Fe203 sol-gel derived optical coatings for electrochromic device Fe203 filmi pripravljeni po sol-gel postopku M. Maček, F.ŠvegI and B. Orel, National Institute of Chemistry, Hajdrihova 19, P.O.Box 30, 61115 Ljubljana, Slovenia Thin iron oxide films (69 nm-1500 nm) were deposited by dip-coating methodfrom iron ion containing sols which have been made from a FeCl} 6Hf) precursor precipitated with ammonium hydroxide. Homogeneous sols were obtained after peptization of precipitates with addition of glacial acetic acid (60 mol %) and no organics (ethylenglycol, glycerol ethyl acetylacetate etc.) were added in order to adjust sol viscosity for depositing the xerogelfilm. In situ UV-VIS spectro-electrochemical measurements revealed that films heat-treated in the temperature range 200°C-350°C exhibited intensive bleaching in 0.001M LiOH, KOH and NaOH electrolytes at cathodic potential (-1.45 V) and became coloured again at anodicpotential (0.6V). Electrochromism has not been obser\>ed for films heat-treated at 500° C when a -Fef)} (hematite) is for med. Key words: Iron (III) okside, electrochromism, infrared spectra, in situ spectroelectrochemistry Filme železovega (III) oksida smo pripravili po sol-gel postopku. Kot izhodno spojino (precursor) za pripravo sola smo uporabili FeCl3 6H20, ki smo ga raztopili v vodi. Pri dodajanju (NHJaq (25%) tej raztopini je prišlo do tvorbe oborine, ki smo jo po večkratnem spiranju (10X) peptizirali z led ocetno kislino (60 mol%). Tako pripravljenemu solu nismo dodali organskih snovi (etileng-likol, glicerol, etil acetilacetat...) za spreminjanje viskoznosti. In situ UV-VIS spektroelektrokemi-jske meritve kažejo, da se filmi žgani v temperaturnem območju 200°C-350°C razbarvajo v 0.001 M LiOH. NaOH, KOH pri potencialu -1.45Vin postanejo ponovno obarvani pri potencialu 0.6V. Filmi žgani na 500°C niso elektrokromni. Pri tej temperaturi namreč že pride do tvorbe hematita a -Fe20 j. Ključne besede: Železov (III) oksid, elektrokromizem, infrardeči spektri, in situ spektroelektro-kemija 1 Introduction Iron oxide belongs to the transition metal oxides and is consideredasapromisingelectrochromicmaterial'. Burke and Lyons 2 first reported that the thick oxide layer grovvn on iron substrate by potential cycling in alkaline medium exhibits an electrochromic transition. The colour of the layer changes from transparent at negative potential to yellow-green at positive potential. With the help of potential modulated reflectance spectroscopic measurements theyfoundthat passivative layeris composedofamixture of a-FeOOH, y -Fe203 (maghemite) and a-Fe203 (hematite). Iron hydrous oxides FeOOH, y -Fe203 and a -Fe2033 have been the subject of countless studies. Conventional preparation methods lack the concept of designing the metal oxide structure at atomic or molecular level. The sol-gel route 4 has recently attracted considerable attention because it has the potential to design the structure of the final compound at the very beginning of the process, i. e. in initial solution. When the substrate (usually glass) is slowly pulled out of the sol the meniscus is formed at the substrate/sol interface. Due to the evaporation of the solvent, the species vvhich are present in the sol start to react. Condensation reactions betvveen sol particles produce the formation of gel. Duringa final stage ofdrying, as the liquid-vapour meniscus recedes into the film mterior, the film collapses and a compact xerogel film is formed. Further heat treatment of the xerogel films (200°C-500°C) produces oxide films with different degree of crystallinity and chemical composition. 2 Experimental Instrumental FT-IR spectroscopic measurements were performed in the spectral range on Digilab FTS-80 FT-IR spectrometer equipped vvith near-normal and variable angle reflection cells (Barnes) for obtaining reflection-absorption spectra (IR RAS) and near-grazing incidence angle FT-IR spectra. VIS and NIR spectra of filnis vvere recorded on Perkin Elnier LAMBDA9 spectrophotometerwithresohition2nm. Thickness measurements vvere performed on a Surface Profiler Alfa Step 200. Cyclic voltammetric and clironocoulometric experiments vvere performed vvith an EG and G PAR model 273 computer controlled potentiostat-galvanostat, driven by a model 270 Electrochemical Analysis softvvare. Single-scan or multi-scan cycling and chronocoulometric measurements vvere performed vvithin and at potentials +0.6 V and -1.45 V, respectively. In situ UV-VIS spectroelectrocheinical measurements vvere performed on a HP 8451A diode array spectrophotom-eter vvith EG and G PAR model 2644A polarographic analyser. The coatings vvere made on the dip coating unit vvith pulling speed of 1 cm/nun. Preparation of Sols, Gels and Coatings Iron oxide vvas prepared from a FeCl, 6H20 precursor vvhich vvas dissolved m deionised vvater (2x) (10 g of FeCl3 6H20 m 100 g H20). 11 ml ( p = 0.91 g/cm3) of 25 % (NH,) vvas added dropvvise into the solution until pH ~ 9 vvasreacfied. Peptization vvasattained by glacialaceticacid (up to 60 mol.%). After filtering and sonification, the viscous sols as formed, vvere used immediately for making films by dip-coating procedure. The corresponding oxides and films vvere obtauied by heat-treatment of xerogels and xerogel films at 300°C and 500°C, respectively. 3 Results and discussion FT-IR Spectroscopy Films. The FT-IR reflection-absorption (IR RAS) spectra of films prepared by heat-treatment of xerogel films at 300°C and 500°C are shovvn in Figure 1.Strong bands at 520 cm" ' and 432 cm"' characterise the phonon spectra of films heat-treated at 300°C. Further heating ofthe same film to 500°C did not change the position ofthe bands at 520 cm"1 and 432 cm1, except that the vvidth of the 520 cm"1 band is decreased. Characteristic bands of films appear at frequencies vvhich nearly coincide vvith the TO modes ofbulk (monocrystalline) a-Fe203, i.e. 520cm1and432cm1,respectively. Thismeans that in the čase of films the po larisation charge effect does not influence the band position, thus confirming that they consist of particles vvhich are too small to exhibit surface modes 6. Similar to the TO modes, it revealed that also LO modes at 675 cm1, 491 cm1 and 371 cm"1 areonly slightly shifted in relation to the LO modes of the bulk a -Fe,03 (Figure2). Beside the position of LO and TO modes also their vvidths strongly support the assumption that films do not contain large agglomerated particles making the film heter- ogeneous. For this reason it is likely that TO phonon mode broadening in the spectra of films heat-treated at 300°C may be considered of the lovver degree of order in the distribution of iron ions m the structure of the film. This feature is characteristic of the y -Fe203 phase vvhich is formed at 250°C-300°C vvith regard to the more ordered a -Fe20, phase obtained at 500°C. Further confirmation vvas obtained from the corresponding LO spectra (Figure 2) vvhich even more clearly exhibited phonon mode broadening. 2500 2000 1500 wavenumbers(cm"1) Figure t: FT-IR reflection-absorption spectra (IR RAS) of Fe203 film (TO modes) prepared at 300°C (a) and 500°C (b- a Fe203). Slika 1: FT-IR refleksijsko-absorbančna spectra (IR RAS) Fe203 filma žganega na 300°C (a) in 500°C (b- a Fe203) -1000 3500 3000 2500 2000 1500 1000 500 wavenumbers (cm*1) Figure 2: NGIA- FT-IR spectra of Fe203 films (LO modes) prepared at 500°C (a-a-Fe203), 300°C (b) and 200°C (c). ♦indicates LO modes of Al-O modes (c.f. -A1203) formed during the temperature- treatment. Slika 2: NGIA-FT-IR spektri Fe,0, filmov (longitudinalna optična nihanja) žganih na 500°C (a-a-Fe2Os), 300°C (b). 200 °C (c). * označuje longitudinalna optična nihanja AI-O vezi. (pri žganju nastane A1,03) Electrochemical Measurements Chronocoulometric measurements showed that the corre-sponding Q were large, being in the range of 10-25 mC/ cm2 and depend on film thickness.(Figure 3) Finallv, when Q was expressed per ran of film thickness it was found that maximal Q/d values were about 0.2 mC/cm2nm. The results of the in situ UV-VIS spectro-electroche-nncal measurements are depicted in Figure4. The most salient feature was a decrease in the absorbance ( A A) in the UV spectral range ( < 400 nm) which appeared at cathodic potential (U = -1.5 V). When voltage was changed in anodic direction, the films became coloured again. The maximal absorption change appears in the spectral range 300 nm-390 nm (Figure 4). This corresponds well to the results obtained by Gutierrez and Beden7. We are not able to confirm that our electrocliromic films also consist of FeOOH because, in addition to OH groups, they also con ta in acetate groups. These groups considerably contribute to the electrochromism of the films, as vvas observed even for films heat- treated at quite low temper-atures ( 135°C). The films retain their electrochromic properties until the temperature of heat-treatment exceeds 350°C, when a -Fe,03 is formed. 10 - _ "s 5 t CJ U 0 |-♦-t-t-t- •=• ' 100 200 300 400 a "b thickness/nm -10 + -15 - V -25--- Figure 3: Cathodic and anodic total charge density (Q) of Fe203 films heat-treated at 300°C as function of film thickness: (a) cathodic scan, (b) anodic scan, electrolyte 10"3 M LiOH, charging potential -1 45V (a) and 0.6V (b) Slika 3: Katodna in anodna celotna gostota naboja (Q) Fe203 filmov (žganih na 300°C ) v odvisnosti od debeline filmov: (a) katodna smer, (b) anodna smer ; elektrolit: 0.001 M LiOH , potencial: -1.45V(a) in 0.6V (b) At the present state of research vve are not able to give ananswer concerning the mechanism which is responsible for the electrochromism of the films. Nevertheless, it is likelv that one of the mechanisms already proposed for the r °J68e5 A/ 4Ds°"t>, Jll Figure 4: Spectroelectrochemical response (cathodic scan-bleaching) of Fe203 film prepared at 300°C (thickness 69 nm) in 0.1 M LiOH. Slika 4: Spekroelektrokemijski odziv (katodna smer-razbarvanje) Fe203 filma zažganega na 300°C (debelina 69nm) v 0.1M LiOH colouring and bleaching of nickel oxide films may also be responsible for electrochromic properties of iron oxide films. 4 Conclusions Thin iron oxide films vvere deposited by dip-coating tech-nique from aqueous colloidal solutions made froma FeCl3 6H20 precursor and precipitated with ammoniuin hydrox-ide. The viscosity of the sol vvas adjusted by the addition of acetic acid which acts as a peptising agent. The films were prepared by heating the xerogel films at various temperatures (135°C, 200°C, 300°C, 500°C). Films prepared at 500°C correspond to a -Fe203 and are not electrochromics. Films prepared by heat-treatment in the temperature range 135°C-350°C exhibit reversible electrochromism. They bleach at cathodic and colour at anodic potential. Electrocromic films very likely consist ofmixed y -Fe,03 and ferrihydrite phases with OH- and acetate inclu-sions. 5 References 1 C M. Lampert, Sol. Energy Mat., 11 (1984) 1. 2 L.D. Burke and M E.G. Lyons, J. Electroanal. Chem., 198 (1986) 347. 3 P.S. Sidhu, Cleays and Clay Minerals, 36 (1988) 31. 4 C J. Brinker, A.J. Hurd, G.C. Frye, P.R. Schunk, C.S. Ashley, in: Chemical Processing of Advanced Materials, Eds. Z. Hensh and J.K. West,John Wiley and Sons, New York, 1992, p. 395. 5 N. Takahashi, N. Kakuta, A. Ueno, K. Yamaguchi, T. Fuji, T. Mizushuna, Y. Udagawa, J. Mat. Sci. 26 (1991) 497. 6 H Xingfang, C. Xiaofeng and M G. Hutchins, SPIE, 1728(1992)73. 7 C. Gutierrez, B. Beden, J. Electroanal. Chem.. 293 (1990) 253.