Synthesis of two-dimensional plasmonic molybdenum oxide nanomaterials by femtosecond laser irradiation

A novel process to synthesize plasmonic MoO_3–X nanosheets is demonstrated, in which MoS₂ powders suspended in ethanol/water are irradiated with pulses from a femtosecond laser, resulting in simultaneous Coulomb explosion, photoexfoliation, and oxidation. The oxidation process is found to start with the formation of hydrogen-bonded molybdenum oxide (H_XMoO₃), followed by the release of −OH₂ groups to create oxygen vacancies, and finally, MoO_3–X is oxidized to MoO₃ after extended irradiation. The formation of H_XMoO₃ is the critical step to create enough oxygen vacancies for localized surface plasmon resonance (LSPR), and this step is attributed to H₃⁺ dissociated from ethanol under femtosecond laser irradiation. It is found that 80–90% ethanol is the optimal concentration to synthesize plasmonic MoO_3–X, where the balance of water facilitates the release of the −OH₂ groups to create the required vacancies. It is shown that different organic solvents like methanol, 1-propanol, and isopropyl alcohol that were reported to generate large amounts of H₃⁺ under femtosecond laser irradiation can also oxidize MoS₂ into plasmonic MoO_3–X. The LSPR properties of the synthesized MoO_3–X are evaluated by UV–vis spectroscopy and photothermal conversion measurements. A photothermal conversion efficiency of 33% is observed under near-infrared irradiation, suggesting a potential application in photothermal cancer therapy.