Surface modification of nanocrystalline metal oxide particles with enediol ligands was found to result in altered optical properties of nanoparticles. The surface modification results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The optical shift is correlated to the dipole moment of the Ti-ligand complexes at the particle surface and decreases with the ligand size. The binding was found to be exclusively characteristic of colloids in the nanocrystalline domain(<20 nm). X-ray near-edge structure measurements at Ti K edge indicate that in this size domain the surface Ti atoms adjust their coordination environment to form undercoordinated sites. These five-coordinated defect sites are the source of novel enhanced and selective reactivity of the nanoparticle toward bidentate ligand binding as observed using IR spectroscopy. Enediol ligands have the optimal geometry for chelating surface Ti atoms, resulting in a five-membered ring coordination complex and restored six-coordinated octahedral geometry of surface Ti atoms. The binding of enediol ligands is enhanced because of the stability gained from adsorption-induced restructuring of the nanoparticle surface. Consistent behavior was found for the three different nanocrystalline metal oxide systems: TiO 2 , Fe 2 O 3 , and ZrO 2 .

The structural and electrochemical properties of nanoparticles were found to be different from those of the corresponding bulk semiconductors. Due to the specific binding of modifiers to “corner defects”, the optical properties of small titania particles were red shifted 1.6 eV compared to unmodified nanocrystallites. It was found using electron paramagnetic resonance (EPR) that, as with organic charge transfer superconductors, these novel nanocrystallites operate with a charge-transfer mechanism, and exhibit semiconducting properties through both constituents (large band gap semiconductor and organic modifier). The EPR spectra were consistent with hole trapping on the surface modifier and electron trapping on shallow interstitial and partially delocalized Ti sites. These systems have an important feature in that charge pairs are instantaneously separated into two phasesthe holes on the donating organic modifier and the electrons in the conduction band of TiO2.

Molecular snapshots obtained by ultrafast X-ray spectroscopy reveal new insight into fundamental reaction mechanisms at single electron and atomic levels.

No abstract