Rapid mixing

APPLIED PHYSICS LETTERS 99, 222501 (2011)

Rapid mixing: A route to synthesize magnetite nanoparticles with high moment Mei Fang,1 Valter Stro¨m,1 Richard T. Olsson,2 Lyubov Belova,1 and K. V. Rao1,a) 1

Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden Department of Fiber and Polymer Technology, Royal Institute of Technology, Stockholm, Sweden

2

(Received 2 October 2011; accepted 30 October 2011; published online 28 November 2011) We demonstrate the impact of rapid mixing of the precursors in a time scale of milliseconds on the reaction rate and magnetic properties of co-precipitated magnetite with a custom-made mixer. The mixed volume is directed into a desk-top AC susceptometer to monitor the magnetic response from the growing particles in real-time. These measurements indicate that the reaction is mostly completed within a minute. The obtained superparamagnetic nanoparticles exhibit a narrow size distribution and large magnetization (87 Am2 kg1). Transmission electron micrographs suggest that rapid mixing is the key for better crystallinity and a more uniform morphology leading to the C 2011 American Institute of Physics. [doi:10.1063/1.3662965] observed magnetization values. V The synthesis of magnetite (Fe3O4) nanoparticles with controlled size and narrow size distribution has captured significant scientific and industrial interest for developing many technological applications, from monodispersive ferrofluids1,2 to magneto-optical devices.3,4 Because of the biocompatibility, superparamagnetic magnetite nanoparticles are also attractive for applications like cellular therapy,5 tissue repair,6 targeted drug delivery,7 magnetic resonance imaging (MRI),8,9 hyperthermia,10 to name a few.11–13 These applications also require high magnetization and numerous chemical methods, like co-precipitation, thermal decomposition, hydrothermal synthesis, sol-gel reactions, polyol methods, etc., have been developed.5,11,14 Aqueous coprecipitation is a facile and convenient method to prepare nano-sized magnetite from an aqueous iron ion source by the addition of a base solution. However, the co-precipitation reaction is complicated and the properties of the nanoparticles are sensitive to the preparation conditions, e.g., the type of iron salts, the ratio of Fe2þ/Fe3þ, the reaction temperature, pH value, and mixing rate, etc.15–18 Besides, the typical saturation magnetization (Ms) of magnetite nanoparticles obtained from co-precipitation (30 – 70 Am2 kg1) is much lower than the theoretical value of 96.4 Am2 kg1 for bulk magnetite,19 which may stem from impurities and surface effects. Table I lists recent reported values of particle size and the magnetic properties [Ms and coercivity (Hc)] of magnetite prepared by different methods. Only limited studies in achieving superparamagnetic magnetite nanoparticles with high magnetization have been reported. In this paper, we report a high-speed method to mix the reactants. Instead of adding the iron ion source to the base solution while stirring—referred to as Classical Mixing (CM)—the reactants flow in two syringes merging into one tube where the two jets (diameter of 0.19 mm) impinge to each other in a laminar flow mode. The flow velocity is high (8 m/s) and tube diameter is small (0.5 mm) in order to achieve a rapid mixing (RM). Particle size, size distribution, and magnetic properties have been studied and are compared a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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with those from particles prepared by CM to address the role of RM. The iron ion source was prepared from Iron (II) chloride tetrahydrate (98%, Fluka) and Iron (III) chloride hexahydrate (98%, Fluka) in de-ionized water with [Fe2þ]:[Fe3þ] ¼ 1:2. Ammonia solution was prepared from 25%–35% ammonium hydroxide (Fluka). The (total) iron ion concentration yielding the highest reaction rate was found to be 0.2 M and was thus chosen for the subsequent experiments. For RM, two syringes with each 0.25 ml iron ion source and 4 M ammonia solution were discharged by a pneumatic cylinder in a laminar mode. The mixed volume was directed into a vial in an AC magnetic susceptometer (custom made) in order to monitor the magnetic response from the growing particles in real time.27 For CM, 10 ml iron ion source was slowly added into the same amount of 4 M ammonia under magnetic stirring at room temperature. After 2 h stirring, the particles were collected for characterization. Figures 1(a) and 1(b) show the custom designed rapidmixer and the classical mixing set up. From x-ray diffractograms (copper anode), see Fig. 1(c), the lattice constant (refined by Celref3) has been determined to be 0.8393 nm for the RM particles, which is close to the theoretical value for magnetite (0.8396 nm), while it is 0.8374 nm for the CM particles which is closer to that of maghemite (0.8352 nm). During mixing, the pH value of the iron ion solution increases from 2 to 11. Since ferric (Fe3þ) ions become unstable and hydrolyze to ferrihydrate at a lower pH than ferrous (Fe2þ) ions do,16 more ferrihydrate will be created from a slow pH transition than from a quick one, i.e., it depends on the mixing rate. We suggest that the occurrence of ferrihydrates complicates the formation of magnetite and is the explanation for the increased reaction speed and the consequent improved properties of RM as compared to CM. Figure 2(a) shows the real-time in-phase (v0 ) and out-ofphase (v00 ) AC susceptibilities for a typical RM experiment. The in-phase susceptibility increases quickly and becomes essentially stable within ca. 1 min, indicating that the reaction is finished. From the almost zero out-of-phase susceptibility, a minute coercivity can be predicted. To seek for optimum iron ion concentration, the reaction rate is used as

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TABLE I. Magnetic properties (Ms, & Hc)of magnetite nanoparticles prepared by different methods. Size (nm) From Theory

T

Ref.

Optimized from:

– 1.5 104 1.2 104

0K RT RT

19

maghemite

96 92 74

14-9 TEM 9.2 XRD 7 TEM 8 TEM 20 TEM 9.9 SEM 24.6 SEM 9.94 XRD 6.47 XRD 19 TEMa 10 TEMa 11.5 TEM 6.8 XRD 11 XRD

64.3 65 49 41.60 49.23 47.1 67.9 58.7 44.5 73.2 72.0 60.1 46.2 60.34

– – – –

RT RT RT RT

18 20 21 22

2.4 102 5.2 103 –

RT

15

RT

16

pH value

300 K 17

Stirring velocity

7.2 TEM 15 TEM

42.1 74.86

6.4 TEM 7.4 TEM 4.46 TEM 3.95 TEM Thermal de8.7 TEM composition 9.1 TEM 17.9 TEM 16 TEM 14 TEM 5.7 TEM 11.0 TEM 8.1 XRD 40 XRD 55 XRD Hydrothermal 9.5 TEM synthesis 38.6 TEM

72 76 87 57 57.5 60.2 84.9 83 77 65 72 60 78.9 85.5 64.1 79 a

80 TEM 12 TEM 40 TEM 150 TEM 6.6 TEM 174 TEM 8 TEM Other methods 37 TEM 9 TEM 11 XRD 7 TEM 10 XRD 20 TEM Hollow solid

58.2 59.8 82.5 75.6 19.5 63.5 21.2 86.6 46.7 75.3 69 62 a 87.4 82.2 78.1

Coprecipitation

magnetite

Ms Hc (Am2kg1) (Am1)b

–

2.72 103 300 K 23 – 300 K 24

Fe2þ/Fe3þ

Alkali addition rate

– 300 K 25 4.8 102a 300 K 26

Ultrasonicassisted 8.5 300 K 27 Mixing rate 33.5 22 300 K This RM work 10 CM 0 77 K 28 Ratio of surfactant 96 to FeO(OH) 6.48 104 – RT 29 – RT 30 – RT 31 – – RT 20 1 104 295 K 32 4.4 103 – RT 33 Conc. of lactase and 6.4 103a sulfate ion 2.15 104 RT 34 – RT 35 8 103a 2.58 104 300 K 23 – 300 K 36 – 300 K 37 Nanocrystal clusters 5.28 103 7.04 102 – – – 1.42 104 –

300 K 38 RT 300 K 295 K 300 K 300 K

FIG. 1. (Color online) Set-ups for RM (a) and CM (b) for co-precipitation. In RM, equal amount of ammonia and iron ion source in the two syringes were discharged by a pneumatic cylinder through the mixer into the sample vial. In CM, the yellowish iron ion source (in 10 ml cylinder) was added into ammonia solution (in the beaker) with magnetic stirring (1000 rpm). (c) The XRD (copper anode) intensity pattern for CM and RM particles.

FIG. 2. (Color online) (a). A typical example for the real-time magnetic responses for RM of iron solutions with ammonia. Left scale is in-phase susceptibility (v0 ) and right scale is out-of phase susceptibility (v00 ). (b) The reaction rate of co-precipitation vs. iron ion concentration used in RM. The reaction rate was taken as the value of v0 determined 10 s after mixing normalized with respect to the amount of iron.

Iron salts

39 40 41 42 43 Micro-spheres

a The b

value is obtained from a figure in the reference. 1ðOeÞ ¼ 1000=4pðAm1 Þ

an indicator. Here, the reaction rate is defined as the value of v0 10 s after mixing and is normalized with respect to amount of iron. Figure 2(b) shows the dependence of reaction rate on

FIG. 3. (Color online) TEM micrographs and the particle size distribution of magnetite nanopartices prepared from (a) and (b) RM compared with those obtained from (c) and (d) CM. The red lines are Gaussian fits of the size distribution (b) and (d), manually determined from þ1000 particles. Concentrations were 0.2 M iron ion solution and 4 M ammonia.

the concentration. Maximum reaction rate was obtained with 0.2 M iron ion solution. For the optimum iron ion concentration, the morphologies of RM and CM nanoparticles were studied by transmission electron microscope (TEM). Figure 3 shows the micrographs and the size distributions. The RM particles are larger and have a narrower size distribution as compared to


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TABLE II. Properties of RM and CM nanoparticles. Sample ID RM CM

Mean Size (nm)

Std. dev.

M (Am2kg1)a

Hc (Am1)

4.46 3.95

1.04 1.20

87 57

22 10

the magnetization (M) is determined at a magnetic field of 500 kAm1.

a

FIG. 4. (Color online) (a) Magnetic hysteresis loops and (b) the magnified M(H) measurements near the origin for RM and CM nanoparticles, respectively.

CM particles, see Table II. The TEM images evidence that the crystalline structure and morphology of the RM particles is significantly better than those of CM particles. Static magnetization data is presented in Fig. 4 (vibrating sample magnetometer EG&G Princeton Research VSM model 155). The magnetization of RM particles is significantly larger than for CM particles, e.g., 87 Am2 kg1 vs. 57 Am2 kg1 at a field of 500 kA m1. Both RM and CM particles show small coercivities, which can only be detected in the low field measurements near the origin [Fig. 4(b)]. Comparing with the reported MS shown in Table I, the magnetization of RM particles, 87 Am2 kg1, is remarkable and not far from the theoretical value (96.4 Am2 kg1). The smaller particle size can be the reason for the lower coercivity for CM particles, but is likely not the reason for the much lower magnetization. We have also prepared RM particles with smaller size (3.8 6 0.9 nm) with similar high magnetization (by synthesizing at a lower temperature). Thus the high magnetization observed is not due to size but due to the preparation method. The reason for the lower magnetization of CM particles appears to be a poor crystalline structure and the possibly disordered surface spins. Rapid mixing of the reactants in co-precipitation results in a quicker chemical reaction, better crystalline structure, a narrower particle size distribution, and a much larger magnetization (87 Am2 kg1) compared to classical mixing (57 Am2 kg1). Maximum reaction rate has been obtained at an iron ion concentration of 0.2 M. Mei Fang is supported by Chinese Scholarship Council (CSC) for her Ph.D. study. A part of this work was funded by the Swedish Agency VINNOVA. 1

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