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Article pubs.acs.org/JPCC

Matchstick-Like Cu2S@CuxO Nanowire Film: Transition of Superhydrophilicity to Superhydrophobicity Cailong Zhou,† Huijing Li,† Jing Lin,‡ Kun Hou,† Zhijie Yang,† Pihui Pi,† Shouping Xu,† Xiufang Wen,† and Jiang Cheng*,† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, People’s Republic of China S Supporting Information *

ABSTRACT: We fabricated a matchstick-like Cu2S@CuxO nanowire film on copper mesh by applying a Cu(OH)2 nanowires template-sacrificial method, which can transformed from superhydrophilic to superhydrophobic just after storage in air for a certain period without any further organic modification. The surface morphology, chemical composition and the wettability were investigated by Scanning Electron Microscopy (SEM), X-ray diffractometer (XRD), Raman, X-ray Photoelectron Microscopy (XPS), and contact angle measurement. Results showed that the change of surface chemical composition and the trapped air among the matchstick-like structures were the decisive factors for the wettability transition. Therefore, on-demand oil/water separation was achieved, which was performed by using the superhydrophilic−underwater superoleophobic mesh for separating light oil/water mixtures and the superhydrophobic one for separating heavy oil/water mixtures.

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INTRODUCTION Wettability, as a macroscopic representation of the interaction between liquid and solid substrate, is a crucial property of solid surfaces from aspects of a theoretical viewpoint and practical applications.1 Inspired by nature, many antiwetting or superwetting surfaces were prepared by controlling the surface free energy and surface micro/nanostructures. The typical applications of these surfaces include self-cleaning,2,3 oil/water separation,4−6 anti-icing and antifogging,7,8 atmospheric water collection,9 anticorrosion,10 antibacterial activities,11 and so on. Because of superior electrical and thermal conductivity, mechanical workability, malleability and relatively noble properties, copper becomes one of the most commonly used engineering materials.12 To obtain special wettability (such as superhydrophilicity and superhydrophobicity) on copper substrate, most efforts chose to create micro/nanostructures on the copper surface based on copper hydroxide or copper oxide, followed with chemical modification at times. For instance, Cheng et al. prepared ball-like structured Cu(OH)2 on copper foil and modified with alkyl thiol to obtain superhydrophobic surfaces with controlled adhesion.13 Zhang © 2017 American Chemical Society

et al. fabricated a superhydrophilic Cu(OH)2 coated copper mesh and realized oil/water separation.14 Liu et al. achieved nonaqueous multiphase liquid separation using a robust Cu(OH)2 nanoneedles decorated copper mesh with tunable wettability.15 Li et al. obtained superhydrophobic CuO surfaces with different topographies on copper plate which could tune wettability between “Locus” state and “Gecko” state.16 Ansah et al. reported various morphologies on copper mesh, such as needle-like, hair-like, arch-like, and pine needle-like structures based on Cu(OH)2, CuO, or their composite. The surfaces could turn to superhydrophobic after treated by 1H,1H,2H,2Hperfluorooctyltriethoxysilane.17 It is worth noticing that a two-step method is usually required to achieve superhydrophobicity, constructing micro/ nanostructures and chemical modification by low surface free energy materials.18 Creating a superhydrophobic surface without any organic modification remains a great challenge in Received: April 18, 2017 Revised: August 22, 2017 Published: August 22, 2017 19716

DOI: 10.1021/acs.jpcc.7b03645 J. Phys. Chem. C 2017, 121, 19716−19726

Article

The Journal of Physical Chemistry C Scheme 1. Schematic of the Preparation of the S-NWM

85 μm was produced by Xiancai Metallic Net Co., Ltd. (Anping, P.R. China). Fabrication of Cu2S@CuxO Film on Copper Mesh. Copper mesh was trimmed into pieces with the size of 3 cm × 3 cm and sequentially washed ultrasonically in ethanol, isopropanol and deionized water for 10 min. It was then immersed in 0.1 M of hydrochloric acid solution to remove surface oxide followed by deionized water rinsing. The preparation of S-NWM was realized via two room-temperature reactions including an alkali assisted oxidation process and a sulfurization process. Briefly, the mesh was first soaked into a mixture of 12.5 mL of 10 M NaOH, 5 mL of 1 M (NH4)2S2O8, and 32.5 mL of H2O for 10 min to form Cu(OH)2 nanowires on the mesh, resulting in a light-blue-color appearance. After being withdrawn from the solution, cleaned with deionized water, and dried in air, the light-blue mesh was further treated by soaking it in 50 mL of 50 mM Na2S solution for certain durations. Finally, the resulting mesh was rinsed with an abundance of deionized water and dried in air. Characterization. The surface morphologies of the samples were observed with scanning electron microscopy (SEM, ZEISS Merlin). The crystalline structure of the nanowires was investigated by a transmission electron microscopy (TEM, FEI Tecnai G2 F20). X-ray diffraction data of samples were obtained using an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Kα radiation. Raman spectra were recorded on a Raman spectrometer (Renishaw inVia) using the laser wavenumber of 514 nm. The surface elements and composition of the film surface were investigated using an X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) with Al Kα radiation. Three-dimensional surface imaging of the films was tested by an atomic force microscopy (AFM, Park Systems XE-100). The wetting properties of the mesh were conducted using a contact angle analyzer (Shanghai Zhongchen Powereach JC2000D1A) with the testing water or oil droplet of 5 μL.

order to extend its durability, especially under harsh conditions in practical application.19 Some previous works have successfully prepared inorganic superantiwetting surfaces on copper substrates by creating a uniform rough structure without further hydrophobization. Electrodeposition, oxidation, and displacement reaction are the main methods to fabricate superhydrophobic surface on copper substrates and CuO seems to be the material most presented in these works. However, CuO has poor resistance to acid which is similar to Cu(OH)2, giving this use a fatal limitation in industrial applications when exposed in acidic environment. Therefore, it is favorable to develop more chemically stable nanostructures to fabricate copper surfaces with special wettability. In order to solve the problem of poor acid-resistance of Cu(OH)2, we prepared previously a more stable CuC2O4 nanoribbons coated copper mesh and achieved oil/water separation.20 However, CuC2O4 is still not an optimal choice under the condition of high acid concentration. Copper sulfides (especially CuS and Cu2S) are known as important p-type semiconductors of versatility, availability, low-toxicity, and stability over a wide range of pH,21 and widely studied in the fields of photocatalysis,22 photoelectrochemistry,23,24 solar cells,25,26 energy storage devices,27 and biomedical applications.28 Previously, we prepared a curled plate-like superhydrophilic-underwater superoleophobic Cu2S coated copper mesh29 and a superhydrophobic poly(dimethylsiloxane) (PDMS) modified Cu2S@Cu2O micro/nanoparticles film on copper mesh30 to realize oil/water separation. In order to obtain ordered copper sulfide structures, we fabricated Cu2S@ CuxO nanowire coated copper mesh (S-NWM) in the present work by immersing Cu(OH)2 nanowire coated copper mesh (H-NWM) into sodium sulfide solution. The freshly prepared S-NWM displayed superhydrophilicity and underwater superoleophobicity, even under harsh environments. Interestingly, the matchstick-like Cu2S@CuxO nanowires film transformed from superhydrophilic to superhydrophobic after a period of storage in air without any organic modifier. According to the transformation of wettability, we could utilize the mesh to realize on-demand oil/water separation on the basis of the density of oil.

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RESULTS AND DISCUSSION

Composition and Morphology of the H-NWM and SNWM. As displayed in Scheme 1, copper mesh was first immersed into an aqueous solution containing of NaOH and (NH4)2S2O8 to form Cu(OH)2 nanowires on the surface. The as-prepared H-NWM was subsequently immersed into a Na2S solution to eventually obtain the S-NWM. Both steps were conducted at room temperature. The mesh turns light blue after alkali-assisted oxidation (Figure S1 in Supporting Information). As shown in Figure 1a, XRD pattern confirms the formation of orthorhombic phase of Cu(OH)2 (JCPDS No. 72-0140) on the substrate surface. After immersion of the HNWM into Na2S solution for 5 min, the intensity of Cu(OH)2 peaks decreased and a slight peak at about 45.95 sprouted,

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EXPERIMENTAL SECTION Materials. Ethanol, isopropanol, hydrochloric acid, nhexane, dichloromethane, chloroform, Methylene Blue, Sudan II, sodium sulfide, sodium chloride, sodium hydroxide, and ammonium persulfate were purchased from Damao Chemical Reagent Co., Ltd. (Tianjin, P.R. China) and used as purchased without further purification. Soybean oil was obtained from a local market. Kerosene was bought from SINOPEC (Beijing, P.R. China). Copper mesh with the average pore size of about 19717


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The Journal of Physical Chemistry C

which could be assigned to the (630) plane of monoclinic phase of Cu2S (JCPDS No. 83-1462). Prolonging the immersion time to 30 min, along with the clear disappearance of Cu(OH)2, the obviously presented characteristic peaks at 2θ equal to 37.60, 46.18, and 48.59 strongly associate with the (034), (630), and (−536) planes of Cu2S (JCPDS No. 831462), respectively. Meanwhile, characteristic peaks marked with rhombuses locate at about 36.48 and 42.55 are assignable to the (111) and (200) planes of the cubic phase of Cu2O (JCPDS No. 05-0667). When increasing the sulfurization time to 180 min, we could clearly observe that the film largely changed into Cu2S. Besides XRD, Raman spectroscopy is also widely adopted to observe the microstructure of nanoscale materials because of its high sensitivity to explore the local atomic arrangements and vibrations of the materials, as well as to provide important information about the structure and bonds of materials.31 Especially for copper compounds, such as CuxO (x = 1, 2), CuxS (x = 1, 1.75, 1.8, 1.95, 2, etc.), and Cu(OH)2, Raman spectroscopy could help to investigate the much lower existence of the above phases compared with the limitation of detection by XRD.32−34 Figure 1b displays the Raman spectra of H-NWM and S-NWM. The peaks at the positions of 293 and 492 cm−1 in the spectrum of H-NWM are well in agreement with the characteristic peaks of Cu(OH)2.35 After the H-NWM was immersed in Na2S solution for 30 min, the peaks of Cu(OH)2 totally disappeared. The most intense peak at about 217 cm−1 was assigned to 2Γ12− in Cu2O, some subpeaks such as 405 and 625 cm−1 belong to 4Γ12−•36

Figure 1. XRD patterns (a) and Raman spectra (b) of the H-NWM and S-NWM.

Figure 2. XPS spectra of the S-NWM prepared by sulfurization for 60 min: (a) full spectrum, (b) Cu 2p, (c) S 2p, and (d) O 1s. 19718


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Figure 3. SEM images of (a, f, k) H-NWM and S-NWM prepared by sulfurization for different durations: (b, g, l) 5, (c, h, m) 30, (d, i, n) 60, and (e, j, o) 180 min. Inset of (k) is the cross-section of the Cu(OH)2 nanowires film. Inset of (n) is a typical picture of a pile of matchsticks. Scale bars in (a, b, c, d, e), (f, g, h, i, j), and (k, l, m, n, o) are 20 μm, 1 μm, and 200 nm, respectively.

Additionally, peaks at about 285 and 331 cm−1 could be distributed to the Raman active modes of Ag and Bg(1) in CuO.37 It should be mentioned that CuO was not found in XRD, possibly due to its low crystallinity. Meanwhile, the Raman scattering possesses a better sensitivity to copper oxide than XRD.38 When the sulfurization time was increased to 60 min, peaks of Cu2O were weakened, indicating the decrease in content of Cu2O (the intensity of Raman scattering may relate directly to the content of products34). The characteristic peaks of CuO and Cu2O were largely suppressed in intensity when the sulfurization time was increased to 180 min. The strong peak at 471 cm−1 is the typical Raman scatting peak of Cu2S.33,39,40 XPS was further conducted to confirm the surface chemical state and composition of the freshly S-NWM which obtained by sulfurization for 60 min. As shown in Figure 2a, only Cu, O, S, and C elements were found in the survey scan spectrum. The Cu 2p3/2 peaks split at 931.3 and 933.0 eV can be assigned to the Cu+/Cu and Cu2+ states, respectively. The two fitting peaks from Cu 2p1/2 at about 951.2 and 953.8 eV verify the existence of Cu+ and Cu2+, respectively. Satellite peaks at 942.7 eV, 940.2 and 961.5 eV confirm the presence of Cu2+ (Figure 2b).41 Apparently, the results demonstrate the existence of CuO on the nanowires surface. The S 2p can be divided into two peaks of 2p3/2 and 2p3/1 at the position of 160.0 and 161.1 eV, respectively (Figure 2c).23 The peaks located at 529.8 and 531.1 eV in the O 1s spectrum belonged to the lattice O of Cu−O and the adsorbed O of −OH, respectively (Figure 2d).42 There was no observation of peak assigning to Cu−O−Cu bond at the position of about 532.0 eV,43 indicating no Cu2O on the mostly outer surface of the nanowires. Figure 3 shows the SEM images of the H-NWM and SNWM. The original copper mesh has an average pore size of

about 85 μm (Figure S2). After alkali-assisted oxidation, an enormous amount of Cu(OH)2 nanowires grew on the mesh surface, with average diameter and length of the nanowires of 200−300 nm and about 20 μm, respectively (Figure 3a,f,k). When the as-prepared H-NWM was immersed into Na2S solution for just 5 min, the nanowires tend to be much rougher with a little increase in diameter because of some nanoscale particles appeared on the surface of the nanowires (Figure 3b,g,l). Increasing the sulfurization time to 30 min, the diameter of the nanowires grew to 400−500 nm (Figure 3c,h). A closeup view of several nanowires shows that they look like the matchsticks with intumescent fronts (Figure 3m). Further increasing the sulfurization time to 60 min, no more obvious variation in the morphology and diameter of nanowires was observed (Figure 3d,i,n). When the sulfurization duration was prolonged to 180 min, intensive change has occurred on the morphology, some nano- and microparticles aggregated on the top of the nanowires, as shown in Figure 3e,j,o, indicating a combination in the fronts of matchsticks with each other, along with the secondary growth of the clusters assembled on the nanowires. Formation Mechanism of the S-NWM. The Cu(OH)2 nanowires were prepared by immersing the Cu substrate into a mixture of NaOH and (NH4)2S2O8 solutions for 20 min (Step I in Scheme 1). The reaction process is mainly formulated as eq 1 shown.44 Cu + 4OH− + 2NH4 + + S2 O82 − → Cu(OH)2 + 2SO4 2 − + 2NH3 ↑ + 2H 2O

(1)

As a self-sacrificial template, the Cu(OH)2 nanowires were sequentially immersed in Na2S solution containing of S2− ions to convert to the matchstick-like nanostructures (Step II in 19719


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The Journal of Physical Chemistry C Scheme 1). The surface content of Cu(OH)2 might be first reduced by S2− to generate Cu2O (eq 2). However, the solubility constant of Cu2O (Ksp = 2.0 × 10−15) is much higher than that of Cu2S (Ksp = 2.5 × 10−48),24,45 the anion-exchange reaction could further occur between O2− and S2− (eq 3),46 making the Cu2O unstable in aqueous solution containing of S2−. 2Cu(OH)2 + S2 − → Cu 2O + S ↓ + 2OH− + H 2O

(2)

H 2O + Cu 2O + S2 − → Cu 2S + 2OH−

(3)

These reactions resulted in an interfacial layer and separating the inner Cu(OH)2 from the outside sulfur sources. The core/ shell structures were consequently formed, leading to a hindrance for the direct sulfurization reaction. Further reactions were contingent on the diffusion of Cu2+ or S2− ions through the interface until the Cu(OH)2 cores depleted.26 As an intermediate, Cu2O might be presented until all converted to Cu2S if the duration of sulfurization incessantly increased according to the analysis of XRD and Raman. Most of the nanowire outer layer nanowires contacted relative higher concentration of S2− than the inner parts did, so more complete reaction of anion-exchange between O2− and S2− might occur on the outer surface, thus, Cu2O was not observed from XPS. Another evidence for eq 2 is the simultaneous generation of S, which could be observed from the yellow sulfur sediments at the bottom of the Na2S solution after the sulfurization treatment (Figure S3). The Cu2S is a monoclinic phase that prefers anisotropic growth, so the surface of the asprepared nanowires was quite rough (also could be seen from TEM image in Figure S4). The top of the nanowires might provide a suitable platform for the growth of Cu2S, thus, leading to an intumescent structure. The formation of CuO might be due to the dehydration of some Cu(OH)2, as shown in eq 4.47

Figure 4. (a) Underwater OCA and OSA of various oils on the 60 min S-NWM sample surface. (b) Underwater OCA and OSA (the testing oil is dichloromethane) of the S-NWM in different corrosive solutions.

sliding angles (OSAs) lower than 10° (Figure 4a). The oil touching-detaching experiment also testifies the ultralow adhesion between oil and the S-NWM surface (Figure S6). The as-prepared S-NWM possesses an excellent anticorrosive property. As displayed in Figure 4b, in some of the corrosive conditions, such as 1 M HCl, 1 M NaOH, and saturated NaCl aqueous solutions, the S-NWM maintained the desirable underwater superoleophobicity. Especially for the acidic environment, the Cu2S@CuxO nanowires demonstrated much better acid-resistance than Cu(OH)2 or CuO nanowires. The Cu(OH)2 could be corroded when been immersed in 1 M HCl just for 1 s (Figure S7). The CuO nanowires coated copper mesh, prepared by annealing the H-NWM at 190 °C for 3 h, was also inevitably corroded within 30 s when soaked it in 1 M HCl (Figure S8). As a comparison, the morphology of the Cu2S@CuxO nanowires remains almost unchanged after soaking in 1 M HCl for 48 h (Figure S9), exhibiting an outstanding acid-resistance. It can be concluded that the presence of Cu2S could efficiently prevent the occurrence of acid corrosion. It is intriguing that, after storage in air for about 2 weeks, all the S-NWM samples changed from superhydrophilic to (super) hydrophobic without any surface modifications (Figure 5a), while the H-NWM kept superhydrophilicity because of the inherent hydrophilicity of Cu(OH)2 (Figure S10). The WCA of S-NWM prepared by 30 and 60 min of sulfurization treatment reached high up to 153 ± 2.4° and 157 ± 1.8°, respectively. In order to investigate the conversion progress of the wetting behavior, the wettability transition of the S-NWM with time was measured as shown in Figure 5b. When the freshly prepared S-NWM stored in air for 7 days, the WCA increased to 91.5°, that is, it has been switched to hydrophobic. After 13 days of storage, the WCA increased to over 150°, enabling the complete transition from hydrophobicity to superhydrophobicity. Several tests have been performed to evaluate the superhydrophobicity of the as-stored S-NWM. As displayed in Figure 6a, the water droplets could stand on the surface showing

Cu(OH)2 + 2OH− → Cu(OH)4 2 − ⇄ CuO + 2OH− + H 2O

(4)

This series of reactions occurred during the process of sulfurization, resulting in a composite of Cu2S and CuxO (x = 1,2) in the matchstick-like nanowires. If the sulfurization was incessantly carried on, the CuO could further react with S2− to form Cu2S (eq 5). This is because the solubility constant of Cu2S is also much lower than that of CuO (Ksp = 1.0 × 10−20),48 and the anion-exchange reaction could further occur between CuO and Cu2S.49 2CuO + 2S2 − + 2H 2O → Cu 2S + S ↓ + 4OH−

(5)

Wettability of the S-NWM. As is well-known, the wetting property of a solid surface generally depends on the surface architecture induced roughness and the chemical composition.50 The pristine copper mesh has a water contact angle (WCA) of about 89 ± 1.5° (Figure S5a), displaying a typical Wenzel state. Meanwhile, the underwater−oil contact angle (OCA) for dichloromethane on the pristine copper mesh was 136 ± 1.3° (Figure S5b). The freshly prepared S-NWM was superhydrophilic with the WCA about 0° (Figure S5c). Figure 4 displays the subsequent underwater−oil repellency testing of the S-NWM prepared by 60 min of sulfurization. For a series of testing oils (including n-hexane, kerosene, soybean oil, dichloromethane, and chloroform), the surface shows good superoleophobicity with the OCAs larger than 150° and the oil 19720


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consequently, a solid−liquid−air interface would be established when submerging the mesh into water, which can prevent the water permeation effectively.51 Figure 6d shows a series of timelapse photographs of a water droplet (5 μL) impacting on the obtained superhydrophobic S-NWM surface employing a high speed camera. The water droplet could easily bounce for four times in 140 ms with no obvious decrease of height, indicating great superhydrophobicity. All the evidence demonstrates that, after stored in air for an adequate duration, the matchstick-like nanowires film has transformed from a superhydrophilic state to the lotus leaf-like state even no organic modifier was used. To better understand the mechanism of wettability transition, the high resolution C 1s XPS spectra of freshly prepared S-NWM and the same sample after storage turning to superhydrophobic were conducted as shown in Figure 7. The C 1s peak resolved into three peaks with binding energies of 283.1 (283.3), 284.3 (284.4), and 287.4 (287.3) eV are corresponding to C−C/CC, C−O, and O−CO, respectively.23,52 The presence of C−C/CC reveals that there are some airborne hydrocarbons generated during the reaction. In the surface of freshly prepared S-NWM, the proportion of the hydrophobic hydrocarbons was 49.7%, and simultaneously, the proportion of C−O and O−CO, which are hydrophilic,53 was 32.9% and 17.4%, respectively. In contrast, the proportion of C−C/CC in the as-stored S-NWM has a dominant position of 69.7%, and the proportion of C−O and O−CO decreased to 18.3% and 12.0%, respectively. So one of the main reasons for the transition of superhydrophilicity to superhydrophobicity is the variation of the adsorbed organic hydrocarbons, which is in good agreement with the previous work by Li et al.54 AFM further indicates a slight increase in average roughness (Ra) from 123 to 142 nm after stored to superhydrophobic state (Figure S11), suggesting the surface morphology changed little after storage, which is in agreement with the SEM result (Figure S12). Though the increase of hydrophobic hydrocarbons provided a significant probability for the increase of WCA, the storageinduced air-pockets in the hierarchical matchstick-like nanowires might be another important factor. Wang et al. demonstrated that the trapped air in the structures placed an important role in the transition of superhydrophobicity (underwater superoleophilicity) to underwater superoleophobicity.55 Barthlott et al. reported that the Salvinia surface, which has an eggbeater-shaped structure, can effectively trap air, thus, causing the special water repellency performance.56 In this work, the surface of freshly prepared S-NWM had almost equal contents of hydrophobic groups and hydrophilic groups from C 1s peaks of XPS, as shown in Figure 7, nevertheless, still displaying superhydrophilicity, which might be due to lack presence of air among the structures. After 2 weeks of storage, the percentage of C 1s peak increased slightly from 27.53% to 27.69%, while both the values of Cu 2p peak and S 2p peak decreased (Figure S13 and Table S1). Considering that the variation of the hydrophilic hydrocarbons (C−O and O−C O) to hydrophobic hydrocarbons (C−C/CC) would result in reducing the amount of O, one can conclude that the additional O may come from the physically adsorbed oxygen molecules (O2) on the nanowire surface during air exposure. Notably, the matchstick-like nanowires structure, to some extent, likes the eggbeater-shaped structure, could provide enough space to store the air. Herein, the airborne hydrophobic hydrocarbons and the special structures cocontribute to the

Figure 5. (a) WCAs of the S-NWM prepared by sulfurization for different durations after storage in air for 2 weeks. (b) WCA variation of the S-NWM prepared by sulfurization for 60 min when stored in air for diverse periods of time.

Figure 6. Superhydrophobic behaviors of the S-NWM after stored in air: (a) Photographs of the deionized water droplets stay on the asstored S-NWM surface. (b) Jet of water bouncing off the as-stored SNWM surface. (c) As-stored superhydrophobic S-NWM was immersed into water by an external force, showing a silver mirror phenomenon. (d) Time-lapse photographs of a water droplet bouncing on the as-stored S-NWM.

almost spheres. A jet of water can easily bounce off the mesh surface with no residues, indicating good water repellency (Figure 6b and video S1). When the superhydrophobic mesh was submerged into water by an external force, it showed a clear silver mirror-like phenomenon (Figure 6c and video S2). This is because an air layer may be trapped on the surface, 19721


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Figure 7. C 1s core level XPS spectra of (a) the freshly prepared 60 min S-NWM and (b) the same sample stored in air for 2 weeks.

Figure 8. (a) Schematic of the relationship between superhydrophilic, underwater superoleophobic, and superhydrophobic surfaces. On-demand oil/ water separation of (b) “water-removal” type and (c) “oil-removal” type. The oils (n-hexane (b) and dichloromethane (c)) and water were dyed with Sudan II and Methylene Blue, respectively. Separation efficiency of (d) underwater superoleophobic mesh for light oils (ρoil < ρwater) and (e) superhydrophobic mesh for heavy oils (ρoil > ρwater) vs the recycle numbers.

water interfaces, respectively. When γOA cos θOA < γWA cos θWA, an oleophobic condition can be guaranteed. Meanwhile, oil surface tension is always much lower than that of water (γOA < γWA), so most of the hydrophilic surfaces (cos θWA > 0) are also underwater oleophobic. Especially for a superhydrophilic surface, could possess underwater superoleophobicity simultaneously (Figure 8a). After storage in air, the water wettability transition can be understood theoretically by the Cassie−Baxter equation:58

final superhydrophobicity even no further chemical modification was adopted. The relationship between superhydrophilicity, underwater superoleophobicity, and superhydrophobicity is illustrated in Figure 8a. In the water−oil−solid interface, the underwater OCA can be expressed as eq 6 according to Young’s equation:57 cos θOW =

γOA cos θOA − γWA cos θ WA γOW

(6)

where θOA, θWA, and θOW are the contact angles of oil in air, water in air, and oil in water, respectively. As well as, γOA, γWA, and γOW are the surface tensions of oil/air, water/air, and oil/

cos θS = f1 cos θ − f2 19722

(7) DOI: 10.1021/acs.jpcc.7b03645 J. Phys. Chem. C 2017, 121, 19716−19726

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The Journal of Physical Chemistry C Table 1. Summary of Some Inorganic Superhydrophobic Films without Organic Modifiers methods

substrates

electrodeposition-based

WO3

oxidation-based

Cu Cu Cu/Cu2O Cu/ZnO/Zn/ CuO/Zn(OH)2/Cu5Zn8 Sn/SnOx Bi/Bi2O3 Ni Ni/Co CuO

tin, copper, or iron disk tin disk copper plate copper disk copper plate

CuO CuO CuO/Cu(OH)2

copper sheet copper plate copper sheet

Cu2O ZnO, ZnO/Zn

copper foil glass slide

Ag/Cu2O Ag Sn ZnO

copper foil copper mesh zinc plate glass wafer

SnO2/SnO2/SiOx

Au-coated Al2O3 substrate copper plate

displacement reaction-based

others

a

chemical composition

sol−gel method followed by a growth process chemical vapor deposition (CVD)

CuO/Cu3Pt/Cu

indium tin oxide (ITO) glass copper sheet copper sheet stainless steel mesh copper sheet

chemical reaction followed by an annealing process atomic layer deposition (ALD)

Y2O3

anodic electrocrystallization

SnO

Si(100) with Si nanowires Tin disk

two-step reactions followed by storage

Cu2S/CuxO

copper mesh

WCAa

morphology

ref

pebble beach-like

151.3 ± 2.9°

61

honeycomb-like pyramid-like coral-like anomalous nanostructures dendritic structures dendritic structures pine cone-like flower-like protuberances with nanowires nanowires sheet-like microflowers on nanoneedles porous film clusters, clusters with nanorods flower-like clusters “Sago cycas” branch-like dendrite-like nanorod arrays

162.1° 151° 163° 170 ± 2°

62 63 64 65

165° 164° 155.7° 163° 156°

66 67 59 52 60

∼153° 158° 159°

68 69 37

∼152° 158°, 155°

70 71

169.2° 159° 156° 161.2 ± 1.3°

72 73 74 75

nanowires with branches

155.8 ± 1.1°

76

coralline-like

170°

77

50 nm thick film

158 ± 4°

78

pompon-like and flowerlike matchstick-like nanowires

153°

79

157 ± 1.8°

this work

WCAs are the maximum values in the corresponding references.

where θ and θs are the CAs of the smooth surface and the SNWM surface, respectively. f1 and f 2 represent the fraction of the solid surface and air in contact with water, respectively. So f1 + f 2 = 1. We can easily deduce that when the f 2 increased and f1 decreased at the same time, θs would increase correspondingly. The trapped air pocket can prevent the penetration of water into the surface effectively (Figure 8a). Therefore, the WCA of the S-NWM would increase gradually with the prolongation of storage time, resulting in a finally superhydrophobic state. Among thousands of papers concerning the superhydrophobic surface, there is just a small quantity of inorganic superhydrophobic films without any organic modification. So, we have summarized some of this type of work, as listed in Table 1. Through an electrodeposition process or a displacement reaction to decorate metal on a substrate is the most popular approach to prepare inorganic superhydrophobic films without artificial modification. Copper oxides seem to be the most studied materials, they can easily form hierarchical structures on copper substrate and achieve superhydrophobicity. Rare earth oxides, in comparison, were relatively less studied. It is notable that, some of the materials need a storage procedure to form enough air pockets among the structure likewise.37,52,59,60 The reported morphologies are diverse, but most of them are 3D structures.

Due to the special wettability of the S-NWM, we could use it to selectively separate water or oil from oil/water mixtures, especially for the oils with different densities. As is well-known, the density of most oils is lower than that of water, so it is a better choice to use superhydrophilic/underwater superoleophobic filters (water-removal type) to separate these types of oil and water mixtures other than superhydrophobic/ superoleophilic filters (oil-removal type). While some of the oils, such as chloroform, dichloromethane, diiodomethane, tetrachloromethane, and 1,2-dichloroethane, have a density greater than water, a superhydrophobic/superoleophilic filter is a relatively optimal choice under this condition.80 When selecting an inappropriate filter for separation of oil/water mixtures, a water (or oil) barrier layer would form on the mesh and prevent the separation from proceeding. For instance, if the density of oil is larger than water, the underwater superoleophobic mesh is not a good choice for separating heavy oil/ water mixtures because the heavy oils will locate between the water and the filter to prevent water permeation (Figure S14). The previously common solution of this problem was to employ an acclivitous separation device, which could make the filter-loving liquid touch the separation membrane.81 However, this approach is time-consuming and hard to control. Hence, we could use the S-NWM with alterable wettability to realize on-demand oil/water separation. Figure 8b and video S3 show the separation process of n-hexane/water mixtures using the 19723



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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21776094), Science and Technology Planning Project of Guangdong Province (2014A010105052), Natural Science Foundation of Guangdong Province (2015A030313506), and One Hundred Steps Climbing Plan Program of SCUT (EC40617155). The authors gratefully thank Dr. Jingyu Wang for the AFM test.

freshly prepared S-NWM. The n-hexane could remain stable on the water-prewetted S-NWM while water quickly permeates through the S-NWM. Furthermore, we can use the superhydrophobic S-NWM to realize the separation of heavy oil/ water mixtures, exhibiting the good flexibility (Figure 8c and video S4). Figure 8d,e display the separation efficiency of the superhydrophilic/underwater superoleophobic and superhydrophobic S-NWMs for separating various oils with different densities. The separation efficiency was calculated using eq 8:82 ES =

M filtrate × 100% M mixture

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(8)

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CONCLUSION In summary, we have demonstrated a facile two-step approach to preparing a matchstick-like Cu2S@CuxO nanowire film on copper mesh. A Cu(OH)2 nanowires film was adopted as a template because of its thermodynamic instability, which could further form Cu2S nanostructure through an anion-exchange reaction. The freshly prepared mesh displayed superhydrophilicity and underwater superoleophobicity even under severe acidic aqueous conditions. The matchstick-like Cu2S@CuxO nanowires coated mesh, nevertheless, can be transformed to superhydrophobic with a water contact angle high up to 157 ± 1.8° after about two weeks of storage in air without any further modification by low surface energy materials. The transition may be attributed to the variation of surface chemical composition as well as the trapped air among the structure. Herein, we can utilize the special wetting behavior of the mesh to realize on-demand oil/water separation on the basis of the density of oil. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03645. Other information on the H-NWM and S-NWM (PDF). A jet of water bouncing off the as-stored S-NWM surface (AVI). The superhydrophobic S-NWM possesses silver mirror phenomenon (AVI). Separation of n-hexane−water mixtures (AVI). Separation of dichloromethane−water mixtures (AVI).

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REFERENCES

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where Mfiltrate and Mmixture are the mass of water (or oil) in the filtrate and original oil/water mixture, respectively. For all the tested oils, the efficiency maintained above 96% after 5 cycles of separation, indicating the good stability.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-20-87112057. ORCID

Xiufang Wen: 0000-0001-5071-8940 Jiang Cheng: 0000-0002-9947-6193 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 19724


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