Optimization of crystalline tungsten oxide nanoparticles for improved electrochromic applications R. Deshpande a,c , S.-H. Lee a , A.H. Mahan a , P.A. Parilla a , K.M. Jones a , A.G. Norman a , B. To a , J.L. Blackburn a , S. Mitra b , A.C. Dillon a, ⁎ a National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, United States b Department of Physics and Engineering Physics, University of Tulsa, OK 74104, United States c Department of Chemical Engineering, University of Tulsa, OK 74104, United States Received 28 July 2006; received in revised form 29 December 2006 Abstract The high-density synthesis of crystalline tungsten oxide nanoparticles employing hot-wire chemical vapor deposition (HWCVD) and enhancement in electrochromic (EC) performance by incorporating these nanoparticles into porous films has been previously reported. Here varying the oxygen concentration during the HWCVD synthesis of these crystalline tungsten oxide (WO x ) nanoparticles is examined in order to better understand the mechanism for the improvement in the EC films. Transmission electron microscopy, Raman spectroscopy, X-ray and electron diffraction are used to determine the particle sizes and crystalline phases of the as-synthesized nanostructures. Nanoparticle films are made employing an electrophoresis deposition technique. Cyclic voltammetry of the nanostructured films show higher charge insertion capacities for the nanoparticles synthesized at comparatively lower oxygen concentrations. Consistent with the electrochemical measurements, optical measurements also indicate a higher coloration efficiency (CE) value of ∼ 42 cm 2 /C for a nanostructured film made using nanoparticles synthesized at lower oxygen concentration (5%) as compared to the CE value of ∼ 24 cm 2 /C for a nanostructured film made using nanoparticles synthesized at higher oxygen concentrations (16%). The CE value of the former is comparable to state-of-the-art amorphous films with the crystalline nanostructures exhibiting significantly improved durability over amorphous films. Notably, the nanoparticle films have been shown to be stable for 3000 cycles in an acidic electrolyte where the amorphous films degrade after only 500 cycles. The optimized EC functional improvements are attributed to a sub-stoichiometric (oxygen deficient) state of WO 3 . © 2007 Elsevier B.V. All rights reserved. Keywords: Crystalline nanoparticles; Electrochromic; Mechanism 1. Introduction Tungsten oxide (WO 3 ) films have a variety of applications including electrochromic devices [1], gas sensors [2,3], and photocatalysis/photoconductivity [4,5]. The electrochromic (EC) effect has been extensively investigated in WO 3 films, since its discovery in 1969 by Deb [6]. Electrochromism represents the ability to induce reversible optical coloration in materials through ion–electron insertion. There are two important parameters, which are considered in selecting EC materials. First is the time constant for the ion insertion reaction, which is determined by the chemical diffusion coefficient and length of the diffusion path. While the former depends on the chemical and crystal structure of the metal oxide, the later is determined by the microstructure [7]. For nanostructures, the smallest dimension determines the diffusion path. Thus, designing a nanostructure with a small dimension and still maintaining the correct crystalline phase is key to obtaining a material with faster insertion kinetics and superior overall device performance. Second, is the coloration efficiency (CE) which is defined as change in optical density (OD) per unit inserted charge (Q), that is: CE = Δ(OD)/ΔQ [8]. Multiple deposition techniques such as sputtering [9], thermal evaporation [10], plasma-enhanced chemical vapor deposition [11], chemical vapor deposition [12], and sol–gel processes [13] Solid State Ionics 178 (2007) 895 – 900 www.elsevier.com/locate/ssi ⁎ Corresponding author. Tel.: +1 303 384 6607. E-mail address: anne_dillon@nrel.gov (A.C. Dillon). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.03.010