Available online at www.sciencedirect.com

2008) 957–966www.elsevier.com/locate/tsf

Thin Solid Films 516 (

Natural and persistent superhydrophilicity of SiO2/TiO2

and TiO2/SiO2 bi-layer films

S. Permpoon a,b, M. Houmard a,b, D. Riassetto b, L. Rapenne b, G. Berthomé a,B. Baroux a, J.C. Joud a, M. Langlet b,⁎

a Laboratoire de Thermodynamique et de Physico-Chimie Métallurgique, ENSEEG-INPG, BP 75, Domaine Universitaire, 38402 Saint Martin d'Hères, Franceb Laboratoire de Matériaux et de Génie Physique, ENSPG-INPG-MINATEC, 3 parvis Louis Néel, BP 257, 38016 Grenoble Cedex 1, France

Received 11 July 2006; received in revised form 19 April 2007; accepted 4 June 2007Available online 13 June 2007

Abstract

Sol–gel SiO2/TiO2 and TiO2/SiO2 bi-layer films have been deposited from a polymeric SiO2 solution and either a polymeric TiO2 mothersolution (MS) or a derived TiO2 crystalline suspension (CS). The chemical and structural properties of MS and CS bi-layer films heat-treated at500 °C have been investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electronmicroscospy. Water contact angle measurements show that MS SiO2/TiO2 and CS TiO2/SiO2 bi-layer films exhibit a natural superhydrophilicity,but cannot maintain a zero contact angle for a long time over film aging. In contrast, CS SiO2/TiO2 bi-layer films exhibit a natural, persistent, andregenerable superhydrophilicity without the need of UV light. Superhydrophilic properties of bi-layer films are discussed with respect to the natureof the TiO2 single-layer component and arrangement of the bi-layer structure, i.e. TiO2 underlayer or overlayer.© 2007 Elsevier B.V. All rights reserved.

Keywords: Superhydrophilicity; Sol–gel thin films; SiO2–TiO2 system; Self-cleaning applications

1. Introduction

It is well known that the photo-induced hydrophilicityof titanium oxide, preferentially in its anatase polymorphicform, confers a self-cleaning functionality to TiO2 surfaces. Thisbehaviour ensues from surface oxygen vacancies (O2⁎), which arecreated through an oxydo-reduction of TiO2 (Ti

4++e−→Ti3+ and2O2−+2h+→O2⁎) induced by photo-generated electron (e

−)/hole(p+) pairs. Surface oxygen vacancies can then be saturated by OHgroups, through a molecular or dissociative adsorption of atmo-spheric water, which yields a superhydrophilic surface, i.e. asurface showing a water contact angle of zero [1,2]. However, thephoto-induced superhydrophilicity does not persist in time in theabsence of UV radiation, which limits its field of applicationbecause in real conditions surfaces are not permanently exposedto UV.

A large number of articles have reported that the additionof SiO2 into TiO2 films enhances the photo-induced super-

⁎ Corresponding author. Tel.: +33 4 56 52 93 22; fax: +33 4 56 52 93 01.E-mail address: Michel.Langlet@inpg.fr (M. Langlet).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.06.005

hydrophilicity, which can be maintained for a certain time in theabsence ofUVradiation [3–8]. The effects of SiO2 addition on thephoto-induced hydrophilicity of TiO2 films have been studied[5,9,10]. Published works suggest that the improved hydrophi-licity of SiO2–TiO2 composite films ensues from an enhancedacidity of Si–O–Ti bonds at the SiO2–TiO2 interfaces, whichwould induce a greater amount of hydroxyl groups at the filmsurface. Several models have been proposed to describe acidity ofSiO2–TiO2 composites, which attribute acidity to the chargeimbalance developed along Si–O–Ti heterolinkages owing to thedifference in the coordination geometry of Si4+ and Ti4+ cations[11–14]. Lewis and/or Bronsted acid sites are thus formed.

Some authors have considered interface interactions inSiO2–TiO2 bi-layer films with respect to the local structures,either on SiO2 or TiO2 overlayers [12,15–17]. Through X-rayphotoelectron spectroscopy (XPS) experiments, Sanz et al.evidenced the formation of cross-linking Ti–O–Si bonds at theTiO2–SiO2 planar interface, which led to significant core levelshifts and changes in the electronic structure of a SiO2 outersurface [15]. Gao and Wachs reported a Raman study on TiO2–SiO2 bi-layer films that confirmed the formation of Ti–O–Si

958 S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

bonds at the planar interface, which were shown to be similar tothose evidenced for SiO2–TiO2 composite films [12]. Otherauthors also compared the interactions at SiO2–TiO2 planarinterfaces for SiO2 and TiO2 overlayers [16,17]. They proposeda similar description for both interfaces, which were formed bythe cross-linking of Ti–O–Si bonds, as determined by angularXPS [16] and Electron Energy Loss Spectroscopy (EELS) [17].All these articles did not report on the hydrophilic properties ofbi-layer films.

More recently, a sustained superhydrophilicity has been re-ported when a TiO2 film is thoroughly covered by a SiO2

overlayer [18,19]. Guan et al. mentioned an enhanced amountof OH groups at the SiO2 outer surface and so-formed Si–OHsurface bonds were thought to be more stable than Ti–OHsurface bonds, which increased in-time persistence of thesuperhydrophilicity [18]. However, Hattori et al. indicated thatthe rate of photo-wettability leading to superhydrophilicitydecreased with increasing the thickness of a SiO2 overlayer[19]. Tada et al. also indicated that a SiOx coated TiO2 filmshowed improved photocatalytic activity only in the case of avery thin SiOx monolayer [20]. This enhanced activity wasattributed to an increase in the electrostatic attraction of adsor-bents at the outer surface, which ensued from a SiO2–TiO2

interfacial charge transfer.In a previous article, we have indicated that sol–gel derived

SiO2–TiO2 composite films could show a natural and persistentsuperhydrophilicity, i.e. a water contact angle of zero without UVradiation [21]. This property had never been reported before forsuch films. According to previous literature, we tentativelyattributed this natural superhydrophilicity to an enhanced acidityat SiO2–TiO2 granular interfaces. We were then encouraged totest whether this unusual property could exist in SiO2–TiO2 bi-layer films. In this work, sol–gel derived bi-layer films depositedfrom two kinds of TiO2 sols have been investigated. The filmhydrophilic properties and their in-time persistence are studiedand discussed with respect to the nature of TiO2 films andarrangement of the bi-layer structures, i.e. SiO2 or TiO2 overlayer.

2. Experimental details

2.1. Sol and film preparations

Sol–gel derived films were deposited from silica andtitania sols. A SiO2 polymeric sol was prepared by dilutingtetraethoxysilane (TEOS) in absolute ethanol, deionized water,and hydrochloric acid (HCl), according to a previously publishedprocedure [22]. A concentrated sol was first prepared with aTEOS concentration of 2.35 M, a H2O/TEOS molar ratio of 2.2,and a pH of 3.5. This solution was aged at 60 °C for 2 days. Then,it was diluted in additional absolute ethanol to get a final TEOSconcentration of 1.5M. TiO2 filmswere deposited from two kindsof TiO2 sols which were prepared using two different sol–gelroutes. A classical method yielded a polymeric mother solution(MS), which was prepared by mixing tetraisopropyl orthotitanate(TIPT) with deionized water, hydrochloric acid, and absoluteethanol as a solvent [23]. TIPT concentration in the solution was0.4 M, and the TIPT/H2O/HCl molar composition was 1/0.82/

0.13. The solutionwas aged at room temperature for 2 days beforedeposition. The second method relied on the preparation of acrystalline suspension (CS) of TiO2 nano-crystallites in absoluteethanol [24]. This suspension was prepared from the mothersolution using a multistep procedure. The mother solution wasfirstly diluted in an excess of deionized water (H2O/TIPT molarratio of 90) and then autoclaved at 130 °C for 6 h. Autoclavingyielded the crystallization of TiO2 particles in the liquid phase. Anexchange procedure was then performed in order to removewaterfrom the sol and to form a crystalline suspension in absoluteethanol. The final TiO2 concentration in ethanol was 0.24 M. Formore data, the whole procedure has been described in a previouspaper [24]. The final sol was composed of TiO2 nano-crystallitesof about 6 nm in diameter. Previous works showed that SiO2 aswell as MS and CS TiO2 sols were very stable, which indicatedthat no gelation took place in polymeric SiO2 and TiO2 (MS) sols,while no significant crystal aggregation took place in CS TiO2

sols. Consequently, all these sols could be used for several weeksin reproducible film deposition conditions.

Single- and bi-layer films were deposited at room tempera-ture on (100) silicon wafers by spin-coating (300 μL of each sol,spin speed of 3000 rpm). Prior to deposition, the substrates wereultrasonically cleaned with ethanol for 3 min, then rinsed withdistilled water, and dried with air spray. Single-layer SiO2 andTiO2 films were first deposited and heat-treated at 500 °C for 2 h.Some single-layer films were studied as-prepared. Bi-layer filmswere produced through the subsequent deposition of a comple-mentary SiO2 or TiO2 single-layer component followed byadditional heat-treatment at 500 °C for 2 h. For comparison,intermediary and final heat-treatments at 110 °C were alsopunctually tested. Heat-treatments were performed in air and thesamples were directly introduced in the pre-heated oven. Afterheat-treatment, the films were cooled to room temperature underambient condition. Hereafter, structures composed of a TiO2

(SiO2) layer deposited on a SiO2 (TiO2) layer are denoted asTiO2/SiO2 (SiO2/TiO2) bi-layer films. In this study, the thicknessof SiO2 and TiO2 components were fixed at around 200 and40 nm, respectively, as shown by ellipsometric measurementsperformed on single-layer films. Some deviations from thesedata will be discussed in Section 3.4.

2.2. Characterizations

The films were characterized by Fourier transform infrared(FTIR) transmission spectroscopy in the range of 4000–250 cm−1 with a resolution of 4 cm−1 using a Bio-Rad FTS-165 spectrometer. Spectra corresponding to 300 scans wererecorded in room atmosphere after purging the measurementchamber with dry air for 15 min. The spectra were analyzedafter subtraction of the bare substrate spectrum. Surface analysiswas performed by XPS using a XR3E2 apparatus from VacuumGenerator employing an Mg Kα source (1253.6 eV). The X-raysource was operated at 15 kV for a current of 20 mA. Beforecollecting data, the samples were put in equilibrium for 24 h inan ultra high vacuum chamber (10−10 mbar). Photoelectronswere collected by a hemispherical analyzer at 30° take-offangle. All spectra were calibrated with C1s peak at 284.7 eV.

Fig. 1. Water contact angle variations vs aging time in the absence of UVradiation for TiO2 (a), SiO2 (b), TiO2/SiO2 (c), and SiO2/TiO2 films (d) heat-treated at 500 °C. TiO2-derived single- and bi-layer films were deposited from aMS sol.

Fig. 2. Water contact angle variations vs aging time in the absence of UVradiation for TiO2 (a), SiO2 (b), TiO2/SiO2 (c), and SiO2/TiO2 films (d) heat-treated at 500 °C, and for a SiO2/TiO2 film heat-treated at 110 °C (e). TiO2-derived single- and bi-layer films were deposited from a CS sol.

959S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

Transmission electron microscope (TEM) studies wereperformed on film cross-sections. For these studies, cross-sectional samples were first thinned down to 5–10 μm by tripodpolishing. Then, thinning was completed by argon ion-millingto obtain a very thin preparation compatible with electrontransmission. In order to avoid irradiation damage during ionthinning, a PIPS-GATAN ion-milling apparatus was used withlow angle (6°) and low voltage (2.5–3 kV). High resolutionTEM (HRTEM) and energy filtered TEM (EFTEM) studieswere performed with JEOL-2010 LaB6 and JEOL-2010 FEFinstruments, respectively, both operating at 200 keV. EFTEMimages were extracted from EELS spectra using a ‘three-window’method that allowed extrapolating the EELS spectrumbackground, for background correction, and filtering a smallenergy window of 10 eV around the Si–L2,3 peak at 108.5 eV.This method yielded EFTEM images that allowed a mapping ofsilicon through the film cross-section.

Surface hydrophilicity of the films was quantified from watercontact angle measurements. Experiments were performed at20 °C in an environmental chamber using a KRUSS G 10goniometer connected with a video camera. Several water drop-lets of 0.5 μL volume were spread on the samples and watercontact angles were measured at different points of the thin filmsurface for statistical purpose. The effects of film aging on thehydrophilic properties were analyzed using a same statisticalprocedure. During aging, the films were stored in a dark place, insuch a way that aging effects only traduced natural hydrophilicproperties of the films, i.e. did not traduce any photo-inducedeffects. In this work, no exposure to UV was performed.

3. Results and discussions

3.1. Hydrophilicity of single- and bi-layer films

The hydrophilic properties of SiO2/TiO2 and TiO2/SiO2 bi-layer films were studied with respect to aging time. Figs. 1 and 2depict in-time variations over aging of the natural water contactangle, i.e. water contact angle measured without UV irradiation,for bi-layer films deposited from MS and CS TiO2 sols, res-pectively. Contact angle variations are also compared to those

measured on SiO2 and TiO2 single-layer films. Films illustrated inFigs. 1a–d and 2a–d were heat-treated at 500 °C, while the bi-layer film illustrated in Fig. 2e was heat-treated at 110 °C.We firstpay attention to films heated at 500 °C. It is observed that freshlyprepared MS and CS TiO2 single-layer films (Figs. 1a and 2a)present hydrophilic properties comparable to those of a SiO2 film(Figs. 1b and 2b). These films exhibit a natural contact angle ofaround 15–20°. After 8 weeks aging in the dark, the water contactangle of both MS and CS TiO2 films has increased by about 35–40°. In contrast, the SiO2 film exhibits a slow contact angleincrease over the same aging period, which illustrates the naturalhydrophilicity of a silica surface and the greater stability of Si–OH surface bonds. TiO2/SiO2 bi-layer films (TiO2 overlayer)exhibit an initial water contact angle of 6° and 0° for MS (Fig. 1c)and CS (Fig. 2c) films, respectively. These values, which aremuch weaker than those measured on TiO2 and SiO2 single-layerfilms, indicate a natural superhydrophilicity that can be related tothe existence of planar TiO2–SiO2 interfaces. As explained inIntroduction, such interfaces are probably characterized by Si–O–Ti heterolinkages. However, the natural superhydrophilicity ofTiO2/SiO2 bi-layer films does not persist in time, i.e. the contactangle of MS and CS TiO2/SiO2 films increases by about 35–40°over an aging period of 8 weeks. Both MS (Fig. 1d) and CS(Fig. 2d) SiO2/TiO2 bi-layer films (SiO2 overlayer) initially showa natural superhydrophilicitywith awater contact angle of zero. Incontrast to TiO2/SiO2 films, this property can maintain for acertain aging period. Natural superhydrophilicity of the SiO2/TiO2 (MS) film persists for a period of 2 weeks. After that, thecontact angle is observed to slowly increase with time, reaching avalue of 13° after aging for 8 weeks. Persistence of the naturalsuperhydrophilicity is much longer for the SiO2/TiO2 (CS) film,since no contact angle increase is observed over an aging period of8 weeks. Here again, these properties can be related to theexistence of a SiO2–TiO2 interface. However, our data indicatethat this interface promotes a better property when the bi-layer isconstituted of a SiO2 overlayer rather than a TiO2 one. Suchnatural and persistent superhydrophilic properties of SiO2/TiO2

bi-layer films have never been reported before.These unusual properties can first be associated to the greater

natural hydrophilicity of a SiO2 surface. However, two arguments

Fig. 3. Variations of the relative intensity of the O–H component, as deducedfrom deconvolution of the XPS O1s peak, for various films deposited from MS(▪) and CS (□) sols and subsequently heat-treated at 500 °C. Insert shows anexample of deconvolution of the O1s peak for a SiO2 film.

960 S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

indicate that superhydrophilic properties of SiO2/TiO2 bi-layerfilms do not entirely rely on the intrinsic properties of theSiO2 overlayer. Firstly, a pure SiO2 film does not exhibit anysuperhydrophilicity. Secondly, for a same structure, a SiO2/TiO2

(MS) film cannot maintain its natural superhydrophilicity for along time, while a SiO2/TiO2 (CS) film exhibits a much morepersistent superhydrophilicity. This difference not only confirmsthat superhydrophilic properties are related to interface effects butalso shows that nature of the TiO2 underlayer influences theseeffects. Let us recall that CS TiO2 films are deposited from asuspension of nano-crystallites. As-deposited CS films are thuswell crystallized, while a heat-treatment at around 500 °C isnecessary to reach a similar crystallization degree in MS films.Previous studies showed that physico-structural properties of CSTiO2 films did not vary significantly with heat-treatment tem-perature up to 500 °C [25]. Thus, a heat-treatment was performedat 110 °C in order to test the wettability properties of a SiO2/TiO2

(CS) bi-layer film processed at low temperature. Fig. 2e showsthat this film exhibits a natural water contact of 65°, which slowlyincreases over aging. It appears therefore that, contrary to a sameSiO2/TiO2 film heat-treated at 500 °C, this film does not presentany natural hydrophilicity. As explained above, such a discrep-ancy cannot be related to any differences in physico-structuralproperties of TiO2 components heat-treated at 110 or 500 °C. It istherefore possible that the formation of a SiO2–TiO2 interfaceable to promote enhanced surface hydrophilicity requires asufficiently high temperature heat-treatment. Such a treatmentwould in turn influence the formation of Si–O–Ti heterolinkagesat the planar interface. Furthermore, Fig. 2b and e show that, for asame SiO2 outer surface, a SiO2/TiO2 film heat-treated at 110 °Cis much less hydrophilic than a SiO2 single-layer film heat-treatedat 500 °C. Alkoxy groups arising from the silicon precursor arestill present in SiO2 films heat-treated at 110 °C, while they arecompletely decomposed after heat-treatment at 500 °C [22]. It cantherefore be inferred that the presence of alkoxy groups stronglyaffect the natural hydrophilicity of a SiO2 surface. Thus, in thefollowing parts we will focus on films heat-treated at 500 °C.

To summarize, wettability measurements suggest that theoccurrence and persistence of natural superhydrophilicity rely ontwo separate aspects. On the one hand, occurrence of a naturalsuperhydrophilicity seems to be governed by the presence andnature (MS or CS) of a SiO2–TiO2 interface. On the other hand, itis interesting to note that all single- and bi-layer films with anouter TiO2 layer lose their hydrophilic properties with acomparable rate, i.e. the contact angle increases by about 35–40° over an aging period of 8 weeks, while for single- and bi-layerfilms with an outer SiO2 layer, the contact angle increases by 15°or less over the same aging period. It suggests that the rate ofhydrophilicity loss follows two kinds of distinct behavioursdepending on the outer surface nature, irrespective of the presenceor not of an interface. Best properties of a SiO2/TiO2 CS filmwould therefore arise from a combination of interface and outerlayer effects. However, even such a film was observed to lose itssuperhydrophilicity after a prolonged aging period. A contactangle value of 5° was measured after 12 weeks aging. This filmwas then submitted to aspersionwith cold (20 °C) deionizedwaterfor 1 min. The contact angle of 0° measured after aspersion

showed that this film could easily recover its natural super-hydrophilicity, which was again observed to persist over anadditional aging period. This behaviour is meaningful since itindicates that the superhydrophilicity of SiO2/TiO2 CS films canbe maintained for a long period of time through a simple periodicwater rinsing. Such a property allows thus envisaging a long-termself-cleaning functionality without the presence of UV radiation.In contrast, this behaviour could not be observed on a SiO2/TiO2

MS film. After a same aging period of 12 weeks, this filmexhibited a contact angle of 16°.Water aspersion yielded a contactangle of 5°, but this angle was observed to increase again im-mediately after aspersion, reaching a value of 16° after furtheraging for 1 week. These data do not mean that, in the latter case,superhydrophilicity cannot be regenerated in more adapted con-ditions, but they confirm that the TiO2 underlayer nature stronglyinfluences the superhydrophilic properties of SiO2/TiO2 bi-layerfilms, best properties being reached from a crystalline suspensionapproach.

3.2. XPS characterization

XPS is a suitable method traditionally used to study surfacechemical properties, which can provide insights in surfacemechanisms yielding a natural superhydrophilicity. In this work,XPS was used for investigating the surface chemical state offreshly deposited single- and bi-layer films, i.e. the films werestudied within the first 24 h following deposition. As mentionedin the Introduction, the hydrophilic properties of SiO2 and TiO2

surfaces rely on the presence of surface OH groups. Thus, wefirst paid attention to these species, which were studied from adeconvolution of the O1s peak located at around 533 and 530 eVfor SiO2 and TiO2 outer surfaces, respectively. The O1s peakcould be decomposed in two components, i.e. O–H and Si–O(or Ti–O) components, using 10% Lorentzian/Gaussian func-tions (see an illustration in insert of Fig. 3). The O–H componentessentially traduces the presence of surface hydroxyl groups.Other possible C–O and H2O components were not accountedfor owing to their very weak intensity. Fig. 3 shows the relativeintensity of the OH component, normalized with respect to thetotal O1s peak intensity, for single- and bi-layer films heat-

Fig. 4. Variations of the C1s peak relative intensity for various films depositedfrom MS (▪) and CS (□) sols and subsequently heat-treated at 500 °C.

Fig. 5. FTIR spectra in the low wavenumber region for TiO2 (a), SiO2 (b), TiO2/SiO2 (c), and SiO2/TiO2 films (d). TiO2-derived films were deposited from a CSsol. All films were heat-treated at 500 °C. Spectra were collected before (dottedlines) and after (continuous lines) aging in the dark for 1 month. Insert shows amagnified view of the 1000–750 cm−1 region for a freshly deposited SiO2/TiO2

film.

961S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

treated at 500 °C. The surface hydroxyl amount is similarly high(35% relative intensity of the O–H component) for the SiO2

single-layer and SiO2/TiO2 bi-layer films, irrespective of theMSor CS TiO2 component nature, while it is similarly much weaker(12% relative intensity) for the TiO2 single-layer and MS or CSTiO2/SiO2 bi-layer films. On the one hand, the greater amount ofsurface OH groups illustrates the fact that a silica surface isnaturally more hydrophilic than a titania one. On the other hand,data of Fig. 3 do not depict any differences in the amount ofsurface OH groups for a SiO2 (TiO2) single-layer film and SiO2/TiO2 (TiO2/SiO2) bi-layer films, which might be correlatedwith differences in their hydrophilic properties, as illustrated inFigs. 1 and 2.

It is known that carbon contamination present at the outersurface can drastically alter the film hydrophilicity. Thus, wealso paid attention to the C1s XPS peak (284.7 eV). Intensity ofthis peak, normalized with respect to the total intensity of O1sand Ti2p (or Si2p) peaks, is presented in Fig. 4 for the same filmsas those illustrated in Fig. 3. Since previous studies have shownthat alkoxy groups, which might contribute to the C1s peak, arenot present in SiO2 and TiO2 films heated at 500 °C [21,22], it isinferred that this peak essentially reflects the amount of carboncontamination at the outer surfaces. Fig. 4 indicates that a TiO2

single-layer and TiO2/SiO2 bi-layer films (MS or CS) suffer asimilarly high contamination, while this contamination is muchweaker and comparable for a SiO2 single-layer and SiO2/TiO2

bi-layer films (MS or CS). Thus, a clear correlation can beobserved between data illustrated in Figs. 3 and 4, i.e. comparedto carbon contamination, the amount of hydroxyl groupsexactly follows reverse trends with respect to film nature. Onthe one hand, this correlation traduces that carbon contaminationis a limiting factor for hydrophilic properties and/or morehydrophilic surface are less prone to be contaminated than lesshydrophilic ones. On the other hand, it confirms that the amountof surface OH groups and the level of carbon contaminationdepicted by XPS are not correlated at all with the water contactangles measured on fresh films.

Let us recall that the films were put in equilibrium for 24 h inultra high vacuum before collecting XPS data. The vacuumatmosphere can influence the surface chemical properties bypromoting partial carbon and water desorption, which in turnrelies on the respective surface reactivity and affinity of each

sample toward carbon and water. Thus, XPS data do notnecessarily correspond to the actual surface properties andcannot strictly be correlated to hydrophilic properties of thefilms, which are measured in ambient atmosphere. It is thereforeinferred that XPS data more presumably traduce the ability ofthe surface to retain OH groups rather than the actual OHamount. Accordingly, a comparison between Figs. 1 2 and 3shows that there is a close correlation between the rate ofhydrophilicity loss, which is related to a lack of retention ofsurface OH groups, and the OH amount depicted by XPS data.All films with a weak and similar surface OH amount (single-and bi-layers with an outer TiO2 layer) lose their hydrophilicproperties with a fast and comparable rate, while films with agreater and similar surface OH amount (single- and bi-layerswith an outer SiO2 layer) do not show any contact anglevariation or follow a very slow rate of contact angle increase. Insummary, data illustrated in Figs. 1–4 seem to confirm that therate of hydrophilicity loss is governed by the nature of theoverlayer, i.e. the superhydrophilicity persistence relies on thepresence of a SiO2 overlayer, irrespective of the presence of aninterface. This SiO2 overlayer influence can be related to theweaker sensitivity of silica surfaces to carbon contamination,compared to titania surfaces, and to the greater stability of Si–OH surface groups compared to Ti–OH surface groups.

3.3. FTIR characterization

According to previous arguments, it is believed that XPSdata do not traduce actual surface chemical states of our films.For this reason, we performed complementary studies usingFTIR spectroscopy. Since FTIR spectra are collected in ambientatmosphere, they should be less influenced by characterizationconditions than XPS data. However, atmospheric water canadsorb at the film surface during spectrum acquisition, whichcan bother the final interpretation of OH absorption bands.Thus, in order to minimize water adsorption, the measurementchamber was systematically purged with dry air for 15 minbefore collecting IR data. FTIR studies were performed on a

Fig. 6. FTIR spectra in the hydroxyl region for TiO2 (a), SiO2 (b), TiO2/SiO2 (c),and SiO2/TiO2 films (d). TiO2-derived films were deposited from a CS sol. Allfilms were heat-treated at 500 °C. Spectra were collected before (dotted lines)and after (continuous lines) aging in the dark for 1 month.

962 S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

SiO2 single-layer film and CS-derived single- and bi-layer filmsheat-treated at 500 °C, before and after aging in the dark for onemonth. As shown in Fig. 5, in the low wavenumber region, i.e.between 1200 and 250 cm−1, single- and bi-layer film spectraexhibit the typical absorption bands of Si–O–Si bonds in silica(1075, 800, and 440 cm−1) and Ti–O–Ti bonds in anatase (425and 260 cm−1). A detailed assignment of these bands has beenmade in our previous article [21]. Fig. 5 shows that silica bandsslightly increase in intensity over aging, this feature beingclearly observed for the most intense band at 1075 cm−1, whileno variation of anatase bands can be observed. These featureswill be discussed below. Aweak band is also observed at around920 cm−1 for the SiO2 film and the bi-layer films (see anillustration in insert of Fig. 5 for a freshly deposited SiO2/TiO2

film). This band is not detected for a TiO2 single-layer film, andcan be attributed to a very small amount of Si–OH groups in theSiO2 component and/or Si–O–Ti heterolinkages at the SiO2–TiO2 interface of bi-layer films [26,27]. However, owing to itsweak intensity, no reliable conclusion could be drawn about theexact assignment of this band and no significant variation couldbe evidenced with aging.

IR absorption spectra in the hydroxyl region (3800–3200 cm−1) are illustrated in Fig. 6. Following observations

Fig. 7. Cross-section TEM images of SiO2/TiO2 (CS) (a), SiO2/TiO2 (M

can be done for freshly deposited films. In the case of a TiO2

film (Fig. 6a), no signal is detected showing not only that thefilm is free of any detectable Ti–OH or water species within itsthickness, but also that the amount of surface OH groups, whichmight account for the moderate hydrophilicity of this film(Fig. 2a), is below the sensitivity threshold of our spectrometer.The IR spectra of SiO2-derived single- and bi-layer films show abroad absorption band between 3800 and 3200 cm−1. For silicasamples, it is well known that this band arises from the over-lapping of stretching vibrations corresponding to different kindsof hydroxyl groups [14,26]. Basically, this band is composed ofthree regions: i/ a large component with an absorption max-imum centered around 3500 cm−1, which is commonly associa-ted to absorbed free water and to Si–OH groups (silanols)linked to molecular water through hydrogen bonds, the lattergiving rise to a component located at higher wavenumbers thanthe former, ii/ an intermediary region around 3650 cm−1, wherepairs of silanols mutually linked through hydrogen bonds aredepicted, and iii/ a high wavenumber peak or shoulder at around3740 cm−1, which corresponds to isolated surface silanols.Literature indicates that similar hydroxyl bands are observedfor titania samples, bands corresponding to free or linked Ti–OH groups being slightly shifted toward lower wavenumberscompared to silica [14,28]. The OH band depicted for a SiO2

single-layer film (Fig. 6b) and a TiO2/SiO2 bi-layer film(Fig. 6c) shows a local maximum at around 3500 cm−1 and apronounced high wavenumber shoulder at around 3740 cm−1,which indicate the presence of water and Si–OH groups. Thesespectra are very similar in their intensity and shape, whichsuggests that both films contain a comparable amount of waterand OH groups. This observation contrasts with XPS dataillustrated in Fig. 3 for same films, which depicted a greateramount of surface OH groups for the SiO2 single-layer. Besides,it is not possible to correlate infrared OH bands with hydro-philicity differences depicted for these films in Fig. 2b and c.However, it should be mentioned that, since spectra are col-lected in transmission, OH bands evidenced by FTIR spec-troscopy do not necessarily correspond to species present at thefilm surface. It cannot be excluded that OH species are presentin the thickness of a SiO2 film. In particular, the presence of

S) (b), and TiO2/SiO2 (CS) (c) bi-layer films heat-treated at 500 °C.

Fig. 8. Cross-section HRTEM images showing the SiO2–TiO2 interfacial region of SiO2/TiO2 (CS) (a), SiO2/TiO2 (MS) (b), and TiO2/SiO2 (CS) (c) bi-layer films heat-treated at 500 °C.

963S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

silanol species would ensue from the incomplete polyconden-sation of hydrolyzed alkoxy groups in the silica network. Thesimilarity between spectra of SiO2 and TiO2/SiO2 films mighttherefore express that the signal of eventual surface OH speciesis very weak and OH bands essentially depict residual waterand/or Si–OH groups present in the film thickness, which arecomparable for both SiO2 film components. Compared to thespectra of SiO2 and TiO2/SiO2 films, that of a SiO2/TiO2 bi-layer film (Fig. 6d) exhibits a more intense OH band, thisdifference being particularly marked in the water region around3500 cm−1. Since the SiO2 single-layer film and the outer SiO2

layer of the SiO2/TiO2 bi-layer film underwent the same heat-treatment at 500 °C for 2 h, it is inferred that the content in waterand Si–OH groups within thickness is comparable for bothfilms. It can therefore be concluded that differences depicted inthe spectra of SiO2 and SiO2/TiO2 films illustrate a much greateramount of surface OH species for the latter, which can in turn berelated to its natural superhydrophilicity, as illustrated inFig. 2d.

These conclusions are supported by FTIR characterizationsperformed on the same films after 1 month aging in the dark.

Fig. 9. HRTEM (a) and EFTEM images (b) of a same CS SiO2/TiO2 cro

The TiO2 film spectrum (Fig. 6a) does not reveal any evolutionover aging, i.e. no OH species are detected in the aged filmspectrum. In contrast, spectra of the SiO2 (Fig. 6b) and TiO2/SiO2 films (Fig. 6c) illustrate significant modifications. Thesilanol-related high wavenumber shoulder is less pronouncedafter aging, while the free water component appears to becomparatively more intense. Such changes possibly traduce adehydroxylation mechanism according to the condensationreaction: Si–OH+Si–OH→Si–O–Si+H2O. This condensa-tion reaction can in turn explain a slight increase of the Si–O–Siband intensities observed in the spectra of aged SiO2 and TiO2/SiO2 films (Fig. 5b and c). However, as previously mentioned, itis difficult for these films to infer on the respective contributionof surface species and species present within the thickness. Thecondensation reaction was also evidenced for the aged SiO2/TiO2 bi-layer film (Fig. 5d). Besides, Fig. 6d indicates anoticeable intensity decrease of the OH band after aging thisfilm for 1 month. This decrease is appreciated in the whole OH(water and silanol) region and the intensity decrease in thesilanol region is more marked than for aged SiO2 and TiO2/SiO2

films. However, this greater decrease does not necessarily imply

ss-sectional area. The CS SiO2/TiO2 film was heat-treated at 500 °C.

964 S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

that the aged SiO2/TiO2 film loses its hydrophilicity faster thanthe other films, but more presumably traduces that this filminitially possesses a much greater amount of surface OH groups,whose variations with aging can thus be appreciated. Accord-ingly, the OH band of the SiO2/TiO2 film aged for 1 month isstill more intense than that depicted for aged SiO2 and TiO2/SiO2 films. This observation probably traduces a greaterpersistence of surface OH groups that can in turn be related tothe persistent superhydrophilicity of this film, as depicted inFig. 2d.

3.4. Structural characterization of SiO2–TiO2 interfaces

Previous XPS and FTIR studies provided complementaryinformation on surface chemical states that could be correlatedwith the hydrophilic properties of bi-layer films. XPS suggeststhat a SiO2 outer surface promotes a greater in-time stability ofsurface OH species, which governs the persistence of hydrophilicproperties. FTIR indicates that, compared to a SiO2 single-layer, aCSTiO2 underlayer induces a greater amount of OH species at thesurface of a SiO2 overlayer, which yields enhanced super-hydrophilic properties of CS SiO2/TiO2 bi-layer films. In thissection, we present structural studies that were performed byTEM to understand how the TiO2 underlayer nature can influenceouter SiO2 surface properties. Fig. 7 shows cross-section imagesof CS SiO2/TiO2, MS SiO2/TiO2, and CS TiO2/SiO2 bi-layerfilms heat-treated at 500 °C. Cross-section of the TiO2 underlayeror overlayer is clearly observed for the three bi-layer films. Astatistical image analysis showed that, forMSSiO2/TiO2 (Fig. 7b)and CS TiO2/SiO2 (Fig. 7c) samples, the TiO2 componentthickness was about 40 nm, which confirmed ellipsometricmeasurements performed on single-layer TiO2 films deposited onsilicon. In contrast, the TiO2 thickness of around 60 nm deducedfrom TEM images for the CS SiO2/TiO2 film was about 30%greater (Fig. 7a). Besides, cross-section images of both formerfilms indicate a marked SiO2–TiO2 interface, while this interfaceis more diffuse for the latter film. When considering brightnesscontrasts with the SiO2 film component, cross-section of the TiO2

film component appears significantly darker for both former bi-layer films than for the latter. These observations are confirmed byHRTEM images presented in Fig. 8, which showmagnified areasof SiO2–TiO2 interfacial regions for the three bi-layer films.Provided that the overall ion-thinned sample thickness iscomparable, which is assessed in Figs. 7 and 8 by a similarbrightness of the SiO2 component for the three bi-layer films,brightness contrasts of HRTEM images are very sensitive toatomic weighs of the elements (Z-contrast). Since the molecularweigh of TiO2 is greater than that of SiO2, the TiO2 film cross-sections logically appear much darker than the SiO2 ones.However, brightness contrasts evidenced in Figs. 7 and 8 wouldindicate that the TiO2 component has a weaker weigh density, i.e.is more porous, in the case of a CS SiO2/TiO2 film (Figs. 7a and8a) than for MS SiO2/TiO2 (Figs. 7b and 8b) and CS TiO2/SiO2

films (Figs. 7c and 8c). Furthermore, bright regions observed inTEM images across the TiO2 film thicknesses, might account forinternal porosity and are much more accentuated for the CS SiO2/TiO2 film.

In a previous paper, we have indicated that CS and MS TiO2

films undergo a thermally activated densification process duringheat-treatment at 500 °C and CS TiO2 films heat-treated at500 °C exhibit a significantly greater pore size (several nano-meters) than MS TiO2 films heat-treated at a same temperature(a few tenths of nanometers) [29]. These data would explaincontrast differences between a CS SiO2/TiO2 bi-layer (Figs. 7aand 8a) with a more porous TiO2 component and a MS SiO2/TiO2 bi-layer (Figs. 7b and 8b) with a denser TiO2 component.However, they cannot account for differences in CS SiO2/TiO2

(Figs. 7a and 8a) and CS TiO2/SiO2 (Figs. 7c and 8c) bi-layerfilms, since the TiO2 component of these bi-layers is a prioricomparable, i.e. it has been deposited from a same sol and heat-treated at a same temperature. We suppose, therefore, that suchdifferences might be induced by arrangement of the bi-layerstructures, i.e. whether the SiO2 film is deposited below orabove the CS TiO2 film. In the latter case, since the CS TiO2

underlayer has rather large intergranular pores, it is likely that,during liquid film deposition of the overlayer, the SiO2 sol canimpregnate pores of the TiO2 film through a certain thickness.During subsequent sol–gel reaction and heat-treatment at500 °C, the impregnated sol would yield a SiO2 componentlocated within pores of the TiO2 underlayer. It would give rise toa SiO2–TiO2 composite underlayer whose HRTEM signature(Figs. 7a, 8a) can differ from that of a CS overlayer (Figs. 7c,8c) owing to differences in weigh densities of SiO2–TiO2

composite and pure TiO2 films. However, it is important notingthat brightness contrasts in TEM images do not only account forweigh density variations or internal porosity [30,31]. Ionthinning performed during sample preparation can locally alterthe sample and induce contrasts due to local thickness varia-tions. Beside, elastic interactions (diffraction) can also affectbrightness contrasts in TEM images.

In order to evidence a possible liquid impregnation and toestimate the impregnation depth, we performed EFTEM imagingon a CS SiO2/TiO2 cross-sectional sample. This imaging methodis based on the extraction and mapping of a particular high energysignature detected in EELS spectra of samples under TEMobservation. When electrons inelastically diffuse through a thinsample, they lose energy that excites different energetic tran-sitions. In the high energy part (typically several tens of eV) of thederived spectra, the so-called core-loss region, each core-lossedge of the spectrum is characteristic for a specific chemicalelement. EFTEM images are very few sensitive to local thicknessvariations or elastic interactions, which can affect brightnesscontrasts in TEM images. Consequently, contrasts in EFTEMonly account for chemical information thus allowing an elementalmapping of the sample [30,31]. In this study, EELS spectra werefiltered around the Si–L2,3 peak. HRTEM and EFTEM images ofa same CS SiO2/TiO2 film cross-sectional area are shown inFig. 9a and b, respectively. Both images show quite similarbrightness contrasts, which indicates that the HRTEM image ofFig. 9a traduces, at least partially, local weigh density variations.In the EFTEM image of Fig. 9b, presence of silicon is depicted bybright regions, which can be appreciated by a grey appearance forSiO2 and a white appearance for the Si substrate, while TiO2

regions appear in black. This image unambiguously shows that

965S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

SiO2 is present across the whole TiO2 film thickness, whichdemonstrates that the SiO2 sol totally impregnated rather largepores of the CS TiO2 underlayer during deposition of theoverlayer. It can therefore be concluded that the underlayer of aCS SiO2/TiO2 bi-layer consists of a SiO2–TiO2 composite filmlocated at the SiO2-substrate interface.

EFTEM imaging has not been performed for MS SiO2/TiO2

and CS TiO2/SiO2 bi-layer films. However it is likely that, forthose two films, liquid impregnation of the underlayer cannottake place in significant extent during deposition of theoverlayer. On the one hand, in the case of a MS SiO2/TiO2

film, the MS TiO2 underlayer has very small intergranular poresthat can hardly be impregnated. On the other hand, in the case ofa CS TiO2/SiO2 film, previous (unpublished) works have shownthat our SiO2 underlayer heat-treated at 500 °C is very dense. Itis thus inferred that occurrence or not of a liquid impregnationcan definitely explain brightness contrasts observed in Figs. 7and 8, since a SiO2–TiO2 composite film has a weaker mole-cular weigh than a pure TiO2 film. Besides, it is possible that, inthe case of a CS SiO2/TiO2 bi-layer film, liquid impregnation ofthe SiO2 sol can modify the arrangement of TiO2 crystallitespresent in the CS TiO2 underlayer, through a mechanism thathas not been elucidated yet, which would in turn explain why inthis case the TiO2 film component has a much greater thicknessthan in both other bi-layer films. We previously indicated thatSi–O–Ti heterolinkages are likely to be formed at SiO2–TiO2

interfaces. Thus, MS SiO2/TiO2 and CS TiO2/SiO2 bi-layerfilms would essentially be composed of Si–O–Ti heterolin-kages located at a planar SiO2–TiO2 interface, while suchheterolinkages would be created at granular interfaces of aSiO2–TiO2 composite film in the case of a CS SiO2/TiO2 bi-layer. It is thus inferred that, owing to a larger SiO2–TiO2

contact surface, the amount of Si–O–Ti heterolinkages is muchgreater in this latter case.

3.5. Superhydrophilic properties of bi-layer films: discussion

In a previous work, we have shown that SiO2–TiO2 com-posite films, constituted of TiO2 crystallites embedded in a SiO2

amorphous matrix, exhibit natural and persistent superhydro-philic properties [21], comparable to those evidenced in thepresent work for CS SiO2/TiO2 bi-layer films. We have postu-lated that, according to well established bibliographic data [11–14], these unusual properties are due to interfacial Si–O–Tiheterolinkages that promote the formation of TiO6

−2 or SiO4+4/3

units inducing charge imbalances at SiO2–TiO2 granularinterfaces. Deprotonated TiO6

−2 and/or protonated SiO4+4/3

units present at the composite film surface can favor adsorptionof H3O

+ and/or OH− ions, thus inducing enhanced molecularor dissociative water adsorption and leading to natural super-hydrophilic properties of composite films. We believe thatsuperhydrophilic properties of bi-layer films might also origi-nate from such depronated or protonated interfacial units, whichcan be discussed as follows.

We have shown that existence and persistence of naturalsuperhydrophilicity in bi-layer films follows the order CS TiO2/SiO2bMS SiO2/TiO2≪CS SiO2/TiO2 and, in same operating

conditions, superhydrophilicity of the latter films can be regene-rated, which is not achieved for both former films. According toprevious studies and arguments, both former films areconstituted of a SiO2–TiO2 planar interface where Si–O–Tiheterolinkages and related negative or positive charge imbal-ances can be present. Since the electrical neutrality of thewhole bi-layer film must be satisfied, such interfacial chargeimbalances are likely to be compensated for by counter chargesat the bi-layer film outer surface. So-formed negative or positivesurface charges would in turn favor the molecular or dissociativewater adsorption, as previously postulated for SiO2–TiO2

composite films, yielding an initial natural superhydrophilicityof CS TiO2/SiO2 and MS SiO2/TiO2 films. Furthermore, a MSSiO2/TiO2 film benefits from the presence of a SiO2 outer layerthat promotes a greater in-time stability of adsorbed OH species.This outer layer would explain a certain persistence of super-hydrophilicity for the MS SiO2/TiO2 film that is not observedfor a CS TiO2/SiO2 film with a TiO2 outer layer. However, inboth cases no long-term persistence of superhydrophilicity isobserved. It can be related to the limited amount of interfacialnegative or positive charges, whose appearance is restrained at aplanar interface, which induces a limited amount of negative orpositive charges at the outer surface.

In contrast, a CS SiO2/TiO2 bi-layer film would not onlybenefit from the existence of a SiO2 outer layer but also fromthe SiO2–TiO2 composite nature of the underlayer. For this bi-layer, Si–O–Ti heterolinkages are not concentrated at a planarSiO2–TiO2 interface but are distributed at granular SiO2–TiO2

interfaces. Thus, owing to a much greater SiO2–TiO2 contactsurface, such a composite layer can promote a noticeably largeramount of intergranular Si–O–Ti heterolinkages, leading to agreater charge imbalance in the interfacial region. According tothe electrical neutrality criterion, it is therefore inferred that theamount of negative or positive charges at the outer surface ofCS SiO2/TiO2 films should be much more important than forother bi-layer films, which would in turn favor a more efficientwater adsorption and, consequently, enhanced natural super-hydrophilic properties.

Finally, it should bementioned thatmorphological features canalso influence the wettability of our bi-layer films, in particularthe surface roughness. Water contact angles measured on ahydrophilic rough surface are usually smaller than those mea-sured on a same non-rough surface. Thus, the roughness of bi-layer films may in principle influence their wettability properties.However, two arguments indicate that, in the present case, rough-ness presumably plays a secondary role. In a previous article, weindicated that the RMS roughness of MS TiO2 and CS TiO2 filmswas about 1 and 8 nm, respectively [29]. In this study, we showedthat these roughness values were presumably too weak to signi-ficantly influence wettability properties of TiO2 films. Further-more, SiO2 films studied in the present work are extremelysmooth, with a RMS roughness of only 0.4 nm. Since existenceand persistence of natural superhydrophilicity in bi-layer filmsfollows the order CS TiO2/SiO2bSiO2/TiO2, it means that themuch smoother SiO2/TiO2 bi-layer film exhibits better wettabilityproperties. It seems, therefore, that roughness effects do notpredominantly act on the superhydrophilicity of our bi-layer

966 S. Permpoon et al. / Thin Solid Films 516 (2008) 957–966

films, whichwould essentially rely on the outer surface nature andeffects of SiO2–TiO2 interfaces.

4. Conclusion

Sol–gel SiO2/TiO2 and TiO2/SiO2 bi-layer films have beendeposited from a polymeric SiO2 solution and either a polymericTiO2 mother solution (MS) or a TiO2 crystalline suspension (CS).The chemical and structural properties of bi-layer films heat-treated at 500 °C have been investigated by FTIR, XPS, andTEM characterizations in relation to their hydrophilic properties.Chemical characterizations suggest that an outer SiO2 surfacepromotes a greater in-time stability of surface OH species, while aCSTiO2 underlayer induces a greater amount of OH species at theouter SiO2 surface than aMSTiO2 underlayer. TEM studies showthat a SiO2/TiO2 composite film is present at the SiO2-substrateinterface of a CS SiO2/TiO2 bi-layer, while this composite filmdoes not exist for other bi-layers under investigation.

These properties can in turn be related to the existence andpersistence of a natural superhydrophilicity of bi-layers, which isnever observed for SiO2 or TiO2 single-layer films. Water contactangle measurements show that existence and persistence ofnatural superhydrophilicity in bi-layer films follows the order CSTiO2/SiO2bMS SiO2/TiO2≪CS SiO2/TiO2 and, in sameoperating conditions, superhydrophilicity of the latter films canbe regenerated, which is not achieved for both former films.Enhanced properties of CS SiO2/TiO2 films are primarily relatedto the presence of an outer SiO2 layer. They also arise fromexistence of a SiO2–TiO2 composite layer in the interfacialregion. Si–O–Ti heterolinkages developing at granular interfacesin the composite layer are believed to be the origin of importantelectrical charge imbalances that would promote negative orpositive charges at the outer SiO2 surface, thus inducing naturalwater adsorption and enhanced natural superhydrophilic proper-ties of CS SiO2/TiO2 bi-layer films. Natural, persistent, andregenarable superydrophilic properties of bi-layer films allowenvisaging long-term self-cleaning applications in conditionswhere UV light is not available.

Acknowledgement

The authors thank D. Lafond from LETI-Grenoble forvaluable help in EFTEM experiments.

References

[1] R. Wang, N. Sakai, A. Fujishima, T. Wanatabe, K. Hashimoto, J. Phys.Chem., B 103 (1999) 2188.

[2] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem., B 105(2001) 3023.

[3] M. Machida, K. Norimoto, T. Watanabe, K. Hashimoto, A. Fujishima,J. Mater. Sci. 34 (1999) 2569.

[4] D. Ren, X. Cui, J. Zhen, Q. Zhang, X. Yang, Z. Zhang, L. Ming, J. Sol–gelSci. Technol. 29 (2004) 131.

[5] K. Guan, Surf. Coat. Technol. 191 (2005) 155.[6] H.J. Lee, S.H. Hahn, E.J. Kim, Y.Z. You, J. Mater. Sci. 39 (2004) 3683.[7] M. Maeda, S. Yamasaki, 204th Electrochem. Soc. Meeting, Orlando, USA,

October 12–16, 2003, extended abstract 1258.[8] J.C. Yu, J. Yu, W. Ho, J. Zhao, J. Photochem. Photobiol., A Chem. 148

(2002) 331.[9] X. Fu, L.A. Clark, Q. Yang, M.A. Anderson, Environ. Sci. Technol. 30

(1996) 647.[10] K. Guan, B. Lu, Y. Yin, Surf. Coat. Technol. 173 (2003) 219.[11] K. Tanabe, T. Sumiyoshi, K. Shibata, T. Kiyoura, K. Kitagawa, Bull. Chem.

Soc. Jap. 47 (1974) 1064.[12] X. Gao, I.E. Wachs, Catal. Today 51 (1999) 233.[13] T. Kataoka, J.A. Dumesic, J. Catal. 112 (1988) 66.[14] Z. Liu, J. Tabora, R.J. Davis, J. Catal. 149 (1994) 117.[15] J.M. Sanz, L. Soriano, P. Prieto, G. Tyuliev, C. Morant, E. Elizalde, Thin

Solid Films 332 (1998) 209.[16] B. Gallas, A. Brunet-Bruneau, S. Fisson, G. Vuye, J. Rivory, J. Appl. Phys.

92 (2002) 1922.[17] N. Brun, C. Colliex, J. Rivory, K.Y. Zhang, Microsc. Microanal. Microstruct.

7 (1996) 161.[18] K.S. Guan, H. Xu, B.J. Lu, Trans. Nonferr. Met. Soc. 14 (2004) 251.[19] A. Hattori, T. Kawahara, T. Uemoto, F. Suzuki, H. Tada, S. Ito, J. Colloid

Interface Sci. 232 (2000) 410.[20] H. Tada, Y. Kubo, M. Akazawa, S. Ito, Langmuir 14 (1998) 2936.[21] S. Permpoon, G. Berthome, B. Baroux, J.C. Joud, M. Langlet, J. Mater.

Sci. 41 (2006) 7650.[22] N. Primeau, C. Vautey, M. Langlet, Thin Solid Films 310 (1997) 47.[23] M. Langlet, M. Burgos, C. Coutier, C. Jimenez, C. Morant, M. Manso,

J. Sol–gel Sci. Technol. 22 (2001) 139.[24] M. Langlet, A. Kim, M. Audier, C. Guillard, J.M. Herrmann, J. Mater. Sci.

38 (2003) 3945.[25] M. Fallet, S. Permpoon, J.L. Deschanvres, M. Langlet, J. Mater. Sci. 41

(2006) 2915.[26] G. Orcel, J. Phallippou, L.L. Hench, J. Non-cryst. Solids 88 (1986) 114.[27] R.M. Almeida, J. Sol–gel Sci. Technol. 13 (1998) 51.[28] C.I. Contescu, J.A. Schwarz, in: K.L. Mittal (Ed.), Acid–Base Interactions,

vol. 2, 2000, p. 245.[29] M. Langlet, S. Permpoon, D. Riassetto, G. Berthomé, E. Pernot, J.C. Joud,

J. Photochem. Photobiol. A 181 (2006) 203.[30] M. Worch, H.J. Engelmann, W. Blum, E. Zschech, Thin Solid Films 405

(2002) 198.[31] R. Pantel, S. Jullian, D. Delille, D. Dutartre, A. Chantre, O. Kermarrec, Y.

Campidelli, L.F.T.Z. Kwakman, Micron 34 (2003) 239.