- Email: [email protected]
α-Alumina membrane having a hierarchical structure of straight macropores and mesopores inside the pore wall
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
α-Alumina membrane having a hierarchical structure of straight macropores and mesopores inside the pore wall ⁎
Hideki Hashimoto , Sumire Kojima, Takashi Sasaki, Hidetaka Asoh Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Anodization α-Alumina membrane Aluminum phosphate Mesopores
We prepared a through-hole α-alumina membrane with a hierarchical porous structure from an anodic porous alumina membrane prepared using phosphoric acid electrolyte as the starting material. By heating the anodic porous alumina membrane to 1400 °C, aluminum phosphate nanoparticles were segregated in the α-alumina matrix at the anion-incorporated outer layer and the high-purity alumina layer forming a cell boundary band sintered as a high-density α-alumina layer at the central core of the pore wall. When the heat-treated membrane was immersed in concentrated hydrochloric acid, a unique hierarchical porous α-alumina membrane structure was formed with straight macropores and mesopores inside the pore wall due to the dissolution of aluminum phosphate nanoparticles. The developed α-alumina membrane can be ultimately used as a multifunctional filter because of its unique hierarchical porous structure and extremely high chemical and thermal durability.
1. Introduction Amorphous alumina films having nanometric straight pores can be formed on an aluminum substrate by anodizing the aluminum in acidic electrolytes. Methods for forming porous oxide films have evolved for nearly a century as essential surface-modification techniques in the aluminum industry because anodic porous alumina films provide various properties to aluminum such as excellent corrosion resistance, hardness, abrasion resistance, and decorative coloration. [1] The cell structure of anodic porous alumina, including the interpore distance, the thickness of the barrier layer, and the pore diameter, is known to depend on the formation voltage;[1,2] a highly ordered cell arrangement can be obtained by anodization at specific self-ordering voltages. [3–5] Further, the pore wall is known to have a distinctive duplex structure that depends on the electrolyte species. [6–11] For example, using a phosphoric acid electrolyte affords a duplex structure having a high-purity alumina layer at the cell boundary band and an anion-incorporated layer occupying 50%–80% of the cell wall at the outer pore wall. [7,8,10,11] Anodization in phosphoric acid is generally conducted at relatively high voltage, yielding a film with a large cell size; for example, an interpore distance of 500 nm can be obtained with a voltage of 195 V. [5] The cell structure of anodic porous alumina is readily controllable using the techniques mentioned above, and the formed amorphous alumina can be easily detached from the aluminum substrate and dissolved using an appropriate etching agent. Therefore, intensive studies
⁎
employing anodic porous alumina as host and template materials for creating functional nanomaterials have been conducted. [1] The high solubility of anodic amorphous alumina may be a disadvantage depending on its intended application, such as in nano-filtration. Therefore, attempts to improve its chemical durability using heat treatment along with crystallization have been investigated. [12–18] Common amorphous alumina changes to thermodynamically stable α-alumina via transition alumina such as γ-, θ-, and η-phases; [19,20] the same phase transition occurs on anodic amorphous alumina through heat treatment. [21–25] Because the membrane film prepared via anodization is generally less symmetrical in vertical direction of the film, it is easily influenced by heat treatment, resulting in thermal deformation such as curving and cracking. Therefore, it is difficult to crystallize anodic alumina membranes to α-alumina membranes while maintaining their morphology and nanopores and while avoiding excess sintering. We have successfully prepared α-alumina membranes having straight pore channels with tunable pore diameters of 30–350 nm by applying appropriate restriction and heat treatment to the amorphous membrane. [15,17,18] We showed that these α-alumina membranes can be used as filters for suspended fluids. [17] Although the main focus for the application of anodic porous alumina is on using the straight pore channels, research on modulating the porous structure, such as pore branching modification and hierarchical control of the number of pores, has also been conducted. [1,26,27] Modulating the porous structure, e.g., introducing fine pores that are smaller than the main straight pore channels, could lead to a significant
Corresponding author. E-mail address: [email protected] (H. Hashimoto).
https://doi.org/10.1016/j.jeurceramsoc.2017.11.032 Received 29 August 2017; Received in revised form 29 October 2017; Accepted 13 November 2017 0955-2219/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Hashimoto, H., Journal of the European Ceramic Society (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.11.032
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al.
increase in the surface area of the membrane, and therefore an improvement in the filtration properties. The discovery of new applications through modulation of the porous structure is highly likely in applications of amorphous and crystalline alumina. In this study, an anodic alumina membrane with a duplex pore wall structure was formed in phosphoric acid and used as a starting material to create an α-alumina membrane with numerous mesopores inside the pore wall through the segregation and subsequent selective dissolution of aluminum phosphate (AlPO4) from an anion-incorporated layer. 2. Materials and methods The anodic porous alumina membrane was prepared using the twostep anodization process and subsequent two-layer anodization. [28] An electropolished high-purity (99.99%) aluminum sheet was first anodized in 0.2 mol dm−3 phosphoric acid at a constant voltage of 180 V at 10 °C for 1 h. A voltage value of 180 V was selected to achieve stable anodization for a long time. The electrolyte solution was stirred at 700 rpm by using a magnetic stirrer to remove generated Joule heat during anodization. The formed film was dissolved in a boiling solution of 6 wt% phosphoric acid and 2 wt% chromic acid for 10 min. The second anodization was performed for 5 h under the same conditions as those used in the first anodization. The film was detached via two-layer anodization as follows. [28] Anodization was conducted under a constant voltage of 180 V in 18 mol dm−3 sulfuric acid for 30 min at 20 °C to form a readily soluble alumina film containing sulfate ions at the interface between the aluminum substrate and the outer film formed due to the phosphoric acid anodization. Subsequent immersion in 2 wt % phosphoric acid at 30 °C for 15 min resulted in selective dissolution of the sacrificial layer generated via the sulfuric acid anodization step, thus detaching the outer film formed by the phosphoric acid anodization from the aluminum substrate. The detached through-hole membrane was carefully washed with deionized water and dried at room temperature. This detached membrane was sandwiched between two quartz plates, heat-treated by elevating the temperature at a rate of 10 °C min−1 to 1400 °C, and then cooled in a furnace. The heat-treated membrane was immersed in 36 wt% hydrochloric acid (HCl aq.) at 50 °C for 1 h to dissolve the AlPO4 particles, leaving a high-purity αalumina membrane. The crystallographic structures of the samples were characterized using X-ray diffractometry (XRD, RINT2500VHF, Rigaku) and the microstructures of the samples were evaluated using scanning electron microscopy (SEM, JSM-6701F, JEOL) and scanning transmission electron microscopy (STEM, JEM-2100F, JEOL) with an energy dispersive X-ray spectrometer (EDS, JED-2300T, JEOL). The surface area and pore size distribution of the obtained membranes were measured using the nitrogen adsorption method at 77 K using a BELSORP-mini-II system (BEL Japan). Prior to the analysis, each membrane was crushed into powder form and degassed under a vacuum for 5 h at 300 °C. Data were analyzed using the Brunauer–Emmet–Teller (BET) method to estimate the surface area and using the Barrett–Joyner–Hallender (BJH) method to determine the pore size distribution.
Fig. 1. I–t curves of the first and second steps of the anodization process conducted in 0.2 mol dm−3 phosphoric acid at 10 °C under an applied voltage of 180 V. The left inset shows the I–t curve for the subsequent anodization using 18 mol dm−3 sulfuric acid at 20 °C under an applied voltage of 180 V. The right inset shows the detached anodic alumina film (φ 25 mm).
step. The current density had a nearly constant value around 50 A m−2. The thickness of the formed film was adjusted to approximately 50 μm via anodization for 5 h. The sample was then re-anodized in 18 mol dm−3 sulfuric acid under a constant applied voltage of 180 V to form the highly soluble sacrificial layer containing sulfate ions below the film formed due to the phosphoric acid anodization. The I–t curve showed a relatively sharp peak of 28 A m−2 at around 7 min, and then showed a steady current value of 9–15 A m−2 (left inset in Fig. 1), suggesting stable formation of a porous film containing sulfate ions beneath the film formed due to the phosphoric acid anodization. Subsequent immersion in 2 wt% phosphoric acid detached this film from aluminum substrate (right inset in Fig. 1) as the sacrificial layer beneath it, which contains sulfate ions, was preferentially dissolved. [28] Thereby, a through-hole, stand-alone membrane film with straight pores was obtained. Fig. 2 shows SEM images of the detached film. The top surface view shows the distinctive nanoporous character of the anodic porous alumina, and the cross-sectional SEM images taken at the top (upper right inset) and bottom (lower left inset) surfaces show the straight, throughhole channels. The interpore distance and pore diameter were estimated to be ∼440 nm and ∼220 nm, respectively, from the SEM image. The values are roughly consistent with those calculated based on the empirical rule that the cell dimensions of anodic porous alumina are
3. Results and discussion Fig. 1 shows current–time (I–t) curves obtained during the first and second anodizing steps using 0.2 mol dm−3 phosphoric acid. An increasing current, which is the sign of pore formation, was observed in the initial stage and again in the second step at an earlier time point than that in the first step. Electropolished flat aluminum was anodized in the first step; the dimple structured surface of aluminum formed by dissolution of the first anodic porous film was anodized in the second step, which induced the pore formation at the depressions within the dimple structure. The applied current concentrated at the depression, prompting pore formation and causing the early current increase associated with pore formation that is seen in the second anodization
Fig. 2. An SEM image of the top surface of the detached film. The upper-right and lowerleft inset images show cross-sectional SEM images taken at the top and bottom surfaces of the film, respectively.
2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al.
Fig. 3. XRD patterns for the as-prepared amorphous alumina membrane (bottom), the membrane after heat-treatment at 1400 °C (middle), and the membrane after heat and HCl treatments (top).
Fig. 5. Pore size distributions of the amorphous alumina membrane as-prepared (open circles) and the membrane after heat and HCl treatments (solid circles).
proportional to the formation voltage: 2.5 nm V−1 interpore distance and 1 nm V−1 pore diameter. [1] The cross-sectional SEM image taken at the top surface (upper right inset in Fig. 2) showed a larger pore size than the image taken at the bottom surface (lower left inset in Fig. 2), indicating chemical dissolution of the pore walls. The XRD pattern of the obtained film showed a halo pattern, which is indicative of an amorphous structure (Fig. 3, bottom). Sharp, intense peaks of α-alumina; very weak peaks assigned to transition alumina such as γ-, θ-, and η-alumina; and relatively sharp peaks from AlPO4 were detected in the heat-treated amorphous alumina membrane at 1400 °C (Fig. 3, middle). To remove the formed AlPO4, the heat-treated membrane was immersed in 36 wt% HCl aq. at 50 °C for 1 h. The sharp XRD peaks of AlPO4 completely disappeared and nearly monophasic αalumina was afforded by the HCl treatment (the top pattern in Fig. 3). The resulting membrane was a crack-free, stand-alone porous α-alumina membrane. To confirm the nano-structural changes during heat and HCl treatments, microscopic analyses were conducted (Fig. 4). The ridge of the dimple structure formed in the first step anodization varied obscure, the surface varied in a ripple-like structure, the cell diameter showed little
change, and the pore diameter increased slightly after heating (Top images in Fig. 4a and b). Observing a cross-section, numerous nanoparticles with 20–50 nm diameters attached onto the pore walls after heating (Fig. 4b, bottom), although the pore wall surface was smooth prior to heating. The uniform contrast, which is considered a signature of high-purity alumina layers, was observed as indicated by the arrow in the bottom image in Fig. 4b. Here the anodic alumina pore wall formed due to phosphoric acid anodization is known to have a duplex structure comprising a high-purity alumina layer located at the cellboundary band and a phosphate-incorporated layer located at the outer pore wall. The fine particles formed due to the heat treatment were densely segregated on the outer pore wall, which is considered a signature of phosphate-incorporated layers, and were not segregated to the interface of the cells, which is considered a signature of high-purity alumina layers (Fig. 4b, bottom). The fine particles could be AlPO4 because XRD measurements detected an AlPO4 phase after heat treatment, as shown in Fig. 3. After the HCl treatment, numerous fine mesopores with pore diameters of 20–80 nm, which may have been formed due to dissolution of AlPO4 nanoparticles, were formed on the
Fig. 4. SEM images of the membrane (a) as-prepared, (b) after the heat treatment at 1400 °C, (c) after the heat and HCl treatments.
3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al.
Fig. 6. (a) HAADF-STEM image, (b) Al Kα and (c) P Kα elemental maps, and (d) a merged image of b (red) and c (green) of the membrane after the heat-treatment at 1400 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
outer pore wall (Fig. 4c). The sintering of α-alumina was found around the cell-boundary band as a central core formed due to the heat treatment (Fig. 4c). The pore size distribution of the formed mesopores and the increase in the surface area were confirmed by analyzing nitrogen adsorption/ desorption isotherms at 77 K using BJH and BET methods, respectively. Fig. 5 shows the pore size distribution of the amorphous alumina membrane as-prepared and the membrane after heating and after HCl treatments. Distinctive mesopores were not observed in the as-prepared membrane, but the membrane after heat and HCl treatments shows distinctive mesopores with diameters in the range of 10–40 nm with the mode around 20 nm, which is roughly consistent with the sizes of segregated AlPO4 particles after heat treatment and with the formed pores after HCl treatment. The BET surface area increased significantly from 3.5 m2 g−1 in the membrane as-prepared to 11.7 m2 g−1 in the heat- and HCl-treated membranes, which represents a nearly threefold increase. Thus, the unique double-layered cell structure comprising a high-density cell-boundary band and the porous outer cell wall was obtained. In other words, the resultant α-alumina membrane has a unique hierarchical structure of highly ordered straight macropores with diameters around 200 nm and mesopores inside the outer pore wall with diameters around 20 nm. Additionally, the obtained α-alumina membrane showed high chemical durability, as it did not dissolve even when placed in 36 wt% HCl aq. at 50 °C for 1 h. To confirm the distribution of AlPO4 nanoparticles on the outer pore wall after heat treatment, the heat-treated sample was analyzed via STEM/EDS (Fig. 6a–d). The heat-treated membrane was crushed to a fine powder and the particles were mounted on a microgrid for TEM observations; the area that is considered a pore wall was observed. The high-angle annular dark-field (HAADF)-STEM image (Fig. 6a) contains low- and high-contrast areas, which result from the density difference between α-alumina (3.95 g cm−3) and AlPO4 (2.57 g cm−3): it is known that the higher the sample density is, the higher the STEM image contrast is. The signal of Al Kα was detected from the whole area of the sample except for the low-contrast, spotty areas (Fig. 6b). On the other hand, the signal of P Kα was strongly detected from the low-contrast, spotty areas observed in the Al Kα map (Fig. 6c). The Al Kα and P Kα maps showed elemental distributions consistent with the high- and lowcontrast areas of the HAADF-STEM image, respectively. The HAADFSTEM image and the merged elemental maps of Al Kα and P Kα (Fig. 6a and Fig. 6d) clearly demonstrate the dispersive segregation of AlPO4 nanoparticles in the α-alumina matrix after heat treatment. These results conclude that the fine particles segregated in the pore wall are AlPO4 nanoparticles and that the removal of these nanoparticles results in a unique double-layered pore wall structure in α-alumina. Using well-known electrolytes, i.e., sulfuric acid or oxalic acid, sulfate or oxalate ions-incorporated films can be formed, respectively. These anions are eliminated from anion-incorporated layer as a gaseous form by heat treatment. [21–25] Because the formation of solid-state AlPO4 particles in an anion-incorporated layer is distinctive characteristic of the anodic porous alumina film prepared in phosphoric
acid electrolyte, formation of mesopores in the pore wall after removal of AlPO4 particles seems a unique phenomenon of phosphoric acid film. Merits of the α-alumina membrane include its chemical and thermal durability, unique porous structure, and high surface area. Therefore, various functional surfaces can be easily added to the membrane under extreme preparation conditions wherein the amorphous alumina membrane is dissolved under high temperature and high pressure using an acidic, basic, or organic solution. Because of its unique hierarchical porous structure and high chemical and thermal durability, the developed α-alumina membrane can be used as a multifunctional membrane filter in various field. 4. Conclusions An α-alumina membrane having unique porous structure was prepared by using an anodic porous alumina membrane as the starting material. The through-hole porous alumina membrane prepared using a two-layer anodization method with phosphoric acid and sulfuric acid as electrolytes was heat treated by increasing the temperature at a rate of 10 °C min−1 to 1400 °C. AlPO4 nanoparticles were segregated in the αalumina matrix at the pore wall. By removing the formed AlPO4 nanoparticles using 36 wt% HCl aq., numerous fine pores formed at the pore wall and high-density sintered α-alumina appeared as a central core at the cell boundary band. The result was a unique hierarchical porous α-alumina membrane structure with straight macropores and mesopores inside the pore walls. Acknowledgements We thank Mr. Hirofumi Inada and Dr. Taigo Takaishi for the measurements associated the nitrogen adsorption method. This study was financially supported by the Kurita Water and Environment Foundation and by the Light Metal Education Foundation of Japan. References [1] W. Lee, S.J. Park, Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures, Chem. Rev. 114 (15) (2014) 7487–7556. [2] S. Ono, N. Masuko, Evaluation of pore diameter of anodic porous films formed on aluminum, Surf. Coat. Technol. 169-170 (2003) 139–142. [3] H. Masuda, F. Hasegawa, S. Ono, Self-ordering of cell arrangement of anodic porous alumina formed in sulfuric acid solution, J. Electrochem. Soc. 144 (1997) L127–L130. [4] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science 268 (5216) (1995) 1466–1468. [5] H. Masuda, K. Yada, A. Osaka, Self-Ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution, Jpn. J. Appl. Phys. 37 (1998) L1340–L1342. [6] Y. Fukuda, Role of oxalate ion in the formation of oxide film of aluminum in oxalic acid electrolyte, Bull. Chem. Soc. Jpn. 10 (1974) 1868–1875. [7] G.E. Thompson, R.C. Furneaux, G.C. Wood, Electron microscopy of ion beam thinned porous anodic films formed on aluminium, Corros. Sci. 18 (1978) 481–498. [8] G.E. Thompson, G.C. Wood, Porous anodic film formation on aluminium, Nature 290 (1981) 230–232. [9] Y. Fukuda, T. Fukushima, Behavior of sulfate ions during formation of anodic oxide
4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al. film on aluminum, Bull. Chem. Soc. Jpn. 53 (1980) 3125–3130. [10] S. Ono, N. Masuko, The duplex structure of cell walls of porous anodic films formed on aluminium, Corros. Sci. 33 (3) (1992) 503–507. [11] F. Le Coz, L. Arurault, L. Datas, Chemical analysis of a single basic cell of porous anodic aluminium oxide templates, Mater. Charact. 61 (3) (2010) 283–288. [12] L. Fernández-Romero, J.M. Montero-Moreno, E. Pellicer, F. Peiró, A. Cornet, J.R. Morante, M. Sarret, C. Müller, Assessment of the thermal stability of anodic alumina membranes at high temperatures, Mater. Chem. Phys. 111 (2-3) (2008) 542–547. [13] K.S. Choudhari, P. Sudheendra, N.K. Udayashankar, Fabrication and high-temperature structural characterization study of porous anodic alumina membranes, J. Porous Mater. 19 (6) (2012) 1053–1062. [14] Y. Chang, Z. Ling, Y. Liu, X. Hu, Y. Li, A simple method for fabrication of highly ordered porous α-Alumina ceramic membranes, J. Mater. Chem. 22 (15) (2012) 7445–7448. [15] S. Ono, M. Nakamura, T. Masuda, H. Asoh, Fabrication of nanoporous crystalline alumina membrane by anodization of aluminum, Mater. Sci. Forum 783-786 (2014) 1470–1475. [16] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Nanoporous α-Alumina membrane prepared by anodizing and heat treatment, Electrochemistry 82 (6) (2014) 448–455. [17] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Fabrication and characterization of single phase α-Alumina membranes with tunable pore diameters, Materials 8 (3) (2015) 1350–1368. [18] H. Asoh, T. Masuda, S. Ono, Nanoporous α-Alumina membranes with pore
diameters tunable over wide range of 30–350 nm, ECS Trans. (2015) 225–233. [19] I. Levin, D. Brandon, Metastable alumina polymorphs: crystal structures and transition sequences, J. Am. Ceram. Soc. 81 (8) (1998) 1998–2012. [20] S.K. Lee, S.B. Lee, S.Y. Park, Y.S. Yi, C.W. Ahn, Structure of amorphous aluminum oxide, Phys. Rev. Lett. 103 (2009) 095501. [21] T. Abe, T. Uchiyama, T. Otuska, Heat-decomposition process of the anions incorporated into anodic oxide films of aluminum, Kinzoku Hyomen Gijutsu 27 (1976) 15–19. [22] K. Okubo, Structure chnages of anodic coatings on aluminum by chemical and heat treatments, Kinzoku Hyomen Gijutsu 28 (1977) 212–215. [23] R. Ozao, H. Yoshida, Y. Ichimura, T. Inada, M. Ochiai, Crystallization of anodic alumina membranes studied by simultaneous TG-DTA/FTIR, J. Therm. Anal. Calorim. 64 (2001) 915–922. [24] P.P. Mardilovich, A.N. Govyadinov, N.I. Mukhurov, A.M. Rzhevskii, R. Paterson, New and modified anodic alumina membranes part I. thermotreatment of anodic alumina membranes, J. Membr. Sci. 98 (1995) 131–142. [25] M.E. Mata-Zamora, J.M. Saniger, Thermal evolution of porous anodic aluminas: a comparative study, Rev. Mex. Fis. 51 (5) (2005) 502–509. [26] J.W. Diggle, T.C. Downie, C.W. Goulding, Anodic oxide films on aluminum, Chem. Rev. 69 (1969) 365–405. [27] J.P. O'Sullivan, G.C. Wood, The morphology and mechanism of formation of porous anodic films on aluminium, Proc. Roy. Soc. Lond. A. 317 (1970) 511–543. [28] T. Yanagishita, H. Masuda, High-Throughput fabrication process for highly ordered through-Hole porous alumina membranes using two-Layer anodization, Electrochim. Acta 184 (2015) 80–85.
5
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
α-Alumina membrane having a hierarchical structure of straight macropores and mesopores inside the pore wall ⁎
Hideki Hashimoto , Sumire Kojima, Takashi Sasaki, Hidetaka Asoh Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Anodization α-Alumina membrane Aluminum phosphate Mesopores
We prepared a through-hole α-alumina membrane with a hierarchical porous structure from an anodic porous alumina membrane prepared using phosphoric acid electrolyte as the starting material. By heating the anodic porous alumina membrane to 1400 °C, aluminum phosphate nanoparticles were segregated in the α-alumina matrix at the anion-incorporated outer layer and the high-purity alumina layer forming a cell boundary band sintered as a high-density α-alumina layer at the central core of the pore wall. When the heat-treated membrane was immersed in concentrated hydrochloric acid, a unique hierarchical porous α-alumina membrane structure was formed with straight macropores and mesopores inside the pore wall due to the dissolution of aluminum phosphate nanoparticles. The developed α-alumina membrane can be ultimately used as a multifunctional filter because of its unique hierarchical porous structure and extremely high chemical and thermal durability.
1. Introduction Amorphous alumina films having nanometric straight pores can be formed on an aluminum substrate by anodizing the aluminum in acidic electrolytes. Methods for forming porous oxide films have evolved for nearly a century as essential surface-modification techniques in the aluminum industry because anodic porous alumina films provide various properties to aluminum such as excellent corrosion resistance, hardness, abrasion resistance, and decorative coloration. [1] The cell structure of anodic porous alumina, including the interpore distance, the thickness of the barrier layer, and the pore diameter, is known to depend on the formation voltage;[1,2] a highly ordered cell arrangement can be obtained by anodization at specific self-ordering voltages. [3–5] Further, the pore wall is known to have a distinctive duplex structure that depends on the electrolyte species. [6–11] For example, using a phosphoric acid electrolyte affords a duplex structure having a high-purity alumina layer at the cell boundary band and an anion-incorporated layer occupying 50%–80% of the cell wall at the outer pore wall. [7,8,10,11] Anodization in phosphoric acid is generally conducted at relatively high voltage, yielding a film with a large cell size; for example, an interpore distance of 500 nm can be obtained with a voltage of 195 V. [5] The cell structure of anodic porous alumina is readily controllable using the techniques mentioned above, and the formed amorphous alumina can be easily detached from the aluminum substrate and dissolved using an appropriate etching agent. Therefore, intensive studies
⁎
employing anodic porous alumina as host and template materials for creating functional nanomaterials have been conducted. [1] The high solubility of anodic amorphous alumina may be a disadvantage depending on its intended application, such as in nano-filtration. Therefore, attempts to improve its chemical durability using heat treatment along with crystallization have been investigated. [12–18] Common amorphous alumina changes to thermodynamically stable α-alumina via transition alumina such as γ-, θ-, and η-phases; [19,20] the same phase transition occurs on anodic amorphous alumina through heat treatment. [21–25] Because the membrane film prepared via anodization is generally less symmetrical in vertical direction of the film, it is easily influenced by heat treatment, resulting in thermal deformation such as curving and cracking. Therefore, it is difficult to crystallize anodic alumina membranes to α-alumina membranes while maintaining their morphology and nanopores and while avoiding excess sintering. We have successfully prepared α-alumina membranes having straight pore channels with tunable pore diameters of 30–350 nm by applying appropriate restriction and heat treatment to the amorphous membrane. [15,17,18] We showed that these α-alumina membranes can be used as filters for suspended fluids. [17] Although the main focus for the application of anodic porous alumina is on using the straight pore channels, research on modulating the porous structure, such as pore branching modification and hierarchical control of the number of pores, has also been conducted. [1,26,27] Modulating the porous structure, e.g., introducing fine pores that are smaller than the main straight pore channels, could lead to a significant
Corresponding author. E-mail address: [email protected] (H. Hashimoto).
https://doi.org/10.1016/j.jeurceramsoc.2017.11.032 Received 29 August 2017; Received in revised form 29 October 2017; Accepted 13 November 2017 0955-2219/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Hashimoto, H., Journal of the European Ceramic Society (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.11.032
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al.
increase in the surface area of the membrane, and therefore an improvement in the filtration properties. The discovery of new applications through modulation of the porous structure is highly likely in applications of amorphous and crystalline alumina. In this study, an anodic alumina membrane with a duplex pore wall structure was formed in phosphoric acid and used as a starting material to create an α-alumina membrane with numerous mesopores inside the pore wall through the segregation and subsequent selective dissolution of aluminum phosphate (AlPO4) from an anion-incorporated layer. 2. Materials and methods The anodic porous alumina membrane was prepared using the twostep anodization process and subsequent two-layer anodization. [28] An electropolished high-purity (99.99%) aluminum sheet was first anodized in 0.2 mol dm−3 phosphoric acid at a constant voltage of 180 V at 10 °C for 1 h. A voltage value of 180 V was selected to achieve stable anodization for a long time. The electrolyte solution was stirred at 700 rpm by using a magnetic stirrer to remove generated Joule heat during anodization. The formed film was dissolved in a boiling solution of 6 wt% phosphoric acid and 2 wt% chromic acid for 10 min. The second anodization was performed for 5 h under the same conditions as those used in the first anodization. The film was detached via two-layer anodization as follows. [28] Anodization was conducted under a constant voltage of 180 V in 18 mol dm−3 sulfuric acid for 30 min at 20 °C to form a readily soluble alumina film containing sulfate ions at the interface between the aluminum substrate and the outer film formed due to the phosphoric acid anodization. Subsequent immersion in 2 wt % phosphoric acid at 30 °C for 15 min resulted in selective dissolution of the sacrificial layer generated via the sulfuric acid anodization step, thus detaching the outer film formed by the phosphoric acid anodization from the aluminum substrate. The detached through-hole membrane was carefully washed with deionized water and dried at room temperature. This detached membrane was sandwiched between two quartz plates, heat-treated by elevating the temperature at a rate of 10 °C min−1 to 1400 °C, and then cooled in a furnace. The heat-treated membrane was immersed in 36 wt% hydrochloric acid (HCl aq.) at 50 °C for 1 h to dissolve the AlPO4 particles, leaving a high-purity αalumina membrane. The crystallographic structures of the samples were characterized using X-ray diffractometry (XRD, RINT2500VHF, Rigaku) and the microstructures of the samples were evaluated using scanning electron microscopy (SEM, JSM-6701F, JEOL) and scanning transmission electron microscopy (STEM, JEM-2100F, JEOL) with an energy dispersive X-ray spectrometer (EDS, JED-2300T, JEOL). The surface area and pore size distribution of the obtained membranes were measured using the nitrogen adsorption method at 77 K using a BELSORP-mini-II system (BEL Japan). Prior to the analysis, each membrane was crushed into powder form and degassed under a vacuum for 5 h at 300 °C. Data were analyzed using the Brunauer–Emmet–Teller (BET) method to estimate the surface area and using the Barrett–Joyner–Hallender (BJH) method to determine the pore size distribution.
Fig. 1. I–t curves of the first and second steps of the anodization process conducted in 0.2 mol dm−3 phosphoric acid at 10 °C under an applied voltage of 180 V. The left inset shows the I–t curve for the subsequent anodization using 18 mol dm−3 sulfuric acid at 20 °C under an applied voltage of 180 V. The right inset shows the detached anodic alumina film (φ 25 mm).
step. The current density had a nearly constant value around 50 A m−2. The thickness of the formed film was adjusted to approximately 50 μm via anodization for 5 h. The sample was then re-anodized in 18 mol dm−3 sulfuric acid under a constant applied voltage of 180 V to form the highly soluble sacrificial layer containing sulfate ions below the film formed due to the phosphoric acid anodization. The I–t curve showed a relatively sharp peak of 28 A m−2 at around 7 min, and then showed a steady current value of 9–15 A m−2 (left inset in Fig. 1), suggesting stable formation of a porous film containing sulfate ions beneath the film formed due to the phosphoric acid anodization. Subsequent immersion in 2 wt% phosphoric acid detached this film from aluminum substrate (right inset in Fig. 1) as the sacrificial layer beneath it, which contains sulfate ions, was preferentially dissolved. [28] Thereby, a through-hole, stand-alone membrane film with straight pores was obtained. Fig. 2 shows SEM images of the detached film. The top surface view shows the distinctive nanoporous character of the anodic porous alumina, and the cross-sectional SEM images taken at the top (upper right inset) and bottom (lower left inset) surfaces show the straight, throughhole channels. The interpore distance and pore diameter were estimated to be ∼440 nm and ∼220 nm, respectively, from the SEM image. The values are roughly consistent with those calculated based on the empirical rule that the cell dimensions of anodic porous alumina are
3. Results and discussion Fig. 1 shows current–time (I–t) curves obtained during the first and second anodizing steps using 0.2 mol dm−3 phosphoric acid. An increasing current, which is the sign of pore formation, was observed in the initial stage and again in the second step at an earlier time point than that in the first step. Electropolished flat aluminum was anodized in the first step; the dimple structured surface of aluminum formed by dissolution of the first anodic porous film was anodized in the second step, which induced the pore formation at the depressions within the dimple structure. The applied current concentrated at the depression, prompting pore formation and causing the early current increase associated with pore formation that is seen in the second anodization
Fig. 2. An SEM image of the top surface of the detached film. The upper-right and lowerleft inset images show cross-sectional SEM images taken at the top and bottom surfaces of the film, respectively.
2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al.
Fig. 3. XRD patterns for the as-prepared amorphous alumina membrane (bottom), the membrane after heat-treatment at 1400 °C (middle), and the membrane after heat and HCl treatments (top).
Fig. 5. Pore size distributions of the amorphous alumina membrane as-prepared (open circles) and the membrane after heat and HCl treatments (solid circles).
proportional to the formation voltage: 2.5 nm V−1 interpore distance and 1 nm V−1 pore diameter. [1] The cross-sectional SEM image taken at the top surface (upper right inset in Fig. 2) showed a larger pore size than the image taken at the bottom surface (lower left inset in Fig. 2), indicating chemical dissolution of the pore walls. The XRD pattern of the obtained film showed a halo pattern, which is indicative of an amorphous structure (Fig. 3, bottom). Sharp, intense peaks of α-alumina; very weak peaks assigned to transition alumina such as γ-, θ-, and η-alumina; and relatively sharp peaks from AlPO4 were detected in the heat-treated amorphous alumina membrane at 1400 °C (Fig. 3, middle). To remove the formed AlPO4, the heat-treated membrane was immersed in 36 wt% HCl aq. at 50 °C for 1 h. The sharp XRD peaks of AlPO4 completely disappeared and nearly monophasic αalumina was afforded by the HCl treatment (the top pattern in Fig. 3). The resulting membrane was a crack-free, stand-alone porous α-alumina membrane. To confirm the nano-structural changes during heat and HCl treatments, microscopic analyses were conducted (Fig. 4). The ridge of the dimple structure formed in the first step anodization varied obscure, the surface varied in a ripple-like structure, the cell diameter showed little
change, and the pore diameter increased slightly after heating (Top images in Fig. 4a and b). Observing a cross-section, numerous nanoparticles with 20–50 nm diameters attached onto the pore walls after heating (Fig. 4b, bottom), although the pore wall surface was smooth prior to heating. The uniform contrast, which is considered a signature of high-purity alumina layers, was observed as indicated by the arrow in the bottom image in Fig. 4b. Here the anodic alumina pore wall formed due to phosphoric acid anodization is known to have a duplex structure comprising a high-purity alumina layer located at the cellboundary band and a phosphate-incorporated layer located at the outer pore wall. The fine particles formed due to the heat treatment were densely segregated on the outer pore wall, which is considered a signature of phosphate-incorporated layers, and were not segregated to the interface of the cells, which is considered a signature of high-purity alumina layers (Fig. 4b, bottom). The fine particles could be AlPO4 because XRD measurements detected an AlPO4 phase after heat treatment, as shown in Fig. 3. After the HCl treatment, numerous fine mesopores with pore diameters of 20–80 nm, which may have been formed due to dissolution of AlPO4 nanoparticles, were formed on the
Fig. 4. SEM images of the membrane (a) as-prepared, (b) after the heat treatment at 1400 °C, (c) after the heat and HCl treatments.
3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al.
Fig. 6. (a) HAADF-STEM image, (b) Al Kα and (c) P Kα elemental maps, and (d) a merged image of b (red) and c (green) of the membrane after the heat-treatment at 1400 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
outer pore wall (Fig. 4c). The sintering of α-alumina was found around the cell-boundary band as a central core formed due to the heat treatment (Fig. 4c). The pore size distribution of the formed mesopores and the increase in the surface area were confirmed by analyzing nitrogen adsorption/ desorption isotherms at 77 K using BJH and BET methods, respectively. Fig. 5 shows the pore size distribution of the amorphous alumina membrane as-prepared and the membrane after heating and after HCl treatments. Distinctive mesopores were not observed in the as-prepared membrane, but the membrane after heat and HCl treatments shows distinctive mesopores with diameters in the range of 10–40 nm with the mode around 20 nm, which is roughly consistent with the sizes of segregated AlPO4 particles after heat treatment and with the formed pores after HCl treatment. The BET surface area increased significantly from 3.5 m2 g−1 in the membrane as-prepared to 11.7 m2 g−1 in the heat- and HCl-treated membranes, which represents a nearly threefold increase. Thus, the unique double-layered cell structure comprising a high-density cell-boundary band and the porous outer cell wall was obtained. In other words, the resultant α-alumina membrane has a unique hierarchical structure of highly ordered straight macropores with diameters around 200 nm and mesopores inside the outer pore wall with diameters around 20 nm. Additionally, the obtained α-alumina membrane showed high chemical durability, as it did not dissolve even when placed in 36 wt% HCl aq. at 50 °C for 1 h. To confirm the distribution of AlPO4 nanoparticles on the outer pore wall after heat treatment, the heat-treated sample was analyzed via STEM/EDS (Fig. 6a–d). The heat-treated membrane was crushed to a fine powder and the particles were mounted on a microgrid for TEM observations; the area that is considered a pore wall was observed. The high-angle annular dark-field (HAADF)-STEM image (Fig. 6a) contains low- and high-contrast areas, which result from the density difference between α-alumina (3.95 g cm−3) and AlPO4 (2.57 g cm−3): it is known that the higher the sample density is, the higher the STEM image contrast is. The signal of Al Kα was detected from the whole area of the sample except for the low-contrast, spotty areas (Fig. 6b). On the other hand, the signal of P Kα was strongly detected from the low-contrast, spotty areas observed in the Al Kα map (Fig. 6c). The Al Kα and P Kα maps showed elemental distributions consistent with the high- and lowcontrast areas of the HAADF-STEM image, respectively. The HAADFSTEM image and the merged elemental maps of Al Kα and P Kα (Fig. 6a and Fig. 6d) clearly demonstrate the dispersive segregation of AlPO4 nanoparticles in the α-alumina matrix after heat treatment. These results conclude that the fine particles segregated in the pore wall are AlPO4 nanoparticles and that the removal of these nanoparticles results in a unique double-layered pore wall structure in α-alumina. Using well-known electrolytes, i.e., sulfuric acid or oxalic acid, sulfate or oxalate ions-incorporated films can be formed, respectively. These anions are eliminated from anion-incorporated layer as a gaseous form by heat treatment. [21–25] Because the formation of solid-state AlPO4 particles in an anion-incorporated layer is distinctive characteristic of the anodic porous alumina film prepared in phosphoric
acid electrolyte, formation of mesopores in the pore wall after removal of AlPO4 particles seems a unique phenomenon of phosphoric acid film. Merits of the α-alumina membrane include its chemical and thermal durability, unique porous structure, and high surface area. Therefore, various functional surfaces can be easily added to the membrane under extreme preparation conditions wherein the amorphous alumina membrane is dissolved under high temperature and high pressure using an acidic, basic, or organic solution. Because of its unique hierarchical porous structure and high chemical and thermal durability, the developed α-alumina membrane can be used as a multifunctional membrane filter in various field. 4. Conclusions An α-alumina membrane having unique porous structure was prepared by using an anodic porous alumina membrane as the starting material. The through-hole porous alumina membrane prepared using a two-layer anodization method with phosphoric acid and sulfuric acid as electrolytes was heat treated by increasing the temperature at a rate of 10 °C min−1 to 1400 °C. AlPO4 nanoparticles were segregated in the αalumina matrix at the pore wall. By removing the formed AlPO4 nanoparticles using 36 wt% HCl aq., numerous fine pores formed at the pore wall and high-density sintered α-alumina appeared as a central core at the cell boundary band. The result was a unique hierarchical porous α-alumina membrane structure with straight macropores and mesopores inside the pore walls. Acknowledgements We thank Mr. Hirofumi Inada and Dr. Taigo Takaishi for the measurements associated the nitrogen adsorption method. This study was financially supported by the Kurita Water and Environment Foundation and by the Light Metal Education Foundation of Japan. References [1] W. Lee, S.J. Park, Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures, Chem. Rev. 114 (15) (2014) 7487–7556. [2] S. Ono, N. Masuko, Evaluation of pore diameter of anodic porous films formed on aluminum, Surf. Coat. Technol. 169-170 (2003) 139–142. [3] H. Masuda, F. Hasegawa, S. Ono, Self-ordering of cell arrangement of anodic porous alumina formed in sulfuric acid solution, J. Electrochem. Soc. 144 (1997) L127–L130. [4] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science 268 (5216) (1995) 1466–1468. [5] H. Masuda, K. Yada, A. Osaka, Self-Ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution, Jpn. J. Appl. Phys. 37 (1998) L1340–L1342. [6] Y. Fukuda, Role of oxalate ion in the formation of oxide film of aluminum in oxalic acid electrolyte, Bull. Chem. Soc. Jpn. 10 (1974) 1868–1875. [7] G.E. Thompson, R.C. Furneaux, G.C. Wood, Electron microscopy of ion beam thinned porous anodic films formed on aluminium, Corros. Sci. 18 (1978) 481–498. [8] G.E. Thompson, G.C. Wood, Porous anodic film formation on aluminium, Nature 290 (1981) 230–232. [9] Y. Fukuda, T. Fukushima, Behavior of sulfate ions during formation of anodic oxide
4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
H. Hashimoto et al. film on aluminum, Bull. Chem. Soc. Jpn. 53 (1980) 3125–3130. [10] S. Ono, N. Masuko, The duplex structure of cell walls of porous anodic films formed on aluminium, Corros. Sci. 33 (3) (1992) 503–507. [11] F. Le Coz, L. Arurault, L. Datas, Chemical analysis of a single basic cell of porous anodic aluminium oxide templates, Mater. Charact. 61 (3) (2010) 283–288. [12] L. Fernández-Romero, J.M. Montero-Moreno, E. Pellicer, F. Peiró, A. Cornet, J.R. Morante, M. Sarret, C. Müller, Assessment of the thermal stability of anodic alumina membranes at high temperatures, Mater. Chem. Phys. 111 (2-3) (2008) 542–547. [13] K.S. Choudhari, P. Sudheendra, N.K. Udayashankar, Fabrication and high-temperature structural characterization study of porous anodic alumina membranes, J. Porous Mater. 19 (6) (2012) 1053–1062. [14] Y. Chang, Z. Ling, Y. Liu, X. Hu, Y. Li, A simple method for fabrication of highly ordered porous α-Alumina ceramic membranes, J. Mater. Chem. 22 (15) (2012) 7445–7448. [15] S. Ono, M. Nakamura, T. Masuda, H. Asoh, Fabrication of nanoporous crystalline alumina membrane by anodization of aluminum, Mater. Sci. Forum 783-786 (2014) 1470–1475. [16] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Nanoporous α-Alumina membrane prepared by anodizing and heat treatment, Electrochemistry 82 (6) (2014) 448–455. [17] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Fabrication and characterization of single phase α-Alumina membranes with tunable pore diameters, Materials 8 (3) (2015) 1350–1368. [18] H. Asoh, T. Masuda, S. Ono, Nanoporous α-Alumina membranes with pore
diameters tunable over wide range of 30–350 nm, ECS Trans. (2015) 225–233. [19] I. Levin, D. Brandon, Metastable alumina polymorphs: crystal structures and transition sequences, J. Am. Ceram. Soc. 81 (8) (1998) 1998–2012. [20] S.K. Lee, S.B. Lee, S.Y. Park, Y.S. Yi, C.W. Ahn, Structure of amorphous aluminum oxide, Phys. Rev. Lett. 103 (2009) 095501. [21] T. Abe, T. Uchiyama, T. Otuska, Heat-decomposition process of the anions incorporated into anodic oxide films of aluminum, Kinzoku Hyomen Gijutsu 27 (1976) 15–19. [22] K. Okubo, Structure chnages of anodic coatings on aluminum by chemical and heat treatments, Kinzoku Hyomen Gijutsu 28 (1977) 212–215. [23] R. Ozao, H. Yoshida, Y. Ichimura, T. Inada, M. Ochiai, Crystallization of anodic alumina membranes studied by simultaneous TG-DTA/FTIR, J. Therm. Anal. Calorim. 64 (2001) 915–922. [24] P.P. Mardilovich, A.N. Govyadinov, N.I. Mukhurov, A.M. Rzhevskii, R. Paterson, New and modified anodic alumina membranes part I. thermotreatment of anodic alumina membranes, J. Membr. Sci. 98 (1995) 131–142. [25] M.E. Mata-Zamora, J.M. Saniger, Thermal evolution of porous anodic aluminas: a comparative study, Rev. Mex. Fis. 51 (5) (2005) 502–509. [26] J.W. Diggle, T.C. Downie, C.W. Goulding, Anodic oxide films on aluminum, Chem. Rev. 69 (1969) 365–405. [27] J.P. O'Sullivan, G.C. Wood, The morphology and mechanism of formation of porous anodic films on aluminium, Proc. Roy. Soc. Lond. A. 317 (1970) 511–543. [28] T. Yanagishita, H. Masuda, High-Throughput fabrication process for highly ordered through-Hole porous alumina membranes using two-Layer anodization, Electrochim. Acta 184 (2015) 80–85.
5