Understanding and Controlling the Aggregative Growth of Platinum Nanoparticles in Atomic Layer Deposition: An Avenue to Size Selection

Abstract

We present an atomistic understanding of the evolution of the size distribution with temperature and number of cycles in atomic layer deposition (ALD) of Pt nanoparticles (NPs). Atomistic modeling of our experiments teaches us that the NPs grow mostly via NP diffusion and coalescence rather than through single-atom processes such as precursor chemisorption, atom attachment, and Ostwald ripening. In particular, our analysis shows that the NP aggregation takes place during the oxygen half-reaction and that the NP mobility exhibits a size- and temperature-dependent scaling. Finally, we show that contrary to what has been widely reported, in general, one cannot simply control the NP size by the number of cycles alone. Instead, while the amount of Pt deposited can be precisely controlled over a wide range of temperatures, ALD-like precision over the NP size requires low deposition temperatures (e.g., T < 100 °C) when growth is dominated by atom attachment.

Low-temperature atomic layer deposition delivers more active and stable Pt-based catalysts

“…The Pt loadings estimated via INAA were 0.5 wt% (∼4 ng cm −2 ), 1.5 wt% (∼12 ng cm −2 ), and 6 wt% (∼48 ng cm −2 ) after 1, 3, and 10 cycles, respectively. As the evolution of the PSD with the number of cycles was already discussed in detail in our previous work, 27 here we now elaborate on the evolution with the deposition temperature of the mass-based PSD of the Pt/GNP obtained after 10 cycles (see Fig. 1), that is, the composites with the highest loading considered here.…”

“…27 The reader is referred to the latter for a comprehensive characterization of the composites obtainable in the cycle and temperature range used here. In brief, such ALD process, by relying on high oxygen partial pressures (i.e., 0.2 bar) and oxygen exposures on the order of minutes, enables the deposition of metallic Pt NPs at temperatures (e.g., 100°C) at which conventional ALD would otherwise lead to negligible deposition without resorting to powerful oxidizers such as ozone and oxygen plasma.…”

“…For example, increasing the number of cycles in thermal ALD of Pt and Pd has been reported to not only vary the average NP size but also broaden the size distribution. 21,[25][26][27] We have recently described the fate of adatoms on supports during ALD, showing that they indeed migrate, form clusters and sinter. 27 These processes are highly size-dependent (large clusters are less mobile), temperature-dependent (as holds for most sintering processes 1,2 ), and crucially depend also on the extent to which both surface reactions have run to completion.…”

“…21,[25][26][27] We have recently described the fate of adatoms on supports during ALD, showing that they indeed migrate, form clusters and sinter. 27 These processes are highly size-dependent (large clusters are less mobile), temperature-dependent (as holds for most sintering processes 1,2 ), and crucially depend also on the extent to which both surface reactions have run to completion. In fact, one can fabricate single-atom catalysts by not running the oxidative removal of precursor ligands to completion.…”

“…21 However, exposing such materials to reaction conditions at higher temperatures burns off the ligands and immediately renders these singleatoms mobile, which can then sinter into larger NPs. 27 The crucial outstanding question is whether ALD techniques can produce supported NPs of desirable narrow size distributions that remain stable in catalytic reaction conditions. Here, we show that Pt nanoparticles deposited by ALD on graphene nanoplatelets (GNP) at low temperatures (100°C) using a fluidized bed operated at atmospheric pressure not only have a narrower size distribution but are also more stable than their high-temperature counterparts.…”