Atomic Layer Deposition of Iron Sulfide and Its Application as a Catalyst in the Hydrogenation of Azobenzenes

Article abstract of DOI:10.1002/anie.201700449

The atomic layer deposition (ALD) of iron sulfide (FeS_x) is reported for the first time. The deposition process employs bis(N,N′-di-tert-butylacetamidinato)iron(II) and H₂S as the reactants and produces fairly pure, smooth, and well-crystallized FeS_x thin films following an ideal self-limiting ALD growth behavior. The FeS_x films can be uniformly and conformally deposited into deep narrow trenches with aspect ratios as high as 10:1, which highlights the broad applicability of this ALD process for engineering the surface of complex 3D nanostructures in general. Highly uniform nanoscale FeS_x coatings on porous γ-Al₂O₃ powder were also prepared. This compound shows excellent catalytic activity and selectivity in the hydrogenation of azo compounds under mild reaction conditions, demonstrating the promise of ALD FeS_x as a catalyst for organic reactions.

Full text of DOI:10.1002/anie.201700449

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International Edition: DOI: 10.1002/anie.201700449
German Edition: DOI: 10.1002/ange.201700449
Atomic Layer Deposition
Atomic Layer Deposition of Iron Sulfide and Its Application as
a Catalyst in the Hydrogenation of Azobenzenes
Youdong Shao+, Zheng Guo+, Hao Li, Yantao Su, and Xinwei Wang*
Abstract: The atomic layer deposition (ALD) of iron sulfide
(FeSx) is reported for the first time. The deposition process
employs bis(N,N’-di-tert-butylacetamidinato)iron(II) and H2S
as the reactants and produces fairly pure, smooth, and well-
crystallized FeSx thin films following an ideal self-limiting
ALD growth behavior. The FeSx films can be uniformly and
conformally deposited into deep narrow trenches with aspect
ratios as high as 10:1, which highlights the broad applicability
of this ALD process for engineering the surface of complex 3D
nanostructures in general. Highly uniform nanoscale FeSx
coatings on porous g-Al2O3 powder were also prepared. This
compound shows excellent catalytic activity and selectivity in
the hydrogenation of azo compounds under mild reaction
conditions, demonstrating the promise of ALD FeSx as
a catalyst for organic reactions.
electrodes, which have shown great promise for energy
conversion and storage applications.[12] Meanwhile, ALD
has also been regarded as an exceptionally powerful tool for
catalyst design.[13] Taking advantage of the atomic precision of
this method, ALD has been used to create catalyst materials
with well-controlled, homogeneous distributions in terms of
their sizes, compositions, and active sites, which can not only
improve the activity, selectivity, and stability of the catalysts,
but also significantly facilitate the elucidation of the reaction
mechanisms and catalyst structure–property relationships.[13]
Moreover, a variety of new catalyst structures for high-
performance multifunctional catalysis have recently been
enabled by ALD.[14] Over the past decades, the ALD
technique itself has been developing very rapidly with
hundreds of different materials having been synthesized by
ALD.[15] However, as reviewed recently,[16] there is still no
ALD process for the synthesis of FeSx materials. Considering
the important and broad applications of FeSx, the absence of
an ALD process for its synthesis can seriously limit the
advancement of FeSx-based nanoscale engineering for elec-
tro- and chemical catalysis. Therefore, the development of
a new ALD process for FeSx is urgently required.
I
ron sulfide (FeSx) is an important earth-abundant material
that has recently received great attention for a variety of
applications, such as solar cells,[1] lithium-ion batteries,[2] and
as a catalyst for hydrogen evolution,[3] oxygen reduction,[4]
CO2 reduction,[5] and organic synthesis.[6] Many of these
applications are based on the use of nanostructured FeSx as
the active material[7] as nanostructured materials usually have
a fairly large surface area and thus expose a large number of
active sites, which can considerably boost the performance of
active materials. To fabricate nanostructured FeSx, many
synthetic methods, including solution-based methods,[2a,c,3c,d,4]
mechanical ball milling,[2b] gas-phase sulfidation,[8] and chem-
ical vapor deposition,[1c,9] have been adopted in the past. On
the other hand, atomic layer deposition (ALD) has recently
gained significant attention as a highly useful deposition
technique for fabricating nanostructured materials.[10] ALD
employs saturated, self-limiting surface-chemistry reactions
and allows the designed materials to grow in a layer-by-layer
fashion[11] so that, in theory, any complex 3D structure can be
conformally and uniformly coated by ALD with excellent
reproducibility and atom-precise control over the film
composition and thickness. These unique features of ALD
have recently enabled a variety of studies[12] on the fabrication
and engineering of nanomaterials, especially for the nano-
scale engineering of the surface of complex 3D scaffold
Herein, we report the first ALD process for FeSx. The
process employed bis(N,N’-di-tert-butylacetamidinato)iron-
(II) (Fe(amd)2, Figure 1) and H2S as the reactants, and
Figure 1. Structure of bis(N,N’-di-tert-butylacetamidinato)iron(II).
followed ideal self-limiting ALD growth behavior over a wide
temperature range to produce fairly pure, smooth, and well-
crystallized FeSx films. The ALD FeSx films were conformally
deposited into deep narrow trenches with aspect ratios as high
as 10:1, which indicated that this ALD process is highly
suitable for preparing conformal and uniform FeSx coatings
on general 3D complex or porous nanostructures. We also
fabricated nanoscale FeSx coatings on porous g-Al2O3 powder
by ALD, and used the prepared FeSx catalyst for the selective
reduction (hydrogenation) of azobenzenes to hydrazoben-
zenes. As hydrazobenzenes generally suffer from facile
[*] Dr. Y. Shao,[+] Z. Guo,[+] H. Li, Dr. Y. Su, Prof. Dr. X. Wang
School of Advanced Materials
Shenzhen Graduate School, Peking University
Shenzhen 518055 (China)
À
reductive cleavage of the NH NH bond to form amines, the
E-mail: wangxw@pkusz.edu.cn
[+] These authors contributed equally to this work.
selective reduction of azo to hydrazo compounds is important
and needs special care.[17] With our FeSx catalyst prepared by
ALD, we achieved the selective reduction (hydrogenation) of
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201700449.
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azobenzenes with electron-withdrawing or -donating groups
with high activity and selectivity under mild reaction con-
ditions.
The temperature dependence of the growth rate is plotted
in Figure 2d. The film thickness, which was used to calculate
the growth rate, was directly measured by X-ray reflectom-
etry (XRR) for films deposited at ꢀ 1608C, but for higher
deposition temperatures (ꢁ 1608C), the deposited films were
too rough (see below) to be analyzed by XRR; therefore, X-
ray fluorescence (XRF) was used instead to estimate the
thickness. XRF measures the areal density of each element in
the film, which can be used to calculate the areal mass of the
deposited film. Assuming a film density of 4.83 gcmÀ3 (the
bulk density of stoichiometric FeS, PDF No. 37-0477), the film
thickness could therefore be calculated. The correctness of
this bulk density assumption was confirmed for the films
deposited at 1608C as the values of the growth rates obtained
by XRR and XRF agreed well with each other (Figure 2d).
The growth rate was found to decrease from 0.40 to 0.19 ꢀ per
cycle as the temperature was increased from 80 to 2008C, and
then started to increase above 2508C, which implies that the
precursors undergo partial decomposition at high temper-
atures.
The film microstructures and crystallinity were examined
by transmission electron microscopy (TEM). As shown in
Figure 3a–d, the ALD FeSx films were well-crystalline for all
deposition temperatures from 80 to 2508C. However, the
crystallinity depended on the deposition temperature as the
electron diffraction patterns for low-temperature (ꢀ 1208C)
and high-temperature (ꢁ 1608C) ALD films were pronoun-
cedly different (Figure 3e). After careful comparison with
reference compounds (see the Supporting Information, Fig-
ure S1), the low- and high-temperature ALD FeSx films were
determined to correspond to mackinawite (tetragonal) and
pyrrhotite (hexagonal or monoclinic) phases, respectively.
The temperature dependence of the phase formation implies
that under our ALD conditions, the pyrrhotite was thermo-
dynamically more stable than the mackinawite. In fact, the
phase transition of FeSx from mackinawite to pyrrhotite is
often observed in geochemistry,[18] where mackinawite is the
The ALD of FeSx was carefully investigated at deposition
temperatures ranging from 80 to 3008C. Unless otherwise
specified, UV/ozone-treated planar Si wafer substrates (with
a ca. 1 nm thick SiO2 surface layer) were used to study the
deposition behavior. Typical self-limiting ALD growth behav-
ior was observed for deposition temperatures of up to 2008C.
Representative thickness saturation curves for films depos-
ited at 1208C by 400 ALD cycles at various precursor dosages
are shown in Figure 2a,b. As shown in Figure 2a, the film
thickness first increased with the Fe(amd)2 exposure, and then
reached saturation as the Fe(amd)2 exposure exceeded about
0.025 Torrs while the amount of H2S was kept fixed at
approximately 0.062 Torrs. Similarly, as shown in Figure 2b,
when the amount of Fe(amd)2 was kept fixed at about
0.025 Torrs, the film thickness also increased initially and then
reached saturation as the H2S exposure exceeded approx-
imately 0.062 Torrs. These results clearly indicate that
saturated, self-limiting film growth could be achieved with
minimum exposures of about 0.025 and 0.062 Torrs for
Fe(amd)2 and H2S, respectively. Under saturation conditions,
the thickness of the deposited film follows an ideal linear
relation with the total number of ALD cycles, suggesting good
linear growth behavior for this ALD process (Figure 2c).
Good growth linearity is of particular importance for ALD as
it could enable one to precisely control the thickness of
deposited films by simply varying the total number of ALD
cycles. In addition, the linear fit shown in Figure 2c also
revealed a negligible intercept, which suggests negligible
nucleation delay in this ALD process.
Figure 2. a) Film thickness with respect to Fe(amd)2 exposure at
a fixed H2S exposure of about 0.062 Torrs. b) Film thickness with
respect to H2S exposure at a fixed Fe(amd)2 exposure of about
0.025 Torrs. Total number of ALD cycles in (a) and (b): 400 cycles.
c) Film thickness as a function of the number of ALD cycles. A
deposition temperature of 1208C was used in (a)–(c). d) Growth rate
as a function of the deposition temperature. The corresponding film
Figure 3. TEM images of approximately 20 nm thick FeSx films depos-
ited at a) 80, b) 120, c) 160, and d) 2508C. Scale bars: 10 nm. e) The
associated electron diffraction patterns. The ALD FeSx films were all
deposited on UV/ozone-treated SiNx grids for TEM. f) Fe/S atomic
ratio as a function of the deposition temperature.
*
*
thickness was measured by XRR ( ) and/or XRF ( ).
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iron sulfide formed first in most ambient environments, and
The surface morphology of the FeSx films was examined
then converts into more stable iron sulfide phases such as
pyrrhotite.[19] The similar phase behavior under the ALD
vacuum conditions and in aqueous geochemistry is remark-
able. In addition, as ALD processes are known for their high
reproducibility, this well-behaved temperature dependence of
the phase of the crystalline film should enable controlling the
highly reproducible formation of uniform phases by simply
adjusting the deposition temperature.
by both scanning electron microscopy (SEM) and atomic
force microscopy (AFM). SEM and AFM images for
approximately 20 nm thick films deposited at various temper-
atures from 80 to 2008C are shown in Figure 5a–j. Granular
The Fe/S atomic ratios in the ALD films were determined
by both XRF and Rutherford backscattering spectroscopy
(RBS). As shown in Figure 3 f, the Fe/S ratio was fairly close
to 1.0 for all deposition temperatures. A stoichiometry of 1:1
had indeed been expected as both Fe and S are divalent in
their respective precursors. However, considering the accu-
racy of the measurements (ca. 5%), we cannot completely
exclude the possibility that the actual atomic ratios deviate
a few percent from the 1:1 stoichiometry, especially consid-
ering that both mackinawite and pyrrhotite phases can
accommodate fairly large deviations from the 1:1 stoichiom-
etry (up to ca. 10%).[18,20]
The purity of the ALD films was further analyzed by X-
ray photoelectron spectroscopy (XPS). Prior to the XPS
measurements, all samples were subjected to Ar+ sputtering
for 150 s to remove adventitious carbon and surface oxide.
Representative survey and high-resolution XPS spectra for an
approximately 20 nm thick FeSx film deposited at 1608C are
shown in Figure 4. The high-resolution Fe 2p spectrum shows
Figure 5. a,c,e,g,i) AFM and b,d,f,h,j) SEM images of approximately
20 nm thick FeSx films deposited at 808C (a,b), 1008C (c,d), 1208C
(e,f), 1608C (g,h), and 2008C (i,j). Scale bars: 200 nm. k) Plot of the
rms roughness as a function of the deposition temperature. l) Cross-
sectional SEM image showing conformal film deposition inside a deep
narrow trench with an aspect ratio as high as 10:1. The deposition was
performed at 1208C with 2000 ALD cycles.
features are clearly visible in all of these images, suggesting
that the as-deposited films were all polycrystalline. The
root mean square (rms) roughness was plotted in Figure 5k
as a function of the deposition temperature. FeSx films
deposited below 1608C were generally smooth as their rms
roughness (2–3 nm) amounted to only about 10–15% of their
film thickness (ca. 20 nm). However, when the deposition
temperature exceeded 1608C, the films started to form flake-
shaped crystallites with increased surface roughness (Fig-
ure 5i,j).
Figure 4. a) XPS survey spectrum for an approximately 20 nm thick
FeSx film deposited at 1608C and the associated b) Fe 2p, c) S 2p, d) C
1s, and e) N 1s high-resolution spectra.
The FeSx films could also be conformally deposited into
deep narrow trenches with high aspect ratios. As demon-
strated by the cross-sectional SEM image in Figure 5l, for
a 1.85 mm deep trench with a high aspect ratio of 10:1, the
deposited FeSx film was able to conformally cover the entire
trench with uniform thickness throughout the trench. This
high conformality is an important feature of a good ALD
process in which the surface chemistry reactions are indeed
self-limiting, and it also indicates that this ALD process can
be employed to uniformly coat FeSx films onto highly porous
structures or complex 3D structures with high aspect ratios.
Thus far, we have shown that with our ALD process, we
can produce pure, smooth FeSx thin films with excellent
conformality and precise thickness control. We were then
interested in using this approach to uniformly coat g-Al2O3
powder with a FeSx thin layer for applications in organic
synthesis. FeSx is assumed to be a good catalyst as many
a pair of spin–orbit-split peaks at 706.9 eV (2p3/2) and
720.3 eV (2p1/2), and the high-resolution S 2p spectrum
shows a pair of spin–orbit-split peaks at 161.8 eV (2p3/2) and
162.7 eV (2p1/2), which are all consistent with the literature
values for iron sulfide.[9b] Possible C and N impurities were
determined to be as low as about 1.6 and 0.5 at%, respec-
tively, which indicates that the purity of the ALD FeSx film
was fairly high. Additional spectra for films deposited at
different temperatures (80–2508C) are provided in Figure S2.
The impurity levels for C and N were even lower than the
detection limit (ca. 0.5 at%) for higher deposition temper-
atures. In addition, a small oxygen peak was observed in the
survey spectrum (Figure 4a), which was due to post-oxidation
of the FeSx surface upon exposure to air. An ALD Co9S8
film[21] was further deposited in situ on top of the FeSx film to
confirm this property (Figure S3).
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À
Table 1: Comparison of the catalytic performance of ALD FeSx/Al2O3 and other
catalysts in the hydrogenation of azobenzene (2.0 mmol) at room temperature.
enzymes contain active Fe S clusters for catalyzing
biological organic transformations.[6a]
To this end, a thin layer of FeSx (ca. 30 nm) was
deposited on activated porous g-Al2O3 powder
(FeSx/Al2O3) with our ALD process at 1608C
(1500 cycles) under the same deposition conditions.
Additional characterization (Figure S4) confirmed
that the FeSx films deposited on Al2O3 substrates
had very similar material properties to those
deposited on SiO2. The deposition produced
a mass load of 1.1 wt% of FeSx, as determined by
dissolving the product powder in acid solution and
measuring the iron amount by atomic emission
spectroscopy. SEM in combination with energy-
dispersive X-ray spectroscopy (EDS) was used to
examine the elemental composition of the ALD-
prepared FeSx/Al2O3 powder. As shown in Figure 6,
benefited from the self-limiting growth behavior, in
the ALD-prepared FeSx/Al2O3 powder, Fe and S
were highly uniformly distributed on the Al2O3
Entry Catalyst
(mol%)[a]
Conv. Selectivity Molar TOF[b] Areal TOF[b]
[%]
[%]
[hÀ1
]
[mmolhÀ1 mÀ2
]
1
2
3
4
5
6
7
8
ALD FeSx/Al2O3 (0.9) 97
–
Al2O3
Fe2O3 (10)
FeCl2 (10)
s-FeSx(m) (10)
s-FeSx(m)+Al2O3 (10) 63
s-FeSx(p) (10) 55
>99
–
–
98
>99
97
96
323
–
–
5.0
7.5
17.8
18.1
16.3
533
–
–
<5
<5
17
25
61
5.5
[c]
–
46.7
47.5
89.6
>99
[a] The amount of catalyst with respect to azobenzene. [b] The molar (areal) TOF
corresponds to the number of moles of converted hydrazobenzene per unit time per
mole (unit area) of catalyst. The calculations are detailed in the Supporting
Information. [c] Soluble in MeOH.
powder, which highlighted the advantageous properties of
ALD for synthesizing uniform catalysts (see also Figures S5
and S6).
for comparison, but at a fairly high catalyst loading
(10 mol%); however, the reaction conversion was much
lower (67%), and an appreciable amount (3%) of over-
reduced byproduct (aniline) was also formed at the same
time. A mixture of solvothermally synthesized FeSx(m) and g-
Al2O3 powder (entry 7) basically gave the same catalytic
performance as the s-FeSx(m) alone, which ruled out any
synergistic effects between the FeSx layer and g-Al2O3
powder. Pyrrhotite FeSx nanowires (s-FeSx(p)) were also
solvothermally synthesized[23] for comparison (entry 8), but
the reaction conversion was even lower (55%) despite the
high catalyst loading (10 mol%). Additional comparisons
with various other reported catalysts are provided in Table S1.
The reaction turnover frequencies (TOFs) were also calcu-
lated with respect to both the molar amount and the surface
area of each catalyst. The ALD FeSx/Al2O3 catalyst gave rise
to the best molar and areal TOFs among the catalysts shown
in Table 1. Therefore, we have clearly demonstrated the
superior catalytic performance of ALD FeSx/Al2O3 in the
hydrogenation of azobenzene.
Figure 6. a) Representative SEM image of an ALD-prepared FeSx/Al2O3
powder. b) O, c) Al, d) S, and e) Fe EDS elemental distributions of the
FeSx/Al2O3 powder imaged in (a). f) The corresponding EDS spectrum.
We further extended the evaluation of the ALD FeSx/
Al2O3 catalyst to a broader range of azobenzene derivatives.
As listed in Table 2, at a low catalyst loading (ca. 1 mol%),
a variety of azobenzenes with methyl, methoxy, chloro, or
trifluoromethyl substituents were selectively (> 99%) re-
duced to the corresponding hydrazobenzenes with fairly good
conversions. These results clearly demonstrate the broad
applicability of this ALD FeSx/Al2O3 catalyst as both
electron-donating and electron-withdrawing substituents
were well tolerated. In addition, the ALD FeSx/Al2O3 catalyst
could also be recycled and reused for quite a few runs. Taking
azobenzene as an example, the recollected catalyst still
showed excellent product selectivity (> 99%) and fairly
high conversions of 91 and 88% in the second and third
reaction run, respectively. Although further mechanistic
studies are needed, the superior catalytic performance of
ALD FeSx/Al2O3 is perhaps related to the special properties
of the ALD FeSx material. Benefitting from the merits of
ALD for highly conformal coatings with superb reproduci-
The ALD FeSx/Al2O3 material was used as a catalyst in
the selective reduction (hydrogenation) of various azoben-
zenes into the corresponding hydrazobenzenes with sodium
borohydride (NaBH4) as the hydrogen source. As the simplest
compound of this class, azobenzene was effectively reduced to
hydrazobenzene at room temperature with 97% conversion
and > 99% selectivity within 20 min with only 0.9 mol% of
the ALD FeSx/Al2O3 catalyst (Table 1, entry 1). To compare
the catalyst performance, various control experiments were
performed in parallel and under the same reaction conditions.
As shown in Table 1, the reactions without any catalyst
(entry 2) or with uncoated g-Al2O3 powder (entry 3) barely
produced any product (< 5% conversion). Commercial Fe2O3
powder (entry 4) and FeCl2 (entry 5) catalyzed the reaction to
some extent, but both of them were much less effective, with
only 17 and 25% conversion, respectively, than our ALD
FeSx/Al2O3 catalyst. Solvothermally synthesized[22] mackina-
wite FeSx nanoparticles (s-FeSx(m), entry 6) were also used
4
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Table 2: Hydrogenation of substituted azobenzenes (2.0 mmol) at room
temperature using ALD FeSx/Al2O3 (ca. 1 mol%) as the catalyst.[a]
Conflict of interest
The authors declare no conflict of interest.
Keywords: atomic layer deposition · hydrogenation · iron ·
supported catalysts · thin films
Entry
1
Product
t [min]
Conv. [%]
93
20
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[a] For all reactions in this Table, the selectivity was >99%.
bility, this catalyst material could be uniformly loaded onto
the support substrate, which highlights the important role of
ALD for the reproducible fabrication of uniform supported
catalysts for organic synthesis.
In summary, we have reported a new ALD process
for the synthesis of FeSx from bis(N,N’-di-tert-butyl-
acetamidinato)iron(II) and H2S. The ALD process follows
ideal self-limiting ALD growth behavior to produce fairly
pure, smooth, and well-crystallized FeSx films. The ALD FeSx
films could also be conformally deposited into deep narrow
trenches with aspect ratios as high as 10:1, which indicates
that this ALD process is highly suitable for preparing
conformal and uniform FeSx coatings on porous or complex
3D nanostructures in general. Therefore, this process will
have a broad scope of applications in nanoscience and
nanoengineering. We further demonstrated its promising
application by the preparation of uniform nanoscale FeSx
coatings on porous g-Al2O3 powder. The ALD FeSx/Al2O3
catalyst exhibited excellent catalytic activity and selectivity in
the hydrogenation of azobenzenes to hydrazobenzene deriv-
atives under mild reaction conditions, which demonstrates the
high promise of ALD FeSx in organic synthesis.
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Manuscript received: January 14, 2017
Final Article published: && &&, &&&&
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Communications
Atomic Layer Deposition
Y. Shao, Z. Guo, H. Li, Y. Su,
X. Wang*
&&&& — &&&&
Atomic Layer Deposition of Iron Sulfide
and Its Application as a Catalyst in the
Hydrogenation of Azobenzenes
Iron sulfide (FeSx) thin films were
obtained by atomic layer deposition.
Nanoscale FeSx coatings on porous g-
Al2O3 powder were also prepared and
showed excellent catalytic activity and
selectivity in azobenzene hydrogenation.
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!