Superhydrophobic silicon surfaces with micro–nano hierarchical structures via deep reactive ion etching and galvanic etching

A comprehensive review on anticorrosive/antifouling superhydrophobic coatings: Fabrication, assessment, applications, challenges and future perspectives

Superhydrophobicity (SHP) is an incredible phenomenon of extreme water repellency of surfaces ubiquitous in nature (E.g. lotus leaves, butterfly wings, taro leaves, mosquito eyes, water-strider legs, etc). Historically, surface exhibiting water contact angle (WCA) > 150° and contact angle hysteresis <10° is considered as SHP. The SHP surfaces garnered considerable attention in recent years due to their applications in anti-corrosion, anti-fouling, self-cleaning, oil-water separation, viscous drag reduction, anti-icing, etc. As corrosion and marine biofouling are global problems, there has been focused efforts in combating these issues using innovative environmentally friendly coatings designs taking cues from natural SHP surfaces. Over the last two decades, though significant progress has been made on the fabrication of various SHP surfaces, the practical adaptation of these surfaces for various applications is hampered, mainly because of the high cost, non-scalability, lack of simplicity, non-adaptability for a wide range of substrates, poor mechanical robustness and chemical inertness. Despite the extensive research, the exact mechanism of corrosion/anti-fouling of such coatings also remains elusive. The current focus of research in recent years has been on the development of facile, eco-friendly, cost-effective, mechanically robust chemically inert, and scalable methods to prepare durable SHP coating on a variety of surfaces. Although there are some general reviews on SHP surfaces, there is no comprehensive review focusing on SHP on metallic and alloy surfaces with corrosion-resistant and antifouling properties. This review is aimed at filling this gap. This review provides a pedagogical description with the necessary background, key concepts, genesis, classical models of superhydrophobicity, rational design of SHP, coatings characterization, testing approaches, mechanisms, and novel fabrication approaches currently being explored for anticorrosion and antifouling, both from a fundamental and practical perspective. The review also provides a summary of important experimental studies with key findings, and detailed descriptions of the evaluation of surface morphologies, chemical properties, mechanical, chemical, corrosion, and antifouling properties. The recent developments in the fabrication of SHP -Cr-Mo steel, Ti, and Al are presented, along with the latest understanding of the mechanism of anticorrosion and antifouling properties of the coating also discussed. In addition, different promising applications of SHP surfaces in diverse disciplines are discussed. The last part of the review highlights the challenges and future directions. The review is an ideal material for researchers practicing in the field of coatings and also serves as an excellent reference for freshers who intend to begin research on this topic.

Fabrication of durable self-cleaning photocatalytic coating with long-term effective natural light photocatalytic degradation performance

The practical application of photocatalytic coating has been greatly challenged in terms of its long-term effective natural light photocatalytic degradation due to its vulnerability and easy contamination caused by poor self-cleaning properties. In this work, photocatalytic coating with self-cleaning properties was prepared by spraying fluorinated dual-scale TiO₂ on the inorganic lithium silicate adhesive, enabling excellent durability and long-term effective photocatalytic degradation performance under natural light. The coating exhibits superhydrophobic properties even after abrasion testing, acid and alkali immersion testing, and UV aging, laying a foundation for the practical use. Moreover, the coating can be applied to various substrates and its excellent self-cleaning properties make it resistant to particulate and liquid contamination that may occur in the environment. Besides, we evaluated the photocatalytic stability of the coating by subjecting it to acidic and alkaline environments and high pollution concentrations. Furthermore, benefiting from the synergistic effect of photocatalytic and self-cleaning properties, the coating achieves long-term effective photocatalytic degradation of dye wastewater under natural light, which still has a high removal rate of 95.8% for methylene blue even after 30 cycles of use. Meanwhile, due to the coating's excellent durability, the long-term quality loss rate of the coating still remained below 0.3%, which avoids the risk of secondary environmental pollution caused by nanoparticle leakage. Therefore, these excellent properties enable the coating to have a broad range of application prospects for the treatment of pollutants in water.

Wafer-Scale Fabrication of Ultra-High Aspect Ratio, Microscale Silicon Structures with Smooth Sidewalls Using Metal Assisted Chemical Etching

Silicon structures with ultra-high aspect ratios have great potential applications in the fields of optoelectronics and biomedicine. However, the slope and increased roughness of the sidewalls inevitably introduced during the use of conventional etching processes (e.g., Bosch and DRIE) remain an obstacle to their application. In this paper, 4-inch wafer-scale, ultra-high aspect ratio (>140:1) microscale silicon structures with smooth sidewalls are successfully prepared using metal-assisted chemical etching (MacEtch). Here, we clarify the impact of the size from the metal catalytic structure on the sidewall roughness. By optimizing the etchant ratio to accelerate the etch rate of the metal-catalyzed structure and employing thermal oxidation, the sidewall roughness can be significantly reduced (average root mean square (RMS) from 42.3 nm to 15.8 nm). Simulations show that a maximum exciton production rate (Gmax) of 1.21 × 1026 and a maximum theoretical short-circuit current density (Jsc) of 39.78 mA/cm2 can be obtained for the micropillar array with smooth sidewalls, which have potential applications in high-performance microscale photovoltaic devices.

Plasma-enabled superhydrophobic coatings on mild steel

This work demonstrates a new pathway to the direct on-surface fabrication of a superhydrophobic surface coating on mild steel. The coating was formed using dielectric barrier discharge (DBD) plasma to convert a liquid small-molecule precursor (1,2,4-tricholorobenzene) to a solid film via plasma-assisted on-surface polymerization. Plasma treatments were performed under a nitrogen atmosphere with a variety of power levels and durations. Samples were analysed by optical and scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), Raman spectroscopy, optical profilometry, contact angle measurement, and potentiodynamic polarisation tests. Wettability of the films varied with the plasma parameters, and through the inclusion of graphene nanoplatelets in the precursor. High-dose plasma exposures of the nanoplatelet-containing precursor created superhydrophobic films with water contact angles above 150°. Potentiodynamic polarisation tests revealed that the superhydrophobic coating provided little or no corrosion protection.

Fabrication of ZnO Nanobrushes by H2-C2H2 Plasma Etching for H2 Sensing Applications

Zinc oxide has widespread use in diverse applications due to its distinct properties. Many of these applications benefit from controlling the morphology on the nanoscale, where for example gas sensing is strongly enhanced for high surface-to-volume ratios. In this work the formation of novel ZnO nanobrushes by plasma etching treatment as a new approach is presented. The morphology and structure of the ZnO nanobrushes are studied in detail by transmission and scanning electron microscopy. It is revealed that ZnO nanobrush structures are fabricated by self-patterned preferential etching of ZnO microtetrapods in a hydrogen-acetylene plasma. The etching process was found to be most effective at 1% C₂H₂ admixture. Nanowire arrays are formed enabled by sidewall passivation due to a-C:H deposition. The nanobrush structures are further stabilized by simultaneous deposition of a SiO_x layer from the opposite direction. Highly sensitive (gas response S = 148), selective, and fast (response time 15 s, recovery time 6 s) hydrogen sensors are fabricated from single nanobrushes. Single nanobrush sensors show enhanced sensing performance in increased gas response S of at least 10 times and improved response as well as recovery times when compared to nonporous single ZnO nanorod sensors due to the small diameters (≈50 nm) of the formed nanowires as well as the strongly enhanced surface-to-volume ratio of the nanobrushes by a factor of more than 10.

Progress in Non-Traditional Processing for Fabricating Superhydrophobic Surfaces

When the water droplets are on some superhydrophobic surfaces, the surface only needs to be inclined at a very small angle to make the water droplets roll off. Hence, building a superhydrophobic surface on the material substrate, especially the metal substrate, can effectively alleviate the problems of its inability to resist corrosion and easy icing during use, and it can also give it special functions such as self-cleaning, lubrication, and drag reduction. Therefore, this study reviews and summarizes the development trends in the fabrication of superhydrophobic surface materials by non-traditional processing techniques. First, the principle of the superhydrophobic surfaces fabricated by laser beam machining (LBM) is introduced, and the machining performances of the LBM process, such as femtosecond laser, picosecond laser, and nanosecond laser, for fabricating the surfaces are compared and summarized. Second, the principle and the machining performances of the electrical discharge machining (EDM), for fabricating the superhydrophobic surfaces, are reviewed and compared, respectively. Third, the machining performances to fabricate the superhydrophobic surfaces by the electrochemical machining (ECM), including electrochemical oxidation process and electrochemical reduction process, are reviewed and grouped by materials fabricated. Lastly, other non-traditional machining processes for fabricating superhydrophobic surfaces, such as ultrasonic machining (USM), water jet machining (WJM), and plasma spraying machining (PSM), are compared and summarized. Moreover, the advantage and disadvantage of the above mentioned non-traditional machining processes are discussed. Thereafter, the prospect of non-traditional machining for fabricating the desired superhydrophobic surfaces is proposed.

Template-Free One-Step Electrodeposition Method for Fabrication of Robust Superhydrophobic Coating on Ferritic Steel with Self-Cleaning Ability and Superior Corrosion Resistance

The corrosion of ferritic steel, a widely used structural material in the power and nuclear industries exposed to humid coastal environments, is a major concern. Here, we present a template-free one-step electrodeposition method for the fabrication of a robust superhydrophobic (SHP) coating on ferritic steel with excellent mechanical stability, enhanced corrosion resistance, and self-cleaning ability. By varying the electrodeposition time and potential, the micronanoscale hierarchical surface structures were optimized. The coated SHP surfaces were characterized by water contact angle measurement, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The coated surfaces showed a characteristic cauliflower morphology of cerium myristate with micronanoscale features. The maximum water contact angle achieved was 162.8 ± 2.4°. Shear abrasion testing showed good mechanical durability for the prepared coatings. The as-prepared SHP coating showed a five order reduction in corrosion current density (4.14 × 10^-11 A/cm²) and corrosion rate (4.63 × 10^-7 mm/y) as compared to the bare sample. Further, a six order enhancement in the polarization resistance (1.55 × 10⁹ Ω) was also observed in agressive chloride solution, which confirmed the excellent corrosion resistance of the SHP coating. Electrochemical impedance spectroscopy (EIS) studies showed a high impedance modulus for SHP coated surfaces due to the presence of a compact protective layer of cerium myristate. This observed impedance modulus of the SHP surface was approximately four orders higher than the reported value on magnesium alloys. This study provides a new platform for obtaining a robust, mechanically stable, and corrosion resistant SHP coating with a self-cleaning ability on ferritic steel substrates that may be adapted for a range of materials in practical applications.

Interaction of Surface Energy Components between Solid and Liquid on Wettability, and Its Application to Textile Anti-Wetting Finish

With various options of anti-wetting finish methods, this study intends to provide basic information that can be applied in selecting a relevant anti-wetting chemical to grant protection from spreading of liquids with different surface energy profiles. With such an aim, the anti-wetting effectiveness of fluorinated coating and silane coating was investigated for liquids having different surface energy components, water (WA), methylene iodide (MI) and formamide (FA). The wetting thermodynamics was experimentally investigated by analyzing dispersive and polar component surface energies of solids and liquids. The role of surface roughness in wettability was examined for fibrous nonwoven substrates that have varied surface roughness. The presence of roughness enhanced the anti-wetting performance of the anti-wetting treated surfaces. While the effectiveness of different anti-wetting treatments was varied depending on the liquid polarities, the distinction of different treatments was less apparent for the roughened fibrous surfaces than the film surfaces. This study provides experimental validation of wetting thermodynamics and the practical interpretation of anti-wetting finishing.

Nanoscale material redistribution induced by spatially modulated femtosecond laser pulses for flexible high-efficiency surface patterning

In this paper, we investigated the material redistribution phenomenon controlled by spatially modulated femtosecond laser pulses on a silicon surface. The intensity distribution was shaped by using a spatial light modulator. The material was first selectively melted and then redistributed by the laser-induced plasma. Thus, complex surface patterns were formed conformal to the laser intensity distribution. Sub-diffraction-limit size can be achieved due to the nanoscale material redistribution. Only one pulse was needed in the surface patterning process, thus greatly favoring the efficiency improvement. Combined with multibeam interference, a large-scale nanostructure array can be fabricated with high efficiency of 1600 μm²/pulse. This method offers a simple, flexible and efficient alternative approach for nanoscale surface patterning applications.

Recent Advances in Superhydrophobic Electrodeposits

In this review, we present an extensive summary of research on superhydrophobic electrodeposits reported in the literature over the past decade. As a synthesis technique, electrodeposition is a simple and scalable process to produce non-wetting metal surfaces. There are three main categories of superhydrophobic surfaces made by electrodeposition: (i) electrodeposits that are inherently non-wetting due to hierarchical roughness generated from the process; (ii) electrodeposits with plated surface roughness that are further modified with low surface energy material; (iii) composite electrodeposits with co-deposited inert and hydrophobic particles. A recently developed strategy to improve the durability during the application of superhydrophobic electrodeposits by controlling the microstructure of the metal matrix and the co-deposition of hydrophobic ceramic particles will also be addressed.

Preparation of mechanical abrasion and corrosion resistant bulk highly hydrophobic material based on 3-D wood template

BH-wood had low free energy microstructures extending throughout the whole volume, and possessed excellent mechanical abrasion and corrosion resistance.

Fabrication of stable and durable superhydrophobic surface on copper substrates for oil-water separation and ice-over delay

We report a simple and rapid method to fabricate superhydrophobic films on copper substrates via Fe(3+) etching and octadecanethiol (ODT) modification. The etching process can be as short as 5 min and the ODT treatment only takes several seconds. In addition, the whole process is quite flexible in reaction time. The superhydrophobicity of as-prepared surfaces is mechanically durable and chemically stable, which have great performance in oil-water separation and ice-over resistance.

Single ZnO nanocactus gas sensor formed by etching of ZnO nanorod

Etching of materials on the nanoscale is a challenging but necessary process in nanomaterials science. Gas sensing using a single ZnO nanocactus (NC), which was prepared by facile isotropic nanoetching of zinc oxide nanorods (NR) grown by chemical vapor deposition (CVD) using an organic photoresist (PR) by a thermochemical reaction, is reported in this work. PR consists of carboxylic acid groups (COOH) and cyclopentanone (C5H8O), which can react with zinc and oxygen atoms, respectively, on the surface of a ZnO NR. The thermochemical reaction is controllable by varying the concentration of PR and reaction time. A gas sensor was fabricated using a single NC. Gas sensing was tested using different gases such as CH4, NH3 and carbon monoxide (CO). It was estimated that the surface area of a ZnO NC in the case of 50% PR was found to increase four-fold. When compared with a single ZnO NR gas sensor, the sensitivity of a ZnO NC was found to increase four-fold. This increase in sensitivity is attributed to the increase in surface area of the ZnO NC. The formed single ZnO NC gas sensor has good stability, response and recovery time.

Robust superhydrophobic silicon without a low surface-energy hydrophobic coating

Superhydrophobic surfaces without low surface-energy (hydrophobic) modification such as silanization or (fluoro)polymer coatings are crucial for water-repellent applications that need to survive under harsh UV or IR exposures and mechanical abrasion. In this work, robust low-hysteresis superhydrophobic surfaces are demonstrated using a novel hierarchical silicon structure without a low surface-energy coating. The proposed geometry produces superhydrophobicity out of silicon that is naturally hydrophilic. The structure is composed of collapsed silicon nanowires on top and bottom of T-shaped micropillars. Collapsed silicon nanowires cause superhydrophobicity due to nanoscale air pockets trapped below them. T-shaped micropillars significantly decrease the water contact angle hysteresis because microscale air pockets are trapped between them and can not easily escape. Robustness is studied under mechanical polishing, high-energy photoexposure, high temperature, high-pressure water shower, and different acidic and solvent environments. Mechanical abrasion damages the nanowires on top of micropillars, but those at the bottom survive. Small increase of hysteresis is seen, but the surface is still superhydrophobic after abrasion.

Ice shear fracture on nanowires with different wetting states

Understanding the function of nanoscale structure morphology in ice adhesion properties is important in deicing applications. The correlation between ice adhesion and nanowire morphology as well as the corresponding ice shear fracture mechanism are presented for the first time. Ice adhesion on nanowires was measured using a tangential ice-detaching instrument that was developed in-house. Stress analysis was performed using a COMSOL software. Nanowire surface shifted from Wenzel to Cassie transition and Cassie wetting states when the nanowire length was increased. Tangential ice-detaching forces were greater on the hydrophilic surface than those on the hydrophobic surface. Ice-ice internal shear fracture occurred on the ice and force probe contact area at the Wenzel state or on the ice and nanowire contact area at Cassie transition and Cassie state. Different lengths of nanowires caused different wetting states; thus, different fracture areas were formed, which resulted in different tangential ice-detaching forces. This paper presents a new way of tailoring surface ice adhesion via rational design of nanowire morphology with different wetting states.

Flexible Teflon nanocone array surfaces with tunable superhydrophobicity for self-cleaning and aqueous droplet patterning

Tunable hydrophobic/hydrophilic flexible Teflon nanocone array surfaces were fabricated over large areas (cm(2)) by a simple two-step method involving the oxygen plasma etching of a colloidal monolayer of polystyrene beads on a Teflon film. The wettability of the nanocone array surfaces was controlled by the nanocone array dimensions and various additional surface modifications. The resultant Teflon nanocone array surfaces were hydrophobic and adhesive (a "gecko" type of surface on which a water droplet has a high contact angle but stays in place) with a contact angle that correlated with the aspect ratio/sharpness of the nanocones. The surfaces switched to a superhydrophobic or "lotus" type of surface when hierarchical nanostructures were created on Teflon nanocones by modifying them with a gold nanoparticle (AuNPs) film. The nanocone array surfaces could be made superhydrophobic with a maximum contact angle of 160° by the further modification of the AuNPs with an octadecanethiol (C18SH) monolayer. Additionally, these nanocone array surfaces became hydrophilic when the nanocone surfaces were sequentially modified with AuNPs and hydrophilic polydopamine (PDA) layers. The nanocone array surfaces were tested for two potential applications: self-cleaning superhydrophobic surfaces and for the passive dispensing of aqueous droplets onto hybrid superhydrophobic/hydrophilic microarrays.

Recent advances in designing superhydrophobic surfaces

The interest in superhydrophobic surfaces has grown exponentially over recent decades. Since the lotus leaf dual hierarchical structure was discovered, researchers have investigated the foundations of self-cleaning behavior. Generally, surface micro/nanostructuring combined with low surface energy of materials leads to extreme anti-wetting properties. The great number of papers on this subject attests the efforts of scientists in mimicking nature to generate superhydrophobicity. Besides the thirst for knowledge, scientists have been driven by the many possible industrial applications of superhydrophobic materials in several fields. Many methods and techniques have been developed to fabricate superhydrophobic surfaces, and the aim of this paper is to review the recent progresses in preparing manmade superhydrophobic surfaces.

Superhydrophobic surfaces by electrochemical processes

This review is an exhaustive representation of the electrochemical processes reported in the literature to produce superhydrophobic surfaces. Due to the intensive demand in the elaboration of superhydrophobic materials using low-cost, reproducible and fast methods, the use of strategies based on electrochemical processes have exponentially grown these last five years. These strategies are separated in two parts: the oxidation processes, such as oxidation of metals in solution, the anodization of metals or the electrodeposition of conducting polymers, and the reduction processed such as the electrodeposition of metals or the galvanic deposition. One of the main advantages of the electrochemical processes is the relative easiness to produce various surface morphologies and a precise control of the structures at a micro- or a nanoscale.