- Email: [email protected]
Hydrothermal synthesis and self-assembly of magnetite (Fe3O4) nanoparticles with the magnetic and electrochemical properties
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 5453–5457
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hydrothermal synthesis and self-assembly of magnetite (Fe3O4) nanoparticles with the magnetic and electrochemical properties Zhong Jie Zhang a,, Xiang Ying Chen b,c,, Bai Nian Wang b,c, Cheng Wu Shi b,c a b c
College of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui 230039, PR China School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei, Anhui 230009, PR China
a r t i c l e in fo
abstract
Article history: Received 24 July 2008 Received in revised form 12 August 2008 Accepted 26 August 2008 Communicated by M. Schieber Available online 4 October 2008
We have developed a simple hydrothermal route to prepare the necklace-shaped superstructures self-assembled by magnetite (Fe3O4) nanoparticles coated with poly (vinyl pyrrolidone) (PVP). The asprepared samples were characterized by means of X-ray powder diffraction (XRPD), Fourier transform infrared (FTIR) spectra, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric analysis (TGA), magnetic hysteresis loop, and cyclic voltammetry. Magnetic measurement results reveal that the necklace-shaped superstructures show smaller magnetization compared with those of randomly distributed magnetite nanoparticles obtained in the absence of PVP. The electrochemical characterization was carried out by cyclic voltammetry, which indicates that the Fe3O4 sample in 1 mol L 1 Na2SO3 aqueous electrolyte is of an excellent electrode material for supercapacitor at the scan rate of 2 mV S 1 in the range of 1.0 to 0.2 V. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.66.Fn 74.70.Dd 77.84.Bw 81.10.Dn Keywords: A1. Nanostructures A2. Growth from solutions B1. Oxides B2. Magnetic materials
1. Introduction One-dimensional (1D) nanostructured materials such as nanorods, nanowires, nanotubes, and nanobelts have attracted much attention during the last decades owing to their potential applications as building blocks in fabricating nanodevices with novel optical, magnetic, and electrical properties [1,2]. As a member of the families of 1D nanostructured materials, necklace-like materials are usually attributed to the oriented assembly of nanoparticles. For example, pearl-necklace porous CdS nanofibers were synthesized using organogel as the template [3]; necklaceshaped mono- and bimetallic nanowires were obtained in hybrid organic–inorganic mesoporous material [4]; necklace-like assembly of inorganic fullerene-like MoS2 nanospheres were fabricated through a micelle-assisted route [5]; pearl-necklace CdTe nanoparticles can spontaneously reorganize into crystalline nanowires
Corresponding author. Also for correspondence at: School of Chemical Engineering, Hefei University
of Technology, Hefei, Anhui 230009, PR China. Tel./fax: +86 551 2901450. E-mail address: [email protected] (X.Y. Chen). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.08.064
upon controlled removal of the protective shell of organic stabilizer [6]. On the other hand, magnetite (Fe3O4) with the space group Fd3m is a cubic crystal of the spinel series. The compound has exhibited unique electric and magnetic properties based on the transfer of electrons between Fe2+ and Fe3+ in the octahedral sites. Owing to its structure, magnetite is a typical semi-metallic material, which has been widely used in the fields of magnetism, photoelectric plot [7], biomedicine, and high-gradient magnetic separation (HGMS) [8]. Especially, properly coated or surfacemodified magnetite nanoparticles can be applied in clinical diagnosis and as a medicine transporter [9]. To date, Fe3O4 with different morphologies, such as nanoparticles [10], nanorods [11], nanowires [12], nanotubes [13], and hollow nanospheres [14] have been prepared through different synthetic routes. However, to our knowledge, there is few report on the synthesis of necklaceshaped Fe3O4 coated with poly (vinyl pyrrolidone) (PVP). In this study, we demonstrate the fabrication of necklaceshaped superstructures self-assembled by Fe3O4 nanoparticles coated with PVP via a simple hydrothermal method. In addition, the magnetic and electrochemical properties of the magnetite nanoparticles were investigated.
ARTICLE IN PRESS 5454
Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
311
2. Experimental section
422
400 222
220
10
511 422
400 222
b
20
30
40 2θ (degree)
440
311
111
220
FeSO4 7H2O (2 mmol), Na2SO3 (4 mmol), and PVP (K30, 0.2 g) were first dissolved in 20 ml distilled water, and 0.2 mol L 1 NaOH solution (20 mL) was dropwise added into the above solution to obtain black colloids. Then the black colloids were transferred into a Teflon-lined stainless steel autoclave, which was degassed with N2 gas for half an hour, and then sealed, and heated at 140 1C for 12 h. After cooling to room temperature, the black products were filtered off, washed with distilled water and absolute ethanol several times, and dried at 50 1C for 4 h in vacuum. For comparison, the parallel experiment was also carried out in the absence of PVP while keeping the other reaction conditions unchanged.
a
111
2.1. Typical procedure for preparing necklace-shaped superstructures self-assembled by Fe3O4 nanoparticles coated with PVP
Intensity (a.u.)
511
440
All the chemical reagents are of analytical grade and used without further purification.
50
60
70
Fig. 1. XRD patterns of the as-prepared Fe3O4 samples: (a) in the presence of PVP and (b) in the absence of PVP.
3. Results and discussion XRPD technique is an effective tool to determine the phase, crystallinity, and purity of samples prepared under various conditions. Fig. 1 is the typical XRPD patterns of the obtained Fe3O4 samples coated with PVP (a) and non-coated with PVP (b). Typically, all the diffraction peaks in Fig. 1a can be indexed as cubic phase Fe3O4, which is in good agreement with the reported values (JCPDS Card no.19-0629). Meanwhile, all the diffraction peaks in Fig. 1b can be indexed as cubic Fe3O4 (JCPDS Card no.190629). The XRPD patterns herein are consistent with the reports on Fe3O4 samples previously by Wang et al. [12]. No obvious impurity can be detected in Fig. 1, indicating the pure phases of samples.
4000
3000
584
583
1065
1451
3500
2853
2924
X-ray powder diffraction (XRPD) patterns were determined using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu Ka radiation (l=1.541874 A˚). Transmission electron microscopy (TEM) images were characterized by Hitachi H-800 transmission electron microscope with a tungsten filament and an accelerating voltage of 200 kV. Highresolution TEM (HRTEM) images were taken on a JEOL 2010 highresolution transmission electron microscope performed at 200 kV. Fourier transform infrared (FTIR) spectra of the as-prepared products were recorded at room temperature with a KBr pellet on a VECTOR-22 (Bruker) spectrometer ranging from 400 to 4000 cm 1. The thermal gravimetric analysis (TGA) was conducted using a Shimadzu TGA-50H analyzer (pure Ar stream 50 mL/min; heating rate 10 1C/min). The magnetic measurement was carried out in a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan). The electrochemical characterization was carried out by cyclic voltammetry (CV) as follows. The electrode was a mixture of Fe3O4/acetylene black/PDFE with weight ratio of 8/1/1. The mixture and N-methyl pyrrolidone were spread on Ti foil and further dried at 60 1C for 12 h in vacuum. Ternary electrode system consisted of SCE as reference electrode, Pt electrode as assistance electrode, and Fe3O4 as the aim electrode. The capacitor performance was conducted at the scan rate of 2 mV s 1 in the range of 1.0 to 0.20 V in 1 mol L 1 Na2SO3 aqueous electrolyte.
Transmittance (%)
2.2. Characterization
2500 2000 1500 Wavenumber (cm-1)
1000
500
Fig. 2. FTIR spectra of the as-prepared Fe3O4 samples: (a) pure PVP; (b) in the presence of PVP; and (c) in the absence of PVP.
In order to further confirm the structure of the sample in Fig. 1, FTIR spectra were recorded at room temperature, which are shown in Fig. 2. First, we pictured the emblematical FTIR spectrum of pure PVP sample as the criteria in Fig. 1a. Second, we pictured the Fe3O4 samples prepared in the presence or absence of PVP to ascertain the detailed phase, which are depicted in Fig. 2b and c, respectively. We can see that the characteristic peaks at about 583 cm 1 exist in Fig. 2b and c, which indicates that pure-phase Fe3O4 products are synthesized under present synthetic conditions, as reported in Ref. [15]. In the meantime, some strong absorption peaks at 1065, 1451, 2853, and 2924 cm 1 exist in Fig. 2b, regarding the sample prepared in the presence of PVP, but these peaks are disappearing in the case of magnetite nanoparticles treated without PVP (Fig. 2c). In terms of the FTIR spectra given in Fig. 2, we can conclude that the surfactant of PVP is probably bonded to the surface of magnetite nanoparticles under the hydrothermal treatment at 140 1C for 12 h. TEM technique has been proved to be a powerful tool to vividly illustrate the sizes and shapes of samples. Herein, the representative TEM images of the as-obtained necklace-shaped Fe3O4 samples coated with PVP are shown in Fig. 3a–f. We can see
ARTICLE IN PRESS Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
5455
Fig. 3. TEM images of the as-obtained Fe3O4 samples coated: (a–f) in the presence of PVP and (g and h) in the absence of PVP. Note: The insets in Fig. 3b, c and f are the enlarged portion of the chain, showing that the necklace-like Fe3O4 nanoparticles are coated and interconnected with PVP.
that Fe3O4 nanoparticles with the average size of 80 nm selfassemble to form a micrometer-scale necklace-like chain in the presence of PVP. The insets in Fig. 3b, c and f are the enlarged portion of the chain, which clearly reveals that the necklace-like Fe3O4 nanoparticles are coated and interconnected with PVP. On the other hand, the necklace-shaped Fe3O4 samples coated with PVP are quite stable in absolute ethanol. In general, samples should be sonicated continuously for more than half an hour to prepare the sample for TEM observation. Herein, the necklaceshaped Fe3O4 samples coated with PVP are not destroyed even if they were sonicated in absolute ethanol for 4 h. However, in the absence of PVP, the resulting Fe3O4 samples contain a lot of randomly distributed tetragonal or hexagonal nanoparticles with the average size of 80 nm, shown in Fig. 3g and h. Therefore, we ascertain that the addition of PVP plays a crucial role in the formation of necklace-shaped Fe3O4 superstructures. On the other hand, we adopted HRTEM technique to acquire the intrinsic structures of the as-prepared samples on the nanometer scale. Fig. 4a is the representative HRTEM image of two neighboring Fe3O4 nanoparticles interconnected with PVP, which clearly demonstrates their single crystalline nature. Fig. 4b and c gives us the magnified portions. We can see in Fig. 4b that the lattice fringe is of 0.24 nm, agreeing with that of the (2 2 2) lattice plane regarding Fe3O4 sample [11]. Meanwhile, Fig. 4c shows the lattice fringe of 0.48 nm, which is consistent with that of the (111) lattice plane according to Laue equation. To further confirm the existence of PVP on the surface of necklace-like Fe3O4 nanoparticles, TGA method was adopted by designating the test temperature in the range of 25–800 1C.
Evidently, the mass profile in Fig. 5 shows a major weight loss of 28% over the temperature range of 25–800 1C. This implies that a portion of PVP sample exists on the surface of Fe3O4 nanoparticles. At the same time, we can see that the weight loss ceases until the high temperature of 650 1C in Fig. 5. Hence, it might be inferred that the connection force between PVP and the surfaces of the Fe3O4 nanoparticles is relatively strong. As a kind of excellent magnetic substance, it is indispensable to describe the magnetic properties of magnetite (Fe3O4). Herein, the typical magnetic hysteresis loops measured at room temperature are depicted in Fig. 6. In the case of Fe3O4 sample coated with PVP in Fig. 6a, it exhibits a ferromagnetic behavior with the saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values of ca. 51.0 emu/g, 12.5 emu/g, and 270.6 Oe, respectively. Regarding the Fe3O4 sample non-coated with PVP in Fig. 6b, it also exhibits a ferromagnetic behavior having the Ms, Mr, and Hc values of ca. 58.2 emu/g, 8.3 emu/g, and 268.8 Oe, respectively. The values of Ms are both lower than that of the corresponding bulk (92 emu/g) [14], which might come from the special characteristics of as-obtained Fe3O4 samples. On the other hand, we can see that the Ms value of Fe3O4 sample coated with PVP (Fig. 6a) is much smaller than that non-coated with PVP (Fig. 6b). This is because the Ms value is found to be proportional to the amount of weight for the same magnetic material [15]. As reported by Wu [16], supercapacitor is usually regarded as unique electrochemical device possessing high power density, high charge–discharge cycle life and high discharge efficiency, leading to meeting the burst power demands in many applications. Besides, some noble metal oxides nanocrystals such as
ARTICLE IN PRESS 5456
Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
60 40
M (emu/g)
20 0 (a) Fe3O4 sample coated with PVP
-20 -40
(b) Fe3O4 sample non-coated with PVP
-60 -10000
0 H (Oe)
-5000
10000
5000
Fig. 6. Room-temperature magnetic hysteresis loops for the Fe3O4 samples: (a) coated with PVP and (b) non-coated with PVP.
4
2
Fig. 4. HRTEM images of two adjacent Fe3O4 nanoparticles (a), as well as their enlarged portions (b and c).
I (mA)
0
-2
(a) Fe3O4 sample coated with PVP
-4
(b) Fe3O4 sample non-coated with PVP
100 95
Weight (%)
-6 90
-1.0
-0.8
-0.6
-0.4 -0.2 E (V)
28 % 85
Fig. 7. CV of electrodes in 1 mol L coated with PVP.
1
0.0
0.2
0.4
Na2SO3: (a) coated with PVP and (b) non-
80 Na2SO3 aqueous electrolyte is of an excellent electrode material for supercapacitor.
75
4. Conclusions
70 100
200
300 400 500 Temperature (°C)
600
700
800
Fig. 5. TGA curve of the necklace-like Fe3O4 sample obtained in the presence of PVP.
RuO2, IrO2 have exhibited pseudo-capacitance under certain conditions. Hereby, exploring the cheap and benign materials for supercapacitor is still of challenge and significance. Herein, Fig. 7 shows the typical CV of the Fe3O4 samples coated with PVP (a) and non-coated with PVP (b) electrodes in 1 mol L 1 Na2SO3 at the scan rate of 2 mV s 1 in the range of 1.0 to 0.20 V. The shapes of CV show the approximate rectangular mirror image, showing the typical characteristic of capacitive behavior [17]. It is almost symmetric between the cathode process and the anode process, which demonstrates that the Fe3O4 nanoparticles in 1 mol L 1
In summary, a simple but efficient hydrothermal method was developed to prepare novel necklace-shaped superstructures, which are self-assembled by magnetite nanoparticles coated with poly (vinyl pyrrolidone) (PVP). The experimental results show that PVP plays the crucial role in obtaining necklace-shaped Fe3O4 samples. The magnetic and electrochemical properties of the asprepared samples were also studied. This method may open an opportunity to synthesize other novel nanoscale superstructures coated with surfactants.
Acknowledgement The authors appreciate the Research Program for Young Teachers in Anhui Provincial Higher Education Institutions (No. 2008jq1014zd) for financial support.
ARTICLE IN PRESS Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
References [1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353. [2] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid Chem. (2003) 5. [3] P.C. Xue, R. Lu, Y. Huang, M. Jin, C.H. Tan, C.Y. Bao, Z.M. Wang, Y.Y. Zhao, Langmiur 20 (2004) 6470. [4] A. Fukuoka, Y. Sakamoto, S.Y. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima K. Hirahara, S. Iijima, M. Ichikawa, J. Am. Chem. Soc. 123 (2001) 3373. [5] Y.J. Xiong, Y. Xie, Z.Q. Li, X.X. Li, R. Zhang, Chem. Phys. Lett. 382 (2003) 180. [6] Z.Y. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237. [7] P.E. Matijeric, Colloid science of composite systems, in: Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 463.
5457
[8] S. Kurinobu, J. Uesugi, Y. Utumi, H. Kasahara, IEEE Trans. Magn. 35 (1999) 4067. [9] L. Fu, V.P. Dravid, D.L. Johnson, Appl. Surf. Sci. 181 (2001) 173. [10] N.R. Jana, Y.F. Chen, X.G. Peng, Chem. Mater. 16 (2004) 3931. [11] J.X. Wan, X.Y. Chen, Z.H. Wang, X.G. Yang, Y.T. Qian, J. Crystal Growth 276 (2005) 571. [12] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. [13] Z.Q. Liu, D.H. Zhang, S. Han, C. Li, B. Lei, W.G. Lu, J.Y. Fang, C.W. Zhou, J. Am. Chem. Soc. 127 (2005) 6. [14] X.Y. Chen, Z.J. Zhang, X.X. Li, C.W. Shi, Chem. Phys. Lett. 422 (2006) 294. [15] K.L. Yao, J. Tao, Z.L. Liu, Q.H. Lu, D. Xi, X.P. Luo, Q. Ning, J. Mater. Sci. Technol. 20 (2004) 417. [16] N.L. Wu, Mater. Chem. Phys. 75 (2002) 6. [17] N.L. Wu, S.Y. Wang, C.Y. Han, D.S. Wu, L.R. Shiue, J. Power Sources (2003) 173.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hydrothermal synthesis and self-assembly of magnetite (Fe3O4) nanoparticles with the magnetic and electrochemical properties Zhong Jie Zhang a,, Xiang Ying Chen b,c,, Bai Nian Wang b,c, Cheng Wu Shi b,c a b c
College of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui 230039, PR China School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei, Anhui 230009, PR China
a r t i c l e in fo
abstract
Article history: Received 24 July 2008 Received in revised form 12 August 2008 Accepted 26 August 2008 Communicated by M. Schieber Available online 4 October 2008
We have developed a simple hydrothermal route to prepare the necklace-shaped superstructures self-assembled by magnetite (Fe3O4) nanoparticles coated with poly (vinyl pyrrolidone) (PVP). The asprepared samples were characterized by means of X-ray powder diffraction (XRPD), Fourier transform infrared (FTIR) spectra, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric analysis (TGA), magnetic hysteresis loop, and cyclic voltammetry. Magnetic measurement results reveal that the necklace-shaped superstructures show smaller magnetization compared with those of randomly distributed magnetite nanoparticles obtained in the absence of PVP. The electrochemical characterization was carried out by cyclic voltammetry, which indicates that the Fe3O4 sample in 1 mol L 1 Na2SO3 aqueous electrolyte is of an excellent electrode material for supercapacitor at the scan rate of 2 mV S 1 in the range of 1.0 to 0.2 V. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.66.Fn 74.70.Dd 77.84.Bw 81.10.Dn Keywords: A1. Nanostructures A2. Growth from solutions B1. Oxides B2. Magnetic materials
1. Introduction One-dimensional (1D) nanostructured materials such as nanorods, nanowires, nanotubes, and nanobelts have attracted much attention during the last decades owing to their potential applications as building blocks in fabricating nanodevices with novel optical, magnetic, and electrical properties [1,2]. As a member of the families of 1D nanostructured materials, necklace-like materials are usually attributed to the oriented assembly of nanoparticles. For example, pearl-necklace porous CdS nanofibers were synthesized using organogel as the template [3]; necklaceshaped mono- and bimetallic nanowires were obtained in hybrid organic–inorganic mesoporous material [4]; necklace-like assembly of inorganic fullerene-like MoS2 nanospheres were fabricated through a micelle-assisted route [5]; pearl-necklace CdTe nanoparticles can spontaneously reorganize into crystalline nanowires
Corresponding author. Also for correspondence at: School of Chemical Engineering, Hefei University
of Technology, Hefei, Anhui 230009, PR China. Tel./fax: +86 551 2901450. E-mail address: [email protected] (X.Y. Chen). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.08.064
upon controlled removal of the protective shell of organic stabilizer [6]. On the other hand, magnetite (Fe3O4) with the space group Fd3m is a cubic crystal of the spinel series. The compound has exhibited unique electric and magnetic properties based on the transfer of electrons between Fe2+ and Fe3+ in the octahedral sites. Owing to its structure, magnetite is a typical semi-metallic material, which has been widely used in the fields of magnetism, photoelectric plot [7], biomedicine, and high-gradient magnetic separation (HGMS) [8]. Especially, properly coated or surfacemodified magnetite nanoparticles can be applied in clinical diagnosis and as a medicine transporter [9]. To date, Fe3O4 with different morphologies, such as nanoparticles [10], nanorods [11], nanowires [12], nanotubes [13], and hollow nanospheres [14] have been prepared through different synthetic routes. However, to our knowledge, there is few report on the synthesis of necklaceshaped Fe3O4 coated with poly (vinyl pyrrolidone) (PVP). In this study, we demonstrate the fabrication of necklaceshaped superstructures self-assembled by Fe3O4 nanoparticles coated with PVP via a simple hydrothermal method. In addition, the magnetic and electrochemical properties of the magnetite nanoparticles were investigated.
ARTICLE IN PRESS 5454
Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
311
2. Experimental section
422
400 222
220
10
511 422
400 222
b
20
30
40 2θ (degree)
440
311
111
220
FeSO4 7H2O (2 mmol), Na2SO3 (4 mmol), and PVP (K30, 0.2 g) were first dissolved in 20 ml distilled water, and 0.2 mol L 1 NaOH solution (20 mL) was dropwise added into the above solution to obtain black colloids. Then the black colloids were transferred into a Teflon-lined stainless steel autoclave, which was degassed with N2 gas for half an hour, and then sealed, and heated at 140 1C for 12 h. After cooling to room temperature, the black products were filtered off, washed with distilled water and absolute ethanol several times, and dried at 50 1C for 4 h in vacuum. For comparison, the parallel experiment was also carried out in the absence of PVP while keeping the other reaction conditions unchanged.
a
111
2.1. Typical procedure for preparing necklace-shaped superstructures self-assembled by Fe3O4 nanoparticles coated with PVP
Intensity (a.u.)
511
440
All the chemical reagents are of analytical grade and used without further purification.
50
60
70
Fig. 1. XRD patterns of the as-prepared Fe3O4 samples: (a) in the presence of PVP and (b) in the absence of PVP.
3. Results and discussion XRPD technique is an effective tool to determine the phase, crystallinity, and purity of samples prepared under various conditions. Fig. 1 is the typical XRPD patterns of the obtained Fe3O4 samples coated with PVP (a) and non-coated with PVP (b). Typically, all the diffraction peaks in Fig. 1a can be indexed as cubic phase Fe3O4, which is in good agreement with the reported values (JCPDS Card no.19-0629). Meanwhile, all the diffraction peaks in Fig. 1b can be indexed as cubic Fe3O4 (JCPDS Card no.190629). The XRPD patterns herein are consistent with the reports on Fe3O4 samples previously by Wang et al. [12]. No obvious impurity can be detected in Fig. 1, indicating the pure phases of samples.
4000
3000
584
583
1065
1451
3500
2853
2924
X-ray powder diffraction (XRPD) patterns were determined using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu Ka radiation (l=1.541874 A˚). Transmission electron microscopy (TEM) images were characterized by Hitachi H-800 transmission electron microscope with a tungsten filament and an accelerating voltage of 200 kV. Highresolution TEM (HRTEM) images were taken on a JEOL 2010 highresolution transmission electron microscope performed at 200 kV. Fourier transform infrared (FTIR) spectra of the as-prepared products were recorded at room temperature with a KBr pellet on a VECTOR-22 (Bruker) spectrometer ranging from 400 to 4000 cm 1. The thermal gravimetric analysis (TGA) was conducted using a Shimadzu TGA-50H analyzer (pure Ar stream 50 mL/min; heating rate 10 1C/min). The magnetic measurement was carried out in a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan). The electrochemical characterization was carried out by cyclic voltammetry (CV) as follows. The electrode was a mixture of Fe3O4/acetylene black/PDFE with weight ratio of 8/1/1. The mixture and N-methyl pyrrolidone were spread on Ti foil and further dried at 60 1C for 12 h in vacuum. Ternary electrode system consisted of SCE as reference electrode, Pt electrode as assistance electrode, and Fe3O4 as the aim electrode. The capacitor performance was conducted at the scan rate of 2 mV s 1 in the range of 1.0 to 0.20 V in 1 mol L 1 Na2SO3 aqueous electrolyte.
Transmittance (%)
2.2. Characterization
2500 2000 1500 Wavenumber (cm-1)
1000
500
Fig. 2. FTIR spectra of the as-prepared Fe3O4 samples: (a) pure PVP; (b) in the presence of PVP; and (c) in the absence of PVP.
In order to further confirm the structure of the sample in Fig. 1, FTIR spectra were recorded at room temperature, which are shown in Fig. 2. First, we pictured the emblematical FTIR spectrum of pure PVP sample as the criteria in Fig. 1a. Second, we pictured the Fe3O4 samples prepared in the presence or absence of PVP to ascertain the detailed phase, which are depicted in Fig. 2b and c, respectively. We can see that the characteristic peaks at about 583 cm 1 exist in Fig. 2b and c, which indicates that pure-phase Fe3O4 products are synthesized under present synthetic conditions, as reported in Ref. [15]. In the meantime, some strong absorption peaks at 1065, 1451, 2853, and 2924 cm 1 exist in Fig. 2b, regarding the sample prepared in the presence of PVP, but these peaks are disappearing in the case of magnetite nanoparticles treated without PVP (Fig. 2c). In terms of the FTIR spectra given in Fig. 2, we can conclude that the surfactant of PVP is probably bonded to the surface of magnetite nanoparticles under the hydrothermal treatment at 140 1C for 12 h. TEM technique has been proved to be a powerful tool to vividly illustrate the sizes and shapes of samples. Herein, the representative TEM images of the as-obtained necklace-shaped Fe3O4 samples coated with PVP are shown in Fig. 3a–f. We can see
ARTICLE IN PRESS Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
5455
Fig. 3. TEM images of the as-obtained Fe3O4 samples coated: (a–f) in the presence of PVP and (g and h) in the absence of PVP. Note: The insets in Fig. 3b, c and f are the enlarged portion of the chain, showing that the necklace-like Fe3O4 nanoparticles are coated and interconnected with PVP.
that Fe3O4 nanoparticles with the average size of 80 nm selfassemble to form a micrometer-scale necklace-like chain in the presence of PVP. The insets in Fig. 3b, c and f are the enlarged portion of the chain, which clearly reveals that the necklace-like Fe3O4 nanoparticles are coated and interconnected with PVP. On the other hand, the necklace-shaped Fe3O4 samples coated with PVP are quite stable in absolute ethanol. In general, samples should be sonicated continuously for more than half an hour to prepare the sample for TEM observation. Herein, the necklaceshaped Fe3O4 samples coated with PVP are not destroyed even if they were sonicated in absolute ethanol for 4 h. However, in the absence of PVP, the resulting Fe3O4 samples contain a lot of randomly distributed tetragonal or hexagonal nanoparticles with the average size of 80 nm, shown in Fig. 3g and h. Therefore, we ascertain that the addition of PVP plays a crucial role in the formation of necklace-shaped Fe3O4 superstructures. On the other hand, we adopted HRTEM technique to acquire the intrinsic structures of the as-prepared samples on the nanometer scale. Fig. 4a is the representative HRTEM image of two neighboring Fe3O4 nanoparticles interconnected with PVP, which clearly demonstrates their single crystalline nature. Fig. 4b and c gives us the magnified portions. We can see in Fig. 4b that the lattice fringe is of 0.24 nm, agreeing with that of the (2 2 2) lattice plane regarding Fe3O4 sample [11]. Meanwhile, Fig. 4c shows the lattice fringe of 0.48 nm, which is consistent with that of the (111) lattice plane according to Laue equation. To further confirm the existence of PVP on the surface of necklace-like Fe3O4 nanoparticles, TGA method was adopted by designating the test temperature in the range of 25–800 1C.
Evidently, the mass profile in Fig. 5 shows a major weight loss of 28% over the temperature range of 25–800 1C. This implies that a portion of PVP sample exists on the surface of Fe3O4 nanoparticles. At the same time, we can see that the weight loss ceases until the high temperature of 650 1C in Fig. 5. Hence, it might be inferred that the connection force between PVP and the surfaces of the Fe3O4 nanoparticles is relatively strong. As a kind of excellent magnetic substance, it is indispensable to describe the magnetic properties of magnetite (Fe3O4). Herein, the typical magnetic hysteresis loops measured at room temperature are depicted in Fig. 6. In the case of Fe3O4 sample coated with PVP in Fig. 6a, it exhibits a ferromagnetic behavior with the saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values of ca. 51.0 emu/g, 12.5 emu/g, and 270.6 Oe, respectively. Regarding the Fe3O4 sample non-coated with PVP in Fig. 6b, it also exhibits a ferromagnetic behavior having the Ms, Mr, and Hc values of ca. 58.2 emu/g, 8.3 emu/g, and 268.8 Oe, respectively. The values of Ms are both lower than that of the corresponding bulk (92 emu/g) [14], which might come from the special characteristics of as-obtained Fe3O4 samples. On the other hand, we can see that the Ms value of Fe3O4 sample coated with PVP (Fig. 6a) is much smaller than that non-coated with PVP (Fig. 6b). This is because the Ms value is found to be proportional to the amount of weight for the same magnetic material [15]. As reported by Wu [16], supercapacitor is usually regarded as unique electrochemical device possessing high power density, high charge–discharge cycle life and high discharge efficiency, leading to meeting the burst power demands in many applications. Besides, some noble metal oxides nanocrystals such as
ARTICLE IN PRESS 5456
Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
60 40
M (emu/g)
20 0 (a) Fe3O4 sample coated with PVP
-20 -40
(b) Fe3O4 sample non-coated with PVP
-60 -10000
0 H (Oe)
-5000
10000
5000
Fig. 6. Room-temperature magnetic hysteresis loops for the Fe3O4 samples: (a) coated with PVP and (b) non-coated with PVP.
4
2
Fig. 4. HRTEM images of two adjacent Fe3O4 nanoparticles (a), as well as their enlarged portions (b and c).
I (mA)
0
-2
(a) Fe3O4 sample coated with PVP
-4
(b) Fe3O4 sample non-coated with PVP
100 95
Weight (%)
-6 90
-1.0
-0.8
-0.6
-0.4 -0.2 E (V)
28 % 85
Fig. 7. CV of electrodes in 1 mol L coated with PVP.
1
0.0
0.2
0.4
Na2SO3: (a) coated with PVP and (b) non-
80 Na2SO3 aqueous electrolyte is of an excellent electrode material for supercapacitor.
75
4. Conclusions
70 100
200
300 400 500 Temperature (°C)
600
700
800
Fig. 5. TGA curve of the necklace-like Fe3O4 sample obtained in the presence of PVP.
RuO2, IrO2 have exhibited pseudo-capacitance under certain conditions. Hereby, exploring the cheap and benign materials for supercapacitor is still of challenge and significance. Herein, Fig. 7 shows the typical CV of the Fe3O4 samples coated with PVP (a) and non-coated with PVP (b) electrodes in 1 mol L 1 Na2SO3 at the scan rate of 2 mV s 1 in the range of 1.0 to 0.20 V. The shapes of CV show the approximate rectangular mirror image, showing the typical characteristic of capacitive behavior [17]. It is almost symmetric between the cathode process and the anode process, which demonstrates that the Fe3O4 nanoparticles in 1 mol L 1
In summary, a simple but efficient hydrothermal method was developed to prepare novel necklace-shaped superstructures, which are self-assembled by magnetite nanoparticles coated with poly (vinyl pyrrolidone) (PVP). The experimental results show that PVP plays the crucial role in obtaining necklace-shaped Fe3O4 samples. The magnetic and electrochemical properties of the asprepared samples were also studied. This method may open an opportunity to synthesize other novel nanoscale superstructures coated with surfactants.
Acknowledgement The authors appreciate the Research Program for Young Teachers in Anhui Provincial Higher Education Institutions (No. 2008jq1014zd) for financial support.
ARTICLE IN PRESS Z.J. Zhang et al. / Journal of Crystal Growth 310 (2008) 5453–5457
References [1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353. [2] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid Chem. (2003) 5. [3] P.C. Xue, R. Lu, Y. Huang, M. Jin, C.H. Tan, C.Y. Bao, Z.M. Wang, Y.Y. Zhao, Langmiur 20 (2004) 6470. [4] A. Fukuoka, Y. Sakamoto, S.Y. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima K. Hirahara, S. Iijima, M. Ichikawa, J. Am. Chem. Soc. 123 (2001) 3373. [5] Y.J. Xiong, Y. Xie, Z.Q. Li, X.X. Li, R. Zhang, Chem. Phys. Lett. 382 (2003) 180. [6] Z.Y. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237. [7] P.E. Matijeric, Colloid science of composite systems, in: Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 463.
5457
[8] S. Kurinobu, J. Uesugi, Y. Utumi, H. Kasahara, IEEE Trans. Magn. 35 (1999) 4067. [9] L. Fu, V.P. Dravid, D.L. Johnson, Appl. Surf. Sci. 181 (2001) 173. [10] N.R. Jana, Y.F. Chen, X.G. Peng, Chem. Mater. 16 (2004) 3931. [11] J.X. Wan, X.Y. Chen, Z.H. Wang, X.G. Yang, Y.T. Qian, J. Crystal Growth 276 (2005) 571. [12] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. [13] Z.Q. Liu, D.H. Zhang, S. Han, C. Li, B. Lei, W.G. Lu, J.Y. Fang, C.W. Zhou, J. Am. Chem. Soc. 127 (2005) 6. [14] X.Y. Chen, Z.J. Zhang, X.X. Li, C.W. Shi, Chem. Phys. Lett. 422 (2006) 294. [15] K.L. Yao, J. Tao, Z.L. Liu, Q.H. Lu, D. Xi, X.P. Luo, Q. Ning, J. Mater. Sci. Technol. 20 (2004) 417. [16] N.L. Wu, Mater. Chem. Phys. 75 (2002) 6. [17] N.L. Wu, S.Y. Wang, C.Y. Han, D.S. Wu, L.R. Shiue, J. Power Sources (2003) 173.