Smart hybridization of Au coupled CdS nanorods with few layered

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Smart hybridization of Au coupled CdS nanorods with few layered MoS nanosheets for high performance photocatalytic hydrogen evolution reaction 2

Rama Krishna Chava, Jeong Yeon Do, and Misook Kang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00249 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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ACS Sustainable Chemistry & Engineering

Smart hybridization of Au coupled CdS nanorods with few layered MoS2 nanosheets for high performance photocatalytic hydrogen evolution reaction

Rama Krishna Chava*, Jeong Yeon Do, Misook Kang**

Department of Chemistry, College of Natural Science, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk-38541, Republic of Korea.

Corresponding Author’s Address: Dr. Rama Krishna Chava (Email: [email protected], [email protected]) Prof. Misook Kang (Email: [email protected]) Department of Chemistry, College of Natural Science, Yeungnam University, 280 DaehakRo, Gyeongsan, Gyeongbuk-38541, Republic of Korea.

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Abstract Fabrications of core-shell heteronanostructures with excellent optical properties and light induced charge separation effects have recently emerged as promising materials for solar to hydrogen conversion. Here, one dimensional CdS-Au/MoS2 hierarchical core/shell heteronanostructures (CSHNSs) have been successfully synthesized by a facile two-step hydrothermal method. Such heteronanostructures exhibit high efficiency and excellent stability towards hydrogen production. The introduction of Au NPs on to CdS nanorods not only making a Schottky junction with strong plasmonic absorption enhancement but also directed the growth of few layered MoS2 nanosheets as a hierarchical protective shell around the CdS nanorods. These CdS-Au/MoS2 hybrid structures possess a large number of edge sites in MoS2 layers which are active sites for hydrogen evolution reaction. As a result, the CdS-Au/MoS2 CSHNSs exhibit outstanding hydrogen evolution performance which is 7 times that of pure CdS nanorods. Also these CdS-Au/MoS2 CSHNSs showed greater stability after a long-time test (16 h), and 90 % of catalytic activity still remained. The enhanced hydrogen evolution activity of CdS-Au/MoS2 CSHNSs was attributed to the improved visible light absorption and the formation of heterojunctions between CdS, Au and MoS2 components, which increases the charge separation efficiency and thereby suppressing the electron-hole recombination.

Keywords: CdS-Au/MoS2, core/shell heteronanostructures, 2D MoS2, photocatalytic H2 evolution.


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Introduction The ever-growing global pollution and the continuous consumption of fossil fuels coupled with rapid depletion of natural resources, has boosted a great interest in procuring the renewable energy sources alternative to fossil fuels.1-4 Thus, to promise a long term and sustainable development of human society, there is an urgent need for the development of environmentally friendly and renewable technologies for green energy production and environmental remediation. Recently, photocatalytic water splitting by using semiconductor materials has long been considered as a promising approach to design a clean, secure, portable, and renewable energy source of hydrogen.5-8 Even though prominent progress in photocatalysis has been accomplished in recent years, the photoconversion efficiency during the photocatalytic reactions is still low and is far from the practical applications due to the quick electron-hole recombination and poor light absorption ability of semiconductors.2,9-12 Thus, tremendous endeavors have been devoted for the development of advanced visible photocatalytic systems with broad spectral absorption and low recombination rates.13-16 Among the various developed semiconductor photocatalysts, cadmium sulfides (CdS) have been demonstrated to be a suitable and highly promising candidate for visible photocatalytic hydrogen (H2) evolution due to its direct and narrow bandgap of 2.4 eV at room temperature. Moreover, the band position of CdS semiconductor perfectly fulfills the thermodynamic requirements of photocatalytic reactions such as water splitting.1,17 Furthermore, the conduction band (CB) edge position of CdS is more negative than that of TiO2, SrTiO3, and ZnO semiconductors,18 demonstrating that the photogenerated electrons from CdS shows stronger reduction power in the photocatalytic reactions. Therefore, CdS has been broadly studied in various photocatalytic fields. Among the different types of CdS nanostructured materials, one dimensional (1D) CdS with high surface-to-volume ratio and excellent electron transport property, are of special



interest in visible photocatalytic hydrogen production reactions.19 Besides this, 1D nanomaterial can benefit the absorption, diffusion, and charge separation steps of photocatalytic systems since the distinctive properties of 1D materials are at these length scales.20-22 Also the diameter and spacing of 1D material could be adjusted in scale as the wavelength of visible light, and light trapping effects are very strong and promote absorption within the semiconductor.22 However, the inherent properties of these 1D CdS materials still limit the efficiency in photocatalytic water splitting reactions and restricts the further realworld applications. This is because the CdS particles generally exhibit small adsorption ability toward reactants, poor photostability, and severe aggregation with each other, which could decrease their surface area and escalate the recombination rate of photogenerated electron-hole pairs, thus leading to a decreased photoactivity.23-25 Recently two dimensional (2D) layered materials have been demonstrated as co-catalysts for the visible photocatalytic systems because they show distinctive physical and chemical properties originating from excellent semiconducting properties and unique thickness.26 Moreover, 2D ultrathin materials with single- or few layer thickness are believed to be ideal catalysts that owns surprising characteristics such as higher surface area, abundant active sites and active edges, greater conductivity, anti-photocorrosion and chemical stability.27,28 Among the 2D layered materials, molybdenum sulfide (MoS2) emerged as an effective cocatalyst and photocatalytic H2 evolution activity of several semiconductors also significantly enhanced by loading MoS2 cocatalyst. MoS2 consists of Mo atoms sandwiched between the two layers of hexagonally close packed sulfur atoms which can be exfoliated to single- or few layer nanosheets.29-31 More recently, several efforts have been made to design the 1D CdS-MoS2 based heteronanostructures for the H2 evolution reactions. The obtained results indicate that the formed CdS-MoS2 heteronanostructures belongs to type-1 junction in which the photoexcited electrons from CdS were transferred to MoS2 sheets which can


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increase the life time of electrons and thereby suppresses the recombination resulting the higher photocatalytic H2 evolution activity.32-37 However, MoS2 nanosheets used in the these reports are having large lateral sizes or multi-layered structures which can greatly restricted the H2 evolution reaction activity due to the low exposure of active edge sites. Also most of these researchers made CdS-MoS2 heteronanostructures by physical mixing of irregular aggregates or stacked multilayers of MoS2 nanosheets on to CdS nanorods (NRs). Hence, it is highly desirable and challenging task to fabricate a core-shell type CdSMoS2 heteronanostructures with few layered MoS2 nanosheets having a large number of active sites and with much enhanced performance. An effective way to improve the performance of 1D CdS photocatalysts is to design the hierarchical heteronanostructures that have exceptional properties including the enhancement of visible light absorption and efficient separation of photogenerated carriers.38 In specific, heterostructured semiconductors with special spatially ordered hierarchical architectures such as 3D branched and core–shell structures have recently been of great interest in H2 evolution reactions due to their unique structural and physicochemical properties.22,39,40 On the other hand, these heterostructured semiconductors with various heterojunctions can enhance the photocurrent and subsequently the artificial photosynthesis efficiency by improving the light absorption capability and charge separation. In addition, the formation of heterojunctions can also improve the chemical stability of the proposed heterostructured semiconductors.41-43 Based on the above facts, herein, we report a novel and effective approach to fabricate of CdS-MoS2 core-shell hierarchical heteronanostructures (CSHNSs) through gold (Au) nanoparticles (NPs) in which Au NPs functionalized CdS NRs are coated with few layered MoS2 nanosheets (CdS-Au/MoS2). In these unique CSHNSs, CdS NRs acts as a core and the deposited Au NPs directs the growth of MoS2 into few layered sheets resulting the hierarchical CSHNSs. The obtained CSHNSs consist of 3 dimensional (3D) hierarchical



configuration with each other which synergistically promote the rapid electron transfer. Compared to the traditional hybrids or heterostructures, the 1D CSHNSs gives rise to increased light absorption and continuity for charge transfer. More importantly, the obtained CSHNS ensures the transfer of photogenerated electrons from the CdS NR not only to the Au metal surface but also to the grown MoS2 nanosheets, promoting the electron-hole separation effectively. Finally, the charge separation efficiency in this unique CdS-Au/MoS2 CSHNSs leads to the improvement in the photocatalytic hydrogen evolution performance.

Experimental section Synthesis of CdS NRs: The CdS nanorods were prepared by a solvothermal method. The pre-calculated amounts of cadmium nitrate and thiourea (CH4N2S) were dissolved in 50 ml of ethane diamine and then transferred to a 100 ml Teflon lined stainless steel autoclave, and maintained at 180oC for 24 hours. After cooling down to room temperature, the obtained precipitate was washed and centrifuged several times with DI water and ethanol respectively for three times. Finally, the obtained product CdS nanorods were dried in air at 70oC for further use. Synthesis of Au nanoparticles: The Au nanoparticles with a particle diameter of 15-20 nm were prepared with the well-known tri sodium citrate reduction method.44,45 Typically, a HAuCl4 solution (100 ml, 100 mM) was heated under stirring until boiling. Next, 5 ml of solution containing 34 mM of Na3Cit was added with rapid stirring. The solution slowly changed to red colour and then resulting solution was kept at the boiling temperature for 15 minutes with constant stirring, and then allowed to cool. The obtained Au NP sol was used directly without further purification. Finally the concentration of Au colloid is calculated as 0.18762 mg/ml. Fabrication of CdS-Au/MoS2 CSHNSs: The CdS-Au/MoS2 CSHNSs were prepared by a hydrothermal method. In a typical procedure, a 20 mg of CdS NRs were dispersed in 50 ml


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DI water by sonication and then stirring. Under constant stirring, 5 ml Au NPs (Au content: 0.94 mg) colloidal solution was injected into the above suspension and kept stirring for 15 minutes. Later a 20 ml solution containing Na2MoO4.5H2O (2 mM) and L-cystein (4 mM) was added and stirred again for 10 minutes. Then the mixture was transferred to a 100 ml Teflon lined stainless steel autoclave and kept in an oven at 200oC for 24 hours. Finally the resultant product was centrifuged, washed with DI water and ethanol and dried at 70oC for overnight. Physical characterization methods: The surface morphology of as synthesized samples was studied by field emission scanning electron microscope (FESEM, HITACHI, S-4800) and transmission electron microscope (TEM, HITACHI, H-7600) images. The high resolution TEM (HR-TEM) images, lattice spacings, selected area electron diffraction patterns (SAED), STEM-HAADF images were taken from FEI company made Titan G2 FE-TEM device at an operating voltage of 200 kV. The high resolution STEM is equipped with HAADF detector and EDAX energy dispersive X-ray to analyze the composition details of the material. Powder X-ray diffraction patterns of all samples were recorded in the range 10-80o using on Panalytical Xpert Pro diffractometer with Cu-Kα radiation, λ=1.5060 Ǻ operated at 40 kV and 30 mA. The surface elemental composition and chemical states of CdS and CdSAu/MoS2 samples were examined using a Thermo Scientific X-ray photoelectron spectroscopy (Al-Kα radiation). The optical absorption properties of the prepared samples were analyzed by Scinco Neosys-2000 UV-VIS diffuse reflectance spectrophotometer. Photoluminescence (PL) emission spectra of CdS and CdS-Au/MoS2 samples were measured with Scinco spectrofluorometer by using the 350 nm line of xenon lamp as excitation source at room temperature. Visible photocatalytic hydrogen evolution studies: The photocatalytic H2 evolution activities of the prepared photocatalysts were studied under simulated sunlight irradiation.



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The photocatalytic H2 evolution reactions were carried out in a 100 ml round bottomed flask closed with a rubber septum. In the present studies, Xenon arc lamp having a UV-cutoff filter (>420 nm) was used as visible-light source. For the photocatalytic hydrogen production experiment, 10 mg of as synthesized H2 evolving photocatalyst powder was suspended in 50 ml aqueous solution containing 10% lactic acid as sacrificial reagent. Before the light irradiation, argon gas was purged into the catalyst solution to remove the dissolved gases and to make the anaerobic conditions in the reaction system. During the reaction, photocatalytic reactor was tightly closed to avoid the gas exchange. The amount of released H2 gas was analyzed with a gas chromatograph (Master GC-2010 Model) equipped with a thermal conductivity detector. Depending on the amounts of hydrogen gas produced during the photocatalytic reaction in an average of one hour, and the AQY was calculated as follow:

ηAQY =

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠

= 2X

𝑋 100%

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑣𝑜𝑙𝑣𝑒𝑑 ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝑔𝑎𝑠 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠

------------ (1) 𝑋 100%

------------ (2)

Transient photocurrent density and electrochemical impedance spectroscopy (EIS) measurements: Photoelectrochemical analysis was performed by a transient photocurrent response curves in a solution containing 0.1 M Na2SO4 by using a three-electrode configuration composed of synthesized samples as working electrode, a Pt wire as counter electrode, and a saturated Ag/AgCl as reference electrode. The working electrode was prepared on fluorine doped tin oxide (FTO) glass substrate which was cleaned by ultrasonication for 30 minutes in DI water and ethanol respectively. After that 10 mg of sample (CdS NRs, CdS-Au/MoS2) was dispersed in 0.5 ml of DI water and ethanol mixture and sonicated for 30 minutes to get slurry. Then the slurry was coated onto FTO glass substrate with a working area of 1 cm2 as working electrode and then dried at 100oC for 2 hours. A solar simulator equipped with a 150 W Xe lamp was used as the light source and the


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photocurrent responses were measured with an external 0.5 V. Net, the electrochemical impedance spectra (EIS) were measured using a IVIUM Technologies workstation with the same three-electrode setup that was used for the transient photocurrent measurements, with a 0.1 M Na2SO4 aqueous solution as the electrolyte. The measurements were carried out in a frequency range of 0.1–105 Hz. A bias voltage of 10mV was utilized for the test.

Results and Discussions Formation mechanism of CdS-Au/MoS2 CSHNSs The synthesis method and the formation mechanism of CdS-Au/MoS2 CSHNSs are given in Scheme 1. In our work, CdS-Au/MoS2 CSHNSs were designed by a two-step hydrothermal process. In first step, high aspect ratios of CdS NRs are synthesized by hydrothermal method using cadmium nitrate and thiourea in the presence of ethane diamine. Subsequently after loading the Au NPs on the surface of CdS NRs, a solution containing the sodium molybdate and L-cysteine was added drop by drop and stirred for few minutes. The resultant mixture was undergone hydrothermal reaction and the formed CdS-Au/MoS2 CSHNSs were washed and then dried. From the controlled experiments it is observed that the loaded Au NPs played a main role for forming and anchoring of few layered MoS2 nanosheets on the surface of CdS NRs. As shown in Figure S1, hierarchical MoS2 nanospheres of size 100-200 nm are formed on CdS NRs when there is no loading of Au NPs. However after loading Au NPs, a clean and few layered MoS2 nanosheets are formed in the reaction and then anchored on the surface of CdS NRs resulting a complete CdS-Au/MoS2 CSHNSs. From this observation we may conclude that the L-cysteine molecules are initially attached to the Au NPs surface effectively (rather than CdS NRs) via the S-Au bonds due to the strong affinity between S and Au atoms.46 During the hydrothermal reaction, the cys-Mo complexes decompose to form MoS2 on Au NPs surface and thereby CdS-Au/MoS2 CSHNSs are successfully achieved.



Scheme 1. Schematic illustration for the fabrication of CdS-Au/MoS2 CSHNSs.

Morphological studies of CdS-Au/MoS2 CSHNSs To understand the changes of samples in each stage we studied the surface morphology of as-prepared CdS NRs, and then CdS-Au@MoS2 CSHNSs by FESEM, TEM and HRTEM images. As shown in Figure 1(a-c), the FESEM images of prepared CdS sample consist of nanorods having a length of 2-3 micrometers. The surfaces of CdS NRs are found to be clean and smooth. After loading the Au NPs, the morphology of CdS-Au NRs was shown in Figure 1(d-f) with different magnifications. As seen from the Figure 1(d-f), small and bright Au nanoparticles are anchored on the surface of CdS NRs. The morphology of CdS NRs is not affected after loading the Au NPs and resulting sample contains CdS-Au NR hybrids. In the next step, the surface morphology of the synthesized CdS-Au/MoS2 CSHNSs was also investigated. As shown in Figure 1(g-i), the significant differences were detected between the initial CdS-Au NRs and the obtained heteronanostructures. After making heterostructures with MoS2, the average size of NRs was increased and it can be clearly seen that the few layered nanosheets with abundant folded edges and active sites on the surface of CdS-Au NRs appeared. Moreover, the high magnified image showed that layered MoS2 nanosheets as a shell around the CdS-Au NRs was formed. From Figure 1(i), we can also observe that there


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Figure 1. FESEM images of as prepared (a-c) CdS NRs, (d-f) CdS-Au nanohybrids, and (g-i) CdS-Au/MoS2 CSHNSs at different magnifications. are CdS NRs associated with Au NPs behind the MoS2 nanosheets. In addition to this, EDAX spectrum (Figure S2) of CdS-Au/MoS2 CSHNSs also recorded and confirms that the prepared sample contains only Cd, S, Au and Mo elements. For comparison, MoS2 nanostructures also prepared by hydrothermal method and FESEM images are given Figure S3. The prepared MoS2 revealed that the sample contains hierarchical spheres of size around 500 nm. To further confirm the formation of CdS-Au/MoS2 CSHNSs, TEM images of as synthesized samples are taken and displayed in Figure 2. As shown in Figure 2(a-c), the obtained CdS sample consists of 1-D NRs having a diameter of 35-40 nm and lengths of about 2-3 micrometers. Figure 2(d-f) represents the morphology of Au NPs loaded CdS NRs and from these images we conclude that Au NPs having a size of 15-20 nm are successfully



Figure 2. TEM images of as prepared samples, (a-c) CdS NRs, (d-f) CdS-Au nanohybrids, and (g-i) CdS-Au/MoS2 CSHNSs. anchored on the surface of CdS NRs with a close contact. Next, Figure 2(g-i) show the typical TEM images of final product, CdS-Au/MoS2 CSHNSs where the few layered MoS2 nanosheets are fully surrounded to the surface of CdS-Au NRs. The formed MoS2 nanosheets are very thin comprises of very few layers. However, the Au NPs cannot be noticed visibly from the TEM observations due to the lower contrast compared with the MoS2 layer. Moreover, the overall 1D structure is well retained with the growth of the MoS2 nanosheets,


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and the average diameter of the formed CdS-Au/MoS2 CSHNSs is about 300 nm (Figure 2i). TEM images clearly show the formation of the CdS-Au/MoS2 CSHNSs. The heterojunctions and sharp interfaces formed between CdS, Au and MoS2 are further studied deeply by HRTEM and STEM-EDS mappings and are displayed in Figure 3. From the Figure 3a, it is observed that the three components are successfully visualized and also each component has a direct contact with the other two components. During the reaction, the produced thin MoS2 nanosheets are uniformly coated on the CdS-Au nanorods. These MoS2 nanosheets are interconnected with each other and forming the 3D nanosheets networks around the CdS-Au nanorods. We believe such a 3D hierarchical structure consists of 1D CdS-Au nanorod and shell of 2D MoS2 nanosheets could favor the charge transfer and suppresses the photoelectron-hole recombination, leading to the enhanced photocatalytic hydrogen production. Figure 3b illustrates the HR-TEM image of CdS-Au/MoS2 CSHNSs, where the lattice separations of CdS, Au and MoS2 are clearly noticed. The (002) plane of hexagonal CdS with a lattice spacing of 0.33 nm is observed in CdS-Au/MoS2 CSHNSs, and the lattice fringes with a d-spacing of 0.23 nm can be assigned to the (111) lattice plane of cubic Au NP. Figure 3b also depicts the edge view of some interconnected MoS2 nanosheets (having less than 7 layers) with noticeable lattice fringes. The measured interplanar distance is about 0.61 nm, which corresponds to the (002) plane of hexagonal MoS2. As shown in Figure 3c, the selected area electron diffraction (SAED) patterns of CdS-Au/MoS2 CSHNSs exhibited the diffraction spots related to (002), (103) and (105) planes of few layered MoS2 nanosheets revealing that MoS2 nanosheets show a certain degree of crystal features, consistent with the analysis of HRTEM. Moreover, it is noticed that CdS, Au, and MoS2 are in close contact with each other which assists the transfer of photogenerated charged carriers among the three components. The presence of Cd, S, Au and Mo elements was also studied by STEM-EDS elemental mapping and the details are given in Figure 3(d-k). From the Figure 3(d-i), it can



Figure 3. High-resolution TEM images (a,b), SAED patterns (c), STEM-EDS elemental mapping details (d-i), and STEM-EDS line scanning profiles of CdS-Au/MoS2 CSHNSs (j,k).


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be seen that Cd, S, Au and Mo elements are coexistent in CdS-Au/MoS2 CSHNSs. Figure 3(e-i) shows the individual elemental mapping profiles of Cd, S, Au and Mo elements. All the elements are well distributed throughout the selected STEM image area (Figure 3d) and the distributions of these elements were fairly uniform. In addition to this, STEM-EDS lines scanning profiles also recorded and are presented in Figure 3(j,k). These STEM-EDS line scanning profiles reveal that the observed peak intensities belong to Cd, S, Au and Mo elements only. Finally, Figure 3(d-k) reveal that the core CdS NR is consists of only Cd and S elements surrounded by Au NPs and the hydrothermal grown MoS2 shell consists of only Mo and S elements. Figure S4 displays the HRTEM elemental composition details in a nanoscale and the projected CdS-Au/MoS2 CSHNSs consists of Cd, S, Au and Mo elements only and are homogeneously distributed in the heterostructures. For clarity, we have also recorded the TEM images of as prepared Au NPs (given in Figure S5) and reveals that the formed Au NPs are having a size of around 25 nm. Structural studies of CdS-Au/MoS2 CSHNSs The crystalline phases and purity of CdS NRs and CdS-Au/MoS2 CSHNSs were analyzed by powder XRD patterns and are given in Figure 4. The diffraction peaks of pure CdS NRs are well matched to the standard peaks of hexagonal phase (a=b= 0.414 nm, c=0.672 nm, JCPDS No: 00-001-0783). After Au functionalization and then making CSHNSs with MoS2, in addition to the peaks of CdS NRs, the diffraction planes of both Au and MoS 2 also observed and are displayed in Figure 4. The characteristic peaks observed at 38.2o, 64.7o, and 77.6 o (* marked) ascribed to the (111), (220) and (311) crystal planes of cubic Au (a=b=c=0.40 nm, JCPDS No: 00-001-1172) respectively. The other reflections observed at 33.1o assigned to (110) crystal plane of hexagonal MoS2 (a=b=0.31 nm, c=1.23 nm, JCPDS No: 00-006-0097). Outwardly, the introduction of Au and MoS2 does not affect the crystal phase and crystallinity of CdS NRs. As seen from Figure 4, in the XRD pattern of CdS-Au/MoS2



CSHNSs, the peak at ~14.4o corresponding to the (002) plane of MoS2 was not appeared. Generally, this diffraction peak is corresponding to the c-plane of MoS2 component and can be useful to analyze the structure of MoS2. In the case of MoS2 structure, molybdenum atoms coordinated with sulfur atoms to form the S-Mo-S sandwich layer. Hence the absence of diffraction peak at ~14.4o reveals that the formed MoS2 nanosheets on the CdS NRs may contain only few layers which are too thin to be detected by XRD.30,47,48 Therefore, the CdSAu NR hybrids might inhibit the formation of MoS2 along the c-axis during the hydrothermal process. Moreover, the formation of three-component CdS-Au/MoS2 system is confirmed by XRD results. Also, the proposed hydrothermal strategy clearly demonstrates the formation of CdS-Au/MoS2 CSHNSs and the results are highly in accordance with the analysis of the FESEM, TEM and HRTEM measurements.

Figure 4. Powder X-ray diffraction patterns of bare CdS NRs and CdS-Au/MoS2 CSHNSs.


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Next, the X-ray photoelectron spectroscopy (XPS) was used to examine the elemental composition and chemical states in the prepared CdS and CdS-Au/MoS2 CSHNSs. The full surface scan XPS spectrum also validates that the prepared photocatalyst is composed of Cd, S, Au and Mo elements. The calculated atomic percentages of Cd, S, Au and Mo elements are 21.35%, 62.46%, 0.46% and 15.74% respectively. Figure 5 shows the high resolution core level spectra of Cd, S, Mo and Au elements. As seen from Figure 5(a,b), the binding energies of Cd-3d5/2, Cd-3d3/2 and S-2p3/2, S-2p1/2 in the pure CdS NRs are observed at 404.70 eV, 411.45 eV and 161.13 eV, 162.29 eV respectively suggesting that Cd element is in +2 state and S is in S2- state.30,37,49 After making heterostructures with MoS2, the binding energies of Cd-3d5/2, Cd-3d3/2 and S-2p3/2, S-2p1/2 energy levels are shifted to 405.42 eV, 412.19 eV and 161.88 eV and 163.07 eV, respectively. These results specify that the transfer

Figure 5. High resolution core-level XPS profiles of (a) Cd-3d, (b) S-2p, (c) Mo-3d and (d) Au-4f elements.



of electrons from CdS to MoS2 happened after making heterostructures, which agrees well with the fact that the electronegativity of Mo is higher than that of Cd (in Pauling scale, 2.16 and 1.69 for Mo and Cd, respectively). Consequently, the formation of heterojunctions, Cd– S–Mo bonds was expected at the interfaces and this remarkable higher binding energy shift could be ascribed to the heterostructure effect between the CdS and MoS2 layered nanosheets.27 The high resolution XPS spectrum of Mo-3d was displayed in Figure 5c, and showed a strong doublet at 229.08 eV and 232.21 eV which are the characteristic binding energies of Mo4+ -3d5/2 and Mo4+ -3d3/2 in MoS2. From Figure 5d, the two binding energy peaks observed at 84.00 eV and 87.72 eV are assigned to Au-4f7/2 and Au-4f5/2energy levels since the Au NPs were loaded on the surface of CdS NRs. It is also observed from Figure 5d, the difference between two binding energies is about 3.7 eV which is characteristic feature of Au metal (Auo) NPs.50-52 From the XPS data and by collaborating with SEM and TEM images, the proposed CdS-Au/MoS2 CSHNSs were successfully designed with strong interaction between different components which can be beneficial for the charge transfer and separation. Optical properties of CdS NRs and CdS-Au/MoS2 CSHNSs To determine the optical properties of as synthesized samples, UV-VIS absorption spectra of CdS, MoS2 and CdS-Au/MoS2 CSHNSs are recorded and are given in Figure 6. As shown in Figure 6a, it can be seen that the CdS NRs shows the significant absorption at 500 nm corresponding to the bandgap of 2.47 eV. The extended absorbance of CdS NRs at the longer wavelength originates from the light scattering effect of the several micrometer sized nanorod structure. The absorption spectrum of MoS2 also displayed in Figure 6a, which shows an absorption band at 695 nm which is an equivalent to the bandgap of 1.78 eV. The absorption of Au NPs is around 520 nm (given in Figure S6), which is referred to the surface plasmon resonance (SPR) band of the Au NPs. In contrast, after making CdS-


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Au/MoS2 CSHNSs, the absorption bands became wider and covered the entire visible region. For CdS-Au/MoS2 CSHNSs, the two peaks are located at 500 nm and 650 nm leading to the increased absorption in the visible light region. When compared to the absorption band at 695 nm of pure MoS2 spheres, the blue shifted (680 nm) absorption band from the MoS2 nanosheets of CdS-Au/MoS2 CSHNSs arises from the strong quantum confinement effect of the nanosheets, which assists the few layered MoS2 nanosheets as an active visible-light photocatalyst.30,53,54 From the Figure 6a, it is also observed that the SPR peak of Au NPs is overlapped with the absorption band of CdS NRs, became a wider and extended up to the region of ~600 nm. Photoluminescence (PL) spectroscopy has been widely used to determine the electronhole separation efficiency which has a direct influence on the overall performance of semiconductor photocatalysts.55-59 Figure 6b shows the PL emission spectra of CdS NRs and CdS-Au/MoS2 CSHNSs measured under the same conditions. An emission band around 525 nm is observed for CdS NRs, which can be assigned to a band–band transition, and the energy of the light is approximately equal to the band-gap energy of CdS. This emission band is related to a sulfur vacancy emission, which can be ascribed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valance band of CdS.60,61 The observed emission peak intensity is very high and represents the high recombination rate of photogenerated charge carriers for CdS NRs. After loading Au NPs and growing few layered MoS2 nanosheets on the surface of CdS NRs, then CdS-Au/MoS2 CSHNSs exhibited very low emission intensity when compared to the CdS NRs. Usually, the PL emission arises from the recombination of charged carriers; the lower emission intensity means the less charge recombination. Compared with CdS NRs, the PL intensity of CdS-Au/MoS2 CSHNSs was much weaker, indicating their low recombination rate of photogenerated electron−hole pairs. These findings indicate that the heterojunction formed between CdS-Au and MoS2 nanosheets could completely decrease the



Figure 6. (a) Optical absorption spectra and (b) Photoluminescence spectra of CdS NRs, and CdS-Au/MoS2 CSHNSs. recombination rate of photogenerated electron−hole pairs and in turn lead to a high photocatalytic H2 evolution performance which will be discussed in the next section. Visible photocatalytic H2 evolution studies of CdS-Au/MoS2 CSHNSs The visible photocatalytic H2 evolution reaction (HER) activities of the prepared samples were measured and displayed in Figure 7a. All the samples were checked under the identical conditions with 10 vol% lactic acid aqueous solution as the sacrificial reagent (hole


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scavenger) under the visible light irradiation (λ> 420 nm). In order to check the effect of Au content on the H2 evolution activity, besides 5 ml Au (0.94 mg) containing CdS-Au/MoS2 CSHNSs, we have also prepared another two samples containing 3 ml (Au content: 0.562 mg) and 10 ml (Au content: 1.88 mg) Au contents. Each photocatalyst sample was irradiated for four hours and the evolved H2 gas values were recorded. As shown in Figure 7a, the CdS5Au/MoS2 CSHNSs (5 ml Au sample) show a maximum H2 evolution of 350.7 μmol, which is higher than that of pure CdS NRs (75.4 μmol), pure MoS2 (15.3 μmol) and CdS-3Au/MoS2 (290.65 μmol), CdS-10Au/MoS2 (310.52 μmol). These results show that the catalytic activity of pure CdS nanorods alone is low, which is possibly due to the rapid charge recombination.33,62,63 Similarly, a very low H2 was noticed when MoS2 alone was used as a photocatalyst, suggesting its very poor photocatalytic activity for HER. However, after few layered MoS2 nanosheets were coated onto CdS-Au nanohybrids, H2 evolution was remarkably improved representing that MoS2 nanosheets works very well as a cocatalyst for H2 generation. In addition to this, it is also noticed that 5 ml Au content was optimized for CdS-Au/MoS2 CSHNSs and hence further studies are carried out by choosing the CdS5Au/MoS2 CSHNSs sample. Further, Figure 7b shows the H2 evolution rates of the photocatalyst samples in which obviously CdS-5Au/MoS2 CSHNSs showed the highest H2 evolution rate (7.01 mmol.g-1.h-1) which is roughly seven times higher than that of pure CdS NRs and other samples. The enhanced photocatalytic H2 evolution rate was attributed to the positive effect arised from the heterostructures. Next, the photostability of CdS-5Au/MoS2 CSHNSs for H2 evolution was also studied by four successive operations and each cycle comprises of four hours light irradiation under the same conditions. As seen from Figure 7c, the proposed heteronanostructured photocatalyst does not show an obvious decrease (only a slight) of hydrogen evolution rate after 12 h of visible light irradiation, demonstrating that the CdS-Au/MoS2 photocatalyst has adequate stability for the photocatalytic H2 production. The



apparent quantum yield (AQY) of the CdS-5Au/MoS2 photocatalyst sample was calculated as 27.85%. Table 1 lists the H2 evolution activity of CdS-MoS2 based nanocomposites; it is obvious that our sample produced 7 mmol.g-1.h-1 of H2 which is comparable to the other reported data. Mostly, the effective charge separation is a crucial step during the photocatalytic process. To check the electron transfer in the prepared samples, transient photocurrent measurements were carried out for several on-off cycles under visible light irradiation. Figure 7d shows the current density-time curves of pure CdS NWs and CdS5Au/MoS2 CSHNSs electrodes with light irradiation at wavelengths longer than 420 nm. Both samples displayed the good reproducibility of the photocurrent. As expected CdSAu@MoS2 CSHNSs showed enhanced current density values of ~32.67 μA.cm-2 which is five times higher than that of pure CdS NRs (~6.84 μA.cm-2) demonstrating that the introduction of MoS2 NSs on to CdS-Au hybrids and the heterostructure formed between CdS-Au hybrids and MoS2 NSs considerably suppresses the recombination of photogenerated charge carriers which leads to the highest photocurrent response and then H2 evolution. In addition to efficient photocurrent generation, a fast response is essential for designing the devices and hence measured the rise and decay times during the light on and off cycles. Transient photocurrent measurements show that CdS-Au/MoS2 CSHNSs taken rise and decay times of 0.35 and 0.21 sec respectively and whereas the bare CdS NRs shown 0.40 and 0.42 sec. Based on the above results, it reveals that the photoinduced charge carriers in CSHNSs are separated efficiently than pure CdS NRs. Electrochemical impedance spectroscopy (EIS) studies: To further understand the charge separation and recombination process, electrochemical impendence spectral (EIS) analysis was utilized. Figure 8 shows the EIS-Nyquist plots of CdS NRs, and CdS-Au/MoS2 CSHNSs respectively. As displayed in Figure 8 through comparing the EIS plots of both photocatalysts, it was clearly observed that the semicircle of CdS-Au/MoS2 CSHNSs is much smaller than


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the CdS NRs, indicating the effective charge separation and transfer of photogenerated charged carriers. The smaller diameter of the semicircle for CSHNSs in comparison to CdS NRs indicates the smaller charge transfer resistance in CSHNSs. Therefore, it was confirmed that making of heterostructures with few layered MoS2 nanosheets could effectively improve the charge transfer and separation of CdS NRs and thus achieving the enhanced H2 generation.

Figure 7. (a) Amount of H2 gas produced during the visible light irradiation of 4 hours, and (b) H2 gas evolution rates of CdS NRs, MoS2 nanospheres and CdS-Au/MoS2 CSHNSs with different Au content; (c) Cycling experiments for CdS-5Au/MoS2 CSHNSs, each cycle of 4 hours; and (d) Transient photocurrent density curves for CdS NRs and CdS-5Au/MoS2 CSHNSs.



Page 24 of 38

---------------------------------------------------------------------------------------------------------------Composite

Co-catalyst

Evolved H2

AQY (%)

Ref.

---------------------------------------------------------------------------------------------------------------CdS-Graphene

Grpahene

1.12 mmol.h-1

22.5

23

CdS-MoS2

MoS2

1472 mmol.h-1.g-1

-

25

CdS-WS2

WS2

1984 mmol.h-1.g-1

-

TiO2-MoS2

MoS2

16.7 mmol.h-1.g-1

-

29

TiO2-MoS2

MoS2

1.6 mmol.h-1.g-1

-

30

CuInS2-MoS2

MoS2

316 μmolh-1g-1

-

31

CdS-MoS2

MoS2

551.3 μmol.h-1.g-1

31.8

32

CdS-MoS2

MoS2

49.80 mmol.h-1.g-1

41.37

33

CdS-MoS2

MoS2

1914 mmol.h-1

46.9

34

CdS/WS2-MoS2 WS2-MoS2

209.79 mmol.h-1.g-1

51.4%

35

CdS-MoS2

MoS2

493.1 μmol h−1

28.5%

36

CdS-MoS2

MoS2

26.14 mmol.h-1.g-1

-

37

CdS-WS2

WS2

1222 μmol.h-1.g-1

28.9

49

CdS-Au-ZnO

Au, ZnO

60.8 μmol h−1

-

50

CdS-Au

Au

601.2 mmol.h-1.g-1

-

65

CdS-Au/MoS2

Au/MoS2

7 mmol.g-1.h-1

27.85

Present work

----------------------------------------------------------------------------------------------------------------

Table 1. Comparison of CdS-MoS2 based heteronanostructures for visible light photocatalytic hydrogen evolution reaction.


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Figure 8. EIS-Nyquist plots for CdS NRs, and CdS-Au/MoS2 CSHNSs

Mechanism for enhanced H2 evolution activity of CdS-Au/MoS2 CSHNSs According to the previous reports on CdS-MoS2 based heteronanostructures, under the visible light irradiation, the valance band electrons of CdS NRs are excited to the conduction band by creating holes in the valance band. These photogenerated charged carriers can recombine very quickly, resulting the low photocatalytic activity of pure CdS NRs.63 After making heteronanostructures with MoS2 nanosheets on the surface of CdS NRs, the MoS2 component can suppress the electron–hole recombination and thereby shows the enhanced H2 evolution activity.33,34 However in the case of CdS-Au/MoS2 CSHNSs, the enhanced H2 production is attributed to the effective electron-hole separation which is promoted by the interaction of Au, CdS and MoS2 under visible light irradiation. When CdS NR is irradiated with visible light, the photoinduced electrons in the conduction band of CdS NRs can move easily to the metallic Au NPs via the Schottky barrier, since the Fermi level of CdS NRs is greater than that of Au NPs.64,65 More specifically, CdS NRs with anchored Au NPs, and



these loaded Au NPs can serve as an efficient electron transmission medium for photoinduced electrons. In addition, the LSPR effect of Au NPs can increase the absorption of incident photons, which strengthens the generation rate of photoinduced charge carriers and assurances the efficient electron transfer. Meanwhile, the CdS-Au heterostructure was then assumed as a whole with SPR effect enhanced H2 generation reaction catalysis, by coupling with few-layered MoS2 nanosheets. With the visible light irradiation, the light absorption by CdS-Au is greatly increased due to the LSPR effect (from Figure S6a), and consequently the SPR facilitated electron injection from Au metal to the MoS2 nanosheets is improved and thus enhanced the H2 evolution performance. The possible mechanism for the enhanced H2 production over CdS-Au@MoS2 CSHNSs under visible light irradiation is presented in Scheme 2. By depositing the Au NPs on the surface of CdS NRs hardly changes the bandgap, but considerably enhances the visible light absorption ability.65 The bandgap values of CdS and MoS2 are 2.4 and 1.8 respectively, and the heterojunction formed between CdS-Au and MoS2 is a Type-1 junction.33,34 When the Au NPs are in conjunction with the CdS semiconductor, electrons from the valence band are excited to the conduction band of CdS under the visible light irradiation and then directly transferred to the Au metal NP via the Schottky barrier formed between CdS and Au NP. The Au NPs existing between CdS and MoS2 increase the light absorption because of the SPR effect and could act as functional hot spots resulting in the efficient photogeneration and spatial separation of electrons and holes. These Au NPs serve as electron relays from CdS to MoS2. During the visible light excitation, the electrons produced in the CdS transfer into the conduction band of the MoS2 nanosheets through the Au NPs driven by the built-in potential in the heterojunction facilitating charge transfer and separation. This electron transfer phenomenon is faster than the recombination of charged carriers, thus a huge amount of stored electrons in Au NPs immediately transfer to the MoS2 nanosheets and contribute to the enhanced hydrogen evolution reaction. MoS2


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nanosheets not only can act as an electron sink to retard the recombination of electron–hole pairs, but also offers abundant catalytic active sites for H2 evolution. Consequently, the proposed CdS-Au/MoS2 CSHNSs system can suppress the electron-hole recombination and prolong the lifetime of the electron-hole pairs leading to the superior photocatalytic activity; meanwhile the holes can be consumed by the sacrificial agents. Since the compound lactic acid in the reaction mixture served as sacrificial electron donor and the consumption of photogenerated holes leads to the oxidation of lactic acid which in turn produces the pyruvic acid.66,67 These results clearly validate the rational and ingenuity of CdS-Au/MoS2 CSHNSs design from CdS-Au to ternary CdS-Au/MoS2 CSHNSs in charge separation and transfer for photochemical conversion applications.

Scheme 2. Schematic illustration for the charge separation and enhanced H2 evolution activity of CdS-Au/MoS2 CSHNSs.

Conclusions In summary, we have successfully demonstrated a facile hydrothermal strategy to synthesize the CdS-Au/MoS2 core/shell heteronanostructures with abundant active sites of few layered MoS2 nanosheets and broad visible light absorption via Au NPs. The well designed heteronanostructures significantly reinforces the 3D contact between the each component of



CdS, Au and MoS2 which is highly beneficial for the charge separation and transfer of photogenerated electron-hole pairs. The obtained CdS-Au/MoS2 CSHNSs showed highest visible photocatalytic hydrogen evolution activity of 7 mmol.g-1.h-1 which is seven fold enhancement of photocatalytic activities of individual CdS and MoS2 components. The excellent performance of the CdS-Au/MoS2 CSHNSs is attributed to the following: the unique structure increases the charge carrier extraction efficiency and broadens the light absorption, along with the matched energy band of CdS-Au/MoS2 heterostructure promoting effective charge separation and thereby suppresses the photoelectron-hole recombination between CdS and MoS2. The enhanced photocatalytic hydrogen evolution verifies the apparent enhancement of charge-separation efficiency in the ternary CdS-Au/MoS2 CSHNSs. We anticipate that the development of this unique nanoarchitecture designed by the combination of plasmonic metal supported 1D semiconductor coated by few layered 2D nanosheets with potential applications of high performance and solar energy conversion photocatalytic devices.

Supporting Information TEM image of CdS-MoS2 heterostructures, SEM-EDX elemental mapping profiles of CdSAu/MoS2 CSHNSs, FESEM images of MoS2 nanospheres, HR-TEM elemental composition details of CdS-Au/MoS2 CSHNSs, TEM image of Au NPs, Optical absorption spectrum of Au NPs.

Notes The authors declare no competing financial interest.


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Acknowledgments This work was supported by the Basic Research Science and Technology Projects through the National Research Foundation (NRF) of Korea grant funded by the Korea government (MSIP) (No: 2018R1A2B6004746), for which authors are very grateful.

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Table of Contents Graphic Synopsis: One dimensional CdS-Au/MoS2 hierarchical core/shell heteronanostructures were successfully fabricated for high performance photocatalytic H2 evolution reaction.


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Smart hybridization of Au coupled CdS nanorods with few layered

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