Superhydrophobic ZnO networks with high water adhesion
© Florica et al.; licensee Springer. 2014
Received: 27 June 2014
Accepted: 1 August 2014
Published: 8 August 2014
ZnO structures were deposited using a simple chemical bath deposition technique onto interdigitated electrodes fabricated by a conventional photolithography method on SiO2/Si substrates. The X-ray diffraction studies show that the ZnO samples have a hexagonal wurtzite crystalline structure. The scanning electron microscopy observations prove that the substrates are uniformly covered by ZnO networks formed by monodisperse rods. The ZnO rod average diameter and length were tuned by controlling reactants' concentration and reaction time. Optical spectroscopy measurements demonstrate that all the samples display bandgap values and emission bands typical for ZnO. The electrical measurements reveal percolating networks which are highly sensitive when the samples are exposed to ammonia vapors, a variation in their resistance with the exposure time being evidenced. Other important characteristics are that the ZnO rod networks exhibit superhydrophobicity, with water contact angles exceeding 150° and a high water droplet adhesion. Reproducible, easily scalable, and low-cost chemical bath deposition and photolithography techniques could provide a facile approach to fabricate such ZnO networks and devices based on them for a wide range of applications where multifunctionality, i.e., sensing and superhydrophobicity, properties are required.
81.07.-b; 81.05.Dz; 68.08.Bc
KeywordsZnO rod networks Chemical bath deposition Ammonia Superhydrophobicity High water adhesion
Zinc oxide, a semiconductor characterized by a direct bandgap (3.37 eV), a large exciton binding energy (60 meV), and a high transmittance of visible light , can be easily engineered to yield functionalities based on its outstanding optical and electrical properties [2–6]. Moreover, as a metal oxide with, probably, the richest family of structural morphology including whiskers, wires, rods, tubes, belts, cages, rings, combs, prisms, etc. [6–11], ZnO may achieve new properties and become a technological key material, its nanostructures representing an interesting choice for the fabrication of electronic and optoelectronic micro/nanodevices. Furthermore, morphology influences other properties such as wettability, another significant characteristic of ZnO-covered surfaces bringing great advantages in a wide variety of applications [12–15]. Recently, special attention has been paid to superhydrophobic ZnO surfaces with high water adhesion [16–18]. The polymorphic properties of ZnO low-dimensional structures triggered different functionalities and therefore enabled different applications. This led to an increased interest in developing new ZnO synthesis methods by various physical (pulsed laser deposition, molecular beam epitaxy, chemical vapor deposition, magnetron sputtering, thermal evaporation) and chemical (chemical bath deposition, electrochemical deposition, hydrothermal, solvothermal, sol-gel, precipitation) techniques [19–24]. Compared to the physical route where harsh conditions such as high temperature or special equipments are usually required and consequently generating high costs, the solution-based chemical approach presents several advantages including the following: easily accessible raw materials, the use of inexpensive equipment, scalability, and control of the morphologies and properties of the final products by changing different experimental parameters. When using low-cost and highly efficient methods, like chemical bath deposition for obtaining desired morphologies, the preparation technique is more and more attractive for mass production.
When designing electronic or optoelectronic micro/nanodevices based on ZnO, a patterning technique such as electron-beam lithography or photolithography is combined with a ZnO preparation method, e.g., hydrothermal growth or chemical bath deposition in order to achieve functionality [25–29]. Photolithography is a conventional patterning approach representing a highly efficient and cost-effective technique of producing metallic electrodes, yielding large patterned surfaces in a short time. On the other hand, the chemical bath deposition is a versatile deposition method with the following main advantages: relatively low process temperature (below 100°C), ambient pressure processing, and the use of inexpensive equipments.
In the present paper, this simple and inexpensive solution process was used to grow ZnO rods quasi-monodispersed in size on Au-patterned SiO2/Si substrate obtained by photolithography. The influence of the reaction parameters, such as reactants' concentration and reaction time, on the morphological, structural, and optical properties of the ZnO rods was studied using scanning electron microscopy, X-ray diffraction, optical spectroscopy, and photoluminescence. In addition, the electrical and the wetting properties of ZnO network rods were investigated. Because the interdigitated electrodes are connected by ZnO networks forming different junctions, no additional lithographical steps are necessary for contacting them. By using two-probe current-voltage measurements, a variation of the ZnO sample resistance was evidenced when these samples were exposed to ammonia. Finally, a superhydrophobic behavior with high water adhesion was observed for all samples regardless of the rod dimensions. Such properties are very helpful for designing devices for sensors, open microfluidic devices based on high adhesive superhydrophobic surface implying no loss of microdroplet reversible transportation , or micro total analysis systems by their synergetic combination.
According to , the ZnO synthesis by chemical bath deposition involves the following chemical reactions:
Zn(NO3)2 → Zn2+ + 2NO3-(a)
(CH2)6N4 + 6H2O → 6HCHO + 4NH3(b)
NH3 + H2O → NH4+ + HO-(c)
Zn2+ + 3NH4+ → [Zn(NH3)4]2+(d)
[Zn(NH3)4]2+ + 2HO- → Zn(OH)2 + 4NH3(e)
Zn(OH)2 → ZnO + H2O(f)
The exact function of the (CH2)6N4 in the ZnO synthesis is still unclear. As a non-ionic cyclic tertiary amine, it can act as a bidentate Lewis ligand capable of bridging two Zn2+ ions in solution . Moreover, it has been suggested  that it is preferentially attached to the non-polar facets of the ZnO crystallite cutting off the access of zinc ions towards those facets, favoring the polar (001) face for growth. (CH2)6N4 is also known as a weak base and pH buffer , being considered a steady source for slow release of HO− ions. All these (CH2)6N4 characteristics influence the nucleation and the growth rates of different ZnO crystal facets, processes responsible for the overall structure and morphology. We investigate the dependence of the ZnO morphology for different reaction parameters varying the precursors' concentration (both reactants with 0.05, 0.1, or 0.2 mM, the Zn(NO3)2/(CH2)6N4 molar ratio was always 1:1) and the deposition time (3 and 6 h). Thus, the synthesized samples were labeled as follows: a (0.05 mM, 3 h), b (0.1 mM, 3 h), c (0.2 mM, 3 h), d (0.05 mM, 6 h), e (0.1 mM, 6 h), and f (0.2 mM, 6 h).
The crystalline phase of the samples was identified by X-ray diffraction (XRD) on a Bruker AXS D8 Advance instrument (Karlsruhe, Germany) with Cu Kα radiation (λ = 0.154 nm). The source was operated at 40 kV and 40 mA and the Kα radiation was removed using a nickel filter.
The optical properties of the ZnO samples were investigated by measuring the total reflection spectra using a PerkinElmer Lambda 45 UV-VIS spectrophotometer (Waltham, MA, USA) equipped with an integrating sphere. The photoluminescence (PL) measurements were performed at 350 nm excitation wavelength using FL 920 Edinburgh Instruments spectrometer (Livingston, UK) with a 450-W Xe lamp excitation and double monochromators on both excitation and emission. All PL spectra were recorded in the same experimental conditions (excitation wavelength = 350 nm, step, dwell time, slits).
The sample morphologies were evaluated using a Zeiss Evo 50 XVP scanning electron microscope (SEM, Oberkochen, Germany).
Electrical measurements were carried out using a Keithley 4200 SCS (Cleveland, OH, USA) and a Cascade Microtech MPS 150 probe station (Thiendorf, Germany). The current-voltage characteristics were obtained by the conventional two-probe method on the samples exposed at different times and at room temperature to ammonia vapors (an area of about 3 mm2 in size contains the patterned metallic stripes and millimeter-sized electrodes).
The wetting properties of the ZnO samples were determined by measuring the static contact angle (CA) with a Drop Shape Analysis System, model DSA100 from Kruss GmbH (Hamburg, Germany) . The sample was placed on a plane stage, under the tip of a water-dispensing disposable blunt-end stainless steel needle with an outer diameter of 0.5 mm. The needle was attached to a syringe pump controlled by a PC for delivery of the water droplet to the test surface. Drop volume was gradually increased until the drop adhered to the surface this being achieved when the volume reached approximately 3 to 4 μl. All the CA measurements were carried out in the static regime at room temperature. The analysis of the CA and of other drop parameters were performed by the PC using the DSA3® software supplied with the instrument. CA was measured by fitting a circle equation to the shape of the sessile drop (due to the sphere-like shape of the drop) and then calculating the slope of the tangent to the drop at the liquid-solid vapor interface line. The camera was positioned in order to observe the droplet under an angle of about 2° to 4° with respect to the plane of the sample surface supporting the droplet. Roll-off angles were measured with a goniometer in order to control the tilt angle. The orthoscopic images were obtained using a commercial photocamera.
Results and discussion
Generally, such high adhesion between a water droplet and a superhydrophobic surface is explained considering the mechanism of the gecko's ability to climb up rapidly smooth, vertical surfaces. Each hair of the gecko's foot produces just a miniscule force through van der Waals' interactions, but millions of hairs collectively create the formidable adhesion . In the present case, the ZnO structure-covered superhydrophobic surface is capable of making close contact with water droplets due to large van der Waals' forces, similar to the effect of the gecko's foot hairs. The high adhesive ability of such a superhydrophobic surface can be applied as a ‘mechanical hand’ in small water droplet transportation without any loss or contamination for microsample analysis [48–51].
Random networks of ZnO rods can be obtained by combining a simple wet chemical route, i.e., chemical bath deposition, with a conventional patterning technique, photolithography. The ZnO rods show a hexagonal wurtzite structure and optical signatures (bandgap value and emission bands) typical for this semiconductor and method of synthesis. The electrical measurements revealed that the ZnO samples can exhibit interesting properties useful for chemical sensing. The contact angle measurements confirm that ZnO structure-covered surfaces present superhydrophobicity, with water contact angles exceeding 150° and a high water droplet adhesion, water volume suspended reaching 20 μl. Such superhydrophobic ZnO rod networks with high water-adhesive force have potential applications in no-loss liquid transportation. Moreover, because of the favorable surface to volume ratio of the 3D interlaced ZnO rods, they are expected to also have potential applications in gas sensors. Therefore, the results described herein regarding multifunctionality of ZnO-covered substrates are of great interest taking into account that the two methods used in sample preparation, chemical bath deposition and photolithography, are low cost and easily scalable, being efficient and suitable techniques for industrial processing.
scanning electron microscope
This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS - UEFISCDI, project number PN-II-RU-TE-2012-3-0148.
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