Abstract
Surface electronic states in aqueous solutions affect many properties of materials used in aqueous solutions, for example, the reactivity of photocatalysts. Therefore, evaluation of the surface electronic states is a coveted objective for developing such materials. Although several methods to evaluate surface electronic states in aqueous solutions have been developed, those methods entail difficulties; electrochemical impedance spectroscopy (EIS) requires electrode fabrication, and photoelectron spectroscopy requires synchrotron radiation. Herein, we present the first successful application of photoemission yield spectroscopy in air (PYSA) as a method to evaluate the surface electronic states of particles in aqueous solution. This method is simple and easily accessible in a laboratory. PYSA can measure the valence band maximum (E (v)) of materials under ambient pressure. Specifically with PYSA, the surface E (v) of hematite (α-Fe(2)O(3)) and molybdenite (MoS(2)) particles dispersed in aqueous solutions was evaluated. This study assessed the effects of pH and solutes in aqueous solution on the surface E (v). When the pH of the dispersion changed, the surface E (v) of hematite and molybdenite shifted linearly with the pH. The correlation between the pH and E (v) evaluated using PYSA was similar to that evaluated using EIS. In addition to the pH, we investigated the effects of solute adsorption on the surface E (v) of hematite. As a solute, phenol and phenol derivatives of five types were used: p-methoxyphenol, p-cresol, p-chlorophenol, p-acetylphenol, and p-nitrophenol. The surface E (v) shifted when the solutes were adsorbed onto hematite. The shift direction was related to the electron-donating and accepting properties of p-substituted functional group of phenol and its derivatives.