(731d) Controlled Production of Nanostructured Noble-Metal Catalysts Using Atomic Layer Deposition

J. Ruud van Ommen, Fabio Grillo, Hao Van Bui, Jacob A Moulijn, Michiel T Kreutzer

Already an established technique for the growth of conformal thin films, atomic layer deposition (ALD) is also an attractive technique for the controlled growth of nanoparticles (NPs). As such, it can play an important role in the synthesis of nanostructured catalysts. Compared to conventional synthesis routes, ALD boasts a number of advantages: atomic-level control over the amount of material being deposited, conformality, scalability to high-surface-area substrates, and its being solvent-free. Yet, bringing the unparalleled precision of ALD of thin films to ALD of NPs is not straightforward. The precision of ALD relies, in fact, on cyclic repetitions of self-saturating surface reactions that lead to the deposition of less than a monolayer per cycle. Hence, if the growth proceeds in a layer-by-layer fashion, as is the case with ALD of thin films, varying the number of cycles translates into tuning the film thickness with atomic-level precision. However, if the as-deposited atoms form into NPs upon deposition, a growth per cycle of less than a monolayer does not necessarily enable atomic-level control over the NP size. This is because the NP morphology (e.g., size, shape and number density) is dictated by atomistic processes other than “ALD reactions” such as surface diffusion and aggregation of atoms and NPs, and atom attachment to and from NPs. Understanding the role of such kinetic processes during the ALD of NPs is therefore crucial to the development of ALD routes for the synthesis of NPs with a well-defined morphology and thus functionality.

Here, we present an atomistic model of thermal ALD of Pt and Pd NPs on oxides nanoparticles and graphene nanoplatelets. In particular, we study the effect of temperature, number of cycles, coreactant partial pressure and exposure time on the evolution of the NP size distribution and metal loading. Atomistic modelling of our experiments shows that the NPs grow mostly due to Smoluchowski aggregation, that is, NP diffusion and coalescence, rather than through single atom processes such as precursor chemisorption, single atom diffusion and attachment, and Ostwald ripening [1]. While the metal loading can be precisely controlled over a wide range of temperatures, atomic-level precision over the NP size is retained only at low deposition temperatures (T≤100 °C) when growth by atom attachment/deposition becomes the dominant process. Furthermore, we show that the coreactant partial pressure and exposure time are far more important parameters for the NP size than the number of cycles. We will demonstrate the favourable effect of optimizing the synthesis conditions on the catalytic performance of the obtained material.

[1] Grillo, F., et al. "Understanding and Controlling the Aggregative Growth of Platinum Nanoparticles in Atomic Layer Deposition: An Avenue to Size Selection." The Journal of Physical Chemistry Letters 8.5 (2017): 975.