Thermally induced solid-state reaction of Fe2(SO4)3 with NaCl or KCl: a route to β-Fe2O3 synthesis

The ferric oxides are a group of interesting and extensively studied materials. The ferric oxides exhibit a structural polymorphism which is responsible for their various remarkable properties. Not including the amorphous ferric oxide, four different ferric oxide crystalline polymorphs, α, β, γ and ε, were reported up to date (Cornell and Schwertmann 2003; Machala et al. 2011). The most common of the polymorphs is α-Fe2O3 (hematite). It founds its use in numerous applications, e.g., pigment production, (Mohapatra and Anand 2011) catalysis (Gregor et al. 2010; Ashraf et al. 2020), gas sensing (Li et al. 2018) or electrochemistry (Sivula et al. 2011). The spinel-structured γ-Fe2O3 (maghemite) is utilized for its superb ferrimagnetic properties, e.g., in biomedicine (Estelrich and Busquets 2018; Dadfar et al. 2019) or data storage applications (Mohapatra and Anand 2011; Ajinkya et al. 2020). The orthorhombic ε-Fe2O3 is investigated for its magnetic properties, specifically for its large coercive field (up to Hc ≈ 2 T at room temperature) (Tuček et al. 2010). The ε-Fe2O3 magnetic thin films are of great interest for its usage as a potential high-density storage media (Tokoro et al. 2020). Lastly, β-Fe2O3 crystallizes in cubic structure (a = 9.39 Å, Ia3 space group) and it is the only ferric oxide polymorph that exhibits a paramagnetic behavior at room temperature (Danno et al. 2013). Recently, it has been studied as a promising candidate in applications including optoelectronics (Lee et al. 2008a, b), anodes for lithium batteries (Carraro et al. 2012) and lately photocatalysis, i.e., H2 evolution (Zhang et al. 2017; Li et al. 2021) and pollutant degradation (Zhang et al. 2019; Fragoso et al. 2022). Additionally, the β-Fe2O3 was also found to be a suitable precursor for the preparation of the interesting χ-Fe5C2 phase (Hagg carbide), which is an active catalyst in Fischer-Tropsch synthesis (Malina et al. 2017). While both α-Fe2O3 and γ-Fe2O3 occur naturally, the other two polymorphs, β-Fe2O3 and ε-Fe2O3 are generally synthesized in laboratory and are considered rare. So far, only a few methods have been reported for a successful preparation of these rare polymorphs in pure forms. For example, a pure ε-Fe2O3 phase was achieved for the first time in 2004 using a combined reverse micelle and sol-gel method (Jin et al. 2004). Preparation of pure polymorphs is usually hindered by (i) polymorphous transitions induced by high temperature or pressure that can occur during the synthesis or (ii) simultaneous formation of two different polymorphs (Machala et al. 2011).

It is known that β-Fe2O3, γ-Fe2O3 and ε-Fe2O3 undergo a polymorphous transition to α-Fe2O3 at elevated temperatures. Owing to its thermal stability, α-Fe2O3 is traditionally the end product of most thermal processes that involve iron oxides (Zboril et al. 2002). The exact temperature of these polymorphous transitions generally depends on various factors such as the particle size, doping, reaction atmosphere, type of precursor, etc. (Machala et al. 2011). Polymorphous transitions including all four polymorphs γ → ε → β → α were observed in the case where the particle growth was restricted by physical barriers, e.g., by a confinement of a silica matrix (Sakurai et al. 2009). Recently, Zhang et. al. observed γ → β transition at 600 °C in the case of their electrophoretically deposited thin layers of γ-Fe2O3 (Zhang et al. 2022). The opposite β → γ transition was observed by Lee et al. in the case of their hollow β-Fe2O3 nanoparticles (Lee et al. 2008a, b). Although β-Fe2O3 was found to be more thermodynamically stable than ε-Fe2O3, the direct transition ε → β remains a question of debate. According to Machala et al. (2011), the coexistence of both phases that was reported in the past (Sakurai et al. 2009) might have been the product of simultaneous formation. The simultaneous formation of α-Fe2O3 and γ-Fe2O3 was reported, for example, in the case of the thermal decomposition of FeC2O4·2H2O, while the simultaneous formation of α-Fe2O3 and β-Fe2O3 was observed during the thermal decomposition Fe4(Fe(CN)6)3 (Hermanek and Zboril 2008; Machala et al. 2013). The ratio of the polymorphs was found to be dependent on the precursor thickness during calcination and particle size, respectively. For more detailed information about polymorphous transitions of ferric oxide the readers are referred to a review by Machala et al. (2011).

Concerning β-Fe2O3, several synthesis methods were reported for the preparation of either pure or high-fraction β-Fe2O3, including chemical vapor hydrolysis (Bonnevie Svendsen 1958; Kumar and Singhal 2007; 2009), condensation (CVC) (Lee et al. 2004; Lee et al 2008a, b), spray pyrolysis (Li et al. 2021), hydrothermal route, (Rahman et al. 2011) microwave-assisted solvothermal route (Ramya and Mahadevan 2012) and solid-state reaction involving the thermal decomposition of a suitable iron salt, e.g. β-Fe2O3 was reported among the products of thermally induced decomposition of iron(III) hexacyanoferrate(II) microcrystals (20-50 μm) or iron(III) sulfate (Machala et al. 2013; Zboril et al. 1999a, b, 2003). However, most of these reaction routes provided β-Fe2O3 in mixture with other ferric oxide polymorphs, e.g. α-Fe2O3.

Another method for the preparation of pure β-Fe2O3 is the solid-state reaction that was first reported by Ikeda et al. (1986) and comprises a calcination of Fe2(SO4)3 and NaCl mixture with subsequent isolation of ferric particles by decantation. According to Ikeda et al. (1986), the Fe2(SO4)3 + NaCl reaction starts with the formation of NaFe(SO4)2 double sulfate. The β-Fe2O3 particles are formed at 500 °C following the equation:

Later, Zboril et al. (1999a, b), who also studied the Fe2(SO4)3 + NaCl solid-state reaction, suggested that β-Fe2O3 formation follows 2 different reaction pathways; (i) between 400‒440 °C β-Fe2O3 is formed directly through the decomposition of Fe2(SO4)3:

and (ii) above 440 °C β-Fe2O3 is also formed by the decomposition of the two double sulfates NaFe(SO4)2 and Na3Fe(SO4)3. According to Zboril et al. (1999a, b), a secondary reaction, i.e., β-Fe2O3 → α-Fe2O3 polymorphous transition, accompanied the primary reaction from its onset at 400 °C and was responsible for the presence of α-Fe2O3 fraction in their samples. More recently, Danno et al. (2010; 2013) reported the preparation of pure β-Fe2O3 particles via solid-state reaction. Contrary to both Ikeda et al. (1986) and Zboril et al. (1999a, b), they calcined a mixture of NaCl and NaFe(SO4)2. Pure β-Fe2O3 was prepared regardless of the reactants ratio already at 350 °C, which further indicated the NaFe(SO4)2 important role in the process.

Although the calcination of Fe2(SO4)3 + NaCl for the preparation of β-Fe2O3 is known, to the best of our knowledge, the exact role of the alkali salt has not been fully explained. In this article we further investigate the role of the alkali ion addition on the preparation of β-Fe2O3 by comparing the solid-state reaction of two different mixtures: Fe2(SO4)3 + NaCl and Fe2(SO4)3 + KCl in the 1:3 ratio at different temperatures. The phase composition and purity of the prepared β-Fe2O3 powder were verified by transmission Mössbauer spectroscopy (TMS) and powder X-ray diffraction (XRD). The reactions processes were also studied by an in situ XRD. We believe the presented study brings further insight into the formation of the rare β-Fe2O3 polymorph as well as the solid-state reactions in general.