8/12/2019 Superhydrophilic and Wetting Behavior of TiO2 Films and Their Surface Morphologies
http://slidepdf.com/reader/full/superhydrophilic-and-wetting-behavior-of-tio2-films-and-their-surface-morphologies 1/4
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: [email protected]© 2012 Chinese Physical Society and IOP Publishing Ltd
088103-1
8/12/2019 Superhydrophilic and Wetting Behavior of TiO2 Films and Their Surface Morphologies
http://slidepdf.com/reader/full/superhydrophilic-and-wetting-behavior-of-tio2-films-and-their-surface-morphologies 2/4
CHIN. PHYS. LETT. Vol.29, No.8 (2012)088103
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).
0
0
250
200
500
400
750
600
1000 0 250 500 750 1000
1000
(nm)
(nm)
( n m )
( n m )
(nm)
(a) (b)
(c)
(e)
(d)
1 0 0 0
7 5 0
5 0 0
2 5 0
0
1 0 0 0
7 5 0
5 0 0
2 5 0
0
1 0 0 0
7 5 0
5 0 0
2 5 0
0
20
10
0
-10
-20
8
4
0
-4
-8
8
4
0
-4
-8
8
4
0
-4
-8
8
4
0
-4
-8
( n m )
( n m )
1 0 0 0
7 5 0
5 0 0
2 5 0
0
1 0 0 0
7 5 0
5 0 0
2 5 0
0
( n m )
( n m )
(nm)
(nm)
(nm)
(nm)
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-
088103-2
8/12/2019 Superhydrophilic and Wetting Behavior of TiO2 Films and Their Surface Morphologies
http://slidepdf.com/reader/full/superhydrophilic-and-wetting-behavior-of-tio2-films-and-their-surface-morphologies 3/4
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-
088103-3
8/12/2019 Superhydrophilic and Wetting Behavior of TiO2 Films and Their Surface Morphologies
http://slidepdf.com/reader/full/superhydrophilic-and-wetting-behavior-of-tio2-films-and-their-surface-morphologies 4/4
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.
References
[1] Dai S Y and Wang K J 2003 Chin. Phys. Lett. 20 953[2] Lan X H, Yang S Q, Zou Y, Wang Z A and Huang N K
2007 Chin. Phys. Lett. 24 3567[3] Zorba V, Chen X B and Mao S S 2010 Appl. Phys. Lett. 96
093702[4] Law W S, Lam S W, Gan W Y, Scott J and Amal R 2009
Thin Solid Films 517 5425[5] Zubkov T, Stahl D, Thompson T L, Panayotov D, Diwald
O and Yates J T 2005 J. Phys. Chem. B 109 15454[6] Cassie A B D and Baxter S 1944 Trans. Faraday Soc. 40
546[7] Wenzel R N 1936 Ind. Eng. Chem. 28 988[8] Barthlott W and Neinhuis C 1997 Planta 202 1[9] Zhai L, Cebeci F Ç, Cohen R E and Rubner M F 2004 Nano
Lett. 4 1349[10] Feng X J and Jiang L 2006 Adv. Mater. 18 3063[11] Dorrer C and Rühe J 2008 Langmuir 24 1959[12] Bormashenko E, Bormashenko Y, Whyman G, Pogreb R
and Stanevsky O 2006 J. Colloid Interface Sci. 302 308[13] Wenzel R N 1949 J. Phys. Chem. 53 1466[14] Stevens N, Priest C I, Sedev R and Ralston J 2003 Lang-
muir 19 3272[15] Borras A and González-Elipe A R 2010 Langmuir 26 15875[16] Song S, Jing L Q, Li S D, Fu H G and Luan Y B 2008
Mater. Lett. 62 3503[17] Katsumataa K I, Nakajimaa A, Shiotaa T, Yoshidab N,
Watanabeb T, Kameshimaa Y and Okadaa K 2006 J. Pho-
tochem. Photobiol. A: Chem. 180 75[18] Sinha S K, Sirota E B and Garoff S 1988 Phys. Rev. B 38
2297
088103-4