Superhydrophilic Nickel Nanoparticles with Core–Shell Structure To

Article pubs.acs.org/JPCC

Superhydrophilic Nickel Nanoparticles with Core−Shell Structure To Decorate Copper Mesh for Efficient Oil/Water Separation Zhi-Yong Luo, Kai-Xuan Chen, Ya-Qiao Wang, Jun-Hui Wang, Dong-Chuan Mo,* and Shu-Shen Lyu* School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, China ABSTRACT: Recently, oil/water separation has attracted intensive attention due to the frequent crude oil spill accidents and the increasing amount of industrial wastewater. Thousands of outstanding achievements have been reported on the basis of membranes with superhydrophilicity and underwater superoleophobicity. However, a universal approach combining cheaper reagents with easier operations to fabricate superhydrophilic materials for effective oil/water separation is still a challenge and greatly encouraged. In the present work, we fabricate the superhydrophilic copper mesh decorated with nickel nanoparticles (Ni-NPs) via electrodeposition in fluorine-containing electrolyte. The size of NiNPs that consists of a metal Ni core and a polar NiO/Ni(OH)2 shell is about 300 nm; what’s more, the Ni-NPs combine with each other to form treelike nickel crystals by varying the experimental parameters. This optimized mesh exhibits remarkable oil/water separation performance. The oil content in separated water is lower than 3 ppm, and the stability in acidic, alkaline, and salt solutions and the properties for repeated use are also excellent. Further, we provide a novel approach to synthesis superhydrophilic nanoparticles with a core−shell structure for coating or other applications such as catalysis, self-cleaning, antifogging, and so on.

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soft when hydrated,18 thus resulting in degenerated efficiency of oil/water separation. Recently, surfactant-decorated and stimuli-responsed membranes19−23 have attracted more and more attention on account of the controllability of separation processes, but the polymeric surfactants tend to degrade in water.15 Fortunately, intrinsic or stimuli-responsive hydrophilic inorganics coated membranes exhibit remarkable stability and high-efficiency in oil/water separation.15,24,25 Hydrophilic copper,26 Cu(OH)2,15 palygorskite,27 titanium dioxides,24 tungsten oxide,28 zeolite,25 and NiOOH29 decorated membranes were reported in the previous works and turned out to be efficient. However, complex operations or exact instruments are required during the preparation. Therefore, a universal approach combining cheaper reagents with easier operations to fabricate stable superhydrophilic materials for effective oil/ water separation is still a challenge and greatly encouraged. Herein, we fabricated superhydrophilic copper mesh decorated with nickel nanoparticles (Ni-NPs) via electrodeposition in fluorine-containing electrolyte; the Ni-NP consists of a metal Ni core and a NiO/Ni(OH)2 shell. This membrane exhibits remarkable superhydrophilicity, underwater ultralow adhesive superoleophobicity as well as excellent performance for oil/water separation. What’s more, the parameters of reaction time and voltage were optimized to

INTRODUCTION Owing to the frequent crude oil spill accidents1,2 and the increasing amount of industrial wastewater, oil/water separation has attracted intensive attention in recent years.3,4 Traditional microfiltration (MF), ultrafiltration (UF),5 nanofiltration (NF),6 and even reverse osmosis (RO)7,8 membranes face enormous challenges due to their energy-intensive separation process and easier to be contaminated.9 Since Jiang et al. first reported the superhydrophobic and superoleophilic membrane for effective oil/water separation in 2004,10 these types of materials have been intensively studied and thousands of outstanding achievements have been reported.11−14 It is a groundbreaking and successful attempt of introducing special wettability into the field of oil/water separation. However, these intrinsic oleophilic materials are easily fouled by adhered or adsorbed oils, thus resulting in a decrease of flux and separation efficiency, as well as secondary pollution.15 In addition, a water barrier layer tends to form between the oil and membrane due to the higher density of water, which is unfavorable to the separation process. Inspired by fish scales,16 superhydophilic and underwater superoleophobic materials, which will redefine the effective oil/ water separation process, come into sight of researchers. In 2011, Feng et al. demonstrated a hydrogel-coated mesh for oil/ water separation, in which the as-prepared meshes can selectively separate water from oil/water mixtures with high separation efficiency and high resistance to oil fouling, and they are easy to recycle.17 Although hydrogel is one of the most typical hydrophilic materials, it is easy to swell and becomes © XXXX American Chemical Society

Received: April 19, 2016 Revised: May 22, 2016

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DOI: 10.1021/acs.jpcc.6b03940 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Characterization of morphology and components of superhydrophilic copper mesh. (a) The photograph of the specimen; the inset photographs show water contact angle and underwater oil contact angle. The SEM images of copper mesh (b) before and (c) after nickel electrodeposition. (d) High magnification of SEM image of Ni-NPs. (e−h) Element mapping of Cu, Ni, O, and C, respectively.

Figure 2. XRD and XPS characterization of superhydrophilic copper mesh prepared at 4 V for 2 h. (a) The XRD analysis of the superhydrophilic copper mesh. (b) The full spectrum, (c) Ni 2p and (d) O 1s spectra of XPS analysis.

was 1.5 cm. The electrolyte consisted of 0.15 wt % NH4F and 99.85 wt % H2O. The bath temperature was kept at 25 °C. To optimize the reaction time of electrodeposition, we synthesized the superhydrophilic copper mesh at 4 V for 2, 3, and 4 h, respectively. On the other hand, in order to optimize the applied voltage, we synthesized specimens at 4, 6, and 8 V for 2 h, respectively. Material Characterization. The morphology of the specimens was characterized by a scanning electron microscope (SEM, JSM-6510LV) and a transmission electron microscope (TEM, FEI Tecnai G2 F30). The components of the specimens were analyzed by an X-ray photoelectron spectrum (XPS, ESCALab250) and an energy dispersive spectrometer (EDS).

improve the performance of the copper mesh for oil/water separation. The stability in acidic, alkaline, and salt solutions and the properties for repeated use were also demonstrated.

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EXPERIMENTAL SECTION Preparation of Superhydrophilic Copper Mesh. Before electrodeposition, copper mesh (purity ≥ 99.5%; size: 3 cm × 3 cm; mesh number: 400) was cleaned in deionized water, ethanol, and deionized water, respectively,30,31 and then immersed in 10 wt % H2SO4 to remove the oxide layer and dried. Nickel deposition was carried out on a two-electrode electrochemical cell in which the copper mesh was used as cathode and a piece of nickel foil (purity: 99.7%; size: 3 cm × 3 cm) as the anode, and the distance between the two electrodes B

DOI: 10.1021/acs.jpcc.6b03940 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

of nanoparticles were made up of metal nickel, nickel oxides, or even hydroxides. After peak split of Ni 2p and O 1s spectra as shown in Figure 2c,d, metal nickel, nickel dioxides, and nickel hydroxides were confirmed at 852.1, 853.6, and 855.5 eV, respectively, which is in accord with previous reports.36,38−40 The surface of Ni-NP is composed of 80.8% Ni(OH)2 and 19.2% NiO. Further, the XPS spectrum of O 1s exhibits the surface oxygen species (Figure 2d). The peak at 529.4 eV is related to typical Ni−O bonds of NiO;41 the peaks at 531.0, 531.6, and 533.0 eV are usually associated with defects and surface species including hydroxyls (Ni-OH or even NiOOH29,35), absorbed oxygen, low-coordinated lattice oxygen, or absorbed water.36,42 The ratio of NiO is 8.8%, meaning that 91.2% of surface oxygen species are polar component, especially Ni(OH)2, which is beneficial to the formation of superhydrophilic surfaces. Ni-NP consisting of a metal Ni core and a NiO/Ni(OH)2 shell is demonstrated, showing excellent superhydrophilicity due to the polar NiO/Ni(OH)2 shell. Furthermore, we analyzed the crystal structure of Ni-NPs via TEM. Ni-NPs show polygonal morphology with clear crystal facets, and the selected area electron diffraction pattern (SAED) of a Ni-NP shows two crystal planes of (111) and (200) of metal nickel (see Figure 3a). As shown in Figure 3b, the lattice parameters in the core are 0.20335,43 and 0.176 nm, related to (111) and (200) planes of metal nickel (JCPDS-040850). What’s more, the (101) crystal plane of Ni(OH)2 (JCPDS-14-0117) is confirmed at the rough shell. Although the NiO is not verified due to its lower content [NiO:Ni(OH)2

The crystal form was confirmed via an X-ray diffractometer (XRD, Empyrean). Droplet Tests. The contact angle and dynamic effect of a 5 μL deionized water droplet spreading out on the interfaces were carried out via a high-speed camera (Vision Research Phantom V.211 capturing at 3000 frames per second). The underwater oil contact angle and slide angle of 5 μL oil were captured via a high-speed camera (Vision Research Phantom V.211 capturing at 100 frames per second); the underwater adhesive test was carried out via a PTFE tube to manipulate the oil droplet. Oil/Water Separation. In this section, hexane, isooctane, petroleum ether (three kinds of saturated oils), paraxylene (a kind of unsaturated oil), and kerosene (a kind of mineral oil), five oil model, were applied for oil/water separation. The superhydrophilic copper meshes were placed between two quartz tubes (d = 2 cm) with a flange, and fastened via screws. The oil/water mixture (water/oil = 10) was separated from the upper tube to the lower tube. After separation, the water (60 mL) was acidized (pH = 1−2) via 1 M HCl, and 2 g NaCl was added to demulsify. Then, it was extracted twice by using 40 mL of CCl4. The extractant was dried with anhydrous Na2SO4. The oil content in CCl4 was measured via an infrared oil content analyzer (OIL-8, China).

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RESULTS AND DISCUSSION Characterization of Materials. As shown in Figure 1a, the copper mesh turns into black and shows excellent superhydrophilicity, ultrahigh water permeability, and underwater superoleophobicity (about 160°) after nickel electrodeposition. Figure 1b shows the photograph of copper mesh before electrodeposition, in which the pore size is about 50 μm and distributed uniformly. The roughness of the mesh was enhanced after Ni-NP coated (Figure 1c). From Figure 1d, we can see that the Ni-NPs exhibit polygonal configurations, which was mainly due to the exposure of crystal facets. In the previous reports,32−34 (001) crystal facet exposure of TiO2, which was mainly due to the existence of fluorine, had been demonstrated. Therefore, fluorine may be the key factor to form the polygonal Ni-NPs as well as the superhydrophilic copper mesh. Element mapping of Cu, Ni, O, and C is shown in Figure 1e−h, respectively. Nickel, the main component of nanoparticles, was distributed uniformly on the Cu mesh. The foreign oxygen and carbon, originating from the surface oxidation during electrodeposition and pollution of carbon species, respectively, were also distributed uniformly. It is noticed that the Ni-NPs were coated on the copper mesh evenly. Figure 2a shows the XRD analysis of the superhydrophilic copper mesh; it consists of Cu and Ni, two kinds of crystal forms, which is in agreement with the values in the standard cards (JCPDS-04-0836 and JCPDS-04-0850). This XRD result indicated that the nanoparticles coated on the copper mesh15 were metal nickel particles.35,36 As we know, wettability of solid interfaces mainly depends on its surface energy and roughness.37 Figure 2b−d shows the XPS analysis of the surface of the copper mesh after it was decorated, and the mesh surface consists of Cu, O, Ni, and C, four kinds of elements. Because of the distribution of Ni-NPs and the negligible content of Cu (see inset table of Figure 2b), the XPS spectra were mainly the characterization of Ni-NPs. Apart from the C, which belonged to the carbon species absorption of the specimen, the surfaces

Figure 3. TEM characterization of Ni-NP. (a) Polygonal nanoparticle with clear crystal facet. The inset photograph is the selected area electron diffraction pattern (SAED) of a Ni-NP. (b) The HR-TEM characterization and (c) STEM analysis and corresponding element mapping of a Ni-NP. C

DOI: 10.1021/acs.jpcc.6b03940 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C = 1:4; see Figure 2c], it provides the direct evidence for the NiNiO/Ni(OH)2 core−shell structure of Ni-NPs. From Figure 3c, we can see that Ni and O distribute evenly. However, the signal intensity of oxygen is weaker, which is mainly owing to the fact that oxygen mainly distributes on the surface of NiNPs. anode Ni → Ni 2 + + 2e−

(1)

cathode H 2O → H+ + OH−

(2)

2H+ + 2e− → H 2

(3)

Ni 2 + + 2e− → Ni

(4)

Ni 2 + + H 2O → NiO + 2H+

(5)

Ni 2 + + 2OH− → Ni(OH)2

(6)

During the electrodeposition, the above eqs 1−6 existed in the electrochemical cell. Nickel foil was dissolved at anode as in eq 1, and the generated Ni2+ migrated to the cathode. At the cathode, two main reactions took place as in eqs 3 and 4, generating Ni-NPs with polygonal configurations in neutral solution. Meanwhile, the H2 was released from the solution, resulting in the solution with alkalescence. The oxidation process proceeded at the surface of nanoparticles as in eq 5 in the alkalescence solution as soon as the polygonal Ni-NPs formed, and generated surface NiO. Then, the solution turned into alkalinity as the reactions progressed, and the H2 is released from the electrochemical cell; eq 6 displaced eq 5 at the surface of Ni-NPs, generating surface Ni(OH)2. The solution changed from neutral to alkaline with the release of H2 during the process, the reactions 4−6 taking place in sequence. By this method, a Ni-NP with a metal Ni core and a NiO/ Ni(OH)2 shell is synthesized. In particular, crystal facets were exposed during the electrodeposition process, thus resulting in the polygonal morphology of the Ni-NP. The facets exposure is the fundament of rapid surface processes such as oxidation or hydro-oxidation.33 Droplet Tests. Droplet tests are the basis of interfacial applications, especially oil/water separation. It is well-known that wettability of solid interfaces mainly relies on their surface energy and roughness, in which surface energy is dependent on the surface components, while surface roughness mainly relies on the nanostructures (size et al.).44 In the present work, the contact angle is dependent on the polar NiO/Ni(OH)2 shell and the nanostructures treelike Ni, and can be reflected by the droplet tests like water contact angle (WCA), underwater oil contact angle (OCA), and oil sliding angle (OSA). Figure 4 shows water and several kinds of oil droplet tests of the superhydrophilic copper mesh with Ni-NP decorated. It is noticed in Figure 4a that the mesh exhibits remarkable superhydrophilicity as well as ultrahigh water permeability (permeates within 15 ms) compared to the previously reported value,24 on which the water will spread out to form a water layer. As we know, surface energy and roughness are two aspects determining the wettability of solid films. The surface energy of the copper mesh is improved after nickel nanopaticles coated owing to the existence of the polar NiO/Ni(ON)2 shell. Meanwhile, granular nickel will enhance the roughness of copper mesh, thus improving the wettability, which can be described by the Wenzel model as in the following formula 7

Figure 4. Droplet tests of copper mesh decorated with Ni-NPs. (a) Dynamic water droplet on copper mesh after electrodeposition. (b) Dynamic effect of underwater adhesive of oil; the oil model is hexane. (c) Underwater oil contact angle and sliding angle. The water and oil droplets are 5 μL.

cos Θ′WCA = r cos Θ WCA

(7)

where Θ′WCA and ΘWCA are the water contact angles on the rough surface and flat surface, respectively, and r is the roughness factor. r > 1, cos Θ′WCA > cos ΘWCA, so Θ′WCA < ΘWCA. Figure 4b shows the underwater oil adhesive effect. We manipulated the oil (hexane) droplet via a PTFE tube; the oil droplet exhibits an ultralow adhesive force and excellent superoleophobicity. OCA and OSA are two key factors affecting oil/water separation performance. In the previous works,15,45 the large OCA (>150°) and small OSA (