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
S
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
© 2013 American Chemical Society
Received: January 25, 2013
Revised: April 27, 2013
Published: May 13, 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.
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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
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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
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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
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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
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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
■
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.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
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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