Electrochemical deposition and superhydrophobic behavior of ZnO nanorod arrays

Vertically aligned ZnO nanorod arrays with different heights are grown on the ZnO seeded indium tin oxide substrate by cathodic electrochemical deposition from zinc nitrate at two temperatures of 60 °C and 80 °C. As-grown ZnO nanorods exhibit wurzite crystal structure and their heights can be well controlled by different deposition times. The fluorination coating tends to induce a superhydrophobicity of ZnO nanorods, i.e., the maximal value of contact angle: 166.9°. The super water repellency can be attributed to the fact that an air layer is confined in the nanorod arrays, and thus leads to water droplets sitting on the ZnO surfaces, referring as Cassie state. Interestingly, their water contact angles are found to vary with the heights of ZnO nanorods, ranged from 99.8 to 746 nm. The superhydrophobicity of ZnO surfaces can be well predicted by a proposed model that is capable of determining the wetted fraction of ZnO pillars. This satisfactory result would shed one light on how the variation of rod height would induce the superhydrophobic behavior of ZnO nanorod arrays.     

金属锌表面超疏水薄膜的制备及其摩擦学性能

通过简单两步法在金属锌表面构筑超疏水薄膜, 锌片首先经N,N-二甲基甲酰胺(DMF)处理在表面构筑微纳结构薄膜, 然后在表面覆盖硬脂酸薄膜以实现超疏水. 采用扫描电子显微镜, 傅里叶红外光谱仪和接触角测量仪等手段表征了超疏水表面的形成机制和表面形貌, 并利用微纳米摩擦磨损试验机研究了超疏水薄膜的减摩耐磨特性. 研究结果发现, 在锌表面形成了一层纳米棒状结构的超疏水薄膜, 水的接触角可达155^o. 超疏水薄膜具有明显的减摩和耐磨特性, 这可归因于DMF处理导致的表面微织构化效应以及脂肪酸自组装薄膜的纳米润滑效应.     

Rapid formation of superhydrophobic surfaces with fast response wettability transition

We have developed a facile and time-saving method to prepare superhydrophobic surfaces on copper sheets. Various surface textures composed of Cu(OH)2 nanorod arrays and CuO microflowers/Cu(OH)2 nanorod arrays hierarchical structure were prepared by a simple solution-immersion process. After chemical modification with stearic acid, the wettability of the as-prepared surfaces was changed from superhydrophilicity to superhydrophobicity. The shortest processing time for fabricating a superhydrophobic surface was 1.5 min. Interestingly, the rapid wettability transition between superhydrophobicity and superhydrophilicity can be realized on the prepared surfaces with ease by the alternation of air-plasma treatment and stearic acid coating. It took just 2 min to complete the whole wettability transition. Additionally, the regeneration of the superhydrophobic surface is also considered regarding its application.     

ZnO/E-51复合涂料上超疏水表面的制备

采用ZnO和环氧树脂机械搅拌制备ZnO/E-51复合涂料,通过真空袋压、室温固化成型,再通过化学刻蚀与表面修饰,在ZnO/E-51复合涂料上制备出超疏水表面。采用扫描电镜和动/静态接触角分析仪,表征表面的形貌和疏水性。结果表明化学刻蚀在复合涂料表面构建了具有微-纳米尺度二元粗糙结构;采用1%(质量分数)的硬脂酸修饰,可改变复合涂料表面微-纳米尺度二元粗糙结构,影响表面的疏水性能,当修饰时间为30min时,其表面与水的平均接触角最高达152.21°。     

Fabrication of ZnS/ZnO hierarchical nanostructures by two-step vapor phase method

A simple two-step vapor phase method is presented to fabricate ZnS/ZnO hierarchical nanostructures in bulk quantities. That is ZnS nanobelts were first synthesized and then used as substrate for growth of ZnO nanorod arrays. Investigation results demonstrate that the polar surfaces of ZnS nanobelts could induce a preferred asymmetric growth of ZnO nanorods on the side surfaces. But it is believed that if the local concentration of ZnO was high enough, ZnO nanorods could also grow symmetrically on the top/bottom surface of the ZnS nanobelts. The optical property of the products was also recorded by means of photoluminescence (PL) spectroscopy.     

Facile fabrication of superhydrophobic Cu(OH)2 nanorod and CuO nanosheet arrays on copper surface

Cu(OH)2 nanorod and CuO nanosheet arrays have been successfully grown on the copper surfaces by a simple one-step solution-immersion process at ambient temperature and pressure. After the chemical modification with 1H, 1H, 2H, 2H-Perfluorodecyltriethoxysilane, the wettability of the copper substrate changed from superhydrophilic to superhydrophobic. Meanwhile, the sliding angle of the superhydrophobic surface is less than 5 degrees. It is confirmed that both the synergic effect of the surface morphology and the surface free energy contribute to this unique surface water repellence. Furthermore, the as-prepared surfaces were stable even after a long-term storage, and retained good superhydrophobicity for corrosive liquids. Such special superhydrophobic properties will greatly extend the applications of copper in many other important industrial fields.     

ZnO纳米棒阵列和薄膜的同步外延生长及其光电化学性能

使用化学气相沉积法在a面蓝宝石衬底上同步外延生长氧化锌(ZnO)竖直纳米棒阵列和薄膜,研究了阵列和薄膜的光电化学性能。结果表明,纳米结构中的竖直单晶纳米棒有六棱柱形和圆柱形,其底部ZnO薄膜使竖直纳米棒互相联通。与ZnO纳米薄膜的比较表明,这种纳米结构具有优异的光电化学性能,其入射光电流效率是ZnO纳米薄膜的2.4倍;光能转化效率是ZnO纳米薄膜的5倍。这种纳米结构优异的光电化学性能,可归因于其高表面积-体积比以及其底部薄膜提供的载流子传输通道。本文分析了这种纳米结构的生长过程,提出了协同生长机理：Au液化吸收气氛中的Zn原子生成合金,合金液滴过饱和后ZnO开始成核,随后在衬底表面生成了ZnO薄膜。同时,还发生了Zn自催化的气-固(VS)生长和Au催化的气-液-固(VLS)生长,分别生成六棱柱纳米棒和圆柱形纳米棒,制备出底部由薄膜连接的竖直纳米棒阵列。     

Charge influence and growth mechanism of ZnO nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystalization and formation of ZnO nanorods.     

Charge influence and growth mechanism of ZnO nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystallization and formation of ZnO nanorods.     

Position and density control in hydrothermal growth of ZnO nanorod arrays through pre-formed micro/nanodots

Position-?and density-controlled ZnO nanorod arrays (ZNAs) were successfully grown on a Si substrate through a low temperature (90?°C) hydrothermal approach assisted by pre-formed ZnO micro/nanodots. The ZnO dots on Si substrates were prepared by a spin-coating technique, through which the pattern and density of the dots could be easily changed. Accordingly, the position-?and density-controlled growth of ZNAs was achieved. For the resulting density-controlled ZNAs, the density could range from (5.6 ± 0.01) × 10(2) to (1.2 ± 0.01) × 10(2)?rods?μm(-2). The room-temperature photoluminescence (PL) spectrum of ZNAs exhibited excellent UV emission. The water wettability measurements of the ZNAs with different density showed good hydrophobicity, and the ZNAs with the lowest density revealed a superhydrophobic characteristic with a water contact angle of 166.1°.     

Positive temperature coefficient of resistance of single ZnO nanorods

ZnO nanorods were synthesized by a simple aqueous solution method. Crystal structures and morphology studies show that the ZnO nanorods are single crystalline with the growth direction aligned with the c axis of ZnO. An Au-ZnO nanorod-Au (metal-semiconductor-metal, MSM) device using the synthesized nanorod was fabricated. An electronic model with two back-to-back Schottky diodes in series with a nanorod was used to describe the electrical transport of the MSM device. A positive temperature coefficient of resistance is observed on a single ZnO nanorod from 383 to 473 K. A simple model has been proposed to explain such an abnormal behavior including the effect of the interface states and the adsorption-desorption of the water/oxygen molecules on the surface of the nanorod.     

A one-step process to engineer superhydrophobic copper surfaces

Superhydrophobic surfaces are conventionally prepared employing two steps: roughening a surface and lowering their surface energy. In the present work, a direct voltage (DC) is applied between two copper plates immersed in a dilute ethanolic stearic acid solution. The surface of the anodic copper electrode transforms to superhydrophobic due to a reaction between copper and stearic acid solution. The fabrication process of superhydrophobic copper surfaces is simplified in just one-step. The surface of the anodic copper is found to be covered with flower-like low surface energy copper stearate films providing the water contact angle of 153 ± 2° with the roll-off properties.     

Highly hydrophilic and superhydrophobic ZnO nanorod array films

The wettability control of solid surfaces is important from the aspects of both science and technology. Herein, we report a surface-modification-induced hydrophilicity to superhydrophobicity transition on well-aligned single-crystalline ZnO nanorod array films (ZnO-NAFs). The ZnO-NAFs were prepared from solution by a hydrothermal method and were characterized by X-ray diffraction and scanning electron microscopy. The surface of transparent ZnO-NAFs was highly hydrophilic with a water contact angle of 9.6 ± 0.8°. However, after being exposed to octadecanethiol solution, the surfaces of the ZnO-NAFs became superhydrophobic with a water contact angle of 156.2 ± 1.8°. The present work offers a technique that has great potentials for preparing two-dimensional micro-patterns with a high wettability contrast for water.     

Electrodeposition of template free hierarchical ZnO nanorod arrays via a chloride medium

We have successfully grown template and buffer free ZnO nanorod films via chloride medium by controlling bath temperature in a simple and cost effective electrochemical deposition method. Thin films of ZnO nano-rods were obtained by applying a potential of ?0.75 V by employing Ag/AgCl reference electrode for 4 h of deposition time. The CV measurements were carried out to determine potential required to deposit ZnO nanorod films whereas chronoamperometry studies were carried out to investigate current and time required to deposit ZnO nanorod films. The formation of ZnO nanorod has been confirmed by scanning electron microscopy (SEM) and Raman spectroscopy. Low angle XRD analysis confirms that ZnO nanorod films have preferred orientation along (101) direction with hexagonal wurtzite crystal structure. The SEM micrographs show nice surface morphology with uniform, dense and highly crystalline hexagonal ZnO nanorods formation. Bath temperature has a little influence on the orientation of nanorods but has a great impact on their aspect ratio. Increase in bath temperature show improvement in crystallinity, increase in diameter and uniform distribution of nanorods. Compositional analysis shows that the amount of oxygen is ~49.35 % and that of Zn is ~50.65 %. The optical band gap values were found to be 3.19 and 3.26 eV for ZnO nanorods prepared at bath temperature 70 and 80 °C respectively. These results indicate that by controlling the bath temperature band gap of ZnO nanorods can be tailored. The obtained results suggest that it is possible to synthesize ZnO nanorod films by a simple, cost effective electrodeposition process which can be useful for opto-electronic devices fabrication.     

Effect of Sn doping on structural, optical, electrical and wettability properties of oriented ZnO nanorod arrays

Herein we present a modified sol gel route for the one step fabrication of oriented ZnO nanorod arrays. The method is seed layer free, and nanorods directly attach to a substrate. We also present the effect of tin (Sn) content on the crystallinity, microstructural, optical and electrical properties of the ZnO nanorod arrays. Thermo gravimetric (TG) curves of gel precursors showed that most of the organic groups and other volatiles were removed at about 450 °C. X-ray diffraction patterns confirmed that the films were polycrystalline in nature with (002) preferred orientation. The texture coefficient, grain size, dislocation density and lattice parameters of the ZnO arrays were determined. The SEM micrographs revealed that the undoped and 1 at.%Sn doped films were composed of nanorods and the concentration of 2 at.%Sn doping hindered the rod like structure growth and modulated into granular nature. UV-visible transmission spectroscopy indicated that the transparency of the films increased with Sn content. On Sn doping, the films also exhibited a red shift and slight shrinkage of band gap. The electrical studies revealed that 1 at.% of Sn doping enhanced electrical conduction in ZnO films and beyond that the distortion caused in the lattice reduced the conductivity. The contact angle of the ZnO nanostructures varied between 91° and 115° depending upon the Sn content. Therefore, 1 at.%Sn doping into ZnO nanorods improves the crystallinity, electrical conductivity and water contact angle.     

One-step electrochemical machining of superhydrophobic surfaces on aluminum substrates

A superhydrophobic surface on an aluminum substrate was fabricated by one-step electrochemical machining using the sodium chloride (NaCl) aqueous solution containing fluoroalkylsilane as the electrolyte. The resulting superhydrophobic surfaces showed a static water contact angle of 166° and a tilting angle of about 1°. The morphological features and chemical compositions were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron probe micro-analyzer (EPMA), and Fourier-transform infrared spectrometer (FTIR). It shows that the binary micrometer–nanometer-scale rough structures and the low surface energy coating were present on the aluminum surfaces. The resulting surfaces have good properties of anti-adhesion and self-cleaning. The durability of the superhydrophobic surfaces on aluminum substrates was also investigated. This preparation method is advantageous as it does not require acid electrolyte or a separate process to lower the surface energy, uses simple steps, and is environmental friendly and highly efficient.     

Superhydrophobic surfaces on cotton textiles by complex coating of silica nanoparticles and hydrophobization

By the complex coating of amino- and epoxy-functionalized silica nanoparticles on epoxy-functionalized cotton textiles to generate a dual-size surface roughness, followed by hydrophobization with stearic acid, 1H,1H,2H,2H-perfluorodecyltrichlorosilane, or their combination, superhydrophobic surfaces were prepared. The static water contact angle of the most superhydrophobic sample as prepared reaches 170° for a 5 μL droplet. The wettability and morphology were investigated by contact angle measurement and scanning electron microscopy. Characterizations by Fourier transformation infrared spectroscopy, and thermal gravimetric analysis were also conducted.     

Design of submicron structures with superhydrophobic and oleophobic properties on zinc substrate

A superhydrophobic and oleophobic surface was demonstrated on Zn substrate via a composite method using chemical etching, hydrothermal reaction, and fluorinated modification. The surface morphology with aligned ZnO rods that grew almost perpendicularly on Zn substrate and had flat hexagonal crystallographic planes played a key role in the achievement of the superhydrophobicity and oleophobicity. The Zn surface with aligned ZnO rods was then made superhydrophobic and oleophobic with maximum distilled water and peanut oil contact angles (CAs) of 152° and 146°, respectively, by further fluorinated modification, and the sliding angle (SA) for distilled water was less than 10°. Moreover, on the basis of the classical models (Wenzel's and Cassie's model), an improved model was established to analyze the influence of the surface morphology on the wettability. The effect of the experimental parameters including the hydrothermal temperature and the concentration of the chemical etching agent on the surface morphology and the wettability were examined, and then the optimum parameters were obtained. This method is simple and inexpensive, and has potential application in depositing Zn coating that can provide both corrosion resistance and oleophobicity to the substrate metals.     

Controllable synthesis of vertically aligned ZnO nanorod arrays in aqueous solution

High-density well-aligned ZnO nanorod array was successfully synthesized on a large-area magnetron sputtering deposited Al doped ZnO film-coated Si (AZO/Si) substrate via a convenient solution method. X-ray diffraction and scanning electron microscopy show that the nanorods are well-oriented perpendicular to the substrate. The influences of the reaction temperature, time, on the size and shapes of the as-prepared ZnO nanorods (ZNs) samples have been studied. The length and diameter of the nanorods became bigger when a longer reaction time was used. When the temperature is elevated to 130 degrees C, a new conical ZNs was synthesized. Room-temperature photoluminescence (PL) spectra of all the ZnO products showed a strong ultraviolet (UV) emission. The photoluminescence from free excitons of the ZNs synthesized at higher temperature reflects the high purity and nearly defect free structure of nanorods. The well-aligned feature of the nanorod array is attributed to the nanorods' epitaxial growth from the AZO films.     

Effects of surface potential on growth of ZnO nanorod arrays investigated by electric force microscopy

The electric and Kelvin force probe microscopy were used to investigate the surface potentials on the ZnO seed layer, which shows a remarkable dependence on the annealing temperature. The optimum temperature for the growth of nanorod arrays normal to the surface was found to be at 600 degrees C, which is in the range of right surface potentials and energy measured between 500 degrees C and 700 degrees C. We demonstrated from both electric and Kelvin force probe microscopy studies that surface potential controls the growth of ZnO nanorods, illustrating the fact that this is a promising technique to visualize the control of ZnO nanorod arrays by studying their surface potentials. This study will provide important understanding of growth of other nanostructures.