Hole scavenger concentration dependent photoreduction pathway of nitrobenzene catalyzed by CdS quantum dots

Hole scavengers are often employed in photocatalytic reactions catalyzed by semiconductors to efficiently extract photogenerated holes, thereby suppressing charge recombination and enhancing the overall catalytic activity. Beyond improving charge separation, the type of hole scavengers can also affect the activity, selectivity, and mechanism of the reaction. Interestingly, our findings in this work reveal that not only the identity but also the amount of hole scavenger plays a significant role, indicating the reaction pathway. Specifically, in nitrobenzene reduction catalyzed by CdS quantum dots (QDs), the concentration of hole scavenger Na₂SO₃ influences the reaction pathway and the final products: low concentrations (2-8 mM) favored the direct reduction pathway and yielded phenylhydroxylamine and aniline, while high concentrations (12-24 mM) favored an indirect (coupling) pathway and produced azoxybenzene. We hypothesized that SO₄˙^- radicals formed at high concentrations of Na₂SO₃ in the presence of dissolved oxygen is responsible for this change in reduction pathway. The existence of SO₄˙^- radicals was validated by Electron Spin Resonance spectroscopy. Quenching of SO₄˙^- radicals using tert-butyl alcohol (TBA) reverted the reaction back to the direct reduction pathway even when a high concentration of Na₂SO₃ was added, confirming the critical role of SO₄˙^-. Density functional theory calculations revealed that the SO₄˙^- adsorbed onto the CdS surface abstracts hydrogen from the reaction intermediates, promoting the indirect coupling reaction. In addition, photoluminescence and femtosecond transient absorption studies showed that rapid hole trapping in CdS QDs occurred within 3-4 picoseconds after excitation, and Na₂SO₃ scavenged trapped holes at a timescale of tens of nanoseconds. These findings highlight the diverse functions of hole scavengers in photocatalysis. Their quantity and the species generated after hole scavenging can direct the reaction pathways and product selectivity.