High-frequency ultrasonic liquid-phase exfoliation of hydrothermally synthesized MoS2 nanoflowers toward monolayer

The development of two-dimensional materials that combine high electrical conductivity, structural stability, and precise thickness control remains a challenge for applications in advanced, high-value-added technologies. Herein, we report a simple and sustainable strategy combining hydrothermal synthesis of MoS 2 nanoflowers with high-frequency ultrasound-assisted liquid-phase exfoliation (80 kHz) to produce oxygen-modified MoS 2 (O-MoS 2 ) monolayers with an exfoliation yield of approximately 60%. Structural characterization by XRD, Raman, FTIR, and XPS demonstrated that oxygen incorporation promotes an expansion of the interlayer spacing from 0.65 to 0.94 nm, facilitating exfoliation while stabilizing the metallic 1 T phase. Quantitative XPS analysis revealed a mixed-phase composition of 72.6 at.% 2H and 27.4 at.% 1 T, together with the formation of Mo-O bonds associated with oxygen incorporation in the metallic domains. Optical measurements evidenced pronounced modifications in the electronic structure, including sub-bandgap absorption and strong photoluminescence quenching, attributed to defect-mediated electronic states and 2H/1 T phase coexistence. Thermal analysis indicated that the metastable 1 T phase remains partially preserved up to ~430 °C. Morphological investigations by SEM, AFM and TEM confirmed the successful exfoliation of nanoflower precursors into monolayers with thicknesses of 0.9-1.0 nm and lateral dimensions approaching 1 μm. The resulting O-MoS₂ monolayers exhibited an electrical conductivity of 9.1 × 10 -3 S/cm, evidencing efficient charge transport enabled by the metallic phase fraction. These findings demonstrate that oxygen-assisted phase engineering combined with high-frequency ultrasonic exfoliation provides an effective route for producing large-area metallic-rich MoS 2 monolayers, opening new opportunities for energy storage, electrocatalysis, and advanced nanoelectronic applications.