Modulating Mid-Gap Electronic States Through Site-Selective Modification in β-Pb _x /β′-Cu _y V ₂ O ₅ /CdS Heterostructures for Photocatalytic Hydrogen Evolution

We interfaced β-Pb_x/β'-Cu_yV₂O₅ compounds, with varying stoichiometries of precisely positioned Pb-ions (x) and Cu-ions (y) in interstitial sites along a tunnel-structured ζ-V₂O₅ framework, with cysteine-capped CdS (cysCdS) quantum dots (QDs) to yield heterostructured photocatalysts. β-Pb_x/β'-Cu_yV₂O₅ compounds exhibit midgap electronic states with orbital contributions from both Cu 3d and stereochemically active Pb 6s states that show distinctive light-initiated reactivity with photoexcited QDs. β-Pb_x/β'-Cu_yV₂O₅/CdS heterostructures were prepared by linker-assisted assembly (LAA). Scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy revealed that cysCdS QDs were deposited onto surfaces of β-Pb_x/β'-Cu_yV₂O₅ via LAA. HAXPES revealed that the site-selective positioning of Pb-ions and Cu-ions promoted close energetic alignment of the midgap states of β-Pb_x/β'-Cu_yV₂O₅ compounds with the valence-band maximum of cysCdS QDs. Transient absorption spectroscopy revealed that photogenerated holes were transferred from CdS QDs to midgap states of β-Pb_x/β'-Cu_yV₂O₅ compounds on time scales <50 ps. Finally, photoelectrochemical and photochemical experiments revealed that β-Pb_x/β'-Cu_yV₂O₅/CdS heterostructures promoted the photocatalytic reduction of H⁺ to H₂. In photoelectrochemical experiments, under oxidative conditions, for all β-Pb_x/β'-Cu_yV₂O₅/CdS heterostructures, H₂ was evolved at a Pt counter electrode while a sacrificial donor was oxidized at the heterostructure-functionalized working electrode. In contrast, under reductive conditions, for β-Pb_0.152V₂O₅/CdS and β-Pb_x/β'-Cu_yV₂O₅/CdS heterostructures, H₂ was evolved at the working electrode. In photochemical experiments, dispersed β-Pb_x/β'-Cu_yV₂O₅/CdS heterostructures promoted the reduction of H⁺ to H₂ under white-light illumination; β'-Cu_0.55V₂O₅/CdS and β-Pb_x/β'-Cu_yV₂O₅/CdS heterostructures, for which midgap states have Cu 3d orbital character, generated 2-fold more H₂ than β-Pb_0.152V₂O₅/CdS heterostructures. Cu-ion insertion thus appends additional acceptor surface states that improve ligand-mediated hole transfer from photoexcited QDs, but such states are intrinsically limited in mediating hole transport to the substrate as a result of the low mobility of holes in narrow Cu 3d-states. Our results reveal that the density and orbital character of midgap states of β-Pb_x/β'-Cu_yV₂O₅ compounds, tunable through recently developed site-selective ion insertion strategies, determine efficiencies of charge-transfer and charge-transport mechanisms that underpin photocatalysis.