Preparation of Superhydrophobic ZnO Films on Zinc Substrate by Chemical Solution Method

Transcription

1 CHEM. RES. CHINESE UNIVERSITIES 2012, 28(3), Preparation of Superhydrophobic ZnO Films on Zinc Substrate by Chemical Solution Method XU Wen-guo, LI Ji-hong, LU Shi-xiang *, DUAN Ya-qiong, MA Cheng-xiang, SHI Xiao-feng, CHEN Yi-ling and YANG Yan-bo Institute for Chemical Physics, School of Science, Beijing Institute of Technology, Beijing , P. R. China Abstract Superhydrophobic surface was prepared on the zinc substrate by chemical solution method via immersing clean pure zinc substrate into a water solution of zinc nitrate hexahydrate[zn(no 3 ) 2 6H 2 O] and hexamethylenetetraamine(c 6 H 12 N 4 ) at 95 C in water bath for 1.5 h, then modified with 18 alkanethiol. The best resulting surface shows superhydrophobic properties with a water contact angle of about 158 and a low water roll-off angle of around 3. The prepared samples were characterized by powder X-ray diffraction(xrd), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy(edx), transmission electron microscopy(tem), and scanning electron microscopy(sem). SEM images of the films show that the resulting surface exhibits flower-shaped micro- and nano-structure. The surfaces of the prepared films were composed of ZnO nanorods which were wurtzite structure. The special flower-like micro- and nano-structure along with the low surface energy leads to the surface superhydrophobicity. Keywords Superhydrophobic; ZnO; Water contact angle; Chemical solution method Article ID (2012) Introduction Wettability is an important property governed by both chemical compositions and geometrical structure of solid surface. A surface having a water contact angle(ca) greater than 150 is commonly considered to be superhydrophobic [1]. Superhydrophobic surface has important application in the national defense, industrial and agricultural production and people daily life, such as antenna, anti-snow doors and windows, reduce of the resistance shell of ships, submarines, prevention of the obstruction of the oil pipes, microinjection needle and so on [2]. In general, the superhydrophobic surfaces are prepared in two ways: one is constructing the rough structure on the hydrophobic material surface; and the other is modificating the low surface energy of the material on the rough surface. Today, many methods have been developed, such as electrospinning [3,4], sol-gel processing [5,6], electrochemical reaction and deposition [7], anodic oxidation [8], physical and chemical vapor deposition [9], plasma etching [10,11], laser etching [12], nano-silica method [13] and so on, to fabricate superhydrophobic surfaces. In the past few years, the superhydrophobic films fabricated on a variety of metallic substrates, such as aluminum alloy [14], copper [15 17], aluminum [18], stainless steel [19], and silver [20] by different methods, have been successfully developed. Zhang et al. [21] also fabricated superhydrophobic coatings on gold substrates by combining assembly technique with electrochemical deposition of dendritic gold aggregates. *Corresponding author. shixianglu@bit.edu.cn Received June 22, 2011; accepted August 30, Supported by the National Natural Science Foundation of China(Nos , ). Recently, the microtextured ZnO surfaces have been introduced to create superhydrophobic surfaces via sol-gel method, chemo- or electro-deposition [22 25]. Ding et al. [26] described a method to prepare superhydrophobic ZnO surfaces by the combination of electrospinning technique with a wet chemical route. However, These ZnO surfaces were not grown on zinc substrate. Hou et al. [27] reported that superhydrophobic ZnO nanorod films on zinc substrate were fabricated by the natural oxidation of zinc metal and subsequent modification with a monolayer of n-octadecyl thiol. Fabricating superhydrophobic surface on metal substrate is of great significance because metal substrates have an important status in the application in modern industry. Considering the wide application of zinc in modern industry and tunable wetting properties to provide zinc substrate with extended functions, we here presented a very simple, inexpensive and time-saving method for fabricating a stable interesting superhydrophobic surface on zinc substrate by chemical solution method. 2 Experimental The sessile drop method was used for CA measurement on a Dataphysics OCA20 CA system at ambient temperature. The water droplet size used for the measurement was 3.0 μl. The average CA value was obtained by measuring more than five different positions for the same sample. The surface morphology was observed under a scanning electron microscope(sem, X650, Hitachi, Japan). The SEM operating voltage was 1.0 kv.

2 530 CHEM. RES. CHINESE UNIVERSITIES Vol.28 The corresponding element distributions on the surface were determined on an energy dispersive X-ray spectrometer(edx) and an X-ray photoelectron spectrometer(xps), respectively. The EDX was carried out on a Hitachi S-4800 at a accelerate voltage of 20 kv and a working distance of 15 mm. The XPS was carried out on a PHI 5300 X-ray photoelectron spectrometer(physical Electronics, USA) with 250 W Mg Kα(hν= ev) X-ray as the excitation source. The XPS spectra were collected in a constant analyzer energy mode at a chamber pressure of 10 7 Pa and a pass energy of at 0.1 ev/step. The texture and morphology of samples were studied via a JEM-2010 Transmission electron microscope(tem). The crystalline structure was investigated by X-ray diffraction(xrd) on an x Pert. Pro. MPD X-ray diffractometer with Ni filtered Cu Kα radiation of nm(40 kv, 40 ma) at a scan rate of 0.05 /s. All the chemicals were analytic grade reagents without further purification that were purchased from Beijing Chemical Company(China). Zinc flakes were obtained from Beijing Nonferrous Metal Research Institute(China). Zinc flakes with a size of 1.0 cm 1.0 cm 0.01 cm were rinsed ultrasonically in distilled water, ethanol, distilled water respectively. The solution of zinc nitrate hexahydrate [Zn(NO 3 ) 2 6H 2 O] and hexamethylenetetraamine(c 6 H 12 N 4 ) was prepared at different concentrations. The clean zinc flakes were immersed into the solution at 95 C in a water bath for 1.5 h, and then the flakes were fetched out from the solution. When the flakes Sample dried, immersing them into 2.0 mol/l 18 alkanethiol anhydrous alcohol solution for 24 h at room temperature. Subsequently, the flakes were rinsed with anhydrous alcohol to remove any dissociation of 18 alkanethiol and dried in air at room temperature. 3 Results and Discussion Table 1 shows different contact angles under different equimolar concentrations and different molar ratios of Zn(NO 3 ) 2 6H 2 O to C 6 H 12 N 4 at a total concentration of 0.04 mol/l. Seen from left column of Table 1, the contact angle was the maximum when the equimolar of Zn(NO 3 ) 2 6H 2 O to C 6 H 12 N 4 reacted to 0.02 mol/l each. The concentration affected the superhydrophobicity of the surface on zinc substrate. With the increase of concentration, the contact angle increased until the concentrations of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 were 0.02 mol/l each. The CA reduced with the increase of the concentrations of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4. Seen from the left column of Table 1, keeping the total concentration of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 at 0.04 mol/l, the molar ratio of Zn(NO 3 ) 2 6H 2 O to C 6 H 12 N 4 had little effect on the superhydrophobicity of the surface on zinc substrate. The contact angles of five different points on each sample were investigated, and the average value was determined; all the values for each sample are in a error range of ±2. Samples 1 5 are prepared using 0.010, 0.015, 0.020, and mol/l Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4, respectively. Table 1 Different contact angles at different equimolar concentrations and different molar ratios of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 Equimolar concentration of Zn(NO 3 ) 2 6H 2 O: Zn(NO 3 ) 2 6H 2 O and Contact angle/( ) Sample C 6 H 12 N 4 (molar ratio) of 0.04 Contact angle/( ) C 6 H 12 N 4 /(mol L 1 ) mol/l total concentration ±2 6 3:1 156± ±2 7 2:1 156± ±2 8 1:1 158± ±2 9 1:2 156± ±2 10 1:3 153±2 Typical XRD patterns of ZnO films grown on zinc substrate prepared via equimolar concentrations of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 are shown in Fig.1. Fig.1 patterns a e are the patterns of samples 1 5, respectively. The peaks attributed to ZnO are at 2θ = (100), (002), (102), (110), (103), (112), (201) and (004). The prepared ZnO shows wurtzite structure according to JCPDS No The growth orientation [002] near is dominant [28]. Fig.2 shows SEM images of the films prepared from equimolar Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4. Fig.2(A) (E) are the surface images of samples 1 5, respectively. It is clearly shown that the resulting surface exhibits flower-like architectures in Fig.2(A 1 ) (E 1 ). The shapes of petalage are different with the change of the concentration of reaction solution. Fig.2(A 2 ) (E 2 ) show the magnified images of samples 1 5. They are rhomb in Fig.2(A 2 ), sheet with hole in Fig.2(B 2 ), nanorod in Fig.2(C), nanorod and flake in Fig.2(D), nanorod with little cuboid tuber. The flowerlike microscopic structures Fig.1 XRD patterns of ZnO films grown on zinc substrate prepared from a equimolar concentration of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 c[zn(no 3 ) 2 6H 2 O]/(mol L 1 ): a ; b ; c ; d ; e over the whole substrate can enhance the surface roughness. Seen from the magnified SEM images of samples 3 5, regular hexagonal structure is presented to our sight. The diameter of

3 No.3 XU Wen-guo et al. 531 ZnO nanorods is about 400 nm and the length of ZnO nanorods is about 4 μm for sample 3. There are holes in the flower-like architectures, and the diameter of the hole is about 7 mm for sample 3. Fig.2(C) also shows the growth of the ZnO in highly oriented alignment of the nanorods over a large substrate area. The lengths of the nanorods of samples 4 and 5 are bigger than that of sample 3. The ZnO film thickness is between 50 and 100 μm measured by means of a vernier caliper. Fig.2 SEM images of the surface of films prepared from samples 1 5(A E), respectively (A 2 E 2 ) are magnified images of (A 1 E 1 ) The formation mechanism of this special zinc oxide microstructure would most likely be explained by the following two steps: (1) The formation of nucleus stage: NH 3 was produced by the decomposition of C 6 H 12 N 4, then NH 3 reacted with H 2 O to produce OH ions, then, ZnO nuclei were produced by the reaction of Zn 2+ ions and OH ions. (2) The crystal growth stage: ZnO nuclei grew along with the orientation [002], then ZnO nanorods formed; while ZnO nanorods arranged on the nuclei in a certain orientation, the flowerlike microstructure formed. The formation of zinc oxide nanorods was expressed by the following chemical reactions [29] : (CH 2 ) 6 N 4 + 6H 2 O 6HCHO + 4NH 3 (1) NH 3 + H 2 O NH OH (2) 2OH + Zn 2+ ZnO(s) + H 2 O (3) Fig.3 shows TEM images of the films prepared from equimolar Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4. Fig.3(A) (F) are the surface images of samples 1, 2, 3, 4(flakes), 4(nanorod) and 5, respectively. The rhombs of sample 1 as well as the nanorods of samples 3 and 4 are single crystals, respectively, while the others are not single crystals from the high-resolution transmission electron microscope(hrtem) and selected area electron diffraction(saed) images. The crystallographic spacing was calculated from SAED patterns of the samples. The distinct lattice fringes of the interplanar spacing d=0.259 nm corresponded to the (002) planes, again conforming the growth orientation of samples 1 and 3, while those of d=0.147 nm corresponded to the (103) planes for sample 4. Fig.3 TEM images of the surface of a film prepared from samples 1(A), 2(B), 3(C), 4[flakes(D), nanorod(e)] and 5(F) Insets are SAED images from samples 1(A), 2(B), 3(C), 4[flakes(D), nanorod(e)] and 5(F), respectively. EDX and XPS were employed to analyze the chemical composition of the surface of the film prepared from equimolar Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 (0.02 mol/l each) and modified with 18 alkanethiol(figs.4 and 5). It is revealed that the surface mainly consists of C, S, O, and Zn elements. Fig.4 shows the EDX spectrum of distributed elements on the zinc substrate.

4 532 CHEM. RES. CHINESE UNIVERSITIES Vol.28 Seen from the EDX spectrum, the surface mainly consists of O and Zn elements at a ratio of close to 1.1:1. It shows the presence of carbon element. The sulfur element was not determined by EDX, so we could not get information of sulfur element from Fig.4. The XPS survey spectrum of the superhydrophobic film shows the presence of sulfur element from Fig.5. The peak located at ev is attributed to S 2p [30], giving the strong evidence of the chemical absorption of 18 alkanethiol. From the above analysis and theoretical calculations, it is clear that 18 alkanethiol had modified the zinc oxide surface, which reduced the free energy of the surface, thus enhancing hydrophobicity [31]. Fig.4 Typical EDX spectrum of a film prepared using a equimolar Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 (0.02 mol/l each) and modified with 18 alkanethiol Fig.5 XPS spectrum of the sulfur element of a film prepared from a equimolar Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 (0.02 mol/l each) before(a) and after(b) modifying with 18 alkanethiol The surface wettability of the as-prepared films was studied by contact angle(ca) measurement. Fig.6(A) is the shape of a water droplet on the surface that was not modified with 18 alkanethiol with a water CA of about 105. Fig.6(B) exhibits the superhydrophobicity with a water CA of about 158. Fig.6(C) is the shape of a sliding water droplet on a surface tilted at 3. Advancing(θ A ) and receding(θ R ) contact angles of the water droplet were 162 and 159, respectively. The difference between θ A and θ R constitutes the contact angle hysteresis. Surfaces with low contact angle hysteresis have a very low water roll-off angle that denotes the angle to which a surface must be tilted for the roll off of water drops (i.e., very low water contact angle hysteresis) [32]. A low roll-off angle, which reflects the difference between θ A and θ R, provides further evidence for the superhydrophobicity of this surface. The contact angle hysteresis is dependent upon micro- and nanoscale effects that control energy dissipation due to the adhesion, kinetic effects and the fine structure of the triple line. Cassie and Baxter proposed an equation to describe the relationship between the water CA on a flat surface(θ) and a rough surface(θ ) composed of a solid and air [Eq.(4)] [33,34]. cosθ =f 1 cosθ f 2 (4) In this equation, f 1 and f 2 are ratios of solid surface and air in contact with liquid, respectively. In other words, f 1 +f 2 =1. It indicates that when a rough surface comes into contact with water, air trapping in the rough area may occur, which would contribute greatly to the increase of hydrophobicity. From Fig.6(A), θ =105 and θ=0, f 1 and f 2 are about and 0.630, respectively. The different contact angle at different concentrations of the equimolar ratios and different molar ratios between Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 has caused the different roughness of ZnO surface. Flower like microstructures formed in the water solution of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 and the exposed hydrophobic crystal planes resulted in the superhydrophobic properties. The flower-like microscopic structures have also been reported not only in zinc oxide but also in other materials [35 38]. The result further confirms that a superhydrophobic surface could be prepared by the combination of reducing the surface energy and enhancing the surface roughness of a solid. The reason why the surface energy reduced via modification with 18 alkanethiols is that self-assembled monolayers were formed spontaneously [39] through chemical bonds on the ZnO surface. The main reaction in the system is shown in Eq.(5): ZnO + 2RSH Zn(SR) 2 + H 2 O (5) 4 Conclusions Fig.6 Shape of a water droplet on the resulting surface (A) Not modified with 18 alkanethiol; (B) modified with18 alkanethiols; (C) a sliding water droplet on a surface tilted at 3. Stable superhydrophobic surface on zinc substrate was obtained by chemical solution method and effective modification. The method is simple, controllable and extendible via controlling the concentration and molar ratio of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4. The concentration affects the superhydrophobicity of the surface on zinc substrate. The contact angle measured was the maximum when Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 reacted at equimolar condition(0.02 mol/l each). The molar ratio of Zn(NO 3 ) 2 6H 2 O to C 6 H 12 N 4 has little effect on the superhydrophobicity of the surface on zinc substrate in some extent. Keeping the total concentration of Zn(NO 3 ) 2 6H 2 O and C 6 H 12 N 4 unchanged at 0.04 mol/l, when

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