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Ultrahydrophobicity of ZnO modified CVD diamond films
Applied Surface Science 270 (2013) 260–266
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Ultrahydrophobicity of ZnO modified CVD diamond films YiZhou Yang a , ChuanXi Wang b , HongDong Li a,∗ , Quan Lin b,∗∗ a b
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China State Key Laboratory of Supermolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 30 August 2012 Received in revised form 25 December 2012 Accepted 26 December 2012 Available online 16 January 2013 Keywords: Ultrahydrophobicity Freestanding polycrystalline CVD diamond ZnO thin films
a b s t r a c t Chemical vapor deposited (CVD) polycrystalline diamond films with an ultrahydrophobic surface were fabricated by constructing a hierarchical structure through sputtering a ZnO layer on diamond grains, with a growth step feature. Under optimized conditions, the combined original diamond with a step structure of the ZnO can achieve a water contact angle (WCA) of as high as 141◦ ± 1◦ . It is proved that WCA decreases as the roughness of ZnO/PDF reduced. It can be concluded that the step structure of diamond grains and ZnO nuclei size have a great influence on the variation of WCA. © 2013 Elsevier B.V. All rights reserved.
1. Introduction A surface with water contact angle (WCA) greater than 90◦ is usually referred to as hydrophobic, and one with WCA higher than 140◦ is qualified as ultrahydrophobic [1,2]. Creating a rough surface that repels water would have potential applications in the areas of self-cleaning coating [3], biocompatible materials [4], and optics [5]. Many researchers have made contributions toward enhancing hydrophobicity [6–12]. Earlier research generally used silicon, glass slide, or quartz as a substrate, which restricts their practical applications due to the substrate’s limited oleophobicity with high contact angle hysteresis, failure upon physical damage, and/or other chemical properties. To address these challenges, we propose diamond as a substrate, given its many unique properties, as a candidate to improve the hydrophobicity performance in extreme environments. However, diamond generally shows a moderate hydrophobicity owing to its high surface tension [13]. In general, surface treatments, such as ion and electron bombardment [14], plasma and glow discharge treatment [15], oxygen and ozone annealing [13], acid and basic attacks [16], and chemical modifications [17], have been used to affect the hydrophobicity. Zinc oxide (ZnO) has been an important semiconductor material due to its wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV. In addition, it is easy to realize ZnO films and nanostructures. ZnO deposition on diamond has been widely investigated to fabricate n-ZnO/p-diamond heterojunction [18,19],
∗ Corresponding author. Tel.: +86 431 85168095; fax: +86 431 85168095. ∗∗ Corresponding author. Tel.: +86 431 85193423; fax: +86 431 85193423. E-mail addresses: [email protected] (H. Li), [email protected] (Q. Lin). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.172
surface acoustic wave (SAW) devices [20], electon field emitter [21], and biosensors [22]. ZnO material has hydrophobicity, but there are few reports on the study of wettability of ZnO/diamond composite structure. In this work, thin layers of ZnO of various grain sizes were sputtered on chemical vapor deposited (CVD) polycrystalline diamond films (PDFs). It is demonstrated that the rough morphology of the PDF and growth steps on diamond grains, as well as the deposited ZnO nanostructure, play critical roles in significantly improving the hydrophobicity of a ZnO/diamond system. This mechanism has been investigated.
2. Experiments 2.1. Preparation of the polycrystalline diamond films (PDFs) The PDFs were synthesized by hot filament chemical vapor deposition (HF-CVD) [23]. The molybdenum (Mo) substrates were abraded by diamond paste for nucleation enhancement and then ultrasonically cleaned in an ethanol solution. A mixed gas consisting of 7 standard cubie centimeter per minute (sccm) methane and 400 sccm hydrogen was used at a constant pressure of 40 Torr. The chemical reaction was activated by a tantalum filament positioned 2 cm above the substrate. Negative bias enhance the nucleation (bias current was 18.5 A) for 15 min with CH4 (4 sccm) and H2 (200 sccm). In our experiments, the temperature of the filament and substrate were kept at ∼2200 and ∼800 ◦ C, respectively, as monitored by an optical pyrometer. The substrate was heated by the hot filament as the plasma was applied. After deposition, the samples were cooled down to room temperature, and the films were
Y. Yang et al. / Applied Surface Science 270 (2013) 260–266
peeled from the substrates utilizing the large difference in thermal expansion coefficient between diamond and Mo. The products were heated in boiling sulfuric acid and nitric acid to remove the non-diamond phases. The thicknesses of the PDFs were ∼380 m. For comparison, diamond films were mechanically polished from the growth side. 2.2. Preparation of PDF/ZnO micro-nanostructure ZnO films were deposited on PDFs by RF magnetron sputtering with a ZnO target (purity: 99.99%). Before deposition, the ZnO target was pre-sputtered in argon atmosphere for about 10 min to remove contaminants from its surface, and then the ZnO seed layer was deposited in argon ambient with an argon flow rate of 60 sccm without heating the substrate. Argon (Ar, 99.999%) and oxygen (O2 , 99.999%) were introduced with flow rates of 54 and 6 sccm, respectively. During the growth processes, the sputtering pressure and power were kept at 1.0 Pa and 100 W, respectively. For comparison, some ZnO-modified samples were annealed at 673 K in air in a horizontal tube furnace under a constant pressure of 6 × 104 Pa. The samples were then cooled to room temperature in the same gas and pressure environment. 2.3. Characterization Scanning electron microscope (SEM) images and energy dispersive spectrometry (EDS) used to investigate the morphology of the PDF were collected on a JEOL JXA-8200 electron probe microanalyzer operated at 15 keV. The crystal structure and properties were examined by means of atomic force microscopy (AFM) and X-ray diffraction (XRD, Rigaku D/max-RA with Cu K␣ radiation). The chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS). The WCA were measured by drop shape analysis (DSA 10MK2, KRUSS) at ambient temperature. Three microliters of deionized water were dropped onto the samples and the static contact angle was determined by averaging at least five measurements taken at the different positions on each sample. 3. Results and discussion Fig. 1 shows the surface images and wettability of the original micro-hill-like PDF and polished mirror-like PDF structures. The polished surface showed a WCA less than 90◦ (Fig. 1(c)), indicating hydrophilia. However, the WCA of the original diamond film increased to up to 101◦ (Fig. 1(a)) indicating hydrophobicity. The change of wetting property from hydrophilic to hydrophobic can be attributed to the difference in roughness of the surfaces. In addition, the smooth ZnO film deposition on a flat diamond surface (Fig. 1(d)) was hydrophobic and had a WCA of 98◦ . However, with the ZnO film covering the original diamond surface (Fig. 1(b)), the WCA reached 141◦ . The results show that all of the PDF/ZnO films exhibit hydrophobic properties, and obviously increased WCAs. The change in WCA between Fig. 1(a) and (b), can be attributed to the ZnO thin film structure. Note that the sample (Fig. 1(b)) maintains ultrahydrophobicity for 40 min before reaching a WCA of 34◦ after 24 h. However, the changes in wettability of the thin films are reversible, after the films were placed in an electric oven at 95 ◦ C for 25 min. A new water droplet was used to measure the resulting surface wettability, and the initial wetting state was obtained again, as shown in the inset in Fig. 1(b). It shows that after water infiltration the sample structure of surface did not change. Additional experiments were performed by varying the sputtering time resulting in a gradual transition in the crystallite dimensions of the ZnO-based nanostructures, which allowed us to investigate the effects of surface geometric structure on the wetting
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behavior. Fig. 2 shows the resulting PDFs after ZnO sputtering for 5–30 min on unpolished diamond. According to the AFM data, the surface of the deposited ZnO after 5, 10, 20 and 30 min exhibited roughness values of 8.6, 16.5, 9.7 and 3.5 nm, respectively. For the 5 min growth, semi-spherical ZnO nuclei with diameters of 3–4 nm were preferentially deposited at the steps, distributed across the terraces and along the boundaries of the diamond grains (Fig. 2(a)) where high density of dangling bonds and low surface energy are present [21,24]. The water droplet experiment results showed that the WCAs were 93◦ ± 1◦ (Fig. 2(e)). After 10 min deposition, a large amount of ZnO nuclei appeared along the typical growth steps of the diamond grains, as shown in Fig. 2(b). At this point, the nuclei still kept a sphere-like morphology, and as a result, the WCA significantly increased to 141◦ ± 1◦ (Fig. 2(f)). This result clearly indicated that ZnO seed layers successfully created a ultrahydrophobic surface on the PDF. As the time increased to 20 min (Fig. 2(c)), ZnO thin films have completely covered diamond steps structure and the WCA correspondingly decreased to 121◦ ± 1◦ (Fig. 2(g)). Finally, after 30 min of sputtering (Fig. 2(d)), the WCA further decreased to 101◦ ± 1◦ (Fig. 2(h)), indicating that the WCA firstly increases before decreasing as with the increase in sputtering time. The XRD spectra of the ZnO modified PDFs are shown in Fig. 3. XRD pattern for the ZnO seed layers film shows a diffraction peak at 34.21◦ corresponding to the (0 0 2) orientation of the wurtzite hexagonal structure of ZnO. In addition to the ZnO peak, the peaks corresponding to diamond were also detected, as labeled in the Fig. 3. The crystallite sizes of the ZnO seed layers films were calculated using the Debye–Scherer formula as shown below in Eq. (1). D=
0.94 ˇ cos
(1)
where D is the crystallite size, is the wavelength (1.546 A˚ for Cu K␣), ˇ is the full-width at half-maximum (FWHM) of the main intensity peak after subtraction of the equipment broadening and is the diffraction angle. The estimated average crystallite size for the ZnO seed layer films deposited for 20 min and 30 min were around 5.4 and 7.2 nm, respectively. XRD patterns show that the crystallinity does not change appreciably among the thin films with different sputter times. Therefore, variations in the initial WCA of the thin films should be primarily attributed to the difference in their surface roughness. The enhancement of hydrophobicity can be ascribed to the increase in the proportion of air/water interface in the rough, solid/air composite surface structure [25], which is positively correlated with the surface roughness of the thin films. Energy dispersive spectrometry (EDS) measurements were performed on the original PDF with the 10 min ZnO sputtering, as shown in Fig. 4. The EDS test showed there are no impurities, only C, Zn and O elements appeared in the sample. The Zn and O signal is clearly observed in the EDS spectrum, although there is no obvious ZnO diffraction peak in the XRD pattern (Fig. 3), which can be attributed to the thinness of the ZnO film sputtered for a short time. The chemical environment of the ZnO-modified surface was examined with XPS. The C 1s, O 1s, Zn 2p3/2 and Zn 2p1/2 can be observed easily and appear at 285.8, 531.1, 1022.23, and 1045.73 eV, respectively. To further study the bonding state of O, Zn and C in the ZnO/PDF film, a high resolution scan of O 1s, Zn 2p3/2 and C 1s peaks is shown in Fig. 5(b), (c) and (d). Three oxidation states for oxygen and two oxidation states for zinc were assigned after peak deconvolution. The assigned binding energies of O 1s are Zn O (530.4 ± 0.1 eV), Zn (OH) (531.7 ± 0.1 eV), and SurfO/CO (533.1 ± 0.1 eV). The assigned binding energies of Zn 2p3/2 are centered at 1021.4 ± 0.1 and 1022.7 ± 0.1 eV for Zn0 and Zn2+ , respectively. The assigned binding energies of O 1s and Zn 2p3/2 are in agreement with previously reported values [26]. The final atomic composition of Zn and O in the PDF film were 29.81% and 37.01%.
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Fig. 1. Typical SEM surface images (left) and their corresponding water droplet images with different WCA (right): (a) original unmodified diamond (WCA 101◦ ), (b) ZnOmodified (10 min ZnO sputter) (WCA 141◦ ), (c) polished diamond (WCA 87◦ ), (d) ZnO-modified polishing diamond (10 min ZnO sputter) (WCA 98◦ ). Diamond step images are the local enlarged view of (a) and (b). Inset in (b): WCA image after being shelved for 24 h (top), and heating treatment after 25 min in an electric oven at 95 ◦ C for 25 min (bottom).
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Fig. 2. 2D (a–d) and 3D (e–h) AFM micrographs of ZnO seed layers grown on unpolished PDF after different growth times show micro-hill-like growth structures. Inset are photos of 3 L water droplets deposited on the surfaces of the corresponding PDFs, showing the different contact angles.
Surface oxygen adsorption may be responsible for the Zn and O ratio not being 1:1. In Fig. 4(d), the four deconvoluted C 1s peaks were assigned as C C (284.6 ± 0.1 eV), C C O (285.6 ± 0.1 eV), C OH (286.4 ± 0.1 eV), and C O (288.4 ± 0.1 eV).
Fig. 6 shows the differences in WCA with the increasing deposition time (i. e. increasing the thickness) for ZnO thin films deposited on the polished and unpolished PDFs. It can be seen that the WCA of polished thin films increase from 97◦ ± 1◦ to 120◦ ± 1◦ with
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Fig. 3. X-ray diffraction patterns of the ZnO films deposited on the PDF substrates at different growth times of 0, 5, 10, 20 and 30 min. Fig. 4. Typical EDS spectrum and the corresponding PDF sputtered by a ZnO thin film.
increasing ZnO layer thickness (with longer sputtering time of 30 min). The original PDF sample displays a strong response, reaching a WCA of 141◦ ± 1◦ , nowever after annealing at 673 K, the WCA decreased to 89◦ ± 1◦ . The weakening hydrophobicity may be ascribed to the decreasing proportion of air/water interface in the rough, solid/air composite surface structure, which is positively correlated with surface roughness of the thin films. At 40 min, the WCA reached a stable point at which further deposition of ZnO no longer affects the WCA.
Wettability is a very important property which can be effected by a combination of surface roughing and lowering surface energy [27,28]. Also, the texture of a surface plays a vital role in the creation of a hydrophobic surface with high WCA [29–31]. Therefore, the structural properties of a material need to be properly tuned in order to synthesize hydrophobic surfaces. To fully utilize the hydrophobic properties of nanostructured surfaces, it is necessary
Fig. 5. (a) Wide range XPS spectrum of original ZnO/PDF thin film and detailed scan of the (b) O 1s, (c) Zn 2p3/2 and (d) C 1s binding energy areas.
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Fig. 7. Schematic illustration of the air effect on ZnO/PDF films.
Fig. 6. Resultant WCA for different PDF treatment processes (polished, unpolished, and annealed) and sputter times.
to study the fundamental relationship between different nanostructures and hydrophobic behavior. Two theoretical models [30,32] of Wenzel model and Cassie–Baxter model form the basic guidelines for the study of hydrophobic surfaces. The basic assumption in Wenzel’s theory is that a water droplet penetrates the asperities while the Cassie–Baxter model predicts the suspension of a water droplet on the top of the asperities. The wetting behavior is described by the Cassie–Baxter and Wenzel models as cos c = fs cos s − fv
(2)
cos c = r cos s
(3)
could be applied in explaining the relationship between surface structure and surface WCA, where c and s represent the WCAs of a rough surface and a native flat surface, while the fs and fv are the fraction of the surface in contact with the liquid droplet and air (fv is between 0 and 1, while fs is more than 0 (i.e., fs + fv = 1), and r is the roughness factor. The cosine results in a negative number, implying that with a smaller fs and larger fv and s , it is possible to create surfaces with a very large WCA. It had been demonstrated in literature that hierarchical structures can effectively increase the roughness of the surface and hence increase the WCA [33]. Thus, according to the Eq. (2), the fv value of the unannealed surface rapidly increased to 0.79, explaining the observed ultrahydrophobicity of the PDF as shown in Fig. 2(b). This rougher surface was favorable for trapping a large amount of air in the valleys on the surface, and thus forming a hydrophobic surface. The WCA is an important parameter in surface science and its measurement provides a simple and reliable technique for the interpretation of surface energies. Generally, the apparent contact angle of a hydrophobic surface increases with its roughness, which is usually defined as the real surface area to the apparent surface area. However, we can see from Fig. 2, the WCA decreases with increasing the sputtering time of ZnO layer, possibly the ZnO seed layer gradually formed flat ZnO films with the for longer sputtering, resulting in a smoother surface. It was reported that if the smooth surface WCA is >90◦ , then the angle on a rough surface will be larger [34]. In Fig. 5, after annealing, ZnO-modified surface changes from a Cassie to Wenzel state because the liquid droplets fill the grooves of the rough substrate, decreasing the apparent WCA. As mentioned in Ref. [25], it is suggested that the amount of trapped air within the microstructures decreases owing to the filling of the valleys with water, and the WCA result changes from hydrophobic to hydrophilic. The
schematic diagram illustrating this mechanism is shown in Fig. 7 for our ZnO/PDF structures. Surface microstructures of solids play a significant role in producing hydrophobic surfaces, and the geometric parameters of the microstructure have a great influence on the wetting behavior. A possible mechanism to achieve the hydrophobic property on the hydrophilic/hydrophobic surfaces is attributed to trapped air within the microstructure, which is isolated from the atmosphere by the liquid. In other words, an increase in surface roughness results in a reduced contact area between solid and water due to an increase in air bubbles trapped at the interface; the more air trapped between the rough surface and the water, the larger the WCA will be. Therefore, for the case of a pitch size of 50 m with sawtooth patterns, the preparation of nanostructures could further decrease the contact fraction of the solid. 4. Conclusions Microtextured diamond surfaces modified by ZnO were fabricated to reach the ultrahydrophobic state. The diamond surfaces were sputtered with ZnO to enhance the wetting properties. When sputtering ZnO for layer with a proper thickness on unpolished diamond surface, the WCA increased to 141◦ ± 1◦ . To produce an ultrahydrophobic surface using a combination of diamond and ZnO seed layers structures, an air trapping suface design is needed. It can be concluded that diamond step structure and ZnO nuclei size have a great influence on the WCA. Future studies will include separation of oil/water solutions. Diamond thin films can be deposited on various substrates. This will allow for hydrophobicity on a wide variety of substrates for self-cleaning surfaces. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) with No. 51072066 and 50772041, the Ph.D. Programs Foundation of Ministry of Education of China with No. 20100061110083. References [1] D. Öner, T.J. McCarthy, Ultrahydrophobic surfaces effects of topography length scales on wettability, Langmuir 16 (2000) 7777. [2] J. Bico, C. Marzolin, D. Quere, Pearl drops, Europhysics Letters 47 (1999) 220. [3] V. Kekkonen, A. Hakola, T. Kajava, E. Sahramo, J. Malm, M. Karppinen, R.H.A. Ras, Self-erasing and rewritable wettability patterns on ZnO thin films, Applied Physics Letters 97 (2010) 044102. [4] B.D. Ratner, A.S. Hoffman, S.R. Hanson, L.A. Harker, J.D. Whiffen, Bloodcompatibility-water-content relationships for radiation-grafted hydrogels, Science Polymer Symposium 66 (1979) 363–375. [5] L.M. Xiao, M.D.W. Grogan, W.J. Wadsworth, R. England, T.A. Birks, Stable lowloss optical nanofibres embedded in hydrophobic aerogel, Optics Express 19 (2011) 764–769. [6] S.T. Wang, Y.L. Song, L. Jiang, Microscale and nanoscale hierarchical structured mesh films with superhydrophobic and superoleophilic properties induced by long-chain fatty acids, Nanotechnology 18 (2007) 015103. [7] S. Shibuichi, T. Onda, N. Satoh, K. Tsuji, Super water-repellent surfaces resulting from fractal structure, Journal of Physical Chemistry 100 (1996) 19512.
266
Y. Yang et al. / Applied Surface Science 270 (2013) 260–266
[8] L. Jiang, Y. Zhao, J. Zhai, A lotus-leaf-like superhydrophobic surface: a porous microsphere/nanofiber composite film prepared by electrohydrodynamics, Angewandte Chemie International Edition 43 (2004) 4338–4341. [9] D. Quéré, Wetting and roughness, Annual Review of Materials Research 38 (2008) 71–99. [10] A. Tuteja, W. Choi, J.M. Mabry, G.H. McKinley, R.E. Cohen, Robust omniphobic surfaces, Proceedings of the National Academy of Sciences, USA 105 (2008) 18200–18205. [11] S.S. Li, J. Hu, J.J. Li, J.H. Tian, Z.T. Han, X.T. Zhou, Y. Chen, Anisotropic wet etched silicon substrates for reoriented and selective growth of ZnO nanowires and enhanced hydrophobicity, Langmuir 27 (2011) 6549–6553. [12] M. Karlsson, P. Forsberg, F. Nikolajeff, From hydrophilic to superhydrophobic: fabrication of micrometer-sized nail-head-shaped pillars in diamond, Langmuir 26 (2010) 889–893. [13] F. Pinzaria, P. Ascarelli, E. Cappelli, G. Mattei, R. Giorgi, Wettability of HF-CVD diamond films, Diamond and Related Materials 10 (2001) 781–785. [14] Y. Inoue, Y. Yoshimura, Y. Ikeda, A. Kohno, Ultra-hydrophobic fluorine polymer by Ar-ion bombardment, Colloids and Surfaces B-Biointerfaces 19 (2000) 257–261. [15] L. Ostrovskaya, V. Perevertalio, V. Ralchenko, A. Dementjev, O. Loginova, Wettability and surface energy of oxidized and hydrogen plasma-treated diamond films, Diamond and Related Materials 11 (2002) 845–850. [16] C.X. Wang, T.J. Yao, J. Wu, C. Ma, Z.X. Fan, Z.Y. Wang, Y.R. Cheng, Q. Lin, B. Yang, Facile approach in fabricating superhydrophobic and superoleophilic surface for water and oil mixture separation, ACS Appl. Mater. Interfaces 1 (2009) 2613–2617. [17] Y. Coffinier, E. Galopin, S. Szunerits, R. Boukherroub, Preparation of superhydrophobic and oleophobic diamond nanograss array, Journal of Materials Chemistry 20 (2010) 10671–10675. [18] C.X. Wang, G.W. Yang, T.C. Zhang, H.W. Liu, Y.H. Han, J.F. Luo, C.X. Gao, G.T. Zou, Fabrication of transparent p-n hetero-junction diodes by p-diamond film and n-ZnO film, Diamond and Related Materials 12 (2003) 1548–1552. [19] D.D. Sang, H.D. Li, S.H. Cheng, Q.L. Wang, Q. Yu, Y.Z. Yang, Electrical transport behavior of n-ZnO nanorods/p-diamond heterojunction device at higher temperatures, Journal of Applied Physics 112 (2012) 036101. [20] J.J. Chen, F. Zeng, D.M. Li, J.B. Niu, F. Pan, Deposition of high-quality zinc oxide thin films on diamond substrates for high-frequency surface acoustic wave filter applications, Thin Solid Films 485 (2005) 257–261.
[21] D.D. Sang, H.D. Li, S.H. Cheng, Growth and electron field emission of ZnO nanorods on diamond films, Applied Surface Science 258 (2011) 333–336. [22] J.W. Zhao, D.H. Wu, J.F. Zhi, A novel tyrosinase biosensor based on biofunctional ZnO nanorod microarrays on the nanocrystalline diamond electrode for detection of phenolic compounds, Bioelectrochemistry 75 (2009) 44–49. [23] L.A. Li, H.D. Li, X.Y. Lv, S.H. Cheng, Q.L. Wang, S.Y. Ren, J.W. Liu, G.T. Zou, Dependence of reaction pressure on deposition and properties of boron-doped freestanding diamond films, Applied Surface Science 256 (2010) 1764–1768. [24] P. Zapol, M. Sternberg, L.A. Curtiss, T. Frauenheim, D.M. Gruen, Tight-binding molecular-dynamics simulation of impurities in ultrananocrystalline diamond grain boundaries, Physical Review B 65 (2001) 045403. [25] C.Y. Kuan, M.H. Hon, J.M. Chou, I.C. Leu, Wetting characteristics on micro/nanostructured zinc oxide coatings, Journal of the Electrochemical Society 156 (2009) J32–J36. [26] S. Sepulveda-Guzman, B. Reeja-Jayan, E. de la Rosa, A. Torres-Castro, V. Gonzalez-Gonzalez, M. Jose-Yacaman, Synthesis of assembled ZnO structures by precipitation method in aqueous media, Materials Chemistry and Physics 115 (2009) 172–178. [27] J. Genzer, K. Efimenko, Creating long-lived superhydrophobic polymer surfaces through mechanically assembled monolayers, Science 290 (2000) 2130–2133. [28] A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Preparation of transparent superhydrophobic boehmite and silica films by sublimation of aluminum acetylacetonate, Advanced Materials 11 (1999) 1365–1368. [29] J.D. Wang, F.B. Liu, H.S. Chen, D.R. Chen, Superhydrophobic behavior achieved from hydrophilic surfaces, Applied Physics Letters 95 (2009) 084104. [30] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Industrial and Engineering Chemistry 28 (1936) 988–994. [31] T.J. Yao, C.X. Wang, Q. Lin, X. Li, X.L. Chen, J. Wu, J.H. Zhang, K. Yuand, B. Yang, Fabrication of flexible superhydrophobic films by lift-up soft-lithography and decoration with Ag nanoparticles, Nanotechnology 20 (2009) 065304. [32] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Transactions of the Faraday Society 40 (1944) 546–551. [33] Y. Li, E.J. Lee, S.O. Cho, Superhydrophobic coatings on curved surfaces featuring remarkable supporting force, Journal of Physical Chemistry C 111 (2007) 14813–14817. [34] X.J. Feng, L. Jiang, Design and creation of superwetting/antiwetting surfaces, Advanced Materials 18 (2006) 3063–3078.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Ultrahydrophobicity of ZnO modified CVD diamond films YiZhou Yang a , ChuanXi Wang b , HongDong Li a,∗ , Quan Lin b,∗∗ a b
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China State Key Laboratory of Supermolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 30 August 2012 Received in revised form 25 December 2012 Accepted 26 December 2012 Available online 16 January 2013 Keywords: Ultrahydrophobicity Freestanding polycrystalline CVD diamond ZnO thin films
a b s t r a c t Chemical vapor deposited (CVD) polycrystalline diamond films with an ultrahydrophobic surface were fabricated by constructing a hierarchical structure through sputtering a ZnO layer on diamond grains, with a growth step feature. Under optimized conditions, the combined original diamond with a step structure of the ZnO can achieve a water contact angle (WCA) of as high as 141◦ ± 1◦ . It is proved that WCA decreases as the roughness of ZnO/PDF reduced. It can be concluded that the step structure of diamond grains and ZnO nuclei size have a great influence on the variation of WCA. © 2013 Elsevier B.V. All rights reserved.
1. Introduction A surface with water contact angle (WCA) greater than 90◦ is usually referred to as hydrophobic, and one with WCA higher than 140◦ is qualified as ultrahydrophobic [1,2]. Creating a rough surface that repels water would have potential applications in the areas of self-cleaning coating [3], biocompatible materials [4], and optics [5]. Many researchers have made contributions toward enhancing hydrophobicity [6–12]. Earlier research generally used silicon, glass slide, or quartz as a substrate, which restricts their practical applications due to the substrate’s limited oleophobicity with high contact angle hysteresis, failure upon physical damage, and/or other chemical properties. To address these challenges, we propose diamond as a substrate, given its many unique properties, as a candidate to improve the hydrophobicity performance in extreme environments. However, diamond generally shows a moderate hydrophobicity owing to its high surface tension [13]. In general, surface treatments, such as ion and electron bombardment [14], plasma and glow discharge treatment [15], oxygen and ozone annealing [13], acid and basic attacks [16], and chemical modifications [17], have been used to affect the hydrophobicity. Zinc oxide (ZnO) has been an important semiconductor material due to its wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV. In addition, it is easy to realize ZnO films and nanostructures. ZnO deposition on diamond has been widely investigated to fabricate n-ZnO/p-diamond heterojunction [18,19],
∗ Corresponding author. Tel.: +86 431 85168095; fax: +86 431 85168095. ∗∗ Corresponding author. Tel.: +86 431 85193423; fax: +86 431 85193423. E-mail addresses: [email protected] (H. Li), [email protected] (Q. Lin). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.172
surface acoustic wave (SAW) devices [20], electon field emitter [21], and biosensors [22]. ZnO material has hydrophobicity, but there are few reports on the study of wettability of ZnO/diamond composite structure. In this work, thin layers of ZnO of various grain sizes were sputtered on chemical vapor deposited (CVD) polycrystalline diamond films (PDFs). It is demonstrated that the rough morphology of the PDF and growth steps on diamond grains, as well as the deposited ZnO nanostructure, play critical roles in significantly improving the hydrophobicity of a ZnO/diamond system. This mechanism has been investigated.
2. Experiments 2.1. Preparation of the polycrystalline diamond films (PDFs) The PDFs were synthesized by hot filament chemical vapor deposition (HF-CVD) [23]. The molybdenum (Mo) substrates were abraded by diamond paste for nucleation enhancement and then ultrasonically cleaned in an ethanol solution. A mixed gas consisting of 7 standard cubie centimeter per minute (sccm) methane and 400 sccm hydrogen was used at a constant pressure of 40 Torr. The chemical reaction was activated by a tantalum filament positioned 2 cm above the substrate. Negative bias enhance the nucleation (bias current was 18.5 A) for 15 min with CH4 (4 sccm) and H2 (200 sccm). In our experiments, the temperature of the filament and substrate were kept at ∼2200 and ∼800 ◦ C, respectively, as monitored by an optical pyrometer. The substrate was heated by the hot filament as the plasma was applied. After deposition, the samples were cooled down to room temperature, and the films were
Y. Yang et al. / Applied Surface Science 270 (2013) 260–266
peeled from the substrates utilizing the large difference in thermal expansion coefficient between diamond and Mo. The products were heated in boiling sulfuric acid and nitric acid to remove the non-diamond phases. The thicknesses of the PDFs were ∼380 m. For comparison, diamond films were mechanically polished from the growth side. 2.2. Preparation of PDF/ZnO micro-nanostructure ZnO films were deposited on PDFs by RF magnetron sputtering with a ZnO target (purity: 99.99%). Before deposition, the ZnO target was pre-sputtered in argon atmosphere for about 10 min to remove contaminants from its surface, and then the ZnO seed layer was deposited in argon ambient with an argon flow rate of 60 sccm without heating the substrate. Argon (Ar, 99.999%) and oxygen (O2 , 99.999%) were introduced with flow rates of 54 and 6 sccm, respectively. During the growth processes, the sputtering pressure and power were kept at 1.0 Pa and 100 W, respectively. For comparison, some ZnO-modified samples were annealed at 673 K in air in a horizontal tube furnace under a constant pressure of 6 × 104 Pa. The samples were then cooled to room temperature in the same gas and pressure environment. 2.3. Characterization Scanning electron microscope (SEM) images and energy dispersive spectrometry (EDS) used to investigate the morphology of the PDF were collected on a JEOL JXA-8200 electron probe microanalyzer operated at 15 keV. The crystal structure and properties were examined by means of atomic force microscopy (AFM) and X-ray diffraction (XRD, Rigaku D/max-RA with Cu K␣ radiation). The chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS). The WCA were measured by drop shape analysis (DSA 10MK2, KRUSS) at ambient temperature. Three microliters of deionized water were dropped onto the samples and the static contact angle was determined by averaging at least five measurements taken at the different positions on each sample. 3. Results and discussion Fig. 1 shows the surface images and wettability of the original micro-hill-like PDF and polished mirror-like PDF structures. The polished surface showed a WCA less than 90◦ (Fig. 1(c)), indicating hydrophilia. However, the WCA of the original diamond film increased to up to 101◦ (Fig. 1(a)) indicating hydrophobicity. The change of wetting property from hydrophilic to hydrophobic can be attributed to the difference in roughness of the surfaces. In addition, the smooth ZnO film deposition on a flat diamond surface (Fig. 1(d)) was hydrophobic and had a WCA of 98◦ . However, with the ZnO film covering the original diamond surface (Fig. 1(b)), the WCA reached 141◦ . The results show that all of the PDF/ZnO films exhibit hydrophobic properties, and obviously increased WCAs. The change in WCA between Fig. 1(a) and (b), can be attributed to the ZnO thin film structure. Note that the sample (Fig. 1(b)) maintains ultrahydrophobicity for 40 min before reaching a WCA of 34◦ after 24 h. However, the changes in wettability of the thin films are reversible, after the films were placed in an electric oven at 95 ◦ C for 25 min. A new water droplet was used to measure the resulting surface wettability, and the initial wetting state was obtained again, as shown in the inset in Fig. 1(b). It shows that after water infiltration the sample structure of surface did not change. Additional experiments were performed by varying the sputtering time resulting in a gradual transition in the crystallite dimensions of the ZnO-based nanostructures, which allowed us to investigate the effects of surface geometric structure on the wetting
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behavior. Fig. 2 shows the resulting PDFs after ZnO sputtering for 5–30 min on unpolished diamond. According to the AFM data, the surface of the deposited ZnO after 5, 10, 20 and 30 min exhibited roughness values of 8.6, 16.5, 9.7 and 3.5 nm, respectively. For the 5 min growth, semi-spherical ZnO nuclei with diameters of 3–4 nm were preferentially deposited at the steps, distributed across the terraces and along the boundaries of the diamond grains (Fig. 2(a)) where high density of dangling bonds and low surface energy are present [21,24]. The water droplet experiment results showed that the WCAs were 93◦ ± 1◦ (Fig. 2(e)). After 10 min deposition, a large amount of ZnO nuclei appeared along the typical growth steps of the diamond grains, as shown in Fig. 2(b). At this point, the nuclei still kept a sphere-like morphology, and as a result, the WCA significantly increased to 141◦ ± 1◦ (Fig. 2(f)). This result clearly indicated that ZnO seed layers successfully created a ultrahydrophobic surface on the PDF. As the time increased to 20 min (Fig. 2(c)), ZnO thin films have completely covered diamond steps structure and the WCA correspondingly decreased to 121◦ ± 1◦ (Fig. 2(g)). Finally, after 30 min of sputtering (Fig. 2(d)), the WCA further decreased to 101◦ ± 1◦ (Fig. 2(h)), indicating that the WCA firstly increases before decreasing as with the increase in sputtering time. The XRD spectra of the ZnO modified PDFs are shown in Fig. 3. XRD pattern for the ZnO seed layers film shows a diffraction peak at 34.21◦ corresponding to the (0 0 2) orientation of the wurtzite hexagonal structure of ZnO. In addition to the ZnO peak, the peaks corresponding to diamond were also detected, as labeled in the Fig. 3. The crystallite sizes of the ZnO seed layers films were calculated using the Debye–Scherer formula as shown below in Eq. (1). D=
0.94 ˇ cos
(1)
where D is the crystallite size, is the wavelength (1.546 A˚ for Cu K␣), ˇ is the full-width at half-maximum (FWHM) of the main intensity peak after subtraction of the equipment broadening and is the diffraction angle. The estimated average crystallite size for the ZnO seed layer films deposited for 20 min and 30 min were around 5.4 and 7.2 nm, respectively. XRD patterns show that the crystallinity does not change appreciably among the thin films with different sputter times. Therefore, variations in the initial WCA of the thin films should be primarily attributed to the difference in their surface roughness. The enhancement of hydrophobicity can be ascribed to the increase in the proportion of air/water interface in the rough, solid/air composite surface structure [25], which is positively correlated with the surface roughness of the thin films. Energy dispersive spectrometry (EDS) measurements were performed on the original PDF with the 10 min ZnO sputtering, as shown in Fig. 4. The EDS test showed there are no impurities, only C, Zn and O elements appeared in the sample. The Zn and O signal is clearly observed in the EDS spectrum, although there is no obvious ZnO diffraction peak in the XRD pattern (Fig. 3), which can be attributed to the thinness of the ZnO film sputtered for a short time. The chemical environment of the ZnO-modified surface was examined with XPS. The C 1s, O 1s, Zn 2p3/2 and Zn 2p1/2 can be observed easily and appear at 285.8, 531.1, 1022.23, and 1045.73 eV, respectively. To further study the bonding state of O, Zn and C in the ZnO/PDF film, a high resolution scan of O 1s, Zn 2p3/2 and C 1s peaks is shown in Fig. 5(b), (c) and (d). Three oxidation states for oxygen and two oxidation states for zinc were assigned after peak deconvolution. The assigned binding energies of O 1s are Zn O (530.4 ± 0.1 eV), Zn (OH) (531.7 ± 0.1 eV), and SurfO/CO (533.1 ± 0.1 eV). The assigned binding energies of Zn 2p3/2 are centered at 1021.4 ± 0.1 and 1022.7 ± 0.1 eV for Zn0 and Zn2+ , respectively. The assigned binding energies of O 1s and Zn 2p3/2 are in agreement with previously reported values [26]. The final atomic composition of Zn and O in the PDF film were 29.81% and 37.01%.
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Fig. 1. Typical SEM surface images (left) and their corresponding water droplet images with different WCA (right): (a) original unmodified diamond (WCA 101◦ ), (b) ZnOmodified (10 min ZnO sputter) (WCA 141◦ ), (c) polished diamond (WCA 87◦ ), (d) ZnO-modified polishing diamond (10 min ZnO sputter) (WCA 98◦ ). Diamond step images are the local enlarged view of (a) and (b). Inset in (b): WCA image after being shelved for 24 h (top), and heating treatment after 25 min in an electric oven at 95 ◦ C for 25 min (bottom).
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Fig. 2. 2D (a–d) and 3D (e–h) AFM micrographs of ZnO seed layers grown on unpolished PDF after different growth times show micro-hill-like growth structures. Inset are photos of 3 L water droplets deposited on the surfaces of the corresponding PDFs, showing the different contact angles.
Surface oxygen adsorption may be responsible for the Zn and O ratio not being 1:1. In Fig. 4(d), the four deconvoluted C 1s peaks were assigned as C C (284.6 ± 0.1 eV), C C O (285.6 ± 0.1 eV), C OH (286.4 ± 0.1 eV), and C O (288.4 ± 0.1 eV).
Fig. 6 shows the differences in WCA with the increasing deposition time (i. e. increasing the thickness) for ZnO thin films deposited on the polished and unpolished PDFs. It can be seen that the WCA of polished thin films increase from 97◦ ± 1◦ to 120◦ ± 1◦ with
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Fig. 3. X-ray diffraction patterns of the ZnO films deposited on the PDF substrates at different growth times of 0, 5, 10, 20 and 30 min. Fig. 4. Typical EDS spectrum and the corresponding PDF sputtered by a ZnO thin film.
increasing ZnO layer thickness (with longer sputtering time of 30 min). The original PDF sample displays a strong response, reaching a WCA of 141◦ ± 1◦ , nowever after annealing at 673 K, the WCA decreased to 89◦ ± 1◦ . The weakening hydrophobicity may be ascribed to the decreasing proportion of air/water interface in the rough, solid/air composite surface structure, which is positively correlated with surface roughness of the thin films. At 40 min, the WCA reached a stable point at which further deposition of ZnO no longer affects the WCA.
Wettability is a very important property which can be effected by a combination of surface roughing and lowering surface energy [27,28]. Also, the texture of a surface plays a vital role in the creation of a hydrophobic surface with high WCA [29–31]. Therefore, the structural properties of a material need to be properly tuned in order to synthesize hydrophobic surfaces. To fully utilize the hydrophobic properties of nanostructured surfaces, it is necessary
Fig. 5. (a) Wide range XPS spectrum of original ZnO/PDF thin film and detailed scan of the (b) O 1s, (c) Zn 2p3/2 and (d) C 1s binding energy areas.
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Fig. 7. Schematic illustration of the air effect on ZnO/PDF films.
Fig. 6. Resultant WCA for different PDF treatment processes (polished, unpolished, and annealed) and sputter times.
to study the fundamental relationship between different nanostructures and hydrophobic behavior. Two theoretical models [30,32] of Wenzel model and Cassie–Baxter model form the basic guidelines for the study of hydrophobic surfaces. The basic assumption in Wenzel’s theory is that a water droplet penetrates the asperities while the Cassie–Baxter model predicts the suspension of a water droplet on the top of the asperities. The wetting behavior is described by the Cassie–Baxter and Wenzel models as cos c = fs cos s − fv
(2)
cos c = r cos s
(3)
could be applied in explaining the relationship between surface structure and surface WCA, where c and s represent the WCAs of a rough surface and a native flat surface, while the fs and fv are the fraction of the surface in contact with the liquid droplet and air (fv is between 0 and 1, while fs is more than 0 (i.e., fs + fv = 1), and r is the roughness factor. The cosine results in a negative number, implying that with a smaller fs and larger fv and s , it is possible to create surfaces with a very large WCA. It had been demonstrated in literature that hierarchical structures can effectively increase the roughness of the surface and hence increase the WCA [33]. Thus, according to the Eq. (2), the fv value of the unannealed surface rapidly increased to 0.79, explaining the observed ultrahydrophobicity of the PDF as shown in Fig. 2(b). This rougher surface was favorable for trapping a large amount of air in the valleys on the surface, and thus forming a hydrophobic surface. The WCA is an important parameter in surface science and its measurement provides a simple and reliable technique for the interpretation of surface energies. Generally, the apparent contact angle of a hydrophobic surface increases with its roughness, which is usually defined as the real surface area to the apparent surface area. However, we can see from Fig. 2, the WCA decreases with increasing the sputtering time of ZnO layer, possibly the ZnO seed layer gradually formed flat ZnO films with the for longer sputtering, resulting in a smoother surface. It was reported that if the smooth surface WCA is >90◦ , then the angle on a rough surface will be larger [34]. In Fig. 5, after annealing, ZnO-modified surface changes from a Cassie to Wenzel state because the liquid droplets fill the grooves of the rough substrate, decreasing the apparent WCA. As mentioned in Ref. [25], it is suggested that the amount of trapped air within the microstructures decreases owing to the filling of the valleys with water, and the WCA result changes from hydrophobic to hydrophilic. The
schematic diagram illustrating this mechanism is shown in Fig. 7 for our ZnO/PDF structures. Surface microstructures of solids play a significant role in producing hydrophobic surfaces, and the geometric parameters of the microstructure have a great influence on the wetting behavior. A possible mechanism to achieve the hydrophobic property on the hydrophilic/hydrophobic surfaces is attributed to trapped air within the microstructure, which is isolated from the atmosphere by the liquid. In other words, an increase in surface roughness results in a reduced contact area between solid and water due to an increase in air bubbles trapped at the interface; the more air trapped between the rough surface and the water, the larger the WCA will be. Therefore, for the case of a pitch size of 50 m with sawtooth patterns, the preparation of nanostructures could further decrease the contact fraction of the solid. 4. Conclusions Microtextured diamond surfaces modified by ZnO were fabricated to reach the ultrahydrophobic state. The diamond surfaces were sputtered with ZnO to enhance the wetting properties. When sputtering ZnO for layer with a proper thickness on unpolished diamond surface, the WCA increased to 141◦ ± 1◦ . To produce an ultrahydrophobic surface using a combination of diamond and ZnO seed layers structures, an air trapping suface design is needed. It can be concluded that diamond step structure and ZnO nuclei size have a great influence on the WCA. Future studies will include separation of oil/water solutions. Diamond thin films can be deposited on various substrates. This will allow for hydrophobicity on a wide variety of substrates for self-cleaning surfaces. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) with No. 51072066 and 50772041, the Ph.D. Programs Foundation of Ministry of Education of China with No. 20100061110083. References [1] D. Öner, T.J. McCarthy, Ultrahydrophobic surfaces effects of topography length scales on wettability, Langmuir 16 (2000) 7777. [2] J. Bico, C. Marzolin, D. Quere, Pearl drops, Europhysics Letters 47 (1999) 220. [3] V. Kekkonen, A. Hakola, T. Kajava, E. Sahramo, J. Malm, M. Karppinen, R.H.A. Ras, Self-erasing and rewritable wettability patterns on ZnO thin films, Applied Physics Letters 97 (2010) 044102. [4] B.D. Ratner, A.S. Hoffman, S.R. Hanson, L.A. Harker, J.D. Whiffen, Bloodcompatibility-water-content relationships for radiation-grafted hydrogels, Science Polymer Symposium 66 (1979) 363–375. [5] L.M. Xiao, M.D.W. Grogan, W.J. Wadsworth, R. England, T.A. Birks, Stable lowloss optical nanofibres embedded in hydrophobic aerogel, Optics Express 19 (2011) 764–769. [6] S.T. Wang, Y.L. Song, L. Jiang, Microscale and nanoscale hierarchical structured mesh films with superhydrophobic and superoleophilic properties induced by long-chain fatty acids, Nanotechnology 18 (2007) 015103. [7] S. Shibuichi, T. Onda, N. Satoh, K. Tsuji, Super water-repellent surfaces resulting from fractal structure, Journal of Physical Chemistry 100 (1996) 19512.
266
Y. Yang et al. / Applied Surface Science 270 (2013) 260–266
[8] L. Jiang, Y. Zhao, J. Zhai, A lotus-leaf-like superhydrophobic surface: a porous microsphere/nanofiber composite film prepared by electrohydrodynamics, Angewandte Chemie International Edition 43 (2004) 4338–4341. [9] D. Quéré, Wetting and roughness, Annual Review of Materials Research 38 (2008) 71–99. [10] A. Tuteja, W. Choi, J.M. Mabry, G.H. McKinley, R.E. Cohen, Robust omniphobic surfaces, Proceedings of the National Academy of Sciences, USA 105 (2008) 18200–18205. [11] S.S. Li, J. Hu, J.J. Li, J.H. Tian, Z.T. Han, X.T. Zhou, Y. Chen, Anisotropic wet etched silicon substrates for reoriented and selective growth of ZnO nanowires and enhanced hydrophobicity, Langmuir 27 (2011) 6549–6553. [12] M. Karlsson, P. Forsberg, F. Nikolajeff, From hydrophilic to superhydrophobic: fabrication of micrometer-sized nail-head-shaped pillars in diamond, Langmuir 26 (2010) 889–893. [13] F. Pinzaria, P. Ascarelli, E. Cappelli, G. Mattei, R. Giorgi, Wettability of HF-CVD diamond films, Diamond and Related Materials 10 (2001) 781–785. [14] Y. Inoue, Y. Yoshimura, Y. Ikeda, A. Kohno, Ultra-hydrophobic fluorine polymer by Ar-ion bombardment, Colloids and Surfaces B-Biointerfaces 19 (2000) 257–261. [15] L. Ostrovskaya, V. Perevertalio, V. Ralchenko, A. Dementjev, O. Loginova, Wettability and surface energy of oxidized and hydrogen plasma-treated diamond films, Diamond and Related Materials 11 (2002) 845–850. [16] C.X. Wang, T.J. Yao, J. Wu, C. Ma, Z.X. Fan, Z.Y. Wang, Y.R. Cheng, Q. Lin, B. Yang, Facile approach in fabricating superhydrophobic and superoleophilic surface for water and oil mixture separation, ACS Appl. Mater. Interfaces 1 (2009) 2613–2617. [17] Y. Coffinier, E. Galopin, S. Szunerits, R. Boukherroub, Preparation of superhydrophobic and oleophobic diamond nanograss array, Journal of Materials Chemistry 20 (2010) 10671–10675. [18] C.X. Wang, G.W. Yang, T.C. Zhang, H.W. Liu, Y.H. Han, J.F. Luo, C.X. Gao, G.T. Zou, Fabrication of transparent p-n hetero-junction diodes by p-diamond film and n-ZnO film, Diamond and Related Materials 12 (2003) 1548–1552. [19] D.D. Sang, H.D. Li, S.H. Cheng, Q.L. Wang, Q. Yu, Y.Z. Yang, Electrical transport behavior of n-ZnO nanorods/p-diamond heterojunction device at higher temperatures, Journal of Applied Physics 112 (2012) 036101. [20] J.J. Chen, F. Zeng, D.M. Li, J.B. Niu, F. Pan, Deposition of high-quality zinc oxide thin films on diamond substrates for high-frequency surface acoustic wave filter applications, Thin Solid Films 485 (2005) 257–261.
[21] D.D. Sang, H.D. Li, S.H. Cheng, Growth and electron field emission of ZnO nanorods on diamond films, Applied Surface Science 258 (2011) 333–336. [22] J.W. Zhao, D.H. Wu, J.F. Zhi, A novel tyrosinase biosensor based on biofunctional ZnO nanorod microarrays on the nanocrystalline diamond electrode for detection of phenolic compounds, Bioelectrochemistry 75 (2009) 44–49. [23] L.A. Li, H.D. Li, X.Y. Lv, S.H. Cheng, Q.L. Wang, S.Y. Ren, J.W. Liu, G.T. Zou, Dependence of reaction pressure on deposition and properties of boron-doped freestanding diamond films, Applied Surface Science 256 (2010) 1764–1768. [24] P. Zapol, M. Sternberg, L.A. Curtiss, T. Frauenheim, D.M. Gruen, Tight-binding molecular-dynamics simulation of impurities in ultrananocrystalline diamond grain boundaries, Physical Review B 65 (2001) 045403. [25] C.Y. Kuan, M.H. Hon, J.M. Chou, I.C. Leu, Wetting characteristics on micro/nanostructured zinc oxide coatings, Journal of the Electrochemical Society 156 (2009) J32–J36. [26] S. Sepulveda-Guzman, B. Reeja-Jayan, E. de la Rosa, A. Torres-Castro, V. Gonzalez-Gonzalez, M. Jose-Yacaman, Synthesis of assembled ZnO structures by precipitation method in aqueous media, Materials Chemistry and Physics 115 (2009) 172–178. [27] J. Genzer, K. Efimenko, Creating long-lived superhydrophobic polymer surfaces through mechanically assembled monolayers, Science 290 (2000) 2130–2133. [28] A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Preparation of transparent superhydrophobic boehmite and silica films by sublimation of aluminum acetylacetonate, Advanced Materials 11 (1999) 1365–1368. [29] J.D. Wang, F.B. Liu, H.S. Chen, D.R. Chen, Superhydrophobic behavior achieved from hydrophilic surfaces, Applied Physics Letters 95 (2009) 084104. [30] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Industrial and Engineering Chemistry 28 (1936) 988–994. [31] T.J. Yao, C.X. Wang, Q. Lin, X. Li, X.L. Chen, J. Wu, J.H. Zhang, K. Yuand, B. Yang, Fabrication of flexible superhydrophobic films by lift-up soft-lithography and decoration with Ag nanoparticles, Nanotechnology 20 (2009) 065304. [32] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Transactions of the Faraday Society 40 (1944) 546–551. [33] Y. Li, E.J. Lee, S.O. Cho, Superhydrophobic coatings on curved surfaces featuring remarkable supporting force, Journal of Physical Chemistry C 111 (2007) 14813–14817. [34] X.J. Feng, L. Jiang, Design and creation of superwetting/antiwetting surfaces, Advanced Materials 18 (2006) 3063–3078.