2012-Nanoscale-Porous Co3O4 nanowires derived from long Co(CO3)0.5(OH)$0.11H2O

Porous Co3O4nanowires derived from long Co(CO3)0.5(OH)$0.11H2O nanowires with improved supercapacitive propertiesBao Wang,ab Ting Zhu,a Hao Bin Wu,a Rong Xu,a Jun Song Chen*ab and Xiong Wen(David)Lou*ab Received2nd December2011,Accepted6th January2012DOI:10.1039/c2nr11897aPorous Co3O4nanowires with large aspect ratio have been obtained by annealing longCo(CO3)0.5(OH)$0.11H2O precursor nanowires synthesized by a facile hydrothermal method.Theresults show that the amount of the additive(urea)has an important impact on the morphology of theas-synthesized cobalt-carbonate-hydroxide intermediate,where the uniformity and the overallstructure can be controlled by changing the urea concentration.After the heat treatment,the as-obtained phase-pure Co3O4nanowires with a well retained structure are applied as the electrodematerial for supercapacitors,and the sample exhibits excellent performance with a high specificcapacitance of240F gÀ1after2000charge/discharge cycles,corresponding to a retention of98%of theinitial capacitance.1.IntroductionIn recent years,nanostructured materials have been intensively studied in manyfields such as batteries,capacitors,catalysis and bionanotechnology.1–5As an important semiconducting transi-tion metal oxide,tricobalt tetraoxide(Co3O4)has attracted enormous research interest because of its applications in many fields including gas sensors,catalysts,supercapacitors and lithium-ion batteries.6–34As previously summarized in a review article,30Co3O4has been engineered into different morphologies via various reaction systems.It is general practice that the nanostructured Co3O4is usually obtained via a two-step approach,where Co-based intermediate compounds,like cobalt carbonate(CoCO3),cobalt hydroxide(Co(OH)2),and cobalt-carbonate-hydroxide(Co(CO3)0.5(OH)$0.11H2O),arefirst synthesized followed by thermal annealing in air,as the control over the morphology of these intermediates is relatively less challenging.8Among all the different Co3O4nanomaterials,one-dimensional(1D)structures,including nanorods,nanotubes,or nanowires,are relatively less reported,13,14,16,17,20,31,35–41compared to two-dimensional nanodiscs or three-dimensional nanocubes. Some previous reports have demonstrated that1D cobalt-carbonate-hydroxide nanostructures can be synthesized by controlling the anisotropic crystal growth of Co(CO3)0.5(OH)$ 0.11H2O.39,40However,well-dispersed Co3O4nanowires with large aspect ratio and a uniform size distribution have hardly been reported before.Electrochemical capacitors(ECs),also called supercapacitors, are fast-developing high-performance electrical energy storage devices which receive enormous research attention worldwide.In general,ECs could be categorized into two types.Electro-chemical double-layer capacitors(EDLCs)store electric energy via accumulation of charges at the surface of the electrode material,and many carbon-based materials possessing a very high specific surface area are usually applied in this type of ECs.42 The other type of ECs is called pseudocapacitors,and they utilize certain fast redox reactions taking place at the surface or near-surface regions of the electrode to store electric charges.5In pseudocapacitors,transition metal oxides and conducting poly-mers are usually employed as electrode materials.Co3O4with a theoretical specific capacitance of3560F gÀ1is considered as a very promising pseudocapacitive electrode material.41 However,the capacitance values obtained from experimental measurements in previous reports are significantly lower than the theoretical one.It is thus highly desirable to develop simple methods to prepare unique Co3O4nanomaterials with improved capacitive performance.In this work,we demonstrate a facile hydrothermal method to synthesize long Co(CO3)0.5(OH)$0.11H2O nanowires and their transformation to porous Co3O4nanowires.Cobalt sulfate (CoSO4)is used as the precursor which is dissolved in a glycerol–water mixed solvent with urea being the additive.The interme-diate compound,cobalt-carbonate-hydroxide(Co(CO3)0.5(OH)$ 0.11H2O),is obtained after the hydrothermal treatment.Inter-estingly,in this hydrothermal synthesis,the morphology of the product can be easily controlled by varying the concentration of urea.These Co(CO3)0.5(OH)$0.11H2O nanowires are subse-quently converted into phase-pure Co3O4nanowires by anneal-ing in air without causing any substantial changes in the morphology.When the as-prepared porous Co3O4nanowires area School of Chemical and Biomedical Engineering,Nanyang Technological University,70Nanyang Drive,Singapore637457,Singapore.E-mail: xwlou@.sgb Energy Research Institute@NTU,Nanyang Technological University,50 Nanyang Drive,Singapore637553,Singapore Dynamic Article Links CNanoscaleCite this:Nanoscale,2012,4,2145/nanoscale PAPERapplied for supercapacitors,they deliver a very high specificcapacitance of240F gÀ1after2000charge/discharge cycles,corresponding to an outstanding cyclic retention of98%.2.Experiment sectionMaterials preparationThe precursor Co(CO3)0.5(OH)$0.11H2O nanowires are synthe-sized by a simple hydrothermal method.All the chemicals wereof analytical grade and used as received without further purifi-cation.In a typical preparation process,0.56g CoSO4$7H2O (Sigma-Aldrich,99.9%)was dissolved in40mL of a mixturecontaining7mL of glycerol and33mL of de-ionized water.Afterstirring for about10min,a transparent solution was obtainedand then a certain amount of urea(0.05–0.2g)was added into theabove solution.After stirring for another30min,the obtainedsolution was transferred into a50mL Teflon-lined stainless steelautoclave,followed by heating at170 C for a period of24h in anelectric oven.After that the autoclave was cooled naturally toroom temperature.The precipitate was collected and washedwith de-ionized water and ethanol for several times by centrifu-gation,then dried at60 C overnight.The Co(CO3)0.5(OH)$0.11H2O product was then calcined at400 C for2h ata ramping rate of0.5 C minÀ1to transform into Co3O4. Materials characterizationThe chemical composition of the samples was determined byX-ray diffraction(XRD,Bruker,D8—Advance X-ray Diffrac-tometer,Cu K a,l¼ 1.5406 A).The morphology of the synthesized products was examined usingfield emission scanningelectron microscopy(FESEM,JEOL,JSM-6700F,5kV)andtransmission electron microscopy(TEM,JEOL,JEM-2010F,200kV).The N2adsorption and desorption isotherm wasobtained using a Quantachrome Instruments Autosorb AS-6B. Electrochemical measurementThe working electrode was prepared by mixing80wt%of theactive material(e.g.,Co3O4nanowires),10wt%of the con-ducting agent(carbon black,super-P-Li),and10wt%of thebinder(polyvinylidene difluoride,PVDF,Aldrich).This mixturewas then pressed onto the nickel foam electrode(X2LabwaresPte.Ltd,99.99%)and dried at60 C.The electrolyte used wasa2M KOH aqueous solution.The capacitive performance of thesamples was tested on a CHI660C electrochemical workstationwith cyclic voltammetry and chronopotentiometry functionsusing a three-electrode cell where Pt foil serves as the counterelectrode and a standard calomel electrode(SCE)as the referenceelectrode.3.Results and discussionFig.1shows the as-synthesized Co(CO3)0.5(OH)$0.11H2Onanowires.It is clear from the panoramic view(Fig.1a)that thesample contains uniform and well-dispersed nanowires tens ofmicrometres in length.Under a higher magnification(Fig.1b),these nanowires are shown to have a uniform width along theaxial direction,and the diameter is approximately several tens of nanometres.Because of the large aspect ratio,the sample also exhibits very high structuralflexibility,resulting in bending of the nanowires.Such an interesting1D nanostructure is also revealed under transmission electron microscope(TEM)with a relatively smooth surface(Fig.1c and d).It has been pointed out that the carbonate and hydroxyl anions provided by the hydrolysis of urea have an important effect on the crystal growth of the product.The reactions involved in the system could be described as follows:40,43CO(NH2)2+H2O/2NH3+CO2[NH3+H2O/NH4++OHÀCO2+H2O/2H++CO32ÀCo2++OHÀ+0.5CO32À+H2O/Co(CO3)0.5(OH)$H2O Fig.1FESEM(a and b)and TEM(c and d)images of long Co(CO3)0.5(OH)$0.11H2O precursor nanowires obtained with0.1g of urea added into the reactionsystem.Fig.2FESEM images of Co(CO3)0.5(OH)$0.11H2O nanowires(a and b)andflower-like spheres(c and d)obtained with0.05and0.2g of urea, respectively.The OH Àand CO 32Àions provided by the slow hydrolysis of urea would probably favor the gradual growth of the nanowires along their longitudinal axis with better crystallinity.However,the direct addition of Na 2CO 3and NaOH into the system will cause fast consumption of Co 2+ions at the initial stage and lead to formation of much shorter nanorods.40We thus investigate the morphology of the samples prepared with different urea concentrations.Fig.2a and b show FESEM images of the sample synthesized with the addition of only 0.05g of urea.Apparently,the obtained sample still assumes a wire-like morphology.However,most of the nanowires are not well dispersed and instead assembled into bundles.With a closer examination (Fig.2b),the 1D nanostructures are actually notuniform.On the other hand,when the amount of urea in the system is increased to 0.2g,the obtained product contains interesting sphere-like structures with a diameter of about 5m m (Fig.2c).These microspheres are composed of very fine wire-like subunits pointing radially outward (Fig.2d).The above results suggest that the concentration of urea has a significant effect on the morphology of the product.We hypothesize that the carbonate ions produced via the hydrolysis of urea not only participate in the formation of the intermediate compound and facilitate the anisotropic growth of the nanowires,but also behave like coordinating agents which help to organize and give rise to a more complex hierarchical structure at a high concen-tration.Furthermore,the control experiment without the addi-tion of glycerol produces a mixture of nanowire bundles and a large amount of irregular nanoparticles,indicating that glyc-erol also plays an important role in the formation of the desired structure of the product.The crystal phase of the as-synthesized product is determined by X-ray diffraction (XRD),with the result shown in Fig.3a.All the identified peaks can be assigned to pure orthorhombic Co(CO 3)0.5(OH)$0.11H 2O (JCPDS card number 48-0083,S.G.:P 2212,a ¼8.79 A,b ¼10.15 A,andc ¼4.43 A).39,40Theabsence Fig.3X-Ray diffraction (XRD)patterns of (a)long Co(CO 3)0.5(OH)$0.11H 2O nanowires and (b)corresponding Co 3O 4nanowires obtained after heattreatment.Fig.4Thermal gravimetric analysis (TGA;a)and the N 2adsorption–desorption isotherm (b)of Co 3O 4nanowires obtained after heat treatment.The inset in (b)is the pore sizedistributions.Fig.5FESEM (a and b)and TEM (c and d)images of Co 3O 4nanowires obtained after heat treatment.The inset in (d)is the high-resolution TEM image of the sample.of peaks due to other phases indicates the high purity of the as-synthesized material.The very high peak intensity suggests that thematerial is well crystallized.The XRD pattern(Fig.3b)confirmsthat the heat treated sample only contains pure cubic phase Co3O4(JCPDS card number42-1467,S.G.:Fd3m,and a o¼8.0837 A).34 This indicates the complete transformation of Co(CO3)0.5(OH)$0.11H2O into the Co3O4phase.Thermal gravimetric analysis(TGA,Fig.4a)shows that the major weight loss of$12.5%takesplace at200–350 C.Thus,this as-synthesized precursorcompound is annealed at400 C for2h.The N2adsorption–desorption isotherm of the sample is depicted in Fig.4b,and thecalcined Co3O4nanowires are shown to have a high surface area of $88m2gÀ1and a total pore volume of0.62cm3gÀ1.The morphology of the calcined product is studied by FESEMand TEM,with the data shown in Fig.5.It is apparent from theFESEM image(Fig.5a)that the well-dispersed wire-like structurecan be relatively well retained,although,as expected,the aspectratio is generally reduced.Under a higher magnification(Fig.5b),some broken wires could be observed,likely due to the smalllateral dimension granting limited structural robustness.TheTEM analysis(Fig.5c)confirms the above results.Interestingly,with a closer examination(Fig.5d),the twin nanowires appear to be highly porous and composed of densely packed nano-particles.40This type of porous1D structure is believed to be very suitable for supercapacitor applications.The inset in Fig.5d shows some visible lattice fringes with an interplanar distance of $0.24nm,corresponding to the(311)plane of Co3O4.Thus,we next investigate the electrochemical properties of these Co3O4nanowires as the electrode material for super-capacitors.Fig.6a shows the representative cyclic voltammogram (CV)curves tested in an aqueous electrolyte containing2M KOH from0.1to0.6V(vs.SCE)at different sweep rates.In general,the current intensities increase with the scan rate.At a low scan rate of 5mV sÀ1,two current peaks can be clearly identified at$0.45V and$0.40V during the cathodic sweep,but only one broad oxidation peak at$0.53V is observed during the anodic sweep. The reactions involved could be described as follows:41Co3O4+H2O+OHÀ43CoOOH+eÀ(1)CoOOH+OHÀ4CoO2+H2O+eÀ(2) The average specific capacitance can then be calculated from the CV curves to be$251,247,239,229,185,and126F gÀ1atthe Fig.6Electrochemical characterizations of the as-prepared Co3O4nanowires.(a)Cyclic voltammetry(CV)curves at various scan rates.(b)Average specific capacitance at various scan rates.(c)Galvanostatic discharge curves at various discharge current densities.(d)Specific capacitance at various discharge current densities.(e)Galvanostatic charge and discharge voltage profiles at a current density of5A gÀ1.(f)Cycling performance at a current density of5A gÀ1.scanning rates of1,2,5,10,20and50mV sÀ1,respectively (Fig.6b).Fig.6c shows the constant-current discharge curves of the as-prepared Co3O4nanowires.A distinct voltage plateau can be observed from0.5to0.4V,corresponding to the redox reaction (eqn(1)).The specific capacitance can also be calculated based on the following equation:C m¼IÂD t/(D VÂm)where C m(F gÀ1)is the specific capacitance,I(A)is the applied current,D t(s)is the time taken during discharge,D V(V)is the voltage window,and m(g)is the mass of the active material. Therefore,the specific capacitance can be calculated to be260, 248,233,213,and171F gÀ1at the discharge current densities of 2,3,5,8,and15A gÀ1,respectively(Fig.6d).Fig.6e illustrates the charge/discharge voltage profiles of the sample at a discharge current density of5A gÀ1for thefirst10cycles,and clearly a Coulombic efficiency of nearly100%can be reached.Fig.6f shows the specific discharge capacitance vs.charge/discharge cycle number at a constant discharge current density of5A gÀ1.It is obvious that the sample can deliver a high specific capacitance of$250F gÀ1at the end of2000charge/discharge cycles.This corresponds to a cyclic retention of$98%.Remarkably,the as-prepared Co3O4nanowires in the present study exhibit superior supercapacitive performance compared to other Co3O4-based electrode materials,reported by us and others,evaluated under similar conditions.33,34,444.ConclusionsIn summary,we have developed a simple hydrothermal method to synthesize long Co(CO3)0.5(OH)$0.11H2O nanowires with a uniform size distribution.This method uses CoSO4as the precursor in a glycerol/water mixed solvent with urea as additive. The morphology of the product can be controlled by changing the concentration of urea.The as-synthesized Co(CO3)0.5(OH)$ 0.11H2O precursor nanowires can be facilely converted into phase-pure porous Co3O4nanowires via thermal annealing. When the as-prepared Co3O4nanowires are evaluated as the electrode material for supercapacitors,they manifest enhanced performance with a specific capacitance of$250F gÀ1and a98% cyclic retention after2000cycles.These encouraging results confirm that properly tailored Co3O4nanomaterials can serve as electrode materials for high performance supercapacitors. 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