Evaluation of the Valence Band Maximum of Hematite Particles Dispersed in Aqueous Solution: An Approach Using Photoemission Yield Spectroscopy in Air

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 (<i>E</i> _v) of materials under ambient pressure. Specifically with PYSA, the surface <i>E</i> _v of hematite (<i>α</i>-Fe₂O₃) and molybdenite (MoS₂) particles dispersed in aqueous solutions was evaluated. This study assessed the effects of pH and solutes in aqueous solution on the surface <i>E</i> _v. When the pH of the dispersion changed, the surface <i>E</i> _v of hematite and molybdenite shifted linearly with the pH. The correlation between the pH and <i>E</i> _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 <i>E</i> _v of hematite. As a solute, phenol and phenol derivatives of five types were used: <i>p</i>-methoxyphenol, <i>p</i>-cresol, <i>p</i>-chlorophenol, <i>p</i>-acetylphenol, and <i>p</i>-nitrophenol. The surface <i>E</i> _v shifted when the solutes were adsorbed onto hematite. The shift direction was related to the electron-donating and accepting properties of <i>p</i>-substituted functional group of phenol and its derivatives.