Efficient electrochromic performance of nanoparticulate WO 3 thin films† Dhanaji S. Dalavi, a Rupesh S. Devan, b Ranjit A. Patil, b Raghunath S. Patil, c Yuan-Ron Ma, b Shivaji B. Sadale, d InYoung Kim, e Jin-Hyeok Kim * e and Pramod S. Patil * ae This report highlights the suitability of electrodeposited nanoparticulate-WO 3 (NP-WO 3 ) electrodes for transmissive electrochromic devices (ECDs). The WO 3 electrodes in the form of thin films are composed of 10–20 nm nanoparticles. An electrochromic (EC) device of dimensions 5  4 cm 2 fabricated using NP- WO 3 showed an Li insertion coefficient (x) of 0.43, which resulted in highest photopic transmittance modulation (88.51%), better Li-ion diffusion coefficient (3.16  10 9 cm 2 s 1 ), fast electrochromic response time (5.2 s for coloration and 3.7 for bleaching) and excellent coloration efficiency (137 cm 2 C 1 ). On reduction of WO 3 , the CIELAB 1931 2  color space coordinates show the transition from colorless to the deep blue state (Y ¼ 97, a* ¼1.93, b* ¼ 0.46 and Y ¼ 10, a* ¼ 1.57, b* ¼41.01) with steady decrease in relative luminance. 1 Introduction Tungsten oxide (WO 3 ) is an intensively studied representative of a group of “chromogenic” materials because of the coloration effects associated with various processes. 1 Even though WO 3 electrochromic thin lms have been studied and manufactured for more than 30 years, new developments are expected to improve the performance of these materials when used for EC displays, electronic displays and color memory devices such as smart windows, mirrors, eyewear, mobile phones, smart cards, and price labels. 2–5 It is an important electrochromic material that can be colored through electro-, photo-, gas-, laser- and thermo- chromic processes. The most crucial parameters in the electro- chromic devices based on inorganic transition metal oxides are their optical modulation, response time, and coloration efficiency. Electrochromism involves the insertion/extraction of ions into/out of EC materials, and it can be achieved effectively by engineering the hierarchical morphologies with a smaller dimension and large specic surface areas which supports faster insertion kinetics and thereby superior overall device performance. 6 In recent years main attention has been focused on nanostructured WO 3 with various surface morphologies such as nanorods, 7 nano-urchins, 8 nano- particles, 9 nanowires, 10 mesopores, 11 nanopores, 12 nanotrees, 13 nanobundles, 14 nanosheets, 15 beads–wires–bers, 16 etc., fabri- cated by using a variety of techniques including hot-wire chemical vapor deposition (HWCVD), 9 electrospinning, 10 electrodeposi- tion, 11 anodization, 12 hydrothermal, 7,13 Langmuir–Blodgett, 14 layer-by-layer (LBL) deposition 15 and pulsed spray pyrolysis. 16 Though these morphologies have already been tested for elec- trochromic performance, to the best of our knowledge, the information received from all these nanostructures was insuffi- cient to boost the overall electrochromic performance based on optical modulation, fast switching time and coloration efficiency. Moreover, in comparison with all other synthesis techniques, electrodeposition (ED) appears to be more attractive because of low temperature and so processing capabilities. Additionally, electrochemical deposition is amenable to the practical fabrica- tion where large scale synthesis can be achieved. In this work, we report on NP-WO 3 thin lms synthesized by a dc electrodeposition method. The preparative parameters were judiciously optimized to obtain high quality NP-WO 3 lms. The lms were characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques and the electrochromic properties were studied using cyclic voltammetry and CIE system of colorimetric analysis. 2 Experimental A Electrosynthesis of NP-WO 3 thin lms A tungsten oxide precursor for the deposition was prepared by dissolving 7.48 g of W metal powder in 80 ml of H 2 O 2 (30%). 11 The reaction being exothermic was conducted between 0 and a Thin Film Materials laboratory, Department of Physics, Shivaji University, Kolhapur-416004, M.S., India. Fax: +91-231-2691533; Tel: +91-231-2609420 b Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan c Department of Physics, The New College, Kolhapur-416012, India d Department of Technology, Shivaji University, Kolhapur, India e Dept. of Materials Science, Chonnam National University, Gwangju, 500-757, South Korea. E-mail: psp_phy@unishivaji.ac.in; jinhyeok@chonnam.ac.kr † Electronic supplementary information (ESI) available: Cross-sectional view of SEM, EDS spectra, and L*a*b* system of colorimetric analysis. See DOI: 10.1039/c3tc30378k Cite this: J. Mater. Chem. C, 2013, 1, 3722 Received 28th February 2013 Accepted 18th April 2013 DOI: 10.1039/c3tc30378k www.rsc.org/MaterialsC 3722 | J. Mater. Chem. C, 2013, 1, 3722–3728 This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry C PAPER Published on 19 April 2013. Downloaded by National Dong Hwa University Library on 15/07/2013 12:18:06. View Article Online View Journal | View Issue