Programming Interfacial Energetic Offsets and Charge Transfer in beta-Pb0.33V2O5/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States
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2016 American Chemical Society. 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/-Pb0.33V2O5 heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/-Pb0.33V2O5 heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/-Pb0.33V2O5 heterostructures relative to CdSe/-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 -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 -Pb0.33V2O5 NWs, along with electron transfer into the conduction band of the -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. (Chemical Equation Presented).