Synthesis and characterization of superhydrophobic wood surfaces

Superhydrophobic films were developed on wood substrates with a wet chemical approach. Growth of zinc oxide (ZnO) nanorods was found differentially in the cross‐sectional walls and inner lumenal surfaces. The surface roughness of the prepared films on the inner lumenal surface conformed to the Cassie–Baxter wetting model, whereas the roughness across the microsurface of the cell wall was in conformity with the hydrophobic porous wetting model. The space between the ZnO nanorods and the microstructure of the wood surface constituted the nanoscale and microscale roughness of the ZnO nanofilm, respectively. The water contact angle of the prepared wood surfaces was up to 153.5°. In the prepared films, monolayers of stearic acid molecules were self‐assembled on the ZnO nanorods, which in turn, were attached to the wood surface via dimeric bonds. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Superhydrophobic surface decorated with vertical ZnO nanorods modified by stearic acid

Static and dynamic wettability of the ZnO nanorod surface prepared by a facile and inexpensive route is reported. The wettability of the ZnO surface was controlled and tuned by post hydrophobization using different stearic acid concentrations. The surface of the ZnO nanorods modified with 8 mM stearic acid showed a static water contact angle of 152° and sliding angle of 9°, which indicates superhydrophobicity. This suggests that the combination of the rough structures achieved by the ZnO nanorods and low surface energy provided by stearic acid modification results in superhydrophobicity and a very low sliding angle. The crystal structure, surface chemical elements, surface morphology, surface roughness, and static and dynamic water contact angles of the ZnO coatings were studied in detail. Further, the surface properties were assessed by calculating the surface free energies and work of adhesion for unmodified and stearic-acid-modified ZnO nanostructure surfaces. These coatings can find potential industrial applications in the electronic industry.     

Sol–gel synthesized zinc oxide nanorods and their structural and optical investigation for optoelectronic application

Nanostructured zinc oxide (ZnO) nanorods (NRs) with hexagonal wurtzite structures were synthesized using an easy and low-cost bottom-up hydrothermal growth technique. ZnO thin films were prepared with the use of four different solvents, namely, methanol, ethanol, isopropanol, and 2-methoxyethanol, and then used as seed layer templates for the subsequent growth of the ZnO NRs. The influences of the different solvents on the structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, and photoluminescence. The obtained X-ray diffraction patterns showed that the synthesized ZnO NRs were single crystals and exhibited a preferred orientation along the (002) plane. In addition, the calculated results from the specific models of the refractive index are consistent with the experimental data. The ZnO NRs that grew from the 2-methoxyethanol seeded layer exhibited the smallest grain size (39.18 nm), largest diffracted intensities on the (002) plane, and highest bandgap (3.21 eV).     

Microcontact Printing of Organic Self-Assembled Monolayers for Patterned Growth of Well-Aligned ZnO Nanorod Arrays and their Field-Emission Properties

This paper describes a simple method for preparing well-aligned ZnO nanorod arrays in a more tunable fashion, which enables the synthesis of nanorods directly in various patterns and the easy control of the array density. This method is based on a combination of the microcontact printing process for patterning and a solution approach for depositing ZnO nanorods. The growth behavior between the contact and noncontact areas is investigated. Different formation mechanisms are proposed, and it is found that the key difference between nanorod and microrod forms was the ZnO seed layer and the van der Waals force at specific conditions. The role of self-assembled monolayers of octadecyl-trichloro-silane in the reaction solution is also discussed. Wettability of the surfaces is assessed by measuring the water contact angle, and the results show significant variation with surface morphology, from 17.6° to 123.6°. The lowest turn-on applied field strength is 4.65 V/μm at the current density of 10 μA/cm², which is achieved by the lowest array density of nanorods. The field-emission characteristics of the nanorods are found to be highly reproducible. The results could be valuable for the application of field-emission-based devices using ZnO nanorod arrays as cathode materials.     

Enhanced wettability performance of ultrathin ZnO nanotubes by coupling morphology and size effects

In this work, we report on the detailed characterization and mechanism analysis of the improved wettability performance of a new type of ZnO nanostructure, the ultrathin ZnO nanotube, whose growth is induced by screw-dislocation. The newly discovered enhanced wettability properties are suggested to be caused by coupling the morphology and size effects of the nanostructured surface. These ultrathin nanotubes with low density and small dimension form a wet-hair-like hierarchical morphology, which shows a further improved superhydrophobic property with an 8.6 ± 1.6° larger contact angle than that of ZnO nanorods due to the morphology effect. In addition, owing to the large surface to volume ratio and increased effective UV-irradiated area of the ultrathin tubular structure, the ZnO nanotubes exhibit ～5 times faster superhydrophobicity to superhydrophilicity conversion speed than nanorods under 254 nm UV illumination. Furthermore, UV light with a wavelength of 254 nm exhibits ～40 times faster wettability conversion speed for nanotubes than that of 365 nm, which is suggested to be a result of the band gap shift at the nanoscale. The combined advantages of enhanced superhydrophobicity, improved sensitivity, and faster conversion speed by coupling morphology and size effects of these ZnO nanotubes should give them broad applications in self-cleaning surfaces and wettability switches.     

水热合成法制备ZnO纳米棒有序阵列薄膜

采用水热合成法在预先生长的ZnO种子层的玻璃衬底上制备出ZnO纳米棒有序阵列薄膜。通过X射线衍射、扫描电镜、透射电镜和选区电子衍射分析表明：所制备的薄膜由垂直于ZnO种子层的纳米棒组成,呈单晶六角纤锌矿ZnO结构,且沿[001]方向择优生长,纳米棒的平均直径和长度分别为10．0nm和3．3μm。     

Efficient piezoelectric ZnO nanogenerators based on Au-coated silica sphere array electrode

We reported ZnO nanorod-based piezoelectric nanogenerators (NGs) with Au-coated silica sphere array as an efficient top electrode. This electrode can readily bend the ZnO nanorods due to its enhanced surface roughness, thus resulting in more increased and regular piezoelectric charge output. Under a low external pushing force of 0.3 kgf, the output current and voltage were increased by approximately 2.01 and 1.51 times, respectively, in comparison with a conventional Au top electrode without silica spheres. Also, the effect of Au-coated silica spheres on the bending radius of ZnO nanorods was theoretically investigated.     

ZnO纳米棒水热法生长与表征

采用两步法在FTO导电玻璃衬底上制备ZnO纳米棒,首先利用浸渍-提拉法在FTO导电玻璃衬底上制备ZnO晶种层,然后把有ZnO晶种层的FTO衬底放入盛有生长溶液的反应釜中利用水热法制备ZnO纳米棒.研究了生长溶液的浓度、生长温度和生长时间对所制备的对ZnO纳米棒阵列的微结构和光致发光性能的影响,利用X射线衍射(XRD)、扫描电子显微镜(SEM)和光致发光谱(PL)研究了ZnO样品的结构、形貌和光学性质.实验结果表明:所制备的ZnO纳米棒呈现六方纤锌矿结构,沿(002)晶面择优取向生长,纳米棒的平均直径约为100 nm,长度约为2.5 μm.所制备的ZnO纳米棒在390 nm附近具有很强的紫外发光峰和在550 nm附近有较弱的宽绿光发光峰.     

Growth and morphology of ZnO nanorods prepared from Zn(NO3)2/NaOH solutions

ZnO nanorods on ZnO-coated seed surfaces were fabricated by a solution chemical method using supersaturated Zn(NO₃)₂/NaOH solution. The seed surfaces were coated on glass substrates by sol–gel processing and PEG addition. The mechanism of crystal growth and the factors affecting the rod growth were elucidated. The morphology and structure of both the seed surface and successive nanorods were analyzed by using SEM, XRD, TEM and SAED. Nucleation on the ZnO seed surface is crucial for rod growth since rods can only be observed on ZnO-coated substrates. Supersaturation is also required for rod growth and the Zn²⁺ ion and NaOH concentrations must be varied synchronously to maintain the high level of supersaturation. The average diameter and length of the ZnO nanorods increase to different degree with increasing precursor concentration. The dependence of rod growth on temperature shows that the maximum rod growth rate at any given concentration of Zn²⁺ occurs at a specific temperature, and the optimal temperature increases with Zn²⁺ ion concentration. Densely thick nanorods oriented perpendicularly to the substrate can be obtained by controlling the seed surface with PEG assistance.     

Hydrothermal Growth of Vertical ZnO Nanorods

Vertically aligned, single crystalline ZnO nanorods with a high packing density and diameter of ∼60 nm have been successfully synthesized via a low-temperature hydrothermal route on glass substrates pre-deposited with a ZnO seeding layer. The seeding layer exhibits an epitaxial effect on the growth and alignment of the ZnO nanorods. This epitaxial effect can arise from two considerations, namely the crystalline orientation and surface roughness of the seeding layer, which can be controlled by the curing temperature. The ZnO seeding layer that was cured at 350°C exhibited a preferred (0002) crystalline orientation of wurtzite hexagonal structure and a low surface roughness. It was demonstrated to promote the vertical growth of ZnO nanorods. The ZnO nanorods grew in an almost linear relationship with hydrothermal time up to 8 h, but thereafter started to dissolve as the reaction time extended beyond 8 h, due to competition from the homogeneous nucleation of ZnO microparticles in the solution.     

Morphological exploration of chemical vapor–deposited P-doped ZnO nanorods for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is beneficial and has received attractive attention due to a greater potential to generate hydrogen and oxygen from water by using plentiful solar light to solve the problem of energy crisis. Various active semiconductor materials are used in PEC water splitting applications. Nevertheless, in past decades, most of the researchers suggested that titanium oxide (TiO₂) is the best photoanode for this type of applications. Now, Zinc oxide (ZnO) is considered a perfect substitution to TiO₂ due to its comparable energy band structure and superior photogenerated electron transfer rate. In this study, bare and phosphorous-doped ZnO nanorods were successfully developed on fluorine-doped tin oxide-coated glass (FTO) substrate by chemical vapor deposition. X-ray diffraction (XRD) pattern authenticated hexagonal structure formation with strong diffraction peak of (101), which showed that ZnO nanorods were perfectly developed along c axis. The optical and morphological properties were analyzed by UV–Vis and scanning electron microscopy images. The energy-dispersive X-ray spectra demonstrated that doping agent phosphorous was present in ZnO nanorods. The PEC properties of the developed ZnO nanorods were further investigated and obtained results suggested that a small amount of phosphorous-doped ZnO nanorods enhances their PEC performance.     

Selective patterning of ZnO nanorods on silicon substrates using nanoimprint lithography

In this research, nanoimprint lithography (NIL) was used for patterning crystalline zinc oxide (ZnO) nanorods on the silicon substrate. To fabricate nano-patterned ZnO nanorods, patterning of an n-octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on SiO₂ substrate was prepared by the polymer mask using NI. The ZnO seed layer was selectively coated only on the hydrophilic SiO₂ surface, not on the hydrophobic OTS SAMs surface. The substrate patterned with the ZnO seed layer was treated with the oxygen plasma to oxidize the silicon surface. It was found that the nucleation and initial growth of the crystalline ZnO were proceeded only on the ZnO seed layer, not on the silicon oxide surface. ZnO photoluminescence spectra showed that ZnO nanorods grown from the seed layer treated with plasma showed lower intensity than those untreated with plasma at 378 nm, but higher intensity at 605 nm. It is indicated that the seed layer treated with plasma produced ZnO nanorods that had a more oxygen vacancy than those grown from seed layer untreated with plasma. Since the oxygen vacancies on ZnO nanorods serve as strong binding sites for absorption of various organic and inorganic molecules. Consequently, a nano-patterning of the crystalline ZnO nanorods grown from the seed layer treated with plasma may give the versatile applications for the electronics devices.     

Fabrication of superhydrophobic surfaces based on PDMS coated hydrothermal grown ZnO on PET fabrics

Abstract

Wetting behavior of Zinc Oxide (ZnO) based nanomaterials has been the subject of intense investigations and is an active research field for various engineering applications and modifying the surface wettability of ZnO is of great interest. In this study, one-dimensional (1?D) semiconducting ZnO nanorods are grown on a superhydrophobic polyethylene terephthalate (PET) fabric using a hydrothermal method. A facile polydimethylsiloxane (PDMS) coating is applied onto the ZnO grown PET fabrics to improve the hydrophobicity. A wide range of characterization techniques such as field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), UV-vis spectroscopy and contact angle measurement are used to explore the morphology and wetting behavior of the as-prepared samples. The measured water contact angle (WCA) is >150° indicating its superhydrophobicity. This study reports an efficient way to obtain highly hydrophobic semiconducting ZnO grown on PET fabric, which can be of great interest for many future applications.     

Structural and optical properties of ZnO nanorods by electrochemical growth using multi-walled carbon nanotube-composed seed layers

We reported the enhancement of the structural and optical properties of electrochemically synthesized zinc oxide [ZnO] nanorod arrays [NRAs] using the multi-walled carbon nanotube [MWCNT]-composed seed layers, which were formed by spin-coating the aqueous seed solution containing MWCNTs on the indium tin oxide-coated glass substrate. The MWCNT-composed seed layer served as the efficient nucleation surface as well as the film with better electrical conductivity, thus leading to a more uniform high-density ZnO NRAs with an improved crystal quality during the electrochemical deposition process. For ZnO NRAs grown on the seed layer containing MWCNTs (2 wt.%), the photoluminescence peak intensity of the near-band-edge emission at a wavelength of approximately 375 nm was enhanced by 2.8 times compared with that of the ZnO nanorods grown without the seed layer due to the high crystallinity of ZnO NRAs and the surface plasmon-meditated emission enhancement by MWCNTs. The effect of the MWCNT-composed seed layer on the surface wettability was also investigated.     

Influence of water content in mixed solvent on surface morphology,wettability, and photoconductivity of ZnO thin films

ZnO thin films have been synthesized by means of a simple hydrothermal method with different solvents. The effect of deionized water content in the mixed solvents on the surface morphology, crystal structure, and optical property has been investigated by scanning electron microscopy, X-ray diffraction, and UV-Vis spectrophotometer. A large number of compact and well-aligned hexagonal ZnO nanorods and the maximal texture coefficient have been observed in the thin film, which is grown in the mixed solvent with x = 40%. A lot of sparse, diagonal, and pointed nanorods can be seen in the ZnO thin film, which is grown in the 40-mL DI water solution. The optical band gap decreases firstly and then increases with the increase of x. Reversible wettability of ZnO thin films were studied by home-made water contact angle apparatus. Reversible transition between hydrophobicity and hydrophilicity may be attributed to the change of surface chemical composition, surface roughness and the proportion of nonpolar planes on the surface of ZnO thin films. Photocurrent response of ZnO thin films grown at different solvents were measured in air. The response duration of the thin film, which is grown in the solvent with x = 40%, exhibits a fast growth in the beginning but cannot approach the saturate current value within 100 s. The theoretical mechanism for the slower growth or decay duration of the photocurrent has been discussed in detail.     

Preparation of a Superhydrophobic ZnO Film on ITO Glass via Electrodeposition Followed by Oxidation. Effect of the Deposition Time

This study investigates the fabrication of a stable superhydrophobic surface with low contact angle (CA) hysteresis using ZnO thin films prepared by cathodic electrodeposition and subsequent gaseous oxidation. The deposition time is a crucial factor in nanostructuring and producing surface roughness of the films. Cathodic electrodeposition for 60 s created a number of nanopillars, which exhibited the highest CA value, i.e., 167.9°. The rough ZnO surface displayed not only enhanced water repellency with low CA hysteresis but also excellent superhydrophobic stability. The application of the Cassie–Baxter model demonstrated that the ZnO nanostructure contributed to increasing the area of a water droplet in contact with air, leading to superhydrophobicity. Such a unique textured surface showed a great potential for the engineering of strong superhydrophobic coatings.     

Farming of ZnO nanorod-arrays via aqueous chemical route for photoelectrochemical solar cell application

A low temperature aqueous chemical route is employed for the synthesis of zinc oxide (ZnO) nanorod arrays onto the soda lime and fluorine-doped tin oxide (FTO) coated glass substrates at various deposition times. Synthesis/farming of ZnO nanorod arrays (ZNRs) consists of the three-step as-ZnO seed forming, ZnO seed sowing followed by ZnO nanorod arrays growing. The length and diameter of ZnO nanorods increased with the reaction time prolonging. The physical, chemical and morphological properties were analyzed by means of X-ray diffraction (XRD), UV–visible spectroscopy (UV–vis), photoluminescence (PL), energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) respectively. The XRD pattern revealed wurtzite crystal structures of ZNRs, preferentially orienting in the (002) direction. SEM micrographs show that the ZnO nanorods grew up perpendicular to the substrate and their length increases with increase in deposition time. Finally, the photoelectrochemical (PEC) performance of ZNRs thin films were studied. The junction quality factor upon illumination (n_l), series and shunt resistance (R_s and R_sh), flat-band-potential (V_FB), fill factor (FF) and efficiency (η) have been estimated.     

Effect of ZnO seed layers on the solution chemical growth of ZnO nanorod arrays

High density ZnO nanorod arrays were grown on Si substrates coated with ZnO seed layers via aqueous solution route. The ZnO seed layers were deposited on the substrate using DC reactive sputtering and RF magnetron sputtering. It was found that ZnO seed layer with (1 0 3) preferred orientation, prepared using DC reactive sputtering, did not facilitate the formation of ZnO nanorods in the solution grown process. Prior seeding of the surface by ZnO layer with (0 0 2) preferred orientation, deposited using RF magnetron sputtering, leads to nucleation sites on which ZnO nanorod arrays can grow in a highly aligned fashion. ZnO nanorods with well-defined hexagonal facets (0 0 2) were grown almost vertically over the entire substrate. The uniformity and alignment of the nanorod arrays are strongly related to the properties of underneath ZnO seed layers.     

Sonochemical synthesis of ZnO‐ZnS core‐shell nanorods for enhanced photoelectrochemical water oxidation

The ultrasonic‐assisted synthesis method provides a fast, simple, and large‐scale route for synthesizing desired materials under ambient conditions. In this work, we report on the facile preparation of ZnO‐ZnS core‐shell nanorods on a fluorine‐doped tin oxide (FTO) substrate. The core‐shell nanorods were synthesized by sequential nanoscale reactions involving the preparation of ZnO nanorods and conversion of the ZnO surface into a ZnS shell on the FTO substrate, using an in situ sonochemical method. The ZnO‐ZnS core‐shell nanorods showed improved photocurrents compared with ZnO nanorods for the water oxidation reaction. During the water oxidation reaction, the ZnS shell passivates the surface‐defects of the ZnO, which results in enhanced charge separation in the ZnO nanorods and higher performance.     

Nucleation and growth of ZnO nanorods on the ZnO-coated seed surface by solution chemical method

ZnO nanorods on ZnO-coated seed surfaces were fabricated by solution chemical method using supersaturated (ZnNO₃)₂/NaOH at 70 °C. The seed surfaces were coated on glass substrates by sol–gel processing, and their texture was dominated by heating temperatures, cooling styles and layer thickness per dipping. The effects of the seed surface on the morphology of the resultant nanorods were primarily discussed. The orientation and morphology of both the seed surface and successive nanorods were analyzed by using XRD and SEM. It is proved that when the seed size increases from 15 to 50 nm with temperature increasing, the average diameter of the resultant nanorods increase from 25 to 50 nm, with a length of 800 nm after growing for 1.5 h. The seed surface prepared by heating at 300–400 °C, fast cooling or drawing at lower speed has better orientation and few surface defects, which leads to higher density of nuclei on the seed surface and thus to the optimal preferred crystal growth of ZnO rods standing perpendicular onto substrates.