controlling factor of self-ordering of anodic porous alumina

Anodic porous alumina film, a typical self-ordered nanohole ma-terial formed by anodizing aluminum in an appropriate acidic solu-tion, is a promising candidate for starting materials of

voltage up to the self-ordering voltage, the ratio of pore diameter to cell diameter dpore/dcell lowered and converged to ⬃0.3

regardless of the electrolyte type Moreover, domains of highly self-ordered pore arrays were found in the film formed during

burning, where extremely high current was locally concentrated This suggests that the condition inducing film growth under high

current density, i.e., high electric field strength is the key controlling factor of self-ordering Based on the above knowledge, a new

self-ordered porous alumina with a 600 nm pore interval was fabricated in citric acid just under the critical voltage of burning.

© 2004 The Electrochemical Society 关DOI: 10.1149/1.1767838兴 All rights reserved.

Manuscript submitted September 9, 2003; revised manuscript received February 9, 2004 Available electronically June 25, 2004.

Anodic porous alumina film, a typical self-ordered nanohole

ma-terial formed by anodizing aluminum in an appropriate acidic

solu-tion, is a promising candidate for starting materials of

nanofabrica-tion of various devices.1-5Except for the pretexturing methods for

an aluminum substrate such as an imprinting process,6,7highly

or-dered self-organizing porous alumina can be obtained only in three

types of electrolyte at individually specified self-ordering voltages,

i.e., sulfuric acid at 25 V, oxalic acid at 40 V, and phosphoric acid at

195 V, giving 63 nm, 100 nm, and 500 nm pore intervals,8-10

respec-tively To widen the applications of anodic porous alumina, the

fab-rication of ordered porous alumina with an arbitrary pore interval is

required

Jessensky et al.11 and Li et al.12 suggested the correlation

be-tween the mechanical stress caused by volume expansion and

self-ordering of porous alumina However, the mechanism has not been

fully confirmed Therefore, we have investigated the self-ordering

behavior of anodic porous alumina formed in three major acid

elec-trolytes by focusing on the current density during oxide growth and

the effect of the ratio of pore diameter to cell diameter The ratio is

controlled by the electric field strength at the barrier layer of porous

alumina during anodizing, as reported previously by Ono and

coworkers.13-16Namely, the ratios of cell diameter dcellto pore

di-ameter dporeobtained at different voltages in different electrolytes

have a linear relationship with log of current density According to

the classical theory of ionic conduction at the high field strength for

the anodic barrier film grown on various metals,17,18the film

thick-ness of each metal is inversely proportional to the logarithm of ionic

current when the film is formed up to the same voltage Thus, it is

indicated that the log of current density log i is proportional to the

electric field strength E, i.e., the formation voltage/film thickness

ratio at the barrier layer.19The purpose of the present study is the

confirmation of the controlling factor of the self-ordering of porous

alumina and the fabrication of a new self-ordering film by applying

the proposed mechanism

Experimental

High-purity共99.99%兲 aluminum sheets were electropolished in a

4:1 mixture of ethanol and 60% perchloric acid at 10°C Anodizing

was performed at constant voltages in 0.3 mol dm⫺3sulfuric acid

solution at 20°C, 0.3 mol dm⫺3oxalic acid solution at 20°C, and 0.2

mol dm⫺3phosphoric acid solution at 0°C-5°C 2 mol dm⫺3citric

acid solution at 20°C was used to fabricate a new self-ordering film

with a large pore interval The pHs of the electrolytes used were 0.1

for sulfuric acid, 1.2 for oxalic acid, and 2.3 for phosphoric acid

To estimate the porosity of anodic alumina, voltage-time (V-t)

curves were measured during re-anodizing of the porous alumina specimens at 5 A m⫺2in a neutral mixed solution of 0.5 mol dm⫺3 boric acid and 0.05 mol dm⫺3sodium tetraborate 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 in-terface by the inward migration of O2 ⫺and the oxide/electrolyte interface by the outward migration of Al3 ⫹ Porosity␣ in the porous layer is calculated using the following equation with transport num-bers of Al3⫹(TAl3⫹) and O2⫺(TO2⫺), which are confirmed to be 0.4 and 0.6, respectively20,21

where

m1is the slope of the V-t curve during re-anodizing of the aluminum specimen having porous alumina layer, and m2is the slope of the V-t curve during the growth of barrier film by anodizing of an aluminum substrate

This method for porosity measurement is well established20,21 and called as a ‘‘Pore-filling’’ technique Under the condition of the present set of work, m2was measured to be 0.2共V/s兲.22The relation between porosity␣, cell diameter dcell, and pore diameter dporecan

be given as

␣ ⫽ 共dpore/dcell兲2

namely, dpore/dcell⫽冑␣ 关3兴 The cell arrangement was evaluated by a field-emission scanning electron microscopy 共FE-SEM兲 observation of the oxide/substrate interface after removing the anodic film by immersion in a boiling mixed solution of 6% phosphoric acid and 2% chromic acid for 10 min in which the selective dissolution of the oxide ensued This is the most simple and accurate method for the evaluation of the cell homogeneity The level of self-ordering can be assessed by the frac-tion of regular hexagonal cells, which neighboring to six cells indi-vidually When self-ordering progresses, the size of the domain con-sist of only regular hexagonal cells increases

Results and Discussion

Current-time transients at constant voltage.—Figure 2 shows

current-time 共I-t兲 transients during constant-voltage anodizing in sulfuric acid solution as a typical case This type of stable I-t curve

is usually obtained when the stable porous film growth proceeds With increasing formation voltage, current density increased When the formation voltage exceeded the value of self-ordering voltage,

i.e., at 27 V, a high current accompanying intense gas evolution at

* Electrochemical Society Active Member.

z E-mail: sachiono@cc.kogakuin.ac.jp

the entire surface was observed In these cases, no film growth at the

entire specimen surface proceeded In the case of oxalic acid

solu-tion, similar phenomenon of a high current was appeared at 45 V As

revealed by the intense gas evolution, electronic current caused by

the electric breakdown at the barrier layer is preferential than the

ionic current While, in the case of phosphoric acid solution, local

film thickening was observed at 200 V accompanying high current

concentration

If local film thickening is observed as a result of such an

ex-tremely high current, it is commonly called ‘‘burning’’ because

black spots of the thickened film often appear in some organic acid

electrolytes.23In the present cases, ‘‘white burning’’ spots appeared

in phosphoric acid solution, while preferential gas evolution at the

entire specimen surface rather than film growth was observed in

sulfuric acid and oxalic acid electrolytes The reason for the

differ-ence in appearing reaction of either local film thickening or gas

evolution at the entire surface is unclear although the phenomena of

the extreme high current and termination of uniform film growth are

similarly observed when the formation voltage exceeds a certain

steady-state current density共upper兲 and the porosity of anodic films 共lower兲 with formation voltage Anodizing was performed at a con-stant voltage for 1 h except for cases of phosphoric acid formed at voltages lower than 90 V In the latter cases, anodizing was per-formed for 2 h to establish sufficient pore development because current reached to a steady state value after more than 1 h It is caused by the low dissolving ability of the electrolyte at low tem-perature Current density increased exponentially for three types of electrolytes when the voltage increased to a value close to the

indi-vidual self-ordering voltage, i.e., sulfuric acid at 25 V, oxalic acid at

40 V and phosphoric acid at 195 V The self-ordering voltage is strongly dependent on the pH of each electrolyte, namely, the oxide dissolution ability At a voltage higher than the individually speci-fied self-ordering voltage, an extremely high current was observed

in the three electrolytes Thus, it is apparent that self-ordering occurs

at a voltage just under the critical voltage which induces extremely high current and prevents uniform film growth

Change in the porosity of anodic alumina with increasing formation voltage.—The high field theory suggests that log of

cur-rent density has a linear relationship with electric field strength We reported previously16that the theory was applicable to the porous film growth and confirmed the linear relationship between log of

current density and the ratio of pore diameter dporeto cell diameter

dcell This suggests that the dpore/dcell ratio is controlled by the

electric field strength E at the barrier layer and the ratio decreased

during the growth of barrier film by anodizing of an aluminum substrate.

Figure 2 Current-time curves of anodizing in 0.3 mol dm⫺3sulfuric acid at

20°C in the voltage range from 5 V to 27 V.

Figure 3 Changes in steady-state current density 共upper兲 and porosity 共lower兲 of anodic films with formation voltage measured at three

electro-lytes Anodizing was carried out at constant voltage.

with increasing E Therefore, it is assumed that self-ordering

pro-ceeds in the film with a low dpore/dcellratio formed under a high

electric field

As shown in the lower part of Fig 3, the porosity of each anodic

film lowered markedly and converged to ⬃0.1 regardless of the

electrolyte type, which corresponded to a dpore/dcellratio of⬃0.3,

when the voltage approached the individual self-ordering voltage

accompanying the exponential current increase

For comparison of the three types of electrolyte, the formation

voltage was normalized to the self-ordering voltage As shown in

Fig 4, porosity was plotted against the ratio of formation voltage Vf

to the self-ordering voltage Vs The three lines corresponding to the

three different electrolytes decreased almost in the same manner

regardless of the electrolyte type and the formation voltage The

minimum porosity of the films obtained just under the critical

volt-age of extremely high current appears to be 0.1 This indicates an

important fact that self-ordering can be attained when the dpore/dcell

ratio approached 0.3 with the increase in electric field strength

re-gardless of the electrolyte type and the formation voltage itself

Thus, the mechanism of self-ordering is assumed to be closely

re-lated to the high electric field strength at the barrier layer during

anodic film growth, rather than to the individual self-ordering

volt-age itself

Nielsch et al.24 suggested recently that three types of

self-ordered porous alumina all gave a porosity value of 0.1 They

ex-plained that the porosity value of⬃0.1, which was produced as a

balance of formation and dissolution of anodic oxide, was

morpho-logically most stable from the viewpoint of mechanical stress They

also claimed that 0.1 was a transitional and optimum porosity value

for self-ordering However, according to the present results, the

po-rosity value of 0.1 for porous alumina was the optimum and also the

minimum value

FE-SEM observation of self-ordering behavior.—When

anodiz-ing voltage was three-fourths of the established self-orderanodiz-ing

volt-age, i.e., 30 V in oxalic acid, the size of cells was not uniform, as

clearly shown in Fig 5a This implies that the irregular film growth

proceeds under the low current density, i.e., low electric field The

homogeneity of cell size could be attained and domains of

self-ordered cell arrays in the same direction appeared when anodizing

voltage increased to 40 V, but just under the critical voltage of

extremely high current共Fig 5b兲 To verify the effects of formation

voltage and film thickness on the self-ordering, anodic film was formed at 30 V for a corresponding time to consume the equivalent electricity for that consumed at 40 V for 1 h, namely, 1 h and 40 min As shown in Fig 5c, the cell homogeneity of anodic film formed at 30 V for prolonged time was somewhat improved, but it was apparently inferior to that associated with 40 V even though the film thickness was similar Thus, the necessity of high current den-sity during anodizing for self-ordering of cell arrangement besides film thickness was clarified The size of the domain continuously increased with increasing anodizing time up to 6 h as shown in Fig 5d, suggesting the necessity of long-term electrolysis to organize the cell arrangement and to form the consequential thick porous layer

A similar self-ordering behavior was also observed in the films formed in sulfuric acid solution The homogeneity of cell size was insufficient for the film formed at 20 V and an ordered domain structure could be obtained when anodizing voltage increased to the

established self-ordering voltage, i.e., 25 V, as shown in Fig 6.

Although the self-ordering voltage in phosphoric acid solution was as high as 195 V, the behavior was almost the same as those of the other two electrolytes, as shown in Fig 7 That is, the homoge-neity of cell arrangement was not sufficient in the film formed at 150

V but the homogeneity was improved at 195 V Figure 7b indicates

a tilting SEM image of the aluminum surface obtained after remov-ing the anodic oxide formed for 1 h Aluminum pillars formed at irregular cell junctions, namely, junctions of four to six cells, were clearly observed, while no such pillars were found at triple cell junctions where self-ordered cell arrays were formed It is apparent

Figure 4 Porosity plotted as a function of the ratio of formation voltage Vf

to self-ordering voltage V s

Figure 5 SEM images of the metal/oxide interface after removal of porous

alumina formed in 0.3 mol dm⫺3oxalic acid at 20°C showing the depen-dence of cell arrangement on formation voltage and anodizing time 共a兲 30 V for 1 h, 共b兲 40 V 2% for 1 h, 共c兲 30 V for 1 h and 40 min, and 共d兲 40 V for

6 h.

Figure 6 SEM images of the metal/oxide interface after removal of porous

alumina formed in 0.3 mol dm⫺3sulfuric acid at 20°C 共a兲 20 V for 1 h, 共b兲

25 V for 1 h, and 共c兲 25 V for 6 h.

In the case of phosphoric acid electrolyte, it was not easy to

continue electrolysis for a long time without burning, namely, local

current concentration The addition of Al3 ⫹ions, vigorous agitation

of electrolyte and repeated experiments were required to perform

electrolysis for more than 1 h at 195 V without burning Moreover,

the homogeneity of cell arrangement is rather inferior to those of

other electrolytes The reason seems to be high anodizing voltage

such as 195 V compared to 25 and 40 V The high voltage tends to

induce local events such as electric breakdown, local thickening of

the barrier layer and pore branching because of the weak acidity

These local events prevent growth of domains of homogeneous cell

arrangement

Self-ordering of anodic porous alumina formed during

burn-ing.—Although anodizing could be carried out at the established

self-ordering voltage, burning occurred frequently in the case of

phosphoric acid as mentioned above Thus, the behavior of porous

film growth during burning was also studied to clarify anodizing that

proceeded under an extremely high current density

A comparison between the current-time curve measured during

burning and that during stable anodizing at 195 V in phosphoric acid

solution is shown in Fig 8 With a rapid current increase, local

thickening of the film was detected Figure 9 shows SEM images of

the surface共upper part: A in Fig 9a兲 and horizontal fracture section

共lower part: B in Fig 9a, as well as Fig 9b兲 of the thickened film

formed during burning The latter fracture section was formed due to

splitting of the outer part of the film as a result of cracking induced

by a strong mechanical stress accompanied by the extremely high

current concentration and the resultant rapid film growth The pore

arrangement at the film surface, which was formed at the initial

anodizing period, is not uniform, however, the horizontal fracture

section shows domains of highly self-ordered pore arrays although

the film is quite thin The dpore/dcellratio of this part was 0.28, i.e.,

⬃0.3 as shown in Fig 10b

Clearly, film thickness is an important factor because the highly

ordered porous structure is only obtained after prolonged anodizing,

as previous studies have indicated However, as shown in Fig 10a,

the self-ordering proceeded instantaneously when current was

con-centrated during the burning Therefore, it can be said that a high

electric field strength is the more significant factor in self-ordering

than the thickening of the anodic alumina itself

Figure 10a shows an SEM image of a burnt area indicating the

protrusion of thickened anodic film with a large number of cracks

The protrusion is divided into three regions:共A兲 center, 共B兲

inter-mediate, and共C兲 outer regions The substrate surface images of the

corresponding regions after the removal of the anodic films are also

shown in Fig 10b-d Apparently, the regularity of the cell

arrange-ment is higher at the center region than that at the outer region

Because the current density seems to be higher at the center, the

regularity of cell arrangement could be further improved Thus, it is

suggested again that the condition of high current density, i.e., high

electric field, is the most important factor that determines the self-ordering of the pore arrangement In addition, the cell size is smaller when the regularity of the cell arrangement is higher Because the voltage dropped to 160 V instantaneously, followed by a rapid cur-rent increase with burning, the average cell size ratio is 1.7 nm/V at the center region, 2.1 nm/V at the intermediate region and 2.36 nm/V at the outer region if the final voltage affects the size of whole cells Compared to the ratio of cell size to applied voltage of 2.5 共nm/V兲 observed for the standard anodic porous films,14,26the ratios obtained here are all lower Therefore, it can be said that the cell size

is affected by the electric field strength E and decreases with in-creasing E under the condition of the same voltage This finding is

similar to that observed for the barrier layer thickness.19As de-scribed details in a separate paper,25the size of the cells at the exact center spot of the protruded area made by burning, where the high current density was most concentrated, was extremely small Thus, the size of self-ordered cells observed at the burning protrusion var-ies to a large extent This implvar-ies the importance of current density

on the self-ordering of porous alumina regardless of the specific

Figure 8 Comparison between current-time curve measured at burning and

that at stable anodizing at 195 V in 0.2 mol dm⫺3phosphoric acid at 0-5°C.

hori-zontal fracture section 共lower part: B in a, as well as b兲 of the ruptured porous film formed during burning in 0.2 mol dm⫺3phosphoric acid at 195

V at 0-5°C.

self-ordering voltage itself The highly self-ordered cells produced

by burning were also found at the film formed in malonic acid.25

To confirm the relationship between current density and electric

field strength, cross sections of the barrier layer of the film were

examined In this experiment, the electric power was switched off

before the voltage drop to maintain the formation voltage of 195 V

As shown in Fig 11, the thickness of the barrier layer near the

center of the burned spot was 150 nm giving the smaller anodizing

ratio such as 0.76, while that of outer region was 216 nm giving the

standard anodizing ratio of 1.1 It was clearly observed that the

barrier layer thickness decreased with increasing distance from the

center of burnt spot

As revealed in the present results, even burning could produce a

highly ordered porous structure Thus, it is verified that the

condi-tion of high current density, i.e., the high electric field strength E at

the barrier layer is a strong controlling factor of the self-ordering of

cell arrangement

Newly developed self-ordered porous alumina with 600 nm pore

interval.—Based on the understanding of high electric field as the

self-ordering condition, a new self-ordering porous alumina with a

pore interval of 600 nm formed in 2 mol dm⫺3citric acid solution at

240 V was developed When a constant anodizing voltage was

ap-plied in the range from 225 to 245 V, current density increased

gradually with increasing voltage, as shown in Fig 12 The film

formed just under the burning voltage of 245 V, namely 240 V,

showed self-ordered cell arrays, as shown in the SEM images of the

substrate surface and the film cross section in Fig 13 Although

anodizing time was 1 h, and the film was as thin as 7␮m, domains

of self-ordered cell arrays were found Thus, the present assumption

in that the high electric field strength induces self-ordering can be

verified Therefore, it can be concluded that self-ordering of

arbi-trary pore intervals by using adequate electrolytes and conditions is

achievable for maintaining a high current condition, i.e., high

elec-tric field on the entire specimen area, while avoiding extremely high

current leading to burning or electric breakdown All other factors

such as aluminum ions, concentration of electrolyte and temperature

would be explicable by that they lead to keep high current density

anodizing without occurring local events such as burning which

in-terrupt the uniform film growth

Conclusions

Self-ordering of the pore arrangement of anodic alumina was observed at the maximum voltage for inducing high current anodiz-ing while avoidanodiz-ing burnanodiz-ing or electric breakdown Such a maximum voltage was identical with the previously established self-ordering voltage

with a large number of cracks The protrusion is divided into three regions:

共A兲 center, 共B兲 intermediate, and 共C兲 the outer 共b,c,d兲 SEM images of the

respective substrate surfaces of the three regions corresponding to A, B, and

C in 共a兲 after the removal of anodic films.

Figure 11 SEM images of the fracture sections of the barrier layer of

po-rous anodic film formed during burning in phosphoric acid at 195 V ob-served at 共a兲 the center region A and 共b兲 the outer region C of the burnt spot.

acid at 20°C in the voltage range from 225 to 245 V.

Concurrently, the ratio of pore diameter to cell diameter

con-verged to 0.3, which corresponded to a porosity value of 0.1

regard-less of the electrolyte type when the formation voltage approached

the individual self-ordering voltage, independent of the formation

voltage itself

Self-ordering of the pore arrangement of anodic alumina was

found even during burning indicating also a dpore /dcellratio of 0.3

This self-ordering at burning was considered to occur under the high

local current concentration and the resultant high electric field

strength in the specific area

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formed in 2 mol dm⫺3citric acid at 240 V for 1 h showing self-ordered cell

arrays.