Kinetic Measurements Using Nanoplasmonic Sensing

Licentiatavhandling, 2010

In this thesis the nanoplasmonic sensing technique was used to study kinetics of (i) the oxidation of Al nanoparticles in air and water and (ii) the solid-liquid phase transition in Sn nanoparticles. The nanoplasmonic sensing technique detects changes of the localized surface plasmon resonance (LSPR) in metal nanoparticles. When light shines on metal nanoparticles, at some wavelength the conduction electrons oscillate in resonance with the light. This resonance is called LSPR. It depends on the electronic structure, size, and shape of the nanoparticle as well as the optical properties of its nanoenvironment. (i) LSPRs in Al nanoparticles were characterized in detail: Extinction, scattering, and absorption efficiencies were determined experimentally for several different nanodisk sizes in the UV-vis-NIR spectral range. The experimental values were shown to be in good agreement with calculations based on the modified long wavelength approximation (MLWA). The oxidation in air of these Al particles was followed for long time periods. The results showed that nanoplasmonic sensing has potential for oxidation studies of metallic nanoparticles. In a more elaborate study, using both the nanoplasmonic sensing technique and quartz crystal microbalance with dissipation monitoring (QCM-D), highly resolved oxidation kinetics were measured for oxidation in water. MLWA model calculations of the LSPR facilitated interpretation of the results. Oxidation of Al nanoparticles in water was found to proceed in several stages, as reported for bulk Al, forming presumably pseudoboehmite (Al2O3·H2O) in the process. (ii) LSPRs in solid and liquid Sn nanoparticles were studied in the temperature range 25-250◦C. Distinct changes of the LSPR features occur when the particles melt and freeze. A large hysteresis between the melting and freezing point is observed. Obviously, nucleation of the solid phase is hindered unless the temperatures is considerably below the melting point. Kinetic measurements of the freezing event at different constant temperatures were performed. Classical nucleation theory with modifications considering the finite nanoparticle dimensions accounts well for the observed behavior.