Introduction

The phase diagram of iron sulfide is complicated and each phase has different magnetic characteristics [1]. Most of the stable iron sulfides are sulfides of Fe(II), while very few iron sulfides with trivalent iron Fe(III) have been reported. Boehm et al. reported the X-ray diffraction (XRD) pattern of Fe2S3; however, the product was heavily contaminated and Mössbauer parameters were not reported [2]. Stiller et al. reported Mössbauer spectra for amorphous Fe2S3 measured at 78 and 4.2 K [3]. Thus, XRD patterns and Mössbauer spectra of well-defined Fe2S3 particles have yet to be reported. The polyol process, which uses a polyhydric alcohol, is a useful method to synthesize particles in solution, where the polyhydric alcohol acts as both a stabilizer and reducing agent [4]. Recently, spinel type γ-Fe2S3 nanoparticles have been synthesized by the polyol process [5]. In this study, iron sulfide particles are synthesized using a polyol method with 1-octadecanethiol (OT) employed as the sulfur source and Mössbauer spectra and XRD patterns are measured.

Experimental

A mixture of ferrocene (2 mmol), oleylamine (30 mL), 1,2-hexadecanediol (HD), and OT was introduced into a three-necked flask. The amounts of ferrocene and oleylamine were kept constant and the amounts of HD and OT were varied (Table 1). The mixture was stirred for 1 h at room temperature and then refluxed for 2 h at 320 °C under an Ar flow. Precipitated particles were washed three times with ethanol and hexane, followed by centrifugation after air-cooling. The resultant particles were investigated using Mössbauer spectroscopy (57Co/Rh source), XRD (Rigaku, RINT2500, Cu Kα), and scanning electron microscopy (SEM; Jeol, JSM7001F).

Table 1 HD/OT ratios for the production of iron sulfide particles and room-temperature Mössbauer parameters for the corresponding products
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Results and discussion

Figure 1 shows selected SEM images of iron sulfide particles (S1, S4, and S7). The particles produced without HD (S1) were hexagonal plate shaped with sizes of 2–3 μm. In contrast, the particles produced with HD were 100–200 nm in size (S4 and S7) and had various rough shapes, rather than the unique plate-like shape. The shape and size of the particles was significantly changed by the presence of HD, whereas the particle size was only slightly changed with varying the amount of OT in the range of 0.5–16 mmol (S2–S6).

Fig. 1
figure 1

SEM images of iron sulfide particles

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Figure 2 shows Mössbauer spectra of the particles (S1–S7) measured at room temperature and the Mössbauer parameters are summarized in Table 1. The Mössbauer spectrum of the S1 particles had three sets of sextets and a small intensity doublet, and was thus assigned to Fe1−x S. The assignment was confirmed by XRD of the S1 sample, as described later. The peak intensity ratio of the sextet was 3:3.4:1:1:3.4:3 due to the shape anisotropy, similar to Mössbauer spectra reported in the literature [6].

Fig. 2
figure 2

Room-temperature Mössbauer spectra for iron sulfide particles produced with various HD/OT ratios. The mixture ratios and the Mössbauer parameters were summarized in Table 1

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The Mössbauer spectra of samples S2–S4 had very similar features. The spectra had a magnetic component and two sets of doublet. The magnetic component was fitted by three sets of sextets, the parameters of which were similar to those of Fe1−x S in S1. Low-spin iron(II) sulfide FeS2 (pyrite, marcasite) has been reported with similar Mössbauer parameters [7], but the XRD patterns of these particles did not correspond with the pattern reported for FeS2 (pyrite: PDF #42-1340, marcasite: PDF #37-0475). The two sets of doublets had small δ values, which may be due to the presence of trivalent iron Fe(III). The intensity ratio of the two sets of doublets in the spectra was always approximately 2:1 and the Mössbauer parameters of the doublets had similar values to those reported for Fe2S3 [3], although the ΔE q value of one of the doublets was smaller than that reported in the literature.

The Mössbauer spectra of the particles produced in solutions containing less than 2 mmol of OT (S5–S7) had a sextet that was assigned to α-Fe, in addition to the two sets of doublets indicative of Fe(III) sulfides. Therefore, it was presumed that the sulfurization reaction did not progress sufficiently fast compared with formation of α-Fe which remains unreacted further. The S5 and S6 particles contained FeS, which was indicated by the presence of divalent Fe(II) species with small ΔE q.

Particle S7 was produced with a very low concentration of OT. The Mössbauer spectrum of S7 had two sets of sextets, which were assigned to Fe3O4 (magnetite). In addition, a component with a broad absorption due to magnetic relaxation and a doublet with a large half-width were observed. The broad absorption may be due to the superparamagnetism of small γ-Fe2O3 (maghemite) particles.

The Mössbauer spectrum of S3 measured at 6 K gave a sextet of Fe1−x S (δ = 0.88 mm/s, 2ε = −0.25 mm/s, H = 314 kOe) and one of the doublets of Fe2S3 (B; δ = 0.53 mm/s, ΔE q = 0.72 mm/s) had similar Mössbauer parameters as those observed at room temperature (Fig. 2), except for the effects of a secondary Doppler shift. However, the other doublet of Fe2S3 (A) showed superparamagnetic relaxation at 6 K (δ = 0.49 mm/s, H = 188 kOe, relaxation time τ = 4.01 × 10−8 s). Fe2S3 has been reported to have a sextet with 253 kOe at 4 K [3]; however, the particle size was too small for the sextet absorption to be observed in the present experiment. The results were similar to those previously reported [3], which indicates an antiferromagnetic character at low temperatures with two distinct iron sites. However, one site showed no magnetic splitting, even at low temperatures, which may be due to the small particle size.

Figure 3 shows XRD patterns for the samples. Assignments of Fe1−x S, α-Fe, and Fe3O4 were made based on the ICDD-PDF database (Fe1−x S: PDF #29-0724, α-Fe: PDF #87-0721, Fe3O4: PDF #19-0629). The relative yields of Fe1−x S, α-Fe, and Fe3O4 were in good agreement with the Mössbauer spectra. Besides these well assigned XRD peaks, unassigned XRD peaks were observed for the S2–S6 particles. The peaks marked ‘X’ in Fig. 3 may correspond to Fe2S3 with a long-range periodic structure. The XRD patterns obtained here are different from those patterns of heavily contaminated samples reported in the literature [2].

Fig. 3
figure 3

XRD patterns for iron sulfide particles obtained by reaction with various HD/OT ratios. Open circles Fe1−x S (PDF #29-0724), filled squares α-Fe (PDF #87-0721), open triangles Fe3O4 (PDF #19-0629), times unidentified peaks

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The Fe2S3 obtained in this work showed two sets of doublets in the room-temperature Mössbauer spectra, and one of the doublets showed broad absorption caused by relaxation at low-temperature. This may be due to the small crystallite size of Fe2S3 in the S3 particles. If the crystallite size is sufficiently large, then the absorption should be a sextet because of the long relaxation time. The S3 particles were annealed for 48 h at 100 °C to enlarge the crystallite size. The annealed particles were then measured using XRD and Mössbauer spectroscopy at room temperature (Fig. 4). The XRD pattern indicated Fe3S4 (greigite); therefore, the S3 particles consisted of Fe1−x S and Fe2S3 that were changed to Fe3S4 by annealing. The Mössbauer spectrum had two sets of doublets (δ = 0.60 mm/s, 2ε = 0.67 mm/s and δ = 0.34 mm/s, ΔE q = 0.67 mm/s) and sextets (δ = 0.60 mm/s, 2ε = 0.01 mm/s, H = 306 kOe and δ = 0.30 mm/s, 2ε = 0.00 mm/s, H = 307 kOe). The sextets were assigned to Fe3S4 and the doublets were interpreted as superparamagnetism of small Fe3S4 particles. The two sets of doublets may due to the spinel structure of Fe3S4. Similar results have been reported, where Fe2S3 changed to Fe3S4 when heated in a vacuum at 150 °C [2], which is consistent with the present results. Thus, the newly found Fe2S3 is stable only when the crystallite size is small.

Fig. 4
figure 4

XRD pattern (left) and room-temperature Mössbauer spectrum (right) of iron sulfide particles heated at 100 °C for 48 h. The XRD pattern for Fe3S4 (JCPDS Card No. 16-0713) is shown for comparison

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Conclusion

Iron sulfide particles were prepared by the polyol method. Hexagonal plate-like Fe1−x S particles of 2–3 μm in size were obtained without HD. In contrast, 100–200 nm sized Fe2S3 particles were produced in the presence of HD. Although the newly found Fe2S3 particles had long-range periodic structure, as indicated by sharp XRD peaks, the Mössbauer spectra of Fe2S3 showed superparamagnetic nature due to its small crystallite size. The Fe2S3 particles were changed to Fe3S4 (greigite) by heating at 100 °C. The polyol method with ferrocene and HD was demonstrated as a practical method for the production of iron sulfide particles, where the Fe2S3 phase is stabilized only in nanoparticles.