Fabrication of superhydrophobic TiO2 quadrangular nanorod film with self-cleaning, anti-icing properties

Fabrication of superhydrophobic TiO2 quadrangular nanorod film with self-cleaning, anti-icing properties

Ceramics International 45 (2019) 11508–11516 Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/loc...

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Ceramics International 45 (2019) 11508–11516

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Fabrication of superhydrophobic TiO2 quadrangular nanorod film with selfcleaning, anti-icing properties

T

Xue Zhou, Sirong Yu∗, Shizhe Jiao, Zhexin Lv, Enyang Liu, Yan Zhao, Ning Cao College of Material Science and Engineering, China University of Petroleum (East China), Qingdao, 266580, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Superhydrophobicity TiO2 quadrangular nanorods Anti-icing property Self-cleaning property

Superhydrophobic TiO2 quadrangular nanorod film was fabricated by hydrothermal reaction and stearic acid modification. X-ray diffractometer and Fourier transform infrared spectrometer were employed to characterize the surface crystal structures and chemical compositions of the superhydrophobic TiO2 film, respectively. The effects of the titanium source (titanium tetraisopropoxide (TTIP)) amount and reaction time on the morphology and wettability of the TiO2 film were studied by scanning electron microscope and contact angle meter. The results show that the diameter of the TiO2 quadrangular nanorods increases and then the water contact angle on modified TiO2 film decreases with the increase of the reaction time and TTIP amount. Moreover, when the TTIP amount is 0.3 mL and solvent is 30 mL, the wetted state of the superhydrophobic TiO2 film surface conforms to an improved Cassie model. Besides, the superhydrophobic TiO2 film shows good low adhesion, self-cleaning and anti-icing properties. Particularly, the anti-icing property decreases with the increase of the reaction time and TTIP amount.

1. Introduction Superhydrophobic surfaces are characterized by a contact angle more than 150° and a sliding angle less than 10°. Recent research has pointed that many organisms in nature have superhydrophobic surfaces, such as lotus leaf [1], strider [1,2], and desert beetle [3], and this helps to promote the development of the bionic superhydrophobic surfaces. The bionic superhydrophobic surfaces have a wide range of practical applications, such as self-cleaning [4–6], anti-corrosion [6], anti-icing [7], oil-water separation [8], and drag reduction [9]. It is reported that two contributing factors to fabricate superhydrophobic surfaces are surface roughness and low surface energy [10]. Thus, various methods are designed to form superhydrophobic surfaces, including sol-gel method [11], vapor deposition method [12], electrochemical method [13] and template method [14], etc. Bionic superhydrophobic surfaces formed by various methods can be achieved by either building rough structure on a material with low energy surface or lowering the surface energy of a rough structure. Titanium dioxide (TiO2) nanomaterials have attracted much interest due to their diverse applications, such as solar cells [15], common products [16], pollutant photodegradation [17], and self-cleaning [18]. Currently, owing to their smaller solid-liquid contact area and large specific surface area, TiO2 nanomaterials have been used to fabricate superhydrophobic surfaces [19]. Wang et al. [20] have prepared ∗

superhydrophobic TiO2 coating with hierarchical morphology constructed by radial nanowires, which is fabricated on the glass substrate by chemical deposition method. Huang et al. [21] have built water repellent TiO2 nanotubes on 316L stainless steel via anodic oxidation, which indicates good corrosion resistance and hemocompatibility. Yang et al. [22] have fabricated fluorinated TiO2 nanoparticles on cotton fabric by sol-gel method with a water contact angle 152.5°. Compared to the above methods, hydrothermal method is widely used with simple procedures. Moreover, it allows to control the microstructure of the TiO2 by adjusting the type and concentration of the reaction solution, pH, reaction time, and temperature, thereby affecting the chemical and physical properties. Karuna P et al. [23] have reported that the length and diameter of TiO2 nanorods increase with the increase of the reaction temperature, and then photoconversion efficiency of the dye-sensitized solar cell (DSSC) is improved. However, few studies have reported the influence of the experimental parameters on the water wettability of the TiO2 nanorod film during the hydrothermal synthesis process. In this paper, superhydrophobic TiO2 quadrangular nanorod film was fabricated on the fluorine-doped tin oxide (FTO) glass by hydrothermal synthesis and modification in ethanol solution of stearic acid. Hydrothermal synthesis was employed to construct rough structure, and stearic acid was utilized to lower the surface energy. The effects of the experimental parameters including the concentration of the

Corresponding author. College of Material Science and Engineering, China University of Petroleum, Qingdao, 266580, China. E-mail address: [email protected] (S. Yu).

https://doi.org/10.1016/j.ceramint.2019.03.020 Received 21 February 2019; Received in revised form 4 March 2019; Accepted 4 March 2019 Available online 14 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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Fig. 1. SEM images of the TiO2 quadrangular nanorod film obtained at different TTIP amount and reaction time. (a), (b) and (c) were 0.3–6, 0.3–8 and 0.3–10 specimens, respectively. (d), (e) and (f) were 0.4–6, 0.4–8 and 0.4–10 specimens, respectively. (g), (h) and (i) were 0.5–6, 0.5–8 and 0.5–10 specimens, respectively.

reaction solution (TTIP amount) and the reaction time on the morphology and wettability of the TiO2 film were studied. Moreover, the low adhesion, self-cleaning property and anti-icing property of the superhydrophobic TiO2 film were tested, and the effects of the TTIP amount and the reaction time on the anti-icing property were also discussed. 2. Experimental methods 2.1. Materials Fluorine-doped tin oxide (F: SnO2 FTO) substrate (20 mm × 20 mm × 10 mm) was purchased from Shenzhen Ode Fu Materials Co. Ltd, Guangzhou, China. Titanium tetraisopropoxide (TTIP) was purchased from Macklin, and concentrated hydrochloric acid (HCl) was obtained from West Long Chemical Co. Ltd in China. Ethanol, acetone and stearic acid (C18H36O2) were received from Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. 2.2. Preparation of superhydrophobic TiO2 film The fabrication of the superhydrophobic TiO2 film took two steps: hydrothermal synthesis and modification. The specific experimental procedure was described as follows: (1) In a typical synthesis process, the TiO2 nanorods were obtained by hydrothermal method. FTO substrate was ultrasonically cleaned in acetone, ethanol and deionized water respectively for 15 min. TTIP was used as the titanium source and dissolved in the mixture of 15 mL HCl and 15 mL deionized water under magnetic stirring for

30 min at room temperature. The TTIP amount was taken 0.3, 0.4, and 0.5 mL, respectively. The final mixture was poured into a 50 mL Teflon liner, and the cleaned FTO substrates were kept at an angle of 45° from the wall of the container with the conducting side facing down to avoid the effect of the sedimentation on the film. The Teflon liner with the final mixture and FTO substrate was placed into a stainless steel autoclave which was maintained at 150 °C for 6, 8 and 10 h, respectively. After cooling down to room temperature, the specimen was cleaned with the deionized water and dried in air. The obtained TiO2 film at 0.3, 0.4, and 0.5 mL TTIP and 6, 8, and 10 h reaction time were named as 0.3–6, 0.3–8, 0.3–10, 0.4–6, 0.4–8, 0.4–10, 0.5–6, 0.5–8, and 0.5–10, respectively. (2) After the hydrothermal synthesis, the FTO substrate coated with the TiO2 film was immersed in a 0.02 mol/L ethanol solution of stearic acid for 2 h in darks and subsequently dried at 90 °C for 30 min. 2.3. Characterizations and tests Field emission scanning electron microscope (FESEM, JEOL, JSM7200F) was used to observe the morphologies. X-ray diffractometer (XRD, X'Pert PRO MPD, PANalytical B.V.) was taken to measure the crystal structures of the specimens. The X-ray source was a Cu target, which was operated at 40 kV and 40 mA within 20–75° range. The infrared spectra of specimen surface were analyzed by Fourier transform infrared spectrometer (FTIR, (Nexus, Thermo Nicolet)). The water contact angle (WCA) of specimen surface was measured using deionized water on a contact angle meter (SL200B, USA, KINO) with digital image analysis software. WCA values were the averages of the contact angles measured at least five different positions on the specimens with a 3 μL water droplet. The sliding angle (SA) was

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measured by gradually tilting the specimen. SA value was the tilt angle of the specimen when a 3 μL water droplet started moving. In order to evaluate the adhesion of the superhydrophobic TiO2 film, the process of a 3 μL water droplet approaching, contacting and leaving the superhydrophobic surface was recorded through moving the needle. Besides, the self-cleaning property of the superhydrophobic film was tested. Fly ash particles with tens of microns in diameter were uniformly sprinkled on the FTO substrate and the superhydrophobic TiO2 film, respectively, which were inclined in a small angle. Then a water droplet was dropped on the two specimens and the sliding process was recorded by photographs. Icing-delay time was a measure of the anti-icing property of the specimen. In order to evaluate the anti-icing property of the superhydrophobic TiO2 film, water droplets (3 μL) were dropped on the different specimens, which were put in a cryogenic box (DW-45W28, China, Jiesheng) (the temperature was controlled at −20 °C). The time that the water droplets changed from transparent to opaque was recorded as the freezing time.

3. Results and discussion 3.1. Microstructure of superhydrophobic TiO2 film The surface morphology of the TiO2 film was controlled by adjusting the experimental parameters. More details of the FTO substrate and as-prepared TiO2 film obtained by SEM analysis were presented in Fig. 1 and Fig. 2, respectively. Fig. 1 showed the TiO2 film obtained at different TTIP amount (0.3 mL, 0.4 mL and 0.5 mL) and reaction time (6 h, 8 h, and 10 h). It could be observed that the TiO2 film was composed of nanorods in the shape of the pillar with quadrangular top facets. Moreover, with the variation of the TTIP amount and the reaction time, the size of the TiO2 nanorods changed. The geometric parameters of the surface morphology were measured by the software of Nano Measurer, and the size was the arithmetic mean of 100 points chose manually. The side length a of the quadrangular top facets of the TiO2 nanorods and the spacing b between the TiO2 nanorods were shown in Table 1. Especially for the TiO2 film formed at 0.5 mL TTIP and more than 8 h reaction time, the side length and the spacing were hard to measure because that the TiO2 nanorods coalesced together seriously. However, it could be learned from Fig. 1(h) and (i) that the side length increased and the spacing decreased with the reaction time. Moreover, as shown in Fig. 2(a), the micro structure of the FTO substrate was like irregular stone which might be the reason why some of the TiO2 quadrangular nanorods grew up at an angle to the substrate surface normal. In this research, low energy modification had no effect on the surface morphology of the film. The main factors affecting the microstructure of the TiO2 film were TTIP amount and the reaction time. Fig. 2(b) and (c) indicated the coverage of the FTO substrate became lager when the reaction time was from 3 h to 6 h. Meanwhile, it could be observed from Fig. 1 and Table 1 that the average diameter of TiO2 quadrangular nanorods increased with the increase of the reaction time.

Table 1 Structural parameters of the TiO2 quadrangular nanorod film obtained at different TTIP amount and reaction time. Specimen code

Side length a (nm)

Spacing b (nm)

0.3–6 0.3–8 0.3–10 0.4–6 0.4–8 0.4–10 0.5–6 0.5–8 0.5–10

78.06 91.69 110.27 93.42 236.16 252.91 213.91 – –

200.88 196.62 193.46 195.97 256.84 203.28 231.74 – –

In particular, when the TTIP amount was 0.4 mL, the average diameter of the TiO2 quadrangular nanorods had the largest growth rate during 6 h–8 h as shown in Fig. 1 and Table 1. When the TTIP amount was 0.5 mL, the average diameter of the TiO2 quadrangular nanorods was too large so that the TiO2 quadrangular nanorods coalesced together as the reaction time increased. Moreover, it appeared in Fig. 1 that when the TTIP amount increased, the TiO2 quadrangular nanorods tended to grow vertically on the substrate. It was because that the density of the nucleation was higher, the TiO2 quadrangular nanorods growing at an angle to the FTO substrate normal ran into adjacent nanorods and stopped growing [24]. On the contrary, when the TTIP amount was low such as 0.3 mL, the nucleation density was lower so that the TiO2 quadrangular nanorods could keep growing at an angle to the FTO substrate in a period of time.

3.2. Crystal structure and chemical compositions of the superhydrophobic TiO2 film The surface crystal structures of the FTO substrate and the superhydrophobic TiO2 film were recorded by the X-ray diffraction. Fig. 3(a) showed the XRD pattern of the FTO substrate (JCPDS No.46–1088, a = b = 4.750, c = 3.198) at 26.01°, 33.59°, 37.64°, 51.46°, 54.48°, 61.43°, 65.49°. As shown in Fig. 3(b) and (c), in addition to the peaks of the FTO substrate, all the peaks were corresponding to the rutile phase with the tetragonal crystal structure (JCPDS No.73–1765, a = b = 4.589, c = 2.954). No characteristic peaks were observed for other polymorphs which revealed the high purity and single phase crystallinity of the rutile TiO2 film. Moreover, the FTO substrate also had the tetragonal rutile structure, and the lattice mismatch between the FTO and the rutile TiO2 was small, which might promote the epitaxial nucleation and growth of rutile TiO2 nanorods on the FTO substrate [25]. Comparing the spectrum line (b) and spectrum line (c), the intensity of the diffraction peaks of the FTO substrate gradually decreased because the crystallinity of TiO2 became higher and the coverage of the substrate was increased. Besides, with the increase of the TTIP amount, the intensity of the two diffraction peaks assigned to the (002) and (101) planes enhanced. It indicated that TiO2 nanorods grew

Fig. 2. SEM images of the (a) FTO substrate and (b, c) the TiO2 quadrangular nanorod film at 0.3 mL TTIP for different reaction time (3 h and 6 h). 11510

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Fig. 3. XRD patterns of the (a) FTO substrate, (b) 0.3–8 TiO2 film and (c) 0.4–8 TiO2 film.

perpendicular to the substrate surface and some leaned toward the azimuthal direction [23,26], which could be observed in Fig. 1. In this study, the TiO2 film was modified by the stearic acid which could effectively reduce the surface energy of the TiO2 film because of the existence of the eCH3 and eCH2 groups. Fig. 4 exhibited the FTIR spectra of the stearic acid and superhydrophobic TiO2 film, and two FTIR spectra showed almost the same peak. The peak at the 1702.84 cm−1 was assigned to the stretching vibration of -COOe groups (Fig. 4(b)), which proved the dehydration reaction between TiO2 film and stearic acid as shown in Fig. 5. Moreover, the peaks at 2917.77 cm−1 and 2850.27 cm−1 were attributed to the vibration of the eCH3 and eCH2 groups (Fig. 4(b)), respectively. Consequently, the existence of these peaks indicated that the stearic acid was successfully combined with the TiO2 film. 3.3. Growth mechanism of TiO2 nanorod film Combining microscopic morphology and chemical reactions that occurred during hydrothermal synthesis, the growth mechanism of the TiO2 nanorod film was described as follows: The formation of the TiO2 nanorod film was divided into two steps.

Fig. 5. Schematic diagram of the dehydration reaction between stearic acid and TiO2 film.

The first step was to form the titanium hydroxide Ti(OH)4 by the hydrolysis of TTIP (Eq. (1)). The second step was the formation of TiO2 through the decomposition of Ti(OH)4 (Eq. (2)). As one of the solvent, HCl played an important role in the formation of TiO2 film. The role of hydrogen ions H+ was to adjust the pH value of the reaction solution and then control the rate of hydrolysis of TTIP. If the concentration of H+ was low, the rate of hydrolysis of TTIP was too fast so that the TiO2 particles were formed which would precipitate on the container. On the other hand, if the concentration of H+ was high, the hydrolysis of TTIP was inhibited and the TiO2 couldn't grow up on the FTO substrate. Thus, the growth of TiO2 nanorod film required appropriate concentration of acidic medium. Moreover, the ability of the chloride Cl− was to retard the diameter growth rate of the TiO2 nanorods while avoiding the side surfaces from coalescing to form a continuous film. The Cl− adsorbed on (110) plane which was a positive polar face and retarded the growth rate of the TiO2 film along (110) plane, which resulted in the TiO2 crystal growth was towards (001) plane [23]. Therefore, the TiO2 crystal was more rapidly grown along the axial direction rather than the radial direction, and it leaded to the formation of rod-like structure. However, if the concentration of the TTIP was too high, the radial size was larger and the TiO2 nanorods still coalesced together, which could be observed in Fig. 1(h) and (i). Fig. 6 exhibited the schematic illustration of the formation of TiO2 nanorod film on the FTO substrate with the reaction time. Ti(OR)4 + 4H2O → Ti(OH)4 + 4ROH

(1)

Ti(OH)4 → TiO2 + 2H2O

(2)

3.4. Wettability of the specimen surface

Fig. 4. FTIR spectra of (a) stearic acid and (b) superhydrophobic TiO2 film.

3.4.1. Contact angles of the specimens under different conditions The contact angles and photographs of 3 μL water droplets of the specimens processed in various conditions were shown in Fig. 7. The contact angle of the FTO substrate increased from 101° to 110° after modification in ethanol solution of stearic acid, which indicated the surface energy was one of the influencing factors to the surface wettability. After hydrothermal synthesis with 0.3 mL TTIP for 8 h, the TiO2 film grew up on the FTO substrate as shown in Fig. 1(b). The 11511

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Fig. 6. Schematic illustration of the formation of TiO2 nanorod film with the reaction time.

unmodified TiO2 film showed hydrophilicity with the WCA about 10°, which was due to the high surface energy and the hydroxyl groups existing on the surface [27]. Moreover, after modification, the surface of the TiO2 film could be changed from a hydrophilic one to a superhydrophobic one with the WCA from 10° to 159°, which could be observed in Fig. 7(c) and (d). It was because that the surface energy was low and the water droplet tended to be spherical. Besides, after modification, the nanorod structure of the TiO2 was benefited to trap air between the surface and water droplet. Thus, microstructure and surface energy were two key factors affecting surface wettability. 3.4.2. Influence of the experiment parameters on the contact angles It could be known from Fig. 1 that different TTIP amount and reaction time had a great influence on the microstructure of TiO2 film. Moreover, the microstructure was the key factor affecting the surface wettability. Therefore, the contact angles of the TiO2 film obtained under different experimental parameters were different. As shown in Fig. 8, when the TTIP amount was constant and the reaction time increased, the contact angle decreased. Besides, when the reaction time was same and the TTIP amount was higher, the contact angle became smaller too. When the TTIP amount was 0.5 mL and the reaction time was 6 h, the TiO2 film was superhydrophobic with the WCA 154.21° and SA 4.5°. But when the reaction time was up to 8 h and 10 h, the WCA was less than 150° and had a greater adhesion to the film. It could be learned from Fig. 1(h) and (i) that most of the TiO2 nanorods coalesced together and the surface became smoother. Thus, the surface roughness was decreased, so that the WCA was lower even less than 150°. For 0.4 mL TTIP, the WCA was still more than 150° when the reaction time was 8 h and 10 h. But compared to the lower TTIP amount (0.3 mL), when the reaction time was same, the contact angles were always lower. To sum up, the greater the concentration of the TTIP, the faster

Fig. 8. Contact angles of the modified TiO2 film synthesized in different TTIP amount and reaction time.

the surface would lost superhydrophobicity with the reaction time. Furthermore, when the TTIP amount was 0.3 mL, although the WCA became smaller as the reaction time increased, the WCA was still greater than 155° and the SA was less than 4° when the reaction time was 10 h. It was closely related to the microstructure. Even when the reaction time was 10 h, the TiO2 nanorods still grew evenly on the surface and few of them coalesced together as shown in Fig. 1(c). Because of the aligned TiO2 nanorods that were almost perpendicular to the substrate and had quadrangular top facets, an improved Cassie

Fig. 7. Optical images of the water droplets (3 μL) on different specimens. (a) FTO substrate. (b) Modified FTO substrate. (c) Unmodified TiO2 film. (d) Modified TiO2 film. 11512

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Fig. 9. A three-dimensional model for the superhydrophobic TiO2 nanorod film with quadrangular prism arrays: (a) quadrangular prism surface along with the structure repeating unit; (b) vertical view of the model; (c) side view of the model.

model could be established to further explain the influence of the reaction time (or microstructure) on the WCA. In the model, as shown in Fig. 9, the side length of the square and height are defined as a and h, respectively. Let b be the spacing of the nearest parallel edges of two squares. For the Cassie model, the air is trapped in the spacing, and the water droplet only contacts the flat top of the quadrangular prism. The area fraction of the liquid-solid contact is f, which is described as:

f=

a2 (a + b)2

(3)

The Cassie model [28] is revised to:

cos θ c = f (cos θ0 + 1) − 1 =

(

⎛ =⎜ ⎜ ⎝

a2 (a + b)2

0



1

(

) (cos θ

(cos θ0 b 2⎟ ⎟ 1+ a ⎠

)

+ 1) − 1 + 1) − 1 (4)

where θ0 is the equilibrium contact angle, which is the WCA (101°) on the modified FTO substrate in this research. In order to reflect the relationship between geometrical parameters and contact angles, Eq. (4) was converted to Fig. 10. As shown in Fig. 10, the WCA was only related to b/a and increased as b/a increased. When the TTIP amount was 0.3 mL and reaction time was 6, 8 and 10 h, the value of b/a of the TiO2 film was 2.57, 2.14 and 1.75, respectively, which was calculated by the value of b and a in Table 1. It could be known that the value of the b/a decreased with the reaction time. Meanwhile the contact angles shown in Fig. 8 also decreased with the decrease of b/a, which had the similar tendencies in the Cassie model. Besides, the theoretical value of the contact angles were 161.50°, 158.95° and 155.92° based on Eq. (4). Moreover, the theoretical contact angles conformed to the experimental contact angles (160.28°, 158.97° and 156.03°). Therefore, when the TTIP amount was 0.3 mL, the wetting state accorded with the improved Cassie model. With the increase of the reaction time, the ratio of solid-liquid contact area increased so that the contact angles decreased. In addition, this improved model could be used to design superhydrophobic surface by adjusting the size of the microstructure, which has a good application prospect in future research.

Fig. 10. Contact angles in the improved Cassie model as the change of b/a.

3.5. Low adhesion and self-cleaning property of the superhydrophobic TiO2 film Due to the weak adhesion between the liquid and solid, liquid droplets can easily slide off and take away the dust on the surface. Fig. 11 showed the process of water droplet (3 μL) approaching, contacting, deforming and separating the superhydrophobic TiO2 film. It could be seen that the water droplet was easily separated from the superhydrophobic TiO2 film after the whole process. It was because that the special nanorod structure of the superhydrophobic film was beneficial to trap air between the liquid and the solid, which was hard for the water to penetrate into the pores of the structure. Even if the water droplet was deformed severely, no visible water remained on the surface after the water droplets left the superhydrophobic TiO2 film. Therefore, the superhydrophobic TiO2 film exhibited low adhesion. Surface self-cleaning is an important application for superhydrophobic surfaces. The self-cleaning test of the FTO substrate and superhydrophobic TiO2 film was shown in Fig. 12. Fig. 12(a) indicated that the water droplet stayed on the substrate surface and didn't roll away. In comparison, the water droplet dropped on the superhydrophobic TiO2 film rolled off quickly and took away the fly ash

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Fig. 11. Approach, contact, deformation, and departure processes of a water droplet suspending on a needle with respect to the superhydrophobic TiO2 film. The arrows represent the moving direction of the needle.

particles deposited on the surface, leaving a clean route on the surface. This was because that the adhesion force between the water droplet and superhydrophobic TiO2 film was smaller. Meanwhile, the water formed a spherical droplet to absorb the fly ash particles and rolled off easily. Consequently, the superhydrophobic TiO2 film with both high WCA and low adhesion displayed excellent self-cleaning ability. 3.6. Anti-icing property of the superhydrophobic TiO2 film At the beginning, the contact angles of the water droplets on the FTO substrate and the superhydrophobic TiO2 film obtained at 0.3 mL TTIP and 8 h reaction time were about 101° and 158.97°, respectively. The water droplets were dropped on the two specimens at the same time in the cryostat. After icing, the water droplets on the specimens would became opaque as shown in Fig. 13(b). Fig. 13(a) showed that the water droplet on the FTO substrate became opaque and frozen after 598 s. However, for the superhydrophobic TiO2 film, the water droplet froze to ice at 2703 s. Furthermore, the frozen water droplet on the superhydrophobic surface was easier peeled off than that on the FTO substrate. The above phenomenon indicated that the superhydrophobic TiO2 film could delay the icing time of the water droplets and had a good anti-icing property. There were two reasons for the phenomenon. Firstly, the superhydrophobic TiO2 nanorod film could trap the air between the water droplet and the solid surface. The trapped air had the heat-isolating effect so that the water droplet freezing rate was reduced. Next, the solid-liquid contact area of the superhydrophobic surface was much smaller than that of the FTO substrate surface. The smaller solid-liquid contact area was benefited to decrease the heat transfer rate and the water droplet was hard to freeze. Besides, after the anti-icing test, the superhydrophobic TiO2 film was removed from the cryostat and naturally dried. The water droplets on the TiO2 film tended to be spherical with a WCA about 158.57° as shown in Fig. 13(c), which meant the superhydrophobicity was almost unaffected.

The solid-liquid contact area was affected by the morphology of the TiO2 film, which was related to the experimental parameters (TTIP amount and reaction time). In order to investigate the relationship between the anti-icing property and the experimental parameters, the water droplets freezing time was tested on the three modified TiO2 film. The three modified TiO2 film were synthesized at 150 °C for 8 h using different TTIP amount (0.3 mL, 0.4 mL and 0.5 mL respectively). The water droplets were dropped on the three specimens at the same time in cryostat. As shown in Fig. 14(c), the water droplet on the 0.5–8 TiO2 film first froze at 1225 s. For the water droplets on the 0.4–8 and 0.3–8 TiO2 film, the freezing time were 2417 s and 2703 s, respectively. Thus, with the decreasing of the TTIP amount, the freezing time was delayed. The main reason was that different solid-liquid contact area would influence the heat transfer rate, which could be explained by the Fourier's Law [29]:

dQ dT = − λA dt dx

(5)

where dQ/dt is the heat transfer rate, λ is the thermal conductivity, and A is the heat transfer area which is the actual water-solid area. dT / dx is the same for all specimens because the environmental conditions in the anti-icing test are the same. Thus, the heat transfer rate dQ/dt is affected by the actual water-solid area A. In order to calculate the value of A, the water droplet is assumed as a segment as shown in Fig. 15. The volume V of the segment is expressed as follows:

V= πH 2 (R − H /3)

(6)

where, R is the radius of the sphere, H is the height of the segment. θ is the contact angle of the water droplet, and r is the radius of the contact area ( A' ). R and H can be represented by r as follows:

R= r /sin θ

(7)

H= R(1 − cos θ) = r /sin θ (1 − cos θ)

(8)

Thus, r can be expressed as follows:

Fig. 12. Dirt remove test on (a) FTO substrate and (b–d) superhydrophobic TiO2 film. 11514

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Fig. 13. Delay times (DT) evaluation of anti-icing property of FTO substrate (left) and superhydrophobic TiO2 film (right) (a); side photograph of frozen water droplets on FTO substrate and superhydrophobic TiO2 film (b); photograph of the water droplets on the superhydrophobic TiO2 film in room temperature after anti-icing test (c).

solid and liquid f in Eq. (11). The actual water-solid area can be calculated in the next equation:

A= fA'

(12)

The contact angles of the three modified TiO2 film (0.3–8, 0.4–8 and 0.5–8) were 158.97°, 155.13° and 148.95°, respectively. With the decrease of the contact angles (90° < θ < 180°), the value of r increased, and then the contact area A' was enlarged (Eq. (9) and Eq. (10)). Meanwhile, it could be learnt from Eq. (11) that the smaller contact angles was, the larger the value of f was. Therefore, as the contact angles decreased, the values of f and A' were both increased so that the value of the actual water-solid area A increased (Eq. (12)). For the 0.3–8 TiO2 film with the largest contact angle, the actual water-solid area A was the smallest, the heat transfer rate dQ/dt was the slowest so that the freezing time was the longest. Hence, the anti-icing property reduced with increasing TTIP amount. Besides, when the TTIP amount was constant, the contact angles decreased with the reaction time so that the anti-icing property also decreased. Fig. 14. Delay times (DT) evaluation of anti-icing property on three modified TiO2 film obtained under different TTIP amount (from left to right): (a) 0.3 mL, (b) 0.4 mL and (c) 0.5 mL.

Fig. 15. The contact model of a water droplet on a superhydrophobic surface. 1

3V ⎞ r= sin θ⎛ 3 ⎝ π ( cos θ − 3 cos θ + 2) ⎠ ⎜

3



(9)

'

4. Conclusions In this work, superhydrophobic TiO2 nanorod film was prepared via a simple method involving hydrothermal reaction and stearic acid modification. Based on the characterization and properties testing results, the following conclusions were drawn: (1) The TiO2 film is composed of nanorods in the shape of the pillar with quadrangular top facets. The average diameter of the TiO2 nanorods increases and even coalesces together with the increase of TTIP amount and reaction time. (2) The contact angle decreases with the increase of TTIP amount and reaction time. When the TTIP amount is 0.5 mL and reaction time is more than 8 h, the modified TiO2 film loses superhydrophobicity. When the TTIP amount is 0.3 mL, the TiO2 nanorods can be assumed as quadrangular prisms and the wetted state of the film surface conforms to the improved Cassie model. (3) The superhydrophobic TiO2 film shows good low adhesion and selfcleaning property. At −20 °C, the icing time of water droplet on the superhydrophobic TiO2 film is delayed, indicating the superhydrophobic TiO2 film has good anti-icing property. Besides, the anti-icing property of the modified TiO2 film reduces with the increase of TTIP amount and reaction time.

And the contact area A is described as: '

A = πr 2

(10)

However, there is trapped air between the water and solid as shown in Fig. 15. According to the Cassie equation [28],

cos θ = f (cos θ0 + 1) − 1

(11)

the actual water-solid area A is affected by the contacting area ratio of

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

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X. Zhou, et al.

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