Electron transfers at semiconductors are widely studied and relevant to technologies ranging from chemical- and photo-catalysis to charge injection in photovoltaic materials. In these processes, protons often play a critical but overlooked role in facilitating charge transfer. The conduction band energies of most metal oxides in contact with an aqueous solution demonstrate a Nernstian pH dependence, an observation that is so pervasive that it cannot be explained by typical 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. Colloidal zinc oxide (ZnO) nanocrystals (NCs) suspended in nonpolar solvents are used as a model system. 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) gives NCs that contain extra electrons in the conduction band, without the 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. In the presence of excess reductant and acid, the number of electrons per NC (&lt;ne–&gt;max) reaches a maximum, beyond which the addition of more acid has no effect. 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.

Proton coupled electron transfer at nanocrystals: Effects of protons on ZnO redox chemistry

251st National Meeting (2016)

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