Nucleation-growth of salicylic acid-oxoTiO2 colloids synthesized by sol-gel processing

• Hybrid oxo-TiO2 nanoparticles modified by salicylic acid (SA) are prepared.
• The synthesis is achieved during the nucleation stage of sol-gel process.
• The formation of SA–oxo-TiO2 charge-transfer complex is evidenced.
• Mechanism responsible for the acceleration of the nanoparticle aggregation kinetics is proposed.
• SA-TiO2 nanoparticles are stable at pH ≤ 7.7 and release SA molecules at pH ≥ 8.5.

In this work, the preparation of hybrid oxo-TiO2 nanoparticles modified by salicylic acid (SA) is reported. The size-selected TiO2 nanoparticles were synthesized in a sol-gel reactor with rapid micromixing of reagents and binding of SA molecules occurs at the nucleation stage. UV-visible absorption, Raman spectroscopy and Light scattering measurements evidence the formation of a SA–oxo-TiO2 charge-transfer complex. In particularly, the CO vibration mode of SA shifts to lower frequencies upon complexation while the C-O vibration modes of both carboxylate and phenolic groups retain their position. The absence of C-O shift comes from two effects that cancel each other, i.e. the hardening of this mode related to the CO mode changes and its softening due to the replacement of hydrogen in COH group by Ti atoms. Kinetic studies of the oxo-TiO2 nanoparticles nucleation and growth in presence of SA show that the acidic ligand at the nanoparticle surface acts as an attractor for another oxo-TiO2 nanoparticle leading to polycondensation. This mechanism is responsible for the acceleration of the nanoparticle aggregation kinetics and shortening of the induction time. The hybrid nanoparticles are stable at pH ≤ 7.7 and release SA molecules at pH ≥ 8.5.