CdS Nanosheets-on-Nanorod Heterostructure for Highly

May 30, 2016 - Quantum dots as enhancer in photocatalytic hydrogen evolution: A review. Debasmita Kandi , Satyabadi Martha , K.M. Parida. Internationa...
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MoS2/CdS Nanosheets-on-Nanorod Heterostructure for Highly Efficient Photocatalytic H2 Generation under Visible Light Irradiation Xing-Liang Yin,†,‡ Lei-Lei Li,§ Wen-Jie Jiang,†,‡ Yun Zhang,† Xiang Zhang,† Li-Jun Wan,†,‡ and Jin-Song Hu*,†,‡ †

ACS Appl. Mater. Interfaces 2016.8:15258-15266. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/05/19. For personal use only.

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Science, 2 North first Street, Zhongguancun, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § MOE Key Laboratory of Cluster Science, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *

ABSTRACT: Semiconductor-based photocatalytic H2 generation as a direct approach of converting solar energy to fuel is attractive for tackling the global energy and environmental issues but still suffers from low efficiency. Here, we report a MoS2/CdS nanohybrid as a noble-metal-free efficient visible-light driven photocatalyst, which has the unique nanosheets-on-nanorod heterostructure with partially crystalline MoS2 nanosheets intimately but discretely growing on single-crystalline CdS nanorod. This heterostructure not only facilitates the charge separation and transfer owing to the formed heterojunction, shorter radial transfer path, and fewer defects in single-crystalline nanorod, thus effectively reducing the charge recombination, but also provides plenty of active sites for hydrogen evolution reaction due to partially crystalline structure of MoS2 as well as enough room for hole extraction. As a result, the MoS2/CdS nanosheets-on-nanorod exhibits a state-of-the-art H2 evolution rate of 49.80 mmol g−1 h−1 and an apparent quantum yield of 41.37% at 420 nm, which is the advanced performance among all MoS2/CdS composites and CdS/noble metal photocatalysts. These findings will open opportunities for developing low-cost efficient photocatalysts for water splitting. KEYWORDS: molybdenum sulfide, cadmium sulfide, nanostructures, solar conversion, HER

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INTRODUCTION Semiconductor-based photocatalytic conversion of solar energy to fuels, such as hydrogen from water, is of growing interest for tackling the global energy and environmental crisis. Over the past decades, tremendous semiconductor materials have been explored as photocatalysts for hydrogen evolution reaction (HER).1,2 But until now, the low photocatalytic activity ascribed to the narrow solar spectral response and high-rate recombination of photoexcited charges in these photocatalysts is still holding back their practical application. Developing the new materials with wide spectral photoresponse and reduced charge recombination is pivotal for the commercialization of photocatalytic H2 generation on a large scale. CdS is a narrow band gap semiconductor with an Eg of 2.4 eV, which enables it to absorb light up to 520 nm. The potential of its conduction band (CB)-edge is more negative than the reduction potential of H+/H2, making it proper for the H2 generation.3−6 However, the photocatalytic activity of CdS itself toward water splitting is still very low because of high-rate charge recombination,7 which can be generally divided into two types: bulk recombination and surface recombination. The efforts to decrease bulk recombination are mainly focused on fabricating nanostructured CdS, such as nanorods,8 nanoparticles,9 nanoflowers,10 nanowires,11 etc., to shorten the © 2016 American Chemical Society

transportation distance of carriers and reduce the impurities and defects in CdS crystals. Several pathways have been investigated to reduce surface recombination, including modifying surface defects,12 constructing heterojunction,13,14 or loading cocatalysts.3,15−17 Among them, combining with noble metals such as Pt, Au as cocatalysts was proved to be effective,15,16 although its further application is limited by the high cost and scarcity of noble metals. Much attention is therefore now being paid on exploiting inexpensive and earthabundant alternative cocatalysts including metals3 or semiconductor compounds.18−20 It is interesting to note that MoS2 and WS2 have demonstrated a higher cocatalytic activity than Pt at the same loading.20−22 Molybdenum disulfide (MoS2), with sandwich structure of three stacked atomic layers (S−Mo−S) linked by van der Waals forces, has been used in supercapacitors, electrocatalysis, photocatalysis, and energy storage.23−29 Density functional theory calculations revealed that the free energy of atomic hydrogen bonding to the MoS2 is close to zero, which is comparable with that of Pt,30 and makes it a promising cheap Received: March 3, 2016 Accepted: May 30, 2016 Published: May 30, 2016 15258

DOI: 10.1021/acsami.6b02687 ACS Appl. Mater. Interfaces 2016, 8, 15258−15266

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facile hydrothermal process using sodium molybdate as molybdenum source and thiourea as sulfur source in deionized water. The preparation details were presented in Experimental Section. The loading of MoS2 can be easily adjusted by changing the amount of its sources. MoS2/CdS nanohybrid with 10 wt % MoS2 (vs CdS) demonstrated the best performance (as discussed later) and was thoroughly characterized. The scanning electron microscopy (SEM) image (Figure S1a) and transmission electron microscopy (TEM) (Figure 2a) image show that the product after the first solvothermal process is composed of nanorods in a diameter of 20−30 nm and a length of hundreds of nanometers. The continuous lattice fringes (Figure 2b) in high-resolution TEM (HRTEM) image and the corresponding fast Fourier transform (FFT) pattern in dot array reveal the single-crystalline nature of these nanorods. The distance of lattice fringes of 0.67 and 0.36 nm matches well with the d-spacing of (001) and (100) crystallographic planes of hexagonal CdS, respectively, indicating it is a CdS nanorod grown along the [0001] direction. Figure 2c and Figure S1b present typical TEM and SEM images of the product after the second hydrothermal process, respectively. It was observed that some sheetlike structures grew on the end of CdS nanorods as marked by red arrows. HRTEM image (Figure 2d and S1c) shows the shortrange continuous lattice fringes on these nanosheets. The distance of 0.27 nm can be indexed to the d-spacing of crystallographic (101) planes of rhombohedral MoS2 (Figure 2e). The zoom-out TEM image shows (Figure S1c) that these lattice fringes are not continuous in long-range, and some amorphous features are also found on the nanosheet, indicating that it is partially crystalline MoS2 nanosheet. The close look at the interface of the nanosheet and the nanorod shows that the continuous lattice fringes of MoS2 directly connected to the continuous lattice fringes with a distance of 0.36 nm (corresponding to (100) planes of hexagonal CdS), implying a good contact (Figure 2e). TEM observation further found that most part of CdS nanorod surface was discontinuously covered by few-layer nanosheets while some part was not (Figure 2f,g). The distance between these layers is 0.61 nm, which is well-consistent with the d-spacing of (003) planes of rhombohedral MoS2, suggesting these nanosheets are also MoS2. After a large number of nanorods were checked under TEM, this happened on every CdS nanorod surface. Furthermore, the energy dispersive X-ray spectroscopy (EDS) mapping (Figure 2h) analysis of a nanorod displays the homogeneous distribution of elemental S and Cd, as well as the relatively weak signal of Mo matching with the sheetlike structure on the surface of CdS nanorod. The chemical states of these elements were further investigated by X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum (Figure 3a) shows the existence of Cd, Mo, S, and O. The high-resolution XPS spectrum in Figure 3b shows two peaks at 412.0 and 405.1 eV corresponding to the characteristic binding energies of Cd2+ 3d3/2 and Cd2+ 3d5/2 in CdS, respectively. Compared with Cd signal at 411.5 and 404.7 eV in pure CdS nanorods, this shift toward higher energy indicates the possible interaction between MoS2 nanosheets and CdS nanorods. XPS spectrum in Figure 3c displays a typical strong doublet at 231.4 and 228.2 eV, which is in good consistency with the binding energies of Mo4+ 3d3/2 and Mo4+ 3d5/2 in MoS2. The peak at 225.9 eV matches well with the binding energy of S 2s in sulfides.43 Together with SEM and TEM observations, these results corroborate that the nanorods are MoS 2 /CdS

and abundant cocatalyst. In addition, both experimental and computational results disclosed that the edges of MoS2 layers were the active sites for HER, but their basal planes were catalytically inert.30,31 Increasing the amount of edge sites will benefit the H2 generation for MoS2. Recently, progress has been made on the development of CdS/MoS2 composites via various approaches such as impregnation,20 ball-milling,32 ultrasound,33 and irradiation,34 which exhibited the enhanced photocatalytic activities in varying degrees compared with CdS, although the activities still need further improvement.35−42 Therefore, it is still highly desirable to develop a simple, mild, cost-effective method to produce new nanostructured MoS2/ CdS composites with much enhanced performance for photocatalytic H2 generation. Photocatalytic H2 generation involves three crucial steps: solar light harvesting, charge separation and transportation, and catalytic H2 evolution reaction on the surface. Here, we proposed and fabricated a new MoS2/CdS nanohybrid through a facile two-step solvothermal approach with cheap raw materials and no addition of any stabilizer. The nanohybrid has a heterostructure of few-layer partially crystalline MoS2 nanosheets discontinuously growing on CdS single-crystalline nanorods. The single-crystalline nanorod benefits both electron and hole transfer due to a shorter radial transfer path and fewer defects for less charge recombination. MoS2 nanosheet acts as a cocatalyst to accept the photogenerated electrons from CdS conduction band to more efficiently catalyze HER. The intimate plane-to-surface contact of MoS2 nanosheets on CdS nanorod facilitates this electron transfer, and thus effectively reduces the surface charge recombination. The partially crystalline structure of MoS2 nanosheets supplies a larger amount of exposed catalytically active sites for HER. The discontinuous coverage of MoS2 nanosheets on CdS nanorod guarantees the effective transfer of holes from CdS to the scavenger. Taking these facts together, the proposed MoS2/ CdS nanosheets-on-nanorod heterostructure exhibits a muchenhanced H2 generation efficiency with a H2 evolution rate of 49.80 mmol g−1 h−1 and an apparent quantum yield of 41.37% at 420 nm, which, to the best of our knowledge, is the highest performance among all MoS2/CdS composites and most of CdS−noble metal photocatalysts in previous reports.6,16,22,33

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RESULTS AND DISCUSSION

As illustrated in Figure 1, the MoS2/CdS nanohybrid was synthesized by using a two-step solvothermal approach. The CdS nanorods were first synthesized through the reaction of thiourea and cadmium nitrate in ethylenediamine. MoS2 nanosheets were then in situ grown onto CdS nanorods via a

Figure 1. Schematic illustration of the fabrication of MoS2/CdS nanosheets-on-nanorods. 15259

DOI: 10.1021/acsami.6b02687 ACS Appl. Mater. Interfaces 2016, 8, 15258−15266

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Figure 2. (a) TEM and (b) HRTEM images of CdS nanorods. (c) TEM, (d−g) HRTEM images, and (h) STEM image and EDS elemental mapping of MoS2/CdS nanosheets-on-nanorod.

during sample preparation in air, which was commonly observed in the previous reports37,44−47and evidenced by the S−O bonding in S 2p XPS spectrum (Figure S3). The crystalline structure of MoS2/CdS nanohybrid with varying loadings of MoS2 from 0 to 60 wt % (vs CdS) were further investigated by X-ray power diffraction (XRD). As shown in Figure 4, all XRD peaks of the sample with no MoS2 (black curve) can be well-indexed to hexagonal CdS (JCPDS No. 65−3414). Comparing with pure CdS, the XRD pattern of the nanohybrid with 10 wt % MoS2 surprisingly shows no obvious diffraction from MoS2, although its features clearly appeared in TEM and SEM images as mentioned above. This

Figure 3. (a) XPS survey spectrum of MoS2/CdS nanosheets-onnanorod heterostructure. High-resolution XPS spectra of (b) Cd 3d in pure CdS nanorods and MoS2/CdS (10 wt %) and (c) Mo 3d in MoS2/CdS (10 wt %).

composite. The trace amount of O (2.6 atom %) is also observed in general scan. The high-resolution XPS signal of O 1s (Figure S2) can be well-deconvoluted into two peaks located at 533.5 and 531.6 eV, corresponding to the surface-adsorbed oxygen species and the oxysulfide due to surface oxidation

Figure 4. XRD patterns of CdS and MoS2/CdS nanohybrids with varying MoS2 loading (vs CdS). 15260

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that the catalytic activity of CdS nanorods alone is very low (0.35 mmol g−1 h−1) probably due to the rapid charge recombination.4 It can also be seen that almost no H2 was detected when MoS2 alone was used as a photocatalyst, suggesting its very poor photocatalytic activity for HER. However, once MoS2 nanosheets were introduced onto CdS nanorods, the HER activity was significantly enhanced even the loading is as low as 0.2 wt %, indicating MoS2 acts very well as a cocatalyst of CdS for HER. The data in Figure 5a also show that the activity of MoS2/CdS catalyst is sensitive to the loading of MoS2. The best performance was achieved at a 10 wt % loading of MoS2 (vs CdS). The highest H2 evolution rate reached 49.80 mmol g−1 h−1, far exceeding 0.35 mmol g−1 h−1 of CdS nanorods alone by a factor of 130.26. The apparent quantum efficiency is 41.37% at 420 nm, which was calculated using the formula listed in Experimental Section. These data, to our best of knowledge, are the highest values among the reported catalysts with MoS2 as a cocatalyst (see Table S1). The high H2 evolution rate was further visualized by a video taken during HER (see Supporting Information), from which it can be clearly seen that a great many bubbles were generated rapidly upon visible-light irradiation in a quartz cell filled with MoS2/CdS composite (10 wt %). To further understand the role of MoS2 nanosheets on the improvement of charge separation efficiency in MoS2/CdS, transient photo-electrochemical measurements were performed on MoS2/CdS (10 wt %) and pure CdS under identical experimental conditions. As shown in Figure 5b, both catalysts show obvious photocurrents with good reproducibility, as expected when they are illuminated by visible light. However, the photocurrent on MoS2/CdS (10 wt %) is ∼12 times higher than that on pure CdS, indicating the introduction of MoS2 nanosheets and the heterostructure formed between MoS2 and CdS significantly suppress the recombination of photoexcited electrons and holes, which should contribute to the enhanced photocatalytic activity on MoS2/CdS. Furthermore, the stability test (Figure 5c) shows that after four cycles and 24 h of tests, the MoS2/ CdS (10 wt %) still retains its original photocatalytic activity in terms of unchanged H2 evolution rate, suggesting its good stability. However, the further increase of MoS2 loading results in the decrease of H2 generation rate. For example, when MoS2 loading reaches to 40 wt %, the average H2 evolution rate dramatically decreases to 5.62 mmol g−1 h−1. The reason can be ascribed to the “shielding effect”48,49 that the excessive MoS2 (Figure S4) will partially block the light absorption of CdS and the hole scavengers. Besides, the excessive MoS2 increases charge transfer resistance and hampers the electron injection for HER, thus deteriorating the charge recombination. This can be well-evidenced by fluorescence analysis, which is an effective measure sensitive to the charge recombination. Figure 5d presents the fluorescence spectra of pure CdS and MoS2/CdS with different MoS2 loadings, recorded under the excitation at 365 nm based on excitation spectra (Figure S8). The weak and broad fluorescence peak appearing at ∼470 and 520 nm correspond to the excitonic and trap-state recombination, respectively.3 Pure CdS exhibits the strongest emission, indicating the highest recombination of photoexcited charges. The emissions from MoS2/CdS hybrids decrease as the increase of MoS2 loading up to 10 wt %. Since the fluorescence originates from the charge recombination, the weaker fluorescence means the less charge recombination. Below optimal loading, the photocatalytically active sites increase with

can be ascribed to the low loading and partial crystalline nature of MoS2 nanosheets on the surfaces of CdS nanorods, as indicated by TEM results. As the MoS2 loading increased to more than 20 wt %, the additional XRD peaks at 14.5°, 33.0°, 34.0°, 38.4°, 41.1°, 58.3°, and 60.5° gradually appeared. All these peaks can be designated to the diffraction of rhombohedral MoS2 (JCPDS No. 17−0744). The apparently broadened peaks implied that MoS2 in the nanohybrids were not well-crystallized. TEM observation confirms that the sheetlike structures in the composites increase as MoS2 loading goes up (Figure S4). The optical properties of MoS2/CdS hybrids were measured by the UV−vis−NIR optical spectroscopy. As shown in Figure S5, pure CdS has an absorption edge at ∼520 nm corresponding to its band gap of 2.4 eV. After the deposition of MoS2 on the surfaces of CdS nanorods, the absorption in vis−NIR region is apparently enhanced, owing to the narrower band gap of MoS2, as indicated by the band gap measurement and discussion in Figures S6 and S7. It is also implied by the color change from light yellow to greenish (inset in Figure S5). The photocatalytic performance for H2 generation of the prepared MoS2/CdS catalysts with a varying content of MoS2, together with pure CdS nanorods and MoS2 prepared in parallel for comparison, were evaluated under visible-light (λ ≥ 420 nm) illumination by using lactic acid (10 vol %) as an environmentally friendly scavenger to consume holes from the valence band (VB) of catalysts. Each catalyst was measured five times, and the average rate of consecutive 4 h of H2 generation was recorded each time to assess the HER photocatalytic performance of the catalysts mentioned in this article. The results plotted in Figure 5a show

Figure 5. (a) Average photocatalytic H2 evolution rate on CdS, MoS2, and MoS2/CdS photocatalysts with varying MoS2 loading. (b) Transient photocurrent responses on CdS and MoS2/CdS (10 wt %) electrodes, recorded at the potential of 0 V vs Ag/AgCl in 1.33 M of lactic acid solution. (c) Time courses of photocatalytic H2 evolution on MoS2/CdS(10 wt %). All above measurements were performed under visible-light irradiation (λ ≥ 420 nm). (d) Photoluminescence spectra of representative MoS2/CdS nanohybrids with various MoS2 loading. 15261

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Figure 6. (a) XRD patterns before (MoS2/CdS) and after (MoS2/CdS-a) annealing. (b, c) HRTEM images of pure MoS2 before (b) and after (c) annealing. The insets are the corresponding SAED patterns. (d) Average H2 generation rate on MoS2/CdS nanosheets-on-nanorods before (MoS2/ CdS) and after (MoS2/CdS-a) annealing, MoS2/a-CdS, MoS2/CdS nanospheres, and MoS2/CdS bulk powder. (e) HRTEM image of CdS nanosphere. (f) STEM image and EDS elemental mapping of S, Cd, and Mo of a single MoS2/CdS nanosphere.

MoS2/CdS-a are very similar (3.73 m2/g vs 3.67 m2/g, Figure S9), but the former delivered a 5.16 times larger H2 evolution rate (18.99 vs 3.68 mmol g−1 h−1). This result unambiguously confirmed that the partial crystallinity of MoS2 nanosheets contributed to the high photocatalytic performance of MoS2/ CdS. Moreover, the morphology of CdS also affects the catalytic activity of MoS2/CdS. CdS nanosphere and CdS bulk powder were selected to synthesize another two control MoS2/CdS catalysts by the same procedure as MoS2/CdS nanosheets-onnanorods (see Experimental Section for details). HRTEM image (Figure 6e) shows that the prepared CdS nanospheres were composed of spherical assemblies of ∼10 nm CdS nanocrystals. Compared with CdS bulk powder precursor, the nanostructured MoS2/CdS nanosheets-on-nanorods and nanospheres exhibit much better photocatalytic performance (Figure 6d). This can be safely attributed to nanosize effects: (1) the smaller size caused larger specific surface area and thus more catalytic sites; and (2) the nanometer-scale size significantly shortened carrier diffuse path and thus reduced the recombination of photogenerated electrons and holes in catalysts. Furthermore, although MoS2/CdS nanospheres (Figure 6f) have a larger BET surface area (66.66 m2/g vs 47.80 m2/g, Figure S10) due to smaller nanocrystal building blocks, MoS2/CdS nanosheets-on-nanorods exhibited much larger photocatalytic H2 evolution rate (49.80 vs 32.85 mmol g−1 h−1). This could be ascribed to the large number of nanocrystal boundaries in the nanospheres (as seen in Figure 6e) that served as carrier recombination centers and thus consumed photogenerated electrons for H2 evolution. The single-crystalline nanorod structure benefits both electron and hole transfer due to a shorter transfer path in radial direction as well as fewer defects for less charge recombination, resulting in the enhanced photocatalytic performance.

the increase of MoS2 loading, which facilitates more electrons to involve in reducing H+ into H2 and thus reduce the charge recombination. This is why the quenching of fluorescence enhanced as the increase of loading before it reaches the optimal loading. However, the emission increases again as the further increase of MoS2 loading, implying the increase of charge recombination. This result is consistent with the above loading-dependent photocatalytic measurements, corroborating the optimal MoS2 loading in our experiments is 10 wt %. It should be mentioned that some reports have demonstrated that the optimal loading amount of MoS2 is less than 2% in their system.20,22 The optimal loading of cocatalyst should be related to the synthetic methods, morphology and structure of the catalysts, as well as the interaction between cocatalyst and semiconductor. It was found that the crystallinity of MoS2 significantly influences the HER photocatalytic activity of MoS2/CdS. It is known that the high-temperature annealing will reduce the defects and improve the crystallinity. MoS2/CdS (10 wt % MoS2) was annealed at 700 °C for 6 h to obtain the sample MoS2/CdS-a. The XRD pattern (Figure 6a) shows that the typical diffraction of (003) planes of MoS2 appeared at 14.5° after annealing, indicating the improvement of its crystallinity. HRTEM images (Figure 6b,c) and the corresponding selected area electron diffraction (SAED) patterns of the pure MoS2 samples prepared before and after annealing clearly corroborated that the annealing process turned partially crystallized MoS2 into well-crystallized one with much less defects. HER experiments show that the H2 evolution rate of the annealed MoS2/CdS-a suffered a dramatic decline in comparison with unannealed one (3.68 vs 49.80 mmol g−1 h−1; Figure 6d). In consideration of the influence from the agglomeration due to annealing, another control catalyst (MoS2/a-CdS) was prepared by annealing CdS nanorods first and then coating MoS2 nanosheets. The BET surface area of MoS2/a-CdS and 15262

DOI: 10.1021/acsami.6b02687 ACS Appl. Mater. Interfaces 2016, 8, 15258−15266

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Simultaneously, the separated holes remaining at VB of CdS transferred to the exposed surface of CdS nanorods and scavenged by the lactic acid.

Further study indicated that the intimate contact between MoS2 and CdS is essential to deliver the high photocatalytic activity. As shown in Figure 7a, the physical mixture of MoS2

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CONCLUSION In summary, a facile two-step solvothermal approach was developed to produce MoS2/CdS nanohybrid with few-layer MoS2 intimately grown on the surfaces of CdS nanorods. The loading-dependent experiments showed that CdS nanosheetson-nanorods with 10 wt % MoS2 loading exhibited the highest H2 generation rate (49.80 mmol g−1 h−1) during photocatalytic HER under visible light, which ranks first in all reported catalysts using MoS2 as cocatalysts. The excellent performance can be attributed to its unique structure in which singlecrystalline nature of the CdS nanorods reduces the bulk recombination, and the partially crystalline MoS2 provides a large number of exposed edges with unsaturated active S atoms as activity sites for photocatalytic HER. Besides, the heterojunction formed between MoS2 and CdS accelerates the charge transportation and suppresses the charge recombination, thus allowing more electrons to become involved in H2 generation. These findings will open opportunities for developing low-cost efficient photocatalysts for water splitting.

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Figure 7. (a) Comparison of H2 evolution rates and (b) photoluminescence spectra of MoS2/CdS nanosheets-on-nanorods and physical mixture (MoS2, CdS). (c) Schematic illustration of charge transfer in MoS2/CdS nanosheets-on-nanorods. Under light irradiation, the electrons are excited upon the CB of CdS, while the holes remain at its VB. The excited electrons will subsequently transfer to the CB of MoS2 due to its lower position, which facilitates the charge separation and suppresses the electron−hole recombination. As a result, more excited electrons can involve in reducing H+ into H2, leading to the enhance H2 evolution rate. (d) Schematic illustration of photocatalytic HER process on MoS2/CdS nanosheets-on-nanorods.

EXPERIMENTAL SECTION

Preparations of CdS Nanorods, Nanospheres, and Bulk Powder. The CdS nanorods were prepared by using solvothermal method.50 In a typical procedure, 16.2 mmol Cd(NO3)2·4H2O and 48.6 mmol NH2CSNH2 were added into a 100 mL Teflon-lined, stainless steel autoclave filled with 80 mL of ethylenediamine. The autoclave was then sealed and heated in an oven at 160 °C for 24 h. After that, the precipitates were collected by centrifugation and rinsed with deionized water and ethanol. The bright yellow powder of CdS nanorods was obtained after drying in a vacuum oven at 60 °C. To fabricate CdS nanospheres, 5 mmol Cd(CH3COO)2 and 50 mmol NH2CSNH2 were dissolved in 100 mL of ethylene glycol. Then the solution was transferred into a 200 mL flask and heated in an oil bath at 160 °C for 6 h. CdS nanospheres were collected by centrifugation, followed by rinsing with deionized water and ethanol and drying in a vacuum oven at 60 °C. To synthesize CdS bulk powder, 25 mmol Na2S was added into a beaker with 40 mL of 0.125 M Cd(NO3)2 solution, and the yellow precipitates instantly formed. The resulting precipitates were filtered, washed with deionized water, and then dried in a vacuum oven at 60 °C. The obtained powder was called CdS bulk powder. Synthesis of MoS2/CdS Composites. MoS2/CdS nanosheets-onnanorods were synthesized in situ by hydrothermal process. Specially, 500 mg of as-obtained CdS nanorods were added into a flask filled with 30 mL of ultrapure water. A varying amount of Na2MoO4 was added into the solution under strong stirring. After 6 h, an appropriate amount of thiourea (NH2CSNH2) with a constant 1:5 molar ratio of Na2MoO4/NH2CSNH2 was added. The solution was further stirred for 1 h and transferred into a Teflon-lined autoclave, followed by heating at 220 °C for 24 h. The precipitate was collected and washed with ultrapure water and absolute ethanol. After they dried in a vacuum oven at 60 °C, MoS2/CdS nanosheets-on-nanorods were obtained. The MoS2 loading can be easily adjusted by changing the amount of Na2MoO4 and thiourea. A series of MoS2/CdS nanosheetson-nanorods with a theoretical MoS2 loading of 0.2, 1, 2, 4, 8, 12, 15, 20, and 40 wt % were prepared. Moreover, CdS nanospheres and CdS bulk powder were also used instead of CdS nanorods to synthesize MoS2/CdS nanospheres and bulk powder, respectively, by following exactly the same procedure mentioned above. Pure MoS2 was also synthesized using the same procedure as mentioned above except for no addition of any CdS support. Preparation of MoS2/CdS-a and MoS2/a-CdS. The photocatalyst MoS2/CdS-a was obtained by annealing MoS2/CdS nanorods

nanosheets and CdS nanorods (9.68 wt % of MoS2 vs CdS, which is also the real loading of MoS2 in MoS2/CdS (10 wt %) nanosheets-on-nanorods determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES)) exhibits very poor HER activity (0.53 mmol g−1 h−1), which is onehundredth of that of MoS2/CdS nanosheets-on-nanorods (49.80 mmol g−1 h−1). It is known that the CB bottom is −0.13 V (vs NHE)33 for few-layer MoS2, but −0.35 V for CdS. As illustrated in Figure 7c, the heterojunction will form at the interfaces of CdS nanorods and MoS2 nanosheets, since they were intimately contacted as evidenced by HRTEM images in Figure 2, which will facilitate the electron transportation from CdS to MoS2, and relieve the charge recombination. This fact can be corroborated by the fluorescence emission (Figure 7b). The physical mixture (MoS2, CdS) exhibited very strong photoluminescence emissions centered at 470 and 520 nm corresponding to the excitonic and trap-state recombination, respectively, while these photoluminescences were dramatically quenched on MoS2/CdS nanosheets-on-nanorods, indicating that the effective carrier separation occurred. On the basis of all above analyses, a tentative mechanism for the photocatalytic HER of MoS2/CdS nanosheets-on-nanorods was proposed in Figure 7d. Under light illumination, electron−hole pairs were generated in CdS nanorods. The electrons were extracted and migrated to the unsaturated Mo−S bond on the surfaces of partially crystalline MoS2 nanosheets intimately grown on CdS nanorods, and then reacted with H2O to generate H2. 15263

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Research Article

ACS Applied Materials & Interfaces Table 1. Theoretical and Actual Loading Amounts of MoS2 on CdS in All Samples theoretical (wt %) actual (wt %) theoretical (wt %) actual (wt %)

0.2 0.13 40 39.83

1 2 0.89 1.92 MoS2/CdS-a (10 wt %) 9.33

4 8 3.83 7.92 MoS2/a-CdS (10 wt %) 9.16

(10 wt %) at 700 °C for 6 h under H2−Ar gas mixture flow. In contrast, the pure CdS nanorods were first annealed at 700 °C for 6 h under H2−Ar gas mixture flow, and then MoS2 nanosheets were deposited on annealed CdS nanorods by using the same procedure as for MoS2/CdS nanosheets-on-nanorods. Note that all loadings in percentage in the manuscript represent the theoretical mass ratios of MoS2 to CdS given that the reactants were completely converted into the products. The actual loading amounts of MoS2 to CdS in all samples were measured using an inductively coupled plasma-atomic emission spectrometry (ICP-AES, ICPE-9000 Shimadzu) and listed in Table 1, which matched well with the theoretical values. Evaluation of Photocatalytic Activities. Photocatalytic H2 evolution experiments were performed in a Pyrex glass cell with a flat, round upside-window for an irradiation area of 38 cm2. A 300 W xenon arc lamp with a 420 nm cutoff filter (PLS-SEX 300, Beijing Trusttech Technology Co., Ltd.) was used to simulate the visible light source. The illumination intensity was adjusted to 100 mW cm−2. The H2 solar system (Beijing Trusttech Technology Co., Ltd.) with a gas chromatogram (GC) equipped with a thermal conductivity detector (TCD), TDX-01 column, and Ar carrier gas, was used to collect and online detect the evolved H2. Photocatalyst (0.2 g) was suspended in a glass cell with 200 mL of solution containing 20 mL of lactic acid as hole scavenger. The cell was kept at 5 °C by using a circulating water system. Before irradiation, the reaction system was pumped to vacuum. The H2 evolution rate was determined by GC. The apparent quantum yield (Ø) was estimated by the following equation: ⌀(%) =

np =

10 12 9.68 11.78 nanospheres (10 wt %) 9.73

15 20 14.69 19.91 powder (10 wt %) 8.37

window for external light incidence. Light on and off was controlled by a shutter installed on a stainless steel black box.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02687. SEM and TEM images, XPS spectra, UV−vis−NIR spectrum, analysis of band gap of MoS2, fluorescence excitation spectra, BET analysis, and photocatalytic performance comparison (PDF) Video (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial supports from the National Key Project on Basic Research of China (2015CB932302), the National Natural Science Foundation of China (21573249), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100).

2nH2 n e− × 100 = × 100 np np

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REFERENCES

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θ I×t×A = hν hν

where Ø is the apparent quantum yield,2,51 ne− is the number of reacted electrons, np is the number of incident photos, nH2 is the number of evolved H2 molecules, θ is the total energy (J) of incident photons, h is the Planck constant (J s−1), ν is the frequency of photo (Hz), I is the illumination intensity (W m−2) determined with a Ray virtual radiation actinometer, t is irradiation time (s), A is the irradiation area (m2). Characterization. Scanning electron microscopy (SEM) images were taken on a JSM-6701F (JEOL, Japan). Transmission electron microscopy (TEM) images were obtained on a JEM 2100F (JEOL, Japan) operated at 200 kV. X-ray powder diffraction (XRD) was performed on a Rigaku D/max-7000 using filtered Cu Kα irradiation. X-ray photoelectron spectroscopy (XPS) was obtained with a VG ESCALab220i-XL electron spectrometer using 300 W Mg Kα radiation. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. The UV−visible absorption spectra were recorded with a UV−visible spectrophotometer (UV2550, Shimadzu, Japan). Luminescence measurements were performed on a PerkinElmer LS55 fluorescence spectrometer. Brunauer− Emmett−Teller (BET) measurements were performed on a Micromeritics’s Tristar 3000. The Mo and Cd contents were measured using an inductively coupled plasma-atomic emission spectrometry (ICPAES, ICPE-9000 Shimadzu). Transient photocurrent responses were recorded on a CHI 760 E electrochemical system (Shanghai, china) using Ag/AgCl as reference electrode and Pt as counter electrode. The working electrode was prepared by coating catalyst ink in ethanol onto ITO/glass with a fixed area of 1.96 × 10−5 m2. The electrolyte is lactic acid solution (1.33 M), which was filled in a quartz cell with a side15264

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