Rethinking Electrochemical Exfoliation: Controlled Cathodic Intercalation at Low Overpotentials for High-Quality, Atomically Thin MoS2 Layers without Gas Evolution-Induced Expansion

The cathodic electrochemical exfoliation of transition metal dichalcogenide (TMD) crystals using bulky tetraheptylammonium ions (THA⁺) is now commonly used to produce high-quality, high-aspect-ratio flakes for diverse applications. However, most studies apply excessively high potential differences (−6 V to −8 V) between a crystal and counter electrode. This study reveals that applying extremely high potentials results in THA⁺ intercalation, while simultaneously causing intercalant decomposition into heptane gas. This gas formation creates additional internal pressure that enhances the macroscopic expansion of the material beyond what would be expected from intercalation alone. To address this, we use a spring-loaded, three-electrode cell to control the working electrode potential. Using this setup, we compare the exfoliation yield and quality of micron-sized MoS₂ powders and large single crystals, both intercalated potentiostatically at –2.5 and −6 V vs the ferrocenium/ferrocene (Fc⁺/Fc) redox couple. At –2.5 V vs Fc⁺/Fc, complete THA⁺ intercalation is achieved with minimal electrolyte decomposition and the associated macroscopic expansion. The intercalates exfoliate with high concentrations (∼0.5–1 mg/mL) of flakes that exhibit narrow thickness distributions (95% below 3 nm), and high aspect ratios (L/t > 150). While all samples exfoliated in our 3-electrode cell exhibit strong photoluminescence and Raman spectra congruent with single-layer MoS₂, materials exfoliated at –2.5 V demonstrate superior quality, with fewer 1T phase impurities and adsorbates related to THA⁺ decomposition as determined by X-ray photoelectron spectroscopy. These findings highlight a pathway to improve the quality of electrochemically exfoliated MoS₂, enhancing its already strong potential for future optoelectronic applications.