Synthesis of Two-Dimensional Plasmonic Molybdenum Oxide Nanomaterials by Femtosecond Laser Irradiation

A novel process to synthesize plasmonic MoO3–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 X MoO3), followed by the release of −OH2 groups to create oxygen vacancies, and finally, MoO3–X is oxidized to MoO3 after extended irradiation. The formation of H X MoO3 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 MoO3–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 MoO3–X . The LSPR properties of the synthesized MoO3–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.