Optical and electrochemical properties of CeO2 and CeO2TiO2 coatings

Solar Energy Materials and Solar Cells 31 (1993) 171-185 North-Holland

Solar Energy Materials and Solar Cells

Optical and electrochemical properties of and CeO2-TiO 2 coatings U. Lavren~.if: S t a n g a r a n d K. K a l c h e r b

a,

CeO 2

B. O r e l a , , I. G r a b e c a, B. O g o r e v c a

a National Institute of Chemistry, Hajdrihova 19, P.O. Box 30, 61115 Ljubljana, Slovenia h Institute of Analytical Chemistry, Karl-Franzens University, Graz, Austria Received 29 January 1993; in revised form 19 April 1993 Thin solid films of CeO 2 and mixed C e O 2 - T i O 2 were prepared by the sol-gel route via the dip-coating technique. Particulate sols of ceria were made from inorganic ((NH4)2Ce(NO3) 6) precursor which were used for preparation of C e O 2 thin solid films while CeO 2 - T i O 2 coatings have been made by using mixed organic-inorganic (Ti(OiPr) 4 and CeCI3.7H 2O) precursors. The solar transmission values (T~-) of both coatings are in the range 0.6-0.8 and depend on coating thickness. Cyclic voltammetric (CV) m e a s u r e m e n t s show that the CeO 2 / L i O H system exhibits higher overall electrochemical reversibility when compared to the C e O 2 - T i O 2 / L i O H system. The CeO 2 / L i O H system is also less sensitive with regard to the coating thickness. Coulometric m e a s u r e m e n t s show that CeO 2 exhibits a larger storage capability which was determined as a function of the coating thickness. " I n situ" U V - V I S spectroelectrochemical m e a s u r e m e n t s which have been performed on CeO 2 and C e O 2 - T i O 2 coatings revealed that both types of samples exhibit electrochromic effect in the spectral range 500 < A < 330 nm but remain unchanged in the visible spectral range.

I. Introduction

During the last decade materials exhibiting etectrochromism have been thoroughly investigated and numerous excellent reviews are available in the literature [1,2]. The best known "chromogenic" material is tungsten trioxide (WO 3) which changes colour from transparent to deep blue when it is in a reduced state, where it forms hydrogen or alkali tungsten bronzes (MxWO3). There are two generally accepted ways for obtaining counter electrodes in electrochromic (transmissive) windows. The first one involves an electrochromic layer which is complementary with the chosen electrochromic material. The combination of WO 3 with NiOxHy is a typical example. The second possibility is an optically passive counter electrode which remains colourless in both oxidized and reduced states. The counter electrode should also exhibit high transparency. Among optically passive counter electrode materials In203 :Sn has been reported as a promising candidate [3]. Although the intercalation reaction is only * Corresponding author.

0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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U. Lat;ren{i( Stangar et al. / Ce02 and CeO 2 TiO 2 coatings

partially reversible, the advantage of this material is that it can also serve as a transparent conducting electrode. Vanadium pentoxide (V205) has high Li + storage capability of up to 0.5 C / I x m cm 2 [4]. The intercalation kinetic for lithium is also reversible but the transmission in its bleached state is low [5]. One of the more promising optically passive counter electrode materials is cerium dioxide (CeO 2) [6]. It has two stable valences available (III and IV) and is transparent in the spectral range from 0.35 Ixm up to 30 ixm where a restrahl band of C e - O appears. It exhibits good reversibility for lithium insertion and extraction, is optically passive but the reaction kinetics is slow [5]. Baudry et al. [5] in their attempts to improve the slow kinetics of pure CeO 2 counter electrode used mixed C e O 2 - T i O 2 thin solid films. Results showed that the storage capability of mixed C e O 2 - T i O 2 is about 10 m C / c m 2. Typically, during insertion and extraction, no coloration was observed. The electrochemical reactions are reversible and the transmissive electrochromic cells using WO 3 as electrochromic material exhibit coloration from blue colour when WO 3 is polarized cathodically to colourless when it is polarized anodically. Other details such as the relationship between the inserted charge and the change in optical properties, film thickness and in particular, correlation between the structure of the thin film and its optical and electrical properties have not been reported. Generally, different techniques are used for the preparation of the thin solid films. The most popular methods are sputtering and electrolytic deposition. The sol-gel process offers a new approach to the synthesis of oxide coatings [7]. Its main advantage is the possibility of being used on large scale. For the preparation of CeO 2 we employed the thermal decomposition method [7] which required metalloid precursors instead of metal alkoxides for sol preparation. Using this method particulate sols are formed from which particles with narrow size distribution can be grown. The method is based on forced hydrolysis which promotes deprotonation of the hydrous metal ion followed by polymerization. Basically, the method consists of the preparation of suitable sols from which thin solid films are prepared via the dip-coating technique. The main advantage of the technique is that the morphology and texture of this coating can be controlled at the very beginning of the preparation process, i.e. in the starting solution. As the aim of this paper was to prepare CeO 2 coatings on glass by dip coating technique and to assess its optical and intercalation properties, a mixed C e O 2 - T i O 2 coating was made to serve as reference. Besides being of different composition they differ in the routes which were followed to prepare them. Mixed C e O z - T i O 2 coatings were produced according to Makishima [8] from CeCI3 • 7 H 2 0 in combination with tetraisopropylorthotitanate (Ti(OiPr)4). Sainz et al. [9] prepared besides C e O 2 - T i O 2 coatings also coatings based on ternary system ( C e O z - T i O 2 - S i O 2) by adding tetraethylorthosilicate (TEOS) into the existing mixture of Ti(OiPr) 4 and CeC13 • 7 H 2 0 sol. Baudry [5] substituted CeC13 with (NH4)2Ce(NO3) 6 and obtained C e O 2 - T i O 2 coatings with extremely good intercalating properties for lithium cations, fast kinetics and appreciable charge inserted and extracted (10 mC/cm2).

U. Lavren6i( Stangar et al. / CeO2 and CeO2- TiO 2 coatings

173

2. Experimental 2.1. Instrumental

SEM micrographs were done on J E O L J X A 840 A electron pulse microanalyser. A Cu coating was applied onto the samples to avoid the charging of the surface. Thickness measurements were performed on a Surface Profiler Alfa Step 200 having maximal resolution of 0.5 n m / 1 0 0 nm. Cyclic voltammetric and chronocoulometric experiments were performed with an E G and G P A R model 273 computer controlled potentiostat-galvanostat, driven by a model 270 Electrochemical Analysis software. A M e t r o h m type voltammetric cell (50 ml) and a three-electrode system were employed. A working (test) electrode consisted of a roughly 2 cm 2 I T O / C e O 2 covered glass slide positioned so that exactly 1 cm 2 (one side) of it was in a physical contact with an electrolyte solution. The electric connection with I T O film was made via a spring clip. A P t rod served as a counter electrode and A g / A g C l / 0 . 2 M KC1 as a reference electrode. All potentials in the present work are quoted against this electrode. In cyclic voltammetry a potential scan rate of 20 m V / s was used throughout. Single scan or multi scan cycling and chronocoulometric measurements were performed within and at potentials + 0.4 V and - 1.3 V, respectively. In situ U V - V I S spectroelectrochemical measurements were performed on H P 8451A diode array spectrophotometer with E G and G P A R model 264A polarographic analyzer. Samples with dimensions ~ 1 × 1 cm 2 were accommodated into spectroelectrochemical cell equipped with Pt counter and A g / A g C 1 reference electrode and L i O H electrolyte which concentration was 0.1 M. Scanning rates used were 20 m V / s . Voltammograms were made only in cathodic direction started at 0.4 and finishing at - 1.3 V. Before repeating the cathodic scan coatings were kept for few minutes at the starting potential (0.4 V) in order to achieve initial electrochemical conditions of the coatings. Reversibility of the observed spectroelectrochemical changes were excellent proving the correctness of the applied procedure. 2.2. Preparation o f sols, gels and coatings

CeO 2 sols were prepared according to a method proposed by Woodhead [10] in which the precursor is obtained by the thermal decomposition of (NH4)2Ce(NO3) 6 (table 1). Peroxo complexes were made by adding H2O 2 into the yellow solution of the starting compound. Brown Ce(IV) peroxo complex decomposed to the yellow gelatinous precipitate which was used after peptization with H N O 3 for making sols and coatings. Sols were stable for few months and the coatings from freshly p r e p a r e d and aged sols were of the same quality. For the C e O 2 - T i O 2 coatings and thin solid films preparation the route proposed by Makishima et al. [8] was used (CeC13 in combination with titanium isopropoxide Ti(OiPr)4). The Ce : Ti mole ratio of 1 : 1 was achieved by this route.

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U. Lauren(i( Stangar et al. / CeO 2 and CeO 2 - TiO 2 coatings

Coatings were made on the dip coating unit with pulling speeds up to 10 cm/min. The supporting glass plates (7.5 × 2.6 x 0.1 cm 3) were previously cleaned in a chrome-sulphuric acid, rinsed with twice distilled water and then dried in air. Substrates were repeatedly dipped up to 10 times. The largest dimension of coatings achieved was 200 × 100 cm 2.

2.3. Electrochemical measurements After a portion (25 ml) of the electrolyte solution (usually 0.1 M LiOH) was transferred into a voltammetric cell, the solution was bubbled through with pure argon (99.99% oxygen free) for about 10 min. Then an I T O / C e O 2 test electrode was mounted and connected as mentioned above. Cyclic voltammetric and chronocoulometric measurements were performed in a still solution and within a given potential range.

3. Results and discussion

3.1. Structure and optical properties of coatings Film thickness of the CeO 2 coating obtained by single dipping is only 0.024 Ixm and increases with the number of dippings as is shown in fig. 1. This values are significantly smaller than those reported by Makishima [8] and Sainz [9] who achieved 0.2-0.3 ixm/dipping. Up to 10 dippings giving the total film thickness about 0.24 ixm were made without deterioration of the coatings quality. Coatings adhere perfectly on glass and it is nearly impossible to remove them with H C I - Z n powder mixture. A single coat looks light brown and takes a yellowish hue with the increasing number of dippings.

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Fig. 1. CeO 2 (1) and CeO 2 - T i O 2 (2) film thickness as a function of n u m b e r of dips at the pulling speed 10 c m / m i n .

U. Lavrengi~ Stangar et al. / CeO 2 and CeO2-TiO 2 coatings

175

SEM micrographs (fig. 2) show that homogeneous, crack free coatings having particulate nature are formed. The longer axis of the particles in the first coat was estimated to be 0.15 I~m and the axis being perpendicular to the substrate had a dimension of 0.024 ~m ( = thickness of the single coat). Subsequent coats are characterized by smaller particle size ( ~ 0.1 i~m). Lower pulling speed (1 c m / m i n ) influences the average particle size making them significantly smaller (0.08 Ixm). Thicker coatings are also crack free which was expected since according to Atkinson [11] the critical thickness of about 0.58 ~ m has not been achieved in our case. As was expected the SEM micrographs of the C e O 2 - T i O 2 coatings (fig. 3) differ significantly from the CeO 2 coatings. They clearly show that the C e O 2 - T i O 2 film

?

Fig. 2. SEM pictures of C e O 2 coatings treated at 500°C: (a) 1 × dip, pulling speed 10 cm/min, (b) 6 × dips, pulling speed 10 cm/min, (c) 1 × dip, pulling speed 1 cm/min (magnifications × 20000).

176

U. L a v r e n ( i ( Stangar et al. / CeO 2 and C e O 2 - TiO 2 coatings

Fig. 2 (continued).

does not have a particulate nature. Film structure is similar albeit more homogeneous to that already reported [8,9]. Transmission spectra of CeO 2 coatings are shown in fig. 4. They are characterized by transmission cut off in the spectral range of 0.3-0.4 ~m which shifts to longer wavelengths with increasing coating thickness. Interference fringes are clearly discernible and indicate highly specular coatings which exhibit high transparency for the visible and near infrared radiation. Solar transmittances (Ts) averaged with regard to the solar spectrum of air mass 2 are reported in table 2. They are high (60-80%) and prove that CeOe could be considered as a very promising optically passive counter electrode for electrochromic transmission devices. Transmission spectra of C e O 2 - T i O 2 coatings look nearly the same as those of CeO 2 coatings (fig. 5). Due to the pronounced interference fringes it is difficult to estimate which of the investigated coatings can be considered as a better optically passive electrode. Nevertheless, comparison of the corresponding T~ values (table 2) (fig. 6) show that CeO 2 coatings, because of their higher Ts values, are preferable for electrochromic application.

3.2. Electrochemical investigations Cyclic voltammetric recordings were performed in order to obtain electrochemical fingerprints of the studied oxide films as a function of different conditions and parameters. The potential in each measurement was swept from anodic region to a chosen cathodic overpotential and then reversed to the initial value. A multi-sweep mode was applied whenever voltammetric stability of a film had to be checked. In figs. 7 and 8 immediate subsequent CV recordings of CeO2 and C e O 2 - T i O 2 films, respectively, illustrate the starting stabilization of the corresponding electro-

177

U. Lavren~i( Stangar et al. / CeO 2 and CeO 2 - TiO 2 coatings

i

Fig. 3. SEM pictures of CeO 2-TiO 2 coatings (1 x dip, pulling speed 10 c m / m i n ) treated at 500°C at various maginfications: (a) × 10000, (b) x 20000.

chemical system when freshly made films were immersed into the electrolyte solution for the first time. In both cases a stable response was achieved after 5 cycles. Whilst C e O 2 / L i O H system exhibited higher overall electrochemical reversibility, C e O 2 - T i O 2 / L i O H system revealed a relative increase in reversibility and no change in peak current density during the stabilization process. A distinctly higher electrochemical reversibility of C e O 2 / L i O H system clearly implies a lower barrier against Li(I) ion intercalation. The dependence of the CV response on the film thickness is depicted in figs. 9 and 10. It can be seen that the influence of the film thickness to the shift of the cathodic peak potential which we express as AU/Ad is for C e O 2 - T i O 2 - 2 . 1

178

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wavelength/rim Fig. 4. Transmission spectra of CeO 2 coatings in the spectral range 0.25-2.0 ~m on ITO covered glass - ITO1 with sheet resistance 4 ~ and thickness 126 nm. Film thicknesses (d) are indicated.

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U. Lauren~i~ Stangar et al.

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CeO 2 and CeO 2 - TiO 2 coatings

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Fig. 7. Multi-cyclic voltammograms of a CeO 2 film (thickness = 97 nm) in 0.1 M LiOH; curves 1-5: immediate subsequent recordings, potential scan rate: 20 m V / s . -1.4 - 1.0

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U. Lauren(i( Stangar et al. / CeO 2 and CeO2-TiO 2 coatings

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m V / n m while for CeO 2 coatings is only - 1 . 2 m V / n m . On the other hand the peak current density changes in greater extent for CeO 2 (from A I / A d = (15 -I 4 ) / ( d 5 - d 4) = - 0 . 6 p . A / n m up to A I / A d = (14 -- I 2 ) / ( d 4 - d2) = - 2 . 9 ~zA/nm, due to the non-linear response of inserted charge into the differently thick coatings, see also fig. 14) than for the CeO2-TiO 2 coatings ( A I / A d = - - 0 . 4 I~A/nm). In fig. 11 the CV responses of two C e O z - T i O 2 films annealed at different temperatures are presented. Although the difference in the thickness of both films is small the peak current density of the film annealed at 300°C is much higher than for the film annealed at 500°C. The difference in film thickness (48 nm (500°C) vs. 57 nm (300°C)) is entirely due to the annealing procedure since the same xerogel films have been used for heat treatment. Our recent results revealed that CeO 2 and CeO2-TiO 2 films are crystalline at 500°C but exhibit more amorphous structure when heat treated at 300°C [12]. A similar behaviour was -1.6 3 --1.2 --0.8 o

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Fig. 10. Cyclic voltammograms of C e O 2 - T i O 2 films at different film thickness: (1) 48 nm, (2) 107 nm, (3) ~ 200 nm; electrolyte and scan rate: as in fig. 7.

U. Lavren~i( Stangar et al. / CeO 2 and CeO2-TiO 2 coatings

181

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found also for the W O 3 coatings where the CV response depends on the crystallinity of the system [13]. Chronocoulometric measurements were performed at constant potentials. These electrode potentials were determined from the sweep voltammetric measurements. Several conclusions can be drawn from the experimental transient data shown in figs. 12 and 13 where the total charge (Q) loaded or unloaded into or out of a film is recorded as a function of time (t). From these Q versus t curves it can be seen that the rate of increase in charge is bigger with a thinner film but the eventual charge storage capability is larger with a thicker film. The time response of both systems is good since about 90% of the charge (shown in figs. 12 and 13) can be

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182

U. Lacren(i{ ,~tangar et al, / CeO 2 and CeO 2 - T i O 2 coatings

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annealing temperature as a function of time: (1) 57 nm (annealing temperature 300°C), (2) 107 nm (annealing temperatire 500°C); cathodic, anodic charging potential and electrolyte: as in fig. 12.

stored in only 25 s. The coulombic efficiency of both films (ratio of anodic to cathodic charge after 20 s) yielded 86% and 98% for CeO 2 and C e O 2 - T i O 2 film, respectively. In fig. 14 charge densities at different current sampling times and film thicknesses are compared for both systems. The increase in the coating thickness of CeOe above 100 nm does not significantly contribute to the storage capability of a film. The reason for this is rather slow kinetics of intercalation process taken place into thick CeO2 films.

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film thickness / n m Fig. 14. Cathodic and anodic total charge density (Q) at different current sampling times as a function of film thickness: (1) CeO2 at 20 s, (2) CeO2 at 70 s, (3) C e O 2 - T i O 2 at 20 s, (4) C e O 2 - T i O 2 at 70 s; cathodic, anodic charging potential and electrolyte: as in fig. 12.

U. Lauren~i~ Stangar et aL / CeO 2 and C e O 2 - TiO 2 coatings

183

.0048~2

ADsOFDance

Fig. 15. Spectroelectrochemical response (cathodic scan) of the CeO 2 film (annealed at 500°C, d = 80 nm); electroNte and scan rate: as in fig. 7.

In situ U V - V I S spectroelectrochemical measurements are shown on figs. 15 and 16. The most salient feature is a decrease of the absorbance (AA) in the UV spectral range (A < 500 nm) which appears at cathodic potentials (U = - 0 . 4 V). Both types of coatings exhibit this phenomenon although absorbance changes are

.0~3~ 9~

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Fig. 16. Spectroelectrochemical response (cathodic scan) of the C e O 2 - T i O 2 film (annealed at 300°C, d = 115 nm); electro~te and scan rate: as in fig. 7.

184

U. Lauren(i? Stangar et al. / CeO 2 and CeO 2- ]riO2 coatings

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smaller for CeO 2 (AA/d = 0.0015 n m - 1 ) than for the C e O 2 - T i O 2 (AA/d = 0.0022 nm -~) coatings. Nevertheless, spectral dependence of absorbance is similar for both compounds starting to decrease approximately at A = 500 rim. From the measured absorbance changes the corresponding changes of transmittance (AT = Tcolo r - Tbleached) were determined giving (AT/d)(Ce02)=O.O03 nm -1 and (AT/d)(CeO2-TiO 2) = 0.004 m - 1. More detailed description of the spectral response of the investigated coatings as a function of the applied potential are shown in figs. 17 and 18 measured at A = 340 nm. Bleaching appears at - 0 . 6 V for CeO 2 and at - 0 . 2 V for C e O 2 - T i O 2 coatings which agrees with the start of current increase.

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Fig. 18. Absorbance change ( ) and cathodic current curve (---) of the CeO 2 - T i O 2 film (see fig. 16) as a function of the applied potential measured at )t = 340 nm.

U. Lavren~i~ Stangar et al. / CeO 2 and CeO 2 - T i O 2 coatings

185

4. Conclusions (1) Ceria sols prepared from (NH4)2Ce(NO3) 6 precursor are stable for few months and the quality of the prepared coatings does not deteriorate with the aging of sol. This is not the case when CeO2-TiO 2 sols are used. (2) The solar transmittances of CeO 2 coatings are comparable to those of CeO2-TiO 2 coatings. (3) CeO 2 coatings exhibit a higher electrochemical reversibility as compared to CeO2-TiO 2 coatings, but time response of the CeO2-TiO 2 coatings is slightly faster than the time response of the CeO 2 coatings. However, the storage capability of CeO 2 coatings for coating thicknesses in the range 0-50 nm, is similar to that of the CeO2-TiO 2 coatings. Above this thicknesses range the CeO 2 coatings show a storage capability which is nearly twice as much as that of the CeO2-TiO 2 coatings. (4) Although both types of coatings are considered as optically passive counter electrodes, they both show electrochromism in the UV spectral range (A < 400 rim).

Acknowledgements The authors wish to thank the Ministry of Science and Technology of the Republic of Slovenia for its financial support. Special thanks to Dr. C.G. Granqvist for initiating this research project and Dr. A. Andersson for performing initial cyclic voltammetric measurements. Authors are also indebted to Ing. A. Demgar for measuring VIS and NIR spectra of coatings.

References [1] C.M. Lampert, Sol. Energy Mater. 11 (1984) 1. [2] C.G. Granqvist, Thin Solid Films 193/194 (1990) 730. [3] R.B. Goldner, R.L. Chapman, G. Foley, E.L. Goldner, T. Haas, P. Norton, G. Seward and K.V. Wong, Sol. Energy Mater. 14 (1986) 195. [4] P. Baudry, M.A. Aegerter, D. Deroo and B. Valla, J. Electrochem. Soc. 138 (1991) 460. [5] P.Baudry, A.C.M. Rodrigues, M.A. Aegerter and L.O. Bulhoes, J. Non-Cryst. Solids 121 (1990) 319. [6] D. Deroo, P. Baudry and H. Arribart, French Patent No. 88 08809, June 1988. [7] C.J. Brinker and G.W. Sherer, Sol-Gel Science (Academic Press, London, 1990) ch. 4. [8] A. Makishima, H. Kubo, K. Wada, Y. Kitami and T. Shimohira, J. Am. Ceram. Soc. 69 (1986) C127. [9] M.A. Sainz, A. Duran and J.M.F. Navarro, J. Non-Cryst. Solids 121 (1990) 315. [10] J.L Woodhead, U.S. Patent 4.231.893, No. 4, 1980. [11] A. Atkinson and R.M. Guppy, J. Mater. Sci. 26 (1991) 3869. [12] U.L. Stangar, B. Orel, I. Grabec, B. Ogorevc, Solid State Ionics (1992), submitted. [13] P. Judenstein and J. Livage, Mater. Sci. Eng. B3 (1989) n l - 2 .