Underwater Superaerophobicity Research Articles

Superhydrophilic/superaerophobic amorphous Ni3S2/NiMoS electrocatalyst for enhanced hydrogen evolution

It is important to develop electrocatalysts that are cheap and have high activity for hydrogen evolution reaction (HER). In this work, Ni3S2/NiMoS with amorphous phase and unique candied-haws shaped nanoarray structure was successfully grown on nickel foam (Ni3S2/NiMoS/NF) as efficient HER catalyst. Combining Ni3S2 with NiMoS resulted in the extension of the heterointerfaces between the materials, which facilitated the HER process in alkaline medium. The amorphous Ni3S2/NiMoS with disordered atom arrangement provided abundant active sites. Also, the unique morphology of the catalytic electrode simultaneously enabled it exhibit superhydrophilicity and underwater superaerophobicity. It is beneficial for the sufficient diffusion of the electrolyte onto the catalyst surface and the fast departure of hydrogen bubbles from the surface. As a result, the activity of Ni3S2/NiMoS/NF was higher than that of Pt/C even at high current densities. It is very valuable for industrial applications that require high current density. The superior stability of Ni3S2/NiMoS/NF compared to Pt/C further demonstrated that this catalytic electrode has potential for industrial applications.

Efficient water boiling evaporation via a laser-architected superhydrophilic, underwater superaerophobic, and high infrared emissivity interface

Water boiling evaporation is crucial for diverse applications ranging from thermal management to power generation. However, current strategies for water boiling evaporation interfaces are restricted to complicated fabrication methods, single function, and low availability. Here, a superhydrophilic (a water contact angle of ∼ 0°), underwater superaerophobic (a bubble contact angle of ∼ 161.5°), and high infrared emissivity (up to 0.9) interface is fabricated via femtosecond laser direct writing technology. The extreme surface wettability and underwater superaerophobicity combined with its high infrared emissivity result in greatly enhanced water temperature ascending speeds and water boiling evaporation efficiency. At the optimum scanning speed, with a heater set at 190 °C and ∼ 0.2 mL volume water, the water on the laser-treated stainless steel surface exhibits the shortest time (∼20 s) required for water boiling evaporation, which is significantly shorter than that of on the pristine stainless steel surface (∼71 s). Moreover, the laser-treated surfaces demonstrate fast water boiling in a heating kettle and rapid water evaporation on an electrical heating element. The temperature of water in the laser-treated stainless steel cup rises faster than that in the pristine stainless steel cup, reaching boiling 1 min earlier and stabilizing ∼ 2 °C higher. Femtosecond laser treatment improves the highest water evaporation efficiency of the electrical heating rod by ∼ 49.1%. Understanding the design parameters for efficient water boiling evaporation using laser-treated surfaces can lead to the development of highly effective technology.

Superwettable Surface-Dependent efficiently electrocatalytic water splitting based on their excellent liquid adsorption and gas desorption

Superwettable surfaces has been applied for electrocatalytic water splitting, however, in this phase conversion reaction, the relationship between wettability and electrochemical processes is still unclear. Herein, we demonstrate superwettable surface dependent liquid adsorption and gas desorption for promotion of electrocatalytic reactions, where significant change of wetting state leads to an obvious decrease in overpotential of water splitting reactions. Through an amorphous coating strategy, an intrinsically non-superwettable Ni3S2 catalyst can be transitioned into superwettable Ni3S2 catalyst (Ni3S2@Ni(II)-TC), where water contact angle (WCA) is 0°, implying superhydrophilicity, and bubble contact angle is 153°, meaning under-water superaerophobicity. The Ni3S2@Ni(II)-TC electrode reduces the overpotential of OER from 267 mV to 186 mV and HER from 262 mV to 166 mV at 10 mA cm−2 compared with non-superwettable Ni3S2. The two-electrode electrolyzer performs water electrolysis at 10 mA cm−2 with the voltage of 1.53 V for 20 h without obvious decay.

Nature-Inspired Superwettability Achieved by Femtosecond Lasers

Wettability is one of a solid surface’s fundamental physical and chemical properties, which involves a wide range of applications. Femtosecond laser microfabrication has many advantages compared to traditional laser processing. This technology has been successfully applied to control the wettability of material surfaces. This review systematically summarizes the recent progress of femtosecond laser microfabrication in the preparation of various superwetting surfaces. Inspired by nature, the superwettabilities such as superhydrophilicity, superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superaerophobicity, underwater superaerophilicity, slippery liquid-infused porous surface, underwater superpolymphobicity, and supermetalphobicity are obtained on different substrates by the combination of the femtosecond laser-induced micro/nanostructures and appropriate chemical composition. From the perspective of biomimetic preparation, we mainly focus the methods for constructing various kinds of superwetting surfaces by femtosecond laser and the relationship between different laser-induced superwettabilities. The special wettability of solid materials makes the femtosecond laser-functionalized surfaces have many practical applications. Finally, the significant challenges and prospects of this field (femtosecond laser-induced superwettability) are discussed.

Electrochemically Fabricated Superhydrophilic/Superaerophobic Manganese Oxide Nanowires at Discontinuous Solid–Liquid Interfaces for Enhanced Oxygen Evolution Performances

AbstractThe design of an electrode surface is critical to the oxygen evolution reaction (OER) during electrochemical water splitting since the catalytic activity depends strongly on surface characteristics, such as the coating uniformity of the catalysts, electrolyte wetting, and the gas‐bubble release ability. Here, nonprecious‐metal electrocatalysts (MnO nanowires) are electrochemically deposited on the surface of a porous gold microgrid. The discontinuous solid–liquid contact lines, which are formed by the microgrids during the liquid‐phase electrochemical reaction, generate MnO nanowires with high coating uniformity. The micro/nanostructure of the electrode induces a superhydrophilic surface, which facilitates the wetting of the alkaline electrolyte and increases the electrochemical active surface area. Simultaneously, the underwater superaerophobicity of the electrode promotes the release of the oxygen products, thus reducing the occupation of the catalytic sites by the gas bubbles. The synergistic effect of electrolyte wetting and gas release ensures that this superhydrophilic/superaerophobic electrode exhibits a current density of 10 mA cm−2 for the OER at an overpotential of 530 mV in 0.1 m KOH, placing the catalyst among the best MnO catalysts for OER. This work avails a new basis for synergistically optimizing the synthesis methods (electrolyte wetting and gas‐bubble release) toward high‐performance OER electrodes.

Rational engineering of superaerophobic CoMoSx electrocatalysts for overall water splitting

Superaerophobic electrocatalyst has recently emerged as the promising solution for tackling the gas bubbles adhesion and departure issues in gas evolution reaction. However, the systematic investigation of the morphology-dependent superaerophobicity and the resulted electrocatalytic performance has been less developed. We herein reported a facile in situ chalcogel method to modulate CoMoSx supported on the nickel foam (NF) with three distinct geometries through varying the synthesis time. The morphologies of supported CoMoSx on NF featuring with different roughness were related to the underwater superaerophobicity. Accordingly, the electrocatalytic performance and the size of the gas bubbles released from the surface varied with the morphology evolution. Furthermore, as bifunctional electrocatalysts, the optimal CoMoSx/NF-1h with hierarchical nanostructure could sustain an extremely high current density of 500 mA/cm2 for overall water splitting at 1.89 V. The current results offered an insight for further engineering superaerophobic electrocatalysts in a practical manner.

Facile and environmentally-friendly fabrication of underwater superaerophobic and superaerophilic metallic surfaces through laser ablation and heat treatment

Underwater superaerophilic and superaerophobic metallic surfaces have gained increasing interest due to their significance in both academic research and industrial applications. However, surface chemical modifications with low-surface-energy reagents are usually demanded to fabricate the underwater superaerophilic metallic surfaces, which would increase production cost, operation difficulty, and environmental risk. In this work, we developed a facile and environmentally-friendly method to fabricate underwater superaerophobic and superaerophilic surfaces on aluminum, H59 brass, 304 stainless steel, and titanium substrates through laser ablation and low-temperature heat treatment. After the laser ablation, the metallic surfaces exhibited underwater superaerophobicity since the great affinity of the surfaces with water makes the gaps of the micro/nano structure be filled with water, which repels the bubbles in water. While, after the heat treatment, the laser ablated metallic surfaces became underwater superaerophilic because the hydrophobic airborne organic compounds adsorbed on the laser ablated metallic surfaces prevent water entering into the roughness grooves and make the gaps of the micro/nano structure be occupied by air, which can absorb the bubbles in water. Additionally, it was demonstrated that such underwater superaerophobic and superaerophilic metallic surfaces can be used to control the passage of the underwater bubbles by combining with fabricating the micro-holes structure.

Superwettability‐based separation: From oil/water separation to polymer/water separation and bubble/water separation

AbstractThe separation of oil/water mixtures, polymer/water mixtures, or bubble/water mixtures has broad applications. In this paper, we aim at extending the well‐developed wettability‐based oil/water separation strategy to separate the mixtures of liquid polymers and water and separate bubbles from water. The micro/nanostructures are simply produced on a stainless steel mesh by femtosecond laser treatment to endow the mesh with superhydrophilicity in the air and superoleophobicity, superpolymphobicity, and superaerophobicity in water. The underwater superoleophobicity, superpolymphobicity, and superaerophobicity enable the structured mesh to have the ability of oil/water separation, polymer/water separation, and bubble/water separation. Water can pass through the mesh due to the superhydrophilicity of the laser‐structured mesh. Whereas, the oils, the liquid polymers, and the bubbles in water are intercepted by the structured mesh because such a water‐wetted mesh has strong repellence to oils (underwater superoleophobicity), liquid polymers (underwater superpolymphobicity), and bubbles (underwater superaerophobicity). As a result, the oil, the liquid polymer, and the bubble are successfully separated from water by using the as‐prepared superwetting mesh. We believe that the diversified wettability‐based separation processes will have wide potential applications in environmental protection, energy utilization, industrial manufacture, agricultural production, and so on.

Relationship and Interconversion Between Superhydrophilicity, Underwater Superoleophilicity, Underwater Superaerophilicity, Superhydrophobicity, Underwater Superoleophobicity, and Underwater Superaerophobicity: A Mini-Review.

Superwetting surfaces have received increasing attention because of their rich practical applications. Although various superwettabilities are independently achieved, the relationship between those superwettabilities is still not well-clarified. In this mini-review, we show that superhydrophilicity, underwater superoleophilicity, underwater superaerophilicity, superhydrophobicity, underwater superoleophobicity, and underwater superaerophobicity can be obtained on a same structured surface by the combination of hierarchical surface microstructures and proper chemistry. The relationship and interconversion between the above-mentioned different superwettabilities are also well-discussed. We believe that the current discussion and clarification of the relationship and interconversion between different superwettabilities has important significance in the design, fabrication, and applications of various superwetting materials.

Hydrogen evolution over N-doped CoS2 nanosheets enhanced by superaerophobicity and electronic modulation

Doping heteroatoms offers an opportunity to tune the electronic structure and promote hydrogen evolution reaction (HER) activity for an electrocatalyst. Herein, N-doped CoS2 nanosheets (N-CoS2 NSs) were deposited on carbon cloth via a hydrothermal and post-sulfuration process. The N-CoS2 NSs intercrossed with each other, exhibiting promising superaerophobicity without any adhesion to underwater bubbles. XPS analyses revealed the presence of electron transfer from Co to N dopant in N-CoS2, further proved by differential charge density and Bader charge analyses of density functional theory calculations. The electron transfer intrinsically altered the electronic structure of N-CoS2 and decreased the Gibbs adsorption energy of hydrogen over Co sites. The metallicity of CoS2 was remained after N doping, ascertaining a fast charge transfer and favorable HER process. The N-CoS2/CC showed an enhanced electrocatalytic HER activity with a Tafel slope of 60.1 mV dec−1 and low overpotential of 112 mV at a current density of 10 mA cm−2. Such promising performance is ascribed to the weak hydrogen adsorption upon N doping as well as the underwater superaerophobicity of N-CoS2/CC electrode. Our work might shed light on designing efficient and inexpensive HER electrocatalysts by tailoring their electronic structure and surface superaerophobicity.

Underwater superoleophobicity of a superhydrophilic surface with unexpected drag reduction driven by electrochemical water splitting

In a fluid transport system, fouling and drag force are key issues because they cause additional expenditure for cleaning and fuel consumption, respectively. Various surface treatments, especially wettability control, have been explored to solve the related problems. However, surface modification has inherent limitations because it has conflicting effects on the anti-fouling and drag force. A superhydrophilic surface has an excellent anti-fouling property underwater because of its excellent oleophobicity, but it also causes drag enhancement due to its high affinity towards water. On the other hand, the superhydrophobic surface shows a drag-reducing effect due to the formation of an air layer underwater, but at the same time, it is easily contaminated by oil due to its high oleophilicity. Herein, we suggest a novel method to overcome these conflicting effects for a superhydrophilic surface for anti-fouling and drag enhancement by employing electrochemical gas generation. Due to its large effective surface area and underwater superaerophobicity, the superhydrophilic surface is ideal for use as an electrocatalyst for electrochemical water splitting (EWS). The small and fine air bubbles (~30 μm sizes) generated on a superhydrophilic electrode surface form an air layer, which lowers the fluid density near the surface and induces a drag reduction efficiency of ~26.5% compared to the flat surface. This value is comparable to the superhydrophobic surface, which has a high drag reduction efficiency. The current study presents an original idea that combines the oleophobicity and superaerophobicity of a superhydrophilic surface for realizing EWS-bubble induced anti-fouling and drag reduction effects simultaneously, which has not been reported before.

Air Bubble Contro: Trapped Air‐Induced Reversible Transition between Underwater Superaerophilicity and Superaerophobicity on the Femtosecond Laser‐Ablated Superhydrophobic PTFE Surfaces (Adv. Mater. Interfaces 17/2019)

Air Bubble Contro: Trapped Air‐Induced Reversible Transition between Underwater Superaerophilicity and Superaerophobicity on the Femtosecond Laser‐Ablated Superhydrophobic PTFE Surfaces (Adv. Mater. Interfaces 17/2019)

Trapped Air‐Induced Reversible Transition between Underwater Superaerophilicity and Superaerophobicity on the Femtosecond Laser‐Ablated Superhydrophobic PTFE Surfaces

AbstractControlling the behavior of underwater bubbles on a solid surface and establishing the contact model between a gas bubble and a solid substrate in a water medium have great significance. Herein, a method is proposed to realize the reversible switching between underwater superaerophilicity and superaerophobicity on the femtosecond laser‐induced rough polytetrafluoroethylene (PTFE) surface by controlling the trapped air layer in water. The original femtosecond laser‐structured PTFE surface is superhydrophobic in air and superaerophilic in water. After vacuum‐pumping treatment, the trapped air layer around the microstructures of the underwater superhydrophobic PTFE surface is removed, leading to the underwater superaerophobicity of the sample surface. The trapped air layer can be recovered and the PTFE surface can regain the underwater superaerophilicity by drying treatment and the immersion of the sample in water again. Such a round‐trip transition can achieve the bubble's “locating capture” and the control of the selective passage of air bubbles through a perforated rough PTFE sheet. It is believed that the resultant underwater superaerophobic and superaerophilic surfaces will have significant applications in manufacturing submarine gas‐collection devices, improving the chemical reaction between liquid reagent and gas, and avoiding the microchannel blockage caused by bubbles in microfluidics and biosensor devices.

Substrate-Independent, Fast, and Reversible Switching between Underwater Superaerophobicity and Aerophilicity on the Femtosecond Laser-Induced Superhydrophobic Surfaces for Selectively Repelling or Capturing Bubbles in Water.

In this paper, the reversible switching between underwater (super-) aerophilicity and superaerophobicity was achieved on various femtosecond (fs) laser-induced superhydrophobic surfaces. A range of materials including Al, stainless steel, Cu, Ni, Si, poly(tetrafluoroethylene), and polydimethylsiloxane were first transformed to superhydrophobic after the formation of surface microstructures through fs laser treatment. These surfaces showed (super-) aerophilicity when immersed in water. In contrast, if the surface was prewetted with ethanol and then dipped into water, the surfaces showed superaerophobicity in water. The underwater aerophilicity of the superhydrophobic substrates could easily recover by drying. The switching between the underwater aerophilicity and superaerophobicity can be fast repeated many cycles and is substrate-independent in stark contrast to common wettability-switchable surfaces based on stimuli-responsive chemistry. Therefore, the as-prepared superhydrophobic surfaces can capture or repel air bubbles in water by selectively switching between underwater superaerophobicity and aerophilicity. Finally, we demonstrated that the underwater bubbles could pass through an underwater aerophilic porous sheet but were intercepted by an underwater superaerophobic porous sheet. The selective passage of the underwater bubbles was achieved by the reversible switching between the underwater aerophilicity and superaerophobicity. We believe that this substrate-independent and fast method of switching air wettability has important applications in controlling air behavior in water.

An all superantiwetting surface in water–oil–air systems

Six superantiwetting states, superhydrophobicity, superoleophobicity, underoil superhydrophobicity, underwater superoleophobicity, and underoil and underwater superaerophobicity, are realized on one surface. The all superantiwetting surface can be extended to polar liquid–nonpolar liquid–air systems and be used for on-demand separation of immiscible organic liquids.

Femtosecond laser-induced superwetting surfaces

The materials with various superwettabilities have attracted increasing interests because of their broad applications, such as water/oil-repellent coating, self-cleaning coating, anti-ice/fog/snow, manipulation of droplets, oil/water separation, antifouling, anti-corrosion, underwater drag reduction, lab chip, cell engineering, fog collection, microfluidic systems, and so on. After millions of years of evolution, natural organisms have evolved perfect structures and multifunctional surfaces. The surfaces of many plants and animals are endowed with special wettability, which inspire scientists to design and fabricate bionic surfaces with superwettability. It is found that the surface wettability is mainly determined by the chemical composition and surface microstructure. Inspired by nature (e.g., lotus leaf, fish scale, pitcher plant, red rose petal, rice leaf, butterfly wings), a large number of superwetting surfaces have been developed by various microfabrication technologies. However, those methods, more or less, are suffered from some inherent limitations, e.g., tight restriction on special materials, lack flexibility, and complex fabrication process. The development of a widely applicable and simple tool that can easily achieve various superwettabilities is still the major trend in this research field. Femtosecond laser has been proven to be a strong tool in advanced micro/nanofabrication. Recently, this tool is also successfully applied to design and change the surface wettability of different solid substrates. The femtosecond laser microfabrication has many advantages, such as small heat-affected zone, precise ablation threshold, non-contact process, and high resolution. In addition, the femtosecond laser can ablate almost all of the materials, including semiconductors, metals, polymers, glasses, and ceramics. By the simple one-step scanning manner, micro/nanoscale hierarchical structures can be directly created on the surfaces of different materials via the femtosecond laser treatment. Taking advantages of these features, femtosecond laser microfabrication have also been a great success in preparing superwetting surfaces. In this review, we summarize the recent developments in femtosecond laser-structured surfaces with special wettability. The article starts with the introduction related to the theoretical basis of wettability and the features of femtosecond laser microfabrication, as the background. Then, different superwettabilities achieved by femtosecond laser treatment are summarized, grouped by the characteristic of wettability. Inspired by nature, those laser-induced surface microstructures have superhydrophobicity, underwater superoleophobicity, underwater superaerophobicity, slippery liquid-infused porous surface, controllable adhesion, and anisotropic wettability, respectively. In each class of properties, the biological surface with such superwettability is introduced in advance. Then, how to use a femtosecond laser to design and fabricate these special superwetting microstructures was discussed, i.e., the preparation principle. According to the formation mechanism of these superwetting states, the internal relation between the femtosecond laser-induced surface microstructure and the macroscopic wettability is revealed; that is, surface microstructure determines the surface wettability. Next, some examples about the practical applications of the femtosecond laser-designed superwetting surfaces are also introduced following various superwettabilities, such as self-cleaning, manipulation of liquid droplets, oil/water separation, cell engineering, fog harvesting, buoyancy enhancement, liquid patterning, underwater gas capture, and detection analysis. Finally, the existing challenges and future prospects of this fast-growing field are discussed. The technology of femtosecond laser-controlled surface wettability is still irreplaceable by other microfabrication methods because of its special advantages. Such technology is better at designing and preparing fine microstructures and complex and heterogeneous wettability. This technology will have superiority in some special applications, such as cell engineering, military facilities, etc. With more and more scientists and engineers getting into this research field and being devoted to fabricating various superwetting surfaces, this research direction will have a bright and exciting future.

Underwater Superaerophobic and Superaerophilic Nanoneedles-Structured Meshes for Water/Bubbles Separation: Removing or Collecting Gas Bubbles in Water.

Water/bubbles separation has great practical significance, which can avoid the harm caused by underwater bubbles or cleverly collect useful gas bubbles in water. Here, a Cu(OH)2‐nanoneedles‐structured rough copper mesh is fabricated by a one‐step chemical reaction. The original rough mesh shows superhydrophilicity in air and superaerophobicity in water. The underwater superaerophobic mesh has great anti/removing‐bubbles ability in water. In contrast, the rough mesh switches to superhydrophobicity in air and superaerophilicity in water after further being modified with fluoroalkylsilane. The underwater superaerophilic mesh can absorb bubbles and allow bubbles to pass through the mesh. Based on the superhydrophilic/superaerophobic mesh and the superhydrophobic/superaerophilic mesh, a strategy to remove gas bubbles from the water pipe is proposed, and an in‐water bubbles‐collection device is also designed. It is believed that these two kinds of mesh will have more applications in controlling the behavior of underwater bubbles.

Water/Bubbles Separation: Underwater Superaerophobic and Superaerophilic Nanoneedles‐Structured Meshes for Water/Bubbles Separation: Removing or Collecting Gas Bubbles in Water (Global Challenges 4/2018)

Water/Bubbles Separation: Underwater Superaerophobic and Superaerophilic Nanoneedles‐Structured Meshes for Water/Bubbles Separation: Removing or Collecting Gas Bubbles in Water (Global Challenges 4/2018)

Reversible switch between underwater superaerophilicity and superaerophobicity on the superhydrophobic nanowire-haired mesh for controlling underwater bubble wettability

Controlling the underwater bubble wettability on a solid surface is of great research significance. In this letter, a simple method to achieve reversible switch between underwater superaerophilicity and underwater superaerophobicity on a superhydrophobic nanowire-haired mesh by alternately vacuumizing treatment in water and drying in air is reported. Such reversible switch endows the as-prepared mesh with many functional applications in controlling bubble’s behavior on a solid substrate. The underwater superaerophilic mesh is able to absorb/capture bubbles in water, while the superaerophobic mesh has great anti-bubble ability. The reversible switch between underwater superaerophilicity and superaerophobicity can selectively allow bubbles to go through the resultant mesh; that is, bubbles can pass through the underwater superaerophilic mesh while are fully intercepted by the underwater superaerophobic mesh in a water medium. We believe these meshes will have important applications in removing or capturing underwater bubbles/gas.

Femtosecond laser induced underwater superaerophilic and superaerophobic PDMS sheets with through microholes for selective passage of air bubbles and further collection of underwater gas.

Controlling underwater bubble behavior on a solid surface is of great research significance, particularly in extreme cases. However, the realization of artificial underwater superaerophobic or superaerophilic surfaces is still a challenge. Herein, a micro/nanoscale hierarchical rough structure was formed on polydimethylsiloxane (PDMS) surface by one-step femtosecond laser ablation. The as-prepared surface showed superhydrophobicity in air and superaerophilicity in water. Interestingly, the wettability of such a PDMS surface could be easily switched to in-air superhydrophilicity and underwater superaerophobicity once it was further irradiated by oxygen plasma because the surface chemistry changed. The original femtosecond laser-structured underwater superaerophilic PDMS surface could absorb/capture bubbles, while the plasma-treated underwater superaerophobic surface had excellent anti-bubble ability in water. A rough through-microhole-array PDMS sheet was prepared by a mechanical drilling process and subsequent femtosecond laser ablation. The sheet could selectively allow bubbles to pass, that is, the porous underwater superaerophilic sheet allowed bubbles to pass through, while the porous underwater superaerophobic sheet was able to intercept bubbles in a water medium. Using the porous underwater superaerophilic PDMS sheet as the core component, a device that has great ability of collecting underwater bubbles/gas was also designed.