A Highly Controllable Electrochemical Anodization Process to Fabricate Porous Anodic Aluminum Oxide Membranes
© Lin et al. 2015
Received: 2 November 2015
Accepted: 14 December 2015
Published: 26 December 2015
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.
Metal anodization has been broadly used in industry as a surface treatment technique to render materials with resistance against uncontrolled oxidation, abrasion, and corrosion. Although this technique has been developed for a long time, it was until 1990s that researchers discovered that highly ordered nanoporous structures can be achieved by properly tuning anodization conditions including electrolyte composition and concentration, temperature, as well as anodization voltage . Among all valve metals that can be anodized, aluminum (Al) and titanium (Ti), particularly Al, can be anodized into nanoporous structures with well-controlled diameter, pitch, and depth. Membranes consist of these nanostructures, i.e., anodic titanium oxide (ATO) and anodic aluminum oxide (AAO), have wide nanoengineering applications that have attracted enormous attention. For example, AAO membranes have been used as templates to directly assemble semiconductor nanowires and nanorods for photodetection  and solar energy conversion [3–5]. A large internal surface area of these oxide nanostructures can also be harnessed to build high-performance energy storage devices such as Li-ion batteries  and supercapacitors [7, 8]. Meanwhile, it is worth pointing out that AAO structures can be engineered into a number of variants via proper combination of wet chemical etching and anodization processes. These variants include nanowells , inverted nanocones [10, 11], nanobowls , nanospikes [13–15], and the integrated nanopillar-nanowell structures . These structures have been used as scaffold of energy harvesting and storage devices in our past works.
There are certainly a number of advantages of using porous anodic nanostructures, particularly AAO membranes for nanoengineering applications, such as large surface area, high regularity and scalable low-cost production . In many applications, precise control of nanostructure shape and geometry is critical. It is known that the geometric features of porous AAO membranes are widely tunable via electrochemical anodization process . Factors such as applied voltage, acid type, and concentration contribute to the formation of the porous nanostructures with various barrier layer thicknesses (D b ), interpore distances (D int), and periodicity [18–20]. Meanwhile, it is noteworthy that the growth rate of porous AAO nanostructures is not constant during the entire growth process, even when the growth starts with a fixed voltage and given electrolyte. This can be attributed to two competing factors. On one hand, AAO growth rate is sensitive to electrolyte temperature. In general, higher temperature expedites growth and lower temperature slows down growth. Therefore, environmental temperature fluctuation leads to the variation of growth rate . Besides, anodization current flow through electrolyte causes temperature increase. This should be also incorporated into consideration as well, particularly for high voltage/high-current anodization. On the other hand, during the anodization process, electrolyte composition is being gradually changed. Specifically, Al cations will be injected into electrolyte and hydrogen evolution at the cathode that reduces proton concentration in electrolyte . The composition and concentration change of electrolyte inevitably affects anodization rate. All these complicated factors pose a challenge to precisely control anodization process, especially when there is a stringent requirement on final membrane thickness. In this work, we revisited the electrochemical reactions during Al anodization. Then a generic electric charge integral approach was developed to monitor the growth pore depth (D p ) of AAO in real time. This approach is based on the discovered linear correlation between measured total electric charge flow through the circuit during stable anodization process and the molar amount of anodic alumina grown in the process. It was found that this linear correlation is rather insensitive to anodization voltage and composition of the electrolyte. This suggests that for a given anodization process, AAO pore depth can be predicted in real time regardless of the temperature variation and electrolyte concentration change. Therefore, a program was coded to visualize the AAO growth process. In this case, not only the AAO growth pore depth can be monitored, but also a target pore depth can be set and the anodization process can be terminated when the projected pore depth reaches the set point. Overall, the revealed correlation between the quantities of electric charge and anodic material is of scientific and practical importance, and it facilitates precise control of AAO anodization in a wide voltage range for a broad applications.
In order to have a more precise control of the anodization process in this work, the voltage and current of the anodization process are monitored and recorded by Keithley 2410 SourceMeter, controlled by a home-built computer program based on LabView. The voltage was increased in a linear manner from zero to the target value with a ramping rate of 10 V/min, so that the current will not rise too fast to avoid overheating of the electrolyte and burning aluminum samples. Meanwhile, we have also discovered that a ramping process which lasted too long will result in forming a fairly smooth anodic oxide film without nanoporous structure. The unnecessarily slow voltage ramping cannot provide high enough electric field for expelling Al3+ into electrolyte, and the Al3+ is retained in oxide bulk. Thus, the oxidization is fairly stable without variation in electric field distribution on the oxide layer and therefore hinders the formation of hemispherical oxide layer and nanoporous structure . Once the ramping process is done, the program maintains target voltage while it is observed that the anodization current falls naturally down to a stable level, which is an indication of inception of stable anodization . In our experiment, a two-step anodization approach is adopted. In the first step, an initial AAO layer was formed after anodization in electrolyte for over 12 h. The first layer of AAO was then removed in a mixture of 6 wt.% phosphoric acid (H3PO4) and 1.8 wt.% chromic acid (H2CrO4) at 98 °C for 30 min, leaving a highly periodic nanoconcave structure on the surface of the aluminum substrate, which forms the initiation sites for the formation of pores in the second anodization step. Interestingly, this nanoconcave structure can be used to enhance light absorption and power conversion efficiency of thin-film solar cells which was reported by us previously . Furthermore, AAO membrane with highly ordered porous nanostructure is produced in the same electrolyte during second anodization step.It is well known that an AAO membrane contains two parts of oxide layers, namely barrier layer and the oxide side wall, as shown in Fig. 1c. AAO is mainly formed at the AAO/Al interface when OH− and O2− diffuse through the barrier layer driven by the electric field . The thickness of the barrier layer remains constant for a particular anodization voltage during the stable anodization process, while the length/height of the oxide side wall continues to grow. In the first step anodization, the nanoimprinted nanoholes on the aluminum surface are the centers for concentrated electric field. During the initial voltage ramping process, an oxide barrier layer with hemispherical shape is formed due to radial alignment of electric field line vectors. When the oxide barrier layer is thin, electric field is strong; thus, the field driven diffusion of OH− and O2− to AAO/Al interface is fast leading to fast oxidation of Al and increase of barrier layer thickness. At the AAO/electrolyte interface, field-assisted etching of AAO and thinning of the barrier layer occur since dissolution of Al2O3 in pH <5 electrolyte . It is obvious that the above barrier layer growth and etching processes are the two competing processes. The etching rate is not only a function of local electric field intensity, local pH value, but also highly depends on temperature. In practice, the temperature is kept at 5~10 °C to slow down the barrier layer dissolution. Generally, the barrier layer growth rate is largely determined by electric field intensity. In the beginning of the barrier layer growth, small oxide thickness leads to high electric field thus, the overall barrier layer thickness increases over the time. However, after a certain period of time, the electric field intensity drops to a level so that a balanced barrier oxide layer dissolution and growth is achieved. Thereafter, the barrier layer thickness remains constant. As a natural outcome of this dynamic process, the AAO/Al interface moves deeper and deeper into Al bulk, leading to a sustainable growth of pore depth. This formation mechanism explains tubular structure of each AAO nanochannel, and the side-wall thickness is roughly twice of that of the barrier layer, since half of the side wall comes from the barrier layer. It is interesting to notice that the hexagonal nanopore arrangement is typically observed even without assistance of nanoimprint, though in a short range. This can be explained by the volumetric expansion of nanopores during formation of oxide which induces stress between neighboring pores . In order to achieve minimal system energy, a close-pack hexagonal ordering is naturally formed.
Results and Discussion
The fitting proved that the ionic current is the main contributor to the growth of AAO, and the ionic current should remain unchanged during stable anodization process. However, we notice that the measured current density has a decaying nature in stable anodization process. This can be ascribed to the gradually reduced concentration of H+ in electrolyte and as a result, the dissolution rate together with oxidation rate is continuously slowed down.
With the achieved understanding on growth constant, the control of AAO membrane thickness can be realized with a programmable source-meter unit. Here, we have performed a set of AAO anodization with various target pore depths. Utilizing the measured growth constant G, pore depth is monitored by a computer connected with Keithley source-meter unit in real time and anodization processes were terminated automatically when reaching target AAO thickness. Figure 4b, c indicates that in a large voltage range and regardless of pore diameters, with this charge integration approach, the AAO pore depths can be precisely controlled with high consistency with the actual thickness measured with SEM. The marginal deviations may possibly come from: (1) inaccuracy in measuring the AAO pore depth with SEM, mainly from viewing angle deviation for cross-section measurement; and (2) deviation in fitting method. All the curves were fitted in concise models such as linear and exponential fitting so as to indicate the primary trend and thus, the measured growth rates derived from the fitting result could have a reasonable deviation.
In this work, a linear correlation between AAO pore depth and integrated charge density during electrochemical anodization process was discovered and utilized to implement precise control of AAO growth process. This method proved to be repeatable and robust regardless of anodization voltage, temperature, and electrolyte composition and concentration variation. Thus, a home-built automatic AAO fabrication setup combining a source-meter unit and software was developed to realize real-time monitoring and automatic anodization termination when reaching a target pore depth. The good reliability in control of the as-fabricate AAO membrane thickness has been verified in a voltage range from 20 to 600 V, indicating its practicality in a broad nanoengineering applications.
This work was partially supported by the General Research Fund (612113) from Hong Kong Research Grant Council, ITS/362/14FP from Hong Kong Innovation Technology Commission, Fundamental Research Project of Shenzhen Science & Technology Foundation JCYJ20130402164725025, National Natural Science Foundation of China under Grants (61574005), and the Priority Academic Program Development of Jiangsu Higher Education Institutions [PAPD].
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