Preparation of 1T′-Phase ReS2 xSe2(1- x) (x = 0-1) Nanodots for Highly Efficient Electrocatalytic Hydrogen Evolution Reaction

Article abstract of DOI:10.1021/jacs.8b04513

As a source of clean energy, a reliable hydrogen evolution reaction (HER) requires robust and highly efficient catalysts. Here, by combining chemical vapor transport and Li-intercalation, we have prepared a series of 1T′-phase ReS_2xSe_2(1-x) (x = 0-1) nanodots to achieve high-performance HER in acid medium. Among them, the 1T′-phase ReSSe nanodot exhibits the highest hydrogen evolution activity, with a Tafel slope of 50.1 mV dec^-1 and a low overpotential of 84 mV at current density of 10 mA cm^-2. The excellent hydrogen evolution activity is attributed to the optimal hydrogen absorption energy of the active site induced by the asymmetric S vacancy in the highly asymmetric 1T′ crystal structure.

Full text of DOI:10.1021/jacs.8b04513

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Article
Preparation of 1T'-Phase ReS2xSe2(1-x) (x=0-1) Nanodots for
Highly Efficient Electrocatalytic Hydrogen Evolution Reaction
Zhuangchai Lai, Apoorva Chaturvedi, Yun Wang, Thu Ha Tran, Xiaozhi Liu, Chaoliang Tan, Zhimin
Luo, Bo Chen, Ying Huang, Gwang-Hyeon Nam, Zhicheng Zhang, Ye Chen, Zhaoning Hu, Bing
Li, Shibo Xi, Qinghua Zhang, Yun Zong, Lin Gu, Christian Kloc, Yonghua Du, and Hua Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04513 • Publication Date (Web): 05 Jun 2018
Downloaded from http://pubs.acs.org on June 5, 2018
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Preparation of 1T'-Phase ReS Se
ficient Electrocatalytic Hydrogen Evolution Reaction
(x=0-1) Nanodots for Highly Ef-
2
x
2(1-x)
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1,†
1,†
2,†
1,†
3,4
Zhuangchai Lai, Apoorva Chaturvedi, Yun Wang, Thu Ha Tran, Xiaozhi Liu, Chaoliang
1
1
1
1
1
1
1
Tan, Zhimin Luo, Bo Chen, Ying Huang, Gwang-Hyeon Nam, Zhicheng Zhang, Ye Chen,
Zhaoning Hu, Bing Li, Shibo Xi, Qinghua Zhang, Yun Zong, Lin Gu,
Yonghua Du, Hua Zhang
1
6
7
4
6
3,4,5
1
Christian Kloc,
7*
1*
1
Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological Universi-
ty, 50 Nanyang Avenue, Singapore 639798, Singapore.
2
Centre for Clean Environment and Energy, School of Environment & Science, Griffith University, Gold Coast cam-
pus, QLD 4215, Australia.
3
School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.
4
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Bei-
jing 100190, China.
5
Collaborative Innovation Center of Quantum Matter, Beijing 100190, China.
6
Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research),
Singapore 138634, Singapore.
7
Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), Singa-
pore 627833, Singapore.
E-mail: hzhang@ntu.edu.sg.
E-mail: du_yonghua@ices.a-star.edu.sg.
ABSTRACT: As a resource of clean energy, a reliable hydrogen evolution reaction (HER) requires robust and highly effi-
cient catalysts. Here, by combining the chemical vapor transport (CVT) and the chemical Li-intercalation, we have pre-
pared a series of 1T’-phase ReS Se
(x=0-1) nanodots for the high-performance HER in acid medium. Among them, the
2(1-x)
2x
-
1
1
T’-phase ReSSe nanodot exhibits the highest hydrogen evolution activity with a Tafel slope of 50.1 mV dec and a low
-
2
overpotential of 84 mV at current density of 10 mA cm . The excellent hydrogen evolution activity is attributed to the
optimal hydrogen absorption energy of the active site induced by the asymmetric S vacancy in the highly asymmetric 1T’
crystal structure.
4
, 18-19, 25-28
INTRODUCTION
active sites.
Besides the size effect, crystal-phase
engineering of TMD nanomaterials has also been proved
as one of the feasible ways to promote their catalytic ac-
tivities. For example, 1T-phase TMD nanosheets with high
electrical conductivity have been confirmed as promising
catalysts for the electrocatalytic HER because of their fast
Rational design and synthesis of efficient electrocata-
lysts for hydrogen evolution reaction (HER) is crucial for
the realization of sustainable hydrogen economy. Recent-
ly, tremendous efforts have been devoted into the explo-
ration of electrocatalysts with low overpotential and high
10, 20, 27, 29-
1-16
charge transport kinetics in the catalytic reaction.
efficiency toward water splitting. The discovery of lay-
3
1
Furthermore, introducing strains into the basal planes
of TMDs can also improve electrocatalytic activities for
ered transition metal dichalcogenide (TMD) nanomateri-
1
7
als, such as MoS , as one of the potential candidates to
2
22, 32-33
HER.
For example, strained 1T-WS nanosheets ex-
2
replace commercial Pt-based catalysts toward the cost-
effective electrocatalytic hydrogen production has trig-
gered increasing research interest on TMD-based cata-
lysts because of their earth abundance and high catalytic
hibited superior electrocatalytic activities toward HER as
2
2
compared to those without strains. Very recently, defect
engineering has been proved as another important strate-
gy to further improve the activity of TMD-based electro-
catalysts for HER. By introducing S vacancies into the
1
8-24
activities.
Recently, the edges of 2H-phase TMD na-
nomaterials have been identified as the active sites to-
1
7
strained 2H-MoS nanosheets, their electrocatalytic activi-
ward the electrocatalytic HER, resulting in the size engi-
2
ty could be greatly enhanced as compared to the 2H-MoS2
neering of TMD nanomaterials in order to expose more
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3
nanosheets with exclusive strain. The vacancy-induced
catalytic performance enhancement is also shown in po-
resolution TEM (HRTEM) image was used to confirm the
crystal structure of ReSSe NDs. As shown in Figure 1d,
the lattice fringe of 0.26 nm is consistent with the (002)
plane of ReSSe. The atomic-resolution high angle annular
dark-field scanning TEM (HAADF-STEM) image further
reveals the distorted 1T’ crystal structure of the ReSSe
NDs (Figure 1e). The EDS in TEM mode was used to
confirm the chemical composition of the ReSSe NDs
(Figure 1f), showing evident signals of Re, S and Se ele-
ments. Moreover, the UV-Vis spectrum of the brown
aqueous solution of prepared ReSSe NDs gives a smooth
1
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rous 1T-MoS nanosheets and 1T-MoSSe nanodots (NDs),
indicating the important role of defects in TMDs for the
2
1
9, 28
HER.
However, due to the symmetric nature of trigo-
nal prismatic 2H structure and octahedral 1T structure,
only one type of anionic vacancies can be formed. Differ-
ent from the 2H and 1T structures, the highly asymmetric
1T’-phase TMDs may result in different types of anionic
vacancies, because there are different types of chalcogen
atoms in the crystal space in terms of their positions.
However, the synthesis of 1T’-phase TMDs still remains a
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line without any obvious absorption peak, which is simi-
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35
36
big challenge. Since the ReS Se
possesses 1T’ phase,
lar with the reported ReS nanosheets (Figure 1g). The
2x
2(1-x)
2
it could be an ideal model catalyst to investigate the effect
of asymmetric vacancies for HER.
Zeta potential of the ReSSe ND solution was measured to
be around -29.7 mV, indicating the negatively charged
surface of ReSSe NDs, which stabilized the NDs in aque-
ous solution (inset in Figure 1g). The atomic force mi-
croscopy (AFM) characterization confirmed that the
height of ReSSe NDs is 1.2 ± 0.6 nm (Figure 1h,i), indicat-
ing their ultrathin thickness.
Here, by combining the chemical vapor transport
(CVT) and the chemical Li-intercalation method, the
large-scale production of water-dispersed, ultrasmall,
inherent 1T’-phase ReS Se
NDs have been prepared.
2(1-x)
2x
As a proof-of-concept application, the prepared ReS Se
2x
2(1-
NDs with high-density exposed active sites were used as
x)
highly efficient catalysts toward the electrocatalytic HER.
Due to the intrinsic distorted 1T’ structure, alloying effect
and the unsaturated coordinated Re sites caused by the
asymmetric S vacancies, the ReSSe NDs exhibit superior
HER performance with a low overpotential of -84 mV at
-
2
current density of 10 mA cm , a Tafel slope of 50.1 mV
-
1
dec , and good long-term stability. To the best of our
knowledge, the electrocatalytic HER performance of the
obtained ReSSe NDs is among the best as compared to
the reported TMD nanomaterials.
RESULT AND DISCUSSIONS
Synthesis and characterizations of 1T’-phase ReSSe
NDs. The ultrasmall ReS Se2(1-x) NDs with distorted 1T’
2x
structure were prepared by the combination of CVT and
chemical Li-intercalation method (Figure 1a). First,
Figure 1. Schematic diagram of the synthetic procedure and
characterizations of ReSSe NDs. (a) Simulated structure of
ReSSe and schematic diagram of the synthetic process of
ReSSe NDs. (b) Low-magnification TEM image of the pre-
pared ReSSe NDs. Inset: Size distribution of ReSSe NDs. (c)
HRTEM image of the prepared ReSSe NDs. (d) HRTEM im-
age of individual ReSSe nanodots. (e) HAADF-STEM image
of a single ReSSe nanodot showing typical 1T’ structure over-
lapped with the simulated structure. (f) EDS spectra of the
prepared ReSSe NDs obtained under TEM mode. (g) UV-Vis
spectra of the diluted solution of ReSSe NDs. Inset: Photo-
graph of the ReSSe ND solution. (h) AFM image of the pre-
pared ReSSe NDs. (i) Statistical analysis of the height of 110
ReSSe NDs measured from AFM images.
ReS Se (x=0-1) bulk crystals were prepared by the
2(1-x)
2x
CVT method (Figure S1), including ReS , ReS Se ,
2
1.8
0.2
ReS Se , ReSSe, ReS Se , ReS Se , and ReSe . In or-
1
.4
0.6
0.6
1.4
0.2
1.8
2
der to get ReS Se NDs in high yield, the prepared big
2(1-x)
2x
ReS Se crystals were subjected to ball-milling to ob-
2(1-x)
2x
tain ReS Se
powder, which was then immersed in n-
2
x
2(1-x)
butyllithium solution to form the Li-intercalated com-
pounds, followed by the sonication process in de-ionized
(
DI) water. Finally, after the high-speed centrifugation,
uniform ultrasmall ReS Se NDs were obtained.
2x
2(1-x)
The as-prepared ReSSe bulk crystals were characterized
by the scanning electron microscopy (SEM) (Figure
S2a,b), showing a small crystal size of a few hundred na-
nometers. Energy dispersive X-ray spectroscopy (EDS)
Besides the ReSSe NDs, other ReS Se NDs, includ-
2(1-x)
2x
(Figure S2c) was used to confirm the chemical composi-
ing ReS , ReS Se , ReS Se , ReS Se , ReS Se and
2
1.8
0.2
1.4
0.6
0.6
1.4
0.2
1.8
tion of the obtained ReSSe bulk crystals, and the result is
close to the designed stoichiometric ratio of Re, S and Se.
The X-ray diffraction (XRD) pattern (Figure S2d) is con-
sistent with the reference XRD patterns of ReS and ReSe .
ReSe , have also been successfully prepared via the similar
method. As shown in Figure S3, all the prepared
2
ReS Se
crystals have similar XRD patterns with a
2(1-x)
2x
2
2
gradual shift of some main peaks between ReS and ReSe
2
2
Transmission electron microscopy (TEM) images (Fig-
ure 1b-c) clearly reveal that the size of the obtained
ReSSe NDs is 1.7 ± 0.4 nm (inset in Figure 1b). The high-
bulk crystals, indicating their good crystallinity and struc-
3
7
ture accuracy. The ground powders of ReS , ReS Se ,
2
1.8
0.2
ReS Se , ReS Se , ReS Se , and ReSe exhibited simi-
1
.4
0.6
0.6
1.4
0.2
1.8
2
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lar crystal sizes with that of ReSSe crystal (Figure S4).
S atoms from ReSSe NDs rather than Se atoms, thus gen-
erating more S vacancies in the basal plane of ReSSe NDs.
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The exact experimental chemical compositions of these
prepared bulk crystals were further characterized by EDS
(see Table S1), showing high consistence with the de-
signed stoichiometric ratios. Using the similar chemical
Li-intercalation method, a series of ReS , ReS Se ,
2
1.8
0.2
ReS Se , ReS Se , ReS Se and ReSe NDs were pre-
pared (see Figure S5). Furthermore, the high-resolution
1
.4
0.6
0.6
1.4
0.2
1.8
2
HAADF-STEM images of ReS NDs clearly reveal the dis-
2
torted 1T’ structure with only the visible Re atoms showed
in the image, resulting from the large Z contrast differ-
ence between Re atom and S atom (Figure S6a,b). While
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for the ReSe NDs, both Re and Se atoms are clearly
2
shown in the HAADF-STEM images, which also exhibit
the similar distorted 1T’ structure (Figure S6c,d).
Moreover, the X-ray photoelectron spectroscopy (XPS),
X-ray absorption near-edge structure (XANES) and ex-
tended X-ray absorption fine structure (EXAFS) were per-
formed to investigate the surface properties and structure
information of the prepared ReSSe NDs. As known, the
chemical Li-intercalation of bulk TMD crystals can pro-
duce many defects, such as S vacancies, during the exfoli-
Figure 2. High-resolution XPS and X-ray absorption charac-
terization of prepared ReSSe bulk crystals and NDs. (a, b) Re
4f (a) and S 2p and Se 3p spectra (b) of ReSSe bulk crystals
and obtained ReSSe NDs. (c, d) XANES spectra (c) and Fou-
27-28
ation process.
Obviously, from the XPS spectra (Fig-
ure 2a), about 0.3 eV lower binding energy of Re 4f in the
ReSSe NDs was observed as compared to the ReSSe bulk
crystals, indicating that the Re atom is partially reduced
in the NDs. However, the binding energies of S 2p and Se
3
rier transformed (FT) k -weighted χ(k)-function of the
EXAFS spectra (d) for Re L -edges of ReSSe bulk crystals and
3
ReSSe NDs.
3
p in the ReSSe NDs and ReSSe bulk crystals are almost
Electrocatalytic activity of ReSSe NDs on HER. Pre-
vious studies demonstrated that the S vacancy and the
unchanged (Figure 2b). It reveals that some anionic va-
cancies, i.e., S and Se vacancies, are formed in ReSSe NDs.
To further investigate the dominant types of vacancy, the
prepared ReSSe NDs were characterized by XANES and
1T/1T’ structure are highly desirable for electrocatalytic
10, 27, 29-30, 33
hydrogen production.
As a proof-of-concept
NDs were also
application, our prepared 1T’-ReS Se
2(1-x)
2x
EXAFS. As shown in the Re L -edge XANES spectra (Fig-
3
3
used for electrocatalytic HER. All the catalytic experi-
ments were carried out on a standard three-electrode
ure 2c) and the corresponding k χ(k) oscillation curve
(Figure S7), the ReSSe NDs exhibit obvious difference as
electrochemical configuration in H -saturated 0.5 M
2
compared to the bulk crystals, which is also verified by
the Fourier transformed EXAFS (FT-EXAFS) (Figure 2d).
As compared to the bulk crystals (2.12 Å), the peak posi-
tion corresponding to the Re-S/Se coordination for ReSSe
NDs slightly shifted to the higher radial distance (2.15 Å),
indicating that there are more Re-Se bonding than Re-S
bonding in the ReSSe NDs. Furthermore, the peak inten-
sity of ReSSe NDs decreased significantly, i.e., around
50%, as compared to the ReSSe bulk crystals. These dif-
ferences between the prepared ReSSe NDs and bulk crys-
tals clearly reveal the structural change and size reduction
of ReSSe NDs. Detailed structure parameters are summa-
rized in Table S2. As compared to the bulk crystals,
which has the same coordination number (N) of 3 for
both Re-S and Re-Se, the Re-S and Re-Se bonds in ReSSe
NDs exhibit unsaturated coordination, giving a lower N of
H SO aqueous solution, using ReS Se ND-modified
2(1-x)
2
4
2x
glassy carbon electrode (GCE), Ag/AgCl (3 M KCl) and
graphite rod as the working electrode, reference electrode
and counter electrode, respectively. The iR-corrected po-
larization curves of ReSSe, ReS and ReSe NDs in com-
2
2
parison with the commercial 10% Pt/C catalyst are
demonstrated in Figure 3a, showing the onset potentials
of -196, -72 and -32 mV for ReS , ReSe and ReSSe NDs,
2
2
respectively. Impressively, the obtained ReSSe NDs
achieve a low overpotential of -84 mV at current densities
-2
of 10 mA cm , which is much better than that of ReS NDs
2
(
-320 mV) and ReSe NDs (-123 mV). The corresponding
2
-1
Tafel slopes of ReSSe NDs is 50.1 mV dec (Figure 3b),
suggesting that the catalytic reaction happened through
the combination of Volmer-Tafel and Volmer-Heyrovsky
mechanisms, in which the Volmer reaction is the rate-
determining step. For comparison, other compositions of
~
1.4 for Re-S and ~1.9 for Re-Se, respectively. The unsatu-
rated coordination and structural distortion of ReSSe NDs
indicate that there are highly exposed Re atoms at edge
sites and vacancies after the vigorous exfoliation process.
Note that in obtained ReSSe NDs, the Re-S bond has a
much lower coordination number (N=~1.4) as compared
to Re-Se bond (N=~1.9), implying that the chemical Li-
intercalation and exfoliation process can partially remove
ReS Se NDs were also subjected to electrochemical
2(1-x)
2x
test under the same conditions (see Figure S8a,b), fur-
ther revealing that ReSSe NDs are the most active catalyst
toward electrocatalytic HER. The electrochemical imped-
ance spectroscopy was also conducted to study the elec-
trode kinetics under HER process (Figure 3c and Figure
S8c,d). The Nyquist plots give the impedances of ReS2
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678.8 Ω), ReS Se (398.7 Ω), ReS Se (55.4 Ω), ReSSe method are described in the Supplementary Infor-
(
(
(
1
.8
0.2
1.4
0.6
1
2
3
4
5
6
7
8
9
14.4 Ω), ReS Se (46.9 Ω), ReS Se (26.9 Ω) and ReSe2
23.1 Ω) NDs at working potential of -0.202 V (vs. RHE),
mation. As proved by the XPS, XANES and FT-EXAFS
results, there are more S vacancies than Se vacancies in
the prepared ReSSe NDs. Therefore, we considered the S
vacancy in ReSSe NDs as the main factor here. Note that
due to the highly asymmetric structure of the 1T’ phase,
there are two possible ways of losing the chalcogenide
atoms to form four types of vacancy site, i.e., the high-site
S vacancy (HS-V), low-site S vacancy (LS-V), high-site Se
vacancy (HSe-V) and low-site Se vacancy (LSe-V) (see Figure
S10). In addition, the total energies of ReS , ReSe and
0
.6
1.4
0.2
1.8
which shows the same trend with their catalytic activities,
indicating that a faster Faradic process results in a superi-
or HER kinetics. Importantly, the ReSSe NDs showed
great long-term stability, with only negligible degradation
on HER performance after continuous 20,000 cycling test
(Figure 3d). The tested ReSSe NDs were then subjected
to the TEM characterization, showing similar dispersity
and crystallinity with the untested ReSSe NDs (Figure
S9a,b). This confirms that ReSSe NDs remained stable
after the long-term stability test. To the best of our
knowledge, the catalytic performance of our obtained
ReSSe NDs is among the best as compared to other re-
ported state-of-the-art TMD-based nanostructured cata-
lysts to date (see Table S3).
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0
1
2
3
4
5
6
7
8
9
0
2
2
ReSSe with different types of vacancies were also calculat-
ed, revealing that ReS , ReSe and ReSSe with low-site
2
2
S/Se vacancy are more stable because they have the lower
energy as compared to the high-site S/Se vacancy coun-
terparts (Table S4). Therefore, the low-site S/Se vacancies
and the edge sites can be the main active sites toward
HER (Figure 4a). On the edge sites, the ΔGH* value is
0
.92, 0.43 and 0.52 eV for ReS , ReSe and ReSSe, respec-
2 2
tively, suggesting that the unsaturated coordinated Re
atom associated with Se atom at the edge sites is most
reactive (Figure 4b). If the H atom adsorbs on the vacan-
cy, the ΔGH* value is further reduced in all systems. This is
because each H atom interacts with only one unsaturated
coordinated Re atom at the edge site, but with two un-
saturated coordinated Re atoms at the vacancy site. As
such, the different structures between edge and vacancy
lead to the much different ΔGH* values. The ΔGH* values
are 0.46, 0.28 and 0.00 eV for ReS L , ReSe L and
Se-V
2
S-V
2
ReSSe LS-V, respectively (Figure 4b). The ΔGH* values for
ReS H , ReSe H and ReSSe HS-V were also calculated
2
S-V
2
Se-V
for comparison, which are -0.33, -0.40 and -0.37 eV, re-
spectively (Figure 4b). Based on the theory proposed by
Norskov and coworkers, the optimal ΔGH* value of desired
3
8
electrocatalysts (e.g., Pt) should be around 0.00 eV. In
this work, it was found that the ΔGH* for H bonded with
the unsaturated coordinated Re atoms in the prepared
ReSSe LS-V site has the optimal value of 0.00 eV, which is
similar as that of Pt, suggesting its highest catalytic activi-
ty toward HER. Additionally, the unsaturated coordinated
Re atoms in ReS have the highest ΔG value, indicating
Figure 3. Electrocatalytic HER performance of ReSSe NDs.
(
a) Polarization curves (iR-corrected) of the commercial 10%
Pt/C, bulk ReSSe, and chemically exfoliated ReS , ReSSe and
2
ReSe NDs used as catalysts in 0.5 M H SO aqueous solu-
2
2
4
tion. (b) The corresponding Tafel slopes of the catalysts de-
rived from (a). (c) Nyquist plots of bulk ReSSe, ReS NDs,
2
2
H*
ReSSe NDs, ReSe NDs at working potential of -0.202 V (vs.
their lowest activity. Our theoretical trend matches the
experimental observations very well. Our results also con-
firm that the alloying effect between S and Se atoms in
ReSSe can enhance the catalytic activity of the basal plane
of 1T’-ReSSe for HER. This can also explain why ReSSe
NDs exhibited the better performance as compared to
ReS and ReSe NDs. Based on the calculation results, it
could be concluded that the significantly enhanced cata-
lytic activity of prepared ReSSe NDs might originate from
the excess of charge densities around the unsaturated
coordinated Re site next to the low-site S vacancy. The
exposed unsaturated coordinated Re atoms endow these
ultrasmall NDs with abundant catalytically active sites for
the electrochemical HER, which may also be found in
other highly asymmetric 1T’-phase TMD nanomaterials.
2
RHE). Insets: (Bottom-left) the corresponding fitting equiva-
lent circuit, where Rs represents the uncompensated re-
sistance, Rp represents the charge transfer resistance, and
CPE is the value of the argument of the constant phase ele-
ment. (Top-right) the enlarged plot of the area indicated
with a red dash square. (d) Durability test of ReSSe NDs. The
polarization curves were recorded before and after 20000
potential cycles in 0.5 M H SO aqueous solution from 0 to -
2
2
2
4
0.202 V (vs. RHE).
Mechanism for the enhanced catalytic activity to-
wards HER. To further verify the catalytically active sites
and understand the mechanism of the high-performance
of ReSSe catalysts, we systematically studied the Gibbs
free energies (ΔG ) of the adsorption of the intermediate
H*
hydrogen atom (H*) on the edges and vacancies of ReS2,
ReSe and ReSSe, by virtue of extensive first-principles
2
density functional theory (DFT) calculations (Figure 4
and Figure S10). More details about the calculation
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logical University, Singapore, for use of their electron mi-
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croscopy (and/or X-ray) facilities. This work was supported
by National Program on Key Basic Research Project
(2014CB921002) and the Strategic Priority Research Program
of Chinese Academy of Sciences (Grant No. XDB07030200)
and National Natural Science Foundation of China (51522212,
5
1421002, and 51672307). The computational research was
undertaken on the supercomputers in National Computa-
tional Infrastructure (NCI) in Canberra, Australia, which is
supported by the Australian Commonwealth Government,
and Pawsey Supercomputing Centre in Perth with the fund-
ing from the Australian government and the Government of
Western Australia.
Figure 4. Theoretical calculation results of ReS , ReSe and
2
2
0
1
2
3
4
5
6
7
8
9
0
1
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3
4
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ReSSe with different types of S/Se vacancies. (a) Simulated
models of an H atom bonded with the ReS edge, ReSe edge,
2
2
ReSSe edge, ReS L , ReSe L and ReSSe LS-V. (b) Calcu-
Se-V
2
S-V
2
lated free energy (ΔG ) versus the reaction coordinate of
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hzhang@ntu.edu.sg
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