Formation of Ordered Nanostructures By Anodization of Ag Substrates with Depression Patterns
Authors: Teshima Sayaka , Yanagishita Takashi 
Published: December 2024

The anodization of metals is an effective technique for preparing metal oxide nanohole arrays on substrate surfaces. It has been reported that nanohole arrays can be formed by anodizing various metals, not only Al and Ti. The variety of metals that can be anodized has increased, and the range of applications for the resulting nanohole arrays has expanded. In various applications using nanohole arrays obtained by the anodization of metals, control of the geometrical structure of the nanohole arrays is essential because its geometrical structure affects the performance of the device. However, structural control of nanohole arrays has not been achieved in the anodization of many metals. We previously reported that nanohole arrays with regularly arranged pores can be obtained by anodizing metal substrates with ordered depression patterns [1–3]. This is because the depressions formed on the metal surface served as the starting point for pore generation. We have previously reported that this method is applicable to various metals such as W, Fe, Cu, and SUS. In this report, we describe the preparation of ordered nanostructures of Ag oxide by the anodization of an Ag substrate with ordered depression patterns.

A depression pattern was formed on the Ag substrate surface by Ar ion milling using an ordered anodic porous alumina mask. An anodic porous alumina mask with an ordered pore arrangement was prepared by a previously reported two-step anodization process. The resulting alumina mask was placed on an Ag substrate, with the barrier layer facing upward and the pore-forming surface facing downward. Ar ion milling was performed for 60 min at 5 kV to penetrate the alumina mask and form depression patterns on the Ag substrate surface. The alumina mask remaining on the Ag substrate surface was dissolved using a mixture of 0.17 M chromic acid and 0.07 M hydrofluoric acid at 20℃, which can dissolve only alumina without dissolving Ag. Ag sheets with the depression pattern were anodized in 0.05–0.13 M NH4F ethylene glycol electrolyte containing 0-0.13 M KOH or 0.1 M CH3COONa at 10℃ under a constant voltage of 2 V. For anodization of the Ag sheet, a two-electrode cell was used with a Pt plate as the counter electrode, and the distance between the electrodes was adjusted to 5 mm. The surface structures of the obtained samples were observed using field-emission scanning electron microscopy (SEM). The compositions and crystallinities of the obtained samples were evaluated using X-ray photoelectron spectroscopy and X-ray diffraction.

Anodization of the Ag substrate with a depression pattern in an ethylene glycol solution containing NH4F and KOH induced pore generation from depressions, resulting in ordered nanohole array structures [4]. In addition, ordered nanopillar array structures were obtained by the anodization of an Ag substrate with depression patterns in an ethylene glycol solution containing NH4F and CH3COONa. This is the first report on the formation of ordered nanohole array structures of Ag oxide by the anodization of an Ag substrate. This is also the first report of the successful fabrication of ordered nanohole and nanopillar array structures from the same metal substrate by changing anodization conditions. The ordered nanostructures of Ag oxides obtained by this method are expected to be applicable in various functional devices, such as sensors, catalysts, and batteries.

References

[1] T. Yanagishita, T. Masuda, T. Kondo, and H. Masuda, Electrochem. Commun., 123, 106916 (2021).

[2] T. Yanagishita, T. Masuda, and H. Masuda, RSC Adv., 12, 6848 (2022).

[3] T. Yanagishita, Y. Osada, T. Masuda, and H. Masuda, J. Electrochem. Soc., 169, 063502 (2022).

[4] S. Teshima and T. Yanagishita, J. Electrochem. Soc., 170, 123503 (2023)