Kinetics and energetic analysis of the slow dispersive electron transfer from nano-TiO to O by diffusion reflectance and Laplace transform.

Electron transfer to O is a universally existing process for the physiochemistry of many materials. Electron transfer to O is also an inevitable process for photocatalytic reactions over TiO and other materials. In the present research, a diffusion reflectance system was developed to measure optical diffusion reflectances caused by photoinduced electrons in nano-TiO under a steady light illumination; absorption decays can be obtained to study the electron transfer from their trapped states to O. It is seen that the kinetics of electron transfer to O 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 (s) are involved in the electron transfer. The effects of O amount, light intensity, and temperature are studied and the results mean the trap-filling effect should be involved in the electron transfer to O. A Laplace transform is used to derive the distributions. It is found that the dispersion shape almost does not change; this indicates that the physical reason causing the 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 O, in additional to the trapping-filling effect. The results indicate that the photocatalytic activity can be increased through a modulation in trap distribution.