Thermodynamics of nanocrystalline Sn _0.586 Ti _0.414 O ₂ rutile solid solution: Comparison with nanocrystalline SnO ₂ and TiO ₂ and with bulk materials

Nanocrystalline Sn _x Ti _1-x O ₂ rutile solid solutions are important materials for a variety of gas sensors and catalysts. Although thermodynamic data are available for nanocrystalline SnO ₂ and TiO ₂ and for bulk (coarse-grained) Sn _x Ti _1-x O ₂ solid solutions, there is a lack of experimental thermochemical data on the energetics of Sn _x Ti _1-x O ₂ nanoparticles. In this work, Sn _0.586 Ti _0.414 O ₂ rutile solid solution nanoparticles were synthesized. The surface energies of anhydrous and hydrated nanoparticles were measured by combining high-temperature oxide melt solution calorimetry molten 2PbO·B ₂O ₃ at 800°C and water adsorption calorimetry. The surface energy of the anhydrous surface is 2.02 ± 0.03 J·m ^-2, and that of the hydrated surface is 1.68 ± 0.03 J·m ^-2. These values lie between the previously reported surface energies of rutile SnO ₂ and TiO ₂. The integral heat of water adsorption is -80 kJ·mol ^-1, with a chemisorbed maximum coverage of ∼6 H ₂O·nm ^-2. These values are also between those for TiO ₂ and SnO ₂ (rutile) reported previously. The strongly positive (unfavorable) energetics of mixing in SnO ₂-TiO ₂ bulk solid solutions are predicted to change little at the nanoscale, and the extensive solid solution seen in the nanophase system prepared near room temperature reflects kinetic hindrance to exsolution of an initially homogeneous precipitate rather than thermodynamic stability.