Appl Phys A(2014)114:853–859
DOI10.1007/s00339-013-7712-5
Structural,optical,and multiferroic properties of single phased BiFeO3
M.Muneeswaran·P.Jegatheesan·M.Gopiraman·
Ick-Soo Kim·N.V.Giridharan
Received:26December2012/Accepted:13April2013/Published online:27April2013
?Springer-Verlag Berlin Heidelberg2013
Abstract A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO3 (BFO)powders.The X-ray diffraction pattern con?rms the perovskite structure of BFO and Rietveld re?nement re-veals the existence of rhombohedral R3c symmetry.Crys-tallite size and strain value are studied from Williamson–Hall(W–H)analysis.The transmission electron microscope (TEM)image shows that the particle size of BFO powders lies between50–100nm.4A1and7E Raman modes have been observed in the range100–650cm?1and a prominent band centered around1150–1450cm?1have also been ob-served corresponding to the two-phonon scattering.Differ-ential Thermal Analysis(DTA)shows the existence of two prominent peaks at330?C and837?C corresponding to the magnetic and ferroelectric ordering,respectively.From the temperature dependent dielectric studies,an anomaly in the dielectric constant is observed at the vicinity of Neel tem-perature(T N)indicating a magnetic ordering.Also,BFO shows antiferromagnetic behavior measured from the mag-netic studies.
1Introduction
Recently,the interest in multiferroics is stimulated by fun-damental physics leading to multiferroism arising from cou-M.Muneeswaran·P.Jegatheesan·N.V.Giridharan( ) Department of Physics,National Institute of Technology, Tiruchirappalli620015,India
e-mail:giri@https://www.doczj.com/doc/fb15253036.html,
Fax:+91-431-2500133
M.Gopiraman·I.-S.Kim
Nano Fusion Technology Research Group,Faculty of Textile Science and Technology,Shinshu University,Ueda,
Nagano386-0015,Japan pling between magnetic and ferroelectric orderings,and
have been extensively studied for their possible technical
applications,including spintronics,microelectronics,mag-
netic memory,and sensors[1].The term“multiferroic”
means coexistence of ferroelectric and magnetic ordering in
one single phase or multiphase materials.However,these
two ordering parameters are mutually exclusive in principle
because ferroelectricity requires empty d shells,while mag-
netism requires partially?lled d shells[2].Several compos-
ite materials,consisting of separate ferroelectric and mag-
netic phases,have been reported to show magnetoelectric
coupling at room temperature[3].However,the availabil-
ity of room-temperature single phase multiferroics is very
limited[4].Among the few room temperature single-phase
multiferroics reported so far[5],BiFeO3(BFO)is an im-
portant multiferroics,which has rhombohedrally distorted
perovskite crystal structure with a space group of R3c at
room temperature[6].It exhibits ferroelectric ordering be-
low T C～1083–1103K,and antiferromagnetic ordering be-low T N～625–643K[7].BFO shows G-type antiferromag-netic structural behavior having modulated spiral spin struc-
ture with long periodicity of62nm in the unit cell[8].In-
terests shown by the researchers to work on these materi-
als in a nanoregime is due to their size dependent proper-
ties compared to the bulk.Nanosized BFO powders have
been reported to exhibit weak ferromagnetism at room tem-
perature,which is different from the magnetic property of
bulk samples[9].One important challenge in the success-
ful synthesis of pure BFO is avoiding the secondary phases
such as Bi2Fe4O9and Bi25FeO39[10].Several techniques
have been developed to prepare pure BFO powders.The
solid state reaction route generally involves a higher pro-
cessing and requires HNO3as a leaching agent to elimi-
nate the secondary phases leading to the coarse nature of
the powders.Nanosized BFO ceramics have been prepared
854M.Muneeswaran et al.
by low-temperature chemical methods such as sol–gel[11], hydrothermal[12],auto combustion[13]and coprecipita-tion[14].But these processes also involve complex solu-tions and acid reagents.Hence,it is still worth in developing a soft chemical approach to obtain single-phase BFO with a homogeneous chemical composition crystallized at rela-tively low temperature.Here,we propose a novel soft chem-ical synthesis of single phase BFO powders relatively at low temperature,without using complex precursor solutions and acid-reagents.The rhombohedral structure of the synthe-sized BFO powders has been con?rmed by Rietveld analy-sis.4A1and7E phonon modes in the lower frequencies and two phonon scattering in the higher frequencies have been observed from Raman-scattering studies.A distinct dielec-tric anomaly observed in the temperature dependent dielec-tric measurements and their antiferromagnetic behavior at room temperature is con?rmed by the Arrott–Belov–Kouvel (ABK)plot.
2Experimental details
The synthesis procedure is as follows.Bi(NO3)3·5H2O and Fe(NO3)3·9H2O was dissolved in200ml of double distilled water and stirred for about20minutes to form a clear solu-tion.A constant pH level of10.8was maintained by syn-chronized dropping of a mixture of ammonia solution and distilled water solution to get the reaction product.This pre-cipitate was kept at room temperature for about24hours and was washed several times with double distilled water to re-move unreacted products and then?ltered.The?nal product was dried in a hot air oven at100?C for about5hours and ?nal sintering was carried out at600?C for2hours.The phase identi?cation was examined on a Rigaku(D/Max ul-tima III)X-ray diffractometer using Cu Kαradiation.XRD data was collected at a slow scan rate of0.05?/min and the simulation of the crystal structure was done based on the measured XRD data and Rietveld crystal structure re?ne-ment software General Structure Analysis System(GSAS). The morphology of the prepared powders was observed us-ing a Field Emission scanning electron microscope(FE-SEM,S-5000,HITACHI,Japan).Transmission electron mi-croscopy(TEM)images of the samples were taken through a JEM-2010electron microscope with an accelerating voltage of200kV.Differential Thermal Analysis(DTA)and Differ-ential scanning colorimetric(DSC)were performed using the SII Nanotechnology Inc.,Japan,and EXSTAR6200,re-spectively.The Raman spectra were recorded with a Raman spectrometer(Hololab5000,Kaiser Optical Systems Inc., (USA)with argon laser(532nm)and a Kaiser holographic edge?lter.Temperature dependant dielectric studies were performed with LCR meter(HIOKI3532-50,Japan)and the magnetic measurement were done with a vibrating sample magnetometer(Lakeshore,USA7404).
3Result and discussion
Figure1(a)shows the XRD patterns(as prepared and sin-tered)of BFO.As prepared powders show amorphous be-havior,while the sintered BFO was found to be well crystal-lized and formed in the rhombohedral structure(R3c)with a clear splitting of(104)and(110)peaks.A small amount of an impurity phase has also been observed due to the kinetics of formation[15].Further,the experimental XRD pattern is simulated to know the structure and the lattice parameters. Figure1(b)shows the results of the Rietveld re?nement of the XRD patterns of BFO.The re?nement is performed us-ing the rhombohedral crystal symmetry.The crystal struc-ture parameters derived from the simulation are listed in Ta-ble1.The Rp and wRp values are found to be higher com-pared to other literatures and may be due to the larger parti-cle size of BFO.The R andχ2values suggest that the sim-ulated XRD patterns agree well with the experimental XRD pattern.
XRD data can also be utilized to evaluate the peak broad-ening in terms of the crystallite size and the lattice strain due to dislocation.Since the breadth of the Bragg peak is the combination of both instrumental and sample dependent effects,it is necessary to collect a diffraction pattern from the line broadening of a standard material such as silicon
Table1Relevant parameters obtained from Rietveld re?nement XRD pattern of BFO powders
Lattice parameters(?)Atom coordinates Bond length(?)Bond angle V olume(?3)
X Y Z Rp wRpχ2
a=5.5947±0.01597Bi6a00012.020.4 1.79Bi–O Fe–O–Fe 2.210161.4?
b=13.9058±0.036508Fe6a000.201Fe–O O–Bi–O376.960 1.88471.2?
O18b0.4610.0330.951Fe–O 2.140
Structural,optical,and multiferroic properties of single phased BiFeO3855
Fig.1(a)X-ray diffraction pattern of BFO powders (rhombohedral with R3c space group).(b)Rietveld re?nement of X-ray diffraction data for BFO.The insets?gure shows that the cells with blue,brown and red spheres correspond to Bi,Fe,and O respectively. (c)W–H plot for BFO
powders
to determine the instrumental broadening[16,17].The in-strumental corrected broadeningβhkl corresponding to the diffraction peak of BFO are estimated by using the rela-tion:
βhkl=
(βhkl)2measured?β2instrumental
1/2
(1)
and strain induced broadening is given byε=βhkl/tanθ. Williamson and Hall(W–H)proposed a method of decon-voluting size and strain from the mathematical expression given by
βhkl cosθ=
kλ
D
+4εsinθ(2)
where“k”is the shape factor,“λ”is the X-ray wavelength,“θ”is the Bragg angle,“D”is the effective crystallite size,εis the strain,andβhkl is the full width at half maximum of the corresponding hkl plane.A plot is drawn between 4sinθalong the x-axis andβhkl cosθalong the y-axis as shown in Fig.1(c).From the linear?t to the data,the value of the strain is calculated from the slope of the line which is 0.00127±0.0003and the calculated crystallite size is42nm derived from the intersection of linear line with the vertical axis.
Figure2shows typical TEM images of the BFO sample. The image indicates[Fig.2(a)]the particle sizes are in be-tween50–100nm,which is in accordance with the particle size calculated from the XRD.Figure2(b)shows images ob-tained from a portion of an individual BFO particle con?rm-ing the good crystalline nature of BFO.Further,by using image analyzer software IMAGE-J on the lattice resolved TEM image,the distance between two parallel planes are found to be～2.35?.
Figure3(a)shows the Differential Thermal Analysis (DTA)curve of BFO for the heating cycle at a rate of 10?C/min.Two distinct peaks have been observed.A broad peak around330?C corresponds to the magnetic order-ing[18].Though a small energy change is associated with the magnetic transition[19],but still it is re?ected in the DTA curve.The same has been con?rmed from the DSC measurements shown in inset?gure.The sharp peak at 837?C corresponds to the ferroelectric to paraelectric tran-sition temperature of BFO[20].
Figure4shows the polarized Raman spectrum of BFO in the frequency range100–1500cm?1.On decomposing the?tted curves into individual Gaussian components,the peak position of each component,i.e.,the natural frequency
856
M.Muneeswaran et al.
Fig.2Transmission electron microscope images of (a )BFO powders.(b )Individual BFO
particle
Fig.3DTA curve of BFO powder and inset ?gure shows DSC mea-surement
(cm ?1)of each Raman active mode is as shown in Fig.4(a)and (b).At room temperature,BFO belongs to rhombohe-dral structure with the R3c space group with two formulas in one primitive cell.According to group theory,rhombohe-dral BFO has 18optical phonon modes [21]:Γopt .,R3c =4A 1+5A 2+9E
The A 1(TO)and E (LO)modes are Raman and in-frared active,while the 5A 2modes are Raman inactive modes [22]
ΓRaman ,R3c =4A 1+9E
where,A 1and E are polar optical modes,which are Ra-man and IR active,and they can split into two modes:Longitudinal Optical (LO)and Transverse Optical modes (TO).Here,A 1-symmetry phonons are therefore longitu-dinal optical A 1(LO)while the E-symmetry phonons are
transverse optical E (TO).It has been reported that Bi–O bonds contribute mostly to A 1modes,?rst-and second-order E (TO)modes,and Fe–O bonds to third-and fourth-order E (TO)modes [23].In polarized Raman scattering,the A 1modes can be observed by parallel polarization,while the E modes can be observed by both parallel and crossed polarizations.Thus,the E mode is associated with the atomic motion in the “a ”and “b ”plane whereas the A 1mode is associated with the atomic motion along the “c ”axis.In our present study [shown in Fig.4(a)],we observe seven E (TO)and four A 1Raman modes,which are mentioned in Table 2along with other literature re-ports [24,25].The Raman scattering data clearly shows three intense peaks of A 1-1,A 1-2,and A 1-3modes ap-pearing at 138,170,and 214cm ?1and a quite weak scat-tering intensity at 470cm ?1corresponding to A 1-4mode;the modes having medium scattering intensities at 254,276,342,418,523,557,and 603cm ?1assigned to E (TO)phonons.According to Yuan et al.[25],the stereo chemi-cal activity of Bi lone electron pair plays the main role in the change of both Bi–O covalent bonds,which is re?ec-tive in ?ve (E 1,A 1,A 2,A 3,and E 2)characteristic modes.These modes are responsible for the ferroelectric nature of the BFO.
Most of the Raman studies on BFO are focused in the low frequency range,since all the A1and E modes fall within this low frequency region.Very few reports are avail-able at higher frequencies.Generally,the origin of the high-frequency modes in the Raman spectra is attributed to elec-tronic Raman scattering or the high-order phonon scattering [26,27].We measured the same and is as shown in Fig.4(c).Three Raman modes,namely,2A 4(LO),2E 8(TO),and 2E 9(TO)are observed at 960cm ?1,1099cm ?1,and 1261cm ?1It has been reported that these high-frequency modes of BFO are overtones of the ?rst-order A 4,E 8,and E 9phonon modes corresponding to 2A 4,2E 8,and 2E 9modes,respec-tively.The modes at 557cm ?1(2E 8)is due to the Fe-O1
Structural,optical,and multiferroic properties of single phased BiFeO3857
Fig.4(a)Polarized Raman spectra of BFO.(b)A magni?ed view of the spectra range between100–650cm?1with their Gaussian?tted curve showing seven E modes and four A1modes.(c)Two phonon scattering observed between 850–1450cm?
1
Table2Observed and reported
Raman modes for BFO samples Raman modes(cm
?1)Yang et al.[24]Yuan et al.[25]Present study
A1-1139152.6138
A1-2172177.5170
A1-3217224.2214
A1-4470–470
E262270254
E275298.8276
E307––
E345354.9342
E396––
E429473.3418
E521–523
E–554.3557
E615618603
bonds and at603cm?1(2E9)assign to Fe–O2bonding, where O1are axial ions and O2are equatorial ions[22]. These two-phonon peaks are associated to the magnetic characters of BFO.The strong contribution of the two-phonon band to the total Raman spectrum has been at-tributed to a resonant enhancement with the intrinsic absorp-tion edge in BFO.This is similar to the two-phonon bands reported in hematite,-Fe2O3,the simplest case of iron ox-ides containing only FeO6octahedra[28].
The temperature dependence of the dielectric constant of BFO measured at different frequencies is shown in Fig.5. The high values of the dielectric constant at low frequen-cies and low values at higher frequencies indicate large dis-persion due to a Maxwell–Wagner type of interfacial polar-
858M.Muneeswaran et al.
ization,in agreement with Koop’s phenomenological the-ory [29].A dielectric anomaly has also been observed in the temperature dependent dielectric studies for the all the fre-quencies around 315?C at the vicinity of the Neel temper-ature (T N )of BFO.This dielectric anomaly may signify the coupling between the polarization and magnetization prop-erty of a multiferroic material.Below T N ,the material is ex-pected to be simultaneously ferroelectric and antiferromag-netic.The vanishing magnetic order on the electric order at the vicinity of T N leads to dielectric anomaly in magneto-electrically ordered systems as explained by the Landau–Devonshire theory of phase transitions [30,31].Similar di-electric anomaly in the vicinity of the Neel temperature for the both bulk BFO and thin ?lms have been reported by sev-eral others [32,33
].
Fig.5Dielectric constant versus temperature plot for the BFO ceram-ics measured at various frequencies
To study the magnetic properties of the BFO,we have measured the magnetization (M )as a function of applying magnetic ?eld (H )at room temperature shown in Fig.6(a).The magnetic hysteresis loop of the BFO shows enhanced antiferromagnetic properties with saturated magnetization (M s ),remanent magnetization (M r ),and coercive ?eld (H c )values of 0.11emu/gm,～0.01emu/gm and ～146.47Oe.To con?rm the antiferromagnetic behavior of BFO,Arrott–Belov–Kouvel (ABK)plots shown in Fig.6(b)are drawn by using M –H data.The ABK plot exhibits a concave nature without any spontaneous magnetization at H =0,indicating a AFM-feature [34,35].
4Conclusions
In summary,a soft chemical coprecipitation method had been proposed for the synthesis of nanosized multiferroic BFO powders.The structural re?nement of BFO reveals R3c crystal symmetry.From TEM analysis,the particle size of the BFO samples found to be between 50–100nm.The Dif-ferential Thermal Analysis (DTA)showed existence of mag-netic and ferroelectric ordering around 330?C and 837?C,respectively.Raman spectra of BFO over the frequency range of 100–1500cm ?1showed 4A 1and 7E modes with the appearance of 2A 4,2E 8,and 2E 9modes corresponding to the two-phonon scattering.From the temperature depen-dent dielectric studies,an anomaly in the dielectric constant was observed at the vicinity of the Neel temperature (T N )indicating a magnetic ordering and coupling between polar-ization and magnetization in BFO.The magnetic studies on the BFO con?rmed the antiferromagnetic behavior at room
temperature.
Fig.6(a )M–H hysteresis loop of BFO sample measured at room temperature.The inset ?gure shows partly enlarged M–H loop.(b )Arrott—Belov–Kouvel (ABK)plots for BFO powder
Structural,optical,and multiferroic properties of single phased BiFeO3859
Acknowledgements The authors would like to thank Dr.R.Na-galakhsmi for Rietveld re?nement analysis and Dr.R.Justin Joseyphus for providing the thermal analysis facility.
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Leakage mechanisms in BiFeO3 thin films Gary W. Pabst, Lane W. Martin, Ying-Hao Chu, and R. Ramesh Citation: Appl. Phys. Lett. 90, 072902 (2007); doi: 10.1063/1.2535663 View online: https://www.doczj.com/doc/fb15253036.html,/10.1063/1.2535663 View Table of Contents: https://www.doczj.com/doc/fb15253036.html,/resource/1/APPLAB/v90/i7 Published by the American Institute of Physics. Related Articles Large photoinduced conductivity reduction in thin films of metallic ferromagnetic manganites Appl. Phys. Lett. 99, 222507 (2011) Temperature-dependent leakage current behavior of epitaxial Bi0.5Na0.5TiO3-based thin films made by pulsed laser deposition J. Appl. Phys. 110, 103710 (2011) Probing the metal-insulator transition of NdNiO3 by electrostatic doping Appl. Phys. Lett. 99, 192107 (2011) Evidence of interface conversion and electrical characteristics improvement of ultra-thin HfTiO films upon rapid thermal annealing Appl. Phys. Lett. 99, 182904 (2011) Examination of insulator regime conduction mechanisms in epitaxial and polycrystalline SmNiO3 thin films J. Appl. Phys. 110, 094102 (2011) Additional information on Appl. Phys. Lett. Journal Homepage: https://www.doczj.com/doc/fb15253036.html,/ Journal Information: https://www.doczj.com/doc/fb15253036.html,/about/about_the_journal Top downloads: https://www.doczj.com/doc/fb15253036.html,/features/most_downloaded Information for Authors: https://www.doczj.com/doc/fb15253036.html,/authors
第45卷第4期2017年4月 硅酸盐学报Vol. 45，No. 4 April，2017 JOURNAL OF THE CHINESE CERAMIC SOCIETY https://www.doczj.com/doc/fb15253036.html, DOI：10.14062/j.issn.0454-5648.2017.04.01 铁酸铋薄膜的阻变效应和导电机制 朱慧1，张迎俏1，汪鹏飞1，白子龙2，孟晓1，陈月圆1，祁琼3 (1. 北京工业大学电子信息与控制工程学院，北京 100124； 2. 复旦大学微电子学院，上海 200433； 3. 中国科学院半导体研究所光电子器件国家工程研究中心，北京 100083) 摘要：针对脉冲激光沉积法制备的铁酸铋薄膜展开研究，利用电流–电压(I–V)特性曲线表征样品的阻变现象，对样品施加不同极性、大小的电压，其I–V曲线呈现出不同高低阻值的变化。通过对I–V曲线拟合，发现样品的导电机制符合空间电荷限制电流。结合正向电压下从高阻到低阻的转变，负向电压下从低阻到高阻的转变规律，验证样品的阻变效应符合陷阱能级的填充和脱陷，即陷阱能级的填充程度不同导致电极与铁酸铋界面势垒高度不同从而导致薄膜阻值的变化。 关键词：阻变效应；导电机制；陷阱填充与脱陷；空间电荷限制电流 中图分类号：TB34 文献标志码：A 文章编号：0454–5648(2017)04–0467–05 网络出版时间：2017–02–24 18:10:49 网络出版地址：https://www.doczj.com/doc/fb15253036.html,/kcms/detail/11.2310.TQ.20170224.1810.001.html Resistive Switching Effect and Conduction Mechanism of BiFeO3 Thin Films ZHU Hui1, ZHANG Yingqiao1, WANG Pengfei1, BAI Zilong2, MENG Xiao1, CHEN Yueyuan1, QI Qiong3 (1. The College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing 100124; 2. The College of Microelectronics, Fudan University, Shanghai 200433; 3. National Engineering Research Center for Optoelectronics Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083) Abstract: The resistive switching behavior of BiFeO3 thin film prepared via pulsed laser deposition was investigated. The current–voltage (I–V) curves were measured at different voltages. The resistance changes from high value to low value at a positive voltage, and the resistance changes from low value to high value at a negative voltage. The s pace charge limited conduction mechanism was analyzed through the fitting of the I–V curves. The resistive effect is attributed to the electric filed induced carrier trapping and detrapping, which results in the variation of the Schottky barrier height at the interface between the film and the electrode. Keywords: resistive effect; conduction mechanism; trapping and detrapping; s pace charge limited conduction 作为下一代最具潜力的非挥发性存储器之一的阻变存储器，因其低功耗、高存储密度、快存储速度、不易被干扰、结构简单等优势而受到广泛关注。目前，已在多种氧化物构成金属–氧化物–金属电容结构中发现阻变现象，如钙钛矿结构氧化物SrZrO3、SrTiO3和BiFeO3等[1–3]。铁酸铋(BiFeO3，BFO)是目前发现的唯一在室温以上同时表现出铁电性和铁磁性的材料，Curie温度(约1 103 K)和Neel温度(约643 K)高[4]，具有良好的电学和阻变特性，成为可应用于非挥发性铁电阻变存储器的材料之一。利用BFO制作的存储器，可通过施加不同的脉冲电压调节高低阻态之间的转换，以达到存储的目的，并且能够显著增加存储密度而不增加存储器尺寸。 BFO的禁带宽度约为2.67 eV，亲和能χ约为3.3 eV，其功函数约为4.7 eV[5]。当选择功函数大于BFO的金属电极时，电子会从BFO向金属电极迁移，直到金属和BFO拥有相同的Fermi能级，失去电子的正电荷在BFO表面形成一个耗尽区，从而在接 收稿日期：2016–06–27。修订日期：2016–12–21。 基金项目：国家自然科学基金(61201046，61306057)；北京市自然科学基金(4162013，2132023)；北京市博士后工作经费资助项目 (2015ZZ–33)；北京市教委科技计划一般项目 (KM201610005005)；教育部留学回国人员科研启动基金。第一作者：朱慧(1980–)，女，博士，副教授。Received date: 2016–06–27. Revised date: 2016–12–12. First author: ZHU Hui (1980–), female, Ph.D., Associate Professor. E–mail: zhuhui@https://www.doczj.com/doc/fb15253036.html,
2011Chinese Journal of Catalysis Vol. 32 No. 4文章编号: 0253-9837(2011)04-0618-06 DOI: 10.3724/SP.J.1088.2011.01210 研究论文: 618~623 粒径可控的纳米铁酸铋的制备及其光催化性能 县涛1,2, 杨华1,2,*, 戴剑锋1,2, 魏智强1,2, 马金元2, 冯旺军2 1 兰州理工大学甘肃省有色金属新材料省部共建国家重点实验室, 甘肃兰州 730050 2 兰州理工大学理学院, 甘肃兰州 730050 摘要：采用改进的聚丙烯酰胺凝胶法制备了 BiFeO3 纳米颗粒, 利用热重-差热、红外光谱及 X 射线衍射等手段研究了干凝胶的热分解及 BiFeO3 的成相过程. 结果表明, 在 600°C 煅烧可制备出高纯的 BiFeO3 纳米颗粒. 同时发现, 随着双丙烯酰胺含量的增加, 所得样品晶粒尺寸逐渐减小, 从而制备出平均粒径约 52~110nm 的系列 BiFeO3 颗粒, 颗粒尺寸分布均匀, 形貌规整, 近似呈球形. 以甲基橙为目标降解物, 研究了 BiFeO3 纳米颗粒的光催化性能. 结果表明, 在紫外光和可见光辐照下该纳米颗粒均表现出良好的光催化活性, 且随着颗粒尺寸减小, 催化活性增加. 适宜的甲基橙初始浓度为 10mg/L, 催化剂用量约为 2.5g/L. 关键词：铁酸铋; 纳米颗粒; 聚丙烯酰胺凝胶法; 晶粒尺寸调控; 光催化; 甲基橙 中图分类号：O643/X7文献标识码：A 收稿日期: 2010-12-07. 接受日期: 2011-01-14. *通讯联系人. Tel: (0931)2973783; Fax: (0931)2976040; E-mail: hyang@https://www.doczj.com/doc/fb15253036.html, 基金来源: 国家自然科学基金(50962009); 教育部科学技术研究重点项目(209130); 甘肃省自然科学基金(1010RJZA041); 兰州理工大学优秀青年基金(Q200902). Preparation and Photocatalytic Performance of Nano-bismuth Ferrite with Tunable Size XIAN Tao1,2, YANG Hua1,2,*, DAI Jianfeng1,2, WEI Zhiqiang1,2, MA Jinyuan2, FENG Wangjun2 1State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, Gansu, China 2School of Science, Lanzhou University of Technology, Lanzhou 730050, Gansu, China Abstract: A modified polyacrylamide gel method was used to fabricate BiFeO3 nanoparticles. Thermogravimetric analysis, differential scan-ning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction were used to investigate the thermal decomposition process of xerogel and the formation of BiFeO3 phase. It is demonstrated that high phase purity BiFeO3 nanoparticles can be prepared at a calcination temperature of 600 °C. Bisacrylamide, which acts as a crosslinking agent, plays an important role in tailoring the grain size of the resulting BiFeO3 nanoparticles. With increasing bisacrylamide content, the grain size decreases gradually. As a result, a series of BiFeO3 samples with average grain size of 52–110 nm were prepared. Scanning electron microscopy reveals that the prepared BiFeO3 nanoparticles are regularly spherical in shape with uniform particle size distribution. The photocatalytic activity of BiFeO3 nanoparticles was investigated by the degra-dation of methyl orange (MO). The experimental results reveal that they exhibit a pronounced photocatalytic activity for the MO decomposi-tion under ultraviolet and visible-light irradiation. With decrease in particle size, the reactive activity increases. The optimum conditions for the photocatalytic decolorization were determined to be initial MO concentration of ~10 mg/L and catalyst amount of ~2.5 g/L. Key words: bismuth ferrite; nanoparticle; polyacrylamide gel method; grain-size tailoring; photocatalysis; methyl orange Received 7 December 2010. Accepted 14 January 2011. *Corresponding author. Tel: (0931)2973783; Fax: (0931)2976040; E-mail: hyang@https://www.doczj.com/doc/fb15253036.html, This work was supported by the National Natural Science Foundation of China (50962009), Key Project of Chinese Ministry of Education (209130), Natural Science Foundation of Gansu Province (1010RJZA041), and Outstanding Youth Fund of Lanzhou University of Technol-ogy (Q200902) . 万方数据
Appl Phys A(2014)114:853–859 DOI10.1007/s00339-013-7712-5 Structural,optical,and multiferroic properties of single phased BiFeO3 M.Muneeswaran·P.Jegatheesan·M.Gopiraman· Ick-Soo Kim·N.V.Giridharan Received:26December2012/Accepted:13April2013/Published online:27April2013 ?Springer-Verlag Berlin Heidelberg2013 Abstract A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO3 (BFO)powders.The X-ray diffraction pattern con?rms the perovskite structure of BFO and Rietveld re?nement re-veals the existence of rhombohedral R3c symmetry.Crys-tallite size and strain value are studied from Williamson–Hall(W–H)analysis.The transmission electron microscope (TEM)image shows that the particle size of BFO powders lies between50–100nm.4A1and7E Raman modes have been observed in the range100–650cm?1and a prominent band centered around1150–1450cm?1have also been ob-served corresponding to the two-phonon scattering.Differ-ential Thermal Analysis(DTA)shows the existence of two prominent peaks at330?C and837?C corresponding to the magnetic and ferroelectric ordering,respectively.From the temperature dependent dielectric studies,an anomaly in the dielectric constant is observed at the vicinity of Neel tem-perature(T N)indicating a magnetic ordering.Also,BFO shows antiferromagnetic behavior measured from the mag-netic studies. 1Introduction Recently,the interest in multiferroics is stimulated by fun-damental physics leading to multiferroism arising from cou-M.Muneeswaran·P.Jegatheesan·N.V.Giridharan( ) Department of Physics,National Institute of Technology, Tiruchirappalli620015,India e-mail:giri@https://www.doczj.com/doc/fb15253036.html, Fax:+91-431-2500133 M.Gopiraman·I.-S.Kim Nano Fusion Technology Research Group,Faculty of Textile Science and Technology,Shinshu University,Ueda, Nagano386-0015,Japan pling between magnetic and ferroelectric orderings,and have been extensively studied for their possible technical applications,including spintronics,microelectronics,mag- netic memory,and sensors[1].The term“multiferroic” means coexistence of ferroelectric and magnetic ordering in one single phase or multiphase materials.However,these two ordering parameters are mutually exclusive in principle because ferroelectricity requires empty d shells,while mag- netism requires partially?lled d shells[2].Several compos- ite materials,consisting of separate ferroelectric and mag- netic phases,have been reported to show magnetoelectric coupling at room temperature[3].However,the availabil- ity of room-temperature single phase multiferroics is very limited[4].Among the few room temperature single-phase multiferroics reported so far[5],BiFeO3(BFO)is an im- portant multiferroics,which has rhombohedrally distorted perovskite crystal structure with a space group of R3c at room temperature[6].It exhibits ferroelectric ordering be- low T C～1083–1103K,and antiferromagnetic ordering be-low T N～625–643K[7].BFO shows G-type antiferromag-netic structural behavior having modulated spiral spin struc- ture with long periodicity of62nm in the unit cell[8].In- terests shown by the researchers to work on these materi- als in a nanoregime is due to their size dependent proper- ties compared to the bulk.Nanosized BFO powders have been reported to exhibit weak ferromagnetism at room tem- perature,which is different from the magnetic property of bulk samples[9].One important challenge in the success- ful synthesis of pure BFO is avoiding the secondary phases such as Bi2Fe4O9and Bi25FeO39[10].Several techniques have been developed to prepare pure BFO powders.The solid state reaction route generally involves a higher pro- cessing and requires HNO3as a leaching agent to elimi- nate the secondary phases leading to the coarse nature of the powders.Nanosized BFO ceramics have been prepared