Thermodynamics of Nanoscale Lead Titanate and Barium Titanate Perovskites


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To explore the surface properties of perovskites with ions of different bond character, the surface and interface enthalpies of nanocrystalline PbTiO 3 and BaTiO 3 perovskites were determined for the first time by a combination of calorimetric, morphological, and structural analyses. PbTiO 3 and BaTiO 3 nanocrystalline samples of varying surface areas and degrees of agglomeration were synthesized by solvothermal and hydrothermal methods, respectively. All synthesized samples were characterized using X-ray diffraction and Raman spectroscopy. Interface areas were estimated by comparing the surface areas measured by N 2 adsorption to the crystallite sizes refined from X-ray diffraction data. The integrated heats of water vapor adsorption on the surfaces of the nanocrystalline phases are −62 ± 4 kJ/mol for PbTiO 3, which is less exothermic than the value −72 ± 9 kJ/mol for the isostructural BaTiO 3, both phases having the same chemisorbed water coverage. Similar behavior is observed for the surface and interface enthalpies. The energies of the hydrous and anhydrous surfaces are 1.97 ± 0.67 J/m2 and 1.11 ± 0.23 J/m2 for PbTiO 3, and 3.69 ± 0.22 J/m2 and 3.99 ± 0.28 J/m2 for BaTiO 3, respectively. The interface energies of the hydrous and anhydrous surfaces are 0.55 ± 0.74 J/m2 and 0.73 ± 0.27 J/m2 for PbTiO 3, and 1.11 ± 0.13 J/m2 for BaTiO 3. These observations suggest that PbTiO 3 has lower surface energy and lower affinity for water adsorption on the surface than BaTiO 3 and that surface energy and hydrophilicity of the surface decrease with increasing covalent character of the ions, as was seen previously in comparing TiO 2 and SnO 2.