Construct hierarchical superhydrophobic silicon surfaces by chemical etching

We present a simple approach for preparing hydrophobic silicon surfaces by constructing silicon nanowire arrays using Ag-assisted chemical etching without low-surface-energy material modification. The static and dynamic wetting properties of the nanostructured surfaces and their dependence on etching conditions were studied. It was revealed that the surface topologies of silicon nanowire arrays and their corresponding wetting properties could be tuned by varying the etching time. Under optimized etching conditions, superhydrophobic surfaces with an apparent contact angle larger than 150 degrees and a sliding angle smaller than 10 degrees were achieved due to the formation of a hierarchical structure. The origin of hydrophobic behavior was discussed based on Wenzel and Cassie models. In addition, the effects of surface modification of Si surface nanostructures on their hydrophobic characteristics were also investigated.     

Wetting behavior and nanotribological properties of silicon nanopatterns combined with diamond-like carbon and perfluoropolyether films

A large number of silicon (Si) patterns consisting of nanopillars of varying diameter and pitch have been fabricated and further coated with diamond-like carbon (DLC) and perfluoropolyether (Z-DOL) films. The wetting behavior and nano-adhesion/friction of the patterns are investigated experimentally in relation to the nanostructures and the hydrophobicity of the materials. Measurements of water contact angle illustrate that the patterning-enhanced wettability of the Si flat surface, along with two distinct wettings which are in good agreement with the Wenzel and hemi-wicking states, depended on the value of the pitch-over-diameter ratio. In the case of the coated patterns, three wetting states are observed: the Cassie-Baxter, the Wenzel, and a transition from the Cassie-Baxter into the Wenzel, which varies with regard to the hydrophobic properties of the DLC and Z-DOL. In terms of tribological properties, it is demonstrated that a combination of the nanopatterns and the films is effective in reducing adhesive and frictional forces. In addition, the pitch and diameter of the patterns are found to significantly influence their adhesion/friction behaviors.     

Monolithic Polymer Nanoridges with Programmable Wetting Transitions

This paper describes polymeric nanostructures with dynamically tunable wetting properties. Centimeter‐scale areas of monolithic nanoridges can be generated by strain relief of thermoplastic polyolefin films with fluoropolymer skin layers. Changing the amount of strain results in polyolefin ridges with aspect ratios greater than four with controlled feature densities. Surface chemistry and topography are demonstrated to be able to be tailored by SF₆‐plasma etching to access multiple wetting states: Wenzel, Cassie–Baxter, and Cassie‐impregnating states. Reversible transitions among the wetting states can be realized in a programmable manner by cyclic stretching and reshrinking the patterned substrates without delamination and cracking.     

Micro/nano engineering on stainless steel substrates to produce superhydrophobic surfaces

Creating micro-/nano-scale topography on material surfaces to change their wetting properties has been a subject of much interest in recent years. Wenzel in 1936 and Cassie and Baxter in 1944 proposed that by microscopically increasing the surface roughness of a substrate, it is possible to increase its hydrophobicity. This paper reports the fabrication of micro-textured surfaces and nano-textured surfaces, and the combination of both on stainless steel substrates by sandblasting, thermal evaporation of aluminum, and aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). Meanwhile, fluorinated carbon films were used to change the chemical composition of the surfaces to render the surfaces more hydrophobic. These surface modifications were investigated to create superhydrophobic surfaces on stainless steel substrates. The topography resulting from these surface modifications was analyzed by scanning electron microscopy and surface profilometry. The wetting properties of these surfaces were characterized by water contact angle measurement. The results of this study show that superhydrophobic surfaces can be produced by either micro-scale surface texturing or nano-scale surface texturing, or the combination of both, after fluorinated carbon film deposition.     

Surface energy-controlled in-plane growth of t-Se nanowires transformed from a-Se colloids

This study investigated the surface energy-controlled transformation of amorphous Se (a-Se) colloids into trigonal Se (t-Se) crystals on solid substrates. Hydrophilic surfaces generated nanowires laterally grown along the surface of the substrates, while on hydrophobic surfaces the nanowires on the colloidal surface were randomly oriented. The ripening was considered to govern the growth of t-Se nanowires. The in-plane growth of the nanowires along the substrates made possible the creation of a chemically interconnected nanowire network because the nanowires made branches on meeting other a-Se colloids and formed chemical junctions on encountering other growing nanowires.     

Hyperhydrophilic rough surfaces and imaginary contact angles

The complete wetting of rough surfaces is only poorly understood, since the underlying phenomena can neither be described by the Cassie‐Baxter nor the Wenzel equation. An experimental accessiblility by the sessile drop method is also very limited. The term “superhydrophilicity” was an attempt to understand the wetting of rough surfaces, but a clear definition is still forthcoming, mainly because non‐superhydrophilic surfaces can also display a contact angle of zero. Since the Wilhelmy balance is based on force measurements, it offers a technology for obtaining signals during the whole wetting process. We have obtained evidence that additional forces occur during the complete wetting of rough surfaces and that mathematically contact angles for a hydrophilicity beyond the contact angle of zero can be defined by imaginary numbers. A hydrophilized TPS‐surface obtained by chemical wettability switching from a superhydrophobic surface has been previously characterized by dynamic imaginary contact angles of 20i°–21i° and near‐zero hysteresis. Here an extremely high wetting rate is demonstrated reaching a virtual imaginary contact angle of Θ_V,Adv > 3.5i° in less than 210 ms. For a rough surface displaying imaginary contact angles and extremely high wetting rates we suggest the term hyperhydrophilicity. Although, as will be shown, the physical basis of imaginary contact angles is still unclear, they significantly expand our methodology, the range of wettability measurements and the tools for analyzing rough hydrophilic surfaces. They may also form the basis for a new generation of rationally constructed medicinal surfaces.     

Friction and Wetting Transitions of Magnetic Droplets on Micropillared Superhydrophobic Surfaces

Reliable characterization of wetting properties is essential for the development and optimization of superhydrophobic surfaces. Here, the dynamics of superhydrophobicity is studied including droplet friction and wetting transitions by using droplet oscillations on micropillared surfaces. Analyzing droplet oscillations by high‐speed camera makes it possible to obtain energy dissipation parameters such as contact angle hysteresis force and viscous damping coefficients, which indicate pinning and viscous losses, respectively. It is shown that the dissipative forces increase with increasing solid fraction and magnetic force. For 10 µm diameter pillars, the solid fraction range within which droplet oscillations are possible is between 0.97% and 2.18%. Beyond the upper limit, the oscillations become heavily damped due to high friction force. Below the lower limit, the droplet is no longer supported by the pillar tops and undergoes a Cassie–Wenzel transition. This transition is found to occur at lower pressure for a moving droplet than for a static droplet. The findings can help to optimize micropillared surfaces for low‐friction droplet transport.     

Facile tuning of superhydrophobic states with Ag nanoplates

GaAs wafers have been decorated with Ag nanoplates through direct galvanic reaction between aqueous AgNO₃ solutions and GaAs, resulting in Ag nanoplate/GaAs composite surfaces with varying hydrophobocity after the Ag nanoplates are coated with self-assembled monolayers of alkyl thiol molecules. By carefully controlling the reaction conditions, such as growth time and concentration of the AgNO₃ solution, the size, thickness, and surface roughness of the individual Ag nanoplates can be tuned in order to produce different topographic structures and roughness of the composite surfaces, which in turn infl uences the hydrophobicity of the surfaces. The as-synthesized composite surfaces have been found to exhibit various levels of hydrophobicity and different wetting states such as the Wenzel wetting state, Cassie impregnating wetting state, and Cassie nonwetting state. The relationship between surface structure and hydrophobic state is also discussed. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

OWL-Based Nanomasks for Preparing Graphene Ribbons with Sub-10 nm Gaps

We report a simple and highly efficient method for creating graphene nanostructures with gaps that can be controlled on the sub-10 nm length scale by utilizing etch masks comprised of electrochemically synthesized multisegmented metal nanowires. This method involves depositing striped nanowires with Au and Ni segments on a graphene-coated substrate, chemically etching the Ni segments, and using a reactive ion etch to remove the graphene not protected by the remaining Au segments. Graphene nanoribbons with gaps as small as 6 nm are fabricated and characterized with atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. The high level of control afforded by electrochemical synthesis of the nanowires allows us to specify the dimensions of the nanoribbon, as well as the number, location, and size of nanogaps within the nanoribbon. In addition, the generality of this technique is demonstrated by creating silicon nanostructures with nanogaps.     

Water transport and purification in nanochannels controlled by asymmetric wettability

Biomimetic asymmetric nanochannels have recently attracted increasing attention from researchers, especially in the aspect of the asymmetric wettability (a hydrophilic-hydrophobic system), which can be utilized to control the wetting behavior of aqueous media and to offer a means for guiding water motion. By using molecular dynamics simulations, a design for a potentially efficient water filter is presented based on (n, n) single-walled carbon nanotubes, where n = 6, 8, 10 and 12, asymmetrically modified with hydrophilic groups (carboxyl, -COOH) at one tip and hydrophobic groups (trifluoromethyl, -CF(3) ) at the other. The reduced water density on the hydrophobic sides of the functionalized nanotubes are observed in both pure water and aqueous electrolyte solution, except for the functionalized (6, 6) tube, due to the change of dipole orientation of the single-file water wire within it. The functionalized (8, 8) tube can significantly maintain the low water density on the hydrophobic side. Both (6, 6) and (8, 8) tubes have relatively high energy barriers at their tips for ion permeation, which can be obtained by calculating the potential of mean force. Such tip functionalization of a nanotube therefore suggests the great possibilities of water transport and filtration, dominated by asymmetric wettability. The functionalized (8, 8) tube could act as a nanofluidic channel for water purification, not only for ion exclusion but also as a stable water column structure.     

Self-assembled ferromagnetic cobalt/yttria-stabilized zirconia nanocomposites for ultrahigh density storage applications

We report on a low-cost, innovative approach for synthesizing prepatterned, magnetic nanostructures, the shapes and dimensions of which can be easily tuned to meet requirements for next-generation data storage technology. The magnetic nanostructures consist of self-assembled Co nanodots and nanowires embedded in yttria-stabilized zirconia (YSZ) matrices. The controllable size and aspect ratio of the nanostructures allows the selection of morphologies ranging from nanodots to nanowires. Co nanowires show strong shape anisotropy and large remanence at 300 K. In contrast, Co nanodots display minimal effects of magnetocrystalline anisotropy and superparamagnetic relaxation above the blocking temperature. These prepatterned magnetic nanostructures are very promising candidates for data storage technology with an ultrahigh density of 1 terabit in(-2) or higher.     

Bicrystalline zinc oxide nanocombs

Bicrystalline ZnO nanocombs have been prepared by zinc powder evaporation at 650 degrees C. Structural analysis showed that as-synthesized samples are composed of two crystals that form a twin structure parallel to the (113) plane with the growth direction of the branching nanowires and the main stem closely parallel to (0001) and (0110), respectively. Due to the unique twin structures, both sides of the main stems could be Zn-terminated ZnO(0001) polar surfaces. The chemically active surfaces make the aligned branching nanowires grow from both sides of the main stems, which is consistent with the structure of the obtained bicrystalline nanomaterials. The growth of bicrystalline ZnO nanocombs can be explained by polar-surface dominated growth and twins induced growth mechanisms.     

Anisotropic Wetting Surfaces with One‐Dimesional and Directional Structures: Fabrication Approaches,Wetting Properties and Potential Applications

This review article provides a brief summary of recent research progress on anisotropic wetting on one‐dimensional (1D) and directionally patterned surfaces, as well as the technical importance in various applications. Inspiration from natural structures exhibiting anisotropic wetting behavior is first discussed. Development of fabrication techniques for topographically and chemically 1D patterned surfaces and directional nanomaterials are then reviewed, with emphasis on anisotropic behavior with topographically (structurally) patterned surfaces. The basic investigation of anisotropic wetting behavior and theoretical simulations for anisotropic wetting are also further reviewed. Perspectives concerning future direction of anisotropic wetting research and its potential applications in microfluidic devices, lab‐on‐a‐chip, sensor, microreactor and self‐cleaning are presented.     

Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High‐Index ZnO Surfaces

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high‐index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c‐axis‐oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO–ZnO core–shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high‐index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO–ZnO core–shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core–shell nanowires are largely non‐responsive to varying O₃ concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect‐rich high‐index polar surfaces.     

Synthesis of Metal/Bimetal Nanowires and Their Applications as Flexible Transparent Electrodes

As a potential alternative to indium oxide (ITO), metal nanowire transparent conductive electrodes (TCEs) have attracted more and more attention. Here, a facile method that can be applied to the synthesis of a variety of metal/bimetallic nanowires has been proposed. Metal/bimetallic nanowires synthesized through this method show high aspect ratios and great dispersibility, which makes them ideal building blocks for transparent electrodes. The synthesis mechanism is discussed in‐depth to give a theoretical basis of morphology control of metal nanostructures in organic synthesizing systems. TCEs with high flexibility, excellent optical–electrical performance as well as outstanding anti‐thermal and anti‐moisture stability are constructed. To the best of our knowledge, this is the first work on synthesizing multiple metal/bimetallic nanowires through one method.     

High-performance photoconductive channels based on (carbon nanotube)-(CdS nanowire) hybrid nanostructures

A photoconductive channel based on hybrid nanostructures comprising carbon nanotubes (CNTs) and CdS nanowires is fabricated by a directed assembly strategy and catalyst-assisted chemical vapor deposition (CVD). The photoconductive channels simultaneously exhibit large photocurrent and fast response speed. Furthermore, it can be easily applied to surfaces that are not flat, such as a glass tube. This is a simple but efficient strategy for various optoelectronic applications.     

A modified Wenzel model for hydrophobic behavior of nanostructured surfaces

A modified Wenzel model was proposed to explore the influence of pore size distributions (PSDs) on water repellency of nanostructured surfaces. Rough surfaces with different porous structures, including surface areas and PSDs, were fabricated by stacking different solid ratios of TiO₂ nanoparticles. These fluorinated surfaces exhibited an excellent hydrophobic performance with the highest value of contact angle ∼ 165°. The PSDs of these surfaces, determined from Dubinin-Stoeckli equation, were found to vary with the solid ratios. The modified Wenzel model incorporated with the PSDs gave a fairly good prediction in describing the variation of contact angle with surface roughness, which is very close to the experimental data. These results demonstrated that the heterogeneity of surfaces caused by different PSDs would induce the hydrophobic behavior.     

Ultrasonic-assisted synthesis of Au nanobelts and nanowires

Au one-dimensional (1D) nanostructures, including nanobelts and nanowires, have been synthesized in an ethylene glycol (EG)/polyvinylpyrrolidone (PVP) system by a simple and convenient seed-mediated growth method. The nanobelts and nanowires have aspect ratios up to 600, a length distribution ranging from several to tens of microns, and an average width of 100 nm. In this method, we used an ultrasonic process to promote the formation of Au seeds, which largely determines the morphology of final product. Additionally, we have found that the ultrasonic process significantly increases the fabrication yield of 1D nanostructures. Further experimental results show strong polarization dependence of Surface-Enhanced Raman Scattering (SERS) on a single Au 1D nanostructure. This convenient, versatile and low-cost synthesis method can be applied to 1D nanostructures composed from a range of materials, making it widely applicable to many areas of modern science and technology.     

Interactive effect of agitation rate and oxidative etching on growth mechanisms of silver nanowires during polyol process

In this work, the interaction between agitation rate and oxidative etching on the growth mechanisms of silver nanowires during polyol process, the most conventional applied method for metallic nanoparticles synthesis, is evaluated. It was found that the main reason for the formation of multidiameter nanowires (MDNWs) is the local accumulation of silver ions in the system, i.e. even at a low concentration of silver ions (94 mM) under the mild agitation conditions, MDNWs were producible. Moreover, it was inferred that the more stress (stemming from the high agitation) applied on the initially formed decahedron Ag particles affects the ‘re-entrant grooves opening’ process which results in the formation and growth of irregular nanostructures. A polynomial equation for tuning aspect ratio of silver nanowires using different concentration of CuCl₂ solution was also proposed. Finally, based on the obtained experimental data together with the thermodynamic considerations, a growth mechanism was proposed which can promisingly be employed for describing the one-dimensional growth of silver nanowires in both surfactant-modified and surfactant-free procedures.     

固体表面润湿性机理及模型

润湿性是固体表面的重要特征之一。人工控制和制备固体表面润湿性已成为研究的热点，而且逐渐被应用于国防、工农业生产和人们日常生活等不同领域。论述了固体表面润湿性的有关理论基础，包括表面张力、表面自由能、润湿过程及其条件。分析了固体表面接触角的Young模型、Wenzel模型和Cassie模型等几种理论模型，以及解释了接触角滞后现象。