Electrochromic Nanostructured Tungsten Oxide Films by Sol-gel: Structure and Intercalation Properties M. Deepa, A. G. Joshi, A. K. Srivastava, S. M. Shivaprasad, and S. A. Agnihotry z National Physical Laboratory, New Delhi 110012, India As-deposited sol-gel derived amorphous tungsten oxide films transform into nanostructured films with an interconnected frame- work of grains and pores and a dominant triclinic crystalline phase upon annealing at 250°C. Transmission electron microscopy and scanning electron microscopy images clearly reveal the annealing-induced microstructural evolution for the film. Subsequent to lithium intercalation, the film annealed at 250°C shows quasi-reversible structural changes, as ascertained by X-ray diffraction and Fourier transform infrared spectral data. Dynamic transmission modulation for film revealed a high optical modulation of 72%   = 650 nm and a coloration efficiency maximum of 132 cm 2 C -1 at 800 nm under a lithium intercalation level of x = 0.20. X-ray photoelectron spectroscopy of the W 4f core levels demonstrated a progressive increase in the W 5+ content at the expense of W 6+ proportion as the insertion coefficient was raised from 0 to 0.25, with 0.20 as the threshold value above which the W 5+ content exceeds the W 6+ proportion. A new W 4+ state also appears which acts to lower the coloration efficiency for x  0.22. The presence of charged oxygen interstitials in the vicinity of electrochemically active tungsten sites is also responsible for the coloration efficiency decline at high ion insertion levels. © 2006 The Electrochemical Society. DOI: 10.1149/1.2184072 All rights reserved. Manuscript submitted June 29, 2005; revised manuscript received November 3, 2005. Available electronically April 3, 2006. Electrochromic windows offer promise for varying transparency or transmittance continuously in response to human input. The input is in the form of a small dc voltage or current, and once the desired optical state is attained the power supply can be interrupted and the state persists for a long time due to an inherent memory effect. 1 Intensified efforts have been focused on electrochromic technology, especially in view of solar heat load reduction and glare attenuation. 2 Nevertheless, the development of electrochromic de- vices for smart window applications with commercial viability is still far from complete. Electrochromic windows usually employ tungsten oxide as the cathodic coloring electrode as no serious con- tenders have appeared  WO 3  despite longstanding efforts. 3 The transmittance and reflectance of WO 3 films can be altered in a re- versible and persistent manner with the intercalation of small cations  H + , Li + , Na +  and electrons into the film host. 1 The reaction is topochemical and can be expressed as WO 3 + xe - Transparent + xM + ⇆ M x WO 3 Blue M = Li, Na, H 1 It is evident that the WO 3 film must bear great impacting effects due to this reversible process of ion insertion and extraction during cy- cling. High-resolution transmission electron microscopy HRTEM analysis on WO 3 films 4 showed that while the phase change of the film is reversible between the colored and bleached states, the mi- crostructural change is not. The latter plays a pivotal role in the degradation of the film. Consequently, cycling stability concerns need to be addressed. Moreover, a wide variety of film growth tech- niques which produce an equally wide variation in material proper- ties further complicates their evaluation. Although there is an ever- increasing number of reports on the primary electrochromic electrode, WO 3 , prepared by conventional vacuum techniques such as reactive dc magnetron sputtering and thermal evaporation, the volume of data decreases off rapidly on the microstructure–optical switching and durability correlation for sol-gel derived films. 5-8 The impetus for the sol-gel deposition of tungsten oxide films stems from the low capital cost of the technique and its adaptability toward large-area deposition of uniform films. 9-13 Reproducibility in terms of porosity, thickness, crystallinity, composition, and stoichiometry for films obtained by sol-gel processing is another major advantage. Early work on sol-gel deposited electrochromic coatings can be found in the reviews by Chemseddine et al., 14 which cover the chemistry of sol-gel precursors, and Cronin et al., 15 which concen- trates on the properties of these coatings. Tungsten oxide films can be fabricated from a diverse range of precursor sols based on col- loidal polytungstic acid, tungsten alkoxides, tungsten chlorides, tungsten oxoalkoxides, and peroxotungstic acid derivatives. 9,16-18 While polytungstic acid sols are characterized by an extremely short lived stability, 19 remnant chlorides are difficult to expel at low tem- peratures and alkoxides are rather cumbersome to handle. 20 Among these starting materials, peroxopolytungstic acid PTA sols or the same complexed with organic acids and ethanol are ideal as they offer several advantages, such as ease of processing metal oxide coatings at low sintering temperatures, long-term shelf life, and good wetting properties. 21,22 Irrespective of the deposition tech- nique, particular attention has been devoted to nanocrystalline WO 3 films, as the synergistic effects of the amorphous and crystalline extrema such as fast optical switching during the double injection/ extraction process and the chemical stability of the optical state, respectively can be easily exploited. 23 Nanostructured WO 3 films, characterized by crystallite sizes in the nanometric range, have a fairly large internal volume or porosity and a reasonably high inter- granular contact. They possess an elevated level of transmission and also show a superior optical modulation as compared to their con- ventional dense polycrystalline or amorphous analoges. 24 Electro- chromic efficiency is observed to have a strong dependence on an open structure and crystallite size. Optimization of grain size in nanocrystalline WO 3 films for high coloration efficiency, by regu- lating the postdeposition annealing temperature, has been previously achieved by us for WO 3 films electrodeposited from an acetylated peroxotungstic acid sol. 25,26 The size of nanograins and film porosity can also be tailored by the choice of an appropriate peroxotungstic acid to organic additive ratio. Our earlier reports on sol-gel derived WO 3 films describe the structural evolution of these films with annealing. 27 Although the optical behavior of amorphous WO 3 films spin-coated from acetylated PTA sols has been extensively investi- gated by us; 28 a similar study on nanostructured WO 3 films prepared from the same sol remains unreported to date. Improving the switch- ing speed while maintaining an acceptable contrast ratio is a major concern and is being actively pursued by researchers worldwide. Therefore, the present paper focuses on the preparation of sols com- prising peroxopolytungstic acid complexed with acetic acid and the characterization of films prepared thereof in terms of their structural, microstructural, morphological, optical, and electrochemical fea- tures as well as their cycling characteristics. The structural evolution of the as-deposited films with annealing has been examined, as it aids in identifying the most appropriate annealing temperature for the film that yields an optimal balance between chemical stability and electrochromic performance. X-ray photoelectron spectroscopy XPS has been extensively used for studying the electrochemical ion insertion properties in WO 3 films fabricated by vacuum evaporation. 29-33 However, sol-gel derived films have rarely been z E-mail: agni@mail.nplindia.ernet.in Journal of The Electrochemical Society, 153 5 C365-C376 2006 0013-4651/2006/1535/C365/12/$20.00 © The Electrochemical Society C365