The Formation Mechanism of 3D Porous Anodized Aluminum Oxide

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The Formation Mechanism of 3D Porous Anodized Aluminum Oxide Templates from an Aluminum Film with Copper Impurities Johannes Vanpaemel, Alaa Abd-Elnaiem, Stefan De Gendt, and Philippe M Vereecken J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508142m • Publication Date (Web): 07 Jan 2015 Downloaded from http://pubs.acs.org on January 13, 2015

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The Journal of Physical Chemistry

The Formation Mechanism of 3D Porous Anodized Aluminum Oxide Templates from an Aluminum Film with Copper Impurities Johannes Vanpaemel1,2, Alaa M. Abd-Elnaiem1,2,3,4, Stefan De Gendt1,5, Philippe M. Vereecken1,2,* 1

imec, Kapeldreef 75, 3001 Heverlee, Belgium

2

Centre of Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium

3

KACST-Intel Consortium Center of Excellence in Nano-manufacturing Applications (CENA), Riyadh, Saudi Arabia

4

Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt

5

Chemistry Department, KU Leuven, Celestijnenlaan 200f, 3001 Heverlee, Belgium

*Corresponding author: [email protected]

Abstract This paper describes the fundamental mechanism for the formation of a 3 dimensional porous template during the anodization of Al with less than 1at. % Cu percentages. It is known that the presence of Cu impurities in an Al film introduces horizontal pores interconnecting the vertically aligned porous structure of the anodized aluminum oxide (AAO) template. We show that the formation of these horizontal pores is accompanied by current density oscillations when the anodization is performed at a constant voltage. The frequency of these oscillations is directly related to the horizontal interpore distance. We propose a mechanism that links the current density oscillations to the Cu accumulation at the metal/oxide interface through the cyclic change in anode potential. The distance between the horizontal pores is found independent on the current density, temperature, and electrolyte concentration. Instead, it was found that the spacing between the vertical pores and thus the anodization voltage determines the spacing between the horizontal pores. A model based on the plastic flow of the alumina barrier layer was suggested to link the spacing between the horizontal and the vertical pores. These results provide important insights in the formation of 3D AAO templates. In

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addition, we show the fabrication of rigid 3D metal nanomeshes by electrochemical deposition into these 3D porous templates.

Keywords Anodization; 3-dimensional; nanoporous template; Al-Cu alloy; metallic nanomesh

Introduction Nanowire arrays have demonstrated tremendous potential in enhancing the properties of various devices. Due to their high effective surface area, these arrays typically have an improved electrical performance, which can be used for example for supercapacitors or microbatteries

1–3

. In practice

however, these improvements are often limited by the lack of stability of the fabricated nanowires. After exceeding a certain length, the free-standing nanowire arrays often collapse or bundle as the result of surface tension during wetting and drying 4. Therefore, strategies are needed to improve the stability of these arrays for example by anchoring individual nanowires to one another. In order to fabricate nanowire structures predefined templates are often used. A typical example of such nano-template with straight vertical pores is anodized aluminum oxide (AAO). By anodizing aluminum in acid media such as sulfuric acid, oxalic acid or phosphoric acid (depending on the voltage range selected), densely packed arrays of pores in a hexagonal unit cell are obtained

5–7

. These AAO

templates have been used to fabricate arrays of 1D nanowires by filling the pores in these templates for example by electrodeposition

8–10

. Interestingly, it was shown that the geometry of the pores can be

altered and tuned by selecting specific anodizing conditions 11–13 or by adding impurities to the Al film 14. For example, Al alloys with a small percentage of Cu (0.05 - 2.7 at.%; in what follows generally indicated as dilute Al-Cu alloys) have been shown to produce additional horizontal pores interconnecting the vertical pore array by anodizing 14,15. These 3D porous templates have been used to fabricate a regular 3D metal nanowire mesh 4,16.


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The behavior of copper containing Al alloys during anodization has already been studied quite extensively by the group of Skeldon and Thompson 17,18. When the anodization is performed in neutral electrolytes, such as H3BO3, a planar oxide is obtained instead of the porous structure. It was shown 18–20 that this planar alumina layer initially contains no copper. Instead, the Cu impurities are plowed down during this initial phase and accumulate right below the planar alumina layer. The enrichment of Cu below this oxide layer is promoted because its free energy of oxide formation is higher than that of Al, which will thus be preferentially oxidized 21,22: 3 + 2 ↔ 3 + ∆ . = −1193.2 /

[1]

Similar plowing and accumulating due to thermodynamic differences in oxide formation with Al was also seen for other impurities such as nickel

23

, gold

24

25

, and tungsten

. The extent of Cu accumulation

initially increases during anodization while the Al activity at the interface decreases. After some time, the copper concentration saturates at a concentration of around 6 x 1015 atoms cm-2 26. The exact number might slightly change with the Cu content in the alloy

27

. It is argued that Cu starts being

incorporated in the oxide layer only after it reaches this critical concentration. Indeed, the presence of Cu2+ 28 in the alumina layer was only found after the accumulated Cu concentration had saturated. After incorporation in the alumina layer, the Cu2+ ions migrate outwards at a faster rate than the Al3+

29

and

finally are ejected in the solution. Typically, this incorporation and ejection of Cu2+ is accompanied with the formation of voids in the oxide layer 29. Porous alumina or AAO with straight vertical pores is obtained by anodization of pure aluminum in the acidic electrolyte solutions. Interestingly, for dilute Al-Cu alloys, the porous template displays both vertical and horizontal pores 14. The spacing between these pores was found to be proportional to the applied voltage 15. At first instance, this seems to contradict the concept of a critical concentration as the spacing between the horizontal features in this case should merely depend on anodized film thickness



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and not on voltage. However, as the interface between the barrier layer and the metal is concave at each pore bottom, the situation is different than that of a planar oxide layer. For example for Al-Au alloys, gold was observed to accumulate preferentially at the triple points resulting in a non-uniform impurity profile across the interface 30. This was argued to be the result of a stress gradient 31 or by local Joule heating

30

that drives the impurities along the interface towards the triple points. After reaching

the sufficient enrichment level, the Cu atoms are oxidized and get incorporated in the barrier layer. This will predominantly take place at the triple points as the enrichment of alloying elements there is expected to be the highest

30

. The generated Cu(II) ions move through the alumina layer under the

influence of the electric field and dissolve into the solution at the pore base. The passage of the Cu2+ ions results in a void in the oxide layer giving the horizontal pore connection at the triple points. Even though the horizontal pore formation can be conceptually explained by the cyclic accumulation and dissolution of copper, the physical parameters that govern the distance between the horizontal connections are less clear. To elaborate on this question, we have systematically studied the influence of the temperature and the electrolyte on the horizontal pore formation in an Al-0.22 at.% Cu alloy. The formation of these horizontal pores is found to be accompanied with periodic oscillations superimposed on the anodic current plateau. We will show that these current density oscillations are linked to a cyclic variation in the electrode potential as a result of the cyclic enrichment and depletion of copper during the anodization process. We will also show that the horizontal and vertical interpore distance are linked and we propose a model to semi-quantitatively explain this relationship.

Experimental For the Al-0.22 at. % Cu films, 70 nm TiN was sputter deposited (ionization metallization plasma or IMP) on n-type Si wafers. Consequently, a 1 - 4 micrometer thick Al-0.22 at. % Cu alloy film was deposited by physical vapor deposition (PVD) on top of the TiN substrate. The 0.22 at.% Cu composition was


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determined with Rutherford Backscattering Spectrometry (RBS) and found to be uniform across the deposited alloy layer. The electrochemical cell for anodization consisted of a two-electrode setup with a titanium sheet as the counter electrode (cathode). A glass cylindrical cell with an exposed substrate area of 4.5 cm2 was used. Anodizations were performed in either 0.3 M H2C2O4 or 0.75 M H3PO4 solutions for applied cell voltages in the range of 20 V – 70 V and 50 V – 100 V, respectively. Unless otherwise mentioned, all experiments consisted of a single anodization step. For the two-step anodization, the aluminum oxide was removed with 1% HF for 1 min before the second step. The temperature of the cell was controlled by immersing the cell in a water bath (Haake C10). The cell voltage was controlled by an Autolab PGSTAT100 potentiostat in combination with a Autolab voltage multiplier allowing an applied voltage between 0 V and 100 V. After anodization, the bottom layer between the pores and TiN was selectively removed with “Standard Clean 1” (SC-1, 1:1:5 NH4OH:H2O2:H20) for 10 minutes at room temperature. In some cases, Ni nanowires were deposited in the pores galvanostatically at -5 mA cm-2 from a Watts type bath. Afterwards, the AAO template was selectively removed with 0.3 M KOH for 20 minutes. The structure of the pores and the nanowires was imaged with a scanning electron microscope (SEM Nova 200, FEI).

Results and Discussion Figure 1 shows the current density during anodization of Al-0.22 at. % Cu at 60 V in 0.3 M H2C2O4 at a fixed temperature of 30°C . The current density decreases at first due to the formation of a dense planar barrier oxide layer. After this initial drop, the current density rises again as a result of pore nucleation 32. Different than during mild anodization of pure Al, damped current density oscillations are observed for this alloy at all anodization voltages above 30 V (see Supplementary Information). The frequency of these oscillations (see inset of Fig. 1 drops from 95 mHz to 25 mHz when lowering the voltage from 70 V to 30 V. Below 30 V, oscillations are no longer observed. Damped oscillations are commonly found during anodization of Si

33,34

, Al (hard anodization)

11

, InP

35

, and Al-2.7 at. % Cu


15

. The oscillating


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behavior in anodic systems is often triggered by the interplay between an inhibiting and an accelerator species that passivates and promotes the growth of the oxide layer respectively 33.The fact that these oscillations are not seen under the same conditions for pure Al films indicates that the presence of Cu atoms in the dilute alloy is causing these oscillations. After the oscillations, a steady-state current jsst is obtained until the barrier layer reaches the underlying TiN after which the current drops (not shown). The steady-state current density jsst increases with anodization voltage very similar to what is found for pure aluminum anodization 36. The resulting pore structure was investigated with cross-section SEM. Figure 2a shows the SEM image of the resulting porous template after anodization in 0.3 M H2C2O4 at 70V. The figure reveals a template with vertical and horizontal pores. This observation is similar as observed by Molchan et al. 14, where horizontal pore branches were visible after anodization of a dilute Al- 0.05 at. % Cu alloy. To better visualize the interconnected structure, nickel nanowires were deposited in the porous template yielding a negative image of the AAO template. First, the bottom layer between the pores and the underlying TiN was selectively removed with SC-1, as taught in 37. After deposition of the Ni nanowires, the template was removed with 0.3 M KOH. Figure 2b clearly reveals the horizontal branches of the Ni nanowires after removal of the alumina template (50 V in 0.3 M oxalic acid). Moreover, it can be seen from Figure 2c that the ~ 4 µm freestanding nanowires did not collapse after template removal due to these horizontal interconnects. The horizontal pore structure parallel to the film surface is shown in Figure 2d after a two-step anodization where the oxide layer after the first anodization step was removed with 1 % HF. A threelegged star can be seen connecting three adjacent pores to a central focal point which has no pore. This is consistent with the horizontal pores being directed towards a triple point of the hexagonal unit cells as also shown in 14. Figure 2e gives a top-down image of the Ni nanowires after template removal. It is


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clear that also the Ni nanowires are horizontally interconnected. Note that these horizontal interconnections prevent the nanowires from collapsing as it is the case for long free-standing nanowires obtained from the vertical AAO pores from pure aluminum films 4. When the electrolyte type is changed to H3PO4, the situation during anodization becomes somewhat different. Figure 3a shows the current density during anodization at 100 V in 0.75 M H3PO4 at 30°C. Similar to H2C2O4, the current density drops initially after which it rises again and current oscillations are observed. The frequency of the oscillations fos remains more or less constant with anodization voltage at 14 mHz. Different than for H2C2O4, periodic spikes appear in the steady-state regime for H3PO4. These spikes are most prominent for voltages above 80 V and are small or absent below 80 V. The frequency of the spikes fsp differs from those of the sine-wave type current oscillations and depends on the anodization voltage. Indeed, the frequency increases from 5.5 mHz to 9 mHz when increasing the voltage from 80 V to 100 V. It appears that both phenomena - the oscillations and the spikes - are not directly connected as they both occur in parallel without any synchronization. The cross-section of the resulting template is shown in Figure 3b. In contrast to the regular and straight porous structure in oxalic acid, the pores are less cylindrical leading to a stacked “molar tooth” like morphology. Interestingly, the vertical pores seem to exhibit an oscillating diameter. Figure 4 shows the center to center distance or pitch between the horizontal pores, dhorizontal, versus anodization potential in H2C2O4 and H3PO4 solutions. The pitch increases linearly with anodization voltage, roughly 1.1 nm V-1 in our case. To relate the observed pitch to the current density oscillations, the charge density between two current density peaks was calculated from, jsst, and the oscillation period, P, by taking into account the anodization efficiency, . This charge density was converted into equivalent Al thickness using Faraday’s law and assuming the molar volume of aluminum. Finally, to



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calculate the equivalent AAO thickness increase, dcharge,, the equivalent Al thickness is multiplied with the empirical expansion factor !:

"#$%&'( =

)**+ ,∙.∙/01 ∙2

[2]

34

with VAl the molar volume of Al, n the amount of electrons involved in the reaction, and F Faraday’s constant. The expansion factor α was determined by measuring the height difference with DEKTAK between the anodized and the initial film. This expansion factor increases with voltage from 1.3 to 1.45 in oxalic acid (30 V – 70 V) and from 1.28 to 1.44 in phosphoric acid (50 V – 100 V). The anodization efficiency 5 was determined by relating the total charge density of anodization qtot to the thickness of the initial film tAlCu using Faraday’s law. For simplicity, the material properties of pure Al were used for these calculations:

5=

3460178 9+:+/01

[3]

For oxalic acid, this value was fairly constant around 0.84 for all cell voltages. In contrast for H3PO4, these values decrease significantly with anodization voltage from 0.69 at 50 V to 0.43 at 100 V. The reduction of anodization efficiency might be related to the spikes observed only for H3PO4 as this decrease was not seen with H2C2O4. The anodization efficiency did not change appreciably when different electrolyte concentrations or temperatures were used. A good correlation between dhorizontal and dcharge is found in Figure 4 validating the relation between the current oscillations and the occurrence of horizontal pores. An excellent agreement between the charge per oscillation and the horizontal pitch was found previously for the Al- 2.7 at.% Cu alloy 15. Interestingly, a similar observation was also found during Si anodization 34, where the charge between oscillations showed a similar trend with the pitch between porous horizontal layers. Note that in this case, the


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oscillations are not impurity driven but due to the cyclic formation and dissolution of a passivating oxide film. The corresponding AAO thickness for the current density spikes, observed for H3PO4 and calculated with equation 2, is significantly larger than dhorizontal and dcharge. Therefore, no relation was found between the horizontal pore formation and the spikes in the current density during anodizing. Instead, there might be a relation between the observed molar tooth like morphology and the current density spikes which were both only visible when the anodization was performed in phosphoric acid. Further study is however required to further elucidate the origin of the spikes during anodization. In the introduction it was discussed how impurities such as copper in aluminum, are plowed downward during anodization giving rise to an impurity-free oxide and a thin interfacial layer at the oxide/alloy interface where the impurities are accumulated. This process continues until a critical impurity concentration is reached. For copper in aluminum this critical concentration was determined at 6 x 1015 atoms cm-2 26. After this point, copper gets oxidized and is incorporated in the aluminum oxide which coincides with void formation. For our Al-0.22 at. % Cu alloy this critical concentration would be obtained after 450 nm of anodization in the case of a planar barrier layer. However, the horizontal pore formation already starts after ~100 - 200 nm. This would seemingly imply that the critical concentration was not reached at pore formation. However, the barrier layer is not planar but has a strongly concave shape. The copper in the interfacial layer is therefore not uniformly distributed. Indeed, initially the dilute copper concentration is the same around the hemispherical oxide/alloy interface (see Figure 5). During anodization, the oxide/metal interface moves from the pore bottom towards the triple point 38. Due to the plastic flow of the barrier oxide layer

31,39

, copper impurities are fixed at the interface and

thus transported to the triple points as well 14. However, because copper cannot enter the oxide, thus also not enter the pore wall at the triple point, copper accumulates at the triple point right underneath the pore wall. In this way, the critical concentration can be reached much faster locally at the triple points compared to a planar barrier layer.



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To elucidate this transport mechanism and the origin of horizontal pore formation, the effect of bath temperature was studied. The anodization voltage was kept constant at 50 V in 0.3 M oxalic acid while the temperature varied from 0°C to 60°C. Figure 6 (left y-axis) shows an Arrhenius plot of the steadystate current density jsst. On the right y-axis, dcharge is plotted according to equation 2. The steady-state current density decreases with temperature in an Arrhenius-like fashion. An activation energy of 0.34 eV is found for the current density and only slightly depends on the anodization voltage. This activation energy is related to the rate-limiting step and may involve contributions from the electric field and chemical dissolution. The charge between the current density oscillations, however, remains fairly constant over the inspected temperature regime. This is because at elevated temperatures a higher oscillation frequency was found to match the increased current density. Also with SEM analysis, the pitch between the horizontal pores was seen not to change significantly with temperature. Consequently, this suggests that the horizontal pore formation is not influenced drastically with temperature. Figure 7 shows the steady-state current density (left), the proton activity (left-grey) and "#$%&'( (right) versus H2C2O4 concentration. The steady-state current density increases significantly with oxalic acid concentration and follows the same trend as the proton activity suggesting jsst is controlled by the pH at the oxide/electrolyte interface. In contrast, "#$%&'( changes only slightly with oxalic acid concentration with a small decrease for increasing [H2C2O4]. The change in pore spacing for different electrolyte concentrations is less than 10 nm and could therefore not be observed conclusively with SEM. Therefore, the electrolyte concentration and acidity do not significantly influence the horizontal pore formation. In addition in Figure 6 and 7, it was shown that the steady-state current density jsst depends strongly on the temperature (Figure 6) and acidity (Figure 7) of the solution. However, in both cases the


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spacing between the horizontal pore was not altered. This implies that also the rate of oxide growth, which is proportional to jsst, does not affect the horizontal pore formation. Figure 8 shows the pitch between the horizontal pores "$;& versus the pitch between the vertical pores "?(&6 . The data is taken from anodizations performed in H2C2O4 and H3PO4. A linear relation is found between "$;& and "?(&6 with a slope equal to around 0.5 and the intercept of 58 nm. The fact that this relation is observed with two different electrolytes indicates a more general mechanism lies behind this observation which will be further discussed in the following section.

Interpretation Current density oscillations during copper accumulation and depletion The current density during anodization of Al-0.22at.%Cu consists of a constant steady-state current density jsst and an oscillating component superimposed on it. The oscillations in current density were found to be linked to the formation of horizontal pores. The steady-state current density was proportional to the acidity of the solution which indicates it is controlled by the dissolution at the oxide/electrolyte interface. In contrast, the charge between the oscillations was independent of the acidity, the temperature and @AA6 . We will argue that the current density oscillates as a result of an oscillating anode potential, B%3;C( as shown in Figure 9. The cell voltage ΔE is fixed in our experiments and equals the voltage drop between the two electrodes: ΔE = FB%3;C(,(9 + |I% |J − FB#%6$;C(,(9 − |I# |J + KLA;> + M;N

[4]

with B#%6$;C(,(9 and B%3;C(,(9 the equilibrium potentials of the cathode and anode respectively, I# and I% the respective overpotentials at the cathode and anode, K the current, LA;> the solution resistance, and M;N the potential drop across the oxide layer. The equilibrium electrode potential of the cathode



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B#%6$;C(,(9 remains more or less fixed, whereas the equilibrium anode potential B%3;C(,(9 will change during anodization thereby affecting the anode electrode potential, B%3;C( = B%3;C(,(9 + I% . The equilibrium anode potential in this case is given by: OP

B%3;C(,(9 = B%3;C(.(9 − 34 lnSTU> V

[5]

with TU> the activity of Al at the metal/oxide interface and B%3;C(,(9 the equilibrium potential at OP

standard conditions where B%3;C(,(9 = BU> ln STU>WX,(9 V with the equilibrium activity of Y WX /U> + 4

in the oxide film an unknown constant. In the case of anodization of pure aluminum metal, TU> = 1 at all times. In the case of the alloy, TU> < 1 when impurities accumulate below the oxide layer. In the accumulation phase, only the aluminum of the alloy can be oxidized as the anode potential is too negative for copper oxidation (see solid line in Figure 9, BU> WX /U> = −1.66 E\]^ versus B_` aX /_` =

0.34 E\]^ ). As a result, the copper remains behind at the alloy/alumina interface during oxidation of the alloy. The gradual accumulation of copper reduces the aluminum activity, TU> , which causes a positive shift in anode potential (downwards in Figure 9). This effect is the most pronounced at the triple point where the copper builds up during pore growth. As the total applied cell voltage in equation 4 is constant, the positive shift in the anode equilibrium potential must be compensated by a decrease in either ηa, ηc, KLA;> , M;N , or a combination of the previous terms. In all cases will this lead to a reduction in current density. Obviously the ohmic drop over the solution KLA;> only decreases with current density. Alternatively, lowering the anode or cathode overpotential results in a lower current density. Finally, a reduction in potential drop across the oxide M;N leads to a decrease in current density when assuming the simplified high-field ionic conduction 11:


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d ~ de exp i

jk:l n 6m

[6]

with de and o material-dependent parameters, and pq the barrier layer thickness. Therefore, the accumulation of copper at the triple points leads to a decrease in current density. The decrease in current density also follows from the anodic branch of the Butler-Volmer equation when correcting it for the copper coverage, r_` , at the oxide/alloy interface: 2 4

t d = d × S1 − r_` V exp i OP I% n

[7]

with i0 the exchange current density and αa the transfer coefficient for the oxidation reaction. A critical concentration of 6 x 1015 cm-2 in a 1 – 2 nm slab corresponds to about 1 – 0.5 monolayer of fcc copper and hence r_` approaches unity. As a result, the current density at the triple points almost entirely disappears upon accumulation of Cu. Because of the decrease in current density, M;N (at the triple point), KLA;> , I# and possibly I% will continue to decrease as well. As a result, according to equation 4 this allows for an increase in the total anode potential until this reaches the potential for Cu oxidation. This can also be seen in Figure 9 with the dashed lines. After Cu is oxidized and enters the oxide layer, the activity of aluminum atoms at the interface restores to its initial concentration and the anode potential decreases again. As a result, the current density starts to increase again. The accumulation and oxidation of copper is thus translated in a decrease and increase in current density respectively. Because the cyclic nature of this process, oscillations in the current density are observed. As a final remark, we would like to point out that M;N does not only decrease with current but also with decreasing oxide thickness. Therefore, the barrier thickness can also oscillate in parallel with the current density oscillations as a result of the potential variation, which was observed for higher cell voltage anodizations in H3PO4.



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The damping of the current oscillations is most probably the effect of non-uniformity in the current distribution over the sample 40. The local current density near the edge (current terminal) is larger than that in the center of the sample resulting in a faster accumulation rate than in the center of the sample. Hence the horizontal pore formation and related current oscillations are ahead of those in the center of the sample. As such different local current density oscillations will cancel each other out and only the average jsst is observed after some time. The origin of the current density spikes which seemed to be unrelated to the current density oscillations is yet unclear. The sudden nature of these events could have parallels with the formation of metallic filaments or shorts as in resistance-random access memory (R-RAM) devices 41. The change in M;N together with modulation in anode potential could indeed cause formation of copper filaments in the oxide. The decrease in current efficiency, ε, for the copper alloys as compared to pure aluminum anodization might be related to the oxidation of O2- anions in the oxide. As the anode equilibrium potential for the alloys shifts more positive as compared to pure aluminum, the oxidation of O2- ions may indeed become energetically favorable. Also the local drop in aluminum oxidation current may shift the advantage towards the parasitic O2- oxidation partial current. The oxidation of O2- anions is often linked to the formation of the voids during Al-Cu anodization as oxygen gas disturbs the oxide 23. Whereas we cannot exclude this mechanism entirely, the passage of Cu2+ cupric ions through the oxide can also account for the preferential etching of the horizontal pores. Upon oxidation the Cu2+ ions quickly drift along the electric field lines; i.e. in the shortest line from the triple point to the pore opening where they dissolve into the solution. The passage of the cupric ions disturb the alumina structure which gives enhanced dissolution also along the ion track. The enhanced dissolution is similar to track etching difference that the track formation and etching occur simultaneously.


42,43

with this

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Relation between distance between horizontal and vertical pores The steady-state formation of porous aluminum oxide can be viewed as the continuous creation (at the bottom) and destruction (at the triple points) of the metal/oxide interface (see Figure 5). In steady-state conditions, the creation of new interface has to be equal to its destruction. During the course of pore growth, an atom at the interface moves along the interface towards the triple points where it disappears into the pore wall. Of course, this is only the case for neutral atoms that are not influenced by the electric field in the oxide layer. Cu atoms at the interface are neutral species and are therefore transported towards the triple points. The distance l across the metal/oxide interface from the pore center to the triple point is proportional to the vertical interpore distance "?(&6 assuming the metal/oxide interface is semicircular 38: u

= v "?(&6

[8]

When the distance between the center of the pore and the triple point increases, Cu atoms need to travel a longer distance to reach the triple point. The horizontal interpore distance "$;& can then be estimated from the oxide growth rate (~ jsst) and the time tCu needed to reach the triple point: "$;& ~ @AA6 ∙ p_` = @AA6 ∙ ?

> wx+yz{t|y

=

u )**+ " v ?wx+yz{t|y ?(&6

[9]

With vinterface the velocity of the interface towards the triple point and "?(&6 the vertical interpore distance. To obtain the direct relation between the horizontal and vertical interpore distance, the ratio between jsst and vinterface has to be constant. Oh et al already argued that for steady-state conditions, it is essential that the velocity of the metal/oxide interface is proportional to the steady-state current density

38

. Therefore, a linear relation between the horizontal and vertical interpore distance is

expected.



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Page 16 of 33

Not only do the Cu atoms have to be transported towards the triple points, the accumulated Cu should also be kept there. For example with Au impurities in a porous structure 30, gold was not incorporated into the barrier layer but entered the space between two adjacent barrier layers in the pore wall. This is striking because incorporation of Au atoms is expected from thermodynamic considerations when a sufficient enrichment is obtained 24. Therefore, to obtain horizontal branching, it is also important that the degree of accumulation at the triple point is not lost before the impurity is incorporated in the barrier layer. The parameters that exactly govern this phenomenon are to this point still unclear, but are most likely related to their low incorporation tendency reflected by their high Gibbs free energy of oxide formation respective with Al

21

and its low solubility in the electrolyte in the absence of a complexing

agent. Additionally, the migration velocity of the impurity species in the barrier layer also has to be sufficiently large for the species to be ejected in the solution. It was shown that slow-migrating species such as W tracers remain in the pore wall 39 while the fast-migrating species are ejected into the solution 44. This is because stress-induced plastic flow in the barrier layer pushes the ions in the oxide toward the pore walls before they could reach the oxide/electrolyte interface. The migration speed of Cu2+ in aluminum oxide was measured to be relatively high and hence the Cu atoms are indeed ejected in the solution. A final requirement to obtain horizontal pores is that the impurity ions have to be soluble in the used electrolyte. Otherwise, either the ions retain in the oxide layer or are precipitated at the oxide/electrolyte interface and the oxide growth will be self-limiting.

Conclusions In summary, the formation mechanism of a 3-dimensional template from a dilute Al-0.22at. % Cu alloy was discussed in this paper. The current density oscillations during anodization are directly related to the distance between the horizontal pores. These oscillations are due to the cyclic oscillations of the


Page 17 of 33

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anode potential as a result of the periodic accumulation and depletion of copper at the metal/oxide interface. For anodizations in H3PO4, additional current density spikes are present resulting in a pore structure that exhibits a more tooth like morphology. The transport time of Cu impurities towards the triple points determines the distance between the horizontal pores. As a result, the distance between the horizontal pores increases with the vertical interpore distance, whereas it remains invariant for changes in the steady-state current density, the temperature, or the electrolyte concentration. Based on a simple model, a qualitative relationship between the horizontal and vertical interpore distance is indeed predicted. The 3 dimensional porous template is used to fabricate non-collapsing 3D Ni nanowires.

Acknowledgements Johannes Vanpaemel gratefully acknowledges the support of a Ph.D. stipend from the Agency for Innovation by Science and Technology (IWT).



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Page 18 of 33

Figure captions Figure 1: Current density vs. time during the initial phase of anodization of Al-0.22 at.% Cu at 30°C using 0.3 M oxalic acid at 60 V. The current density remained the same after 150 s and only dropped when the pore front reached the underlying TiN. The inset shows the frequency of the oscillations for different anodization voltages. Figure 2: SEM images of (a) X-section of 3D porous AAO template at 70 V in 0.3 M H2C2O4, (b) and (c) Ni nanowires after etching of the AAO template (50 V in 0.3M H2C2O4). (d) Top view of the template after two step anodization (50 V in 0.3 M oxalic acid for 1 C, 1 minute of 1% HF clean, 50 V in 0.3 M oxalic until bottom). (e) Top view of nickel nanowires after etching of the template. Figure 3: (a) Current density vs. time during anodization of Al-0.22 at.% Cu at 30°C using 0.75 M phosphoric acid at 100 V. The inset shows the frequency of the spikes fsp for different anodization voltages.(b) Crosssection of template after anodization at 90 V in 0.75 M H3PO4 at 30°C. Figure 4: Distance between horizontal pores observed with SEM (dhorizontal) and calculated from the current oscillations (dcharge) versus anodization potential for 0.3 M oxalic acid and 0.75 M phosphoric acid. Figure 5: Schematic view of the AAO template during anodization. Copper impurities are initially distributed equally across the oxide/metal interface. As a result of interface motion, the copper impurities are dragged along towards the triple points of the structure. The dotted lines represent their motion during pore growth. Because Cu initially cannot incorporate into the barrier layer, it accumulates at the triple points. Figure 6: Arrhenius plot of the steady-state current density (left y-axis) and dcharge (right y-axis) for anodization at 50 V using 0.3 M H2C2O4. The error bars for the current density range between 2 – 5 %. Figure 7: Steady-state current density, proton activity (left y-axis) and dcharge (right y-axis) versus oxalic acid concentration for anodizations at 50 V. The proton activity was measured with a calibrated pH meter. Figure 8:


Page 19 of 33

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Pitch between the horizontal pores versus the pitch between the vertical pores for anodizations in 0.3 M H2C2O4 (30 V – 70 V) and 0.75 M H3PO4 (80 V – 100 V). Figure 9: Potential distribution across the electrochemical cell (anode, oxide, solution, and cathode) starting from the triple point at the oxide/metal (O/M) interface. For simplicity, the potential across each interface was assumed to be sharp. The solid line represents the situation before accumulation of Cu at the triple and the dashed line corresponds to the situation after Cu accumulation. The potential difference between the anode and cathode at all times equals the cell voltage ∆E.



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Page 20 of 33

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W. Wang, M. Tian, A. Abdulagatov, S. M. George, Y.-C. Lee, R. Yang, Three-Dimensional Ni/TiO2 Nanowire Network for High Areal Capacity Lithium Ion Microbattery Applications. Nano Lett. 2012, 12, 655–660

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P. Skeldon, G. E. Thompson, G. C. Wood, X. Zhou, H. Habazaki, K. Shimizu, Evidence of Oxygen Bubbles Formed within Anodic Films on Aluminium-Copper Alloys. Philos. Mag. A 1997, 76, 729– 741

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Graphical abstract


Page 24 of 33

Page 25 ofJournal 33 of Physical Chemistry 25 The jsst

ji/imAicm-2

1 20 Voltagei(V) fi(mHz) jssti(mAicm-2) 2 30 25.2 5.7 3 15 40 9.1 37.8 4 50 13.5 40.3 5 10 60 21.25 72.5 6 30 95.2 70 7 o C O ati30 C 8 5 ACS Paragon60Viini0.3MiH 2 2 4 Plus Environment 9 0 25 50 75 100 125 150 10 Timei/is 11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SEM images of (a) X-section of 3D porous AAO template at 70 V in 0.3 M H2C2O4, (b) and (c) Ni nanowires after etching of the AAO template (50 V in 0.3M H2C2O4). (d) Top view of the template after two step anodization (50 V in 0.3 M oxalic acid for 1 C, 1 minute of 1% HF clean, 50 V in 0.3 M oxalic until bottom). (e) Top view of nickel nanowires after etching of the template. 233x63mm (150 x 150 DPI)


Page 26 of 33

20

Page 27 of 33

jn/nmAncm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

a


Voltagen(V) fosn(mHz) fspn(mHz) 5.5 13.9 80 90 100

15

14.7 14.5

7.5 9.0

10

5

100Vninn0.75MnH3PO4 atn30oC 0

500

1000

1500


Timen/ns

b

250

dhorizontal )Vdcharge /Vnm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

225 200


Page 28 of 33

SEMVpoxalicVacids ChargeVpoxalicVacids SEMVpphosphoricVacids ChargeVpphosphoricVacids

175 150

4 O P

H3

125 100

O 4 C 2 H2

75 20

40

60 ACS Paragon Plus Environment

80

AnodizationVvoltageV/VV

100

Cu impurity

The Page Journal 29 ofof 33Physical Chemistry

1 2 3 4 5 6 7 ACS 8 Paragon Plus Environment 9 10 Al-0.22at.%Cu

10

jsst / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40


160

Page 30 of 33

dcharge jsst

140 120

101

100 80

34

36

38

40

(kBT)-1 / (eV)-1 ACS Paragon Plus Environment

42

60

dcharge / nm

2

jsst / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

25

aH + 0.20

20 0.16


160

aH+ jsst

140

dcharge 120

15 0.12 100 10

0.08

80 5

0.04

0.0

0.2

0.4

0.6


H2C2O4 / M

0.8

1.0

60

dcharge / nm

Page 31 of 33


2 5 0

P itc h h o r iz o n ta l p o r e s / n m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Page 32 of 33

2 0 0

1 5 0

1 0 0

5 0 1 0 0

2 0 0

P itc h v e r tic a l p o r e s / n m ACS Paragon Plus Environment

3 0 0

Page 33 TheofJournal 33 Oxide of Physical Sol. Chemistry Anode Cathode 1 U 3+ No accumulation 2 Al /Al Accumulation 3 4 5U 2+ ox Cu /Cu 6 7 8 9 IRsol 10 11 ACS Paragon Plus Environment 12 Ucathode 13 14

φ

ΔV

The Formation Mechanism of 3D Porous Anodized Aluminum Oxide

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