Appl Phys A (2008) 92: 615–620 DOI 10.1007/s00339-008-4546-7
Self-cleaning characteristics on a thin-film surface with nanotube
arrays of anodic titanium oxide
Chien-Chon Chen· Jin-Shyong Lin · Eric Wei-Guang Diau· Tzeng-Feng Liu
Received: 15 January 2008 / Accepted: 17 April 2008 / Published online: 15 May 2008 © Springer-Verlag Berlin Heidelberg 2008
Abstract Self-cleaning of a surface of nanotube arrays of anodic titanium oxide (ATO) is demonstrated. The ATO was prepared in fluoride ion containing sulfate electrolytes with a structure of 0.4 µm length, 100 nm pores diameter, 120 nm interpore distance, 25 nm pore wall thickness, a 8× 109 pores cm−2 pore density, and 68.2% porosity. Prepared as thin films either directly from a Ti foil or on a glass sub-strate, these arrays have the property that water drops spread quickly over the surface of the films without irradiation. In contrast, a flat anatase TiO2 film requires irradiation
with UV light for several minutes before the contact an-gle decreases to zero. The observed self-cleaning behavior of the ATO thin films is due to the capillary effect of the nanochannel structure and the superhydrophilic property of the anatase TiO2surface inside the tube.
PACS 96.15.Lb· 61.46.-W · 61.46.Fg
1 Introduction
From ancient times rutile titania (TiO2)has had a common
use as a white pigment because it is economical, chemically stable, and harmless [1], but anatase TiO2 becomes active
C.-C. Chen· E.W.-G. Diau (
)Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu, Taiwan 30010, Taiwan
e-mail:[email protected] J.-S. Lin· T.-F. Liu
Department of Material Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010, Taiwan
under irradiation with ultraviolet (UV) light. This charac-teristic has been extensively applied for self-cleaning, an-tifogging, antibacterial activity [2], conversion of solar en-ergy [3], splitting of water [4], and storage of enen-ergy [5–8]. When TiO2is illuminated with UV light, a photocatalytic
effect occurs on the TiO2 surface such that the contact
an-gle at the boundary between TiO2 and H2O decreases to
zero. Surfaces that are rough on a nanoscale tend to be more hydrophobic than smooth surfaces because of the reduced contact area between the water and solid. The nanoscale roughness is also essential to the self-cleaning effect—on a smooth hydrophobic surface, water droplets slide rather than roll. It has been demonstrated that the nanoscale surface roughness causes superhydrophobicity. This prevents wet-ting, causes such a surface to suspend small liquid droplets, and causes the contact angle to approach 180oleading to al-most spherical droplets. If such droplets had a chance to so-lidify, then they would be likely to form solid spherical par-ticles. From Patankar’s [9] simulation, the contact angle of a drop is a function of the roughness of a solid surface and is described by the equation, (H /W ) >−(1 + cos θ)/2 cos θ, where θ is the contact angle between the liquid and solid, H is the height and W is the width of the groove. Also, Cassie’s analysis [10] for a porous surface of heterogeneous surfaces states that cos θ= σ1cos θ− σ2, where θ is the
contact angle, σ1is the solid surface area, and σ2represents
air spaces. The rate of hydrophilic conversion depends on the experimental conditions; for example, this rate increases with increased intensity of UV light, a decreased UV wave-length, and an increased period of UV illumination [11]. The control of surface wettability induced by argon-ion (Ar+) [12], iron-ions (Fe3+) [13] sputtering, and surface modified by octadodecyldimethylchlorosilane (ODS) [14] have also been observed on thin films of TiO2. A self-cleaning surface
616 C.-C. Chen et al.
Fig. 1 Optical microscope images of a titanium surface after (a) me-chanical polishing and (b) electrolytic polishing
is near 0° or 180°. For instance, water forms a drop (180°) on a lotus leaf (a hydrophobic or lotus effect) or forms a thin aqueous film (so zero angle) on anatase TiO2 (a
hy-drophilic effect). Because a contact angle at 0° or 180° im-pedes the retention of water on the solid surface, the water slides from the solid, removing dust. According to Young’s equation [15],
γLcos θ= γS− γSL, (1)
the contact angle θ is related to the surface tension γL of
the liquid, the surface tension γS of the solid and the
sur-face tension γSLbetween the liquid and solid. According to
Girifalco and Good [16], the latter quantity is expressible as γSL= γS+ γL− φ(γS/γL)0.5, (2)
in which φ is a constant (typically in a range 0.6–1.1). From (1) and (2), the cosine of the contact angle becomes ex-pressed as
cos θ= C√γS− 1 (3)
Fig. 2 SEM images of a TiO2thin film produced on annealing
(sam-ple 1): (a) a top view shows the compact TiO2film; (b) a cross-section
view shows the film thickness 900 nm
in which C is a constant. An increased surface tension on the solid hence causes a decreased contact angle so that the solid tends to become hydrophilic.
The roughness of a solid surface also affects the contact angle. A rough hydrophilic surface tends to be more hy-drophilic, whereas a rough hydrophobic surface tends to be more hydrophobic than the corresponding smooth surfaces [17,18]. Because TiO2is a hydrophilic material, roughening
the surface of a porous TiO2film causes it to become
super-hydrophilic. Balaur et al. [19,20] demonstrated the wetta-bility of a TiO2 film with a nanotubular structure modified
by self-assembled organic monolayers. Through illumina-tion with a UV light, the wetting properties of a TiO2
sur-face are thus controllable from superhydrophobic to super-hydrophilic [19].
In the present work, we demonstrate that self-cleaning is achieved with a structure of nanotube arrays of anodic tita-nium oxide (ATO) on the surface of thin films of anatase TiO2. We find that the ATO nanostructural films display
Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodic titanium oxide 617
Fig. 3 SEM images of a TiO2 thin film on a Ti-foil substrate
(sample 2): (a) a top view shows the nanochannel structure; (b) a cross-section view shows the film thickness 400 nm. The film was produced on anodization in HF (0.5 vol.%)+ H2SO4(10 vol.%)
electrolyte
quickly over the ATO surface without illumination. In con-trast, for compact anatase TiO2films, the contact angle
de-creases to zero after UV radiation was applied on the surface for several minutes.
2 Experiments
To fabricate TiO2films we used titanium substrates of three
types: sample 1 was a titanium plate (purity 99.995%, thick-ness 10 mm), sample 2 was a commercial titanium foil (pu-rity 99.7%, thickness 0.127 mm), and sample 3 was a thin film of titanium (purity 99.995%, thickness 500 nm) formed on a glass substrate by sputtering. The area of each tita-nium substrate was 2 cm× 2 cm. To obtain a homogeneous α-Ti structure, we annealed samples 1 and 2 in an air fur-nace for 1 h, whereas sample 3 was annealed in an evac-uated chamber (P < 10−4Pa). The annealing temperature
Fig. 4 Enlarged (a) SEM and (b) TEM images showing the ring-packed nanotube structure of sample 2
was controlled below the temperature of the β-phase trans-formation at 850°C. For samples 1 and 2, a clean and flat sur-face was obtained on mechanical and electrolytic polishing; the electrolytic polishing involved a platinum sheet (2 cm× 2 cm) as a cathode, titanium plates as an anode, perchlo-ric acid (HClO4, 5 vol.%, 70%)+ ethandiol monobutylether
(HOCH2CH2OC4H9, 53 vol.%, 95%)+ methanol (H3COH,
42 vol.%, 99%) as an electrolyte at 15°C, with 52 V applied for 1 min followed by 28 V for 13 min, and with constant stirring at 200 rpm.
After a clean and plate-shaped α-Ti surface was formed, a compact anatase TiO2film resulted from heating sample 1 to
450°C for 3 h in air in a furnace. The nanoporous TiO2films
were formed on anodization followed by annealing at 450◦C for 3 h. Sample 2 was anodized with hydrofluoric acid (HF, 0.5 vol.%, 55%)+ sulfuric acid (H2SO4,10 vol.%, 98%)
electrolyte at 25◦C, with 20 V applied for 10 min. Sample 3 was anodized with HF (0.15 vol.%)+ H2SO4 (10 vol.%)
electrolyte at 2°C, with 10 V applied for 1 min.
The micromorphology of the α-Ti and TiO2 surfaces
Olym-618 C.-C. Chen et al.
Fig. 5 SEM images of a TiO2thin film on a glass substrate (sample 3):
(a) a top view shows the nanoporous structure; (b) a cross-section view shows the film thickness 200 nm. The film was produced on anodiza-tion in HF (0.15 vol.%)+ H2SO4(10 vol.%) electrolyte
pus BJ-51), a scanning electron microscope (SEM, JEOL 6500, operating voltage 15 kV), and a transmission electron microscope (TEM, JEOL 2000, operating voltage 200 kV). For SEM images, the surface morphology of the ATO films was observed directly; for TEM images, the ATO films were scraped and collected on carbon/formvar films supported by Cu grids. The anatase TiO2was detected with X-ray
diffrac-tion (XRD, Philips X’Pert Pro, solid-state detecdiffrac-tion using filtered Cu KR radiation). The contact angle of H2O on the
TiO2 surface was determined in an equilibrium condition
with a contact-angle meter (Phoenix 600) at 25◦C. Optical images were obtained with a CCD camera interfaced to a computer; the contact angle was evaluated according to the equipment program.
3 Results and discussion
Figures 1a and1b present optical micrographs of an α-Ti surface after mechanical and electrolytic polishing,
respec-Fig. 6 XRD patterns of a TiO2thin film with an ATO nanochannel
structure on Ti foil (sample 2): (a) before annealing, the features show only an α-Ti phase (TiO2is amorphous); (b) after annealing at 450°C
for 3 h, the features show both the α-Ti phase and an anatase TiO2
phase. T and A represent titanium metal and anatase TiO2, respectively
tively. The ductility of titanium made it difficult to obtain a non-scraped surface by mechanical polishing alone. Fig-ure1a shows that an α-Ti surface retains scrapes even after grinding with SiC paper (#4000) and polishing with Al2O3
powder (50 nm), whereas Fig.1b shows that electrolytic pol-ishing produces a non-scraped surface. Such a surface would form a TiO2 film of high quality. If left in air near 23°C,
a surface of titanium metal is known to become sponta-neously covered with a transparent film of titania of thick-ness 1–10 nm. The thickthick-ness of the film is increased on ei-ther anodization or annealing. Figure 2 shows the results of annealing sample 1 for a compact TiO2 film (thickness
900 nm) produced on the Ti plate after annealing at 450°C for 3 h.
For sample 2, Fig. 3 presents SEM images of ATO nanotube arrays on a Ti foil that was anodized in HF (0.5 vol.%)+ H2SO4(10 vol.%) electrolyte. The long-range
ordered nanochannel structure has a length 400 nm, pores of diameter 100 nm, an interpore distance 120 nm, a pore wall of thickness 25 nm, a pore density 8× 109pores cm−2, and a porosity 68.2%. Detailed images of ATO nanotubes of sample 2 appear in Fig.4. The SEM (Fig.4a) and TEM (Fig. 4b) images reveal the ring-packed structure for the ATO nanotubes. During anodization of Ti, the gases TiF(g),
TiF2(g), TiF3(g), TiF4(g), TiOF(g)and TiOF2(g)are formed in
the Ti–F–O electrochemical system [21,22]. When the tita-nium is anodized, these gases and H2(g)escape from the
ti-tanium substrate through the titania film, leaving nanopores and forming an ATO nanochannel structure.
Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodic titanium oxide 619
Fig. 7 A water drop on the surface of a compact thin film of TiO2
(sample 1) after ultraviolet illumination (350 nm, 2 mW cm−2) for (a) 0, (b) 1, (c) 6, and (d) 20 min; the contact angles are 51.5°, 28.3°, 12° and 0°, respectively
Figure5 shows images resulting from sample 3, which had pore size 40 nm and tube length 200 nm. The ATO nanostructures were formed on the glass substrate. To pre-vent the ATO thin film from flaking off the glass substrate, we controlled the HF concentration, electrolyte temperature, applied voltage and the duration of anodization to be sig-nificantly smaller than those conditions for sample 2. As a result, the surface morphology of the former (Fig. 5) was poorer than that of the latter (Fig.3).
Figure6shows XRD patterns of sample 2, a large-scale TiO2thin film with ATO nanotubes on a Ti foil. Before
an-nealing, the feature (denoted T) in Fig.6a indicates the pres-ence of only the α-Ti phase (TiO2is amorphous); after the
specimen was annealed at 450°C for 3 h, the XRD patterns
Fig. 8 A water drop on the surface of a thin film of TiO2(sample 2)
with ATO nanochannel arrays showing zero contact angle without ul-traviolet illumination
shown in Fig.6b reveal both the α-Ti phase (denoted T) and the anatase TiO2phase (denoted A).
To determine the contact angle at the interface between the TiO2 and H2O, we placed a drop of H2O on the
sur-face of TiO2 films. Figure 7a shows that the initial
con-tact angle of sample 1 was 51.5°. After ultraviolet illumi-nation (350 nm, 2 mW cm−2) for 1 min and 6 min, these an-gles decreased to 28.3° and 12.0°, respectively, as shown in Fig.7b and7c. With the duration of illumination increased to 20 min, the contact angle decreased to zero, as shown in Fig.7d.
For both samples 2 and 3 for which ATO nanochannel arrays were produced on a Ti foil and a glass surface, the contact angles between TiO2 and H2O were zero without
ultraviolet illumination, as shown in Fig.8. The observation of zero contact angle is rationalized according to a capillary effect, for which the height of the liquid inside the tube is expressible as [23]
h= (2γ cos θ)/(ρgr) (4)
in which appear symbols for liquid-air surface tension γ, contact angle θ , density ρ of liquid, acceleration g due to gravity, and radius r of the tube. For H2O,
val-ues of these parameters are γ = 0.072 J m−2, θ = 20°, ρ= 997 kg m−3, and g= 9.8 m s−2. For a tube of diame-ter 100 nm (r= 50 nm), the height h of a liquid column is estimated to be 280 m; H2O therefore becomes readily
ad-sorbed inside these TiO2pores.
4 Conclusion
When the surface of a compact anatase TiO2film with water
drops was irradiated with UV light (350 nm, 2 mW cm−2) for 20 min, the contact angle of water on the surface de-creased from 51.5° to zero, but the contact angle became
620 C.-C. Chen et al. zero immediately when water drops were placed on an ATO
nanochannel surface without UV light illumination. The or-dered ATO nanostructural thin films with a great surface area exhibit a superhydrophilic nature as a prospective mate-rial for self-cleaning applications with beneficial properties of being robust, economical, chemically stable, transparent, and harmless.
Acknowledgements National Science Council of Republic of China (contract 96-2628-M-009-018-MY2) and the MOE-ATU program pro-vided financial support.
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