Directional Charge Transfer Mediated by Mid-Gap States: A Transient Absorption Spectroscopy Study of CdSe Quantum Dot/beta-Pb0.33V2O5 Heterostructures
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2016 American Chemical Society. For solar energy conversion, not only must a semiconductor absorb incident solar radiation efficiently but also its photoexcited electron - hole pairs must further be separated and transported across interfaces. Charge transfer across interfaces requires consideration of both thermodynamic driving forces as well as the competing kinetics of multiple possible transfer, cooling, and recombination pathways. In this work, we demonstrate a novel strategy for extracting holes from photoexcited CdSe quantum dots (QDs) based on interfacing with -Pb0.33V2O5 nanowires that have strategically positioned midgap states derived from the intercalating Pb2+ ions. Unlike midgap states derived from defects or dopants, the states utilized here are derived from the intrinsic crystal structure and are thus homogeneously distributed across the material. CdSe/-Pb0.33V2O5 heterostructures were assembled using two distinct methods: successive ionic layer adsorption and reaction (SILAR) and linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate that, for both types of heterostructures, photoexcitation of CdSe QDs was followed by the transfer of electrons to the conduction band of -Pb0.33V2O5 nanowires and holes to the midgap states of -Pb0.33V2O5 nanowires. Holes were transferred on time scales less than 1 ps, whereas electrons were transferred more slowly on time scales of 2 ps. In contrast, for analogous heterostructures consisting of CdSe QDs interfaced with V2O5 nanowires (wherein midgap states are absent), only electron transfer was observed. Interestingly, electron transfer was readily achieved for CdSe QDs interfaced with V2O5 nanowires by the SILAR method; however, for interfaces incorporating molecular linkers, electron transfer was observed only upon excitation at energies substantially greater than the bandgap absorption threshold of CdSe. Transient absorbance decay traces reveal longer excited-state lifetimes (1-3 s) for CdSe/-Pb0.33V2O5 heterostructures relative to bare -Pb0.33V2O5 nanowires (0.2 to 0.6 s); the difference is attributed to surface passivation of intrinsic surface defects in -Pb0.33V2O5 upon interfacing with CdSe.