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SciencePress

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Trans. Nonferrous Met. SOC. China 16(2006) s681-s684

'kansactions ofNonferrous MetalsSocietyof Chinawww.csu.edu.cnJysxbl

Electrochemical modification process of anodic alumina membraneW Mei(T %), LIU Jian-hua($l]&%), LI Song-mei(%fi&)

School of Materials Soience and Engineering, Beijing University of Aeronautics and Astronautics,Beijing 100083, China

Received 10April 2006; accepted 25 April 2006Abstract: The modification procedure of anodic alumina membrane(AAM) was studied. The AAM structure after modification wascharacterized by nickel nanowires prepared in AAM. Scanning electron microscopy was used to characterize the topography andstructure properties of the AAM and nickel nanowires. The transformation of the current during the voltage reduction was studied.The mechanism of current and structure change during modification was discussed. The results show that a root structure producesafter the AAM modification. The length of the root structure depends on the velocity of the voltage reduction. Slow voltage reductionleads to a large length of the root structure, otherwise, a short length of the root structure. At the end of the modification, the barrierlayer is thin enough to be passed by electrons. Hence, the direct electrodeposition of one-dimensional nanowires can be carried outon theAAM with barrier layer and aluminum matrix successfully without any other treatments.Key words: anodic alumina membrane; modification; electrodeposition; nanowires

1 IntroductionThe synthesis of one-dimension nanomaterials

attracted more and more interests because of their basicconcepts and potential technology application, such ashigh density perpendicular magnetic recording mediaand nanosensor[l-31. The AAM played an importantrole in the synthesis of one-dimension nanomaterials dueto their advantages[3,4]. There are many approachesbased on AAM to fabricate one-dimension nanomaterials,such as alternating current electrodeposition[5], directcurrent electrodeposition[6-81, sol-gel[9], electrolessdeposition[ 101 and vapor-phase deposition technique[111.Among the different approaches to the fabrication ofone-dimension nanomaterials, direct current electro-deposition based synthetic method was used widely. Theideal AAM consists of three parts of nanopores alumina,barrier layer (in the middle of nanopores alumina andaluminum matrix) and aluminum matrix. The generalprocesses in direct current electrodeposition are that thealuminum matrix is dissolved at first and then etched toremove the compact layer. Then, a conductive metallayer is sputterdeposited on one side of the AAM to serveas the working electrode during the electrodeposition.These processes are inconvenient and the pore diameter

of AA M is not easily controlled after the etching.In this paper, an easy synthesis approach wassuggested without removing the barrier layer and

aluminum matrix of the AA M before theelectrodeposition. The thickness of the barrier layer wasmodified by additional electrochemical process aftercompleting the anodizing step of the AAM. After themodification of the AAM, the preparation ofone-dimension nanomaterials based on direct currentelectrodeposition was carried out successfully withoutother treatments. The anodizing voltage of modificationprocesses decreased at a certain velocity. Thetransformation of the current during the decreasingprocesses of the voltage was also studied.

Through the modification of the AAM, nickel (Ni)nanowires were prepared successfully using the directcurrent electrodeposition in the AAM with barrier layerand aluminum substrate without any other treatments. Itis known that the topography of the nanowires is similarwith the pores of "6 8, 91. Therefore, the change ofthe AAM morphology based on the modification wascharacterized by that of the nickel nanowires.

2 ExperimentalThe AAM templates were prepared in 0.3 mol/L

Correspondingauthor: LIU Jian-hua; TeVFax: +86-10-82317103;E-mail: Iiujh@buaa,edu.cn

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s682 YU Mei, et aVTrans. Nonferrous Met. SOC. hina 16(2006)oxalic acid so lution by a two-step anodizing process [ 121.The pure aluminum (Al) sheet (99.999%) plates weredegreased in acetone, and then annealed in the furnace at500 'C for 1.5 h to remove the mechanical stress andrecrystallize. The A1 plates were electropolished under aconstant current of 10 mA/cm2 for 1 min. Electropoli-shing solution was a mixture of HC104 and C ~H SO H iththe volume ratio of 1 : 4.

In the first step of anodic oxidation process, thepretreated aluminum sheets were exposed to the preparedacid solution in the electrochemical cell at voltage of 40V for 3 h. During anodization, electrochemical cell wasput in a water bath to keep temperature at the roomtemperature. Then, the plates were immersed in amixture solution of phosphoric acid (0.4 mol/L) andchromic acid (0.2 mol/L) at 60 "C or 3 h to remove thealumina layer formed in the first step of anodizationprocess.

The parameters of the second step of anodizationprocesses were the same with that of the first step ofanodization process. Between two steps, the aluminaplates were washed with distilled water.

After the second step of anodizing procedure, themodification process was carried out by decreasing theanodizing voltage at 1-1.5 V/s. The decreasing processof the voltage was not stopped until the anodizingvoltage was smaller than the electrodeposition voltageapplied. And also, the anodizing voltage was separatelyreduced from 40 V to 35 V (sample A) and from 40 V to30 V (sample B) to study the change of the current withthe reduction of anodizing voltage.

The direct current electrodeposition of the nickelnanowires was performed in a bath containing NiS04(200 g/L), NiCl (50 g/L), H 3B 03 (45 g/L) at the roomtemperature after the AAM modification. The pH valueof the bath w as adjusted to 4.4-5.2.

Scanning electron microscopy (SEM XL30 S-FEG)was used to characterize the topography and structureproperties of the AAM and the nickel nanowires. Aunanoparticles were sputtered on the AAM sample surfacebefore SEM measurement and increased the conductivity.The AAM was removed with 6 mol/L NaOH .3 Results and discussion

Fig.1 shows SEM images of AAM. As shown inFig.1 a), the AAM has a highly ordered porous structureand the pores organize in an almost precise hexagonalstructure. From Fig. 1 b), the pores are straight, uniformand parallel to each other in the whole length. Themicroparticles on the surface shown in the micrographare Au caused by the sputtering Au before SEMdetermination. Au microparticles were too large to findthe detail of the transformation of the AAM structure

after modification. Because the morphology ofnanowires prepared by AAM template method wassimilar with that of the nanopores in the used AAM, themorphology of nickel nanowires was also characterizedby SEM to investigate the change of the AAM structureafter the m odification.

Fig.1 SEM images of AAM: (a) Top-view; (b) Cross-sectionviewFig.2 shows the SEM image of nickel nanowires

fabricated within the nanopores of AAM aftermodification of the AAM. Fig.2(b) shows themagnification graph of the nanowires bottomcorresponding to Fig.2(a). It is shown that there are manyrooted structure at the bottom of nanowires and all thenanowires in area B (marked in Fig.2(a)). But, nanowiresin area A (marked in Fig.2(a)) are almost smooth surfacecontrast to area B. Because the size of the nanowires issimilar with that of the used AAM, it can be concludedthat the structure of the AAM changes with theprocedures of modifying. There are root structures at thebase of AAM nanopores.

The transformation of the an odizing current densitywith the voltage decreasing is shown in Fig.3. It indicatesthat the current steep reduces when the voltage decreasesrapidly from 40 V to 35 V (sample A) and from 40 V to30 V (sample B) at 60 s . The current comes tostabilization with the anodization process performing.Sample A has a shorter time to reach steady state contrastto sample B.The changes of the anodizing current and the AAMstructure with the voltage reduction were explained byfollowing model[13]. Column (a) is the schematicdiagram of barrier layer seen from the arrow a s shown in

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Fig.2 SEM images of N i nanowires: (a) Large scale area; (b )Magnification graph of area marked in (a)

Time/sFig3 Changes curves of anodizing current with voltage(SampleA from 40 V to 35 V, sample B from 40 V to 30 V)Fig.4. Column (b) in Fig.4 is the schematic diagram ofthe AAM cross section. The thickness of the barrierdepends on the anodizing voltage (1-1.2 nmN)[14]. Thebarrier will dissolve with the anodizing voltagedecreasing (step 11 ) until the thickness adapts to thelater voltage (step 111). With the barrier dissolving, thenew smaller pores are produced at the base of previouslarger pores. When the voltage decreases rapidly, thebarrier will dissolve, so there are no current flowing inthe anodizing electric circuit. It can explain the currentchanges in Fig.3.

The modification of AA M in this paper is acontinuous voltage reduction instead of rapid reduction.

Fig.4 Schematic diagram of AAM change during anodizingvoltage reducingWhen the voltage decreases at a certain velocity, thebarrier is dissolving. Smaller pores are produced at thebarrier layer. Then, there are smaller pores produced atthe base of the new pores and it causes a root structure atbase of pores. Fig.5 illustrates the structure of the poresafter AAM modifying and also explains the results ofFig.2. The length of the root structure depends on thevelocity of the voltage reduction. High velocity leads toshort time modification and short length of the rootstructure. Otherwise, slow velocity leads to modificationof long time and increases the length of the root structure.

Length of

Barrier layerFig.5 Schematic diagram of AAM structure after modification

The thickness of the barrier layer reduces with thevoltage reducing. At the end of AAM modification, theanodizing voltage will be smaller than the voltage planedto apply during electrodeposition. Hence, electron canpass the thin barrier layer at the electric field producedby the electrodeposition voltage when the electro-deposition performs. It also is proved by the successfulsynthesis of nickel nanowires in our work.

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4 ConclusionsThe AAM templates are modified successfilly by

the anodizing voltage reduction. The m odification of thenanopores AAM simplifies the synthesis process of one-dimensional nanomaterials. The AA M structure ch angesafter the modification. A root structure produces at thebase of the pores. The length of root structure depends onthe velocity of the voltage reduction. The thickness ofthe barrier layer becomes smaller and smaller with themodification process performing. When theelectrodeposition voltage planed to apply is larger thanthe voltage at end of the modification procedure, directcurrent electrodeposition can perform successfullywithout any other treatments.References

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