Search Results (31)

K-ion batteries (KIB) are considered the future energy storage and conversion technology due to their remarkable performance. In this work, a high-temperature solid-state process was used to effectively synthesize KMnO₂, a promising cathode material for KIBs. The materials were examined using X-ray powder diffraction (XRPD), Raman and infrared spectroscopies, electron microscopy analysis, optical, and impedance spectroscopies. Rietveld refinement of X-ray diffraction data confirmed that the compound crystallizes in the monoclinic system with the P-2₁/m space group. Fourier transform infrared and Raman spectroscopies revealed the vibrational modes of the KMnO₂ compound and proved the existence of the octahedral environment MO₆ (M = Mn, K), which affirms structural configuration. The morphological distribution and grain size of the titled compound were examined using SEM studies. A direct band gap of around 3.12 eV was found by optical studies using UV–Vis spectroscopy, confirming the semiconducting nature of KMnO₂ and indicating its applicability for optoelectronic and energy-related applications. The characteristics of this material were further examined using impedance spectroscopy at temperatures between 343 and 443 K and a frequency range of 10⁻¹ Hz to 10⁶ Hz. The DC conductivity and relaxation time exhibited Arrhenius behavior, with a significant shift in activation energy at 373 K, suggesting a change in the conduction mechanism. The frequency behavior of AC conductivity, σ_ac, was analyzed using the universal Jonscher law. The findings of the charge transportation study on KMnO₂ indicate that this material follows a non-overlapping small polaron tunneling (NSPT) for T < 383 K and correlated barrier hopping (CBH) above for T > 383 K. A correlation between the ionic conductivity and the crystal structure was established and discussed. Full article

The development of stable and efficient solid electrolytes is essential for advancing solid-state battery technologies. In this study, we present a comparative study of three sulfide-based electrolytes, Li₆PS₅Cl (LPSCl), Li₆PS₅Br (LPSBr), and Li₁₀GeP₂S₁₂ (LGPS), combining Density Functional Theory (DFT) and hybrid (HSE06) simulations for electrochemical, charge carrier transport, and structural characterization. DFT and HSE06 simulations revealed semiconductor-like direct band gaps for LPSCl, with a 2.45 eV (DFT) −3.30 eV (HSE06) and 2.32 eV (DFT) −3.34 eV (HSE06) for LPSBr, and indirect band gap with 2.13 eV (DFT) −3.22 eV (HSE06) for LGPS, along with work functions of 3.40 eV for the argyrodites and 3.67 eV for LGPS. Scanning Kelvin Probe (SKP) analyses, performed at both micrometric and nanometric resolution, showed consistently negative surface potentials and interfacial polarons associated with electron tunneling through the surface of the electrolyte. Potentiostatic electrochemical impedance spectroscopy (PEIS) and cyclic voltammetry (CV) confirmed enhanced ionic conductivity with increasing temperature. While LPSCl and LGPS exhibited stable behavior at almost all temperatures, from −20 to 60 °C, LPSBr displayed noise-like activity at 0 °C with Au symmetric electrodes. This integrated experimental/theoretical approach highlights differences in electronic structure, interfacial charge distribution, and electrochemical stability, all showing affinity to react with lithium, providing key insights for the design and optimization of solid electrolytes for next-generation batteries. Full article

The field of developing effective catalysts for heterogeneous catalysis has recently focused on controlling the structures of catalysts themselves to optimise the density and energy of crystal lattice defects. This can significantly influence catalytic activity in terms of both reaction rates and reaction mechanisms, and thus the selective production of desired substances as well. In many cases, these crystal lattice defects manifest themselves as so-called electron traps (ETs) and thus significantly influence charge transfer between the catalyst and reactants. ETs provide the missing electronic link between atomic-scale defects and macroscopic performance in heterogeneous catalysis. Therefore, the importance of ETs for catalysis is particularly evident in areas where charge transfer plays a fundamental role in the reaction mechanism, such as photocatalysis and electrocatalysis. In the field of thermally initiated reactions, the importance of ETs in heterogeneous catalysis has not yet been fully appreciated. However, several studies have already addressed the importance of ETs for this type of reaction. This review consolidates and extends the concept of ETs to purely thermal-initiated reactions, with a focus on CO₂ hydrogenation using typical transition metal catalysts. Firstly, in this review, ETs are defined as band gap states associated with internal and external defects, and their depth, density, spatial location, and dynamics are then coupled with key steps in thermocatalytic cycles, including charge storage/release, reactant activation, intermediate stabilisation, and redox turnover. Secondly, electron trap detection is reviewed based on advanced spectroscopic techniques, including reversed double-beam photoacoustic spectroscopy (RDB-PAS), thermally stimulated current (TSC), deep-level transient spectroscopy (DLTS), thermoluminescence (TL), electron paramagnetic resonance (EPR), and photoluminescence (PL), highlighting how each method describes trap energetics and populations under realistic operating conditions. Finally, case studies on the application of metal oxides and supported metals are discussed, as these are typical catalysts for the reaction mentioned above. This review highlights how oxygen vacancies (OVs), polarons, and metal–support interfacial sites act as robust electron reservoirs, lowering the barriers for CO₂ activation and hydrogenation. By reframing thermocatalysts through the lens of ET chemistry, this review identifies ETs as actionable targets for the rational design of next-generation materials for CO₂ hydrogenation and related high-temperature transformations. Full article

We investigate how grain growth, strain relaxation, and vacancy chemistry shape the near-edge optical response of nanocrystalline Ce${\mathrm{O}}_{2-\delta }$ prepared by a chymosin-assisted Pechini route from nitrate–citrate precursors. Rietveld line-profile analysis shows that phase-pure Ce${\mathrm{O}}_{2-\delta }$ forms after calcination between 400 and 1000 °C. Over this range, the average crystallite size increases from ≈3.4 to ≈57 nm, while the microstrain decreases from 0.79% to 0.05%, with size–strain scaling consistent with interface-controlled grain growth that follows a normal growth law with exponent $m=2$ and activation energy $Q\approx 155$ kJ ${\mathrm{mol}}^{-1}$. Raman spectroscopy tracks the sharpening of the $F_{2g}$ mode and the fading of defect-related bands, X-ray photoelectron spectroscopy reveals a nonmonotonic evolution of the surface ${\mathrm{Ce}}^{3+}$ fraction and separates lattice from adsorbed oxygen species, and electron paramagnetic resonance detects vacancy-bound ${\mathrm{Ce}}^{3+}$ polarons that weaken at high temperature. Diffuse-reflectance UV–Vis spectra show a modest blue shift of the apparent band gap from $E_g\approx 2.78$ to 2.95 eV as crystallites coarsen, while the Urbach energy $E_u$ follows the ${\mathrm{Ce}}^{3+}$ content and sub-gap tailing. The structural, spectroscopic, and optical results together map out a quantitative connection between grain growth, vacancy populations, and near-edge optical properties in ${\mathrm{CeO}}_{2-\delta }$ nanoparticles. Full article

This study presents the electrical and optical properties of 35P₂O₅–xV₂O₅–(65–x) Sb₂O₃ glasses for 0 ≤ x ≤ 65 mol%. The direct current (DC) resistivity was measured by the Van der Pauw method and optical absorption spectra were taken in the Ultraviolet–Visible-Near-Infrared (UV–VIS–NIR) range. Electrical transport is attributed to simultaneous hopping of small polarons (SPs) between V⁴⁺ and V⁵⁺ (vanadium ion) sites and small bipolarons (SBPs) between the Sb³⁺ and Sb⁵⁺ (antimony ion) sites. The resistivity exhibits a non-linear dependence on the ionic fraction of vanadium (n_v), whereas the resistivity exhibits a minimum in the composition range 0 ≤ n_V ≤ 0.3, and a resistivity maximum was observed in the range 0.3 ≤ n_V ≤ 0.5. On further increasing n_v, the resistivity exhibits a monotonic decline. In the composition range 0 ≤ n_V ≤ 0.3, where the hopping distance between V ions decreases, while that between the Sb ions increases, the resistivity minimum has been shown to be the consequence of decreasing tunneling distance of SPs between the V⁴⁺ and V⁵⁺ ion sites. In the composition range 0.3 ≤ n_V ≤ 0.5, the resistivity, activation energy for DC conduction, glass transition temperature, and density exhibit their respective maxima even though the separation between the V⁴⁺ and V⁵⁺ sites continues to decrease. This feature is explained by enhanced localization of electrons on account of increased disorder (entropy) among the SPs and SBPs, like that of Anderson localization. This argument is further supported by a shift in the polaronic optical absorption bands associated with the SPs and SBPs toward higher energies. The transport behavior of all the glasses except the x = 0 composition has been explained by adiabatic transport, principally, by the SPs on V ions while the Sb ions contribute little to the total transport process. The results provide a clear relation between composition, polaron/bipolaron contributions, and conduction in these glasses. Full article

Three composite materials, made by inserting the same amount of BiFeO₃/Bi₂₅FeO₄₀ powders (each powder having a different concentration of the secondary phase, Bi₂₅FeO₄₀: 10%, 20%, and 30%) into a silicone rubber (SR) matrix, were investigated to understand their electrical properties. Electrical conductivity measurements of the composite samples were carried out over a frequency range from 0.5 kHz to 2 MHz. The resulting conductivity spectra revealed two distinct regions: a low-frequency plateau corresponding to DC conductivity and a high-frequency region where AC conductivity increases with frequency. Some key electrical parameters, such as DC conductivity and band gap energy, were calculated using these measurements. An increase in Bi₂₅FeO₄₀ concentration resulted in a rise in DC conductivity from 5.61 × 10⁻⁵ S/m to 7.67 × 10⁻⁵ S/m across the composite samples. To gain further insight into the mechanisms of charge transport, both Jonscher’s universal response and the correlated barrier hopping (CBH) model were applied. The polaron model was also used to calculate the energy barrier for electrical conduction, but for higher temperatures (where the samples exhibit conductor behavior). The last part of the study was an aging analysis that showed a degradation of the investigated sample, as reflected by a decline in their conductive properties over time. Having no endothermic or exothermic events in the DTA curves, it is clear that the observed variation in conductive properties is not related to phase transitions, but it can be attributed to microstructural mechanisms, such as defects, microcracks, or structural disorders. These results can help in designing composite materials with desirable conductive properties by optimizing their filler concentration and processing conditions. Full article

The direct discharge of cationic surfactants into environmental matrices has exponentially increased due to their wide application in many products. These compounds and their degraded products disrupt microbial dynamics, hinder plant survival, and affect human health. Therefore, there is an urgent need to develop electroanalytical assessment techniques for their identification, determination, and monitoring. In our study, ZnO-PANI nanocomposites were electrodeposited on a glassy carbon electrode (GCE), followed by the immobilization of laccase enzymes and the electrodeposition of polypyrrole (PPy), to form a biosensor that was used for the detection of CTAB. A UV-Vis analysis showed bands corresponding to the π-π* transition of benzenoid and quinoid rings, π-polaron band transition and n-π*polaronic transitions associated with the extended coil chain conformation of PANI, and the presence and interaction of ZnO with PANI and type 3 copper in the laccase enzymes. The FTIR analysis exhibited peaks corresponding to N-H and C-N stretches and bends for amine, C=C stretches for conjugated alkenes, and a C-H bend for aromatic compounds. A high-resolution scanning electron microscopy (HRSEM) analysis proved that PANI and ZnO-PANI were deposited as fibres with hairy topography resulting from covalent bonding with the laccase enzymes. The modified electrode (PPy-6/GCE) was used as a platform for the detection of CTAB with three linear ranges of 0.5–100 µM, 200–500 µM, and 700–1900 µM. The sensor displayed a high sensitivity of 0.935 μA μM⁻¹ cm⁻², a detection limit of 0.0116 µM, and acceptable recoveries of 95.02% and 87.84% for tap water and wastewater, respectively. Full article

Amorphous ZrO₂ thin films with increasing Mg content were deposited on quartz substrates, by dip coating method. The films are transparent in the visible domain and absorbent in UV, with an optical band gap that decreases with the increase of Mg content, from 5.42 eV to 4.12 eV. The temperature dependent conductivity measurements showed typical semiconductor comportment. The decrease of the electrical conductivity by Mg doping was related to the increase of the OH groups (37% to 63%) as seen from X-ray Photoelectron Spectroscopy. It was found out that the electrical conductivity obeys the Meyer-Neldel rule. This rule, previously reported for different disordered material systems is obtained for ZrO₂ for the first time in the literature. Exploring novel aspects of Mg-doped ZrO₂, the present study underscores the origin of the Meyer-Neldel rule explained by the small-polaron hopping model in the non-adiabatic hopping regime. Determination of the presence of such a conduction mechanism in the samples hold promise for comprehending the important aspects, which might be a concern in developing various devices based on Mg-doped ZrO₂. Full article

In this study, we explored the effect of either Nb or Sc doping at a concentration range of 0.0–1.0 at.% on the physical–chemical and photoelectrochemical behavior of TiO₂ anatase electrodes. This behavior was characterized by work function, flat band potential, donor density, spectral dependence of photocurrent and stationary photocurrent measurements. All experimental results are interpreted in terms of the formation of the shallow delocalized polaron states in the case of Nb doping and deep acceptor states induced by Sc doping on TiO₂ anatase. Full article

Copper (Cu²⁺)-and-magnesium (Mg²⁺)-doped Zinc aluminate ZnAl₂O₄ is a promising material with diverse applications in electronic and energy storage devices. In this study, the synthesis of Zn_0.9M_xAl₂O₄ (M = Cu²⁺ and Mg²⁺; x = 0.00 and 0.10) was conducted via the sol–gel combined combustion technique. The structural, spectral, optical and dielectric parameters of the synthesized spinel aluminates were analysed to explore the substitution effect of Cu²⁺ and Mg²⁺ content. The formation and crystallinity analyses of the single-phase cubic spinel structure in the synthesized spinel aluminates were confirmed using XRD patterns. The lattice parameter and grain size were ascertained from the XRD data. The crystallite size of Cu²⁺ and Mg²⁺ substituted into ZnAl₂O₄ using Scherrer’s formula was found to be around 22 nm. The spinel structure formations in the prepared spinel aluminates were ascertained through an FT-IR study. The UV-Vis spectra exhibited a broad absorption band in the UV-Vis region, indicating the presence of electronic transitions. The band gap energy of the prepared aluminates was estimated from the absorption edge, with values varying between 2.90 eV and 3.03 eV, revealing its suitability for optoelectronic applications. Measurement of the dielectric parameters was performed in the frequency range of 100 Hz to 20 MHz at temperatures ranging from 30 °C to 250 °C. The dielectric constant (ε′) and dielectric loss (ε”) were determined as a function of frequency at different temperatures. The results showed that the dielectric constant decreased with increasing frequency for all the observed temperatures, while the dielectric loss exhibited a peak at a specific temperature. The conductivity results indicate that the conduction mechanism occurred due to polaron hopping. The Arrhenius relation was adopted to calculate the activation energies E_a for all the samples, and the values were between 0.70 eV and 0.38 eV. The obtained results were discussed and interpreted. These findings contribute to our understanding of the electrical behaviour of doped zinc aluminate materials and their useful applications in different electronic and energy systems. Full article

The oxygen reduction reaction (ORR) activity of a Cu-doped Ba_0.5Sr_0.5FeO_3−δ (Ba_0.5Sr_0.5Fe_1−xCu_xO_3−δ, BSFCux, x = 0, 0.05, 0.10, 0.15) perovskite cathode was investigated in terms of oxygen vacancy formation and valence band structure. The BSFCux (x = 0, 0.05, 0.10, 0.15) crystallized in a cubic perovskite structure ($Pm\overline{3}m$). By thermogravimetric analysis and surface chemical analysis, it was confirmed that the concentration of oxygen vacancies in the lattice increased with Cu doping. The average oxidation state of B-site ions decreased from 3.583 (x = 0) to 3.210 (x = 0.15), and the valence band maximum shifted from −0.133 eV (x = 0) to −0.222 eV (x = 0.15). The electrical conductivity of BSFCux increased with temperature because of the thermally activated small polaron hopping mechanism showing a maximum value of 64.12 S cm⁻¹ (x = 0.15) at 500 °C. The ASR value as an indicator of ORR activity decreased by 72.6% from 0.135 Ω cm² (x = 0) to 0.037 Ω cm² (x = 0.15) at 700 °C. The Cu doping increased oxygen vacancy concentration and electron concentration in the valence band to promote electron exchange with adsorbed oxygen, thereby improving ORR activity. Full article

We have established that luminescence in lithium niobate crystals both congruent and near-stoichiometric (R ≈ 1) is due to point defects in the cationic sublattice and intraconfigurational transitions in the oxygen-octahedral NbO₆ clusters. We have also determined that the main contribution to the luminescence in the visible and near IR regions is made by luminescence centers with the participation of Nb_Li defects: the Nb_Li-Nb_Nb bipolaron pair and the Nb_Li-O defect in a congruent crystal. The minimum intensity of bipolaron luminescence has been observed in stoichiometric crystals obtained using different technologies. Weak luminescence of the Nb_Li-Nb_Nb bipolaron pair indicates a small number of Nb_Li defects in the crystal structure. The number of Nb_Li defects in the crystal structure indicates a deviation of the crystal composition from stoichiometry. Full article

Aligned polymer nanofibres are prepared by means of the electrospinning of a chlorobenzene solution containing regioregular poly(3-hexyltiophene-2,5-diyl), P3HT, and poly(ethylene oxide), PEO. The PEO scaffold is easily dissolved with acetonitrile, leaving pure P3HT fibres, which do not show structural modification. Polymer fibres, either with or without the PEO supporting polymer, are effectively doped by exposure to iodine vapours. Doping is monitored following the changes in the doping-induced vibrational bands (IRAVs) observed in the infrared spectra and by means of Raman spectroscopy. Molecular orientation inside the fibres has been assessed by means of IR experiments in polarised light, clearly demonstrating that electrospinning induces the orientation of the polymer chains along the fibre axis as well as of the defects introduced by doping. This work illustrates a case study that contributes to the fundamental knowledge of the vibrational properties of the doping-induced defects—charged polarons—of P3HT. Moreover, it provides experimental protocols for a thorough spectroscopic characterisation of the P3HT nanofibres, and of doped conjugated polymers in general, opening the way for the control of the material structure when the doped polymer is confined in a one-dimensional architecture. Full article

The optoelectronic properties of transition metal oxide semiconductors depend on their oxygen vacancies, nanostructures and aggregation states. Here, we report the synthesis and photoluminescence (PL) properties of substoichiometric tungsten oxide (WO_3-x) aggregates with the nanorods, nanoflakes, submicro-spherical-like, submicro-spherical and micro-spherical structures in the acetic acid solution without and with the special surfactants (butyric or oleic acids). Based on theory on the osmotic potential of polymers, we demonstrate the structural change of the WO_3-x aggregates, which is related to the change of steric repulsion caused by the surfactant layers, adsorption and deformation of the surfactant molecules on the WO_3-x nanocrystals. The WO_3-x aggregates generate multi-color light, including ultraviolet, blue, green, red and near-infrared light caused by the inter-band transition and defect level-specific transition as well as the relaxation of polarons. Compared to the nanorod and nanoflake WO_3-x aggregates, the PL quenching of the submicro-spherical-like, submicro-spherical and micro-spherical WO_3-x aggregates is associated with the coupling between the WO_3-x nanoparticles and the trapping centers arising from the surfactant molecules adsorbed on the WO_3-x nanoparticles. Full article

The existence of thermally-activated quasiparticles in amphiboles is an important issue, as amphiboles are among the main hydrous complex silicate minerals in the Earth’s lithosphere. The amphibole structure consists of stripes of 6-membered TO₄-rings sandwiching MO₆ octahedral slabs. To elucidate the atomistic origin of the anomalous rock conductivity in subduction-wedge regions, we studied several Fe-containing amphiboles with diverse chemistry by using in situ, temperature-dependent, polarised Raman spectroscopy. The occurrence of resonance Raman scattering at high temperatures unambiguously reveal temperature-activated small polarons arising from the coupling between polar optical phonons and electron transitions within Fe²⁺O₆ octahedra, independently of the amphibole chemical composition. The FeO₆-related polarons coexist with delocalised H⁺; that is, at elevated temperatures Fe-bearing amphiboles are conductive and exhibit two types of charge carriers: electronic polarons with highly anisotropic mobility and H⁺ cations. The results from density-functional-theory calculations on the electron band structure for a selected amphibole compound with a relatively simple composition are in full agreement with experimental data. The polaron activation temperature, mobility, and polaron-dipole magnitude and alignment can be controlled by varying the mineral composition, which makes amphiboles attractive “geo-stripes” that can serve as mineral-inspired technology to design thermally-stable smart materials with anisotropic properties. Full article