Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Wafer-Scale Sulphur Vacancies-Rich Monolayer MoS for Massive Hydrogen Production 2

Ce Hu, Zhenzhen Jiang, Wenda Zhou, Manman Guo, Ting Yu, Xingfang Luo, and Cailei Yuan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01399 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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The Journal of Physical Chemistry Letters

Wafer-Scale Sulphur Vacancies-Rich Monolayer MoS2 for Massive Hydrogen Production Ce Hu,‡ Zhenzhen Jiang,‡ Wenda Zhou, Manman Guo, Ting Yu, Xingfang Luo, Cailei Yuan* Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Communication and Electronics, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang 330022, Jiangxi, China

‡These

authors contributed equally to this work.

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

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ABSTRACT: As one of promising low-cost and high-efficient catalysts for electrochemical hydrogen evolution reaction (HER), it is well known that there are both tiny exposed catalytic active edge sites and large-area inert basal plane in two-dimensional MoS2 structures. For enhancing its HER activity, extensive works have been made to activate the inert basal plane of MoS2. In this article, wafer-scale (2-inch) continuous monolayer MoS2 films with substantial insitu generated sulphur vacancies are fabricated by employing laser molecular beam epitaxy process benefitted from ultra-high vacuum growth condition and high substrate temperature. The intrinsic sulphur vacancies throughout the wafer-scale basal plane present an ideal electrocatalytic platform for massive hydrogen production. The fabricated vacancies-rich monolayer MoS2 can achieve a current density of −10 mA/cm2 at an overpotential of −256 mV. The wafer-scale fabrications of sulphur vacancies-rich monolayer MoS2 provide great leaps forward in the practical application of MoS2 for massive hydrogen production.

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Since the discovery of graphene, properties of various two-dimensional (2D) layered materials as well as their heterostructures have been extensively studied.1-3 A brilliant example is the transition-metal dichalcogenide molybdenum disulfide (MoS2), and in particular its monolayer structure, has attracted tremendous attentions because of its remarkable optical and electrical properties, as well as its high catalytic activity and electrochemical stability for the hydrogen evolution reaction (HER).4-6 Owing to its earth abundance and outstanding catalytic performance, MoS2 have been acknowledged as one of the ideal substitutes to Pt-group metals for hydrogen generation.7-10 For monolayer MoS2 film, the exposed crystal edges that just occupy a tiny percentage of the surface area than the inactive basal plane, have been affirmed as its active catalytic sites. Therefore, extensive works have been made to activate the inert basal plane of MoS2 for enhancing its HER activity. Defects formation by post-synthesis strategies such as hydrogen or argon plasma treatment have been applied to generate defects to activate the inert basal plane of MoS2,11 but the defect type and density are difficult to control, especially for large-area and atomically thin MoS2.12,13 Differing from the post-synthesis of defects, the insitu generation of defects in MoS2 is milder and more feasible. However, this strategy has rarely been reported and it's still a challenge that need to be developed for studying and optimizing the catalytic performance of MoS2 nanomaterials. For practical production and utilization of hydrogen energy, despite optimal catalytic activities, large-scale fabrication of MoS2 catalysts are highly desirable as well, which enables hydrogen production on a massive scale, and thus lower the production costs. However, MoS2 layers demonstrated in the past studies were obtained primarily by mechanical or chemical exfoliation,1,2,14 the process of which is not scalable and does not enable a systematic control over the thickness and size of MoS2 layers. Most recently, chemical vapor deposition (CVD) based


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methods were proposed to synthesize large area monolayer MoS2,15,16 which, however was also not wafer-scale fabrication, and suffered from the limitations of non-uniformity in growth conditions, and thus non-uniformity in material stoichiometry and properties.17-19 Therefore, it still presents a big challenge to fabricate wafer-scale monolayer MoS2 combining uniform material composition and high catalytic activity. In this work, we meet the challenges by employing laser molecular beam epitaxy (L-MBE) process for insitu generating sulphur vacancies in wafer-scale monolayer MoS2. The intrinsic sulphur vacancies throughout the wafer-scale basal planes present an ideal electrocatalytic platform for hydrogen production. The as-synthesized vacancies-rich monolayer MoS2 can achieve a current density of −10 mA/cm2 at an overpotential of −256 mV without any performanceenhancing additives or processes. The wafer-scale fabrications of sulphur vacancies-rich, uniform, and high-quality monolayer MoS2 provide great leaps forward in the practical application of MoS2 for massive hydrogen production to really tackle the energy shortage. Monolayer MoS2 with substantial sulphur vacancies were fabricated on 300 nm Si/SiO2 wafers (2-inch) by employing a scalable L-MBE process benefitted from ultra-high vacuum growth condition and high substrate temperature. It can be seen continuity and uniformity covering a large area under optical microscopes (Figure S1). To quantitatively evaluate the film continuity and uniformity of the as-deposited (pristine) monolayer MoS2 over a wafer-scale, we divided the MoS2 wafer into seven concentric rings (Figure 1(a) inset). Numerous Raman spectra have been measured on each ring, and the representative results are plotted in Figure 1(a). As the characteristic Raman modes of MoS2, the Raman peaks at ~407.1 and ~387.3 cm-1 are respectively identified as A1g (out-of-plane) and E12g (in-plane) vibration modes.20 The measured frequency difference between the two peaks (Δk=19.8 cm-1) can be used as an indicator of monolayer MoS2.


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We can see that the peak positions and full width at half maximum of the E12g and A1g vibration modes are consistent for all the spectra, indicating that the fabricated film is a uniform monolayer with similar crystal quality across the whole wafer. To furtherly confirm the continuity and uniformity, sufficient Raman mappings and spectra were undertaken on the monolayer MoS2 wafer. The typical Raman intensity mappings are shown in Figure 1(b) and (c), which were collected from a randomly selected area on the wafer. Moreover, the total 140 Raman spectra were collected from 140 randomly selected points on the wafer, which are shown in Figure 1(d). Obviously, uniform composition distribution crossing the continuous monolayer MoS2 wafer was confirmed by the Raman spectra and intensity mappings.

Figure 1. (a) Seven spectra collected from locations evenly distributing along radical direction across the wafer. The inset is an optic image of a representative as-synthesized MoS2 on a 2-inch Si/SiO2 wafer, which is symmetric w.r.t. the central point O. The dashed lines serve to visualize the constant Raman peaks (E12g at 387.3 cm-1 and A1g at 407.1 cm-1) across entire wafer. Raman


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intensity mappings of (b) E12g (red) at 387.3 cm-1, (c) A1g (blue) at 407.1 cm-1. (d) Raman spectra collected from 140 randomly selected exciting points (inset, ~140 curves). The fabricated pristine monolayer MoS2 was furtherly characterized using atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. As shown by the AFM image in Figure 2(a), the thickness of the L-MBE fabricated MoS2 film is about 0.75 nm, which is consistent with previously reported thickness of monolayer MoS2.4,19 Another evidence for the fabrication of monolayer MoS2 comes from the analysis of the PL spectra, as shown in Figure 2(b). Obviously, no PL peak can be observed at 1.63 eV, which is related to the indirect-band-gap transition. In contrast, a strong peak at ~1.87 eV (A) and a weak peak at ~2.04 eV (B) can be found in this representative PL spectrum, which are related to the direct-band-gap transition at K and Kʹ points of the Brillouin zone for the monolayer MoS2.21 To evaluate the structural properties of the monolayer MoS2 film, high-resolution transmission electron microscope (HRTEM) studies were undertaken, the results of which are shown in Figure 2(c)-(e). As shown by the TEM image at lowmagnification in Figure 2(c), monolayer MoS2 present as planar structure.22 To furtherly confirm the morphology of fabricated MoS2 film, as shown in Figure 2(d), HRTEM measurements were undertaken around the edge areas of the MoS2 film, which clearly demonstrate the fabrication of monolayer MoS2 structure. To determine the crystalline quality of the monolayer MoS2, selected area electron diffraction (SAED) patterns were also performed with selected-area apertures from the MoS2 suspended onto a vacant micrometer-sized hole on TEM grid. As shown in Figure 2(e), only one set of hexagonal diffraction pattern was detected, which indicates that the suspended monolayer MoS2 film is single crystalline material with well-ordered hexagonal crystal structure. Therefore, as demonstrated by the systemic analyses of Raman, PL, AFM and HRTEM data, the fabrication of continuous wafer-scale (2-inch) monolayer MoS2 by using L-MBE technique can


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be confirmed. X-ray photoelectron spectroscopy (XPS) characterizations were also implemented to study the material stoichiometry and chemical composition of the pristine monolayer MoS2 film (Figure 2(f)). The S 2p exhibits two peaks at 164.3 and 163.1 eV, which are respectively identified as the doublet of S2- 2p1/2 and S2- 2p3/2. In the Mo 3d region, the doublet peaks for Mo4+ 3d3/2 and Mo4+ 3d5/2 appear at 233.5 and 230.3 eV, respectively. The shapes and positions of the S 2p and Mo 3d doublets validate that the fabricated monolayer MoS2 film presents only as the semiconducting 2H phase.23 According to the XPS characterizations, the atomic concentration ratio of Mo to S is calculated to be 1:1.48, indicating the fabrication of wafer-scale sulphur vacancies-rich monolayer MoS2 (see Figure S2 for the detailed Raman and XPS characterizations of pristine monolayer MoS2 which suggested that the sulphur vacancies are generated, as evidenced by electron paramagnetic resonance (EPR) spectrum (Figure S3)), which may provide an ideal electrocatalytic platform for massive hydrogen production.


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Figure 2. (a) AFM images of the pristine monolayer MoS2 deposited on 300 nm Si/SiO2 substrate. The cross-sectional step height profile of monolayer MoS2 along the black dash line indicated in AFM image shown in the inset of (a). (b) PL spectrum of pristine monolayer MoS2. (c) Lowmagnification TEM image, (d) HRTEM image, and (e) SAED pattern of pristine monolayer MoS2, respectively. The inset in (e) show the area (film suspended above a vacant micro-hole) where the SAED pattern was collected. (f) XPS spectrum of the pristine monolayer MoS2. The pristine continuous wafer-scale (2-inch) monolayer MoS2 can be transferred to arbitrary substrates using a PMMA-assisted wet transfer process for characterizations and electrocatalytic HER experiments (Figure S4). Apart from the well-known catalytically active edge sites of monolayer MoS2, sulphur vacancies in the basal plane provide another type of major active sites


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for HER.12,13,24 As a result, it can be expected that the wafer-scale vacancies-rich monolayer MoS2 will present an excellent efficiency of hydrogen production. We estimate the active site numbers of pristine monolayer MoS2 using the underpotential deposition (UPD) method.25 Figure 3(a) displays the linear sweep voltammetry (LSV) curves for the stripping of Cu deposited at different overpotentials from 0.12 V to 0.40 V, and the regions for overpotential deposition (OPD), UPD and their stripping can be observed. The charges required to strip off the Cu deposited on MoS2 catalysts at different underpotentials with a sweep rate of 5 mV/s are summarized in Figure 3(b). The charges quantity at the plateau allows us to estimate the active site numbers from the Cu deposited during the UPD cycle by the formula QCu×NA/2/F/S (NA: Avogadro constant, F: Faraday constant, S: Sample area). The active sites density for pristine monolayer MoS2 is estimated 1.8×1016 sites/cm2, thus guaranteeing the outstanding catalytic performance as we discussed below. Figure 3(c) and 3(d) show the polarization curves and corresponding Tafel plots of the pristine monolayer MoS2. The polarization curve and corresponding Tafel plot of bare glassy carbon electrode are also illustrated, indicating the poor catalytic activity of glassy carbon for HER. The pristine monolayer MoS2 can provide a current density of −10 mA/cm2 at an overpotential of −256 mV with a Tafel slope of 93 mV/dec, which demonstrates that the pristine monolayer MoS2 compare favorably among the state-of-art MoS2 catalysts6,12,26-32 while combining with a waferscale monolayer synthesis process. More importantly, beyond high catalytic activity, as shown in the inset of Figure 3(c), the pristine monolayer MoS2 shows consistent HER characteristics under continuous operations of 10 000 cycles, which indicates the remarkable catalytic stability of these insitu generated sulphur vacancies in the wafer-scale monolayer MoS2. Note that we examined the Raman and XPS (Figure S5) of the monolayers after catalytic reaction, and found that its composition and crystalline structure did not change.24


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Figure 3. (a) The LSV for the stripping of Cu deposition at various potentials and (b) the electrical charge as a function of various deposited potentials of pristine monolayer MoS2. The red dashed lines in (b) represent the plateaus, which indicate the charge numbers of Cu at UPD. (c) Polarization curves of bare glassy carbon electrode, pristine monolayer MoS2, and 20 % Pt/C. The inset is stability characterizations by recording the polarization curve for pristine monolayer MoS2 before and after 10 000 cyclic voltammograms. (d) Tafel plots of the polarization curves in (c). The white lines indicate linear fitting for the plots. Moreover, electrochemical impedance spectroscopy (EIS) measurement (Figure 4(a)) indicates that the efficient charge transfers between electrolyte and the active basal plane as affirmed by the exceedingly low charge transfer resistance (Rct) (Table S1). Meanwhile, as suggested in Figure 4(b) and 4(c), the pristine monolayer MoS2 possesses an effective electrochemically active basal


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plane deduced from the electric double-layer capacitance (EDLC) measurements. To further understand the catalytic properties of the pristine vacancies-rich monolayer MoS2, we suppress the sulphur vacancies of the monolayer MoS2 by executing rapid thermal annealing in the atmosphere of nitrogen as we have previously developed.33 With the sulfur vacancies healed by nitrogen atoms, the active sites number was significantly reduced to 7.4×1014 sites/cm2 (Figure S6). Consequently, the catalytic activity of the nitrogen-doped monolayer MoS2 is distinctly weakened and the Tafel slope increases significantly (Figure S7). In addition, the double-layer capacitance reduces and the charge transfer resistance increases remarkably (Figure S8 and Table S1) after nitrogen doping. These consequences after nitrogen doping indicate that the prominent electrocatalytic activity of fabricated pristine wafer-scale monolayer MoS2 actually originate from the substantial intrinsic sulphur vacancies in the activated basal plane.

Figure 4. (a) Nyquist plots of pristine MoS2 at 0.235 V (vs RHE) in the frequency range of 0.1 Hz to 105 Hz with an amplitude of 5 mV (the inset is an equivalent circuit to fit the EIS of the HER process). (b) EDLC measurements, linear fitting of the capacitive currents (data obtained from the LSV in part c) of the samples against the scan rate to fit a linear regression. (c) The cyclic voltammograms measurements with various scan rates for pristine monolayer MoS2 in 0.5 M H2SO4 solution.


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In summary, we have introduced a practicable method to fabricate continuous wafer-scale monolayer MoS2 with substantial insitu generated sulphur vacancies by employing L-MBE process. The intrinsic sulphur vacancies throughout the wafer-scale basal plane present an ideal electrocatalytic platform for massive hydrogen production. The vacancies-rich monolayer MoS2 can achieve a current density of −10 mA/cm2 at an overpotential of −256 mV without any performance-enhancing additives or processes. Our proposed method not only provides a waferscale production of monolayer MoS2 with high uniformity and crystallinity, but also insitu generates substantial sulphur vacancies in the basal plane, as we expected, and thus significantly enlarges its practicality for massive hydrogen production.

Experimental Methods L-MBE sample fabrication: Monolayer MoS2 films were grown using L-MBE process. Briefly, a MoS2 round target (purity 99.99%, diameter D=40 mm) was ablated by KrF pulsed laser (248 nm) in ultra-high vacuum chamber with base pressure of 1×10-9 Torr. During the deposition, the substrate temperature was kept at 600 oC. After deposition, the films were naturally cooled to room temperature in growth pressure. Layer transfer: The monolayer MoS2 films were coated with a thin layer of polymethyl methacrylate (PMMA, Alfa Aesar) by spin-coating at 3000 rpm for 1 minute, followed by baking at 100 oC for 1 minute. After etching the Si/SiO2 substrate underneath with KOH solution (5 wt. %) at 80 oC, the PMMA/MoS2 films were transferred to deionized water, and were floated on the water to wipe off the etchant residue. Then, the films were picked up by TEM grids or other


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substrates for subsequent characterization and analysis. Finally, the upper layers of PMMA were dissolved by acetone. The whole process was schematically illustrated in Figure S4. Characterizations: The optical microscope figures were taken with 10× objective lens. AFM topographies were recorded with Park Systems XE7 in tapping mode. A scratch was deliberately made for the measurement of thickness of continuous MoS2 film. Raman and PL characterizations were undertaken with HORIBA Scientific LabRAM HR Evolution systems using a 514 nm laser. The spectra were taken with a 1800 grooves/mm grating and a 100× objective lens. The power of incident laser on the film surface is ~100 μW and the diameter of the laser spot size is ~1.5 µm. Raman mapping images were taken by scanning a 35×35 μm2 area with a step of 3 μm and the acquisition time was 20 s at each spot. The Raman and PL spectra were collected under vacuum condition at room temperature. The structural properties and elemental compositions of the MoS2 monolayers were evaluated using HRTEM (JEOL 2010 microscope) equipped with an energy dispersive X-ray spectrometer. XPS spectra were collected with Kratos Axis Ultra DLD spectrometer using Al Kα radiation. To obtain high resolution of the spectra, the electron energy analyzer was performed with a pass energy of 20 eV. The binding energies were corrected for specimen charging by referencing the C 1s peak. The EPR spectrum at X-band frequency (~9.85 GHz) was obtained with a Magnettech A300 EPR spectrometer at room temperature. Electrochemical measurements: A CH Instrument electrochemical analyzer (model CHI660E) was used to electrochemically characterize the monolayer MoS2 films with a glassy carbon working electrode, a saturated mercuric sulfate reference electrode (SMSE), and a Pt counter electrode. The potential shift of SMSE was calibrated to be −0.68 V vs HER (Figure S9). The HER was measured using a linear sweep from +0.1 to −0.6 V (vs HER) with a scan rate of 2 mV/s. The scanning rate of cyclic voltammetry was 100 mV/s while the perform range was −0.1 to −0.6 V


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(vs HER). EIS was used to characterize the double-layer capacitance of electrocatalyst and the electrolyte resistance. The measurement range of AC impedance frequency is 105 to 0.1 Hz, and the perturbation voltage amplitude is 5 mV (Figure 4). The interpretation of the EIS measurements is usually done by the correlation between the impedance data and the equivalent circuit which representing the physical processes taking place in the system. The Randles equivalent circuit has a wide application in many electrochemical systems. In this circuit, Rs represents the impedance of the solution and Rct is the charge transfer resistance. Since the presence of non-ideal behavior of the capacitor, the constant phase element (CPE) is introduced to the equivalent circuit for improving the accuracy of determination of capacitance and resistance. We use the equivalent Randles circuit model (inset in Figure 4(a)) to fit the data in order to identify the capacitance and resistance of the system. The measurements above were performed in Ar saturated 0.5 M H2SO4 electrolyte solution at room temperature. The UPD method was used to gauge the active site numbers of MoS2 monolayers, which was based on the assumption that the active sites for proton reduction are also responsive to the Cu2+ reduction at an underpotential. The active site density was estimated by the exchanged charges during the oxidative stripping of the Cu obtained in UPD. And the LSV scans were carried out in the solution with 0.5 M H2SO4, 1 mM CuSO4 and 14 mM NaCl. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51871115, 51561012, 51661012, 51761017 and 61664005), the Excellent Youth Science Foundation of Jiangxi Province of China (Grant No. 20171BCB23033) and the Natural Science Foundation of Jiangxi Province of China (Grant No. 20181BAB206001). ASSOCIATED CONTENT


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Supporting Information Available: Additional details of sample preparations, characterizations, electrochemical measurements, and analyses. The following files are available free of charge. Optical image of pristine monolayer MoS2 (Figure S1, Docx) Detailed Raman and XPS characterizations of pristine monolayer MoS2 (Figure S2, Docx) EPR spectrum of pristine monolayer MoS2 (Figure S3, Docx) Illustration of the transfer process (Figure S4, Docx) Raman and XPS spectrum of pristine monolayer MoS2 after catalytic reaction (Figure S5, Docx) Active sites measurements of nitrogen-doped monolayer MoS2 (Figure S6, Docx) Electrochemical measurements of nitrogen-doped monolayer MoS2 (Figure S7, Docx) EIS measurement of nitrogen-doped monolayer MoS2 (Figure S8, Docx) Calibration of reference electrode (Figure S9, Docx) Rs and Rct of pristine and nitrogen-doped monolayer MoS2 (Table S1, Docx) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Cailei Yuan: 0000-0002-8088-0313 Author Contributions ‡C.H.

and Z.J. contributed equally to this work.

Notes The authors declare no competing financial interest. REFERENCES (1) Lv, Q.; Yan, F.; Wei, X.; Wang, K. High-Performance, Self-driven Photodetector Based on


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Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive

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