From Single Atoms to Nanoparticles: Autocatalysis and Metal Aggregation in Atomic Layer Deposition of Pt on TiO₂ Nanopowder

Abstract

A fundamental understanding of the interplay between ligand-removal kinetics and metal aggregation during the formation of platinum nanoparticles (NPs) in atomic layer deposition of Pt on TiO nanopowder using trimethyl(methylcyclo-pentadienyl)platinum(IV) as the precursor and O as the coreactant is presented. The growth follows a pathway from single atoms to NPs as a function of the oxygen exposure (P × time). The growth kinetics is modeled by accounting for the autocatalytic combustion of the precursor ligands via a variant of the Finke-Watzky two-step model. Even at relatively high oxygen exposures (<120 mbar s) little to no Pt is deposited after the first cycle and most of the Pt is atomically dispersed. Increasing the oxygen exposure above 120 mbar s results in a rapid increase in the Pt loading, which saturates at exposures >> 120 mbar s. The deposition of more Pt leads to the formation of NPs that can be as large as 6 nm. Crucially, high P (≥5 mbar) hinders metal aggregation, thus leading to narrow particle size distributions. The results show that ALD of Pt NPs is reproducible across small and large surface areas if the precursor ligands are removed at high P .

Thermal atomic layer deposition of gold nanoparticles: controlled growth and size selection for photocatalysis

“…This figure is very much in line with what has been reported for a similar process: Pt ALD on P25, where Δn Pt after one cycle at satur- ation is about ∼1 at nm −2 . 29,33 The deposition of less than a monolayer per cycle further attests to the ALD-nature of the precursor chemisorption on TiO 2 . 35 Furthermore, we estimate about 0.57 g of Au to be fed to the reactor in 60 min (saturation time), this amount results in Au to TiO 2 weight fraction of 2.8 wt% which is very close to the experimentally obtained saturation value (2.7 wt%).…”

“…Atomic layer deposition (ALD) is a vapor-phase deposition technique for the synthesis of supported nanoparticles as it boasts high precursor utilization efficiency, low degree of contamination of the final product, and an excellent degree of control over both the metal loading and the particle size. 16,[23][24][25][26][27][28][29] While there exist over seventy ALD processes for the deposition of noble metals, only limited success with regards to Au has been reported. 10 Two successful studies on ALD of Au include a plasma process at 120°C using (trimethylphosphino)trimethylgold(III) as metal precursor; and a thermal process at 180°C, with dimethylgold(III)(diethylthiocarbamato) as metal precursor.…”

Boron Nitride as a Novel Support for Highly Stable Palladium Nanocatalysts by Atomic Layer Deposition

“…The control of NPs dimensions is very important for heterogeneous catalysis applications. Many different parameters such as pressure, substrate temperature, and co-reactant exposure are involved in their growth but the formation of metallic NPs by ALD is mainly based on the difference in the surface energies between the metal and the substrate surface, diffusion processes, and the ALD precursors’ chemistries [ 56 , 74 , 75 , 84 ].…”

Single‐Atom Catalytic Materials for Advanced Battery Systems

“…The self‐limiting nature of ALD provides control (e.g., by the number of cycles) and accuracy of synthesis of single‐atom catalysts, [ 57 ] while this method is limited by expensive equipment and rigorous manipulation of time like many vapor deposition techniques such as chemical vapor deposition (CVD). This method has been utilized to realize coordination designs by reactions between precursor ligands and absorbed oxygen, [ 41,58 ] and to achieve spatial confinement by creating nanocavities or depositing metal atoms into nanocubes. [ 59,60 ]…”

“…For example, the sample undergoing 50 ALD cycles (1.52 wt% Pt) displayed dominant individual Pt atoms with occasionally observed subnanometer clusters. Grillo et al ascribed the successful fabrication of SACMs to short oxygen exposures (2 s) and low oxygen pressures (<1 mbar), [ 58 ] suggesting both the reaction atmosphere and the initial state of support surface were dominant in achieving atomic‐scale metal dispersion. Apart from Pt, isolated single‐atom Pd can also be realized by TiO 2 ALD.…”