Photonic crystals for applications in photoelectrochemical processes Photoelectrochemical solar cells with inverse opal topology I. Rodriguez a,1 , P. Atienzar b , F. Ramiro-Manzano a , F. Meseguer a, * , A. Corma b, * , H. Garcia b a Centro Tecnolo ´gico de Ondas-Unidad Asociada CSIC-UPV, Universidad Polite ´cnica de Valencia, 46022 Valencia, Spain b Instituto de Tecnologı ´a Quı ´mica, CSIC-UPV, Universidad Polite ´cnica de Valencia, 46022 Valencia, Spain Received 3 August 2005; received in revised form 23 September 2005; accepted 25 September 2005 Available online 14 October 2005 Abstract Photonic crystal structures, that present strong light localization effects near photonic band gap frequency regions, can be very useful to maximize chemical processes of phototoactive materials. One example is the use of photonic crystals to improve solar energy harvesting in photoelectrochemical solar cells. Here, we describe the optical monitoring synthesis of macroporous materials, with inverse opal topology, made of transition metal and rare-earth oxide nanoparticles. Through the optical properties we can obtain information concerning both infiltration and over layer growth. Finally, we report on the efficiency improvement of photoelectrochemical cells when titania inverse opal topology is used. # 2005 Elsevier B.V. All rights reserved. PACS: 42.70.Qs; 81.05.Rm; 81.29.n; 84.60.j Keywords: Colloidal crystals; Opals; 3D photonic crystals; Rare-earth oxide nanoparticles; Transition metal oxide nanoparticles; Optical monitoring; Photonic crystals based solar cells 1. Introduction Porous materials exhibiting a high surface area, have received great attention owing to their impact on catalysis and photochemical processes [1]. A high degree of control over their structural and textural properties is quite desirable to obtain ordered porous materials tailored for each specific application. As a result from the intense research work carried out by the scientific community, it is now possible to build up 3D structures on multiple length scales, from few Amstrongs to macroscopic dimensions [2,3]. 3D photonic crystals (PCs) are also microporous materials and therefore they can be of application in chemistry. As a consequence of the appearance of optical gaps, one can also control the propagation and localization of photons involved in photochemical reactions. This effect is of enormous interest in photochemical processes where PCs can be used to harvest photons to maximize chemical reactions in areas such as photocatalysis, and photoelectrochemical solar cells. One example of this application is the use of PC www.elsevier.com/locate/photonics Photonics and Nanostructures – Fundamentals and Applications 3 (2005) 148–154 * Corresponding authors. Fax: +34 963 879849 (F. Meseguer); fax: +34 963 877809 (A. Corma). E-mail addresses: fmese@fis.upv.es (F. Meseguer), acorma@itq.upv.es (A. Corma). 1 Tel.: +34 963 879843. 1569-4410/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.photonics.2005.09.009