Reversible carrier-type transition in gas-sensing oxides and nanostructures

Despite many important applications of a-Fe2O3 and Fe doped SnO2 in semiconductors, catalysis, sensors, clinical diagnosis and treatments, one fundamental issue that is crucial to these applications remains theoretically equivocal- the reversible carrier-type transition between n- and p-type conductivities during gas-sensing operations. Here, we give unambiguous and rigorous theoretical analysis in order to explain why and how the oxygen vacancies affect the n-type semiconductors, a-Fe2O3 and Fe doped SnO2 in which they are both electronically and chemically transformed into a p-type semiconductor. Furthermore, this reversible transition also occurs on the oxide surfaces during gas-sensing operation due to physisorbed gas molecules (without any chemical reaction). We make use of the ionization energy theory and its renormalized ionic displacement polarizability functional to reclassify, generalize and to explain the concept of carrier-type transition in solids, and during gas-sensing operation. The origin of such a transition is associated to the change in ionic polarizability and the valence states of cations in the presence of (a) oxygen vacancies and (b) physisorped gas molecules.Comment: To be published in ChemPhysChe

A first-principles study of helium storage in oxides and at oxide--iron interfaces

Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides He migration is significantly slower and He--He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe|MgO, Fe|FeO|MgO). In both cases the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.Comment: 13 pages, 10 figures, 4 table

In situ N-doped graphene and Mo nanoribbon formation from Mo2Ti2C3 MXene monolayers

Since the advent of monolayered 2D transition metal carbide and nitrides (MXenes) in 2011, the number of different monolayer systems and the study thereof have been on the rise. Mo2Ti2C3 is one of the least studied MXenes and new insights to this material are of value to the field. Here, the stability of Mo2Ti2C3 under electron irradiation is investigated. A transmission electron microscope (TEM) is used to study the structural and elemental changes in situ. It is found that Mo2Ti2C3 is reasonably stable for the first 2 min of irradiation. However, structural changes occur thereafter, which trigger increasingly rapid and significant rearrangement. This results in the formation of pores and two new nanomaterials, namely, N-doped graphene membranes and Mo nanoribbons. The study provides insight into the stability of Mo2Ti2C3 monolayers against electron irradiation, which will allow for reliable future study of the material using TEM. Furthermore, these findings will facilitate further research in the rapidly growing field of electron beam driven chemistry and engineering of nanomaterials.Web of Scienceart. no. 190711

Defect Chemistry of Novel Transparent Conductors

The combination of optical transparency and metallic-like conductivity in a single material is unusual, such that only a handful of transparent conductors have been discovered in the last century. However, they play a crucial rôle as transparent electrodes in the operation of modern devices such as display screens and solar cells. The increasing ubiquity of opto-electronic technology therefore motivates the continued development and discovery of transparent conductors. This work uses computational methods to investigate the defect chemistry and rationalise the performance of a wide variety of transparent conducting materials. The insights from this project will guide the optimisation of existing technologies through alternative doping strategies, and act as a stepping stone towards the development of new materials for transparent conducting applications. An overview of the electronic structure requirements and some applications of transparent conducting materials are offered in Chapter 1. Next, the computational theory that enables the calculation of electronic structure is presented in Chapter 2, followed by a section on methodology and implementation in Chapter 3. Chapter 4 introduces a selection of post-transition metal oxides (ZnO, In₂O₃, Ga₂O₃ and ZnSb₂O₆) as n-type transparent conductors and investigates doping strategies for achieving metallic-like conductivity. This is followed by Chapter 5, which examines CaCuP and CuI as potential p-type transparent conductors, with a focus on their defect chemistry (specifically the copper vacancy) and charge transport behaviour. Finally, p-type doping strategies are considered in the transparent perovskite BaSnO₃ in Chapter 6, followed by a summary of the results and suggestions of future work in Chapter 7

Origin of Weaker Fermi Level Pinning and Localized Interface States at Metal Silicide Schottky Barriers

The Schottky barriers of transition metal silicides on silicon are characterized by two anomalous features, a face dependence of Schottky barrier heights (SBHs) and a weaker than expected dependence of SBHs on work function or “weaker Fermi level pinning.” Density functional supercell calculations reported here find that these features arise from the occurrence of localized gap states at interfacial coordination defects, in addition to the usual metal-induced gap states (MIGSs), and these lead to pinning energies that increase sequentially across the Si gap from PtSi2 to YbSi2. The interfacial gap states vary in shape with face orientation and cause the unusual face-dependent SBHs. The localized interface defect states are a key missing addition to the MIGS model, which are needed to describe fully the interface bonding such as face orientation or coordination defects. This anomalous Fermi level pinning does not reduce gap state densities but could be used to better control SBHs by creating specific configurations with near band edge pinning energies, thus giving low contact resistances in highly scaled silicon devices or 2D semiconductors

Boosting water oxidation through in situ electroconversion of manganese gallide: an intermetallic precursor approach

For the first time, the manganese gallide (MnGa4) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnOx‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnOx minerals with distinct structures and induced defects: birnessite δ‐MnO2, feitknechtite β‐MnOOH, and hausmannite α‐Mn3O4. The abundance and intrinsic stabilization of MnIII/MnIV active sites in the three MnOx phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa4 precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm−2 with a durability of more than five days.DFG, 390540038, EXC 2008: UniSysCatTU Berlin, Open-Access-Mittel - 201