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*₂Co (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*₂Co can be increased to over 15 electrons per NC. The weaker reductant Cp*₂Cr transfers electrons only to ZnO NCs in the presence of protons. Cp*₂M⁺ counterions are much less effective than protons at stabilizing reduced NCs. With excess Cp*₂Co or Cp*₂Cr, the extent of reduction increases roughly linearly with the number of protons added. Some of the challenges in understanding these results are discussed.