Tailoring the optoelectronic properties of MoOX nanoparticles: a novel microwave-assisted synthesis for near-infrared absorbing polyoxometallic clusters.

The emergence of novel catalytic, electrochemical, and biomedical applications of nanomaterials requires an understanding of their structural basis for stimuli-responsive performance. The chemistry of polyoxometallic nanomaterials, with a variety of interesting properties, remains poorly explored. In this study, microwave-assisted non-aqueous sol-gel synthesis was used for the first time to prepare nanoparticles based on polyoxomolybdates. Their optoelectronic properties, focusing on laser-triggered photothermal response, were investigated in detail depending on the synthesis temperature. Significant differences were observed between products prepared from the same precursor via a fast protocol by varying the synthesis temperature. Only low-temperature synthesis (≤ 90 °C) yielded near-infrared (NIR) photothermally active MoOX nanoparticles. The regular packing with large lattice defects of these clusters, along with a low reduction degree, allows water molecules to penetrate and interact with surface Mo = O bonds, forming intermediate electron states within the bandgap. These intermediate electron states are responsible for the NIR laser response suitable for photothermia. Additionally, the NIR response can be modulated in a controlled manner even after synthesis through electrochemical impedance spectroscopy. These findings have direct implications for MoOX photothermal therapy, targeted defect engineering of polyoxomolybdate structures, and their electrochemical and biological applications.