Dispersive photoconductivity in the layered perovskite Nd2Ti3O9

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 (E_is) 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 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 O₂, in additional to the trapping-filling effect. The results indicate that the photocatalytic activity can be increased through a modulation in trap distribution.

The effect of Cu dopants on electron transfer to O2 and the connection with acetone photocatalytic oxidations over nano-TiO2

Modifying TiO₂ with the Cu element has been shown to be useful for photocatalysis. Although it had been known that Cu species could trap electrons from TiO₂, whether they can affect the kinetics of electron transfer and how this can contribute to photocatalysis still remain unknown. In the current research, Cu-TiO₂ samples were firstly prepared with a hydrothermal reaction and characterized in detail. It was shown that Cu elements were doped in the TiO₂ lattice in +1/0 valence states and have a minor effect on the TiO₂ structure. By means of photoconductances, it is shown that the Cu dopants could catalyze the electron transfer from TiO₂ to O₂ by reducing the apparent activation energy (E_app) by about 2 times. The photocatalytic experiments conducted at different temperatures showed that the E_app of the acetone photocatalytic oxidations could be decreased by ∼2 times; this implies that the Cu dopants change the photocatalytic pathway. First-principles computation showed that the surface Cu dopants, along with the compensated oxygen vacancies, can mediate both of the electron and hole transfer. By combining other studies, we proposed that the Cu sites could act as Lewis acid and base pairs that could combine with acetone and O₂ molecules under UV light illumination; this allows electron transfer to O₂via the Cu sites that then react with acetone. As compared to pure TiO₂ surfaces, the different chemical environment of the Cu sites leads to the decrease in the E_app of photocatalysis.