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

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.

💡 Research Summary

In this work, the authors present a systematic study of molecular beam epitaxy (MBE) growth of molybdenum disulfide (MoS₂) thin films directly on silicon (Si) substrates, aiming to optimize the material for the hydrogen evolution reaction (HER). Traditional MoS₂ catalysts rely on edge sites for activity, while the basal planes remain inert. The authors address this limitation by precisely controlling three independent growth parameters: annealing temperature, deposition cycle number, and the Mo-to-sulfur (S) thickness ratio.

First, annealing temperature was varied from 300 °C to 600 °C. Higher temperatures improve crystallinity, as confirmed by sharper X‑ray diffraction (XRD) (002) peaks and clearer Raman A₁g/E₂g mode separation, indicating well‑ordered 2H‑MoS₂ layers. However, excessive annealing reduces sulfur vacancy concentration and eliminates residual metallic Mo, leading to lower electrical conductivity and fewer active edge sites, which diminishes HER performance.

Second, the number of deposition cycles (4, 8, 12, 16) was adjusted to tune film thickness and continuity. Few cycles produce ultrathin, discontinuous layers that strongly couple to the Si substrate, facilitating charge transfer but lacking sufficient conductive pathways. Many cycles generate thick, continuous MoS₂ films with improved conductivity but cause edge sites to be buried within the bulk, reducing the electrochemical surface area (ECSA).

Third, the Mo/S thickness ratio was deliberately made sulfur‑deficient (ratios of 1:1, 1:0.9, 1:0.85, 1:0.8). Under sulfur‑deficient conditions, X‑ray photoelectron spectroscopy (XPS) and X‑ray absorption spectroscopy (XAS) reveal the coexistence of metallic Mo⁰ and Mo⁴⁺ species, indicating that metallic Mo nanoclusters remain embedded within a MoS₂ matrix rich in sulfur vacancies. This mixed‑phase structure activates the basal plane by providing additional electronic states and conductive pathways, effectively turning the otherwise inert basal plane into an active catalytic surface.

Electrochemical testing in 0.5 M H₂SO₄ demonstrates that the optimal combination—annealing at ~450 °C, 8–12 deposition cycles, and a Mo:S ratio of 1:0.85—delivers a low overpotential of –0.33 V at –10 mA cm⁻², an enlarged ECSA of approximately 8 cm², and a mass‑based turnover frequency (TOF) exceeding 23 mmol H₂ g⁻¹ s⁻¹. This performance is more than double that of stoichiometric, highly crystalline MoS₂ films, which typically achieve TOFs around 10 mmol H₂ g⁻¹ s⁻¹ under similar conditions. Long‑term stability tests (10 h at 20 mA cm⁻²) show less than 5 % current decay, confirming the structural robustness of the mixed Mo/MoS₂ domains.

The study highlights sulfur stoichiometry and growth kinetics as powerful levers to balance crystallinity, defect density, and conductivity. By integrating MoS₂ directly onto Si, the authors achieve strong interfacial coupling that minimizes charge‑transfer resistance, a crucial advantage for semiconductor‑integrated catalytic architectures. The findings suggest a broader strategy for engineering layered catalysts at the atomic scale, where controlled incorporation of metallic phases and anion vacancies can simultaneously enhance conductivity and catalytic activity. This approach opens pathways toward hybrid devices that combine electronic functionality (e.g., field‑effect control) with efficient electrocatalysis, potentially impacting photoelectrochemical water splitting, integrated fuel‑cell electrodes, and next‑generation energy‑conversion platforms.