Multifunctional bandgap-reduced ZnO nanocrystals for photocatalysis, self-cleaning, and antibacterial glass surfaces

Abstract

A set of ZnO nanocrystals assembly with tuneable reduced bandgaps was prepared via thermal decomposition of ZnO2 precursor. A detailed Raman and EPR analysis revealed ZnO rich in oxygen vacancies with concentration varying upon annealing temperature. The structural defect features corroborated with the bandgap variations indicate photocatalytic response in the visible-light, which was evaluated by using monochromatic LEDs (377, 401, and 429 nm wavelengths) for a correct assessment of the photocatalytic activities of samples in the close vicinities of their bandgaps. It was revealed, that bandgap reduced ZnO exhibits only little yet negligible photocatalytic activity towards Methylene Blue discoloration under 429 nm diode. The Photocatalytic experiments using scavengers support the model that oxygen vacancies easily trap photo-excited electrons (whether VO+ and VO++ state) and, if the energy level of this donor state is close enough, the trapped electron could easily thermalize to the conduction band as well as recombine with photo-excited holes. Furthermore, oxygen vacancies rich ZnO nanocrystals treated glasses were prepared and their photocatalytic-induced self-cleaning property, wettability, and antibacterial activities were evaluated under both UV and visible-light. Besides excellent antibacterial activities against both S. aureus and E. coli, which was achieved even in dark, both photocatalytically-induced self-cleaning ability and wettability provided yet another indirect evidence of the inability of bandgap-reduced ZnO to become activated under visible-light irradiation.