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Data and materials availability: All data are available in the manuscript or supplementary information. All materials are available upon request to L.D. Methods Solution-phase synthesis of pure 2D halide perovskite sheets In this study, ten types of pure 2D halide perovskite sheets were synthesized via a quaternary solvent method.

This article presents a systematic study of the formation and thermal stability of Pt oxide species on sizeselected Pt nanoparticles (NPs) supported on SiO 2 , ZrO 2 , and TiO 2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex situ transmission electron microscopy and atomic force microscopy. The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO 2 species are formed on all supports investigated after O 2 -plasma exposure. A two-step thermal decomposition (PtO 2 f PtO f Pt) is observed from 300 to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO 2 under annealing treatments in O 2 . Strong NP/support interactions and the formation of Pt-Ti-O alloys are detected for Pt/TiO 2 upon annealing in UHV above 550 K but not under an identical treatment in O 2 . Furthermore, thermal treatments in both environments above 700 K lead to the encapsulation of Pt by TiO x . The final shape of the micellar Pt NPs is influenced by the type of underlying support as well as by the post-deposition treatment. Spherical Pt NPs are stable on SiO 2 , ZrO 2 , and TiO 2 after in situ ligand removal with atomic oxygen at RT. However, annealing in UHV at 1000 K leads to NP flattening on ZrO 2 and to the diffusion of Pt NPs into TiO 2 . The stronger the nature of the NP/support interaction, the more dramatic is the change in the NP shape (TiO 2 > ZrO 2 > SiO 2 ).

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