Hydrothermal phase transformation of hematite to magnetite

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.


Background
The more stable phases in iron oxides are hematite and magnetite. Hematite can be used in a lot of applications, such as sensors [1], water photooxidation [2], drug delivery [3], lithium ion battery [4], pigmentation [5], solar cell [6], etc., and magnetite can be utilized in biomedicine [7][8][9][10][11], magnetic devices [12], 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 Fe 2+ and Fe 3+ in the octahedral sites [15].
Several robust methods have been developed for phase transformation of iron oxides. α-Fe 2 O 3 can be transformed to Fe 3 O 4 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-Fe 2 O 3 to Fe 3 O 4 particles can be achieved [32]. 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-Fe 2 O 3 hexagonal plates to octahedral Fe 3 O 4 particles triggered by the addition of hydrazine which is used as an antioxidant [35] 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-Fe 2 O 3 to Fe 3 O 4 was provided. The phase and shape variations with the addition of potassium hydroxide (KOH), EDA, and KOH and EDA were investigated and compared.

Methods
Ferric nitrate (Fe(NO 3 ) 3 · 9H 2 O), 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.    Figure 1a shows the α-Fe 2 O 3 hexagonal plates which were obtained with the addition of KOH, and Figure 1b shows the α-Fe 2 O 3 hexagonal bipyramid particles obtained when EDA was added into the system. Figure 1c shows the Fe 3 O 4 polyhedral particles obtained with the addition of both KOH and EDA into the reaction system. (When NaOH substitutes for KOH, a similar reaction would occur.) The crystal structure of these iron oxide particles was analyzed by XRD and is shown in Figure 1d. The phase can be identified to be α-Fe 2 O 3 when either KOH or EDA alone was added to the reaction system despite different morphologies. The diffraction peaks match the JCPDS card no. 33 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-Fe 2 O 3 . However, the results show that the particles that we obtained have a different phase, Fe 3 O 4 , and, surely, a different shape.
The transmission electron microscopy images and the corresponding selected area electron diffraction (SAED) patterns of iron oxide particles are shown in Figure 3. The diffraction patterns of the particles confirmed the results of the XRD diffractions. In Figure 3b, the zone axis of the hexagonal plate is [0001] and the six directions normal to the edge are < 2 1 À 1 À 0 > and its other five equivalent directions. In Figure 3d, the hexagonal bipyramid shows that the pyramid is pointed in the direction of <0001>. According to the literatures, the bipyramidal structure was enclosed by 10 1 À 1 f g crystal planes [41]. In Figure 3f  shows that, after 2 h of growth, the main phase of the particles is α-Fe 2 O 3 hexagonal plates. The edge of the hexagonal plate is not as straight as that obtained for the reaction system with KOH only. As the reaction time increased to 5 h, as shown in Figure 4b, small octahedron particles were observed and the original hexagonal plate started to dissolve and no longer maintained the hexagonal shape. As the reaction time continued to increase to 7 h, more polyhedron particles were observed with larger sizes and only a small amount of plate-like particles still existed, as shown in Figure 4c. At the reaction time of 9 h, the observed particles are mainly polyhedron ones, as shown in Figure 4d. The first observation in this sequence of experiment is that KOH can rapidly transform iron hydroxides to hematite. The second observed phenomenon is that the α-Fe 2 O 3 hexagonal plates were dissolved to become irregular plates during the transformation process.
The result implied that phase transformation evolved in four steps: (1) the reaction systems rapidly transformed Fe   To further understand the role of NO 3 − ions on the phase transition process, the precursor of FeNO 3 was substituted by FeCl 3 with the same hydrothermal conditions. Two cases were investigated, one with the addition of KOH only and the other with the addition of both KOH and EDA under the same hydrothermal condition of 200°C for 9 h. Figure 5a shows that the α-Fe 2 O 3 hexagonal plates were obtained when the reaction system consists of FeCl 3 and KOH, while the phase transformation from α-Fe 2 O 3 hexagonal plates to Fe 3 O 4 polyhedral particles still occurred when the reaction system consists of FeCl 3 , KOH, and EDA, as shown in Figure 5b. The shape of the polyhedral particles is more irregular in this case. The XRD patterns, shown in Figure 4c, confirmed the related phases. Notice that the α-Fe 2 O 3 plates were not completely reduced to Fe 3 O 4 particles. Thus, NO 3 − ions are not directly involved in the reduction process of Fe 3+ to Fe 2+ . However, the transformation process is faster with the presence of NO 3 − ions in the reaction system than that of Cl − ions.
We further explore the role that NO 3 − ions play on the phase transition. The pre-synthesized α-Fe 2 O 3 hexagonal plates of 9 mg were added to the same KOH and EDA medium as above but with different amounts of HNO 3 and heated to 200°C for 7 h. As shown in Figure 6, the results show that the phase transition rates were slow when the solution contained large and small amounts of HNO 3 ; the optimal amount of HNO 3 for phase transition is 0.19 ml. The slow phase transition rate observed for small amount of HNO 3 may be attributed to the limiting dissolution of α-Fe 2 O 3 which produced Fe 3+ ion in the solution for further reduction to Fe 2+ . Thus, the rate of phase transformation is slow. At large amount of HNO 3 ,

the NO 3
− ions can be the oxidant in the reaction [29] and the pH value of the reaction system is changed toward a less basic solution. Hence, the reduction process can be again suppressed. Thus, there is a proper amount of HNO 3 that induces the maximum rate for phase transformation.
A similar in situ reduction capability of EDA in neutral and basic solutions for the reduction of uranium from U 6+ to U 4+ has been reported by Jouffret et al. [42]. In our study, the phase transition process should be similar. The EDA maintains stable and chelates with Fe 3+ ions that were released by α-Fe 2 O 3 hexagonal plates upon dissolving, and the reduction of Fe 3+ ions to Fe 2+ ions occurred. Figure 7 shows the curve of transformed fraction of magnetite (α) as a function of reaction time. The fraction of α-Fe 2 O 3 and Fe 3 O 4 was determined by XRD measurement in conjunction with the Rietveld method. By using the Avrami equation, α = 1 − exp(−kt n ), where k is the reaction constant, t is the reaction time, and n is the exponent of reaction, we can fit, relatively well, the experiment data of the magnetite fraction obtained by hydrothermal treatment at 200°C for different times. The value of n is about 4 obtained in this case. From this curve, we can further investigate the kinetic behavior of phase transformation in the reaction condition in the future.
The magnetic properties of iron oxide particles followed the phase transition process from α-Fe 2 O 3 hexagonal plates to Fe 3 O 4 polyhedral particles, as shown in Figure 8. After a short reaction time of 2 h, the α-Fe 2 O 3 hexagonal plates show weak ferromagnetic behaviors with a coercive force of 90 Oe at room temperature and the saturation magnetization is yet to reach the maximum in the range of the applied magnetic field, as shown in Figure 7a. Prolonging the reaction time to 5~7 h, the fraction of Fe 3 O 4 polyhedral particles as well as the particle size of Fe 3 O 4 increases gradually. As shown in Figure 7b,c, the values of saturation magnetization increase to 55 and 66 emu/g and the coercive forces decrease to 6.5 and 5.4 Oe for the reaction time of 5 and 7 h, respectively. Finally, the phase transition was completed at the reaction time of 9 h. The Fe 3 O 4 polyhedral particles show strong ferromagnetic behaviors with the highest saturation magnetization of 80 emu/g and the lowest coercive force of 5 Oe, as shown in Figure 7d. The magnetic properties of α-Fe 2 O 3 hexagonal plates and Fe 3 O 4 polyhedral particles are similar to the previous reports [27,43].
Conclusions α-Fe 2 O 3 nano/microhexagonal plates can be successfully reduced to octahedral Fe 3 O 4 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-Fe 2 O 3 hexagonal plates triggered by KOH, (2) the dissolution of the α-Fe 2 O 3 hexagonal plates, (3) the reduction of Fe 3+ to Fe 2+ , and (4) the nucleation and growth of new Fe 3 O 4 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.