MOF-Derived ZnO Nanoparticles Covered by N ... - ACS Publications

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MOF-Derived ZnO Nanoparticles Covered by N-doped Carbon Layers and Hybridized on Carbon Nanotubes for Lithium-Ion Battery Anodes Hui Zhang, Yunsong Wang, Wenqi Zhao, Mingchu Zou, Yijun Chen, Liusi Yang, Lu Xu, Huaisheng Wu, and Anyuan Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12095 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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ACS Applied Materials & Interfaces

MOF-Derived ZnO Nanoparticles Covered by N-doped Carbon Layers and Hybridized on Carbon Nanotubes for Lithium-Ion Battery Anodes Hui Zhang,1 Yunsong Wang,1 Wenqi Zhao,1,2 Mingchu Zou,1 Yijun Chen,1Liusi Yang,1 Lu Xu,1 Huaisheng Wu,1 Anyuan Cao1* 1 2

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China

* Corresponding author: [email protected]

Abstract Metal-organic frameworks (MOFs) have many promising applications in energy and environmental areas such as gas separation, catalysis, supercapacitors and batteries; the key toward those applications is controlled pyrolysis which can tailor the porous structure, improve electrical conductivity and expose metal ions in MOFs. Here, we present a systematic study on the structural evolution of zeolitic imidazolate frameworks hybridized on carbon nanotubes (CNTs) during the carbonization process. We show that a number of typical products can be obtained, depending on the annealing time, including 1) CNTs wrapped by relatively thick carbon layers, 2) CNTs grafted by ZnO nanoparticles which are covered by thin nitrogen-doped carbon layers, and 3) CNTs grafted by aggregated ZnO nanoparticles. We also investigated the electrochemical properties of those hybrid structures as freestanding membrane electrodes for lithium ion batteries, and the second one (CNT-supported ZnO covered by N-doped carbon) shows the best performance with a high specific capacity (850 mAh/g at a current density of 100 mA/g) and excellent cycling stability. Our results indicate that tailoring and optimizing the MOF-CNT hybrid structure is essential for developing high-performance energy storage systems. Keywords: Metal-organic framework (MOFs), carbon nanotube, hybrid structure, carbonization process, lithium-ion battery anodes.

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Introduction Zeolitic imidazolate frameworks (ZIFs) are a subclass of porous metal-organic frameworks (MOFs) in which divalent metal cations are linked by imidazolate anions into tetrahedral units with a zeolite topology.1-8 Some guest-free ZIFs, for example, ZIF-8, possess large surface area, high porosity and exceptional chemical and thermal stability, with a wide range of applications in gas storage/separation, oil-water separation, filtration, and as a porous carrier for catalysis.9-12 Recently, there is an emerging interest in directly using nitrogen-rich ZIFs as catalytic precursors by carbonization,13-15 since nitrogen species could form chemical bonds with metal nanoparticles and serve as heteroatoms in the N-doped nanocarbon,16 which are reported to be effective electrocatalysts for oxygen reduction reaction (ORR).17-19 Pyrolysis of µm-size MOF crystals produces oxide nanoparticles (ZnO, or Co-doped ZnO) encapsulated in amorphous carbon matrix, resulting in significantly enhanced anode capacity in lithium-ion batteries (LIBs).20,21 Similar strategy has been utilized to fabricate high capacity, stable LIB anodes based on organic-coated ZIF-8 nanocomposites (with a discharge capacity of 750 mAh/g at a current density of 50 mA/g) and ZnO nanorod/quantum dot arrays grown on carbon cloth (699 mAh/g after 100 cycles at 500 mA/g).22,23 For applications as LIB anodes, pyrolysis is usually involved as a key process for MOF or ZIF-based sample preparation since this thermal treatment could produce (more conductive) carbon shells, tailor the porous structure (including micropores and mesopores) and surface area, and also expose inner metal ions in the form of carbon-coated nanoparticles that will be available for electrochemical reaction (Li-ion intercalation). Among many relevant structural parameters, there are two critical factors having direct influence on the LIB performance, including 1) a uniform 2


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dispersion of ZnO nanoparticles or quantum dots (for reaching high capacity) and 2) a porous carbon matrix or thin carbon coating encapsulating ZnO nanoparticles (for achieving excellent cycling stability), as indicated by previous studies.20 Both the ZnO nanoparticles and carbon coating are formed after the collapse of original regular frameworks during the pyrolysis process, at predefined target temperatures for a certain duration. Despite that a good electrode structure could be inferred by its electrochemical behavior, the structure-performance relationship and the optimized configuration in terms of metal and carbon have not been clearly interpreted. In addition, most of MOF-derived metal@carbon porous structures are in the powder form (as individual micro to macro-scale carbonized crystals).20-22 As a result, carbon black and polymer adhesives are added to glue the powders and increase through-sample electrical conductivity, creating a significant portion (20 wt.% or more) of inactive or less active mass in the as-fabricated electrode, consequently leading to a reduction of specific capacity and energy density. In a recent work, carbon cloth was used as a flexible substrate to make the anode, however, carbon fibers have limited conductivity and a complex process was adopted to first synthesize ZnO nanorod arrays as sacrificial templates and then coat ZIF-8 around the nanorod to form the core-shell structure.23 An alternative approach to freestanding membrane electrodes is to introduce carbon nanotubes (CNTs), a one-dimensional nanostructure with superior mechanical and electrical properties, to serve as conductive interconnects for powder-form MOF crystals. For examples, ZIF-8 nanocrystals were synthesized in situ from a solution environment containing dispersed CNTs, resulting in the formation of ZIF-8@CNT hybrid structures.24,25 Although ZIF-8 grafted CNT films have shown good properties in CO2 adsorption and binder-free supercapacitor electrodes, their potential application in LIBs remains un-explored and the active metallic species in ZIFs has not been utilized. 3



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Here, we present a systematic study on the structural evolution of ZIF-8@CNT hybrid films through controlled pyrolysis, particularly focusing on the growth and aggregation process of metal nanoparticles and simultaneous formation (or disappearance) of carbon shells which are nitrogen-doped. By tailoring the pyrolysis period, we obtain three typical hybrid structures including 1) CNTs wrapped by relatively thick carbon layers, 2) CNTs grafted by ZnO nanoparticles and those nanoparticles are covered by thin N-doped carbon layers, and 3) CNTs grafted by aggregated and exposed (without carbon coating) ZnO nanoparticles. We further tested their electrochemical behavior as LIB anodes, and find that the second structure exhibits the best performance in terms of capacity and cycling stability. The structure-performance relationship, as revealed in our work, provides a reference for tailoring and optimizing MOF-CNT hybrid structures as porous conductive energy storage electrodes. Experimental Section Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, AR) and nitric acid (HNO3, AR) were purchased from Xilong Chemical Co., Ltd. 2-methylimidazole (C4H6N2, ≥99%) and methanol (CH3OH, GR) were provided by Being Bailingwei Science and technology Co. and Being Tongguang Fine Chemical Co., Ltd. Respectively. All the chemicals were analytical grade without further purification. Fabrication of ZIF-8@CNTs CNTs were obtained from ultrasonication of CNT sponges which were synthesized by chemical vapor deposition using ferrocene and 1,2-dichlorobenzene as catalyst and carbon precursor.26 Before the synthesis of ZIF-8@CNT composites, CNTs were acidized by concentrated nitric acid at 120 °C 4


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for 12 h, recovered by filtration, washed with deionized water, and freeze dried. Finally, well-dispersed CNTs were obtained with oxidized carboxylic groups on the outer walls. The fabrication of ZIF-8@CNT is similar to the solvothermal procedure of ZIF-8,27 typically CNTs (0.12 g) were dispersed in Hmim (0.649 g) in 20 ml methanol by repeated ultrasonication and stirring followed by adding Zn(NO3)2·6H2O (0.239 g) methanol (20 ml) solution in dropwise manner. The mixture was magnetically stirred for 15 min before placed in a 100 ml Teflon-line autoclave and heated to 90 °C for 6 h. After the hydrothermal reaction, the ZIF-8@CNT composites were separated by centrifugation and washed with methanol for at least three times. The composites were finally suction filtrated into a thin film and dried at 40 °C under vacuum for subsequent use. Pyrolysis of ZIF-8@CNTs ZIF-8@CNT thin films were transferred to a porcelain boat and placed in a tube furnace. Subsequently, pyrolysis of ZIF-8@CNT thin films were processed at 500 °C under Argon atmosphere with a heating rate of 5 °C/min for 2, 3 and 4 hours. After the heating process, Argon atmosphere is immediately closed, and the cooling process is conducted in air atmosphere. ZIF-8 derived ZnO nanoparticles coated on CNTs were obtained. These pyrolyzed samples were labeled as Zn@NC@CNTs (2 h), ZnO@NC@CNTs (3 h), and ZnO@CNTs (4 h), respectively. For comparison, ZIF-8@CNT thin films were heated to 1000 °C under Argon atmosphere with a heating rate of 10 °C/min, and Argon atmosphere was not stopped until the temperature is dropped to the room temperature to achieve the CNT networks wrapped by nitrogen-doped carbon layers marked as NC@CNTs. Material Characterization 5



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The morphology and structure of the prepared samples were analyzed by SEM (Hitachi S4800) and TEM (FEI Tecnai F20, 200 kV). High-angle annular dark-field scanning transmission electron microscopy and the electron energy loss spectroscopy (EELS) mapping were recorded on a TEM (FEI Tecnai F30, 300 kV). The pore structure was determined by N2 adsorption at 150 °C by using an ASAP 2020 volumetric sorption analyzer. The Brunauer-Emmett-Teller (BET) surface area was calculated from the isotherm by using the BET equation. The pore size distribution was calculated according to the nonlocal density functional theory (NLDFT) which could acquire the size distribution of micro and meso pores over the entire range, precisely, on the basis of the desorption branch. X-ray diffraction (XRD) measurements were performed with a Bruker D8 Focus X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ =1.54178 Å). X-ray photoelectron spectroscopy (XPS) analysis were performed on an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and low-energy electron flooding for charge compensation. Thermogravimetric (TGA) curves were acquired from a thermogravimetric analyzer (TA instruments) at a heating rate of 10 °C/min in nitrogen or air. Raman spectra were conducted using HORIBA iHR550 with 532 nm laser to analyze the surface chemical structures of the samples. Cell assembly The original CNT film and pyrolyzed samples Zn@NC@CNTs (2 h), ZnO@NC@CNTs (3 h), ZnO@CNTs (4 h), and NC@CNTs were used as freestanding LIB anodes, while a lithium metal foil as the counter-electrodes and a polypropylene (PP) film (Celgard 2400) as the separators. The electrolyte was a mixture of ethyl methylcarbonate (EMC), ethylene carbonate (EC), and dimethyl 6


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carbonate (DMC) with a volume ratio of 1: 1: 1, and 1 M LiPF6. Coin-type (CR 2032) cells were assembled in an argon-filled glove box, and the mass of the electrodes is in the range of 1-2 mg. Electrochemical characterization A galvanostatic cycling test of the assembled cells was carried out on a Neware system in the potential range of 0.01–3 V at a discharge/charge current density of 100 mA/g. Cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on CHI660E electrochemical workstation. CV curves were acquired by sweeping the potential from 0.01 to 3 V (vs Li/Li+) at a scan rate of 0.05 mV/s. EIS curves were obtained by applying a sine wave with amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz.

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Results & Discussions We adopted an in situ growth and thermal pyrolysis process to synthesize ZIF-8@CNT hybrid materials with tailored microstructures (illustrated in Figure 1, and see Experimental). Multi-walled nanotubes with average diameters of 40 nm were dispersed into the precursor solution to act as the substrate for ZIF growth. They have an oxidized surface (grafted by functional groups) which facilitate the nucleation of ZIF nanocrystals with strong adhesion. This solvothermal method yields nanoscale crystals that can attach to the CNT surface securely to form hybrid structure. In the subsequent annealing step, we chose a relatively lower temperature (500 °C) at which ZIF-8 nanocrystals start to decompose (Figure S1) compared to a target temperature of 650 °C (without heating duration) in previous reports.23,25 It allows us to carry out the annealing process for a longer period (2 to 4 hours) and thus more convenient observation and better control on the resulting hybrid structure. Typically, we obtain three different structures depending on the transformation products of ZIF-8 nanocrystals during pyrolysis. These include: 1) thick nitrogen-doped (NC) carbon layers (containing un-exposed Zn) wrapping around CNTs for a short duration (2 hours), termed as Zn@NC@CNTs, 2) thin carbon shells coating on ZnO nanoparticles which are grafted on CNTs, formed after a moderate duration (3 h) and termed as ZnO@NC@CNTs, and 3) aggregated ZnO nanoparticles (without carbon shell) on CNTs after prolonged duration (4 h), ZnO@CNTs. Therefore, our study enables a systematic study on the structural evolution of ZIF-8@CNTs hybrid structures and correspondingly their electrochemical properties. Grafting ZIF-8 nanocrystals onto CNTs allows us to fabricate freestanding carpet-like films by filtration. Here, the CNTs have an average length of ∼20 µm, which is sufficiently long for making 8


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stable filtration membranes like previously reported bucky-papers.28,29 These films can be rolled and folded without cracking, indicating high flexibility and acceptable mechanical property for electrochemical tests and cell assembly (Figure 2a). Figure 2b shows the SEM image of pristine CNTs stacking into a porous network. In the ZIF-8@CNTs film, the initially clean and smooth surface of CNTs become rough, with many small crystals grafted uniformly along individual CNTs throughout the entire film (Figure 2c). The ZIF-8 nanocrystals have sizes in the range of 20 to 50 nm consistent with the statistic size distribution reported in literature,27 and the diameters of ZIF-8@CNTs skeletons increase to 60-80 nm, indicating the formation of a core-shell configuration. The CNTs form an interconnected conductive network while maintaining the porous structure. TEM image clearly shows that many ZIF-8 nanocrystals are well adhered to the CNT surface even after long time sonication (Figure 2d). On some CNTs, those nanocrystals are closely arranged into a single-layer distribution. Previously, ZIF-8 crystals were grown on bucky-papers via Zn2+ adsorption and seeding.30 Here, our acid treatment produced functional groups to form Zn-O chemical bonds, resulting in good interface between the as-grown ZIF-8 nanocrystals and CNTs, which is favorable for subsequent LIB applications. Pyrolysis is a necessary step to create functional hybrid structures for electrochemical applications. Here, pyrolysis of ZIF-8 nanocrystals (in which Zn is the metallic center) could produce Zn species embedded within a porous carbon matrix, and Zn could be oxidized into ZnO in the presence of air at high temperature. Starting from the ZIF-8@CNTs films, we have investigated the structural change upon thermal annealing in Ar flow and under 500 °C for 2 to 4 hours (to let ZIF-8 collapses) and a subsequent cooling stage in air (to form ZnO by oxidation of Zn species embedded in the collapsed ZIF-8). The resulting annealed films show different morphologies and 9



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microstructures as revealed by SEM and TEM characterization (Figure 3 and 4). For an intermediate annealing time (3 h), there are homogeneous ZnO nanoparticles (NPs) (about 10 nm in diameter) distributed uniformly throughout the CNT network (Figure 3a). TEM images show that these NPs are grafted on the surface of CNTs, and there is a thin N-doped carbon coating (2-5 nm thick) covering on the CNTs and NPs due to the collapsing of ZIF-8 (Figure 4a and b). High-resolution TEM image shows the lattice fringes of ZnO NPs with a lattice space of 0.26 nm, corresponding to (002) plane of zinc oxides (Figure 4c). Correspondingly, the SAED displays a typical hexagonal wurtzite crystal structure of ZnO, possessing well-defined lattice planes of (200) (110) (102) (101) (002) (100). Furthermore, the Fourier transform image from a single ZnO NP clearly shows two symmetric diffraction spots from the lattice plane (002), in consistent with the lattice fringes in the high-resolution TEM and SAED results in ZnO@NC@CNTs (Figure S2). Thus the ZnO NPs are strongly fixed on the CNT networks by an external carbon shell, forming a ZnO@NC@CNTs hybrid structure. For a relatively short time (2 h), ZIF-8 nanocrystals also have disappeared leaving a residual irregular carbon layer with thicknesses up to 20 nm adhering on CNTs (Figure 3b, 4d-f). At this stage, most of inner Zn species were protected by the thicker carbon layer from oxygen access and oxidation, therefore the resulting product is denoted as Zn@NC@CNTs. When the annealing process was further prolonged (4 h), initially dispersed ZnO NPs have aggregated into an increased size (>20 nm) (Figure 3c, 4g-i). The carbon shell outside ZnO NPs has evaporated completely, leaving exposed NPs with a clean surface. In addition to direct observation by TEM, we also carried out EELS-mapping on the regions of ZnO NPs in the ZnO@CNTs sample, in which the carbon signal is very weak, to further confirm the absence of carbon shell on the ZnO NPs (Figure S3). At the same time, the average diameter of CNTs decreases from 40 to about 20 nm due to long time 10


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heating and gasification. This sample is denoted as ZnO@CNTs. For all the above three samples, both the macroscopically flexible film and the microscopically porous network structure are well maintained after pyrolysis. XRD patterns show that the peaks of ZIF-8 have disappeared, and sharp Bragg peaks from crystalline ZnO NPs emerge in the annealed three samples (Figure 5, S4). The relative intensity ratio between the ZnO peaks and the carbon peak (coming from CNTs) increases consistently with the annealing time (from 2 to 4 hours), especially for peaks located in the 2θ range of 45° to 75° which are more pronounced in ZnO@NC@CNTs and ZnO@CNTs than those of Zn@NC@CNTs. In the Zn@NC@CNTs, the ZnO peaks are relatively weak indicating that some modest oxidation occurs even most of the Zn species are protected by the thick carbon layer. After a longer pyrolysis (3 h), the carbon layer becomes thin and the oxidation of Zn species is enhanced, resulting in ZnO nanoparticles that can be clearly seen in SEM and TEM. In addition, we have performed Raman study on ZIF-8@CNT-derived hybrids after different pyrolysis conditions (Figure S5). All three samples show pronounced D and G bands, but with different relative intensity ratios (ID/IG). For the two samples with relatively short pyrolysis time (Zn@NC@CNTs and ZnO@NC@CNTs), ID/IG >1, indicating the amorphous nature of the carbon shell from ZIF-8 nanocrystals. The ID/IG value of ZnO@NC@CNTs is slightly lower than that of Zn@NC@CNTs, which might be attributed to improved crystallinity over increasing pyrolysis time. For the third sample without carbon shell on ZnO (ZnO@CNTs), ID/IG

MOF-Derived ZnO Nanoparticles Covered by N ... - ACS Publications

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