Nikolaos Kelaidis

Post-lithium (Li)-ion batteries are presently being investigated by the science community to improve materials for energy storage applications. The use of alternative ions such as sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca) as charge carriers has potential benefits that will revolutionize the field. Fast diffusion is a key advantage of Li-ions, and these alternative ions need to compete in this regard, particularly in anode materials. Here, we use density functional theory calculations to study the diffusion properties of K, Mg, and Ca dopants in Na2Ti3O7. It is calculated that Na self-diffusion is faster. Codoping with trivalent or tetravalent cations can lower the migration energies of K, Mg, and Ca dopants in Na2Ti3O7 significantly. Nevertheless, Na self-diffusion is prevalent, and only K codoped with Ga leads to a lower migration energy. When codoping Na with Al, the migration energy is reduced from 0.69 to 0.39 eV, representing a significant reduction of ∼43.5%. These findings provide a clear understanding of ion migration mechanisms and offer practical guidance for lattice engineering strategies and doping to optimize ion transport in layered titanates for energy storage applications.

Semiconducting clathrates are a distinct class of inclusion compounds with considerable interest for thermoelectric applications. We report here the synthesis, crystal structure and thermoelectric properties of Sn₃₈Sb₈I₈. The compound was synthesized via planetary ball milling of the corresponding elements for 6 h and then sintering of amorphous mixture at 620 K for 3 days. The crystal structure of the polycrystalline product was determined via X-ray powder diffraction and Rietveld refinement as a type-I clathrate (a = 12.0390(2), space group Pm-3n, No. 223) with mixed-occupied Sn/Sb framework sites and fully occupied I guest sites. Further analysis on the chemical composition, nanomorphology and vibrational modes of the material was carried out via Induced-Coupled-Plasma–Mass Spectrometry, SEM/EDX microscopy and Raman spectroscopy, respectively. Thermoelectric measurements were performed on hot-pressed samples with ca. 98% of the crystallographic density. The clathrate compound behaves as an n-type semiconductor with a band gap of 0.737 eV and exhibits a maximum ZT of 0.0016 at 473 K. Theoretical calculations on the formation enthalpy, electron density of states and transport properties provide insights into the experimentally observed physical behavior.

There is intensive research by the community to improve materials for energy storage applications beyond lithium-ion batteries. The use of sodium as the charge carrier has benefits and can revolutionize the field. The wide use of sodium-ion batteries is partially hindered by the slow diffusion of sodium ions in the anode. Here, we use density functional calculations to investigate the diffusion properties of sodium ions in Na2Ti3O7. We introduce trivalent [aluminum (Al), gallium, and scandium] and tetravalent (silicon, germanium, and tin) substitutional dopants to calculate their impact on the Na self-diffusion process. Considering a potential migration pathway, sodium atoms migrate via the vacancy mechanism with an energy barrier of 0.69 eV in undoped Na2Ti3O7. This is substantially reduced by 0.3 eV when an Al atom substitutes for a titanium atom in the pathway of migration.

Semiconducting clathrates have attracted considerable interest in the field of thermoelectric materials. We report here a computational study on the crystal structure, the enthalpy of formation, and the physical properties of the following type-I clathrates: (a) experimentally studied Cs₈Sn₄₄ and hypothetical Cs₈Sn₄₆ and (b) hypothetical (NH₄)₈Sn_46−x (x = 0 or 2). The ab initio VASP calculations for the nominal stoichiometries include the geometry optimization of the initial structural models, enthalpies of formation, and the electronic and phonon density of states. Comparison of the chemical bonding of the structural models is performed via the electron localization function. The results show that the presence and distribution of defects in the Sn framework for both Cs₈Sn_46−x and (NH₄)₈Sn_46−x systems significantly alters the formation energy and its electrical properties, ranging from metallic to semiconducting behavior. In particular, one defect per six-membered Sn ring in a 3D spiro-network is the thermodynamically preferred configuration that results in the Cs₈Sn₄₄ and (NH₄)₈Sn₄₄ stoichiometries with narrow-band gap semiconducting behavior. Moreover, the rotation of the ammonium cation in the polyhedral cavities is an interesting feature that may promote the use of ammonium or other small molecular cations as guests in clathrates for thermoelectric applications; this is due to the decrease in the lattice thermal conductivity.

Tin dioxide (SnO₂) is an important transparent conductive oxide (TCO), highly desirable for its use in various technologies due to its earth abundance and non-toxicity. It is studied for applications such as photocatalysis, energy harvesting, energy storage, LEDs, and photovoltaics as an electron transport layer. Elemental doping has been an established method to tune its band gap, increase conductivity, passivate defects, etc. In this study, we apply density functional theory (DFT) calculations to examine the electronic and optical properties of SnO₂ when doped with members of the oxygen family, namely S, Se, and Te. By calculating defect formation energies, we find that S doping is energetically favourable in the oxygen substitutional position, whereas Se and Te prefer the Sn substitutional site. We show that S and Se substitutional doping leads to near gap states and can be an effective way to reduce the band gap, which results in an increased absorbance in the optical part of the spectrum, leading to improved photocatalytic activity, whereas Te doping results in several mid-gap states.

Tin dioxide (SnO₂) is one of the most used materials for sensing applications operating at high temperatures. Commonly, “undoped SnO₂” is made by precursors containing elements that can have a deleterious impact on the operation of SnO₂ sensors. Here, we employ experimental and theoretical methods to investigate the structural properties and electronic structure of the rutile bulk and surface SnO₂, focusing on unintentional doping due to precursors. Unintentional doping from precursors as well as intrinsic doping can play an important role not only on the performance of gas sensors, but also on the properties of SnO₂ as a whole. The theoretical calculations were performed using density functional theory (DFT) with hybrid functionals. With DFT we examine the changes in the electronic properties of SnO₂ due to intrinsic and unintentional defects and we then discuss how these changes affect the response of a SnO₂-based gas sensor. From an experimental point of view, we synthesized low-cost SnO₂ thin films via sol–gel and spin-coating processes. To further enhance the performance of SnO₂, we coated the surface with a small amount of platinum (Pt). The crystalline structure of the films was analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM), while for the determination of the elements contained in the sample, X-ray photoelectron spectroscopy (XPS) measurements were performed. Furthermore, we investigated the effect of temperature on the band structure of SnO₂ in air, in a vacuum and in nitrogen and hydrogen chemical environments. To optimize the response, we used current–voltage characterization in varying environments. The aim is to associate the response of SnO₂ to various environments with the changes in the band structure of the material in order to gain a better understanding of the response mechanism of metal oxides in different pressure and temperature environments. We found that the resistance of the semiconductor decreases with temperature, while it increases with increasing pressure. Furthermore, the activation energy is highly affected by the environment to which the thin film is exposed, which means that the thin film could respond with lower energy when exposed to an environment different from the air.

Growth of PbO for energy applications and conversion of SnO ₂ /PbO into SnO ₂ /PbO ₂ .

Titanium dioxide (TiO₂) has a strong photocatalytic activity in the ultra-violet part of the spectrum combined with excellent chemical stability and abundance. However, its photocatalytic efficiency is prohibited by limited absorption within the visible range derived from its wide band gap value and the presence of charge trapping states located at the band edges, which act as electron–hole recombination centers. Herein, we modify the band gap and improve the optical properties of TiO₂ via co-doping with hydrogen and halogen. The present density functional theory (DFT) calculations indicate that hydrogen is incorporated in interstitial sites while fluorine and chlorine can be inserted both as interstitial and oxygen substitutional defects. To investigate the synergy of dopants in TiO₂ experimental characterization techniques such as Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray and ultra-violet photoelectron spectroscopy (XPS/UPS), UV–Vis absorption and scanning electron microscopy (SEM) measurements, have been conducted. The observations suggest that the oxide’s band gap is reduced upon halogen doping, particularly for chlorine, making this material promising for energy harvesting devices. The studies on hydrogen production ability of these materials support the enhanced hydrogen production rates for chlorine doped (Cl:TiO₂) and hydrogenated (H:TiO₂) oxides compared to the pristine TiO₂ reference.