Copyright © 2017 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 17, 1–9, 2017 www.aspbs.com/jnn Kinetics of Light-Driven Oxygen Evolution at Nanostructured Hematite Semiconductor Electrodes Ricardo Schrebler 1 , Luis A. Ballesteros 1 , Humberto Gómez 1 , Paula Grez 1 , Ricardo Córdova 1 , Eduardo Muñoz 1 , Rodrigo Schrebler 2 , Gustavo Sessarego 1 , Francisco Martín 3 , José R. Ramos-Barrado 3 , and Enrique A. Dalchiele 4 ∗ 1 Instituto de Química, Facultad de Ciencias, Pontificia Universidad Católica de Valparaíso, Av. Brasil, 2950, Valparaíso, Chile 2 Escuela de Ingeniería Química, Facultad de Ingeniería, Pontificia Universidad Católica de Valparaíso, Av. Brasil, 2950, Valparaíso, Chile 3 Laboratorio de Materiales y Superficies (Unidad Asociada al CSIC). Departamento de Física Aplicada & Ing. Química, Universidad de Málaga, E29071 Málaga, Spain 4 Instituto de Física & CINQUIFIMA, Facultad de Ingeniería, Julio Herrera y Reissig 565, C. C. 30, 11000 Montevideo, Uruguay Hematite nanostructures have been electrochemically grown by ultrasound-assisted anodization of iron substrates in an ethylene glycol based medium. This hematite nano-architecture has been tuned from a 1-D nanoporous layer (grown onto a bare iron foil substrate) to a high aspect self- organized nanotube one (grown onto a pretreated iron foil). Well-developed hematite nanotube arrays perpendicular to the substrate with a 1 m in length have been obtained. The nanoporous sample was characterized by pores of a mean diameter of 30 nm and an interpore distance of 150 nm, whereas the self-organized nanotube layer consisted of nanotube arrays with a single tube inner diameter of approximately 50 nm and average spacing of approximately 90 nm. The wall thickness of the hematite nanotubes was of approximately 30 nm. A comparative study of the photoelectrochemical properties of these two different hematite nanostructures under water-splitting conditions have been studied through EIS and PEIS methods. The strong correlation between the C SS increase with the R SS ct decrease and the photocurrent development as the potential is made more anodic, indicated that holes transfer for the water splitting reaction takes place through the surface states and not directly from valence band holes. From the PEIS spectra the rate constants of the elementary reactions responsible for the competing processes of interfacial charge transfer (k tr  and electron–hole recombination (k rec  have been determined. A better photoresponse kinetic was observed from the hematite nanotubular structure as compared to the nanoporous one. The last indicates that in the hematite nanotubular structure it exists a very well length scale matching between the nanotube wall thickness and the hole diffusion length (maximize light absorption while maintaining the bulk within hole collection length), diminishing then the recombination processes. Keywords: Hematite, Nanotubes, Nanostructures, Water Splitting, Ultrasound-Assisted Anodization, Impedance, EIS, PEIS. 1. INTRODUCTION Oxygen and hydrogen production by solar-driven water- splitting (photoelectrolysis) at semiconductor electrodes is an attractive means to convert intermittent solar radiation into storable, non-polluting fuels. 1 2 However, low-cost efficient water splitting using solar energy remains one of ∗ Author to whom correspondence should be addressed. the major scientific challenges for photoelectrochemistry. 2 In fact, the efficiency and stability of semiconducting pho- toelectrodes used in photoelectrochemical (PEC) devices must be substantially improved to make this process eco- nomical viable. 1 Since 1972, when the pioneering work on PEC water splitting was reported by Fujishima and Honda, 3 a wide range of materials has been investigated. In fact, J. Nanosci. Nanotechnol. 2017, Vol. 17, No. xx 1533-4880/2017/17/001/009 doi:10.1166/jnn.2017.14193 1