Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST
Article
Fluorine and Nitrogen Co-doped MoS2 with a Catalytically Active Basal Plane Yuanzhe Wang, Shanshan Liu, Xian-Feng Hao, Junshuang Zhou, Dandan Song, Dong Wang, Li Hou, and Faming Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06795 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Fluorine and Nitrogen Co-doped MoS2 with a Catalytically Active Basal Plane Yuanzhe Wang†, Shanshan Liu†, Xianfeng Hao†, Junshuang Zhou, Dandan Song, Dong Wang, Li Hou, Faming Gao* Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, P. R. China. *Corresponding author. E-mail:
[email protected]. †These authors contributed equally to this work. KEYWORDS:Fluorine and nitrogen, MoS2, active basal plane, electrocatalyst, hydrogen evolution reaction
ABSTRACT: Two-dimensional molybdenum disulphide (2D MoS2) has drawn persistent interests as one of the most promising alternatives to Pt catalysts for the hydrogen evolution reaction (HER). It is generally accepted that the edge sites of 2D MoS2 are catalytically active but the basal planes are inert. Activating the MoS2 basal plane is an obvious strategy to enhance the HER activity of this material. However, few approaches have sought to activate the basal plane. Here, for the first time, we demonstrate that the inert basal planes can be activated via the synergistic effects of nitrogen and fluorine co-doping. Our first-principles calculations reveal that nitrogen in the basal plane of the fluorine and nitrogen co-doped MoS2(NF-MoS2) can act as a new active and further tuneable catalytic site. The as-prepared NF-MoS2 catalyst exhibited an enormously enhanced HER activity compared to pure MoS2 and N-doped MoS2 due to the chemical co-doping effect. This work will pave a novel pathway for enhancing the HER activity using the synergistic effects of chemical co-doping.
INTRODUCTION Due to its environmental friendliness and high energy density, hydrogen is one of the most ideal energy candidates to replace the fossil fuels.1 Hydrogen production by electrochemical water splitting is one sustainable route to a potential source of clean energy.2 In water-splitting devices, a high-performance catalyst for the hydrogen evolution reaction (HER) is necessary.3 The most efficient catalysts are Pt-based. However, the large-scale utilization of platinum for hydrogen production is seriously hindered by its costliness and the limited Pt resources.4 Thus, it remains a crucial task for ongoing research to uncover various innovative approaches for the pursuit of highly active HER catalysts based on the low-cost, earth-abundant materials. Over the past few decades, various earth-abundant HER electrocatalysts, such as non-noble metal alloys, phosphides, chalcogenides, borides, carbides, ni-
trides and oxides have been continually explored.5 Recently, nanosized MoS2 electrocatalysts have received tremendous interests, due to their 2D permeable channels for ion transportation and their potential catalytic activity.6 Both experimental and theoretical studies have indicated that the HER activity of nanosized MoS2 correlates with the number of the unsaturated Mo and S sites along the edges, whereas their basal planes are catalytically inert.7,8 Therefore, exposing the maximum number of edge sites and improving the intrinsic activity of the edge sites by chemical doping have become main strategies to enhance the HER activity of MoS2. Few studies have focused on making use of the inert basal plane. To fully utilize the basal plane,the development of activating approaches for the basal plane of 2D MoS2 materials is highly desirable.9 Once the inert basal plane sites are activated, the catalytic activity of 2D MoS2 will be considerably enhanced. Very recently, the experimental attempt by Li et al. found that the inert basal plane of MoS2 could be activated by introducing sulfur vacancies.10 Liu et al. demonstrate that P dopants could be the new active sites in the basal plane of MoS2.11 However , the synergy effect between different non-metallic elements for MoS2 is few studied . Herein, we report the first successful activation of the inert basal plane of 2D MoS2 via chemical co-doping.
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 1. (a) Schematic illustration of the active sites in MoS2; (b) HER freeenergy diagrams for different sites in the basal plane; (c) Partial chargedensity distribution of a single N-MoS2 monolayer with H adsorption; (d) Partial charge-density distribution of a single N,F-MoS2 monolayer with H adsorption; (e) Band structures of pristine MoS2; (f) Band structures of N,F-doped MoS2
Recently, we found that fluorine and nitrogen co-doped carbon microspheres (CM-NF) possess higher specific volumetric capacitance than nitrogen doped carbon microspheres.12 The ultrahigh capacitance of the CM-NF electrode was attributed to the synergistic effects of fluorine and nitrogen doping on the electronacceptor/donor properties. Furthermore, Rao et al. demonstrate substitution of N and F in oxides brings about major changes in the electronic structure and properties, exhibiting good visiblelight induced hydrogen evolution activity.13-16 To probe the synergistic effects of fluorine and nitrogen doping in the HER of 2D MoS2, systematic density functional theory (DFT) calculations are performed (calculation methods and models described in the Supporting Information). First, the 2H-MoS2 is used as a model and the negative formation energies of N- and F- substituted S on the same Mo atom were small, suggesting that N and F can easily substitute S on the same Mo atom ( Table S1, S2 , Figure S1). Theoretically, the hydrogen adsorption free energy (∆GH*) is a good indicator for HER catalysts, as defined in the Experimental section. An optimum ∆GH* value close to zero suggests high catalytic activity. To obtain deep insights on the functions of N and F on the inert basal plane of MoS2, we calculated the free energy change (∆GH*) of hydrogen adsorption on the basal plane of pristine MoS2 , N-doped MoS2 and NF-MoS2, as shown in Figure. 1b , Figure. S2 and Table. S3. The ∆GH* values of the Mo site and S site on the basal plane of pristine MoS2 monolayer were -0.938
Page 2 of 7
eV and 1.885 eV, respectively. These ∆GH* values are far away from zero, indicating difficult hydrogen desorption processes on the Mo site and the S site on the basal plane. The ∆GH* values of the Mo site, S site and N site on the basal plane of a N-doped MoS2 monolayer were -0.902 eV ,0.728 eV and -1.539 eV, respectively, which are also far away from zero. Thus, we concluded that the basal planes of pristine MoS2 and N-doped MoS2 are inactive in HER catalysis, which is in agreement with previous research.17 For the abovementioned NF-MoS2 structure, after substituting the S atoms surrounding the same Mo atom by N and F, the Mo site and the surrounding S sites on the basal plane presented greater ∆GH* values of -0.766 eV,1.745 eV,1.701 eV, indicating the Mo site and the S sites were not catalytically active sites yet. Surprisingly, the calculated ∆GH* value for the N site on the basal plane of NF-MoS2 was only -0.145 eV, which is close to zero, much better than the other atoms on the basal plane (Figure. 1b) ,indicating that the N site on the basal plane may provide fast proton/electron adsorption as well as fast hydrogen release. This case is in sharp contrast to that of the N site on the basal plane of N-doped MoS2 .17 As seen in Figure. 1c,d, the optimized charge distribution results in an ideal value of ∆GH* in NF-MoS2 , owing to its different electronegativity.18Our theoretical calculations indicated a new activating mechanism: F atom induce the activation of the N site on the basal planes. In other words, the basal plane of NF-MoS2 may be regarded as being catalytically activated by the synergistic effects of fluorine and nitrogen co-doping. Furthermore,it can be expected that a greater dosage of N and F atoms will result in higher HER activity. At this point, we have theoretically predicted that nitrogen on the basal plane of NFMoS2 can act as a new active and tuneable catalytic site. In addition, as seen in Figure. 1e,f and Figure S15, the NF-MoS2 showed relatively narrow bandgap compared with pristine MoS2, indicating that fluorine and nitrogen co-doping in MoS2 nanosheets could lead to higher intrinsic conductivity.17 Furthmore, the N-2p states contributed to the top of the valence bands, as presented by the computed density of states for N-doped and N,F-codoped MoS2 (Figure S15). Therefore, fluorine and nitrogen co-doping in MoS2 nanosheets could be an effective solution to activate the inert basal plane for HER. To experimentally verify the synergistic effects of fluorine and nitrogen co-doping on the HER activity of MoS2 nanosheets, we synthesized the few-layer 2D NF-MoS2 by a one-pot hydrothermal method. A distinct enhancement of NF-MoS2 HER activity was achieved compared to pristine MoS2 and N-doped MoS2. The novel strategy of harnessing the synergistic effects between different non-metallic elements can broaden our horizons in the design of new highly efficient catalysts of two-dimensional transition metal sulfides for hydrogen evolution.
ACS Paragon Plus Environment
Page 3 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Figure 2. SEM images of (a) pristine MoS2, (b) N-MoS2 and (c) N,FMoS2. TEM images of (d) pristine MoS2, (e) N-MoS2 and (f) N,F-MoS2. High-resolution images of the edges of (g) pristine MoS2, (h) N-MoS2 and (i) N,F-MoS2.
no impurity peaks of N-MoS2 and NF-MoS2, indicating that the N and F atoms are effectively doped into the MoS2 matrix without forming other phases or changing the crystal structure. To further confirm their structure, Raman spectroscopy was utilized to investigate the vibrational characteristics. As seen in Figure. S8b, the pristine MoS2, N-MoS2 and NF-MoS2 exhibit two characteristic peaks at 378 cm-1 and 402 cm-1, which can be indexed to the inplane Mo-S phonon (E12g) and out-of-plane Mo-S (A1g) vibrational modes,19 respectively. These results are in good agreement with the range of the reference data, indicating the 2H-phase is not damaged by doping.The pristine MoS2, N-MoS2 and NF-MoS2 were further investigated by combined nitrogen adsorptiondesorption isotherms. The specific area of NF-MoS2 determined by the BET method is 27.15 m2 g-1, which is very close to that of the pristine MoS2 (27.76 m2 g-1) and slightly lower than that of NMoS2(31.32 m2 g-1), indicating that the incorporated N and F atoms had little effect on the specific surface area. The pore-size distribution curve of pristine MoS2, N-MoS2 and NF-MoS2 were analyzed using the Barrett-Joyner-Halenda (BJH) method (Figure. S8c). The results demonstrate the existence of mesopores in NMoS2 and NF-MoS2.
Few-layer pristine MoS2, N-MoS2 and NF-MoS2 nanosheets were prepared via a one-pot hydrothermal method (described in details in the Experimental section). Scanning electron microscopy (SEM) images show an overall view of the morphologies of pristine MoS2(Figure. 2a), N-MoS2（ Figure. 2b） and NFMoS2(Figure. 2c). It was found that all the as-prepared samples have similar flower-like morphologies composed of intercrossed nanoflakes with thicknesses of several thin platelike nanosheets. No distinct morphology differences among the three samples were observed, demonstrating that the introduction of N atoms or F atoms does not affect significantly the morphology of the MoS2. Transmission electron microscopy (TEM, Figure. 2d,e,f) images further showed that flower-like microspheres were assembled from a large number of thin rugose-shaped MoS2 nanosheets. HRTEM images of pristine MoS2, N-MoS2 and NF-MoS2 in side view were shown in Figure. 2g,h,i. The N-MoS2 and NFMoS2 consist of similar layers stacking together in the edge areas. Furthermore, the images of N-MoS2 and NF-MoS2 in the basal planes can be observed from the top-view images(Figure. S4a,b), displaying similar morphologies. In addition, the corresponding energy-dispersive X-ray spectroscopy (EDX) mapping identifies the elements and visualizes their distributions in the samples. (Figure. S3,S4 and Table S4)The elements Mo, S, N and F were distributed over the nanosheets, suggesting that N or N and F were successfully introduced and spread in the product. To analyse the charge states and chemical contents, X-ray photoelectron spectroscopy (XPS) was carried out(Figure.S5,S6,S7).20 XPS demonstrates the successful substitutional doping of N and F at the S sites in MoS2. In addition, the F:N ratio is not 1:1, where F is apparently more abundant that N, it may be due to that fluorine substitution is favored in the presence of nitrogen and there were interstitial F atoms presence in the basal plane.16 The crystal structures of pristine MoS2,N-MoS2 and NF-MoS2 were confirmed by XRD measurements (Figure. S8a). There were
Figure 3. (a) Polarization curves of pristine MoS2, N-MoS2, N,F-MoS2, commercial 1T and 2H MoS2 and commercial 20% Pt/C catalysts. (b) Corresponding Tafel plots. (c) Nyquist plots of the different samples over the frequency range of 100 kHz to 0.01 Hz. (d) Cycling stability of N,FMoS2 before and after 1500 CV measurements.The inset shows the timedependent current density curve of the N,F-MoS2 nanosheets under static overpotential for 30,000 s.
To investigate the effects of N and F co-doping in the HER activity in the MoS2 system, the HER performance of NF-MoS2 was investigated in 0.5 M H2SO4 ( saturated with N2) using a standard three-electrode setup. For comparison, the HER performances of the N-MoS2, 20%Pt/C(Alfa Aesar),1T-2H MoS2(Nanjing XFNANO Materials Tech Co.,Ltd 1T:2H=3:2) and pristine MoS2 samples were also shown in Figure. 3a. The NF-MoS2 nanosheets exhibited a small onset overpotential of 110 mV for HER. This onset overpotential is much smaller than that of the other doped nanostructured MoS2 sample and the pristine MoS2, which suggests that the NF-MoS2 product has superior catalytic activity.
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The results above demonstrate that the presence of F atoms in the nanosheets significantly improves the HER performance. Furthermore, the HER activity of NF-MoS2 synthesized at different temperatures(250-450°C) and with different dosages of NH4F were studied. The NF-MoS2 synthesized at 350°C with 0.7 g NH4F showed the best HER activity (Figure. S9 and S10). The HER activity of NF-MoS2 increased with an increasing dosages of NH4F. Moreover, the HER activity of F-MoS2(Figure.S12 and Table S5), NH4Cl-MoS2, NH4Br-MoS2 and NH4I-MoS2 were also studied(Figure. S11 ). A smaller Tafel slopes indicates faster kinetics for efficient HER.21 As shown in Figure. 3b, the Tafel slope of NF-MoS2, NMoS2, and pristine MoS2 were 57, 84 and 164 mVdec-1, respectively. According to the mechanism, the overall HER reaction of NF-MoS2 and N-MoS2 occurring in acidic media could proceed via a discharge step (Volmer-reaction, 120 mV dec-1) followed by either an ion or atom reaction (Heyrovsky reaction,40 mV dec-1) or a combination reaction (Tafel reaction, 30 mV dec-1).NF-MoS2 exhibited the smallest Tafel slope among the three samples. The HER performance of NF-MoS2(onset potential of 110 mV, Tafel slope of 57 mV dec-1) is better than those of transition-metaldoped MoS2 samples ( Figure. S13 and Table S6), indicating that the interaction between the nitrogen and fluorine atoms is helpful to improving the HER performance of MoS2. To investigate the electrode kinetics of the catalysts under HER, electrochemical impedance spectroscopy (EIS) measurements were carried out (Figure. 3c). It is clear that the diameters of the semicircles in the Nyquist plots show the following trend:Rct （ NF-MoS2） ≈ Rct （ N-MoS2）
