Tavakoli by xiaoyounan

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									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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