Cationic and anionic surface binding sites on nanocrystalline zinc oxide: surface influence on photoluminescence and photocatalysis.

To probe the influence of the surface on the overall nature of zinc oxide nanocrystals (ZnO NCs) this paper examines the effects of surface modifiers: cobalt Co(II) and trimethylsilanolate, on the properties of ZnO NCs. A clear relationship between the surface, photocatalytic (PC), and photoluminescent (PL) character of ZnO is observed. With potassium trimethylsilanolate and cobalt(II) acetate we have determined that anionic binding sites occupied by silanolate contribute to 40% of the PL character of the defect emission (500-550 nm). Photodegradation of 4-nitrophenol was used as a probe to evaluate the effect of surface modification on the PC performance of ZnO NCs. At maximum silanolate modification the PC activity of ZnO was reduced by 50%. Modification of ZnO NCs with Co(II) resulted in the transfer of photoexcited electrons to the cobalt center where consequent nonradiative recombination, at energies lower than required for PC, was observed via a comparable decrease in both PL and PC activity. These results are critical for using ZnO NCs in sensory, photocatalytic, and electronic applications.