Proton-coupled electron transfer from photo-excited CdS nanoparticles

Polyoxometalate (POM) cluster anions form monolayers on metal(0) nanoparticles (NPs) in water, serve as protecting ligands for binary-salt nanocrystals (such as AgCl), and as covalently attached ligands on anatase TiO₂ nanocrystals. We now show that the lacunary-Keggin ion [α-AlW₁₁O₃₉]⁹⁻ (1) binds strongly to Cd²⁺ in water, providing control over the growth and stability of CdS nanoparticles (NPs) that form upon addition of sulfide. When reduced by a single electron, the already highly negatively charged POM, 1 is protonated by water, and 1-protected CdS NPs were used as visible-light driven electron donors to assess whether combined reduction and protonation of 1 occurred via sequential electron- and proton-transfer steps (an ETPT mechanism), or simultaneously, via concerted proton-electron transfer (CPET). Comparison of the kinetic profiles for reduction of 1 in D₂O and in H₂O showed the absence of a kinetic isotopic effect (KIE), characteristic of ETPT mechanisms.