CHINESE JOURNAL OF PHYSICS VOL. 52, NO. 5 October 2014 ZnO Hierarchical Nanostructures as a Powerful Photocatalyst for the Degradation of P-Nitrophenol S. Safa, 1, ∗ R. Azimirad, 2 R. Hejazi, 3 and M. Rabbani 3 1 Young Researchers and Elite Club, South Tehran Branch, Islamic Azad University, Tehran, Iran 2 Malek-Ashtar University of Technology, Tehran, Iran 3 Department of Chemistry, Iran University of Science and Technology, Tehran, Iran (Received March 26, 2014; Revised May 4, 2014) The ZnO nanoflower morphology was successfully fabricated by the hydrothermal method. The samples were characterized by scanning electron microscopy (SEM), X-ray diffractom- etry (XRD), Fourier transform infrared spectroscopy (FT-IR), and diffuse reflectance spec- trophotometry (DRS). Photocatalytic activity of the nanostructures against P-nitrophenol was studied under ultra-violet (UV) and visible light irradiation. The more porous nanoflower powder exhibits higher visible light absorbance, lower optical band gap (∼ 3.07 eV), substan- tially more surface area, and, consequently, stronger photocatalytic activity. The kinetic- constant of the photocatalytic degradation of ZnO nanoflowers follows from the pseudo-first- order kinetic model and was found to be 0.0198 (under UV) and 0.0038 (under visible light), which were ∼ 1.6 times stronger than that of commercial ZnO nanopowder. DOI: 10.6122/CJP.20140319 PACS numbers: 81.16.-c I. INTRODUCTION Among various organic pollutants, p-nitrophenol (PNP) is extensively considered, because its considerable toxicity to the ecosystem and human health is added its environ- mentally stability [1, 2]. In recent years, numerous studies have been conducted to degrade nitrophenol based compounds of industrial waste water using semiconducting nanostruc- tures, such as TiO2 [3], CuO [4], and ZnO [5], through photocalytic reactions. Among them, zinc oxide (ZnO) as an important multifunctional semiconductor with appropriate optical properties and great chemical stability, is being considered to be one of the most applica- ble candidates [5]. Concerning the photocatalytic activity of ZnO materials, Sabbaghan et al. [6] reported that the size and morphology of ZnO nanopowder act as important param- eters on the (photo)catalytic activity of ZnO powder, and, in their experiments, the ZnO nanorods with the highest surface area showed the optimized state for photodecolorization of methylene blue. Wang et al. [7] observed that the photo-catalytic degradation of typical persistent organic pollutants (POPs) by chrysanthemum-like ZnO nanostructures is higher than that of rod-like ZnO nanostructures, and those are all better than the case of com- mercial ZnO. The photocatalytic degradation of 4-cholrophenol (4-CP) carried out by [8] * Electronic address: sda.safa@gmail.com,saeed.safa@modares.ac.ir http://PSROC.phys.ntu.edu.tw/cjp 1612 c  2014 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA