FeS2-doped MoS2 nanoflower with the dominant 1T-MoS2 phase as an excellent electrocatalyst for high-performance hydrogen evolution

Accepted Manuscript Title: FeS2 -doped MoS2 nanoflower with the dominant 1T-MoS2 phase as an excellent electrocatalyst for high-performance hydrogen evolution Authors: Xue Zhao, Xiao Ma, Qingqing Lu, Qun Li, Ce Han, Zhicai Xing, Xiurong Yang PII: DOI: Reference:

S0013-4686(17)31631-6 http://dx.doi.org/doi:10.1016/j.electacta.2017.08.004 EA 30008

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

23-4-2017 1-8-2017 1-8-2017

Please cite this article as: Xue Zhao, Xiao Ma, Qingqing Lu, Qun Li, Ce Han, Zhicai Xing, Xiurong Yang, FeS2-doped MoS2 nanoflower with the dominant 1TMoS2 phase as an excellent electrocatalyst for high-performance hydrogen evolution, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

FeS2-doped MoS2 nanoflower with the dominant 1T-MoS2 phase as an excellent electrocatalyst for high-performance hydrogen evolution Xue Zhaoa, b, Xiao Maa, b, Qingqing Lua, b , Qun Lia, b , Ce Hana, Zhicai Xinga, Xiurong Yanga,* a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China b

University of Chinese Academy of Sciences, Beijing 100049, PR China

* E-mail address: [email protected]

ABSTRACT: Well-established methods to improve the hydrogen evolution reaction (HER) performances include, but are not limited to, tailoring the morphology and electronic structure of transition metal dichalcogenides (TMDs), and doping of earth abundant chemicals such as iron pyrite FeS2 into existing TMDs. In this work, MoS2 nanoflowers with the majority being octahedral MoS2 (1T-MoS2) and doped with FeS2 were prepared and applied to HER. The as-prepared catalysts were characterized by X-ray absorption fine structure at the K-edge of Mo, S, and Fe to probe the local electronic structures. The resulting nanomaterial was identified to be FeS2 doped MoS2 nanoflower (denoted as Fe-MoS2NF) with 66 % 1T-MoS2 which was the metallic phase and could drastically boost the HER properties. The Fe-MoS2NF exhibited high HER performance with a Tafel slope of 82 mV dec-1 and it needs 136 mV to achieve a current density of 10 mA cm-2. The synthesis of Fe-MoS2NF with refined morphology and active electronic structure is expected to open a new era for improving the catalytic activity and stability of MoS2.

KEYWORDS: magnetic microspheres; nanoflower; octahedral MoS2; hydrogen evolution; FeS2

1. Introduction Hydrogen, a key alternative to coal, oil, and natural gas, has been utilized in a wide range of applications. The electrocatalytic hydrogen evolution reaction (HER) is of great importance for the hydrogen production using electricity [1]. Pt-based materials are known to be excellent HER catalysts. However, the low abundance and high cost

of Pt have limited its large-scale applications [2]. To overcome this issue, various noble-metal-free catalyst materials, such as transition-metal dichalcogenides (TMDs= MoS2 [3], WS2 [4], TaS2 [5], CoS2 [6], FeS2 [7] etc.) and their composites (Co-Mo-Sx [8], MoxW1-x(SySe1-y)(2) [9], Fe0.9Co0.1S2/CNT [10] etc.) have been developed to be potential substitutes of Pt to catalyze the conversion of H2O (alkaline) and H3O+ (acid) to H2 [11-12]. TMDs have unique layered structures [13], tunable band gaps, [14]

and

remarkable

electrochemical

properties

[15].

These

non-precious

electrocatalysts not only exhibit high reactivity close to that of Pt, but also are abundant throughout the earth. However, a big challenge remains as to simultaneously improve their catalytic efficiency and stability to meet the requirements for practical application [8]. Tuning morphology and/or regulating the electronic structure of TMDs are common ways to improve their HER performance. In terms of morphology tuning, the approaches have been widely researched, such as adding graphene as the alternation to form layer-by-layer assembled nanocomposite [16] and adding surfactants to assist the formation of nanoflowers [17]. Recently, enormous efforts have been made to regulate the electronic structure of TMDs in order to endow them with new functionalities or further enhance their intrinsic physical and chemical properties. Well-known approaches to engineering the intrinsic electronic structure of 2D nanomaterials include crystal phase tuning [18], surface functionalization [19-20], sulphur

vacancies

introduction [3],

defect-rich

engineering

[2],

molecular

titanium-oxide incorporation [21-22], and heteroatom hybridizing [23], and so on.

These investigations have enabled effective preparation of numerous TMDs with high HER performance. However, TMD nanomaterials with well-refined morphology, active crystal phase and proper doping prepared by well-designed methods were barely reported [24]. Herein, MoS2 was chosen as prototypical TMD and acted as an excellent model system to explore the targeted synthesis of 2D TMDs. In this work, we developed a novel MoS2-based hydrogen evolution electrode with high catalytic activity and stability by simply incorporating magnetic microspheres (means, Fe3O4) into the hydrothermal synthesis reaction of MoS2. In this case, Fe3O4 was originally anticipated to regulate the morphology of MoS2, affect crystal phase of MoS2, as well as provide Fe source to generate FeS2 [25]. The resulting nanomaterial, denoted as Fe-MoS2NF, not only had defined nanoflower-like morphology but also possessed the unique electronic structure that originated from its particular crystal phase. The X-ray photoelectron spectroscopy (XPS), Raman, as well as X-ray absorption near-edge structure (XANES) spectra showed that the electronic structure of the Fe-MoS2NF was different with that of pure MoS2. The as-prepared Fe-MoS2NF exhibited a Tafel slope of 82 mV dec−1. This electrode needs an overpotential of as low as 136 mV to afford a current density of 10 mA cm−2 and maintain its catalytic activity for at least 24h. To the best of our knowledge, this manuscript is the first report of research to incorporate both octahedral MoS2 (1T-MoS2) and FeS2 into the MoS2 for catalyzing HER. 2. Experimental

2.1. Preparation of Fe-MoS2NF and pure FeS2. In a typical synthesis, 0.30 g of Na2MoO4·2H2O, 0.40 g NH2CSNH2 were ultrasonic dissolved in 30 mL deionized water. Then the obtained solution was transferred into a Teflon-lined stainless autoclave (50 mL) together with 1 mL Fe3O4 microspheres (0.5% w/ V). The autoclave was sealed and maintained at 220 °C for 24 h in an electric oven. After cooling naturally, the Fe-MoS2NF was collected and washed with water thoroughly before vacuum dried at 40 °C overnight. To investigate the catalytic performance of FeS2, the control sample of pure FeS2 was prepared by the same reaction condition with the as-prepared Fe-MoS2NFs without adding the Na2MoO4·2H2O. 2.2. Electrochemical measurements All the electrochemical measurements were conducted using a CHI660E potentiostation (CH Instruments, China) in a typical three-electrode setup at room temperature, using the carbon cloth supported Fe-MoS2NF as the working electrode, a graphite rod as the counter electrode and saturation Ag/AgCl as the reference electrode. Polarization curves were obtained using LSV with a scan rate of 2 mV s -1 and no activation was used before recording the curves. The long-term durability test was performed using chronopotentiometric measurements. All currents presented are after iR-corrected. 2.3. Characterization Transmission electron microscopy (TEM) images were taken using a Tecnai G2 F20 with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM)

images were recorded on an FE-SEM XL30 ESEM-FEG at an accelerating voltage of 20.0 kV. Energy-dispersive X-ray spectroscopy (EDS) analysis and elemental mapping of the material was performed along with TEM and SEM. Raman spectra were collected on a Smart System HR Evolution (Horiba Jobin Yvon, France) using 532 nm laser source. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer. X-ray powder diffraction (XRD) patterns of the various samples were recorded using a D8-Advance system (Bruker, Germany) with Cu Kα radiation (λ =0.154 nm). X-Ray photo-electron spectroscopy (XPS) analysis was carried out on an ESCALAB MKIIX-ray photoelectron spectrometer using monochromated Al Kα X-rays and the energy values were calibrated by using the C1s level of 284.8 eV. X-ray absorption fine structure (XAFS) spectroscopy experiments at Mo K-edge and Fe K-edge were carried out at 1W2B end station, Beijing Synchrotron Radiation Facility (BSRF). The spectra were collected at room temperature in transmission mode. The S K-edge X-ray absorption near-edge (XANES) spectra were measured at the 4W7B beamline of the BSRF in the total electron yield mode. 3 Results and discussion Na2MoO4 and H2NCSNH2 were dispersed in aqueous solution and with the Fe3O4 microspheres as initial material. After the hydrothermal reaction, Fe-MoS2NF nanocomposites were obtained. The morphology and size of both pure MoS 2 and Fe-MoS2NF were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. It is well known that the regular

morphology of MoS2 crystal is nanosheet containing a multitude of MoS2 layers (Fig. 1A-C). Therefore, it is necessary to purposely engineer MoS 2 nanostructure to maximize the exposure of active sites, as have been demonstrated theoretically [26] and experimentally [27] to be the uncoordinated sulfur edge sites. By strong contrast, in the presence of Fe3O4 microspheres (Fig. S1), the resulted Fe-MoS2NF coalesced into elegant three-dimensional nanoflower through the exact same reaction condition. As is shown in Fig. 1D-F, the nanoflower is composed of tens to hundreds of slightly curved petals which grow perpendicularity to the center, providing more edges and corner atoms. The lateral size of a single nanoflower and the thickness of each petal are measured to be 1.3 um and 16.7 nm, respectively (Fig. 1D and 1E). The average neighboring-layer distances of pure MoS2 and Fe-MoS2NF are measured to be 0.62 nm and 0.65 nm, respectively, which match the interlayer spacing range between the (002) planes for MoS2 [25]. It is noted that adding Fe3O4 microspheres as additional reactants could endow MoS2 with flexural edges, further increasing the specific surface area, which was confirmed by Brunauer-Emmett-Teller (BET) analysis of Fe-MoS2NF. (Fig. S2) In detail, the Fe-MoS2NF has a high specific surface area of 172.73 m2·g−1 and desorption average pore volume of 0.29 cm3 g-1. The data corroborate the wide existence of the microporous and mesoporous structure, which ultimately provided more adsorption and reactive sites on the catalyst surface [28]. The drastic morphology difference highlights the importance of Fe3O4 microsphere in mediating the growth of MoS2. The Fe atoms were perfectly overlapped along the distribution of

Mo and S atoms, as shown in the element mappings (Fig. 1G-K and Fig. S3) and line-scanning of Fe-MoS2NF (Fig. S4). The atomic ratio of Mo: S: Fe in Fe-MoS2NF was measured to be 1: 1.87: 0.033 by ICP. The Mo/S ratio was consistent with the stoichiometry of MoS2, and the major building blocks in Fe-MoS2NF were MoS2. Also, the atomic ratio of Mo: S in pure MoS2 was about 1: 2.1 by ICP. Additional characterization was performed to understand the electronic structure of the Fe-MoS2NF, from which deeper insight into the electrocatalytic performance may be gained. Raman spectrum revealed the characteristic peaks of MoS2 at 283, 378, 404 and 454 cm-1 corresponding to the E1g, E12g, A1g and longitudinal acoustic phonon modes of MoS2, respectively (Fig. 2A) [29]. In addition, the representative J1, J2, and J3 regions in the lower frequency were observed in the Raman spectrum of Fe-MoS2NF, which represented the modes active only in 1T-MoS2 [4]. This phenomenon tentatively means the existence of 1T-MoS2 in Fe-MoS2NF. The characteristic peak of FeS2 at ~342 as well as ~ 378 cm-1 were not obvious because of the low percentage of FeS2 and/or overlapping by the peaks of MoS2 [7]. However, it was challenging to use powder X-ray diffractograms (PXRD) to characterize the 1T-MoS2 and hexagonal MoS2 (2H-MoS2) nanostructures. The differences between the XRD patterns of Fe-MoS2NF with pure MoS2 indicate the crystal structure change of Fe-MoS2NF from pure MoS2 after the adding of Fe3O4 microspheres as a reactant. The intensity of (002) peak of pure MoS2 was higher than that of Fe-MoS2NF, which signified a well-stacked layered structure in pure MoS2 as well as the fewer layered

MoS2 did exist in Fe-MoS2NF. (Fig. 2B) [30]. The XRD patterns of Fe-MoS2NF were identical with the simulated 1T-MoS2 in the work by Cai et al [31]. In order to further verify the incorporation of 1T-MoS2, we employed the X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) characterizations to probe the local electronic structures of Fe-MoS2NF. The crystal phase of pure MoS2 is 2H [32], in accordance with this, and the XPS spectra of pure MoS2 absolutely identified to 2H-MoS2 (Fig. 3A). Based on the XPS results, the binding energies peaks at around 229 and 232 eV of pure MoS 2 corresponded to Mo 3d5/2 and 3d3/2 orbitals, which were relaxed to lower energy side by 0.65 eV respectively, as to the situation of Fe-MoS2NF (Fig. 3A). This phenomenon was first observed by Eda et al. for 1T-MoS2 [33], and the difference value (0.65 eV) was similar to that of 1T-MoS2 reported in other research [18, 31]. It can be inferred that 1T-MoS2 was successfully introduced into the 2H-MoS2 matrix, forming a crystal-phase-coexistence MoS2. In order to determine the concentration of 1T-MoS2 in the Fe-MoS2NF, Mo 3d data were deconvoluted and revealed additional peaks at the lower energy along with the initial peaks of 2H-MoS2. According to the peak area, the relative content of 1T-MoS2 was estimated to be 66% (Fig. 3B). Furthermore, XAFS of Fe-MoS2NF was measured and the distinct spectral features were resolved by Mo K-edge XANES (Fig. 3C). There are four obvious peaks in the XANES spectra of both standard 2H-MoS2 and pure MoS2, which are in contrast not existed in that of Fe-MoS2NF. This result was identical with the published literature, in which the distinct spectral features above could be reproduced by XANES

calculations using FEFF8.2 code to prove the presence of 1T-MoS2 [31]. Furthermore, the Fourier transforms (FTs) of the extended X-ray absorption fine structure (EXAFS) simulations of pure MoS2, Fe-MoS2NF, and standard 2H-MoS2 were showed in Figure S5A. Different from standard 2H-MoS2, the oscillation of the Fe-MoS2NF in the second coordination decreased obviously. Similarly to the Mo 3d orbits, the S 2p3/2 and 2p1/2 peaks of Fe-MoS2NF were located at 161.2 and 162.3 eV, respectively, which are also ~1 eV lower than the corresponding peaks in 2H-MoS2 (Fig. S5) [18, 34-35]. Sulphur K-edge XANES spectra (Fig. 3D) were also conducted to understand the influences by the incorporation of 1T-MoS2 into 2H-MoS2. Both pure MoS2 and Fe-MoS2NF showed three characteristic XANES peaks at ∼2471eV, ∼2479 eV and ∼2482 eV [36].The peak at 2471eV is known to arise from the electron transition of S 1s electrons to unoccupied hybridized orbitals of S 3p and Mo 4d. This peak showed lower amplitude in the curve of Fe-MoS2NF than pure MoS2. Besides, the curve of Fe-MoS2NF strikingly appears a shoulder peak a1 at the position (∼2470 eV), implying the existence of a new energy level (means 1T-MoS2) near the Fermi level of 2H-MoS2 [33]. These phenomena above had clearly proved that new crystal phase was integrated into MoS2 matrix. As was reported in the literature, 1T phase TMD would convert to the 2H phase after annealing. Annealing experiments were conducted to further identify the phase compositions [4, 37]. The XPS and XANES were used to monitor the evolution of the MoS2 phase from the 1T to 2H. The as-prepared Fe-MoS2NF was annealed at 200 °C

under the argon atmosphere and denoted as Fe-MoS2NF-200. As shown in Fig. 4, the XPS, and XANES at Mo K-edge spectra of Fe-MoS2NF-200 were different with that of Fe-MoS2NF, but resembled that of 2H-MoS2. Based on the linear fitting of XANES curves, the 1T phase percentage of Fe-MoS2NF-200 was 30% to that of Fe-MoS2NF, indicating that ~70% of the 1T phase was converted to 2H after annealing at 200 °C, which was similar to the reported in the literature [4]. The annealed experiments solidly confirmed that the 1T-MoS2 was successfully implanted in the 2H-MoS2 nanosheet matrix onto Fe-MoS2NF. We further study the local electronic structure of Fe atom in Fe-MoS2NF based on the deconvolution of the core region of Fe 2p and S 2s. In the XPS spectra of Fe 2p, the binding energies peaks at about 707.3 eV and 720.5 eV can be attributed to FeS2. The XPS spectra of Fe 2p can be all attributed to FeS2 except the satellite peak (Fig. S6). In the XPS spectra of S 2p, the peaks at 162.3 eV and 161.1 eV indicated the existence of FeS2 (Fig. S6) [38]. Furthermore, the XANES and FT-XANES of Fe-MoS2NF at Fe K-edge were performed (Fig. S7). As shown in Fig. S7, the curve of Fe-MoS2NF was in line with the FeS2, but was very different with that of Fe3O4 microsphere. Bearing the corresponding element mapping, line scan and ICP results in mind, we can draw the conclusion that the FeS2 building blocks were formed into the as-prepared Fe-MoS2NF. During the reaction process, the Fe3O4 microspheres were completely converted into FeS2. FeS2, one of the earth-abundant metal pyrites, could exhibit efficient hydrogen evolution property [6-7]. Catalysis by FeS2 alone is of activity for HER. Futhermore, it is reported that the degradation effect from the

introduction of sulfide to natural FeS2 is less susceptible than that of platinum [39].It is reported that p-dopants such as Nb and Ta on bulk TMDs have a negative electrocatalytic effect on the HER, which contradicts the general consensus [40]. Although doping could lead to positive and negative results, it is anticipated that this subtle doping of FeS2 could enhance the electrocatalytic HER performance of Fe-MoS2NF. Catalysis activity of 1T MoS2 was first discussed by Martin Pumera’s group [41]. As is known to all, the presence of 1T-TMDs may lead to good catalytic properties [4, 42]. The unique morphology and electronic structure of Fe-MoS2NF were believed to boost the electrocatalytic performances in the HER. In order to maintain the morphology and facilitate the electrocatalytic test of the catalysts, we added a piece of carbon cloth to the hydrothermal reaction to fabricate the corresponding HER electrodes. The SEM image of carbon cloth supported Fe-MoS2NF was shown and its morphology was still defined nanoflower (Fig. S8). The mass loading of Fe-MoS2NF and pure-MoS2 were calculated to be 8.9 mg cm-2 and 12 mg cm-2 normalized to the geometric area (0.25 cm2), respectively. 20 % Pt/C was deposited on carbon cloth with the same area. The HER performances of the electrodes above were measured through a three-electrode cell in 0.5 M H2SO4. As shown in Fig. 5A and 5B, the polarization curves and corresponding Tafel plots of Fe-MoS2NF and pure-MoS2 compared with those of Pt/C. Obviously, the obtained Fe-MoS2NF exhibited superior activity for the HER, the onset potential was 100 mV and the overpotential to achieve a current density of 10 mA cm-2 was as low as 136

mV. This observed overpotential for hydrogen production was comparable to the value

of

the

previously

reported

HER

electrocatalysts,

for

example,

MoS2+x/NCNTs/CP need 160 mV to achieve the current density of 10 mA cm-2 [43]. (Table S1) Based on the calculation of i-V data from polarization curves, the Tafel slopes were 33, 82, 135 mV per decade for Pt/C, Fe-MoS2NF and pure-MoS2, respectively. For the control group, when lowering the loading of the Fe-MoS2NF on carbon cloth by reducing the reactants dosage, the control sample shows lower current density than the as prepared Fe-MoS2NF (Fig. S9). Polarization curve of Fe-MoS2NF during 0.2~ -0.58V shows large current density under hydrogen evolution condition (Fig. S10). As a control example, we synthesized pure FeS2 and measured its morphology as well as HER property under the same test condition with Fe-MoS2NF (Fig. S11). FeS2 exhibited catalytic properties for HER, but was inferior to Fe-MoS2NF. The long-term stability of the Fe-MoS2NF electrode under electrochemical condition was investigated. After 1,000 cycles of continuous operation, the Fe-MoS2NF exhibited inconspicuous decay of the current density (Fig. 5C). Based on i-t curve, this electrode maintains its catalytic activity for at least 24 h (Fig. 5D). The 24 h HER stability of Fe-MoS2NF was particularly interesting considering the generally accepted acid instability of HER catalysts [44]. 4. Conclusions In summary, Fe-MoS2NF with refined morphology, the majority being 1T-MoS2 and doped with FeS2, was successfully prepared. These factors were identified to synergistically enhance the HER performance of Fe-MoS2NF. The Fe-MoS2NF

exhibited exceptional performances as an effective HER catalyst, including an onset potential of 100 mV and a Tafel slope of 82 mV dec−1, as well as a large cathode current density. This electrode needs an overpotential of only 136 mV to afford a current density of 10 mA cm−2 and maintains its catalytic activity for at least 24 h. The catalytic activity of MoS2 for HER is enhanced by incorporation of Fe3O4, indicating that Fe3O4 is an excellent dopant for modified MoS2 to improve the HER performance.

Acknowledgements

The authors appreciate Key Research Program of Frontier Sciences, Chinese Academy of Sciences (No. QYZDY-SSW- SLH019) and the National Natural Science Foundation of China (No. 21627808, 21435005, and 21603215) for financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in the online version, at http://dx.doi. Org/10.1016/

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Fig. 1 SEM images, TEM images and HRTEM images of the pure MoS2 (A-C), and as-prepared Fe-MoS2NF (D-F). HAADF-STEM image (G) and corresponding element mapping analysis (H-K) of a single nanoflower which represents Fe-MoS2NF and indicates the homogeneous distribution of Mo, S, Fe.

Fig. 2 Raman spectra (A) and XRD patterns (B) of pure MoS2 versus Fe-MoS2NF were shown. Fig. 3 (A) Mo 3d XPS spectra of pure MoS2 versus Fe-MoS2NF; (B) deconvolution of the core region of Mo 3d for Fe-MoS2NF; (C) Mo K-edge XANES spectra of pure MoS2, Fe-MoS2NF, and standard 2H-MoS2; (D) S K-edge XANES spectra of pure MoS2 and Fe-MoS2NF.

Fig. 4 XPS of Mo 3d (A), S 2s (B) of Fe-MoS2NF, Fe-MoS2NF-200 °C, and pure MoS2, XANES (C) and FT-EXAFS (D) of Fe-MoS2NF, Fe-MoS2NF-200 °C and standard 2H-MoS2. Fig. 5 HER activity of the synthesized Fe-MoS2NF. (A) Polarization curves of the Fe-MoS2NF, pure-MoS2 and Pt/C in 0.5 M H2SO4. (B) Corresponding Tafel plots obtained from the polarization curves. (C) Polarization curves of the Fe-MoS2NF after 1 and 1,000 cycles. (D) i-t curve recorded at 0.46 V vs. RHE.

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