Surfactant-free scalable synthesis of hierarchically spherical Co3O4 superstructures and their

enhanced lithium-ion storage performances

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Nanotechnology 23 (2012) 465401 (7pp) doi:10.1088/0957-4484/23/46/465401

Surfactant-free scalable synthesis ofhierarchically spherical Co3O4superstructures and their enhancedlithium-ion storage performances

Xiaohui Guo1, Weiwei Xu1, Sirong Li2, Yanping Liu1, Maolin Li1,Xiaoni Qu1, Chaochao Mao1, Xianjin Cui3 and Chunhua Chen2

1 Key Lab of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, and TheCollege of Chemistry and Materials Science, Northwest University, Xi’an 710069,People’s Republic of China2 Department of Material Science, University of Science and Technology of China, Hefei 230026,People’s Republic of China3 Division of Imaging Sciences and Biomedical Engineering, King’s College London, SE1 7EH, UK

E-mail: guoxh2009@nwu.edu.cn

Received 15 May 2012, in final form 14 September 2012Published 23 October 2012Online at stacks.iop.org/Nano/23/465401

AbstractUnique hierarchically porous spherical Co3O4 superstructures were synthesized via asurfactant-free hydrothermal process followed by a calcination treatment, in which theconcentration of reactant cobalt (II) nitrate hexahydrate is a key factor affecting themorphology of products. X-ray powder diffraction, electron microscopies (TEM and SEM),and thermogravimetric analysis were employed to investigate the formation of Co3O4spherical superstructures. Our results suggest that they formed from numerous cubic Co3O4nanocrystals via an oriented attachment mechanism. These superstructures exhibit a highspecific capacity of 1750 mA h g−1 after the first charge–discharge cycle, and the capacityretention remains at a constant of 1600 mA h g−1 at 0.2 C after 50 cycles. The facile, scalable,energy-efficient and environmentally friendly nature of the presented approach renders itparticularly attractive from a technological standpoint. In addition, this scalable and facilesynthesis method could be extended to the preparation of other transition metal oxides withspecific morphologies and surface textures.

S Online supplementary data available from stacks.iop.org/Nano/23/465401/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Nanostructured materials with tunable characteristics suchas chemical composition, relative size, and morphologicalstructure are interesting from both the fundamental andtechnological viewpoints [1, 2]. With regards to the latter,the ability to manipulate the structure, morphology, andfunctionality of nanostructural materials is currently at the

forefront of materials science research, enabling their rapiddeployment into new technologies [3].

In recent decades, lithium-ion batteries (LIBs) haveemerged as a most promising rechargeable battery technologyowing to their potentially high energy density and improvedefficiency and lifetime in comparison with many alterna-tives [4, 5]. There is increasing demand for low-cost batteriesthat exhibit a high specific energy density as well as goodcycling performance. Consequently, the further development

10957-4484/12/465401+07$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 23 (2012) 465401 X Guo et al

of new electrode materials with improved capability ofstoring and deliver energy efficiently remains both timely andwarranted [4–6].

Recently, transition metal oxide nanostructures have beenextensively investigated as potential stable and high energydensity anodes for LIBs [7–9]. In such devices, the transitionmetal oxides are reduced to small metal clusters and thepreviously bound oxygen reacts with Li+ to form Li2O. Underelectrochemical cycling at high current densities, appreciablevolume changes of the electrode material with potentialconcomitant structural degradation often accompany theaforementioned reactions [5, 10–12]. A significant challengeto the advancement of LIB technology is the preparationof well-defined hierarchical superstructures under relativelymild synthetic conditions that exhibit durable and enhancedreversible capacity as well as rate capability.

Co3O4 is a transition metal oxide that has been exten-sively investigated due to its potential in such applicationsas lithium-ion batteries, gas sensing, magnetic devices, andelectrochromic devices [13]. Significant progress pertainingto the controlled synthesis of cobalt oxide nanomaterialshas indeed been made over the past two decades [14].In particular, Rahman et al prepared Co3O4 nanoparticlesby a hydrothermal process under a pulsed magnetic fieldand subsequently examined their performance as anodes inlithium-ion batteries [15]. Askarinejad et al reported onthe catalytic performance of Co3O4 nanocrystals that weresynthesized by a sonochemical method in the presence ofstyrene and cyclooctene [16]. Interestingly, urchin-like Co3O4hierarchical micro/nanostructures, produced via calcinationof the hydrothermally formed urchin-like precursor CoCO3have recently been reported [17]. In addition, Yang et alsynthesized flower-like Co3O4 via a hydrothermal processin a mixed organic solvent system with subsequent calcina-tion [18]. Specifically, Co3O4 thin films with a unique hollowparticle microstructure obtained from a two-step syntheticroute exhibited a high reversible capacity of 1000 mA h g−1

for up to 50 cycles with good rate capability [19]. Finally,a class of porous Co3O4 hollow rods has recently beenprepared via a high-temperature process in the presenceof templating bacteria. The rods displayed an enhancedCoulombic efficiency as well as an excellent reversiblespecific capacity of 903 mA h g−1 after 20 cycles [20].

However, each of the aforementioned synthetic methodsinvolves the use of surfactants, which hinder scalability aswell as contribute to increased environmental contamination.In contrast, the approach we describe in detail below isadvantageously surfactant-free and makes use of readilyavailable cobalt nitrate as the cobalt precursor, and wateras the reaction medium. The presented synthetic methodis particularly interesting from a technological standpointbecause it is simple, affordable, environmentally friendly, andscalable.

In this study, we report an environmentally friendlyand scalable synthesis approach for hierarchical sphericalCo3O4 superstructures. Significantly improved lithium-ionstorage performances, such as better cyclic capacity retentionand greatly enhanced rate capability, were observed.

Unexpectedly, the as-synthesized Co3O4 samples display anexcellent capacity retention of around 1750 mA h g−1 amongthe available current densities of 0.2–2 C, which is superiorto most reported cobalt oxide-based anode nanomaterials [13,19, 21].

2. Experimental section

Typically, suitable amounts of cobalt (II) nitrate hexahydrate(Co(NO3)2·6H2O) were dissolved in a mixture solvent of20 ml water and 20 ml ethylene glycol (EG), under strongmagnetic stirring. The resultant solution was transferred toa 80 ml stainless Teflon vessel and kept at 160 ◦C for 24 h.The sample was collected by centrifugation at 8000 rpm for10 min, and washed three times with water and absoluteethanol. The final product was obtained by calcinationtreatment at 400 ◦C oven for 6 h with a heating rate of1 ◦C min−1. Different concentrations (e.g., 0.930, 0.465, and0.186 M) of cobalt (II) nitrate hexahydrate were used toprepare sample 1, sample 2, and sample 3, respectively.

Characterization. X-ray powder diffraction (XRD) patternswere recorded on a Bruker D4 x-ray diffractometer(Germany) with Ni-filtered Cu KR radiation (40 kV,40 mA). The morphology and structure of the cobalt-based products were characterized by field emission scanelectron microscopy (FE-SEM; JEOL, JSM-6701) andhigh-resolution transmission electron microscopy (HRTEM,JEM-2010) with an accelerating voltage of 200 kV. TheBrunauer–Emmett–Teller (BET) method was used to analyzethe specific surface area, pore volume, and pore size ofcobalt oxide samples on a Micromeritics ASAP-2020 nitrogenabsorption analyzer. The thermal behavior of the samples wasinvestigated by thermogravimetric analysis (TGA, ShimadzuDRG-60) with a heating rate of 10 ◦C min−1 under a nitrogenatmosphere (200 ml min−1) using α-alumina crucibles.

Electrochemical test. The electrochemical analyses wereperformed on coin-type cells (CR2032). Working electrodeswere prepared by pasting a mixture of Co3O4, acetyleneblack and polyvinylidene fluoride (PVDF) at a weight ratioof 40:40:20 onto Al foil. A 1 M solution of LiPF6 inethylene carbonate (EC)/diethylcarbonate (DEC) (1:1, v/v)was used as the electrolyte solution. The cells were assembledin argon-filled glove boxes. A Celgard 2400 microporouspolypropylene membrane was used as the separator. The cellsthus fabricated were cycled galvanostatically in the voltagerange from 0.01 to 3.0 V at varying current densities with amultichannel battery test system (NEWARE BTS-610).

3. Results and discussion

When different concentrations of cobalt nitrate precursor ascobalt resource are used, various unique spherical aggregateswere obtained after hydrothermal treatment at 160 ◦C for24 h (figures S2(a)–(c) available at stacks.iop.org/Nano/23/465401/mmedia). XRD results indicate that these crystallineaggregates can be assigned to the β-cobalt hydroxide phase(figure S1, supporting information available at stacks.iop.org/

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Figure 1. XRD patterns of the obtained Co3O4 samples at differentconcentrations (M) of cobalt (II) nitrate hexahydrate precursor, viahydrothermal reaction and sequent calcination treatment processes,(a) 0.93; (b) 0.465; (c) 0.186, the XRD results can be referred tostandard JCPDS card No. 78-1969.

Nano/23/465401/mmedia). Further treatments of the preparedβ-cobalt hydroxide intermediates at 400 ◦C for 6 h leads tothe formation of cobalt oxide; Co3O4 is achieved according tothe following reaction:

3β-Co(OH)2 + 1/2 O2 → Co3O4 + 3H2O. (1)

The obtained samples can be assigned to the pure cubicCo3O4 phase according to standard JCPDS card no: 78-1969,as shown in figure 1. Herein, it is clearly seen that the threesamples display good crystalline features and well-defineddiffraction peaks. No impurity peaks can be detected, asclearly shown by XRD pattern in figure 1. In particular,sample 3 exhibits diffraction peaks broader than those ofthe other two samples, indicating its smaller nanocrystalblocks. Furthermore, a high yield of hierarchically sphericalaggregates of sample 1 was obtained when 0.930 M nitratecobalt was employed as the cobalt resource (figure 2and figure S3 available at stacks.iop.org/Nano/23/465401/mmedia). These spherical aggregates consist of numerousnanorod-like intermediates, with an average diameter of ca10µm and a rough surface (figures 2(b) and (c)). TEM imagesin figure 2(d) suggest that specific nanorod-like structuresare made of 30 nm nanocrystals via an oriented attachmentmechanism. A crystal lattice stripe was observed on theHRTEM image of individual nanocrystals, and the latticespacing of 4.62 A was associated with the (111) planes(figure 2(e)). The electron diffraction (ED) of the as-preparedsample displays a well-defined diffraction circle pattern,indicating its polycrystalline feature (figure 2(f)).

Meanwhile, a class of self-similar polycrystallinespherical Co3O4 superstructures with a mean diameter of10 µm is obtained in the case of sample 2, as shownin figure 3. It was observed from figure 3 that theobtained spherical aggregates were actually composed ofnumerous smaller nanocrystals ranging from 30 to 60 nmin diameter (figure 3(d)). In addition, the mean diameter

of the individual spherical aggregates was measured to be10 µm (figures 3(b) and (c)). Its spacing was 4.63 A,as obtained by the HRTEM image shown in figure 3(e),corresponding to {111} planes. The electron diffraction (ED)of the as-prepared sample displays an obvious symmetriccircle pattern, which is a reflection of the polycrystallinesample (figure 3(f)). However, a class of porous dumbbell-likeCo3O4 aggregates as well as some broken spherical patchescan be formed when the concentration of cobalt (II) nitratehexahydrate is 0.186 M. Results are shown in supportinginformation figure S2(d) (available at stacks.iop.org/Nano/23/465401/mmedia). Therein, sample 3 displays relativelyloose porous stacking compared to that of sample 1 andsample 2. Moreover, it is noteworthy that the aforementionedobservations are confirmed by previous studies [13c, 22].

We also investigate the porous features of sphericalsamples by the isothermal N2 adsorption/desorption test.The isothermal N2 adsorption–desorption curves of sphericalCo3O4 samples 1 and 2 were observed (figures 4(a) and(b)). The corresponding BET specific surface area and poresize of sample 1 are measured to be ∼53.84 m2 g−1 and15.8 nm (the inset in figure 4(a)), respectively. The nitrogenadsorption/desorption isotherms of sample 2 display a typicaltype-IV isothermal behavior (figure 4), suggesting uniformmesoporous feature. The specific BET surface area and porevolume for sample 2 were measured to be 44.31 m2 g−1 and0.18 cm3 g−1, respectively. According to the correspondingBarrett–Joyner–Halenda (BJH) pore size distributions (theinset in figure 4(b)), the mean pore size is measured to be14.4 nm.

In order to further study the formation process forspherical Co3O4 samples, it is necessary to perform TGAtests for the β-Co(OH)2 intermediates formed in the caseof the 0.465 M cobalt (II) nitrate hexahydrate system, asshown in figure 5. It was clearly seen that the TGA curveshows a significant loss ranging from room temperature to400 ◦C, indicating the decomposition of β-cobalt hydroxideintermediates. The whole weight loss percentage wasmeasured to be around 34 wt% during the calcinationprocess. The proposed formation process of hierarchicallyspherical Co3O4 superstructures was schematically illustratedin scheme 1. Initially, the cobalt source decomposes to formnucleation sites that simply consist of small nanocrystals(scheme 1(a)), and then grow along the c-axis directionsby the oriented attachment mechanism, due to a hydrogen-bonding interaction among surface hydroxide groups oncobalt hydroxide intermediates. Similar results were foundin previous reports [23]. Plate- or rod-shaped structuresformed finally (schemes 1(b) and (c)). In addition, dueto the higher surface free-energy of the rod-like buildingblocks, they can facilitate further aggregation to form morestable spherical aggregates, and then hierarchically sphericalsuperstructures. The cobalt hydroxide spherical intermediatescan be completely decomposed into crystalline cobalt oxideby a calcination treatment process, and then the obtainedcobalt oxide samples display similar spherical morphologiesto those of original cobalt hydroxide (schemes 1(f) and (g),figure 2 and figure S4 available at stacks.iop.org/Nano/23/

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Figure 2. Typical SEM and TEM images of the porous spherical Co3O4 superstructure formed in the case of 0.93 M cobalt (II) nitratehexahydrate precursor via hydrothermal reaction and sequent calcination treatment processes, (a)–(c) different magnifications of SEMimages; (d) TEM image; (e) HRTEM image and (f) ED pattern.

Figure 3. Typical SEM and TEM images of the porous spherical Co3O4 superstructure formed in the case of 0.465 M cobalt (II) nitratehexahydrate precursor via hydrothermal reaction and sequent calcination treatment processes, (a)–(c) different magnifications of SEMimages; (d) TEM image; (e) HRTEM image and (f) ED pattern.

465401/mmedia). Verification of the formation of β-cobalthydroxide and Co3O4 products throughout the entire reactionprocess can be obtained from FTIR measurements (figure S5available at stacks.iop.org/Nano/23/465401/mmedia).

Electrochemical properties of the prepared porous spher-ical Co3O4 samples were performed. Figure 6 represents thetypical charge–discharge voltage profiles of spherical Co3O4samples at current density of 0.2 C. The discharge capacity

of sample 1 was measured to be around 1530 mA h g−1 after1 cycle, and it quickly decreased to 1135 mA h g−1 after 2cycles, indicating 75% capacity retention (figure 6(a)). Thespecific capacity of the Co3O4 sample 2 was measured tobe around 1285 mA h g−1 after 1 cycle, and it likewiselydecreased to 968 mA h g−1 after 2 cycles (figure 6(b)), higherthan their theoretical value (890 mA h g−1). However, thespecific capacity of the obtained Co3O4 sample 3 was around

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Figure 4. Isothermal N2 adsorption–desorption curves of spherical Co3O4 samples formed in the presence of different concentrations ofcobalt (II) nitrate hexahydrate, (a) 0.93 M; (b) 0.465 M, the insets in the isothermal plots report the pore size distribution curves.

Figure 5. TGA curve of the β-cobalt hydroxide intermediatesformed in the case of the 0.465 M cobalt (II) nitrate hexahydratesystem in a nitrogen atmosphere with a heating rate of 10 ◦C min−1.

1593 mA h g−1 after 1 cycle, and almost 1145 mA h g−1 after2 cycles (figure 6(c)). The decrease of the specific capacity forthe three samples after two cycles could be closely associatedwith the electrolyte decomposition and the formation of anirreversible CoO component.

Scheme 1. The proposed possible formation processes for thehierarchically porous spherical Co3O4.

According to previous studies [11], the whole elec-trochemical reaction mechanism of lithium-ion storage forporous spherical Co3O4 systems can be described as thefollowing:

Co3O4 + 8Li+ + 8e− ↔ 4Li2O+ 3Co0 (2)

8Li↔ 8Li+ + 8e− (3)

Co3O4 + 8Li↔ 4Li2O+ 3Co0. (4)

The formation of Li2O and Co is thermodynamicallyfavorable during the discharge process, but the extraction of

Figure 6. The charge–discharge voltage profiles of the obtained spherical Co3O4 sample using different concentrations (M) of cobalt (II)nitrate hexahydrate as precursors, (a) 0.93; (b) 0.465; (c) 0.186, the voltage window ranged from 0.01 to 3.0 V at a current density of 0.2 C.

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Figure 7. The rate-capability curves (a) and cycling performances (b) for the obtained spherical Co3O4 samples in the presence of varyingconcentrations (M) of cobalt (II) nitrate hexahydrate as precursors while the voltage window was ranged from 0.01 to 3 V, The appliedcurrent density for the three samples is 0.2 C during the cycling test.

Li from Li2O during the charge process is relatively difficult,indicating a certain extent of irreversible reaction.

More interestingly, the rate performance test was alsoperformed for samples 1–3 at varying current rate. Specially,it was observed that the capacity of the sample 1 wasobviously increased after 12 cycles at a current density of0.2 C, its capacity can be retained at around 1115 mA h g−1.Similar results were obtained for sample 2 (figure 7(a)).Unexpectedly, the discharge capacity for sample 3 wasretained at 1326 mA h g−1 at 0.2 C after 12 cycles, whichmay be ascribed to a possible electrode materials activationprocess [9, 14]. It is worth noting that the spherical Co3O4electrodes can display enhanced rate capability at varyingcurrent densities. The discharge capacities for sample 1 underdifferent rate conditions were 1168 mA h g−1 at 0.5 C,1043 mA h g−1 at 1 C, and 1230 mA h g−1 at 2 C, andthen back to above 1158 mA h g−1 at 0.2 C. Similar behaviorwas observed for sample 2. Under the same test conditions,the discharge capacities for sample 2 were measured tobe 1210 mA h g−1 at 0.5 C, 1205 mA h g−1 at 1 C,1425 mA h g−1 at 2 C, and 1375 mA h g−1 at 0.2 C.Interestingly, the discharge capacities for sample 3 weremeasured to be 1392 mA h g−1 at 0.5 C, 1395 mA h g−1

at 1 C and 1750 mA h g−1 at 2 C, and then up to around1860 mA h g−1 at 0.2 C after 50 cycles (figure 7(a)).These results demonstrate that the Co3O4 superstructures weobtained have superior electrochemical properties comparedto other cobalt materials [13, 24]. This improvement of therate capability of sample 3 can be attributed to a moreloose porous feature and higher accessible interfacial area orsuitable void space, which enhances its electrode activationefficiency and shortens the electron/ion diffusion pathway [8,25].

Based on the results we present above, it can beconcluded that a possible reversible activation processoccurs during their charge/discharge process. As a result,it was assumed that the solid electrolyte interface (SEI)layer formation is responsible for the enhanced irreversiblecapacity [11].

The cycling performances of the porous spherical Co3O4samples with a voltage window from 0.01 to 3 V at a current

density of 0.2 C were also investigated. It was clear that thedischarge capacity of around 1510 mA h g−1 can be retainedafter 50 cycles for sample 1 (figure 7). Similarly, the dischargecapacity for sample 2 was retained to 1385 mA h g−1

after 50 cycles, which is higher than its initial capacityof 1285 mA h g−1, indicating its 93% capacity retention(figure 7(b)). Nevertheless, the discharge capacity for sample3 was changed to 1054 mA h g−1 after 50 cycles. The specificcapacities for the three Co3O4 samples can be greatly retained,even slightly enhanced after 50 cycles, which is closelyassociated with the addition of much more acetylene blackactive materials and larger accessible surface/interfacial areas,as well as the specific porous spherical superstructures [24,26]. It can also improve the electrode stability by effectivelymitigating the internal mechanical stress induced by the largevolume change of the electrode upon cycling [25]. Moreimportantly, the nanoscale interspaces among these nanorodscan dramatically enhance the Li-ion storage possibilities,and the oriented self-assembly of nanorods can ensure mostCo3O4 building blocks are involved in the electrochemicalreaction process. Meanwhile, the void space from the poroussphere can accommodate the volume change after undergoinga charge/discharge process [8, 27].

4. Conclusions

In summary, we have demonstrated the preparation of aunique class of hierarchically spherical Co3O4 superstructuresvia facile hydrothermal reaction and solid phase transitionprocesses. Hierarchically, porous spherical Co3O4 aggregateswere obtained by simply adjusting the quantity of cobaltnitrate precursor employed in the reaction. Furthermore, thesespherical aggregates were composed of numerous smallersize nanocrystals via a specific oriented attachment mode.These spherical Co3O4 samples show a great potential asanode electrode materials. Results demonstrated that theporous spherical samples possess a high specific capacity of1750 mA h g−1 at 2 C. More importantly, the dischargecapacity for Co3O4 samples was maintained, even partlyenhanced after 50 cycles, and meanwhile the rate-capability

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was significantly improved. Therefore, the presented resultsclearly demonstrate the significant potential of the preparedporous spherical Co3O4 superstructures for use in a range ofapplications.

Acknowledgments

We acknowledge the funding support from the NationalScience Foundation of China (NSFC) (Nos 21001087,21173167 and 21075098), the Education Committee of Shan-nxi Province (Grant Nos 2010JK870, 2010JS115), the OpenProject of Inorganic Synthesis and Preparation ChemistryNational Key Lab of Jilin University (No. 2012-09).

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