Probing into the Electrical Double Layer Using a Potential Nano-Probe

Transcription

1 A3 Foresight Program, , 26 Probing into the Electrical Double Layer Using a Potential Nano-Probe Heon Kang ( 姜憲 ) Department of Chemistry, Seoul National University, Republic of Korea ( surfion@snu.ac.kr) Coworkers: Dae-Ha Woo (Pohang University of Science and Technology) Tae-Ho Shin (SNU) Young-Hwan Yoon (SNU) Prof. Il-Cheol Jeon (Jeonbuk National University) Prof. Su-Moon Park (Pohang University of Science and Technology) Funding: Korea Research Foundation Single-molecule devices in ambient environments contain not only the bridging molecule but also other components, such as contact resistance and interfacial capacitance whose electrical operties are still veiled. 1

2 Electrochemical Cell Nernst equation RT E = E o - ln Q nf Is thermodynamics all that controls electrochemical cells? IE(Cu) = 7.72 ev IE(Cu + ) = 2.3 ev in gas phase 2 Ag + (aq) + Cu(s) 2 Ag(s) + Cu 2+ (aq) Very strong electric fields are required to ionize metal atoms even in solution. The electrical double layer (EDL) at the electrode/electrolyte interface controls the rates, direction, and even the nature of electrochemical reactions. Electrical Double Layer Extremely thin ( ~ 1 nm) Strong electric field ( ~ 1 7 volt/m ) Electric field-induced chemistry The inner potential has not yet been directly measured. 2

3 Gouy-Chapman Model of a Diffuse Layer Thickness of the GC diffuse layer at 25 tanh( e / 4kT ) exp( x) tanh( e / 4kT ) e C (M) 1/ (A ) Previous Studies Measurement of forces between two overlapping EDLs Inverse decay length = (3.29 x 1 7 )C o 1/2 The Surface Force Apparatus Israelachvili & Adams, J.Chem. Soc. Faraday Trans. I (1978) Molecular layering of water on mica Israelachvili & Pashley, Nature (1983) 3

4 Silica colloidal particle attached to the tip of an atomic force microscope Vibrational frequency shift of molecules adsorbed on electrode surfaces (vibrational Stark effect) reveals strong electric fields inside the EDL. Korzeniewski (1985) Weaver (1993) Oklejas, Sjostrom, Harris, J. Am. Chem. Soc. (22) Hillier, Kim, Bard, J. Phys. Chem. (1996) Probing into the Electrode/Electrolyte Interface Using a Potential Nano-Probe 4

5 Potential Nano-Probe / STM Potential Nano-obe Potentiostat Anion Cation Water Au Tip CE RE Potential Probe STM Bias Voltage WE Piezo Actuator Au Electrode Fabrication of a Gold Nano-Probe ~ 1.7 V Au wire etching Varnish coating Pulse etching of the apex ~ 4 ns i l in na current in na mm HCl 4 ns, 1.65 V pulse i l E in V (vs. Ag/AgCl) number of pulse Cutting down of a gold apex by pulse etching CV limiting current (i lim ) decreases as the apex is ogressively etched by ultrashort (4 ns) pulses. Charging time of UME with r =1 nm : τ = RC ~ 1 MΩ x 2fF ~2ns CV :.1 M K 4 Fe(CN) 6 / 1. M KCl(aq) D. H. Woo et al., Anal. Chem. (23) 5

6 SEM Image of Etched Gold Tips (a) : The whole shape of an electrochemically etched gold tip. (b) and (c) : Its apex in a side and in a top view. The radius of curvature at the apex is about 25 nm. (d) and (e) : The flattened apex of a gold tip after pulse etching seen from the side and from the top. The apex radius is about 1 nm. Ideal Potential Probe (i = with R input = ) E F metal Φ m vacuum Φ s E F soln Φ Φ m E metal F Φ s E F soln obe solution separated in contact Upon contact of an inert metal with a solution, E F metal = E f soln or μ m electrochem = μs electrochem (The electrochemical potentials for the charged species at the obe and in the solution near the obe are equal.) μ e m - e o φ m = μ e s - e o φ s (for electrons) μ i m + z i e o φ m = μ i o,s + RT ln a is + z i e o φ s (for charged species i) At equilibrium (if net current i = ), EDL is absent at the obe/solution interface. The obe can directly read the electrochemical potential of the local environment. 6

7 A Real Potential Probe with a Voltage Follower (i input 1 fa with R VF ~ 1 14 Ω) A small current flows across the obe/solution interface due to the finite input resistance of VF (R VF ~ 1 14 Ω, i input ~ 1 fa ~ 6x1 4 ions s -1 ). A voltage may develop across the interface (and therefore, an EDL may be formed), when the obe has a very small area. The obe feels the electrochemical potential of the local environment through this EDL. Characterization of a Potential Probe in a Bulk Electrolyte Solution RE Potential WE - o (mv) -2-4 GND Time (a) 2 mm, r ~ 2 nm (b) 2 mm, r ~ 5 nm (c) 1 mm, r ~ 5 nm c b a 1 2 Time (s) Probe : 2-5 nm radius Solution : 2-1 mm concentration of ferri-/ferro-cyanide The solution potential is changed in a step function, and the dynamic response of a obe is measured. 7

8 Equivalent Circuit of Potential Probe -25 V () V (oo) simulation V () C C C VF V (mv) experiment Time V ( ) C RVF R R C VF 1 R 1 R VF VF C C VF V s (t) i i vf R R vf V Equivalent circuit of the potential obe R vf ~5x1 13-2x1 14 Ω C vf ~ 1x1-11 F R ~ Ω C ~ 1x1-11 F Au(111) Surface of WE : Cyclic Voltammogram and STM image t ( A ) Current Au(111), 1 mm NaClO 4 scan rate =.5 V/s cm-2 j / Ac M HClO4.1M NaClO E WE ( V vs. Ag/AgCl ) E/V (vs. Ag/AgCl) Experiments were done in the double layer region (red CV curve) in the absence of gold oxidation or specific adsorption of electrolytes. E WE (vs. Ag/AgCl RE) = E WE (vs. gold RE) +.33 ±.2 V, WE surface area =.56 cm 2. STM image 8

9 Potential Profile across the Electrode/Electrolyte Interface : Measurements at Various Electrode Potentials WE : Au(111) - in V mm NaBF 4 E WE =.4 V ( ).2 V ( ).12 V( ). V ( ) -.2 V ( ) 1 2 distance to the bulk in nm WE EDL region bulk solution For E WE =.4 V (O) GRD.1 V E WE.4 V φ.12 V E RE φ : bulk potential measured at the obe Woo et al., Bull. Korean Chem. Soc. 24, 25, 577 Potential Profile at Various Electrolyte Concentrations Electron tunneling region EDL overlapping Tail of a diffuse layer The outer part of a potential ofile is fitted to the GC model of a diffuse layer. ze ze tanh tanh e 4kT 4kT x 9

10 Electrochemical Potential Microscopy (ECPM): Equi-potential Mapping of a Patterned Gold Surface = -25 mv, x 14A STM topography = -2 mv, x 3 A Electrode Bias Potentials for the Potential Feedback NaClO 4, 1 mm solution Bulk, 7-8 mv Ground, mv -2 mv -25 mv o exp(- x) = 25 mv : x 1.4 nm = 2 mv : x 3 nm -5 mv RE -3 mv WE Radius of the obe 33 nm Scan speed = 1 μm /sec 1

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