- Email: [email protected]
Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy
Author's Accepted Manuscript
Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy Yingdong Li, Songmei Li, You Zhang, Mei Yu, Jianhua Liu
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02151-X http://dx.doi.org/10.1016/j.matlet.2014.11.148 MLBLUE18136
To appear in:
Materials Letters
Received date: 5 August 2014 Accepted date: 29 November 2014 Cite this article as: Yingdong Li, Songmei Li, You Zhang, Mei Yu, Jianhua Liu, Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.11.148 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy Yingdong Li, Songmei Li1∗, You Zhang, Mei Yu and Jianhua Liu School of Materials Science and Engineering, Beihang University, Beijing 100083, China Abstract Layered double hydroxide (LDH) films with different metal cations, such as Mg2+, Co2+, Ni2+ and Zn2+, were fabricated on anodized aluminum 2198 alloy. The structure, morphology and composition of the LDH films were investigated. The results showed that different metal cations greatly influenced the growth and crystallization procedure of the LDH nanocrystals, which led to different morphologies of the LDH films. After surface treatment with 1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (PFDTMS) solution, all of the four types of LDH films showed excellent superhydrophobic property. Keywords: Layered double hydroxide; In situ growth; Multilayer structure; Superhydrophobic; Thin films. 1. Introduction In recent years, superhydrophobic surfaces with water contact angle (CA) larger than 150° have aroused great interest because of their significant potential practical applications as photoresponsive devices [1], microchemical sensor [2], chemical microreactors [3] and biosensors [4]. Different methods including polymerization [5], sol-gel processing [6], chemical etching [7] and anodic oxidation [8] have been ∗ Corresponding author. Tel.: +86 010 82317103; fax: +86 010 82317103. E-mail address: [email protected] (Songmei Li)
1
employed in the fabrication of superhydrophobic materials. Very recently, synthesis of layered double hydroxides (LDHs) as superhydrophobic surfaces has received increasing attention. LDHs are a class of synthetic anionic clays that consist of positively charged layers containing alternatively distributed divalent and trivalent cations in the sheets and charge balancing anions between the layers [9]. Generally, structure of LDHs can be represented by the generic formula [M’1-xM’’ x(OH)2]
X+
An−x/n · mH2O, where M’ is a divalent metal cation such as Mg2+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, Zn2+ and Ga2+; M’’ is a trivalent metal cation such as Al3+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+ and La3+; and An− represents an m-valence interlayer anion such as CO32-, OH-, NO3-, SO42- and ClO4- [10]. As a result, a large family of isostructural materials with varied physicochemical properties can be obtained by changing the identity of the metal cations of M’ and M’’ as well as the interlayered anions of An−. Yan Zhan [11] prepared hybrid Mg-Al LDH nanoplatelet for rendering cotton fabrics with dual factions of superhydrophobic and UV-blcking. Dutta. K. [12] synthesized a cone-shaped Ca-Al LDH intercalated with dodecyl sulphate and the measured contact angle was 140°. In comparison with LDH films arranged in a random fashion, the oriented LDH films always show better superhydrophobic behavior. An oriented structure can dramatically decrease the area of contact between the liquid and solid, which reduces the adhesion of a liquid droplet to the solid surface and sliding angle. X. Duan [13] fabricated oriented Ni-Al LDH film on PAO/Al substrate, and the contact angle reached to 163°after hydrophobic treatment with a sodium laurate solution. W. Li [14] 2
prepared Zn-Al LDH nanowalls on aluminum substrates in situ with contact angle as high as 167.3°. However, few works use the practical aluminum alloy as substrate. In the present work, M-Al LDH (M=Mg, Co, Ni and Zn) films were prepared in situ on anodized
aluminum
2198
alloy
and
modified
with
1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (PFDTMS). The aim of this work is to investigate influence of different anions on the structure, morphology and surperhydrophobic property of the LDH films. 2. Experimental Aluminum 2198 alloy plates (major elements, wt.%: Cu 2.9~3.5, Li 0.8~1.1, Zn 0.35, Mg 0.25~0.8, Mn 0.50, Fe 0.10, Ti 0.10, Zr 0.04~0.18, Si 0.08, Cr 0.05), was anodized in a sulfuric acid electrolyte for 30 min with applied voltage of 15 V. The M-Al LDH layers were fabricated by immersion of the anodized samples in 0.05 M M(NO3)2 salt (M=Mg, Co, Ni and Zn) and 0.3 M NH4NO3 mixture solution with a pH in the neutral range by adding diluted ammonia. The synthesis was carried out at 45°C for several hours. After the reaction, the filmed samples were washed with deionized water and dried at room temperature. Then, the as-prepared films were immersed vertically in the mixed solvents (60 mL deionized water and 40 mL methanol) with the addition of 0.6 g PFDTMS (n-CF3(CF2)7CH2CH2Si(OC2H5)3) for 2 h, followed by washing with deionized water and drying in an oven at 60 ;
for 2 h.
3. Result and discussion The glancing angle X-ray diffraction (GAXRD) patterns of the samples obtained at a glancing angle of 1°are depicted in Fig. 1. In these samples, well-resolved reflections of typical LDH phases are observed along with the reflections intrinsic to 3
the aluminum. For Mg-Al LDH sample, positions of the characteristic LDH reflections of (003) and (006) are clearly found and the (111) and (200) reflections are associated with pure aluminum. For Co-, Ni- and Zn-Al LDH films, the diffraction peaks show a decreased (003) reflection intensity and the peak of (006) plane almost extinguishes. Generally, intensity of the (003) reflection is contributed by diffraction from both M-Al-OH layers constituting the LDH ‘host’ structure and the ‘guest’ anions layer. All of the fabricated LDHs have the same ‘guest’ interlayer of NO3anions. Hence, the observed lower intensity of the (003) reflection certainly results from a higher atomic scattering factor for the M-Al-hydroxide (M=Co, Ni and Zn) ‘host’ layer, probably originated from the loss of crystallinity [15]. Meanwhile, the broadening reflection of (003) observed for Zn-Al LDH film may arise for the same reason. Fig. 2 gives the photographs and scanning electron microscope (SEM) images combined with energy dispersive spectrum (EDS) analysis of the LDH samples. The photographs show that the filmed samples obtained with Mg2+, Co2+, Ni2+ and Zn2+ display various colors of white, pink, light green and gray, respectively. The SEM images indicate that the types of metal cations greatly influence the morphology of the samples. For Mg-Al LDH, a flower-like structure is observed on the top layer and each “flower” consists of nano-flake petals. For Co-Al LDH, most LDH microcrystals have an obvious blade-like morphology. The flakes are preferentially oriented perpendicular to the substrate, which is correlated with the faster crystal growth rate in the direction of bulk solution. The Ni-Al LDH exhibits flat area and the layer is 4
much more compact with fine flakes. Meanwhile, several clusters observed above the compact layer, probably due to the presence of intermetallics of the substrate alloy [16]. The EDS images show that the Mg-Al LDH displays the highest Mg/Al ratio and this is related to the relatively high crystallinity mentioned in the GAXRD analysis. The adhesion strength of the films between the LDH and the anodic aluminum oxide (AAO) layer could be determined by a stretch test. A radial stress was imposed along the filmed samples and the different elastic modulus of the separate layer led to generate different strains. It is shown in Fig. 3 that for Mg- and Co-Al LDH, an obvious delamination is observed on the site even though the AAO layers underneath only exhibit minor cracks. This indicates that the parallel bonding strength within LDH layer is stronger than the vertical bonding strength between LDH layer and AAO layer. In contrast, Ni- and Zn-Al LDH display tight adhesion with the AAO layer. The LDH layer ruptured with the AAO film, and from the fracture, the layered structure of LDH-AAO-Al alloy is clearly observed. Then, it is assumed that the bonding strength of Ni- and Zn-Al LDH with the AAO substrate is stronger than Mgand Co-Al LDH. The variation of morphology and adhesion strength of LDH layer with different metal cations could be rationalized as follows. Compared with Mg2+ and Co2+ cations, cations of Ni2+ and Zn2+ adsorb more strongly on the substrate for the higher thermodynamic stability [17]. It has been reported in literature that release of Al3+ cations from the alumina substrate is influenced by the adsorption of metal cations 5
M2+ [18]. Thus, during the formation process of Ni- and Zn-Al LDH, cations of Ni2+ and Zn2+ preferred to adsorb on the AAO substrate, and afterward, the nucleation of Ni- and Zn-Al LDH occurred only at the substrate/solution interface and the nanocrystals growth proceeded towards the substrate until the crystals eventually bonded with the substrate. In contrast, nucleation of Mg and Co species tended to occur in the bulk solution and the nuclei then grew into small crystals. Once the interaction between the small nuclei and the substrate surface became favorable, it was energetically preferred for these nuclei to become adsorbed on the substrate. That is the reason why Mg- and Co-Al LDH show larger size of crystals and lower adhesion strength with AAO substrate. After hydrophobic treatment with PFDTMS, the wettability of LDH films was greatly improved. The water CA tests on the four surfaces were measured and the results are presented in the Fig. 4. For large area, many water droplets (15-20 L) present round shapes with large CAs on the PFDTMS modified LDHs. The measured CAs are 168.8°, 169.6°, 165.8° and 164.2° for Mg-, Co-, Ni- and Zn-Al LDH films, respectively, displaying excellent superhydrophobic property. That is because the oriented and hierarchical micro/nanostructure provides sufficient roughness and with the presence PFDTMS, a large fraction of air could be trapped within the structure. The trapped air can change the contact state from liquid−solid to liquid−air−solid and prohibit water penetrating the grooves of the surface. Thus, porous layer (Mg- and Co-Al LDH films) are prone to trap larger amount of air and show higher water CA than compact layer (Ni- and Zn-Al LDH films). 6
4. Conclusion
In the present work, different cations were used to fabricate LDH films in situ on anodized practical aluminum 2198 alloy. The types of metal cations showed great influence on the morphologies of the prepared LDH films. Compared with Mg- and Co-Al LDH films, the Ni- and Zn-Al LDH films were more compact and displayed tighter adhesion with the AAO layer. After simple hydrophobic treatment with PFDTMS solution, all of the four types of LDH films showed excellent superhydrophobic property. Furthermore, the Mg- and Co-Al LDH films show better surperhydrophobic property than Mg- and Co-Al LDH films, owing to the more porous structure. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.51271012). References [1] Y. Wu, Z. Liu, Y. Liang, X. Pei, F. Zhou, Q. Xue, Photoresponsive superhydrophobic coating for regulating boundary slippage, Soft matter, 10 (2014) 5318-5324. [2] D. Borin, M. Melli, S. Dal Zilio, V. Toffoli, G. Scoles, G. Toffoli, M. Lazzarino, How to engineer superhydrophobic micromechanical sensors preserving mass resolution, Sensors and Actuators B-Chemical, 199 (2014) 62-69. [3] M.B. Oliveira, A.I. Neto, C.R. Correia, M.I. Rial-Hermida, C. Alvarez-Lorenzo, J.F. Mano, Superhydrophobic chips for cell spheroids high-throughput generation and drug screening, ACS applied materials & interfaces, 6 (2014) 9488-9495. [4] D. Ishii, M. Shimomura, Preparation of Biomimetic High Adhesive Superhydrophobic Polymer Pillar Surfaces with Crown-Like Metal Microstructures, J. Nanosci. Nanotechnol., 14 (2014) 7611-7613. [5] Y. Lei, Q. Wang, J. Huo, Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International, 40 (2014) 10907-10914. [6] W.-H. Huang, C.-S. Lin, Robust superhydrophobic transparent coatings fabricated by a low-temperature sol-gel process, Applied Surface Science, 305 (2014) 702-709. [7] Y. Qi, Z. Cui, B. Liang, R.S. Parnas, H. Lu, A fast method to fabricate superhydrophobic surfaces on zinc substrate with ion assisted chemical etching, Applied Surface Science, 305 (2014) 716-724. 7
[8] B.-Y. Jeong, E.-H. Jung, J.-H. Kim, Fabrication of superhydrophobic niobium pentoxide thin films by anodization, Applied Surface Science, 307 (2014) 28-32. [9] X. Li, J.P. Liu, X.X. Ji, J.A. Jiang, R.M. Ding, Y.Y. Hu, A.Z. Hu, X.T. Huang, Ni/Al layered double hydroxide nanosheet film grown directly on Ti substrate and its application for a nonenzymatic glucose sensor, Sensors and Actuators B-Chemical, 147 (2010) 241-247. [10] T.W. Kim, M. Sahimi, T.T. Tsotsis, The Preparation and Characterization of Hydrotalcite Thin Films, Ind. Eng. Chem. Res., 48 (2009) 5794-5801. [11] Y. Zhao, Z. Xu, X. Wang, T. Lin, Superhydrophobic and UV-blocking cotton fabrics prepared by layer-by-layer assembly of organic UV absorber intercalated layered double hydroxides, Applied Surface Science, 286 (2013) 364-370. [12] K. Dutta, A. Pramanik, Synthesis of a novel cone-shaped CaAl-layered double hydroxide (LDH): its potential use as a reversible oil sorbent, Chem. Commun., 49 (2013) 6427-6429. [13] H.Y. Chen, F.Z. Zhang, S.S. Fu, X. Duan, In situ microstructure control of oriented layered double hydroxide monolayer films with curved hexagonal crystals as superhydrophobic materials, Adv. Mater., 18 (2006) 3089-+. [14] W. Li, X. Zhang, J. Yang, F. Miao, In situ growth of superbydrophobic and icephobic films with micro/nanoscale hierarchical structures on the aluminum substrate, J. Colloid Interface Sci., 410 (2013) 165-171. [15] A.N. Salak, J. Tedim, A.I. Kuznetsova, M.L. Zheludkevich, M.G.S. Ferreira, Anion exchange in Zn–Al layered double hydroxides: In situ X-ray diffraction study, Chemical Physics Letters, 495 (2010) 73-76. [16] J. Tedim, M.L. Zheludkevich, A.C. Bastos, A.N. Salak, J. Carneiro, F. Maia, A.D. Lisenkov, A.B. Oliveira, M.G.S. Ferreira, Effect of Surface Treatment on the Performance of LDH Conversion Films, ECS Electrochem. Lett., 3 (2014) C4-C8. [17] R.k. Allada, E. Peltier, A. Navrotsky, W.H. Casey, C.A. Johnson, H.T. Berbeco, D.L. Sparks, CALORIMETRIC
DETERMINATION
OF
THE
ENTHALPIES
OF
FORMATION
OF
HYDROTALCITE-LIKE SOLIDS AND THEIR USE IN THE GEOCHEMICAL MODELING OF METALS IN NATURAL WATERS, Clays and Clay Minerals, 54 (2006) 409-417. [18] A. Voegelin, R. Kretzschmar, Formation and dissolution of single and mixed Zn and Ni precipitates in soil: evidence from column experiments and extended X-ray absorption fine structure spectroscopy, 2005.
Figure captions Figure 1. GAXRD patterns of (a) Mg-Al, (b) Co-Al, (c) Ni-Al and (d) Zn-Al LDH films fabricated on aluminum 2198 alloy. Figure 2. Photographs of (a) Mg-Al, (b) Co-Al, (c) Ni-Al, (d) Zn-Al LDH films respectively and corresponding SEM morphologies of (e)-(h) with EDS analysis of (i)-(l). Figure 3. Cross section views of (a) Mg-Al, (b) Co-Al, (c) Ni-Al, (d) Zn-Al LDH films after stretch test. Figure 4. CA of LDH filmed samples of (a) Mg-Al, CA=168.8°; (b) Co-Al, CA=169.6°; (c) Ni-Al, CA=165.8°; (d) Zn-Al, CA=164.2° with surface modification with PFDTMS and the corresponding photographs of water droplets on the surfaces.
8
highlights 1. LDH films with different metal cations were prepared for the first time on anodized practical aluminum 2198 alloy. 2. Compared with Mg- and Co-Al LDH films, the Ni- and Zn-Al LDH films were more compact and displayed tighter adhesion with the AAO layer. 3. All of the four types of LDH films showed excellent superhydrophobic property after surface treatment with PFDTMS.
9
Figure 1
Figure 2
Figure 3
Figure 4
Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy Yingdong Li, Songmei Li, You Zhang, Mei Yu, Jianhua Liu
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02151-X http://dx.doi.org/10.1016/j.matlet.2014.11.148 MLBLUE18136
To appear in:
Materials Letters
Received date: 5 August 2014 Accepted date: 29 November 2014 Cite this article as: Yingdong Li, Songmei Li, You Zhang, Mei Yu, Jianhua Liu, Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.11.148 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy Yingdong Li, Songmei Li1∗, You Zhang, Mei Yu and Jianhua Liu School of Materials Science and Engineering, Beihang University, Beijing 100083, China Abstract Layered double hydroxide (LDH) films with different metal cations, such as Mg2+, Co2+, Ni2+ and Zn2+, were fabricated on anodized aluminum 2198 alloy. The structure, morphology and composition of the LDH films were investigated. The results showed that different metal cations greatly influenced the growth and crystallization procedure of the LDH nanocrystals, which led to different morphologies of the LDH films. After surface treatment with 1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (PFDTMS) solution, all of the four types of LDH films showed excellent superhydrophobic property. Keywords: Layered double hydroxide; In situ growth; Multilayer structure; Superhydrophobic; Thin films. 1. Introduction In recent years, superhydrophobic surfaces with water contact angle (CA) larger than 150° have aroused great interest because of their significant potential practical applications as photoresponsive devices [1], microchemical sensor [2], chemical microreactors [3] and biosensors [4]. Different methods including polymerization [5], sol-gel processing [6], chemical etching [7] and anodic oxidation [8] have been ∗ Corresponding author. Tel.: +86 010 82317103; fax: +86 010 82317103. E-mail address: [email protected] (Songmei Li)
1
employed in the fabrication of superhydrophobic materials. Very recently, synthesis of layered double hydroxides (LDHs) as superhydrophobic surfaces has received increasing attention. LDHs are a class of synthetic anionic clays that consist of positively charged layers containing alternatively distributed divalent and trivalent cations in the sheets and charge balancing anions between the layers [9]. Generally, structure of LDHs can be represented by the generic formula [M’1-xM’’ x(OH)2]
X+
An−x/n · mH2O, where M’ is a divalent metal cation such as Mg2+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, Zn2+ and Ga2+; M’’ is a trivalent metal cation such as Al3+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+ and La3+; and An− represents an m-valence interlayer anion such as CO32-, OH-, NO3-, SO42- and ClO4- [10]. As a result, a large family of isostructural materials with varied physicochemical properties can be obtained by changing the identity of the metal cations of M’ and M’’ as well as the interlayered anions of An−. Yan Zhan [11] prepared hybrid Mg-Al LDH nanoplatelet for rendering cotton fabrics with dual factions of superhydrophobic and UV-blcking. Dutta. K. [12] synthesized a cone-shaped Ca-Al LDH intercalated with dodecyl sulphate and the measured contact angle was 140°. In comparison with LDH films arranged in a random fashion, the oriented LDH films always show better superhydrophobic behavior. An oriented structure can dramatically decrease the area of contact between the liquid and solid, which reduces the adhesion of a liquid droplet to the solid surface and sliding angle. X. Duan [13] fabricated oriented Ni-Al LDH film on PAO/Al substrate, and the contact angle reached to 163°after hydrophobic treatment with a sodium laurate solution. W. Li [14] 2
prepared Zn-Al LDH nanowalls on aluminum substrates in situ with contact angle as high as 167.3°. However, few works use the practical aluminum alloy as substrate. In the present work, M-Al LDH (M=Mg, Co, Ni and Zn) films were prepared in situ on anodized
aluminum
2198
alloy
and
modified
with
1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (PFDTMS). The aim of this work is to investigate influence of different anions on the structure, morphology and surperhydrophobic property of the LDH films. 2. Experimental Aluminum 2198 alloy plates (major elements, wt.%: Cu 2.9~3.5, Li 0.8~1.1, Zn 0.35, Mg 0.25~0.8, Mn 0.50, Fe 0.10, Ti 0.10, Zr 0.04~0.18, Si 0.08, Cr 0.05), was anodized in a sulfuric acid electrolyte for 30 min with applied voltage of 15 V. The M-Al LDH layers were fabricated by immersion of the anodized samples in 0.05 M M(NO3)2 salt (M=Mg, Co, Ni and Zn) and 0.3 M NH4NO3 mixture solution with a pH in the neutral range by adding diluted ammonia. The synthesis was carried out at 45°C for several hours. After the reaction, the filmed samples were washed with deionized water and dried at room temperature. Then, the as-prepared films were immersed vertically in the mixed solvents (60 mL deionized water and 40 mL methanol) with the addition of 0.6 g PFDTMS (n-CF3(CF2)7CH2CH2Si(OC2H5)3) for 2 h, followed by washing with deionized water and drying in an oven at 60 ;
for 2 h.
3. Result and discussion The glancing angle X-ray diffraction (GAXRD) patterns of the samples obtained at a glancing angle of 1°are depicted in Fig. 1. In these samples, well-resolved reflections of typical LDH phases are observed along with the reflections intrinsic to 3
the aluminum. For Mg-Al LDH sample, positions of the characteristic LDH reflections of (003) and (006) are clearly found and the (111) and (200) reflections are associated with pure aluminum. For Co-, Ni- and Zn-Al LDH films, the diffraction peaks show a decreased (003) reflection intensity and the peak of (006) plane almost extinguishes. Generally, intensity of the (003) reflection is contributed by diffraction from both M-Al-OH layers constituting the LDH ‘host’ structure and the ‘guest’ anions layer. All of the fabricated LDHs have the same ‘guest’ interlayer of NO3anions. Hence, the observed lower intensity of the (003) reflection certainly results from a higher atomic scattering factor for the M-Al-hydroxide (M=Co, Ni and Zn) ‘host’ layer, probably originated from the loss of crystallinity [15]. Meanwhile, the broadening reflection of (003) observed for Zn-Al LDH film may arise for the same reason. Fig. 2 gives the photographs and scanning electron microscope (SEM) images combined with energy dispersive spectrum (EDS) analysis of the LDH samples. The photographs show that the filmed samples obtained with Mg2+, Co2+, Ni2+ and Zn2+ display various colors of white, pink, light green and gray, respectively. The SEM images indicate that the types of metal cations greatly influence the morphology of the samples. For Mg-Al LDH, a flower-like structure is observed on the top layer and each “flower” consists of nano-flake petals. For Co-Al LDH, most LDH microcrystals have an obvious blade-like morphology. The flakes are preferentially oriented perpendicular to the substrate, which is correlated with the faster crystal growth rate in the direction of bulk solution. The Ni-Al LDH exhibits flat area and the layer is 4
much more compact with fine flakes. Meanwhile, several clusters observed above the compact layer, probably due to the presence of intermetallics of the substrate alloy [16]. The EDS images show that the Mg-Al LDH displays the highest Mg/Al ratio and this is related to the relatively high crystallinity mentioned in the GAXRD analysis. The adhesion strength of the films between the LDH and the anodic aluminum oxide (AAO) layer could be determined by a stretch test. A radial stress was imposed along the filmed samples and the different elastic modulus of the separate layer led to generate different strains. It is shown in Fig. 3 that for Mg- and Co-Al LDH, an obvious delamination is observed on the site even though the AAO layers underneath only exhibit minor cracks. This indicates that the parallel bonding strength within LDH layer is stronger than the vertical bonding strength between LDH layer and AAO layer. In contrast, Ni- and Zn-Al LDH display tight adhesion with the AAO layer. The LDH layer ruptured with the AAO film, and from the fracture, the layered structure of LDH-AAO-Al alloy is clearly observed. Then, it is assumed that the bonding strength of Ni- and Zn-Al LDH with the AAO substrate is stronger than Mgand Co-Al LDH. The variation of morphology and adhesion strength of LDH layer with different metal cations could be rationalized as follows. Compared with Mg2+ and Co2+ cations, cations of Ni2+ and Zn2+ adsorb more strongly on the substrate for the higher thermodynamic stability [17]. It has been reported in literature that release of Al3+ cations from the alumina substrate is influenced by the adsorption of metal cations 5
M2+ [18]. Thus, during the formation process of Ni- and Zn-Al LDH, cations of Ni2+ and Zn2+ preferred to adsorb on the AAO substrate, and afterward, the nucleation of Ni- and Zn-Al LDH occurred only at the substrate/solution interface and the nanocrystals growth proceeded towards the substrate until the crystals eventually bonded with the substrate. In contrast, nucleation of Mg and Co species tended to occur in the bulk solution and the nuclei then grew into small crystals. Once the interaction between the small nuclei and the substrate surface became favorable, it was energetically preferred for these nuclei to become adsorbed on the substrate. That is the reason why Mg- and Co-Al LDH show larger size of crystals and lower adhesion strength with AAO substrate. After hydrophobic treatment with PFDTMS, the wettability of LDH films was greatly improved. The water CA tests on the four surfaces were measured and the results are presented in the Fig. 4. For large area, many water droplets (15-20 L) present round shapes with large CAs on the PFDTMS modified LDHs. The measured CAs are 168.8°, 169.6°, 165.8° and 164.2° for Mg-, Co-, Ni- and Zn-Al LDH films, respectively, displaying excellent superhydrophobic property. That is because the oriented and hierarchical micro/nanostructure provides sufficient roughness and with the presence PFDTMS, a large fraction of air could be trapped within the structure. The trapped air can change the contact state from liquid−solid to liquid−air−solid and prohibit water penetrating the grooves of the surface. Thus, porous layer (Mg- and Co-Al LDH films) are prone to trap larger amount of air and show higher water CA than compact layer (Ni- and Zn-Al LDH films). 6
4. Conclusion
In the present work, different cations were used to fabricate LDH films in situ on anodized practical aluminum 2198 alloy. The types of metal cations showed great influence on the morphologies of the prepared LDH films. Compared with Mg- and Co-Al LDH films, the Ni- and Zn-Al LDH films were more compact and displayed tighter adhesion with the AAO layer. After simple hydrophobic treatment with PFDTMS solution, all of the four types of LDH films showed excellent superhydrophobic property. Furthermore, the Mg- and Co-Al LDH films show better surperhydrophobic property than Mg- and Co-Al LDH films, owing to the more porous structure. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.51271012). References [1] Y. Wu, Z. Liu, Y. Liang, X. Pei, F. Zhou, Q. Xue, Photoresponsive superhydrophobic coating for regulating boundary slippage, Soft matter, 10 (2014) 5318-5324. [2] D. Borin, M. Melli, S. Dal Zilio, V. Toffoli, G. Scoles, G. Toffoli, M. Lazzarino, How to engineer superhydrophobic micromechanical sensors preserving mass resolution, Sensors and Actuators B-Chemical, 199 (2014) 62-69. [3] M.B. Oliveira, A.I. Neto, C.R. Correia, M.I. Rial-Hermida, C. Alvarez-Lorenzo, J.F. Mano, Superhydrophobic chips for cell spheroids high-throughput generation and drug screening, ACS applied materials & interfaces, 6 (2014) 9488-9495. [4] D. Ishii, M. Shimomura, Preparation of Biomimetic High Adhesive Superhydrophobic Polymer Pillar Surfaces with Crown-Like Metal Microstructures, J. Nanosci. Nanotechnol., 14 (2014) 7611-7613. [5] Y. Lei, Q. Wang, J. Huo, Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International, 40 (2014) 10907-10914. [6] W.-H. Huang, C.-S. Lin, Robust superhydrophobic transparent coatings fabricated by a low-temperature sol-gel process, Applied Surface Science, 305 (2014) 702-709. [7] Y. Qi, Z. Cui, B. Liang, R.S. Parnas, H. Lu, A fast method to fabricate superhydrophobic surfaces on zinc substrate with ion assisted chemical etching, Applied Surface Science, 305 (2014) 716-724. 7
[8] B.-Y. Jeong, E.-H. Jung, J.-H. Kim, Fabrication of superhydrophobic niobium pentoxide thin films by anodization, Applied Surface Science, 307 (2014) 28-32. [9] X. Li, J.P. Liu, X.X. Ji, J.A. Jiang, R.M. Ding, Y.Y. Hu, A.Z. Hu, X.T. Huang, Ni/Al layered double hydroxide nanosheet film grown directly on Ti substrate and its application for a nonenzymatic glucose sensor, Sensors and Actuators B-Chemical, 147 (2010) 241-247. [10] T.W. Kim, M. Sahimi, T.T. Tsotsis, The Preparation and Characterization of Hydrotalcite Thin Films, Ind. Eng. Chem. Res., 48 (2009) 5794-5801. [11] Y. Zhao, Z. Xu, X. Wang, T. Lin, Superhydrophobic and UV-blocking cotton fabrics prepared by layer-by-layer assembly of organic UV absorber intercalated layered double hydroxides, Applied Surface Science, 286 (2013) 364-370. [12] K. Dutta, A. Pramanik, Synthesis of a novel cone-shaped CaAl-layered double hydroxide (LDH): its potential use as a reversible oil sorbent, Chem. Commun., 49 (2013) 6427-6429. [13] H.Y. Chen, F.Z. Zhang, S.S. Fu, X. Duan, In situ microstructure control of oriented layered double hydroxide monolayer films with curved hexagonal crystals as superhydrophobic materials, Adv. Mater., 18 (2006) 3089-+. [14] W. Li, X. Zhang, J. Yang, F. Miao, In situ growth of superbydrophobic and icephobic films with micro/nanoscale hierarchical structures on the aluminum substrate, J. Colloid Interface Sci., 410 (2013) 165-171. [15] A.N. Salak, J. Tedim, A.I. Kuznetsova, M.L. Zheludkevich, M.G.S. Ferreira, Anion exchange in Zn–Al layered double hydroxides: In situ X-ray diffraction study, Chemical Physics Letters, 495 (2010) 73-76. [16] J. Tedim, M.L. Zheludkevich, A.C. Bastos, A.N. Salak, J. Carneiro, F. Maia, A.D. Lisenkov, A.B. Oliveira, M.G.S. Ferreira, Effect of Surface Treatment on the Performance of LDH Conversion Films, ECS Electrochem. Lett., 3 (2014) C4-C8. [17] R.k. Allada, E. Peltier, A. Navrotsky, W.H. Casey, C.A. Johnson, H.T. Berbeco, D.L. Sparks, CALORIMETRIC
DETERMINATION
OF
THE
ENTHALPIES
OF
FORMATION
OF
HYDROTALCITE-LIKE SOLIDS AND THEIR USE IN THE GEOCHEMICAL MODELING OF METALS IN NATURAL WATERS, Clays and Clay Minerals, 54 (2006) 409-417. [18] A. Voegelin, R. Kretzschmar, Formation and dissolution of single and mixed Zn and Ni precipitates in soil: evidence from column experiments and extended X-ray absorption fine structure spectroscopy, 2005.
Figure captions Figure 1. GAXRD patterns of (a) Mg-Al, (b) Co-Al, (c) Ni-Al and (d) Zn-Al LDH films fabricated on aluminum 2198 alloy. Figure 2. Photographs of (a) Mg-Al, (b) Co-Al, (c) Ni-Al, (d) Zn-Al LDH films respectively and corresponding SEM morphologies of (e)-(h) with EDS analysis of (i)-(l). Figure 3. Cross section views of (a) Mg-Al, (b) Co-Al, (c) Ni-Al, (d) Zn-Al LDH films after stretch test. Figure 4. CA of LDH filmed samples of (a) Mg-Al, CA=168.8°; (b) Co-Al, CA=169.6°; (c) Ni-Al, CA=165.8°; (d) Zn-Al, CA=164.2° with surface modification with PFDTMS and the corresponding photographs of water droplets on the surfaces.
8
highlights 1. LDH films with different metal cations were prepared for the first time on anodized practical aluminum 2198 alloy. 2. Compared with Mg- and Co-Al LDH films, the Ni- and Zn-Al LDH films were more compact and displayed tighter adhesion with the AAO layer. 3. All of the four types of LDH films showed excellent superhydrophobic property after surface treatment with PFDTMS.
9
Figure 1
Figure 2
Figure 3
Figure 4