Effective utilization of spray pyrolyzed CeO 2 as optically passive counter electrode for enhancing optical modulation of WO 3 A.K. Bhosale a , N.L. Tarwal b , P.S. Shinde b , P.M. Kadam c , R.S. Patil d , S.R. Barman e , P.S. Patil b, ⁎ a Raje Ramrao Mahavidhyalaya, Jath-416 005, India b Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, Maharashtra, India c Kasturbai Walchand College, Sangli-416 416, Maharashtra, India d The New College, Kolhapur-416 004, Maharashtra, India e UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore-452 017, Madhya Pradesh, India abstract article info Article history: Received 10 February 2009 Received in revised form 12 May 2009 Accepted 17 July 2009 Keywords: Cerium oxide Spray pyrolysis XPS Ion storage capacity Electrochromism Nanocrystalline cerium oxide (CeO 2 ) thin films were deposited onto the fluorine doped tin oxide coated glass substrates using methanolic solution of cerium nitrate hexahydrate precursor by a simple spray pyrolysis technique. Thermal analysis of the precursor salt showed the onset of crystallization of CeO 2 at 300 °C. Therefore, cerium dioxide thin films were prepared at different deposition temperatures from 300 to 450 °C. Films were transparent (T ~ 80%), polycrystalline with cubic fluorite crystal structure and having band gap energy (Eg) in the range of 3.04–3.6 eV. The different morphological features of the film obtained at various deposition temperatures had pronounced effect on the ion storage capacity (ISC) and electrochemical stability. The larger film thickness coupled with adequate degree of porosity of CeO 2 films prepared at 400 °C showed higher ion storage capacity of 20.6 mC cm -2 in 0.5 M LiClO 4 + PC electrolyte. Such films were also electrochemically more stable than the other studied samples. The Ce 4+ /Ce 3+ intervalancy charge transfer mechanism during the bleaching–lithiation of CeO 2 film was directly evidenced from X-ray photoelectron spectroscopy. The optically passive behavior of the CeO 2 film (prepared at 400 °C) is affirmed by its negligible transmission modulation upon Li + ion insertion/extraction, irrespective of the extent of Li + ion intercalation. The coloration efficiency of spray deposited tungsten oxide (WO 3 ) thin film is found to enhance from 47 to 53 cm 2 C -1 when CeO 2 is coupled with WO 3 as a counter electrode in electrochromic device. Hence, CeO 2 can be a good candidate for optically passive counter electrode as an ion storage layer. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Cerium oxide exhibits two structures mainly cubic fluorite cerium dioxide or ceria (CeO 2 ) (due to tetravalent Ce) [1,2] and a hexagonal sesquioxide Ce 2 O 3 (due to trivalent Ce) Both the forms are refractory, which are optically transparent in the visible region and highly absorbing in the UV region [1,3]. However, the most common of cerium oxides is CeO 2 , which is stable even in the substoichiometric form (CeO 2 - x ,0 b x b 0.4) [4] and thus has been easily produced by several deposition techniques. The fluorite structure of ceria consists of four coordinate oxygen atoms and the eight coordinate cerium atoms; one cerium atom is surrounded by eight oxygen atoms with space group Fm3m. Ceria has recently attracted much interest due to its unique prop- erties which make it suitable for wide range of applications in cat- alysts, fuel cell, phosphor/luminescence, and glass/ceramic [5] and these utilizations are mainly based on cerium's high thermodynamic affinity of cerium for oxygen (i.e. ceria being oxygen reservoir) and its potential redox chemistry involving Ce(III)/Ce(IV). It is used in opto- electronic devices due to its attracting physical properties such as wide band gap (3.2–3.6 eV), large dielectric constant (~ 26) and lattice parameter (0.541 nm, which matches well with Si) [6]. It is also used as an intermediate buffer layer at the interface owing to its high chemical stability [7]. In electrochemistry, CeO 2 thin film can be employed as a counter electrode in electrochromic devices (ECDs) [8–14] because of its high optical transparency in the visible region and its ability to insert/ extract large charge densities [15,16]. Several synthesis techniques such as sol–gel [8,11,13,14,17,18], electron-beam pulsed vapor deposition [9,10], sputtering [14,18], MOCVD [19], flash evaporation [20] and spray pyrolysis [21–27] have been employed to elaborate CeO 2 thin films with superior crystalline quality, chemical stability and high optical transmission. Among these, spray pyrolysis process is essentially a continuous and allows facile fabrication of large-area coatings both dense and porous at a low cost. It offers advantages of controlling the composition and microstructure Solid State Ionics 180 (2009) 1324–1331 ⁎ Corresponding author. Tel.: +91 231 2609230; fax: +91 231 2691533. E-mail addresses: pravinshindephysics@gmail.com (P.S. Shinde), psp_phy@unishivaji.ac.in (P.S. Patil). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.07.007 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi