Slow Equilibration between Spectroscopically Distinct Trap States in Reduced TiO₂ Nanoparticles

Understanding the nature of charge carriers in nanoscale titanium dioxide is important for its use in solar energy conversion, photocatalysis, and other applications. UV-irradiation of aqueous, colloidal TiO₂ nanoparticles in the presence of methanol gives highly reduced suspensions. Two distinct types of electron traps were observed and characterized by EPR and optical spectroscopies. The relative populations of the states depend on temperature, indicating a small energy difference, ΔH° = 3.0 ± 0.6 kcal/mol (130 ± 30 meV). Interconversion between the electron traps occurs slowly over the course of minutes to hours within the temperature range studied here, 0-50 °C. The slow time scale implies that interconversion involves changes in structure or stoichiometry, not just the movement of electrons. This occurrence of slow structural modification with changes in trap state occupancy is likely a general feature of reduced TiO₂ systems at thermodynamic equilibria or photostationary states and should be considered in the design of TiO₂-containing devices.