Photocatalytic properties of zinc oxide nanostructures synthesized using synergistic pulse laser ablation and hydrothermal methods

Abstract

Mild growth conditions based novel techniques are essential for the controlled synthesis of zinc oxide (ZnO) nanostructure (ZNS) with desired properties. Most of the existing synthesis methods of ZNSs are limited by complicated growth conditions such as high vacuum, expensive devices, high temperature, long growth time and requirement of template. The freshwater pollution due to residual industrial organic dyes is presently being a major environmental concern that needs efficient photocatalytic materials to overcome it. To achieve these goals, this work synthesized good quality ZNS films using two different methods and evaluated the photodegradation of industrial methylene blue (MB) dye by these ZNS films under sunlight irradiation. Such films were grown on glass and silicon (Si) substrates via a simple hydrothermal method in the absence of any catalyst. Prepared samples were characterized using various techniques to determine their physical and optical properties. The effects of growth time and temperature, substrate type, nutrient pH and concentration on the morphology, size, crystallinity, and the emission properties of as-grown ZNS films were evaluated. Results showed that the morphology of these ZNSs was strongly influenced by the growth time, nutrient pH and concentration. A good quality ZNS was achieved within 5 min whereby nutrient solution with lower pH was appropriate for the growth of 1D ZNSs. However, higher pH values produced 3D flower-like ZNSs. Variation of growth temperature from 90-110 oC allowed good size-control of ZNSs. Growth conditions and substrate type dependent emission spectra were used to evaluate the optical band-gap energy. To get better control of the ZNS growth and evolving morphology, a novel catalyst-free and rapid preparation technique was adopted. In this method a mixture of precursor solution and ZnO nanoparticles (ZNPs) colloid/or only ZNPs colloid as a nutrient solution was used. Pulse laser ablation in liquid (PLAL) was combined with hydrothermal (H) method to develop PLAL-H growth technique. A Q-switched Nd:YAG laser with wavelength 532 nm, 8 ns pulse duration, and 10 Hz repetition rate was employed as the irradiation source. Metallic zinc target of 1 mm thick was used to produce colloidal ZNPs. The effects of varying growth time of 0.5, 5, 30 and 60 min and ablation energy of 200-400 mJ on the physical and optical properties of the grown ZNSs were examined. Four types of ZNSs with varying sizes and shapes were obtained on Si substrate at 110 oC for 5 min duration. Increasing ablation energy led to a substantial change of ZNS morphology and promoted the structure quality together with photoluminescence emission intensity. ZNSs synthesized under prolonged growth time of 60 min exhibited remarkable morphology alteration from rod/flower-like ZNSs to ZNPs with higher crystallinity and enlarged bandgap due to increase of nutrient pH of 10.5. Finally, the photocatalytic activities of the optimal ZNS films were assessed via sunlight driven photodegradation of MB dye. Experimental findings verified that the ZNPs prepared by PLAL-H technique possessed excellent photocatalytic efficiency (97.4%) towards degradation of MB dye. The observed boost in the photocatalytic activities was ascribed to the synergism of the improved surface area and band-gap modification. It was established that the proposed novel PLAL-H growth strategy is not only cost-effective but greatly useful for the rapid production of different quality of ZNSs at low temperature in a controlled way. This may overcome the shortcomings involving the effective exploitation of sunlight source towards practical photocatalytic applications of ZNSs.