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 O-2 is a universally existing process for the physiochemistry of many materials. Electron transfer to O-2 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 O-2. It is seen that the kinetics of electron transfer to O-2 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 (E(i)s) are involved in the electron transfer. The effects of O-2 amount, light intensity, and temperature are studied and the results mean the trap-filling effect should be involved in the electron transfer to O-2. A Laplace transform is used to derive the E-i distributions. It is found that the E-i dispersion shape almost does not change; this indicates that the physical reason causing the E-i 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-2, in additional to the trapping-filling effect. The results indicate that the photocatalytic activity can be increased through a modulation in trap distribution.

Automated summary

The electron transfer to O-2 is a persistent and dispersive process in the photocatalysis over TiO2, lasting for several minutes and following a stretched exponential kinetics. The variable apparent energy barriers (E(i)s) involved in the electron transfer are affected by O-2 amount, light intensity, and temperature, indicating the trap-filling effect plays a role. The slow kinetics of the electron transfer is dependent on the slow rate for an electron transferring from a trap to O-2, and the modulation in trap distribution can increase the photocatalytic activity.