Int. J. Appl. Ceram. Technol., 8 [4] 848–853 (2011) DOI:10.1111/j.1744-7402.2010.02516.x
Ceramic Product Development and Commercialization
Superhydrophobic Nanostructured Boehmite Coatings Prepared by AlN Powder Hydrolysis Andraz Kocjan,* Ales Dakskobler, and Tomaz Kosmac Engineering Ceramics Department, Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia
A simple method that exploits the hydrolysis of AlN powder was used to deposit a nanostructured boehmite coating onto a polished sintered alumina substrate. The coating consists of interconnected polycrystalline nanoporous lamellas and exhibits a large specific surface area. Heat treatment of the as-deposited coating at 5001C transformed it to g-alumina, and then to d-alumina at 9001C, with no substantial change in the morphology. However, the nanoporosity of the lamellas disappeared after the heat treatment at 9001C. After a subsequent chemical modification of the coatings with various low-energy-surface chemicals, that is, carboxylic acids, heptadecafluoro-1,1,2,2-tetrahydrodecyl trimethoxy-silane (FAS), the initially highly hydrophilic coatings were transformed into a hydrophobic ones. Carboxylic acids proved to be less-effective low-energy-surface chemicals because they produce hydrophobic surfaces with a water droplet contact angle of only 1351. A superhydrophobic surface, exhibiting a water-droplet contact angle of 1551, was prepared by modifying the boehmite or g-alumina coating with FAS.
Introduction Mimicking nature in the production of highly hydrophobic and self-cleaning surfaces, such as the legs of a water strider1 and the leaves of the lotus plant,2 has become an important scientific and technological research topic. Superhydrophobic and self-cleaning surfaces are of great interest in many practical applications,
varying from the prevention of the adhesion of snow on antennas,3 self-cleaning windshields,4 and antibiofouling paints for boats, to the inhibition of surface oxidation and electrical conductivity.5 In general, two models describe the wetting of a rough surface. Wenzel6 proposed a theoretical model describing the contact angle y of a liquid wetting the entire area of a rough surface: cos y0 ¼ r cos y
*
[email protected] r 2010 The American Ceramic Society
ð1Þ
where r is a roughness factor, defined as the ratio of the actual area of a rough surface to the geometrical projected
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Superhydrophobic Nanostructured Boehmite Coatings
area, and y is the contact angle for a flat surface. The equation states that surface roughness enhances the hydrophilicity of a hydrophilic surface (yo901) and the hydrophobicity of a hydrophobic surface (y4901) compared with a flat surface with the same chemical composition (Wenzel’s state). When air is trapped in the pockets of a hierarchically textured surface, a large water/air interface will be formed below a water droplet.7 In this case, the wetting can be described by the Cessie–Baxter equation, in which the apparent contact angle is expressed as the sum of all the contributions of the different phases, such as the water/air and water/solid interfaces (Cessie’s state)8: cos y0 ¼ f1 cos y f2
ð2Þ
where f1 is the fraction of solid material in contact with the liquid, y is the contact angle of the pure solid material, and f2 is the fraction of air in contact with the liquid. When the surface is rough but not porous, f2 is zero, and equation (2) reduces to Wenzel’s equation for the apparent contact angle of a rough surface with the roughness factor f1 [Eq. (1)]. Therefore, an increased surface roughness, preferably with a nanostructured morphology, in combination with a chemical modification that lowers the surface energy, is needed in order to obtain a water-repellant and superhydrophobic effect, exhibiting water contact angles in excess of 1501.9–11 There are numerous methods for making rough surfaces, such as mechanical stretching, laser/plasma/ chemical etching, lithography, sol–gel casting, layer-bylayer and colloidal assembling, electrical/chemical reaction and deposition, electrospinning and chemical vapor deposition.10 Among the low-surface-energy chemicals, long-chain carboxylic acids and semi-fluorinated silanes are some of many, with the latter being the most frequently used.12 Recently, it was shown by Krnel et al.13 that a nanostructured boehmite coating can be deposited onto a substrate using a new and simple method that exploits the hydrolysis of AlN powder. In the present work, this process was applied to a sintered alumina ceramic substrate in order to prepare a nanostructured boehmite coating that has a large specific surface area. After a subsequent chemical modification of the boehmite coating with various low-energy-surface chemicals, that is, carboxylic acids, heptadecafluoro-1,1,2,2-tetrahydrodecyl trimethoxy-silane (FAS), the initially highly hydro-
849
philic boehmite coating was transformed into a superhydrophobic one.
Experimental Procedure The substrates used in this work were fabricated in the form of discs (f 5 15.5 mm, h 5 2 mm) from commercially available A16 powder (Alcoa, Pittsburgh, PA), sintered at 15501C for 4 h in air. One side of each disc was ground and polished using a standard metallographic procedure. The substrates were then inserted into preheated (901C) deionized water and after 30 s of tempering the AlN powder was added to the water so that a diluted suspension containing 3 wt% of AlN in deionized water was obtained. After 7 min of the hydrolysis reaction the substrates were removed from the suspension, rinsed with deionized water, dried and then stored for subsequent analyses. Some of the as-prepared boehmite-coated substrates (labeled B) were heat treated in a resistance oven in dry air at 5001C (labeled A-g) and 9001C (labeled A-d). The heating rate was 101C/min, with a dwell time of 1 h at the final temperature. The as-prepared (boehmite) and heat-treated (alumina) coatings were modified with decanoic acid (labeled C10; Sigma-Aldrich, Taufkirchen, Germany), arashidic acid (labeled C20; Sigma-Aldrich) and with heptadecafluoro-1,1,2,2-tetrahydrodecyl trimethoxysilane (labeled FAS; CF3(CF2)7CH2CH2Si(OCH3)3; ABCR, Karlsruhe, Germany). The decanoic and arashidic solutions were prepared by mixing 1.2 g of acid and 50 mL ethanol for 1 h. The FAS solution was prepared by mixing 1 mL of FAS and 50 mL of ethanol for 1 h. Afterwards, the coated substrates were immersed for 1 h in the solution and dried, followed by annealing for 1 h at 1001C for the decanoic acid, 1201C for the arashidic acid and 1801C for the FAS. For comparison, decanoic acid, arashidic acid and FAS were also applied to polished substrates, that is, substrates without the coating. The morphologies of the coatings were analyzed using scanning electron microscope (FEG-SEM; Carl Zeiss, Supra 35LV, Oberkochen, Germany) and atomic force microscope (AFM; Solver PRO, NT-MDT, Moscow, Russia) analyses. The root mean square (RMS) surface roughness, Sq [nm], was measured on a 4 4 mm2 surface. The RMS surface roughness is the
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most widely used amplitude roughness parameter that gives the standard deviation of the height.14 A transmission electron microscope (TEM; JEOL, JEM-2100, Tokyo, Japan) analysis was employed in order to determine the morphology and the crystal structure of the lamellas before and after the heat treatment. The water contact angles were measured using a system of a microliter syringe for releasing the water droplet and a laboratory-made optical system connected to a computer for the data analysis. The size of the water droplet for the measurement was 2 mL.
Results and Discussion The morphology of the as-prepared boehmite coating is presented in Fig. 1a. The coating consists of nanostructured, interconnected, boehmite lamellas. According to the SEM analysis (Fig. 1a), the individual lamellas are 5–10 nm thick and 200–300 nm long. They are interlocking in various directions and most of them are oriented perpendicular to the substrate surface.13 Heat treatment of the boehmite coatings resulted in a topotactic transformation of the boehmite to transitional aluminas15–17 and the morphology of the coating remained basically unchanged up to 9001C, as illustrated in Fig. 1b. Characteristic 2D height AFM image of the boehmite coating is presented in Fig. 2. The results of the roughness measurements (RMS) for the boehmite and alumina coatings varied from 34 to 38 nm, irrespective of the heat treatment applied. The results of the contact-angle measurements for the water on different surfaces are presented in Table I. After lowering the surface energy of the initially hydro-
(a)
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philic (yYo901) polished alumina surface, exhibiting a water contact angle of 671, by modifying the surface with C10, C20 and FAS, the measured angles increased to 921, 981 and 1031, respectively, showing a slightly hydrophobic character (yY4901). In contrast, increasing the surface roughness of the initially hydrophilic polished alumina surface by depositing the nanostructured boehmite coating (B) by exploiting the AlN powder hydrolysis, and subsequently heat treating the coatings (A-g, A-d), resulted in an even more hydrophilic surface, which is in agreement with Eq. (1). The water-droplet contact angles of the samples B, A-g and A-d, which were not modified with low-surface-energy chemicals, could not be accurately measured with the apparatus because of their very low values, that is, the water droplet was too flat. Nevertheless, the contact angle was estimated to be around 101. However, when the rough surface of sample B was subsequently chemically modified, that is, the surface energy was lowered, with C10, C20 and FAS, the water contact angles dramatically increased. A water contact angle of 1051 (Fig. 3a) for B-C10 indicates that the droplet on this surface is in Wenzel’s state [Eq. (1)]. As a consequence, the liquid almost fills the entire pore area of the coating, being in a wet contact mode, and as a result the water droplet is unable to slide on the surface, according to Wang et al.18 Sample B-C20 was even more hydrophobic than sample B-C10, with the measured water contact angle being 1351 (Fig. 3b). This situation could be described as a transitional state between Wenzel’s and Cassie’s states with some air trapped in the voids and pores of the B coating. As a consequence, the water droplets can still slide, provided the surface is tilted at a certain angle.18 Because the hydrophobicity of the carboxylate groups increases with the chain length,19 the
(b)
Fig. 1. (a) SEM micrographs of the precipitated boehmite coating on the polished alumina surface using AlN powder hydrolysis at 901C for 7 min and (b) after a heat treatment in dry air at 9001C for 1 h.
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Table I. Measured Water-Droplet Contact Angles for the Polished Al2O3, Boehmite-, c-, and d-Alumina-Coated Alumina Surfaces for Differently Prepared Samples Modified with Various LowEnergy-Surface Chemicals C10w C20w FASw
Polished Al 2 O3
B
A-c
A-d
921 981 1031
1051 1351 1551
1261 1341 1551
1211 1301 1421
B, boehmite; A-g, gamma-alumina; A-d, delta-alumina. w
C10, decanoic acid; C20, arashidic acid; FAS, (heptadecafluoro1,1,2,2-tetrahydrodecyl) trimethoxy-silane.
Fig. 2. 2D AFM height image (4 mm 4 mm) of a boehmite coating on the polished alumina surface using AlN powder hydrolysis at 901C for 7 min (RMS 5 36 mn).
higher water contact angle for B-C20 compared with BC10 can be attributed to the chain-length difference between C10 and C20. On the other hand, applying the FAS to the boehmite coating (B-FAS, Fig. 3c) transformed this coating into a superhydrophobic one, where a water contact angle of 1551 was measured and the water droplet was in Cassie’s state [Eq. (2)]. It seems, therefore, that a large water/air interface below a water droplet was obtained in the case of B-FAS, and the water droplet adopted a non-wet-contact mode on the B-FAS sample surface and could roll off easily owing to its low adhesive force.18 The coatings that were heat treated at 5001C exhibited similar water-droplet contact angles compared with B samples, except for the A-g-C10 sample, which exhibited a slightly higher contact angle, as compared with B-C10 (Table I). The morphology and roughness of the boehmite coating after the heat treatment at 5001C for 1 h remained basically unchanged and this should be the reason for similar values of the measured water-droplet contact angles. As for the A-g-C10 sample, the increase in the water contact angle from 1051 (B-C10 sample) to 1261 can be ascribed to the dehydration of the boehmite into g-alumina during the heat treatment, because after a heat treatment at 5001C the topotactic transformation of the boehmite to g-alumina occurred.15–17 The structure of colloidal and/or lamellar
boehmite was reported to contain up to 30 wt% of water in its crystal structure, depending on the crystallite size and the synthesis procedure.17,20,21 Therefore, the length of the C10 does not seem to be long enough to prevent an interaction between the water droplet and the water molecules from the structure of the boehmite.22 As a result, the water droplet on the B-C10 sample was in the Wenzel’s state, whereas in the case of the A-g-C10 sample the droplet changed its state through Wenzel’s state to the transitional state. Finally, the water contact angles for samples heat treated at 9001C for 1 h, when d-alumina was formed,15–17 were measured and gave slightly lower contact angles compared with the A-g samples, that is, 1211 for C10-A-d, 1301 for C20A-d, and 1421 for FAS-A-d. Notice that the superhydrophobicity of the FAS-A-d sample was lost and the coating was transformed into a hydrophobic coating. According to the SEM and AFM measurements, the morphology and roughness factor (RMS) of the coating heat treated for 1 h at 9001C (d-alumina) were similar to those of the as-prepared boehmite and to coating heat treated for 1 h at 5001C (g-alumina). Therefore, in order to explain the decrease in the water-droplet contact angles, in the A-d samples compared with the B and A-g samples, high-resolution TEM with SAED was performed to further analyze a single lamella of the various prepared coatings. The results of the TEM analyses of the as-prepared precipitates as well as heat-treated samples are presented in Fig. 4. The single polycrystalline lamella of the as-prepared coating consists of individual nano-rods stacked together. The imperfect stacking of the rods results in spaces between stacks of crystallites,16 thereby exhibiting a nanoporosity of single lamella (Fig. 4a). The crystal structure of the as-prepared coating indeed corresponds to boehmite
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Fig. 3.
(b)
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(c)
Pictures of water-droplet contact angles measured for (a) B-C10 sample, (b) B-C20 sample and (c) B-FAS.
(inset of Fig. 4a). The nanoporosity of the lamellas changed to larger, circular nanopores to a certain extent due to the dehydration after the lamellas were subjected to a heat treatment at 5001C for 1 h, having the crystal structure of g-alumina (Fig. 4b). The stacked nano-rods forming a single lamella can still be observed. The TEM micrograph of a lamella that was heat treated at 9001C for 1 h (Fig. 4c) revealed that the nano-rods which originated from the as-prepared boehmite disappeared and crystallized into well-defined, uniform single-crystal lamellas during the heat-treatment process. The nanoporosity disappeared as well and the lamella has the structure of d-alumina, as confirmed by the SAED pattern (inset of Fig. 4c). The SAED pattern corresponds to a (a)
polycrystalline material, because it was obtained from the larger area consisting of agglomerates of lamellas pointing in all directions. The disappearance of the nanoporosity of the lamellas after the heat-treatment process is probably the main reason that the water-droplet contact angles measured for the A-d samples were lower compared with the B and A-g. There is less air trapped in the substrate material and also a smaller number of available binding sites for the molecules of the low-surface-energy chemicals. A similar observation was made by Zhang et al.23 They showed in their study that the surface roughness of a boehmite film prepared by the sol-gel method and subsequently modified with FAS, lost its micro/nano-details after heat treatment at 9001C for 30 min, resulting in a (b)
(c)
Fig. 4. TEM micrographs and the corresponding SAED patterns of the lamellas forming: (a) the as-prepared B coating, (b) the A-g coating heat treated at 5001C for 1 h and (c) the A-d coating heat treated at 9001C for 1 h.
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decreased water-contact angle due to the phase change from boehmite to transition alumina. The above results imply that long-chain carboxylic acids can be used as low-surface-energy chemicals, even though the superhydrophobic effect was obtained only with the FAS. The chain length of the C10 is insufficiently long and the water droplet interacted with the water from the boehmite structure (B-C10 sample). With the C20, a moderate water-droplet contact angle of 1351 was measured for the B-C20, which is below the superhydrophobic boundary of 1501. On the other hand, the FAS, a fluorosilane, has a shorter chain length compared with the C20, but it is more hydrophobic and oleophobic compared with the alkane-based carboxylic acid, being a more suitable low-energy-surface agent.
This work was supported by Slovene Research Agency.
Conclusions
8.
References 1. 2. 3.
4. 5.
6. 7.
9.
The hydrolysis of AlN powder was exploited for the preparation of nanostructured boehmite coatings on sintered and polished alumina ceramics. The coating exhibited a high roughness and nanoporosity. Heat treatment of the coating resulted in the transformation of boehmite into g-alumina at 5001C, and into d-alumina at 9001C, with no substantial change in the morphology and roughness. However, after the heat treatment at 9001C the nanoporosity of the lamellas disappeared. The C10 proved to be a less-effective low-energysurface chemical due to the insufficiently long chain length. On the other hand, lowering the surface energy of the boehmite and g-alumina coatings with C20 resulted in hydrophobic surfaces with a water-droplet contact angle of 1351. The superhydrophobic surface was prepared by modifying the boehmite or g-alumina coating with the FAS, exhibiting a water-droplet contact angle of 1551. The d-alumina coating modified with the FAS lost its superhydrophobic nature due to the transformation of the boehmite and/or g-alumina polycrystalline nanoporous lamellas consisting of stacked nanorods into the more densely crystallized lamellas of the d-alumina.
10. 11.
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20. 21.
22.
Acknowledgement 23.
The authors are grateful to Ita Junkar for the water contact angle measurements.
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