Hierarchically Assembled Porous ZnO Nanoparticles:  Synthesis, Surface Energy, and Photocatalytic Activity

We show an effective experimental method to prepare hierarchically porous zinc oxide (ZnO) spherical nanoparticles through a self-assembly pathway using surface-modified colloidal ZnO nanocrystallites as the building blocks and P-123 copolymers as the template in aqueous solution. Copolymers are thoroughly removed by Soxhlet extraction with ethanol and calcination at 400 °C. The final products have been characterized by X-ray powder diffraction (XRD) pattern, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and photoluminescence (PL) spectroscopy. On the basis of calorimetric measurements reported separately, the surface enthalpy (γ) of the hydrated porous ZnO is 1.42 ± 0.21 J/m2, in good agreement with that of ZnO nanoparticles. The calorimetric results support the presence of self-assembled ZnO nanocrystallites in the nanoporous ZnO. Photocatalytic activitiy of porous ZnO nanoparticles has been tested on the photodegradation of phenol under ambient condition, indicating that porous ZnO nanoparticles show superior activity to TiO2 nanoparticles (PC-500).