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

Abstract

Mesoporous thin films of TiO 2 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 Ag 0 , 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.

HAADF‐STEM characterization and simulation of nanoparticle distributions in an inhomogeneous matrix

“…; Liu et al . ). These films are deposited on a glass substrate, their thickness is around 200 nm and the average pore size of the mesoporous TiO 2 matrix is around 8 nm.…”

“…D) depending on the illumination conditions (Liu et al . ). We suggest the interested reader to refer to (Destouches et al .…”

“…The motivation for the present work grew out of the study of photosensitive Ag:TiO 2 thin films where Ag NP distributions with an average diameter smaller than 3 nm should be accurately measured and distinguished from a mesoporous TiO 2 matrix, in order to properly characterize the light‐induced physico‐chemical mechanisms, which give rise to changes in the NP size under laser exposure (Liu et al . ; Liu et al . ).…”

Three-Dimensional Self-Organization in Nanocomposite Layered Systems by Ultrafast Laser Pulses

“…After rinsing and drying, they are exposed to UV light (400 μW.cm −2 at a wavelength of 254 nm) for 30 min. This initiates the growth of a high density of small Ag NPs with typically two size distributions, as characterized in reference 42 centred at 3 nm for nanoparticles located in the 100 nm upper part of the film and at 1.5 nm for nanoparticles located in the rest of the film.…”

“…4b). 21,42 They occur only above a temperature threshold and their yield increases non-linearly with temperature. Therefore, when using fs laser pulses, thermal accumulation needs to be exploited in order to enable the NP growth to large sizes.…”