Programming Interfacial Energetic Offsets and Charge Transfer in β-Pb_0.33V₂O₅/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States

Abstract

Semiconductor heterostructures for solar energy conversion interface light-harvesting semiconductor nanoparticles with wide-band-gap semiconductors that serve as charge acceptors. In such heterostructures, the kinetics of charge separation depend on the thermodynamic driving force, which is dictated by energetic offsets across the interface. A recently developed promising platform interfaces semiconductor quantum dots (QDs) with ternary vanadium oxides that have characteristic midgap states situated between the valence and conduction bands. In this work, we have prepared CdS/β-Pb0.33V2O5 heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/β-Pb0.33V2O5 heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/β-Pb0.33V2O5 heterostructures relative to CdSe/β-Pb0.33V2O5 heterostructures suggest a positive shift in the VB edge potential and, therefore, an increased driving force for the photoinduced transfer of holes to the midgap state of β-Pb0.33V2O5. This approach facilitates a ca. 0.40 eV decrease in the thermodynamic barrier for hole injection from the VB edge of QDs suggesting an important design parameter. Transient absorption spectroscopy experiments provide direct evidence of hole transfer from photoexcited CdS QDs to the midgap states of β-Pb0.33V2O5 NWs, along with electron transfer into the conduction band of the β-Pb0.33V2O5 NWs. Hole transfer is substantially faster and occurs at <1-ps time scales, whereas completion of electron transfer requires 530 ps depending on the nature of the interface. The differentiated time scales of electron and hole transfer, which are furthermore tunable as a function of the mode of attachment of QDs to NWs, provide a vital design tool for designing architectures for solar energy conversion. More generally, the approach developed here suggests that interfacing semiconductor QDs with transition-metal oxide NWs exhibiting intercalative midgap states yields a versatile platform wherein the thermodynamics and kinetics of charge transfer can be systematically modulated to improve the efficiency of charge separation across interfaces.

Hole Extraction by Design in Photocatalytic Architectures Interfacing CdSe Quantum Dots with Topochemically Stabilized Tin Vanadium Oxide

“…We next turn our attention to determining whether the observed optimal interfacial energetics indeed promote efficacious or rapid hole extraction from the VB of photoexcited QDs using transient absorbance (TA) spectroscopy measurements, as would be expected from energy offset diagrams (Figures 3G, 3H, S9). 11,12 4A and C) are characterized by two broad induced absorption bands at 500−600 nm and 650−750 nm as well as a bleach centered at ca. 465 nm.…”

“…465 nm. We have previously conducted TA spectroscopy studies of charge transfer processes in the structurally related β-Pb x V 2 O 5 and employed spectroelectrochemical measurements to assist in the interpretation of the nature of the observed induced absorption features; 11,12 accordingly, the short-wavelength induced absorption feature observed here for β-Sn 0.23 V 2 O 5 can be ascribed to transitions from deep VB states into the midgap states following initial photoexcitation of electrons from the midgap states (and thus is indicative of oxidation of the nanowires). The induced absorption band observed at longer-wavelengths can in turn be assigned to excitation of electrons in the conduction band of β-Sn 0.23 V 2 O 5 to higher energy states (corresponding to the reduction of the nanowires).…”

“…Given that the only time-resolvable event is electron transfer, hole-transfer from photoexcited CdSe QDs to the midgap states of β-Sn 0.23 V 2 O 5 occurs within the instrument response time on ultrafast subpicosecond time scales, 11,12 whereas the electron transfer process can be resolved within the first few picoseconds. The excellent transfer kinetics observed for β-Sn 0.23 V 2 O 5 /CdSe are consistent with those expected based on the quantitative band alignments shown in Figure 1B.…”

“…The TA recovery and decay traces have been fitted using multiexponential functions since multiple relaxation pathways are operational, including multiple trap-state-mediated recombination processes. 10,12 The fitting parameters used to describe the decay and recovery kinetics are listed in Table S7. Decay traces obtained for the β-Sn 0.23 V 2 O 5 /CdSe heterostructures are well described using biexponential decay functions.…”

Nanoengineered lone-pair active photocatalysts for more efficient water splitting

“…7 To illustrate the possibility of engineering the thermodynamic barrier for efficient charge separation using this paradigm, we replaced CdSe QDs with CdS. 9 In so doing, we achieved a 0.4eV reduction of the hole-barrier height for hole injection (as confirmed by XPS). Additionally, complementary transient absorption spectroscopy of CdS QD/Pb x V 2 O 5 NWs confirmed fast (<1 picosecond) hole transfer from the dot to the NW, representing a speed that is orders of magnitude faster than the competing electron transfer (i.e., which occurs simultaneously and is detrimental to the hole transfer process).…”