slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
S canning E lectro c hemical M icroscopy (SECM) PowerPoint Presentation
Download Presentation
S canning E lectro c hemical M icroscopy (SECM)

Loading in 2 Seconds...

play fullscreen
1 / 48

S canning E lectro c hemical M icroscopy (SECM) - PowerPoint PPT Presentation


  • 679 Views
  • Uploaded on

S canning E lectro c hemical M icroscopy (SECM). Heterogeneous reactions Industrial applications - heterogeneous catalysts combinatorial chemistry. Heterogeneous reactions. Aristoteles : „ corpora non agunt nisi fluida seu soluta “ Compounds that are not fluid or dissolved, do not react

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'S canning E lectro c hemical M icroscopy (SECM)' - royal


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Scanning Electrochemical

Microscopy (SECM)

slide2

Heterogeneous reactions

Industrial applications - heterogeneous catalysts

combinatorial chemistry

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.“

slide3

D

D

Reactions at interfaces

  • Assumptions
  • Mass transport is limited to diffusion
  • Diffusion constants are equal for both, educt and product
  • Adsorption, desorption, and reaction are not distinguished
slide4

Forward reaction rate

Backward reaction rate

Electron transfer reactions

Electrode reaction

slide5

Dependence of kf and kb on the interfacial potential difference

Current-potential characteristic (Butler-Volmer model)

Electron transfer reactions

slide6

Current-potential characteristic

Electron transfer reactions

Exchange current

At equilibrium i = 0

slide7

Nernst-equation

Electron transfer reactions

Testing the equation

slide8

ia

ic

Calculation of i0 starting from ic

Electron transfer reactions

Exchange current

slide9

ScanningElectrochemical

Microscopy (SECM)

slide13

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

slide14

Planar and radial diffusion at electrodes

Fick‘s second law in one dimension

Concentration profiles at disk electrodes

1 s after starting a diffusion-controlled electrolysis

r0 = 3 mm

r0 = 30 µm

r0 = 300 µm

rad > 6 = UME

vert

rad

slide15

Spherical diffusion at an UME

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

slide16

Hemispherical diffusion at UME

Planar diffusion

(Cottrell-equation)

Planar and radial diffusion at electrodes

Current-time curves

r0 = 5 µm

r0 = 12.5 µm

r0 = 1.5 mm

slide17

r0

rA

Preparation of UME

Pt

glass

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

slide18

UME probe

Ultramicroelectrode

5 < RG < 20

slide20

Approach curves

Tip far away from surface

Tip close to the surface

Current depends on distance between tip and sample

slide21

i/ i

i/i

d/r0

Approach curves

slide22

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

slide23

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

slide24

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

slide26

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

slide27

Feedback mode

Positive feedback

Negative feedback

i/i

i/i

d/r0

d/r0

=> Reactivity of flat surfaces

=> Topography of inactive surfaces

slide28

Enzyme mediated positive feedback mode

Enzyme is immobilized on surface

Enzyme catalyzes the reduction of the oxidized species

slide29

Oxidation of [Fe(CN)6]4-at the Pt-UME

[Fe(CN)6]4- [Fe(CN)6]3-+ e-

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-

slide30

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

slide31

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

slide32

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!

slide33

F = 1 nN => x = 140 nm

Tip

Sizelength l =100-500 µm

thickness t = 0.3-5 µm

width w = 10-50 µm

Material Si or Si3N4 (E = modulus of elasticity)

Spring constant

Example

ESi= 179 GPa, l = 200 μm, w = 10 μm, t = 0.5 μm

=> k = 0.007 N/m

slide34

Which forces can occur?

  • Van-der-Waals forces
  • Coulomb forces
  • Repulsive forces
  • Hydrophobic entropic forces
slide35

Mirror

PSD

LED

Cantilever

with tip

sample

z-Signal

Scanning electronics

Piezo

Scanner

Setup of a scanning force microscope

Contact mode - constant height mode

slide37

Mirror

PSD

LED

Cantilever

with tip

sample

setpoint

z-Signal

Control unit

Scanning electronics

Piezo

Scanner

Setup of a scanning force microscope

Contact mode - constant force mode

slide39

Characterization of SECM-AFM tips

Spring constant

Example

EPt= 17 GPa , l = 1200 μm, w = 200 μm, t = 5 μm

=> k = 0.06 N/m

slide40

Approximation of tip radius

Linear sweep voltammetry

i∞ = 0.8 nA

Hemispherical geometry

D(IrCl63-) = 7.5 ∙ 10-6 cm2 s-1

c*(IrCl63-) = 0.01 M

=> r0 = 180 nm

slide41

Determination of the tip geometry

Approach curve

Cantilever deflection

Contact point

b = 2

Contact point

b = 1, 1.5, 2, 2.5, 3

Cone-like geometry

r0

h

slide43

Imaging polycarbonate membranes

AFM image (constant force mode)

Diffusion profile

SECM image

slide44

Imaging polycarbonate membranes

AFM image (constant force mode)

SECM image

slide46

Imaging polycarbonate membranes

AFM image (constant force mode)

SECM image

slide47

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.

slide48

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