Photocatalytic Hydrogen Evolution from a Transparent Aqueous Dispersion of Quantum-Sized SnO₂ Nanoparticles: Effect of Electron Trap Density within One Particle

A facile methodology of preparing a transparent aqueous dispersion of SnO2 (rutile) nanoparticles through sol/gel propagative hydrolysis/condensation of SnCl4 was reported. The transparent aqueous dispersion of SnO2 using wet nanoparticles (wet-NPs) as prepared exhibited an enormous enhancement of the photocatalytic H2 evolution reaction upon UV light irradiation, while NPs once dried from wet-NPs (dry-NPs) afforded only a turbid aqueous dispersion with completely silent reactivity against H2 evolution. Although wet- and dry-NPs of SnO2 have the same quantum-sized crystallites, the striking difference of the reactivity was ascribable to the different sizes of wet-NPs (6 nm) in a monomeric form and dry-NPs (58 nm) as agglomerated clusters in the aqueous dispersion. Detailed analysis of electron trap density by means of reversed double-beam photoacoustic spectroscopy (RDB-PAS) to reveal the energy-resolved distribution of electron traps and conduction-band bottom (ERDT/CBB) pattern indicated that the extremely small number of electron traps within one particle of wet-NPs enabled the enormous enhancement of H2 evolution, while a much larger number of electron traps within clusters of dry-NPs could efficiently capture the carrier electron to undergo dominant charge recombination, leading to being silent against H2 evolution.