Thin Solid Films 520 (2011) Contents lists available at SciVerse ScienceDirect. Thin Solid Films

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1 Thin Solid Films 520 (2011) Contents lists ville t SciVerse ScienceDirect Thin Solid Films journl homepge: Effect of compressive stress inducing nd gp nrrowing on the photoinduced ctivities of sol gel TiO 2 films Mohmed N. Ghzzl,,, N. Choui c, M. Genet d, Eric M. Gigneux, D. Roert e Lortoire de Chimie et Methodologie pour l'evironnement, Université Pul Verline de Metz, rue Victor Demnge, Sint-Avold, Frnce Institute of Condensed Mtter nd Nnoscience Molecules, Solids nd rectivity (IMCN/MOST), Université ctholique de Louvin, Croix du Sud 2/17, B-1348 Louvin-l-Neuve, Belgium c Lortoire de Spectrométrie de Msse et de Chimie Lser, Université Pul Verline de Metz, Boulvrd rgo, F Metz Cedex 3, Frnce d Institute of Condensed Mtter nd Nnoscience Molecules, Division of Bio nd Soft Mtter (IMCN/BSMA), Université ctholique de Louvin, Croix du Sud 2/17, B-1348 Louvin-l-Neuve, Belgium e Lortoire des Mtériux, Surfce et Procédés pour l Ctlyse (LMSPC) CNRS-UMR7515, Antenne de Sint-Avold, UPV-Metz, Rue Victor Demnge Sint-Avold, Frnce rticle info strct Article history: Received 28 April 2011 Received in revised form 26 August 2011 Accepted 26 August 2011 Aville online 2 Septemer 2011 Keywords: Titnium oxide Thin film Photoctlysis Photoinduced hydrophilicity Compressive stress Bnd gp TiO 2 thin films grown on different kinds of sustrtes were otined y sol gel process. X-ry diffrction reveled tht the TiO 2 lttice prmeter c decresed continuously, indicting continuous vrition in the compressive stress, negligile compressive stress of the film grown onto Sod-Lime Glss (SLG), medium compressive stress of the film grown onto BoroSilicte Glss (BSG) nd lrge compressive stress of the film deposited onto the Qurtz Sustrte (QS). UV Vis sornce spectr exhiited red-shift of the sornce edge of the TiO 2 films suggesting lowering of the nd gp, which is direct consequence of the increse of the compressive stress. X-ry photoelectron spectroscopy reveled tht the surfce composition of titni films ws similr except for sodium-ion concentrtion. The rte oserved during the photo-oxydtion of the steric cid on TiO 2 /QS ws twice s high s tht of TiO 2 /BSG nd out 1000 times superior to tht of. The photoinduced wettility shows n identicl dependence of the compressive stress. According to these results, the compressive stress could e used to tune the nd gp of the titnium oxide in order to enhnce the photoinduced properties Elsevier B.V. All rights reserved. 1. Introduction Photoctive TiO 2 semiconductor using low-energy UV hs led to widespred scientific nd technologicl interest. Besides the photo-oxidtive ctivity of TiO 2 surfces, UV-induced hydrophilicity effects on TiO 2 present second extrordinry chrcteristic. Tking dvntge of the comintion of these two phenomen self-clening surfces constitute n re where the trnsition from promising technology to glol mrket product rnges hs occurred. Since the pioneering work of Fujishim nd Hond [1], the photoctlytic mechnism of the TiO 2 surfce ws lrgely studied [2 4]. The prevlent mechnism is sed on photogenerted electron-holes pirs tht re le to migrte to the surfce nd eventully interct with oxygen nd dsored hydroxyls groups thus prticipting in the photoctlytic rection. In contrst, the mechnism for photoinduced hydrophilicity still remins uncler. Most of the proposed models for this effect involve structurl chnge of the photoctlyst surfce. On the other hnd, the dissocitive dsorption of H 2 O molecules on the oxygen vcncies leds to n increse in the Corresponding uthor t: Institute of Condensed Mtter nd Nnoscience Molecules, Solids nd rectivity (IMCN/MOST), Université ctholique de Louvin, Croix du Sud 2/17, B-1348 Louvin-l-Neuve, Belgium. Tel.: ; fx: E-mil ddress: mohmed.ghzzl@uclouvin.e (M.N. Ghzzl). numer of the surfce hydroxyl groups of the semiconductor during UV light irrdition [5,6]. It hs een found tht in the drk the hydroxyl groups newly formed on the TiO 2 surfce fter UV irrdition cn e oxidized y O 2 nd tht the oxygen vcncies cn e recovered thus grdully turning the surfce hydrophoic [5,7]. White et l.[8] hve reveled y temperture progrmmed desorption mesurement of TiO 2 (110) single crystl tht neither H 2 O dissocition nor oxygen vcncies re relted to the hydrophilic conversion. Tkeushi et l. [9] proposed tht the photoinduced hydrophilic conversion of TiO 2 surfce originted from the desorption of H 2 O molecules from the photoctlyst surfce cused y heting of the surfce during irrdition, nd y the photoctlytic decomposition of dsored hydrocrons. Others works reported tht the hydrophilicity ws simply consequence of the clenup of the surfce through photoctlytic oxidtion of hydrophoic thin contmintion lyers [10 12]. Mny efforts hve een mde to improve the photoinduced ctivity of titnium films. In the cse of photoctlytic ctivity, reducing the numer of photogenerted electron-hole pirs hs een extensively explored. To do so, one possiility consists on coupling lrge nd gp semiconductor with smller nd gp semiconductor, the ltter presenting suitle potentil energies [13,14]. The process not only helps for chrge seprtion y isolting electrons nd holes into two distinct prticles ut, t the sme time, it llows the extension of the photoresponse of the photoctlyst in the visile rnge /$ see front mtter 2011 Elsevier B.V. All rights reserved. doi: /j.tsf

2 1148 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) On the other hnd, severl studies reported tht it is possile to improve the photoctlytic ctivity of TiO 2 y doping with foreign elements [15 17]. Doping of TiO 2 ffects its ulk electronic structure, nmely, the position of the Fermi energy level, the formtion of new energy levels y the interction of n interstitil dopnt with the semiconductor lttice, the electron conductivity in the ulk semiconductor, the surfce properties of the thickness of the spce chrge lyer. However, some dopnts, such s metllic ctions, hve the disdvntge tht they cn give rise to loclized d-levels deep in the nd gp of TiO 2 which often serve s recomintion centers for photogenerted chrge crriers. Thus, nionic non-metl dopnts, especilly nitrogen, pper to e more pproprite for extending the photoctlytic ctivity of TiO 2 to the visile light region since their p sttes contriute to the nrrowing of the nd gp y mixing with O 2p sttes [18]. In fct, the efficiency of the hydrophilic conversion of the titnium surfce ws improved y incresing the hydroxyl groups t the surfce. Gun et l. [19] reported tht when mixing SiO 2 nd TiO 2 they form single oxide prticles in the films, ut with prt of complex oxide my e formed. They suggest tht Ti\O\Si onds increse the surfce cidity which induces n increse of hydroxyl content in the composite films. Recently, Houmrd et l. [20] reported tht TiO 2 SiO 2 composite films increse the rtio of dsored hydroxyl groups t the surfce thus leding to cotings with nturl super-hydrophilicity persistence nd esy photoregenertion. Titnium film grown onto sustrte often requires het tretment followed y cooling step in order to otin the photoctive phse; c.. ntse. During these steps, the structurl properties cn e strongly influenced. The films were, often, suject to residul constrin induced y the discrepncy of the therml expnsion coefficient etween the film nd sustrte. Shit et l. [21,22] controlled the residul stress in titni films, deposited vi the RF rective mgnetron sputtering method onto different kinds of sustrtes, to induce surfce structurl chnge nd then to enhnce the rte of the hydrophilic conversion. Aury et l. [23] reported tht the titnium film grown y RF rective mgnetron sputtering onto glss sustrte nd clcined t incresing tempertures shows photoinduced ctivity optimum t 400 C. They supposed tht, t other tempertures, structurl defects induced y internl (tensile or compressive) stress could crete energy levels in the nd gp of the TiO 2 semiconductor, fvoring the chrge crrier cpture nd thus their recomintion. Other studies lso hve shown tht strin cn e used to enhnce the TiO 2 photoelectrochemicl efficiency [24,25]. However, little ttention hs een pid to the influence of the residul stress in the structurl properties of TiO 2 films nd its reltionship with photoctivity. In this pper, we investigte the effect of the residul stress in the photoinduced properties of the TiO 2 film. We show tht residul stress induces physicl modifiction on the TiO 2 structure nd improves its photoinduced ilities. The titni films were prepred vi sol gel method nd grown onto different kinds of sustrtes in order to introduce vrile residul stress. The photoctive properties of ech film were evluted. The effects of the residul stress on the structurl nd opticl properties of titnium film were lso investigted. 2. Experimentl section The procedure used for the preprtion of the titni films is descried in detil elsewhere [26]. Briefly, titnium(iv) isopropoxide ws used s precursor to synthesize the titni sol vi n cid ctlyzed sol gel process t room temperture in the following wy: 10 ml of Ti(OC 3 H 7 ) 4 ws dissolved in 50 ml of solute ethnol under mgnetic stirring nd 1,3 ml of HCl ws dded to ctlyze the hydrolysis. After stirring during 30 min, the otined solution ws hydrolyzed y the ddition of mixture of wter (0,6 ml) nd solute ethnol (50 ml) drop-wise keeping stirring for nother 2 h. The synthesis ws crried out under rgon tmosphere. The resultnt TiO 2 sol ws cler nd stle for some weeks. The ph of the prepred TiO 2 sol ws close to 1. To prepre the TiO 2 thin films iming to otin different photoctlytic ctivity, the films were grown onto three kinds of glss sustrtes (75 mm 25 mm 1 mm) of vrile therml expnsion coefficient (see Tle 1). The following sustrtes were used: Sod- Lime Glss (SLG), BoroSilicte Glss (BSG) nd Qurtz Sustrte (QS). The sustrtes were dipped into nd pulled out from the solution t velocity of 11,5 cm.min 1 nd then they were dried in ir t 70 C for 5 min. The dip-coting process ws repeted 36 times t mient tmosphere. The s-prepred films were dried overnight t 80 C nd then heted in ir for 2 h t 450 C using heting rte of 5 C.min 1. The sustrtes were clened with detergent nd with ethnol under ultrsoniction for hlf n hour, nd then rinsed with distilled wter nd dried for 1 h t 100 C prior to use. The contct ngle mesured for ll sustrtes fter the clening procedure ws inferior to 10. At these conditions perfect wetting of the sol is llowed whtever the sustrte nture, nd it is possile to improve the ffinity etween the surfce of the glsses nd the TiO 2 solprecursor during the coting process. The coting morphology ws investigted y scnning electron microscopy (SEM) using Philips XL30 SFEG equipped with field effect gun operting t 5 kv. The structurl properties were investigted y X-ry diffrction (XRD) y mens diffrctometer Siemens D500 in Brgg Brentno configurtion using Co-Kα (λ=0,17889 nm) rdition t grzing incidence of 0,05. Silicon powder ws dispersed on the coting surfce in order to clirte the diffrctogrms ccording to the procedure descried y Aury et l. [23]. Diffuse reflectnce spectr of titni films were recorded y mens of UV Vis spectrophotometer (Perkin-Elmer) equipped with n integrting sphere. The seline ws clirted y BSO 4 in the diffuse reflectnce mode. The spectr were recorded t room temperture in the spectrl rnge nm. X-ry photoelectron spectroscopy (XPS) ws performed on Krtos Axis Ultr spectrometer (Krtos Anlyticl, Mnchester, U.K.). The smples were fixed on the support using doule-sided dhesive conducting tpe. The residul pressure in the nlysis chmer ws lower thn 10 6 P. The monochromtic Al X-ry source ws powered t 15 kv nd the emission current ws 10 ma. The chrge stiliztion ws chieved y using the Krtos Axis device. Anlyses were performed in the hyrid lens mode with the slot perture nd the iris drive position set t 0,5 inch; the resulting nlyzed re ws Tle 1 Comprison of d-spcing, cell prmeters, therml expnsion coefficients nd residul stress for ech smple. Sustrte α s x10 6 / C σ th (MP) d(101) σd(101) (MP) d(004) σ d(004) (MP) (nm) c (nm) SLG 8,6 0 0, , , ,946 BSG , , , ,9384 QS 0,5 888,16 0, , , ,936 * σ th ws therml stress cused y imetl effect etween the film nd the sustrte, which ws determined y the following eqution σ th =E f (α f α s ) ΔT. Where E f is Young's modulus for the film, nd α f nd α s re therml expnsion coefficients of the film nd the sustrte, respectively.

3 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) μm 300 μm 2. A pss energy of the nlyzer ws set t 160 ev for the survey scn nd 40 ev for the nrrow scns. The inding energy (BE) vlues were referred to the C-(C, H) contriution of the C1s pek fixed t 284,8 ev. The dt tretment ws performed with the Cs XPS softwre (Cs Softwre Ltd., UK). The peks were decomposed using liner seline, nd component shpe defined y the product of Guss nd Lorentz function, in the 70:30 rtio, respectively. Molr concentrtion rtios were clculted using pek res normlized ccording to the cquisition prmeters nd the reltive sensitivity fctors nd trnsmission function provided y the mnufcturer. The photoctlytic degrdtion of the orgnic hydrocron model nd the kinetics of the hydrophilic conversion of the TiO 2 surfces were mesured seprtely s function of irrdition time. A Solr ox ATLAS Suntest CPS+pprtus provided with Xenon lmp simulting nturl rdition ws used for ll experiments. The pprtus ws lso equipped with n IR cut-filter nd ir cooling system. The temperture inside the ox did not exceed 25 C s mesured y thermocouple. The power density in the UVA nd the UVB rnge ws 0,25 mw.cm 2 nd 0,016 mw.cm 2 s mesured y powermeter (Delt Ohm 2101). The photoctlytic performnces were estimted y following the degrdtion rte of Steric Acid (SA herefter) for ech system (, TiO 2 /BSG nd TiO 2 /QS). The photoctlyst surfces were recovered y SA y dip coting the smples in methnolic solution of SA (0,02 mol.l 1 ) nd then drying in ir. The disppernce of SA from the surfce ws chieved y mesuring the integrted sornce over the cm 1 infrred rnge. The necessry infrred mesurements were mde with Fourier trnsform infrred (FTIR) spectrometer (BIO-RAD, FTS 165). The contct ngle of sessile wter drops on test sustrtes (TiO 2 / Sustrte) ws mesured using homemde pprtus. The system comprises SONY AVC-D5/D7CE cmer interfce vi n dptor with computer loded with n pproprite PEGASE softwre. The softwre pckge ssocited with the instrument llowed clcultion of the contct ngle mde y the wter droplet deposited on the sustrte through curve-fitting of the droplet imge outline. The experiments were crried out under mient conditions with temperture rnging etween 21 nd 24,5 C nd reltive humidity rnging etween 46 nd 54 %. The droplet size used for the mesurements ws 3 μl. c Crcks 500 nm 500 nm 3. Results nd discussions 3.1. Structure nd surfce morphology 500 nm In Fig. 1 1c typicl SEM imges of surfce-films nneled t 450 C re shown. The surfce imges of the film dip-coted on SLG reveled the presence of crcks whose orienttion seems to e similr ll over the film. These crcks cn e explined y considering the lrge difference in the therml expnsion coefficient etween the sustrte nd the titni films s it hs een frequently reported nd suggested y severl uthors. In one hnd, it hs een recognized tht residul compressive stresses my cuse film delmintion from the sustrte wheres tensile stress my cuse surfce crcks in the film [27,28]. In other hnd, relted the method employed in our experiment, dehydrtion or solvent trp cn occurs. During the clcintions step, dehydrtion removes surfce-ound Ti-OH groups nd increses shrinkge into the film (formtion of Ti O Ti). In generl, the verge thickness of the film ws reduced fter nneling. This shrinkge in thickness signifies the dehydrtion of wter from the TiO 2 film nd could induce crck formtion [29]. Also the solvent cn e trpped within the close porosity of the film during the condenstion. The het tretment increses the pressure under the pore nd n internl stress is creted leding to the distortion of the film nd liertion of the solvent leding to crcks formtion. However, in our cse there ws no difference etween the expnsion coefficient Fig. 1. SEM imges of the TiO 2 films surfce (), () TiO 2 /BSG nd (c) TiO 2 /QS fter clcintion t 450 C. of the film nd the SLG sustrte (α SLG =α TiO2 =8, /C ) nd no difference ws oserved etween the morphology of TiO 2 /QS nd tht of TiO 2 /BSG. The films grown on the FS nd BSG presented no evidence of surfce crcking, nd they were neither ffected y the solvent evportion nor y the dehydrtion upon heting (even using the sme procedure). This suggests tht the origin of the crcks proly results from the internl strin induced y the diffusion of N + ions throughout grin oundries. It is well known tht during the clcintion step of TiO 2 films grown on SLG (contining pproximtely 14 % of sodium mong other lkli elements of NO), lkli ions, prticulrly N +, re likely to diffuse from the sustrte to the coting [30 32]. The films presented grnulr structure with grins sizes of nm for nd finer nm for TiO 2 /FS nd TiO 2 /BSG.

4 1150 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) Intensity (r.unit.) (nm) 0,379 0,378 0,377 0,376 0,375 X-ry diffrction (XRD) mesurements were used to investigte the crystl structure of the sol gel films. XRD ptterns confirmed tht the films re crystlline nd predominnce of the ntse structure with preferentil (101) orienttion (Fig. 2). The pek position of the (101) direction ws slightly shifted compred to the initil position (JCPDS File No ). The oserved shift proves tht the post-tretment, needed to otin the ntse phse, induces residul stress due to the difference in the therml expnsion coefficient, especilly in the cse of TiO 2 /BSG nd TiO 2 /QS. The TiO 2 deposited on SLG shows wek residul stress since d-spcing long the (101) direction (see Tle 1) ws slightly inferior from the theoricl vlue (d (101) = 0,352 nm, JCPDS File No ). However, the shift ws more noticele for QS. The texturl chnge in the titni films grown on vriles sustrtes llows determining the cell prmeters using the following formule for tetrgonl symmetry: c ¼ 4 d 004 (101) θ ( ) ¼ q d ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 101 4d 004 : ð2þ 16d d Si (004) theoricl θ ( ) (200) Fig. 2 shows the cell prmeters of the TiO 2 coting for different systems. In comprison with the theoreticl vlue, ll cell prmeters were lower (=0,3785 nm nd c=0,9514 nm from the JCPDS crd numer), indicting tht the cotings re sumitted to compressive stress. The low temperture sol gel processed films (105) TiO 2 /BSG TiO 2 /QS (204) SLG BSG FS Si () () (c) c theoricl 0,950 c 0,945 0,940 0,935 Fig. 2. () Grzing incidence X-ry diffrction pttern of the TiO 2 films deposited on the different glsses () SLG, () BSG nd (c) QS. () Evolution of the ntse cell prmeters with the nture of the sustrte. c (nm) ð1þ possess shorter out-of plne lttice prmeter due to n in-plne residul stress [33] hence corroorting our results. In order to understnd the strin effect on oth nd c cell prmeters, the residul stress ws clculted from the lttice spcing of the ntse films for the (101) nd the (004) direction. The dependence of the lttice strin ε of the ntse on the direction nd on the residul stress σ of the films were defined s follows [21]: ε ¼ d hkl d hkl d hkl σ ¼ Eε 2ν : Where, d (hkl) nd d (hkl) re the lttice spcing in the film nd in the ulk in respect to the (hkl) plne. E is Young's modulus, nd ν is Poisson's rtio of the film. To roughly estimte the residul stresses of the films, Poisson's rtio nd Young's modulus for the ntse phseweressumedtoe0,26nd258gp[21], respectively. The clculted therml stresses nd oserved residul stresses re summrized in Tle 1. A reltively good correltion ws oserved etween the evolution of the cell prmeters nd the corresponding residul stress. The cell prmeters remined constnt long the (101) direction for SLG nd BSG, wheres it ws twice higher for QS. In turn, the greter the compressive stress in the film long the (004) direction ws the lower the c cell prmeter ws. The decrese in the c lttice prmeter under compressive stress suggests shortening of the lttice distnce etween the Ti Ti toms long the (004) direction. The evolution of the cell prmeters vried independently from the therml stress, therefore corroorting the evolution of the residul strin. These results show tht the difference in the therml expnsion coefficient is most likely not responsile for the significnt modifiction of nnocrystlline ntse structure oserved in the sol gel nneled films. The residul stress in the titni films ws the min dominnt effect, prticulrly long the (004) direction. These results diverge from those reported y Shit et l. [21]. These uthors demonstrted tht the therml stress cn e used to control the residul stress of titni films grown on different kinds of sustrtes thence to the evolution of the d-spcing. The difference cn e explined y considering the procedure dopted for the elortion of the mterils. Contrry to Shit et l. who use physicl deposition to otin the ntse films, the pyrolysis nd densifiction in the sol gel process derived film led to the evolution of in-plne residul stress [33]. Furthermore, the reduction of the out-of-plne lttice prmeter suggests tht the densifiction ws more vigorous [34,35] nd leds to the increse of the residul stress for SLG, BSG nd QS respectively. Fig. 3 showstheuv Vis sorption spectr of TiO 2 films grown onto different sustrtes. The sorption edge of ws oserved t out 340 nm wheres tht of TiO 2 films grown onto BSG nd QS were t out 360 nd 375 nm, respectively. Therefore red-shift of the dsorption edge of the titnium film ws clerly identified. Tht the compressive stress extended the sornce specter of the TiO 2 films to higher wvelengths suggests tht lowering of the nd gp of the semiconductor hs occurred. In this sense, the present results were in ccordnce with other studies [25,36,37] reporting tht the nd gp of conventionl semiconductor is usully sensitive to externl pressure or volume deformtion. The nd gp ws determined y plotting (αhν) 2 versus (hν) [38].Wehvessumedthtthesensitivityαd(α:eingthesorption coefficient nd d eing the film thickness) should e of the order of unity or d 1/α nd tht the scttering ws negligile. Fig. 3 shows nd gp nrrowing from 3,68 ev for with negligile compressive stress, 3,49 ev for TiO 2 /BSG for medium compressive stress to 3,35 ev for TiO 2 /QS for lrge compressive stress. ð3þ ð4þ

5 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) Asornce (%) TiO 2 /BSG TiO 2 /QS Tle 2 Surfce composition content of titni films s determined y XPS expressed y t %. Smples O Ti Ti 2p O=C.O-C.-OH C-(C.H) Cl 2p Si 2p Mg 2p N 1 s 37,3 19,3 12,4 13 0,8 1,3 4,8 7,7 TiO 2 /BSG 32,4 18,7 12,8 25 0,7 5,8 1 TiO 2 /QS 21,8 13,1 15,7 32 0,1 0,9 4,3 1 (αhυ) 2 (cm -2 ev 2 ) Wvelength (nm) 2,0 2,5 3,0 3,5 4,0 Photon Energy (ev) Fig. 3. () UV Vis sorption results of films grown on different sustrte, () vrition of (αhυ) 2 versus hυ of the titni thin films. Ellipsometry mesurements (not included) reveled tht the films hve thickness vlues of 41 nm nd 42 nm for TiO 2 /BSG nd TiO 2 /QS, respectively [26]. An opticl index of 2,17 t 630 nm ws found for oth smples. This vlue is in good greement with the vlues reported in other works for sol gel ntse thin films [39]. The film thickness ws estimted from the crcks oserved on the SEM imges. This feture llowed to roughly estimte the film thickness t nm. An exmple of generl XPS spectrum of thin film is shown in Fig. 4. Whtever the system, the chrcteristic peks of titnium, oxygen nd cron cn e oserved. Furthermore, other species such s Si 2p, Mg 2p, Cl 2p (C 2p, K 2p s trces 0,1 %) nd N 1 s were detected. It is well known tht during the nneling step lkli elements, such s N, diffuse from the sustrte to rech the coting [31]. The concentrtion of ech element is summrized in Tle 2 for ll smples. XPS nlysis indictes n O/Ti tomic rtio etween 1,6 nd 1,9, with cron content rnging from 12 to 30 %. In Fig. 4, the high resolution XPS spectrum of Ti 2p shows intense peks for Ti 2p3/2 nd Ti 2p1/2 centered t the inding energy vlues of 458,48 ev nd 463,16 ev, respectively. The mesured seprtion etween the Ti 2p3/2 nd Ti 2p1/2 peks ws 5,7 ev nd it corresponded to the inding energy seprtion oserved for stoichiometric TiO 2 [40]. The rther TiO 2 crystlline structure ws confirmed y XRD. The C 1 s ws deconvoluted into three peks (Fig. 4c). The pek centered t 284,8 ev ws ttriuted to the hydrocron dsored t the surfce nd the peks t 286,3 nd 288,5 ev were scried to cron linked to oxygen in different forms. The symmetric O 1 s region ws deconvoluted into two O1s Ti 2p Mg2p Si2p C1s Ti2p N1s Ti 2p 1/2 Ti 2p 3/ c C 1s d N 1s K 2p 1/2 C-O C=O, O-C-O K 2p 3/2 C-(H,C) TiO 2 /SLG TiO 2 /BSG TiO 2 /FS Fig. 4. XPS spectr of the surfce of TiO 2 film.

6 1152 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) NK1LL23 O=C,O-C,-OH O-Ti NK1LL23 O=C,O-C,-OH O-Ti c 3.2. Titni films photoctivity The photoctlytic ctivity of the films ws quntittively evluted y compring the degrdtion rte of SA. The overll process cn e summrized y the following rection eqution [30,43]: CH 3 ðch 2 Þ 16 CO 2 H þ 26O 2 hν E BGþTiO 2 18CO 2 þ 18H 2 O: ð5þ It ws ssumed tht the glol rection rte of the degrdtion of the SA monitored y FTIR cn e descried y the kinetic model corresponding to pseudo first order rection s suggested y Suwunym et l. [44]. The photoclytic ctivity ws expressed in terms of pprent kinetic constnt K pp. This constnt ws otined y plotting ln([sa] 0 / [SA]), where [SA] is the concentrtion t given time s function of time on strem. Fig. 6 illustrtes typicl FTIR dsorption spectr collected periodiclly versus illumintion time for TiO 2 /QS system. From the dt in Fig. 6, it is cler tht s illumintion time incresed the SA molecules dispper, with peks in the rnge cm 1. Fig. 6 shows comprison of the pprent constnt rte for the degrdtion of SA using the vrious titni films. The degrdtion rte oserved for TiO 2 /QS ws 1000 times higher thn tht registered for nd out 50 % higher thn tht for TiO 2 /BSG. The chnges of the contct ngle under UV irrdition cn e followed from the results presented in Fig. 7. The initil vlues of the contct ngle were 37,6±0,8, 31,6±0,9, nd 37,4±0,8 for TiO 2 / SLG, TiO 2 /BSG nd TiO 2 /QS, respectively. The difference in the initil vlues of the contct ngle etween the TiO 2 /BSG nd TiO 2 /QS is considered to e cused y difference in the concentrtion of surfce cron contmintion of the films, s reveled y XPS nlysis. However, the high contct ngle vlue oserved for should e O-Ti O=C,O-C,-OH NK1LL Asornce (%) 0,016 0,012 0,008 0,004 Fig. 5. XPS peks of the O 1 s region for ech titnium film (), () TiO 2 /BSG nd (c) TiO 2 /QS. Irrdition time 0,000 5min 15min 25min 35min Wvenumer (cm -1 ) 2700 peks (Fig. 5 5c). The min pek centered t the inding energy of 529,8 ev ws ttriuted to the Ti\O onds of TiO 2. The shoulder on the left-hnd side of the O 1 s pek nd centered t 531,4 ev ws ssigned to oxygen linked to cron nd to the O\H onds of the hydroxyl groups of TiO 2. XPS nlysis t O 1 s region, efore UV-irrdition, did not revel ny structurl chnge. The result suggests tht the compressive stress hs no influence on the hydroxyl species dsored t the surfce. The sodium content ws found to vry from less thn 1 % to 7,8 % depending on the sustrte (Fig. 4d). Indeed, the N + content ws less thn 1 % for BSG nd QS, nd ws higher for SLG sed sustrtes. This result ws in ccordnce with erlier studies discussing the diffusion of sodium-ions from the sustrte to the titni films during the nneling step [31,41,42]. However, it ws difficult to quntify the sodium-ion content t the surfce of the films grown on BSG nd QS, ecuse of the low sodium-ion content t the titni surfce nd lso to the superposition of the N 1 s pek with tht of the TiKL 1 L 1 Auger pek. K pp (SA).10-4 (min -1 ) SLG BSG QS Fig. 6. () FTIR sornce versus wvenumer spectr recorded for smple of TiO 2 / FS coted with steric cid, s function of irrdition time nd () disppernce rte (pprent kinetic constnt K pp ) of the SA coted, TiO 2 /BSG nd TiO 2 /QS.

7 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) Contct ngle (deg) K' (deg -1.min -1 ) ,05 0,04 0,03 0,02 0,01 0,00 TiO 2 /BSG TiO2/QS Irrdition time (min) SLG BSG QS Fig. 7. () Chnges in wter contct ngles of titni thin films grown on different kind of sustrte, () the rte constnt of the hydrophilic conversion of, TiO 2 /BSG nd TiO 2 /QS. due to the surfce roughness. The contct ngle decresed with UV irrdition time for ll smples. A similrity etween the photoinduced hydrophilic conversion efficiency nd the photoctlytic ctivity of the titni films ws oserved. Wheres the hydrophilic conversion of the photoctlyst surfce grown on SLG ws the slowest, those grown on BSG nd QS showed fster rte conversion versus the irrdition time. The kinetic constnt of the hydrophilic conversion is showed in Fig. 7, ccording to the work previously reported y Ski et l. [45]. Indeed, the hierrchicl order estlished for the kinetic degrdtion of SA ws well correlted to the hydrophilic conversion of the titni surfces. These results clerly suggest the existence of dependency of the photoctive properties of the TiO 2 films on the sustrte compressive stress. Let us discuss the dependence of the photoctive properties on the sustrte. The min difference oserved etween the films properties could e the compressive stress. Indeed, the rte of photoctlytic ctivity nd the hydrophilic conversion exhiits higher performnce when the residul stress increses in the film (see Tle 2). However, in the cse of the, neither the smll residul stress, nor the smllest sorption edge llows explining the wek photoctive performnce. Indeed, it ws reported tht the photoctlytic ctivity of sol gel titni films grown on sodium ion rich sustrtes ws sensitive to the diffusion of the N + ion [30 32,42] to the surfce. Nm et l. [31] reveled tht N + ions diffused from the SLG sustrte, rised the temperture required for ntse formtion nd gve rise the prticle size of the TiO 2 thin films, hence resulting in reduction of the photoctlytic ctivity. Wtne et l. [42] reported tht the sodium ion cts s recomintion center of the photogenerted e - /h + pirs in order to explin the deteriortion of the photoctlytic ctivity of the titni films. In the sme study, they showed tht the photoinduced hydrophilic conversion is not influenced y the diffusion of the sodium-ion. In contrst, we oserved dependence of the sodium ion concentrtion on oth the photoctlytic ctivity nd the photoinduced hydrophilic conversion. The sodium ion rich film (7,8 %, XPS dt) hd grin in size greter thn tht of the TiO 2 /BSG nd of the TiO 2 /QS sodium ion poor film (1%, XPS dt). This result suggests tht the diffusion of the sodium ion through the grin oundry rises the crystlliztion temperture hence the size of the grins. The ltter leds to modifiction of the surfce re of the films (t the moleculr level) nd cuses deteriortion of their photoctive properties. Actully, titni films grown on BSG nd QS exhiited comprle grin size, thickness, surfce re nd chemicl surfce composition. Nevertheless, they showed vrile photoctive properties nd vrile residul stress; prticulrly, medium compressive stress for TiO 2 /BSG nd lrge compressive stress for TiO 2 /QS. The results show tht s the compressive stress increses, the photoctive performnce ws significntly improved. Contrry to the findings presented herein, Shit et l. [21] reported tht the photoctlytic oxidtive decomposition of cis-9-octdecenoic cid shows no stress dependence while the photoinduced hydrophilic conversion shows remrkle stress dependence [21,22]. The uthors explined their results y considering tht seprte nd n independent mechnism occurred during the photoctlytic degrdtion nd the photoinduced hydrophilicity. In our cse, the distortion of the c lttice prmeter under compressive stress rther suggests reduction of the lttice distnce of Ti Ti toms. This compressive stress reduces the TiO 2 nd gp nd extends the photoresponse of the photoctlyst t higher wvelength (Fig. 3). The energy of the excittion of the TiO 2 photoctlyst ecomes lower for TiO 2 /QS thn tht for TiO 2 /BSG nd. To explin the enhncement of the rte of photoctlytic degrdtion of SA, oth the nd gp reduction nd the wel crrier loss cn e considered. The compressive stress reduces the nd gp energy nd extends the sorption edge of TiO 2 tohigherwvelength,whichinturnllowsthe photoctlyst soring more energy from the UV rnge. Since the degrdtion of SA could result of direct rection with photogenerted electrons/holes nd/or photooxidtion vi the formtion of highly rective oxidnt species. These ltter re products of the rection involving the dsorption of species (H 2 OndO 2 ) on the TiO 2 surfce. In the sis of the photoctlytic mechnism, the numer of the holes photogenerted t the TiO 2 /QS surfce cn e estimted to e pproximtely hlf of tht on the TiO 2 /BSG surfce, since the surfce properties were nerly identicl, nd the degrdtion rte of SA ws nerly hlf of tht of TiO 2 /BSG. This result seems resonle, since TiO 2 /QS photoctlyst with lrge compressive stress might hve more photogenerted crriers s compred to the TiO 2 /BSG photoctlyst possessing medium compressive stress due to the decrese in the surfce defect. Bsed on oth oservtion nd the work of Yin et l. [24], we suggest tht the compressive stress keeps the mteril defect-free, thus, improve its crrier collection. The photogenerted holes rect with the dsored wter producing more OH rdicls tht oxidize SA. These fetures cn help rtionlizing the resons ehind the improvement of the photoctlytic ctivity with the compressive stress ut not with the rte of the hydrophilic conversion. In one hnd, the power density reched for the photoctlyst surfce ws out (0,25+0,016) mw.cm 2. This vlue ws four time lower thn tht used y Tkeushi et l. [9] (1 mw.cm 2 ). Since the modelproposedytkeushiwssedonthedesorptionofh 2 Omolecules from the TiO 2 surfce during UV irrdition y heting effect of the light source, the decrese of the mount of H 2 O molecules leds to reduction of the distriution of the hydrogen onds resulting in the formtion of H 2 O thin lyer. The low flux used in our study void heting of the photoctlyst surfce nd llowed to exclude prtilly tht model to explin the photoinduced wettility since ny vrition of the temperture t the photoctlyst surfce (mesured y thermocouple) ws oserved during UV-irrdition. In other hnd, the comintion of the two mechnisms descriing the hydrophilic conversion should e involved somehow. The photoctlytic oxidtion of the hydrocron monolyer t the surfce of the films is responsile for the reduction of

8 1154 M.N. Ghzzl et l. / Thin Solid Films 520 (2011) the contct ngle t the high rnge (from ~30 to ~10 ) [8,10,11], nd the evolution of the compressive stress contriutes ctively to tht process. However, elow the vlue of 10 the mechnism for inducing the chnge in contct ngle ecomes complex. The UV illumintion proly induces structurl chnge on the photoctlyst surfce nd n dsorption of metstle OH groups occurs. This mechnism ws contemplted in order to explin this phenomenon. This concept ws reviewed y Fujishim et l. [46]. On the sis of Fujishim's concept, we cn ssume tht the compressive stress improves the crrier collection tht rects with the dsored wter nd leds to ond rekge, which might e improved y the hydroxyl group of the titnium surfce. 4. Conclusion Titni films with vrile compressive stress on different sustrtes were otined vi dip-coting of precursor sol. After tretment t 450 C, distortion of the lttice prmeters ws confirmed y XRD ptterns t different levels. 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