Hydrothermal phase transformation of hematite to magnetite
© Lu and Tsai; licensee Springer. 2014
Received: 7 March 2014
Accepted: 13 April 2014
Published: 13 May 2014
Different phases of iron oxide were obtained by hydrothermal treatment of ferric solution at 200°C with the addition of either KOH, ethylenediamine (EDA), or KOH and EDA into the reaction system. As usually observed, the α-Fe2O3 hexagonal plates and hexagonal bipyramids were obtained for reaction with KOH and EDA, respectively. When both KOH and EDA were added into the reaction system, we observed an interesting phase transformation from α-Fe2O3 to Fe3O4 at low-temperature hydrothermal conditions. The phase transformation involves the formation of α-Fe2O3 hexagonal plates, the dissolution of the α-Fe2O3 hexagonal plates, the reduction of Fe3+ to Fe2+, and the nucleation and growth of new Fe3O4 polyhedral particles.
The more stable phases in iron oxides are hematite and magnetite. Hematite can be used in a lot of applications, such as sensors , water photooxidation , drug delivery , lithium ion battery , pigmentation , solar cell , etc., and magnetite can be utilized in biomedicine [7–11], magnetic devices , etc. Therefore, studies about the nano/microstructures of iron oxides and their properties, which are related to the intrinsic structure and crystal shapes, have been intensively engaged, especially for hematite and magnetite. The bandgap of hematite is 2.0 to 2.2 eV which makes it useful in applications that involve visible light absorption [13, 14]. Magnetite has unique electric and magnetic properties because its intrinsic crystal structure allows electrons to be transferred between Fe2+ and Fe3+ in the octahedral sites .
Many researches have demonstrated the capability of using chemical syntheses to control particle morphologies of iron oxide by surfactants [16–18]. Morphologies like wires , rods , tubes , rings , disks , cubes , spheres , hexagonal plates of α-Fe2O3 [26, 27], and polyhedral particles of Fe3O4 [28, 29] have been synthesized successfully.
Several robust methods have been developed for phase transformation of iron oxides. α-Fe2O3 can be transformed to Fe3O4 at high temperature under a reducing ambient, such as hydrogen ambient [30, 31]. Yanagisawa and Yamasaki also showed that by controlling the mineralizer solutions, temperatures, and partial pressures of hydrogen in a hydrothermal system, phase transformation from α-Fe2O3 to Fe3O4 particles can be achieved . The result indicated that high temperature and high pressure of hydrogen can accelerate the reduction reaction.
Phase transition of iron oxides can also take place by hydrothermal reaction with a reducing agent [33, 34]. Sapieszko and Matijewic had observed a similar phase transformation from α-Fe2O3 hexagonal plates to octahedral Fe3O4 particles triggered by the addition of hydrazine which is used as an antioxidant  during hydrothermal process.
In this experiment, we explore the role of ethylenediamine (EDA or en in ligand form) on the phases of iron oxide in hydrothermal condition. EDA is usually considered to be the chelating agent or to function as a ligand to facilitate the growth of particles under hydrothermal reaction [36, 37]. However, phase transformation of iron oxide was observed when EDA was added into the alkaline solution. Thus, a special low-temperature route for the transformation of α-Fe2O3 to Fe3O4 was provided. The phase and shape variations with the addition of potassium hydroxide (KOH), EDA, and KOH and EDA were investigated and compared.
Ferric nitrate (Fe(NO3)3 · 9H2O), 1 mmol, was dissolved in 10 ml of distilled water to form a transparent yellow solution. Next, three different mineralizing agents were added into the ferric solution. First is 5 ml of 10.67 M KOH aqueous solution. The solution was added dropwisely into the ferric solution. Second is 1 ml of EDA. The EDA was added gradually into the ferric solution. Third is the combination of KOH and EDA. The 10.67 M KOH solution, 5 ml, was added first followed by the addition of 1 ml of EDA. After adding these mineralizing agents, a brown Fe(OH)3 suspension was obtained. Then, these solutions were all stirred for 30 min before transferring the mixture into a Teflon-lined stainless steel autoclave (DuPont, Wilmington, DE, USA) of 40-ml capacity and followed by heat treatments at 200°C for 9 h. After that, the autoclave was cooled down to room temperature in air. The precipitates were collected by centrifugation, washed with deionized water and ethanol several times to remove organic and impurities, and finally dried in air at 80°C for 12 h.
The as-synthesized powder was characterized by X-ray diffraction (XRD) with Cu-Kα radiation, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The magnetic properties were measured by a vibrating sample magnetometer (VSM) with a maximum magnetic field of 1.5 kOe.
Results and discussion
The α-Fe2O3 hexagonal plates have an average size of about 10 μm in edge length and about 500 nm in thickness. The average lateral size of the α-Fe2O3 particles with the shape of a hexagonal bipyramid is about 120 nm. The Fe3O4 polyhedral particles with mainly octahedral shape have an average lateral size in the range of 5 to 25 μm. The particles obtained from the reaction system with the addition of KOH and EDA alone have the same phase but different shapes. One would assume that the reaction system with the addition of both KOH and EDA would produce particles with maybe different shapes but still maintain the phase of α-Fe2O3. However, the results show that the particles that we obtained have a different phase, Fe3O4, and, surely, a different shape.
The result implied that phase transformation evolved in four steps: (1) the reaction systems rapidly transformed Fe(OH)3 or FeOOH to α-Fe2O3 hexagonal plates under the hydrothermal conditions, (2) the α-Fe2O3 hexagonal plates dissolved gradually, (3) the reduction process causes valence transition of Fe3+ to Fe2+, and (4) the Fe3O4 particles started to nucleate and then finally grew to form polyhedral particles.
A similar in situ reduction capability of EDA in neutral and basic solutions for the reduction of uranium from U6+ to U4+ has been reported by Jouffret et al. . In our study, the phase transition process should be similar. The EDA maintains stable and chelates with Fe3+ ions that were released by α-Fe2O3 hexagonal plates upon dissolving, and the reduction of Fe3+ ions to Fe2+ ions occurred.
α-Fe2O3 nano/microhexagonal plates can be successfully reduced to octahedral Fe3O4 particles with EDA in an alkaline solution under a low-temperature hydrothermal process. In general, the transformation consists of four stages: (1) the formation of α-Fe2O3 hexagonal plates triggered by KOH, (2) the dissolution of the α-Fe2O3 hexagonal plates, (3) the reduction of Fe3+ to Fe2+, and (4) the nucleation and growth of new Fe3O4 polyhedral particles. The Avrami equation can be used to describe the transformation kinetics. As the phase transformation proceeded, the magnetic properties of the sample gradually transformed from weak ferromagnetic behaviors to strong ferromagnetic behaviors.
JFL is a Ph.D. student at National Tsing Hua University. CJT holds a professor position at National Tsing Hua University.
The authors acknowledge the support from the National Science Council through grant no. 101-2221-E-007-061-MY2.
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