Co-precipitation in aqueous solution synthesis of magnetite nanoparticles using iron(III) salts as precursors

1 Introduction

Synthesis, characterization and applications of iron oxide nanocrystals have received tremendous attention in recent years due to their potentials for information storage devices, rotary shaft sealing, position sensing (Raj and Moskowitz, 1990), as well as medical and pharmaceutical applications (Xie et al., 2008). Several methods are known in the art for the synthesis of magnetite, Fe₃O₄. Cornwell and Schwertmann (Cornwell and Schwertmann, 2003) have reported several synthetic methods all of which require more than one iron component as precursors, several chemical reagents, inert atmosphere, special apparatus and/or restricting conditions. Exemplary methods are shown as follows:

1. (1)
   $${\text{FeSO}}_4+\text{KOH}+{\text{KNO}}_3\underset{{\text{N}}_2\text{gas}}{\overset{{90}^{°}\text{C}}{\to }}{\text{Fe}}_3{\text{O}}_4$$ (David and Welch, 1956).

2. (2)
   $${\text{FeSO}}_4+\text{NaOH}\underset{\text{gas}}{\overset{{\text{N}}_2}{\to }}\text{Fe}{(\text{OH})}_2\underset{{\text{N}}_2\text{gas}}{\overset{{100}^{°}}{\to }}{\text{Fe}}_3{\text{O}}_4+2{\text{H}}_2\text{O}+{\text{H}}_2$$ (produces complicating side effect) (Schikorr, 1929).

3. Reduction of hematite at 400 °C in an atmosphere of 5% H₂/95% Ar, saturated with water vapour free of O₂ (Regazzoni et al., 1981).

4. Reaction of a 1:2 Fe(II)/Fe(III) solution, under alkaline conditions at 80 °C under N₂ (Regazzoni et al., 1981).

5. Reaction at 85 °C of Fe(II) ammonium solution (buffered to pH 7 to 8 with sodium acetate) with hydroxylamine sulfate; the suspension is held under N₂ gas (Ardizzone and Formaro, 1983), as shown thus

   (3)
   $$3{\text{Fe}}^{2+}+{\text{NH}}_3{\text{OH}}^{+}+3{\text{H}}_2\text{O}\to {\text{Fe}}_3{\text{O}}_4+{\text{NH}}_4^{+}+6{\text{H}}^{+}$$

6. Reductive transformation in a sealed ampoule of an akaganeite suspension in the presence of hydrazine at pH 9.5 to 11.5 and 100 °C (Blesa and Maroto, 1986).

   (4)
   $$12\mathrm{\beta }-\text{FeOOH}+{\text{N}}_2{\text{H}}_2\to 4{\text{Fe}}_3{\text{O}}_4+8{\text{H}}_2\text{O}+{\text{N}}_2$$

7. Decomposition of an alkaline (0.2–0.4 MOH) solution of Fe(III) NTA at 217 °C in an autoclave (Booy and Swaddle, 1978).

8. Heating of iron hydroxide acetate at 200–260 °C under N₂ (Pinheiro et al., 1987).

9. Boiling a mixture of Fe(II) sulfate and bispyridoxylidene hydrazine phthalazine for 10 min at pH 7 (Sarel et al., 1989).

10. Thermal decomposition of Fe(II) sulfide in air at 500 °C, which is environmentally unfriendly (Robl, 1958).

    (5)
    $$3{\text{FeS}}_2+5{\text{O}}_2\to {\text{Fe3O}}_4+3\text{S}+3{\text{SO}}_2$$

11. Holding a solution of Fe(III) acetylacetonate in 1-propanol under N₂ in an autoclave at 300 °C for several hours (Kominami et al., 1999).

12. Reduction of nitrobenzene to aniline produces Fe₃O₄ (Laux, 1925) as shown thus:

Further, several Fe₃O₄ synthetic methods were patented (Michael et al., 2004; Shen, 2002; Yoshito et al., 1984; Tomio and Horoyuki, 1999; Shouheng, 2002; Maurice et al., 1978; Naoyoshi and Kenzo, 2000).

The most popular methods for the synthesis of the spinel structured Fe₃O₄ include co-precipitation in aqueous solution by adding hydroxides into iron salt solutions (Hui et al., 2008; Fried et al., 2001) and the thermal decomposition of iron organometallic compounds in high boiling organic solutions in the presence of stabilizers (Sun and Zeng, 2002; Xu et al., 2010).

Recently, Xu et al. (2010) reported on the organic phase synthesis of mono dispersed iron oxide nano crystals using iron chloride as a precursor.

In this paper, a report was made on an innovative invention (Khalil, 2012) that relates to a process of preparing magnetite nanocrystals by the co-precipitation method using iron(III) chloride or nitrate salts as precursors. The objective of this invention was to provide a process of preparing magnetite (Fe₃O₄) and derivatives thereof, not reported herein, which overcomes the drawbacks of the prior art, especially a process which only requires one iron compound as the starting precursor, a limited number of additional chemical reagents, and a process which can be carried out under simple reaction conditions, preferably at room temperature, with easy work-up of the product obtained.

2 Experimental

3 Results and discussion

Instead of using two iron precursors, that is Fe(III) and Fe(II) mixed in a 2:1 mol ratio for the preparation of magnetite, one can start with only an aqueous iron(III) salt solution and reduce it by potassium iodide to maintain the appropriate mole ratio according to Eq. (7):

Thereafter, filtering out the iodine precipitate and then hydrolyzing the filtrate by 25% ammonium or sodium hydroxide solution at pH 9–11 were used to obtain black magnetite nanoparticles. The percentage yield of magnetite and iodine was 99% and 86%, respectively.

This innovative method for the preparation of magnetite is economical and green, achieving a high selectivity and atom economy percents (Lancaster, 2002). Eqs. (8) and (9) imply that both magnetite and iodine are desired products.

The FTIR spectrum of the as-synthesized magnetite from anhydrous FeCl₃ (Fig. 1) exhibited vibrational bands at 634 sh., 582 s, 397 m cm⁻¹ characteristic of magnetite (Cornwell and Schwertmann, 2003; Zheng et al., 2011; Liese, 1967). The intense band at 582 cm⁻¹ could be assigned to the IR active T_1u mode corresponding to the vibration of the Fe²⁺–O^2- functional group. The splitting-up peak at 634 cm⁻¹ is due to the symmetry degeneration on octahedral B sites (Zheng et al., 2011). Rowan and Patterson (Rowan and Patterson, 2009) have reported that for monoclinic unit cell at low temperature, the IR modes in the Fe²⁺–O²⁻ stretching vibrational band would be induced as obvious split pairs for fields along the x and y directions.

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Figure 1

FTIR spectrum of the as-synthesized magnetite.

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The EDS intensity peaks (Fig. 2) show the exact iron percent in Fe₃O₄. The Cu signals were from the Cu grid. No other signal was detected within the detection limits of EDS which confirms the purity of the Fe₃O₄ nanoparticles.

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Figure 2

EDS pattern of the as-synthesized magnetite.

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The crystallinity of the magnetite sample was investigated by XRD as shown in Fig. 3. The results show that the sample has six peaks at 2θ of 30.36°, 35.74°, 43.52°, 53.95°, 57.34° and 63.0° representing the corresponding indices of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) respectively Cornwell and Schwertmann, 2003; Xu et al., 2010.

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Figure 3

XRD pattern of the as-synthesized magnetite.

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The crystal system is cubic with Z value of 8.0, a = b = c = 8.3199 Å, α = β = y = 90° belonging to space group Fd-3m.

A typical scanning electron microscopic (SEM) image of the as-synthesized iron oxide nanoparticles is shown in Fig. 4. It is evident from the picture that the particles are in a nano scale.

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Figure 4

SEM image of the as-synthesized magnetite.

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TEM image (Fig. 5) of the synthesized magnetite indicates the formation of nanocubes and nanorods composite.

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Figure 5

TEM image of the as-synthesized magnetite.

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The selected area electron diffraction (SAED) pattern of the as-synthesized iron oxide is shown in Fig. 6(a). The high resolution TEM image of a single iron oxide cube (Fig. 6b) shows lattice fringes with an interfringe distance of 0.292 nm, which is close to the interplane distance (2 2 0) of the planes in the cubic spinel structured iron oxide (Xu et al., 2010).

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Figure 6

HRTEM image of a single magnetite cube; (a) SAED pattern and (b) Lattice fringes.

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Information of the mean size and standard deviation (SD) was calculated from measurements of the nano cubes and nanorods in random fields of view and presented in Fig. 7. The mean ± SD of the nanocubes was 7.8 ± 0.05 nm while the mean ± SD of the nanorods was 6.3 ± 0.2 nm in diameter and 46.2 ± 0.9 nm in length.

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Figure 7

Mean size and standard deviation of: (a) Fe₃O₄ nano cubes; (b and c) Fe₃O₄ nano rod.

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Liquid nitrogen temperature (77 K) and room temperature (300 K) ⁵⁷Fe Mössbauer spectra of one magnetite-loaded PVA dry hydrogel sample are presented in Fig. 8(a) and (b). The 77 K recorded spectrum, Fig. 8(a), is similar to that of an assembly of 8–10-nm noninteracting magnetite particles published by Morup et al. (1980) and Szabó et al. (2000). The RT ⁵⁷Fe Mössbauer spectrum consists of two sextets with isomer shifts of 0.15 mm/s and a hyperfine field of 48.6T for Fe³⁺ in the A sites and 0.56 mm/s and a hyperfine field of 45.7T for the Fe^2.5+ average valence on the B sites (Dzési et al., 2008). A quadrupole splitting of 0.00 and −0.01 mm/s is indicative of cubic symmetry. The absence of a paramagnetic component indicates that the transition to paramagnet occurs above 300 K.

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Figure 8

Mössbauer spectra of a disk-shaped dried ferrogel at 77 K (a) and 300 K (b).

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Detailed ⁵⁷Fe Mössbauer effect study will be conducted and published elsewhere.