MoS2 heterostructures for efficient photocatalytic hydrogen evolution

Accepted Manuscript Glucose-assisted synthesize 1D/2D nearly vertical CdS/MoS2 heterostructures for efficient photocatalytic hydrogen evolution Yue Li, Longlu Wang, Tao Cai, Shuqu Zhang, Yutang Liu, Yuze Song, Xueru Dong, Liang Hu PII: DOI: Reference:

S1385-8947(17)30507-7 http://dx.doi.org/10.1016/j.cej.2017.03.139 CEJ 16735

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 January 2017 13 March 2017 28 March 2017

Please cite this article as: Y. Li, L. Wang, T. Cai, S. Zhang, Y. Liu, Y. Song, X. Dong, L. Hu, Glucose-assisted synthesize 1D/2D nearly vertical CdS/MoS2 heterostructures for efficient photocatalytic hydrogen evolution, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.03.139

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Glucose-assisted

synthesize

1D/2D

nearly

vertical

CdS/MoS2

heterostructures for efficient photocatalytic hydrogen evolution Yue Li1*§, Longlu Wang2,3§, Tao Cai2, Shuqu Zhang2, Yutang Liu 2*,Yuze Song1, Xueru Dong1, Liang Hu2. 1

School of Materials and Chemical Engineering, Henan Institute of Engineering,

Zhengzhou, Henan 451191, P. R. China 2

College of Environmental Science and Engineering, Hunan University, Changsha

410082, P. R. China 3

State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,

Changsha 410082, P. R. China. Corresponding Authors *Y. Li, Tel.: +86 0371-67718909. Fax: +86 0371-67718909. E-mail address: [email protected] *Y.T. Liu, Tel.: +86 0731-88821429. Fax: +86 0731-88821429. E-mail address: [email protected]. Author Contributions §Y. Li and L.L. Wang contributed equally to this work.

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ABSTRACT： ： Molybdenum disulfide (MoS2) is a promising non-precious-metal cocatalyst to replace scarcity and cost Pt, but still suffers from less active sites and poor electrical transport due to the unoptimized geometrical configuration of MoS2 on the semiconductor surface. Here, two different morphological and structural CdS/MoS2 heterostructures were successfully prepared by a simple hydrothermal reaction with glucose-assisted or not. With the help of glucose, the MoS2 nanosheets can nearly vertical stand on the CdS nanowires, which do not have the characteristic stacked layer structure of MoS2. This unique configuration leads to a high exposure of the active edge sites and increases charges separation and transfer rate. As expected, the 1D/2D nearly vertical CdS/MoS2 heterostructures exhibited excellent photocatalytic activity and good durability even after being reused five times. Simultaneously, an apparent quantum yield can be reached to 60.3% at 420 nm, which was the advanced performance among all CdS/MoS2 composites. This work will provide an intriguing and effective approach on design of low-cost and efficient photocatalysts for enhanced HER performance. Keywords: MoS2, CdS, nearly vertical, nanostructures, hydrogen production

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1. Introduction Photocatalytic hydrogen production from water on semiconductor-based photocatalysts has attracted increasing attention due to their potential to resolve energy and environmental issues[1-9]. Among the photocatalysts, CdS is one of the most promising visible light-responsive material for hydrogen evolution reaction (HER) according to the relatively narrow band gap of 2.4 eV and proper position of the valence band and conduction band[10-15]. However, the intrinsic properties of high-rate recombination of photoexcited charges and photocorrosion limit the maximum efficiency of water splitting[16-18]. An attractive way to solve the above problem is that loading cocatalysts on CdS to form heterostructures, which could promote the separation of photoexcited electrons and holes, offer the low activation potentials for H2 evolution, as well as inhibit the photocorrosion of CdS[19-21]. Molybdenum disulfide(MoS2), consisting of a single layer of Mo atoms sandwiched between two layers of hexagonal close packed sulfur atoms, has been proved to be an excellent cocatalyst to combine with CdS, which could significantly enhance the separation of photoexcited charges and suppress the photocorrosion of CdS[22-24]. It has been reported that the activity of producing H2 on CdS/MoS2 is comparative or even higher than that of CdS/Pt under the same reaction conditions[25], thereby it is a potential substitute for scarcity and cost Pt. However, researches show only the edges have high activities for photocatalytic H2 evolution, simultaneously the basal plane is catalytically inert[5, 9]. Frustratingly, MoS2 often exist in the form of stacked layers to minimize surface energy, greatly obstructing the

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exposure of the catalytic edge active sites and increasing the charge transfer resistance from one S-Mo-S sheet to another. As a result, it is necessary to maximum expose the catalytic edge sites, reduce the number of layers and electrons transport resistance by engineering the morphology of MoS2. Up to now, considerable efforts have been placed in preparing various CdS/MoS2 heterostructures, such as branches[23], core-shell[24], flowers[26], balls[27] and so on[18, 28]. All of these hybrid photocatalysts with different morphology and structure inhibited the stacking of MoS2 nanosheets in a certain extent, as a result in remarkable enhancement in H2 evolution. However, researches found that the vertically aligned 2D nanosheets onto 1D nanomaterials allow the exposure of almost the entire surface, thereby maximally exposing the edge active sites[29-31]. In addition, connecting the edges of few-layered MoS2 nanosheets with the 1D nanomaterials makes a better electronic contact and an optimized electron transport pathway, resulting from the less electrons transport resistance and more conductive on the edges of MoS2 than its basal plane[5, 30, 32, 33]. To the best of our knowledge, easy preparation of the unique 1D/2D vertically structures with enhanced HER performance has still been a challenge. Herein, we reported the rational design and fabrication of 1D CdS nanowire/2D MoS2 nanosheet nearly vertical heterostructures by a simple glucose-assisted hydrothermal reaction. Notably, 1D hexagonal CdS nanowire were chosen as scaffold to composite MoS2 nanosheet for two reasons: On the one hand, 1D nanomaterials have been regarded as the smallest-dimension structures, which is benefit for efficient

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transport of charges, optical excitation, and suppression of unexpected back reaction[34-36]; On the other hand, a hexagonal crystal phase of CdS, sharing the same crystalline structure with MoS2, which endows that a high-quality, intimate heterojunction between CdS and MoS2[27, 29]. In addition, we found that when glucose was introduced in the preparation process of CdS/MoS2 heterostructures, it can make the MoS2 nanosheets as thin as possible, and at the same time, serve as a binder to help the MoS2 nanosheets growing along the longitudinal axis in the formation of 1D/2D nearly vertical CdS/MoS2 heterostructures. As a proof-ofconcept, the synthesized catalysts exhibited an excellent performance as a highly efficient and stable photocatalyst without requiring the Pt cocatalys.

2. Experimental 2.1. Synthesis of CdS nanowires The CdS nanowires were prepared by using solvothermal method[29]. Firstly, 80 mg Cadmium acetate dehydrate and 16 mg Sulfur powder were dissolved into 9 mL ethylenediamine under stirring for 30 min, getting a yellow dispersion. Then it was transferred into a 10 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 3 h. After the reaction, the autoclave was cooled down to room temperature in air and the products washed with ethanol and deionized water for several times. The solid product was dried at 60 °C for 12 h to obtain CdS nanowire. 2.2. Synthesis of CdS/MoS2(G) and CdS/MoS2(W) heterostructures The CdS/MoS2 heterostructures were synthesized by a simple hydrothermal reaction with or without glucose-assisted. The former labeled as CdS/MoS2(G), and

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the later labeled as CdS/MoS2(W). Typically, sodium molybdate (Na2MoO4·2H2O, 45 mg) and thioacetamide (C2H5NS, 90 mg) were dissolved in glucose solution (20 mL, 0.05 M) by ultrasonication for 30 min to form a transparent solution. Then, 270 mg of as-obtained CdS nanowires were added into the above solution and stirred to get a suspension. After stirring the suspension for 1 h at room temperature, the suspension was transferred into a Teflon-lined autoclave, followed by heating at 220 °C for 24 h. Finally, the precipitate (containing 10wt% of MoS2) was washed with ethanol and deionized water for several times. The final CdS/MoS2(G) samples were obtained after drying at 60 °C for 12 h. The CdS/MoS2(G) composites with different weight ratios of MoS2 (1%, 2%, 3%, 5%, 10% and 15%) can be tuned by adjusting the amount of CdS nanowire. In this paper, only the optimized CdS/MoS2(G) sample with 10 wt% MoS2 loading was discussed in detail unless otherwise stated. For comparison, the CdS/MoS2(W) samples or pure MoS2 nanosheets were also prepared under the same method without the addition of glucose or CdS nanowires. 2.3. Characterization The morphology of the as-prepared samples was examined with an S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan) and a transmission electron microscope (TEM, JEOL JEM-2100F). The EDS mapping images were captured on a Tecnai G2 F20 S-TWIN atomic resolution analytical microscope. The crystal-phase properties of the samples were analyzed with an X-ray diffractometer with Cu-Kα radiation (XRD, M21X, MAC Science Ltd., Japan). The binding energies of S, Mo, and O of the heterostructures were tested by X-ray photoelectron

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spectroscopy using an Al-Kα X-ray source (XPS, K-Alpha 1063, Thermo Fisher Scientific, England). The UV−visible diffuse reflectance spectra (DRS) were recorded with a UV–vis spectrophotometer (Cary300, USA) with an integrating sphere. The photoluminescence (PL) spectra were obtained using Hitachi F-2500 fluorescence spectrophotometer at an excitation wavelength of 514 nm. 2.4. Photoelectrochemical measurements Photocurrent studies were performed on a CHI 660D electrochemical workstation, using a three-electrode configuration where FTO electrodes deposited with the samples as working electrode, Pt as counter electrode, and a saturated calomel electrode(SCE) as reference. The electrolyte was 0.35 M/0.25 M Na2S– Na2SO3 aqueous solution. For the fabrication of the working electrode, 0.25 g of the sample was grinded with 0.06 g polyethylene glycol (PEG, molecular weight: 20000) and 0.5 ml ethanol to make a slurry. Then, the slurry was spread onto a 1 cm×4 cm FTO glass by the doctor blade technique, and then allowed to dry in air. A 300 W xenon arc lamp equipped with a filter to cut off light of wavelength below 420 nm (λ> 420 nm) was used as visible light source (Perfectlight, PLS-SXE 300C, Beijing, China). The incident light intensity was tuned to be 160 mW/cm2 measured by NOVA Oriel 70260 with a thermodetector. 2.5. Photocatalytic hydrogen production tests The photocatalytic hydrogen evolution experiments were performed in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. A 300 W xenon arc lamp equipped with a filter to cut off light of wavelength below 420 nm

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(λ> 420 nm) was used as visible light source (Perfectlight, PLS-SXE 300C, Beijing, China). The illumination intensity on the flask was 160 mW cm−2. In a typical photocatalytic experiment, 20 mg of photocatalysts were suspended in an 80 mL mixed aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 as sacrificial agents (pH = 13.2). The cell was kept at low temperature by using a circulating water system. The evolved H2 were collected and online analyzed by a H2 solar system (Beijing Trusttech Technology Co., Ltd.) with a gas chromatogram equipped with a thermal conductivity detector (TCD), 5 A molecular sieve column, and nitrogen as the carrier gas. The apparent quantum efficiency(AQE) is calculated according to the equation 1:

Number of reacted electrons AQE [%] =

=

Number of incident photons

× 100

Number of evolved H2 molecules × 2 Number of incident photons

× 100

(1)

3. Results and discussion Scheme 1 shows the overall synthetic procedure of CdS/MoS2(G) and CdS/MoS2(W) composite photocatalysts. The two samples with different morphology and structures were both synthesized by using a two-step solvothermal approach, the difference is adding glucose or not in the second prepared step. With the help of glucose, the MoS2 nanosheets preferred to nearly vertical grown on CdS nanowires, while these nanosheets just connected discrete with CdS nanowires in the absent of glucose. As we have been discussed, the activity of MoS2 stems from the expose active edges and electron transport pathway (the basal planes are catalytically inert), it

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is expected that the glucose-assisted synthesize nearly vertical CdS/MoS2 heterostructures can enhance the efficiency of separation and transfer of photoexcited electron− hole pairs and prolong the lifetime of charge carriers. The different microstructures of CdS/MoS2(G) and CdS/MoS2(W) were revealed by SEM images as shown in Fig. 1. The pure CdS nanowires (Fig. 1a) shows highly uniform 1D morphology with diameter ranging from 30 to 50 nm, and its surface is smooth with nothing adhered to it. Fig. 1b revels that without using the CdS nanowires as growth templates, the MoS2 nanosheets were clustered to form micronsized platelet-like structures. However, when only CdS nanowires were added, the ideal 1D/2D nearly vertical structure still can not be obtained until with the assistance of glucose. As seen in Fig. 1c, the thick MoS2 nanosheets were grown separately rather than onto the CdS nanowires. Fig. 1d shows the MoS2 nanosheets became thinner and nearly vertical distribution on the surface of CdS, and the detail structure will be discussed later. This is agreement with the recent reports that glucose plays a crucial role in the formation of MoS2/CNT and TiO2/MoS2 nearly vertical structure[31, 37]. Therefore, it can be concluded that the glucose not only enables MoS2 nanosheets with smaller size and thickness but also serves as a binder to help the MoS2 nanosheets to grow along the longitudinal axis. The loading of MoS2 can be turned by changing the adding amount of CdS nanowires. In the photocatalytic H2 evolution experiments (as discussed later), the CdS/MoS2(G) with 10% loading MoS2 demonstrated the best performance and was thoroughly characterized unless otherwise stated.

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The structure of CdS/MoS2(G) was further investigated by transmission electron microscope(TEM) as shown in Fig. 2. Fig. 2a and b show that the transparent and flexible MoS2 nanosheets nearly vertical grown on the outer surface of CdS nanowires. Such a morphology would be an ideal structure for HER, because the nearly vertical and open structure can provide an optimized electron transport pathway and high density of active sites, thus enhancing the electron–hole pairs separation and photocatalytic performance[38]. The top-view high resolution TEM (HRTEM) images of CdS/MoS2(G) (Fig. 2c and d) reveal the layer number of MoS2 nanosheet is almost 1-2 layers as shown by the arrows, indicating the trend of MoS2 nanosheets agglomeration is reduced(Fig. S1). The selected area in Fig. 2c and d as marked by white square were further magnified on Fig. 2e and f. The lattice spaces of 0.330 nm and 0.630 correspond to the (002) plane of hexagonal CdS and the (002) plane of MoS2, respectively. The yellow arrows indication of 1-2 layered MoS2 confirmed the low crystallinity nature of the nanosheets, which would provide more active sites for photocatalytic HER. However, the SAED pattern shown in Fig. S2 was only agreed to the hexagonal CdS, and there was no clearly diffraction spots attributed to MoS2 which maybe due to their low relatively content and poor crystallinity. The energy dispersive X-ray spectroscopy (EDS) mapping (Fig. 2g) analysis of a nanowire displays the mapping width of S and Mo are larger than that of Cd, suggesting the MoS2 nanosheets have grown on the CdS nanowires successfully. The uniform color and luster illustrate the homogeneous distribution of elemental S and Mo. Besides, it was also found the coexistence of C element in the EDS mapping and EDX

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spectrum(Fig. S3), which would improve the electrical conductivity and thus enhance photocatalytic H2 rate[37]. The high intensity of C peak maybe come from the residual glucose hydrocarbon groups in the CdS/MoS2(G) sample. The XRD patterns of the as-prepared CdS, MoS2, CdS/MoS2(G) were shown in Fig. 3. For the pure CdS sample, the detected peaks fit well with hexagonal phase (JCPDS No. 65-3414), revealing no obvious indication of impurities. For the pure MoS2 sample, the diffraction peaks at 14°, 33°, 39°, and 59° assigned to the (002), (100), (103) and (110) planes of hexagonal phase MoS2 (a=b=0.316 nm, c=1.230 nm, JCPDS card No. 37-1492). Here, the diffraction peak of (002) was corresponding to the periodicity in c-axis of MoS2 plane, indicating the S–Mo–S sandwich layer where Mo atoms coordinated with S atoms were formed in MoS2 nanosheets. After formation of the CdS/MoS2 heterostructures, there was no obvious characteristic diffraction peaks for MoS2, which can be attributed to their relatively low diffraction intensities although the features have been clearly investigated in TEM and HRTEM images as mentioned above[26]. To precisely identify the chemical state of each element in the synthesized CdS/MoS2(G) sample, an XPS study was performed as illustrated in Fig. 4. The survey spectrum shows the coexistence of Mo, Cd, and S elements (Fig. 4a), and the extra C 1s peaks is ascribed to inevitable carbon contamination within the nanocomposite. Fig. 4b displays a high resolution XPS spectrum of Cd 3d, which gives two peaks at 412.0 and 405.1 eV corresponding to the Cd 3d 3/2 and Cd 3d5/2 in CdS, respectively, which is indicative of the typical divalent Cd of CdS[28]. The

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peaks of Mo 3d appear at 232.2 and 228.8 eV are ascribed to the binding energy of Mo 3d3/2 and 3d5/2 orbits for the dominant Mo 4+ oxidation state(Fig. 4c). The small peak locates at approximately 235.3 eV is observed due to the small amount of Mo6+. In addition, the typical characteristic peaks at 226.1, 162.8 and 161.7 eV are in good consistency with the binding energies of S 2s, S 2p 1/2 and S 2p3/2, suggesting the existence of S2-(Fig. 4d). The XPS analysis provides strong evidence that the synthesized composites comprise CdS and MoS2. As an effective photocatalyst, it should possess the ability of harvesting the whole solar-energy spectrum as much as possible. Fig. 5 shows the UV− vis diffuse reflectance (DRS) spectra of CdS, MoS2, CdS/MoS2(W) and CdS/MoS2(G) samples. Compared with the CdS alone, all the MoS2-based samples exhibit the inherent visible-light absorption due to the strong light scattering and trapping effect from MoS2 nanosheets. In addition, the light harvesting ability of CdS/MoS2(G) samples was higher than CdS/MoS2(W) samples in both UV and visible region, displaying that the better attachment of MoS2 nanosheets on CdS nanowires and the open nearly vertical structure of CdS/MoS2(G) samples make limited obstruction on the light absorption of CdS core[33]. Fig.6a shows the photocatalytic H2 production activities of pure CdS and CdS/MoS2(G) catalysts with different MoS2 loading under visible light irradiation (λ > 420 nm) in a mixed aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 as sacrificial agents. It is shown that the rates of H2 evolution on the CdS/MoS2(G) heterostructures were significantly enhanced compared to that of the pure CdS

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nanowires even the MoS2 loading was 1 wt %. The low activity of CdS alone may due to the rapid electrons and holes recombination. As the MoS2 content in the heterostructures increased, the H2 evolution rate initially increased and the best performance was achieved on a 10wt % loading of MoS2, corresponding to a maximum of 9.73 mmol h 1g 1 which was 36.04 times greater than that of the pure −

−

CdS nanowires. The apparent quantum efficiency of the optimized CdS/MoS2(G) catalysts was 60.3% at 420 nm, which was calculated according to the formula listed in supplementary information, exhibiting one of the best MoS2 or CdS based hybrid HER catalysts (Table S1) reported in literature. To the best of our knowledge, the CdS/MoS2(G) catalyst performs the best H2 production rate because the nearly vertical nanostructure relative to those of the referred CdS/MoS2 catalysts. On the one hand, the nearly vertical nanostructure almost exposes its entire surface, thereby proving more edge active sites to generate H2. On the other hand, the MoS2 edges direct connect with the CdS nanowires, which makes a better electronic contact and an optimized electron transport pathway. However, when the content of MoS2 was more than 10 wt%, the excessive MoS2 maybe partially block the light absorption of CdS, thus leading to a low photoactivity[18, 39]. Besides, the highly overlapping each other of MoS2 nanosheeets also reduced active site exposure, thus fading the photocatalytic H2 generation. For comparison, a control experiment was conducted to evaluate the photocatalytic activity of optimized CdS/MoS2(G) and CdS/MoS2(W) with the same MoS2 loading, physical mixture of CdS and MoS2, pure CdS, and pure MoS2 under

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visible light(Fig. 6b). It is remarkable to find that the H2 evolution rate of CdS/MoS2(G) is much higher than the CdS/MoS2(W), and the pure CdS and MoS2 almost have no activity for hydrogen evolution. The results consolidated our standpoint that the nearly vertical MoS2 nanosheets on CdS/MoS2(G) maximally expose HER-active edges, while the MoS2 nanosheets on CdS/MoS2(W) have a random orientation and serious aggregation which limited photogenerated charges efficiently migrate. In addition, the physical mixture of CdS nanowires and MoS2 nanosheets shows poor HER activity, indicating the intimate contact between MoS2 and CdS is essential to deliver the high hydrogen production. The above results can be further explained by photoluminescence (PL) test and photoelectrochemical responses of CdS, CdS/MoS2(G) and CdS/MoS2(W), which can be used to investigate the efficiency of electron–hole separation, immigration and transfer[40, 41]. The lower PL intensity is, the higher the e--h+ separation efficiency is, and consequently the enhanced photocurrent densities. As shown in Fig. 7a, it can be found that the CdS/MoS2(G) exhibited lowest PL intensity among all the samples, suggesting the lowest electron–hole pairs recombination rate and increased lifetime of the charge carriers there. As excepted, the CdS/MoS2(G) exhibited the highest photocurrent density owing to the enhanced electron–hole separation efficiency (Fig. 7b). The value of photocurrent density of CdS/MoS2(G) (23.5 µA/cm2) was roughly 2.24, 5.59 times higher than that of CdS/MoS2(W) and pure CdS nanowires, respectively. These results elucidate that the e--h+ separation efficiency for CdS/MoS2 catalysts can be considerably controlled by engineering the morphology of samples.

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A mechanism was proposed to explain the outstanding photocatalytic hydrogen evolution of the CdS/MoS2(G) heterostructures, as illustrated in Fig. 8. Base on the previous study that the band gap of MoS2 can be increased to approximately 1.8 eV along with the decreasing layer-number of MoS2[28]. Therefore, the CdS/MoS2(G) catalysts with reduced layer number have a large driving force for the HER compared to the CdS/MoS2(W) catalysts[42]. The schematic illustration of photoinduced electron–hole pairs separation of CdS/MoS2(G) is presented in Fig. 8a. Under visible light illumination, the electrons of both CdS and MoS2 are excited from their valence bands (VB) to their conduction bands (CB). Due to the lower CB energy level of MoS2 than that of CdS, the photogenerated electrons are immediately transferred from CdS to MoS2 through the intimate interfacial contacts. Then, the accumulated electrons on the MoS2 nanosheets react with H2O to generate H2. Thermodynamically, the holes accumulate in the VB of CdS/MoS2(G) are trapped by the sacrificial agent. In addition to the above, the nearly vertical structural advantages of the CdS/MoS2(G) heterostructures also contribute to the excellent production H2 rate compared to CdS/MoS2(W) sample. First, the surface energy of the MoS2 edges is reported to be 100 times larger than the basal plane surface energy (250 mJ m-2), it is commonly believed that the MoS2 nanosheets would like to grow along its basal plane in the formation of stacked layer structure to minimize surface energy[30]. However, the basal plane surface energy could be lower by adsorbing the glucose, thus the MoS2 nanosheets can nearly vertical stand on the CdS nanowires. Such nearly vertical configuration leads to a high exposure of the active edge sites, thus provides more

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catalytic active sites contributing for H2 production. Second, previous studies have shown that the resistance of electron transport in such lay-down configuration of MoS2 flakes is ~2200 times higher than that of the electron moving in the same layer[30, 32]. Therefore, the photoexcited electrons on CdS transfer to MoS2 along the direction as shown in Fig. 8b would be quickly, thereby facilitating the transfer and separation of photoexcited electron−hole pairs and prolong the lifetime of charge carriers, as a result in the photocatalytic H2 evolution activity of CdS/MoS2(G) being higher than that of CdS/MoS2(W). Third, the more conductive of MoS2 edges than its basal plane is another critical factors for the excellent photocatalytic performance, resulting from better electronic contact and an optimized the electron transport pathway on the CdS/MoS2(G)[29]. The stability test of 10 wt% CdS/MoS2(G) heterostructures was also evaluated in the five reaction cycles for 30 h under visible light irradiation. The data in Fig. 9 shows that the hybrid photocatalyst still retained its original photocatalytic activity without noticeable degradation in the repeated run, demonstrating its exceptional photocatalytic stability. Notably, the good stability of the CdS/MoS2(G) also indicated that the introduction of MoS2 having an additional function of inhibiting CdS photocorrosion. This result was consisted with previous reports, which can be explained by the holes transfer from CdS to MoS2 resulting from the matched energy bands of the CdS/MoS2 heterostructures, thus reducing the photocorrosion of CdS[16, 23]. 4. Conclusions

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In conclusion, we have controllable synthesized 1D CdS nanowire/2D MoS2 nanosheet nearly vertical heterostructures via a simple glucose-assisted hydrothermal reaction. The CdS/MoS2(G) catalysts showed high hydrogen production rated up to 9.73 mmol h−1 g−1 and high apparent quantum efficiency of 60.3% at 420 nm for the sample with 10% MoS2 loading. It is believed that the unique nearly vertical structure not only maximally exposes the edge active sites for effective HER, but also regulates the photogenerated electrons transport along MoS2 basal plane after separating holes and electrons from CdS, which improve the electrons transfer rate and thus suppress the holes and electrons recombination. Besides, the optimized CdS/MoS2(G) catalysts exhibited excellent photostability after a continuous test for 30 h. We hope these findings can shed light on the rational design of highly efficient and stable noblemetal-free catalysts with specified structures for photocatalytic H2 evolution. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51608175, 51672077 and 51378187), the Key Project of Science and Technology of the Education Department of Henan Province (17B610004, 2011A150006), the Doctor Foundation of Henan Institute of Engineering (D2015020). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/ j.cej.XXXX.XX.XX References [1] M.A. Lukowski, A.S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin, Enhanced

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Figure captions Scheme 1. Schematic illustration of the growth processes of CdS/MoS2 heterostructures under different conditions (a) without glucose and (b) with glucose. Fig. 1. SEM images of (a) CdS nanowires, (b) MoS2 nanosheets synthesized without glucose and CdS nanowires, (c) CdS/MoS2(W) composites prepared without the addition of glucose and (d) CdS/MoS2(G) composites prepared with the addition of glucose. Fig. 2. (a) and (b) TEM images show the nearly vertical structure of CdS/MoS2(G) composites, (c) and (d) HRTEM images of CdS/MoS2(G) composites, (e) and (f) amplified HRTEM images of designated square part in (d) and (c), respectively, and (g) EDS elemental mappings of CdS/MoS2(G) composites. Fig. 3. XRD patterns of (a) pure CdS, (b) pure MoS2, and (c) CdS/MoS2(G) composites. Fig. 4. XPS spectra of the samples: (a) XPS full survey spectrum of CdS/MoS2(G) composites, (b) Cd 3d spectrum, (c) Mo 3d spectrum, and (d) S 2p spectrum. Fig. 5. UV−vis spectra of MoS2, CdS, CdS/MoS2(W) and CdS/MoS2(G) samples. Fig. 6. (a) Average photocatalytic H2 evolution rate over CdS/MoS2(G) loaded with different amount of MoS2; (b) comparison of H2 evolution rate on CdS, CdS/MoS2(G)10%, and CdS/MoS2(W)10%, MoS2 nanosheets, and the physical mixture of 10 wt% MoS2 and 90 wt% CdS. All above measurements were performed under visible-light irradiation (λ ≥ 420 nm) in the mixed aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 as sacrificial agents.

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Fig. 7. (a) PL spectra of CdS, CdS/MoS2(G)10%, and CdS/MoS2(W)10%; (b) photocurrent density–time curves of CdS, CdS/MoS2(G)10%, and CdS/MoS2(W)10% recorded at zero bias potential under time-dependent light irradiation(λ ≥ 420 nm). Fig. 8. (a) Schematic illustration of the energy band structure and photogenerated electrons

transport

of

CdS/MoS2(G)

samples；(b)

Schematic

picture

of

photogenerated electrons transport along MoS2 basal plane on CdS/MoS2(G) samples. Fig. 9. Cycling test of photocatalytic H2 evolution for CdS/MoS2(G) samples.

26

Scheme 1. Schematic illustration of the growth processes of CdS/MoS2 heterostructures under different conditions (a) without glucose and (b) with glucose.

27

(a)

(b)

(c)

(d)

Fig. 1. SEM images of (a) CdS nanowires, (b) MoS2 nanosheets synthesized without glucose and CdS nanowires, (c) CdS/MoS2(W) composites prepared without the addition of glucose and (d) CdS/MoS2(G) composites prepared with the addition of glucose.

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Fig. 2. (a) and (b) TEM images show the nearly vertical structure of CdS/MoS2(G) composites, (c) and (d) HRTEM images of CdS/MoS2(G) composites, (e) and (f) amplified HRTEM images of designated square part in (d) and (c), respectively, and (g) EDS elemental mappings of CdS/MoS2(G) composites.

29

Intensity (a.u.)

a (002)

b c

10

20

30

40

50

2 theta (deg.)

60

70

80

Fig. 3. XRD patterns of (a) pure CdS, (b) pure MoS2, and (c) CdS/MoS2(G) composites.

30

Cd

600 400 200 Binding energy (e V)

0

234 231 228 Binding energy (eV)

Intensity (a.u)

Intensity (a.u)

S 2s

237

412 408 404 Binding energy (eV)

(d)

Mo 3d 3/2

240

416

Mo 3d 5/2

(c)

Cd 3d3/2

Intensity (a.u)

Mo S C

800

Cd 3d5/2

(b)

Intensity (a.u)

(a)

400

S 2p3/2 S 2p1/2

166 165 164 163 162 161 160 159 158 Binding energy (eV)

225

Fig. 4. XPS spectra of the samples: (a) XPS full survey spectrum of CdS/MoS2(G) composites, (b) Cd 3d spectrum, (c) Mo 3d spectrum, and (d) S 2p spectrum.

31

1.6

CdS MoS2 CdS/MoS2 (W)

Absorbance (a.u.)

1.4 1.2

CdS/MoS2 (G)

1.0 0.8 0.6 0.4 0.2 400

500 600 700 Wavelength (nm)

800

Fig. 5. UV−vis spectra of MoS2, CdS, CdS/MoS2(W) and CdS/MoS2(G) samples.

32

Rate of H2 evolution (mmol g-1h-1)

Rate of H2 evolution (mmol g-1h-1)

10

10 (b)

(a)

8 6 4 2 0 0

2 4 6 8 10 12 14 Amount of MoS2 loaded on CdS

16

MoS2/CdS(G)

8 MoS2/CdS(W)

6 4 2

MoS2 + CdS CdS

MoS2

0 photocatalyst

Fig. 6. (a) Average photocatalytic H2 evolution rate over CdS/MoS2(G) loaded with different amount of MoS2; (b) comparison of H2 evolution rate on CdS, CdS/MoS2(G)10%, and CdS/MoS2(W)10%, MoS2 nanosheets, and the physical mixture of 10 wt% MoS2 and 90 wt% CdS. All above measurements were performed under visible-light irradiation (λ ≥ 420 nm) in the mixed aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 as sacrificial agents.

33

CdS CdS/MoS2(W) CdS/MoS2(G)

Photocurrent (uA/cm2)

Intensity (a.u.)

(a)

35 30 25 20 15 10 5 0

500

550 600 Wavelength (nm)

CdS CdS/MoS2(W) CdS/MoS2(G)

(b)

0

50

100

150 200 Time (s)

250

300

Fig. 7. (a) PL spectra of CdS, CdS/MoS2(G)10%, and CdS/MoS2(W)10%; (b) photocurrent density–time curves of CdS, CdS/MoS2(G)10%, and CdS/MoS2(W)10% recorded at zero bias potential under time-dependent light irradiation(λ ≥ 420 nm).

34

Fig. 8. (a) Schematic illustration of the energy band structure and photogenerated electrons

transport

of

CdS/MoS2(G)

samples；(b)

Schematic

picture

of

photogenerated electrons transport along MoS2 basal plane on CdS/MoS2(G) samples.

35

Amount of H2 evolution (mmolg-1)

70 60 50 40 30 20 10 0 0

5

10

15 20 Time (h)

25

30

Fig. 9. Cycling test of photocatalytic H2 evolution for CdS/MoS2(G) samples.

36

Graphical abstract

CdS/MoS2

37


1D/2D nearly vertical CdS/MoS2 heterostructures was prepared
Glucose plays a curial role on the formation of vertically heterostructures.
High HER rate and apparent quantum efficiency
A probable photocatalytic mechanism over the 1D/2D nearly vertical structure is proposed

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