No abstract

The scarce inventory of compounds that allow for diffusion of multivalent cations at reasonable rates poses a major impediment to the development of multivalent intercalation batteries. Here, we contrast the thermodynamics and kinetics of the insertion of Li, Na, Mg, and Al ions in two synthetically accessible metastable phases of V2O5, ζ- and ε-V2O5, with the relevant parameters for the thermodynamically stable α-phase of V2O5 using density functional theory calculations. The metastability of the frameworks results in a higher open circuit voltage for multivalent ions, exceeding 3 V for Mg-ion intercalation. Multivalent ions inserted within these structures encounter suboptimal coordination environments and expanded transition states, which facilitate easier ion diffusion. Specifically, a nudged elastic band examination of ion diffusion pathways suggests that migration barriers are substantially diminished for Na- and Mg-ion diffusion in the metastable polymorphs: the predicted migration barriers for Mg ions in ζ-V2O5 and ε-V2O5 are 0.62–0.86 and 0.21–0.24 eV, respectively. More generally, the results indicate that topochemically derived metastable polymorphs represent an interesting class of compounds for realizing multivalent cation diffusion because many such compounds place cations in “frustrated” coordination environments that are known to be useful for realizing low diffusion barriers.

The invention of rechargeable batteries has dramatically changed our landscapes and lives, underpinning the explosive worldwide growth of consumer electronics, ushering in an unprecedented era of electric vehicles, and potentially paving the way for a much greener energy future. Unfortunately, current battery technologies suffer from a number of challenges, e.g., capacity loss and failure upon prolonged cycling, limited ion diffusion kinetics, and a rather sparse palette of high-performing electrode materials. Here, we discuss the origins of diffusion limitations in oxide materials using V2O5 as a model system. In particular, we discuss constrictions in ionic conduction pathways, narrow energy dispersion of conduction band states, and the stabilization and self-trapping of polarons as local phenomena that have substantial implications for introducing multiscale compositional and phase heterogeneities. Strategies for mitigating such limitations are discussed such as reducing diffusion path lengths and the design of metastable frameworks yielding frustrated coordination and decreased barriers for migration of polarons.

The known crystal structures of solids often correspond to the most thermodynamically stable arrangement of atoms. Yet, oftentimes there exist a richly diverse set of alternative structural arrangements that lie at only slightly higher energies and can be stabilized under specific constraints (temperature, pressure, alloying, point defects). Such metastable phase space holds tremendous opportunities for nonequilibrium structural motifs and distinctive chemical bonding and ultimately for the realization of novel function. In this Feature Article, we explore the challenges with the prediction, stabilization, and utilization of metastable polymorphs. We review synthetic strategies that allow for trapping of such states of matter under ambient temperature and pressure including topochemical modification of more complex crystal structures; dimensional confinement wherein surface energy differentials can alter bulk phase stabilities; templated growth exploiting structural homologies with molecular precursors; incorporation of dopants; and application of pressure/strain followed by quenching to ambient conditions. These synthetic strategies serve to selectively deposit materials within local minima of the free-energy landscape and prevent annealing to the thermodynamic equilibrium. Using two canonical early transition-metal oxides, HfO 2 and V 2 O 5 , as illustrative examples where emerging synthetic strategies have unveiled novel polymorphs, we highlight the tunability of electronic structure, the potential richness of energy landscapes, and the implications for functional properties. For instance, the tetragonal phase of HfO 2 is predicted to exhibit an excellent combination of a high dielectric constant and large band gap, whereas ζ-V 2 O 5 has recently been shown to be an excellent intercalation host for Mg batteries. Despite recent advances, the discipline of metastable periodic solids still remains substantially dependent on empiricisms given current inadequacies in structure prediction and limited knowledge of energy landscapes. The close integration of theory and experiment is imperative to transcend longstanding chemical bottlenecks in the prediction, rationalization, and realization of new chemical compounds outside of global thermodynamic minima.

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable β-Sn 0.23 V 2 O 5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and β-Sn 0.23 V 2 O 5 /CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The β-Sn 0.23 V 2 O 5 /CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.

New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.

The disproportionation of H2O into solar fuels H2 and O2, or water splitting, is a promising strategy for clean energy harvesting and storage but requires the concerted action of absorption of photons, separation of excitons, charge diffusion to catalytic sites and catalysis of redox processes. It is increasingly evident that the rational design of photocatalysts for efficient water splitting must employ hybrid systems, where the different components perform light harvesting, charge separation and catalysis in tandem. In this topical review, we report on the recent development of a new class of hybrid photocatalysts that employs M x V2O5 (M = p-block cation) nanowires in order to engineer efficient charge transfer from the photoactive chalcogenide quantum dots (QDs) to the water-splitting and hydrogen evolving catalysts. Herein, we summarize the oxygen-mediated lone pair mechanism used to modulate the energy level and orbital character of mid-gap states in the M x V2O5 nanowires. The electronic structure of M x V2O5 is discussed in terms of density functional theory and hard x-ray photoelectron spectroscopy (HAXPES) measurements. The principles of HAXPES are explained within the context of its unique sensitivity to metal 5(6)s orbitals and ability to non-destructively study buried interface alignments of quantum dot decorated nanowires i.e., M x V2O5/CdX (X = S, Se, Te). We illustrate with examples how the M x V2O5/CdX band alignments can be rationally engineered for ultra-fast charge-transfer of photogenerated holes from the quantum dot to the nanowires; thereby suppressing anodic photo-corrosion in the CdX QDs and enabling efficacious hydrogen evolution.

No abstract