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CHIN. PHYS. LETT.   Vol.29, No.8 (2012)088103

Superhydrophilic and Wetting Behavior of TiO2   Films and their Surface

Morphologies   ∗

WANG Wei(王伟)1, ZHANG Da-Wei(张大伟)1∗∗, TAO Chun-Xian(陶春先)1, WANG Qi(王琦)1,WANG Wen-Na(王文娜)1, HUANG Yuan-Shen(黄元申)1, NI Zheng-Ji(倪争技)1,

ZHUANG Song-Lin(庄松林)1, LI Hai-Xia(李海霞)2, MEI Ting(梅霆)31Engineering Research Center of Optical Instrument and System (Ministry of Education), Shanghai Key Lab of 

Modern Optical System, University of Shanghai for Science and Technology, Shanghai 200093 2Department of Information Science and Technology, Shandong University of Politics Science and Law, Jinan 2500143Laboratory of Nanophotonic Functional Materials and Devices, Institute of Optoelectronic Materials and Technology,

South China Normal University, Guangzhou 510631

(Received 16 February 2012)

TiO2  films, showing superhydrophilic behavior, are prepared by electron beam evaporation. Atomic force mi-croscopy and the contact angle measurement were performed to characterize the morphology and wetting behavior of the TiO2   films. Most studies attribute the wetting behavior of TiO2   surfaces to their physical characteristics rather than surface chemistry. These physical characteristics include surface morphology, roughness, and agglom-

erate size. We arrange these parameters in order of effectiveness. Surface morphologies are demonstrated to be the most important. TiO2   films with particular morphologies show superhydrophilic behavior without external stimuli, and these thin films also show stable anti-contamination properties during cyclical wetting and drying.

PACS: 81.05.Rm, 81.40.Ef, 68.60.Wm   DOI: 10.1088/0256-307X/29/8/088103

TiO2  has attracted great interest due to its opti-cal, electronic[1,2] and wetting properties. There havebeen many studies on the particular physical charac-teristics of TiO2   films that affect wetting behavior,i.e., surface morphology, roughness, and agglomeratesize.[3,4] Zubkov   et al  .[5] demonstrated that the su-perhydrophilicity of TiO2   films is caused by the at-

tractive interaction of water with clean TiO2   insteadof any changes in surface chemical composition re-sulting from UV-light irradiation. Furthermore, earlyworks[6,7] related to the wetting phenomenon to gen-eral surface properties have been confirmed by an in-creasing number of studies.[8−12] The superhydrophilicstate can be induced by fine tuning of surface physicalcharacteristics, circumventing the need for any exter-nal stimuli.[3] Determination of the relative levels of effectiveness of these physical characteristics remainsto be carried out. In the present study, we produceTiO2  films with controlled variations in surface mor-

phology that plays key roles in wetting behavior, andthis indicate the possibility of manufacturing surfaceswith controlled wettability.

A simple model described by Wenzel[13] charac-terized the influence of surface roughness on the hy-drophilicity of solid surface, and he proposed thata sufficiently rough surface texture would be readilywetted. Thus, a large amount of inorganic particu-late matter adhering to the surface attenuates wetting

behavior by filling the asperities that allow a roughsurface to be wetted, especially when the films aresubject to cyclical wetting and drying. However, su-perhydrophilic thin films generally have high valuesof surface roughness, typically in the range of 20–80 nm.[4,14,15] Therefore, controlling morphology andreducing surface roughness below a critical level could

be a useful route to the production of superhydrophilicTiO2  films.

To determine the relative effectiveness of the phys-ical parameters and the influence of low rms surfaceroughness, TiO2  films with various surface morpholo-gies, degrees of roughness, and agglomerate sizes (alla direct consequence of the film thickness) were pro-duced. Using the test result of these materials wedemonstrate that surface morphology is the predom-inant factor controlling the wetting behavior of TiO2

films.TiO2  films were prepared by electron beam evapo-

ration without the use of ion beams. The thickness of the films was monitored by quartz crystal oscillationduring the depositing process and confirmed by us-ing a step-height profiler after the depositing process.The films were divided into the followed six thicknessranges: 100nm, 200 nm, 300 nm, 500 nm, 1000 nm and2000 nm. BK7 glass was chosen for the substrates. Af-ter depositing the TiO2  material onto the glass sub-strates, the coated substrates were annealed in a muf-

∗Supported by the National Natural Science Foundation of China (60908021), the National Key Technologies R&D Program(2011BAF02B00), the National Science Instrument Important Project (2011YQ15004), Singapore National Research Foundation

(CRP Award No NRF-G-CRP 2007-01), and the Leading Academic Discipline Project of Shanghai Municipal Government (S30502).∗∗Corresponding author. Email: dwzhang@usst.edu.cn© 2012 Chinese Physical Society  and   IOP Publishing Ltd

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fle furnace at 500◦C for one hour. Before and after thisprocess, the topography of the TiO2  surfaces (Figs. 1,2(a), and 2(b)) were scanned with a nanoscope atomicforce microscope (AFM) (PSIA Inc.) in tapping mode.A silicon scanning probe microscopy cantilever was

used to draw the topography of the surface. The con-tact angle measurements were performed using thesessile drop method, in which water droplets (3–5µL)were gently deposited on the TiO2   surfaces with amicro-injector. A progressive scan camera capturedthe digital images of the dispersed droplet which areshown in Figs. 2(c) and 2(d).

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Fig. 1.   Morphological images of unannealed TiO2   sur-

faces: (a) 150nm, (b) 2000nm. Morphological imagesof annealed films: (c) 150 nm, (d) 2000 nm. (e) Three-dimensional (3D) image of (d) an annealed TiO2   film inthickness 2000 nm.

Figures 2(a) and 2(d) show the results of wetting.Thicker TiO2 films (500–2000 nm), although they havelower surface roughness (2.0–2.3), show more super-hydrophilic wetting behavior than the thinner TiO2

films (150–300 nm) after annealing. This observationis at odds with previous work[16,17] where a higherdegree of roughness increases superhydrophilic behav-ior on a TiO2  film surface. Hence, surface roughness

does not completely determine superhydrophilic wet-ting behavior. In addition, all of our TiO2  films werefabricated under the same conditions, and neither UV

irradiation nor visible light was applied to stimulatethe hydrophilic behavior. Based on Zubkov’s theory,[5]

we can exclude the effects of chemical reactions. Thuswe attribute the hydrophilic behavior specifically tothe morphological transformation which corresponds

to crystallization (anatase formation) after annealing,as shown in Fig. 3.

                       

         

         

         

     

          

     

    

     

       

          

     

         

         

      

       

     

      

      

     

    

      

       

      

    

      

    

     

               

         

Fig. 2.   (a) Variation of rms roughness with increasingfilm thickness. (b) Variation of contact angles with filmthickness. Contact angle measurements were made withwater droplets placed in four separate positions on thefilm. Data shown in (b) are the average values of thecontact angles and captured after 1 s. (c) The state of awater droplet on a 1000-nm-thick TiO2  film after 1 s. (d)A water droplet on a 150-nm-thick TiO2  film after 1 s.

       

         

       

         

Fig. 3.  XRD patterns of TiO2   films with thicknesses (a)2000 nm and (b) 150 nm.

The three-dimensional image in Fig. 1(e) showsthat, after annealing, the surface morphology waschanged, with more recessed areas. Thus, the crystal-lization can generate a more porous morphology. Thesuperhydrophilic wetting behavior can be explainedby highly accessible pores on the TiO2   films, whichcan enhance the diffusion within the film structure,and subsequently allow water droplets to penetratethrough the voids. Furthermore, the nanocapillary

effect within the porous network could also be incharge of the superhydrophilic wetting behavior. Formore advanced morphological parameters of the sam-

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CHIN. PHYS. LETT.   Vol.29, No.8 (2012)088103

ples, we calculate the height-height correlation func-tions from height data in the AFM examination. Theheight-height correlation function  H (ρ)   is defined byH (ρ) = 2[w2−Rh(ρ)] where Rh(ρ) = ⟨h(r0)h(r0 +ρ)⟩is the autocorrelation function of the rough surface,  ρ

is the correlation separation and  w   is the rms rough-ness. For the quantitative analysis, we use the roughself-affine fractal surface model, of which H (ρ) is givenby the phenomenological function[18]

H (ρ) = 2w2{1 − exp[−(ρ/ξ )2α]},

where ξ  is the correlation length of the surface, and αis the roughness exponent related to the surface frac-tal dimension  Df    by   α   =   d −  Df    with   0  ≤   α  ≤   1and   d   being the embedded dimension. In Figs. 4(a)and   4(b), the scattered curves calculated from theAFM image data give  H (ρ)   versus  ρ   for the samples

given in Figs. 1(a) and 1(d). The curve fits from thefunction of  H (ρ) are also given in Figs. 4(a) and 4(b).The values of both  ξ  and  α  are shown respectively inthe figure legends. We see that for the 150-nm-thicksample,  α   increases from 0.665 to 0.758, which indi-cates a decrease in the fractal dimension  Df . How-ever, for the 2000-nm-thick sample,  α  decreases from0.620 to 0.571, due to the irregular short-range gran-ular structure seen in the magnified image Fig. 1(e),which may lead to a decrease in  α  or an increase inDf . For the 150-nm-thick sample, ξ   increases signifi-cantly from 22.05 nm to 52.74 nm, while for the 2000-

nm-thick sample, ξ  does not significantly change. Thisconfirms that more 3-D voids are present in the 2000-nm-thick TiO2   films after annealing. This in turndemonstrates that it is not the surface roughness butthe surface topography that is the dominant factorintensifying the hydrophilicity of anatase TiO2.

           

    

       

           

                               

                       

             

           

                       

         

      

    

      

       

     

    

       

      

    

      

       

     

    

      

      

     

     

      

    

      

     

    

      

       

    

     

     

     

       

          

      

     

    

       

    

       

       

                               

                       

       

           

                               

                       

Fig. 4.   (a) The calculated height-height correlation func-

tion curves and (b) self-affine fractal fits of the sample inthickness (a) 150 nm and (b) 2000 nm.

The thickness of the unannealed TiO2  film was in-

creased from 500 nm to 2000 nm, with the result thatthe morphologies of these films increasingly approx-imated to the hierarchical topography proposed byZorba et al .[3] Particularly, films with an average thick-ness of 2000 nm (shown in Fig. 1(b)) have exactly the

same hierarchical topography. However, in our ex-periment, the size of the particles which agglomeratetogether to form the large clusters is much smallerthan that in the hierarchical porous TiO2   surface de-scribed by Zorba   et al  .[3] Thus, although having asimilar morphology, the small agglomerate size meansthat these unannealed samples exhibit poor wettingbehavior (contact angle (CA) is 40◦). Therefore, thissupports the conclusion[4] that the size of the parti-cles is still a parameter in retaining wetting behavior.We therefore conclude that surface roughness and ag-glomerate size have a less significant effect than mor-

phologies on superhydrophilic wetting behavior.To demonstrate that TiO2   films with low rough-ness are effective when used for decontamination of inorganic chemical particles, we subjected the films tosuccessive wetting and drying cycles. The results areshown in Figs. 5(a) and 5(b). After five cycles of wet-ting and drying, the contact angle of TiO2  films withlow rms values increase from 2◦ to 30◦ and the con-tact angle of TiO2 films with high rms values increasesfrom 10◦ to 50◦.

           

         

         

     

      

       

     

          

    

     

     

      

       

      

      

    

     

          

     

         

Fig. 5.   Comparison of the decontamination ability of films with rms roughnesses 6 nm (a) and 2.3 nm (b) af-ter 5 cycles of wetting and drying. Both the images werecaptured by a high powered objective lens mounted on theAFM. (c) Comparison of the antifogging abilities of a glasssubstrate (left) and a TiO2  coated glass substrate (right).(d) Transmittance of a TiO2  coated glass as a function of wavelength.

Although the thicker TiO2  films exhibit superhy-drophilic wetting behavior, the transmittance of thesefilms can also reach satisfactory levels (82%) as shownin Fig. 5(d). To demonstrate the antifogging potentialof our TiO2  films, we exposed an untreated glass sub-

strate and a TiO2 film with thickness of 1000nm to thehumid air after the samples were chilled in a freezerat approximately  −17◦C. Figure 5(c) shows the com-

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CHIN. PHYS. LETT.   Vol.29, No.8 (2012)088103

parison of the antifogging properties of a glass sub-strate (left) and an annealed TiO2  coated with glasssubstrate (right). The reason for the antifogging be-havior is that a super hydrophilic surface allows thecomplete spreading of the water droplet on the surface

to form a uniform transparent film.In conclusion, physical parameters, such as sur-face morphology, roughness, and agglomerate size, to-gether affect the wetting phenomenon of TiO2   films.It is important to find the most important physical pa-rameter that affects wetting. In our study, we demon-strate that surface morphology plays a major role onretaining the superhydrophilicity of TiO2   films. Thistype of TiO2  surface is also able to alleviate contam-ination by inorganic particles because of low surfaceroughness. Although an antifogging TiO2   surface isthick for a film of this type, typical transmittance re-

quirements can still be reached.

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