Change in Magnetic Properties upon Chemical Exfoliation of FeOCl

Article abstract of DOI:10.1021/acs.inorgchem.9b02856

The development of novel, intrinsic two-dimensional (2D) antiferromagnets presents the opportunity to vastly improve the efficiency of spintronic devices and sensors. The strong intrinsic antiferromagnetism and van der Waals layered structure exhibited by the bulk transition-metal oxychlorides provide a convenient system for the synthesis of such materials. In this work, we report the exfoliation of bulk FeOCl into and subsequent characterization of intrinsically antiferromagnetic thin-layer FeOCl nanosheets. The magnetic properties of bulk FeOCl, its lithium intercalate, and its nanosheet pellet are measured to determine the evolution of magnetic properties from the three-dimensional to the quasi-two-dimensional system. This work establishes FeOCl and isostructural compounds as a source for the development of two-dimensional intrinsic antiferromagnets.

Full text of DOI:10.1021/acs.inorgchem.9b02856

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Change in Magnetic Properties upon Chemical Exfoliation of FeOCl
†
†
†
†
‡
Austin M. Ferrenti, Sebastian Klemenz, Shiming Lei, Xiaoyu Song, Pirmin Ganter,
Bettina V. Lotsch,‡ and Leslie M. Schoop*
,§
,†
†
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany
‡
*
S Supporting Information
ABSTRACT: The development of novel, intrinsic two-dimensional (2D) antiferromagnets presents the opportunity to vastly
improve the efficiency of spintronic devices and sensors. The strong intrinsic antiferromagnetism and van der Waals layered
structure exhibited by the bulk transition-metal oxychlorides provide a convenient system for the synthesis of such materials. In
this work, we report the exfoliation of bulk FeOCl into and subsequent characterization of intrinsically antiferromagnetic thin-
layer FeOCl nanosheets. The magnetic properties of bulk FeOCl, its lithium intercalate, and its nanosheet pellet are measured
to determine the evolution of magnetic properties from the three-dimensional to the quasi-two-dimensional system. This work
establishes FeOCl and isostructural compounds as a source for the development of two-dimensional intrinsic antiferromagnets.
INTRODUCTION
produces only small yields of fragile, non-single-layer nano-
10,11
■
sheets.
Recently, increased attention has been paid to the develop-
ment of novel intrinsically magnetic nanosheets for spintronic
applications and the production of novel heterostructure
Recent developments in this field have focused largely on
the production of novel magnetic 2D analogues of layered
3
5
4
transition-metal trihalides, such as CrI , CrCl , RuCl , and
1
−8
3
3
3
materials.
Candidate parent compounds for the production
6
VCl3, as well as of layered dichalcogenides by use of
mechanical exfoliation methods. The chemical exfoliations
of RuCl , H CrS , and MnO , some of the first two 2D
of such thin layers are typically magnetic materials which
exhibit only weak interlayer attractive forces and thus are
typically considerably easier to exfoliate into appreciably
12
4
7
8
3
x
2
2
magnetic systems produced in this manner, have established a
baseline for the future production of less fragile 2D magnets.
In contrast to the transition-metal halide family of
compounds, the transition-metal oxyhalide family of magnetic,
layered vdW compounds has thus far received far less
attention, despite the unusual magnetic properties exhibited
1
uniform nanosheets than nonlayered compounds. Magnetic
van der Waals (vdW) compounds are of particular interest, due
to the wide range of magnetic, electronic, and physical
properties exhibited by their bulk forms and the ease with
2
which they may be delaminated into uniform nanostructures.
13,14
The recent development of liquid-based exfoliation methods
has drawn significant attention for the ease by which such vdW
compounds with very weak interlayer interactions may be
uniformly delaminated into monolayer or few-layer sheets in
by many of the bulk compounds.
Three such compounds
were recently predicted by Miao et al. to exhibit robust
intrinsic ferromagnetic (CrOCl) and antiferromagnetic (VOCl,
15
TiOCl) character in the two-dimensional monolayer. Here
9
large quantities. This represents a significant improvement
over classical mechanical exfoliation with adhesive tape, which
typically requires extended periods of tedious labor and often
Received: September 25, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02856
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Inorganic Chemistry
Article
we study 2D FeOCl, the bulk of which has over time been
extensively studied for various catalytic applications, as well for
16,17
its potential use as a cathode material.
Iron oxychloride
and isostructural materials comprise the FeOCl structure type,
which is defined by metal−oxygen double layers sandwiched
between anionic layers, which are separated by van der Waals
1
7,18
gaps.
Beyond its potential in various catalytic systems, iron
oxychloride has also been extensively studied for its magnetic
properties. Bulk FeOCl has been characterized as an
antiferromagnet that deviates from typical Curie−Weiss
19
paramagnetic behavior. The observed susceptibility is nearly
temperature independent, even slightly increasing, between 90
and 300 K. The observed room-temperature magnetic moment
of bulk FeOCl has been reported by Kauzlarich et al. to be
2
.76 μ , significantly lower than the 5.96 μ expected for high-
B
B
spin Fe(III), which the authors attribute to strong anti-
ferromagnetic Fe−Fe coupling, even above the Neel temper-
́
2
0
ature (T = 90 K).
N
Over the years, a significant number of guest molecules have
20−23
been intercalated into the interlayer space of FeOCl.
Figure 1. pXRD patterns of synthesized FeOCl and Li FeOCl
x
Previous reports have shown that lithium-intercalated FeOCl
typically shows a mixture of two distinct lithiated and
nonlithiated regions, as well as a small but universal shift to
samples, in comparison to the simulated pXRD pattern for FeOCl.
simulated pattern for the bulk material; no impurity peaks are
1
7,24
lower angles in the pXRD diffraction pattern.
The
visible. The pXRD pattern of the synthesized Li FeOCl
x
intercalation of various species into the FeOCl host structure
is also reported to produce a uniform increase in magnetic
powder is shown to exhibit a small shift to lower angles,
relative to FeOCl, for all major reflections, in agreement with
previous studies (see enlarged view in Figure S1 in the
24
susceptibility values.
Thin sheets of FeOCl have previously been synthesized and
17
Supporting Information). All reflections also exhibit broad-
25−27
studied for their potential in catalytic applications.
The
ening and a further reduction in intensity after lithiation. When
lithiation conditions are particularly harsh (in terms of either
increased reaction temperature or increased n-butyllithium
concentration), there is a reduction in the intensity of the
first attempt at exfoliation of a sodium benzoate-intercalated
FeOCl compound was reported in 2013, and the resulting
nanosheets, for which no thickness or other properties were
+
28
reported, were found to be composed of FeO . Since then,
numerous groups have attempted to synthesize monolayer
FeOCl nanosheets through a variety of methods, but the
presented characterization using transmission electron micros-
copy (TEM) and atomic force microscopy (AFM) did not
provide definitive proof for sample thickness below the few-
[
010] reflection, and an additional reflection often appears in
the pXRD pattern around 2θ = 3.0°. This reflection
corresponds to a d value of 13.6 Å, which relative to the d
value of 7.90 Å for the [010] reflection suggests a pattern of
FeOCl bilayers separated by lithium.
Analysis by scanning electron microscopy (SEM) and
energy-dispersive X-ray spectroscopy (EDX) of a representa-
tive FeOCl sample indicates the presence of a single-phase
sample with a 1:1:1 Fe:O:Cl composition (see Figure S2).
2
5−27,29,30
layer state.
In addition, the magnetic properties of
exfoliated FeOCl samples have not yet been studied, but the
likelihood that exfoliated FeOCl will prove to be a source for a
new 2D magnetic material warrants further investigation. Here,
we report the exfoliation of FeOCl into uniform, very thin
nanosheets. We study the magnetic properties of the exfoliated
sheets by fabricating a pellet containing a few milligrams of the
dried nanosheets. The nanosheet pellet, the magnetic proper-
ties of which differ from those of bulk FeOCl, also shows
indications of frustrated magnetism. These results suggest that
chemically exfoliated FeOCl nanosheets are a source for a
novel 2D magnetic material.
SEM/EDX analysis of a representative Li FeOCl sample
x
indicates uniform elemental composition after lithium
intercalation (Figure S3). Inductively coupled plasma mass
spectrometry measurements performed on a representative
Li FeOCl show a roughly 1:1:1:1 Li:Fe:O:Cl elemental
x
composition.
Nanosheet Characterization. Liquid exfoliation of
Li FeOCl was achieved through the addition of the sample
x
powder to dry MeOH in a 1 mg to 2 mL ratio under argon
flow and subsequent shaking for several days. For details,
please see the Supporting Information.
RESULTS AND DISCUSSION
Bulk Synthesis and Characterization. Phase-pure bulk
FeOCl powder was synthesized by a modified version of the
■
TEM and EDX analysis of representative Li FeOCl samples
x
exfoliated under low (∼20%) relative humidity indicate the
presence of very thin, well-defined FeOCl nanosheets, shown
in Figure 2a. The uniform elemental composition of the
exfoliated nanosheets is seen in Figure 3 and shows no region
of clear Cl deficiency, indicating that they are still composed of
Fe, O, and Cl after exfoliation. A well-defined diffraction
pattern could be obtained for the FeOCl nanosheets (Figure
2b), indicating that the orthorhombic symmetry is retained.
The observed intensity differences between the experimental
31
thermal decomposition method reported by North et al.
Unless they were stored under an inert gas atmosphere or in a
desiccator, FeOCl samples were found to readily hydrolyze,
eventually decomposing to hydrated FeCl . Lithium inter-
3
calation was achieved through a modified version of the n-
17
butyllithium method. Further experimental details are given
in the Supporting Information.
Figure 1 shows the powder X-ray diffraction (pXRD) data
obtained for FeOCl powder, which is in agreement with the
and simulated [1
̅
01] and [101] peaks are likely due to the
B
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Figure 2. (a) TEM imaging of representative stacked FeOCl nanosheets. (b) Selected area electron diffraction pattern of a representative FeOCl
nanosheet along the [010] zone axis. (c) Simulation of bulk FeOCl based on the space group Pmmn referenced in the literature.
Figure 3. (a) TEM imaging and (b) Fe, (c) O, and (d) Cl EDX elemental analysis of representative FeOCl nanosheets.
experimental diffraction pattern being taken on a thicker slab
of stacked nanosheets (as seen in Figure 2a), which were
slightly offset from each other. The experimentally derived d
values are found to be consistent within 0.12 Å of the predicted
values (Figure S4). Over a period of several days, the
nanosheet suspension was observed to change color, where
AFM characterization of representative Li FeOCl samples
x
exfoliated under low (∼20%) relative humidity shows relatively
thin-layer nanosheets, roughly 0.5−1.0 μm across (Figure S5).
Due to slightly uneven substrate height, a definite nanosheet
thickness could not be reliably determined, but topography
profiles obtained suggest a thickness on the order of 2.0−2.4
nm, in comparison to the expected thickness of a true FeOCl
monolayer, 0.79 nm. While the larger than expected thickness
could be due to residual methanol coating of an FeOCl
monolayer, it is more likely that the exfoliated nanosheets are
bilayers, possibly with a small contribution from residual
the initial black Li FeOCl suspension gradually turned purple-
x
brown and eventually became brown-red after a period of more
than 4 days in suspension. TEM/EDX analysis of Li FeOCl
x
samples in suspension for a period of more than 6 days
provided no evidence for the existence of FeOCl nanosheets,
instead indicating only the presence of amorphous iron oxide
particles.
4
methanol. Samples of the nanosheet suspension drop-casted
C
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Article
Figure 4. Temperature-dependent ZFC (blue) and FC (orange) magnetic susceptibility and (insets) magnetization at 2 K (red) and at 300 K
green) of (a) FeOCl, (b) Li FeOCl, and (c) an FeOCl nanosheet pellet in an applied field of H = 10000 Oe.
(
x
3
2
19
onto a silicon wafer in moist air were observed to decompose
within a period of a few minutes.
been reported to be in the range of roughly 84 K to 92 K.
In this study, the 1/χ vs T plot (Figure S9a) shows a broad
transition centered at 84.6 K, which is where the dχ/dT plot
(Figure S10) crosses zero. The broad transition from
antiferromagnetic character to paramagnetic behavior observed
in this region is likely related to the powder nature of the
samples studied here, as the transition has been observed to be
AFM characterization of representative Li FeOCl samples
x
exfoliated under higher (∼90−100%) relative humidity shows
thicker, likely decomposed nanoparticles (Figure S6). The
exfoliated sample is seen to consist of a relatively flat but
fragmented surface on the order of 5.0 nm thick, with
numerous holes, indicating decomposition. As bulk FeOCl is
known to be extremely moisture sensitive, exposure to high
humidity is likely to cause the decomposition and agglomer-
ation of FeOCl nanosheets, contributing to the increased
thickness and reduced uniformity of the sample surface
observed.
Inert-atmosphere thermogravimetric analysis (TGA) char-
acterization of a representative FeOCl nanosheet pellet,
produced by the centrifugation and subsequent compaction
of the nanosheet suspension, suggests that the nanosheets are
relatively stable up to at least 523 K, albeit with a much lower
crystallinity than in the original sample (see Figure S7b).
Through fast heating to 523 K, the nanosheet pellet sample
experienced a 9% mass loss, likely due to the evaporation of
residual methanol present after centrifugation and drying
32,33
sharper in previous single-crystal studies.
The nonlinear
behavior of the inverse susceptibility at high temperatures,
which violates the Curie−Weiss law, also agrees with the
reported magnetic data of bulk FeOCl (Figure S9a). The
observed magnetic moment at room temperature was found to
be 2.79 μ (Figure S11), similar to that reported previously
B
3
+
(2.76 μ ) but lower than that expected for high-spin Fe (5.96
B
20
μ ). However, the large susceptibility observed at temper-
B
32,33
atures below 50 K disagrees with published results
and
likely originates from a minor iron oxide impurity phase. This
impurity is not apparent in the XRD patterns or EDX spectra
of representative bulk FeOCl samples, suggesting that it is
quite small and largely amorphous. The presence of such a
strongly magnetic impurity may also conceal the antiferro-
magnetic to paramagnetic transition, as well as produce the
slight deviations from reported behavior at higher temper-
atures. This is also supported by field-dependent magnet-
ization measurements on the bulk, polycrystalline FeOCl. At 2
K, there is a slight curvature to the magnetization curve,
indicating the presence of a small ferromagnetic impurity, but
at 300 K no such curvature is evident, suggesting that it is not
ferromagnetic at room temperature (Figure 4a). This minor
impurity is removed during the centrifugation step of the
exfoliation and therefore does not affect the studies of the
nanosheet pellet.
(
Figure S7a). Over time, the nanosheet pellet experienced a
33
gradual but minimal mass loss on the order of 5%. As the
decomposition temperature of bulk FeOCl in inert atmosphere
is roughly 543 K, it is likely that the observed mass loss is
caused by the proximity of the testing conditions to this
3
1
temperature. While the observed mass loss is significant, the
post-heating pXRD pattern of the nanosheet pellet (Figure
S7b) aligns well with that of Li FeOCl, suggesting that
x
decomposition of the nanosheet pellet itself was not extensive,
albeit the degree of crystallinity was lowered.
When it was heated in air under high relative humidity, the
nanosheet suspension gradually changed color from the purple-
brown observed for representative FeOCl nanosheets to red,
further indicating decomposition of the nanosheets under high
relative humidity. When it was heated in air under low relative
humidity, the nanosheet suspension was also observed to
change color from purple-brown to red, but over a much
longer period of time than the high relative humidity exfoliated
suspension. The pXRD pattern of the decomposed nanosheet
solution after drying shows that the sample decomposed to
Fe O (Figure S8).
Upon lithiation, the magnetic character of FeOCl is found to
change significantly, exhibiting a new magnetic transition in the
ZFC curve around 9 K, above which Li FeOCl is shown to
x
display a decrease in magnetic susceptibility more linear than
previously reported. Although linear in the higher-temperature
region, the lithiated material does not follow the Curie−Weiss
law (Figure S9b), although Curie−Weiss behavior could be
obscured by the presence of a small ferromagnetic impurity.
The magnetic moment at room temperature was found to be
5.58 μ , unlike that in bulk FeOCl, and agrees better with the
B
3
+
20
expected magnetic moment of high-spin Fe (5.96 μB).
2
3
Magnetic Characterization. Zero-field-cooled (ZFC) and
field-cooled (FC) magnetic susceptibility measurements of
polycrystalline FeOCl are in relative agreement with previously
However, since the magnetic moment is so strongly temper-
ature dependent (Figure S12) and decreases with temperature
(similar to that in bulk FeOCl), we believe that the magnetic
moment for this sample cannot be derived reliably. Impurities
may also have an effect on the observed magnetic moment, as
32,33
published results (Figure 4a).
below which FeOCl should order antiferromagnetically, has
The Neel temperature,
́
D
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Inorganic Chemistry
Article
is supported by the field-dependent magnetization data shown
affected. However, since the observed magnetic moment
agrees with that observed for bulk FeOCl, and the
susceptibility remained constant over several sample batches
(whereas it was found to be much higher for the decomposed
sheets), we believe this to be unlikely. The large negative Weiss
temperature indicates that the nanosheet pellet is magnetically
frustrated. The frustration index, f = θCW/T (where θ is the
below. The ZFC magnetic behavior of the lithiated material
24
roughly agrees with the previous investigation by Sagua et al.
In this study, the authors saw a slight reduction in
susceptibility below 25 K, which is higher than the transition
temperature observed in the current study. Furthermore, in our
study a more pronounced drop in susceptibility is observed.
Similar to the previous study, we find an increase in magnetic
susceptibility upon intercalation. While previously it was
argued that this increase can be explained by the reduction
of Fe(III) to Fe(II) upon Li intercalation, we doubt that this
explanation is warranted. The susceptibility increases by 1
order of magnitude, which is much too high to be explained by
a change in Fe oxidation state. We find three more plausible
scenarios: (1) The increase in susceptibility could be caused by
a change in the magnetic ground state from AFM to canted
AFM, which would result in a net moment. This is supported
by the observed divergence of the FC and ZFC susceptibility
curves. (2) Partial decomposition of the sample upon
treatment with n-butyllithium, resulting in less antiferromag-
netic FeOCl and more of the ferromagnetic impurity being
present in the sample, could cause an increase in susceptibility.
This option is also supported by the field-dependent
magnetization discussed below. (3) A smaller particle size in
the lithiated sample in comparison to the unlithiated sample,
which could be caused by the aggressive n-butyllithium
method of lithium intercalation, could also contribute to a
N
CW
Curie−Weiss temperature derived from the Curie−Weiss
fitting and T is the transition observed at around 14 K), for
N
this system is 24.29 ± 0.71; compounds with a frustration
index greater than 10 are generally considered to be
35
magnetically frustrated.
Field-dependent magnetization measurements on the FeOCl
nanosheet pellet at 2 K exhibit hysteresis not present in
measurements conducted at 300 K, but of a lesser magnitude
in comparison to that observed in the Li FeOCl sample at 2 K
x
(Figure 4c). The lack of hysteretic character in the 300 K M vs
H loop would suggest that centrifugation of the exfoliation
suspension was sufficient to remove the strongly magnetic
impurity observed in both the bulk and lithiated FeOCl
samples. The hysteresis exhibited by the pellet at 2 K, as well as
the branching of the FC and ZFC curves at low temperatures,
also support that canted antiferromagnetic character may be
intrinsic to the nanosheet pellet.
All magnetic measurements conducted on the FeOCl
nanosheet pellet could be reproduced across different batches
of samples and differ significantly from those conducted on a
representative decomposed FeOCl nanosheet pellet (Figure
S13). While the magnetic properties observed for the
nanosheet pellet are not necessarily representative of those
intrinsic to an FeOCl monolayer, our results point to the
existence of magnetic order in the sheets as well. As the pellet
is turbostratically disordered, and thus interlayer interactions
are weakened, its magnetic properties would be expected to
more closely resemble those of a monolayer nanosheet than
34
higher observed moment. While we also observe an increase
in susceptibility upon lithiation, the total increase is
significantly smaller than that reported by Sagua et al., which
hints at the presence of different amounts of impurity phases or
different particle sizes in the lithiated samples. Field-dependent
magnetization measurements on the polycrystalline Li FeOCl
x
at 2 K exhibit hysteresis not present in measurements
conducted at 300 K (Figure 4b), suggesting some
ferromagnetic character of the material when it is cooled to
this temperature. At 300 K, the magnetization loop is seen to
be S-shaped, to an extent greater than that seen in the 300 K
magnetization loop for the bulk FeOCl, again indicating the
presence of a ferromagnetic impurity.
4
those of the bulk.
CONCLUSION
■
We report the synthesis and physical characterization of thin-
layer, roughly 0.5−1.0 μm across FeOCl nanosheets, as well as
magnetic characterization of the nanosheet pellet. TEM and
AFM measurements show that when they are prepared under
low relative humidity, relatively uniform, crystalline, thin-layer
nanosheets may be produced. Magnetic measurements show
that the nanosheet pellet retains the antiferromagnetic
character intrinsic to the bulk FeOCl system and has a similar
magnetic moment but, unlike its bulk parent, follows the
Curie−Weiss law, has a lower transition temperature, and
likely exhibits magnetic frustration.
To investigate the magnetic properties of the FeOCl
nanosheets, we conducted FC and ZFC magnetic susceptibility
measurements of the FeOCl nanosheet pellet, similarly to
those performed in other studies of chemically exfoliated
4
,8
magnetic nanosheets.
We observe a reduction in the
magnitude of the magnetic susceptibility, roughly to the scale
of that observed for bulk FeOCl, suggesting that the impurity
previously present was removed by centrifugation. A new
magnetic transition around 14 K is visible in the ZFC magnetic
susceptibility curve but absent in the FC curve, a behavior that
is often indicative of canted antiferromagnetism. Both the FC
and ZFC measurements suggest Curie−Weiss behavior for the
pellet (Figure S9c). Curie−Weiss fits show that the FeOCl
nanosheet pellet has a Curie constant of C = 1.10 ± 0.06, a
Weiss temperature of θ = −340.1 ± 9.8 K, and an effective
magnetic moment of μ = 2.98 ± 0.07 μ . This moment is
Further studies on this system should focus on the
understanding of the magnetic properties of single nanosheets.
A similar synthetic approach may be used in the future toward
the production of additional two-dimensional magnetic
systems from the transition-metal oxychloride family.
eff
B
ASSOCIATED CONTENT
close to that of bulk FeOCl, suggesting that the sheets might
exhibit similar short-range magnetic order. This also supports
the removal of the previously present impurity. The errors for
these values were determined using the results of multiple
measurements on different exfoliated samples. Nonetheless, we
wish to point out that, if an impurity would still be present in
the sample, the Weiss temperature could be significantly
■
*
S
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02856.
Experimental details, bulk FeOCl and Li FeOCl SEM
x
data, additional TEM data, AFM data, TGA data,
E
DOI: 10.1021/acs.inorgchem.9b02856
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Inorganic Chemistry
decomposed FeOCl nanosheet pellet XRD data, and
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AUTHOR INFORMATION
■
*
Corresponding Author
E-mail for L.M.S.: lschoop@princeton.edu.
ORCID
(14) Komarek, A.; Taetz, T.; Fernandez-Díaz, M.; Trots, D.; Moller,
́
̈
Xiaoyu Song: 0000-0003-3384-0527
Bettina V. Lotsch: 0000-0002-3094-303X
Leslie M. Schoop: 0000-0003-3459-4241
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ACKNOWLEDGMENTS
̈
2
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3
2
■
̈
arative Darstellung von Eisenoxychlorid. Z. Anorg.
The authors acknowledge Marie-Luise Schreiber of the Max
Planck Institute for Solid State Research for assistance with
ICPMS analysis. This research was supported by the Princeton
Catalysis Initiative (PCI). The authors thank the Max Planck
Society for financial support. The authors also acknowledge the
use of Princeton’s Imaging and Analysis Center, which is
partially supported by the Princeton Center for Complex
Materials, a National Science Foundation (NSF)-MRSEC
program (DMR-1420541). We also acknowledge financial
support from the German Research Foundation (DFG) under
Germany's Excellence Strategy - EXC 2089/1-390776260, and
the Center for Nanoscience.
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