Salt effects on the physical properties of magnetite nanoparticles synthesized at different NaCl concentrations

Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 41–46

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Salt effects on the physical properties of magnetite nanoparticles synthesized at different NaCl concentrations Ja Young Park a , Daksha Patel a , Eun Sook Choi a , Myung Ju Baek b , Yongmin Chang b,c , Tae Jeong Kim b,d , Gang Ho Lee a,b,∗ a

Department of Chemistry, College of Natural Sciences, Kyungpook National University, Taegu 702-701, South Korea Department of Nanoscience and Nanotechnology, Kyungpook National University, Taegu 702-701, South Korea Department of Diagnostic Radiology, School of Medicine, Kyungpook National University and Hospital, Taegu 702-701, South Korea d Department of Applied Chemistry, College of Engineering, Kyungpook National University, Taegu 702-701, South Korea b c

a r t i c l e

i n f o

Article history: Received 29 April 2010 Received in revised form 12 June 2010 Accepted 14 June 2010 Available online 22 June 2010 Keywords: Fe3 O4 Nanoparticle Physical properties Salt effect Homogeneous nucleation model

a b s t r a c t Magnetite nanoparticles were synthesized at a wide range of NaCl solutions (c = 0.01–3.0 M). The particle diameter (d) was reduced from 7.7 to 6.8 nm and the colloidal stability dropped with increasing c. The salt effect on the d was successfully elucidated with an aid of a homogeneous nucleation model. The saturation magnetization (Ms ) reduction and the lattice contraction (a) from −0.015 to −0.038 Å with increasing c were observed due to the decrease of the d. From the a, the surface tension () enhancement from 0.192 to 0.614 N/m with increasing c was estimated. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Although it is well-known that the colloidal stability decreases with increasing salt ion concentration [1], the salt effect on the formation and the growth of magnetite (Fe3 O4 ) nanoparticles in aqueous solution and the resulting changes in physical properties are very poorly known. A few studies showed that the particle diameter and the chemical composition of iron oxide nanoparticles could be altered by varying the solution pH and the temperature [2]. The particle diameter could be altered by changing the alkalinebase addition rate [3]. The different morphologies could be obtained by using different types of salt solutions [4]. These previous studies certainly show the salt effect on the physical properties of Fe3 O4 nanoparticles. In this work, we synthesized Fe3 O4 nanoparticles at a wide range of NaCl concentrations (c = 0.01–3.0 M) and investigated the salt effect on the physical properties of Fe3 O4 nanoparticles. The physical properties studied include the particle diameter (d), the colloidal stability, the saturation magnetization (Ms ), the lattice contraction

∗ Corresponding author at: Department of Chemistry, College of Natural Sciences, Kyungpook National University, 1370 Sankyuk-dong, Taegu 702-701, South Korea. Tel.: +82 53 950 5340; fax: +82 53 950 6330. E-mail address: [email protected] (G.H. Lee). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.06.014

(a), and the surface tension (). A simple formula which satisfactorily explains the observed salt effect on the d, is proposed from a homogeneous nucleation model [5–7]. From this, the changes in the other physical properties, i.e. the Ms drop, the a, and the  enhancement with increasing c, could be explained. The origin for the salt effect on the d is ascribed to the adsorption of salt ions on Fe3 O4 nanoparticles, thus hindering both the formation and the growth of Fe3 O4 nanoparticles in NaCl solution. 2. Experimental 2.1. Synthesis Fe3 O4 nanoparticles were synthesized at five different NaCl concentrations (c = 0.01, 0.15, 1.0, 2.0, 3.0 M). The chemicals used include FeCl3 ·6H2 O (>99%), FeCl2 ·4H2 O (>99%), NaCl (>99%), and NH4 OH solution (∼30.0%) and were all purchased from Aldrich and used without further purification. Ar gas (99.999%) was used as a flowing gas. Both 0.1 M ferric and 0.2 M ferrous chloride stock solutions were prepared in a 2.0 M aqueous HCl solution. Five NaCl concentrations were prepared. In a typical experiment, 25 ml of 0.1 M ferric solution and 6.25 ml of 0.2 M ferrous solution were added into 100 ml of NaCl solution in a three neck flask. The reaction solution was mechanically stirred while Ar gas flowed during the reaction. The solution temperature was raised to ∼80 ◦ C, and the

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Fig. 1. TEM micrographs of Fe3 O4 nanoparticles synthesized in c= (a) 0.01 M, (b) 0.15 M, (c) 1.0 M, (d) 2.0 M, and (e) 3.0 M NaCl solutions, and (f) the plot of the davg as a function of c.

NH4 OH solution was then slowly added to the reaction solution by using a syringe (addition rate: 0.1–0.5 ml/s) until the solution pH of 6–7 was reached [8]. A vigorous stirring continued for additional 2 h. After the reaction was completed, nanoparticles were isolated by applying a permanent magnet. The top solution was decanted. Nanoparticles were washed three times with the same NaCl solutions used for the synthesis. For each sample, three kinds of samples were prepared: a solution sample in the corresponding NaCl solution, a powder sample dried in air, and a salt free powder sample dried in air after the solution sample was washed with distilled water three times.

infrared (FTIR) absorption spectrometer (Mattson Instruments, Inc., Galaxy 7020A) was used to record the FTIR absorption spectra between 400 and 4000 cm−1 . For the measurement, pellets were made by pressing a mixture of a powder sample and KBr. A superconducting quantum interference device (SQUID) (Quantum Design, MPMS-7) was used to measure the magnetic properties. The M–H curves (−5 ≤ H ≤ 5 T) at T = 300 K were recorded. For the measurement, a known mass of each salt free powder sample was loaded into a nonmagnetic capsule.

2.2. Characterization

3.1. TEM micrograph

A transmission electron microscope (TEM) (Philips, CM 200) operated at 200 kV was used to measure the d. For the measurement, the salt free Fe3 O4 nanoparticles were added into methanol and then, sonicated for 10 min for dispersion. The dispersed Fe3 O4 nanoparticles were then deposited on a copper grid covered with an amorphous carbon membrane. An inductively coupled plasma atomic emission spectrometer (ICPAES) (Thermo Jarrell Ash Co., IRIS/AP) was used to check the salt free Fe3 O4 nanoparticles. An X-ray diffraction (XRD) spectrometer (Philips, X-PERT) was used to measure the crystal structure. The XRD patterns were recorded by using a CuK␣ ( = 1.54184 Å) radiation between 2 = 20–90◦ . A zeta potential measurement device (Malvern Instrument, Zetasizer nano ZS) was used to measure the zeta potential of colloidal Fe3 O4 nanoparticles in 0.01–3.0 M NaCl solutions. A Fourier transform

TEM micrographs of Fe3 O4 nanoparticles synthesized in 0.01–3.0 M NaCl solutions are shown in Fig. 1(a)–(e). The salt free Fe3 O4 nanoparticles were used for the measurement. The measured average particle diameters (davg ) are provided in Table 1 and also plotted as a function of c in Fig. 1(f), showing that the davg slightly and gradually decreased with increasing c. As discussed below, the XRD measurement confirmed this observation.

3. Results and discussion

3.2. XRD pattern XRD patterns are shown in Fig. 2. All samples showed a good crystallinity of a face centered-cubic (fcc) structure like the bulk material. This indicates no salt effect on the crystallinity. The additional peaks assigned as “*” arise from the NaCl. The lattice constant

Table 1 The average particle diameter (davg ) estimated from TEM and XRD, the lattice constant (a), the lattice contraction (a), the saturation magnetization (Ms ), the zeta potential (), the surface tension () of Fe3 O4 nanoparticles, and the free energy barrier (G(r*)) for the Fe3 O4 nanoparticle formation at different NaCl concentrations (c). c (M)

0.01 0.15 1.0 2.0 3.0

davg (nm) TEM

XRD

7.7 7.5 7.3 7.0 6.8

9.62 9.57 9.43 9.38 9.23

a (Å)

a (Å)

Ms (emu/g)

 (mV)

 (N/m)

G(r*) × 1021 (J/particle)

8.381 8.374 8.372 8.358 8.342

−0.015 −0.022 −0.024 −0.038 −0.054

83.1 82.8 82.6 82.5 80.2

−34.1 −16.8 −5.2 1.7 1.9

0.192 0.275 0.292 0.444 0.614

8.87 26.26 31.31 110.73 292.09

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lattice contraction (a). The a had been also observed in various nanoparticles such as Al [10], Pd [11], Pt [12], Fe3 O4 [3], NiFe2 O4 [13], ZnFe2 O4 nanoparticles [14], and also theoretically discussed [15]. According to these previous studies, the a arises from the particle size effect, i.e. it increases with decreasing d. The a of a nanoparticle is defined as a = a(nanoparticle) − a(bulk material (= 8.396 Å )[16]) =−

Fig. 2. XRD patterns of Fe3 O4 nanoparticles synthesized at different NaCl concentrations. The assignments are the Miller indices (h k l). The additional sharp peaks (*) in the 1.0 M XRD pattern arise from the NaCl and are applied to other XRD patterns.

(a) was estimated from the most intense (3 1 1) peak in each XRD pattern and provided in Table 1. The davg of each sample was estimated by using the full width at half maximum (FWHM) of the (3 1 1) peak and the Scherrer’s formula [9] and is provided in Table 1. The decrease of the davg with increasing c is consistent with the TEM result as mentioned before. Note that XRD peak positions are gradually shifted toward higher 2 values with increasing c. As a representative, the XRD patterns of the (3 1 1) peaks are plotted in an expanded 2 scale in Fig. 3(a). The (3 1 1) peak centers are also plotted as a function of c in Fig. 3(b) to clearly show the peak shift. This shift indicates the

4a 3

d

ˇ

(1)

in which ˇ and  are the compressibility of the bulk material (=5.38 × 10−12 (N/m2 )−1 ) [17] and the surface tension (N/m) of a Fe3 O4 nanoparticle, respectively [15]. Both a and  are estimated from Eq. (1) and provided in Table 1. They are also plotted as a function of c in Fig. 4(a) and (b), respectively, clearly showing the gradual changes with increasing c. These arise from particle size effect as mentioned above. 3.3. Particle formation and growth: a homogeneous nucleation model In order to explain the salt effect on the d, a homogeneous nucleation model [5–7] is employed. In this model, the free energy (G(r)) of a nanoparticle with a particle radius (r) is given by G(r) = −

4 3

r 3

1 V

kB T ln(S) + 4r 2

(2)

in which V, kB , T, and S are the molecular volume (m3 ) of Fe3 O4 , the Boltzman constant (J/K), the absolute temperature (K), and the supersaturation value (defined below), respectively [5]. Lets assume that Fe3 O4 nanoparticles are formed through the following

Fig. 3. Plots of (a) the (3 1 1) peaks in an expanded 2 scale and (b) the 2 values of the (3 1 1) peak centers as a function of c.

Fig. 4. Plots of (a) the a and (b) the  as a function of c.

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reaction Fe2+ + 2Fe3+ + 8OH−  Fe(OH)2 · 2(Fe(OH)3 ) → (FeO)Fe2 O3 (i.e. Fe3 O4 ) + 4H2 O

(3)

If the S is approximately defined as S = [Fe2+ ][Fe3+ ]2 [OH− ]8 /[Fe2+ ]eq [Fe3+ ]eq 2 [OH− ]eq 8 = [Fe2+ ][Fe3+ ]2 [OH− ]8 /K SP (Fe(OH)2 )K SP 2 (Fe(OH)3 )

(4)

in which KSP is the solubility constant, the S is then estimated to be 1.171 × 1024 by using the V of 7.41 × 10−29 m3 (=molecular weight of Fe3 O4 /NA  in which NA is the Avogadro number and  the density (=5.19 g/cm3 ) of the bulk material [18]), the initial concentrations of [Fe2+ ] = 0.01 M and [Fe3+ ] = 0.02 M, and [OH− ] = ∼10−8 M from the Fig. 6. Plot of J/Jmax as a function of c.

experimental condition, and KSP = 4.79 × 10−17 and 2.67 × 10−39 for Fe(OH)2 and Fe(OH)3 [19], respectively. By using the S value estimated above, the T of ∼353 K from the experimental condition, and the  estimated from Eq. (1), the G(r) of all samples are estimated and then, plotted in Fig. 5(a)–(c) in three different r scales. The G(r) initially increases, reaches the free energy barrier (G(r*)), and then, falls down as the r keeps increasing. The G(r*) is obtained to be (4/3)(r*)2 by putting r = r* into G(r) in which the critical radius r* was obtained to be 2V/kB T ln(S) by setting d[G(r)]/dr = 0. The values of G(r*) are given in Table 1. Note that the G(r*) values increase with increasing c, indicating that both the formation and the growth of Fe3 O4 nanoparticles become harder with increasing c. Then, the formation rate J of a nanoparticle is approximately written in terms of G(r*) such as J=

dN = dt

 2D  h5

exp

 −G(r∗)  kB T

= Jmax exp

 −G(r∗)  kB T

(5)

in which N, D, and h are the number density of Fe3 O4 nanoparticles with the r*, the diffusion coefficient of a Fe3 O4 nanoparticle, and the molecular diameter of Fe3 O4 , respectively [5]. The Jmax (=2D/h5 ) corresponds to the maximum formation rate at which the G(r*) is zero. The J/Jmax is estimated and then, plotted as a function of c at T = 353 K in Fig. 6, showing that the J/Jmax decreases with increasing c. We finally write that d ≈ Jq

(6)

in which q is the reaction time that is expected to be nearly the same for all c because Fe3 O4 nanoparticles were well dispersed by the mechanical stirring during the reaction. Therefore, this simple formula approximately predicts that the d should decrease with increasing c because J decreases with increasing c, which satisfactorily explains the observed salt effect on the d. 3.4. FTIR absorption spectra

Fig. 5. Plots of the G(r) for the formation of a Fe3 O4 nanoparticle synthesized at different NaCl concentrations at three different r scales.

Fe3 O4 nanoparticles can be confirmed by two fundamental Fe–O lattice vibrations, which include the strong 1 and weak 2 vibrations, occurring at 570 and 375 cm−1 , respectively, for the bulk material [20–27]. They belong to the active F2 normal mode vibrations which had been in detail pictured by Waldron [20]. The 1 vibration is blue-shifted from that of the bulk material, as shown in Fig. 7(a) and (b), indicating that the Fe–O bond strength of Fe3 O4 nanoparticles increased with respect to the bulk material due to the particle size effect. Note that the intensity of the 1 vibration gradually decreased with increasing c as shown in Fig. 7(a). This supports the fact that Cl− ions specifically adsorb on Fe3 O4 nanoparticles through the replacement of the surface Fe–OH with

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Fig. 7. FTIR absorption spectra of Fe3 O4 nanoparticles synthesized at different NaCl concentrations (a) before and (b) after washing of Fe3 O4 nanoparticles with distilled water. The peaks labelled together with a vertical bar indicate the Fe–O 1 vibration.

the Fe–Cl [28–31]. However, the 1 vibrational intensity was nearly constant for all samples (see Fig. 7(b)) after being washed with distilled water, implying the recovery of the surface Fe–OH. 3.5. Colloidal stability As expected, a large salt effect on the colloidal stability was observed from the measurement of the zeta potential (). Note that the  directly reflects the surface charge of Fe3 O4 nanopar-

Fig. 8. (a) Zeta potential curves of Fe3 O4 nanoparticles synthesized at different NaCl concentrations and (b) a series of photographs showing the sedimentation of Fe3 O4 nanoparticles (i) 0 min, (ii) 15 min, (iii) 40 min, and (iv) 2.5 days after dispersion.

ticles [28]. It was observed that the  values of Fe3 O4 nanoparticles decreased with increasing c (Fig. 8(a) and Table 1), clearly indicating that the colloidal stability decreased with increasing c. Here, the broadening of the zeta potential curves with increasing c is due to the decrease in the diffuse double layer thickness with increasing c. This gradual colloidal stability decrease with increasing c is supported by the sedimentation period of Fe3 O4 nanoparticles in solution (see Fig. 8(b)). That is, the sedimentation period got decreased with increasing c. Based on the previous other researchers’ reports [28–32], Cl− ions specifically adsorb on Fe3 O4 nanoparticles through the replacement of the surface Fe–OH by the Fe–Cl [28–31], which was confirmed in our FTIR spectra as mentioned before but Na+ ions, however, nonspecifically adsorb on Fe3 O4 nanoparticles through the ion pair formations such as Fe–OH− Na+ and Fe–Cl− Na+ [28,32]. These specific and nonspecific adsorptions certainly reduced the surface charge and thus, the colloidal stability of Fe3 O4 nanoparticles. Similar decreases in colloidal stability were also observed in several nano-colloidal systems as the ionic strength increased [33–35]. In these systems, zeta potentials decreased as the ionic strength increased and were used in estimating the repulsion energy between nano-colloids by using the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory [36,37]. According to the DLVO theory, the thickness of the diffuse double layer and thus, the repulsion energy between nanocolloids decrease as the ionic strength increases, resulting in the decrease of colloidal stability (i.e. particle aggregation).

Fig. 9. M–H curves at T = 300 K of Fe3 O4 nanoparticles synthesized at different NaCl concentrations. Inserted are those in an expanded H scale.

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3.6. Magnetic properties Hysteresis loops (M–H curves) at T = 300 K were recorded by using the salt free Fe3 O4 nanoparticles (Fig. 9). All of the M–H curves show the zero coercivity and the zero remanence at T = 300 K within an experimental error limit, showing that Fe3 O4 nanoparticles are superparamagnetic at room temperature. The Ms estimated from the M–H curves decreased from 80 to 83 emu/g with increasing c (Table 1). This is due to the particle size effect [3]. 4. Conclusion Fe3 O4 nanoparticles were synthesized at a wide range of NaCl concentrations (c = 0.01–3.0 M) and their physical properties were investigated. The gradual changes in them were observed. The salt effect on the d was successfully elucidated with an aid of a homogeneous nucleation model. From this, the changes in the other physical properties such as the a, the , and the Ms with increasing c could be explained. (1) The davg of Fe3 O4 nanoparticles measured from both TEM and XRD slightly and gradually decreased with increasing c. By using a homogeneous nucleation model, a simple formula satisfactory for the observed salt effect on the d was suggested such that d ≈ Jq in which the J decreased with increasing c (q is nearly constant for all c). (2) Both the  and the a increased with increasing c because the d decreased with increasing c. (3) The Ms decreased with increasing c because the d decreased with increasing c. (4) No salt effect on the crystal structure was observed. That is, a fcc structure of the bulk material was observed for all samples. (5) The colloidal stability of Fe3 O4 nanoparticles rapidly decreased with increasing c. This was confirmed from both the  and sedimentation period. The origin for the salt effect on the d is ascribed to the adsorption of Na+ and Cl− salt ions on Fe3 O4 nanoparticles, thus hindering both the formation and the growth of Fe3 O4 nanoparticles in NaCl solution. Acknowledgments This work was supported by the Regional Technology Innovation Program of the Ministry of Commerce, Industry, and Energy (No. RTI04-01-01) and by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0002436). We thank the Korea Basic Science Institute for allowing us to use their XRD. References [1] R.M. Cornell, U. Schwertmann, The Iron Oxides, VCH, Weinheim, 1996, p. 207. [2] G. Gnanaprakash, S. Mahadevan, T. Jayakumar, P. Kalyanasundaram, J. Philip, B. Raj, Effect of initial pH and temperature of iron salt solutions on formation of magnetite nanoparticles, Mater. Chem. Phys. 103 (2007) 168–175. [3] G. Gnanaprakash, J. Philip, T. Jayakumar, B. Raj, Effect of digestion time and alkali addition rate on physical properties of magnetite nanoparticles, J. Phys. Chem. B 111 (2007) 7978–7986. [4] C.Y. Wang, G.M. Zhu, Z.Y. Chen, Z.G. Lin, The preparation of magnetite Fe3 O4 and its morphology control by a novel arc-electrodeposition method, Mater. Res. Bull. 37 (2002) 2525–2529. [5] J.A. Dirksen, T.A. Ring, Fundamentals of crystallization: kinetic effects on particle size distributions and morphology, Chem. Eng. Sci. 46 (1991) 2389–2427.

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