Highly ordered nanoporous Ta2O5 formed by anodization of Ta at high temperatures in a glycerol/phosphate electrolyte

doi:10.1016/j.elecom.2011.03.005

Electrochemistry Communications

Volume 13, Issue 6, June 2011, Pages 542-545

https://doi.org/10.1016/j.elecom.2011.03.005 Get rights and content

Abstract

In the present work, we investigate the anodization of Ta in a K₂HPO₄ containing glycerol electrolyte at elevated temperatures (180 °C). Under optimized conditions, uniformly over the entire anodized surface, self-organized porous Ta₂O₅ structures can be formed, that consist of highly aligned pores of 25 nm diameter and several tens of micrometers in length.

Research Highlights

► We form a highly self-organized porous Ta₂O₅ layers by a simplebut optimized anodization process. ► We show that key is using a hot glycerol/K₂HPO₄ electrolyteatcontrolled anodization voltages. ► The layer thickness and pore diameters are controllable by the anodization conditions.

Introduction

The formation of highly ordered self-organized nanoporous oxide structures has stimulated substantial research, ever since Masuda et al. [1], [2] reported hexagonally ordered porous alumina structures that can be grown on Al by anodization in acidic solutions such as oxalic acid, phosphoric acid or sulfuric acid. Such defined oxide structures offer a wide potential for use, owing to their morphological and physical properties. Except for direct applications for filtration or photonics, well-aligned alumina nanoporous structures have been widely used as templates for the growth of ordered arrays of one-dimensional nanostructures [3].

In 1999, Zwilling et al. reported on the formation of TiO₂ nanotube structures when anodizing Ti in a fluoride containing electrolyte [4]. This process since then has been optimized to form permanently increased degrees of self-organization and geometries of nanotubes [5]. In the meantime, anodization in fluoride electrolytes has been transferred to several metals to form ordered oxide nanoporous or nanotube structures (Ti [4], [6], Nb [7], Zr [8], Hf [9], W [10] and Ta [11]).

Very recently, we reported the formation of various TiO₂ nanostructures such as mesosponge [12] or fishbone [13] structures that can be achieved by anodization of Ti in glycerol electrolytes containing K₂HPO₄ at temperatures between 160 and 220 °C. The morphologies and diameters of these structures can be controlled by the applied voltage and temperature during anodization. The advantages of these structures over TiO₂ nanotubes are a considerably higher surface area, the possibility to obtain highly crystallized material directly in the anodization process, a well connected structure as well as a high degree of mechanical stability and flexibility [12], [13].

For tantalum anodization this electrolyte was used already in 1998 by Melody et al. [14] and later by Habazaki et al. [15] to form an anodic oxide layer without a drop of growth rate over time (so called non-thickness-limited (NTL) growth) [14].

In the present work, we explore the feasibility to use this electrolyte to achieve, by a variation in the anodization parameters, the establishment of self-organizing conditions — i.e. to grow instead of a morphologically ill-defined thick oxide, a well-defined structure of aligned channels (pores).

Section snippets

Experimental

Tantalum foils (0.1 mm thick, 99.9% purity, Advent, England) were degreased by sonication in acetone, ethanol and isopropanol, successively followed by rinsing with deionized water; finally the samples were dried with nitrogen gas.

The electrolyte for anodization was 10 wt.% K₂HPO₄ (Sigma-Aldrich) in anhydrous glycerol (99.8% purity, < 1% H₂O, Fluka). Prior to anodization, the electrolyte was held at 200 °C for 4 h to reduce the water content. This is very important as only compact oxide can be

Results and discussion

After a series of preliminary parameter screening experiments (voltage and temperature), the most promising conditions for obtaining a self-organized porous Ta₂O₅ layers were found to be an electrolyte temperature at 180 °C and 20 V (Fig. 1(a)). The electrolyte temperature and applied voltage mainly influence the growth rates of oxide layer and the pore ordering. For lower (150 °C) or higher (200 °C) temperatures, the oxide layers grew thinner and much more irregular pores were formed.

For lower

Conclusions

We examined the formation of porous Ta₂O₅ structures in glycerol electrolyte containing K₂HPO₄. The anodic oxide morphology is strongly affected by electrolyte temperature and applied voltage. In the present work, we show that highly ordered self-organized porous structures with an overall pore diameter of approximately 25 nm and a thickness of several tens micrometers can be obtained under optimized anodization conditions.

As Ta₂O₅ is very well established in electronic industry and other fields

Acknowledgments

The authors would like to acknowledge DFG and the Erlangen DFG Cluster of Excellence for financial support, and Helga Hildebrand for valuable technical help.

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Highly ordered nanoporous Ta2O5 formed by anodization of Ta at high temperatures in a glycerol/phosphate electrolyte

Abstract

Research Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Electrochem. Commun.

Corros. Sci.

Electrochem. Commun.

Electrochem. Commun.

Electrochem. Commun.

Electrochem. Acta.

Thin Solid Films

Appl. Surf. Sci.

Thin Solid Films

J. Colloid. Interf. Sci.

Highly ordered nanoporous Ta₂O₅ formed by anodization of Ta at high temperatures in a glycerol/phosphate electrolyte