Besides the photochromic glasses based on silver halides especially the cuprous halide containing materials are of practical importance. Due to their high absorption coefficient the cuprous halides are particularly suited for the preparation of thin photochromic films.
Using the sol-gel technique silicon alkoxide based coating solutions with continuously distributed photoactive components were prepared. From these sols single or multilayer coatings were produced by dipping. The precipitation of the cuprous halide microcrystals as carriers of the photochromic properties of the coatings proceeded during a subsequent heat treatment (temperature range of 100 to 800 °C). Thus it was possible for the first time to produee photochromic coatings with a pronounced darkening and with complete reversibility of the photochromic reactions at room temperature

Bürger, Herbert

Fachet, Regina

Kriltz, Antje

Müller, Matthias

English

Repositorium für Naturwissenschaften und Technik

Original PaperSol-gel derived coatings with completely reversible photochromismRegina Fachet, IVIatthias Müller and Herbert BürgerOtto-Schott-Institut für Glaschennie, Friedrieh-Schiller-Universität, Jena (Gernnany)Antje KriltzInstitut für Physikalische Chemie, Friedrieh-Schiller-Universität, Jena (Germany)Besides the photochromic glasses based on silver halides especially the cuprous halide containing materials are of practical import-ance. Due to their high absorption coefficient the cuprous halides are particularly suited for the preparation of thin photochromicfilms.Using the sol-gel technique Silicon alkoxide based coating Solutions with continuously distributed photoactive components wereprepared. From these sols single or multilayer coatings were produced by dipping. The preeipitation of the cuprous halide microcrys-tals as carriers of the photochromic properties of the coatings proceeded during a subsequent heat treatment (temperature ränge of100 to 800 °C). Thus it was possible for the first time to produee photochromic coatings with a pronounced darkening and withcomplete reversibility of the photochromic reactions at room temperature.Vollständig reversible Fotochromie in Sol-Gel-SchichtenNeben den fotochromen Gläsern auf Silberhalogenidbasis besitzen die silberfreien, kupferhalogenid-enthaltenden Gläser besondereBedeutung. Aufgrund des höheren Absorpdonskoeffizienten der Kupferkolloide sind solche Materialien zur Herstellung dünnerfotochromer Schichten gut geeignet.Mit Hilfe der Sol-Gel-Technik wurden Beschichtungslösungen auf Siliziumalkoxidbasis mit homogen verteilten fotoaktiven Komponenten erhalten. In den daraus hergestellten Tauchschichten erfolgte die Abscheidung der Kupferhalogenidkristalle während derWärmebehandlung im Temperaturbereich von 100 bis 800 °C. Auf diese Weise gelang es erstmals, tief eindunkelnde Materialien mitvollständiger Reversibilität des fotochromen Prozesses bei Raumtemperatur zu erzeugen.1. IntroductionIn contrast to the manufactur ing of mol ten photo-chromic glasses, the prepara t ion via sol-gel process is a fairly new field of research. The investigations concen-trate on the preeipitation of noble metal halide crystalsin an oxide glass. In this way materials can be createdwhich show a certain darkening under the influence ofactinic radiation.The greatest problem up to now was the control ofthe fading reacüons. So far, the fading of materials, e.g.the restoration of the initial State, was possible only byan addit ional temperature t reatment [1 to 3]. In this publication we present a way to produee photochromic coat-ings via sol-gel process with completely reversible photochromic reactions at r oom temperature.Eingegangen am 29 February 2000.Presented in German at: 73rd Annual Meedng of the GermanSociety of Glass Technology (DGG) on 2 June 1999 in Halle(Germany).P h o t o c h r o m i s m is defined as a l ight induced, revers-ible change of the absorp t ion State of a mate r ia l . T h e r eare var ious classes of substances showing p h o t o c h r o m i cpropert ies . T h e m o s t i m p o r t a n t ones are, o n the o n eh a n d , organic Compounds hke spiropyranes , ace tani l idesor fulgides [4 to 6]. O n the o ther h a n d , there a re thenanocrys ta l s embedded in an inorganic mat r ix . A m o n gthem, the noble meta l hahdes are of special i m p o r t a n c e .T h e p r e p a r a t i o n of such mater ials is poss ible b o t h bymel t ing a n d by sol-gel process. T h e sol-gel process showssome specific advantages [7 to 9], above all t he lowersynthesis tempera tures , connected wi th smal ler evapora t ion losses of the photoac t ive substances. F u r t h e r m o r e ,the sol-gel route ahows a wider Va r i a t i on of the c o m p o -sit ion of the basic glass a n d makes it easier to modi fythe mater ia l propert ies.Because of the comparat ively low process t e m p e r a -tures, n o stabilization of the pho toac t ive subs tances inthe System is necessary and so they can be i n c o r p o r a t e din to the sol-gel mater ia ls in considerably h igher concen------Variationofrw> Γι. pHSi-alkoxidessolventwateracidAl-alkoxidesormocers®h a l i d e c o n n p o u n d sCu^^^^-conripoundsc o a c t i v a t o r ssoldip-coatingwet coatingheat t reatment at 100 8 0 0 ° C , (N2)coating withC u X - microcrystalsFigure  1 . General preparation pattern of photochromic copperhalide containing coatings; R^: molar ratio of water to alkoxide,ΓΙ'. molar ratio of solvent to alkoxide.t ra t ions . For this reasons, the sol-gel process is suited forthe addi t iona l surface modif icat ion wi th th in coatings,whereas Systems with cuprous hal ide crystals exhibit a pa r t i cu la r suitability [10 a n d 11]. Thei r h igher absorp-t ion coefficients c o m p a r e d to silver hal ides lead to a pro-n o u n c e d gradient in darkening . T h e subsurface layersshow a very s t rong darken ing a n d pro tec t deeper areasfrom U V light. Therefore, it should be possible usingc o p p e r hal ide conta in ing mater ia ls to achieve a deepda rken ing at comparat ively low film thicknesses.In o rder to possess p h o t o c h r o m i c propert ies , a mate-rial ha s to fulfil special demands , such as an inert matrixa r o u n d the p h o t o c h r o m i c crystals which shows no reac-t ion wi th the p roduc t s of the p h o t o d a r k e n i n g . The crys-tal size should a m o u n t from to 10 ... 50 n m in diametera n d the crystals should be d isordered, e.g. by implantingof foreign ions or by forming of mixed crystals.2. ExperimentalFigure 1 shows the general pa t t e rn of the synthesis ofcup rous hal ide conta in ing coat ings via sol-gel process.T h e basic compos i t ion consists of par t ia l ly subsdtutedSilicon alkoxides in a suited solvent. T h e hydrolysis isacid-catalyzed. T h e sol compos i t ion , especially thea m o u n t s of water a n d solvent, as well as the p H valuecan be widely varied.To modify the film propert ies , it is possible to addo the r alkoxides as well as inorganic-organic hybrid poly-mers , the so-called O R M O C E R S ® . T h e copper comEcc1 0 0 - r - L7 5 -5 0 -2 5 -150 2 0 0 250 Temperature in °C  Figure 2. Diameter of preeipitated photochromic crystals as a function of the temperature of heat treatment.pounds may be introduced either in form of complexesor directly as dissolved copper(I) or copper(II) Com-pounds. Besides chlorine- and bromine-subst i tuted organic substances we used hydrochloric and hydrobromicacid and their mineral salts, respeetively. Afterwards theaddit ion of coact ivadng agents is carried out.F rom the sol received in this way single or multilayercoatings on various Substrates can be prepared. Α following heat t reatment in a temperature ränge between100 and 800 C is necessary to obtain cuprous halidemicrocrystal conta ining photochromic coatings.3. Results and discussionΑ better handl ing of the sols and coatings as well as a Controlling of the photochromic properties requires thatthe formation of crystals in the layers proeeeds no t during the synthesis but only during the subsequent heatt reatment .Figure 2 shows investigations of the crystallizationin cuprous halide containing dip-coatings. The crystaldiameter, calculated from X-ray diffraction pat tern, wasplot ted versus the temperature of heat t reatment . It canbe seen that the crystal preeipitation Starts as desiredonly at heating temperatures of more than 150°C. Atfirst, the crystals grow slowly with increasing tempera-ture. Wi th further temperature rise, evaporat ion losseslead to a cont inuous reduction in the crystal size. Besidesthe crystal size, the a m o u n t of preeipitated cuprous halide influences the photochromic properties. Figure 3 pre-sents the areas of the 100 % peaks of the X-ray pat ternsas a function of the temperature of heat t reatment . Αpreeipitat ion m a x i m u m was found at a temperature ofabout 250 C. With increasing temperature a growingsurface opalescence is to be observed.— —--° --° -03<1 0 0 1 5 0 2 0 0 2 5 0T e m p e r a t u r e In C  3 0 0Figure 3. Amount of preeipitated copper halide as a functionof the temperature of heat treatment.ΕcοοοΕcοο n m 1 0 0 0 0 n m 2 0 0 0 0 n mFigure 4. AFM of the surface texture of a copper halide containing coating without coactivators, heat-treated at 150 C.Besides electron microscopy, the atomic force mi-croscopy is especially suited for the characterization ofthe coating surfaces. Figure  4 shows an atomic forcemicrograph ( A F M ) of the surface texture of a copperhalide containing dip-coating which was prepared froma sol without coactivators. The sample was heat-treatedat 150°C for 10 min, e.g. in the iniüal stage of the crystalpreeipitation. The micrograph shows the photochromiccrystals embedded in a silica matrix. The crystals ju t outfrom the coating surfaee to about 200 nm. The averagecrystal size amounts to about 70 nm. It can be seen thatin these eondit ions the matrix content is large comparedwith the content of crystals.With rising temperature of heat t reatment , the crystalpreeipitation increases, as already stated for the X-raydiffraedon measurements. In figure 5, an atomic forcemicrograph is presented for a sample which was heat-0 n m 10 0 0 0 nm 2 0 0 0 0 nmFigure 5. AFM of the surface texture of a copper halide containing coating without coactivators, heat-treated at 200 °C.0 nm- η •1000 nm 2000 nmFigure 6. AFM of the habit of a monocrystal sample withoutcoactivators, heat-treated at 200 C.t rea ted at 200 C for 10 min. It demons t ra t e s tha t b o t hcrystal size a n d a m o u n t are distinctly increasing wi th ris-ing tempera ture . Besides the spherical g rowth the crys-tals exhibit an increasing six-fold s y m m e t r y In figure 6 the habi t of a monocrys ta l (same sample as in figure 5)is presented .A d d i n g small a m o u n t s of coact ivat ing agents d u r i n gthe sol synthesis, we found s t rong influences o n the sur-face crystall ization. The crystal sizes of coact iva ted a n dnoncoac t iva ted samples differ only slightly, bu t we o bserved  a s t rong increase in the degree of crystal l ini ty of03ΦQ .° —--° ° ° -0 n m 10 000 n m 20 000 n mFigure 7. AFM of the surface texture of a sample with lowcontent of coactivators, heat-treated at 200 °C.0 n m 10 000 n m 20 000 n mFigure 8. AFM of the surface texture of a sample with doubledcontent of coactivators, heat-treated at 200 °C.the coat ing surface of coact ivated samples. Figure 7 shows the a tomic force micrograph of a copper halidesample with a low conten t of coact ivat ing agents. Thecoated Substrate was heat- t reated at 200 C for 10 min.After the heat t rea tment the coat ing surface is nearlydensely crystallized, only at high magn i tudes small par t sof the mat r ix are perceptible. Us ing h igher amounts ofcoact ivat ing agents, the coat ing surfaces are completelycovered with pho toch romic crystals after the heating.This is shown in figure 8 for a sample wi th doubled co-35000 30000 25000 20000Wave number in cm i 15000Figure 9. Darkening as function of Irradiation dme of a samplewith doubled content of coactivators.activator content . The crystals grow more and morespirally and have one or more growth centers. Matr ixpar ts a round the crystals are not detectable.The investigation of the photochromic propert ies ofthe coatings proved that they are a function of bo th theheat t reatment condi t ions and the content of coactivat-ing agents. Figure 9 illustrates the darkening behaviourof a coating with a higher coactivator content . Thesample was heated up to 150°C and kept at this temperature for 10 min. U n d e r the influence of actinic radi-ation electron-hole pairs, so called excitons, are formedin the cuprous halide crystals. In the absorbance spectrathe bands at 25660 and 24390 c m  ^ are caused by theabsorpt ion of these excitons. The U V Irradiation leadsto the formation of copper coUoids on the crystal sur-face, which are responsible for the characteristic colourof the darkened samples. The absorpt ion bands of thecoUoids are detected at 20370 and 16480 c m - ^ The pre-sented spectra show a strong dependence of the absorbance of the coatings on the Irradiation time. Theabsorbance time behaviour resembles that of normalphotochromic glasses. In the ease of the describedsample the transmission lowering amoun t s to about to45 % after 5 min of irradiation and to more than 50 % after 15 min.For the first ü m e to our knowledge the characteriz-ation of the fading behaviour of the developed materialsshows a complete fading at room temperature wi thoutany addi t ional temperature treatment or t rea tment withoxidizing agents.Figure 10 illustrates the transmission behaviour ofthe described sample dur ing one irradiation cycle. As described above, the coatings show a fast darkening reac-tion under the effect of UV-irradiation. The so calledhalf-darkening time amounts to about 60 s. After a 15 min irradiat ion the coatings reach transmission val-ues of less than 5 %. Tha t corresponds to a t ransmissiondecrease of more than 50 % compared with the initialState. The fading process Starts at room temperature im- — -"--° --cοCOCOΕCOcCO15Tinne in nninFigure 10. Photochromic behaviour of a sample with doubledcontent of coactivators during one irradiation cycle.mediately after the switch off of the irradiation. Already5 min after s topping the U V i r radiaüon, the sampleshows a noticeable fading, in the case of the describedsample this makes about 10 % increase in transmission.Α complete fading can be reached after about 10 h.The described photochromic propert ies of thesamples are reproducible, the manyfold r epeüüon of thedarkening and fading processes did no t show any signifi-cant changes in the photochromic behaviour.4. SummaryUsing the new materials, we succeeded for the first timein producing photochromic coatings via sol-gel processwith complete reversibility of the photochromic reac-tions at room temperature. The carriers of the photo-chromic properties are cuprous halide microcrystals pre-eipitated preferably on the coat ing surfaces. As de-manded , the preeipitation of crystals proeeeds only dur-ing a heat treatment. Both crystallization behaviour andphotochromic properties depend on the content of coac-tivating agents.U n d e r U V ir radia t ion, the da rken ing of the coa t ingsa m o u n t s to a b o u t 45 % for the first 5 m i n a n d t o m o r et h a n 50 % for the first 15 min . T h e half d a r k e n i n g t imea m o u n t s to a b o u t 60 s.After the switch off of the i r radia t ion, the samples fadeal ready at r o o m tempera ture , tha t m e a n s w i t h o u t anyadd i t iona l hea t t rea tment . In the first 5 m i n the fadinga m o u n t s to abou t 10 %. Α comple te fading takes p lacewi th in a few hours .We wish to thank the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, for supporting this work.5. References[1] Kriltz, Α.; Müller, M.; Fachet, R. et al.: Cuprous halidemicrocrystals containing photochromic sol-gel-glass. J. SolGel Sei. Technol. 11 (1998) p. 197-201.[2] Fachet, R.; Müller, M.; Bürger, H. et al.: Photochromicmaterials via sol-gel process. Glastech. Ber. Glass Sei.Technol. 71 (1998) no. 8, 243-246.[3] Mennig, M.; Fink, C ; Schmidt, H.: Α sol-gel derived AgClphotochromic coating on glass. Bol. Soc. Esp. Ceram. Vid.31-C(1992) p. 281-286.[4] Granqvist, C. G : Chromogenic materials for transmittancecontrol of large-area W i n d o w s . Grit. Rev. Solid State Mater.Sei. 16 (5) (1990) p. 291 308.[5] Levy, D.; del Monte, F.; Lopez-Amo, M. et al.: Opticallytunable fiber optie delay generator utilizing photochromicdoped sol-gel gel-glass delay line. J. Appl. Phys. 77 (1995)no. 6, p. 2804-2805.[6] Hou, L.; Mennig, M.; Schmidt, H.: An organic-inorganichybrid glass hosting photochromic dyes. Chim. Chron. 23(1994) nos. 2 and 3, p. 175-180.[7] Hench, L. L.; West, J. K.: The sol-gel process. Chem. Rev.90 (1990) p. 33 -72 .[8] Sakka, S.: Glasses and glass-ceramics from gels. J. Non-Cryst. Solids 73 (1985) p. 651-660.[9] Dislich, H.: Glassy and crystalline Systems from gels:Chemical basis and technical application. J. Non-Cryst.Solids 57 (1983) p. 371-388.[10] Müller, M.; Reiß, H.: Zur induzierten Absorption in foto-chromen Gläsern. Jenaer Rundsch. 35 (1990) p. 3 8 - 3 9 .[11] Kriltz, Α.: Untersuchungen zur Koaktivierung silberfreierfotochromer Gläser. Universität Jena, PhD thesis 1989.Addresses of the authors:R. Fachet, M. Müller, H. BürgerOtto-SchoU-Institut f GlaschemieFriedrieh-Schiller-UniversitätFraunhoferstraße 6 D-07743 JenaA. KriltzInstitut f Physikalische ChemieFriedrieh-Schiller-UniversitätLessingstraße 10D-07743 Jena--

Sol-gel derived coatings with completely reversible photochromism

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Sol-gel derived coatings with completely reversible photochromism

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Repositorium für Naturwissenschaften und Technik