Fabrication of Porous Anodic Alumina Templates with

Sub-20nm Pores

Shaud TavakoliSands Research Group

Advisor: Manuel DaSilva

Backgroundo Oxide film can be grown on

certain metals via anodizationo Aluminum, niobium, tantalum,

titanium, tungsten, zirconiumo Aluminum and titanium unique –

thick oxide coating with high density of tiny pores possibleo Other metals – only see formation of

barrier oxideo Anodized alumina referred to as

Porous Anodic Alumina (PAA)

Properties of PAAo Electrically insulatingo Optically transparent over wide energy bando Chemical and thermal stabilityo Factor of 2 volume expansion from aluminum

to aluminao Alumina often thinner than original Al due to

chemical dissolution of alumina during anodizationo Pore diameter 4-250nmo Density of pores ranging from 108 to 1012 pores/cm2

o Thickness up to 300µmo Brittle/fragile

Applications of PAAo Electronic and optoelectronic

deviceso Magnetic storageo Chemical sensorso Biochemical membraneso Carbon nanotubeso Catalystso Metallic/semiconducting nanowires

and nanorods

Geometry of PAAo Ideally

o Honeycomb structureo Close-packed array of

columnar hexagonal cells

o Each cell – central pore normal to substrate

o Realityo Usually cells irregular

polygonso Pores often unordered

Masuda et al. J. Electrochem. Soc., Vol. 144, No. 5, May 1997

A. Metzger et al. IEEE Transaction on Magnetics, Vol. 36, No. 1, January 2000

B. Nielsch et al. Nano Letters Vol. 2, No. 7, July 2002

Relevant Reactionso Overall anodization reaction: 2Al + 3H2O Al2O3 + 3H2

o Sum of reactions at each electrodeo Metal/oxide interface:

2Al + 3O2- Al2O3 + 6e-

o Oxygen atoms react with metalo Oxide/electrolyte interface:

Al3+ + 3H2O Al2O3 + 6H+

o Aluminum anions react with watero Reaction at cathode:

6H+ + 6e- 3H2o Hydrogen gas evolution

Our Procedureo Electropolish sample

o Removes thin native oxideo Eliminates roughnesso Provides a shiny surface

finish

o Two-step anodizationo Anodize onceo Strip aluminao Anodize second timeo Pore order develops during

1st anodization!

o Characterize sample using field emission scanning electron microscopy

Yuan et al. Chem. Mater. 2004, 16, 1841-1844

Experimentso Adjusting conditions:

oacid concentrationoanodization timeotemperatureovoltage

o Using different electrolytes/voltages for 1st and 2nd anodizationoOxalic for 1st anodizationoSulfuric for 2nd anodization

o Three-step anodizationo Pore shrinking

Concentration

0.313 M Sulfuric Acid

72 mM Sulfuric Acid

0.625 M Sulfuric Acid

Conditions:•15V•8 hr. 1st anodization•4oC

Anodization Time

•1st Anodization: 8h•2nd Anodization: 22h•Avg. pore diameter: ~20nm

•1st Anodization: 20h•2nd Anodization: 50h•Avg. pore diameter ~20.5nm

•Maybe result of etching

Anodized at 4o C and 15V in 72mM sulfuric acid. Anodized at 4o C and 15V in 72mM sulfuric acid.

Voltage

o 15V sample avg. pore diameter ~20 nmo 20V sample larger pores than 15V

15V Sulfuric Acid 20V Sulfuric AcidAnodized for 8 and 22 hrs. at 4o C in 72mM sulfuric acid.Anodized for 8 and 22 hrs. at 4o C in 72mM sulfuric acid.

Two SolutionPhosphoric 104V/Sulfuric 10V•~280nm cell size•100+ pores/cell

Sulfuric 25V/Sulfuric 10V•~60nm cell size•4-5 pores/cell

Anodized for 5 hrs. at 4o C in 1M phosphoric acid, then 19 hrs. at 4o C in 0.313M sulfuric acid.

Anodized for 8 hrs. at 4o C in 0.313M sulfuric acid, then 21 hrs. at 4o C in 0.313M sulfuric acid.

Three-Step Anodization

o Improved cell order with three-step

o Cell order slightly decreased during 3rd anodization

Oxalic 40V/Sulfuric 10V Oxalic 40V/Oxalic 40V/Sulfuric 10V

Oxalic 40V/Oxalic 40V/Sulfuric 10V/ Sulfuric 10V

All samples anodized at 4o C. Oxalic acid concentrations: 0.3M; Sulfuric acid concentrations: 0.313M.

Two Solution Results

o Cell order and uniformity possibleo Approx. 7 pores per cello Pore order within cells not observed

Oxalic 35V/Oxalic 35V/Sulfuric 10V• ~85nm cell size• 8-9 pores/cell

Oxalic 30V/Oxalic 30V/Sulfuric 10V• ~75nm cell size• 6-7 pores/cell

Samples anodized for 8 and 12 hrs. at 4o C in 0.3M oxalic acid, then 16 hrs. at 4o C in 0.313M sulfuric acid.

Pore Shrinkingo Put sample in boiling water to

convert alumina to aluminum oxy-hydroxide (Al(O)OH)o Optimize pore ordero May develop irregular pores

Myung et al. Nanotechnology 15 (2004) 833-838

Pore Shrinking Results

1 min.

5, 10, 20 min.

o Conditions:o 4oC, 40Vo 0.3M oxalic acido Anodized 9 hr.; 12hr.

o 1 min. boilo Avg. pore diameter

40 nmo 20% reduction at

surface

o Longer heat treatmento Samples ruined

Pore Shrinking Results

O sec. 30 sec.

6O sec. 9O sec.

Conditions:•4oC, 30V•Anodization times

•8hr; 12hr; 12hr•0.3M oxalic acid

0 sec.•~25nm pores

30 sec.•~22nm pores

60 sec.•~22nm pores

90 sec.•~21nm pores

THE END.