MORPHOLOGY CONTROL OF NANOSTRUCTURED ZNO AND ZNO/AL2O3 PHOTOCATALYSTS TOWARD PHOTOPHYSIC PROPERTIES AND PHOTODEGRADATION KINETICS

• Main Author: Prasetyo Benu, Didi
• Format: Dissertations
• Language: Indonesia
• Subjects: Kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/76749
• Institution: Institut Teknologi Bandung

Zinc oxide (ZnO) is a semiconductor material that has attracted much attention in recent decades. ZnO is widely used in optoelectronic devices, batteries, sensors, catalysts, photocatalysts, etc. Various studies have been conducted to improve the performance of ZnO-based devices. As a photocatalyst, the photocatalytic activity of ZnO dramatically depends on the exposed crystal facets. Additionally, the low surface area and accessibility of ZnO are also challenges in its application as a photocatalyst. Therefore, a comprehensive study is needed on the strategies that can be employed to overcome the shortcomings of ZnO as a photocatalyst. This research synthesized ZnO by exposing specific crystal facets using a mixture of water and alcohol solvents. Nanostructured ZnO with a high surface area was synthesized using water-ethylene glycol solvent and macroemulsion media. Furthermore, the decoration of ZnO nanoparticles onto the surface of a high surface area material (?-Al2O3) was also conducted. The photocatalytic activity of synthesized particles was tested for photodegradation of rhodamine B dye. The water-alcohol mixture facilitated the formation of ZnO nanocrystals with different shapes and morphologies. Even-chain alcohols result in ZnO with flowerlike morphology, while odd-chain alcohols produce ZnO nanocrystals with clearly exposed crystal facets. ZnO_methanol has a hexagonal rod shape with tapered ends, ZnO_isopropanol has a truncated hexagonal pyramid shape, and ZnO_pentanol has a hexagonal plate shape. The ZnO_methanol sample exhibits the best photocatalytic activity with a photodegradation rate of 0.0093 min-1 and a maximum photodegradation efficiency of 90.40% over 30 minutes. The high photocatalytic activity of ZnO_methanol is attributed to the high percentage of exposed {101?0} facets (71.89%). In the synthesis using a water and ethylene glycol mixture, urea plays a crucial role in forming nanostructured ZnO. During the hydrothermal process, Zn(NO3)2.6H2O reacts with urea to form zinc hydroxide carbonate (Zn5(CO3)2(OH)6/ZHC). ZHC then decomposes into ZnO at a temperature of 500 o C. The addition of surfactants CTAB, SDS, and PVP successfully inhibits the growth of ZnO crystals in specific crystallographic directions, resulting in ZnO particles with different morphologies and porosities. The synthesized sample using PVP surfactant has the highest surface area and porosity, making it suitable for use as a photocatalyst. This sample exhibits the best photocatalytic activity with a rate constant of 0.1349 min-1 , a degradation efficiency of 92.23% for 30 minutes, and activation energy (Ea) of 19.26 kJ mol-1 . It maintains a degradation efficiency of 50.81% after three time uses. Macroemulsion media plays a role in producing unique morphologies in the synthesized ZnO particles. Under acidic conditions, the resulting particles have a porous sheet-like morphology, while synthesis under alkaline conditions yields rodshaped particles with a fibrous surface. Synthesis using different precursors under alkaline conditions shows that zinc acetate precursor produces particles with a more uniform morphology (ZnO_Ac_U2 sample) than zinc nitrate precursor (ZnO_Nit_U2 sample). The uniform morphology in the ZnO_Ac_U2 sample contributes to its high surface area, narrower bandgap energy, and high density of crystal defect. Photocatalytic activity testing for the degradation of rhodamine B shows that the ZnO_Ac_U2 sample has a degradation efficiency of 72.57% for 30 minutes, a rate constant of 0.1962 min-1 , and maintains a degradation efficiency of 50.54% after three uses. In addition to the synthesis of ZnO nanocrystals and nanostructured ZnO photocatalysts, the synthesis of ZnO nanoparticles decorated on the surface of ?AlOOH and ?-Al2O3 was also conducted. ?-AlOOH material synthesized using reverse micelle media has a flower-like morphology. This material transformed to ?-Al2O3 at a temperature of 550 o C, accompanied by changes in the optical properties of the material. ?-Al2O3 material has narrower bandgap energy and a higher density of crystal defects. The photocatalytic activity of ?-Al2O3 decorated with ZnO depends on the concentration of zinc salt precursor. Sample with the highest concentration of zinc salt precursor (ZnO_D/?-Al2O3) has the highest photocatalytic activity with rate constant (k) is 0.1154 min-1 . This rate constant value is nineteen times higher than that of pristine ?-Al2O3 photocatalyst. Furthermore, the ZnO_D/?-Al2O3 photocatalyst can degrade rhodamine B solution up to 84.08% in 30 minutes. A narrower bandgap energy, high crystal defect density, and the formation of a heterojunction between ZnO and ?-Al2O3 facilitate the high activity of the ZnO_D/?-Al2O3 photocatalyst. The heterojunction between ZnO and ?-Al2O3 increases electron lifetime and minimizes the electron-hole recombination rate