Type-II heterostructures of α-V2O5 nanowires interfaced with cadmium chalcogenide quantum dots: Programmable energetic offsets, ultrafast charge transfer, and photocatalytic hydrogen evolution

Abstract

We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V2O5 nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V2O5/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V2O5 NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10−12–10−11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10−6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V2O5/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H2 with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V2O5 NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V2O5/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.

Elucidating the Mechanistic Origins of Photocatalytic Hydrogen Evolution Mediated by MoS2/CdS Quantum-Dot Heterostructures

“…The resulting charge-separated states need to be long-lived and the separated electrons/holes further need to be rapidly transported along potential gradients to catalytic sites to mediate redox reactions . Staggered, type-II, band alignments are particularly desirable to drive rapid interfacial charge separation of photogenerated electrons and holes and to enable their localization at opposite ends of binary heterostructures. − Interfacing MoS 2 with other semiconductor nanocrystals provides a means to tune interfacial energetic offsets and charge transfer dynamics, thereby enabling electron injection and effective solar energy conversion. ,, As an example of this approach, Hong et al have demonstrated that upon photoexcitation of MoS 2 /WS 2 heterostructures, charge separation is achieved within 50 fs, enabling a substantially increased photocurrent density and incident-photon-to-current-efficiency in a photovoltaic device. , II–VI semiconductors are attractive candidates for coupling with MoS 2 by dint of their high absorption cross sections in the visible region of the electromagnetic spectrum and the size and compositional tunability of their bandgaps. , …”

Lone but Not Alone: Precise Positioning of Lone Pairs for the Design of Photocatalytic Architectures

“…These heterostructures have type-II interfaces that drive electron transfer from photoexcited CdSe to the conduction band of α-V 2 O 5 . 29 The chargeseparation mechanism at the α-V 2 O 5 /CdSe interface, QD-to-NW electron transfer, is thus the reverse of the mechanism for M x V 2 O 5 /QD heterostructures. Under prolonged illumination of dispersions, HER from LAA-derived α-V 2 O 5 /CdSe was 20-fold more efficient relative to colloidal cys-CdSe QDs, illustrating that interfacial charge separation indeed promotes redox photocatalysis.…”

“…We attributed the slower HER at SILARderived α-V 2 O 5 /CdSe heterostructures to rapid electron−hole recombination at interfaces with direct contact between CdSe and α-V 2 O 5 , whereas, for LAA-derived heterostructures, the presence of cysteine as a molecular linker further separates electrons and holes and provides a barrier to recombination. 29 In an effort to eliminate the need for solvated cocatalyst, and toward the goal of preparing ternary heterostructure photocatalysts, we have recently synthesized MoS 2 /CdS heterostructures, via SILAR deposition of CdS onto MoS 2 nanoplatelets, and characterized their electronic structure, excitedstate charge-transfer reactivity, and photocatalytic performance. 30 with Faradaic efficiency approaching 100% (Figure 8E).…”

“…In recent work, we have synthesized M x V 2 O 5 /QD heterostructures of varying compositions through both SILAR and LAA as illustrated in Figure . ,,, These techniques each have distinct advantages and disadvantages. SILAR yields direct M x V 2 O 5 /QD interfaces and controllable loadings of QDs encompassing almost complete core/shell geometries but affords relatively less control over the size and size distribution of deposited QDs.…”

“…Several generations of M x V 2 O 5 /QD heterostructures have been designed and characterized in terms of their electronic structure, excited-state charge-transfer dynamics, and performance in photocatalytic proton reduction. ,,,,,, By developing high-throughput topochemical synthetic methods, we have recently prepared and characterized a palette of new ternary M x V 2 O 5 and quaternary M x M y ′ V 2 O 5 compounds with composition- and structure-dependent electronic properties, which are ripe for incorporation into heterostructure photocatalysts. This Perspective summarizes the distinctive electronic structure of M x V 2 O 5 /QD heterostructures, highlights the potential to control both the thermodynamics and kinetics of excited-state charge-transfer processes central to photocatalysis, provides an overview of synthetic strategies, and addresses future prospects for the heterostructure photocatalysts.…”