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Facile Hydrothermal Synthesis and Growth Kinetics of Fe-Based Magnetic Nanoparticles
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ACTA METALLURGICA SINICA (ENGLISH LETTERS)
www.arns,org.cn
Acta Metall. Sin. (Engl. Lett.) Vol.20 No. 6 pp434-440 Dec. 2007
FACILE HYDROTHERMAL SYNTHESIS AND GROWTH KINETICS OF Fe-BASED MAGNETIC NANOPARTICLES C . X . You, J . C . Zhang*, Y. Shen, and Z.W. Song Shoo1 of Material Science and Engineering, Shanghai University, Shanghai 200072, China Manuscript received 17 May 2007
The facile hydrothermal method was used to synthesize F r o , nanoparticles with an average diameter of I Inm. The pure bodv-centered cubic (bcc)-Fe nanoparticles were prepared by reduction of Fe 0,nanoparticlespowder in H2 atmosphere. The structure, tnorphoiogy and mugnetic properties of the products were characterized by X-ray powder diffraction (XRD). trunsmission electron microscopy (TEM), thermrigravimetric analysis-diyerential scanning calorimetn, (TGA-DSC) and vibrating sample magnetometer (VSM). The results showed thar the as-prepared Fe,O, nanoparticles had a relatively homogeneous size. The particle diameters became bigger with the increuse of reaction time. The growth kinetics of the FePI nunoparticles was also brieflv discussed. The products exhibited superparamagnetic properties at room temperature and the spec@ saturation magnetization w m dependent on the particle sizes. KEY WORDS Fe-based nunoparticle; hydrothermal; specijc saturation tnagnetization; kinetics
1. Introduction Compared with bulk materials, magnetic nanoparticles exhibit unique physical, electrical, magnetic and chemical They have attracted great deal of attention because of their various technological applications, for example, magnetic recording media, catalysis, medicine transporter, bio-medi~ine[~-'] and so on. Magnetic nanoparticles have been prepared by a variety of methods, such as the coprecipitation of an aqueous solution of ferrous and ferric ions by a sol-gel[lol,sonochemistry["', microwave plasma synthesis[l21,nonaqueous route['"'41,gas-phase methods[l5]etc.. These methods could be used to synthesize magnetic nanoparticles with grain size larger or less than 20nm, but they require many expensive experiment instruments or poisonous raw materials, or rigorous experiment conditions. In this article, hydrothermal method was employed to synthesize Fe-based magnetic nanoparticles using sodium bis (2-ethylhexyl) sulfosuccinate (Aerosol OT) as dispersant and NzH4 HzO as a reagent for
-
'Corresponding author. Tel. : +86 21 56334743. E-mail address : [email protected] .cn ( J . C . Zhang)
. 435
*
adjusting the pH value of the solution['61,respectively. The experimental conditions were simple, no expensive instruments and poisonous raw materials were used. Fe304magnetic nanoparticles with an average grain size of 1 lnm have been synthesized by this approach, and the particle diameters were relatively homogeneous. The pure bcc-Fe nanoparticles were obtained by reduction of Fe30, nanoparticles powder in H2 atmosphere. The magnetic properties of the as-prepared Fe-based nanoparticles were investigated. The growth kinetics was also briefly discussed.
2. Experimental All the reagents were of analytical grade and purchased from Shanghai Chemical Reagents Company, China National Medicines (Group) and used without hrther purification. The process can be divided into the following two steps: preparation of Fe304nanoparticles, preparation of Fe203and bcc-Fe nanoparticles. (1) Process 1. Typically, 54mmol N2H4.H20was mixed with 20mLofO. 1 1 M sodium bis (2-ethylhexyl) sulfosuccinate (Aerosol OT) solution in a beaker. AFter magnetic stirring for some time, 2mmol FeC13* 6H20was added into the previous solution. Then, the stirring and supersonic dispersal was carried on for another 15min to make the reaction sufficient. The mixture was then transferred into the Teflon-lined stainless autoclave. The autoclave was put in an oven at 150°C for 1Oh and then slowly cooled to room temperature naturally. The black Fe304nanoparticles were precipitated by centrifugation, washed and dried in air. (2) Process 2. The red-brown Fe,O, nanoparticles were assembly obtained from O2oxidation of black Fe304particles at 420°C for 6h. After being annealed under H2at 450"C, the Fe304nanoparticles were assembly reduced to bcc-Fe nanoparticles. (3) Characterization. Differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA) were carried out with a NETZSCH STA 409 PC simultaneous TG-DSC apparatus with a heating rate of 10'C/min in flowing air. X-ray diffraction (XRD) data were collected at room temperature using a Rigaku D/max 2200V X-ray difiactometer with high-intensity Cu K , radiation (A = 0.154056nm). For measuring the diameters and morphology of magnetite nanoparticles, the transmission electron microscopy (TEM) images were taken with FEI Tecnai 20 transmission electron microscopes. The magnetic properties of the nanoparticles were measured by an EG&G Princeton applied research vibrating sample magnetometer (VSM) at room temperature.
3. ResultsandDiscussion 3.1 TEM micrograph Fig.1 shows the average particle size of the Fe304nanoparticles to be about 1 lnm and the distribution of the particle diameters is relatively homogenous. 3.2 XRD patterns and the growth kinetics analysis Fig.2 shows the XRD spectrum of the obtained magnetic Fe304nanoparticles from the various hydrothermal reaction time. The curves
Fig. 1 TEM image of Fe304nanoparticles from hydrothermal reaction time of 1Oh at 150'C.
10
20
30
40
50
60
70
80
90
2 a, deg.
Fig.2 XRD patterns of the obtained magnetic Fe,O, nanoparticles from different hydrothermal reaction time (curve a. 3h, curve b. 5h, curve c. 7h, curved. 10h, and curve e. 13h). Table 1 The estimated average particle diameter according to the Scherrer‘sformula Sample
Reaction time, h
2 8 , deg.
FWHM, deg.
Average diameter, nm
a
3
35.4
0.74
11.1
b
5
35.4
0.72
11.4
C
7
35.4
0.70
11.7
d
10
35.4
0.68
12.1
e
13
35.4
0.66
12.5
Notes: FWHM mcans full width half maximum.
a-e in Fig.2 represent the products of 3, 5 , 7 , 10 and 13h, respectively. The XRD patterns exhibit a group of spinel-type diffi-actionpeaks, similar to the reported data (JCPDS #72-2303), which can be indexed as face-centered cubic Fe1O4structure with a lattice parameter a = 0.840nm. On the basis of Scherrer’s formula[”],the average crystallite sizes of the powder are calculated using the strongest phases of the diffraction plane 3 1 1 in Fig.2. Listed in Table 1, the estimated results are consistent with that determined by the statistical analysis of the TEM images. According to Fig.2 and Table 1, it can be concluded that the particle size is dependent on time (Fig.3), but the influence is not very distinctive. Assuming that the particles are spherical and the growth rate is determined by a difhsion process of the solute, we employ the simplified growth rate law derived fiom the Lifshitz-Slyozov-Wagner (LSW) m ~ d e l [ ’ ~as. ’in ~ ]Eq.( 1). This equation can be used to hrther develop a quantitative understanding of the growth kinetics of the FelO, nanoparticles
where r is the average particle radius at time t, ro is the initial particle radius, c,=, is the equilibrium concentration at a flat surface, k.,the bulk solubility (because the concentration difference is relatively small
(2y Vm/(R T r ) << l), the approximation was made that c , =c,according to Gibbs-Thomson A is aconstant, y is the interfacial energy, V , is the molar volume of the solid phase, R is the gas constant, T is the temperature and D is the diffision coefficient. Fig.3 shows the cube of average particle radius plotted time according to Table 1. The curve is roughly linear. The slope of the line (the value ofA in Eq.( 1)) is about 57.5nm3/h. Therefore, it can be inferred that the increase of the particle size is dominated by diffusion-limited growth at this temperature. The intercept at t=O (ro) is about 10.8nm, so the average particle radius can be extrapolated to longer reaction time according to Eq.( 1). The difision coefficient (D)can be further estimated by consideration of the constant A according to Eq.(l). Values for the interfacial energy (y)‘”] and the equilibrium concentration at a flat surface (cr=-) are assumed to be 0.035mJ/m2(here the bulk solubility was used as y ) and 1.5 x lO-’mol/L, respectively. Taking V, = 29.7mL/mol, T = 423K and A = 57.5nm3/h, the approximation of D is obtained, that is about 1.4 x 1O“cm’/s, consistent with the typical value for ions in solution. The difision coefficient for Fe” obtained from the LSW model can be compared with the StokesEinstein model for ionic diffusion as V.P.
where q is the viscosity of the solvent, a is the hydrodynamic radius of the solute, and kT has the usual meaning. The viscosity of water is 0.284 x 1O”kg /(m * s) and the hydrodynamic ra~ ] . the difision dius for Fe” is 0.5 1n ~ n ‘ ~Hence, coefficient (D)obtained from Eq.(2) is on the same order of 104cm2/s. The result shows good agreement with that from LSW model and further provides confirmation of the LSW model for the growth kinetics of the particles.
3.3 DSC and TGA curves The thermal behavior of the Fe30, nanoparticles is investigated with TG and DSC analysis in air (Fig.4). The TGA curve shows that the sample starts to volatilize at about 50°C. Weight loss before 180°C (about 6.7%) is possibly attributed to the volatilization of water and surfactants in the sample. The major weight loss (about 2 1.2%) from 180 to 450’C is caused by the transformation of Fe,04 into Fe203.The DSC curve shows two primary endothermal peaks located at 160.9 and 413°C. The positions of the peaks fit well with that of the weight loss in the TGA curve. On the basis
1800
-
-L
I 0
I 2
4
6 8 Time, h
1
0
1
2
1
4
Fig.3 Plotted as the particle average radius cubed I J S . the growth time at 150”C, in accordance with the LSW model as described in Eq.( I )
-- I
I
160 9 C
75 0
200
400
600
800
Temperature, C
Fig.4 TG-DSC curves of Fe304 nanoparticles from hydrothermal reaction time of 10h at 150°C.
of the TG and DSC analysis, 450°C was chosen as heat-treatment temperature to ensure the complete reduction of the Fe,O, into bcc-Fe nanoparticles in H2.All the results can be demonstrated by the XRD characterization (Fig.5). The curve a represents the phase structure of the Fe304nanoparticles before the thermal treatment. The curve b represents the phase structure of the FQO, nanoparticles obtained fiom the oxidation of Fe,O, in O2at 420°C for 6h. Compared with curve a, the large-angle peaks in curve b shift slightly to higher angles, and additional small peaks appear at lower angles. Curve b matches well with the index of the cubic Fe203phase (JCPDS #39-1346). Curve cc represents represents the the phase phase structure structure of of Fe Fe nanoparticles nanoparticles 0 obtained obtained fiom fiom Fe304 Fe304annealed annealed under under Hz H2at at 450°C. 450°C. ItIt can #87can be be indexed indexed to to the the bcc-Fe bcc-Fe phase phase (JCPDS (JCPDS #87-
-
3
072 0721). 1).
-
m
2
3.4 3.4 Magnetic Magnetic properties properties
c
C
E
Figs. Figs.6a-c 6a-c shows shows the the hysteresis hysteresis loops loops of of the the asassynthesized synthesized Fe304 Fe304nanoparticles nanoparticles fiom fiom different different hyhydrothermal reaction time and of the as-reduced bcc-Fe nanoparticles (Fig.6d) measured at room temperature. The results indicate that all the particles are possibly superparamagnetic at room temperature,
N
0 0 N
.-
7
1
m
_ c
Fig.5 XRD patterns of nanoparticles (curve a. Fe304; curve b. Fe103;curve c. bcc-Fe).
20
-20
c
-601
I
'
-10000
-5000
I
0
5000
10000
I
I -10000
-5000
H. Oe
__
0
5000
10000
5000
10000
H, Oe
I
I -10000
-5000
0 H, Oe
5000
10000
-10000
-5000
0 H, Oe
Fig.6 Hysteresis loops of the magnetic nanoparticles assembly measured at room temperature:Fe,O, nanoparticles at 150°C from hydrothermal reaction time of 3h (a), 10h (b), and 13h (c); (d) bcc-Fe nanoparticles.
because the remanence of the particles is zero in the absence of an external magnetic field and the coercivity is almost negligible. The reason for the nanoparticles having superparamagnetic property is explained as follows: the characteristic physical lengths related to magnetism (such as the size of the single magnetic domain and the critical size of superparamagnetism e t c . ) are approximately on the order of 1 I OOnm. When the particle size is corresponding to or smaller than the characteristic physical length, the particles will show abnormal magnetic property. As the critical size of superparamagnetism for Fe304at room temperature is 16nm and the average diameter of the as-prepared sample is 1 lnm, the Fe304particles exhibit superparamagnetism. For Fe304 nanoparticles from a hydrothermal reaction time of 1Oh at 150"C, the value of specific saturation magnetization (as)is 55.8emu/g, smaller than that of the Fe304 bulk materials (us= 88emdg )[231. According to the report in the literature, the decrease of usresults from the noncollinear structure of the surfactants on the surface of the Meanwhile, us decreases with the particles size (the value of asfor the Fe304nanoparticles fi-om reaction time of 3, 10 and 13h are 49.5, 55.8 and 62.lemdg, respectively). Hence, the decrease of G, here is most likely attributed to the existence of surfactants on the surface of the Fe304nanoparticles and their much smaller size. The value of a, for the bcc-Fe nanocrystallite is about 204.6emu/g, similar to that of bulk (as= 208emu/g )L231 at room temperature, because the surfactants have possibly volatilized through the reduction process at high temperature and the average size slightly increased caused by particle aggregation. In the mean time, it can also be concluded that compared with Fe304nanoparticles, the specific saturation magnetization of bcc-Fe particles is obviously enhanced.
4. Conclusions Fe-based nanoparticles were successfidly prepared by using facile hydrothermal method. The average diameter of the as-prepared Fe304nanoparticles is 1 1nm. The particle sizes are relatively homogeneous and become bigger with increasing of reaction time. The increase in particle sizes is dominated by diffhionlimited growth at this temperature and the diffusion coefficient (D)is about 1.4 x I04cm2/s. The products exhibit superparamagnetic behavior at room temperature. Specific saturation magnetizations (a,) of Fe304 and bcc-Fe nanoparticles are 55.8 and 204.6emu/g, respectively.
Acknowledgements-This work was supported by the National Naturul Science Foundation of China (No. 9020601 7), National High Technical Reasearch and Devdopment Programme of China (No 2003BA301 A21 ).
REFERENCES 1 A.P. Alivisatos, Science 271 (1996) 933.
2 R.C. Ashoori, Ntititre 379 (1996) 413. 3 C.Y. Hong, C.C. Wu, Y.C. Chiu, S.Y. Yang, H.E. Hong, and H.C. Yang, A p p l . Phys. I,ett. 88 (2006) 212512 4 R. Abu-Reziq, H. Alper, D. Wang, and M.L. Post, J. A m . Cliern. Sor. 128 (2006) 5279. 5 J. Gas, P. Poddar, J. Almand, S. Srinath, and H. Srikanth, A h . Funct. Mnter. 16 (2006) 71. 6 L. Fu, V.P. Dravid, and D.L. Johnson, Appl. SIL& Sci. 181 (2001) 173. 7 K. Woo and H. Lee, J. Mugn. Mrgn. Mater. 272 (2004) 1 155.
8 L. Shen, P.E. Laibinis, and T.A. Hatton, Lnrigmuir 447 (1 999) 7. 9 T. Fried, G. Shemer, and G. Markovich, Adv. Mater. 13 (2001) 1158. 10 J. Xu, H.B. Yang, and W.Y. Fu, J. Magn. Magn. Maker. m ( 2 0 0 6 ) 307. 11 R.V. Kumar, Y. Koltypin, Y. Cohen, D. Aurbach, 0. Palchik, I. Felner, and A. Gedanken, J. Mruer. Cliem. 10
(2000) 1125. D. Vollath and D.V. Szabo, J. Mater. Res. 12 (1997) 2175. N. Pinna, S. Grancharov, P. Beato, P. Bonville, and M. Antonietti-Niederberger, Chem. Mater. 17 (2005) 3044. Z. Li, H. Chen, H. Bao, and M. Gao, Chem. Mater. 16 (2004) 1391. S. Gangopadhyay, G.C. Hadjipanayis, C.M. Sorensen, K.J. Klabunde, V. Papaefthymiou, and A. Kostikas, Phys. Rev. B 4S (1992) 9778. 16 X.M. Ni, X.B. Su, and H.G. Zheng, J. Crys. Grouj/h275 (2005) 548. 17 B.D. Cullity, Elements of X - R r q Diffmction (Vol. 41, Addison-Wesley: Reading, 1978) p.102. 18 G. Oskam, A. Nellore, R.L. Penn, and P.C. Searson, J. Phys. Chem. B 107 (2003) 1734. 19 M. Yin and S. OBrien, J. Am. Chem. Soc. 125 (2003) 10180. 20 X. Peng, J. Wickham, and A.P. Alivisatos, J Am. Chem. Soc. 120 (1998) 5343. 21 D.W. Marr and A.P. Gast, Lungmuir 10 (1994) 1348. 22 R. Lovas, G. Macri, and S. Petrucci, J. A m. Chem. Soc. 92 (1 970) 6502. 23 H.G. Cha, Y.H. Kim, C.W. Kim, and Y.S. Kang, Sensors md Actuators B: Chemical 126(1) (2007) 221. 24 S.H. Sun, H. Zeng, D.B. Robinson, S. Raoux, P.M. Rice, S.X. Wang, and G.X. Li, J. Am. Chem. Soc. 126 (2004) 273. 12 I3 14 15
?ScienceDirect
ACTA METALLURGICA SINICA (ENGLISH LETTERS)
www.arns,org.cn
Acta Metall. Sin. (Engl. Lett.) Vol.20 No. 6 pp434-440 Dec. 2007
FACILE HYDROTHERMAL SYNTHESIS AND GROWTH KINETICS OF Fe-BASED MAGNETIC NANOPARTICLES C . X . You, J . C . Zhang*, Y. Shen, and Z.W. Song Shoo1 of Material Science and Engineering, Shanghai University, Shanghai 200072, China Manuscript received 17 May 2007
The facile hydrothermal method was used to synthesize F r o , nanoparticles with an average diameter of I Inm. The pure bodv-centered cubic (bcc)-Fe nanoparticles were prepared by reduction of Fe 0,nanoparticlespowder in H2 atmosphere. The structure, tnorphoiogy and mugnetic properties of the products were characterized by X-ray powder diffraction (XRD). trunsmission electron microscopy (TEM), thermrigravimetric analysis-diyerential scanning calorimetn, (TGA-DSC) and vibrating sample magnetometer (VSM). The results showed thar the as-prepared Fe,O, nanoparticles had a relatively homogeneous size. The particle diameters became bigger with the increuse of reaction time. The growth kinetics of the FePI nunoparticles was also brieflv discussed. The products exhibited superparamagnetic properties at room temperature and the spec@ saturation magnetization w m dependent on the particle sizes. KEY WORDS Fe-based nunoparticle; hydrothermal; specijc saturation tnagnetization; kinetics
1. Introduction Compared with bulk materials, magnetic nanoparticles exhibit unique physical, electrical, magnetic and chemical They have attracted great deal of attention because of their various technological applications, for example, magnetic recording media, catalysis, medicine transporter, bio-medi~ine[~-'] and so on. Magnetic nanoparticles have been prepared by a variety of methods, such as the coprecipitation of an aqueous solution of ferrous and ferric ions by a sol-gel[lol,sonochemistry["', microwave plasma synthesis[l21,nonaqueous route['"'41,gas-phase methods[l5]etc.. These methods could be used to synthesize magnetic nanoparticles with grain size larger or less than 20nm, but they require many expensive experiment instruments or poisonous raw materials, or rigorous experiment conditions. In this article, hydrothermal method was employed to synthesize Fe-based magnetic nanoparticles using sodium bis (2-ethylhexyl) sulfosuccinate (Aerosol OT) as dispersant and NzH4 HzO as a reagent for
-
'Corresponding author. Tel. : +86 21 56334743. E-mail address : [email protected] .cn ( J . C . Zhang)
. 435
*
adjusting the pH value of the solution['61,respectively. The experimental conditions were simple, no expensive instruments and poisonous raw materials were used. Fe304magnetic nanoparticles with an average grain size of 1 lnm have been synthesized by this approach, and the particle diameters were relatively homogeneous. The pure bcc-Fe nanoparticles were obtained by reduction of Fe30, nanoparticles powder in H2 atmosphere. The magnetic properties of the as-prepared Fe-based nanoparticles were investigated. The growth kinetics was also briefly discussed.
2. Experimental All the reagents were of analytical grade and purchased from Shanghai Chemical Reagents Company, China National Medicines (Group) and used without hrther purification. The process can be divided into the following two steps: preparation of Fe304nanoparticles, preparation of Fe203and bcc-Fe nanoparticles. (1) Process 1. Typically, 54mmol N2H4.H20was mixed with 20mLofO. 1 1 M sodium bis (2-ethylhexyl) sulfosuccinate (Aerosol OT) solution in a beaker. AFter magnetic stirring for some time, 2mmol FeC13* 6H20was added into the previous solution. Then, the stirring and supersonic dispersal was carried on for another 15min to make the reaction sufficient. The mixture was then transferred into the Teflon-lined stainless autoclave. The autoclave was put in an oven at 150°C for 1Oh and then slowly cooled to room temperature naturally. The black Fe304nanoparticles were precipitated by centrifugation, washed and dried in air. (2) Process 2. The red-brown Fe,O, nanoparticles were assembly obtained from O2oxidation of black Fe304particles at 420°C for 6h. After being annealed under H2at 450"C, the Fe304nanoparticles were assembly reduced to bcc-Fe nanoparticles. (3) Characterization. Differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA) were carried out with a NETZSCH STA 409 PC simultaneous TG-DSC apparatus with a heating rate of 10'C/min in flowing air. X-ray diffraction (XRD) data were collected at room temperature using a Rigaku D/max 2200V X-ray difiactometer with high-intensity Cu K , radiation (A = 0.154056nm). For measuring the diameters and morphology of magnetite nanoparticles, the transmission electron microscopy (TEM) images were taken with FEI Tecnai 20 transmission electron microscopes. The magnetic properties of the nanoparticles were measured by an EG&G Princeton applied research vibrating sample magnetometer (VSM) at room temperature.
3. ResultsandDiscussion 3.1 TEM micrograph Fig.1 shows the average particle size of the Fe304nanoparticles to be about 1 lnm and the distribution of the particle diameters is relatively homogenous. 3.2 XRD patterns and the growth kinetics analysis Fig.2 shows the XRD spectrum of the obtained magnetic Fe304nanoparticles from the various hydrothermal reaction time. The curves
Fig. 1 TEM image of Fe304nanoparticles from hydrothermal reaction time of 1Oh at 150'C.
10
20
30
40
50
60
70
80
90
2 a, deg.
Fig.2 XRD patterns of the obtained magnetic Fe,O, nanoparticles from different hydrothermal reaction time (curve a. 3h, curve b. 5h, curve c. 7h, curved. 10h, and curve e. 13h). Table 1 The estimated average particle diameter according to the Scherrer‘sformula Sample
Reaction time, h
2 8 , deg.
FWHM, deg.
Average diameter, nm
a
3
35.4
0.74
11.1
b
5
35.4
0.72
11.4
C
7
35.4
0.70
11.7
d
10
35.4
0.68
12.1
e
13
35.4
0.66
12.5
Notes: FWHM mcans full width half maximum.
a-e in Fig.2 represent the products of 3, 5 , 7 , 10 and 13h, respectively. The XRD patterns exhibit a group of spinel-type diffi-actionpeaks, similar to the reported data (JCPDS #72-2303), which can be indexed as face-centered cubic Fe1O4structure with a lattice parameter a = 0.840nm. On the basis of Scherrer’s formula[”],the average crystallite sizes of the powder are calculated using the strongest phases of the diffraction plane 3 1 1 in Fig.2. Listed in Table 1, the estimated results are consistent with that determined by the statistical analysis of the TEM images. According to Fig.2 and Table 1, it can be concluded that the particle size is dependent on time (Fig.3), but the influence is not very distinctive. Assuming that the particles are spherical and the growth rate is determined by a difhsion process of the solute, we employ the simplified growth rate law derived fiom the Lifshitz-Slyozov-Wagner (LSW) m ~ d e l [ ’ ~as. ’in ~ ]Eq.( 1). This equation can be used to hrther develop a quantitative understanding of the growth kinetics of the FelO, nanoparticles
where r is the average particle radius at time t, ro is the initial particle radius, c,=, is the equilibrium concentration at a flat surface, k.,the bulk solubility (because the concentration difference is relatively small
(2y Vm/(R T r ) << l), the approximation was made that c , =c,according to Gibbs-Thomson A is aconstant, y is the interfacial energy, V , is the molar volume of the solid phase, R is the gas constant, T is the temperature and D is the diffision coefficient. Fig.3 shows the cube of average particle radius plotted time according to Table 1. The curve is roughly linear. The slope of the line (the value ofA in Eq.( 1)) is about 57.5nm3/h. Therefore, it can be inferred that the increase of the particle size is dominated by diffusion-limited growth at this temperature. The intercept at t=O (ro) is about 10.8nm, so the average particle radius can be extrapolated to longer reaction time according to Eq.( 1). The difision coefficient (D)can be further estimated by consideration of the constant A according to Eq.(l). Values for the interfacial energy (y)‘”] and the equilibrium concentration at a flat surface (cr=-) are assumed to be 0.035mJ/m2(here the bulk solubility was used as y ) and 1.5 x lO-’mol/L, respectively. Taking V, = 29.7mL/mol, T = 423K and A = 57.5nm3/h, the approximation of D is obtained, that is about 1.4 x 1O“cm’/s, consistent with the typical value for ions in solution. The difision coefficient for Fe” obtained from the LSW model can be compared with the StokesEinstein model for ionic diffusion as V.P.
where q is the viscosity of the solvent, a is the hydrodynamic radius of the solute, and kT has the usual meaning. The viscosity of water is 0.284 x 1O”kg /(m * s) and the hydrodynamic ra~ ] . the difision dius for Fe” is 0.5 1n ~ n ‘ ~Hence, coefficient (D)obtained from Eq.(2) is on the same order of 104cm2/s. The result shows good agreement with that from LSW model and further provides confirmation of the LSW model for the growth kinetics of the particles.
3.3 DSC and TGA curves The thermal behavior of the Fe30, nanoparticles is investigated with TG and DSC analysis in air (Fig.4). The TGA curve shows that the sample starts to volatilize at about 50°C. Weight loss before 180°C (about 6.7%) is possibly attributed to the volatilization of water and surfactants in the sample. The major weight loss (about 2 1.2%) from 180 to 450’C is caused by the transformation of Fe,04 into Fe203.The DSC curve shows two primary endothermal peaks located at 160.9 and 413°C. The positions of the peaks fit well with that of the weight loss in the TGA curve. On the basis
1800
-
-L
I 0
I 2
4
6 8 Time, h
1
0
1
2
1
4
Fig.3 Plotted as the particle average radius cubed I J S . the growth time at 150”C, in accordance with the LSW model as described in Eq.( I )
-- I
I
160 9 C
75 0
200
400
600
800
Temperature, C
Fig.4 TG-DSC curves of Fe304 nanoparticles from hydrothermal reaction time of 10h at 150°C.
of the TG and DSC analysis, 450°C was chosen as heat-treatment temperature to ensure the complete reduction of the Fe,O, into bcc-Fe nanoparticles in H2.All the results can be demonstrated by the XRD characterization (Fig.5). The curve a represents the phase structure of the Fe304nanoparticles before the thermal treatment. The curve b represents the phase structure of the FQO, nanoparticles obtained fiom the oxidation of Fe,O, in O2at 420°C for 6h. Compared with curve a, the large-angle peaks in curve b shift slightly to higher angles, and additional small peaks appear at lower angles. Curve b matches well with the index of the cubic Fe203phase (JCPDS #39-1346). Curve cc represents represents the the phase phase structure structure of of Fe Fe nanoparticles nanoparticles 0 obtained obtained fiom fiom Fe304 Fe304annealed annealed under under Hz H2at at 450°C. 450°C. ItIt can #87can be be indexed indexed to to the the bcc-Fe bcc-Fe phase phase (JCPDS (JCPDS #87-
-
3
072 0721). 1).
-
m
2
3.4 3.4 Magnetic Magnetic properties properties
c
C
E
Figs. Figs.6a-c 6a-c shows shows the the hysteresis hysteresis loops loops of of the the asassynthesized synthesized Fe304 Fe304nanoparticles nanoparticles fiom fiom different different hyhydrothermal reaction time and of the as-reduced bcc-Fe nanoparticles (Fig.6d) measured at room temperature. The results indicate that all the particles are possibly superparamagnetic at room temperature,
N
0 0 N
.-
7
1
m
_ c
Fig.5 XRD patterns of nanoparticles (curve a. Fe304; curve b. Fe103;curve c. bcc-Fe).
20
-20
c
-601
I
'
-10000
-5000
I
0
5000
10000
I
I -10000
-5000
H. Oe
__
0
5000
10000
5000
10000
H, Oe
I
I -10000
-5000
0 H, Oe
5000
10000
-10000
-5000
0 H, Oe
Fig.6 Hysteresis loops of the magnetic nanoparticles assembly measured at room temperature:Fe,O, nanoparticles at 150°C from hydrothermal reaction time of 3h (a), 10h (b), and 13h (c); (d) bcc-Fe nanoparticles.
because the remanence of the particles is zero in the absence of an external magnetic field and the coercivity is almost negligible. The reason for the nanoparticles having superparamagnetic property is explained as follows: the characteristic physical lengths related to magnetism (such as the size of the single magnetic domain and the critical size of superparamagnetism e t c . ) are approximately on the order of 1 I OOnm. When the particle size is corresponding to or smaller than the characteristic physical length, the particles will show abnormal magnetic property. As the critical size of superparamagnetism for Fe304at room temperature is 16nm and the average diameter of the as-prepared sample is 1 lnm, the Fe304particles exhibit superparamagnetism. For Fe304 nanoparticles from a hydrothermal reaction time of 1Oh at 150"C, the value of specific saturation magnetization (as)is 55.8emu/g, smaller than that of the Fe304 bulk materials (us= 88emdg )[231. According to the report in the literature, the decrease of usresults from the noncollinear structure of the surfactants on the surface of the Meanwhile, us decreases with the particles size (the value of asfor the Fe304nanoparticles fi-om reaction time of 3, 10 and 13h are 49.5, 55.8 and 62.lemdg, respectively). Hence, the decrease of G, here is most likely attributed to the existence of surfactants on the surface of the Fe304nanoparticles and their much smaller size. The value of a, for the bcc-Fe nanocrystallite is about 204.6emu/g, similar to that of bulk (as= 208emu/g )L231 at room temperature, because the surfactants have possibly volatilized through the reduction process at high temperature and the average size slightly increased caused by particle aggregation. In the mean time, it can also be concluded that compared with Fe304nanoparticles, the specific saturation magnetization of bcc-Fe particles is obviously enhanced.
4. Conclusions Fe-based nanoparticles were successfidly prepared by using facile hydrothermal method. The average diameter of the as-prepared Fe304nanoparticles is 1 1nm. The particle sizes are relatively homogeneous and become bigger with increasing of reaction time. The increase in particle sizes is dominated by diffhionlimited growth at this temperature and the diffusion coefficient (D)is about 1.4 x I04cm2/s. The products exhibit superparamagnetic behavior at room temperature. Specific saturation magnetizations (a,) of Fe304 and bcc-Fe nanoparticles are 55.8 and 204.6emu/g, respectively.
Acknowledgements-This work was supported by the National Naturul Science Foundation of China (No. 9020601 7), National High Technical Reasearch and Devdopment Programme of China (No 2003BA301 A21 ).
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