Accepted Manuscript
Tailoring catalytic activities of transition metal disulfides for water splitting Seokhoon Choi, Ki Chang Kwon, Soo Young Kim, Ho Won Jang
PII: S2452-2627(17)30064-8
DOI: http://dx.doi.org/10.1016/j.flatc.2017.06.010
Reference: FLATC 31
To appear in: FlatChem
Received Date: 30 April 2017 Revised Date: 25 June 2017 Accepted Date: 25 June 2017
Please cite this article as: S. Choi, K.C. Kwon, S.Y. Kim, H.W. Jang, Tailoring catalytic activities of transition metal disulfides for water splitting, FlatChem (2017), doi: http://dx.doi.org/10.1016/j.flatc.2017.06.010
Tailoring catalytic activities of transition metal
disulfides for water splitting
Seokhoon Choi1,‡, Ki Chang Kwon1,2,‡, Soo Young Kim2,*, Ho Won Jang1,*
1Department of Materials Science and Engineering, Research Institute of Advanced Materials,
Seoul National University, Seoul 08826, Republic of Korea
2School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974,
Republic of Korea
‡These authors contributed equally.
KEYWORDS
Transition metal disulfide, hydrogen evolution reaction, water splitting, catalytic, molybdenum
Abstract
Development of clean and renewable energy to attain a sustainable society is one of the most
urgent tasks at present. In this respect, the production of hydrogen (H2) through electrochemical
or photoelectrochemical water splitting is a promising way to utilize sustainable energy source
such as sunlight. Recently, transition metal disulfides (TMDs) have received tremendous
attention as hydrogen evolution reaction (HER) catalysts due to their favorable chemical and
catalytic properties. Theoretical and experimental demonstrations have revealed that TMD-based
catalysts exhibit excellent catalytic activity at edge sites. However, there is limitation that the
basal plane which constitutes most of the TMD material remains chemically inert. Herein, we
overview recent progress in tailoring catalytic activities of TMDs for electrochemical and
photoelectrochemical hydrogen production and discuss the future strategies to overcome the
1. Introduction
The potential greenhouse gas emission of fossil fuels as well as their uncertain reserves has
triggered the need for renewable energy development to create a sustainable society. It has been
an urgent issue to find out renewable and carbon-neutral energy sources in research field.
Hydrogen has the highest energy density than any other chemical fuels and produces water for
combustion products. Therefore, hydrogen can be utilized as future energy carrier by storing
energy in a chemical bond between hydrogen atoms [1-3]. Since the atmosphere contains little
hydrogen, it must be separated from other sources. The most economically competitive method
for generating hydrogen is steam reforming of the fossil fuels [4], but there is a severe problem
of the greenhouse gas emission such as carbon dioxides. The other method for hydrogen
production is electrolysis of water by electrochemical (EC) [5, 6] or photoelectrochemical (PEC)
[1, 7, 8] water splitting. For practical purpose, the storing solar energy in the form of hydrogen
via water splitting is the ultimate goal to meet the growing global energy demand with reducing
dependence on fossil fuels. However, under standard conditions, the free energy change of ΔG =
237.2 kJ/mol is required to decompose one molecule of H2O to H2 and 1/2O2. Furthermore, the
charge transfer processes at solid/liquid interface are sluggish due to the high activation energy
of water splitting reaction. These process could be accelerated by loading electrocatalysts which
can reduce activation energy [9-11]. Noble metal catalysts, such as Pt, Pd, Ru, and Ir, exhibit
outstanding hydrogen evolution reaction (HER) performance [12-14]. However, the high cost of
noble-metal catalysts due to their scarcity limits their widespread utilization. Hence, it is
significant to develop the noble-metal-free electrocatalysts which are composed of earth
abundant materials for the sustainable water splitting devices.
During the past several years, there have been a surge in the development of noble-metal-free
[15, 16] and hydroxides [17, 18], carbon based materials and their hybrids [19-21]. Even though
these electrocatalysts display excellent improvement for HER, they still show lower HER
performance compared to Pt-based catalysts.
2-dimensional transition metal disulfides (TMDs) which have fascinating optical and
electrical properties became one of the most extensively investigated materials as HER catalysts
in the last decade [6, 22]. There are various types of bulk TMDs including insulators such as
HfS2, semiconductors such as MoS2 and WS2, to true metals such as NbS2 [23, 24]. Each layer of
TMDs which have the general formula of MS2 (M = transition metal) is composed of three
atomically stacked layer (S-M-S) and interconnected by weak van der Waals forces. When the
bulk TMDs become atomically thin 2D films, the 2D TMDs not only maintain their original
characteristics but also obtain additional properties due to quantum confinement effects [23].
Since 2005, MoS2 was spotlighted as a promising HER electrocatalyst by the study of the
Norskov’s group. They found that the binding free energy of atomic hydrogen to Mo-edge in the
MoS2 is close to zero which is comparable to Pt [25, 26]. To the best of our knowledge, it is the
first time that the edge sites of TMDs are identified as electrochemically active site.
Subsequently, other 2D TMDs (WS2, TiS2, and TaS2) were investigated by experimental and
theoretical studies [27-30]. Such studies demonstrated that other 2D TMDs could be also
efficient electrocatalysts for hydrogen production. However, one crucial shortcoming of a
TMD-based catalyst is the limited number of catalytically active edge sites since the most of the
surface consists of the electrochemically inactive basal plane in its presented form.
In this review, electrocatalysts based on 2D TMDs are covered in comprehensive manner
with major emphasis on the active site engineering. We summarize the major strategies for
tailoring catalytic activity of 2D TMDs with synthetic method. While the electrocatalysts based
obstacles that impede practical utilization. We will discuss the challenges to realize TMD based
catalysts comparable to noble metal catalysts such as Pt.
2. Tailoring catalytic activity
2.1. By nanostructuring
The layered chalcogenide materials including MoS2 show poor catalytic activity for the
HER in its bulk form [31]. Recent work showed MoS2 to be a promising electrocatalyst for the
HER. Both computational and experimental results confirmed that the HER activity stemmed
from the Mo-edges of MoS2 plates, while their basal planes were catalytically inert [25, 32]. For
these reasons, the nanosized MoS2 synthesized by various methods such as lithiation [33, 34] and
liquid exfoliation [35] should be more active for the HER catalysis than materials in bulk form.
The nanoparticles of layered materials usually exhibit platelet-like morphology, in which the
basal planes exposed, due to the anisotropic bonding and the general tendency to minimize its
surface energy. The increasing of the edge dimension is challenging for improving the catalytic
activity of layered chalcogenide materials [36]. Furthermore, the anisotropic conductivity of
MoS2 poses a challenge for high surface area structures as the conductivity of MoS2 is poor
along certain crystallographic directions [37].
Recently, the synthesis of nanostructured MoS2 have been extensively reported to improve
the HER catalytic activity of MoS2 [32, 38-40].As shown in Fig. 1A–D, Chen et al. [41] have
developed a core-shell MoO3-MoS2 nanowire catalyst with the following design criteria: (1) a
partially reduced MoO3 core with high conductivity, (2) a MoS2 shell (surface) with high
catalytic activity and stability in acids, and (3) a morphology of vertically aligned core-shell
nanowires to provide a high aspect ratio and a high surface area. The core-shell MoO3-MoS2
with a various temperature from 200 to 700 °C as shown in the scanning electron microscopy
(SEM) and transmission electron spectroscopy (TEM) images. The shapes and morphologies of
nanowires are gradually changed to the more blade-like (grass-like) surface. Nanowire with
synthesis temperature of 200 °C exhibits an onset for the HER at the overpotential at 1 mA/cm2
of approximately 150–200 mV, consistent with earlier results seen for nanostructured MoS2
particles [32, 38, 39]. The nanowire synthesized with the higher temperatures shows the lower
HER catalytic activity due to the excess sulfurization in the MoO3 core structure. The MoO3–
MoS2 core-shell nanowire could maintain the almost 100 % of its initial current density and
overpotential value at 10 mA/cm2 even after 10000 cycles. The partially reduced MoO3 core
serves as a nanostructured and conductive core, while the ultrathin and conformal MoS2 shell
serves as both a HER catalyst and a passivation layer.
In order to enhance the conductivity and the electrochemically active surface area (ECSA) it
can be an effective way to combine TMDs with conductive scaffolds. Li et al. [42] reported the
MoS2/reduced graphene oxide (RGO) composite nanostructures which exhibited excellent
electrocatalytic activity for HER. The MoS2/RGO catalyst was synthesized through facile
solvothermal method as illustrated Fig. 1E. The RGO plays a role as a nucleation sites for MoS2
nanosheets, enabling well dispersed growth of the MoS2 nanosheets. In contrast, the MoS2
nanosheets grown without RGO aggregate to spherical particles (see Fig. 1F). HRTEM image of
Fig. 1G shows highly dispersed MoS2 nanosheets on RGO template. As shown in Fig. 1H,
electrical coupling of MoS2 nanosheets with relatively more conducting RGO template
significantly reduced the charge-transfer impedance, leading to a small Tafel slope of 41 mV/dec.
Meng et al. [43] also reported p-MoS2/n-rGO composite nanostructures as a photocatalyst. The
nanoscale pn junction greatly enhanced PEC performance, shifting the onset potential, increasing
most semiconductor-based electrodes insufficiently absorb the entire solar spectrum and have a
limitation on recombination of electron-hole pairs. TMDs can enlarge the light absorption
spectrum of photocatalyst materials. Moreover, TMDs can be applied to reduce the activation
energy of hydrogen evolution and improve the separation of photogenerated electron-hole pairs
[44]. Liang et al. [45] demonstrated vertical VS2 nanoplate arrays on carbon paper grown by
facile hydrothermal method. The VS2 nanoplate arrays as a HER electrocatalyst exhibited
excellent catalytic activity with a small Tafel slope of 36 mV/dec, a high exchange current
density of 0.955 mA/cm2
, and a low overpotential of 42 mV at a current density of -10 mA/cm2 .
Because the VS2 which is the intrinsic metallic TMDs (e.g., VS2, NbS2, and TaS2) is catalytically
active in both basal planes and the edges, compared with MoS2 and WS2 that have catalytically
inert basal planes. Furthermore, the directly grown VS2 nanoplate arrays on conductive scaffold
(i.e., carbon paper) enabled the fast charge transport and also facilitated the detachment of
evolved hydrogen bubbles from the surface.
To increase the active edge sites and dangling bond on its surface, Kong et al. [46] have
demonstrated that the vertically aligned MoS2 and MoSe2 layers to maximally expose the
catalytic active sites by using a rapid sulfurization/selenization process as displayed in Fig. 2A.
Although the edge-terminated film has been considered unstable, it could be synthesized on
diverse substrates through a kinetically controlled rapid growth method. The chemical
conversion of the Mo thin films to the MoS2/MoSe2 occurs much faster than the diffusion across
the Mo thin film at the high temperature due to the anisotropic structure of MoS2/MoSe2 layer.
The edge-terminated thin films of MoS2 and MoSe2 layer was synthesized by controlling the
kinetics and temperature gradient as shown in Fig. 2B and C. The layers tend to be perpendicular
to the substrate, with exposed van der Waals gaps for fast reaction. They have evaluated the
The overpotential at 1 mA/cm2 of both materials are similar to each other with value of
approximately 350 mV. Interestingly, the turn over frequency (TOF) of MoS2 is very close to the
reported value of 0.016 s-1 obtained from combined scanning tunneling microscopy (STM)
studies and in situ electrochemical measurements on MoS2 nano-platelet catalysts [32]. It
confirmed that nearly all of the surface sites on synthesized edge-terminated films are
catalytically active. Due to the increased number of edge sites, the exchange current densities of
edge-terminated films were about ten times higher than previous MoS2 nanoparticle-based
electrodes and compare favorably to most common metal catalysts [32, 47].
Kwon et al. [48] demonstrated the wafer-scale fabrication of MoS2/p-Si heterojunction
using a thin-film transfer method for high-performance PEC hydrogen production.
Thickness-controlled MoS2 layers were synthesized by the thermolysis of a solution precursor layer on
SiO2/Si substrate as shown in Fig. 2E. The synthesized 2H-MoS2 layers contain a-domains with
layer-by-layer stacked (001) planes and c-domains with vertically stacked (100) planes. The
serrated surface of the synthesized 2H-MoS2 thin film provides a larger surface area than the
atomically flat surface of a single-crystalline MoS2 monolayer. Furthermore, the partially rotated
MoS2 basal planes, Moiré fringes, roughen the surface of the transferred MoS2 thin film as
observed in Fig. 2F. The films mainly contain a-domains with the preferred out-of-plane (001)
orientation of the 2H-MoS2 phase and c-domains with the preferred in-plane (001) orientation of
the 2H-MoS2 phase. The 13-nm-thick MoS2 layer heterojunction with p-Si photocathode have
displayed the high photocurrent density (24.6 mA/cm2 at 0 V vs. RHE), the large shifts in
overpotential (0.79 V at 10 mA/cm2), and the long-term stability (over 10000 s) as shown in Fig.
2G. The overpotential at 10 mA/cm2 of this heterojunction photocathode is lower than the those
at 0 V vs. RHE of this sample shows the highest value among the similar materials system.
These improved PEC performances compared to the atomically flat MoS2 layer originate from
the vertically stacked c-domains in synthesized thin films which can act as efficient catalytic
active region. They also have demonstrated the versatility of the synthesize thin films, the
various p-type III-V semiconductors were employed as photoelectrodes. When the 13-nm-thick
MoS2 films are transferred onto the p-InP, p-GaAs, and p-GaP photocathodes, the onset
potentials are shifted toward the anodic direction without notable losses in the saturation
photocurrents.
According to these previous reports, the nanostructured MoS2 could act as not only the
efficient catalyst but also the passivation layer for the stable electrolysis. The increase of
catalytic active edge sites is still main challenging issue for the efficient hydrogen production by
utilizing the TMD catalyst.
2.2 By anion doping
The electronic nature of TMDs can be manipulated by adjusting chemical composition. For
TMD nanosheets, the dopants are easily exposed on the surface of TMDs and take part in the
HER. Jin’s group [49] reported the amorphous MoQxCly (Q = S, Se) electrocatalyst for
electrochemical and photoelectrochemical hydrogen production. The catalytic activity of the
amorphous MoQxCly electrocatalysts is comparable to the state-of-art MoS2 based electrocatalysts.
According to ultraviolet photoelectron spectroscopy (UPS), the valence band edge position of the
amorphous MoQxCly is shifted close to the Fermi level, compared to the crystalline MoS2.
Obviously, the doping of Cl changes the electronic structures of MoS2 due to generation of defect
states into the band gap. Moreover, the additional Cl atoms increase disorder of the amorphous
MoQxCly electrocatalyst intergrating with n+pp+ Si micropyramid (MP) absorber. The n+pp+ Si
MPs were utilized in order to improve the light absorption of planar p-Si by scattering effect.
Furthermore, a built-in pn junction of n+p Si can enlarge the photovoltageof Si MPs by
modifying the limited p-Si/electrolyte junction [51]. The extra p+ layer on the backside of n+p Si
could improve the majority carrier (hole) collection, resulting in enhanced fill factor [52]. It is
also important to achieve high photocurrent density (see Fig. 3A). A uniform MoQxCly thin films
were deposited on the surface of the Si MPs using CVD method. As shown in Fig. 3B, MoSxCly
thin film exhibited a smooth surface, while MoSexCly thin film formed a nanogranular surface.
MoQxCly/n+pp+ Si MPs photocathodes showed a significant enhancement in the onset potential
and the photocurrent density as illustrated in Fig. 3C. The onset potential of MoSxCly/n+pp+ Si
MPs phothocathode showed cathodic shift of 0.14 V with respect to the MoSxCly/planar p-Si
photocathode due to the band bending of the n+pp+ structure. The onset potential of
MoSexCly/n+pp+ Si MPs also exhibited the cathodic shift of 0.18 V compared to MoSexCly/planar
p-Si. Furthermore, at 0 V vs. RHE, the photocurrent density of Si MPs photocathodes
significantly showed two- or three-fold increase compared to planar p-Si due to the improved
light-trapping property of Si MPs.
In recent reports, the phosphorus (P), which places on the near sulfur (S) in the periodic
table, was considered as a substitutional dopant for improving the catalytic activity to efficiently
generate the hydrogen [53-55]. Ye et al. [56] have reported the one-step approach to improve the
HER activity of MoS2 and molybdenum phosphide (MoP) via formation of a MoS2(1-X)PX (X = 0
to 1) solid solution. The bulk particulate MoS2 (diameter 6 m) was annealed at 750 °C with
different amounts of red phosphorus in Ar/H2 flow. They have calculated the hydrogen
adsorption Gibbs free energy by using the Projector Augmented Wave pseudopotentials, an
initio Simulation Package [57-61]. A supercell composed of 6 × 6 MoS2 primitive cells was
used to model the P-alloying and H adsorption as shown in Fig. 3D. They found that ΔGH = 2.2
eV for the pristine MoS2 surface and ΔGH < 0.8 eV for the neighnoring S atoms in the P-alloying
MoS2 surface. The drastic enhanced H adsorption on the P-alloying MoS2 surface could be stated
by using the “state-filling” model. The work required to accommodate the H electron in much
lower in the surface of the P-alloyed MoS2 than that of the pristine MoS2, leading to a stronger H
binding. To observe the morphology of the pristine and P-alloyed MoS2 particles, they measured
the SEM images and Brunauer-Emmett-Teller (BET) analysis. As shown in the Fig. 3E, the size
of the P-alloyed MoS2 particles became the larger than that of the pristine MoS2 after the
phosphorization reaction. Furthermore, the BET surface area MoS2(1-X)PX increased gradually
from 1.3 to 9.4 m2/g as a function of increase of X. To clarify the substitution of S by P in the
MoS2 lattice, the energy-dispersive-X-ray-spectroscopy (EDS)-scanning transmission electron
microscopy (STEM) of MoS0.94P0.53 was conducted as shown in the Fig. 3E. The Mo, P, and S
are mostly homogeneously distributed among the sample, except the region with low P intensity
which is marked by circle in the Fig. 3E. The P atoms could be easily substituted into the MoS2
lattice due to the similarity of the Mo–S and Mo–P bond lengths, indicating that the MoS2(1-X)PX
particles could be easily synthesized without breakage of Mo–S bonding. The HER activities of
MoS2(1-X)PX are gradually enhanced until the X value reaches at 0.53 (see Fig. 3F). It showed the
overpotential of 150 mV at the current density of 10 mA/cm2. However, the lower HER activities
were observed in the samples with the larger P ratio than S. Also, the Tafel slopes of these
samples have shown the similar tendency. The reaction with phosphorus significantly reduced
the Tafel slope from 131 mV/dec (MoS2) to 57 mV/dec (MoS0.60P0.70), indicating that the HER
stability with 20000 cylces clearly show that the P-alloyed MoS2 particles could endure the
acidic nature of measurement system, while the pristine MoS2 were degraded. Based on these
results, the P-alloyed MoS2 particles could act as an efficient catalyst with high acidic stability.
Kwon et al. [62] also synthesized sulfur-doped molybdenum phosphide (S:MoP) thin film
through simple thermolysis method by using [(NH4)2MoS4] solution and powder precursors with
different sulfur/phosphorus weight ratios. The S:MoP thin film catalyst, which is transferred on
p-Si substrates, showed a high photocurrent density (33.13 mA/cm2 at 0 V vs. RHE), large
overpotential shift (0.9 V at 10 mA/cm2) and long-term stability. Similar to above mentioned
report, DFT calculations indicated that the surface of S:MoP thin film absorb hydrogen better
than that of MoS2 thin film. Cabán-Acevedo et al. [63] studied pyrite-type cobalt
phosphosulphide (CoPS) as a HER catalyst. According to their DFT calculations, ΔGH can be
controlled by tuning the anion constituents of sulfur and phosphorus. After spontaneous H
adsorption at opened P sites, the ΔGH of the adjacent Co sites becomes very close to that of Pt.
Nanostructured CoPS only needed an overpotential of 45mV to achieve 10 mA/cm2 with long
term stability. The CoPS/n+pp+ Si micropyramids photocathode achieved photocurrent density up
to 35 mA/cm2 at 0 V vs. RHE.
The anions doping in MoS2 or other TMDs could be the one of the possible approach to
improve the catalytic inactivity of MoS2 in basal planes. The substituted anions facilitate the
decrease of the hydrogen adsorption Gibbs free energy in the neighboring S atoms in the MoS2
lattice by filling previously unoccupied electronic states within the substrate. The strong H
binding in the substitutional anion doped structure might be useful to improve the catalytic
2.3. By cation doping
Two-dimensional (2D) MoS2 nanomaterials have exhibited potential, but catalytic activity
only originates from their limited edge sites with massive, useless in-plane domains. It is great
interest to explore strategies to stimulate the catalytic activity of inert region which is in-plane
atoms of 2D hexagonal structure of MoS2, but it still remains as a great challenge due to the lack
of efficient experimental techniques and theoretical approaches [42, 64]. From the previous
technological approaches in graphene, the electronic properties of the carbon atoms neighboring
the doped heteroatoms can be significantly modulated thereby altering the catalytic activity of
in-plane carbon atoms [65-67]. To facilitate the useless in-in-plane domains for the catalysts, the
single-atom replacing ‘Mo’ atoms in MoS2 is one of the efficient way to enhance its catalytic
activity. The doped metal atoms could tune the adsorption behavior of H atoms on the
neighboring S sites. From this system, a significance enhanced HER activity on the MoS2 surface
could be anticipated by density-functional theory (DFT) calculations in recent reports.
Deng et al. [68] have demonstrated that the catalytic activity of in-plane S atoms of MoS2
can be triggered via single-atom metal (Pt, Ni, Co) substitutional doping into the Mo site. They
used a one-pot chemical reaction to synthesize the single Pt atom-doped, few-layer MoS2
nanosheets by using (NH4)6Mo7O24, H2PtCl6 and CS2 as precursors. From sub-angstrom
resolution high-angle annular dark field-scanning transmission electron microscopy
(HAADF-STEM) images, the single Pt atoms uniformly dispersed in the 2D MoS2 nanosheets were
observed as shown in Fig. 4A. The single Pt atoms could exactly occupy the positions of the Mo
atoms. The DFT calculations were carried out to characterize the catalytic process in Pt-MoS2.
Based on the optimized DFT calculations results, the in-plane S atoms, in the presence of Pt
and thereby the introduction of Pt atoms into the MoS2 can significantly enhance the HER
activity, as shown in Fig. 4B. Furthermore, as predicted in the volcano plot for varied
single-atom metal-doped MoS2, the non-noble-metal dopants such as Co and Ni could also be the good
alternative of Pt to tune the catalytic activity of MoS2. The HER polarization curves for the
Ni-MoS2, Co-MoS2, and Pt-MoS2 is displayed in Fig. 4C. In the case of Co-MoS2 nanosheets, the
obvious enhanced HER activities are observed by reducing the overpotential of about 20 mV
compared to flower-like MoS2 at the current density of 10 mA/cm2. The trend of HER activity in
these substitutional dopants is Pt > Co > Ni, which is good agreement with the calculated in
volcano plot as shown in Fig. 4B.
In MoS2, the Mo-edge has been demonstrated to be active sites for HER whereas the S-edge
is inert. Facilitating the catalytic inert S-edge sites is the one of challenging issues to improve the
catalytic activity of MoS2. In the recent report, Wang et al. [69] have predicted that the S-edge
site could also become HER active sites by incorporating transition metal atoms (Fe, Co, Ni, or
Cu) on the vertically standing, edge-terminated MoS2 nanofilms. From the DFT calculations, all
doped S-edge sites display the closer hydrogen adsorption Gibbs free energy value to
thermos-neutral than the that of S-edge of pristine MoS2 layer. Doping with Co, Fe, Ni, and Cu could
elevate the catalytic activity of S-edge sites to make it comparable to or better than pristine
Mo-edge sites. As shown in Fig. 4D, the transition metal-doped MoS2 thin films could be synthesized
by sulfurization process of the thin transition metal film/Mo film layer. In the kinetics controlled
synthesis condition, the Mo film changed to vertically aligned structure with many catalytic
active sites on its surface. The thin transition metal film could also be substituted into the top
layer of the MoS2 layer. The schematic illustration of synthesized nanofilms is shown in Fig. 4D.
pristine and Co-doped MoS2 nanofilms. No particles or clusters are also observed, indicating that
the Co atoms are uniformly distributed on the surface edge sites without formation of cobalt
sulfide. The electrochemical characterization of the synthesized nanofilms are displayed in Fig.
4H. The pristine MoS2 nanofilm reaches 0.6 mA/cm2 at 300 mV overpotential, while the Fe, Co,
Ni and Cu-doped MoS2 nanofilms achieve 2.3, 3.5, 2.4, and 2.6 mA/cm2 at 300 mV,
respectively. The catalytic activity was enhanced about 4 times compared to the pristine one.
Although the current density value at the 300 mV is drastically improved compared to the
pristine one, the value of Tafel slope is comparable with each other, regardless of kinds of
dopants. The unaffected Tafel slopes suggested that the rate-determining step is not changed
after the doping process, and thus the comparison of the exchange current densities as well as the
turn over frequency per active site is meaningful. They calculated the exchange current densities
and the turn over frequencies of the synthesized materials. Based on the theoretical and
experimental trends, the order of catalytic activity should be Fe-doped MoS2 > Co-doped MoS2 >
pristine MoS2, Cu-doped MoS2 > Ni-doped MoS2. This results paved the way to improve the
electrochemical catalysts by incorporating promoters into particular atomic sites, and for using
well-defined systems in order to understand the origin of the promotion effects.
The catalytic activity of materials is mainly decided by its structure, the number of edge
sites, and elements, but there are some promising way to improve the catalytic activity by using
various approaches. The substitutional doping method is one of the promising technique with
changing the catalytic inactive element to the other catalytic active element.
2.4 By phase transformation
2D TMDs has diverse polymorphs. Above all, 2H-TMDs mostly exhibit semiconducting
maximizing the exposure of edge sites to improve HER performance. However, 2H-TMDs have
one drawback to improve their performance due to the limited number of active edge sites. It has
been recently demonstrated that phase transition of semiconducting 2H-TMDs into metallic
1T-TMDs could enhance the HER performance [34]. Joensen et al. [33] showed that single layer
MoS2 nanosheets could be exfoliated to stable colloidal suspension by Li intercalation. Moreover,
chemical exfoliation via Li intercalation leads to transformations of the crystal structure owing to
the electron transfer from Li compounds to MoS2 nanosheets. According to the calculation work
[70], octahedral coordination is desirable in order to accept these surplus electrons in the d
orbitals of the transition metal. The metallic property of 1T-TMDs has come of these results.
Ambrosi et al. [71] explored the electrocatalytic activities of MoS2, MoSe2, WS2, and WSe2
for HER using similar exfoliation method of Li intercalation. They showed that the degree of
phase transformation and exfoliation are apparently different for the four TMD materials. The
differences in catalytic activities originated in the portion of the 1T phase which exist in the
exfoliated TMDs. Among them, WS2 was the most efficient catalyst for HER as the WS2
nanosheets exhibited the lowest overpotential. Lukowski et al. [72] demonstrated the facile
procedure to obtain 1T-TMD (1T-WS2) through a microwave-assisted reaction. The exfoliated
1T-WS2 nanosheets showed rapid kinetics, metallic property and increased number of active sites,
which enable a dramatic improvement in HER performance compared to 2H-WS2.
To elucidate the HER mechanism of two different phase MoS2 (2H and 1T), Voiry et al. [73]
partially oxidized the 2H-MoS2 and 1T-MoS2 nanosheets over few days in aqueous solution
which was saturated by oxygen. The oxidation was identified by the confirmation of several new
XPS peaks. The partial oxidation of the edges was also confirmed by TEM (see Fig. 5A) that the
edges of MoS2 nanosheets are very disordered after oxidation process, preserving the basal plane
showed large overpotential over 250 mV and low current densities due to the limited number of
edge site. On the other hand, chemically exfoliated 1T-MoS2 nanosheets showed significantly
enhanced electrochemical activity with reduced overpotentials of ~ 100 mV and low Tafel slopes
of ~ 40mV/dec (iR-corrected). After oxidation, HER activity of 2H-MoS2 nanosheets was
drastically deteriorated, while 1T-MoS2 exhibited thoroughly unaffected HER activity as shown
in Fig. 5B and C. 1T-MoS2 nanosheets showed no change in HER performance regardless of
edge oxidation, suggesting that the primary active site of 1T-MoS2 is located in the basal plane.
Furthermore, the metallic edge sites have quite smaller contribution on the HER in contrast with
2H-MoS2 nanosheets. Wang et al. [74] investigated the d-spacing, oxidation state and the ratio of
phase transformation of MoS2 catalysts using the battery testing system (see Fig. 5D). They
found that when the phase transformation from 2H to 1T takes place, the electrocatalytic
properties are already enhanced as the oxidation state of Mo is lowered. The decreased oxidation
states of Mo might affect the electronic structure which dramatically changes the hydrogen
adsorption free energy, ∆GH, and reduce the activation energy, resulting in the improvement of
HER performance. As the applied voltage vs. Li+
/Li is continuously reduced from 2.1 V to 1.1 V,
the first-order phase transition of 2H-MoS2 to 1T-MoS2 occurred between 1.2 V and 1.1 V. This
explains the superior HER activity of 1.1 V lithiated MoS2 as shown in Fig. 5F which well
accords with other recent reports [34, 75]. Gao et al. [40, 76] investigated the phase transition of
monolayer MoS2 from 2H to 1T and to 1T′ (distorted 1T) by density functional theory (see Fig.
5G). The atomic and electronic structures of 2H-, 1T- and 1T′-MoS2 are illustrated in Fig. 5H.
They found that the phase transition of MoS2 monolayer is seriously affected by the negative
charge. In general, the 1T phase MoS2 is not stable and easily transform to 2H-MoS2 or 1T′-MoS2.
The negative charge could thermodynamically stabilize 1T-MoS2 in comparison with the
eV when 4 electrons are supplied per MoS2 unit. At the same time, 1T-MoS2 is transformed to
1T′-MoS2. According to calculation (see Fig. 5I), the hydrogen adsorption free energy, ∆GH, on
(1010) Mo-edge of 1T′-MoS2 is as low as the (1010) Mo-edge of 2H-MoS2. This result suggest
that the electrocatalytic activity of 1T′-MoS2 exhibits comparable to the 2H-MoS2. Considering
the charge transfer, the 1T′-MoS2 would exhibit higher catalytic activity than the 2H-MoS2. Tang
et al. [77] also conducted DFT calculations to demonstrate the HER mechanism of the basal
planes of metallic 1T-MoS2 nanosheets. Compared to the catalytically inert basal plane of
2H-MoS2, the superior catalytic activity of the basal plane of 1T-MoS2 originated in its affinity for H
at the S sites on the surface. They found that the optimum HER on the basal plane of 1T-MoS2
will occur at the surface H coverage of 12.5 % ~ 25 %. Within this coverage, their results
suggest that HER could mainly occur on the basal plane of 1T-MoS2 through a combination of
the Volmer-Heyrovsky mechanism rather than the Volmer-Tafel mechanism. Moreover, they
discovered that the electrocatalytic activity of 1T-MoS2 can be further improved by substituting
other metal atoms (e.g., Mn, Cr, Cu, Ni, Fe) for Mo sites in the MoS2 lattice.
2.5 By defects generation
The catalytically active edge sites of 2H-TMDs are attributed to the unsaturated sulfur sites
of the edge. This resulted in the fabrication of amorphous TMD in which a myriad of sulfur was
unsaturated [40, 78]. Active edge sites could be also increased through the structural control of
the TMD nanosheets [79, 80]. Xie et al. [79] demonstrated defect-rich MoS2 nanosheets
synthesized by using high concentrated precursors and controlling the amounts of thiourea. The
defect-rich structure is formed when thiourea is used in excess, because excess thiourea adsorbs
defects resulting from the partial cleavage of the catalytically inactive basal planes leads to the
creation of extra active edge sites. A corresponding HRTEM images of top and side view are
displayed in Fig. 6B. The top-view HRTEM image shows the d-spacing of 2.7 Å which well
coincides with the d-spacing of the 2H-MoS2 (001) planes. Through the side view of HRTEM
image, numerous dislocations and distortions of defect-rich structures could be recognized. As
shown in Fig. 6C, the defect-rich MoS2 nanosheets show high HER performance with low onset
potential of 120 mV and the low Tafel slope of 50 mV/dec by virtue of the creation of extra
active edge sites. Ye et al. [81] also reported the structural defect-induced monolayer MoS2 via
oxygen plasma exposure and hydrogen annealing. The 2H-MoS2 monolayers are grown by CVD
method, similar to their previous report [82]. They exposed the 2H-MoS2 monolayers to oxygen
plasma for 10, 20, 30 s, reported as a successful method to generate defects into the monolayer
MoS2 [83-85]. After 10 s of oxygen plasma treatment, short and individual cracks could be
recognized as illustrated in Fig. 6D that begin to grow in the basal plane of 2H-MoS2
monolayers. The lowest Tafel slope of 171 mV/dec and the smallest onset potential (Fig. 6F) are
obtained from the sample which exposed to oxygen plasma for 20 s. Moreover, they also
intentionally generated structural defects through another approach using hydrogen annealing.
The hydrogen annealing has been utilized to create defects on graphene such as hexagonal holes
or fractal patterns [86, 87]. These analogous approach was applied to create additional active
sites by etching the catalytically inert basal plane of MoS2 monolayers. Unlike the case of
producing defects through oxygen plasma, the triangular holes created by hydrogen annealing
are non-uniform and omnipresent on basal plane as displayed in Fig. 6H. Samples treated with
hydrogen annealing apparently exhibit reduced onset potential and increased current density as
shown in Fig. 6G. Since the increase in current density is proportional to the exposed edge length
plasma method in improving the catalytic activity of MoS2 monolayers. Yin et al. [88] proposed
a novel hydrothermal method which simultaneously control crystal phase and disorder of
partially crystallized 1T-MoSe2 nanosheets. The phase transition of 2H-phase to 1T-phase, using
an excess content of reductant (NaBH4), enhanced the intrinsic conductivity and HER activity.
Moreover, a lower reaction temperature made disordered structure of abundant unsaturated
defects which play a role as active sites. These two synergistic factors led to an outstanding HER
performance with a low onset potential of 120 mV to exhibit – 10 mA/cm2 and the low Tafel
slope of 52 mV/dec.
As mentioned above, most approaches focus on optimizing the edge sites by generating
structural defects. Meanwhile, there are a few approaches that activate the basal planes which
occupy the majority of 2H-MoS2. Li et al. [89] reported that the inert basal plane of 2H-MoS2
monolayer could be activated and optimized by generating S-vacancies and exerting strains as
illustrated in Fig. 6H. They first used DFT calculations to determine the hydrogen adsorption
free energy, ∆GH. The maximum catalytic activity can be expected when the ∆GH approaches 0 eV
[90]. This is because the lower ∆GH let the atomic hydrogen more strongly bound, making it
difficult to desorb molecular hydrogen, while higher ∆GH hinder the adsorption of atomic
hydrogen on the catalyst. For basal planes of pristine 2H-MoS2, ∆GH is around 2 eV, which
exhibits an inert property. As S-vacancies are introduced, the atomic hydrogen is stabilized on
the S-vacancies and results in the decrease of ∆GH as shown in Fig. 6I. Then, they explored the
effect of the strained 2H-MoS2. Regardless of the S-vacancies concentration, the elastic strains
lead to more negative ∆GH by strengthening hydrogen adsorption [75, 91]. The decrease of ∆GH by
introducing S-vacancies and elastic strains can be explained through investigating its electronic
structure. As S-vacancies are generated, new electronic bands are created in the gap close to the
closer to the Fermi level. In order to identify the results from DFT calculations, they synthesized
monolayer 2H-MoS2. Then, tensile strain was applied by capillary force using a patterned Au
nanocone and the S-vacancies were introduced on basal plane of 2H-MoS2 through exposure to
Ar plasma. Fig. 6J displays linear sweep voltammetry (LSV) curves of Au electrode, Pt electrode,
Pristine MoS2 without strain and S-vacancy, strained MoS2 without S-vacancy (S-MoS2),
S-vacant MoS2 without strain (V-MoS2), and strained MoS2 with S-vacancies (SV-MoS2). Firstly,
the Au electrode exhibits little HER activity and the Pt electrode has the highest HER activity as
in previous studies [34, 73, 92-94]. The pristine MoS2 has very low current density which well
coincides with the nature of inert basal plane. The HER activity is slightly increased for S-MoS2
while the HER activity is considerably increased for V-MoS2 in accordance with the prediction
for ∆GH. Moreover, the SV-MoS2 exhibits much higher current density at fixed potential. These
strategies of creating S-vacancies with elastic strain can be a universal post-treatment method for
other 2H-TMD catalysts that could maximize catalytic activity by activating inert basal planes of
2H-TMDs. Cheng et el. [95] also created S-vacancies to activate the basal plane of MoS2
monolayer using remote hydrogen plasma. They experimentally and theoretically examined the
effect of S-vacancies on the basal planes of the MoS2 monolayer. They demonstrated that the
HER activity of the MoS2 monolayer can be controlled by tuning the period of hydrogen plasma.
By using hydrogen plasma, the stoichiometry of the MoS2 monolayer can be controlled from
MoS1.97 to MoS1.20, providing relatively wide range of Mo-S atomic ratios with preserving the
original basal plane of MoS2 monolayer. Yin et al. [96] fully explored the catalytic nature of
MoS2 for HER in terms of the phase, edge exposure, and sulfur vacancies. They used a series of
representative samples including 2H and 1T phase, porous 2H and 1T phase, and
sulfur-compensated 2H phase MoS2 nanosheets. Among them, they reported that the excellent HER
and S-vacancies than pristine 1T-MoS2. This result is not only because the crystal phase, as
mentioned in Chapter 2.4, plays a major role in HER properties, but also because the synergistic
effects of both increased edge sites and S-vacancies contributed greatly to the catalytic activity of
porous 1T-MoS2 nanosheets.
2.6 By heterojunctions
Heterojunction is one of the promising approaches to make the best use of TMDs as
electrocatalyst. Xiang et al. [97] synthesized TiO2/MoS2/graphene heterojunctions (see Fig. 7A)
via two-step hydrothermal process. Fig. 7B shows TEM images of the synthesized
TiO2/MoS2/graphene heterostructure composite, which represents uniform distribution of TiO2
nanoparticles on MoS2/graphene support. The photocatalytic activity of TiO2 nanoparticles
without any support exhibited a negligible photocatalytic activity due to the fast recombination
of photogenerated electron-hole pairs. When the TiO2 nanoparticle make heterojunction with
MoS2, the TiO2/MoS2 composite showed a modest photocatalytic hydrogen production rate of
36.8 µmol hmol h−1. This relatively low photocatalytic activity is attributed to the mismatched
heterostructure of TiO2/MoS2 that form Schottky barrier at the interface [98], hindering the
transfer of photogenerated charge. With introduction of graphene, the photocatalytic hydrogen
production activity was significantly enhanced to 165.3 µmol hmol h−1
as shown in Fig. 7C. Since the
redox potential of graphene is located between the conduction band minimum of TiO2 and H+ /H2
potential, the photogenerated electrons in the TiO2 nanoparticles could be easily transferred to
MoS2 nanosheets through the conductive graphene “highway”. Then, the transferred electrons
could efficiently reduce H+
ions to H2 at the active edge site of MoS2 nanosheets. This synergetic
effect of TiO2/MoS2/graphene heterostructure is illustrated in Fig. 7A. Zong et al. [99] reported
The MoS2/CdS heterostructure showed even higher rate of hydrogen evolution than Pt/CdS
heterostructure. This result originates in the structural and electronic coupling of MoS2/CdS
heterostructure. Chang et al. [100] demonstrated highly efficient CdS/MoS2/graphene
heterostructure photocatalyst. The introduction of graphene prevents the aggregation of MoS2,
resulting in the few layer MoS2/graphene heterostructure. Furthermore, the incorporation of
graphene enable the rapid charge transfer of electrons which were generated from CdS. Thus, the
photogenerated electrons could be directly transferred from CdS to the edge sites of MoS2 or via
graphene to reduce H+
ion to H2. The schematic illustration of CdS/MoS2/graphene
heterostructure is shown in Fig. 7E and the corresponding HRTEM image is displayed in Figure
7D. The photocatalytic activity of CdS/MoS2/graphene heterostructure was greatly enhanced in
lactic acid solution. As shown in Fig. 7F, the hydrogen production rate of the
CdS/MoS2/graphene heterostructure system reached 2.0mmol h -1
which was higher value than
that of Pt/CdS heterostructure system (0.3 mmol h-1
). This result suggests that the charge transfer
is enhanced and the recombination of photogenerated electron-hole pairs is suppressed with
3. Conclusion and perspectives
Although the 2D TMDs have been extensively focused as a catalyst for evolving hydrogen,
the catalytic activity of 2D TMDs are still not enough to replace the Pt catalyst because of its
catalytic inactive in their basal planes. Both computational and experimental studies revealed
that the edge sites of 2H-TMDs exhibit the good catalytic activity for the HER, while the basal
planes were catalytically inert due to weak hydrogen binding capability. In this review, we have
categorized the tailoring methods to improve the catalytic activity in 2D TMDs based on their
dimensionalities and atomic structures. We have summarized the major approaches to improve
the catalytic activity of 2D TMDs in EC and PEC catalysis including i) core-shell nanostructures,
ii) substitutional anion and cation doping, iii) phase transformation from 2H to 1T, iv) effective
sulfur vacancies and the strain on its surface, and v) the heterostructure with other materials.
These tailoring methods have been reported to exhibit the enhanced catalytic activity in HER,
but the several issues still remained for applying these techniques to the practical application. To
increase the likelihood of industrialization, not laboratory-level performance, the TMD-based
catalysts have to be synthesized in the largescale via cost-competitive method. Also, the
long-term stability in actual application should be ensured. To date, the research has focused only on
edge sites and defects as active sites for 2H-TMDs.
One approach of our suggestions to achieve the highly efficient catalysts is heterojunction
between atomically thin TMD films. This approach using atomically thin TMD pn junctions to
reduce the overpotential of HER provides a very innovative way for realizing a highly efficient
and stable PEC water splitting because it allows us to overcome the fundamental limitations of
the conventional 2H-TMDs. According to the idealized volcano curve shown in Fig. 8, the
exchange current density of existing catalysts is determined intrinsically by the Gibbs free
each other to make a junction, carriers diffuse to reach the equilibrium and thus space charge
(depletion) region spatially confined in the 2D atomic planes of the junction is formed. Since the
depletion width is atomically thin, the induced electrical field would be extremely strong. Such a
field significantly promotes the transfer of injected carriers from p-Si into water. In other words,
the TMD pn junction can act as an interface dipole layer to preferentially adjust energy level
alignment at the electrode/electrolyte interface, as analogous to the n+p Si which has internal
built-in potential to modify the electrode/electrolyte junction. That is, an atomically thin TMD pn
junction plays a role as an electrostatic catalyst, which allows us to overcome the limitations of
the catalytically inert basal plains of TMDs, enabling the realization of dipole-induced
self-activating water splitting by TMD pn junctions. In addition, the TMD pn junctions should enable
the ultrafast exciton dissociation [101, 102], which helps transfer and separation of
photogenerated carriers injected from underlying light absorbing semiconductor without notable
losses. Therefore, the tailored TMD monolayer (ML) pn junctions can be highly efficient HER
catalysts. Furthermore, since diverse TMD materials with sizable band gaps and work functions
are available, there are a large number of untested combinations to form pn junctions. In order to
realize this TMD pn junction electrocatalyst, extensive studies in various combinations of TMD
and their alloys should be performed to identify a clear descriptor for HER, rather than being
Acknowledgements
This work was supported by the Samsung Research Funding Center of Samsung Electronics.
Ki Chang Kwon acknowledges the Global Ph. D Fellowship Program of the National Research
Foundation of Korea funded by the Ministry of Education.
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Fig. 1. (A) Schematic of core-shell nanowires with substiochiometric MoO3 cores of 20–50 nm and conformal MoS2 shells of 2–5 nm for the PEC hydrogen generation. (B) External morphology of synthesized core (MoO3) – shell (MoS2) structure at 200 °C. The inset figure shows the 2 × 3 cm2
samples on the SiO2 substrate. (C) Internal morphology of nanowires sulfurized at 200 °C. The inset image shows the magnified TEM images. (D) The PEC performance of the nanowires sulfurized at 200 and 300 °C on the TEC15FTO glass [41]. Copyright © 2011, American Chemical Society. Schematic solvothermal synthesis (E) with GO sheets to afford the MoS2/RGO hybrid and (F) without any GO sheets, free MoS2 particles. (G) High-resolution TEM images of folded edges of MoS2 particles on RGO in the hybrid. (H) Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.28 mg/cm3
Fig. 2. (A) Schematic of the synthesis setup in a horizontal tube furnace and the volume-rendered reconstructed TEM tomogram of a MoS2 film grown by rapid sulfurization, which resembles the ideal edge-terminated structure. The TEM image of (B) MoS2 and (C) MoSe2 thin film produced by rapid sulfurization, clearly showing exposed edges. (D) The polarization curves of edge-terminated MoS2 and MoSe2 films as well as a blank glassy carbon substrate showing H2 evolution [46]. Copyright © 2013, American Chemical Society. (E) The schematic diagram of simple thermolysis method by using thermal chemical vapor deposition system. (F) The TEM image of the synthesized MoS2 thin film with partially vertical stacked c-domains (001) and Moiré fringes. (G) PEC performances shown as polarization J–V curves of MoS2 films of
different thicknesses. The 13 nm-thick MoS2/p-Si photocathode shows 24.6 mA/cm2 at 0 V and a
significant potential value shift at 10 mA/cm2 from0.69 V for
p-Si to 0.082 V [48]. Copyright ©
Fig. 3. (A) Schematic of MoQxCly catalysts coated on n+ pp+
Fig. 5. (A) Edge-oxidized MoS2 nanosheets. High-resolution TEM and HAADF STEM of edge-oxidized MoS2 nanosheets showing corrugated edges caused by the chemical oxidization. Scale bars: 5 and 1 nm, respectively. And schematic of the oxidation process and partial restoration of the nanosheet edges after several voltammetric cycles. (B) Polarization curves of 1T and 2H MoS2 nanosheet electrodes before and after edge oxidation. iR-corrected polarization curves from 1T and 2H-MoS2 are shown by dashed lines. (C) Corresponding Tafel plots obtained from the polarization curves. Tafel slopes of ∼40 and 75−85 mV/dec have been measured for 1T and 2H MoS2, respectively. After oxidation, the Tafel slopes of 45 and 186 mV/dec for 1T and 2H-MoS2, respectively, were obtained [73]. Copyright © 2013, American Chemical Society. (D) Schematic of the battery testing system. The cathode is MoS2 nanofilm with molecular layer perpendicular to the substrate, where the green and yellow colors represent the edge sites and the terrace sites, respectively. The anode is the Li foil. (E) TEM image of MoS2 by Li electrochemical intercalation to 1.1 V vs. Li+
Highlights
Transition metal disulfides (TMDs) have received tremendous attention as hydroge n evolution reaction (HER) catalysts
Recent progress in tailoring catalytic activities of TMDs for electrochemical and p hotoelectrochemical hydrogen production is reviewed.
Obstacles for practical applicability are discussed to mention challenges to be mad e for realizing a catalyst comparable to Pt.
