Studies of Zinc Oxide Nanocrystals: Quantification of Capping Ligands and the Coupling of Protons and Electrons
Valdez, Carolyn Nicole
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The energetics of semiconductors are widely relevant to technologies ranging from chemical- and photo-catalysis to charge injection in photovoltaic materials. In these processes involving electron transfer, protons often play a critical but overlooked role in facilitating charge transfer. For example, the conduction band energies of most metal oxides in contact with an aqueous solution demonstrate a Nernstian pH dependence, an observation that cannot be explained by surface protonation models. Given that a Nernstian dependence is typically attributed to proton coupled electron transfer (PCET), we are interested in determining if the reduction of metal oxides can also be described by PCET. Zinc oxide (ZnO) nanocrystals (NCs) were chosen as a model system given the broad range of previous research on bulk and nanocrystalline forms of ZnO, the relative ease of synthesis and characterization, and their use in developing a fundamental understanding of interfacial electron transfer. We demonstrate that photochemically reduced NCs react with hydrogen-atom acceptors, indicating that both electrons and protons are transferred by the NCs. To isolate the influence of a proton coupled to the extra electron in the conduction band, the NCs have also been reduced chemically. Addition of an excess of the one-electron reductant CoCp*2 (Cp* = pentamethylcyclopentadienyl, -1.94 V vs. Fc/Fc+) gives NCs that contain extra electrons in the conduction band, without protons that arise from photoreduction. Protons can also be individually added stoichiometrically to the NCs by either a photoreduction/oxidation sequence or by addition of acid. Using these methods, we have shown that the presence of one extra proton drastically alters the redox potential of the NCs. With the addition of acid the NC orbitals are lowered, allowing the systematic variation of driving force for electron transfer from the reductant to the NCs. In the presence of excess reductant and acid, the number of electrons per NC (<ne–>max) reaches a maximum, beyond which the addition of more acid has no effect. This <ne–>max varies with the NC radius with an r3 dependence, so the density of electrons (<Ne–>max) is constant over a range of NC sizes. The approximately 1:1 relationship of <ne–> with protons per NC, and the dramatic dependence of <Ne–>max on the nature of the cation (H+ vs. MCp*2+) suggest that the protons intercalate into the NCs under these conditions. These studies illustrate the strong coupling between protons and electrons in ZnO NCs and show that proton activity is a key parameter in nanomaterial energetics.
- Chemistry