In this feature article, we explore the electronic and structural phase transformations of ternary vanadium oxides with the composition MxV2O5 where M is an intercalated cation. The periodic arrays of intercalated cations ordered along quasi-1D tunnels or layered between 2D sheets of the V2O5 framework induce partial reduction of the framework vanadium atoms giving rise to charge ordering patterns that are specific to the metal M and stoichiometry x. This periodic charge ordering makes these materials remarkably versatile platforms for studying electron correlation and underpins the manifestation of phenomena such as colossal metal-insulator transitions, quantized charge corrals, and superconductivity. We describe current mechanistic understanding of these emergent phenomena with a particular emphasis on the benefits derived from scaling these materials to nanostructured dimensions wherein precise ordering of cations can be obtained and phase relationships can be derived that are entirely inaccessible in the bulk. In particular, structural transformations induced by intercalation are dramatically accelerated due to the shorter diffusion path lengths at nanometer-sized dimensions, which cause a dramatic reduction of kinetic barriers to phase transformations and facilitate interconversion between the different frameworks. We conclude by summarizing numerous technological applications that have become feasible due to recent advances in controlling the structural chemistry and both electronic and structural phase transitions in these versatile frameworks.

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...