Morphological variations of hydrothermally synthesised ZnO nanostructures and its impact on optical properties and photocatalytic degradation of methylene blue

Abstract

Zinc oxide nanostructures of different morphology were prepared through a facile low-temperature hydrothermal method in the presence of ionic capping agents, cetyltrimethylammonium bromide (ZnO-CTAB) and sodium dodecyl sulfate (ZnO-SDS) and in absence of capping agents (ZnO-1). The synthesis follows an environmentally safer, simpler, and cost-effective method. The present work highlights the morphological transition and its impact on the optical and photocatalytic properties of ZnO nanostructures. Results from X-ray diffraction spectroscopy (XRD) indicate that ZnO is in a single phase with a hexagonal wurtzite structure in all three samples. Photoluminescence spectra showed a green emission mediated by the presence of defects. The UV–vis absorption spectra revealed a marked blueshift in absorbance for capped zinc oxide nanostructures compared to uncapped ones. Raman spectroscopy was carried out to evaluate the vibrational modes of the molecules. FESEM images showed that the morphology changed from nanorods (ZnO-1) to rounded morphology (ZnO-CTAB) and petal-like structure (ZnO-SDS) upon capping with ionic surfactants. Under solar irradiation, the capped nanostructures ZnO-CTAB and ZnO-SDS have effectively decolorized the methylene blue dye compared to uncapped ones and showed improved efficiency. The presence of oxygen vacancies in the nanostructures was confirmed by the emission bands in photoluminescence (PL) studies and by Raman spectral analysis. The synergistic effect of defects in the sample and the morphological transition played an important role in increasing the photocatalytic degradation of the capped ZnO samples. The photocatalytic reaction obeyed pseudo first-order kinetics and the degradation mechanism was investigated using radical scavengers such as tertiary-butyl alcohol (OH radical scavenger), ammonium oxalate (h+ scavenger), and p-benzoquinone (superoxide radical scavenger) to identify the primary species involved in the photodegradation process. The mechanism of photodegradation and the recyclability of capped ZnO nanostructures were studied. The present study suggests the potential of this simple method to prepare desired nanostructures of different morphology using a variety of capping agents that can act as a promising candidate for dye degradation.