JP_2011_Zhu_Cobalt Oxide Nanowall Arrays on Reduced Graphene Oxide

8/2/2019 JP_2011_Zhu_Cobalt Oxide Nanowall Arrays on Reduced Graphene Oxide

http://slidepdf.com/reader/full/jp2011zhucobalt-oxide-nanowall-arrays-on-reduced-graphene-oxide 1/7

Published: April 01, 2011

r 2011 American Chemical Society 8400 dx.doi.org/10.1021/jp2002113| J. Phys. Chem. C 2011, 115, 8400–

8406

ARTICLE

pubs.acs.org/JPCC

Cobalt Oxide Nanowall Arrays on Reduced Graphene OxideSheets with Controlled Phase, Grain Size, and Porosity for

Li-Ion Battery Electrodes Jixin Zhu,† Yogesh Kumar Sharma,† ,‡ Zhiyuan Zeng,† Xiaojun Zhang,† Madhavi Srinivasan,† ,‡

Subodh Mhaisalkar,† ,‡ Hua Zhang,† Huey Hoon Hng,† and Qingyu Yan* ,† ,‡

†School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore‡Energy Research Institute@NTU, Nanyang Technological University, Singapore 637459, Singapore

bS Supporting Information

’ INTRODUCTION

Controlled synthesis processes of nanomaterials to deliverdesired structure, shape, composition, and size have led to many promising applications, e.g., photovoltaic devices,1 field eff ecttransistors,2,3 thermoelectric modules,4À7 and electrodes forsupercapacitors8,9 or Li-ion batteries.10,11 For applications aselectrodes of Li-ion batteries, it is important to achieve high

specific surface area, high electrical conductivity, and an eff ectiveLi-ion diff usion process. Preparation of nanostructured electrodematerials with designed shapes, crystals size and surface area becomes a considerable strategy to improve the capacity andcyclability of Li-ion batteries.12,13 However, due to the highreactivity and large volume swings during the Li-ion intercalationprocess, nanostructured electrode materials may still degradeduring charge/discharge cycling.14 Normally for electrode ma-terials, high surface area with high crystalinity can expose reactivesite and allow eff ective Li-ion insertion/extraction to the host, while small grain size canshorten the Li-ion diff usion path, whichleads to high specific capacities.

Hybridizing nanostructures with conducting matrices, e.g.,amorphous carbon shell,15À17 carbon nanotubes,18 or graphenesheets,19À24 to form complex structures have been reported to bean eff ective route to overcome these problems. Especially,nanocomposites of reduced graphene oxide (rGO) sheets at-tached with metal oxide (MO) nanoparticles have been shown toexhibit high specific capacities and stable charge/discharge

cycling performances.22,23,25,26

The rGO sheets can off er aconductive scaff old to maintain the reliable contact betweenthe electrode materials (e.g., Co3O4) and current collectorsduring the charge/discharge process, which results in stablecycling performance. However, it is worth pointing out thatoptimization of the individual rGO/MO hybrid system is re-quired in order to achieve its ideal Li storage performances. Thisshould involve adjusting their phases, grain sizes, and even

Received: January 8, 2011Revised: March 17, 2011

ABSTRACT: A facile chemical approach has been developed toproduce nanohybrids with ultrathin Co oxides nanowall arrays onreduced graphene oxide (rGO) sheets. The Co oxides exhibited

porous structure. The porosity of the Co oxide/rGO nanohybridsand the grain size of the Co oxides could be tailored by varying theannealing temperature, which directly aff ected their performanceas Li-ion battery electrodes. When tested as anode materials for Li-ion batteries, these Co oxide/rGO nanohybrids showed structural-process-dependent performances. For example, Co3O4/rGO hybrids obtained by annealing R-Co(OH)2/rGO at 350 °C showed ahigh specificcapacityof673mAhgÀ1 after 100 cycles at a dischargecurrent density of 180 mA gÀ1 (0.2 C), which was better thanCo3O4/rGO samples obtained at other annealing temperatures.Similarly, CoO/rGO hybrids obtained by pyrolysis of R-Co-(OH)2/rGO at 350 °C showed optimum performance, as com-pared to that of CoO/rGO samples obtained at other annealing temperatures, with a capacity of 732 mAh gÀ1 after 100 cycles at adischarge current density of 150 mA gÀ1 (0.2 C). Although many metal oxide/rGO hybrid systems have been investigated as

electrode materials for Li-ion batteries, thisstudy indicates thatoptimizationof suchnanohybrids by adjustingthe phases, grainsizes,and porosities is necessary to achieve ideal Li storage performances.



8401 dx.doi.org/10.1021/jp2002113 | J. Phys. Chem. C 2011, 115, 8400–8406

The Journal of Physical Chemistry C ARTICLE

porosity of MO nanostructures in the rGO/MO hybrids, whichhave not been well investigated.

Herein, we showed a facile chemical approach to producenanohybrids with ultrathin Co oxides nanowall arrays attachedon rGO sheets. The Co oxide/rGO hybrids were derived fromR-Co(OH)2/rGO precursors produced from a hydrothermalprocess. Through a controlled annealing process, e.g., under

diff

erent gas environments and diff

erent temperature, we couldadjust the phase, grain size of the Co oxides, and even the totalsurface area of the Co oxide/rGO hybrids. When tested as anodematerials for Li-ion batteries, these Co oxide/rGO nanohybridsshowed structural-process-dependent performances. Althoughthe optimized samples showed very promising performances asLi-ion battery anodes, e.g., a capacity of 732 mAh gÀ1 after 100cycles at a discharge current density of 150 mA gÀ1 (0.2C),itwasalso revealed that samples without proper structural propertiesadjustment showed poor performances. It indicated that tuningthe detailed structural properties of such attractive rGO/MOhybrids should be considered of utmost importance in order torender their advanced performances as Li-ion battery electrodes.

’EXPERIMENTAL SECTION

Synthesis of the Graphite Oxide. Graphite oxide wassynthesized from natural graphite (SP-1) by a modifiedHummer’s method.19,27À29 In brief, 1.5 g of graphite powder was added into a mixture of 10 mL of 98% H2SO4 , 1.25 g of K 2S2O8 , and 1.25 g of P2O5 , and the solution was maintained at80 °C for 4.5 h. The resulting preoxidized product was cleanedusing water and dried in a vacuum oven at 50 °C. After it wasmixed with 60 mL of 98% H2SO4 and a slowly added 7.5 g of KMnO4 at a temperature below 20 °C, then 125 mL of H2O wasadded. After 2 h, an additional 200 mL of H2Oand10mLof30%H2O2 were slowly added into the solution to completely react with the excess KMnO4. After 10 min, a bright yellow solution

was obtained. The resulting mixture was washed with dilutedHCl aqueous (1/10 v/v) solution and H2O. The graphite oxide was obtained after drying in a vacuum oven at 30 °C.

Synthesis of Co3O4 or CoO/rGO Nanowalls. Cobalt pre-cursornanowalls: 30 mg of graphite oxide was dispersedin 40 mL99.9% ethanol by ultrasonication to obtain graphene oxide(GO). Then, 292 mg of Co(NO3)2 3 6H2O and 100 μL of 25%ammonia were added into the solution. The mixture was sealedin a 50 mL Teflon-lined autoclave and maintained at 170 °C for5 h. After it was cooled to room temperature, the precipitate wascollected and washed using ethanol.

Co3O4/rGO nanowalls were obtained by thermal pyrolysis of the as-prepared cobalt precursor at selected temperatures (e.g.,250, 350, 450°C)for30minataheatingrateof10 °C/min in air.

The obtained Co3O4/rGO hybrids were correspondingly namedas Co3O4/rGO-250, Co3O4/rGO-350, and Co3O4/rGO-450according to the reaction temperatures of 250, 350, and450 °C. CoO/rGO nanowalls were obtained under the sameconditions except for the replacement of air for Ar gas. TheCoO/rGO hybrids were named as CoO/rGO-250, CoO/rGO-350, and CoO/rGO-450, respectively, according to the reactiontemperatures of 250, 350, and 450 °C.

Materials Characterization. The morphology of the samples were investigated by using a field-emission scanning electronmicroscopy (FESEM) system (JEOL, model JSM-7600F), andthe nanostructures of the samples were characterized by using atransmission electron microscopy (TEM) system (JEOL, Model

JEM-2100) operating at 200 kV. To investigate the samples viaTEM, a suspension of the MO/rGO nanohybrids in ethanol wasdrop-casted onto carbon-coated copper grids and dried underambient conditions. Crystal phases of samples were identifiedusing a Scintag PAD-V X-ray diffractometer with Cu K R irradia-tion. Thermogravimetry analysis (TGA, Q500) was carried outin the temperature of 30to 800 °Cataheatingrateof10KminÀ1

in air. Raman spectra were obtained with a WITec CRM200confocal Raman microscopy system with a laser wavelength of 488 nm and a spot size of 0.5 mm.

Nitrogen adsorption/desorption isotherms were measured ona Micromeritics TriStar 3000 porosimeter (mesoporouscharacterization) and Micromeritics ASAP 2020 (microporouscharacterization) at 77 K. All samples were outgassed at 100 °Cfor 6 h under vacuum before measurements were recorded. Thespecific surface areas were calculated using the BrunauerÀEmmettÀTeller (BET) method.

Electrochemical Measurements. 80 wt % active material(Co3O4/rGO or CoO/rGO), 10 wt % acetylene black (Super-P), and 10 wt % polyvinylidene fluoride (PVDF) binder weremixed into N-methyl-2-pyrrolidinone (NMP). The obtained

slurry was coated onto Cu foil disks to form the workingelectrodes, which were then dried in vacuum at 50 °C for 12 hto remove the solvent. Electrochemical measurements werecarried out on the CR2032 (3 V) coin-type cells with lithiummetal as the counter/reference electrode, Celgard 2400 mem- brane as the separator, and electrolyte solution obtained by dissolving 1 M LiPF6 into a mixture of ethylene carbonate (EC)and dimethyl carbonate (DMC) (EC/DMC, 50:50 wt/wt). Thecoin cells were assembled in an Ar-filled glovebox with concen-trations of moisture and oxygen below 1.0 ppm. The charge/discharge tests were performed with a NEWARE battery tester ata voltage window of 0.01À3.0 V for both samples. Cyclic voltammetry (CV; 0.01À3 V, 0.5 mV sÀ1) was performed withan electrochemical workstation (CHI 660C). Electrochemical

impedance spectroscopy (EIS) measurements were carried outin the frequency range from 10 kHz to 0.1 Hz at open circuitpotential with an alternating current (ac) perturbation of 10 mV with the help of an impedance spectrum analyzer (Solatron, SI1255B Impedance/grain-phase analyzer and computer softwareZView).

’RESULTS AND DISCUSSION

The morphology of the as-prepared samples formed by reacting Co(NO3)2 with GOs through the solvothermal processas described above were characterized by scanning electronmicroscopy (SEM) and TEM (see Supporting InformationFigure S1aÀc). The images revealed that the hybrids consisted

of highly dense nanowall arrays attached onto two-dimensionalnanosheets. The thicknesses of the nanowalls were 3À5 nm. The X-ray diff raction (XRD) pattern of the sample (see SupportingInformation Figure S1d) indicated that the nanowalls wereR-Co(OH)2. The reduction of the GOs to rGO was confirmed by Raman spectroscopy (see Supporting Information FigureS1e) and electrical conductivity measurements. The Ramanspectroscopy results showed the increase in the intensity ratioof the D band (located at 1350 cmÀ1) to the G band (located at1580 cmÀ1), e.g., I D/ I G , from 0.9 to 1.2 upon the reduction of GOs through the solvothermal process, which is consistent withprevious reports.30 Meanwhile, the four-point-probe measure-ments showedthat the GOfilms on glass were insulating, and the





as-prepared nanohybrids depicted a high electrical conductivity of 200 S mÀ1. Here, it was found that the phase and morphology of the Co compound were dependent on the presence of GOs. Without the addition of GOs during the solvethermal process,the resulting precipitates were agglomerated Co3O4 nanoparti-cles with an average diameter of 100 nm (see SupportingInformation Figure S2a-c).

The R-Co(OH)2 attached rGO samples were annealed toconvert them into Co oxide/rGO hybrids. It was found that thephases, grain sizes, and surface area were highly dependent on theannealing temperatures andgasenvironments.For example, anneal-ing the R-Co(OH)2/rGO samples in air at diff erent temperaturesdid not significantly change the morphology of the nanowalls (seeFigure 1aÀc). However, it was noted that there were pores

generated in theannealednanowalls,especiallyfor samplesannealedat higher temperatures (see Figure 1b,c). The pore sizes in theannealed nanowalls were larger for samples annealed at highertemperatures as illustrated by the TEM images (see Figure 1dÀf).The pore size increased from 3À5 nm for samples annealed at250 °Cto10À20 nm for samples annealed at 450 °C. The phase of the nanowalls was face-centered-cubic (fcc) Co3O4 (JCPDS78-1970) as confirmed by the selected-area electron diff raction(SAED) pattern and high-resolution TEM (HRTEM) image (seeSupporting Information Figure S3). Furthermore, nitrogen adsorp-tion/desorption isotherms for these porous Co3O4/rGO hybridsshowed type H3 hysteresis loops (see Supporting InformationFigure S4), which are commonly observed for plate-like particles

with slit-shaped pores.8 The specific surface areas were calculatedusingthe BET method andwere determined to be 172.8,133.6, and72.3 m2 gÀ1 for Co3O4/rGO samples after annealing at 250, 350,and 450 °C (named as Co3O4/rGO-250, Co3O4/rGO-350, andCo3O4/rGO-450), respectively.

Figure 2 shows the XRD patterns of the Co3O4/rGO samples.The XRD results of the as-prepared nanohybrids confirmed theformation of the fcc Co3O4 phase (JCPDS78-1970), which wasconsistent with the HRTEM and SAED results. No impurity phase was detected. By analyzing the peak width of the XRDpatterns using Scherrer’s equation, the crystallites of Co3O4/rGO-250, Co3O4/rGO-350, and Co3O4/rGO-450 were esti-mated to be 4.2, 6.3, and 9.7 nm, respectively, which were

consistent with the TEM and HRTEM observations (seeFigure 1). The broad hump at 2θ of about 25° in the XRDpatterns was attributed to the glass sample holder.

The phase of the as-prepared hybrid samples was found to beable to be controlled by changing the annealing gas environmentfrom airto Ar.FESEM andTEMimages revealed that theproductsshowednanowallshapewith porous structures after annealing in Arat diff erent temperatures (see Figure 3). The SAED and HRTEManalysis (see Supporting Information Figure S5) revealed that thenanowalls were polycrystalline fcc CoO phase (JCPDS71-1178).The nitrogen adsorption/desorption isotherms (see SupportingInformation Figure S6) for these porous CoO/rGO hybrids werealso tested, and the specific surface areas were determined by BETmethod to be 84.1, 103.3, and 61.5 m2 gÀ1 for samples obtained at

250 °C, 350 and 450 °C (named as CoO/rGO-250, CoO/rGO-350 and CoO/rGO-450), respectively. The corresponding XRDpatterns of these samples (see Figure 4) indicated no detectableimpurity phase except for the fcc CoO phase (JCPDS71-1178).The crystal sizes of CoO were estimated from the peak width usingScherrer’s equation to be 3.3, 5.1, and 9.1 nm for CoO/rGO-250,CoO/rGO-350, and CoO/rGO-450, respectively.

These two types of porous Co oxide nanowall hybrids weretested as anodes for Li-ion batteries, and a series of electrochemi-cal measurements were carried out based on the half cell confi-guration.19,26TheCVofCo3O4/rGO electrodes at a scanrate of 0.5mV sÀ1 for the first, second, and third cycles were carried out (seeSupporting Information Figure S7). All Co3O4/rGO electrodes

Figure 1. FESEM and TEM images of Co3O4/rGO obtained at diff erent annealing temperatures in air: (a,d) 250 °C for 30 min; (b,e) 350 °C for 30min; (c,f) 450 °C for 30 min; red circles indicate pores.

Figure 2. XRD patterns of Co3O4/rGO obtained by thermal pyrolysisof the precursors at 250, 350, and 450 °C for 30 min in air.





depicted similar CV profiles. During the first cycle, two cathodicpeaks were observed at 1.30 and 0.69 V, corresponding to theelectrochemical lithiation processes of Co3O4.

31 During the secondcycle, the main reduction peak was shifted to 1.15 V, which involvedthe conversion reaction: Co3O4þ 8Liþþ 8eÀT 3Coþ 4Li2O.32

The charge/discharge voltage profiles of Co3O4/rGO hybrids at acurrent density of 180 mA/g (0.2 C) were also examined (seeSupporting Information Figure S7). It was observed that there weretwo voltage plateaus at 0.75 and 1.12 V for the first discharge step,corresponding to reactions between Liþ with Co3O4.

33 There wasno obvious voltage plateau observed for rGO.34 The first dischargeandcharge capacities were 1236 mAh gÀ1 and 707 mAh gÀ1 , whichgave a low Coulombic efficiency of 57.2%. The low Coulombicefficiency was mainly attributed to the incomplete conversion

reaction and irreversible lithium loss due to the formation of solidelectrolyte interface (SEI)film during the firstcycle. This resulted inthe low Coulombic efficiency for the first cycle. The electrodedepicted discharge and charge capacities of 750 mAh gÀ1 and 693mAh gÀ1 during the second cycle, which resulted in a higherCoulombic efficiency of 92.4%. The Coulombic efficiency increasedand retained at ∼98% in the subsequent cycles (see SupportingInformation Figure S8).

The discharge/charge cycling performance of porous Co3O4

nanowall/rGO hybrids were evaluated (see Figure 5a) up to 100cycles in the voltage range of 0.01À3.0 V and at a current rate of 180 mAh gÀ1 (0.2 C). The Co3O4/rGO-250 electrode showed apoor cycling stability. Its discharge capacity increased during the

first 10 cycles due to an activation process and then decreased to320 mAh gÀ1 after 100 cycles. The Co3O4/rGO-350 electrodeshowed much improved charge/discharge capacities and cyclingstabilities. It depicted a dischargecapacity of 884mAh gÀ1 duringthe second cycle, which slightly reduced and maintained at 673mAh gÀ1during the 100th cycle. Such performance was betterthan those reported for Co3O4.

31,35À40 The Co3O4/rGO-450sample showed a discharge capacity of 802 mAh/g during thesecond cycle, which decreased to 582 mAh gÀ1 during the 100thcycle. The lower discharge capacity of Co3O4/rGO-450 ascompared to that of Co3O4/rGO-350 is possibly due to thereduced specific surface area as a result of the collapsing of thepores upon annealing at high temperature. We also prepared

pure Co3O4 nanoparticles (see Supporting Information FigureS2), which depicted a low capacity of 266 mAh gÀ1 during the100th cycle (see Supporting Information Figure S9).

A similar series of electrochemical measurements were alsocarried out for the CoO/rGO hybrids. The CVs of the CoO/rGO electrodes were also obtained at a scan rate of 0.5 mV sÀ1 forthe first, second, and third cycles (see Supporting InformationFigure S10). For the firstcyclecurve, therewere threepeaks at 1.42,0.86, and 0.42 V. The chargeÀdischarge voltage profiles of theCoO/rGOelectrodes(seeSupportingInformation Figure S8) wereevaluated for the first two cycles at a current rate of 150 mA gÀ1

(0.2 C, where 1 C is defined as 716 mA gÀ1). For example, theinsertion process in CoO/rGO-350 led to a discharge specificcapacity of 1200 mAh gÀ1 with a low Coulombic efficiency of

70.6% for the first cycle, while the discharge capacity decreased to863mAhgÀ1 witha corresponding charge capacity of803mAh gÀ1 ,leading to a much higher Coulombic efficiency of 93.1%. The longcharacteristic plateau around 0.73 V observed in the first dischargecurves wasassociatedwith the conversion reaction: CoOþ 2Liþþ2eÀT Li2Oþ Co.33 Here, the capacity of CoO/rGO electrode ishigher than the theoretical value,whichis possibly due to the specialporousstructure and synergistic eff ect betweentheflexible rGOandCoO nanowall arrays as reported.40

Figure 5b shows the charge/discharge cycling performance between0.01 and 3.0 V at0.2C (150mA gÀ1) for the CoO/rGOsamples. The CoO/rGO-250 electrode delivered a dischargecapacity of 914 mAh gÀ1 during the second cycle. However, the

Figure 3. FESEM and TEM images of CoO/rGO obtained at diff erent annealing temperature in Ar: (a,d) 250 °C for 30 min; (b,e) 350 °C for 30 min,and (c,f) 450 °C for 30 min; red circles indicate pores.

Figure 4. XRD patterns of CoO/rGO obtained by thermal pyrolysis of the precursor at 250, 350, and 450 °C for 30 min in Ar.





discharge capacity gradually decreased to 380 mAh gÀ1 after 100cycles.BothCoO/rGO-350andCoO/rGO-450electrodes showedgood capacity retentionupon cycling. It was found that CoO/rGO-350 sample depicted higher discharge capacities, which increased

gradually during the first 20 cycles to 998 mAh gÀ1

and decreasedslightly to 732 mAh gÀ1 during the 100th cycle. The CoO/rGO-450 sample delivered a high capacity of 890 mAh gÀ1 during thesecond cycle. However, the capacity faded gradually to 663 mAhgÀ1 during the 100th cycle. Here, the calculated weight percentageof Co3O4 in Co3O4/rGO is ∼89.3%, and that of CoO in CoO/rGO is ∼85.8% (see Supporting Information Figure S11).

EIS was used to understand the relevance of morphology andsurface area of the synthesized CoO/rGO and Co3O4/rGO withthe electrochemical performance in terms of the total internalelectrochemical impedances of a cell. The characteristic impe-dance curves (Nyquist plots) for the CoO/rGO and Co3O4/rGO samples annealed at diff erent temperatures are shown inFigure 6. In impedance spectroscopy, high frequency activity is

attributed to charge transfer phenomenon, whereas the low frequency region of the spectrum is ascribed to the mass transferprocess.41,42 The Nyquist plots for the samples annealed atdiff erent temperatures for the two hybrid systems were similarexcept for the diameters of the semicircles, and thereby theassociated impedance values. This means that the diameter of thesemicircle is strongly dependent on the synthesis temperature,surface area, and grain/grain interfaces, which may have a directimpact on the Li storage performance.43À48 In order to quantify these respective values, a theoretical model consisting of resis-tance, capacitance, Warburg impedance, and intercalation im-pedance was used to fit the experimental data (see SupportingInformation, Figure S12 and Table 1).41,42 The surface film and

inter grain/grain boundary resistance are important factors thatdetermine the electrochemical performance of a given sample.The thickness of surface film over active electrode aff ects theperformance of a given electrode, since a very thick layer may

prevent the eff ective charge transfer and diff usion process from/or to the electrolyte/electrode. Meanwhile, the number of grainsand thereby grain boundaries give rise to the high charge transferimpedance and thereby a continuous capacity fading upon along-term cycling.44,45 The impedance curves obtained for theCoO/rGO and Co3O4/rGO samples were identical. However, a variation in the diameter of the semicircles with annealingtemperatures was observed.This was attributed to thediff erencesin the surface area and grain size/grain boundaries interfaces inthe various samples. From the analysis of the data obtained forthe hybrid composites (see Supporting Information, Figure S9andTable 1),it was noted that several factors (surface area, grain-size, interparticle/grain interfaces) were competing with eachother, which might aff ect the Li-ion storage performance. For

example, the 250 °C annealed samples (CoO/rGO-250 orCo3O4/rGO-250) showed largersemicircles,whichwas attributedto their higher surface resistance or interparticle impedance withintheelectrode.Thediameter of thesemicircles reducedsignificantly for the samples annealed at 450 °C (CoO/rGO-450 or Co3O4/rGO-450), which indicated that the surface film and intergrain boundary were smaller. Higher annealing temperature led to largergrain size, which corresponded to longer diff usion length anddegraded Li-ion storage performance (see Figure 5). The sampleannealed at 350 °C was found to be appropriate in terms of theinternal impedance of the cell, with the smallest semicircleindicating the lowest surface film resistance and optimizedgrain/grainÀgrain interface to achieve optimum Li-ion storage

Figure 5. (a) Cycling performance of Co3O4/rGO electrodes at a current density of 180 mA gÀ1 (0.2 C) within a voltage window of 0.01À3.0 V. Here,1 C is equal to 891 mA gÀ1. (b) Cycling performance of CoO/rGO electrodes at a current density of 150 mA gÀ1 (0.2 C) within a voltage window of 0.01À3.0 V. Here, 1 C is equal to 716 mA gÀ1.

Figure6. Nyquist plots of Co3O4/rGO andCoO/rGOelectrodes obtained by applying a sine wave with amplitudeof 10.0 mV over thefrequency range10 kHz to 0.1 Hz.





performance. The above results indicated that the Li storageperformance of the samples depended on the surface area, thecrystalinity, and the size of the grain/particles, which compete toeach other. For samples annealed at a low temperature such as250 °C, their specific surface area is high but their crystalinity ispoor. In this case, the diff usionoftheLiinthegrainisnotefficient.For samples annealed at a high temperature such as 450 °C, their

crystalinity is good, but their specifi

c surface area is low. Thus, theLi intercalation site/area is limited. Only for samples annealed at350 °C do they possess large specific surface area and the goodcrystalinity, which demonstrate better Li storage performance.

’CONCLUSION

We have developed a facile chemical approach to producenanohybrids with porous Co oxides nanowall arrays onto rGOsheets. The composition, grain size, and surface area of Co oxide/rGO hybrids were tunable through controlled annealing processes,e.g., under diff erent gas environments and diff erent temperatures.These Co oxide/rGO nanohybrids showed structural-process-dependent performances as anode materials for Li-ion batteries.

Both Co3O4/rGO and CoO/rGO hybrid samples annealed at350 °C showed optimized Li storage performance with highdischarge capacitiesandgood cyclingstabilities. This study indicatedthat tuning the detailed structural properties of attractive rGO/MOhybrids should be considered of utmost importance in order torender their advanced performances as Li-ion battery electrodes.

’ASSOCIATED CONTENT

bS Supporting Information. Additional figures and table asdescribed in the text. This material is available free of charge viathe Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail correspondence to [email protected].

’ACKNOWLEDGMENT

The authors gratefully acknowledge AcRF Tier 1 RG 31/08of MOE (Singapore), NRF2009EWT-RP001-026 (Singapore), A*STAR SERC Grant 1021700144 and AcRF Tier 2 (MOE2010-T2-1-017) from MOE (Singapore). H.Z. thanks the sup-port of AcRF Tier 2 (ARC 10/10, No. MOE2010-T2-1-060)from MOE (Singapore) and the New Initiative fund FY 2010(M58120031) from NTU, Singapore.

’REFERENCES

(1) Shu,Q. K.; Wei,J. Q.; Wang, K.L.;Song,S. A.; Guo,N.;Jia, Y.; Li, Z.; Xu, Y.; Cao, A. Y.; Zhu, H. W.; Wu, D. H. Chem. Commun. 2010 , 46 , 5533.

(2) Burghard, M.; Klauk, H.; Kern, K. Adv. Mater. 2009 , 21 , 2586.(3) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates,

B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. Adv. Mater. 2003 , 15 , 353.(4) Zhou, W.W.; Zhu,J. X.; Li, D.; Hng,H. H.; Boey, F.Y. C.; Ma, J.;

Zhang, H.; Yan, Q. Y. Adv. Mater. 2009 , 21 , 3196.(5) Yan, Q.; Chen, H.; Zhou, W.; Hng, H. H.; Boey, F. Y. C.; Ma, J.

Chem. Mater. 2008 , 20 , 6298.(6) Purkayastha, A.; Yan, Q. Y.; Raghuveer, M. S.; Gandhi, D. D.; Li,

H. F.; Liu, Z. W.; Ramanujan, R. V.; Borca-Tasciuc, T.; Ramanath, G. Adv. Mater. 2008 , 20 , 2679.

(7) Purkayastha, A.; Yan, Q. Y.; Gandhi, D. D.; Li, H. F.; Pattanaik,G.; Borca-Tasciuc, T.; Ravishankar, N.; Ramanath, G. Chem. Mater.2008 , 20 , 4791.

(8) Zhang, X. J.; Shi, W. H.; Zhu, J. X.; Zhao, W. Y.; Ma, J.;Mhaisalkar, S.; Maria, T. L.; Yang, Y. H.; Zhang, H.; Hng, H. H.; Yan,Q. Y. Nano Res. 2010 , 3 , 643.

(9) Wang, H. L.; Casalongue, H. S.; Liang, Y. Y.; Dai, H. J. J. Am.Chem. Soc. 2010 , 132 , 7472.

(10) Liang, M. H.; Zhi, L. J. J. Mater. Chem. 2009 , 19 , 5871.(11) Chen, J. S.; Li, C. M.; Zhou, W. W.; Yan, Q. Y.; Archer, L. A.;

Lou, X. W. Nanoscale 2009 , 1 , 280.(12) Liu, J. P.; Li, Y. Y.; Fan, H. J.; Zhu, Z. H.; Jiang, J.; Ding, R. M.;

Hu, Y. Y.; Huang, X. T. Chem. Mater. 2010 , 22 , 212.(13) Hassoun, J.; Derrien, G.; Panero, S.; Scrosati, B. Adv. Mater.

2008 , 20 , 3169.(14) Chan, C. K.; Zhang, X. F.; Cui, Y. Nano Lett. 2008 , 8 , 307.(15) Zhu, J. X.; Sun, T.; Chen, J. S.; Shi, W. H.; Zhang, X. J.; Lou,

X. W.; Mhaisalkar, S.; Hng, H. H.; Boey, F.; Ma, J.; Yan, Q. Y. Chem. Mater. 2010 , 22 , 5333.

(16) Cui, L. F.; Yang, Y.; Hsu, C. M.; Cui, Y. Nano Lett. 2009 , 9 ,3370.

(17) Cho, J. Electrochim. Acta 2008 , 54 , 461.(18) Reddy, A.L. M.; Shaijumon, M.M.; Gowda, S.R.; Ajayan, P. M.

Nano Lett. 2009 , 9 , 1002.(19) Zhu, J. X.; Zhu, T.; Zhou, X. Z.; Zhang, Y. Y.; Lou, X. W.; Chen, X. D.; Zhang, H.; Hng, H. H.; Yan, Q. Y. Nanoscale 2011 , 3 , 1084.

(20) Shi, W. H.; Zhu, J. X.; Sim, D. H.; Tay, Y. Y.; Lu, Z. Y.; Zhang, X. Y.; Sharma, Y. K.; Srinivasan, M.; Zhang, H.; Hng, H. H.; Yan, Q. Y. J. Mater. Chem. 2010 , 21 , 3422.

(21) Paek, S. M.; Yoo, E.; Honma, I. Nano Lett. 2009 , 9 , 72.(22) Wang, H.;Cui,L.-F.;Yang, Y.;Sanchez Casalongue, H.;Robinson,

J. T.; Liang, Y.; Cui, Y.; Dai, H. J. Am. Chem. Soc. 2010 , 132 , 13978.(23) Zhou, G. M.; Wang, D. W.; Li, F.; Zhang, L. l.; Li, N.; Wu, Z. S.;

Wen, L.; Lu, G. Q.; Cheng, H. M. Chem. Mater. 2010 , 22 , 5036.(24) Huang,X.;Yin, Z.Y.;Wu, S.X.;Qi,X. Y.; He, Q.Y.;Zhang,Q. C.;

Yan, Q. Y.; Boey, F.; Zhang, H. Small 201110.1002/smll.201002009.(25) Wang, D. H.; Choi, D. W.; Li, J.; Yang, Z. G.; Nie, Z. M.; Kou,

R.; Hu, D. H.; Wang, C. M.; Saraf, L. V.; Zhang, J. G.; Aksay, I. A.; Liu, J.

ACS Nano 2009 , 3 , 907.(26) Wang, D. H.; Kou, R.; Choi, D.; Yang, Z. G.; Nie, Z. M.; Li, J.;Saraf, L. V.; Hu, D. H.; Zhang, J. G.; Graff , G. L.; Liu, J.; Pope, M. A.;

Aksay, I. A. ACS Nano 2010 , 4 , 1587.(27) Hummers, W.S.; Off eman,R.E. J. Am.Chem.Soc. 1958 , 80 ,1339.(28) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc.

2008 , 130 , 5856.(29) Zhou, X. Z.; Huang, X.; Qi, X. Y.; Wu, S. X.; Xue, C.; Boey,

F.Y.C.;Yan,Q.Y.;Chen,P.;Zhang,H. J.Phys. Chem.C 2009 , 113 , 10842.(30) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.;

Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff , R. S. Carbon2007 , 45 , 1558.

(31) Wang, G. X.; Chen, Y.; Konstantinov, K.; Lindsay, M.; Liu,H. K.; Dou, S. X. J. Power Sources 2002 , 109 , 142.

(32) Yao, W. L.; Yang, J.; Wang, J. L.; Nuli, Y. J. Electrochem. Soc.2008 , 155 , A903.

(33) Do, J. S.; Weng, C. H. J. Power Sources 2006 , 159 , 323.(34) Pan, D. Y.; Wang, S.; Zhao, B.; Wu, M. H.; Zhang, H. J.; Wang,

Y.; Jiao, Z. Chem. Mater. 2009 , 21 , 3136.(35) Li, W. Y.; Xu, L. N.; Chen, J. Adv. Funct. Mater. 2005 , 15 , 851.(36) Zheng, J.; Liu, J.; Lv, D. P.; Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.;

Huang, R. B.; Zheng, L. S. J. Solid State Chem. 2010 , 183 , 600.(37) Rahman, M. M.; Wang, J. Z.; Deng, X. L.; Li, Y.; Liu, H. K.

Electrochim. Acta 2009 , 55 , 504.(38) Ryu, J.; Kim, S.W.; Kang, K.; Park, C.B. ACS Nano 2010 , 4 , 159.(39) Xiao, A.; Yang, J.; Zhang, W. J. Porous Mater. 2010 , 17 , 583.(40) Yang, S. B.; Feng, X. L.; Ivanovici, S.; Mullen, K. Angew. Chem.,

Int. Ed. 2010 , 49 , 8408.(41) Sharma, Y.; Sharma, N.; Rao, G. V. S.; Chowdari, B. V. R. Solid

State Ionics 2008 , 179 , 587.





(42) Sharma,Y.; Sharma,N.; Rao, G. V. S.; Chowdari, B. V. R. Chem. Mater. 2008 , 20 , 6829.

(43) Yuan, Z.; Huang, F.; Feng, C.; Sun, J.; Zhou, Y. Mater. Chem. Phys. 2003 , 79 , 1.

(44) Hassan, M. S.; Akhtar, M. S.; Shim, K. B.; Yang, O. B. NanoscaleRes. Lett. 2010 , 5 , 735.

(45) La Mantia, F.; Vetter, J.; Novak, P. Electrochim. Acta 2008 , 53 ,4109.

(46) Needham, S. A.; Wang, G. X.; Konstantinov, K.; Tournayre, Y.;Lao, Z.; Liu, H. K. Electrochem. Solid State Lett. 2006 , 9 , A315.

(47) Shaju, K. M.; Kumar, V. G.; Rodrigues, S.; Munichandraiah, N.;Shukla, A. K. J. Appl. Electrochem. 2000 , 30 , 347.

(48) Connor, P. A.; Irvine, J. T. S. Electrochim. Acta 2002 , 47 , 2885.

Documents

JP_2011_Zhu_Cobalt Oxide Nanowall Arrays on Reduced Graphene Oxide