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...

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).

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/β-Pb0.33V2O5 heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/β-Pb0.33V2O5 heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/β-Pb0.33V2O5 heterostructures relative to CdSe/β-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 β-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 β-Pb0.33V2O5 NWs, along with electron transfer into the conduction band of the β-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.

Photon upconversion is a photophysical process in which two lowenergy photons are converted into one high-energy photon. Photon upconversion has broad appeal for a range of applications from biomedical imaging and targeted drug release to solar energy harvesting. Current upconversion nanosystems, including lanthanide-doped nanocrystals and triplet−triplet annihilation molecules, have achieved upconversion quantum yields on the order of 10−30%. However, the performance of these materials is hampered by inherently narrow absorption cross sections and fixed energy levels originating in atomic, ionic, or molecular states. Semiconductors, on the other hand, have inherently wide absorption cross sections. Moreover, recent advances enable the synthesis of colloidal semiconductor nanoparticles with complex heterostructures that can control band alignments and tune optical properties. We synthesize and characterize a three-component heterostructure that successfully upconverts photons under continuous-wave illumination and solarrelevant photon fluxes. The heterostructure is composed of two cadmium selenide quantum dots (QDs), an absorber and emitter, spatially separated by a cadmium sulfide nanorod (NR). We demonstrate that the principles of semiconductor heterostructure engineering can be applied to engineer improved upconversion efficiency. We first eliminate electron trap states near the surface of the absorbing QD and then tailor the band gap of the NR such that charge carriers are funneled to the emitting QD. When combined, these two changes result in a 100-fold improvement in photon upconversion performance.

Reported herein is the study of the preparation, characterization, and electrochemical activity of a silver-polymer-carbon composite electrode in a nonaqueous cell. An enhanced oxygen reduction activity for the composite electrode in a nonaqueous, aprotic solvent is demonstrated, relative to uncoated glassy carbon or silver disk electrodes. The improvement of oxygen reduction activity increases the current capability and power output of the air electrode, facilitating future development of small, lightweight, long-life power sources.Metal-air batteries are unusual because the electroactive cathode material ͑O 2 ͒ is provided by ambient air, so the only electroactive material contained within metal-air batteries is the anode. Although aqueous metal-air batteries have been successfully deployed in consumer-as well as defense-related applications, 1 the development of practical nonaqueous metal-air batteries 2 remains a challenge. The investigation of nonaqueous metal-air batteries is receiving increased attention as a research focus area, with several recent studies focusing on various aspects of electrolyte formulation, 3-6 cathode, 7 and air battery design. 8,9 The search for new materials to promote oxygen reduction has been an area of research interest. 10 The primary focus to date has been metal oxides. 3,[11][12][13] This article is the first study of the electrochemical reduction of O 2 at a silver-polymer-carbon electrode in a nonaqueous cell. The preparation, characterization, and electrochemical activity of a novel composite electrode containing silver on a polypyrrole ͑PPy͒-coated carbon substrate are described here. An enhanced oxygen reduction activity for the composite electrode is observed relative to uncoated glassy carbon ͑GC͒ or silver disk electrodes. The improvement of the cathode oxygen reduction activity increases the current capability and power output of the air electrode, facilitating future development of small, lightweight, long-life power sources. ExperimentalCH Instruments potentiostats and electrodes were used for the deposition, oxygen reduction, and ac impedance experiments. Platinum auxiliary electrodes were used for all experiments. Reference electrodes were purchased from CH Instruments. For aqueous measurements, a silver/silver chloride reference was used, whereas for nonaqueous measurements, a silver/silver nitrate reference electrode was used. All potentials reported are relative to the reference electrodes used. A Thermo Fisher Scientific ICAP 6000 inductively coupled plasma spectrophotometer was used for silver analysis. Scanning electron microscopy ͑SEM͒ images were recorded using a Hitachi S-800.Temperature was maintained at 25°C throughout all electrochemical experiments. Silver deposition was conducted in a method consistent with that described by Palomar-Pardavé and co-workers. 14 PPy was deposited using a methodology similar to that described by Wallace, Ralph, and co-workers. 15,16 Oxygen reduction was measured using an electrolyte of 0.1 M tetrabutylammonium hexafluor...