Fluoroalkylsilane Modification Research Articles

Synthesis of mechanically robust epoxy cross-linked silica aerogel membranes for CO 2 capture

Abstract Mesoporous SiO2 aerogels are multi-functional porous materials with diverse applications. However, the fragile characteristics and low mechanical strength of mesoporous SiO2 aerogels limits their development for use in industrial applications. The mechanical strength of SiO2 aerogels can be greatly enhanced by the addition of tri-epoxy cross-linkers. Tri-epoxy cross-linked SiO2 aerogels with tri-epoxy concentrations of 7.5%, 15%, 30% and 45% were coated on macroporous Al2O3 membrane supports. The surface of the tri-epoxy cross-linked SiO2 aerogel membranes became superhydrophobic after fluoroalkylsilane (FAS) modification. Compared to native SiO2 aerogel membranes in the absence of tri-epoxy cross-linker, the pore size of the tri-epoxy cross-linked SiO2 aerogel membranes with tri-epoxy concentrations of 15% and 30% increased to approximately 6 nm, resulting in an increase in the CO2 absorption flux. The highest CO2 absorption flux of 1.4 mmol/m2s was reached for the tri-epoxy cross-linked SiO2 aerogel membrane with a tri-epoxy concentration of 15%. The durability of the as-prepared SiO2 aerogel membranes for CO2 absorption is also demonstrated in this work. Consequently, the mechanically robust tri-epoxy cross-linked SiO2 aerogel membranes have potential applications in membrane contactor systems for CO2 capture.

Electrochemical machining of superhydrophobic surfaces on mold steel substrates

Superhydrophobic surfaces on mold steel substrates were fabricated via simple and environment-friendly electrochemical technology followed by fluoroalkylsilane (FAS) modification. The microstructure, crystal structure, chemical composition, and wettability of the sample surfaces were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and optical contact angle measuring instrument. The influences of current density and electrochemical machining time on wettability were also studied. The results show that, after the electrochemical processing, the mold steel substrates were covered with a layer of passive films, which have micro/nano binary rough structures composed of rugged plateau structures and rupture-like shapes. Before fluorination, the obtained sample surfaces showed superhydrophilicity (the water contact angles were almost 0°), while the sample surfaces became superhydrophobic (the water contact angles were 167.2° and the water rolling angles were 4.3°) after fluorination. The proposed method is simple, economical and environmental-friendly, and has great potential for large-scale industrial applications.

A Cycle‐Etching Approach Toward the Fabrication of Superamphiphobic Stainless Steel Surfaces With Excellent Anticorrosion Properties

In this paper, a facile approach is developed for preparing superamphiphobic surfaces with excellent anticorrosion properties on 304 stainless steel (SS) substrates. This proposed methodology involves fabrication of micro/nano‐scale binary structures on SS substrates by a cycle‐etching approach (cycles of hydrofluoric acid etching and ultrasonic cleaning), and final fluoroalkylsilane modification. Wettability measurements reveal that the fabricated SS surfaces become super‐repellent toward water and peanut oil. By varying the times of cycle‐etching, the authors compare the relationship between morphological change and wettability. The potentiodynamic polarization curves and electrochemical impedance spectroscopy suggest that the fabricated SS surfaces have superior corrosion resistance.

Preparation of multifunctional Al alloys substrates based on micro/nanostructures and surface modification

Aluminum alloys (Al alloys) are vulnerable to penetrating and peeling failures in seawater. Preparing a coating to isolate the substrate from corrosive mediums is an effective anticorrosion method. Enlightened by the lotus leaf effect, a liquid-repelling Al alloy surface with enhanced corrosion resistance has been prepared via a facile one-step deposition process. Results indicate that the fabricated flower-like micro/nanostructures with nano-ridges on petals can achieve excellent superamphiphobicity after fluoroalkylsilane modification. The as-prepared surface is also found with good chemical stability. Furthermore, superior anticorrosion behavior of the resultant sample in seawater is investigated by electrochemical tests. Due to trapped air in such structures, the solid-air-liquid interfaces can help to resist the seawater penetration by reducing interface interactions between corrosive ions and the superamphiphobic surface. Finally, an optimized two-layer perceptron artificial neural network (ANN) is set up to model and predict the relationship between preparation conditions and anticorrosion parameters. This work provides a great potential to extend the applications of Al alloys in many areas especially for marine engineering fields.

Lotus-leaf-like microstructures on tungsten surface induced by one-step nanosecond laser irradiation

Abstract A method of one-step nanosecond laser scanning irradiating was employed to produce microstructures on tungsten surfaces. The effect of the laser fluence, scanning speed and scanning interval on the morphology of tungsten surfaces was investigated. The results indicated that higher laser fluence along with smaller scanning interval and lower scanning speed was more beneficial for obtaining lotus-leaf-like hierarchical microstructures. When the laser fluence was > 1.8 J/cm 2 at the scanning speed of 10.0 mm/s and interval of 1.0 μm, lotus-leaf-like hierarchical microstructures were produced on tungsten surfaces, on which the WCAs of 5 μl water droplets were measured as low as 0°. Superhydrophobic surfaces with a maximum WCA of 162° and a minimum rolling angle of 1.0° were further obtained from fluoroalkylsilane modification on the superhydrophilic surfaces. The availability of tungsten surfaces with controllable wettability from superhydrophilicity to superhydrophobicity was meaningful for the application of tungsten-based components in various fields.

Facile fabrication of wear-resistant multifunctional surfaces on titanium alloy substrate by one-step anodization and modification with silicon dioxide nanoparticles

This paper describes a simple, facile, low-cost method of fabricating a wear-resistant surface on titanium alloy substrates by single-step anodization and modification with silicon dioxide particles. The method produced a TiO2–SiO2 composite surface with micro- to nanohierarchical structure, which became multifunctional after fluoroalkylsilane modification. The surface not only showed excellent superhydrophobicity [contact angles (CAs) > 150°, sliding angles (SAs) ≤ 8°] but also exhibited better repellency of complex whole blood (CAs = 153° ± 2°, SAs = 9°). Furthermore, the wear resistance of the surface was investigated by sandpaper abrasion test. The results showed that the mechanical wear resistance of the TiO2–SiO2 composite surface was more than three times that of the surface prepared only by the electrochemical technique, and the surface still maintained the capacity of hydrophobicity even after 33.6 m of wear under conditions of 3400 Pa and 360# sandpaper. The method mentioned in this paper could be a candidate for the production of abrasion-resistant and super-repellent titanium alloy.

Hydrophobic Light-to-Heat Conversion Membranes with Self-Healing Ability for Interfacial Solar Heating.

Self-healing hydrophobic light-to-heat conversion membranes for interfacial solar heating are fabricated by deposition of light-to-heat conversion material of polypyrrole onto a porous stainless-steel mesh, followed by hydrophobic fluoroalkylsilane modification. The mesh-based membranes spontaneously stay at the water-air interface, collect and convert solar light into heat, and locally heat only the water surface for enhanced evaporation.

Fabrication of superhydrophobic surface with improved corrosion inhibition on 6061 aluminum alloy substrate

This work has developed a simple and low-cost method to render 6061 aluminum alloy surface superhydrophobicity and excellent corrosion inhibition. The superhydrophobic aluminum alloy surface has been fabricated by hydrochloric acid etching, potassium permanganate passivation and fluoroalkyl-silane modification. Meanwhile, the effect of the etching and passivation time on the wettability and corrosion inhibition of the fabricated surface has also been investigated. Results show that with the etching time of 6min and passivation time of 180min the fabricated micro/nano-scale terrace-like hierarchical structures accompanying with the nanoscale coral-like network bulge structures after being modified can result in superhydrophobicity with a water contact angle (CA) of 155.7°. Moreover, an extremely weak adhesive force to droplets as well as an outstanding self-cleaning behavior of the superhydrophobic surface has also been proved. Finally, corrosion inhibition in seawater of the as-prepared aluminum alloy surface is characterized by potentiodynamic polarization curves and electrochemical impedance spectroscopy. Evidently, the fabricated superhydrophobic surface attained an improved corrosion inhibition efficiency of 83.37% compared with the traditional two-step processing consisting of etching and modification, which will extend the further applications of aluminum alloy especially in marine engineering fields.

Facile fabrication of superhydrophilic/superhydrophobic surface on titanium substrate by single-step anodization and fluorination

A facile and scalable technique for preparation of superhydrophilic/superhydrophobic titanium (Ti) surface by single-step anodization and fluorination is presented in this paper. The Ti substrates were anodized to produce micro-nano hierarchical structure which is essential for superhydrophilic surface. The water contact angles (WCAs) of 5μl water droplets on the anodized Ti surfaces were measured as low as 0°. Capillary rise measurement was used to evaluate the superhydrophilicity on Ti surfaces anodized at different conditions. Results show that higher anodization voltage can yield stronger superhydrophilicity on Ti surface, but the influence of electrolyte temperature on the superhydrophilicity has a close correlation with the anodization voltages. At 20V and 40V anodization voltages, the increase of electrolyte temperature can improve the surface superhydrophilicity, but this trend will be reversed when the voltages rise to 60V and 80V. Superhydrophobic surfaces were further obtained from fluoroalkylsilane (FAS) modification on the anodized Ti substrates. It was observed that appropriate anodization voltages and electrolyte temperatures can balance the growth and dissolution of the micro-nano hierarchical surface structure, thereby obtaining the desired superhydrophobic Ti surface. The WCA, rolling angle and contact angle hysteresis of water droplets on the best superhydrophobic Ti surface were respectively recorded as 160°, 2° and 1.7° in this work. Furthermore, the superhydrophilic and superhydrophobic Ti surfaces fabricated in this research also show satisfactory stability in acidic, neutral and alkaline aqueous solutions as well as ambient conditions.

Superhydrophobic Polyimide via Ultraviolet Photooxidation: The Evolution of Surface Morphology and Hydrophobicity under Different Ultraviolet Intensities

Ultraviolet (UV) photooxidation has recently been developed to fabricate superhydrophobic polyimide (PI) films in combination with fluoroalkylsilane modification. However, it remains unclear whether the surface morphology and hydrophobicity are sensitive to technical parameters such as UV intensity and radiation environment. Herein, we focus on the effects of UV intensity on PI surface structure and wettability to gain comprehensive understanding and more effective control of this technology. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results showed that UV intensity governed the evolutionary pattern of surface morphology: lower UV intensity (5 mW/cm2) facilitated in‐plane expansion of dendritic protrusions while stronger UV (10 and 15 mW/cm2) encouraged localized growth of protrusions in a piling‐up manner. Surface roughness and hydrophobicity maximized at the intensity of 10 mW/cm2, as a consequence of the slowed horizontal expansion and preferred vertical growth of the protrusions when UV intensity increased. Based on these results, the mechanism that surface micro/nanostructures developed in distinct ways when exposed to different UV intensities was proposed. Though superhydrophobicity (water contact angle larger than 150°) can be achieved at UV intensity not less than 10 mW/cm2, higher intensity decreased the effectiveness. Therefore, the UV photooxidation under 10 mW/cm2 for 72 h is recommended to fabricate superhydrophobic PI films.

Superhydrophobic Polyimide via UV Photo-Oxidation and Fluoroalkyl Silanization: Changes in Surface Chemistry

Topographically rich superhydrophilic surfaces can be generated by UV photo-oxidation of originally flat polyimide. Then stable superhydrophobic PI films can be achieved after fluoroalkylsilane (FAS) modification. Present work describes the changes in surface chemistry during the preparation processes. After UV treatment in air, PI was oxidized accompanied by the generation of OH and amide groups as well as N-O bondings. Moreover, the oxidized polymer tended to aggregate as protrusions, gathering of polar groups such as OH groups. The OH groups could further react with FAS, combining fluoroalkyl chains with irradiated PI through O-Si bondings (silanization). The chemical bindings contributed to the stability of superhydrophobicity.

Preparation of stable superamphiphobic surfaces on Ti-6Al-4V substrates by one-step anodization

Superamphiphobic surfaces on titanium alloy substrates were successfully fabricated via one-step anodization and subsequent fluoroalkylsilane modification. The prepared superamphiphobic titanium alloy surfaces display contact angles with water, glycerol and hexadecane are 166.4°±1.8°, 158.4°±2.1° and 152.5°±1.9°, respectively, corresponding sliding angles are all within 10°, which can be attributed to the combination of reentrant micro/nano structures with low surface energy modification. In order to obtain the optimal superamphiphobicity, the relationship between reaction time with wettability of the superamphiphobic surfaces is analyzed in detail. Furthermore, the robustness of the superamphiphobicity is also investigated by UV light test, immersion test and abrasion test, which show that the created superamphiphobic surfaces possess good stability under harsh conditions. The developed approach is simple and has potential to be applied into the commercial production of large-area superamphiphobic titanium alloy surface.

Multifunctional TiO2‐Based Particles: The Effect of Fluorination Degree and Liquid Surface Tension on Wetting Behavior

A systematic investigation into the influence of the degree of fluorination on the static and dynamic wetting behavior of TiO2‐based nanobelt (TNB) particles with various liquids is described. The effect of the degree of fluorination and the surface tension of the liquid on the occurrence and stability of liquid marbles, foams or dispersions are studied and the wetting behavior and arrangement of particles at the air–liquid surface are observed. Using contact angle (θ) measurements, the relation between the type of particle‐stabilized material and θ is established. For liquids of relatively high tension like water or formamide which do not wet the fluorinated particles, a powder‐like material (marble) is formed. For polar oils of intermediate tension (35–50 mN m−1), which partially wet the fluorinated particles, stable air‐in‐oil foams can be prepared in which particles form a close‐packed layer enveloping air bubbles. Liquids of relatively low tension, e.g., ethanol or polydimethylsiloxane, wet the particles forming a uniform dispersion and partial sedimentation. By contrast, the as‐prepared hydrophilic TNB particles are rapidly wetted by all the liquids as expected due to their high surface energy. The stable cross‐stacked TNB particles with fluoroalkylsilane (FAS) modification could be a versatile platform in a wide range of applications, especially for fluidic devices (e.g., biofluids, gas sensing, and lab‐on‐a‐chip devices). In a proof‐of‐concept study, the oil–water separation performance of fabrics with chemically stable TNB/FAS coating and the liquid isolation by a TNB/FAS shell for highly sensitive gas sensing or reagent assays are investigated.

Surface modification of polypyrrole-coated foam for the capture of organic solvents and oils

The low stability and complicated fabrication procedures seriously hindered practical applications of superhydrophobic and superoleophilic materials. Here, we present a simple method for preparing the novelly three-dimensional material based on commercially available nickel foams functionalized with electrodepositing of sub-micrometer polypyrrole (PPy) particles, followed by modification of low-surface-energy material such as fluoroalkylsilane (FAS), which can efficiently separate oils and organic solvents from water. The formation of nanostructured surface roughness of PPy onto the nickel foam by combination with FAS modification would contribute to the excellent superhydrophobic and superoleophilic performance, as is the evidence of the water CA of 155° and oil CA of ca. 0° for FAS-treated PPy foam. As a separating membrane, organic solvents and oils could be easily removed without obvious absorption of water, which has great potential over traditional treatment techniques and is of technological significance as a promising and efficient absorbent material for separation of organic contaminates and oils from water.

Superhydrophobic polyimide films with high thermal endurance via UV photo-oxidation

UV photo-oxidation was first applied to fabricate superhydrophobic polyimide (PI) films in combination with fluoroalkylsilane (FAS) modification. During prolonged UV irradiation, commercial flat PI films evolved to form unique micro/nanostructures. Meanwhile, the root mean square (RMS) surface roughness increased from 1.74 to 53.70 nm, leading to a gradual increase of WCA from 105.1 to 159.2° after FAS treatment. After 72 h of UV radiation exposure, the micro/nano-structured and FAS-modified PI films exhibited superhydrophobicity with water contact angle (WCA) larger than 150° and sliding angle (SA) less than 5°. The superhydrophobicity remained even after annealing at 350°C, which enabled stable utilization under elevated temperature. Stable micro/nanostructures and chemical bonding of FAS were found to contribute to the high thermal endurance. Moreover, the formation mechanism of the superhydrophobic PI films was investigated. The proposed UV photo-oxidation method provides a new route for the industrial fabrication of thermally stable superhydrophobic PI films.

Fabrication of Low-Adhesive Superhydrophobic Al Surfaces via Self-Assembled Primary Cell Assisted Etching

In this work, we present a novel etching method to fabricate the superhydrophobic Al surfaces in a salt solution. Hierarchical rough structures composed of micrometer-scale pits, protrusions, and rectangle plateaus and nanometer-scale step-like structures are formed on the Al surfaces by the self-assembled primary cell assisted etching and the preferential corrosion of grain boundaries and dislocations. After fluoroalkylsilane modification, the superhydrophobic Al surfaces with a 166.4° water contact angle and a 1° rolling angle are obtained. The developed method does not use any strong acids and has a smaller harm to the environment and operators.

Tuning the wettability of an aluminum surface via a chemically deposited fractal dendrite structure

We have developed a straightforward method to tune the wettability of an aluminum substrate within a contact angle (CA) range from 2(°) to 170(°) by chemical deposition in CuCl2 solution and fluoroalkylsilane (FAS) modification. The CA of the as-deposited surface decreases with deposition time due to the growth of fractal copper dendrites, which enhance the surface roughness significantly. After subsequent modification with FAS, a superhydrophobic surface with CA 170(°) and sliding angle less than 5(°) has been obtained. With the increase of CA, the maximum spreading of water droplets is reduced. A bouncing behavior is observed for droplets impinging on the superhydrophobic substrate, suggesting its potential application as a self-cleaning surface.

Biomimetic fabrication and tunable wetting properties of three-dimensional hierarchical ZnO structures by combining soft lithography templated with lotus leaf and hydrothermal treatments

Three-dimensional hierarchical ZnO films with lotus-leaf-like micro/nano structures were successfully fabricated via a biomimetic route combining sol–gel technique, soft lithography and hydrothermal treatments. A PDMS mold replicated from a fresh lotus leaf was used to imprint microscale pillar structures directly into a ZnO sol film. Hierarchical ZnO micro/nano structures were subsequently fabricated by a low-temperature hydrothermal growth of secondary ZnO nanorod arrays on the micro-structured ZnO film. The morphology and size of ZnO hierarchical micro/nano structures can be easily controlled by adjusting the hydrothermal reaction time. Wettability of hierarchical ZnO thin films was found to convert from superhydrophilicity to hydrophobicity after a low-surface-energy fluoroalkylsilane modification. Improved wetting properties from hydrophobic to superhydrophobic can be tuned by increasing the growth of ZnO nanorod structures.

Electrochemical machining of super-hydrophobic Al surfaces and effect of processing parameters on wettability

Super-hydrophobic aluminum (Al) surfaces were successfully fabricated via electrochemical machining in neutral NaClO3 electrolyte and subsequent fluoroalkylsilane (FAS) modification. The effects of the processing time, processing current density, and electrolyte concentration on the wettability, morphology, and roughness were studied. The surface morphology, chemical composition, and wettability of the Al surfaces were investigated using scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), white-light interferometry, roughness measurements, X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FTIR), and optical contact angle measurements. The results show that hierarchical rough structures and low surface energy films were present on the Al surfaces after electrochemical machining and FAS modification. The combination of the rough structures and the low surface energy materials plays a crucial role in achieving super-hydrophobicity. Compared with the anodic oxidation and chemical etching method, the method proposed in our work does not require strong acid or alkali, and causes less harm to the environment and operators but with high processing efficiency. The rough structures required by the super-hydrophobic surfaces were obtained at 30-s processing time and the best super-hydrophobicity with 164.6∘ water contact angle and 2∘ tilting angle was obtained at 360 s. The resulting super-hydrophobic Al surfaces have a long-time stability in air and an excellent resistance to corrosive liquids.

Fabrication of superhydrophobic surfaces on Mg alloy substrates via primary cell corrosion and fluoroalkylsilane modification

AbstractThe present work reports a simple and safe two‐step process to render magnesium (Mg) alloy surfaces superhydrophobic via primary cell corrosion and subsequently cover it with a fluoroalkylsilane (FAS) film. The surfaces were characterized by scanning electron microscopy (SEM), optical microscopy, energy‐dispersive X‐ray spectroscopy (EDS), Fourier‐transform infrared spectrophotometry (FTIR), X‐ray diffraction (XRD), and optical contact angle measurements. The power generated via the primary cell corrosion of copper and Mg alloys was also measured using a digital multimeter. The results show that micro/nanometer‐scale binary rough structures and an FAS film with a low surface energy were present on the Mg alloy surfaces, both of which confer good superhydrophobicity with a water contact angle of 162.8° and a tilting angle of 2°. The micro/nanometer‐scale binary rough structures consisted of micrometer‐scale grains, cluster‐like structures composed of nanometer‐scale needles, and network‐like structures composed of nanometer‐scale sheets. Superhydrophobicity was analyzed by the Cassie–Baxter theory. Findings show that only about 6.3% of the water surface was in contact with the Mg alloy substrates, while the remaining 93.7% was in contact with the air cushion. The unique advantage of the proposed method is that power can be generated during the machining process of the superhydrophobic surfaces on the Mg alloy substrates.