Hydrated MoO3 nanoparticles and -MoO3 nanosheets synthesis by fs laser irradiation

We present a study of the synthesis and characterization of molybdenum oxide nanostructures produced by laser ablation of solids in liquids (LASL). A high purity molybdenum target was ablated in deionized water using femtosecond laser pulses. We found that both nanoparticles and nanosheets composed of molybdenum oxide were formed. The colloid suspension optical absorbance changes with time, as the colloid ages it changes color; a blue shift is observed within a few days of aging. It results in a band gap increase from 2.55 eV up to 2.9 eV for nearly a month of aging, which we computed through the Tauc method. Raman characterization of the nanostructures shows that the synthesized material turns into a hydrated molybdenum oxide (MoO3.nH2O). This is consistent with the band gap evolution, which for early aging corresponds to MoO3 to later achieve the band gap value of a hydrated molybdenum oxide. The nanostructures were also characterized by transmission electron microscopy (TEM) showing that the nanoparticles have a core-shell structure with a raspberry-like shell for medium and small nanoparticles and a smooth homogeneous shell for large nanoparticles. The size distribution peaks at 30-40 nm, with the smallest nanoparticles at 10 nm and the largest at 120 nm. The nanosheets are decorated by tiny nanoparticles. High resolution transmission electron microscopy (HRTEM) images allowed measuring the interplanar distances at the nanosheets, we found the nanosheets are composed by the orthorhombic stable-phase alpha-MoO3. SAED measurements confirmed the formation of this molybdenum oxide phase. An absorption band at 830 nm that develops in the few first days of aging of the colloid is of interest for pho-tothermal therapy, which gives the reported nanostructures a good potential for biomedical applications.