The suitability of WO3 for dark photocatalysis is observed by irradiating a suspension of nanoparticles with a broad-spectrum light source in the presence of 2-propanol as a hole acceptor. The suspension turns blue due to the build up of electrons on the WO3 nanoparticles (pictured) and the colour remains for several hours after the removal of the light source, indicating the long-lived nature of the trapped electrons. After pre-charging the suspension with light, a series of quenchers were added in the dark, each revealing information about the nature of the electrons by monitoring the electron signal decay using ultraviolet–visible spectroscopy. The immediate quenching of the signal with the addition of Ag+ as electron scavenger suggests that the trapped electrons are stored on the surface of the WO3 and that the charges have low mobility. When the one-electron oxidant methyl viologen is added, a single exponential decay is observed, which supports the case that all trapped electrons involved in reactivity are equivalent — a fundamental assumption in much of photocatalysis. The same result is obtained regardless of a high or low concentration of trapped electrons, indicating that electron repulsion does not play a major role in reactivity. By contrast, the introduction of the dye methyl orange, a four-electron oxidant, results in biexponential decay kinetics, which agrees with the requirement of high electron density to support multielectron reduction with a shift to a slower rate occurring as electron density decreases.
The researchers suggest that such systems could be useful for the treatment of continuously released pollutants. More broadly, the study reveals that dark photocatalysis can be a powerful tool for gaining insight into photocatalytic and charge storage materials.
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