Iron Sulfide (FeS) Nanotubes Using Sulfurization of Hematite

Letter pubs.acs.org/NanoLett

Iron Sulfide (FeS) Nanotubes Using Sulfurization of Hematite Nanowires Dustin R. Cummins,† Harry B. Russell,† Jacek B. Jasinski,† Madhu Menon,‡ and Mahendra K. Sunkara*,† †

Department of Chemical Engineering and Conn Center for Renewable Energy Research, University of Louisville, Louisville, Kentucky 40292, United States ‡ Center for Computational Sciences, University of Kentucky, Lexington, Kentucky 40506-0045, United States S Supporting Information *

ABSTRACT: We report the phase transformation of hematite (αFe2O3) single crystal nanowires to crystalline FeS nanotubes using sulfurization with H2S gas at relatively low temperatures. Characterization indicates that phase pure hexagonal FeS nanotubes were formed. Time-series sulfurization experiments suggest epitaxial growth of FeS as a shell layer on hematite. This is the first report of hollow, crystalline FeS nanotubes with NiAs structure and also on the Kirkendall effect in solid−gas reactions with nanowires involving sulfurization. KEYWORDS: FeS, troilite, nanotubes, Kirkendall effect, optical properties, sulfurization, hematite nanowire arrays

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sulfur atmosphere.14 These wires appear to be both phase pure and single crystalline, but their growth mechanism is not welldescribed and understood. Morrish et al. attempted sulfurization of large diameter iron oxide “nanorods” using H2S plasma, but it appears from the very limited characterization, that only a polycrystalline film of agglomerated particles, composed predominantly of marcasite, was formed.15 In this paper, we investigated the synthesis of pure phase, iron sulfide nanowires by sulfurization of hematite nanowire arrays. The synthesis of highly oriented, single crystal hematite nanowire arrays by rapid atmospheric plasma oxidation of an iron foil is well-understood and easily accessible.16 Thermodynamic analysis suggests that it is possible to form many phases of iron sulfide, such as pyrite and marcasite, at low reaction temperatures (300 °C). Therefore, it is not clear whether phase pure iron sulfide formation is possible through the sulfurization reaction. In addition, earlier studies with nitridation of metal oxide nanowires to form nitride nanowires have suggested that it is possible to create single crystal nanowire to single crystal nanowire transformations.17 In the case of sulfurization, the atomic sulfur species is larger than the atomic nitrogen species and, therefore, is expected to form only core−shell structures, due to diffusion limits.18 So, a systematic study is conducted to determine the effect of nanowire diameter on the sulfurization reaction and the mechanistic details using time-series sulfurization experiments.

ynthesis of transition metal compounds, particularly dichalcogenides, has been researched extensively due to the interesting properties of these sulfide compounds, especially for use in photoelectrochemistry and photovoltaics. Iron sulfide, particularly pyrite phase FeS2, is of considerable interest in solar applications1−3 and lithium batteries4 due to its low cost, favorable band gap (0.95 eV), and high light absorption (6 × 105 cm2/mol). Due to iron sulfide’s complicated phase diagram, there are a multitude of possible stoichiometries and crystal structures for iron sulfides, dependent upon temperature, pressure, and sulfur concentrations.5 As such, phase pure, single crystal pyrite nanowires have not been reported until recently. Many groups have attempted synthesis of pyrite FeS2 thin films and particles using a variety of techniques, including MOCVD,6,7 sulfurization of iron8 and iron oxide films, sputtering, spray pyrolysis,3 hydrothermal,9 solvothermal,10,11 surfactant-assisted hot injection,12 and so forth. While some have shown the synthesis of single crystal, pyrite nanoparticles, the majority of thin films and particles are polycrystalline and show phase impurities. The most common impurities are orthorhombic FeS2 (marcasite) and FeS (troilite), which have band gaps of 0.34 and 0.04 eV, respectively. Even trace amounts of these impurities will degrade the performance of pyrite. The first reported iron sulfide nanowires synthesized in pyrite phase were in 2008 by Wan et al. This synthesis route involved the use of an aluminum oxide template and the sulfurization of electro-deposited Fe.13 These nanowires were not thoroughly characterized, however, and no evidence of phase purity and crystallinity was provided. In 2012, Jin et al. grew pyrite phase nanowires directly from an iron foil in a © XXXX American Chemical Society

Received: January 25, 2013 Revised: April 27, 2013

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Iron oxide nanowire arrays are prepared on iron foil substrates by exposing the iron to an atmospheric pressure oxygen plasma flame. The plasma heats the iron to temperatures between 580 and 740 °C for 2−15 min, yielding vertically oriented, single crystal hematite nanowire arrays of varying diameters (∼5−300 nm), lengths (∼0.5−2 μm), and nanowire coverage densities.16,19 Hematite nanowire arrays were reacted at 300 °C in a 15 Torr H2S atmosphere for 2 h. Under this higher sulfur condition, the hematite nanowires were completely converted to iron sulfide nanostructures. Using transmission electron microscopy (TEM), it was found that the nanostructures formed were hollow iron sulfide nanotubes with preferential orientation, which will be discussed further. These nanotubes had diameters in the 100−300 nm range, wall thicknesses of ∼60 nm, and an average length of 3 μm. Additional experiments were performed at 200 and 250 °C with 15 Torr H2S atmosphere for varying time lengths to further understand the mechanism and kinetics of the hematite to iron sulfide transformation on the nanoscale. The morphology of the iron sulfide nanowires was characterized using the FEI Nova 600 NanoLab scanning electron microscope (SEM). A Bruker D8 powder X-ray diffraction (XRD) system was used for crystal phase analysis. High-resolution transmission electron microscopy (HR-TEM), nanoprobe electron diffraction (nano-ED), energy dispersive Xray spectroscopy (EDS), and scanning transmission electron microscopy (STEM) measurements were performed using FEI Tecnai F20 TEM. The optical band gap was determined by collecting diffuse reflectance spectra using the Perkin-Elmer Lambda 950 UV−vis spectrometer. Rapid atmospheric plasma oxidation of iron foils yielded densely packed, vertically oriented iron oxide nanowire arrays, without the use of catalyst or template. An SEM image of a typical hematite nanowire array is shown in Figure 1A, along with an HR-TEM of a typical hematite nanowire, showing single crystallinity and phase purity. These wires are almost completely free of extended defects such as dislocations, stacking faults, and so forth, which could contribute to their facile transformation to iron sulfide. These hematite nanowire arrays were placed in a vacuum chamber and heated to 300 °C for 2 h in a 15 Torr H2S atmosphere. After the reaction, the nanowires retain their morphology, as shown in the SEM image in Figure 1B. HR-TEM and EDS analysis show that the resulting iron sulfide nanostructures are hollow one-dimensional structures of crystalline iron sulfide (nanotubes). EDS shows that these nanotubes were completely converted from hematite to iron sulfide, with no oxygen remaining in the wire. Bright field (BF) TEM imaging clearly shows a hollow iron sulfide nanotube (Figure 2A). It also indicates, as further supported by electron diffraction, that the shell is single crystalline radially, but there are low angle grain boundaries along the length of the wire, with elongated grains in the wire direction with sizes in the range of 100 nm. Figure 2B shows a HR-TEM image of one of such grains with three sets of crystallographic planes clearly resolved (as indicated in the image). These planes represent two sets of d-spacing, namely, of 2.97 Å and 2.65 Å, which corresponds to the (100) and (101) planes of hexagonal FeS, respectively. Due to the polycrystalline nature of the iron sulfide shell, nano probe electron diffraction was used to obtain easily interpretable diffraction data. An example of three nanodiffraction patterns obtained from the same grain at three

Figure 1. (A) SEM image of iron oxide, hematite, nanowire array with inset showing HR-TEM of single crystal hematite nanowire. (B) SEM image of iron sulfide nanowire array after sulfurization reaction, showing that nanomorphology was maintained.

different zone axes are shown in Figure 2C, D, and E. These patterns were obtained from the grain shown in HR-TEM image in Figure 2B and the pattern in Figure 2C was collected at the [1−21−3] zone axis; that is, at the same zone as the HRTEM image. The diffraction patterns can be consistently indexed between these multiple zone axes using the crystallographic parameters for hexagonal FeS with the NiAs structure (PDF 00-001-1247) and lattice parameters a = 3.43 Å and c = 5.68 Å. The experimentally measured d-spacings obtained from nanoprobe electron diffraction patterns agree within less than 0.5% with the d-spacings of this hexagonal FeS phase, which is a definitive evidence for the identification. B

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Figure 2. (A) Bright-field TEM image of the iron sulfide nanotube, synthesized at 300 °C for 2 h. (B) HRTEM of iron sulfide crystal, showing dspacings of 2.97 Å and 2.65 Å. These correspond to the [1−21−3] zone axis for hexagonal FeS. Electron nanoprobe diffraction patterns from multiple zone axes (C) [1−21−3], (D) [01−10], and (E) [01−11] all confirm the FeS hexagonal phase.

coefficient (α) was calculated (Figure 4A). This shows an average absorption on the order of 105 cm2 mol−1. When the absorbance data is represented in a Tauc plot (Figure 4B); it shows an indirect band gap of ∼0.92 eV. The most common report of band gap for bulk FeS troilite is 0.04 eV, but the optical properties of the NiAs structure phase FeS have not been adequately reported. One group reported a direct band gap for troilite FeS of 2.07 eV and claim that this drastic band gap increase resulted from “smaller particle size (∼8 nm)” compared to the bulk.20 This is a poorly supported conclusion and does not seem reliable. Despite this claim, theoretical calculations have been done on the NiAs structure phase FeS and show a significant increase in band gap during the phase transition from hexagonal troilite to hexagonal NiAs structure FeS; it is reported as a p-type semiconductor21 with a direct band gap comparable to that of Fe7S8 (0.8 eV)22 and FeS2 pyrite (0.95 eV). Due to the lack of reliable data, first principle calculations were done for NiAs structure FeS (a = 3.43 Å, c = 5.34 Å), and the results are shown in Figure 5. For the theoretical study of FeS systems we used first-principles density functional theory (DFT) in the generalized gradient approximation (GGA) and the Perdew−Burke−Ernzerhof (PBE)23 augmented by including Hubbard-U corrections (GGA+U formalism)24 based on Dudarev’s approach25 as implemented in the Vienna Ab initio Simulation Package (VASP).26 These first principle calculations show a direct band gap of ∼0.6 eV and high absorption in the visible region. This theoretical band gap calculation matches closely to the experimentally measured band gap of 0.92 eV and high visible absorption. It is interesting that the sulfurization reaction yielded hollow tubes with a crystalline wall, rather than a core−shell morphology. To better understand the mechanism of the phase transformation, the hematite nanowires were reacted

XRD analysis of the iron sulfide nanowire arrays confirms the formation of FeS with the NiAs structure, Figure 3A. Since the nanowire array was grown on a thick iron foil, even after sulfurization, a thick layer composed of hematite (Fe2O3) and magnetite (Fe3O4) remains on the substrate. The measured XRD peak intensities indicate a preferential orientation compared to the intensities expected for a polycrystalline film with randomly oriented grains. The ratio of intensities between the (100) and (102) planes for FeS (PDF 00-001-1247) should be 0.33 for a random distribution, but the nanowires show a ratio ∼1.2, which indicates that a large volume fraction of the FeS shell shows oriented growth with the (100) planes parallel to the substrate. This preferential orientation was further investigated using HR-TEM and electron diffraction (Figure 3B and C) on a nanowire with a thin iron sulfide shell. Moiré fringes with ∼1.5 nm spacing normal to the nanowire direction were observed in many areas where the core and the FeS shell overlap with each other in HRTEM images, as is the case shown in Figure 3B. In addition, SAED patterns obtained from such areas showed that the (100) planes of FeS were parallel to the (210) planes of hematite core. Using the d-spacing values for these two sets of planes yields the estimated spacing of a translation Moiré pattern very close to that measured from HRTEM images. Therefore, the HRTEM and SAED clearly show an epitaxial relationship between the hematite core (210) and FeS (100) shell. As the reaction progresses, the strain accumulated due to large lattice mismatch leads to defect formation (dislocations, stacking faults, twinning, etc.), which is the most likely cause of the low angle grain boundaries along the axial direction in the FeS shell. UV−visible diffuse reflectance spectroscopy was performed on the iron sulfide nanowire array to observe optical properties. The diffuse reflectance data were converted to absorbance data using Kubelka−Munk transformation, and the absorption C

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Figure 3. (A) XRD of iron sulfide nanowire arrays. (B) HRTEM of thin FeS shell on hematite core showing epitaxial growth. (C) Electron diffraction of thin FeS shell on hematite core showing epitaxial relationships between hematite (210) and FeS (100).

under the same conditions, but for shorter time periods (30 min and 1 h). Under these shorter reaction times, the formation of an iron sulfide shell on an oxide core is observed. STEM images, along with EDS elemental line scans show the progression of the sulfurization reaction (Figure 6). There is a clear correlation between reaction time and sulfurization of the iron oxide wire, which is to be expected. After 30 min, the sulfurization reaction only proceeds an average of 40 nm, while the core of the nanowire remains iron oxide. After 1 h, the sulfide thickness increases to an average of 50−60 nm, and accumulation of voids at the center of the wire, as well as at the sulfur reaction interface, can be observed. After 2 h, all of the iron oxide is reacted, leaving a crystalline iron sulfide shell. The diffusion rates and mechanisms in bulk, natural iron sulfide are well-researched, due to the importance of pyrite in

geological and mining applications. Condit et al. extensively measured the self-diffusion of atomic iron in substoichiometric iron sulfides using iron isotopes as a radiotracer. At 300 °C, an average diffusion coefficient for Fe in Fe1−xSx, which takes into account differing crystal planes, was measured on the order of 9 × 10−12 cm2 s−1.27 In those analyses, the diffusion of sulfur into iron sulfide was assumed to be negligible, since the diffusion rate is significantly lower than iron. Recently, sulfur isotopes were used as a radiotracer in natural pyrite.28 The temperature dependence of the self-diffusion coefficient of sulfur species in pyrite was calculated, experimentally, to follow an Arrhenius form of: DS = 1.75 × 10−14 exp( −132 100/RT )

where DS has units of m2 s−1, R is the ideal gas constant (8.314 J mol−1 K−1), and T is the absolute temperature. Extrapolating D

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Figure 4. Absorption measurement and band gap determination by UV−visible diffuse reflectance spectroscopy. (A) Absorption coefficient (α) of pyrite nanowire array, showing absorption on the order of 105 cm2 mol−1. (B) Tauc plot using diffuse reflectance data converted using Kubelka− Munk transformation. It shows an indirect band gap of 0.92 eV. The raw reflectance data was smoothed using adjacent-averaging.

Figure 5. First-principles calculations for FeS with the NiAs structure. The left panel shows the optimized structure obtained using first principles calculations. The top right panel shows the optical spectra for the two polarization directions, while the bottom right panels show band structure and partial density of states for the optimized structure, respectively.

the data, the self-diffusion coefficient of sulfur in pyrite is 1.6 × 10−22 cm2 s−1 at 300 °C, which is 10 orders of magnitude lower than the self-diffusion of iron. This large difference in diffusion rates leads to the hollowing of the nanowire during the sulfurization reaction per the Kirkendall effect. The Kirkendall effect, first described in 1947, involves nonequal diffusion of two species, with different diffusion rates, at an interface. The faster diffusing component will quickly diffuse out, toward the interface, resulting in a balancing inward diffusion of vacancies,

which agglomerate into voids or are annihilated at dislocations or grain boundaries.29 By observing the differences in reaction progression of sulfurization of hematite nanowires at varying temperatures and times, an experimental determination of the activation energy for diffusion can be calculated. Figure 7 shows an Arrhenius relationship, leading to an experimental activation energy of 59 ± 7 kJ/mol. This calculated value corresponds well with measured activation energies for iron diffusion through vacancies in iron sulfides.27,30 This further confirms E

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Figure 6. STEM images and corresponding EDS spectra showing reaction progression of sulfurization in time at 300 °C.

Figure 7. Experimental determination of diffusion activation energy in sulfurization reaction. This leads to an activation energy for diffusion of 59 ± 7 kJ mol−1, which corresponds to the diffusion of iron via vacancies.

reaction of H2S with Fe2O3, there are four diffusion processes, a schematic of which can be seen in Figure 8. At the nanowire surface, the H2S dissociates to form H2 and S. These hydrogen molecules and sulfur atoms are diffusing into the nanowire, while the iron and oxygen are diffusing out toward the surface. The hydrogen reacts with the iron oxide, carrying the oxygen out of the nanowire as H2O, leaving a reduced Fe2+. The Fe2+ diffuses out faster than S diffuses in, which causes a diffusion of vacancies opposite to the Fe diffusion. As the Fe diffuses to the surface, these vacancies diffuse inward, accumulating at the very center of the hematite nanowire and at the oxide/sulfide interface to form voids. These voids continue to accumulate to form a hollow tube structure, with the iron sulfide forming the shell. When observing the STEM image, Figure 6, of the nanowire at different points of synthesis, it can be clearly seen that voids are accumulating at the center of the nanowire, but

that iron diffusion is the dominant process in phase transformation of iron oxide to iron sulfide. An early report on the exploitation of Kirkendall effect for the formation of nanocrystals was by Alivisatos et al. in 2004. They utilized a nanocrystalline dispersion of cobalt in a solvent and exposed it to reactants in solution, forming hollow, crystalline nanospheres of CoO, Co3S4, and CoSe.31 Many works have applied this hollowing effect to form many metal oxide nanostructures, using metallic nanowires and nanoparticles as a starting material. This includes NiO nanotubes,32 MgO nanotubes,33 Al2O3 and Cu2O nanospheres,34 and many others. The Kirkendall effect has been observed in the transformation of metal nanowires to metal oxide tubes. This effect can be used to understand the phase transformation of metal oxide nanowires to metal sulfide nanotubes. In the case of the F

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Figure 8. Schematic showing sulfurization reaction and Kirkendall effect.

reported. By comparing experiments with different reaction times and temperatures, a mechanism has been proposed involving the epitaxial formation of an iron sulfide shell on the surface of the hematite and subsequent decomposition of the hematite core due to faster diffusion rate of iron in the lattice compared to sulfur, a form of the Kirkendall effect. HR-TEM and electron nanodiffraction confirm that hexagonal (NiAs structure) FeS is formed with lattice parameter a = 3.43 Å and c = 5.68 Å. UV−visible diffuse reflectance spectroscopy showed an optical band gap of 0.92 eV, which appears to be the first report of the optical band gap of (NiAs) FeS. This proposed mechanism of phase transformation can potentially be applied to other chalcogenide systems and will lead to interesting nanomorphologies.

also at the sulfide/oxide interface. As the reaction proceeds, the iron oxide core is being depleted by the sulfurization reaction, and the voids continue to accumulate. Once the iron oxide core is completely depleted, all that remains is the iron sulfide shell and the accumulated voids at the center. Based on electron diffraction analysis of nanowires in various points during the sulfurization reaction, there appears to be, at least at the initial stages of the reaction, an epitaxial relationship between the hematite nanowire and FeS shell. The hematite nanowire are vertically oriented, growing in the (210) direction. As the sulfurization reaction begins, the (100) planes of the FeS crystal are epitaxially related to the (210) planes of hematite. As the reaction proceeds, strain accumulates due to lattice mismatch and then starts to relax through the formation of structural defects, leading to the formation of misoriented FeS grains. As a result, thicker FeS shells show low-angle grain boundaries axially between single crystal grains. However, even for thicker shells, XRD analysis shows a high degree of preferential growth of the FeS shell. In this study, we report the first successful synthesis of phase pure, highly oriented FeS nanotubes. Phase transformation of single crystal hematite nanowire arrays to phase pure FeS nanotubes was carried out by reacting the nanowires in an H2S atmosphere at relatively low temperatures. Iron monosulfide (FeS) is a promising candidate as a high current density material for Li batteries,35,36 a catalyst for H2O2 reduction and sensing,37 and as an absorber layer and photocathode for solar cells.20,21 Despite its promising applications, the electronic and optical properties of FeS have not been well-characterized, due to the complex crystal nature of iron sulfide. To date, this is the first report of FeS nanotubes, though nonstoichiometric iron sulfides (Fe7S8) nanowires have been reported,38,39 as well as other interesting nanomorphologies.40 Predominantly, the work done with FeS as thin films and particles report hexagonal FeS, troilite, with lattice parameters a = 5.96 Å, c = 11.74 Å. Troilite is antiferromagnetic and has a small band gap of 0.04 eV.41 At slightly higher temperatures, there is small movement of Fe and S atoms to form the more symmetric NiAs structure of FeS, which is now nonmagnetic and has an increased band gap,42 but that band gap has not been well-researched or reliably

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectra of FeS nanowire arrays, XRD spectra of FeS nanotube arrays, electron diffraction ring pattern of FeS nanotubes, HR-TEM images and corresponding electron diffraction pattern of hematite and FeS epitaxy; details of DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author would like to acknowledge the Conn Center for Renewable Energy Research for facilities and access to characterization equipment and acknowledge support from DOE-EPSCoR (DE-FG02-07ER46375).

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