Boundary activated hydrogen evolution reaction on monolayer MoS2

Abstract

Recently, monolayer molybdenum disulphide (MoS₂) has emerged as a promising and non-precious electrocatalyst for hydrogen evolution reaction. However, its performance is largely limited by the low density and poor reactivity of active sites within its basal plane. Here, we report that domain boundaries in the basal plane of monolayer MoS₂ can greatly enhance its hydrogen evolution reaction performance by serving as active sites. Two types of effective domain boundaries, the 2H-2H domain boundaries and the 2H-1T phase boundaries, were investigated. Superior hydrogen evolution reaction catalytic activity, long-term stability and universality in both acidic and alkaline conditions were achieved based on a multi-hierarchy design of these two types of domain boundaries. We further demonstrate that such superior catalysts are feasible at a large scale by applying this multi-hierarchy design of domain boundaries to wafer-scale monolayer MoS₂ films.

Fig. 1

Activity of 2H–2H domain boundaries and 2H–1T-phase boundaries for HER. a Optical image of the type-I MoS₂ with individual domain size of few hundreds microns transferred onto a TEM grid. b HRTEM images of as-grown type-II MoS₂ film with highly orientated domains of few microns for the domain sizes. c False-color dark-field TEM image of type-III MoS₂ with high-density 2H–2H domain boundaries. d, e Optical microscope image of a individual MoS₂ single-crystal domain without 2H–2H domain boundaries (d) and polycrystalline ML–MoS₂ with domain boundaries (e). Dashed regions indicate the HER window opened on the basal plane. f Polarization curves of the pristine type-I MoS₂ (without any domain boundaries), pristine type-III MoS₂ (with 2H–2H domain boundaries), heterophase type-I MoS₂ (with 2H–1T-phase domain boundaries), and heterophase type-III MoS₂ (with both 2H–2H and 2H–1T domain boundaries), respectively. g Tafel plots of the corresponding curves in f

Fig. 2

Hydrogen adsorption at 2H–1T-phase boundaries. a STM topography of as-treated MoS₂ showing mixed 2 H (bright) and 1 T (dark) domains. b Zoom-in image of a domain boundary as denoted by a dashed square in a, where 2H and 1T phases are rendered with yellow and purple colors, respectively. The lattice grid of 2H phase is superimposed on the image to highlight the lateral sliding of S atoms in 1T phase. The black arrows indicate the sliding direction [120]. The two in-plane primitive vectors are shown in the upper left of b. c dI/dV spectra taken on the 2H and 1T phases, suggesting the semiconducting 2H phase with a 2.5-eV bandgap and metallic 1T phase. d STM image of the MoS₂ after hydrogenation, showing prominent depression features. The sample bias was chosen such that the apparent heights of 2 H and 1 T phases become similar, to highlight the depression features. e Zoom-in image of the depression features as denoted by a dashed square in d, indicating that the depression features are located exactly at the boundary of 2H and 1T phases. Set points of the STM images: a: 1.5 V, 50pA; b: 1 V, 10 pA; d: 2 V, 10 pA; e: 0.5 V, 50pA. Set point of dI/dV spectra:1.5 V, 50 pA c. f Projected density of states (PDOS) of S and H atoms at the phase boundary before and after hydrogenation. Blue curve: PDOS of S atoms before hydrogenation, black curve: PDOS of S atoms with H bonded, red curve: PDOS of absorbed H atoms. Scale bar: 2 nm a, 0.7 nm b, 1.5 nm d, 0.7 nm e

Fig. 3

Theoretical simulations. a, b The top views of the atomic model of the zigzag type 2H–1T-phase boundaries and four kinds of 2H–2H boundaries (4|8, 6|8, 4|4, 5|7). c A comparison of the Gibbs free energies of the adsorbed H on 2H-phase of MoS₂, 1T-phase of MoS₂, Pt(111) surface, 2H–1T-phase boundaries (PBs) and four kinds of 2H–2H boundaries in the context of HER

Fig. 4

Multi-hierarchy monolayer MoS₂ catalysts for HER. a Schematic structure of the multi-hierarchy MoS₂ catalysts with both high density of domain and phase boundaries. b Polarization curves for pristine type-II samples, pristine type-III samples, a series of heterophase type-III samples with different phase boundary densities and Pt. c Tafel plots of the corresponding curves in b. d HER performance of a multi-hierarchy MoS₂ catalyst in 0.5-M H₂SO₄ (red curve) and 1-M KOH (purple curve). e Time-dependent current density curve for a multi-hierarchy MoS₂ catalyst under static overpotential of 150 mV for 200 h. f Demonstration of the catalytic HER activity in 0.5 M H₂SO₄ from a multi-hierarchy MoS₂ catalyst with a size of 4 inches in diameter. Inset: photograph of pristine as-grown wafer-scale MoS₂ on sapphire substrate