Candle soot coated nickel foam for facile water and oil mixture separation

Abstract

Nickel foam with superhydrophobic and superoleophilic surface has been successfully fabricated via a facile two-step process. The hierarchical structure was prepared on the commercial nickel foam surface by deposition of a soot layer. After being solidified by polydimethylsiloxane, the as-prepared nickel foam showed both superhydrophobic and superoleophilic properties simultaneously. This as-prepared nickel foam as a facile device could then be applied in oil and water mixture separation.

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Introduction

The Gulf of Mexico oil spill in 2010 is one of the most serious pollution incidents in recent decades in the world. The cost of the oil spill, including the damage to the coastal ecosystem and the loss of an entire fishing and tourism season, is still being tabulated.¹ The frequent oil spill accidents caused great concern and worry. As a result, many methods are currently employed to solve oil spill problems, including coagulation/flocculation, in situ burning, skimming, centrifugation and gravity separation. Although many oil-absorbing materials have been developed for cleanup of oil spill, they all exhibit shortcomings including low separation efficiency, high operation costs, large size and the generation of secondary pollutants.² On the other hand, owing to the increasing environmental pollution accompanying the global industrial development in recent years, there is a growing need for functional membrane materials in the fields of separation of oily wastewater.³ Various approaches such as selective etching of solid or polymer templates, self-assembly of block co-polymers, layer-by-layer assembly, electrospinning of nanofibres and replica moulding against porous templates, have been used in research for the preparation of functional membranes.^4–10 However, all of the above methods have limitations for practical use because of the need to withstand harsh conditions, multistep procedures for implementation or limitations in substrate size. Therefore, facile strategies to synthesize highly efficient and inexpensive oil cleanup equipment are indispensable.

Wettability is an important characteristic of solid surfaces and is controlled by the chemical composition and the geometrical structure of the surface.¹¹ Nature always gives us inspirations to design functional interfaces with special wettability.¹² For example, learning from the self-cleaning lotus leaf, Jiang and co-workers prepared superhydrophobic surfaces with low hysteresis of sliding angle.¹³ In addition, learning from the antifogging eyes of a mosquito, the same group discovered a novel biological anti-fogging strategy by using a superhydrophobic approach.¹⁴ Because of the specific surface properties of superoleophilicity and superhydrophobicity, superhydrophobic materials have extended applications to the research area of oil spill cleanup. For example, Athanassiou and co-workers prepared a novel composite material based on commercially available polyurethane foams functionalized with colloidal superparamagnetic iron oxide nanoparticles and submicrometer polytetrafluoroethylene particles, which can efficiently separate oil from water. In addition, the functionalized foams exhibited also magnetic responsivity.¹⁵ Zhang and co-workers fabricated a superhydrophobic CNTs-PTFE bulk material by a simple approach. The simultaneous superhydrophobicity and superoleophilicity of the bulk material enables them to use it for removing oil from water.¹⁶ Among them, superhydrophobic 3D porous materials are considered as promising high-capacity absorbents due to their larger surface area and well-developed pores. Nickel foam is a kind of commercially available 3D porous material, which has been used for fabricating superhydrophobic absorbents aimed at oil spill cleanup.^17,18 However, the complex synthesis processes involved in creating the reported absorbent materials hamper the use of these materials in large-scale applications. Therefore, it is of great importance to develop a low-cost efficient material for oil/water separation. Polydimethylsiloxane (PDMS), which is inert, nontoxic, nonflammable, and highly flexible, is one of the most frequently used materials.^19–22 Herein, we reported a facile strategy in the preparation of superhydrophobic and superoleophilic nickel foam surface through candle soot coated and sequential modification with PDMS. The as-prepared nickel foam could be used for the selective and effective separation of water and oil through simple, time-saving, and inexpensive process without any extra power.

2. Experimental

2.1 Chemicals and apparatus

Nickel foam was purchased from Kunshan Dessco Electron Co., Ltd., China. The Sylgard 184 PDMS elastomer kit was purchased from Dow Corning. Candle was acquired from supermarket (no. 10203363, Decorvilla, China). Other reagents were of analytical grade and were used as received without further purification. All aqueous solutions were prepared with Milli-Q water (18.2 MΩ.cm). Field emission scanning electron microscope (FESEM) images were obtained on a Hitachi S-4800. The static water contact angles (WCAs) on the surfaces of superhydrophobic and superoleophilic nickel foam were measured at room temperature and 60% relative humidity using a contact angle goniometer (DSA, KRUSS GMBH, Germany) by the sessile drop method with a 3 μL water droplet. The WCA values were recorded after 3 s from droplet deposition.

2.2 Fabrication of the superhydrophobic and superoleophilic nickel foam

The superhydrophobic and superoleophilic surface was fabricated by candle soot coated and sequential modification with PDMS. Nickel foam was cleaned by alternate ultrasonication in ethanol and Milli-Q water for three times and then dried in oven. Afterward, the surface to be coated, in our case a nickel foam, is held above the flame of a paraffin candle. About 5 min after the ignition of the candle, a steady flame was obtained with a total flame height of 4 cm. Ni foam (5 cm × 5 cm) was mounted in the flame envelope 3.5 cm above the candle. The growth process lasted for 2 min for one side and then for another 2 min for the other side with Ni foam moving back and forth, after which the Ni foam was taken out from the flame. Deposition of a soot layer turns the nickel foam black. The candle soot coated nickel foam was placed into a petri dish in which a mixture of hexane, PDMS prepolymer and a thermal curing agent was poured in a ratio of 100 : 10 : 1 by weight. The candle soot coated nickel foam with the absorbed mixture were then cured at 80 °C for 4 hours.

2.3 Separating hexane from water

In order to distinguish hexane from water, we dyed hexane to a red color with Oil Red O. The labeled hexane and water were added into a beaker forming the oil and water mixture solution. After the oil and water mixture solution were poured into the superhydrophobic and superoleophilic nickel foam box, dyed hexane infiltrated in rapidly. Then we collected the water in the box within anther beaker.

3. Results and discussion

The overall synthesis procedure of superhydrophobic and superoleophilic nickel foam is shown in Fig. 1. First, the candle soot coating was obtained by holding nickel foam above the flame of a paraffin candle. Deposition of a soot layer turns the nickel foam black. However, the structure of soot layer is fragile because the particle–particle interactions are only physical and are weak. When water rolls off the surface, the drop carries soot particles with it until almost all of the soot deposit is removed. Second, in order to enhance the binding degree of the candle soot on the surface of nickel foam, we developed a technique to coat the soot layer with a PDMS shell. The candle soot coated nickel foam was placed into a petri dish in which a mixture of hexane, PDMS prepolymer and a thermal curing agent was poured in a ratio of 100 : 10 : 1 by weight. After coating, the PDMS shell was sufficiently robust to completely resist water impact.

Fig. 1 Illustration of the procedure for preparing superhydrophobic and superoleophilic nickel foam.

Fig. 2A shows a photograph of pristine nickel foam (left) and as-prepared superhydrophobic and superoleophilic nickel foam (right). A uniform growth of soot layer on the nickel foam was observed that the nickel foam surface completely turned into black. It was also observed that the soot layer were still well adhered to the nickel foam substrate even after they were subjected to a water flushing, indicating that it was a reliable material for the separation of water and oil. As shown in Fig. 2B, pristine nickel foam will sink beneath the surface of water owing to its characteristics of hydrophilicity. While the as-prepared superhydrophobic and superoleophilic nickel foam floated on the water surface when in contact with water, which can be attributed to its superhydrophobicity. The as-prepared superhydrophobic and superoleophilic nickel foam appeared as silver mirror-like surfaces when they were totally immersed in water by an external force owing to a continuous air layer between the superhydrophobic surface and water (Fig. 2C). After releasing the external force, the superhydrophobic nickel foam could float immediately on water without the absorption of water. This implies that the superhydrophobic nickel foam maybe have an important applications in oil/water separation.

Fig. 2 (A) The picture of pristine nickel foam (left) and as-prepared superhydrophobic and superoleophilic nickel foam (right). (B) The picture of the as-prepared superhydrophobic and superoleophilic nickel foam floated on water owing to its superhydrophobicity and pristine nickel foam submerged in water. (C) The picture of the as-prepared superhydrophobic and superoleophilic nickel foam immersed in the water bath by an external force.

The difference in the geometric structures and chemical compositions is reflected in the surface wettability of the samples. The contact-angle images of nickel foam with and without treatment are shown in Fig. 3. For the pristine nickel foam (Fig. 3A), the water contact-angle was about 86° and water drop penetrated through the nickel foam over time, exhibiting its intrinsic hydrophilicity. Modified by soot layer and PDMS, the nickel foam surfaces exhibited superhydrophobicity, and the water contact-angle was about 152° (Fig. 3B). Water droplets deposited on the surface form almost perfect spheres. The surface wettability transformation from hydrophilicity to superhydrophobicity can be attributed to the induced roughness by the soot layer. The surface morphologies of nickel foam before and after deposition of soot layer and PDMS were characterized by using FESEM. Fig. 4A and B show representative FESEM images of the pristine nickel foam at different magnification, it is clearly seen that the pristine nickel foam surface is smooth before soot layer and PDMS growth. There are no obvious micrometer scale or nanometer scale structures on the pristine nickel foam surfaces. After soot layer and PDMS growth, the surface of the whole nickel foam becomes rough as revealed in Fig. 4C, indicating the growth of the target materials over a large area. High magnification FESEM image reveals that the soot layer consists of carbon particles coated with PDMS with a typical diameter of 60–80 nm, forming a loose, fractal-like network (Fig. 4D). FESEM results demonstrated that the soot layer growth results in the formation of micro-nanoscale hierarchical structures on the nickel foam surface. It is well known that wettability of solid surfaces is controlled by the chemical composition and the geometrical structure. According to the Wenzel model and the Cassie-Baxter model, the introduction of a proper microstructure could make a flat hydrophobic surface to be more hydrophobic or even superhydrophobic owing to the introduction of an air cushion beneath the water droplet, whereas a flat oleophilic surface becomes more oleophilic or even superoleophilic owing to the capillary effect.²³ In our case, the hierarchical structure was prepared on the commercial nickel foam surface by deposition of a soot layer. After being modified by PDMS, the fragile structure of soot layer was solidified, which make the nickel foam have both superhydrophobic and superoleophilic property simultaneously. As expected, the superhydrophobic and superoleophilic nickel foam separated oil and water easily. In order to clarify the separation capacity of the superhydrophobic and superoleophilic nickel foam, the separation experiment was carried out as shown in Fig. 5. Hexane was dyed red using an Oil Red O ink for easy observation. As shown in Fig. 5C, hexane permeated through the box while keeping water in the box. In the next step, water was poured into another beaker (Fig. 5D). This demonstrates excellent characteristics of the superhydrophobic and superoleophilic nickel foam box as a separation filter for oil and water separation.

Fig. 3 Optical images of a water droplet on the surface of the pristine nickel foam (A) and the as-prepared superhydrophobic and superoleophilic nickel foam (B).

Fig. 4 Typical FESEM images of pristine nickel foam (A and B) and as-prepared superhydrophobic and superoleophilic nickel foam (C and D) at different magnifications.

Fig. 5 Experiment process images of the water purification using superhydrophobic and superoleophilic nickel foam: (A) Oil Red O hexane solution and water before separation. (B) Simple instrument fabricated by ourselves. (C) Only the Oil Red O hexane solution stay in beaker after passing through the nickel foam box. (D) Oil Red O hexane solution and water in two separate beakers.

4. Conclusions

We have developed a simple as well as eco-friendly process to prepare superhydrophobic and superoleophilic nickel foam that is easy to handle for practical use. The as-prepared nickel can be applied as effective devices for the separation of water and oil mixture. Compared to other methods for preparation of separation devices, the reported process was simple, time-saving and inexpensive. Because the coating of candle soot can be applied to a wide variety of heat-resistant surfaces, such as metal, metal oxide, or glass, this reported process expects to have the strong potential for practical production.

Acknowledgements

Partial support of this work by the National Science Foundation of china (no. U1304210) and the Science and Technology Foundation of Henan Province (122102310519) is grateful acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46476h

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