Potential Modulated Spectroscopy in Electrochemical Systems

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1 Potential Modulated Spectroscopy in Electrochemical Systems David J. Fermín Action D36

2 Overview Combining potential modulation and spectroscopy? Electroreflectance Potential Modulated Spectroscopy of Molecular Systems Mapping Surfaces with Potential Modulated Spectroscopy Take home message

3 Potential Modulated Spectroscopy (PMS) 3 Electrochemical Impedance Spectroscopy (EIS) accurate determination of charge transfer/transport kinetics EIS Chemically non-specific (poor at establishing mechanisms) Combining EIS Spectroscopy: dynamic responses specific to chemical species PMS Applicable to molecular systems and solid state materials

4 Potential Modulated Spectroscopy (PMS) 4 Transmission/Reflectance/Fluorescence Electroreflectance/absorption Spectroscopy Infrared Spectroscopy (Dr. A. Rodes and Dr. P. Rodriguez) Microwave Reflectivity

5 Electroreflectance 5 I 0 R0 + R sin( ωt + φ ) E0 + E sin( ωt )

6 Potential Modulated Spectroscopy 6 T. Sagara, Adv. Electrochem. Sci. Eng. Vol. 9

7 Electroreflectance 7 Three layer model: 1 bulk electrolyte, electrode/electrolyte boundary, 3 bulk electrode Buried layer: adsorbates, upd layers, electron spill-over λ ε 1 ϕ ˆ ε = ε ' + iε '' ˆ ε = ε ' + iε '' d

8 Electroreflectance 8 Dielectric function of Au films and reflectivity spectrum perpendicularly to the surface

9 Electroreflectance 9 Assuming the thickness of the buried layer d << λ R 8π dn ϕ ε ε 1cos ˆ ˆ = Im 3 R λ ε1 ˆ ε3 ( 1 ˆ ˆ3 )( ˆ ˆ3 ) ( )( ) R 8π dn1 cosϕ ˆ ε 1 sin ˆ ε ε ε ε ε + ε ϕ Im 3 R = λ ε1 ˆ ε3 1 1 ˆ ε3 ε1 + ˆ ε3 sin ϕ

10 Electroreflectance ER signal arising from surfaces states at Ag (100) electrodes 1 0 ER signal dependence on E 0 : population of surface states D. Kolb, Phys. Rev. Lett. 1981

11 Electroreflectance ER signal arising from surfaces states (A and B) in Ag (100) electrodes 1 1 ER signal dependence on E 0 : population of surface states Sharp potential drop across the interface! D. Kolb, Phys. Rev. Lett. 1981

12 Electroreflectance Polycrystalline Au in the presence and absence of CO in 0.1 M H SO 4. Features A and B: transition from d orbitals to surface states 1 D. Kolb, Electrochim. Acta 31 (1986) 99 CO: weak effect on the electroreflectance signal A. Cuesta et al., Electrochim. Acta 48 (003) 949

13 Electroreflectance 1 3 Polycrystalline Au in the presence and absence of CO in 0.1 M NaOH. Surface states: populated at -0.9 V Strong effect of CO adsorption in alkaline solution CO high coverage at negative potentials. A. Cuesta et al., Electrochim. Acta 48 (003) 949

14 Potential Modulated Spectroscopy 1 4 I 0 R0 + R sin( ωt + φ ) E0 + E sin( ωt )

15 PMS Thin Molecular layers 1 5 H N Anodic Cathodic current absorbance Oxidation of the polymer phase generates changes in the absorbance (60 nm) No hysteresis in the absorbance vs. charge curve Kalaji and Peter, J. Chem. Soc. Faraday Trans. 87 (1991) 853

16 PMS Thin Molecular layers I0 T T I Surface concentration ( σ ) = exp Γ 0 n n Absorption cross-section 1 6 At two different potentials a and b ( ) ( ) ( ) ( n n ) T b T a T a = T T = exp Γ σ 1 Considering a small potential perturbation, T T = Γ σ Changes in the surface concentration are proportional to the faradaic charge Q F [ σ σ ] [ σ σ ] R O R O = F = T T F nf Q nf I dt n n Kalaji and Peter, J. Chem. Soc. Faraday Trans. 87 (1991) 853

17 PMS Thin Molecular layers The total ac current (I tot ) and charge (Q tot ): Itot = V Z = V Y Q = i dt = V Ydt = C V tot tot Complex capacitance 1 7 The relationship between the complex optical and electrical parameters: ( ) ( σ σ ) Copt = nf T T R O V ( ) [ σ σ ] ( ω ) R O d T T dt = IF t nf { F ( σ σ ) } ( ) Yopt = n R O V d T T dt ac optical responses are 90 phase shift with respect to the ac faradaic current Optical and faradaic responses are proportional if non-faradaic responses are negligible Kalaji and Peter, J. Chem. Soc. Faraday Trans. 87 (1991) 853

18 PMS Thin Molecular layers Distributed impedance associated with thin conducting polymers Useful to display several complex representation 1 8 R sheet = 0 Ω cm -1 R contact = 10 Ω R solution = 0. Ω cm C polymer = 0.5 mf cm - Impedance + Admittance Capacitance Kalaji and Peter, J. Chem. Soc. Faraday Trans. 87 (1991) 853

19 PMS Thin Molecular layers Complex impedance and optical response for a 6 nm PANI film at various dc potentials V V Good correlation between the electrical and optical relaxation constants. Confirm that the current responses are primarily associated with faradaic processes. Deconvoluting charging current (non-faradaic) from redox processes Kalaji and Peter, J. Chem. Soc. Faraday Trans. 87 (1991) 853

20 Electroreflectance Microscopy Microscopic surface characterisation employing ER 0 Changes in the composition at the surface of semiconductors (e.g. corrosion) P. Salvador, J. Phys. Chem. 1981

21 Electroreflectance Microscopy Evolution of the ER signal of RuS during electrochemical etching 1 15 Hz 300 Hz EER-b EER-s Contrast in the ER arises from inhomogeneous doping of the RuS electrode M. Turrion, J. Electroanal. Chem. 1998

22 Potential Modulated Fluorescence - PMF Potential induced adsorption and transfer of ionic species Fluorescence responses in total internal reflection water Linearly polarised excitation beam DCE Orientation of molecules adsorbed at the interface

23 PMF Molecular interfaces Complex ion transfer at polarisable interfaces between two immiscible liquids 3 Ag Ag SO 4 LiSO 4 Dye (aq) BTPPATPFB (DCE) BTPPACl LiCl AgCl Ag RE aq (Ag/Ag SO 4 ) aq. phase LiSO4 piston burette CE aq (Pt wire) DCE phase Bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate RE org (Ag/AgCl) / LiCl + Bis(triphenylphosphoranylidene) ammonium chloride cw He-Cd laser (44 nm) CE org (Pt wire) optical wave guide PMT H. Nagatani, J. Phys. Chem. B 104 (000) 6869

24 PMS Molecular interfaces Potential induced transfer of Ru(bipy) 3 + from water to DCE Voltammograms exhibits a quasi-reversible behaviour 4 ν =, 5, 10 and 0 mv s -1 Water to DCE DCE to Water H. Nagatani, J. Phys. Chem. B 104 (000) 6869

25 PMS Molecular interfaces Fluorescence is proportional to the amount of transferred ions 5 εφi0 εφi0 F = QF ( t ) = IF ( t ) dt zfa zfa The differential fluorescence: df dt = εφi0 zfa I F ( t ) The ac-fluorescence is 90 out of phase with respect to the faradaic admittance: εφi E zfa 0 iω F = YF H. Nagatani, J. Phys. Chem. B 104 (000) 6869

26 PMS Molecular interfaces Admittance and potential modulated fluorescence responses 6 Im E = 10 mv rms f = 6 Hz Re E = 10 mv rms f = 6 Hz Re Im Admittance: faradaic and non-faradaic contributions PMF: Only contributions linked to the fluorescent ion PMF independent of polarisation of the incident beam H. Nagatani, J. Phys. Chem. B 104 (000) 6869

27 PMS Molecular interfaces Transfer of ZnTMPyP 4+ from water to 1,-dichloroethane 7 Complex electrochemical responses: adsorption + transfer ν =, 5, 10 and 0 mv s -1 E = 10 mv rms f = 6 Hz Re E = 10 mv rms f = 6 Hz Re Im Im PMF responses: adsorption prior and after the transfer step 0.1< φ w o 0.1< φ < 0.1 w o 0.1< φ < 0. oφ 0. w > ZnTMPyP 4+ (aq) ZnTMPyP 4+ (w/o) ZnTMPyP 4+ (o/w) ZnTMPyP 4+ (o) w o H. Nagatani, J. Phys. Chem. B 104 (000) 6869

28 PMS Molecular interfaces Transfer of ZnTMPyP 4+ from 1,-dichloroethane to water 8 PMF signal as function of the polarisation of the excitation beam E = 10 mv rms f = 6 Hz Porphyrin transition dipoles: 78 with respect to the interface normal (neglecting orientation distribution) H. Nagatani, J. Phys. Chem. B 104 (000) 6869

29 Take Home Message 9 Correlating impedance with chemically specific information Effect of electrode potential on surface electronic structure Interfacial surface states and molecular organisation Highly sensitive tool for studying adsorption and charge transfer

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