Carbon Encapsulated Hollow Co3O4 Composites Derived from

Aug 23, 2018 - Carbon Encapsulated Hollow Co3O4 Composites Derived from Reduced Graphene Oxide Wrapped Metal–Organic Frameworks with ...
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Carbon Encapsulated Hollow Co3O4 Composites Derived from Reduced Graphene Oxide Wrapped Metal−Organic Frameworks with Enhanced Lithium Storage and Water Oxidation Properties Yana Men, Xiaochen Liu, Fulin Yang, Fusheng Ke,* Gongzhen Cheng, and Wei Luo* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China

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S Supporting Information *

ABSTRACT: Transition metal oxides have received great attention for boosting the performances for lithium-ion batteries and oxygen evolution reaction (OER). Here, hollow Co3O4 nanoparticles encapsulated in reduced graphene oxide (rGO) (h-Co3O4@rGO) were synthesized through a two-step annealing process of graphene oxide wrapped zeolitic imidazolate framework-67 (ZIF-67@GO) precursors. By taking advantage of the enhanced conductivity, high dispersity, high surface area, and unique hollow morphology derived from the GO-wrapped protecting annealing strategy, the as-synthesized h-Co3O4@rGO composite not only exhibits a reversible capacity as high as 1154.2 mAh g−1 at 500 mA g−1 after 100 cycles and high rate performance (746 mAh g−1 at 3000 mA g−1) but also displays superior OER performance with an overpotential of 300 mV to obtain 10 mA cm−2.

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INTRODUCTION With the growing demand for energy consumption as well as rising environmental concerns, clean and efficient energy storage and conversion systems, such as lithium-ion batteries (LIBs) and water electrolysis, have received enormous attention.1−5 However, the commercial LIBs, based on graphite anode, cannot reach the energy and power requirements of next-generation LIBs due to their low energy density and poor rate performance.6 In addition, IrO2/RuO2 are still considered as the benchmark electrocatalysts toward oxygen evolution reaction (OER), the rate-limiting step for water electrolysis.7 Consequently, considerable efforts have been devoted to designing alternative electrode materials for LIBs and precious metal free OER catalysts for over two decades.8−11 Recently, cobalt oxide (Co3O4) has been regarded as a promising anode material for LIBs and OER electrocatalyst, due to its high theoretical storage capacity (890 mAh g−1), high electrocatalytic activity, and environmental benignity.12−14 Unfortunately, their practical applications are limited by the low electrical conductivity and volumetric variation, which lead to poor stability in LIBs and water electrolyzers.15,16 Therefore, improving the activity and stability of Co3O4 toward LIBs and OER is highly desirable, but still remains challenging. Morphology control and carbon coating have been regarded as an efficient method to increase the performance of transition metal oxides (TMOs).17−20 Owing to relatively high specific surface area and intriguing structures, metal−organic frameworks (MOFs) have been used as versatile templates for preparing TMOs with unique structures for broad applications in energy storage and conversion.21−24 However, pyrolysis at © XXXX American Chemical Society

high temperature often causes the irreversible aggregation of MOF-derivatives and decrease in the number of active sites, resulting in poor electrocatalytic performance and low stability.25 To address this issue, MOF arrays with superstructures were successfully fabricated on various substrates (such as Ni foam, carbon cloth, etc.) to prevent aggregation and maintain the oriented arrangement during the annealing process.26−28 In addition, coating MOFs with a protecting shell such as silica was also developed to prevent aggregation of MOFs under high temperature pyrolysis.29 Despite that significant progress has been made, most of these approaches always involve a complicated synthetic procedure, such as fabricating MOF-precursors on the substrates and/or protecting shell removing after pyrolysis.30 Therefore, developing convenient approaches for morphology-control synthesis of TMOs with superior performance and stability toward LIBs and OER is highly desirable. In this work, we report the successful fabrication of carbon encapsulated hollow Co 3O4 composites using reduced graphene oxide (rGO)-wrapped zeolitic imidazolate framework-67 (ZIF-67), as both the precursors and self-sacrificing templates. Benefiting from the high dispersity, high surface area, unique hollow morphology, and good electrical conductivity derived from rGO wrapping, the resulting rGO encapsulated hollow Co3O4 (h-Co3O4@rGO) composite exhibits remarkable lithium storage and superior OER activity. Received: May 14, 2018

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DOI: 10.1021/acs.inorgchem.8b01309 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic illustration of the preparation of h-Co3O4@rGO hollow composite by using ZIF-67@GO as precursor, followed by a two-step annealing strategy: (Step 1) The formation of the GO-wrapped ZIF-67 (ZIF-67@GO) through a solvothermal treatment process. (Step 1) The obtained ZIF-67@GO was then subjected to an annealing process in N2 with 5 vol % H2 at 750 °C to generate the rGO-wrapped Co@CoO composite with yolk−shell structure (Co@CoO@rGO). (Step 2) Annealing the Co@CoO@rGO composite in air at 350 °C to obtain the rGOwrapped Co3O4 composite with hollow structure (h-Co3O4@rGO).

Figure 2. (a, b) SEM images, (c) TEM image of ZIF-67@GO. The arrows indicate GO sheets (b, c). (d, e) SEM images, (f−h) TEM images of hCo3O4@rGO. The arrows in (e, f) indicate the rGO sheets. (f−h) TEM images of h-Co3O4@rGO and the magnified HRTEM image in (i): the lattice spacings of 0.24 and 0.46 nm are attributed to the (311) plane and (111) plane of Co3O4, respectively.

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RESULTS AND DISCUSSION

(SEM) (Figure 2a,b) and transmission electron microscopy (TEM) images (Figure 2c), it is unambiguously observed that the surface of ∼1.2 μm ZIF-67 is coated with relatively thick layers of GO sheets, resulting in a spherical morphology. However, without GO, ZIF-67 exhibits a dodecahedron morphology with a relatively smooth surface as shown in the SEM images from Figure S1. This result indicates that GO sheets are successfully coated on the surface of ZIF-67. In

The rGO-wrapped protecting calcination strategy is illustrated in Figure 1. In the first step, solvothermal treatment of Co(NO3)2·6H2O, 2-methylimidazole, polyvinyl pyrrolidone (PVP), and GO was used to obtain the GO-wrapped ZIF-67 (ZIF-67@GO). For comparison, ZIF-67 was synthesized without adding GO. As shown in scanning electron microscope B

DOI: 10.1021/acs.inorgchem.8b01309 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Electrochemical performance of h-Co3O4@rGO composite: (a) Charge−discharge voltage profiles at 500 mA g−1, (b) cycling performance and Coulombic efficiency, and (c) rate performance.

Co3O4@rGO) was obtained. As shown in Figure 2d−f, the highly dispersed spherical morphology of the h-Co3O4@rGO composites is well maintained during the oxidation process. The wrinkles of rGO sheets are clearly observed on the surface of Co3O4. In addition, Raman spectroscopy of the samples was also studied. First, we prepared rGO by calcining GO at 750 °C in N2 with 5 vol % H2 for 2 h. Then, we oxidized the rGO at 350° in air for 2 h to obtain the rGO-350. As shown in Figure S7, three characteristic bands of graphene located at 1350 cm−1 (D-band), 1580 cm−1 (G-band), and 2700 cm−1 (2D-band) were observed in rGO.34 After oxidizing the rGO at 350°, three characteristic bands of graphene can also be observed clearly in the rGO-350, suggesting the existence of rGO after oxidizing at 350°. As a contrast, the Co@CoO@ rGO and h-Co3O4@rGO were also analyzed by Raman spectroscopy. As can be seen clearly, three characteristic bands of graphene at 1350 cm−1 (D-band), 1580 cm−1 (Gband), and 2700 cm−1 (2D-band) are also observed in Co@ CoO@rGO and h-Co3O4@rGO, which further indicated the existence of reduced graphene oxide in the h-Co3O4@rGO. Furthermore, as shown in the TEM images (Figure 2g,h), the h-Co3O4@rGO displays a hollow petal-shaped morphology.35−37 It has been reported that the hollow structures could provide higher surface area, shorter diffusion pathways and effectively alleviate the stress-induced structural variation during long-term electrochemical reactions, resulting in enhanced LIBs and electrocatalytic performances.38−40 In addition, the high-resolution TEM (HRTEM) image of the h-Co3O4@rGO (Figure 2i) clearly shows metal lattice fringes with interspaces of about 0.24 and 0.46 nm, corresponding to the (311) and (111) planes of Co3O4, respectively. Meanwhile, XRD analysis of the h-Co3O4@rGO (Figure S6) clearly shows

addition, a series of like peaks located at 7.3°, 10.4°, 12.7°, and 18.1° are observed from the powder X-ray diffraction (XRD) patterns of ZIF-67@GO and pure ZIF-67 (Figure S2), corresponding to the (011), (002), (112), and (222) diffraction peaks of ZIF-67,31,32 respectively. The obtained ZIF-67@GO was then subjected to an annealing process in N2 with 5 vol % H2 at 750 °C to obtain an rGO-wrapped Co@CoO composite with yolk−shell structure (Co@CoO@rGO). Unexpectedly, only metallic Co was observed by annealing the pure ZIF-67 under the same condition. The SEM and TEM images (Figure S3) indicate that highly dispersed spherical architecture of Co@CoO@rGO composites are obtained after the annealing process. rGO could be unambiguously observed from the rougher and wrinkled surface of Co@CoO@rGO composites. Meanwhile, the TEM images (Figure S3c,d) show that the Co@CoO@ rGO displays a yolk−shell morphology, probably due to the Kirkendall effect.33 Two peaks located at 44.2° and 51.5°, corresponding to (111) and (200) planes of Co (PDF# 150806), are observed from the XRD patterns, respectively (Figure S4). Meanwhile, the other three peaks located at 36.5°, 42.4°, and 61.5° can be assigned to (111), (200), and (220) crystal planes of CoO (PDF# 48-1719), respectively. However, without GO coating, the MOF-derived hybrids are ready to aggregate as shown in Figure S5a,b. Compared with Co@ CoO@rGO, no peak of CoO is observed from the XRD pattern as shown in Figure S5c. Only typical peaks located at 44° and 52° belonging to the (111) and (200) planes of Co (PDF# 15-0806) are observed, indicating the formation of metallic Co. After the subsequent annealing of Co@CoO@rGO in air at 350 °C, the rGO encapsulated hollow Co3O4 composite (hC

DOI: 10.1021/acs.inorgchem.8b01309 Inorg. Chem. XXXX, XXX, XXX−XXX

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discharge−charge cycles. Furthermore, as shown in the highfrequency region of the Nyquist plot (Figure S12), the hCo3O4@rGO electrode exhibits a semicircle relatively smaller than that of pure Co3O4, indicating that the h-Co3O4@rGO composite possesses the smaller charge-transfer resistance and high electrical conductivity. The cycling performance and the corresponding Coulombic efficiency (CE) of the h-Co3O4@rGO composite electrode are presented in the Figure 3b. At 500 mA g−1, the first charge and discharge capacities are about 1287.5 and 1962.7 mAh g−1, respectively, corresponding to a CE of 65.6%, which are much higher than those of most reported the Co3O4-based materials (Table S1). The capacity of the as-synthesized h-Co3O4@rGO composite is higher than the theoretical capacity of either cobalt or carbon species, which might be derived from the larger electrochemical active surface area of graphene and the grain boundary area of the Co3O4 nanoparticles. In addition, the reversible formation/dissolution of the polymeric surface layer coated outside the inorganic solid-electrolyte interface (SEI) layer may also contribute to additional capacity.49 However, in accordance with CV analysis, the relatively large irreversible capacity could be ascribed to the decomposition of electrolyte and formation of the SEI film. The h-Co3O4@rGO composite exhibits stabilized specific capacity after the gradual capacity decay in the first 5 cycles, and nearly 100% CE is maintained during the subsequent cycling process. The hCo3O4@rGO can present a reversible capacity of 1154.2 mAh g−1 with CE of 99.7% after 100 cycles. For comparison, pure Co3O4 without rGO coating, physically mixed Co3O4 and rGO (Co3O4-rGO), ZIF-67@rGO, ZIF-67, and rGO were also investigated (Figures S13 and S14). As shown in Figure S13, it is worth mentioning that, without rGO coating, the first discharge and charge capacities of pure Co3O4 are 1430.7 and 801.2 mAh g−1, respectively. However, after 100 cycles, only 373.5 mAh g−1 of reversible capacity is left, probably due to the loss of active sites during volume change and aggregation. The rate performance of the h-Co3O4@rGO composite was also evaluated. As shown in Figure 3c, the h-Co3O4@rGO exhibits specific capacities of 1385, 1232, 1061, 874, and 746 mAh g−1 at 200, 500, 1000, 2000, and 3000 mA g−1, respectively. Furthermore, a high capacity of about 1310 mAh g−1 can be delivered when the current density is reduced back to 200 mA g−1, suggesting the excellent reversibility of hCo3O4@rGO for lithium storage. In contrast, the rate performance of the pure Co3O4 without rGO coating, Co3O4-rGO, ZIF-67@rGO, ZIF-67, and rGO were also tested (Figures S15 and S16). Particularly, the pure Co3O4 exhibits only 806, 680, 548, 377, and 292 mAh g−1 at the same current densities, which are much lower than those of h-Co3O4@rGO, especially at high current density. These results indicate that the rGO-coating can not only effectively prevent the aggregation of Co3O4 nanomaterials and keep the spherical structure intact, which is beneficial to alleviate the volume expansion effect during the lithium storage process, but also increase the electrical conductivity, resulting in the enhancement of the electrochemical activity. Furthermore, the hollow structure of h-Co3O4@rGO composites could provide higher surface area, shorter diffusion path and also alleviate the stressinduced structural variation during the lithium storage process, which further boost the performance. The OER activity of h-Co3O4@rGO was also studied in 1 M KOH solution using a rotating disk electrode. For comparison, the OER performances of the pure Co3O4 without rGO

the typical peaks at 31.3°, 36.8°, 44.8°, 59.4°, and 65.3°, corresponding to the (220), (311), (400), (511), and (440) planes of Co3O4 (PDF# 15-0806), respectively. For comparison, the Co3O4 was also prepared by annealing of metallic Co derived from pure ZIF-67 in air. Obviously, the SEM and TEM images (Figure S8) reveal that, without coating of GO, the Co3O4 composites aggregate seriously, which might gravely impede the exposure of active sites and hinder the reaction of lithium ion (vide infra). The diffraction peaks in the XRD pattern (Figure S8d) indicate the formation of Co3O4 as well. These results indicate that the GO shells play critical roles in preparation of MOF-derived catalysts with high dispersity without aggregation during high temperature pyrolysis process. The obtained h-Co3O4@rGO composites were further studied by X-ray photoelectron spectroscopy (XPS). As shown in Figure S9a, the signals of Co 2p1/2 peaks that appear at 779.5 and 780 eV are corresponding to Co3O4. Similarly, the signals of Co 2p3/2 peaks that appear at 794.6 and 796.2 eV are also corresponding to Co3O4. In addition, the peaks that appear at 786.6 and 803.7 eV can be assigned to the satellite peaks of Co.41,42 There is no signal of Co, which indicates that the Co@CoO is totally converted to Co3O4 successfully. The O 1s XPS spectra of the h-Co3O4@rGO composite (Figure S9b) exhibits two peaks at 529.8 and 531.3 eV, which can be assigned to O 1s of Co3O4.43,44 The C 1s (284.6 eV) spectrum is deconvoluted into three different peaks, C−C sp2 (284.6 eV), C−O (286.2.5 eV), and C−N (288.8 eV), respectively (Figure S9c).45 Meanwhile, pyrrolic-N (399.6 eV), graphitic-N (401.7 eV), and oxidized-N (404.4 eV) could be observed in the high resolution N 1s spectrum (Figure S9d).46 The obtained h-Co3O4@rGO composite and Co3O4 were further characterized by nitrogen adsorption and desorption measurements. The Brunauer−Emmett−Teller (BET) surface area of Co3O4 is measured to be 8.9 m2 g−1. However, the h-Co3O4@rGO exhibits the specific surface area of 46.5 m2 g−1, which is almost 5 times higher than that of Co3O4 (Figure S10). The higher surface area of Co3O4@rGO might provide more active sites for the redox reaction and shorten the ion diffusion length for charge and discharge of lithium ion (vide infra). The h-Co3O4@rGO composites were further explored for potential use as advanced anode materials for LIBs. Figure 3a presents discharge−charge voltage profiles of h-Co3O4@rGO at a current density of 500 mA g−1. In the first discharge curve, an inconspicuous plateau at around 1.2 V and an obvious plateau at about 1.0 V can be observed, which are associated with the reduction of Co3O4 to CoO and then further to Co.47 In the first charge curve, the plateau at about 2.2 V is assigned to the reaction of Co and lithium oxide to form Co3O4. In the second discharge curve, the main voltage plateau shifts to about 1.3 V.48 However, the voltage profiles are well overlapping even in the fifth cycle, suggesting that the hCo3O4@rGO composites have gradually built good stability for reversible lithium storage after the initial cycle. Similarly, the cyclic voltammetry (CV) curves of h-Co3O4@rGO composites agree well with the discharge−charge voltage profiles, suggesting a multistep electrochemical process during lithiation of Co3O4 (Figure S11). Obviously, after the initial cycle, the oxidation peak at about 2.2 V and reduction peak at about 1.3 V are well overlapping, suggesting the good stability for reversible lithium storage. However, the redox peaks for the pure Co3O4 electrode weaken gradually, probably due to the aggregation of active materials and volume change during D

DOI: 10.1021/acs.inorgchem.8b01309 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) LSV curves of Co3O4, h-Co3O4@rGO, physically mixed Co3O4 and rGO (Co3O4-rGO), and IrO2 at scan rate of 1 mV/s. (b) The corresponding Tafel plots of Co3O4, Co3O4-rGO, h-Co3O4@rGO, and IrO2 for OER in 1 M KOH solution. (c) Nyquist plots of Co3O4, h-Co3O4@ rGO at an overpotential of 300 mV (Rct refers to charge-transfer resistance, Rs refers to the resistance of the solution, CPE refers to the constant phase element). (d) The continuous CV performance of h-Co3O4@rGO at scan rate of 5 mV/s toward OER.

theoretical yields, suggesting the nearly 100% Faraday efficiencies of h-Co3O4@rGO composites. The long-term durability of h-Co3O4@rGO was also investigated. As shown in the chronopotentiometry test (Figure S20), the current density is well maintained even over 120 000 s test. Moreover, even after 1000 cycles of CV test, negligible changes of overpotential are observed compared to the first cycle (Figure 4d). The h-Co3O4@rGO catalyst after OER test (abbreviated as A-Co3O4@rGO) was further characterized by XRD and TEM. As shown in Figure S21, the peaks of A-Co3O4@rGO match well with those of Co3O4 (PDF# 19-0364). Furthermore, the morphology of A-Co3O4@ rGO is still maintained well as shown in Figure S22. However, as expected, the Co3O4 derived from pure ZIF-67 without GO coating exhibits serious decrease even after 8000 s, as shown in Figure S23, highlighting the rGO coating in preventing aggregation and maintaining the stability of the desired materials.

coating, Co3O4-rGO, ZIF-67@rGO, ZIF-67, rGO, and commercial IrO2 were also tested (Figure S17). From the linear sweep voltammetry (LSV) curves (Figure 4a), hCo3O4@rGO exhibits a superior catalytic activity with an overpotential of 300 mV at 10 mA cm−2, which is quite close to that of IrO2 (320 mV) and lower than most of the reported values of Co-based materials (Table S2). As expected, Co3O4 derived from ZIF-67 without rGO protecting and Co3O4-rGO exhibit much inferior catalytic activity with overpotentials of 380 and 371 mV at 10 mA cm−2, respectively. The Tafel slope of h-Co3O4@rGO (Figure 4b) is measured to be 58 mV dec−1, which is lower than those of pure Co3O4 (73 mV dec−1), Co3O4-rGO (70 mV dec−1), and IrO2 (66 mV dec−1), suggesting a more favorable OER kinetics of h-Co3O4@rGO. In addition, the electrochemical impedance spectroscopy (EIS) was also studied at the overpotential of 300 mV. As shown in Figure 4c, h-Co3O4@rGO possesses a much smaller diameter of semicircle than that of Co3O4, indicating lower electron and charge-transfer resistance and faster electrode kinetics of hCo3O4@rGO. Furthermore, the electrochemical active site numbers of h-Co3O4@rGO and Co3O4 were measured to intensively study the OER activity difference between them. Through integration to the CV plots (Figure S18), the ratio of electrochemical active site number of h-Co3O4@rGO was measured to be 13 times higher than those of Co3O4, indicating the large electrochemically active area of h-Co3O4@ rGO (more calculation details can be seen in the Supporting Information). Furthermore, the Faraday efficiency of O2 production was also determined at a current of 50 mA. Figure S19 indicates the obtained gas moles are fitted well with the

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CONCLUSION In summary, well-dispersed rGO encapsulated hollow Co3O4 composites have been successfully synthesized through a GOwrapped protecting pyrolysis strategy. Thanks to the high dispersity, large surface area, and enhanced conductivity derived rGO coating, as well as unique hollow morphology, the obtained h-Co3O4@rGO composites exhibit superior electrochemical properties and stability both in lithium storage and in oxygen evolution reaction (OER). The as-synthesized hCo3O4@rGO composites show a high reversible discharge capacity of 1287.5 mAh g−1 at 500 mA g−1 with superior rate capability and long cycle life over 100 cycles as an anode E

DOI: 10.1021/acs.inorgchem.8b01309 Inorg. Chem. XXXX, XXX, XXX−XXX

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(9) Ji, L.; Lin, Z.; Alcoutlabi, M.; Zhang, X. Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries. Energy Environ. Sci. 2011, 4, 2682−2699. (10) Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M. Advanced materials for energy storage. Adv. Mater. 2010, 22, E28. (11) Gao, W.; Xia, Z.; Cao, F.; Ho, J. C.; Jiang, Z.; Qu, Y. Comprehensive Understanding of the Spatial Configurations of CeO2 in NiO for the Electrocatalytic Oxygen Evolution Reaction: Embedded or Surface-Loaded. Adv. Funct. Mater. 2018, 28, 1706056. (12) Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.; Wang, D.; Kisailus, D.; Zhao, H.; Tang, Z. Accurate control of multishelled Co3O4 hollow microspheres as high-performance anode materials in lithium-ion batteries. Angew. Chem. 2013, 125, 6545− 6548. (13) Wu, Z.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Cheng, H. M.; Zhou, G.; Li, F. Graphene anchored with Co3O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance. ACS Nano 2010, 4, 3187−3194. (14) Ni, B.; Wang, K.; He, T.; Gong, Y.; Gu, L.; Zhuang, J.; Wang, X. Mimic the Photosystem II for Water Oxidation in Neutral Solution: A Case of Co3O4. Adv. Energy Mater. 2018, 8, 1702313. (15) Wu, Z.-S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Cheng, H.-M.; Zhou, G.; Li, F. Graphene anchored with Co3O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance. ACS Nano 2010, 4, 3187− 3194. (16) Hu, H.; Guan, B.; Xia, B.; Lou, X. Designed formation of Co3O4/NiCo2O4 double-shelled nanocages with enhanced pseudocapacitive and electrocatalytic properties. J. Am. Chem. Soc. 2015, 137, 5590−5595. (17) Zhang, G.; Hou, S.; Zhang, H.; Zeng, W.; Yan, F.; Li, C.; Duan, H. High-Performance and Ultra-Stable Lithium-Ion Batteries Based on MOF-Derived ZnO@ ZnO Quantum Dots/C Core−Shell Nanorod Arrays on a Carbon Cloth Anode. Adv. Mater. 2015, 27, 2400−2405. (18) Luo, Y.; Kong, D.; Jia, Y.; Luo, J.; Lu, Y.; Zhang, D.; Yu, T.; Qiu, K.; Li, C. M. Self-assembled graphene@ PANI nanoworm composites with enhanced supercapacitor performance. RSC Adv. 2013, 3, 5851−5859. (19) Cheng, C.; Liu, J.; Yang, H.; Cong, C.; Li, C. A general strategy toward graphene@ metal oxide core−shell nanostructures for highperformance lithium storage. Energy Environ. Sci. 2011, 4, 4954−4961. (20) Meng, J.; Liu, X.; Li, J.; Li, Q.; Zhao, C.; Xu, L.; Liu, F.; Niu, C.; Mai, L.; Wang, X.; Yang, W.; Liu, Z.; Xu, X. General oriented synthesis of precise carbon-confined nanostructures by low-pressure vapor superassembly and controlled pyrolysis. Nano Lett. 2017, 17, 7773−7781. (21) deKrafft, K. E.; Wang, C.; Lin, W. Metal-Organic Framework Templated Synthesis of Fe2O3/TiO2 Nanocomposite for Hydrogen Production. Adv. Mater. 2012, 24, 2014−2018. (22) Kim, T. K.; Lee, K. J.; Cheon, J. Y.; Lee, J. H.; Joo, S. H.; Moon, H. R. Nanoporous metal oxides with tunable and nanocrystalline frameworks via conversion of metal−organic frameworks. J. Am. Chem. Soc. 2013, 135, 8940−8946. (23) Xu, X.; Cao, R.; Jeong, S.; Cho, J. Spindle-like mesoporous αFe2O3 anode material prepared from MOF template for high-rate lithium batteries. Nano Lett. 2012, 12, 4988−4991. (24) Yin, D.; Huang, G.; Na, Z.; Wang, X.; Li, Q.; Wang, L. CuO Nanorod Arrays Formed Directly on Cu Foil from MOFs as Superior Binder-Free Anode Material for Lithium-Ion Batteries. ACS Energy Lett. 2017, 2, 1564−1570. (25) Cao, X.; Zheng, B.; Rui, X.; Shi, W.; Yan, Q.; Zhang, H. Metal Oxide-Coated Three-Dimensional Graphene Prepared by the Use of Metal−Organic Frameworks as Precursors. Angew. Chem. 2014, 126, 1428−1433. (26) Sun, Y.; Yang, F.; Wei, Q.; Wang, N.; Qin, X.; Zhang, S.; Li, J.R.; Wang, B.; Nie, Z.; Ji, S.; Yan, H. Oriented Nano−MicrostructureAssisted Controllable Fabrication of Metal−Organic Framework Membranes on Nickel Foam. Adv. Mater. 2016, 28, 2374−2381.

material for lithium-ion batteries. In addition, the composites also exhibit outstanding OER catalytic activity with an overpotential of 300 mV at 10 mA cm−2 and long-term stability. This work offers a new strategy for designing welldispersed TMO nanocomposites derived from MOFs with high electrochemical activity, and opens up a new way for construction of carbon-rich materials such as transition metal phosphides (TMPs), transition metal dichalcogenides (TMDs), and transition metal nitrides (TMNs) derived from MOFs for more applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01309. Experimental data and additional characterizations, including Figures S1−S23 and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 86-27-68752366 (F.K.). *E-mail: [email protected]. Tel.: 86-27-68752366 (W.L.). ORCID

Wei Luo: 0000-0002-7807-1373 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21571145, 21633008), and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University

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DOI: 10.1021/acs.inorgchem.8b01309 Inorg. Chem. XXXX, XXX, XXX−XXX