Charge Carrier Transport in Nanostructured Anatase TiO₂ Films Assisted by the Self-Doping of Nanoparticles

Abstract

The photoelectrochemical behavior of nanostructured TiO 2 (consisting mainly of anatase) films, with thicknesses ranging from ca. 0.6 to 45 µm, was examined under the band gap UV illumination. The shape of the spectral photoresponses exhibiting a maximum at 300 nm, irrespective the film thickness, and the excellent photocurrent-voltage characteristics of the thickest films are inconsistent with the previously proposed models aimed at describing electron transport in such nanoporous semiconductor networks. It was found that even electrophoretically deposited nanoparticulate films, simply dried at room temperature, behaved essentially as the high-temperature-annealed films. An explanation of the particular behavior of the nanostructured anatase films, in terms of the self-doping occurring at the initial stages of the photocurrent flow, is proposed. This initial film charging causes an insulator-metal (Mott) transition in a donor band of anatase, accompanied by a sharp rise in conductivity of the nanoparticles. Such a self-doping appears as a special feature of nanostructured semiconductor films filled with the electrolyte and characterized by large surface-area-tovolume ratios. This offers a convenient way of compensating the excess charge within the semiconductor by the adjustment of the ion concentration in the Helmholtz layer.

Theoretical Verification of Photoelectrochemical Water Oxidation Using Nanocrystalline TiO2 Electrodes

“…These facts suggest that self-organization of Yamashita/Jono model will give photoconductive nc-TiO2 electrodes when the PEC cell is kept at negative oxidation potential at short circuit conditions and energized by band gap excitation. In fact, the sharp rise in photoconductivity of nc-TiO2 electrodes was reported and discussed as an insulator-metal (Mott) transition in a donor band of anatase TiO2 [11]. It is also worth noting that in studies on dye-sensitized nc-TiO2 solar cells (DSC), adsorption of cationic species like tetrabutylammonium cation and sensitizing dye molecules enhanced electron transport in nc-TiO2 electrodes [12,13].…”

Tuning Photoelectrochemical and Photocatalytic Properties through Electronic Semiconductor–Support Interaction

“…than the space-charge region, not band-bending but competition between IFET and secondary recombination (process 5) is responsible for charge separation, as supported by the photooxidation of alcohols at titania powder electrodes. [31,32] A ±. + h r + ® A (1)…”

Photoactive titania nanostructured thin films: Synthesis and characteristics of ordered helical nanocoil array

“…Nanostructured titania thin films having a columnar-type growth [15,16], a nanowire [17], nanorods [18] or helical ribbon [19] microstructure, or formed by an ordered array of tapered, conical-shaped nanotubes [20,21] have been synthesized. These thin films, with respect to those prepared by conventional techniques such as dip-coating, have been reported to show enhanced properties in terms of charge separation, photocurrent, visible light response and reactivity connected to the peculiar nanostructure characteristics [11,[21][22][23][24][25].…”