Atomically-Precise Synthesis of Platinum catalysts on Strontium Titanate using Atomic Layer Deposition.

摘要

This dissertation examines growth of platinum nanoparticles from atomic layer deposition (ALD) on SrTiO3 using a characterization approach that combines imaging techniques and other complementary analytical methods. The primary suite of characterization probes includes high resolution transmission electron microscopy (HRTEM), high angular annual dark field-scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and The Fourier transform infrared (FTIR). All imaging techniques reveal that Platinum nanoparticle grown via ALD are ultrafine with very narrow size distribution and well dispersed on either single crystal or nanocuboid shape strontium titanate (SrTiO3, STO) substrates.;The fundamental growth mechanism of Pt nanoparticles is investigated in details using HAADF-STEM, XPS and ICP-AES. During the initial cycle of ALD the deposition process begins with nucleation followed by growth, both occurring on a fast time scale relative to the deposition time. The final size is determined by net Pt deposition which is affected by reaction temperature. For multiple cycles, the particle size increases with the number of ALD cycles which also relies on net Pt deposition during that cycles. The increase in size per cycle is significantly lower than first cycle. This effect is due to carbonaceous material left on the surface from decomposition of the MeCpPtMe3 ligands. A negligible change of particle density during multiple cycle ALD deposition suggests a minimum secondary nucleation or particle coalescence.;The influence of different ALD parameters, e.g. reaction temperature, number of cycles, substrate, reagents and type of ALD methods, to acquired Pt nanoparticles are investigated and discussed individually. Particle size, density and loading vary with reaction temperature, number of cycles and type of ALD method. Pt growth orientation and shape can be changed by using different substrates. Application of Winterbottom construction to the observed shape of Pt nanoparticles on strontium titanate nanocuboids revealed that the surface structure of substrates and particle shape, growth orientation are interlinked, with control over one allowing equally precise control over the other. And changing the 2nd reagent can leads to different chemical states and compositions of formed particles. Thus, atomic level controlling synthesis in particle size, density, loading, chemical composition, growth orientation and thermodynamic shape of deposited Pt nanoparticles can be achieved by tuning these ALD parameters. Combining these observations will allow one to understand and improve catalytic performance when these particles are used for catalytic purposes, as selectivity and reactivity are often heavily dependent on catalyst size, shape, dispersion, exposed surfaces and chemical state. By choosing CO oxidation as a probe reaction, we revealed that Pt catalyst performance is strongly depends on their supporting material and size, as changing of which will alter the number or type of active sites of Pt catalysts which will change their catalytic performance.