1T-2H Crx-MoS2 Ultrathin Nonosheets for Durable and Enhanced

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1T-2H Crx-MoS2 Ultrathin Nonosheets for Durable and Enhanced Hydrogen Evolution Reaction Juan Jian, He Li, Xuejiao Sun, Dechen Kong, Xinghui Zhang, Le Zhang, Hongming Yuan, and Shouhua Feng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00229 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Sustainable Chemistry & Engineering

1T-2H Crx-MoS2 Ultrathin Nonosheets for Durable and Enhanced Hydrogen Evolution Reaction Juan Jian†, He Li†, Xuejiao Sun†, Dechen Kong†, Xinghui Zhang†, Le Zhang†, Hongming Yuan*,† and Shouhua Feng† † State

Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, Qianjin Street 2699, P.R. China * Corresponding author’s E-mail: [email protected] KEYWORDS: 2H- and 1T-MoS2, Hydrogen Evolution Reaction, Cr doping, ultrathin nanosheets

ABSTRACT: MoS2-based materials for hydrogen evolution reaction (HER) were widely reported in recent years. However, the poor conductivity limited the application of MoS2. In this paper, we introduced the heteroatom Cr3+ into the MoS2 with different weight percent. The property for HER was largely enhanced, in special the Cr0.06-MoS2. The overpotential of Cr0.06MoS2 (0.20 V) was much lower than that of the un-doped MoS2 (0.44 V) at current density of 10 mA cm-2. What’s more, the Cr0.06-MoS2 also with long time stability (> 60 h) and almost 100% Faraday efficiency for water reduction. The enhanced HER activity was mostly due to the increased exposed active sites and shortened pass way of electron transfer. For that the ultrathin nanosheets structure of Cr0.06-MoS2 displayed a larger BET surface area (165.52 m2 g-1) than the un-doped MoS2 (130.31 m2 g-1). Besides, Cr0.06-MoS2 showed coexistence of 2H- and 1T-MoS2 phase after Cr3+ doping and resulted much lower data of EIS (16.55 Ω) than the un-doped MoS2 (51.85 Ω). Therefore, the 1T-2H Cr0.06-MoS2 showed enhanced HER activity than the un-doped MoS2 and was a supplementary of the theoretical calculation.

INTRODUCTION Hydrogen, wind and solar energy are regarded as pollution-free and renewable energy sources,1,2 for that the HER (2H+ + 2e-→ H2) gives the green energy H2 for main product, which consistent with the sustainable development of environment. Therefore, it was paid lots of attention and great amount of electro/photo-catalysts have been exploded over these years.3 It is widely accepted that the Pt-based metals/alloys are the effective HER catalysts.4 However, the application has been largely limited due to their low reserves and high price.5 Thus, more and more research focus on the discovering of lowcost and high-efficient catalyst for HER. For example that the metal-carbide,6 -phosphide,7 -oxide,8 -selenide9 and sulfide10 were widely reported. Among of them, the MoS2 was paid lots of attention. For that the atomic-thickness layer structure of MoS2 is similar to that of the graphene structure. Besides, the theoretical calculation shows that the binding energy of edge to hydrogen atom for MoS2 is similar to that of Pt.11 As a result, MoS2 has been applied to different fields,12 especially the research on water splitting.

Materials with high-efficient HER property always with following characters: 1) good conductivity; 2) large number of exposed efficient active sites.13 As for improving the conductivity of catalyst, most research focus on combining catalyst with the conductive material (e.g. compounding with graphene, 14,15 reduced graphene oxide (rGO),16 carbon black,17,18 carbon cloth,19 doped carbon20 and etc.). As we know that the widely reported MoS2 has three-phase (1T-, 2H- and 3R-MoS2), the 1T-MoS2 cannot be found in the nature but can be gained by some way.21 What’s more, the active sites of 1T- and 2H-MoS222 are mainly located on the plane and at the S-edge, respectively.23 About increasing the active sites of a catalyst, the following two ways have been widely accepted: i) improving the activity of MoS2 itself, for instance, ball milling, ultrasonic or even ion intercalation;24 ii) introducing heteroatom (non-metal or metal) to improve the HER activity of material. About the non-metal doped MoS2-based materials, for example, the introduction of P activated the S-edge of MoS2 and finally increased the active sites of P-MoS2.25 As for metal-doped materials, both Sn-26 and Co-MoS227 showed increased hydrogen adsorption active sites at the S-edge after doping. Rh-MoS228 with even better activity than the Pt/C, for that

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Rh-doping promoted the adsorption activity of hydrogen to the catalyst, and accelerated the departure of hydrogen reacted during the HER. As for V-MoS2,29 the improved HER performance was attribute to the largely increased conductivity after V-doping. In brief, the introduction of heteroatom can enhance the HER activity of MoS2. Cr-α-Fe2O330 and Cr-SrTiO331 have ever been reported with an enhanced property for photo-catalysis. As for CrMoS2, several articles have reported for its structure, spectroscopy, 32 semi-metallicity33 and magnetism34 properties, however, no report for the application of water splitting. Jiang’s group has ever proposed the theoretical calculation of Cr-doped 1-T MoS2.23 They supposed that the Cr-doped 1-T MoS2 has a zero closed ∆GH and may resulted an enhanced HER activity. Herein, in this paper, we give the systematic nature of Cr-doped MoS2 for practical application of the water reduction. We synthesized a series of Crx-MoS2 (x is the mass content, x=0.02, 0.04, 0.06, 0.08, 0.10 g), and find that the Cr0.06-MoS2 has the best property for HER. Which has the ultrathin nanosheets structure (0.63 nm), and the Cr has doped into the MoS2 uniformly. The HER performance of Cr0.06-MoS2 was remarkable, for that when the current density reached to 10 mA cm-2, the required overpotential of Cr0.06-MoS2 was 0.20 V, much lower than that of the undoped MoS2 (0.44 V). Besides, the Tafel slope of Cr0.06-MoS2 (41.6 mV dec-1) was smaller than that of the un-doped MoS2 (94.1 mV dec-1). What’s more, the Cdl of Cr0.06-MoS2 (5.5 mF cm-2) is over 17-times larger than that of the un-doped MoS2 (0.315 mF cm-2), which also confirmed the increased exposed active sites with Cr doping.

EXPERIMENTAL SECTION Preparation of Crx-MoS2, MoS2 and other contrast samples. The synthesis of the Crx-MoS2 was almost same with our ever reported.26 Briefly, the Cr(NO3)3·9H2O with different mass ratio [the weight ranges from 1 to 10 mg, and labeled as Crx-MoS2 (x=0.02, 0.04… 0.1)], H2NCSNH2 (2.0 g), (NH4)6Mo7O24·4H2O (0.36 g) and g-C3N4 (4.0 g) were mixed and grinded evenly. Put the mixed powder into the porcelain boat and reacted in a nitrogen atmosphere at 600 oC for 4 h, then heat up to 800 oC, then cooling down to room temperature. After that, washed the as synthesized powder with HCl (0.5 M) for 10 min, and then washed with distilled water for several times. Finally, the Crx-MoS2 was gained. The synthesis process of the MoS2 was similar to the CrxMoS2, just without the present of the Cr(NO3)3·9H2O. The blank contrast test was just to determine the residual amount of the g-C3N4 after the whole process. Grinded the g-C3N4 (4.0 g) evenly and put it into the porcelain boat to undergo the same temperature programmed and cooling process.

Electrochemical measurements The electrochemical performance was tested using the three-electrode system on an electrochemical station (CHI

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660C electrochemical analyzer, CH Instruments, Inc. Shanghai).13 The working electrode was glass carbon electrode (surface area of the GCE is 0.07065 cm2), which was decorated with sample on the surface, directly. The reference and counter electrode using the Hg/Hg2Cl2 and Platinum wire electrode, respectively. What’s more, the reference electrode (Hg/Hg2Cl2) was calibrated with the follow equation: E(RHE) = E(Hg/Hg2Cl2) + 0.24 + 0.591pH

(1)

The electrochemical tests were carried out by CV (cyclic voltammetry) and LSV (linear sweep voltammetry) in N2bublled 0.5 M H2SO4 with a scan rate of 5 mV s-1. It is worth to mention that before the measurement of HER performance, all the working electrodes were tested by the CVs for dozens of cycles (more than 30) at a much larger scan rate (e.g. 50 mV s-1) until the electerochemical signals were relatively stabilized. The Tafel slopes were plotted from the LSV curves and obtained with the fitted equation as follows: Ƞ= a + b log j

(2)

(“Ƞ” is the overpotential, “j” is the current density, “a” is the intercept and relative to the exchange current density jo, “b” is the Tafel slope).13

Estimation of electrochemically active surface area (ECSA) The effective surface area of the electrode material is proportional to the double-layer capacitance (Cdl).13 ECSA = S * Cdl / Cs

(3)

(S is the area of the electrode, here the S = 0.07065 cm2; Cdl is related to the different catalysis material and the Cs = 0.25 mF cm-2 for the glass carbon electrode.35) Cdl was measured under the non-Faraday overpotential, a series of CV tests were performed at various scan rates (10, 20... 100 mV s-1) from the region of 0.01 to 0.07 V (vs. RHE), and the sweep segments were set to 30 during the test.13 By plotting the current density (∆j = janode - jcathode) and scan rate at the overpotential of 0.04 V, the resulted data of the slope is twice as much as that of the Cdl for a material.

Measurements of spectroscopy (EIS)

electrochemical

impedance

EIS measurements were also performed with the threeelectrode cell system in 0.5 M H2SO4. The frequency range is from 0.1 to 10000 Hz, the amplitude is 5 mV. All the LSV curves were without of iR-correction.

Calculation of the TOF (Turnover Frequency) The TOF was calculated follow the equation:1,36 TOF (s-1) = (j ∗ A)/(2 ∗ n ∗ F)

(4)

[Where the j (mA cm-2) is the current density; A (cm2) is the surface area of the material, here A = 0.07065 cm2; “2” means that it needs two-electrons to transfer during the formation of 1 mol H2 ; F is the Faraday’s constant, the value is 96485 C mol-1; “n” (mol) is calculated from the equation: n = m / M, here the m = 354 µg cm-2 * 0.07065 cm2, for that the content of Cr is little, so we considered the M = M(MoS2).]


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RESULTS AND DISCUSSION Cr-doped MoS2 for electrocatalysis water reduction application has never been reported. In order to certify the reasonable content of doped Cr, we synthesized a series of compounds for HER.

Figure 1. a) The XRD spectrums of the MoS2 and the doped Crx-MoS2 (x= 0.02, 0.04, 0.06, 0.08 and 0.10), b) the XRD spectra of MoS2 and Cr0.06-MoS2 (the PDF#37-1492 belongs to the 2H-MoS2).

caused by the smaller atomic of Cr has partly doped at the Mo site. What’s more, the AFM height sensor results (Figure S5c, d) also certified that the layer space is wider than 0.61 nm, and height data of 2.47 and 3.79 nm means that Cr0.06-MoS2 with 4- and 6-layers of S-(Mo, Cr)-S, respectively.29,38 Apart from the (002) plane, we can see that the HRTEM image of Cr0.06-MoS2 has both 1T- and 2HMoS2 phase (Figure 2c).39 The HRTEM graph of MoS2 was different with the Cr0.06-MoS2, and showed the 2H-phase only (Figure S6). The EDX-mapping of Cr0.06-MoS2 (Figure 2d) displayed a well distribution of S, Mo and Cr, this was consisting with the ICP results (Table S1). All of these demonstrated that the Cr has doped into the MoS2, uniformly.

We cannot find the spectrum of the template (g-C3N4) at 30o (Figure S1). Besides, both the result of thermogravimetry (TG, Figure S2) and the blank test of gC3N4 confirmed that the g-C3N4 has decomposed during the high temperature (800 oC) synthesis process, completely. This is consistent with ever reported.26 The XRD spectra (Figure 1a) of doped Crx-MoS2 (x=0.02, 0.04, 0.06, 0.08 and 0.10) are almost the same with that of the un-doped MoS2. The 2ϴ peak (Figure 1b) at 14.1, 32.6, 33.4, 39.7, 49.5 and 58.5o are assigned to the (002), (100), (101), (103), (105) and (110) planes of MoS2 (PDF#37-1492 belongs to the 2H-MoS2), respectively. The diffraction peaks positions of the Cr0.06-MoS2 are almost the same with that of the un-doped MoS2, so we suppose that the Cr has successfully doped into the MoS2. As for the broadened peak of (002), it’s largely due to the less layers of MoS2.37 The SEM image of Cr0.06-MoS2 shows an ultrathin nanosheets structure (Figure S3d), which is quite different with the template of g-C3N4 (Figure S1b) and the block MoS2 ever reported.27 This is mostly due to the decomposition of the g-C3N4, which leaving a large number of gas passages. Un-doped MoS2 also has the nanosheets structure, however, not as thin as that of the Crx-MoS2 (x=0.02, 0.04, 0.08, Figure S3). The ultrathin nanosheets morphology of Cr0.06-MoS2 greatly increased the specific surface area, which can be confirmed by the N2-adsorption and desorption curves (Figure S4). The BET surface area data of MoS2 (130.31 m2 g-1) is much lower than that of the Cr0.06-MoS2 (165.52 m2 g-1), this will insure expose more active sites and favorable for the channel leaving of the gas (H2) generated during the HER process. The TEM results confirmed that the Cr0.06-MoS2 has fewer layers structure (Figure S5b), this is consistent with the broadened peak of (002) in the XRD result. The Cr0.06MoS2 has the same fewer-layer structure with the un-doped MoS2. However, the former is not as straight as that of the latter (Figure 2a, b). For that the layer spacing of the MoS2 is 0.61 nm (Figure 2a), which is 0.02 nm thinner than that of the Cr0.06-MoS2 (0.63 nm, Figure 2b). This is mainly

Figure 2. a) The STEM images of a) MoS2 and b) Cr0.06MoS2. c) HRTEM pictures of the Cr0.06-MoS2, a mixture of 1T- (left) and 2H-MoS2 (right), d) the EDS-Mapping results of the Cr0.06-MoS2, S (red), Mo (green) and Cr (orange). The Raman results (Figure 3a) showed that the peaks around the wave number of 400 cm-1 are the characteristic Raman modes of MoS2. For that, A1g and E2g are the in plane and out of plane mode of MoS2, respectively.40 However, peaks at 147.7, 221.8, 286.1 and 337.2 cm-1 of Cr0.06-MoS2 are only active in the 1T-MoS2,35,37,41 which confirmed the coexist of 1T- and 2H-phase in the Cr0.06-MoS2. The chemical states of the element in the compound were investigated by the X-ray photoelectron spectroscopy (XPS) analysis. The element spectrums of the Cr0.06-MoS2 were showed in the Figure 3b-d. The main peaks at 586.1 and 576.9 eV were assigned to the Cr 2p1/2 and 2p3/2 (Figure 3b), respectively.30,31 This confirmed that the Cr3+ has doped into the MoS2, successfully. The peaks at 228.7 and 232.3 eV were corresponding to the 1H-phase. Besides, the binding energy of 228.3 and 231.5 eV were assigned to the 1T-phase (Figure 3c).41, 42 All of these confirmed the appearance of



Mo4+ in the Cr0.06-MoS2. The peak at 226.5 eV was owing to the S 2s.35 As for the spectrum of S, the binding energy at 161.8 and 162.9 eV were corresponding to the 2H-phase, and the BE at 161.3 and 162.5 eV were the peaks of 1T-phase, these were consistent with ever reported.37, 41 What’s more, both 2p1/2 and 2p3/2 (Figure 3d) of S were responses to the S2- in the compound. As for the XPS result of the MoS2 (Figure S7), only showed the 2H-MoS2. As a result, the Cr3+ has doped into the MoS2 and the Cr0.06-MoS2 with a coexistence of 2H- and 1T-phase.

Figure 3. a) The Raman results of MoS2 (black) and the Cr0.06-MoS2 (red). The high resolution XPS spectrum of element b) Cr 2p, c) Mo 3d and d) S 2p of the Cr0.06-MoS2. Water Reduction Performance The HER activity of Cr0.06-MoS2 is evaluated in the N2saturated 0.5 M H2SO4. Using the three-electrode system and with the scan rate of 5 mV s-1. The loading on the electrode has lots of effects on the property of the HER (Figure S8). Take Cr0.06-MoS2 for example; in order to achieve the same current densities, the electrode that loaded with 354 µg cm-2 needed the lowest overpotential but had the best activity for HER (Figure S8d). This was not only applied to the Cr0.06-MoS2, but also fitted to the undoped MoS2 and other Crx-MoS2 (Figure S8a, b, c, e and f). Therefore, we use the most suitable loading for all the tested electrodes except the blank glassy carbon electrode (GCE). The LSV curves of different mass content of Crx-MoS2 (Figure 4a) showed that the Cr0.06-MoS2 has the best performance for HER, adding more Cr(NO3)3·9H2O cannot improve the activity of catalyst. In order to arrive the same current densities of -10, -30 and -50 mA cm-2, the required overpotentials for Cr0.06-MoS2 (0.20, 0.28 and 0.34 V) were much lower than that of the MoS2 (0.44, 0.55 and 0.62 V), respectively (Figure S9a). When the overpotentials are 0.2, 0.3 and 0.4 V, that the current densities of Cr0.06-MoS2 (10, 35.8 and 70.4 mA cm-2) are almost over 11-times larger than that of the un-doped MoS2 (0.035, 1.08 and 6.20 mA cm-2),

respectively (Figure S9b). Above all, the property for HER has been greatly enhanced with the introduction of Cr3+. These are also corresponding to the calculated Tafel slopes (Figure 4b). The Tafel slope of Cr0.06-MoS2 is 41.6 mV dec-1, which is much lower than the 94.1 mV dec-1 of undoped MoS2. The much lower data of Tafel slope demonstrates the better HER kinetic process of Cr0.06-MoS2 electrode.36 What’s more, the series of Crx-MoS2 with the data of Tafel slope in the range of 40~50 mV dec-1, which is consist with ever report that the Tafel slope of 1T- and 2HMoS2 are ∼40 and 75-85 mV dec-1, respectively.43 Therefore, the Crx-MoS2 was a mixture of 2H- and 1T-MoS2.

Figure 4. a) LSV curves of Crx-MoS2 (x=0, 0.02, 0.04, 0.06, 0.08 and 0.10), g-C3N4, GCE and 20wt% Pt/C, b) corresponding Tafel slopes Pt/C, MoS2 and Crx-MoS2 (x=0.02, 0.04, 0.06, 0.08 and 0.10). c) The electrochemical impedance spectroscopy (EIS) of un-doped MoS2 and Cr0.06-MoS2 at overpotential of 0.22 V. Insert of it is the simulate circuit diagram of the EIS curves, d) i-t curves of Cr0.06-MoS2 for over 60 h (Ƞ=0.20 V). These were also corresponding to the electrochemical conductivity (Figure 4c, S10). Both the Rct (resistance of charge transfer in the catalyst, 16.55 Ω) and Rad36 (adsorption resistance on the surface of the electrode, 57.30 Ω) of the Cr0.06-MoS2 are much lower than that of the MoS2 (51.85 and 569.6 Ω, Table S2), respectively. This means that the doping of Cr has greatly shortened the transfer path of the electron and finally improved the conductivity of the catalyst. This was responded to ever report that the Crdoped MoS2 has the semimetal property.33 As for the durability, that the LSV curves maintains well even after 1000th CV tests (Figure S11). When the overpotential is 0.20 V, the i-t curve maintains at current density of -10 mA cm2 for over than 60 h (Figure 4d). All of these confirmed that the stability of Cr0.06-MoS2 is well enough even for long time tests. In order to evaluate the ECSA of material and certify that the active sites of Cr0.06-MoS2 has increased, we gained a series of CV curves using different scan rates from 10 to 100 mV s-1 to calculate the data of Cdl. We find that the ∆j (difference between positive and negative values at the


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fixed voltage) of Cr0.06-MoS2 are much larger than that of the un-doped MoS2 at different scan rates. This is consistent with the data of Cdl (Figure 5a and S12), that the Cdl of Cr0.06-MoS2 (5.5 mF cm-2) is over 17-times higher than that of the un-doped MoS2 (0.315 mF cm-2). Other CV curves of Crx-MoS2 are also given (Figure S12); the higher Cdl data confirms the larger active surface area (Table S3), as well as more active sites of catalyst. Both the EIS and Cdl data confirms that introduce more Cr cannot improve the HER performance. For that more Cr-doping will not shorten the transfer of electron during the HER process and cannot expose more active sites. As a result, the Cr0.06-MoS2 exposed more active sites and with the best activity for HER. The remarkable HER activity also corresponding to the theoretical calculation, that the Crdoped 1T-MoS2 has the reasonable ∆GH.23 In order to learn the efficient of the active sites, we also calculated the turn over frequency (TOF) of the H2 (s-1).44 The TOF data of Cr0.06-MoS2 were much higher than that of the MoS2 at different overpotentials (Figure S13). This also confirmed that the doping of Cr has greatly increased the efficient active sites in catalyst. As a result, the intrinsic HER activity of the Cr0.06-MoS2 has been hugely improved and among the best reported HER catalysts (Table S4). What’s more, the water reduction was very efficient, for that the faraday efficiency of the Cr0.06-MoS2 for HER was ≈100% (Figure 5b). The H2 was collected by the drainage method during the HER process.13

Figure 5. a) The relationship between the scan rate and the ∆j of MoS2 (black) and Crx-MoS2 (x=0.02, 0.04…0.10). b) The Faraday efficiency of H2 for the Cr0.06-MoS2, the insert image is the i-t test for over 60 min. The greatly improved property of Cr0.06-MoS2 for HER was mainly attribute to following reasons: 1) the decompose of g-C3N4 at high temperature leaving number of pass channels, this enlarged the BET surface area and did good to the bubbles (H2) leaving during the water reduction; 2) the introduction of Cr caused the coexistence of 1T- and 2H-MoS2 phase, which exposed more active sites (both on the plane and at the edges) and extremely decreased the impedance of Cr0.06-MoS2; 3) the enhanced efficiency of each active site greatly improved the HER activity; 4) As theoretical calculated that the 1T-MoS2 has the appropriate ∆GH for water reduction, which may partly enhance the H-atom-absorption property and promote the HER process. All of these reasons leading to the remarkable HER activity of the Cr-doped MoS2.

In summary, we have synthesized a series of the CrxMoS2 and used them for enhanced water reduction application, successfully. The Cr0.06-MoS2 has an ultrathin nanosheets structure, much larger adsorption/desorption capacity and more exposed efficient active sites. For that the Cdl of Cr0.06-MoS2 is nearly 17-times higher than that of the MoS2. Furthermore, the much lower impedance of Cr0.06-MoS2 has greatly shortened the electron transfer. These in turn enhanced the intrinsic activity of the catalyst for water reduction. For instance, the current densities of Cr0.06-MoS2 were almost 11-times larger than that of the undoped MoS2 at some different potential. The remarkable enhanced properties for water reduction through Crdoping not only a supplement to the theoretical calculation, but also will open more opportunities in discovering more efficient catalysts for the water splitting application.

ASSOCIATED CONTENT Supporting Information. Supplementary figures for the XRD of g-C3N4. Nitrogen adsorption-desorption curve of MoS2 and Cr0.06-MoS2. The SEM pictures of g-C3N4, Crx-MoS2 (x=0, 0.02, 0.04, 0.06, 0.08 and 0.10) and the CV curves related to the data of Cdl for Crx-MoS2. Height sensor results of AFM for the Cr0.06-MoS2, HER performance showed by LSV curves that affected by the electrode-loading. TOF (H2) of Cr0.06-MoS2. Supplementary tables of ICP, EIS, ECSA data and a summary of the HER properties for various electro-catalysts are available. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Follow the journal’s guidelines on what to include in the Acknowledgement section.

REFERENCE

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For Table of Contents Use Only

Non-noble-metal doped 1T-2H Cr0.06-MoS2 shows enhanced HER activity and lower EIS, which will beneficial to the sustainable development of environment.

8 ACS Paragon Plus Environment

1T-2H Crx-MoS2 Ultrathin Nonosheets for Durable and Enhanced

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