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Article Cite This: ACS Omega 2019, 4, 7565−7573

http://pubs.acs.org/journal/acsodf

Facile Preparation of Porous Rod-like CuxCo3−xO4/C Composites via Bimetal−Organic Framework Derivation as Superior Anodes for Lithium-Ion Batteries Li Hou, Xinyu Jiang, Yang Jiang, Tifeng Jiao,* Ruiwen Cui, Shuolei Deng, Jiajia Gao, Yuanyuan Guo, and Faming Gao*

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Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, 438 West Hebei Street, Qinhuangdao 066004, China S Supporting Information *

ABSTRACT: To meet growing demand of energy, lithiumion batteries (LIBs) are under enormous attention. The development of well-designed ternary transition metal oxides with high capacity and high stability is important and challengeable for using as electrode materials for LIBs. Herein, a new and highly reversible carbon-coated Cu-Co bimetal oxide composite material (CuxCo3−xO4/C) with a one-dimensional (1D) porous rod-like structure was prepared through a bimetal−organic framework (BMOF) template strategy followed by a morphology-inherited annealing treatment. During the annealing process, carbon derived from organic frameworks in situ fully covered the synthesized bimetal oxide nanoparticles, and a large number of porous spaces were generated in the MOF-derived final samples, thus ensuring high electrical conductivity and fast ion diffusion. Benefiting from the synergetic effect of bimetals, the unique 1D porous structure, and conductive carbon network, the as-synthesized CuxCo3−xO4/C delivers a high capacity retention up to 92.4% after 100 cycles, with a high reversible capacity still maintained at 900 mA h g−1, indicating an excellent cycling stability. Also, a good rate performance is demonstrated. These outstanding electrochemical properties show us a concept of synthesis of MOF-derived bimetal oxides combining both advantages of carbon incorporation and porous structure for progressive lithium-ion batteries.

1. INTRODUCTION As one of the most promising technologies to solve the heavy consumption of fossil fuel resources and serious environmental problems, Li-ion batteries (LIBs) have been used as an advanced and portable source of electricity for various applications.1−5 However, the specific energy of conventional LIBs is not sufficient for these applications due to the limited specific capacity of the conventional graphite anode (372 mA h g−1).6−8 Many research studies have been carried out to find new electrode materials with high energy density or high capacity.9,10 Among those materials, Co3O4, a transition metal oxide (TMO), owing to its complex electron transfer reaction upon cycling, exhibits excellent potential for its high specific capacities (nearly 1000 mA h g−1).11,12 Recently, in consideration of the nonenvironmentally friendliness, expensiveness, and high-lithium redox potential (2.2−2.4 V vs Li/Li+) of Co, some other eco-friendly and cheaper transition metal atoms, such as Zn,13,14 Mn,15−17 Ni,18 and Cu, have been used to replace Co partially to form ternary metal oxides. By replacing Co with cheaper and more conductive Cu, the obtained Cu-Co bimetal oxides present an improved electronic conductivity and higher electrochemical activity than the single copper or cobalt oxide and © 2019 American Chemical Society

thus proved to be excellent electrode materials for LIBs. Sharma et al. synthesized CuCo2O4 as an anode material for LIBs with a reversible capacity of 755 mA h g−1 at a current of 60 mA g−1.19 Sun et al. reported a mesoporous CuCo2O4 structure; after six cycles, it still retained its capacity of 900 mA h g−1 at 60 mA g−1.20 Although these reported CuCo2O4 electrodes present a much higher capacity than that of the conventional graphite, due to the large volume expansion with cycling and relatively poor electrical conductivity, the cycling stability, Coulombic efficiencies, and rate performance of CuCo bimetal oxides need to be further improved. So far, researchers have done much effort to solve these problems. One effective method is to produce rational porous nanostructures, which can provide additional channels for fast diffusion of lithium ions. Meanwhile, it can alleviate the structural strain induced by the lithium insertion/extraction process.21 Second, combining the transition metal oxides with more conductive carbon materials is another well-established strategy for enhancing the electrochemical performance of Received: March 21, 2019 Accepted: April 16, 2019 Published: April 25, 2019 7565

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Scheme 1. Schematic Illustration of the Fabrication of Porous Rod-like CuxCo3−xO4/C Composites

100 cycles and a high capacity retention up to 92.4% with respect to the initial discharge capacity of 974.1 mA h g−1) and excellent rate capability when evaluated as anode materials.

TMOs. Since carbon is in the composite, the volume expansion of TMOs could be restrained, and the cycling stability of the electrode material could be enhanced effectively. Moreover, the improved electrical conductivity of the electrode usually leads to an enhanced rate capability. Kang et al. indicated that the capacity of the CuCo2O4 nanocube encapsulated by reduced graphite oxide sheet was almost two times higher than that of pure CuCo2O4 nanocube.22 Recently, Zhang et al. utilized an electrospinning method to synthesize CuCo2O4/C nanofibers, which present an excellent cycling stability with a high capacity of 865 mA h g−1 after 400 cycles.23 Motivated by these progresses, combining the above two strategies to fabricate carbon incorporated Cu-Co bimetal oxide composite materials with a well-designed porous structure will be a promising method to improve the electrochemical performance. However, synthesis of such porous structured composite materials is still facing challenges. In this work, we report a rational design and fabrication of CuxCo3−xO4/C composite with a porous rod-like structure by using bimetal−organic frameworks (BMOFs) as the precursors and then calcination at a certain temperature. Metal−organic frameworks (MOFs) are constructed from supramolecular assembly of metal ions or clusters and organic ligands, which are well known for their diverse porosity and architectural structures.24−26 Inspired by their excellent configuration, MOFs are usually considered as the multifunctional templates and precursors for the preparation of new porous nanostructured materials with high surface area and graded pore distribution, basically through thermal decomposition. In addition, by accurately adjusting and controlling MOF precursors and calcination conditions, the structure and composition of MOF-derived nanomaterials can be easily adjusted and optimized to achieve excellent electrochemical properties as LIB anodes.27−30 Specially, we made use of the organic frameworks to generate a microcrystal precursor with a well-designed rod-like structure, which then transformed to morphology-kept bimetal oxides during annealing in insert atmosphere. Attributed to the annealing process, a large number of porous spaces were generated in the MOF-derived final products because of the carbonization or graphitization of organic ligands in MOFs during annealing treatment. Most importantly, carbonized and graphitizated carbon originated from organic frameworks would in situ fully cover these metal oxide nanoparticles, which can not only improve the electronic conductivity of electrode materials but also protect the electrode materials against destruction during the charge/ discharge process. The obtained CuxCo3−xO4/C particles connect each other and form a porous rod-like structure, ensuring fast diffusion of Li+/electrons. Owing to the unique structural and compositional features, the CuxCo3−xO4/C presents a high-lithium storage capacity (900 mA h g−1 after

2. RESULTS AND DISCUSSION 2.1. Characterization of CuxCo3−xO4/C Nanocomposites. The overall strategy for the fabrication of porous rod-like CuxCo3−xO4/C is clarified in Scheme 1. First, a facile solvothermal method was used to synthesize Cu-Co-BTC precursors. Owing to the presence of a carboxyl functional group (H3BTC), Co2+ and Cu2+ ions can easily connect it by complexation or adsorption; thus, metal ions are uniformly distributed in these rod-like MOF precursors, providing the metal sources for further preparation of bimetal oxides. After annealing these MOF microcrystal precursors in an inert atmosphere, the morphology-kept bimetal oxide/C composites were obtained. The formation of porous rod-like structure is mainly due to carbon and oxygen in organic ligands reacting to generate gas to cause holes during the thermal decomposition of rod-like Cu-Co-BTC precursor. Meanwhile, these synthesized bimetal oxide nanocrystals are simultaneously covered by carbon derived from in situ carbonization of the organic ligands. Combining the advantages of Cu-Co bimetal oxide nanocrystals with a 1D rod-like structure, high porosity, and well-conductive carbon network, the as-synthesized CuxCo3−xO4/C has great potential to exhibit outstanding electrochemical lithium storage performance as an anode material for LIBs. The precursors of Cu and Co with different molar proportions were easily synthesized by a solvothermal reaction, which are denoted as Cu-Co-BTC-n (n represents the molar ratio of Cu to Co). Figure 1a shows a representative fieldemission scanning electron microscopy (FESEM) image of Cu-Co-BTC-0.3. It can be seen that Cu-Co-BTC-0.3 presents a rod-like structure with a diameter around 1 μm and a length close to 10 μm. High-magnification SEM image in Figure 1b reveals the detailed structure of the rod, which actually consists of four prismatic planes and a relatively smooth surface.

Figure 1. (a, b) FESEM images, (c) TEM image, and (d−h) corresponding elemental mapping images of Cu-Co-BTC-0.3. 7566

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nanocrystals could be found. Besides, a lattice fringe with an interplanar spacing of 0.28 nm could be clearly seen in the HRTEM image, which is corresponding to the (220) lattice plane of spinel CuCo2O4. The selected area electron diffraction (SAED) pattern (Figure S4) also demonstrates the crystalline feature of these particles. The elemental mapping shows the homogeneous coexistence and highly uniform distribution of Cu, Co, O, and C within one particle, confirming the composition of CuxCo3−xO4/C. On the basis of these observations, it can be safely concluded that the as-prepared Cu-Co MOF rods transformed into CuxCo3−xO4/C composites at 600 °C in a N2 atmosphere. The carbon shell is derived from carbonization of organic ligands, and the crystallized bimetal oxide particles are encapsulated by the carbon wall, thus forming a core−shell structure for each CuxCo3−xO4/C particle. Since the carbon framework still maintains a 1D structural characteristic, the CuxCo3−xO4/C particles interconnect each other and form a porous rod-like structure. Nitrogen adsorption−desorption measurement was used to investigate the specific porous nature of CuxCo3−xO4/C rods. As shown in Figure S5, the CuxCo3−xO4/C-0.3 exhibits a type IV isotherm with a distinct hysteresis loop in the relative pressure region of 0.5−1.0, indicating a mesoporous feature. Barrett−Joyner−Halenda (BJH) desorption analyses reveal a relatively wide pore size range from 1 to 50 nm (inset in Figure S5). The origination of pores may be attributed to the escape of gases via the calcination of organic ligands. In addition, the composite is assembled from small particles; these stacked particles may cause some pores on the surface of the material. A high Brunauer−Emmett−Teller (BET) surface area of 98.0 m2/g was also confirmed. As we know, such a mesoporous structure could ensure efficient diffusion of the electrolyte, and a high surface area may provide more electrochemical active sites. We also studied the influences of Cu/Co molar ratio on the structure of the product. These SEM results strongly support that idea that the morphology of CuxCo3−xO4/C-n keeps the same character as that of Cu-Co-BTC-n precursors (Figures S2 and S6). As for the obtained CuxCo3−xO4/C-0.8, it clearly displays a porous rod-like structure composed of nanoparticles. While it cannot be ignored that there are many disorderly packed CuxCo3−xO4/C particles, they appear due to the destruction of the rod-like framework. Actually, the thicker the rod, the easier it is to destroy the structure during calcination. XRD patterns of the obtained CuxCo3−xO4/C-n samples are shown in Figure 3. For all CuxCo3−xO4/C-n products, there is a main phase of spinel CuCo2O4, and a secondary phase CuO (JCPDS card no. 80-1917) could be observed. The relatively sharp reflections located at 18°, 31°, 36°, 38°, 44°, 55°, 58°, and 64° can be well indexed to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of CuCo2O4 (JCPDS card no. 78-2177), respectively, revealing a high crystallization. The cubic lattice parameter, assessed by least-squares fitting of 2θ and (hkl), is a = 8.139(4) Å, which is in good agreement with the value of 8.133(2) Å. Since the atomic diameter of Cu is bigger than that of Co, the little higher diameter of the unit cell demonstrates that the Cu atoms were completely incorporated into the Co3O4 lattice. No reflection peaks originated from the graphitized carbon could be found, owing to the small amount of carbon. Meanwhile, no other redundant peaks are observed, suggesting a high purity of metal oxides. X-ray photoelectron spectroscopy (XPS) measurements were applied to indicate the elemental composition and

Transmission electron microscopy (TEM) investigation as shown in Figure 1c also reveals a solid structure for the Cu-CoBTC-0.3 rods. The formation of bimetallic Cu-Co-BTC-0.3 is confirmed by elemental mapping images (Figure 1d−h), which present a uniform distribution of elements of Co, Cu, C, and O along the rods. Meanwhile, the amorphous nature of Cu-CoBTC-0.3 rods is confirmed by powder X-ray diffraction (XRD) pattern, which is common for MOFs (Figure S1). We also took FESEM investigations for other Cu-Co-BTC-n samples with different Cu/Co molar ratios, and the results are shown in Figure S2. Notably, when the Cu/Co molar ratio is less than 0.1, the obtained Cu-Co-BTC still maintains a typical spherelike morphology as the reported single-metal Co-BTC.31,32 As the molar ratio increases, a rod-like structure for the Cu-CoBTC appears, and the diameter size increases accordingly, from 1 μm for Cu-Co-BTC-0.3 to around 1.5−2 μm for Cu-CoBTC-0.8. These results reveal that the amount of Cu source plays an important role in the formation of 1D rod-like Cu-CoBTC-n. The thermal behavior of the Cu-Co-BTC-0.3 precursor was investigated with thermogravimetric analysis (TGA). As shown in Figure S3, below 200 °C, there is a weight loss of 20%, which may be attributed to the escape of DMF molecules and adsorbed water. The subsequent weight loss of 49% under 400−550 °C can be attributed to the oxidation of the BTC ligand and the decomposition of the skeleton. To ensure complete conversion of precursors to oxide products, 600 °C was selected as the calcination temperature in the next process. The morphologies of CuxCo3−xO4/C samples obtained via calcination of Cu-Co-BTC precursors were observed with FESEM and TEM. Figure 2 shows the morphology features of

Figure 2. Structural characterization of CuxCo3−xO4/C-0.3. (a, b) FESEM images, (c) TEM image, (d) HRTEM image, and (e−i) EDS elemental mapping images.

CuxCo3−xO4/C-0.3. It can be seen that the annealed products typically maintain almost the same size and rod-like morphology as that of the MOF precursors (Figure 2a). Careful morphology observation by high-magnification SEM (Figure 2a (inset),b) reveals that the CuxCo3−xO4/C rods are composed of particles with sizes varying from dozens to hundreds of nanometers. The surface of these rods is highly rough and porous, which can be attributed to the carbonization of organic ligands in the Cu-Co-BTC precursor during the annealing process. The structure features of these CuxCo3−xO4/C-0.3 rods were further examined by TEM. The microstructure displayed in Figure 2c shows that the particles interconnects each other and form a 1D rod-like structure, which corresponds to the SEM results (Figure 2a,b). Figure 2d shows a typical high-magnification TEM image taken from the edge of CuxCo3−xO4/C nanoparticle. A very thin carbon layer coated on the surface of the bimetal oxide 7567

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indicating the existence of copper in the oxidation state of Cu(II) in CuxCo3−xO4.33 The peaks of O 1s present three characteristic locations at 529.3, 531.0, and 531.9 eV (Figure 4d).34 All these results indicate the successful synthesis of copper cobalt oxides. The deconvolution of C 1s peak (Figure S7) indicates that carbon exists mainly in the form of graphitized carbon, which is dominant on the surface of CuxCo3−xO4/C. 2.2. Electrochemical Performances of CuxCo3−xO4/C Nanocomposites. To explore the benefits of rational design for the porous rod-like CuxCo3−xO4/C composites as electrode materials of LIBs, the electrochemical behavior of CuxCo3−xO4/C as the anode was explored by the cyclic voltammetry (CV) technique. The CV curves for the first four cycles of the CuxCo3−xO4/C-0.3 electrode at the scan rate of 0.5 mV s−1 are shown in Figure 5a. Two obvious peaks can be found in the first cathodic cycle at 0.95 and 0.6 V. The intense peak located at around 0.95 V can be attributed to the decomposition of the CuxCo3−xO4/C-0.3 and the formation of Cu and Co nanoparticles in an amorphous matrix Li2O, as reported in earlier studies.20 As stated in eq 1, during the initial discharge cycle, the irreversible structure destruction process of CuxCo3−xO4 consumes 8 mol of Li per mole of CuxCo3−xO4, and thus, electrochemically active Co and Cu nanoparticles are formed in the matrix of Li2O. The relatively weak peak located at around 0.6 V in the first cathodic scan and then disappearing in the subsequent cycles is due to the formation of the solid electrolyte interface (SEI) at the interface of the electrolyte and electrode surface.35 In the first anodic process, the broad oxidation peak located at around 2.1 V can be assigned to the

Figure 3. XRD patterns of CuxCo3−xO4/C-n. (a) CuxCo3−xO4/C-0.1, (b) CuxCo3−xO4/C-0.3, and (c) CuxCo3−xO4/C-0.8.

chemical bond of CuxCo3−xO4/C-0.3 (Figure 4). The survey spectrum illustrated unique peaks of Cu, Co, O, and C (Figure 4a), demonstrating the successful doping of Cu in Co3O4, and no other peaks of elements could be found in the survey. The Co 2p3/2 peak of the CuxCo3−xO4/C-0.3 composite reveals two characteristic peaks at 779.2 and 780.1 eV (Figure 4b), corresponding to Co(III) and Co(II) species, respectively.22 Figure 4c shows the Cu 2p XPS spectrum, which exhibits two major peaks at 934.5 and 954.6 eV for Cu 2p3/2 and Cu 2p1/2, respectively, and a small satellite peak between them,

Figure 4. XPS spectra of (a) survey spectrum, (b) Cu 2p, (c) Co 2p, and (d) O 1s for CuxCo3−xO4/C-0.3. 7568

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Figure 5. Electrochemical performances of the as-prepared CuxCo3−xO4/C-0.3 electrode. (a) CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01−3.0 V. (b) Charge−discharge profiles at a current density of 100 mA g−1. (c) Comparison of rate capabilities at various rates of 0.1, 0.2, 0.5, 1, and 0.1 A g−1. (d) Cycling performance at a current rate of 100 mA g−1. (e) Nyquist plot of CuxCo3−xO4/C-0.3 electrode with the equivalent circuit used to model the impedance spectra (inset). (f) Z′ versus ω−0.5 plots in the low-frequency range.

oxidation of Cu and Co to their corresponding metal oxides, as seen in eqs 2−4.19 During the next cycling process, it is observed that the reduction peaks are found to be shifted to a higher potential of around 1.10 V with respect to the first cycle (0.95 V), indicating a different reaction mechanism with Li compared to the first discharge reaction. Meanwhile, there is a

slight potential shift in the oxidation peaks in the second cycle, which can be attributed to the reorganization of the active materials.36 After the second cycle, the intensity of the redox peaks changes slightly, indicating good cycling stability and high reversibility of the CuxCo3−xO4/C-0.3 electrode. 7569

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porous CuxCo3−xO4/C rods, thus leading to a slow electrochemical activation and reversible reactions of active materials.22 Another suggestion is that the formed metal nanoparticles during the discharge processes can promote the reversible transformation of some SEI components. Also, it allows more accessible active sites available for Li+ insertion, thus leading to an increase of capacity.33 Over the prolonged cycling, the CuxCo3−xO4/C-0.3 electrode shows outstanding capacity retention, and the reversible discharge capacity remains as high as 900 mA h g−1 after 100 cycles, indicating a high capacity retention of 92.4% with respect to the initial discharge capacity of 974.1 mA h g−1. In addition, the Columbic efficiency increases gradually and stabilizes at above 96% even at 100% after the first four cycles, indicating good cycling stability. For comparison, the cycling performance of other CuxCo3−xO4/C-n electrodes at a current density of 100 mA g−1 was also investigated. Owing to the synergetic effect of two types of metal elements, compared to the corresponding monometal oxides, the introduction of copper is an important factor for enhanced LIB performance. The conductivity of Cu is better than that of Co, while the theoretical specific capacity of CuO (670 mA h g−1) is much lower than that of Co3O4 (around 1000 mA h g−1). So, with the increase of Cu in CuxCo3−xO4/C-n, the conductivity will be enhanced, but the capacity may decrease. As shown in Figure S8, CuxCo3−xO4/C0.1 shows a high initial capacity (1115.2 mA h g−1) but decays quickly with cycling. For CuxCo3−xO4/C-0.8, the capacity drops from the first discharge capacities of 836.7 mA h g−1 to less than 300 mA h g−1 after 100 cycles, which is much lower than that of CuxCo3−xO4/C-0.3. The bad cycling performance may also be attributed to the destruction of the rod-like structure, as revealed in Figure S6. Combining both capacity advantage and morphological integrity, CuxCo3−xO4/C-0.3 presents the best cycling performance. 2.3. Possible Reasons for High Performance. Electrochemical impedance spectroscopy (EIS) was carried out to explain the superior electrochemical performance of CuxCo3−xO4/C-0.3. The Nyquist plot is shown in Figure 5d. Obviously, a semicircle in the high-frequency region is closely related to the charge transfer resistance of the electrode/ electrolyte interface. Meanwhile, an inclined line in the lowfrequency region is associated with the lithium-ion diffusion process.40 The intersection point of axis in the high-frequency region is correlated with electrolyte resistance. The classic equivalent circuit model of the investigated system is depicted in the inset of Figure 5d to represent the internal resistance of the battery. Re stands for the internal resistance of the battery; Rsl represents the resistance of migration, and Csl represents the capacity of the layer in the high-frequency semicircle; Cdl and Rct are related with the double-layer capacitance and charge transfer resistance, respectively; Zw stands for the diffusioncontrolled Warburg impedance. The Zsimp Win computer program was used to calculate the main electrical parameters in the equivalent circuit. The Rct value of CuxCo3−xO4/C-0.3 electrode is 9.5 Ω, which is much smaller than that of the reported pristine CuCo2O4 electrode (≈31 Ω),22 suggesting that the carbon coating on CuxCo3−xO4 nanoparticles derived from carbonization of organic ligands significantly lowers contact and charge transfer impedances. Additionally, the Li-ion diffusion coefficients of CuxCo3−xO4/C-0.3 were determined with the low-frequency Warburg contribution of the impedance. The diffusion

Figure 5b shows the galvanostatic charge−discharge lines of electrode at a current density of 100 mA g−1 in a voltage window of 0.01−3.0 V. A large voltage plateau located at ∼1.2 V followed by a sloping profile is observed in the first discharge curve. The plateau can be attributed to the reduction of CuxCo3−xO4 to metal Co and Cu and the formation of Li2O at the same time. The slopping part is generally regarded as capacitive behavior of surface storage of lithium. The phenomenon has been widely observed in mesoporous materials with high BET surface area and some other nanostructured materials.37 The first charge process presents a smoothly varying profile followed by a plateau at around 2.1 V, which is attributed to the oxidation of Cu and Co to corresponding metal oxides. These results are well consistent with the CV measurements. According to previous studies, the lithium storage mechanism of the CuxCo3−xO4/C could be considered as follows CuxCo3 − x O4 + 8Li+ + 8e− → xCu + 3 − xCo + 4Li 2O (1) +

Cu + Li 2O ↔ CuO + 2Li + 2e

−

(2)

Co + Li 2O ↔ CoO + 2Li+ + 2e−

(3)

3CoO + Li 2O ↔ Co3O4 + 2Li+ + 2e−

(4)

6C + Li+ + e− ↔ LiC6

(5)

As shown in Figure 5b, the first discharge and charge capacities are 974.1 mA h g−1 and 616.9 mA h g−1, respectively, corresponding to a Coulombic efficiency of 63.3%. The irreversible capacity loss can be attributed to the formation of a solid electrolyte interface (SEI) layer, decomposition of electrolyte, and some unresolved Li2O phases, which agree with most anode materials.38 It is notable that the Coulombic efficiency increases gradually and maintains at above 96% after the first four cycles. For the 10th and 50th cycles, the curves demonstrate a high Coulombic efficiency of nearly 100%, indicating the increasing reversibility and good cycling stability. The rate capability of the CuxCo3−xO4/C-0.3 electrode was measured with different current densities after 50 cycles. As shown in Figure 5c, the electrode exhibits a good rate capability with the average discharge capacities of 885, 800, 620, and 507 mA h g−1 at current densities of 100, 200, 500, and 1000 mA g−1, respectively. It is noteworthy that a high capacity of 866 mA h g−1 can be recovered when the specific current returns back to 100 mA g−1 after dozens of heavy-duty cycles, which still closes to the initial capacity at 100 mA g−1. This result shows that CuxCo3−xO4/C-0.3 has a great potential as high-rate anode materials for LIBs. Cycling stability is another important factor in LIB applications. Figure 5d shows the cycling performance of the CuxCo3−xO4/C-0.3 electrode at a current density of 100 mA g−1. Interestingly, the capacity of the electrodes decreases from the first discharge of 974.1 mA h g−1 to less than 600 mA h g−1 during the first few cycles, then increases gradually with cycling, and finally maintains stability at a certain value. There is a common phenomenon in other transition metal oxide electrodes, which is rationally attributed to the reversible formation of polymeric/gel-like films originating from kinetic activation in the electrode.39 The porous structures and conductive carbon shell are other important factors for the increasing capacities since the electrolyte gradually penetrates into the inner part of the 7570

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treatment. The synthesized CuxCo3−xO4 nanoparticles were encapsulated in carbon coating, which may restrain the volume expansion and improve the electronic conductivity of electrode materials. The MOF-derived 1D porous structure provides the efficiently accessible lithium storage active sites for high capacity and the enhanced pathway for ion diffusion in the charge−discharge process. Combining the advantages of the well-designed structure and incorporation of carbon, the obtained CuxCo3−xO4/C-0.3 presents superior electrochemical performance, including high reversible capacity of 900 mA h g−1 even after 100 cycles and excellent cycling stability and rate capability. It is believed that the facile preparation process and high electrochemical performance of the porous rod-like CuxCo3−xO4/C composite would pave the way for its practical LIB application.

coefficient of Li ions (DLi+) can be calculated using the following equations41 Z′ = R e + R ct + σω−0.5 DLi =

(RT )2 2(An2F 2C Liσ )2

(6)

(7)

As shown in Figure 5f, Z′ corresponds to the real impedance, ω(2πf) represents the angular frequency in the low-frequency region; meanwhile, Re + Rct and the Warburg coefficient (σ) can be calculated from the fitting line, and Z′ has a linear relationship with ω−0.5. In eq 7, T, R, A, and F represents the absolute temperature, gas constant, surface area of the electrode, and Faraday’s constant, respectively. CLi represents the molar concentration of Li ions. n is the number of electrons transfer per mole of the active material involved in the electrode reaction. According to the fitting linear equation in Figure 5f, the lithium-ion diffusion coefficients of CuxCo3−xO4/C-0.3 is calculated to be approximately 3.17 × 10−12 cm2 s−1, which is much better than another reported CuCo2O4/carbon composite (2.23 × 10−13 cm2 s−1).22 These results further confirm that both the charge transfer and Li-ion diffusion kinetics of the porous rod-like CuxCo3−xO4/C are improved significantly, thus leading to superior electrochemical properties. The high reversible capacity, superior rate performance, and excellent cycling stability of the porous rod-like CuxCo3−xO4/ C-0.3 electrode may be attributed to several factors. First, the CuxCo3−xO4 particles are derived from Cu-Co-BTC precursors, accompanying with the carbonization of organic ligands to form graphitized carbon covered on the bimetal oxide particles. There is a strong and well-established interfacial connection with carbon, thus ensuring a high conductivity required. Second, the contact area between the active material and electrolyte could be enlarged for the mesoporous structure. Compared with the spherical morphology for CuxCo3−xO4/C0.1, the unique rod-like porous structure for CuxCo3−xO4/C0.3 can offer direct channels for efficient electron transport. However, 1D nanostructures still suffer from interfacial kinetic problems in contact areas. Once the 1D nanostructure combines with carbon, it can effectively solve interfacial kinetic problems. Particularly, the pores in the 1D rods allow a high utilization efficiency of active sites. Third, the typical onedimensional structure and existence of carbon materials would mitigate the volume expansion, protect the active materials against pulverization during the charge/discharge process, and thus improve the stability of the electrode. The comparison of the electrochemical performance of the obtained CuxCo3−xO4/ C-0.3 with earlier reports of other bimetal oxide for lithium-ion batteries is summarized in Table S1. Results confirm that the porous rod-like CuxCo3−xO4/C composite electrode with large reversible capacity and good cycling ability may accelerate the development of high-performance LIBs. Thus, the present research work demonstrated the important basis and wide applications in various self-assembled nanocomposites and functionalized nanostructures.42−57

4. EXPERIMENTAL SECTION 4.1. Synthesis of Cu-Co-BTC MOFs. In a typical synthesis, 1 g of Cu(NO)3·6H2O and Co(NO)3·6H2O with different Cu/Co ratios, 1 g of polyvinylpyrrolidone (PVP), and 0.1 g of trimesic acid (H3BTC) were dissolved in 30 mL of DMF solution. After magnetically stirring for 5 min, the mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 120 °C for 12 h. The obtained precipitate was washed several times with ethanol and deionized water and then dried at 70 °C for 12 h in vacuum. The final obtained powers are named Cu-Co-BTC-n (n represents the molar ratio of Cu to Co). 4.2. Preparation of CuxCo3−xO4/C Rods. The obtained Cu-Co-BTC-n powder was annealed in a nitrogen atmosphere at 600 °C for 2 h with a heating rate of 5 °C min−1. The powder samples were collected after cooling down to room temperature and named as CuxCo3−xO4-n (n = 0.1, 0.3 and 0.8). 4.3. Characterization. The structure, morphology, and composition of samples were characterized with X-ray diffraction spectroscopy (XRD, Rigaku 2550), X-ray photoelectron spectroscopy (XPS, Escalab 250Xi), scanning electron microscopy (SEM, Carl Zeiss Sigma HD), transmission electron microscopy (TEM, JEM-2100F), high-resolution TEM (HRTEM) imaging, and selected area electron diffraction (SAED). Thermogravimetric analysis (TGA) was carried out by a STA449C thermal analyzer with a heating rate of 10 °C min−1 under Ar. 4.4. Electrochemical Measurements. The active materials, carbon black, and poly(vinylidene fluoride) with a weight ratio of 80:10:10 were mixed in N-methyl-2-pyrrolidinone (NMP) solution to prepare the working electrode. The obtained slurry was coated onto the copper foil current collector and dried in vacuum at 70 °C for the whole night. The coin cells were assembled in a glove box filled with argon. The pure lithium foil, a Celgard 2400 membrane, and 1 M LiPF6 in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 in volume) were used as the counter electrode, separator, and the electrolyte, respectively. Before electrochemical measurement, the cells were placed for 8 h. The electrochemical measurements were carried out in a CT3008 battery testing system at room temperature. An electrochemical workstation (CHI660e) was used to test the cyclic voltammetry (CV) with a scan rate of 0.1 mV s−1 and a voltage window of 0.01−3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were implemented at 5 mV amplitude ranging from 100 kHz to 0.01 Hz.

3. CONCLUSIONS In summary, the porous rod-like CuxCo3−xO4/C composite materials with diverse Cu/Co ratios have been synthesized with a solvothermal method to fabricate bimetal−organic frameworks followed by a morphology-inherited annealing 7571

DOI: 10.1021/acsomega.9b00787 ACS Omega 2019, 4, 7565−7573

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embedded in hierarchically porous carbon nanofibers as advanced electrodes for lithium storage. Adv. Funct. Mater. 2016, 26, 6188− 6196. (10) Wang, H.-G.; Yuan, S.; Ma, D.-L.; Zhang, X.-B.; Yan, J.-M. Electrospun materials for lithium and sodium rechargeable batteries: from structure evolution to electrochemical performance. Energy Environ. Sci. 2015, 8, 1660−1681. (11) Zhang, C.-L.; Lu, B.-R.; Cao, F.-H.; Yu, Z.-L.; Cong, H.-P.; Yu, S.-H. Hierarchically structured Co3O4@carbon porous fibers derived from electrospun ZIF-67/PAN nanofibers as anodes for lithium ion batteries. J. Mater. Chem. A 2018, 6, 12962−12968. (12) Wang, Z.; Qu, S.; Cheng, Y.; Zheng, C.; Chen, S.; Wu, H. Facile synthesis of Co3O4 spheres and their unexpected high specific discharge capacity for Lithium-ion batteries. Appl. Surf. Sci. 2017, 416, 338−343. (13) Chen, C.; Lam, K.; Liu, B.; Hou, X.; Wu, Y. Chemical integration of reduced graphene oxide sheets encapsulated ZnCo2O4 quantum dots achieving excellent capacity storage for lithium-ion batteries. Electrochim. Acta 2017, 245, 672−684. (14) Deng, J.; Yu, X.; He, Y.; Li, B.; Yang, Q.-H.; Kang, F. A sliced orange-shaped ZnCo2O4 material as anode for high-performance lithium ion battery. Energy Storage Mater. 2017, 6, 61−69. (15) Kong, X.; Zhu, T.; Cheng, F.; Zhu, M.; Cao, X.; Liang, S.; Cao, G.; Pan, A. Uniform MnCo2O4 Porous dumbbells for lithium-ion batteries and oxygen evolution reactions. ACS Appl. Mater. Interfaces 2018, 10, 8730−8738. (16) Luo, Y.; Chen, C.; Chen, L.; Zhang, M.; Wang, T. 3D reticular pomegranate-like CoMn2O4/C for ultrahigh rate lithium-ion storage with re-oxidation of manganese. Electrochim. Acta 2017, 241, 244− 251. (17) Zhang, L.; Tang, Q.; Chen, X.; Fan, B.; Xiao, K.; Zhang, S.; Deng, W.; Hu, A. Self-assembled synthesis of diamond-like MnCo2O4 as anode active material for lithium-ion batteries with high cycling stability. J. Alloys Compd. 2017, 722, 387−393. (18) Wu, L.-L.; Wang, Z.; Long, Y.; Li, J.; Liu, Y.; Wang, Q.-S.; Wang, X.; Song, S.-Y.; Liu, X.; Zhang, H.-J. Multishelled NixCo3‑xO4 Hollow Microspheres Derived from Bimetal-Organic Frameworks as Anode Materials for High-Performance Lithium-Ion Batteries. Small 2017, 13, 1604270. (19) Sharma, Y.; Sharma, N.; Rao, G. V. S.; Chowdari, B. V. R. Lithium recycling behaviour of nano-phase-CuCo2O4 as anode for lithium-ion batteries. J. Power Sources 2007, 173, 495−501. (20) Sun, S.; Wen, Z.; Jin, J.; Cui, Y.; Lu, Y. Synthesis of ordered mesoporous CuCo2O4 with different textures as anode material for lithium ion battery. Microporous Mesoporous Mater. 2013, 169, 242− 247. (21) Sun, Y.; Zhang, P.; Wang, B.; Wu, J.; Ning, S.; Xie, A.; Shen, Y. Hollow porous CuO/C nanorods as a high-performance anode for lithium ion batteries. J. Alloys Compd. 2018, 750, 77−84. (22) Kang, W.; Tang, Y.; Li, W.; Li, Z.; Yang, X.; Xu, J.; Lee, C.-S. Porous CuCo2O4 nanocubes wrapped by reduced graphene oxide as high-performance lithium-ion battery anodes. Nanoscale 2014, 6, 6551−6556. (23) Zhang, H.; Tang, Z.; Zhang, K.; Wang, L.; Shi, H.; Zhang, G.; Duan, H. Pseudo-solid-solution CuCo2O4/C nanofibers as excellent anodes for lithium ion batteries. Electrochim. Acta 2017, 247, 692− 700. (24) Zhao, M.; Wang, Y.; Ma, Q.; Huang, Y.; Zhang, X.; Ping, J.; Zhang, Z.; Lu, Q.; Yu, Y.; Xu, H.; Zhao, Y.; Zhang, H. Ultrathin 2D metal−organic framework nanosheets. Adv. Mater. 2015, 27, 7372− 7378. (25) Genna, D. T.; Pfund, L. Y.; Samblanet, D. C.; Wong-Foy, A. G.; Matzger, A. J.; Sanford, M. S. Rhodium hydrogenation catalysts supported in metal organic frameworks: influence of the framework on catalytic activity and selectivity. ACS Catal. 2016, 6, 3569−3574. (26) Li, Y.-W.; Xu, J.; Li, D.-C.; Dou, J.-M.; Yan, H.; Hu, T.-L.; Bu, X.-H. Two microporous MOFs constructed from different metal cluster SBUs for selective gas adsorption. Chem. Commun. 2015, 51, 14211−14214.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00787. XRD and SAED patterns for structure analysis, FESEM images for morphology information, TGA curve for thermal behavior, XPS spectrum for elemental composition and chemical bond, N2 adsorption/desorption isotherm for the specific porous nature, cycling performance of samples at current density of 100 mA g−1, and table of comparison of cycling performance between this work and other bimetal oxides electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.J.). *E-mail: [email protected] (F.G.). ORCID

Tifeng Jiao: 0000-0003-1238-0277 Faming Gao: 0000-0001-8711-7153 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (grant nos. 21371149, 21671168, and 21872119), Hebei Youth Top-notch Talent Support Program, and the Natural Science Foundation of Hebei Province (grant nos. 17964403D and B2016203498).

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REFERENCES

(1) Feng, X.; Ouyang, M.; Liu, X.; Lu, L.; Xia, Y.; He, X. Thermal runaway mechanism of lithium ion battery for electric vehicles: A review. Energy Storage Mater. 2018, 10, 246−267. (2) Shang, H.; Zuo, Z.; Li, L.; Wang, F.; Liu, H.; Li, Y.; Li, Y. Ultrathin graphdiyne nanosheets grown in situ on copper nanowires and their performance as lithium ion battery anodes. Angew. Chem. Int. Ed. 2018, 57, 774−778. (3) Son, Y.; Sim, S.; Ma, H.; Choi, M.; Son, Y.; Park, N.; Cho, J.; Park, M. Exploring critical factors affecting strain distribution in 1D silicon based nanostructures for lithium ion battery anodes. Adv. Mater. 2018, 30, 1705430. (4) Liu, L.; Yang, X.; Lv, C.; Zhu, A.; Zhu, X.; Guo, S.; Chen, C.; Yang, D. Seaweed-derived route to Fe2O3 hollow nanoparticles/Ndoped graphene aerogels with high lithium ion storage performance. ACS Appl. Mater. Interfaces 2016, 8, 7047−7053. (5) Shen, L.; Che, Q.; Li, H.; Zhang, X. Mesoporous NiCo2O4 Nanowire arrays grown on carbon textiles as Binder-Free flexible electrodes for energy storage. Adv. Funct. Mater. 2014, 24, 2630− 2637. (6) Huang, B.; Li, X.; Pei, Y.; Li, S.; Cao, X.; Massé, R. C.; Cao, G. Novel carbon encapsulated porous SnO2 anode for lithium ion batteries with much improved cyclic stability. Small 2016, 12, 1945− 1955. (7) Yan, C.; Chen, G.; Zhou, X.; Sun, J.; Lv, C. Template-Based engineering of Carbon-Doped Co3O4 Hollow nanofibers as anode materials for Lithium-Ion batteries. Adv. Funct. Mater. 2016, 26, 1428−1436. (8) Nan, X.; Liu, C.; Zhang, C.; Ma, W.; Wang, K.; Li, Z.; Cao, G. A new anode material for high performance lithium-ion batteries: V2(PO4) O/C. J. Mater. Chem. C 2016, 4, 9789−9796. (9) Xia, G.; Zhang, L.; Fang, F.; Sun, D.; Guo, Z.; Liu, H.; Yu, X. General synthesis of transition metal oxide ultrafine nanoparticles 7572

DOI: 10.1021/acsomega.9b00787 ACS Omega 2019, 4, 7565−7573

ACS Omega

Article

(27) Liu, T.; Wang, W.; Yi, M.; Chen, Q.; Xu, C.; Cai, D.; Zhan, H. Metal−organic framework derived porous ternary ZnCo2O4 nanoplate arrays grown on carbon cloth as binder-free electrodes for lithium-ion batteries. Chem. Eng. J. 2018, 354, 454−462. (28) Liu, K.; Abass, M.; Zou, Q.; Yan, X. Self-assembly of biomimetic light-harvesting complexes capable of hydrogen evolution. Green Energy Environ. 2017, 2, 58−63. (29) Yan, X. Bio-inspired photosystem for green energy. Green Energy Environ. 2017, 2, 66. (30) Liu, K.; Xing, R.; Chen, C.; Shen, G.; Yan, L.; Zou, Q.; Ma, G.; Möhwald, H.; Yan, X. Peptide-induced hierarchical long-range order and photocatalytic activity of porphyrin assemblies. Angew. Chem. Int. Ed. 2015, 54, 500−505. (31) Li, C.; Lou, X.; Shen, M.; Hu, X.; Guo, Z.; Wang, Y.; Hu, B.; Chen, Q. High anodic performance of Co 1,3,5-benzenetricarboxylate coordination polymers for Li-ion battery. ACS Appl. Mater. Interfaces 2016, 8, 15352−15360. (32) He, T.; Xu, X.; Ni, B.; Lin, H.; Li, C.; Hu, W.; Wang, X. Metal− organic framework based microcapsules. Angew. Chem. Int. Ed. 2018, 130, 10305−10309. (33) Jiang, F.; Su, Q.; Li, H.; Yao, L.; Deng, H.; Du, G. Growth of ultrafine CuCo2O4 nanoparticle on graphene with enhanced lithium storage properties. Chem. Eng. J. 2017, 314, 301−310. (34) Li, Z.; Shao, M.; Zhou, L.; Zhang, R.; Zhang, C.; Wei, M.; Evans, D. G.; Duan, X. Directed growth of Metal-Organic frameworks and their derived Carbon-Based network for efficient electrocatalytic oxygen reduction. Adv. Mater. 2016, 28, 2337−2344. (35) Jadhav, H. S.; Pawar, S. M.; Jadhav, A. H.; Thorat, G. M.; Seo, J. G. Hierarchical mesoporous 3D flower-like CuCo2O4/NF for highperformance electrochemical energy storage. Sci. Rep. 2016, 6, 31120. (36) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Long-term cycling studies on 4V-cathode, lithium vanadium fluorophosphate. J. Power Sources 2010, 195, 5768−5774. (37) Bai, J.; Li, X.; Liu, G.; Qian, Y.; Xiong, S. Unusual formation of ZnCo2O4 3D hierarchical twin microspheres as a High-Rate and Ultralong-Life Lithium-Ion battery anode material. Adv. Funct. Mater. 2014, 24, 3012−3020. (38) Peng, C.; Chen, B.; Qin, Y.; Yang, S.; Li, C.; Zuo, Y.; Liu, S.; Yang, J. Facile ultrasonic synthesis of CoO quantum dot/graphene nanosheet composites with high lithium storage capacity. ACS Nano 2012, 6, 1074−1081. (39) Fu, Y.; Wei, Q.; Zhang, G.; Wang, X.; Zhang, J.; Hu, Y.; Wang, D.; Zuin, L.; Zhou, T.; Wu, Y.; Sun, S. High-performance reversible aqueous Zn-ion battery based on porous MnOx Nanorods coated by MOF-derived N-doped carbon. Adv. Energy Mater. 2018, 1801445. (40) Liu, S.; Zhang, S.; Xing, Y.; Wang, S.; Lin, R.; Wei, X.; He, L. Facile synthesis of hierarchical mesoporous CuxCo3‑xO4 nanosheets array on conductive substrates with high-rate performance for Li-ion batteries. Electrochim. Acta 2014, 150, 75−82. (41) Sheng, L.; Liang, S.; Wei, T.; Chang, J.; Jiang, Z.; Zhang, L.; Zhou, Q.; Zhou, J.; Jiang, L.; Fan, Z. Space-confinement of MnO nanosheets in densely stacked graphene: Ultra-high volumetric capacity and rate performance for lithium-ion batteries. Energy Storage Mater. 2018, 12, 94−102. (42) Wang, C.; Sun, S.; Zhang, L.; Yin, J.; Jiao, T.; Zhang, L.; Xu, Y.; Zhou, J.; Peng, Q. Facile preparation and catalytic performance characterization of AuNPs-loaded hierarchical electrospun composite fibers by solvent vapor annealing treatment. Colloids Surf. A 2019, 561, 283−291. (43) Sun, S.; Wang, C.; Han, S.; Jiao, T.; Wang, R.; Yin, J.; Li, Q.; Wang, Y.; Geng, L.; Yu, X.; Peng, Q. Interfacial nanostructures and acidichromism behaviors in self-assembled terpyridine derivatives Langmuir-Blodgett films. Colloids Surf. A 2019, 564, 1−9. (44) Huang, X.; Jiao, T.; Liu, Q.; Zhang, L.; Zhou, J.; Li, B.; Peng, Q. Hierarchical electrospun nanofibers treated by solvent vapor annealing as air filtration mat for high-efficiency PM2.5 capture. Sci. China Mater. 2019, 62, 423−436. (45) Xu, Y.; Ren, B.; Wang, R.; Zhang, L.; Jiao, T.; Liu, Z. Facile preparation of rod-like MnO nanomixtures via hydrothermal

approach and highly efficient removal of methylene blue for wastewater Treatment. Nanomaterials 2019, 9, 10. (46) Zhan, F.; Wang, R.; Yin, J.; Han, Z.; Zhang, L.; Jiao, T.; Zhou, J.; Zhang, L.; Peng, Q. Facile solvothermal preparation of Fe3O4−Ag nanocomposite with excellent catalytic performance. RSC Adv. 2019, 9, 878−883. (47) Huang, X.; Wang, R.; Jiao, T.; Zou, G.; Zhan, F.; Yin, J.; Zhang, L.; Zhou, J.; Peng, Q. Facile Preparation of Hierarchical AgNPLoaded MXene/Fe3O4/Polymer Nanocomposites by Electrospinning with Enhanced Catalytic Performance for Wastewater Treatment. ACS Omega 2019, 4, 1897−1906. (48) Chen, K.; Jiao, T.; Li, J.; Han, D.; Wang, R.; Tian, G.; Peng, Q. Chiral Nanostructured Composite Films via Solvent-Tuned SelfAssembly and Their Enantioselective Performances. Langmuir 2019, 35, 3337−3345. (49) Wang, C.; Yin, J.; Wang, R.; Jiao, T.; Huang, H.; Zhou, J.; Zhang, L.; Peng, Q. Facile preparation of self-assembled polydopamine-modified electrospun fibers for highly effective removal of organic dyes. Nanomaterials 2019, 9, 116. (50) Guo, R.; Wang, R.; Yin, J.; Jiao, T.; Huang, H.; Zhao, X.; Zhang, L.; Li, Q.; Zhou, J.; Peng, Q. Fabrication and highly efficient dye removal characterization of beta-cyclodextrin-based composite polymer fibers by electrospinning. Nanomaterials 2019, 9, 127. (51) Feng, Y.; Jiao, T.; Yin, J.; Zhang, L.; Zhang, L.; Zhou, J.; Peng, Q. Facile Preparation of Carbon Nanotube-Cu2O Nanocomposites as New Catalyst Materials for Reduction of p-Nitrophenol. Nanoscale Res. Lett. 2019, 14, 78. (52) Chen, K.; Yan, X.; Li, J.; Jiao, T.; Cai, C.; Zou, G.; Wang, R.; Wang, M.; Zhang, L.; Peng, Q. Preparation of Self-Assembled Composite Films Constructed by Chemically-Modified MXene and Dyes with Surface-enhanced Raman Scattering Characterization. Nanomaterials 2019, 9, 284. (53) Chen, K.; Li, J.; Zhang, L.; Xing, R.; Jiao, T.; Gao, F.; Peng, Q. Facile synthesis of self-assembled carbon nanotubes/dye composite films for sensitive electrochemical determination of Cd(II) ions. Nanotechnology 2018, 29, 445603. (54) Guo, R.; Jiao, T.; Li, R.; Chen, Y.; Guo, W.; Zhang, L.; Zhou, J.; Zhang, Q.; Peng, Q. Sandwiched Fe3O4/carboxylate graphene oxide nanostructures constructed by layer-by-layer assembly for highly efficient and magnetically recyclable dye removal. ACS Sustainable Chem. Eng. 2018, 6, 1279−1288. (55) Liu, K.; Yuan, C.; Zou, Q.; Xie, Z.; Yan, X. Self-Assembled Zinc/Cystine-Based Chloroplast Mimics Capable of Photoenzymatic Reactions for Sustainable Fuel Synthesis. Angew. Chem. Int. Ed. 2017, 56, 7876−7880. (56) Liu, K.; Xing, R.; Li, Y.; Zou, Q.; Möhwald, H.; Yan, X. Mimicking primitive photobacteria: sustainable hydrogen evolution based on peptide-porphyrin co-assemblies with a self-mineralized reaction center. Angew. Chem. Int. Ed. 2016, 55, 12503−12507. (57) Yin, Y.; Ma, N.; Xue, J.; Wang, G.; Liu, S.; Li, H.; Guo, P. Insights into the role of poly(vinylpyrrolidone) in the synthesis of palladium nanoparticles and their electrocatalytic properties. Langmuir 2019, 35, 787−795.

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