[Paper Review] Controlling Mixed Mo/MoS$_2$ Domains on Si by Molecular Beam Epitaxy for the Hydrogen Evolution Reaction

Eunseo Jeon, Vincent Masika Peheliwa+3 more|arXiv (Cornell University)|Jan 23, 2026

2D Materials and Applications0 citations

TL;DR

The paper shows how molecular beam epitaxy can tailor MoS2 on Si by adjusting annealing, cycles, and Mo/S ratios to create Mo- and MoS2-rich domains, optimizing HER activity.

ABSTRACT

Molybdenum disulfide (MoS$_2$) 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 MoS$_2$ thin films. The successful epitaxial growth of atomically uniform MoS$_2$ 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 electrical conductivity, leading to diminished hydrogen evolution reaction (HER) activity. In contrast, intermediate cycle numbers and sulfur-deficient growth conditions yield heterostructures composed of MoS$_2$ 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 cm$^2$, and mass-based turnover frequencies exceeding 23 mmol H$_2$ 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 MoS$_2$ and point toward a broader strategy for engineering layered catalysts at the atomic scale.

Motivation & Objective

• Understand how annealing temperature, deposition cycle number, and Mo/S thickness ratio influence the structure and electronic properties of MoS2 grown on Si.
• Investigate how interfacial coupling and charge transfer affect catalytic performance for the hydrogen evolution reaction.
• Identify defect-engineering strategies (Mo residues, S vacancies) that activate basal planes and improve conductivity.
• Assess how sulfur stoichiometry and growth kinetics tune the balance between crystalline order and catalytic activity.

Proposed method

• Systematic MBE growth of MoS2 on Si with independent variation of annealing temperature, deposition cycle number, and Mo/S thickness ratio.
• Characterization using X-ray diffraction, Raman spectroscopy, and X-ray absorption analyses to correlate structure with properties.
• Evaluation of electrochemical performance for HER, including overpotential, electrochemical surface area (ECSA), and turnover frequency.
• Analysis of the trade-offs between crystallinity, edge-site density, and electrical conductivity on catalytic activity.

Experimental results

Research questions

• RQ1 How do annealing temperature, deposition cycle number, and Mo/S thickness ratio affect crystallinity and interfacial coupling in MoS2 films on Si?
• RQ2 How does sulfur stoichiometry and defect content (e.g., Mo residues, S vacancies) influence HER activity and conductivity?
• RQ3 What growth conditions optimize the balance between crystalline order and catalytic defect engineering for MoS2/Si hybrids?
• RQ4 Can mixed Mo/MoS2 domains enhance charge transfer and lower HER overpotentials compared to stoichiometric MoS2?

Key findings

• Higher annealing temperatures and excessive deposition cycles increase crystallinity but reduce edge-site density and electrical conductivity, lowering HER activity.
• Intermediate cycle numbers and sulfur-deficient growth yield heterostructures with residual metallic Mo and S vacancies that activate basal planes and provide conductive pathways.
• Defect-engineered MoS2 films achieve overpotentials as low as -0.33 V at -10 mA cm^-2.
• Electrochemical surface area (ECSA) reaches up to 8.0 cm^2, with mass-based turnover frequencies exceeding 23 mmol H2 g^-1 s^-1.
• Defect engineering and sulfur stoichiometry emerge as levers to tune order versus catalytic activity in MBE-grown MoS2 on Si.