DaltonTransactions

PAPER

Cite this: DOI: 10.1039/c3dt50333j

Received 1st February 2013,Accepted 5th March 2013

DOI: 10.1039/c3dt50333j

www.rsc.org/dalton

Facile preparation of a cobalt hybrid/graphenenanocomposite by in situ chemical reduction: highlithium storage capacity and highly efficient removalof Congo red

Lixia Wang,a,b Jianchen Li,a Chunsheng Mao,a Lishu Zhang,a Lijun Zhao*a andQing Jianga

We report a facile approach to prepare a cobalt hybrid/graphene (Co/G) nanocomposite via a general

one-pot hydrothermal synthesis. NaBH4 is used as the reducing agent. Co/G nanocomposite possesses

narrow size-distribution and good dispersion, providing tremendous potential for energy and environ-

ment applications. As a proof of concept, we demonstrate the use of the Co/G nanocomposite in a

lithium-ion battery and an adsorbent for Congo red (CR), respectively. More importantly, more than 97%

of capacity retention (605 mAh g−1) is retained after 50 cycles, indicative of high charge/discharge rever-

sibility of the Co/G nanocomposite electrode. Furthermore the CR removal ability of the Co/G nanocom-

posite can reach 934.9 mg g−1.

Introduction

Due to the increase demanding, in recent year, of rechargeablebatteries of higher specific energy density, the presence of met-allic Co in the active materials of Li ion batteries has attractedextensive attention for them as anode in Li-ion battery.1–4

First, the metallic Co in the active materials of Li ion batteriescan greatly increase the electronic conductivity of the electrodematerials. Second, the catalytic effect of Co facilitates thereversible decomposition of Li2O and the solid-electrolyteinterface (SEI).5,6 Third, metal Co portion can provide a favor-able electrical contact between adjacent particles and greatlyimproves the efficiency of the electronic connection betweenthe active material and the current collector. However, puremetal Co nanoparticles as anode active material in Li-ion bat-teries are few and not deeply researched, because a pure Coelectrode suffers severely from poor cycle ability due to mech-anical fatigue caused by volume change during lithium inser-tion and extraction processes. Therefore, the electrochemicalperformance of Co nanoparticles was not excellent.4 In orderto improve the electrochemical performance of Co nanoparti-cles, some important factors should be considered, including

high stability, small particle size and good dispersion, and soon. To solve the above-mentioned problems, it would be aneffective measure to integrate Co nanoparticles with graphene(G) nanosheets. The unusual attributes of G nanosheetsinclude excellent thermal conductivity and fracture strength,high specific surface area,7–9 and fascinating transport pheno-mena such as the quantum Hall effect.10,11 G nanosheets incomposite materials could also act as electronic conductivechannels, which could improve the electronic conductivity ofthe nanocomposite and greatly decrease the Ohmic loss.1

Furthermore, the unique structure and excellent flexibility of Gnanosheets could not only provide enough void spaces tobuffer the volume change and decrease the strain associatedwith the volume variation, but also prevent the aggregation ofCo particles during cycling, which could alleviate the degrad-ing of the electrode. Of course, the electrostatic attractionbetween electron-rich Co atoms can enable Co to be firmlyanchored on the G surface and enhance the conjunction stab-ility of the composite. Most importantly, the performance ofthe G also could be further enhanced through decoration withCo nanoparticles. G nanosheets usually suffer from seriousagglomeration and restacking during utilization, due to theπ–π interactions between neighboring sheets. The presence ofCo nanoparticles between G nanosheets effectively preventsthe agglomeration of G nanosheets and consequently keepstheir high active surface area, which is favorable for increasingthe Li storage capacity of G in the nanocomposite.2 Incorporat-ing G within Co nanoparticles may be a suitable route to

aKey Laboratory of Automobile Materials (Jilin University), Ministry of Education and

School of Materials Science and Engineering, Jilin University, Changchun 130022,

China. E-mail: lijunzhao@jlu.edu.cn; Fax: +86 431 85095876; Tel: 86 431 85095878bSchool of Mechanical Science and Engineering, Northeast Petroleum University,

Daqing 163318, China

This journal is © The Royal Society of Chemistry 2013 Dalton Trans.

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article OnlineView Journal

obtain an unusual Co anode active material in Li-ion battery,possessing both high reversible lithium-ion storage capacityand Coulombic efficiency. Such Co/G nanocomposite is envi-saged to synergistically offer unique properties, providing tre-mendous opportunities for improving and expandingapplications.

In addition to the Co/G composite use as an anode materialin Li-ion batteries, the removal ability of Co/G for a dye froman aqueous solution was also investigated in detail. Rapidindustrialization has led to an increased discharged waste-water containing dyes, which have detrimental effects on theenvironment and human health. Among those dyes, Congored (CR) is a commonly identified contaminant because of itshigh toxicity and mobility.12–14 The magnetic properties of theCo/G nanocomposite allow high-efficient separation afterremoval of CR from the liquid suspension. And the largespecific surface area of the Co/G nanocomposite facilitates theadsorption of CR from the aqueous solution.

Herein, we report a facile approach to prepare a Co/G nano-composite via a one-pot hydrothermal synthesis. In thisapproach, NaBH4 is used as reducing agent, and GO and CoCl2are reduced simultaneously during the hydrothermal process.Such composites with the well-dispersed Co nanoparticlesanchored firmly on the G nanosheets are advantageous forinhibiting aggregation, and consequently providing a higheravailable surface area and enhancement of lithium storagecapacity and adsorption capacity.

Materials and methodsPreparation of GO

The graphite oxide (GO) was synthesized from natural graphitepowder based on a modified Hummers method,15 where a9 : 1 mixture of concentrated H2SO4–H3PO4 (120 : 13.3 mL) wasadded to a mixture of graphite flakes (1.0 g, 1 wt equiv.) andKMnO4 (6.0 g, 6 wt equiv.). The reaction was then heated to50 °C and stirred for 12 h. The reaction was cooled to roomtemperature and poured onto ice (400 mL) with 30% H2O2

(3 mL). The resultant was centrifuged (10 000 rpm for 20 min)and washed several times with 5% HCl aqueous solution, thenby deionized water until the pH of the supernatant becameneutral. Finally, a homogeneous GO aqueous dispersion wasobtained and used for further characterizations and thechemical reduction.

Preparation of the Co/G nanocomposite

A one-pot method for water-phase synthesis of the Co/G nano-composite with NaBH4 reduction was demonstrated. In thefirst step, 15 mg of oxidized graphite powder was dispersed in15 mL of doubly distilled water and sonicated for 30 min usingan ultrasonic cleaner to exfoliate oxidized graphite particles toGO sheets. After a volume of 5 mL of mixture solution contain-ing 0.1 g of CoCl2·6H2O was added, the mixture solution wassonicated for 1 hour. Then, a volume of 5 mL aqueous solutioncontaining 0.1 g of NaBH4 was gradually added to the mixture

solution of GO and CoCl2. After that, the mixture solution wasstirred for 30 min at room temperature. Subsequently, thesolution was transferred into a 30 mL Teflon-lined stainlesssteel autoclave. The autoclave was sealed and maintained at200 °C for 12 h and then cooled to room temperature naturally.After the completion of the reaction, the solid product was col-lected by magnetic filtration and washed several times withdeionized water and absolute ethanol, respectively. The finalproduct was dried in a vacuum oven at 60 °C for 12 hours. Ablack Co/G nanocomposite was prepared.

Material characterizations

The phases were identified by means of X-ray diffraction (XRD)with a Rigaku D/max 2500pc X-ray diffractometer with Cu Kαradiation (λ) 1.54156 (Å) at a scan rate of 0.02°/1 (s), mor-phologies were characterized by a JEOL JSM-6700F field emis-sion scanning electron microscope (FESEM) operated at anacceleration voltage of 10.0 kV. Transmission electronmicroscopy (TEM) and high-resolution TEM (HRTEM) obser-vations were carried out on a JEOL 2100F with an emissionvoltage of 200 kV. The hysteresis loops were measured on aVSM-7300 vibrating sample magnetometer (VSM) (Lakeshore,USA) at room temperature. N2 adsorption–desorption iso-therms were measured at the liquid nitrogen temperature(77 K) using a JW analyzer (JW-RB224, Beijing). Samples weredegassed at 120 °C overnight before measurements. IR spectraof the samples were characterized using a FTIR spectro-photometer (NEXUS, 670) in KBr pellets. An Agilent Cary 50UV-vis spectrophotometer was used for determination of theCR concentration in the solutions.

Electrochemical measurements

The working electrodes were prepared by mixing 80 wt% activematerial (Co/G nanocomposite), 10 wt% acetylene black, and10 wt% polyvinylidene fluoride (PVDF, 5 wt%) binder dissolvedin N-methyl-2-pyrrolidinone. The slurry was uniformly pastedon a Cu foil. Such prepared electrode sheets were dried at100 °C for 12 h in a vacuum oven. The CR2025-type half-coincells were assembled in an argon-filled glove box with H2Oand O2 contents below 1 ppm. Metallic lithium foil was usedas the counter and reference electrode. The electrolyte consistsof a solution of 1 M LiPF6 in a mixture of ethylene carbonate(EC), ethyl methyl carbonate (EMC) and dimethyl carbonate(DMC) with an EC : EMC : DMC volume ratio of 1 : 1 : 1.Charge–discharge performance was evaluated by a LANDCT2001A battery instrument at a constant current density inthe voltage range of 0.001–3.0 V at room temperature. Cyclicvoltammetry measurements were carried out on a CHI650Delectrochemical workstation over the potential range 0.001–3.0V vs. Li/Li+ at a scanning rate of 0.1 mV s−1.

Batch adsorption experiments

Congo red (CR) (sodium salt of benzidinediazobis-1-naphthyl-amine-4-sulfonic acid) is metabolized to benzidine, a knownhuman carcinogen and exposure to this dye can cause someallergic responses.16 CR is a typical example of anionic

Paper Dalton Transactions

Dalton Trans. This journal is © The Royal Society of Chemistry 2013

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online

secondary diazo dyes, which is very difficult to degrade photo-catalytically because of its large and complicated structure,which contains one central biphenyl group and two symmetricnaphthalenic groups.17 Its structure is illustrated in Fig. 4a.

The stock solution of CR (1 g L−1) was prepared in deio-nized water and desired concentrations of the dye wereobtained by diluting the same with deionized water. The cali-bration curve of CR was prepared by measuring the absor-bance of different predetermined concentrations of thesamples at λmax = 497 nm using UV-vis spectrophotometer (CRhas a maximum absorbency at wavelength 497 nm on a UV-visspectrophotometer). The amount of adsorbed CR (mg g−1) wascalculated based on a mass balance equation as given below:

qe ¼ ðC0 � CeÞ � VW

ð1Þ

where qe is the equilibrium adsorption capacity per gram dryweight of the adsorbent, mg g−1; C0 is the initial concentrationof CR in the solution, mg dm−3; Ce is the final or equilibriumconcentration of CR in the solution, mg dm−3; V is the volumeof the solution, dm3; and W is the dry weight of the hydrogelbeads, g.

Take one adsorption of CR for example. Standard solutionwith initial concentrations of 100 mg L−1 was prepared. Then,5 mg of the Co/G nanocomposite was added to 50 mL of abovesolution under stirring. After a specified time, the solid andliquid were separated by a magnet.

Results and discussion

The protocol for the synthesis of the Co/G nanocomposite isillustrated in Scheme 1. Firstly, graphite oxide (GO) was syn-thesized from natural graphite using the Hummers method,which is widely used to prepare GO. Before reduction, the GOdispersion was light brown, as shown in Scheme 1a. Secondly,Co2+ were anchored covalently to GO through the surface func-tional groups of GO.18,19 Thirdly, the Co2+–GO was reduced to

Co/G nanocomposite by NaBH4 after heating at 200 °C for12 h, and a black Co/G nanocomposite dispersion wasobserved, as shown in Scheme 1b. It is powerful evidence thatGO is indeed reduced by NaBH4 into G, because the colors ofthe dispersion are changed from light brown to dark black.

Structural and morphological characterization

Briefly, the Co/G nanocomposite was prepared using a veryfacile hydrothermal method. The Co nanoparticles with amean diameter of about 60 nm are closely and homogeneouslyanchored on the surface of G nanosheets (Fig. 1a and 1b). Afolding nature of G is clearly visible from Fig. 1c, the relativelydark area is due to the stacking of several G nanosheets. AHRTEM image (Fig. 1d) further reveals that the Co nanoparti-cles are the mixture of hcp- and fcc-Co. The two sets of inter-planar spacing are measured to be 0.218 and 0.205 nm,consistent with the (100) of hcp-Co and (111) of fcc-Co. Inaddition, the corresponding Fourier transform pattern (insetin Fig. 1d) clearly demonstrates the polycrystalline nature ofCo NPs in the Co/G nanocomposite. The crystal structure wasfurther investigated by XRD technique. Fig. 1e and 1f show thetypical XRD patterns of graphite, GO and Co/G nanocompo-site. The diffraction peaks of graphite at 26.4° and 54.6° couldbe attributed to (002) and (110) planes of the graphite (Fig. 1e).Oxidation treatment results in a decrease in the peak (002)intensity of graphite and the appearance of the diffractionpeak of the GO at 2θ = 11.4° (Fig. 1e). Compared with that ofpure Co NPs, an additional small and very broad (002) diffrac-tion peak appears at 2θ = 26.0°, which can be indexed into thedisorderly stacked G nanosheets (Fig. 1f).20 Nevertheless,the broad nature of the reflection indicates poor ordering ofthe nanosheets along the stacking direction, implying that thesample is composed of mostly few layers of G nanosheets.21,22

The diffraction peaks for the as-formed sample at 2θ = 41.63,44.57, 47.51, and 75.97°, which match the reflection planes of(100), (002), (101), and (110), respectively, could be indexed toa pure hcp-Co (JCPDS 05-0727). The diffraction peaks for theas-formed sample at 2θ = 44.20, 51.54 and 75.89°, whichmatch the reflection planes of (111), (200) and (220), respect-ively, could be indexed to a pure fcc-Co (JCPDS 15-0806). Theseresults indicate that the nanocomposite consists of disorderlystacked G nanosheets and Co nanoparticles with hcp and fccmixing structure, which is consistent with the crystal charac-terization using HRTEM.

The specific surface area of the as-prepared Co/G nano-composite is calculated to be 86.9 m2 g−1 which is muchhigher than the value of bare Co (10.2–21.1 m2 g−1),23 on thebasis of the Brunauer–Emmett–Teller (BET) analysis of thenitrogen absorption/desorption isotherm. Of more practicalsignificance, the improving surface area of Co/G nanocompo-site can facilitate its potential electrochemical and adsorptionapplications.

Magnetic properties

VSM technique was employed to study the magnetic propertiesof the Co/G nanocomposite. Fig. 2 shows a typical hysteresis

Scheme 1 Schematic illustration of the synthetic procedure for the fabricationof the Co/G nanocomposite.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2013 Dalton Trans.

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online

cure of the Co/G nanocomposite measured at room tempera-ture, indicating a saturation magnetization (Ms) of 74.2 emug−1 and a coercivity of (Hc) of 667.6 Oe. The Ms of Co/G nano-composite is much lower than that of Co nanoparticles, butthe Hc of Co/G nanocomposite is much higher than that of Conanoparticles.24,25 The decreasing Ms value may be due thepresence of the non-magnetic G. The obviously increasing Hc

value can be explained by considering the reduced influenceof Co nanoparticles on the rotation of magnetic dipolesbecause of the restraint of the G nanosheets. As a result, anincreased amount of energy (or higher magnetic field strength)was required to change the magnetization direction of thesealigned dipoles. Thus, the Co/G nanocomposite exhibited arelatively higher Hc.

Fig. 1 Co/G nanocomposite: (a) SEM image; (b) low-magnification TEM; (c) high-magnification TEM; (d) HRTEM images. Inset in (d) is the corresponding Fouriertransform pattern of Co/G nanocomposite; (e) XRD patterns of graphite and GO; (f ) XRD pattern Co/G nanocomposite.

Paper Dalton Transactions

Dalton Trans. This journal is © The Royal Society of Chemistry 2013

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online

Evaluation of the electrochemical performances

The electrochemical performances of the as-prepared Co/Gnanocomposite were evaluated by charge/discharge cycling inthe voltage range of 0.001–3.0 V (vs. Li/Li+) at a current densityof 100.0 mA g−1 for up to 50 cycles. Fig. 3a shows the charge/discharge curves of the Co/G electrode in the 1st, 10th, 30thand 50th cycles, respectively. In the first discharge step, a longvoltage plateau presents at 1.26 V for the Co/G nanocomposite,followed by a sloping curve down to the cutoff voltage of 0.001V. The first discharge and charge capacities are 920.3 and

587.7 mAh g−1 for Co/G nanocomposite electrode, with thefirst Coulombic efficiency of 63.8%. The following cycles arenearly overlapped indicating good capacity retention of theCo/G nanocomposite electrode after the 1st cycle. Since thesecond cycle, however, the Co/G nanocomposite electrode pre-sents much better electrochemical lithium storage perform-ance. After ten discharge/charge cycles, it exhibits a highreversible capacity of 583.8 mAh g−1. The Coulombic efficiencyrapidly rises from 63.8% in the first cycle to 97.40% in the 7thcycle and then remains above 97% in the following cycles(Fig. 3b).

Compared with capacity of Co (440 mAh g−1)4 and graphite(372 mAh g−1),2 the extra discharge capacity of the Co/G nano-composite may be attributed to the high intrinsic electronicconductivity and increasing specific surface area of the Co/Gnanocomposite. Another possible reason is that a largenumber of lattice defects in the typical nanostructure providemore active sites for the insertion of Li ions. On the otherhand, the reversible capacity, which is higher than the theo-retical value, can be attributed to the catalytic effect of Co,which facilitates the reversible decomposition of Li2O and thesolid electrolyte interface (SEI) layer.1,4 Compared with singleG or Co powder electrodes, the Co/G nanocomposite exhibits amuch better cycling performance. It can be seen that thereversible capacities of G and Co powder decrease from 378 to250 mAh g−1 1 and from 440 to only 280 mAh g−1,4 respectively,up to 30 cycles. More importantly, the reversible capacity of

Fig. 2 Hysteresis loop of the Co/G nanocomposite measured at room tempera-ture; the inset is the photograph showing the magnetic separation.

Fig. 3 (a) The charge/discharge curves of the Co/G electrode for 1st, 10th, 30th and 50th cycles at a current density of 100.0 mA g−1, respectively; (b) cyclingbehavior of the Co/G nanocomposite. Cycling took place between 0.001 and 3.0 V vs. Li/Li+ at a cycling rate of 100.0 mA g−1; (c) rate capability of the Co/G nano-composite at various current densities between 100.0 and 500.0 mA g−1; (d) cyclic voltammograms (CV) of the Co/G electrode at scan rate of 0.1 mV s−1 at 20 °C.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2013 Dalton Trans.

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online

the Co/G nanocomposite slightly increases with cycling andreaches 603 mAh g−1 after 50 cycles (Fig. 3b). It is important tonote that there is a strong synergistic effect between Co nano-particles and G nanosheets, which plays a central role in theexcellent cyclic performance of the Co/G nanocomposite. Thesynergy of Co and G can greatly avoid the degradation of elec-trode, improving its excellent electrochemical performance.

Furthermore, the Co/G nanocomposite exhibits a better ratecapability at various rates between 100 and 500 mA g−1

(Fig. 3c). For comparison, the Co/G nanocomposite keeps areversible capacity of 608 mAh g−1 after the 10th cycle at acurrent density of 100 mA g−1, whereas the reversible capacityof the Co/C electrode rapidly drops from 586 to 500 mAh g−1.3

The following reversible capacities of the Co/G nanocompositeand Co/C electrodes at other various rates are 597 and480 mAh g−1 for the 20th cycle at 200 mA g−1, 560 and358 mAh g−1 for the 30th cycle at 500 mA g−1, and 585 and410 mAh g−1 for the 40th cycle at 200 mA g−1, respectively,with the initial reversible capacity of 60.9% for Co/G and only55% for Co/C electrodes. It is noteworthy that the size and dis-persion of the Co nanoparticles on G nanosheets are crucialfactors for improving the cell performance, because small par-ticle size plus good dispersion can endow the nanocompositeelectrode a superior high surface area to buffer the volumechange of the metal, and it could also bring the required con-ductivity to individual nanoparticles and shorten the diffusionlength for Li ions, which are beneficial for high lithiumstorage and rate capability.

Fig. 3d shows the first three cyclic voltammogram (CV)curves of the Co/G nanocomposite at a scan rate of 0.1 mV s−1

at 20 °C. The initial three cycles almost overlap, indicatinggood capacity retention of the Co/G nanocomposite electrode.There are no strong peaks at the CV curves, because Co is con-sidered as an inert material. However, nanosized Co showshigh catalytic activity which facilitates the reversible decompo-sition of Li2O. It can be seen that the three curves have a broadpeak at 0.6 V in the cathodic scan, corresponding the for-mation of amorphous Li2O. The broad peak at 2.1 V in thecathodic scan curves corresponds to the Li insertion from theG nanosheets. Upon charge, two current peaks appeared at 2.4and 1.4 V. The anodic peaks at a potential of 2.4 V are assigned

to the decomposition of Li2O by Co,5 while peaks at 1.4 V areattributed to Li extraction from the G nanosheets.

In brief, the Co/G nanocomposite may be a possible futureanode active material in Li-ion batteries, possessing both highreversible lithium-ion storage capacity and Coulombicefficiency.

Evaluation of CR removal ability

For water treatment application, the Co/G nanocomposite isdesired to exhibit (1) high BET surface area; (2) good magneticproperties. An excellent magnetic performance can ensure ahigh-efficient magnetic separation after water treatment. ThisCo/G nanocomposite not only is used as anodes for Li ion bat-teries, but also can show potential uses as adsorbent. Adsorp-tion is commonly considered to be a fast and relatively cost-effective technology for water treatment. It normally occurs viaboth physical and chemical pathways, like activated carbon,which offers the advantage of its porous structure and surface-modified functionalities. Similarly, graphene-based materialshave been envisaged to have a high specific surface area,leading to its potential in the environmental field as aneffective choice for pollutant elimination or environmentalremediation.

Herein, the Co/G nanocomposite was used to remove CRfrom an aqueous solution. CR removal is relatively difficultbecause of its good water solubility and large and complicatedstructure. The CR removal abilities of the Co/G nanocompositewere shown in Fig. 4a. Aqueous solutions with an initial CRconcentration of 100 mg L−1 was used for the experiment atneutral pH = 7, adsorption different times from 0 to240 minutes, and T = 20 °C. The final CR removal ability of theCo/G nanocomposite is 934.9 mg g−1. The maximum adsorp-tion capacity (qmax) of the Co/Graphene nanocomposite for CRis listed in Table 1 with literature values of qmax of other car-bonaceous materials for CR adsorption.26–29 The adsorptionefficiency is far higher than the values reported in the litera-tures. In addition, the Co/G nanocomposite loaded with CRcan be quickly separated by a magnet, and then leaves a clearand colorless solution.

In order to understand the CR removal process, FTIR wasused to investigate the surface changes of the Co/G

Fig. 4 (a) Adsorption capacity of CR on Co/G nanocomposite; (b) FTIR spectra. Inset in (a) is the molecular structure of CR.

Paper Dalton Transactions

Dalton Trans. This journal is © The Royal Society of Chemistry 2013

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online

nanocomposite before and after CR removal, as shown inFig. 4b. For GO (green line), the spectrum exhibited thecharacteristic peaks of O–H (νO–H at 3427 cm−1), CvC (νCvC at1631 cm−1), C–O (νC–O at 1061 cm−1), CvO (νCvO at1730 cm−1) and deformation vibration absorption peaks ofO–H (νO–H at 1385 cm−1). This shows that GO is basically asingle atomic layer of carbon covered with epoxy, hydroxyl, car-bonyl, and carboxyl groups. Similar results have been reportedfor GO.20,30 For the FTIR spectrum of Co/G (red line), there areno bonds of metal oxides, indicating that the obtained cobaltnanoparticles are not oxidized in the synthesis process. Therewas a dramatic decrease in the intensities of the characteristicabsorption bands of oxygen functionalities (νO–H, νCvO, andνC–O). This further suggests that GO had been reduced to gra-phene sheets, which is consistent with the XRD and TEMresults. As seen from the Co/G + CR spectrum (black line), theweak bond at 579 cm−1 is believed to be associated with thestretching vibrations Co2+–O2−. Hence, oxidation andreduction reactions took place during the CR removal process.The weak bond at 3432 cm−1 can be attributed to N–H stretch-ing vibration. The new band at 1627 cm−1 can be attributed toCvC stretching vibration in benzene. New signals around1382 and 1175 cm−1 are due to the framework vibration ofbenzene rings. Furthermore, the SvO stretching vibration inCR is still observed from the Co/G + CR spectrum, but it shiftsto a low wave number of 1046 cm−1.31,32 Compared with FTIRspectroscopy of CR (blue line), the obvious decrease in the aro-matic ring absorption peak of SvO bond after treatment indi-cated the destruction of the CR structure by the Co/Gnanocomposite. By analysis of the FTIR spectra, we are certainthat CR molecular were loaded on the surface of the Co/Gnanocomposite by the means of physical adsorption andchemical reduction at the same time. In addition, all the

particles can be easily separated after adsorption by a magnet, asshown in Scheme 2. Quickly magnetic separation is an impor-tant feature which makes the Co/G nanocomposite suitable foreconomic operations in industrial wastewater treatment.

Conclusion

In summary, we have presented a facile one-pot hydrothermalmethod for the synthesis of a Co/G nanocomposite based on asimple reduction process using NaBH4 as reducing agent. Conanoparticles are embedded in the G matrix homogeneouslywhich both enhances electrical conductivity of electrodeeffectively and catalyzes Li-ion insertion and extraction in theelectrochemical process. Therefore, as-prepared Co/G nano-composite is an attractive anode material for high-perform-ance lithium ion battery. Furthermore, the magneticproperties and high specific surface area of the Co/G nano-composite enable its potential application in the field of watertreatment.

Acknowledgements

This work was financially supported by the Social Develop-ment Projects of Jilin Province (20120406) and the YouthResearch Foundation of Jilin Province (201101060).

References

1 Y. S. He, D. W. Bai, X. W. Yang, J. Chen, X. Z. Liao andZ. F. Ma, Electrochem. Commun., 2010, 12, 570.

2 Z. S. Wu, W. C. Ren, L. Wen, L. B. Gao, J. P. Zhao,Z. P. Chen, G. M. Zhou, F. Li and H. M. Cheng, ACS Nano,2010, 4, 3187.

3 J. C. Yue, X. T. Zhao and D. G. Xia, Electrochem. Commun.,2012, 18, 44.

4 D. Y. Kim, H. J. Ahn, J. S. Kim, I. P. Kim, J. H. Kweon,T. H. Nam, K. W. Kim, J. H. Ahn and S. H. Hong, Electron.Mater. Lett., 2009, 5, 183.

5 Y. M. Kang, M. S. Song, J. H. Kim, H. S. Kim, M. S. Park,J. Y. Lee, H. K. Liu and S. X. Dou, Electrochim. Acta, 2005,50, 3667.

6 J. R. González, R. Alcántara, F. Nacimiento and J. L. Tirado,Electrochim. Acta, 2011, 56, 9808.

7 J. L. Zhang, H. J. Yang, G. X. Shen, P. Cheng, J. Y. Zhangand S. W. Guo, Chem. Commun., 2010, 46, 1112.

8 J. Gao, F. Liu, Y. L. Liu, N. Ma, Z. Q. Wang and X. Zhang,Chem. Mater., 2010, 22, 2213.

9 C. Z. Zhu, S. J. Guo, Y. X. Fang and S. J. Dong, ACS Nano,2010, 4, 2429.

10 Y. Zhang, Y. W. Tan, H. L. Stormer and P. Kim, Nature,2005, 438, 201.

11 S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217.12 L. X. Wang, J. C. Li, Y. Q. Wang and L. J. Zhao, J. Hazard.

Mater., 2011, 196, 342.

Table 1 Adsorption capacities of CR dye on carbonaceous materials

Type of adsorbent qmax (mg g−1) Reference

Fe3O4@graphene nanocomposite 33.66 26Bael shell carbon 98.03 27Commercial activated carbon 493.80 28Bamboo dust carbon 101.90 28Coconut shell carbon 188.40 28Groundnut shell carbon 110.80 28Rice husk carbon 237.80 28Straw carbon 403.70 28Co/graphene nanocomposite 934.90 This study

Scheme 2 Schematic adsorption process on the Co/G nanocomposite.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2013 Dalton Trans.

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online

13 L. X. Wang, J. C. Li, Y. Q. Wang, L. J. Zhao and Q. Jiang,Chem. Eng. J., 2012, 181–182, 72.

14 X. M. Liang and L. J. Zhao, RSC Adv., 2012, 2, 5485.15 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii,

Z. Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour,ACS Nano, 2010, 4, 4806.

16 M. K. Purkait, A. Maiti, S. DasGupta and S. De, J. Hazard.Mater., 2007, 145, 287.

17 S. Chatterjee, M. W. Lee and S. H. Woo, Bioresour. Technol.,2010, 101, 1800.

18 H. M. Sun, L. Y. Cao and L. H. Lu, Nano Res., 2011, 4, 550.19 M. Zhang, D. N. Lei, X. M. Yin, L. B. Chen, Q. H. Li,

Y. G. Wang and T. H. Wang, J. Mater. Chem., 2010, 20, 5538.20 D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner,

G. H. B. Dommett, G. Evmenenko, S. T. Nguyen andR. S. Ruoff, Nature, 2007, 448, 457.

21 P. Kundu, C. Nethravathi, P. A. Deshpande, M. Rajamathi,G. Madras and N. Ravishankar, Chem. Mater., 2011, 23,2772.

22 A. V. Murugan, T. Muraligant and A. Manthiram, Chem.Mater., 2009, 21, 5004.

23 Q. H. Wang, L. F. Jiao, H. M. Du, Q. N. Huan, W. X. Peng,D. W. Song, Y. J. Wang and H. T. Yuan, J. Mater. Chem.,2011, 21, 14159.

24 L. F. Duan, S. S. Jia, R. M. Cheng and L. J. Zhao, Chem. Eng.J., 2011, 173, 233.

25 L. F. Duan, S. S. Jia and L. J. Zhao, Eur. J. Inorg. Chem.,2010, 1957.

26 Y. G. Yao, S. D. Miao, S. Z. Liu, L. P. Ma, H. G. Sun andS. B. Wang, Chem. Eng. J., 2012, 184, 326.

27 R. Ahmad and R. Kumar, Appl. Surf. Sci., 2010, 257,1628.

28 K. Nagarethinam and M. Mariappan, Water, Air, Soil Pollut.,2002, 138, 289.

29 M. K. Purkait, A. Maiti, S. DasGupta and S. De, J. Hazard.Mater., 2007, 145, 287.

30 Y. Z. Pan, H. Q. Bao and L. Li, ACS Appl. Mater. Interfaces,2011, 3, 4819.

31 S. D. Deng, X. H. Li and H. Fu, Corros. Sci., 2011, 53,760.

32 G. S. Zhang, J. H. Qu, H. J. Liu, A. T. Cooper and R. C. Wu,Chemosphere, 2007, 68, 1058.

Paper Dalton Transactions

Dalton Trans. This journal is © The Royal Society of Chemistry 2013

Dow

nloa

ded

by J

ilin

Uni

vers

ity o

n 14

/04/

2013

03:

38:5

7.

Publ

ishe

d on

06

Mar

ch 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3D

T50

333J

View Article Online