Thermochemistry of nanoparticles on a substrate: Zinc oxide on amorphous silica
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Joseph A. Libera and Jeffrey W. Elam Argonne National Laboratory, Argonne, Illinois 60439 (Received 3 February 2008; accepted 21 March 2008)
Crystalline samples of zinc oxide on a mesoporous amorphous silica substrate were prepared by 5 to 15 atomic layer deposition cycles with diethyl zinc and water at 150 °C. Samples were characterized by x-ray diffraction, thermogravimetry, and nitrogen adsorption–desorption isotherms. High-temperature oxide melt solution calorimetry and water adsorption calorimetry experiments were performed to measure surface enthalpy for crystalline ZnO particles supported on the substrate. The measured enthalpies 1.23 ± 0.35 and 2.07 ± 0.59 J/m2 for hydrous and anhydrous surfaces, respectively, are in agreement with previously reported measurements for unsupported ZnO nanoparticles. Feasibility of thermochemical characterization of complex system of atomic layer deposition (ALD) prepared particles on a substrate was demonstrated.
I. INTRODUCTION
The nonnegligible contribution of surface and interfacial terms is a characteristic feature of any thermodynamic description of nanomaterials. High-temperature oxide melt solution calorimetry has proved to be one of the most versatile methods to measure surface enthalpies of unsupported metal oxide nanoparticles.1–10 One of the most significant findings of such calorimetric experiments is strong evidence that changes in the bulk crystal structure of nanophases (polymorphism) observed for alumina,2 zirconia,3 and titania4 are caused by the lower surface enthalpies of the metastable phase overriding its bulk instability. In thermodynamic analyses of nanostructures, three main factors must be addressed: (i) effect of synthesis method and surface morphology on surface energies, (ii) contribution from interfacial energies, and (iii) effect of adsorption of H2O or other molecules on the surface or interface. Nanoparticles in both natural and synthetic settings often form complex structures: core shell particles, adsorbed layers of molecules or particles on the surface of parent nanoparticles, and corrosion layers. Their structures contain several kinds of interfaces: particle–air, particle–water, and particle–particle. To understand these complex interactions, it is instructive to study a model
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0237 J. Mater. Res., Vol. 23, No. 7, Jul 2008
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system, intermediate in complexity between isolated nanoparticles and real-world multicomponent and multiphase nanophase systems. We report a study of zinc oxide deposited by atomic layer deposition (ALD)11 on a commercial amorphous silica substrate (Silicycle). There is widespread use of amorphous SiO2 as a substrate for catalysis12–14 and application of ZnO to catalyze methanol synthesis.15–17 This system is appropriate for calorimetric study for several reasons. First, there are no polymorphic transformations or oxidation state changes during heating ZnO up to
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