Energetics of ZnO nanoneedles: Surface enthalpy, stability, and growth
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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/m2, 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.
I. INTRODUCTION
ZnO nanoneedles, roughly one-dimensional (1D) materials with a wide band gap (3.37 eV), have potential application as ultraviolet (UV)-emitting and even visible luminescent materials.1,2 ZnO 1D structures attract great attention because of their semiconducting and piezoelectric properties, as well as desirable thermal stability and lack of oxidation or phase change.3,4 Recently, the field-emission properties of ZnO 1D nanocrystals were studied intensively.5,6 Since sharp tips can dramatically increase the field enhancement factor and reduce the threshold field, nanoneedle arrays have promising potential in application to field-emission displays.7 Various approaches have been used in the preparation of ZnO nanoneedles, such as electrochemical methods,8,9 chemical vapor deposition (CVD),10,11 direct oxidation of zinc foil,12 hydrothermal synthesis,13 and room-temperature solution methods.14 Thermodynamic data on the surface energies of different crystal planes are essential for understanding the synthesis and application of nanomaterials. Recently, high-temperature oxide melt solution calorimetry15 has been shown to be a powerful tool to study the surface enthalpies of nanophase oxides, such as nanoparticles of Al2O3,16 TiO2,17 ZrO2,18 Fe2O3,19 and Y2O3.20 We also determined surface enthalpies of ZnO nanoparticles, nanoporous composites, nanorods, and nanotetrapods.21 Surface enthalpies of ZnO nanocrystals with different morphologies depend on the nature of their exposed planes. A ZnO nanoneedle is a tapered nanorod with a
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0216 1652 J. Mater. Res., Vol. 23, No. 6, Jun 2008 http://journals.cambridge.org Downloaded: 11 Dec 2014
wurtzite structure grown along the c axis.13 The surface structure of ZnO nanoneedles has not been reported in detail. The study of surface enthalpies of nanoneedles, the precursor of nanorods and nanowires during synthesis,14 is essential for understanding the thermodynamic driving forces for synthesis of nanocrystals having different morphologies. In this work, the surface enthalpy of nanoneedles has been measured using high-temperature oxide melt solution calorimetry. A series of nanoneedle samples with various surface areas and sizes were prepared, and their heats of drop solution in molten sodium molybdate at 700 °C were determined. Their surface enthalpy was derived from their drop solution enthalp
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