Energetics of Nanoarchitectured TiO2−ZrO2 and TiO2−MoO3 Composite Materials

High-temperature drop solution calorimetry in sodium molybdate solvent was performed on nanoarchitectured materials using a Calvet twin microcalorimeter at 975 K. Nanocrystalline TiO2−ZrO2 samples (mesoporous structure) and nanocrystalline TiO2−MoO3 samples (core−shell structure) were investigated. With the use of the measured drop solution enthalpies, the transformation enthalpies to macroscopic stable crystalline phases have been calculated. The enthalpy of transformation from amorphous to monoclinic ZrO2 obtained from this work, −50.08 ± 4.92 kJ/mol, is in reasonable agreement with the value of −58.60 ± 3.30 kJ/mol determined by Molodetsky et al. (2000). The enthalpy of transformation from amorphous to crystalline MoO3 derived from these data is −51.25 ± 2.01 kJ/mol. The enthalpy of transformation of nanophase anatase to bulk macrocrystalline rutile TiO2 is −7.02 ± 0.96 kJ/mol. The linear variation of energetics with the composition of both series, TiO2−ZrO2 and TiO2−MoO3, suggests that these materials constitute a nanoscopic two-phase mixture, with their energetics being a weighted sum of end-member enthalpies. The energetic effects of surface area and of phase transformations could not be separated because the surface area (m2/mol) varies approximately linearly with mole fraction. Thermogravimetric analysis and differential thermal analysis corroborate the proposed energetic model. Thermodynamic stability issues of these nanoarchitectured materials are discussed along with structural aspects.