Tavakoli by xiaoyounan


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

                                  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
o Characterize sample using
  field emission scanning
  electron microscopy
o Adjusting conditions:
  oacid concentration
  oanodization time
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

                                 72 mM Sulfuric Acid

         0.313 M Sulfuric Acid

•8 hr. 1st anodization
                                 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

             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
  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.
  o 20% reduction at surface
o Longer heat treatment
  o Samples ruined

                               5, 10, 20 min.
               Pore Shrinking Results
•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.

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