Nb2O5 coatings prepared by the sol-gel route using the dip coating technique with sols prepared from alkoxide or chloroalkoxide (two different methods: sonocatalytic and conventional) precursors are a promising alternative for WO3 electrochromic coatings. The crystalline films (TT phase) sintered at 560 and 600 degrees C are transparent and present a deep blue colour under Li+ ion insertion. Electrochemical stability is excellent as these systems are fully reversible and stable and no change in colour, amount of charge exchanged and corrosion effect could be observed after 2000 voltammetry cycles between 2 and - 1.8 V versus Ag. The electrochemical and optical properties of these coatings have been determined as a function of sintering temperature. The superficial structure was visualized by atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the Li+ diffusion coefficient was determined for both kinds of coatings

Aegerter, Michel A.

Avellaneda, César O.

Pawlicka, Agnieszka

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Two methods of obtaining sol gel Nb2O5 thin films for electrochromic devices

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Author to whom correspondence should be addressed.0022—2461  1998 Chapman & Hall 2181JOURNAL OF MATERIALS SCIENCE 33 (1998) 2181—2185Two methods of obtaining sol–gel Nb2O5 thin filmsfor electrochromic devicesC. O. AVELLANEDA, A. PAWLICKA*Departamento de Fı´sica e Cieˆncia dos Materiais, Instituto de Fisica de Sa o Carlos, and*Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica de Sa o Carlos, Universidade de Sa oPaulo, C.P. 780, CEP 13560-970 Sa o Carlos (SP) BrazilM. A. AEGERTERInstitut fu¨r Neue Materialien, Im Stadtwald, Geba¨ude 43, D 66-123 Saarbru¨cken, GermanyNb2O5 coatings prepared by the sol—gel route using the dip coating technique with solsprepared from alkoxide or chloroalkoxide (two different methods: sonocatalytic andconventional) precursors are a promising alternative for WO3 electrochromic coatings. Thecrystalline ﬁlms (TT phase) sintered at 560 and 600 °C are transparent and present a deepblue colour under Li ion insertion. Electrochemical stability is excellent as these systemsare fully reversible and stable and no change in colour, amount of charge exchanged andcorrosion effect could be observed after 2000 voltammetry cycles between 2 and !1.8 Vversus Ag. The electrochemical and optical properties of these coatings have beendetermined as a function of sintering temperature. The superﬁcial structure was visualizedby atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the Lidiffusion coefﬁcient was determined for both kinds of coatings. © 1998 Chapman & Hall1. IntroductionA wide class of materials, called chromogenic mater-ials, change their optical properties (transmission, ab-sorption, etc.) in response to a change in the ambientcondition. If their colours change under an appliedpotential in an electrochemical cell they are calledelectrochromicmaterials. This property is observed bya large number of metal oxides, such as TiO, WO,VOand NbO. These materials are interestingbecause of their use in various solid state electro-chemical devices, including batteries, and electrodesfor photoelectrochemical and electrochromic devices[1, 2]. The electrochemical properties are due to Hand Li insertion in the layer, which in the case ofTiO, WOand NbOcause changes in opticaltransmissivity from a quasi-transparent state (trans-mission ¹:85%) to less than ¹:20%.The electro-optical performances of electrochromiccoatings are strongly dependent on their structuralmorphology. Outstanding performances have recentlybeen obtained using materials constituted of aggre-gates of nanometre-sized particles of CeO—TiO[3—6], TiO[7] and NbO[8, 9]. The sol—gel processis a particularly well adapted method for achievingsuch morphology. Moreover the dip coating tech-nique, a common method of deposition in this ﬁeld,allows deposition of large area coatings at low cost.Recently some studies have reported on the elec-trochromic properties of NbO. Blue colourationwas found in an opaque NbOlayer grown thermallyat &500 °C on a niobiummetallic disk [10, 11]. Alves[12] has conﬁrmed the possibility of inserting Liions in a NbOceramic prepared from a commercialpowder sintered at &800 °C. The ﬁrst attempt tofabricate sol—gel NbOfor electrochemical purposeswas reported by Lee and Crayston [13], using a solmade of a mixture of NbCldissolved in ethanol.However, the 5—10 m thick ﬁlm presented substan-tial cracking and peeling due to an important shrink-age during drying. More recently, we have obtainedhomogeneous ﬁlms without cracks and defects thatpresent good and promising electrochromic properties[8, 9, 14].In this work we report on the electrochemical andmicroscopic characterization and electrochromicproperties of NbOsol—gel coatings prepared by twodiﬀerent sol—gel methods and deposited by a dip-coat-ing technique on indium-tin oxide ITO-coated glass.2. Experimental procedureTwo alkoxy methods were used to prepare a transpar-ent and stable niobium alkoxide solution. Sol 1 wasprepared by a sonocatalytic process [14, 15] and al-lowed coatings with good optical qualities and inter-esting electrochromic properties to be obtained. Thestarting solution to produce NbOﬁlms was pre-pared by dissolving NbClpowder (CBMM, Brazil)(1.3 g, 0.005 mol) in butanol (15 ml, 0.16 mol) andacetic acid (3 ml, 0.05 mol). The solution mixture wassubmitted for a few minutes to the action of ultrasonicirradiation (Sonicator W385 Heat System-Ultrasonic,Inc. at 20 kHz) resulting in a transparent and viscoussolution stable for several months at room temper-ature. The main precursors were probably chloroal-koxides of the type NbCl(OBu)[16].The second precursor, Sol 2, was prepared by syn-thesizing Nb pentabutoxide, Nb(OBu), by dissolvingNbCl(CBMM, Brazil) in butanol, to which was ad-ded sodium butoxide under reﬂux. The process led tothe formation of Nb(OBu)and NaCl. The last com-pound was separated by centrifugation. The ﬁnal solwas prepared by mixing this precursor with glacialacetic acid (CHCOOH) resulting in a sol stable at20 °C for several months [8, 9]. In this method we didnot use ultrasonic irradiation.The coatings were deposited on an ITO-coatedglass substrate (Asahi-Glass, 14  ) previouslycleaned and rinsed with bidistilled water, ethanol andthen dried at room temperature. The substrates weredipped into the solution in ambient atmosphere andwithdrawn at a rate of 12 cm min. The samples weresubsequently dried in air at room temperature for15 min. The uniform gel ﬁlms were then calcined attemperatures up to 600 °C using a heating rate of10 °C min. The resulting coatings were then kept atthe ﬁnal temperature from 10 min to 3 h and werefound to be transparent and homogeneous witha thickness of about 300 nm after three dips.The surface morphology of the ﬁlms was visualizedby AFM (Topometrix) and SEM (Zeiss 960) analyses.The electrochemical characteristics (voltammog-rams) were determined using a Solartron 1286 ana-lyser and a conventional three electrode cell placed ina dry-box under dry N. The counter-electrode wasa platinum foil of 1 cm area and the quasi-referenceelectrode was a silver wire. The electrolyte was a 0.1 Msolution of LiClOdissolved in a propylene carbonate(PC). The cell was previously purged with dry Ngas.The thickness of a ﬁlm, measured with a Taylor—Hobson Talystep was about 100 nm for a one-diplayer. Optical spectra in the ultraviolet—visual(u.v.—vis) range were recorded in situ with a Cary 2315spectrophotometer. The coated substrate was placedin a special electrochemical cell built with two ﬂatfused quartz windows and the u.v.—vis spectra weremeasured before and after insertion of Li cations atﬁxed potentials.X-ray diﬀraction measurements of xerogels ob-tained with the same sols and heat treated at 560 °Cfor 10 min following the same protocol, indicate thatthe powder material is crystalline, with a very-low-temperature form TT structure [14, 17].3. Results and discussionFig. 1a, b presents typical cyclic voltammograms ofcoatings obtained from Sols 1 and 2 measured ata speed of 50 mV s in the scan range #2 to!1.8 V. The increase of the current in the cathodicpotential sweep is associated with the Li insertionFigure 1 Typical cyclic voltammetry of an NbOﬁlm preparedfrom Sols 1(a) and 2(b), deposited on Asahi ITO-coated glass, heattreated at (- - - ) 600 and (——) 560 °C for 2—3 h in an Oatmo-sphere. Electrolyte (0.1 M) LiClO—PC, rate 50 mV s.process. After reversal of the potential scan at!0.95 V an anodic current peak is clearly observed.This peak is attributed to the extraction of Li. Aninteresting feature of this result is that the coatingsshow small diﬀerences when heated at diﬀerent tem-peratures. It was found that the NbOﬁlms preparedfrom Sol 1 were better when calcined at 600 °C(Fig. 1a) while those prepared from Sol 2 were betterwhen calcined at 560 °C (Fig. 1b). Both samples pre-pared in these conditions were very stable and success-ive cycles (tested for 2200 cycles for Sols 1 and 2)repeated the original current—potential curve (Fig. 2a,b). The charge exchanged during the insertion—extrac-tion processes decreased only very slightly from20 mC cm to 18 mC cm for Sol 1 and increasedfrom 13 to 15 mC cm for Sol 2 (Fig. 3). In all casesa cathodic scan to !1.8 V turned the ﬁlm dark blueand after an anodic scan to #2.0 V the ﬁlm againbecame transparent (slightly yellow).Fig. 4a,b shows typical cyclic voltammogramsregistered by a three-layer NbOﬁlm heat treatedat 600 and 560 °C for 2—3 h in an Oatmospherefor three diﬀerent scanning rates 50, 100 and200 mVs. The maximum value of the anodic peakincreases with scanning rate, and the quantity ofcharge inserted decreases from 19 (v"50 mV s)to 6.9 mC cm (v"200 mV s) for Sol 1 and from20 (v"50 mV s) to 8.8 mC cm (v"200 mV s)for Sol 2.2182Figure 2 Voltammograms of an NbOﬁlm as a function of num-ber of cronoamperometric cycles. NbOprepared from: (a) Sol1 (heat treated at 600 °C) and (b) Sol 2 (heat treated at 560 °C) for2—3 h in an Oatmosphere. Electrolyte (0.1 M) LiClO—PC, rate50 mV s . (——) 1°, (- - - ) 1100°.Figure 3 Cathodic (, ) and anodic (, ) charge density, Q asa function of the number of chronoamperometric cycles with poten-tial steps at every 10 s between !1.8 and 2 V. Closed symbols, Sol1; open symbols, Sol 2.The diﬀusion coeﬃcient, D, of Li ions in the elec-trolyte was calculated from the formula [18]D"i2.7210 nCvwhere D is the diﬀusion coeﬃcient in centimetressquared per second, Cthe concentration of active ionin solution (moles per cubic centimetre), v the sweepFigure 4 Voltammograms of NbOcoatings deposited on AsahiITO-coated glass heat treated at 560 and 600 °C for 2—3 h in an Oatmosphere. Electrolyte (0.1 M) LiClO—PC, scan rate (——) 50,(- - -) 100, (— — —) 200 mV s ; (a) Sol 1 and (b) Sol 2.rate (volts per second), n the number of Electron 1, andithe anodic peak of current density (amps per cen-timetre squared) that is read from the cyclic voltam-mograms (Fig. 1a, b). The values were for Sols 1 and 2,respectively, D"1.410	 and 3.610	 cm s.In both cases the values obtained were higher thanthose reported for WOﬁlms i.e. D"2.9310cm s [18], but similar to the value D"510 cm s reported by Mohapatra [19] ob-tained from potentiostatic measurements.Fig. 5a,b presents micrographs of ﬁlms of NbOprepared in the two ways and deposited on glass. Thestructure of the ﬁlm, Sol 1 (Sol 2 not presented here),visualized by the AFM picture, is formed by a uniformagglomeration of small particles with an average sizeof about 30—70 nm (Fig. 6). These particles are notvisible in the SEM micrographs, but they conﬁrm thatthe surface of the ﬁlms prepared by the two diﬀerentmethods have excellent microstructure without visiblecracks and defects.Fig. 7 shows transmission spectra of NbOﬁlmsprepared from Sols 1 and 2 and deposited onto an ITOelectrode. The u.v.—vis spectra were registered in situ inthe reduced and oxidized state for three-dip layers ata ﬁxed potential of #2.0 (bleached state) and !1.8 V(coloured state). The insertion of Li changes thetransmission in the visible—near infrared range fromabout 80 to less than 20% for &300 nm thick (three-dip) coatings prepared by the two diﬀerent routes(Sols 1 and 2).2183Figure 5 SEM micrographs of NbOﬁlms: (a) ﬁlm prepared from Sol 1 heat treated at 600 °C, and (b) ﬁlm prepared from Sol 2 heat treatedat 560 °C.Figure 6 AFM picture of a ﬁlm prepared from Sol 1 heat treated at 600 °C for 1 h. Imaging mode AFM super-tip topography (forward) withthree-dimensional perspective.Figure 7 U.v.—vis spectral variation observed during electroch-romic switching of thrice dipped NbOﬁlms of Sols 1 (——) and2 (- - -) before (a) and after (b) polarization at 1.8 V.4. ConclusionsWe have shown that ﬁlms of NbOprepared eitherby sonocatalytic or conventional methods are promis-ing candidates for electrochromic devices using lith-ium electrolytes. The powders calcined at 560—600 °Cin an Oatmosphere have a crystalline TT structure.The coatings present good, reversible and fast inser-tion—extraction for Li ions. AFM measurementsshow that the ﬁlms are formed by uniform agglomer-ation of nanometre-sized particles (20—70 nm). Thecharge density inserted at a voltammetric scanningrate of 50 mVs in the ﬁlms prepared from Sol1 (about 18—20 mC cm) is similar to the chargeinserted in the ﬁlm prepared from Sol 2 (15—20 mC cm). Small diﬀerences were found in thevalue of the Li diﬀusion coeﬃcient, where theD value from Sol 2 coatings was about 2.6 timessuperior than that for coatings prepared from Sol 1.However, the sonocatalytic route was easier andquicker in preparing sols than the classic method andthis route avoided the possibility of Na residue.AcknowledgementsThe authors are indebted to Dr Ulf Werner (INM-Saarbru¨cken) for assistance with the AFM measurements2184and acknolwedge ﬁnancial support from FAPESPand CNPq (Brazil).References1. M. A. AEGERTER, in ‘‘Sol—gel chromogenic materials anddevices, structure and bonding,’’ Vol. 85, edited by R. Reisfeldand C. K. Jorgensen (Springer Verlag, Berlin, 1996) p. 149.2. C. M. LAMPERT and C. G. GRANQUIST (eds), ‘‘Large-areachromogenics: materials and devices for transmittance con-trol,’’ Vol. IS4, (SPIE, Bellingham, WA, 1990) p. 2.3. P. BAUDRY, A. C. M. RODRIGUES, M. A. AEGERTERand L. O. S. BULHO ES, J. Non-Cryst. Solids 121 (1990) 319.4. J . C. L. TONAZZI, B. VALLA, M. A. MACE DO, P.BAUDRY and M. A. AEGERTER, in ‘‘Proceedings on sol—geloptics,’’ Vol. 1328, edited by J. D. Mackenzie (SPIE, Belling-ham, WA, 1990) p. 375.5. M. A. MACE DO, L. H. DALL’ANTONIA and M. A.AEGERTER, in ‘‘Proceedings on sol—gel optics II’’, Vol. 1758,edited by J. D.Mackenzie (SPIE, Bellingham,WA, 1992) p. 320.6. D. KE OMANY, J . -P. PETIT and D. DEROO, in ‘‘Proceed-ings on optical material technology for energy eﬃciency andsolar energy conversion XIII’’, Vol. 2255, edited by V. Wittwer,C. G. Granquist and C. M. Lampert (SPIE, Bellingham, WA,1994) p. 363.7. A. HAGFELDT, N. VLACHOPOULOS and M. GRA TZEL,J. Electrochem. Soc. 141 (1994) L82.8. C. O. A. AVELLANEDA, M. A. MACE DO, A. O. FLOREN-TINO and M. A. AEGERTER, in ‘‘Proceedings on opticalmaterials technology for energy eﬃciency and solar-energyconversion XIII,’’ Vol. 2255, edited by V. Wittwer, C. G.Granquist and C. M. Lampert (SPIE, Bellingham, WA, 1994)p. 38.9. C. O. AVELLANEDA, M. A. MACE DO, A. O. FLOREN-TINO, D. A. BARROS FILHO and M. A. AEGERTER, in‘‘Proceedings on sol—gel optics III’’, Vol. 2288, edited by J. D.Mackenzie (SPIE, Bellingham, WA, 1994) p. 422.10. B. REICHMAN and A. J. BARD, J. Electrochem. Soc. 127(1980) 241.11. M. A. B. GOMES, L. O. S . BULHO ES, S .C. CASTRO andA. J . DAMIA O, ibid. 137 (1990) 3067.12. M. C. ALVES, MSc thesis, Federal University of Sa o Carlos,Sa o Carlos, Brazil (1989).13. G. R. LEE and J. A. CRAYSTON, J. Mater. Chem. 1 (1991)381.14. A. PAWLICKA, M. ATIK and M. A. AEGERTER, J. Mater.Sci. ¸ett. 14 (1995) 1568.15. M. ATIK and J. ZARZYCKI, ibid. 13 (1994) 1301.16. G. R. LEE and J . A. CRAYSTON, J. Chem. Soc. Dalton ¹rans.(1991) 3073.17. E. I . KO and J . G. WEISMANN, Catalysis ¹oday 8 (1990)27.18. M. A . HABIB and D. GLUECK, Solar Energy Mater. 18(1989) 127.19. S. K. MOHAPATRA, J. Electrochem. Soc. 125 (1978) 145.20. SPIE The International Society for Optical Engineering.Received 13 September 1996and accepted 5 December 19972185

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Two methods of obtaining sol gel Nb2O5 thin films for electrochromic devices

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