Origin of the green luminescence band in ZnO nanoparticles synthesized by sol-gel route

Structural and optical properties of ZnO nanoparticles of diameter ~ 5 nm, which are synthesized in a sol-gel route, have been studied using a variety of experimental techniques. The photoluminescence (PL) study that has been carried out on these particles in the atmospheric and vacuum conditions shows a suppression of the defect related green luminescence (GL) band and a simultaneous enhancement of the near-band-edge ultra violet luminescence (UVL) when the surroundings of the nanoparticles are evacuated. This finding clearly suggests that GL is originating from certain groups which are physisorbed on the surface of the nanoparticles. Fourier transform infrared spectroscopy (FTIR) that has also been conducted at the vacuum and atmospheric conditions reveals the presence of the hydroxyl and the acetate groups in these nanoparticle samples. These groups are also found to be removed upon evacuation, suggesting that they are physically adsorbed at the surface of the nanoparticles. When the PL spectrum is recorded again at the atmospheric condition, the GL intensity recovers almost up to its original value. Since there are plentiful of water molecules in air, which can source the hydroxyl groups, while the acetate groups are not expected to be abundant in air, this finding further suggests that the hydroxyl groups rather than the acetate groups are the likely cause for the GL emission observed in this system.