Effect of Protons on the Redox Chemistry of Colloidal Zinc Oxide Nanocrystals

Electron transfer (ET) reactions of colloidal 3-5 nm diameter ZnO nanocrystals (NCs) with molecular reagents are explored in aprotic solvents. Addition of an excess of the one electron reductant Cp*Co-2 (Cp* = pentamethylcyclopentadienyl) gives NCs that are reduced by up to 1-3 electrons per NC. Protons can be added stoichiometrically to the NCs by either a photoreduction/oxidation sequence or by addition of acid. The added protons facilitate the reduction of the ZnO NCs. In the presence of acid, NC reduction by Cp*Co-2 can be increased to over 15 electrons per NC. The weaker reductant Cp*Cr-2 transfers electrons only to ZnO NCs in the presence of protons. Cp*M-2(+) counterions are much less effective than protons at stabilizing reduced NCs. With excess Cp*Co-2 or Cp*Cr-2, the extent of reduction increases roughly linearly with the number of protons added. Some of the challenges in understanding these results are discussed.