Electrochromic Performance of Viologen-Modified Periodic Mesoporous Nanocrystalline Anatase Electrodes Sung Yeun Choi, † Marc Mamak, † Neil Coombs, † Naveen Chopra, ‡ and Geoffrey A. Ozin* ,† Materials Chemistry Research Group, Department of Chemistry, 80 St. George Street, UniVersity of Toronto, Toronto, Ontario, Canada M5S 3H6, and Xerox Research Centre of Canada, 2660 Speakman DriVe, Mississauga, Ontario, Canada L5K 2L1 Received April 6, 2004; Revised Manuscript Received May 20, 2004 ABSTRACT We report for the first time on the electrochromic performance of viologen-modified periodic mesoporous nanocrystalline anatase, denoted meso-nc-TiO 2 -V(2+). Electrodes fashioned from meso-nc-TiO 2 -V(2+) were found to display enhanced color contrast yet have similar conduction band edge energy level and electron percolation ability as electrodes made from viologen-modified nanocrystalline titania, nc-TiO 2 -V(2+). This performance can be attributed to the uniform and ordered mesopore architecture, the nanocrystalline anatase wall structure, and the large accessible surface area for tethering viologen molecules. With the explosive growth of broadband communications and wireless technologies, the demand for inexpensive thin film lightweight display devices with paper-like readability and ultralow power consumption has increased. One promis- ing technology is the electrochromic (EC) device based upon dye-modified semiconductor electrodes. Such a device has advantages compared to intercalation EC electrodes, based on materials such as tungsten oxide, due to much improved time response and enhanced color contrast especially for display technologies such as electronic paper or billboards. 1-3 The heart of the device is a working electrode composed of a nanocrystalline semiconductor, usually an n-type metal oxide, which has been modified with an electrochromophoric molecular species, most often a redox active viologen derivative, chemically tethered to the surface of the nano- crystalline electrode. The efficiency of this system is based upon fast interfacial electron transfer between the nanocrys- talline electrode and the anchored chromophore along with optical amplification brought about by the large surface area of the nanocrystals, which enables high chromophore loading per unit area of the electrode. 1d Nanocrystalline titania (nc-TiO 2 ) has been successfully used for this purpose, resulting in a high efficiency electro- chromic device mainly due to the intrinsic properties of the nanocrystalline titania electrode, such as transparency to visible light, electronic conductivity, high surface affinity toward certain ligands, and large surface area. 1-4 In this context, mesoporous titania (meso-TiO 2 ) is of considerable interest because of its high surface area and uniform and well-ordered mesopores, which should facilitate the entry of dye molecules into pores and anchoring of them to anatase nanocrystals that comprise the channel walls. 5 There are many reports for the synthesis and photocatalytic/photoelec- trochemical properties of meso-TiO 2 , 6,7 however, no reports yet exist concerning its electrochromic (EC) behavior. In this communication, we report for the first time on the EC performance of a new generation of viologen-modified periodic mesoporous nanocrystalline titania, meso-nc-TiO 2 - V(2+), thin film electrodes, particularly concerning color contrast and color switching speed, electrochemical redox potentials and reversibility, transient absorption change during cycling, and color cycling stability. The uniquely robust channel walls of meso-nc-TiO 2 support a relatively large average anatase crystallite size, which favorably impacts electrochromic reversibility and overall cell perfor- mance. The following abbreviations will be used throughout the text to clearly distinguish the three types of titania that we are concerned with, namely: nc-TiO 2 , conventional nanocrystalline anatase titania; meso-TiO 2 , periodic meso- porous titania calcined at 350 °C; meso-nc-TiO 2 , periodic mesoporous titania calcined at 400 °C. Viologen-modified analogues of these materials are denoted as nc-TiO 2 -V(2+), meso-TiO 2 -V(2+), and meso-nc-TiO 2 -V(2+), respectively. Meso-nc-TiO 2 thin film electrodes were prepared using procedures described in a recent report. 8 Meso-nc-TiO 2 synthesized by this method utilizes 1-butanol as solvent and has a 2-D structure with hexagonally close-packed mesopores * Corresponding author E-mail: gozin@chem.utoronto.ca. † University of Toronto. ‡ XRCC. NANO LETTERS 2004 Vol. 4, No. 7 1231-1235 10.1021/nl049484d CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004