The Wavelength and Potential Dependency of Spatially Resolved Photoelectrochemical Measurements on TiO2

Abstract

Spatially resolved (on a micron scale) photoelectrochemical measurements on anodically prepared titanium dioxide and single-crystal rutile titanium dioxide, using the photoelectrochemical laser imaging (PECLI) technique, show a wavelength dependency. In the PECLI system, a pulsed laser beam is focused onto the working electrode to generate the photoresponse. Depending on whether the photon energy used to generate the map is above or below the bandgap energy, the spatial distribution of different optoelectronic information is obtained. Based on experimental results, it is proposed that the subbandgap photocurrent measured under these conditions is caused by a two-step excitation mechanism utilizing a localized state in the bandgap as an intermediate. Thus, maps made with subbandgap photons generate an image which reflects the distribution of subbandgap states. Anodically prepared oxide films on titanium having formation potentials, Uf, between 2 and 90 V are studied. Under 10 V, the PECLI images possess a uniform photocurrent distribution, while for U~ values above 10 V, large variations of the photocurrent are observed. The potential dependency of photocurrent transients for the anodically prepared TiO2 films is also examined.The application of photoelectrochemical techniques in the study of passive films has been well established. 1 An emerging technique utilizes spatially resolved photoelectrochemical maps to examine (to within a few microns) optoelectronic properties of the passive films. 2 Thus far, the most popular passive film to be studied by spatially resolved photoelectrochemical means is the anodically grown oxide film on titanium, and this has been done by a number of groups.The earliest reference to a spatially resolved photoelectrochemical map of the titanium oxide was by Butler. 3' 4 The system utilized a chopped krypton laser source where the photocurrent was extracted by a lock-in amplifier. Two photon energies were used, 2.6 and 3.5 eV, and differences in the photoelectrochemical maps were observed. 3 Peaks (with a 150 to 300 ~m base) identified in the maps made with the low energy photons were found to exist in areas where no peaks had been observed for the higher energy photons. This characteristic was attributed to the presence of regions of increased trap density and that "the traps observed in the below bandgap photoelectrochemical map do not contribute significantly to the recombination rate." ~ Using an oxide film with a formation potential of 2.2 V (NHE) and photons with an energy of 3.5 eV, a correlation between the peaks observed in the resulting photoelectrochemical maps of the surface (300 to 750 ~m wide) and the underlying crystallographic faces of the titanium substrate were established.Stimming et al. used a chopped krypton laser beam in conjunction with a lock-in amplifier to study the spatially resolved corrosion of anodically prepared titanium oxide films in HBr. ~ Using a "photoelectrochemical microscopy" (PEM) technique, based on a lock-in technique, Kozlowski et aI. ~ observe...

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