Tailoring the Optoelectronic Properties of MoOX Nanoparticles: A Novel Microwave-Assisted Synthesis for Near-Infrared Absorbing Polyoxometallic Clusters

Emergence of novel catalytic, electrochemical and biomedical applications of nanomaterials necessitates an understanding of the structural basis of their stimuli-responsive performance. Chemistry of polyoxometallic nanomaterials with variety of interesting properties is still poorly explored. In this study, the microwave-assisted non-aqueous sol-gel synthesis was used for the first time to prepare nanoparticles based on polyoxomolybdates. Their optoelectronic properties with the focus on laser-triggered photothermal response were investigated in detail depending on the synthesis temperature. Striking differences were observed between the products prepared from the same precursor according to fast protocol by varying the temperature of synthesis. Only low-temperature synthesis (≤90°C) provided near-infrared (NIR) photothermally active MoOX nanoclusters. The regular packing with large lattice defects of these clusters and low reduction degree allow penetration of water molecules into the cluster and their interactions with surface Mo=O bonds along with formation of intermediate electron states in the bandgap. The intermediate electron states are responsible for the NIR-laser response suitable for photothermia. In addition, the NIR response can be modulated in a controlled manner even after the complete synthesis by the electrochemical impedance spectroscopy. These results have direct implications for MoOX photothermal therapy, tailored defect engineering of polyoxomolybdate structures and their electrochemical and biological applications.