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Colorful nanostructured TiO2 film with superhydrophobic–superhydrophilic switchable wettability and anti-fouling property
Journal of Alloys and Compounds 798 (2019) 257e266
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Colorful nanostructured TiO2 film with superhydrophobicesuperhydrophilic switchable wettability and antifouling property Xue Zhou, Sirong Yu*, Jie Zang, Zhexin Lv, Enyang Liu, Yan Zhao College of Material Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2019 Received in revised form 20 May 2019 Accepted 21 May 2019 Available online 24 May 2019
Superhydrophobic nanostructured TiO2 film was successfully fabricated on the Ti plate by anodic oxidation and lauric acid modification. By varying the applied voltage, TiO2 films with various nanostructures and colors were prepared. The water contact angles (WCA) on the modified TiO2 film increased as the applied voltage increased, and the highest WCA on the TiO2 nanovesuvianite film was about 161.23 . When the applied voltage was 5 V and 20 V, the obtained TiO2 nanopores and TiO2 nanotubes could be used to build two improved Cassie models for designing superhydrophobic surface. Moreover, the superhydrophobic TiO2 film showed good anti-fouling property, particularly it could maintain superhydrophobicity both when immersed in and taken out from the organic solvents. The superhydrophobic film could be changed into superhydrophilic under ultraviolet (UV) irradiation, and the superhydrophilic film recovered to hydrophobic or superhydrophobic gradually at 60 C in the dark. Additionally, the superhydrophilic film showed underwater superoleophobicity, which exhibited excellent anti-fouling property by degrading the oil under UV light. © 2019 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobic Nanostructured TiO2 film Superhydrophilic Underwater superoleophobic Anti - fouling property
1. Introduction Wettability is an important characteristic of solid surface, which depends on the chemical compositions and surface structures. Superhydrophobic surface with a contact angle more than 150 and superhydrophilic surface with a contact angle less than 5 have received particular attention due to their important practical applications [1e3]. The common methods to fabricate superhydrophobic and superhydrophilic surface are almost same, such as vapor deposition [3,4], etching [5], hydrothermal treatment [6,7], and electrochemical anodization [8] and so on, by which various rough structure can be created on the surface. Besides, another step is needed to construct a superhydrophobic surface that the deposition of self-assembled monolayers of hydrophobic compounds, such as stearic acid [9] and triethoxyoctylsilane [10], etc. However, the surface, which only shows superhydrophobicity or superhydrophilicity, can't satisfy the needs of industrial development. Recently, smart surfaces with reversible switchable wettability under external stimuli have attracted a great deal of attention
* Corresponding author. E-mail address: [email protected] (S. Yu). https://doi.org/10.1016/j.jallcom.2019.05.259 0925-8388/© 2019 Elsevier B.V. All rights reserved.
because they can take the strong points of both superhydrophobicity and superhydrophilicity [11]. Various external stimuli have been reported to realize the reversible switchable wettability, such as temperature [12], pH [13], light [14], and magnetic field [15] and so on. To realize the reversible switching between superhydrophobicity and superhydrophilicity, external stimuli responsive materials, such as TiO2, ZnO, SiO2, SnO2, WO3 and V2O5, have been developed by ultraviolet (UV) irradiation. Among them, TiO2 has been widely researched, owing to its specific physical and chemical properties, for many potential applications such as photocatalysis [16], solar cell [17] and chemical sensor [18], etc. Particularly, superhydrophobic TiO2 materials have been constructed in recent years due to the small solid-liquid contact area on the rough structure of TiO2 nanomaterials [19]. Since the initial discovery of photo-induced superhydrophilicity on TiO2 surface in 1997 [20], various methods are developed to create TiO2-based surface which shows switchable wettability. Lyons et al. [21] have prepared a superhydrophobic TiO2-polyethylene surface with hierarchical structures that indicates UV-induced reversible wettability and selfcleaning property by template lamination method. Nishimoto et al. [22] have fabricated the superhydrophobicesuperhydrophilic
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pattern with an extremely high wettability contrast by UV irradiation on porous TiO2 layers by hydrothermal-based method. Qing et al. [23] have achieved superhydrophobic modified TiO2/polyvinylidene fluoride composite surface on Cu substrate through dipcoating process, which exhibits reversible switchable wettability by UV light irradiation and heating. TiO2 materials tend to be hydrophilic and easily convert to superhydrophilic under UV light. However, the construction of superhydrophobic TiO2 surface requires suitable roughness structures, such as nanotubes and nanorods, which are easier to trap air between liquid and solid. Compared to the above methods, anodic oxidation is an efficient and highly controllable method to obtain TiO2 nanoporous/nanotube structure by adjusting the voltage, which has an important effect on the wettability. In addition, the anti-fouling or self-cleaning property of the surface with reversible switchable wettability is always tested when the surface shows superhydrophobicity, but few studies report the property when the smart surface shows superhydrophilicity or underwater superoleophobicity. In this paper, superhydrophobic TiO2 film was prepared on the Ti plate by anodic oxidation and modification in ethanol solution of lauric acid. The effects of the applied voltages on the microstructure, hydrophobicity and color of the TiO2 film were discussed, respectively. Among them, for the obtained TiO2 nanopores and nanotubes, two improved Cassie models were established to design the superhydrophobic surface. Moreover, the conversion wettability between superhydrophobicity and superhydrophilicity was analyzed in detail, and the superhydrophilic surface showed underwater superoleophobicity. The anti-fouling properties of the superhydrophobic surface and superhydrophilic surface (underwater superoleophobic surface) were tested. 2. Experimental 2.1. Fabrication of TiO2 film TA2 titanium plates (40 mm 20 mm 1 mm) were obtained from Shenzhen Ode Fu Materials Co. Ltd, Guangzhou, China. Before the anodic oxidation, surface impurities on the Ti plate were removed by being immersed in an aqueous solution that was composed of 15 mL deionized water, 12 mL HNO3 and 3 mL HF for 30 s at ambient temperature. The Ti plate was then ultrasonically cleaned by acetone, ethanol and deionized water for 10 min respectively, and dried at room temperature. The pretreated Ti plate and graphite plate (40 mm 20 mm 3 mm) were used as anode and cathode respectively, whose spacing was 40 mm. Electrolytic solution consisted of 1 vol% HF (40 wt%, Sinopharm) aqueous solution. Anodic oxidation was performed for 2 h with a DC power. So as to investigate the effects of the applied voltage on the structure and wettability of the TiO2 film, the applied voltage was 5, 10, 15, 20, 25, and 30, respectively. After the anodic oxidation, the specimen was cleaned by deionized water and dried at ambient temperature. 2.2. Fabrication of superhydrophobic TiO2 film To lower the surface energy, Ti plate coated with TiO2 was immersed in a 0.02 mol/L ethanol solution of lauric acid for 24 h and subsequently dried at 60 C for 20 min. Because TiO2 always shows photocatalytic property which is introduced in Section 3.6, the surface modification process was carried out in the dark to avoid the effect of light. 2.3. Characterizations and tests The surface morphology of the as-prepared specimens was observed using a field-emission scanning electron microscope
(FESEM, NovaNano SEM 450, FEI). The surface chemical compositions of the specimens were studied by X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha). A contact angle meter (SL200B, USA, KINO) with digital image analysis software was used to measure the contact angle (CA) at room temperature. The value reported was the average at least five measurements performed at different positions with 3 mL water droplets or 3 mL oil droplets. Dichloromethane was used as model oil for the test and the CA oil/ water was measured under deionized water environment. The error bars in the plots of contact angles indicated the test error. Water and oil wettability changes of the TiO2 film were tested by UV illumination. A 250 W mercury lamp was devoted to be the UV light source which was 20 cm away from the specimens. Besides, in order to prove the low adhesion between oil droplet and underwater superoleophobic surface, the 3 mL oil droplet was contacted and separated with the underwater superoleophobic surface through the movement of the needle, which was recorded by the contact angle meter. In the test of photocatalytic property of TiO2 film, UV light was carried out with a 250 W mercury lamp. An ultravioletevisible spectrophotometer (UVeVis) (Hitachi Ue3900H, Japan) was used to measure the UVevis spectra of the methylene blue (MB) absorption peaks. 3. Results and discussion 3.1. Microstructure of the TiO2 film The applied voltage decides the electric field intensity, thus affecting the migration of ions and ultimately the microstructure of the TiO2 film. Fig. 1 showed the microstructure of the TiO2 film fabricated at different voltages. It could be learnt from Fig. 1(a) that neat nanopores appeared on the TiO2 film when the applied voltage was 5 V. With the increase of the voltage, nanopores gradually evolved into nanotubes. As shown in Fig. 1(b), when the applied voltage was 10 V, part of the TiO2 film between the pores was etched, thus TiO2 nanopores and TiO2 nanotubes coexisted on the surface. When the applied voltage increased to 15 V and 20 V, closepacked TiO2 nanotubes and separated TiO2 nanotubes were formed on the Ti plates respectively (Fig. 1(c) and (d)). However, when the applied voltage reached up to 25 V, a nanovesuvianite structure was synthesized as shown in Fig. 1(e). On the surface, large numbers of nanoscale protrusions were randomly distributed and there were holes existing among these. The similar structure appeared in Fig. 1(f) when the applied voltage was 30 V. Besides, it could be found from Fig. 1 that the internal diameter of the TiO2 nanopores or nanotubes was affected by the applied voltage. The internal diameter was measured by the software of Nano Measurer that the value was the arithmetic mean of 100 points chose manually. The nanovesuvianite structure was not appropriate to be measured when the applied voltage was 25 V and 30 V. The distribution range and proportion of the internal diameters for different voltages were exhibited in Fig. 2(aed) that the internal diameters of the nanostructured TiO2 were relatively uniform. Moreover, the average internal diameters were shown in Fig. 2(e) that the internal diameters increased with the increase of the applied voltage. When the applied voltages were 5 V and 20 V, the average internal diameters were 17.00 nm and 81.10 nm, respectively. Accordingly, with the increase of the applied voltage, the microstructure of the TiO2 film changed from nanopore structure to nanotube structure and then the nanovesuvianite structure, and the internal diameters increased too. It was because that the higher applied voltage would enhance the electro field and ionic mobility and both TiO2 growth and dissolution were improved, leading to a larger pore size and pore spacing. Especially, when the applied
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Fig. 1. SEM images of the TiO2 film obtained under different voltages. (a) 5 V, (b) 10 V, (c) 15 V, (d) 20 V, (e) 25 V and (f) 30 V.
Fig. 2. Distribution range and proportion of the internal diameters (aed) and average internal diameters (e) of nanostructured TiO2 for 5 V, 10 V, 15 V and 20 V, respectively.
voltage was too high such as 25 V, the irregular microstructure would appear because the TiO2 growth and dissolution were uncoordinated. 3.2. Superhydrophobicity of the TiO2 film Microstructure and low surface energy are two key factors in the construction of superhydrophobic surfaces. After anodic oxidation, TiO2 film having a certain microstructure was grown on the surface of the Ti plate and showed hydrophilicity with a water contact angle less than 90 . In order to obtain the superhydrophobicity, TiO2 film was subsequently modified by lauric acid to lower the surface energy, which had no effect on its microstructure. It could be seen from Fig. 1 that the microstructure of TiO2 was various at different voltages, and therefore the surface wettability was also affected. Fig. 3 showed the water contact angles (WCA) on the modified TiO2 film fabricated under different voltages. The WCA increased with the increase of the applied voltage. When the
applied voltage was 5 V that the TiO2 film was composed of nanopore structures, the modified TiO2 film exhibited hydrophobicity with the WCA about 143.35 . For the other applied voltages, the modified TiO2 film showed superhydrophobicity with the WCA more than 150 . Especially, when the applied voltages were 20 V and 30 V, the WCA on the TiO2 nanotube film and TiO2 nanovesuvianite film were 157.43 and 161.23 , respectively. The main reason for the above phenomenon was the difference of the water-solid contact area which could be calculated by the Cassie law [24]:
cosqc ¼ f ðcosq0 þ 1Þ 1 c
(1)
where q is the WCA on the modified TiO2 film; f is the area fraction of the liquid-solid contact; q0 is the equilibrium contact angle which is the WCA (101 ) of the modified Ti plate. When the applied voltages were 5 V, 20 V, and 30 V that the TiO2 film consisted of nanopore, nanotube and nanovesuvianite structure respectively,
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f ¼
p ðD2 =2 þ hÞ2 ðD2 =2Þ2
(4)
L2 2
Then the Cassie model is revised to:
0 cosq ¼ @ c
p ðD2 =2 þ hÞ2 ðD2 =2Þ2 L2 2
1 Aðcosq0 þ 1Þ 1
(5)
¼ pðh = L2 ÞððD2 = L2 Þ þ ðh = L2 ÞÞðcosq0 þ 1Þ 1
Fig. 3. WCA of the modified TiO2 film obtained under different voltages.
the values of f were calculated to be 24.43%, 9.46% and 6.57%, respectively. The results showed that the solid-liquid contact area decreased with the increase of applied voltages. Nanopore structure (5 V) and nanotube structure (20 V) consisted of pores but differed in that the interspaces between pores were filled with solid in nanopore structure but the air pockets in nanotube structure. During the increase of voltage from 5 V to 20 V, the solid-liquid contact mode changed from “area contact” to “line contact”, which was more conducive to forming superhydrophobic surface for TiO2 film. When the applied voltages were higher (25 V or 30 V), the solid-liquid contact area was lower. For the nanovesuvianite structure, the pores were rough and uneven, forming a 3D random nanoporous network. The special nanostructure satisfied typical liquidesolid ‘‘point-contact’’ which could trap large air between solid and water while forming a good superhydrophobic surface. Thus the various microstructure had a great influence on the wettability and the above results offered important information for designing superhydrophobic surface. The nanopores and nanotubes were almost perpendicular to the substrate, and assumed to be uniform in size. Therefore, two improved Cassie model could be established for designing superhydrophobic surface as shown in Fig. 4(a and b). For nanopore structure, let L1 be the center distance between adjacent pores, and D1 be the inside diameter of the pores. Generally, the water droplet only contacts the flat top of the surface and the diameter of the water droplet is much larger than L1. Thus the area fraction of the liquid-solid contact f can be defined as:
f ¼
L1 2 pðD1 =2Þ2
(2)
L1 2
Then the Cassie model is revised to: c
cosq ¼
L1 2 pðD1 =2Þ2 L1 2
! ðcosq0 þ 1Þ 1
p ¼ 1 ðD1 =L1 Þ2 ðcosq0 þ 1Þ 1 4
(3)
For nanotube structure, let the inside diameter of the tubes be D2 and the wall thickness be h. Let L2 be the center distance between adjacent tubes. Thus f can be represented as:
In order to reflect the relationship between contact angle qc and geometric parameters intuitively, the Eqs. (3) and (5) are converted to Fig. 4(c) and (d), respectively. For the nanopore structure, Fig. 4(c) indicated the contact angle qc is only related to D1 =L1 , and increases with the increase of the D1 =L1 (90 < q0 < 180 ). While for the nanotube structure, the contact angle qc is related to both D2 =L2 and h =L2 , and decreases with the increase of D2 =L2 and h =L2 (90 < q0 < 180 ) as shown in Fig. 4(d). It can be concluded that the wettability of the surface can be controlled by adjusting geometric parameters of the microstructure. Therefore, the superhydrophobic surface can be fabricated by designing the geometric parameters of the hydrophobic surface. 3.3. Chemical compositions of the superhydrophobic TiO2 film Chemical compositions of superhydrophobic TiO2 film were investigated by XPS analysis. Fig. 5(a) showed the XPS survey spectra of the modified TiO2 film, in which elements of Ti, O, F and C were detected on the surface corresponding to peaks at binding energies of approximately 459.26 eV, 531.26 eV, 686.26 eV and 286.26 eV, respectively. Besides, the high-resolution XPS spectra of Ti 2p, F 1s, C 1s and O 1s were measured. As shown in Fig. 5(b), at binding energies of 458.6 eV and 464.3 eV, two Ti 2p peaks were observed corresponding to Ti 2p (3/2) and Ti 2p (1/2), respectively, indicating the formation of TiO2 film on Ti plate [25]. Moreover, the peak at 459.8 eV was due to the highly electronegative hexafluorotitanate complex [TiF6]2- in the anodic film [26], which was also confirmed by the high-resolution XPS spectrum of F 1s (Fig. 5(c)). The competition between the formation of TiO2 and its dissolution was a key factor in determining the TiO2 structure, and the [TiF6]2- was the reaction product of TiO2 and F in electrolyte. Fig. 5(d) exhibited that C 1s peaks could be decomposed into three peaks at 288.4 eV, 285.8 eV and 284.6 eV, attributed to C]O, CeO, and CeH and CeC groups, respectively. It indicated lauric acid was successfully self-assembled on the TiO2 film. After modified in ethanol solution of lauric acid, the microstructure of the TiO2 film was unaffected (Fig. 6(a)) and the surface energy could be effectively reduced due to the existence of the CeH groups on lauric acid. As shown in Fig. 6(b), lauric acid was self-assembled on the TiO2 film through the dehydration reaction between the hydroxyl groups of the TiO2 film and carbonyl groups of the lauric acid. The TiO2 film anodic oxidation always leaded to the formation of hydroxyl groups on the surface which was the reason why the TiO2 film showed hydrophilic before modification [27]. In the highresolution XPS spectrum of O 1s (Fig. 5(e)), in addition to the TieO groups at 530.1 eV, there was a small amount of TieOH groups existing at 531.5 eV that didn't react with lauric acid [28]. 3.4. Coloration and anti-fouling property of the superhydrophobic TiO2 film The color of the TiO2 film is always affected by applied voltage. It could be leant from Fig. 7 that TiO2 film anodized at different voltages had various colors. In general, when light hits the surface
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Fig. 4. Top view of the (a) nanopore model and (b) nanotube model. (c) Contact angles in the nanopore model as the change of D1/L1. (d) Contact angles in the nanotube model as the change of D2/L2 and h/L2.
Fig. 5. XPS spectra of the superhydrophobic TiO2 film: (a) survey spectrum; (b) Ti 2p; (c) F 1s; (d) C 1s; (e) O 1s.
of the Ti specimen coated with TiO2 film, the light reflected from the interface between TiO2 film and air interferes with the light which is reflected from the interface between Ti plate and TiO2 film. The light of different wavelengths is mixed, so the surface of the metal titanium exhibits a beautiful interference color [29]. As shown in Fig. 1, the microstructure of the TiO2 film obtained at
different voltages was different, therefore the luminous flux of the TiO2 film and the refractive index and reflectance of the light were changed, thereby exhibiting different colors. With a natural light (indoor), Fig. 7 was taken from a 90 viewing angle at a distance of 30 cm. Moreover, the color of the film was diverse at different angles and different light sources. The colorful Ti specimens coated
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Fig. 6. (a) Morphological evolution of the Ti plate surface after anodic oxidation and modification. (b) Formation mechanism of the self-assembled lauric acid on the rough TiO2 film.
Fig. 7. Macroscopic view of TiO2 film anodized at different voltages.
with TiO2 film can be used as decoration in applications of building materials and daily necessities, and the color can be controlled by adjusting applied voltage. However, the TiO2 film was typically hydrophilic and lipophilic that was easy to absorb contaminants and then break the film color. Thus the superhydrophobic surface with anti-fouling property could avoid contamination and keep the film color for long-term use. In this research, anti-fouling property of the superhydrophobic TiO2 film was tested as shown in Fig. 8. The unmodified and superhydrophobic TiO2 film were immersed in sewage consisting of soil and sand and then taken out. For unmodified TiO2 film, the surface was wetted and contaminated as shown in Fig. 8(c). However, for superhydrophobic TiO2 film, the surface was clean as before immersing. It was because that the nanostructure of the
superhydrophobic TiO2 film was benefited to trap air between liquid and TiO2 film and then showed liquid repellence. The liquid could flow down and take away solid contaminants on the superhydrophobic surface. Besides, the superhydrophobic TiO2 film also showed anti-fouling property for the organic solvents that had low boiling point such as hexane, toluene and chloroform. The superhydrophobic specimen was immersed in hexane as shown in Fig. 9(aee). A MB dyed water droplet was dripped on the surface and rolled down in a spherical shape, indicating that the superhydrophobic TiO2 film could maintain the superhydrophobicity when immersed in organic solvent. However, the dyed water droplet spread on the unmodified TiO2 film as shown in Fig. 9(f). What's more, when the superhydrophobic TiO2 film was taken out from the organic solvent, the WCA was unaffected as the initial
Fig. 8. Anti-fouling test of unmodified TiO2 film (aec) and superhydrophobic TiO2 film (def).
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Fig. 9. (aee) A dyed water droplet rolling on the superhydrophobic TiO2 film when immersed in organic solvents. (f) A dyed water droplet spreading on the unmodified TiO2 film when immersed in organic solvents.
state. This indicated that the low boiling point organic solvent didn't damage the microstructure and compositions of the superhydrophobic TiO2 film, which had the similar result with the previous report [30]. 3.5. Superhydrophobicesuperhydrophilic switchable wettability of the TiO2 film Fig. 10(a) showed the process of superhydrophobic surface (the applied voltage was 30 V) converting into superhydrophilic surface under UV irradiation with time. Different contact angles could be controlled by adjusting UV irradiation time, and then hydrophobic surface and hydrophilic surface could also be obtained during this process. In addition, the distance between the specimen and light and the power of the light also had a certain influence on the change process of the contact angles on the surface. The stronger the UV intensity, the faster the contact angle decreased. A TiO2 film surface with superhydrophobicity and superhydrophilicity at the same time
could be fabricated by selective UV irradiation. As shown in Fig. 10(b), the water droplets tended to be spherical on the superhydrophobic surface before UV irradiation, and the water spread fully on the irradiated part. Even if the irradiated area was filled with a large amount of water, there was a clear boundary between superhydrophobic and superhydrophilic surface. The surface with extremely high wettability contrast can be used in the offset printing, thus various superhydrophobic-superhydrophilic patterns could be designed by area-selective UV irradiation for this application [22]. During the UV irradiation, it required two steps to convert a superhydrophobic surface to superhydrophilic surface. Firstly, When the TiO2 film was irradiated by UV, electron-hole pairs were generated. The lauric acid combined on the surface was decomposed into CO2 and H2O, meanwhile the holes reacted with the oxygen ions on the surface to form oxygen vacancies [31]. Secondly, H2O molecules adsorbed to oxygen vacancy and dissociated to hydroxyl groups. The hydroxyl groups could easily combine with water molecules to make the film superhydrophilic [32].
Fig. 10. (a) The change of the WCA on superhydrophobic TiO2 film under UV irradiation with time going. (b) Images of water droplets dropped on superhydrophobic TiO2 film (left) and a water droplet and layer on partly UV-irradiated TiO2 film (right).
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superhydrophobicity with a WCA about 153.80 , and the WCA increased to the maximum recovery contact angle using the shortest time. During the heat treatment in the dark, the hydroxyl groups on the surface were replaced by the adsorption of atmospheric oxygen, and the TiO2 surface recovered to the original state. The lauric acid combined on the surface was decomposed and small amount of hydroxyl groups were still on the surface, the hydrophobicity was weaker than that of the modified TiO2 film [32]. Besides, the TiO2 nanovesuvianite film (the applied voltage was 30 V) was more conducive to trap the air between the water and film than the TiO2 nanopore film and nanotube film, so the WCA was higher after recovery. 3.6. Anti-fouling property of the underwater superoleophobic TiO2 film
Fig. 11. Recovery contact angles and time of three different specimens.
Fig. 12. (a, b) Photographs of water and oil droplet shape on the superhydrophobic TiO2 film with contact angle about 158 and 3 , respectively. (c, d) Photographs of oil droplets shape on the superhydrophilic TiO2 film in air and under water with contact angle about 4 and 165 , respectively.
Interestingly, the WCA on the irradiated surface (superhydrophilic surface) recovered gradually when the specimen was heated at 60 C in the dark. Fig. 11 showed the WCA on three typical TiO2 film after being heated at 60 C in the dark for different time. It could be learned that the irradiated TiO2 film obtained at 5 V and 20 V showed hydrophobicity with the WCA about 135.43 and 143.54 , respectively. Moreover, after the heat treatment, the TiO2 film obtained at 30 V recovered from superhydrophilicity to
Before the UV irradiation, the superhydrophobic surface exhibited superhydrophobicity and superlipophilicity in the air as shown in Fig. 12 (a, b). After the UV irradiation the superhydrophilic surface also showed the superlipophilicity in the air but indicated the underwater superoleophobicity with a high CA oil/water about 165 (Fig. 12 (c, d)). Moreover, the underwater oil adhesion of the underwater superoleophobic surface was tested. Fig. 13 exhibited the process that a 3 mL oil droplet approached, contacted, pressed and departed to the underwater superoleophobic surface. It could be seen that there was no visual oil leaving on the surface after the oil droplet being pressed seriously and departed from the underwater superoleophobic surface (video S1). Thus, the underwater superoleophobic surface showed low adhesion for oil, which is important for the anti-fouling property underwater. However, the superhydrophilic surface showed superlipophilicity in the air, which meant the surface was easy to be polluted by the oil. It is known that TiO2 has good photocatalytic property for organics [33]. To test the photocatalytic property of the superhydrophilic TiO2 film, the ability of the film to degrade the MB (5 mg/L) solution was tested. In contrast, the ability of the Ti plate to degrade the MB was also measured. Moreover, before the test, the specimens were immersed in MB solution under magnetic stirring for 30 min in the dark to establish adsorption/desorption equilibrium. According to the Beer-Lambert Law [34], the absorbance of MB is proportional to its concentration, thus the concentration of MB is corresponding to the magnitude of MB absorption peak at 665 nm wavelength. According to Fig. 14(a), Compared to the Ti plate, almost all of MB was degraded by TiO2 film under UV irradiation about 6 h. Besides, the magnitude of MB absorption peak decreased with the time as shown in Fig. 14(b), which meant that the TiO2 film had the photocatalytic property under UV light. Supplementary video related to this article can be found at https://doi.org/10.1016/j.jallcom.2019.05.259. The CA oil/water on the superhydrophilic TiO2 film decreased from 165 to 16 after being immersed in the oleic acid. The photocatalytic property of the TiO2 film was always used to degrade the oily contaminants. The CA oil/water recovered to 165 after 10 h UV irradiation, and even after five cycles of the oleic acid immersion and UV irradiation, the CA oil/water could be recovered as the initial
Fig. 13. Underwater oil adhesion test on the superhydrophilic surface. The arrows represent the moving direction of the needle.
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Fig. 14. UVevis spectra of (a) the MB absorption peaks after being degraded by Ti plate and TiO2 film for 6 h and (b) the MB absorption peaks being degraded by TiO2 film with time.
superhydrophilic. After heating at 60 C in the dark, the WCA of the irradiated film recover gradually, and the restored contact angle of the TiO2 nanovesuvianite film is the largest about 153.80 . Besides, the irradiated TiO2 film also has underwater superoleophobicity and shows low adhesion for oil underwater. The underwater superoleophobic surface also indicates good anti-fouling property that oleic acid can be degraded by the TiO2 film under UV irradiation and the CA oil/water is not affected. Acknowledgements This work was supported by the Postgraduate Innovation Project (No. 17CX06051) of China University of Petroleum (East China), China; the Open Fund (No. OGE201702-07) of Key Laboratory of Oil & Gas Equipment, Ministry of Education of China (Southwest Petroleum University), China; the Natural Science Foundation of Shandong Province (No. ZR2017LEM004), China.
Fig. 15. Reversible underwater superoleophobicity-lipophilicity switching of the TiO2 film under oleic acid pollution and UV irradiation.
state, as shown in Fig. 15. This indicated that the TiO2 film had good anti-fouling property by photocatalytic degradation of pollutants under UV irradiation. The underwater superoleophobicity and antifouling property of the TiO2 film have a good application prospect in oil-water separation.
4. Conclusion In summary, nanostructured TiO2 film is fabricated on the Ti plate by anodic oxidation and then lauric acid is utilized to make the film superhydrophobic. TiO2 film with various TiO2 nanostructures are prepared at different applied voltages, among which three typical porous structures that nanopores, nanotubes and nanovesuvianite are obtained at voltage of 5 V 20 V and 30 V, respectively. Moreover, the WCA on the modified TiO2 film increase with the increase of applied voltages, and two improved Cassie models are established for the nanopore structure and nanotube structure. The superhydrophobic TiO2 film exhibits different colors at different applied voltages, and shows excellent anti-fouling property to pollutants especially for organic solvents that have low boiling point. Under UV irradiation, the surface can be changed from superhydrophobic to hydrophobic, hydrophilic and
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[22] S. Nishimoto, M. Becchaku, Y. Kameshima, Y. Shirosaki, S. Hayakawa, A. Osaka, M. Miyake, TiO2-based superhydrophobicesuperhydrophilic pattern with an extremely high wettability contrast, Thin Solid Films 558 (2014) 221e226. https://doi.org/10.1016/j.tsf.2014.02.060. [23] Y. Qing, C. Yang, N. Yu, Y. Shang, Y. Sun, L. Wang, C. Liu, Superhydrophobic TiO2/polyvinylidene fluoride composite surface with reversible wettability switching and corrosion resistance, Chem. Eng. J. 290 (2016) 37e44. https:// doi.org/10.1016/j.cej.2016.01.013. [24] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546e551. https://doi.org/10.1039/TF9444000546. [25] B. Gao, X. Du, Y. Liu, B. Song, S. Wei, Y. Li, Z. Song, Candle soot as a template for fabricating superhydrophobic titanium dioxide film by magnetron sputtering, Vacuum 159 (2019) 29e36. https://doi.org/10.1016/j.vacuum.2018.10.014. [26] D. Regonini, C.R. Bowen, A. Jaroenworaluck, R. Stevens, A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotubes, Mater. Sci. Eng. R Rep. 74 (2013) 377e406. https://doi.org/10.1016/j.mser.2013.10.001. [27] L. Wermuth, M. Kolb, T. Mertens, T. Strobl, D. Raps, Superhydrophobic surfaces based on self-organized TiO2-nanotubes, Prog. Org. Coating 87 (2015) 242e249. https://doi.org/10.1016/j.porgcoat.2015.05.010. [28] M.A. Rasheed, K. Ahmad, N. Khaliq, Y. Khan, M.A. Rafiq, A. Waheed, G. Ali, Effect of electrochemical reduction on the structural and electrical properties of anodic TiO2 nanotubes, Curr. Appl. Phys. 18 (2018) 297e303. https://doi. org/10.1016/j.cap.2018.01.001. [29] S. Kim, M. Jung, M. Kim, J. Choi, Bi-functional anodic TiO2 oxide: nanotubes for wettability control and barrier oxide for uniform coloring, Appl. Surf. Sci. 407 (2017) 353e360. https://doi.org/10.1016/j.apsusc.2017.02.166. [30] X. Zhang, Y. Guo, P. Zhang, Z. Wu, Z. Zhang, Superhydrophobic and superoleophilic nanoparticle film: synthesis and reversible wettability switching behavior, ACS Appl. Mater. Interfaces 4 (2012) 1742e1746. https://doi.org/10. 1021/am201856j. [31] E. Balaur, J.M. Macak, L. Taveira, P. Schmuki, Tailoring the wettability of TiO2 nanotube layers, Electrochem. Commun. 7 (2005) 1066e1070. https://doi.org/ 10.1016/j.elecom.2005.07.014. [32] Y. Yin, R. Huang, W. Zhang, M. Zhang, C. Wang, Superhydrophobicesuperhydrophilic switchable wettability via TiO2 photoinduction electrochemical deposition on cellulose substrate, Chem. Eng. J. 289 (2016) 99e105. https://doi.org/10.1016/j.cej.2015.12.049. [33] B. Szczepanik, Photocatalytic degradation of organic contaminants over clayTiO2 nanocomposites: a review, Appl. Clay Sci. 141 (2017) 227e239. https:// doi.org/10.1016/j.clay.2017.02.029. [34] Z. Huang, R.S. Gurney, T. Wang, D. Liu, Environmentally durable superhydrophobic surfaces with robust photocatalytic self-cleaning and selfhealing properties prepared via versatile film deposition methods, J. Colloid Interface Sci. 527 (2018) 107e116. https://doi.org/10.1016/j.jcis.2018.05.004.
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Colorful nanostructured TiO2 film with superhydrophobicesuperhydrophilic switchable wettability and antifouling property Xue Zhou, Sirong Yu*, Jie Zang, Zhexin Lv, Enyang Liu, Yan Zhao College of Material Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2019 Received in revised form 20 May 2019 Accepted 21 May 2019 Available online 24 May 2019
Superhydrophobic nanostructured TiO2 film was successfully fabricated on the Ti plate by anodic oxidation and lauric acid modification. By varying the applied voltage, TiO2 films with various nanostructures and colors were prepared. The water contact angles (WCA) on the modified TiO2 film increased as the applied voltage increased, and the highest WCA on the TiO2 nanovesuvianite film was about 161.23 . When the applied voltage was 5 V and 20 V, the obtained TiO2 nanopores and TiO2 nanotubes could be used to build two improved Cassie models for designing superhydrophobic surface. Moreover, the superhydrophobic TiO2 film showed good anti-fouling property, particularly it could maintain superhydrophobicity both when immersed in and taken out from the organic solvents. The superhydrophobic film could be changed into superhydrophilic under ultraviolet (UV) irradiation, and the superhydrophilic film recovered to hydrophobic or superhydrophobic gradually at 60 C in the dark. Additionally, the superhydrophilic film showed underwater superoleophobicity, which exhibited excellent anti-fouling property by degrading the oil under UV light. © 2019 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobic Nanostructured TiO2 film Superhydrophilic Underwater superoleophobic Anti - fouling property
1. Introduction Wettability is an important characteristic of solid surface, which depends on the chemical compositions and surface structures. Superhydrophobic surface with a contact angle more than 150 and superhydrophilic surface with a contact angle less than 5 have received particular attention due to their important practical applications [1e3]. The common methods to fabricate superhydrophobic and superhydrophilic surface are almost same, such as vapor deposition [3,4], etching [5], hydrothermal treatment [6,7], and electrochemical anodization [8] and so on, by which various rough structure can be created on the surface. Besides, another step is needed to construct a superhydrophobic surface that the deposition of self-assembled monolayers of hydrophobic compounds, such as stearic acid [9] and triethoxyoctylsilane [10], etc. However, the surface, which only shows superhydrophobicity or superhydrophilicity, can't satisfy the needs of industrial development. Recently, smart surfaces with reversible switchable wettability under external stimuli have attracted a great deal of attention
* Corresponding author. E-mail address: [email protected] (S. Yu). https://doi.org/10.1016/j.jallcom.2019.05.259 0925-8388/© 2019 Elsevier B.V. All rights reserved.
because they can take the strong points of both superhydrophobicity and superhydrophilicity [11]. Various external stimuli have been reported to realize the reversible switchable wettability, such as temperature [12], pH [13], light [14], and magnetic field [15] and so on. To realize the reversible switching between superhydrophobicity and superhydrophilicity, external stimuli responsive materials, such as TiO2, ZnO, SiO2, SnO2, WO3 and V2O5, have been developed by ultraviolet (UV) irradiation. Among them, TiO2 has been widely researched, owing to its specific physical and chemical properties, for many potential applications such as photocatalysis [16], solar cell [17] and chemical sensor [18], etc. Particularly, superhydrophobic TiO2 materials have been constructed in recent years due to the small solid-liquid contact area on the rough structure of TiO2 nanomaterials [19]. Since the initial discovery of photo-induced superhydrophilicity on TiO2 surface in 1997 [20], various methods are developed to create TiO2-based surface which shows switchable wettability. Lyons et al. [21] have prepared a superhydrophobic TiO2-polyethylene surface with hierarchical structures that indicates UV-induced reversible wettability and selfcleaning property by template lamination method. Nishimoto et al. [22] have fabricated the superhydrophobicesuperhydrophilic
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pattern with an extremely high wettability contrast by UV irradiation on porous TiO2 layers by hydrothermal-based method. Qing et al. [23] have achieved superhydrophobic modified TiO2/polyvinylidene fluoride composite surface on Cu substrate through dipcoating process, which exhibits reversible switchable wettability by UV light irradiation and heating. TiO2 materials tend to be hydrophilic and easily convert to superhydrophilic under UV light. However, the construction of superhydrophobic TiO2 surface requires suitable roughness structures, such as nanotubes and nanorods, which are easier to trap air between liquid and solid. Compared to the above methods, anodic oxidation is an efficient and highly controllable method to obtain TiO2 nanoporous/nanotube structure by adjusting the voltage, which has an important effect on the wettability. In addition, the anti-fouling or self-cleaning property of the surface with reversible switchable wettability is always tested when the surface shows superhydrophobicity, but few studies report the property when the smart surface shows superhydrophilicity or underwater superoleophobicity. In this paper, superhydrophobic TiO2 film was prepared on the Ti plate by anodic oxidation and modification in ethanol solution of lauric acid. The effects of the applied voltages on the microstructure, hydrophobicity and color of the TiO2 film were discussed, respectively. Among them, for the obtained TiO2 nanopores and nanotubes, two improved Cassie models were established to design the superhydrophobic surface. Moreover, the conversion wettability between superhydrophobicity and superhydrophilicity was analyzed in detail, and the superhydrophilic surface showed underwater superoleophobicity. The anti-fouling properties of the superhydrophobic surface and superhydrophilic surface (underwater superoleophobic surface) were tested. 2. Experimental 2.1. Fabrication of TiO2 film TA2 titanium plates (40 mm 20 mm 1 mm) were obtained from Shenzhen Ode Fu Materials Co. Ltd, Guangzhou, China. Before the anodic oxidation, surface impurities on the Ti plate were removed by being immersed in an aqueous solution that was composed of 15 mL deionized water, 12 mL HNO3 and 3 mL HF for 30 s at ambient temperature. The Ti plate was then ultrasonically cleaned by acetone, ethanol and deionized water for 10 min respectively, and dried at room temperature. The pretreated Ti plate and graphite plate (40 mm 20 mm 3 mm) were used as anode and cathode respectively, whose spacing was 40 mm. Electrolytic solution consisted of 1 vol% HF (40 wt%, Sinopharm) aqueous solution. Anodic oxidation was performed for 2 h with a DC power. So as to investigate the effects of the applied voltage on the structure and wettability of the TiO2 film, the applied voltage was 5, 10, 15, 20, 25, and 30, respectively. After the anodic oxidation, the specimen was cleaned by deionized water and dried at ambient temperature. 2.2. Fabrication of superhydrophobic TiO2 film To lower the surface energy, Ti plate coated with TiO2 was immersed in a 0.02 mol/L ethanol solution of lauric acid for 24 h and subsequently dried at 60 C for 20 min. Because TiO2 always shows photocatalytic property which is introduced in Section 3.6, the surface modification process was carried out in the dark to avoid the effect of light. 2.3. Characterizations and tests The surface morphology of the as-prepared specimens was observed using a field-emission scanning electron microscope
(FESEM, NovaNano SEM 450, FEI). The surface chemical compositions of the specimens were studied by X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha). A contact angle meter (SL200B, USA, KINO) with digital image analysis software was used to measure the contact angle (CA) at room temperature. The value reported was the average at least five measurements performed at different positions with 3 mL water droplets or 3 mL oil droplets. Dichloromethane was used as model oil for the test and the CA oil/ water was measured under deionized water environment. The error bars in the plots of contact angles indicated the test error. Water and oil wettability changes of the TiO2 film were tested by UV illumination. A 250 W mercury lamp was devoted to be the UV light source which was 20 cm away from the specimens. Besides, in order to prove the low adhesion between oil droplet and underwater superoleophobic surface, the 3 mL oil droplet was contacted and separated with the underwater superoleophobic surface through the movement of the needle, which was recorded by the contact angle meter. In the test of photocatalytic property of TiO2 film, UV light was carried out with a 250 W mercury lamp. An ultravioletevisible spectrophotometer (UVeVis) (Hitachi Ue3900H, Japan) was used to measure the UVevis spectra of the methylene blue (MB) absorption peaks. 3. Results and discussion 3.1. Microstructure of the TiO2 film The applied voltage decides the electric field intensity, thus affecting the migration of ions and ultimately the microstructure of the TiO2 film. Fig. 1 showed the microstructure of the TiO2 film fabricated at different voltages. It could be learnt from Fig. 1(a) that neat nanopores appeared on the TiO2 film when the applied voltage was 5 V. With the increase of the voltage, nanopores gradually evolved into nanotubes. As shown in Fig. 1(b), when the applied voltage was 10 V, part of the TiO2 film between the pores was etched, thus TiO2 nanopores and TiO2 nanotubes coexisted on the surface. When the applied voltage increased to 15 V and 20 V, closepacked TiO2 nanotubes and separated TiO2 nanotubes were formed on the Ti plates respectively (Fig. 1(c) and (d)). However, when the applied voltage reached up to 25 V, a nanovesuvianite structure was synthesized as shown in Fig. 1(e). On the surface, large numbers of nanoscale protrusions were randomly distributed and there were holes existing among these. The similar structure appeared in Fig. 1(f) when the applied voltage was 30 V. Besides, it could be found from Fig. 1 that the internal diameter of the TiO2 nanopores or nanotubes was affected by the applied voltage. The internal diameter was measured by the software of Nano Measurer that the value was the arithmetic mean of 100 points chose manually. The nanovesuvianite structure was not appropriate to be measured when the applied voltage was 25 V and 30 V. The distribution range and proportion of the internal diameters for different voltages were exhibited in Fig. 2(aed) that the internal diameters of the nanostructured TiO2 were relatively uniform. Moreover, the average internal diameters were shown in Fig. 2(e) that the internal diameters increased with the increase of the applied voltage. When the applied voltages were 5 V and 20 V, the average internal diameters were 17.00 nm and 81.10 nm, respectively. Accordingly, with the increase of the applied voltage, the microstructure of the TiO2 film changed from nanopore structure to nanotube structure and then the nanovesuvianite structure, and the internal diameters increased too. It was because that the higher applied voltage would enhance the electro field and ionic mobility and both TiO2 growth and dissolution were improved, leading to a larger pore size and pore spacing. Especially, when the applied
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Fig. 1. SEM images of the TiO2 film obtained under different voltages. (a) 5 V, (b) 10 V, (c) 15 V, (d) 20 V, (e) 25 V and (f) 30 V.
Fig. 2. Distribution range and proportion of the internal diameters (aed) and average internal diameters (e) of nanostructured TiO2 for 5 V, 10 V, 15 V and 20 V, respectively.
voltage was too high such as 25 V, the irregular microstructure would appear because the TiO2 growth and dissolution were uncoordinated. 3.2. Superhydrophobicity of the TiO2 film Microstructure and low surface energy are two key factors in the construction of superhydrophobic surfaces. After anodic oxidation, TiO2 film having a certain microstructure was grown on the surface of the Ti plate and showed hydrophilicity with a water contact angle less than 90 . In order to obtain the superhydrophobicity, TiO2 film was subsequently modified by lauric acid to lower the surface energy, which had no effect on its microstructure. It could be seen from Fig. 1 that the microstructure of TiO2 was various at different voltages, and therefore the surface wettability was also affected. Fig. 3 showed the water contact angles (WCA) on the modified TiO2 film fabricated under different voltages. The WCA increased with the increase of the applied voltage. When the
applied voltage was 5 V that the TiO2 film was composed of nanopore structures, the modified TiO2 film exhibited hydrophobicity with the WCA about 143.35 . For the other applied voltages, the modified TiO2 film showed superhydrophobicity with the WCA more than 150 . Especially, when the applied voltages were 20 V and 30 V, the WCA on the TiO2 nanotube film and TiO2 nanovesuvianite film were 157.43 and 161.23 , respectively. The main reason for the above phenomenon was the difference of the water-solid contact area which could be calculated by the Cassie law [24]:
cosqc ¼ f ðcosq0 þ 1Þ 1 c
(1)
where q is the WCA on the modified TiO2 film; f is the area fraction of the liquid-solid contact; q0 is the equilibrium contact angle which is the WCA (101 ) of the modified Ti plate. When the applied voltages were 5 V, 20 V, and 30 V that the TiO2 film consisted of nanopore, nanotube and nanovesuvianite structure respectively,
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f ¼
p ðD2 =2 þ hÞ2 ðD2 =2Þ2
(4)
L2 2
Then the Cassie model is revised to:
0 cosq ¼ @ c
p ðD2 =2 þ hÞ2 ðD2 =2Þ2 L2 2
1 Aðcosq0 þ 1Þ 1
(5)
¼ pðh = L2 ÞððD2 = L2 Þ þ ðh = L2 ÞÞðcosq0 þ 1Þ 1
Fig. 3. WCA of the modified TiO2 film obtained under different voltages.
the values of f were calculated to be 24.43%, 9.46% and 6.57%, respectively. The results showed that the solid-liquid contact area decreased with the increase of applied voltages. Nanopore structure (5 V) and nanotube structure (20 V) consisted of pores but differed in that the interspaces between pores were filled with solid in nanopore structure but the air pockets in nanotube structure. During the increase of voltage from 5 V to 20 V, the solid-liquid contact mode changed from “area contact” to “line contact”, which was more conducive to forming superhydrophobic surface for TiO2 film. When the applied voltages were higher (25 V or 30 V), the solid-liquid contact area was lower. For the nanovesuvianite structure, the pores were rough and uneven, forming a 3D random nanoporous network. The special nanostructure satisfied typical liquidesolid ‘‘point-contact’’ which could trap large air between solid and water while forming a good superhydrophobic surface. Thus the various microstructure had a great influence on the wettability and the above results offered important information for designing superhydrophobic surface. The nanopores and nanotubes were almost perpendicular to the substrate, and assumed to be uniform in size. Therefore, two improved Cassie model could be established for designing superhydrophobic surface as shown in Fig. 4(a and b). For nanopore structure, let L1 be the center distance between adjacent pores, and D1 be the inside diameter of the pores. Generally, the water droplet only contacts the flat top of the surface and the diameter of the water droplet is much larger than L1. Thus the area fraction of the liquid-solid contact f can be defined as:
f ¼
L1 2 pðD1 =2Þ2
(2)
L1 2
Then the Cassie model is revised to: c
cosq ¼
L1 2 pðD1 =2Þ2 L1 2
! ðcosq0 þ 1Þ 1
p ¼ 1 ðD1 =L1 Þ2 ðcosq0 þ 1Þ 1 4
(3)
For nanotube structure, let the inside diameter of the tubes be D2 and the wall thickness be h. Let L2 be the center distance between adjacent tubes. Thus f can be represented as:
In order to reflect the relationship between contact angle qc and geometric parameters intuitively, the Eqs. (3) and (5) are converted to Fig. 4(c) and (d), respectively. For the nanopore structure, Fig. 4(c) indicated the contact angle qc is only related to D1 =L1 , and increases with the increase of the D1 =L1 (90 < q0 < 180 ). While for the nanotube structure, the contact angle qc is related to both D2 =L2 and h =L2 , and decreases with the increase of D2 =L2 and h =L2 (90 < q0 < 180 ) as shown in Fig. 4(d). It can be concluded that the wettability of the surface can be controlled by adjusting geometric parameters of the microstructure. Therefore, the superhydrophobic surface can be fabricated by designing the geometric parameters of the hydrophobic surface. 3.3. Chemical compositions of the superhydrophobic TiO2 film Chemical compositions of superhydrophobic TiO2 film were investigated by XPS analysis. Fig. 5(a) showed the XPS survey spectra of the modified TiO2 film, in which elements of Ti, O, F and C were detected on the surface corresponding to peaks at binding energies of approximately 459.26 eV, 531.26 eV, 686.26 eV and 286.26 eV, respectively. Besides, the high-resolution XPS spectra of Ti 2p, F 1s, C 1s and O 1s were measured. As shown in Fig. 5(b), at binding energies of 458.6 eV and 464.3 eV, two Ti 2p peaks were observed corresponding to Ti 2p (3/2) and Ti 2p (1/2), respectively, indicating the formation of TiO2 film on Ti plate [25]. Moreover, the peak at 459.8 eV was due to the highly electronegative hexafluorotitanate complex [TiF6]2- in the anodic film [26], which was also confirmed by the high-resolution XPS spectrum of F 1s (Fig. 5(c)). The competition between the formation of TiO2 and its dissolution was a key factor in determining the TiO2 structure, and the [TiF6]2- was the reaction product of TiO2 and F in electrolyte. Fig. 5(d) exhibited that C 1s peaks could be decomposed into three peaks at 288.4 eV, 285.8 eV and 284.6 eV, attributed to C]O, CeO, and CeH and CeC groups, respectively. It indicated lauric acid was successfully self-assembled on the TiO2 film. After modified in ethanol solution of lauric acid, the microstructure of the TiO2 film was unaffected (Fig. 6(a)) and the surface energy could be effectively reduced due to the existence of the CeH groups on lauric acid. As shown in Fig. 6(b), lauric acid was self-assembled on the TiO2 film through the dehydration reaction between the hydroxyl groups of the TiO2 film and carbonyl groups of the lauric acid. The TiO2 film anodic oxidation always leaded to the formation of hydroxyl groups on the surface which was the reason why the TiO2 film showed hydrophilic before modification [27]. In the highresolution XPS spectrum of O 1s (Fig. 5(e)), in addition to the TieO groups at 530.1 eV, there was a small amount of TieOH groups existing at 531.5 eV that didn't react with lauric acid [28]. 3.4. Coloration and anti-fouling property of the superhydrophobic TiO2 film The color of the TiO2 film is always affected by applied voltage. It could be leant from Fig. 7 that TiO2 film anodized at different voltages had various colors. In general, when light hits the surface
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Fig. 4. Top view of the (a) nanopore model and (b) nanotube model. (c) Contact angles in the nanopore model as the change of D1/L1. (d) Contact angles in the nanotube model as the change of D2/L2 and h/L2.
Fig. 5. XPS spectra of the superhydrophobic TiO2 film: (a) survey spectrum; (b) Ti 2p; (c) F 1s; (d) C 1s; (e) O 1s.
of the Ti specimen coated with TiO2 film, the light reflected from the interface between TiO2 film and air interferes with the light which is reflected from the interface between Ti plate and TiO2 film. The light of different wavelengths is mixed, so the surface of the metal titanium exhibits a beautiful interference color [29]. As shown in Fig. 1, the microstructure of the TiO2 film obtained at
different voltages was different, therefore the luminous flux of the TiO2 film and the refractive index and reflectance of the light were changed, thereby exhibiting different colors. With a natural light (indoor), Fig. 7 was taken from a 90 viewing angle at a distance of 30 cm. Moreover, the color of the film was diverse at different angles and different light sources. The colorful Ti specimens coated
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Fig. 6. (a) Morphological evolution of the Ti plate surface after anodic oxidation and modification. (b) Formation mechanism of the self-assembled lauric acid on the rough TiO2 film.
Fig. 7. Macroscopic view of TiO2 film anodized at different voltages.
with TiO2 film can be used as decoration in applications of building materials and daily necessities, and the color can be controlled by adjusting applied voltage. However, the TiO2 film was typically hydrophilic and lipophilic that was easy to absorb contaminants and then break the film color. Thus the superhydrophobic surface with anti-fouling property could avoid contamination and keep the film color for long-term use. In this research, anti-fouling property of the superhydrophobic TiO2 film was tested as shown in Fig. 8. The unmodified and superhydrophobic TiO2 film were immersed in sewage consisting of soil and sand and then taken out. For unmodified TiO2 film, the surface was wetted and contaminated as shown in Fig. 8(c). However, for superhydrophobic TiO2 film, the surface was clean as before immersing. It was because that the nanostructure of the
superhydrophobic TiO2 film was benefited to trap air between liquid and TiO2 film and then showed liquid repellence. The liquid could flow down and take away solid contaminants on the superhydrophobic surface. Besides, the superhydrophobic TiO2 film also showed anti-fouling property for the organic solvents that had low boiling point such as hexane, toluene and chloroform. The superhydrophobic specimen was immersed in hexane as shown in Fig. 9(aee). A MB dyed water droplet was dripped on the surface and rolled down in a spherical shape, indicating that the superhydrophobic TiO2 film could maintain the superhydrophobicity when immersed in organic solvent. However, the dyed water droplet spread on the unmodified TiO2 film as shown in Fig. 9(f). What's more, when the superhydrophobic TiO2 film was taken out from the organic solvent, the WCA was unaffected as the initial
Fig. 8. Anti-fouling test of unmodified TiO2 film (aec) and superhydrophobic TiO2 film (def).
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Fig. 9. (aee) A dyed water droplet rolling on the superhydrophobic TiO2 film when immersed in organic solvents. (f) A dyed water droplet spreading on the unmodified TiO2 film when immersed in organic solvents.
state. This indicated that the low boiling point organic solvent didn't damage the microstructure and compositions of the superhydrophobic TiO2 film, which had the similar result with the previous report [30]. 3.5. Superhydrophobicesuperhydrophilic switchable wettability of the TiO2 film Fig. 10(a) showed the process of superhydrophobic surface (the applied voltage was 30 V) converting into superhydrophilic surface under UV irradiation with time. Different contact angles could be controlled by adjusting UV irradiation time, and then hydrophobic surface and hydrophilic surface could also be obtained during this process. In addition, the distance between the specimen and light and the power of the light also had a certain influence on the change process of the contact angles on the surface. The stronger the UV intensity, the faster the contact angle decreased. A TiO2 film surface with superhydrophobicity and superhydrophilicity at the same time
could be fabricated by selective UV irradiation. As shown in Fig. 10(b), the water droplets tended to be spherical on the superhydrophobic surface before UV irradiation, and the water spread fully on the irradiated part. Even if the irradiated area was filled with a large amount of water, there was a clear boundary between superhydrophobic and superhydrophilic surface. The surface with extremely high wettability contrast can be used in the offset printing, thus various superhydrophobic-superhydrophilic patterns could be designed by area-selective UV irradiation for this application [22]. During the UV irradiation, it required two steps to convert a superhydrophobic surface to superhydrophilic surface. Firstly, When the TiO2 film was irradiated by UV, electron-hole pairs were generated. The lauric acid combined on the surface was decomposed into CO2 and H2O, meanwhile the holes reacted with the oxygen ions on the surface to form oxygen vacancies [31]. Secondly, H2O molecules adsorbed to oxygen vacancy and dissociated to hydroxyl groups. The hydroxyl groups could easily combine with water molecules to make the film superhydrophilic [32].
Fig. 10. (a) The change of the WCA on superhydrophobic TiO2 film under UV irradiation with time going. (b) Images of water droplets dropped on superhydrophobic TiO2 film (left) and a water droplet and layer on partly UV-irradiated TiO2 film (right).
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superhydrophobicity with a WCA about 153.80 , and the WCA increased to the maximum recovery contact angle using the shortest time. During the heat treatment in the dark, the hydroxyl groups on the surface were replaced by the adsorption of atmospheric oxygen, and the TiO2 surface recovered to the original state. The lauric acid combined on the surface was decomposed and small amount of hydroxyl groups were still on the surface, the hydrophobicity was weaker than that of the modified TiO2 film [32]. Besides, the TiO2 nanovesuvianite film (the applied voltage was 30 V) was more conducive to trap the air between the water and film than the TiO2 nanopore film and nanotube film, so the WCA was higher after recovery. 3.6. Anti-fouling property of the underwater superoleophobic TiO2 film
Fig. 11. Recovery contact angles and time of three different specimens.
Fig. 12. (a, b) Photographs of water and oil droplet shape on the superhydrophobic TiO2 film with contact angle about 158 and 3 , respectively. (c, d) Photographs of oil droplets shape on the superhydrophilic TiO2 film in air and under water with contact angle about 4 and 165 , respectively.
Interestingly, the WCA on the irradiated surface (superhydrophilic surface) recovered gradually when the specimen was heated at 60 C in the dark. Fig. 11 showed the WCA on three typical TiO2 film after being heated at 60 C in the dark for different time. It could be learned that the irradiated TiO2 film obtained at 5 V and 20 V showed hydrophobicity with the WCA about 135.43 and 143.54 , respectively. Moreover, after the heat treatment, the TiO2 film obtained at 30 V recovered from superhydrophilicity to
Before the UV irradiation, the superhydrophobic surface exhibited superhydrophobicity and superlipophilicity in the air as shown in Fig. 12 (a, b). After the UV irradiation the superhydrophilic surface also showed the superlipophilicity in the air but indicated the underwater superoleophobicity with a high CA oil/water about 165 (Fig. 12 (c, d)). Moreover, the underwater oil adhesion of the underwater superoleophobic surface was tested. Fig. 13 exhibited the process that a 3 mL oil droplet approached, contacted, pressed and departed to the underwater superoleophobic surface. It could be seen that there was no visual oil leaving on the surface after the oil droplet being pressed seriously and departed from the underwater superoleophobic surface (video S1). Thus, the underwater superoleophobic surface showed low adhesion for oil, which is important for the anti-fouling property underwater. However, the superhydrophilic surface showed superlipophilicity in the air, which meant the surface was easy to be polluted by the oil. It is known that TiO2 has good photocatalytic property for organics [33]. To test the photocatalytic property of the superhydrophilic TiO2 film, the ability of the film to degrade the MB (5 mg/L) solution was tested. In contrast, the ability of the Ti plate to degrade the MB was also measured. Moreover, before the test, the specimens were immersed in MB solution under magnetic stirring for 30 min in the dark to establish adsorption/desorption equilibrium. According to the Beer-Lambert Law [34], the absorbance of MB is proportional to its concentration, thus the concentration of MB is corresponding to the magnitude of MB absorption peak at 665 nm wavelength. According to Fig. 14(a), Compared to the Ti plate, almost all of MB was degraded by TiO2 film under UV irradiation about 6 h. Besides, the magnitude of MB absorption peak decreased with the time as shown in Fig. 14(b), which meant that the TiO2 film had the photocatalytic property under UV light. Supplementary video related to this article can be found at https://doi.org/10.1016/j.jallcom.2019.05.259. The CA oil/water on the superhydrophilic TiO2 film decreased from 165 to 16 after being immersed in the oleic acid. The photocatalytic property of the TiO2 film was always used to degrade the oily contaminants. The CA oil/water recovered to 165 after 10 h UV irradiation, and even after five cycles of the oleic acid immersion and UV irradiation, the CA oil/water could be recovered as the initial
Fig. 13. Underwater oil adhesion test on the superhydrophilic surface. The arrows represent the moving direction of the needle.
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Fig. 14. UVevis spectra of (a) the MB absorption peaks after being degraded by Ti plate and TiO2 film for 6 h and (b) the MB absorption peaks being degraded by TiO2 film with time.
superhydrophilic. After heating at 60 C in the dark, the WCA of the irradiated film recover gradually, and the restored contact angle of the TiO2 nanovesuvianite film is the largest about 153.80 . Besides, the irradiated TiO2 film also has underwater superoleophobicity and shows low adhesion for oil underwater. The underwater superoleophobic surface also indicates good anti-fouling property that oleic acid can be degraded by the TiO2 film under UV irradiation and the CA oil/water is not affected. Acknowledgements This work was supported by the Postgraduate Innovation Project (No. 17CX06051) of China University of Petroleum (East China), China; the Open Fund (No. OGE201702-07) of Key Laboratory of Oil & Gas Equipment, Ministry of Education of China (Southwest Petroleum University), China; the Natural Science Foundation of Shandong Province (No. ZR2017LEM004), China.
Fig. 15. Reversible underwater superoleophobicity-lipophilicity switching of the TiO2 film under oleic acid pollution and UV irradiation.
state, as shown in Fig. 15. This indicated that the TiO2 film had good anti-fouling property by photocatalytic degradation of pollutants under UV irradiation. The underwater superoleophobicity and antifouling property of the TiO2 film have a good application prospect in oil-water separation.
4. Conclusion In summary, nanostructured TiO2 film is fabricated on the Ti plate by anodic oxidation and then lauric acid is utilized to make the film superhydrophobic. TiO2 film with various TiO2 nanostructures are prepared at different applied voltages, among which three typical porous structures that nanopores, nanotubes and nanovesuvianite are obtained at voltage of 5 V 20 V and 30 V, respectively. Moreover, the WCA on the modified TiO2 film increase with the increase of applied voltages, and two improved Cassie models are established for the nanopore structure and nanotube structure. The superhydrophobic TiO2 film exhibits different colors at different applied voltages, and shows excellent anti-fouling property to pollutants especially for organic solvents that have low boiling point. Under UV irradiation, the surface can be changed from superhydrophobic to hydrophobic, hydrophilic and
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