In the past decades, a variety of self-aligned functional oxides have successfully
been grown on a wide range of metals using electrochemical, self-organizing anodization.
The earliest reports on highly ordered oxide structures included porous aluminum oxide
layers grown by optimized anodization of Al in oxalic acid.1 These nanoporous oxides
found a considerable number of direct applications, such as size-exclusion filters2
and waveguide structures,3 or sacrificial uses, such as templates for secondary material
deposition for the production of nanowires and tubes.4, 5
A very versatile self-organizing anodization approach was introduced in 1999 by Zwilling
et al., which used fluoride-containing electrolytes for the fabrication of ordered
TiO2 nanotube arrays on Ti.6 These fluoride-based electrolytes were optimized over
the last ten years to enable the growth of self-organized oxide layers on many metals
and alloys, including Ti,7 Zr,8, 9 Hf,10 Nb,11, 12 Ta,13 W,14 Ti–W,15 and Ti–Nb.16
A detailed overview can be found in Ref. 7.
In 2005, Masuda et al., followed by others,16–19 showed that by using perchlorate
or chloride electrolytes, another type of nanotubes, the so-called rapid-breakdown
anodization (RBA) nanotubes could be grown on Ti and W surfaces. This process was
later extended to Ti–Nb, Ti–Zr and Ti–Ta alloys to form mixed-oxide nanostructures.20,
21 In this anodization approach, the formation of tubes occurs with a high current
flow, and tubes grow as bundles from a specific surface site on the metal into the
electrolyte.17–21 Due to the localized nature of the process and high current densities,
mechanistically, the formation process was attributed to repeated anodic breakdown
events of the surface oxide layer.18
Over the past ten years, considerable efforts have been directed toward the finding
of other electrolyte types that would lead to the formation of self-organized nanostructured
metal oxides. While nitrate-based electrolytes were used to etch Ti through a porous
alumina template and, thus, can form etch channels,22 we recently showed that nitrate-based
electrolytes also may be a promising new route to achieve truly self-organized oxide
structures in the context of Ti and Ta anodization.23 In the present work, we explore
the use of nitrate-containing electrolytes for the formation of self-organized (template-free)
oxide structures on Ti, Nb and Ti–Nb alloys.
Ordered TiO2-based nanoscale structures are particularly interesting in terms of applications
in catalysis,24 solar cells,25 photolysis,26 sensing,27 and electrochromic devices.16
Nb is an important element in combination with Ti, since composite oxides can be formed,
or TiO2 can be Nb-doped for an alteration of the electronic properties.28 For TiO2
nanotubes, it has been shown that in large concentrations the incorporation of Nb
leads to lattice widening16 and is, therefore, beneficial in ion insertion devices
(e.g., electrochromic applications and ion intercalation batteries). In smaller concentrations,
Nb acts as a donor species to enhance the performance of TiO2-based solar cells and
water splitting reactions.28–30 Herein, we demonstrate that anodization in nitrate-based
electrolytes can be tuned to form ordered, nanoporous oxide structures, not only on
Ti, but also on Nb and Ti–Nb alloys. Moreover, in contrast to any other previously
reported electrolyte types, this nitrate-based anodization leads directly to a through-hole
morphology for all investigated structures, i.e., where the pores are open at the
top and bottom.
A series of preliminary anodization experiments for all the metals in various aqueous
and ethylene glycol-based nitrate electrolytes were carried out, screening parameters
being concentration, pH, and water content. The results showed that on Ti, Nb and
Ti45–Nb, ordered porous layers could be grown (Figure 1). In aqueous electrolytes,
a sufficiently high anodization voltage had to be applied to initiate the growth of
a porous layer with an aligned pore structure. For the three investigated materials,
the conditions to achieve a defined layer growth are different in each case. For Ti,
well-ordered pores could be observed for anodization in HNO3. The example shown in
Figure 1 a resulted in an oxide-layer thickness of approximately 10 μm. The inset
pictures in Figure 1 show that regular pore channels with a diameter of 10–20 nm and
a through-hole morphology could be obtained. Using Nb, a stable compact oxide film,
instead of a porous oxide layer, was formed in all explored aqueous nitric acid electrolytes.
However, when anodization was carried out in an organic nitrate electrolyte, well-defined
porous layers could be grown. Figure 1 b shows the cross section of a self-organized
nanoporous Nb oxide layer formed in an ammonium nitrate electrolyte. The resulting
layer thickness was approximately 4 μm, and the pore diameter of the through-hole
morphology was approximately 10–15 nm.
Figure 1
SEM images of cross sections. a) Nanoporous TiO2 with a through-hole morphology prepared
in 0.1 m HNO3 at 40 V for 180 s. The insets show the top, middle and bottom part of
the cross section and the top view. b) Nanoporous Nb2O5 prepared in 0.2 m NH4NO3 and
3 % (v/v) H2O in ethylene glycol at 40 V for 300 s. The insets show the middle and
bottom part of the cross section and the top view. c) Nanoporous Ti45–Nb alloy with
through-hole morphology prepared in 0.1 m HNO3 at 40 V for 180 s. The insets show
the top and the bottom view. d) The Ti45–Nb alloy with well-aligned pores anodized
in 0.2 m NH4NO3 and 3 % (v/v) H2O in ethylene glycol for 1800 s. The insets show the
top and the middle part of the layer.
A defined and self-organized pore morphology was obtained for the Ti45–Nb alloy in
both types of electrolytes—aqueous and organic nitrate. Examples of the highly aligned
pore channels and the through-hole morphology for aqueous and organic electrolytes
are shown in Figure 1 c and d. Whereas a pore diameter of 10–20 nm, comparable to
Ti, was found in the aqueous electrolyte, a pore diameter of 20–50 nm was measured
in the organic electrolyte. Compared to pure Ti, a thinner layer of approximately
4 μm was obtained with the alloy in the aqueous electrolyte, which indicates that
the alloyed Nb reduces the oxide growth rate. The oxide layer thickness (∼10 μm) grown
in the organic electrolyte under the same conditions, on the other hand, is comparable
to pure Nb (Figure 1 b and d).
Figure 2 shows current density–time curves observed during anodization, illustrating
the different anodizing behaviors of the various substrates in aqueous and organic
electrolytes. In general, the current density in the organic electrolyte is lower
than in the aqueous electrolyte by an order of magnitude. In the aqueous electrolyte,
very high current densities are directly observed with a gradual decay for Ti and
the Ti–Nb alloy. Ordered porous layers were formed with both substrates with the thickness
increasing with time. This type of behavior was seen for applied voltages in the range
of 15–40 V. However, when anodization was carried out in ethylene glycol electrolytes,
much higher currents were observed, and self-organized porous oxide layers could be
grown in the voltage range of 15–60 V. In the organic electrolyte, the alloy showed
an increase in current up to 10 mA cm−2, followed by a decay. A similar trend was
observed in the aqueous electrolyte, where highly self-organized nanoporous Ti–Nb
oxide layers could also be observed in a potential range of 20–40 V. Overall, regarding
the electrochemical conditions under which ordered, porous layers were formed, it
may be deduce that in NO3
−-based electrolytes and under the investigated conditions, a mechanism lying between
the high-field case,7, 31–33 which is responsible for self-ordered nanoporous structures
on Al in oxalic acid or fluoride-induced self-ordered structures, and the breakdown
mechanism, which forms RBA tubes,15 is responsible for order and oxide growth.
Figure 2
Current transient of a) Ti (▪), Nb (▵) and the Ti45–Nb alloy (○) in aqueous 0.1 m
HNO3 at 40 V for 180 s, and b) Nb at 30 (▪) and 40 V (□) and the Ti–Nb alloy at 40
V (▿) in 0.2 m NH4NO3 and 3 % (v/v) H2O in ethylene glycol.
To obtain information on the structure and composition of the oxide layers, X-ray
diffraction (XRD) and energy dispersive X-ray (EDX) measurements were carried out.
The XRD patterns in Figure 3 a–c show the as-formed nanoporous oxide layers on Ti,
Nb and the Ti–Nb alloy to be amorphous. EDX measurements confirmed the as-formed layer
on Ti to correspond to TiO2, and the layer on Nb to correspond to Nb2O5. The structures
could be crystallized by an appropriate annealing treatment. Crystallization was carried
out at 450 °C in air for TiO2 and at 550 °C in an N2 atmosphere for Nb2O5.34, 36 As
seen from the XRD measurements, after annealing, the nanoporous oxide layers were
present as anatase TiO2 with small amounts of rutile TiO2 in the case of Ti,34 and
as monoclinic Nb2O5 for pure Nb.35 Upon annealing, the layer on the alloy showed conversion
to a composite oxide (Figure 3 c), that is, individual anatase TiO2 and monoclinic
Nb2O5 phases could be detected. EDX measurements of the annealed layer grown on the
Ti45–Nb alloy in the aqueous or organic electrolyte yielded a composition of approximately
14 % Ti, 33 % Nb, 48 % O, 5 % C and 0 % N in both cases (Figure 4 a and b). The results
are in line with the formation of a TiO2/Nb2O5 layer on top of the Ti–Nb substrate.
It must be mentioned that the substrate is clearly contributing to the EDX signal.
From this result, it becomes apparent that in the oxide structures, Nb2O5 is enriched
compared with TiO2. This is in line with the electrochemical behavior observed in
Figure 2 and literature reports that state TiO2 to be much more prone to dissolution
in NO3
−-based electrolytes than Nb2O5.30
Figure 3
XRD pattern of the as-formed and annealed oxide layer: a) Ti anodized in 0.1 m HNO3,
showing distinct anatase TiO2 peaks; b) Nb anodized in 0.2 m NH4NO3, resulting in
the formation of Nb2O5; c) Ti45–Nb alloy anodized in 0.2 m NH4NO3, where a mixed oxide
consisting of anatase TiO2 and Nb2O5 is observed. Anatase TiO2 (A); rutile TiO2 (R);
Ti metal (Ti); Nb2O5 (N); Nb metal (Nb).
Figure 4
EDX spectra after annealing of the Ti45–Nb alloy grown in a) the nitric acid electrolyte,
and b) the organic nitrate electrolyte.
Overall, the finding that nitrate-based electrolytes can be used to achieve self-organized
oxide growth on metals with a very different electrochemical behavior, such as Ti
and Nb (and their alloys), indicates that these electrolytes are very versatile. It
is therefore likely that this approach can be applied to a wide range of metals to
form ordered, through-hole morphologies, which might be, valuable as, for example,
flow-through membranes or templates for the deposition of secondary materials.
The present work demonstrates that plain nitrate electrolytes can be successfully
used to grow through-hole, self-organized oxide nanopore (or nanochannel) layers on
Ti, Nb and their alloys. The observed pore diameter is in the range of 10–50 nm, and
the layers typically have a thickness of 10 μm. Different optimized electrolyte conditions
are needed to achieve successful growth of these structures, with key parameters being
the water content and applied voltage. These findings suggest that, based on nitrate
electrolytes, a wide range of metals can be anodized to form self-organized, through-hole
porous oxide structures. Considering the plethora of applications of TiO2 and doped
TiO2 nanoscale structures in particular, we believe that the present finding represents
a novel platform for the fabrication of doped and well-defined metal oxide nanostructures.
Experimental Section
Titanium and niobium foils (0.1 mm, 99.6 % Ti, 99.9 % Nb purity; Advent Materials,
Oxford, UK) and a Ti–Nb alloy with a composition of 55 wt % Ti and 45 wt % Nb (ATI
Wah Chang, Albany, USA) were used as substrates for anodization experiments. Prior
to electrochemical treatments, the samples were degreased by sonication in acetone
and ethanol, subsequently rinsed with deionized water and finally dried in a nitrogen
stream. The samples were contacted with a copper plate and pressed against an o-ring
in an electrochemical cell (1 cm2 exposed to the electrolyte). Anodization was carried
out in 0.1 m HNO3 in a potential range of 10–40 V, or, alternatively, in ethylene
glycol containing 0.2 m NH4NO3 with 3 % (v/v) H2O at potentials of 10–60 V. For the
electrochemical experiments, a high-voltage potentiostat (Jaissle IMP 88) was used
in a conventional three-electrode configuration with a platinum sheet as a counter
electrode and a platinum wire as the pseudo reference electrode. All electrolytes
were prepared from reagent grade chemicals.
A scanning electron microscope (Hitachi FE-SEM S4800) was employed for the morphological
characterizations. The composition of the porous anodic oxide layers was investigated
by energy dispersive X-ray (EDX) spectroscopy (Genesis system from EDAX). For structural
characterization, X-ray diffraction (XRD) measurements were performed (X′Pert XRD
system from Philips). Annealing of the samples was carried out in a furnace at 450
°C in air for Ti and at 550 °C in an N2 atmosphere for 1 h for Nb and the Ti45–Nb
alloy.