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Formation and microstructures of unique nanoporous AAO films fabricatedby high voltage anodization

Li Yi, Ling Zhiyuan,* Hu Xing, Liu Yisen and Chang Yi

Received 22nd February 2011, Accepted 19th April 2011

DOI: 10.1039/c1jm10781j

Nanoporous anodic aluminium oxide (AAO) films with a large-range tunable interpore distance (Dint)

and unique microstructures have been fabricated by high voltage anodization (140–400 V) in 0.3 M

oxalic acid electrolyte. The influences of anodization conditions (e.g., anodization voltage (Ua) and

current density (ia)) on the microstructures of AAO films have been investigated in detail. Experimental

results show that there is a linear relationship between Ua and Dint under relatively low Ua. With the

increase of Ua, the influences of ia and dehydration of aluminium hydroxide on the Dint may not be

ignored, thus resulting in a nonlinear relationship between Ua and Dint. When the anodization is

performed at an excessively highUa of 400 V, an interesting competitive growth of nanochannels inside

the AAO film tends to occur, thus forming an AAO film with unique nano/micro morphology at the

barrier layer side.

1. Introduction

As a typical self-assembly material, nanoporous anodic

aluminium oxide (AAO) films formed by the anodization process

have been investigated for many years,1–7 and have become one

of the most commonly used templates for the fabrication of

various nanostructured materials such as nanowires, nanotubes

and nanopores,8–11 since AAO films have a highly ordered 2D

nanoporous array with controllable interpore distance (Dint),

pore size (Dpore) and aspect ratio by varying the anodization

conditions.12–15

Clearly, the microstructural parameters of AAO films (e.g.,

Dint and Dpore) will directly influence their performance in all

these applications. Considering that the Dpore can be adjusted

and controlled by a further chemical etching process, and the

maximum Dpore is dependent on the Dint, developing proper

methods for the fabrication of AAO films with a large-range

tunable Dint has attracted much attention.15–21 As we know, the

Dint of AAO films increases with the anodization voltage

(Ua).15–21 So performing the anodization under a Ua as high as

possible is an effective way to obtain a larger Dint. In addition,

high voltage anodization processes can also provide a faster

AAO film growth rate and a wider self-organized window.15–21

However, there are still some drawbacks in these processes: (1)

under fixed anodization conditions (e.g., the voltage increasing

process, electrolyte concentration and temperature), there is

always an upper limit value of Ua, over which the ‘‘burning and

Department of Electronic Materials Science and Engineering, College ofMaterials Science and Engineering, South China University ofTechnology, Guangzhou, 510640, People’s Republic of China. E-mail:imzyling@scut.edu.cn; Fax: +86 20 8711 1224; Tel: +86 20 8711 1224

This journal is ª The Royal Society of Chemistry 2011

breakdown’’ phenomenon will occur during the voltage

increasing process,15 (2) the cell separation phenomenon easily

occurs, resulting in a relatively weaker mechanical robustness of

the as-prepared AAO film,16,19,20 (3) the relationship between Ua

and Dint is usually nonlinear, thus it is difficult to control the

microstructural parameters of the AAO film. Therefore, finding

proper methods for the fabrication of designated AAO films with

accurately controllable microstructural parameters by high

voltage anodization, is significant in both scientific research and

commercial applications. In the present work, a simple and

reliable approach is proposed for the fabrication of AAO films

with a large-range tunable Dint in 0.3 M oxalic acid electrolyte at

a high Ua of 140–400 V. By using an improved voltage increasing

process, the ‘‘burning and breakdown’’ phenomenon can be

effectively avoided. Based on the experimental results, the

influences of anodization conditions (e.g., anodization voltage

and current density (ia)) on the microstructural parameters of

AAO films have been investigated in detail. To explain the AAO

cell separation phenomenon and the nonlinear relationship

betweenUa andDint, the formation and dehydration processes of

aluminium hydroxide at the AAO cell walls have also been dis-

cussed. Moreover, a new type of AAO film with a unique nano/

micro microstructure has also been realized via a competitive

nanochannel growth process under an excessively high Ua of

400 V, widening the potential applications of AAO films.

2. Experimental

Highly pure aluminium sheets (99.99%) with a thickness of 1 mm

were used as the starting materials. The sheets were electro-

polished at 0–5 �C and 21 V for 5 min in a mixture solution of

perchloric acid and ethanol (1 : 4, v/v) to diminish the roughness

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of their surface. Then the sheet was carefully washed and put into

a tailor-made holder with a square area of 1 cm2 exposed to the

electrolyte. Anodization was conducted in a 0.3 M oxalic acid

electrolyte at 0–5 �C and 140–400 V, with vigorous magnetic

stirring. A powerful low temperature bath, together with an

electrolysis cell (1 L) was used to maintain the low temperatures

required for the anodization. In addition, an improved voltage

increasing process was used: (1) the Ua was increased at a rate of

1 V s�1 until the ia reached 500 A m�2, (2) a constant-current

process was applied, by which the Ua can be smoothly increased

to the high target value (140–400 V), (3) the anodization was

conducted at a designated Ua with a constant-voltage mode. All

the obtained samples were immersed into a saturated CuCl2solution to remove the residual aluminium. The microstructures

of the as-prepared specimens were examined by a field-emission

scanning electron microscope (FE-SEM: LEO 1530 VP). The

anodization voltage and current were measured by multimeters

(Keithley 2010 and 2410).

Fig. 1 SEM images of the barrier layer side of highly ordered AAO films

fabricated in 0.3 M oxalic acid electrolyte (a) 140 V, 160 Am�2, (b) 165 V,

170 A m�2, (c) 190 V, 210 A m�2, (d) 215 V, 200 A m�2, (e) 260 V,

190 A m�2, (f) 310 V, 150 A m�2. Scale bars ¼ 300 nm.

Fig. 2 The evolution of average interpore distance (Dint) as a function of

anodization voltage (Ua). The data marked by filled black squares were

obtained in our experiment, corresponding to the samples shown in Fig. 1.

The datamarked by open black circles were proposed in ref. 15. The black

solid line represents the proportionality constant (z) of 2.0 nm V�1.

3. Results and discussion

3.1 Interpore distance of AAO films

3.1.1 The influence of Ua and ia. In the former studies, the

most commonly used AAO film fabricated in oxalic acid elec-

trolyte has been prepared at 40 V with a solution concentration

of 0.3 M.6 To suppress the burning and breakdown phenomenon

at a highUa (>100 V), Lee et al. found that the voltage increasing

process should be improved, and the maximum Ua can thus be

increased to 155 V in the same electrolyte.15Moreover, it was also

found that the ia plays an important role in the anodization

process.7,20–25 Generally speaking, ia can be divided into two

parts: ionic current density (ii) and electronic current density (ie).

The ii is used to form AAO and the ie can directly influence the

stability of the anodization process: the burning and breakdown

phenomenon will occur when the ie reaches a critical value.24

Considering that the ie is only a part of the ia (ia ¼ ii + ie), keeping

a steady ia is an effective way to obtain a controllable ie during

the initial voltage increasing process, thus avoiding the burning

and breakdown phenomenon under a high Ua. Accordingly,

a constant-current nonlinear voltage increasing process has been

performed in the present work, by which the maximumUa can be

successfully increased to 400 V in 0.3 M oxalic acid electrolyte.

Fig. 1 shows the SEM images of the barrier layer side of AAO

films fabricated under a Ua of 140–310 V with different ia. It can

be seen that their hexagonal structural cells are arranged with

a representative localized ordering, which has a similar

morphology to that reported before.15 In order to investigate the

influencing factors on the Dint, the evolution ofDint as a function

of Ua was measured (Fig. 2). In Fig. 2, it can be seen that the Dint

increases with the Ua, and the data proposed by Lee et al. (<140

V) agrees well with the proportionality constant (z) of 2.0 nm

V�1.15 However, it may only be a special case which occurs under

a special self-organized regime, depending on the anodization

conditions such as anodization voltage, current density and

electrolyte concentration. Obviously, the Dint obtained under

a Ua > 140 V shows a relatively gentle rise as the Ua increases. In

addition, the z exhibits a divergence under certain values of the

Ua. It was found that the Dint is not only dependent on the Ua,

9662 | J. Mater. Chem., 2011, 21, 9661–9666

but also influenced by the ia, and the corresponding equation can

be written as follows:20,21

Dint

Ua

¼ Aþ Be

�iaC : (1)

Where A, B and C are constants which are dependent on the

anodization conditions. According to eqn (1), the Dint and z will

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increase with the decrease of the ia. Therefore, it is not surprising

to see that the Dint obtained at 140 V in our experiment (160 A

m�2) is relatively larger than that reported before (230 A m�2).15

In addition, the divergence of the data that deviate from the

linear relationship of z ¼ 2.0 nm V�1 (<190 V) can also be well

elucidated by eqn (1). However, it can be seen that the z under the

condition of Ua > 215 V increases with the ia, which cannot be

reasonably elucidated by eqn (1), but may be ascribed to the

formation and dehydration of aluminium hydroxide at the AAO

cell walls during high voltage anodization.

3.1.2 The influence of hydroxide. It has been known that the

formation and dehydration of aluminium hydroxide (e.g., Al

(OH)3) during the high voltage anodization process can directly

influence the microstructural parameters of the as-prepared

AAO.19,26 In fact, a large amount of O2� and OH� can be

produced under a high enough Ua (Fig. 3a), according to the

hydrolysis equation shown as follows:7,14,25

Al2O3 þ nH2O/2Al3þ þ ð3þ n� xÞO2�

þxOH� þ ð2n� xÞHþ (2)

For these two types of anions, most of the O2� will be consumed

and released in the form of O2, due to the extremely high ieaccompanied by a high voltage anodization (Fig. 3b).24,26 As

a result, most of the Al3+ tends to combine with the OH� instead

of O2�, thus forming Al(OH)3 in the as-prepared AAO (Fig. 3b).

Considering that the dissociation energy of Al(OH)3 is relatively

small (3.80 kcal mol�1) and its dehydration reaction is endo-

thermic, Al(OH)3 will dehydrate easily and result in the forma-

tion of aluminium oxyhydroxide (AlOOH) under a high

temperature (i.e., high ia) accompanied by high voltage anod-

ization (Fig. 3c).19,26–28 This dehydration process will lead to the

volume contraction of the AAO cell walls, and a smaller Dint can

thus be obtained under the repulsive forces between the

Fig. 3 Schematic diagrams of the Al(OH)3 dehydration and volume

contraction process which occurs inside the AAO film during the high

voltage anodization process.

This journal is ª The Royal Society of Chemistry 2011

neighboring cells which are generated inside the AAO film during

the anodization process (Fig. 3c).29 It should be noted that more

Al(OH)3 can be formed and dehydrated under a higherUa during

high voltage anodization, thus resulting in a more apparent

volume contraction of the AAO cell walls. Accordingly, it is not

surprising to see that the z under the condition of Ua > 215 V

increases with the ia, but decreases with the Ua (Fig. 2), which

further means that the nonlinear relationship between Ua and

Dint caused by Ua cannot be ignored when a high enough Ua is

applied. In addition, a larger ia will lead to a higher temperature

in the AAO, thus resulting in a more apparent volume contrac-

tion and a smaller Dint under the same Ua (e.g., 140 V in Fig. 2).

According to this argument, the nonlinear relationship between

Ua and Dint caused by ia variation (i.e., eqn (1)) can also be well

explained.

3.2 A general growth model of AAO

In former studies, anodic aluminium oxide (AAO) and anodic

titanium oxide (ATO) are two nanoporous structural materials

which have been extensively studied in anodization processes. It

has been known that there are three representative morphologies

of the ATO: continuous nanoporous film, smooth separated

nanotubes and nanotubes with wall thickness variations.30,31 By

adjusting and controlling the anodization conditions, these

representative morphologies can also be realized and observed in

AAO.6,16,19,20,26 It is believed that the basic formation mecha-

nisms of AAO and ATO should be similar. However, it is still

difficult to reasonably explain the evolution of their micro-

structures and the z variation by using classical growth models

proposed before.27,29 Herein, we develop a general growth model

of AAO, and related schematic diagrams are shown in Fig. 4.

(I): In this type, the as-prepared AAO is formed as a ‘‘contin-

uous nanoporous film’’, usually corresponding to a relatively

lower Ua and ia. As a consequence, little Al(OH)3 is formed, and

the volume contraction caused by Al(OH)3 dehydration is rela-

tively slight. Therefore, a comparatively larger z of 2.5 nm V�1

can be obtained, which corresponds to the so called ‘‘mild

anodization’’ process.6,12,13With the increase ofUa, more O2� and

OH� can be produced, and more Al(OH)3 can be formed inside

the AAO film because of the increasing ie. As a result, a relatively

smaller z of approximately 2.0 nm V�1 can be obtained under the

coactions of Al(OH)3 dehydration (i.e., volume contraction) and

the repulsive forces between the neighboring cells inside the AAO

film, corresponding to the so called ‘‘hard anodization’’

process.15 It should be noted that in type I, most of the Al(OH)3 is

formed and dehydrated at the position near the pores instead of

the cell boundaries (see the arrows in Fig. 4a), since the migration

abilities of O2� and OH� are different.19,24,26 In this case, the as-

prepared AAO film is made up of anion-contaminated alumina

(near the pores) and relatively pure anion-free alumina (at the

cell boundaries),5,26,32,33 and the cleavage appears along the

pores.6,15

(II): In this type, the as-prepared AAO is formed as ‘‘smooth

separated nanotubes’’. With the further increase of Ua, most of

the O2� is consumed and released in the form of O2,24,26 and more

OH� can be driven by the extremely high electrical field to reach

the oxide/metal interface, thus forming an Al(OH)3 layer at the

cell boundaries.19,26 The dehydration of the Al(OH)3 layer will

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Fig. 4 Schematic diagrams of three representative morphologies of the

AAO: (a) Type I, continuous nanoporous film, (b) Type II, smooth

separated nanotubes, (c) Type III, nanotubes with wall thickness varia-

tions, the cell boundaries are marked by arrows shown in (a) and (b).

Fig. 5 SEM images of the AAO film fabricated in 0.3 M oxalic acid

electrolyte under a final Ua and ia of 400 V and 180 A m�2, respectively:

(a) the cross-sectional image of the upper and lower AAO films, (b)

a partially enlarged image of the fracture plane shown in (a), (c) the cross-

sectional image of the lower AAO film, (d) a partially enlarged image of

(c). Scale bars: 10 mm for (a), 1 mm for (b), 5 mm for (c), and 2 mm for (d).

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lead to volume contraction at the cell boundaries, resulting in the

formation of separated AAO nanotubes.16,19,20,26 Considering

that the Al(OH)3 layer is relatively thinner, the volume

contraction is slight, and separated AAO nanotubes with

a smooth external wall can be obtained.16,20 In this case, the

cleavage appears along the cell boundaries.16,20

(III): In this type, the as-prepared AAO is formed as ‘‘nano-

tubes with wall thickness variations’’. Compared with type II, the

Al(OH)3 layer at the cell boundaries is relatively thicker, and

the volume contraction is more apparent. Considering that the

direction of volume contraction of the Al(OH)3 layer can be

parallel to the growth direction of the AAO, separated AAO

nanotubes with wall thickness variations can be obtained, and

the cleavage also appears along the cell boundaries.19,26

Moreover, it is worth mentioning that the morphologies of the

AAO fabricated in different electrolyte systems are quite

different even under the same Ua. For example, under a Ua of

40 V, the AAO fabricated in oxalic acid and sulfuric acid elec-

trolytes will appear as ‘‘continuous nanoporous film’’ and

‘‘smooth separated nanotubes’’, respectively.6,16,20 Further

systematic studies are still needed to improve this general growth

model of AAO from a qualitative model to a quantitative one.

3.3 Unique nanoporous AAO films

Interestingly, a phenomenon accompanied by high voltage

anodization can be observed under the condition that a high

9664 | J. Mater. Chem., 2011, 21, 9661–9666

enough Ua is applied. Fig. 5 shows the SEM images of the AAO

film fabricated under a final Ua and ia of 400 V and 180 A m�2 in

0.3 M oxalic acid electrolyte. Apparently, a fracture plane can be

observed, and the AAO film is divided into two parts (Fig. 5a). It

can be seen that the bottom side of the upper AAO film is rela-

tively smooth, which has a similar morphology with that fabri-

cated via traditional anodization processes (Fig. 5a).6,12,13

However, the lower AAO film exhibits a novel morphology: its

barrier layer surface is rough, and made up of many microscale

(10–30 mm) hemispherical structures. Fig. 5b shows a partially

enlarged image of the fracture plane in Fig. 5a. It can be seen that

the AAO film at the fracture plane is made up of separated AAO

nanotubes (the circular region in Fig. 5b). Fig. 5c–d shows the

partially enlarged images of the lower AAO film shown in

Fig. 5a. It can be seen that each hemispherical structure consists

of many nanochannels developed in radial directions (Fig. 5c–d),

and branched nanochannels can also be observed (Fig. 5d), thus

resulting in a novel nano/micro structure. Fig. 6 shows the cross-

sectional schematic diagram of the AAO film shown in Fig. 5.

To explain these experimental phenomena one may examine

the Ua and ia transients during the anodization process (Fig. 7).

Apparently, two modes are applied in the whole anodization

process: first, a constant current (CC) mode is used during the

voltage increasing process, thus obtaining a high enough Ua, and

then a constant voltage (CV) mode is performed for the fabri-

cation of the AAO film under designated Ua and ia. In addition,

apparent oscillations of the Ua and ia can be observed during the

CC and CV modes, respectively. Similar phenomena can also be

observed in our former research.19,26 Moreover, the ia–time

transients show a decrease as a function of the anodization time,

which can be explained by the diffusion-limited anodization at

the pore bottom.15

Based on the experimental results and above arguments, the

formation process of the AAO film shown in Fig. 5 can be

illustrated as follows. At a certain stage of the voltage increasing

This journal is ª The Royal Society of Chemistry 2011

Fig. 6 The cross-sectional schematic diagram of the AAO film, corre-

sponding to the sample shown in Fig. 5.

Fig. 7 The evolutions of anodization current density (ia) and anodiza-

tion voltage (Ua) as a function of anodization time (t), corresponding to

the sample shown in Fig. 5. Two modes were used during the anodization

process: constant current (CC) mode and constant voltage (CV) mode.

Fig. 8 Schematic diagrams of the unsteady-state nanochannel growth

inside the AAO film, corresponding to the lower AAO film shown in

Fig. 5.

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process under the CCmode, a continuous nanoporous AAO film

can be obtained, just like those shown in Fig. 1. When the Ua is

increased over a critical value (Uc), the proportion of ie in the

whole ia will raise sharply, and a large enough ie can be obtained.

As a result, the formation and dehydration of Al(OH)3 at the

AAO cell boundaries will result in the formation of separated

AAO nanotubes,24,26 corresponding to the interface layer shown

in Fig. 6. It has been known that the AAO film which is made up

of separated AAO nanotubes always has a weaker mechanical

robustness, compared with the continuous nanoporous AAO

film,16,20,26,34 thus resulting in the occurrence of the fracture plane

under external mechanical forces (Fig. 5a–b). It is worth

mentioning that the Uc under our experimental conditions is

expected to be higher than 310 V since the fracture plane cannot

be observed at this Ua (Fig. 1f). When the anodization process is

This journal is ª The Royal Society of Chemistry 2011

performed under the CVmode, the ia and ie will decrease with the

anodization time (Fig. 7). As a consequence, the formation of Al

(OH)3 in the AAO film is decreased, and the morphology of the

as-prepared AAO will change from separated nanotubes to

continuous nanoporous film (Fig. 5–6).

In fact, the growths of nanochannels inside the AAO film are

not completely synchronous because of the heterogeneity of the

aluminium sheet (Fig. 8a). When the growth rate is relatively

slow, the growth processes of the nanochannels can be regulated

by the self-organized regime effectively, thus resulting in the

synchronous growth and ordering of the nanochannels, corre-

sponding to the steady-state anodization process.29 When the

growth rate is fast, the self-organized regime cannot play an

effective role. The faster-growth nanochannels will expand and

branch in a radial direction, and take over the original area of

neighboring nanochannels, since the bottom of each nano-

channel is similar to a hemisphere (Fig. 8b).35 During this growth

process, the nanochannels can develop based on some specific

relationships involving Dint, Ua, ia and other anodization

conditions (e.g., eqn (1)),20,21,35,36 and the growths of some

nanochannels will stop (Fig. 8c–d). Similar processes will

continue to occur, and the AAO film with a novel nano/micro

microstructure can be fabricated (Fig. 5–6 and Fig. 8e). This

unsteady-state nanochannel growth is quite different from that

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which occurs in conventional steady-state anodization

processes,6,12,13 but similar to the ‘‘competitive nanochannel

growth’’ reported before.35,37 It worth mentioning that such

a nano/micro AAO structure has rarely been reported in former

investigations, and may have diverse potential applications in

many fields, such as filters, superhydrophobic surfaces, etc.

4. Conclusions

In summary, nanoporous anodic aluminium oxide (AAO) films

with diverse microstructures and a large-range tunable interpore

distance (Dint) have been fabricated in 0.3 M oxalic acid elec-

trolyte under an anodization voltage (Ua) of 140–400 V. The

relationship between AAO microstructural parameters and

anodization conditions has been investigated in detail. Experi-

mental results show that both the Ua and anodization current

density (ia) can lead to the nonlinear relationship betweenUa and

Dint. The formation and dehydration of Al(OH)3 inside the AAO

film can play a very important role during the high voltage

anodization process, thus resulting in AAO films with diverse

microstructures (e.g., continuous nanoporous AAO film or

separated AAO nanotubes). In addition, the AAO film with

a novel nano/micro microstructure has been fabricated under

aUa of 400 V via an unsteady-state nanochannel growth process.

The AAO films obtained in this approach may have important

use in many related fields, and similar methods may also be

applied in other electrolyte systems to fabricate AAO films with

diverse microstructures.

Acknowledgements

This work was supported by the Fundamental Research Funds

for the Central Universities (2009ZM0095), the China Post-

doctoral Science Foundation funded project (20100470913), and

the Doctoral Fund of Ministry of Education of China

(20100172110011).

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