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High-Content Metallic 1T Phase in MoS-Based Electrocatalyst for Efficient Hydrogen Evolution Liang Cai, Weiren Cheng, Tao Yao, Yuanyuan Huang, Fumin Tang, Qinghua Liu, Wei Liu, Zhihu Sun, Fengchun Hu, Yong Jiang, Wensheng Yan, and Shiqiang Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03103 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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The Journal of Physical Chemistry

High-Content Metallic 1T Phase in MoS2-Based Electrocatalyst for Efficient Hydrogen Evolution Liang Cai†, Weiren Cheng†, Tao Yao, Yuanyuan Huang, Fumin Tang, Qinghua Liu*, Wei Liu, Zhihu Sun, Fengchun Hu, Yong Jiang, Wensheng Yan*, and Shiqiang Wei National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China

†These authors contributed equally to this work. *E-mail: [email protected]; [email protected] Abstract Realizing high efficiency hydrogen evolution in cost-effective and long-lasting electrocatalysts is critical for global production of clean and sustainable chemical fuels. Here, via modulating the metallic 1T phase in 2H MoS2 nanosheets, we have greatly improved the conductivity and effective active sites for highly efficient electrocatalytic hydrogen evolution. The as-synthesized 1T-2H MoS2 electrocatalyst with high 1T-phase content of 50% can significantly increase the charge concentration by an order of magnitude and triple the effective active surface sites, successfully boosting hydrogen evolution reaction (HER) at a quite low overpotential of 126 mV at 10 mA/cm2 and a small Tafel slope of 35 mV/dec. Ultraviolet photoelectron spectroscopy and electrochemical characterizations reveal that the valence band edges of 1T-2H MoS2 are upshifted by 0.15–0.36 eV, which obviously enhances the charge transfer ability of the surface active sites in the basal plane of MoS2 for high HER performance.

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Introduction Electrochemical reduction of water to hydrogen has been regarded as one of the most attractive ways to produce clean and sustainable fuel, especially driven by the renewable solar-resource-derived electricity.1-4 To enhance the electrochemical efficiency for practical applications, the key challenge lies in the development of efficient and durable electrocatalysts to promote the hydrogen evolution reaction (HER).3,5,6 Platinum (Pt) and other precious metals have been identified as the most active HER electrocatalysts; however, their low abundance and high cost seriously limit their large-scale commercial applications. To address these issues, considerable efforts have been conducted to develop active electrocatalyst based on the earth-abundant materials through modulating their electron structure of active sites, so as to decrease the overpotential for the HER.3,5-11 Recently, molybdenum disulfide (MoS2) has been identified as a promising noble-metal-free electrocatalyst for hydrogen evolution, owing to its high electron mobility and tunable band structure.12,13 Further functionalizing the MoS2-based electrocatalysts with enhanced electronic property would be an effective pathway to maximizing the HER activity. To enhance the HER activity, tremendous efforts have been devoted to improving the electron structure of surface active sites for MoS2-based electrocatalyts.14-21 For example, a highly ordered double-gyroid MoS2 bicontinuous network with nanoscaled pores at the surface has been designed to preferentially expose edge active sites, and the sample exhibits an onset for the HER at an overpotential of 150–200 mV.16 To further improve the catalytic active sites of MoS2 electrocatalysis, the strained sulphur 2


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vacancies has been sought to control the basal plane of monolayer 2H-MoS2, which significantly reduces the HER overpotential.20 Recently, the incorporation of metallic 1T phase in the semiconducting 2H basal plane of MoS2 has been reported to simultaneously enhance the conductivity and improve the in-plane active sites for high HER activity.21 Indeed, the coexistence of 1T-2H phase is an effective way to significantly activate HER for MoS2 electrocatalysts. However, it is extremely difficult to precisely control the formation of 1T phase during generic solvothermal synthesis process, resulting in relatively lower 1T-phase contents .21-23 To optimize the 1T phase concentration, it is highly imperative to explore alternative strategy to rationally manipulate the 1T phase transformation during wet-chemical synthesis process for maximizing the HER performance of MoS2-based electrocatalysts. Herein, we present a template-assisted strategy by reduced graphene oxide (RGO) to modulate the metallic 1T phase in 2H MoS2 for efficient HER performance. We choose RGO as template as it can effectively donate electrons to promote phase transition in MoS2. The obtained MoS2 nanosheets electrocatalysts with 50% metallic 1T phase incorporated into the basal plane of 2H phase could significantly enhance the electron carrier concentration by an order of magnitude and effectively increase the surface active sites by three fold. As a result, the as-synthesized MoS2 electrocatalyst with 50% of 1T phase concentration exhibits a low HER overpotential of 126 mV at 10 mA/cm2 current density and an excellent Tafel slope of 35 mV/dec, comparable to the reported MoS2 electrocatalysts so far. Our findings provide prospective insights into the design of high performance electrocatalysts for hydrogen evolution. 3



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Experimental section Synthesis of 1T-2H MoS2 nanosheets. The high concentration 1T phase incorporated MoS2 nanosheets were obtained by a graphene-oxide-template assisted two-step hydrothermal synthetic strategy. Typically, graphene oxide was synthesized from natural graphite by the modified Hummer’s method, and then was ultrasonic dispersed into distilled water at room temperature (25 ºC) for 2h. Afterwards, (NH4)6 Mo7O

24·4H2O

and thiourea were added into the above solution under vigorous stirring. After being stirred for 1 h, the solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 200 °C for 20 h. Then the reaction system was allowed to cool down to room temperature naturally. The obtained products were collected by centrifugation and washed with ethanol. Then the stoichiometric MoS2 nanosheets were subjected to centrifugation and ultrasonation in ethanol solution and once more autoclavation under 220 °C for 6 h to form the final products and dried at 60 °C under vacuum. The low concentration 1T phase incorporated MoS2 nanosheets were synthesized in similar preparation process without graphene oxide template. The 2H MoS2 nanosheets were obtained via liquid ultrasonic exfoliation of the bulk MoS2 and was marked as ex-MoS2. Morphology characterizations of 1T-2H MoS2 nanosheets. Atomic force microscopy (AFM) was performed by means of Veeco DI Nano-scope MultiMode V system. The transmission electron microscopy (TEM) was carried out on a JEM-2100F field emission electron microscope at an acceleration voltage of 200 kV. The high-resolution TEM (HRTEM) and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were performed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. XRD patterns were recorded by using a Philips X’Pert Pro Super 4


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diffractometer with Cu Kα radiation (λ=1.54178 Å). Electronic structure characterization. The S K-edge XANES spectra were measured at the 4W7B beamline of the Beijing Synchrotron Radiation Facility (BSRF), China, in the total electron yield (TEY) mode by collecting sample drain current under a vacuum better than 5×10–8 Pa. X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) were acquired on an ESCALAB MKII with Mg Kα (hυ = 1253.6 eV) as the excitation source. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.5 eV. Electrochemical measurements. Electrochemical measurements were performed using an electrochemical workstation (Model CHI760D, CH instruments, Inc., Austin, TX) with a three-electrode system, operated with the modified glassy carbon disk electrode as working electrode, platinum mesh as the counter electrode, and saturated Ag/AgCl (3M KCl) as reference electrode. All the electrochemical measurements were conducted in 0.5 M H2SO4 (aq) electrolytes continuously purged with 99.999% N2 (Praxair) and at a sweep rate of 5 mV/s. The glassy carbon disk electrodes with a diameter of 3 mm covered by various catalyst films are used as the working electrodes. Typically, 4 mg of catalysts and 30 µL Nafion solution (5 wt%, Sigma Aldrich) were dispersed in 1 mL water–ethanol solution with volume ratio of 3:1 by sonicating for 2 h to form a homogeneous ink. Then 5 µL of the dispersion (containing 20 µg catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter (loading ca. 0.285 mg/cm2). Finally, the as-prepared catalyst film was dried at room temperature. All polarization curves were corrected for iR losses. Electrochemical impedance spectroscopy (EIS) dates were recorded with frequency range of 0.1–100 kHz at overpotential of 200 mV vs RHE. 5



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Figure 1. Characterization of the 1T-2H MoS2 with various 1T contents. TEM image for 50% (a) and 15% (b) 1T phase. Insets are the corresponding SEM images. Scale bar: 200 nm. HRTEM images for 50% (c) and 15% (d) 1T phase. (e) XPS spectra, where the pink and green dot curves represent the contributions of 1T and 2H phases to the Mo 3d peaks in 1T-2H MoS2 nanosheets, respectively. (f) XRD pattern. (g) Raman spectra.

Results and discussion The successful synthesis of the phase-incorporated MoS2 nanosheets (hereafter denoted as 1T-2H MoS2) can be verified by the TEM, HRTEM images and XPS measurements as shown in Figure 1a-e. Seen from the TEM, SEM image shown in Figure 1a and b, there are a number of homogenous MoS2 nanosheets grown atop the surface of RGO for 50% 1T phase sample, while only free-standing sheet-like MoS2 is observed for 15% 1T phase sample. The HRTEM image of the 1T-2H MoS2 nanosheets in Figure 1c shows distinct lattice fringes of 0.27 nm with 60° angles attributed to the (100) and (010) planes of 2H-MoS2.24,25 Notably, the implanted 1T-MoS2 phase in the 2H MoS2 matrix could be clearly visualized as marked by the circles in Figure 1c and d. In addition, the corresponding HRTEM images evidently display some trigonal lattice 6


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area of the 1T phase, besides the common honeycomb lattice area for the 2H phase as schematically represented in Figure 1b. Furthermore, the deconvolution of Mo XPS peaks of 1T-2H MoS2 in Figure 1e reveals that the binding energies of Mo4+ 3d5/2 and 3d3/2 electrons in 2H MoS2 are shifted to lower energy side by ~0.5 eV due to the incorporated 1T phase, and the relative contents of 1T phase in the samples of Figure 1c and d are estimated to be ~50% and 15%, respectively.26,27 Moreover, the XRD results in Figure 1f show that all peaks for the ex-MoS2 sample can be well indexed to those of pure MoS2, and the peaks of (002), (103) and (110) for 1T-2H MoS2 are boarder relative to ex-MoS2, suggesting the co-existence of 1T and 2H phase. Seen from Raman spectra results (Figure 1g), all characteristic peaks at 283, 379, 404, and 454 cm−1 for ex-MoS2 are originated from the E1g, E12g, A1g and longitudinal acoustic phonon modes of 2H MoS2, and two additional peaks at 219 and 335 cm−1 for 15% and 50% 1T-2H MoS2 can be attributed to the 1T phase MoS2. These results strongly demonstrate that 1T-MoS2 has been successfully embedded in the 2H MoS2 matrix, and the contents of 1T phase MoS2 can be further tuned by the RGO. To evaluate the HER activity of 1T-2H MoS2 nanosheets, the electrochemical performance of the electrocatalyst was measured using the standard three-electrode configuration in 0.5 M H2SO4 electrolyte (see details in the experimental section). As shown in Figure 2a, the polarization curve (J–V) recorded with the 50% 1T-2H MoS2 reveals a markedly small onset potential of 90 mV for the HER, beyond which the cathodic current rises fast by applying a smaller overpotential (from 2.1 mA/cm2 at 95 mV to 20 mA/cm2 at 160 mV). It is specially noted that the cathodic current rapidly 7



reaches to 10 mA/cm2 at a quite low overpotential of 126 mV. In contrast, ex-2H MoS2 nanosheets exhibit a much larger overpotential of 348 mV at 10 mA/cm2 current density. The outstanding HER activity of the 1T-2H MoS2 nanosheet is also reflected by its small Tafel slope of 35 mV/dec (Figure 2b). This Tafel slope is comparable to that of the state-of-the-art Pt catalyst (30 mV/dec, see Figure 2b), corresponding to the more effective HER kinetics of Volmer-Tafel mechanism.10

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Figure 2. Electrochemical measurements of 1T-2H MoS2 nanosheets. (a) Polarization curves, (b) Corresponding Tafel plots obtained from the polarization curves in (a), (c) Electrochemical impedance spectroscopy (EIS) Nyquist plots, and (d) Accelerated cyclic voltammetric test.

Figure 2c presents the electrochemical impedance spectroscopy (EIS) results for various samples under HER process. It can be seen from the Nyquist plots that the charge transfer resistance for the 50% 1T-2H MoS2 nanosheets is as low as 12 ohm, one

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order of magnitude smaller than that of the ex-2H MoS2 nanosheets (370 ohm). Notably, the charge transport in the 1T-2H MoS2 exhibits a characteristic transmission line behavior under low resistance of ~2 ohm (inset of Figure 2c), suggesting a convenient interlayer electron exchange throughout the interior electrode. To verify the operation stability of the 1T-2H MoS2 nanosheet electrocatalyst, the accelerated degradation polarization measurements were conducted. As shown in Figure 2d, the polarization curve after 1000 cycles almost overlaps the J–V curve of the first cycle. Therefore, it is concluded that the 1T-2H MoS2 nanosheets exhibits great potential in the HER as a highly active and long-term stable catalyst for practical applications. (b)

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Figure 3. The electronic structure characterization and thermogravimetric analyses of the 1T-2H MoS2 nanosheets. (a) UPS spectra, (b) S K-edge XANES spectra, and differential scanning calorimeter (DSC) and thermogravimetric (TG) measurements for 50% (c) and 15% (d) 1T MoS2 nanosheets.

To explore the electron structure and formation mechanism of 1T-2H MoS2, we 9



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performed electronic structure characterizations and thermogravimetric analyses on 1T-2H MoS2 nanosheets. First, the ultraviolet photoelectron spectra (UPS) in Figure 3a show that the valence-band maximum (VBM) of 1T-2H MoS2 is upshifted by 0.36 and 0.15 eV toward the Fermi level relative to 2H MoS2 for the 50% and 15% 1T phase samples, respectively, indicating strong electron coupling after 1T phase incorporation. This result confirms that the electronic structures of MoS2 are evidently modified by 1T phase incorporation.28 Moreover, the S K-edge X-ray absorption near-edge structure (XANES) spectra in Figure 3b show three characteristic peaks of a (~2471 eV), b (~2479 eV), and c (~2482 eV).29 Compared with 2H MoS2 nanosheet, the amplitude of peak a for 1T-2H MoS2 is lowered and a shoulder peak a1 appears at the low-energy side, which means larger densities of unoccupied antibonding S 3p-like states located in the valence bands.30 Therefore, these results reveal that the electron density of 1T-2H MoS2 is redistributed and some new energy levels around the Fermi level of the material are formed. The redistributed electron density and new forming energy level of 1T-2H MoS2 evidently demonstrate a charge transfer from RGO to MoS2. This electron transfer behave would greatly disorder the local lattice of 2H MoS2 via charge Coulomb effect and then induce the transformation of 1T phase in micro-domains, finally resulting in the formation of 1T-2H MoS2 with increasing 1T content.31,32 Besides the influence of electron injection, the intercalation of guest ions or molecules may also lead to the transformation of metallic 1T phase in 2H MoS2. Seen from the DSC curve of 50% 1T-2H MoS2 (Figure 3c), the endothermic peaks at ~74 and ~354 °C are ascribed to the 10


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release of adsorbed H2O/CO2 and the deintercalation of inserted NH4+, respectively, and the exothermic peak at ~280 °C is attributed to the structural transformation between 1T and 2H phase. Similarly, seen from Figure 3d, the temperatures for adsorbed H2O/CO2 release, structural transformation and inserted NH4+ deintercalation were ~115, ~265 and ~318 °C for 15% 1T-2H MoS2, respectively. These results indicate the presence of inserted NH4+ ions in both 15% and 50% 1T-2H MoS2. For the inserted NH4+, it could greatly change the local electronic structure of 2H MoS2 and then induce the transformation of 1T from 2H phase, which is revealed by previous work.33,34 Therefore, this inserted NH4+ may have certain contribution on the formation of 1T phase for both 15% and 50% 1T-2H MoS2. As the TG curves shown in Figure 3c and d, the loss of the inserted NH4+ for 50% 1T-2H MoS is ~9 wt%, close to that of 15% 1T-2H MoS2 (~7 wt%). It is noted that in spite of similar NH4+content, the 1T phase content of 50% 1T MoS2 is three times that of 15%. Thereby, we consider that the inserted NH4+ ions are not the main reason for the increasing 1T content. Based on the above analyses, it can be drawn that the electron injection from RGO into MoS2 may be the crucial factor for the formation of increasing 1T phase in 50% 1T-2H MoS2. For an in-depth understanding the influence of electron redistribution on HER activity of 1T-2H MoS2, the Mott-Schottky plots and electrochemical double-layer capacitance measurements were performed. As shown in Figure 4a, all samples exhibit positive slopes, indicating the n-type behavior of both ex-2H and 1T-2H MoS2. Moreover, the charge carrier concentration, inversely proportional to the slope of Mott-Schottky plots, is slightly enhanced for 15% 1T MoS2 compared with that of 11



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ex-2H MoS2. Notably, the Mott-Schottky slope of 50% 1T MoS2 is much smaller relative to 15% 1T MoS2, corresponding to an order of magnitude increase of the charge carrier after large amount of 1T phase incorporation. The enhanced charge concentration is not only beneficial for electron transfer but also helpful for catalytic activity of surface reductive sites, which could be confirmed by the electrochemical double-layer capacitance (Cdl) results. Seen from Figure 4b, the 50% 1T MoS2 possess the largest Cdl of ~670 µF/cm2, 10 and 30 times that of 15% 1T and ex-2H MoS2, respectively. These results demonstrate larger effective active sites of 50% 1T MoS2 after more 1T phase incorporation for high efficient HER performance.

Figure 4. (a) Mott-Schottky plots. (b) The differences in current density variation at 50 mV vs RHE plotted against scan rate for estimation of double-layer capacitance (Cdl). (c) Schematic for the hydrogen evolution process at the surface active sites of 1T-2H MoS2 nanosheets.

In summary, the preparation of 1T phase MoS2 in hydrothermal methods, reported 12


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by previous work, was mainly under pressurized conditions or with assistance of guest ions and molecules.33-35 In this work, by taking advantages of RGO-template, high 1T content 1T-2H MoS2 catalysts with 50% 1T phase were synthesized in a facile hydrothermal process. Importantly, via varying the addition content of RGO, the 1T phase content of 1T-2H MoS2 catalysts can be tuned from 15% to 50 %, as confirmed by the regulating experiment results. The formation and stabilization of high 1T phase in these fewer layers 1T-2H MoS2 catalysts may be ascribed to the charge transfer from RGO to MoS2 and the strong interaction of 1T-2H phase as verified by the UPS and S

K-edge XANES results, which will greatly boost the electrocatalytic hydrogen evolution activity for 50% 1T-2H MoS2. Indeed, the obtained high content of 50% 1T-2H MoS2 significantly reduces the HER overpotential to 126 mV at 10 mA/cm2, which is considerably lower than that of 2H MoS2 nanosheets by about 200 mV (348 mV at 10 mA/cm2 for ex-2H MoS2 nanosheets). As for the electronic structure of 1T-2H MoS2, the valence bands of 1T-2H MoS2 are changed evidently with strong electron densities introduced at the Fermi level. Such an electronic feature plays critical roles in enhancing the HER activity of 1T-2H MoS2 electrocatalyst as shown in Figure 4c. Firstly, the valence bands of 1T-2H MoS2, upshifted by about 0.36 eV toward the Fermi level introduced by the coupling effect of 1T-2H phase, will strengthen the hybridization of Mo d states and S p states and the interlaminar interaction as revealed by the UPS and XANES results. The significantly enhanced interlayer coupling effect could unambiguously facilitate charge transport through 1T-2H MoS2 nanosheet, contributing to the improved surface HER 13



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acvtivity.36-38 Secondly, the newly introduced electron densities at the Fermi level could induce effective electron hybridization of the active sites and the H+ adsorbates. As is known, in the HER process, the strong electron hybridization between the active sites and H+ adsorbates can reduce the electron carriers transfer resistance by about one order of magnitude as demonstrated by the EIS measurement results (Figure 2c), and thus evidently decreases the ovpertential for the HER.39 Finally, the significant electron density distributions near the Fermi level in MoS2 can effectively promote the charge transfer across the 1T-2H MoS2 interfaces, decreasing its Gibbs free-energy of the intermediate state, |∆GH*|.3,10,11,40 This decreased Gibbs free-energy |∆GH*| of 1T-2H MoS2 typically corresponds to a two-electron transfer process following a more efficient Volmer-Tafel mechanism,10,38 as confirmed by the small Tafel slope of 35 mV/dec. In this Volmer-Tafel mechanism, the thermodynamics of H+ adsorption and H* desorption on the catalytic sites could be effectively equilibrated, leading to a moderate kinetic energy barrier of hydrogen evolution pathway.8,10,40 This also in turn favors the electrochemical desorption of H* and leads to a relatively moderate Mo–H binding strength, thus resulting in significant enhancement of the HER.38,39

Conclusion In summary, we have reported a phase-incorporated MoS2 nanosheets as an efficient HER electrocatalyst. The 1T-2H MoS2 electrocatalyst with strong phase coupling and electron transfer ability could effectively decrease the charge transfer resistance to 12 Ω and lower the Gibbs free-energy of H* near zero, triggering high 14


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HER activity with a small Tafel slope of 35 mV/dec and a low overpotential of 126 mV at 10 mA/cm2 current density. Electronic structure and electrochemical characterizations reveal that the significant electron density distribution near the Fermi level of 1T-2H MoS2 promotes the formation of H* intermediates and decreases the activation barrier of HER. Our design may open up opportunities for the design of efficient and cost-effective two-dimensional HER electrocatalysts.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 21533007, 11621063, 21603207, U1532265, 11435012, 11305174, and 11422547), and the Fundamental Research Funds for the Central Universities (WK2310000054), and the China Postdoctoral Science Foundation (2016M590581).

References [1] Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. [2] Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. [3] Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.; Norskov, J. K. Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nature Mater. 2006, 5, 909-913. [4] Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. [5] Lu, Q. P.; Yu, Y. F.; Ma, Q. L.; Chen, B.; Zhang, H. 2D Transition-MetalDichalcogenide-Nanosheet-Based

Composites

for

Photocatalytic

and

Electrocatalytic Hydrogen Evolution Reactions. Adv. Mater. 2016, 28, 1917-1933. 15



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

Page 16 of 20

[6] Faber, M. S.; Jin, S. Earth-Abundant Inorganic Electrocatalysts and Their Nanostructures for Energy Conversion Applications. Energy Environ. Sci. 2014, 7, 3519-3542. [7] Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. First-Row Transition Metal Dichalcogenide Catalysts for Hydrogen Evolution Reaction. Energy Environ. Sci. 2013, 6, 3553-3558. [8] Hinnemann, B.; Moses, P. G.; Bonde, J.; Jorgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Norskov, J. K. Biornimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. [9] Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102. [10] Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. [11] Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the Computational Design of Solid Catalysts. Nature Chem. 2009, 1, 37-46. [12] Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics

and

Optoelectronics

of

Two-Dimensional

Transition

Metal

Dichalcogenides. Nature Nanotechnol. 2012, 7, 699-712. [13] Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nature Chem. 2013, 5, 263-275. [14] Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. [15] Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation. Science 2012, 335, 698-702. 16


Page 17 of 20

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


[16] Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis.

Nature Mater. 2012, 11, 963-969. [17] Kibsgaard, J.; Jaramillo, T. F.; Besenbacher, F. Building an Appropriate Active-Site Motif into a Hydrogen-Evolution Catalyst with Thiomolybdate [Mo3S13]2- Clusters. Nature Chem. 2014, 6, 248-253. [18] Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25, 5807-5013. [19] Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013,

135, 17881-17888. [20] Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J.; Han, H. S.; Manoharan, H. C.; Abild-Pedersen, F.; et al. Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution Through the Formation of Strained Sulphur Vacancies. Nature Mater. 2016, 15, 48-53. [21] Zhang, J.; Wang, T.; Liu, P.; Liu, Y.; Ma, J.; Gao, D. Enhanced Catalytic Activities of Metal-Phase-Assisted 1T@2H-MoSe2 Nanosheets for Hydrogen Evolution. Electrochim. Acta 2016, 217, 181-186. [22] Yang, J.; Wang, K.; Zhu, J.; Zhang, C.; Liu, T. Self-Templated Growth of Vertically Aligned 2H-1T MoS2 for Efficient Electrocatalytic Hydrogen Evolution.

ACS Appl. Mater. Interfaces 2016, 8, 31702-31708. [23] Du, P.; Zhu, Y.; Zhang, J.; Xu, D.; Peng, W.; Zhang, G.; Zhang, F.; Fan, X. Metallic 1T Phase MoS2 Nanosheets as a Highly Efficient Co-catalyst for the Photocatalytic Hydrogen Evolution of CdS Nanorods. RSC Adv. 2016, 6, 74394-74399. [24] Jiang, T.; Liu, H.; Huang, D.; Zhang, S.; Li, Y.; Gong, X.; Shen, Y. R.; Liu, W. T.; Wu. S. Valley and Band Structure Engineering of Folded MoS2 Bilayers. Nature

Nanotechnol. 2014, 9, 825-829. 17



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

[25] Yan, A.; Velasco, J.; Kahn, S.; Watanabe, K.; Taniguchi, T.; Wang, F.; Crommie, M. F.; Zettl, A. Direct Growth of Single- and Few-Layer MoS2 on h-BN with Preferred Relative Rotation Angles. Nano Lett. 2015, 15, 6324-6331. [26] Yu, L.; Lee, Y.-H.; Ling, X.; Santos, E. J. G.; Shin, Y. C.; Lin, Y.; Dubey, M.; Kaxiras, E.; Kong, J.; Wang, H.; et al. Graphene/MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics. Nano Lett. 2014, 14, 3055-3063. [27] Cai L.; He J. F.; Liu Q. H.; Yao T.; Chen L.; Yan W. S.; Hu F. C.; Jiang Y.; Zhao Y. D.; Hu T. D.; et al. Vacancy-Induced Ferromagnetism of MoS2 Nanosheets. J. Am.

Chem. Soc. 2015, 137, 2622-2627. [28] Eda, G.; Fujita, T.; Yamaguchi, H.; Voiry, D.; Chen, M.; Chhowalla, M. Coherent Atomic and Electronic Heterostructures of Single-Layer MoS2. ACS

Nano 2012, 6, 7311-7317. [29] Hunt, B.; Sanchez-Yamagishi, J. D.; Young, A. F.; Yankowitz, M.; LeRoy, B. J.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; et al. Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure. Science 2013, 340, 1427-1430. [30] Guay, D.; Divigalpitiya, W. M. R.; Belanger, D.; Feng, X. H. Chemical Bonding in Restacked Single-layer MoS2 by X-Ray-Absorption Spectroscopy. Chem.

Mater. 1994, 6, 614-619. [31] Voiry, D.; Mohite, A.; Chhowalla, M. Phase Engineering of Transition Metal Dichacogenides. Chem. Soc. Rev., 2015, 44, 2702-2712. [32] Lin, Y. C.; Dumcenco, D. O.; Huang, Y. S.; Suenaga, K. Atomic Mechanism of the Semiconducting-to-Metallic Phase Transition in Single-Layered MoS2. Nat.

Nanotech., 2014, 9, 391-396. [33] Geng, X. M.; Sun, W.W.; Wu, W.; Chen, B. J.; Al-Hilo, A.; Benamara, M.; Zhu, H. L.; Watanabe, F.; Cui, J. B.; Chen, T. Pure and Stable Metallic Phase Molybdenum Disulfide Nanosheets for Hydrogen Evolution Reaction. Nature.

Commun., 2016, 7:10672. [34] Wang, D.; Xiao, Y. Y.; Luo, X. N.; Wu, Z. Z.; Wang, Y. J.; Fang, B. Z. Swollen Ammoniated MoS2 with 1T/2H Hybrid Phases for High-Rate Electrochemical 18


Page 18 of 20

Page 19 of 20

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


Energy Storage. Acs Sustainable Chem. Eng., 2017, 5, 2509-2515. [35] Wang, D. Z.; Zhang, X. Y.; Bao, S. Y.; Zhang, Z. T.; Fei, H.; Wu, Z. Z. Phase Engineering of A Multiphasic 1T/2H MoS2 Catalyst for Highly Efficient Hydrogen Evolution. J. Mater. Chem. A, 2017, 5, 2681-2688. [36] Zubavichus, Y. V.; Slovokhotov, Y. L.; Schilling, P. J.; Tittsworth, R. C.; Golub, A. S.; Protzenko, G. A.; Novikov, Y. N. X-Ray Absorption Fine Structure Study of the Atomic and Electronic Structure of Molybdenum Disulfide Intercalation Compounds with Transition Metals. Inorg. Chim. Acta 1998, 280, 211-218. [37] Hang, S.; Liang, L.; Ling, X.; Puretzky, A. A.; Geohegan, D. B.; Sumpter, B. G.; Kong, J.; Meunier, V.; Dresselhaus, M. S. Low-Frequency Interlayer Raman Modes to Probe Interface of Twisted Bilayer MoS2. Nano Lett. 2016, 16, 1435-1444. [38] Purtzky, A. A.; Liang, L.; Li, X.; Xiao, K.; Sumpter, B. G.; Meunier, V.; Geohegan, D. B. Twisted MoSe2 Bilayers with Variable Local Stacking and Interlayer Coupling Revealed by Low-Frequency Raman Spectroscopy. ACS

Nano 2016, 10, 2736-2744. [39] Chen W. F. ; Wang, C. H.; Sasaki, K.; Marinkovic, N.; Xu, W.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Highly Active and Durable Nanostructured Molybdenum Carbide Electrocatalysts for Hydrogen Production. Energy Environ. Sci. 2013, 6, 943-951. [40] Tsai, C.; Abild-Pedersen, F.; Norskov, J. K. Tuning the MoS2 Edge-Site Activity for Hydrogen Evolution via Support Interactions. Nano Lett. 2014, 14, 1381-1387.

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High-Content Metallic 1T Phase in MoS2-Based ... - ACS Publications

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