In-Situ X-ray Spectroscopy of the Electric Double Layer around TiO₂ Nanoparticles Dispersed in Aqueous Solution: Implications for H₂ Generation

We report an experimental observation of a significant amount of hydroxide (OH–) created upon water dissociation and subsequently trapped around TiO2 nanoparticles dispersed in NH4OH aqueous solution. The hydroxide species is identified and quantified by a combination of photoemission and photon emission X-ray spectroscopies conducted on liquid samples using a liquid microjet. Unlike previous X-ray studies that observed only a few monolayers of water coverage on TiO2 surfaces and found maximally submonolayer of OH–, the true aqueous environment adopted in this study enables ion mobility and the separation of the water dissociation products H+/OH–. This facilitates the formation of OH– diffused multilayer in which the trapped OH– ions are discovered to coordinate with three water molecules to form a tetrahedral hydration configuration. The negatively charged diffuse layers, together with the positive NH4+ Stern layers, constitute >0.8 nm thick electric double layers around the TiO2 nanoparticles. The large observed amount of hydroxide indicates a high efficiency of water dissociation for the TiO2 catalyst, a promising result for H2 generation in true aqueous environments.