Fabrication of an α-MoO₃ nanobelt membrane showing a three-dimensional cross-linked nano-scale network structure for water and oil mixture separation

Abstract

α-MoO₃ nanobelts with amphiphilic properties have been successfully fabricated via a hydrothermal method. The nanobelt membrane with a three-dimensional cross-linked nano-scale network structure was prepared on a filter paper surface by a suction filtration process. After being modified with water, the wetted membrane could be applied for the separation of nonpolar liquids and water, especially in emulsions. The separation process was simple, without employing any extra power, and the nanobelt membrane was easily recyclable. Based on this study, a separation mechanism was proposed. Our results suggest an innovative inorganic material that has excellent oil/water selectivity in the cleanup of oil from water.

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1. Introduction

In recent years, with increasing amounts of industrial oil-containing wastewater and frequent oil spill accidents, oil/water separation has become a worldwide challenge.¹ Because oil/water separation is an interfacial problem, using special wettability to design novel materials is an effective and facile way to separate oil from water, or water from oil. Materials with both hydrophobic and oleophilic properties have received broad attention due to their wide applications for both fundamental and potential research.^2–5 In real applications, the separation techniques usually need to use a certain kind of polymer or organic matter to make the surface super-hydrophobic or super-hydrophilic. Therefore, various materials had been developed to effectively separate oil/water mixtures, such as nanofibrous membranes,⁶ carbon-based materials,⁷ cellulose aerogel,⁸ superhydrophobic/superoleophilic film⁹ and so on.^10–14 However, they are easily fouled or even blocked up by oils because of their intrinsic oleophilic properties.¹⁵ Adhered or absorbed oils are hard to remove, which results in secondary pollution during the post-treatment process.¹⁶ To some extent, those approaches always involve special equipment, multi-step procedures and energy-consuming processes. Therefore, it is of great importance to develop novel materials for oil/water separation with high separation efficiency, excellent recyclability and anti-pollution ability.

One-dimensional (1D) nanostructures have attracted much attention in recent years due to their importance in mesoscopic physics and technological application in nanoscale device generation. Although significant progress has been made in the area of fabricating 1D nanostructures in the past several years, the development of appropriate methodologies to apply these nanostructures has been lagging behind.^17–19 Here, we report the fabrication of α-MoO₃ nanobelts via a hydrothermal method. A nanobelt membrane was obtained in a suction filtration process. It presents three-dimensional porous nanostructures, which can form hydrogen bonds with the water molecules on the surfaces of the nanobelts throughout the membrane and can effectively prevent the transport of nonpolar liquids, for example, cyclohexane, carbon tetrachloride, diesel, and even oil/water emulsions. During the separation process, the membrane can prevent the oil phase from passing through the woven web fabricated by water molecules. However, water permeates through the membrane under the drive of gravity without any extra energy. After being used, the membrane can be re-dispersed in a nanobelt-suspension in water by ultrasonic treatment and then a new membrane can be re-formed by suction filtration. The membrane can be recycled and re-used numerous times without any damage.

2. Experimental

In a typical procedure, 1.24 g of ammonium molybdate powder (1.00 mmol) was added into a beaker containing 20 mL of deionized water under strong stirring. After stirring for 10 min at room temperature, 10 mL of nitric acid (2.4 mol L⁻¹) was dropped slowly into the solution and a clarified solution was obtained. Subsequently, the transparent solution was transferred into a Teflon-lined stainless steel autoclave (50 mL) and treated at 180 °C for 18 h. After cooling to room temperature naturally, a white α-MoO₃ powder was collected after being washed with distilled water and centrifuged several times, and then dried in a vacuum oven at 60 °C for 8 h.

The separation experiment of a water (colored by Cu(NO₃)₂ for clarity) and oil (cyclohexane) emulsion system was performed using a simple instrument fabricated by ourselves. A wet α-MoO₃ membrane, prepared on a filter paper and wetted with water for only one second, was mounted into a dead-end cell equipped with a syringe and then a certain amount of a fresh water–cyclohexane emulsion was poured into the syringe.

The morphology and microstructures of the obtained samples were examined using SEM (Hitachi S-4800) with an accelerating voltage of 10 kV. TEM observations and SAED experiments were carried out with a JEM-2100F. The crystallographic information of the as-prepared samples was established using powder X-ray diffraction (XRD, D8 DISCOVER with the GADDS version of the BRUKER Company, Germany) with graphite monochromatic high-intensity Cu Kα. FT-IR spectrometry was performed with a Bruker TENSOR 37 FT-IR analyzer. Microphotographs were taken with an optical microscope BX51 produced by OLYMPUS. The efficiency of the oil–water separation was determined by ¹H nuclear magnetic resonance (¹H NMR). Samples prepared in CD₃OD were recorded on a DRX-500 spectrometer operating at 300 MHz (Bruck Co. Ltd.). The wettability of the as-prepared nanobelt membrane was measured by dynamic contact angle measurements using the drop shape analysis system (Kruss DSA 100, Germany) at room temperature. The BET surface area was evaluated from the nitrogen adsorption and desorption isotherms (Micromeritics, ASAP 2020).

3. Results and discussion

The structure and morphology of the obtained products were investigated by XRD and SEM analyses. As shown in Fig. 1a, all of the diffraction peaks can be perfectly indexed to orthorhombic MoO₃ (JCPDS 05-0508), which is in good agreement with the literature.²⁰ The strong and sharp diffraction peaks indicate that the as-synthesized products are highly crystalline. No peaks of any other phase are detected, indicating the high purity of MoO₃. Fig. 1b shows the corresponding SEM images of α-MoO₃, demonstrating that the sample consists entirely of nanobelts. A panoramic view also reveals that the sample shows a three-dimensional cross-linked nano-scale network structure. These nanobelts have an average length of tens of micrometers and their width is around 80–150 nm.

Fig. 1 (a) XRD pattern of the MoO₃ nanobelts. (b) SEM characterization of the nanobelts. (c)–(e) Recycling process for the fabrication and utilization of the α-MoO₃ membrane.

The cyclic fabrication and utilization process of the membrane is shown in Fig. 1c–e. As presented, the nanobelts that form the membrane structure can be re-suspended in the solution and subsequently re-form the original morphology over many cycles by suction filtration. TEM was also carried out to further investigate the inner structure of the nanobelts. A TEM image and the corresponding selected area electron diffraction (SAED) pattern of an individual α-MoO₃ nanobelt are shown in Fig. 2a and b. A typical TEM image for a free-standing α-MoO₃ nanobelt is presented in Fig. 2a and a rectangle-like cross-section is clearly visible. The HRTEM image in Fig. 2b indicated interplanar distances of 0.39 and 0.36 nm for the (100) and (001) lattice planes, respectively. The SAED pattern of the sample (inset of Fig. 2b) indicates a single crystalline structure of α-MoO₃. The surface area of α-MoO₃ was determined to be 4.11 m² g⁻¹ with the nitrogen adsorption isotherms using the Brunauer–Emmett–Teller (BET) method.

Fig. 2 (a) TEM image of a single α-MoO₃ nanobelt and (b) HRTEM image of an enclosed nanobelt area showing interplanar distances along the (001) and (100) directions. The inset of (b) is the corresponding SAED pattern.

The separation experiment was realized as shown in Fig. 3. From Fig. 3, we can see that the original milky blue solution was gradually separated into two kinds of transparent liquid. Water permeated through the membrane under the drive of gravity while the cyclohexane was rejected by the membrane. Moreover, a control experiment was carried out in which an ordinary filter paper without α-MoO₃ nanobelts was used for the emulsion separation mentioned above. The emulsion passed smoothly through the filter paper and neither water nor cyclohexane was obtained. As a result, it can be said that the α-MoO₃ nanobelt membrane plays an indispensable role in this separation process.

Fig. 3 The process of separating oil and water.

The separation efficiency is so high that nearly no visible oil existed in the permeated water. As shown in Fig. 4a and b, the separation of this mixed system was observed under an optical microscope. We can clearly see that the state of separation changed from the oil/water mixed phases (the oil phase is marked by arrows) to the water phase only. In order to further investigate the separation efficiency for cyclohexane from permeated water, the probable content of cyclohexane after separation was tested using ¹H nuclear magnetic resonance (¹H NMR, Fig. 4c). The sample was prepared in CD₃OD. The peak associated with the permeated products at 3.325 ppm comes from the solvent residual peak of MeOD. The singlet peak at 4.981 ppm and the peak at 4.720 ppm are representative of the HOD and H₂O protons. From the ¹H NMR spectrum, the absence of the cyclohexane peak (∼1.45 ppm) indicates the high separation efficiency of the membrane.²¹

Fig. 4 Microphotographs for the emulsion formed with oil/water before (a) and after (b) the separation. The insets of Fig. 4a and b are the images of the corresponding solutions. (c) ¹H NMR spectrum of the permeated solution.

The wettability of the as-prepared nanobelt membrane was investigated, as shown in Fig. 5a. A water droplet of about 0.5 μL was allowed to come into contact with the surface of the dried membrane, and the water contact angle reached 0° after only 0.567 s. The same phenomenon occurred for the cyclohexane/dried membrane system. This amphiphilic feature is quite different from the traditional hydrophobic and oleophilic membranes, which usually have specific hydrophilic or hydrophobic surface properties. A novel filter membrane was fabricated from α-MoO₃ nanobelts by a facile suction filtration process (Fig. 5b(1)). The nanobelt membrane in its dry state remained permeable for both polar and nonpolar liquids. Interestingly, it was found that the transport properties of this membrane could be easily tuned by a facile wetting process with H₂O. However, when the water-wetted membrane was treated with polar liquids, the polar liquids would immediately mix with the water cluster system forming a new cluster system and letting the polar liquid pass through smoothly (Fig. 5b(2)).²² A higher magnification image of the randomly arranged coated-nanobelts is shown in Fig. 5b(3). The water films formed in the pores reflect a mutual interaction between H₂O and α-MoO₃. The results of the FTIR spectra of the dried nanobelts compared with the those of the wetted ones provide insight concerning the nature of the interaction between H₂O and the α-MoO₃ framework. In Fig. 5c, the FTIR spectrum of the pristine α-MoO₃ nanobelts exhibits three typical peaks at 998, 864, and 557 cm⁻¹, which agree well with those in the literature.^23,24 In contrast, in the spectrum of the α-MoO₃/H₂O system, an obvious peak at 3370 cm⁻¹ reveals the existence of water in the α-MoO₃ nanobelts. The aforesaid peak at 557 cm⁻¹ in pristine α-MoO₃ shifts down to 549 cm⁻¹, and displays attenuation of its intensity. Most likely, it is a hydrogen bond formed between the oxygen atom of Mo–O–Mo and the hydrogen atom of H–O–H in H₂O.^25,26 The extensive H-bonding network film in the pores throughout the membrane and the whole structure is stabilized by these hydrogen bonds.

Fig. 5 (a) Spreading and permeating behaviors of a droplet on the nanobelt membrane. (b) Schematic diagrams of the separation mechanism. (c) FTIR spectra of the α-MoO₃ nanobelts before (I) and after (II) wetting.

4. Conclusions

In summary, we have demonstrated a novel and efficient approach to separate oil/water emulsions using an α-MoO₃ nanobelt membrane, formed by a facial wetting process. The existing water molecules distribute evenly throughout the α-MoO₃ nanoscale membrane and form a hydrogen bond network film in the pores. Furthermore, the FTIR results show that the hydrogen atoms of water are H-bonded with the oxygen atoms in the Mo–O–Mo bonds of the α-MoO₃ nanobelts. The coated membrane shows perfect water permeation with high separation efficiency, with resistance to nonpolar liquids using an energy-saving filtration method. This study may prove particularly useful in the design of recyclable separation materials. It is a new attempt to design next-generation materials for oil/water separation.

Notes and references

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