Perturbation of the Electron Transport Mechanism by Proton Intercalation in Nanoporous TiO₂ Films

Abstract

This study addresses a long-standing controversy about the electron-transport mechanism in porous metal oxide semiconductor films that are commonly used in dye-sensitized solar cells and related systems. We investigated, by temperature-dependent time-of-flight measurements, the influence of proton intercalation on the electron-transport properties of nanoporous TiO(2) films exposed to an ethanol electrolyte containing different percentages of water (0-10%). These measurements revealed that increasing the water content in the electrolyte led to increased proton intercalation into the TiO(2) films, slower transport, and a dramatic change in the dependence of the thermal activation energy (E(a)) of the electron diffusion coefficient on the photogenerated electron density in the films. Random walk simulations based on a microscopic model incorporating exponential conduction band tail (CBT) trap states combined with a proton-induced shallow trap level with a long residence time accounted for the observed effects of proton intercalation on E(a). Application of this model to the experimental results explains the conditions under which E(a) dependence on the photoelectron density is consistent with multiple trapping in exponential CBT states and under which it appears at variance with this model.

Dynamics of Electron Recombination and Transport in Water-Splitting Dye-Sensitized Photoanodes

“…44 These intercalated protons can form long-lived trap states and significantly decrease the diffusion coefficient for electrons within TiO 2 . 19 We have previously identified intercalated protons as a major contributor to the poor performance of WS-DSPECs. 18 Furthermore, the low charge carrier density of WS-DSPEC photanodes is likely to modify the transport behavior within the nanocrystalline TiO 2 .…”

Electron trapping induced electrostatic adsorption of cations: a general factor leading to photoactivity decay of nanostructured TiO2

“…This viewpoint can be supported by recent reports although they did not investigate the mechanism of the proton uptake. 42,43 The existence of surface defect states is inevitable for nanostructured inorganic semiconductors, which is why R-TiO 2 still shows slow photocurrent decay despite of its single crystal nature. Moreover, proton charge compensation uptake is also common for many other metal oxides.…”

Toward tailored functionality of titania nanotube arrays: Interpretation of the magnetic-structural correlations

“…6,7 Thus, investigation of these properties is anticipated to lead to a better understanding of factors promoting higher photocatalytic efficiencies. [8][9][10] In this study, ordered arrays of amorphous titania NTs are synthesized by room-temperature electrochemical anodization of titanium foil in the presence of fluorine ions, followed by elevated-temperature crystallization treatments (200-450°C) in oxygen-rich (O 2 ), oxygen-deficient (Ar) and reducing (H 2 ) environments, to modify the NTs' crystal structure, morphology and electronic properties. The crystal structure, morphology and magnetic properties of the NTs in their as-anodized and annealed states were examined by x-ray diffractometry (XRD), scanning electron microscopy (SEM) and superconducting quantum interference device (SQUID) magnetometry.…”