Integrating β-Pb_0.33V₂O₅ Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer

Abstract

Achieving directional charge transfer across semiconductor interfaces requires careful consideration of relative band alignments. Here, we demonstrate a promising tunable platform for light harvesting and excited-state charge transfer based on interfacing β-Pb x V 2 O 5 nanowires with CdSe quantum dots. Two distinct routes are developed for assembling the heterostructures: linker-assisted assembly mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker is found to bind to the quantum dot surfaces, whereas a protonated amine moiety interacts electrostatically with the negatively charged nanowire surfaces. In the alternative SILAR route, the surface coverage of CdSe nanostructures on the β-Pb x V 2 O 5 nanowires is tuned by varying the number of successive precipitation cycles. High-energy valence band X-ray photoelectron spectroscopy measurements indicate that "mid-gap" states of the β-Pb x V 2 O 5 nanowires derived from the stereoactive lone pairs on the intercalated lead cations are closely overlapped in energy with the valence band edges of CdSe quantum dots that are primarily Se 4p in origin. Both the midgap states and the valence-band levels are in principle tunable by variation of cation stoichiometry and particle size, respectively, providing a means to modulate the thermodynamic driving force for charge transfer. Steady-state and time-resolved photoluminescence measurements reveal dynamic quenching of the trapstate emission of CdSe quantum dots upon exposure to β-Pb x V 2 O 5 nanowires. This result is consistent with a mechanism involving the transfer of photogenerated holes from CdSe quantum dots to the midgap states of β-Pb x V 2 O 5 nanowires. ■ INTRODUCTIONTuning interfaces between disparate semiconductors, between molecules and semiconductor surfaces, and between semiconductors and metals remains of paramount importance for electronics, optoelectronics, photocatalysis, photovoltaics, and electrochemical energy storage. 1−4 Interfaces assume special significance for nanostructures given their high surface-tovolume ratios. Nanoscale heterostructures are of particular interest for photocatalysis owing to the tunability of the energies of the valence and conduction band edges of semiconductors as a function of finite size and doping, which allows for different components performing discrete functions to be assembled within modular platforms to facilitate sequential light-harvesting, charge transfer, and catalytic processes. 4,5 To enable programmable cascades of directional charge transfer reactions, heterostructures need to be designed keeping in mind several considerations such as the nature of the interface, the thermodynamics of band alignments between different components, and the kinetics of charge transfer. In this work, we have sought to design nanoscale heterostructures to exploit the availability of midgap states energetically positioned between the valence and conduction bands of a transition metal oxi...

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

“…CdSe was deposited directly onto β-Pb 0.33 V 2 O 5 NWs via SILAR using a previously reported method. 13 In a single SILAR cycle, CdSe QDs were grown directly onto β-Pb 0.33 V 2 O 5 NWs by sequentially mixing the NWs with a cadmium precursor solution (Cd(NO 3 ) 2 in ethanol), followed by a selenium precursor (Na 2 Se in ethanol). The NWs were washed with ethanol following each mixing step to remove excess ions.…”

“…Cys-CdSe(sm)/β-Pb 0.33 V 2 O 5 and Cys-CdSe(lg)/β-Pb 0.33 V 2 O 5 heterostructures were prepared via LAA as reported previously. 13 An aqueous dispersion of β-Pb 0.33 V 2 O 5 NWs was added to an aqueous dispersion of purified Cys-CdSe(sm) or Cys-CdSe(lg) QDs and allowed to equilibrate overnight (12-16 h). CdSe-functionalized NWs were recovered by centrifugation, rinsed with DI water to remove excess QDs, and dried at room temperature before characterization.…”

“…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.…”

Nanoengineered lone-pair active photocatalysts for more efficient water splitting

“…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. 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).…”

“…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.…”