Iridium-Triggered Phase Transition of MoS2 Nanosheets Boosts

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Iridium-Triggered Phase Transition of MoS2 Nanosheets Boosts Overall Water Splitting in Alkaline Media Shuting Wei, Xiaoqiang Cui, Yanchao Xu, Bo Shang, Qinghua Zhang, Lin Gu, Xiaofeng Fan, Lirong Zheng, Changmin Hou, Haihua Huang, Sisi Wen, and Weitao Zheng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01840 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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Iridium-Triggered Phase Transition of MoS2 Nanosheets Boosts Overall Water Splitting in Alkaline Media Shuting Wei,† Xiaoqiang Cui,*,† Yanchao Xu,† Bo Shang,† Qinghua Zhang,‡ Lin Gu,*,‡ Xiaofeng Fan,† Lirong Zheng,§ Changmin Hou,¶ Haihua Huang,† Sisi Wen,¶ and Weitao Zheng*,† †Key

Laboratory of Automobile Materials of MOE and School of Materials Science and

Engineering, Jilin University, Changchun 130012, P.R. China ‡Laboratory

of Advanced Materials and Electron Microscopy, Beijing National Laboratory for

Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China §Beijing

Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100049, P.R. China ¶State

Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P.R. China

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ABSTRACT Metallic phase molybdenum disulfide (1T-MoS2), with its fast carrier mobility and highly abundant active sites, plays a vital role in the field of catalysis. However, the development of a simple and efficient strategy for the preparation of stabilized 1T-MoS2 remains a great challenge. Herein, we report the spontaneous phase transformation of MoS2 from the 2H to the 1T phase, caused by the strong metal-support interaction during iridium (Ir) adsorption. The resulting Ir/MoS2 heterostructures show higher catalytic activity for overall water splitting than those of commercial Pt/C and IrO2 in alkaline medium. We believe that the spontaneous phase transformation of this material not only opens up a new perspective for developing advanced catalysts for alkaline water splitting but also presents an efficient and intriguing method for the phase engineering of two-dimensional materials.


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Metallic MoS2 (1T-MoS2), a polymorphs of MoS2, has been broadly applied in the fields of catalysis,1-4 energy storage,5-7 sensors8 and optoelectronic devices.9-11 Although 1T-MoS2 exhibits higher electrochemical performance than the semiconducting counterpart 2H-MoS2,12, 13 its application is severely hindered by its instability. Traditionally, 1T-MoS2 was obtained by strategies such as chemical exfoliation,14,

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strain,16,

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electron radiation,18 plasmonic hot

electron injection19 and substitutional doping of 2H-MoS2,20,

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which are complicated, time

consuming and technically difficult. In particular, 1T-MoS2 prepared by the most popular method, the intercalation of guest ions (Li+, Na+ and K+), is unstable and easily devolves back into 2H-MoS2. Therefore, it is desirable to find an efficient preparation method for stable 1TMoS2. Noble metallic nanostructure-decorated MoS2 nanohybrids have been applied in electrochemical catalysis, batteries and sensors, in which MoS2 serves as a two-dimensional support to enhance the performance of the metal nanoparticles by preventing their aggregation.2226

However, the strong interaction of the counterpart metal species on the MoS2 substrate and the

consequent effect of the resultant catalyst on alkaline catalytic performance have never been explored. In this work, we reported that the adsorption of Ir on the surface of MoS2 triggers a phase transition of MoS2 from 2H to 1T. The mechanism of this phase transition was revealed using density functional theory (DFT) and further confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the regions of MoS2 surrounded by Ir nanocrystals were converted into the 1T phase. The Ir/MoS2 heterostructures exhibit excellent catalytic activity with a low overpotential of -44 mV for the hydrogen evolution reaction (HER) and 330 mV for the oxygen evolution reaction (OER)


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at 10 mA cm-2, values superior to those of commercial Pt/C and IrO2 in 1.0 M KOH, respectively. A two-electrode cell assembled with Ir/MoS2 as bifunctional catalysts shows good performance for overall water splitting with a cell voltage of 1.57 at 10 mA cm-2. Therefore, our study provides a prospect for developing catalytic materials for water splitting by phase engineering.

Figure 1. The theoretical model (a) and energy differences (b) between Ir/1T-MoS2 and Ir/2HMoS2 as a function of the atomic concentration of the adsorbed Ir. (c) Raman spectra of MoS2 and Ir/MoS2 with different amounts of Ir precursor. (d) Mo 3d and (e) Ir 4f XPS spectra of Ir black and Ir/MoS2 with different amounts of Ir precursor. DFT calculations were first performed to evaluate the phase transition of MoS2 depending on the atomic concentration of adsorbed Ir. The total energy difference between Ir/2H-MoS2 and Ir/1T-MoS2 (ΔET-H / formula cell) as a function of Ir atom concentration is the criterion for phase transition. As shown in Figure 1a and 1b, the ΔET-H / formula cell gradually decreases with increasing Ir atom concentration. When the Ir atom concentration is above 20%, the ΔET-H / formula cell becomes negative, indicating that Ir/1T-MoS2 is more stable than Ir/2H-MoS2. The


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above results indicated that the phase transformation of MoS2 may occur when high-density Ir atoms are adsorbed on the surface of MoS2. Inspired by these theoretical calculations, we prepared Ir/MoS2 heterostructures by reducing the chloroiridic acid precursor on MoS2 nanosheets in ethylene glycol.27 Transmission electron microscopy (TEM) images show that ultrasmall Ir nanocrystals with an average diameter of 2.01 nm are uniformly dispersed on the MoS2 support, and X-ray diffraction (XRD) reveals that the crystalline structure of MoS2 is well retained (Figure S1 and S2). The loading amount of Ir crystals is highly dependent on the precursor concentration (Figure S3). Raman spectroscopy was performed to investigate the structural evolution of Ir/MoS2 prepared by adding different amounts of Ir precursor (Figure 1c). As the amount of Ir in the Ir/MoS2 heterostructure increases, three new vibration peaks at 146, 283 and 335 cm-1, which correspond to the Mo-Mo stretching vibrations (J1), the E1g phonon mode and the J3 phonon mode of 1T-MoS2, respectively, gradually emerge and grow in intensity,3, 28 indicating the phase change in the Ir/MoS2 heterostructure. The pristine MoS2 nanosheets show two peaks at 378 and 404 cm-1, which correspond to the in-plane E12g and the out-of-plane A1g, respectively. The onephonon forbidden vibration at 454 cm-1 is ascribed to the A2u or E1g + XA band, which is derived from the resonance between the indirect band gap of MoS2 and the 633 nm excitation.29 The band at 187 cm-1 is attributed to the acoustic out-of-plane mode ZA (M).30 The phase transition was confirmed by Mo 3d X-ray photoelectron spectroscopy (XPS) spectra of Ir/MoS2 heterostructures with different Ir contents (Figure 1d). The Mo 3d binding energy of Ir/MoS2 depends on the concentration of the precursor and gradually shifts to lower energy by 0.50 eV. A negative shift in the Mo 3d binding energy results from the appearance of the 1T phase, suggesting that the MoS2 support undergoes a phase transition when the Ir atoms


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become attached to MoS2. Meanwhile, Ir 4f binding energy is also dependent on the loading amount of Ir in Ir/MoS2. As the amount of Ir precursor decreases, the Ir 4f binding energy gradually shifts by 0.55 eV to higher energy than that of commercial Ir black (Figure 1e), indicating that the valence state of Ir in Ir/MoS2 has an obvious trend in variation from the metallic state to the high oxidation state. The XPS results show the strong interaction between Ir and MoS2 induces the phase transition of MoS2 to 1T from 2H phase, which was further confirmed by the results of ultroviolet photoelectron spectrometer (UPS) (Figure S4), indicating a strong charge transfer between Ir and MoS2. Pt/MoS2 was also prepared by using the same synthetic method as the Ir/MoS2. Raman and XPS spectra show that the introduction of Pt only causes a weak phase transition of MoS2 (Figure S5).


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Figure 2. (a) HAADF-STEM image of Ir/MoS2 heterostructure. (b) TEM and EDX mapping of Ir/MoS2 heterostructure (scale bar: 200 nm). (c, d) High-magnification HAADF-STEM images of the areas in (a). (e, f) Filtered magnified images of the areas marked by boxes in (c) and (d). (g, h) Intensity distributions along the dashed lines in (e) and (f). (i) XANES and (j) FT-EXAFS spectra of the Mo K-edge region before and after Ir decoration. The inset in (i) shows the Mo K-


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edge XANES spectra of MoS2 and Ir/MoS2 at the rising edge. (k) XANES and (l) FT-EXAFS spectra of the Ir L3-edge region of Ir foil, Ir/MoS2 and IrO2. The atomic structure of Ir/MoS2 (prepared by adding 45 μL of Ir precursor) was characterized by aberration-corrected TEM. A HAADF-STEM image clearly shows that the bright Ir nanocrystals are dispersed on the surface of the MoS2 nanosheets (Figure 2a). TEM and the corresponding elemental mapping images show the homogeneous distribution of Mo, Ir and S (Figure 2b). Attention has been focused on the two regions of Ir/MoS2 in Figure 2a: an area with a high loading amount of Ir nanocrystals (Figure 2c) and an area with no Ir loading (Figure 2d). Interestingly, the regions surrounded by Ir nanocrystals are all 1T MoS2 with octahedral coordination and an atomic stacking sequence of ABC (Figure 2e, Figure S6 and S7); however, the regions without Ir nanocrystals are all pristine 2H-MoS2 with trigonal prismatic coordination and an atomic stacking sequence of ABA (Figure 2f, Figures S6 and S7). Figure 2g and 2h show the intensity distributions along the dashed lines in Figure 2e and 2f. The intensity ratios between Mo and S atoms in Figure 2g and 2h are consistent with the previous simulation of 1T and 2H MoS2, respectively.13, 31 The Mo-Mo distance in Figure 2g is 4.5 Å, which is smaller than the simulated Mo-Mo distance of 5.1 Å in 1T MoS2, indicating that compressive strain from the surrounding Ir nanocrystals may exist. XAS was performed to obtain structural information about Ir/MoS2 (prepared by 45 µL H2IrCl6), as shown in Figure 2i-l. X-ray absorption near-edge spectroscopy (XANES) of the Mo K-edge of Ir/MoS2 shows a new peak emerging at 20022 eV and the peak at 20010 eV disappearing, indicating the presence of 1T-MoS2.32 The rising edge of the XANES spectrum of Ir/MoS2 shifts to lower energy compared with MoS2, suggesting strong electronic coupling between Ir species and MoS2 in Ir/MoS2 (the inset in Figure 2i). The extended X-ray absorption fine structure (EXAFS) was used to analyse the local coordination environment and bond


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distance of Ir/MoS2. The Fourier transform (FT) curves in R-space display two main peaks at 1.96 Å and 2.85 Å assigned to the nearest Mo-S and Mo-Mo bond, respectively (Figure 2j). The peak intensity of the Mo-S bond decreases slightly because of the newly formed vacancy or interfacial effect between Ir species and MoS2.20, 33 The dramatic decrease in the intensity of the Mo-Mo bond in Ir/MoS2 indicates structural disorder, which is attributed to the atomic rearrangement after introducing Ir species. The ARTEMIS module was performed to fit these FT curves (Figure S8 and Table S2). Ir/MoS2 shows a first-shell Mo-S bond with a coordination number of 5.7 and a second-shell Mo-Mo bond with a coordination number of 3.8, lower than the original coordination numbers of 6.0 in 2H-MoS2. The decrease of nearly half in the coordination number of the Mo-Mo bond is a typical characteristic of 1T-MoS2, supporting the occurrence of the phase transition in Ir/MoS2.34 The chemical state and coordination information of Ir species were further analysed by XAS spectroscopy. As shown in Figure 2k, the intensity of the white line of the normalized XANES spectrum of Ir/MoS2 is distinctly higher than that of the reference Ir foil but lower than that of the reference IrO2, indicating that the valence state is between Ir0 and Ir4+, which is consistent with the XPS results. FT-EXAFS spectroscopy was performed to identify the coordination information and bond length (Figure 2l). The curve of Ir/MoS2 is clearly different from those of Ir foil and IrO2, demonstrating the different local atomic coordination. Two distinct peaks in the Ir/MoS2 curve at 1.90 and 2.45 Å are assigned to the nearest Ir-S bond and Ir-Ir bond, respectively. The corresponding FT curves are fitted to obtain the quantitative parameters of the local structure near the adsorbed Ir by using the ARTEMIS module (Figure S9 and Table S3). The nearest Ir-S bond was perfectly fitted with a bond length of 2.29 Å and a coordination number of 2.3. The Ir-Ir bond distance of 2.68 Å with a coordination number of 4.0 in Ir/MoS2 is


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clearly different from that of bulk Ir (an Ir-Ir bond distance of 2.74 Å with a coordination number of 12.0), which is attributed to the surface contraction of Ir species in the cluster structure.35, 36

Figure 3. Electrochemical performance of Ir/MoS2. (a) Polarization curves of different catalysts tested in 1.0 M KOH. (b) Overpotentials of different catalysts at 10 mA cm–2 in 1.0 M KOH. (c) Tafel plots of different catalysts calculated by extrapolation methods. (d) Polarization curves of Ir/MoS2 before and after different numbers of CV cycles (inset: time-dependent current density for Ir/MoS2 at a static overpotential of -44 mV). (e) Comparison of catalytic activity for Ir/MoS2 heterostructure before and after CO poisoning in 1.0 M KOH. (f) Proposed schematic illustration of HER in alkaline solution. S (blue), Mo (red), Ir (green), H (white) and O (orange). The HER electrocatalytic activity of as-synthesized Ir/MoS2 heterostructures was evaluated by a three-electrode system in 1.0 M KOH. For comparison, commercial Pt/C and Ir nanoparticles loaded on carbon black (Ir/C, Figure S10) were also investigated under the same conditions. The concentration of the precursor had an important effect on the HER catalytic performance, and the Ir/MoS2 heterostructure prepared with 90 μL of precursor was determined to be the optimal candidate for HER (Figure S11). The contact angle test demonstrated that the surface of Ir/MoS2 possesses higher hydrophilicity than pristine MoS2 (Figure S12), which promotes water adsorption and subsequent electrolytic processes. The optimized Ir/MoS2 ACS Paragon Plus Environment

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heterostructure displays a low overpotential of -44 mV at 10 mA cm-2, which is better than those of commercial Pt/C (-68 mV@10 mA cm-2), Ir/C (-157 mV@10 mA cm-2) and the most recently reported alkaline HER catalysts (Figure 3a-b and Table S4). The Tafel slope of Ir/MoS2 (32 mV dec-1) is lower than those of commercial Pt/C (36 mV dec-1) and Ir/C (59 mV dec-1), indicating fast reaction kinetics for HER (Figure 3c). The Ir/MoS2 heterostructures exhibit a higher exchange current density of 1.40 mA cm-2 than Pt/C (0.57 mA cm-2) and Ir/C (0.25 mA cm-2), as determined by applying the extrapolation method from the Tafel plot (Figure S13), which implies high intrinsic catalytic activity. The interfacial properties and the charge transfer dynamics in the HER process were also investigated by electrochemical impedance spectroscopy (EIS). The Ir/MoS2 heterostructure shows a smaller charge transfer resistance (Rct) of 19 Ω than Ir/C (30 Ω) and MoS2 (810 Ω), revealing fast charge transfer dynamics during the HER process (Figure S14). The Ir/MoS2 heterostructure dropped on carbon fiber paper (CF) also shows remarkable stability according to polarization curve testing before and after 9000 CV cycles (Figure 3d). The long-term stability measurement shows that the current density remained 96.3% even after continuous measurements of 18 h in alkaline media (the inset of Figure 3d). To explore the active sites of the Ir/MoS2 heterostructure, a poisoning experiment was performed by using carbon monoxide (CO) as a toxic agent because of its strong binding ability with noble metals (Figure 3e).37 After the electrode was saturated with CO, the catalytic activity of the Ir/MoS2 heterostructure exhibited a significant reduction. The overpotential shifted to -116 mV from -44 mV at 10 mA cm-2, indicating that the Ir species in the Ir/MoS2 heterostructure are the catalytic sites for HER. However, the catalytic activity of the poisoned Ir/MoS2 heterostructure was still much better than that of pristine MoS2, demonstrating that the 1T-MoS2 support induced by Ir species also contains vital active sites for HER.


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The proposed schematic illustration for HER in alkaline solution is shown in Figure 3f. Specifically, water molecules are first adsorbed on Ir active sites, and H-OH bonds are cracked to form the intermediates H* and OH-. Subsequently, the OH- is adsorbed on Ir sites, and the resulting intermediate H* is transferred to the S sites of neighbouring 1T-MoS2. Finally, nearby S sites promote the combination of intermediate H* and the desorption of H2. Hence, we believe that the superior catalytic activity of Ir/MoS2 may be mainly attributed to the synergistic effect at the interface of Ir species and 1T-MoS2.

Figure 4. Electrochemical performance of Ir/MoS2 in 1.0 M KOH. (a) Polarization curves of different catalysts for OER tested in 1.0 M KOH. (b) Tafel plots of Ir/MoS2 and IrO2. (c) The long-term stability for OER. (d) Polarization curves of different catalysts for overall water splitting in 1.0 M KOH.


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The electrochemical performance of the Ir/MoS2 heterostructures for OER was also investigated by a typical three electrode system in 1.0 M KOH. As a comparison, commercial IrO2 and pristine MoS2 were also tested under identical conditions, respectively. The Ir/MoS2 heterostructures exhibit excellent catalytic activity for OER with an overpotential of 330 mV@10 mA cm-2 and a low Tafel slope of 44 mV dec-1, values superior to those of commercial IrO2 (365 mV@10 mA cm-2 and a Tafel slope of 50 mV dec-1) (Figure 4a -b),indicating the fast reaction kinetics for OER. EIS shows that the Ir/MoS2 has a small charge transfer resistance (Rct) of (20 Ω) than IrO2 (47 Ω) and MoS2 (433 Ω) (Figure S15), signifying that Ir/MoS2 possesses the high electrical conductivity to accelerate charge transfer between the electrode and electrolyte. Ir/MoS2/CF also displays good long-term stability with slight deterioration after 12 h (Figure 4c). The possible reason for the good OER performance of Ir/MoS2 was attributed to the following features. (1) The Ir nanocrystals possess the excellent OER activity in light of previous reports.38, 39

(2) The contribution from the two-dimensional MoS2, which facilitates the better distribution

and reduces the aggregation of Ir species. (3) The phase transformation from 2H-MoS2 to 1TMoS2 highly improved the conductivity of the catalyst. Encouraged by the excellent HER and OER performance of the Ir/MoS2 heterostructures, a two-electrode electrolyser with Ir/MoS2/CF as both the anode and the cathode was constructed to assess its catalytic activity for overall water splitting (Figure 4d). The alkaline electrolyser can achieve current densities of 10 and 20 mA cm2

at cell voltages of only 1.57 and 1.62 V, respectively, which are superior to the values of

IrO2(+)//Pt/C(-) (10 mA [email protected] V and 20 mA [email protected] V). This result demonstrates that Ir/MoS2/CF acts as a promising candidate in the overall water splitting device. In summary, we have demonstrated that the adsorption of Ir atoms on MoS2 triggers the phase transition of MoS2 by both theoretical calculations and experiments. The performance of


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Ir/MoS2 depends heavily on the loading amount of Ir. The optimized Ir/MoS2 heterostructure shows excellent catalytic activity, superior to those of commercial Pt/C and IrO2. The excellent catalytic performance of the Ir/MoS2 heterostructure can be attributed to the following reasons. First, 1T-MoS2 induced by Ir species has good electrical conductivity and an activated basal plane compared to 2H-MoS2. Second, the surface of Ir/MoS2 possesses higher hydrophilicity than that of pristine MoS2, which promotes water adsorption and subsequent electrolytic processes. Third, the high dispersion and ultrasmall size of Ir species results in a large number of heterojunction interfaces and many active sites. Fourth, the strong synergistic and coupling effect between Ir and 1T-MoS2 promotes water dissociation, fast charge transfer at the interface and the recombination of the intermediate H*, thus accelerating the reaction kinetics and improving the catalytic activity for HER in alkaline solution. The phase transition and interface engineering in this work provide a new perspective for the design of efficient catalysts for water splitting and chlorine-alkali electrolysers. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx. Experimental methods, material synthesis and characterization, electrochemical measurements, DFT calculations; SEM, TEM images and more HAADF-STEM images; XRD pattern; FTEXAFS fitting and analysis; contact angle test; electrochemical impedance spectra. AUTHOR INFORMATION Corresponding Authors Prof. X. Q. Cui (E-mail: [email protected])


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Prof. L. Gu (E-mail: [email protected]) Prof. W. T. Zheng (E-mail: [email protected]) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2016YFA0200400), the National Natural Science Foundation of China (51571100, 51872116, 51602305, 51522212, 51421002), Program for JLU Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09) and the Fundamental Research Funds for the Central Universities.

REFERENCES (1) Yu, Y.; Nam, G. H.; He, Q.; Wu, X. J.; Zhang, K.; Yang, Z.; Chen, J.; Ma, Q.; Zhao, M.; Liu, Z.; et al. High Phase-Purity 1T'-MoS2- and 1T'-MoSe2-Layered Crystals. Nat. Chem. 2018, 10, 638-643. (2) Attanayake, N. H.; Thenuwara, A. C.; Patra, A.; Aulin, Y. V.; Tran, T. M.; Chakraborty, H.; Borguet, E.; Klein, M. L.; Perdew, J. P.; Strongin, D. R. Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS2 for the Hydrogen Evolution Reaction. ACS Energy Lett. 2018, 3, 7-13. (3) Geng, X.; Sun, W.; Wu, W.; Chen, B.; Al-Hilo, A.; Benamara, M.; Zhu, H.; Watanabe, F.; Cui, J.; Chen, T. P. Pure and Stable Metallic Phase Molybdenum Disulfide Nanosheets for Hydrogen Evolution Reaction. Nat. Commun. 2016, 7, 10672. (4) Wang, L.; Liu, X.; Luo, J.; Duan, X.; Crittenden, J.; Liu, C.; Zhang, S.; Pei, Y.; Zeng, Y.; Duan, X. Self-Optimization of the Active Site of Molybdenum Disulfide by an Irreversible Phase Transition During Photocatalytic Hydrogen Evolution. Angew. Chem. Int. Ed. 2017, 56, 76107614. (5) Lei, Z.; Zhan, J.; Tang, L.; Zhang, Y.; Wang, Y. Recent Development of Metallic (1T) Phase of Molybdenum Disulfide for Energy Conversion and Storage. Adv. Energy Mater. 2018, 8, 1703482. (6) Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T Phase MoS2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotechnol. 2015, 10, 313-318. (7) Zhang, Y.; Mu, Z.; Yang, C.; Xu, Z.; Zhang, S.; Zhang, X.; Li, Y.; Lai, J.; Sun, Z.; Yang, Y.; et al. Rational Design of Mxene/1T-2H MoS2-C Nanohybrids for High-Performance Lithium–Sulfur Batteries. Adv. Funct. Mater. 2018, 1707578.


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ACS Energy Letters

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