Structural, thermal and optical characterization of TiO2

Structural,thermal and optical characterization of TiO 2:ZrO 2thin films prepared by sol –gel methodH.Bensouyad a ,H.Sedrati a ,H.Dehdouh a ,M.Brahimi a ,F.Abbas a ,H.Akkari b ,R.Bensaha a ,⁎a Laboratoire de Céramiques,UniversitéMentouri de Constantine,Route Ain El-Bey,25000Constantine,Algeria bDépartement des Sciences Fondamentales,Universitédu 20Août 1955,Skikda,B.P:26,21000Skikda,Algeriaa b s t r a c ta r t i c l e i n f o Article history:Received 19January 2010Received in revised form 9July 2010Accepted 15July 2010Available online 23July 2010Keywords:Thin films TiO 2–ZrO 2Sol –gel Anatase BrookiteOptical properties Structural properties Thermal propertiesWe have studied the structural and optical properties of thin films of TiO 2,doped with 5%ZrO 2and deposited on glass substrate (by the sol –gel method).The dip-coated thin films have been examined at different annealing temperatures (350to 450°C)and for various layer thicknesses (63–286nm).Refractive index and porosity were calculated from the measured transmittance spectrum.The values of the index of refraction are in the range of 1.62–2.29and the porosity is in the range of 0.21–0.70.The coef ficient of transmission varies from 50to 90%.In the case of the powder of TiO 2,doped with 5%ZrO 2,and aged for 3months in ambient temperature,we have noticed the formation of the anatase phase (tetragonal structure with 14.8nm grains).However,the undoped TiO 2exhibits an amorphous phase.After heat treatments of thin films,titanium oxide starts to crystallize at the annealing temperature 350°C.The obtained structures are anatase and brookite.The calculated grain size,depending on the annealing temperature and the layer thickness,is in the range (8.58–20.56nm).©2010Elsevier B.V.All rights reserved.1.IntroductionTitanium and zirconium oxides are very promising candidates for future technology of thin layers because of their good mechanical,thermal and chemical properties.Titanium oxide (TiO 2)is a cheap,non toxic and biodegradable material that is widely used in industry [1].This material is a semiconductor that is insensitive to visible light because of its band gap (3.2eV);it absorbs in the near ultraviolet region [2].It can be sensitized by a great number of dyes;some of them allow a conversion rate of incident photon –electron approach-ing unity.The signi ficant uses of TiO 2thin films are in solar cells [3],photo-catalytic systems [4]and electro-chromic systems [5].In other words,they are mainly used in optics.Zirconium oxide (ZrO 2)has good dielectric and optical properties [6,7]:it has a high refraction index [8].Additionally,it has a very good transparency on a broad spectral domain [9],a great chemical stability and a high threshold of resistance to laser flux.All these properties led to miscellaneous applications such as optical filters,laser mirrors [10],barrier layers to heat [11].ZrO 2films are also employed as buffer layer for super-conducting ceramics [12,13],as biomaterial for prostheses [14,15],as gas sensor [16]or as components in combustible batteries [17].ZrO 2is an insulating direct wide gap metal oxide,with an optical band gap in the range (5.0–5.85eV)[18].The solutions of deposits,prepared by sol –gel process,have a strong potential for the development of thin layers.The main advantages of sol –gel processes are low cost,excellent control on the film purity,homogeneity,simultaneous coating of two faces and possibility of forming multi and mixed layers.TiO 2–ZrO 2composite films are also of signi ficant scienti fic and technological importance.The most attractive properties of composite materials can be easily tailored by a simple control on the composition of the systems.Moreover,the composite materials often exhibit enhanced mechanical and thermal properties than both participating components TiO 2and ZrO 2.Indeed,various investigations have been devoted to sol –gel preparation of TiO 2,ZrO 2and TiO 2–ZrO 2composite materials.Most studies have focused on their properties for use as catalysts [19–22];however,little attention has been paid to their applications in optical coatings.In this paper,we report the study of structural,thermal and optical properties of TiO 2,doped with ZrO 2thin films deposited by the sol –gel process.Several experimental techniques were used to characterize structural and optical properties resulting from different annealing treatments and different layer thicknesses:X-ray powder diffraction,Raman spectroscopy,differential scanning calorimetric (DSC)and UV spectroscopy.2.ExperimentsThe TiO 2,doped with ZrO 2,thin films were prepared by dip coating,which is based on the hydrolysis of alkoxides,in alcoholicThin Solid Films 519(2010)96–100⁎Corresponding author.E-mail address:bensaha@yahoo.fr (R.Bensaha).0040-6090/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.tsf.2010.07.062Contents lists available at ScienceDirectThin Solid Filmsj o u r n a l h o m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /ts fsolutions,in the presence of an acid catalyst,in three steps.Thefirst step: the dissolution of1mol of butanol(C4H9OH)as solvent,4mol of acetic acid(CH3COOH),1mol of distilled water and1mol of tetrabutyl-orthotitanate(C4H9O)4Ti[23].In the second step,the solution of ZrO2 was prepared from the dissolution of1mol of zirconium oxychloride salt(ZrOCl2·8H2O)in distilled water and2mol of ethanol(95%)as catalyst.In the third step,the solutions of TiO2were doped with5%of ZrO2solution.Then,the resultant yellowish transparent solutions were ready for use.Glass substrates with refractive index equal to1.52and thickness of2mm were carefully cleaned before use.The substrates were dip-coated in the solutions at a constant rate of6.25cm s−1.After each dipping,thinfilms were dried for30min at a distance of40cm from a500W light source.The drying temperature of the light source is approximately equal to100°C.Subsequently,thinfilms were heat-treated in the temperature range(350–450°C),with a temperature increase rate of5°C min−1,for2h in the furnace.The powders obtained from the xerogel were prepared in room temperature and under air atmosphere.To determine the transformation points,the obtained powdered xerogels were analyzed by differential scanning calorimetry(DSC) using a SETARAM DSC-92analyzer,equipped with a processor and a measuring cell.The thermal cycle applied consists of heating from room temperature to520°C,holding for5min at this temperature andfinally cooling back to room temperature.X-ray powder dif-fraction was performed by Siemens D5005diffractometer,using a CuKα1radiation.The patterns were scanned at room temperature,over the angular range(10–70°2θ),with a step length of0.1°2θand counting time of1s step−1.The UV absorption studies were carried out using UV–VIS double-beam spectrophotometer SHIMADZU (UV3101PC).Its useful range is between190and3200nm.The treatment of the spectra was performed using the UVPC software.A surface profiler DEKTAK3ST AUTO1(VEECO)was used to determine film thicknesses.Raman spectra were recorded in a back scattering configuration with a Jobin Yvon micro Raman spectrometer coupled to a DX40Olympus microscope.The samples of doped and undoped TiO2thinfilms were excited with a632.8nm wavelength with an output of20mW.3.Results and discussion3.1.Structural properties3.1.1.Thermal analysisThe differential scanning calorimetric(DSC)curves of undoped TiO2 and5%ZrO2doped TiO2xerogels are shown in Fig.1.It is interesting to note that both doped and undoped xerogels showed a similar thermal behavior in the temperature range(20–250°C).Generally,weight loss corresponds to the evaporation of water,thermal decomposition of butanol as well as carbonization or combustion of acetic acid and other organic compounds which constitute metal alkoxides[24–26].Hence, the above thermal events were represented by an endothermic peak spreading from50to250°C.A broad exothermic peak in the290–410°C temperature range of TiO2xerogel can be attributed to the crystallization of titanium oxide as anatase phase[27].The addition of5%of zirconium oxide led to a shift of exothermic peak of anatase phase towards lower tempera-tures(286°C).This may be due to the speeding up of the crystallization of titanium oxide(anatase)compared to the undoped one.Another broad exothermic peak was observed close to386°C and it can be attributed to the decomposition and burning of organic groups[24,25].3.1.2.Crystalline phases(XRD,Raman)Fig.2a and b shows the X-ray diffraction(XRD)patterns of TiO2 xerogels of undoped(Fig.2a)and doped with5%ZrO2(Fig.2b).The XRD pattern evolution of titanium xerogel(Fig.2a)obtained after the evaporation of the organic compounds during3months of aging at ambient temperature shows that it is an amorphous phase as reported in reference[27].It has been reported that the used acid catalyst,during sol–gel preparation,plays a crucial role for determining the TiO2phase,e.g., Mechiakh et al.[27]and Ivanda et al.[28],found that powder is amorphous when they use acetic acid as catalyst.However,when using formic acid Ivanda et al.found that,in addition to amorphous phase,there is an amount of the anatase nanoparticles.Whereas, Music et al.[29]obtained that TiO2powder crystallizes in both anatase and brookite phases.This analysis of the doped TiO2xerogel exhibits that the addition of 5%ZrO2(Fig.2b)would be largely sufficient to form nanoparticles of anatase which crystallizes with(101)plane.It is interesting to note that the addition of a minor amount of ZrO2starts crystallizationof Fig.1.Differential scanning calorimetric curves of xerogels(a)undoped TiO2and(b)5% ZrO2doped TiO2.Fig.2.Evolution of XRD patterns of xerogels(a)undoped TiO2and(b)5%ZrO2doped TiO2.97H.Bensouyad et al./Thin Solid Films519(2010)96–100anatase.Whereas,A.Kitiyanan et al.[30]and B.Neppolian et al.[31] reported that addition of ZrO2has no effect on morphology of TiO2 oxide.Fig.3shows XRD patterns of both thinfilm oxides doped and undoped obtained after2dippings and various annealing temperatures at350°C,400°C and450°C.Clearly,titanium oxide starts to crystallize starting from annealing at350°C.Furthermore,all XRD patterns show a peak corresponding to(101)plane which is attributed to anatase whatever the annealing temperature.In addition to anatase phase,the presence of brookite can be observed,i.e.it crystallizes with(121)plane parallel to powder surface.Peak intensities corresponding to charac-teristic planes of anatase(101)and brookite(121)phases are obviously increased with the increase of annealing temperature.3.1.2.1.Surface morphology and grain size.The crystallite size L of TiO2 doped with ZrO2thinfilms can be deduced from XRD line broadening using Scherrer equation[32]:L=0:94×λffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiΔ2hkl−Δ2instrÀÁq1cosθ:λis the wavelength of X-ray beam(Cu Kα=1.5406Å),Δhkl is the full width at half maximum(FWHM)of(hkl)diffraction peak,Δinstr isthe FWHM corresponding to the instrumental limit,andθis the Bragg angle.The computed values of grain sizes,given in Table1,were calculated for different temperatures of annealing with the same thickness.Thus,the obtained grain sizes of anatase and brookite increase from8.58nm to20.56nm and from17.50nm to18.06nm, respectively.In fact,as annealing temperature increases grain sizes also increases,and doping with the ZrO2increases the crystallite size L in comparison with the undoped titanium oxide.The Raman spectra(Fig.4)display various peaks related to titanium oxide as anatase and brookite phases.These spectra exhibit bands at around138(strong),235(weak),514(weak)and632cm−1 (medium)for the thin layers of ZrO2doped TiO2.The above bands can be assigned to anatase phase except the band235cm−1which is due to the crystallization of brookite phase.While bands of144,188and 651cm−1can be assigned to both anatase and brookite phases [33,34].A slight shift of the most intense peak,E g,to smaller wavenumber is observed for all TiO2thinfilms doped with ZrO2.Similar displacements have been previously reported and they can be correlated with the confinement effects in nano-structured anatasecrystallites.Fig.3.Evolution of diffraction patterns of undoped and5%ZrO2doped TiO2thinfilms;obtained at various annealing temperatures(350,400,450°C)for thesame thickness.Table1Crystallite size L(nm)of the xerogels and the thinfilms for different annealingtemperatures and the same thickness.Phase L(nm)(hkl)Undoped TiO2Xerogel3months at T ambient Amorphous5%ZrO2dopedTiO2Xerogel3months at T ambient Anatase14.80(101)SamethicknessAnnealed at350°C Anatase08.58(101)Brookite17.50(121)Anatase16.66(112)Anatase14.74(200)Anatase16.33(105)Annealed at400°C Anatase10.09(101)Brookite17.61(121)Anatase17.27(112)Anatase15.57(200)Anatase18.71(105)Annealed at450°C Anatase13.92(101)Brookite18.06(121)Anatase19.09(112)Anatase18.63(200)Anatase20.56(105)Undoped TiO2Annealed at400°C Anatase09.21(101)Brookite16.37(121)Anatase13.78(112)Anatase16.05(200)Anatase18.24(105)Fig.4.Raman spectrum of undoped and5%ZrO2doped TiO2thinfilms at450°Cannealing temperature;A=anatase,B=brookite.98H.Bensouyad et al./Thin Solid Films519(2010)96–1003.2.Optical properties3.2.1.UV absorption analysisFig.5a and b displays diffused scattering UV –VIS transmittance spectra of TiO 2thin films undoped and doped with ZrO 2,for different annealing temperatures and different numbers of dipping in the wavelength range 300–800nm.Transmission of titanium oxide thin films decreases with the increase of annealing temperature and with the number of dipping.This can be due to the formation stage of anatase and with the increase in the grain size [35].The bands caused by the interference color of the film appeared in the wavelength range of 350–800nm.As can be seen in Fig.5,the presence of interference fringes is due to re flections on the levels of both film/substrate and film/air interfaces.The occurrence of such fringes means that our films are suf ficiently thick (see Table 2).A regular increase of the thickness with the number of dipping is clearly observed.A slight shift of transmission curves to higher wavelengths is observed for curves of doped thin films in comparison with those undoped.The refractive index of TiO 2thin films was calculated from measured UV –VIS transmittance spectrum.The evaluation method used in this work is based on the analysis of UV –VIS transmittance spectrum of a weakly absorbing film deposited on a non-absorbingsubstrate [36].The refractive index n(λ)over the spectral range is calculated by using the envelopes that are fitted to the measured extreme:n ðλÞ=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiS +ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiS 2−n 20ðλÞn 2S ðλÞq r S =1n 20ðλÞ+n 2S ðλÞ +2n 0n S T max ðλÞ−T min ðλÞmax ðλÞ×T min ðλÞwhere n 0is the refractive index of air,n s is the refractive index of the film,T max is the maximum envelope,and T min is the minimum envelope.The thickness of the films was adjusted to provide the best fits to the measured spectra.In this study,all the deposited films are assumed to be homogeneous.The porosity of the thin films is calculated using the following equation [37]Porosity =1−n 2−1n d!×100ð%Þwhere n d is refractive index of pore-free anatase (n d =2.52)[38],andn is refractive index of porous thin films.The results of the computed refractive index (n)and porosity (p)are shown in Table 3.It is noted that the refractive index of thin films of doped titanium oxide increases with increasing annealing temperature and number of dipping;however,the porosity decreases.It may be due to phase transition (anatase,anatase –brookite),which increases grain sizes and/or the density of layers.4.ConclusionIn this study,structural and optical properties of 5%ZrO 2doped TiO 2thin films,prepared by sol –gel method using dip-coating technique were studied.Analyses of doped TiO 2xerogel show that addition of 5%ZrO 2would be largely suf ficient to form nanoparticles of anatase phase by contrast to that of undoped TiO 2.X-ray diffraction and Raman spectroscopy analyses show that doped thin films obtained from annealing at 350°C crystallize in both anatase and brookite phases.Calculation of grain sizes by Scherrer's formula,gives sizes ranging from 8.58to 20.56nm for all structures.Analysis of UV –VIS transmission spectra shows that the 5%ZrO 2doped TiO 2thin films are transparent in the visible range and opaque in the UV region,whatever are the annealing temperature and the number of dipping.Refractive index of the thin films of titanium oxide increaseswithFig.5.UV –VIS spectra of undoped TiO 2and 5%ZrO 2doped TiO 2thin films,for various layers and at different annealing temperatures (a)350and (b)450°C.Table 2Variation of the film thicknesses d (nm)for different annealing temperatures and different numbers of layers.The standard deviation of the thickness is (8nm)from the average value for three measurements on five samples.T (°C)Film thickness d (nm)2layers3layers 4layers 6layers 8layers 350°C 6392147206275400°C 7598136235272450°C79108160212286Table 3Variation of refractive index (n)and porosity (p)for different annealing temperatures and different numbers of layers.The standard deviation of refractive index (n)is 0.5and of porosity is 0.02from the average value for three measurements on five samples.T (°C)4layers 6layers 8layers nPorosity n Porosity n Porosity 350°C 1.620.70 2.850.54 2.180.30400°C 1.970.46 2.130.34 2.210.27450°C2.170.312.230.262.290.2199H.Bensouyad et al./Thin Solid Films 519(2010)96–100increasing annealing temperature and number of dipping,but the porosity decreases.It may be due to phase transition(anatase, anatase–brookite)which increases grain sizes and/or density of 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