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Ultra-slow growth rate: Accurate control of the thickness of porous anodic aluminum oxide films
Electrochemistry Communications 109 (2019) 106602
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Ultra-slow growth rate: Accurate control of the thickness of porous anodic aluminum oxide films
T
Jingcheng Wu, Yi Li , Zhengxiang Li, Shize Li, Le Shen, Xing Hu, Zhiyuan Ling ⁎
Department of Electronic Materials Science and Engineering, College of Materials Science and Engineering, South China University of Technology, Guangzhou, China
ARTICLE INFO
ABSTRACT
Keywords: Anodization Porous anodic aluminum oxide PAAO Low temperature Growth rate
Porous anodic aluminum oxide (PAAO) films have attracted widespread attention because they can be used as porous templates for the preparation of various nanomaterials. However, there are still some challenges. For example, although fine control of the thickness of AAO films is generally quite easy to achieve, it is difficult to control the thickness of extremely thin films (e.g. < 1 µm). This is important for practical applications where accurate control of film thickness is necessary. In the present work, PAAO films have been prepared in an electrolyte composed of a mixture of ethanol and oxalic acid under 40–50 V (at −20 to 0 °C), attaining a minimum growth rate (νa) of ~1.4 nm min−1 (at −20 °C). The influence of anodization conditions on νa, the regularity of structural cells, and the interpore distance has been studied in detail. The results show that νa can be reduced effectively by using an extremely low anodization temperature. Ultra-thin PAAO films with precisely controlled thicknesses have been fabricated using this approach.
1. Introduction Films of porous anodic aluminum oxide (PAAO), a widely used porous material, have attracted considerable research attention [1–12]. In 1995, Masuda et al. first reported the preparation of PAAO films with highly ordered pores using a two-step anodization process [4]. Since then, PAAO films have been used as porous templates for the synthesis of a variety of functional nanomaterials [13–18]. Moreover, PAAO films can be used directly as functional materials in many application areas, e.g., photonics [19], sensors [20], and biotechnology [21]. In recent years, the preparation of large, ordered nanodot arrays using PAAO films as a mask has been widely studied [17,18,22,23]. In order to form high quality nanodot arrays, the aspect ratio of the PAAO films (thickness/pore size) should be less than 10 [22]. For example, if we need to prepare nanodots with a diameter of 5 nm, the thickness of the PAAO film used should be less than 50 nm. In addition, ultra-thin nanoporous films with a thickness of tens of nanometers can also be used in single-molecule detection and ultrafast sequencing of DNA [24,25]. Therefore, developing appropriate anodization conditions to obtain ultra-thin PAAO films is important. Since the thickness is directly related to the growth rate (νa) of PAAO films and anodization time, an ultra-slow νa is essential for preparation of ultra-thin PAAO films with precisely controlled thickness. It is known that the value of νa in mild anodization is about 2000–10000 nm h−1 [5,10]. For hard anodization,
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the νa is even larger (> 50000 nm h−1) [5]. The growth rate can be slowed by reducing the concentration of the electrolyte [26,27]. However, in this method νa has a limiting value; as the electrolyte concentration decreases, νa cannot be reduced continuously [27]. There remain practical and fundamental questions regarding the minimum νa. In the present work, PAAO films have been prepared using 40–50 V in an ethanol and oxalic acid electrolyte (−20 to 0 °C), obtaining a νa of 83–165 nm h−1. An ultra-slow νa of ~1.4 nm min−1 was obtained by using both an extremely low anodization temperature (Ta) of −20 °C and adding ethanol to the oxalic acid electrolyte. The minimum νa obtained is approximately four times slower than that reported previously [27]. The thickness of PAAO films can therefore be precisely controlled by adjusting the anodization time. This is the first time that PAAO films have been prepared at such a low temperature. The results and findings should be helpful in expanding the application range of PAAO films. 2. Materials and methods Aluminum sheets (Research Institute of Nonferrous Metals and Rare Earth Applications, Beijing, China) with high purity (99.999%) were electropolished in a mixed solution of ethanol and perchloric acid (4:1, V/V) at 0–5 °C under 21 V for 5 min. Then the sheets were cleaned and put into a holder with a window 1 cm2 square. Anodization was
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.elecom.2019.106602 Received 13 October 2019; Received in revised form 6 November 2019; Accepted 6 November 2019 Available online 07 November 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Electrochemistry Communications 109 (2019) 106602
J. Wu, et al.
Fig. 1. The evolution of Ja as a function of anodization time under different conditions: ① 40 V, ~5 °C; ② 50 V, −20 °C; ③ 45 V, −20 °C; ④ 40 V, 0 °C; ⑤ 40 V, −20 °C; ① Electrolyte II; ②−⑤ Electrolyte I. The left inset shows an SEM image of the top surface of PAAO corresponding to ⑤, Scale bar = 100 nm. The right inset shows the relationships between the stable Ja, Ua, and Ta in Electrolyte I.
Fig. 3. (a) Cross-sectional SEM images of PAAO films prepared under 40 V with different Ta; (b) The evolution of thickness and va as a function of Ta. Scale bars = 1 μm.
conducted in an ethanol–oxalic acid electrolyte (Electrolyte I, 0.3 M oxalic acid:ethanol = 1:1, V/V) or 0.3 M oxalic acid solution (Electrolyte II) at –20 to 0 °C under 40–50 V. The anodization voltage was applied directly, and the anodization time was varied between 4 and 24 h to prepare PAAO films with different thicknesses. The residual aluminum substrates were removed using saturated CuCl2 solution at room temperature. The reagents (analytically pure) were produced by Guangzhou Chemical Reagent Factory, including oxalic acid dihydrate (99.5 wt%), perchloric acid (70–72 wt%), copper chloride dihydrate (99 wt%), ethanol (99.7 wt%). The microstructure of the PAAO films were examined using a field-emission scanning electron microscope (Carl Zeiss Merlin). A Keithley 2450 was used in the anodization processes; both the anodization current and the voltage could be monitored and recorded in real time. The deionized water was produced using a water purification system (Ultra pure, Hitech, Shanghai, China). A cooling system (DAWOXI DW-LS-2500 W) was also used in the present work. 3. Results and discussion Fig. 1 shows the evolution of anodization current density (Ja) as a function of anodization time under different conditions. It can be observed that Ja decreases exponentially with anodization time, which is different from traditional mild anodization [5,10]. Generally, Ja goes through four typical stages at the beginning of mild anodization (curve ①): (I) instant surge (high electronic current density caused by electrolysis of water) and sharp decrease (a rapid decrease in electronic current density due to the formation of compact alumina), (II) moderate
Fig. 2. Cross-sectional SEM images of PAAO films prepared under 40–50 V (–20 °C) in Electrolyte I with different anodization times. Scale bars = 100 nm.
2
Electrochemistry Communications 109 (2019) 106602
J. Wu, et al.
Fig. 4. SEM images of the barrier layer surface of PAAO films prepared under 40–50 V (−20 to 0 °C): (a)–(c) 40 V; (d)–(f) 45 V; (g)–(i) 50 V; (a), (d) and (g) 0 °C; (b), (e) and (h) −10 °C; (c), (f) and (i) −20 °C. Scale bars = 100 nm.
decreases with decreasing Ta, resulting in a reduction in the number of ions passing through the barrier layer per unit time (ionic current density). As a result, the total charge transferred decreases, and a lower Ja can be obtained [32]. In addition, the second inset (right) in Fig. 1 shows the relationship between the steady Ja, Ua and Ta. The values of the steady Ja were obtained after an anodization time of 6000 s. It can be observed that the steady Ja obtained at 50 V is larger than that obtained at 40 V, when the same Ta is applied [5,10]. Moreover, the steady Ja increases with Ta, when Ua is unchanged [33]. Fig. 2 shows cross-sections of the PAAO films prepared at –20 °C in Electrolyte I with different anodization times and values of Ua. The resulting film thicknesses are about 665 nm (40 V, 8 h), 1660 nm (40 V, 20 h), 900 nm (45 V, 8 h), 2270 nm (45 V, 20 h), 1195 nm (50 V, 8 h), and 3605 nm (50 V, 20 h). The average νa is 83 nm h−1 (40 V), 113 nm h−1 (45 V), and 165 nm h−1 (50 V). The thickness and average νa increase with Ua and anodization time. A minimum νa of ~1.4 nm min−1 has been obtained. This value is significantly slower than those obtained via traditional anodization processes (34–50 nm min−1) [5,10,34]. Considering that the stable Ja is only about 0.08 mA cm−2, 0.19 mA cm−2 and 0.24 mA cm−2 for PAAO films prepared under 40 V, 45 V and 50 V (–20 °C), the extremely slow νa can be reasonably explained, since νa is directly related to Ja [5,10,34]. The concentration and freezing temperature of the electrolyte can be
Table 1 The regularity value of the samples prepared under different Ua and Ta. Ta (°C)
Regularity (40 V)
Regularity (45 V)
Regularity (50 V)
0 −20
0.8945 0.8339
0.8826 0.8140
0.8694 0.7802
increase (tiny pores begin to form, resulting in thinning of the barrier layer, i.e., decreasing the resistance of the barrier layer), (III) slight decrease (some tiny pores stop growing, i.e., increasing the resistance of the barrier layer), (IV) steady Ja (equilibrium of self-organization process) [5,10,28–31]. The left inset in Fig. 1 shows the top surface of PAAO prepared under 40 V at –20 °C. Shallow tiny pores which have stopped growing can still be seen. This means that the thickness of the compact alumina might not reach its maximum when the tiny pores began to grow. The growth of compact alumina, the initiation and selected growth of tiny pores occur almost simultaneously. As a result, the four stages overlap with each other to produce the exponential decrease Ja shown in Fig. 1. Moreover, Ja increases with anodization voltage (Ua) or Ta for all the samples. The stable Ja of curve ④ is smaller than that obtained in 0.3 M oxalic acid electrolyte without addition of ethanol (curve ①) [5,10]. The stable Ja can be further reduced by using a lower Ta (curve ⑤). It is known that the diffusion rate of ions in the electrolyte 3
Electrochemistry Communications 109 (2019) 106602
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(Table 1). Measurements show that the regularity values of the samples are 0.8945 (40 V), 0.8826 (45 V) and 0.8694 (50 V) at 0 °C [36]. It is higher than the values obtained at –20 °C (0.8339, 0.8140 and 0.7802). In fact, the regularity of the cells is greatly influenced by va [36–38]. The regularity of the cells is low in the initial stage of anodization, since the surface of the aluminum sheets has not been pre-patterned. When va is slow, it is difficult to obtain a proper volume expansion during anodization [37,38]. As a result, the self-ordering regime will not work, resulting in low regularity of the cells [36–38]. In order to obtain PAAO with both precisely controlled thickness and excellent cell regularity, a modified two-step anodization process was used. The first anodization was performed in Electrolyte II under 50 V at 5 °C for 12 h. Then, the PAAO formed on the aluminum sheet was selectively removed using a mixed solution of CrO3 (30 g L−1) and H3PO4 (50 mL L−1) at ~65 °C for 1 h, forming highly ordered shallow pits on the surface of the aluminum sheet. The second anodization was conducted in Electrolyte I under 50 V at –20 °C for 8 h. The SEM images of the resulting PAAO are shown in Fig. 5. It can be seen that the cells show excellent regularity on both the top and the barrier layer surface of the PAAO. A proper volume expansion can be obtained by applying suitable anodization conditions, resulting in mechanical stress in PAAO films. It is proposed that the stress causes repulsive forces between the neighboring structural cells, facilitating the self-ordering behavior of the cells [38]. Therefore, a highly ordered cell arrangement can be obtained after a sufficiently long period of anodization (usually a few hours or more). The calculated regularity value is 0.9670; it is clearly better than that shown in Fig. 4i. Measurements show that the interpore distances in Fig. 5a and b are both ~120 nm, which agrees well with the “2.5 nm V−1” rule between Ua and interpore distance in mild anodization [5,10]. Under the current experimental conditions, the interpore distance is almost unchanged (~120 nm) at the same Ua, even when using a different electrolyte system and Ta. It can be seen that the modified two-step anodization process is simple and effective for the accurate control of the structural parameters of the PAAO film and produces excellent regularity, facilitating the use of the resulting PAAO films in many applications, e.g., template synthesis of quantum dot arrays, single-molecule detection and ultrafast sequencing of DNA or RNA.
Fig. 5. SEM images of the PAAO film prepared under 50 V using the modified two-step anodization process: (a) barrier layer surface; (b) top surface. Scale bars = 100 nm.
reduced by adding ethanol, facilitating a low value of Ja. In fact, the viscosity of the electrolyte is also an important factor affecting ion migration and Ja. The viscosity will increase with the decrease in Ta, helping to reduce the value of Ja [34]. Fig. 3a shows cross-sections of the PAAO films prepared under 40 V (24 h) in Electrolyte I with different values of Ta. Measurement results show that the resulting films are about 4150 nm (–10 °C), 8990 nm (–5 °C), and 12,000 nm (0 °C) thick. Fig. 3b shows the evolution of thickness and va as a function of Ta. It can be seen that thickness and va increase with Ta. A non-linear relationship between va and Ta is obtained, which agrees well with the modified Arrhenius equation reported previously [35]. In addition, it can be observed that the pore channels have low regularity, and that branched channels can be found (Figs. 2 and 3a). It is known that the regularity of pore channels will directly influence the applications of PAAO films. Therefore, it is also necessary to study the regularity of the PAAO films prepared at extremely low temperatures. Fig. 4 shows SEM images of the barrier layer surface of the PAAO films prepared under different Ua and Ta conditions. Ordered cells can be observed in some regions of the samples prepared under 40–50 V at 0 °C. It is known that the ordering or regularity of the structural cells of PAAO films will have a direct impact on their practical applications. Finding an appropriate method to quantify the regularity is significant from the viewpoints of both fundamental science and commercial applications. Recently, an improved method of calculating regularity has been proposed by our group [36]. The regularity value of the PAAO films prepared in the present work was also calculated using this method. The regularity of the cells worsens as the value of Ta decreases
4. Conclusions PAAO films have been prepared in an oxalic acid–ethanol electrolyte at 40–50 V at a minimum Ta of −20 °C. A minimum νa of ~1.4 nm min−1 which corresponds to a Ja of 0.08 mA cm−2 has been achieved. The νa increases with Ta when the other anodization conditions are unchanged. PAAO films with accurately controllable thickness and excellent cell regularity have been prepared using a modified twostep anodization process. The findings reported in the present work could effectively widen the applications of PAAO films, either as highprecision nanoporous templates or as ultrathin functional materials. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (51202071), Guangdong Natural Science Foundation (S2011040005425), Fundamental Research Funds for the Central Universities of China. In addition, many thanks for the strong support from Jasmine Li and Riana Feng. 4
Electrochemistry Communications 109 (2019) 106602
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References
[21] Y. Wang, A. Santos, G. Kaur, A. Evdokiou, D. Losic, Structurally engineered anodic alumina nanotubes as nano-carriers for delivery of anticancer therapeutics, Biomaterials 35 (2014) 5517–5526, https://doi.org/10.1016/j.biomaterials.2014. 03.059. [22] Y. Lei, W.P. Cai, G. Wilde, Highly ordered nanostructures with tunable size, shape and properties: a new way to surface nano-patterning using ultra-thin alumina masks, Prog. Mater Sci. 52 (2007) 465–539, https://doi.org/10.1016/j.pmatsci. 2006.07.002. [23] M.H. Wu, L.Y. Wen, Y. Lei, S. Ostendorp, K. Chen, G. Wilde, Ultrathin alumina membranes for surface nanopatterning in fabricating quantum-sized nanodots, Small 6 (2010) 695–699, https://doi.org/10.1002/smll.200902038. [24] A. Singer, M. Wanunu, W. Morrison, H. Kuhn, M. Frank-Kamenetskii, A. Meller, Nanopore based sequence specific detection of duplex DNA for genomic profiling, Nano Lett. 10 (2010) 738–742, https://doi.org/10.1021/nl100058y. [25] E.C. Yusko, J.M. Johnson, S. Majd, P. Prangkio, R.C. Rollings, J. Li, J. Yang, M. Mayer, Controlling protein translocation through nanopores with bio-inspired fluid walls, Nat. Nanotechnol. 6 (2011) 253–260, https://doi.org/10.1038/nnano. 2011.12. [26] X.F. Qin, J.Q. Zhang, X.J. Meng, L.F. Wang, C.H. Deng, G.Q. Ding, H. Zeng, X.H. Xu, Effect of ethanol on the fabrication of porous anodic alumina in sulfuric acid, Surf. Coat. Technol. 254 (2014) 398–401, https://doi.org/10.1016/j.surfcoat.2014.06. 050. [27] K. Surawathanawises, X. Cheng, Nanoporous anodic aluminum oxide with a longrange order and tunable cell sizes by phosphoric acid anodization on pre-patterned substrates, Electrochim. Acta 117 (2014) 498–503, https://doi.org/10.1016/j. electacta.2013.11.144. [28] Y. Li, Z.Y. Ling, X. Hu, Y.S. Liu, Y. Chang, Investigation of intrinsic mechanisms of aluminium anodization processes by analyzing the current density, RSC Adv. 2 (2012) 5164–5171, https://doi.org/10.1039/c2ra01050j. [29] Y. Li, Y.Y. Qin, S.Y. Jin, X. Hu, Z.Y. Ling, Q.H. Liu, J.F. Liao, C. Chen, Y.H. Shen, L. Jin, A new self-ordering regime for fast production of long-range ordered porous anodic aluminum oxide films, Electrochim. Acta 178 (2015) 11–17, https://doi. org/10.1016/j.electacta.2015.07.119. [30] K. Zhang, S.K. Cao, C. Li, J.R. Qi, L.F. Jiang, J.J. Zhang, X.F. Zhu, Rapid growth of TiO2 nanotubes under the compact oxide layer: Evidence against the digging manner of dissolution reaction, Electrochem. Commun. 103 (2019) 88–93, https:// doi.org/10.1016/j.elecom.2019.05.015. [31] W.Q. Huang, H.Q. Xu, Z.R. Ying, Y.X. Dan, Q.Y. Zhou, J.J. Zhang, X.F. Zhu, Split TiO2 nanotubes − evidence of oxygen evolution during Ti anodization, Electrochem. Commun. 106 (2019) 106532, , https://doi.org/10.1016/j.elecom. 2019.106532. [32] C.V. Manzano, J. Martín, M.S. Martín-González, Ultra-narrow 12 nm pore diameter self-ordered anodic alumina templates, Microporous Mesoporous Mater. 184 (2014) 177–183, https://doi.org/10.1016/j.micromeso.2013.10.004. [33] M. Almasi Kashi, A. Ramazani, The effect of temperature and concentration on the self-organized pore formation in anodic alumina, J. Phys. D: Appl. Phys., 38 (2005), 2396–2399, doi:10.1088/0022-3727/38/14/015. [34] S. Shingubara, O. Okino, Y. Sayama, H. Sakaue, T. Takahagi, Ordered two-dimensional nanowire array formation using self-organized nanoholes of anodically oxidized aluminium, Jpn. J. Appl. Phys. 36 (1997) 7791–7795, https://doi.org/10. 1143/jjap.36.7791. [35] B. Sellarajan, M. Sharma, S.K. Ghosh, H.S. Nagaraja, H.C. Barshilia, P. Chowdhury, Effect of electrolyte temperature on the formation of highly ordered nanoporous alumina template, Microporous Mesoporous Mater. 224 (2016) 262–270, https:// doi.org/10.1016/j.micromeso.2015.12.045. [36] Z.X. Li, Y. Li, S.Z. Li, J.C. Wu, X. Hu, Z.Y. Ling, L. Jin, A modified quantitative method for regularity evaluation of porous AAO and related intrinsic mechanisms, J. Electrochem. Soc. 165 (2018) E214–E220, https://doi.org/10.1149/2. 0841805jes. [37] T.T. Kao, Y.C. Chang, Influence of anodization parameters on the volume expansion of anodic aluminum oxide formed in mixed solution of phosphoric and oxalic acids, Appl. Surf. Sci. 288 (2014) 654–659, https://doi.org/10.1016/j.apsusc.2013.10. 091. [38] O. Jessensky, F. Müller, U. Gösele, Self-organized formation of hexagonal pore arrays in anodic alumina, Appl. Phys. Lett. 72 (1998) 1173–1175, https://doi.org/10. 1063/1.121004.
[1] F. Keller, M.S. Hunter, D.L. Robinson, Structural features of oxide coatings on aluminum, J. Electrochem. Soc. 100 (1953) 411–419, https://doi.org/10.1149/1. 2781142. [2] J.W. Diggle, T.C. Downie, C.W. Goulding, Anodic oxide films on aluminium, Chem. Rev. 69 (1969) 365–405, https://doi.org/10.1021/cr60259a005. [3] G.E. Thompson, R.C. Furneaux, G.C. Wood, J.A. Richardson, J.S. Goode, Nucleation and growth of porous anodic films on aluminium, Nature 272 (1978) 433–435, https://doi.org/10.1038/272433a0. [4] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science 268 (1995) 1466–1468, https://doi.org/10.1126/science.268.5216.1466. [5] W. Lee, R. Ji, U. Gösele, K. Nielsch, Fast fabrication of long-range ordered porous alumina membranes by hard anodization, Nat. Mater. 5 (2006) 741–747, https:// doi.org/10.1038/nmat1717. [6] Y. Li, Z.Y. Ling, S.S. Chen, X. Hu, X.H. He, Novel AAO films and hollow nanostructures fabricated by ultra-high voltage hard anodization, Chem. Commun. 46 (2010) 309–311, https://doi.org/10.1039/b914703a. [7] G.E.J. Poinern, N. Ali, D. Fawcett, Progress in nano-engineered anodic aluminum oxide membrane development, Materials 4 (2011) 487–526, https://doi.org/10. 3390/ma4030487. [8] Y. Song, L.F. Jiang, W.X. Qi, C. Lu, X.F. Zhu, H.B. Jia, High-field anodization of aluminum in concentrated acid solutions and at higher temperatures, J. Electroanal. Chem. 673 (2012) 24–31, https://doi.org/10.1016/j.jelechem.2012.03.017. [9] X.F. Zhu, Y. Song, D.L. Yu, C.S. Zhang, W. Yao, A novel nanostructure fabricated by an improved two-step anodizing technology, Electrochem. Commun. 29 (2013) 71–74, https://doi.org/10.1016/j.elecom.2013.01.018. [10] W. Lee, S.-J. Park, Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures, Chem. Rev. 114 (2014) 7487–7556, https:// doi.org/10.1021/cr500002z. [11] M.S. Yu, C. Li, Y.B. Yang, S.K. Xu, K. Zhang, H.M. Cui, X.F. Zhu, Cavities between the double walls of nanotubes: evidence of oxygen evolution beneath an anioncontaminated layer, Electrochem. Commun. 90 (2018) 34–38, https://doi.org/10. 1016/j.elecom.2018.03.009. [12] S.Z. Li, Y. Li, S.Y. Jin, J.C. Wu, Z.X. Li, X. Hu, Z.Y. Ling, Fabrication of crystallized porous anodic aluminum oxide under ultra-high anodization voltage, J. Electrochem. Soc. 165 (2018) E623–E627, https://doi.org/10.1149/2.0141813jes. [13] E.M. Palmero, M. Méndez, S. González, C. Bran, V. Vega, M. Vázquez, V.M. Prida, Stepwise magnetization reversal of geometrically tuned in diameter Ni and FeCo bisegmented nanowire arrays, Nano Res. 12 (2019) 1547–1553, https://doi.org/10. 1007/s12274-019-2385-9. [14] A. Yadav, M.S. Bobji, S.J. Bull, Controlled growth of highly aligned Cu nanowires by pulse electrodeposition in nanoporous alumina, J. Nanosci. Nanotechnol. 19 (2019) 4254–4259, https://doi.org/10.1166/jnn.2019.16868. [15] A. Mezni, T. Altalhi, N.B. Saber, A. Aldalbahi, S. Boulehmi, A. Santos, D. Losic, Sizeand shape-controlled synthesis of well-organised carbon nanotubes using nanoporous anodic alumina with different pore diameters, J. Colloid Interface Sci. 491 (2017) 375–389, https://doi.org/10.1016/j.jcis.2016.12.035. [16] M. Alsawat, T. Altalhi, A. Santos, D. Losic, Carbon nanotubes–nanoporous anodic alumina composite membranes: influence of template on structural, chemical, and transport properties, J. Phys. Chem. C 121 (2017) 13634–13644, https://doi.org/ 10.1021/acs.jpcc.7b01257. [17] T. Yanagishita, H. Masuda, Facile preparation of porous alumina through-hole masks for sputtering by two-layer anodization, AIP Adv. 6 (2016) 085108, , https:// doi.org/10.1063/1.4961126. [18] M. Jung, J.H. Kim, Y.H. Choi, Preparation of anodic aluminum oxide masks with size-controlled pores for 2D plasmonic nanodot arrays, J. Nanomater. 2018 (2018) 6249890, https://doi.org/10.1155/2018/6249890. [19] L.K. Acosta, F. Bertó-Roselló, E. Xifre-Perez, A. Santos, J. Ferré-Borrull, L.F. Marsal, Stacked nanoporous anodic alumina gradient-index filters with tunable multispectral photonic stopbands as sensing platforms, ACS Appl. Mater. Interfaces 11 (2019) 3360–3371, https://doi.org/10.1021/acsami.8b19411. [20] Z.Y. Ling, S.S. Chen, J.C. Wang, Y. Li, Fabrication and properties of anodic alumina humidity sensor with through-hole structure, Chin. Sci. Bull. 53 (2008) 183–187, https://doi.org/10.1007/s11434-008-0008-z.
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Ultra-slow growth rate: Accurate control of the thickness of porous anodic aluminum oxide films
T
Jingcheng Wu, Yi Li , Zhengxiang Li, Shize Li, Le Shen, Xing Hu, Zhiyuan Ling ⁎
Department of Electronic Materials Science and Engineering, College of Materials Science and Engineering, South China University of Technology, Guangzhou, China
ARTICLE INFO
ABSTRACT
Keywords: Anodization Porous anodic aluminum oxide PAAO Low temperature Growth rate
Porous anodic aluminum oxide (PAAO) films have attracted widespread attention because they can be used as porous templates for the preparation of various nanomaterials. However, there are still some challenges. For example, although fine control of the thickness of AAO films is generally quite easy to achieve, it is difficult to control the thickness of extremely thin films (e.g. < 1 µm). This is important for practical applications where accurate control of film thickness is necessary. In the present work, PAAO films have been prepared in an electrolyte composed of a mixture of ethanol and oxalic acid under 40–50 V (at −20 to 0 °C), attaining a minimum growth rate (νa) of ~1.4 nm min−1 (at −20 °C). The influence of anodization conditions on νa, the regularity of structural cells, and the interpore distance has been studied in detail. The results show that νa can be reduced effectively by using an extremely low anodization temperature. Ultra-thin PAAO films with precisely controlled thicknesses have been fabricated using this approach.
1. Introduction Films of porous anodic aluminum oxide (PAAO), a widely used porous material, have attracted considerable research attention [1–12]. In 1995, Masuda et al. first reported the preparation of PAAO films with highly ordered pores using a two-step anodization process [4]. Since then, PAAO films have been used as porous templates for the synthesis of a variety of functional nanomaterials [13–18]. Moreover, PAAO films can be used directly as functional materials in many application areas, e.g., photonics [19], sensors [20], and biotechnology [21]. In recent years, the preparation of large, ordered nanodot arrays using PAAO films as a mask has been widely studied [17,18,22,23]. In order to form high quality nanodot arrays, the aspect ratio of the PAAO films (thickness/pore size) should be less than 10 [22]. For example, if we need to prepare nanodots with a diameter of 5 nm, the thickness of the PAAO film used should be less than 50 nm. In addition, ultra-thin nanoporous films with a thickness of tens of nanometers can also be used in single-molecule detection and ultrafast sequencing of DNA [24,25]. Therefore, developing appropriate anodization conditions to obtain ultra-thin PAAO films is important. Since the thickness is directly related to the growth rate (νa) of PAAO films and anodization time, an ultra-slow νa is essential for preparation of ultra-thin PAAO films with precisely controlled thickness. It is known that the value of νa in mild anodization is about 2000–10000 nm h−1 [5,10]. For hard anodization,
⁎
the νa is even larger (> 50000 nm h−1) [5]. The growth rate can be slowed by reducing the concentration of the electrolyte [26,27]. However, in this method νa has a limiting value; as the electrolyte concentration decreases, νa cannot be reduced continuously [27]. There remain practical and fundamental questions regarding the minimum νa. In the present work, PAAO films have been prepared using 40–50 V in an ethanol and oxalic acid electrolyte (−20 to 0 °C), obtaining a νa of 83–165 nm h−1. An ultra-slow νa of ~1.4 nm min−1 was obtained by using both an extremely low anodization temperature (Ta) of −20 °C and adding ethanol to the oxalic acid electrolyte. The minimum νa obtained is approximately four times slower than that reported previously [27]. The thickness of PAAO films can therefore be precisely controlled by adjusting the anodization time. This is the first time that PAAO films have been prepared at such a low temperature. The results and findings should be helpful in expanding the application range of PAAO films. 2. Materials and methods Aluminum sheets (Research Institute of Nonferrous Metals and Rare Earth Applications, Beijing, China) with high purity (99.999%) were electropolished in a mixed solution of ethanol and perchloric acid (4:1, V/V) at 0–5 °C under 21 V for 5 min. Then the sheets were cleaned and put into a holder with a window 1 cm2 square. Anodization was
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.elecom.2019.106602 Received 13 October 2019; Received in revised form 6 November 2019; Accepted 6 November 2019 Available online 07 November 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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Fig. 1. The evolution of Ja as a function of anodization time under different conditions: ① 40 V, ~5 °C; ② 50 V, −20 °C; ③ 45 V, −20 °C; ④ 40 V, 0 °C; ⑤ 40 V, −20 °C; ① Electrolyte II; ②−⑤ Electrolyte I. The left inset shows an SEM image of the top surface of PAAO corresponding to ⑤, Scale bar = 100 nm. The right inset shows the relationships between the stable Ja, Ua, and Ta in Electrolyte I.
Fig. 3. (a) Cross-sectional SEM images of PAAO films prepared under 40 V with different Ta; (b) The evolution of thickness and va as a function of Ta. Scale bars = 1 μm.
conducted in an ethanol–oxalic acid electrolyte (Electrolyte I, 0.3 M oxalic acid:ethanol = 1:1, V/V) or 0.3 M oxalic acid solution (Electrolyte II) at –20 to 0 °C under 40–50 V. The anodization voltage was applied directly, and the anodization time was varied between 4 and 24 h to prepare PAAO films with different thicknesses. The residual aluminum substrates were removed using saturated CuCl2 solution at room temperature. The reagents (analytically pure) were produced by Guangzhou Chemical Reagent Factory, including oxalic acid dihydrate (99.5 wt%), perchloric acid (70–72 wt%), copper chloride dihydrate (99 wt%), ethanol (99.7 wt%). The microstructure of the PAAO films were examined using a field-emission scanning electron microscope (Carl Zeiss Merlin). A Keithley 2450 was used in the anodization processes; both the anodization current and the voltage could be monitored and recorded in real time. The deionized water was produced using a water purification system (Ultra pure, Hitech, Shanghai, China). A cooling system (DAWOXI DW-LS-2500 W) was also used in the present work. 3. Results and discussion Fig. 1 shows the evolution of anodization current density (Ja) as a function of anodization time under different conditions. It can be observed that Ja decreases exponentially with anodization time, which is different from traditional mild anodization [5,10]. Generally, Ja goes through four typical stages at the beginning of mild anodization (curve ①): (I) instant surge (high electronic current density caused by electrolysis of water) and sharp decrease (a rapid decrease in electronic current density due to the formation of compact alumina), (II) moderate
Fig. 2. Cross-sectional SEM images of PAAO films prepared under 40–50 V (–20 °C) in Electrolyte I with different anodization times. Scale bars = 100 nm.
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Fig. 4. SEM images of the barrier layer surface of PAAO films prepared under 40–50 V (−20 to 0 °C): (a)–(c) 40 V; (d)–(f) 45 V; (g)–(i) 50 V; (a), (d) and (g) 0 °C; (b), (e) and (h) −10 °C; (c), (f) and (i) −20 °C. Scale bars = 100 nm.
decreases with decreasing Ta, resulting in a reduction in the number of ions passing through the barrier layer per unit time (ionic current density). As a result, the total charge transferred decreases, and a lower Ja can be obtained [32]. In addition, the second inset (right) in Fig. 1 shows the relationship between the steady Ja, Ua and Ta. The values of the steady Ja were obtained after an anodization time of 6000 s. It can be observed that the steady Ja obtained at 50 V is larger than that obtained at 40 V, when the same Ta is applied [5,10]. Moreover, the steady Ja increases with Ta, when Ua is unchanged [33]. Fig. 2 shows cross-sections of the PAAO films prepared at –20 °C in Electrolyte I with different anodization times and values of Ua. The resulting film thicknesses are about 665 nm (40 V, 8 h), 1660 nm (40 V, 20 h), 900 nm (45 V, 8 h), 2270 nm (45 V, 20 h), 1195 nm (50 V, 8 h), and 3605 nm (50 V, 20 h). The average νa is 83 nm h−1 (40 V), 113 nm h−1 (45 V), and 165 nm h−1 (50 V). The thickness and average νa increase with Ua and anodization time. A minimum νa of ~1.4 nm min−1 has been obtained. This value is significantly slower than those obtained via traditional anodization processes (34–50 nm min−1) [5,10,34]. Considering that the stable Ja is only about 0.08 mA cm−2, 0.19 mA cm−2 and 0.24 mA cm−2 for PAAO films prepared under 40 V, 45 V and 50 V (–20 °C), the extremely slow νa can be reasonably explained, since νa is directly related to Ja [5,10,34]. The concentration and freezing temperature of the electrolyte can be
Table 1 The regularity value of the samples prepared under different Ua and Ta. Ta (°C)
Regularity (40 V)
Regularity (45 V)
Regularity (50 V)
0 −20
0.8945 0.8339
0.8826 0.8140
0.8694 0.7802
increase (tiny pores begin to form, resulting in thinning of the barrier layer, i.e., decreasing the resistance of the barrier layer), (III) slight decrease (some tiny pores stop growing, i.e., increasing the resistance of the barrier layer), (IV) steady Ja (equilibrium of self-organization process) [5,10,28–31]. The left inset in Fig. 1 shows the top surface of PAAO prepared under 40 V at –20 °C. Shallow tiny pores which have stopped growing can still be seen. This means that the thickness of the compact alumina might not reach its maximum when the tiny pores began to grow. The growth of compact alumina, the initiation and selected growth of tiny pores occur almost simultaneously. As a result, the four stages overlap with each other to produce the exponential decrease Ja shown in Fig. 1. Moreover, Ja increases with anodization voltage (Ua) or Ta for all the samples. The stable Ja of curve ④ is smaller than that obtained in 0.3 M oxalic acid electrolyte without addition of ethanol (curve ①) [5,10]. The stable Ja can be further reduced by using a lower Ta (curve ⑤). It is known that the diffusion rate of ions in the electrolyte 3
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(Table 1). Measurements show that the regularity values of the samples are 0.8945 (40 V), 0.8826 (45 V) and 0.8694 (50 V) at 0 °C [36]. It is higher than the values obtained at –20 °C (0.8339, 0.8140 and 0.7802). In fact, the regularity of the cells is greatly influenced by va [36–38]. The regularity of the cells is low in the initial stage of anodization, since the surface of the aluminum sheets has not been pre-patterned. When va is slow, it is difficult to obtain a proper volume expansion during anodization [37,38]. As a result, the self-ordering regime will not work, resulting in low regularity of the cells [36–38]. In order to obtain PAAO with both precisely controlled thickness and excellent cell regularity, a modified two-step anodization process was used. The first anodization was performed in Electrolyte II under 50 V at 5 °C for 12 h. Then, the PAAO formed on the aluminum sheet was selectively removed using a mixed solution of CrO3 (30 g L−1) and H3PO4 (50 mL L−1) at ~65 °C for 1 h, forming highly ordered shallow pits on the surface of the aluminum sheet. The second anodization was conducted in Electrolyte I under 50 V at –20 °C for 8 h. The SEM images of the resulting PAAO are shown in Fig. 5. It can be seen that the cells show excellent regularity on both the top and the barrier layer surface of the PAAO. A proper volume expansion can be obtained by applying suitable anodization conditions, resulting in mechanical stress in PAAO films. It is proposed that the stress causes repulsive forces between the neighboring structural cells, facilitating the self-ordering behavior of the cells [38]. Therefore, a highly ordered cell arrangement can be obtained after a sufficiently long period of anodization (usually a few hours or more). The calculated regularity value is 0.9670; it is clearly better than that shown in Fig. 4i. Measurements show that the interpore distances in Fig. 5a and b are both ~120 nm, which agrees well with the “2.5 nm V−1” rule between Ua and interpore distance in mild anodization [5,10]. Under the current experimental conditions, the interpore distance is almost unchanged (~120 nm) at the same Ua, even when using a different electrolyte system and Ta. It can be seen that the modified two-step anodization process is simple and effective for the accurate control of the structural parameters of the PAAO film and produces excellent regularity, facilitating the use of the resulting PAAO films in many applications, e.g., template synthesis of quantum dot arrays, single-molecule detection and ultrafast sequencing of DNA or RNA.
Fig. 5. SEM images of the PAAO film prepared under 50 V using the modified two-step anodization process: (a) barrier layer surface; (b) top surface. Scale bars = 100 nm.
reduced by adding ethanol, facilitating a low value of Ja. In fact, the viscosity of the electrolyte is also an important factor affecting ion migration and Ja. The viscosity will increase with the decrease in Ta, helping to reduce the value of Ja [34]. Fig. 3a shows cross-sections of the PAAO films prepared under 40 V (24 h) in Electrolyte I with different values of Ta. Measurement results show that the resulting films are about 4150 nm (–10 °C), 8990 nm (–5 °C), and 12,000 nm (0 °C) thick. Fig. 3b shows the evolution of thickness and va as a function of Ta. It can be seen that thickness and va increase with Ta. A non-linear relationship between va and Ta is obtained, which agrees well with the modified Arrhenius equation reported previously [35]. In addition, it can be observed that the pore channels have low regularity, and that branched channels can be found (Figs. 2 and 3a). It is known that the regularity of pore channels will directly influence the applications of PAAO films. Therefore, it is also necessary to study the regularity of the PAAO films prepared at extremely low temperatures. Fig. 4 shows SEM images of the barrier layer surface of the PAAO films prepared under different Ua and Ta conditions. Ordered cells can be observed in some regions of the samples prepared under 40–50 V at 0 °C. It is known that the ordering or regularity of the structural cells of PAAO films will have a direct impact on their practical applications. Finding an appropriate method to quantify the regularity is significant from the viewpoints of both fundamental science and commercial applications. Recently, an improved method of calculating regularity has been proposed by our group [36]. The regularity value of the PAAO films prepared in the present work was also calculated using this method. The regularity of the cells worsens as the value of Ta decreases
4. Conclusions PAAO films have been prepared in an oxalic acid–ethanol electrolyte at 40–50 V at a minimum Ta of −20 °C. A minimum νa of ~1.4 nm min−1 which corresponds to a Ja of 0.08 mA cm−2 has been achieved. The νa increases with Ta when the other anodization conditions are unchanged. PAAO films with accurately controllable thickness and excellent cell regularity have been prepared using a modified twostep anodization process. The findings reported in the present work could effectively widen the applications of PAAO films, either as highprecision nanoporous templates or as ultrathin functional materials. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (51202071), Guangdong Natural Science Foundation (S2011040005425), Fundamental Research Funds for the Central Universities of China. In addition, many thanks for the strong support from Jasmine Li and Riana Feng. 4
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References
[21] Y. Wang, A. Santos, G. Kaur, A. Evdokiou, D. Losic, Structurally engineered anodic alumina nanotubes as nano-carriers for delivery of anticancer therapeutics, Biomaterials 35 (2014) 5517–5526, https://doi.org/10.1016/j.biomaterials.2014. 03.059. [22] Y. Lei, W.P. Cai, G. Wilde, Highly ordered nanostructures with tunable size, shape and properties: a new way to surface nano-patterning using ultra-thin alumina masks, Prog. Mater Sci. 52 (2007) 465–539, https://doi.org/10.1016/j.pmatsci. 2006.07.002. [23] M.H. Wu, L.Y. Wen, Y. Lei, S. Ostendorp, K. Chen, G. Wilde, Ultrathin alumina membranes for surface nanopatterning in fabricating quantum-sized nanodots, Small 6 (2010) 695–699, https://doi.org/10.1002/smll.200902038. [24] A. Singer, M. Wanunu, W. Morrison, H. Kuhn, M. Frank-Kamenetskii, A. Meller, Nanopore based sequence specific detection of duplex DNA for genomic profiling, Nano Lett. 10 (2010) 738–742, https://doi.org/10.1021/nl100058y. [25] E.C. Yusko, J.M. Johnson, S. Majd, P. Prangkio, R.C. Rollings, J. Li, J. Yang, M. Mayer, Controlling protein translocation through nanopores with bio-inspired fluid walls, Nat. Nanotechnol. 6 (2011) 253–260, https://doi.org/10.1038/nnano. 2011.12. [26] X.F. Qin, J.Q. Zhang, X.J. Meng, L.F. Wang, C.H. Deng, G.Q. Ding, H. Zeng, X.H. Xu, Effect of ethanol on the fabrication of porous anodic alumina in sulfuric acid, Surf. Coat. Technol. 254 (2014) 398–401, https://doi.org/10.1016/j.surfcoat.2014.06. 050. [27] K. Surawathanawises, X. Cheng, Nanoporous anodic aluminum oxide with a longrange order and tunable cell sizes by phosphoric acid anodization on pre-patterned substrates, Electrochim. Acta 117 (2014) 498–503, https://doi.org/10.1016/j. electacta.2013.11.144. [28] Y. Li, Z.Y. Ling, X. Hu, Y.S. Liu, Y. Chang, Investigation of intrinsic mechanisms of aluminium anodization processes by analyzing the current density, RSC Adv. 2 (2012) 5164–5171, https://doi.org/10.1039/c2ra01050j. [29] Y. Li, Y.Y. Qin, S.Y. Jin, X. Hu, Z.Y. Ling, Q.H. Liu, J.F. Liao, C. Chen, Y.H. Shen, L. Jin, A new self-ordering regime for fast production of long-range ordered porous anodic aluminum oxide films, Electrochim. Acta 178 (2015) 11–17, https://doi. org/10.1016/j.electacta.2015.07.119. [30] K. Zhang, S.K. Cao, C. Li, J.R. Qi, L.F. Jiang, J.J. Zhang, X.F. Zhu, Rapid growth of TiO2 nanotubes under the compact oxide layer: Evidence against the digging manner of dissolution reaction, Electrochem. Commun. 103 (2019) 88–93, https:// doi.org/10.1016/j.elecom.2019.05.015. [31] W.Q. Huang, H.Q. Xu, Z.R. Ying, Y.X. Dan, Q.Y. Zhou, J.J. Zhang, X.F. Zhu, Split TiO2 nanotubes − evidence of oxygen evolution during Ti anodization, Electrochem. Commun. 106 (2019) 106532, , https://doi.org/10.1016/j.elecom. 2019.106532. [32] C.V. Manzano, J. Martín, M.S. Martín-González, Ultra-narrow 12 nm pore diameter self-ordered anodic alumina templates, Microporous Mesoporous Mater. 184 (2014) 177–183, https://doi.org/10.1016/j.micromeso.2013.10.004. [33] M. Almasi Kashi, A. Ramazani, The effect of temperature and concentration on the self-organized pore formation in anodic alumina, J. Phys. D: Appl. Phys., 38 (2005), 2396–2399, doi:10.1088/0022-3727/38/14/015. [34] S. Shingubara, O. Okino, Y. Sayama, H. Sakaue, T. Takahagi, Ordered two-dimensional nanowire array formation using self-organized nanoholes of anodically oxidized aluminium, Jpn. J. Appl. Phys. 36 (1997) 7791–7795, https://doi.org/10. 1143/jjap.36.7791. [35] B. Sellarajan, M. Sharma, S.K. Ghosh, H.S. Nagaraja, H.C. Barshilia, P. Chowdhury, Effect of electrolyte temperature on the formation of highly ordered nanoporous alumina template, Microporous Mesoporous Mater. 224 (2016) 262–270, https:// doi.org/10.1016/j.micromeso.2015.12.045. [36] Z.X. Li, Y. Li, S.Z. Li, J.C. Wu, X. Hu, Z.Y. Ling, L. Jin, A modified quantitative method for regularity evaluation of porous AAO and related intrinsic mechanisms, J. Electrochem. Soc. 165 (2018) E214–E220, https://doi.org/10.1149/2. 0841805jes. [37] T.T. Kao, Y.C. Chang, Influence of anodization parameters on the volume expansion of anodic aluminum oxide formed in mixed solution of phosphoric and oxalic acids, Appl. Surf. Sci. 288 (2014) 654–659, https://doi.org/10.1016/j.apsusc.2013.10. 091. [38] O. Jessensky, F. Müller, U. Gösele, Self-organized formation of hexagonal pore arrays in anodic alumina, Appl. Phys. Lett. 72 (1998) 1173–1175, https://doi.org/10. 1063/1.121004.
[1] F. Keller, M.S. Hunter, D.L. Robinson, Structural features of oxide coatings on aluminum, J. Electrochem. Soc. 100 (1953) 411–419, https://doi.org/10.1149/1. 2781142. [2] J.W. Diggle, T.C. Downie, C.W. Goulding, Anodic oxide films on aluminium, Chem. Rev. 69 (1969) 365–405, https://doi.org/10.1021/cr60259a005. [3] G.E. Thompson, R.C. Furneaux, G.C. Wood, J.A. Richardson, J.S. Goode, Nucleation and growth of porous anodic films on aluminium, Nature 272 (1978) 433–435, https://doi.org/10.1038/272433a0. [4] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science 268 (1995) 1466–1468, https://doi.org/10.1126/science.268.5216.1466. [5] W. Lee, R. Ji, U. Gösele, K. Nielsch, Fast fabrication of long-range ordered porous alumina membranes by hard anodization, Nat. Mater. 5 (2006) 741–747, https:// doi.org/10.1038/nmat1717. [6] Y. Li, Z.Y. Ling, S.S. Chen, X. Hu, X.H. He, Novel AAO films and hollow nanostructures fabricated by ultra-high voltage hard anodization, Chem. Commun. 46 (2010) 309–311, https://doi.org/10.1039/b914703a. [7] G.E.J. Poinern, N. Ali, D. Fawcett, Progress in nano-engineered anodic aluminum oxide membrane development, Materials 4 (2011) 487–526, https://doi.org/10. 3390/ma4030487. [8] Y. Song, L.F. Jiang, W.X. Qi, C. Lu, X.F. Zhu, H.B. Jia, High-field anodization of aluminum in concentrated acid solutions and at higher temperatures, J. Electroanal. Chem. 673 (2012) 24–31, https://doi.org/10.1016/j.jelechem.2012.03.017. [9] X.F. Zhu, Y. Song, D.L. Yu, C.S. Zhang, W. Yao, A novel nanostructure fabricated by an improved two-step anodizing technology, Electrochem. Commun. 29 (2013) 71–74, https://doi.org/10.1016/j.elecom.2013.01.018. [10] W. Lee, S.-J. Park, Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures, Chem. Rev. 114 (2014) 7487–7556, https:// doi.org/10.1021/cr500002z. [11] M.S. Yu, C. Li, Y.B. Yang, S.K. Xu, K. Zhang, H.M. Cui, X.F. Zhu, Cavities between the double walls of nanotubes: evidence of oxygen evolution beneath an anioncontaminated layer, Electrochem. Commun. 90 (2018) 34–38, https://doi.org/10. 1016/j.elecom.2018.03.009. [12] S.Z. Li, Y. Li, S.Y. Jin, J.C. Wu, Z.X. Li, X. Hu, Z.Y. Ling, Fabrication of crystallized porous anodic aluminum oxide under ultra-high anodization voltage, J. Electrochem. Soc. 165 (2018) E623–E627, https://doi.org/10.1149/2.0141813jes. [13] E.M. Palmero, M. Méndez, S. González, C. Bran, V. Vega, M. Vázquez, V.M. Prida, Stepwise magnetization reversal of geometrically tuned in diameter Ni and FeCo bisegmented nanowire arrays, Nano Res. 12 (2019) 1547–1553, https://doi.org/10. 1007/s12274-019-2385-9. [14] A. Yadav, M.S. Bobji, S.J. Bull, Controlled growth of highly aligned Cu nanowires by pulse electrodeposition in nanoporous alumina, J. Nanosci. Nanotechnol. 19 (2019) 4254–4259, https://doi.org/10.1166/jnn.2019.16868. [15] A. Mezni, T. Altalhi, N.B. Saber, A. Aldalbahi, S. Boulehmi, A. Santos, D. Losic, Sizeand shape-controlled synthesis of well-organised carbon nanotubes using nanoporous anodic alumina with different pore diameters, J. Colloid Interface Sci. 491 (2017) 375–389, https://doi.org/10.1016/j.jcis.2016.12.035. [16] M. Alsawat, T. Altalhi, A. Santos, D. Losic, Carbon nanotubes–nanoporous anodic alumina composite membranes: influence of template on structural, chemical, and transport properties, J. Phys. Chem. C 121 (2017) 13634–13644, https://doi.org/ 10.1021/acs.jpcc.7b01257. [17] T. Yanagishita, H. Masuda, Facile preparation of porous alumina through-hole masks for sputtering by two-layer anodization, AIP Adv. 6 (2016) 085108, , https:// doi.org/10.1063/1.4961126. [18] M. Jung, J.H. Kim, Y.H. Choi, Preparation of anodic aluminum oxide masks with size-controlled pores for 2D plasmonic nanodot arrays, J. Nanomater. 2018 (2018) 6249890, https://doi.org/10.1155/2018/6249890. [19] L.K. Acosta, F. Bertó-Roselló, E. Xifre-Perez, A. Santos, J. Ferré-Borrull, L.F. Marsal, Stacked nanoporous anodic alumina gradient-index filters with tunable multispectral photonic stopbands as sensing platforms, ACS Appl. Mater. Interfaces 11 (2019) 3360–3371, https://doi.org/10.1021/acsami.8b19411. [20] Z.Y. Ling, S.S. Chen, J.C. Wang, Y. Li, Fabrication and properties of anodic alumina humidity sensor with through-hole structure, Chin. Sci. Bull. 53 (2008) 183–187, https://doi.org/10.1007/s11434-008-0008-z.
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