Atomic Plane-Vacancy Engineering of Transition-Metal

Jun 21, 2019 - Tafel plots (Figure 4c) show that the S vacancies created on the basal plane reduce the Tafel slope from 213 to 64 mV/dec. The electroc...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25264−25270

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Atomic Plane-Vacancy Engineering of Transition-Metal Dichalcogenides with Enhanced Hydrogen Evolution Capability Cong Wei,† Wenzhuo Wu,† Hao Li,† Xiangcheng Lin,† Tong Wu,† Yida Zhang,‡ Quan Xu,*,‡ Lipeng Zhang,§ Yonghao Zhu,§ Xinan Yang,∥ Zheng Liu,⊥ and Qun Xu*,† †

College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China State Key Laboratory of Petroleum Resources and Prospecting, Harvard SEAS-CUPB Joint Laboratory on Petroleum Science, 29 Oxford Street, Cambridge, Massachusetts 02138, United States § College of Engineering, Beijing University of Chemical Technology, Beijing 100029, China ∥ National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ⊥ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

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S Supporting Information *

ABSTRACT: Introducing anion vacancies on two-dimensional transition-metal dichalcogenides (TMDs) would significantly improve their catalytic activity. In this work, we proposed a solid-phase reduction (SPR) strategy to simultaneously achieve efficient exfoliation and controlled generation of chalcogen vacancies on TMDs. Consecutive sulfur vacancies were successfully created on the basal plane of the bulk MoS2 and WS2, and their interlamellar distances were distinctly expanded after the SPR treatment (about 16%), which can be conveniently exfoliated by only gentle shaking. The S-vacancy significantly increases the hydrogen-evolution reaction activity of the MoS2 and WS2 nanosheets, with overpotential of −238 and −241 mV at 10 mA cm−2, respectively. We anticipate that our SPR strategy will supply a general platform for the development of TMD-based electrocatalysts for industrial water splitting and hydrogen production in the near future. KEYWORDS: transition-metal dichalcogenides, solid-phase reduction, vacancy engineering, scalable exfoliation, hydrogen-evolution reaction

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INTRODUCTION Electrochemical water splitting is one of the easiest and cleanest methods for high-purity hydrogen production and an effective way to store the intermittent renewable energy, such as sunlight and wind.1−3 To facilitate adequate water splitting, efficient catalysts for hydrogen-evolution reaction (HER) are essential.4−6 Two-dimensional (2D) transition-metal dichalcogenide (TMD)-based catalysts, with molybdenum disulfide as the most well-known example, are considered as the ideal candidates due to their earth abundance, low cost, chemical stability, and high catalytic activity.7−10 But, for the stable TMDs, only their edge sites exhibit catalytic activity for the hydrogen evolution, while their basal planes, which usually constitute the bulk of the material, are nearly inert.11−13 Recently, Zheng et al. have successfully activated the MoS2 basal plane by creating sulfur vacancies14,15 and the subsequent theoretical calculations indicated that this strategy is also suitable for many other TMDs, such as WS2, MoSe2, WSe2, ZrSe2, etc.16−19 Anion-vacancy-contained TMDs are predicted to exhibit excellent performance toward the HER, even approaching that of platinum.20−22 Subsequently, considerable © 2019 American Chemical Society

efforts including Ar plasma treatment, H2 annealing, and electrochemical reduction have been devoted to generate the chalcogen vacancies in the basal plane of TMDs.23−25 However, Ar plasma treatment14 and H2 annealing23 need delicate equipment, multistep procedure, and high temperature, and are thus unsuitable for large-scale synthesis. Electrochemical reduction was demonstrated an effective desulfurization approach for the MoS2 catalysts but can hardly be extended to other TMD catalysts for its limited operating potential windows and complex side reactions.15 To create sufficient anion vacancies in TMD catalysts for commercial applications, a facile, low-cost, and scalable route to exclude chalcogen atoms is highly needed.26−29 Herein, we employ a solid-phase reduction (SPR) strategy for the fabrication of MoS2 nanosheets (NSs) with sulfur vacancies in the basal plane. NaBH4, a commercially available reducing agent,20 was used to reduce Mo4+ into Moδ+ (δ < 4) Received: May 5, 2019 Accepted: June 21, 2019 Published: June 21, 2019 25264

DOI: 10.1021/acsami.9b07856 ACS Appl. Mater. Interfaces 2019, 11, 25264−25270

Research Article

ACS Applied Materials & Interfaces and exclude the sulfur atoms. Just by changing the applied amount of the reductant, the concentration of S-vacancies can be efficiently tuned. During the desulfurization process, sodium ions would diffuse and enter into the interlayer spaces of MoS2, gaining distinct interlayer expansion of 16%. As a result, the treated MoS2 crystals can be exfoliated into 2D sulfur-vacancy MoS2 NSs by only shaking in the hydrochloric acid solution in the yield that is 5−10 times higher than that from the conventional ultrasonic-assisted process. The as-prepared SvMoS2 exhibit excellent HER electrocatalytic activity with an overpotential of −238 mV at 10 mA cm−2, which is comparable and even superior to generated from the Ar plasma and electrochemically. To demonstrate the generality of the SPR strategy, we also investigated the structure and HER activity of the SPR-generating S-vacancies on WS2 NSs, and significant improvement of HER activity was also demonstrated. Our SPR strategy paves the way for the engineering of scalable defects of 2D TMDs materials, which is anticipated to promote the application of 2D TMD materials in the next-generation energy devices.

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Figure 1. Experimental design and creation of S vacancies in atomiclayered 2H-MoS2. (a) Schematic illustration of the desulfurization process of the MoS2 prepared via SPR strategy. (b) Process of bulk MoS2 was reduced to expanded bulk crystals that can be fast exfoliated into few-layer NSs (shown in the bottle at middle bottom) by only hand-shaking. (c) XRD spectra of MoS2 before (black) and after (blue) SPR treatment with NaBH4.

EXPERIMENTAL SECTION

Synthesis of Sv-MoS2 NSs. The preparation of Sv-MoS2 from bulk MoS2 materials was carried out via a three-step solid-phase reduction and liquid-phase exfoliation process using NaBH4 as the reductant and HCl as the terminating agent. In step I, 1 g of commercial bulk MoS2 and NaBH4 was mixed together via grinding, then dried, and degassed at 60 °C for 30 min under inert atmosphere. Then, the mixture was heated to 380 °C with a heating rate of 10 °C per min for 2 h. In step II, the resultant mixture was gradually poured into 0.1 M HCl solution under vigorous stirring for several minutes. After exfoliation in the acid solution, the dispersions were centrifuged at 5000 rpm for 30 min and the top supernatant was collected by pippette. The collected supernatant was centrifuged again at 20 000 rpm to isolate the exfoliated MoS2 nanosheets. Then, the product was washed with deionized water for 5 times and dried in a vacuum oven at 60 °C. Here, we carried out reactions using different ratios of NaBH4 (x = 0, 0.5, 1, 2, and 3) at 380 °C. Electrochemical Characterizations. Electrochemical measurements were implemented with a standard three-electrode setup (CHI660D) using graphite rod and Ag/AgCl electrode (3.5 M KCl) as counter and reference electrodes, respectively. A glassy carbon electrode with a geometric area of 0.19625 cm2 was used as the working electrode. For HER experiments, 4.0 mg of catalyst was ultrasonically dispersed for 30 min in 2 mL of ethanol containing 0.1 wt % Nafion, and 20 μL of homogeneous catalyst ink was then transferred onto the glassy carbon electrode. The catalyst loading was 0.2 mg/cm2. The linear sweep voltammograms (LSV) and cyclic voltammograms (CV) were measured at a scan rate of 5 mV s−1 in 0.5 M H2SO4 solution. The turnover frequency (TOF) is calculated following a reported method.30 We assume that all of the electrocatalyst MoS2 NSs are monolayers, thus giving an upper bound for the density of S vacancies and hence the lower bound of TOFMo.

MoS2 particles obviously expanded and their lateral size decreased after being treated by NaBH4. The structural characteristics of the SPR-treated sample were investigated with X-ray diffraction (XRD) analysis. As shown in Figure 1c, after annealing, the dominant (002) peak of MoS2 reduced and shifted from 14.4 to 12.4°, indicating an expansion of the lattice along the c-axis (6.15− 7.12 Å). The apparent expansion is attributed to the formation of MoS2 intercalation compounds (NaxMoS2).31,32 No expansion occurred in the case of annealed bulk MoS2 without NaBH4 (Figure S2). Moreover, after the exposure of SPRtreated MoS2 to the ambient atmosphere, extra peaks of (002)′1, (002)′2, (004)′1, and (004)′2 were found at 7.5, 9.7, 15.1, and 19.5°, respectively, which is a result of the hydration of the intercalated Na+ (Figure S3).32 These results confirm the intercalation of Na+ into the MoS2 layers during the SPR treatment using NaBH4 as the reductant. This distinct expansion will effectually weaken the van der Waals interactions between the MoS2 layers, thus enabling the exfoliation of bulk MoS2 via gentle routes without any highenergy driving forces.33 The mass yield of Sv-MoS2 nanosheets prepared by commercial bulk MoS2 with various NaBH4 doses in the SPR process was provided in Figure S4, from which it can be found that the yield of Sv-MoS2 nanosheets climbed notably to ∼24%. The exfoliation process video in the Supporting Information (Video S1) shows that once the asreduced sample was poured into the acid solution, the MoS2 bulk crystals were exfoliated into the few-layer MoS2 NSs and dispersed uniformly in a water solution, along with massive gases release (detailed schematic illustration is provided in Figure 1b). Transmission electron microscopy (TEM) image reveals (Figure S5) the ultrathin nanosheet morphology of the asprepared Sv-MoS2 NSs, with integral lateral sizes of about 100−200 nm that are comparable with those of the ultrasonic

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RESULTS AND DISCUSSION Our SPR approach is schematically illustrated in Figure 1a. Thermal annealing with NaBH4 is an intriguing redox process, and S atoms in MoS2 would be removed as H2S and Na2S, leading to the formation of S vacancies on the basal plane. In a typical synthesis, the commercial bulk 2H-MoS2 and NaBH4 were mixed well via grinding and subsequently annealed at 380 °C for 2 h. After the thermal treatment, the color of the mixture distinctly changed from gray to black (Figure 1b), suggesting the successful removal of S atoms in the SPR desulfurization process. SEM images in Figure S1 show that 25265

DOI: 10.1021/acsami.9b07856 ACS Appl. Mater. Interfaces 2019, 11, 25264−25270

Research Article

ACS Applied Materials & Interfaces exfoliated nanosheets. As shown in Figure 2a,b, the Sv-MoS2 NSs are composed of 3−5 monolayers (with thickness of about

Figure 3. (a, b) XPS spectra showing Mo 3d (a) and S 2p (b) corelevel peak regions for the ultrasonic exfoliated 2D-MoS2 and Sv-MoS2 NSs. (c) The S:Mo atomic ratio decreases with increasing mass ratio of NaBH4:MoS2 for the reduction treatment, as obtained from XPS measurements. (d) Representative Raman spectra for the ultrasonic exfoliated 2D-MoS2 and SPR produced Sv-MoS2 NSs. Dotted lines indicate the positions of the 1E2g and 1Ag peaks.

components of 2H-MoS2, respectively.39 Notably, the Sv-MoS2 sample exhibits two additional Mo 3d peaks at 228.9 and 232.0 eV, which are ascribed to the lower oxidation state, Moδ+ (δ < 4), indicating the reduction of Mo4+ during the SPR treatment. To evaluate the loss of S atoms in Sv-MoS2, the area ratio of all Mo 3d peaks decomposed from the pristine ultrasonic exfoliated 2D-MoS2 (P-MoS2) and Sv-MoS2 NSs was used as a reference to normalize the XPS peaks. The decrease of the obtained S 2p peaks in Figure 3b indicates that the S:Mo atomic ratio of Sv-MoS2 is 1.81, lower than the stoichiometric number 2 for P-MoS2 NSs. Furthermore, the XPS spectra of PMoS2, 0.5-, 2-, and 3-Sv-MoS2 (the as-prepared samples were labeled as x-Sv-MoS2, where x represents the mass ratio of NaBH4 to MoS2) are provided in Figure S8, and the average Svacancy percentage increased from a minimum of 4.0% to a maximum of 24.5% (Figure 3c), suggesting that the reduction degree of MoS2 NSs could be tuned in a controlled manner. Raman spectroscopy of 1-Sv-MoS2 shows that the crystal structure of the original material remained (in accordance with the SAED pattern in Figure S5a), with the Raman 1E2g (the inplane vibration mode) and 1Ag (the out-offline vibration mode) peaks at 383.5 and 409 cm−1, respectively (Figure 3d and Figure S9).40 The obvious red shift and broadening of the 1 E2g and 1Ag peaks is clearly observed due to the damage of the crystal symmetry induced by S vacancies.41,42 Next, we examined the effects of different S-vacancy ratios on the catalytic HER activity of MoS2 (see the Experimental Section in the SI). For bulk MoS2, as shown in Figure S10, there occurred an obvious increase of current density, which confirmed the catalytic activity improvement of the bulk MoS2 after being treated with NaBH4. Figure 4a shows the representative LSV for P-MoS2 NSs (obtained from the routine sonication technique), 20% Pt/C electrode, and the asreduced Sv-MoS2 (0.5, 1, 2, and 3-Sv-MoS2) NSs. Pristine ultrasonic exfoliated MoS2 sample shows poor electrochemical catalytic activity with an overpotential of −521 mV at 10 mA

Figure 2. Morphology and structure characterizations of Sv-MoS2 NSs. (a, b) TEM image (a) and HRTEM image (b) of Sv-MoS2. (c) STEM image of specific area of (a). (d) STEM of the basal plane of the S-vacancy region in Sv-MoS2. (e) Intensity profile of the selected line (bright cyan) in (d). (f) Atomic model of the S-vacancy region in Sv-MoS2.

2−3 nm). The corresponding selected-area electron diffraction (SAED) pattern show a distinct sixfold symmetry diffraction spots (Figure S5a, inset), revealing that the crystallinity and hexagonal lattice of MoS2 were preserved during the SPR and exfoliation process,34 which can be further confirmed by the well-matched XRD pattern of the as-prepared Sv-MoS2 NSs (Figure S6). Unlike the ultrasonic exfoliated MoS2 NSs (Figure S7), dark regions (around 2−10 nm2) were observed on the aberration-corrected STEM image of the SPR-generated MoS2 NSs (Figure 2c), suggesting the removal of S atoms and the successful formation of consecutive S vacancy.35,36 Magnified image of the atom loss region is provided in Figure 2d. The characteristic hexagonal pattern of alternating intensity is associated with 2H-MoS2. Intensity profile analysis of the ADF image37 taken from the indicated line (Figure 2e) demonstrates that the integral intensity of Mo and S atoms has a decrease of around 20% from perfect to atom-loss plane of SvMoS2 NSs, indicating the formation of consecutive S vacancies via the SPR process.15,38 The intensity profile of the selected region is in agreement with the atoms configuration in mimic of bilayer MoS2 (Figure 2f), which means that the S vacancies are created in the basal plane of the MoS2 nanosheets. High-resolution XPS measurements are performed in Figure 3a,b. The Mo 3d spectrum of Sv-MoS2 shows Mo4+ peaks at 229.4 and 232.5 eV corresponding to the 3d5/2 and 3d3/2 25266

DOI: 10.1021/acsami.9b07856 ACS Appl. Mater. Interfaces 2019, 11, 25264−25270

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overpotential to facilitate a current density of 10, 20, and 50 mA cm−2 within the Sv-MoS1.81 (∼9.5% S-vacancies) catalyst reduced to 238, 279, and 382 mV, respectively (Figure 4b), which is comparable with that reported previously for Sv-MoS2 (Table S1, SI). Tafel plots (Figure 4c) show that the S vacancies created on the basal plane reduce the Tafel slope from 213 to 64 mV/dec. The electrochemical double-layer capacitance (Cdl) is achieved by the CV data in Figure S11. As shown in Figure 4d, Cdl values for 0.5-, 1-, 2-, and 3-Sv-MoS2 increased to 0.75, 1.25, 1.46, and 1.65 mF cm−2, respectively, which were obviously higher than 0.05 mF cm−2 for P-MoS2 NSs. This result indicates that the Sv-MoS2 samples display a higher exposure of catalytically active sites and also suggests that the introduced basal planar S vacancies are indeed active catalytic sites.43 Besides, the TOF value of 1-Sv-MoS2 NSs reaches 6.44 s−1 at 200 mV, which is much higher than that of 2H-MoS2 NSs (P-MoS2 NSs, close to 0.4 s−1) (Figure 4e).5 Furthermore, the durability yof the as-prepared Sv-MoS2 was tested by 10 000 CV sweeps (Figure 4f) to accelerate the degradation, and Sv-MoS2 was proved to be a durable nonprecious HER electrocatalyst. The Gibbs free energy of the intermediate state (H*), |ΔGH*|, is a powerful method to represent the catalytic activity of the sample.44 Using DFT calculations, we evaluate the HER activity of the S-vacancy sites on the basal plane (computational cell shown in Figure 5a−c).45 The order of catalytic activity in Figure 5d indicates that two sulfur vacancies (|ΔGH*S| = 1.18 eV) < one sulfur vacancy (|ΔGH*S| = 1.29 eV) < no vacancy (|ΔGH*S| = 1.75 eV), which demonstrate S atoms in S vacancy are activated, consistent with the experimental results (Table S2, Figure S12). Moreover, the electron distribution and spin density also changed due to the formation of sulfur vacancy (Figure 5e−g), indicating that the electrons are transferred from circumambient sulfur to contrapuntal sulfur atom after the formation of a sulfur vacancy (Figure 5h).14,15 To demonstrate the generality of the SPR strategy, we applied the SPR desulfurization process to the commercial bulk WS2 crystals. As shown in Figure S13, the diffraction peak at 2θ of 14.3° in the XRD patterns corresponds to the (002) facet of the 2H-WS2,46 which shifted notably toward the lower

Figure 4. Electrochemical characterization of Sv-MoS2 for HER catalysis. (a) J−V curves show the catalytic performance of various SvMoS2 samples in comparison to a Pt wire and ultrasonic exfoliated 2D-MoS2 (P-MoS2) NSs. (b) Overpotential at current densities of 10, 20, and 50 mA cm−2. (c) Tafel plots for the data presented in (a). (d) Plots showing the extraction of Cdl for different Sv-MoS2 samples. (e) TOF plots of P-MoS2, 0.5-, 1-, 2-, and 3-Sv-MoS2 NSs. (f) Electrochemical stability test of Sv-MoS2 after 10 000 cycles CV.

cm−2, while the current density of Sv-MoS2 was significantly increased. The current density increment first improves and then decreases as as the %S-vacancy increases, indicating that there is a volcano relation between the %S-vacancy and HER performance. Compared with P-MoS2 NSs, the required

Figure 5. (a−c) Computational unit cell of 2H-MoS2, (a) without vacancy, (b) with one S vacancy, and (c) with two S vacancies. Dashed circles label the S-vacancy sites. (d) Free energy versus the reaction coordinates of HER for different S-vacancy ratios. (e−g) The calculated spin density of (e) without vacancy structure and one sulfur vacancy structure with (f) lateral view and (g) top view; the yellow zone presents the spin up, the blue zone is spin down. (h) The Bader charge distribution on the atom around the single S vacancy; the read dash circle is contrapuntal of the vacancy and the green dash circle represents the nearest sulfur atom from the vacancy. 25267

DOI: 10.1021/acsami.9b07856 ACS Appl. Mater. Interfaces 2019, 11, 25264−25270

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commercially available bulk materials. NaBH4 was used as the solid reductant to remove the sulfur atoms from the basal plane of MoS2 and WS2. Sodium ions diffused and entered into their interlayer spaces concurrently, expanding their lattices in the caxis direction distinctly. Therefore, large-scale 2D Sv-MoS2 and Sv-WS2 NSs were achieved within several seconds by mild shaking. Moreover, the obtained 2D Sv-MoS2 and Sv-WS2 NSs exhibit excellent HER activity compared to their bare counterpart. The DFT calculation shows that S atoms in the basal-plane S vacancies were activated. We expect the solidphase reduction strategy would provide a scalable and general route to develop higher-efficiency nonprecious TMD-based electrocatalysts in the near future.

angles (12.5°) after the NaBH4 treatment, with the interlamellar spacing increasing from 6.18 to 7.12 Å. Ultrathin 2D WS2 can also be obtained by only gentle shaking in the hydrochloric acid solution (Figure 6a). As shown in Figure 6b,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07856. The PDF file includes additional figures (Figures S1− S16), tables (Tables S1−S3) (PDF) Exfoliation process video (Video S1) (MP4)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.X.). *E-mail: [email protected] (Q.X.). ORCID

Cong Wei: 0000-0002-4131-0553 Quan Xu: 0000-0003-2195-2513 Zheng Liu: 0000-0002-8825-7198 Qun Xu: 0000-0002-2264-0266

Figure 6. Morphology and electrochemical characterizations of SvWS2 for HER catalysis. (a) TEM image of reduced Sv-WS2 NSs. (b) STEM image of reduced Sv-WS2 NSs. (c) J−V curves show the catalytic performance of various Sv-WS2 samples in comparison to a Pt wire and 2D-WS2 NSs. (d) Tafel plots for the data presented in (c). (e) Plots showing the extraction of the Cdl for different Sv-MoS2 samples. (f) Electrochemical stability test after 10 000 cycles CV.

Author Contributions

Dr. C. Wei and W. Z. Wu contributed equally to this work. C. Wei designed the experiments, analyzed the data, and helped to revise the manuscript. W. Z. Wu performed the experiments, collected and analyzed the data, and wrote the manuscript. H. Li, X. C. Lin, and T. Wu helped to perform the experiments. Q. Xu carried out the simulations and calculations and improved the manuscript. L. P. Zhang, Y. J. Guo, and Z. Liu designed the experiments and improved the manuscript. X. A. Yang took the TEM images. Q. Xu conceived the project, designed the experiments, and analyzed the data.

the atom loss regions were found in the basal plane of the obtained WS2 NSs, consistent with the result of Sv-MoS2. The high-resolution XPS measurements (Figure S14) further verify the successful desulfurization of the WS2 in the SPR process.47 As shown in Figure S15, the current density of bulk WS2 has an obvious enhancement after being treated by NaBH4. The HER catalytic activity of the as-prepared Sv-WS 2 shows a pronounced improvement, with the required overpotential at 10 mA cm−2 reduced from 613 (P-WS2) to 241 mV (Figure 6c). The Tafel slopes greatly decreased due to the desulfurization in WS2 (from 242 to 78 mV dec−1) (Figure 6d). The results are comparable or even better than the reported WS2 nanomaterials (Table S3). The Cdl value of SvWS2 increased about 200 times than that of the pristine ones (Figure 6e, Figure S16). Besides, the obtained Sv-WS2 catalysts showed negligible degradation after 10 000 cyclic voltammetric (CV) sweeps (Figure 6f), exhibiting outstanding electrochemical stability. These results verify the generality of the SPR strategy, which might be applicable to other sulfides, oxides, selenides, phosphides, and nitrides as well.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Nos. 21773216, 51173170, 21571157, and 21805246), the financial support from the Innovation Talents Award of Henan Province (114200510019), the Scientific & Technology Program of Henan Province (182102410073), and the Key program of science and technology (121PZDGG213) from Zhengzhou Bureau of science and technology.

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REFERENCES

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CONCLUSIONS In summary, we have proposed a scalable solid-phase reduction strategy to produce 2D chalcogen-vacancy TMD NSs from the 25268

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.9b07856 ACS Appl. Mater. Interfaces 2019, 11, 25264−25270