Добірка наукової літератури з теми "Zn2Mo3O8"

A hierarchical Zn₂Mo₃O₈ nanodots–porous carbon composite has been successfully synthesized via the ingenious combination of ion-exchange and molten salt strategies, and the composite exhibits remarkable performance as an anode material for lithium-ion batteries.

Analysis of Bragg diffraction is the normal route to the structure of crystalline materials. Here we demonstrate the use of total neutron diffraction in determining the local structure in the disordered lanthanum molybdate LaMo2O5. An average structure with space-group symmetry P63/mmc accounts for the Bragg scattering and shows that the compound contains the rare Mo6O18 cluster and a unique type of Mo—Mo bonded sheet. However, this gives an incomplete picture of the structure, since it does not reveal how the sites with fractional occupancy are occupied at a local level. Two models describing possible local structures are constructed by removing symmetry elements present in the average structure. Total correlation functions, T(r), calculated from these structures, with space-group symmetry P63 mc and P3¯m1, are compared with the experimental T(r) to show the validity of these local structures. The close relationship between the T(r)'s of the component structures gives an insight into why disorder occurs in LaMo2O5. The calculated and experimental T(r)'s for a model compound, Zn2Mo3O8, are compared to show the agreement expected from an ordered crystalline material. Remaining discrepancies between our model and the experimental T(r) give an insight into the origin of additional disorder in LaMo2O5.

Zinc-ion batteries (ZIBs) have received attention as one type of multivalent-ion batteries due to their potential applications in large-scale energy storage systems. Here we report a prototype of rocking-chair ZIB system employing Zn2Mo6S8 (zinc Chevrel phase) as an anode operating at 0.35 V, and K0.02(H2O)0.22Zn2.94[Fe(CN)6]2 (rhombohedral zinc Prussian-blue analogue) as a cathode operating at 1.75 V (vs. Zn/Zn2+) in ZnSO4 aqueous electrolyte. This type of cell has a benefit due to its intrinsic zinc-dendrite-free nature. The cell is designed to be positive-limited with a capacity of 62.3 mAh g−1. The full-cell shows a reversible cycle with an average discharge cell voltage of ~1.40 V, demonstrating a successful rocking-chair zinc-ion battery system.

In the recent years, oxides have been the focus of numerous theoretical and experimental studies. This is because of a wide variety of exotic physical phenomenon, such as multiferroicity, charge ordering, metal-insulator transitions, high-Tc superconductivity etc that have been observed in these materials. Moreover, most oxides are earth-abundant, stable, non-toxic and easy to produce in a wide range of environmental conditions. As a result, they have also been used in a variety of technological applications. In this thesis, we study bulk oxides, interfaces between different oxides, and defects in bulk oxides. We use first-principles methods to calculate different properties of these systems as discussed below. These state-of-the- art methods based on density functional theory (for ground-state properties) and many-body perturbation theory (for excited-state properties) have been shown to predict properties that are in excellent agreement with experiments. Our study of bulk properties of oxides is motivated by the possibility of constructing an efficient all-oxide solar cell. We explore two ferroelectric transition metal oxides, YMnO3 and Zn2Mo3O8, as potential candidates for photoabsorbers. We calculate the electronic structure and optical properties of these materials and compare our results with available experiments. A technologically and fundamentally interesting phenomenon at oxide interfaces is the formation of a two-dimensional electron gas (2DEG). We propose a novel oxide heterostructure system, consisting of two materials with chemical formula A2Mo3O8 (A = Zn, Mg, Cd), which has the potential to host a 2DEG. Our calculations predict the formation of 2DEG at this interface with electron densities and localization comparable to that of other well-known 2DEG systems. In the last part of the thesis, we investigate the electronic structure and optical properties of the oxygen vacancies (F-centers) in -alumina. -Alumina or sapphire is a widely used and well-studied material. We propose a modi fication of the existing method for calculation of defect charge transition levels (CTLs) in solids. Using this modi fication we calculate CTLs for F-centers in -alumina. We show that our modi fication improves the accuracy of the results signifi cantly. Furthermore, we calculate excited state properties of these F-centers to understand and explain photoluminescence experiments performed on these systems

Several oxides are non-toxic, stable, and can be deposited using inexpensive techniques on a variety of substrates. There is a compelling case for use of oxide in photovoltaics, referred to as “all-oxide photovoltaics”. Traditional photovoltaics technologies often use oxides, e.g. as transparent electrodes (SnO2: F. In2O3: SnO2, etc.) or carrier-selective contacts (TiO2, MoOx, etc.). However, oxides are rarely used as light-absorbers because oxides tend to have large bandgap (> 2 eV), low hole mobility (< 10-4 cm2/Vs), and low carrier diffusion lengths (<100 nm). Despite considerable effort, few oxides have been demonstrated as efficient solar absorbers. One class of oxides still poorly investigated, are the multi-cation transition oxides. This is a very large library, so it is conceivable that good solar absorbers are waiting to be discovered. In this work, we present a study of three multi-cation transition metal oxides for their application in solar cells: Zn2Mo3O8, Mn2V2O7, and Ag2CrO4. Zn2Mo3O8 is a non-centrosymmetric oxide with a low-bandgap of 2.1 eV. Polycrystalline films deposited at room temperature are n-type with Hall mobility of 0.6 - 0.7 cm2V-1s-1. DFT calculations suggest that the valence band is composed of both O 2p and Mo 3d orbitals – which could lead to higher hole mobility than that typical oxides. The valence band composition was experimentally determined using resonant photoelectron spectroscopy, which confirms this assertion. Unfortunately, DFT calculations also show that ZMO has high energy Frenkel excitons (0.78 eV), which will not dissociate at room-temperature, leading to reduced voltage in solar cells. Au/ZMO/TiO2 Schottky diodes with ZMO films deposited at room temperature show photoresponse but no photovoltage. This shows that ZMO is a carrier conducting semiconductor that can be used in a photodetector but not as a solar absorber. Mn2V2O7(MVO) was previously demonstrated as a photocatalyst with a low bandgap of 1.6 eV. MVO also has a d-contribution to the valence band which leads to high hole mobility. MVO has a low melting point of ≈1080 oC, allowing deposition by PLD at high homologous temperatures which result in films with a large lateral grain size >10 μm on SiO2 and ~ 1 μm on SrRuO3. The conductivity of MVO changes across five orders of magnitude by changing the deposition pressure. The ability to deposit ‘conductive’ MVO allows us to make rectifying devices, but the photovoltaic performance of solar cells is poor. The film deposited at higher temperatures is p-type with high Hall mobility (>200 cm2/V-s). An interdigitated back- contacted cell with carrier-selective TiO2 contact shows improved performance with an open- circuit voltage of 0.33 V and a short circuit current of 0.2 mA/cm2, with an efficiency of 0.017%. N2 -annealing is shown to reduce the Urbach energy from 264 meV to 76 meV, comparable to CIGS which is a commercial solar absorber. The bandgap and crystallinity of MVO are unaltered. Annealed MVO film shows significant solar cell performance with a short circuit current of 0.46 mA/cm2 and an open-circuit voltage of 0.21 V with an efficiency of 0.024% in lateral top contacted solar cells. The annealed film also shows photoluminescence(PL) at low temperatures – another indicator of enhanced electronic quality. The PL is a doublet with a low FWHM of just 100 μeV and separation of 380 μeV at 4.6 K. Analysis of the temperature-dependent PL reveals a pair of shallow dopant levels: a donor at 5.6 meV and acceptor at 48.9 meV from the corresponding band edges, which led to p-type intrinsic doping. Ag2CrO4(ACO) is a known photocatalyst with low bandgap (1.8 eV) and a low melting point (660 oC). The latter leads to highly crystalline films, even when deposition temperatures are low. DFT shows that ACO has a valence band composed of O 2p and Ag 4d, which could lead to higher hole mobility. Schottky solar cells using ACO show typical photovoltaic behavior, with a short circuit current of 12 μA/cm2 and open-circuit voltage of 0.2 V. However, the power conversion efficiency is just 0.0007%, like other emerging oxide absorbers. An investigation into the possible reasons shows that the Urbach tail at the bandgap is very large (885 meV), a sign of significant electronic disorder in the film. Just like oxide-absorbers, p-type oxides are also quite uncommon. There are few known p-type wide-bandgap oxides, like Cu2O, but band-alignment and interfaces are often the limiting factors for their integration in devices. Here we successfully integrated-type Cu2O with a silicon absorber to make hole-selective contact. Photoelectron spectroscopy measurements reveal that the band-alignment between Cu2O and Si blocks the flow of electrons from silicon to Cu2O but allows the passage of holes. Interface recombination was reduced by integrating an ultra-thin SiO2 layer between Cu2O and Si. The p-Cu2O/n-Si heterojunction with the passivating SiO2 interlayer showed an open-circuit voltage of 0.53 V, which at the time was a record among cells without back surface passivation. In summary, the properties of transition metal oxide thin films for were investigated for application in photovoltaics, out of which MVO and ACO proved to be potential candidates as oxide absorbers, and Cu2O a candidate for a hole-blocking layer. This work is expected to contribute to the development of efficient all-oxide-solar cells.