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Superhydrophobic coating deposited directly on aluminum
Applied Surface Science 305 (2014) 774–782
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Superhydrophobic coating deposited directly on aluminum Ana M. Escobar ∗ , Nuria Llorca-Isern CPCM, Departament de Ciència dels Materials i Enginyeria Metal·lurgica, Facultat de Química, Universitat de Barcelona, Marti-Franqués 1, 08028 Barcelona, Spain
a r t i c l e
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Article history: Received 18 June 2013 Received in revised form 6 March 2014 Accepted 31 March 2014 Available online 12 April 2014 Keywords: Superhydrophobicity Aluminum Lauric and myristic acid Simple process Coating Water contact angle
a b s t r a c t This study develops an alternative method for enhancing superhydrophobicity on aluminum surfaces with an amphiphilic reagent such as the dodecanoic acid. The goal is to induce superhydrophobicity directly through a simple process on pure (99.9 wt%) commercial aluminum. The initial surface activation leading to the formation of the superhydrophobic coating is studied using confocal microscopy. Superhydrophobic behavior is analyzed by contact angle measurements, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The highest contact angle (approaching 153◦ ) was obtained after forming hierarchical structures with a particular roughness obtained by grinding and polishing microgrooves on the aluminum surface together with the simultaneous action of HCl and dodecanoic acid. The results also showed that after immersion in the ethanol-acidic-fatty acid solutions, they reacted chemically through the action of the fatty acid, on the aluminum surface. The mechanism is analyzed by TOF-SIMS and XPS in order to determine the molecules involved in the reaction. The TOF-SIMS analysis revealed that the metal and its oxides seem to be necessary, and that free-aluminum is anchored to the fatty acid molecules and to the alumina molecules present in the medium. Consequently, both metallic aluminum and aluminum oxides are necessary in order to form the compound responsible for superhydrophobicity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surfaces have received considerable attention from researchers because the surface wettability of materials is a very important property that affects the final applications of products containing those materials [1,2]. Superhydrophobic surfaces show very high water contact angles (larger than 150◦ ) [3–5] and these surfaces are also of special interest because they have anti-adhesive properties and self-cleaning effects. They can be very useful in the aeronautical industry [6] and civil engineering [7]. Traditionally, superhydrophobic surfaces are obtained via multistep processes [8–25]: a certain rough pattern is first created by different processes such as chemical deposition [26], a sol–gel method or electrodeposition on an aluminum plate with further use of an immersion bath [27] to provide a prepared surface (clean and polished, etc.). In all these processes the first step is to roughen the surface, which is followed by its activation (in different steps) using fatty acids or inorganic salts [1]. In single-step processes, substrates with an initial roughness require a certain final roughness for superhydrophobicity to be produced in a single reaction. Saleema [28] produced superhydrophobic aluminum alloys in a basic bath; while
∗ Corresponding author. Tel.: +34 934021299. E-mail address: [email protected] (A.M. Escobar). http://dx.doi.org/10.1016/j.apsusc.2014.03.196 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Peng’s work [29] is based on initial electrolytic anodizing so that the alumina formed induces superhydrophobicity. In the present study the process is carried out in an electroless immersion bath and superhydrophobicity results from a chemical reaction. One of the goals of this study is to identify the growth step actors using TOFSIMS and XPS techniques, so close attention was paid to the analysis of the mechanism of the chemical reaction. Two main factors seem to characterize the behavior: the contact angle and the existence of pores at the interface. According to Brushan and Jung [4], the contact angle measured for hydrophobic surfaces with microstructures, nanostructures and hierarchical structures increases if the roughness of the surface increases. Another important factor is the existence of pores that hold air and ensure that water droplets do not come into contact with the solid surface. Two main models describe the possible wetting regimes: the homogenous Wenzel model [30], which predicts that a water droplet can penetrate the asperities; and the composite Cassie–Baxter model [31], in which a droplet may be suspended above the asperities since a gaseous phase may be trapped in the cavities of a rough surface (Fig. 1) [32]. The hydrophobicity of ceramic materials has been widely studied, whereas metallic surfaces have not been studied much with the exception of copper and aluminum. Both, multistep and singlestep processes have been proposed [28–32,34]. Aluminum surfaces are always covered by a thin protective aluminum oxide film (passivation film). Saleema et al. [35] used this alumina film (in
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Fig. 1. Wetting regimes on rough surfaces: (A) Cassie–Baxter heterogeneous wetting on a hydrophobic surface; (B) Wenzel wetting; (C) Cassie–Baxter on a hydrophilic surface; and (D) mixed wetting [33].
a basic medium) to produce a fluoroalkyl-silane compound and claimed that this combination was responsible for superhydrophobic behavior. In the present study, a simple method was used to prepare superhydrophobic surfaces directly on metallic aluminum using organic acids, by combining a facile acid etching with a polymer coating in a single reagent bath. The reaction requires an acid medium, provided by HCl which is used here to remove the oxide layer partially. The acid also ensures that the roughness produced in preparation is not lost, which facilitates the entrapment of air. In addition, the present study had three main objectives: to identify the molecules responsible for superhydrophobicity, the mechanism by which superhydrophobicity is produced, and consequently the influence of variables such as reactant composition and substrate surface. Preparation of the substrate surface is sometimes not considered an independent step (Ramadhianti et al. [21] and Song et al. [24]) although any surface treatment needs a previously prepared surface (e.g. it must be clean, degreased, etc.).
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the surface preparation: D1 corresponds to grinding with SiC abrasive paper from 63 m average particle diameter (P220 grade) to (P600 grade) “extra fine” 35.8 m average particle diameter; D2 stands for grinding as for D1, followed by a “super fine” grinding with 15.3 m mean particle diameter SiC paper (P1200 grade); D3 corresponds to “fine” grinding as for D2, followed by fine polishing with a 6 m diamond polishing suspension (Buehler, USA); and finally, D4 are samples to which ultrafine polishing with a 1 m diamond polishing suspension (Buehler, USA) followed the D3 procedure. Before and between this preparation, the samples were ultrasonically cleaned with ethanol for 10 min followed by deionized water for another 10 min. Finally, they were dried with forced air. The cleaned samples were then immersed vertically into a fresh ethanol solution of hydrochloric acid (30%) and lauric acid (20 g/L). The etching time and temperature for all the samples were 10 min and 50 ◦ C. After etching, the samples were immediately rinsed with ethanol and deionized water, then left in an oven at 80 ◦ C for 30 min. Another set of samples was prepared to optimize the amount of organic acid necessary to produce superhydrophobicity on samples with the optimized condition D2. Electron microscopy characterization of the specimen surface was carried out with an SEM Leica S-360 and an FE-SEM Hitachi S-4100 in order to study their morphological features. AFM measurements were performed using a Dimension 3100 microscope attached to Nanoscope IV electronics (Bruker) using Si probes with a nominal spring constant of 40 nN/nm (T300R-W, Vistaprobes). All topographic measurements were performed in tapping mode at a scan rate of 0.3 Hz and 512 × 512 pixels resolution. Samples were imaged in air and at 55% RH. The stability of the superhydrophobicity was studied over time at room temperature. The initial roughness measurements were performed by confocal microscopy using LeicaScan DCM 3D on a surface of 1.21 mm × 0.91 mm. The chemical composition of the surface was analyzed by X-ray photoelectron spectroscopy in a PHI 5500 Multitechnique System (Physical Electronics) with a monochromatic X-ray source (Aluminum K␣ line of 1486.6 eV energy and 350 W) on 0.8 mm diameter areas. All measurements were made in an ultra-high vacuum (UHV) chamber pressure between 5 × 10−9 and 2 × 10−8 Torr. The carbon 1s line was to calibrate the bindingenergy scale for XPS measurements, for which a binding energy of 284.8 eV was assumed. High-resolution mass spectra of positive and negative secondary ions were obtained using TOF-SIMS IV (ION-TOF, Munster, Germany) equipment operated at a pressure of 5 × 10−9 mbar and a 25 keV pulsed Bismuth liquid-metal ion source (Bi3 ++ ). Secondary ions were detected with a reflector TOF analyzer, multichannel plates (MCPs), and a time-to-digital converter (TDC). Measurements were performed with a typical acquisition time of 10 s, at a TDC time resolution of 200 ps and 100 s cycle time. Secondary ion spectra were acquired from randomly rastered sample surface areas of 100 m × 100 m. Mass spectral acquisition was performed using the ION-TOF Ion Spec software (version 4.1). Finally, contact angles were measured with a Contact Angle Measuring System DSA 100 from KRÜSS with 5 L of deionized water at room temperature.
2. Materials and methods 3. Results Commercial pure (99.9 wt%) aluminum 8 mm × 15 mm × 10 mm substrates were used for this study. As an initial study, different metallographic grinding and polishing conditions were applied to the substrates to produce a specific roughness; its influence on the coating step was then evaluated. Samples were ground and polished to four different metallographic conditions and were labeled as follows: the first letter refers to the fatty acid used in the reaction bath (L, for lauric acid; M, for myristic acid), the following digit is the acid concentration in g/L and finally, the “D” number indicates
Samples were immersed in the fatty acid solution after being prepared using the procedure outlined above. One factor that promotes superhydrophobicity can be attributed to the roughness of the surface on which superhydrophobicity will appear. In order to study this factor in aluminum, the four different surface conditions described above (D1, D2, D3, and D4) were evaluated with the same reaction bath conditions (20 g/L lauric acid in a (30/70) HCl/ethanol solution at 50 ◦ C for 10 min) to study only
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Fig. 2. SEM images from top to bottom: (A) column, as-polished, in different preparation conditions: D1, D2, D3 and D4 samples, (B) column, aluminum substrates with D1, D2, D3 and D4 initial condition immersed into (30:70) HCl/ethanol bath for 10 min at 50 ◦ C (D1 HCl, D2 HCl, D3 HCl and D4 HCl), (C) column, coated samples with 20 g/L lauric acid in ethanol/HCl bath, L20D1, L20D2, L20D3 and L20D4 samples. Measured RMS roughness is included.
the influence of the pretreatment. A set of samples labeled L20D1, L20D2, L20D3 and L20D4 were obtained after the coating step. Another set of samples with these four surface conditions was evaluated with the HCl/ethanol bath at 50 ◦ C for 10 min, in order to observe only the effect of the HCl reaction on the aluminum surfaces. SEM-micrographs of the differently treated surfaces were characterized and are shown in Fig. 2, from top to bottom: (A) column, aluminum samples with different surface conditions, D1, D2, D3 and D4 samples; (B) column, aluminum substrates with D1, D2, D3 and D4 initial condition immersed into (30:70) HCl/ethanol bath for 10 min at 50 ◦ C (D1 HCl, D2 HCl, D3 HCl and D4 HCl); and finally (C) column, coated samples with 20 g/L lauric acid in ethanol/HCl bath (L20D1, L20D2, L20D3 and L20D4). Confocal
microscopy was used to measure the roughness as RMS before and after coating; the values are included in Fig. 2. Parallel stripes of different thickness are showed in the D1, D2 and D3 surface images according to the metallographic conditions performed. However, D4 has stripes in different directions. In contrast, the surfaces of L20D1, L20D2, and L20D3 are fully covered by terraced features, even inside the pores; the L20D2 surface has larger cavities or pores. Wu and Liang [36] obtained similar results with a different HCl treatment. Sample L20D4 had a homogeneous appearance; the terraced growth seems less ordered on L20D4 than on L20D1, L20D2 and L20D3. The roughness measured by confocal microscope is included in Fig. 2, the highest RMS value of an uncoated sample is for D1 (0.46 m); decreasing RMS is obtained
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Fig. 3. Different wettability on aluminum samples: D2 sample: hydrophilic (left); D2 HCl sample: hydrophilic (center) and L20D2 sample: hydrophobic (right).
with increasing D number: the lowest being for D4 (0.04 m). After being treated with the lauric acidic bath, the RMS values showed a different tendency from those for the uncoated samples. L20D1 and L20D3 showed similar RMS values (4.56 m and 4.36 m, respectively). Roughness is also similar for L20D2 and L20D4 (3.13 m and 3.34 m, respectively). As HCl will produce terraced features on treated surfaces and carboxylic acid also grows again producing terraced features, the initial tendency is modified. The measurement of the contact angle (CA) of the samples resulted in hydrophobic behavior only on L20D2; so the surface treatment of this sample was considered the best and the rest of the samples to be studies were produced using the D2 surface treatment. Different wettability of the D2, D2 HCl and L20D2 surfaces is shown in Fig. 3 in order to visualize that hydrophobicity is only obtained when the surface is coated with the carboxylic acid. D2 and D2 HCl surfaces show hydrophilic behavior. The grinding and polishing carried out on the Al surface of the samples produces a particular surface activation that favors the later terraced growth. This growth is seen to be parallel to the surface for L20D1 and L20D3; and perpendicular to the surface in
L20D4, whereas mixed growth appears in L20D2. The mixed growth seems to be more efficient in leading to the superhydrophobicity of the surface. The growth perpendicular to the surface promotes greater roughness on L20D4 than the parallel growth observed on the surface of L20D1 and L20D3, but the mixed growth and the pores on L20D2 produced the highest contact angle. FE-SEM micrographs showed the surface morphology of the samples treated with different fatty acid concentrations (Fig. 4); as explained, all of them now in the D2 initial condition. Small pores and faceted features are clearly visible in all the samples. It is noteworthy that many micropores are distributed among the terraced structures, so that a large fraction of air can be trapped in them. This is essential for the formation of a solid–liquid–air composite interface leading to superhydrophobicity. Differences between the samples treated with different lauric acid concentrations were studied. The surface of the specimen L30D2 (immersed in 30 g/L lauric acid solution) was completely covered by terraces and showed a large amount of tiny pores. This morphology is very similar to that observed on the L20D2 and M20D2 specimens for which the mixed parallel and
Fig. 4. FE-SEM micrographs showing the terraced features of the final compound on aluminum specimens coated with 20 g/L myristic acid solution with D2 initial condition (top left) and at different concentrations of lauric acid with D2 initial condition (top right = 10 g/L, bottom left = 15 g/L and bottom right = 30 g/L).
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Fig. 5. AFM images of (A) aluminum substrate with D2 condition, (B) M1D2, 20 g/L myristic acid, (C) L30D2, 30 g/L lauric acid, (D) L20D2, 20 g/L lauric acid, (E) L15D2, 15 g/L lauric acid, and (F) L10D2, 10 g/L lauric acid.
perpendicular growth was detected, as mentioned in Section 2, myristic acid was studied only in one condition (D2). However, the surface features of the specimen treated with 15 g/L lauric acid showed overlapping terraces in a disordered arrangement. Decreasing the concentration of lauric acid (10 g/L) produced a morphology similar to that observed for the L30D2 specimen (30 g/L lauric acid) but with smaller terraces. AFM images (3 m × 3 m area) were produced to compare the final roughness of the coated specimen together with the aluminum substrate using RMS roughness as a comparison parameter. The results are shown in Fig. 5, in order to identify each AFM image with its corresponding sample, consecutive letters have been assigned to each of the images: the aluminum substrate D2 was named A; the sample treated with 20 g/L myristic acid was named B; the image named C is of the L30D2 sample (lauric acid) and the rest of the specimens have been assigned in the following row in descending order of the amount of lauric acid. The surface roughness resulting from the AFM analysis (from here on AFMRMS) of the myristic acid coated sample (B), 96 ± 11 nm does not present significant changes with respect to the uncoated substrate (A, uncoated sample), 104 ± 12 nm. Samples (C) and (D) have AFMRMS of 300 ± 37 nm and 247 ± 17 nm, respectively. Samples (E) and (F) are both in the range of the highest AFM-RMS values, 490 ± 90 and 383 ± 116 nm. It is important to emphasize the large standard deviation of the L15D2 and L10D2 samples because they are
more heterogeneous than L20D2; these heterogeneous surfaces do not allow roughness to be correlated with lauric acid concentration. However, the best superhydrophobicity result was obtained by combining low roughness and a large amount of lauric acid. AFM roughness reveals the relationship between a large number of C C chains with lower roughness as compared with the same quantity of organic acid (247 nm, L20D2 and 96 nm, M20D2) but they have similar static contact angle. The static contact angle was measured according to the Wenzel and Cassie–Baxter models. The model developed by Wenzel is described in Eq. (1) [30], where r is the roughness factor (for a rough surface r > 1), W is the apparent contact angle on a rough surface, and Y is the ideal contact angle on a smooth surface of identical chemistry: cos W = r × cos Y
(1)
The apparent contact angle in the approach developed by Cassie–Baxter [31] given by Eq. (2), is the sum of all the contributions of the different phases, in which f is the surface fraction, since f = 1 − f2 , where f2 is the area fraction of the air on the surface. cos W = f × (1 + cos Y ) − 1
(2)
Static contact angle was measured on the samples using 5 l of deionized water and the results are summarized in Table 1. According to results calculated using Eq. (1), the Wenzel approach is not
Table 1 Measured contact angle, measured roughness (AFM-RMS) and theoretical model factor. The samples are M1D2 = 20 g/L myristic acid, L30D2 = 30 g/L lauric acid, L20D2 = 20 g/L lauric acid, L15D2 = 15 g/L lauric acid, L10D2 = 10 g/L lauric acid. Sample Substrate D2 M1D2 L30D2 L20D2 L15D2 L10D2
Contact angle 40 149 153 146 128 143.9
± ± ± ± ± ±
◦
2 2◦ 3◦ 2◦ 2◦ 0.6◦
AFM-RMS (nm) 104 96 300 247 490 383
± ± ± ± ± ±
12 11 37 17 90 116
f (Cassie–Baxter)
Superhydrophobicity
0.08 0.06 0.1 0.22 0.11
No Yes Yes No No No
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Fig. 6. Secondary ion mass spectra of positive ions from an L30D2 surface (30 g/L lauric acid).
valid [37]. The relationship between the surface wettability and the surface can be well described by Cassie–Baxter equation (Eq. (2)). The surface fraction values are also included in Table 1. Sample L30D2 is in contact with 6% contact of the deionized water drop and the remaining 94% is in contact with air; therefore the contact angle is the highest and the L30D2 sample should be considered superhydrophobic because the contact angle is greater than 150◦ [3,4]. In contrast, the L15D2 sample has the lowest contact angle because the solid fraction showed a contact value of 22% of the droplet area. The rest of the samples showed similar contact angles. A chemical reaction will be associated with each part of the process to produce the change to superhydrophobicity of the aluminum surface. HCl provided the acid medium was so the oxide layer removal process could be improved with additional chemical effects. Cl− ions from the HCl/ethanol solution may partially dissolve the protective aluminum oxide film, but once the aluminum surface is exposed to the etchant a redox reaction would take place (Scheme 1):
Scheme 1.
Redox reaction:
2Almetal + 6HCl → 2AlCl3 + 3H2 It is worth noting too that the HCl is an acid that preferentially etches the dislocation sites of an aluminum surface [38]. The Al3+ ions released will in turn combine with myristate or laureate anions present in the solution following Scheme 2: Scheme 2. reaction.
Aluminum ions combine with fatty acids; addition
Al3+ + 2CH3 (CH2 )n COO− → [CH3 (CH2 )n COO]2 Al-O− where n = 10 (lauric acid) or n = 12 (myristic acid). Secondary ion mass spectrometry was used to study the chemical reactions and lateral distributions of fatty acids enhanced to react on commercial aluminum surfaces. On Al surfaces, a direct interaction with the acid and the surface aluminum oxide is observed by the detection of a molecular ion that corresponds
Fig. 7. XPS spectrum of the L30D2 sample (30 g/L lauric acid) within high-resolution spectra of C 1s (right) and O 1s (left) for the substrate and the L30D2 sample.
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Fig. 8. Snapshots of a 10 l droplet of deionized water: first, growth of the drop (top left); then, drop falls on surface (top right); the droplet bounces on the surface (bottom left); and finally, the droplet is suspended on the surface (bottom right).
to the mass of the laureate anion and AlO− ; the latter characteristic of aluminum oxide. Since TOF-SIMS can detect and discriminate specific chemical reaction products, it was used to identify reaction products formed between fatty acid molecules and metallic aluminum surfaces. The positive secondary ion mass spectra of the L30D2 samples (Fig. 6) show peaks attributable to the presence of lauric (or myristic) ions on the Al surface. The spectrum contains a peak that is due to the formation of a fatty acid union with aluminum, to which more molecules of the former are then bonded. The peaks represent mass of: 349 units, which corresponds to NaCH(CH2 )10 COO-Al (Al2 O3 ), and 451 units, for NaCH((CH2 )10 COO-Al (Al2 O3 ))2 and also NaCH(CH2 )7 COO-Al (Al2 O3 ))2 . (Na is considered a contaminant that is easily detected with SIMS.) The presences of these species on the Al surface was also confirmed by the XPS Al 2p and O 2s signals. The alumina is due to the action of the HCl which partially dissolved the aluminum oxide. In addition, as H2 is produced, the reducing medium impedes the formation of further aluminum oxide via oxidation of metallic aluminum. The XPS and TOF-SIMS analyses are used to obtain the surface composition of the samples. XPS analysis of the L30D2 specimen (30 g/L lauric acid and superfine grinding) is shown in Fig. 7 with inserted high-resolution spectra of C 1s (right) and O 1s (left) for the substrate and the L30D2 sample, where the peaks corresponding to Al, C, O and Cl are shown. The peaks located at 76.0 and 120.8 eV are attributed to the Al 2p of Al O and Al Al bonds and to the Al 2s of Al Cl bonds, respectively. The identification of the Al 2p peak comes from the deconvolution of the 76.0 eV peak, which gives rise to two components: 76.5 and 73.7 eV. They can be assigned to oxidized aluminum Al O bonds and to metallic aluminum Al Al bonds, respectively. The peak located at 534.4 eV is assigned to the O 1s (carbonyl of the lauric and myristic acids [39]). Deconvolution of this 534.4 eV peak gives rise to three components: 534.6 and 533.4 eV assigned to C O bonds in an ester group; and 532.0 eV assigned to Al O bonds. The peak at 287.3 eV corresponds to C 1s of the C O bonds in an ester molecule [40,41] and finally, the O 2s peak at 27.5 eV is attributed
to O Al bonds in AlO(OH). High resolution XPS spectra of C 1s (right) and O 1s (left) for the substrate and the L30D2 sample have been inserted into Fig. 7. The C1s spectrum of the substrate is small due to the low number of counts/s and can mainly be considered as contamination of XPS analysis and ethanol cleaning, as opposed to the coated sample (L30D2) in which the carbon comes from the chemical reaction. The remnant oxygen of the substrate belongs to the alumina as confirmed by TOF-SIMS analysis. The XPS spectra of the aluminum coated samples show the peaks attributable to the presence of AlCl3 and CH3 (CH2 )10 COO-Al-O-OH. Miller et al. [42] detected a molecular ion corresponding to the mass of dodecylamine and AlO , characteristic of aluminum oxide, when organic molecules with alcohol, ester and amine functional groups undergo specific chemical reactions with oxidized Al–Mg alloy surfaces. Summarizing, the M1D2 and L30D2 samples (surfaces treated with 20 g/L myristic acid and 30 g/L lauric acid, respectively, and with the same surface preparation before immersion), presented superhydrophobic surfaces. In their research, Zhang and Wu [1] found superhydrophobicity using myristic acid and longer-chain fatty acids on Al–Mg alloy. In the present research, lauric acid (a shorter-chain fatty acid) concentration and surface preparation were optimized; the best results were obtained when using 30 g/L of the lauric acid on a surface prepared in a specific way, thereby resulting in superhydrophobicity due to the mixed growth of a terraced forming coating. 4. Discussion In the present study, the best initial surface roughness is that showing a confocal-RMS value of 0.22 m (D2) because when coated, this surface was full of pores and it had mixed terraced growth, showing the highest contact angle (153◦ ). Final roughness does not follow that of the initial surfaces, due to many factors: firstly, natural alumina has been partially destroyed by HCl with a slow reaction [43], in contrast, the removal of aluminum substrate is a very fast reaction and coating is a competitive reaction for both
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them; and finally, different initial sample roughness promotes various growth situations, as also reflected in the research by Aurongzeb et al. [44] and Montero et al. [45]. Successive metallographic treatments reduce surface roughness, consequently, they promote differential HCl etching of the aluminum surface and also, perpendicular terraced growth of the coating as seen on the L20D4 sample. A different situation is that of the L20D1 sample and the L20D3 sample, with growth parallel to the surface. L20D3 is rougher than L20D2, as seen in the SEM images (Fig. 2). L20D4 is similar to L20D2 in roughness, but the L20D4 sample does not have large pores; this is because HCl mainly attacks aluminum dislocation sites [38], sites left unprotected because alumina has been removed in D4 samples. Specimens treated with 30 and 10 g/L lauric acid (L30D2 and L10D2, respectively) showed a surface morphology with highly ordered terraces occupying the interior of the pores and the entire surface evenly. This is very noteworthy as the pores are essential for the formation of a solid–liquid–air composite interface. The contact angle measurements allowed us to optimize the lauric acid concentration and ensuring a superhydrophobic pure aluminum surface. The larger contact angle on the coated surface was shown by the L30D2 surface; consequently, it has a smaller surface energy than the aluminum substrate and these specimens presented a small solid-water contact area (6% fraction). This procedure can be carried out for multiple coating processes. The following images (Fig. 8) show a sequence of photographs of the contact between a water droplet and the superhydrophobic surface of a sample treated with 30 g/L of lauric acid. The results obtained by XPS showed the presence of AlCl3 and CH3 (CH2 )10 COO)-AlO; the Al O bond of aluminum oxide or aluminum hydroxide can promote the latter. This chemical product was also observed by Miller et al. [42] in research on aluminum–magnesium alloys using SIMS equipment. In the present study, our TOF-SIMS results let us confirm the bonding between fatty acids and Al, as well as the bonding of alumina to the Al-fatty acid previously formed. The latter is the result of the partial etching of the oxide by the HCl. From these findings, in order to ensure superhydrophobicity, different factors are needed: on the one hand, metallic aluminum, aluminum oxide and fatty acid in an acidic and reducing medium; and, on the other hand, an activated surface on which the reaction products can grow in parallel and the perpendicular terraces. The complexity of this product growth decreases the contact area between the surface and the liquid enhancing superhydrophobicity. 5. Conclusions In conclusion, superhydrophobic surfaces on commercial pure (99.9 wt%) aluminum are developed via a very simple procedure consisting of immersing a cleaned and microgrooved metal surface in the selected carboxylic/chloride/ethanol bath. SEM analysis confirms the creation of three types of terraced reaction products: parallel to the surface, perpendicular to the surface and a mixture of both types. Superhydrophobic aluminum surfaces are obtained when mixed growth of the covering compounds takes place. TOFSIMS and XPS analysis allowed the identification of the species responsible of the superhydrophobicity on the pure aluminum substrate. The compounds identified are CH3 (CH2 )10 COOAl(Al2 O3 ) and (CH3 (CH2 )10 COO)2 Al(Al2 O3 ). These corroborate the need for metal and metal oxide to be present in order for the reaction with the carboxylic acid to proceed and to ensure superhydrophobicity. Finally, the use of TOF-SIMS combined with XPS analysis can be extended to provide a better understanding of the interactions of fatty acids and metal surfaces.
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Superhydrophobic coating deposited directly on aluminum Ana M. Escobar ∗ , Nuria Llorca-Isern CPCM, Departament de Ciència dels Materials i Enginyeria Metal·lurgica, Facultat de Química, Universitat de Barcelona, Marti-Franqués 1, 08028 Barcelona, Spain
a r t i c l e
i n f o
Article history: Received 18 June 2013 Received in revised form 6 March 2014 Accepted 31 March 2014 Available online 12 April 2014 Keywords: Superhydrophobicity Aluminum Lauric and myristic acid Simple process Coating Water contact angle
a b s t r a c t This study develops an alternative method for enhancing superhydrophobicity on aluminum surfaces with an amphiphilic reagent such as the dodecanoic acid. The goal is to induce superhydrophobicity directly through a simple process on pure (99.9 wt%) commercial aluminum. The initial surface activation leading to the formation of the superhydrophobic coating is studied using confocal microscopy. Superhydrophobic behavior is analyzed by contact angle measurements, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The highest contact angle (approaching 153◦ ) was obtained after forming hierarchical structures with a particular roughness obtained by grinding and polishing microgrooves on the aluminum surface together with the simultaneous action of HCl and dodecanoic acid. The results also showed that after immersion in the ethanol-acidic-fatty acid solutions, they reacted chemically through the action of the fatty acid, on the aluminum surface. The mechanism is analyzed by TOF-SIMS and XPS in order to determine the molecules involved in the reaction. The TOF-SIMS analysis revealed that the metal and its oxides seem to be necessary, and that free-aluminum is anchored to the fatty acid molecules and to the alumina molecules present in the medium. Consequently, both metallic aluminum and aluminum oxides are necessary in order to form the compound responsible for superhydrophobicity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surfaces have received considerable attention from researchers because the surface wettability of materials is a very important property that affects the final applications of products containing those materials [1,2]. Superhydrophobic surfaces show very high water contact angles (larger than 150◦ ) [3–5] and these surfaces are also of special interest because they have anti-adhesive properties and self-cleaning effects. They can be very useful in the aeronautical industry [6] and civil engineering [7]. Traditionally, superhydrophobic surfaces are obtained via multistep processes [8–25]: a certain rough pattern is first created by different processes such as chemical deposition [26], a sol–gel method or electrodeposition on an aluminum plate with further use of an immersion bath [27] to provide a prepared surface (clean and polished, etc.). In all these processes the first step is to roughen the surface, which is followed by its activation (in different steps) using fatty acids or inorganic salts [1]. In single-step processes, substrates with an initial roughness require a certain final roughness for superhydrophobicity to be produced in a single reaction. Saleema [28] produced superhydrophobic aluminum alloys in a basic bath; while
∗ Corresponding author. Tel.: +34 934021299. E-mail address: [email protected] (A.M. Escobar). http://dx.doi.org/10.1016/j.apsusc.2014.03.196 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Peng’s work [29] is based on initial electrolytic anodizing so that the alumina formed induces superhydrophobicity. In the present study the process is carried out in an electroless immersion bath and superhydrophobicity results from a chemical reaction. One of the goals of this study is to identify the growth step actors using TOFSIMS and XPS techniques, so close attention was paid to the analysis of the mechanism of the chemical reaction. Two main factors seem to characterize the behavior: the contact angle and the existence of pores at the interface. According to Brushan and Jung [4], the contact angle measured for hydrophobic surfaces with microstructures, nanostructures and hierarchical structures increases if the roughness of the surface increases. Another important factor is the existence of pores that hold air and ensure that water droplets do not come into contact with the solid surface. Two main models describe the possible wetting regimes: the homogenous Wenzel model [30], which predicts that a water droplet can penetrate the asperities; and the composite Cassie–Baxter model [31], in which a droplet may be suspended above the asperities since a gaseous phase may be trapped in the cavities of a rough surface (Fig. 1) [32]. The hydrophobicity of ceramic materials has been widely studied, whereas metallic surfaces have not been studied much with the exception of copper and aluminum. Both, multistep and singlestep processes have been proposed [28–32,34]. Aluminum surfaces are always covered by a thin protective aluminum oxide film (passivation film). Saleema et al. [35] used this alumina film (in
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Fig. 1. Wetting regimes on rough surfaces: (A) Cassie–Baxter heterogeneous wetting on a hydrophobic surface; (B) Wenzel wetting; (C) Cassie–Baxter on a hydrophilic surface; and (D) mixed wetting [33].
a basic medium) to produce a fluoroalkyl-silane compound and claimed that this combination was responsible for superhydrophobic behavior. In the present study, a simple method was used to prepare superhydrophobic surfaces directly on metallic aluminum using organic acids, by combining a facile acid etching with a polymer coating in a single reagent bath. The reaction requires an acid medium, provided by HCl which is used here to remove the oxide layer partially. The acid also ensures that the roughness produced in preparation is not lost, which facilitates the entrapment of air. In addition, the present study had three main objectives: to identify the molecules responsible for superhydrophobicity, the mechanism by which superhydrophobicity is produced, and consequently the influence of variables such as reactant composition and substrate surface. Preparation of the substrate surface is sometimes not considered an independent step (Ramadhianti et al. [21] and Song et al. [24]) although any surface treatment needs a previously prepared surface (e.g. it must be clean, degreased, etc.).
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the surface preparation: D1 corresponds to grinding with SiC abrasive paper from 63 m average particle diameter (P220 grade) to (P600 grade) “extra fine” 35.8 m average particle diameter; D2 stands for grinding as for D1, followed by a “super fine” grinding with 15.3 m mean particle diameter SiC paper (P1200 grade); D3 corresponds to “fine” grinding as for D2, followed by fine polishing with a 6 m diamond polishing suspension (Buehler, USA); and finally, D4 are samples to which ultrafine polishing with a 1 m diamond polishing suspension (Buehler, USA) followed the D3 procedure. Before and between this preparation, the samples were ultrasonically cleaned with ethanol for 10 min followed by deionized water for another 10 min. Finally, they were dried with forced air. The cleaned samples were then immersed vertically into a fresh ethanol solution of hydrochloric acid (30%) and lauric acid (20 g/L). The etching time and temperature for all the samples were 10 min and 50 ◦ C. After etching, the samples were immediately rinsed with ethanol and deionized water, then left in an oven at 80 ◦ C for 30 min. Another set of samples was prepared to optimize the amount of organic acid necessary to produce superhydrophobicity on samples with the optimized condition D2. Electron microscopy characterization of the specimen surface was carried out with an SEM Leica S-360 and an FE-SEM Hitachi S-4100 in order to study their morphological features. AFM measurements were performed using a Dimension 3100 microscope attached to Nanoscope IV electronics (Bruker) using Si probes with a nominal spring constant of 40 nN/nm (T300R-W, Vistaprobes). All topographic measurements were performed in tapping mode at a scan rate of 0.3 Hz and 512 × 512 pixels resolution. Samples were imaged in air and at 55% RH. The stability of the superhydrophobicity was studied over time at room temperature. The initial roughness measurements were performed by confocal microscopy using LeicaScan DCM 3D on a surface of 1.21 mm × 0.91 mm. The chemical composition of the surface was analyzed by X-ray photoelectron spectroscopy in a PHI 5500 Multitechnique System (Physical Electronics) with a monochromatic X-ray source (Aluminum K␣ line of 1486.6 eV energy and 350 W) on 0.8 mm diameter areas. All measurements were made in an ultra-high vacuum (UHV) chamber pressure between 5 × 10−9 and 2 × 10−8 Torr. The carbon 1s line was to calibrate the bindingenergy scale for XPS measurements, for which a binding energy of 284.8 eV was assumed. High-resolution mass spectra of positive and negative secondary ions were obtained using TOF-SIMS IV (ION-TOF, Munster, Germany) equipment operated at a pressure of 5 × 10−9 mbar and a 25 keV pulsed Bismuth liquid-metal ion source (Bi3 ++ ). Secondary ions were detected with a reflector TOF analyzer, multichannel plates (MCPs), and a time-to-digital converter (TDC). Measurements were performed with a typical acquisition time of 10 s, at a TDC time resolution of 200 ps and 100 s cycle time. Secondary ion spectra were acquired from randomly rastered sample surface areas of 100 m × 100 m. Mass spectral acquisition was performed using the ION-TOF Ion Spec software (version 4.1). Finally, contact angles were measured with a Contact Angle Measuring System DSA 100 from KRÜSS with 5 L of deionized water at room temperature.
2. Materials and methods 3. Results Commercial pure (99.9 wt%) aluminum 8 mm × 15 mm × 10 mm substrates were used for this study. As an initial study, different metallographic grinding and polishing conditions were applied to the substrates to produce a specific roughness; its influence on the coating step was then evaluated. Samples were ground and polished to four different metallographic conditions and were labeled as follows: the first letter refers to the fatty acid used in the reaction bath (L, for lauric acid; M, for myristic acid), the following digit is the acid concentration in g/L and finally, the “D” number indicates
Samples were immersed in the fatty acid solution after being prepared using the procedure outlined above. One factor that promotes superhydrophobicity can be attributed to the roughness of the surface on which superhydrophobicity will appear. In order to study this factor in aluminum, the four different surface conditions described above (D1, D2, D3, and D4) were evaluated with the same reaction bath conditions (20 g/L lauric acid in a (30/70) HCl/ethanol solution at 50 ◦ C for 10 min) to study only
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Fig. 2. SEM images from top to bottom: (A) column, as-polished, in different preparation conditions: D1, D2, D3 and D4 samples, (B) column, aluminum substrates with D1, D2, D3 and D4 initial condition immersed into (30:70) HCl/ethanol bath for 10 min at 50 ◦ C (D1 HCl, D2 HCl, D3 HCl and D4 HCl), (C) column, coated samples with 20 g/L lauric acid in ethanol/HCl bath, L20D1, L20D2, L20D3 and L20D4 samples. Measured RMS roughness is included.
the influence of the pretreatment. A set of samples labeled L20D1, L20D2, L20D3 and L20D4 were obtained after the coating step. Another set of samples with these four surface conditions was evaluated with the HCl/ethanol bath at 50 ◦ C for 10 min, in order to observe only the effect of the HCl reaction on the aluminum surfaces. SEM-micrographs of the differently treated surfaces were characterized and are shown in Fig. 2, from top to bottom: (A) column, aluminum samples with different surface conditions, D1, D2, D3 and D4 samples; (B) column, aluminum substrates with D1, D2, D3 and D4 initial condition immersed into (30:70) HCl/ethanol bath for 10 min at 50 ◦ C (D1 HCl, D2 HCl, D3 HCl and D4 HCl); and finally (C) column, coated samples with 20 g/L lauric acid in ethanol/HCl bath (L20D1, L20D2, L20D3 and L20D4). Confocal
microscopy was used to measure the roughness as RMS before and after coating; the values are included in Fig. 2. Parallel stripes of different thickness are showed in the D1, D2 and D3 surface images according to the metallographic conditions performed. However, D4 has stripes in different directions. In contrast, the surfaces of L20D1, L20D2, and L20D3 are fully covered by terraced features, even inside the pores; the L20D2 surface has larger cavities or pores. Wu and Liang [36] obtained similar results with a different HCl treatment. Sample L20D4 had a homogeneous appearance; the terraced growth seems less ordered on L20D4 than on L20D1, L20D2 and L20D3. The roughness measured by confocal microscope is included in Fig. 2, the highest RMS value of an uncoated sample is for D1 (0.46 m); decreasing RMS is obtained
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Fig. 3. Different wettability on aluminum samples: D2 sample: hydrophilic (left); D2 HCl sample: hydrophilic (center) and L20D2 sample: hydrophobic (right).
with increasing D number: the lowest being for D4 (0.04 m). After being treated with the lauric acidic bath, the RMS values showed a different tendency from those for the uncoated samples. L20D1 and L20D3 showed similar RMS values (4.56 m and 4.36 m, respectively). Roughness is also similar for L20D2 and L20D4 (3.13 m and 3.34 m, respectively). As HCl will produce terraced features on treated surfaces and carboxylic acid also grows again producing terraced features, the initial tendency is modified. The measurement of the contact angle (CA) of the samples resulted in hydrophobic behavior only on L20D2; so the surface treatment of this sample was considered the best and the rest of the samples to be studies were produced using the D2 surface treatment. Different wettability of the D2, D2 HCl and L20D2 surfaces is shown in Fig. 3 in order to visualize that hydrophobicity is only obtained when the surface is coated with the carboxylic acid. D2 and D2 HCl surfaces show hydrophilic behavior. The grinding and polishing carried out on the Al surface of the samples produces a particular surface activation that favors the later terraced growth. This growth is seen to be parallel to the surface for L20D1 and L20D3; and perpendicular to the surface in
L20D4, whereas mixed growth appears in L20D2. The mixed growth seems to be more efficient in leading to the superhydrophobicity of the surface. The growth perpendicular to the surface promotes greater roughness on L20D4 than the parallel growth observed on the surface of L20D1 and L20D3, but the mixed growth and the pores on L20D2 produced the highest contact angle. FE-SEM micrographs showed the surface morphology of the samples treated with different fatty acid concentrations (Fig. 4); as explained, all of them now in the D2 initial condition. Small pores and faceted features are clearly visible in all the samples. It is noteworthy that many micropores are distributed among the terraced structures, so that a large fraction of air can be trapped in them. This is essential for the formation of a solid–liquid–air composite interface leading to superhydrophobicity. Differences between the samples treated with different lauric acid concentrations were studied. The surface of the specimen L30D2 (immersed in 30 g/L lauric acid solution) was completely covered by terraces and showed a large amount of tiny pores. This morphology is very similar to that observed on the L20D2 and M20D2 specimens for which the mixed parallel and
Fig. 4. FE-SEM micrographs showing the terraced features of the final compound on aluminum specimens coated with 20 g/L myristic acid solution with D2 initial condition (top left) and at different concentrations of lauric acid with D2 initial condition (top right = 10 g/L, bottom left = 15 g/L and bottom right = 30 g/L).
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Fig. 5. AFM images of (A) aluminum substrate with D2 condition, (B) M1D2, 20 g/L myristic acid, (C) L30D2, 30 g/L lauric acid, (D) L20D2, 20 g/L lauric acid, (E) L15D2, 15 g/L lauric acid, and (F) L10D2, 10 g/L lauric acid.
perpendicular growth was detected, as mentioned in Section 2, myristic acid was studied only in one condition (D2). However, the surface features of the specimen treated with 15 g/L lauric acid showed overlapping terraces in a disordered arrangement. Decreasing the concentration of lauric acid (10 g/L) produced a morphology similar to that observed for the L30D2 specimen (30 g/L lauric acid) but with smaller terraces. AFM images (3 m × 3 m area) were produced to compare the final roughness of the coated specimen together with the aluminum substrate using RMS roughness as a comparison parameter. The results are shown in Fig. 5, in order to identify each AFM image with its corresponding sample, consecutive letters have been assigned to each of the images: the aluminum substrate D2 was named A; the sample treated with 20 g/L myristic acid was named B; the image named C is of the L30D2 sample (lauric acid) and the rest of the specimens have been assigned in the following row in descending order of the amount of lauric acid. The surface roughness resulting from the AFM analysis (from here on AFMRMS) of the myristic acid coated sample (B), 96 ± 11 nm does not present significant changes with respect to the uncoated substrate (A, uncoated sample), 104 ± 12 nm. Samples (C) and (D) have AFMRMS of 300 ± 37 nm and 247 ± 17 nm, respectively. Samples (E) and (F) are both in the range of the highest AFM-RMS values, 490 ± 90 and 383 ± 116 nm. It is important to emphasize the large standard deviation of the L15D2 and L10D2 samples because they are
more heterogeneous than L20D2; these heterogeneous surfaces do not allow roughness to be correlated with lauric acid concentration. However, the best superhydrophobicity result was obtained by combining low roughness and a large amount of lauric acid. AFM roughness reveals the relationship between a large number of C C chains with lower roughness as compared with the same quantity of organic acid (247 nm, L20D2 and 96 nm, M20D2) but they have similar static contact angle. The static contact angle was measured according to the Wenzel and Cassie–Baxter models. The model developed by Wenzel is described in Eq. (1) [30], where r is the roughness factor (for a rough surface r > 1), W is the apparent contact angle on a rough surface, and Y is the ideal contact angle on a smooth surface of identical chemistry: cos W = r × cos Y
(1)
The apparent contact angle in the approach developed by Cassie–Baxter [31] given by Eq. (2), is the sum of all the contributions of the different phases, in which f is the surface fraction, since f = 1 − f2 , where f2 is the area fraction of the air on the surface. cos W = f × (1 + cos Y ) − 1
(2)
Static contact angle was measured on the samples using 5 l of deionized water and the results are summarized in Table 1. According to results calculated using Eq. (1), the Wenzel approach is not
Table 1 Measured contact angle, measured roughness (AFM-RMS) and theoretical model factor. The samples are M1D2 = 20 g/L myristic acid, L30D2 = 30 g/L lauric acid, L20D2 = 20 g/L lauric acid, L15D2 = 15 g/L lauric acid, L10D2 = 10 g/L lauric acid. Sample Substrate D2 M1D2 L30D2 L20D2 L15D2 L10D2
Contact angle 40 149 153 146 128 143.9
± ± ± ± ± ±
◦
2 2◦ 3◦ 2◦ 2◦ 0.6◦
AFM-RMS (nm) 104 96 300 247 490 383
± ± ± ± ± ±
12 11 37 17 90 116
f (Cassie–Baxter)
Superhydrophobicity
0.08 0.06 0.1 0.22 0.11
No Yes Yes No No No
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Fig. 6. Secondary ion mass spectra of positive ions from an L30D2 surface (30 g/L lauric acid).
valid [37]. The relationship between the surface wettability and the surface can be well described by Cassie–Baxter equation (Eq. (2)). The surface fraction values are also included in Table 1. Sample L30D2 is in contact with 6% contact of the deionized water drop and the remaining 94% is in contact with air; therefore the contact angle is the highest and the L30D2 sample should be considered superhydrophobic because the contact angle is greater than 150◦ [3,4]. In contrast, the L15D2 sample has the lowest contact angle because the solid fraction showed a contact value of 22% of the droplet area. The rest of the samples showed similar contact angles. A chemical reaction will be associated with each part of the process to produce the change to superhydrophobicity of the aluminum surface. HCl provided the acid medium was so the oxide layer removal process could be improved with additional chemical effects. Cl− ions from the HCl/ethanol solution may partially dissolve the protective aluminum oxide film, but once the aluminum surface is exposed to the etchant a redox reaction would take place (Scheme 1):
Scheme 1.
Redox reaction:
2Almetal + 6HCl → 2AlCl3 + 3H2 It is worth noting too that the HCl is an acid that preferentially etches the dislocation sites of an aluminum surface [38]. The Al3+ ions released will in turn combine with myristate or laureate anions present in the solution following Scheme 2: Scheme 2. reaction.
Aluminum ions combine with fatty acids; addition
Al3+ + 2CH3 (CH2 )n COO− → [CH3 (CH2 )n COO]2 Al-O− where n = 10 (lauric acid) or n = 12 (myristic acid). Secondary ion mass spectrometry was used to study the chemical reactions and lateral distributions of fatty acids enhanced to react on commercial aluminum surfaces. On Al surfaces, a direct interaction with the acid and the surface aluminum oxide is observed by the detection of a molecular ion that corresponds
Fig. 7. XPS spectrum of the L30D2 sample (30 g/L lauric acid) within high-resolution spectra of C 1s (right) and O 1s (left) for the substrate and the L30D2 sample.
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Fig. 8. Snapshots of a 10 l droplet of deionized water: first, growth of the drop (top left); then, drop falls on surface (top right); the droplet bounces on the surface (bottom left); and finally, the droplet is suspended on the surface (bottom right).
to the mass of the laureate anion and AlO− ; the latter characteristic of aluminum oxide. Since TOF-SIMS can detect and discriminate specific chemical reaction products, it was used to identify reaction products formed between fatty acid molecules and metallic aluminum surfaces. The positive secondary ion mass spectra of the L30D2 samples (Fig. 6) show peaks attributable to the presence of lauric (or myristic) ions on the Al surface. The spectrum contains a peak that is due to the formation of a fatty acid union with aluminum, to which more molecules of the former are then bonded. The peaks represent mass of: 349 units, which corresponds to NaCH(CH2 )10 COO-Al (Al2 O3 ), and 451 units, for NaCH((CH2 )10 COO-Al (Al2 O3 ))2 and also NaCH(CH2 )7 COO-Al (Al2 O3 ))2 . (Na is considered a contaminant that is easily detected with SIMS.) The presences of these species on the Al surface was also confirmed by the XPS Al 2p and O 2s signals. The alumina is due to the action of the HCl which partially dissolved the aluminum oxide. In addition, as H2 is produced, the reducing medium impedes the formation of further aluminum oxide via oxidation of metallic aluminum. The XPS and TOF-SIMS analyses are used to obtain the surface composition of the samples. XPS analysis of the L30D2 specimen (30 g/L lauric acid and superfine grinding) is shown in Fig. 7 with inserted high-resolution spectra of C 1s (right) and O 1s (left) for the substrate and the L30D2 sample, where the peaks corresponding to Al, C, O and Cl are shown. The peaks located at 76.0 and 120.8 eV are attributed to the Al 2p of Al O and Al Al bonds and to the Al 2s of Al Cl bonds, respectively. The identification of the Al 2p peak comes from the deconvolution of the 76.0 eV peak, which gives rise to two components: 76.5 and 73.7 eV. They can be assigned to oxidized aluminum Al O bonds and to metallic aluminum Al Al bonds, respectively. The peak located at 534.4 eV is assigned to the O 1s (carbonyl of the lauric and myristic acids [39]). Deconvolution of this 534.4 eV peak gives rise to three components: 534.6 and 533.4 eV assigned to C O bonds in an ester group; and 532.0 eV assigned to Al O bonds. The peak at 287.3 eV corresponds to C 1s of the C O bonds in an ester molecule [40,41] and finally, the O 2s peak at 27.5 eV is attributed
to O Al bonds in AlO(OH). High resolution XPS spectra of C 1s (right) and O 1s (left) for the substrate and the L30D2 sample have been inserted into Fig. 7. The C1s spectrum of the substrate is small due to the low number of counts/s and can mainly be considered as contamination of XPS analysis and ethanol cleaning, as opposed to the coated sample (L30D2) in which the carbon comes from the chemical reaction. The remnant oxygen of the substrate belongs to the alumina as confirmed by TOF-SIMS analysis. The XPS spectra of the aluminum coated samples show the peaks attributable to the presence of AlCl3 and CH3 (CH2 )10 COO-Al-O-OH. Miller et al. [42] detected a molecular ion corresponding to the mass of dodecylamine and AlO , characteristic of aluminum oxide, when organic molecules with alcohol, ester and amine functional groups undergo specific chemical reactions with oxidized Al–Mg alloy surfaces. Summarizing, the M1D2 and L30D2 samples (surfaces treated with 20 g/L myristic acid and 30 g/L lauric acid, respectively, and with the same surface preparation before immersion), presented superhydrophobic surfaces. In their research, Zhang and Wu [1] found superhydrophobicity using myristic acid and longer-chain fatty acids on Al–Mg alloy. In the present research, lauric acid (a shorter-chain fatty acid) concentration and surface preparation were optimized; the best results were obtained when using 30 g/L of the lauric acid on a surface prepared in a specific way, thereby resulting in superhydrophobicity due to the mixed growth of a terraced forming coating. 4. Discussion In the present study, the best initial surface roughness is that showing a confocal-RMS value of 0.22 m (D2) because when coated, this surface was full of pores and it had mixed terraced growth, showing the highest contact angle (153◦ ). Final roughness does not follow that of the initial surfaces, due to many factors: firstly, natural alumina has been partially destroyed by HCl with a slow reaction [43], in contrast, the removal of aluminum substrate is a very fast reaction and coating is a competitive reaction for both
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them; and finally, different initial sample roughness promotes various growth situations, as also reflected in the research by Aurongzeb et al. [44] and Montero et al. [45]. Successive metallographic treatments reduce surface roughness, consequently, they promote differential HCl etching of the aluminum surface and also, perpendicular terraced growth of the coating as seen on the L20D4 sample. A different situation is that of the L20D1 sample and the L20D3 sample, with growth parallel to the surface. L20D3 is rougher than L20D2, as seen in the SEM images (Fig. 2). L20D4 is similar to L20D2 in roughness, but the L20D4 sample does not have large pores; this is because HCl mainly attacks aluminum dislocation sites [38], sites left unprotected because alumina has been removed in D4 samples. Specimens treated with 30 and 10 g/L lauric acid (L30D2 and L10D2, respectively) showed a surface morphology with highly ordered terraces occupying the interior of the pores and the entire surface evenly. This is very noteworthy as the pores are essential for the formation of a solid–liquid–air composite interface. The contact angle measurements allowed us to optimize the lauric acid concentration and ensuring a superhydrophobic pure aluminum surface. The larger contact angle on the coated surface was shown by the L30D2 surface; consequently, it has a smaller surface energy than the aluminum substrate and these specimens presented a small solid-water contact area (6% fraction). This procedure can be carried out for multiple coating processes. The following images (Fig. 8) show a sequence of photographs of the contact between a water droplet and the superhydrophobic surface of a sample treated with 30 g/L of lauric acid. The results obtained by XPS showed the presence of AlCl3 and CH3 (CH2 )10 COO)-AlO; the Al O bond of aluminum oxide or aluminum hydroxide can promote the latter. This chemical product was also observed by Miller et al. [42] in research on aluminum–magnesium alloys using SIMS equipment. In the present study, our TOF-SIMS results let us confirm the bonding between fatty acids and Al, as well as the bonding of alumina to the Al-fatty acid previously formed. The latter is the result of the partial etching of the oxide by the HCl. From these findings, in order to ensure superhydrophobicity, different factors are needed: on the one hand, metallic aluminum, aluminum oxide and fatty acid in an acidic and reducing medium; and, on the other hand, an activated surface on which the reaction products can grow in parallel and the perpendicular terraces. The complexity of this product growth decreases the contact area between the surface and the liquid enhancing superhydrophobicity. 5. Conclusions In conclusion, superhydrophobic surfaces on commercial pure (99.9 wt%) aluminum are developed via a very simple procedure consisting of immersing a cleaned and microgrooved metal surface in the selected carboxylic/chloride/ethanol bath. SEM analysis confirms the creation of three types of terraced reaction products: parallel to the surface, perpendicular to the surface and a mixture of both types. Superhydrophobic aluminum surfaces are obtained when mixed growth of the covering compounds takes place. TOFSIMS and XPS analysis allowed the identification of the species responsible of the superhydrophobicity on the pure aluminum substrate. The compounds identified are CH3 (CH2 )10 COOAl(Al2 O3 ) and (CH3 (CH2 )10 COO)2 Al(Al2 O3 ). These corroborate the need for metal and metal oxide to be present in order for the reaction with the carboxylic acid to proceed and to ensure superhydrophobicity. Finally, the use of TOF-SIMS combined with XPS analysis can be extended to provide a better understanding of the interactions of fatty acids and metal surfaces.
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