Directional Charge Transfer Mediated by Mid-Gap States: A Transient Absorption Spectroscopy Study of CdSe Quantum Dot/β-Pb_0.33V₂O₅ Heterostructures

Abstract

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 β-Pb 0.33 V 2 O 5 nanowires that have strategically positioned mid-gap states derived from the intercalating Pb 2+ -ions. Unlike mid-gap 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/β-Pb 0.33 V 2 O 5 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 electrons and holes can be transferred from the photoexcited CdSe QDs to the conduction and mid-gap states, respectively, of β-Pb 0.33 V 2 O 5 nanowires for both types of heterostructures. Holes were transferred on time scales less than 1 ps, whereas electrons were transferred more slowly on time scales of approximately 2 ps. In contrast, for analogous heterostructures consisting of CdSe QDs interfaced with V 2 O 5 nanowires (wherein mid-gap states are absent), only electron transfer was observed. Interestingly, electron transfer was readily achieved for CdSe QDs interfaced with V 2 O 5 nanowires by the SILAR method; however, for interfaces incorporating molecular linkers, electron transfer was observed only upon excitation at energies substantially greater than the band-gap absorption threshold of CdSe. Transient absorbance decay traces reveal longer exciton lifetimes (1-3 µs) for CdSe/β-Pb 0.33 V 2 O 5 heterostructures relative to bare β-Pb 0.33 V 2 O 5 nanowires (0.2-0.6 µs); the difference is attributed to surface passivation of intrinsic surface defects in β-Pb 0.33 V 2 O 5 upon interfacing with CdSe (290 words).

Programming Interfacial Energetic Offsets and Charge Transfer in β-Pb0.33V2O5/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States

“…The increased population of holes and electrons is attributed to charge transfer from the photoexcited CdS, as schematically illustrated in Figure , wherein electrons are transferred from the photoexcited QDs to the conduction band of the β-Pb 0.33 V 2 O 5 NWs, whereas holes are transferred to the midgap states of the β-Pb 0.33 V 2 O 5 NWs. Both this charge-transfer mechanism and the transient absorption spectral features are consistent with previously reported results for CdSe/β-Pb 0.33 V 2 O 5 heterostructures …”

“…The following spectral features are distinguished in all three spectra: a transient bleach at 440–480 nm, an induced absorption band centered at 550–650 nm, and another lower-energy induced absorption band at 650–750 nm. The spectral features have been assigned based on spectroelectrochemical studies of β-Pb 0.33 V 2 O 5 NWs, which indicate that the TA spectrum for this material is captured well by a summation of spectra for electrochemically oxidized and reduced species. Considering unfunctionalized β-Pb 0.33 V 2 O 5, the intense 550–650-nm induced absorption band can be ascribed to the excitation of electrons from the valence band to empty midgap states (and is thus characteristic of a hole situated on the β-Pb 0.33 V 2 O 5 framework) .…”

“…1,[11][12] Recently, we demonstrated a promising tunable platform for light-harvesting and excited-state charge transfer derived from interfacing β-Pb 0.33 V 2 O 5 nanowires (NWs) with CdSe QDs. [13][14] This platform exploits a distinctive feature in the electronic structure of ternary vanadium oxide bronzes: the presence of intrinsic midgap states, derived from the intercalating cations, that are situated between the valence and conduction band edges. [15][16] In the case of β-Pb 0.33 V 2 O 5 , midgap states are derived from Pb 6s-O 2p antibonding interactions.…”

Faster Electron Injection and More Active Sites for Efficient Photocatalytic H2 Evolution in g‐C3N4/MoS2 Hybrid

“…Representative decay traces transient absorption at 950 nm is illustrated in Figure D and is fitted by a multiexponential function: Δ A = Δ A 0 +where A and τ refer the amplitudes and lifetimes, respectively . The smallest number of decay lifetimes, τ i , which resulted in the minimum χ 2 , was used for each fit . Kinetic parameters for these three samples are listed in Table 1 .…”

Nanoengineered lone-pair active photocatalysts for more efficient water splitting

“…6 Further, attaching CdSe QDs to the Pb x V 2 O 5 NWs resulted in the transfer of photogenerated holes from the quantum dots to the nanowires. 7,8 Holes can then be transported between the catalytic sites by the nanowires, as shown in Figure 1. We were able to confirm this favorable band alignment (i.e., the thermodynamic barrier) at the buried QD/NW interface for hole transfer via high-energy x-ray photoelectron spectroscopy (XPS) measurements.…”