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 H generation in true aqueous environments.