CdSe/beta-Pb0.33V2O5 heterostructures: Nanoscale semiconductor interfaces with tunable energetic configurations for solar energy conversion and storage

This dissertation focuses on the formation and characterization of semiconductor heterostructures, consisting of light-harvesting cadmium selenide quantum dots (CdSe QDs) and single crystalline lead vanadium oxide nanowires (beta-Pb0.33V2O5 NWs), for the purpose of excited-state charge transfer and photocatalytic production of solar fuels. We reported two distinct routes for assembling CdSe/beta-Pb0.33V2O5 heterostructures: linker-assisted assembly (LAA) mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker, cysteine (Cys) is found to bind to the QD surface, whereas a protonated amine moiety interacts electrostatically with the negatively charged NW surface. In the alternative SILAR route, the surface coverage of CdSe on the beta-Pb0.33V2O5 NWs is tuned by varying the number of successive precipitation cycles. Hard X-ray photoelectron spectroscopy (HAXPES) measurements revealed that the mid-gap states of beta-Pb0.33V2O5 NWs are closely overlapped in energy with the valence band edges of CdSe QDs, suggesting that hole transfer from the valence band of CdSe into the mid-gap states is possible. Preliminary evidence of hole transfer was obtained through photoluminescence quenching experiments. Steady-state and time-resolved photoluminescence measurements on Cys-CdSe dispersions, mixed dispersions of Cys-CdSe QDs and beta-Pb0.33V¬2O5 NWs, and mixed dispersions of Cys-CdS QDs and V2O5 revealed a greater extent of quenching of the emission of Cys-CdSe QDs by beta Pb0.33V¬2O5 relative to V2O5. V2O5, devoid of mid-gap states, is unable to accept holes from CdSe and therefore should not quench emission to the same extent as beta-Pb0.33V¬2O5. The additional quenching was dynamic, consistent with a mechanism involving the transfer of photogenerated holes from CdSe QDs to the mid-gap states of beta Pb0.33V2O5. Transient absorption spectroscopy (TA) was used to probe the dynamics of interfacial charge transfer of CdSe/beta-Pb0.33V¬2O5 and CdSe/V2O5 heterostructures. TA measurements indicate that, for both types of heterostructures, photoexcitation of CdSe QDs was followed by a transfer of electrons to the conduction band of beta-Pb0.33V¬2O5 and holes to the mid-gap states of beta-Pb0.33V¬2O5. Ultrafast transient absoprtion measurements revealed that holes actually transferred before electrons, on time scales of ca. 2 ps. In contrast, for analogous heterostructures consisting of CdSe QDs interfaced with V2O5, only electron transfer was observed. In addition, electron transfer was readily achieved for SILAR-prepared heterostructures; however, for LAA-prepared heterostructures, electron transfer was observed only upon excitation at energies substantially greater than the bandgap absorption threshold of CdSe. Transient absorbance decay traces revealed longer excited-state lifetimes (1beta3 betas) for CdSe/beta Pb0.33V2O5 heterostructures relative to bare beta-Pb0.33V2O5 NWs (0.2 to 0.6 betas); the difference was attributed to surface passivation of intrinsic surface defects in beta-Pb0.33V2O5 upon interfacing with CdSe. In an effort to improve the energetic offset in QD/beta-Pb0.33V2O5 heterostructures, cadmium sulfide (CdS) QDs were used in place of CdSe QDs. X-ray photoelectron spectroscopy (XPS) valence band spectra of CdS/beta-Pb0.33V2O5 and CdSe/beta-Pb0.33V2O5 revealed a greater binding energy onset for CdS compared to CdSe. Binding energy onsets of 1.33 (+/- 0.03) and 0.92 (+/- 0.02) eV were determined for Cys-CdS/beta Pb0.33V2O5 and Cys-CdSe/beta Pb0.33V2O5, respectively; suggesting a 0.41 (+/-0.04) eV decrease in the free energy (betaG) needed for hole transfer from the valence band edge of the QDs to the mid-gap states. Linear sweep voltammetry was employed to measure the photocatalytic activity of CdSe/beta Pb0.33V2O5 heterostructures in electrolytes containing ascorbic acid as a sacrificial proton donor. Preliminary photoelectrochemical measurements on CdSe/beta-Pb0.33V2O5 electrodes revealed reductive photocurrents at applied potentials ca. 450 mV positive of the dark proton reduction onset. Importantly, no reductive photocurrents were measured on bare beta-Pb0.33V2O5 electrodes. These results are consistent with a mechanism in which photoinduced hole transfer from CdSe QDs to the mid-gap states of beta Pb0.33V2O5 NWs facilitates the reduction of protons, as the charge-separated state allows proton reduction to compete with exciton recombination. This avenue of research is ongoing.