Enhanced optical and hydrophilic properties of V and La co-doped ZnO thin films

V and La co-doped and undoped ZnO thin films were deposited on a glass substrate via the sol–gel method to investigate the structural, optical, and wettability of ZnO thin film by changing the V-doping concentration. Microstructure and water contact angles of the films were measured by SEM and contact angle goniometer, respectively. SEM studies revealed that the grain size and surface roughness of the film were changed by doping concentration. In addition, the contact angles were studied to find the possible effects of doping on the hydrophilicity of the film, indicating that the ZnO films were hydrophobic in nature. Finally, a good correlation was observed between the SEM micrographs and contact angle results, and the nature of ZnO film was found to be changed from hydrophobic to hydrophilic.     

Superhydrophobic surfaces via controlling the morphology of ZnO micro/nano complex structure

ZnO micro/nano complex structure films, including reticulate papillary nodes, petal-like and flake-hole, have been self-assembled by a hydrothermal technique at different temperatures without metal catalysts. The wettability of the above film surfaces was modified with a simple coating of heptadecafluorodecyltrimethoxy-silane in toluene. After modifying, the surface of ZnO film grown at 50~${^\circ}$C was converted from superhydrophilic with a water contact angle lower than 5$^{\circ}$ to superhydrophobic with a water contact angle of 165$^{\circ}$. Additionally, the surface of reticulate papillary nodes ZnO film grown at 100~${^\circ}$C had excellent superhydrophobicity, with a water contact angle of 173$^{\circ}$ and a sliding angle lower than 2$^{\circ}$. Furthermore, the water contact angle on the surface of petal-like and flake-hole ZnO films grown at 150~${^\circ}$C and 200~${^\circ}$C were found to be 140$^{\circ}$ and 120$^{\circ}$, respectively. The wettability for the samples was found to depend strongly on the surface morphology which results from the growth temperature.     

Superhydrophobic surfaces based on dandelion-like ZnO microspheres

This study presents a simple method to fabricate superhydrophobic surface based on ZnO nanoneedles. ZnO nanoneedles had been constructed on zinc layers by immersing in an aqueous NH₄OH solution at 80 °C. The ZnO films were characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The ZnO films exhibited excellent superhydrophilicity (contact angle for water was 0°), while they changed wettability to superhydrophobicity with a water contact angle greater than 150° after further chemical modification with n-dodecanoic acid. The procedure reported here only needs readily available reagents and laboratory equipments, which can be applied to various substrates of any size and shape.     

Wetting characteristics of ZnO smooth film and nanowire structure with and without OTS coating

ZnO is an important material that is used in a variety of technologies including optical devices, sensors, and other microsystems. In many of these technologies, wettability is of great concern because of its implications in numerous surface related interactions. In this work, the effects of surface morphology and surface energy on the wetting characteristics of ZnO were investigated. ZnO specimens were prepared in both smooth film and nanowire structure in order to investigate the effects of surface morphology. Also, a hydrophobic octadecyltrichlorosilane (OTS) coating was used to chemically modify the surface energy of the ZnO surface. Wettability of the surfaces was assessed by measuring the water contact angle. The results showed that the water contact angle varied significantly with surface morphology as well as surface energy. OTS coated ZnO nanowire specimen had the highest contact angle of 150°, which corresponded to a superhydrophobic surface. This was a drastic difference from the contact angle of 87° obtained for the smooth ZnO film specimen. In addition to the initial contact angle, the evolution of the water droplet with respect to time was investigated. The wetting state of water droplet was analyzed with both Wenzel and Cassie-Baxter models. Spontaneous and gradual spreading, together with evaporation phenomenon contributed to the changing shape, and hence the varying contact angle, of the water droplet over time.     

The conversion of wettability in transparent conducting Al-doped ZnO thin film

We report on the systematic changes of surface wettability in one of the most promising transparent conducting oxide materials, Al-doped ZnO (AZO) thin films. It was revealed that the characteristic surface wettability, which would make a key role in adhesion with other layers of optoelectronic device, can be largely changed by Al concentrations and film growth temperature. Keeping the electrical conductivity constant, the water contact angle (WCA) of a 2 mol% AZO film was changed by about 50 ^°C depending on the surface roughness. In the samples grown at 300 ^°C, the roughness enhancement was large and a hydrophobic surface formed, whereas in the samples grown at 500 ^°C a hydrophilic surface formed. We attributed the variation in surface wettability with growth temperature to changes in surface morphology. This result suggests that 2 mol% Al doping concentration can be considered as a critical concentration in changing of surface morphology of AZO as well as in electrical properties.     

Growth of hierarchical based ZnO micro/nanostructured films and their tunable wettability behavior

Hierarchical zinc oxide (ZnO) micro/nanostructured thin films were grown onto as-prepared and different annealed ZnO seed layer films by a simple two step chemical process. A cost effective successive ionic layer adsorption and reaction (SILAR) method was employed to grow the seed layer films at optimal temperature (80 °C) and secondly, different hierarchical based ZnO structured thin films were deposited over the seed layered films by chemical bath deposition (CBD). The influence of seed layer on the structural, surface morphological, optical and wettability behavior of the ZnO thin films were systematically investigated. The XRD analysis confirms the high crystalline nature of both the seed layer and corresponding ZnO micro/nanostructured films with a perfect hexagonal structure oriented along (0 0 2) direction. The surface morphology revels a complex and orientated hierarchical based ZnO structured films with diverse shapes from plates to hexagonal rod-like crystal to tube-like structure and even much more complex needle-like shapes during secondary nucleation, by changing the seed layer conditions. The water contact angle (WCA) measurements on hierarchical ZnO structured films are completely examined to study its surface wettability behavior for its suitability in future self-cleaning application. Photoluminescence (PL) spectra of the ZnO structured film exhibit UV and visible emissions in the range of 420-500 nm. The present approach demonstrates its potential for low-temperature, large-scale, controlled synthesis of crystalline hierarchical ZnO nanostructures films.     

Extreme wettability due to dendritic copper nanostructure via electrodeposition

Dendritic copper film with convertible extreme wettability is prepared on metal surface via electrodeposition. With field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and electrochemical measurement, the morphology, composition and formation mechanism of dendritic copper film were studied. It is found that the film is mainly composed of metallic copper. Also some residual cuprous oxide and chloride exist in the deposit. The single micron-sized dendrite consists of a main stem with side branches, on which the higher-order branches with the dimension of tens of nanometers grow. A hydrophobic modification can induce the conversion of the apparent wettability of film from super-hydrophilicity (with apparent water contact angle of 5 ± 3°) to super-hydrophobicity (with apparent water contact angle of 154.1 ± 3°), which is due to the capillary effect. The method proposed in this paper is time-saving and facile to operate, and it offers a promising technique to prepare metallic surface with a high wettability contrast for water.     

Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification

A superhydrophobic ZnO nanorod array film on cotton substrate was fabricated via a wet chemical route and subsequent modification with a layer of n-dodecyltrimethoxysilane (DTMS). The as-obtained cotton sample was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning probe microscope (SPM) and X-ray photoelectron spectroscopy (XPS), respectively. The wettability of the cotton fabric sample was also studied by contact angle measurements. The modified cotton fabrics exhibited superhydrophobicity with a contact angle of 161° for 8 μL water droplet and a roll-off angle of 9° for 40 μL water droplet. It was shown that the proper surface roughness and the lower surface energy both played important roles in creating the superhydrophobic surface, in which the Cassie state dominated.     

Wettability of ZnO: A comparison of reactively sputtered; thermally oxidized and vacuum annealed coatings

We have studied the wettability of sputter deposited ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. The X-ray diffraction patterns showed the formation of hexagonal-wurtzite structure of ZnO, which was further confirmed by micro-Raman spectroscopy data. The X-ray photoelectron spectroscopy data indicated that the sputter deposited ZnO coatings were more stoichiometric than thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. The wettability measurements indicated that water contact angles of 158.5° and 155.2° with sliding angles of 2° and 4° were achieved for thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings, respectively. The superhydrophobicity observed in thermally oxidized Zn-ZnO and vacuum annealed coatings is attributed to the nanorod cluster like morphology along with the presence of high fraction of micron scale air pockets. The water droplet on such surfaces is mostly in contact with air pockets rather than solid surface, leading to high contact angle. Whereas, the sputter deposited ZnO coatings exhibited a maximum water contact angle of 110.3°. This is because the sputter deposited ZnO coatings exhibited a densely packed nanograin-like microstructure without any air pockets. The work of adhesion of water was very low for thermally oxidized Zn-ZnO (5.06 mJ/m²) and vacuum annealed ZnO coatings (6.71 mJ/m²) when compared to reactively sputtered ZnO coatings (90.41 mJ/m²). The apparent surface free energy (SFE) for these coatings was calculated using Neumann method and the SFE values for sputter deposited ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings were 32.95, 23.21 and 18.78 mJ/m², respectively.     

Super-hydrophobic nickel films with micro-nano hierarchical structure prepared by electrodeposition

Super-hydrophobic nickel films were prepared by a simple and low cost electrodepositing method. The surface morphologies of the films characterized by scanning electronic microscope exhibit hierarchical structure with micro-nanocones array, which can be responsible for their super-hydrophobic characteristic (water contact angle over 150°) without chemical modification. The wettability of the film can be varied from super-hydrophobic (water contact angle 154°) to relatively hydrophilic (water contact angle 87°) by controlling the size of the micro-nanocones. The mechanism of the hydrophobic characteristic of nickel films with this unique structure was illustrated by several models. Such micro-nanostructure and its special wettability are expected to be applied in the practical industry.     

Tunable surface wettability of ZnO nanorods prepared by two-step method

Zinc oxide (ZnO) nanorods were deposited on silicon substrate by two-step method. The crystal structure, surface morphology and water contact angle (WCA) were measured by X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), and water contact angle apparatus. It is demonstrated that the WCA of the as-grown ZnO nanorods varies between 136° and 43° and the contact angle reduction rate of ZnO nanorods changes rapidly with increasing growth time. The variation of contact angle in the as-grown samples and contact angle reduction rate has been attributed to the combined effects of the proportion of nonpolar planes in the outermost surface, the area fraction of vapor on the surface and the increase of surface energy of ZnO nanorods.     

ZnO/SiO2 复合薄膜的光学性能

采用溶胶-凝胶法在玻璃衬底上制备ZnO/SiO₂复合薄膜,分别用XRD、TEM、SEM对样品的结构和形貌进行表征,并研究了不同ZnO含量对复合薄膜透过率及荧光特性的影响。结果表明,样品经500 ℃退火处理生成了SiO₂和ZnO,其晶粒尺寸为18.7 nm,薄膜具有双层结构。复合薄膜的透过率随着其中ZnO含量的增加而降低,禁带宽度减小,光学吸收边红移。样品在355 nm波长激发下产生了384 nm的紫外发射峰和440 nm的蓝光发射带,并随ZnO含量的增加而增强,它们分别来自ZnO的电子-空穴复合发光和缺陷发光,及ZnO/SiO₂复合薄膜双层结构的缺陷发光。     

Characterization of electrodeposited Bi2S3 thin films by holographic interferometry

Optical non destructive evaluation methods, using lasers as the object illumination source, include holographic interferometry. It is widely used to measure stress, strain, and vibration in engineering structures. Double exposure holographic interferometry (DEHI) technique is used to determine thickness and stress of electrodeposited bismuth trisulphide (Bi₂S₃) thin films for various deposition times. The same is tested for other concentration of the precursors. It is observed that, increase in deposition time, increases thickness of thin film but decreases stress to the substrate. The structural, optical and surface wettability properties of the as deposited films have been studied using X-ray diffraction (XRD), optical absorption and contact angle measurement, respectively. The X-ray diffraction study reveals that the films are polycrystalline with orthorhombic crystal structure. Optical absorption study shows the presence of direct transition with band bap 1.78 eV. The water contact angle measurement shows hydrophobic nature of Bi₂S₃ thin film surface.     

Residual stress induced wetting variation on electric brush-plated Cu film

Nanocrystalline Cu film with a mirror surface finishing is prepared by the electric brush-plating technique. The as- prepared Cu film exhibits a superhydrophilic behavior with an apparent water contact angle smaller than 10°. A subsequent increase in the water contact angle and a final wetting transition from inherent hydrophilicity with water contact angle smaller than 90° to apparent hydrophobicity with water contact angle larger than 90° are observed when the Cu film is subjected to natural aging. Analysis based on the measurement of hardness with nanoindentation and the theory of the bond-order-length-strength correlation reveals that this wetting variation on the Cu film is attributed to the relaxation of residual stress generated during brush-plating deposition and a surface hydrophobization role associated with the broken bond polarization induced by surface nanostructure.     

Synthesis and characterization of superhydrophobic functionalized Cu(OH)2 nanotube arrays on copper foil

Superhydrophobic functionalized cupric hydroxide (Cu(OH)₂) nanotube arrays were prepared on copper foils via a facile alkali assistant surface oxidation technique. Thus nanotube arrays of Cu(OH)₂ were directly fabricated on the surface of copper foil by immersing in an aqueous solution of NaOH and (NH₄)₂S₂O₈. The wettability of the surface was changed from surperhydrophilicity to superhydrophobicity by chemical modification with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS). The morphologies, microstructures, crystal structure, chemical compositions and states, and hydrophobicity of the films on the copper foil substrates were analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. It was found that the rough structure of the surface helped to magnify the wettability. The static contact angle (CA) for water is larger than 160° and the contact angle hysteresis (CAH) is lower than 5° on the modified surface. The high roughness of the nanotube arrays along with the generated C-F chains by chemical modification contributed to the improved superhydrophobicity. The present research is expected to be significant in providing a new strategy for the preparation of novel multifunctional materials with potential industrial applications on copper substrates.     

ZnO纳米线薄膜的合成参数、表面形貌和接触角关系研究

水浴法合成ZnO纳米线薄膜的工艺参数直接影响其表面形貌, 并使其接触角及润湿性能发生变化. 本文仿真分析了轮廓算数平均偏差、偏斜度、峭度、相关长度等特征参数对随机粗糙表面特性的影响规律; 改变生长时间、种子层溶液和生长液的浓度, 批量制备了表面形貌不同的ZnO纳米线薄膜; 提出了取样长度的确定方法, 并基于扫描电镜图像和Matlab图像处理算子对ZnO纳米线薄膜表面形貌的特征参数进行了提取; 将表面形貌高度和水平方向的特征参数引入Wenzel模型, 分析了合成参数、表面形貌特征参数与接触角的影响关系. 结果表明, 合成参数变化时, 选择取样长度5.0 μm为宜; 生长液浓度大于0.125 mol/L时, ZnO纳米线之间发生重结晶, 并呈现疏水性; 改变种子层溶液浓度和生长时间, 均得到超亲水表面. 上述结论可用于不同氧化酶、细胞等在ZnO纳米线薄膜上的有效吸附及相应传感器测试性能的进一步提高. 关键词： ZnO纳米线 水浴合成 表面形貌 接触角     

Rapid fabrication of superhydrophobic surfaces on copper substrates by electrochemical machining

Hierarchical micrometer-nanometer-scale binary rough structures were fabricated on copper substrates by electrochemical machining in a neutral NaCl electrolyte. The rough structures are composed of the micrometer scale potato-like structures and the nanometer scale cube-like structures. After modified by the fluoroalkylsilane, the copper surfaces reached superhydrophobicity with a water contact angle of 164.3° and a water tilting angle less than 9°. This method has a high processing efficiency which can take just 3 s to fabricate the roughness required by the superhydrophobic surface. The effect of the processing time on wettability of the copper surfaces was investigated in this paper. The possible mechanism of the formation of the hierarchical roughness was also proposed, and the wettability of the copper surfaces was discussed on the basis of the Cassie-Baxter theory.     

Angle-sensitive and fast photovoltage of silver nanocluster embeded ZnO thin films induced by 1.064-μm pulsed laser

Silver nanocluster embedded ZnO composite thin film was observed to have an angle-sensitive and fast photovoltaic effect in the angle range from –90o to 90o, its peak value and the polarity varied regularly with the angle of incidence of the 1.064-μ m pulsed Nd:YAG laser radiation onto the ZnO surface. Meanwhile, for each photovoltaic signal, its rising time reached ～2 ns with an open-circuit photovoltage of ～2 ns full width at half-maximum. This angle-sensitive fast photovoltaic effect is expected to put this composite film a candidate for angle-sensitive and fast photodetector.     

Superhydrophobic poly(vinylidene fluoride) film fabricated by alkali treatment enhancing chemical bath deposition

Based on the lotus effect principle, the superhydrophobic poly(vinylidene fluoride) (PVDF) film was successfully prepared by the method of alkali treatment enhancing chemical bath deposition. The surface of PVDF film prepared in this work was constructed by many smooth and regular microreliefs. Oxygen-containing functional groups were introduced in PVDF film by treatment with aqueous NaOH solution. The nano-scale peaks on the top of the microreliefs were implemented by the reaction between dimethyldichlorosilane/methyltrichlorosilane solution and the oxygen-containing functional groups of PVDF film. The micro- and nano-scale structures, similar to the lotus leaf, was clearly observed on PVDF film surface by scanning electronic microscopy (SEM) and atomic force microscope (AFM). The water contact angle and sliding angle on the fabricated lotus-leaf-like PVDF film surface were 157° and 1°, respectively, exhibiting superhydrophobic property and self-cleaning property.     

Preparation of superhydrophobic surface with a novel sol-gel system

Sol-gel method is a simple and cheap way to prepare superhydrophobic coatings or films, however, most of the researches on sol-gel focus on silica or ZnO sol-gel. The present paper proposes a novel sol-gel which is made from hydrolysis and condensation of the by-product of polymethylhydrosiloxane (PMHS) reacting with γ-aminopropyltriethoxysilane (KH550). The mechanism of formation of the by-product and the sol-gel is discussed and the by-product is characterized by FT-IR. The mass ratio of KH550/PMHS of the sol-gel influences the water contact angle (WCA) and water sliding angle (WSA) of the film made of spraying the sol-gel to microscope glass. When the mass ratio of KH550/PMHS of the sol-gel reaches 0.25, WCA of the corresponding film is 157° and WSA of it is less than 1°. The mechanism of formation of the sol-gel is discussed, and the size of the sol-gel is characterized by polarization microscope as well. The morphology of the film made of the sol-gel is analyzed by means of scanning electron microscope (SEM). It was found that the diameter of the particle of the superhydrophobic film is about 40 μm, nevertheless, from the larger magnification picture, the particle is found to be composed of micro-balls whose diameter is about 2 μm, and the micro-ball is composed of nano-sphere whose diameter is less than 200 nm.