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reduced graphene oxide hybrid for enhanced hydrogen evolution
Accepted Manuscript Title: Mn doped MoS2 /reduced graphene oxide hybrid for enhanced hydrogen evolution Authors: Liqian Wu, Xiaobing Xu, Yuqi Zhao, Kaiyu Zhang, Yuan Sun, Tingting Wang, Yuanqi Wang, Wei Zhong, Youwei Du PII: DOI: Reference:
S0169-4332(17)31878-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.223 APSUSC 36423
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-5-2017 19-6-2017 21-6-2017
Please cite this article as: Liqian Wu, Xiaobing Xu, Yuqi Zhao, Kaiyu Zhang, Yuan Sun, Tingting Wang, Yuanqi Wang, Wei Zhong, Youwei Du, Mn doped MoS2/reduced graphene oxide hybrid for enhanced hydrogen evolution, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.223 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.
Manuscript number: APSUSC-D-17-04931 Revision (the parts with red words have been revised)
Mn doped MoS2 / reduced graphene oxide hybrid for enhanced hydrogen evolution
Liqian Wua, Xiaobing Xua, b, Yuqi Zhaoa, Kaiyu Zhanga, Yuan Suna, Tingting Wanga, Yuanqi Wanga, Wei Zhonga,* and Youwei Dua a) Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing, 210093, China. b) College of electronic Engineering, Nanjing Xiaozhuang University, Nanjing, 210017, China. *
Corresponding author, E-mail: [email protected]
Graphical abstract
Highlights:
In this study, hybrid catalyst of Mn doped MoS2 / reduced graphene oxide(Mn-MoS2/rGO) was easily synthesized by the hydrothermal method.
Effect of Mn-MoS2/rGO hybrid for the HER was systematically investigated for the first time.
The hybrid material, as an electro-catalyst, exhibits excellent catalytic activity
and good stability for the HER in acidic medium.
The excellent HER performance is due not only to high conductivity mainly from reduced graphene oxide, and more importantly to fundamental catalytic activity enhancement generated by Mn doping into MoS2. The hybrid material of Mn-doped molybdenum disulfide / reduced graphene
oxide (Mn-MoS2/rGO) is successfully fabricated through a one-pot hydrothermal method. The hybrid material, evaluated as electrochemical catalyst for the hydrogen evolution reaction (HER), exhibits improved catalytic activity and good stability for the HER in acidic medium with small overpotential (approximately 110 mV). Through analyses, it can be concluded that the improvement for electrochemical HER performance of the hybrid catalyst is ascribed not only to high conductivity mainly from reduced graphene oxide, but also to fundamental catalytic activity enhancement generated by Mn ions doping into the S-edge of MoS2 with a portion of Mo ions replacement. These results verify the facilitation effect of Mn-doping on HER activities, and the Mn-MoS2/rGO hybrid prepared in this work could be employed as a promising alternative to noble metal catalysts in HER.
Keywords: HER, Molybdenum disulfide, Mn-MoS2/rGO hybrid, S-edge of MoS2 1. Introduction With global warming and the urgent need for renewable energy sources as viable alternatives to fossil-fuel-based technologies, hydrogen as a clean energy carrier has gained growing attention [1, 2], and it can be sustainably produced through the electrochemical splitting of water [3, 4]. The hydrogen evolution reaction (HER) acts as a remarkable technique in electrochemical water splitting, and Pt-group metals are recognized as the best electrocatalysts. However, due to that the high cost of Pt-group metals hindered their wide application in hydrogen production, efficient and cheap earth-abundant HER catalysts are needed for intensive investigation [5-6]. Recently, the two-dimensional (2D) layered transition-metal dichalcogenide
semiconductors (especially MoS2, which Gibbs free energy of adsorbed atomic hydrogen is close to that of Pt-group metals (i.e., ΔGH ≈ 0)), have been widely studied as promising hydrogen evolution catalysts [7-9]. Generally, there are three strategies to improve the electrocatalytic ability of MoS2 catalysts [10]: (i) increasing the number of exposed active sites, (ii) improving the electrical contact to active sites and (iii) enhancing the intrinsic activity of each active site. Following these strategies, a great number of efforts have been made for improving the HER activities of MoS2. For example, on the account of that the active sites of MoS2 are located at the edges sites and the basal plane is inert, Yan et al designed ultrathin MoS2 nanoplates developed by a facile solvent-dependent control route to expose active edge sites, leading to a great improvement of the HER activity [11]. Also, Xie et al prepared defect-rich MoS2 ultrathin nanosheets [12] and MoS2 nanowall [13]. Due to that rich defects can make partial cracking of the catalytically inert basal planes, further resulting in exposure of additional active edge sites, both the defect-rich MoS2 ultrathin nanosheets and nanowall exhibit excellent HER activity. Considering the intrinsic low electrical conductivity of MoS2, efforts were adopted to engineer hybrids with other highly conductive materials, including carbon cloth [14], carbon nanotubes [15], carbon nanopapers [16], carbon nanofibers [17], graphene [18], MoC2 [19], which can improve the electron transfer ability. Furthermore, combine above two key factors concerning the active sites and conductivity of MoS2, the synthesis of oxygen-incorporated MoS2 ultrathin nanosheets with a moderate degree of disorder was also realized [20]. The disordered structure can offer abundant unsaturated sulfur atoms as active sites for HER, while the oxygen incorporation can effectively regulate the electronic structure and further improve the intrinsic conductivity, which contribute to the dramatically enhanced HER activity. Likewise, in order to enhance the intrinsic activity of each active site, various promoters can also be considered. Since the Mo-edge was predicted to be the active edge for HER [21], transition metal (such as Co, Ni and Fe) doping into the S-edge of MoS2 was proposed as one effective method for modifying the edge site activity for HER [22]. And it has proven theoretically and experimentally that the introduction of
Co, Ni, Fe or Cu into MoS2 could improve its HER electrocatalytic activity [23-26]. Recently, Tsai et al reported that the DFT-calculated Gibbs free energy of adsorbed atomic hydrogen(ΔGH) of Mn-doping into MoS2 S-edges is closer to thermo-neutral than the non-doping MoS2 Mo-edges [27], and it is therefore approached that Mn-doped MoS2 could also improve the HER activity over pristine MoS2. Accordingly, inspired by these, we rationally designed and successfully synthesized hybrid electrocatalyst of Mn-MoS2/rGO by one-pot hydrothermal method for the first time. The results show that the hybrid is indeed an efficient catalyst for electrochemical hydrogen evolution and the effects of Mn-doping have also been systematically investigated. 2. Experimental Section 2.1 . Reagents and materials All chemicals were reagent grade and used as received without further purification. Manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea ((NH2)2CS), ethanol, and graphene oxide (GO) were purchased from Aladdin Reagent (Shanghai, China), Macklin Biochemical Reagent (Shanghai, China) and advanced material supplier, respectively. High purity distilled water was used in this work. 2.2 . Synthesis of Mn-MoS2/rGO hybrid As shown in Scheme 1, the Mn-MoS2/rGO hybrid was synthesized by a simple general hydrothermal reaction of Mn(CH3COO)2·4H2O, (NH4)6Mo7O24·4H2O and (NH2)2CS in GO aqueous solution.
In a typical batch, 0.1 mmol of
(NH4)6Mo7O24·4H2O/ Mn(CH3COO)2·4H2O
and 3 mmol of (NH2)2CS were
dissolved into 30 mL of 0.2 mg/mL GO dispersion with vigorous stirring for 1 h. Afterwards, the resulting mixture was transferred into a 50 mL Teflon-lined autoclave, which was heated at 220 °C for 24 h. After cooling to room temperature naturally, the black products were collected and washed carefully with distilled water and ethanol for several times. Finally, the products were dried in a vacuum oven at 60 °C for 12 h. For aforementioned samples, non-doped and doped MoS2/rGO hybrids were labelled as MoS2/rGO, Mn-MoS2/rGO, respectively. Additionally, for comparison, the pristine
MoS2 was prepared by the same experiment procedure neither Mn(CH3COO)2·4H2O nor GO dispersion in the hydrothermal solution, and the pristine MnS was also prepared by the same experiment procedure neither (NH4)6Mo7O24·4H2O nor GO dispersion. 2.3. Characterizations In this work, Powder diffraction data were collected from 10° to 80° in 2θ using an X-ray diffractometer (TD-3500, China) with Cu Kα (λ = 1.5418 Å) radiation. The morphology of the samples was examined by transmission electron microscopy (TEM) (Model JEOL-2010, Japan) and field-emission scanning electron microscopy (FE-SEM) (FEI Helios600i, USA). The element mapping was carried out by an energy dispersive spectrometer (EDS) (Oxford X-MAX 50, UK). X-ray photoelectron spectrum (XPS) was measured on a PHI 5000 VersaProbe (UIVAC-PHi, Japan) using Al Kα radiation (1486.6 eV). The specific surface area of sample was investigated by nitrogen adsorption/desorption measurement on a TriStar 3000 surface area analyzer (Micromeritics, USA) using Brunauer-Emmett-Teller (BET). 2.4. Electrochemical measurements Electrochemical measurements were carried out in a standard three-electrode system on an electrochemical workstation (CHI660D) at room temperature. The system consists of a glassy carbon electrode (GCE, 3 mm in diameter) as the working electrode, Pt wire electrode as counter and Ag/AgCl (in 2.5 M KCl solution) as reference electrode, respectively. For the glassy carbon (GC) electrode modified by Mn-MoS2/rGO as the working electrode, 5 mg of products were dispersed in 1 mL of N, N-dimethylformamide (DMF) with 40 μL of Nafion solution (0.5 wt%) by sonication for 1h to form a homogeneous ink. Then, 5 μL of the dispersion was loaded onto the GC until the suspension was dried. Linear sweep voltamperometry (LSV) was performed in 0.5 M H2SO4 solution with a scan rate of 5 mV s-1 to evaluate the HER performance of different catalysts. Cyclic voltammetry (CV) was conducted with different scan rates (20-200 mV s-1) in the potential range from 0.1-0.2 V (vs. the reversible hydrogen electrode (RHE)) to investigate the electrochemical double-layer capacitance. Electrochemical impedance spectroscopy
(EIS) was performed by applying an ac amplitude of 0.5 mV in the frequency from 106 to 10-2 Hz. Furthermore, all tests were carried out in N2-saturated 0.5 M H2SO4 solution to get rid of the dissolved oxygen. And all the potentials reported in our manuscript were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.209+0.059 pH) V. 3. Results and discussion The XRD patterns of the pristine MoS2, un-doped and Mn-doped MoS2 / rGO hybrids are showed in Fig. 1. It can be seen that all as-synthesized samples show nearly identical characteristics peaks, which can be indexed to hexagonal MoS2 (2H-MoS2, JCPDS 37-1492). However, the (002) diffraction peaks of the graphene at 2=24.5 in the XRD patterns of the hybrids can be also hardly detected, which indicates that the graphene nanosheets seldom stack during the hydrothermal process [28], while they act as novel substrates for the nucleation and subsequent growth of MoS2 [29]. Furthermore, remarkably, with the addition of Mn, there are no remarkable changes in the XRD patterns and no additional peak indexed to MnS (JCPDS 65-0891) or to any other secondary phases can be identified, indicating that Mn might be doped into the MoS2 crystals. Additionally, as a comparison, the as-prepared MnS is displayed in Fig. S1, and from Fig. S1, it is observed that the MnS possesses pure phase and perfectly matches with those recorded in PDF standard card of JCPDS 65-0891. Based on FE-SEM images, the morphology of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO were investigated. As shown in Fig. 2a, the pristine MoS2 exhibits a flower-like morphology with serious agglomeration and stacking of MoS2 sheets, which leads to less exposed active sites and inferior activity [30]. Fig. 2b displays that, in MoS2/rGO, MoS2 nanosheets crosslink with each other growing on the surfaces of graphene sheets. And, in Mn-MoS2/rGO hybrid (shown in Fig. 2c), MoS2 nanosheets with interconnected ripples and curved edges are anchored on the graphene sheets; furthermore, MoS2 nanosheets possess smaller sizes, which would be conducive to the exposure of more active edge sites. In order to know accurately actual surface area
of the samples, low temperature N2 adsorption and desorption isotherms are carried out as shown in Fig. S2. The BET special surface area of the Mn-MoS2/rGO hybrid is 63.65 m2 g-1, which is slightly higher than that of MoS2/rGO (50.08 m2 g-1), much larger than that of MoS2 (17.22 m2 g-1). The experimental results of the specific surface areas are consistent with those of the morphology observed by FE-SEM (shown in Figs. 2 b and c). For further observation of Mn-MoS2/rGO hybrid, TEM and high-resolution TEM (HRTEM) analyses were performed, as shown in Figs. 2d and e. It can be observed that ripples and corrugations are rather well-defined with MoS2 represented by the dark-contrast objects, whereas rGO by the gray background, forming highly integrated structure, which can improve electron transfer in electrochemical reactions and the efficiency of catalytic. For HRTEM image of Mn-MoS2/rGO hybrid, it clearly shows that the interlayer distance of (002) is about 0.61 nm, which is close to the standard value of MoS2 (0.62 nm) [25]. It is also in accordance with the experimental result reported by Wang et al that inserting a small amount of Mn ions in MoS2 cannot cause the distortion of MoS2 layered structures [31]. Similarly, the crystallinity of the samples changes little before and after Mn doping via the contrast between the HRTEM image of Mn-doped MoS2 (Fig. 2e) and that of none-doped MoS2 (Fig. S3) grown on rGO, which is consistent with the XRD observation that there are no remarkable changes for the diffraction peaks of samples before and after Mn doping. In addition, the EDX-mapping (Fig. S4) validates the existence of C, Mo, S, O, and Mn elements, and all the elements have a homogeneous distribution in Mn-MoS2/rGO hybrid. The aforementioned (the HRTEM image and EDX-mapping) experiment results all confirm manganese atoms are successfully incorporated into MoS2, including its surface edge sites, which may affect the HER activity of the hybrid. X-ray photoelectron spectroscopic (XPS) measurements were carried out to further investigate the chemical composition of Mn-MoS2/rGO hybrid. As is shown in the survey spectrum in Fig. 3a, the elements of C, Mo, S, and O can be clearly identified. Fig. 3b shows that the Mn 2p XPS spectra of Mn-MoS2/rGO hybrid, and the characteristic peak at 641.2 eV, corresponding to Mn 2p3/2, ascribes to Mn2+. Fig.
3c depicts Mo 3d and S 2s XPS spectra of Mn-MoS2/rGO hybrid, where the characteristic peaks located at 229.7 and 232.8 eV are ascribed to Mo 3d5/2 and 3d3/2 of tetravalence molybdenum ion (Mo4+), respectively, and the emergence of the two peaks at 232.8 and 235.9 eV are attributed to the existence of a small amount of Mo6+ due to the explosion of MoS2 in air [18]. Otherwise, the peak at 226.9 eV corresponds to S 2s of MoS2. For S 2p region, which is shown in Fig. 3d, the peaks at 162.3 and 163.4 eV can be assigned, respectively, to the S 2p3/2 and 2p1/2 orbitals of divalent sulfide ions (S2-), while the other two peaks at 162.7 and 163.9 eV manifest the presence of bridging S22-, which may result from incorporation of manganese atoms and is considered as an important factor to improve the HER activity [25-26]. In addition, the quantification of the XPS peaks reveals that the atomic ratio of Mn/Mo in the Mn-MoS2/rGO is 1/32. For evaluating electrochemical performance of Mn-doped serial samples, electrochemical workstation was used to investigate the HER activity by a typical three-electrode system. Polarization curves of all the samples were shown in Fig. 4a. As a reference, the Pt/C catalyst exhibits high HER activity with an extraordinary low overpotential and a high current density, while the bare GC catalyst embodys catalytically inactive HER activity with a trivial current provided. For the HER performance of serial samples, Mn-MoS2/rGO catalyst exhibits better HER activity (with a small onset overpotential () of 110 mV) than those of the pristine MoS2 and MoS2/rGO, as shown in Table 1. Moreover, Mn-MoS2/rGO catalyst also displays a cathodic current density of 4.52 mA cm-2 at 200 mV, which is 4 times larger than that of MoS2/rGO (i.e. 1.15 mA cm-2), and 17 times larger than that of pristine MoS2 (i.e. 0.27 mA cm-2), respectively, as shown in Table 1. The corresponding Tafel plots of these catalysts derived from the polarization curves (shown in Fig. 4a) fit well with the Tafel equation (η=b log (j0) + a, where j0 is the exchange current density and b is the Tafel slope) in different overpotential ranges, and only the linear portions are selected to give a clear comparison. As shown in Fig. 4b, Mn-MoS2/rGO exhibits a smallest Tafel slope (about 76 mV per decade), except the Pt/C electrode (41 mV per decade). At the same time, the Tafel slopes of the
pristine MoS2, MoS2/rGO are 135 and 105 mV per decade, respectively. It is well known that the smaller Tafel slope means a faster increase of HER rate with the increasing potential [32], and thus Mn-MoS2/rGO catalyst displays the best activity. In addition, the Tafel slope is also an inherent property of the catalyst that is determined by the rate-limiting step of the HER. In acidic aqueous, three possible reaction steps have been suggested for the HER [33]. First is the discharge reaction (Volmer reaction): H3O++e-Hads+H2O (1). Second is electrochemical the desorption step (Heyrovsky reaction): Hads +H++e-H2 (2), or the recombination reaction (Tafel reaction): Hads+ HadsH2 (3). When a fast discharge reaction (1) is followed by a rate limiting combination reaction (3), the Tafel slope is 30 mV dec-1. When (1) is fast and followed by a slow electrochemical desorption (2), a Tafel slope of 40 mV dec-1 is obtained. Furthermore, if (1) is rate-limiting, the Tafel slope amounts to 120 mV dec-1. In our present work, the observed smallest Tafel slope values is 76 mV dec-1 for Mn-MoS2/rGO catalyst, which suggests primary discharge and electrochemical desorption are the rate-limiting step. By fitting j-E data to the Tafel equation [33], apart from Tafel slope above mentioned, the exchange current density j0, which can signify the most inherent measure of activity for the HER, is also obtained. As is shown in Table 1, Mn-MoS2/rGO catalyst, as the most active catalyst, shows the largest exchange current density of 24.2 A cm-2. And not only that, compared with previously reported MoS2 based composite catalysts (see Table 2 for details), the sample of Mn-MoS2/rGO exhibits excellent electrochemical properties in our work. The effect of Mn content on the activity of HER has also been discussed and the results are shown in Fig. S5. As shown in the polarization curves and Tafel plots, with the increasing Mn content, the HER activity of Mn-doped MoS2 / rGO samples is firstly increased, and reaches the maximum value at Mn : Mo= 1 : 32, then drops down afterwards, in accordance with the effect of other transition-metal (Co, Ni)
content on the HER activity [25,26]. To understand better effect of Mn-MoS2/rGO hybrid as enhanced electrocatalyst for the hydrogen evolution reaction, the turnover frequency (TOF) of H2 molecules evolved per second (represented as units of s-1) for each active site was measured, and can be calculated by the Jaramillo's method [7, 36]. For this direct site-to-site comparison, the TOF values of the pristine MoS2 and MoS2/rGO (as shown in Table 1) are calculated to be 0.029 and 0.042 s-1, respectively. Whereas, the Mn-MoS2/rGO shows the highest TOF value of 0.50 s-1, which means that the latter possesses a higher intrinsic activity of each active site. Fe, Co, Ni, and Cu doped MoS2 catalysts, exhibiting more excellent HER than pristine MoS2, have been demonstrated to preferentially substitute at the S-edge with 100% replacement of the Mo atoms in many cases, theoretically and experimentally [27, 37]. The enhancement of the intrinsic activity of each active site is also possibly attributed to Mn ions doping into the S-edge of MoS2 with a portion of the Mo ions replacement, as shown in Fig. 5, which is also accordance with the DFT-calculated results of Tsai et al [27]. Besides, the catalytic activity of as-prepared MnS is much less than that of the other catalysts in this work (as shown in Fig. S5). It excludes the influence of the formation of MnS phase on HER activity of Mn-MoS2/rGO hybrid catalyst, and further demonstrates that the significant enhance of HER activity of Mn-MoS2/rGO hybrid should be attributed to Mn ions doping into MoS2. In brief, the normally less active S-edges become more active in the presence of Mn, while the total amount of active sites also increases, which result in improving HER activity for hybrid. These results also verify experimentally that Mn doped MoS2 can promote the HER activity, apart from other transition metals such as Fe, Co, Ni, and Cu.
Furthermore, the double layer capacitances (Cdl) were measured to evaluate the effective active surface area of catalysts [38], as shown in Fig. 6a. By plotting the △j= ja-jc at a given potential (0.15 V vs. RHE) against the CV scan rates, the double layer capacitances (Cdl) for each samples are extracted. As shown in Fig. 6b and Table 1, the Cdl for the Mn-MoS2/rGO exhibits the largest Cdl (17.37 mF cm-2). It indicates the
highest exposure of effective active edge sites from the Mo-edge or the Mn-doped S-edge, and thus catalytic performance of Mn-MoS2/rGO could greatly improves. In order to further evaluate the performance of the series catalysts in HER processes, EIS measurements were also performed for various composite catalysts. The corresponding Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, are shown in Fig. 6c, while we expanded the scale pertaining to that portion of intersection of the semi-circle on the x-axis shown in the insert of Fig. 6c. It is clearly revealed that MoS2/rGO, and Mn-MoS2/rGO catalysts show much smaller radius of semicircle in the Nyquist plots, compared with MoS2. These reveal that hybrids possess higher conductivity, which is ascribed to the synergetic effect between MoS2 and rGO by mutual interface. Furthermore, due to the effect of Mn doping, the charge transfer resistance (Rct) of Mn-MoS2/rGO (1.5 ), is smaller than that of MoS2/rGO (8.5 ), as shown in Table 1, which further improves the electron transport ability and enhances the HER activity of Mn-MoS2/rGO. In addition to the excellent HER activity and relevant analysis mentioned above, durability of catalysts, as another important parameter to assess electrocatalyst, was measured. Mn-MoS2/rGO catalyst was cycled continuously for 1000 cycles, and in the end of cycling, the cathodic current density of Mn-MoS2/rGO catalyst decreased slightly, as shown in Fig. 6d, indicating that the Mn-MoS2/rGO composite has stable HER activity. 4. Conclusions In conclusion, Mn-MoS2/rGO hybrid catalyst has been successfully prepared, exhibiting excellent HER activity with a small overpotential of 110 mV, large cathodic currents, and a Tafel slope as small as 76 mV dec-1, superior to those of the pristine MoS2 and MoS2/rGO. The prominent HER performance is due not only to high conductivity mainly from reduced graphene oxide, more importantly to fundamental catalytic activity enhancement generated by Mn doping into MoS2. These results verify that the Mn-MoS2/rGO hybrid prepared in this work can be employed as a promising alternative to noble metal catalysts in HER.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.11474151), the National Key Project for Basic Research (Grant No. 2012CB932304), and PAPD, People’s Republic of China.
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5087-5092.
Figure captions Fig. 1. XRD patterns of the pristine MoS2, un-doped and Mn-doped MoS2/rGO hybrids. Fig. 2. FE-SEM images of the pristine MoS2 (a), MoS2/rGO (b), Mn-MoS2/rGO (c), TEM(d) and corresponding HRTEM(e) images of as-prepared Mn-MoS2/rGO. Fig. 3. (a) XPS survey spectrum, (b) high-resolution Mn 2p, (c) Mo 3d-S 2s, and (d) S 2p of the Mn-MoS2/rGO hybrid. Fig. 4. (a) Polarization curves, (b) corresponding Tafel plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO. Fig. 5. The ball model of Mn-MoS2/rGO. Color code for the ball model: S, yellow; Mo, green; Mn, purple; C, black; O, red; H, white. S included by red bracket represent the S-edge of Mn-doped MoS2; Mo included by yellow bracket represent the Mo-edge of MoS2. Fig. 6. Cyclic voltammograms of (a) Mn-MoS2/rGO, at different scan rates (20-200 mV s-1) used to (b) estimate the Cdl and relative electrochemically active surface area. (c) Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, (d) the durability test for Mn-MoS2/rGO hybrid.
(002) (103)
MoS2
Intensity(a.u.)
MoS2/rGO Mn-MoS2/rGO
JCPDS #37-1492 MoS2 JCPDS #65-0891 MnS
10
20
30
40
50
2(degree)
60
70
80
Fig. 1. XRD patterns of the pristine MoS2, un-doped and Mn-doped MoS2/rGO hybrids
(a)
(b)
1 μm
1 μm
(c)
1 μm
(e)
rGO
(d)
0.61 nm
Fig. 2. FE-SEM images of the pristine MoS2 (a), MoS2/rGO (b), and Mn-MoS2/rGO (c); TEM (d) and corresponding HRTEM (e) images of as-prepared Mn-MoS2/rGO.
(a)
(b) Mn 2p
Mo 3d
Mn 2p3/2
O 1s
C 1s
Mn 2p
1000
800
Intensity (a.u.)
Intensity (a.u.)
Mo 3p
S 2p
600 400 Binding Energy (eV)
200
0
(c) Mo 3d
648
646
644 642 640 Binding Energy (eV)
638
636
(d) S 2p Mo
4+
2-
Mo 3d3/2
Intensity (a.u.)
Intensity (a.u.)
S Mo 3d5/2
6+
Mo
238
236
S 2s
234 232 230 228 226 Binding Energy (eV)
224
222
S 2p1/2 S2
166
165
S 2p3/2
2-
164 163 162 Binding Energy (eV)
161
160
Fig. 3. (a) XPS survey spectrum, (b) high-resolution Mn 2p, (c) Mo 3d-S 2s, and (d) S 2p of the
Mn-MoS2/rGO hybrid.
(a) 0
(b) 0.4 Pt/C
-5
135 mV dec
MoS
-1
2
MoS /rGO 2
-10 -15
MoS2/rGO
-20
2
0.2 76 mV dec
41 mV dec
0.0 -0.4
-0.3 -0.2 -0.1 Potential ( V vs.RHE)
-1
0.1
Mn-MoS2/rGO
-25 -30
Mn-MoS /rGO
105 mV dec-1
Pt/C Bare GC MoS2
(V)
Current (mAcm-2)
0.3
0.0
-0.4
0.0
-1
0.4 0.8 1.2 lg(-j(mA cm-2))
1.6
2.0
Fig. 4. (a) Polarization curves, (b) corresponding Tafel plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO.
Fig. 5. The ball model of Mn-MoS2/rGO. Color code for the ball model: S, yellow; Mo, blue; Mn, purple; C, black; O, red; H, white. S included by red bracket represent the S-edge of Mn-doped MoS2; Mo included by yellow bracket represent the Mo-edge of MoS2. Fig. 5. The ball model of Mn-MoS2/rGO. Color code for the ball model: S, yellow; Mo, green; Mn, purple; C, black; O, red; H, white. S included by red bracket represent the S-edge of Mn-doped MoS2; Mo included by yellow bracket represent the Mo-edge of MoS2.
(a)
(b)
Mn-MoS2/rGO
4
MoS2/rGO
3
3.0
2
Mn-MoS2/rGO -2
17.37 mF cm
2.5
j0.15V(mA cm-2)
j (mA cm-2)
MoS2
3.5
200mV/s
1 0
2.0
20mV/s
-1
1.5
-2
-2
8.55 mF cm
1.0
-3
0.5
-4 -5 0.10
-2
1.18 mF cm
0.0 0.12
(c) 120
0.20
0.18
0.16 0.14 Potential (V vs RHE)
0
20
40
60
80 100 120 140 160 180 200 220 Scan Rate(mV/s)
(d) 0
MoS2 MoS2/rGO
100
Mn-MoS2/rGO
60
Current (mAcm-2)
-Z''
80 25
20
40
15
10
20
-20
initial after 1000 cycles
5
0
0
-10
0
20
40
5
60 Z'
10
15
80
20
100
25
30
120
-30 -0.4
-0.3
-0.2 -0.1 Potential ( V vs.RHE)
0.0
0.1
Fig. 6. (a) Cyclic voltammograms of Mn-MoS2/rGO at different scan rates (20-200 mV s-1), (b) estimated the Cdl and relative electrochemically active surface area, (c) Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, (d) the durability test for Mn-MoS2/rGO hybrid. Fig. 6. Cyclic voltammograms of (a) Mn-MoS2/rGO, at different scan rates (20-200 mV s-1) used to (b) estimate the Cdl and relative electrochemically active surface area. (c) Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, (d) the durability test for Mn-MoS2/rGO hybrid
Scheme 1. The fabrication process of Mn-MoS2/rGO hybrid. Scheme 1 The fabrication process of Mn-doped MoS2/rGO hybrids.
Table 1. Electrochemical parameters of the pristine MoS2, MoS2/rGO, Mn-MoS2/rGO. sample
ja
Tafel slope
j0
TOF
Cdl
Rct
(mV)
(mA cm-2)
(mV dec-1)
(10-5A cm-2)
) -1) (S
(mF cm-2)
()
135
cm-2) 1.38
0.029
1.18
70
2.00
0.042
8.55
8.5
2.42
0.050
17.37
1.5
MoS2
270
0.27
MoS2/rGO
165
1.15
Mn-MoS2/rGO
110
4.52
a
105 76
Cathodic current density was recorded at = 200 mV.
Table 2. Comparison of HER activity of the as-prepared Mn-MoS2/rGO composite catalyst with other reported similar works. Sample
Onset potential μ(mV)
Tafel slope (mV dec-1)
Reference
MoS2/rGO
125
98
[34]
MoS2/CNT-GO
140 potential(mV)
100
[35]
MWCNTs@Cu@MoS2
146
62
[32]
Cu-MoS2/rGO
126
90
[28]
Mn-MoS2/rGO
110
76
This work
S0169-4332(17)31878-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.223 APSUSC 36423
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-5-2017 19-6-2017 21-6-2017
Please cite this article as: Liqian Wu, Xiaobing Xu, Yuqi Zhao, Kaiyu Zhang, Yuan Sun, Tingting Wang, Yuanqi Wang, Wei Zhong, Youwei Du, Mn doped MoS2/reduced graphene oxide hybrid for enhanced hydrogen evolution, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.223 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.
Manuscript number: APSUSC-D-17-04931 Revision (the parts with red words have been revised)
Mn doped MoS2 / reduced graphene oxide hybrid for enhanced hydrogen evolution
Liqian Wua, Xiaobing Xua, b, Yuqi Zhaoa, Kaiyu Zhanga, Yuan Suna, Tingting Wanga, Yuanqi Wanga, Wei Zhonga,* and Youwei Dua a) Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing, 210093, China. b) College of electronic Engineering, Nanjing Xiaozhuang University, Nanjing, 210017, China. *
Corresponding author, E-mail: [email protected]
Graphical abstract
Highlights:
In this study, hybrid catalyst of Mn doped MoS2 / reduced graphene oxide(Mn-MoS2/rGO) was easily synthesized by the hydrothermal method.
Effect of Mn-MoS2/rGO hybrid for the HER was systematically investigated for the first time.
The hybrid material, as an electro-catalyst, exhibits excellent catalytic activity
and good stability for the HER in acidic medium.
The excellent HER performance is due not only to high conductivity mainly from reduced graphene oxide, and more importantly to fundamental catalytic activity enhancement generated by Mn doping into MoS2. The hybrid material of Mn-doped molybdenum disulfide / reduced graphene
oxide (Mn-MoS2/rGO) is successfully fabricated through a one-pot hydrothermal method. The hybrid material, evaluated as electrochemical catalyst for the hydrogen evolution reaction (HER), exhibits improved catalytic activity and good stability for the HER in acidic medium with small overpotential (approximately 110 mV). Through analyses, it can be concluded that the improvement for electrochemical HER performance of the hybrid catalyst is ascribed not only to high conductivity mainly from reduced graphene oxide, but also to fundamental catalytic activity enhancement generated by Mn ions doping into the S-edge of MoS2 with a portion of Mo ions replacement. These results verify the facilitation effect of Mn-doping on HER activities, and the Mn-MoS2/rGO hybrid prepared in this work could be employed as a promising alternative to noble metal catalysts in HER.
Keywords: HER, Molybdenum disulfide, Mn-MoS2/rGO hybrid, S-edge of MoS2 1. Introduction With global warming and the urgent need for renewable energy sources as viable alternatives to fossil-fuel-based technologies, hydrogen as a clean energy carrier has gained growing attention [1, 2], and it can be sustainably produced through the electrochemical splitting of water [3, 4]. The hydrogen evolution reaction (HER) acts as a remarkable technique in electrochemical water splitting, and Pt-group metals are recognized as the best electrocatalysts. However, due to that the high cost of Pt-group metals hindered their wide application in hydrogen production, efficient and cheap earth-abundant HER catalysts are needed for intensive investigation [5-6]. Recently, the two-dimensional (2D) layered transition-metal dichalcogenide
semiconductors (especially MoS2, which Gibbs free energy of adsorbed atomic hydrogen is close to that of Pt-group metals (i.e., ΔGH ≈ 0)), have been widely studied as promising hydrogen evolution catalysts [7-9]. Generally, there are three strategies to improve the electrocatalytic ability of MoS2 catalysts [10]: (i) increasing the number of exposed active sites, (ii) improving the electrical contact to active sites and (iii) enhancing the intrinsic activity of each active site. Following these strategies, a great number of efforts have been made for improving the HER activities of MoS2. For example, on the account of that the active sites of MoS2 are located at the edges sites and the basal plane is inert, Yan et al designed ultrathin MoS2 nanoplates developed by a facile solvent-dependent control route to expose active edge sites, leading to a great improvement of the HER activity [11]. Also, Xie et al prepared defect-rich MoS2 ultrathin nanosheets [12] and MoS2 nanowall [13]. Due to that rich defects can make partial cracking of the catalytically inert basal planes, further resulting in exposure of additional active edge sites, both the defect-rich MoS2 ultrathin nanosheets and nanowall exhibit excellent HER activity. Considering the intrinsic low electrical conductivity of MoS2, efforts were adopted to engineer hybrids with other highly conductive materials, including carbon cloth [14], carbon nanotubes [15], carbon nanopapers [16], carbon nanofibers [17], graphene [18], MoC2 [19], which can improve the electron transfer ability. Furthermore, combine above two key factors concerning the active sites and conductivity of MoS2, the synthesis of oxygen-incorporated MoS2 ultrathin nanosheets with a moderate degree of disorder was also realized [20]. The disordered structure can offer abundant unsaturated sulfur atoms as active sites for HER, while the oxygen incorporation can effectively regulate the electronic structure and further improve the intrinsic conductivity, which contribute to the dramatically enhanced HER activity. Likewise, in order to enhance the intrinsic activity of each active site, various promoters can also be considered. Since the Mo-edge was predicted to be the active edge for HER [21], transition metal (such as Co, Ni and Fe) doping into the S-edge of MoS2 was proposed as one effective method for modifying the edge site activity for HER [22]. And it has proven theoretically and experimentally that the introduction of
Co, Ni, Fe or Cu into MoS2 could improve its HER electrocatalytic activity [23-26]. Recently, Tsai et al reported that the DFT-calculated Gibbs free energy of adsorbed atomic hydrogen(ΔGH) of Mn-doping into MoS2 S-edges is closer to thermo-neutral than the non-doping MoS2 Mo-edges [27], and it is therefore approached that Mn-doped MoS2 could also improve the HER activity over pristine MoS2. Accordingly, inspired by these, we rationally designed and successfully synthesized hybrid electrocatalyst of Mn-MoS2/rGO by one-pot hydrothermal method for the first time. The results show that the hybrid is indeed an efficient catalyst for electrochemical hydrogen evolution and the effects of Mn-doping have also been systematically investigated. 2. Experimental Section 2.1 . Reagents and materials All chemicals were reagent grade and used as received without further purification. Manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea ((NH2)2CS), ethanol, and graphene oxide (GO) were purchased from Aladdin Reagent (Shanghai, China), Macklin Biochemical Reagent (Shanghai, China) and advanced material supplier, respectively. High purity distilled water was used in this work. 2.2 . Synthesis of Mn-MoS2/rGO hybrid As shown in Scheme 1, the Mn-MoS2/rGO hybrid was synthesized by a simple general hydrothermal reaction of Mn(CH3COO)2·4H2O, (NH4)6Mo7O24·4H2O and (NH2)2CS in GO aqueous solution.
In a typical batch, 0.1 mmol of
(NH4)6Mo7O24·4H2O/ Mn(CH3COO)2·4H2O
and 3 mmol of (NH2)2CS were
dissolved into 30 mL of 0.2 mg/mL GO dispersion with vigorous stirring for 1 h. Afterwards, the resulting mixture was transferred into a 50 mL Teflon-lined autoclave, which was heated at 220 °C for 24 h. After cooling to room temperature naturally, the black products were collected and washed carefully with distilled water and ethanol for several times. Finally, the products were dried in a vacuum oven at 60 °C for 12 h. For aforementioned samples, non-doped and doped MoS2/rGO hybrids were labelled as MoS2/rGO, Mn-MoS2/rGO, respectively. Additionally, for comparison, the pristine
MoS2 was prepared by the same experiment procedure neither Mn(CH3COO)2·4H2O nor GO dispersion in the hydrothermal solution, and the pristine MnS was also prepared by the same experiment procedure neither (NH4)6Mo7O24·4H2O nor GO dispersion. 2.3. Characterizations In this work, Powder diffraction data were collected from 10° to 80° in 2θ using an X-ray diffractometer (TD-3500, China) with Cu Kα (λ = 1.5418 Å) radiation. The morphology of the samples was examined by transmission electron microscopy (TEM) (Model JEOL-2010, Japan) and field-emission scanning electron microscopy (FE-SEM) (FEI Helios600i, USA). The element mapping was carried out by an energy dispersive spectrometer (EDS) (Oxford X-MAX 50, UK). X-ray photoelectron spectrum (XPS) was measured on a PHI 5000 VersaProbe (UIVAC-PHi, Japan) using Al Kα radiation (1486.6 eV). The specific surface area of sample was investigated by nitrogen adsorption/desorption measurement on a TriStar 3000 surface area analyzer (Micromeritics, USA) using Brunauer-Emmett-Teller (BET). 2.4. Electrochemical measurements Electrochemical measurements were carried out in a standard three-electrode system on an electrochemical workstation (CHI660D) at room temperature. The system consists of a glassy carbon electrode (GCE, 3 mm in diameter) as the working electrode, Pt wire electrode as counter and Ag/AgCl (in 2.5 M KCl solution) as reference electrode, respectively. For the glassy carbon (GC) electrode modified by Mn-MoS2/rGO as the working electrode, 5 mg of products were dispersed in 1 mL of N, N-dimethylformamide (DMF) with 40 μL of Nafion solution (0.5 wt%) by sonication for 1h to form a homogeneous ink. Then, 5 μL of the dispersion was loaded onto the GC until the suspension was dried. Linear sweep voltamperometry (LSV) was performed in 0.5 M H2SO4 solution with a scan rate of 5 mV s-1 to evaluate the HER performance of different catalysts. Cyclic voltammetry (CV) was conducted with different scan rates (20-200 mV s-1) in the potential range from 0.1-0.2 V (vs. the reversible hydrogen electrode (RHE)) to investigate the electrochemical double-layer capacitance. Electrochemical impedance spectroscopy
(EIS) was performed by applying an ac amplitude of 0.5 mV in the frequency from 106 to 10-2 Hz. Furthermore, all tests were carried out in N2-saturated 0.5 M H2SO4 solution to get rid of the dissolved oxygen. And all the potentials reported in our manuscript were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.209+0.059 pH) V. 3. Results and discussion The XRD patterns of the pristine MoS2, un-doped and Mn-doped MoS2 / rGO hybrids are showed in Fig. 1. It can be seen that all as-synthesized samples show nearly identical characteristics peaks, which can be indexed to hexagonal MoS2 (2H-MoS2, JCPDS 37-1492). However, the (002) diffraction peaks of the graphene at 2=24.5 in the XRD patterns of the hybrids can be also hardly detected, which indicates that the graphene nanosheets seldom stack during the hydrothermal process [28], while they act as novel substrates for the nucleation and subsequent growth of MoS2 [29]. Furthermore, remarkably, with the addition of Mn, there are no remarkable changes in the XRD patterns and no additional peak indexed to MnS (JCPDS 65-0891) or to any other secondary phases can be identified, indicating that Mn might be doped into the MoS2 crystals. Additionally, as a comparison, the as-prepared MnS is displayed in Fig. S1, and from Fig. S1, it is observed that the MnS possesses pure phase and perfectly matches with those recorded in PDF standard card of JCPDS 65-0891. Based on FE-SEM images, the morphology of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO were investigated. As shown in Fig. 2a, the pristine MoS2 exhibits a flower-like morphology with serious agglomeration and stacking of MoS2 sheets, which leads to less exposed active sites and inferior activity [30]. Fig. 2b displays that, in MoS2/rGO, MoS2 nanosheets crosslink with each other growing on the surfaces of graphene sheets. And, in Mn-MoS2/rGO hybrid (shown in Fig. 2c), MoS2 nanosheets with interconnected ripples and curved edges are anchored on the graphene sheets; furthermore, MoS2 nanosheets possess smaller sizes, which would be conducive to the exposure of more active edge sites. In order to know accurately actual surface area
of the samples, low temperature N2 adsorption and desorption isotherms are carried out as shown in Fig. S2. The BET special surface area of the Mn-MoS2/rGO hybrid is 63.65 m2 g-1, which is slightly higher than that of MoS2/rGO (50.08 m2 g-1), much larger than that of MoS2 (17.22 m2 g-1). The experimental results of the specific surface areas are consistent with those of the morphology observed by FE-SEM (shown in Figs. 2 b and c). For further observation of Mn-MoS2/rGO hybrid, TEM and high-resolution TEM (HRTEM) analyses were performed, as shown in Figs. 2d and e. It can be observed that ripples and corrugations are rather well-defined with MoS2 represented by the dark-contrast objects, whereas rGO by the gray background, forming highly integrated structure, which can improve electron transfer in electrochemical reactions and the efficiency of catalytic. For HRTEM image of Mn-MoS2/rGO hybrid, it clearly shows that the interlayer distance of (002) is about 0.61 nm, which is close to the standard value of MoS2 (0.62 nm) [25]. It is also in accordance with the experimental result reported by Wang et al that inserting a small amount of Mn ions in MoS2 cannot cause the distortion of MoS2 layered structures [31]. Similarly, the crystallinity of the samples changes little before and after Mn doping via the contrast between the HRTEM image of Mn-doped MoS2 (Fig. 2e) and that of none-doped MoS2 (Fig. S3) grown on rGO, which is consistent with the XRD observation that there are no remarkable changes for the diffraction peaks of samples before and after Mn doping. In addition, the EDX-mapping (Fig. S4) validates the existence of C, Mo, S, O, and Mn elements, and all the elements have a homogeneous distribution in Mn-MoS2/rGO hybrid. The aforementioned (the HRTEM image and EDX-mapping) experiment results all confirm manganese atoms are successfully incorporated into MoS2, including its surface edge sites, which may affect the HER activity of the hybrid. X-ray photoelectron spectroscopic (XPS) measurements were carried out to further investigate the chemical composition of Mn-MoS2/rGO hybrid. As is shown in the survey spectrum in Fig. 3a, the elements of C, Mo, S, and O can be clearly identified. Fig. 3b shows that the Mn 2p XPS spectra of Mn-MoS2/rGO hybrid, and the characteristic peak at 641.2 eV, corresponding to Mn 2p3/2, ascribes to Mn2+. Fig.
3c depicts Mo 3d and S 2s XPS spectra of Mn-MoS2/rGO hybrid, where the characteristic peaks located at 229.7 and 232.8 eV are ascribed to Mo 3d5/2 and 3d3/2 of tetravalence molybdenum ion (Mo4+), respectively, and the emergence of the two peaks at 232.8 and 235.9 eV are attributed to the existence of a small amount of Mo6+ due to the explosion of MoS2 in air [18]. Otherwise, the peak at 226.9 eV corresponds to S 2s of MoS2. For S 2p region, which is shown in Fig. 3d, the peaks at 162.3 and 163.4 eV can be assigned, respectively, to the S 2p3/2 and 2p1/2 orbitals of divalent sulfide ions (S2-), while the other two peaks at 162.7 and 163.9 eV manifest the presence of bridging S22-, which may result from incorporation of manganese atoms and is considered as an important factor to improve the HER activity [25-26]. In addition, the quantification of the XPS peaks reveals that the atomic ratio of Mn/Mo in the Mn-MoS2/rGO is 1/32. For evaluating electrochemical performance of Mn-doped serial samples, electrochemical workstation was used to investigate the HER activity by a typical three-electrode system. Polarization curves of all the samples were shown in Fig. 4a. As a reference, the Pt/C catalyst exhibits high HER activity with an extraordinary low overpotential and a high current density, while the bare GC catalyst embodys catalytically inactive HER activity with a trivial current provided. For the HER performance of serial samples, Mn-MoS2/rGO catalyst exhibits better HER activity (with a small onset overpotential () of 110 mV) than those of the pristine MoS2 and MoS2/rGO, as shown in Table 1. Moreover, Mn-MoS2/rGO catalyst also displays a cathodic current density of 4.52 mA cm-2 at 200 mV, which is 4 times larger than that of MoS2/rGO (i.e. 1.15 mA cm-2), and 17 times larger than that of pristine MoS2 (i.e. 0.27 mA cm-2), respectively, as shown in Table 1. The corresponding Tafel plots of these catalysts derived from the polarization curves (shown in Fig. 4a) fit well with the Tafel equation (η=b log (j0) + a, where j0 is the exchange current density and b is the Tafel slope) in different overpotential ranges, and only the linear portions are selected to give a clear comparison. As shown in Fig. 4b, Mn-MoS2/rGO exhibits a smallest Tafel slope (about 76 mV per decade), except the Pt/C electrode (41 mV per decade). At the same time, the Tafel slopes of the
pristine MoS2, MoS2/rGO are 135 and 105 mV per decade, respectively. It is well known that the smaller Tafel slope means a faster increase of HER rate with the increasing potential [32], and thus Mn-MoS2/rGO catalyst displays the best activity. In addition, the Tafel slope is also an inherent property of the catalyst that is determined by the rate-limiting step of the HER. In acidic aqueous, three possible reaction steps have been suggested for the HER [33]. First is the discharge reaction (Volmer reaction): H3O++e-Hads+H2O (1). Second is electrochemical the desorption step (Heyrovsky reaction): Hads +H++e-H2 (2), or the recombination reaction (Tafel reaction): Hads+ HadsH2 (3). When a fast discharge reaction (1) is followed by a rate limiting combination reaction (3), the Tafel slope is 30 mV dec-1. When (1) is fast and followed by a slow electrochemical desorption (2), a Tafel slope of 40 mV dec-1 is obtained. Furthermore, if (1) is rate-limiting, the Tafel slope amounts to 120 mV dec-1. In our present work, the observed smallest Tafel slope values is 76 mV dec-1 for Mn-MoS2/rGO catalyst, which suggests primary discharge and electrochemical desorption are the rate-limiting step. By fitting j-E data to the Tafel equation [33], apart from Tafel slope above mentioned, the exchange current density j0, which can signify the most inherent measure of activity for the HER, is also obtained. As is shown in Table 1, Mn-MoS2/rGO catalyst, as the most active catalyst, shows the largest exchange current density of 24.2 A cm-2. And not only that, compared with previously reported MoS2 based composite catalysts (see Table 2 for details), the sample of Mn-MoS2/rGO exhibits excellent electrochemical properties in our work. The effect of Mn content on the activity of HER has also been discussed and the results are shown in Fig. S5. As shown in the polarization curves and Tafel plots, with the increasing Mn content, the HER activity of Mn-doped MoS2 / rGO samples is firstly increased, and reaches the maximum value at Mn : Mo= 1 : 32, then drops down afterwards, in accordance with the effect of other transition-metal (Co, Ni)
content on the HER activity [25,26]. To understand better effect of Mn-MoS2/rGO hybrid as enhanced electrocatalyst for the hydrogen evolution reaction, the turnover frequency (TOF) of H2 molecules evolved per second (represented as units of s-1) for each active site was measured, and can be calculated by the Jaramillo's method [7, 36]. For this direct site-to-site comparison, the TOF values of the pristine MoS2 and MoS2/rGO (as shown in Table 1) are calculated to be 0.029 and 0.042 s-1, respectively. Whereas, the Mn-MoS2/rGO shows the highest TOF value of 0.50 s-1, which means that the latter possesses a higher intrinsic activity of each active site. Fe, Co, Ni, and Cu doped MoS2 catalysts, exhibiting more excellent HER than pristine MoS2, have been demonstrated to preferentially substitute at the S-edge with 100% replacement of the Mo atoms in many cases, theoretically and experimentally [27, 37]. The enhancement of the intrinsic activity of each active site is also possibly attributed to Mn ions doping into the S-edge of MoS2 with a portion of the Mo ions replacement, as shown in Fig. 5, which is also accordance with the DFT-calculated results of Tsai et al [27]. Besides, the catalytic activity of as-prepared MnS is much less than that of the other catalysts in this work (as shown in Fig. S5). It excludes the influence of the formation of MnS phase on HER activity of Mn-MoS2/rGO hybrid catalyst, and further demonstrates that the significant enhance of HER activity of Mn-MoS2/rGO hybrid should be attributed to Mn ions doping into MoS2. In brief, the normally less active S-edges become more active in the presence of Mn, while the total amount of active sites also increases, which result in improving HER activity for hybrid. These results also verify experimentally that Mn doped MoS2 can promote the HER activity, apart from other transition metals such as Fe, Co, Ni, and Cu.
Furthermore, the double layer capacitances (Cdl) were measured to evaluate the effective active surface area of catalysts [38], as shown in Fig. 6a. By plotting the △j= ja-jc at a given potential (0.15 V vs. RHE) against the CV scan rates, the double layer capacitances (Cdl) for each samples are extracted. As shown in Fig. 6b and Table 1, the Cdl for the Mn-MoS2/rGO exhibits the largest Cdl (17.37 mF cm-2). It indicates the
highest exposure of effective active edge sites from the Mo-edge or the Mn-doped S-edge, and thus catalytic performance of Mn-MoS2/rGO could greatly improves. In order to further evaluate the performance of the series catalysts in HER processes, EIS measurements were also performed for various composite catalysts. The corresponding Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, are shown in Fig. 6c, while we expanded the scale pertaining to that portion of intersection of the semi-circle on the x-axis shown in the insert of Fig. 6c. It is clearly revealed that MoS2/rGO, and Mn-MoS2/rGO catalysts show much smaller radius of semicircle in the Nyquist plots, compared with MoS2. These reveal that hybrids possess higher conductivity, which is ascribed to the synergetic effect between MoS2 and rGO by mutual interface. Furthermore, due to the effect of Mn doping, the charge transfer resistance (Rct) of Mn-MoS2/rGO (1.5 ), is smaller than that of MoS2/rGO (8.5 ), as shown in Table 1, which further improves the electron transport ability and enhances the HER activity of Mn-MoS2/rGO. In addition to the excellent HER activity and relevant analysis mentioned above, durability of catalysts, as another important parameter to assess electrocatalyst, was measured. Mn-MoS2/rGO catalyst was cycled continuously for 1000 cycles, and in the end of cycling, the cathodic current density of Mn-MoS2/rGO catalyst decreased slightly, as shown in Fig. 6d, indicating that the Mn-MoS2/rGO composite has stable HER activity. 4. Conclusions In conclusion, Mn-MoS2/rGO hybrid catalyst has been successfully prepared, exhibiting excellent HER activity with a small overpotential of 110 mV, large cathodic currents, and a Tafel slope as small as 76 mV dec-1, superior to those of the pristine MoS2 and MoS2/rGO. The prominent HER performance is due not only to high conductivity mainly from reduced graphene oxide, more importantly to fundamental catalytic activity enhancement generated by Mn doping into MoS2. These results verify that the Mn-MoS2/rGO hybrid prepared in this work can be employed as a promising alternative to noble metal catalysts in HER.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.11474151), the National Key Project for Basic Research (Grant No. 2012CB932304), and PAPD, People’s Republic of China.
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Figure captions Fig. 1. XRD patterns of the pristine MoS2, un-doped and Mn-doped MoS2/rGO hybrids. Fig. 2. FE-SEM images of the pristine MoS2 (a), MoS2/rGO (b), Mn-MoS2/rGO (c), TEM(d) and corresponding HRTEM(e) images of as-prepared Mn-MoS2/rGO. Fig. 3. (a) XPS survey spectrum, (b) high-resolution Mn 2p, (c) Mo 3d-S 2s, and (d) S 2p of the Mn-MoS2/rGO hybrid. Fig. 4. (a) Polarization curves, (b) corresponding Tafel plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO. Fig. 5. The ball model of Mn-MoS2/rGO. Color code for the ball model: S, yellow; Mo, green; Mn, purple; C, black; O, red; H, white. S included by red bracket represent the S-edge of Mn-doped MoS2; Mo included by yellow bracket represent the Mo-edge of MoS2. Fig. 6. Cyclic voltammograms of (a) Mn-MoS2/rGO, at different scan rates (20-200 mV s-1) used to (b) estimate the Cdl and relative electrochemically active surface area. (c) Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, (d) the durability test for Mn-MoS2/rGO hybrid.
(002) (103)
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Fig. 1. XRD patterns of the pristine MoS2, un-doped and Mn-doped MoS2/rGO hybrids
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Fig. 2. FE-SEM images of the pristine MoS2 (a), MoS2/rGO (b), and Mn-MoS2/rGO (c); TEM (d) and corresponding HRTEM (e) images of as-prepared Mn-MoS2/rGO.
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Fig. 3. (a) XPS survey spectrum, (b) high-resolution Mn 2p, (c) Mo 3d-S 2s, and (d) S 2p of the
Mn-MoS2/rGO hybrid.
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1.6
2.0
Fig. 4. (a) Polarization curves, (b) corresponding Tafel plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO.
Fig. 5. The ball model of Mn-MoS2/rGO. Color code for the ball model: S, yellow; Mo, blue; Mn, purple; C, black; O, red; H, white. S included by red bracket represent the S-edge of Mn-doped MoS2; Mo included by yellow bracket represent the Mo-edge of MoS2. Fig. 5. The ball model of Mn-MoS2/rGO. Color code for the ball model: S, yellow; Mo, green; Mn, purple; C, black; O, red; H, white. S included by red bracket represent the S-edge of Mn-doped MoS2; Mo included by yellow bracket represent the Mo-edge of MoS2.
(a)
(b)
Mn-MoS2/rGO
4
MoS2/rGO
3
3.0
2
Mn-MoS2/rGO -2
17.37 mF cm
2.5
j0.15V(mA cm-2)
j (mA cm-2)
MoS2
3.5
200mV/s
1 0
2.0
20mV/s
-1
1.5
-2
-2
8.55 mF cm
1.0
-3
0.5
-4 -5 0.10
-2
1.18 mF cm
0.0 0.12
(c) 120
0.20
0.18
0.16 0.14 Potential (V vs RHE)
0
20
40
60
80 100 120 140 160 180 200 220 Scan Rate(mV/s)
(d) 0
MoS2 MoS2/rGO
100
Mn-MoS2/rGO
60
Current (mAcm-2)
-Z''
80 25
20
40
15
10
20
-20
initial after 1000 cycles
5
0
0
-10
0
20
40
5
60 Z'
10
15
80
20
100
25
30
120
-30 -0.4
-0.3
-0.2 -0.1 Potential ( V vs.RHE)
0.0
0.1
Fig. 6. (a) Cyclic voltammograms of Mn-MoS2/rGO at different scan rates (20-200 mV s-1), (b) estimated the Cdl and relative electrochemically active surface area, (c) Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, (d) the durability test for Mn-MoS2/rGO hybrid. Fig. 6. Cyclic voltammograms of (a) Mn-MoS2/rGO, at different scan rates (20-200 mV s-1) used to (b) estimate the Cdl and relative electrochemically active surface area. (c) Nyquist plots of the pristine MoS2, MoS2/rGO, and Mn-MoS2/rGO, (d) the durability test for Mn-MoS2/rGO hybrid
Scheme 1. The fabrication process of Mn-MoS2/rGO hybrid. Scheme 1 The fabrication process of Mn-doped MoS2/rGO hybrids.
Table 1. Electrochemical parameters of the pristine MoS2, MoS2/rGO, Mn-MoS2/rGO. sample
ja
Tafel slope
j0
TOF
Cdl
Rct
(mV)
(mA cm-2)
(mV dec-1)
(10-5A cm-2)
) -1) (S
(mF cm-2)
()
135
cm-2) 1.38
0.029
1.18
70
2.00
0.042
8.55
8.5
2.42
0.050
17.37
1.5
MoS2
270
0.27
MoS2/rGO
165
1.15
Mn-MoS2/rGO
110
4.52
a
105 76
Cathodic current density was recorded at = 200 mV.
Table 2. Comparison of HER activity of the as-prepared Mn-MoS2/rGO composite catalyst with other reported similar works. Sample
Onset potential μ(mV)
Tafel slope (mV dec-1)
Reference
MoS2/rGO
125
98
[34]
MoS2/CNT-GO
140 potential(mV)
100
[35]
MWCNTs@Cu@MoS2
146
62
[32]
Cu-MoS2/rGO
126
90
[28]
Mn-MoS2/rGO
110
76
This work