CoFe2O4submicrospheres with tunable magnetic and electrocatalytic properties

Colloids and Surfaces A:Physicochem.Eng.Aspects 423 (2013) 170–177

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A:Physicochemical and

Engineering

Aspects

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c o l s u r f

a

Facile synthesis of cobalt ferrite submicrospheres with tunable magnetic and electrocatalytic properties

Lijun Cui a ,b ,Peizhi Guo a ,b ,?,Guoliang Zhang a ,b ,Qun Li a ,b ,Rongyue Wang a ,b ,Meng Zhou a ,b ,Lina Ran a ,b ,X.S.Zhao a ,b ,c

a

Laboratory of New Fiber Materials and Modern Textile,the Growing Base for State Key Laboratory,Qingdao University,Qingdao 266071,PR China b

School of Chemistry,Chemical Engineering and Environment,Qingdao University,Qingdao 266071,PR China c

School of Chemical Engineering,The University of Queensland,St Lucia,QLD 4072,Australia

h i g h l i g h t s

CoFe 2O 4spheres with controlled sizes are synthesized by solvother-mal method.

Crystalline nature of CoFe 2O 4submi-crospheres can be regulated by iron precursors. Polycrystal CoFe 2O 4submicro-spheres show superparamagnetic behavior.

CoFe 2O 4spheres display electro-chemical sensing properties to detect biomolecules.

g r a p h i c a l

a b s t r a c

t

a r t i c l e

i n f o

Article history:

Received 4November 2012Accepted 24January 2013

Available online 14 February 2013

Keywords:Cobalt ferrite Submicrosphere Magnetization Electrocatalysis

Solvothermal synthesis

a b s t r a c t

Cobalt ferrite (CoFe 2O 4)submicrospheres with tunable structures were synthesized by a one-pot solvothermal method through adjusting iron precursors.CoFe 2O 4submicrospheres were characterized by a series of technologies including X-ray diffraction (XRD),scanning electron microscopy (SEM),trans-mission electron microscopy (TEM),Fourier transform infrared spectrometry (FTIR),thermal gravity analysis (TGA)and vibrating sample magnetometry (VSM).It was found that single crystal CoFe 2O 4submicrospheres were prepared from the synthesis system containing ferric chloride and showed fer-romagnetic behavior at room temperature.However,polycrystal CoFe 2O 4submicrospheres composed of CoFe 2O 4nanocrystals were obtained from ferric nitrate system and displayed superparamagnetic behavior with a magnetization saturation value up to 45.2emu/g.CoFe 2O 4spheres obtained from fer-ric chloride system showed better electrocatalytic performance toward simultaneous determination of dopamine and uric acid than polycrystal CoFe 2O 4spheres.The formation mechanisms of CoFe 2O 4sub-microspheres as well as the structure–property relationship of the spheres were discussed based on the experimental results.

? 2013 Elsevier B.V. All rights reserved.

1.Introduction

Magnetic nanocrystals of spinel ferrites have received exten-sive attention due to their potential applications in magnetic

?Corresponding author.Tel.:+8653283780378;fax:+8653285955529.E-mail addresses:guopz77@https://www.360docs.net/doc/86962042.html, ,pzguo@https://www.360docs.net/doc/86962042.html, (P.Guo).

materials,energy,catalysis and biomedicine [1,2].The synthetic methodology of ferrite nanomaterials has been developed on the basis of the rational design of synthesis systems and proce-dures [3–9].Various ferrite nanomaterials have been synthesized through a series of techniques,such as hydrothermal/solvothermal method [3],sol–gel technique [4],electrochemical synthesis [5],template [6],co-precipitation [7],seed-mediated growth [8],and mechanical alloying [9],etc.The targeted nanostructures including

0927-7757/$–see front matter ? 2013 Elsevier B.V. All rights reserved.https://www.360docs.net/doc/86962042.html,/10.1016/j.colsurfa.2013.01.064

L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects423 (2013) 170–177171

nanocrystals[10],core–shell nanoparticles[11],nanospheres[12], nanorods/nanowires[13,14]and nanotubes[15]are prepared. Among these synthetic methods,solvothermal method is usu-ally used to synthesize targeted nanomaterials with controlled morphology and structure due to its easily tunable experimental parameters such as the diversity of organic solvents and synthesis time and temperature[3,12].

Generally,magnetic behavior of ferrite materials can be adjusted through changing the size and structure of nano-/micrometer scaled structures[10].Various magnetic ferrite nanostructures with the size scales from several nanometers to submicrome-ter have been prepared controllably in the past decades[16–18]. It is known that magnetic nanocrystals with the size scales less than a critical size can display superparamagnetic behavior at room temperature while magnetic nanocrystals with a larger size usually show ferromagnetic characteristic.The magnetization sat-uration(Ms)value also relates to the size and structure of magnetic nanocrystals,in which small nanocrystals with superparamag-netism usually have a small Ms value or vice versa.Recently,it is becoming more interesting to prepare superparamagnetic nano-structures with suitable size(up to submicron scale),shape and properties which could meet the need of,for example,biomedical applications[19,20].

Cobalt ferrite(CoFe2O4),a well known ferrimagnetic spinel, is one of the most commercially signi?cant members of the magnetic ferrite class because of its strong magnetic anisotropy, moderate magnetization,and high coercivity at room temperature [7–9,21].Moreover,CoFe2O4material is an excellent candidate to study and regulate magnetic properties at atomic level through chemical manipulation[20].Commonly,bivalent cation(Co2+) and trivalent ferric ion(Fe3+)can distribute at both tetrahe-dral(A sites)and octahedral(B sites)coordination sites,and the type of the cationic distribution affects the magnetic prop-erties of spinel ferrites[22].So it has been attracted much attention to systematically regulate magnetic properties of fer-rite by changing the identity of the divalent cation or by partial substitution while the basic crystal structure is maintained.On the other hand,as the sizes of CoFe2O4particles approach to the nanometer scale,their physicochemical properties will become different from the corresponding bulk counterparts.It is reported that CoFe2O4nanostructures exhibit superparamag-netic property when the size of the materials decreases to a critical size of9nm or smaller[23].CoFe2O4primary crystals with large size would induce superparamagnetic–ferromagnetic transition and display large Ms https://www.360docs.net/doc/86962042.html,rge-scale superpara-magnetic nanoparticles could enlarge their potential applications such as biomedicine partly due to their large Ms values. However,how to synthesize superparamagnetic nanostructures with a relative large size still needs to be further devel-oped.

In this work,nearly monodisperse CoFe2O4submicrospheres with tunable crystalline nature have been synthesized con-trollably by a one-pot solvothermal method through simply adjusting the synthesis microenvironments.It is found that single crystal CoFe2O4submicrospheres are obtained from fer-ric chloride systems and show ferromagnetic behavior with Ms value of63.1emu/g and coercivity(Hc)value of574.1Oe. However,submicron polycrystal CoFe2O4spheres showing the room-temperature superparamagnetic property are prepared using ferric nitrate as the iron precursor.Furthermore,elec-trochemically sensing properties of CoFe2O4spheres toward the simultaneously electrochemical determination of dopamine and uric acid have been explored.The formation mecha-nism of these cobalt ferrite submicrospheres as well as their structure–property relationship was discussed based on the exper-imental results.2.Experiments

2.1.Materials

Ferric chloride(FeCl3·6H2O),iron nitrate(Fe(NO3)3·9H2O), iron sulfate(Fe2(SO4)3·6H2O),cobalt acetate(Co(CH3COO)2·4H2O), ethanol,ethylene glycol(EG),uric acid(UA),ascorbic acid(AA) and polyvinylpyrrolidone(PVP,M=30,000)were purchased from Sinopharm Chemical Reagent Co.,Ltd.Dopamine(DA)was pur-chased from Aldrich.All chemicals were analytical grade and used without any further puri?cation.

2.2.Synthesis of CoFe2O4

In a typical synthesis,PVP(0.5g)was added into the homogeneous ethylene glycol solution(30mL)containing Co(CH3COO)2·4H2O(1mmol)and FeCl3·6H2O or Fe(NO3)3·9H2O (2mmol)under stirring.Then,the mixture was transferred into a 40ml Te?on-lined stainless autoclave.The autoclave was sealed and maintained at200?C for6h or12h.The solid products were collected by centrifugation,washed separately with distilled water and ethanol for several times,and then dried in an oven at60?C for6h.The prepared products were abbreviated by CoFe–C6, CoFe–C12,CoFe–N6and CoFe–N12,where C and N represented the synthesis systems containing FeCl3and Fe(NO3)3,respectively, numbers6and12denoted the synthesis time of the corresponding samples.

2.3.Characterization

Powder X-ray diffraction(XRD)measurements were recorded using Bruker D8advance X-ray diffractometer equipped with graphite monochromatized Cu K?radiation( =0.15418nm). Scanning electron microscopy(SEM)examinations were per-formed with a JSM-6390LV scanning electron microscope operated at20kV.Transmission electron microscopy(TEM)images were obtained with a JEM-2000EX transmission electron microscope operated at160kV.Room temperature magnetic properties were analyzed by a LDJ9500vibrating sample magnetometer.Fourier transformation infrared(FTIR)were carried out with a Nicolet 5700FTIR on KBr pellets.Thermogravimetric analysis(TGA)was conducted on a Thermogravimetric analyzer TGA822e(Mettler Toledo)with a heating rate of10?C min?1under an air?ow rate of100ml min?1.

2.4.Bioelectrochemical sensing performance of CoFe2O4spheres

Electrochemical measurements were performed on a CHI760C electrochemical workstation using a three-electrode cell with plat-inum foil as counter electrode and saturated calomel electrode as reference electrode in aqueous phosphate buffer solutions(PBS, 0.1M,pH7.4)containing DA or UA with the concentration of 0.1mM.Bare glass carbon electrode(GCE,3mm in diameter)and ferrite-modi?ed GCE were used as the working electrodes.After ultrasonicating ferrite suspensions(1mg mL?1)for30min,a15?L aliquot suspension was uniformly cast onto the GCE surface.The modi?ed GCEs were dried under ambient conditions before use. The phosphate buffer solution was in thoroughly anaerobic con-ditions by bubbling with high-purity nitrogen for15min.All the potentials were referred to a saturated calomel electrode(SCE). 3.Results and discussion

3.1.Crystal structure

Fig.1shows the XRD patterns of the products prepared from the synthesis systems containing ferric chloride or ferric nitrate.

172L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 423 (2013) 170–

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Fig.1.XRD patterns of the products:(a)CoFe–C6,(b)CoFe–C12,(c)CoFe–N6and (d)CoFe–N12.

It can be seen that all the diffraction peaks of each sample match well with cubic CoFe 2O 4crystals (JCPDS card NO.22-1086).The observed interplanar spacing d values of the peak (311)for samples CoFe–C6,CoFe–C12,CoFe–N6and CoFe–N12are 2.5335?,2.5438?,2.5078?,2.5288?,respectively,close to that of cubic CoFe 2O 4(JCPDS card NO.22-1086,d (311)=2.5310?)and the estimated values of lattice constants for these four samples are 8.4026?,8.4451?,8.3174?and 8.3871?,respectively,close to the bulk CoFe 2O 4(8.395±0.005?).The differences between the as-made CoFe 2O 4samples and bulk CoFe 2O 4materials may be resulted from the different distribution of metal cations.For FeCl 3systems,the diffraction peaks become sharper and stronger with reaction time extended from 6h to 12h (Fig.1a and 1b),indicating the growth of crystals and improvement of crystallinity.The same trend is also observed from Fe(NO 3)3systems.The broadening of the diffraction peaks of samples CoFe–N6and CoFe–N12indicates that the prod-ucts should consist of small nanocrystals with the sizes of about 4nm and 5nm [24],respectively,calculated from (311)peaks using Scherrer equation.

3.2.Morphology and nanostructure

The size and morphology of all the above samples were investi-gated by SEM.As depicted in Fig.2,all products appear sphere-like shape and the sizes are about 130nm,170nm,140nm and 110nm for samples CoFe–C6,CoFe–C12,CoFe–N6and CoFe–N12,respectively.For the synthesis system containing FeCl 3,the size of spheres increases with reaction time prolonged from 6h to 12h.Opposite phenomenon is observed for the samples pre-pared from the synthesis system containing Fe (NO 3)3,in which the size of spheres decreases with increasing the synthesis time from 6h to 12h.It can also be seen that CoFe 2O 4submicro-spheres synthesized from chloride systems are separated better than those obtained from nitrate systems under the same condi-tions.

Fig.3a–d shows the TEM images of CoFe–C6and CoFe–C12,in which well separated spheres are observed.The sizes of the spheres are in accord with those SEM results with narrow size distributions (Fig.S1).The selected area electron diffraction (SAED)patterns of a single sphere of CoFe–C6and CoFe–C12,as shown in the insets of Fig.3b and d,respectively,indicate that both samples are single crystals.Moreover,wormlike nanopores across sbumicrospheres can also be seen in samples CoFe–C6and CoFe–C12.It is pro-posed that the aggregation of primary nanoparticles into compact spheres should be inhibited because the PVP chains are absorbed onto the primary nanocrystals [20].However,for samples CoFe–N6and CoFe–N12,TEM micrographs show the rough,irregular spher-ical shapes consisting of agglomerated 4–6nm nanocrystals which are in good agreement with the values calculated from the XRD patterns.Furthermore,samples CoFe–N6and CoFe–N12are not as separate as samples CoFe–C6and CoFe–C12,which could also be observed in SEM images (Fig.2).These may be ascribed to the magnetic dipole–dipole interaction with neighboring particles and elevated synthesis temperature [25].As depicted in Fig.3e–h,the spheres become more compact with the prolongation of synthesis time.These phenomena are essentially the same as those results of submicron-meter magnetite and manganese ferrite nanocrystal clusters [26].Five diffraction rings can be clearly observed in

the

Fig.2.SEM images of samples CoFe–C6(a),CoFe–C12(b),CoFe–N6(c)and CoFe–N12(d).

L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects423 (2013) 170–177

173

Fig.3.TEM images of samples CoFe–C6(a,b),CoFe–C12(c,d),CoFe–N6(e,f)and CoFe–N12(g,h).The insets are the SAED patterns of the corresponding samples. corresponding SAED patterns,indicating the polycrystalline nature

of submicrometer CoFe–N6and CoFe–N12spheres.

3.3.FTIR analysis

The FTIR absorption spectra of four samples are shown in Fig.4.

The vibration bands at398,405,399and403cm?1are ascribed to

the vibration modes of octahedral groups(Fe3+O2?)and the bands

at580,596,604and600cm?1are assigned to the stretching vibra-

tions of the tetrahedral groups(Fe3+O2?or Co2+O2?bond)[11]

for samples CoFe–C6,CoFe–C12,CoFe–N6and CoFe–N12,respec-

tively.The bands at3500–3300cm?1and1620cm?1are attributed

respectively to the stretching and bending vibration of the O H

groups,indicating the existence of free and absorbed water on

the surface of cobalt ferrite nanoparticles.The weak bands at890,

1060,1390,2890and2952cm?1in the spectra indicate the exis-

tence of a few organic molecules adsorbed on the surface of the

spheres.

The cation distribution on A-and B-sites which affects the mag-

netic properties can be evaluated non-quantitatively from the FTIR

absorption spectra.It can be seen that the band corresponding to

A-sites of samples CoFe–C12,CoFe–N6shifts to high wave num-

bers,indicating that the contents of Fe3+ions on A-sites of samples

CoFe–C12and CoFe–N6are larger than those of samples CoFe–C6

and CoFe–N12respectively,because the radius of Fe(III)ion(0.64

?)

Fig.4.FTIR spectra of samples(a)CoFe–C6,(b)CoFe–C12,(c)CoFe–N6and(d) CoFe–N12.is less than that of Co(II)ion(0.74?)[27].Likewise,the con-tents of Fe3+ions on B-sites of samples CoFe–C12and CoFe–N12 are larger than those of samples CoFe–C6and CoFe–N6,respec-tively.

3.4.TGA analysis

Thermogravimetric analysis(TGA)of samples CoFe–C12and CoFe–N12have been operated under air?ow.As shown in Fig.5, the TGA traces of both samples show similar multistep mass loss processes,indicating similar changes of samples during heating processes.However,the weight loss of CoFe–C12is decreased much less than that of CoFe–N12with the increase of temperature.Take sample CoFe–N12as an example,the?rst step of mass loss starts from room temperature to about150?C related to the elimination of adsorbed water molecules.However,there is a gentle step between 80?C and110?C probably resulting from two forming water.One is free water on the surface of the agglomerated particles.The other is entrapped inside the inner space among the nanocrystallites[28]. In the second step at300–400?C,a partial decomposition of organic compounds takes place,which can also be veri?ed from FTIR results in Fig.4.The?nal step with a slow decline until reaching a plateau at650?C suggests further decomposition of the partially decom-posed sample to form monophasic CoFe2O4[29].Obviously,the weight loss rate of sample CoFe–N12is larger than that of sample CoFe–C12,the reason may be that certain amounts of

micropores Fig.5.TGA curves of samples CoFe–C12(a)and CoFe–N12(b).

174L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 423 (2013) 170–

177

Fig.6.TEM images of the intermediates prepared from chloride (a–c)and nitrate (d–f)system at the synthesis time of 1h (a,d),3h (b,e)and 4h (c,f).

existing in the agglomerated particles and higher surface energy of nanocrystals make it possible for physical absorption.

3.5.Evolution of the intermediates

In order to understand the formation mechanism of CoFe 2O 4submicrospheres,time-dependent intermediates were collected from the systems containing ferric chloride,ferric nitrate or ferric sulfate.It is found that pure cobalt ferrite phase cannot be obtained when Fe 2(SO 4)3is used as the reagent under the controlled con-ditions.Thereafter,the intermediates from the synthesis systems containing ferric chloride or ferric nitrate have been studied care-fully.The color changes of these two series of intermediates are obviously different.Clearly,yellowish-brown and dust color pre-cipitates are obtained in chloride and nitrate systems,respectively,when the synthesis time is 1h.The corresponding XRD results (Fig.S2)show that hematite and cobalt ferrite phases are appeared respectively for the chloride and nitrate synthesis systems.For the synthesis system containing ferric nitrate,black products are obtained with the synthesis time up to 3h.However,black prod-ucts cannot be collected until the reaction time up to 4h when the synthesis solutions contain ferric chloride.As we know,the mag-netization values are very small for hematite,but large for cobalt ferrite.So it is easy to use a coin-like magnet to simply detect the magnetization of the precipitates.It is found that all the products except the yellowish-brown one can be attracted obviously by the magnet.

The structural evolution of the intermediates is investigated by TEM,as shown in Fig.6.For ferric chloride system,quasi one dimensional structure is formed when the synthesis time is 1h

(Fig.6a).With the synthesis time elongated to 3h,aggregated nano-structures are observed in Fig.6b and cobalt ferrite phase can be detected.Spherical cobalt ferrite particles with the size of about 120±30nm are gradually formed with reaction time up to 6h,as shown in Fig.6c.However,cobalt ferrite primary crystals with the size of several nanometers are observed for nitrate synthe-sis system when the solvothermal time is 1h (Fig.6d).Increasing the synthesis time to 3h leads to the aggregation of nanoparticles (Fig.6e).Clearly,spherical aggregates can be observed with the time up to 4h (Fig.6f).When the reaction time is prolonged to 6h,spherical nanocrystal clusters can be obtained (Fig.2c).Based on these results,it is proposed that the solubility of ferric chloride and ferric nitrate in ethylene glycol with the order of Fe(NO 3)3>FeCl 3should play an important role in the formation of the targeted prod-ucts [30].The lower solubility of ferric chloride may cause lower reaction rate that is crucial for the growth of one dimension struc-ture of hematite phase [31].Higher solubility of ferric nitrate in the solvent may give a more homogeneous phase reaction than that of chloride system,which should have an important effect on the formation of CoFe 2O 4in ferric nitrate system.It is proposed that the quick formation of CoFe 2O 4nanocrystals combined with the existence of adsorbed PVP on the nanocrystal surfaces should con-tribute to the ultimate formation of polycrystal submicron spheres with the elongation of the synthesis time [32].

3.6.Magnetic properties

The room-temperature magnetization measurements of CoFe 2O 4spheres are shown in Fig.7.It is observed that the hysteresis loop of samples CoFe–C6and CoFe–C12(Fig.7a and

L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 423 (2013) 170–177

175

Fig.7.Magnetic curves of samples CoFe–C12(a),CoFe–C6(b),CoFe–N6(c)and CoFe–N12(d).

b)get saturated at about 53.6and 63.1emu/g with the Hc values of 424.8and 574.1Oe,respectively.These data are well within the range of bulk reported values [33].The squareness ratio,namely remnant/magnetization saturation (Mr/Ms),gives a rough way to study the exchange interaction and mangetocrystalline anisotropy.The Mr/Ms values of samples CoFe–C6and CoFe–C12are 0.41and 0.34,respectively,implying that the particles are predominantly not in the single-domain state and consist of pseudo-single-domain particles [34].Clearly,the Ms values are lower than that of bulk cobalt ferrite reported about 80emu/g [32].Commonly,the grain boundaries and dead layer shell thickness can result in the decrease of the Ms value [35].Other factors including surface effects,cation redistribution in the nanoregime,cation distribution on both sites as well as the possible presence of Co 3+and Fe 2+ions should also contribute to the reduction of the Ms values of samples CoFe–C6and CoFe–C12.

However,submicrometer spheres prepared from Fe(NO 3)3sys-tem display superparamagnetic behavior at room temperature as shown in the curves c–d in Fig.7.The size of CoFe 2O 4nanocrys-tals is about 4–6nm lower than the critical size (about 9nm),the thermal activation energy can overcome the magnetocrystalline anisotropy energy (E A )without more energy behaving superpara-magnetic property [23].The Ms values of samples CoFe–N6and CoFe–N12are 45.2and 37.8emu/g,respectively,which is larger than those of nanocrystals with similar crystal size [36,37].These should be ascribed to the unique structure of these samples,in which oriented attachment among nanocrystals should be adopted [28,38].As is known to us,the distribution of divalent and triva-lent cations on A-and B-sites determines the magnetocrystalline anisotropy,which is directly in?uenced by solvothermal time [34].On the one hand,the Ms value becomes larger with increase of the

nanocrysal size [27,39].It can also be concluded from Fig.1that the degree of crystallization and the size of nanocrystals increase with increasing synthesis time from 6h to 12h.On the other hand,when the size of nanoparticles decreases into nanometer scale,surface effects would become more signi?cant due to the increased volume fraction of surface atoms within the whole particle,which also con-tributes to the increase of Ms value [40].The in?uence of atoms at the surface does more contribution to the total magnetic response than the in?uence of the increased nanocrystals size.Therefore,sample CoFe–N6with smaller crystalline size has a higher Ms value than sample CoFe–N12.

3.7.Electrocatalytic performance

Metal oxide nanomaterials have been successfully used to prepare chemical sensors in many areas including environmental protection and bioelectrochemical detection [41,42].Cobalt ferrite spheres are suggested to be used for the electrochemical catalysis of soluble molecules,which would be expected to display the struc-tural effect of ferrite spheres on their electrocatalytic performance.Samples CoFe–C12and CoFe–N12are selected to prepare mod-i?ed GCEs named as CoFe–C/GCE and CoFe–N/GCE,respectively,and then their electrocatalytic performances toward simultaneous determination of DA and UA are explored.Fig.8shows the CV curves of CoFe–C/GCE and CoFe–N/GCE in the electrolyte solutions under different conditions.It can be seen that the oxidation peaks of DA are about 0.209V and 0.301V for CoFe–C/GCE and CoFe–N/GCE,respectively,indicating that CoFe–C/GCE shows better electrocat-alytic performance toward the electrochemical oxidation of DA than CoFe–N/GCE.Furthermore,the oxidation peaks of DA are narrower and higher than that of UA,indicating a quick electron transfer kinetic [43],which results from the difference in molecular structures of DA and UA.The separations of the peak potentials of DA and UA are 0.252V for CoFe–C/GCE and 0.182V for CoFe–N/GCE.Interestingly,two separated peaks corresponding to the oxidation of DA and UA can be explicitly observed in the mixed electrolyte solutions for CoFe–C/GCE,as shown in Fig.8a,indicating the realiza-tion of the simultaneous determination of DA and UA.However,two broad and overlapped peaks are obtained for CoFe–N/GCE,which cannot distinguish the oxidation of DA or UA in the mixed solutions (Fig.8b).These should attribute to the unique structural character-istic of CoFe–C12with excellent crystalline nature,which shows the higher electron transfer rate than CoFe–N12[44].It should also be pointed out that the electrochemical response of AA cannot be observed and the experimental results show that the existence of AA in the mixed solutions containing DA and UA do not disturb the determination of DA or

UA.

Fig.8.CV curves of CoFe–C/GEC (a)and CoFe–N/GEC (b)in the electrolytes containing DA (1),UA (2)or DA +UA (3)as well as the blank solutions (4).Scan rate:50mV/s.

176L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects423 (2013) 170–177

4.Conclusions

CoFe2O4spheres with controlled structures have been synthe-sized by solvothermal method through adjusting iron source and reaction time.The products obtained from ferric chloride system display single crystallite nature and ferromagnetic behavior with Ms values of53.6and63.1emu/g,respectively,for the synthesis time of6h and12h.Polycrystal CoFe2O4spheres are obtained when ferric nitrate is used as iron source,and the superpara-magnetic behavior is ascribed to the smaller primary nanocrystals assembling to the submicron-scale spheres.The intermediates investigation reveals that the quick formation of cobalt ferrite in ferric nitrate system is ascribed to the larger solubility of ferric nitrate in EG.And the aggregation of primary nanocrystals into spheres can reduce surface energy.Furthermore,CoFe2O4spheres prepared after12h from ferric chloride system have better elec-trochemical sensing performance to simultaneously determine DA and UA than those obtained from ferric chloride system.These results should be helpful for the controlled synthesis of targeted nanomaterials through the rational design of intermolecular inter-actions.

Acknowledgements

This work was?nancially supported by the National Natural Science Foundation of China(No.21143006and U1232104),the Foundation of Qingdao Municipal Science and Technology Com-mission(11-2-4-2-(8)-jch).

Appendix A.Supplementary data

Supplementary data associated with this article can be found,in the online version,at https://www.360docs.net/doc/86962042.html,/10.1016/j.colsurfa. 2013.01.064

References

[1]T.Hyeon,Y.Chung,J.Park,S.S.Lee,Y.W.Kim,B.H.Park,Synthesis of highly

crystalline and monodisperse cobalt ferrite nanocrystals,J.Phys.Chem.B106 (2002)6831–6833.

[2]S.Ida,K.Yamada,T.Matsunaga,H.Hagiwara,Y.Matsumoto,T.Ishihara,Prepa-

ration of p-type CaFe2O4photocathodes for producing hydrogen from water,J.

Am.Chem.Soc.132(2010)17343–17345.

[3]X.F.Chu,D.G.Jiang,Y.Guo,C.M.Zheng,Ethanol gas sensor based on CoFe2O4

nano-crystallines prepared by hydrothermal method,Actuat.B,Sens.Actuat.B 120(2006)177–181.

[4]S.P.Zhang,D.W.Dong,Y.Sui,Z.G.Liu,H.X.Wang,Z.N.Qian,W.H.Su,Preparation

of core–shell particles consisting of cobalt ferrite and silica by sol–gel process, J.Alloys Compd.415(2006)257–260.

[5]S.D.Sartale,C.D.Lokhande,A room temperature two-step electrochemical pro-

cess for large area nanocrystalline ferrite thin?lms deposition,J.Electroceram.

15(2005)35–44.

[6]Y.Xu,J.Wei,J.L.Yao,J.L.Fu, D.S.Xue,Synthesis of CoFe2O4nanotube

arrays through an improved sol–gel template approach,Mater.Lett.62(2008) 1403–1405.

[7]S.H.Sun,H.Zeng,D.B.Robinson,S.Raoux,P.M.Rice,S.X.Wang,G.X.Li,Monodis-

perse MFe2O4(M=Fe,Co,Mn)nanoparticles,J.Am.Chem.Soc.126(2004) 273–279.

[8]Q.Song,Z.J.Zhang,Shape control and associated magnetic properties of spinel

cobalt ferrite nanocrystals,J.Am.Chem.Soc.126(2004)6164–6168.

[9]E.Manova,B.Kunev,D.Paneva,I.Mitov,L.Petrov,Mechano-synthesis,charac-

terization,and magnetic properties of nanoparticles of cobalt ferrite,CoFe2O4, Chem.Mater.16(2004)5689–5696.

[10]B.-G.Veronica,S.-P.Regino,J.T.-F.María,Superparamagnetism and interparti-

cle interactions in ZnFe2O4nanocrystals,J.Mater.Chem.22(2012)2992–3003.

[11]Y.Shen,Q.D.Zhao,X.Y.Li,Y.Hou,G.H.Chen,Surface photovoltage property of

magnesium ferrite/hematite heterostructured hollow nanospheres prepared with one-pot strategy,Colloids Surf.A:Physicochem.Eng.Aspects403(2012) 35–40.

[12]N.Z.Bao,L.M.Shen,Y.-H.A.Wang,J.X.Ma,D.Mazumdar,A.Gupta,Controlled

growth of monodisperse self-supported superparamagnetic nanostructures of spherical and rod-like CoFe2O4nanocrystals,J.Am.Chem.Soc.131(2009) 12900–12901.[13]E.Tirosh,G.Markovich,Control of defects and magnetic properties in colloidal

HfO2nanorods,Adv.Mater.19(2007)2608–2612.

[14]Z.L.Wang,X.J.Liu,M.F.Lv,P.Chai,Y.Liu,X.F.Zhou,J.Meng,Preparation of one-

dimensional CoFe2O4nanostructures and their magnetic properties,J.Phys.

Chem.C112(2008)15171–15175.

[15]H.M.Fan,J.B.Yi,Y.Yang,K.W.Kho,H.R.Tan,Z.X.Shen,J.Ding,X.W.Sun,

M.C.Olivo,Y.P.Feng,Single-crystalline MFe2O4nanotubes/nanorings synthe-sized by thermal transformation process for biological applications,ACS Nano 3(2009)2798–2808.

[16]R.Abu-Much,A.Gedanken,Sonochemical synthesis under a magnetic?eld:

structuring magnetite nanoparticles and the destabilization of a colloidal mag-netic aqueous solution under a magnetic?eld,J.Phys.Chem.C112(2008) 35–42.

[17]N.Feltin,M.P.Pileni,New technique for synthesizing iron ferrite magnetic

nanosized particles,Langmuir13(1997)3927–3933.

[18]T.Hyeon,Y.Chung,J.Park,S.S.Lee,Y.W.Kim,B.H.Park,Synthesis of highly

crystalline and monodisperse cobalt ferrite nanocrystals,J.Phys.Chem.B106 (2002)6831–6833.

[19]J.J.Zhao,M.Deng,J.W.Zeng,Z.B.Huang,G.F.Yin,X.M.Liao,J.W.Gub,J.J.Huang,

Preparation of Fe3O4and CoFe2O4nanoparticles with cellular compatibility via the histidine assistance,Colloids Surf.A:Physicochem.Eng.Aspects401(2012) 54–60.

[20]C.Cannas,A.Ardu,A.Musinu,D.Peddis,G.Piccaluga,Spherical nanoporous

assemblies of iso-oriented cobalt ferrite nanoparticles:synthesis,microstruc-ture,and magnetic properties,Chem.Mater.20(2008)6364–6371.

[21]C.H.Chia,S.Zakaria,M.Yusoff,S.C.Goh,C.Y.Haw,Sh.Ahmadi,N.M.Huang,

H.N.Lim,Size and crystallinity-dependent magnetic properties of CoFe2O4

nanocrystals,Ceram.Int.36(2010)605–609.

[22]R.A.McCurrie,in:Ferromagnetic materials:structure and properties,Academic

Press,London,1994.

[23]T.E.Quickel,V.H.Le,T.Brezesinski,S.H.Tolbert,On the correlation between

nanoscale structure and magnetic properties in ordered mesoporous cobalt ferrite(CoFe2O4)thin?lms,Nano Lett.10(2010)2982–2988.

[24]X.F.Qu,Q.Z.Yao,G.T.Zhou,S.Q.Fu,J.L.Huang,Formation of hollow magnetite

microspheres and their evolution into durian-like architectures,J.Phys.Chem.

C114(2010)8734–8740.

[25]Z.B.Zhang,H.F.Duan,S.H.Li,Y.J.Lin,Assembly of magnetic nanospheres into

one-dimensional nanostructured carbon hybrid materials,Langmuir26(2010) 6676–6680.

[26]P.Hu,L.J.Yu,A.H.Zuo,C.Y.Guo,F.L.Yuan,Fabrication of monodisperse mag-

netite hollow spheres,J.Phys.Chem.C113(2009)900–906.

[27]L.J.Zhao,H.J.Zhang,Y.Xing,S.Y.Song,S.Y.Yu,W.D.Shi,X.M.Guo,J.H.

Yang,Y.Q.Lei,F.Cao,Studies on the magnetism of cobalt ferrite nanocrys-tals synthesized by hydrothermal method,J.Solid State Chem.181(2008) 245–252.

[28]P.Z.Guo,G.L.Zhang,J.Q.Yu,H.L.Li,X.S.Zhao,Controlled synthesis,magnetic and

photocatalytic properties of hollow spheres and colloidal nanocrystal clusters of manganese ferrite,Colloids Surf.A:Physicochem.Eng.Aspects395(2012) 168–174.

[29]N.N.Mallikarjuna,https://www.360docs.net/doc/86962042.html,gashetty,A.Venkataraman,Cobalt ferrite from citrate

precursor by self-propagating combustion reaction,J.Therm.Anal.Calorim.74 (2003)819–826.

[30]D.Zhao,X.Wu,H.Guan,E.Han,Study on supercritical hydrothermal synthesis

of CoFe2O4nanoparticles,J.Supercrit.Fluids42(2007)226–233.

[31]Z.G.Jia,D.P.Ren,Y.C.Liang,R.S.Zhu,A new strategy for the preparation of

porous zinc ferrite nanorods with subsequently light-driven photocatalytic activity,Mater.Lett.65(2011)3116–3119.

[32]Y.D.Yin, A.P.Alivisatos,Colloidal nanocrystal synthesis and the

organic–inorganic interface,Nature437(2005)664–670.

[33]M.V.Limaye,S.B.Singh,S.K.Date, D.Kothari,V.R.Reddy, A.Gupta,V.

Sathe,R.J.Choudhary,S.K.Kulkarni,High coercivity of oleic acid capped CoFe2O4nanoparticles at room temperature,J.Phys.Chem.B113(2009) 9070–9076.

[34]P.C.R.Varma,R.S.Manna,D.Banerjee,M.R.Varma,K.G.Suresh,A.K.Nigamc,

Magnetic properties of CoFe2O4synthesized by solid state,citrate precursor and polymerized complex methods:a comparative study,J.Alloys Compd.453 (2008)298–303.

[35]S.Ayyappan,S.Mahadevan,P.Chandramohan,M.P.Srinivasan,J.Philip,B.Raj,

In?uence of Co2+ion concentration on the size,magnetic properties,and purity of CoFe2O4spinel ferrite nanoparticles,J.Phys.Chem.C114(2010)6334–6341.

[36]K.E.Mooney,J.A.Nelson,M.J.Wagner,Superparamagnetic cobalt ferrite

nanocrystals synthesized by alkalide reduction,Chem.Mater.16(2004) 3155–3161.

[37]Y.Ahn,E.J.Choi,S.Kimb,H.N.Ok,Magnetization and M?ssbauer study of cobalt

ferrite particles from nanophase cobalt iron carbonate,Mater.Lett.50(2001) 47–52.

[38]J.P.Ge,Y.X.Hu,M.Biasini,W.P.Beyermann,Y.D.Yin,Superparamagnetic

magnetite colloidal nanocrystal clusters,Angew.Chem.Int.Ed.46(2007) 4342–4345.

[39]Y.Q.Qu,H.B.Yang,N.Yang,Y.Z.Fan,H.Y.Zhu,G.T.Zou,The effect of

reaction temperature on the particle size,structure and magnetic prop-erties of coprecipitated CoFe2O4nanoparticles,Mater.Lett.60(2006) 3548–3552.

[40]C.R.Vestal,Z.J.Zhang,Effects of surface coordination chemistry on the magnetic

properties of MnFe2O4spinel ferrite nanoparticles,J.Am.Chem.Soc.125(2003) 9828–9833.

L.Cui et al./Colloids and Surfaces A:Physicochem.Eng.Aspects423 (2013) 170–177177

[41]X.L.Hu,J.C.Yu,J.M.Gong,Q.L.Li,G.S.Li,?-Fe2O3nanorings prepared by a

microwave-assisted hydrothermal process and their sensing properties,Adv.

Mater.19(2007)2324–2329.

[42]X.B.Lu,H.J.Zhang,Y.W.Ni,Q.Zhang,J.P.Chen,Porous nanosheet-based

ZnO microspheres for the construction of direct electrochemical biosensors, Biosens.Bioelectron.24(2008)93–98.

[43]H.Q.Bi,Y.H.Li,S.F.Liu,P.Z.Guo,Z.B.Wei,C.X.Lv,J.Z.Zhang,X.S.Zhao,Carbon-

nanotube-modi?ed glassy carbon electrode for simultaneous determination of

dopamine,ascorbic acid and uric acid:The effect of functional groups,Sens.

Actuat.B171–172(2012)1132–1140.

[44]P.Wang,Y.X.Li,X.Huang,L.Wang,Fabrication of layer-by-layer modi?ed

multilayer?lms containing choline and gold nanoparticles and its sensing application for electrochemical determination of dopamine and uric acid, Talanta73(2007)431–437.