Influence of Ti⁴⁺ on the energetics and microstructure of SnO₂ nanoparticles

Nanocrystalline Sn_1-xTi_xO₂ rutile-structured solid solutions have shown promise as gas sensors and photocatalysts. A fuller understanding of their microstructure and thermodynamics is necessary to improve the performance of the device. In this work, Sn_1-xTi_xO₂ (0.00 ≤ x ≤ 0.50) rutile-structured nanoparticles were synthesized by Pechini method at 500°C for 15 h. Upon increasing the Ti⁴⁺ content, both crystallite size determined by XRD and particle size determined by N₂ adsorption decrease. Surface energies and solid-solid interface energies were calculated by combining water adsorption calorimetry and high temperature oxide melt solution calorimetry. Botìi surface energy and solid-solid interface energy decreases with the addition of Ti⁴⁺. It is proposed that the stabilization of the particle and the crystallite size are a consequence of the decrease in the surface and interface energies caused by the Ti⁴⁺ surface segregation, which is supported by EELS. Given that the surface energy of pure TiO₂ rutile reported in the literature is higher than the surface energy of pure SnO₂, one may expect that the segregation of Ti⁴⁺ in the surface would increase the surface energy contradicting the results. However, it is suggested that the surface of the nanocrystalline Sn_1-xTi_xO₂ has a similar structure to TiO₂ anatase, which has a lower surface energy compared to both SnO₂ and TiO₂ rutile.