Nanostructure Study of TiO2 Films Prepared by Dip Coating Process

J. Matcr. Sci. Techno.. Vol.I8 No.1, 20023Nanostructure Study of TiO2 Films Prepared by DipCoating ProcessS.M.Atlia, Jue WANG, Guangmning WUt, Jun SHEN and Jianhua AAPohl Instiule of Solid State 'lhysics. Tongji I'niversity. Shanghai 200092. Chinal Manuscripi received March 30. 2001. in revised form \lay 31. 2001|TB3 AThe microstructure properties of the sol-ge1 derived TiO2 filmns were studied by the atomic force microscopy (AFM).The films were prepared by dip coating process. The optical properties of the filmns were explained on the basis ofthe microstructure of the films.KEY WORDS: Atomic force microscopy. TiO2 thin fiIm. Sol-gel process .1. Introductionstralcs. The thickncss of the substrate was 5 mm. First.they were cleaned ultrusonically in ether. rinsed with deion-The structural property of the thin filt is importart sinceized water. then dried al 70°C for 1 h.Alterr cooling,it can give an explanation for the behavior of the optical prop-substrates were dipped in the solution, then withdrawn inerties of the thin films. The roughness of the surface has andifferent speeds ranged from 3.66 to 22 cm/min. Then theimportant ffct on the transmission and reflection spectrafilms were dried in air at 70°C for 30 min. The films wereof the film. An antireflertion coating can be approached bycrack-Free. Finally, the 6lms were anncaled for half an hourembussing of surfaces with a microstructure of dimensionsat different tenperaturcs. 200 and 450°C in order to studysmaller than the wavelength of light-2l. The mnicrostructuretheir opt ical propertien and nicrostructure.can be investigated by atoumic force inicroscopy (AFM). TheAFM probes the surface of a sample with a sharp tip, which2.3 Measuremnentsis located at the free end of a cantilever. Forces between the2.3.1 X-ray diffractionTiO2 powders were characterized bytip andsaunple surface cause the cantilever to bend, orX-ray diffraction (XRD) using Philips Pw1700 X-raifrac-deflect. A deteclor measures the cantilever defection as thetip is scanned over the samplc, or the sample is scanned unuder2.3.2 EllipsometryThe thickness and refractive indexthe tip. The measured cant ilever deflections allow a conputerof the films were ineasured by a monochromnatic ellipsometerto generate a nap of surface topography.(type TP-77) at a wavelength of 632.8 nm.TiO2 flms are widely used for optical and electrical appli-2.3.3 AFM The topographies of the surface were iaves-cations due to their optical and dielectric property!-:. TiO2tigated by AFM (Nanoscope II with A-head, Digital Instru-thin films have many applications such a electrochroinicments. Inc.. Santa Barbara, CA, USA) operating in air inswitchesI. electrochrornic displays可l, smarwindowsl! .cuntact mode and tapping mode. For cuntact mode opera-photocatalysts", ceramic membranes-8, and in the synthe-tion, the standard silicon nitride tip was used with a radius ofsis of photoelectrochemical solar ells9l.10 11n and spring collstant of 0.01 N/m. The resolution of theIn this paper we present an AI'M study on the infuce ofimage was 512x512 points. For the tapping mode operation,the withdrawing speed and annealing temperat ure on the op-the standard silion tip was used with a radius of 5 nm andtical property of the sol-gel derived TiO2 thin filns preparedspring conust,ant of 100 N/m. The resolution of the image wasby dip coating technique.512 x512 points. The scanning area was around I pm2。2. Experimental3. Results and Discussion2.1 Prcparation of the Ti02 solution3.1 X-ray difractionDifferent TiO2 thin fiIms were prepared by the solgelX-ray difraction patterns of TiO2 powders annealed atmethod at diferent anncaling temperatures. The detailsdiferent temperatures of 200 and 400°C are showed in Fig.1.of the preparation of the starting solution was describedThe XRD pattern of the sample annealed at 200°C showedelsewhere101. The starting solntion was prepared by dissolv-no rrystalographic feature and the sample was amorphous.ing 1 mole of Ti(OCjHg); in 20 mole of ethyI alcohol andThe sample treated at 400°C showed anatase features only.adding 0.2 mole of acety| ucetune. 'T'len 1.5 mole of aceticThe change from amorphous state lo crystalline state affectsacid was dissolved in 3 mole of II2() and 20 mole of ethylthe optical property and microstructure of the filns.alcohol, and added to the first solution during stirring. Thestirring of the solution was continued for half an hour, while3.2 AF'M image of the surface of TiO2 flmsthe veessel was covered well to prevent the reaction of the so-The norphologics of the surfaces are shown in Hig.2. Thetotplution with the moisture in air. Then the solution was mixedobservation of the filn treated at 200 C in contact mode wasultrasonically for half an hour. Acetyl aretone was added to :impossible.slow down the speed of hydrolysis and condensation reactionscarried out in tapping mode. Figure 2(a) shows the morphol-and to forn a stable sol. The solutions were kept for 1 weekogv of the surface of the film. The withdrawing speed of theat room ternperature. TiO: gel was prepared by drying thefilm was 22 cm/min. The surface of the film is stnooth. Theinitial solution at 70°C in air. 'TiO2 powders were preparedroot mean square (RMS) roughness of the surface was of theby annealing the gel for 1 b at diferent temperatures 200.urder 0.245 nm.400°C.中国煤化工e surface of thefilm ainnealeced of the film2.2 Coating process(h) wias 1cIY;vas 22 cm/ rnin,Glass plates of dimensions 6 cmx 12 cnl were used as sutb-The surface. CNM H Grasthe preesence .of large particles. Theiameteroft: particles ranged from+To whom correspondence should be addresserd30 to 100 nn. whilr the maximum height was 14 nn. TheE-mail: gmingwngonliesl.cn, Ph.D.. AssoC. Pruf.321 Mater. Sri. Technol.. Vol.I8 No.1. 2002(4 nml") with that of the samples. it is clear that the TiO2laver reduces signifirantly the roughness of the siuhstrate. TheAFMI resu:!Ls are in good agreenent with the others(12.13] .3.3 Ellipsonery :1 hue dependence of the thickness of TiO2 thin filmus pre-美是美pared by dip coating procss on the wit hdrawing speed isshown il Fig,3.It is shown thut the thickess increasied non-201'0linearly with increasing the withdrawing sperd. It rangedfrom 510 80 11 while the withdrawing specd ranged from3.66 to 22 cm/ min. This berhavior agrees with t.hat reported20by Iandau aund Levih'l4l, wbere the thickness of the wet film壮depis given by(n1:6)9/2Fig.1 X-ray difraction pattern fur the TO2 powders al-d- 0.94-nealed at 200 aund 400*('°(ρ.9)1/2where li is the withdrawing speed, η is the viscosity, r is theRMS roughn:ss of the film wabs of the order 0.8 um.liquid-vaupor surfare tension, p is the density of the liquid, andIhe R.\IS roughuess of the film (cus rloseto I un. Forg is the gravitational constant. After heal treatinent the filmsthis film another image witlh surface areu 100 pum2' is sthuwn became denser and the thickness decreased.in Fig.2(d). The image shows the prescence uf large paricles.Figure 4 shows the dependence of the refractive index ofThe diameler uf the particles ranged frou 200 t0 600 n Trio2 thin fius pepared by dip coating proces on the with-with maximun height 17 nn. The presence of these grains drawing speed. It is shown that the refractive index increasedin Fig2(b.c) where the ancaling temperalure was 150°C. linearly with incresing the withdruwing speed. It rangedwas due tn the erystallizatiou of the flm al anealing tern- fromn 1.9 to 1.94 while the wit hdrawing speed ranged fromperatures higher than 400°C. (onparing the AF'MI images of 3.66 to 22 cm/min. The dependence of the refructive indexFigs.2(L) and 2(d), one can see that the mumber of crysalliteO11 the withdraw ing apeed can be explained on the basis of theparticles per unit area increased wilth increasing withdrawingmicrostructure of the surface of the films and their porosities.speed.As ilustrated in the previous section the nunber of crystalliteBy conmparing the RMS roughuess of glass substrate particles per unit area on the surface of the film increased(a)(b)/00000.6200."(C)(d).5~1.0中国煤化工MHCNMH GFig.2 AFM images of the TiO2: (2) the flm annealed at 200"C, witbdrawing speed is 22 cm/ mnin (tapping mode),(b) the filn annealed at 450*C and the withdrawing speed is 44 cm/ min (contact mode), (c,d) the flmannealed at 450*C and the withdrawing speed is 22 (imn/ min (contact mode) at different scauning areasJ. \latcr. Sci. ltethuol, Vil.18 No.1. 20023:with the withdrawing sperd It i slown thar the porositydeereases with increasing the witdrawing speed. This may“厂be lue to the microst rurture of the surfacr of the film.or4. SummaryTiO2 films wre prepard by dipping glass substrate in asolution of TiO2. The filns were prpared at difterent witb-drawing speeds range form 3.6 to 22 en/nin. X ray difrac-tion paltr wus useul tu characterize the crystal structureof the Ti()2 powder. The refrctive index and thickmess ofthe flIns wore measired by ellipsonueter. AFM was used toWindawng spedr ctm mn'sl udy the morpholngy of the films. T'le results slowed thatas the annealing temperalure increused the TiO2 changedFig.3 Dependence of the thickness of TiO: tilnuns prepuredfrom anwurphous to cryslalline phase where the anat ase phaseby dip coatings procrsss on the withdrawing spedwas obscrvcd ut around 400*C. As the wit hdrawing speed in.rreased bot h the refractive index and the thickness of the fimincreased, and the lumber of eryst alite particles per unit areaon the surfare o[ the film increased leading to the increase ofthe refractive index. The porosity deereased with increasingthe withdrawing speed.Acknowledgemen!We are very grateful to the National Natural Scicnee of Foun-dation of Chint (No. 69978017 and 59802007) and Shangbai Ed-ucation onmittee (No. JW99- 1.1-03) for their help and finaucialSuUpIpuorth.Fig.4 Dependence uf the refractive index of TiO; films prrHEFFRENCESpared by dip couting process on the wit hdrawingspeed中! C.G.Graqvisl and V.Wittwer: Sol. Energy Matr. Sol. Cells,1998, 39, 54..2) V.Witwer. A.CGomnbert, W Grar and M.Kohl: SPIE. 1995, 2,[3] P.Babclon， A .S.J)equiede，HMosterfa-Sba. S Bourgeois,P.Sibillot and M.Siarilotti: Thin Solid Films, 1998. 63, 322.[4] A.Hagfeldt, N.V lachopoulxs, S.Cillbert and M.Ciratsel: SPIE,1994. 297, 2255.5 ] A.Hagfeldt, L.Walder and M.Gratzel: SPIE, 1995, 60, 2531.」R.Cinnsenlarh, G Boschloo, S.N.Rao and D.F itznaurice: Sol.Energy Mater. Sul. C'ells. 1999, 107, 57riN.N.Negishi, K.Takeuchi iane T,lbusuki: Sol-Gel Sei. Technol,1998, 691, 13.8] D.S.Bac. K.S.Han and SII.Choi: Soulid State lonics. 1998. 239,109windaing see m m'[9J.Hagen. w Schaffrath. P.OLschik and D.Haarer: Sol.Finergy" slater, Sicrl. Cells. 199 167. 56.Fig.5 Variation of the porosity of Ti02 films with the with-|10] S.M.Auria. J.Wang. J.Sthen G.A1.Wu. Q.Zhang, 7.Lai anddrawing speed[| RJ.Marin-Palma. L.Vazquez, J.M.M1-Duart and M.Riera: Sol.Energy Nlater. SolCells.1998,55.53.with increasing the withdrawing speed. This leads to all in-12] M.Zaharescu and M.Crisan: Sol-Gen Sci Technol,. 1998, 079,creasc of the refractive index. As reported bv Wuet a1.l th:effective relractive index of the film is related to the porosity13↓Y.Du. E.Radlein aud G.H.Frischat: J. Sol-Ciel Sci. Technol,p of the film by the relation.1998。 7G3, 13[1川] C.J.Bruker. A.J.1lurd. P R.Sc lunk. G.C.irye and CS. Ashely:Non Cryst. Sulids, 1992. 424，147/118.[15] Guangmning WT. Jue WANG. Jun SHEN, Tianbe YANG,Qinvu ZHANG, Bin ZHOI' and Zhongsheng DENG: ActaPh's. Sin.. 2001, 175. 50, 1. (in Chinese)where n。is the refractive index of the pore-free anatase16| L.Hu. T. Yoko. HL.Kosuku und S.Sakka: Thiu Solid Pilms, 1992,(ng =2.5206). Figure 5 shows the variation of the porusit!'18. 219. .中国煤化工MYHCNMHG