Emily Tsui

Controlling Electrons and Reduction Potentials in Inorganic Clusters and Semiconductor Nanocrystals

Understanding the principles that govern the reactivity and electrochemical behavior of molecules and materials allows chemists to develop new catalysts, reactions, and devices. Taking a molecular approach, a series of heterometallic manganese oxido clusters was prepared as structural models of the oxygen-evolving complex (OEC), which is responsible for water oxidation during photosynthesis. Electrochemical studies demonstrated that the reduction potentials of the clusters are correlated to the Lewis acidity of the incorporated redox-inactive metal. Reactivity studies show that varying the redox-inactive metals in isostructural compounds also affects the rates of oxygen atom transfer from the clusters as well as hydrogen atom transfer to the clusters. These studies suggest that the Ca²⁺ ion plays a role in modulating the reduction potential and oxygen atom transfer activity of the OEC during catalysis. To study the behavior of electrons in larger materials, colloidal semiconductor nanocrystals (NCs) were reduced using a photochemical electronic doping method. In CdSe NCs, chemically reduced surface selenium sites quench valence-band holes during irradiation, resulting in excess delocalized conduction-band electrons. These photodoped n-type CdSe NCs undergo slow electron trapping that appears to be governed by an as-yet unidentified surface migration process. Electron trapping occurs on time scales relevant to photoluminescence blinking, and can be tuned by variations in nanocrystal size, surface composition, and sample temperature.