Fabrication of Porous Anodic Alumina Templates with Sub-20nm Pores Shaud Tavakoli Sands Research Group Advisor: Manuel DaSilva

Background o Oxide film can be grown on certain metals via anodization o Aluminum, niobium, tantalum, titanium, tungsten, zirconium o Aluminum and titanium unique – thick oxide coating with high density of tiny pores possible o Other metals – only see formation of barrier oxide o Anodized alumina referred to as Porous Anodic Alumina (PAA)

Properties of PAA oElectrically insulating oOptically transparent over wide energy band oChemical and thermal stability oFactor of 2 volume expansion from aluminum to alumina oAlumina often thinner than original Al due to chemical dissolution of alumina during anodization oPore diameter 4-250nm oDensity of pores ranging from 10 8 to pores/cm 2 oThickness up to 300µm oBrittle/fragile

Applications of PAA o Electronic and optoelectronic devices o Magnetic storage o Chemical sensors o Biochemical membranes o Carbon nanotubes o Catalysts o Metallic/semiconducting nanowires and nanorods

Geometry of PAA o Ideally o Honeycomb structure o Close-packed array of columnar hexagonal cells o Each cell – central pore normal to substrate o Reality o Usually cells irregular polygons o 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 Reactions o Overall anodization reaction: 2Al + 3H 2 O  Al 2 O 3 + 3H 2 o Sum of reactions at each electrode o Metal/oxide interface: 2Al + 3O 2-  Al 2 O 3 + 6e - o Oxygen atoms react with metal o Oxide/electrolyte interface: Al H 2 O  Al 2 O 3 + 6H + o Aluminum anions react with water o Reaction at cathode: 6H + + 6e -  3H 2 o Hydrogen gas evolution

Our Procedure o Electropolish sample o Removes thin native oxide o Eliminates roughness o Provides a shiny surface finish o Two-step anodization o Anodize once o Strip alumina o Anodize second time o Pore order develops during 1 st anodization! o Characterize sample using field emission scanning electron microscopy Yuan et al. Chem. Mater. 2004, 16,

Experiments o Adjusting conditions: o acid concentration o anodization time o temperature o voltage o Using different electrolytes/voltages for 1 st and 2 nd anodization o Oxalic for 1 st anodization o Sulfuric for 2 nd anodization o Three-step anodization o Pore shrinking

Concentration M Sulfuric Acid 72 mM Sulfuric Acid M Sulfuric Acid Conditions: 15V 8 hr. 1 st anodization 4 o C

Anodization Time 1 st Anodization: 8h 2 nd Anodization: 22h Avg. pore diameter: ~20nm 1st Anodization: 20h 2nd Anodization: 50h Avg. pore diameter ~20.5nm Maybe result of etching Anodized at 4 o C and 15V in 72mM sulfuric acid.

Voltage o 15V sample avg. pore diameter ~20 nm o 20V sample larger pores than 15V 15V Sulfuric Acid20V Sulfuric Acid Anodized for 8 and 22 hrs. at 4 o C in 72mM sulfuric acid.

Two Solution Phosphoric 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 4 o C in 1M phosphoric acid, then 19 hrs. at 4 o C in 0.313M sulfuric acid. Anodized for 8 hrs. at 4 o C in 0.313M sulfuric acid, then 21 hrs. at 4 o C in 0.313M sulfuric acid.

Three-Step Anodization o Improved cell order with three-step o Cell order slightly decreased during 3 rd anodization Oxalic 40V/Sulfuric 10V Oxalic 40V/Oxalic 40V/Sulfuric 10V Oxalic 40V/Oxalic 40V/ Sulfuric 10V/ Sulfuric 10V All samples anodized at 4 o C. Oxalic acid concentrations: 0.3M; Sulfuric acid concentrations: 0.313M.

Two Solution Results o Cell order and uniformity possible o Approx. 7 pores per cell o 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 4 o C in 0.3M oxalic acid, then 16 hrs. at 4 o C in 0.313M sulfuric acid.

Pore Shrinking o Put sample in boiling water to convert alumina to aluminum oxy-hydroxide (Al(O)OH) o Optimize pore order o May develop irregular pores Myung et al. Nanotechnology 15 (2004)

Pore Shrinking Results 1 min. 5, 10, 20 min. o Conditions: o 4 o C, 40V o 0.3M oxalic acid o Anodized 9 hr.; 12hr. o 1 min. boil o Avg. pore diameter 40 nm o 20% reduction at surface o Longer heat treatment o Samples ruined

Pore Shrinking Results O sec.30 sec. 6O sec. 9O sec. Conditions: 4 o C, 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

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