Abstract
We discuss the influence of mass and heat transfer on the morphologies of Al, Ti, and Zr nanochannel arrays during anodization process. When these metals are anodized, the nanopores are firstly formed at the metal surface, and the nonuniform distribution of mass transfer in the pores results in the increase of pore depth. The nonuniform temperature distribution and the downward movement of reaction interface lead to the temperature changes and the generation of microcracks inside the pore wall, which results in the conversion of nanopores into nanotubes. The low-valency oxides also make the middle of the pore wall crack easily. The morphologies during metal anodization depend greatly on the temperature at the reaction interface. At low interface temperature, it appears to form the nanopores more easily, and, at high interface temperature, it is more propitious to form the nanotube structure. Many factors including resistivity, thermal conductivity, oxidizing reaction heat, and electric field strength (or current density) affect the reaction interface temperature.
1. Introduction
Metal oxide nanochannel structures including alumina nanopore arrays [1], titania nanotube arrays [2, 3], zirconia nanotube arrays [4, 5], and so forth have been successfully fabricated by the anodization method [6–11] in recent years, and the formation mechanism of these nanochannel structures have been proposed [12–15]. The nanochannel structure may improve and enhance the performance of metal oxides and have vast application prospects [16, 17]. For example, titania nanotube arrays have the outstanding properties in photocatalysis [18], water decomposition [19], solar energy cell [20], catalyst [21], gas sensitivity [22–24], photoelectroactivity [25], and so forth.
Generally, the nanotube arrays are mainly formed when titanium and zirconium are anodized while the nanopore arrays are mainly formed when aluminum is anodized. There are various explanations why the nanopore or nanotube arrays are formed. In this paper, the mechanism why the different morphologies of metal oxide are formed during anodization is discussed in detail from the viewpoint of the mass and heat transfer.
2. Experimental Section
The metal foil was anodized in electrolytes while Pt foil was used as cathode and the electrode distance was kept at 2 cm. During the experiments, the solutions were stirred using a magnetic stirrer. After anodization, the samples were rinsed in the deionized water, dried and characterized through a field emission scanning electron microscope (SEM) (XL 30 w/Tmp, Philips).
3. Results and Discussion
3.1. Formation of the Nanopores and Nanotubes
Figure 1 shows the oxide images during the anodization of Al, Ti, and Zr. The nanopore arrays were formed when Al was anodized in 0.3 M oxalic acid at 40 V for 12 h (Figures 1(a) and 1(b)). The nanopores were formed when Ti was anodized in the electrolytes of DMSO + 2 wt% HF at 40 V for 7 hours (Figure 1(c)). When the anodization time was 37 hours, the nanotube arrays were formed (Figure 1(d)). When Zr was anodized in 1 M (NH4)2SO4 + 0.1% NH4F at 20 V for 5 h, nanopores were formed at the surface (Figure 1(e)). When anodization was conducted in 1 M (NH4)2SO4 + 1.0% NH4F at 20 V for 3 h, zirconia nanotube arrays were formed (Figure 1(f)).
(a)
(b)
(c)
(d)
(e)
(f)
Previous studies show that the oxide nanopore arrays as well as the nanotube arrays can be formed at the surface of Al [26–29], Ti-Al [30], and Zr [31–33] under different anodization conditions. When titanium foil was anodized for the second time, the nanopores were formed at the surface, and the nanotubes were formed under the pore mouth [34]. All these experimental results indicate that when Al, Ti, and Zr are anodized, oxides nanopores are firstly formed at the metal surface and subsequently converted into the nanotubes under the specific conditions.
3.2. Influence of Mass and Heat Transfer
The mass transfer distribution during metal anodization after the formation of nanopores is illustrated in Figure 2. The mass transfer driving force comes from the action of electric field on the charged ions and the concentration gradient of the solution constituents. Since the pore walls obstruct the flow of electrolytes, the mass transfer velocity at the pore axis centre is much faster than that near the walls. The large mass transfer velocity could accelerate the dissolving of the oxides; consequently, dissolving action is much larger at the pore bottom than that at the pore wall, which leads to the increase of pore depth.
Metal anodization is an exothermic reaction. The fast mass transfer is favorable to the transmission of heat outwards, and the heat at the position with low mass transfer velocity would accumulate easily. Consequently, the nonuniform mass transfer would lead to the nonuniform temperature distribution at the reaction interface (Figure 2). The temperature at the center of pore bottom is lowest (Figure 2, point t), while the temperature at the wall-metal interface is highest (Figure 2, point h). During the anodization process, along with the downward movement of the reaction interface, the newly formed pore wall at high temperature would be cooled due to the heat diffusion and mass transfer inside the nanopores. The temperature decrease shrinks the pore wall and produces the inner stress inside the pore wall, which would lead to the formation of microcracks in the pore wall and cause the conversion of nanopores into nanotubes (Figure 3). The formation of microcracks mainly lies on the magnitude of inner stress which is directly related with the temperature variation scope at the pore wall. If the temperature at reaction interface is high and temperature variation scope of the newly formed pore wall is large, large inner stress would be generated and lead to the formation of microcracks inside the pore wall; in this case, the nanotube structure tends to be formed. Otherwise, the nanopore structure will be formed.
The temperature at the reaction interface mainly depends on the metal properties and the anodization reaction rate. It can be expressed as： where is the temperature at the reaction interface, is the resistivity of the metal, is the current density which reflects the reaction rate, is the oxidation reaction heat, is the coefficient of heat conductivity, is the constant term and the positive parameters , , and are the influence coefficients.
In addition, the stress would generate at the bottom of the nanopores due to volume expansion along with the conversion of metal into oxides. The stress analysis at the bottom of nanopores is illustrated in Figure 4. The stress F at point p can be resolved into and . The longitudinal component force () would push the oxides layer upward, while the lateral component force () would increase the inner stress inside the pore wall and contribute to the conversion of nanopores into nanotubes. For this reason, the larger the volume expansion along with the conversion of metal into oxides is, the more favorable the formation of nanotube is.
O2- ions of the metal oxide come from the electrolytes, so the mass transfer would influence the compositions of the oxide fabricated by anodizing a metal. Insufficient O2- ions may result in the formation of suboxides (i.e., low-valency oxides) [35]. As it can be seen from Figure 2, the mass transfer velocity at point h in Figure 5 is lowest, and, therefore, the metal at h may be not anodized completely and turn into the suboxides. Along with the reaction proceeding, the pore bottom will move downwards, and the suboxides at point h would constitute the centre part of porous walls (Figure 5). Compared with the high-valency oxides, the low-valency oxides have less intermolecular cross-linking. Therefore, the pore walls may crack easily in the middle position under the stress action.
The morphologies of the metal oxides formed during anodization are the consequence of the joint action of above-mentioned factors. When the temperature at reaction interface is high and the volume expansion during the conversion of metal into oxides is large, the nanotube structure tends to be formed. Otherwise, the nanopore structure will be formed. From Table 1 and (1), we know that the resistivity of Al is very low, which means that the less heat would be generated when the electric current passes through; the oxidation reaction heat is also low, and, therefore, the less heat would be released during the anodization process; the coefficient of heat transfer of Al is high, and, thus, the heat would be transferred and diffused around quickly. In addition, the volume expansion ratio of Al is low. Consequently, the nanopore structure tends to be formed during the anodization of Al. Compared with Al, the resistivity of Ti and Zr is 15 times as large as that of Al, while the coefficient of heat conductivity of Ti and Zr is only 1/10 of that of Al. At the same time, the oxidation reaction heat and the volume expansion rate of Ti and Zr is much larger than that of Al. For these reasons, the nanotube structure tends to be formed during the anodization of Ti and Zr.
Furthermore, the morphologies of metal oxides can be changed through controlling the anodization reaction rate. When the anodization is conducted at a low voltage or low current density, the low reaction rate and interface temperature would more likely result in the formation of the nanopore structure. Otherwise, the nanotube structure would be more likely formed. For example, the alumina nanotube structure has been fabricated through the anodization process of Al at the high voltages and current densities [28, 29]. Another example is that the nanopore structure was fabricated through anodization process of Ti-Al alloy at the low voltages (10, 20 V), while the nanotube structure was fabricated at a high voltage (40 V) [30].
4. Conclusions
During the anodization process of Al, Ti, and Zr, oxides nanopores are firstly formed at the metal surface. The nonuniform distribution of mass transfer velocity inside the nanopores result in the increase of pore depth and nonuniform temperature distribution at the reaction interface. The temperature at the pore bottom is lowest while temperature at the interface between the pore wall and metal is highest. Along with the downward movement of reaction interface, the pore wall formed at a high temperature would be cooled down due to the heat diffusion and mass transfer inside the nanopores. The decrease of temperature shrinks the pore wall and produces stress inside the pore wall, which would lead to the formation of microcracks in the pore wall and the conversion of nanopores into nanotubes. In addition, the stress would be generated in the pore wall due to the large volume expansion during the conversion of metal into oxides. In the meantime, the formation of low-valency oxides makes the pore wall crack easily in the middle position. The morphologies of the oxides during anodization process mainly depend on the temperature at the reaction interface. The low interface temperature is favorable to the formation of nanopore structure while the high interface temperature is favorable to the formation of nanotube structure. The main parameters that influence the interface temperature include resistivity, coefficient of heat conductivity, reaction heat of oxidation, and voltages (current densities), and so forth.
Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 50972036) and Support Program for Hundred Excellent Innovation Talents from the Universities and Colleges of Hebei Province.