Size Reduction of CdSe/ZnS Core-Shell Quantum Dots Photosensitized by Benzophenone: Where Does the Cd(0) Go?

The size of core shell CdSe/ZnS quantum dots can be decreased by using the combined action of an n,pi* aromatic ketone and UVA light. Energy-dispersive X-ray spectroscopy as well as X-ray photoelectron spectroscopy techniques gave information on the photosensitization mechanism and the eventual destiny of Cd2+ and Se2- core ions. Our data support the electron transfer from the BP ketyl radical to Cd2+, leading to Cd-0 and H+, as well as to the recovery of benzophenone. Elemental Cd remains on the core and, eventually, can be oxidized to CdO. In addition. Se(2-)counterions disperse inside the solution mainly attached to protonated amine ligands. The Se2- combines with H. leading to SeH2, which is finally oxidized to Se-0 by oxygen. Therefore, quantum dots illumination in the presence of benzophenones brings about a profound nanoparticle reconstruction which takes place after dark storage; this agrees with the drastic quenching of the quantum dot emission detected immediately after illumination as well as the slow recovery in the dark.