(6gn) Nanostructured Anodic Oxides of Metals: From Corrosion Protection to Nanotechnology and Emerging Applications

Research Interests:

Anodic oxidation of metals, especially Al and Cu

Fundamentals of Al and Cu anodizing

Nanostructured materials

AAO-templates nanofabrication

Electrodeposition

CO₂ reduction reaction on CuO_x nanostructures

Corrosion protection

Teaching Interests:

Materials chemistry, physical chemistry, nanotechnology, corrosion protection, characterization techniques in corrosion, spectroscopy, electron microscopy, X-ray diffraction

Nanostructured Anodic Oxides of Metals: from Corrosion Protection to Nanotechnology and Emerging Applications

Wojciech J. Stępniowski^*, Kuo-Kuang Wang, Wojciech Z. Misiolek

Loewy Institute andDepartment of Materials Science and Engineering, Lehigh University

5 East Packer Ave., Bethlehem, PA 18015 USA

Anodizing is a metal passivation technique, contributing mainly to aluminum alloys corrosion protection, known for over century. It allows to form a compact, dense, adherent, dielectric layer on a protected metal, hindering galvanic coupling. In industrial practice, a basic, one-step approach is applied. As an effect, a porous oxide film is obtained and consequently the pores are being chemically sealed. Corrosion performance of such films depends strongly on the applied electrolyte, voltage, temperature, but also chemicals applied for pore sealing. Numerous approaches have been developed and are patented all over the world.

A milestone research was published by Masuda and Fukuda in 1995 [1]. Till then, there was a belief that anodizing provides layers with disordered pores and the only application is corrosion protection. Masuda and Fukuda reported a totally new approach, resulting in one of the most popular, nowadays nanomaterials. After formation of the anodic aluminum oxide (AAO), the formed nanoporous oxide has to be chemically removed in a mixture of chromic and ortophosphoric acid. Then, pre-patterned aluminum surface layer is exposed. Finally, re-anodization of so pre-patterned aluminum allows to obtain, after the second step of anodization, highly-ordered anodic aluminum oxide, with hexagonally arranged, parallel and uniform pores (Fig. 1a). Since this milestone publication, AAO attracted attention of numerous researchers, becoming one of the most popular template in nanofabrication. Thanks to the application of AAO, a large variety of nano-architectures, including nanowires, nanotubes, nanodots and nanoporous arrays made of diverse materials, including metals, alloys, semiconductors, superconductors, salts, carbon materials and polymers was possible to achieve. In order to form such nanostructures, various techniques, from simple ones like electrodeposition, to advanced ones like atomic layer deposition were applied (Fig. 1 b-c) [2]. The method we have developed, cuts down costs and the number of steps of metallic nanowires fabrication: we have demonstrated that even anodizing of technical purity aluminum and subsequent stepwise voltage decrease, results in a through-hole AAO template with conductive bottoms of the pores. This enabled electrodeposition of copper from standard CuSO₄ bath, and allowed to form nanowires (Fig. 1 b-c).

Figure 1. Top-view of anodic aluminum oxide (AAO) formed in sulfuric acid (a) and copper nanowires formed by electrodeposition into AAO (b-c) [2].

Research on AAO triggered experiments with other metals. It was found that majority of transition metals can be successfully subjected to anodizing. For example, anodization of titanium allows to form nanotubes and nanoporous arrays made of amorphous TiO₂. Such formed TiO₂ found numerous applications, including formation of back-side illuminated dye sensitized solar cells, electrochromic devices, photocatalysis and as an additional protective layer in biomaterials. Other anodic oxides also form nanoporous or nanotubular layers. It is worth mentioning that anodic WO₃ is a promising H₂O splitting photocatalyst, while anodic ZrO₂ puts contribution in surface enhanced Raman spectroscopy.

Our recent study focuses on copper anodization. The first reason is fundamental understanding of the formation mechanism. Other oxides have fixed stoichiometry, are amorphous (except ZrO₂) and form nanoporous arrays or nanotubes (except nanowires made of ZnO). Products of copper anodic oxidation, due to the complexity of the occurring phenomena on the electrodes, are diverse: Cu₂O, CuO and Cu(OH)₂ are major, but often not the only products of the reactions. Additionally, the as-formed anodic films are crystalline: major components are cuprite and tenorite, while usually Cu(OH)₂ in an amorphous shell of the nanostructures. What is also interesting, anodization of copper results in the formation of nanowires (Fig. 2). Thus, so formed anodic films, with high-surface area 1D-nanostructures shall be perfect for catalytic applications [3]. Recently, Cu₂O and CuO are being reported as catalysts for electrochemical carbon dioxide reduction reaction (CO₂RR), thus so formed anodic films can be considered as a perfect candidate for such application. In opposite to so far reported catalysts, the anodic oxides are well-adherent to the metallic substrate and employ facile, easy to scale-up technique of fabrication. Nonetheless, still much fundamental issues of copper anodizing mechanism have to be explored and explained.

The most obvious approach in searching for the formation of nanostructures on metals, is the linear polarization of the metal in electrolyte that provides passivity at certain potential ranges. In the first attempts in the formation of copper oxides nanostructures, voltammetric scans in such solutions as KOH was done, resulting in the formation of the nanostructures (Fig. 2a). Majority of the researchers in this field also apply hydroxide solutions as the electrolytes [3]. Although, Pourbaix diagram shows passivity even at more neutral values of pH [3]. Our recent approaches utilize sodium and potassium carbonates and bicarbonates as electrolytes. Research conducted in K₂CO₃ shed some light on the phenomena occurring when copper is anodized. It was found that the diameter of the obtained nanowires increases linearly with the applied voltage [4]. Furthermore, among expected products of copper anodizing: CuO, Cu₂O and Cu(OH)₂, chemical compounds like Cu₄O₃ and Cu₂CO₃(OH)₂ were detected using X-ray diffraction (XRD). Thus, identification of malachite shows incorporation of carbonate anions into the growing oxide, what is analogous phenomenon to the incorporation of acidic anions into AAO.

Further anodization research, conducted in NaHCO₃ showed, for the optimized conditions, formation of close-packed nanowires with diameter down to 20 nm (Fig. 2 b-c). Also, in this, unpublished research, the nanowires are crystalline and their diameter increase linearly with the voltage.

In current research, the influence of various additives into electrolyte has been studied. After anodization in 0.01 M KHCO₃ at voltages ranging from 10 to 60 V as the reference, the influence of NaCl as additive was investigated. On one hand, anodizing in KHCO₃ without additives allowed to form nanowires. On another hand, there are reports showing that anodic polarization of copper, in NaCl-rich electrolytes, results in the formation of Cu₂O and CuO powders, depending on the operating conditions. In our research, we have studied the gap between these two opposite situations: the anodization of copper in KHCO₃ with various amounts of NaCl. Registered current curves showed the expected impact on the process: the greater the NaCl concentration, the greater the conductivity of the electrolyte and consequently, the greater the registered current densities. Of course, this had a significant impact on the formed nanostructures. For optimized conditions, hemi-spheres made of nanowires thicker than ones formed in KHCO₃ without additives were formed. Another studied additive was ethylenediaminetetraacetic acid (EDTA). The following assumption was made: water-soluble Cu²⁺ species, like Cu(OH)₄^2- are formed during copper anodizing and contribute by re-deposition process in the formation of the nanostructures. Thus, chelating them with EDTA would prevent the re-deposition and influence the phase composition and morphology of the grown nanowires. Currently, the in-depth characterization, including optical properties of the grown nanostructures and spectra of the electrolytes after anodizing is being performed.

Figure 2. Copper oxides formed by passivation in three-electrode system in KOH (a) and two-electrode system in NaHCO₃ (b-c).

To sum up, anodization of metals allows to form various nanostructural oxides. Anodic aluminum oxide, consisting of hexagonally arranged nanopores, serves often as a template for further nanofabrication. In that way, for example, nanowires made from metals can be formed by electrodeposition. Another example of the nanostructures formation by anodizing are nanowires grown by copper electrochemical oxidation. Still much of the fundamental work has to be performed to reveal new anodizing regimes, improving control of the morphology and composition, such nanostructures can successfully serve as catalysts for electrochemical CO₂ reduction.

References

[1] H. Masuda, K. Fukuda, Science 268 (1995) 1466-1468.

[2] W.J. Stępniowski, M. Moneta, K. Karczewski, M. Michalska-Domańska, T. Czujko, J.M.C. Mol, J.G. Buijnsters, J. Electroanal. Chem. 809 (2018) 59-66.

[3] W.J. Stępniowski, W.Z. Misiołek, Nanomaterials 8 (2018) 379.

[4] W.J. Stępniowski, D. Paliwoda, Z. Chen, K. Landskron, W.Z. Misiolek, Mater. Lett. 252 (2019) 182-185.