Understanding the Growth Mechanisms of Ag Nanoparticles Controlled by Plasmon-Induced Charge Transfers in Ag-TiO₂ Films

Mesoporous thin films of TiO₂ doped with silver can undergo spectacular microstructural modifications upon laser scanning at visible wavelengths through the excitation of a localized surface plasmon resonance in Ag nanoparticles (NPs). The latter can result in competitive physicochemical mechanisms, leading either to the shrinkage or to the growth of NPs depending on the exposure conditions. Contrary to intuition, we provide evidence that the speed of the laser scan controls the size of NPs as follows: low speeds lead to silver oxidation and a decrease in the NP size, whereas high speeds induce rapid temperature rises and a spectacular growth of NPs. Both regimes are separated by a speed threshold that depends on extrinsic and intrinsic parameters such as laser power, beam diameter, and initial size of Ag NPs. We propose here a comprehensive model based on a set of coupled differential equations describing the transformations of silver under laser excitation between the Ag⁰, Ag⁺, and metallic NP states, which provides a convincing physicochemical explanation of the experimental findings. This study constitutes a significant advance in the understanding of oxidation–reduction processes involved during laser exposure of metallic NPs and opens new directions to control their growth rate and their final size