ABC s of Electrochemistry: X-Ray Photoelectron Spectroscopy (XPS) Madhivanan Muthuvel

Transcription

1 ABC s of Electrochemistry: X-Ray Photoelectron Spectroscopy (XPS) Madhivanan Muthuvel Center for Electrochemical Engineering Research (CEER) Chemical and Biomolecular Engineering Ohio University Athens, Ohio November 17, 2011

2 Outline What is XPS? Background Principle Instrumentation Analysis of XPS Data Applications Facility at Ohio University Summary 2

3 What is XPS? X-ray photoelectron spectroscopy (XPS) is also known as Electron Spectroscopy for Chemical Analysis (ESCA) XPS is a surface analytical technique Widely used to determine the chemical information in addition to elemental information of the samples Related techniques are Auger electron spectroscopy (AES) and Ultra-violet photoelectron spectroscopy (UPS) 3

4 Background In 1887, Heinrich Hertz observed the photoelectric effect In 1905, Albert Einstein explained the photoelectric effect with a simple mathematical description, which lead to Nobel Prize in Physics E = h - E k E k and E b is the kinetic energy and binding energy of the photoelectron, respectively, and hν is the energy of the incident beam b 4

5 Background Prof. Kai Siegbahn from the University of Uppsala, Sweden, utilized the photoelectric effect to develop an analytical technique During the mid-1960 s, Prof. Kai Siegbahn and his co-workers developed the analytical technique known as X-ray photoelectron spectroscopy (XPS) He coined the term Electron spectroscopy for chemical analysis (ESCA) In 1981, Prof. Kai Siegbahn was awarded the Nobel Prize in Physics for the development of the XPS technique 5

6 Principle Conducting Sample E = h E k b sample Φ sample is the work function of the sample Work function is the energy difference between Fermi level and Vacuum level University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/xps Class 99.pps] 6

7 Principle Analysis of sample with the XPS instrument e - Sample Spectrometer Free Electron Energy E k (1s) E k (1s) Vacuum Level, E v hv sample spec Fermi Level, E f E b (1s) E 1s E = h E k b spec University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/xps Class 99.pps] 7

8 Instrumentation Photo of a XPS Instrument Kratos Axis Ultra model in Surface Science Western Laboratory at The University of Western Ontario 8

9 Instrumentation Schematic for a XPS instrument Hemispherical Energy Analyzer Computer System Outer Sphere Magnetic Shield Inner Sphere Analyzer Control X-ray Source Electron Optics Sample Lenses for Energy Adjustment (Retardation) Lenses for Analysis Area Definition Position Sensitive Detector (PSD) University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/xps Class 99.pps] Multi-Channel Plate Electron Multiplier Resistive Anode Encoder Position Computer Position Address Converter 9

10 Instrumentation Ultra high vacuum (UHV) chamber Typical pressure: torr Reason to have UHV condition Maintain sample surface integrity Minimize scattering of the photoelectrons Maximize mean free path of the photoelectrons Helpful to use tungsten filament or other electron source in the X-ray source cathode 10

11 Instrumentation Ultra high vacuum (UHV) chamber XPS instrument with facility to perform electrochemical experiments C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski, Journal of Power Sources 195 (2010)

12 Energy of the x-ray beam depends on the anode material in the x-ray source High intensity x-ray beam with a narrow line width gives best spectroscopic result Commonly Mg Kα ( ev) and Al Kα ( ev) are used Dual-anode x-ray source (Al/Mg, Mg/Zr, and Al/Zr) are also used X-ray beam lines from synchrotron facility can be used for XPS analysis Instrumentation X-ray source 12

13 Instrumentation X-ray source Diameter of X-ray beam ranges from 5 mm to 1-5 µm X-ray penetration depth ~ 1 µm Sampling depth depends on wavelength of the x-ray beam and sample material For Al Kα, sampling depth is generally 10 nm and 10 atomic layers for heavier elements D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York,

14 Instrumentation Electron energy analyzer Cylindrical mirror analyzer (CMA) X-Rays Source Electron Pathway through the CMA Slit 0 V 0 V +V +V +V +V Sample Holder 0 V 0 V Concentric hemispherical analyzer (CHA) Used in XPS and AES instruments Detector University of Western Ontario [mmrc.caltech.edu/ss_xps/xps_ppt/xps_slides.pdf] 14

15 Instrumentation Sample size depends on the instrument Evans Analytical Group (EAG) can handle samples up to 8 in diameter and thickness till 1 Generally, sample s lateral size cannot exceed 1 and thickness within 0.5 Any solid sample (conducting and non-conducting) can be analyzed Sample has to be compatible in the ultra high vacuum (10-9 torr) condition Sample preparation Samples for the XPS analysis Degrease before loading in the holder Use conductive tape for attachment 15

16 Analysis of XPS Data Example of XP spectrum XPS Spectra, CasaXPS 16

17 Analysis of XPS Data Identify Auger peaks in XP spectrum Cu XP spectra illustrating the shift in auger peak positions with the change from Mg to Al anodes in the X-ray source D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York,

18 Analysis of XPS Data Peak quantification in XP spectrum XPS Spectra, CasaXPS 18

19 Chemical Effects in XPS Chemical shift: change in binding energy of a core electron of an element due to a change in the chemical bonding of that element Withdrawal of valence electron charge Addition of valence electron charge increase in Binding energy decrease in Binding energy 19

20 Chemical Effects in XPS Charges are withdrawn from Ti to form Ti 4+, which results in higher Binding energy for the Ti 2p orbitals Chemical shift information very powerful tool for functional group, chemical environment, and oxidation state University of Western Ontario [mmrc.caltech.edu/ss_xps/xps_ppt/xps_slides.pdf] 20

21 Chemical Effects in XPS Chemical shift for Gold (Au) 4f 7/2 peak University of Western Ontario [mmrc.caltech.edu/ss_xps/xps_ppt/xps_slides.pdf] 21

22 Chemical Effects in XPS Curve fitting for Carbon 1s peak C 1s region XP spectrum for polymethylmethacrylate (PMMA) D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York,

23 Depth Profile Examples of XPS spectrum Ar + sputtering of the sample results in layer-by-layer removal of the sample using Ion gun XP spectrum of the sample surface was collected after each step of Ar + sputtering XPS Spectra, CasaXPS 23

24 Depth Profile Multi layer SiO / TiO 2 sample The set of O 1s spectra measured during a depth profiling experiment XPS Spectra, CasaXPS 24

25 Depth Profile Architectural Glass Coating sample Atomic concentration for the elements found in the Architectural Glass Coating sample from depth profiling experiment University of Western Ontario [mmrc.caltech.edu/ss_xps/xps_ppt/xps_slides.pdf] 25

26 Strengths of X-ray Photoelectron Spectroscopy Surface sensitive technique (top 10 nm) Chemical state identification on surfaces Identification of all elements except for H and He Quantitative analysis, including chemical state differences Applicable for a wide variety of materials, including non conducting samples (paper, plastics, and glass) Depth profiling with matrix-level concentrations Oxide thickness measurements 26

27 Limitations for X-ray Photoelectron Spectroscopy Detection limits typically ~ 0.1% atomic Smallest analytical area ~ 10 µm diameter Limited organic information (short-range bonding only) Samples must be ultra high vacuum compatible Samples that decompose under X-ray irradiation cannot be studied 27

28 Applications Analyzing the composition of powders and debris Determining contaminant sources Examining polymer functionality before and after processing Bonding and adhesion issues Obtaining depth profiles of thin film stacks (both conducting and non-conducting) for matrix level constituents Identifying stains and discolorations Characterizing cleaning processes Assessing the differences in oxide thickness between samples 28

29 Applications Industries using XPS technique Aerospace Automotive Biomedical / Biotechnology Data Storage Defense Displays Electronics Lighting Pharmaceutical Photonics Polymer Semiconductor Solar Photovoltaics Telecommunications 29

30 Applications Analysis of Pigment from Mummy Artwork Egyptian Mummy 2nd Century AD World Heritage Museum University of Illinois Pb 3 O 4 O C PbO Binding Energy (ev) P b Pb Binding Energy (ev) N Ca Na Cl Pb XPS analysis showed that the pigment used on the mummy wrapping was Pb 3 O 4 rather than Fe 2 O 3 30

31 N(E)/E Applications Analysis of Carbon Fiber Polymer Composite material -C-C- -C-O Woven carbon fiber composite -C=O Binding energy (ev) XPS analysis identifies the functional groups present on composite surface. Chemical nature of fiber-polymer interface will influence its properties. University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/xps Class 99.pps] 31

32 Applications Analysis of Nanoparticle catalysts used in DMFC Nanoparticles were less than 4 nm in size. (a) Pt/Ni (1:1), (b) Pt/Ni (3:1), (c) Pt/Ru/Ni (5:4:1), and (d) Pt/Ru (1:1). The dotted line is the Pt 4f 7/2 peak position for pure Pt. The peaks were shifted from 0.17 ev for (c) Pt/Ru/Ni (5:4:1) to 0.35 ev for (a) Pt/Ni (1:1) and 0.36 ev for (b) Pt/Ni (3:1). Metallic Ni content in catalyst (a) is 11.8%, catalyst (b) is 33.7%, and catalyst (c) is 14.4%. These shifts were interpreted to result from modification of the Pt electronic structure by electron transfer from Ni to Pt. C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski, Journal of Power Sources 195 (2010)

33 Facility at Ohio University Instrument and Location XPS instrument is located in the W. M. Keck Thin Film Analysis Facility John E. Edwards Accelerator Laboratory (across Clippinger building) 4.5 MV Tandem Accelerator 33

34 Facility at Ohio University Contact Personnel At John E. Edwards Accelerator Laboratory Prof. David C. Ingram Accelerator Lab Chairman At Center for Electrochemical Engineering Research (CEER) Madhivanan Muthuvel Ph.D. John Goettge 34

35 Facility at Ohio University XPS analysis of Pt/Ir catalyst developed at CEER Dr. Madhi' samples. April 27th 2007 XPS Results: No sputtering Pt Sample C only Sample PtIr1 sample PtIr2 sample PtIr3 sample PtIr4 sample PtIr5 sample April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 Mass Mass Mass Mass Mass Mass Mass Component Conc % Conc % Conc % Conc % Conc % Conc % Conc % Iridium(Ir) Platinum (Pt) Carbon ( C ) Total Binding energies for Pt and Ir element (NIST database) Pt-Ir sample Platinum (Pt) Iridium (Ir) 4d 5/ ev ev 4f 7/ ev ev 35

36 Summary X-ray photoelectron spectroscopy (XPS) is a surface analytical technique This technique is used to identify elemental and chemical information on the surface (~ 10 nm) of the sample The strengths of the XPS technique is extensively used by various industries for there research and development 36

37 References Power point presentations X-ray Photoelectron Spectroscopy (XPS), Center for Microanalysis of Materials, University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0- ppt/xps Class 99.pps] X-ray Photoelectron Spectroscopy, R. Smart et. al., Surface Science Western, University of Western Ontario [mmrc.caltech.edu/ss_xps/xps_ppt/xps_slides.pdf] X-ray Photoelectron Spectroscopy, D. Torres, University of Texas at El Paso [nanohub.org/resources/2011/download/x-ray photoelectron spectroscopy (xps).ppt] Journal C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski, Journal of Power Sources 195 (2010)

38 References Web sites XPS Spectra, CasaXPS X-ray Photoelectron Spectroscopy (XPS), Evans Analytical Group (EAG) John E. Edwards Accelerator Laboratory, Ohio University edwards1.phy.ohiou.edu/~oual/ Books D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York, F. A. Settle, Handbook of Instrumental Techniques for Analytical Chemistry, Prentice-Hall, New Jersey,

39 Further Reading Web site X-ray Photoelectron Spectroscopy (XPS) Reference Pages xpsfitting.blogspot.com Books A. T. Hubbard, The Handbook of Surface Imaging and Visualization, CRC Press, Boca Raton, Florida, T. L. Barr, Modern ESCA: The principles and practice of X-ray Photoelectron Spectroscopy, CRC Press, Boca Raton, Florida, J. Chastain, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS data, Perkin Elmer Corporation,

40 Thank You Questions? Contact: Madhivanan Muthuvel at