Metallic 1T phase MoS2 nanosheets as a highly efficient co-catalyst for the photocatalytic hydrogen evolution of CdS nanorods

Article abstract of DOI:10.1039/c6ra10170d

Conversion of solar energy to hydrogen requires efficient, low-cost and earth-abundant H-producing photocatalysts. Herein, a novel noble metal-free photocatalyst 1T-MoS₂/CdS NRs was prepared via a simple method. The 1T-MoS₂/CdS NRs hybrid showed the highest photocatalytic activity when the optimized weight ratio of metallic 1T-MoS₂ to CdS NRs is 10 wt% in a lactic acid solution under visible light. A 794.93 μmol h^-1 H₂ evolution rate of the 1T-MoS₂/CdS NRs hybrid reached not only 35 times improvement than pure CdS NRs but also 4.5 times enhancement than the 2H-MoS₂/CdS NRs hybrid. The relative mechanism has been investigated. 1T-MoS₂/CdS NRs interfaces could form ohmic contact with very low contact resistances to promote photoexcited electron transfer, which was different from the p/n junction on the 2H-MoS₂/CdS NRs hybrid.

Full text of DOI:10.1039/c6ra10170d

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Metallic 1T phase MoS2 nanosheets as a highly
efficient co-catalyst for the photocatalytic
hydrogen evolution of CdS nanorods†
Cite this: RSC Adv., 2016, 6, 74394
Ping Du, Yuanzhi Zhu, Junyang Zhang, Danyun Xu, Wenchao Peng, Guoliang Zhang,
*
Fengbao Zhang and Xiaobin Fan
Conversion of solar energy to hydrogen requires efficient, low-cost and earth-abundant H-producing
photocatalysts. Herein, a novel noble metal-free photocatalyst 1T–MoS2/CdS NRs was prepared via
a simple method. The 1T–MoS2/CdS NRs hybrid showed the highest photocatalytic activity when the
optimized weight ratio of metallic 1T–MoS2 to CdS NRs is 10 wt% in a lactic acid solution under visible
light. A 794.93 mmol hꢀ1 H2 evolution rate of the 1T–MoS2/CdS NRs hybrid reached not only 35 times
improvement than pure CdS NRs but also 4.5 times enhancement than the 2H–MoS2/CdS NRs hybrid.
The relative mechanism has been investigated. 1T–MoS2/CdS NRs interfaces could form ohmic contact
with very low contact resistances to promote photoexcited electron transfer, which was different from
the p/n junction on the 2H–MoS2/CdS NRs hybrid.
Received 20th April 2016
Accepted 21st July 2016
DOI: 10.1039/c6ra10170d
www.rsc.org/advances
For practical applications, however, a noble metal-free co-
catalyst with high efficiency is more desired for economic
1. Introduction
reasons.
MoS2 was recently found to be an efficient noble metal-free co-
Photocatalytic hydrogen production is one of the most
attractive strategies to solve the energy crisis and environ-
mental issues.1,2 For the past few decades, many semi-
conductors have been developed at the forefront of these
efforts owing to their electronic properties and advances in
their synthesis.3–7 Among them, the n-type semiconductor
CdS can be used not only as an excellent sensitizer8,9 but also
as a promising visible-light-driven photocatalyst,10 attributed
to its narrow but direct band gap (about 2.4 eV) and a suitable
conduction band potential for the reduction of H+ to H2.11 In
particular, CdS nanorods (CdS NRs) have shown better
activity than the other CdS counterparts in the sacricial
donor assisted photocatalytic water splitting reaction,
because they can provide a more efficient pathway for elec-
tron transport.12,13 However, pure CdS, including the CdS
NRs, exhibits relatively low photocatalytic activity, because of
the easy recombination of photoexcited electrons and
holes.14 To enhance the separation of the photogenerated
charge and provide additional surface reaction sites, loading
of noble metals or their oxides as co-catalysts is an efficient
way to improve the photocatalytic activity.15–17 For example,
about 200 times of enhancement in the activity have been
observed in the Pt/CdS when compared with pure CdS.18,19
catalyst for many semiconductor photocatalyst.20,21 Actually, it
has been reported by many groups that MoS2 combined with CdS
could show enhanced photocatalytic activity.22,23 These improve-
ments are usually attributed to the formation of p/n junction
which can reduce the charge recombination, considering the
p-type semiconductor nature of the MoS2 employed.24,25 However,
the MoS2 is polymorphic and can adopt structures with different
electronic properties. For example, the intercalation of bulk MoS2
with lithium will destabilize the original trigonal prismatic
coordination of Mo, and a metastable 1T phase with octahedral
coordination and metallic properties is found in the exfoliated
MoS2 nanosheets.26 Interestingly, the metallic 1T phase MoS2
nanosheets have showed much higher electrocatalytic activity in
hydrogen evolution reaction, despite the specic mechanisms
involved remain controversial.27–29 Therefore, we wonder whether
the 1T phase MoS2 nanosheets can be used as a high efficient co-
catalyst for the photocatalytic hydrogen evolution of CdS.
To test this idea, we prepared the 1T–MoS2/CdS NRs hybrid
photocatalyst by a simple mixing strategy and evaluated its
performance in the photocatalytic hydrogen evolution under
visible light irradiation. Preliminary experiments revealed that
optimum loading of the 1T–MoS2 co-catalyst is around 10 wt%,
and further increase of the co-catalyst will result in the gradual
decrease of the photocatalytic performances. As comparison, we
also prepared the 2H–MoS2/CdS NRs hybrid using the dispersion
of 2H–MoS2 nanosheets that were converted from the 1T phase
counterpart according to a recent study by Chou et al.30
School of Chemical Engineering and Technology, State Key Laboratory of Chemical
Engineering, Collaborative Innovation Center of Chemical Science and Engineering,
Tianjin University, Tianjin 300072, China. E-mail: xiaobinfan@tju.edu.cn
† Electronic supplementary information (ESI) available: Supplementary gures.
See DOI: 10.1039/c6ra10170d
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ratios of MoS2 to the CdS NRs were 0, 5%, 10%, 20%, 50% and
100%.
2. Experimental
2.1. Materials and synthesis of the photocatalyst
2.2. Characterization
Materials. Commercially available molybdenum sulphide
(MoS2) powder (<2 mm, 99%), n-butylithium (n-BuLi, 2.5 M
hexane solution), Cd(OAC)2$4H2O, NH2CSNH2, ethylenedi-
amine (99%), ODCB, CTAB are purchased from Aladdin
Chemistry Co., Ltd., Shanghai, China. Lactic acid was provided
by J&K Scientic Ltd., Beijing, China. All the reagents were of
analytical grade and were used without further purication.
Deionized water was used in all experiments.
Synthesis of CdS NRs. CdS NRs were synthesized using
a modied method according to the reported literature.31,32
2.4 mmol Cd(OAC)2$4H2O and 4.8 mmol NH2CSNH2 were
added into a Teon-lined, stainless steel autoclave which had
been lled with 90% ethylenediamine to 60% of its capacity
(150 mL). The autoclave was maintained at 120 ꢁC for 20 h and
then cooled down to room temperature. The yellow precipitate
was collected by ltration and washed three times with distilled
water to remove the residues of organic solvent.
Exfoliation of 1T phase MoS2. Typically, the intercalation of
MoS2 was carried out by our previous methods.33 1 g MoS2
crystals with 10 mL BuLi/hexane solution was sonicating in
a 50 mL Schlenk ask for 3 h. Excess BuLi was recovered by
ltration with hexane, and the intercalated samples were
retrieved by centrifugation and washed three times with hexane
to remove excess lithium and organic residues in 30 minutes.
Standard air-free techniques were employed by using a Schlenk
line under the protection of Ar. Exfoliation was achieved by
immediately sonicating the freshly intercalated samples in
water for about 5 min, and the exfoliated samples were puried
by centrifugation several times.
The samples were characterized by scanning electron micros-
copy (SEM) (Hitachi S-4800), transmission electron microscopy
(TEM) (Philips Tecnai G2 F20), Raman (Renishaw inVia reex,
Raman spectrometer with 532 nm laser excitation), X-ray
diffraction (XRD) (Bruker-Nonius D8 FOCUS diffractometer),
diffusion reectance UV-visible (UV-vis) spectra (Unico UV-
2800), X-ray photoelectron spectra (XPS) (PHI5000VersaProbe)
and electrochemical impedance spectroscopy (EIS) (Chenhua
ChI660E). The photocatalytic reaction results were measured by
GC (GC-7900 FID system).
2.3. Photocatalytic experiments
The reactions were carried out in a Pyrex reaction cell connected
to a closed gas circulation and evacuation system. 20 mg of
catalyst was dispersed in a 100 mL 10 vol% aqueous solution of
lactic acid (10 mL) and water (90 mL). The solution was thor-
oughly degassed before irradiation. Visible light was generated
by a 300 W Xe lamp which is equipped with a UV cutoff lter (l >
420 nm). A continuous magnetic stirrer ensured the photo-
catalysts in suspension and uniform irradiation during the
whole process. The temperature of the reaction solution was
maintained at 6–10 ꢁC by a ow of cooling water. The amount of
evolved H2 was determined with a gas chromatograph. For the
consecutive reactions, 3 mL lactic acid was added into the
photocatalytic system aer each cycle. The activities of different
catalysts were compared by the average rate of the H2 produc-
tion in the rst 8 h.
The phase transition of MoS2. The phase transition of MoS2
were obtained by Chou's methods.30 A biphasic mixture was
formed in a separation ask consisting of the ODCB spiked with
2.5% oleylamine in one phase, and exfoliated MoS2 nanosheets
in water in the other. The mixture was vigorously stirred, and
then allowed to separate for 2 h. The process was repeated until
the sufficient quantities are transferred. The organic portion
was then collected, and residual water was removed. Then MoS2
nanosheets in organic solvent were transferred to a dried
Schlenk ask with a condenser column and a magnetic stirring
bar. While stirring at 500 rpm, the mixture was then purged of
air using Ar (>30 min). Heating rate of z10 ꢁC minꢀ1 was used
to reach 180 ꢁC. The mixture was held for 4 h with stirring. Aer
annealing, large aggregates were removed by low speed centri-
fugation at 1000 rpm. Later, the remaining mixture was washed
with hexane, ethanol and distilled water three times each via
centrifugation. The precipitate was dried in a freeze dryer for
12 h. Then it was redispersed in the distilled water.
2.4. Fabrication of lm electrodes and electrochemical
impedance spectra (EIS) measurements
1T–MoS2/CdS and 2H–MoS2/CdS powders were fabricated as
lm electrodes by a modied method reported in previous
work.35,36 First, the powders and ethanol were mixed homoge-
neously (10 mg mLꢀ1), then the obtained mixture was then
drop-casting on the conducting uorine-doped SnO2 glass
substrate (FTO, <15 U per square). The obtained lm has an
active area of 1 cm2. The EIS measurements were carried out at
an electrochemical workstation (Chenhua ChI660E) by using
a typical three-electrode conguration with a Pt wire as counter
electrode and Ag/AgCl (saturated KCl) electrode as the reference
electrode. The as-prepared lm electrode served as the working
electrode. The measurement was performed in the presence of
a 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1 : 1) mixture as a redox
probe in 0.1 M KCl solution under 300 W Xe illumination (l >
420 nm, 50 mW cmꢀ2). EIS Nyquist plots were collected at open-
circuit potential, with the frequency ranging from 100 kHz to
0.01 Hz and the modulation amplitude of 5 mV.
Fabrication of MoS2/CdS NRs photocatalyst. 0.1 g CdS NRs
were ultrasonically dispersed in 100 mL 0.027 M CTAB aqueous
solution for 0.5 h, a certain volume of MoS2 solution was added.
Then, the mixture was sonicated for more than 2 h.34 Large
amount of water was used to remove the excess of CTAB by
ltration. The precipitate was freeze-dried for 12 h. The weight
3. Results and discussion
As illustrated in the typical scanning electron microscope (SEM)
images of both hybrids (Fig. 1a and b), abundant CdS NRs are
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Fig. 1 Typical SEM and TEM images of the 1T–MoS2/CdS NRs (a and c)
and 2H–MoS2/CdS NRs (b and d).
Fig. 2 (a) Raman spectra of CdS NRs, 1T–MoS2/CdS NRs and 2H–
MoS2/CdS NRs. The high-resolution XPS spectra of Mo(3d) of (b) 1T–
MoS2/CdS NRs and (c) 2H–MoS2/CdS NRs.
well-dispersed on the surfaces of the exfoliated MoS2 nano-
sheets. On the contrary, pure CdS NRs show heavily aggregated
morphology (Fig. S2a and b, ESI†). The CdS NRs with 100 nm in
length and a length-to-diameter ratio of ꢂ10 are vivid on the
translucent MoS2 nanosheets under transmission electron
microscopy (TEM) in both cases (Fig. 1c and d). The close
contact is indicated by the invariably lying of CdS NRs on the
nanosheets and should facilitate the transport of the photo-
generated charge, as supported by the electrochemical imped-
ance spectroscopy (EIS) below.
To evaluate the performances of 1T–MoS2 nanosheets co-
catalyst, the same amount of 1T–MoS2/CdS NRs and 2H–
MoS2/CdS NRs hybrids, as well as the pure CdS NRs, 1T–MoS2
and 2H–MoS2 were subjected to the photocatalytic hydrogen
production by using lactic acid as the sacricial agent. Control
experiments show that no appreciable H2 evolution in the
absence of either irradiation or a catalyst, while both 1T–MoS2
and 2H–MoS2 are inert for the photocatalytic reaction (Fig. 3a).
In line with previous studies,39 repeated experiments by using
pure CdS NRs show limited photocatalytic activity (22.51 mmol
hꢀ1). Whereas the 1T–MoS2/CdS NRs hybrid with 10 wt%
loading of 1T–MoS2 show approximately 35 times higher activity
(794.93 mmol hꢀ1) than the pure CdS NRs under the same
conditions. It should be noted that further increasing the 1T–
MoS2 nanosheets will result in gradual decrease in the perfor-
mances (Fig. S1, ESI†), probably due to the decrease of the CdS
NRs photocatalyst, as well as the shielding of light by MoS2.
Although parallel experiments on the 2H–MoS2/CdS NRs hybrid
shows enhanced activity (175.82 mmol hꢀ1) as expected, the 1T–
MoS2/CdS NRs hybrid obviously outperforms its 2H phase
counterpart. To evaluate the photocatalytic durability, consec-
utive reactions (each for 4 h) were also carried out (Fig. 3b). We
found that the amount of produced H2 steadily increased with
the irradiation time in each cycle, and the rate of H2 evolution
decreased with every reaction cycle. Aer 12 h of irradiation, the
CdS NRs, 1T–MoS2/CdS NRs and 2H–MoS2/CdS NRs hybrid
remained 40.3%, 89.5% and 78.9% of their catalytic activity,
respectively. These results show that the photocorrosion of CdS
Raman spectroscopy reveals that the longitudinal optical
phonons of CdS NRs37 at 297 and 595 cmꢀ1 slightly shi to
lower frequencies aer loaded on the both the 1T and 2H phase
MoS2 (Fig. 2a). This phenomenon can be explained by the
increased tensile strain38 when CdS NRs closely contact with the
MoS2 nanosheets. Except the characteristic peaks of CdS NRs,
the Raman spectra of 1T–MoS2/CdS NRs and 2H–MoS2/CdS NRs
hybrids show weak in-plane E12g (at ꢂ380 cmꢀ1) and out-of-plane
A
1g (at ꢂ400 cmꢀ1) modes of MoS2. However, the additional J1,
J2, J3 and E1g peaks of 1T phase MoS2 are overwhelmed by the
strong CdS NRs background below 350 cmꢀ1 (Fig. S3, ESI†). To
provide a direct evidence for the phase differences of the MoS2
nanosheets in the two hybrids, X-ray photoelectron spectros-
copy (XPS) was also employed. As shown in Fig. 2b and c, the
Mo(3d) XPS spectra of the two hybrids show obvious differences
in both shapes and intensities at the Mo(3d) region, despite the
presence of the strong adjacent S(2s) peak mainly from the CdS
NRs. Similar to our previous study on 1T–MoS2 nanosheets,33
the Mo4+ 3d5/2 and Mo4+ 3d3/2 peaks of the 1T–MoS2/CdS NRs
hybrid can be deconvoluted into two independent components
with a separation of binding energy around 0.5 eV, corre-
sponding to the dominant 1T phase and the mixed 2H phase.
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Fig. 4 (a) Diffuse reflectance UV-vis spectra of CdS NRs, 2H–MoS2/
CdS NRs, 1T–MoS2/CdS NRs, 2H–MoS2 and 1T–MoS2. (b) Corre-
sponding plot of transformed Kubelka–Munk function versus the
energy of light.
Fig. 5 (a) The Nyquist plot of 1T–MoS2/CdS NRs and 2H–MoS2/CdS
NRs electrodes in 0.1 M KCl solution with 2.5 mM K3[Fe(CN)6]/
K4[Fe(CN)6] (1 : 1) mixture as a redox probe under 300 W Xe illumi-
nation (l > 420 nm, 50 mW cmꢀ2), at open circuit potential vs. Ag/
AgCl; the inset shows the equivalent circuit. (b) Bode phase plots
measured for 1T–MoS2/CdS NRs and 2H–MoS2/CdS NRs
photocatalyst.
Fig. 3 (a) The hydrogen evolution rate of different photocatalysts. (b)
Cycling test of photocatalytic H2 evolution for CdS NRs, 1T–MoS2/CdS
NRs and 2H–MoS2/CdS NRs.
NRs should account for the reduction of the catalytic activity.
Actually, the photocorrosion is a major problem for metal interfacial charge transfer resistance across the electrode/elec-
sulde photocatalysts.40 However, the introduction of the 1T– trolyte.43 It is found that the 1T–MoS2/CdS NRs shows a much
MoS2 and 2H–MoS2 did alleviate the photocorrosion and extend smaller Rct (359.3 U) when compared with that of the 2H–MoS2/
the lifetime when compared with pure CdS NRs.
CdS NRs (620.8 U) (Table 1). A lower value of Rct indicates more
To probe the mechanisms involved, diffuse reectance UV- efficient charge transfer across the electrode/electrolyte inter-
vis spectroscopy was rstly employed to study the optical face.44 Therefore, it is more difficult for the electron recombi-
properties of the two hybrids. In both the cases, the loading of nation from the photocatalyst to the electrolyte, and thus
MoS2 nanosheets can lead to the shi of the absorption edge of enhancing the photocatalytic activity.
the CdS NRs towards the visible light range (Fig. 4a). According
Besides, the electrons life time was also determined from the
to the Kubelka–Munk method (Fig. 4b), the bandgap of the pure Bode phase plots (Fig. 5b) by using the equation (s ¼ 1/2fmax),
CdS NRs, 1T–MoS2/CdS NRs and 2H–MoS2/CdS NRs can be where s is the lifetime of electrons in the photocatalysts, and
roughly estimated to be 2.44, 2.36 and 2.37 eV, respectively. The fmax is mid frequency peak in Bode plots.45,46 The frequency peak
slight decrease in the bandgap should benet the light of the 1T–MoS2/CdS NRs and 2H–MoS2/CdS NRs are at 175.78
absorption to enhance the photocatalytic performances,41 and 214.84 Hz, corresponding to electron life time of 0.91 and
indicating the electronic communication between the MoS2 and 0.74 ms, respectively (Table 1). These results demonstrate that
CdS NRs in both the two hybrids.42
Electrochemical impedance spectroscopy (EIS) was carried
out to further study the charge transfer processes on the MoS2/
Table 1 EIS results and electron life time of photocatalysts
CdS NRs interface. As shown in Fig. 5a, the Nyquist plot is tted
Photocatalyst
Rct (U)
fmax (Hz)
s (ms)
to the equivalent Randle circuit (inset in Fig. 5a), where Rs is the
electrolyte solution resistance, CPE is the constant phase
element for the electrode/electrolyte interface, and Rct is the
1T–MoS2/CdS NRs
2H–MoS2/CdS NRs
359.3
620.8
175.78
214.84
0.91
0.74
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based photocatalysts. Different from the p/n junctions on the
2H–MoS2/CdS NRs interface, thorough characterizations
suggest an ohmic contact with very low contact resistances is
formed on the 1T–MoS2 and CdS NRs interfaces. This ohmic
contact can signicantly reduce the recombination of photo-
excited electrons and contribute to the extraordinary photo-
catalytic activity of the hybrid. This work provides novel
mechanistic insights into the MoS2/CdS photocatalyst and an
attractive noble metal-free photocatalyst for the visible-light-
driven photocatalytic hydrogen evolution.
Fig. 6 Schematic illustration of the mechanism of the photocatalytic
reaction.
Acknowledgements
This study is supported by the National Natural Science Funds
for Excellent Young Scholars (No. 21222608), Foundation for the
Author of National Excellent Doctoral Dissertation of China (No.
201251), Program for New Century Excellent Talents in
University (No. NCET-12-0392) and the Program of Introducing
Talents of Discipline to Universities (No. B06006).
1T–MoS2/CdS NRs photocatalyst has a longer electron life time
than 2H–MoS2/CdS NRs, leading to a better photocatalytic
activity.47
Based on all of the above results, the extraordinary photo-
catalytic activities of the 1T–MoS2/CdS NRs can be explained by
the proposed mechanisms in Fig. 6. Under visible light irradi-
ation, the whole photocatalytic reaction can be considered as
three steps:48 (I) the electrons are excited from the valence band
of CdS NRs to their conduction band. (II) The photo-generated
electrons transfer from CdS NRs to 1T–MoS2. (III) The electrons
react with H+ in lactic acid aqueous solution to produce H2.
Meanwhile, the holes are mainly consumed by the lactic acid to
generate CO2. The second step of the photocatalytic reaction is
different between 1T–MoS2/CdS NRs and 2H–MoS2/CdS NRs
hybrid because of their diverse contact ways between MoS2 and
CdS NRs. The p/n junctions will be formed at the 2H–MoS2/CdS
NRs interfaces, according to previous studies.25,42 On the other
hand, Chhowalla's recent study49 pointed out that low-work-
function metals with energy levels close to the conduction
band of the contact semiconductors would enhance the device
performance as a result of the efficient injection of carriers,
which in turn decrease the contact resistance. In our study, the
work function of the metallic 1T–MoS2 is about 4.2 eV, which is
similar to the conduction band energy of CdS NRs. Therefore,
an ohmic contact with low contact resistance should be formed
at the interfaces between CdS NRs and 1T–MoS2 nanosheets.
The photo-excited electrons can readily transfer from CdS NRs
to the metallic 1T–MoS2 nanosheets through the ohmic contact,
which enables the energetic electrons to diffuse farther from the
interface and have longer life time.
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