How Hot Plasmonic Heating Can Be: Phase Transition and Melting of P25 TiO _<b>2</b> 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 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 reach is unclear. Herein, we study the dissipation of plasmonic heat generated by resonantly excited gold (Au) NPs dispersed on a P25 TiO₂ NP porous film in air. Under resonant 532 nm continuous wave (CW) laser irradiation at the surface of Au-TiO₂, the surface evaporation and the aggregation of Au NPs were observed at moderate laser power. This process is accompanied by the phase transition of TiO₂. More importantly, the TiO₂ NP film melted, forming melt pools and a molten TiO₂ matrix. This indicates that the temperature of 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 increase that melted TiO₂. Our results suggest that the photothermal effect could be a significant mechanism in the plasmon-assisted photocatalytic reactions.