Size-Dependent Structures and Properties of Metallic Particles and Thin Films

Fundamental aspects of metallic nanoparticles, especially size-dependent properties and their interaction with their surrounding have been investigated. Several conclusions were reached as follows, (1) Au and Pt particles form dense SAMs on glass. Interparticle resonance is absent for 15 nm Au particles, but present for 30 nm ones. A new strategy was developed to deposit silica insulation layers in between Au particle monolayers. (2) Au particles, 1.5-20 nm in size, were encapsulated in silica shells. Their melting point was determined and we show that it decreases significantly as particle size decreases, leading to increased self-diffusion coefficient of the Au atoms. (3) Au core particles of different sizes were synthesized and Ag shells of different thickness were deposited on them. XAFS measurements show that Au/Ag alloy is spontaneously formed for the particles with small core size (2.5 nm). The alloy formation is size-dependent and molecular dynamics calculations demonstrate that vacancies at the bimetallic boundary dramatically enhance the rate of mixing. (4) EPR spectroscopy was used to study the interactions between stable free radicals and gold nanoparticles. The EPR signal is reduced upon adsorption of the radicals onto Au particle surface. We propose that the reduction in signal intensity arises from exchange interactions between the unpaired electrons of the adsorbed radicals and conduction-band electrons of the metallic particles. Catalytic autoxidation of TEMPAMINE to TEMPO was also observed and a mechanism for this unexpected reaction is proposed. (5) Redox/galvanic exchange reactions between Au and Pt nanoparticles and Ag(CN)2- were investigated. For Au particles, the exchange reaction is size dependent. 2.5 nm Au particles form an alloy with Ag and the extinction coefficient of the alloy particle linearly depends on the Au/Ag mole fraction. The full exchange for both 2 and 8 nm Pt particles indicates that the atom diffusion rate within particles is very high and such high rate is not an intrinsic property of these Pt particles.