X-ray Induced Photocurrents at the Metal/Solution Interface Yuriy V. Tolmachev, In Tae Bae, and Daniel A. Scherson Department of Chemistry and Ernest B. Yeager Center for Electrochemical Sciences, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078 ReceiVed: January 27, 2000; In Final Form: May 7, 2000 A technique is herein described for the acquisition of X-ray absorption spectra of electrodes fully immersed in an electrolyte solution by monitoring the energy dependence of X-ray induced photocurrents. This novel strategy made it possible to obtain in situ Au L III -edge X-ray absorption spectra of solid Au electrodes displaying virtually identical features to those recorded simultaneously via X-ray fluorescence. The dependence of the X-ray induced photocurrents on chopping frequency and electrode potential was found to be consistent with a temperature modulation of the rate of the interfacial electron transfer (faradaic heating currents), rather than electron photoemission, as the primary source of the observed effects. Introduction The development and implementation of spectroscopic and structural probes of condensed-phase interfaces may be expected to have pronounced impact in areas of fundamental and technological importance, including adhesion, corrosion and a variety of electrochemical phenomena. Methods that rely on tunable, high-intensity X-ray sources hold extraordinary promise for the study of such interfaces. In particular, the photon energies associated with element-specific, core-electron excitations of moderate to high atomic number elements, are, in many instances, sufficiently large to penetrate rather deeply into matter, and surface specificity can be achieved by using anomalous surface scattering, 1,2 external total reflection at grazing incidence angles 3,4 and/or electron yield (EY) detection. 5 The latter technique relies on the emission of electrons following photon absorption, which can then be detected either under vacuum or high pressure conditions utilizing an externally applied electric field to prevent electron recapture by the emitting source and to increase the collector current as in conversion electron yield (CEY) X-ray absorption spectroscopy (XAS). The lifetime of excess electrons in condensed media is short and, therefore, implementation of EY detection for condensed-phase interfaces requires at least one of the constituent phases to be thin enough for electrons to escape into the gas phase. Such conditions have been realized in the case of solid-liquid interfaces by partial emersion of electrodes from the electrolyte enabling photoemit- ted electrons to cross the thin layer of electrolyte into the detector placed directly above the solution. 4 A different approach for the measurement of sustained photocurrents, developed in the field of UV-visible photoelec- tron emission at electrode-solution interfaces, involves the use of electron scavengers in the ionically conducting media. As explained in detail elsewhere, 6-8 electrons at the Fermi level of the electrode are excited by UV-visible radiation into the conduction band of the solvent, undergoing subsequent ther- malization and solvation in the ps time scale. Solvated electrons localized a few nanometers away from the surface can then diffuse back and be recaptured by the electrode within tens of nanoseconds. 6,7 The main role of scavengers is to react with such electrons, yielding products that cannot be reoxidized at the electrode, thereby preventing the return of the photoemitted charge. Somewhat surprisingly, this elegant and potentially powerful detection scheme has not as yet been attempted for the acquisition of in situ XAS spectra. This paper describes an experimental approach for the acquisition of XAS spectra of fully immersed electrodes originally conceived to exploit this scavenging scheme. As will be shown, implementation of this technique made it possible to obtain in situ Au L III -edge XAS spectra of solid Au electrodes displaying virtually identical features to those recorded simul- taneously via X-ray fluorescence yield. More detailed studies of the X-ray induced photocurrents, however, revealed a behavior that could not be explained by electron photoemission but was instead largely consistent with heating as primarily responsible for the observed effects. Experimental Section Gold was selected for these initial studies because of its large electrochemical potential window, 9 and also because photons with energies about Au L III -edge (E edge ), i.e., 11 919 eV, can penetrate through the solution without significant attenuation. Furthermore, Au has been recently examined by gas-phase CEY- XAS, 10 allowing comparisons to be made with the results to be presented in this work. Persulfate was chosen as the scavenger, a species that may allow a doubling of the photocurrent via the following reaction sequence: where e s - represents an electron in solution and e m - an electron in the metal electrode. The overall experimental arrangement employed in these studies is shown schematically in Figure 1. The electrochemical cell comprises a bent Au wire working electrode cast in epoxy resin exposing an area 0.5 × 5 mm 2 to the aqueous electrolyte, placed in front of an X-ray transparent Kapton window at a distance of ca. 1 mm. A Au foil and a saturated calomel (SCE) were used as counter and reference electrodes, respectively. Except where otherwise noted all solutions were deaerated by purging with nitrogen. A PAR 173 potentiostat/galvanostat was employed to control the potential S 2 O 8 2- + e s - ) SO 4 2- + SO 4 •- SO 4 •- + e m - ) SO 4 •2- 7663 J. Phys. Chem. B 2000, 104, 7663-7667 10.1021/jp000312o CCC: $19.00 © 2000 American Chemical Society Published on Web 07/19/2000