)

Samples

Eonset（V vs. RHE）

Electrolyte

references

Co3O4/N-rmGO

0.92

~0.80

-5.5

0.1 M KOH

34

Cu2ZnSnS4-AuAg

0.85

0.73

-5.32

0.1 M KOH

35

Co@Co3O4@C-CM

0.85

0.70

-4.6

0.1 M KOH

36

Co@Co3O4-NC

0.91

0.74

-4.5

0.1 M KOH

37

N-GNR/ Co3O4

0.89

0.83

-5.8

0.1 M KOH

38

Ag@Co3O4/C

0.94

0.799

-3.8

0.1 M KOH

39

S-rGO

0.74

0.6

-2.3

0.1 M KOH

40

N-doped 3D rGO

0.894

0.695

-3.9

0.1 M KOH

41

PdNiCu/N-rGN

-0.1(Ag/Agcl)

-0.21(Ag/Agcl)

-3.5

0.1 M KOH

42

PdNiSn/N-rGN

-0.1(Ag/Agcl)

-0.21(Ag/Agcl)

-3.9

0.1 M KOH

42

N-CDC

0.80

0.90

-

0.1 M KOH

43

AuPC

0.95

0.83

-3.61

0.1 M KOH

44

C-FeZIF-900-0.84

0.95

0.77

-5.54

0.1 M KOH

45

Co3O4/Au

0.99

0.83

-5.3

0.1 M KOH

This work

Indeed, structural features of the composite were believed to exert significant contributions to the excellent catalytic activity of Co3O4/Au. Therefore, structures of the composite were readily tuned to elucidate their relationship with the activities. Firstly, the catalysts annealed under different temperature were dispended onto the GCE. From the CV and LSV curves shown in Figure 6a and b, we can see that Co3O4/Au-250 exhibits very poor ORR activity with an onset potential of about 0.61 V vs. RHE. After the increase of calcination temperture, the product of Co3O4/Au-350 shows a much more positive ORR onset potential (0.90 V) than Co3O4/Au-250, but lower than Co3O4/Au-300 (0.98 V). In the diffusion controlled region, the limited current densities 11



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follow the same trend of the onset potentials, suggesting that Co3O4/Au-300 owns best ORR catalytic activity (LSV curves of Co3O4/Au-250 and Co3O4/Au-350 under different rotating speed and corresponding K-L plots are shown in Figure S6 and S7). This result may be because that when the precursor ZIF-67/Au3+ was annealed under 250℃, most of ZIF-67/Au3+ hasn’t been transformed into Co3O4/Au yet. While the precursor was annealed under 350℃, the framework of ZIF-67/Au3+ was crashed down. This is supported by the SEM and TEM image shown in Figure S8 and S9. That is to say the porous framework of the heterostructure provides a flexible channel for the electron transfer. The macroporous channels can be observed and tested by TEM and BJH pore size distribution analysis as Figure S10. Secondly, varied loading content of Au to Co3O4 was obtained by simply adjusting the initial amount of NaAuCl4. These products after calcination at 300 ℃ with different contents of Au to Co3O4 are defined as Co3O4, Co3O4/Au-0.03, Co3O4/Au-0.06,

Co3O4/Au-0.08,

Co3O4/Au-0.10,

Co3O4/Au-0.12,

and

Co3O4/Au-0.15,

respectively. The mass fraction of Au within the Co3O4/Au was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Table S1). The value of Au mass fraction with different loading contents was in the range of 1.5-3.8%, which is quite consistent with that determined by EDX analysis. CV and LSV measurements are carried out to study the ORR catalytic activity of the series of Co3O4/Au catalysts (Figure 6c and d). It is obvious that both the onset potential and potential of the cathodic peak current increase with the raising of Au content from 0.03 to 0.12 mmol. After the addition of Au over 0.12 mmol, the onset potential begins to decrease. The trend of limiting current density (DLCD) from LSV curves at 1600 rpm is also consistent with the results of CV tests (LSV curves under different rotating speed and corresponding K-L plots are shown in Figure S11-15), and Eonset, E1/2 and DLCD of the series 12


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catalysts to ORR in O2-saturated 0.1 M KOH solutions were listed in Table S2. According to the results, 0.12 mmol is the best Au content to ZIF-67 precursor and the mass fraction of Au within the Co3O4/Au was determined by XPS analysis to be 3.35 wt%. The optimal Au content maybe resulted from the synergy effect between gold nanocrystals and Co3O4, while high content of Au would lead the decrease of active materials in electrode materials.

Figure 6. CV (a) and LSV at 1600 rpm (b) curves of Co3O4/Au annealed under different temperature in 0.1M O2-saturated (solid line) and Ar-saturated (dash line) KOH solution. CV (c) and LSV at 1600 rpm (d) curves of Co3O4/Au with different loading amount of Au in O2-saturated 0.1M KOH.

To further quantify the ORR electron transfer pathway, we employed a rotating ring-disk electrode (RRDE) technique to calculate the amount of HO2- at the disk electrode. From Figure 7a and b, the measured HO2- yield for Co3O4 is about 20% and the electron transfer number (n) 13



ranges in 3.60-3.70, while the measured HO2- yield for Co3O4/Au is about 10% and n ranges in 3.75-3.85. All of these results suggest that the ORR process of Co3O4/Au follows a four-electron (4e−) pathway with the high selectivity of OH− as the main product, which is in accordance with the RDE tests. Taken together, both of the porous york- shell structure and the interaction between Au nanocrystals and framework contribute to the excellent electrocatalytic activity of Co3O4/Au heterostructures.

Figure 7. (a) RRDE curves of Co3O4 and Co3O4/Au in 0.1 M O2-saturated KOH solution at a rotation rate of 1600 rpm with the ring potential kept at 1.45 V vs. RHE. (b) Calculated electron transfer number and yield of the peroxide species on the Co3O4 and Co3O4/Au electrodes.

Why Co3O4/Au heterostructure own such good performance for ORR electro-catalysis? We present the following discussion. Firstly, Figure 4d shows 4f XPS spectra of Au in Au nanocrystals and Co3O4/Au heterostructure. As displayed, the binding energies of the 4f5/2 and 4f7/2 of Au for Co3O4/Au have an obvious shift toward 0.43 eV (positive value) compared with that of the 4f XPS spectra for Au nanocrystals, suggesting an electron-donating effect from the Au to the neighboring Co3O4 in the Co3O4/Au heterostructures. As we know, the low ORR activities of metals such as Ag and Au, with low d-band centers, are mainly induced by the slow kinetics of the O-O bond 14


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breakage. So the charge transfer will show a d-band vacancy for Au and raise the d-band center, which is favorable for the O-O bond split during ORR. Secondly, the kinetics and interfacial processes of electrode reactions in electrochemical filed was investigated by the electrochemical impedance spectroscopy (EIS). The Nyquist plots of ZIF-67, Co3O4 and Co3O4/Au catalysts are shown in Figure 8a. It is obvious that the radius, which reflects the contact and charge transfer resistance, of ZIF-67, Co3O4 and Co3O4/Au catalysts is getting smaller and smaller. The results indicate the introduction of Au will make the Co3O4/Au possess better conductivity and faster electron transfer ability, which will shorten the diffusion length of the charges, further enhancing the ORR activity.

Figure 8. (a) Nyquist plots for ZIF-67, Co3O4 and Co3O4/Au, respectively. (b) The N2 adsorption-desorption isotherm of Co3O4 and Co3O4/Au.

Thirdly, porosity and specific surface area of the Co3O4 and Co3O4/Au were characterized by the BJH pore size distribution and N2 adsorption-desorption isotherms analysis to further discuss the reasons. As shown in Figure 8b, both of the samples show a typical type IV adsorption isotherm and at a relative pressure of 0.35-1.0, show a H3-type hysteresis loop for Co3O4 and Co3O4/Au, indicating the mesoporous structure exists20. Besides, the specific surface area of 15



Co3O4/Au (234 m2·g-1) is far higher than that of Co3O4 (122 m2·g-1) due to the existence of small Au nanoparticles and the changes of pore structure (Figure S10c,d). These quantities were consistent to those of other porous structure derived from MOFs[46,47]. The special porous structure and high specific surface area of Co3O4/Au will also be beneficial for oxygen adsorption and reduction. The durability of the catalysts is a key factor for electrocatalysis process [48-51]. The durability of Co3O4/Au heterostructures was measured by chronoamperometric method in 0.1 M O2-saturated KOH solution, which is shown in Figure 9a. After continuous operation for 40,000 s, about 96.4 % of current density was reserved. However, the current density of commercial 20% Pt/C dropped to 84.9%. The stability of Co3O4/Au was further assessed in O2-saturated 0.1 M KOH electrolyte by cycling the catalyst between -0.05 V and 1.45 V vs. RHE at 200 mV s-1 (Figure S16). After 6000 continuous cycles, the Co3O4/Au modified electrode showed a 15 mV negative shift in E1/2, but no shift in Eonset. The result combined with chronoamperometric measurement indicated the good stability of Co3O4/Au heterostructures.

Figure 9. The durability (a) and methanol-tolerance evaluation (b) of Co3O4/Au and 20% Pt/C catalysts by the chronoamperometric responses.

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Furthermore, with the addition of 3 M methanol to electrolyte, the current density of Co3O4/Au heterostructures catalyst increased slightly and steadied quickly compared to the 20% Pt/C catalyst (Figure 9b). Therefore Co3O4/Au showed better methanol tolerance than 20% Pt/C catalyst. This is an important factor for practical applications. These results strongly suggest that Co3O4/Au heterostructures can work in the presence of methanol for a long time.

CONCLUSION In conclusion, by a simple one-step pyrolysis process we have synthesized Co3O4/Au heterostructure in which Au nanocrystals are evenly dispersed on the Co3O4 YSNCs. The special structure owns high specific surface area and macroporous channel for the fast transport of electrons. Combined with the electronic coupling between Au and Co3O4 in the composite, the Co3O4/Au heterostructure displays excellent electrocatalytic performance for ORR, with the onset potential of 0.99V vs. RHE, half-wave potential of 0.83 V vs. RHE and limit current density of (5.3 mA cm-2), which is close to the performance of commercial 20 wt% Pt/C (Table S3). Besides, the Co3O4/Au heterostructure shows good methanol tolerance and stability. These results may open up a new way for the controlled design of metal oxide heterostructures with interesting architectures and tailored functionalities.

EXPERIMENTAL SECTION Synthesis of ZIF-67 precursor: according to the literature report, 1.164g Co(NO3)2·6H2O and 1.312g 2-methylimidazole were dissolved in 100 mL of methanol respectively, then the two solutions were mixed rapidly with vigorous stirring and aged at room temperature for 24 h. The precipitate was washed by ethanol for 4 times and vacuum dried overnight to get ZIF-67 precursor. 17



Preparation of ZIF-67/Au3+ precursor: 50.0 mg ZIF-67 was added to 2 mL ethanol contained 0.12 mmol NaAuCl4·2H2O, then at room temperature, the varia was rocked for 20 min. The precipitate was separated by centrifugation, washed with ethanol three times, and dried in vacuum. To investigate the influence of Au content on their electrocatalytic properties, different amount of NaAuCl4·2H2O including 0.03, 0.06, 0.08, 0.1, 0.15 mmol were added into the solution and treated with the same process, and the resulting products were denoted as ZIF-67/Au3+-0.03, ZIF-67/Au3+-0.06, ZIF-67/Au3+-0.08, ZIF-67/Au3+-0.10, ZIF-67/Au3+-0.15, Preparation of Co3O4/Au heterostructures: the purple ZIF-67/Au3+ precursor was heated to 300 °C for 2 h with a heating rate of 1 ℃·min-1. After the calcination, the purple precursor turns out black powder. To find the optimal temperature for precursor, different calcination temperatures was carried out containing 0.12 mmol NaAuCl4·2H2O, and the resulting products were denoted as Co3O4/Au-250, Co3O4/Au-300, and Co3O4/Au-350, respectively. Preparation of Au nanoparticles: A certain amount of HAuCl4·3H2O was firstly dissolved in 99 mL of ultrapure water under vigorous stirring. Then sodium borohydride (6.0 mL, 1 wt%) was added in it, and the mixture was further stirred for 30 min. Finally, the Au nanocrystals were obtained by washing step. Characterization:

TG measurement was carried out with a SDT Q600 analyzer. PXRD was

used to analysis the structure and purity of the sample with a Bruker D8 operating at 40 kV and 40 mA. FTIR spectroscopy was recorded on Bruker EQUINOX55 version analyzer. The specific surface area was determined by N2 adsorption-desorption and Barrett-Joyner-Halenda (BJH) analysis with Micromeritics TRISTAR Ⅱ3020.The morphology of the sample was examined by scanning electron microscope and HR-TEM (SEM, TEM, Hitachi S-8100). EDS were recorded on 18


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Hitachi S-8100. XPS was used to analysis the surface chemistry by with AXIS ULTRA DLD analyzer. Electrochemical measurements: CV, LSV, and electrochemistry experiments were performed on an electrochemical work station (CHI 760E, Shanghai Chenhua, China) with a three-electrode system. A glassy carbon rotating disk electrode (GC RDE, 4 mm in diameter) was used as the working electrode and a platinum wire was used as the counter electrode. An Ag/AgCl/KCl electrode calibrated with respect to the reversible hydrogen electrode (+0.949 V vs. RHE) was used as the reference electrode. All the given potentials were converted in terms of the RHE potential. The working electrode was prepared by dropping each of the catalyst solution onto a GC RDE. Typically, the catalysts were mixed with carbon black with a weight ratio of 3:7 (catalysts/C). The mixture (5 mg) were dispersed in the mixture of 0.25 mL isopropanol, 0.75 mL deionized water and 3 µL Nafion (0.5 wt%) and under ultrasonic for 1 h. Finally, 10 µL of solution, with a loading density of 0.4 mg cm-2, was deposited on the GC RDE and dried in air overnight for electrochemical characterization. For comparison, a GC RDE coated with commercial 20 wt% Pt/C was also fabricated by the same procedure. The Electrochemical Impedance Spectroscopy (EIS) was tested in 0.1 M KOH electrolyte with continuous O2 flow at a1600 rpm rotating speed. Impedance data was recorded at 0.071 V vs Ag/AgCl (0.815 V vs RHE) for ORR. 0.01-100000 Hz frequency range was used and the signal amplitude is 5 mV.

ASSOCIATED CONTENT Supporting Information. FTIR and XRD patterns of ZIF-67 and ZIF-67/Au3+, The size distribution of supported Au nanocrystals, EDX elemental mapping of Co3O4/Au heterostructures, 19



Page 20 of 29

RRDE data of commercial Pt/C, RDE data of Au nanoparticles, SEM and TEM of Co3O4/Au-250 and Co3O4/Au-350, TEM, pore size distributions of the Co3O4 and Co3O4/Au, Rotating-disk voltammograms of Co3O4/Au-250 and Co3O4/Au-350 (loading: 0:4 mg cm-2) with a sweep rate of 5mV s-1 at the different rotation rates indicated and corresponding Koutecky-Levich plots (J-1 versus ω -0.5) at different potentials in O2-saturated 0.1 M KOH solutions, Rotating-disk voltammograms

of

Co3O4/Au-0.03,

Co3O4/Au-0.06,

Co3O4/Au-0.08,

Co3O4/Au-0.10,

Co3O4/Au-0.15, (loading: 0:4 mg cm-2) with a sweep rate of 5mV s-1 at the different rotation rates indicated and corresponding Koutecky-Levich plots (J-1 versusω-0.5) at different potentials in O2-saturated 0.1 M KOH solutions, durability tests of Co3O4/Au heterostructure, ICP-AES and EDS of Au content in Co3O4/Au heterostructure. Comparison of ORR performance in O2-saturated 0.1 M KOH solutions with different catalysts.

AUTHOR INFORMATION Corresponding Author: *

Qin Wei. E-mail address: [email protected]

*

Xuelian Yu. E-mail address: [email protected]

*

Yihe Zhang. E-mail address: [email protected];

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51572246) and Fundamental Research Funds for the Central Universities (2652015086), the National Key Scientific Instrument and Equipment Development Project of China (No.21627809)，National 20


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Natural Science Foundation of China (No. 21575050，21777056, 21505051）, and QW thanks the Special Foundation for Taishan Scholar Professorship of Shandong Province (No. ts20130937) and UJN..

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TOC

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Au Heterostructure as A

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