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Synthesis of Ni9S8/MoS2 hetero-catalyst for Enhanced Hydrogen Evolution Reaction Adnan Khalil, Qin Liu, Zahir Muhammad, Muhammad Habib, Rashid Khan, Qun He, Qi Fang, Hafiz Tariq Masood, Zia ur Rehman, Ting Xiang, Chuan Qiang Wu, and Li Song Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017

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Synthesis of Ni9S8/MoS2 hetero-catalyst for Enhanced Hydrogen Evolution Reaction

Adnan Khalil,a Qin Liu,a Muhammad Zahir,a Muhammad Habib,a Rashid Khan,a Qun He,a Qi Fang,a Hafiz Tariq Masood,b Zia ur Rehman,a Ting Xiang,a ChuanQiang Wua and Li Song*a

a

National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui, 230029, P.R. China.

b

Hefei National Laboratory for Physical Science at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

Abstract: We demonstrate a hetero-structure Ni9S8/MoS2 hybrid with tight interface synthesized via an improved hydrothermal method. As compared to pure MoS2, the increased surface area and the shorten charge transport pathway in the layered hybrid significantly promote the photocatalytic efficiency for hydrogen evolution reaction (HER). In particularly, the optimized Ni9S8/MoS2 hybrid with 20 wt% Ni9S8 exhibits the highest photocatalytic activity with HER value of 406 µmolg-1h-1, which is enhanced by 70% compared to that of pure MoS2 nanosheets (285.0 µmolg-1h-1). Moreover, the value is 4 times more than the commercial MoS2 (92.0 µmolg1 -1

h ), indicating the high potential of the hybrid in the catalytic fields.

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Keywords: In-situ growth; Hetero-Structure; Co-Catalyst; Hydrothermal; H2 Evolution.

1. Introduction: As human need increases, it is necessary to discover more renewable sources of clean energy such as solar energy, which is evergreen source of energy.1-2 Thus, it is beneficial to bring solar energy into its useful form according to the demand of modern era. Recently photocatalytic water splitting appeared to be one of the finest technique used for the large-scale and sustainable hydrogen evolution and oxygen evolution.3-4 In modern scientific world the society has been attracted towards semiconductor-based photocatalysis for the functions of solar energy conversion and environmental refining.5 The photocatalytic system for HER, usually needs a highly efficient photocatalyst and a cocatalyst.6 The semiconductors photocatalyst materials have two salient features such as levels between conduction and valence band as well as band gap width. The conduction band’s bottom level should be lower than the redox potential of H+/H2 (0 V vs. NHE) and upper level of valance band should be higher as compared to redox potential of O2/H2O (1.23 V). Thus, theoretically water splitting minimum band gap width is 1.23 eV which coincide with light of about 1100 nm.7 Various materials like oxides8, sulfides9-10 and nitrides11 contain suitable band structures for water splitting and dye sensitized solar cell.12 Although, there are some materials which have suitable bands positions and band gaps with visible light responses, still they are inactive for water splitting, because of photo-corrosion.7These materials play an active role for water splitting when they are used with a co-catalyst. A noble metal i.e. Pt and their composites are found to be a very useful co-catalyst and other applications.13-15 However, due to its high cost and insufficient abundance, their use as co-catalyst is limited. That's why this is indispensable to look for new noble metal co-catalysts which are highly dynamic, stable, and inexpensive


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especially from cost and material abundance point of view. To understand the HER techniques and nature of active sites present in the catalyst material for H2 evolution, more considerable developments have been carried out in designing and artificial production of analogues active sites of hydrogenases, like catalysts i.e. cobalt, nickel, and iron based complexes.16 Currently, two-dimensional layered transition metal dichalcogenides nanosheets (LTMD), such as graphene analogues, which are effectively and technologically attractive. MS2 (M=Mo/W) are well known LTMD, which display several fascinating features like electronic, mechanical, electrochemical, optical and thermal properties. Due to such properties they are used in different applications like photoluminescence17, electrocatalysts18, chemical cell19, field effect transistors20, photothermal cancer therapy21, and supercapacitor.22 The bulk MoS2 has semiconducting nature with an indirect bandgap of 1.2 eV23 which can intellect it into optoelectronics, photovoltaic and photocatalytic materials. Each layer of MoS2 structure possess 6.5 Å thickness, where each layer of Mo atoms is placed between two layers of S atoms exhibiting strong in-plane bonding while vertically stacked by weak Vander Waals interaction. Polymorphism with its various electronic features is one of the promising characteristics of MoS2. According to the theoretical reports MoS2 is predicted to be exhibits two kinds of symmetries in accordance with the configuration of S atoms, i.e. 2H semiconducting phase and 1T metallic phase.24-25 The 1-T octahedral symmetry is achieved from the 2-H trigonal phase by 60o rotation (along c-axis) of one of the sulphur basal planes. Both of these phases show totally distinct electronic structures. Chemically exfoliated 1T-MoS2 monolayers are considered to be very effective for electrochemical HER because of the existence of metallic 1T phase, in which, edges and basal planes both are engaged in catalytic activity while on the other hand in 2H-MoS2 basal planes remain inert and only edges play active role for catalytic reaction.26-28


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Herein, we used one-step hydrothermal method for the synthesis of stable MoS2 nanosheets. Interestingly, we found that metallic phase also exists in MoS2 structure, by the intercalation of ammonium ions. Afterwards we synthesized Ni9S8/MoS2 hybrid to investigate the photocatalytic water splitting properties of these structures. It exhibits substantially higher H2 evolution efficiency.

2. Experimental: Synthesis of MoS2 Nanosheets: All the reagents were analytical grade and used without additional cleaning. Hydrothermal method was used to synthesize MoS2 nanosheets in a closed autoclave system. To synthesized, 1 mmol (NH4)6Mo7O24·4H2O and 15 mmol Thiourea {n(Mo) : n(S) = 7 : 15} were mixed in 35 mL deionized (DI) water under intense stirring to make a uniform solution. Then obtained solution was shifted to a 45 mL

autoclave (Teflon-lined

stainless steel), kept at 200 °C for 24 hours, and gets cool to room temperature. The final black precipitates were successively washed using ethanol for many times until all impurities were removed and finally dried at 60 °C under vacuum. Synthesis of the Ni9S8/MoS2 Hybrid: The as synthesized MoS2 nanosheets were dissolved in 30.0 mL distilled water and loaded 3.0 µL of hydrazine (N2H4). Then, 1.0 M of a Ni(NO3)2 solution was added drop wise into the MoS2 suspension under dynamic stirring. The weight ratios of Ni(NO3)2 to MoS2 were 5, 10, 20 and 50 wt%, in the same way. The Ni9S8/MoS2 hybrid was purified with the similar method to the nanosheets. Lastly, the precipitate was dried out at 60 °C under vacuum and was further characterized.


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Photo-catalytic Hydrogen Evolution Measurements: The photocatalytic hydrogen reactions were carried out in a Pyrex reaction cell associated with a closed gas circulation and evacuation system. 20 mg of Ni9S8/MoS2 powder was dispersed in 45 ml of aqueous solution. Then it was loaded with 0.3 M Na2S and 0.3 M Na2SO3. The suspension was then properly degassed and irradiated by a Xe lamp (300 W). The amount of hydrogen produced was examined using an online gas chromatography (Agilent 7890A). The results of different catalysts were compared with each other on the basis of the average rate of H2 evolution in first 5 hours. The experimental error was less than 5%. Characterizations: Samples were characterized by X-ray powder diffraction (XRD) method by a Philips X’Pert Pro Super diffractometer equipped with Cu Kα radiations (λ = 1.54178 Å). Field emission scanning electron microscopy (FE-SEM) images were taken via JEOL JSM-6700F SEM. JEM-2100F field emission electron microscopy (TEM) with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a VG ESCALAB MK II X-ray photoelectron spectrometer equipped with an Mg Kα = 1253.6 eV source. The binding energies obtained in the XPS spectral range were corrected for specimen charging effects using the C 1s level at the energy of 284.5 eV as a reference. Raman spectra were detected by a Renishaw RM3000 Micro-Raman system with a 514.5 nm Ar laser.

3. Results and discussions: Figure 1(a) indicates the XRD spectrum of the as-synthesized hybrid of MoS2 nanosheets and the 20 wt% Ni9S8 nanobelts. From the XRD result it is seen that at 14.38° of pattern (002) peak shows the pristine MoS2 same as (002) new peak of the 1T-MoS2 which can show both 1T and


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2H phases. The value of d spacing which corresponds to (002) peak was 3.35 Å which is agreeable with the diameter of hydrogen-bonding in ammonium ions present in the metal disulfides.29

Figure 1. (a) XRD patterns and (b) EDS spectrum of the as-synthesized Ni9S8/MoS2 hybrid. The 34.19° (203) and 37.93° (330) peaks of Ni9S8 with additional peak of the (002) plane of MoS2 is also present in the XRD spectrum (see Figure S3 in ESI†). Upon the integration of the Ni9S8 nanobelts onto the MoS2 nanosheets, there is no clear transformation in the MoS2 diffraction pattern, which is also confirmed by the Raman spectrum (see Figure S1 in ESI†). During the process, the MoS2 was partially decomposed that resulted in the release of sulfur. This released sulfur acted as a source that caused the formation of Ni9S8. In EDS spectra the elemental ratios of the synthesized Ni9S8/MoS2 hybrid were characterized as shown in Figure 1(b). It can be seen that molybdenum, sulfur, and nickel are present in the grown layered heterostructure Ni9S8/MoS2 hybrid. Figure 2 (a, b), shows typical TEM images of Ni9S8/MoS2 hybrid. The TEM images show a nanosheets and nanobelts morphology of MoS2 and Ni9S8 respectively. Figure S2 shows the SEM image which also confirms the flower like sheets of pure MoS2. HRTEM were carried out to examine the morphology and the interfacial configuration between MoS2 nanosheets and Ni9S8


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nanobelts hybrid sample as shown in Figure 2 (c). The obtained HRTEM image shows fringes with lattice spacing of ca. 6.7 Å, which correspond to the (002) plane of 2H-MoS2. The measured interlayer distances of Ni9S8 nanobelts is found to be 2.62 Å and 2.37 Å as observed from the HRTEM image which corresponds to (203) and (330) diffraction peaks that is well in agreement of XRD results. The intimate connection between Ni9S8 nanobelts and MoS2 nanosheets favors the charge separation and hence boosts the photocatalytic activity, which are discussed in our experimental measurements.

Figure 2. (a, b) Typical TEM images of Ni9S8/MoS2 hybrid and (c) its corresponding HRTEM analysis. (d) Chemical structure of layered MoS2 and Ni9S8, along with its hybrid.


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XPS was used to study the chemical structure of Ni9S8/MoS2 hybrid. From Figure 3 (a) we found two Mo doublets 3d5/2 and 3d3/2 at 229.35 and 232.65 eV respectively, which represent 2H phase of MoS2.30 The S 2p band in Figure 3 (b) shows a main peak at 162.46 eV, comprising two S 2p1/2 and S 2p3/2 doublets. Figure 3 (c) shows Ni 2p XPS spectrum of Ni9S8/MoS2. The Ni 2p peaks display 2p3/2 electronic configuration at 853.6 and 856.7 eV that confirms the presence of divalent and trivalent states. Similarly peak located at 870.7 eV refers to the Ni 2p1/2 band.31 XPS survey report of Ni9S8/MoS2 hybrid structure confirms the bonding between Mo and Ni as evident from Mo, 3p3/2 and 3p1/2 in Figure 3 (d).

Figure 3. XPS analysis of the obtained Ni9S8/MoS2 hybrid. (a) Mo 3d, (b) S 2p, (c) Ni 2p spectra and (d) XPS profile of Ni9S8/MoS2 hybrid.


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We further evaluate the catalytic performance of the obtained materials. Figure 4(a) depicts the results of photocatalytic hydrogen evolution drive by visible-light for pure MoS2 and Ni9S8/MoS2 hybrid. In addition to the synthesis and characterization, we aimed to compare the photocatalytic activity of the MoS2 and its different nanostructures. We have used MoS2 nanosheets as the platform to create Ni9S8/MoS2 hybrid model for water splitting. The production of hydrogen gas from commercially prepared MoS2 is reported to be 92.0µmolg-1h-1 while that of the MoS2 nanosheets is 285µmolg-1h-1.

Figure 4. (a) Yield of H2 for different synthesized samples. (b) Time dependent photocatalytic H2 production rate for MoS2 nanosheets, 10 wt% Ni9S8/MoS2 and 20 wt% Ni9S8/MoS2 hybrid. Upon introducing Ni9S8 nanobelts onto the MoS2 nanosheets, there is a significant enhancement in the hydrogen generation rate for the entire hybrid. The HER outcomes after the pure MoS2 nanosheets were integrated with co-catalyst Ni9S8 nanobelts. The increment is attributed to the drop of electron, where Ni9S8 nanobelts may act as a “pond” to collect photo excited electrons of the MoS2 nanosheets and hence overcome the flaw of recombination.32 These photo excited electrons are utilized to generate the photocatalytic reaction to improve the


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hydrogen production rate. After comparison 20wt% Ni9S8/MoS2 was appeared to be more prominent sample. It was observed that 20wt% Ni9S8/MoS2 exhibits almost 70% enhancement in hydrogen production rate (406 µmolg-1h-1) as compared to as-synthesized MoS2 nanosheets (285 µmolg-1h-1), and 4 times more value as compare to commercial MoS2 (92.0 µmolg-1h-1). However, it was seen that the HER performance decreases with the increase in percentage of Ni9S8, such as 50 wt% Ni9S8/MoS2. Moreover, Figure 4(b) indicates the result of visible-light driven photocatalytic H2 production of MoS2 nanosheets, 10wt% Ni9S8/MoS2 and 20wt% Ni9S8/MoS2 for 5 hour. It indicates comparable stability of the layered hetero-structure Ni9S8/MoS2 hybrid. In order to better understand the mechanism, we give a schematic diagram of photocatalytic process for the Ni9S8/MoS2 hybrids, as shown in scheme 1.Figure S4 shows the UV-VIS absorption measurements (ahν)2 verses photon energy hν of Ni9S8/MoS2 hybrid. Estimated value of the bandgap is 1.64eV.

Scheme 1. Schematic diagram of charge separation for HER using Ni9S8/MoS2 hybrid as photocatalyst.


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Considering of previous works, we suggest that the HER enhancement of the layered Ni9S8/MoS2 hybrid is due to the significant transfer of photo excited electrons facilitated by the co-catalyst Ni9S8 nanobelts. The enhanced photocatalytic performance of MoS2 nanosheets is ascribed to its larger surface area with access of active sites along the edges compared to commercial MoS2. Moreover, the hybrid material that appears in sheets which are relatively thinner give the high hydrogen production rate. The reason behind is their shorter carrierdiffusion length which facilitate the charge transport from the inner to the solid-water interface. The contribution from these factors has firmly provoked the reduction at the interface between the layered nanosheets and water for resulting in an enhanced hydrogen evolution rate.

4. Conclusion: In summary, we have developed a facile hydrothermal method to synthesize a heterostructure Ni9S8/MoS2 hybrid. Its practical application in water splitting hydrogen evolution reaction was evaluated. The photocatalytic evaluation indicated that hydrogen gas production depends on Ni9S8 amount in the hybrid. The hybrid with 20 wt% Ni9S8/MoS2 exhibited the maximum value with a 70% enhancement in HER as compared to that of as-synthesized MoS2 nanosheets. Moreover, hydrogen amount of 20 wt% Ni9S8/MoS2 is 4 times higher than commercial MoS2. The possible mechanism of the enhanced HER performance was originated from the presence of Ni9S8 co catalyst. This low-cost, scalable bottom-up aqueous growth may offer a possibility to synthesize other stable LTMD like MoSe2, WS2 and WSe2 which can provide a chance to reveal their uses in different potential application such as, electronics, optics and catalysts.


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Corresponding Author *Li Song Email: [email protected]; Tel.: +86-551-6360-2102 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Financial supported by the MOST (2014CB848900), NSFC (U1532112, 11375198, 11574280), CUSF (WK2310000053) and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) Nankai University. ACKNOWLEDGMENT We acknowledge MCD and Catalysis, photoemission and Surface Science End stations at NSRL for help in Synchrotron Radiation-based characterizations. L.S. thanks the recruitment program of global experts and the CAS Hundred Talent Program. A.K. thanks the CSC fellowship. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ABBREVIATIONS MoS2, molybdenum disulphide; HER, hydrogen evolution reaction; LTMD, layered transition metal dichalcogenides; SEM, scanning electron microscope; TEM, transmission electron microscope; HRTEM, high resolution transmission electron microscope; XPS, X-ray photoelectron spectroscopy; XRD, x-ray diffraction; UV vis, ultraviolet visible; DI, deionized; EDS, energy dispersive spectroscopy.


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31. Xiao, J.; Wan, L.; Yang, S.; Xiao, F.; Wang, S., Design hierarchical electrodes with highly conductive NiCo2S4 nanotube arrays grown on carbon fiber paper for high-performance pseudocapacitors. Nano letters 2014, 14 (2), 831-838. 32. Cheah, A.; Chiu, W.; Khiew, P.; Nakajima, H.; Saisopa, T.; Songsiriritthigul, P.; Radiman, S.; Hamid, M. A. A., Facile synthesis of a Ag/MoS2 nanocomposite photocatalyst for enhanced visible-light driven hydrogen gas evolution. Catalysis Science & Technology 2015, 5 (8), 4133-4143.


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Abstract Graphic


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MoS2 heterocatalyst for ... - ACS Publications

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