Silver nanoparticles exhibit size, shape and the surrounding dielectric medium dependent optical properties (scattering and absorption) in the visible spectral region. These optical properties can be explained in terms of the collective oscillation of the "free conduction band electrons" or "surface plasmons" induced by an external electromagnetic field. Dipole and Quadrupole Plasmon resonances have been observed for silver particles less than 100nm in diameter prepared using various synthetic techniques. However higher order resonances have been elusive due to lack of experimental techniques to prepare colloidal solutions of large silver nanoparticles in the size range above 100nm. Controlled hydrogen reduction of silver (I) oxide at 70˚C resulted in the formation of a colloidal solution of submicron silver particles. These particles exhibit the ability to optically excite higher order multipoles of the plasmon resonances, i.e. octupole and hexadecapole in its extinction spectra and the features match well with the Mie extinction calculations.
Core-shell particles represent a distinct class of nanomaterial with collective physicochemical properties that are unique and different from the individual components. Metal silver cores coated with amorphous semiconductor titania shell hybrid nanoparticles were prepared using controlled hydrolysis and condensation of titanium(IV) butoxide (sol-gel technique). Hydrothermal treatment at 350˚C resulted in the conversion of the amorphous titania shell into its crystalline anatase form. Progressive red shifts in the plasmon resonance peaks in the extinction spectra were observed for amorphous and crystalline anatase titania coated silver nanoparticles.
The anatase form of titania is one of the most effective semiconductor photocatalyst when excited with UV light (λ≤ 380nm). Metal ion doping into anatase titania is one means to shift its band gap transition to the visible spectral region for its practical application in the solar spectral region. A modified sol-gel technique using titanium (IV) butoxide and iron(III) nitrate nonahydrate precursor followed by heat treatment in air was employed to prepare iron (III) ion doped anatase titania nanocomposites with the onset of band gap transition red-shifted (~λ= 475nm) to the visible spectral region. The direct photocatalytic effect was observed in the degradation of dye pollutant sulforhodamine-B in the presence of iron(III) ion doped titania nanocomposites upon visible light irradiation.
Hydrogen reduction of silver(I) oxide in the presence of ~200nm polystyrene microspheres (Polysciences Inc.) at 70˚C led to the formation of silver nanoparticles attached to polystyrene microspheres. Extinction spectra of the resulting colloidal solution revealed a suppressed quadrupolar plasmon resonance peak when compared to the resonance observed for colloidal silver nanoparticles in the same size range without the presence of polystyrene beads. Electron microscopy images support the above observation with the polystyrene microspheres attached to one of the crystalline surface of the particle. Further acetone treatment lead to the encapsulation of the silver nanoparticles within the polystyrene microspheres as observed in the electron microscopy images and red shifts in the plasmon resonance peaks with the evolution of a prominent quadrupolar resonance peak in the extinction spectra. These silver-polystyrene particles represent a unique metal-polymer core shell nanostructure.