(166e) Synthesis and Characterization of ZnO Photocatalysts with Different Morphologies

In recent years, pollution by persistent organic pollutants (POPs) is becoming one of the severe environmental issues which cause damage to human health. Currently, although ozone treatment has been employed for POPs removal as advanced treatment, it has disadvantages of high costs and production of harmful byproducts. Photocatalysts have been attracting attention as one of the innovative POPs removal methods. The photocatalyst is a semiconductor that triggers an oxidation-reduction reaction by light exposure on the particle surface and can achieve mineralization of pollutants. Although TiO₂ is a typical photocatalyst, ZnO also has been paid attention because of its low costs. Many researchers reported synthesis and characterization of various ZnO particles with different particle shapes and morphologies using different synthesis methods. However, comparison of their photocatalytic activity under the same experimental condition is very limited.

Therefore, in this study, we synthesized three different ZnO particles with different particle shapes and morphologies such as flowerlike, multi-shell and rodlike and compared their particle properties and photocatalytic activities for the same degradation target compound under the same degradation condition. Samples were synthesized by a solvothermal method using Zn(NO₃)₂ï½¥6H₂O as a raw material. The particle shape was controlled by adding appropriate structure-directing agents. The flowerlike, multi-shell and rodlike samples were synthesized by adding hexamethylenetetramine, L(-)-proline and ethanolamine, respectively. The photocatalytic activity was evaluated by degradation of 2,4-dinitrophenol (DNP) as model POPs.

Morphologies of the resulting samples were observed by a scanning electron microscope (FE-SEM, JSM-7500F, JEOL). These observations were performed with an acceleration voltage of 5-15 kV. Based on the SEM observation, it was confirmed that flowerlike, multi-shell and rodlike samples were obtained as expected. X-ray diffraction (XRD) measurement was performed by an X-ray diffractometer (D8 ADVANCE, Bruker) using CuKα radiation (0.154021 nm) generated at 40 kV and 40 mA. Measurement was carried out in the range of 2θ = 20 – 80° with step size of 0.05° and scan rate of 0.1 s/step. The result shows that the peaks at 2θ =31°, 34°, 36°, 48°, 57°, 63°, 68°, and 69° can be attributed to ZnO. This result indicates that ZnO is successfully obtained by all the synthesis methods. The results also showed that the intensity of strongest diffraction peak of the three samples varies depending on the ZnO samples in the following order: flowerlike < multi-shell < rodlike, and the rodlike sample showed significantly strong peak intensity among the samples (approximately 15 times larger than that of the flowerlike).

On the other hand, the BET specific surface area exhibited opposite order: rodlike < multi-shell < flowerlike, and the surface area of the flowerlike ZnO was more than 5 times larger than that of the other two samples. These results showed that the strongest peak intensity in XRD measurement and the specific surface area for the three samples are in the trade-off relationship. Finally, photocatalytic activity test was carried out using 10 ppm DNP under UV light irradiation. The DNP is widely used as indicator of the photocatalytic activity. The treated water was sampled periodically, and the DNP concentration change as a function of time was determined by measuring the absorption at 357 nm with a UV–Vis spectrophotometer (V-650; JASCO). It was found that the DNP degradation fitted the first-order kinetics well for all the samples and the degradation rate was in the following order: rodlike < multi-shell < flowerlike. This order is same as the order for the specific surface area and opposite to that of the strongest peak intensity. These results suggest that highest photocatalytic activity for DNP degradation in the flowerlike shape ZnO is mainly contributed by a very high specific surface area and not by crystallinity of ZnO.