4, ALBA Synchrotron, Cerdanyola del Vallès, , Spain
2, Synchrotron SOLEIL, Saint-Aubin, , France
3, ESRF, Grenoble, , France
Supported noble metal nanoparticles are widely used in heterogeneous catalysis. It is well established that the performance of catalytic nanoparticles is closely related to their size, shape and interparticle distance. Synthesis methods that can tailor the structural properties of noble metal nanoparticles are therefore attractive to elucidate performance-structure relationships and tune the catalytic activity, selectivity and thermal stability. In this regard, there is an increasing interest in Atomic Layer Deposition (ALD), a vapor-phase deposition method which proved its efficiency in dispersing noble metal nanoparticles on complex high surface area supports with atomic-scale control over the metal loading (atoms per cm2) and nanoparticle size . However, an improved understanding of how the deposition parameters influence the formation and growth of the noble metal nanoparticles is required to fully exploit the tuning potential of ALD.
We recently designed a synchrotron-compatible high-vacuum setup that enables in situ X-ray fluorescence (XRF) and grazing incidence small-angle X-ray scattering (GISAXS) monitoring during thermal and plasma-enhanced ALD . Using this setup, we resolved the dynamics of Pt and Pd nanoparticle formation and growth on planar SiO2 substrates. In situ XRF was used to quantify the metal loading, while analysis of the GISAXS patterns allowed us to correlate the amount of deposited material with the evolution of structural parameters such as cluster shape, average size and areal density. Firstly, we investigated how the choice of reactant (O2 gas, O2 plasma, N2 plasma, NH3 plasma) affects the island growth and morphology during ALD of Pt with the MeCpPtMe3 precursor at 300 °C. It was found that O2 induces atom and cluster surface diffusion and promotes the ripening of the Pt nanoparticles, while diffusion phenomena seem to be suppressed during N2 and NH3 plasma-based ALD. This insight provided the ground for the development of a tuning strategy that is based on combining the O2-based and N2 plasma-based ALD processes and offers independent control over the Pt nanoparticle size and coverage . Secondly, we carried out a systematic study of plasma-enhanced Pd nanoparticle ALD with the Pd(hfac)2 precursor at 150°C, comparing a purely reducing chemistry (H2 plasma as reactant) with a three-step process that includes an oxidizing agent (sequential dosing of H2 plasma and O2 plasma ). In contrast to the Pt system, it was found that Pd ALD is characterized by a static nanoparticle growth, even when an O2 plasma step is included in the deposition process. This knowledge is vital to enable efficient synthesis of supported catalysts.
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