Mechanisms of Semiconducting 2H to Metallic 1T Phase Transition in

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mechanisms of Semiconducting 2H to Metallic 1T Phase Transition in Two-Dimensional MoS Nanosheets 2

Qiu Jin, Ning Liu, Biao-Hua Chen, and Donghai Mei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10256 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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The Journal of Physical Chemistry

Mechanisms of Semiconducting 2H to Metallic 1T Phase Transition in Two-dimensional MoS2 Nanosheets Qiu Jin,†,‡ Ning Liu, †,§ Biaohua Chen,*,†,§ Donghai Mei*,‡ †State

Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

‡Institute

of Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

§College

of Environmental and Energy Engineering, Beijing University of Technology, Beijing, 100022, China

ABSTRACT In the present work, phase transition mechanisms from semiconducting 2H phase to metallic 1T phase in MoS2 nanosheets were studied using density functional theory (DFT) method. Various 2H→1T phase transition mechanisms that consist of nucleation and propagation steps, which simulated by collective rotational and rotational/translational movements, single atom translational movement, as well as the gliding movement of one row for sulfur (S) atoms, on both the basal plane and Mo- and S-edges with different S coverages were investigated. On the perfect basal plane, the 1T phase nucleation is unlikely due to the extremely high barrier of 2.25 eV/atom. Whereas the presence of defective S vacancies on the basal plane dramatically facilitate the 1T phase nucleation and propagation around the defective sites by the collective rotational movement of three S atoms. On the 2H phase basal plane with two S vacancies, the kinetic barriers for the 1T phase nucleation are as low as of 0.66~0.77 eV/atom. Like the promoting effect of S vacancies on the phase transition over the basal plane, DFT results suggest that the S coverage on the Mo- and S-edges will affect the 1T phase nucleation and propagation. The 1T phase nucleation starting with the translational movement of single S atom on the bare Mo-edge and the gliding movement of an entire row of S atoms on the S-edge with 50%S coverage are kinetically favorable. While the 1T phase formation at the Mo-edge with 50%S coverage and the S-edge with 100%S coverage are unlikely. The present work not only confirms the important role of S vacancies/coverages in the 2H-1T phase transition, but also provides new insight into how and where the 2H-1T phase transition occurs at the atomic level, 1

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which also sheds light on the general phase transition mechanism for two-dimensional transition metal dichalcogenide materials.

* Corresponding authors: [email protected] (B. Chen); [email protected] (D. Mei)

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1. INTRODUCTION Molybdenum disulfide (MoS2), a two-dimensional (2D) transition metal dichalcogenides (TMDCs), has drawn significant attention due to their numerous potential applications such as catalysis, electronic devices, optoelectronics, and energy storage.1-8 The MoS2 bulk is composed of sandwiched S-Mo-S monolayers that are bonded with weak van der Waals interactions. Depending on the arrangement of six sulfur (S) atoms associated with the Mo atom, there are two typical MoS2 phases, i.e., thermodynamically stable 2H phase with trigonal prismatic coordinated S atoms, and metastable 1T phase with octahedral coordinated S atoms, which are shown in Figure 1. With completely different electronic structures, the 2H phase is semiconducting with a direct band gap of 1.2~1.9 eV while the 1T phase is metallic with high electrochemical activity.9-10 Transformation of 2H to 1T phase in MoS2 can be achieved by various chemically synthesized approaches,11-12 in which alkali metal intercalation is one of the most effective and widely used methods.13-14 The alkali cations such as lithium, sodium, potassium, etc. have been intercalated between S-Mo-S monolayers, inducing a phase transition from the 2H bulk phase to 1T phase nanosheets.15-18 Cai et al. reported a two-step hydrothermal method to synthesize the 1T phase inside the 2H phase nanosheets.19 They proposed that the generation of sulfur vacancies play an important role in the 2H→1T phase transformation, which is the same as the alkali intercalation. Metallic 1T phase can also be obtained from 2H phase using physical methods including strain effect20-22 and charge injection.23 For example, Gao et al. found that the kinetic barrier for the 2H→1T phase transition was dramatically lowered from 1.59 to 0.23 3



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eV when an amount of 4e electrons was injected into the 2H MoS2 system.23 The formed metastable 1T phase can then be stabilized by doping other metal (Re, Tc, Mn) atoms.24-25 Huang et al. found the single-layer 2H→1T phase transition of MTe2 (M=Mo and W) could be modulated by the biaxial and uniaxial strains, resulting the dynamical phase transition with a low kinetic barrier of ~0.9 eV.21 Despite all those macroscopic phase transition phenomena were observed, the atomic-level understanding of the 2H→1T phase transition is limited. The specific evolution of intermediate structures between 2H and 1T phases through atomic mechanisms is not clear. How and where the phase transition occurs with the thermodynamically stable 2H phase is not fully described yet. Using in situ scanning transmission electron microscopy, Lin et al. studied the 2H→1T phase transition processes of MoS2 nanosheets at 600 oC.24 On the basis of obtained atomic resolution images, an atomic mechanism involving both sulfur (S) and Mo atom gliding movement was proposed. An intermediate phase (α-phase) which consisted of three to four constricted Mo zigzag chains was formed as a precursor, followed by the migration of  and  boundaries via both S and Mo collective gliding movement. This experimentally observed phase transition mechanism was further elucidated in a recent theoretical study, which suggested that the phase transition from 2H to 1T phase needs to overcome a kinetic barrier of 1.59 eV per unit within a MoS2 bulk only by S plane gliding.23 By creating different patterns and sizes of nucleated 1T phases on the perfect 2H phase basal plane, Zhao and Ding studied the stabilities of various interfacial boundaries between 2H and 1T phases in MoS2 nanosheets using density functional theory (DFT) based thermodynamic method.26 They found that the 4


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nucleation barrier for the 2H→1T phase transition was too high to occur on the prefect 2H phase basal plane without S vacancies. Therefore, it is reasonably assumed that the observed 2H→1T phase transition in MoS2 could be initialized by the S vacancies or the edge sites of 2H phase.19, 27 However, the detailed atomic mechanisms involve a number of elementary steps of atomic movements has not been clearly elucidated. In the present work, the atomic nucleation and growth processes of 2H→1T phase transition on the perfect basal plane, and both Mo- and S-edges, for the first time, were studied using first principles DFT calculations. Herein, we have to mention that the transformed 1T phase from the 2H phase also includes 1T’ phase, which is one of derivatives of the 1T phase.28-30 Both 1T and 1T’ phases are metastable, compared to the stable 2H phase. Various atomic mechanisms via translational, rotational, and collective rotational/translational movements involving different number of S atoms in the nucleation and propagation of the 1T phase over 2H phase domain of MoS2 nanosheets were examined. In particular, the effects of S vacancies on the basal plane, and of S coverages on the Mo- and S-edges on the 2H1T phase transition were investigated.

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Figure 1. Schematic structures of 2H and 1T phases in MoS2 nanosheets. Mo and S atoms are in light green and yellow, respectively.

2. COMPUTATIONAL METHOD All geometrical optimizations and transition state calculation were carried out using the QUICKSTEP program within the CP2K code,31 employing a mixed Gaussian and planewave basis sets. Core electrons were represented with norm-conserving Goedecker-Teter-Hutter pseudopotentials,32-34 and the valence electron wavefunction was expanded in a double zeta basis set with polarization functions (DZVP-MOLOPT-SR-GTH)35 along with an auxiliary plane wave basis set with an energy cutoff of 500 Ry. The generalized gradient approximation exchange-correlation functional of Perdew, Burke, and Enzerhof (PBE)36 was used. To account for the van der Waals (vdW) interaction between periodic S-Mo-S layers, the DFT-D3 scheme23 with an empirical damped potential term was added into the energies obtained from exchange-correlation functional in the calculations. The BFGS algorithm with SCF convergence criteria of 1.0× 10-8 au was used in geometrical optimizations. Test calculations showed that the energy change was negligible (

Mechanisms of Semiconducting 2H to Metallic 1T Phase Transition in

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