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Materials Chemistry and Physics 104 (2007) 306–311

Preparation of transparent anodic alumina with ordered nanochannels bythrough-thickness anodic oxidation of aluminum sheet

M. Mehmood ∗, A. Rauf, M.A. Rasheed, S. Saeed, J.I. Akhter 1, J. Ahmad, M. AslamNational Centre for Nanotechnology, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad 45650, Pakistan

Received 31 May 2006; received in revised form 16 February 2007; accepted 16 March 2007

bstract

Through-thickness anodic oxidation of aluminum sheet has been possible to form 100–400 �m thick transparent alumina in 0.3 M oxalic acidt 30–70 V. Transmission at 900–1100 nm was 20–60%, the higher being at lower anodizing voltages. Two-step anodizing after electropolishingn Brytal solution resulted in best hexagonal order at 50 V. The alumina layers with ordered porosity growing from the opposite faces met at the

iddle of the sample without changing the orientation of the pores, except at the last stage when a number of pores meet to form Y-, S- and I-typeunctions. Three-dimensional compressive stresses around aluminum nanowires (nanoparticles) seem to be responsible for re-orienting the pores.n case of thicker barrier layers formed at higher anodizing voltage, the pores growing from opposite side do not meet and are separated by a barrierype compact oxide at the middle of the sample. 2007 Elsevier B.V. All rights reserved.

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eywords: Ordered porosity; Nanochannels; Anodic alumina; Transparency; In

. Introduction

Nanostructured materials exhibit interesting properties in aide range of spectrum including catalytic activity [1], corro-

ion resistance [2], optical properties [3] and magnetic properties4], etc. Since 1995 [5], nanoporous anodic alumina has sig-ificantly attracted the attention of scientists and researchers.his is because of the self-organization of vertical (cylindri-al) pores to form hexagonal array that has provided controllednd narrow distribution of pore diameters and inter-pore dis-ances in addition to the possibility of forming the pores withxtremely high aspect ratio [5–9]. This has opened a wide rangef applications of anodic alumina, particularly as a template toorm nanowires, nanotubes, nanodots, and composites for catal-sis, emitters, rechargeable batteries, magnetic storage devices,tc. [10–16]. The superlattice of pores and nanowires in anodiclumina has also been found extremely useful to exploit andtudy magnetic interactions, dielectric properties, and optical

nterference [17–19]. The other useful properties are photo- andlectro-luminescence, and transparency with absorption wave-engths higher than corundum [20].

∗ Corresponding author. Tel.: +92 519290273; fax: +92 519223727.E-mail address: mazhar@pieas.edu.pk (M. Mehmood).

1 Address: PINSTECH, PO Nilore, Islamabad, Pakistan.

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The anodic porous alumina is composed of two layers, i.e.,he top thick layer with vertical pores and a barrier layer thats in turn supported by aluminum [21]. For obtaining self-tanding alumite with ordered porosity, anodizing has alwayseen performed from one side of aluminum substrate; variousost-treatments have been necessary to dissolve remaining alu-inum with or without the barrier layer. However, the chemicals

sed are either hazardous to the environment or aggressive tolumina. Hence, special care is required, which may be cumber-ome and sometimes less reliable [22–26]. Therefore, obtaininglumite with ordered porosity by anodizing aluminum sheetrom both sides may be interesting. This may reduce the timeequired for anodizing to half in comparison to the case, inhich, anodizing is performed from one side of the aluminum

ubstrate. In addition, it may be of fundamental interest tobserve the structure formed when the two alumite layers grow-ng from opposite sides meet each other. Investigations have alsoeen performed to determine the degree of ordering as a functionf anodizing voltage.

. Experimental

High purity aluminum sheet (99.99%, 0.5 mm thick) was used as the startingaterial. C2H2O4·2H2O (Riedel, 97.5%), Na2CO3 (Panreac, 99.5%), Na3PO4

2H2O (Riedel, 98%), CrO3·2H2O (Merk, 99%), and H3PO4 (BDH, 98%) wereurchased from commercial resources and used without further processing.

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M. Mehmood et al. / Materials Chem

After annealing at 500 ◦C for more than 100 h, the aluminum samples wereegreased in acetone ultrasonically for 15 min. Electropolishing and anodizingere performed using Stabilized Power Supply (FARNELL, TSV70 MK.2) with

wo electrode configuration; the counter electrode being a platinum plate.Electropolishing was carried out in Brytal solution (15 wt.% Na2CO3 and

wt.% Na3PO4) at 80 ◦C at a voltage of 2 V for 1 h or more, which resultedn a smooth surface. The electropolished samples were anodized at constantoltage, ranging from 30 to 70 V, in 0.3 M oxalic acid at 0–1 ◦C. First anodizingas performed for about 3–12 h. The resulting anodic oxide was selectivelyissolved in a mixture of chromic acid (0.2 M CrO3·H2O) and phosphoric acid0.4 M H3PO4) at 70 ◦C for 3 h. Then second anodizing was performed underhe same conditions as the first anodizing until a transparent region appearedndicating through-thickness oxidation over about 0.5–1 cm2 area of the sampleor necessary measurements.

The samples were characterized using Scanning Electron Microscope (SEM,EO-441-I, UK), Field Emission Scanning Electron Microscope (FE-SEM,eck, LEO 15150) and UV–visible spectrometer.

. Results and discussion

Fig. 1 shows typical top view SEM images of the surfacesf anodized samples prepared at different voltages. The poresend to arrange in a hexagonal pattern forming the domainsf long-range order in case of anodizing at 40 and 50 V, as

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Fig. 1. SEM images of the surfaces, after preparing the samples at: (a)

and Physics 104 (2007) 306–311 307

hown in Fig. 1(a) and (b). It may be noticed, however, that therdering is better at 50 V than 40 V, as revealed by the parallelrrangement of hexagonal cells, larger domain size and narrowomain boundaries in Fig. 1(b). The sample prepared at a stilligher anodizing voltage, i.e., 60 V (Fig. 1(c)), exhibits mixedharacter and ordered domains are dispersed in the order-lessatrix. The ordered domains cover approximately 40% area.he order is totally lost accompanied by a wide dispersion inore sizes in case of the sample prepared at 70 V, as shown inig. 1(d).

Fig. 2(a) shows cell size as a function of anodizing volt-ge. The average cell size is approximately 103, 123–125, and69 nm after anodizing at 40, 50 and 60 V, respectively. The ratiof cell size to anodizing voltage is shown in Fig. 2(b). The ratios minimum at 50 V. This behavior has also been confirmed at0 ◦C for the anodizing voltage of 40 and 50 V. It seems worth-entioning here that the order was also better at 50 V both in

he case of 10 and 0 ◦C, as partly shown in Fig. 1.

It is known that elastic strains (or stresses) develop at

xide–metal interface due to volume expansion during oxida-ion. The oxide–metal interface has protrusion below the poreips. This stress distributes in such a way that it concentrates

40 V, (b) 50 V, (c) 60 V and (d) 70 V; the temperature was ∼0 ◦C.

308 M. Mehmood et al. / Materials Chemistry and Physics 104 (2007) 306–311

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hteaalahdvaFhbblast stage of anodization. Fig. 4(d) and (e) present two regionsselected from Fig. 4(c) to show the interface at larger magnifica-tion. The parallel pores growing from both sides tend to join thepores of the other side and a change in the orientation of verti-

Fig. 2. Hexagonal cell size (a), and ratio of cell size to

elow the pore tips and relaxed below the pore wall [27]. Theseegions of high stress (below the pore tips) seem to repel eachther depending on the magnitude of stress associated with thendividual pore and the distance between them, i.e., cell size.his repulsive force should be proportional to the strain associ-ted with the individual stress/strain center at the pore tip whichn turn proportional to the barrier layer thickness and thus thenodizing voltage. On the other hand the repulsive force shoulde strengthened by decrease in the cell size, i.e., the distanceetween the stress centers. Therefore, the repulsive force shouldncrease with a decrease in the ratio of cell size to the anodiz-ng voltage. We consider that the repulsive forces are responsibleor the hexagonal ordering, and the best order should be attainedhen we have the strongest repulsive forces, which should be

ttained at the lowest ratio of the cell size to the anodizing volt-ge. It appears, therefore, that the best self-ordering at 50 Vn comparison with other anodizing voltages is attributable totronger interfacial strain interactions as a result of smaller ratiof interpore distance to anodizing voltage. Furthermore, elec-ropolishing in Brytal solution seems to produce more conduciveonditions for better self-ordering at 50 V in 0.3 M oxalic acid,n contrast to perchlorate solution that is known to favor 40 Vor the best ordering under similar anodizing conditions. Thexact reason for the role of electropolisihing solution is undernvestigation.

The samples were exposed to the radiations in the wavelengthange of 900–1100 nm for estimating transmission through theamples. Fig. 3 shows the percentage transmission, as a functionf anodizing voltage. Transmission has been possible between0 and 60%, although it varies with anodizing voltages. Alu-inum is known to be optically very active metal for extremely

igh absorbance. Therefore, the presence of extremely thin alu-inum film (a few nanometers) should be very effective in

educing the transmission. Accordingly, the aluminum has beenostly consumed through the thickness, although an extremely

hin aluminum film (few nanometer-thick) may still be present

esulting in some loss of transmission. Decrease in transmissionith an increase in anodizing voltage may suggest that more effi-

ient consumption of aluminum takes place at smaller anodizingoltages.

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izing voltage (b), as a function of anodizing voltage.

Typical cross-sectional views of the sample anodized at 50 Vave been shown in Fig. 4. As shown in Fig. 4(a), almost 100 �mhick alumite layers have grown from opposite sides to meetach other at the middle of the sample. Aluminum has beenpparently consumed completely through the thickness exceptt the islands, which exhibit thin unconsumed aluminum. Theoss in transmission, as mentioned in Fig. 3 may partly bettributable to these islands. Fig. 4(b) shows the cross-section atigher magnification. The terraces in the image show longitu-inal view of pores, while the edges exhibit the cross-sectionaliew. Uniformity of the inter-pore spacing and extremely par-llel arrangement of high aspect ratio pores is clearly evident.ig. 4(c) is a high magnification image of the rectangular regionighlighted in Fig. 4(a). The position of the interface formedetween the alumite layers have been marked in the figure. It maye noticed that pores remain extremely parallel even close to the

ig. 3. Average percentage transmission of light at a wavelength range of00–1100 nm through the transparent regions of the sample anodized forhrough-thickness oxidation.

M. Mehmood et al. / Materials Chemistry and Physics 104 (2007) 306–311 309

F c–e) The middle position of the section in order to reveal the interface of the meetings of the pores.

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ig. 4. (a–e) Typical cross-sectional views of a sample prepared at 50 V; (a andides of the alumite, while (b) focuses the longitudinal and cross-sectional view

ally growing pores in order to facilitate this joining can also beeen in Fig. 5(d). A few pores from one side are joining with twoores on the opposite side forming Y-junctions. Such pores haveeen explored and found very interesting for preparing physicalanodevices [10,14,17]. In contrast to Fig. 4(d), a few aluminumarticles (or the sections of aluminum wires) are still presentn Fig. 4(e). This region is closer to the island of unconsumedluminum. Accordingly, aluminum is consumed gradually andt forms particles (or wires) at the last stage, which are alsoonsumed as the growth of the continuous oxide proceeds. Theontinuity of a few pores in proximity to the aluminum particlean also be seen indicating that the pore joining occurs beforeomplete consumption of aluminum, i.e., as a result of anodicxidation. Furthermore, I-type pore junctions can also be seen inig. 5(e). These junctions have been formed between the pores

lready heading towards each other.

The formation of numerous Y-junctions, at a specified loca-ion with same diameter of the pores has not been reportedarlier, as far as our knowledge is concerned. Using such pores

Fig. 5. A schematic diagram of the way the pores from the opposite sides tendto meet each other depending on their relative position (a number of pores fromopposite sides join each other to form Y-type, S-type and I-type junctions.).

310 M. Mehmood et al. / Materials Chemi

Fig. 6. FESEM cross sectional view of the sample prepared at 60 V, showingtwo alumite layers grown from opposite sides separated by barrier layer (a fewrm

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ounded white spots at the middle of the barrier layer, as indicated by arrow)ay be particles of unconsumed aluminum).

s template can extend the applications of porous alumina foraking nano-devices.The growth of pores and their ordering is believed to depend

n complex interaction of dissolution and formation conditions,on-uniform electric filed across the barrier layer, and stresstate in the barrier layer and the underlying aluminum [27–29].luminum with an inhomogeneous thickness due to protrusionselow the pores tips is finally converted to a mesh of thin wirest the thicker sections of aluminum below the cell walls. Therrangement of these wires (or the nanoparticles when viewedlong a given cross-section) should be complex because twoifferent hexagonal cells of the opposite sides meet each otherith various degrees of coherency from one position to another,

s schematically shown in Fig. 5(a) and (b). When these wiresor particles) are oxidized by anodizing, the three-dimensionaltress state, electric field distribution, formation and dissolutiononditions of alumina must be totally different from that onhe bulk aluminum due to change in geometrical features at thelumina–aluminum interface, which may alter the path of theore tips. In our opinion, the expansion around the aluminumanoparticles due to the conversion of aluminum to aluminaeems to exert the compressive stresses that push the barrierayer. Thus, a pore tip changes its path when it is heading towardsn aluminum particle; otherwise it keeps itself on a straight path,s schematically shown in Fig. 5(a) and (b), respectively. Theormer phenomenon results in Y-type or S-type junctions andhe later one favors the I-junction (straight) joints, as indicatedn Fig. 4(d) and (e).

Fig. 6 shows a cross-sectional view of the samples preparedt anodizing voltage of 60 V. The barrier layer is still present athe middle of the section. This suggests that thicker barrier layeremain intact and is not pierced-through by the pores. A fewarticles of aluminum can be seen sandwiched between the bar-

ier layers of the opposite side. Although not revealed by SEM,he presence of an extremely thin layer of aluminum cannot beuled out in other parts of the barrier layer, as well.

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stry and Physics 104 (2007) 306–311

As far as our knowledge is concerned, the formation of annterface between alumite layers grown from opposite sides hasot been explored before. The porous layers joining each other inhis way may be explored for physical devices based on fillinghe pores with a material of choice to form, e.g., Y-junctionsith equal diameter of wires; the wires separated at the endy a dielectric material, i.e., compact (barrier layer of) aluminaepending on the preparation conditions of the alumite.

. Conclusions

(i) Anodizing of aluminum sheet from opposite sides hasbeen performed in oxalic acid to achieve through-thicknessoxidation to form 100–400 �m thick transparent alumina.Transmission in the near infrared range, i.e., 900–1100 nm,ranges between 20 and 60%. The higher transmission isachieved at lower voltages, which may be due to betterconsumption of aluminum at lower voltages.

(ii) Electropolishing at 2 V in Brytal solution at 80 ◦C after heattreatment and subsequent two-step anodizing in oxalic acidhas resulted in the best hexagonal order at 50 V. The poresgrow straight from opposite sides and meet at middle ofthe sample. A number of pores join to form Y-type, S-typeand I-type junctions. The three-dimensional compressivestress state around the nanoparticles, at the last stages ofanodizing, seems responsible for pushing the pore tips awayand diverting their path to ensure smooth joining.

iii) In case of thicker barrier layers at higher anodizing voltage,e.g., 60 V, the pores do not meet and are separated by abarrier type oxide.

cknowledgements

The authors are grateful to Prof. Hiroki Habazaki (Hokkaidoniversity) for useful comments and suggestions.

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