a comparison between self - ordering of nanopores in aluminium oxide

b 68.37.Hk 78.55.Mb 82.45.Cc Keywords: Nano-scale materials Anodic aluminum oxide Nanopore Self ordered Three step a b s t r a c t We report on the enhancement of naturally-occurred self

A comparison between self-ordering of nanopores in aluminium oxide

films achieved by two- and three-step anodic oxidation

Department of Materials Engineering, Sahand University of Technology, Tabriz 53317-11111, Iran

a r t i c l e i n f o

Article history:

Available online 31 August 2008

PACS:

61.46 w

81.07 b

68.37.Hk

78.55.Mb

82.45.Cc

Keywords:

Nano-scale materials

Anodic aluminum oxide

Nanopore

Self ordered

Three step

a b s t r a c t

We report on the enhancement of naturally-occurred self-ordering of nanopores in anodic aluminium oxide (AAO) membrane by performing three step anodic oxidation Two and three step anodic oxidation methods were used to achieve self ordering of nanopores and the ordering of nanopores obtained by the methods were compared The current–time curves, recorded during anodization, elucidate an almost similar features for all three steps in both methods Scanning electron micrographs (SEM) show hexago-nally arranged nanopores in a way which forms highly ordered areas or ordered domains Domains are placed beside each other like a polycrystalline structure after two and three step anodizing, while larger ones are clearly observed over the surface after three step anodizing More uniform nanopores with nar-rower size distribution are observed for AAO films of three step anodic oxidation

Ó 2008 Elsevier B.V All rights reserved

1 Introduction

The template synthesis is known as an elegant electrochemical

approach for the fabrication of nanomaterials such as nanowires

and can be considered as an alternative choice to the other

sophis-ticated methods such as molecular beam epitaxy and

nanolithog-raphy[1] The fabrication method may include electrochemically

synthesis of thin fibrils into cylindrical pores of the template[2]

Among the frequently used templates, anodic aluminium oxide

films provide highly ordered pore arrangements due to the

self-organizing process, introduced by Masuda et al under the

applica-tion of anodic oxidaapplica-tion process for two steps[3–6] AAO

mem-branes with pore density of 1010–1012cm 2, are formed by

anodizing Al in appropriate acidic electrolytes such as oxalic acid

[4,5] The typical structure of the film consists of a thin non-porous

barrier oxide layer which lies adjacent to the Al substrate and a

porous layer on the top[7–8] The top layer contains an array of

hexagonally close-packed cylindrical cells at the center of which

a straight hole is located The formation of the porous structure

re-sults from the effect of a dynamic mechanical equilibrium between

the film growth at the aluminium-oxide interface and the

field-as-sisted oxide dissolution at the oxide–electrolyte interface[6,7]

Most of the reports on obtaining highly ordered AAO have been focused on using two step anodizing process in which the duration

of the first step has a considerable effect on the formation of highly ordered polycrystalline domains As the duration of the first step increases, the domains in the bottom of porous oxide layer form

on larger areas[3,7] Therefore, the effect of the first step of anod-ization on the ordering of nanopores is remarkable to achieve bet-ter cell arrangements of nanopores In this paper, we aim to demonstrate the effect of three step anodization of aluminium on the ordering of the nanopores in AAO films

2 Experiments High purity Al foil (99.999%) was used as substrate after cutting into 1  1.5 cm2 sized rectangular samples After annealing at

500 °C for 5 h, Al substrates were surface prepared under a se-quence of chemical surface treatments in KOH 200 g/lit, Na2CO3

50 g/lit and HNO350% (in vol.) solutions Then, Al foils were eloc-tropolished in a mixture of HClO4and ethanol (1:4 in vol.) at 15

Vdcbelow 5 °C Anodization was conducted under constant poten-tial of 40 Vdcin 0.3 M oxalic acid electrolyte The temperature of the electrolyte was maintained at 0 ± 2 °C during anodization using

a cooling system The solution was stirred vigorously in order to accelerate the dispersion of the heat that evolved from samples For anodizing more than once, the oxide layer formed in the 1567-1739/$ - see front matter Ó 2008 Elsevier B.V All rights reserved.

* Corresponding author.

E-mail addresses: f_nasirpouri@yahoo.com , nasirpouri@sut.ac.ir (F Nasirpouri).

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previous steps was removed by wet chemical dissolution in a

mix-ture of 0.2 M chromic acid and 0.4 M phosphoric acid at 60 °C for

an appropriate time depending on the anodizing time Either the

second or the third anodization steps were carried out under the

same condition as the first step for different durations which will

be described later For each step, the variation of the anodizing

cur-rent was recorded against time In addition, ordered polycrystalline

domains and the diameter of the nanopores were characterized

using SEM on the top and cleaved surfaces of the AAOs The

histo-grams of pore diameter demonstrating the size distribution of

pro-duced pores were then derived based on SEM images The domain

areas were determined by first outlining the boundaries on SEM

micrographs, counting the number of pores for several domains,

converting these numbers to areas, and finally averaging

3 Results and discussion

In order to investigate the effect of the third step anodic

oxida-tion, series of samples were fabricated by two and three step

ano-dic oxidation and their growth mechanisms and structures were

studied Figs 1 and 2 show the current transients recorded for

the two and the three step anodic oxidation processes Figs 1a

and2a illustrate the variation of anodizing current in

potentiostat-ic mode at a constant voltage of 40 V for the first steps As shown,

there exists a region associated with almost uniform and constant

values of current without any possible expected drop in the graphs

Any drop in current transients at the first stages of anodic

oxida-tion process may occure due to the formaoxida-tion of an oxide layer

The observed smooth variation of current may be explained by

the formation of an oxide layer on aluminium, naturally produced

in the atmosphere on the samples during the rest time in the lab after electropolishing The pre-formed oxide film may act as a bar-rier layer and further anodic oxidation develops the porous layer of AAO films The current transient curves show plateaus regions in a range of 0.6–0.9 mA/cm2during first step which may be an evi-dence of the growth of porous film on the substrate The current transients recorded for the second (Figs.1b and2b) and the third (Fig 2c) steps just after dissolving the oxide layer show a drastic drop in the beginning of these steps Since the second and the third steps were conducted immediately after dissolving the grown oxide layer at the first step, the sudden drop followed by a raise

0

0.25

0.5

0.75

1

Time (min)

2)

0

0.4

0.8

1.2

1.6

2

Time (min)

2)

Fig 1 Typical current transients recorded during the anodizing Al for two steps: (a)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Time (min)

2 )

0 0.4 0.8 1.2 1.6 2

Time (min)

2)

0 0.4 0.8 1.2 1.6 2

Time (Sec)

2 )

Fig 2 Typical current transients recorded during the anodizing Al for three steps: (a) first step for 15 min, (b) second step for 6 h and (c) third step for 5 min.

of current up to the same value of the first step (0.6–0.9 A/cm2), can be attributed to the formation of barrier oxide layer on oxide-free surface of substrate[7,9]

The fabrication process and durations of steps were chosen so that the effect of the first step on the size of the ordering can be investigated in both two and three step anodic oxidation methods

Fig 3illustrates a typical SEM image the of AAO films after two step anodic oxidation and the histogram of pore diameters formed Although the structure shows that the ordering happens in the form of hexagonally arranged structures in the ordered domains, the histogram reveals a wide distribution of pore diameters with

a considerable contribution from pores with diameters of 15–

50 nm The pore alignments are different in the domains More-over, some other types of defects such as point defects and misfit dislocations can be seen in the topological studies

Once anodic oxidation is conducted for one more step, the third step, a different degree of ordering can be achieved The results shown in Figs 4 and 5correspond to AAO film anodized three times with a shorter and an equivalent anodization time at the first step to the film seen inFig 3, respectively As seen, there is a con-siderable change of pore size distribution and also ordered domain size The histograms show reasonable enhancement of size distri-bution Not only histograms show a tighter pore size distribution which falls into a range of 40–45 nm, but also confirms a more uni-form pore size distribution by increasing the first step anodization time which could be completely understood with the formation of better ordered barrier layer after dissolving the layers[7,8] Conse-quently, the ordered domain size also varies as a function of time The ordered domain size of AAO was figured out from the regions on SEM images Measurements reveal that the average

do-Fig 3 (a) Topography and (b) histogram of the distribution of pore diameters in

AAO films anodized for two steps including 1 h in both in the first and the second

steps.

Fig 4 Topographical information including (a) SEM image, (b) the identified domains and (c) the histogram of pore diameter, obtained from AAO film anodized for 0.5 h, 6 h

main size for three step anodized samples with first step duration

of 0.5 and 1 h are approximately 0.47 and 2.81lm2, respectively

On the other hand, using the equation D = 0.52t0.7, in which D is

the domain size in square micrometer and t is the first step

anod-ization time in hours[7,9], the domain size of two step anodized

samples with the same first step anodization times are estimated

to be 0.308 and 0.52lm2, respectively Thus, the larger areas are

located in the ordered domain for the three step films These

esti-mations qualitatively explain that third step of anodic oxidation

enhance the ordering of nanopores

4 Conclusion

In conclusion, based on the study, the three step anodic

oxida-tion of aluminium has a considerable effect on the self-ordering of

nanopores in AAO films The higher ordering can be achieved at

shorter anodization time, when the process is repeated for three

times Current transients for both two and three step anodic

oxida-tion methods demonstrate the same behavior including: (i) flat

current variation against time during the first step and (ii) sudden

drop of current once the anodic oxidation occurs for second and

the third steps It can be understood by considering the mechanism

with elucidating the formation of ordered barrier layer, followed

by an increase in current showing the growth of hexagonally

or-dered nanopores arrays Pore size distribution is enhanced for

the three step anodized films and a larger domain size can be formed at the same or possibly shorter anodization times in three step method

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Fig 5 Topographical information including (a) SEM image, (b) the identified domains and (c) the histogram of pore diameter, obtained from AAO film anodized for 1 h, 6 h and 10 min in the first, the second and the third steps, respectively.