Energetics of ZnO nanoneedles: Surface enthalpy, stability, and growth

The surface enthalpy of ZnO nanoneedles has been measured by oxide melt solution calorimetry of samples with different surface areas. Water adsorption calorimetry was carried out to characterize the stabilization effect of surface hydration. The surface enthalpies of hydrated and anhydrous surfaces (8.21 +/- 0.67 and 9.81 +/- 0.69 J/m(2), respectively) are larger than those of nanorods. The less stable surface of nanoneedles provides a driving force for the transformation of nanoneedles into nanorods during aging. The formation of bushlike assemblies of nanoneedles is also discussed.