on the growth sequence of highly ordered nanoporous

The ordering characteristics ordered pore domains, average pore diameter size and through-pore arrangement of anodic aluminium oxide films, obtained in different growth sequences, were ide

On the growth sequence of highly ordered nanoporous

anodic aluminium oxide

M Ghorbani a, F Nasirpouri a,*, A Iraji zad b, A Saedi a

a Department of Materials Science and Engineering, Sharif University of Technology, P.O Box: 11365-9466, Tehran, Iran

b Department of Physics, Sharif University of Technology, Tehran, Iran Received 7 October 2004; accepted 15 February 2005

Available online 14 April 2005

Abstract

Anodic aluminium oxide films were fabricated by well known two-step anodizing process in oxalic acid electrolyte The ordering characteristics (ordered pore domains, average pore diameter size and through-pore arrangement) of anodic aluminium oxide films, obtained in different growth sequences, were identified by microscopic analysis such as ex situ contact-mode atomic force microcopy and scanning electron microscopy Flattened areas in which some pits are seen mostly cover the electropolished surface of alumin-ium Single anodizing of aluminium produces a broad distribution of nanopore size, whereas induces a highly ordered hemispherical pattern, which plays the ordered nucleation sites for the second anodizing step Moreover, a quasi-linear growth behavior exists for the ordered domain growth versus the duration of first step anodizing The through-pore arrangement of ideally grown membranes

is not influenced by increasing the duration of second step anodizing

 2005 Elsevier Ltd All rights reserved

Keywords: Nanomaterials; Nanoporous; Aluminium oxide; Anodic oxidation

1 Introduction

Anodic aluminium oxide (AAO) has attracted much

more interests recently due to its self-organizing

nano-porous structure, introduced by Masuda and Fukuda

[1] This kind of self-organization of nanoporous anodic

aluminium oxide is based on naturally occurring

long-range ordering, in which a highly regular

poly-crystal-line pore structures occurs only for a quite small

pro-cessing window, whereas an amorphous pore structure

can be obtained for a very wide range of parameters

without substantial change in morphology [2–7] Over

the last decade, these highly ordered nanoporous films

have been used as templates for fabricating metal and

semiconductor nanostructures in magnetic and

opto-electronic applications[8–11]

On the mechanism of self-ordering in AAO nanopor-ous materials, so far ÕSullivan and Wood presented a model which was based on an electric field distribution

at the pore tip This model is able to give microscopic explanations for the dependence of, e.g., pore diameters and inter-pore distances on applied voltage or electro-lyte composition, but cannot easily explain the self-ordering behavior The self-organized arrangement of neighboring pores in hexagonal arrays can be explained

by any repulsive interaction between the pores [12] A possible origin of these forces between neighboring pores is the mechanical stress, which is associated with the expansion during oxide formation interface and leads to form curved shape metal/oxide interface It is claimed that the pores are formed during electropolish-ing and/or anodizelectropolish-ing on the aluminium surface and can become hexagonally ordered at certain voltages and times of the initial electropolishing[13]or by long-term anodization and reanodization[1]or also by a dynamic

0261-3069/$ - see front matter  2005 Elsevier Ltd All rights reserved.

doi:10.1016/j.matdes.2005.02.018

*

Corresponding author Tel./fax: +98 21 6005717.

E-mail address: nasirpouri@mehr.sharif.edu (F Nasirpouri).

www.elsevier.com/locate/matdes Materials and Design 27 (2006) 983–988

Materials

& Design

process depending on the mobility of ions within the

barrier oxide and of Al atoms within the metal[14]

The self-ordered anodic aluminium oxide (AAO)

membranes have been generally characterized by

remov-ing the aluminium substrate and the structure has been

observed from the bottom view of grown layer, in which

the pore ordering can be achieved easily than the top

surface of grown films which is more important in

appli-cation [15–18] In this study, we have investigated the

topographical properties of AAO films, which

contrib-ute into ordering the nanopore domains Atomic Force

Microcopy (AFM) is used as a useful method to control

the nanopore topographical characteristics of anodic

aluminium oxide AFM allows the assessment of the

ordering in nanopore domains, pore density and the

external shape of AAO films However, It must be taken

into account that the internal diameter and shape of

nanopores, which are defined as through-pore

arrange-ment, cannot be evaluated by this method This

struc-tural characteristic of AAO membranes is easily

characterized by scanning electron microscopy across

the cleavage surfaces of films

2 Method

Two-step anodizing process was used to fabricate

AAO templates with 40 nm pore diameter High purity

(99.999%) aluminium foils were annealed at 450C for

4 h to avoid remaining any residual stress in the

alumin-ium substrates Then, Al plates were electropolished in a

mixture of perchloric acid (60%) and ethanol (1:4 in

vol-ume) under 20 V below 5C for approximately 10 min

Anodization was conducted under constant cell

poten-tial in oxalic acid electrolyte The temperature of

electro-lyte was maintained at 0C (between 2 and +2C) during anodization using a cooling system However, the solution was stirred vigorously in order to accelerate the dispersion of the heat that evolved from the samples The first and second anodization steps were conducted

in the same condition as mentioned above Meanwhile, the oxide layer formed in the first step was removed

by wet chemical dissolution in a mixture of 0.2 M chro-mic acid and 0.4 M phosphoric acid at 60C for an appropriate time depending on the anodizing time To facilitate the observation of pore arrangement on the

Fig 1 AFM images of electropolished surface of aluminium in 1 HClO /4 EtOH below 5 C for 10 min.

Fig 2 Topography of the single anodized aluminium in 0.3 M oxalic acid for 6 h at 40 V.

surface (topography), the samples were etched in 5%wt

phosphoric acid in 35C for 30 min

In order to characterize the AAO films, the structural

parameters including ordered pore domains, average

pore diameter size and through pore arrangement of

AAO films, obtained in different growth sequences, were

identified by microscopic analysis such as ex situ

con-tact-mode AFM and scanning electron microscopy

The domain areas were determined by first outlining

the boundaries of several domains on scanning electron

microscopy (SEM) micrographs, counting the number

of pores for several domains, converting these numbers

to areas, and finally averaging Moreover, the

through-pore configuration was observed across the fracture

sur-face of grown films

3 Results and discussion

Fig 1illustrates AFM images of electropolished alu-minium surface in HClO4/EtOH solution After electro-polishing, Al has an almost flat surface, exhibiting small etch pits and bumps, which could be seeds for pore nucleation [13] Consequently, the electropolished alu-minium was anodized for first time It has been shown

[14]that the pits or pores nucleate on the natural barrier layer or in the bottom of porous layer during the ini-tial stages of anodization However, it is assumed that the pits formed in the electropolishing contributing in the nucleation of pores on the aluminium in the order

of 1010 to 1012 [18] Fig 2 shows the topography of anodized aluminium for 6 h in 0.3 M oxalic acid at

Fig 3 AFM images of aluminium surface after removing the first oxide layer in: (a) 2 lm · 2 lm and (b) 0.5 lm · 0.5 lm scan areas.

0C, obtained by contact-mode AFM As it can be seen,

pores occur on the top surface randomly and have a

broad size distribution During the first step anodization

process, the pores nucleate on the electropolished surface at almost random positions, i.e., lattice imper-fections or pits formed by electropolishing As the anod-ization time is increasing the pores merge and form the curved metal/oxide interface due to stress inducing by volume expansion Because of the random nucleation positions of initial pores, the hexagonally ordering of pores is just achieved in the first stages of anodizing at the bottom of porous oxide layer and cannot grow up

to the thick anodized layers After removing the first anodized oxide layer, the curved shape interface remains

on the aluminium substrate This structure is shown in

Fig 3, which demonstrates the AFM images of alumin-ium surface after removing the first oxide layer The uni-form hemispherical shape of barrier layer covers the substrate surface

Anodizing the sample for the second time develops the pore growth exactly on the concave pattern created during the first step anodization As the duration of the first step anodizing increases, the hexagonally or-dered pore areas, domains, in the bottom of porous oxide layer occur in the larger surfaces In fact, nanop-ores exactly grow upon the relevant hemispheres and form direct pillars, which can be detected on top sur-faces of AAO films Fig 4 shows typical topography

of AAO films, anodized in the different first step anodiz-ing times The pore alignments are different in the do-mains and can be found out by domain boundaries, along which the pores gradually merge Moreover, some other kinds of defects such as point defects and misfit dislocations can be seen in the topological studies The misfit dislocation of the pores interrupts the periodic arrangement of the pores As another important result, the domains size is changed as a function of time (Fig 5) Two kinds of data fitting methods have been

Fig 4 SEM micrographs of identified ordered domains on top

surfaces of AAO films obtained in oxalic acid after: (a) 4 h first step

and 45 min second anodization step; (b) 15 h first step and 45 min

second anodization step.

Fig 5 The ordered domain growth versus first-step anodizing time Data represent the average domain size based on the identified areas of SEM micrographs.

applied to our experimental data The linear method

(R2= 0.9792) gives a function as: D = 0.55 t, where D

is the domain size in square micrometer and t is the

duration of the first step anodizing in hours Li et al

[14]have shown a linear behavior in their investigations

We have also fitted our data to the parabolic form:

D = 0.52 t0.7 with R2= 0.9988 This method provides

a better fitting accuracy and is very similar to grain

growth behavior in metals and alloys For metals and

al-loys, the driving force of grain growth is the grain

boundary energy per unit area For grain growth at a

fixed temperature, the average radius R of the grain is

a function of the time t: R = Bn, where B is a

tempera-ture-dependent parameter and n is about 0.4–0.5 As a

result, we considered a quasi-linear growth of ordered

domain size versus the first step anodizing time It

im-plies that the ordered domain size changes linearly in

the short durations of first step anodizing and a

para-bolic growth behavior exists in ordered domain size

ob-tained by anodizing for long periods

Consequently, the effect of second step anodizing

time has been investigated In Fig 6, the fracture

sur-faces of highly ordered AAO films, obtained in different

reanodizing time, elucidate the same through-pore

arrangement Thus, as the pore ordering takes place,

increasing the second step anodizing time does not affect

the achieved arrangement The thicknesses of AAO films were measured approximately 1 and 6 lm after 45 min and 6 h anodizing in second step, respectively

4 Conclusion Contact-mode atomic force microscopy confirmed the existence of concave pattern on the aluminium sub-strate after removing the first step oxide layer More-over, the ordered domain size depends on the first step anodizing time as a quasi-linear behavior and the do-main size does not change significantly when the first step anodizing extends for very long periods Also, thor-ough-pore arrangement of ideally grown of AAO films does not depend on second step anodizing time

Acknowledgment The authors wish to acknowledge the high technology center of Iranian ministry of industries for the financial support

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