Photocatalytic CO <sub>2</sub> Reduction by Ternary Heterostructures of Sb <sub>2</sub> VO <sub>5</sub> Nanorods, CdS Quantum Dots, and a Zinc(II) Porphyrin Complex
Udani K. Wijethunga, Marissa A. Harrell, Jaime R. Ayala, Alice R. Giem, Karoline E. García-Pedraza, Daoyang Zhang, Anindya Pakhira, Matthew R. Crawley, Yuguang Li, Sarbajit Banerjee, Timothy R. Cook, David F. Watson
Abstract
We synthesized ternary heterostructures of Sb 2 VO 5 nanorods, CdS quantum dots (QDs), and zinc(II) 5-(3-carboxyphenyl)-10,15,20-(triphenyl)porphyrin (Zn3MCP), hereafter referred to as Sb 2 VO 5 /CdS-Zn3MCP triads, and evaluated their performance as CO 2 -reduction photocatalysts. The photocatalytic CO 2 -reduction reaction (CO 2 RR), which converts CO 2 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 Sb 2 VO 5 /CdS-Zn3MCP triads, photoexcited CdS QDs would transfer electrons to Zn3MCP and holes to midgap electronic states of Sb 2 VO 5, (b) that charge separation would enable subsequent redox half-reactions, and (c) that Zn3MCP would selectively catalyze CO 2 RR. To evaluate these hypotheses, we characterized excited-state mechanisms and photocatalytic reactivity of Sb 2 VO 5 /CdS-Zn3MCP triads, as well as CdS-Zn3MCP dyads and Sb 2 VO 5 /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 CO 2 RR, to yield CO and CH 4, and HER. Sb 2 VO 5 /CdS heterostructures likewise promoted both CO 2 RR and HER but with H 2 as the primary photoproduct. Remarkably, Sb 2 VO 5 /CdS-Zn3MCP triads promoted photocatalytic CO 2 RR, to yield CO and CH 4, with 100% selectivity for CO 2 RR over HER. As the loading of Zn3MCP increased, the yield of CH 4, the 8-electron, 8-proton CO 2 RR product, increased, suggesting cooperativity between porphyrin centers. This photocatalytic reactivity is consistent with the targeted charge-separation mechanism. Our results reveal that Sb 2 VO 5 /CdS-Zn3MCP triads are promising CO 2 -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.