Non-fluorinated superhydrophobic and micro/nano hierarchical Al doped ZnO film: the effect of Al doping on morphological and hydrophobic properties

Abstract

This work reports on the systematic comparison of the crystalline structural, morphological and hydrophobic properties of ZnO and Al-doped ZnO (AZO) thin films fabricated by atomic layer deposition and the hydrothermal method. It was revealed that the surface wettability can be largely modified by Al doping in the zinc oxide film growth process. With Al doping, the morphology of the AZO films became more complex and rough. The water contact angle of a flower-like hierarchical ZnO film (123 ± 4°) was improved by about 40° via Al doping to 160 ± 4°. We attributed the variation in surface hydrophobicity with Al doping to changes in the bond angle and distance between ZnO–H₂O molecule. The computational simulations have been employed to verify the interfacial distinction between two main crystal orientations of AZO. This result suggests that Al doping can be considered a critical factor in changing the surface morphology of AZO as well as the hydrophobic properties. It is believed that the present route holds promise in the design and application of practical superhydrophobic materials.

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Introduction

During the past decade, superhydrophobic coatings, the apparent contact angles of water (CA) above 150°, have attracted considerable attention among many researchers and experts due to their advantages such as self-cleaning, anti-fogging, anti-icing, anti-corrosion and anti-bacterial properties in the commercial surface industry.^1–3

As is well known, superhydrophobicity is based on the combination of suitable rough topography and low surface energy. The most common superhydrophobic surface can be fabricated via chemically modifying with low surface free energy organic perfluorinated materials.^4,5 However, the organic modified coatings are expensive, unstable and easily contaminated in their practical applications.⁶ Therefore, increasing attention is drawn to fabricating superhydrophobic surfaces without any low energy organic materials, such as inorganic material films with specific hierarchical rough surfaces.^7–11

Being a wide band-gap semiconductor material, zinc oxide (ZnO) possesses various optical and electrical applications. Recently, with the development of optoelectronic devices, the surface wettability of ZnO and its metal doping oxide has aroused great interest and been realized by combination of the micro-nanostructured topography and low surface energy.^12–14 However, to the best of our knowledge, nearly all of the applications are based on its photoelectric and acoustic properties. Those works are mostly focusing on the morphologies, optical and electrical properties of Al doping ZnO (AZO) materials, in which Al doping is a critical factor in changing of the optical and electrical properties of the films.^15,16 However, few studies have reported how doping can affect the superhydrophobic properties of the surface.

In the present work, a layer to layer strategy combining atomic layer deposition (ALD) and hydrothermal methods have been involved in fabricating superhydrophobic flower like ZnO and AZO films. The differences in the structural, morphology and superhydrophobic properties between ZnO and AZO hydrothermal films have been investigated. In spite of the similar flower like hierarchical structures obtained in the ZnO and AZO materials, the AZO films present excellent superhydrophobicity (CA = 160 ± 4°) compared to ZnO films (CA = 123 ± 4°), which indicates that Al doping can dramatically modify the superhydrophobicity of the surface. The modified mechanism of the Al atom on the superhydrophobic property of AZO films had been discussed. The computational simulation was performed using the plane wave code CASTEP,¹⁷ as implemented in the Materials Studio. The results indicated the variation in surface hydrophobicity with Al doping can be attributed to changes in the bond angle and distance between ZnO–H₂O. It is believed that this non-fluorinated micro- and nano-hierarchical superhydrophobic AZO film could be a potential candidate for photoelectric fields, and the atom doping shows a good strategy to modify the chemistry of the surface.

Experimental

A glass slide with a ZnO thin film layer was used as a seed layer for the fabrication of the ZnO and AZO films, respectively. An ALD system (TALD-150 Kemicro Jiaxing) was employed to deposit the ZnO seed layer. Diethyl zinc [DEZn, Zn(C₂H₅)₂] and distilled-water (H₂O) were used as precursors of Zn and O, respectively.

In a typical synthesis process, Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O and C₆H₁₂N₄ (HMTA) were used as starting materials. The Zn²⁺ (or 97% Zn²⁺ + 3% Al³⁺) and HMTA were dissolved in aqueous solution with a 1 : 1 mole ratio. The solution and the substrate was placed on the bottom of a 50 mL stainless steel Teflon-lined autoclave and heated at 140 °C for 4 h. The autoclave was cooled down to room temperature and the sample was washed repeatedly with ethanol, deionized water and dried with nitrogen.

The crystalline structures of the films was tested by X-ray diffraction at room temperature. X-ray diffraction (XRD) was carried out using an X-ray diffractometer with the Cu-Kα wavelength (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS, TSC K-Alpha, AlKα) was adopted to investigate the elemental states of Zn, O and Al. The morphological structure of the ZnO and AZO films was characterized by Helios Nanolab 600i scanning electron microscopy and transmission electron microscopy (TEM, JEM-2100F) was used to provide selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) images. Water CA measurement was carried out with a CA101 contact angle meter.

The strategy for preparing the superhydrophobic hierarchical AZO thin films is schematically presented in Scheme 1.

Scheme 1 Schematics of two step process for growing ZnO and AZO nanostructures.

Results and discussion

Fig. 1 shows the FE-SEM images of the hydrothermal ZnO (lower) and AZO (upper) nanostructures formed on the seed layer coated glass substrates, respectively. The flower-like ZnO nanosheets and AZO nano fluffy-clew were obtained. The average sizes of the micro-flower are around 5 μm for ZnO and 3 μm for AZO. The smaller sized AZO micro-flower may be attributed to the different growth rate of the crystals.

Fig. 1 SEM top-images at low magnification (a) and high magnification (b) for the hydrothermal AZO films; SEM top-images at low magnification (c) and high magnification (d) for the hydrothermal ZnO films; the inset of (a) and (c) show the photo of a droplet on hydrothermal AZO and ZnO films, respectively.

The nano rod-like array and micro flowers were two kinds of typical morphology for hydrothermal AZO, which can be obtained under low and high growth temperature conditions, respectively.¹⁸ In most research works, the surface of the ZnO micro-nanorods array is still hydrophilic, and thus the fluorination method needs to be used for the formation of the superhydrophobic surface texture.¹⁶ Herein, the flower-like micro/nano hierarchical AZO had been investigated in this work. The wettability of the as-prepared hydrothermal AZO and ZnO films was evaluated by the water contact angle measurement. The inset of Fig. 1a and c depict the shapes of water droplet on different micro/nanostructure films, showing the obvious difference between the AZO and ZnO films. The CA angles for AZO and the ZnO films were equal to 160 ± 4° and 123 ± 4°, respectively.

In order to investigate the chemical state of AZO hydrothermal nanostructures, XPS measurement was employed. The survey XPS spectrum of the AZO hydrothermal film is shown in Fig. 2a, which indicates the representative peaks of Zn and O. Fig. 2b–d demonstrates the high resolution XPS spectra of the Zn 2p, O 1s, and Al 2p core levels and their Gaussian-resolved results for the AZO micro flowers, respectively. The Gaussian-resolved result for the Zn 2p spectra in Fig. 2b shows two main peaks located at 1020.9 eV and 1044.0 eV, which were assigned to Zn 2p_3/2 and Zn 2p_1/2 electronic states respectively. There is no significant difference in the chemical shift and signal for Zn 2p_1/2. The binding energy of Zn 2p_3/2 is smaller than the value of Zn 2p_3/2 in ZnO films (1022.40 ± 0.1 eV).¹⁹ The Gaussian-resolved result for O 1s spectra in Fig. 2c shows oxygen peak can be consistently fitted from a high binding energy to a low binding energy by three nearly Gaussian components, centered at 532.10, 531.15 and 530.29 eV. Generally, those binding energy can be ascribed to the adsorbed H₂O or O₂, O²⁻ in the oxygen deficient regions and O²⁻ ions on the wurtzite structure of ZnO, respectively.²⁰ Fig. 2d indicates that an Al–O bond has been formed at the interface region of ZnO according to the appearance of Al 2p peak at 74.1 eV, which suggests that the Al elements have been successfully incorporated into in Zn²⁺ sites of ZnO.¹⁹

Fig. 2 (a) The survey XPS spectrum of the AZO hydrothermal film. (b–d) XPS spectra (color map for experimental data; solid line for Gaussian-resolved result) of the Zn 2p, O 1s and Al 2p core level regions, respectively.

To profoundly understand the dramatic transition from the hydrophobic to superhydrophobic state of the ZnO films, the surface free energy and the surface roughness, which are two main factors governing the surface wettability, have been considered.

It is well-known that ZnO with a high (002) plane intensity has a lower surface free energy.²¹ However, compared to organic material coatings such as fluorosilane and poly-propylene, the surface free energy of metallic oxide is higher and hydrophobicity or superhydrophobicity could not be obtained without significant roughness of the nanostructured oxide so that enough air could be trapped in it.^14,22,23 Here, the AZO films demonstrate a significant higher water contact angle which is attributed to the higher roughness. These results agree with the study performed by Pierre Colson²⁴ and Th. Pauporte.²⁵

Flower-like micro/nanostructure of ZnO and AZO films possess double-scale roughness which plays an important role in perfect superhydrophobic behaviour. Compared to the ZnO structure, the Al-doped ZnO morphology is similar to lotus leaf-like double-scale roughness surface, showing superhydrophobic with a water CA about 160°.

Apparently, the surface roughness served as an important factor for superhydrophobic states. Therefore, it is worth discussing the growth mechanism of the flower-like micro/nanostructure of AZO films to support its superhydrophobic properties. As is well-known, ZnO is a polar crystal where the c-axis is the polar axis. The asymmetric distribution of the zinc and oxygen atoms along the c-axis gives the crystal a positive and negative side. During the growth process, the negative growth units, Zn(OH)₄^2−, could easily be superimposed on the positive Zn-terminated (002) polar surface, forming nanorods. HMTA slowly released OH⁻ groups at a low reaction temperature,²⁶ while the higher hydrothermal temperature could speed up the release rate of OH⁻ groups from HMTA. More OH⁻ groups could also absorb at the positive Zn-terminated (002) polar surface, leading to the disappearing of the (002) polar surface, which is favourable for nanosheet growth.²⁷ In the process of fabricating the AZO micro flower structures, Al(OH)₄⁻ as one of the growth units of the ZnO crystal, has the ability to deteriorate the (002) orientation growth of ZnO.^19,28

The crystallographic structures of the ZnO and the AZO hydrothermal film were characterized using XRD, as shown in Fig. 3. The main diffraction peaks of the Al doped ZnO thin films at the 2 theta values of 31.91°, 34.64° and 36.45° were indexed as the (100), (002) and (101) planes, respectively, with a hexagonal wurtzite structure of ZnO (JCPDS 36-1451). Compared with the undoped ZnO, the (002) peak position of AZO was slightly shifted to higher angle direction, as shown in the inset of Fig. 3, which revealed that the inter-planar distance between the (002) planes were decreased due to Zn sites being replaced by the Al atoms, since the ionic radius of Al³⁺ was smaller than that of Zn²⁺. In addition, compared with the low (100) peak intensities of ZnO, the (100) peak of AZO is greatly increased.^15,29 P. Banerjee¹⁵ indicated that the Al³⁺ ions may suppress the (002) phase and disturb the charge neutrality of the (100) plane, affecting its surface energy and causing its preferential growth.³⁰ Hence it can be determined that the difference in the ratio of (100) to (002) for ZnO and AZO films has been induced by the doping of Al³⁺ ions.

Fig. 3 The XRD patterns of ZnO (lower) and AZO (upper) hydrothermal films. Inset shows the zoomed in view of 31–37 deg.

The appearance of the (100) plane in AZO and the distinction on the ratio of crystal direction for ZnO and AZO films also could be verified by high-resolution TEM (HRTEM) and SAED. The micrographs of bright field TEM and HRTEM images of AZO (upper) and ZnO (lower) were shown in Fig. 4. It should be noted that the AZO still remained flower like hierarchical structure even after a long ultrasound time during the preparation of the TEM specimen, indicating the structural durability of AZO hydrothermal films.

Fig. 4 The TEM information of AZO (upper) and ZnO (lower); (a and c) TEM image and high-resolution TEM image (inset), and (b and d) selected area electron diffraction (SAED).

From the HRTEM observation, it can be seen that there are AZO polycrystalline grains with crystalline plane distances of 0.28 nm (inset Fig. 4a), which correspond to the interplanar spacing of (100) planes for AZO films. In contrast, only (002) planes with 0.26 nm can be obtained in the ZnO HRTEM (inset Fig. 4c), which is in agreement with the XRD peak measurement. The selected area electron diffraction (SAED) images were shown in Fig. 4b and d for AZO and ZnO, respectively. The presence of the diffused rings and regular spots further confirms the above results.

Overall, in Cassie–Baxter’s theory, the liquid droplet will not completely interact with the whole solid surface due to the AZO nano fluffy-clew structure and thus the micro- and nano-scale structure traps air inside, forming a solid–air–liquid interface. The micro-scale structure increases the area of trapped air compared to a single nano-scale structure and therefore increases the contact angle.

Besides the surface roughness, the surface energy of AZO film is another important factor for hydrophobic analysis. To the best of our knowledge, most works focus on the energy of the macroscopic surface or the coating material. In present work, we attribute the superhydrophobicity of AZO to its interfacial electronic structure, which is due to the coexistence of the (100) and (002) lattice planes. The O(ZnO)–H(H₂O) bond distance for different planes become an important consideration factor to determine the surface wettability.

To validate our hypothesis on the hydrophobicity of AZO films, our calculations were performed using the plane wave code CASTEP, as implemented in the Materials Studio. For structural relaxations, we employed the Perdew–Burke–Ernzerhof (PBE)³¹ functional with the ultrasoft pseudopotentials. The Brillouin-zone integrations were performed on a dense Monkhorst–Pack 3 × 3 × 1 k-point grid for ZnO surfaces with or without water molecule (5 × 5 × 4 k-point grid for ZnO wurtzite bulk). The kinetic energy cutoff for plane waves was set to 340 eV (380 eV for ZnO wurtzite bulk). The convergence criterion for the electronic self-consistent loop was set to 10⁻⁶ eV. Based on the fully relaxed ZnO wurtzite bulk, we built two facets, namely (002) and (100) facets. The (100) and (002) facets are the two most stable facets for ZnO wurtzite bulk. A slab with four layers and a (2 × 2)-ZnO (002) and (100) surface super-cell was used to study their interactions with water molecules, as seen in Fig. 5 ((b), (c) for the (002) facet & (d), (e) for the (100) facet). During the structural relaxations of ZnO surfaces with or without water molecules, the vacuum regions were at least 15 Å to ensure the periodic images are well separated, and the last two layers of ZnO were fixed to mimic the bulk phase. The remaining atoms were relaxed until the Hellmann–Feynman forces were smaller than 0.01 eV Å⁻¹. This approach balances the accuracy and computational cost.

Fig. 5 (a) The wurtzite structure of ZnO; the slab with four layers and a ZnO (002) surface super-cell ((b) for top view, (c) for side view) and ZnO (100) surface super-cell ((d) for top view, (e) for side view).

Fig. 6 shows the optimized geometries of most stable structures of single water molecule adsorbed on ZnO surfaces. First, the bonding sites of water molecule on ZnO (002) and (100) surfaces are dramatically different. On ZnO (002) surface, the exposed oxygen atoms on the surface bonds with the hydrogen atoms of the water molecules (Fig. 6a and b). The O(ZnO)–H(H₂O) distance is ca. 1.893 Å, and the O(ZnO)–H(H₂O)–O(H₂O) angle is ca. 172.2°. On ZnO (100) surfaces, it is found that water molecule strongly bonds with Zn atom, which makes the interacted Zn atom regain tetrahedral-coordination (Fig. 6c and d). The Zn(ZnO)–O(H₂O) distance is ca. 2.094 Å, and the Zn(ZnO)–O(H₂O)–H(H₂O, up) angle is ca. 118.0°. A similar study of methanol adsorption on ZnO (100) was reported by Metiu and co-workers,^32,33 and the orientation of water molecules next to the surface was similar to the explanation about the hydrophobicity of La₂O₃ and Al₂O₃ surface, reported by G. Amizi.³⁴ The surface possessing longer bond distances and smaller bond angles between ZnO and H₂O molecules will display a better hydrophobic property. With a different ratio between the (100) and (002) facets in ZnO and AZO, we can conclude the behaviour of water adsorption on ZnO and AZO would be quite different.

Fig. 6 The optimized geometries of most stable structures of single water molecule adsorbed on surfaces. The view of water molecules on the (002) facet ((a) top view, (b) side view) and the (100) facet ((c) top view, (d) side view).

Conclusions

In summary, nano fluffy-clew-like AZO superhydrophobic film have been fabricated by ALD and the hydrothermal method. The subtle role of Al in controlling the morphology and wettability of AZO film have been discussed. With Al doping, the morphology of AZO became more complex and rough. The water contact angle of a flower-like hierarchical ZnO film (123 ± 4°) was improved by about 40° via Al doping to 160 ± 4°. The bond distance of ZnO (100)–H₂O (2.094 Å) is longer than the distance for ZnO (002)–H₂O (1.893 Å), which makes a better hydrophobic property. This result suggests that Al doping can be considered as a critical factor in changing the surface morphology of AZO as well as its hydrophobic properties.

Acknowledgements

The financial supports of the National Natural Science Foundation of China (Grant No. 51173033 and 51572060), the Excellent Youth Foundation of Heilongjiang Scientific Committee (No. JC2015010), the program for Innovation Research of Science in Harbin Institute of Technology (B201413) and the Doctoral Foundation of Heilongjiang province (LBH-Z14103) are gratefully acknowledged. J. Z. thanks the Fundamental Research Funds for the Central Universities of China (Grant No. AUGA5710013115). Computer time is made available by the National Supercomputing Center of China in Shenzhen (Shenzhen Cloud Computing Center).

Notes and references

1. A. Walther, I. Bjurhager, J. M. Malho, J. Pere, J. Ruokolainen, L. A. Berglund and O. Ikkala, Nano Lett., 2010, 10, 2742–2748.
2. P. Guo, Y. Zheng, M. Wen, C. Song, Y. Lin and L. Jiang, Adv. Mater., 2012, 24, 2642–2648.
3. P. Ragesh, V. Anand Ganesh, S. V. Nair and A. S. Nair, J. Mater. Chem. A, 2014, 2, 14773.
4. J. El-Maiss, T. Darmanin and F. Guittard, RSC Adv., 2015, 5, 37196–37205.
5. L. Barbieri, E. Wagner and P. Hoffmann, Langmuir, 2007, 23, 1723–1734.
6. I.-K. Oh, K. Kim, Z. Lee, K. Y. Ko, C.-W. Lee, S. J. Lee, J. M. Myung, C. Lansalot-Matras, W. Noh, C. Dussarrat, H. Kim and H.-B.-R. Lee, Chem. Mater., 2015, 27, 148–156.
7. J. Malm, E. Sahramo, M. Karppinen and R. H. A. Ras, Chem. Mater., 2010, 22, 3349–3352.
8. L. Huang, S. P. Lau, H. Y. Yang, E. S. P. Leong, S. F. Yu and S. Prawer, J. Phys. Chem. B, 2005, 109, 7746–7748.
9. Y. Gao, I. Gereige, A. El Labban, D. Cha, T. T. Isimjan and P. M. Beaujuge, ACS Appl. Mater. Interfaces, 2014, 6, 2219–2223.
10. I. Das, M. K. Mishra, S. K. Medda and G. De, RSC Adv., 2014, 4, 54989–54997.
11. L. J. Li, T. Huang, J. L. Lei, J. X. He, L. F. Qu, P. L. Huang, W. Zhou, N. B. Li and F. S. Pan, ACS Appl. Mater. Interfaces, 2015, 7, 1449–1457.
12. H. J. Wang, Z. Yang, J. Yu, Y. Z. Wu, W. J. Shao, T. T. Jiang and X. L. Xu, RSC Adv., 2014, 4, 33730–33738.
13. C. Mondal, M. Ganguly, A. K. Sinha, J. Pal and T. Pal, RSC Adv., 2013, 3, 5937–5944.
14. X. J. Feng, L. Feng, M. H. Jin, J. Zhai, L. Jiang and D. B. Zhu, J. Am. Chem. Soc., 2004, 126, 62–63.
15. P. Banerjee, W. J. Lee, K. R. Bae, S. B. Lee and G. W. Rubloff, J. Appl. Phys., 2010, 108, 043504.
16. S. D. Shinde, S. K. Date, A. V. Deshmukh, A. Das, P. Misra, L. M. Kukreja and K. P. Adhi, RSC Adv., 2015, 5, 24178–24187.
17. M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark and M. C. Payne, J. Phys.: Condens. Matter, 2002, 14, 2717–2744.
18. B. J. Li, L. J. Huang, M. Zhou and N. F. Ren, Appl. Surf. Sci., 2013, 286, 391–396.
19. J. T. Chen, J. Wang, R. F. Zhuo, D. Yan, J. J. Feng, F. Zhang and P. X. Yan, Appl. Surf. Sci., 2009, 255, 3959–3964.
20. J. C. C. Fan and J. B. Goodenough, J. Appl. Phys., 1977, 48, 3524.
21. H. Deng, J. J. Russell, R. N. Lamb, B. Jiang, Y. Li and X. Y. Zhou, Thin Solid Films, 2004, 458, 43–46.
22. H. Liu, L. Feng, J. Zhai, L. Jiang and D. B. Zhu, Langmuir, 2004, 20, 5659–5661.
23. B. Su, M. Li, Z. Y. Shi and Q. H. Lu, Langmuir, 2009, 25, 3640–3645.
24. P. Colson, A. Schrijnemakers, B. Vertruyen, C. Henrist and R. Cloots, J. Mater. Chem., 2012, 22, 17086–17093.
25. T. Pauporte, G. Bataille, L. Joulaud and F. J. Vermersch, J. Phys. Chem. C, 2010, 114, 194–202.
26. X. L. Zhang, H. T. Dai, J. L. Zhao, S. G. Wang and X. W. Sun, Cryst. Res. Technol., 2014, 49, 220–226.
27. Z. W. Chen, G. H. Zhan, Y. P. Wu, X. H. He and Z. Y. Lu, J. Alloys Compd., 2014, 587, 692–697.
28. B. Wang, M. J. Callahan, C. Xu, L. O. Bouthillette, N. C. Giles and D. F. Bliss, J. Cryst. Growth, 2007, 304, 73–79.
29. Y. Geng, L. Guo, S. S. Xu, Q. Q. Sun, S. J. Ding, H. L. Lu and D. W. Zhang, J. Phys. Chem. C, 2011, 115, 12317–12321.
30. S. Y. Pung, K. L. Choy, X. Hou and C. X. Shan, Nanotechnology, 2008, 19, 435609.
31. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868.
32. R. G. S. Pala and H. Metiu, J. Phys. Chem. C, 2007, 111, 8617–8622.
33. R. G. S. Pala and H. Metiu, J. Catal., 2008, 254, 325–331.
34. G. Azimi, R. Dhiman, H. M. Kwon, A. T. Paxson and K. K. Varanasi, Nat. Mater., 2013, 12, 315–320.

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