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 (HXMoO3), 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 HXMoO3 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.