The effects of aging on the luminescence of PEG-coated water-soluble ZnO nanoparticle solutions

Water-soluble ZnO nanoparticles coated with polyethylene glycol biscarboxymethyl (PEG(COOH)(2)) were prepared in ethanol/water solutions. The ZnO nanoparticles have a hexagonal structure with an average size of 10 nm. Three different luminescence bands are observed from the nanoparticle solutions: green emission at 530 nm from surface states or defects, UV emission at 380 nm from the ZnO exciton, and an emission band at around 338 nm from the PEG(COOH)(2). The fresh as-prepared samples have very strong green emission at 530 nm from surface states or defects but very weak excitonic emission at 380 nm. After dilution with ethanol, the green emission decreases in intensity and the excitonic emission increases. In the diluted samples, the excitonic luminescence intensity increases with storage time. This intensity increase is attributed to surface passivation by CH3COO- ligands resulting from precursor reactions in the ethanol solvent.