Reactions of TiO₂ excess electron in nanocrystallite aqueous solutions studied in pulse and gamma-radiolytic systems

Reactions of excess electrons in TiO₂ nanocrystallites produced by radiolysis have been studied by both steady state and pulse radiolysis techniques. The TiO₂ electrons were produced by γ or pulse radiolysis of 2-propanol or t-butanol aqueous solutions in acid pH's. The effects of various materials, including such that are often present in photocatalytic systems such as hydrogen peroxide, oxygen, nitrate and perchlorate on the decay of the TiO₂ electron absorption was followed, using two nanocrystallite sizes with average diameters of 1.0 and 4.7nm. The rates of electron reactions depend on particle size. Several scavengers including Cu²⁺, ClO₂^-, ClO₃^-, NO₂^-, and NO₃^- show decay of the TiO₂ electron predominantly by single pseudo-first order process. The rate of reaction of the above ions in the large nanocrystallites systems is 2-10 times faster than in the respective small particle systems. Hydrogen peroxide and oxygen show a multi-exponential decay, which is supposedly related to different kinds of adsorption. Large particles show reduction rates 2-3 orders faster for H₂O₂ and 10-20 times faster for O₂, compared to the small particles. The reaction of the electron with ClO₄^- ions is first order in [ClO₄^-] but second order with respect to the TiO₂ electron. In the large particle system the rate was found to depend on the number of electrons per particle. (No similar information is available for the small particles). The rates of reduction of the different scavengers, tend to increase with the driving force, although not always. Deviations from linear dependency of logk on the driving force are large. The mechanisms and role of adsorption are discussed.