Greigite (Fe3S4) particles, with strong ferrimagnetic behavior, have been found to have desirable uses in the areas of biomedical and environmental applications. Size-dependent magnetic properties of greigite can play a crucial role in efficiency of its applications. This study reviews two synthetic approaches to producing such particles. The methods tested within this study include a coprecipitation synthesis and a hydrothermal process. While the coprecipitation method proved to be ineffective at producing greigite, the hydrothermal process showed promise after consistently producing greigite. For the hydrothermal process, the effects of synthesis time, autoclave atmosphere, and polyvinylpyrrolidone (PVP) addition as a capping agent to control particle growth were investigated using X-ray diffractometry (XRD) with Rietveld refinement, vibrating sample magnetometry (VSM), and scanning electron microscopy (SEM). The results show that, while pyrite (FeS2) forms as an impurity phase, increasing the synthesis time up to 18 hours increases the greigite fraction up to 70 wt% and saturation magnetization up to ∼ 35 emu/g for the powder synthesized in argon. The SEM micrographs of this sample reveal a mean greigite particle size of ∼ 700 nm. It was found that adding PVP brings about a much more uniform microstructure of agglomerated plate-shape particles with nano-sized features.
Greigite (Fe3S4), first reported by Skinner et al.,1 is an iron sulfide mineral, analogous to magnetite, with an inverse spinel crystal structure that generally occurs naturally due to the actions of sulfate-reducing bacteria. Greigite is a strong ferrimagnet, and pure greigite can have a saturation magnetization of 59 emu/g.2 As a nanoparticle, this iron sulfide has been shown to have properties favorable for various applications, including gas sensing,3 battery anodes,4–6 cancer hypothermia,7 and radionuclide sorbants.8 However, to our knowledge, greigite is not readily available commercially as nanoparticles. Therefore, an effective method of synthesizing greigite nanoparticles with controlled properties is desirable.
In the area of cancer research, the use of magnetic nanoparticles in a method of hyperthermia cancer treatment is theorized to be a successful way to kill or weaken tumor cells.9 This method involves injecting magnetic nanoparticles directly into a tumor and heating them via an alternating magnetic field. Furthermore, greigite is a viable candidate for this hyperthermia cancer treatment due to its magnetic properties and ability to be synthesized as a nanoparticle.7
Environmentally, greigite is insoluble in most solvents, and has been found to have heavy metal adsorption capabilities, which would prove to be useful in the capacity to immobilize hazardous or radioactive materials via a magnetic separation.8,10,11 Specifically, in nuclear waste treatment, this application would be helpful in the immobilization of technetium-99 (99Tc) because greigite can reduce Tc (VII) in TcO4- to Tc (IV) forming TcO2, which can be sequestered via a sulfate reduction.12 99Tc is a long-lived beta emitter with a half-life of 213,000 years and it is commonly found as the pertechnetate anion (TcO4-), which is highly soluble and environmentally mobile. TcO4- often contaminates ground water at nuclear waste reprocessing sites and is capable of entering the food chain.13 Considering these properties and tendencies, 99Tc must be considered a key radionuclide determining the long-term environmental impact of the nuclear fuel cycle. Due to the difficulty of removing Tc from the supernatant that constitutes low activity waste (LAW) some solutions have been proposed that involve capturing Tc in ion exchange media14 or incorporating the Tc as part of a LAW cement waste form. The former solution puts off the disposal of Tc to another entity, and the latter has been shown to result in leached Tc from cement pore water.15,16 Given the lack of a robust separation and immobilization procedure, a method of using greigite particles to adsorb, magnetically remove and ultimately dispose of Tc from the LAW has been proposed.
With regard to magnetic separation, nanoparticles offer a larger surface area to volume ratio, therefore, theoretically maximizing the amount of Tc that can be adsorbed by greigite. Also, when the diameter of magnetic particles decreases past a critical size the particles can lose their coercivity and become superparamagnetic. This superparamagnetic property is desirable for both hyperthermia, due to the maximum energy absorption by biological tissue,17 and magnetic separations, because it would prevent particles from aggregating after a magnetic field is removed.18 The critical room temperature superparamagnetic diameter for greigite spheres was computed from the first (effective) anisotropy constant (2.1-3.1×104 Jm-3 for greigite19), and the usual equations for critical blocking volume,20 to be between 15 and 20 nm, depending on assumptions.
The methods analyzed in this work involved coprecipitation and hydrothermal processes. The coprecipitation synthesis was adapted from Chang, et al.7 The hydrothermal process was a modified method from Zhang, et al21 by adding a capping agent. The role of a capping agent is to coat particles as a surfactant in order to prevent the excessive growth and aggregation of the particles.22 Polyvinylpyrrilidone (PVP) was chosen in this work as the capping agent after considering a wet chemistry synthesis of greigite from Lyubutin et al.23 PVP is often used in hydrothermal syntheses,24,25 and is compatible with syntheses that use ethylene glycol (EG) as a reducing agent,26 which is a key reagent used by Zhang et al.21 in their hydrothermal synthesis.
The purpose of this study is to determine an effective method of producing greigite particles. The work reported herein is preliminary and is not yet fully optimized. These findings are meant to aid in the improvement of techniques to synthesize greigite nanoparticles.
II. MATERIALS AND METHODS
All chemicals used within the hydrothermal synthesis were used as received without any further purification.
A. Coprecipitation synthesis
Within an Ar glove tent in a 200 mL beaker with 100 mL of DI water (18.2 mΩ, 25°C), FeSO4·7H2O (ACROS Organics, 99+%, 1.3893 g) was added and stirred until dissolved, resulting in a clear and colorless solution. To this solution, Na2S·9H2O (Sigma-Aldrich, >98%, 1.2023 g) was added and stirred until a black solution was obtained with fine dark solute. This was stirred for 5 minutes. Acetic acid (MilliporeSigma, glacial grade, >99.7%) was added dropwise to obtain various pH levels between 3-5. The solution was poured into 15 mL flasks and was centrifuged for 5 minutes at 5000 rpm, resulting in a fine black powder, which was dried using a watch glass and a drying oven at 80°C.
B. Hydrothermal synthesis
FeCl3·6H2O (Sigma-Aldrich, >98%, 1.0812 g) and thiourea (Alfa Aesar, >99%, 0.609 g) were dissolved in mixed solvents of 40 mL ethylene glycol (EG, VWR, reagent grade, >99%) + 20 mL H2O. The solution was poured into an autoclave (200 mL Teflon-lined stainless-steel autoclave, Parr) and placed in an electrical oven at 180°C for variable time (6, 12, 18, or 24 hours). The black powder formed was filtered and washed several times using water and ethanol and transported to a vacuum oven to dry at 60°C for 2-3 hours. This synthesis was later altered to include letting the solution sit under Ar within the glove tent before pouring into the autoclave to prevent the formation of magnetite (Fe3O4).
The above procedure was later used with the addition of PVP (Sigma Aldrich, MW=55000, 2.5 g) during the dissolution of the reagents in EG and H2O. The PVP-assisted hydrothermal procedure was done for 18 hours. The resulting solution was jet-black and black powders were obtained by filtering and centrifuging the solution. Ethanol was added to the solution which was then sonicated and centrifuged at 15000 rpm for about 5 minutes.
X-ray diffraction (XRD, PANalytical X’pert Pro) scans were performed from 5-90° 2θ with a step size of 0.5°, a time per step of 10 seconds and with Co Kα radiation (λ = 0.1789 nm) at 40 kV and 40 mA. Five scans were sequentially performed and summed to produce the final XRD pattern. Phase quantification on the crystalline fraction was performed by Rietveld refinement using HighScore Plus software (PANalytical, Netherlands). A vibrating sample magnetometer (VSM, PMC3900, Lakeshore Cryotronics, Westerville, OH) with maximum applied field of 18 kOe was used to determine room temperature magnetic properties of the samples. A scanning electron microscope (SEM, FEI Sirion 200) was employed to capture images for particle size and morphology analyses.
III. RESULTS AND DISCUSSION
Most of the coprecipitation trials at various pH levels rendered a black precipitate that quickly became brown/orange after being produced. One successful synthesis was accomplished at pH 3.5, producing ∼0.5 mg of black precipitate. XRD (not shown here), conducted on the particles in solution, revealed weak peaks of mackinawite (Fe1-xS), but no other phase. It is possible that the precipitate was largely x-ray amorphous. This method was therefore ineffective in synthesizing greigite and did not produce viable material for XRD and VSM testing.
XRD results of the hydrothermally-synthesized samples (Figure 1–a) showed that greigite, pyrite, and magnetite are the main crystalline phases within most samples. However, in the sample synthesized under Ar (18 hr- Ar), magnetite was successfully eliminated. Figure 1–b presents the fraction (wt %) of the phases obtained from Rietveld refinement analysis for various hydrothermal reaction times. The results show that the fraction of formed greigite increases with increasing reaction time. As seen in Figure 1–b, the fraction of magnetite is similar regardless of reaction time; in the case of the Ar synthesis, no magnetite is observed. Also, as the fraction of greigite increases with reaction time, that of pyrite decreases. This may be an indication that pyrite is a precursor to greigite in this synthesis method. Interestingly, this is the reverse of that reported by Hunger and Benning,27 who suggested a mackinawite→greigite→pyrite in hydrothermal synthesis, at least in the presence of excess dissolved sulfur. Furthermore, in Figure 1–b, as the reaction time increases, a saturation in the production of greigite occurs, which suggests 18 hours to be an effective reaction time as the relative fraction of greigite produced by the 24-hour sample is slightly smaller.
Figure 2 shows the room temperature magnetic hysteresis loops of the samples synthesized at various reaction times, and Table I lists the hysteresis parameters obtained from the loops. The saturation magnetization (MS) increased with the reaction time, which directly correlated with the increase in greigite fraction within the samples. The saturation magnetization of the 18-hour Ar sample was lower than the sample made without Ar, because it does not contain magnetite, which had a saturation magnetization of 92 emu/g,2 larger than that of greigite. Note that pyrite is a paramagnetic phase with a very small specific magnetization compared to that of greigite or magnetite. While the samples prepared in air atmosphere showed similar coercivities (HC), the 18-hour Ar sample revealed an increased coercivity. This is likely due to particle size effects on coercivity and/or the absence of magnetite phase in the Ar sample compared to the other ones. The SEM images (Figure 3) revealed an estimated mean particle size of 700 nm for all hydrothermally-synthesized samples (i.e., Figure 3 a–e). Nonetheless, at least two distinct particle morphologies were observed in the SEM micrographs during the microstructure investigations. Since all present phases have cubic structures, it is unclear whether the octahedral morphology relates to pyrite, greigite, or magnetite. However, based on the particles’ abundances in the microstructure, and comparing that to the corresponding XRD and Rietveld results, the faceted particles were likely pyrite whereas the small plate-like agglomerates are likely greigite. Figure 3–e clearly showed the two morphologies. Several other studies27–29 have reported similar morphologies for griegite and/or pyrite particles. Szczepanik and Sawlowicz30 discussed morphologies for pyrite, greigite, and mackinawite (Fe1+xS, tetragonal) minerals in biogenic remains. In their study, pyrite particles grew larger idiomorphic euhedra, while greigite forms smaller particles within framboids.
|Sample .||HC (Oe) .||MSa (emu/g) .||Mr (emu/g) .|
|18 hr - Ar||400.0||35.1||14.8|
|Sample .||HC (Oe) .||MSa (emu/g) .||Mr (emu/g) .|
|18 hr - Ar||400.0||35.1||14.8|
MS is the maximum magnetization at 1.8 T applied field.
The hydrothermal synthesis method proved to be an effective way to produce greigite. The PVP-Assisted synthesis was adapted from this method in an attempt to produce greigite particles with controlled size in an effective manner. However, the powder synthesized with PVP for 18 hours of reaction time yielded, according to XRD (not shown here), pyrite as the only crystalline phase albeit with a large amorphous XRD background. No greigite characteristic peaks were identified. On the other hand, the hysteresis loop of this sample (not shown here) indicated a ferri-(or ferro-) magnetic behavior with increased coercivity, which could not be from the pyrite (paramagnetic) phase. Therefore, there was likely other magnetic phase(s) not detected by XRD. These magnetic phases were likely greigite and/or magnetite with very small particle size. The SEM image of the sample synthesized by PVP-assisted method (Figure 3–f) displayed a uniform microstructure of plate-like agglomerates. The nano-sized plates formed within uniformly-shaped agglomerates with sizes between about 2 to 5 µm. This morphology was similar to some of the features observed in the previous hydrothermally synthesized samples. We were unable to produce sub-100 nm unagglomerated particles, though previous reports have succeeded in producing particles as small as 50 nm by hydrothermal methods,31 ∼10 nm by a polyol process using PVP capping23 or a trinuclear complex with thiourea,32 or even <5 nm under certain conditions with stabilizing agents.33
A coprecipitation synthesis and a hydrothermal process with and without PVP as a capping agent were tested within this study to determine their relative ability in producing greigite particles. The coprecipitation synthesis proved to be an ineffective method because the particles obtained degraded shortly after being produced. This degradation rendered it non-viable for XRD and VSM results. The hydrothermal process showed consistent production of stable particles. The results confirmed the presence of a large fraction of greigite with an average particle diameter of 700 nm. A new method involving the same hydrothermal process with the addition of PVP as a capping agent was also performed. This method was successful in producing very uniform microstructure of nano-sized plates, though it failed to produce a measurable concentration of greigite.
This research is being performed using funding received from the DOE Offices of Nuclear Energy and Environmental Management through the Nuclear Energy University Program under the award DE-NE0008597. The authors would like to thank Sam Karcher for his help with SEM imaging.