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XPS & ARXPS for Fluid Interfaces

XPS & ARXPS for Fluid Interfaces. Geok Mei CHONG Master Candidate of Advanced Spectroscopy in Chemistry University of Leipzig, ASC Network 4 th December 2009. Outline. Principle of XPS & ARXPS Instrumentation Depth Profile by ARXPS XPS and ARXPS applied to fluid analysis

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XPS & ARXPS for Fluid Interfaces

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  1. XPS & ARXPS for Fluid Interfaces Geok Mei CHONG Master Candidate of Advanced Spectroscopy in Chemistry University of Leipzig, ASC Network 4th December 2009

  2. Outline • Principle of XPS & ARXPS • Instrumentation • Depth Profile by ARXPS • XPS and ARXPS applied to fluid analysis • Experimental setup for fluid analysis • Application of in research • Surfactant • Water • Biological molecules

  3. Principle of XPS & ARXPSPhotoelectric effect

  4. Required spectrometer components

  5. Electron energy analyzer & detector Radius of curvature is dependent on kinetic energy of electron. Channel electron multipliers

  6. What information is learned from XPS? • Elemental Identification • Chemical State Identification • Quantification • Mapping • Depth profile • ARXPS

  7. Depth profile How is z related to Is ? z = depth λ = mean free path θ = emission angle λ’ = observation depth Io attenuated exponentially according to Beer Lambert law λ’ = λ cos θ

  8. Angular resolved XPS the observed depth information varies with photoelectron detection angle θ λ’ = λ cos θ z = depth λ = mean free path θ = emission angle (relative to surface normal) λ’ = observation depth

  9. Angular resolved XPS λ’ = λ cos θ z = depth λ = mean free path θ = emission angle (relative to surface normal) λ’ = observation depth the observed depth information varies with photon energy

  10. Angular resolved XPS Quantification The Observed photoelectron intensity of element A: A fitting process Ical -> Iobs

  11. XPS & ARXPS applied to fluid H. Siegbahn, K. Siegbahn, J. Electron Spectrosc. Rel. Phenomena 2 1973, 319 • First performed by H. Siegbahn, K. Siegbahn and colleagues. • Complete separation between PE signals from liquid and vapour using a beam of liquid formamide.

  12. XPS & ARXPS applied to fluidA challenging investigation Needs for producing “well-behaved” liquid beam in vacuum • Liquids of sufficient low vapour pressure (< 1 Torr). • Cooled to -40 0C • Droplet formation for high vapour pressure • Loss of PE when absorbed by the vapor • Surface smoothness • Sample charging effect

  13. XPS & ARXPS applied to fluidHow to produce “well behaved” liquid beam? Liquid lamella Rotating metal disc • Produced flat liquid surface • Allowed studies of liquids with low vapour pressure.

  14. XPS & ARXPS applied to fluidHow to produce “well behaved” liquid beam? Liquid microjet The size of the jet was reduced to the μsize range. • Vacuum jet consists of a smooth continuous region of liquid water, which decays into droplet at a distance of approximately 5mm. • Allowed studies of liquids with higher vapour pressure, example: water. • However, using HeI radiation, only the outer valence region could be probed.

  15. Angular resolved XPS Quantification A fitting process Ical -> Iobs The Observed photoelectron intensity of element A: (Eq 1) Requires accurate knowledge of photoionization cross section and angular characteristics of emission direction

  16. Application of ARXPS in researchSurfactant F.Eschen, M. Heyerhoff, H. Morgner, J. Vogt, J. Phys. Condens. Matter 7 (1995) 1961 Concentration depth profile of TBAI in FA from C 1s • Used chemical shift to evaluate the relative intensities due to TBAI and FA. • The contributions from TBAI, FAliq and FAgas are separated. • The ratio of the peak area of TBAI to that of FAliq are determined for many combinations of photonenergies and observation angles.

  17. Application of ARXPS in researchSurfactant F.Eschen, M. Heyerhoff, H. Morgner, J. Vogt, J. Phys. Condens. Matter 7 (1995) 1961 Concentration depth profile of TBAI in FA from C 1s • Single molecular layer is assumed to be 1.5 Å thick. • Large decrease in salt conc. after 3rd layer. • The thickness of the enhanced salt conc. was estimated to be about 12 Å. • Given diameter of TBA+ is 9.5 Å, the thickness of enhanced salt corresponds to 1 monolayer of salt. • TBA+ ions have preferred orientation near the surface

  18. Application of ARXPS in researchBehaviour of hydroxide at the water interface Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247 O1s XPS (microjet) spectra of NaOH 0.2 – 2M aqueous solutions • Spectral contributions from H2O(gas), H2O(aq), and OH-(aq) @ 600eV were assigned. • Zoom into the OH-(aq) 2pπ. • Fully quantitative of OH-intensity was not visible here as the intensity of O1s peak was small.

  19. Application of ARXPS in researchBehaviour of hydroxide at the water/vapour interface Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247 Oxygen 1s XPS spectra of NaOH 0.2 – 2 M aqueous solutions • OH-(aq) 2pπ and OH-(aq) O1s photoelectron signal as function of OH- conc. • Linear dependence of the interfacial OH- density on bulk conc. • MD results support PE experiments findings.

  20. Application of ARXPS in researchBehaviour of hydroxide at the water/vapour interface Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247 Experimental and computational calculations suggest that: • OH- do not have any special surface binding site. • There is linear dependence of the interfacial OH- signal on its bulk concentration. Some earlier studies suggest that OH- strongly accumulates within the interfacial region (cluster?). The debates are still on going …

  21. Applications in biological Molecules D. Nolting, E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 129 (2007) 14068 N1s PE spectral of 0.5m lysine at diff. pH • Biological molecules in water environment is very challenging in monitoring local charge density. • Microscopic structure of aa is sensitive to pH

  22. Applications in biological Molecules D. Nolting, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 130 (2008) 8150. N1s PE spectral of 2m imidazole aqueous at diff. pH • Structural changes can be faster than time resolution of NMR (10-5 s). • At high pH, proton exchange between the 2 N site on time scale of 10-12 s. • The 2 chemically pseudo-equivalent N atoms resolved.

  23. Conclusion • ARXPS is highly surface sensitive. Possible to probe depth profile as small as 1.5 nm. • ARXPS is very sensitive to study interfacial at various depths at microscopic scale. • Still challenging to deal with fluid samples, especially high vapor pressure solution. An interesting and challenging field …

  24. Quiz ! Which peak is caused by inelastic scattering? Why XPS is surface sensitive? What is the main factor that affect the spatial resolution of XPS?

  25. References • H. Siegbahn, K. Siegbahn, J. Electron Spectrosc. Rel. Phenomena, 2 (1973), 319 • H. Siegbahn, S. Svensson and M. Lundholm, J. Electron Spectrosc. Rel. Phenomena 24 (1981), p. 205 • Eschen F, Heyerhoff M, Morgner H and Vogt J (1995) J. Phys.: Condens. Matter 7 1961 • Faubel M and Steiner B Ber. Bunsenges. Phys. Chem. 96 (1992)1167 • Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247 • B. Winter, M. Faubel, Chem. Rev. 106 (2006) 1176 • D. Nolting, E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 129 (2007) 14068

  26. Thank you for your attention! Discussion session

  27. Quiz again? Just kidding 

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