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Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization
Thin Solid Films 520 (2011) 1554–1558
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization C.K. Chung ⁎, M.W. Liao, H.C. Chang, C.T. Lee Department of Mechanical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e
i n f o
Available online 23 August 2011 Keywords: Anodic aluminum oxide Hybrid pulse anodization One-step anodization
a b s t r a c t Many conventional anodic aluminum oxide (AAO) templates were performed using two-step direct current anodization (DCA) at low temperature (0–5 °C) to avoid dissolution effects. This process is relatively complex. Pulse anodization (PA) by switching between high and low voltages has been used to improve wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum or hard anodization for current nanotechnology. However, there are only few investigations of AAO by hybrid pulse anodization (HPA) with normal-positive and small-negative voltages, especially for the one-step anodization, to shorten the running time. In this article, the effects of temperature and voltage modes (DCA vs. HPA) on one-step anodization have been investigated. The porous AAO films were fabricated using one-step anodization in 0.5 M oxalic acid in different voltage modes including the HPA and DCA and the environment temperature were varied at 5–15 °C. The morphology, pore size and oxide thickness of AAO films were characterized by high resolution field emission scanning electron microscope. The pore size distribution and circularity of AAO films can be quantitatively analyzed by image processing of SEM. The pore distribution uniformity and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperatures. On the other hand, increasing environment temperature can increase the growth rate and enlarge the pore size of AAO films. The results of one-step anodization by hybrid pulse could promote the AAO quality and provide a simple and convenient fabrication compared to DCA. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Anodic aluminum oxide (AAO) can be classified into two types of barrier and porous ones according to different structures. The barrier type with thin and compact packed structure has been widely used in protection and dielectric capacitors in [1]. While the porous type has received much attention since the characteristic of high-ordered nanopore arrangement was discovered [2]. Recently, many researches focused on the nanostructured materials due to its significant physical properties in last decades [3]. Although several techniques like photolithography, etching or gas phase synthesis can fabricate the nanowires or nanotubes [4], template-assisted growth using nanoporous AAO was considered as one of the most prominent methods because the advantages of controllable diameter, high aspect ratio, and economic. The AAO template has been used in various applications such as multiple quantum wells [5], photonic crystal [6], light-emitting diodes [7], humidity sensors [8], nanomaterials synthesis [9] and super capacitors [10]. Nowadays, the typical electrochemical method for producing AAO film was two-step anodization proposed by Masuda and Fukuda in 1995 [2]. The two-step anodization on Al foil at constant
⁎ Corresponding author. Tel.: + 886 6 2757575 62111; fax: + 886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.08.053
voltage can result in the well-ordered AAO configuration compared to one-step anodization. Moreover, the electrolyte temperature usually keeps at 0–5 °C to reduce the dissolution rate of alumina during anodization process. However, low temperature causes the decrease of the growth rate. Thus, there are some researchers devoted to anodize aluminum at relatively high temperatures [11,12]. On the other hand, we had demonstrated that the two-step hybrid pulse anodization (HPA) could reduce the resistive heating effect of anodizing current and form nanopores at room temperatures [13–15]. The heat generation is harmful to nanostructures because it accumulates randomly all over the AAO structure especially the discontinuous geometry to enhance the dissolution effect. Pulse technology has widely been applied to increase wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum [16] and fabricated novel threedimensional nanostructures in porous AAO [17]. However, most of the investigations focused on the high and low positive voltage pulsing instead of positive and negative voltage. One of the reasons is that the negative voltage in AC pulse anodizing lead to attract excess hydrogen ions that tend to harm the porous AAO structure [18]. HPA with high positive voltage and low negative voltage may improve this problem. Moreover, pulse reverse with an equal value of long positive and short negative currents (/voltages) has been reported to helpfully increase the uniformity of pore size of AAO [19]. It is thus of interest to further study more details about the effect of low
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negative voltage of HPA on different anodized conditions, especially for one-step anodization, which has usually been used to form the Si-base porous AAO nanostructure because the aluminum film deposited on Si wafer is too thin for proceeding two-step anodization [20,21]. For industrial applications, the one-step anodization offers the advantages of low cost and time saving process. It is noted that little attention has been paid to study the behavior of one-step anodized Al foil because of nonuniform and disordered pore structure [22,23]. In this paper, we have investigated the one-step anodization of Al foil in 0.5 M oxalic acid by HPA and direct current anodization (DCA) at environment temperature of 5–15 °C. The film thickness and nanopore size distribution was analyzed to understand the effect of temperature and voltage mode on AAO performance between HPA and DCA. The results of one-step anodization by hybrid pulse could provide a convenient process for synthesis of nanoporous AAO. 2. Experimental procedures The high-purity aluminum foil (99.997%, Alfa Aesar, USA) was used for one-step anodization. It was cut into pieces of 1.5 cm × 1.0 cm in size and then electropolished in a mixture of HClO4 and ethanol (1:4 in volumetric ratio) at 20 V for 30 s at room temperature (RT). The one-step anodization experiments were performed in 0.5 M oxalic acid by HPA and DCA at environment temperatures of 5–15 °C for 1 h. The applied hybrid pulse was constructed from a positive square wave followed by another negative square wave with the duty ratio of 1:1. The voltage of 40 V was applied in the HPA positive pulse and all the DCA duration, while the negative one was fixed at −2 V for the HPA negative pulse. The period of a hybrid pulse was 2 s (1 s:1 s). Formation of AAO was performed by means of the potentiostat (Jiehan 5000, Taiwan) and the three-electrode electrochemical cell with the platinum mesh as the counter electrode, the gold coated silicon as the working electrode, Ag/AgCl as the reference electrode. In order to further study the behavior of anodization process, the realtime time–current curves were recorded. After the anodization, the specimens were immersed in 5 wt.% phosphoric acid at RT for 30 min to remove a part of AAO top and enlarge the pore diameter. The morphology and pore characteristics of AAO films were examined by High Resolution Field Emission Scanning Electron Microscope (HR-FESEM, JEOL JSM-7001, Japan). In order to illustrate the pore distributions in details, SEM micrographs of the AAO films at different parameters were further analyzed by gray-scale imaging analysis. The diameter and amount of pores can be estimated using commercial software (ImageJ) by collecting the adequate range in gray scale of nanopores which are in circular dark region. The pores per μm 2 (pore density) can be calculated by dividing amount of pores into whole area (~4.2 μm 2) of a SEM micrographs.
Fig. 1. SEM micrographs of AAO formed by one-step (a) DCA and (b) HPA from 99.997% Al foil in 0.5 M oxalic acid at 5 °C for 1 h and then immersed in phosphoric acid 5 wt.% for 30 min.
with negligible current in negative voltage period. Therefore, higher Joule-heat-dissolution in DCA results in the larger size of pores than HPA. On the other hand, the distribution uniformity can be calculated by the ratio of the pores located at these main peaks to the whole amount of pores. In the case of HPA, around 92.6% pores are formed within the range of 35 ± 5 nm while only 82.9% pores are achieved by DCA. The better uniformity is also obtained by HPA than DCA. It implies that the lower Joule-heat dissolution in HPA is beneficial for the uniformity of pores distribution. Pore circularity is another important factor to determine the quality of porous AAO besides the pore size. The definition of circularity can be expressed as [24]
circularity ¼ 4π
Α S2
3. Results and discussion Fig. 1a and b show the SEM micrographs of AAO formed by onestep DCA and HPA from 99.997% Al foil for 1 h at low temperature of 5 °C and then immersed in phosphoric acid 5 wt.% for 30 min, respectively. The porous AAO nanostructure is clearly seen and obtained for both cases. Moreover, the morphology of AAO by HPA method exhibits higher ordered pore arrangements, more uniform pore size, and more circularity than that by DCA. The pore size distribution of AAO films can be further quantitatively analyzed by imaging processing of SEM images. Fig. 2 shows the relationship between the amount of pores per μm 2 and pore diameters of AAO films by onestep DCA (dash-line, triangles) and HPA (solid-line, squares) in Fig. 1. The range of 40 ± 5 nm pores occupies the main distribution in DCA while the range of 35 ± 5 nm is observed in HPA. It indicated that the pore diameter of AAO by DCA is larger than HPA which may be attributed to Joule-heat-dissolution effect [13]. The continuous heat accumulation in DCA has higher Joule heat that that in HPA
Fig. 2. The relationship between the amount of pores per μm2 area and pore diameters of AAO formed by one-step DCA (dash-line, triangles) and HPA (solid-line, squares) from 99.997% Al foil in 0.5 M oxalic acid at 5 °C for 1 h.
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Fig. 3. Pore circularity of AAO formed by (a) DCA and (b) HPA from 99.997% Al foil in 0.5 M oxalic acid for 1 h at 5 °C.
where A and S represent the area and perimeter of each pore of AAO. A circularity value of 1.0 indicates a perfect circle. As the value approaches 0, it indicates an increasingly elongated polygon. Fig. 3a and b show the distribution of pore circularity of AAO film formed by DCA and HPA, respectively, corresponding to Fig. 1. It reveals that the value of pore circularity of AAO films by HPA is much higher than DCA. The main fraction of pore circularity of AAO by HPA ranges from 0.7 to 0.8 which is consistent with the SEM image (Fig. 1b). In contrast, the pore circularity of AAO by DCA is lower than 0.6 corresponding to more irregular pores. Indeed, most of pore arrangements in the one-step anodization were not well except for long anodizing time of several dozens of hours [24]. It evidences that HPA is beneficial for improving the uniformity and circularity of nanopores distribution in the one-step anodized alumina. Regarding temperature effect on AAO quality, Fig. 4a and b show the SEM micrographs of top-view and cross-section of AAO formed by onestep HPA from 99.997% Al foil for 1 h at relatively high temperature of 15 °C, respectively. The AAO nanostructures formed by one-step HPA were still good at the elevated temperature. But the AAO nanostructures were ruined by DCA at 15 °C due to temperature-enhanced dissolution. In our experiments, the unexpected rising anodizing current was easily occurred especially in high potential DCA. These unexpected rising current could be attributed to some defects of high-purity aluminum foil like grain boundaries or dislocations and it leads to a great deal of Joule's heat (P= I2 × R, P: power, I: current, R: resistance). Moreover, the drastically local increasing temperature increases dissolution rate. Finally, the unbalance of formation and dissolution rate of alumina causes the electric breakdown at the barrier layer [25] then ruin the whole AAO nanostructure. That is one of the reasons for most of conventional AAOs being performed at low temperature (0–10 °C). However, this phenomenon can be avoided in HPA because the negative period with nearly zero cathodic current provides sufficient cooling effect [13].
Fig. 4. (a) Plane view and (b) cross section of SEM micrographs; and (c) pore diameter distribution and (d) circularity of AAO formed by HPA from 99.997% Al foil in 0.5 M oxalic acid at 15 °C for 1 h.
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The morphology of barrier layer is shown in the cross-section of SEM micrographs in Fig. 4b. It is noted that the well-formed semicircle structure without cracks or voids at the interface between AAO and Al foil is obtained. It indicates that HPA is advantageous to the one-step AAO formation at the elevated temperature compared to DCA. Fig. 4c and d show the results of quantitative analysis of the pore size distribution as a function of diameter and the pore circularity of AAO films by one-step HPA at 15 °C for 1 h, respectively. The main fraction of pore diameter of AAO by one-step HPA at 15 °C is in the rage of 45± 5 nm and around 88.6% pores are formed within the range of diameter. Compared to the results of HPA at 5 °C (35 ± 5 nm, Fig. 2), it reveals that the pore size increases with increasing environment temperatures but the distribution uniformity decreases at the same time. It is attributed to temperature-enhanced dissolution to widen the pore and smear the distribution uniformity. However, it is noticed that not only pore diameter but also the uniformity of one-step HPA at 15 °C is higher than one-step DCA at 5 °C (40 ± 5 nm and 82.6%). In addition, the main fraction of pore circularity by one-step HPA ranging from 0.5 to 0.6 is higher than one-step DCA at 5 °C. Although the pore uniformity and pore circularity slightly decrease at the elevated temperature in one-step HPA, the quality of porous AAO films is still better than DCA at low temperature. One advantage of increasing environment temperature is dramatical increase of growth rate of AAO. Fig. 5a and b show the SEM micrographs of the whole cross-section of AAO formed by the same one-step HPA condition at 5 °C and 15 °C (note the different scale bar), respectively. The thickness of AAO film estimated is 1.2 μm for 5 °C and greatly increased to 4.1 μm for 15 °C, respectively. From industrial point of view, it is more economic for fabricating AAO by HPA at relatively high temperature. Let us consider the mechanism of the hybrid pulse action on onestep AAO growth during porous anodizing. In general, formation and dissolution of aluminum oxide during electrochemical reaction can be expressed in formulas (1) and (2), respectively [13,26]. þ
2AlðsÞ þ 3H2 O → Al2 O3 ðsÞ þ 6H ðaqÞ þ 6e
−
þ
1557 3þ
Al2 O3 ðsÞ þ 6H ðaqÞ → 2Al ðaqÞ þ 3H2 O
ð2Þ
When the potential was applied to the electrochemical cell, both reactions of formulas (1) and (2) reacted with the aluminum specimen and reached a balanced condition. The releasing electrons in formula (1) produce anodizing current. At the beginning of anodizing, surface fluctuation causes varied local field distributions. The dissolution of the aluminum oxide can be enhanced by local high field and pores are created first in high field region. After a period of anodizing, the AAO structures at the pore bottom become stable growth and the regularity of pores configuration is adjusted with increasing anodizing time. However, the AAO structure on the top was still rough and distorted due to these anodizing processes only occur in the barrier layer at the bottom of AAO. That is the reason for removing the first anodized alumina and performing the second anodization for better configuration in the conventional two-step anodization [2]. In the case of HPA process, the small negative potential (−2 V) is applied after a duration of anodizing positive potential. And the hydrogen ions with positive charge are attracted to the surface at the same time as shown in the schematic diagram of Fig. 6. The attracted hydrogen ions lead to the dissolution of AAO on the top. Therefore, the rough and distorted AAO structure can be improved. Moreover, there is no electron in dissolution reaction of alumina so that no extra Joule's heat generates when applying negative potential. The small negative voltage resulting in the nearly zero current at the negative duration is good for the uniformity and circularity of pore distribution as mentioned previously. It is noted that too large negative potential may lead to negative current by electrical breakdown and destroy the whole AAO structure.
ð1Þ
Fig. 5. Cross-section of AAO formed by HPA from 99.997% Al foil in 0.5 M oxalic acid for 1 h at (a) 5 °C and (b) 15 °C.
Fig. 6. Schematic diagram of the effect of negative potential applied in one-step HPA anodization during AAO growth. The positive hydrogen ions attracted to the surface lead to the dissolution of the top of AAO for improving the rough and distorted structure by one-step DCA.
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4. Conclusion The one-step AAO films with high uniformity and circularity of pore distribution have been successfully fabricated using hybrid pulse anodization method. The pore distribution uniformity and circularity of AAO by HPA in 0.5 M oxalic acid from 99.997% Al foil at 5 °C for 1 h reach 92.6% and 0.7, respectively. While the results in DCA at the same experiment conditions are just 82.9% and 0.3, respectively. When the environment temperature increases to 15 °C, the AAO nanostructures formed by one-step HPA are still good. In contrast, the AAO nanostructures are ruined by DCA at 15 °C due to temperature-enhanced dissolution. Moreover, the pore diameter, uniformity, and circularity of one-step HPA at 15 °C are still higher than one-step DCA at 5 °C. Although the slight decrease of pore uniformity and pore circularity at the elevated temperature in one-step HPA is seen, the growth rate is still dramatically increased from 1.2 μm/h for 5 °C and 4.1 μm/h for 15 °C. It is more economical for fabricating the AAO by one-step HPA at relatively high temperatures. In HPA process, the negative period with nearly zero cathodic current can provide effective cooling for promoting AAO quality. On the other hand, the positive hydrogen ions attracted to the surface lead to the dissolution of the top of AAO. It can improve the rough and distorted AAO structure by one-step DCA. The advantages of effective cooling and extra dissolution in HPA are helpful for fabricating AAO by one-step anodization at relatively high temperatures. Acknowledgments This work is partially sponsored by the National Science Council under contract no. NSC99-2221-E-006-032-MY3. We also would like to thank the Center for Micro/Nano Science and Technology in National Cheng Kung University, National Nano Device Laboratories (NDL), and
National Center for High-performing Computing (NCHC) for the access of process and analysis equipments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25] [26]
J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. H. Masuda, F. Fukuda, Science 268 (1995) 1466. H. Gleiter, Acta Mater. 48 (2000) 1. S. Kawakami, T. Kawashima, T. Sato, Appl. Phys. Lett. 74 (1999) 463. K.Y. Zang, Y.D. Wang, H.F. Liu, S.J. Chua, Appl. Phys. Lett. 89 (2006) 171921. V. Mizeikis, I. Mikulskas, R. Tomasiunas, S. Juodkazis, S. Matsuo, H. Misawa, Jpn. J. Appl. Phys. 43 (2004) 3643. Y.D. Wang, K.Y. Zang, S.J. Chua, S. Tripathy, P. Chen, C.G. Fonstad, Appl. Phys. Lett. 87 (2005) 251915. P.K. Nahar, Sens. Actuator B-Chem. 63 (2000) 49. Z. Zhang, T. Shimizu, S. Senz, U. Gosele, Adv. Mater. 21 (2009) 2824. K.B. Shelimov, D.N. Davydov, M. Moskovits, Appl. Phys. Lett. 77 (2000) 1722. M. Almasi Kashi, A. Ramazani, J. Phys. D: Appl. Phys. 38 (2005) 2396. G.D. Sulkaa, W.J. Stepniowski, Electrochim. Acta 54 (2009) 3683. C.K. Chung, R.X. Zhou, T.Y. Liu, W.T. Chang, Nanotechnology 20 (2009) 055301. C.K. Chung, W.T. Chang, M.W. Liao, H.C. Chang, C.T. Lee, Electrochim. Acta 56 (2011) 6489. C.K. Chung, W.T. Chang, M.W. Liao, H.C. Chang, Thin Solid Films 519 (2011) 4754. K. Yokoyama, H. Konno, H. Takahashi, M. Nagayama, Plat. Surf. Finish 69 (1982) 62. W. Lee, K. Schwirn, M. Steinhart, E. Pippel, R. Scholz, U. Gosele, Nat. Nanotechnol. 3 (2008) 234. S. Wernick, R. Pinner, P.G. Sheasby, The Surface Treatment and Finishing of Aluminum and Its Alloys, 5. Ed. Finshing Publications LTD., Teddington, Middlesex, England, 1987. K. Okubo, S. Suyama, Y. Sakura, J. Surf. Finish. Soc. Jpn. 40 (1989) 1366. S. Inoue, S.Z. Chu, K. Wada, D. Li, H. Haneda, Sci. Technol. Adv. Mater. 4 (2003) 269. M.T. Wu, I.C. Le, M.H. Hon, J. Mater. Res. 19 (2004) 888. A.O. Araoyinbo, M.N.A. Fauzi, S. Sreekantan, A. Aziz, Sains Malays. 38 (2009) 521. K. Hou, J.P. Tu, X.B. Zhang, Chin. Chem. Lett. 13 (2002) 689. W.B. Yang, X.H. Tang, Mater. Sci. Forum 663–665 (2011) 272. Leszek Zaraska, Grzegorz D. Sulka, Marian Jaskuła, Surf. Coat. Technol. 204 (2010) 1729. Z. Wu, C. Richter, L. Menon, J. Electochem. Soc. 154 (2007) E8.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization C.K. Chung ⁎, M.W. Liao, H.C. Chang, C.T. Lee Department of Mechanical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e
i n f o
Available online 23 August 2011 Keywords: Anodic aluminum oxide Hybrid pulse anodization One-step anodization
a b s t r a c t Many conventional anodic aluminum oxide (AAO) templates were performed using two-step direct current anodization (DCA) at low temperature (0–5 °C) to avoid dissolution effects. This process is relatively complex. Pulse anodization (PA) by switching between high and low voltages has been used to improve wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum or hard anodization for current nanotechnology. However, there are only few investigations of AAO by hybrid pulse anodization (HPA) with normal-positive and small-negative voltages, especially for the one-step anodization, to shorten the running time. In this article, the effects of temperature and voltage modes (DCA vs. HPA) on one-step anodization have been investigated. The porous AAO films were fabricated using one-step anodization in 0.5 M oxalic acid in different voltage modes including the HPA and DCA and the environment temperature were varied at 5–15 °C. The morphology, pore size and oxide thickness of AAO films were characterized by high resolution field emission scanning electron microscope. The pore size distribution and circularity of AAO films can be quantitatively analyzed by image processing of SEM. The pore distribution uniformity and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperatures. On the other hand, increasing environment temperature can increase the growth rate and enlarge the pore size of AAO films. The results of one-step anodization by hybrid pulse could promote the AAO quality and provide a simple and convenient fabrication compared to DCA. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Anodic aluminum oxide (AAO) can be classified into two types of barrier and porous ones according to different structures. The barrier type with thin and compact packed structure has been widely used in protection and dielectric capacitors in [1]. While the porous type has received much attention since the characteristic of high-ordered nanopore arrangement was discovered [2]. Recently, many researches focused on the nanostructured materials due to its significant physical properties in last decades [3]. Although several techniques like photolithography, etching or gas phase synthesis can fabricate the nanowires or nanotubes [4], template-assisted growth using nanoporous AAO was considered as one of the most prominent methods because the advantages of controllable diameter, high aspect ratio, and economic. The AAO template has been used in various applications such as multiple quantum wells [5], photonic crystal [6], light-emitting diodes [7], humidity sensors [8], nanomaterials synthesis [9] and super capacitors [10]. Nowadays, the typical electrochemical method for producing AAO film was two-step anodization proposed by Masuda and Fukuda in 1995 [2]. The two-step anodization on Al foil at constant
⁎ Corresponding author. Tel.: + 886 6 2757575 62111; fax: + 886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.08.053
voltage can result in the well-ordered AAO configuration compared to one-step anodization. Moreover, the electrolyte temperature usually keeps at 0–5 °C to reduce the dissolution rate of alumina during anodization process. However, low temperature causes the decrease of the growth rate. Thus, there are some researchers devoted to anodize aluminum at relatively high temperatures [11,12]. On the other hand, we had demonstrated that the two-step hybrid pulse anodization (HPA) could reduce the resistive heating effect of anodizing current and form nanopores at room temperatures [13–15]. The heat generation is harmful to nanostructures because it accumulates randomly all over the AAO structure especially the discontinuous geometry to enhance the dissolution effect. Pulse technology has widely been applied to increase wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum [16] and fabricated novel threedimensional nanostructures in porous AAO [17]. However, most of the investigations focused on the high and low positive voltage pulsing instead of positive and negative voltage. One of the reasons is that the negative voltage in AC pulse anodizing lead to attract excess hydrogen ions that tend to harm the porous AAO structure [18]. HPA with high positive voltage and low negative voltage may improve this problem. Moreover, pulse reverse with an equal value of long positive and short negative currents (/voltages) has been reported to helpfully increase the uniformity of pore size of AAO [19]. It is thus of interest to further study more details about the effect of low
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negative voltage of HPA on different anodized conditions, especially for one-step anodization, which has usually been used to form the Si-base porous AAO nanostructure because the aluminum film deposited on Si wafer is too thin for proceeding two-step anodization [20,21]. For industrial applications, the one-step anodization offers the advantages of low cost and time saving process. It is noted that little attention has been paid to study the behavior of one-step anodized Al foil because of nonuniform and disordered pore structure [22,23]. In this paper, we have investigated the one-step anodization of Al foil in 0.5 M oxalic acid by HPA and direct current anodization (DCA) at environment temperature of 5–15 °C. The film thickness and nanopore size distribution was analyzed to understand the effect of temperature and voltage mode on AAO performance between HPA and DCA. The results of one-step anodization by hybrid pulse could provide a convenient process for synthesis of nanoporous AAO. 2. Experimental procedures The high-purity aluminum foil (99.997%, Alfa Aesar, USA) was used for one-step anodization. It was cut into pieces of 1.5 cm × 1.0 cm in size and then electropolished in a mixture of HClO4 and ethanol (1:4 in volumetric ratio) at 20 V for 30 s at room temperature (RT). The one-step anodization experiments were performed in 0.5 M oxalic acid by HPA and DCA at environment temperatures of 5–15 °C for 1 h. The applied hybrid pulse was constructed from a positive square wave followed by another negative square wave with the duty ratio of 1:1. The voltage of 40 V was applied in the HPA positive pulse and all the DCA duration, while the negative one was fixed at −2 V for the HPA negative pulse. The period of a hybrid pulse was 2 s (1 s:1 s). Formation of AAO was performed by means of the potentiostat (Jiehan 5000, Taiwan) and the three-electrode electrochemical cell with the platinum mesh as the counter electrode, the gold coated silicon as the working electrode, Ag/AgCl as the reference electrode. In order to further study the behavior of anodization process, the realtime time–current curves were recorded. After the anodization, the specimens were immersed in 5 wt.% phosphoric acid at RT for 30 min to remove a part of AAO top and enlarge the pore diameter. The morphology and pore characteristics of AAO films were examined by High Resolution Field Emission Scanning Electron Microscope (HR-FESEM, JEOL JSM-7001, Japan). In order to illustrate the pore distributions in details, SEM micrographs of the AAO films at different parameters were further analyzed by gray-scale imaging analysis. The diameter and amount of pores can be estimated using commercial software (ImageJ) by collecting the adequate range in gray scale of nanopores which are in circular dark region. The pores per μm 2 (pore density) can be calculated by dividing amount of pores into whole area (~4.2 μm 2) of a SEM micrographs.
Fig. 1. SEM micrographs of AAO formed by one-step (a) DCA and (b) HPA from 99.997% Al foil in 0.5 M oxalic acid at 5 °C for 1 h and then immersed in phosphoric acid 5 wt.% for 30 min.
with negligible current in negative voltage period. Therefore, higher Joule-heat-dissolution in DCA results in the larger size of pores than HPA. On the other hand, the distribution uniformity can be calculated by the ratio of the pores located at these main peaks to the whole amount of pores. In the case of HPA, around 92.6% pores are formed within the range of 35 ± 5 nm while only 82.9% pores are achieved by DCA. The better uniformity is also obtained by HPA than DCA. It implies that the lower Joule-heat dissolution in HPA is beneficial for the uniformity of pores distribution. Pore circularity is another important factor to determine the quality of porous AAO besides the pore size. The definition of circularity can be expressed as [24]
circularity ¼ 4π
Α S2
3. Results and discussion Fig. 1a and b show the SEM micrographs of AAO formed by onestep DCA and HPA from 99.997% Al foil for 1 h at low temperature of 5 °C and then immersed in phosphoric acid 5 wt.% for 30 min, respectively. The porous AAO nanostructure is clearly seen and obtained for both cases. Moreover, the morphology of AAO by HPA method exhibits higher ordered pore arrangements, more uniform pore size, and more circularity than that by DCA. The pore size distribution of AAO films can be further quantitatively analyzed by imaging processing of SEM images. Fig. 2 shows the relationship between the amount of pores per μm 2 and pore diameters of AAO films by onestep DCA (dash-line, triangles) and HPA (solid-line, squares) in Fig. 1. The range of 40 ± 5 nm pores occupies the main distribution in DCA while the range of 35 ± 5 nm is observed in HPA. It indicated that the pore diameter of AAO by DCA is larger than HPA which may be attributed to Joule-heat-dissolution effect [13]. The continuous heat accumulation in DCA has higher Joule heat that that in HPA
Fig. 2. The relationship between the amount of pores per μm2 area and pore diameters of AAO formed by one-step DCA (dash-line, triangles) and HPA (solid-line, squares) from 99.997% Al foil in 0.5 M oxalic acid at 5 °C for 1 h.
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Fig. 3. Pore circularity of AAO formed by (a) DCA and (b) HPA from 99.997% Al foil in 0.5 M oxalic acid for 1 h at 5 °C.
where A and S represent the area and perimeter of each pore of AAO. A circularity value of 1.0 indicates a perfect circle. As the value approaches 0, it indicates an increasingly elongated polygon. Fig. 3a and b show the distribution of pore circularity of AAO film formed by DCA and HPA, respectively, corresponding to Fig. 1. It reveals that the value of pore circularity of AAO films by HPA is much higher than DCA. The main fraction of pore circularity of AAO by HPA ranges from 0.7 to 0.8 which is consistent with the SEM image (Fig. 1b). In contrast, the pore circularity of AAO by DCA is lower than 0.6 corresponding to more irregular pores. Indeed, most of pore arrangements in the one-step anodization were not well except for long anodizing time of several dozens of hours [24]. It evidences that HPA is beneficial for improving the uniformity and circularity of nanopores distribution in the one-step anodized alumina. Regarding temperature effect on AAO quality, Fig. 4a and b show the SEM micrographs of top-view and cross-section of AAO formed by onestep HPA from 99.997% Al foil for 1 h at relatively high temperature of 15 °C, respectively. The AAO nanostructures formed by one-step HPA were still good at the elevated temperature. But the AAO nanostructures were ruined by DCA at 15 °C due to temperature-enhanced dissolution. In our experiments, the unexpected rising anodizing current was easily occurred especially in high potential DCA. These unexpected rising current could be attributed to some defects of high-purity aluminum foil like grain boundaries or dislocations and it leads to a great deal of Joule's heat (P= I2 × R, P: power, I: current, R: resistance). Moreover, the drastically local increasing temperature increases dissolution rate. Finally, the unbalance of formation and dissolution rate of alumina causes the electric breakdown at the barrier layer [25] then ruin the whole AAO nanostructure. That is one of the reasons for most of conventional AAOs being performed at low temperature (0–10 °C). However, this phenomenon can be avoided in HPA because the negative period with nearly zero cathodic current provides sufficient cooling effect [13].
Fig. 4. (a) Plane view and (b) cross section of SEM micrographs; and (c) pore diameter distribution and (d) circularity of AAO formed by HPA from 99.997% Al foil in 0.5 M oxalic acid at 15 °C for 1 h.
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The morphology of barrier layer is shown in the cross-section of SEM micrographs in Fig. 4b. It is noted that the well-formed semicircle structure without cracks or voids at the interface between AAO and Al foil is obtained. It indicates that HPA is advantageous to the one-step AAO formation at the elevated temperature compared to DCA. Fig. 4c and d show the results of quantitative analysis of the pore size distribution as a function of diameter and the pore circularity of AAO films by one-step HPA at 15 °C for 1 h, respectively. The main fraction of pore diameter of AAO by one-step HPA at 15 °C is in the rage of 45± 5 nm and around 88.6% pores are formed within the range of diameter. Compared to the results of HPA at 5 °C (35 ± 5 nm, Fig. 2), it reveals that the pore size increases with increasing environment temperatures but the distribution uniformity decreases at the same time. It is attributed to temperature-enhanced dissolution to widen the pore and smear the distribution uniformity. However, it is noticed that not only pore diameter but also the uniformity of one-step HPA at 15 °C is higher than one-step DCA at 5 °C (40 ± 5 nm and 82.6%). In addition, the main fraction of pore circularity by one-step HPA ranging from 0.5 to 0.6 is higher than one-step DCA at 5 °C. Although the pore uniformity and pore circularity slightly decrease at the elevated temperature in one-step HPA, the quality of porous AAO films is still better than DCA at low temperature. One advantage of increasing environment temperature is dramatical increase of growth rate of AAO. Fig. 5a and b show the SEM micrographs of the whole cross-section of AAO formed by the same one-step HPA condition at 5 °C and 15 °C (note the different scale bar), respectively. The thickness of AAO film estimated is 1.2 μm for 5 °C and greatly increased to 4.1 μm for 15 °C, respectively. From industrial point of view, it is more economic for fabricating AAO by HPA at relatively high temperature. Let us consider the mechanism of the hybrid pulse action on onestep AAO growth during porous anodizing. In general, formation and dissolution of aluminum oxide during electrochemical reaction can be expressed in formulas (1) and (2), respectively [13,26]. þ
2AlðsÞ þ 3H2 O → Al2 O3 ðsÞ þ 6H ðaqÞ þ 6e
−
þ
1557 3þ
Al2 O3 ðsÞ þ 6H ðaqÞ → 2Al ðaqÞ þ 3H2 O
ð2Þ
When the potential was applied to the electrochemical cell, both reactions of formulas (1) and (2) reacted with the aluminum specimen and reached a balanced condition. The releasing electrons in formula (1) produce anodizing current. At the beginning of anodizing, surface fluctuation causes varied local field distributions. The dissolution of the aluminum oxide can be enhanced by local high field and pores are created first in high field region. After a period of anodizing, the AAO structures at the pore bottom become stable growth and the regularity of pores configuration is adjusted with increasing anodizing time. However, the AAO structure on the top was still rough and distorted due to these anodizing processes only occur in the barrier layer at the bottom of AAO. That is the reason for removing the first anodized alumina and performing the second anodization for better configuration in the conventional two-step anodization [2]. In the case of HPA process, the small negative potential (−2 V) is applied after a duration of anodizing positive potential. And the hydrogen ions with positive charge are attracted to the surface at the same time as shown in the schematic diagram of Fig. 6. The attracted hydrogen ions lead to the dissolution of AAO on the top. Therefore, the rough and distorted AAO structure can be improved. Moreover, there is no electron in dissolution reaction of alumina so that no extra Joule's heat generates when applying negative potential. The small negative voltage resulting in the nearly zero current at the negative duration is good for the uniformity and circularity of pore distribution as mentioned previously. It is noted that too large negative potential may lead to negative current by electrical breakdown and destroy the whole AAO structure.
ð1Þ
Fig. 5. Cross-section of AAO formed by HPA from 99.997% Al foil in 0.5 M oxalic acid for 1 h at (a) 5 °C and (b) 15 °C.
Fig. 6. Schematic diagram of the effect of negative potential applied in one-step HPA anodization during AAO growth. The positive hydrogen ions attracted to the surface lead to the dissolution of the top of AAO for improving the rough and distorted structure by one-step DCA.
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4. Conclusion The one-step AAO films with high uniformity and circularity of pore distribution have been successfully fabricated using hybrid pulse anodization method. The pore distribution uniformity and circularity of AAO by HPA in 0.5 M oxalic acid from 99.997% Al foil at 5 °C for 1 h reach 92.6% and 0.7, respectively. While the results in DCA at the same experiment conditions are just 82.9% and 0.3, respectively. When the environment temperature increases to 15 °C, the AAO nanostructures formed by one-step HPA are still good. In contrast, the AAO nanostructures are ruined by DCA at 15 °C due to temperature-enhanced dissolution. Moreover, the pore diameter, uniformity, and circularity of one-step HPA at 15 °C are still higher than one-step DCA at 5 °C. Although the slight decrease of pore uniformity and pore circularity at the elevated temperature in one-step HPA is seen, the growth rate is still dramatically increased from 1.2 μm/h for 5 °C and 4.1 μm/h for 15 °C. It is more economical for fabricating the AAO by one-step HPA at relatively high temperatures. In HPA process, the negative period with nearly zero cathodic current can provide effective cooling for promoting AAO quality. On the other hand, the positive hydrogen ions attracted to the surface lead to the dissolution of the top of AAO. It can improve the rough and distorted AAO structure by one-step DCA. The advantages of effective cooling and extra dissolution in HPA are helpful for fabricating AAO by one-step anodization at relatively high temperatures. Acknowledgments This work is partially sponsored by the National Science Council under contract no. NSC99-2221-E-006-032-MY3. We also would like to thank the Center for Micro/Nano Science and Technology in National Cheng Kung University, National Nano Device Laboratories (NDL), and
National Center for High-performing Computing (NCHC) for the access of process and analysis equipments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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