Revealing the Relative Electronic Landscape of Colloidal ZnO and TiO2 Nanoparticles via Equilibration Studies

Interest in metal oxide semiconductors for energy processes has increased due to their prominent roles in photocatalysis, electrical energy storage, and conversion. However, an understanding of the thermochemistry of electron transfer (ET) reactions of these systems has lagged behind photophysical studies. This report investigates ET equilibria between reduced forms of well-characterized, ligated ZnO and TiO2 nanoparticles (NPs) suspended in toluene. Multiple electrons were added to each type of NP, either photochemically or with a chemical reductant. Equilibration experiments monitoring these added electrons are used to construct a qualitative band diagram. Surprisingly, the difference between the “reducible” oxide TiO2 and the formally “nonreducible” ZnO is reflected not in the relative band energies but rather in the relative width of the bands (the density of trap and/or band states). Moreover, the position of the electron equilibrium shifts upon addition of excess dodecylamine or oleic acid capping l...