ARTICLE pubs.acs.org/JPCC
Nanometric Multiscale Rough CuO/Cu(OH)2 Superhydrophobic Surfaces Prepared by a Facile One-Step Solution-Immersion Process: Transition to Superhydrophilicity with Oxygen Plasma Treatment Archana Chaudhary and Harish C. Barshilia* Surface Engineering Division, Council of Scientific and Industrial Research, National Aerospace Laboratories, Bangalore 560017, India ABSTRACT: An inexpensive and facile one-step method to develop a superhydrophobic coating on the copper surface is reported. Superhydrophobic CuO/Cu(OH)2 surfaces were prepared by a simple solution-immersion process at room temperature, without using a low surface energy material. The structure and composition of as-prepared CuO/Cu(OH)2 hierarchical structure were confirmed by X-ray diffraction, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The growth stage was carefully examined by field emission scanning electron microscopy (FESEM), and it was observed that initially Cu(OH)2 nanoneedle arrays were formed on the copper surface and subsequently the CuO microflowers formed on the nanoneedle arrays. The contact angle as a function of immersion time was studied using a contact angle goniometer. The correlation between the microstructure of the immersed copper surface and the contact angle was examined carefully using FESEM and atomic force microscopy (AFM). Our results based on FESEM and AFM studies show that the CuO/Cu(OH)2 coatings demonstrate superhydrophobicity only for an optimal combination of the solid region (i.e., microflowers and nanoneedles) and air pockets (i.e., voids). The maximum static water contact angle on the prepared surface was 159°. The wettability transition of the CuO/Cu(OH)2 surface from superhydrophobicity to superhydrophilicity was studied by the alteration of oxygen plasma treatment and dark storage. The FESEM, AFM, and XPS studies showed that this transformation was mainly due to the morphological changes that occur in addition to the chemical changes taking place on the CuO/Cu(OH)2 surface under the influence of oxygen plasma. XPS analysis demonstrated that the incorporation of oxygen species by oxygen plasma activation accounted for the highly hydrophilic character of the surface.
1. INTRODUCTION The wettability property of solid material surfaces is extremely important and has several applications. Contact angle (CA) and sliding angle are two criteria for the evolution of wetting ability. Several studies have confirmed that superhydrophobicity is a combination of micrometric-scale and nanometric-scale roughnesses.1,2 There is much interest in the development of superhydrophobic surfaces as it is expected that innovations and inventions in this area will improve industrial processes such as cleaning, glass coating, building materials, textiles, ship building, corrosion protection, anti-bio fouling, etc.35 Nature has provided us a plethora of plants and insects, such as lotus leaves,6 water strider legs, and cicada wings7 that have superhydrophobic surfaces with a water contact angle greater than 150°. Lotus leaves exhibit an unusual type of superhydrophobicity and selfcleaning property called the “Lotus Effect”. It is a product of the cooperative effect of the hydrophobic waxy coating and a binary rough surface.6 To mirror this effect, researchers have fabricated artificial superhydrophobic surfaces through two different processes: by creating hierarchical micro-/nanostructures on hydrophobic substrates8,9 or by chemically modifying a micro-/nanostructured surface with low surface energy materials.1012 On the basis of these processes, various methods have been reported for creating superhydrophobic surfaces such as plasma processing, electrospinning, solgel processing, magnetron sputtering, spray pyrolysis, and laser irradiation.1317 However, all these r 2011 American Chemical Society
techniques require either specific equipment or have difficult process controls. Copper oxide is a narrow band gap (1.2 eV) p-type semiconductor, which crystallizes in the monoclinic crystal structure. CuO is primarily used in electronics, in gas sensors, as a catalyst, and for transforming solar energy.1820 Combining the properties listed above with superhydrophobicity opens up new possibilities for the use of CuO in diverse fields. CuO, through synthesis, has enabled the creation of structures such as nanorod arrays, nanowires, nanoleaves, nanoribbon arrays, microcabbages, rectangles, seed-, belt-, and sheet-like nanostructures, etc.2125 Here we report a simple, time-saving, and inexpensive, facile one-step method for the preparation of superhydrophobic CuO/Cu(OH)2 coating on a copper sheet at room temperature. The fabrication of a hierarchical structure and the low percentage of hydrophilic hydroxyl group are the critical factors in the preparation of the superhydrophobic surface in our study. To the best of our knowledge, no one has yet reported a superhydrophobic CuO/Cu(OH)2 hierarchical structure by a simple immersion process without using a low surface energy material. The reversible wetting behavior for “smart surfaces” has been extensively researched for their potential applications in Received: May 12, 2011 Revised: August 5, 2011 Published: August 11, 2011 18213
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self-cleaning surfaces, intelligent microfluidic devices, lab-on-a-chip systems, and so on.2629 As per our knowledge, only a few papers have reported transition between superhydrophobicity and superhydrophilicity.3032 In this study, we report reversible wettability transition of the CuO/Cu(OH)2 coating. Oxygen plasma treatment of the CuO/Cu(OH)2 coating resulted in superhydrophilicity, whereas superhydrophobicity was observed by placing plasma-treated CuO/Cu(OH)2 coating in dark storage. We expect that the present fabrication technique will make possible large-scale production of superhydrophobic engineering materials. Also the rapid reversible wettability transition property will find application in various smart devices.
2. EXPERIMENTAL DETAILS 2.1. Materials Used. A copper sheet, sodium hydroxide (NaOH), ammonium persulfate ((NH4)2S2O8), acetone, and ethanol (analytical grade) were used for the experiments reported in the present work. 2.2. Synthesis. CuO/Cu(OH)2 nanoneedle arrays with microflower shapes were prepared on a copper sheet by alkali assistant surface oxidation at room temperature. The copper sheets were ultrasonically cleaned in acetone and deionized water for about 15 min, followed by a 3060 min immersion in an aqueous solution of 2.0 M sodium hydroxide and 0.15 M ammonium persulfate at room temperature. The color of the copper sheet surface turned gradually from faint blue to light blue to faint black and finally light black during the immersion process. The copper sheet was then removed, bathed with deionized water and absolute ethanol, and dried at room temperature. 2.3. Reversible Wettability Transition. The wettability of the as-prepared superhydrophobic surface was changed by oxygen plasma treatment for different durations, ranging from 30 s to 3 min. For 3 min oxygen plasma treatment, the as-prepared coating exhibited superhydrophilic nature. This superhydrophilic coating restored superhydrophobicity after dark storage for 3 days under ambient conditions. The reversible wettability transition between superhydrophobicity and superhydrophilicity was observed by switching between oxygen plasma treatment and dark storage alternatively. 2.4. Characterization. The static contact angle was measured according to the sessile-drop method using a contact angle analyzer (Phoenix 300 Goniometer) with water. The system mainly consists of a CCD video camera with a resolution of 768 576 pixels. The drop image was stored by the video camera, and an image analysis system was used to calculate the left and right angles from the shape of the drop. The droplet size of the fluid was about 5 μL; therefore, the gravitational effect can be neglected. The contact angle of the samples was measured at three different places, and the values reported herein are the average of three measurements. The chemical structure of the coatings was studied using micro-Raman spectroscopy. A DILOR-Jobin-Yvon-SPEX integrated micro-Raman spectroscopy was used for the present study. Three-dimensional surface imaging of the coatings was carried out using Surface Imaging System atomic force microscopy (AFM). The microstructure of the coatings was studied using field emission scanning electron microscopy (FESEM, Supra 40VP, Carl Zeiss). The X-ray diffraction (XRD) patterns of the coatings were recorded in an X-ray diffractometer system (JEOL, JDX-8030). The X-ray source was a Cu Kα radiation (λ = 0.15418 nm), which was operated at 35 KV and 20 mA. The bonding structure of the coatings was characterized by X-ray
Figure 1. FESEM images of: (a) unmodified Cu sample and (bf) the sample surface after immersion in the solution for: (b) 2, (c) 5, (d) 10, (e) 30, and (f) 100 min. The inset shows the FESEM images at higher magnification.
photoelectron spectroscopy (XPS) using an ESCA 3000 (V. G. Microtek) system with a monochromatic Al Kα X-ray beam (energy = 1486.5 eV and power = 150 W).
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The surface morphology of the sample after immersion in the alkaline aqueous solution of NaOH and (NH4)2S2O8 for different immersion times was examined by FESEM and is shown in Figure 1. Also shown is the surface morphology of the pristine copper substrate (Figure 1(a)). As seen in Figures 1(b) and (c), after 2 and 5 min of immersion, the surface of the copper sheet is uniformly covered with nanoneedle arrays. When the immersion time is prolonged to 10 min and above, the hierarchical structures of microflowers and nanoneedle arrays on the copper substrate were formed. It was observed that the microflowers were formed over the nanoneedle arrays and the density, and the size of microflowers increased as the immersion time was prolonged (see Figures 1(d) and (e)). For an immersion time greater than or equal to 100 min, the hierarchical structure of the microflowers/nanoneedle arrays got converted into a cluster of ribbons (Figure 1(f)). These microflowers and merged nanoneedle structures gave rise to different scales of nanometric roughness in the coating, which were responsible for the superhydrophobic behavior. The XRD analysis was carried out to study the crystal structure of the as-prepared CuO/Cu(OH)2 coating on the copper substrate. Figure 2(a) shows the XRD plot of Cu(OH)2 nanoneedle arrays (the corresponding FESEM image of which is shown in Figure 1(b)). The peak marked with an asterisk (*), observed at 2θ = 43.3°, is from the substrate. The weak peaks centered at 2θ = 38° and 38.7° correspond to (041) and (022) planes of orthorhombic Cu(OH)2 (JCPDS card No. 80-0076). 18214
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Figure 3. Micro-Raman spectrum of the as-prepared CuO/Cu(OH)2 sample surface.
Figure 2. XRD patterns of: (a) Cu(OH)2 nanoneedle arrays and (b) an optimized coating on the copper substrate.
Figure 2(b) displays the XRD pattern of the as-fabricated surface on the copper sample prepared at an immersion time of 30 min. The peaks observed at 2θ = 32.3°, 35.5°, and 38.8° correspond to (110), (111), and (200) planes of monoclinic CuO (JCPDS card No. 4-0836). A low intensity peak centered at 2θ = 34.0° corresponds to the (002) plane of orthorhombic Cu(OH)2 (JCPDS card No. 80-0076). The two peaks marked with asterisks (*), positioned at the 2θ value of 43.3° and 50.3°, are from the copper substrate. From the XRD study, it can be concluded that the nanoneedle arrays and the hierarchical structure of the microflowers/nanoneedle arrays were composed of Cu(OH)2 and CuO/Cu(OH)2, respectively. The synthesis and growth of flower-like copper monoxide has been studied extensively.31,34 On the basis of FESEM and XRD analysis of the experimental data shown in Figures 1 and 2, the suggested chemical reactions are33,35 Cu þ 4NaOH þ ðNH4 Þ2 S2 O8 f CuðOHÞ2 þ 2Na2 SO4 þ 2NH3 v þ 2H2 O
ð1Þ
CuðOHÞ2 þ 2OH f CuðOHÞ4 2 T CuO þ 2OH þ H2 O
ð2Þ The top surface of the copper sheet gets rapidly oxidized to Cu2+ by the oxidant (NH4)2S2O8 when the copper sheet is immersed in alkaline aqueous solution. Equation 1 accounts for the formation of Cu(OH)2 nanoneedle arrays and is basically an oxidation process. The concentration of NaOH and (NH4)2S2O8 influences the formation of Cu(OH)2 nanoneedle arrays, and the growth rate depends upon the reaction time.15 Due to the metastable nature of copper hydroxide, it can be easily transformed into more stable copper oxide. In the presence of hydroxide ions (OH), kinetics of transformation is very fast because the divalent copper ions are dissolved in the form of tetrahydroxocuprate (II) anions Cu(OH)42.35 When the immersion time was increased, some microflowers were synchronously
Figure 4. Variation of the water contact angle with immersion time.
deposited from the bulk solution because of tetrahydroxocuprate (II) anions Cu(OH)42.36 These Cu(OH)4 anions act as a precursor that exists for a short period of time during the preparation of CuO, thus the needle-like morphology of the subunits in our study is strongly associated with the layered structure of orthorhombic Cu(OH)2, as has been discussed comprehensively in already published work.3537 It is noteworthy to mention here that (NH4)2S2O8 is not essential for the formation of hierarchical structure. Namely, a copper sheet immersed into concentrated NaOH alone can also be oxidized by dissolved oxygen, although it takes longer time.10 The chemical structure of the coating was studied by microRaman spectroscopy (Figure 3). CuO is a monoclinic unit cell and belongs to the space group C62h (C2/c) with two molecules per primitive cell.38 There are 12 possible normal modes (4Au + 5Bu + Ag + 2Bg) because the primitive cell has four atoms, and only three modes (Ag + 2Bg) are Raman active (297 cm1 (Ag), 344 cm1 (Bg), and 629 cm1 (Bg)). The peak observed at 293.8 cm1 is attributed to the Ag mode of CuO,38,39 and peaks centered at 344.1 and 631.2 cm1 are attributed to the Bg1 and Bg2 modes of CuO.38,39 A very low intensity peak observed at 485.2 cm1 is attributed to Cu(OH)2.40 This confirms the presence of CuO and Cu(OH)2 in the chemically modified Cu surface. 3.2. Optimization of Immersion Time. The immersion time plays a vital role in determining the surface structure. The immersion time was optimized as a function of the contact angle. The variation of contact angle with immersion time is shown in Figure 4. It can be seen that the optimized immersion time of 30 min resulted in a contact angle greater than 150°. It is believed that microflowers and nanoneedle arrays on the copper surface can trap a large fraction of air within the grooves, which is responsible for the superhydrophobicity. The CuO/Cu(OH)2 coating exhibited a maximum contact angle of 159° at the optimized immersion time (30 min). Further increase in the immersion time resulted in a decrease in the contact angle, possibly due to 18215
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Figure 5. FESEM images of the sample after immersion in the solution for (a) 10, (b) 30, (c) 80, and (d) 100 min and the corresponding optical photographs of water droplet are shown in the inset. Marked region represents the variation of intensity and size of the air pockets.
the change in the surface roughness. To explain the variation of contact angle with immersion time, the CuO/Cu(OH)2 coating was carefully studied using FESEM. FESEM images at different immersion times along with the corresponding contact angles (inset) are shown in Figure 5. As we can see from Figures 5(ad), with an increase in the immersion time, the fraction of solid components (i.e., white regions) increases, whereas the fraction of air pockets (i.e., dark regions highlighted in red) reduces. Therefore, a water droplet placed on the coating is in contact with a comparatively higher fraction of the solid region than the air pockets, which results in the reduction of the contact angle. With the help of the CassieBaxter equation, we can explain how a contact angle will reduce as the surface area fraction increases41 cos θapp ¼ f1 cos θ1 þ f2 cos θ2
ð3Þ
where θapp is the apparent contact angle on a rough surface; f1 and f2 are the area fractions of the rough and smooth surfaces, respectively; and f1 + f2 = 1. θ1 and θ2 are the contact angles on the rough and smooth surface, respectively. Here we have assumed air as the smooth surface and the prepared coating as the rough surface. The contact angle for air is 180°. Thus, the modified equation for an apparent contact angle is cos θapp ¼ f1 ðcos θ1 þ 1Þ 1
ð4Þ
It can be seen that as the value of f1 increases the corresponding value of θapp decreases. Therefore, in the present study, the contact angle is highest at 30 min of immersion time because of reduction in the fraction of solid regions. To cross check the above observation, the roughnesses of the asprepared coatings were measured. It is well-known that improving the surface roughness is a crucial factor for the fabrication of superhydrophobic surfaces.42 The three-dimensional AFM images of the coated samples for different immersion times are shown in Figures 6(ad). The average surface roughness (Ra) values for 10, 30, 80, and 100 min immersion times were 192, 226, 132, and 65.2 nm, respectively. Figure 7 represents the schematic description
Figure 6. AFM images of the sample surface after immersion in the solution for: (a) 10, (b) 30, (c) 80, and (d) 100 min.
Figure 7. Schematic representation of the water droplet on: (a) rough surface and (b) smooth surface. The corresponding roughness profiles are shown in (c) and (d), respectively.
of the aforementioned analysis. Figures 7(a) and (b) represent the water droplet on a rough surface and a relatively smooth surface, respectively. The corresponding roughness profiles are shown in Figures 7(c) (Ra = 226 nm) and (d) (Ra = 65.2 nm). The change in the roughness value is consistent with the observation recorded using FESEM images. Examination of the CuO/Cu(OH)2 coating prepared at the optimized immersion time at low magnification (Figure 8(a)) showed needle- and flower-like microstructure along with air pockets. The needles and the flowers examined at very high magnification (Figures 8(b) and (c)) showed that they consisted of fused individual structures, which generated a textured or a patterned surface, attributing to a lower scale of roughness (believed to be in the range of a few tens of nanometers). The combination of solid components and air grooves acts as the higher or coarser-scale roughness, which is about 226 nm, as measured by AFM (Figure 6(b)). The textured microflowers/ nanoneedle arrays give rise to the multiscale roughness in the CuO/Cu(OH)2 coating, which was responsible for the superhydrophobic behavior. The multiscale nanometric rough nature of the coating can be depicted schematically as shown in Figure 7, 18216
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Figure 8. (a) FESEM image of the as-prepared sample at lower magnification. Figures (b) and (c) are FESEM images of areas marked by a rectangle and a circle in (a), at higher magnification.
Figure 9. (a) Optical photographs of the water droplet before and after oxygen plasma treatment. (b) Reversible superhydrophobic superhydrophilic transition of the as-prepared film under the alteration of plasma treatment and dark storage. (c) Variation of the water contact angle of the surface with the plasma processing time.
wherein the higher-scale roughness is due to the postlike microstructure, while the lower-scale roughness is due to the roughness around or on the top of the post. 3.3. Reversible Conversion of Surface Wettability. The reversible wettability transition between superhydrophobicity and superhydrophilicity of the inorganic oxide films has been observed in the recent past.3032 By alternatively switching oxygen plasma treatment and dark storage, we found reversible wetting behavior in the CuO/Cu(OH)2 coating. The plasma treatment of the CuO/Cu(OH)2 coated sample was done by using the four-cathode reactive unbalanced dc magnetron sputtering system. The plasma etching of the CuO/Cu(OH)2 coating was carried out in an Ar + O2 atmosphere at a pressure of ∼1.5 Pa. The process parameters for plasma treatment were: substrate bias = 800 V, Ar flow rate = 20 standard cubic centimeter per minute (sccm), oxygen flow rate = 2 sccm, and duration = 30 s to 3 min. The estimate of wettability was made by measuring the water contact angle of the prepared coating. Figure 9(a) (left) shows the optical image of the water droplet with contact angle of 159° on the prepared surface. Post oxygen plasma treatment, the
Figure 10. FESEM images of the as-prepared CuO/Cu(OH)2 coating before oxygen plasma treatment at different magnifications [(a), (b)]. The corresponding FESEM images of the CuO/Cu(OH)2 coating after oxygen plasma treatment at different magnifications are shown in (c) and (d), respectively.
water droplet spread out on the film, resulting in a contact angle
