Controlled Fabrication of Nanoporous Oxide Layers on Zircaloy by Anodization
© Park et al. 2015
Received: 18 August 2015
Accepted: 21 September 2015
Published: 29 September 2015
We have presented a mechanism to explain why the resulting oxide morphology becomes a porous or a tubular nanostructure when a zircaloy is electrochemically anodized. A porous zirconium oxide nanostructure is always formed at an initial anodization stage, but the degree of interpore dissolution determines whether the final morphology is nanoporous or nanotubular. The interpore dissolution rate can be tuned by changing the anodization parameters such as anodization time and water content in an electrolyte. Consequently, porous or tubular oxide nanostructures can be selectively fabricated on a zircaloy surface by controlling the parameters. Based on this mechanism, zirconium oxide layers with completely nanoporous, completely nanotubular, and intermediate morphologies between a nanoporous and a nanotubular structure were controllably fabricated.
Electrochemical anodization has been widely used to produce oxide nanostructures on the surfaces of various metals [1–6] and alloys [7–10] due to its simplicity and low cost. The morphology of the metal oxide layer fabricated by anodization is normally nanoporous or nanotubular structure. The main difference between nanoporous and nanotubular structures is the presence of gaps between pores: the pores are interconnected without gaps in nanoporous structures but the pores are splitted due to the gaps in nanotubular structures. Nanotubular metal oxides are useful for the applications to catalysts [11, 12], solar energy conversion [13, 14], hydrophilic surface [15, 16], and sensors [17, 18]. Nanoporous metal oxides are used for corrosion resistance [19, 20], decoration , and templates for nanomaterial fabrication [22, 23]. For certain applications, controlled synthesis of the oxide layer to have only nanoporous or only nanotubular structure is crucial. One example is anti-corrosion of a metal using a metal oxide nanostructure . If a nanotubular oxide layer is formed on a metal surface, the layer does not exhibit a good anti-corrosion property. This is because many small gaps exist between the pores and furthermore large cracks are also formed on the nanotubular oxide layers. Water or moisture can then interact with a metal underneath the tubular oxide layer after penetrating into the gaps and cracks, resulting in the corrosion of the metal. However, if a nanoporous oxide layer is created on a metal surface, water or moisture hardly directly meets a metal underneath the oxide layer because no gaps or cracks exist on the nanoporous oxide layer. Consequently, a nanoporous oxide layer can act as a good protective layer for metal corrosion while a nanotubular oxide layer might not show such a good corrosion-resistant behavior.
It has been claimed that the morphologies of the metal oxide nanostructures produced by anodization is dependent on the material: porous morphologies are formed if Al, Ta, and Nb are anodized while tubular structures are formed in the case of Ti, Zr, and Hf when they are anodized in fluoride-containing electrolytes . On the other hand, it has been shown that the resulting morphology of the produced oxide nanostructure is affected by certain anodization parameters [26–28]. So far, a few mechanisms explaining the anodization-induced nanostructure formation and the morphology evolution have been proposed; [22, 26, 28–31] however, the current mechanisms are still controversial.
Here, we propose a mechanism on the morphology evolution of anodic oxide layer to porous or tubular structure and experimentally demonstrate that nanoporous or nanotubular oxide layer can be controllably and selectively fabricated by anodization of zirconium alloys, or zircaloy. The reason behind using zircaloy in the present experiments is to apply this anodization technique to improve the safety of a nuclear power plant under severe accidental conditions. In fact, zircaloy is the most widely used nuclear fuel cladding material in nuclear reactors. When zircaloy comes in contact with high-temperature steam, the zircaloy metal atoms become oxidized from the presence of oxygen molecules in steam. As a result, hydrogen gas is formed from water molecules that have lost the oxygen atoms. If an excessive amount of hydrogen gas is produced, explosion, similar to the Fukushima nuclear power plant accident , could occur. However, we suggest that the hydrogen production rate can be dramatically reduced if nanoporous oxide layer is preformed on the zircaloy cladding through anodization. Since zircaloy surface is pre-oxidized through anodization, the oxide layer hinders further oxidation of the zircaloy cladding even though the cladding contacts with steam, thereby preventing hydrogen production due to water splitting. However, for this purpose, the oxide layer prepared on a zircaloy cladding should not have a nanotubular but a nanoporous morphology because nanotubular oxide layer having many gaps and cracks cannot protect the interaction of steam with zircaloy base metal. Therefore, for the application to nuclear reactors, nanoporous oxide structures without gaps and cracks are to be fabricated on the surface of zircaloy cladding.
Zircaloy plates (KEPCO Nuclear Fuel Company Ltd., 10 × 40 × 0.7 mm3) were used for the anodization experiments. They were cleaned by sonicating in acetone and isopropyl alcohol, followed by rinsing with deionized (DI) water and drying in air. Anodization was carried out using a two-electrode system with a platinum sheet (15 × 40 × 0.5 mm3) as a counter electrode and a zircaloy plate as a working electrode. The distance between the two electrodes was 10 mm. Ethylene glycol (95 % purity, Junsei) and glycerol (95 % purity, Junsei) containing ammonium fluoride (NH4F, Sigma-Aldrich Corporation, St. Louis, MO, USA) and DI water were used as an electrolyte of the anodization process. All the chemicals and materials were used in their as-received forms without any further purification. A direct current power supply with a maximum capacity of 1000 V and 1 Å was used for the electrochemical treatment. The anodization experiments were performed in a dry glove box at room temperature. After the experiments, the samples were rinsed with DI water and subsequently dried in air.
The structural morphologies of the anodized samples were examined by a field emission scanning electron microscope (FESEM, Nova230, FEI, USA). For the measurement, the anodized oxide layer was mechanically cracked on purpose. The chemical composition of the sample was characterized using energy dispersive X-ray spectroscopy (EDS) (EDAX Genesis attached to the FESEM). The crystalline structure was identified with the help of glancing angle X-ray diffractometer (GAXRD, D/MAX 2500 V, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (k = 1.5406 Å).
Results and Discussion
In the reaction (1), oxygen ions that are produced from water combine with a metal cation to form a metal oxide. In the reaction (2), fluorine ions that are produced from an electrolyte react with the preformed metal oxide, forming soluble compounds. As a result, the preformed metal oxide is etched and nanometer-sized pores are created on the metal oxide. These two reactions are enhanced by an electric field that is generated by an anodization voltage because anions are involved in the reactions. Etching is further accelerated through the reaction (3), where a metal cation is directly ejected to the electrolyte due to the electric field. The oxidation (1) and etching (2, 3) reactions compete with one another during the anodization process, and an oxide layer with nanometer-sized pores is formed.
Due to the reactions (1)–(5), self-organized metal oxide nanostructures are fabricated if a metal is anodized. The morphologies of the resulting oxide layers are normally nanotubes or nanopores. Here, we propose that porous or tubular nanostructures can be controllably fabricated by tuning the reaction rates. The reactions (2) and (3) result in pores on a metal substrate because a metal substrate and a metal oxide are etched away, and we call these reactions as pore etching reactions. The rate of pore formation is determined by the pore etching rate, which is proportional to the anodization current . A high anodization current reflects a high pore etching rate and a fast pore formation. The magnitude of the anodization current generally decreases with time , suggesting that pores are rapidly formed during an initial anodization stage but grow slowly as anodization proceeds. The pore formation rate can be increased by increasing the anodization voltage because the pore etching reactions are strongly affected by the applied electric field. The average length of the created pore is determined by the total charge flowing through the anode , which is calculated by integrating the anodization current with time. In contrast, reaction (5) leads to the change in the morphology of the oxide layer from nanopores to nanotubes because the interpore regions are chemically dissolved. We call the reaction (5) as interpore dissolution reaction. Reaction (5) suggests that the interpore dissolution rate is influenced by water and fluoride contents in an electrolyte. We confirmed that nanoporous morphologies were not changed when the anodized samples were placed in F− free electrolytes such as in NH4Cl and NH4NO3 solutions, which demonstrates that the presence of F− ions is crucial for the transformation in the aging process. Therefore, the pore etching rate or the interpore dissolution rate can be independently or selectively adjusted by the anodization parameters, and then, the morphology of the resulting metal oxide can be controlled to be a porous or a tubular nanostructure.
It should be noted that anodization voltage is not a critical parameter to determine that the morphology of the resulting oxide layer is nanopores or nanotubes. Higher anodization voltage leads to higher anodization current. As a consequence, pore etching rate and correspondingly the length of pores is increased. However, whether the final morphology of the oxide layer is nanoporous or nanotubular is determined mainly by how much the interpore regions are dissolved. Since the interpore dissolution reaction (5) is a chemical reaction that is not affected by an electric field, anodization voltage does not affect the porous or tubular morphology. As demonstrated previously, if anodization time is sufficiently small or if water content is sufficiently low, the dissolution of the interpore regions can be negligible, and thus, the resulting oxide layer has a completely nanoporous structure (Figs. 1a, c, 4a, and 5a). However, if the product of anodization time and water content is not so small and not so large, an intermediate structure comprising a top nanotube layer and a bottom nanopore layer is created (Fig. 4). If the product of anodization time and water content is large enough, a completely nanotubular oxide layer is fabricated (Figs. 1b, d, 4b, and 5b).
In conclusion, we have proposed a model to explain why the morphologies of anodized metal oxide layers are evolved to nanoporous or nanotubular structures. Nanoporous structures are initially formed by anodization, but the degree of interpore dissolution determines that the final morphology of the oxide layer is nanoporous or nanotubular. The degree of the interpore dissolution can be tuned by changing the anodization parameters such as anodization time and water content in an electrolyte as well as aging time. Consequently, nanoporous or nanotubular oxide morphology can be selectively fabricated on a metal surface by controlling the parameters. This model was demonstrated through the anodization experiments of zircaloy. We suggest that the proposed model can also be applied to other metals and alloys for the preparation of morphology-controlled oxide nanostructures on the surface. Therefore, the anodic metal oxides can exhibit enhanced performances for various applications
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013M2A8A1041415) and the KUSTAR-KAIST Institute, KAIST, Korea.
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