Synchrotron X-ray surface scattering techniques. Joanne E. Stubbs Center for Advanced Radiation Sources, GeoSoilEnviroCARS, University of Chicago

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1 Synchrotron X-ray surface scattering techniques Joanne E. Stubbs Center for Advanced Radiation Sources, GeoSoilEnviroCARS, University of Chicago

2 Mineral Surfaces Dynamic Geochemical Microenvironments Dissolution Growth Sorption Fundamentally surface-mediated processes that influence: Compositions of natural waters Biogeochemical cycling of elements Kantishna, Denali NP, AK Chemical weathering of rocks & soil formation Oak Ridge, TN Ground & surface water contamination and remediation Croal et al., Ann. Rev. Genetics (2004) 38, Surface structures are essential for Understanding reaction mechanisms Developing robust conceptual, quantitative and predictive models

3 Synchrotron Surface Science Techniques Crystal Truncation Rod (CTR) X-ray Diffraction 3D atomic structures at crystalline surfaces & interfaces Resonant Anomalous X-ray Reflectivity (RAXR) (aka Resonant Interface Diffraction Spectroscopy (RIDS) Element-specific CTR X-ray Reflectivity Surface roughness, thin film (e.g. biofilm) characterization Long-period X-ray Standing Waves (XSW) Depth profiling of elements & speciation Grazing-incidence X-ray Absorption Spectroscopy (GI-XAS) and Powder XRD (GI-XRD) Medium/long-range structures of adsorbed species and reaction products Oxidation states of surface species Total Reflection X-ray Fluorescence (TXRF) Determine elemental composition of top layers of surface Trainor, et al. (2006) J. Electr. Spectr. Relat. Phenom. 150, 66-85

4 Scattering theory 101 Master Equation for X-ray Scattering I F 2 n f a,n e i Q r n 2 sum over all n atoms at r n f a,n are atomic scattering factors k i Q (r) r k f Q momentum transfer k k k i f incident wavevector final wavevector 1

5 Scattering theory 101 X-ray Source (plane wave) k i Q k f 2 (-1,1) (-1,0) (-1,-1) Q/2 (0,1) (1,1) b * (1,0) a * (0,-1) (1,-1) sample Q as a vector in real space Q k f - k i 4 Q sin(2 / 2) Q as a vector in reciprocal space Q * * * 2 G 2 H a K b L c Q 2 G 2 d HKL 2d sin

6 Crystal truncation rod (CTR) x-ray diffraction CTR L Bragg peak Q H K k i k f Surface cell Bulk cell Scattering intensity: in-phase summation of truncated bulk and surface I F bulk, c F CTR ( L) F surf, c 2

7 CTR lets us Measure 3-D surface/interface structure Dependence on chemical/physical conditions Growth/dissolution mechanisms and kinetics Structure/binding modes of adsorbates Structure reactivity relationships Determine/distinguish between terminations Work in situ Atmospheric pressure Under liquids Buried interfaces Growth chambers Tanwar et al., 2007; Lo et al., 2007; Catalano et al., 2007; Aboud et al., 2011

8 Practical details 1. Sample requirements - High quality single crystal - Sample sizes from 1mm to several cm - Low miscut - Low roughness Roughness kills intensity Mosaic spreads out intensity nˆ Scattering between different height features causes destructive interference c* L 1, rms roughness a* F Å 1 Å 10 Å 50 Å Miscut surface tilts rods L I.K. Robinson, Crystal truncation rods and surface roughness, Phys. Rev. B 33(6):

9 Practical details 2. Facilities - Synchrotron - Goniometer - Sample environment Entrance Flight Path Detector Arm Sample Environment Multi-axis goniometer allows high degree of flexibility to access surface scattering features Sample motions control direction of rod Detector motions control Q Six circle Kappa geometry diffractometer (GSECARS, Sector 13 APS)

10 Trainor et al., J. Elect. Spectr. Relat. Phenom. 150: In-situ liquid cells: Scattered x- ray Incident x- ray (a) Transmission and (b) thin film cells (Fenter 2002, Rev. Mineral. Geochem 49:149) Thin membrane cell Adjustable membrane gap for rapid changes in bulk chemistry and flow experiments. Traps 1 m or less of liquid for scattering and spectroscopy measurements.

11 in situ gas and liquid cells for radioactive material and atmospheresensitive interface studies Multiple containment layers Satisfies radiation safety requirements Shields crystal surfaces against air exposure Remote liquid control (fully contained) X-ray scattering compatible Gas port, inner dome Sample puck Containment domes Gas port, outer dome Secondary Containment Dome Primary Containment Dome Sample Sample Puck Liquid Membrane Liquid Control and Handling O-Ring Seals Liquid Inlet / Outlet Hermetic Electrical Feedthrough Secondary Containment He Gas Purge Schmidt, Eng, Stubbs, Fenter, and Soderholm (2011) Rev. Sci. Inst. 82:075105

12 Practical details 3. Before your experiment - Bulk crystal structure, coordinate system for surface, allowed Bragg peaks, etc. - Hypotheses for possible terminations, positions of sorbates, etc. - Simulate before you measure

13 Example: Surface-Mediated Formation of Pu nanoparticles M. Schmidt, S. Lee, R.E. Wilson, K.E. Knope, F. Bellucci, P.J. Eng, J.E. Stubbs, L. Soderholm, P. Fenter, Surface-Mediated Formation of Pu(IV) Nanoparticles at the Muscovite-Electrolyte Interface, ES&T 47(24) (2013) Pu(III) adsorption on muscovite basal plane Specular (00L) CTR Grazing-incidence XANES Continuous increase in plutonium coverage observed in situ with time

14 Example: Surface-Mediated Formation of Pu nanoparticles RAXR Spectra Energy scans across Pu L III edge at fixed Q Fit parameters: phase and amplitude

15 Example: Surface-Mediated Formation of Pu nanoparticles Pu has broad vertical distribution, peaked at 10.5 Å and extends >70 Å Total plutonium coverage >9 Pu per muscovite unit cell (0.77µg Pu/cm²) Presence of discrete nanoparticles confirmed by high resolution atomic force microscopy.

16 Example: Surface-Mediated Formation of Pu nanoparticles The combined X-ray scattering and AFM results suggest the surface-mediated formation of nm-sized Pu(IV) aggregates on muscovite. Pu(IV) nanoparticles from an otherwise stable Pu(III) solution: 1. Pu(IV) formation in solution is supported by Pu(III) Pu(IV) redox equilibrium (< 2%) 2. The surface acts to concentrate Pu-ion species 3. Pu(IV) has strong proclivity to form polymeric species

17 Conclusions CTR is a powerful technique for determining atomic structures at crystalline surfaces and interfaces Surface scattering results can be supplemented and enhanced by resonant scattering and grazing-incidence spectroscopies

18 Acknowledgements GeoSoilEnviroCARS, University of Chicago Peter Eng, Mark Rivers, Steve Sutton, Matt Newville, Tony Lanzirotti, Vitali Prakapenka, Yanbin Wang, Tony Yu GSECARS Users NSF Earth Sciences DOE Basic Energy Sciences Geosciences