Nano-sized ZnO and its composites with TiO2 were prepared via sol-gel and hydrothermal methods. The prepared nano-sized ZnO and its composites were characterized by laser induced breakdown spectroscopy (LIBS), scanning electron microscope (SEM), transmission electron microscope (TEM) and x-ray diffraction (XRD) methods. Sol-gel method has provided nano-rod ZnO in the presence of surfactant as capping agents while thermal/hydrothermal methods have provided spherical shaped nano-ZnO with minor amount of rod-like ZnO. The size and shape of the prepared nano-ZnO were confirmed either by SEM or TEM. The shape of ZnO-TiO2 nanocomposite was almost spherical. BET isotherm was used for measuring the surface area of the nanomaterials. Reflectance spectrometer was used to estimate the band gap energy of the nanomaterials and found in a range from 3.15 to 3.20 eV. At least five to six organic dyes (both azo- and non-azo) were used to investigate the photocatalytic activities of the nanomaterials under irradiation of solar light along with other light sources such as tungsten filament, fluorescent and UV-light. Results show that nano-rod ZnO can efficiently degrade both the azo- and non-azo dyes in aqueous system under solar light irradiation. However, very poor catalytic activities were observed to degrade both types of the dyes in aqueous system under UV, tungsten and fluorescent light irradiation. While spherical-shaped nano-ZnO showed significant photocatalytic activities to degrade the dyes in aqueous system under UV irradiation. The spherical ZnO-TiO2 nanocomposite exhibited better photocatalytic activities to degrade organic dyes in aqueous system under visible light compared to spherical nano-ZnO alone or spherical TiO2 (anatage) alone. The nano-rod ZnO was also applied to industrial wastewater to see its catalytic activities in complex system and found azo-part is completely disappeared in the UV-visible spectrum of the waste water. Further attempt was taken to implant the nano-rod ZnO on transparent plastic substrates and used for mineralization of the organic dyes. A complete degradation (about 98%) of OG and RB was observed under solar light irradiation with longer degradation times. Therefore, it may conclude that the implanted nano-rod ZnO on plastic substrate can be used at effluent treatment plant (ETP) of textile/dyeing/ball-point pen industries to de-pollute their wastewater by utilization of the solar light.

Md Ahsan Habib is currently working as a Full Professor in the Department of Chemistry (Analytical and Environmental Section), University of Dhaka (Bangladesh) from where he completed his BSc (Hons) in 1991 and MSc in 1993. Dr. Habib completed his MPhil from the Department of Chemistry, Bangladesh University of Engineering and Technology in 2001. In 2004, Dr. Ahsan Habib obtained his PhD from the Department of Chemistry, Saga University (Japan) and he completed four post doctoral researches along with JSPS Invitation fellowship at five Japanese Universities in various fields in Chemistry. Prof. Habib has also worked as a visiting professor at Ningbo University and Guangzhou Institute of Geochemistry (GIG), China. Dr. Habib published 45 papers in various international journals and delivered talk at many international conferences.