Superhydrophobic Copper Oxide Research Articles

Robust superhydrophobic copper oxide layer with high-low staggered structure on polymers for strong anti-icing and all-day de-icing

Recently, superhydrophobic materials with photothermal and electrothermal conversion functions have received much attention in solving icing problems. However, the currently emerging methods still suffer from the complexity of the preparation method, poor hydrophobicity, or poor abrasion resistance. Herein, a robust copper oxide layer with a high-low staggered structure was prepared on the polymer surface by laser-induced selective metallization (LISM), which can be used for anti-icing and all-day (daytime and night) de-icing. The high-low staggered structure of the superhydrophobic copper oxide surface gives the surface excellent hydrophobicity, light trapping capability, and high electrical resistance, resulting in the surface with superior delayed icing, photothermal de-icing, and electrothermal de-icing capabilities. The copper oxide layer has a water contact angle (CA) of up to 161.3° and a surface delayed icing time of 1374 s, which is 8.23 times longer than the conventional copper layer prepared by LISM. In addition, the surface temperatures of the copper oxide layer are 67.6 °C and 57.3 °C, respectively, under 1.0 sun irradiation or at 5 V applied voltage. Under these conditions, the copper oxide surface’s photothermal and electrothermal de-icing times are 159 and 186 s. In addition, the high-low staggered structure of the prepared superhydrophobic copper oxide surface makes the surface less susceptible to abrasion, resulting in excellent mechanical robustness of the copper oxide layer surface. The proposed method is suitable for low-cost large-scale manufacturing, which guides the preparation of copper oxide superhydrophobic materials for anti-icing and all-day de-icing, and has good application prospects.

Fluorine-Free Photothermal Superhydrophobic Copper Oxide Micro-/Nanostructured Coatings for Anti-icing/De-icing Applications

Excessive ice accumulation in low-temperature environments can lead to serious economic losses and unexpected safety hazards. Therefore, effective measures must be developed to impede icing and to quickly de-ice. Superhydrophobic coatings with low surface energy and rough structure can effectively retard ice formation. However, in low temperature and high humidity environments, superhydrophobic coatings may form interlocking structures with ice, which increases the adhesion strength of ice to the coating and makes it difficult to remove ice. Photothermal coatings utilize the photothermal effect of photothermal materials to achieve effective photothermal conversion for photothermal de-icing. So photothermal de-icing is a good solution to the drawbacks of superhydrophobic anti-icing. At the same time, fluorine-containing materials used to prepare superhydrophobic surfaces are potentially hazardous to humans and the environment. Therefore, we show a method to achieve a combination of photothermal de-icing and superhydrophobic anti-icing to obtain a fluorine-free photothermal superhydrophobic coating (PSC). The substrate surface is electrolessly plated with copper and then etched to obtain the superhydrophobic structure with photothermal effect. Under simulated irradiation conditions, the PSC reached a surface temperature of 82.0 °C in 10 min and exhibited an extremely short photothermal de-icing time, about one-third that of the substrate. In addition, the self-cleaning, antifouling, and alkali resistance of PSC could help the coating in long-term outdoor applications. The robust copper oxide (CuO) micro-/nanostructures allow the PSC to maintain significant photothermal and hydrophobic properties after friction and water impact, contributing to improving environmental resilience. The synergy effect of superhydrophobicity and photothermal properties greatly improves anti-/de-icing performance, providing a more convenient and environmentally friendly approach for the design of anti-/de-icing coatings.

Fabrication of CuO mesh for efficient oil/water separation

In this work, we have fabricated superhydrophobic copper oxide mesh by simply annealing copper mesh followed by surface modification with octadecanoic acid. The XRD analysis of the sample revealed the presence of cubic Cu2O and monoclinic CuO phases of copper oxide. The FESEM analysis of the samples demonstrated the presence of wire-like structures of copper oxide. The successful deposition of octadecanoic acid was confirmed by FT-IR analysis. The wettability study has confirmed that the sample modified with octadecanoic acid showed excellent superhydrophobic behavior with a water contact angle of 156° and superoleophilic behavior with oil contact angles of 0°. Owing to the excellent superhydrophobic-superoleophilic behavior, the modified copper oxide mesh allows oil to penetrate while restricting the passage of water through its surface, which results in the separation of the oil/water mixture. The modified mesh showed ∼98% separation efficiency and high flux for different oil/water mixtures. Moreover, this mesh exhibited excellent recyclability by maintaining its high efficiency even after 10 separation cycles. Owing to its high separation efficiency (∼98%), recyclability and high flux, the modified copper oxide mesh meets the current need for the separation of oil/water mixtures in real-world applications.

Single-step fabrication of superhydrophobic urchin-like copper oxide nanopowders: Effect of structure-directing agents

The use of superhydrophobic metal oxide nanoparticles has been considered for the construction of self-cleaning, anti-corrosion, anti-icing, anti-fogging, and oil/water separation surfaces, but the low chemical and thermal stabilities of applied organic modifiers restrict their wide applications. In this study, the superhydrophobic copper oxide nanoparticles with the hierarchical urchin, flower and spherical-like architectures were fabricated via a simple and facile single-step hydrothermal method without any post chemical modification. Taguchi experimental design was used to evaluate and optimize the synthesis parameters such as structure-directing agent concentration and its type, synthesis temperature, and the molar ratio of the complexing agent to the initial copper (II) ions. The wettability characteristics of synthesized samples were assessed based on the chemical composition of powders and their morphological structures. The scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) analyses were used to characterize the resultant samples. The wettability properties of samples were determined by measuring the water contact angle (WCA) and contact angle hysteresis (CAH). The results confirmed that the synthesized sample under the optimum condition had an excellent superhydrophobic property with a WCA of 158.4⁰±2⁰ and CAH of 1.5⁰. The surface wettability evaluations also revealed that the morphology and surface free energy of the powders could be controlled simultaneously by alterations of type and concentration of the structure-directing agent, in order to enhance the anti-wetteing characteristics.

Copper-based nanoribbons fabricated on a copper substrate by a liquid-solid reaction and their corrosion performance

In this study, copper oxide nanoribbons were fabricated on a Cu substrate by a simple liquid-solid reaction. The superhydrophobic copper oxide surface was formed by the immersion of the sample in an ethanol solution of stearic acid. The water contact angle on this surface was 153.6°. A slippery copper oxide surface was prepared by the immersion of superhydrophobic copper oxide in DuPont Krytox. Electrochemical measurement revealed that the corrosion resistance of the slippery surface was more stable than those of bare Cu and superhydrophobic surface. The inhibition efficiency reached up to 92.68%. This study provided a possible method for preventing copper corrosion.

Droplet Jumping on Superhydrophobic Copper Oxide Nanostructured Surfaces

Condensation has been widely explored because of its importance in numerous applications including water desalination, water harvesting, power generation, and thermal management. Previous studies have shown that the enhancement in condensation heat transfer can be achieved with the engineering of surface wettability by promoting dropwise condensation that has lower thermal resistances than film condensation. In this paper, we explore microscopic-level droplet dynamics using various copper surfaces by employing a simple two-step chemical method. Nanostructured copper surfaces are prepared by a chemical immersion method using alkaline solution and are further functionalized by using dodecanoic acid that provides the hydrophobicity. The wetting properties and corresponding condensation processes are examined using an optical microscope and environmental scanning electron microscope by capturing the real-time phase change process. The measurements confirm that the surface morphology with the largest feature ratio promotes droplet jumping events. The understanding of condensation behaviors associated with nanostructured copper oxide surfaces can provide design rules for efficient surface structures for numerous condensation applications.

Solar-driven high-temperature steam generation at ambient pressure

This paper demonstrates a facile solar steam generator for the generation of high-temperature steam at ambient pressure. The steam generator consists of a coiled copper tube (CCT) that serves as both solar collector and vapor heater. Both the inner and outer surfaces of the CCT are coated with black superhydrophobic copper oxide layer to improve the solar absorption, and such superhydrophobic layer prevents the condensation and clogging of water vapor during the heating process. Solar vapor is generated at the inlet of CCT through interfacial evaporation and subsequently heated inside the CCT as the vapor travels from the inlet to the outlet. By employing this approach, steam temperature that exceeds 100 °C under 1-sun illumination is demonstrated. Furthermore, steam temperature could reach as high as 250 °C under 10-sun illumination at ambient pressure. Successful sterilization experiment was also performed in this work to demonstrate the potential application of the high temperature steam generated. Such demonstrated approach helps open up the potential usage of solar-driven interfacial evaporation in applications that involve high-temperature steam, including medical sterilization, chemical manufacturing and food processing.

Enhanced Coalescence-Induced Droplet-Jumping on Nanostructured Superhydrophobic Surfaces in the Absence of Microstructures.

Superhydrophobic surfaces are receiving increasing attention due to the enhanced condensation heat transfer, self-cleaning, and anti-icing properties by easing droplet self-removal. Despite the extensive research carried out on this topic, the presence or absence of microstructures on droplet adhesion during condensation has not been fully addressed yet. In this work we, therefore, study the condensation behavior on engineered superhydrophobic copper oxide surfaces with different structural finishes. More specifically, we investigate the coalescence-induced droplet-jumping performance on superhydrophobic surfaces with structures varying from the micro- to the nanoscale. The different structural roughness is possible due to the specific etching parameters adopted during the facile low-cost dual-scale fabrication process. A custom-built optical microscopy setup inside a temperature and relative humidity controlled environmental chamber was used for the experimental observations. By varying the structural roughness, from the micro- to the nanoscale, important differences on the number of droplets involved in the jumps, on the frequency of the jumps, and on the size distribution of the jumping droplets were found. In the absence of microstructures, we report an enhancement of the droplet-jumping performance of small droplets with sizes in the same order of magnitude as the microstructures. Microstructures induce further droplet adhesion, act as a structural barrier for the coalescence between droplets growing on the same microstructure, and cause the droplet angular deviation from the main surface normal. As a consequence, upon coalescence, there is a decrease in the net momentum in the out-of-plane direction, and the jump does not ensue. We demonstrate that the absence of microstructures has therefore a positive impact on the coalescence-induced droplet-jumping of micrometer droplets for antifogging, anti-icing, and condensation heat transfer applications.

Microscopic droplet formation and energy transport analysis of condensation on scalable superhydrophobic nanostructured copper oxide surfaces.

Utilization of nanotechnologies in condensation has been recognized as one opportunity to improve the efficiency of large-scale thermal power and desalination systems. High-performance and stable dropwise condensation in widely-used copper heat exchangers is appealing for energy and water industries. In this work, a scalable and low-cost nanofabrication approach was developed to fabricate superhydrophobic copper oxide (CuO) nanoneedle surfaces to promote dropwise condensation and even jumping-droplet condensation. By conducting systematic surface characterization and in situ environmental scanning electron microscope (ESEM) condensation experiments, we were able to probe the microscopic formation physics of droplets on irregular nanostructured surfaces. At the early stages of condensation process, the interfacial surface tensions at the edge of CuO nanoneedles were found to influence both the local energy barriers for microdroplet growth and the advancing contact angles when droplets undergo depinning. Local surface roughness also has a significant impact on the volume of the condensate within the nanostructures and overall heat transfer from the vapor to substrate. Both our theoretical analysis and in situ ESEM experiments have revealed that the liquid condensate within the nanostructures determines the amount of the work of adhesion and kinetic energy associated with droplet coalescence and jumping. Local and global droplet growth models were also proposed to predict how the microdroplet morphology within nanostructures affects the heat transfer performance of early-stage condensation. Our quantitative analysis of microdroplet formation and growth within irregular nanostructures provides the insight to guide the anodization-based nanofabrication for enhancing dropwise and jumping-droplet condensation performance.

Jumping-droplet electrostatic energy harvesting

Micro- and nanoscale wetting phenomena have been an active area of research due to its potential for improving engineered system performance involving phase change. With the recent advancements in micro/nanofabrication techniques, structured surfaces can now be designed to allow condensing coalesced droplets to spontaneously jump off the surface due to the conversion of excess surface energy into kinetic energy. In addition to being removed at micrometric length scales (∼10 μm), jumping water droplets also attain a positive electrostatic charge (∼10–100 fC) from the hydrophobic coating/condensate interaction. In this work, we take advantage of this droplet charging to demonstrate jumping-droplet electrostatic energy harvesting. The charged droplets jump between superhydrophobic copper oxide and hydrophilic copper surfaces to create an electrostatic potential and generate power during formation of atmospheric dew. We demonstrated power densities of ∼15 pW/cm2, which, in the near term, can be improved to ∼1 μW/cm2. This work demonstrates a surface engineered platform that promises to be low cost and scalable for atmospheric energy harvesting and electric power generation.

Condensation on Superhydrophobic Copper Oxide Nanostructures

Condensation is an important process in both emerging and traditional power generation and water desalination technologies. Superhydrophobic nanostructures promise enhanced condensation heat transfer by reducing the characteristic size of departing droplets via coalescence-induced shedding. In this work, we investigated a scalable synthesis technique to produce functionalized oxide nanostructures on copper surfaces capable of sustaining superhydrophobic condensation and characterized the growth and departure behavior of the condensed droplets. Nanostructured copper oxide (CuO) films were formed via chemical oxidation in an alkaline solution resulting in dense arrays of sharp CuO nanostructures with characteristic heights and widths of ≈1 μm and ≈300 nm, respectively. To make the CuO surfaces superhydrophobic, they were functionalized by direct deposition of a fluorinated silane molecular film or by sputtering a thin gold film before depositing a fluorinated thiol molecular film. Condensation on these surfaces was characterized using optical microscopy and environmental scanning electron microscopy to quantify the distribution of nucleation sites and elucidate the growth behavior of individual droplets with characteristic radii of ≈1–10 μm at supersaturations ≤1.5. Comparison of the measured individual droplet growth behavior to our developed heat transfer model for condensation on superhydrophobic surfaces showed good agreement. Prediction of the overall heat transfer enhancement in comparison to a typical dropwise condensing surface having an identical nucleation density suggests a restricted regime of enhancement limited to droplet shedding radii <~2.5 μm due to the large apparent contact angles of condensed droplets on the fabricated CuO surfaces. The findings demonstrate that superhydrophobic condensation typified by coalescence-induced droplet shedding may not necessarily enhance heat transfer and highlights the need for further quantification of the effects of surface structure on nucleation density and careful surface design to minimize parasitic thermal resistances.