Thin Solid Films 403 – 404 (2002) 312–319 0040-6090/02/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž 01 . 01509-7 Comparison of optical and electrochemical properties of anatase and brookite TiO synthesized by the sol–gel method 2 M. Koelsch , S. Cassaignon *, J.F. Guillemoles , J.P. Jolivet a a, b a Laboratoire de Chimie de la Matiere Condensee, Universite Pierre et Marie Curie, Paris VI (CNRS UMR 7574) Tour 54 E4, case 174, a ` ´ ´ 4 place Jussieu, 75252 Paris Cedex 05, France Laboratoire d’Electrochimie et de Chimie Analytique, ENSCP, Paris VI (CNRS UMR 7575), 11 rue Pierre et Marie Curie, b 75232 Paris Cedex 05, France Abstract Three polymorphs of TiO , rutile, anatase and brookite, are well known. Each variety has its own physical properties, bandgap, 2 surface states, etc. For photovoltaic applications, anatase is the phase most used. Up to now, the third polymorph of TiO , 2 brookite, could not be obtained by soft chemistry methods. We have now synthesized nanometric particles of pure brookite by ‘chimie douce’ in aqueous solution. For the first time, this phase will be characterized in view of its possible application in dye- sensitized cells. We discuss the potential of brookite as a good candidate for photovoltaic devices in comparison to other phases, in terms of its morphology, bandgap and electrochemical properties in water and acetonitrile before sensitization.  2002 Elsevier Science B.V. All rights reserved. Keywords: TiO2; Anatase; Brookite; Synthesis; Optical properties; Electrochemistry 1. Introduction Dye-sensitized nanoporous semiconductors are an ele- gant answer to the problem of solar energy conversion with photoelectrochemical systems, providing for effi- cient w1x, stable and potentially low-cost technology. This hybrid system relies on a fortunate combination, unsurpassed so far, of a nanoporous semiconductor (anatase form of TiO ), a dye (Ru-bipyridyl based) and 2 an electrolyte (I y I in acetonitrile)w2,3x. The precise y 2 reasons why this combination is so successful and the existence of better systems are still a matter of active research. Our study aimed to explore the role of nanoporous semiconductors by comparing the activity of two poly- morphs of TiO : anatase and brookite. We need to stress 2 that this study is the first, to the best of our knowledge, reporting on brookite films prepared by ‘chimie douce’. The first part of this study, presented here, investigates * Corresponding author. E-mail address: cassai@ccr.jussieu.fr (S. Cassaignon). the morphology, optical properties and electrochemical behavior of the bare interfaces (before sensitization) of the two aforementioned phases. 2. Experimental 2.1. Synthesis of TiO 2 2.1.1. Synthesis of anatase Pure TiCl diluted in 3 mol l hydrochloric acid y1 4 (HCl) was introduced into distilled water. The pH was adjusted with NaOH to between 2 and 5. Precipitation was instantaneous and complete. The solution was heat- ed and aged at 608C over 24 h. The precipitate crystal- lized to anatase (with a low proportion of brookite). The solid obtained was treated with 3 mol l nitric y1 acid (HNO )(after centrifugation and washing with 3 water) in order to peptize the surface of the particles. The solid was centrifuged and dispersed in water. A stable sol of nanoparticles of anatase was obtained. The final concentration of titanium was 0.177 mol l . The y1 details of this synthesis are reported elsewhere w4x.