Kinetics and energetic analysis of the slow dispersive electron transfer from nano-TiO2 to O2 by in situ diffusion reflectance and Laplace transform.

Electron transfer to O2 is a universally existing process for the physiochemistry of many materials. Electron transfer to O2 is also an inevitable process for photocatalytic reactions over TiO2 and other materials. In the present research, a diffusion reflectance system was developed to measure in situ optical diffusion reflectances caused by photoinduced electrons in nano-TiO2 under a steady light illumination; in situ absorption decays can be obtained to study the electron transfer from their trapped states to O2. It is seen that the kinetics of electron transfer to O2 is persistent and dispersive; this lasts for several minutes and approximately agrees with a stretched exponential kinetics. The result implies that variable apparent energy barriers (Eis) are involved in the electron transfer. The effects of O2 amount, light intensity, and temperature are studied and the results mean the trap-filling effect should be involved in the electron transfer to O2. A Laplace transform is used to derive the Ei distributions. It is found that the Ei dispersion shape almost does not change; this indicates that the physical reason causing the Ei dispersion is the same for different experimental conditions and possibly comes from the trap-filling effect. It is shown that the slow kinetics of the electron transfer is also dependent on the slow rate for an electron transferring from a trap to O2, in additional to the trapping-filling effect. The results indicate that the photocatalytic activity can be increased through a modulation in trap distribution.