Determination of electron diffusion lengths in nanostructured oxide electrodes from photopotential maps obtained with the scanning microscope for semiconductor characterization Teresa Lana-Villarreal, Damia ´n Monllor-Satoca, Roberto Go ´mez * , Pedro Salvador 1 Institut Universitari d’Electroquı ´mica i Departament de Quı ´mica Fı ´sica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain Received 29 June 2006; received in revised form 1 August 2006; accepted 7 August 2006 Available online 7 September 2006 Abstract The scanning microscope for semiconductor characterization, a scanning laser-spot technique, has been used to obtain laterally resolved open circuit photopotential measurements of anatase nanostructured electrodes in contact with acidic solutions of catechol (or salicylic acid). The oxide nanoporous films were supported on a conducting glass plate with a groove cut in it as to remove a strip of the conducting layer, thus defining two electrically isolated areas. Upon 514.5-nm light illumination, the electrons photoinjected from adsorbed catechol spread over the nanoporous anatase thin film. The photopotential of one of the isolated conducting parts was mon- itored as a function of the laser spot location. A quantitative model is developed to simulate the photoresponse based on the validity of the diffusion equation for electron transport. Accordingly, a value of 0.4 mm was found for the electron diffusion length in the presence of catechol, which probably results from a high average lifetime of the photogenerated electrons. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Scanning laser-spot microscopy; Catechol; Photopotential; Diffusion length; Anatase; Nanoporous electrode 1. Introduction The study of the electronic and (electro)chemical properties of nanostructured semiconductor thin films is nowadays rising a considerable interest [1]. Besides its intrinsic scientific relevance, it is directly related to impor- tant applications. For instance, such samples can be used as photocatalytic substrates [2] or as photoanodes in Dye-Sensitized Solar Cells (DSSC) [3]. The transport of photogenerated carriers through the semiconductor matrix determines to a great extent the behavior of such samples. In contrast with typical bulk semiconductor photoelectrodes, where migration is of great importance in the space charge layer, diffusion is the main mechanism of electron transport within a nanoporous layer [4]. Several electrochemical techniques have been applied to study electron transport. On most occasions, short circuit conditions have been employed [5], such as in Intensity Modulated Photocurrent Spectroscopy (IMPS) [6] and pulsed laser-induced photocurrent transient measurements [7]. The short circuit condition entails a continuous transfer of electrons from the nanoporous structure to the conduct- ing substrate. It is believed that an electric field gradient could exist near the semiconductor-substrate interface, which would complicate the analysis of the experimental results on the basis of the diffusion equation [8]. Although photopotential measurements would be free of such com- plications, only in a few reports they have been employed for studying electron transport. Concretely, some work has been done with both Intensity Modulated photoVolt- age Spectroscopy (IMVS) [6] and pulsed laser-induced photovoltage transients [8]. 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.08.008 * Corresponding author. Fax: +34 96 5903537. E-mail address: Roberto.Gomez@ua.es (R. Go ´ mez). 1 On leave from the Instituto de Cata ´lisis y Petroleoquı ´mica, CSIC, Spain. www.elsevier.com/locate/elecom Electrochemistry Communications 8 (2006) 1784–1790