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CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation
international journal of hydrogen energy xxx (xxxx) xxx
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One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation Lei-Lei Li a, Xing-Liang Yin a,*, Dong-Hui Pang a, Xin-Xin Du a, Han Xue b, Hua-Wei Zhou a, Qing-Xia Yao a, Huai-Wei Wang a, Jun-Chao Qian c, Jie Yang a, Da-Cheng Li a, Jian-Min Dou a a
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, PR China b College of Chemical Engineering, Daqing Normal University, Daqing, 163712, PR China c Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, Suzhou, 215009, PR China
highlights The
graphical abstract
MoS2/CdS
nano-
heterostructures were synthesized by using one-pot approach. The H2 evolution rate of MoS2/CdS is 162 times higher than that of pristine CdS. The extraordinary performance is attributed
to
the
well-defined
heterostructures. The
photocatalytic
mechanism
was detailedly investigated.
article info
abstract
Article history:
Exploration of cost-efficient and high-performance photocatalyst for H2 evolution by using
Received 4 June 2019
a facile approach is of great importance. In this manuscript, a facile one-pot method was
Received in revised form
employed to fabricate MoS2/CdS heterostructures with MoS2 intimately grown on the
28 August 2019
surface of CdS resulting in the formation of well-defined heterostructures. Screen experi-
Accepted 10 October 2019
ment reveals the optimized photocatalytic H2 evolution performance of MoS2/CdS is far
Available online xxx
exceeding that of pristine CdS by a factor of more than 162. The outstanding performance can be ascribed to the formation of heterostructures which accelerate charge trans-
Keywords:
portation and separation, and the MoS2 serving as a cocatalyst for the decrease of H2
One-pot synthesis
overpotential. Moreover, the photocatalytic mechanism of the MoS2/CdS was carefully
Heterostructures
* Corresponding author. E-mail address: [email protected] (X.-L. Yin). https://doi.org/10.1016/j.ijhydene.2019.10.056 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Photocatalysis
investigated, which contributes to the deep understanding of the photocatalytic process
MoS2/CdS
and the designation of other low-cost and high-efficient photocatalyst.
H2 generation
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photocatalytic H2 evolution from water splitting has been considered as a sustainable and promising way to solve the worldwide energy crisis and environmental problems [1e5]. Since the first report on TiO2-supporting Pt electrodes for photoinduced H2 production by Fujishima and Honda in 1972, a large number of semiconductors, such as TiO2, CdS, ZnxCd1xS, WO3, and g-C3N4 have been developed for photocatalytic water splitting [6e10]. Among them, CdS as a visible-light response semiconductor has attracted considerable attention owing to its superior light absorption and proper band structure for photocatalytic H2 evolution [11e14]. However, pristine CdS still suffers from low-efficiency and bad stability caused by ultrafast charge recombination and photocorrosion. To address these issues, loading cocatalysts on CdS surface not only accelerates photogenerated charge separation but also inhibits CdS from photocorrosion [15,16]. Noble metals, such as Ru, Pd, Pt, and Au, are excellent cocatalysts for CdS, but high-cost and difficult to be recycled still resist their application. Therefore, it is desirable to explore low-cost and highly efficient cocatalysts to further facilitate the H2 production. Recently, cheap and earth-abundant two-dimensional transition metal dichalcogenides (2D-TMDs) have been proved to be a kind of promising low-cost cocatalysts [17e19]. As a member of 2D-TMDs, MoS2 has aroused deep interest among scientists owing to its narrow band gap and wonderful morphologies [20e22]. The catalytic system of MoS2/CdS has been intensively studied since Zong et al. first reported MoS2 as a good cocatalyst for H2 generation [23]. In the past ten years, the researchers generally paid attentions on several aspects as follows: morphology control [24,25], increasing active sites [26,27], novel fabrication approaches [28,29], and photocatalytic mechanism [30]. Although those excellent works significantly promote the development of this catalytic system, how to combine those excellent strategies into a work is still somewhat of a challenge. In addition, the nanostructure MoS2/CdS usually suffers from severe agglomeration which deteriorates mass transfer and results in the charge recombination occurred at contacted surface, but this drawback in the past was not carefully overcome. Currently, the materials with secondary nanostructures, such as pomegranate- and urchin-like, have attracted increasing attention [8,31e33]. If the catalyst is self-assembled by the sub-nano structures and porous between them, the issue of agglomeration may be well addressed. With these considerations, in this work the porous MoS2/ CdS heterostructures, which consist of big sphere selfassembled by sub-nanosphere (~3.6 nm) (see Fig. 1 and Fig. S2), was synthesized for the first time via a facile one-pot
solvothermal approach. This work involves morphology control, enhanced active sites and using a gentle fabrication approach with low-energy and time consumption. The asprepared MoS2/CdS heterostructures with MoS2 intimately grown on the CdS reveal extraordinary high H2 evolution rate of 37.31 mmol g1 h1, which is 162 folds higher than that of the pristine CdS. This excellent performance can be attributed to the formation of well-defined heterostructures which accelerate charge transportation and separation as verified by various characterizations in this work. Besides, the favourable mass transfer and agglomeration-resistant structure, lower H2 overpotential of MoS2 and a great number of active sites offered by amorphous MoS2 collaboratively contribute to the remarkable photocatalytic performance. Moreover, the MoS2/ CdS nanosphere heterostructures exhibit good stability with no obvious deterioration in four cycles of 24 h reaction. Furthermore, we carefully investigated the photocatalytic mechanism which is not only beneficial for the further understanding the H2 generation process in this work but also the design and synthesis of other catalytic systems with lowcost but high efficiency.
Experimental Chemicals Cadmium acetate dihydrate (Cd(CH3COO)2$2H2O, 98%), sodium molybdate dihydrate (Na2MoO4$2H2O, 99%), thiourea (CN2H4S, 99%), ethylene glycol (C2H6O2, 99%) and polyvinylpyrrolidone (PVP, Mw ¼ 40000) were purchased from Aladdin Co. Ltd. All chemicals were used directly without further purification. Deionized water was used in all the experiments, and was obtained by a Milli-Q apparatus (Millipore).
Synthesis of MoS2/CdS heterostructures The MoS2/CdS heterostructures were successfully synthesized by using a simple one-pot solvothermal approach [34]. Typically, PVP (0.4 g), Cd(CH3COO)2$2H2O (0.2665 g, 1 mmol) and a varying amount of Na2MoO4$2H2O and CS(NH2)2 with a molar ratio of 1:5 for (Cd(NO3)2$4H2O þ Na2MoO4$2H2O) versus CS(NH2)2 were mixed in a certain amount of ethylene glycol. Then the mixture was transferred into 25 mL Teflon-lined stainless steel autoclaves with filling coefficient of 0.6. The autoclaves were heated at 170 C for 10 h, and then automatically heated to 210 C for 24 h. After that, the assynthesized samples were collected by centrifugation and washed with deionized water and ethanol several times. The MoS2/CdS powders were ultimately obtained after drying at 60 C for 10 h.
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Synthesis of the control samples
Electrochemical characterization
As a control, the pristine CdS and MoS2 were also fabricated by using a similar process to that of the MoS2/CdS heterostructures except for no addition of the starting materials of Na2MoO4$2H2O or Cd(CH3COO)2$2H2O.
All the electrochemical measurements in this manuscript were conducted in a typical three-electrode system with AgeAgCl and Pt as reference and counter electrodes. The corresponding tests were performed on a CHI 760 C electrochemical workstation. For the tests of electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV), the working electrodes were fabricated as follows: 5 mg of photocatalyst was ultrasonically dispersed in 0.5 mL ethanol and then deposited on a glassy carbon electrode (1.5 mm in radius) with 3 mL of the solution. The measurement of EIS was performed in a 0.5 M Na2SO4 solution with a frequency range of 0.1e105 Hz and an ac amplitude of 2 mV in the dark. The LSV test was carried out in a 0.5 M H2SO4 solution in the dark with a 50 mV/s scan rate. For the transient photocurrent measurement, the working electrode was prepared by dispensing sample suspension in ethanol onto ITO/glass with a fixed area of 0.196 cm2. The lactic acid solution (1.33 M) worked as an electrolyte which was filled in a quartz cell with a side window for external light incidence. Light on and off was controlled by a baffle installed on a stainless steel black box.
Evaluation of photocatalytic activities The photocatalytic H2 evolution experiments were performed in a Pyrex glass Cell. A 300 W Xenon arc lamp was used to simulate the visible light equipped with a 420 nm cut-off filter (CEL-HXF 300, Beijing China Education Au-light Co., Ltd). The illumination intensity was adjusted to 100 mW cm2. In general, 20 mg photocatalyst was dispersed in 80 mL aqueous solution containing 8 mL lactic acid as a hole scavenger. The reaction system was kept at 5 C by using a circulating water system and was pumped to vacuum before irradiation. The evolved H2 was quantified by a gas chromatogram (GC), equipped with a thermal conductivity detector (TCD), Ar carrier gas and TDX-01 column. The apparent quantum yield (Ø) was estimated by the following equation: ∅% ¼
np ¼
ne 2nH2 100 ¼ 100 np np
q Its ¼ hn hn
where Ø is the apparent quantum yield, ne- is the number of reacted electrons, np is the number of incident photos, nH2 is the number of evolved H2 molecules, q is the total energy of incident photos (J), h is the Planck constant (J s1), n is the frequency of light (Hz), I is the illumination intensity (W m2) determined with a ray virtual radiation actinometer, t is the irradiation time (s), s is the irradiation area (m2).
Characterization The morphology investigation of the samples in a wide-fieldof-view was performed on a Gemini microscope (Zeiss Ltd., Germany). The detailed microstructure of the samples was investigated by using a transmission electron microscope (TEM, JEM 2100F). The crystal phase properties of the asprepared products were tested on an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Ka radiation (l ¼ 1.5418 A). The X-ray photoelectron spectroscopy (XPS) analysis was recorded on a VG ESCALab220i-XL photoelectron spectroscopy with 300 W Mg Ka radiation, in which the binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The UVevis absorption spectra were obtained by a UVevis spectrophotometer (PerkinElmer Lambda 750), in which BaSO4 was used as the internal reflectance standard. The Cd and Mo contents were measured by using inductively coupled plasma atomic emission spectrometry (ICP-AES, ICPE-9000 Shimadzu). The specific surface area measurement was analyzed by ASAP 2460 (Micromeritics, USA). Surface photovoltage spectroscopy analysis was performed on a CELSPS 1000 (Beijing China Education Au-light Co., Ltd).
Results and discussion The MoS2/CdS heterostructures were synthesized by using a facile one-pot solvothermal approach. The colors of the endproducts gradually darken with the increase of the Na2MoO4 preliminarily suggesting the formation of CdS based hybrids compared with the yellow of typical CdS (See Fig. S1). The assynthesized samples were marked by MoS2/CdS-1, 3, 5, 10, 15, and 20, respectively for the calculated MoS2 contents of 1, 3, 5, 10, 15, and 20 wt% versus CdS. The actual MoS2 contents in the heterostructures were confirmed by the ICP-AES and summarized in Table S1. From it, the actual MoS2 contents approach to the calculated ones owing to the excessive thiourea resulting in the Mo and Cd elements thoroughly transferred into the resulting products. Phase structures of MoS2/CdS with MoS2 contents from 5 to 20 wt%, as well as pristine CdS and MoS2, were investigated by X-ray diffraction (XRD). As shown in Fig. 1, the obtained sample without MoS2 deposition displays six strong peaks centered at 24.8 , 26.5 , 28.2 , 43.7 , 47.9 , and 51.8 which are well matched to the (100), (002), (101), (110), (103), and (112) crystal planes of greenockite CdS (JCPDS Card No. 41e1049). However, for the hybrid samples, no additional peaks were observed even the theoretical MoS2 content was increased to 20 wt%. This result can be ascribed to the low crystallinity of the MoS2 as verified by the XRD patterns of pristine MoS2 with widened and dispersed peaks observed (Fig. 1). It is well known that the active sites for H2 evolution reaction are located at edges of MoS2 as reported in the previous studies [35,36]. Thus this low-crystallinity MoS2 with numerous edges exposure is beneficial for the acceleration of H2 generation. Moreover, the patterns of MoS2/CdS display no peak shift compared with that of pristine CdS indicating the
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 1 e The XRD patterns of pristine CdS, MoS2, and MoS2/ CdS heterostructures with MoS2 contents from 5 to 20 wt%.
MoS2 in heterostructures in the form of deposition rather than doping. Owing to the excellent H2 evolution performance of MoS2/ CdS-5 (see the section of photocatalytic activity evaluation in details), as a representative it was thoroughly characterized in this manuscript. Surface chemical states of MoS2/CdS-5 were measured by the X-ray photoelectron spectroscopy (XPS) and the results were plotted in Fig. 2. Elemental signals of Cd, Mo, and S can be observed from the survey spectrum (Fig. 2a). Two strong doublets (Fig. 2b) appeared at 230.6 and 227.4 eV are assigned to the binding energies of Mo 3d3/2 and Mo 3d5/2, suggesting the existence of Mo4þ species in the heterostructures. The high-resolution spectrum (Fig. 2c) of Cd 3d displays two peaks centered at 410.8 and 404.1 eV, which agree well with that of the Cd 3d3/2 and Cd 3d5/2 in the form of Cd2þ. Peak shift about 0.3 eV toward high binding energy is observed compared with that of the pristine CdS indicating the interaction of the composites, which may facilitate the welldefined heterostructures formation. For the XPS signal of S 2p, two fitted peaks at 161.6 and 160.4 eV, are corresponding to the S 2p1/2 and S 2p3/2 in the form of S2 (Fig. 2d), respectively. The mentioned-above XPS analysis further reveals the existence of CdS and MoS2 in the obtained heterostructures. The morphologies of MoS2/CdS were investigated by TEM. A typical TEM image shown in Fig. 3a reveals the MoS2/CdS is composed of nanospheres with a diameter in the range of 30e55 nm. Further enlarged TEM image (inset of Fig. S2) of it shows the nanosphere consisted of sub-nanosphere with an average diameter of 3.6 nm. The whole nanosphere exhibits porous structure. This typical structure not only has potential to shorten charge transfer path but also promote mass transfer and thus promote H2 generation. HRTEM image (Fig. 3b) of MoS2/CdS reveals the lattice fringe of 0.34 nm corresponding to the (0 0 2) plane of the hexagonal CdS. Whereas, the lattice fringe of loaded nanosheets outside of CdS displays disordered structure further indicating the low
crystallinity of MoS2. This result agrees well with the mentioned-above XRD characterization. The intimate interface marked by dashed line between CdS and MoS2 suggests the well-defined heterostructure construction. This good contact interface was further validated by the SEM images of MoS2/CdS heterostructures with MoS2 contents of 5 and 20 wt %. As shown in Fig. S3, no separated phase was observed for both detected samples further indicating the intimate contact between MoS2 and CdS. The intimate contact in combination with the band-alignment of MoS2 and CdS favours the formation of the built-in electric field and thus the well-defined heterostructure. With the assist of heterostructure, the excited charges are effectively transported and separated, and therefore the charge recombination is significantly inhibited and the photocatalytic performance is dramatically enhanced. Moreover, the energy dispersive X-ray spectroscopy (EDS)-mappings (Fig. 3d-f) of the randomly selected region of MoS2/CdS demonstrate the homogeneous distribution of the Mo, Cd, and S elements which match well with the morphology of the STEM image (Fig. 3c) further implying the formation of the MoS2/CdS heterostructures. Photocatalytic H2 evolution performance investigation was carried out in a tight-gas system (Fig. S4). In this manuscript, three batches of samples were synthesized and tested at the identical conditions. The test performed in dark reveals no H2 generation indicating that the mechanocatalytic water splitting did not occur in this system. Similarly, pristine MoS2 shows no photocatalytic activity under visible light irradiation (Fig. 4a) suggesting it not a photoactive for water splitting being the same to the results of the early reports [37,38]. In the absence of MoS2, pure CdS shows low H2 generation rate of 0.23 mmol g1 h1 owing to the ultrafast charge recombination (Fig. 4a). However, once the MoS2 was deposited on the CdS surface, the H2 evolution activity was significantly enhanced even with low MoS2 content. As shown in Fig. 4a, take 1 wt% of MoS2 versus CdS for example, the optimized H2 evolution rate of it is 105 times higher than that of CdS (24.33 vs. 0.23 mmol g1 h1) indicating MoS2 a good cocatalyst for H2 generation. Screening experiments (Fig. 4a) shows there exists an optimized MoS2 content of 5 wt% with highest H2 evolution rate of 37.31 mmol g1 h1 far exceeding that of CdS by a factor of more than 162. The corresponding apparent quantum generation (see the equation listed in the experimental section) is 29.37% under 420 nm monochromatic light irradiation. This optimized activity of the MoS2/CdS-5 is comparable to that of the recent works related to CdS based heterostructures further suggesting the good cocatalytic performance of the MoS2 (please see Table S2). When the MoS2 contents in heterostructures are less than 5 wt%, the photocatalytic activity enhanced with the MoS2 content increment, which can be explained by the increase of the active sites. But the MoS2 content exceeding 5 wt% could result in the deterioration of the photocatalytic performance owing to the shielding effect caused by the excess coverage of black MoS2. The shielding effect prohibits the light absorption of photoactive CdS and impedes the extraction of the holes, and therefore renders the low activity for the H2 generation. To detect the effect of the heterostructures, physical mixture sample with the same MoS2 content to that (4.36 wt%) of the MoS2/CdS-5 was fabricated. Obviously, the photocatalytic performance of the
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 2 e (a) XPS spectrum of survey scan of MoS2/CdS (5 wt%) heterostructures. (b, d) High-resolution XPS spectra of (b) Mo 3d and (d) S 2p in MoS2/CdS heterostructures. (c) High-resolution XPS spectra of Cd 3d in pristine CdS and MoS2/CdS (5 wt%) heterostructures.
Fig. 3 e (a) TEM, (b) HRTEM, (c) STEM image, and EDS elemental mappings of (d) S, (e) Cd, and (f) Mo for the MoS2/CdS (5 wt%). Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 4 e (a) Photocatalytic activities of samples with MoS2 contents from 0 to 100 wt% and the physical mixture sample of MoS2 þ CdS. (b) Nitrogen adsorption-desorption isotherms with inset of the corresponding pore-size distribution curves of CdS and MoS2/CdS-5. (c, d) Time courses of photocatalytic H2 evolution on (c) MoS2/CdS-5 and (d) pristine CdS.
physical mixture sample is inferior to that of the heterostructures (37.31 vs. 2.08 mmol g1 h1) indicating the heterostructures play a key role toward boosting activity (Fig. 4a). Moreover, BET results (Fig. 4b) reveal the specific surface area of MoS2/CdS-5 is slightly higher than that of pristine CdS (23.9 vs. 17.8 cm3/g), which may contribute to the activity site exposure and thus the enhanced hydrogen evolution performance. Long-term photocatalytic test reveals no obvious activity decrement for the MoS2/CdS-5 in four cycles of 24 h reaction indicating its good stability and potential in the application (Fig. 4c). The stability of the heterostructures was further evidenced by the XRD and TEM characterizations of the cycled products with no obvious phase and morphology changes compared with that of the fresh ones (Figs. S5 and 6). As a control, the pristine CdS shows consecutive photocatalytic performance decrease with the reaction proceeding (Fig. 4d). The excellent stability of the heterostructures can be ascribed to the deposition of the MoS2 which prohibits the corrosion and passivates the surface trap states of the CdS [39,40]. From the previous reports, the MoS2 was an excellent catalyst for electrocatalytic H2 generation from water with low H2-overpotential. In consideration of the similar mechanism for the electrocatalytic and photocatalytic water splitting with both processes concerning with the proton reduction by the electrons, the H2-overpotentials of MoS2/CdS and CdS were detected. As shown in Fig. 5a, the MoS2/CdS heterostructures
show lower H2-overpotential compared with that of the CdS indicating the MoS2 grown on the CdS can significantly promote H2 generation. To gain insight into the charge transfer and recombination process of the MoS2/CdS heterostructures, the electrochemical impedance spectra of MoS2/CdS and pristine CdS were measured. The corresponding arc radiuses of EIS Nyquist plots are shown in Fig. 5b. The smaller radius for the spectrogram represents the lower impedance of charge transportation. Clearly, the impedance of MoS2/CdS is lower than that of CdS. This result suggests the formation of the heterostructures greatly improve the charge transfer and separation. The positive effect of heterostructures for the charge transfer and separation was further certified by the transient photocurrent measurements. Photocurrent density is correlated to the number of the photogenerated charges. The higher of the photocurrent density manifests the higher efficiency for the charge transfer and separation. As shown in Fig. 5c, in dark no photocurrent was detected indicating no charges generated. However, once light irradiation, the photocurrent was rapidly generated and subsequently stabilized at a certain value. This phenomenon reveals the light is the key factor determining the photoelectron generation. As expected, the photocurrent density of MoS2/CdS is much higher than that of the pristine CdS implying more charges were extracted from the MoS2/CdS and thus more electrons have potential involving in the Hþ reduced into H2 being good consistent with the corresponding photocatalytic H2 evolution
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 5 e (a) LSV, (b) EIS, (c) Transient photocurrent, and (d) SPV spectra for CdS and MoS2/CdS-5 heterostructures.
performance. As another good technique, the surface photovoltage spectrum (SPV) has been employed to the charge separation efficiency measurements for the MoS2/CdS heterostructures. It is known that the SPV signal stems from the variations of the surface potential barriers under light irradiation. The stronger of the SPV identifies the higher efficiency of the charge separation efficiency. The results shown in Fig. 5d reveal a broad positive SPV response ranging from 300 to 600 nm for the MoS2/CdS, but no signal was detected for the pristine CdS indicating the higher charge separation efficiency for the MoS2/CdS heterostructures. The mentioned above LSV, ESI, transient photocurrent, and SPV results collaboratively verify the positive effect of MoS2/CdS heterostructures for charge transfer and separation. To further shed light on the photocatalytic mechanism of the MoS2/CdS heterostructures, the band energy structures of CdS and MoS2 were detailedly analyzed. Fig. 6a shows the band gap energy of 2.3 eV for the pristine CdS with n ¼ 1/2 as a typical indirect semiconductor. The semiconductor type of MoS2 is significantly determined by the layer number of MoS2 as demonstrated by early reports [41,42]. However, the actual layer numbers of MoS2 in this work was difficult to be identified. Therefore, to be cautious, we selected n ¼ 1/2 and 2 respectively to calculate the energy band gap of MoS2. As shown in Fig. 6b, the band gap of MoS2 was 1.51 and 1.72 eV for n ¼ 1/2 and 2, respectively. VB-XPS spectra of CdS and MoS2
reveal that the VB maximum energy level was located at 1.91 and 1.50 eV (Fig. 6c, d), respectively. The Fermi levels of CdS and MoS2 were determined as 0.41 and 0.03 V reckoned by Motto-Schottky (Fig. S7). Referencing to Fermi levels, the VB energy levels were ultimately determined as 1.50 and 1.47 V for the CdS and MoS2, respectively. Correspondingly, the CB minimum energy levels of CdS and MoS2 can be calculated as 0.80 and 0.04~-0.25 V according to the equation listed as follows. Eg ¼ EVBðmaxÞ ECBðminÞ Based on the mentioned-above analyses, the tentative photocatalytic mechanism of MoS2/CdS heterostructures can be illustrated in Fig. 7. Under light illumination, the charges are generated with the electrons excited to the respective CB from VB for both CdS and MoS2. Owing to the more negative CB energy level of CdS compared with that of the MoS2 (0.80 vs. 0.04~-0.25 V), the electrons at the CB of CdS tend to be transferred to the CB of MoS2. As mentioned-above analyses of band structures, the MoS2 with enough redox potential for the proton reduction (0 V for Hþ/H2 without consideration of the overpotential) makes for the electrons accumulated at its CB involving in the H2 generation. Simultaneously, the holes located at the VB of CdS and MoS2 are captured by the hole scavengers. As a result, the charge recombination is significantly inhibited and thus more H2 is generated.
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 6 e (a, b) Tauc plots of (a) CdS and (b) MoS2. (c, d) Valence-band XPS spectra of (c) CdS and (d) MoS2.
that of pristine CdS by more than 162 times. The excellent performance can be attributed to the formation of the welldefined heterostructures which accelerate charge transportation and separation as verified by various approaches in this manuscript. Therefore the unfavorable charge recombination was significantly inhibited and accordingly the H2 generation rate was enhanced. The advantages presented by this work will inspire similar heterostructure construction and highly efficient catalysts with low-cost may be received.
Acknowledgements Fig. 7 e Schematic illustration of the band structures of CdS and MoS2 and the photocatalytic mechanism for the MoS2/ CdS heterostructures.
Conclusion In summary, we have successfully synthesized MoS2/CdS nanosphere heterostructures by using a facile one-pot approach with low time and energy consumption. The obtained heterostructures with MoS2 grown in-situ on CdS exhibit H2 evolution rate of 37.31 mmol g1 h1 exceeding
This work was financially supported by National Natural Science Foundation of China (Grant No. 21801106), Shandong Province Natural Science Foundation (Grant No. ZR2018PB001, ZR2017PB002, ZR2016BQ35), Research Fund for the Doctoral Program of Liaocheng University (Grant No. 318051640, 318051643), Heilongjiang Provincial Department of Education (Grant No. 12523006), Innovation and Entrepreneurship Program for College Students (Grant No. CXCY2018132).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.056.
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation Lei-Lei Li a, Xing-Liang Yin a,*, Dong-Hui Pang a, Xin-Xin Du a, Han Xue b, Hua-Wei Zhou a, Qing-Xia Yao a, Huai-Wei Wang a, Jun-Chao Qian c, Jie Yang a, Da-Cheng Li a, Jian-Min Dou a a
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, PR China b College of Chemical Engineering, Daqing Normal University, Daqing, 163712, PR China c Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, Suzhou, 215009, PR China
highlights The
graphical abstract
MoS2/CdS
nano-
heterostructures were synthesized by using one-pot approach. The H2 evolution rate of MoS2/CdS is 162 times higher than that of pristine CdS. The extraordinary performance is attributed
to
the
well-defined
heterostructures. The
photocatalytic
mechanism
was detailedly investigated.
article info
abstract
Article history:
Exploration of cost-efficient and high-performance photocatalyst for H2 evolution by using
Received 4 June 2019
a facile approach is of great importance. In this manuscript, a facile one-pot method was
Received in revised form
employed to fabricate MoS2/CdS heterostructures with MoS2 intimately grown on the
28 August 2019
surface of CdS resulting in the formation of well-defined heterostructures. Screen experi-
Accepted 10 October 2019
ment reveals the optimized photocatalytic H2 evolution performance of MoS2/CdS is far
Available online xxx
exceeding that of pristine CdS by a factor of more than 162. The outstanding performance can be ascribed to the formation of heterostructures which accelerate charge trans-
Keywords:
portation and separation, and the MoS2 serving as a cocatalyst for the decrease of H2
One-pot synthesis
overpotential. Moreover, the photocatalytic mechanism of the MoS2/CdS was carefully
Heterostructures
* Corresponding author. E-mail address: [email protected] (X.-L. Yin). https://doi.org/10.1016/j.ijhydene.2019.10.056 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Photocatalysis
investigated, which contributes to the deep understanding of the photocatalytic process
MoS2/CdS
and the designation of other low-cost and high-efficient photocatalyst.
H2 generation
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photocatalytic H2 evolution from water splitting has been considered as a sustainable and promising way to solve the worldwide energy crisis and environmental problems [1e5]. Since the first report on TiO2-supporting Pt electrodes for photoinduced H2 production by Fujishima and Honda in 1972, a large number of semiconductors, such as TiO2, CdS, ZnxCd1xS, WO3, and g-C3N4 have been developed for photocatalytic water splitting [6e10]. Among them, CdS as a visible-light response semiconductor has attracted considerable attention owing to its superior light absorption and proper band structure for photocatalytic H2 evolution [11e14]. However, pristine CdS still suffers from low-efficiency and bad stability caused by ultrafast charge recombination and photocorrosion. To address these issues, loading cocatalysts on CdS surface not only accelerates photogenerated charge separation but also inhibits CdS from photocorrosion [15,16]. Noble metals, such as Ru, Pd, Pt, and Au, are excellent cocatalysts for CdS, but high-cost and difficult to be recycled still resist their application. Therefore, it is desirable to explore low-cost and highly efficient cocatalysts to further facilitate the H2 production. Recently, cheap and earth-abundant two-dimensional transition metal dichalcogenides (2D-TMDs) have been proved to be a kind of promising low-cost cocatalysts [17e19]. As a member of 2D-TMDs, MoS2 has aroused deep interest among scientists owing to its narrow band gap and wonderful morphologies [20e22]. The catalytic system of MoS2/CdS has been intensively studied since Zong et al. first reported MoS2 as a good cocatalyst for H2 generation [23]. In the past ten years, the researchers generally paid attentions on several aspects as follows: morphology control [24,25], increasing active sites [26,27], novel fabrication approaches [28,29], and photocatalytic mechanism [30]. Although those excellent works significantly promote the development of this catalytic system, how to combine those excellent strategies into a work is still somewhat of a challenge. In addition, the nanostructure MoS2/CdS usually suffers from severe agglomeration which deteriorates mass transfer and results in the charge recombination occurred at contacted surface, but this drawback in the past was not carefully overcome. Currently, the materials with secondary nanostructures, such as pomegranate- and urchin-like, have attracted increasing attention [8,31e33]. If the catalyst is self-assembled by the sub-nano structures and porous between them, the issue of agglomeration may be well addressed. With these considerations, in this work the porous MoS2/ CdS heterostructures, which consist of big sphere selfassembled by sub-nanosphere (~3.6 nm) (see Fig. 1 and Fig. S2), was synthesized for the first time via a facile one-pot
solvothermal approach. This work involves morphology control, enhanced active sites and using a gentle fabrication approach with low-energy and time consumption. The asprepared MoS2/CdS heterostructures with MoS2 intimately grown on the CdS reveal extraordinary high H2 evolution rate of 37.31 mmol g1 h1, which is 162 folds higher than that of the pristine CdS. This excellent performance can be attributed to the formation of well-defined heterostructures which accelerate charge transportation and separation as verified by various characterizations in this work. Besides, the favourable mass transfer and agglomeration-resistant structure, lower H2 overpotential of MoS2 and a great number of active sites offered by amorphous MoS2 collaboratively contribute to the remarkable photocatalytic performance. Moreover, the MoS2/ CdS nanosphere heterostructures exhibit good stability with no obvious deterioration in four cycles of 24 h reaction. Furthermore, we carefully investigated the photocatalytic mechanism which is not only beneficial for the further understanding the H2 generation process in this work but also the design and synthesis of other catalytic systems with lowcost but high efficiency.
Experimental Chemicals Cadmium acetate dihydrate (Cd(CH3COO)2$2H2O, 98%), sodium molybdate dihydrate (Na2MoO4$2H2O, 99%), thiourea (CN2H4S, 99%), ethylene glycol (C2H6O2, 99%) and polyvinylpyrrolidone (PVP, Mw ¼ 40000) were purchased from Aladdin Co. Ltd. All chemicals were used directly without further purification. Deionized water was used in all the experiments, and was obtained by a Milli-Q apparatus (Millipore).
Synthesis of MoS2/CdS heterostructures The MoS2/CdS heterostructures were successfully synthesized by using a simple one-pot solvothermal approach [34]. Typically, PVP (0.4 g), Cd(CH3COO)2$2H2O (0.2665 g, 1 mmol) and a varying amount of Na2MoO4$2H2O and CS(NH2)2 with a molar ratio of 1:5 for (Cd(NO3)2$4H2O þ Na2MoO4$2H2O) versus CS(NH2)2 were mixed in a certain amount of ethylene glycol. Then the mixture was transferred into 25 mL Teflon-lined stainless steel autoclaves with filling coefficient of 0.6. The autoclaves were heated at 170 C for 10 h, and then automatically heated to 210 C for 24 h. After that, the assynthesized samples were collected by centrifugation and washed with deionized water and ethanol several times. The MoS2/CdS powders were ultimately obtained after drying at 60 C for 10 h.
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Synthesis of the control samples
Electrochemical characterization
As a control, the pristine CdS and MoS2 were also fabricated by using a similar process to that of the MoS2/CdS heterostructures except for no addition of the starting materials of Na2MoO4$2H2O or Cd(CH3COO)2$2H2O.
All the electrochemical measurements in this manuscript were conducted in a typical three-electrode system with AgeAgCl and Pt as reference and counter electrodes. The corresponding tests were performed on a CHI 760 C electrochemical workstation. For the tests of electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV), the working electrodes were fabricated as follows: 5 mg of photocatalyst was ultrasonically dispersed in 0.5 mL ethanol and then deposited on a glassy carbon electrode (1.5 mm in radius) with 3 mL of the solution. The measurement of EIS was performed in a 0.5 M Na2SO4 solution with a frequency range of 0.1e105 Hz and an ac amplitude of 2 mV in the dark. The LSV test was carried out in a 0.5 M H2SO4 solution in the dark with a 50 mV/s scan rate. For the transient photocurrent measurement, the working electrode was prepared by dispensing sample suspension in ethanol onto ITO/glass with a fixed area of 0.196 cm2. The lactic acid solution (1.33 M) worked as an electrolyte which was filled in a quartz cell with a side window for external light incidence. Light on and off was controlled by a baffle installed on a stainless steel black box.
Evaluation of photocatalytic activities The photocatalytic H2 evolution experiments were performed in a Pyrex glass Cell. A 300 W Xenon arc lamp was used to simulate the visible light equipped with a 420 nm cut-off filter (CEL-HXF 300, Beijing China Education Au-light Co., Ltd). The illumination intensity was adjusted to 100 mW cm2. In general, 20 mg photocatalyst was dispersed in 80 mL aqueous solution containing 8 mL lactic acid as a hole scavenger. The reaction system was kept at 5 C by using a circulating water system and was pumped to vacuum before irradiation. The evolved H2 was quantified by a gas chromatogram (GC), equipped with a thermal conductivity detector (TCD), Ar carrier gas and TDX-01 column. The apparent quantum yield (Ø) was estimated by the following equation: ∅% ¼
np ¼
ne 2nH2 100 ¼ 100 np np
q Its ¼ hn hn
where Ø is the apparent quantum yield, ne- is the number of reacted electrons, np is the number of incident photos, nH2 is the number of evolved H2 molecules, q is the total energy of incident photos (J), h is the Planck constant (J s1), n is the frequency of light (Hz), I is the illumination intensity (W m2) determined with a ray virtual radiation actinometer, t is the irradiation time (s), s is the irradiation area (m2).
Characterization The morphology investigation of the samples in a wide-fieldof-view was performed on a Gemini microscope (Zeiss Ltd., Germany). The detailed microstructure of the samples was investigated by using a transmission electron microscope (TEM, JEM 2100F). The crystal phase properties of the asprepared products were tested on an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Ka radiation (l ¼ 1.5418 A). The X-ray photoelectron spectroscopy (XPS) analysis was recorded on a VG ESCALab220i-XL photoelectron spectroscopy with 300 W Mg Ka radiation, in which the binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The UVevis absorption spectra were obtained by a UVevis spectrophotometer (PerkinElmer Lambda 750), in which BaSO4 was used as the internal reflectance standard. The Cd and Mo contents were measured by using inductively coupled plasma atomic emission spectrometry (ICP-AES, ICPE-9000 Shimadzu). The specific surface area measurement was analyzed by ASAP 2460 (Micromeritics, USA). Surface photovoltage spectroscopy analysis was performed on a CELSPS 1000 (Beijing China Education Au-light Co., Ltd).
Results and discussion The MoS2/CdS heterostructures were synthesized by using a facile one-pot solvothermal approach. The colors of the endproducts gradually darken with the increase of the Na2MoO4 preliminarily suggesting the formation of CdS based hybrids compared with the yellow of typical CdS (See Fig. S1). The assynthesized samples were marked by MoS2/CdS-1, 3, 5, 10, 15, and 20, respectively for the calculated MoS2 contents of 1, 3, 5, 10, 15, and 20 wt% versus CdS. The actual MoS2 contents in the heterostructures were confirmed by the ICP-AES and summarized in Table S1. From it, the actual MoS2 contents approach to the calculated ones owing to the excessive thiourea resulting in the Mo and Cd elements thoroughly transferred into the resulting products. Phase structures of MoS2/CdS with MoS2 contents from 5 to 20 wt%, as well as pristine CdS and MoS2, were investigated by X-ray diffraction (XRD). As shown in Fig. 1, the obtained sample without MoS2 deposition displays six strong peaks centered at 24.8 , 26.5 , 28.2 , 43.7 , 47.9 , and 51.8 which are well matched to the (100), (002), (101), (110), (103), and (112) crystal planes of greenockite CdS (JCPDS Card No. 41e1049). However, for the hybrid samples, no additional peaks were observed even the theoretical MoS2 content was increased to 20 wt%. This result can be ascribed to the low crystallinity of the MoS2 as verified by the XRD patterns of pristine MoS2 with widened and dispersed peaks observed (Fig. 1). It is well known that the active sites for H2 evolution reaction are located at edges of MoS2 as reported in the previous studies [35,36]. Thus this low-crystallinity MoS2 with numerous edges exposure is beneficial for the acceleration of H2 generation. Moreover, the patterns of MoS2/CdS display no peak shift compared with that of pristine CdS indicating the
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 1 e The XRD patterns of pristine CdS, MoS2, and MoS2/ CdS heterostructures with MoS2 contents from 5 to 20 wt%.
MoS2 in heterostructures in the form of deposition rather than doping. Owing to the excellent H2 evolution performance of MoS2/ CdS-5 (see the section of photocatalytic activity evaluation in details), as a representative it was thoroughly characterized in this manuscript. Surface chemical states of MoS2/CdS-5 were measured by the X-ray photoelectron spectroscopy (XPS) and the results were plotted in Fig. 2. Elemental signals of Cd, Mo, and S can be observed from the survey spectrum (Fig. 2a). Two strong doublets (Fig. 2b) appeared at 230.6 and 227.4 eV are assigned to the binding energies of Mo 3d3/2 and Mo 3d5/2, suggesting the existence of Mo4þ species in the heterostructures. The high-resolution spectrum (Fig. 2c) of Cd 3d displays two peaks centered at 410.8 and 404.1 eV, which agree well with that of the Cd 3d3/2 and Cd 3d5/2 in the form of Cd2þ. Peak shift about 0.3 eV toward high binding energy is observed compared with that of the pristine CdS indicating the interaction of the composites, which may facilitate the welldefined heterostructures formation. For the XPS signal of S 2p, two fitted peaks at 161.6 and 160.4 eV, are corresponding to the S 2p1/2 and S 2p3/2 in the form of S2 (Fig. 2d), respectively. The mentioned-above XPS analysis further reveals the existence of CdS and MoS2 in the obtained heterostructures. The morphologies of MoS2/CdS were investigated by TEM. A typical TEM image shown in Fig. 3a reveals the MoS2/CdS is composed of nanospheres with a diameter in the range of 30e55 nm. Further enlarged TEM image (inset of Fig. S2) of it shows the nanosphere consisted of sub-nanosphere with an average diameter of 3.6 nm. The whole nanosphere exhibits porous structure. This typical structure not only has potential to shorten charge transfer path but also promote mass transfer and thus promote H2 generation. HRTEM image (Fig. 3b) of MoS2/CdS reveals the lattice fringe of 0.34 nm corresponding to the (0 0 2) plane of the hexagonal CdS. Whereas, the lattice fringe of loaded nanosheets outside of CdS displays disordered structure further indicating the low
crystallinity of MoS2. This result agrees well with the mentioned-above XRD characterization. The intimate interface marked by dashed line between CdS and MoS2 suggests the well-defined heterostructure construction. This good contact interface was further validated by the SEM images of MoS2/CdS heterostructures with MoS2 contents of 5 and 20 wt %. As shown in Fig. S3, no separated phase was observed for both detected samples further indicating the intimate contact between MoS2 and CdS. The intimate contact in combination with the band-alignment of MoS2 and CdS favours the formation of the built-in electric field and thus the well-defined heterostructure. With the assist of heterostructure, the excited charges are effectively transported and separated, and therefore the charge recombination is significantly inhibited and the photocatalytic performance is dramatically enhanced. Moreover, the energy dispersive X-ray spectroscopy (EDS)-mappings (Fig. 3d-f) of the randomly selected region of MoS2/CdS demonstrate the homogeneous distribution of the Mo, Cd, and S elements which match well with the morphology of the STEM image (Fig. 3c) further implying the formation of the MoS2/CdS heterostructures. Photocatalytic H2 evolution performance investigation was carried out in a tight-gas system (Fig. S4). In this manuscript, three batches of samples were synthesized and tested at the identical conditions. The test performed in dark reveals no H2 generation indicating that the mechanocatalytic water splitting did not occur in this system. Similarly, pristine MoS2 shows no photocatalytic activity under visible light irradiation (Fig. 4a) suggesting it not a photoactive for water splitting being the same to the results of the early reports [37,38]. In the absence of MoS2, pure CdS shows low H2 generation rate of 0.23 mmol g1 h1 owing to the ultrafast charge recombination (Fig. 4a). However, once the MoS2 was deposited on the CdS surface, the H2 evolution activity was significantly enhanced even with low MoS2 content. As shown in Fig. 4a, take 1 wt% of MoS2 versus CdS for example, the optimized H2 evolution rate of it is 105 times higher than that of CdS (24.33 vs. 0.23 mmol g1 h1) indicating MoS2 a good cocatalyst for H2 generation. Screening experiments (Fig. 4a) shows there exists an optimized MoS2 content of 5 wt% with highest H2 evolution rate of 37.31 mmol g1 h1 far exceeding that of CdS by a factor of more than 162. The corresponding apparent quantum generation (see the equation listed in the experimental section) is 29.37% under 420 nm monochromatic light irradiation. This optimized activity of the MoS2/CdS-5 is comparable to that of the recent works related to CdS based heterostructures further suggesting the good cocatalytic performance of the MoS2 (please see Table S2). When the MoS2 contents in heterostructures are less than 5 wt%, the photocatalytic activity enhanced with the MoS2 content increment, which can be explained by the increase of the active sites. But the MoS2 content exceeding 5 wt% could result in the deterioration of the photocatalytic performance owing to the shielding effect caused by the excess coverage of black MoS2. The shielding effect prohibits the light absorption of photoactive CdS and impedes the extraction of the holes, and therefore renders the low activity for the H2 generation. To detect the effect of the heterostructures, physical mixture sample with the same MoS2 content to that (4.36 wt%) of the MoS2/CdS-5 was fabricated. Obviously, the photocatalytic performance of the
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 2 e (a) XPS spectrum of survey scan of MoS2/CdS (5 wt%) heterostructures. (b, d) High-resolution XPS spectra of (b) Mo 3d and (d) S 2p in MoS2/CdS heterostructures. (c) High-resolution XPS spectra of Cd 3d in pristine CdS and MoS2/CdS (5 wt%) heterostructures.
Fig. 3 e (a) TEM, (b) HRTEM, (c) STEM image, and EDS elemental mappings of (d) S, (e) Cd, and (f) Mo for the MoS2/CdS (5 wt%). Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 4 e (a) Photocatalytic activities of samples with MoS2 contents from 0 to 100 wt% and the physical mixture sample of MoS2 þ CdS. (b) Nitrogen adsorption-desorption isotherms with inset of the corresponding pore-size distribution curves of CdS and MoS2/CdS-5. (c, d) Time courses of photocatalytic H2 evolution on (c) MoS2/CdS-5 and (d) pristine CdS.
physical mixture sample is inferior to that of the heterostructures (37.31 vs. 2.08 mmol g1 h1) indicating the heterostructures play a key role toward boosting activity (Fig. 4a). Moreover, BET results (Fig. 4b) reveal the specific surface area of MoS2/CdS-5 is slightly higher than that of pristine CdS (23.9 vs. 17.8 cm3/g), which may contribute to the activity site exposure and thus the enhanced hydrogen evolution performance. Long-term photocatalytic test reveals no obvious activity decrement for the MoS2/CdS-5 in four cycles of 24 h reaction indicating its good stability and potential in the application (Fig. 4c). The stability of the heterostructures was further evidenced by the XRD and TEM characterizations of the cycled products with no obvious phase and morphology changes compared with that of the fresh ones (Figs. S5 and 6). As a control, the pristine CdS shows consecutive photocatalytic performance decrease with the reaction proceeding (Fig. 4d). The excellent stability of the heterostructures can be ascribed to the deposition of the MoS2 which prohibits the corrosion and passivates the surface trap states of the CdS [39,40]. From the previous reports, the MoS2 was an excellent catalyst for electrocatalytic H2 generation from water with low H2-overpotential. In consideration of the similar mechanism for the electrocatalytic and photocatalytic water splitting with both processes concerning with the proton reduction by the electrons, the H2-overpotentials of MoS2/CdS and CdS were detected. As shown in Fig. 5a, the MoS2/CdS heterostructures
show lower H2-overpotential compared with that of the CdS indicating the MoS2 grown on the CdS can significantly promote H2 generation. To gain insight into the charge transfer and recombination process of the MoS2/CdS heterostructures, the electrochemical impedance spectra of MoS2/CdS and pristine CdS were measured. The corresponding arc radiuses of EIS Nyquist plots are shown in Fig. 5b. The smaller radius for the spectrogram represents the lower impedance of charge transportation. Clearly, the impedance of MoS2/CdS is lower than that of CdS. This result suggests the formation of the heterostructures greatly improve the charge transfer and separation. The positive effect of heterostructures for the charge transfer and separation was further certified by the transient photocurrent measurements. Photocurrent density is correlated to the number of the photogenerated charges. The higher of the photocurrent density manifests the higher efficiency for the charge transfer and separation. As shown in Fig. 5c, in dark no photocurrent was detected indicating no charges generated. However, once light irradiation, the photocurrent was rapidly generated and subsequently stabilized at a certain value. This phenomenon reveals the light is the key factor determining the photoelectron generation. As expected, the photocurrent density of MoS2/CdS is much higher than that of the pristine CdS implying more charges were extracted from the MoS2/CdS and thus more electrons have potential involving in the Hþ reduced into H2 being good consistent with the corresponding photocatalytic H2 evolution
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 5 e (a) LSV, (b) EIS, (c) Transient photocurrent, and (d) SPV spectra for CdS and MoS2/CdS-5 heterostructures.
performance. As another good technique, the surface photovoltage spectrum (SPV) has been employed to the charge separation efficiency measurements for the MoS2/CdS heterostructures. It is known that the SPV signal stems from the variations of the surface potential barriers under light irradiation. The stronger of the SPV identifies the higher efficiency of the charge separation efficiency. The results shown in Fig. 5d reveal a broad positive SPV response ranging from 300 to 600 nm for the MoS2/CdS, but no signal was detected for the pristine CdS indicating the higher charge separation efficiency for the MoS2/CdS heterostructures. The mentioned above LSV, ESI, transient photocurrent, and SPV results collaboratively verify the positive effect of MoS2/CdS heterostructures for charge transfer and separation. To further shed light on the photocatalytic mechanism of the MoS2/CdS heterostructures, the band energy structures of CdS and MoS2 were detailedly analyzed. Fig. 6a shows the band gap energy of 2.3 eV for the pristine CdS with n ¼ 1/2 as a typical indirect semiconductor. The semiconductor type of MoS2 is significantly determined by the layer number of MoS2 as demonstrated by early reports [41,42]. However, the actual layer numbers of MoS2 in this work was difficult to be identified. Therefore, to be cautious, we selected n ¼ 1/2 and 2 respectively to calculate the energy band gap of MoS2. As shown in Fig. 6b, the band gap of MoS2 was 1.51 and 1.72 eV for n ¼ 1/2 and 2, respectively. VB-XPS spectra of CdS and MoS2
reveal that the VB maximum energy level was located at 1.91 and 1.50 eV (Fig. 6c, d), respectively. The Fermi levels of CdS and MoS2 were determined as 0.41 and 0.03 V reckoned by Motto-Schottky (Fig. S7). Referencing to Fermi levels, the VB energy levels were ultimately determined as 1.50 and 1.47 V for the CdS and MoS2, respectively. Correspondingly, the CB minimum energy levels of CdS and MoS2 can be calculated as 0.80 and 0.04~-0.25 V according to the equation listed as follows. Eg ¼ EVBðmaxÞ ECBðminÞ Based on the mentioned-above analyses, the tentative photocatalytic mechanism of MoS2/CdS heterostructures can be illustrated in Fig. 7. Under light illumination, the charges are generated with the electrons excited to the respective CB from VB for both CdS and MoS2. Owing to the more negative CB energy level of CdS compared with that of the MoS2 (0.80 vs. 0.04~-0.25 V), the electrons at the CB of CdS tend to be transferred to the CB of MoS2. As mentioned-above analyses of band structures, the MoS2 with enough redox potential for the proton reduction (0 V for Hþ/H2 without consideration of the overpotential) makes for the electrons accumulated at its CB involving in the H2 generation. Simultaneously, the holes located at the VB of CdS and MoS2 are captured by the hole scavengers. As a result, the charge recombination is significantly inhibited and thus more H2 is generated.
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Fig. 6 e (a, b) Tauc plots of (a) CdS and (b) MoS2. (c, d) Valence-band XPS spectra of (c) CdS and (d) MoS2.
that of pristine CdS by more than 162 times. The excellent performance can be attributed to the formation of the welldefined heterostructures which accelerate charge transportation and separation as verified by various approaches in this manuscript. Therefore the unfavorable charge recombination was significantly inhibited and accordingly the H2 generation rate was enhanced. The advantages presented by this work will inspire similar heterostructure construction and highly efficient catalysts with low-cost may be received.
Acknowledgements Fig. 7 e Schematic illustration of the band structures of CdS and MoS2 and the photocatalytic mechanism for the MoS2/ CdS heterostructures.
Conclusion In summary, we have successfully synthesized MoS2/CdS nanosphere heterostructures by using a facile one-pot approach with low time and energy consumption. The obtained heterostructures with MoS2 grown in-situ on CdS exhibit H2 evolution rate of 37.31 mmol g1 h1 exceeding
This work was financially supported by National Natural Science Foundation of China (Grant No. 21801106), Shandong Province Natural Science Foundation (Grant No. ZR2018PB001, ZR2017PB002, ZR2016BQ35), Research Fund for the Doctoral Program of Liaocheng University (Grant No. 318051640, 318051643), Heilongjiang Provincial Department of Education (Grant No. 12523006), Innovation and Entrepreneurship Program for College Students (Grant No. CXCY2018132).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.056.
Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056
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Please cite this article as: Li L-L et al., One-pot synthesis of MoS2/CdS nanosphere heterostructures for efficient H2 evolution under visible light irradiation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.056