Journal article

Charge separation and charge transport in nanostructured TiO2 film electrodes

Photoelectrochem. measurements have been performed on film electrodes consisting of linked nano-sized TiO2 colloids. The film thickness ranged from 1-40 μm. The film network was attached to a thin transparent conducting layer of SnO2 allowing for photogenerated electrons to be collected in an outer elec. circuit. By illuminating electrodes of different thicknesses with monochromatic light from either sides, it was possible to induce charge sepn. in different regions of the film network. In this way, it was proved that electrons have different probabilities of reaching the back contact depending on the location in the film where they are created. The results also illustrate the importance of the redox species in the cavities of these porous electrodes. By adding acceptors to the electrolyte it was possible to alter the conditions for charge transfer in the nanocryst. film. It was shown that electron acceptors such as oxygen or iodine in the soln. strongly affects the rate of charge transfer at the particle-electrolyte interface and the transport of electrons throughout the TiO2 film-network. Modification of the semiconductor-electrolyte interface with surface adsorbed pyridine, induced major changes in the charge transfer events at the interface. The photocurrent yields was greatly improved by this surface treatment. The effect of pH in soln. was also investigated. The rate of charge transfer at the particle-electrolyte interface was changed at high surface d. of OH--ions. This was explained due to the change of the surface energy causing different driving forces for redox reactions, but also due to the more neg. electrostatic surface potential of the particles preventing the encounter of neg. charged redox species with the colloid surface. Phototransient measurements indicated a depletion of redox species in the pores of the film. Thereby it was pointed out that the dynamics of the redox species in the confined cavities of the film is a limiting factor for the charge sepn. efficiency in nanocryst. film. The photovoltage in anaerobic solns. sustained for very long periods, indicating that the linked particles may work as reservoirs for photoexcited electrons if the access to electron acceptors in the soln. is choked. It was concluded that surface processes are favored at the small semiconductor particles used in this study.


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