Exploring the Formation of CoO/ZnO Heterostructure to Enhance Electrocatalytic Oxygen Evolution Reaction

Revealing the electrocatalytic microstructure is the basis for finding more efficient and stable electrode materials for clean energy conversion devices. However, there is still a lack of direct understanding of the phase transition process of the sample during heating, which makes the synthesis conditions of the catalyst often rely on experienced guidance. Herein, ZIF‐8@α‐Co(OH)2 core‐shell structure and uniformly coated α‐Co(OH)2 nanosheets on the surface of ZIF‐8 are synthesized. The effect of temperature on microstructure evolution is studied by in situ heating technique with high‐resolution transmission electron microscopy. Based on understanding the relationship between temperature, microstructure, and OER properties, a simple annealing synthesis strategy is designed effectively. During the annealing process, ZnO@CoO heterogeneous structure is formed at 400 °C, and the material reached a stable hydroxyl oxide structure after electrochemical cycle voltammetry in an alkaline solution. The OER performance test shows that the commercial IrO2 catalyst has comparable catalytic performance.


Introduction
Electrochemical water splitting is a promising chemical method for the development of clean and sustainable energy to solve the increasing depletion of fossil fuels. The oxygen evolution reaction (OER) is a four-electron process that is pyrolysis products were obtained by in situ heating under an in situ electron microscope. During the annealing process, ZnO@CoO heterostructure was formed at 400 °C, and a stable hydroxyl oxidation structure was achieved by electrochemical cyclic voltammetry in an alkaline solution, showing excellent catalytic performance. [19][20][21][22] It plays an effective guiding role in the synthesis of electrocatalytic oxygen evolution catalysts in the future.

Synthesis and Morphology Characterization
In this work, the catalysts with ZnO@CoO heterostructures prepared by in situ pyrolysis process are shown in Figure 1. Figure 1a illustrates the synthesis process of our samples. First, we synthesized ZIF-8 dodecahedron by simple solution www.advmatinterfaces.de method. According to the scanning electron microscopy test, the particle size is uniformly ≈200 nm ( Figure S1a, Supporting Information). Then, α-Co(OH) 2 nanosheets were formed by hydrolysis of Co 2+ and propylene oxide, which was uniformly coated on the surface of ZIF-8. [23] Finally, the composites were annealed at different temperatures under the protection of an inert atmosphere to obtain the products. Meanwhile, we synthesized pure α-Co(OH) 2 nanosheets for comparison. The XRD patterns of ZIF-8, ZIF-8@α-Co(OH) 2, and α-Co(OH) 2 are shown in Figure 1b, indicate that the α-Co(OH) 2 nanosheets are successfully doped into the material. [24] The FTIR spectra of ZIF-8 and ZIF-8@α-Co(OH) 2 in the absorption region 400-4000 cm −2 are given in Figure S2 (Supporting Information), in which several bands are observed. For example, absorption bands at 2930 and 3135 cm −1 were attributed to the aromatic and aliphatic CH stretch of the imidazole, respectively. [25] The band at 1586 cm −1 could be assigned as the C≐N stretch mode. The absorption band at 420 cm −1 was observed for the ZnN stretching mode, while those in the 1100-1400 cm −1 region were associated with the CN stretch. At the same time, In the spectrum of ZIF-8@α-Co(OH) 2 , the sharp band at 3553 cm −1 is related to the ν(OH) stretching vibration of free CoOH groups on the inner interlayer surface. [26] To further understand the morphology and micro-structure of samples, highresolution transmission electron microscopy was employed. As shown in Figure 1c,d, the α-Co(OH) 2 nanosheet was uniformly coated on the surface of ZIF-8 without affecting the integrity of ZIF-8. The chemical composition of ZIF-8@α-Co(OH) 2 hybrid was further probed by energy dispersive spectroscopy element mapping. Figure 1e indicates that the purple cobalt and green oxygen are mainly distributed in the α-Co(OH) 2 of the outer layer of ZIF-8, while the zinc and carbon elements that makeup ZIF-8 are mainly distributed in the wrapped ZIF-8. Figure S3 (Supporting Information) shows the morphology and element distribution of α-Co(OH) 2 nanosheets. Typically, the prepared ZnO@CoO heterostructure was annealed at 400 °C. [27] As shown in Figure 1f, pyrolysis of α-Co(OH) 2 in the outer layer occurred after annealing. The annealed TEM images of pure α-Co(OH) 2 and the elemental Mapping images are also compared, as shown in Figure S4a-d (Supporting Information). Further, through high-resolution image analysis, the lattice fringe of 0.25 nm corresponds to the (101) crystal plane of ZnO and the lattice fringe of 0.21 nm corresponds to the (200) crystal plane of CoO (Figure 1g). Based on XRD and high-resolution image analysis, nano heterostructures of CoO and ZnO were successfully prepared. The integrity of the target material and the contrast material can be further observed in Figure S1 (Supporting Information). [28]

Structural Characterization
The annealing conditions of ZIF-8@α-Co(OH) 2 were determined according to the microstructure evolution of in situ TEM heating experiment in Figure 2. In situ heating process carefully restored the sample phase change process. In order to further confirm and study the phase changes of the materials during the heating process, samples collected at different annealing temperatures were characterized by ex situ XRD. [29] As shown in Figure S5a (Supporting Information), to avoid the effects of electron irradiation, the material was electron irradiated for 10 min and no significant change was found in the TEM image. The heating rate was 10 °C per minute and the material was held for 30 min for every 100 °C rise in temperature. XRD pattern of samples annealed at 200 °C show that the peak of α-Co(OH) 2 has disappeared, indicating that α-Co(OH) 2 has begun to amorphous. As shown in Figure S5b,S6a (Supporting Information), the outer α-Co(OH) 2 sheet of ZIF-8@α-Co(OH) 2 produced micropores and weak cracks. As shown in Figure S6a (Supporting Information), when the temperature reaches 300 °C, ZIF-8 begins to decompose according to the XRD pattern, and poor crystalline CoO is generated in the heating process. As can be seen from the results of in situ TEM in Figure 2a, ZnO nanocrystals are formed on the surface of the ZIF-8 core, while CoO is formed on the outer layer of α-Co(OH) 2 . When the temperature increased to 400 °C, Figure 2b shows that with further increase in temperature we found the presence of ZnO in the outer layers further away from the ZIF-8 core and with CoO forming a ZnO@CoO heterostructure further away from the ZIF-8 core. This is due to Zn ions evaporating into the outer layer and participating in the formation of ZnO@CoO heterogeneous tissues. Figure 2c shows HRTEM images of in situ experiments at temperatures up to 500 °C showed that ZIF-8@α-Co(OH) 2 was further pyrolyzed to produce more ZnO as the temperature increased further, while the original ZnO@CoO heterostructure was separated, and interestingly, cobalt nanocrystalline formation was detected at this temperature. According to the ex situ XRD pattern, diffraction peaks representing Co particles appear, indicating that the cobalt oxide is reduced to Co metal. As shown in Figure 2d, the reduction process of CoO to Co nanoparticle was captured in the in situ heating experiment when the temperature rose to 600 °C. Interestingly, the Co nanoparticles swam around as they grew, while catalyzing the formation of CNTs(carbon nanotubes) on their tracks. Combined with the XRD pattern in Figure S6d (Supporting Information) showed that cobalt nanoparticle was synthesized. As shown in Figure 2e-h, the temperature continues to increase from 700 °C to 900 °C, and it can be observed from TEM images that the ZIF-8 skeleton structure as a carbon source supports the continuous generation of CNTs. When the temperature is increased to 900 °C, the zinc oxide disappears due to the sublimation of the Zn. Only cobalt particles and carbon nanotubes are present. The ex situ XRD results are also consistent with the in situ experimental data. In order to exclude the influence of ZIF-8 during in situ heating, we deliberately performed a set of in situ heating experiments containing only ZIF-8 and found that ZIF-8 does not undergo a very pronounced structural evolution during the warming process, but only a volumetric collapse, as shown in Figure S7 (Supporting Information). The in situ heating electron microscopy very visually observed a series of microstructural evolution of ZIF-8@α-Co(OH) 2 with increasing temperature, which, combined with the XRD characterization of the material annealed under ex situ, provided a detailed analysis of the physical phase evolution of ZIF-8@α-Co(OH) 2 with temperature. [30] To further investigate the phenomena observed under in situ transmission and to clarify the valence changes of the substances that form heterostructures at 400 °C and complete www.advmatinterfaces.de reactions after 900 °C, we have carried out ex situ heating experiments and characterized the X-ray photoelectron spectroscopy of these intermediate substances in Figure 3a-d shows TEM images of ZIF-8@α-Co(OH) 2 from room temperature to 400 °C, 700 °C and then to 900 °C in the ex situ heating experiment. It can clearly show the formation process of core-shell structure transformation to Co nanoparticles and then catalyzed carbon nanotubes. As shown in the XPS spectrum in Figure 3e, due to the splitting of the spin orbitals, the Zn 2p spectrum in ZIF-8@α-Co(OH) 2 has two peaks, Zn 2p 3/2 with a binding energy of 1021.9 eV and Zn 2p 1/2 with a binding energy of 1044.98 eV, respectively. It indicates the formation of ZIF-8 by the binding of zinc ions with the 2-methylimidazole ligand. It can be seen that a comparison with the XPS spectra of Zn elements in our prepared ZIF-8@α-Co(OH) 2 shows that α-Co(OH) 2 wrapped around ZIF-8 did not destroy the physical phase structure of ZIF-8. Comparison of the XPS spectra of the samples of ZIF-8@α-Co(OH) 2 after annealing at 400 °C shows that the Zn 2p spectra is essentially unchanged from that in ZIF-8@α-Co(OH) 2 . This shows that there is no valence change of zinc ion and the form of ZnO exists. The XPS result of the sample after annealing at 900 °C showed no obvious peak

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profile, indicating that the sample did not contain Zn elements at this time, indicating that the Zn ions evaporated at high temperature, also proving the results of XRD and HRTEM. As shown in Figure 3f, the XPS profiles of the original ZIF-8@α-Co(OH) 2 show two peaks in the Co 2p profile, Co 2p 3/2 with a binding energy of 781.3 eV and Co 2p 1/2 with a binding energy of 797.3 eV. The fitting peak of Co indicates that the valence state of Co is a positive bivalent state and exists in the form of α-Co(OH) 2 . When annealed at 400 °C, the XPS fitting peak of Co still exists in the form of Co 2+ , and the comparison with the ex situ XRD pattern shows that Co 2+ exists in the form of CoO, as shown in Figure S6 (Supporting Information). The XPS spectra annealed at 900 °C show mainly the Co 2p 3/2 fitted peak with a binding energy of 779 eV for the zero valent state and the Co 2p 1/2 fitted peak with a binding energy of 794.8 eV, demonstrating that CoO reduced to Co metal after annealing at 900 °C. As shown in Figure 3g, the XPS fitted peaks for C of ZIF-8@α-Co(OH) 2 were divided into CC bonds at 284.74 eV and CN bonds at 285.42 eV, which were mainly present in ZIF-8. After annealing at 400 °C showed that the XPS of the samples were basically the same as those of the unannealed samples, and the binding energies of the fitted peaks were 284.90 and 286.15 eV respectively. It indicates that the CC and CN bonds were still present after annealing at 400 °C. After annealing at 900 °C, it shows two fitted peaks of CC at 284.81 eV and CO at 285.65 eV. Combined with Figure 2h shows that the formation of carbon nanotubes, and the C1s peak is sharp and thin, indicating that carbon nanotubes containing a large number of CC bonds are prevalent and well crystallized at this time. Meantime, XANES spectra of the Co L edge were investigated, which involved the excitation of electrons from the 2p level to the partially unoccupied 3d states that are very sensitive to the valence and spin states of the absorbing atom. Figure 3h shows the normalized Co L edge XANES spectra of three samples. There is no significant change of Co element in ZIF-8@α-Co(OH) 2 after annealing at 400 °C, corresponding to the bivalent chemical state. However, after annealing at 900 °C, the position of the peak is blue-shifted and reduced to a metal state. [31][32][33] This is consistent with the results of XPS and XRD characterization.

Verification of OER Performance
Using in situ heating experiment can make us clearly understand what interesting structures will produced during the annealing process, and it is convenient to determine the experimental scheme quickly. The OER performance is summarized in Figure 4. As shown in Figure 4a, the faradaic pseudocapacitance is shown to become larger with increasing CV curves, suggesting that the CoO@ZnO heterostructure undergoes continuous reconfiguration under conditions of applied current. In order to explore the electrocatalytic OER properties of substances formed under different temperature conditions, we compared the electrocatalytic OER activity of ZIF-8@α-Co(OH) 2 annealed at different temperatures by linear sweep voltammetry scanning using a glassy carbon electrode with a sweep rate of 10 mV s −1 along with iR compensation, as shown in Figure S8a (Supporting Information). It was confirmed that ZIF-8@α-Co(OH) 2 had the lowest overpotential on annealing treatment at 400 °C. As shown in Figure 4b, the LSV polarisation curve of ZIF-8@α-Co(OH) 2 at 400 °C annealing treatment was significantly improved compared to ZIF-8@α-Co(OH) 2 . Comparison of the LSV polarisation curves of the CoO@ZnO Figure 3. a-d) TEM images of ZIF-8@α-Co(OH) 2 and annealed at 400, 700, and 900 °C, respectively. e-h) High-resolution XPS images of Zn 2p, Co 2p, and C 1s at room temperature, heated to 400 and 900 °C, respectively, and Co L edge XANES of synchrotron radiation absorption spectra.

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heterostructure of ZIF-8@α-Co(OH) 2 after annealing treatment at 400 °C with that of a single CoO indicates that the high OER activity is produced by the synergistic effect of CoO with ZnO. In Figure 4c, the Tafel slope of the ZIF-8@α-Co(OH) 2 electrocatalyst at 400 °C annealings is 65.4 mV dec −1 , which indicates faster OER kinetics on the heterogeneous structure. According to previous studies, the ECSA(electrochemical surface area) increases during the reconstruction process. The increasing ECSA implies the exposure of more accessible sites for redox reactions, as shown in Figure S8c-f (Supporting Information). The ZIF-8@α-Co(OH) 2 annealed at 400 °C reached a steady state after ≈100 cycles of cyclic voltammetry testing. In comparison to the commercial catalyst IrO 2 , Figure S8b (Supporting Information) shows that the current density of the ZIF-8@α-Co(OH) 2 annealed at 400°C is higher as the applied voltage increases. At a current density of 10 mA cm −2 , the overpotential was 322 mV, which is very close to that of the most advanced IrO 2 catalysts currently available on the market. Also, at a current density of 82 mA cm −2 , the overpotentials of the two materials are identical. Electrochemical impedance spectroscopy (EIS) measurement was also an important index to measure OER activity and was further carried out to study the OER behavior of the ZIF-8@α-Co(OH) 2 annealed at 400 °C. The corresponding Nyquist plots obtained in a frequency range from 0.01 to 100 kHz with an AC potential amplitude of 5 mV at 1.5 V (RHE) was shown in Figure S8g (Supporting Information). For a better understanding of the electrochemical impedance, the corresponding equivalent circuit diagram of ZIF-8@α-Co(OH) 2 annealed at 400 °C consisting of an electrolyte resistance (R1), a charge-transfer resistance (R2), an electrochemical reaction diffusion impedance (W) and a constant-phase element (CPE) was displayed in the inset of Figure S8g

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The OER performance can be explained by comparing the diameter of the semicircle in EIS curves. As shown in Figure 4d, it is clearly observed that ZIF-8@α-Co(OH) 2 annealed at 400 °C exhibits a smaller semicircle than other catalysts, suggesting a much faster electron transfer and better OER catalytic activity. In order to explore the stability of the sample, a long performance test conducted in an alkaline environment. As shown in Figure 4e, under the overvoltage of 0.4 V, the current remains at or near 10 mA cm −2 , showing good stability after a 10 h test, and the LSV curves before and after the stability test are almost identical. At the same time, we collected the materials formed by electrocatalytic reconfiguration. As shown in Figure 4f, the XRD pattern of ZIF-8@α-Co(OH) 2 annealed at 400 °C after the electrochemical reaction is composed of cobalt oxide, zinc oxide, and cobalt oxyhydroxide. It is proved that cobalt oxide is partially converted to cobalt oxyhydroxide in an electrochemical reaction. The TEM image in Figure 4g shows that the original core-shell structure remains after the electrochemical reaction, and the lamellaeares composed of nanoparticles. The HRTEM image shows that the CoO nanoparticle was reconstituted to CoOOH corresponding to (110) planar spacing of 0.23 nm. At the same time, zinc oxide is retained in the heterostructure, which can stabilize the structure and maintain the stability of the catalytic process. The element mapping images show that Zn, Co, N, and C are distributed throughout the material. Therefore, we successfully optimized the maximum catalytic performance of the sample by guiding the synthesis of the sample rationality.

Conclusion
The in situ heating transmission electron microscopy technique reveals the phase transition of the sample during heating very intuitively. It provides direct guidance for the synthesis of catalytic materials. We synthesized ZIF-8@α-Co(OH) 2 coreshell structure and uniformly coated α-Co(OH) 2 nanosheets on the ZIF-8 surface. In situ transmission electron microscopy was used to study the evolution process of annealing temperature on the microstructure, revealing the relationship between temperature, microstructure and the properties of OER, and the ZnO@CoO heterogeneous structure was designed and synthesized. The stable hydroxyl oxidation structure was obtained by electrochemical cyclic voltammetry in basic solution. The catalytic performance and stability after 10 h are comparable to that of industrial IrO 2 catalyst.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.