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Heat-induced structural transformations of anodic porous alumina formed in phosphoric acid
Accepted Manuscript Heat-induced structural transformations of anodic porous alumina formed in phosphoric acid Hideki Hashimoto, Yoshihito Shigehara, Sachiko Ono, Hidetaka Asoh PII:
S1387-1811(18)30008-8
DOI:
10.1016/j.micromeso.2018.01.008
Reference:
MICMAT 8733
To appear in:
Microporous and Mesoporous Materials
Received Date: 15 November 2017 Revised Date:
5 January 2018
Accepted Date: 8 January 2018
Please cite this article as: H. Hashimoto, Y. Shigehara, S. Ono, H. Asoh, Heat-induced structural transformations of anodic porous alumina formed in phosphoric acid, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.01.008. 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 proof before it is published in its final 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.
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Heat-Induced Structural Transformations of Anodic Porous Alumina Formed in Phosphoric Acid
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Hideki Hashimoto,* Yoshihito Shigehara, Sachiko Ono, Hidetaka Asoh
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2665-1 Nakano, Hachioji, Tokyo 1982-0015, Japan
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Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University,
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*Corresponding author. Tel. & fax.: +81 426284537; E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract An anodic porous alumina prepared in phosphoric acid electrolyte was heat-treated under various conditions and
structural transformations were investigated. Heating the anodic
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porous alumina resulted in crystallization from amorphous phase to transition alumina at ~900 °C; subsequently, aluminum phosphate was formed in the anion-incorporated layer at
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~1000 °C, and finally, phase transition from transition alumina to α-alumina occurred above
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1350 °C. The aluminum phosphate particles grew, and sintering of the alumina and phase transition to α-alumina was promoted with increasing temperature and total quantity of heat. When the temperature and total quantity of heat were further increased, aluminum phosphate passed into the gaseous phase and the gas migrated through straight pores to the membrane
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surface along with sintering of the α-alumina. By removing the aluminum phosphate particles, the introduction of mesopores inside the pore walls and expansion of the surface area can be
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easily achieved.
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KEYWORDS: Anodization, Alumina, Phosphoric acid, Structural transformations, α-Alumina membrane, Aluminum phosphate
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ACCEPTED MANUSCRIPT 1. Introduction Anodization of aluminum in acidic electrolytes generates amorphous alumina film with straight pore channels, and this method allows high controllability of the cell structure, whose
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dimensions, such as the thickness of the barrier layer, the interpore distance, and the pore diameter of the anodic porous alumina, can be easily controlled via the formation voltage [1,
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2]. . Therefore, anodic porous alumina has attracted much attention from nanotechnology
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researchers as an attractive host and template material for creating nanomaterials and nanodevices [1].
Ease of solubility of anodic alumina may be a disadvantage depending on the application field, such as filtration. Research has been conducted on the crystallization of anodic porous
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alumina membrane by heating to achieve high chemical stability [3-9]. Anodic amorphous alumina crystallizes to thermally and chemically stable α-alumina via transition alumina such
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as γ-, δ-, orθ-alumina by heat treatment. Because the upper part of a thick anodic porous
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alumina film is chemically dissolved by electrolyte during prolonged anodization, the film is generally less symmetrical in the vertical direction. Therefore, it is easily influenced by heat treatment, undergoing thermal deformation such as curving and cracking, and it is difficult to crystallize α-alumina while retaining the membrane morphology and distinctive porous structure. We have successfully prepared α-alumina membrane films with tunable pore diameters of 30–350 nm by crystallizing anodic porous alumina while applying appropriate restrictions and thermal treatment, and we showed that the prepared α-alumina membrane can 3
ACCEPTED MANUSCRIPT be used to filter suspended fluids [6-9]. The application of anodic porous alumina mainly focuses on achieving straight pores, whereas some attempts to modulate porous structures with respect to pore branching and pore
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size have been conducted [1, 10-16]. Such attempts are important for expanding the application fields of anodic porous alumina in both amorphous and crystalline phases. In
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recent years, we focused on a duplex structure of anodic porous alumina to give a modulated
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porous structure. The pore wall structure of anodic porous alumina is known to be a distinctive duplex structure depending on the electrolyte species [17-22]. For example, when phosphoric acid is used as an electrolyte for anodization, a duplex structure composed of an anion-free alumina layer forming a cell boundary band and an anion-incorporated layer that
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occupies 50–80% of the outer cell wall is formed [18, 19, 21, 22].
Recently, using an anodic porous alumina formed in phosphoric acid, we prepared an
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α-alumina membrane having numerous mesopores inside its pore walls by dissolving
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aluminum phosphate nanoparticles formed by heat treatment [23]. The developed membrane can be used as a multifunctional filter because of its unique hierarchical porous structure—a combination of straight pore channels from the top to the bottom surface of the membrane and numerous mesopores inside the pore walls. However, there is little information on the segregation behavior of aluminum phosphate by heat treatment and the porous structure of alumina obtained after removal of the aluminum phosphate. In this study, an anodic porous alumina prepared in phosphoric acid electrolyte was 4
ACCEPTED MANUSCRIPT heat-treated under various conditions and the phase transitions of alumina and the decomposition behavior of the incorporated phosphate ions were investigated. In addition, segregation behavior of aluminum phosphate, and the variation in the porous structure of the
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crystalline alumina after removal of the aluminum phosphate was investigated.
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2. Materials and methods
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2.1 Preparation of Amorphous Alumina Membrane. The anodic porous alumina membrane was prepared using a two-step anodization and subsequent two-layer anodization [24, 25]. The detailed preparation process of the membrane using phosphoric acid was summarized in our previous report [23] and Supplementary materials.
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2.2 Heat treatment of the powder sample. The as-detached membrane was crushed into fine powder, and thermogravimetry-differential thermal analysis (TG-DTA) was performed on
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Rigaku Thermo plus EVO equipment at a heating rate of 10 °C min–1 to 1400 °C. The powder
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was heat-treated at a heating rate of 10 °C min–1 to 800 –1400 °C and cooled in a furnace. The crystallographic structure and chemical binding state of the heat-treated samples were evaluated by X-ray diffractometry (XRD, RINT2500VHF, Rigaku) and Fourier transform infrared spectroscopy (FTIR, FT-IR-4600, JASCO), respectively. FTIR spectra of γ-alumina (Kanto Chemical, 99%), α-alumina (Soekawa Chemicals, 99.9%), and aluminum phosphate (Kanto Chemical, 95%) were obtained for comparison. 2.3 Heat treatment of the membrane. The as-detached membrane was sandwiched between 5
ACCEPTED MANUSCRIPT two quartz plates, heat-treated at a heating rate of 1–30 °C min–1 to 1400 °C, and cooled in a furnace. For comparison, heat treatment was also conducted at an average heating rate of 1.5 °C min–1 to 1400 °C, kept for 4 h, and cooled at a rate of 0.83 °C min–1 to room
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temperature [8]. Each heat-treated membrane was immersed in 36 wt% hydrochloric acid at 50 °C for 1 h to dissolve aluminum phosphate particles [23]. The prepared membranes were
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evaluated by XRD and scanning electron microscopy (SEM, JSM-6701F, JEOL). The
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acid-treated samples were measured by the nitrogen adsorption method (BEL Japan, Belsorp Mini II) at −196 °C using a BELSORP-mini-II system (BEL Japan) to determine the pore size distribution and surface area. Prior to measurement, all samples were degassed under vacuum for 5 h at 300 °C. Data were analyzed by the Barrett-Joyner-Halender method to obtain the
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pore size distribution and the Brunauer-Emmett-Teller (BET) method to estimate the surface
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area.
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3. Results and discussion3.1 Preparation of amorphous alumina membrane. A through-hole membrane film with straight pores was obtained by two-step anodization using 0.2 mol dm−3 phosphoric acid and subsequent detachment by two-layer anodization using 18 mol dm−3 sulfuric acid. The current-time curves for two-step anodization (Fig. S1) and structural analysis (Fig. S2) of as-prepared membrane was summarized in Supplementary materials. 3.2 Structural Transformation of the Powder Sample. Figure 1 shows TG-DTA curves of 6
ACCEPTED MANUSCRIPT powdered phosphoric acid film. Curves of powdered oxalic acid film is displayed for comparison [7]. It is well known that anodic oxide film contains anions that originate from the electrolyte, and anion decomposition occurs just after crystallization by heating, producing a
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significant weight loss. However, powdered phosphoric acid film showed almost no weight change. The absence of weight loss after crystallization is consistent with previous reports [26,
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27]. Anion decomposition on the phosphoric acid film will be described later. For the DTA
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curve of powdered oxalic acid film, a sharp exothermic peak due to crystallization is observed at ~890 °C, and a weak exothermic peak due to phase transition to α-alumina is detected at ~1150 °C. However, for the DTA curve of powdered phosphoric acid film, three exothermic peaks were detected at ~860, ~1020, and ~1340 °C. Comparing both DTA curves, the
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exothermic peaks observed at ~860 and ~1340 °C could be assigned to crystallization of transition alumina and phase transition to α-alumina, respectively. Coz et al. assigned the
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three exothermic peaks to phase transition of γ- to δ-alumina, δ- to θ-alumina, and θ- to
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α-alumina [27]. To confirm the reactions related to the three exothermic peaks, XRD and FTIR measurements were conducted. Figure 2 shows XRD patterns of heat-treated powder samples. A broad diffraction profile was observed for the unheated powder sample and heat-treated powder samples up to 800 °C, indicating amorphous characteristics. Sharp Bragg peaks assigned to transition alumina were observed from 900 °C. Because the crystallization temperature was almost consistent with the first exothermic peak, the reaction was assigned to crystallization. A broad peak of the 7
ACCEPTED MANUSCRIPT amorphous phase became weaker, and diffraction peaks of transition alumina intensified from 900 °C to 1050 °C. A very broad diffraction peak of aluminum phosphate was detected at around 2θ = 20 ° at 1200 °C, and the diffraction peak of aluminum phosphate at 1300 °C was
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sharpened and intensified. Phosphate ions in the anion-incorporated layer could react with alumina to give aluminum phosphate crystals at around 1200 °C. At 1350 °C, diffraction
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peaks of aluminum phosphate and transition alumina further sharpened and diffraction peaks
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assigned to α-alumina appeared. Because this temperature region was almost consistent with the third exothermic peak, this was assigned to phase transition to α-alumina. Further increasing the temperature to 1400 °C completely transformed transition alumina to α-alumina, and the diffraction peaks of the crystalline phase sharpened.
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Figure 3a shows FTIR spectra. Spectra of γ-alumina, α-alumina, and aluminum phosphate were obtained for comparison. Although the entire spectrum profile of the unheated sample
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resembled that of γ-alumina, the profile was broader than that of γ-alumina, and an absorption
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band was detected at around 1100 cm-1. Comparing the spectra below 1000 cm-1, the broad profile seen in the spectrum of the unheated sample changed to resemble the spectral profile of γ-alumina at 900 °C, and it was almost consistent with the spectral profile of α-alumina above 1350 °C. This result is consistent with above XRD results. On the other hand, focusing on the absorption band near 1100 cm-1, the position and profile systematically changed depending on temperature. A similar band was detected in the spectrum of aluminum phosphate. Because this absorption band was assigned to the P–O stretching vibration mode 8
ACCEPTED MANUSCRIPT of tetrahedral PO43– [28], the absorption band observed near 1100 cm-1 in the spectra can be assigned to the P–O stretching vibration. The band top position of the P–O stretching vibration was plotted against the heat
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treatment temperature (Fig. 3b). The position gradually shifted to lower wavenumbers up to ~1000 °C, significantly shifted to lower wavenumbers at 1050 –1100 °C, and showed a stable
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value above 1150 °C. The result implies that a drastic local structural change around the
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phosphate ion occurred at 1050 –1100 °C. Because the crystal phase of aluminum phosphate was detected at around 1200 °C in the XRD pattern, the peak shift observed at 1050 –1100 °C in the FTIR spectra might be a sign of nucleation of aluminum phosphate before crystallization. This temperature region was almost consistent with the second exothermic
aluminum phosphate.
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peak observed in the DTA curve. The exothermic reaction could be related to the formation of
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3.3 Structural Transformation of the Membrane Sample. Heat treatment was conducted
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by sandwiching the as-prepared membrane film between two quartz plates to retain the membrane morphology. To vary the total quantity of heat that the membrane received, it was heat-treated at rates of 1–30 °C min–1 to 1400 °C and then cooled in a furnace. In addition, the as-prepared membrane was also heat-treated at an average rate of 1.5 °C min–1 to 1400 °C for 4 h and then cooled at a rate of 0.83 °C min–1. 3.3.1 XRD measurements. Figure 4 shows XRD patterns and photographs of heat-treated membranes. The membrane morphology was retained andthe membrane slightly whitened 9
ACCEPTED MANUSCRIPT after heating. The total quantity of heat was lower and higher when the heating rate increased and decreased, respectively. For the highest heating rate of 30 °C min–1, only diffraction peaks of transition alumina were detected; these peaks intensified and sharpened, and diffraction
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peaks of aluminum phosphate appeared at a heating rate of 20 °C min–1. Diffraction peaks of α-alumina appeared at a heating rate of 15 °C min–1. Transition alumina almost transformed to
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α-alumina, resulting in concomitance of α-alumina and aluminum phosphate at heating rates below 10 °C min–1. Diffraction peaks of α-alumina and aluminum phosphate intensified and
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sharpened at low heating rates below 10 °C min–1. When these heat-treated membranes were immersed in 36 wt% hydrochloric acid at 50 °C for 1 h, complete removal of aluminum phosphate was confirmed. The XRD pattern of the membrane heat-treated at 1 °C min–1 then
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acid treated is shown in the topmost part of Fig. 4 as a representative result of acid treatment. After acid treatment, diffraction peaks of aluminum phosphate were not detected and only the
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diffraction peaks of α-alumina were observed.
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3.3.2 SEM/EDS measurements. Figure 5 shows SEM top-view images and cross-sectional EDS elemental mapping images of heat-treated membranes. At a high heating rate of 30 °C min–1, the sample receives a low quantity of heat, and SEM images were identical to those of unheated membranes (Figs. 5a and d). When the heating rate is decreased to 1 °C min–1, the sample receives a higher quantity of heat, and an uneven pattern showing contrasting white and black areas was observed in a low magnification SEM image (Fig. 5b). In a high magnification SEM image, a porous alumina structure was observed and a low-contrast black 10
ACCEPTED MANUSCRIPT substance covered the pores (Fig. 5e). On the membrane heat-treated at 1400 °C for 4 h, which receives the highest quantity of heat, even contrast was observed in a low
black substance covered most of the alumina pores (Fig. 5f).
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magnification SEM image (Fig. 5c). In the high magnification SEM image, the low-contrast
When applying EDS analyses to cross sections of heat-treated membranes, a P Kα signal
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was evenly detected from the entire area at a high heating rate of 30 °C min–1 (Fig. 5g), while a P Kα signal was slightly concentrated near the top and bottom surfaces at a low heating rate
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of 1 °C min–1 (Fig. 5h). For the membrane heat-treated at 1400 °C for 4 h, a P Kα signal was strongly detected from the top and bottom surfaces (Fig. 5i). This was treated with hydrochloric acid and SEM observations were conducted. The black substance covering the
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alumina pores (Fig. 5f) was removed and the pores were clearly observed (inset to Fig. 5f). Because aluminum phosphate can be removed by acid treatment according to the XRD
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pattern (Fig. 4), the low-contrast black substance covering the alumina pores (Figs. 5e and f)
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was found to be aluminum phosphate crystals. The above results indicate that aluminum phosphate crystals formed at the top and bottom surfaces of the heat-treated membrane as a result of prolonged high temperature heating. Here, the as-prepared membrane was heat-treated at 1400 °C for 4 h without the restrictive quartz plates. Based on SEM measurements as shown in Fig. S3 in the Supplementary Materials and EDS analyses, the formation of aluminum phosphate was unconfirmed and the concentration of phosphorous in the membrane substantially decreased. Therefore, the formed aluminum 11
ACCEPTED MANUSCRIPT phosphate could pass into the gaseous form, and the gas could migrate through the straight alumina pores to be deposited outside the membrane. In other words, the reason why aluminum phosphate concentrated at the membrane surface after prolonged heating at
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1400 °C is that the quartz plates stopped the release of gaseous aluminum phosphate at the membrane surface by functioning as a lid. Almost no weight change was observed by TG
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measurements (Fig. 1), presumably because gaseous aluminum phosphate released from the
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anion-incorporated layer might adsorb onto an alumina sample holder and/or the surface of the powder sample.
3.3.3 SEM observations of segregated aluminum phosphate particles. To reveal the segregation of aluminum phosphate and microstructural change by crystallization of alumina,
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surface and cross-sectional structures of heat-treated membranes before and after acid treatment were observed by SEM (Fig. 6). After heating, the microstructure mainly reflected
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the segregated morphology of aluminum phosphate, while after acid treatment, it mainly
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consisted of a framework of alumina with the segregated morphology of aluminum phosphate. Because the particles observed after heat treatment (Figs. 6f–j) were completely removed by acid treatment (Figs. 6a–e), these were confirmed to be aluminum phosphate crystals. Surface images showed that aluminum phosphate was segregated at a ridge at the cell boundary (the black arrow in Fig. 6a), which was formed after the first anodization, and morphological changes were observed at the cell wall (the slope area of the depression, the white arrow in Fig. 6a) due to segregation of aluminum phosphate as the heating rate decreased, which 12
ACCEPTED MANUSCRIPT increased the quantity of heat received by the sample (the upper images of Figs. 6a–d). For the membrane heat-treated at 1400 °C for 4 h, remarkable segregation of aluminum phosphate was observed, and pores that were completely covered with aluminum phosphate were
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confirmed in some areas (the upper image of Fig. 6e). When observing cross sections of heat-treated membranes, cracks that were considered to be crystal grain boundaries of
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transition alumina were found at 30 °C min–1 and the crystal grain size was estimated to be
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30–200 nm (the lower image of Fig. 6a). The crystal grains disappeared and aluminum phosphate particles with diameters of 20–50 nm were formed on the entire cell wall at 10 °C min–1 (the lower image of Fig. 6b). At a heating rate of 3 °C min–1 that the sample received a higher quantity of heat, the particle size of the aluminum phosphate formed on the cell wall,
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which is considered to be an anion-incorporated layer, increased to 30–130 nm in diameter (the black arrow in the lower image of Fig. 6c). Cross-sectional images of a cell wall showed
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sintering of the cell boundary band, which is considered to be an anion-free alumina layer (the
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white arrow in the lower image of Fig. 6c). With further heat treatment at 1 °C min–1, aluminum phosphate particles grew and aggregated near the membrane surface (the middle image of Fig. 6d), although the number of particles decreased and the particle size was almost unchanged in the central layer of the membrane (the lowest image of Fig. 6d). For membranes heat-treated at 1400 °C for 4 h, a remarkable concentration of aluminum phosphate appeared at the membrane surface, covering some of the pores (the middle image of Fig. 6e), although the amount of segregated aluminum phosphate decreased and the cell walls became thinner in 13
ACCEPTED MANUSCRIPT the central layer of the membrane (the lowest image of Fig. 6e). 3.3.4 Morphological features of the membrane heat-treated then acid treated. The surfaces and cross-sectional structures of the membranes were also observed after acid
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treatment. Surface images showed an increase in the pore diameter as the heating rate decreased (the upper images of Figs. 6f–j). Because the variation in cell size caused by
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heating was small, expansion of the pore diameter could occur due to segregation of
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aluminum phosphate and volume contraction resulting from crystallization. Numerous fine pores attributed to segregation of aluminum phosphate at an anion-incorporated layer were confirmed at the pore walls (the gray arrow in the lower image of Fig. 6g), and their size increased as the heating rate decreased; finally, the pores disappeared after heating at 1400 °C
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for 4 h, and concomitantly the pore wall was sintered to form a thin dense wall. Cross-sectional images indicated only a small change for 30 °C min–1 compared to before acid
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treatment (the lower image of Fig. 6f). For 10 °C min–1, the cell wall was composed of a
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high-density cell boundary band (the black arrow in the lower image of Fig. 6g), and an outer layer having fine mesopores (the white arrow in the lower image of Fig. 6g). At an even lower heating rate, the central core of the cell wall was densified, the number of mesopores decreased, and the mesopores expanded (the lower images of Figs. 6h and i). For a membrane heat-treated at 1400 °C for 4 h, sintering of α-alumina proceeded, the mesopores disappeared, and the pore wall became smoother (the lower image of Fig. 6j). 3.3.5 Nitrogen adsorption measurements. To evaluate the size distribution of the mesopores 14
ACCEPTED MANUSCRIPT introduced at the cell walls and the surface area of the resultant membranes, nitrogen adsorption measurements were conducted. Figure 7 shows the pore size distributions of an as-prepared membrane and the heat-treated samples with mesopores introduced after acid
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treatment. Although the as-prepared membrane and the sample heat-treated at 30 °C min–1 then acid treated (Fig. 6f) showed no distinctive pores below 100 nm, a relatively sharp pore
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size distribution of 10–40 nm with a peak top of ~20 nm was observed in the sample
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heat-treated at 10 °C min–1 then acid treated (Fig. 6g). Note that the hierarchical porous structure was combined with large straight pore channels running from the top to the bottom of the membrane, and the mesopores inside the pore walls were easily introduced by segregation of aluminum phosphate via heat treatment and its subsequent removal by acid
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treatment. At 3 °C min–1 (Fig. 6h), a pore size distribution of 20–70 nm with a peak top of ~30 nm was observed, the pore volume was slightly reduced, and the pore size distribution
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increased. Pore sizes estimated using the nitrogen adsorption method were nearly consistent
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with the size of segregated aluminum phosphate and mesopores estimated from SEM measurements. Although the BET surface area of the sample heat-treated at 30 °C min–1 then acid treated was the same as that of the as-prepared membrane, the BET surface area significantly increased from 3.5 m2 g–1 for the as-prepared membrane to 11.7 m2 g–1 for the membranes heat-treated at 10 °C min–1 and 8.5 m2 g–1 for that treated at 3 °C min–1, i.e., 3.4 times and 2.5 times larger than the as-prepared membrane, respectively. This significant increase in the BET surface area was achieved mainly by the introduction of numerous 15
ACCEPTED MANUSCRIPT mesopores inside the pore wall. The above results indicate that the size of the mesopores and the surface area can be controlled by varying the heating conditions for the membrane.
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4. Conclusions
In contrast with the membranes prepared in sulfuric or oxalic acid, for the membrane prepared
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in phosphoric acid electrolyte, aluminum phosphate crystals was formed in an
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anion-incorporated layer after crystallization of transition alumina by heat treatment. Crystal grain boundaries and aluminum phosphate were formed in the transition alumina matrix at the anion-incorporated layer after crystallization. The particle size of aluminum phosphate increased, and sintering of alumina and phase transition to α-alumina was promoted as
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temperature and total quantity of heat increased. Sintering of the anion-free alumina layer located at a cell boundary proceeded because there were no aluminum phosphate particles.
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Further increasing the quantity of heat induced transition of solid-state aluminum phosphate
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to the gaseous phase, and the gas went out through straight macropores of the membrane along with sintering of the α-alumina framework, resulting in smooth pore walls.By exploiting the formation of aluminum phosphate crystals and subsequent removal of aluminum phosphate particles, numerous mesopores were formed in the pore wall and surface area was significantly increased. Because the anodization process has been used for an industrial surface treatment of aluminum for nearly a century, it could be relatively easy to produce crystalline alumina membranes for industrial production scale. The hierarchical 16
ACCEPTED MANUSCRIPT porous membranes prepared in this study are composed of α-alumina having high chemical stability and thermostability, therefore, the application for multifunctional reusable filters will
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be achieved.
Acknowledgments
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We thank Mr. Hirofumi Inada and Dr. Taigo Takaishi for the measurements of nitrogen
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adsorption method. A part of this study was financially supported by Kurita Water and
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Environment Foundation and the Light Metal Education Foundation of Japan.
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[24] H. Masuda, M. Satoh, Fabrication of Gold Nanodot Array Using Anodic Porous Alumina as an Evaporation Mask, Jpn. J. Appl. Phys., 35 (1996) L126-L129. [25] 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. [26] M.E. Mata-Zamora, J.M. Saniger, Thermal Evolution of Porous Anodic Aluminas: A Comparative Study, Rev. Mex. Fis., 51 (2005) 502-509. 20
ACCEPTED MANUSCRIPT [27] F. Le Coz, L. Arurault, S. Fontorbes, V. Vilar, L. Datas, P. Winterton, Chemical Composition and Structural Changes of Porous Templates Obtained by Anodising Aluminium in Phosphoric Acid Electrolyte, Surf. Interface Anal., 42 (2010) 227-233.
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[28] F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero, G. Colon, J.A. Navio, M. Macias, Structure, Texture, Surface Acidity, and Catalytic Activity of AlPO4–
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ZrO2 (5–50 wt% ZrO2) Catalysts Prepared by a Sol–Gel Procedure, J. Catal., 179 (1998)
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483-484.
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Figures
Figure 1. TG-DTA curves for films prepared using 0.2 mol dm-3 phosphoric acid and 0.3 mol
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heating rate of 10 °C min-1 to 1400 °C.
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dm-3 oxalic acid electrolytes. The films were crushed into fine powders and heat-treated at a
Figure 2. XRD patterns of unheated and heat-treated powder samples.
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Figure 3. (a) FTIR spectra of unheated and heat-treated powder and reference samples. (b)
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The band top position at ~1100 cm-1 was plotted against the treatment temperature.
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Figure 4. Photographs of unheated membrane (left) and the membrane heat-treated at 10 °C
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min–1 (right). XRD patterns of the unheated and heat-treated membranes. The topmost pattern
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is an XRD pattern of the membrane heat-treated at 1 °C min–1 then acid treated.
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Figure 5. SEM images of membranes heat-treated at (a, d) 30 °C min-1 and (b, e) 1 °C min-1. (c, f) SEM images of the membrane heat-treated at 1400 °C for 4 hours. The inset of Fig. 7f is
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an SEM image of the membrane heat-treated at 1400 °C for 4 hours then acid treated.
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Figure 6. Top surface and cross-sectional SEM images of membranes heat-treated at (a–e) 30, 10, 3, 1 °C min-1, and at 1400 °C for 4 hours, respectively. SEM images of membranes
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respectively.
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heat-treated at (f–j) 30, 10, 3, 1 °C min-1, and at 1400 °C for 4 hours then acid treated,
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Figure 7. Pore size distributions of as-prepared membrane and membranes heat-treated at 30,
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10, and 3 °C min-1 then acid treated (open circle, solid triangle, solid circle, and solid diamond,
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respectively).
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ACCEPTED MANUSCRIPT Heat-induced structural transformation of anodic porous alumina was investigated. Heat and acid treatments yielded α-alumina with unique hierarchical porous structure. The α-alumina has straight macropore channels and mesopores inside the pore wall.
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The surface area and size of mesopores can be tunable by heat treatment conditions.
S1387-1811(18)30008-8
DOI:
10.1016/j.micromeso.2018.01.008
Reference:
MICMAT 8733
To appear in:
Microporous and Mesoporous Materials
Received Date: 15 November 2017 Revised Date:
5 January 2018
Accepted Date: 8 January 2018
Please cite this article as: H. Hashimoto, Y. Shigehara, S. Ono, H. Asoh, Heat-induced structural transformations of anodic porous alumina formed in phosphoric acid, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.01.008. 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 proof before it is published in its final 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.
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Heat-Induced Structural Transformations of Anodic Porous Alumina Formed in Phosphoric Acid
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Hideki Hashimoto,* Yoshihito Shigehara, Sachiko Ono, Hidetaka Asoh
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2665-1 Nakano, Hachioji, Tokyo 1982-0015, Japan
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Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University,
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*Corresponding author. Tel. & fax.: +81 426284537; E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract An anodic porous alumina prepared in phosphoric acid electrolyte was heat-treated under various conditions and
structural transformations were investigated. Heating the anodic
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porous alumina resulted in crystallization from amorphous phase to transition alumina at ~900 °C; subsequently, aluminum phosphate was formed in the anion-incorporated layer at
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~1000 °C, and finally, phase transition from transition alumina to α-alumina occurred above
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1350 °C. The aluminum phosphate particles grew, and sintering of the alumina and phase transition to α-alumina was promoted with increasing temperature and total quantity of heat. When the temperature and total quantity of heat were further increased, aluminum phosphate passed into the gaseous phase and the gas migrated through straight pores to the membrane
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surface along with sintering of the α-alumina. By removing the aluminum phosphate particles, the introduction of mesopores inside the pore walls and expansion of the surface area can be
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easily achieved.
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KEYWORDS: Anodization, Alumina, Phosphoric acid, Structural transformations, α-Alumina membrane, Aluminum phosphate
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ACCEPTED MANUSCRIPT 1. Introduction Anodization of aluminum in acidic electrolytes generates amorphous alumina film with straight pore channels, and this method allows high controllability of the cell structure, whose
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dimensions, such as the thickness of the barrier layer, the interpore distance, and the pore diameter of the anodic porous alumina, can be easily controlled via the formation voltage [1,
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2]. . Therefore, anodic porous alumina has attracted much attention from nanotechnology
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researchers as an attractive host and template material for creating nanomaterials and nanodevices [1].
Ease of solubility of anodic alumina may be a disadvantage depending on the application field, such as filtration. Research has been conducted on the crystallization of anodic porous
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alumina membrane by heating to achieve high chemical stability [3-9]. Anodic amorphous alumina crystallizes to thermally and chemically stable α-alumina via transition alumina such
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as γ-, δ-, orθ-alumina by heat treatment. Because the upper part of a thick anodic porous
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alumina film is chemically dissolved by electrolyte during prolonged anodization, the film is generally less symmetrical in the vertical direction. Therefore, it is easily influenced by heat treatment, undergoing thermal deformation such as curving and cracking, and it is difficult to crystallize α-alumina while retaining the membrane morphology and distinctive porous structure. We have successfully prepared α-alumina membrane films with tunable pore diameters of 30–350 nm by crystallizing anodic porous alumina while applying appropriate restrictions and thermal treatment, and we showed that the prepared α-alumina membrane can 3
ACCEPTED MANUSCRIPT be used to filter suspended fluids [6-9]. The application of anodic porous alumina mainly focuses on achieving straight pores, whereas some attempts to modulate porous structures with respect to pore branching and pore
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size have been conducted [1, 10-16]. Such attempts are important for expanding the application fields of anodic porous alumina in both amorphous and crystalline phases. In
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recent years, we focused on a duplex structure of anodic porous alumina to give a modulated
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porous structure. The pore wall structure of anodic porous alumina is known to be a distinctive duplex structure depending on the electrolyte species [17-22]. For example, when phosphoric acid is used as an electrolyte for anodization, a duplex structure composed of an anion-free alumina layer forming a cell boundary band and an anion-incorporated layer that
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occupies 50–80% of the outer cell wall is formed [18, 19, 21, 22].
Recently, using an anodic porous alumina formed in phosphoric acid, we prepared an
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α-alumina membrane having numerous mesopores inside its pore walls by dissolving
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aluminum phosphate nanoparticles formed by heat treatment [23]. The developed membrane can be used as a multifunctional filter because of its unique hierarchical porous structure—a combination of straight pore channels from the top to the bottom surface of the membrane and numerous mesopores inside the pore walls. However, there is little information on the segregation behavior of aluminum phosphate by heat treatment and the porous structure of alumina obtained after removal of the aluminum phosphate. In this study, an anodic porous alumina prepared in phosphoric acid electrolyte was 4
ACCEPTED MANUSCRIPT heat-treated under various conditions and the phase transitions of alumina and the decomposition behavior of the incorporated phosphate ions were investigated. In addition, segregation behavior of aluminum phosphate, and the variation in the porous structure of the
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crystalline alumina after removal of the aluminum phosphate was investigated.
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2. Materials and methods
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2.1 Preparation of Amorphous Alumina Membrane. The anodic porous alumina membrane was prepared using a two-step anodization and subsequent two-layer anodization [24, 25]. The detailed preparation process of the membrane using phosphoric acid was summarized in our previous report [23] and Supplementary materials.
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2.2 Heat treatment of the powder sample. The as-detached membrane was crushed into fine powder, and thermogravimetry-differential thermal analysis (TG-DTA) was performed on
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Rigaku Thermo plus EVO equipment at a heating rate of 10 °C min–1 to 1400 °C. The powder
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was heat-treated at a heating rate of 10 °C min–1 to 800 –1400 °C and cooled in a furnace. The crystallographic structure and chemical binding state of the heat-treated samples were evaluated by X-ray diffractometry (XRD, RINT2500VHF, Rigaku) and Fourier transform infrared spectroscopy (FTIR, FT-IR-4600, JASCO), respectively. FTIR spectra of γ-alumina (Kanto Chemical, 99%), α-alumina (Soekawa Chemicals, 99.9%), and aluminum phosphate (Kanto Chemical, 95%) were obtained for comparison. 2.3 Heat treatment of the membrane. The as-detached membrane was sandwiched between 5
ACCEPTED MANUSCRIPT two quartz plates, heat-treated at a heating rate of 1–30 °C min–1 to 1400 °C, and cooled in a furnace. For comparison, heat treatment was also conducted at an average heating rate of 1.5 °C min–1 to 1400 °C, kept for 4 h, and cooled at a rate of 0.83 °C min–1 to room
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temperature [8]. Each heat-treated membrane was immersed in 36 wt% hydrochloric acid at 50 °C for 1 h to dissolve aluminum phosphate particles [23]. The prepared membranes were
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evaluated by XRD and scanning electron microscopy (SEM, JSM-6701F, JEOL). The
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acid-treated samples were measured by the nitrogen adsorption method (BEL Japan, Belsorp Mini II) at −196 °C using a BELSORP-mini-II system (BEL Japan) to determine the pore size distribution and surface area. Prior to measurement, all samples were degassed under vacuum for 5 h at 300 °C. Data were analyzed by the Barrett-Joyner-Halender method to obtain the
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pore size distribution and the Brunauer-Emmett-Teller (BET) method to estimate the surface
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area.
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3. Results and discussion3.1 Preparation of amorphous alumina membrane. A through-hole membrane film with straight pores was obtained by two-step anodization using 0.2 mol dm−3 phosphoric acid and subsequent detachment by two-layer anodization using 18 mol dm−3 sulfuric acid. The current-time curves for two-step anodization (Fig. S1) and structural analysis (Fig. S2) of as-prepared membrane was summarized in Supplementary materials. 3.2 Structural Transformation of the Powder Sample. Figure 1 shows TG-DTA curves of 6
ACCEPTED MANUSCRIPT powdered phosphoric acid film. Curves of powdered oxalic acid film is displayed for comparison [7]. It is well known that anodic oxide film contains anions that originate from the electrolyte, and anion decomposition occurs just after crystallization by heating, producing a
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significant weight loss. However, powdered phosphoric acid film showed almost no weight change. The absence of weight loss after crystallization is consistent with previous reports [26,
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27]. Anion decomposition on the phosphoric acid film will be described later. For the DTA
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curve of powdered oxalic acid film, a sharp exothermic peak due to crystallization is observed at ~890 °C, and a weak exothermic peak due to phase transition to α-alumina is detected at ~1150 °C. However, for the DTA curve of powdered phosphoric acid film, three exothermic peaks were detected at ~860, ~1020, and ~1340 °C. Comparing both DTA curves, the
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exothermic peaks observed at ~860 and ~1340 °C could be assigned to crystallization of transition alumina and phase transition to α-alumina, respectively. Coz et al. assigned the
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three exothermic peaks to phase transition of γ- to δ-alumina, δ- to θ-alumina, and θ- to
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α-alumina [27]. To confirm the reactions related to the three exothermic peaks, XRD and FTIR measurements were conducted. Figure 2 shows XRD patterns of heat-treated powder samples. A broad diffraction profile was observed for the unheated powder sample and heat-treated powder samples up to 800 °C, indicating amorphous characteristics. Sharp Bragg peaks assigned to transition alumina were observed from 900 °C. Because the crystallization temperature was almost consistent with the first exothermic peak, the reaction was assigned to crystallization. A broad peak of the 7
ACCEPTED MANUSCRIPT amorphous phase became weaker, and diffraction peaks of transition alumina intensified from 900 °C to 1050 °C. A very broad diffraction peak of aluminum phosphate was detected at around 2θ = 20 ° at 1200 °C, and the diffraction peak of aluminum phosphate at 1300 °C was
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sharpened and intensified. Phosphate ions in the anion-incorporated layer could react with alumina to give aluminum phosphate crystals at around 1200 °C. At 1350 °C, diffraction
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peaks of aluminum phosphate and transition alumina further sharpened and diffraction peaks
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assigned to α-alumina appeared. Because this temperature region was almost consistent with the third exothermic peak, this was assigned to phase transition to α-alumina. Further increasing the temperature to 1400 °C completely transformed transition alumina to α-alumina, and the diffraction peaks of the crystalline phase sharpened.
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Figure 3a shows FTIR spectra. Spectra of γ-alumina, α-alumina, and aluminum phosphate were obtained for comparison. Although the entire spectrum profile of the unheated sample
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resembled that of γ-alumina, the profile was broader than that of γ-alumina, and an absorption
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band was detected at around 1100 cm-1. Comparing the spectra below 1000 cm-1, the broad profile seen in the spectrum of the unheated sample changed to resemble the spectral profile of γ-alumina at 900 °C, and it was almost consistent with the spectral profile of α-alumina above 1350 °C. This result is consistent with above XRD results. On the other hand, focusing on the absorption band near 1100 cm-1, the position and profile systematically changed depending on temperature. A similar band was detected in the spectrum of aluminum phosphate. Because this absorption band was assigned to the P–O stretching vibration mode 8
ACCEPTED MANUSCRIPT of tetrahedral PO43– [28], the absorption band observed near 1100 cm-1 in the spectra can be assigned to the P–O stretching vibration. The band top position of the P–O stretching vibration was plotted against the heat
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treatment temperature (Fig. 3b). The position gradually shifted to lower wavenumbers up to ~1000 °C, significantly shifted to lower wavenumbers at 1050 –1100 °C, and showed a stable
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value above 1150 °C. The result implies that a drastic local structural change around the
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phosphate ion occurred at 1050 –1100 °C. Because the crystal phase of aluminum phosphate was detected at around 1200 °C in the XRD pattern, the peak shift observed at 1050 –1100 °C in the FTIR spectra might be a sign of nucleation of aluminum phosphate before crystallization. This temperature region was almost consistent with the second exothermic
aluminum phosphate.
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peak observed in the DTA curve. The exothermic reaction could be related to the formation of
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3.3 Structural Transformation of the Membrane Sample. Heat treatment was conducted
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by sandwiching the as-prepared membrane film between two quartz plates to retain the membrane morphology. To vary the total quantity of heat that the membrane received, it was heat-treated at rates of 1–30 °C min–1 to 1400 °C and then cooled in a furnace. In addition, the as-prepared membrane was also heat-treated at an average rate of 1.5 °C min–1 to 1400 °C for 4 h and then cooled at a rate of 0.83 °C min–1. 3.3.1 XRD measurements. Figure 4 shows XRD patterns and photographs of heat-treated membranes. The membrane morphology was retained andthe membrane slightly whitened 9
ACCEPTED MANUSCRIPT after heating. The total quantity of heat was lower and higher when the heating rate increased and decreased, respectively. For the highest heating rate of 30 °C min–1, only diffraction peaks of transition alumina were detected; these peaks intensified and sharpened, and diffraction
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peaks of aluminum phosphate appeared at a heating rate of 20 °C min–1. Diffraction peaks of α-alumina appeared at a heating rate of 15 °C min–1. Transition alumina almost transformed to
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α-alumina, resulting in concomitance of α-alumina and aluminum phosphate at heating rates below 10 °C min–1. Diffraction peaks of α-alumina and aluminum phosphate intensified and
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sharpened at low heating rates below 10 °C min–1. When these heat-treated membranes were immersed in 36 wt% hydrochloric acid at 50 °C for 1 h, complete removal of aluminum phosphate was confirmed. The XRD pattern of the membrane heat-treated at 1 °C min–1 then
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acid treated is shown in the topmost part of Fig. 4 as a representative result of acid treatment. After acid treatment, diffraction peaks of aluminum phosphate were not detected and only the
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diffraction peaks of α-alumina were observed.
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3.3.2 SEM/EDS measurements. Figure 5 shows SEM top-view images and cross-sectional EDS elemental mapping images of heat-treated membranes. At a high heating rate of 30 °C min–1, the sample receives a low quantity of heat, and SEM images were identical to those of unheated membranes (Figs. 5a and d). When the heating rate is decreased to 1 °C min–1, the sample receives a higher quantity of heat, and an uneven pattern showing contrasting white and black areas was observed in a low magnification SEM image (Fig. 5b). In a high magnification SEM image, a porous alumina structure was observed and a low-contrast black 10
ACCEPTED MANUSCRIPT substance covered the pores (Fig. 5e). On the membrane heat-treated at 1400 °C for 4 h, which receives the highest quantity of heat, even contrast was observed in a low
black substance covered most of the alumina pores (Fig. 5f).
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magnification SEM image (Fig. 5c). In the high magnification SEM image, the low-contrast
When applying EDS analyses to cross sections of heat-treated membranes, a P Kα signal
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was evenly detected from the entire area at a high heating rate of 30 °C min–1 (Fig. 5g), while a P Kα signal was slightly concentrated near the top and bottom surfaces at a low heating rate
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of 1 °C min–1 (Fig. 5h). For the membrane heat-treated at 1400 °C for 4 h, a P Kα signal was strongly detected from the top and bottom surfaces (Fig. 5i). This was treated with hydrochloric acid and SEM observations were conducted. The black substance covering the
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alumina pores (Fig. 5f) was removed and the pores were clearly observed (inset to Fig. 5f). Because aluminum phosphate can be removed by acid treatment according to the XRD
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pattern (Fig. 4), the low-contrast black substance covering the alumina pores (Figs. 5e and f)
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was found to be aluminum phosphate crystals. The above results indicate that aluminum phosphate crystals formed at the top and bottom surfaces of the heat-treated membrane as a result of prolonged high temperature heating. Here, the as-prepared membrane was heat-treated at 1400 °C for 4 h without the restrictive quartz plates. Based on SEM measurements as shown in Fig. S3 in the Supplementary Materials and EDS analyses, the formation of aluminum phosphate was unconfirmed and the concentration of phosphorous in the membrane substantially decreased. Therefore, the formed aluminum 11
ACCEPTED MANUSCRIPT phosphate could pass into the gaseous form, and the gas could migrate through the straight alumina pores to be deposited outside the membrane. In other words, the reason why aluminum phosphate concentrated at the membrane surface after prolonged heating at
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1400 °C is that the quartz plates stopped the release of gaseous aluminum phosphate at the membrane surface by functioning as a lid. Almost no weight change was observed by TG
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measurements (Fig. 1), presumably because gaseous aluminum phosphate released from the
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anion-incorporated layer might adsorb onto an alumina sample holder and/or the surface of the powder sample.
3.3.3 SEM observations of segregated aluminum phosphate particles. To reveal the segregation of aluminum phosphate and microstructural change by crystallization of alumina,
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surface and cross-sectional structures of heat-treated membranes before and after acid treatment were observed by SEM (Fig. 6). After heating, the microstructure mainly reflected
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the segregated morphology of aluminum phosphate, while after acid treatment, it mainly
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consisted of a framework of alumina with the segregated morphology of aluminum phosphate. Because the particles observed after heat treatment (Figs. 6f–j) were completely removed by acid treatment (Figs. 6a–e), these were confirmed to be aluminum phosphate crystals. Surface images showed that aluminum phosphate was segregated at a ridge at the cell boundary (the black arrow in Fig. 6a), which was formed after the first anodization, and morphological changes were observed at the cell wall (the slope area of the depression, the white arrow in Fig. 6a) due to segregation of aluminum phosphate as the heating rate decreased, which 12
ACCEPTED MANUSCRIPT increased the quantity of heat received by the sample (the upper images of Figs. 6a–d). For the membrane heat-treated at 1400 °C for 4 h, remarkable segregation of aluminum phosphate was observed, and pores that were completely covered with aluminum phosphate were
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confirmed in some areas (the upper image of Fig. 6e). When observing cross sections of heat-treated membranes, cracks that were considered to be crystal grain boundaries of
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transition alumina were found at 30 °C min–1 and the crystal grain size was estimated to be
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30–200 nm (the lower image of Fig. 6a). The crystal grains disappeared and aluminum phosphate particles with diameters of 20–50 nm were formed on the entire cell wall at 10 °C min–1 (the lower image of Fig. 6b). At a heating rate of 3 °C min–1 that the sample received a higher quantity of heat, the particle size of the aluminum phosphate formed on the cell wall,
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which is considered to be an anion-incorporated layer, increased to 30–130 nm in diameter (the black arrow in the lower image of Fig. 6c). Cross-sectional images of a cell wall showed
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sintering of the cell boundary band, which is considered to be an anion-free alumina layer (the
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white arrow in the lower image of Fig. 6c). With further heat treatment at 1 °C min–1, aluminum phosphate particles grew and aggregated near the membrane surface (the middle image of Fig. 6d), although the number of particles decreased and the particle size was almost unchanged in the central layer of the membrane (the lowest image of Fig. 6d). For membranes heat-treated at 1400 °C for 4 h, a remarkable concentration of aluminum phosphate appeared at the membrane surface, covering some of the pores (the middle image of Fig. 6e), although the amount of segregated aluminum phosphate decreased and the cell walls became thinner in 13
ACCEPTED MANUSCRIPT the central layer of the membrane (the lowest image of Fig. 6e). 3.3.4 Morphological features of the membrane heat-treated then acid treated. The surfaces and cross-sectional structures of the membranes were also observed after acid
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treatment. Surface images showed an increase in the pore diameter as the heating rate decreased (the upper images of Figs. 6f–j). Because the variation in cell size caused by
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heating was small, expansion of the pore diameter could occur due to segregation of
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aluminum phosphate and volume contraction resulting from crystallization. Numerous fine pores attributed to segregation of aluminum phosphate at an anion-incorporated layer were confirmed at the pore walls (the gray arrow in the lower image of Fig. 6g), and their size increased as the heating rate decreased; finally, the pores disappeared after heating at 1400 °C
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for 4 h, and concomitantly the pore wall was sintered to form a thin dense wall. Cross-sectional images indicated only a small change for 30 °C min–1 compared to before acid
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treatment (the lower image of Fig. 6f). For 10 °C min–1, the cell wall was composed of a
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high-density cell boundary band (the black arrow in the lower image of Fig. 6g), and an outer layer having fine mesopores (the white arrow in the lower image of Fig. 6g). At an even lower heating rate, the central core of the cell wall was densified, the number of mesopores decreased, and the mesopores expanded (the lower images of Figs. 6h and i). For a membrane heat-treated at 1400 °C for 4 h, sintering of α-alumina proceeded, the mesopores disappeared, and the pore wall became smoother (the lower image of Fig. 6j). 3.3.5 Nitrogen adsorption measurements. To evaluate the size distribution of the mesopores 14
ACCEPTED MANUSCRIPT introduced at the cell walls and the surface area of the resultant membranes, nitrogen adsorption measurements were conducted. Figure 7 shows the pore size distributions of an as-prepared membrane and the heat-treated samples with mesopores introduced after acid
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treatment. Although the as-prepared membrane and the sample heat-treated at 30 °C min–1 then acid treated (Fig. 6f) showed no distinctive pores below 100 nm, a relatively sharp pore
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size distribution of 10–40 nm with a peak top of ~20 nm was observed in the sample
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heat-treated at 10 °C min–1 then acid treated (Fig. 6g). Note that the hierarchical porous structure was combined with large straight pore channels running from the top to the bottom of the membrane, and the mesopores inside the pore walls were easily introduced by segregation of aluminum phosphate via heat treatment and its subsequent removal by acid
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treatment. At 3 °C min–1 (Fig. 6h), a pore size distribution of 20–70 nm with a peak top of ~30 nm was observed, the pore volume was slightly reduced, and the pore size distribution
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increased. Pore sizes estimated using the nitrogen adsorption method were nearly consistent
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with the size of segregated aluminum phosphate and mesopores estimated from SEM measurements. Although the BET surface area of the sample heat-treated at 30 °C min–1 then acid treated was the same as that of the as-prepared membrane, the BET surface area significantly increased from 3.5 m2 g–1 for the as-prepared membrane to 11.7 m2 g–1 for the membranes heat-treated at 10 °C min–1 and 8.5 m2 g–1 for that treated at 3 °C min–1, i.e., 3.4 times and 2.5 times larger than the as-prepared membrane, respectively. This significant increase in the BET surface area was achieved mainly by the introduction of numerous 15
ACCEPTED MANUSCRIPT mesopores inside the pore wall. The above results indicate that the size of the mesopores and the surface area can be controlled by varying the heating conditions for the membrane.
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4. Conclusions
In contrast with the membranes prepared in sulfuric or oxalic acid, for the membrane prepared
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in phosphoric acid electrolyte, aluminum phosphate crystals was formed in an
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anion-incorporated layer after crystallization of transition alumina by heat treatment. Crystal grain boundaries and aluminum phosphate were formed in the transition alumina matrix at the anion-incorporated layer after crystallization. The particle size of aluminum phosphate increased, and sintering of alumina and phase transition to α-alumina was promoted as
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temperature and total quantity of heat increased. Sintering of the anion-free alumina layer located at a cell boundary proceeded because there were no aluminum phosphate particles.
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Further increasing the quantity of heat induced transition of solid-state aluminum phosphate
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to the gaseous phase, and the gas went out through straight macropores of the membrane along with sintering of the α-alumina framework, resulting in smooth pore walls.By exploiting the formation of aluminum phosphate crystals and subsequent removal of aluminum phosphate particles, numerous mesopores were formed in the pore wall and surface area was significantly increased. Because the anodization process has been used for an industrial surface treatment of aluminum for nearly a century, it could be relatively easy to produce crystalline alumina membranes for industrial production scale. The hierarchical 16
ACCEPTED MANUSCRIPT porous membranes prepared in this study are composed of α-alumina having high chemical stability and thermostability, therefore, the application for multifunctional reusable filters will
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be achieved.
Acknowledgments
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We thank Mr. Hirofumi Inada and Dr. Taigo Takaishi for the measurements of nitrogen
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adsorption method. A part of this study was financially supported by Kurita Water and
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Environment Foundation and the Light Metal Education Foundation of Japan.
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ACCEPTED MANUSCRIPT References [1] W. Lee, S.J. Park, Porous Anodic Aluminum Oxide: Anodization and Templated Synthesis of Functional Nanostructures, Chem. Rev., 114 (2014) 7487-7556.
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[2] S. Ono, N. Masuko, Evaluation of Pore Diameter of Anodic Porous Films Formed on
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Morante, M. Sarret, C. Müller, Assessment of the Thermal Stability of Anodic Alumina Membranes at High Temperatures, Mater. Chem. Phys., 111 (2008) 542-547. [4] K.S. Choudhari, P. Sudheendra, N.K. Udayashankar, Fabrication and High-Temperature Structural Characterization Study of Porous Anodic Alumina Membranes, J. Porous Mater.,
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19 (2012) 1053-1062.
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Ordered Porous α-Alumina Ceramic Membranes, J. Mater. Chem., 22 (2012) 7445-7448.
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[6] 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.
[7] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Nanoporous α-Alumina Membrane Prepared by Anodizing and Heat Treatment, Electrochemistry, 82 (2014) 448-455. [8] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Fabrication and Characterization of Single Phase α-Alumina Membranes with Tunable Pore Diameters, Materials, 8 (2015) 1350-1368. 18
ACCEPTED MANUSCRIPT [9] H. Asoh, T. Masuda, S. Ono, Nanoporous α-Alumina Membranes with Pore Diameters Tunable over Wide Range of 30-350 nm, ECS Trans.2015, pp. 225-233. [10] J.W. Diggle, T.C. Downie, C.W. Goulding, Anodic Oxide Films on Aluminum, Chem.
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Rev., 69 (1969) 365-405.
[11] J.P. O'Sullivan, G.C. Wood, The Morphology and Mechanism of Formation of Porous
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Anodic Films on Aluminium, Proc. Roy. Soc. Lond. A., 317 (1970) 511-543.
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[12] J. Broughton, G.A. Davies, Porous Cellular Ceramic Membranes: A Stochastic Model to Describe the Structure of an Anodic Oxide Membrane, J. Membr. Sci., 106 (1995) 89-101. [13] J. Li, C. Papadopoulos, J.J. Xu, Growing Y-junction carbon nanotubes, Nature, 402 (1999) 253-254.
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[14] J.M. Montero-Moreno, M. Belenguer, M. Sarret, C.M. Müller, Production of alumina templates suitable for electrodeposition of nanostructures using stepped techniques,
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Electrochim. Acta, 54 (2009) 2529-2535.
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[15] S. Ono, S. Chiaki, T. Sato, Micro-Structure of Anodic Oxide Film on Aluminum Formed in Chromic Acid Solution, J. Surf. Finish. Soc. Jpn., 26 (1975) 456-460. [16] S. Ono, T. Sato, Difference of Structural Changes of Anodic Oxide Films on Aluminum during Current Recovery, Hyomen Gijutsu, 33 (1982) 249-255. [17] 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. [18] G.E. Thompson, R.C. Furneaux, G.C. Wood, Electron Microscopy of Ion Beam Thinned 19
ACCEPTED MANUSCRIPT Porous Anodic Films Formed on Aluminium, Corros. Sci., 18 (1978) 481-498. [19] G.E. Thompson, G.C. Wood, Porous Anodic Film Formation on Aluminium, Nature, 290 (1981) 230–232.
Film on Aluminum, Bull. Chem. Soc. Jpn., 53 (1980) 3125-3130.
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[20] Y. Fukuda, T. Fukushima, Behavior of Sulfate Ions during Formation of Anodic Oxide
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on Aluminium, Corros. Sci., 33 (1992) 503-507.
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[21] S. Ono, N. Masuko, The Duplex Structure of Cell Walls of Porous Anodic Films Formed
[22] F. Le Coz, L. Arurault, L. Datas, Chemical Analysis of a Single Basic Cell of Porous Anodic Aluminium Oxide Templates, Mater. Charact., 61 (2010) 283-288.
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[23] H. Hashimoto, S. Kojima, T. Sasaki, H. Asoh, α-Alumina Membrane Having a Hierarchical Structure of Straight Macropores and Mesopores Inside the Pore Wall, J. Eur.
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Ceram. Soc., In Press (2017).
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[24] H. Masuda, M. Satoh, Fabrication of Gold Nanodot Array Using Anodic Porous Alumina as an Evaporation Mask, Jpn. J. Appl. Phys., 35 (1996) L126-L129. [25] 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. [26] M.E. Mata-Zamora, J.M. Saniger, Thermal Evolution of Porous Anodic Aluminas: A Comparative Study, Rev. Mex. Fis., 51 (2005) 502-509. 20
ACCEPTED MANUSCRIPT [27] F. Le Coz, L. Arurault, S. Fontorbes, V. Vilar, L. Datas, P. Winterton, Chemical Composition and Structural Changes of Porous Templates Obtained by Anodising Aluminium in Phosphoric Acid Electrolyte, Surf. Interface Anal., 42 (2010) 227-233.
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[28] F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero, G. Colon, J.A. Navio, M. Macias, Structure, Texture, Surface Acidity, and Catalytic Activity of AlPO4–
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ZrO2 (5–50 wt% ZrO2) Catalysts Prepared by a Sol–Gel Procedure, J. Catal., 179 (1998)
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483-484.
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Figures
Figure 1. TG-DTA curves for films prepared using 0.2 mol dm-3 phosphoric acid and 0.3 mol
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heating rate of 10 °C min-1 to 1400 °C.
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dm-3 oxalic acid electrolytes. The films were crushed into fine powders and heat-treated at a
Figure 2. XRD patterns of unheated and heat-treated powder samples.
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Figure 3. (a) FTIR spectra of unheated and heat-treated powder and reference samples. (b)
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The band top position at ~1100 cm-1 was plotted against the treatment temperature.
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Figure 4. Photographs of unheated membrane (left) and the membrane heat-treated at 10 °C
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min–1 (right). XRD patterns of the unheated and heat-treated membranes. The topmost pattern
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is an XRD pattern of the membrane heat-treated at 1 °C min–1 then acid treated.
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Figure 5. SEM images of membranes heat-treated at (a, d) 30 °C min-1 and (b, e) 1 °C min-1. (c, f) SEM images of the membrane heat-treated at 1400 °C for 4 hours. The inset of Fig. 7f is
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an SEM image of the membrane heat-treated at 1400 °C for 4 hours then acid treated.
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Figure 6. Top surface and cross-sectional SEM images of membranes heat-treated at (a–e) 30, 10, 3, 1 °C min-1, and at 1400 °C for 4 hours, respectively. SEM images of membranes
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respectively.
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heat-treated at (f–j) 30, 10, 3, 1 °C min-1, and at 1400 °C for 4 hours then acid treated,
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Figure 7. Pore size distributions of as-prepared membrane and membranes heat-treated at 30,
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10, and 3 °C min-1 then acid treated (open circle, solid triangle, solid circle, and solid diamond,
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ACCEPTED MANUSCRIPT Heat-induced structural transformation of anodic porous alumina was investigated. Heat and acid treatments yielded α-alumina with unique hierarchical porous structure. The α-alumina has straight macropore channels and mesopores inside the pore wall.
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The surface area and size of mesopores can be tunable by heat treatment conditions.