Molybdenum oxide (MoO 3 ) thin film based electrochromic cell characterisation in 0·1M LiClO 4 .PC electrolyteby R. Sivakumar, K. Shanthakumari, A. Thayumanavan, M. Jayachandran, C. Sanjeeviraja

Surface Engineering

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Molybdenum oxide (MoO3) thin film based electrochromic cell characterisation in 0?1M

LiClO4.PC electrolyte

R. Sivakumar1, K. Shanthakumari2, A. Thayumanavan3, M. Jayachandran4 and

C. Sanjeeviraja*1

Electrochromic thin films of molybdenum oxide (MoO3) were prepared on transparent conducting oxide substrates, i.e. fluorine doped tin oxide coated (FTO or SnO2:F) glass substrates by electron beam evaporation technique using pure MoO3 (99?99%) pellets at various substrate temperatures (i.e. Tsub5room temperature (30uC), 100 and 200uC) under the vacuum of 161025 mbar. The room temperature prepared films were further annealed Tanne at 200 and 300uC for about one hour in the vacuum environment. The electrochemical nature of the films was studied by the cyclic voltammetry technique using a three electrode electrochemical cell in 0?1M

LiClO4.PC electrolyte. The performance of the films was also tested by making electrochromic cells. The films produced at higher substrate temperature show lesser modulation in the visible spectrum, compared with the films produced at lower temperatures. A maximum colouration efficiency of 66 cm2 C21 was observed in the infrared region for the films prepared at room temperature.

Keywords: Smart windows, Molybdenum oxide films, Electrochemical analysis, Electron beam evaporation technique, Colouration efficiency

Introduction

Electrochromism is well known in semiconducting transition metal oxides. Among the transition metal oxides, molybdenum oxide (MoO3) has been found to be a versatile material for electrochromic devices, owing to its excellent electrochromic behaviour.1 Molybdenum oxide that can undergo reversible lithium intercalation/ deintercalation process at ambient temperature is of great technical interest as cathode material for secondary lithium batteries,2 because it exhibits two dimensional van der Waal bonded layered structure in orthorhombic phase (a-phase). With ion uptake, the interlayer spacing d of the MoO3 host structure changes, usually increasing, but the guest ions are largely accommodated without disrupting the primary Mo–O bonding within the individual MoO3 type layers. 2

Because the intercalation of MoO3 is generally accompanied by pronounced changes in colouration and is subject to varying extent of reversibility, this oxide has attracted interest for a number of applications, including electrochromic devices and optical switching devices.1

Optical switching devices can be used for windows in a variety of applications where optical and thermal modulation is required. The purpose of this material is to control the flow of light and heat into and out of a window, according to personal comfort or an energy management scheme. The basic property of an optical switching material or smart window is that it shows a large change in optical properties upon a change in light intensity, spectral composition, temperature, electric field or injected charge. Another important feature is that MoO3 film acts as either a negative or positive photoresist3 with high contrast capability for focused ion exposure depending on the preparation condition.

The intercalation and deintercalation of Liz ions in non-aqueous media has been of particular interest, primarily owing to its relation to secondary battery applications.

It is well recognised that the performance of an electrochromic device depends strongly on the technique that is used to prepare the active electrochromic electrode thin film. Apart from the available various preparation techniques of molybdenum oxide, the electron beam evaporation technique is finding ever increasing application in high technology solutions for preparing device quality thin films.4,5 Establishing optimum parameters or a special technique for thin film preparations may permit films to be prepared with specific microstructures that are relevant to a particular application.6 Hence the present work has adopted the 1(Formerly Department of Physics, Alagappa University, Karaikudi 630 003, India) Department of Chemical Engineering, National Taiwan

University, Taipei 10617, Taiwan 2Department of Physics, SASTRA University, Thirumalai Samudram 613 402, India 3AVVM Sri Pushpam College, Poondy 613 503, India 4ECMS Division, Central Electrochemical Research Institute, Karaikudi 630 006, India *Corresponding author, email sanjeeviraja@rediffmail.com  2009 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 6 May 2007; accepted 2 August 2007 548 Surface Engineering 2009 VOL 25 NO 7 DOI 10.1179/174329408X282523 electron beam evaporation technique for the preparation of MoO3 films. The authors have reported the detailed preparation procedures for MoO3 films by electron beam evaporation technique and also reported the structural, morphological, compositional and optical properties of MoO3 films in the authors’ previous papers.4,5,7 The present work describes the detailed characterisation of the electrochromic properties of molybdenum oxide thin film based device by inserting/ extracting the Liz ions. The colouration/decolouration process in 0?1M LiClO4.PC electrolyte is investigated.

The effect of substrate temperature and post-annealing temperature on the electrochromic properties of the films were studied. The complete description for the cyclic voltammetry analysis and the construction of electrochromic cell concerning the intercalation/deintercalation of Hz and Kz ions is elaborately discussed in the authors’ earlier reported works.8,9

Experimental

Thin MoO3 films were prepared by electron beam evaporation of dry MoO3 pellets under a chamber pressure of 161025 mbar on fluorine doped tin oxide coated glass substrates (FTO or SnO2:F) (Rsh<15 V22) at different substrate temperatures Tsub like 30uC (room temperature, RT), 100 and 200uC. Later on, the MoO3 films deposited at room temperature were subsequently annealed for one hour in vacuum atmosphere Tanne at 200 and 300uC. The electrochemical analysis was performed using an EG&G Princeton applied research 273A potentiostat. During the electrochemical measurement using cyclic voltammetry technique, the working electrode was the FTO bearing electrochromic oxide films (MoO3), while counter electrode was the platinum foil. Saturated calomel electrode (SCE) was used as the reference electrode (RE). Whereas in the electrochromic cell analysis, the FTO plate was used as the counter electrode and the platinum wire was acting as pseudoRE and was placed at equidistant between the working electrode and counter electrode. In both the analysis, 0?1M LiClO4 in propylene carbonate (PC) was used as the electrolyte. The optical transmittance of coloured and bleached MoO3 films was carried out by Hitachi3400 UV-Vis-NIR spectrophotometer. The infrared (IR) investigations of the films were performed by using a