How Hot Plasmonic Heating Can Be: Phase Transition and Melting of P25 TiO2 from Plasmonic Heating of Au Nanoparticles

Plasmonic heating has been utilized in many applications, including photocatalysis, photothermal therapy, and photocuring. However, the heat dissipation process of the plasmonic nanoparticles (NPs) and the surrounding matrix is complex. How high the temperature of the matrix that surrounds the plasmonic NPs, such as the catalyst and substrate, can be reached is unclear. Herein, we study the dissipation of plasmonic heat generated by resonantly excited gold (Au) NPs dispersed on P25 TiO2 NP porous film in air. Under resonant 532 nm continuous wave (CW) laser irradiation at the surface of the Au-TiO2, the surface evaporation and the aggregation of Au NPs were observed at moderate laser power (10 mW/um2). This process is accompanied by the phase transition of TiO2. More importantly, the TiO2 nanoparticle film melted, forming melt pools and molten TiO2 matrix. This indicates that the temperature of the TiO2 reached as high as its melting point of 1830 °C. When Au/TiO2 was irradiated with an off-resonance laser at 638 nm, no phase transformation or melting of TiO2 was observed. The temperature calculation showed that the heating generated by Au NPs is not localized. The collective heating from an ensemble of Au NPs in the irradiated area produced a global temperature rise that melted TiO2. Our results suggest that the photothermal effect could be a significant mechanism in the plasmon-assisted photocatalytic reactions. The melting of the supporting metal oxide film by plasmonic heating at relatively low laser power suggests new applications for utilizing plasmonic heating, such as additive manufacturing.