Photocatalytic CO₂ Reduction by Ternary Heterostructures of Sb₂VO₅ Nanorods, CdS Quantum Dots, and a Zinc(II) Porphyrin Complex

We synthesized ternary heterostructures of Sb₂VO₅ nanorods, CdS quantum dots (QDs), and zinc(II) 5-(3-carboxyphenyl)-10,15,20-(triphenyl)porphyrin (Zn3MCP), hereafter referred to as Sb₂VO₅/CdS-Zn3MCP triads, and evaluated their performance as CO₂-reduction photocatalysts. The photocatalytic CO₂-reduction reaction (CO₂RR), which converts CO₂ to fuels and feedstocks, is challenging because it competes with the hydrogen evolution reaction (HER) and because it can proceed through several different multielectron, multiproton mechanisms. We hypothesized (a) that, within Sb₂VO₅/CdS-Zn3MCP triads, photoexcited CdS QDs would transfer electrons to Zn3MCP and holes to midgap electronic states of Sb₂VO₅, (b) that charge separation would enable subsequent redox half-reactions, and (c) that Zn3MCP would selectively catalyze CO₂RR. To evaluate these hypotheses, we characterized excited-state mechanisms and photocatalytic reactivity of Sb₂VO₅/CdS-Zn3MCP triads, as well as CdS-Zn3MCP dyads and Sb₂VO₅/CdS heterostructures as two-component analogues. Within CdS-Zn3MCP dyads, photoexcited CdS QDs transferred energy to Zn3MCP. In photochemical experiments, CdS-Zn3MCP dyads promoted both CO₂RR, to yield CO and CH₄, and HER. Sb₂VO₅/CdS heterostructures likewise promoted both CO₂RR and HER but with H₂ as the primary photoproduct. Remarkably, Sb₂VO₅/CdS-Zn3MCP triads promoted photocatalytic CO₂RR, to yield CO and CH₄, with 100% selectivity for CO₂RR over HER. As the loading of Zn3MCP increased, the yield of CH₄, the 8-electron, 8-proton CO₂RR product, increased, suggesting cooperativity between porphyrin centers. This photocatalytic reactivity is consistent with the targeted charge-separation mechanism. Our results reveal that Sb₂VO₅/CdS-Zn3MCP triads are promising CO₂-reduction photocatalysts and, more generally, that light-initiated mechanisms of heterostructures can be tuned with composition and interfacial properties to engender new reactivity and selectivity in photocatalysis.