Observation of Spallation in an Isolated Aluminum Nanoparticles and 2D Molybdenum Oxide System by Rapid Plasmonic Heating

Spallation of aluminum nanoparticles (Al NPs) in the presence of abundant oxidizing species, molybdenum trioxide (MoO3) 2D flakes, is initiated by rapid photothermal heating. Al NPs of various sizes, 50, 80, and 120 nm diameters, and alumina shell thicknesses from 2-4 nm were studied. Photothermal heating was accomplished by coupling of focused blue laser (446 nm wavelength) to an optical grating substrate and to the plasmonic resonance of the Al particles. The heating rate was estimated through COMSOL simulations to be 10(8) K/s, sufficient for the occurrence of the Melt Dispersion Mechanism (MDM). In some cases, additional heating was provided by the reaction of Al NPs and MoO3. Scanning electron Microscope (SEM) images provided observation of reaction products at reaction sites on the molybdenum oxide flake. Evidence of reactions was seen at both the original particle location and at the secondary sites where spalled particles were located. Product particles were observed with a wide range of sizes, implying that the reaction is a complex process resulting in spallation, and secondary reactions and sintering of spall particles. The inclusion of reaction heating enables spallation of Al NPs with less photothermal heating, and a greater spatial distribution of spalled particles, implying a more energetic reaction mechanism. This mechanism is important for mesoscale nanothermite reactions.