structure of alumina nanowires synthesized by chemical etching

Physica E 36 (2007) 140–146 Structure of alumina nanowires synthesized by chemical etching of anodic alumina membrane J.K. Han a , J. Kim a , Y.C. Choi a , K S. Chang b , J. Lee c , H.J. Youn c , S.D. Bu a,Ã a Department of Physics, Research Institute of Physics and Chemistry (RINPAC), Chonbuk National University, Jeonju 561-756, Korea b Department of Chemistry, Korea Air Force Academy, Cheongwon 363-840, Korea c Jeonju Center, Korea Basic Science Institute, Jeonju 561-756, Korea Received 7 December 2005; received in revised form 27 September 2006; accepted 2 October 2006 Available online 13 November 2006 Abstract We report the structure of alumina nanowires (ANWs) synthesized by the chemical etching of an anodic alumina membrane (AAM). The AAM was prepared by the two-step anodization method followed by the lift-off process. The field emission transmission electron microscopy analyses showed that the AAM consists of a gel-like structure, an outer layer, and an inner layer, which are distinguished by their anion impurity concentration. The fabricated ANWs appeared to have a two-oxide-layer structure, similar to the core-shell structure in a coaxial cable. The inner oxide layer may be composed of relatively pure alumina and the outer oxide layer of the byproducts of the reaction between alumina and the anodization or etching solution. The fabricated ANWs have a flexible nature, with some of them being sufficiently malleable to form L-shaped ANWs. One possible formation process of the two-oxide-layer structured ANW is discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 77.84.Dy; 81.20.Fw Keywords: Alumina nanowires; Anodic alumina membrane; Two-oxide-layer structure 1. Introduction Alumina nanowires (ANWs) have become an exciting field of study because their large surface area and highly electropositive surface make them to be an ideal candidate for water purification filter technology [1].Theyrapidly absorb dissolved heavy metals including mercury, gold, silver, cadmium, lead, and uranium. Their highly electro- positive surface attracts and retains the pathogens and viruses present in water, which are principally electronega- tive as a result of the difference in electrical potential between the water and the particle phase [2]. In addition, the flow capacity of ANW-based filters is several orders of magnitude greater than that of alumina membrane filters. A major challenge is to synthesize high aspect-ratio ANWs with a uniform and smaller diameter, in order to maximize the density of the positively charged sites, and to immo- bilize them into the non-woven filters for improved adapt- ability. The simple chemical etching of an anodic aluminum oxide membrane (AAM) is considered to be an easy and cost- effective way to fabricate ANWs with high aspect ratios [3]. In our previous work [4], we proposed a possible mechanism of formation of ANWs produced by the droplet chemical etching process. Our previous observation revealed that the mechanism of formation of ANWs could be explained by the different etching rates of the various oxide layers of the AAM, as determined by their anion impurity concentration. In this work, we report the structure of ANWs synthesized by the chemical etching of an AAM. 2. Experimental procedure AAMs were fabricated using the two-step anodization method and subsequent lift-off process by applying a ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.10.003 Ã Corresponding author. Tel.: +82 63 270 4264; fax: +82 63 270 3320. E-mail address: sbu@chonbuk.ac.kr (S.D. Bu). saturated HgC l 2 solution [5]. A pure aluminum sheet (99.999%) is degreased in acetone and electropolished at a voltage of 20 V and a temperature of 5 1C under stirring. The electropolished aluminum sheet is then anodized in a 0.3 M oxalic acid at a voltage of 40 V for 12 h at 5 1C. In order to remove the irregular porous oxide layer formed during the first anodization, it is etched in an aqueous mixture solution of phosphoric acid (6 wt%) and chromic acid (1.8 wt%). The subsequent second anodization, which is performed under the same conditions as the first anodization but with a different period of time depending on the desired thickness of the oxide layers, yields a highly ordered hexagonal nanopore array of anodic aluminum oxide on aluminum, i.e. an AAO template. A saturated HgCl 2 solution is then applied to separate the AAM from the AAO template. The separated AAM is rinsed with de- ionized water several times. Finally, an additional pore widening process is conducted in 5 wt% phosphoric acid at 30 1C in order to obtain the desired pore diameter. Prior to the droplet chemical etching, the AAM is annealed at 650 1C to remove the hydroxyl groups (OH À ) [6]. The annealed AAM is attached to double-sided Ca tape on a Pt-coated silicon substrate to hold the ANWs during the etching process and to prevent the electron charging phenomenon during the field emission scanning electron microscopy (FESEM) observation. 0.1 ml of an aqueous NaOH or H 3 PO 4 solution is applied to the surface of the AAM for 10 min using a micropipette. The etched AAM is rinsed several times with de-ionized water and ethanol, and dried overnight at 70 1C prior to the FESEM observation. 3. Results and discussion AAMs with suitable pore diameters ranging from 20 to 100 nm were prepared by adjusting the electrolytes, anodization time and temperature, applied voltage, and pore widening treatment. The best quality AAM was obtained using 0.3 M oxalic acid as the electrolyte at 40 V, which corresponds to the optimum anodization conditions suggested by Masuda et al., [5]. A typical SEM image of an AAM fabricated by the two-step anodization method is shown in Fig. 1. The surface view of the AAM in Fig. 1(a) shows that it has a perfect hexagonal pore distribution within a domain size of one to two micro meters, which is separated from neighboring domains having a pore lattice with a different orientation. Its average pore diameter is about 60 nm and the inter-pore distance, that is, the distance from pore to pore, is about 100 nm. The inter-pore distance could be controlled by adjusting the applied voltage. Fig. 1 (b) is a cross-sectional view SEM image of the AAM and it shows the nature of the straight and parallel nan opore morphology. The thickness of the AAM could be controlled by varying the second anodization time. Fig. 1(c) shows an atomic force microscopy (AFM) image of the AAM. The configuration of the AAM can be thought of as a pore surrounded by six small hills, which we call triple points, and are identified by the black circle in Fig. 1(c). The average diameter of the triple points is about 40 nm and their height is about 2 nm. AAMs are known to have a duplex structure, which results from the different degrees of anion contamination during the anodic anodization [7]. Fig. 1(d) is a field emission transmission electron microscopy (FETEM) image showing the duplex structure of an AAM. The darker gray layer comprising hexagonal cells is the inner layer and the relatively bright gray layer is the outer layer. The inner layer is only about 10 nm in thickness, which is a typical result under the anodization condition of 0.3 M oxalic acid at 40 V [8]. The two oxide layers are expected to have different compositions: the inner layer is known to be composed of relatively pure Al 2 O 3 and the outer layer contains several othe r elements such as C and H in the case of oxalic acid based anodization [9]. The scanning trans- mission electron microscopy (STEM) image in Fig. 1(e) suggests that the inner layer and the outer layer are indeed composed of different elements, which can be notice d from the different contrasts in the image. The white contrast should represent the inner layer observed in the FETEM image of Fig. 1(d). Fig. 1(f) shows a high-resolution TEM (HRTEM) image of the AAM. It should be noted that this image is quite different from the one in Fig. 1(d) and shows that the AAM is composed of three regions. This contradicts the usual experimental results obtained for AAMs consisting of two oxide layers [7,8,10]. The inner layer is blurred and cannot be clearly distinguished from the outer layer. This can be explained by there being a gradual change of the anion impurity concentration, rather than a uniform distribution. The anions that could possibly contaminate the outer layer in our fabri cation method are C 2 O 4 2À , HC 2 O 4 À ,HC 2 O 3 À ,O 2À ,andOH À . In fact, Choi et al. [9] reported that the anions are not uniformly distributed over the outer layer, but that the concentration of the contaminating ions is the highest in the middle of the outer layer, because of the different mobilities of among such anions. In addition, tube-like configurations can be observed at the interface between the pores and the outer layer, as marked by the white arrows in Fig. 1(f). The tube- like configurations are clearly different in appearance from the rest of the cells and their lighter contrast suggests that they are closely related to the gel-like structure Thompson and Wood [7] proposed. We found that the droplet chemical etching of the AAM creates four distinctive regions with respect to the different surface morphologies on AAM, as shown in Fig. 2,as compared with the general immersion etching method. When a droplet of aqueous NaOH solution is applied to the AAM, a spreading phenomenon occurs because of the high surface energy of the AAM, while the aqueous NaOH solution exhibits low surface energy [11,12]. This spreading phenomenon results in different volumes of aqueous NaOH solution per unit area on the AAM in the radial direction, which in turn results in different degrees of etching in the radial direction. The center region, marked ARTICLE IN PRESS J.K. Han et al. / Physica E 36 (2007) 140–146 141 as A, has the highest volume of aqueous NaOH solution per unit area, so this region would be etched the most. Most of the ANWs in this region were lying on the Ca tape, as is usually observed in the general immersion etching method with a longer etching time [13]. Such an observa- tion indicates that this region is analogous to that corresponding to the longer period of etching time in the general immersion etching method. By scanning outw ard from the center, in the A–D direction, the degree of etching can be seen to have been weakened due to the dwindling volume of aqueous NaOH solution per unit area. In the edge region of the etched area denoted by D, the nanopores are slightly widened, so that this region may correspond to that observed after a very short etching time in the general immersion etching method. The border region, where the precursor film of the aqueous NaO H solution woul d be formed, can be divided into two regions; one showing the bundles of ANWs (region B), and the other showing the formation process of the ANWs (region C). ARTICLE IN PRESS Fig. 2. Surface view SEM image of an AAM etched by a dilute NaOH solution, which shows four distinctive regions. Fig. 1. Microscopic images of a typical AAM: (a) the surface view SEM image, (b) the cross-sectional view SEM image, (c) the surface view AFM image, (d) the surface view FETEM image, (e) the surface view STEM image, and (f) the surface view HRTEM image. J.K. Han et al. / Physica E 36 (2007) 140–146142 Fig. 3 shows the serial plane view SEM images obtained from the C regions in Fig. 2. AAM with distorted nanopores, indicated by an arrow, is illustrated in Fig. 3(a), which is considered as the first stage in the formation of the ANWs. The inset of Fig. 3(a) clearly shows that the pore diameter of the AAM has been broadened and that the pore morphology has evolved from the initially smooth circular pores of the as-prepared AAM shown in Fig. 1 to one of distorted circles. The distorted nanopores suggest that the etching behavior of the AAM by the NaOH solutions is anisotropic. This anisotropic nature of the etching behavior in the AAM nanopores becomes manifest in Fig. 3(b), which shows that the thickness of the thin walls has been further etched, while the triple points appear less affected by ch emical attack. The inset of Fig. 3(b) clearly shows that the trip le points are distinctively noticeable. Fig. 3(c) shows the fabricated ANWs, whose diameters range from 30 to 40 nm, which is in accordance with the hill diameter mentioned above. Since the diameter and length of the ANWs are closely related to the hill diameter and thickness of the AAM, respectively, the diameter and length of the ANWs is controllable by adjusting the physical characteristics of the AAM that is used. The FESEM image in Fig. 4 shows the longitudinal view of a naturally formed ANW bundle, which can be observed in region B of Fig. 2. The presence of this naturally formed ANW bundle indicates that it is created in the natural course of etching as the aqueous NaOH precursor penetrates in the direction from top to bottom. When the etching solution reacts with the AAM, the upper parts of the AAM transform into ANWs, as is evident in Figs. 3(a) and (b). At the same time, these ANWs lose their support from the neighboring thin walls. Consequently, they tend to lean against other ANWs formed nearby, so as to form natural ANW bundles. We found that the ANWs on the outside of the naturally formed bundle are etched further than those on the inside of the bundle. Figs. 4(a) and (b) show the outside of the bundle, and Figs. 4(c)–(f) show the inside of the bundle. The magnified image of Fig. 4(a), that is Fig. 4(b), shows that the ANWs are not completely separated from each other. In addition, the presence of many lumps on the surface of the ANWs indicates that the outer ANWs of the bundles are etched further. This can be explained by the difference in the OH À concentration on the outside and inside of the bundles. Hydroxide ions would be expected to be trapped inside the bundles and their concentration dwindle with increasing reaction time, while the outer ANWs would likely be supplied with enough fresh OH À ions. Since the ANWs on the outside of the bundle may be excessively etched, they are not likely to possess the original surface morphology of the ANWs. In order to observe the morphology of the ANWs inside the bundle, the etched AAM was flushed with water so as to expose the initially hidden ANWs inside the bundles. Fig. 4(c) shows the basin created in the bundle of ANWs obtained by ARTICLE IN PRESS Fig. 3. Serial plane view SEM images of the formation process from an AAM to ANWs: (a) is the first stage after the dissolution of the gel-like structure, (b) shows distinctively noticeable triple points, and (c) shows the fabricated ANWs. J.K. Han et al. / Physica E 36 (2007) 140–146 143 flushing it with water. As expected, the surface morphology of the artificially expo sed ANWs inside the bundle differs from that of the ANWs outside the bundle, as shown in Figs. 4(d)–(f). In contrast to conventional bul k alumina ceramics, ANWs are highly flexible as shown in Fig. 4(d). Some can be sufficiently flexible to form L-shaped ANWs, as identified by a yellow solid line in Fig. 4(d). Note that some oxide materials also exhibit such a flexible nature when they are constructed on the nanoscale [14]. The detailed FESEM analysis of the ANWs in Fig. 4(e) demonstrates that they have fissures on their surface. These fissures in the ANWs may be caused by mechanical stress, which most likely occurs when the ANWs collapse because of their losing the support from the wall when the wall is etched away. Fig. 4(f) is the magnified view of Fig. 4(e) and it shows that some of the ANWs appear to consist of two layers, as shown in the schematic diagram, which is usually observed in the corrosion process [15]. In order to understand more clearly the structure of the ANWs, X-ray diffraction (XRD), TEM, and X-ray photoelectron spectroscopy (XPS) analyses were per- formed. In the XRD analysis, the inner oxide layer was found to be a metastable form of aluminum oxide [16]. Fig. 5 shows the dependence of the XRD pattern of the ANWs on the etching time. As the etching time was increased from 0 to 15 min, a new peak appeared at a 2y value of 42.61 and its intensity increased. This peak was ARTICLE IN PRESS Fig. 4. Longitudinal view SEM images of ANWs: (a) ANWs on the outside of the ANW bundle, (b) a magnified image of Fig. 4(a), (c) ANWs inside the bundle, (d) a magnified image of Fig. 4(c), which shows flexible ANWs, (e) a magnified image of Fig. 4(c), which shows fissures in the ANWs, and (f) a magnified image of Fig. 4(e), which shows two-oxide- layer structured ANWs along with a corresponding schematic diagram. Fig. 5. XRD patterns of ANWs with etching times of 0, 5, and 15 min. Fig. 6. TEM image of a typical ANW. It clearly shows the presence of the inner and outer oxide layer of the ANW. J.K. Han et al. / Physica E 36 (2007) 140–146144 identified as the (2 0 2) reflection of metastable aluminum oxide [JCPDS]26-0031]. These results strongly suggest that the inner oxide layer is composed of relatively pure aluminum oxide. In addition, the TEM image in Fig. 6 clearly shows the presence of the inner and outer oxide layer. Fig. 7 shows the XPS spectra of the ANWs, which were synthesized by the chemical etching of the AAM in dilute NaOH and H 3 PO 4 solutions. The Al, O, Na, P, C, and N peaks are clearly observed. The peak at 74.4 eV can be assigned to an Al-oxide, not metallic Al. The C 1s peak shows the presence of strong carbon contamination at the surface. It should be noted that Na 1s and P 2p peaks were observed, which indicates the strong presence of sodium and phosphorous compounds. These results suggest that the outer oxide layer may be composed of the byproducts of the reaction between alumina and the anodization or etching solution. Based on the results of this study, we suggest a possible mechanism of formation of the two-oxide-layer structured ANWs from the AAM membrane, which is described in the schematic diagram of Fig. 8. Fig. 8(a) shows a schematic diagram of the AAM characterized by the different anion concentrations, as shown in Fig. 1. The concentric ring-like parts of the diagram in Fig. 8(a) describe the gel-like structure, which may contain a high concentration of anion impurities [7]. The inner layer is the darkest part of the hexagon. The outer layer is situated in between the gel-like layer and the inner layer. The gradually increasing contrast ARTICLE IN PRESS Fig. 8. Schematic illustration of the mechanism of formation of the ANWs from the AAM: (a) pore structure of the AAM with three oxide layers, (b) AAM after gel-like structure is dissolved, (c) AAM after further etching, which describes the triple points surrounded by the byproducts of the reaction between alumina and NaOH, and (d) core-shell structured ANWs. Fig. 7. XPS spectra of ANWs. The ANWs were synthesized by the chemical etching of the AAM with NaOH and H 3 PO 4 etching solutions. J.K. Han et al. / Physica E 36 (2007) 140–146 145 from light gray to dark gray shows the transition from the gel-like structure through the outer layer to the inner layer. In the first step of the formation process, the gel-like structure would be dissolved within a very short period of time, as shown in Fig. 8(b) and mentioned in Fig. 3(a). Secondly, Fig. 8(c) shows that the triple points may be mostly composed of relatively pure alumina and the light gray part may be the byproducts from the reaction between alumina and the anodization or etching solution. This diagram corresponds to Fig. 3(b) showing that the triple points are not significantly affected by the course of etching, because the relatively pure alumina has better resistance to chemical attack [17].Lastly,Fig. 8(d) shows that the triple points become two-oxide-layer structured ANWs, similar to core-shell ANWs, when the thin walls are completely etched, as observed in Figs. 4(e) and (f). That is, the relatively pure alumina becomes the core of the ANWs, which are surrounded by the byproducts of the reaction between alumina and the anodization or etching solution. 4. Conclusion ANWs synthesized by the chemical etching of an AAM are found to have a two-oxide-layer structure. The inner layer may be composed of relatively pure alumina and the outer layer of the byproducts of the reaction between alumina and the anodization or etching solution. Acknowledgments This work was supported by the Korea Research Foundation funded by the Korean Government (MOEHRD) through Grant nos. R05-2004-000-10989-0 and KRF-2005- 005-J07501 to one of the authors (S.D.B.). K.S.C. acknowledges the support provided by the Dong-Hwan Research Foundation at the Korea Air Force Academy. References [1] F. Tepper, M. Lerner, D. Ginley, Am. Ceram. Soc. Bull. 80 (2001) 57. [2] F. Tepper, L. Kaledin, C. Hartmann, Water Cond. Purif. 47 (2005) 55. [3] Z. Yuan, H. Huang, S. Fan, Adv. Mater. 14 (2002) 303. 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