1T Phase Transition of Multilayer MoS2 by Electrochemical

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2H/1T Phase Transition of Multilayer MoS2 by Electrochemical Incorporation of S vacancies Xiaorong Gan, Huimin Zhao, Tsz Wing Lo, Kwun Hei Willis Ho, Lawrence Yoon Suk Lee, Dangyuan Lei, and Kwok-Yin Wong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00875 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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ACS Applied Energy Materials

2H/1T Phase Transition of Multilayer MoS2 by Electrochemical Incorporation of S vacancies Xiaorong Gan, Huimin Zhao, Tsz Wing Lo, Kwun Hei Willis Ho, Lawrence Yoon Suk Lee, Dang Yuan Lei*, Kwok-yin Wong* Xiaorong Gan, Lawrence Yoon Suk Lee, Prof. Kwok-yin Wong Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic University, Hong Kong, 999077, China [email protected] Tsz Wing Lo, Kwun Hei Ho, Dr. Dang Yuan Lei Department of Applied Physics The Hong Kong Polytechnic University, Hong Kong, 999077, China [email protected] Prof. Huimin Zhao Key Laboratory of Industrial Ecology and Environmental Engineering School of Environmental Science and Technology Dalian University of Technology, Dalian, 116024, China

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ABSTRACT: The phase transition of multilayer MoS2 nanosheets from semiconducting 2H to metallic 1T (2H/1T) has been realized mainly by chemical methods (e.g., Li intercalation). Here, we develop a simple yet effective method cyclic voltammetry - to successfully tune the 2H/1T phase transition of multilayer MoS2 nanosheets without using intercalation species. The phase transition is triggered by the electrochemical incorporation of S vacancies (obtained by electrochemical etching), which on the one hand injects electrons into the framework of S-Mo-S and on the other hand facilitates the sliding of S planes. Density functional theory calculations show that O doping in the framework of S-Mo-S decreases the energy barrier for forming S vacancies and stabilizes the 1T-phase by occupying the 4d orbital of Mo. Our calculations further show that the presence of S vacancies and O incorporation not only reduces the bandgap of MoS2, indicating an increased conductivity, but also decreases the hydrogen adsorption free energy, implying significant improvement of hydrogen evolution reaction (HER) activity. Indeed, the overpotential and Tafel plot of the electrochemically treated MoS2 nanosheets are decreased respectively by 174 mV and 25 mV/dec at a cathodic current density of 10 mA cm−2 compared with pristine 2H-MoS2 nanosheets. The HER experiment also reveals the order of catalytical activity for the studied phases and structural defects: 1T-MoS2 > S vacancies > O doping > 2H-MoS2. Our study has provided a new route to control the phase transition of multilayer MoS2 nanosheets with promising applications potentially in catalysis and optoelectronics. KEYWORDS: multilayer MoS2 nanosheets, phase transition, S vacancies,1T-MoS2, 2H-MoS2


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1. Introduction In addition to chemical composition and atomic arrangement, dimensionality plays a crucial role in determining the fundamental properties of materials. Two-dimensional (2D) layered nanomaterials, including graphene and transition metal dichalcogenides (TMDCs), are such an excellent expample.1-3 Among them, 2D molybdenum disulfide (MoS2) has recently trigged intensive research interest due to the intriguing physicochemical properties that stem from quantum size effects due to its atomic thickness.4-6 It exhibits great potential applications in electronics, optoelectronics, sensors, catalysis, gas separation, energy storage and conversion, water remediation, and biomedicine.4-10 Particularly in such applications as field effect transistors (FETs), phototransistors and logic circuits, 2D MoS2 is expected to complement or even surpass graphene (with a zero band gap).5 2D MoS2 has three polytypic structures including 2H (trigonal prismatic coordination), 1T (octahedral symmetry), and 1T’ (distorted 1T).4, 11-12 As a matter of fact, 1T-MoS2 is unstable without no extra electron injection into its framework of S-Mo-S, because it tends to aggregate and then transform into stable 2H-MoS2 due to S–S van der Walls interaction.12 Under certain conditions, the phase transition can occur between prototypes of MoS2, such as 2H-MoS2→1T-MoS2 and 2H-MoS2→ 1T’-MoS2.11-12 In 2H-MoS2, the d orbitals split into three bands, namely, d z2 , d x 2 -y 2 ,xy , and d x z,y z , separated by an energy gap.11 Therefore, 2H-MoS2 exhibits

semiconductor characteristics and poor capability of charge transfer (a bandgap of ∼1.9 eV and charge carrier mobility of 0.1–10 cm2 V-1 s-1 for monolayer 2H-MoS2),5-6



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which severely limits the applications in electrochemical catalysis,13-14 FETs,15-16 thermoelectric energy harvesting,16 and electrochemical sensing.5 Due to the localization behavior of the d-band electrons in transition metals, the electronic properties of 1T-MoS2 are dramatically different from 2H-MoS2. Specifically, the partially filled d xz,yz,zx orbital leads to a metallic character in the band structure of 1T phase, 107 times more conductive than the semiconducting 2H phase.11,

16

Such

metallic phase eliminates the Schottky barrier upon contact with metals and thus improves

charge

transfer

rate,11

which

provides

obvious

advantages

in

electrochemical applications. For example, compared with 2H-MoS2, 1T-MoS2 has low overpotential and Tafel slope in hydrogen evolution reactions (HER),17 high sensitivity for constructing sensors (e.g., gas sensors),5,

16

high power factor for

thermoelectric energy harvesting (73.1 µW/mK2 at room temperature),16 small contact resistance for FETs (200-300 Ω·µm at zero gate bias for 1T-MoS2 and 0.7-10 kΩ·µm for 2H-MoS2).15 Although a great deal of endeavors to improve the conductivity of 2H-MoS2 were performed by simple combination of 2H-MoS2 with various conductive supports, such as graphene/MoS2 and polyaniline/MoS2, these methods usually generate significant contact resistance between 2H-MoS2 and the substrates.9 In contrast, conducting 1T-MoS2 nanosheets obtained through the 2H/1T phase transition (2H-MoS2→1T-MoS2) represents a more effective and desirable means.11 In general, the 2H/1T phase transition of MoS2 can be realized with chemical and physical methods.11, 18 Thereinto, the chemical methods are mainly based on using intercalation species, such as Li+ and Na+,19 to promote the 2H/1T phase transition by


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electron donation from corresponding atoms (Li and Na) into the van der Waals gap of 2H-MoS2, and generate a stable 1T phase. However, almost all chemical methods involve a rather long reaction time (typically over 3 days) and the use of dangerous reagents (n-butyl lithium or Na electrode).20-21 The physical methods are mainly based on strain effects and charge injection.11 So far, in experiment the strain-effect-induced 2H/1T

phase

transition

has

only

been

realized

in

WS2,

and

the

charge-injection-induced phase transition has been predicted by density functional theory (DFT) calculations but not yet observed in experiment. In theory, it is demonstrated that injection of 4e− in a (4 × 4 × 1) supercell of 2H-MoS2 decreases the phase transition barrier energy by 0.27 eV so that the 1T-MoS2 is more stable than 2H-MoS2 in the total energy by 0.30 eV.

11

In essence, both ion intercalation and

charge transfer share the same mechanism in the context of electron donation. Structurally, the 2H/1T phase transition of MoS2 involves one of the S atoms in the unit cell moving from one pyramidal position to the other, that is, a transversal displacement of one of the S planes (Scheme 1).19, 22 Therefore, S vacancies could decrease the energy barrier of 2H/1T phase transition, and as electron donors, promote the phase transition by weakening Mo-S bonds.11-12 On the other hand, S vacancies act as the nucleation sites of 1T phase which subsequently extends to whole flakes to complete the 2H/1T phase transition.12,

23-26

Previous reports on generating S

vacancies were focused on monolayer 2H-MoS2 by combining argon (Ar) plasma exposure and H2 annealing.27 Recently, electrochemical desulfurization has also been performed under proper negative potentials to achieve the 2H/1T phase transition in



2H-MoS2.27 Considering the fact that multilayer 2H-MoS2 nanosheets are the main product of typical synthesis methods such as liquid-phase exfoliation (LPE),5, 28-29 here we develop a simple, scalable and effective method based on electrochemical cycling to realize the 2H/1T phase transition in multilayer 2H-MoS2 nanosheets by forming a large amount of S-vacancy without using intercalation species. We demonstrate for the first time that the formation of S vacancies in multilayer 2H-MoS2 nanosheets can be realized by applying positive voltage for electrochemical etching. In addition, certain amount of S atoms are substituted by O atoms under the positive overpotentails. Comprehensive spectroscopy and imaging characterizations jointly confirm the formation of S vacancies, O doping and 1T phase in the 2H host. Thorough DFT calculations demonstrate that the generation of S vacancies significantly reduces the band gap energy of MoS2, and the O doping lowers the energy barrier for the formation of S vacancies. These results are consistent with the HER activity observations with the multilayer 2H-MoS2 nanosheets before and after electrochemical treatments. Scheme 1. Atomic mechanism of the 2H/1T phase transition in MoS2. The S plane glides over a distance equivalent to a/√3 (a = 3.16 Å), which results in the 2H→1T phase transition.18 Blue, Mo; yellow, S.


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2. Results and Discussion 2.1.

Raman characterizations for 2H/1T phase transition

The pristine multilayer 2H-MoS2 nanosheets used in this work were prepared by the LPE method with commercially available bulk MoS2. The as-prepared nanosheets have an average thickness of ~10 nm (16 layers) and a lateral size around 0.29 µm as jointly determined by performing TEM imaging and UV-Vis absorption spectroscopy (see Figure S1 in Supporting Information).30 To observe the 2H/1T phase transition, the as-formed pristine multilayer 2H-MoS2 nanosheets were treated by cyclic voltammetry (CV) with different cycling times and potentials in 0.5 M H2SO4 after removing the dissolved oxygen (Figure S2). The samples were treated by CV with different potential ranges (0~x, x=0.8, 1.6, and 2.2 V) for 160 cycles, labeled as Sx-160. Because the 2H/1T phase transition is sensitive to the exposure of electron radiation in TEM characterizations,19 Raman spectra were first carried out to characterize both the pristine and treated 2H-MoS2 nanosheets. Figure 1a shows that the influences of the scan potentials on the 2H/1T phase transition. For the pristine MoS2 nanosheets, the vibration modes located at 282, 380, 406, and 450 cm−1 correspond to E1g, E12g,



A1g modes, and the longitudinal acoustic phonon mode of 2H-MoS2, respectively.31 Due to the forbidden selection rule, generally the E1g mode is quite weak and hardly observed in the backscattering geometry on the surface perpendicular to the c axis. The E12g mode is attributed to the in-plane opposite vibration of two S atoms with respect to the Mo atom, and the A1g mode is for out-of-plane vibration of only S atoms in opposite directions. The asymmetric Raman peak at 450 cm-1 arises from a second-order process involving the longitudinal acoustic phonons at M point (LA(M)), which, compared with bulk MoS2 (460 cm-1), shifts slightly due to decreased electron-phonon coupling.32 As the applied potential is 0.8 V, several new vibration modes in the low frequency range could be observed for S0.8-160 at 151, 221, and 326 cm-1, corresponding to J1, J2, and J3 peaks of 1T-MoS2.32-33 Here the mode at 151 cm-1 is an in-plane shearing mode of one stripe of an atom with respect to the other inside a chain. The J2 mode corresponds to the shift of the S-atom layers with respect to the Mo-atom layers, and the J3 mode is for the stretching of one side of a zig-zag chain relative to the other with a slightly out-of-plane component.25 The presence of stable 1T phase in S0.8-160 implies that the S vacancies have been generated, because S vacancies as electron donors promote the 2H/1T phase transition.12 When the applied potential is further increased from 0.8 V to 1.6 V and 2.2 V, the three peaks of 1T-MoS2 become more obvious. However, compared with the four modes of 2H-MoS2, J1, J2, and J3 peaks are relatively broad and weak, indicating that the as-formed 1T-phase should be the minority component, in contrast to 2H-phase. In addition, for S1.6-160 and S2.2-160, new high-frequency modes at 794, 847, and 880 cm-1


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correspond to the hydrous MoO3, such as [MoO4]2-(aq) and [Mo7O24]6-(aq).34 With the increase of applied potential range, the intensity of MoO3 gradually enhances, indicating of the enhanced level of O-doping. Also, the amount of 1T-MoS2 in the treated MoS2 gradually increases, mainly due to the increased amount of S vacancies that benefit the 2H/1T phase transition. The frequencies of E12g and A1g modes exhibit slight blue shift, especially as a large amount of MoO3 are formed. In general, with the increase of thickness, the E12g vibration softens (red shifts), while the A1g vibration stiffens (blue shifts). Therefore, this result again suggests that the incorporation of S vacancies and O-doping could affect the intralayer coupling and lattice dynamics of 2H-MoS2, especially the long-range Coulombic interlayer interactions.31-32, 35 Similar to the applied potential, with the increase of the cycle times, the same trend could be observed, that is, the peak intensities of 1T-MoS2 and MoO3 become more obvious. The increased cycle times is equal to prolonging the reaction time of the selective electrochemical extraction and oxidation processes. In essence, there is no any difference in the components of the resultant formed by tuning the scan potential or cycling times.



Figure 1. Raman spectra of the multilayer (~ 16 layers) 2H-MoS2 nanosheets treated under varied scanning potential (vs. SCE) range (a) and cycling times (b). The samples were loaded on a Si substrate, contributing to the oserved characteristic peaks of Si and SiO2. 2.2 Morphology characterizations about the enhanced structural disorder Because the oxidation of 2H-MoS2 nanosheets is inevitable in electrochemical treatments even at the stage of LPE process due to the appearance of hydroperoxide intermediates on the plane edges,36 we did the morphology characterization of S2.2-160. Figure 2 compares the morphological and structural features of the pristine and treated MoS2 (S2.2-160). The HRTEM image of the pristine MoS2 nanosheets in Figure 2a reveals clear crystal lattice arrangement with a measured d-spacing of 0.28 nm (see the inset in Figure 2a),13 consistent with the (100) atomic planes of 2H-MoS2. The corresponding fast Fourier transform (FFT) generated selected-area electron


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diffraction (SAED) pattern shows two sets of well-defined diffraction spots, corresponding to the (100) and (200) planes of 2H-MoS2, respectively. The six diffraction spots in each set can be divided into groups: ka = {(1 110), (10 1 0), (0 1 10)} and kb = -ka, 37 indicating good crystalline quality without the presence of the 2H/1T phase transition. Compared to the pristine sample (Figure 2a and Figure S3a), the electrochemically modified MoS2 exhibits obvious differences in the morphology and crystal structure, especially for the thin regions of MoS2 nanosheets (Figure 2b and Figure S3b). First, the HAADF-STEM (Figure 2c) of the thin regions and HRTEM image of specific hole (the inset in Figure 2b) reveal 2D porous structures with an average pore size less than 10 nm, indicating of the presence of more defects that promoted the localized electrochemical corrosion to occur at a faster rate. As a result, a large amount of S atoms were preferentially removed by selective electrochemical etching,38 because S atoms possess a higher reactivity (the Hirschfeld charge (χ): χ(Mo) = 0.223, χ(S) = -0.112) and less stability (the formation energy (Ef): Ef(Mo) = -3977.387 Ha, Ef(S) = -398.115 Ha), according to our DFT calculations. Second, the SAED pattern of the treated

sample

(Figure

1d)

shows

six

diffraction

rings

(rather

than

hexagonally-arranged six diffraction spots as observed for the pristine sample), indicating an increased structural disorder, possibly due to the presence of quasi-periodic arrangement of nanodomains with varied amounts of defects (see the inset in Figure 2b). The structural disorder of these regions was quantitatively evaluated to be 50.2% by the angle (30.1°) formed by the two end points of a



diffraction arc and the central spot (Figure 2d).13 The appearance of additional spots (indicated by dashed circles in Figure 2d) between the two diffraction rings is an important signature of the 2H/1T phase transition.24 In addition, S vacancies, 1T-MoS2, 2H-MoS2, and the intermediate (precursor) phases (α-phase) are clearly observed in Figure 2e and Figure 2f. After electrochemical treatments, a series of S vacancies, rather than individual vacancies (see the down inset of Figure 1e and Figure S4), were found, because S vacancies tend to form in clusters to stabilize the system.27, 39 The clusters of S vacancies lead to the formation of the intermediate (precursor) phases, α-phase, possibly via local compression of Mo-Mo distance, and finally form the 1T phase due to the movement of defects and the electron injection from S vacancies.11, 19 In addition, the major part of basal planes still exhibits the periodic high-quality crystalline structure of 2H and 1T hybrid phases, while the partial or complete structural disorder appears at the grain boundaries due to their high surface energy (Figure 2e). The coexistence of 1T (octahedral) and 2H (trigonal prismatic) phases with arranging atoms in orderly fashion is similar to the observation for the mixed phase structure synthesized by lithium intercalation.26 According to the results indicated by Figure 1 and Figure 2, the enhanced structural disorder should be derived from the incorporation of S vacancies and O-doping, which could result in the sliding and structural distortion.19, 40 The induced disorder should gradually propagate from the grain boundaries and plane edges to the surroundings and the center of basal planes as the electrochemical treatments proceed, and finally generate the porous structure (Figure 2b, c).


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Figure 2. Morphology characterization of multilayer 2H-MoS2 nanosheets before and after electrochemical treatments. (a) HRTEM image of the pristine nanosheets. (b) TEM, (c) HAADF-STEM, (d) selected-area electron diffraction (SAED) pattern, and (e, f) HRTEM images of the modified S2.2-160 sample. Here, S, 1T, 2H, and α represent S vacancies, 1T-MoS2, 2H-MoS2, and the intermediate (precursor) phases of 2H/1T transformation process, respectively. The schematic models of 1T and 2H phases did not cover all corresponding regions in selected parts (white squares) of (e) and (f). EDS mapping and TEM-EDX spectroscopy were carried out to further figure out the influences of electrochemical treatments on the formation of S vacancies. The HAADF-STEM image and corresponding EDS mapping analyses in Figure 3a reveal a homogeneous distribution of Mo and S in the pristine multilayer 2H-MoS2 nanosheets. Quantitative element analysis of the STEM-EDX spectrum shows that the atomic ratio of Mo:S for the pristine sample is 1:2.09, slightly smaller than that of bulk MoS2, possibly because of the segregation of S atoms on the basal planes.41 On the other hand, the trace amount of O on the surface of MoS2 is derived from the adsorption of dimethyl formamide (DMF) and its hydroperoxide intermediates during the LPE synthesis process.36 After the electrochemical treatments, however, the relative amount of S is dramatically decreased with Mo:S atomic ratio of 11:1 (Figure 3b), while the amount of O is significantly increased from ~0 to 32.92%, evidencing



that a large number of S atoms were selectively removed or oxidized (replaced by O atoms). According to the atomic ratio of Mo, S, and O, the concentration of S vacancies is approximately 46%, much larger than the theoretically predicted optimal S-vacancy concentration of 11% that can maintain the surface stability.39 Therefore, the excessive extraction of S results in the enhanced structural disorder and the mixture of crystalline and non-crystalline regions.

Figure 3. Element distribution and analysis. HAADF-STEM image, corresponding EDS mapping, STEM-EDX spectra of the pristine 2H-MoS2 nanosheets (a) and the treated S2.2-160 sample (b). The inset tables summarize corresponding quantitative element analysis. 2.3. Influences of enhanced disorder on electronic structures


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The enhanced structural disorder (S vacancy, O incorporation, and 1T phase) was further confirmed by comparing the absorption spectra of multilayer 2H-MoS2 nanosheets before and after the electrochemical treatments. The UV-Vis absorption spectrum of the pristine MoS2 sample (denoted as Pristine) exhibits four well-defined absorption peaks (Figure 4a). Thereinto, A and B peaks correspond to the excitonic transitions occurring at the K/K′ points in the first Brillouin zone, and C and D for the optical transitions from the deep valence band to the conduction band.6 After electrochemical treatments, these four absorption peaks are less resolved, again suggesting the presence of structural disorder.42 Compared with pristine multilayer 2H-MoS2 nanosheets, all absorption peaks of S2.2-160 (the treated sample) shift to low wavelength (red shift), indicating of the decreased exciton binding energy (Ebind), characterized by,43 E bind =

m* e 4 e2 = 2εrB* 2εh 2

(Equation 1)

where rB* is the exciton Bohr radius, ε is the dielectric constant, and m* is the exciton reduced mass. From this Equation 1, the exciton binding energy is expected to be decreased by enlarging the dielectric constant, or decreasing the excitonic reduced mass (i.e., effective masses of the carriers). DFT calculations demonstrate that the effective mass becomes large (Table S1), implying that the enhanced disorder could result in the strong electron scattering. On the other hand, MoS2 has a small relative dielectric constant ∼5, while α-MoO3 possesses much larger relative dielectric constant >500.44 Therefore, the enlarged dielectric constant leads to the decreased exciton binding energy. Compared with the pristine one, S2.2-160 exhibits the enhanced



absorption in the near-IR region, indicating the enhanced light-matter interactions, because the band gap is reduced after the introduction of structural disorder. Furthermore, XPS spectra (Figure 4b, c) suggest that for the pristine MoS2 nanosheets, most of the Mo signal arises from a 3d5/2 peak at 229 eV and a 3d3/2 peak at 232 eV characteristic of a +4 oxidation state. Two weak shoulder peaks corresponding to the higher +5 and +6 oxidation states remain, probably due to the formation of some surface oxide.45 After the electrochemical treatments, the peaks of +5 and +6 oxidation states of Mo disappear, indicating that the surface oxide is the adsorbed organics, such as DMF and its hydroperoxide intermediates.36 As the electrochemical etching proceeds, the surface S atoms are also removed and form S vacancies, which is again confirmed by XPS results of S (2s) that the peak of SII (sp 3/2) decreases and broadens, and the peak of SII (sp 1/2) almost disappears (Figure 4c). Deconvolution of the broad Mo 3d5/2 and 3d3/2 peaks in both spectra presents two independent Mo 3d regions, with a separation of binding energy around 0.8 eV, again indicating the coexistence of 2H- and 1T- phases in the treated MoS2 nanosheets with the 2H phase being dominant (Figure 4b). We also observe that the peak intensity ratio of MoIV 3d3/2 to MoIV 3d5/2 is significantly increased after the electrochemical treatments, indicating that the number of equivalent electrons of MoIV 3d5/2 available to ionize is decreased, possibly due to the incorporation of O atoms. The calculated PDOS also confirms that for (3×3×1) 2H-MoS2 containing 3.7% O (Figure 4d and Figure 4e), more electrons could be distributed in the states with high energies (indicated by an arrow), which would relatively decrease the number of electrons available at the


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ground state for the generation of photoelectrons. In addition, the peaks of both MoIV 3d3/2 and MoIV 3d5/2 shift towards the lower binding energy, indicating of the reduced electronic band gap (Eg) that is caused by the formation of structural disorder. DFT calculations further confirm that Eg decreases from 1.90 eV (the pristine monolayer) to 1.62 eV of 3.7% O-doping, and 0.99 eV of 3.7% S-vacancy (Figure 4f), which is in good agreement with the results of UV-vis absorption spectra (Figure 4a). Of note, before and after the incorporation of S vacancies and O atoms, the conduction-band minimum (CBM) and the valence-band maximum (VBM) are located at Γ(0, 0, 0) in Brillouin zone. The red shift of MoIV 3d3/2 and MoIV 3d5/2 is also consistent with the observation of LixMoS2 (x = 0.02~0.85) compound synthesized by electrochemical intercalation of Li+ ions into the van der Waals gap of MoS2 nanosheets.7 Therefore, we believe that there is the electron injection into the framework of S-Mo-S, which is derived from the presence of S vacancies, similar to the Li intercalation. According to XPS and DFT calculations, the influences of the enhanced structural disorder on the behaviors of photoexcited electrons of 2H-MoS2 include two aspects: the reduced band gap and the introduced gap states between VBM and CBM. 7, 39



Figure 4. (a) UV-vis absorption spectra, (b, c) high-resolution XPS spectra of the pristine 2H-MoS2 nanosheets (Pristine) and the S2.2-160 sample (Treated). (d) Optimized structures, (e) partial density of states (PDOS) of Mo-d, (f) calculated band structures of (3×3×1) 2H-MoS2 containing 3.7% S vacancies or 3.7% O atoms, and pristine one (from top to down). The Fermi level is set to zero in PDOS (red dotted line). Cyan, Mo; yellow, S; red, O. The dotted circle represents S-vacancy. The Mo atom with an arrow is investigated for PDOS.

2.4. Influences of O-doping on the formation of S vacancies The instability of 1T-MoS2 is derived from S-S van der Walls interactions.12 According to the crystal field theory, the S-vacancies on MoS2 surface, which act as electron donors, promote the phase transition by both weakening Mo-S bonds and decreasing the energy barrier of phase transition.11-12, 19 STEM-EDX, Raman, and XPS spectra have demonstrated that the formation of S vacancies is accompanied by the O-doping. To some extent, the O-doping makes S-S van der Walls interactions weak; therefore, we further investigate the influences of the O-doping on the


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formation of S vacancies including the formation energies (Ef) and the electronic band structures of (3×3×1) super cell of 2H-MoS2. The model is built based on the fact that the phase transition and incorporation of S vacancies and O-doping occur mainly on the basal plane of 2H-MoS2. The formation energy of O-doping in the nanosheets can be calculated by Ef = Etot(x O-incorporated*) + x µ(S) – Etot*– x µ(O)

(Equation 2)

and the formation energy of S vacancies in the nanosheets is given by Ef = Etot(x S-vacancy*) – Etot* + x µ(S)

(Equation 3)

where Etot(x O-incorporated*), Etot(x S-vacancy*), and Etot* are the total energy of MoS2 with the incorporation of x oxygen atoms, the total energy of MoS2 containing x sulphur vacancies, and the total energy of pristine MoS2, respectively. µ(S) and µ(O) are the chemical potentials of sulphur and oxygen atoms. As shown in Figure 5a, the formation of 3.7% S vacancies (Ef = 0.06 eV) in (3×3×1) 2H-MoS2 requires much more energy than that of 3.7% O atoms (Ef = -0.08eV). For the mixture of S vacancies and O atoms, to some extent, the O incorporation can activate the formation of S vacancies because Ef (S vacancies) becomes more negative with the increase of O atoms. For example, Ef = -0.17 eV is for S vacancies in the mixture of 11.1% O atoms and 3.7% S vacancies. Therefore, certain amount of S atoms, especially the unbound S atoms at high energy sites such as plane edges and defects, would be firstly replaced by O atoms before the formation of S vacancies. In addition, the computed projected density of states (PDOSs) of (3×3×1) 2H-MoS2 with 3.7% O incorporation (Figure 5b) points out that the O incorporation could result in the strong hybridization between



the O 2p states and the neighboring Mo 4d states. As a result, additional electrons from O atoms occupy the Mo 4d orbitals, which can increase the stability of the 1T-phase.18 The calculated electron density difference and Hirshfeld charge analysis demonstrate that compared with the pristine 2H-MoS2, the incorporation of S vacancies (3.7%) in 2H-MoS2 could lead to the elastic strains (~5.7%). As such, it led to the movement of other S atoms and the formation of the intermediate (precursor) α-phase, which are consistent with the results obtained in Figure 2. In addition, DFT calculations demonstrate that the presence of S vacancies can cause the electron injection of 0.04e and 0.02e into the left Mo atoms (Figure 5c), which could promote the phase transition because the electron injection could destabilize the 2H-MoS2.11 Some report has demonstrated that the extra electron entrancing into nonbonding d-orbitals of MoS2 makes it an isoelectronic of group 7 transition metal dichalcogenides, which exhibit a metallic character of 1T-MoS2. Therefore, we believe that in this study, the formation of S vacancies could not only inject electrons into the framework of S-Mo-S, but also facilitate the sliding of S planes, both which promote the 2H/1T phase transition. The electronic band structures show that the band gap of (3×3×1) 2H-MoS2 with O-doping and S vacancies (0.80 eV, Figure 5d) is much smaller than that of pure O-doping (1.62 eV) or S vacancies (0.99 eV). Therefore, the simultaneous incorporation of O atoms and S vacancies leads to a synergetic effect in improving the conductivity of the MoS2 nanosheets.


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Figure 5. (a) Calculated formation energies of (3×3×1 super cells) 2H-MoS2 containing 3.7% S vacancies, 3.7% O atoms, and mixture of S vacancies and O atoms at varied atomic ratio (1:1, 1:2, and 1:3). The 2H-MoS2 containing 3.7% O atoms was used to determine the formation energy of O atoms, and the rest for the formation energy of S vacancies. (b) Projected density of states of (3×3×1) 2H-MoS2 with 3.7% O. (c) Electron density difference of Mo atoms in the pristine 2H-MoS2 and 2H-MoS2 containing 3.7% S vacancies and 3.7% O atoms. Compared with the pristine one, there are additional 0.04e and 0.02e of injection into the left Mo atoms in (3×3×1) 2H-MoS2 containing 3.7% S vacancies and 3.7% O atoms. (d) Band structures of (3×3×1) 2H-MoS2 with 3.7% O atoms and 3.7% S vacancies. The presence of S vacancies introduces new bands (red curves) in the gap, near the Fermi level.

2.5. HER catalytic activity As a proof of application, we investigated the electrocatalytic HER activity of the multilayer 2H-MoS2 nanosheets before and after the electrochemical treatments. Figure 6a shows representative linear sweep voltammograms (LSV) for the multilayer 2H-MoS2 nanosheets before and after electrochemical treatments with different scan potentials. The pristine sample exhibits an overpotential value (η) of 373 mV with low current density because of low



edge concentration (16-layer thickness) and poor conductivity (Eg=1.2 eV).10 In contrast, the treated samples exhibit a significantly improved electrocatalytic activity with low overpotentials, which are determined as 333 mV for S0.8-160, 26 mV for S1.6-160, and 133 mV for S2.2-160. The phenomena are consistent with the previous reports that the electrochemical treatments of MoS2 could dramatically improve its HER activity due to the presence of S vacancies or 1T phase.27,

46

From their Raman spectra (Figure 1a), the presence of certain

amount of 1T-MoS2 is responsible for the decreased η of S0.8-160. For S1.6-160, the current density was obviously smaller than that of S0.8-160 or S2.2-160, an indication of the catalyst poisoning of S1.6-160, which decreases the number of active sites (unsaturated S atoms at the plane edges) of S1.6-160 for HER.10, 46-47 Moreover, the abnormal overpotential value (26 mV) of S1.6-160 is derived not from the HER, but from the reduction of the multilayer 2H-MoS2 nanosheets after the electrochemical oxidation of plane edges, because there is an obvious Faradaic current even at the potential of 0 V.47 As a matter of fact, the incorporation of O atoms into MoS2 nanosheets could play opposite roles dependent on the amount and location of incorporated O atoms.10, 13 A small amount of O atoms incorporated at the plane edges of 2H-MoS2 will reduce the HER activity of 2H-MoS2 (S1.6-160),10 while a proper amount of incorporated O atoms, especially in the basal plane, not at the plane edges, could can effectively regulate the electronic structure and further improve the intrinsic conductivity.13 As such, we could conclude that the synergistic improvement of


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both active sites and electrical conductivity results in the enhanced HER activity of S1.6-160 and S2.2-160, which could be proved by the reduced energy gap (Figure 4 and Figure 5) and the enhanced structure disorder (Figure 1).13, 48 The three inflection points observed in the polarization curve of S2.2-160, suggest that the HER activity of 1T-MoS2, O atoms, S vacancies, and 2H-MoS2 should be different.13, 24, 49 From the extrapolation of the linear region of overpotential versus Log j (Figure 6b), we obtained Tafel slopes of 289, 159, 650, and 264 mV/dec for the pristine 2H-MoS2 nanosheets, S0.8-160, S1.6-160, and S2.2-160, respectively. The dramatic enhancement in the catalytic activity of S0.8-160 has demonstrated that the phase transition into the metallic 1T polymorph is as important as active edge sites in enhancing HER activity when the main phase is 2H-MoS2.24 The HER rate (Tafel slope) of S2.2-160 is improved in contrast to the pristine 2H-MoS2 nanosheets, but lower than that of S0.8-160 because of the mass loss after electrochemical corrosion for a long time. After increasing the loading amount of S2.2-160 from 1.59 to 3.18 mg/cm2, the onset of the catalytic activity shifted to a much lower overpotential (107 mV) with a lower Tafel slope of 199 mV/dec (Figure S5). In order to shed light on the independent influences of 1T-MoS2, O atoms, S vacancies, and 2H-MoS2 on HER activity, we systemically investigated the polarization curves of 2H-MoS2 after electrochemical treatments with different cycling times (Figure 6c). Even though the low scan rate could improve the HER activity (Figure S6), especially at high scan potential range of 0~2.2 V, some detailed information



was lost about the roles of 1T-MoS2, O atoms, S vacancies in affecting HER activity. As shown in Figure 6c, the polarization curve of the sample (S2.2-20) obtained after 20 cycles, exhibits a clear and new reduction peak current (one inflection point), which should be ascribed to the presence of 1T-MoS2 when compared its Raman spectrum with that of the pristine 2H-MoS2 (Figure 1). Furthermore, the polarization curves of S2.2-40 and S2.2-80 exhibit three inflection points, which are similar to that of S2.2-160. The phenomenon implies that S2.2-40 and S2.2-80 also formed S vacancies, O atoms, and 1T phase. The overpotentials for S2.2-40, S2.2-80, and S2.2-160 are 222, 264, and 309 mV for the cathodic current density of 10 mA cm−2. With the increase of cycling times, the overpotential becomes large mainly due to the mass loss of the catalysts.

Figure 6. Polarization curves (left) and corresponding Tafel plots (right) of the MoS2 nanosheets after electrochemical treatments with CVs at different scan potential ranges (a


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and b) and cycling times (c and d). The sample was labeled as Sx-y, in which x and y mean the scan potential range of 0~x V vs. SCE and cycling times of y, respectively.

The Gibbs free-energy of the adsorption atomic hydrogen ( ∆G 0H ) (Figure S7a) shows that, as the S atoms at plane edges are replaced by O atoms (3.7%),

∆G 0H is 0.255 eV, bigger than that of pristine 2H-MoS2 ( ∆G 0H =0.08 eV).50 The result demonstrates that the incorporation of O atoms at plane edges could enlarge the binding energy of hydrogen adsorbed on 2H-MoS2, and consequently result in the catalyst poisoning, which is consistent with the observations in S1.6-160. In addition, the formation of S vacancies (3.7%) and O atoms (7.4%) in MoS2 could dramatically decrease ∆G 0H at active sites of either S vacancies ( ∆G 0H =-0.132 eV) or O atoms ( ∆G 0H =-0.196 eV). Compare with O-doping, S vacancies are still the major active sites for the HER by the introduction of these new localized gap states (Figure 5), which could improve the DOS near the Fermi level.51 With the increase of O atoms from 3.7 to 7.4%,

∆G 0H at active sites of S vacancies are changed from -0.191 to -0.132 eV, indicating that S vacancies and O-doping could synergistically improve the HER activity. According the conclusions above, the inflection points in polarization curves could be attributed to the reduction of adsorbed H (H*) on 1T-MoS2, S vacancies, O atoms, and 2H-MoS2, respectively, in the order of overpotential. The corresponding Tafel slopes (Figure 6d) show that the increase of cycling times could improve the HER rate (decreased Tafel slope). Furthermore, the treated MoS2 (S2.2-160) could possess certain stability confirmed by the plot of i-t test for 9 h (Figure S7b). Of note, the HER activity



Page 26 of 35

of S2.2-160 can be further improved and optimized by changing the loading amount,

scan

rate,

and

cycling

times.

Electrochemical

double-layer

capacitances (Cdl) demonstrate that the treated MoS2 exhibits much larger Cdl of 25.6 mF/cm-2 than that (2.6 mF/cm-2) of the pristine 2H-MoS2 nanosheets (Figure S8a-c). The EIS experiment (Figure S8d) of the treated MoS2 shows nearly a line, indicating of the low charge transfer resistance (Rct) and internal resistance, in contrast to the pristine sample (Rct = 1.9×104). The observations about Cdl and Rct prove that the 2H/1T phase transition and the incorporation of S vacancies and O atoms could significantly improve the electrocatalytic activity.9, 13 3. Conclusion In summary, we developed a simple yet effective method to successfully tune the 2H/1T phase transition by incorporation of S vacancies. The stable 2H/1T phase transition has been obtained for the first time without using intercalated species. The S-vacancies, as electron donors, are the critical factor for the 2H/1T phase transition by both weakening Mo-S bonds and decreasing the energy barrier of phase transition. On the other hand, the O incorporation activates the formation of S vacancies, and benefit the stability of the 1T-phase by occupying the Mo 4d orbitals. The 2H/1T phase transition and the enhanced structural disorder enhance the conductivity and decrease the hydrogen adsorption free energy. Compared with the pristine multilayer 2H-MoS2, the hybrid phase MoS2 electrocatalyst exhibits remarkably improved HER activity with lowered onset overpotential and Tafel plot. The electrochemical


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treatments enable synergistically modulating both the structural and electronic properties of the multilayer 2H-MoS2 nanosheets. 4. Experimental Methods Materials. Molybdenum sulfide (2H-MoS2, 99.5%) and dimethyl formamide (DMF) were provided by Aladdin Industrial Corporation (Shanghai, China). All other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received without additional purification. Electrochemical Treatments of Multi-layer 2H-MoS2 and Electrochemical Characterizations. Firstly, the multilayer 2H-MoS2 nanosheets were synthesized by LPE that used 1 mg/mL suspension of bulk MoS2 and DMF in a sonic bath of 5 h. The supernatant was centrifuged and washed with cyclohexane and anhydrous methanol for several times, and then vacuum-dried to obtain the 2H-MoS2 nanosheet powder. The 2H/1T phase transition and the incorporation of S vacancies and O atoms were realized by the electrochemical treatments of the resultant multilayer 2H-MoS2 nanosheets by cyclic voltammetry (CV) with different potential scan ranges (positive) and cycling times. For simplicity, the resultant samples were labeled as Sx-y, in which x and y mean the scan potential range of 0~x V vs. SCE and cycling times of y, respectively. If no additional explanation, the scan rate was 100 mV/s. The electrochemical treatments of the multilayer 2H-MoS2 nanosheets and the following electrochemical characterizations were performed in a standard three-electrode electrochemical cell using an electrochemical workstation (CHI660D) in argon gas-saturated 0.5 M H2SO4. A saturated calomel reference electrode (SCE), a glassy



carbon electrode (GCE), and a graphite rod were used as reference, working, and counter electrodes, respectively. 10 µL (5 mg/mL) suspension of the multilayer 2H-MoS2 nanosheets in ethanol were dropped on a clean GCE. The loading amount on the surface of GCE was 1.59 mg/cm2. The potential shift of the SCE was calibrated to be -0.256 V vs. RHE.10 Typical electrochemical characterizations were performed using linear sweeping from 0 to -0.7 V (vs. RHE) with a scan rate of 5 mV/s. The electrolyte resistance and capacitance of the electrocatalysts were characterized using electrochemical impedance spectroscopy (EIS) in the frequency range from 0.001 Hz to 100 kHz with 5 mV as the amplitude. 5 µL of 0.05 wt % Nafion solution was pipetted onto the catalyst modified GCE only for the stability test using chronoamperograms. Calculations Details. Spin-polarized density functional theory (DFT) computations employed an all-electron method within a generalized gradient approximation (GGA) for the exchange-correlation term, as implemented in the DMol3. The double numerical plus polarization function (DNP) basis set and PBE functional were adopted. The van der Waals interaction was described by using the empirical correction in Grimme’s scheme. The electron wave functions were expanded by plane waves with cutoff energies of 500 eV, and the convergence tolerance for the residual force and energy on each atom during structure relaxation were set to be 0.005 eV/Å and 10−5 eV, respectively. The vacuum space in the z-direction was 20 Å, which was enough to prevent the interaction between periodical images. The Brillouin zone was sampled with the Monkhorst-Pack mesh with a K-point of 9 × 9 × 1 for the unit cell


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and 5 × 5 × 1 for the supercell in reciprocal space during geometry optimization.52 The Gibbs free-energy of the adsorption of an atomic hydrogen ( ∆G 0H ) is a key quantity to describe the HER activity of an electrocatalyst.11 The stability of hydrogen adsorption is described by the differential hydrogen chemisorption energy ∆E H , which is calculated as follows:

∆E H = E(*MoS2+nH) – E(*MoS2+(n-1)H) –1/2 E(H2)

(Equation 4)

where E(*MoS2+nH) is the total DFT energy for an S vacancy- or O- incorporated MoS2 system with n hydrogen atoms, and E(H2) is the DFT energy for a hydrogen molecule in the gas phase. The differential binding free energy can thus be determined as:13

∆G 0H = ∆E H + 0.29eV

(Equation 5)

Apparatus. The morphologic characterizations were obtained using a JEM-2100F field-emission transmission electron microscope (TEM) (JEOL USA, Peabody, MA, USA) operating at 300 kV and an upgraded JEOL JSM-633F field-emission scanning electron microscope (SEM) (JEOL USA, Peabody, MA, USA). The high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were performed on a JEM-2100F TEM/STEM. Raman spectra of the samples on the substrate of SiO2/Si were recorded with a confocal Raman spectrometer (NT-MDT, NTEGRA Spectra) equipped with a 488 nm semiconductor laser and a charge-coupled device (DU420A, Andor) detector. The chemical structure information was investigated using Fourier transform infrared



spectrophotometry (FT-IR) (Bruker, Model VERTEX 70) with KBr as the reference sample. Ultraviolet-visible spectrophotometry (UV-vis) absorption spectra were recorded on a Shimadzu Model UV-2450 spectrophotometer. X-ray photoelectron spectroscopy (XPS) data were obtained with VG ESCALAB 250 spectrometer, using a nonmonochromatized Al Kα X-ray source (1486.6 eV). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Electrochemical treatments of multilayer 2H-MoS2 nanosheets, TEM images, UV-vis absorption spectra, stability test, electrochemical double-layer capacitance, EIS, HER activity, effective mass, and free energy diagram of multilayer 2H-MoS2 nanosheets before and after electrochemical treatments. AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected] *E-mail: [email protected] ORCID Xiaorong Gan: 0000-0003-4733-1184 Huimin Zhao: 0000-0003-4733-1184 Dangyuan Lei: 0000-0002-8963-0193 Kwok-Yin Wong: 0000-0003-4984-7109 Acknowledgements This study was supported by the Hong Kong Research Grants Council (ECS Grant No. 509513), the Innovation and Technology Commission SAR, and The Hong Kong Polytechnic University (Grant No. 1-ZVH9). REFERENCES (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. (2) Gupta, A.; Sakthivel, T.; Seal, S., Recent Development in 2D Materials beyond


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1T Phase Transition of Multilayer MoS2 by Electrochemical

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