Surface modification of zinc oxide nanoparticles for improved photocatalytic properties

Abstract

In this dissertation, I have focused on ZnO surface modification to improve the photocatalytic degradation performance. The studies have been divided to three sections for different strategies to improve photocatalytic properties of ZnO including to nonmetal and metal doping as well as specific charge and polar molecule functionalization.

In first section, the research studies focused on nonmetal doping. Zinc oxide/reduced graphene oxide nanocomposites (ZnO/rGO) are synthesized via a simple one-pot solvothermal technique. The nanoparticle–nanorod tunability was achieved with the increase in GO additive, which was necessary to control the defect formation. The optimal defect in ZnO/rGO not only increased ZnO/rGO surface and carrier concentration, but also provided the alternative carrier pathway assisted with rGO sheet for electron–hole separation and prolonging carrier recombination. These properties are ideal for photodetection and photocatalytic applications. For photosensing properties, ZnO/rGO shows the improvement of photosensitivity compared with pristine ZnO from 1.51 (ZnO) to 3.94 (ZnO/rGO (20%)). Additionally, applying bending strain on ZnO/rGO enhances its photosensitivity even further, as high as 124% at r= 12.5 mm, due to improved surface area and induced negative piezoelectric charge from piezoelectric effect. Moreover, the photocatalytic activity with methylene blue (MB) was studied. It was observed that the rate of MB degradation was higher in presence of ZnO/rGO than pristine ZnO. Therefore, ZnO/rGO became a promising material for different applications.

In second section, the research studies focused on metal doping. Cerium-oxide-nanoparticle-decorated zinc oxide was successfully prepared using a simple one-pot hydrothermal technique with different weight% Ce doping. It was found that an increase in Ce doping has an effect on the optical energy band-gap tunability. Ce dopant provides electron trapping on Ce/ZnO nanocomposites and also acts as a surface defect generator during hydrothermal processing. Additionally, a bi-metal oxide heterojunction forms, which acts as a charge separator to obstruct charge recombination and to increase the photodegradation performance. It was found that the methyl orange (MO) degradation performance improved with an increase in Ce doping. The decomposition of MO went from 69.42% (pristine ZnO) to 94.06% (7% Ce/ZnO) after 60 min under fluorescent lamp illumination.

In last section, the research studies focused on Specific charge and polar molecule functionalization. Functionalization of aniline molecules on zinc oxide (ZnO) nanoparticles is reported using a simple impregnation technique. As-prepared samples were systematically characterized based on morphology, surface and optical properties, and photocatalytic performance towards methyl orange (MO). Aniline functionalization increases the surface charge of the modified ZnO. Compared to pristine ZnO, the aniline-functionalized ZnO shows faster photodegradation of MO, degrading 98.29% of MO in 60 min. These results indicate that the improvement of photocatalytic degradation is ascribed to opposite charge-induced surface adsorption. Hence, aniline, as a positively charged molecule, is expected to increase the surface adsorption of MO (as an anionic dye) onto the ZnO nanoparticles, thereby increasing their photodegradation ability.