Electrochemical Photocurrent X-Ray Spectroscopy: A Novel Analytical Surface Technique

Abstract

X-ray absorption spectroscopy (XAS) is a powerful tool for characterizing the chemical nature and environment of atoms in molecules. However, in situ techniques are limited to transmission and fluorescence techniques and are not surface sensitive. A novel in situ technique that uses synchrotron radiation to produce photoelectrons was developed to provide information on the surface and near surface regions. The results obtained from this technique, Electrochemical Photocurrent X-ray Spectroscopy (EPXS) were compared transmission and fluorescence X-ray absorption spectroscopy (XAS). Surface sensitive information was obtained by collecting X-ray induced photocurrent from a polarized electrode. An Au working electrode in 0.1 N H2SO4 was polarized to 300 mVSCE and exposed to high energy X-rays. The photocurrent response was recorded during Au L3 edge scans while X-ray fluorescence yield data was collected concurrently. The photocurrent data showed good agreement with fluorescence data and transmission data collected on Au foils (Figure 1). Investigations using electrochemical impedance spectroscopy and cyclic voltammetry we conducted to insure the photocurrent response was not due to double layer charging or solution radiolysis. Minor differences in peak characteristics were observed between photocurrent and fluorescence data. We believe this to be due to the surface sensitivity of the photocurrent technique. A comparison of the surface sensitivity of fluorescence and photocurrent techniques was accomplished by evaluating signal response to X-ray beam intensity changes. The results show that the photocurrent technique, as expected, was more surface sensitive than fluorescence. Current work is focusing on EXAFS analysis of photocurrent spectra to quantify the techniques surface sensitivity. This novel technique has the potential to provide information on electron distribution on conductive surface atoms and changes in charge of surface atoms as a function of electrochemical potential. It could significantly impact fields where measurements of surface electron distribution during electrochemical charge transfer are of primary importance, such as corrosion, catalysts and semiconductor research. Figure 1- XAS spectrum of Au foil using transmission (left) and a spectrum of Au in 0.1N H2SO4 polarized to 300 mVSCE using the photocurrent technique. Figure 1