Elsevier

Journal of Solid State Chemistry

Volume 217, September 2014, Pages 50-56
Journal of Solid State Chemistry

Investigations on Bi25FeO40 powders synthesized by hydrothermal and combustion-like processes

https://doi.org/10.1016/j.jssc.2014.05.006Get rights and content

Highlights

  • Two simple syntheses routes for stoichiometric Bi25FeO40 powders using starch as polymerization agent.

  • Monitoring the phase evolution and crystallite growth kinetics during the syntheses.

  • Determination of the optical band gap and melting point.

  • Investigations of the magnetic behaviour of Bi25FeO40 powders.

  • Influence of amorphous iron oxide and a non-stoichiometric Bi/Fe ratio on the magnetic behaviour.

Abstract

The syntheses of phase-pure and stoichiometric iron sillenite (Bi25FeO40) powders by a hydrothermal (at ambient pressure) and a combustion-like process are described. Phase-pure samples were obtained in the hydrothermal reaction at 100 °C (1), whereas the combustion-like process leads to pure Bi25FeO40 after calcination at 750 °C for 2 h (2a). The activation energy of the crystallite growth process of hydrothermally synthesized Bi25FeO40 was calculated as 48(9) kJ mol−1. The peritectic point was determined as 797(1) °C. The optical band gaps of the samples are between 2.70(7) eV and 2.81(6) eV. Temperature and field-depending magnetization measurements (5−300 K) show a paramagnetic behaviour with a Curie constant of 55.66×10−6 m3 K mol−1 for sample 1 and C=57.82×10−6 m3 K mol−1 for sample 2a resulting in magnetic moments of µmag=5.95(8) µB mol−1 and µmag=6.07(4) µB mol−1. The influence of amorphous iron-oxide as a result of non-stoichiometric Bi/Fe ratios in hydrothermal syntheses on the magnetic behaviour was additionally investigated.

Graphical abstract

Bi25FeO40 powders were prepared by a hydrothermal method and a combustion process. The optical band gaps and the peritectic point were determined. The magnetic behaviour was investigated depending on the synthesis and the initial Bi/Fe ratios. The influence of amorphous iron-oxide on the magnetic properties was examined.

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Introduction

Sillenite materials are of great importance because of their photorefractive, piezoelectric, electrooptic, and photoconductive properties [1], [2], [3]. Bi25FeO40 is a sillenite-type material which crystallizes in the cubic space group I23 with cell parameter of about 1.018 nm [4], [5]. In the literature two possible chemical formulas Bi25FeO40 and Bi25FeO39 can be found [6], [7]. Craig and Stephenson[4] have shown that one Bi ion per unit cell exists in the oxidation state +5 resulting in the formula Bi24III(BiVFeIII)O40. Investigations by Devalette et al. [8], Soubeyroux et al. [9], and Wang et al. [10] also confirmed the formation of Bi5+. In the sillenite structure of Bi25FeO40 the Bi3+ ions occupy the octahedral positions forming a cage of corner-connected BiO5E-polyhedrons (E=inert 6 s2 electron pair), whereas Bi5+ and Fe3+ ions are sharing the tetrahedral positions within the cage [4], [7]. On the other hand studies by Radaev et al. [11], [12] suggest Bi3+ in the tetrahedral position accompanied by oxygen vacancies. In this paper we use the formula Bi25FeO40.

The magnetic properties of Bi25FeO40 samples prepared by soft-chemistry methods have been only rarely investigated and deviating results have been reported. Cheng et al. [13] found a paramagnetic-like behaviour at room temperature, whereas a superparamagnetic behaviour was also reported by other research groups [5], [14], [15], [16]. In contrast a clear hysteresis loop at room temperature was also found pointing to a ferro- or ferrimagnetic behaviour [17]. In addition the values of the specific magnetization at room temperature vary significantly [5], [13], [14], [15], [17]. Moreover, a spin-glass freezing behaviour [17] as well as a possible ferromagnetic transition at low temperatures [5] were also reported. In contrast, magnetic measurements between 5 and 950 K on Bi25FeO40 powders prepared by the conventional mixed-oxide method show a paramagnetic behaviour [18], [19], [20]

Bi25FeO40 shows photocatalytic activity under ultraviolet and visible light irradiation in the degradation of methyl orange, methyl violet, and pentachlorophenol [15], [21], [22], [23]. Sun et al. [24], [13] used Bi25FeO40-graphene and Bi25FeO40-carbon-nitride composite materials for the degradation of methylene blue. It was reported that hydrothermally prepared Bi25FeO40 samples can be recycled by magnetic separation [15], [16], [24]. Borowiec et al. [25] found that single crystals of iron sillenite are strongly photochromic. Furthermore, Bi25FeO40 is a typical byproduct in the synthesis of BiFeO3 [26].

In addition to the classical solid state synthesis [7], various hydrothermal syntheses were performed to obtain Bi25FeO40 powders. The reported hydrothermal processes were mostly carried out with non-stoichiometric initial Bi/Fe ratios of 1, despite of the Bi/Fe ratio of 25 according to the chemical formula of iron sillenite [5], [13], [14], [15], [17], [21], [27], [28]. A solvothermal synthesis by Dai and Yin [23] with an initial ratio of Bi/Fe=24 led to Bi25FeO40 with small amounts of Bi2O3.

In this paper we present both a simple one-pot hydrothermal and a combustion-like synthesis of phase-pure Bi25FeO40 with an initial Bi/Fe ratio of 25. The hydrothermal process was carried out in aquous NaOH solution at 100 °C. For the combustion-like reaction starch was used as gellant and fuel, because starch is an eco-friendly and cheap abundant biopolymer. Phase evolution and the crystallite growth kinetics were monitored by XRD and the magnetic behaviour and the optical band gap were studied. Moreover, the influence of a non-stoichiometric initial Bi/Fe ratio in the hydrothermal process on the magnetic properties was investigated.

Section snippets

Bi25FeO40-Hydrothermal method at ambient pressure (sample 1)

The reaction was carried out in a PFA (perfluoroalkoxycoplomyer) round bottom flask in argon atmosphere to avoid the formation of carbonates and the leaching out of silicon from a glass flask. Bi(NO3)3·5H2O (0.012 mol, Alfa Aesar) and Fe(NO3)3·9H2O (0.00048 mol, Merck) were dissolved in 10 ml water and 1 ml HNO3 (65%). 70 ml of a 10 M NaOH was heated to about 80 °C and the (Bi25Fe) solution was added slowly. The resulted solution was refluxed for 6 h at ambient pressure under vigorous stirring.

Hydrothermal process

Fig. 1a shows the XRD pattern of the yellow powder 1 synthesized by the hydrothermal method. The pattern shows only reflections of Bi25FeO40 [35] and no secondary phases were observed. The volume-weighted average crystallite size (size of a coherent scattering domain) was calculated as 75 nm. SEM images (Fig. 2a) reveal an agglomerated nature of powder 1. Agglomerates up to 30 µm with a tetrahedron-like shape can be observed, which consist of smaller particles between 0.2 µm and 2 µm. Thermal

Conclusion

Stoichiometric Bi25FeO40 powders were synthesized both by a simple hydrothermal synthesis at 100 °C and by a combustion-like reaction using starch as a complexing agent and gellant. The hydrothermal synthesis leads to a yellow sample (powder 1) showing only reflections of Bi25FeO40. The colour of the powder changes to permanent green–grey upon irradiation with sunlight at room temperature or heating to at least 400 °C. The combustion-like process leads to phase-pure Bi25FeO40 after calcining the

Acknowledgments

Financial support by the German Science Foundation within the Collaborative Research Centre (SFB 762) Functionality of Oxide Interfaces is gratefully acknowledged.

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