Chemical Triggers Behind Pt Nanoparticle Growth During Post-Deposition O₂ Annealing by In Situ Near-Ambient Pressure X-Ray Photoelectron Spectroscopy

Nanoparticles (NPs) are crucial in a manifold of applications, owing to their plasmonic, magnetic and catalytic properties. In the past decades, increasingly advanced fabrication methods have been developed – such as ALD^[1] – to atomically tailor the structure of NPs, thereby fine-tuning their functional properties. However, during application, as-deposited NPs can undergo significant restructuring, such as NP growth or shape changes^[2], thereby perturbing the initial performance. In such cases, the as-deposited NPs evolve from their metastable initial state to the global minimum of the Gibbs free energy landscape. As a result, rationalizing and controlling the nanoparticle changes during operation is equally important as advancing the tailoring precision of nanofabrication techniques.<br/><br/>Herein, we explore<i> in situ</i> synchrotron-based near-ambient pressure X-ray photo-electron spectroscopy (NAP-XPS) to understand the chemical triggers behind Pt NP growth during post-synthesis O₂ annealing (50–500 °C, 1 mbar O₂). Such study finds relevance in the field of catalysis, where high temperature/pressure gas treatments induce atomic mobility and NP growth during the operation of catalysts, such as archetypal SiO₂-supported Pt NPs. From NAP-XPS, the chemical state of the NPs is probed from spectral signatures at relevant edges (Pt 4f, O 1s, C 1s, N 1s), while the relative coverage of the Pt NPs on the surface oxidized Si substrate is extracted from the Si 2p intensity. Before annealing, Pt NPs with controlled size and spacing are deposited on a SiO₂/Si wafer by applying the MeCpPtMe₃-O₂ and MeCpPtMe₃-N₂ plasma (N₂*) ALD processes^[3] at 300 °C.<br/><br/>When annealing the Pt NPs deposited with the O₂-based Pt ALD process, the initial C-layer on the Pt NP surface is burned around 200 °C. This yields metallic Pt NPs terminated with chemisorbed O-adatoms (~0.25 ML coverage). From 400–500 °C, the Pt oxidation state increases gradually but significantly from Pt⁰ to Pt^+x, resulting from thermally-activated Pt oxidation deeper inside the Pt NP sub-surface, yielding a Pt-PtO_x core-shell structure. However, surprisingly, Pt oxidation beyond 400 °C does not lead to atomic mobility and concomitant NP growth, which was expected from mobile (partially) oxidized PtO_x species. Quite the opposite, mild Pt NP growth takes place gradually at the start of annealing at 50 °C, even upon the presence of the C-surface layer.<br/><br/>The annealing process of the Pt NPs deposited by the N₂*-based ALD process behaves markedly different. First, the as-deposited state of the Pt NPs consists of oxidized Pt^+x species with CN-type ligands – still resulting from the N₂*-based ALD process. This metastable state decomposes upon thermal activation at 300 °C in O₂, yielding more stable metallic Pt NPs with chemisorbed O-adatoms (~0.25 ML coverage). Subsequently, as for the NPs of the O₂-based ALD process, these metallic Pt NPs transform into Pt-PtO_x core-shell particles by sub-surface Pt oxidation beyond 400 °C. During this annealing process, sudden atomic mobility of Pt and NP growth is observed upon fast decomposition of the oxidized Pt-CN phase into metallic Pt NPs. Again, no significant NP growth is observed upon Pt-PtO_x core-shell formation, identical to the mechanism observed for Pt NPs deposited by the O₂-based Pt ALD process.<br/><br/>This study shows that the chemical nature of the as-deposited phase by ALD can strongly depend on the ALD processes applied for its fabrication, in casu yielding metal Pt NPs with C-layer versus oxidized Pt-CN-type NPs. The initial, metastable state of these NPs will determine its pathway through free energy space, and hence result in different chemical mechanisms to evolve to a global minimum of the Gibbs free energy landscape.<br/><br/>References<br/>[1] S. M. George, <i>Chem. Rev.</i> <b>2010</b>, <i>110</i>, 111.<br/>[2] M. Filez et al., <i>Angew. Chem. Int. Ed.</i> <b>2019</b>, <i>58</i>, 13220.<br/>[3] J. Dendooven et al., <i>Nat. Commun.</i> <b>2017</b>, <i>8</i>, 1074.