Simultaneous measurements of solid–liquid interfaces and bulk liquids using soft X-ray absorption spectroscopy

We have developed a method for making simultaneous soft X-ray absorption spectroscopy (XAS) measurements of solid-liquid interfaces and bulk liquids. By controlling the thickness of the liquid layer, we obtained the O K-edge XAS spectrum of bulk H₂O from a liquid H₂O layer on a thin Au film using the transmission method, and we used the electron-yield method to obtain the XAS spectrum of the H₂O/Au interface by measuring the drain currents from the Au surface following soft X-ray absorption. This method for obtaining simultaneous XAS measurements of solid-liquid interfaces and bulk liquids can be utilized to investigate the mechanisms of a variety of catalytic, electrochemical, and biological reactions involving solid-liquid interfaces.

Water molecules at solid-liquid interfaces play important roles in various catalytic, electrochemical, and biological reactions. Soft X-ray absorption spectroscopy (XAS) is an element-specific method for investigating the electronic structures of liquid water and organic molecules. In this study, we developed a method for simultaneously obtaining XAS measurements of a solid-liquid interface, using the electron-yield method, and of the bulk liquid, using the transmission method. In the present work, we measured the XAS spectra while precisely controlling the thickness of the liquid layer in the range from 20 nm to 40 μm in a liquid cell for the transmission of soft X-rays. The XAS spectra acquired in transmission mode are derived mainly from the bulk liquid because the contributions from the solid-liquid interfaces are smaller than those from the bulk liquid. In contrast, the XAS spectra of solid-liquid interfaces are obtained by detecting Auger electrons, which originate mostly from those interfaces because they escape only from shallow depths.

We performed our XAS experiments at the soft X-ray beamline BL-13A of the Photon Factory (KEK-PF). As shown in Figure 1(a), in a liquid cell we sandwiched the liquid layer between two 100 nm thick Si₃N₄ membranes, with the lower membrane coated with a 20 nm thick Au layer and a 5 nm thick Cr layer. In transmission mode, the XAS spectra reflect the absorption spectra of bulk H₂O. We simultaneously obtained the electron-yield XAS spectra by measuring the drain current from the Au/Cr/Si₃N₄ membrane, which compensates for the H₂O⁺ cations generated by the emission of Auger electrons following soft X-ray absorption. Most electron-yield XAS spectra are derived from the H₂O/Au interface because the effective attenuation length of electrons in liquid H₂O is small at the O K-edge.

Figure 1(b) shows the O K-edge XAS spectra of liquid H₂O on the Au/Cr/Si₃N₄ membrane. The O K-edge XAS spectrum of bulk H₂O is consistent with those reported in previous studies, where the pre-edge, main-edge, and post-edge peaks appear around 534.7, 537, and 540 eV, respectively. The pre-edge peak at the H₂O/Au interface merges with the shoulder of the main-edge peak because it is shifted to a higher energy by interactions between the H₂O molecules and the Au surface.

We expect the method we have developed to be very useful for investigating the mechanisms of catalytic reactions involving different catalytic interfaces. In particular, the electron-yield XAS measurements can be used to investigate electrocatalyst interfaces during reactions. In addition, for biological reactions at solid-liquid interfaces, XAS measurements of membrane proteins can be performed using lipid bilayers that incorporate them. Consequently, this method for obtaining simultaneous XAS measurements of solid-liquid interfaces and bulk liquids can be utilized to investigate the mechanisms of various catalytic, electrochemical, and biological reactions involving solid-liquid interfaces.
 

Information of the paper:

Authors: Fumitoshi Kumaki and Masanari Nagasaka
Journal Name: Journal of Synchrotron Radiation
Journal title: "Simultaneous measurements of solid-liquid interfaces and bulk liquids using soft X-ray absorption spectroscopy"
DOI: 10.1107/S1600577526004637