New Fabrication Strategies for Ordered Nanoporous Alumina Kathrin Schwirn MPI of Microstructure Physics Halle/Saale
New Fabrication Strategies for OrderedNanoporous Alumina
Kathrin SchwirnMPI of Microstructure Physics
Halle/Saale
Outline
• Alumina - Initial point for applications• Ordered nanoporous alumina
– applications– self-ordering
• Tasks• High-voltage anodization of H2SO4
– lattice parameters– mechanism of pore growth– ordering conditions
• Outlook
Alumina - Initial point for Applications
• 1898 first patent on porous Al2O3– Corrosion passivation and upgrading of Al-components
Disordered porous alumina
• Condenser, rectifier• Absorber• Catalyst• Carrier material
Applications:
Ordered nanoporous alumina
• First report in 1995• Electrochemical anodization• Hexagonally ordered pores• Polydomain degree of order• High aspect ratio
– up to 1:4000
Al2O3
AlH. Masuda et al., Science 268, 1466 (1995)
Self-ordered porous alumina
Applications
• Nanotubes and nanorods– Electrochemical deposition of metals– Template wetting with polymers
• Hybrid systems– Aligned 1D nanostructures inside matrix
(e.g., high storage media)
• Nanobiotechnology– Separation of molecules– Infiltration of peptides– Cell growth– 1D diffusion of single fluorescence molecules
J. Choi et al., Chem. Mater. 15 776 (2003)
M. Steinhart et al., Science 296 1997 (2002)courtesy of P.Göring
Self-ordering during electrochemical anodization
+-
O 2-/OH -Al3+
Electrode
Electrolyte
Al2O 3
Aluminium
+-
O 2-/OH -Al3+
Electrode
Electrolyte
Al2O 3
Aluminium
1.
EAl3+O2-
EAl3+O2-
+-
Electrode
+-
Electrode
4.
EE+-
Electrode
+-
Electrode
3.
EE
+-
Electrode
+-
Electrode
2.
O. Jessensky, J.Electrchem. Soc. 145, 3735 (1998)
1. Formation of barrier layer
2. Fluctuation in surfaceenhanced electric field
3. Pore nucleationenhancement of electric field
4. End of excess poresStable pore growth
0.00
0.02
0.04
0.06
0.08
0.10
0 25 50 75 100 125 150 175 2000
5
10
15
20
25
Icu
rren
t [A
]
volta
ge [V
]
time [s]
V
1.
2. 4.3.
Parameters that determine self-ordering
• Applied voltage– Interpore distance
(dint)=2,5nm/V*U
• Type and concentration ofelectrolyte– Threshold of pore formation– pH value determines pore
diameter
• Temperature– Removing local heat– Preventing formed alumina
from attack of acid
0 50 100 150 2000
75
150
225
300
375
450
525
inte
rpor
e di
stan
ce [n
m]
applied voltage [V]
sulfuric acid oxalic acid phosphoric acid d
int=2.5*U
K. Nielsch et al., Nano Letters 2, 677 (2002)
Tasks
• Extension of latticeparameters– close the gap
• Increase long-range order• Decrease of porosity
0 50 100 150 2000
75
150
225
300
375
450
525
inte
rpor
e di
stan
ce [n
m]
applied voltage [V]
sulfuric acid oxalic acid phosphoric acid d
int=2.5*U
K. Nielsch et al., Nano Letters 2, 677 (2002)
?
SZ.Chu et al. Adv. Mater 17, 2115 (2005)
Solution:
Anodization with high voltage under high current limitation
High-voltage anodization with H2SO4
• Extension of latticeparameters
• Interpore distance from70nm to 145nm
0 20 40 60 80 1000
50
100
150
200
250
inte
rpor
e di
stan
ce [n
m]
applied voltage [V]
sulfuric acid oxalic acid d
int=2.5*U
sulfuric acid (high voltage)
anodization with 80V
0 300 600 900 1200 1500 1800
25
30
35
40
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
curr
ent [
A]
volta
ge [V
]
time [s]
Mechanism of high-voltage anodizationwith H2SO4
30 40 50 60 70 80 900.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
coun
ts/#
of p
ores
distance [nm]
26V dint
=55nm 32V d
int=65nm
30 40 50 60 70 80 900.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
coun
ts/#
of p
ores
distance [nm]
26V dint
=55nm
30 40 50 60 70 80 900.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
coun
ts/#
of p
ores
distance [nm]
26V dint
=55nm 32V d
int=65nm
40V dint
=72nm
30 40 50 60 70 80 900.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
coun
ts/#
of p
ores
distance [nm]
26V dint=55nm 32V d
int=65nm
40V dint
=72nm 40V d
int=73nm
Evolution of the distribution of nearestneighbor distance
25
30
35
40
0 500 1000 15000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
V
vo
ltage
[V]
curr
ent [
A]
time [s]
I
Mechanism of high-voltage anodization with H2SO4
• Periodicity of 120-150nm• Max/min ratio 1.5-1.7
• Pronounced die off ofpores
• Slight change of interporedistance with increasing voltage
• Sporadic ends of pores
40 50 60 70 80 900.0
0.1
0.2
0.3
0.4
coun
ts/#
of p
ores
pore distance [nm]
25V standard 40V
30 40 50 60 70 80 900.0
0.4
0.8
1.2
1.6
2.0
2.4
coun
ts/#
of p
ores
angle [°]
Local pair distribution functions
Local angular distribution functions
Distributions ofnearest neighbordistance
Distributions of anglebetween three nearestneighbors (expected forideal hexagonal lattice:sharp peak at 60°)
Comparison of lattice order
• High-voltage anodizationimproves the local order
Sample 25V HWHM 40V HWHMdint 62nm 3.5nm 66nm 3.0nmangle 60° 10.5° 60° 9.0°
Comparison of lattice order
0 200 400 600 800 10000.0
0.2
0.4
0.6
0.8
1.0
1.2 25V standard 40V
coun
ts/#
of p
ores
interpore distance [nm]
Pair distribution functions (long-range)
Distributions of following neighbor distances
• High-voltage anodizationleading to sooner loss of longrange order
• High-voltage process:correlation up to 850 nm
• Standard process: correlationup to more than 1000 nm
• Anodization under stress• Brittle membranes,
occurrence of cracks• Chemically more instable
Conclusions and Problems
• Extension of accessiblerange of lattice parameters
• Improved local order
Outlook
• Chemical composition of the oxide ?
• When/why takes high voltage anodization place ?
• Transition between standard and high voltageanodization ?
• How to produce mechanically stable membranes byhigh-voltage anodization ?
• Is it possible to reduce the porosity ?