Chemical synthetic strategy for single-layer transition-metal chalcogenides

Article abstract of DOI:10.1021/ja5079943

A solution-phase synthetic protocol to form two-dimensional (2D) single-layer transition-metal chalcogenides (TMCs) has long been sought; however, such efforts have been plagued with the spontaneous formation of multilayer sheets. In this study, we discovered a solution-phase synthetic protocol, called "diluted chalcogen continuous influx (DCCI)", where controlling the chalcogen source influx (e.g., H₂S) during its reaction with the transition-metal halide precursor is the critical parameter for the formation of single-layer sheets as examined for the cases of group IV TMCs. The continuous influx of dilute H₂S throughout the entire growth period is necessary for large sheet formation through the exclusive a- and b-axial growth processes. By contrast, the burst influx of highly concentrated H₂S in the early stages of the growth process forms multilayer TMC nanodiscs. Our DCCI protocol is a new synthetic concept for single-layer TMCs and, in principle, can be operative for wide range of TMC nanosheets.

Full text of DOI:10.1021/ja5079943

Communication
pubs.acs.org/JACS
Chemical Synthetic Strategy for Single-Layer Transition-Metal
Chalcogenides
Dongwon Yoo,† Minkyoung Kim,† Sohee Jeong, Jeonghee Han, and Jinwoo Cheon*
Department of Chemistry, Yonsei University, Seoul 120-749, Korea
S
* Supporting Information
keeping them in a single-layer has been one of the challenges to
overcome.
ABSTRACT: A solution-phase synthetic protocol to form
two-dimensional (2D) single-layer transition-metal chalco-
genides (TMCs) has long been sought; however, such
efforts have been plagued with the spontaneous formation
of multilayer sheets. In this study, we discovered a
solution-phase synthetic protocol, called “diluted chalc-
ogen continuous influx (DCCI)”, where controlling the
chalcogen source influx (e.g., H2S) during its reaction with
the transition-metal halide precursor is the critical
parameter for the formation of single-layer sheets as
examined for the cases of group IV TMCs. The continuous
influx of dilute H2S throughout the entire growth period is
necessary for large sheet formation through the exclusive
a- and b-axial growth processes. By contrast, the burst
influx of highly concentrated H2S in the early stages of the
growth process forms multilayer TMC nanodiscs. Our
DCCI protocol is a new synthetic concept for single-layer
TMCs and, in principle, can be operative for wide range of
TMC nanosheets.
Here, we successfully developed a solution-phase synthetic
protocol, DCCI, which provides the formation of single-layer
sheet with a large lateral dimension for group IV TMCs. One
key observation is that the slow and continuous influx of the
chalcogen source (e.g., H2S) into the reaction medium makes
selective growth in the lateral (a-, b-axis) direction (Scheme 1).
Scheme 1. Single- vs Multilayer Transition-Metal Sulfide
a
Nanosheets
wo-dimensional (2D) single-layer transition-metal chalco-
Tgenides (TMCs) have recently received tremendous
attention after the observation of unique material properties
which are different from multilayer or bulk phase TMCs.1 Their
physicochemical properties, such as conductivity, absorption,
and quantum efficiency, are greatly affected by the number of
layers2 and, in particular, electronic band structure changes
from indirect for bulk TMCs to direct for single-layer TMCs.3
Single-layer TMCs are considered as next generation 2D
materials especially for applications such as optoelectronics,
photocatalysis, and solar energy harvesting.4 Currently, gas-
phase chemical vapor deposition (CVD) and mechanical
exfoliation techniques are the two most established approaches
for the preparation of single-layer TMC nanosheets; however,
neither are perfect yet and tremendous efforts are currently
being made to improve issues such as controllability over size
and thickness, reproducibility, and mass production.5
Although solution-phase synthetic methods are advantageous
for their simple synthetic procedures and relatively easy scale-
up as proven in earlier reports,6 their synthetic implementation
for single-layer 2D TMCs has been challenging. Recent reports
suggest new possibilities for obtaining single-layer TMC
nanosheets;7,8 however, these methods also reveal their
limitations. Notably, single-layer 2D TMCs being present
during the early growth period are readily forming multilayer
2D nanosheets. Continuous lateral growth of TMCs while
a
Single-layer TMCs with 1T structure consisting of chalcogen−
metal−chalcogen triatomic layers. Finding a kinetic growth regime,
where lateral growth in high-energy surface (a-, b-axis) continues,
while the low-energy surface (c-axis) growth is prohibited, is key to
generate single-layer sheets. Upon its reaction with the metal halide
precursors (MXn; X= halogen), a dilute and continuous supply of the
chalcogen source throughout the entire reaction yields single-layer
nanosheets, whereas a rapid and highly concentrated supply of the
chalcogen source during the beginning of the reaction generates
multilayer structures (see vide infra).
As a representative case of this study, we choose ZrS2 among
the various TMCs to examine the possibility of single-layer
sheet formation. Each layer of ZrS2 comprises a S−Zr−S
triatomic octahedral structure corresponding to 1T-ZrS2
(Scheme 1). We use 1-dodecanethiol (DDT, 5.0 mmol) as
sulfur precursor for the in situ formation of H2S, a reactive
chalcogen source, which reacts with ZrCl4 (0.3 mmol) in
oleylamine at 245 °C (Figure 1a). The reaction is maintained
for 10 h and centrifuged to isolate the ZrS2 nanosheets. TEM
analysis shows that the products are single-layer nanosheets
Received: August 4, 2014
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
at 245 °C in the presence of oleylamine is collected into the
reaction mixture of N,N-dimethyl-1,4-phenylenediamine and
FeCl3 in a 6 M aqueous HCl solution to convert the H2S into
methylene blue (Figure 2a). The generation of methylene blue
Figure 1. Formation of single-layer ZrS2 nanosheets and multilayer
nanodiscs with the use of different chalcogen precursors. (a)
Characterization of the single-layer ZrS2 nanosheets: (i) TEM image
of the single-layer ZrS2 nanosheets; (ii) pseudocolor image of a small
area (L: layer); (iii) HRTEM image and FFT pattern (inset); (iv)
SAED pattern; and (v) AFM image. (b) Characterization of the
multilayer ZrS2 nanodiscs: (i) TEM image of ZrS2 nanodiscs and (ii)
high-magnification TEM image of the ZrS2 nanodiscs side view. DDT:
1-dodecanethiol, OLA: oleylamine.
Figure 2. In situ H2S generation kinetics from DDT and CS2. (a)
Methylene blue assay is used for the quantification of H2S. (b) Time-
dependent cumulative H2S generation profile from DDT (black dots)
and the H2S concentration for a given time interval (40 min, red bars),
which indicates that H2S is continuously generated over a long
reaction period (10 h). (c) Time-dependent cumulative H2S profile
from CS2 (black dots) and H2S concentration for a given time interval
(5 min, blue bars), where the H2S generation is initially very high but
stops after 20 min. (d) First-order reaction kinetics of H2S generation
from DDT. (e) Second-order reaction kinetics of H2S generation using
CS2 and OLA.
with ∼200−500 nm in lateral size (Figure 1a(i)). A
pseudocolor image of a selected area is provided to clearly
visualize the some overlay between the single-layer ZrS2
nanosheets (Figure 1a(ii)). A top-view high-resolution trans-
mission electron microscopy (HRTEM) image of the ZrS2
nanosheet reveals the periodic atomic arrangement by showing
lattice fringes with interplanar spacings of 3.1 and 1.8 Å
corresponding to the (100) and (110) planes, respectively, in
the hexagonal 1T-ZrS2 (Figure 1a(iii)). The fast Fourier
transform (FFT) image of the ZrS2 nanosheet shows hexagonal
patterns corresponding to the (100) and (110) reflections from
the top view (Figure 1a(iii) inset). The selected area electron
diffraction (SAED) pattern for the ZrS2 nanosheets confirms
their high crystallinity (Figure 1a(iv)). The atomic force
microscopy (AFM) image shown in Figure 1a(v) indicates a
nanosheet with a height of ∼0.88 nm, which is consistent with
the formation of a single layer. Additional XRD analysis shows
the absence of (001) peak, indicating the formation of single-
layer ZrS2 (Figure S2a).
By contrast, when we performed an identical reaction except
using CS2 as an alternative sulfur precursor, multilayer ZrS2
nanodiscs resulted; such an outcome is consistent with previous
studies (Figure 1b).9 A top-view TEM image of the synthesized
ZrS2 nanodiscs shows a uniform, free-standing disc-like
morphology with ∼20 nm in lateral size (Figure 1b(i)), and a
side-view image indicates that the number of layers is three
(Figure 1b(ii)).
is then monitored by using a UV−vis absorption spectrometer.
The total amount H2S produced by DDT gradually increases
(black dots, Figure 2b) up to ∼100 mM for the entire reaction
period (10 h). For each measured time segment (40 min), the
H2S influx holds steady between 12 and 3 mM from 1 to 10 h
period after a slightly higher H2S influx (∼19 mM) during the
first 40 min (red bars, Figure 2b).
In case of CS2, the amount of generated H2S increases
drastically at the initial temporal point and reaches a maximum
of ∼120 mM in 20 min with a plateau afterward (black dots,
Figure 2c). After the burst influx of H2S during the first 15 min
(blue bars, Figure 2c), the H2S influx drops down to zero from
20 to 180 min time period. The amount of generated H2S is 92
mM in the first 5 min. We observed that H2S generation from
the thermolysis of DDT is a unimolecular reaction following
first-order kinetics with a reaction rate constant of 2.8 × 10−3
s−1 (Figure 2d). By contrast, the kinetic plot presented in
Figure 2e shows that H2S generation from CS2 follows second-
order kinetics, confirming that H2S generation is a bimolecular
reaction between CS2 and oleylamine with a reaction rate
constant of 2.4 × 10−1 L mol−1s−1. The in situ H2S generation
rate from CS2 is ∼86 times faster than that from DDT.
The growth kinetics of colloidal nanostructures is governed
by a number of parameters, and one of them is the
concentration of monomers.13 As shown here, the distinct
kinetic differences between DDT and CS2 for H2S influx into
the reaction medium can be the determining factor for different
For detailed information on the in situ generation of H2S, we
11
investigated the H2S generation kinetics for DDT10 and CS2
under identical crystal growth conditions via the standard
methylene blue method.12 H2S generated from each precursor
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crystal growth patterns. Because the surface energies of the a-,
b-axis edge facets, such as (100) and (110), are approximately
4−6 times higher than that of the c-axis planar facet (001),8b,9
exclusive growth at the edges can be possible, but only when
the growth proceeds under specific conditions that can keep
their facet energy differences.14 This study shows that the dilute
and continuous influx of H2S from DDT satisfies such a kinetic
growth window for preferential 2D lateral growth. By contrast,
the burst H2S influx from CS2 generates overly concentrated
monomers and disrupts the facet selectivity so that
simultaneous growth in both the lateral and vertical directions
can take place.15 Similar growth pattern has been observed for
metallic prisms with high monomer concentrations.16 When we
examined another H2S precursor, thioacetamide, which has
similar H2S generation kinetics with CS2, multilayer ZrS2
nanodics are consistently observed (Figure S1).
Figure 4. Single-layer TiS2 nanosheets through DCCI method. (a)
Single-layer TiS2 nanosheets formed by using DDT. (b) (i) Low-
magnification TEM image; (ii) pseudocolor image for a selected area;
(iii) HRTEM image and FFT pattern (inset); (iv) SAED pattern; and
(v) AFM image of a single-layer TiS2 nanosheet.
For the validation of such H2S influx effects on the growth
patterns, H2S gas is directly introduced into a solution of ZrCl4
and oleylamine (Figure 3a). Rapidly injecting a fixed amount of
the top view (Figure 4b(iii) inset). Figure 4b(iv) is the SAED
pattern for the TiS2 nanosheets confirming the high
crystallinity. The AFM image in Figure 4b(v) shows a single-
layer 2D sheet with a height of 0.87 nm that is close to the 0.9
nm reported in the literature.5a,18 XRD analysis confirms single-
layer TiS2 formation (Figure S2b). This DCCI protocol is also
effective for single-layer HfS2 nanosheet formation (Figure 5).
Figure 3. Direct H2S gas flow experiment. (a) Diagram of the
experimental setup consisting of (i) H2S gas tank; (ii) pressure gauge;
(iii) shutoff valve; (iv) flowmeter; and (v) reaction vessel. (b,c) TEM
images of the ZrS2 nanodiscs (b) and nanosheets (c) produced by
using rapid and slow H2S supply, respectively.
H2S gas in a short time (20 min) following an identical H2S
generation profile of CS2 provides multilayer ZrS2 nanodiscs of
∼25 nm in diameter (Figure 3b). By contrast, the low and
prolonged influx of H2S gas for 600 min, mimicking the
dodecanethiol-mediated reaction, affords much larger ZrS2 (few
hundred nanometers) with a sheet morphology (Figure 3c).
Our control study indicates that the lateral size and thickness of
the nanosheets are clearly dependent on the H2S influx kinetics.
As a note, the use of gas-phase H2S does not provide high-
quality nanosheets relative to the in situ solution-generated
cases, possibly due to inhomogeneous mixing and reaction
between the gas- and solution-phases during the nucleation and
growth processes;17 however, such direct introduction of
chalcogen source also has the potential to generate high quality
single-layer TMCs upon further optimization processes.
Our DCCI method is extended for its suitability with other
group IV TMC nanosheet, TiS2 and HfS2. First, the same
synthetic procedures are performed with the introduction of
DDT (5.0 mmol) to a mixture of TiCl4 (0.26 mmol) and
oleylamine (18.7 mmol) at 230 °C (Figure 4a). The resulting
products are single-layer TiS2 nanosheets ranging between
∼300 nm and ∼1 μm in lateral size (Figure 4b(i)). The
pseudocolor image shows single-layer TiS2 nanosheets (Figure
4b(ii)). A top-view HRTEM image of the TiS2 nanosheet
indicates the periodic atom arrangements of the lattice fringes
with interplanar spacings of 2.9 and 1.7 Å for the (100) and
(110) hexagonal 1T-TiS2 planes, respectively (Figure 4b(iii)).
The FFT image of the TiS2 nanosheet shows a hexagonal
pattern corresponding to the (100) and (110) reflections from
Figure 5. Synthesis of single-layer HfS2 nanosheets. (a) Chemical
reaction for HfS2. (b) (i) TEM image; (ii) pseudocolor image for a
selected area; and (iii) SAED pattern.
Here, we have developed a solution-based synthetic protocol,
DCCI, for the formation of single-layer nanosheets of group IV
metal sulfides. The control of key synthetic parameters, which
renders the continuous growth of lateral facets, is critical, and
the dilute and continuous influx of H2S plays such a role. Our
study provides a new synthetic principle that could be
potentially useful for the single-layer sheet formation of other
2D layered materials; however, the specific reaction conditions
and parameters should be designed prior for each case.
ASSOCIATED CONTENT
* Supporting Information
Detailed synthetic methods and characterizations. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jcheon@yonsei.ac.kr
■
C
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Journal of the American Chemical Society
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Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was financially supported by the National Creative
Research Initiatives Program (2010-0018286).
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