# Controlling Mixed Mo/MoS2 Domains on Si by Molecular Beam Epitaxy for the Hydrogen Evolution Reaction

## TL;DR

Researchers used molecular beam epitaxy to control the structure of MoS2 films on silicon, improving their performance in hydrogen production.

## Contribution

A method to engineer defect-rich MoS2 films via MBE, enhancing catalytic activity for hydrogen evolution.

## Key findings

- Defect-engineered MoS2 films achieved overpotentials as low as −0.33 V at −10 mA cm–2.
- Electrochemical surface area reached up to 8.0 cm2 with turnover frequencies exceeding 23 mmol H2 g–1 s–1.
- Sulfur-deficient growth conditions activated basal planes and improved conductivity.

## Abstract

Molybdenum disulfide
(MoS2) is a prototypical layered
transition-metal dichalcogenide whose electrocatalytic performance
is governed by a delicate balance between crystallinity, defect density,
and electronic conductivity. Here we report a systematic molecular
beam epitaxy (MBE) study in which annealing temperature, deposition
cycle number, and Mo/S thickness ratio were independently varied to
control the structural and electronic properties of MoS2 thin films. The successful epitaxial growth of atomically uniform
MoS2 directly on Si substrates enables strong interfacial
coupling and efficient charge transfer, offering a viable route toward
semiconductor-integrated catalytic architectures. X-ray diffraction,
Raman spectroscopy, and X-ray absorption analyses reveal that higher
annealing temperatures and excessive deposition cycles enhance crystallinity
but reduce edge-site density and electronic conductivity, leading
to diminished hydrogen evolution reaction (HER) activity. In contrast,
intermediate cycle numbers and sulfur-deficient growth conditions
yield heterostructures composed of MoS2 with residual metallic
Mo and sulfur vacancies, which activate otherwise inert basal planes
while providing conductive pathways. These defect-engineered films
deliver the best catalytic performance, achieving overpotentials as
low as −0.33 V at −10 mA cm–2, enlarged
electrochemical surface area (ECSA) up to 8.0 cm2, and
mass-based turnover frequencies exceeding 23 mmol H2 g–1 s–1, more than double those of
stoichiometric counterparts. Our findings establish sulfur stoichiometry
and growth kinetics as powerful levers to tune the interplay between
structural order and catalytic activity in MBE-grown MoS2 and point toward a broader strategy for engineering layered catalysts
at the atomic scale.