Article pubs.acs.org/JPCC
CuO Necklace: Controlled Synthesis of a Metal Oxide and Carbon Nanotube Heterostructure for Enhanced Lithium Storage Performance Yin Zhang,†,‡ Minwei Xu,*,† Fei Wang,† Xiaoping Song,*,† YuHuang Wang,‡ and Sen Yang† †
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an JiaoTong University, Xi’an 710049, China ‡ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: Metal oxide and nanocarbon heterostructures provide a powerful strategy to materials innovation because judicious combination of different materials may afford collective properties that are otherwise unattainable. However, design and fabrication of spatially patterned heterostructures has been still challenging. Here, we develop a facile approach to synthesize a CuO and carbon nanotube necklace-like heterostructure in solution for enhanced lithium storage performance. Observations confirm that propagative sidewall alkylcarboxylation of carbon nanotubes in liquid ammonia allows the CuO nanospheres to grow on the functional defect sites introduced by the functionalization. Such a method does not require surfactants and is highly scalable. The resulting materials show high lithium ion storage capacities (stable at 500 mAh g−1) and enhanced cycling performance as a lithium-ion battery anode. The improved performance is attributed to the necklace-like nanostructure, which provides an electrical channel as well as excess room to accommodate the volume changes of CuO during electrochemical cycling.
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INTRODUCTION Recently, heterostructures of carbon nanotubes (CNTs) and metal oxides present great promise and opportunity for new generation materials with improved properties as well as fresh technological applications, such as lithium batteries, super capacitor electrodes, and gas sensing,1 rendering it significant. Metal oxide nanostructures are ideal systems for exploring novel phenomena at nanoscale and the dimensionality dependence of nanostructure properties for potential applications.2 The metal oxide nanostructures not only inherit the well-known properties from their bulk form that are proven useful as catalyst, sensors, and photodetection but also possess properties associated with their various morphologies and size confinement.3 The novel properties originate from the more complex electronic properties and crystal structures of metal oxides compared to common materials. Cupric oxide (CuO), for instance, is an important p-type of 3d transition-metal oxide that has been widely investigated owing to its potential applications in sensors, catalysts, and energy storage.4 It has aroused special attention as promising anode materials in lithium ion batteries (LIBs) because of its high theoretic capacity (670 mAh g−1). CNTs are a remarkable new class of nanomaterials, because essentially, they are quasi 1D tubelike materials with a reproduced honeycomb structure.5 Their outstanding electrical, optical, and mechanical properties have received steadily growing interests, generating diverse applications.6 It is then expected that the addition of CNTs in the © XXXX American Chemical Society
dispersion of the oxide nanostructures presents a new strategy to achieve collective properties that are otherwise unattainable. Several methodologies have been exploited toward the design of CNTs and metal oxide heterostructures during the past decade. For instance, ZnO quantum dots can self-assemble on the surface of acid-treated multiwalled carbon nanotubes (MWNTs) via a solution-process method in dimethylformamide (DMF) using zinc acetate dehydrate as the precursor under wild reaction conditions.7 Large-scale NiO/MWNT nanocomposites have been prepared by a direct thermal decomposition method using N-methyl-2-pyrrolidone (NMP) as solvent and being refluxed at 180 °C, and the obtained amorphous nickel oxide nanoparticles uniformly coat on the MWNT.8 It also has been reported that, by electrolytic oxidation of TiCl3 solution, the single-walled carbon nanotube bundles are covered by nanocrystal TiO2.9 However, a majority of the methodologies fail to control the morphology of the heterostructures. Lots of methods also require either a nonaqueous solvent (such as DMF or ethylene glycol) or using a surfactant to facilitate dispersions, which may have a safety issue and introduce unnecessary impurities. These problems in utilization are particularly essential for CNTs since raw CNTs are inherently difficult to disperse in solvents Received: March 15, 2013 Revised: May 10, 2013
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and chemical inert.10 Though massive chemical and physical modifications have been attempted to solve these problems, the fundamental understanding of nanotubes is still quite insufficient. We have previously shown that Billups−Birch sidewall alkylcarboxylation of CNTs11 occurs predominately through a defect-activated propagation mechanism: the mobile electrons are preferentially trapped around the defect sites. Meanwhile, the trapped electrons further promote local reactions at the vicinity of the defect, and the propagation of the reaction fronts quickly becomes confined in the tubular direction, leading to a clustered distribution of the covalently attached functional groups. This propagation chemistry allows one to not only control the spatial pattern on the disturbed surface of CNTs differing from the intact regions but also render the individual CNTs highly soluble in water, reducing the difficulty in processing CNTs subsequently. Here, we describe a facile synthesis of CuO necklaces containing CuO nanospheres on CNT strings for improved electrochemical performance. Propagative sidewall alkylcarboxylation of carbon nanotubes introduces spatially controlled functional groups that serve as the nucleation sites for the growth of CuO nanospheres in solutions. Additionally, attached carboxyl groups largely render carbon nanotubes soluble in water, making it possible to grow CuO nanospheres on the nanotubes’ sidewalls without the addition of any surfactant. The improved electrochemical performance of the heterostructure as anode materials for LIBs has also been investigated here. It exhibits much higher reversible capacities and more a stable cyclability than the pure CuO nanostructures without CNT networks.
The morphology of CuO necklaces was characterized using a JSM-7000F (JEOL) scanning electron microscope (SEM) at 30 kV. High-resolution transmission electron microscopy (HRTEM) was carried out using a JEM-2100 (JEOL) at 200 keV. An X-ray diffraction (XRD) pattern was obtained on a Bruke D8-Advanced X-ray diffractometer with Cu Kr radiation. Data were collected from 20° to 80° with an interval of 0.01° per step. The Raman spectrum was taken on a LabRAMHR800 (HORIBA JOBIN YVON) using a 633 nm He−Ne laser. Each sample of data was collected and averaged at three random selected spots.
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RESULTS AND DISCUSSION From the SEM image of Figure 1a, it was clearly observed that the f-MWNTs and CuO heterostructure ( f-MWNT@CuO)
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METHODS In a typical procedure, a Billups−Birch sidewall alkylcarboxylation of carbon nanotubes was employed as previously reported.12 The reaction was placed in a 250 mL sized flask cooled by a mixture of dry ice and acetone. A 50 mg portion of raw MWNTs was first dispersed in 70 mL of liquid ammonia, followed by the addition of 145 mg of sodium. After rigorous stirring for 10 min, 6-bromo-hexanoic acid (1.625 g) was added into the blue homogeneous solution and allowed to react for 50 min. To get highly functionalized MWNTs ( f-MWNTs), sodium and 6-bromo-hexanoic acid were alternatingly added to the mixture repeatedly to react for 10 and 50 min, respectively. The solution remained stirring overnight to dry until the liquid ammonia evaporated. The residual was then washed by filtration and sonication to remove impurities and some excess salts. The f-MWNT water solution was prepared by repeated extractions from hexane using basic water (NaOH solution, PH = 11). The CuO necklace heterostructuress were prepared by a simple liquid method.13 Briefly, concentrated ammonia solution (5 mL, 25−28%) and deionized water (5 mL) were first mixed into a 100 mL sized beaker. CuCl2 solution (0.1 M, 100ul) was added dropwise to the beaker to generate the Cu(NH3)42+ complex. The basic aqueous solution of f-MWNTs (5 mL) obtained previously was added finally. After shaking to homogeneity, the mixture was placed in a water bath at 70 °C for 1.5 h (no stirring was applied). During the reaction, the solution gradually changed from light blue to colorless, and a black precipitate was then observed. The remaining precipitate was washed with ethanol and centrifuged several times. Finally, it was collected and dried in an oven at 55 °C overnight.
Figure 1. SEM images of f-MWNT@CuOs obtained by ammonia induced at 1.5 h.
with a homogeneous morphology were successfully synthesized by our two-step wet chemistry. All of the CuO nanospheres were observed growing on the surface of f-MWNTs. Instead of aggregation, the CuO nanospheres were well-dispersed and aligned in sequences along the nanotubes. The CuO nanospheres show an average diameter of 140 nm, with a Gauss distribution (Figure S2, Supporting Information). The highmagnification SEM image (Figure 1b) shows that a certain part of the f-MWNTs’ sidewalls was completely coated by spherical CuO particles. On the contrary, some other regions of the carbon nanotubes’ surface were naked without any growth of CuO, which might suggest that the nucleation of CuO nanospheres on the f-MWNTs’ sidewalls was spatially controlled. To further reveal the formation mechanism of fMWNT@CuOs, we had observed some enlightening images, which are shown in Figure 1c,d. In Figure 1c, the small-sized CuO particles, which are less than 80 nm, were found attaching to only one side of the f-MWNTs’ surface. In the red circle of B
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Figure 1c, the locations of each primary CuO particles are apparently very close. It seems that they are likely to aggregate at the same axial direction rather than being uniformly distributed along the tube. For the larger particles in Figure 1d, except for a small gap left (pointed by the arrows), the fMWNT was almost wrapped by the CuO nanoparticles. In addition, sonication was applied during the washing and preparation of samples, suggesting that there is a strong adhesive between CuO and the f-MWNTs’ sidewalls in our case. A representative image (Figure 2a) clearly confirms the necklace-like heterostructure. The CuO nanospheres were
Figure 3. Raman spectra of raw-MWNTs, f-MWNTs, and f-MWNT@ CuOs, respectively; the spectra were normalized at the G peak (1597 cm−1).
Normally, a Cu(NH3)n2+ complex is first generated in the solution containing CuCl2 and excess ammonia. With a continuous decrease of pH and evaporation of ammonia, Cu(NH3)n2+ hydrolyze to Cu(OH)2, which then dehydrate to CuO in a dynamic environment.13 In our study, the major driving force for the generation of the heterostructure is believed to be the electrostatic force between the functional groups on the sidewalls of f-MWNTs and mobile metal cations in the solution. The schematic reaction mechanism of the synthesis of f-MWNT@CuOs is depicted in Figure 4. As shown in the schematic drawn in Figure 4, Birch reduction was applied to functionalize the raw MWNTs to introduce a surfactant-free path for synthesis of f-MWNT@ CuOs. In comparison, Birch reduction is easily able to assist carboxylic groups attaching on the defect sites of CNTs as well as maintain the most pristine sp2 structures. Followed by the attachment on the defects, the functional groups begin to propagate around the defect centers with continuation of the reaction.11 Meanwhile, the mobile surface electrons move and localize near these functional groups. Previously, through applying a gold substrate enhanced SEM technique, we had successfully resolved the intact and functionalized regions alternatively distributed on the functionalized single-walled carbon nanotubes.17 Beyond that, this chemistry further allows us to obtain highly water-soluble and stable individual CNTs in solution without the addition of surfactant.18 In the f-MWNT solution, Cu(NH3)n2+ cations form and aggregate at the alkylcarboxyl functional bands on f-MWNTs due to the electrostatic interactions between Cu(NH3)n2+ cations and −COO− groups instead of moving randomly in the solution. It is also known that carboxyl groups have exhibited the strongest biosorption to the metal ions (Cu2+, Cr3+, and Cd2+) during the cation exchange process.19 With the evaporation of ammonia, Cu(NH3)n2+ gradually dehydrates to form spherical CuO, which was owing to the strong ammonia passivation effect under excess ammonia conditions.13 For these reasons, we believe that CuO is nucleated on the functional regions of fMWNTs. Three major chemical reactions occur during this process, as follows:
Figure 2. (a) TEM image of one individual f-MWNT@CuO. (b) HRTEM image of the interface of CuO nanospheres and f-MWNTs.
selectively growing and wrapping at some certain locations of the sidewalls of f-MWNTs. In Figure 2b, the HRTEM image reveals that the CuO nanospheres tightly decorated the surface of CNTs, evidenced by the lattice orientations of graphite and CuO. The observation confirms that the lattice fringe spacing of CuO is 0.247 nm, which is in agreement with the [1̅11] direction of the monoclinic structure CuO.14 The XRD pattern (Figure S1, Supporting Information) identifies that the products belong to the monoclinic phase CuO (JCPDF 801268). It is also suggestive of the high purity of the CuO nanostructures in our study. Raman spectra (Figure 3) show that the integrated area of the disorder-induced mode (D band) near 1350 cm−1 with respect to that of the tangential mode (G band) close to 1600 cm−1, which is the so-called D/G ratio, resulted in a significant increase after functionalization. The increased D/G ratio suggests that the covalent chemistry has successfully introduced the sp3 defect centers, and allows the functional groups attaching on these sites. In addition, two new peaks at 301 and 597 cm−1 arise after synthesis of the heterostructure. The two peaks correspond to the phonon vibrations from the Raman signal of the monoclinic structure CuO with a nanometer size,15 which also provides strong empirical support for the successful synthesis of necklace-like f-MWNT@CuOs It is noteworthy that no additional chemicals, such as surfactants or reducing agents, were employed in this process, which highlights the fundamental difference and extraordinary advantage of our approach over the other studies.16 Here, on the basis of the experimental observation and previous studies, we propose that CuO nanospheres were nucleated on the defect sites in the presence of carboxyl functional groups and subsequently grew into spherical nanoparticles.
Cu 2 + + n NH3 = Cu(NH3)n 2 + C
(1)
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Figure 4. Schematic illustration for the procedure and formation mechanism of f-MWNT@CuOs.
Figure 5. SEM images of different spots, showing that the CuO nanospheres were generated at the ends of CNTs.
Cu(NH3)n 2 + + 2OH− = Cu(OH)2 + n NH3
(2)
Cu(OH)2 = CuO + H 2O
(3)
nanoparticles are formed, the surface energy and surface passivation of ammonia (the initial ratio of NH3/Cu2+ is more than 8) will dominate the growth mechanism in the growth. When the size of f-MWNT@CuO is larger than 140 nm and the majority of the ammonia is evaporated, CuO begins to coat on the unfunctionalized sidewalls of f-MWNTs serving as a bridge connecting the two CuO nanospheres rather than continuing growth on the spheres. We believe that the surface energy is no longer the main driving force at the last stage, which makes CuO grow along the intact sidewalls. As anode materials, common CuO materials experience a large volume change associated with Li+ insertion and extraction processes, which cause the pulverization of the electrode, leading to rapid deterioration in capacity.20 To address these issues, it is critically important to reduce the dimensionality of active materials or introduce other media. As we know, introducing carbon materials (such as CNTs and graphene) is considered as a prospective strategy, because the carbon materials are capable of accommodating the irreversible volume changes as well as providing a 3D matrix with a continuous electron transport channel. Multiple experiments have been employed to synthesize a CuO/carbon nanocomposite, achieving improved lithium storage properties.16 Nevertheless, to the best of our knowledge, the highly controlled synthesis of such a unique heterostructure is obtained for the first time. To examine the potential of CuO necklaces as anode materials, we fabricated two-electrode Swagelok-type cells to test their electrochemical performance. The working electrodes were prepared by mixing the CuO necklace, polyvinylidene fluoride (PVDF), and carbon black at a weight ratio of 70:15:15 in NMP. The details for the test cells were described in our previous report.21 Figure 6a displays the selected discharge−charge curves of the f-MWNT@CuOs measured between 0.01 and 3.0 V at a current density of about 160 mA g−1. Three plateaus at the potentials of 2.0−2.5, 1.5−2.0, and 0.5−0.8 V were observed in the discharge curves. The three plateaus arise from the
Instead of homogeneously coating on the entire surface, the CuO grew around alkylcarboxyl functional bands. As a result, it transformed into a necklace-like heterostructure in which CuO nanospheres act as the jewelry and f-MWNTs as the chains. The CuO nanospheres completely wrap along the disturbed sidewalls and leave the intact sections alone. This alternating (jewelry and chain) spatial pattern is associated with the alternating contrast (dark and bright) image observed using the functionalized single-walled carbon nanotubes.17 Moreover, since the functional groups were no longer exposed in water, the f-MWNT@CuOs were expected to be precipitated at the end of the reaction as anticipated. Further observation discovered that numerous CuO nanospheres were also nucleated at the ends of f-MWNTs (see Figure 5). It is wellknown that the defect density is higher at the ends of CNTs due to the higher reactivity. This is also consistent with the defect imaging observed by SEM. The necklace-like f-MWNT@CuOs were observed when a reproduced experiment reacted at 1.5 h. Surprisingly, a new morphology was found when the reaction time increased to 2 h (Figure S3a, Supporting Information). In Figure S3b (Supporting Information), the carbon nanotubes have been completely coated by CuO, but the shape of CuO nanospheres also can be observed. This observation suggests that the CuO nanospheres do not grow larger when the reaction time is increased. Instead, the newly generated CuO begins to grow from the edges of CuO nanospheres and coat on the sidewalls of f-MWNTs along the intact sections. It is known that the crystal growth is a complex process and mainly determined by the nucleation site, driving force, and the surface energy. At the initiation of the reaction, functional groups play a major role in the formation of the heterostructure. Once the CuO primary D
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Figure 7. Nyquist plots of the fresh cells containing f-MWNT@CuO and CuO electrodes, respectively. The inset presents the equivalent circuit model.
Rct are the Ohmic resistance and the charge-transfer resistance, respectively. CPE represents the constant phase element, and Zw is the Warburg impedance. According to the fitted results, it is found that these two electrodes show a similar Re (about 6 ohm). However, the Rct of f-MWNT@CuO (120 ohm) is much smaller than that for pure CuO (400 ohm), indicating enhanced electron and lithium transport for f-MWNT@CuO. We believe that the enhanced electrochemical performance of the f-MWNT@CuO is primarily attributed to the unique architecture. During the lithium insertion−extraction cycles, the necklace-like structure could efficiently accommodate the volume change of CuO as well as good electronic conductivity in the active materials, which thus prevent the pulverization of the electrode. The “bridge” f-MWMTs can be regarded as electron carriers due to their extraordinary conductivity. Unfortunately, the f-MWNT@CuO anode showed a continued fading of capacity when more cycles were tested (Figure S5, Supporting Information). The inset of Figure S5 shows the SEM image of the f-MWNT@CuO electrode after 100 cycles. Compared with Figure 1, it can be seen that the morphology of the CuO necklace only partially remained after lithiation/ delithiation cycles (marked by arrows in Figure S5, Supporting Information). Further works focus on the rational fabrication of other nanostructured CuO hybrids with CNTs, which is an urgent demand.
Figure 6. (a) The 1st, 2nd, 5th, and 10th charge/discharge voltage profiles of f-MWNT@CuOs. (b) Cycle performance of f-MWNT@ CuOs.
formation of an intermediate phase, the reductive reaction from CuO to Cu2O, and further decomposition to metal Cu, respectively.22 In the charge curves, the voltage increases gradually, which can be attributed to the reversible oxidation of metal Cu to CuO. These results are consistent with previously reported CuO anode materials for LIBs.20 The electrochemical reaction progress can be expressed as CuO + 2Li+ + 2e− ↔ Cu + Li 2O
(4)
In addition, the high degree overlapping of the latter discharge/charge curves demonstrated an improved cyclic stability. The cycling performance of the f-MWNT@CuOs is shown in Figure 6b. The initial discharge and charge capacities of the fMWNT@CuO anode were 1020 and 701 mAh g −1 , respectively. The superior initial discharge capacity, which is 300 mAh g−1 more than the theoretical value of CuO, could have contributed to the decomposition of electrolyte and formation of solid electrolyte interphase (SEI) film.23 Subsequently, the CuO necklace reveals a slow capacity fading, and a high capacity above 500 mAh g−1 is still retained after 50 cycles. Moreover, our facile product exhibits a nearly perfect Coulombic efficiency, which increases at first and then steadily keeps higher than 99%. The cycle performance is significantly improved in comparison with the similarly prepared CuO sample without addition of f-MWNT (Figure S4, Supporting Information). Figure 7 shows the Nyquist plots of the f-MWNT@CuO and CuO anode before cycling. These impedance data were analyzed by fitting to an equivalent electrical circuit, similar to the circuit employed for other oxide electrodes.24,25 Re and
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CONCLUSION In summary, we have proposed a surfactant-free approach to synthesize necklace-like f-MWNT@CuO on the basis of a propagative chemical functionalization mechanism. By introducing carboxylic functional bands on f-MWNTs, the CuO grew on the defects sites, forming a necklace-like heterostructure. The heterostructures significantly improved the electrochemical performance as anode materials for lithium batteries, exhibiting ideal Columbic efficiency and good cycling performance by a factor of nearly 3, in comparison with the CuO control.
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ASSOCIATED CONTENT
S Supporting Information *
XRD pattern of products, distributions of obtained CuO nanospheres, SEM images of f-MWNT@CuO with different reaction times, variation in charge capacity versus cycle number for f-MWNT@CuOs and CuO, and 100 cycles of the fE
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(15) Xu, J. F.; Ji, W.; Shen, Z. X.; Li, W. S.; Tang, S. H.; Ye, X. R.; Jia, D. Z.; Xin, X. Q. Raman Spectra of CuO Nanocrystals. J. Raman Spectrosc. 1999, 30 (5), 413−415. (16) Zheng, S.-F.; Hu, J.-S.; Zhong, L.-S.; Song, W.-G.; Wan, L.-J.; Guo, Y.-G. Introducing Dual Functional CNT Networks into CuO Nanomicrospheres toward Superior Electrode Materials for LithiumIon Batteries. Chem. Mater. 2008, 20 (11), 3617−3622. (17) Zhang, Y.; Wang, Y. H. Gold-Substrate-Enhanced Scanning Electron Microscopy of Functionalized Single-Wall Carbon Nanotubes. J. Phys. Chem. Lett. 2011, 2 (8), 885−888. (18) Deng, S. L.; Brozen, A. H.; Zhang, Y.; Piao, Y. M.; Wang, Y. H. Diameter-Dependent, Progressive Alkylcarboxylation of Single-Walled Carbon Nanotubes. Chem. Commun. 2011, 47 (2), 758−760. (19) Chojnacka, K.; Chojnacki, A.; Gorecka, H. Biosorption of Cr3+, Cd2+ and Cu2+ Ions by Blue−Green Algae Spirulina sp.: Kinetics, Equilibrium and the Mechanism of the Process. Chemosphere 2005, 59 (1), 75−84. (20) Xiang, J. Y.; Tu, J. P.; Zhang, L.; Zhou, Y.; Wang, X. L.; Shi, S. J. Self-Assembled Synthesis of Hierarchical Nanostructured CuO with Various Morphologies and Their Application as Anodes for Lithium Ion Batteries. J. Power Sources 2010, 195 (1), 313−319. (21) Xu, M. W.; Wang, F.; Zhao, M. S.; Yang, S.; Song, X. P. Molten Hydroxides Synthesis of Hierarchical Cobalt Oxide Nanostructure and Its Application as Anode Material for Lithium Ion Batteries. Electrochim. Acta 2011, 56 (13), 4876−4881. (22) Debart, A.; Dupont, L.; Poizot, P.; Leriche, J.; Tarascon, J. A Transmission Electron Microscopy Study of the Reactivity Mechanism of Tailor-Made CuO Particles toward Lithium. J. Electrochem. Soc. 2001, 148 (11), A1266−A1274. (23) Laruelle, S.; Grugeon, S.; Poizot, P.; Dolle, M.; Dupont, L.; Tarascon, J. On the Origin of the Extra Electrochemical Capacity Displayed by MO/Li Cells at Low Potential. J. Electrochem. Soc. 2002, 149 (5), A627−A634. (24) Zhang, C.; Peng, X.; Guo, Z.; Cai, C.; Chen, Z.; Wexler, D.; Li, S.; Liu, H. Carbon-Coated SnO2/Graphene Nanosheets as Highly Reversible Anode Materials for Lithium Ion Batteries. Carbon 2012, 50 (5), 1897−1903. (25) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Subba Rao, G. V.; Chowdari, B. V. R. α-Fe2O3 Nanoflakes as an Anode Material for Li-Ion Batteries. Adv. Funct. Mater. 2007, 17 (15), 2792− 2799.
MWNT@CuO heterostructure. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (M.X.)
[email protected]. (X.S.) E-mail: xpsong@ mail.xjtu.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (Nos. 51222104 and 51071116), and the Fundamental Research Funds for Central Universities and the Science. Y.Z. acknowledges a fellowship support provided by the State Scholarship Council of China. Y.H.W acknowledges support of the University of Maryland and the ACS Petroleum Research Fund.
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