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 o Electrically insulating o Optically transparent over wide energy band o Chemical and thermal stability o Factor of 2 volume expansion from aluminum to alumina o Alumina often thinner than original Al due to chemical dissolution of alumina during anodization o Pore diameter 4-250nm o Density of pores ranging from 108 to 1012 pores/cm2 o Thickness up to 300µm o Brittle/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 Masuda et al. J. Electrochem. Soc., Vol. 144, No. 5, May 1997 o Usually cells irregular polygons o Pores often unordered 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 + 3H2O Al2O3 + 3H2 o Sum of reactions at each electrode o Metal/oxide interface: 2Al + 3O2- Al2O3 + 6e- o Oxygen atoms react with metal o Oxide/electrolyte interface: Al3+ + 3H2O Al2O3 + 6H+ o Aluminum anions react with water o Reaction at cathode: 6H+ + 6e- 3H2 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 Yuan et al. Chem. Mater. 2004, 16, 1841-1844 o Strip alumina o Anodize second time o Pore order develops during 1st anodization! o Characterize sample using field emission scanning electron microscopy Experiments o Adjusting conditions: oacid concentration oanodization time otemperature ovoltage o Using different electrolytes/voltages for 1st and 2nd anodization oOxalic for 1st anodization oSulfuric for 2nd anodization o Three-step anodization o Pore shrinking Concentration 72 mM Sulfuric Acid 0.313 M Sulfuric Acid Conditions: •15V •8 hr. 1st anodization •4oC 0.625 M Sulfuric Acid Anodization Time Anodized at 4o C and 15V in 72mM sulfuric acid. Anodized at 4o C and 15V in 72mM sulfuric acid. •1st Anodization: 8h •1st Anodization: 20h •2nd Anodization: 22h •2nd Anodization: 50h •Avg. pore diameter: ~20nm •Avg. pore diameter ~20.5nm •Maybe result of etching Voltage 15V Sulfuric Acid 20V Sulfuric Acid Anodized 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. o 15V sample avg. pore diameter ~20 nm o 20V sample larger pores than 15V Two Solution Phosphoric 104V/ Sulfuric 10V •~280nm cell size •100+ pores/cell Anodized for 5 hrs. at 4o C in 1M phosphoric acid, then 19 hrs. at 4o C in 0.313M sulfuric acid. Sulfuric 25V/ Sulfuric 10V •~60nm cell size •4-5 pores/cell 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 Oxalic 40V/Sulfuric 10V Oxalic 40V/Oxalic 40V/Sulfuric 10V o Improved cell order with three-step o Cell order slightly decreased during 3rd anodization Oxalic 40V/Oxalic 40V/ All samples anodized at 4o C. Oxalic acid concentrations: 0.3M; Sulfuric acid concentrations: 0.313M. Sulfuric 10V/ Sulfuric 10V Two Solution Results Oxalic 35V/Oxalic 35V/Sulfuric 10V Oxalic 30V/Oxalic 30V/Sulfuric 10V • ~85nm cell size • ~75nm cell size • 8-9 pores/cell • 6-7 pores/cell o Cell order and uniformity possible o Approx. 7 pores per cell o Pore order within cells not observed 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 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) 833-838 Pore Shrinking Results o Conditions: o 4oC, 40V o 0.3M oxalic acid o Anodized 9 hr.; 12hr. o 1 min. boil o Avg. pore diameter 40 1 min. nm o 20% reduction at surface o Longer heat treatment o Samples ruined 5, 10, 20 min. Pore Shrinking Results Conditions: •4oC, 30V •Anodization times •8hr; 12hr; 12hr •0.3M oxalic acid 0 sec. •~25nm pores 30 sec. O sec. 30 sec. •~22nm pores 60 sec. •~22nm pores 90 sec. •~21nm pores 6O sec. 9O sec. THE END.
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