Anion Engineering on Free-Standing Two-Dimensional MoS2

Sep 20, 2017 - School and Electronic Information, Huaibei Normal University, Huaibei, Anhui 235000, P. R. China. ‡ High Magnetic Field Laboratory, C...
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Anion Engineering on Free-Standing Two-Dimensional MoS2 Nanosheets toward Hydrogen Evolution Qilong Liu,† Qiangchun Liu,*,† and Xiangkai Kong*,†,‡ †

School and Electronic Information, Huaibei Normal University, Huaibei, Anhui 235000, P. R. China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China

‡

S Supporting Information *

design and explore MoS2-based electrocatalysts on a large scale, with intrinsically improved conductivity and boosting active sites via a simple method. Electronic configuration control plays a pivotal role in strengthening the catalytic performance. However, simultaneous optimization of both the conductivity and active sites for MoS2 electrocatalysts is hindered because of the contradictory relationship between the active sites and conductivity.19 Despite the successful observation of enhancing HER behaviors caused by introducing Co,20 Pt,21 V,22 and other cations, reports about anion engineering, especially on free-standing 2D MoS 2 nanosheets, remain scare.19,23−25 Recently, Yang’s group have reviewed the binary nonmetal−transition metal compounds for HER.26 Meanwhile, Xie’s group have demonstrated that MoN could exhibit efficient hydrogen evolution activity, with high electric conductivity and also classic 2D morphology.27 Keeping this in mind, it is anticipated that nitrogen incorporation may be an effective way of regulating the electronic structure of MoS2, providing the opportunity to modulate the intrinsic conductivity of free-standing 2D MoS2 electrocatalysts. Meanwhile, we envisage that the active sites may also be enriched. On the one hand, the newly formed local active MoN component is active for HER. On the other hand, more edge S atoms could access electrons and be activated because the enhanced conductivity could guarantee more electrons to transport to the lowcoordinated S sites, making them electrochemically active. Density functional theory (DFT) calculations were first implemented on pristine and N-doped MoS2 nanosheets to directly visualize and understand the effect of anion engineering on the HER behavior. Parts A and B of Figure S1 depict the optimized structure for an N-incorporated catalyst, which was constructed from Zakharov’s MoS2 model.28 The S atom adjacent to the doped N atom was selected as the H-adsorbed site (marked in Figure S1B). Figure S2 addresses the calculated density of states for these two catalysts, where N-doped MoS2 possessed a much narrower band gap of 1.24 eV with respect to that of the pristine case (1.45 eV), implying that N incorporation could lead to improved conductivity. The Gibbs free energy change for H (ΔGH*) was calculated to be −0.52 eV on an anionengineered MoS2 nanosheet, which was significantly less negative than −1.15 eV on its pristine counterpart (Figure S1C). |ΔGH*| was a good descriptor for estimating HER, and the optimal value should be close to 0.29 Thus, the theoretical results provided great hope and design guidance for us to study the influence of

ABSTRACT: On the basis of theoretical predictions, nitrogen was designed and incorporated into free-standing two-dimensional MoS2 nanosheets. Both the amount of electrochemical active sites on the surface and its intrinsic conductivity could be significantly increased as a result of anion engineering, which can extremely improve the electrocatalytic kinetics toward hydrogen evolution.

A

s a sustainable, highly efficient, and carbon-neutral energy carrier, hydrogen is fascinating to the scientific community and is recognized as one of the most promising alternatives to diminishing fossil fuels.1,2 Catalytic steam reforming, partial oxidation, and coal gasification are the current major commercial processes to make hydrogen, but the processes are highly complicated and commonly suffer from CO2 emissions, causing serious environmental problems.3 Water electrolysis has gained recognition rapidly for generating hydrogen, which is environmentally benign and can be an efficient aid for good electrocatalysts toward hydrogen evolution reaction (HER).4,5 Although platinum-group materials are the state-of-the-art catalysts for HER, their scarcity and high cost largely limit their practical applicability.6 Hence, exploitation of active earthabundant electrocatalysts that can efficiently expedite sluggish processes and initiate HER at low overpotentials has become the central research attempt.7−9 MoS2 possesses an intrinsically two-dimensional (2D) structure and is of high abundance, with the sandwich-like S− Mo−S layers serving as building blocks.10,11 During the past few decades, MoS2-based nanomaterials have received tremendous attention and thus been intensively studied for HER behavior.12 Unfortunately, the poor electron conductivity of pristine MoS2 causes a significant ohmic drop and decreased electrocatalysis efficiency. Henceforth, conductive-substrate-supported13 or constructed vertically aligned films14 have been designed to enhance the in-plane conductivity and reduce the out-of-plane ohmic loss. Meanwhile, transformation from a 2H phase to a metallic 1T phase is also confirmed to be an effective strategy for improving the HER activity.15,16 Furthermore, the low active site content on the MoS2 surface also causes sluggish kinetics because Chorkendorff and his co-workers have demonstrated that only edge-site S atoms with low coordination could serve as active sites for HER.17 Chen’s group have fabricated defect-rich MoS2/ C hierarchical spheres by a simple microemulsion procedure, which showed a highly active and stable performance toward HER.18 At this juncture, it would be more desirable if one could © 2017 American Chemical Society

Received: July 25, 2017 Published: September 20, 2017 11462

DOI: 10.1021/acs.inorgchem.7b01886 Inorg. Chem. 2017, 56, 11462−11465

Communication

Inorganic Chemistry

incorporation.27 Furthermore, positive shifts were obvious for the (002) peak, and slight negative shifts in the (110) peak were observed, revealing induced lattice distortion, which probably offers more active sites for hydrogen generation. To better characterize the product, Figure S6B addresses the representative Raman spectra, where two peaks located at 379 and 406 cm−1 of the pristine MoS2 are assigned to the in-plane E2g and out-ofplane A1g vibrational modes, respectively.30 Slight negative shifts of 3 cm−1 are observed for both, indicating the influence of N engineering on the crystal structure. Meanwhile, the distance between these two peaks remains unchanged, implying unchanged thickness of the sample. After that, XPS was performed to confirm the chemical state and composition. As displayed in Figure S6C, the characteristic peak from the Mo 3d orbital is located at 229 eV, indicating a dominant 4+ oxidation state. In addition, most S signals arose from a 2p peak around 162 eV, corresponding to a 2− valence state.31 High-resolution XPS was further carried out to verify the introduced N component. Figure S6D addresses the distinct differences for N detection, where an obvious peak appears at 402.3 eV for N-doped MoS2, with a 1− valence state,32 affirming the successful incorporation of an anion into the MoS2 substrate, which is in line with the above discussion. Of note, in order to introduce N into the matrix and, in the meantime, to keep free-standing 2D morphology, it was highly desirable but remained challenging to rationally design the anion engineering technique. First pristine MoS2 nanosheets have been prepared via the previously reported method,19 and then posttreatment was tried via the hydrothermal method. Beyond that, NH3 was employed through a heat treatment at 550 °C. However, neither of them could give a reasonable product. Besides, a one-step preparation also required attention because it might result in nanoparticles if not well-controlled.23 To investigate the effect of anionic N incorporation on MoS2 nanosheets, techniques of linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were used to estimate the intrinsic HER activity. Figure 2A displays HER LSV curves recorded by different catalysts in a 0.5 M H2SO4 electrolyte, with a mass loading of 0.1415 mg cm−2. Sure enough, the blank electrode was almost inactive for HER. Meanwhile, pristine MoS2 showed remarkably weak activity for hydrogen production. Impressively, with the aid of N incorporation, the polarized current density increased rapidly and the HER kinetics exhibited a significant enhancement. The N-doped catalyst illustrated a relatively low onset overpotential of 110 mV, much smaller than that of the pristine case (220 mV). For achieving a current density of 10 mA cm−2, more than 350 mV overpotential was required on the pristine material. In comparison, the anion-engineered case needed a much smaller value of 195 mV. Furthermore, under an overpotential of 250 mV, the reactive current density of the Ndoped case could reach −36 mA cm−2, exhibiting a 35-fold enhancement relative to the pristine counterpart. Moreover, it would be more impressive when correcting the raw data with iR losses. As illustrated by the dashed polarization curves, the cathodic current density could reach −62 mA cm−2 at 250 mV overpotential, which was 60-fold larger compared with the pristine counterpart, verifying the excellent kinetics accelerated by anion engineering toward hydrogen evolution. Control experiments with different amounts of N have also been carried out, with their corresponding HER performance shown in Figure S7. All of the anion-engineered materials exhibited largely improved activity toward electrochemical

catalytic behavior, caused by anion engineering into the MoS2 matrix. Therefore, the incorporation of anionic N into MoS2 nanosheets was carried out by a one-pot hydrothermal process with ammonium molybdate tetrahydrate, thiourea, and hexamethylenetetramine as the Mo, S, and N precursors, respectively. The detailed preparation can be found in the Supporting Information, and the fabrication process is illustrated in Figure S3. The resultant material was denoted as N-doped MoS2. A post-treatment strategy was also tried, but it was difficult to give an ideal product. The amount of typically sample obtained was more than 1 g, as shown in Figure S4; meanwhile, a larger scale could be produced with increased precursors. Energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and elemental analysis data are collected in Table S1 for compositional analysis, providing evidence for N incorporation and revealing that the introduced anion component was ca. 2.7% in atomic ratio. Figure 1A presents a representative scanning electron microscopy (SEM) image of the N-doped MoS2 nanosheets.

Figure 1. (A) SEM, (B) TEM, and (C and D) HRTEM characterizations on the N-doped MoS2 nanosheets.

Some ripples and corrugations could be observed, suggesting a thin feature. Transmission electron microscopy (TEM) characterization was also performed (Figure 1B), demonstrating its free-standing 2D nature with clear edges and uniform size in the range of 10−20 nm. The nanosheets were deposited onto a fresh silicon substrate to obtain the thickness by atomic force microscopy (AFM). Height profiles showed that the nanosheets ranged from about 2 to 4 nm in thickness for individual ones (Figure S5). High-resolution TEM (HRTEM) characterization of the individual flat nanosheets illustrated good crystallinity (Figure 1C,D), which would facilitate fast electron transport and contribute to good stability. As marked, lattice d spacings of 0.310 and 0.268 nm were measured from the parallel lattice fringes, which were close to the 0.307 and 0.274 nm spacings of pristine MoS2, corresponding to (004) and (100) planes, respectively. The slight variation should be a consequence of the distorted structure caused by anion incorporation, implying that more active sites might be induced compared to the pristine case. As shown in Figure S6A, X-ray diffraction (XRD) patterns of 2D MoS2 nanocrystals with and without N modifications are compared to investigate the structural information. The pristine sample exhibited a 2H phase, whose XRD pattern matched well with that of reference MoS2 (JCPDS 73-1508).19 It was worth noting that the pattern basically remained during anion engineering and exhibited evident disparity relative to MoN, suggesting that the 2H phase was not affected by N 11463

DOI: 10.1021/acs.inorgchem.7b01886 Inorg. Chem. 2017, 56, 11462−11465

Communication

Inorganic Chemistry

activated because of the improved conductivity, facilitating more active sites to be accessed by electrons. In summary, on the basis of DFT calculations, we presented a facial strategy for preparing MoS 2 nanosheets with N incorporation on a large scale, which exhibited efficient HER behavior. The enhanced activity could be attributed to three aspects: (1) improved conductivity was caused by N incorporation; (2) more electrons could transfer to and reach edge S atoms, activating the initially inactive sites; (3) a newly formed local MoN component was also active and contributed to HER. This method could be extended to other anion engineering on functional nanomaterials, stimulating their applications in energy and catalysis.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01886. Experimental and calculated methods, schematic illustration, AFM characterizations, LSV tests, and elemental composition analysis (PDF)

Figure 2. Electrochemical measurements. (A) LSV, (B) Tafel plots, and (C) EIS tests on the prepared samples. (D) Durability test at 10 mA cm−2 and (E) CV curves at different scan rates on N-incorporated MoS2. (F) Calculated double-layer capacitance for the samples.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

hydrogen generation. Among them, N-doped MoS2 nanosheets prepared with a molar ratio of 0.2 between N and S precursors displayed the best performance, and thus we focus our discussion on them below. Tafel analysis was helpful for an understanding of the electrocatalytic mechanism. Meanwhile, it was more advantageous for HER catalysis because the current density underwent a sharper increase with a smaller overpotential increase. It showed a Tafel slope of 116 mV dec−1 for pristine MoS2, corresponding to the Volmer mechanism (Figure 2B). When N was incorporated, a much smaller value of 58 mV per decade was obtained, indicating the Volmer−Heyrovsky mechanism, and the chemical desorption process was the rate-limiting step. The enhanced performance was supported by the EIS data, with the Nyquist plots acquired in Figure 2C. An obvious declined chargetransfer resistance (Rct) was observed for N doping compared to that without N incorporation, suggesting the dramatically smaller interfacial Rct and significantly improved conductivity. Meanwhile, the anion-engineered sample exhibited a gradual decay of the potential for maintaining 10 mA cm−2 via a chronopotentiometry test, with 11% loss after 8 h of reaction, indicating good stability of the catalyst (Figure 2D). The electrochemically active surface area (ECSA) was effective for estimating efficient active sites and was measured by running CV at different rates in the region of −0.09 to −0.01 V in the HER process and thus calculated by the extracted current difference at −0.04 V against the potential scan rate (Figure 2E,F). Flat CV curves were obtained, suggesting little occurrence of Faraday reactions, and the collected currents were primarily attributed to the charging/discharging capacitance. Pristine MoS2 had a low double-layer capacitance of 0.027 mF cm−2, which increased by an order of magnitude to reach 0.241 mF cm−2 when N was introduced. Of note, the increased capacitance should be caused by two factors: the incorporated N would induce new active sites because MoN was also a good HER catalyst,27 and a larger party of chemically inactive sites could be

ORCID

Xiangkai Kong: 0000-0002-0552-5557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grant 51602116), Anhui Natural Science Foundation (Grant 1708085QB40), China Postdoctoral Science Foundation (Grant 2016M600492) (to X.K.), and Anhui Provincial Natural Science Foundation (Grant 1508085ME100), Anhui Provincial Natural Science Fund for Colleges and Universities (Grant KJ2017ZD31), key technical project of Huaibei City (Grant 20140311) (to Q.L.). The calculations were completed on the supercomputing system in the Supercomputing Center of USTC.

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DOI: 10.1021/acs.inorgchem.7b01886 Inorg. Chem. 2017, 56, 11462−11465