A Highly Controllable Electrochemical Anodization Process to Fabricate

Lin et al. Nanoscale Research Letters (2015) 10:495 DOI 10.1186/s11671-015-1202-y NANO EXPRESS Open Access A Highly Controllable Electrochemical Anodization Process to Fabricate Porous Anodic Aluminum Oxide Membranes Yuanjing Lin1, Qingfeng Lin1, Xue Liu1, Yuan Gao1, Jin He2, Wenli Wang3,4* and Zhiyong Fan1* Abstract Due to the broad applications of porous alumina nanostructures, research on fabrication of anodized aluminum oxide (AAO) with nanoporous structure has triggered enormous attention. While fabrication of highly ordered nanoporous AAO with tunable geometric features has been widely reported, it is known that its growth rate can be easily affected by the fluctuation of process conditions such as acid concentration and temperature during electrochemical anodization process. To fabricate AAO with various geometric parameters, particularly, to realize precise control over pore depth for scientific research and commercial applications, a controllable fabrication process is essential. In this work, we revealed a linear correlation between the integrated electric charge flow throughout the circuit in the stable anodization process and the growth thickness of AAO membranes. With this understanding, we developed a facile approach to precisely control the growth process of the membranes. It was found that this approach is applicable in a large voltage range, and it may be extended to anodization of other metal materials such as Ti as well. Keywords: Anodic aluminum oxide, Nanoporous structure, Integrated charge density, Controllable electrochemical anodization Background aluminum oxide (AAO), have wide nanoengineering ap- Metal anodization has been broadly used in industry as plications that have attracted enormous attention. For a surface treatment technique to render materials with example, AAO membranes have been used as templates resistance against uncontrolled oxidation, abrasion, and to directly assemble semiconductor nanowires and corrosion. Although this technique has been developed nanorods for photodetection [2] and solar energy for a long time, it was until 1990s that researchers dis- conversion [3–5]. A large internal surface area of covered that highly ordered nanoporous structures can these oxide nanostructures can also be harnessed to be achieved by properly tuning anodization conditions build high-performance energy storage devices such including electrolyte composition and concentration, as Li-ion batteries [6] and supercapacitors [7, 8]. temperature, as well as anodization voltage [1]. Among Meanwhile, it is worth pointing out that AAO struc- all valve metals that can be anodized, aluminum (Al) tures can be engineered into a number of variants via and titanium (Ti), particularly Al, can be anodized into proper combination of wet chemical etching and an- nanoporous structures with well-controlled diameter, odization processes. These variants include nanowells pitch, and depth. Membranes consist of these nanostruc- [9], inverted nanocones [10, 11], nanobowls [12], tures, i.e., anodic titanium oxide (ATO) and anodic nanospikes [13–15], and the integrated nanopillar- nanowell structures [16]. These structures have been * Correspondence: [email protected]; [email protected] used as scaffold of energy harvesting and storage de- 3College of Textile and Clothing Engineering, Soochow University, Suzhou vices in our past works. 215021, China 1Department of Electronic and Computer Engineering, The Hong Kong There are certainly a number of advantages of using University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong porous anodic nanostructures, particularly AAO mem- SAR, China branes for nanoengineering applications, such as large Full list of author information is available at the end of the article © 2015 Lin et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Lin et al. Nanoscale Research Letters (2015) 10:495 Page 2 of 8 surface area, high regularity and scalable low-cost pro- anodization in a wide voltage range for a broad duction [17]. In many applications, precise control of applications. nanostructure shape and geometry is critical. It is known that the geometric features of porous AAO membranes are widely tunable via electrochemical anodization Methods process [10]. Factors such as applied voltage, acid type, The fabrication of porous AAO membranes with nano- and concentration contribute to the formation of the structures mainly follows a two-step electrochemical anod- porous nanostructures with various barrier layer thick- ization process [22] after proper pretreatments [23, 24]. The aluminum foils are initially covered by natural oxide nesses (Db), interpore distances (Dint), and periodicity [18–20]. Meanwhile, it is noteworthy that the growth layers and surface roughness caused by thermal and mech- rate of porous AAO nanostructures is not constant dur- anical processes [25]. To minimize the impact of these ing the entire growth process, even when the growth defects on the fabrication of highly ordered porous AAO starts with a fixed voltage and given electrolyte. This membranes, pretreatments such as pressing and electro- can be attributed to two competing factors. On one polishing play an essential role in removing particles and hand, AAO growth rate is sensitive to electrolyte smoothing the surface of aluminums. Normally, the two- temperature. In general, higher temperature expedites step anodization of aluminum leads to formation of AAO growth and lower temperature slows down growth. nanopores with short-range hexagonal ordering [26]. D Therefore, environmental temperature fluctuation leads Typically, int is simply proportional to the applied voltage U D ðÞnm to the variation of growth rate [19]. Besides, anodization ( ) withÀÁ a linear constant of 2.5 nm/V, namely int nm current flow through electrolyte causes temperature in- ¼ 2:5 V Â UVðÞ[18]. Meanwhile the depth of the nano- crease. This should be also incorporated into consider- pores and their diameter can be controlled by anodization ation as well, particularly for high voltage/high-current time and the subsequent wet chemical etching. In some anodization. On the other hand, during the anodization applications, highly ordered structures are required. For process, electrolyte composition is being gradually example in nanophotonic applications, both ordering and changed. Specifically, Al cations will be injected into periodicity may affect light-structure interaction [9]. More- electrolyte and hydrogen evolution at the cathode that over, when utilizing nanopores to directly integrate semi- reduces proton concentration in electrolyte [21]. The conducting nanowires for nanoelectronic applications, composition and concentration change of electrolyte in- each nanowires needs to be individually addressable which evitably affects anodization rate. All these complicated also requires perfect ordering. In the past, we have inten- factors pose a challenge to precisely control anodization sively explored fabrication of AAO membranes with per- process, especially when there is a stringent require- fect hexagonal and square orderings with an area up to ment on final membrane thickness. In this work, we tens of centimeter square, and the periodicity ranging from revisited the electrochemical reactions during Al anodi- 500 nm to 3 μm [9, 10, 13, 14, 27]. In order to achieve zation. Then a generic electric charge integral approach long-range regular structures, hexagonally or squarely was developed to monitor the growth pore depth (Dp) ordered nanohole arrays are produced on electrochem- of AAO in real time. This approach is based on the dis- ically polished aluminum surface with nanoimprint, using covered linear correlation between measured total elec- a silicon stamp mold with ordered short pillars on the sur- tric charge flow through the circuit during stable face. Then the aluminum sample undergoes anodization anodization process and the molar amount of anodic process yielding a perfectly ordered AAO nanopore array alumina grown in the process. It was found that this lin- on the surface. Note that it is important to satisfy a match- ear correlation is rather insensitive to anodization volt- ing condition between the periodicity of the nanoimprinted age and composition of the electrolyte. This suggests nanoholes and the anodization voltage, governed by that for a given anodization process, AAO pore depth the linear constant of 2.5 nm/V. Figure 1a, b shows the can be predicted in real time regardless of the scanning electron microscopic (SEM) images of an as- temperature variation and electrolyte concentration fabricated AAO membrane demonstrating ideally regular change. Therefore, a program was coded to visualize configuration of the straight and parallel holes. In this case, the AAO growth process. In this case, not only the the Al foil was anodized with an applied voltage of 400 V AAO growth pore depth can be monitored, but also a in 230 ml 1:1(v/v) mixture of 4 wt.% citric acid (C6H8O7) target pore depth can be set and the anodization and ethylene glycol (EG) with an extra 15 ml 0.1 wt.% process can be terminated when the projected pore phosphoric acid (H3PO4)atatemperatureof10°Cfor depth

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