Hydrothermal phase transformation of hematite to magnetite
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NANO EXPRESS
Open Access
Hydrothermal phase transformation of hematite to magnetite Jie-feng Lu and Cho-Jen Tsai*
Abstract 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. Keywords: Iron oxides; Hydrothermal; Phase transformation
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 [7-11], 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 Fe2+ and Fe3+ in the octahedral sites [15]. Many researches have demonstrated the capability of using chemical syntheses to control particle morphologies of iron oxide by surfactants [16-18]. Morphologies like wires [19], rods [20], tubes [21], rings [22], disks [23], cubes [24], spheres [25], hexagonal plates of α-Fe2O3 [26,27], and polyhedral particles of Fe3O4 [28,29] have been synthesized successfully.
* Correspondence: [email protected] Department of Material Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan
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 [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-Fe2O3 hexagonal plates to octahedral Fe3O4 particles trigge
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