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Platinum Nanoparticles in Transition Alumina: An Atomic Resolution Study of Interfaces
The phases of transition alumina are scientifically interesting due to their high specific surface area, favorable acid/ base characteristics, and chemical and hydrothermal stability which make this metallic oxide a prime candidate for use as a support for metallic nanoparticles (NPs) in heterocatalysis. Interactions at alumina/Pt interfaces are important for catalysis, but poorly understood due to the microstructural complexity of transition alumina and challenges associated with characterization of interfaces. The goal of this work is to compare experimentally determined atomic-level structural information from aberration-corrected scanning transmission electron microscopy (STEM) with density functional theory (DFT) based structural models and quantum mechanical STEM image simulations to characterize the chemistry and atomic-level structure of Pt/γ and Pt/θ-interfaces.
In this work, dense transition-alumina layers are formed using high-energy ion implantation to amorphize near surface layers of α-alumina wafers which are then recrystallized into the desire alumina form via thermal annealing. Self-implanted alumina wafers are used to determine viability of producing oriented γ-alumina layers. Pt-implanted alumina is processed to produce Pt/γ-alumina and Pt/θ-alumina interfaces in the form of faceted Pt NPs embedded in alumina. Conventional transmission electron microscopy (TEM), image-corrected TEM, and x-ray diffraction (XRD) are used to characterize the microstructure of γ-alumina and Pt/transition-alumina systems. The recrystallization of amorphized alumina occurs epitaxially to the α-alumina with parallel alignment close-packed O planes of (0006)α and (111)γ or (201). Faceted Pt NPs possess a (111)Pt||(111)γ;[110]||[110] orientation in γ-alumina and a (201) ||(111)Pt; [132]θ|| [110] orientation in θ-alumina. In both systems the NPs are primarily bound by {111}Pt facets. Probe-corrected STEM was used to collect atomic resolution images of the faceted interfaces to determine the structure and chemical terminations of γ- and θ-alumina at the interfaces. This is facilitated by comparison of the experimental images to quantum mechanical scanning (S)TEM simulations of density functional theory (DFT) based model structures. Oxygen termination of the alumina is found to be the dominant chemical termination for both (111)γ||(111)Pt and ( 01)||(111)Pt interfaces
Orientation and morphology of Pt nanoparticles in γ-alumina processed via ion implantation and thermal annealing
Structure and chemistry of metal/metal-oxide interfaces are critical for many catalytic processes and sensing. Pristine interfaces of Pt and γ -Al2O3 were fabricated using high-energy ion implantation and thermal processing. Amorphous regions of alumina develop in single crystal α-alumina during Pt+ implantation and an 800 °C thermal treatment crystalizes amorphized alumina to γ -Al2O3 and allows Pt ions to precipitate within the developing γ -alumina, yielding Pt nanoparticle tetrahedra terminated by {111} surfaces. The phase of alumina that developed and the distribution, morphology, and orientation of Pt nanoparticles was determined using x-ray diffraction, Rutherford backscattering spectrometry, transmission electron microscopy and scanning transmission electron microscopy