Proton-Controlled Reduction of ZnO Nanocrystals: Effects of Molecular Reductants, Cations, and Thermodynamic Limitations

Charge carriers (electrons) were added to ZnO nano crystals (NCs) using the molecular reductants CoCp2* and CrCp2* [Cp* = eta(5)-pentamethylcyclopentadienyl]. The driving force for electron transfer from the reductant to the NCs was varied systematically by the addition of acid, which lowers the energy of the NC orbitals. In the presence of excess reductant, the number of electrons per NC (< n(e)(-)>) reaches a maximum, beyond which the addition of more acid has no effect. This < n(e)(-)>(max) varies with the NC radius with an r(3) dependence, so the density of electrons (< N-e(-)>(max)) is constant over a range of NC sizes. < N-e(-)>(max) = 4.4(1.0) X 10(20) cm(-3) for CoCp2* and 1.3(0.5) X 10(20) cm(-3) for the weaker reducing agent, CrCp2*. Up until the saturation point, the addition of electrons is linear with respect to protons added. This linearity contrasts with the typical description of hydrogen atom-like states (S, P, etc.) in the conduction band. The 1:1 relationship of < n(e)(-)> with protons per NC and the dramatic dependence of < N-e(-)>(max). on the nature of the cation (H+ vs MCp2*(+)) suggest that the protons intercalate into the NCs under these conditions. The differences between the reductants, the volume dependence, calculations of the Fermi level using the redox couple, and a proposed model encompassing these effects are presented. This study illustrates the strong coupling between protons and electrons in ZnO NCs and shows that proton activity is a key parameter in nanomaterial energetics.