Surface, interface and bulk sensitive X-ray absorption spectroscopy probed by total electron yield in liquid cell Daniela Schön, Jie Xiao, Ronny Golnak, Marc F. Tesch, Emad F. Aziz Helmholtz-Zentrum Berlin Freie Universität Berlin MRS Fall Meeting 2016 November 29, 2016

Introduction Motivation Probe of interfacial and bulk species Detection schemes: Partial Fluorescence Yield Total Electron Yield Chemical reactions occur at solid-liquid interface Understanding of processes at the interface Velasco-Velez Science Vol. 346, Issue 6211, pp. 831-834 (2014) Winter Nature Chemistry 7,192–194 (2015)

TEY_Au only membrane; TEY_BE membrane and electrode Flow cell structure Keithley 6514 Distance of electrode to membrane: 1 mm TEY_Au only membrane; TEY_BE membrane and electrode

Surface vs Bulk sensitivity Features II and III are from the bulk, feature I is not. Intensity ratio depends on either different relaxation channels or embedded oxygen. Golnak et al Scientific Reports 6, 24659 (2016) Golnak J. Phys. Chem. A, 2016, 120 (18), pp 2808–2814  TEY_Au spectrum is surface senstive

Analysis of the gold coated Membrane  Cr is not detectable with TEY_Au in liquid cell

Analysis of the gold coated Membrane  Short mean-free-path makes TEY_Au surface sensitive

Interface Sensitivity  TEY_BE is interface sensitive

TEY_Au: Auger electron is seperated due to Au dipole Mechanism of the current production at the Water/gold Interface Under X-ray exposure TEY_Au: Auger electron is seperated due to Au dipole

TEY_Au generates no current  recombination Mechanism of the current production at the Water/gold Interface Under X-ray exposure TEY_Au generates no current  recombination

TEY_BE: Auger electron induces potential  current Mechanism of the current production at the Water/gold Interface Under X-ray exposure TEY_BE: Auger electron induces potential  current

TEY_BE ion migrates to backside electrode  recombination Mechanism of the current production at the Water/gold Interface Under X-ray exposure TEY_BE ion migrates to backside electrode  recombination

Bulk has no contribution due to short mean-free-path Mechanism of the current production at the Water/gold Interface Under X-ray exposure Bulk has no contribution due to short mean-free-path

backside electrode connected Mechanism of the current production at the Water/gold Interface Under X-ray exposure TEY_BE membrane and backside electrode connected TEY_Au only membrane connected TEY_BE: Interface signal / TEY_Au: Surface signal

i-TEY signal  Bulk sensitivity Mn is not at interface. Resonant absorption of Mn depletes the production of charge pairs at the water/gold interface  i-TEY signal. Gotz, J. Phys. Chem. Lett., 2012, 3 (12), pp 1619–1623 i-TEY signal  Bulk sensitivity

Summary TEY_Au: surface sensitive. TEY_BE: interface or bulk sensitive, yielding intensity peaks or dips, respectively, thanks to interfacial dipole layer and short mean-free-path of photoelectrons. Bulk detection of dissolved ions through i-TEY. TEY_BE exploits photon penetration depth and electron yield probability, maybe capable of probing low concentration samples.

Thank you for your attention Questions? Helmholtz-Zentrum Berlin Institute of Methods for Material Development