Facile synthesis of superhydrophobic surface of ZnO nanoflakes: chemical coating and UV-induced wettability conversion
© Yao et al; licensee Springer. 2012
Received: 18 October 2011
Accepted: 13 April 2012
Published: 13 April 2012
This work reports an oriented growth process of two-dimensional (2D) ZnO nanoflakes on aluminum substrate through a low temperature hydrothermal technique and proposes the preliminary growth mechanism. A bionic superhydrophobic surface with excellent corrosion protection over a wide pH range in both acidic and alkaline solutions was constructed by a chemical coating treatment with stearic acid (SA) molecules on ZnO nanoflakes. It is found that the superhydrophobic surface of ZnO nanoflake arrays shows a maximum water contact angle (CA) of 157° and a low sliding angle of 8°, and it can be reversibly switched to its initial superhydrophilic state under ultraviolet (UV) irradiation, which is due to the UV-induced decomposition of the coated SA molecules. This study is significant for simple and inexpensive building of large-scale 2D ZnO nanoflake arrays with special wettability which can extend the applications of ZnO films to many other important fields.
KeywordsZnO nanoflakes Chemical coating Superhydrophobic Corrosion protection UV irradiation
Wettability of solid surfaces has been regarded as one of the most important morphology-dependent characteristics from both fundamental and practical viewpoints, and tremendous scientific interests are concentrated on functional surfaces with special wettability due to their excellent advantages over some particular fields. A superhydrophobic surface with a water contact angle (CA) greater than 150° and a water sliding angle less than 10° has been expected to inhibit snow sticking, contamination, erosion, and even current conduction [1–3]. While superhydrophilic surface with a water CA close to 0° has also prompted extensive interests such as fluid microchips  and papers in ink-jet printing . Recently, with the development of smart devices, such as intelligent microfluidic switch and lab-on-chip systems, reversibly controlling the surface wettability has aroused great interest and been realized by chemical coating the surface with stimuli-responsive organic compounds. Various external inducement have been investigated to trigger this kind of conversion including ultraviolet (UV) light irradiation and dark storage [6, 7], temperature , and electric field .
Being an important semiconductor, ZnO is a direct and wide bandgap (3.37 eV at room temperature), and it has been widely considered as a great electronic and photonic material used in UV photo detector, photocatalyst, gas sensors, solar cells, and others [10–17]. However, nearly all of the efforts are focused on the preparation of one-dimensional (1D) ZnO nanostructured arrays using kinds of approaches but few reports on the design of growing two-dimensional (2D) ZnO nanostructured arrays directly on special substrates [18, 19] at low temperature and studies of their controllable wetting behavior.
In this paper, we report the oriented growth of 2D ZnO nanoflakes on bare aluminum substrate through low temperature hydrothermal route and reveal a detailed evolution of surface morphologies during the growth process. After surface coating with stearic acid (SA) monolayer molecules, the as-grown superhydrophilic surface of ZnO nanoflakes shows superhydrophobic property in the pH range from 2.3 to 12.1, which denotes that water contact angles are larger than 150° for not only pure water but also corrosive liquids, such as acidic and basic solutions, and the sliding angle is as low as about 8°. Also, we investigate the UV-induced chemical decomposition of the coated SA monolayer molecules on the ZnO nanoflake surface by means of X-ray photoelectron spectroscopy (XPS) analysis and CA measurement, and an opposite conversion from hydrophobicity to hydrophilicity is observed under UV irradiation. Therefore, the wettability of this kind of inorganic oxide films can be reversibly switched by alternation of UV irradiation and surface chemical coating with SA molecules.
Fabrication of oriented ZnO nanoflakes and chemical coating
Solution-based hydrothermal growth of 2D ZnO nanoflakes was achieved by dipping aluminum substrates in a capped Pyrex glass bottle filled with 16 mMol zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 16 mMol hexamethylenetetramine (HMT, C6H12N4); all chemicals were of reagent grade. The Pyrex glass bottle was sealed and maintained at a constant temperature of 90°C in a regular laboratory oven, and the reaction time was kept from 5-90 min to study the detailed growth process. Subsequently, aluminum substrates were taken out the solution, thoroughly washed with deionized water to eliminate any residual salts and dried by a nitrogen stream. SA molecules were chemisorbed on the ZnO nanoflake surfaces by immersing the sample (reaction time of 90 min) in an ethanol solution of 8 mMol SA (C18H36O2) for 24 h, followed by rinsing it in absolute ethanol to remove excess reactants, and then dried naturally.
Analysis techniques and UV irradiation
Surface morphology was characterized by field emission scanning electron microscope (FE-SEM, Philips Sirion 200, Philips, Holland, The Netherlands). The X-ray diffraction (XRD) experiment was carried out with a D/max-2200/PC type diffraction, using CuKα radiation (λ = 1.5418 Å). Fourier transform infrared spectrum (FTIR) was measured by a spectrometer (Spectrum 100 FTIR, PerkinElmer, Waltham, MA, USA). An optical contact-angle meter system (Data Physics Instrument GmbH, Filderstadt, Germany) was used for static CA measurement at ambient temperature; liquid droplets of volume approximately 5 μl were suspended with needletube and brought in contact with ZnO nanoflake surface using a computer-controlled device. The sliding angle which reflects the relationship between advancing and receding contact angles was measured by tilting the sample platform of the optical contact-angle meter system until the water droplet rolled off the fixed sample. Surface chemical composition was analyzed by XPS (Kratos AXIS Ultra DLD, Shimadzu Corporation, Hadano, Kanagawa, Japan) at room temperature, the binding energies are calibrated with respect to the signal for adventitious carbon (284.8 eV).
To investigate the effect of UV irradiation on surface wettability of SA-coated ZnO nanoflake arrays, the sample was placed under the UV lamp (ZF-1 UV, Gu Cun, Shanghai, China), which emits UV light with a center wavelength of 254 nm, and light intensity was maintained at about 40 μW/cm2. Water contact angles and XPS peak intensities were recorded at different UV irradiation times at ambient temperature.
Results and discussions
Two sharp peaks at about and indicate the existence of long-chain aliphatic groups and successful coating of SA molecules. The characteristic peak of the COOH group at 1,713 cm-1 disappears after chemisorption, whereas two peaks appear at 1,453 cm-1 and 1,539 cm-1 were assigned to the symmetric and antisymmetric carboxylate ion COO- stretching modes.
After UV irradiation over 5 h, a hydrophilic surface with CA less than 90° can be observed. When UV irradiation time is increased to 20 h, a water CA of about 61° shows. The wettability conversion indicates that UV irradiation efficiently decomposes the alkyl chains of SA on ZnO nanoflake surface in air. It has been revealed that alkylsiloxane monolayers can be slowly decomposed by OH radical and atomic oxygen made from UV dissociation of ozone, which is photogenerated from air . A UV-induced decomposition mechanism of the self-assembled alky chains is also proposed based on the gas-phase oxidation mechanism of alkanes. OH radical and atomic oxygen firstly abstract hydrogen from alkyl chains and then produce alkyl radicals. Then alkyl radicals react to form alkoxy radicals, further producing reactive carbonyls through oxidation. Finally, these carbonyl groups dissociate through photodecomposition or attacked by radicals with the loss of carbon and thereby, gradually reduce the carbon chain length. Another possible decomposition mechanism is proposed on the photocatalytic effects . As we all know, ZnO is a great semiconductor photocatalyst for organic compound degradation [7, 11]. When ZnO nanoflakes are irradiated by UV light with photo energy higher than or equal to its bandgap, electrons in the valence band can be excited to the conduction band with the same amount of holes simultaneously generate in the valence band. The created holes and electrons will migrate to the surface of ZnO nanoflakes and initiate redox reactions with water and oxygen, leading to the decomposition of the alkyl chain of SA molecules in our experimental case. However, due to the weak UV light intensity (40 μW/cm2), CA can not reduce to 0° in a short time. If the UV irradiation time is increased to more than 80 h, a contact angle of less than 10° can be also acquired in this condition.
The wide scan XPS spectrum of the as-grown ZnO nanoflakes (sample A), SA-coated ZnO nanoflakes (sample B), and SA-coated ZnO nanoflakes with UV irradiation of 20 h (sample C) (the peak intensities of samples A and B are added by 40,000 C/s and 20,000 C/s, respectively). Asterisk denotes the detected Al 2p and Al 2 s peaks from the aluminum substrates.
In conclusion, we have demonstrated the oriented growth process of 2D ZnO nanoflakes on aluminum substrate through a low-temperature hydrothermal route; the growth mechanism was proposed on the basis of the Al(OH)4- passivating agent formed by the chemical reaction between OH- and aluminum substrate and then presumably attaches to Zn2+-terminated (001) surface. After chemical coating with SA monolayer molecules onto ZnO nanoflake arrays, surface wettability was converted from superhydrophilicity to superhydrophobicity with a maximum CA up to 157° and a low sliding angle close to 8°. This super water repellent surface revealed a stable property over a wide pH range; however, an opposite wettability conversion to hydrophilicity was observed under UV irradiation because of the cooperation of the surface photosensitivity and special chemical structure of SA molecules. This method possesses the advantages of being both simple and inexpensive, and special wettability of 2D ZnO nanoflakes can be reversibly switched by alternation of chemical coating and UV irradiation which is certainly significant for future industrial applications.
This work was supported by the Natural Science Foundation of China (grant numbers 10874115, 11174197, and 10734020), National Major Basic Research Project of 2010CB933702, Shanghai Nanotechnology Research Project of 0952 nm01900, National 863 Program 2011AA050518863.
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