23. - 25. 10. 2012, Brno, Czech Republic, EU

PREPARATION OF NANOSTRUCTURED SURFACES ON TANTALUM AND NIOBIUM

Hynek MORAVEC, Jaroslav FOJT

ICT Prague, Prague, Czech Republic, Eu, moravech@vscht.cz

Abstract

Electrochemical processes leading to formation of an ordered nanostructures are currently intensively

studied. Such modified surfaces can be used in many practical applications - medicine, catalysis,

photovoltaics, etc. The surface of titanium can be covered by an ordered layer of nanotubes when

experimental conditions are carefully selected. According to the theory could be similarly modified surface of

other metals. However, the literature dealing with this issue offers much less information than it is in the case

of titanium and also indicates that the nanostructure formation occurs at significantly different experimental

conditions. The work was focused on creating nanostructures on tantalum and niobium.

Measurements were carried out with the potentiostat-galvanostat Jaissle Potentiostat IMP 88 PC-200V.

Experiments were done in mixtures of hydrofluoric acid with sulphuric, phosphoric acid etc. The formed

nanostructures were evaluated by the scanning electron microscope.

At lower scan rates, 25 to 75 mV·s-1

, and all exposure lengths the surface of tantalum was covered by

nanostructure. Final state of surfaces was not arranged layer of nanotubes, but the structure with an irregular

distribution of pores with sizes of tens of nanometers. The results of measurements on niobium lead to the

conclusion that neither the potential value nor the length of potentiostatic delay do not allowed for the desired

surface state achievement.

Based on the results we can conclude that on tantalum a layer of numerous nano-pores was formed.

Desired nanostructures on niobium were not achieved by any combination of experimental conditions.

Keywords:

Tantalum, niobium, nanostructures, electrochemical oxidation, pores

1. INTRODUCTION

Tantalum and niobium have very similar corrosion behaviour based on passivity – creation a thin oxide layer

characterized by high stability. Tantalum is resistant to almost all mineral acids in a wide range of

concentrations and temperatures [1]. Tantalum is attacked at room temperature by the sulfur trioxide,

respectively oleum, as well as by hydrofluoric or oxalic acid. Niobium resistance is slightly lower in

comparison with tantalum, but still acceptable for practical use. If fluoride ions are present, negative

influence on the corrosion resistance appears. Passive layer is broke-down and soluble complexes are

formed [1].

Significant corrosion resistance of tantalum is used in the production of chemical apparatus where

conventional materials fail. Because of high cost it is often applied as a coating. Tantalum is used for

production of special surgical instruments or prostheses and in electronics industry for capacitors and

resistors [2-4]. Fuel elements in the nuclear industry are made of niobium [5].

Current studies are focused on the composition and surface morphology leading to improvement of utility

properties of materials. A new generation of materials with structure or surface parts in the nanometer scale

shows different properties in comparison with standard materials [6]. Frequently used procedures are based

on electrochemical oxidation. In the case of suitable experimental conditions, an ordered layer of tubes can

be created on the surface. Regular distribution and morphology, large active surface and relatively easy

23. - 25. 10. 2012, Brno, Czech Republic, EU

preparation procedure would potentially guarantee utilization in various fields such as catalysis, solar cells,

sensors etc. [7].

Electrolyte must contain aggressive components which attack a passive layer. Fluoride ions from hydrofluoric

acid fulfill this role for tantalum and niobium. Hydrofluoric acid is often used in a mixture with inorganic acids

such as H2SO4, H3PO4 [8, 9]. Composition of the electrolyte, pH, viscosity, or temperature can significantly

affect the resulting surface morphology [10].

There is lack of data in literature dealing with surface nanoscale modifications of tantalum or niobium. In this

work we try to find a set of suitable experimental conditions for creation of nanostructures on tantalum and

niobium. Moreover the influence of different parameters of the electrochemical oxidation on surface state

was studied.

2. EXPERIMENT

All experiments were carried out on tantalum and niobium samples with diameter 8 mm. All specimens were

mechanically ground (final grinding on paper P 2500) and polished (diamond paste, grain size 3 µm).

Samples were rinsed in demineralised water and subsequently ultrasonically cleaned in acetone and

isopropanol. Than the samples were put against an O-ring in an electrochemical cell, leaving 18 mm2

exposed to electrolytes. Following electrolytes were selected: a mixture of 1 mol/l H2SO4 and 2% wt. HF for

nanostructuring of tantalum, a mixture of 1 mol/l H3PO4 + 1% wt. HF for niobium [8, 9].

Electrochemical measurements were carried out with potentiostat-galvanostat Jaissle Potentiostat IMP 88

PC-200V with control unit PGU-AUTO Extern. The electrochemical procedure consisted of potential sweep

from the open-circuit potential to desired potential, followed by delay at the final potential. Electrochemical

measurements were carried out with a three-electrode system with an Ag/AgCl (3 mol/l KCl) as reference

electrode and pair of graphite electrodes as counter electrodes. Samples were rinsed with a deionized water

and acetone and dried in air stream after exposure. All measurements were realised in a Faraday‘s cage at

room temperature and without stirring. Surface of exposed samples was characterized by scanning electron

microscope TESCAN VEGA3 SBU.

3. RESULTS AND DISCUSSION

3.1. Tantalum - Effect of potential sweep rate

a) 25 mV/s b) 50 mV/s c) 75 mV/s

Fig. 1: Effect of potential sweep rate on surface state

23. - 25. 10. 2012, Brno, Czech Republic, EU

The influence of potential sweep rate was tested at first because of significant influence on the surface state

was supposed. As it is apparent from the Fig. 1 metal surface was covered with a nanostructure after

exposure. The final surface could be described as the system with numerous pores of irregular distribution.

The typical current-time dependence during electrochemical process is shown in the Fig. 2. An oxide layer

with defined thickness was formed in potentiodynamic part of exposure. Then the passive layer was attacked

by hydrofluoric acid. This process was followed by rapid decrease of current. An oxidic layer with pores was

formed.

Fig. 2: Current-time curves during electrochemical oxidation (sweep rate 75 mV/s)

3.2. Tantalum - Effect of the length of potentiostatic exposure

3000 s 4000 s 5000 s

Fig. 3: Effect of the length of potentiostatic exposure on surface condition

23. - 25. 10. 2012, Brno, Czech Republic, EU

The surfaces of tantalum samples were covered by the pores in all experiments. The number of pores

increased with the time of potentiostatic polarization. Simultaneously, decrease of their average size was

noticed. A completely different morphology was obtained when the system was polarized for 5000 seconds.

In this case longer exposure resulted in slight etching of the surface and formation of new arrays resembling

the shape of tubes.

3.3. Niobium - Effect of potential

7.5 V 10 V 12.5 V

Fig 4: Effect of potential on surface condition

Observation of surface state of niobium led to the conclusion that the desired nanostructures were not

achieved. Degraded and etched structure was obtained with no signs of pore formation in all cases.

3.4. Niobium - Effect of the length of potentiostatic exposure

1800 s 3600 s 7200 s

Fig 5: Effect of anodization duration on surface condition

The aim of this experiment was to determine the effect of exposure duration on the formation of

nanostructures. When the duration of exposure was 1800 and 3600 s the signs of pore formation on the

surface were observed. With increasing anodization time (7200 s) the signs of porous structure disappeared.

23. - 25. 10. 2012, Brno, Czech Republic, EU

4. CONCLUSION

Nanostructure on tantalum surface was observed after using polarization sweep rates 50 and 75 mV/s at all

exposure times. However, surface state corresponded to irregular and random distribution of pore sizes in

the order of tens of nanometres. Desired nanostructure was not achieved using any combination of

experimental conditions in the case of niobium.

ACKNOWLEDGEMENTS

The work was realized with the support of a grant project TE 01020390 - Centre for development of

advanced metallic biomaterials for medical implants.

LITERATURE

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[2] Robin, A., et al., Corrosion behavior of Ti-xNb-13Zr alloys in Ringer's solution. Mater. Corros., 2008. 59(12): p.

929-933.

[3] Kaufmann, D., et al., Size dependent mechanical behaviour of tantalum. International Journal of Plasticity, 2011. 27(3): p. 470-478.

[4] Albrecht, S., et al., Tantalum and Tantalum Compounds. 2011.

[5] Albrecht, S., C. Cymorek, and J. Eckert, Niobium and Niobium Compounds. 2011.

[6] Variola, F., et al., Improving biocompatibility of implantable metals by nanoscale modification of surfaces: an overview of strategies, fabrication methods, and challenges. Small, 2009. 5(9): p. 996-1006.

[7] Berger, S., et al., Transparent TiO2 Nanotube Electrodes via Thin Layer Anodization: Fabrication and Use in Electrochromic Devices. Langmuir, 2009. 25(9): p. 4841-4844.

[8] Sieber, I.V. and P. Schmuki, Porous Tantalum Oxide Prepared by Electrochemical Anodic Oxidation. Journal of the Electrochemical Society, 2005. 152(9): p. C639-C644.

[9] Choi, J., et al., Porous niobium oxide films prepared by anodization in HF/H3PO4. Electrochimica Acta, 2006. 51(25): p. 5502-5507.

[10] 1Lee, K. and P. Schmuki, Highly ordered nanoporous Ta2O5 formed by anodization of Ta at high temperatures in a glycerol/phosphate electrolyte. Electrochemistry Communications, 2011. 13(6): p. 542-545.