Thermodynamics of nanoscale lead titanate and barium titanate perovskites

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/m ² and 1.11 ± 0.23 J/m ² for PbTiO ₃, and 3.69 ± 0.22 J/m ² and 3.99 ± 0.28 J/m ² for BaTiO ₃, respectively. The interface energies of the hydrous and anhydrous surfaces are 0.55 ± 0.74 J/m ² and 0.73 ± 0.27 J/m ² for PbTiO ₃, and 1.11 ± 0.13 J/m ² 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.