Comparison Between Ultrasonic and Thermal Exfoliation for Molybdenum Disulfide Nanoparticles-A Multifunctional Study

The transition metal dichalcogenides (MX₂, TMD) are a family of two-dimensional compounds, and MoS₂ is perhaps the most representative material among them. MoS₂, in its bulk form, is semiconductive, with an indirect band gap of around 1.2-1.3 eV. Monolayer MoS₂, nevertheless, exhibits a direct band gap of 1.8-1.9 eV. For zero-dimensional MoS₂ (quantum dots), the band gap would be even wider. These changes are mainly due to quantized energy levels arising from geometric confinement. Quantum effects directly yield distinct optoelectronic properties, such as photoluminescence, which are absent in the material's bulk form. In this study, we developed an economical, scalable one-pot synthesis of MoS₂ nanoparticles at atmospheric pressure, eliminating the need for high-pressure autoclaves commonly employed in hydrothermal synthesis at around 150-270 °C. This one-pot synthesis was tested by two exfoliation methods-ultrasonication and thermal heating at 80 °C in three solvents: ethanol, deionized water, and N-methyl-2-pyrrolidone. Our focus is on the empirical study of nanoparticles' photoluminescence and photothermal effects. The material characterization includes transmission electron microscopy for morphology and crystal structure, X-ray photoelectron spectroscopy for chemical bonding energies, and a UV-visible-NIR spectrometer for optical absorption. Photoluminescent spectroscopy of the nanoparticles was assessed using laser excitation between 300 and 400 nm, and photothermal effects were measured as temperature variations in DI water under continuous irradiation by an 808 nm laser. Results show that MoS₂ nanoparticles synthesized in ethanol exhibit an additional broad optical absorption in the red-infrared range, in addition to the usual high absorption in the UV-blue range. This photothermal absorption would raise the temperature of DI water to 50 °C in 300 s. A simplified numerical analysis by finite elements was also carried out as a numerical confirmation of the photothermal test. On the cell biological side, both in vitro cultures with NIH/3T3 fibroblasts and antibacterial against E. coli show that critical tolerance of MoS₂ is no more than 0.25 mg/mL in the media.