1. Probing Buried Electrochemical Interfaces with Multiplex Broadband Sum Frequency Generation Spectroscopy Alexei Lagutchev, Guo-Qiang Lu, Tomohiro Takeshita, Rachel Behrens, Andrzej Wieckowski and Dana Dlott School of Chemical Sciences, University of Illinois at Urbana-Champaign

2. SFG in electrochemistry electrode Main challenge in monitoring electrode surface: get through electrolyte electrolyte window SFG:sensitive to vibrations at the surface (+) produces visible response (+) lots of background signals are generated (-)

3. Electrochemical cell with gap SFGVIS IR Electrolyte F MgF 2 I

4. Cell design counter electrode to solution delivery system to reference bridge MgF 2 window O-ring Kel-F base bottom view side view electrode gas out syringe barrel plunger gas in

5. CO on Pt: SFG spectrum

6. CO on Pt: cyclic voltammetry

7. CO on Pt: wavelength vs. potential

8. CO on Pt: bandwidth vs. potential

9. SFG vs. cyclic voltammetry

10. 2D phase transition of CO on Pt(111) (2  2)  3CO (  19  19)  13CO atop, 3-fold atop, bridge

11. SFG detection CO saturated solution  26 cm -1 V -1 amplitude (a.u.) frequency (cm -1 ) width (cm -1 ) 2060 2070 2080 0.00.40.8 0 10 20 “”“” (2 x 2) to (  19 x  19) *SFG intensity jumps by 3.1 *no Stark shift after transition

12. Atop vs. multifold sites 0.022 -0.173 V 0.277 0.517 0.532 0.547 atop bridge 3-fold 1600180020002200 0.562 0.607 0.637 0.502 wavenumber (cm -1 ) SFG intensity (a.u.)

13. SFG intensity and surface coverage Normally one expects I sfg ~N 2 Experiment: From structure: When corrected for dipole-dipole interaction and changes in electron back-donation from Co to Pt

14. CO from other source on Pt(100) 1800190020002100 SFG intensity (a.u.) wavenumber (cm -1 ) 0 s after oxidation 145 275 365 435 500 0.1M HCOOH + 0.1M H 2 SO 4 @ -200 mV vs. Ag/AgCl Low potential - low coverage CO favors bridge sites Coverage effect

15. Jumping to higher potential 1800190020002100 SFG intensity (a.u.) wavenumber (cm -1 ) 0 s 2 5 25 230 500 0.1M HCOOH + 0.1M H 2 SO 4 @ 25 mV vs. Ag/AgCl Eventually CO favors atop sites Potential effect + coverage effect

16. Higher E - noticeable oxidation current 1800190020002100 SFG intensity (a.u.) wavenumber (cm -1 ) 0 s after oxidation 2 8 32 315 500 Lower steady state coverage - bridge sites remain populated Coverage effect 0.1M HCOOH + 0.1M H 2 SO 4 @ 225 mV vs. Ag/AgCl

17. CO on Pt black on Au 1800190020002100 SFG intensity (a.u.) wavenumber (cm -1 ) -0.19 V 0.08 0.16 0.30 0.45 0.64 0.56 0.1M H 2 SO 4, CO saturated CO on Au - oxidizes first CO on Pt

18. Polycrystalline Ni 1800190020002100 SFG intensity (a.u.) wavenumber (cm -1 ) -0.975V -0.855 -0.735 -0.615 -0.465 -0.405 0.1M NaOH, CO saturated

19. Background vs. delay

20. Less obvious background effect

21. Time domain picture

22. Conclusions and future work SFG is uniquely suitable to study electrochemical interfaces - vibrational spectroscopy least restrictive for kinetics Accurate quantitative analysis of electrode coverage is possible provided local field corrections are accounted for Time domain manipulations can be successfully used to suppress background signals CO on noble metals (Pt, Pd, Au) may be used as ultrathin detection layer in pump-probe experiments - fast shocking, heating. CO adsorption on nanoparticles can be studied - direct application to electrocatalysis