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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/m 2 , 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 TiO 2 nanoparticles (PC-500).

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A direct calorimetric measurement of the dependence of the surface enthalpy of nanophase ZnO on morphology is reported. Nanoparticles, nanoporous composites, nanorods, and nanotetrapods were prepared with various sizes, and their surface enthalpies were derived from their drop solution enthalpies in molten sodium molybdate. Water adsorption calorimetry for nanoparticles and nanorods was carried out to characterize the stabilization effect of surface hydration. The surface enthalpies of hydrated surfaces for nanoparticles, nanoporous composites, nanorods, and nanotetrapods are 1.31 ± 0.07, 1.42 ± 0.21, 5.19 ± 0.56, and 5.77 ± 2.50 J/m2, respectively, whereas those of the anhydrous surfaces are 2.55 ± 0.23, 2.74 ± 0.16, 6.67 ± 0.56, and 7.28 ± 2.50 J/m2, respectively. The surface enthalpies of nanoparticles are the same as those of nanoporous composites and are much lower than those of nanorods and nanotetrapods, which also are close to each other. The dependence of surface enthalpy on morphology is discussed in terms of exposed surface structures. This is the first time that calorimetry on nanocrystalline powders has been able to detect differences in surface energetics of materials having different morphologies.