Surfactant-free hydrothermal synthesis of submicron BiFeO3 powders

Appl Phys A(2011)103:511–516

DOI10.1007/s00339-010-6024-2

Surfactant-free hydrothermal synthesis

of submicron BiFeO3powders

Jianhong Peng·Mirabbos Hojamberdiev·

Baowei Cao·Juan Wang·Yunhua Xu

Received:1April2010/Accepted:16August2010/Published online:10September2010?Springer-Verlag2010

Abstract Submicron BiFeO3powders were successfully

synthesized via a simple hydrothermal process with the as-

sistance of mineralizer(NaOH)at150–190°C,using FeCl3

and Bi(NO3)3·5H2O as reactants.The effects of mineral-izer concentration,reaction temperature and time on the

phase evolution and crystal morphology of the resulting

samples were investigated.X-ray diffraction(XRD),trans-

mission electron microscopy(TEM),thermogravimetry and

differential scanning calorimetry(TG-DSC),and vibrating

sample magnetometry(VSM)were used to characterize the

as-synthesized samples.The experimental results revealed

that a pure BiFeO3phase could be formed at a tempera-

ture ranging from170to190°C for4–20h in the pres-

ence of0.03–0.12M NaOH.It was found that the miner-

alizer concentration,reaction temperature and time played

a key role in controlling the growing speed of nuclei and

formation of BiFeO3crystallites.The possible formation

mechanisms of submicron BiFeO3powders with different

morphologies were presented.The magnetization of BiFeO3

powders showed a weak ferromagnetic behavior at room

temperature.

1Introduction

Recently,multiferroic materials exhibiting simultaneous

magnetic and ferroelectric order have been receiving inten-

sive scienti?c attention[1,2].Because of the spontaneous

J.Peng( )·M.Hojamberdiev·B.Cao·J.Wang·Y.Xu

Shaanxi Key Laboratory of Nano-materials and Technology,

Xi’an University of Architecture and Technology,Xi’an710055,

PR China

e-mail:pjhhj@https://www.360docs.net/doc/5013658841.html,

Fax:+86-29-82202886electric polarization(magnetization),which can be switched

by an applied electric?eld(magnetic?eld),and intrinsic

coupling of ferroelectric and magnetic ordering,multifer-

roic materials have a wide range of applications,including

data storage,transducer,actuator,non-volatile memory[3],

and gate ferroelectrics in?eld-effect transistors[4].So far,

considerable efforts have been made in?nding new multi-

ferroic materials and investigating the multiferroic property

in known oxides,such as BiMnO3[5],BiFeO3[6],BiScO3

[7],BiCoO3[8],and rare earth manganates[9–12].

BiFeO3(BFO)is one of the most important multifer-

roic materials,which exhibits simultaneously ferroelectric

(T c～1103K)and antiferromagnetic ordering(T n～643K) [13].Owing to its magnetoelectric coupling,BFO-based

systems could be used to develop novel applications in the

?eld of radio,television,microwave and satellite commu-

nication,bubble memory devices,audio-video and digital

recording[14–16].Up to date,various kinds of fabrica-

tion methods have been applied in order to obtain a pure

phase BFO with enhanced properties,such as solid-state

[17],sonochemical and microemulsion[18],sol-gel[19],

solution-combustion[20,21],the soft chemical route[22],

co-precipitation[23]and hydrothermal[24].Among them,

the hydrothermal method has been widely used to fabri-

cate?ne powders with narrow particle size and controlled

stoichiometry.Some researchers have applied mineralizer-

[25],polymer-[26],and surfactant-assisted[27]hydrother-

mal and mechanochemical[28]methods in order to obtain

nano-scaled BFO possessing unique physical properties due

to potential quantum con?nement effect and low dimension-

ality.

It is of interest to investigate the phase evolution and

crystal morphology of the BFO powders under hydrother-

mal process as a function of mineralizer(NaOH)concen-

tration,reaction temperature and time.In the present report,

512J.Peng et al.

we describe the effects of mineralizer(NaOH)concentra-

tion,reaction temperature and time on the formation of BFO

crystallites with different morphologies.

2Experimental

All the reagents in the present work were analytical grade

and used without further puri?cation.In a typical synthesis

procedure,20mmol Bi(NO3)3·5H2O and20mmol FeCl3 were dissolved in5mL ethylene glycol.Aqueous ammo-

nia was added dropwise to the solution to adjust pH to9–10

and an orange-yellow-colored precipitate was eventually ob-

tained.The precipitate was repeatedly washed with distilled

water and then a30mL precursor containing NaOH solution

was transferred into a Te?on-lined stainless steel autoclave

with a?lling capacity of75%.The autoclave was sealed and

maintained at140–200?C for4–20h.After hydrothermal

treatment,the autoclave was cooled down to room tempera-

ture.The products were collected,washed several times with

distilled water and dried at80?C for4h in an oven.

The phase composition of the as-prepared powders was

characterized by X-ray powder diffraction(XRD;Model

D/MAX2550,Rigaku Co.,Japan)with Cu Kαradiation

(λ=1.5406?)at40kV and50mA.Transmission elec-

tron microscopy(TEM),high-resolution transmission elec-

tron microscopy(HRTEM),and selected area electron dif-

fraction pattern(SAED)observations were performed on a

JEM-2100electron microscope(JEOL,Japan)with an ac-

celerating voltage of200kV.Thermogravimetry and dif-

ferential scanning calorimetry(TG-DSC)analysis was con-

ducted on a Q600SDT(TA Co.Ltd.,USA)at a temperature

range of400–1200?C in nitrogen ambient at a scan rate of

10?C/min.Magnetic properties of BFO powders were mea-

sured using a vibrating sample magnetometer(VSM,7303,

Lake Shore,USA)at room temperature with a maximum

magnetic?eld of10KOe.

3Results and discussion

Figure1shows the XRD patterns of the BFO powders pre-

pared at170?C for16h using different NaOH concen-

trations under hydrothermal process.As can be seen,the

phase evolution of BFO crystallites was greatly affected

by NaOH concentration.That is,a NaOH concentration

of≤0.03and≥0.12M resulted the formation of impu-

rity phases,Bi2Fe4O9and Bi25FeO40.Only within0.03M

and0.12M NaOH concentrations the single-phase BFO

crystallites were obtained.All the diffraction peaks in the

XRD pattern,shown in Fig.1c,of BFO powders synthe-

sized at170?C for16h using NaOH concentration of0.07M

can be indexed to a pure hexagonal phase of BFO with

a Fig.1XRD patterns of the BFO powders synthesized at170?C for 16h using different NaOH concentrations:0.01M(a),0.03M(b), 0.07M(c),0.10M(d),0.12M(e),0.15M(f),and0.20M(g) space group of R3c.The calculated lattice parameters are a=b=0.5587nm and c=1.3880nm,which are con-sistent with the previous reported data for a bulk BiFeO3 (JCPDS card no.86-1518).The in?uences of the hydrother-mal reaction temperature ranging from140to200?C and time in the range of4–20h were also investigated in the presence of0.07M NaOH in the synthesizing system.As summarized in Table1,the pure phase BFO powders could be obtained through a hydrothermal process at the different temperatures ranging from150to190?C.However,the BFO powders synthesized beyond this range were formed along with impurity phases of Bi2Fe4O9and Bi25FeO40.It can be concluded that the formation of the secondary phases was strongly favored by140?C and200?C,0.01M and0.20M NaOH,and reaction time of6h.

Figure2displays the TEM images of the BFO powders prepared at150?C for16h using different NaOH concen-trations.The BFO powders synthesized with0.03M NaOH possess nearly regular chickpea-like morphology with the diameter of about200nm(Fig.2a).As the NaOH concen-tration in the synthesis solution was increased to0.07M, irregular particles in the size of about250nm were formed in the?nal product(Fig.2b).When the NaOH concentration was further increased to0.10M,the BFO powders consist-ing of a large amount of irregular submicron particles with a size in the range of200–400nm were formed,as shown in Fig.2c.

Figure3represents the TEM images of the BFO pow-ders prepared at170?C for16h using different NaOH con-centrations.As can be observed in Fig.3a,the?nal prod-uct synthesized with0.03M NaOH is composed of irregular submicron particles of BFO with the size of about250nm. When the NaOH concentration was increased to0.07M,ir-regular shape submicron particles were formed along with

Surfactant-free hydrothermal synthesis of submicron BiFeO3powders513

Table1The in?uences of the processing parameters on the formation of BFO powders under hydrothermal process Concentration of Time,h Temperature,?C Formed crystal phase NaOH,M

0.0116170Bi2Fe4O9,BFO

0.0316170BFO

0.074170Bi2Fe4O9,BFO

0.076170BFO

0.0710170BFO

0.0716140Bi25FeO40,BFO

0.0716150BFO

0.0716170BFO

0.0716190BFO

0.0716200Bi2Fe4O9,BFO

0.0720170BFO

0.1016170BFO

0.1216170BFO

0.1516170Bi2Fe4O9,BFO

0.2016170Bi2Fe4O9,BFO,Bi25FeO40

Fig.2TEM images of the BFO powders synthesized at150?C for16h using different NaOH concentrations:0.03M(a),

0.07M(b),and0.10M(c

)

514J.Peng et al.

Fig.3TEM images of the BFO powders synthesized at170?C for16h using different NaOH concentrations:0.03M(a),

0.07M(b),0.10M(c);HRTEM image of the rod-like BFO particles(d)with a SAED pattern(inset

)

a few rod-like particles with the size of250nm in width and400nm in length(Fig.3b).As the NaOH concentration was further increased to0.10M,almost uniform rod-like submicron particles with the length of max.500nm were obtained,as shown in Fig.3c.Similar results were obtained by Hojamberdiev et al.[29]using0.05M LiOH.

Figure3d shows a representative HRTEM image at the end of a rod-like particle.The image reveals that the submi-cron particle is single crystalline with inter-planar spacing of about0.279nm,which corresponds to the(110)crystalline plane of the hexagonal phase BFO bulk crystal.The selected area electron diffraction(SAED)pattern taken from the rod-like particle,inserted as an inset in Fig.3d,con?rms that the observed BFO particle is single crystalline.It should be noted that with increasing hydrothermal reaction tempera-ture170?C,particles transformed to rod-like shape rather than irregular at lower temperature.

In general,the formation of inorganic crystallites un-der hydrothermal conditions is controlled either by situ for-mation or dissolution-recrystallization-crystal growth mech-anisms[30].It is well known that the amorphous phase with the same chemical composition as crystalline phase should normally have higher free energy.In order to in-crease the surface energy,the amorphous precursors were aggregated together.When the hydrothermal treatment was performed in a relative low alkali(i.e.,a large number of water molecules and a few[Na(H2O)n)]+cation)and low-temperature conditions,the amorphous precursors could not dissolve at all.Therefore,the main formation mechanism could be considered as an in situ transformation process,in which amorphous precursors crystallized into small parti-cles by removal of structure water.As the reaction tempera-ture and the alkali concentration(i.e.,a few water molecules and a large number of[Na(H2O)n)]+cation)increased, the dissolving rate of the amorphous precursors increased, which would accelerate the dissolution-recrystallization ki-netic processes.Once the solution is supersaturated,nucle-ation and crystallization take place in the solution,and?-nally self-organize into high-aspect ratio of submicron rods due to its high anisotropic crystal structure.Therefore,the formation of the submicron BFO rods synthesized at170?C for16h with0.10M NaOH could be considered as a result of dissolution and recrystallization mechanism.Be-cause BFO has a rhombohedrally distorted perovskite struc-

Surfactant-free hydrothermal synthesis of submicron BiFeO 3powders

515

Fig.4TG-DSC curves of the BFO powders synthesized at 170?C for 16h with 0.07M NaOH

ture with a high anisotropy,it can lead to an anisotropic growth of BFO particles under a suitable hydrothermal con-dition.

Figure 4shows the TG-DSC curves of the BFO powders synthesized at 170?C for 16h with 0.07M NaOH.The sig-ni?cant two endothermic effects were observed at 827.89?C and 964.07?C.The TG curve shows no obvious change at the temperature between 600?C to 900?C.These endother-mic effects agree very well with the phase transitions indi-cated by the phase diagram,reported by Koizumi et al.[31].The endothermic effects at about 827.89?C and 964.07?C are attributed to the ferroelectric to paraelectric phase trans-formation and the melting temperature of BFO phases,re-spectively.

It was considered that the size effect of BFO structures might be responsible for the magnetic ordering by compar-ing the magnetic property of BFO ?lms with that of BFO nanowires [32].The magnetization of the BFO powders with an applied ?eld at room temperature was also investigated.As shown in Fig.5,a weak ferromagnetism (0.3emu /g)is observed for the submicron BFO powders at room tempera-ture,which is different from the linear M–H behavior in bulk BFO [33].It may be that the cycloid structure in bulk BFO is partially destroyed in the BFO nanostructures,which leads to the weak ferromagnetism behavior at room temperature.

4Conclusions

In summary,single-phase submicron BiFeO 3powders have been successfully synthesized by hydrothermal synthesis at the temperature of 150–190?C with NaOH concentration of 0.03–0.12M.X-ray diffraction indicates that bismuth fer-rite is hexagonal phase in structure with lattice constant a =b =0.5587nm,c =1.3880nm.The NaOH concen-tration,reaction temperature and time had a great effect

on

Fig.5M–H curve measured at room temperature for the BFO pow-ders synthesized at 170?C for 16h with 0.07M NaOH

the phase composition and morphology of the resultant pow-ders.In addition,weak ferromagnetism of the BFO powders was observed at room temperature due possibly to the small particle size,which was different from the magnetic prop-erty of bulk BFO.

Acknowledgements The authors are thankful to Science Foundation of Shaanxi Provincial Education Department for the ?nancial support (No.08JZ38).

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