Surface enthalpies of nanophase ZnO with different morphologies

A direct calorimetric measurement of the dependence of the surface enthalpy I 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/m(2), 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/m(2), 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.