Transcript Folie 1 - uni-regensburg.de

Scanning Electrochemical
Microscopy (SECM)
1
Heterogeneous reactions
Aristoteles: „corpora non agunt nisi fluida seu soluta“
Compounds that are not fluid or dissolved, do not react
J. B. Karsten (1843): „Philosophy of Chemistry“
The reaction of two heterogeneous, solid, and under certain conditions reactive
compounds can only occur if one of them can be transformed into a fluid induced by
the interaction between the two compounds at a given temperature or due to
pressure increased temperature, which then will induce the fluid state in the other
compound.“
Heterogeneous reactions
Industrial applications - heterogeneous catalysts
combinatorial chemistry
2
Reactions at interfaces
Assumptions
D
• Mass transport is limited to diffusion
• Diffusion constants are equal for both,
educt and product
• Adsorption, desorption, and reaction are
not distinguished
D
3
Electron transfer reactions
Electrode reaction
Ox z  n e
kf
kb
Redz '
Forward reaction rate
ic
nFA
z  n  z'
Backward reaction rate
v b  kbcred  0, t  
ia
nFA
v net  v f  v b  k f cox  0, t   kbcred  0, t  
i
nFA
v f  k f cox  0,t  
i  i c  i a  nFA k f cox  0, t   kbcred  0, t 
4
Electron transfer reactions
Dependence of kf and kb on the interfacial potential difference
  nF

k f  k0 exp 
E  E 0'  

 RT



 1    nF
0' 
kb  k0 exp 
E  E 
RT




i  nFA k f cox  0, t   kbcred  0, t 
Current-potential characteristic (Butler-Volmer model)




 1    nF
  nF
0' 
0' 
i  nFAk0 exp 
E

E
c
0,
t

exp
E

E
c
0,
t




 ox  

 red  
RT
RT








5
Electron transfer reactions
Exchange current
At equilibrium
*
cox 0,t   cox
i=0
*
cred 0,t   cred
E  Eeq
Current-potential characteristic




 1    nF
  nF
0' 
0' 
i  nFAk0 exp 
E  E   cox  0, t   exp 
E  E   cred  0, t 


RT
RT













 1    nF
  nF
0'  * 
0'  *
nFAk0 exp 
E

E
c

nFAk
exp
E

E
c




 ox 

 red 
eq
0 
eq
RT
RT

 







6
Electron transfer reactions
Testing the equation





 1    nF
  nF
0'  * 
0'  *
nFAk0 exp 
Eeq  E   cox   nFAk0 exp 
Eeq  E   cred 


 RT
 





 RT



 1    nF
  nF
0'  *
0'  *
exp 
Eeq  E   cox  exp 
Eeq  E   cred


RT
RT






*
cox
 nF
0' 

exp
E

E


eq
*
cred
RT



*
RT cox
Eeq  E 
ln *
nF cred
0'

Nernst-equation

7
Electron transfer reactions
Exchange current


 1    nF
  nF
0' 
*
0' 
nFAk c exp 
Eeq  E    nFAk0cred exp 
Eeq  E  


RT
RT






*
0 ox
ic
ia
i0  ia  ic
Calculation of i0 starting from ic
  nF

*
i0  nFAk0cox
exp 
Eeq  E 0'  

 RT

*
 cox

 * 
 cred 
*
cox
 nF
0' 

exp
E

E


eq
*
cred
 RT


*
*  cox 
i0  nFAk0cox  * 
 cred 



 nF

 exp 
Eeq  E 0 ' 
 RT






* 
 nFAk0cox
1 
*
cred
*
*
cox
 cred
c
i0  nFAk0c
8
Scanning Electrochemical
Microscopy (SECM)
9
Scanning probe microscopy (SPM) techniques
10
Principle of scanning probe techniques
11
Scanning electrochemical microscope
12
Ultramicroelectrodes (UME)
Essential concept
At least in one dimension (the “characteristic dimension”), the size of the
electrode surface is smaller than the diffusion length of the redox active species
(during the time period of the experiment)
Spherical or hemispherical UME
Disk UME
Cylindrical UME
Band UME
13
Planar and radial diffusion at electrodes
Fick‘s second law in one dimension
c
 2c
D 2
t
x
Concentration profiles at disk electrodes
1 s after starting a diffusion-controlled electrolysis
r0 = 3 mm
r0 = 300 µm
r0 = 30 µm
 rad 
DR/Ot
r02
rad > 6 = UME
vert
rad
14
Planar and radial diffusion at electrodes
Chronoamperometric experiments
Applying a constant potential E
diffusion controlled transport of the electroactive species
monitoring the time-dependent current that depends on the concentration gradient
How does the concentration gradient of cR/O change?
Planar diffusion at a conventional electrode
Spherical diffusion at an UME
15
Planar and radial diffusion at electrodes
Current-time curves
Hemispherical diffusion at UME
r0 = 5 µm
r0 = 12.5 µm
*
DR/O *
nFDR/OcR/O
j t   nF
cR/O 
t
r0
*
nFDR/OcR/O
j 
r0
Planar diffusion
r0 = 1.5 mm
j t   nF
DR/O *
cR/O
t
(Cottrell-equation)
16
Preparation of UME
Pt
glass
rA
RG 
rA
r0
r0
Melting of wires into glass tubes => large RG-values
Pulling wire-glass tube with a pipet puller => decrease of RG value
Etching of platinum wires and isolation with electrodeposition paint
17
UME probe
Ultramicroelectrode
5 < RG < 20
18
Scanning electrochemical microscope
19
Approach curves
Tip far away from surface
Tip close to the surface
*
i  2 nFDR/OcR/O
r0
Current depends on distance between tip and sample
20
Approach curves
i/ i
i/ i
d/r0
21
Modi of SECM
Generation-collection mode (GC)
Sample-generation/tip-collection mode (SG/TC)
Tip-generation/sample-collection mode (TG/SC)
=> Constant height
Feedback mode (FB)
Negative feedback
Positive feedback
=> Constant current
22
Generation-collection mode
Sample-generation/tip-collection mode (SG/TC)
Tip is scanned across the surface at constant height
Generator:
Heterogeneous reaction
Mass transport through a pore
23
Generation-collection mode
Disadvantages
Diffusion layer larger than the tip => determines lateral resolution
Electrical isolation of SECM-tip limits diffusion of educts to the generator
In case of large generator areas a continuously increasing background
signal is observed due to the formation of product
Advantages
In the beginning of the measurement no background signal occurs as there
is no product produced
24
Generator: glucose oxidase
NH2
FAD
O
O
N
N
O
P
HO
N
O
O
H2C
N
P
O
O
O
H
HO
H
H
OH
H
OH
HO
CH2
N
O
N
NH
N
O
H OH
H OH
H O
H O
HO
H
H
OH
OH
OH
H
+ O2
+
HO
H
H
OH
OH
O
H2O2
25
Generator: glucose oxidase
-D-Glucose + Glucoseoxidase/FAD  Glucono--Lacton + Glucoseoxidase/FADH2
Glucoseoxidase/FADH2 + O2  Glucoseoxidase/FAD + H2O2
Oxidation of H2O2 at the Pt-UME
H2O2  2 H+ + 2 e- + O2
26
Feedback mode
Positive feedback
Negative feedback
i/ i
i/ i
d/r0
=> Topography of inactive surfaces
d/r0
=> Reactivity of flat surfaces
27
Enzyme mediated positive feedback mode
Enzyme is immobilized on surface
Enzyme catalyzes the reduction of the oxidized species
28
Enzyme mediated feedback mode: glucose oxidase
-D-Glucose + Glucoseoxidase/FAD  Glucono--Lactone + Glucoseoxidase/FADH2
Glucoseoxidase/FADH2 + O2  Glucoseoxidase/FAD + H2O2
Glucoseoxidase/FADH2 + [Fe(CN)6]3- Glucoseoxidase/FAD + [Fe(CN)6]4-
Oxidation of [Fe(CN)6]4- at the Pt-UME
[Fe(CN)6]4-  [Fe(CN)6]3- + e-
29
Enzyme mediated positive feedback mode
Disadvantages
Redox mediator has to be a cofactor of the enzyme, which limits the possible
enzymes to oxidoreductases
As the mediator concentration is rather low, the signals are also small
Enzymes need to be immobilized on inactive surfaces. Active surfaces would
lead to a large background signal, larger than that of the enzyme
The probe-sample distance has to be small => possible damage of the UME
Advantages
Lateral resolution is better than in GC mode
30
Combination of SECM and AFM
Samples can show variations in both reactivity and topography.
Thus, it is difficult to resolve these two components with conventional
SECM measurements
New strategies are required to determine sample topography and
reactivity independently
A) Addition of a second electroactive marker to provide information on the
topography of the sample
B) Vertical tip position modulation
C) Shear force damping of the UME
=>
Absolute sample-tip-distance is not known
Combination of SECM and AFM
31
Principle of AFM
Binnig, Quate, and Gerber 1986, Phys. Rev. Lett. 56, 9
Detection of atomic forces to monitor tip-sample distances
10-7-10-11N!
32
Tip
Size
length l =100-500 µm
thickness t = 0.3-5 µm
b
h
width w = 10-50 µm
Material
l
Si or Si3N4 (E = modulus of elasticity)
wt 3
Spring constant k  E 3
4l
Example
ESi = 179 GPa, l = 200 μm, w = 10 μm, t = 0.5 μm
=> k = 0.007 N/m
F  k  x
F = 1 nN
=> x = 140 nm
33
Which forces can occur?
•
•
•
•
Van-der-Waals forces
Coulomb forces
Repulsive forces
Hydrophobic entropic forces
34
Setup of a scanning force microscope
Mirror
PSD
LED
Contact mode - constant height mode
Cantilever
with tip
sample
z-Signal
Piezo
Scanner
Scanning electronics
35
Contact mode / constant height
36
Setup of a scanning force microscope
Mirror
PSD
LED
Contact mode - constant force mode
Cantilever
with tip
sample
z-Signal
Piezo
Scanner
Scanning electronics
setpoint
Control unit
37
Preparation of SECM-AFM tips
38
Characterization of SECM-AFM tips
wt 3
Spring constant k  E 3
4l
Example
EPt = 17 GPa , l = 1200 μm, w = 200 μm, t = 5 μm
=> k = 0.06 N/m
39
Approximation of tip radius
Linear sweep voltammetry
i∞ = 0.8 nA
Hemispherical geometry
*
i  2 nFDR/OcR/O
r0
D(IrCl63-) = 7.5 ∙ 10-6 cm2 s-1
c*(IrCl63-) = 0.01 M
=> r0 = 180 nm
40
Determination of the tip geometry
Cantilever deflection
Approach curve
Contact point
b=2
Contact point
b = 1, 1.5, 2, 2.5, 3
Cone-like geometry
r0
h
b
h
r0
41
Experimental setup
42
Imaging polycarbonate membranes
AFM image (constant force mode)
Diffusion profile
SECM image
43
Imaging polycarbonate membranes
AFM image (constant force mode)
SECM image
44
Experimental setup II
45
Imaging polycarbonate membranes
AFM image (constant force mode)
SECM image
46
References
Bard, A. J., Faulkner, L. R. (2001) Electrochemical methods.
Fundamentals and applications. John Wiley & Sons, Inc., New York
Kranz, C., Wittstock, Wohlschläger, H. Schuhmann, W. (1997)
Imaging of microstructured biochemically active surfaces by means
of scanning electrochemical microscopy. Electrochimica Acta, 42,
3105-3111.
Macpherson, J. V., Unwin, P. R. (2000) Combined scanning
electrochemical-atomic force microscopy. Anal. Chem. 72, 276-285
Macpherson, J. V., Jones, C. E., Barker, A.L., Unwin, P. R. (2002)
Electrochemical imaging of diffusion through single nanoscale
pores. Anal. Chem. 74, 1841-1848.
47
Prof. Wolfgang Schuhmann
Anal.Chem.-Electroanalytik & Sensorik,
Ruhr-University Bochum
"Microelectrochemistry – from materials to biological applications"
Wednesday, June 18, 2003
17.00 h
Lecture room: Biol. 5.2.38
For further information see http://www.uni-regensburg.de/GK/SP
48