Super-Hydrophobic Photothermal Copper Foam for Multi-Scenario Solar Desalination: Integrating Anti-Icing, Self-Cleaning, and Mechanical Durability

Abstract

Solar desalination is widely regarded as an effective way to solve freshwater scarcity. However, the balance between the costs of micro-nanostructures, thermal regulation, and the durability of interface evaporators must all be considered. In this study, a super-hydrophobic copper foam with hierarchical micro-nanostructures exhibited temperatures greater than 66 °C under solar illumination of 1 kW·m⁻². Significantly, the modified copper foam acting as a solar interface evaporator had a water harvesting efficiency of 1.76 kg·m⁻²·h⁻¹, resulting from its good photothermal conversion and porous skeleton. Further, the anti-deicing, self-cleaning, and anti-abrasion tests were carried out to demonstrate its durability. The whole fabrication of the as-prepared CF was only involved in mechanical extrusion and spray-coating, which is suitable for large-scale production. This work endows the interface evaporator with super-hydrophobicity, photo-thermal conversion, anti-icing, and mechanical stability, all of which are highly demanded in multi-scenario solar desalination.

1. Introduction

Freshwater resources are crucial for human survival and social development. However, with population growth and environmental changes, freshwater scarcity is becoming increasingly severe [1,2,3,4,5]. To obtain freshwater resources from marine environments, researchers have developed a variety of devices with biomimetic structures usable for seawater desalination [6,7,8,9,10]; typically, the pure water extracted from high salinity seawater through interface evaporators under solar illumination. Some evaporator surfaces are equipped with gold (Au) [11], silver (Ag) [12,13] nanoparticles [14,15,16], or MXenes materials [17,18,19,20] that feature high photo-thermal conversion efficiency. However, these high-cost raw materials are not suitable for large-area preparation, designing surface micro-nanostructures with subwavelength scale and light capture effects is a feasible solution to enhance photothermal conversion capability.

It is acknowledged that inverted pyramids [21], bionic plant stem structures [22,23], and nanoscale structures [24,25] are typical anti-reflection structures, showing a light adsorption rate of no less than 98%. Li et al. [26] processed aluminum by picosecond laser in point–line–plane scanning mode to prepare anti-reflection solar desalination materials, and achieved an evaporation efficiency of 2.325 kg·m⁻²·h⁻¹ under a solar irradiation level of 0.95 kW·m⁻². Wang et al. [27] prepared microchannel hydrogels with a tree structure to achieve an evaporation efficiency of 4.12 kg·m⁻²·h⁻¹ at a light intensity of 1 kW·m⁻². But, the structural formation originates from photolithography and laser ablation, involving a low fabrication efficiency and sophisticated treatment. Inspired by plant transpiration, Zheng et al. [28] design a three-dimensional array of pore-structured evaporators using natural wood. The evaporator could achieve a freshwater yield of 4.8 L·m⁻²·h⁻¹ under a solar intensity of 1 kW·m⁻². But, the corrosion damage of the wood immersed in seawater is concerning. Based on the above, the question of how to balance satisfactory evaporation efficiency with the facile and low-cost fabrication of anti-reflective micro-nanostructures is still a considerable issue.

Outside of photothermal conversion capability, thermal management at the evaporation interface and water vapor transport channels also play an important role in evaporation efficiency [29,30]. Moreover, durability and environmental adaptability are critical features that determine stability and service life during seawater desalination, e.g., self-cleaning, anti-fouling, and anti-icing ability, which need to be evaluated for evaporators working in outdoor environments.

Herein, a porous copper foam (CF) decorated with photothermal super-hydrophobic nanoparticles was developed by facile mechanical extrusion and a spray-coating method. The copper foam features a microscale porous structure coupled with nanoscale spikes, which act as water vapor transport channels and enhance the interaction between solar energy and the surface. The water harvesting efficiency, ranging from 0.91 kg·m⁻²·h⁻¹ to 4.5 kg·m⁻²·h⁻¹ for the solar desalination copper foam (SDCF), has been demonstrated under solar irradiation from 0.5 to 3.0 kW·m⁻². In order to adapt to the harsh outdoor working environment, the decorated nanoparticles make the water contact angle (WCA) exhibited by CF reach 150.9° and the rolling angle 2°, achieving an outstanding self-cleaning and anti-icing capacity. Moreover, the metal microcolumns, as the sacrifice layer on the SDCF surface, can resist sever abrasion in tests, ensuring the maintenance of the skeleton and functional coatings even facing the potential external forces. The as-prepared solar evaporator provides an effective strategy to deal with seawater desalination, due to its simple fluorine-free and eco-friendly preparation process, and reliable durability.

2. Materials and Methods

2.1. Materials

Copper foam (CF) was purchased from Suzhou Keshenghe Co., Ltd. Terminated hydroxyl carbon nanotubes (THCNTs) were purchased from Guangdong Dazhan Nano Co., Ltd. Ethanol (AR, ≥99.7%), n-decyltriethoxysilane (DTES, 98%), sodium hydroxide (NaOH, AR, ≥99.7%), ammonium persulfate, tetrahydrofuran (THF), and ammonia were purchased from Maclean Reagent Co., Ltd. (Shanghai, China). Polystyrene foam board, non-woven fabric (NWF) and acrylic board were purchased from local market.

2.2. Preparation of Super-Hydrophobic Carbon Nanotubes (SHB-CNTs)

A total of 1 g of THCNTs was added to 60 mL of ethanol. After ultrasonic dispersion, 2 mL of ammonia and 1 g of DTES were added and stirred at 50 °C for 6 h. After washing the sample twice with alcohol, it was then filtered and dried to obtain SHB-CNTs.

2.3. Preparation of Copper Foam with Micro-Nano Structure

Porous molds were prepared by processing acrylic sheets using a laser. CF was placed under the porous mold and embossed to prepare a micro-pillar micro-groove structure. Then, the CF was put into a solution of NaOH and ammonium persulfate for alkali corrosion to corrode the CF into copper hydroxide to prepare the nanospike structure. The copper hydroxide was then heated using an alcohol lamp to turn the unstable copper hydroxide into copper oxide. SHB-CNTs were sprayed on the surface of copper foam to make the surface of copper foam super-hydrophobic.

2.4. Preparation of SDCF

The preparation of SDCF is shown in Figure 1. Firstly, the CF was placed under a porous mold and imprinted to prepare the micropillar structure, and then the CF was placed in NaOH solution for alkaline etching to generate a nanospike structure. The copper hydroxide is heated by an alcohol lamp to change the copper hydroxide into a stable copper oxide. Then, SHB-CNTs were sprayed on the surface of copper foam to obtain the SDCF.

2.5. Anti-Deicing Experiment

The anti-deicing experiments were all carried out using a freezing platform at a temperature of 15 °C and a humidity of 30%. The samples were placed horizontally on the freezing platform, and then 10 μL drop of ultrapure water was added to the surface. The temperature of the freezing platform was set to −15 °C. The freezing platform was activated, and a high-speed camera was switched on to record the icing process. After freezing, the samples were placed under a solar intensity of 1 kW·m⁻² irradiation to record the melting process of the ice droplets.

The adhesion strength of the ice was measured using a cooling platform and a digital force transducer (accuracy 0.01 N). After the water droplets were completely frozen, the digital force sensor feed was controlled by a rolling screw, and the maximum peak force of the sensor was recorded after the ice was smoothly removed.

2.6. Characterization

The microscopic morphology of the samples was observed by scanning electron microscopy (SEM, Merlin, Zeiss, Oberkochen, Germany), and the elemental distribution of the samples was analyzed by energy dispersive spectroscopy (EDS) in SEM. The chemical composition of the samples was characterized using a Fourier Transform Infrared Spectrometer (FTIR, model WD6901A, manufactured by Tianjin Energy Spectrum Technology Co., Ltd., Tianjin, China). The contact and roll angles of the sample surfaces were tested using a contact angle system OCA 20 (Dataphysics, Filderstadt, Germany). The delayed icing time of the droplets was recorded by high-speed camera (FASTCAM MINI UX100TYPE 200K-M, Photron, Tokyo, Japan); the photothermal deicing process was recorded using a mobile phone camera. Sample temperature changes were recorded using an infrared camera.

3. Results and Discussion

3.1. Morphological Characterization of SDCF

The micro-morphology evolution during the preparation was characterized as shown in Figure 2. The pristine copper foam is smooth and flat. After alkali treatment, the surface of copper foam was covered with a nanospike structure. Then, was is heated by alcohol lamp to generate copper oxide; as shown in Figure 2c, the copper oxide nanometer diameter spike structure still existed. At subwavelength scales, nano-spike structures boosted light absorption through an anti-reflection mechanism. Figure 2d shows the surface of the copper foam coated with SHB-CNTs, which was uniformly distributed on the surface of the copper foam. The SDCF has a water CA = 150.9° and a water RA < 2°, with excellent super-hydrophobicity.

Figure 2e shows the FTIR spectra, SHB-CNTs compared with THCNTs, and a new absorption peak at 1116 cm⁻¹ corresponding to the stretch adsorption of Si-O-C [31]. The two new characteristic peaks at 2850 cm⁻¹ and 2917 cm⁻¹ in the profile of SHB-CNT are the telescopic vibration absorption peaks of -CH₂ and -CH₃ [32], proving that the DTES was successfully grafted with THCNTs. The absorption peak of -OH centered at 3440 cm⁻¹ was weakened in SHB-CNTs, owing to the grafting with DTES. Figure 2f is the EDS spectra of SDCF, and it can be clearly seen that silicon elements from SHB-CNTs are evenly distributed on the SDCF surface. This proves that the photothermal super-hydrophobic surface was successfully prepared.

3.2. Solar Desalination Capacity of the SDCF

The schematic of the solar desalination process is shown in Figure 3a. Initially, a thermal insulation foam (TIF) is placed on the water surface to provide buoyancy and minimize heat transfer to the bulk water. Then, the non-woven fabric (NWF) is uniformly spread over the TIF, and both of its sides are immersed in water to transport water to the surface by capillary force. Finally, the SDCF is placed on the NWF as a photothermal interface to provide thermal energy for solar desalination. Figure 3b shows the comparisons of mass change for four cases under the same illumination: bulk water, TIF and NWF, TIF and NWF and CF, and TIF and NWF and SDCF. Notably, the incorporation of CF on the TIF unexpectedly reduces the evaporation efficiency. This phenomenon can be attributed to the smooth surface morphology of pristine CF (Figure 2a) and its weak photothermal conversion capability, which, when put together, results in significant light reflection and inefficient solar–thermal conversion. In contrast, the TIF and NWF case improves the enhancement in evaporation rate by approximately 1.35 times compared to bulk water, confirming the positive effect of thermal management on desalination performance. Remarkably, when SDCF is integrated with the TIF, the system achieves a substantially improved evaporation rate, reaching a stable value of 1.76 kg·m⁻²·h⁻¹, demonstrating the superior performance of this anti-reflective evaporator combined with thermal insulation.

As shown in Figure 3c, the SDCF has good desalination performance under 0.5 to 3 kW·m⁻², which can be attributed to its superior photothermal conversion efficiency. As shown in Figure 3d, SDCF with nanospikes and photothermal particles has an extremely strong photothermal performance. After only 5 min of exposure to illumination, the surface temperature of SDCF surpasses that of untreated CF by nearly 30 °C. This remarkable thermal enhancement is the fundamental reason for the outstanding desalination performance observed in the TIF and NWF and SDCF case. Further, we cycled the solar evaporation of the SDCF for 20 times, and the results are shown in Figure 3e, where the evaporation efficiency remains stable near 1.76 kg·m⁻²·h⁻¹. And, there is no significant change in the chemical elements and morphological features (Figure 2f). This shows that SDCF has excellent durability in desalination applications.

3.3. Anti-Deicing Performance and Durability of the SDCF

In areas with significant day/night temperature differences or cold regions, anti-icing capability is key to ensuring water enrichment efficiency. Super-hydrophobic surfaces possess micro-nano scale structures that prolong icing time due to air cushions isolating the solid–liquid contact. As shown in Figure 4a, by comparing the icing times of SDCF, CF, stainless steel, and glass surfaces at 15 °C and 45% RH, we confirmed that SDCF has good anti-icing ability. Moreover, SDCF showed excellent photothermal deicing performance. As shown in Figure 4b,c, the deicing experiments of CF and SDCF were conducted under 1 kW·m⁻², and the results showed that the CF surface only melted the surface frost in a time period of 120 s. In contrast, the ice on the surface of SDCF was completely melted. And, after the completion of ice melting, SDCF still had good self-cleaning performance, as shown in Figure 4d.

Even after the surface icing, due to the super-hydrophobic property of SDCF, the surface liquid will not completely destroy the air pockets in the micro-nano scale structures during the icing process, which reduces the ice adhesion to SDCF. As shown in Figure 5a, the ice adhesion strength of SDCF is significantly lower than that of CF, stainless steel, and glass. In order to verify the anti-deicing durability of SDCF, we conducted ten ice adhesion strength experiments and deicing experiments, respectively. The results are shown in Figure 5b, where it can be seen that the anti-deicing performance of SDCF remains stable. The ice adhesion strength ranged from 63.21 to 71.23 kPa and the freezing times ranged from 440 to 461 s. As shown in Figure 5c, the evaporation performance and super-hydrophobicity of SDCF were not affected after icing. The evaporation efficiency ranged from 1.71 kg·m⁻²·h⁻¹ to 1.8 kg·m⁻²·h⁻¹ and the contact angle ranged from 150.4° to 151°.

Compared with the evaporators developed in recent years, the evaporation performance of SDCF has advantages (Figure 5d). Long-term use of SDCF for desalination may result in the accumulation of salt crystals on the surface. Interestingly, the super-hydrophobic surface of SDCF has a self-cleaning property, which can effectively remove the surface salt crystals, as shown in Figure 5e.

To evaluate the durability of SDCF in harsh environments where mechanical abrasion may occur, we conducted systematic wear resistance tests. Figure 5f presents the evolution of evaporation efficiency as a function of abrasion distance. While mechanical abrasion was found to compromise the evaporation efficiency of SDCF to some degree, the material maintained remarkably high evaporation performance post-abrasion. The micropillars function as an effective sacrificial layer due to their limited surface coverage and protrusive morphology, which enables them to preferentially bear external mechanical stresses while preserving the underlying super-hydrophobic regions. Figure 5g presents a side view of the micropillar array, demonstrating that these approximately 200 µm high structures completely absorbed the abrasion damage during testing, thereby safeguarding the majority of the functional SDCF surface. This protective mechanism is further corroborated by SEM characterization (Figure 5h) of a sample subjected to 5 m of abrasion on 2000 mesh sandpaper. This reveals the following two key observations: the original machining marks on the micropillars were smoothed by wear, and the critical superhydrophobic microgrooves remain completely intact, confirming their effective protection by the sacrificial micropillar architecture.

These findings collectively demonstrate that SDCF retains excellent water harvesting capability after mechanical abrasion, owing to the effective damage mitigation provided by the hierarchical microstructure. The strategically designed micropillars bear the brunt of the mechanical abrasion experienced, shielding the functional microgrooves that are essential for maintaining super-hydrophobicity and solar desalination performance.

4. Conclusions

We developed a super-hydrophobic photothermal copper foam with good self-cleaning, anti-icing, and mechanical durability. The SDCF exhibited composite structures with a porous skeleton, nanospikes (around 200 nm in diameter), and CNT nanomaterials, remarkably enhancing the photothermal conversion; e.g., the surface temperature of SDCF increased from 24.2 °C to 66.7 °C within 5 min under solar irradiation of 1 kW·m⁻², compared with pristine copper foam with weak heating performance. Then, the seawater desalination performance was demonstrated by assembling SDCF with a thermal insulation layer and capillary wetting layer, revealing that the evaporation efficiency reached to 1.76 kg·m⁻²·h⁻¹ under solar irradiation of 1 kW·m⁻². Further, an icing time of 450 s and deicing strength of 67.7 kPa on the SDCF was achieved under 15 °C. At the same time, the self-cleaning effect remained after a photothermal (1 kW·m⁻²) deicing process of 120 s. The abrasion test showed that the designed micropillars play an important role in protecting the functional coatings, and the evaporation efficiency almost has no change even after an abrasion distance of 5 m. Therefore, the SDCF is shown to be a reliable interface evaporator for long-term use in seawater desalination.