Electrochemical and optical properties of Ce02 and mixed Ce02/Sn02 coatings
Elektrokemijske in optične lastnosti Ce02 in mešanih Ce02/Sn02 tankih filmov
Z. Crnjak Orel, National Institute of Chemistry, Hajdrihova 19, 61115 Ljubljana, Slovenia
Ion storage Ce02 and Ce0/Sn02 coatings were prepared by sol-gel dip-coating method using aqueous-basedprocess. The influence of addedSn02 in the Ce02 oxide coatings on the inserted/extracted charge was determined by chronocoulometric measurements. It wasfound that for 60 nm thickfilm the inserted/extracted charge was two times larger (0= 10mC/cm2) for films containing 17 mol% SnO: if compared to pure Ce02. The addition of Sn02 to the mixed oxides coatings on their optical properties and structural characteristics were studied.
Key words: sol-gel, counter electrode, charge density
Ionske hranilnike Ce02 in Ce0/Sn02 v tankih filmih smo pripravili iz anorganskih prekurzorjev po sol gel postopku. Vpliv dodanega Sn02 v Ce02filmih na interkalirani/deinterkalirani naboj smo določali s kronokulometijskimi meritvami. Ugotovili smo, da je za 60 nm debele filme interkalirani/deinterkalirani naboj najmanj dvakrat večji po dodatku 17 mol % Sn02 (0= 1 OmC/cm2). Vpliv dodanega Sn02 na optične in strukturne lastnosti smo študirali s spekrofotometričnimi metodami.
Ključne besede: sol-gel, ionski hranilnik, gostota naboja
1 Introduction
Cerium dioxide (Ce02) has been considered as a useful material with high-refractive index film in single- and multilayered optical coating. Beside that, films are highly efficient for ab-sorbing ultraviolet radiation and glass with 2-4% Ce02 is valuable in protecting light-sensitive materials1. Cation doped ceria is potential electrolyte for solid oxide fuel celi due to its high oxygen ionic conductivity2.
Sol-gel deposited Ce02 based films were studied3,4 as counter electrode in electrochromic devices. To improve slow kinetics of pure Ce02 prepared by the evaporation technique they substitute cerium atoms by another element of a smaller radius as titanium. Cathodic and anodic peaks for so prepared mixed Ce02/Sn02 films showed reversible insertion for litium ions and the charge inserted and extracted at sweep rate 10 mV was approximately 10 mC/cm2 for three dippings3.
According to Baudry et al. 3 in our study tin was used (atomic radius 0.71 A) instead of titanium (atomic radius 0.68 A) and mixed Sn02-Ce02 films were prepared by dip-coating method via sol-gel route. In the present work we investigated in which extent ion storage capability in Ce02 coatings could be lmproved by the addition of Sn02 previously investigated 5,6,7 in the mixed oxide coatings and its influence on optical and structural properties.
2 Experimental
Preparation of an aqueous dispersion of Ce02 and mixed Ce02/Sn02 started from solution of metalic salts Ce(NH4)3(N03)6 and SnCl4. Precipitates were obtained by addition of NH4OH until pH = 9 was reached After washing the precipitate in order to remove residual NH4+, Cl", NO3" with distilled water the peptization was performed by addition of equimolar quantity of HN03 (pH a 1). The coloidal sols were aged at temperature up to 90°C for 20 min, giving yel-lowish semitransparent sol.
Films were prepared with dip coating methods with pull-ing speed 10 cm/min on ITO glass plates with repeated dip-ping of 8 times. EG & PAR - Mod 273 potentiostat galvanostat with electrochemical analysis software was used for cyclic voltammetric and chronocoulometric experiments. A Ramp acquisition mode was employed in ali cyclic voltammetric measurements. A Pt rod and Ag/AgCl/0.2 M KC1 served as counter and reference electrode, respectively. The vvorking electrode area was 1 cm2 in electrolyte solution (30 ml 0.1 M LiOH). Scanning rate was 20 mV/sec. Cycling and chronocoulometric measurements were performed at potentials of +0.4 V and -1.3 V.
X-ray diffraction analyses of oxide povvders and dip coated films were performed on Philips PW 1710 x-ray diffractome-ter.
Transmittance spectra, in the spectral range 0.3-2.5 pm were measured vvith Perkin-Elmer Lambda 9 spectrometer at resolution 2nm. Obtained T(X.) values vvere used for the calculation of normal šolar transmittance according to the equation
2/jn
\T(x)Ps{Z.)dX
rrt _ 0.3//71_
~ 2/m
0.3/jn
where Ps(/.) is šolar spectrum at air mass of 2.
Surface Profiler Alfa Step 200 vvas used for thickness measurements.
used and vvith the number of dipping. The charge density vvas high if the starting concentration of used sol was high and the number of dipping vvas large.
According to results obtained the concentration of 7.2 10"3 mol vvas chosen for the preparation of pure Ce02 and mixed Ce02/Sn02 dip coated films. It vvas found that for one laver charge density vvas about 4 Q (mC/cnr2). The concentration of Sn02 added vvas maximally 50 mol percents. For ali of them, cathodic and anodic total charge density Q (mC) after 60 s, vvere determined. In figure 1 the influence of added Sn02 up to 17 mol% is presented. For that concentation the amount of charge density (Q) vvas about 10 mC/cm2 for one layer and about 22 mC/cm2 for 8 layers. The number of dipped layers is closely connected vvith charge density. If the concentration of added Sn02 is higher than 17 mol % the charge density of mixed Ce02/Sn02 starts to decrease.
No. nT dipped ]ayeis
Figure 1: Cathodic and anodic total charge density Q (mC/cm2) as a function of added Sn02 and a number of dipped layers: o - pure Sn02, * - 3.2 mol% Sn02, x - 6.25mol% Sn02, + - 9.1 mol% Sn02, • - 17mol % Sn02.
Slika 1: Katodna in anodna gostota naboja Q (mC/cm2) kot funkcija dodanega Sn02 in števila plasti; o - čisti Sn02, * - 3.2 moI% Sn02, X - 6.25mol% Sn02, + - 9.1 mol% Sn02, • -17mol% Sn02.
3 Results and discussion
Cyclic voltammetry vvas employed to investigate the useful-ness of Ce02 and Ce02/Sn02 dip-coated films as a counter electrode for electrochromic device for "smart vvindovv". This technique measures the capability of the films to reversibly intercalate and deintercalate mobile ions. In our vvork ali measurements vvere performed for Li+ ions. The dependence of cyclic voltammogram (CV) for Ce02 dip coated films prepared from different sols concentrations vvas studied. The obtained CV revealed that the electrochemical response depended on the concentration used. The peak current density of the films, prepared from the 7.2 10'3 mol and 9.6 10"3 mol in 20 ml (H20) starting sols, vvas very similar and much higher than for the film prepared from 4.8 10"3 mol in 20 ml H20. Chronocou-lometric measurements vvere also performed for Ce02 films prepared from the same concentration of sols vvith different number of dipped layers. Charge capacity (mC/cm2) refers to the amount of charge stored or extracted per cycle for an electrode and vvere recorded as a function of time (to 60 s) for each film. For three different concentrations it vvas found that charge density vvas in close connection vvith the concentration
Figure 2: Cylic voitammograms of Ce02 thin film (one layer) at 17mol%
added Sn02. l(fifth cycle), 2(400thcycle). Slika 2: Ciklični voltamogram tankega filma (ena plast) Ce02 s 17mol% dodanega Sn02. 1 (peti cikel), 2(400. cikel).
The best sample vvith only one layer, prepared bv addition of 17 mol% of Sn02 vvas tested for voltammetric stability. Four hundred svveep mode was applied and C V for initial and last cycle (400,h cycle) are presented on figure 2. The film after cycling looks the same, vvithout any visible changes.
K
"V, 2
Figure 3: X-ray diffraction patterns of Ce02 powder and thin dip-coatings films heated at 500°C: a) pure Ce02, b) Ce02/Sn02 =1, 1 -powder, 2-films.
Slika 3: Rentgenski diffaktogrami vzorcev, gretih na 500°C: a) čisti Ce02, b) Ce02/Sn02 =1, 1-prašek, 2-film.
X-ray diffraction measurement of powders and thin films, heated at 500°C, showed formations of crystaline cerianite. According to X-ray measurements (Figure 3), the addition of Sn02, did not influence the possition of peaks at 26= 28.6, 33.34, 47.5, and 56.5, only increased the broadening of diffraction lines with increased amount of Sn02, is observed.
Inml
Figure 4: Transmittance (UV-VIS-NIR) spectra of Ce02 with different concentrations of added Sn02: 1-3.2 mol% Sn02, 2-6.25. mol% Sn02, 3-9.1 mol% Sn02, 4-25 mol% Sn02, 5-33 mol% Sn02, 6-50 mol% Sn02.
Slika 4: Transmisijski (UV-VIS-NIR) spektri tankih filmov Ce02 pri različnih koncentracijah dodanega Sn02: 1-3.2 mol% SnOj, 2-6.25 mol % Sn02, 3-9.1 mol% Sn02, 4-25 mol% Sn02, 5-33 mol% Sn02, 6-50 mol% Sn02.
Transmission spectra of Ce02 and Ce02/Sn02 > 1 for one layer on ITO cover glass previously treated at 500°C are pre-sented in figure 4. The transmission cut-off in the spectral range at 30CM00 nm is observed. Spectra obtained for pure Ce02 thin films and by addition 17 mol% of Sn02 are very similar. Calculated šolar transmittance Ts for ali samples in the spectral range 0.3-2(im are high (Ts > 63%). In table 1, Ts for approximately the same thicknesses of applied pure Ce02 and Ce02 with 17 mol% of Sn02 are 20% lower than calculated Ts of pure ITO on the glass.
Table 1: Šolar transmitance Ts of Ce02 (EC1 -3) and Ce02 vvith 17 mol% of Sn02 (EC551-3) on ITO covered glass
5	AcknowIedgement
This work was supported by Ministry of Science and Techno-
logy, Republic of Slovenia, research contract Pl-5012-104-93.
6	Rcferences
1	C. A. Hampel: Cerium in the glass industrv, The Glass In-dustry, 41 (1960) 82-113.
2	K. Eguchi, T. Setoguchi, T. Inone and H. Azar: Electrical properties of ceria-based oxides and their application to oxide fuel cells, Solid State Ionics, 52 (1992) 165-172.
3	P. Baudry, A.C.M. Rodrigues and M.A. Aegerter: Dip-coated Ti02-Ce02 films as transparent caunter electrode for transmissive electrochromic device, J. Non-Cryst. Sol-ids, 121 (1990) 319-322.
4	U.Lavrenčič Štangar, B.Orel, I.Grabec and B.Ogorevc: Optical and electrochemical properties of Ce02 and Ce02-Ti02 coatings, Proc. SPIE, 1728-33, (1992).
5	Z.Crnjak Orel, B.Orel, M.Hodošček and V.Kavčič: Con-ductive SnO/Sb powder:preparation and optical properties, J. Mat. sci. 27 (1992) 313-318.
6	Z.Crnjak Orel, B.Orel and M.Klanjšek Gunde: Speetrallv selective Sn02:F film on glass and black enamelled steel substrate: Sprav pyrolitical deposition and optical properties, Šolar Energy matenals 26 (1992) 105-116.
7	U.Lavrenčič Štangar, B.Orel, Z.Crnjak Orel, P.Bukovec and M.Kosec: Optical and structural properties of Sn02:Sb gels and thin films prepared by dip-coating metod, Proc. SPIE, 1727(1992) 166-178.
Sample	thickness fnml	Ts 0.3-2.0pm
EC1	120	0.645
EC2	180	0.655
EC3	560	0.656
EC551	120	0.648
EC552	200	0.665
EC553	560	0.639
ITO	30	0.80
4 Conclusions
Aqueous-based process was successfuly used for the prepara-tion of Ce02 and Ce02/Sn02 dip-coatmg films for counter electrode. The best sample obtained at 17 mol% Sn02 has a very good possibility for the intercalation/deintercalation of Li+ ions. Cathodic and anodic charge density of lOmC and 22mC was obtained for one- and eight layers, respectively.