3D Hierarchical Mesoporous Flowerlike Cobalt Oxide Nanomaterials

Article pubs.acs.org/JPCC

3D Hierarchical Mesoporous Flowerlike Cobalt Oxide Nanomaterials: Controllable Synthesis and Electrochemical Properties Peng Liu, Qingli Hao,* Xifeng Xia, Lei Lu, Wu Lei, and Xin Wang Key Laboratory for Soft Chemistry and Functional Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China S Supporting Information *

ABSTRACT: The controllable synthesis of 3D inorganic nanomaterials has attracted much attention. Here, 3D hierarchical mesoporous cobalt oxide (Co3O4) nanomaterials with different morphologies are successfully prepared by a facile and surfactant-free solvothermal synthesis of cobalt carbonate hydroxide and subsequent calcination. The morphology of the precursor can be well controlled by adjusting the experimental conditions, such as the proportion of solvent, the amount of urea, and reaction time, thus controlling the structures of the cobalt oxide. Two different plausible growth mechanisms of the red-headed calliandra-like and dandelion-flower-like precursors have been investigated by transmission electron spectroscopy (TEM) of the timedependent products, respectively. The hierarchical mesoporous materials are tested as the anode materials for lithium ion batteries (LIBs). In particular, the hierarchical mesoporous dandelion-flower-like cobalt oxide shows good rate performance and high specific capacities of 1298 mA h g−1 at the first cycle and 1204 mA h g−1 over 20 cycles, whereas those of the red-headed calliandra-like mesoporous Co3O4 are 1340 and 833 mA h g−1, respectively. In addition, the as-prepared mesoporous cobalt oxides are evaluated as the catalyst for the oxygen reduction reaction (ORR). hierarchical Fe3O4−Co3O4 yolk−shell nanostructures, in which no templates or surfactants were used.4 Moreover, other hierarchical metal oxides with special morphologies were also fabricated by the solvothermal method.11−13 It is an alternative approach to form the hierarchical metal oxides. Although many hierarchical metal oxides with various morphologies were prepared, a fine controllable and facile synthesis of novel structure still remains a big challenge. As an important magnetic p-type semiconductor, spinel cobalt oxide (Co3O4) has attracted great attention because it is a very promising material with broad practical applications in many fields, such as magnetic materials,14 electrochromic devices,15 catalysts,16 sensors,17 oxygen reduction reaction,18,19 supercapacitors,6,20−23 and Li ion batteries.5,24−33 Various approaches, covering microemulsion,34 spray pyrolysis,35 sol− gel,36 and hydro/solvo-thermal method,11 have been adopted to control the synthesis of cobalt oxide with special structures. As we all know, not only the preparation methodology but also the morphologies and structures, such as crystallite sizes, shapes, assemblies, and porosity, would strongly affect the performances of as-prepared metal oxides. In particular, the mesoporous structure has been intensively studied because of

1. INTRODUCTION Controlling the structure and morphology of nanomaterials to tailor their chemical and physical properties has become an essential issue in materials science. As a special part of micro-/ nanomaterials, transition metal oxides with three-dimensional (3D) hierarchical architecture are always attractive for functional materials owing to their unique structure-dependent properties, which have a wide application in various fields, such as drug delivery, catalysis, optical devices, water treatment, sensors, lithium ion batteries, and supercapacitors.1−9 The general methodology to achieve 3D hierarchical structures is template-assisted.5,7−10 For instance, three-dimensional composites consisting of carbon nanotube and various metal oxide nanoparticles have been fabricated by utilizing the vertically aligned carbon nanotube patterns as the template.7 Wang and co-workers reported the synthesis of hierarchical multishell Co3O4 hollow microspheres based on the carbon microspheres as the sacrificial template.5 Zhong et al. reported a flowerlike hierarchical iron oxide by using the tetrabutylammonium bromide as a soft template.10 These template-assisted methods have demonstrated effectiveness in forming hierarchical metal oxides with various unique shapes. However, most of the procedures are tedious as a result of adding or removing the templates from the reaction system, which makes the specification of the reaction process more challenging. Recently, Ye and co-workers reported a simple solvothermal synthesis of © XXXX American Chemical Society

Received: February 8, 2015 Revised: April 2, 2015

A

DOI: 10.1021/acs.jpcc.5b01315 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) Low- and (b) high-magnification SEM images and (c) low- and (d) high-magnification TEM images of red-headed calliandra-like precursor Co0:40. (e) HRTEM image of the square marked in part d and the corresponding FFT patterns are shown in image e1 and e2. (f) Low- and (g) high-magnification SEM images and (h) low- (i) high-magnification TEM images of dandelion-flower-like precursor Co20:20. (j) HRTEM image of the square marked in part i and the corresponding FFT pattern is shown in image j1. Image b and g are the magnifying photos of the dot elliptic region in image a and f, respectively. Insets of parts a and f show red-headed calliandra and dandelion flower found in nature, respectively.

high specific capacity. Moreover, the oxygen reduction reaction catalytic activities of the as-prepared mesoporous Co3O4 hierarchical architectures were also investigated.

the high surface area and very active site, which could improve the properties.17,18,22,28,37 Importantly, once the mesoporous materials are applied in supercapacitors or Li ion batteries, they would exhibit good electrochemical performance attributed to the fact that the mesoporous structure may facilitate the penetration of electrolyte and the ion diffusion. For example, mesoporous cobalt oxide nanowires showed a capacitance of 240 F g−1 and excellent cycle performance.38 Mesoporous Co3O4 nanobelts achieved reversible capacities of 1400 mA h g−1.24 However, the most mesoporous cobalt oxides are 1D or 2D micro-/nanostructures. Although there are few reports on the three-dimensional hierarchical mesoporous Co3O4, the resulting structures are limited to cubes, cages, and capsules.28,37,39,40 Therefore, fabrication of novel 3D mesoporous Co3O4 micro/nano architectures is continuously pursued. Herein, the 3D mesoporous Co3O4 hierarchical architectures with red-headed calliandra-like and dandelion-flower-like morphologies were successfully obtained after calcining cobalt carbonate hydroxide at 400 °C for 3 h in air. The cobalt carbonate hydroxide precursors with different subunits were prepared by a facile and surfactant-free solvothermal method at a relatively low temperature (90 °C). By controlling the ratio of ethylene glycol and water, the morphology of the precursors can be decorated from red-headed calliandra-like spheres to dandelion flowers. The relationship between the precursors’ morphology and different reaction parameters has been discussed in detail. Two different formation mechanisms have been proposed to understand the growth process of the precursors with different morphologies. When evaluated as an anode material for lithium ion batteries, the as-prepared hierarchical Co3O4 with mesoporous structure manifested

2. EXPERIMENTAL SECTION 2.1. Synthesis. All the chemicals were analytical grade and used in the present work without further purification. In a typical procedure, 2 mmol of Co(NO3)2·6H2O and 10 mmol of urea were dissolved in 40 mL of mixed solvent of ethylene glycol (20 mL) and deionized water. After stirring for 15 min, the as-obtained transparent solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 90 °C for 12 h. After cooling to room temperature, the product was collected by centrifugation, rinsed with deionized water several times, and dried in an oven at 60 °C for 6 h to obtain the precursor. Finally, the precursor was annealed at 400 °C for 3 h to yield the Co3O4. Here, the precursor was denominated as Coa:b, where a:b refers to the value of ethylene glycol/water (volume/volume) in the reaction system; the total volume was fixed at 40 mL. Further calcination of the Coa:b precursor at 400 °C for 3 h in air produced the Co3O4, which were assigned as Coa:b-annealed. 2.2. Characterization. The morphology and structure were observed by a field emission scanning electron microscope (FESEM, Hitachi S-4800) and a JEOL JEM-2100 transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra were collected on a MB154S-FTIR spectrometer using pressed KBr pellets. The powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å). The Brunauer−Emmett− Teller (BET) tests were determined by using a Micrometrics TriStar II 3020 nitrogen adsorption apparatus at 77 K. B


Article


(JCPDS card no. 48-0083). The corresponding fast Fourier transformation (FFT) patterns (Figure 1e1, e2) and highly clear crystal lattice demonstrate the nanoneedles of the novel 3D red-headed calliandra-like spheres are highly crystalline. As shown in Figure 1j, the spacing of the clear lattice fringes for Co20:20 is 0.51 nm, corresponds to the (020) plane of orthorhombic Co(CO3)0.5(OH)·0.11H2O (JCPDS card no. 48-0083). The relevant well-ordered dot pattern in the FFT pattern of Co20:20 (Figure 1j1) further demonstrates the characteristic of a single crystal. To further confirm the structure of the as-synthesized sample, XRD was also conducted. From the XRD pattern in Figure 2a, we can see that Co0:40 and Co20:20 have the same

Thermogravimetric analysis (TGA) was carried out under air flow with a temperature ramp of 10 °C min−1. 2.3. Electrochemical Measurement for the Lithium Storage Property. Electrochemical tests were examined using CR2016 coin type cells. The working electrodes were composed of active material, conductive carbon black (SuperP-Li), and polyvinylidene fluoride (PVDF) in a weight ratio of 7:2:1. Cells were assembled in an argon-filled glovebox with lithium foil as both the counter and reference electrodes, and a polypropylene membrane (Celgard 2400) served as the separator. The electrolyte used was 1 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume). The charge−discharge tests were performed on a multichannel battery testing system (LAND CT2001A) between 0.01 and 3.00 V. 2.4. Catalytic Measurements for the Oxygen Reduction Reaction (ORR). Electrochemical measurements were carried out with a CHI 760D electrochemical workstation. A conventional, three-electrode cell consisting of a glassy carbon electrode (GCE) with a diameter of 4 mm as the working electrode, Pt wire as the counter electrode, and Hg/HgO (1.0 M KOH) as the reference electrode was used. The working electrode was modified with a catalyst layer by dropping a suitable amount of catalyst ink on the GCE. The catalyst ink was fabricated by ultrasonically dispersing 1 mg of the annealed samples in a 500 μL solution (250 μL of ethanol, 237.5 μL of water and 12.5 μL of 5 wt % Nafion solution) for 30 min to form a homogeneous solution. Ten microliters of the catalyst ink was loaded onto the GCE, which was then dried in air. CV curves were carried out in a 0.1 M KOH solution at a scan rate of 50 mV s−1. The linear sweep voltammetry (LSV) was recorded at a scan rate of 5 mV s−1 with a rotating glassy carbon disk electrode (RRDE-3A) at a rotating speed of 1600 rpm.

Figure 2. (a) XRD patterns of Co0:40, Co20:20. (b) XRD patterns of Co0:40-annealed, Co20:20-annealed.

main diffraction peaks, which can be indexed to the orthorhombic Co(CO3)0.5(OH)·0.11H2O (JCPDS card no. 48-0083). Supporting Information (SI) Figure S1 presents the thermal gravimetric analysis of Co20:20. As shown in SI Figure S1, the curve shows that the major weight loss process at ∼250 °C is probably due to the decompositions of the carbonate and hydroxyl groups;42 meanwhile, the oxidative decomposition with a total weight loss of 26.6% is nearly complete around 400 °C. Therefore, the as-synthesized precursors are annealed at 400 °C for 3 h in air to obtain cobalt oxide. Figure 2b shows the XRD patterns of Co0:40-annealed and Co20:20-annealed; the main diffraction peaks at 2θ = 31.3°, 36.9°, 38.6°, 44.9°, 59.5°, and 65.3° can be indexed to the (220), (311), (222), (400), (511), and (440) planes of the cubic phase Co3O4 (JCPDS card no. 42-1467), respectively. FT-IR measurements were carried out, as shown in Figure 3, which further confirms the composition of the four products. In Figure 3a and b, the strong peak at 3494 cm−1 is assigned to the O−H stretching mode, which is the feature of hydrogen-band O−H groups and molecular water. As we know, the band at 1632 cm−1 is the bending mode of water molecules. The bands centered at about 1516, 831, and 681 cm−1 can be indexed to stretching vibrations ν(OCO2), δ(CO3), and ρ(OCO), respectively.43 The peak at ∼518 cm−1 can be attributed to the ρw(Co−OH) bending vibration.42 These results indicate that both Co0:40 and Co20:20 are cobalt carbonate hydroxide. After calcination, however, only two very strong peaks at 663 and 568 cm−1 were observed in Figure 3c and d which are characteristic of Co3O4, except that the bands at 3421 and 1632 cm−1 for adsorbed water. In addition, the morphology and structure of the annealed products were studied by SEM and TEM. As we can see in Figure 4a, c, f, and h, the red-headed calliandra-like and dandelion-flower-like structures are conserved during the

3. RESULTS AND DISCUSSION The novel red-headed calliandra-like Co0:40 and dandelionflower-like Co20:20 precursors were produced. The morphologies of Co0:40 and Co20:20 are observed by TEM and SEM. As shown in Figure 1a and b, Co0:40 is composed of red-headed calliandra-like spheres assembled from needles as building blocks with lengths of ∼3 μm. From Figure 1b and d, we can find that the 1D nanoneedles have relatively smooth surfaces. Furthermore, the width of the nanoneedles at the middle part is much wider than that at the front part, indicating trait of the lateral growth during their formation.41 The TEM image (Figure 1c) demonstrates that the diameter of the red-headed calliandra like spheres is about 6 μm, which can also be observed in Figure 1a. Interestingly, it is found that the volume ratio of ethylene glycol/water change to 20:20 will produce product with distinct morphology. The SEM and TEM images (Figure 1f, g, h, and i) indicate that the novel 3D dandelionflower-like Co20:20 consists of subunits with petal-like structure assembled from several smooth needles, and the width at the middle part and length of the petal are 150−250 nm and 1−1.5 μm, respectively. The said morphologies are similar to that of red-headed calliandra and dandelion flower found in nature (insets in Figure 1a and f, respectively). To investigate the detailed structural features of the two precursors, high-resolution transmission electron microscopy (HRTEM) was also carried out. The HRTEM image (Figure 1e) of Co0:40 indicates two sets of lattice fringes with interplanar spaces of 0.51 and 0.89 nm, corresponding to the (020) and (100) planes of orthorhombic Co(CO3)0.5(OH)·0.11H2O C


Article


shows the N2 adsorption−desorption isotherms of these porous products, and the insets depict the corresponding Barrett− Joyner−Halenda (BJH) pore-size distribution plots. Both of the isotherms show a type IV trait with a type H3 hysteresis loop, confirming the presence of a mesoporous structure.46 The pore-size distribution plots of Co0:40-annealed as well as Co20:20annealed reveal a bimodal characteristic with a narrow distribution centered at ∼2 nm and a wide distribution centered at ∼20 nm. The result is consistent with that from TEM. In addition, the BET surface areas of Co0:40-annealed and Co20:20-annealed were 36.1 and 38.1 m2 g−1, respectively. To further investigate the effect of the relative amount of ethylene glycol and water in the reaction system on the precursors’ morphology, other experiments were conducted by changing the volume of ethylene glycol in the system while the other conditions were not changed. Figure 6a shows the morphology of Co15:25 synthesized with 15 mL of ethylene glycol in the reaction system. The SEM image (Figure 6a) suggests that the precursors are flowerlike architecture, but the petals of Co15:25 are a little smaller and sparser than those of Co20:20 (Figure 1f). However, when the amount of ethylene glycol in the reaction solution is increased to 25 mL, we can obtain the tremella-like products, which consist of many curved and thin sheets (Figure 6b). Moreover, the morphologies of Co15:25-annealed and Co25:15-annealed are retained after annealing, which can be found in the SEM images of Figure S2a and b (SI). From Figure 6c, it can be seen that when the amount of ethylene glycol is further increased to 30 mL, the product possesses nearly the same morphology as tremella-like Co25:15, with a smaller size of the assembled tremella-like structure. As shown in SI Figure S2c and d, the morphology of Co30:10-annealed is same as that of Co30:10, whereas the annealed product possesses a porous structure. The corresponding diffraction peaks of the annealed products in the XRD

Figure 3. FTIR spectra of (a) Co0:40, (b) Co20:20, (c) Co0:40-annealed, and (d) Co20:20-annealed.

thermal treatment process. It indicates the products are quite thermally stable. Figure 4b and g indicate that both Co0:40annealed and Co20:20-annealed are composed of many interconnected nanoparticles, which is consistent with the previous reports.38,44,45 From Figure 4d and i, we can observe that the average sizes of the nanoparticles in Co0:40-annealed and Co20:20-annealed are about 50 and 30 nm, respectively, and the irregular mesoporous pores with diameters of 2−50 nm are dispersed among the nanoparticles. As shown in the HRTEM images (Figure 4e and j), the interfringe spacing of Co0:40annealed as well as Co20:20-annealed is 0.47 nm, corresponding well to the (111) plane of cubic phase Co3O4 (JCPDS card no. 42-1467). Both the highly clear crystal lattices and well-ordered dot pattern of the FFT patterns (Figure e1 and j1) demonstrate the high-quality single-crystalline characteristic of Co3O4. To further investigate the porosity and specific surface area of Co0:40-annealed and Co20:20-annealed, the N2 adsorption− desorption measurements were carried out at 77 K. Figure 5

Figure 4. (a) Low- and (b) high-magnification SEM images and (c) low- and (d) high-magnification TEM images of Co0:40-annealed. (e) HRTEM image of the square marked in part d and the corresponding FFT pattern is shown in image e1. (f) Low- and (g) high-magnification SEM images and (h) low- and (i) high-magnification TEM images of Co20:20-annealed. (j) HRTEM image of the square marked in part i and the corresponding FFT pattern is shown in image j1. Images b and g are the magnified photos of the dot elliptic region in images a and f, respectively. D


Article


Figure 5. BET isotherm plots and corresponding BJH pore size distributions (insets) of (a) Co0:40-annealed and (b) Co20:20-annealed.

Figure 6. FESEM images of (a) Co15:25, (b) Co25:15, and (c) Co30:10.

mmol, the flowerlike product with zigzag petals is obtained. And each petal with almost sheetlike architecture is obviously different from that of Co20:20 (shown in Figure 1f). As shown in Figure 7b, however, when the amount of urea is increased to 20 mmol, we can obtain the product that consists of subunits with the analogous morphology as Co20:20. The reason for the different morphologies between them may be that the much lower concentration of urea in the system guarantees the slower formation of products. It is consistent with the above hypothesis, in which the slower reaction can accelerate the formation of sheetlike subunits. To understand the formation of the red-headed calliandralike Co0:40 and dandelion-flower-like Co20:20, time-dependent experiments were performed. After reacting for 3 h, it can be seen that the products of precursor Co0:40 consist of several sheaflike bundles with a diameter of about 1 μm (Figure 8a). As shown in Figure 8b and c, the morphology of the products further changed into red-headed calliandra-like spheres and six half-sheaves after increasing the reaction time to 4 h. From Figure 8c, we can also discover that the individual sheaf has a much bigger size than it does in Figure 8a. Several references report that 1D nanorods can grow into self-assembled, sheaflike bundles depending on a crystal-splitting mechanism.48−50 It may be similar to the growth mechanism in the formation of red-headed calliandra-like Co0:40 in the current work. The nanorods first are assembled into bundles, and then the bundles gradually grow into six sheaves and red-headed calliandra-like spheres after further splitting. Interestingly, the formation process of precursor Co20:20 is distinct from the red-headed calliandra-like Co0:40. The growth mechanism may be similar to the previous report,51 in which the product was prepared via a multistep splitting−in situ dissolution−recrystallization growth process. As shown in Figure 8d, when the reaction time is 2.5 h, the nanowires with a length of about a few hundred nanometers are connected at one point to form a unit, and the units are also connected to each other, especially in the white dot elliptic region. It could be attributed to the fast growth of precursor generates nanowires, then after subsequent splitting, growth can take place at the two

patterns (Figure S2e in the SI) are assigned to cubic Co3O4 (JCPDS card no.42-1467). All of the above observations (including red-headed calliandra-like Co0:40 and dandelion-flower-like Co20:20) indicate that the relative amount of ethylene glycol and water in the reaction system can control the morphology of the precursors to a certain extent. As a result, with the increase in the amount of ethylene glycol in the reaction solution, the morphology of the subunits is changed from the needles to sheets. It may be the reason that the ethylene glycol with much higher viscosity in the reaction solution can limit the diffusion of ions (Co2+, CO32−, and OH−). On the other hand, ethylene glycol can coordinate to the Co2+ to form complexes in the reaction solution, which also hinders the combination of cobalt ions and anions (CO32− and OH−).12,13 Both of the above reasons show that the ethylene glycol can slow the reaction. The higher the concentration of ethylene glycol in the reaction system was, the slower the formation of precursors was. Therefore, we hypothesize that the slow reaction contributes to the formation of the sheetlike subunit. Moreover, the carbonate anions may also direct the growth of nanoneedles.47 As a synergy of ethylene glycol and carbonate anions, the specific subunits’ morphology from nanoneedles to sheets could be obtained. Urea as a precipitator was also investigated for its effect on the morphology of the precursors, while the other conditions were consistent with the synthesis of precursor Co20:20. As shown in Figure 7a, when the amount of urea is decreased to 5

Figure 7. FESEM images of the precursors obtained with different amounts of urea: 5 mmol (a) and 20 mmol (b). The other reaction condition was kept the same with the formation of precursor Co20:20. E


Article


h, the petals (Figure 8g) become more obvious, but there still exist a few nanowires (Figure 8h) between the petals. With a further increase in the reaction time to 8 h, the flowerlike product (Figure 8i) composed of petal-like architecture is obtained; meanwhile, the interlaced nanowires disappear completely. The disappearance of interlaced nanowires and formation of petals are probably the result of dissolution and recrystallization.51,52 The plausible formation process of dandelion-flower-like Co20:20 is shown schematically in Figure 8j. The first nucleating, initial splitting growth of nanorods and further splitting growth of nanowires and subsequent in situ dissolution−recrystallization, result in the formation of a dandelion-flower-like architecture, which can be seen in our TEM images. In recent years, cobalt oxide (Co3O4), as an anode electrode material for lithium secondary batteries, has attracted much attention because of its high theoretical capacity of ∼890 mA h g−1 compared with that of conventional graphite-based anodes (372 mA h g−1) as well as lower virulence and cost. To study the potential of the as-prepared Co3O4 with different morphology as an anode material in a Li ion battery, the lithium storage properties were measured by using the standard Co3O4/Li half-battery configuration. Figure 9a shows the first two discharge/charge curves of two samples (Co0:40-annealed and Co20:20-annealed) at a current density of 100 mA g−1 in the potential range from 0.01 to 3.0 V. The first-discharge profiles are qualitatively similar to each other, which indicates the same electrochemical behaviors of both. It is consistent with previous reports.25,27−29,37,53 For all samples, the two clear potential plateaus at around 1.2 and 1.0 V vs Li+/Li can be found from the first discharge curves, which are attributed to the reduction processes to CoO and metallic Co, respectively.25 Moreover, the potential plateau shifted to about 1.25 V vs Li+/Li in the second discharge curves. The first discharge capacities of Co0:40-annealed and Co20:20-annealed are 1340 and 1298 mA h g−1, respectively. All of them are larger

Figure 8. TEM images of the products synthesized at different reaction times of precursor Co0:40: (a) 3 and (b, c) 4 h; TEM images of the products obtained with different times of precursor Co20:20: (d) 2.5, (e) 3, (f) 4, (g, h) 6, and (I) 8 h. (j) Schematic illustration of the growth mechanism for dandelion-flower-like precursor Co20:20.

heads of the nanowires (shown by white arrows in Figure 8d). Because of further splitting growth, the units between themselves are connected tightly after reacting for 3 h (Figure 8e). With the reaction time prolonged to 4 h, however, the curved ribbonlike or petal-like architecture (Figure 8f) appeared in a radiating form. We can also find that nanowires are still intertwined closely and irregularly. After reacting for 6

Figure 9. (a) Charge−discharge curves of Co0:40-annealed and Co20:20-annealed for first and second cycles at a current density of 100 mA g−1. (b) Cycling performance of Co0:40-annealed, Co15:25-annealed, Co20:20-annealed, Co25:15-annealed, and Co30:10-annealed at a current density of 100 mA g−1. (c) Representative CV curves for the initial five cycles of the Co20:20-annealed at a scan rate of 0.1 mV s−1. (d) Discharge capacity of Co20:20annealed at different current densities between 3.00 and 0.01 V. F


Article

The Journal of Physical Chemistry C than the theoretical capacity (890 mA h g−1), which can be ascribed to irreversible reactions to form a solid electrolyte interphase (SEI) layer and possible interfacial lithium storage.29,54 The first charging curves with a similar shape result in capacities of 940 and 944 mA h g−1, respectively, and the corresponding Coulombic efficiency can be calculated as 70.1% and 72.7%. The discharge capacity versus cycle number curves at a current density of 100 mA g−1 in the voltage range from 0.01 to 3.0 V are depicted in Figure 9b. As can be observed, most of samples show almost no fading from the 2nd to 20th cycles, which implies the relative stability of their electrochemical properties. The Co25:15-annealed showed the highest capacity values in the first cycle, but its capacity decreased gradually during the first four cycles, then kept almost constant until 20 cycles, with the capacity of 939 mA h g−1. In addition, Co0:40annealed, Co15:25-annealed, and Co30:10-annealed also exhibited good performance with discharge capacities of 833, 995, and 987 mA h g−1 after 20 cycles, respectively. These values exceed the theoretical capacity of 890 mA h g−1, except for that of Co0:40-annealed. After the third cycle, the Co20:20-annealed maintained the highest capacity until 20 cycles and exhibited a high discharge capacity of 1204 mA h g−1 in the 20th cycle, which is higher than those previously reported in the literature.25−29,55 Therefore, the Co20:20-annealed demonstrates the best cycling performance and the highest discharge capacity. According to the morphologies of these samples shown in Figure 4 and SI Figure S2, we can find that the cobalt oxides with sheetlike subunits synthesized in the mixed solvents possess higher values than the Co0:40-annealed with a nanoneedlelike structure. Moreover, the Co20:20-annealed with reasonable distribution of sheet- and needlelike subunits exhibits the best cycling performance. The drop in capacity of Co25:15-annealed during the first four cycles is not very clear, probably due to the slow growth of SEI layers on the surface of this large-size tremella-like Co25:15-annealed (Seen in Figure 6 and SI Figure S2). Figure 9c depicts the CV curves of the Co20:20-annealed for the first five cycles at a scanning rate of 0.1 mV s−1. Two obvious cathodic peaks exist in the first cycle; the shoulder peak at around 1.17 V could be ascribed to the reduction of Co3O4 to CoO and Li2O. The cathodic peak at around 0.9 V can be attributed to the further reduction of CoO into metallic cobalt, Li2O, and the irreversible formation of a passivating SEI film.56 During the subsequent anodic scan, the anodic peak at around 2.08 V could be attributed to the reversible oxidation of Co to Co3O4. In the following cycles, the cathodic peak shifts to around 1.09 V, and the anodic peak locates at around 2.09 V. The rate discharge capacity of Co20:20-annealed was also evaluated and shown in Figure 9d, due to the highest discharge capacity during the cycle test. From the plot, it can be calculated that the average discharge capacities are about 980, 772, 656, and 533 mA h g−1 at 100, 200, 500, and 1000 mA g−1, and then the discharge capacity shows 796 mA h g−1 near to the theoretical capacity of Co3O4 at 100 mA g−1. These prominent lithium storage properties may be ascribed to the unique structure and morphology of these cobalt oxides. Typically, both the 3D hierarchical and mesoporous structure facilitate permeation of electrolyte and transfer of Li+ ions in the electrode. Meanwhile, such a porous structure could alleviate the stress caused by volume variation during the cycle process. The cobalt oxide is thought to be a potential alternate for noble Pt and its alloys to ORR catalyst.18,19 The catalytic

activities of the Co0:40-annealed, Co20:20-annealed, and Co25:15annealed for the ORR were studied by cyclic voltammetry. As shown in Figure 10a, the three CV curves obtained in N2-

Figure 10. (a) CV curves of Co0:40-annealed, Co20:20-annealed, and Co25:15-annealed on glassy carbon electrodes in O2-saturated (solid line) or N2-saturated 0.1 M KOH (dash line). (b) LSV plots of commercial Pt/C, Co0:40-annealed, Co20:20-annealed, and Co25:15annealed at a rotation speed of 1600 rpm and a scan rate of 5 mV s−1 in an O2-saturated 0.1 M KOH solution. (c) CV curves of Co20:20annealed in O2-saturated 0.1 M KOH with or without 10% (in volume) methanol.

saturated 0.1 M KOH electrolyte are all featureless. In contrast, remarkable reduction currents and well-defined cathodic peaks are observed in O2-saturated electrolyte. This result could be attributed to the catalytic activity of the cobalt oxide for the ORR. To further assess the catalytic activity for ORR, LSV measurements on a rotating disk electrode were conducted in O2-saturated 1 M KOH solution (Figure 10b). The reference commercial Pt/C catalyst manifested the onset potential of ∼0.058 V, which was similar to previously reported.57 The Co20:20-annealed with an onset potential of ∼−0.167 V showed a relatively positive onset potential compared with those of Co0:40-annealed (∼−0.172 V) and Co25:15-annealed (∼−0.204 V), suggesting the higher ORR catalytic activity than the others. Moreover, the onset potential of Co20:20-annealed is more positive than that of the Co3O4 nanoparticles reported previously.57 In addition, the difference (∼0.225 V) of the onset potential between the Co20:20-annealed and commercial Pt/C in our study is compared with that (∼0.23 V) in the previous report.18 From Figure 10b, we can also observe that the peak current of Co20:20-annealed is the largest in the three as-prepared samples, which also demonstrates the relatively high catalytic activity of Co20:20-annealed. In general, the morphology and surface area could greatly affect the catalytic activity. Indeed, according to the BET result in Figure 5, the dandelion-flower-like Co20:20-annealed has a highest surface area of 38.1 m2 g−1 contributing to the best electrocatalytic activity, compared with the red-headed calliandra-like Co0:40annealed (36.1 m2 g−1) and tremella-like Co25:15-annealed (28.3 m2 g−1, SI Figure S3). In addition, the durability of the Co0:40G



■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21103092); Program for NCET-120629; the Fundamental Research Funds for the Central Universities (No. 30920130111003); the Ph.D. Programs Foundation of the Ministry of Education of China (No. 20133219110018); the Qing Lan Project and Six Major Talent Summit (XNY-011); the Science and Technology Support Plan (No. BE2013126); and PAPD of Jiangsu Province, China.

annealed, Co20:20-annealed, and Co25:15-annealed were measured by the long-term chronoamperometric experiments, and the Co20:20-annealed exhibits the best durability in comparison with the Co0:40-annealed and Co25:15-annealed (see SI Figure S4). Although the ORR activity of Pt/C catalyst is very high, the methanol is poisonous for Pt/C during the ORR.18,19 To investigate the methanol tolerance of Co20:20-annealed, CV curves were measured in O2-saturated 0.1 M KOH with and without the addition of 10% methanol by volume. As shown in Figure 10c, the two curves are nearly overlapped, indicating the catalyst of Co20:20-annealed possesses a strong ability to avoid the influence of methanol. All the results above demonstrate the Co20:20-annealed shows the best electrocatalytic activity to ORR and the highest lithium storage performance, compared with the other cobalt oxide samples in this work.

■

ASSOCIATED CONTENT

S Supporting Information *

TG curve of precursor Co20:20; FESEM images of Co15:25annealed and Co25:15-annealed; TEM images of Co30:10 and Co30:10-annealed; XRD patterns of Co15:25-annealed, Co25:15annealed, and Co30:10-annealed; BET isotherm plots and pore size distributions of Co25:15-annealed; chronoamperometric responses (percentage of current retained versus operation time) of Co0:40-annealed, Co20:20-annealed, and Co25:15annealed. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Seisenbaeva, G. A.; Moloney, M. P.; Tekoriute, R.; HardyDessources, A.; Nedelec, J. M.; Gun’ko, Y. K.; Kessler, V. G. Biomimetic Synthesis of Hierarchically Porous Nanostructured Metal Oxide MicroparticlesPotential Scaffolds for Drug Delivery and Catalysis. Langmuir 2010, 26, 9809−9817. (2) Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Groen, J. C. Hierarchical Zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by Advances in Materials Design. Chem. Soc. Rev. 2008, 37, 2530−2542. (3) Park, C.; So, H.-M.; Jeong, H. J.; Jeong, M. S.; Pippel, E.; Chang, W. S.; Lee, S.-M. Hierarchically Structured ZnO/Petal Hybrid Composites with Tuned Optoelectronic and Mechanical Properties. ACS Appl. Mater. Interfaces 2014, 6, 16243−16248. (4) Ye, Y.; Kuai, L.; Geng, B. A Template-Free Route to a Fe3O4− Co3O4 Yolk−Shell Nanostructure as a Noble-Metal Free Electrocatalyst for ORR in Alkaline Media. J. Mater. Chem. 2012, 22, 19132− 19138. (5) Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.; Kisailus, D.; Zhao, H.; Tang, Z.; Wang, D. Accurate Control of Multishelled Co3O4 Hollow Microspheres as High-Performance Anode Materials in Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2013, 52, 6417−6420. (6) Xiao, Y.; Liu, S.; Li, F.; Zhang, A.; Zhao, J.; Fang, S.; Jia, D. 3D Hierarchical Co3O4 Twin-Spheres with an Urchin-Like Structure: Large-Scale Synthesis, Multistep-Splitting Growth, and Electrochemical Pseudocapacitors. Adv. Funct. Mater. 2012, 22, 4052−4059. (7) Mazloumi, M.; Shadmehr, S.; Rangom, Y.; Nazar, L. F.; Tang, X. Fabrication of Three-Dimensional Carbon Nanotube and Metal Oxide Hybrid Mesoporous Architectures. ACS Nano 2013, 7, 4281−4288. (8) Lee, J. H. Gas Sensors Using Hierarchical and Hollow Oxide Nanostructures: Overview. Sens. Actuators, B 2009, 140, 319−336. (9) Hu, J. S.; Zhong, L. S.; Song, W. G.; Wan, L. J. Synthesis of Hierarchically Structured Metal Oxides and Their Application in Heavy Metal Ion Removal. Adv. Mater. 2008, 20, 2977−2982. (10) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. Self-Assembled 3D Flowerlike Iron Oxide Nanostructures and Their Application in Water Treatment. Adv. Mater. 2006, 18, 2426−2431. (11) Yang, H.; Dai, H.; Deng, J.; Xie, S.; Han, W.; Tan, W.; Jiang, Y.; Au, C. T. Porous Cube-Aggregated Co3O4 Microsphere-Supported Gold Nanoparticles for Oxidation of Carbon Monoxide and Toluene. ChemSusChem 2014, 7, 1745−1754. (12) Yu, X. Y.; Xu, R. X.; Gao, C.; Luo, T.; Jia, Y.; Liu, J. H.; Huang, X. J. Novel 3D Hierarchical Cotton-Candy-Like CuO: Surfactant-Free Solvothermal Synthesis and Application in As(III) Removal. ACS Appl. Mater. Interfaces 2012, 4, 1954−1962. (13) Yang, L. X.; Zhu, Y. J.; Li, L.; Zhang, L.; Tong, H.; Wang, W. W.; Cheng, G. F.; Zhu, J. F. A Facile Hydrothermal Route to FlowerLike Cobalt Hydroxide and Oxide. Eur. J. Inorg. Chem. 2006, 2006, 4787−4792. (14) Nethravathi, C.; Sen, S.; Ravishankar, N.; Rajamathi, M.; Pietzonka, C.; Harbrecht, B. Ferrimagnetic Nanogranular Co3O4 through Solvothermal Decomposition of Colloidally Dispersed Monolayers of α-Cobalt Hydroxide. J. Phys. Chem. B 2005, 109, 11468−11472.

4. CONCLUSIONS In summary, we have developed a facile and surfactant-free approach for synthesis of novel 3D mesoporous red-headed calliandra-like and dandelion-flower-like Co3O4. It is found that the reaction time and the relative amount of ethylene glycol in the reaction system could dramatically affect the growth of cobalt carbonate hydroxide precursors and further control the morphology of Co3O4. The growth processes of the novel flowerlike precursors (cobalt carbonate hydroxide) were investigated, and plausible mechanisms were proposed. When tested as potential anode materials for lithium ion batteries, the as-prepared Co3O4 products with different unique structures showed good cycling performance and high lithium storage capacities. Moreover, when evaluated for the catalytic activity for ORR, the 3D mesoporous dandelion-flower-like Co3O4 exhibited the highest activity and excellent methanol tolerance. It is believed that the current strategy could be extended to other metal oxide or hydroxide micro-/nanomaterials with controllable structure and shape; meanwhile, these unique 3D hierarchical Co3O4 with mesoporous structures may have promising applications in anode material for Li ion batteries, chemical sensors, and catalysts for the oxygen reduction reaction.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-25-84315943. Fax: +86-25-84315190. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest. H


Article

The Journal of Physical Chemistry C (15) Xia, X. H.; Tu, J. P.; Zhang, J.; Huang, X. H.; Wang, X. L.; Zhang, W. K.; Huang, H. Enhanced Electrochromics of Nanoporous Cobalt Oxide Thin Film Prepared by a Facile Chemical Bath Deposition. Electrochem. Commun. 2008, 10, 1815−1818. (16) Xie, X. W.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. J. LowTemperature Oxidation of CO Catalysed by Co3O4 Nanorods. Nature 2009, 458, 746−749. (17) Li, C. C.; Yin, X. M.; Wang, T. H.; Zeng, H. C. Morphogenesis of Highly Uniform CoCO3 Submicrometer Crystals and Their Conversion to Mesoporous Co3O4 for Gas-Sensing Applications. Chem. Mater. 2009, 21, 4984−4992. (18) Sa, Y. J.; Kwon, K.; Cheon, J. Y.; Kleitz, F.; Joo, S. H. Ordered Mesoporous Co3O4 Spinels as Stable, Bifunctional, Noble Metal-Free Oxygen Electrocatalysts. J. Mater. Chem. A 2013, 1, 9992−10001. (19) Li, S. S.; Cong, H. P.; Wang, P.; Yu, S. H. Flexible NitrogenDoped Graphene/Carbon Nanotube/Co3O4 Paper and Its Oxygen Reduction Activity. Nanoscale 2014, 6, 7534−7541. (20) Deori, K.; Ujjain, S. K.; Sharma, R. K.; Deka, S. Morphology Controlled Synthesis of Nanoporous Co3O4 Nanostructures and Their Charge Storage Characteristics in Supercapacitors. ACS Appl. Mater. Interfaces 2013, 5, 10665−10672. (21) Zhu, T.; Chen, J. S.; Lou, X. W. Shape-Controlled Synthesis of Porous Co3O4 Nanostructures for Application in Supercapacitors. J. Mater. Chem. 2010, 20, 7015−7020. (22) Xiong, S.; Yuan, C.; Zhang, X.; Xi, B.; Qian, Y. Controllable Synthesis of Mesoporous Co3O4 Nanostructures with Tunable Morphology for Application in Supercapacitors. Chem.Eur. J. 2009, 15, 5320−5326. (23) Zhang, F.; Yuan, C.; Zhu, J.; Wang, J.; Zhang, X.; Lou, X. W. Flexible Films Derived from Electrospun Carbon Nanofibers Incorporated with Co3O4 Hollow Nanoparticles as Self-Supported Electrodes for Electrochemical Capacitors. Adv. Funct. Mater. 2013, 23, 3909−3915. (24) Tian, L.; Zou, H.; Fu, J.; Yang, X.; Wang, Y.; Guo, H.; Fu, X.; Liang, C.; Wu, M.; Shen, P. K.; et al. Topotactic Conversion Route to Mesoporous Quasi-Single-Crystalline Co3O4 Nanobelts with Optimizable Electrochemical Performance. Adv. Funct. Mater. 2010, 20, 617− 623. (25) Lou, X. W.; Deng, D.; Lee, J. Y.; Feng, J.; Archer, L. A. SelfSupported Formation of Needlelike Co3O4 Nanotubes and Their Application as Lithium-Ion Battery Electrodes. Adv. Mater. 2008, 20, 258−262. (26) Guo, B.; Li, C. S.; Yuan, Z. Y. Nanostructured Co3O4 Materials: Synthesis, Characterization, and Electrochemical Behaviors As Anode Reactants in Rechargeable Lithium Ion Batteries. J. Phys. Chem. B 2010, 114, 12805−12817. (27) Li, W. Y.; Xu, L. N.; Chen, J. Co3O4 Nanomaterials in LithiumIon Batteries and Gas Sensors. Adv. Funct. Mater. 2005, 15, 851−857. (28) Yan, N.; Hu, L.; Li, Y.; Wang, Y.; Zhong, H.; Hu, X. Y.; Kong, X. K.; Chen, Q. W. Co3O4 Nanocages for High-Performance Anode Material in Lithium-Ion Batteries. J. Phys. Chem. B 2012, 116, 7227− 7235. (29) Li, Y.; Tan, B.; Wu, Y. Mesoporous Co3O4 Nanowire Arrays for Lithium Ion Batteries with High Capacity and Rate Capability. Nano Lett. 2008, 8, 265−270. (30) Liu, Y.; Mi, C.; Su, L.; Zhang, X. Hydrothermal Synthesis of Co3O4 Microspheres as Anode Material for Lithium-Ion Batteries. Electrochim. Acta 2008, 53, 2507−2513. (31) Chen, J. S.; Zhu, T.; Hu, Q. H.; Gao, J.; Su, F.; Qiao, S. Z.; Lou, X. W. Shape-Controlled Synthesis of Cobalt-Based Nanocubes, Nanodiscs, and Nanoflowers and Their Comparative Lithium-Storage Properties. ACS Appl. Mater. Interfaces 2010, 2, 3628−3635. (32) Lou, X. W.; Deng, D.; Lee, J. Y.; Archer, L. A. Thermal Formation of Mesoporous Single-Crystal Co3O4 Nano-Needles and Their Lithium Storage Properties. J. Mater. Chem. 2008, 18, 4397− 4401. (33) Wu, H. B.; Chen, J. S.; Hng, H. H.; Wen Lou, X. Nanostructured Metal Oxide-Based Materials as Advanced Anodes for Lithium-Ion Batteries. Nanoscale 2012, 4, 2526−2542.

(34) Boyen, H. G.; Kästle, G.; Zürn, K.; Herzog, T.; Weigl, F.; Ziemann, P.; Mayer, O.; Jerome, C.; Möller, M.; Spatz, J. P.; et al. A Micellar Route to Ordered Arrays of Magnetic Nanoparticles: From Size-Selected Pure Cobalt Dots to Cobalt−Cobalt Oxide Core−Shell Systems. Adv. Funct. Mater. 2003, 13, 359−364. (35) Son, M. Y.; Hong, Y. J.; Kang, Y. C. Superior Electrochemical Properties of Co3O4 Yolk−Shell Powders with a Filled Core and Multishells Prepared by a One-Pot Spray Pyrolysis. Chem. Commun. 2013, 49, 5678−5680. (36) Drasovean, R.; Monteiro, R.; Fortunato, E.; Musat, V. Optical Properties of Cobalt Oxide Films by a Dipping Sol-Gel Process. J. NonCryst. Solids 2006, 352, 1479−1485. (37) Liu, J.; Xia, H.; Lu, L.; Xue, D. Anisotropic Co3O4 Porous Nanocapsules toward High-Capacity Li-Ion Batteries. J. Mater. Chem. 2010, 20, 1506−1510. (38) Wang, B.; Zhu, T.; Wu, H. B.; Xu, R.; Chen, J. S.; Lou, X. W. D. Porous Co3O4 Nanowires Derived from Long Co(CO3)0.5(OH)· 0.11H2O Nanowires with Improved Supercapacitive Properties. Nanoscale 2012, 4, 2145−2149. (39) Liu, X.; Long, Q.; Jiang, C.; Zhan, B.; Li, C.; Liu, S.; Zhao, Q.; Huang, W.; Dong, X. Facile and Green Synthesis of Mesoporous Co3O4 Nanocubes and Their Applications for Supercapacitors. Nanoscale 2013, 5, 6525−6529. (40) Lü, Y.; Zhan, W.; He, Y.; Wang, Y.; Kong, X.; Kuang, Q.; Xie, Z.; Zheng, L. MOF-Templated Synthesis of Porous Co3O4 Concave Nanocubes with High Specific Surface Area and Their Gas Sensing Properties. ACS Appl. Mater. Interfaces 2014, 6, 4186−4195. (41) Rui, X.; Tan, H.; Sim, D.; Liu, W.; Xu, C.; Hng, H. H.; Yazami, R.; Lim, T. M.; Yan, Q. Template-Free Synthesis of Urchin-Like Co3O4 Hollow Spheres with Good Lithium Storage Properties. J. Power Sources 2013, 222, 97−102. (42) Xie, X. W.; Shang, P. J.; Liu, Z. Q.; Lv, Y. G.; Li, Y.; Shen, W. J. Synthesis of Nanorod-Shaped Cobalt Hydroxycarbonate and Oxide with the Mediation of Ethylene Glycol. J. Phys. Chem. B 2010, 114, 2116−2123. (43) Klissurski, D.; Uzunova, E. Synthesis of Nickel Cobaltite Spinel from Coprecipitated Nickel−Cobalt Hydroxide Carbonate. Chem. Mater. 1991, 3, 1060−1063. (44) Sharma, S.; Garg, N.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. Design of Anisotropic Co3O4 Nanostructures: Control of Particle Size, Assembly, and Aspect Ratio. Cryst. Growth Des. 2012, 12, 4202−4210. (45) Wang, H. T.; Zhang, L.; Tan, X. H.; Holt, C. M. B.; Zahiri, B.; Olsen, B. C.; Mitlin, D. Supercapacitive Properties of Hydrothermally Synthesized Co3O4 Nanostructures. J. Phys. Chem. B 2011, 115, 17599−17605. (46) Kruk, M.; Jaroniec, M. Gas Adsorption Characterization of Ordered Organic−Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13, 3169−3183. (47) Xu, R.; Zeng, H. C. Dimensional Control of Cobalt-HydroxideCarbonate Nanorods and Their Thermal Conversion to OneDimensional Arrays of Co3O4 Nanoparticles. J. Phys. Chem. B 2003, 107, 12643−12649. (48) Tang, J.; Alivisatos, A. P. Crystal Splitting in the Growth of Bi2S3. Nano Lett. 2006, 6, 2701−2706. (49) Shen, X. F.; Yan, X. P. Facile Shape-Controlled Synthesis of Well-Aligned Nanowire Architectures in Binary Aqueous Solution. Angew. Chem., Int. Ed. 2007, 46, 7659−7663. (50) Li, L.; Sun, N.; Huang, Y.; Qin, Y.; Zhao, N.; Gao, J.; Li, M.; Zhou, H.; Qi, L. Topotactic Transformation of Single-Crystalline Precursor Discs into Disc-Like Bi2S3 Nanorod Networks. Adv. Funct. Mater. 2008, 18, 1194−1201. (51) Bai, J.; Li, X. G.; Liu, G. Z.; Qian, Y. T.; Xiong, S. L. Unusual Formation of ZnCo2O4 3D Hierarchical Twin Microspheres as a HighRate and Ultralong-Life Lithium-Ion Battery Anode Material. Adv. Funct. Mater. 2014, 24, 3012−3020. (52) Jin, R. C.; Chen, G.; Wang, Q.; Sun, J. X.; Wang, Y. A Facile Solvothermal Synthesis of Hierarchical Sb2Se3 Nanostructures with I


Article

The Journal of Physical Chemistry C High Electrochemical Hydrogen Storage Ability. J. Mater. Chem. 2011, 21, 6628−6635. (53) Zhan, F.; Geng, B.; Guo, Y. Porous Co3O4 Nanosheets with Extraordinarily High Discharge Capacity for Lithium Batteries. Chem.Eur. J. 2009, 15, 6169−6174. (54) Wang, X.; Wu, X. L.; Guo, Y. G.; Zhong, Y.; Cao, X.; Ma, Y.; Yao, J. Synthesis and Lithium Storage Properties of Co3O4 NanosheetAssembled Multishelled Hollow Spheres. Adv. Funct. Mater. 2010, 20, 1680−1686. (55) Chou, S. L.; Wang, J. Z.; Liu, H. K.; Dou, S. X. Electrochemical Deposition of Porous Co3O4 Nanostructured Thin Film for LithiumIon Battery. J. Power Sources 2008, 182, 359−364. (56) Zhang, B.; Zhang, Y.; Miao, Z.; Wu, T.; Zhang, Z.; Yang, X. Micro/Nano-Structure Co3O4 as High Capacity Anode Materials for Lithium-Ion Batteries and the Effect of the Void Volume on Electrochemical Performance. J. Power Sources 2014, 248, 289−295. (57) Xiao, J.; Kuang, Q.; Yang, S.; Xiao, F.; Wang, S.; Guo, L. Surface Structure Dependent Electrocatalytic Activity of Co3O4 Anchored on Graphene Sheets toward Oxygen Reduction Reaction. Sci. Rep. 2013, 3, 2300−2304.

J


3D Hierarchical Mesoporous Flowerlike Cobalt Oxide Nanomaterials

Recommend Documents