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Free keywords:
Electric double layer; TiO2 nanoparticles; Water dissociation; Liquid microjet; Photoelectron spectroscopy; X-ray absorption spectroscopy
Abstract:
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 sub-monolayer 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 a multilayer OH- diffuse layer 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, constitutes > 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.
