Thermochemical Study of Photoinduced Processes at TiO2 Nanoparticles in Aqueous Suspension Containing Br- or Cl-. Photodeaggregation and Subsequent Processes

Laser pulse-induced optoacoustic calorimetry (LIOAC) was applied to determine enthalpy changes in aerated aqueous suspensions of TiO2 nanoparticles (2.4 nm mean diameter) at pH = 1 in the absence and presence of the halide ions X- = Br- or Cl-. LIOAC demonstrates that in the system TiO2|H2O, O2 10% of the absorbed optical energy are stored for at least a few μs, which can be assigned to the decomposition of water. In the systems containing the X-, the overall photoprocess was exothermic although the oxidation of X- to - and the concurrent decomposition of water are endothermic. As an explanation we suggest a model in which the photodeaggregation of TiO2 nanoparticle aggregates produces additional photocatalyst surface area. The subsequent establishment of the thermodynamic equilibria on the additional surface comprises protonation of the TiO2 particles, halide adsorption, and formation of the electrochemical double layer. All these processes are exothermic. Adsorption of X- is best described by a Temkin-i...