- Email: [email protected]
copper stearate nanocoating
Accepted Manuscript Excellent floating and load bearing properties of superhydrophobic ZnO/copper stearate nanocoating Gokulraja Thangaiyanadar Suyambulingam, Kadarkaraithangam Jeyasubramanian, Vimal Kumar Mariappan, Pandiyarasan Veluswamy, Hiroya Ikeda, Karthikeyan Krishnamoorthy PII: DOI: Reference:
S1385-8947(17)30398-4 http://dx.doi.org/10.1016/j.cej.2017.03.052 CEJ 16648
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
31 August 2016 8 March 2017 14 March 2017
Please cite this article as: G.T. Suyambulingam, K. Jeyasubramanian, V.K. Mariappan, P. Veluswamy, H. Ikeda, K. Krishnamoorthy, Excellent floating and load bearing properties of superhydrophobic ZnO/copper stearate nanocoating, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.03.052
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Excellent floating and load bearing properties of superhydrophobic ZnO/copper stearate nanocoating Gokulraja Thangaiyanadar Suyambulingam1, Kadarkaraithangam Jeyasubramanian1#, Vimal Kumar Mariappan1, Pandiyarasan Veluswamy2,3, Hiroya Ikeda2,3, and Karthikeyan Krishnamoorthy4* 1
Centre for Nanoscience and Technology, Department of Mechanical Engineering, Mepco Schlenk Engineering College, Sivakasi, India-626005.
2
3
Research Institute of Electronics, Shizuoka University, Hamamatsu, 4328018, Japan.
Graduate School of Science and Technology, Shizuoka University, Hamamatsu, 4328018, Japan.
4
Nanomaterials and System Lab, Department of Mechatronics Engineering, Jeju National University, Jeju 690-756, South Korea.
Email: #[email protected] (Dr. KJ) *[email protected] (Dr. KK)
ABSTRACT The development of superhydrophobic materials is rapidly increasing due to their diverse applications such as self cleaning, oil-water separation, aquatic robotics, and so on. In this study, we developed a superhydrophobic coating comprising of porous ZnO nanoparticles and copper stearate (CuSA2) via a cost effective spray coating method. Physico-chemical characterizations such as X-ray diffraction analysis, Fourier transform infrared spectroscopy, scanning electron microscope, and elemental mapping were used to understand the crystallinity, bonding nature, morphology and distribution of the ZnO/CuSA2 coatings. Superhydrophobic nature of the bare CuSA2 coating measured through goniometer was found to be more than 151°, further it was enhanced to around 161° by increasing the weight percentage of ZnO. The ZnO/CuSA2 superhydrophobic coatings showed excellent super buoyancy characteristic as evidenced by their floating nature in water over a prolonged time. Further, experiments on maximum load bearing capacity of the ZnO/CuSA2 coatings were examined by placing stapler pins on their surface and the effect of deposition time on the load bearing capacity has been investigated in detail. The experimental results showed that the superhydrophobic nature of ZnO/CuSA2 coating with excellent floating characteristics and higher load bearing capacity, may find a wide range of future applications in man-made water floating robots. Keywords Superhydrophobic coatings; porous ZnO; copper stearate; spray coating technique; floating ability; load bearing capability.
1. Introduction Nature has empowered many surfaces with water repellent properties including lotus leaf, water strider’s leg, dames fly wings, butterfly wings, etc [1–4]. In 1977, the “Lotus effect” was discovered by Barthlott and Ehler in which they revealed the presence of micron-sized structures (papillae) and low surface energy coating (like cuticular waxes) over the surface were responsible for such an imperative property [5,6]. Nowadays, many researchers have been working to mimic this property via engineering the physics and chemistry of surfaces and materials through various routes. Different kinds of superhydrophobic structures such as coatings, membranes, and textiles, are examined towards diverse applications such as self cleaning, friction reducing, anti-corrosion, oil-water separation, etc [7–10]. Water repellent property of any surface mainly depends on their surface energy and structural linearity. Any surface with a surface energy lower than that of water (~ 0.07 Nm-1) will float and get repelled from the surface which is the major reason for hydrophobicity [11]. Additionally, if the surface has higher roughness value, the repelled water will get rolled on the surface even tilting at lower angles [12]. To imitate this self-cleaning property, chemistry in the form of low surface energy materials and physics in the form of roughness over the surface are highly essential. Therefore, engineering the combination of these two factors will surely make any type of surface into superhydrophobic nature. In general, superhydrophobic coatings can be useful in variety of applications including self cleaning coatings, corrosion protection, biofouling mitigation, oil water separation, aquatic devices (microboats), water pollution monitoring, water quality surveillance and so on [13-16]. Among these, floating and loading capacities of superhydrophobic coatings are attracted mainly due to their application in aquatic systems. Considering the recent efforts taken by the
researchers on superhydrophobic coatings with these properties, Pan et al. demonstrated the fabrication of superhydrophobic coatings using n-dodecanoic acid coated copper mesh with good load bearing capacity [17]. Zhang et al. developed an aquatic microrobot using n-dodecanoic acid coated copper wires which is capable of walking on water surface mimicking a water strider [18]. Yong et al. demonstrated the fabrication of planar superhydrophobic microboats using surface engineering polydimethylsiloxane irradiated by focused femtosecond laser [19]. Choi et al. presented a highly buoyant free-standing superhydrophobic composite structure embedded with nanoparticle shelled bubbles and these composite structures are able to float in water with a load bearing capacity of 200 times heavier than the structure itself [20]. Later on He et al. demonstrated the fabrication of superhydrophobic coating on copper substrate using electrodeposition and the substrate retained the floating state over several months [21]. Among the materials used for developing superhydrophobic surface, low surface energy materials like n-dodecanethiol and 1H, 1H, 2H, 2H-perfluorooctylsilane were used initially [22,23]. However, their practical applications are limited due to their high cost and hence, there is a need for alternative cost effective materials. In this scenario, low cost metallic stearate has been used for creating superhydrophobic surfaces by some researchers. Among them, copper stearate (CuSA2) is attracting since it has low critical surface tension (24 dynes/cm) with a water contact angle of ~115°[24]. Besides low surface energy material coating, surface roughness in the order of nano-micron sized with hierarchical structure are also a very prominent factor for achieving superhydrophobic surface. The nano-micro hierarchies and a low surface energy material made the desired surface into superhydrophobic nature [29,30]. In order to achieve the nanoscaled surface roughness, nanoparticles were used as fillers along with some additives recently. The use of nanosized particles (over conventional particles) with high porosity might
create the roughness to the expected level and enhances the water contact angle (WCA) of the surface as well as it augments the water droplet against the spreading ability [31]. There are different methods including electrodeposition, etching, growing nanowires/rods, spin coating, spray pyrolysis, creating negative replica from biotemplates, and etc., were used for the fabrication of superhydrophobic surface [25-28]. This work demonstrates the conversion of superhydrophilic glass substrate into a superhydrophobic surface via one-step spray coating method. The spray coating used here is not only to create rough surface as found in lotus leaves but also to develop a low surface energy coating over a glass substrate using a mixture of CuSA2 and porous ZnO nanoparticles. Among the different kinds of nano metal oxides, ZnO nanoparticles prepared using combustion method have numerous amounts of pores can influence greatly the required properties like higher WCA and excellent floating characteristics. In this study, the superhydrophobic properties of porous ZnO/CuSA2 coating with different mass loadings of ZnO was examined and their interesting properties such as floating nature and load bearing capabilities were investigated in detail. Further, the effect of deposition time on the load bearing capability of the prepared ZnO/CuSA2 superhydrophobic coating is also discussed in detail. 2. Materials and Methods 2.1 Materials Copper chloride (Loba chemicals, 99%), hexamine (Merck chemicals, 99%), zinc nitrate (Merck chemicals, 96%), ammonia (Emplura Merck, 25%), stearic acid (SD Fine chemicals limited, 90%), poly ethylene glycol (PEG) (Merck chemicals, M.W-3500 to 4000g/mol), Submicron sized ZnO (Spectrum Chemicals, India), and ethanol (Changshuyan Gyuan, China)
were used as such without any further purification. The ZnO nanoparticles used in this report were prepared by the precipitation method as described in our earlier report [32]. 2.1 Preparation of copper stearate A precipitation method was used to obtain CuSA2 powders using the precursors viz. copper chloride, ammonia, and stearic acid [33]. Breifly, 4 g of copper chloride was dissolved in 200 mL of water via magnetic stirring followed by the dropwise addition of 3.5 mL ammonia and the stirring process was continued until it gets completely dissolved. Immediately, the colour of the solution turned from green to light blue which indicates the formation of hydroaquacopper (II) complex precipitate. The obtained precipitate was kept in a water bath at 60 °C for one hour, after that it was separated via filtration and washed thoroughly with distilled water. The wet precipitate was carefully collected and mixed with 200 mL of water and kept in a magnetic stirrer at 60 °C. Meanwhile, 2.5 g of stearic acid was dissolved in 25 mL of water via heating over a hot plate at a temperature of 90 °C for 10 minutes. After the complete dissolution of stearic acid, the copper hydroxide precipitate was slowly mixed with fast stirring. After 45 minutes, blue colored particles formed and floated over the surface of the liquid. The obtained CuSA2 was separated via filtration, washed thoroughly with cold water for 4 times and dried in hot air oven at 50 °C for 24 hours and stored in vacuum. 2.2 Preparation of porous ZnO nanoparticles The porous ZnO nanoparticles were prepared via combustion method using zinc nitrate, PEG, and hexamine as starting precursors. Breifly, 4 g of zinc nitrate was dissolved in distilled water (100 mL) followed by the addition of PEG (3 g), hexamine (3g) and allowed to vigorous stirring for 30 minutes at 60 °C. Then, ammonia solution was added drop wise until the pH of the solution was reached upto 9. After that, the solution was left in the hot plate at a temperature of
90 °C until a dark brown gel was formed. After 4 hours, the dried brown colour gel was transferred into a silica crucible and calcined at 500 °C in a muffle furnace for 2 hours. The resulting white colored porous ZnO nanoparticles were stored in vacuum. 2.3 Spray coating of ZnO/copper stearate films The ZnO/CuSA2 thin films were deposited using spray coating method with different weight ratio of ZnO. Breifly, CuSA2 (0.32 g) was dispersed in ethanol (25 mL) using ultrasonication process for 10 minutes. Then, different weight percentage of ZnO nanoparticles (0.06, 0.08, 0.10, 0.12, and 0.14 g) were added to the CuSA2 solution and allowed to ultrasound irradiation for 30 minutes using a probe type sonicator. The resulting green colored suspension was taken in a nebulizer bowl and sprayed on a pre-cleaned glass substrate (over a hot plate maintained at 60 °C) at a pressure of 4 psi. The coating process was performed at different time duration ranging from 30 seconds to 30 minutes. The experimental setup of the spray coating system is provided in Fig. S1 (supplementary material). The spray coated films containing porous ZnO, submicron sized ZnO, and ZnO nanoparticles impregnated CuSA2 were mentioned as ZnO/CuSA2 coatings, submicron sized ZnO/CuSA2 coatings, and ZnO nanoparticle/CuSA2 coatings, respectively. 2.4 Instrumentation The sonication process was employed using Probe type sonicator (Sonics-VCX600-USA) with Ti horn. Spray pyrolysis was performed using a homemade setup comprising compressor and nebulizer with specifications (nozzle diameter - 2mm, distance between the nozzle and glass plate – 5 cm). The Fourier transform infrared (FT-IR) spectral analysis was obtained using an FT-IR spectrometer (ALPHA-E) -Bruker Optics, Germany. The X ray diffraction studies were performed using Cu Kα radiation (λ= 1.5406 Ȧ) in an X-Ray diffractometer (D8 Advance ECO
XRD Systems with SSD160 1 D Detector, Bruker Instruments). The porous nature and morphology of the prepared ZnO nanoparticles was examined using field emission-scanning electron microscope (FE-SEM, JEOL JSM 7001F) working at 15 kV accelerating voltage with Energy Dispersive X-ray Spectrometer (EDS) and JEM-2100Plus high resolution transmission electron microscope (HR-TEM). The chemical and surface electronic states of elements present in ZnO nanoparticles were examined using X-photoelectron spectroscopy (XPSESCA Shimadzu 3100 instruments). The surface morphology of the super-hydrophobic coatings was carried out on scanning electron microscope (Carl Zeiss EVO 18) and energy dispersive X-ray spectrometer (Quantax 200 with X-Flash – Bruker). The surface topography of the coatings is examined using Atomic force microscopy (Scanning Probe Microscope XE 70, Park Systems, Suwan, South Korea). The surface area of the prepared porous ZnO nanoparticles with its average pore size, diameter, area and volume were measured with a Micromeritics Gemini 2380 Surface area analyzer. Water contact angle (WCA) measurements were carried out on an OCA15 WCA Goniometer (Geotferd dataphysics Instruments, GmbH, Germany) employing 3 µL of ultrapure water. The WCA presented in this study were the average values of three independents readings measured at different locations of the coatings. 3. RESULTS AND DISCUSSION In this study, a spray coating method was used to develop the superhydrophobic coating comprised of porous ZnO nanoparticles and CuSA2. At first, the formation of CuSA2 was confirmed using FT-IR spectroscopy and XRD analysis. In general, stearic acid and CuSA2 differs only in one place especially in the carboxylate end, where Cu replaces the hydrogen atom of carboxylic acid. The formation of CuSA2 from stearic acid was confirmed by FT-IR analysis as shown in Fig. 1(a and b). The band arised due to presence of COO- in the stearic acid
normally exhibit its characteristic vibration at 1702 cm–1 gets shifted to lower wave number (1583 cm–1) due to the formation of CuSA2 [34]. The sharp bands corresponding to OH vibration (3352 cm–1), C-O stretching (1046 cm–1) was strong for stearic acid and it was diminished in CuSA2. The other characteristic bands observed in CuSA2 are CH2 (2847 cm-1), C-H (2917 cm– 1
), C-C (1441cm–1), CH2 rocking (720 cm–1), CH3 rocking (880 cm–1), and anti-symmetric CH2
rocking vibrations (1046 and 1087 cm–1), respectively [33]. The XRD pattern of CuSA2 (Fig. 1(c)) showed the presence of diffraction peaks at angles in the range of 5° to 20° which is well matched with the literatures [35,36]. The XRD pattern of the porous ZnO nanoparticles was shown in Fig. 2(a) which revealed the presence of sharp diffraction peaks at 2θ = 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8° and 67.9° corresponding to the reflections of (100), (002), (101), (102), (110), (103) and (112) planes confirming the formation of ZnO with wurtzite structure [37]. The observed diffraction pattern and interplanar spacing of the porous ZnO is matched well with the standard diffraction pattern of ZnO (JCPDS No. 89-7102). The laser Raman spectrum of the prepared porous ZnO nanoparticles was shown in Fig. 2(b) which revealed the presence of a major band at 437 cm-1 corresponding to E2 (high) mode, suggesting the formation of wurtzite ZnO [38]. The X-ray photoelectron spectroscopic analysis (XPS) was used to determine the chemical and surface states of elements present in the prepared ZnO nanoparticles. The XPS survey spectra of the ZnO nanoparticles is shown in Fig. S2 which evidenced the presence of Zn, and O groups at binding energies around 1030 and 532 eV, respectively. Figure 2(c) represents the high resolution spectra of Zn 2p which indicated the presence of two peaks around 1022.45 and 1045.73 eV respectively, corresponding to the Zn 2p3/2 and Zn 2p1/2 states of Zn in the ZnO nanoparticles [39]. The difference between the binding energy in between these two states is around 23 which
is closer to the reported values and also confirmed that the oxidation state of Zn in the prepared ZnO is about 2 [39]. Figure 2(d) represents the O 1s spectrum suggesting the presence of broad band centered at 532 eV, which can be assigned to the oxygen content (O2- ions) in the Zn-O bonding of the wurtzite ZnO [39]. The FE-SEM micrographs of ZnO with different magnifications were shown in Fig. 3(a and b) which represents the presence of uniformly distributed pores in the prepared ZnO nanoparticles. The presence of honeycomb like ZnO nanoparticles with pore size in the range of 200 to 400 nm was evidenced from the high magnification micrograph shown in Fig. 3 (b). The EDS spectrum shown in Fig. 3(c) confirmed the presence of Zn and O in the prepared ZnO nanoparticles. Further, the HR-TEM micrograph of ZnO nanoparticles was provided in Fig. 3(d) which revealed the presence of ZnO nanoparticles with size in the range of 20 to 30 nm. Herein, ZnO nanoparticles were used as filler in CuSA2 to increase the superhydrophobic nature of the CuSA2 via creating nanoscale roughness with high porosity. The surface area and porous nature of as obtained porous ZnO nanoparticles were analyzed through Brunauer-Emmet-Teller (BET) analysis and Barrett-Joyner-Halenda (BJH) method respectively. The N2 adsorption/desorption isotherm of the ZnO nanoparticles is given supplementary material (Fig. S3) which showed the presence of curve similar to type IV isotherm with a hysteresis. The size, surface area, and pore volume of the porous ZnO nanoparticles are found to be 11.77 nm, 9.769 m2/g, and 0.028 cm3/g, respectively. The prepared ZnO nanoparticles were found to be mesoporous nature according to IUPAC (International Union of Pure and Applied Chemistry) standard. The high surface area was achieved in the prepared ZnO nanoparticles which is due to the use of PEG in the synthesis method, in which PEG act as host matrix to hold the Zn2+ ions (guest) via guest-host chemistry.
Upon calcination at high temperature, the PEG molecule decomposes which results in the formation of porous ZnO nanoparticles. The bonding nature and the functional groups present in the ZnO/CuSA2 coating was examined using FT-IR spectroscopy as shown in Fig. 4 (a and b). The characteristics bands of ZnO were observed at 437 and 518 cm–1 which corresponds to the stretching vibrations of Zn & O atoms [39]. The FT-IR spectrum of ZnO/CuSA2 shows all characteristic vibration bands corresponding to the ZnO and CuSA2 with slight deviation in their band position, thus indicating the interaction of ZnO with CuSA2. Figure 4(c) shows the X-ray diffraction pattern of the spray coated ZnO/CuSA2 coatings. It showed the presence of diffraction peaks originating from ZnO nanoparticles in the coatings and the peaks corresponding to CuSA2 are not observed. This might be due to the poor crystalline nature of CuSA2 compared to ZnO and it has also highlighted the uniform distribution of ZnO nanoparticles in the prepared coatings. The surface morphology of the porous ZnO/CuSA2 coatings in comparison with coatings of bare CuSA2, ZnO nanoparticle/CuSA2 and submicron sized ZnO/CuSA2 were examined using SEM analysis. The micrograph of bare CuSA2 coatings revealed that surfaces were not smooth and the presence of leaf like structures emerged from surface of thin films with large voids (See Fig. S4, supplementary material). Figure 5(a) represents the micrograph of ZnO/CuSA2 coatings which show the homogenous distribution of ZnO nanoparticles in the coatings. The presence of porous ZnO nanoparticles in the coatings was not distinguishable suggesting the better dispersibility of the porous ZnO in the CuSA2 matrix via ultrasonication. Further, the surface morphology of these coatings shows the presence of hierarchical and rough surfaces with more pores or voids which is attributed to the porous nature of the ZnO. The size of the pores is in the range of 100-300 nm which was measured using ImageJ software [40]. The SEM micrograph of
submicron sized ZnO/CuSA2 and ZnO nanoparticle/CuSA2 were provided in supplementary material (Fig. S5), which showed irregular surfaces and the presence of ZnO (submicron and nanoparticles) in the CuSA2 matrix. This highlights the importance of porous ZnO nanoparticles which provide the macromolecular dispersion in the CuSA2 matrix. The composition and spatial distribution of the porous ZnO nanoparticles in the CuSA2 matrix of the coatings were examined using EDX mapping analysis as shown Fig. 5 (b-d). The energy graph of the ZnO/CuSA2 coating was given in Fig. S6 (supplementary material) clearly indicated the presence of copper at 8.046, 8.904, 0.928, and 0.947 eV respectively. The presence of ZnO was confirmed by their characteristic peak of Zn (8.637, 9.570, 1.012 and 1.035 eV) and oxygen around at 0.525 eV, respectively. The EDAX mapping of the ZnO/CuSA2 coating highlighted the presence of Zn, O, and Cu elements (as shown in Fig. 5(b-d)) and it was further evidenced the homogenous distribution of both ZnO and CuSA2 in the ZnO/CuSA2 coatings. The Zn mapping on the examined area (Fig. 5(b)) suggested the homogenous distribution of nano ZnO throughout the coating. The O mapping of the coating (Fig. 5(c)) generally arised from ZnO and organic groups from the CuSA2 revealed the presence of array like pattern which might be due to the ordered polymeric chain like structure of CuSA2 in the coating. The Cu mapping image (Fig. 5(d)) revealed the random distribution of Cu atom in the prepared coatings since the Cu present at the end of CuSA2 chain and more probably the Cu end was attached to the surface of the glass substrate, thus leaving the hydrophobic methyl group on the outermost periphery. Figure 6 represents the two and three dimensional topographic atomic force micrograph of the ZnO/CuSA2 coating. The 2D profile shown in Fig. 6(a) clearly evidenced the porous nature of the coatings and the 3D topographic profile (Fig. 6(b)) revealed the presence of porous and rough surface of the as deposited ZnO/CuSA2 coatings. In comparison with the atomic force
micrograph of bare CuSA2 coating (shown in Fig. S7, supplementary material), the topography of ZnO/CuSA2 coatings is of high porous which can be attributed due to the presence of porous ZnO nanoparticles in the as deposited films. The superhydrophobic properties of the prepared ZnO/CuSA2 coatings with different loading of ZnO were examined via WCA measurements and the results were shown in Fig. 7. The WCA of plain glass substrate was about 27.4° revealing its hydrophilic nature due to the presence of silica in them [27]. The WCA of the bare CuSA2 coating was found to be 152.4° suggesting the hydrophobic nature of the coatings which is due to the low surface energy of the CuSA2. Further improvements in the hydrophobicity were achieved by the addition of porous ZnO nanoparticles as filler in the prepared coatings. Until a weight percentage of 0.01 g of ZnO, the superhydrophobic effect was not obtained in the coatings. With an increase in weight percentage of ZnO (0.14 g), the WCA of the ZnO/CuSA2 coatings was enhanced and reaches upto 161° indicating the superhydrophobicity of the coatings. The achieved superhydrophobicity was attributed to the improved roughness of the ZnO/CuSA2 coatings due to the porous nature of ZnO nanoparticles. The obtained results can be explained using Cassie-Baxter model which relies on the solid fraction’s effect towards the change in WCA of droplet [41]. Thus, a theoretical estimation of the solid fraction in contact with the liquid droplet calculated using the Cassie-Baxter equation given below [42]: cos θ* = φs cos θ + φs – 1 …………(1) where θ*, ϕs and θ are apparent contact angle, solid fraction in contact with liquid, and Young’s contact angle respectively. The solid fraction of the bare CuSA2 coating was found to be 0.197 and with an increase in ZnO (wt %), the solid fraction decreases (Table S1, supplementary material) and finally reaches to a value of about 0.094 which was due to the increase in
roughness of the coating owing to the presence of porous ZnO nanoparticles. Such an effect was not obtained in the coatings comprised of submicron or nanosized ZnO (See Fig. S8 and Table S1 in the supplementary material). A better superhydrophobicity of about 161° was achieved for the ZnO/CuSA2 coatings and this coating was used for the analysis of floating and load bearing properties. Floating nature and load bearing capability are some of the important parameters useful for the real time applications of superhydrophobic coatings such as aquatic devices (microboats), water pollution monitoring, water quality surveillance and so on [17,18]. The sinking ability of the objects in water was well established by the theory proposed by Archimedes [43,44]. However, small object on water such as a jump clip or pin (which is much denser than water) will float on water. In fact, water strider leg having hierarchical fibrous like architecture with super hydrophobic coating is a well-known example found in nature [45]. Apart from floating, they also tend to stride freely over the surface of water [46]. In order to evaluate the super floating nature of ZnO/CuSA2 coatings, they were laid down in water as shown in Fig. 8 (a). The bare glass substrates drowned immediately in the water as shown in Fig. S9 (supplementary material) due to the Archimedes’ principle. The basic principle of floating characteristic of the body is that the buoyant force exerted by the body should be higher than the drowning force acting on it. Interestingly, ZnO/CuSA2 coated glass substrates floated on the surface of water over a prolonged time without sinking. In addition to the buoyant force, an additional curvature force on the surface of the ZnO/CuSA2 coating will act against the drowning force which makes them to float on the water [21]. The trapped air film on the outer surface of the ZnO/CuSA2 coating prevents them from wetting by water as well as provides additional displaced volume of
water and thus resulting super-buoyancy effect [21]. This clearly evidences the floating characteristics of the ZnO/CuSA2 coating which might be due to the plastron effect. The load bearing capacity of a denser floating object in water mainly depends on the superhydrophobic nature of the surface that is in contact with water [17,47]. Herein, we made an attempt to examine the load bearing capacity of the as prepared ZnO/CuSA2 coatings by loading stapler pins (mass of one stapler pin is 0.0193 g) on the top surface as shown in Fig. 8 (b and c). After loading certain number of stapler pins, the ZnO/CuSA2 coatings did not showed any signs of sinking and continues to float. The effect of load bearing capability of ZnO/CuSA2 coating deposited at different times from 3 to 30 min is shown in Fig. 9. It depicts the thick coatings (with 0.062 g of weight) float even after loading 19 stapler pins (of weight 0.3667 g) without any sign of sinking. Figure 9 and Table S2 revealed that the ratio of total floating weight to the weight of ZnO/CuSA2 coating is decreasing with increase in the deposition time, which might be due to more inert layers in the superhydrophobic coatings deposited in 30 min. However, the amount of load bearing is increased for the coatings deposited at 30 seconds which stay floating even after loading of 52 stapler pins (almost 333 times heavier than its weight). These excellent floating and load bearing properties of the prepared ZnO/CuSA2 coatings can be useful for the future applications such as water floating robots, and environmental surveillance. Conclusions: The collective findings of this study demonstrated the potential use of ZnO/CuSA2 superhydrophobic coatings via one step spray coating process. The presence of well dispersed porous ZnO nanoparticles in the CuSA2 matrix of the coatings results in the superhydrophobic effect. The deposited ZnO/CuSA2 coatings on glass slides exhibit a very high WCA of 161° indicating the superhydrophobic nature. The ZnO/CuSA2 coatings also possess good floating
nature (more than 4 months) with excellent load bearing capability of nearly 333 times heavier than their weight. The load bearing properties of the ZnO/CuSA2 coatings is decreased with the increase in coating thickness. The experimental results ensure that the spray coated ZnO/CuSA2 superhydrophobic coatings may find wide range of applications especially in the water floating robots. Acknowledgements The authors would like to thank the Management and the Principal, Mepco Schlenk Engineering College for their constant support and encouragement. One of the author, Mr. TSG thanks DRDO, ER & IPR, New Delhi for providing financial support to carry out the project work. Electronic supplementary material: The design of spray coating process, XPS survey spectrum of porous ZnO nanoparticles, BET surface area analysis of nanoporous ZnO particles, EDAX analysis of ZnO/CuSA2 coatings, SEM and Atomic force micrograph of CuSA2 coating, SEM and WCA of commercial ZnO/CuSA2 coatings and ZnO NPs/CuSA2coatings, Digital photograph of the drowned glass substrate after immersed in water are provided in supplementary information. Further, a video demonstration of floating and load bearing properties of porous ZnO/CuSA2coating are also provided in MP4 format. References: 1. X.-M. Li, D. Reinhoudt, M. Crego-Calama, What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces, Chem. Soc. Rev. 36 (2007) 1350-1368. 2. T. Darmanin, F. Guittard, Recent advances in the potential applications of bioinspired
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Figure captions: Fig. 1. Fourier transform infra-red spectrum of (a) stearic acid and (b) CuSA2, and (c) represents the X-ray diffraction pattern of CuSA2. Fig. 2. (a) X-ray diffraction pattern, (b) laser Raman spectrum, (c) Zn 2p core level X-ray photoelectron spectrum and (d) O 1s core level photoelectron spectrum of porous ZnO nanoparticles. Fig. 3. The field emission scanning electron micrographs of porous ZnO nanoparticles with (a) low and (b) high magnification, (c) represents the energy dispersive spectrum (EDS), and (d) high resolution transmission electron micrograph of porous ZnO nanoparticles. Fig. 4. Fourier transform infra-red spectrum of (a) porous ZnO nanoparticles and (b) porous ZnO/ CuSA2 coatings. (c) X-ray diffraction pattern of porous ZnO/CuSA2coatings. Fig. 5. Field emission scanning electron micrograph of (a) porous ZnO/CuSA2 coatings and (b-d) represents the elemental maps of Zn, O and Cu composition in the porous ZnO/CuSA2 coatings. Fig. 6. Atomic force micrograph of porous ZnO/copper stearate coatings presented in two dimensional view (a) and three dimensional view (b). Fig. 7. Water contact angle of porous ZnO/CuSA2 coating with different ZnO loadings. Fig. 8. Floating characteristics of porous ZnO/CuSA2 coating. (a) Digital photograph represent the floating nature of the porous ZnO/CuSA2 coated glass, (b) Load bearing properties of the porous ZnO/CuSA2 coatings and (c) shows the top view of Fig. 7(b.)
Fig. 9. Load bearing capability of porous ZnO/CuSA2 superhydrophobic (SH) coating indicating the decrease ratio of total weight to SH coating weight with a corresponding increase in SH coating weight.
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Graphical abstract
Highlights Superhydrobic coating comprised of ZnO/copper stearate was fabricated. The ZnO/copper stearate nanocoatings showed a water contact angle of 161°. The ZnO/copper stearate nanocoatings displayed excellent floating characteristics. The superhydrophobic nanocoatings showed superior load bearing properties.
S1385-8947(17)30398-4 http://dx.doi.org/10.1016/j.cej.2017.03.052 CEJ 16648
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
31 August 2016 8 March 2017 14 March 2017
Please cite this article as: G.T. Suyambulingam, K. Jeyasubramanian, V.K. Mariappan, P. Veluswamy, H. Ikeda, K. Krishnamoorthy, Excellent floating and load bearing properties of superhydrophobic ZnO/copper stearate nanocoating, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.03.052
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Excellent floating and load bearing properties of superhydrophobic ZnO/copper stearate nanocoating Gokulraja Thangaiyanadar Suyambulingam1, Kadarkaraithangam Jeyasubramanian1#, Vimal Kumar Mariappan1, Pandiyarasan Veluswamy2,3, Hiroya Ikeda2,3, and Karthikeyan Krishnamoorthy4* 1
Centre for Nanoscience and Technology, Department of Mechanical Engineering, Mepco Schlenk Engineering College, Sivakasi, India-626005.
2
3
Research Institute of Electronics, Shizuoka University, Hamamatsu, 4328018, Japan.
Graduate School of Science and Technology, Shizuoka University, Hamamatsu, 4328018, Japan.
4
Nanomaterials and System Lab, Department of Mechatronics Engineering, Jeju National University, Jeju 690-756, South Korea.
Email: #[email protected] (Dr. KJ) *[email protected] (Dr. KK)
ABSTRACT The development of superhydrophobic materials is rapidly increasing due to their diverse applications such as self cleaning, oil-water separation, aquatic robotics, and so on. In this study, we developed a superhydrophobic coating comprising of porous ZnO nanoparticles and copper stearate (CuSA2) via a cost effective spray coating method. Physico-chemical characterizations such as X-ray diffraction analysis, Fourier transform infrared spectroscopy, scanning electron microscope, and elemental mapping were used to understand the crystallinity, bonding nature, morphology and distribution of the ZnO/CuSA2 coatings. Superhydrophobic nature of the bare CuSA2 coating measured through goniometer was found to be more than 151°, further it was enhanced to around 161° by increasing the weight percentage of ZnO. The ZnO/CuSA2 superhydrophobic coatings showed excellent super buoyancy characteristic as evidenced by their floating nature in water over a prolonged time. Further, experiments on maximum load bearing capacity of the ZnO/CuSA2 coatings were examined by placing stapler pins on their surface and the effect of deposition time on the load bearing capacity has been investigated in detail. The experimental results showed that the superhydrophobic nature of ZnO/CuSA2 coating with excellent floating characteristics and higher load bearing capacity, may find a wide range of future applications in man-made water floating robots. Keywords Superhydrophobic coatings; porous ZnO; copper stearate; spray coating technique; floating ability; load bearing capability.
1. Introduction Nature has empowered many surfaces with water repellent properties including lotus leaf, water strider’s leg, dames fly wings, butterfly wings, etc [1–4]. In 1977, the “Lotus effect” was discovered by Barthlott and Ehler in which they revealed the presence of micron-sized structures (papillae) and low surface energy coating (like cuticular waxes) over the surface were responsible for such an imperative property [5,6]. Nowadays, many researchers have been working to mimic this property via engineering the physics and chemistry of surfaces and materials through various routes. Different kinds of superhydrophobic structures such as coatings, membranes, and textiles, are examined towards diverse applications such as self cleaning, friction reducing, anti-corrosion, oil-water separation, etc [7–10]. Water repellent property of any surface mainly depends on their surface energy and structural linearity. Any surface with a surface energy lower than that of water (~ 0.07 Nm-1) will float and get repelled from the surface which is the major reason for hydrophobicity [11]. Additionally, if the surface has higher roughness value, the repelled water will get rolled on the surface even tilting at lower angles [12]. To imitate this self-cleaning property, chemistry in the form of low surface energy materials and physics in the form of roughness over the surface are highly essential. Therefore, engineering the combination of these two factors will surely make any type of surface into superhydrophobic nature. In general, superhydrophobic coatings can be useful in variety of applications including self cleaning coatings, corrosion protection, biofouling mitigation, oil water separation, aquatic devices (microboats), water pollution monitoring, water quality surveillance and so on [13-16]. Among these, floating and loading capacities of superhydrophobic coatings are attracted mainly due to their application in aquatic systems. Considering the recent efforts taken by the
researchers on superhydrophobic coatings with these properties, Pan et al. demonstrated the fabrication of superhydrophobic coatings using n-dodecanoic acid coated copper mesh with good load bearing capacity [17]. Zhang et al. developed an aquatic microrobot using n-dodecanoic acid coated copper wires which is capable of walking on water surface mimicking a water strider [18]. Yong et al. demonstrated the fabrication of planar superhydrophobic microboats using surface engineering polydimethylsiloxane irradiated by focused femtosecond laser [19]. Choi et al. presented a highly buoyant free-standing superhydrophobic composite structure embedded with nanoparticle shelled bubbles and these composite structures are able to float in water with a load bearing capacity of 200 times heavier than the structure itself [20]. Later on He et al. demonstrated the fabrication of superhydrophobic coating on copper substrate using electrodeposition and the substrate retained the floating state over several months [21]. Among the materials used for developing superhydrophobic surface, low surface energy materials like n-dodecanethiol and 1H, 1H, 2H, 2H-perfluorooctylsilane were used initially [22,23]. However, their practical applications are limited due to their high cost and hence, there is a need for alternative cost effective materials. In this scenario, low cost metallic stearate has been used for creating superhydrophobic surfaces by some researchers. Among them, copper stearate (CuSA2) is attracting since it has low critical surface tension (24 dynes/cm) with a water contact angle of ~115°[24]. Besides low surface energy material coating, surface roughness in the order of nano-micron sized with hierarchical structure are also a very prominent factor for achieving superhydrophobic surface. The nano-micro hierarchies and a low surface energy material made the desired surface into superhydrophobic nature [29,30]. In order to achieve the nanoscaled surface roughness, nanoparticles were used as fillers along with some additives recently. The use of nanosized particles (over conventional particles) with high porosity might
create the roughness to the expected level and enhances the water contact angle (WCA) of the surface as well as it augments the water droplet against the spreading ability [31]. There are different methods including electrodeposition, etching, growing nanowires/rods, spin coating, spray pyrolysis, creating negative replica from biotemplates, and etc., were used for the fabrication of superhydrophobic surface [25-28]. This work demonstrates the conversion of superhydrophilic glass substrate into a superhydrophobic surface via one-step spray coating method. The spray coating used here is not only to create rough surface as found in lotus leaves but also to develop a low surface energy coating over a glass substrate using a mixture of CuSA2 and porous ZnO nanoparticles. Among the different kinds of nano metal oxides, ZnO nanoparticles prepared using combustion method have numerous amounts of pores can influence greatly the required properties like higher WCA and excellent floating characteristics. In this study, the superhydrophobic properties of porous ZnO/CuSA2 coating with different mass loadings of ZnO was examined and their interesting properties such as floating nature and load bearing capabilities were investigated in detail. Further, the effect of deposition time on the load bearing capability of the prepared ZnO/CuSA2 superhydrophobic coating is also discussed in detail. 2. Materials and Methods 2.1 Materials Copper chloride (Loba chemicals, 99%), hexamine (Merck chemicals, 99%), zinc nitrate (Merck chemicals, 96%), ammonia (Emplura Merck, 25%), stearic acid (SD Fine chemicals limited, 90%), poly ethylene glycol (PEG) (Merck chemicals, M.W-3500 to 4000g/mol), Submicron sized ZnO (Spectrum Chemicals, India), and ethanol (Changshuyan Gyuan, China)
were used as such without any further purification. The ZnO nanoparticles used in this report were prepared by the precipitation method as described in our earlier report [32]. 2.1 Preparation of copper stearate A precipitation method was used to obtain CuSA2 powders using the precursors viz. copper chloride, ammonia, and stearic acid [33]. Breifly, 4 g of copper chloride was dissolved in 200 mL of water via magnetic stirring followed by the dropwise addition of 3.5 mL ammonia and the stirring process was continued until it gets completely dissolved. Immediately, the colour of the solution turned from green to light blue which indicates the formation of hydroaquacopper (II) complex precipitate. The obtained precipitate was kept in a water bath at 60 °C for one hour, after that it was separated via filtration and washed thoroughly with distilled water. The wet precipitate was carefully collected and mixed with 200 mL of water and kept in a magnetic stirrer at 60 °C. Meanwhile, 2.5 g of stearic acid was dissolved in 25 mL of water via heating over a hot plate at a temperature of 90 °C for 10 minutes. After the complete dissolution of stearic acid, the copper hydroxide precipitate was slowly mixed with fast stirring. After 45 minutes, blue colored particles formed and floated over the surface of the liquid. The obtained CuSA2 was separated via filtration, washed thoroughly with cold water for 4 times and dried in hot air oven at 50 °C for 24 hours and stored in vacuum. 2.2 Preparation of porous ZnO nanoparticles The porous ZnO nanoparticles were prepared via combustion method using zinc nitrate, PEG, and hexamine as starting precursors. Breifly, 4 g of zinc nitrate was dissolved in distilled water (100 mL) followed by the addition of PEG (3 g), hexamine (3g) and allowed to vigorous stirring for 30 minutes at 60 °C. Then, ammonia solution was added drop wise until the pH of the solution was reached upto 9. After that, the solution was left in the hot plate at a temperature of
90 °C until a dark brown gel was formed. After 4 hours, the dried brown colour gel was transferred into a silica crucible and calcined at 500 °C in a muffle furnace for 2 hours. The resulting white colored porous ZnO nanoparticles were stored in vacuum. 2.3 Spray coating of ZnO/copper stearate films The ZnO/CuSA2 thin films were deposited using spray coating method with different weight ratio of ZnO. Breifly, CuSA2 (0.32 g) was dispersed in ethanol (25 mL) using ultrasonication process for 10 minutes. Then, different weight percentage of ZnO nanoparticles (0.06, 0.08, 0.10, 0.12, and 0.14 g) were added to the CuSA2 solution and allowed to ultrasound irradiation for 30 minutes using a probe type sonicator. The resulting green colored suspension was taken in a nebulizer bowl and sprayed on a pre-cleaned glass substrate (over a hot plate maintained at 60 °C) at a pressure of 4 psi. The coating process was performed at different time duration ranging from 30 seconds to 30 minutes. The experimental setup of the spray coating system is provided in Fig. S1 (supplementary material). The spray coated films containing porous ZnO, submicron sized ZnO, and ZnO nanoparticles impregnated CuSA2 were mentioned as ZnO/CuSA2 coatings, submicron sized ZnO/CuSA2 coatings, and ZnO nanoparticle/CuSA2 coatings, respectively. 2.4 Instrumentation The sonication process was employed using Probe type sonicator (Sonics-VCX600-USA) with Ti horn. Spray pyrolysis was performed using a homemade setup comprising compressor and nebulizer with specifications (nozzle diameter - 2mm, distance between the nozzle and glass plate – 5 cm). The Fourier transform infrared (FT-IR) spectral analysis was obtained using an FT-IR spectrometer (ALPHA-E) -Bruker Optics, Germany. The X ray diffraction studies were performed using Cu Kα radiation (λ= 1.5406 Ȧ) in an X-Ray diffractometer (D8 Advance ECO
XRD Systems with SSD160 1 D Detector, Bruker Instruments). The porous nature and morphology of the prepared ZnO nanoparticles was examined using field emission-scanning electron microscope (FE-SEM, JEOL JSM 7001F) working at 15 kV accelerating voltage with Energy Dispersive X-ray Spectrometer (EDS) and JEM-2100Plus high resolution transmission electron microscope (HR-TEM). The chemical and surface electronic states of elements present in ZnO nanoparticles were examined using X-photoelectron spectroscopy (XPSESCA Shimadzu 3100 instruments). The surface morphology of the super-hydrophobic coatings was carried out on scanning electron microscope (Carl Zeiss EVO 18) and energy dispersive X-ray spectrometer (Quantax 200 with X-Flash – Bruker). The surface topography of the coatings is examined using Atomic force microscopy (Scanning Probe Microscope XE 70, Park Systems, Suwan, South Korea). The surface area of the prepared porous ZnO nanoparticles with its average pore size, diameter, area and volume were measured with a Micromeritics Gemini 2380 Surface area analyzer. Water contact angle (WCA) measurements were carried out on an OCA15 WCA Goniometer (Geotferd dataphysics Instruments, GmbH, Germany) employing 3 µL of ultrapure water. The WCA presented in this study were the average values of three independents readings measured at different locations of the coatings. 3. RESULTS AND DISCUSSION In this study, a spray coating method was used to develop the superhydrophobic coating comprised of porous ZnO nanoparticles and CuSA2. At first, the formation of CuSA2 was confirmed using FT-IR spectroscopy and XRD analysis. In general, stearic acid and CuSA2 differs only in one place especially in the carboxylate end, where Cu replaces the hydrogen atom of carboxylic acid. The formation of CuSA2 from stearic acid was confirmed by FT-IR analysis as shown in Fig. 1(a and b). The band arised due to presence of COO- in the stearic acid
normally exhibit its characteristic vibration at 1702 cm–1 gets shifted to lower wave number (1583 cm–1) due to the formation of CuSA2 [34]. The sharp bands corresponding to OH vibration (3352 cm–1), C-O stretching (1046 cm–1) was strong for stearic acid and it was diminished in CuSA2. The other characteristic bands observed in CuSA2 are CH2 (2847 cm-1), C-H (2917 cm– 1
), C-C (1441cm–1), CH2 rocking (720 cm–1), CH3 rocking (880 cm–1), and anti-symmetric CH2
rocking vibrations (1046 and 1087 cm–1), respectively [33]. The XRD pattern of CuSA2 (Fig. 1(c)) showed the presence of diffraction peaks at angles in the range of 5° to 20° which is well matched with the literatures [35,36]. The XRD pattern of the porous ZnO nanoparticles was shown in Fig. 2(a) which revealed the presence of sharp diffraction peaks at 2θ = 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8° and 67.9° corresponding to the reflections of (100), (002), (101), (102), (110), (103) and (112) planes confirming the formation of ZnO with wurtzite structure [37]. The observed diffraction pattern and interplanar spacing of the porous ZnO is matched well with the standard diffraction pattern of ZnO (JCPDS No. 89-7102). The laser Raman spectrum of the prepared porous ZnO nanoparticles was shown in Fig. 2(b) which revealed the presence of a major band at 437 cm-1 corresponding to E2 (high) mode, suggesting the formation of wurtzite ZnO [38]. The X-ray photoelectron spectroscopic analysis (XPS) was used to determine the chemical and surface states of elements present in the prepared ZnO nanoparticles. The XPS survey spectra of the ZnO nanoparticles is shown in Fig. S2 which evidenced the presence of Zn, and O groups at binding energies around 1030 and 532 eV, respectively. Figure 2(c) represents the high resolution spectra of Zn 2p which indicated the presence of two peaks around 1022.45 and 1045.73 eV respectively, corresponding to the Zn 2p3/2 and Zn 2p1/2 states of Zn in the ZnO nanoparticles [39]. The difference between the binding energy in between these two states is around 23 which
is closer to the reported values and also confirmed that the oxidation state of Zn in the prepared ZnO is about 2 [39]. Figure 2(d) represents the O 1s spectrum suggesting the presence of broad band centered at 532 eV, which can be assigned to the oxygen content (O2- ions) in the Zn-O bonding of the wurtzite ZnO [39]. The FE-SEM micrographs of ZnO with different magnifications were shown in Fig. 3(a and b) which represents the presence of uniformly distributed pores in the prepared ZnO nanoparticles. The presence of honeycomb like ZnO nanoparticles with pore size in the range of 200 to 400 nm was evidenced from the high magnification micrograph shown in Fig. 3 (b). The EDS spectrum shown in Fig. 3(c) confirmed the presence of Zn and O in the prepared ZnO nanoparticles. Further, the HR-TEM micrograph of ZnO nanoparticles was provided in Fig. 3(d) which revealed the presence of ZnO nanoparticles with size in the range of 20 to 30 nm. Herein, ZnO nanoparticles were used as filler in CuSA2 to increase the superhydrophobic nature of the CuSA2 via creating nanoscale roughness with high porosity. The surface area and porous nature of as obtained porous ZnO nanoparticles were analyzed through Brunauer-Emmet-Teller (BET) analysis and Barrett-Joyner-Halenda (BJH) method respectively. The N2 adsorption/desorption isotherm of the ZnO nanoparticles is given supplementary material (Fig. S3) which showed the presence of curve similar to type IV isotherm with a hysteresis. The size, surface area, and pore volume of the porous ZnO nanoparticles are found to be 11.77 nm, 9.769 m2/g, and 0.028 cm3/g, respectively. The prepared ZnO nanoparticles were found to be mesoporous nature according to IUPAC (International Union of Pure and Applied Chemistry) standard. The high surface area was achieved in the prepared ZnO nanoparticles which is due to the use of PEG in the synthesis method, in which PEG act as host matrix to hold the Zn2+ ions (guest) via guest-host chemistry.
Upon calcination at high temperature, the PEG molecule decomposes which results in the formation of porous ZnO nanoparticles. The bonding nature and the functional groups present in the ZnO/CuSA2 coating was examined using FT-IR spectroscopy as shown in Fig. 4 (a and b). The characteristics bands of ZnO were observed at 437 and 518 cm–1 which corresponds to the stretching vibrations of Zn & O atoms [39]. The FT-IR spectrum of ZnO/CuSA2 shows all characteristic vibration bands corresponding to the ZnO and CuSA2 with slight deviation in their band position, thus indicating the interaction of ZnO with CuSA2. Figure 4(c) shows the X-ray diffraction pattern of the spray coated ZnO/CuSA2 coatings. It showed the presence of diffraction peaks originating from ZnO nanoparticles in the coatings and the peaks corresponding to CuSA2 are not observed. This might be due to the poor crystalline nature of CuSA2 compared to ZnO and it has also highlighted the uniform distribution of ZnO nanoparticles in the prepared coatings. The surface morphology of the porous ZnO/CuSA2 coatings in comparison with coatings of bare CuSA2, ZnO nanoparticle/CuSA2 and submicron sized ZnO/CuSA2 were examined using SEM analysis. The micrograph of bare CuSA2 coatings revealed that surfaces were not smooth and the presence of leaf like structures emerged from surface of thin films with large voids (See Fig. S4, supplementary material). Figure 5(a) represents the micrograph of ZnO/CuSA2 coatings which show the homogenous distribution of ZnO nanoparticles in the coatings. The presence of porous ZnO nanoparticles in the coatings was not distinguishable suggesting the better dispersibility of the porous ZnO in the CuSA2 matrix via ultrasonication. Further, the surface morphology of these coatings shows the presence of hierarchical and rough surfaces with more pores or voids which is attributed to the porous nature of the ZnO. The size of the pores is in the range of 100-300 nm which was measured using ImageJ software [40]. The SEM micrograph of
submicron sized ZnO/CuSA2 and ZnO nanoparticle/CuSA2 were provided in supplementary material (Fig. S5), which showed irregular surfaces and the presence of ZnO (submicron and nanoparticles) in the CuSA2 matrix. This highlights the importance of porous ZnO nanoparticles which provide the macromolecular dispersion in the CuSA2 matrix. The composition and spatial distribution of the porous ZnO nanoparticles in the CuSA2 matrix of the coatings were examined using EDX mapping analysis as shown Fig. 5 (b-d). The energy graph of the ZnO/CuSA2 coating was given in Fig. S6 (supplementary material) clearly indicated the presence of copper at 8.046, 8.904, 0.928, and 0.947 eV respectively. The presence of ZnO was confirmed by their characteristic peak of Zn (8.637, 9.570, 1.012 and 1.035 eV) and oxygen around at 0.525 eV, respectively. The EDAX mapping of the ZnO/CuSA2 coating highlighted the presence of Zn, O, and Cu elements (as shown in Fig. 5(b-d)) and it was further evidenced the homogenous distribution of both ZnO and CuSA2 in the ZnO/CuSA2 coatings. The Zn mapping on the examined area (Fig. 5(b)) suggested the homogenous distribution of nano ZnO throughout the coating. The O mapping of the coating (Fig. 5(c)) generally arised from ZnO and organic groups from the CuSA2 revealed the presence of array like pattern which might be due to the ordered polymeric chain like structure of CuSA2 in the coating. The Cu mapping image (Fig. 5(d)) revealed the random distribution of Cu atom in the prepared coatings since the Cu present at the end of CuSA2 chain and more probably the Cu end was attached to the surface of the glass substrate, thus leaving the hydrophobic methyl group on the outermost periphery. Figure 6 represents the two and three dimensional topographic atomic force micrograph of the ZnO/CuSA2 coating. The 2D profile shown in Fig. 6(a) clearly evidenced the porous nature of the coatings and the 3D topographic profile (Fig. 6(b)) revealed the presence of porous and rough surface of the as deposited ZnO/CuSA2 coatings. In comparison with the atomic force
micrograph of bare CuSA2 coating (shown in Fig. S7, supplementary material), the topography of ZnO/CuSA2 coatings is of high porous which can be attributed due to the presence of porous ZnO nanoparticles in the as deposited films. The superhydrophobic properties of the prepared ZnO/CuSA2 coatings with different loading of ZnO were examined via WCA measurements and the results were shown in Fig. 7. The WCA of plain glass substrate was about 27.4° revealing its hydrophilic nature due to the presence of silica in them [27]. The WCA of the bare CuSA2 coating was found to be 152.4° suggesting the hydrophobic nature of the coatings which is due to the low surface energy of the CuSA2. Further improvements in the hydrophobicity were achieved by the addition of porous ZnO nanoparticles as filler in the prepared coatings. Until a weight percentage of 0.01 g of ZnO, the superhydrophobic effect was not obtained in the coatings. With an increase in weight percentage of ZnO (0.14 g), the WCA of the ZnO/CuSA2 coatings was enhanced and reaches upto 161° indicating the superhydrophobicity of the coatings. The achieved superhydrophobicity was attributed to the improved roughness of the ZnO/CuSA2 coatings due to the porous nature of ZnO nanoparticles. The obtained results can be explained using Cassie-Baxter model which relies on the solid fraction’s effect towards the change in WCA of droplet [41]. Thus, a theoretical estimation of the solid fraction in contact with the liquid droplet calculated using the Cassie-Baxter equation given below [42]: cos θ* = φs cos θ + φs – 1 …………(1) where θ*, ϕs and θ are apparent contact angle, solid fraction in contact with liquid, and Young’s contact angle respectively. The solid fraction of the bare CuSA2 coating was found to be 0.197 and with an increase in ZnO (wt %), the solid fraction decreases (Table S1, supplementary material) and finally reaches to a value of about 0.094 which was due to the increase in
roughness of the coating owing to the presence of porous ZnO nanoparticles. Such an effect was not obtained in the coatings comprised of submicron or nanosized ZnO (See Fig. S8 and Table S1 in the supplementary material). A better superhydrophobicity of about 161° was achieved for the ZnO/CuSA2 coatings and this coating was used for the analysis of floating and load bearing properties. Floating nature and load bearing capability are some of the important parameters useful for the real time applications of superhydrophobic coatings such as aquatic devices (microboats), water pollution monitoring, water quality surveillance and so on [17,18]. The sinking ability of the objects in water was well established by the theory proposed by Archimedes [43,44]. However, small object on water such as a jump clip or pin (which is much denser than water) will float on water. In fact, water strider leg having hierarchical fibrous like architecture with super hydrophobic coating is a well-known example found in nature [45]. Apart from floating, they also tend to stride freely over the surface of water [46]. In order to evaluate the super floating nature of ZnO/CuSA2 coatings, they were laid down in water as shown in Fig. 8 (a). The bare glass substrates drowned immediately in the water as shown in Fig. S9 (supplementary material) due to the Archimedes’ principle. The basic principle of floating characteristic of the body is that the buoyant force exerted by the body should be higher than the drowning force acting on it. Interestingly, ZnO/CuSA2 coated glass substrates floated on the surface of water over a prolonged time without sinking. In addition to the buoyant force, an additional curvature force on the surface of the ZnO/CuSA2 coating will act against the drowning force which makes them to float on the water [21]. The trapped air film on the outer surface of the ZnO/CuSA2 coating prevents them from wetting by water as well as provides additional displaced volume of
water and thus resulting super-buoyancy effect [21]. This clearly evidences the floating characteristics of the ZnO/CuSA2 coating which might be due to the plastron effect. The load bearing capacity of a denser floating object in water mainly depends on the superhydrophobic nature of the surface that is in contact with water [17,47]. Herein, we made an attempt to examine the load bearing capacity of the as prepared ZnO/CuSA2 coatings by loading stapler pins (mass of one stapler pin is 0.0193 g) on the top surface as shown in Fig. 8 (b and c). After loading certain number of stapler pins, the ZnO/CuSA2 coatings did not showed any signs of sinking and continues to float. The effect of load bearing capability of ZnO/CuSA2 coating deposited at different times from 3 to 30 min is shown in Fig. 9. It depicts the thick coatings (with 0.062 g of weight) float even after loading 19 stapler pins (of weight 0.3667 g) without any sign of sinking. Figure 9 and Table S2 revealed that the ratio of total floating weight to the weight of ZnO/CuSA2 coating is decreasing with increase in the deposition time, which might be due to more inert layers in the superhydrophobic coatings deposited in 30 min. However, the amount of load bearing is increased for the coatings deposited at 30 seconds which stay floating even after loading of 52 stapler pins (almost 333 times heavier than its weight). These excellent floating and load bearing properties of the prepared ZnO/CuSA2 coatings can be useful for the future applications such as water floating robots, and environmental surveillance. Conclusions: The collective findings of this study demonstrated the potential use of ZnO/CuSA2 superhydrophobic coatings via one step spray coating process. The presence of well dispersed porous ZnO nanoparticles in the CuSA2 matrix of the coatings results in the superhydrophobic effect. The deposited ZnO/CuSA2 coatings on glass slides exhibit a very high WCA of 161° indicating the superhydrophobic nature. The ZnO/CuSA2 coatings also possess good floating
nature (more than 4 months) with excellent load bearing capability of nearly 333 times heavier than their weight. The load bearing properties of the ZnO/CuSA2 coatings is decreased with the increase in coating thickness. The experimental results ensure that the spray coated ZnO/CuSA2 superhydrophobic coatings may find wide range of applications especially in the water floating robots. Acknowledgements The authors would like to thank the Management and the Principal, Mepco Schlenk Engineering College for their constant support and encouragement. One of the author, Mr. TSG thanks DRDO, ER & IPR, New Delhi for providing financial support to carry out the project work. Electronic supplementary material: The design of spray coating process, XPS survey spectrum of porous ZnO nanoparticles, BET surface area analysis of nanoporous ZnO particles, EDAX analysis of ZnO/CuSA2 coatings, SEM and Atomic force micrograph of CuSA2 coating, SEM and WCA of commercial ZnO/CuSA2 coatings and ZnO NPs/CuSA2coatings, Digital photograph of the drowned glass substrate after immersed in water are provided in supplementary information. Further, a video demonstration of floating and load bearing properties of porous ZnO/CuSA2coating are also provided in MP4 format. References: 1. X.-M. Li, D. Reinhoudt, M. Crego-Calama, What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces, Chem. Soc. Rev. 36 (2007) 1350-1368. 2. T. Darmanin, F. Guittard, Recent advances in the potential applications of bioinspired
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Figure captions: Fig. 1. Fourier transform infra-red spectrum of (a) stearic acid and (b) CuSA2, and (c) represents the X-ray diffraction pattern of CuSA2. Fig. 2. (a) X-ray diffraction pattern, (b) laser Raman spectrum, (c) Zn 2p core level X-ray photoelectron spectrum and (d) O 1s core level photoelectron spectrum of porous ZnO nanoparticles. Fig. 3. The field emission scanning electron micrographs of porous ZnO nanoparticles with (a) low and (b) high magnification, (c) represents the energy dispersive spectrum (EDS), and (d) high resolution transmission electron micrograph of porous ZnO nanoparticles. Fig. 4. Fourier transform infra-red spectrum of (a) porous ZnO nanoparticles and (b) porous ZnO/ CuSA2 coatings. (c) X-ray diffraction pattern of porous ZnO/CuSA2coatings. Fig. 5. Field emission scanning electron micrograph of (a) porous ZnO/CuSA2 coatings and (b-d) represents the elemental maps of Zn, O and Cu composition in the porous ZnO/CuSA2 coatings. Fig. 6. Atomic force micrograph of porous ZnO/copper stearate coatings presented in two dimensional view (a) and three dimensional view (b). Fig. 7. Water contact angle of porous ZnO/CuSA2 coating with different ZnO loadings. Fig. 8. Floating characteristics of porous ZnO/CuSA2 coating. (a) Digital photograph represent the floating nature of the porous ZnO/CuSA2 coated glass, (b) Load bearing properties of the porous ZnO/CuSA2 coatings and (c) shows the top view of Fig. 7(b.)
Fig. 9. Load bearing capability of porous ZnO/CuSA2 superhydrophobic (SH) coating indicating the decrease ratio of total weight to SH coating weight with a corresponding increase in SH coating weight.
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Graphical abstract
Highlights Superhydrobic coating comprised of ZnO/copper stearate was fabricated. The ZnO/copper stearate nanocoatings showed a water contact angle of 161°. The ZnO/copper stearate nanocoatings displayed excellent floating characteristics. The superhydrophobic nanocoatings showed superior load bearing properties.