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Influence of silanes on the wettability of anodized titanium
Applied Surface Science 292 (2014) 650–657
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Influence of silanes on the wettability of anodized titanium S.C. Vanithakumari, R.P. George, U. Kamachi Mudali ∗ Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
a r t i c l e
i n f o
Article history: Received 29 August 2013 Received in revised form 6 December 2013 Accepted 6 December 2013 Available online 17 December 2013 Keywords: Surface modification Superhydrophobicity Water contact angle Anodization Silane Titania
a b s t r a c t A facile method was adapted to make superhydrophobic (SHP) titanium in which a synergistic combination of surface roughness and surface chemistry was utilized. In the first step, titanium was mechanically polished and pickled followed by anodization. The next step was to dip coat the samples with silane solution and then were cured at 110 ◦ C. Influence of different synthesis parameters such as silane concentration, number of dip coating and curing temperature on water contact angle (WCA) was studied and conditions were optimized to achieve a WCA of 150◦ . The wetting properties of the samples were elucidated using contact angle meter and the water just rolled off the modified titanium surface with a slight tilting. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the morphology and surface roughness of the silane coated titanium samples. Grazing incidence X-ray diffraction (GIXRD), energy dispersive spectroscopy (EDS), attenuated total reflection-infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to analyze the chemical composition of the coatings which confirmed the presence of silicon along with titanium and oxygen. Immersion studies in sea water and nitric acid medium for 15 days indicated the stability of the coatings with minimal variations in contact angle. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nature has its own ‘Nanotechnology’ revealed in different objects for different purposes. A specific example is the lotus leaf which is considered as a sacred flower in East Asian countries. Its peculiarity arises from the fact that it grows in murky waters and yet the lotus leaves are free from impurities. Water droplets falling on the lotus leaf bead up and roll off taking away all the dirt. This phenomenon is called the ‘Lotus effect’ and the lotus leaves are said to be superhydrophobic. When the WCA is above 150◦ , the surface is superhydrophobic, water drops simply bounce-off the surface. This phenomenon is exhibited by a wide range of plants and a few insects for self-cleaning purpose. In nature, the leaves of plants like Nelumbo nucifera [1], Colocasia esculenta, Brassica oleracea, the wings of butterflies, and the legs of water striders are all superhydrophobic. A detailed microscopic examination of the lotus leaves revealed a micro-nano rough feature namely papillae and a wax-like coating [2]. They have papillose epidermal cells and an additional layer of epicuticular waxes that provide the water repellence. As air is trapped between the micro-nano features, the
∗ Corresponding author at: Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Tel.: +91 44 27480121; fax: +91 44 27480121. E-mail addresses: [email protected], [email protected] (U. Kamachi Mudali). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.027
water drops rest only on the tip of the surface microstructures, thereby minimizing the interfacial area between the surface and the water drop. But without micro-nano architecture there is also a possibility of exhibiting superhydrophobicity as in the case of the wings of butterflies. Superhydrophobicity could be generated in two steps: (i) creating micro-nano roughness and (ii) coating with a low surface energy material. These steps could be achieved by a number of experimental techniques and materials. The first step is usually achieved by mechanical polishing or electro polishing and then a suitable technique is used to create micro-nano roughness followed by dip coating or spin coating the low surface energy materials. At the same time, a number of sophisticated experimental techniques are also employed to create superhydrophobic surfaces; plasma enhanced chemical vapor deposition, magnetron sputtering, self-assembly, laser treatment, photo catalytic lithography, anodization, electrospinning, electrostatic spinning and spraying, chemical vapor deposition, electroless replacement deposition, wet chemical reaction, sol–gel method, polymer replication method, electroless galvanic deposition, plasma arc deposition, polymerization, and atomic layer deposition are some of the techniques used to roughen the surfaces or to coat a low surface energy material, or both [3,4]. On the other hand, some techniques are used to roughen the surfaces alone (the materials need not necessarily be low surface energy materials) including mechanical polishing, laser/plasma/chemical etching, lithography, sol–gel processing,
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Fig. 1. (a and b) SEM images of silane coated titanium anodized in 0.3 M H2 SO4 and (c) the corresponding EDS spectrum.
layer-by-layer or colloidal assembly, electrical/chemical reaction and deposition, electrospinning, and chemical vapor deposition to name a few. Having created the necessary roughness by these techniques, the next step is to coat them with low surface energy materials like myristic acid, stearic acid [5–9], silica coatings [10], octadecyltrichlorosilane (OTS) [11,12], perfluorooctyltriethoxysilane (PFOTES) [13], polydimethylsiloxanes (PDMS) [14,15], non-fluorinated alkylsilane [16], silicone nanofilament [17], porous polymer coatings [18], nano-titania coatings [19,20], polymethylmethacrylate (PMMA) coatings [21], carbon nanotube based composite coatings [3,22,23], tetrafluoroethylene (Teflon) coating [24], and polystyrene [25] which are some of the common materials coated on the mechanically polished surfaces to yield superhydrophobicity. In addition, superhydrophobic surfaces were also prepared using one-step processes predominantly on copper substrates [9], which involve longer process time ranging from a few hours to few days, special substrate preparation, and stringent experimental conditions. A bio-inspired approach is also utilized for the one step surface modification of substrates using soft lithography and polymer coating which in turn mimic the adhesive pads of the Mussels [26]. In the present study, the fabrication of superhydrophobic titanium is prepared using a simpler and faster two step method which neither requires sophisticated instruments nor laborious experimental procedures. The ‘lotus effect’ has been successfully employed for superhydrophobic surface modification of titanium [5,6], a significant material used in nuclear power plants and reprocessing plants. In the case of stearic acid coated titanium anodized in H2 SO4 , the maximum WCA obtained was 149◦ . To further improve the uniformity and stability of the coatings as well as the tilting angle on titanium, stearic acid coating was replaced by silane coating. A two step surface modification involving anodization and coating with silane was employed for titanium in the present work. Though a considerable amount of work was reported in the literature on silanes, titanium modified using silanes was the first of its kind and the
experimental parameters were optimized especially for titanium. The morphology, surface roughness, contact angle measurements, and chemical composition analysis were carried out for surface modified titanium using silanes. In addition, the superhydrophobic coatings developed on titanium were tested in nitric acid and sea water for long term stability, and the morphology variations and changes in WCA before and after immersion were recorded.
2. Experimental 2.1. Sample preparation Commercially pure grade 2 titanium (1.5 cm × 1.5 cm × 0.05 cm) coupons were used for the present investigation. Highly pure analytical grade chemicals such as hydrofluoric acid (HF, 40%), sulfuric acid (H2 SO4 , 99%), hydrochloric acid (HCl, 90%), nitric acid (HNO3 , 90%), ethanol (90%), and perfluorooctyltriethoxysilane (PFOTES, 97%, Alfa Aesar) were used. Deionized (DI) water was used for the preparation of the electrolytes. The titanium coupons were mechanically polished on both sides using SiC paper upto 1000 grit. After polishing, the samples were consecutively washed in soap solution and DI water and degreased with acetone. The polished titanium samples were pickled in 400 g/L HNO3 , and 40 g/L HF followed by washing in DI water and dried in air at room temperature. 2.2. Superhydrophobic surface modification 2.2.1. Step 1 Anodization Anodization was carried out using a two electrode set up with polished Ti coupon as the anode and stainless steel (SS) as the cathode. The electrodes were connected to a DC voltage source (Aplab, Model L3230). The experiments were carried out at room temperature in 0.3 M H2 SO4 electrolyte at 30 V for 1 h duration. The anodized samples were washed in DI water and dried in air.
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Fig. 2. (a) AFM image of the surface modified titanium sample using silane, (b and c) represent the corresponding line profile and the 3D image.
2.2.2. Step 2 Dip coating in silane solution The silane chosen for surface modification is 1H, 1H, 2H, 2HPerfluorooctyltriethoxysilane and the molecular formula is:
The organosilanes have one organic substituent and three hydrolyzable substituents. The fluorinated hydrocarbon substituents are the hydrophobic entities which enable the silane to provide the hydrophobicity to the sample on which they are coated
[27]. The alkoxy groups of the trialkoxysilanes are hydrolyzed to form silanol-containing species. Reaction of these silanes involves four steps: initially, hydrolysis of the three alkoxy groups takes place followed by condensation to oligomers. The oligomers then form hydrogen bond with the OH groups of the substrate. Finally, during drying or heat treatment, a covalent linkage is formed with the substrate with loss of water. These four steps can occur simultaneously after the initial hydrolysis step. At the substrate-silane interface, there will be only one bond from silicon of the organosilane to the substrate interface. The two remaining silanol groups are present either in condensed or free form. The R group remains available for covalent reaction or physical interaction with other phases [28].
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With this input, the next step was to prepare the silane solution by mixing the required wt.% (0.5, 1.0, 1.5, and 2.0) of PFOTES in 88 wt.% methanol, 10 wt.% DI water, and 1.5 wt.% of 0.1 N HCl. The mixture was stirred at room temperature for 4 h. In this process, there are few steps including hydrolysis, condensation, and polymerization as explained earlier. Then the anodized titanium samples were dip coated in the silane solution using single dip coater (SDC 2007 C, Apex Instruments, India). The dipping and lifting speed of the samples was maintained at 50 mm/min and the dip coating was carried out in ambient conditions. The sample was immersed in the solution for 3 min before withdrawal. The dip coated samples were initially dried in air followed by heat treatment at 110 ◦ C for 30 min in a hot air oven.
2.3. Role of various experimental parameters on wettability To check the role of silane concentration, curing temperature, and number of dip coating on the wetting behavior of titanium samples, the following procedures were carried out. The silane concentration was varied as 0.5, 1.0, 1.5, and 2.0 wt.%, and all other experimental parameters were kept unchanged. The silane dip coating was carried out for 1 to 7 times on the anodized titanium substrates followed by heat treatment. To elucidate the role of curing temperature, one set of dip coated samples were cured at room temperature and another set at 110 ◦ C for 30 min.
2.4. Sample characterization 2.4.1. Morphology and surface studies SEM (Desktop Mini-SEM, SNE 3000 M, Korea) and AFM (NTMDT Solver Pro ECAFM) were used to study the morphology of the silane coated titanium samples. The surface roughness of the samples was calculated from the AFM images using NOVA Image Analysis Software (1.0.26.1443). XPS (SPECS XPS with Al K␣ PSOI BOS-150, Germany) and energy dispersive spectrometer (EDS) coupled with SEM were used to analyze the chemical composition of the coatings. GIXRD (Grazing incidence X-ray diffraction, Bruker D8 DISCOVER) patterns were recorded for anodized titanium and anodized titanium coated with silane. ATR-IR (PerkinElmerSpectrum two spectrophotometer) spectra were recorded for both silane solution and surface modified titanium samples using silane. The surface profile of the superhydrophobic titanium samples were examined using a profilometer (Taylor Hobson, Model-Talysurf C1000).
2.4.2. Wettability studies The wetting properties of the superhydrophobic surface modified samples were elucidated using contact angle meter (OCA15EC, DataPhysics Instruments, Germany). The contact angle (CA) measurements were chosen as the benchmark for optimizing the role of various experimental parameters on the surface modification. The dosing volume and the dosing rate were chosen as 5 L and 1 L/s respectively, for all the experiments. The average CA values were obtained by measuring the CA at five different places on the same sample and DI water was used in these experiments.
2.4.3. Stability studies The superhydrophobic surface modified titanium samples were tested for their stability in sea water and 1 N HNO3 . The samples were immersed for 360 h at room temperature. After immersion, the changes in morphology and water contact angles were recorded and compared with the values before immersion.
Fig. 3. XRD patterns of (a) anodized titanium and (b) anodized titanium coated with silane.
3. Results and discussion 3.1. Surface morphology SEM images of titanium anodized in 0.3 M H2 SO4 and coated with silane are shown in Fig. 1a and b. The morphology can be correlated to a lotus leaf like surface texturing. Micron sized pores and flower like eruptions are present throughout the surface and similar morphology is reported in the literature for titanium anodized in sulfuric acid electrolyte [29,30]. A typical EDS spectrum recorded from the same sample revealed the presence of silicon around 2 at % along with titanium and oxygen peaks. The EDS results are further validated by the XPS results discussed below. The surface topography of anodized and silane coated titanium is shown in Fig. 2. The 2D and 3D AFM images (Fig. 2a and b) of the sample show hill and valley like TiO2 grains coated with silane distributed throughout the surface modified titanium. The root mean square (rms) roughness is about 300 nm and average roughness is 240 nm in the 10 m × 10 m scan area. The roughness histogram showed a Gaussian like distribution for the surface modified titanium with the maximum height of 765 nm for the features found on the sample. The corresponding line profile of the surface modified titanium using silane is shown in Fig. 2c. From the morphological investigation using SEM and AFM, it is clear that the complex and heterogeneous surface morphology of the modified titanium together with an appropriate surface roughness contributed for its superhydrophobic property. 3.2. Surface composition GIXRD patterns of the titanium sample anodized in H2 SO4 electrolyte, and another titanium sample anodized and coated with silane shown in Fig. 3a and b assisted in identifying the phase and composition. In both cases, the majority of the peaks are indexed to hexagonal and cubic phase titanium (JCPDS # 89-4913 and # 89-2762) which originated from the titanium substrate. The other peaks in the XRD pattern are indexed to anatase TiO2 according to JCPDS # 89-4921. It is evident from the patterns shown in Fig. 3a and b that the intensity and the width of the peaks have not changed before and after coating with silane. Therefore, the crystallinity of the TiO2 anodized surface remain unchanged possibly due to the low temperature heat treatment and the amorphous nature of the fluorosilane used as the low surface energy material coating on the anodized Ti sample. The interaction between the anodized titanium
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Fig. 4. XPS survey scan, C1s, Ti2p, Si2p, F1s, and deconvoluted O1s spectra of anodized titanium coated with silane.
and silane coating may be only at the surface level and therefore, the TiO2 grain size and crystallinity remained the same [31]. The surface chemical composition of surface modified superhydrophobic titanium was identified by XPS. Fig. 4a shows the XPS survey scan and Fig. 4b–f show the high resolution spectra of the elements present in the anodized titanium sample modified by the silane derivatives. The modified surface showed significant peaks at 285.75 eV which correspond to the C 1s i.e. carbon singly bonded to an oxygen atom [32]. The O 1s region is deconvoluted
into two peaks, the peaks at 530.99 and 532.76 eV correspond to Ti O in TiO2 , hydroxyl groups (OH− ) and Ti O Si in the sample [33]. Ti O Si chemical bond is the one which binds the silane and the TiO2 layer of the sample whereas, the peak at 103.4 eV correspond to Si 2p peak, which is an indication of silicon atom singly bonded to oxygen atom [33]. The peaks at 459.48 and 465.26 eV correspond to Ti 2p1/2 and Ti 2p3/2 [32]. The peak at 689 eV corresponds to F1s from the perfluoro group in the silane used for surface modification [31]. The results strongly suggest that the surface of the
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Fig. 5. ATR-IR spectrum of surface modified titanium coated with silane.
titanium sample was coated with silane. Surface chemical modification of the titanium was carried out by forming a self-assembled monolayer (SAM) of organosilane derivatives. After coating the surface with silane, the water contact angle increased considerably; the presence of hydrophobic molecules on the surface is proof of the increase in the contact angle as compared to the polished titanium samples without silane coating. ATR-IR results in Fig. 5 show the strong and broad peak in the region 800–400 cm−1 which is the Ti O stretching band correspond to the IR active mode of TiO2 [34]. The peaks at 1 4 3 0, 1 2 4 0, and 1 1 4 4 cm−1 correspond to the vibrations of CF2 and CF3 groups in PFOTES [35]. The ATR-IR results confirm the presence of hydrophobic alkyl chains formed on the substrate from the silane solution. 3.3. Surface wettability A typical water contact angle measured on silane coated titanium was around 150◦ and the tilting angle was less than 5◦ . Since the as prepared titanium showed a very high static contact angle and a very low roll-off angle, it is expected to have the self cleaning property in addition to superhydrophobicity [1,36]. Fig. 6 represents a schematic diagram where the different pretreatments and the corresponding contact angles are shown. In the case of polished titanium, the surface was smooth and hydrophilic with a WCA of 30◦ . It is worthy to note here that the contact angle measured on polished titanium coated with silane and without performing the anodization step was only 114◦ . When titanium was anodized in sulfuric acid, it resulted in surface heterogeneities as evident from
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SEM and AFM images shown in Figs. 1 and 2, but the surface was still hydrophilic with a contact angle of 70◦ . But anodized titanium surface coated with silane was superhydrophobic with a contact angle of 150◦ . Therefore, it could be concluded that heterogeneous surface morphology in combination with the low surface energy material alone could lead to superhydrophobic behavior; a similar trend was also observed for anodized titanium modified with stearic acid [30]. The role of silane concentration, curing temperature, and number of dip coating on the surface modification of titanium were studied systematically. When the silane dip coating on titanium was repeated for 1 to 7 times, the CA varied from 150 to 128◦ as represented in Fig. 7. The contact angle was maximum for 1 to 2 repetitions of dip coating after which the contact angle reduced drastically to 128◦ for the trial in which dip coating was repeated for 7 times. In order to explain the reduction in contact angle with the number of dip coatings in silane, the surface roughness values are obtained from the AFM analysis for the surface modified titanium samples. The average surface roughness value of the sample was found to be around 300 nm for single dip coating in silane, 270 nm for 2 dip coating, an average of 200 nm for 3–6 repetitions of dip coating, and 50 nm for 7 repetitions of dip coating. The above results show that after the first two repetitions of dip coating which resulted in highest contact angle, the surface roughness started to decrease for 3 to 6 dip coating with the reduction in contact angle compared to 1–2 dip coating and finally, the surface roughness drastically reduced for 7 repetitions of dip coating which resulted in a contact angle of 128◦ . It is worthy to recall here that superhydrophobicity is a result of both surface roughness and surface chemical composition. The critical surface roughness required for superhydrophobicity is achieved with 1–2 repetitions of dip coating. When the dip coating is performed more than 2 times, the surface gradually lost its roughness required for air entrapment according to Cassie–Baxter model; therefore, contact angle is reduced [37]. The silane concentration was varied as 0.5, 1.0, 1.5, and 2.0 wt.% in the solution, and the maximum contact angle of 151◦ was obtained for 2.0 wt.% silane in the dip coating solution as shown in the inset of Fig. 7. The increase in silane concentration lead to the presence of more hydrophobic groups on the surface resulting in increased contact angle [28]. Similarly, one set of samples were cured at room temperature for 24 h and the other set of samples were heat treated at 110 ◦ C for 30 min in hot air oven. In the case of heat treated samples, the WCA was 1 to 2◦ more as compared to the samples cured at room temperature [28]. The surface modified titanium samples were immersed in 1 N nitric acid and sea water media for 360 h. After immersion, the samples were again tested for variation in contact angle, and the dissolution of the coatings if any is checked by using SEM and the images are shown in Fig. 8. There was less than 10◦ reduction in WCA after the exposure. By comparing the morphology of the coatings before immersion as shown in the SEM images in
Fig. 6. Schematic representing the topography of titanium after different surface treatments and the corresponding contact angles measured on these surfaces.
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coating with silane and curing. AFM and SEM revealed the heterogeneous surface morphology of the anodized and silane coated titanium surface. Water contact angle measured on such a surface was around 150◦ and the water just rolled off the surface with a slight tilting. The variation in WCA with respect to silane concentration, number of dip coating in silane, and curing temperature was recorded. It is concluded that the silane concentration of around 2%, and a single dip coating of silane and curing at 110 ◦ C for 30 min is adequate to impart a contact angle of ∼150◦ on the anodized titanium surface modified with silane. XRD analysis showed the anatase phase of TiO2 projections formed on the titanium substrate. EDS, XPS, and ATR-IR studies confirmed the presence of silane on the anodized titanium surface. The superhydrophobic titanium samples modified with silane were found to be stable in sea water and mild nitric acid media after long duration exposure with minimal reduction in contact angle. Further work is in progress to improve the stability of the coatings and to employ the superhydrophobic titanium in in-service conditions. Fig. 7. Water contact angles are plotted against number of dip coating performed on titanium samples anodized in 0.3 M H2 SO4 . Inset shows the variation in water contact angle with respect to the silane concentration.
Acknowledgements The authors acknowledge Dr. S. Rajagopalan and Dr. M. Kamruddin of Materials Science Group, IGCAR for GIXRD and FESEM characterization, and Dr. N. Rajendran, Anna University for ATR-IR facility. One of the authors (SCV) acknowledges Mr. Nanda Gopala Krishna for XPS characterization. References
Fig. 8. SEM images of surface modified titanium after 360 h immersion in (a) 1 N HNO3 and (b) sea water media.
Fig. 1 and after immersion as shown in Fig. 8, there were minimal changes in the morphology which implies that the silane based superhydrophobic coatings are intact even after 360 h immersion. 4. Conclusions Superhydrophobic surface modification of titanium using silanes was achieved by employing the following steps: (i) mechanical polishing, (ii) pickling, (iii) anodization in 0.3 M H2 SO4 , and (iv)
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Influence of silanes on the wettability of anodized titanium S.C. Vanithakumari, R.P. George, U. Kamachi Mudali ∗ Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
a r t i c l e
i n f o
Article history: Received 29 August 2013 Received in revised form 6 December 2013 Accepted 6 December 2013 Available online 17 December 2013 Keywords: Surface modification Superhydrophobicity Water contact angle Anodization Silane Titania
a b s t r a c t A facile method was adapted to make superhydrophobic (SHP) titanium in which a synergistic combination of surface roughness and surface chemistry was utilized. In the first step, titanium was mechanically polished and pickled followed by anodization. The next step was to dip coat the samples with silane solution and then were cured at 110 ◦ C. Influence of different synthesis parameters such as silane concentration, number of dip coating and curing temperature on water contact angle (WCA) was studied and conditions were optimized to achieve a WCA of 150◦ . The wetting properties of the samples were elucidated using contact angle meter and the water just rolled off the modified titanium surface with a slight tilting. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the morphology and surface roughness of the silane coated titanium samples. Grazing incidence X-ray diffraction (GIXRD), energy dispersive spectroscopy (EDS), attenuated total reflection-infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to analyze the chemical composition of the coatings which confirmed the presence of silicon along with titanium and oxygen. Immersion studies in sea water and nitric acid medium for 15 days indicated the stability of the coatings with minimal variations in contact angle. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nature has its own ‘Nanotechnology’ revealed in different objects for different purposes. A specific example is the lotus leaf which is considered as a sacred flower in East Asian countries. Its peculiarity arises from the fact that it grows in murky waters and yet the lotus leaves are free from impurities. Water droplets falling on the lotus leaf bead up and roll off taking away all the dirt. This phenomenon is called the ‘Lotus effect’ and the lotus leaves are said to be superhydrophobic. When the WCA is above 150◦ , the surface is superhydrophobic, water drops simply bounce-off the surface. This phenomenon is exhibited by a wide range of plants and a few insects for self-cleaning purpose. In nature, the leaves of plants like Nelumbo nucifera [1], Colocasia esculenta, Brassica oleracea, the wings of butterflies, and the legs of water striders are all superhydrophobic. A detailed microscopic examination of the lotus leaves revealed a micro-nano rough feature namely papillae and a wax-like coating [2]. They have papillose epidermal cells and an additional layer of epicuticular waxes that provide the water repellence. As air is trapped between the micro-nano features, the
∗ Corresponding author at: Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Tel.: +91 44 27480121; fax: +91 44 27480121. E-mail addresses: [email protected], [email protected] (U. Kamachi Mudali). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.027
water drops rest only on the tip of the surface microstructures, thereby minimizing the interfacial area between the surface and the water drop. But without micro-nano architecture there is also a possibility of exhibiting superhydrophobicity as in the case of the wings of butterflies. Superhydrophobicity could be generated in two steps: (i) creating micro-nano roughness and (ii) coating with a low surface energy material. These steps could be achieved by a number of experimental techniques and materials. The first step is usually achieved by mechanical polishing or electro polishing and then a suitable technique is used to create micro-nano roughness followed by dip coating or spin coating the low surface energy materials. At the same time, a number of sophisticated experimental techniques are also employed to create superhydrophobic surfaces; plasma enhanced chemical vapor deposition, magnetron sputtering, self-assembly, laser treatment, photo catalytic lithography, anodization, electrospinning, electrostatic spinning and spraying, chemical vapor deposition, electroless replacement deposition, wet chemical reaction, sol–gel method, polymer replication method, electroless galvanic deposition, plasma arc deposition, polymerization, and atomic layer deposition are some of the techniques used to roughen the surfaces or to coat a low surface energy material, or both [3,4]. On the other hand, some techniques are used to roughen the surfaces alone (the materials need not necessarily be low surface energy materials) including mechanical polishing, laser/plasma/chemical etching, lithography, sol–gel processing,
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Fig. 1. (a and b) SEM images of silane coated titanium anodized in 0.3 M H2 SO4 and (c) the corresponding EDS spectrum.
layer-by-layer or colloidal assembly, electrical/chemical reaction and deposition, electrospinning, and chemical vapor deposition to name a few. Having created the necessary roughness by these techniques, the next step is to coat them with low surface energy materials like myristic acid, stearic acid [5–9], silica coatings [10], octadecyltrichlorosilane (OTS) [11,12], perfluorooctyltriethoxysilane (PFOTES) [13], polydimethylsiloxanes (PDMS) [14,15], non-fluorinated alkylsilane [16], silicone nanofilament [17], porous polymer coatings [18], nano-titania coatings [19,20], polymethylmethacrylate (PMMA) coatings [21], carbon nanotube based composite coatings [3,22,23], tetrafluoroethylene (Teflon) coating [24], and polystyrene [25] which are some of the common materials coated on the mechanically polished surfaces to yield superhydrophobicity. In addition, superhydrophobic surfaces were also prepared using one-step processes predominantly on copper substrates [9], which involve longer process time ranging from a few hours to few days, special substrate preparation, and stringent experimental conditions. A bio-inspired approach is also utilized for the one step surface modification of substrates using soft lithography and polymer coating which in turn mimic the adhesive pads of the Mussels [26]. In the present study, the fabrication of superhydrophobic titanium is prepared using a simpler and faster two step method which neither requires sophisticated instruments nor laborious experimental procedures. The ‘lotus effect’ has been successfully employed for superhydrophobic surface modification of titanium [5,6], a significant material used in nuclear power plants and reprocessing plants. In the case of stearic acid coated titanium anodized in H2 SO4 , the maximum WCA obtained was 149◦ . To further improve the uniformity and stability of the coatings as well as the tilting angle on titanium, stearic acid coating was replaced by silane coating. A two step surface modification involving anodization and coating with silane was employed for titanium in the present work. Though a considerable amount of work was reported in the literature on silanes, titanium modified using silanes was the first of its kind and the
experimental parameters were optimized especially for titanium. The morphology, surface roughness, contact angle measurements, and chemical composition analysis were carried out for surface modified titanium using silanes. In addition, the superhydrophobic coatings developed on titanium were tested in nitric acid and sea water for long term stability, and the morphology variations and changes in WCA before and after immersion were recorded.
2. Experimental 2.1. Sample preparation Commercially pure grade 2 titanium (1.5 cm × 1.5 cm × 0.05 cm) coupons were used for the present investigation. Highly pure analytical grade chemicals such as hydrofluoric acid (HF, 40%), sulfuric acid (H2 SO4 , 99%), hydrochloric acid (HCl, 90%), nitric acid (HNO3 , 90%), ethanol (90%), and perfluorooctyltriethoxysilane (PFOTES, 97%, Alfa Aesar) were used. Deionized (DI) water was used for the preparation of the electrolytes. The titanium coupons were mechanically polished on both sides using SiC paper upto 1000 grit. After polishing, the samples were consecutively washed in soap solution and DI water and degreased with acetone. The polished titanium samples were pickled in 400 g/L HNO3 , and 40 g/L HF followed by washing in DI water and dried in air at room temperature. 2.2. Superhydrophobic surface modification 2.2.1. Step 1 Anodization Anodization was carried out using a two electrode set up with polished Ti coupon as the anode and stainless steel (SS) as the cathode. The electrodes were connected to a DC voltage source (Aplab, Model L3230). The experiments were carried out at room temperature in 0.3 M H2 SO4 electrolyte at 30 V for 1 h duration. The anodized samples were washed in DI water and dried in air.
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Fig. 2. (a) AFM image of the surface modified titanium sample using silane, (b and c) represent the corresponding line profile and the 3D image.
2.2.2. Step 2 Dip coating in silane solution The silane chosen for surface modification is 1H, 1H, 2H, 2HPerfluorooctyltriethoxysilane and the molecular formula is:
The organosilanes have one organic substituent and three hydrolyzable substituents. The fluorinated hydrocarbon substituents are the hydrophobic entities which enable the silane to provide the hydrophobicity to the sample on which they are coated
[27]. The alkoxy groups of the trialkoxysilanes are hydrolyzed to form silanol-containing species. Reaction of these silanes involves four steps: initially, hydrolysis of the three alkoxy groups takes place followed by condensation to oligomers. The oligomers then form hydrogen bond with the OH groups of the substrate. Finally, during drying or heat treatment, a covalent linkage is formed with the substrate with loss of water. These four steps can occur simultaneously after the initial hydrolysis step. At the substrate-silane interface, there will be only one bond from silicon of the organosilane to the substrate interface. The two remaining silanol groups are present either in condensed or free form. The R group remains available for covalent reaction or physical interaction with other phases [28].
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With this input, the next step was to prepare the silane solution by mixing the required wt.% (0.5, 1.0, 1.5, and 2.0) of PFOTES in 88 wt.% methanol, 10 wt.% DI water, and 1.5 wt.% of 0.1 N HCl. The mixture was stirred at room temperature for 4 h. In this process, there are few steps including hydrolysis, condensation, and polymerization as explained earlier. Then the anodized titanium samples were dip coated in the silane solution using single dip coater (SDC 2007 C, Apex Instruments, India). The dipping and lifting speed of the samples was maintained at 50 mm/min and the dip coating was carried out in ambient conditions. The sample was immersed in the solution for 3 min before withdrawal. The dip coated samples were initially dried in air followed by heat treatment at 110 ◦ C for 30 min in a hot air oven.
2.3. Role of various experimental parameters on wettability To check the role of silane concentration, curing temperature, and number of dip coating on the wetting behavior of titanium samples, the following procedures were carried out. The silane concentration was varied as 0.5, 1.0, 1.5, and 2.0 wt.%, and all other experimental parameters were kept unchanged. The silane dip coating was carried out for 1 to 7 times on the anodized titanium substrates followed by heat treatment. To elucidate the role of curing temperature, one set of dip coated samples were cured at room temperature and another set at 110 ◦ C for 30 min.
2.4. Sample characterization 2.4.1. Morphology and surface studies SEM (Desktop Mini-SEM, SNE 3000 M, Korea) and AFM (NTMDT Solver Pro ECAFM) were used to study the morphology of the silane coated titanium samples. The surface roughness of the samples was calculated from the AFM images using NOVA Image Analysis Software (1.0.26.1443). XPS (SPECS XPS with Al K␣ PSOI BOS-150, Germany) and energy dispersive spectrometer (EDS) coupled with SEM were used to analyze the chemical composition of the coatings. GIXRD (Grazing incidence X-ray diffraction, Bruker D8 DISCOVER) patterns were recorded for anodized titanium and anodized titanium coated with silane. ATR-IR (PerkinElmerSpectrum two spectrophotometer) spectra were recorded for both silane solution and surface modified titanium samples using silane. The surface profile of the superhydrophobic titanium samples were examined using a profilometer (Taylor Hobson, Model-Talysurf C1000).
2.4.2. Wettability studies The wetting properties of the superhydrophobic surface modified samples were elucidated using contact angle meter (OCA15EC, DataPhysics Instruments, Germany). The contact angle (CA) measurements were chosen as the benchmark for optimizing the role of various experimental parameters on the surface modification. The dosing volume and the dosing rate were chosen as 5 L and 1 L/s respectively, for all the experiments. The average CA values were obtained by measuring the CA at five different places on the same sample and DI water was used in these experiments.
2.4.3. Stability studies The superhydrophobic surface modified titanium samples were tested for their stability in sea water and 1 N HNO3 . The samples were immersed for 360 h at room temperature. After immersion, the changes in morphology and water contact angles were recorded and compared with the values before immersion.
Fig. 3. XRD patterns of (a) anodized titanium and (b) anodized titanium coated with silane.
3. Results and discussion 3.1. Surface morphology SEM images of titanium anodized in 0.3 M H2 SO4 and coated with silane are shown in Fig. 1a and b. The morphology can be correlated to a lotus leaf like surface texturing. Micron sized pores and flower like eruptions are present throughout the surface and similar morphology is reported in the literature for titanium anodized in sulfuric acid electrolyte [29,30]. A typical EDS spectrum recorded from the same sample revealed the presence of silicon around 2 at % along with titanium and oxygen peaks. The EDS results are further validated by the XPS results discussed below. The surface topography of anodized and silane coated titanium is shown in Fig. 2. The 2D and 3D AFM images (Fig. 2a and b) of the sample show hill and valley like TiO2 grains coated with silane distributed throughout the surface modified titanium. The root mean square (rms) roughness is about 300 nm and average roughness is 240 nm in the 10 m × 10 m scan area. The roughness histogram showed a Gaussian like distribution for the surface modified titanium with the maximum height of 765 nm for the features found on the sample. The corresponding line profile of the surface modified titanium using silane is shown in Fig. 2c. From the morphological investigation using SEM and AFM, it is clear that the complex and heterogeneous surface morphology of the modified titanium together with an appropriate surface roughness contributed for its superhydrophobic property. 3.2. Surface composition GIXRD patterns of the titanium sample anodized in H2 SO4 electrolyte, and another titanium sample anodized and coated with silane shown in Fig. 3a and b assisted in identifying the phase and composition. In both cases, the majority of the peaks are indexed to hexagonal and cubic phase titanium (JCPDS # 89-4913 and # 89-2762) which originated from the titanium substrate. The other peaks in the XRD pattern are indexed to anatase TiO2 according to JCPDS # 89-4921. It is evident from the patterns shown in Fig. 3a and b that the intensity and the width of the peaks have not changed before and after coating with silane. Therefore, the crystallinity of the TiO2 anodized surface remain unchanged possibly due to the low temperature heat treatment and the amorphous nature of the fluorosilane used as the low surface energy material coating on the anodized Ti sample. The interaction between the anodized titanium
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Fig. 4. XPS survey scan, C1s, Ti2p, Si2p, F1s, and deconvoluted O1s spectra of anodized titanium coated with silane.
and silane coating may be only at the surface level and therefore, the TiO2 grain size and crystallinity remained the same [31]. The surface chemical composition of surface modified superhydrophobic titanium was identified by XPS. Fig. 4a shows the XPS survey scan and Fig. 4b–f show the high resolution spectra of the elements present in the anodized titanium sample modified by the silane derivatives. The modified surface showed significant peaks at 285.75 eV which correspond to the C 1s i.e. carbon singly bonded to an oxygen atom [32]. The O 1s region is deconvoluted
into two peaks, the peaks at 530.99 and 532.76 eV correspond to Ti O in TiO2 , hydroxyl groups (OH− ) and Ti O Si in the sample [33]. Ti O Si chemical bond is the one which binds the silane and the TiO2 layer of the sample whereas, the peak at 103.4 eV correspond to Si 2p peak, which is an indication of silicon atom singly bonded to oxygen atom [33]. The peaks at 459.48 and 465.26 eV correspond to Ti 2p1/2 and Ti 2p3/2 [32]. The peak at 689 eV corresponds to F1s from the perfluoro group in the silane used for surface modification [31]. The results strongly suggest that the surface of the
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Fig. 5. ATR-IR spectrum of surface modified titanium coated with silane.
titanium sample was coated with silane. Surface chemical modification of the titanium was carried out by forming a self-assembled monolayer (SAM) of organosilane derivatives. After coating the surface with silane, the water contact angle increased considerably; the presence of hydrophobic molecules on the surface is proof of the increase in the contact angle as compared to the polished titanium samples without silane coating. ATR-IR results in Fig. 5 show the strong and broad peak in the region 800–400 cm−1 which is the Ti O stretching band correspond to the IR active mode of TiO2 [34]. The peaks at 1 4 3 0, 1 2 4 0, and 1 1 4 4 cm−1 correspond to the vibrations of CF2 and CF3 groups in PFOTES [35]. The ATR-IR results confirm the presence of hydrophobic alkyl chains formed on the substrate from the silane solution. 3.3. Surface wettability A typical water contact angle measured on silane coated titanium was around 150◦ and the tilting angle was less than 5◦ . Since the as prepared titanium showed a very high static contact angle and a very low roll-off angle, it is expected to have the self cleaning property in addition to superhydrophobicity [1,36]. Fig. 6 represents a schematic diagram where the different pretreatments and the corresponding contact angles are shown. In the case of polished titanium, the surface was smooth and hydrophilic with a WCA of 30◦ . It is worthy to note here that the contact angle measured on polished titanium coated with silane and without performing the anodization step was only 114◦ . When titanium was anodized in sulfuric acid, it resulted in surface heterogeneities as evident from
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SEM and AFM images shown in Figs. 1 and 2, but the surface was still hydrophilic with a contact angle of 70◦ . But anodized titanium surface coated with silane was superhydrophobic with a contact angle of 150◦ . Therefore, it could be concluded that heterogeneous surface morphology in combination with the low surface energy material alone could lead to superhydrophobic behavior; a similar trend was also observed for anodized titanium modified with stearic acid [30]. The role of silane concentration, curing temperature, and number of dip coating on the surface modification of titanium were studied systematically. When the silane dip coating on titanium was repeated for 1 to 7 times, the CA varied from 150 to 128◦ as represented in Fig. 7. The contact angle was maximum for 1 to 2 repetitions of dip coating after which the contact angle reduced drastically to 128◦ for the trial in which dip coating was repeated for 7 times. In order to explain the reduction in contact angle with the number of dip coatings in silane, the surface roughness values are obtained from the AFM analysis for the surface modified titanium samples. The average surface roughness value of the sample was found to be around 300 nm for single dip coating in silane, 270 nm for 2 dip coating, an average of 200 nm for 3–6 repetitions of dip coating, and 50 nm for 7 repetitions of dip coating. The above results show that after the first two repetitions of dip coating which resulted in highest contact angle, the surface roughness started to decrease for 3 to 6 dip coating with the reduction in contact angle compared to 1–2 dip coating and finally, the surface roughness drastically reduced for 7 repetitions of dip coating which resulted in a contact angle of 128◦ . It is worthy to recall here that superhydrophobicity is a result of both surface roughness and surface chemical composition. The critical surface roughness required for superhydrophobicity is achieved with 1–2 repetitions of dip coating. When the dip coating is performed more than 2 times, the surface gradually lost its roughness required for air entrapment according to Cassie–Baxter model; therefore, contact angle is reduced [37]. The silane concentration was varied as 0.5, 1.0, 1.5, and 2.0 wt.% in the solution, and the maximum contact angle of 151◦ was obtained for 2.0 wt.% silane in the dip coating solution as shown in the inset of Fig. 7. The increase in silane concentration lead to the presence of more hydrophobic groups on the surface resulting in increased contact angle [28]. Similarly, one set of samples were cured at room temperature for 24 h and the other set of samples were heat treated at 110 ◦ C for 30 min in hot air oven. In the case of heat treated samples, the WCA was 1 to 2◦ more as compared to the samples cured at room temperature [28]. The surface modified titanium samples were immersed in 1 N nitric acid and sea water media for 360 h. After immersion, the samples were again tested for variation in contact angle, and the dissolution of the coatings if any is checked by using SEM and the images are shown in Fig. 8. There was less than 10◦ reduction in WCA after the exposure. By comparing the morphology of the coatings before immersion as shown in the SEM images in
Fig. 6. Schematic representing the topography of titanium after different surface treatments and the corresponding contact angles measured on these surfaces.
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coating with silane and curing. AFM and SEM revealed the heterogeneous surface morphology of the anodized and silane coated titanium surface. Water contact angle measured on such a surface was around 150◦ and the water just rolled off the surface with a slight tilting. The variation in WCA with respect to silane concentration, number of dip coating in silane, and curing temperature was recorded. It is concluded that the silane concentration of around 2%, and a single dip coating of silane and curing at 110 ◦ C for 30 min is adequate to impart a contact angle of ∼150◦ on the anodized titanium surface modified with silane. XRD analysis showed the anatase phase of TiO2 projections formed on the titanium substrate. EDS, XPS, and ATR-IR studies confirmed the presence of silane on the anodized titanium surface. The superhydrophobic titanium samples modified with silane were found to be stable in sea water and mild nitric acid media after long duration exposure with minimal reduction in contact angle. Further work is in progress to improve the stability of the coatings and to employ the superhydrophobic titanium in in-service conditions. Fig. 7. Water contact angles are plotted against number of dip coating performed on titanium samples anodized in 0.3 M H2 SO4 . Inset shows the variation in water contact angle with respect to the silane concentration.
Acknowledgements The authors acknowledge Dr. S. Rajagopalan and Dr. M. Kamruddin of Materials Science Group, IGCAR for GIXRD and FESEM characterization, and Dr. N. Rajendran, Anna University for ATR-IR facility. One of the authors (SCV) acknowledges Mr. Nanda Gopala Krishna for XPS characterization. References
Fig. 8. SEM images of surface modified titanium after 360 h immersion in (a) 1 N HNO3 and (b) sea water media.
Fig. 1 and after immersion as shown in Fig. 8, there were minimal changes in the morphology which implies that the silane based superhydrophobic coatings are intact even after 360 h immersion. 4. Conclusions Superhydrophobic surface modification of titanium using silanes was achieved by employing the following steps: (i) mechanical polishing, (ii) pickling, (iii) anodization in 0.3 M H2 SO4 , and (iv)
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