| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Monday Sessions |
| Session TF-MoE |
| Session: | Nanostructured Surfaces & Thin Films I |
| Presenter: | Fabio Grillo, Delft University of Technology,The Netherlands |
| Authors: | F. Grillo, Delft University of Technology,The Netherlands H. Van Bui, Delft University of Technology,The Netherlands J.R. van Ommen, Delft University of Technology, The Netherlands |
| Correspondent: | Click to Email |
Atomic layer deposition (ALD) is proving to be an attractive technique for depositing nanoparticles (NPs) in a controlled fashion. By relying on self-limiting surface reactions, ALD lends itself to the decoration of high-surface-area substrates with NPs for various applications such as enhanced catalysts and batteries. However, the mechanisms underlying the nucleation and growth of NPs during ALD are still poorly understood. Understanding the evolution of the particle size distribution of NPs during ALD is essential for fabricating supported NPs with predictable morphology and functionality. ALD growth is generally explained in terms of self-limited layer-by-layer growth, and although such picture applies to the deposition of a variety of metal oxides, especially at low temperature, it does not apply to the nucleation and growth of metal NPs. In fact, the evolution of the particle size distribution (PSD) of NPs is governed by kinetic processes such as surface diffusion, aggregation and coalescence. Here, we study the evolution of the PSD of Pt and Pd NPs deposited in time scales relevant to thermal ALD based on oxidative chemistries on high-surface-area substrates. To unravel the relevant processes underlying the evolution of the shape of the PSD, we devise a kinetic model based on the Smoluchowski equation that incorporates the generation of adatoms on both the substrate and the pre-existing NPs due to surface reactions, surface diffusion, adatom capture and NP aggregation. In doing so, we show that, during the first cycles, NP growth proceeds mostly due to NP diffusion and coalescence. The evolution of the PSD during dedicated sintering experiments of the ALD-grown NPs suggests that NPs aggregation takes place mostly during the oxygen exposure. In fact, the oxygen exposure not only regenerates the active surface for the subsequent precursor exposure but also mediates the metal-support interaction. Finally, we show that by varying the deposition temperature, while adjusting the exposure times to retain saturation conditions, the determining mechanism for NPs growth shifts from accretion due single-atom capture, at low temperatures (T<100 °C), to NPs diffusion and coalescence at high temperatures (T>150 °C). The insights obtained in this study open up new avenues to the rational design of novel composite materials based on supported metals NPs.