AAO ALUMINUM POROUS

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Applied Physics A

Materials Science & Processing

ISSN 0947-8396

Appl. Phys. A

DOI 10.1007/s00339-014-8242-5

Controllable fabrication of highly orderedthin AAO template on Si substrate for

electrodeposition of nanostructures

Khaled M. Chahrour, Naser M. Ahmed,

M. R. Hashim, Nezar G. Elfadill &

M. A. Qaeed


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Controllable fabrication of highly ordered thin AAO templateon Si substrate for electrodeposition of nanostructures

Khaled M. Chahrour Naser M. Ahmed

M. R. Hashim Nezar G. Elfadill M. A. Qaeed

Received: 30 November 2013 / Accepted: 8 January 2014

Springer-Verlag Berlin Heidelberg 2014

Abstract In this work, simple fabrication of hexagonally

highly ordered porous anodic aluminum oxide (AAO) of Althin film (1 lm) on Si substrate is described using two-step

anodization method for electrochemical synthesis of

nanostructures. In this method, the templates were prepared

under the controllable conditions of the parameters, which

give rise to the possibility of highly ordered nanopore

arrays with a well aspect ratio. Pore widening was then

fulfilled in 5 wt% phosphoric acid solution at 25 C. The

pore diameter and spacing are proportional to the applied

voltage, which is due to the mechanical stress associated

with the volume expansion of the aluminum during the

anodization according to the mechanical stress model.

Pore-widening solution adjusted the pore diameter and

thinned the AAO barrier layer at room temperature under

the control of etching time. As an application, Cu nanorods

arrays embedded in anodic alumina (AAO) template were

fabricated by dc electrodeposition. The characterization of

the AAO templates and the Cu nanorods produced was

made by X-ray diffraction, field emission scanning

microscope, energy dispersive X-ray spectroscopy and

atomic force microscope (AFM). The images of AFM

show that porous AAO template under constant voltage is

40 V which presents the optimum ordering.

1 Introduction

Template technique is one of the most successful approa-

ches for obtaining size-controllable nanomaterials [1].

Recently, porous anodic aluminum oxide (AAO) templates

have received considerable attention in synthetic nano-

structure materials due to particular characters such as

controllable pore diameter and periodicity [2]. The porous

AAO template fabrication process and mechanisms of pore

formation have been studied [3, 4]. Using porous AAO

templates for nanostructure deposition needs no costly

nanolithography. There are two major kinds of porous AAO

templates, the first-type of porous AAO template is grown

on a bulk pure aluminum foil [5], while the second-type is

grown on a substrate such as silicon [3]. The second-type of

the template is preferable due to fact that this silicon sub-

strate usually functions as an electrode as well as a

mechanical support [6]. Most of the developed methods for

producing porous AAO templates generally yield highly

ordered arrays on bulk Al foil [7]. Until now, the fabrication

of highly ordered thin films of porous AAO on Si substrate

is difficult to be formed mainly due to the complicated

surface states (roughness and crystallite sizes) and non-

uniformity of the deposited Al film [8,9] and still remains a

major challenge from the scientific and technological point

of review [10]. In spite of the wide range of promising

applications of the AAO templates, the biggest problem

remains is the barrier layer formed at the bottom of the AAO

pores, which prevents direct physical and electrical contact

to the substrate [11]. To remove the barrier layer, various

techniques such as pore widening, cathodic polarization,

voltage drop, plasma assisted etching, etc. were employed

[1214]. However, there are some challenges posed by the

thin film AAO templates, including achieving highly-

ordered AAO templates with open-through pore structure.

K. M. Chahrour (&) N. M. Ahmed M. R. Hashim

N. G. Elfadill M. A. Qaeed

Nano-Optoelectronics Research and Technology Laboratory,

School of Physics, Universiti Sains Malaysia, 11800 Penang,

Malaysia

e-mail: [email protected]

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Appl. Phys. A

DOI 10.1007/s00339-014-8242-5


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In this work, we show that highly ordered thin porous

AAO templates can be grown on Si substrate under con-

trollable condition sets of anodizing process with thin Al

films using two-step anodizing process [15], leading to

perfect hexagonal pore structure, since the ordering of the

pore arrangement of porous AAO templates control the

regularity of further nanostructures fabricated using AAO

templates as host material [16]. This work focused on theinfluence of voltage and pore-widening time on the struc-

ture, order of thin porous AAO template. The advantages

of our work over the past studies are that we used a simple

pore-widening solution for adjusting the pore diameter and

thinning the AAO barrier layer at room temperature under

control of etching time for only few minutes to prepare

nanostructures scaffolds with desirable pore diameters that

we need.

2 Experimental method

P-type (100) Si substrate with area of (1.5 cm 9 1.5 cm)

was cleaned with (RCA) method prior to deposit 20 nm of

Ti film using RF sputtering, subsequently an Al (purity

99.99 %) was evaporated by e-beam on Ti film with

thickness of 1 lm. All samples were annealed at 500 C

for 2 h in a conventional furnace under nitrogen ambient.

Our experimental results show that the use of a Ti adhesion

layer prevented the pealing-off the Al film from the Si

wafer during anodization. The anodizing process was per-

formed in a special design electrochemical cell using a

platinum rod as a cathode. The electric contact was made at

the backside of the Si substrate. The samples were anod-

ized in acidic aqueous solution of 0.3 M oxalic acid under

different voltages for a certain time. After the finishing ofthe first anodizing, an aluminum oxide layer was removed

by wet etching in aqueous solution of 6 wt% phosphoric

acid and 1.8 wt% chromic acid at 60 C for 30 min. The

second anodizing was carried out with the same control-

lable conditions of the first anodizing until all of the

residual aluminum was oxidized. Pore widening is con-

trolled by immersing the AAO template into 5 wt%

phosphoric acid solution at 25 C for different times. The

electrochemical cell was cooled with circulating water bath

system to ensure a constant temperature below 20 C.

During anodizing process, the electrolyte was vigorously

stirred. Some as-synthesized AAO templates are furtherannealed at 800 C for 5 h in atmosphere.

As an application, copper nanorods were deposited po-

tentiostatically, with AAO template serving as working

electrode, platinum rod and Ag/AgCl(sat)as the counter and

reference electrodes, respectively, using a EDAQ Model

potentiostat. The electrolyte was 0.45 M of CuSO2

Fig. 1 aFESEM images show top view and a cross-sectional view of

the AAO templates. AAO templates were prepared under parameter

conditions; time of first anodization is 10 min and anodizing voltage

40, 45, 50 V, respectively. Pore-widening time is 20 min, b curve

correlating mean pore diameter with voltage, andc AFM images (i )

AAO prepared under 50 V and (ii) AAO prepared under 40 V

K. M. Chahrour et al.

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dissolved in 3 M lactic acid. The solutions PH was

adjusted to 9 using sodium hydroxide. The electrodepos-

ition was performed at an applied potential of -1 V for

5 min. The temperature was maintained at 60 C, with

constant stirring throughout electrodeposition. Upon com-

pletion, the AAO template was rinsed by deionized water

and dried. The surface morphology and structure of the

fabricated AAO templates and the deposited Cu nanorodswere studied using FESEM, EDX, AFM and XRD.

3 Results and discussion

The morphology of the fabricated AAO templates was

characterized by FESEM and AFM images. Figure1a

shows typical top view and a cross-sectional view of the

AAO templates under 40, 45 and 50 V, respectively. The

results show distinctly that an ordered honeycomb structure

with uniformity in pore diameter and spacing can be fab-

ricated with two-step anodization under controllable con-ditions; also Fig.1a illustrates that the pore diameter and

interpore distance increase with increasing voltage. The

mean of pore diameter is 65 nm and mean of interpore

distance is 110 nm for AAO template prepared under 40 V.

AAO template prepared under 45 V has a mean pore size of

70 nm and a mean interpore distance of 115 nm, while

AAO template prepared under 50 V has a mean pore size of

85 nm and a mean interpore distance of 128 nm. A curve

correlating mean pore diameter with anodizing voltage

described in Fig.1b illustrates that the mean pore diameter

is increased with increasing the voltage. In addition, AAOsample prepared under lower voltage is more regular as

shown in AFM images (Fig. 1c). The mechanism of self-

organization is not fully understood, even though it can be

explained by the mechanical stress model proposed by

Jessensky et al. [4]. Pores are first formed at certain micro-

rough region where the current density is concentrated on

after the formation of steady oxide layer and then the pores

grow vertically to surface with equilibrium of field-

enhanced oxide dissolution at the oxide/electrolyte interface

and oxide growth at the metal/oxide interface, and the

horizontal growth is performed simultaneously. In this

process, the compressive stress between the pores which areassociated with the volume expansion of the aluminum

during anodization impulses the structural adjustment to

form honeycomb like pore arrays. With the enhancement of

Fig. 2 FESEM images show top view of the AAO templates. AAO

templates were prepared under parameter conditions; time of first

anodization is 10 min and anodizing voltage 40 V. Time of pore

widening t= 10, t= 15 and t= 20 min, respectively. Curve corre-

lating mean pore diameter with pore-widening time

Controllable fabrication of highly ordered thin AAO template on Si substrate

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anodizing voltage, the horizontal growth of the pores

increases leading to the enlargement of the pore diameter

and spacing. At the same time, the arrangement among the

pores cannot keep up with the growth velocity of AAO and

the ordering of this structure might decrease.

Although AAO template prepared after two-step

anodization possesses controllable pore diameter and

periodicity, further pore adjustment can be actualized byexposure to phosphoric acid, which is called pore wid-

ening. The FESEM images of the AAO templates were

prepared under 40 V after pore widening for 10, 15 and

20 min at 25 C as shown in Fig. 2a. The pore diameter

increases, while the pore spacing almost keeps unchanged.Fig. 3 XRD spectra of AAO templatea with annealing andbwithout

annealing

Fig. 4 a, b FESEM image shows the deposition of Cu inside of AAO template and EDX spectra. c, d FESEM image shows a cross-sectional

view of Cu nanorods perpendicular to the Si substrate after removal of AAO template and EDX spectra, respectively

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A curve correlating mean pore diameter with widening

time described in Fig.2b illustrates that the mean pore

diameter is monotonously increased with increasing the

pore-widening time. The increase of pore diameter after

the thinning of the outer and inner surface of AAO tem-

plate might be due to the wet etching effect of phosphoricacid. This provides a convenient route to prepare AAO

template of any desired pore diameter that we need. Then,

any desired nanostructure diameter can be grown in its

pore. Figure3 shows XRD spectra of AAO template after

annealing, the reflection peak of (222) corresponding to

the Al2O3 phase appear [17], which implies that the AAO

transforms from amorphous to crystalline after annealing

at 800 C.

With the aid of AAO template and electrodeposition

process, highly ordered and vertical arrays of Cu nanorods

can be prepared. Figure4a, b shows the plane view for

AAO template on a Si substrate after deposition of Cu,almost all the nanopores were filled by Cu and confirmed

with the EDX spectra. The lengths of Cu nanorods grown

inside the AAO template were longer than the depth of

AAO template whereby all the nanorods showed sign of

overgrowth. Figure4c shows the Cu nanorods remained

vertically to the Si substrate and could be observed clearly.

It is worth noting that the heights of the nanorods were

uniform and remained separated from each other after the

removal of the AAO template. The length of the nanorods

was 1.25 lm and the approximate diameter was 50 nm.

Note that some of the Cu nanorods near the front edge were

broken during the splitting process for the preparation ofcross-sectional FESEM samples. Figure4d shows EDX

spectra that confirm the compositions of the nanorods on Si

substrate after the removal of the AAO template. Figure5

shows the X-ray diffraction patterns of copper nanorods

embedded in AAO template. The nanorods were poly-

crystalline with cubic structure indicated by the presence of

two prominent peaks close to 2h angles of 43.41 and

50.60, corresponding to Cu (111) and Cu (200) diffrac-

tions, respectively.

4 Conclusions

Highly ordered pore arrays in AAO templates with uniform

pore size and vertically aligned nanotubes with well aspect

ratio were successfully fabricated by anodization of thin Al

film on Si wafer. Anodizing voltage and time of pore

widening are explored in our experimental conditions.

FESEM analysis show that the pore diameter depends onboth anodizing voltage and time of pore widening, also the

AAO template prepared under 40 V present the optimum

ordering as shown in the AFM images. The relation

between the ordering of the pore arrays and anodizing

voltage is explained by a growthdissolution model. A

simple wet etching process is used for thinning the bottom

barrier layer of AAO template and widening of any desired

pore diameter that we need under a control of etching time.

FESEM analysis show that highly orderly self-aligned Cu

nanorods have been prepared on AAO template/Si sub-

strate using electrochemical process. Finally, stirring and

maintaining the electrolyte below 20 C are critical steps inobtaining ordered pore arrays.

Acknowledgments We gratefully acknowledge the support of the

School of Physics, University Saince Malaysia under short term Grant

No. 304/PFIZIK/6312076.

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Fig. 5 XRD spectra of copper nanorods embedded in AAO template

Controllable fabrication of highly ordered thin AAO template on Si substrate

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