Controllable fabrication of porous alumina templates for ...

Materials Chemistry and Physics 122 (2010) 295–300

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Controllable fabrication of porous alumina templates for nanostructures synthesis Junping Zhang ∗ , Jerrold E. Kielbasa, David L. Carroll ∗ Center for Nanotechnology and Molecular Materials and Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA

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Article history: Received 28 April 2009 Received in revised form 18 January 2010 Accepted 19 February 2010 Keywords: Porous alumina templates Anodization AC electrochemical deposition Silver nanorods/wires

a b s t r a c t Porous alumina templates (AAO) has attracted significant interest due to the fact that they are readily fabricated through a simple procedure and are extremely popular templates in nanoscience studies. In this paper, the effects of different pore-widening treatments on the pore quality of the AAO templates were investigated. Results show that, through a highly controllable chemical pore-widening process at low temperature, different pore dimensions and diameters of the AAO templates can be easily achieved in a nanometer-scale way without changing the interpore distance. Combining with anodization voltage control, AAO templates with desired size distribution can be obtained, which will be extremely useful in template technology and masks for lithographic application. Also, silver nanorods/wires of different dimensions have been fabricated from above AAO templates after pore diameter adjustments. Such nanostructure materials hold high potential for electronics, optics, mechanics and sensing technology. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Nanoscale species, such as nanorods, nanowires, have attracted significant research interests [1–5] because of their unique electronic, optical, magnetic, thermoelectrical and chemical properties. However, the physical properties of nanorods/wires are highly influenced by their morphology and size distributions [6,7]. To explore more potential applications of the nanorods/wires, it is desirable to develop fabrication methods which can better control the size distributions of the nanorods/wires and organize them into highly ordered arrays abrications of such nanoscale materials [8,9]. AAO is a self-ordered nanoporus template that consists of a hexagonal array of cells with uniform and parallel straight cylindrical nanopores perpendicular to the template surface [10]. Nanorods/wires can be fabricated onto these AAO templates by pressure injection [11,12], vapor deposition [13,14] and chemical electrodeposition methods [15–17]. AAO templates can be obtained easily by a two-step electrochemical anodization process of aluminum sheet was anodized in some electrolytes [8]. The diameter of the pores, the pore density and the thickness of the AAO template can be controlled by changing their anodization conditions, such as anodization voltage, types of electrolyte acids, temperature of the electrolyte solution, etc. The

∗ Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (D.L. Carroll). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.02.023

formed AAO templates on aluminum sheet have a double-layered structure: a porous AAO layer forms on the top and a thick barrier film forms as a semi-spherical oxide layer on the bottom of the pore. when fabricate nanomaterials into the AAO pores with electrodeposition method, the thick barrier layer at the bottom of the pores in the AAO template provides strong electrical resistance of AAO templates and thus it is not possible to deposit metal inside the pores directly by electrochemical method.Pore-widening technique is mainly used to modify the pore quality of the AAO templates, and it is also an effective way to prepare AAO templates with different pore size arrays that cannot be obtained by other techniques. During the pore-widening process, the cell pore wall can be tailored and the pore diameter can be tuned without changing the pore density of the AAO template [18]. Moreover, such porewidening treatment also thins the barrier layer on the bottom of the AAO pore array and improves the conductivity for the metal nucleation during electrodeposition process. Thus optimizing the pore-widening time is a key to get high quality AAO pore arrays and to facilitate uniform electrodeposition of nanomaterials onto AAO templates. However, lots of research has been focused on optimizing anodization conditions to get highly ordered AAO templates [19–22], only few works have been studied to improve the chemical pore-widening procedure. Usually, pore-widening processes run at above 30 ◦ C [18], and nanopore arrays in AAO template are easily destroyed. During our research, we tried to improve the pore-widening process by differing pore widen the same AAO sample in same acid at different temperature and with different techniques. It is found that the

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pore-widening process done at low temperature could be highly controlled without damaging the AAO pore array. Results also indicate that it is possible to fabricate highly ordered AAO templates of desired pore dimensions and diameters by controllable chemical pore-widening process at low temperature. To our knowledge, there is no pore-widening process of AAO templates have been done under the room temperature.

2. Experimental The AAO templates were prepared with a modified two-step anodization process. The high purity aluminum sheets (99.999%) were cleaned, degreased, annealed at 500 ◦ C for 3 h, and then electropolished in a 1:3 volume mixture of perchloric acid and ethanol at a current density of 100 mA cm−2 and a temperature of 1 ◦ C for 2 min. In the first anodization step, the electropolished aluminum sheet was anodized in a 0.3 M oxalic acid solution at 40 V and at 0 ◦ C for different time. A preliminary oxidization layer was obtained and then was removed by immersing the sample in the mixture solution of H3 PO4 (6 wt%) and CrO3 (1.8 wt%) until the AAO was totally etched and a shiny aluminum surface was appeared. Subsequently, a second anodization was carried out at 0 ◦ C for about 1 h under the same conditions as the first anodizaion step. A highly ordered porous AAO template could be obtained by this two-step anodization process. Afterward, the AAO template was immersed into a 5 wt% phosphoric acid solution to widen the nanochannels. The pore-widening time were controlled for different time. Silver nanorods and nanowires were electrodeposited into the pores of AAO by AC electrodeposition with peak-to-peak modulation currents of ±20 mA and delay time of 3 ms. The electrolyte was aqueous solution composed of AgNO3 2 g L−1 and boric acid 20 g L−1 . The electrolyte was buffered to pH 2.5 with H2 SO4 solution. The length of Ag structures deposited inside the AAO pores was controlled by the electrodeposition time. After electrodeposition, the AAO template with silver nanorods/wires was detached from the aluminum substrate with solution of saturated HgCl2 . 1.0 M NaOH solution was used to dissolve the AAO template and free the silver nanowires. The pore size of the AAO template for each controlled widening time and the silver nanowires were examined using a Scanning Electron Microscope (JEOL, JSM-6330F).

3. Results and discussion 3.1. Formation of AAO templates The usual electrochemical method for fabricating AAO templates is the anodization of single crystal or high purity Al foils at constant voltage. In this paper, the pretreated Al foils were first anodized in 0.3 M oxalic acid solution at 0 ◦ C under a constant voltage of 40 V. Fig. 1(a) gives the SEM image of the AAO template after first anodization for 10 min. It can be seen that the pores nucleate on the Al surface at random, the pore size and the thickness of pore cell walls are not uniform, and some sub-holes also appear under the main holes. The pore size, pore density and the thickness of the AAO template are approximately proportional to the anodization voltage, but the quality of the ordering in the AAO structure also highly depends on the first anodization time. After first anodization, the anodization layer was removed by wet chemical etching with a mixture solution of phosphorus acid and chromic acid, and then anodized again under the same condition as the first anodization. As the first anodization proceeds, the depth of pores increased and the pore domain structure become more regular. Fig. 1(b)–(d) show the top views of the secondly anodized AAO pore arrays with different first anodization time. For Fig. 1(b), the fist anodization time is only 4 h, the nanopores on AAO are still disordered. Fig. 1(c) shows a little more ordered nanopore array after increasing the first anodization time to 12 h, but the pore array is still not very regular and pore sizes are not uniform. With the first anodization time up to 40 h, the AAO array after second anodization is highly ordered and almost all the pores are of uniform size, as shown in Fig. 1(d). It should be point out that all the AAO templates in Fig. 1(a)–(d) had been pore widened in a 5 wt% phosphoric acid at 19 ◦ C for 20 min after the second anodization.

Fig. 1. SEM images of porous AAO templates made from aluminum foil. (a) The anodized AAO template was anodized for 10 min in 0.3 M oxalic acid at 0 ◦ C at 40 V. (b) Top view of non-ordered AAO nanopores array after the second anodization for 30 min (first anodization time is 4 h). (c) Nanopores array after the second anodization for 1 h (first anodization time is 12 h). (d) Ordered nanopores array after the second anodization for 1 h (first anodization time is 40 h). (All the samples were pore widened in a 5 wt% phosphoric acid at 19 ◦ C for 20 min after second anodization). Scale bar: 500 nm.


3.2. AAO template with pore-widening treatments Many studies have been done to optimize the pore parameters of AAO. It has been reported before that the thickness and pore diameter of the AAO can be controlled by changing different anodization parameters [8,22]. But for certain electrolytes with same concentration, the interpore distance of the AAO template only depends on the anodization voltage, not on other anodization parameters [22]. The diameter of the pores can be adjusted by wet chemical etching process without changing the pore density, which is known as “the pore-widening process”. The pore-widening process is the dissolution of aluminum oxide in the pore wall in an acid solution. During the pore-widening process, the pore wall distances decrease; the pores increase to a uniform size and present a hexagonal structure. Li et al. [18] found the optimal pore-widening condition for the AAO template prepared with 0.3 M oxalic acid at 40 V was carried out in a 5 wt% phosphoric acid at 35 ◦ C for 30 min. In our research, we tried to pore widen the same AAO sample in 5 wt% phosphoric acid at different temperature. It was found that nanopore arrays in

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AAO templates were easily damaged over 35 ◦ C, even though the pore-widening process had been carried out only for 25 min. While decreasing the pore-widening temperature to room temperature or even lower, the pore-widening process can be easily controlled without destroying the pore arrays. In this paper, all of the AAO samples were pore widened at 19 ◦ C. Fig. 2 shows the effect of the pore-widening time on the diameter of AAO pores at low temperature. The AAO sample was prepared by two-step anodization procedure at 40 V in 0.3 M oxalic acid, the first anodization time was 20 h and then the sample was anodized for 1 h under the same conditions as the first anodization. Pore widenings were done in a 5 wt% phosphoric acid at 19 ◦ C for different times after second anodization. The microstructure of the AAO template before pore-widening is shown in Fig. 2(a), the interpore distance is 100 nm, the mean pore diameter is about 28 nm and the double-cell wall thickness is about 72 nm. Fig. 2(b)–(d) show the top views of the highly ordered AAO arrays with pore widening for different time. It can be seen clearly that, after pore widening for about half an hour, the surface of the pore structures are very

Fig. 2. SEM images of AAO array before and after selectively etching the Al substrate in 5Wt% H3PO4 at 19 ◦ C for different time. (a) Before the pore-widening process; (b) pore widening for 30 min at 19 ◦ C; (c) pore widening for 37 min at 19 ◦ C; (d) pore widening for 42 min at 19 ◦ C; (e) three-dimensional image of the AAO pore array (left) and one-dimensional pore array with hexagonal structure (right); (f) schematic diagram of two-dimensional hexagonally ordered AAO pore array. di = interpore distance, dp = pore diameter, dc = 2 × cell wall thickness. Scale bar: 200 nm.

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Fig. 3. The pore diameter (a) and pore cell wall thickness (b) changes with the chemical pore-widening time. (c) The pore structure was destroyed when the pore-widening time was over 47 min. Scale bar: 1 ␮m.

smooth and the AAO template exhibits two-dimensional array with a regular hexagonal pore pattern, the diameters of the pore increase to about 67 nm (see Fig. 2(b)). Along with the pore-widening time prolong, uniformity is increased and the hexagonal pore patterns become more and more apparent (Fig. 2(c) and (d)). After pore widening the sample for about 42 min, the pore wall thickness decrease to 7 nm while the corresponding pore size increase to 93 nm. In Fig. 2(e), the left image shows the three-dimensional image of the AAO pore array and the right image shows the hexagonal pore structure formed with six bright spots. It is obvious that the hexagonal pore patterns in the AAO template are highly oriented and with a uniform size. The schematic diagram in Fig. 2(f) shows the AAO pore array and some pore parameters. From above results, it can be confirmed that highly ordered nanoporous AAO templates with different pore diameters can be prepared by the chemical pore-widening process at low temperature. As is known, AAO templates with various interpore distances can be fabricated in different acids by changing the anodization voltage. By control-

lable pore widening at low temperature, much better controlled dimensional AAO templates with different pore sizes might be obtained. Fig. 3(a) and (b) shows the changes of the pore diameter and the thickness of the pore cell wall with the pore-widening time, it can be seen that the pore-widening processes slowly in the beginning and becomes very effective after 20 min. But for long time the pore-widening process will destroy the nanopores array, as shown in Fig. 3(c). The pore array was damaged after pore widening over 47 min. Thus optimizing the pore-widening time is a key to get high quality AAO pore arrays, which is also a way to prepare different pore size arrays. Fig. 4 shows the cross-sectional view of AAO pore array with pore-widening treatments for different time, respectively, which clearly confirms the increase of the pore diameter and the thinning of the pore cell wall. Moreover, such pore-widening treatments also thins the barrier layer on the bottom of the AAO pore array. As known, barrier layer thinning is essential to increase the electrical contact between the electrode and the AAO and is very

Fig. 4. The cross-sectional image of the AAO samples with different pore-widening time: (a) for 25 min and (b) for 35 min at 19 ◦ C. Scale bar: 200 nm.


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Fig. 5. SEM image of silver nanowires after AAO templates were dissolved. (a) Silver nanorods separated from AAO template with 35 min pore-widening treatment. (b) and (c) are silver nanowires separated from AAO template with 30 min (b) and 22 min (c) pore-widening treatments, respectively. Scale bar: 1 ␮m.

important to facilitate uniform electrodeposition of nanomaterials onto AAO templates [15,23].

3.3. Fabrication of nanorod/nanowires Methods that involve direct pore filling of the AAO template by chemical electrodeposition technology are considered to be the most straightforward and versatile technique for nanowire fabrication. However, during the growth of the porous AAO template, a semi-spherical oxide layer at the bottom of the pore, known as the barrier layer, is also formed [10]. This thin barrier layer underneath the pores of AAO template has high resistivity and direct current cannot pass through the barrier layer [24]. Usually, there are two ways to improve the shortcoming caused by the barrier layer, one is to detach the AAO template from the aluminum substrate and followed by evaporating a metal layer on one side of the freestanding AAO template. The drawback of this technique is that the requirement of the freestanding AAO template must be strong enough to tolerate manipulation since the template is so fragile without the support of the aluminum substrate. An alternative way is to grow metallic nanowires involves thinning the barrier layer and directly pores fill various metal nanowires by a. c. pulse electrodeposition, which is used in our work. In AC pulse electrodeposition, alternating current is applied between the template and other electrodes. The anodic half cycle and cationic half cycle alternate with each other and let the electrodeposition proceed [15]. Various metal nanorods and nanowires of different size can be fabricated into AAO templates which have been pore widened by AC pulse electrodeposition. Fig. 5 shows the SEM micrographs of the free silver nanowires with different length and width after dissolving the AAO template. For the silver nanorods shown in Fig. 5(a), the supporting AAO template had been pore widened in phorphorus acid for 35 min. The mean diameter of

pores is about 80 nm. The consequently prepared silver nanorods are 80 nm. Fig. 5(b) and (c) is the silver nanowires separated from AAO template with pore-widening treatment for 30 and 22 min, respectively. The diameter of the silver nanowires in Fig. 5(b) is about 67 nm while that in Fig. 5(c) is 55 nm. Results indicate, again, that an ordered AAO template with different pore dimensions and diameteres can be obtained by the controllable pore-widening treatments. Thus nanomaterials of different sizes can be obtained even using the same original AAO template anodized under the same condition.

4. Conclusions Highly ordered AAO pore array can be fabricated by anodizing high purity Al foils at constant voltage. While the anodization voltage, electrolyte and temperature affect the growth rate of the pores and the interpore distance on AAO template, the first anodization time also strongly influences the order of pore arrays on AAO template. For the same AAO templates prepared in 0.3 M oxalic acid under same anodization condition in a two-step anodization process, their pore diameters can be tuned between 28 and 93 nm by controlling pore widening in 5 wt% phosphorus acid solution at low temperature. These results can be used to make ordered AAO templates of various pore dimensions and diameters without changing the interpore distance in the template. Moreover, the interpore distances of AAO template can be adjusted by changing anodic oxidization parameters, followed by controllable pore-widening process. Further much more AAO templates with desired pore dimensions and diameters might be obtained for different applications, such as masks for lithographic purpose, size selective filtration and duplication matrixes. Consequently, various nanomaterials (such as nanorods, nanowires and nanotubes) with different size distributions can be fabricated on such AAO tem-

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plates. These nanomaterials exhibit unique electrical, magnetic, optical, thermoelectric and chemical properties and have potential applications in the fields of electronics, optics, biomedical and sensor devices. Acknowledgment This work was supported by the United States Air Force Office of Scientific Research under MURI grant FA9550-06-1-0337. References [1] [2] [3] [4] [5]

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