up to 700 nm was formed by constant voltage anodizing in 1% HF solution for 1 hour. ... work, m2 was measured to be 8.2 (V/m) at a current density of 5Am. -2.
in Pits and Pores III: Formation, Properties, and Significance for Advanced Materials/2004, P. Schmuki, D. J. Lockwood, Y. H. Ogata, M. Seo, H. S. Isaacs, Editors, PV 2004-19, p. 123-133, The Electrochemical Society Proceedings Series, Pennington, NJ (2006).
FABRICATION OF POROUS NIOBIA BY ANODIZING OF NIOBIUM Sachiko Ono, Takumi Nagasaka, Hiroki Shimazaki and Hidetaka Asoh Department of Applied Chemistry, Faculty of Engineering, Kogakuin University 1-24-2 Nishi-shinjuku, Tokyo 163-8677, Japan ABSTRACT Three types of porous niobia having sufficient high porosity were fabricated by anodizing of niobium. Porous structure was dependent on types of electrolyte used. Lava-like porous type film with the pore size of 1 m was obtained in NaOH solution, and cracked type film was obtained in H2SO4 solution associated with electric breakdown. Porosity of the films increased with increasing breakdown time. Porous films formed by breakdown were composed of crystalline Nb2O5. Nano-porous amorphous film could be obtained at low current density in a solution containing HF with the pore size of 10nm and the cell size of 30nm. The film thickness up to 700 nm was formed by constant voltage anodizing in 1% HF solution for 1 hour. Dimension of nano-porous film was independent of formation voltage except for the thickness of the barrier layer. INTRODUCTION Anodic oxide film formed on niobium is expected as a new dielectric oxide for electrolytic capacitor because of its exploitation in capacitor industry as an alternative material to tantalum anodic oxide. Niobium is more abundant than tantalum and the density of niobium is half to that of tantalum. In addition, the anodic oxide film formed on niobium has high permittivity. However, little information on anodizing behavior of niobium has been given [1-5] in contrast to numerous studies of anodizing of aluminum. We have investigated the anodizing behavior of niobium and oxide film structure with focusing on the dielectric properties [6-8]. Recently, fabrication of porous anodic films on various semiconductors such as titanium and indium phosphate has been received significant attention because of its exploitation as photocatalyst and chemical sensors [9, 10]. On the other hand, fabrication of mesoporous oxide of niobium has also especial importance in the field of acid catalyst [11] in addition to its potential utilization as a novel capacitor and devise applications. Therefore, an attempt has been done to fabricate porous niobia having high specific surface area by anodizing of niobium, which was rarely reported. EXPERIMENTAL The pure (99.93%) niobium foils were chemically polished in a 1:4 mixture of HF and HNO3 solution before anodizing. Anodic films were formed at a constant current density in the range of 1 Am-2 to 50 Am-2 or constant voltage in various electrolytes such as H3PO4, H2SO4, NaOH, H2C2O4, (NH4)2C2O4, (NH4)2C4H4O6, (NH4)2B4O7 and HF.
Voltage
m1 m2
Vj
Vj
Nb Re-anodizing time
Fig.1 Schematic representation of “Pore filling” in a neutral solution. Dotted area corresponds to the oxide layer formed during re-anodizing showing the slope of m1. m2: The slope of the V-t curve during the growth of barrier film by anodizing of an metal substrate.
To estimate the porosity of anodic niobium oxide, voltage - time (V-t) curves were measured during re-anodizing of the porous niobia specimens at constant current density in 0.1 mol phosphoric acid at 20°C, as shown schematically in Fig.1. This measurement is based on the fact that the anodic barrier film growth proceeds both at the oxide/metal interface by the inward migration of O2- and the oxide/electrolyte interface by the outward migration of Nb5+. Porosity in the porous layer is calculated using the following equation with transport numbers of Nb5+ (TNb5+) and O2- (TO2-), which are confirmed to be 0.26 and 0.74, respectively [6]. = ( TNb5+) / (1 - TO2-), [1] where = m2 / m1. [2] m1 : The slope of the V-t curve during re-anodizing of the niobium specimen having porous niobia layer m2: The slope of the V-t curve during the growth of barrier film by anodizing of an niobium substrate This method for porosity measurement is well established [12-15] for anodic alumina and called as a “pore-filling” technique. Under the condition of the present set of work, m2 was measured to be 8.2 (V/m) at a current density of 5Am-2 and 16.1 for 10 Am-2. Anodic film structure was observed by a field emission scanning electron microscope (FE-SEM, HITACHI S-4200) and thin film X-ray diffraction analysis (TF-XRD). Glow discharge optical emission spectroscopy (GD-OES, Jobin-Yvon JY5000RF) was used for measuring depth-profiles of constituent elements in the films accompanied by argon ion sputtering at a power of 40W. RESULTS AND DISCUSSION Voltage – time curves of anodizing in various electrolytes
300 250
NaOH （20℃）
H3PO4
200 150
(COOH)2
100
H2SO4 NaOH
50
Voltage / V
Voltage / V
300
(NH4)2B4O7
200
100
H2SO4 （40℃）
0 0
10
20
0
Time / min
0
Fig.2 Voltage – time curves of anodizing at 10 Am-2 in various 0.1 moldm-3 electrolytes at 20℃.
20
40 Time / min
60
Fig.3 Voltage – time curves of anodizing at 10 Am-2 in 0.1 moldm-3 NaOH at 20℃ and 0.1 moldm-3 H2SO4 at 40℃.
Voltage – time curves of anodizing in various 0.1 moldm-3 electrolytes at 10 Am-2 were shown in Fig.2. The slope of the voltage increase became larger in the order of H3PO4, (NH4)2B4O7, H2SO4, H2C2O4 and NaOH solutions. V-t curves for (NH4)2C2O4 and (NH4)2C4H4O6 are found between (NH4)2B4O7 and H2C2O4. Breakdown voltage was in the range of 220V to 280V. After prolonged anodizing accompanied by breakdown for 10 min (Fig.3) with varying electrolyte temperature, porosity of the films was measured. The porosity higher than 0.01, i.e., a volume ratio of 1%, was obtained only for the films formed in 0.1 moldm-3 H2SO4 and NaOH solutions even after the effect of electrolyte concentration and temperature changes was examined.
a
b
c
5m
SEM images of anodic films formed in H2SO4 (40℃) at 10Am-2 with the change in breakdown time. a: 0min, b: 10min, c: 20min Fig.4
5m
Morphology of the surface and porosity changes When niobium was anodized in 0.1 moldm-3 H2SO4 at 10 Am-2 up to the breakdown voltage, many cracks caused by breakdown events appeared and the number of cracks increased with increasing breakdown time as shown in Fig.4. The flower-like shape of these cracks was produced by the peeling of the outer amorphous oxide layer of
the film accompanying rapid volume expansion of the inner crystallized oxide layer formed at breakdown points similar to that found in anodic films on tantalum [16].
a
b
c
Fig .1 Cu rre nt – 5m tim e curfilms formed in NaOH (20℃) at 10Am-2 with the change Fig.5 SEM images of anodic in breakdown time. a: 0min,ves b: 10min, c: 20min me asu red dur ing bur nin g co mp are d wit h to tha 1m t dur ingfilms formed on niobium in NaOH at 10Am -2 after electric Fig.6 SEM images of anodic breakdown for 20min. left: sta cracked film right: lava-like pores ble an odi size of approximately 1m appeared when anodizing While, many pores in the zin -3 was performed in 0.1mol dm g NaOH at 40℃ (Fig.5). Circular cracks similar to that formed in H2SO4 were sporadically at found. Cross section of the cracks (Fig.6, left) show 19 the thick crystalline layer composed of fine particles underneath the amorphous outer layer of the film. Porous oxide 5V layer produced in NaOH (Fig.6, right) shows many pores in having different diameter in the ph range from 0.2m to 1 m. The maximum pore size os appears to increase with breakdown time as well as pore number. A lava-like shape of the ph porous layer is similar to that formed on other valve metals such as aluminum and ori titanium as a result of breakdown accompanied by minute sparking and gas evolution. c TF-XRD analysis indicated that aci the both films formed in H2SO4 and NaOH were composed of crystalline Nb2O5d.as shown in Fig.7. From these XRD spectra, it is shown
5 m
that the film formed in NaOH is thicker and has less crystallinity. The difference in the film structure after breakdown between H2SO4 and NaOH would be due to the difference in dissolution ability of electrolytes to the niobium oxide.
The porosity change with breakdown time was measured by “pore-filling technique”, i.e., the V-t transient measurement at re-anodizing of the specimen as shown in Fig.8. The porosity of the both films formed in H2SO4 and NaOH solutions increased with increasing breakdown time. It was in the order of 0.04 after 20min for the films formed in H2SO4 and 0.06 for the films formed in NaOH although the porous shape was different.
Intensity / a.u.
Nb Nb2O5
H2SO4
NaOH 20
30
40
50
60
70
80
2θ［degree］ Fig.7 TF-XRD patterns of anodic films formed on niobium in NaOH and H2SO4 at 10Am-2 after electric breakdown for 10min.
0min ＝0 Voltage / V
200
20min ＝0.027
100
30min α＝0.039
0
1 2 3 4 Re-anodizing time / min
0min ＝0.006
NaOH (20℃)
200
100
10min α＝0.018
10min α＝0.014
0
300
H2SO4 (40℃)
Voltage / V
300
30min 20min α＝0.059 α＝0.042
0
5
0
1
2 3 4 Re-anodizing time / min
Fig.8 Voltage-time curves obtained by re-anodizing at 1 Am-2 of anodized specimens. Anodic films were formed in 0.1mol dm -2 H2SO4 and NaOH at 10Am-2 for different breakdown time. α: Estimated porosity from the V-t slope.
5
Fig.9 FE-SEM images of anodic films formed in NaOH after breakdown for 10min with the effect of current density.
Effect of current density on surface morphology FE-SEM images of anodic films formed in NaOH after breakdown for 10min were shown in Fig.9 with the effect of current density. When current density was lower than 5 Am-2, crack-like holes were only formed. With the increase in current density, porosity increased. However, when current density was as high as 50 Am-2, the number of pores decreased and the number of crack-type holes increased again. Therefore, suitable current density for porous niobia formation seems to be around 10 Am-2. Anodizing in a H2SO4 / HF mixed solution For the fabrication of porous niobia having high surface area, HF was added to 1 moldm H2SO4 to enhance the dissolution ability of the electrolyte against the oxide film. Figure 10 shows voltage - time curves of anodizing in 0.3wt% HF / 1 moldm-3 H2SO4 mixed solution at various current densities. Breakdown voltage was about 100V. As shown in Fig.11, the surface structure of the films were similar to that obtained in NaOH solution when current density was higher than 5 Am-2. While, fine pores in the size of 10nm - 20nm were formed when current density was lower than 2 Am-2. Then, anodizing at constant voltage was performed. Current density - time transients indicate the similar curve to that associates with a porous film formation of aluminum, although the current is quite low (Fig.12) and relatively independent of formation voltage. FE-SEM images of cross sections shown in Fig.13 clearly indicate nano-porous niobia films with the cell size of 30nm to 40nm, the pore size of 10nm to 20nm, and the film thickness of approximately 400nm. The barrier layer thickness was dependent on formation voltage with the anodizing ratio of 2.0 (nm/V). It is slightly thinner than 2.5 of barrier type film [6] similar to the case of anodic alumina film, i.e., 1.0 for a porous type film and 1.4 for a barrier type film [17]. -3
200
50A m-2 20A m-2 10A m-2
Voltage / V
150
5A m-2
100
2A m-2 50
0 0
10
20
30
Time / min
voltage - time curves of anodizing in 0.3wt% HF / 1 moldm-3 H2SO4 mixed solution at various current densities. Fig.10
Fig.11 FE-SEM images of anodic films formed in 0.3wt% HF / 1 moldm-3 H2SO4 mixed solution showing the effect of current density. Fine pores in the size of 10nm - 20nm were formed when current density was lower than 2 Am-2.
Current density / A m-2
3
2
70V 1
0 0
20V
50V
10V
60 Anodization time / min
120
Fig.12 Current - time curves of anodizing in 0.3wt% HF / 1 moldm-3 H2SO4 mixed solution at various formation voltage.
Fig.13 FE-SEM images of cross sections of the films formed in 0.3wt% HF / 1 moldm-3 H2SO4 mixed solution at various formation voltage.
10V
70V
150
300
60min
90min
40min
60min
Voltage / V/ Voltage V
Voltage / V
100
200
20min 50
100
30min
120min Blank
10min Blank
0 0
5 Re-anodizing time / min Re-anodization time / min
30min
0 10
0
5 Anodizing time / min
10
Re-anodization time / min
Fig.14 Voltage – time curves at re-anodizing of the specimen after anodizing at 10V (left) and 70V (right) with the change in formation time.
Figure 14 shows voltage – time curves at pore-filling, i.e., re-anodizing of the
specimen after anodizing at 10V (left) and 70V (right) with the change in formation time. At the anodizing at 10V, the thickness of the film increased with time. Porosity was estimated to be in the order of 0.15 to 0.2. When anodizing voltage increased and anodizing time was prolonged, the resistively of the barrier layer of the film was lost as detected in Fig.13. This is due to dissolution of the barrier layer by easy fluorine penetration into the substrate. Electrolysis in HF solution Figure 15 shows current – time transients of anodizing at 20V in a HF solution with the effect of electrolyte concentration. Current density increased with increasing concentration of HF. FE-SEM images of the film cross sections shown in Fig.16 indicate the growth of porous layer proportional to the current density. The thickness of the film formed in 1% HF for one hour was 700nm. However, the film was extensively dissolved after anodizing for 2 hour.
Current density / A m-2
5 4
1wt% 3 2
0.5wt%
1
0.1wt%
0 0
30 Anodizing time / min
60
Fig.15 current – time transients of anodizing at 20V in a HF solution with the effect of electrolyte concentration.
Fig.16 FE-SEM images of cross sections of the films formed in HF solution with the effect of electrolyte concentration.
Thus, porous niobia having nano-scale pores could be fabricated by anodization. Further attempt to improve the specific surface area is under investigation.
CONCLUSIONS (1) Porous niobia was fabricated by anodizing of niobium for the first time. (2) Porous structure of the films was dependent on electrolyte types. A lava-like porous film was obtained in NaOH solution and a cracked type film was obtained in H2SO4 solution at breakdown voltage. Porosity of the films increased with increasing breakdown time. (3) Porous films formed by breakdown were composed of crystalline Nb2O5. (4) No porous film having sufficient porosity could be obtained in other electrolytes such as H3PO4, H2C2O4, NaCl, (NH4)2C2O4, (NH4)2C4H4O6 and (NH4)2B4O7 at any anodizing condition used in this experiment. (5) In the electrolytes containing HF, nano-porous amorphous films with the cell size of approximately 30nm, pore size of 10nm and maximum film thickness of 700nm could be formed at relatively low current density.(6) Dimension of nano-porous film was independent of formation voltage except for the thickness of barrier layer.
ACKNOWLEDGEMENTS Parts of this work were financially supported by the Promotion and Mutual Aids Corporation for Private Schools of Japan and the Light Metal Education Foundation of Japan. Thanks are also due to CBMM Asia Co.,Ltd and Dr. M. Imagunbai for their help to this work.
REFERENCES 1. 2. 3. 4.
Y.M.Li and L.Young, J. Electrochem. Soc. 147, 1344 (2000). F. Di Quarto, S. Piazza and C. Sunseri, Corrosion Sci., 31, 267(1990). F.M. Al-Kharafi and W.A. Badawy, Electrochimica Acta, 40, 2623 (1995). K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson and G.C. Wood, Electrochimica Acta, 45, 1805 (2000). 5. S.L. de Albuquerque Maranhao and R.M. Torresi, Electrochimica Acta, 43, 257 (1998). 6. S. Ono, M. Baba, M. Shimoyama and H. Asoh, The ECS Meeting, Surface Oxide Films, Proceeding of the International Symposium, PV.2003-25, 133 (2004) 7. H, Odate, H. Asoh and S. Ono, 204th Meeting of the Electrochemical Society, Abstr. 34 (2003) 8. S. Ono, J. Surf. Finish. Soc. Jpn., 54, 447 (2003) 9. D. Gong, C. A. Grimes, O. K. Varghese, W. Hu, R. S. Singh, Z. Chen and E. C. Dickey, J. Mater. Res., 16, 3331 (2001) 10. R. Beranek, H. Hildebrand and P. Schmuki, Electrochem. and Solid-State Lett., 6, B12 (2003) 11. I.Nowak and M.Ziolek, Chem. Rev., 99, 3603 (1999) 12. A. Dekker and A. Middelhoek, J.Electrochem. Soc., 117, 440 (1970). 13. H. Takahashi and M. Nagayama, Corros. Sci., 18, 911 (1978).
14. S. Ono and K. Takeda, Proce. 8th Int. Symp. Passivity of Metals and Semi-conductors, pp.931-938 (2000). 15. S. Ono and N. Masuko, Surf. and Coat. Tech., 169-170, 139 (2003) 16. N. F. Jackson; J. Applied Electrochem., 3, 91 (1973) 17. S. Ono, F. Mizutani, M. Ue, and N. Masuko, in Corrosion and Corrosion Protection, PV. 2001-22, p.1129 The Electrochemical Society Proceedings Series, Pennington, NJ (2001).
Key words: niobium, anodizing, nano-porous film, breakdown, porosity
