Seedless hydrothermal growth of hexagonal prism ZnO for photocatalytic degradation of methylene blue: the effect of pH and post-annealing treatment

In this paper, we study the effect of solution pH in the hydrothermal synthesis and post-annealing treatment on the photocatalytic performance of hexagonal prism ZnO grown without a seed layer. By varying the precursor molarity ratio, the solution obtained was 6.88 (ZnO-2), 7.00 (ZnO-1), and 7.58 (ZnO-3). The three samples show hexagonal prism ZnO with wurtzite structures based on scanning electron microscope and x-ray diffraction analysis. ZnO-1 samples could degrade methylene blue as high as 65.9% for a UV irradiation time of 5 h, better than ZnO-2 (51.80%) and ZnO-3 (57.55%). Post-annealing treatment of ZnO-1 with the best photodegradation efficiency was carried out at 200 °C (ZnO-4) and 400 °C (ZnO-5) to alter the structure. The post-annealing treatment changes the domination of crystal orientation from (002) to (100) plane. Also, the morphology of ZnO-5 changed significantly to become smaller rods with a diameter of 2.79 μm, as compared to ZnO-1 (2.83 μm) and ZnO-4 (3.12 μm). It is due to ionic rearrangements occurring at higher temperatures. The ZnO-5 sample reduces methylene blue by 82.91%, which is better than ZnO-1 (65.9%) and ZnO-4 (64.39%). Interestingly, we found a relation between smaller rod diameters and higher photocatalytic activity. The results show the importance of the solution pH and the annealing treatment in improving the photocatalytic performance of hexagonal prism ZnO without the seed layer.