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A novel approach for high-yield solid few-layer MoS2 nanosheets with effective photocatalytic hydrogen evolution
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 6 6 3 9 e1 6 6 4 7
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A novel approach for high-yield solid few-layer MoS2 nanosheets with effective photocatalytic hydrogen evolution Jun Wan a,b, Ruimiao Wang b, Lin Liu a,*, Jun Fan b,**, Enzhou Liu b, Xiaoming Gao a, Feng Fu a a
College of Chemistry & Chemical Engineering, Yan'an University, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an 716000, PR China b School of Chemical Engineering, Northwest University, Xi'an 710069, PR China
article info
abstract
Article history:
Few-layer molybdenum disulfide (MoS2) nanosheets are well applied in many field, but the
Received 1 February 2019
lack of simple methods for the preparation of solid few-layer MoS2 nanosheets with high
Received in revised form
yield and quality has greatly restricted their development. In this work, a facile sol-
8 April 2019
vothermal treatment coupled with the liquid exfoliation strategy was conducted to pro-
Accepted 15 April 2019
duce solid monodispersed few-layer MoS2 nanosheets from the MoS2 stack, and the output
Available online 19 May 2019
can reach as high as approximately 0.3 g/g. The few-layer features were confirmed by characterizations of SEM, TEM, Raman spectra, UVevis absorption spectrum and PL
Keywords:
spectrum. The obtained MoS2 nanosheets exhibit fantastic dispersity and stability in an
2D nanomaterials
NMP solution, which can remain uniform even after one year. In general, pure MoS2 cat-
Few-layer MoS2 nanosheets
alysts show no or poor activity for photocatalytic hydrogen evolution as reported in the
Photocatalysis
literature, however, the prepared MoS2 nanosheets in this work display excellent photo-
Hydrogen evolution
catalytic H2 evolution performance of 1241.3 mmol g1 h1 due to the synergistic structural and electronic modifications, including a bigger specific surface area, additional exposed active edge sites, superior charge separation and transfer efficiency, and higher reduction potential. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Since the discovery of graphene in 2004, analogous twodimensional (2D) layered nanomaterials, such as transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), hexagonal boron nitride (h-BN) and carbon nitride, have attracted tremendous attention in many potential
applications due to their fantastic optical, electronic, thermal and mechanical properties [1e5]. MoS2 is a typical TMD material with a sandwich-like structure consisting of an atomic plane of molybdenum between two planes of sulfur atoms, and the individual sandwiched S-Mo-S layers are kept together via weak van der Waals force, making it possible to exfoliate the layers to few-layer or mono-layer nanosheets from bulk MoS2 [6e9]. It has been demonstrated that single- or
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (J. Fan). https://doi.org/10.1016/j.ijhydene.2019.04.150 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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few-layered MoS2 exhibits much better properties in comparison with the bulk material due to its direct band gap, more exposed active sites and higher specific surface area, which is superior for electrocatalytic hydrogen evolution reaction (HER) and photocatalysis applications [10e14]. However, the current lack of methods to prepare single- or few-layered MoS2 nanosheets in large quantity and high quality still restricts the development of these materials. To date, a number of techniques have been employed to prepare 2D ultrathin MoS2 nanosheets [15e20], including mechanical exfoliation, liquid-phase exfoliation with sonication, chemical or electrochemical exfoliation via lithium intercalation, and chemical vapor deposition (CVD) growth, but the above methods all have some drawbacks. Mechanical exfoliation has a very low throughput, the condition requirements of the CVD method are very strict, and the method is expensive. The shortcomings of chemical and electrochemical exfoliation are the extraordinary sensitivity to the environment, the phase and structural deformation of MoS2, and the small horizontal size of nanosheets; moreover, it is difficult to remove the residual Li in the MoS2 nanosheets. Compared with other methods, liquid-phase exfoliation is a promising route for the production of few-layer MoS2 nanosheets due to its low cost and simple operation [21e23]. However, for the direct exfoliation of bulk materials via ultrasonication in solvents, the harvest is very low. It can only obtain a dispersion liquid of MoS2 nanosheets with low concentration but cannot obtain sufficient solid-state few-layer MoS2 nanosheets, thus greatly limiting the applications of 2D MoS2 nanosheets. The reason comes from the stubborn bulk structure feature of original MoS2, some especial pretreatment process must introduced before ultrasonication. In addition, the aggregation of the layers would occur during the slow solvent evaporation process. Actually, monolayers are not necessary in many practical applications, and dispersed fewlayer nanosheets are sufficient. Therefore, developing a simple method for obtaining solid few-layer MoS2 nanosheets with high yield and good properties is of great significance. In this study, we developed a facile and reliable solvothermal treatment coupled with the exfoliation strategy to produce solid monodispersed few-layer MoS2 nanosheets. The few-layer features of MoS2 including the morphology, crystallinity, optical property and band structure were testified using various characterizations. Meanwhile, the photocatalytic hydrogen evolution performances of obtained the 2D MoS2 nanosheets were also investigated and compared to the origin MoS2 stack.
Experimental section Synthesis of solid few-layer MoS2 nanosheets MoS2 stack The origin MoS2 was first synthesized via a facile hydrothermal method [12]. In detail, 1 mmol of (NH4)6Mo7O24$4H2O and 30 mmol of N2H4CS were dissolved in 70 mL of water to form a homogeneous solution under magnetic stirring. The solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 220 C for 18 h. After cooling down to
room temperature naturally, the obtained black precipitate was collected by centrifugation and washed repeatedly with deionized water and then dried at 60 C.
MoS2 nanoflower Before exfoliation, the MoS2 stack was pretreated by a solvothermal process with special solvent to expand the microstructure. In a typical synthesis, 1 g of MoS2 stack was dispersed in 70 mL of N-methyl-2-pyrrolidone (NMP, C5H9NO). Then the dispersion was transferred into a Teflon-lined autoclave and heated at 220 C for 18 h once again. The reactor was cooled to room temperature naturally, the resulting sample was collected and washed with DI water and ethanol several times and dried at 60 C.
Few-layer MoS2 nanosheets The final product was obtained from liquid exfoliation by probe ultrasonic processing and suction filtration. 1 g of heat treated MoS2 (MoS2 nanoflower) was well dispersed in 500 mL of NMP by magnetic stirring, followed by ultrasonic treatment in an Ultrasonic Processor (Sonics VCX800) for 6 h and was allowed to stand for 12 h. Then, the suspension was filtered through porous polyvinylidene fluoride (PVDF) (0.45 mm nominal pore size) membranes. The solid MoS2 product was washed by DI water and ethanol several times and dried at 60 C.
Characterization The morphologies and microstructures were observed by field emission scanning electron microscopy (FE-SEM, Carl Zeiss SIGMA) and transmission electron microscopy (TEM, Tecnai G2 F20S-TWIN). Powder X-ray diffraction (XRD) was performed on a Shimadzu XRD-6000 powder diffractometer with Cu Ka radiation. The specific surface areas of the samples were measured using a Quantachrome NOVA 2000e and calculated by the Brunauer-Emmett-Teller (BET) method. The UVevis absorption spectrum was recorded on a Shimadzu UV-3600 UV/vis/NIR spectrophotometer. The photoluminescence (PL) spectrum was obtained on a Hitachi F-7000 fluorescence spectrophotometer.
Photoelectrochemical measurements The photoelectrochemical measurements were also carried out on an electrochemical workstation (CHI-660E, Chenhua) with a three-electrode system that consisted of a Pt wire as the counter electrode and saturated calomel electrode as the reference electrode. The working electrode was prepared as follows: 10 mg photocatalyst was dispersed in 1.0 mL a mixed solution (0.5 mL water and 0.5 mL ethanol) with ultraphonic dispersion for 30 min. Then the slurry was dropped onto a fluoride tin oxide (FTO) glass electrode and then dried naturally in ambient air and heated at 200 C for 2 h. The illumination area of the working electrode was 0.785 cm2. A 0.5 M Na2SO4 aqueous solution was employed as the electrolyte in the photoelectrochemical test system. A 300 W Xenon lamp (MICROSOLAR300UV, Beijing Perfect light) with a UV cutoff filter was employed as the light source.
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Evaluation of the photocatalytic performance The photocatalytic performance of the samples was evaluated by photocatalytic water splitting for H2 evolution, which was performed in a Pyrex top-irradiated reactor with an online gas-closed circulation system. A 300 W Xe lamp was used as the light source. In a typical experiment, 50 mg of photocatalyst powder was dispersed in 100 mL of aqueous solution under magnetic stirring containing 0.35 M Na2S and 0.25 M Na2SO3 as the scavenger. Prior to irradiation, the air in the reaction system was completely removed by a vacuum pump. The temperature of the reaction solution was controlled at 6 C by a flow of cooling water (DC-0506, Shanghai Hengpin Technology). The H2 amount was in situ recorded periodically using an online gas chromatography (GC7900, Tech-comp Shanghai) with high-purity nitrogen as the carrier gas.
Results and discussion The structure and morphology of the as-prepared MoS2 samples in three steps were first investigated by FE-SEM. As shown in Fig. 1a and b, the MoS2 obtained from the hydrothermal process is accumulated by large amounts of interconnected nanosheets, while the excessive stack results in the generation of thick bulk structures, which is not beneficial to its further applications. However, the ultrathin nature of the nanosheets still can be observed in the stack system with a uniform lateral size and corrugate character (Fig. 1b). Summarizing the reported reference, it can be found that the direct exfoliation of bulk MoS2 is usually insufficient and of low output due to the difficulty of breaking the tight bulk structure, and only a low-concentration liquid MoS2 nanosheets dispersion can be obtained [12,22e24]. To obtain few-layer nanosheets of high yield and quality, we creatively conduct a two-times solvothermal process towards the above stack structures in NMP solution. As a result, the stack structure of MoS2 is broken up owing to the effect of high temperature and pressure during the reaction and the suitable surface energy of NMP [21]. Fig. 1c and d shows that MoS2 after heat treatment exhibits a uniform and well dispersed flower-like morphology, and this loose nanoflower structure can be a good intermediate state for further exfoliation. After the efficient liquid exfoliation via probe-type ultrasonication, the final product is separated from the NMP solvent by vacuum filtration. The output of the solid product can reach approximately 0.3 g/g, and the rest of MoS2 can be used for exfoliation again. The SEM images (Fig. 1e and f) clearly reveal that the MoS2 nanoflowers are effectively broken into monodispersed ultrathin nanosheet morphology with a lateral size of 0.3～1.5 mm and thickness of 2～10 nm. XRD and BET characterizations were further carried out to investigate the structural changes of various samples. As shown in the XRD pattern (Fig. 2a), all the diffraction peaks of the three samples can be well indexed to hexagonal MoS2 (JCPDS card No. 73-1508), and the broadening of the diffraction peaks indicates the nanoscale of the crystal structure [10]. In addition, it can be observed that the XRD patterns of the MoS2 stack and MoS2 nanoflower have no difference, suggesting that
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the heat treatment mainly changes the morphology of MoS2 but not the crystal structure. In comparison, the peaks intensites of MoS2 nanosheets dramatically decrease, especially the major diffraction peak at 14 corresponding to the (002) plane of interlayer stacking along the c axis, the peak area ratio of (002)/ (100) of MoS2 stack, MoS2 nanoflower and MoS2 nanosheets decrease from 1.75 to 1.58 and 1.35, suggesting the significant decrease of the layer number of MoS2 nanosheets and efficient exfoliation to a few-layers [24,25]. In addition, the BET specific surface area of the MoS2 nanosheets shows an obvious increase due to its smaller nano size and 2D structure, which is evaluated to be 46.58 m2/g, and it is 1.37 and 1.71 times higher than that of MoS2 nanoflower (33.93 m2/g) and MoS2 stack (27.19 m2/ g). Furthermore, as shown in Fig. 2bec and Table 1, the MoS2 stack and MoS2 nanoflower have a similar hysteresis loops due to their intertwined flower-like structure and narrow pore size distribution, which is mainly concentrated at 3.8 nm, and the average pore diameters are about 10.2 nm and 12.0 nm, respectively. In comparison, when the nanoflower is broken by ultrasonication and further exfoliated into few-layer MoS2 nanosheets, the adsorption-desorption isotherms and hysteresis loops of the MoS2 nanosheets exhibit great differences correspond to their loose laminar stacked structure and more dispersive pore size distribution, and the average pore diameter is about 15.1 nm. To better characterize the few-layer feature of the obtained ultrathin MoS2 nanosheets, the nanostructure was further characterized by TEM, Raman spectra, UVevis absorption spectrum and PL spectrum. The TEM images in Fig. 3a and b also verify the ultrathin nanosheet morphology with obvious ripples and corrugations of the MoS2 product. The side- and top-view HRTEM images are shown in Fig. 3c and d, directly evidencing that a few -layers can be observed from the folded edge. The obtained MoS2 nanosheets show layers thickness and the interlayer spacing is determined to be 0.685 nm, which is larger than the theoretical layer-to-layer distance of bulk MoS2 (0.62 nm) along the c-axis direction [26]. It is worth noting that different interplanar spacing of 0.249 nm, 0.273 nm and 0.307 nm are observed from the top-view image, which are consistent with the (102), (100) and (004) planes of hexagonal MoS2, respectively. In addition, the directions of these lattice fringes are not the same and obvious cracks can be observed at the interface of different lattice planes, suggesting the disordered atomic arrangement on the basal surface. This interesting phenomenon indicates that the ultrasonic exfoliation not only decreases the layer number of MoS2 nanosheets but can also create rich defects on the surface of MoS2 nanosheets, which will offer a large amount of unsaturated sulfur atoms as exposed active edge sites for the photocatalytic hydrogen evolution reaction [14]. The Raman spectra of the MoS2 stack, the MoS2 nanoflower and the MoS2 nanosheet were obtained (Fig. 4a), and the characteristic vibration modes of in-plane E12g and out-ofplane A1g are observed in all curves. It is known that the frequency difference of the two modes can be used to confirm the existence of the few-layer nanostructure and estimate the layer number of few-layer lamellae [6]. The two Raman peaks of the MoS2 stack are located at approximately 378.0 and 402.9 cm1, and the frequency difference of approximately
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Fig. 1 e (a) SEM images of (a, b) MoS2 stack, (c, d) MoS2 nanoflower and (e, f) few-layer MoS2 nanosheets.
Fig. 2 e (a) XRD diffraction patterns, (b) BET nitrogen adsorption-desorption isotherms and (c) pore diameter distribution of the samples. 24.9 cm1 is smaller than that of the bulk structure in previously reported results [14,23]. The MoS2 nanoflower shows similar vibration peaks to those of the MoS2 stack without an
apparent shift, while the MoS2 nanosheets exhibit an appreciable redshift of the E12g vibration and blueshift of the A1g vibration. A reduced frequency difference of 23.6 cm1
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Table 1 e BET characteristic data of MoS2 stack, MoS2 nanoflower and few-layer MoS2 nanosheet. Sample
MoS2 stack MoS2 nanoflower MoS2 nanosheet
Specific surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
27.19
0.069
10.15
33.93
0.102
12.02
46.58
0.176
15.10
indicates that the layer number of MoS2 nanosheets is successfully decreased to a few-layers [6,23]. Moreover, the absorption spectrum of few-layer MoS2 nanosheets in NMP is illustrated in Fig. 4b. The two well-known characteristic absorption peaks at approximately 620 nm and 672 nm are observed and ascribed to the direct transition from the top of the valence band at the K-point to the conduction band, which are usually regarded as the symbol of few-layer or mono-layer MoS2 [15,24]. The band gap of MoS2 nanosheets was calculated to be 1.89 eV (Fig. 4c), which is larger than that of bulk MoS2
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(1.2 eV) and close to that of single layer MoS2. Fig. 4d shows the PL spectrum of the MoS2 nanosheets in NMP solution, and the two corresponding characteristic emission peaks at 646 nm and 661 nm are related to the direct excitonic transitions at the K-point of the Brillouin zone, indicating that the MoS2 nanosheets are a direct gap semiconductor. All the above observations clearly reveal the successful synthesis of 2D fewlayer MoS2 nanosheet. In addition, the light harvesting efficiency (LHE, the fraction of absorbed light) of different samples was also calculated from the measured reflection (R%) and transmission (T%) [27]: LHE (%) ¼ 100% - R (%) - T (%) in Supporting Information Fig. S1, MoS2 nanosheets display an obvious higher light absorption ability than the two others. Furthermore, the dispersibility and the stability of the obtained solid MoS2 nanosheets were investigated, and the same amount of solid MoS2 stack and MoS2 nanosheets was dispersed in NMP solution under ultrasonication in an ultrasonic cleaning instrument. Fig. 4e displays the digital photo of the MoS2 dispersion liquid after some times. It can be observed that most of the MoS2 stack was precipitated after only one day, and the solution was completely clarified after two days. For the MoS2 nanosheets, the black dispersion was
Fig. 3 e (a, b) TEM and (c, d) HRTEM images of the as-prepared few-layer MoS2 nanosheets.
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Fig. 4 e (a) Raman spectra of MoS2 stack, MoS2 nanoflower and MoS2 nanosheets; (b) UVevis absorption spectrum, (c) the calculation of the direct bandgap based on the (ahn)2-hn curve, and (d) photoluminescence spectrum of few-layer MoS2 nanosheets in NMP; (e) digital photo of different MoS2 samples dispersed in NMP after some times. kept uniform and almost no precipitation was found even after standing for one year. This phenomenon indicates that the obtained few-layer MoS2 nanosheets possess great dispersibility and stability in solution, which will greatly expand their application fields. The photoelectrochemical properties of the different MoS2 samples were examined in a three-electrode electrochemical workstation. Fig. 5a displays a comparison of the transient photocurrent responses for the MoS2 stack, MoS2 nanoflower and MoS2 nanosheet under repeated on/off illumination cycles. It can be clearly seen that the transient photocurrent density of MoS2 nanosheets is much higher than that of the MoS2 stack and MoS2 nanoflower, implying a better photoelectric response capacity and higher photogenerated charge carriers separation efficiency of the few-layer MoS2 nanosheets, which is beneficial for superior photocatalytic activity. Same result also can be obtained from photoluminescence spectra of MoS2 with different structure (Fig. S2). Meanwhile, the Mott-Schottky measurement was also provided to
investigate the change of the band structure and carrier density of the different MoS2 samples. As shown in Fig. 5b and according to the intercept of the tangent line on the axis, the flat-band potentials of the MoS2 stack, MoS2 nanoflower and MoS2 nanosheet are 0.07 eV, 0.12 eV and 0.24 eV, respectively. As the conduction band (CB) potential for an ntype semiconductor is approximately 0.3 eV higher than the flat-band potential [28], an apparent negative movement can be concluded in the comparison of the CB potentials, the MoS2 nanosheets have a higher CB potential than the two other, which leads to a more negative potential and better reducing capacity for photocatalytic hydrogen evolution. In addition, the carrier density can be calculated based on the slope of the M S plot according to the following equation [29,30]: ND ¼
2 dE 2 1 ¼ εε0 e d C12 εε0 e slope
where ND is the carrier density for n-type semiconductors, ε is the dielectric constant of the semiconductor (6.3 for MoS2),
Fig. 5 e (a) Transient photocurrent responses, (b) Mott-Schottky plots and (c) EIS Nyquist plots of the as-prepared samples.
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Fig. 6 e (a) The amount of H2 production over time and (b) comparison of the H2 production rate of different samples, (c) stability testing over MoS2 nanosheet for H2 generation under the same condition. ε0 is the vacuum permittivity (8.85 1014 F cm2), and e is the electronic charge (1.6 1019 C). The calculated carrier density of MoS2 nanosheets (5.18 1020 cm3) is higher than that of MoS2 nanoflower (4.31 1020 cm3) and MoS2 stack (3.42 1020 cm3), suggesting that a larger number of electron donors exist in the MoS2 nanosheets under photoexcitation, which is in line with the transient photocurrent responses. In addition, the electrochemical impedance spectroscopy (EIS) measurement was performed to characterize the charge transfer resistance (Rct) of the materials. The corresponding Nyquist plots were recorded in Fig. 5c. All samples display a semicircle at high frequencies, and the reaction resistance is indicated by the relative diameter size of the semicircle. It can be obviously observed that MoS2 nanosheets exhibit the smallest semicircle diameter compared to the other nanostructures, suggesting that MoS2 nanosheets have a superior conductivity for promoting the transfer of photogenerated electrons in the catalytic reaction. Fig. 6a and b shows the photocatalytic hydrogen evolution of the as-synthesized MoS2 samples under simulated light irradiation without noble metal cocatalysts but using mixed Na2S-Na2SO3 as the sacrificial reagents, which works as electron donors and final oxidized into SO2 4 . The good linear increase of H2 production over the entire time range indicates the good photostability of all samples during the photocatalytic water splitting, and the MoS2 nanosheets display a distinctly better photocatalytic hydrogen evolution activity than the MoS2 stack and MoS2 nanoflower. In detail, the hydrogen gas production amount of MoS2 nanosheets is
measured to be 1241.3 mmol g1 h1, which exhibits a 39.4% and 72.7% increment compared to that of the MoS2 nanoh1) and MoS2 stack flower (890.7 mmol g1 1 1 (718.7 mmol g h ), respectively. The results are consistent with the photoelectrochemical analyses. Durability is another significant criterion for HER catalysts, and it was investigated by cycling photocatalytic experiments as shown in Fig. 6c, the H2 evolution rate of the MoS2 nanosheet is stable after longterm test of five sequential cycles, and there is an inappreciable change in the phase structure of MoS2 nanosheet before and after cycling H2 evolution experiments according to the XRD pattern (Fig. S3), implying its good structural stability. Moreover, it is worth mentioning that the MoS2 is usually used as cocatalysts for CdS-based or other photocatalysts in hydrogen evolution [31e33], pure MoS2 samples in the literature always show poor or even no photocatalytic H2 evolution activities [14,34]. However, compared to the reported MoS2 catalysts for photocatalytic H2 evolution [14,34e39], as shown in Table 2, the obtained few-layer MoS2 nanosheets in this work exhibit the highest evolution rate of hydrogen. On the basis of the above results, the structural changes during the preparation and the probable mechanism for the enhanced photocatalytic H2 evolution performance of the few-layer MoS2 nanosheets were analyzed and illustrated in Fig. 7. First, the two-times solvothermal treatment of MoS2 stack plays a critical role in the synthesis of high-yield solid monodispersed few-layer MoS2 nanosheets. The tight MoS2 stack is effectively expanded to a uniform flower structure with unfolded ultrathin MoS2 nanosheets, which greatly
Table 2 e Comparison of the photocatalytic hydrogen evolution data of pure MoS2 in the literature reports. Photocatalyst MoS2 nanosheets MoS2 nanosheets MoS2 nanoflakes MoS2 nanosheets MoS2 quantum dots MoS2 sub-micrometer sphere MoS2 nanosheets Few-layer MoS2 nanosheets
Synthetic method
Activity (mmol g1 h1)
Reaction conditions
Ref.
hydrazine assisted liquid exfoliation formation hydrothermal method hydrothermal method hydrothermal method ultrasonic bath hydrothermal method
0
lactic acid scavengers,150 W Xe lamp
14
0 24.5 40 200 220
Na2S-Na2SO3 scavengers, 300 W Xe lamp Na2S-Na2SO3 scavengers, 300 W Xe lamp lactic acid scavengers, solar simulator Pt scavengers, 450 W Xe lamp Na2S-Na2SO3 scavengers, 300 W Xe lamp
34 35 36 37 38
hydrothermal method heat treatment coupled liquid exfoliation method
240 1241.3
methanol scavengers, 300 W Xe lamp Na2S-Na2SO3 scavengers, 300 W Xe lamp
39 This work
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Fig. 7 e (a) Schematic representation of the liquid exfoliation process by ultrasonication, (b) the illustration of the disordered structure on few-layer MoS2 nanosheets and the HER process at the active sites driven by photocatalysis. reduce the exfoliation difficulty and increase the output of the final few-layer MoS2 nanosheets. In addition, the high photocatalytic HER activity of the optimized MoS2 nanosheets can be attributed to four aspects as follows: (i) during the ultrasonication process, the flower-like structures are broken into isolated nanosheets, and a much bigger specific surface area can be obtained to provide more exposed active sites for HER; (ii) as shown in Fig. 7, the ultrasonication effect can not only decrease the layer number of MoS2 nanosheets but also damage the atoms structure of basal surface in some degree, some cracks appear on the inert basal planes of the nanosheets, resulting in the exposure of additional active edge sites; (iii) the few-layer MoS2 nanosheets possess much better overall conductivity, leading to more efficient charge separation and transfer in the H2 evolution reaction; (iv) a more negative CB potential was obtained for the MoS2 nanosheets due to the structure change, which leads to a higher reduction potential for superior HER performance.
Conclusions In summary, a facile heat treatment coupled with an exfoliation method was developed for the preparation of solid monodispersed few-layer MoS2 nanosheets with high yield and superior properties. The obtained MoS2 nanosheets show few-layer and direct band gap features, and additional lattice cracks were observed on the basal surfaces of the nanosheets. The solid nature and excellent dispersibility and stability in solution of the MoS2 nanosheets can give them greater potential in many applications. In addition, enhanced photoelectrochemical properties and effective photocatalytic hydrogen evolution were achieved according to the synergistic structural and electronic modifications. The superior photocatalytic HER activity originates from the large specific surface area, excellent charge transfer capability, more negative reduction potential and rich active edge sites, and these properties have been proven by the BET surface area, HRTEM and photoelectrochemical characterizations. This work will provide new insights for few-layer MoS2 nanosheets in the application of photocatalytic hydrogen evolution and pave a new pathway for preparing other solid 2D nanomaterials with high yield.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21476183, 21676213, 21663030, 21766039), Startup Foundation for Doctors of Yan'an University (YDBK2018-41, YDBK2018-42), and Scientific Research Foundation of the Education Department of Shaanxi Provincial Government, China (No. 18JK0873, 16JS120).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.150.
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A novel approach for high-yield solid few-layer MoS2 nanosheets with effective photocatalytic hydrogen evolution Jun Wan a,b, Ruimiao Wang b, Lin Liu a,*, Jun Fan b,**, Enzhou Liu b, Xiaoming Gao a, Feng Fu a a
College of Chemistry & Chemical Engineering, Yan'an University, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an 716000, PR China b School of Chemical Engineering, Northwest University, Xi'an 710069, PR China
article info
abstract
Article history:
Few-layer molybdenum disulfide (MoS2) nanosheets are well applied in many field, but the
Received 1 February 2019
lack of simple methods for the preparation of solid few-layer MoS2 nanosheets with high
Received in revised form
yield and quality has greatly restricted their development. In this work, a facile sol-
8 April 2019
vothermal treatment coupled with the liquid exfoliation strategy was conducted to pro-
Accepted 15 April 2019
duce solid monodispersed few-layer MoS2 nanosheets from the MoS2 stack, and the output
Available online 19 May 2019
can reach as high as approximately 0.3 g/g. The few-layer features were confirmed by characterizations of SEM, TEM, Raman spectra, UVevis absorption spectrum and PL
Keywords:
spectrum. The obtained MoS2 nanosheets exhibit fantastic dispersity and stability in an
2D nanomaterials
NMP solution, which can remain uniform even after one year. In general, pure MoS2 cat-
Few-layer MoS2 nanosheets
alysts show no or poor activity for photocatalytic hydrogen evolution as reported in the
Photocatalysis
literature, however, the prepared MoS2 nanosheets in this work display excellent photo-
Hydrogen evolution
catalytic H2 evolution performance of 1241.3 mmol g1 h1 due to the synergistic structural and electronic modifications, including a bigger specific surface area, additional exposed active edge sites, superior charge separation and transfer efficiency, and higher reduction potential. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Since the discovery of graphene in 2004, analogous twodimensional (2D) layered nanomaterials, such as transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), hexagonal boron nitride (h-BN) and carbon nitride, have attracted tremendous attention in many potential
applications due to their fantastic optical, electronic, thermal and mechanical properties [1e5]. MoS2 is a typical TMD material with a sandwich-like structure consisting of an atomic plane of molybdenum between two planes of sulfur atoms, and the individual sandwiched S-Mo-S layers are kept together via weak van der Waals force, making it possible to exfoliate the layers to few-layer or mono-layer nanosheets from bulk MoS2 [6e9]. It has been demonstrated that single- or
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (J. Fan). https://doi.org/10.1016/j.ijhydene.2019.04.150 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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few-layered MoS2 exhibits much better properties in comparison with the bulk material due to its direct band gap, more exposed active sites and higher specific surface area, which is superior for electrocatalytic hydrogen evolution reaction (HER) and photocatalysis applications [10e14]. However, the current lack of methods to prepare single- or few-layered MoS2 nanosheets in large quantity and high quality still restricts the development of these materials. To date, a number of techniques have been employed to prepare 2D ultrathin MoS2 nanosheets [15e20], including mechanical exfoliation, liquid-phase exfoliation with sonication, chemical or electrochemical exfoliation via lithium intercalation, and chemical vapor deposition (CVD) growth, but the above methods all have some drawbacks. Mechanical exfoliation has a very low throughput, the condition requirements of the CVD method are very strict, and the method is expensive. The shortcomings of chemical and electrochemical exfoliation are the extraordinary sensitivity to the environment, the phase and structural deformation of MoS2, and the small horizontal size of nanosheets; moreover, it is difficult to remove the residual Li in the MoS2 nanosheets. Compared with other methods, liquid-phase exfoliation is a promising route for the production of few-layer MoS2 nanosheets due to its low cost and simple operation [21e23]. However, for the direct exfoliation of bulk materials via ultrasonication in solvents, the harvest is very low. It can only obtain a dispersion liquid of MoS2 nanosheets with low concentration but cannot obtain sufficient solid-state few-layer MoS2 nanosheets, thus greatly limiting the applications of 2D MoS2 nanosheets. The reason comes from the stubborn bulk structure feature of original MoS2, some especial pretreatment process must introduced before ultrasonication. In addition, the aggregation of the layers would occur during the slow solvent evaporation process. Actually, monolayers are not necessary in many practical applications, and dispersed fewlayer nanosheets are sufficient. Therefore, developing a simple method for obtaining solid few-layer MoS2 nanosheets with high yield and good properties is of great significance. In this study, we developed a facile and reliable solvothermal treatment coupled with the exfoliation strategy to produce solid monodispersed few-layer MoS2 nanosheets. The few-layer features of MoS2 including the morphology, crystallinity, optical property and band structure were testified using various characterizations. Meanwhile, the photocatalytic hydrogen evolution performances of obtained the 2D MoS2 nanosheets were also investigated and compared to the origin MoS2 stack.
Experimental section Synthesis of solid few-layer MoS2 nanosheets MoS2 stack The origin MoS2 was first synthesized via a facile hydrothermal method [12]. In detail, 1 mmol of (NH4)6Mo7O24$4H2O and 30 mmol of N2H4CS were dissolved in 70 mL of water to form a homogeneous solution under magnetic stirring. The solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 220 C for 18 h. After cooling down to
room temperature naturally, the obtained black precipitate was collected by centrifugation and washed repeatedly with deionized water and then dried at 60 C.
MoS2 nanoflower Before exfoliation, the MoS2 stack was pretreated by a solvothermal process with special solvent to expand the microstructure. In a typical synthesis, 1 g of MoS2 stack was dispersed in 70 mL of N-methyl-2-pyrrolidone (NMP, C5H9NO). Then the dispersion was transferred into a Teflon-lined autoclave and heated at 220 C for 18 h once again. The reactor was cooled to room temperature naturally, the resulting sample was collected and washed with DI water and ethanol several times and dried at 60 C.
Few-layer MoS2 nanosheets The final product was obtained from liquid exfoliation by probe ultrasonic processing and suction filtration. 1 g of heat treated MoS2 (MoS2 nanoflower) was well dispersed in 500 mL of NMP by magnetic stirring, followed by ultrasonic treatment in an Ultrasonic Processor (Sonics VCX800) for 6 h and was allowed to stand for 12 h. Then, the suspension was filtered through porous polyvinylidene fluoride (PVDF) (0.45 mm nominal pore size) membranes. The solid MoS2 product was washed by DI water and ethanol several times and dried at 60 C.
Characterization The morphologies and microstructures were observed by field emission scanning electron microscopy (FE-SEM, Carl Zeiss SIGMA) and transmission electron microscopy (TEM, Tecnai G2 F20S-TWIN). Powder X-ray diffraction (XRD) was performed on a Shimadzu XRD-6000 powder diffractometer with Cu Ka radiation. The specific surface areas of the samples were measured using a Quantachrome NOVA 2000e and calculated by the Brunauer-Emmett-Teller (BET) method. The UVevis absorption spectrum was recorded on a Shimadzu UV-3600 UV/vis/NIR spectrophotometer. The photoluminescence (PL) spectrum was obtained on a Hitachi F-7000 fluorescence spectrophotometer.
Photoelectrochemical measurements The photoelectrochemical measurements were also carried out on an electrochemical workstation (CHI-660E, Chenhua) with a three-electrode system that consisted of a Pt wire as the counter electrode and saturated calomel electrode as the reference electrode. The working electrode was prepared as follows: 10 mg photocatalyst was dispersed in 1.0 mL a mixed solution (0.5 mL water and 0.5 mL ethanol) with ultraphonic dispersion for 30 min. Then the slurry was dropped onto a fluoride tin oxide (FTO) glass electrode and then dried naturally in ambient air and heated at 200 C for 2 h. The illumination area of the working electrode was 0.785 cm2. A 0.5 M Na2SO4 aqueous solution was employed as the electrolyte in the photoelectrochemical test system. A 300 W Xenon lamp (MICROSOLAR300UV, Beijing Perfect light) with a UV cutoff filter was employed as the light source.
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Evaluation of the photocatalytic performance The photocatalytic performance of the samples was evaluated by photocatalytic water splitting for H2 evolution, which was performed in a Pyrex top-irradiated reactor with an online gas-closed circulation system. A 300 W Xe lamp was used as the light source. In a typical experiment, 50 mg of photocatalyst powder was dispersed in 100 mL of aqueous solution under magnetic stirring containing 0.35 M Na2S and 0.25 M Na2SO3 as the scavenger. Prior to irradiation, the air in the reaction system was completely removed by a vacuum pump. The temperature of the reaction solution was controlled at 6 C by a flow of cooling water (DC-0506, Shanghai Hengpin Technology). The H2 amount was in situ recorded periodically using an online gas chromatography (GC7900, Tech-comp Shanghai) with high-purity nitrogen as the carrier gas.
Results and discussion The structure and morphology of the as-prepared MoS2 samples in three steps were first investigated by FE-SEM. As shown in Fig. 1a and b, the MoS2 obtained from the hydrothermal process is accumulated by large amounts of interconnected nanosheets, while the excessive stack results in the generation of thick bulk structures, which is not beneficial to its further applications. However, the ultrathin nature of the nanosheets still can be observed in the stack system with a uniform lateral size and corrugate character (Fig. 1b). Summarizing the reported reference, it can be found that the direct exfoliation of bulk MoS2 is usually insufficient and of low output due to the difficulty of breaking the tight bulk structure, and only a low-concentration liquid MoS2 nanosheets dispersion can be obtained [12,22e24]. To obtain few-layer nanosheets of high yield and quality, we creatively conduct a two-times solvothermal process towards the above stack structures in NMP solution. As a result, the stack structure of MoS2 is broken up owing to the effect of high temperature and pressure during the reaction and the suitable surface energy of NMP [21]. Fig. 1c and d shows that MoS2 after heat treatment exhibits a uniform and well dispersed flower-like morphology, and this loose nanoflower structure can be a good intermediate state for further exfoliation. After the efficient liquid exfoliation via probe-type ultrasonication, the final product is separated from the NMP solvent by vacuum filtration. The output of the solid product can reach approximately 0.3 g/g, and the rest of MoS2 can be used for exfoliation again. The SEM images (Fig. 1e and f) clearly reveal that the MoS2 nanoflowers are effectively broken into monodispersed ultrathin nanosheet morphology with a lateral size of 0.3～1.5 mm and thickness of 2～10 nm. XRD and BET characterizations were further carried out to investigate the structural changes of various samples. As shown in the XRD pattern (Fig. 2a), all the diffraction peaks of the three samples can be well indexed to hexagonal MoS2 (JCPDS card No. 73-1508), and the broadening of the diffraction peaks indicates the nanoscale of the crystal structure [10]. In addition, it can be observed that the XRD patterns of the MoS2 stack and MoS2 nanoflower have no difference, suggesting that
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the heat treatment mainly changes the morphology of MoS2 but not the crystal structure. In comparison, the peaks intensites of MoS2 nanosheets dramatically decrease, especially the major diffraction peak at 14 corresponding to the (002) plane of interlayer stacking along the c axis, the peak area ratio of (002)/ (100) of MoS2 stack, MoS2 nanoflower and MoS2 nanosheets decrease from 1.75 to 1.58 and 1.35, suggesting the significant decrease of the layer number of MoS2 nanosheets and efficient exfoliation to a few-layers [24,25]. In addition, the BET specific surface area of the MoS2 nanosheets shows an obvious increase due to its smaller nano size and 2D structure, which is evaluated to be 46.58 m2/g, and it is 1.37 and 1.71 times higher than that of MoS2 nanoflower (33.93 m2/g) and MoS2 stack (27.19 m2/ g). Furthermore, as shown in Fig. 2bec and Table 1, the MoS2 stack and MoS2 nanoflower have a similar hysteresis loops due to their intertwined flower-like structure and narrow pore size distribution, which is mainly concentrated at 3.8 nm, and the average pore diameters are about 10.2 nm and 12.0 nm, respectively. In comparison, when the nanoflower is broken by ultrasonication and further exfoliated into few-layer MoS2 nanosheets, the adsorption-desorption isotherms and hysteresis loops of the MoS2 nanosheets exhibit great differences correspond to their loose laminar stacked structure and more dispersive pore size distribution, and the average pore diameter is about 15.1 nm. To better characterize the few-layer feature of the obtained ultrathin MoS2 nanosheets, the nanostructure was further characterized by TEM, Raman spectra, UVevis absorption spectrum and PL spectrum. The TEM images in Fig. 3a and b also verify the ultrathin nanosheet morphology with obvious ripples and corrugations of the MoS2 product. The side- and top-view HRTEM images are shown in Fig. 3c and d, directly evidencing that a few -layers can be observed from the folded edge. The obtained MoS2 nanosheets show layers thickness and the interlayer spacing is determined to be 0.685 nm, which is larger than the theoretical layer-to-layer distance of bulk MoS2 (0.62 nm) along the c-axis direction [26]. It is worth noting that different interplanar spacing of 0.249 nm, 0.273 nm and 0.307 nm are observed from the top-view image, which are consistent with the (102), (100) and (004) planes of hexagonal MoS2, respectively. In addition, the directions of these lattice fringes are not the same and obvious cracks can be observed at the interface of different lattice planes, suggesting the disordered atomic arrangement on the basal surface. This interesting phenomenon indicates that the ultrasonic exfoliation not only decreases the layer number of MoS2 nanosheets but can also create rich defects on the surface of MoS2 nanosheets, which will offer a large amount of unsaturated sulfur atoms as exposed active edge sites for the photocatalytic hydrogen evolution reaction [14]. The Raman spectra of the MoS2 stack, the MoS2 nanoflower and the MoS2 nanosheet were obtained (Fig. 4a), and the characteristic vibration modes of in-plane E12g and out-ofplane A1g are observed in all curves. It is known that the frequency difference of the two modes can be used to confirm the existence of the few-layer nanostructure and estimate the layer number of few-layer lamellae [6]. The two Raman peaks of the MoS2 stack are located at approximately 378.0 and 402.9 cm1, and the frequency difference of approximately
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Fig. 1 e (a) SEM images of (a, b) MoS2 stack, (c, d) MoS2 nanoflower and (e, f) few-layer MoS2 nanosheets.
Fig. 2 e (a) XRD diffraction patterns, (b) BET nitrogen adsorption-desorption isotherms and (c) pore diameter distribution of the samples. 24.9 cm1 is smaller than that of the bulk structure in previously reported results [14,23]. The MoS2 nanoflower shows similar vibration peaks to those of the MoS2 stack without an
apparent shift, while the MoS2 nanosheets exhibit an appreciable redshift of the E12g vibration and blueshift of the A1g vibration. A reduced frequency difference of 23.6 cm1
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Table 1 e BET characteristic data of MoS2 stack, MoS2 nanoflower and few-layer MoS2 nanosheet. Sample
MoS2 stack MoS2 nanoflower MoS2 nanosheet
Specific surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
27.19
0.069
10.15
33.93
0.102
12.02
46.58
0.176
15.10
indicates that the layer number of MoS2 nanosheets is successfully decreased to a few-layers [6,23]. Moreover, the absorption spectrum of few-layer MoS2 nanosheets in NMP is illustrated in Fig. 4b. The two well-known characteristic absorption peaks at approximately 620 nm and 672 nm are observed and ascribed to the direct transition from the top of the valence band at the K-point to the conduction band, which are usually regarded as the symbol of few-layer or mono-layer MoS2 [15,24]. The band gap of MoS2 nanosheets was calculated to be 1.89 eV (Fig. 4c), which is larger than that of bulk MoS2
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(1.2 eV) and close to that of single layer MoS2. Fig. 4d shows the PL spectrum of the MoS2 nanosheets in NMP solution, and the two corresponding characteristic emission peaks at 646 nm and 661 nm are related to the direct excitonic transitions at the K-point of the Brillouin zone, indicating that the MoS2 nanosheets are a direct gap semiconductor. All the above observations clearly reveal the successful synthesis of 2D fewlayer MoS2 nanosheet. In addition, the light harvesting efficiency (LHE, the fraction of absorbed light) of different samples was also calculated from the measured reflection (R%) and transmission (T%) [27]: LHE (%) ¼ 100% - R (%) - T (%) in Supporting Information Fig. S1, MoS2 nanosheets display an obvious higher light absorption ability than the two others. Furthermore, the dispersibility and the stability of the obtained solid MoS2 nanosheets were investigated, and the same amount of solid MoS2 stack and MoS2 nanosheets was dispersed in NMP solution under ultrasonication in an ultrasonic cleaning instrument. Fig. 4e displays the digital photo of the MoS2 dispersion liquid after some times. It can be observed that most of the MoS2 stack was precipitated after only one day, and the solution was completely clarified after two days. For the MoS2 nanosheets, the black dispersion was
Fig. 3 e (a, b) TEM and (c, d) HRTEM images of the as-prepared few-layer MoS2 nanosheets.
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Fig. 4 e (a) Raman spectra of MoS2 stack, MoS2 nanoflower and MoS2 nanosheets; (b) UVevis absorption spectrum, (c) the calculation of the direct bandgap based on the (ahn)2-hn curve, and (d) photoluminescence spectrum of few-layer MoS2 nanosheets in NMP; (e) digital photo of different MoS2 samples dispersed in NMP after some times. kept uniform and almost no precipitation was found even after standing for one year. This phenomenon indicates that the obtained few-layer MoS2 nanosheets possess great dispersibility and stability in solution, which will greatly expand their application fields. The photoelectrochemical properties of the different MoS2 samples were examined in a three-electrode electrochemical workstation. Fig. 5a displays a comparison of the transient photocurrent responses for the MoS2 stack, MoS2 nanoflower and MoS2 nanosheet under repeated on/off illumination cycles. It can be clearly seen that the transient photocurrent density of MoS2 nanosheets is much higher than that of the MoS2 stack and MoS2 nanoflower, implying a better photoelectric response capacity and higher photogenerated charge carriers separation efficiency of the few-layer MoS2 nanosheets, which is beneficial for superior photocatalytic activity. Same result also can be obtained from photoluminescence spectra of MoS2 with different structure (Fig. S2). Meanwhile, the Mott-Schottky measurement was also provided to
investigate the change of the band structure and carrier density of the different MoS2 samples. As shown in Fig. 5b and according to the intercept of the tangent line on the axis, the flat-band potentials of the MoS2 stack, MoS2 nanoflower and MoS2 nanosheet are 0.07 eV, 0.12 eV and 0.24 eV, respectively. As the conduction band (CB) potential for an ntype semiconductor is approximately 0.3 eV higher than the flat-band potential [28], an apparent negative movement can be concluded in the comparison of the CB potentials, the MoS2 nanosheets have a higher CB potential than the two other, which leads to a more negative potential and better reducing capacity for photocatalytic hydrogen evolution. In addition, the carrier density can be calculated based on the slope of the M S plot according to the following equation [29,30]: ND ¼
2 dE 2 1 ¼ εε0 e d C12 εε0 e slope
where ND is the carrier density for n-type semiconductors, ε is the dielectric constant of the semiconductor (6.3 for MoS2),
Fig. 5 e (a) Transient photocurrent responses, (b) Mott-Schottky plots and (c) EIS Nyquist plots of the as-prepared samples.
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Fig. 6 e (a) The amount of H2 production over time and (b) comparison of the H2 production rate of different samples, (c) stability testing over MoS2 nanosheet for H2 generation under the same condition. ε0 is the vacuum permittivity (8.85 1014 F cm2), and e is the electronic charge (1.6 1019 C). The calculated carrier density of MoS2 nanosheets (5.18 1020 cm3) is higher than that of MoS2 nanoflower (4.31 1020 cm3) and MoS2 stack (3.42 1020 cm3), suggesting that a larger number of electron donors exist in the MoS2 nanosheets under photoexcitation, which is in line with the transient photocurrent responses. In addition, the electrochemical impedance spectroscopy (EIS) measurement was performed to characterize the charge transfer resistance (Rct) of the materials. The corresponding Nyquist plots were recorded in Fig. 5c. All samples display a semicircle at high frequencies, and the reaction resistance is indicated by the relative diameter size of the semicircle. It can be obviously observed that MoS2 nanosheets exhibit the smallest semicircle diameter compared to the other nanostructures, suggesting that MoS2 nanosheets have a superior conductivity for promoting the transfer of photogenerated electrons in the catalytic reaction. Fig. 6a and b shows the photocatalytic hydrogen evolution of the as-synthesized MoS2 samples under simulated light irradiation without noble metal cocatalysts but using mixed Na2S-Na2SO3 as the sacrificial reagents, which works as electron donors and final oxidized into SO2 4 . The good linear increase of H2 production over the entire time range indicates the good photostability of all samples during the photocatalytic water splitting, and the MoS2 nanosheets display a distinctly better photocatalytic hydrogen evolution activity than the MoS2 stack and MoS2 nanoflower. In detail, the hydrogen gas production amount of MoS2 nanosheets is
measured to be 1241.3 mmol g1 h1, which exhibits a 39.4% and 72.7% increment compared to that of the MoS2 nanoh1) and MoS2 stack flower (890.7 mmol g1 1 1 (718.7 mmol g h ), respectively. The results are consistent with the photoelectrochemical analyses. Durability is another significant criterion for HER catalysts, and it was investigated by cycling photocatalytic experiments as shown in Fig. 6c, the H2 evolution rate of the MoS2 nanosheet is stable after longterm test of five sequential cycles, and there is an inappreciable change in the phase structure of MoS2 nanosheet before and after cycling H2 evolution experiments according to the XRD pattern (Fig. S3), implying its good structural stability. Moreover, it is worth mentioning that the MoS2 is usually used as cocatalysts for CdS-based or other photocatalysts in hydrogen evolution [31e33], pure MoS2 samples in the literature always show poor or even no photocatalytic H2 evolution activities [14,34]. However, compared to the reported MoS2 catalysts for photocatalytic H2 evolution [14,34e39], as shown in Table 2, the obtained few-layer MoS2 nanosheets in this work exhibit the highest evolution rate of hydrogen. On the basis of the above results, the structural changes during the preparation and the probable mechanism for the enhanced photocatalytic H2 evolution performance of the few-layer MoS2 nanosheets were analyzed and illustrated in Fig. 7. First, the two-times solvothermal treatment of MoS2 stack plays a critical role in the synthesis of high-yield solid monodispersed few-layer MoS2 nanosheets. The tight MoS2 stack is effectively expanded to a uniform flower structure with unfolded ultrathin MoS2 nanosheets, which greatly
Table 2 e Comparison of the photocatalytic hydrogen evolution data of pure MoS2 in the literature reports. Photocatalyst MoS2 nanosheets MoS2 nanosheets MoS2 nanoflakes MoS2 nanosheets MoS2 quantum dots MoS2 sub-micrometer sphere MoS2 nanosheets Few-layer MoS2 nanosheets
Synthetic method
Activity (mmol g1 h1)
Reaction conditions
Ref.
hydrazine assisted liquid exfoliation formation hydrothermal method hydrothermal method hydrothermal method ultrasonic bath hydrothermal method
0
lactic acid scavengers,150 W Xe lamp
14
0 24.5 40 200 220
Na2S-Na2SO3 scavengers, 300 W Xe lamp Na2S-Na2SO3 scavengers, 300 W Xe lamp lactic acid scavengers, solar simulator Pt scavengers, 450 W Xe lamp Na2S-Na2SO3 scavengers, 300 W Xe lamp
34 35 36 37 38
hydrothermal method heat treatment coupled liquid exfoliation method
240 1241.3
methanol scavengers, 300 W Xe lamp Na2S-Na2SO3 scavengers, 300 W Xe lamp
39 This work
16646
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 6 6 3 9 e1 6 6 4 7
Fig. 7 e (a) Schematic representation of the liquid exfoliation process by ultrasonication, (b) the illustration of the disordered structure on few-layer MoS2 nanosheets and the HER process at the active sites driven by photocatalysis. reduce the exfoliation difficulty and increase the output of the final few-layer MoS2 nanosheets. In addition, the high photocatalytic HER activity of the optimized MoS2 nanosheets can be attributed to four aspects as follows: (i) during the ultrasonication process, the flower-like structures are broken into isolated nanosheets, and a much bigger specific surface area can be obtained to provide more exposed active sites for HER; (ii) as shown in Fig. 7, the ultrasonication effect can not only decrease the layer number of MoS2 nanosheets but also damage the atoms structure of basal surface in some degree, some cracks appear on the inert basal planes of the nanosheets, resulting in the exposure of additional active edge sites; (iii) the few-layer MoS2 nanosheets possess much better overall conductivity, leading to more efficient charge separation and transfer in the H2 evolution reaction; (iv) a more negative CB potential was obtained for the MoS2 nanosheets due to the structure change, which leads to a higher reduction potential for superior HER performance.
Conclusions In summary, a facile heat treatment coupled with an exfoliation method was developed for the preparation of solid monodispersed few-layer MoS2 nanosheets with high yield and superior properties. The obtained MoS2 nanosheets show few-layer and direct band gap features, and additional lattice cracks were observed on the basal surfaces of the nanosheets. The solid nature and excellent dispersibility and stability in solution of the MoS2 nanosheets can give them greater potential in many applications. In addition, enhanced photoelectrochemical properties and effective photocatalytic hydrogen evolution were achieved according to the synergistic structural and electronic modifications. The superior photocatalytic HER activity originates from the large specific surface area, excellent charge transfer capability, more negative reduction potential and rich active edge sites, and these properties have been proven by the BET surface area, HRTEM and photoelectrochemical characterizations. This work will provide new insights for few-layer MoS2 nanosheets in the application of photocatalytic hydrogen evolution and pave a new pathway for preparing other solid 2D nanomaterials with high yield.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21476183, 21676213, 21663030, 21766039), Startup Foundation for Doctors of Yan'an University (YDBK2018-41, YDBK2018-42), and Scientific Research Foundation of the Education Department of Shaanxi Provincial Government, China (No. 18JK0873, 16JS120).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.150.
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