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

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 Uf 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.