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Materials Chemistry and Physics93(2005)251–254Materials science communicationSuper-hydrophilic amorphous titanium dioxide thinfilm deposited by cathodic electrodepositionSubbian Karuppuchamy∗,Jae Mun JeongMolecular Engineering Institute,Kinki University,11-6Kayanomori,Iizuka,Fukuoka820-8555,JapanReceived15September2004;received in revised form11March2005;accepted13April2005AbstractA super-hydrophilic amorphous titanium dioxide(TiO2)thinfilm was prepared by simple electrodeposition method from aqueous peroxo-titanium complex solution.The surface structure was characterized by scanning electron microscopy(SEM).The hydrophobic amorphous TiO2film was converted into super-hydrophilic by the irradiation of ultraviolet(UV)light.The super-hydrophilicity of thefilms was controlled reversibly by alternate UV irradiation and storage in the dark.©2005Elsevier B.V.All rights reserved.Keywords:Electrodeposition;Titanium dioxide;Amorphous thinfilms;Nanostructures;Hydrophilicity1.IntroductionNanostructured semiconductor materials have been the prime focus of scientific research[1,2]due to their unusual optical[3],chemical[4],photoelectrochemical[5]and elec-tronic properties[6].Therein,nanocrystalline titanium diox-ide(TiO2)is one of the most investigated oxide materials owing to its important applications in environmental cleanup [7],photocatalysis[8],dye-sensitized solar cells[9–11]and gas sensors[12].Furthermore,the hydrophilic coating(0◦water contact angle)has emerged as a new and very attrac-tive application of TiO2[13–20].Since,the TiO2films are having amphiphlic properties under ultraviolet(UV)light irradiation and transparent na-ture,it can be used in various kinds of applications such as self-cleaning and anti-fogging materials used in motor cars, buildings,and household glazing[13–17].TiO2films show hydrophobicity and super-hydrophilicity at different condi-tions,and the wettability can be switched reversibly by al-ternation of ultraviolet light illumination of the surface and dark storage.This unique property is believed to be due to the synergistic effect of the surface photosensitivity and the ∗Corresponding author.Tel.:+81948227336;fax:+81948227336.E-mail address:kskchamy@(S.Karuppuchamy).geometrical structure of thefilms.Therefore,the suitable geo-metrical surface of thefilm is necessary to achieve improved hydrophilic property[17,20].The preparation method can play an important role to prepare the suitable geometrical surface for better hydrophilic TiO2.Regarding the surface morphology related to hydrophilicity of TiO2film,the prepa-ration method of many research studies has usually been sol–gel[13–17]or sputtering[21,22].Recently,Gao et al. has reported that the preparation of amorphous TiO2from peroxotitanate solution[23]and their super-hydrophilicity. However,the above method is a time consuming process because it requires several hours to120h for the prepa-ration of TiO2thinfilm.Electrodeposition technique of-fers a convenient and versatile route to deposit TiO2thin films from aqueous solutions.The electrodeposition tech-nique[11]has distinct advantages over the other techniques due to rigid control offilm thickness,uniformity,and depo-sition rate,careful regulation over reaction parameters such as solution concentration and composition and facile adapta-tion to conformal deposition on substrates of complicated shape and geometry.However,to the best of our knowl-edge,there is no report on the studies of hydrophilicity of cathodically electrodeposited amorphous TiO2thinfilm.In this communication,we report the super-hydrophilicity of an amorphous TiO2films by UV illumination,which were0254-0584/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.matchemphys.2005.04.015252S.Karuppuchamy,J.M.Jeong/Materials Chemistry and Physics93(2005)251–254fabricated by simple cathodic electrodeposition from aque-ous solution.This study indicates that the super-hydrophilicTiO2surface created by our simple and environmentallyfriendly process is a promising candidate for self-cleaninglayers.2.Experimental2.1.Preparation of TiO2thinfilmsAll chemicals were purchased from commercial sourcesand were the highest purity available.They were usedwithout further purification.Doubly distilled and ion ex-changed water was used in all the experiments.Opti-cally transparent indium tin oxide coated glass plates(ITO,10 /sq.,obtained from Musashino Fine Glass)were used as the substrates.Prior tofilm deposition,the ITO substrates were ultrasonically cleaned sequen-tially in acetone,2-propanol and water each for15min.A standard single compartment,three-electrode electro-chemical cell was used for thefilm preparation.A Ptfoil counter electrode and an Ag/AgCl reference elec-trode,along with ITO glass substrate as the working elec-trode,completed the cell set up.Electrodeposition wascarried out on a potentio-galvanostat(HOKUTO DENKO,HABF501).TiO2thinfilms were cathodically electrodeposited froman aqueous peroxo-titanium solution that has been describedin one of the author’s previous paper[11].Briefly,thebath consisted of0.05M TiOSO4,30%H2O2and0.1MNH4NO3and the deposition bath was kept at10◦C.Ca-thodic electrodeposition was carried out at−1.2V(ver-sus Ag/AgCl),which led to the formation of hydratedTiO2or amorphous state(TiO(OH)2·x H2O)film on the electrode.The as-depositedfilm was amorphous and crys-tallized in to anatase after heating at300◦C for1h inair.2.2.CharacterizationThe crystallization behavior of the electrodeposited TiO2films was analyzed by X-ray diffraction(XRD)using aRIGAKU RAD-2R diffractometer with Cu K␣radiation.The surface morphological features of the samples were ob-served using a scanning electron microscope(SEM,TOP-CON ABT-150FS).Film thickness was determined by usinga KOSAKA SE-2300surface profilometer.The photoin-duced hydrophilicity was evaluated by the water contactangle measurement,which was performed at ambient con-ditions(i.e.,25◦C,relative humidity(RH)65%)usinga commercial contact angle meter(Kyowa Kaimen Ka-gaku).Ultraviolet illumination was carried out using anHg lamp with an opticalfiber coupler,employing afil-ter to obtain light with a wavelength centered at365nm.The water droplet size used for the measurements was10␮L.3.Results and discussion3.1.Structural characterizationThe X-ray diffraction analysis was carried out for the elec-trodepositedfilms and found that the as-depositedfilms were amorphous(figure is not shown).The surface morphology of the as-deposited amorphous TiO2film is shown in Fig.1.The surface morphology of thefilm reveals an open porous struc-ture with the grain size falling mostly in the submicron range. The surface morphology of the stored amorphous TiO2film also has similar structure with that of the as-depositedfilm (figure is not shown).3.2.SuperhydrophilicityThe UV-light induced hydrophilicity of the electrode-positedfilms(ca.0.5␮m)was investigated by measurements of the water contact angle during thefilm exposure to light irradiation.To ensure fair comparison,twofilms were pre-pared under the same conditions and onefilm to be denoted as(a)was used for the measurement immediately after the deposition and the other one to be denoted as(b)was kept under dark at room temperature for2weeks.Irradiation of the amorphousfilms(a)and(b)by UV light at5mW cm−2 intensity in atmospheric air induced the changes of the wa-ter contact angle.Fig.2shows the water droplet shape on the TiO2films before and after UV light illumination.Prior to the light illumination,thefilms(a)and(b)were yield a water contact angles of12◦and118◦,respectively.It indi-cates that the as prepared(a)and stored(b)TiO2films are having hydrophilic and hydrophobic surfaces,respectively. Upon UV light illumination,the water droplet spread out on thefilm,resulting in a CA of about0◦for both thefilms. These results clearly demonstrate that the wettability changes from hydrophobicity to super-hydrophilicity.It should be no-tified that the CA for the as-deposited(a)film could still be measured,suggesting that thefilm surface is notcompletely Fig.1.SEM photograph of electrodeposited amorphous TiO2thinfilm.S.Karuppuchamy,J.M.Jeong/Materials Chemistry and Physics93(2005)251–254253Fig.2.Photographs of water droplet shape on the electrodeposited amorphous TiO2thinfilm(a)and the electrodepositedfilm which was stored in the dark condition(b)before(left)and after(right)UV light irradiation.wetted.It is well known that the organic contaminants are accumulates at high-energy surfaces[24].Since,TiO2also have high-energy surface,obviously it will adsorbs the con-taminants[25].The adsorbed contaminants are decomposed under the UV light irradiation of the TiO2surface and it be-comes more wettable due to the clean nature.During storage in the laboratory atmosphere,contaminants are gradually ad-sorbing at the surface,thus,making it more hydrophobic.It should be noted here that there are several reports says that the titania surfaces remain hydrophilic only for a few min-utes after cleaning[26]and easily contaminated within few days[25].The reasons for the increase of CA of the electrode-posited stored TiO2film(b)may also be due to the adsorption of contaminants during the storage and or due to the release of water molecules.After the UV irradiatedfilms were placed in the dark for 12h,again wettability of the surface was measured using a new water droplet and the hydrophobicity was achieved again and the contact angle values are39◦and115◦for the sam-ples(a)and(b),respectively.The sample(a)shows slightly higher contact angle compared to its previous value,indicates that the remaining water molecules are released during the storage.However,the contact angle of zero was still obtained after the irradiation of UV light for certain time.Hence,these films fulfill the cycle from hydrophobic to super-hydrophilic and vice versa.Fig.3shows the changes of water contact angle of thefilms (a)and(b)depends upon the UV light irradiation time.The as-depositedfilm(a)converted to super-hydrophilic with the shorter irradiation time compared to thefilm(b).The sim-ilar behavior was observed for both the samples after kept in the dark condition for12h(result not shown).Thefilm (a)has relatively low hydrophobicity due to the presence of water compared to thefilm(b)and that could be the reason for the fastest change to super-hydrophilic state right after 20min of UV light illumination.The presence of mixture of sub-micro and nanostructures of the electrodeposited TiO2 thinfilm surface should be responsible for these unusual hy-drophobic properties[27].The present results suggests that crystalline TiO2is not always necessary for a self-cleaning or antifogging surface;that is the crystallization process can be eliminated to achieve super-hydrophilicity by heating the amorphousfilm to high temperatures.3.3.Formation mechanism of SuperhydrophilicityIt is well known that TiO2films have a hydrophilic surface induced by UV light irradiation irrespective of their crys-talline nature[13,16,21,23].The mechanism of the photoin-duced hydrophilic conversion has been intensivelystudied Fig.3.Variations of water contact angle as they were induced by the UV light illumination(5mW cm−2)of the as-depositedfilm(a)and thefilm stored in the dark condition for2weeks(b)in air at RH of65%and temperature of 25◦C.254S.Karuppuchamy,J.M.Jeong/Materials Chemistry and Physics93(2005)251–254[13–20]and explained that the photoinduced hydrophilici-ties of TiO2is due to the structural changes of the surfaces. It is assumed that surface Ti4+sites were reduced to Ti3+ states via the photo-generated electrons,and oxygen vacan-cies were generated through the oxidation of the bridging O2−species to oxygen via the photogenerated holes.Suc-cessively,dissociated water adsorption on the vacancy sites, created hydrophilic OH groups on the surface.This photoin-duced hydrophilic process is always reversed slowly in the dark.The oxygen in the air was responsible for oxidizing of the hydrophilic surface via Ti3+to Ti4+state during storage in the dark[14].Hence,it can be assumed that during dark storage oxygen healing could be a possible candidate for hy-drophilic/hydrophobic conversion mechanism.4.ConclusionsThe surface wettability of the electrodeposited amorphous TiO2thinfilms was investigated.Reversible hydrophobicity to super-hydrophilicity transition was observed and nicely controlled by alternation of UV light irradiation and dark storage.The contact angle of the as-deposited amorphous film has greatly increased(about120◦)when it was stored in air at room temperature for a certain time,however,the super-hydrophilic surfaces with0◦contact angle was achieved after the irradiation of UV light.The present study clearly indicates that the electrodeposited amorphous TiO2is a promising ma-terial for self-cleaning layers.References[1]L.J.Guo,X.Cheng,C.F.Chou,Nano Lett.4(2004)69.[2]Q.Xu,B.D.Gates,G.M.Whitesides,J.Am.Chem.Soc.126(2004)1332.[3]A.Krolikowska,A.Kudelski,A.Michota,J.Bukowska,Surf.Sci.532–535(2003)227.[4]A.Kumar,S.Mandal,P.R.Selvakannan,R.Pasricha,A.B.Mandale,M.Sastry,Langmuir19(2003)6277.[5]N.Chandrasekharan,P.V.Kamat,J.Phys.Chem.B104(2000)10851.[6]G.Peto,G.L.Molnar,Z.Paszti,O.Geszti,A.Beck,L.Guczi,Mater.Sci.Eng.C19(2002)95.[7]M.Andersson,L.¨Osterlund,S.Ljungstr¨o m,A.Palmqvist,J.Phys.Chem.B106(2002)10674.[8]Y.Matsumoto,Y.Ishikawa,M.Nishida,S.Ii,J.Phys.Chem.B104(2000)4204.[9]B.O’Regan,M.Gr¨a tzel,Nature353(1991)737.[10]G.Benk¨o,P.Myllyperki¨o,J.pan,A.P.Yartsev,V.Sundstr¨o m,J.Am.Chem.Soc.125(2003)1118.[11]S.Karuppuchamy,K.Nonomura,T.Yoshida,T.Sugiura,H.Mi-noura,Solid State Ionics151(2002)19.[12]A.Rothschild,A.Levakov,Y.Shapira,N.Ashkenasy,Y.Komem,Surf.Sci.532(2003)456.[13]R.Wang,K.Hashimoto,A.Fujishima,M.Chikuni,E.Kojima,A.Kitamura,M.Shimohigoshi,T.Watanabe,Nature388(1997)431.[14]N.Sakai,R.Wang,A.Fujishima,T.Watanabe,K.Hashimoto,Lang-muir14(1998)5918.[15]R.Wang,K.Hashimoto,A.Fujishima,M.Chikuni,E.Kojima,A.Kitamura,M.Shimohigoshi,T.Watanabe,Adv.Mater.10(1998) 135.[16]R.Wang,N.Sakai,A.Fujishima,T.Watanabe,K.Hashimoto,J.Phys.Chem.B103(1999)2188.[17]T.Watanabe,A.Nakajima,R.Wang,M.Minabe,S.Koizumi,A.Fujishima,K.Hashimoto,Thin Solid Films351(1999)260. 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