Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 47 (2014) 055001 (13pp)
doi:10.1088/0022-3727/47/5/055001
Structural and magnetic properties of sonoelectrocrystallized magnetite nanoparticles S Mosivand1,2 , L M A Monzon1 , K Ackland1 , I Kazeminezhad2 and J M D Coey1 1 2
Physics Department, SNIAMS Building, Trinity College Dublin, Dublin 2, Ireland Physics Department, Faculty of Science, Shahid Chamran University, Ahvaz 61357-43337, Iran
E-mail:
[email protected] Received 3 July 2013, revised 11 November 2013 Accepted for publication 3 December 2013 Published 31 December 2013 Abstract
The effect of ultrasound power on the morphology, structure and magnetic properties of magnetite nanoparticles synthesized from iron electrodes by the electro-oxidation method was investigated. Samples made in aqueous solution in the absence or presence of an organic stabilizer (thiourea, tetramethylammonium chloride, sodium butanoate or β-cyclodextrine) were characterized by x-ray diffraction, transmission and scanning electron microscopy, magnetometry and M¨ossbauer spectrometry. The iron is almost all in the form of 20–85 nm particles of slightly nonstoichiometric Fe3−δ O4 , with δ ≈ 0.10. Formation of a paramagnetic secondary phase in the presence of sodium butanoate or β-cyclodextrine is supressed by ultrasound. Specific magnetization of the magnetite nanoparticles ranges from 19 to 90 A m2 kg−1 at room temperature, and it increases with particle size in each series. The particles show no sign of superparamagnetism, and the anhysteretic and practically temperature-independent magnetization curves are associated with a stable magnetic vortex state throughout the size range. The spin structure of the particles and the use of magnetization measurements to detect magnetite in unknown mixtures are discussed. Keywords: sonoelectrooxidation, magnetite, magnetic nanoparticles, ultrasound power, magnetization curves, magnetic vortex state (Some figures may appear in colour only in the online journal)
that can be conveniently collected by high-gradient magnetic separation. They also serve as contrast agents in magnetic resonance imaging [9, 10], and they are useful for hyperthermia [11, 12] and other, drug-based cancer treatments [13, 14]. Further areas of application include water treatment [15]. Magnetite nanoparticles are often present in significant quantities in the atmosphere, and they can be synthesized by various physical and chemical methods [16–22]. One of the more recent is electro-oxidation, where fine magnetite powder is produced electrolytically from iron electrodes [23]. We recently characterized the nanoparticles produced by this method in the presence of a wide range of organic additives [24, 25], and process conditions [26]. Here we investigate the effect of ultrasonic power in the electrochemical bath.
1. Introduction
Magnetic oxide nanoparticles are useful for investigating physical phenomena such as superparamagnetism [1], supermagnetism [2], spin structures [3] and quantum size effects in reduced dimensions. They carry the thermoremanent magnetism in rocks [4], which was critical for establishing the theory of global plate tectonics. They are also the key constituent of ferrofluids, which have spawned the subfield of ferrohydrodynamics [5]. Acicular magnetic oxide particles enjoyed an 80-year innings as magnetic recording media [6]. More recent applications have been in the biomedical area [7, 8] where superparamagnetic iron oxide nanoparticles (SPIONS) can be functionalized and used as magnetic labels 0022-3727/14/055001+13$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
S Mosivand et al
Acoustic power (W)
In comparison with other energy sources, ultrasound (US) creates rather extraordinary reaction conditions in solution, with high pressures and possible acoustic cavitation [27–29]. US influences nucleation, and it decreases the diffusion layer thickness so that overall mass transport is enhanced. Particle dispersion can be improved by the use of organic surfactants. Cabrera et al [30] have described the sonoelectrochemical synthesis of magnetite nanoparticles with tetramethylammonium chloride. They found that Fe3 O4 generated with the assistance of US had a higher magnetization than material prepared in silent conditions. In this work, we analyse the impact of the ultrasonic power on the structural and magnetic properties of magnetite nanoparticles produced by electrooxidation, and the influence of four organic additives. The method allows us to obtain magnetite with high specific magnetization values without the need to perform the reaction in an inert atmosphere [25, 30]. We focus on the magnetic structure of the nanoparticles, which are in the 20–85 nm size range. We are especially interested in evaluating the use of magnetic methods to detect the presence of small quantities of these particles in a nonmagnetic matrix. Our results cast doubt on the conventional view that magnetite nanoparticles larger than the blocking diameter are stable single-domain, colinearferrimagnetic particles.
2.4
0.012
2.0
0.010
1.6
0.008
1.2
0.006
0.8
0.004
0.4
0.002
0.0
0
20
40
60
80
100
Acoustic power density (W cm-3)
J. Phys. D: Appl. Phys. 47 (2014) 055001
0.000
US Amplitude (%) Figure 1. A plot of the absorbed acoustic power and acoustic power density values as a function of applied US amplitude.
an ultrasonic bath, this system allows the US to be directed to the electrode surface and provides efficient power and pulse control [27]. The sonoelectrosynthesis was carried out for 30 min at 60 ◦ C. A consequence of the US was a temperature increase of up to ∼10 ◦ C during the reaction. The black precipitates were separated from the electrolyte solution using a Nd–Fe–B permanent magnet, washed several times with a copious amount of deionized water and left to dry. A control group with no organic additive was prepared with the same range of 0–100% of maximum ultrasonic amplitude. For some samples, the magnetite was diluted with up to 100 times its mass of ∼15 nm γ -Al2 O3 nanoparticles, and the mixture was thoroughly ground with an agate mortar and pestle. The acoustic power absorbed by the aqueous solution for each amplitude was calibrated by the thermal probe method [31, 32], which involves measuring the rate of change of temperature dT /dt of a mass of solvent used to absorb the acoustic power Pa , given by the equation:
2. Materials and methods 2.1. Materials
Iron sheet (purity 99.5%) was supplied by Advent Research Materials. Thiourea (Tu), sodium butanoate (Bu), βcyclodextrine (β-CD), tetramethylammonium chloride (TMA) and sodium sulfate anhydrous were purchased from Sigma-Aldrich Chemical Co. Sodium sulfate anhydrous and nonmagnetic γ -Al2 O3 nanoparticles with a particle size of ∼15 nm were purchased from BDH. 2.2. Methods
Pa = mC dT /dt
The electrochemical cell contains 200 ml of an aqueous solution of 0.25M Na2 SO4 salt with 0.04M of one of the four organic stabilizers and the pH is almost neutral. A 1 × 1 cm2 and a 1 × 4 cm2 iron plate were used as sacrificial anode and cathode respectively. The electrodes, which were separated by 1 cm, were polished with fine-grain emery paper and cleaned with ethanol in an ultrasonic bath. Then the reaction was conducted potentiostatically with an applied potential of 5 V [23, 24]. The initial current density was about 0.8 A cm−2 , and the iron anode is oxidized to Fe2+ and Fe3+ as the reaction proceeds. Water is reduced to hydrogen and hydroxyl anions at the cathode, resulting in an increase in pH. The Fe2+ , Fe3+ and OH− ions meet in solution to produce an orange–brown iron hydroxide which then dehydrates to form a black magnetite (Fe3 O4 ) precipitate. The electrode separation controls the pH profile in the cell and it is critical for the formation of magnetite. A 100 W Hielscher UP100H ultrasonic processor with a 0.5 mm MS 0.5 sonorode and a working frequency of 30 kHz was used to sonicate the electrolyte during synthesis. Unlike
(1)
where m and C are the mass and the heat capacity of the solvent, in this case 200 ml of deionized water. The acoustic power density is plotted as a function of applied amplitude in figure 1. The absorbed acoustic power is about 2% of the nominal electrical power of the ultrasonic device. The crystal structure of the products was analysed using a Philips X-pert diffractometer, using Cu Kα radiation (λ = 154.05 pm). In order to confirm the existence of organic molecules at the surface of the nanoparticles, a Perkin-Elmer Fourier transformation infrared (FT-IR) spectrophotometer was used. An FEI Titan high-resolution transmission electron microscope (HRTEM) and a Carl Zeiss Ultra Plus scanning electron microscope (SEM) were employed to examine the morphology, particle size, nanostructure and dispersion of the Fe3 O4 particles. Magnetic measurements were conducted using a vibrating-sample magnetometer with a 1.1 T permanent magnet flux source, or a 5 T SQUID superconducting magnetometer. M¨ossbauer spectra in transmission geometry were recorded for all samples using a source of Co57 in Rh, at room temperature. 2
β -CD, 20% US
Transmittance (a.u)
β -CD, Without US
(642) (731) (800)
(440) (531) (620) (533) (622) (444)
(422)
(400) (331)
(222)
(111)
(511)
Y obs Y cal Y obs -Y cal Bragg position
(220)
(a)
Intensity (a.u.)
S Mosivand et al
(311)
J. Phys. D: Appl. Phys. 47 (2014) 055001
Bu, 20% US Bu, Without US Tu, 20% US Tu, Without US TMA, 20% US Without Organic
Without organic, Without US Without organic, Amplitude 100% Tu, without US Tu, Amplitude 100% TMA, without US TMA, Amplitude 100%
Intensity (a.u.)
(b)
3500
2000
1500
1000
-1
Figure 3. FT-IR spectra of samples synthesized in the presence of TMA, Tu, Bu and β-CD at silent condition and in the presence of US with a 20% amplitude.
silent conditions and in the presence of US with a 100% amplitude. Based on these results, all products have the ¯ cubic spinel Fe3 O4 crystal structure with space group Fd3m and lattice parameters in the range 837.1–839.5 pm, which may be compared with the reference value for stoichiometric magnetite of 839.6 pm. In stoichiometric magnetite, iron is distributed among the tetrahedral A-sites and octahedral B-sites of the spinel structure in a ratio 1 : 2. Small peaks marked with an asterisk, observed in the x-ray diffractograms of samples prepared with Bu and β-CD in silent conditions, correspond to secondary phases formed between Fe3+ and β-CD or Bu [25]. These extra reflections are eliminated by using US in the case of Bu, and greatly reduced in the case of β-CD.
Bu, Amplitude 100% β-CD, Without US β-CD, Amplitude 100%
Intensity (a.u.)
2500
Wave number (cm )
Bu, Without US
(c)
3000
TMA, Without US
* *
*
3.2. Infrared spectra **
**
The FT-IR spectra of samples prepared in the presence of different organic molecules or without organic were recorded to obtain some information about the interaction between the iron oxide surface and the organic materials. Figure 3 shows the FT-IR spectra for the reference sample without any organic and different samples prepared in the presence of Tu, TMA, Bu, β-CD, at silent condition and with a 20% amplitude of US. The FT-IR bands corresponding to vibration of the β-CD moieties were identified in a previous article [25]. What is important to highlight here is that the intensity of these bands, especially a band at ∼1650 cm−1 assigned to C–C stretching of polysaccharides, and those between 1200–1000 cm−1 characteristic of symmetric stretching of glycosidic C–O–C and C–O bonds in polysaccharides [25, 33–35] is diminished in samples produced by US. This could be related to the influence of US on the structure of this organic or decreasing the particles surface area as a result of increasing their size (section 3.2). Therefore the reflections from organic molecules attached at the particle surface decreases. The
*
20
30
40
50
60
70
80
90
100
2θ (degree) Figure 2. A typical XRD pattern with Rietveld analysis of magnetite nanoparticles (a), x-ray pattern of samples prepared without and with US (amplitude 100%) with no organic, Tu and TMA (b), Bu and β-CD (c).
3. Results 3.1. XRD results
A typical XRD pattern with Rietveld analysis of a sample prepared with no additive in the cell is shown in figure 2(a). Figures 2(b) and (c) compare the x-ray patterns of samples obtained with Tu, TMA, Bu, β-CD and without organic under 3
J. Phys. D: Appl. Phys. 47 (2014) 055001
Tu, Without US
S Mosivand et al
Tu, Amplitude 20%
Tu, Amplitude 100%
500 nm
500 nm
500 nm
TMA, Without US
TMA, Amplitude 20%
TMA, Amplitude 100%
500 nm
Bu, Without US
500 nm
β-CD, Without US
500 nm
Without organic, Without US
500 nm
500 nm
500 nm
Bu, Amplitude 20%
Bu, Amplitude 100%
500 nm
500 nm
β-CD, Amplitude 20%
500 nm
β-CD, Amplitude 100%
500 nm
Without organic, Amplitude 20%
500 nm
Without organic, Amplitude 100%
500 nm
Figure 4. Typical SEM images of magnetite nanoparticles prepared with no organic and in the presence of Tu, TMA, Bu and β-CD in silent conditions and with 20% and 100% US amplitude.
FT-IR spectra of the samples produced when TMA is present in the electrochemical cell do not show any reflections from this organic. This indicates that although TMA may interfere with the nucleation process of Fe3 O4 nanoparticles, it does not specifically attach to the nanoparticle surface. Tu shows weak bands in the region 1090–1020 cm−1 which can correspond to the C–N stretching of primary amines. The presence of Bu at the particle surface was confirmed by the out-of-plane deformation bands at around 1120 and 1034 cm−1 , together with the characteristic asymmetric and symmetric stretching vibration bands of carboxylic groups at around 1624 cm−1 and 1448 cm−1 , respectively [25, 33].
3.3. SEM and HRTEM images
The particle size was determined by analysing the electron microscope images of at least 100 particles of each sample using Image-J like measurement software. Figures 4 and 5 compare typical SEM and TEM images of magnetite nanoparticles synthesized in silent conditions (left column) and with 20% (middle column) and 100% (right column) of maximum US amplitude. The mean particle size dav and its standard deviation are plotted as a function of US amplitude for nanoparticles prepared with Tu, TMA, Bu, β-CD, and without organic in figure 6. 4
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
Tu, Without US
Tu, Amplitude 20%
Tu, Amplitude 100%
TMA, Without US
TMA, Amplitude 20%
TMA, Amplitude 100%
Bu, Without US
Bu, Amplitude 20%
Bu, Amplitude 100%
β - CD, Without US
β - CD, Amplitude 20%
β - CD, Amplitude 100%
Figure 5. Typical TEM images of magnetite nanoparticles prepared in the presence of Tu, TMA, Bu and β-CD in silent conditions and with 20% and 100% US amplitude.
The mean particle size is in the range 20–85 nm for all samples. In the absence of organics, the effect of US is initially to decrease dav , and then to improve the particle dispersion. The Fe3 O4 particles made with Tu are the largest, with a mean particle size of ∼85 nm that decreases to 50 nm
with increasing ultrasonic power. SEM images show some sheet-like structures for samples prepared at 20% amplitude. For TMA, dav is 50 nm without US, and it goes through a minimum of 35 nm at 40%. Curious hexagonal aggregates of particles about 10 nm in size are seen in both TEM and SEM 5
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
Mean particle size Specific magnetization 100
160 140
140
40
40
80 40
60 40
20
Tu Bu
140
120 -1 2
60
80 40
60 40
β−CD
US Amplitude (%) 80
120 100
20
60
80 40
60 40
20
20
20 0
20
40
60
80
100
0
0
0
20
US Amplitude (%)
80
100
0
100
Without organic 80
120 100
60
2
Mean particle size (nm)
60
US Amplitude (%)
160 140
40
-1
0
0 100
2
Mean particle size (nm)
80 100
TMA
0 160
0 100
-1
0 160 140
20 20
σs (Am kg )
20
-1
60
σs (Am kg )
60
100
2
80
80 120
σs (Am kg )
Mean particle size (nm)
2
60
-1
100
σs (Am kg )
120
σs (Am kg )
Mean particle size (nm)
80
Mean particle size (nm)
100
160
80 40
60 40
20 20 0
0
20
40
60
80
100
0
US Amplitude (%)
Figure 6. The dependence of mean particle size and σs as a function of US amplitude for all samples.
images of the sample prepared at the highest amplitude, and some noodle-like structures were also observed. The effect of Bu is to reduce the particle size to 30 nm, which increases up to about 50 nm with increasing US amplitude; there are some intermixed sheet-like structures for those samples prepared at an amplitude greater than 60%. The smallest size, dav ≈ 20 nm, are found for particles prepared with β-CD, but US produces an abrupt increase to 50–65 nm. These results all demonstrate that tuning the US amplitude produces significant changes in the size, morphology and agglomeration of the Fe3 O4 nanoparticles formed. Figure 7 shows two typical high-resolution transmission electron microscopy (HRTEM) images of the Fe3 O4 nanoparticles prepared (a) in the cell without organic and (b) in
the presence of Bu with 20% amplitude. The lattice striations confirm the well-crystalized nature of the nanoparticles, and the observed separations of the lattice planes correspond to those expected for Fe3 O4 . 3.4. Magnetometry
Magnetization curves for all the Fe3 O4 nanoparticle samples are shown in figure 8. Everyone is magnetically soft with a specific magnetization, σs that depends on the US amplitude. Figure 6 includes σs data in the plots of for all five groups of samples. There is a clear correlation between σs and dav in each group, which is summarized in figure 9. The magnetization data for each organic additive lie either above (TMA, Bu) 6
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
0.102 nm
0.257 nm 0.237 nm
0.302 nm
0.302 nm 0.237 nm
0.257 nm
0.237 nm
0.204 nm 0.302 nm 0.176 nm
(a)
0.135 nm
0.163 nm
(b)
Figure 7. Two typical HRTEM image of the Fe3 O4 nanoparticles (a) in the cell without organic and (b) in the presence of Bu with 20% amplitude.
or below (Tu, β-CD) those for particles produced without organic additives. The smallest particles made with Bu or β-CD are outliers, because not all the iron there is present in the magnetically ordered magnetite phase, as discussed in the next section. All the magnetization curves shown in figure 8 are broadly similar, with no significant hysteresis in any case. At room temperature the coercivity µ0 Hc ≈ 4–9 mT, and the remanence ratio is ≈5–11%. Some of the samples have also been measured at 4 K, where there is a slight increase in σs and µ0 Hc , but the low field shape of the magnetic curve is unchanged. The curves are all fitted to an empirical expression: M = Ms tanh(H /H0 )
samples in figure 11(a) where the 3% level is indicated by the dashed line. In stoichiometric magnetite, the B-site spectrum is due to an average electronic configuration of Fe2.5+ , which arises from fast electron hopping among the iron on the octahedrally coordinated sites. Hence the ratio of the areas of the Fe3+ (A-site) to Fe2.5+ (B-site) spectra is expected to be 1 : 2, assuming equal recoilless fractions for both A- and B-sites [36]. Magnetite can exhibit a range of stoichiometry with a deficit of iron represented by the formula Fe3−δ O4 , where the extremes are δ = 0 (stoichiometric magnetite) and δ = 0.33 (γ -Fe2 O3 ). For nonstoichiometric magnetite, the vacancies are on B-sites, and charge balance is preserved by a greater proportion of Fe3+ ions there [37]. The two six-line subspectra are not attributed simply to A- and B-site iron, but to iron in an Fe3+ configuration (A-site and some B-site iron) and iron in an Fe2.5+ configuration involved in electron hopping (the remaining B-site iron). For all samples R the ratio of Fe2.5+ to Fe3+ absorption and also the δ parameter in the formula Fe3−δ O4 as a function of US amplitude are shown in figures 11(b) and (c), respectively. The following formula is used for δ: δ = (2 − f R)/(6 + 5R) (3)
(2)
to determine the saturation magnetization Ms and the initial susceptibility Ms /H0 . We have also taken samples prepared at silent conditions and with 100% amplitude of US, and diluted them with increasing mass of γ -Al2 O3 nanoparticles. The magnetization decreases linearly with dilution as expected, but H0 is unchanged.
where f is the ratio of A-site to B-site recoilless fractions. Previously, we have taken this ratio to be unity [25, 26], but here we prefer to use the measured value of 1.06 [38], which has the effect of slightly reducing the calculated nonstoichiometry parameter δ. For samples produced in the presence of β-CD, δ decreases from 0.3 without US to 0.1 with increasing amplitude, indicating a tendency for US to favour the formation of more stoichiometric magnetite in this case. With Bu, however, US tends to make the magnetite a little more oxidized, and less stoichiometric. The sonoelectrocrystallized samples prepared with no additive are practically stoichiometric for an intermediate amplitude of 40% (figure 11(c)).
¨ 3.5. Mossbauer spectroscopy
Figure 10 shows the experimental and fitted room-temperature M¨ossbauer spectra for Fe3 O4 nanoparticles prepared with different US amplitude and no organic, or in the presence of Tu, TMA, Bu or β-CD. The spectra are well fitted by two magnetic sextets associated with magnetite, and a paramagnetic doublet, which accounts for the presence of paramagnetic secondary phases associated with organometallic complexes containing Fe3+ and Bu or β-CD. The doublet rapidly decreases with US amplitude, but there is some residual intensity in the case of β-CD; and in the case of Tu the doublet increases slightly with US. Another very weak component, a paramagnetic doublet with isomer shift 0.09 mm s−1 and quadrupole splitting 0.35 mm s−1 comprising about 3% of the total absorption, is an artefact associated with iron in the beryllium window of the proportional counter. The dependence of the paramagnetic doublet contribution on US amplitude is presented for all
4. Discussion
The chemical effects of US in sonoelectrochemistry are generally a consequence of acoustic cavitation and transient 7
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
100
100 Without US
80 60
Amplitude 60%
Amplitude 40% Amplitude 60%
40
Amplitude 80%
20
Amplitude 20%
Amplitude 80%
20
Amplitude 100%
Amplitude 100%
2
-1
Without US
60
Amplitude 40%
40
σs (Am kg )
80
Amplitude 20%
0
0
-20
-20
-40
-40
-60
-60
-80
-80
TU
-100 100 Without US
Amplitude 60%
Amplitude 40% Amplitude 60%
40
-1
Amplitude 80%
20
2
σs (Am kg )
Amplitude 20%
60
Amplitude 40%
40
Without US
80
Amplitude 20%
60
0
0
-20
-20
-40
-40
-60
-60
Amplitude 100%
Amplitude 80%
20
-80
Amplitude 100%
β −CD
-80
Bu
-100 100
-100 100
80
Without US
60
Amplitude 40%
1:0, Fe3O4:γ −Al2O3
80
1:1, Fe3O4:γ −Al2O3
Amplitude 20%
60
Amplitude 60%
40
Amplitude 80%
-1
0
0
-20
-20
-40
-40
-60
-60
Amplitude 100%
-80 0.0
0.5
±
±
Without organic Amplitude 100%
-80
Without organic -0.5
1:100, Fe3O4:γ −Al2O3
20
20
-1.0
1:10, Fe3O4:γ −Al2O3
40
2
σs(Am kg )
TMA
-100 100
80
-100
± ±
-100
1.0
µ0H (T)
-1.0
-0.5
0.0
0.5
1.0
µ0H (T)
Figure 8. Room-temperature magnetization curves of all samples.
stable phase between Fe3+ and carboxylate groups is inhibited. The formation of these phases requires the establishment of coordinating bonds between the terminal carboxylate groups and iron. It would seem that in this case, the US breaks these intermolecular interactions, leaving the ions free to precipitate with the electrochemically generated hydroxide, although β-CD fragments remain associated with the nanoparticle surface (figure 3). US has a marked, but rather unpredictable influence on the mean magnetite particle size, and to a lesser extent on the magnetite stoichiometry (figures 6 and 11(c)). The mean
localized hot spots which improve the mass transport, mixing the molecules or ions and increasing the speed of chemical reactions [30, 38]. In addition, organics and solute molecules present within the cavitation bubbles may be decomposed and generate some highly reactive radicals [28, 29, 39]. The clearest effect of US in our data relates to the samples prepared with Bu and β-CD, which show secondary phases involving complexes of paramagnetic Fe3+ , as seen in the M¨ossbauer spectra. The quantity of secondary phase is eliminated by US in the case of Bu, and greatly reduced in the case of β-CD. It appears that the formation of a 8
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
80
fragments bound at the surface of the particles is to dilute the specific magnetization of magnetite by adding nonmagnetic mass which shows up in the FT-IR spectrum (figure 3). If there are n groups attached per unit surface area, then
70
σs (d) = σs0 (1 − 6t/d)(1 − 6nMu/ρd)
100
2
-1
σs (Am kg )
90
where u is the atomic mass unit, M = 1135 g mol−1 is the atomic weight of the molecule and ρ = 5195 kg m−3 is the density of magnetite. Taking t = 1.8 nm and σs0 = 93 A m2 kg−1 , as before, we find n = 5.5 molecules nm−2 . The other surfactant with a carboxylate group, Bu, only has M = 110 g mol−1 , so there is little effect on the specific magnetization. IR spectra (figure 3) shows the presence of some of this organic at the surface of the oxide particles. The other organics mainly influence the nucleation and growth of the magnetite nanoparticles, TMA leads to smaller particle size and a thinner dead layer, whereas Tu inhibits nucleation and produces larger particles (figure 9). One can often read in textbooks and reviews that with reducing particle size, there is a sequence of magnetic configurations (figure 12(a)):
60 50 40 TMA Tu Bu β -CD Without organic
30 20 10
20
30
40
50
60
70
80
90
Mean particle size (nm) Figure 9. A plot of magnetization as a function of mean particle size for all iron oxide nanoparticles.
particle size decreases as the nucleation rate increases, and it increases when the growth rate increases. The balance between these two effects is delicate, and the net outcome depends both on the system and the ultrasonic power. The modification of the average bath temperature is probably also a factor, but rather than engaging in speculative discussion, we will simply regard US as a method of modifying the particle size, dav and consider how this and the organic additive influence the magnetic properties of the magnetite. Our experiments provide us with an extended set of magnetization data on magnetite nanoparticles with little variation in oxide stoichiometry, where the average size varies from 20 to 85 nm. The magnetization curves are all practically anhysteretic, although there is no sign of superparamagnetism in any but the smallest particles. The M¨ossbauer spectra are well-resolved magnetic hyperfine patterns, and the magnetization curves are largely independent of temperature below 300 K, with little hysteresis, even at 4 K. First we consider the magnetization of the particles. In figure 9 it is clear that σs is always less than the value σs0 = 93 A m2 kg−1 expected for bulk stoichiometric magnetite at room temperature, and it decreases with decreasing particle size. This variation is not caused by the organics, since it is also found for the series made without any additive. The lack of saturation in ferromagnetic oxide nanoparticles is usually attributed to a shell of misaligned spins at the surface [40]. If the thickness of the shell is t and the particle diameter is d, the magnetization σs (d) should vary as σs (d) = σs0 (1 − 6t/d).
(5)
Multidomain → Single-domain → Superparamagnetic. Superparamagnetism sets in below a critical blocking diameter db or blocking temperature Tb which depend on the characteristic timescale of the measurement [41]. In the standard picture, based on the Stoner–Wohlfarth model, each particle is a single magnetic domain. The superparamagnetic fluctuations are coherent rotations of the particle’s moment, regarded as a macrospin, between two states where the moment is aligned in opposite directions along an axis, known as the easy axis of the particle. Each particle has its own easiest axis of anisotropy, and the two lowest energy states are separated by an energy barrier = Ku V , where V = π d 3 /6 and Ku is the uniaxial anisotropy, which may be either magnetocrystalline in origin, or due to a nonspherical particle shape. Superparamagnetic particles fluctuate rapidly on the timescale of the measurement when the magnetic nanoparticles are above the blocking temperature or below the blocking radius. The usual criterion for blocking in magnetization measurements is /kT = 25, or Ku Vb = 0.6 eV when the blocking temperature Tb is room temperature. Superparamagnetic magnetization curves M(H ) are necessarily anhysteretic, and data are expected to collapse onto a single Langevin function curve when M/Ms is plotted as a function of H /T , provided the initial susceptibility is not limited by the demagnetizing field. This is the recognized ‘litmus test’. The model should apply to well-dispersed nanoparticles. Weak magnetic interactions among particles smaller than the blocking radius can suppress the superparamagnetism and increase their effective magnetic volume [1, 42]. Magnetite has cubic magnetocrystalline anisotropy, with K1c = −13 kJ m−3 , where the negative sign signifies that the four cubic 1 1 1 directions are easy. The barrier to reversal is then = −K1c V /12, and the blocking diameter for a spherical magnetite particle at room temperature is therefore
(4)
The dashed line for the sonoelectrocrystallized samples prepared with no additive, in figure 9 corresponds to σs0 = 93 A m2 kg−1 and t = 1.8 nm, or a magnetic dead layer two unit cells thick. The one dataset in figure 9 that is clearly distinct from the others is β-CD. The molecule breaks up into many fragments in solution, and the carboxylic groups may attach themselves to the surface of the iron oxide nanoparticles. The effect of the
db ≈ (1800kT /π K1c )1/3 ≈ 50 nm. 9
(6)
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
101
99
% Transmission
100
100
100
99
99
98
97
96
97
Fit Fe
94
Fe Para
3+
Fe
96
2.5+
Tu, Without US
100
3+
96
Fe
95
Fe Para
2.5+
2.5+
Fe Para
Tu, Amplitude 20%
95
Tu, Amplitude 100%
100
100
99
% Transmission
Fit
Fit
3+
95
93
99
99
98
98
98
97
97 97
Fit
96
3+
Fe
96
2.5+
95
Fit
Fit
3+
Fe
Fe Para
TMA, Without US
94
100
99
99
98
98
3+
96
Fe
95
Fe Para TMA, Amplitude 100%
2.5+
2.5+
Fe Para
TMA, Amplitude 20%
95
100
% Transmission
98
98
97
100 99 98 97
97
97
Fit
3+
3+
Fe
96
Fit
Fit Fe
96
2.5+
Fe Para
95
Fe Para
2.5+
Fe Para
Bu, Amplitude 20%
95
100
Fe
2.5+
Bu, Without US
95
3+
96
100
100
99
99
Bu, Amplitude 100%
% Transmission
99 98 97
98
98
Fit
96
Fit
Fe
97
3+
95
Fit
3+
3+
Fe
97
2.5+
Fe Para
β -CD, Without US
94
2.5+
Fe Para
β-CD, Amplitude 20%
96
96
100
100
Fe Para
100
β-CD, Amplitude 100% -1
Velocity (mm s )
% Transmission
99 99
98
99
97 98
Fit
96
Fit
3+
94
Fe Para -10
-8
-6
-4
-2
3+
Fe
2.5+
95
Fit
98
3+
Fe
Without organic, Without US
0
2
4 -1
Velocity (mm s )
6
8
10
Fe
2.5+
Fe Para
97 -10
-8
-6
-4
-2
2.5+
Without organic, Amplitude 20%
0
2
-1
4
Velocity (mm s )
6
8
10
97
Fe Para -10
-8
-6
-4
-2
Without organic, Amplitude100%
0
2
4
6
8
10
-1
Velocity (mm s )
Figure 10. The experimental and fitted room-temperature M¨ossbauer spectra for Fe3 O4 nanoparticles prepared with US, for all samples prepared in silent condition and by applying 20% and 100% amplitude.
If the magnetite particle is nonspherical, shape anisotropy Ks = µ0 Ms2 (1–N)/4 defines an easy direction, where N is the demagnetizing factor (N = 1/3 for a sphere). The value of m0 Ms2 /4 for stoichiometric magnetite is 72 kJ m−3 , so a slight asphericity is sufficient to make shape the dominant contribution to the anisotropy barrier. For example, N = 0.2 reduces db to 20 nm. These considerations, and the tendency
for our particles to agglomerate are consistent with the observation that only one case, the 20 nm sample (β-CD, silent), shows any evidence of a significant superparamagnetic fraction. We emphasize that our magnetite is not superparamagnetic. The particles are clearly blocked on the M¨ossbauer timescale, since they exhibit well-resolved magnetic hyperfine 10
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
45 40
(a)
Tu TMA Bu β−CD Without organic Beryllium window
Doublet (%)
35 30 25 20 15
(a)
10 5 0 2.0
(b)
A(Fe
2.5+
3+
)/A(Fe )
1.6 1.2
(b) 0.8 0.4 0.0 0.35
(c)
f =1.06
15 nm, and interparticle interactions will reduce this value. Other experimental estimates are 25–30 nm [4]. If the particles are not superparamagnetic, they do not seem to be single domain or multidomain either. Single-domain nanoparticles smaller than the coherence diameter dcoh should behave as Stoner–Wohlfarth particles, and exhibit square hysteresis loops with appreciable coercivity, up to 2Ku /Ms or 70 mT. The coherence diameter for magnetite is about 50 nm. The observed coercivity does not exceed 18 mT, even at 4 K; it is usually much less. The 20–85 nm magnetite particles are likely to adopt the more stable vortex structure, originally envisaged by N´eel for a soft ferromagnetic sphere [43]. Multidomain particles would be expected to show anhysteretic initial magnetization curves with H0 ∼ Ms /3, but they should be considerably larger than the domain wall width, which is 73 nm for magnetite [41]. The observed values of Ms /H0 are 4–5. The conventional magnetite sequence shown in figure 12(a) applies when the magnetocrystalline anisotropy is strong, Ku >∼ 1 MJ m−3 . It does not necessarily apply to magnetite, nor to many other soft magnetic particles, such as iron or permalloy. Particles of these materials tend to minimize their dipolar self-energy by adopting a vortex configuration where the magnetization lies parallel to the surface, and there is a singular core of spins lying along one diameter. The magnetization process is not coherent rotation of the uniform magnetization of the whole particle into the direction of the applied field, but a continuous unwinding of the vortex so that the magnetic configuration is always in equilibrium with the field. This is in marked contrast to the hysteretic magnetization process of a Stoner–Wohlfarth particle. The minimum particle size required for the vortex state to form is estimated by equating the exchange energy of the vortex to the magnetostatic
Tu TMA Bu β−CD Without organic
0.30 0.25
δ
Figure 12. Sequence of magnetic states expected for ferromagnetic particles with strong uniaxial anisotropy (a) and weak anisotropy, such as near-spherical magnetite particles (b).
Tu TMA Bu β−CD Without organic
0.20 0.15 0.10 0.05 0.00 0
20
40
60
80
100
US Amplitude (%) Figure 11. The dependence of the paramagnetic doublet contribution on US amplitude for all samples. The dashed line shows the amplitude of the doublet due to the beryllium window of the proportional counter (a), the ratio of Fe2.5+ to Fe3+ (b), and the nonstoichiometry parameter δ as a function of US amplitude for all samples (c).
patterns, which indicate a relaxation time >10−7 s [40]. The lack of temperature dependence of the initial susceptibility of the magnetization curves at and below room temperature shows that the magnetic relaxation time is >100 s, because if the particles were superparamagnetic in magnetization measurements, their initial susceptibility should vary as 1/T , when the temperature is sufficiently high. The absence of superparamagnetism is not unexpected. The blocking diameter for magnetite at room temperature is often found to be about 11
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
chemical analysis for iron may be necessary to eliminate the possibility of magnetite or iron contamination when H0 lies in these ranges.
10000
N =1/6
β -CD coated nanoparticles with
40% paramagnetic contribution
N =1/3
5. Conclusion
-1
Ms (kA m )
1000
100
Ultrasound has a significant influence on magnetic properties of magnetite nanoparticles produced by electro-oxidation in the presence or absence of organic additives as it influences the morphological, structural and chemical properties of nanoparticles obtained. Based on structural characterization, it is possible to improve the structure of samples by using ultrasound treatment, depending on the applied amplitude and organic agent employed. The particle size and morphology were found to be easily controlled by adjusting the power. Magnetometry showed that the specific magnetizations of the Fe3 O4 nanoparticles range from 19 to 90 A m2 kg−1 , depending on the type of organic molecules and ultrasonic power, but much of this variation is due to the mass of attached, nonmagnetic organic material. Analysis of M¨ossbauer spectra showed that ultrasonic treatment is very effective to remove a strong paramagnetic doublet, associated with organometallic complexes containing Fe3+ which were present in samples prepared with β-CD and Bu. The magnetite nanoparticles produced by electrocrystallization in the presence of organic surfactants are composed of slightly substoichiometric Fe3−δ O4 with δ ≈ 0.1. Synthesis in the presence of ultrasound modifies the particle size and improves the purity of magnetite phase. The practically anhysteretic and temperature-independent magnetization is attributed to a vortex magnetic configuration, which is expected to be a general feature of magnetite nanoparticles larger than about 20 nm in diameter. We find no evidence of single-domain Stoner–Wohlfarth behaviour in the size range 20–85 nm. Further investigation of the slight coercivity and remanence of those magnetite vortex particles is needed to understand the magnetization of magnetite nanoparticles found in the natural environment and in rocks.
10
Magnetite nanoparticles Magnetite: γ-Al2O3, 1:1
1
Magnetite: γ-Al2O3, 1:10 Magnetite: γ-Al2O3, 1:100 0.1 10
100
1000 -1
H0 (kA m ) Figure 13. A scatter plot of Ms versus H0 for all the magnetite nanoparticles and some samples diluted with different amount of γ -Al2 O3 nanoparticles.
energy µ0 Ms2 V /6 of the uniformly magnetized spherical particle in its own demagnetizing field. For magnetite, this gives a minimum diameter of about 10 nm. We conclude that the magnetite particles we are studying are all in a vortex state, where each particle is magnetically ordered, but it has no net moment in the absence of an applied field. The sequence of magnetic states with increasing particle size is illustrated in figure 12(b). We can therefore understand the anhysteretic, temperatureindependent magnetization curves of our particles. The field H0 for all the particles lies in the range 40–100 kA m−1 . The average value is 81±11 kA m−1 , where the error is one standard deviation. Since the magnetization of our nonstoichiometric magnetite is around σs = 70 A m2 kg−1 or Ms = 360 kA m−1 it follows that the effective demagnetizing factor is the ratio of these two, Neff ∼ 0.2. A similar result has been found for metallic iron nanoparticles [44]. Finally, we discuss the problem of detecting magnetite impurity in unknown mixtures. The magnetization data on a set of diluted magnetite nanoparticles were shown in figure 8. The mass dilution by γ -Al2 O3 were 1 : 1, 1 : 10 and 1 : 100. These points are included with all the others in the Ms versus H0 plot in figure 13. The obvious effect of dilution is to reduce the sample magnetization in proportion to the quantity of nonmagnetic γ -Al2 O3 , but it can be seen that dilution has almost no effect on the value of H0 . If each magnetite particle were an isolated monodomain with a spontaneous magnetization equal to the saturation magnetization of the undiluted magnetite, the effect of reducing the dipolar interactions between the particles should be to increase the remanence to >Ms /2, as expected for isolated particles with cubic anisotropy. In fact H0 and Mr are unchanged by dilution, as expected for soft magnetic particles with a vortex structure [44]. This means that a value of H0 ≈ 80 kA m−1 can be associated with dilute nanoparticles of magnetite. Likewise values of H0 ≈ 300 kA m−1 may be associated with iron particles with a vortex structure. Precise
Acknowledgments
The work was supported by Science Foundation Ireland as part of the NISE project, contract number NISE 10/IN1.I3006. We are grateful to Dr M Venkatesan for the SQUID measurements and Dr S T Ali Shah for help with the FT-IR analysis. Some of the work was enabled by the CRANN Advanced Microscopy Laboratory (AML), Trinity College Dublin. S M thanks Shahid Chamran University for financial support. References [1] Mørup S, Hansen M F and Frandsen C 2011 Magnetic nanoparticles Comprehensive Nanoscience and Technology vol 1 ed D L Andrews et al (Amsterdam: Elsevier) pp 437–91, chapter 14 [2] Bedanta S and Kleemann W 2009 Supermagnetism J. Phys. D: Appl. Phys. 42 013001 12
J. Phys. D: Appl. Phys. 47 (2014) 055001
S Mosivand et al
[3] Coey J M D 1987 Noncollinear spin structures Can. J. Phys. 65 1210–32 ¨ ¨ 1997 Rock Magnetism, [4] Dunlop D J and Ozdemir O Fundamentals and Frontiers (Cambridge: Cambridge University Press) [5] Rosencwaig R E 1985 Ferrohydrodynamics (New York: Cambridge University Press) [6] Jorgensen F 1995 The Complete Handbook of Magnetic Recording (New York: McGraw-Hill) chapters 11–15 [7] Pankhurst Q A, Connolly J, Jones S K and Dobson J 2003 Applications of magnetic nanoparticles in biomedicine J. Phys. D: Appl. Phys. 36 R167–81 [8] Pankhurst Q A, Thanh N T K, Jones S K and Dobson J 2009 Progress in applications of magnetic nanoparticles in biomedicine J. Phys. D: Appl. Phys. 42 224001 [9] Br¨ahler M, Georgieva R, Buske N, M¨uller A, M¨uller S, Pinkernelle J, Teichgr¨aber U, Voigt A and B¨aumler A 2006 Magnetite-loaded carrier erythrocytes as contrast agents for magnetic resonance imaging Nano Lett. 6 2505–9 [10] Basti H, Ben Tahar L, Smiri L S, Herbst F, Vaulay M J, Chau F, Ammar S and Benderbous S 2010 Catechol derivatives-coated Fe3 O4 and γ -Fe2 O3 nanoparticles as potential MRI contrast agents J. Colloids Interface Sci. 341 248–54 [11] Laurent S, Dutz S, H¨afeli U O and Mahmoudi M 2011 Magnetic fluid hypothermia: focus on superparamagnetic iron oxide nanoparticles Adv. Colloids Interface Sci. 166 8–23 [12] Qu J, Liu G, Wang Y and Hong R 2010 Preparation of Fe3 O4 –chitosan nanoparticles used for hyperthermia Adv. Powder Technol. 21 461–7 [13] Goya G F, Grazu V and Ibarra M R 2008 Magnetic nanoparticles for cancer therapy Curr. Nanosci. 4 1–16 [14] Kievit F M and Zhang M 2011 Surface engineering of iron oxide nanoparticles for targeted cancer therapy Acc. Chem. Res. 44 853–62 [15] Shen Y F, Tang J, Nie Z H, Wang Y D, Ren Y and Zuo L 2009 Tailoring size and structural distortion of Fe3 O4 nanoparticles for the purification of contaminated water Bioresources Technol. 100 4139–46 [16] Gubin S P, Koksharov Y A, Khomutov G B and Yurkov G Y 2005 Magnetic nanoparticles: preparation, structure and properties Russ. Chem. Rev. 74 489–520 [17] Guardia P, Batlle-Brugal B, Roca A G, Iglesias O, Morales M P, Serna C J, Labarta A and Batlle X 2007 Controlled synthesis of iron oxide nanoparticles over a wide size range J. Magn. Magn. Mater. 316 756–9 [18] Iida H, Takayanagi K, Nakanishi T and Osaka T 2007 Synthesis of Fe3 O4 nanoparticles with various sizes and magnetic properties by controlled hydrolysis J. Colloid Interface Sci. 314 274–80 [19] Gao G, Huang P, Zhang Y, Wang K, Qin W and Cui D 2011 Gram scale synthesis of superparamagnetic Fe3 O4 nanoparticles and fluid via a facile solvothermal route Cryst. Eng. Commun. 13 1782–5 [20] Frey N A, Peng S, Cheng K and Sun S 2009 Magnetic nanoparticles: synthesis, functionalization, and applications in bioimaging and magnetic energy storage Chem. Soc. Rev. 38 2532–42 [21] Chen D, Ni S and Chen Z 2007 Synthesis of Fe3 O4 nanoparticles by wet milling iron powder in a planetary ball mill Chin. Particuol. 5 357–8 [22] Amara D, Felner I, Nowik I and Margel S 2009 Synthesis and characterization of Fe and Fe3 O4 nanoparticles by thermal decomposition of triiron dodecacarbonyl Colloids Surf. A 339 106–10 [23] Cabrera L, Gutierrez S, Menendez N, Morales M P and Herrasti P 2008 Magnetite nanoparticles: electrochemical
[24] [25]
[26]
[27] [28] [29] [30] [31]
[32] [33] [34]
[35]
[36]
[37] [38] [39] [40] [41] [42] [43] [44]
13
synthesis and characterization Electrochim. Acta 53 3436–41 Kazeminezhad I, Mosivand S and Farbod M 2011 Effect of growth parameters on structure of electrooxidized Fe3 O4 magnetic nanoparticles Curr. Nanosci. 7 819–24 Mosivand S, Monzon L M A, Ackland K, Kazeminezhad I and Coey J M D 2013 The effect of organics on the structure and magnetization of electro-synthesised magnetite nanoparticles J. Nanopart. Res. 15 1795 Mosivand S, Monzon L M A, Kazeminezhad I and Coey J M D 2013 Influence of growth conditions on magnetite nanoparticles electro-crystallized in the presence of organic molecules Int. J. Mol. Sci. 14 10383–96 S´aez V and Mason T J 2009 Sonoelectrochemical synthesis of nanoparticles Molecules 14 4284–99 Bang J H and Suslick K S 2010 Applications of ultrasound to the synthesis of nanostructured materials Adv. Mater. 22 1039–59 Okitsu K, Ashokkumar M and Grieser F 2005 Sonochemical synthesis of gold nanoparticles: effects of ultrasound frequency J. Phys. Chem. B 109 20673–5 Cabrera L, Guti´errez S, Herrasti P and Reyman D 2010 Sonoelectrochemical synthesis of magnetite Phys. Procedia 3 89–94 Raso J, Ma˜nas P, Pag´an R and Sala F J 1999 Influence of different factors on the output power transferred into medium by ultrasound Ultrason. Sonochem. 5 157–62 Kobus Z and Kusi´nska E 2008 Influence of physical properties of liquid on acoustic power of ultrasonic processor TEKA Kom. Mot. Energ. Roln.–OL PAN 8a 71–8 Socrates G 2001 Infrared and Raman Characteristic Group Frequencies 3rd edn (Chichester: Wiley) Ge X, He J, Qi F, Yang Y, Huang Z, Lu R and Huang L 2011 Inclusion complexation of chloropropham with β-cyclodextrin: preparation, characterization and molecular modelling Spectrochim. Acta A 81 397–403 Zhang S, He B, Ge J, Li H, Luo X, Zhang H, Li Y, Zhai C, Liu P, Liu X and Fei X 2010 Extraction, chemical analysis of Angelica sinensis polysaccharides and antioxidant activity of the polysaccharides in ischemia–reperfusion rats Int. J. Biol. Macromol. 47 546–50 Sawatzky G A, van der Woude F and Morrish A H 1969 Recoilless-fraction ratios for Fe57 in octahedral and tetrahedral sites of a spinel and a garnet Phys. Rev. 183 383–6 Coey J M D, Morrish A H and Sawatzky G A 1971 A M¨ossbauer study of conduction in magnetite J. Phys. Colloq. 32 C1:271–3 Wu W, He Q, Chen H, Tang J and Nie L 2007 Sonochemical synthesis, structure and magnetic properties of air-stable Fe3 O4 /Au nanoparticles Nanotechnology 18 145609 Zin V, Pollet B G and Dabal`a M 2009 Sonoelectrochemical (20 kHz) production of platinum nanoparticles from aqueous solutions Electrochim. Acta 54 7201–6 Chuev M A 2012 Multilevel relaxation model for describing the Mossbauer spectra of nanoparticles in a magnetic field J. Exp. Theor. Phys. 114 609–30 Coey J M D 2010 Magnetism and Magnetic Materials (Cambridge: Cambridge University Press) Mørup S, Hansen M F and Frandsen C 2010 Magnetic interactions between nanoparticles Beilstein J. Nanotechnol. 1 182–90 N´eel L 1947 Propri´et´es d’un ferromagn´etique cubique en grains fins C. R. Acad. Sci. 224 1488–90 Grace P, Venkatesan M, Alaria J, Coey J M D and Naaman R 2009 The origin of magnetism of etched silicon Adv. Mater. 21 71–4
