Screening Effects in Probing the Electric Double Layer by Scanning Electrochemical Potential Microscopy (SECPM)

Transcription

1 Presented at the COMSOL Conference 2009 Milan Screening Effects in Probing the Electric Double Layer by Scanning Electrochemical Potential Microscopy (SECPM) R. Fayçal Hamou MaxPlanckInstitut für Eisenforschung GmbH Interface Chemistry and Surface Engineering Department Atomistic Modeling Group (AMG) 1

2 V e V IHP 2 0

3 Experimental techniques Atomic force microscopy Scanning tunneling microscopy Scanning electrochemical microscopy : ECSTM, SECPM Allow electrochemists to learn more about the structure of the double layer at the atomic level. On the theoretical side, the new numerical methods of calculations provide a possibility to simulate, all the changes within the double layer. 3

4 Scanning Electrochemical Potential Microscopy (SECPM) Probing the potential profile of the EDL SECPM :Patented in

5 SECPM probe EDL Potential Profiling Electrolyte Electrode 5

6 Previous experimental results Potential profile In the SECPM experiments presented here, the decay length of the potential profile was always smaller than the Debye length from the Gouy Chapman Stern theory Debye length depends on the applied potential. Cedric Hurth; Chunzeng Li; Allen J. Bard; J. Phys. Chem. C 2007, 111,

7 SECPM probe Scanning electron micrographs of a PtIr tip prepared by the procedure described earlier in the text. at low (a) and high (b) magnification. PhD Thesis C. M. Hurth 2005 under the supervision of Prof. Allen J. Bard, The University of Texas at Austin 7

8 SECPM simulation Metallic probe Coating Electrolyte Electrode Metallic apex Using the PoissonBoltzmann for simulating the EDL (by including a Stern layer) Poisson equation to model the dielectric coating Suitable boundaries, Moving mesh, time dependent simulation: Probe moving at 10 nm/s Using Comsol Multiphysics software 8

9 3D distribution of the electric field and potential 9

10 Effect of the metallic apex geometry : protruding probe Length protrusion effect 15 nm 23 nm Geometry 1 Geometry 2 11 nm Geometry 3 Quasi flat surface 2.5nm 10

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14 Effect of the metallic apex geometry : protruding probe 14

15 Effect of the metallic apex geometry : protruding probe Potential profile between the Probe and the electrode for different separation distances. 15

16 Effect of the metallic apex geometry : protruding probe Geometry 2 Geometry 1 Surface charge density on the metallic protrusion 16

17 Effect of the exposed metallic surface r=8nm r=15nm r=22nm r=60nm r=30nm 17

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20 Effect of the exposed metallic surface Variation of the electric flux passing through the exposed metallic tip during the approach 20

21 Effect of the exposed metallic surface Variation of the tip surface charge density during the approach 21

22 The effect of the Open Circuit Potential: Positively charged 22

23 The effect of the Open Circuit Potential: Positively charged ocp=0.148 V 23

24 The effect of the Open Circuit Potential: Positively charged ocp=0.046 V 24

25 The effect of the Open Circuit Potential: Positively charged 25

26 The effect of the Open Circuit Potential: Positively charged 26

27 The effect of the Open Circuit Potential: Positively charged 27

28 The effect of the Open Circuit Potential: Positively charged 28

29 The effect of the Open Circuit Potential: Positively charged 29

30 The effect of the Open Circuit Potential: Positively charged 30

31 The effect of the Open Circuit Potential: Positively charged Variation of the electric flux passing through the exposed metallic tip during the approach 31

32 The effect of the Open Circuit Potential: Positive charge Variation of the tip surface charge density during the approach 32

33 The effect of the Open Circuit Potential: Negatively charged ocp=0.149 V 33

34 The effect of the Open Circuit Potential: Negatively charged ocp=0.149 V 34

35 The effect of the Open Circuit Potential: Negatively charged ocp=0.045 V 35

36 The effect of the Open Circuit Potential: Negatively charged 36

37 The effect of the Open Circuit Potential: Negatively charged 37

38 The effect of the Open Circuit Potential: Negatively charged 38

39 The effect of the Open Circuit Potential: Negatively charged 39

40 The effect of the Open Circuit Potential: Negatively charged 40

41 The effect of the Open Circuit Potential: Negatively charged 41

42 The effect of the Open Circuit Potential: Negatively charged Variation of the electric flux passing through the exposed metallic tip during the approach 42

43 The effect of the Open Circuit Potential: Negatively charged Variation of the tip surface charge density during the approach 43

44 Conclusion In this investigation it was shown that the tip geometry has an influence on the probed potential. A sharp protrusion distorts the charge distribution, which can effect the probed potential considerably. A clear electrostatic screening effects was observed in probing the double layer. This effect depends on the strength of the double layer at the probe. This simulation will be extended for the surface potential mapping in order to comprehend better the importance of the effects mentioned above. 44