Solid‐state reaction as a mechanism of 1T ↔ 2H transformation in MoS₂ monolayers

Abstract

Monolayers of molybdenum disulfide MoS2 are considered to be prospective materials for nanoelectronics and various catalytic processes. Since in certain conditions they undergo 1T ↔ 2H phase transitions, studying these phase changes is an urgent task. We present a DFT research of these transitions to show that they can proceed as a solid-state reaction. Two transition states were discovered with energy barriers 1.03 and 1.40 eV. Sulfur atoms in the transition states are shown to be displaced relative to molybdenum atoms so that a tendency of one structural modification to transform into the other modification is seen. This kind of displacements agrees with electron microscopy data reported earlier. The energy parameters indicate that 1T → 2H reactions are exothermic for both transition states and can possibly proceed in a self-sustained manner when initially activated by some external energy impact.

Thermodynamics of H-T phase transition in MoS2 single layer

“…5). The energy barriers estimated using our brute-force scan of the MoS 2 energy map are equal to 1.66 eV for H→T transition, 0.83 eV and 1.06 eV for T→H and T →H transitions, respectively, which are in-between the values found from the DFT-based NEB approach [29] and from the DFT analysis of consecutive transition states [26]. However, the energy path established here demonstrates the existence of a flattened part of the energy barrier, related to the S-MoS 2 phase as the transition state.…”

“…Hence, a deep understanding of the phase transition from a stable 2H b -MoS 2 polymorph to a metastable 1T-MoS 2 polymorph is required. The mechanism of the H-T phase transition was suggested earlier, as the one involving the gliding of atomic S planes [22,[26][27][28][29]. The magnitude of the energy barrier for H→T and T(T )→H transitions was estimated using the DFT calculations with the nudged elastic band (NEB) method as 1.5-1.9 and 0.7-1.0 eV, respectively.…”

“…In this paper, we propose another mechanism for the H-T phase transitions in MoS 2 layer, which might be realized not only under electron doping, but under shock-wave propagation, too. Namely, the H-T transition is considered as an in-plane compression of a monolayer along armchair and/or zigzag directions via formation of an intermediate metastable "square" phase (S-phase) not yet disclosed on the minimum energy paths using DFT NEB method [26,27,29]. In addition, a relationship is established between the different scenarios of subsequent structure relaxations of S-phase into H-phase and the experimentally observed grain boundaries within the H-phase [30][31][32][33][34][35].…”

A comparative study on the photocatalytic degradation of organic dyes using hybridized 1T/2H, 1T/3R and 2H MoS2 nano-sheets

“…4 The higher conductivity and the more active basal plane of the 1T phase enabled a superior performance in energy storage and HER applications making it, thus, quite desirable. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Studies have shown that domains of 1T and 2H phases can coexist in single layers of hybridized MoS 2 . 22 The activity of MoS 2 in electrochemical and photocatalytic water splitting was found to be substantially improved by increasing the density of active sites via introducing defects or increasing conductivity in composites with carbon based materials or other semiconductors.…”

“…3,[23][24][25][26][27] The electronic structure of 1T/2H phases has been the focus of several computational studies. 13,[28][29][30][31][32] The origin of the improved performance in HER catalysis by 1T/2H phase materials has been attributed to the enhanced charge mobility in the material, due to the enhanced metallic conductivity of 1T phases. 25 As an efficient semiconducting nanomaterial, MoS 2 has been employed in environmental engineering, for treatment of organic pollutants via both, adsorption or photo-catalytic degradation.…”