Non-bridging Oxygen Research Articles

Thermal Conductivities of Molten Na2O–SiO2 Slags from the Perspective of Depolymerization of the Silicate Network

The thermal conductivities of molten KNO3, molten NaCl, and molten xNa2O–(100−x)SiO2 (x = 20, 33, and 40) (mol%) are measured over the temperature ranges between 623–723 K, 1173–1473 K, 1173–1573 K, respectively, by nonstationary hot‐wire method to elucidate the relationship between thermal conductivities of molten Na2O–SiO2 slags and silicate network structures. 1) The thermal conductivity measurements of molten KNO3 reveal that the measurements are successfully made without the influence of electrical leakage from the bare metallic wire to the melt as the present data are in good agreement with the reported data measured by the nonstationary hot‐wire method with ceramic‐coated hot‐wires. 2) The thermal conductivity measurements of molten NaCl measured at 1173 and 1273 K agree well with the recommended data. However, the data at 1373 and 1473 K are much smaller than the recommended data due to some electrical noise affecting the voltage changes ΔV between the potential leads. 3) The thermal resistivities of molten Na2O–SiO2 at liquidus temperatures ρLT increase with an increase in the number of nonbridging oxygens (NBO) per tetrahedrally coordinated atoms (e.g., Si, Al), i.e., NBO/T, and have a linear relationship between ρLT and NBO/T.

Microwave Synthesis of Luminescent Recycled Glass Containing Dy2O3 and Sm2O3

This research studied the recycling of borosilicate glass wastes from damaged laboratory glassware. The luminescent glasses were prepared by doping glass waste powder with rare earth ions, namely, dysprosium ions (Dy3+) and samarium ions (Sm3+), as well as co-doping with Dy3+ and Sm3+ at a concentration of 2% by weight. The sintering process was conducted in a microwave oven for a duration of 15 min. The photoluminescence spectra of the doped glasses were obtained under excitation at 401 nm and 388 nm. The results showed that the emission characteristics depended on the doping concentrations of Dy3+ and Sm3+ and the excitation wavelengths. Upon excitation at 401 nm, the co-doped glasses exhibited the maximum emission peak of Sm3+ at 601 nm (yellowish and orange region in the CIE chromaticity diagram) due to the energy transition from 4G5/2 to 6H7/2. When excited at 388 nm, however, the emission spectra of the co-doped glasses were similar to the characteristic emission peaks of Dy3+ (white region in the CIE chromaticity diagram), but the peak position exhibits a red shift. This could be attributed to an increase in the amount of non-bridging oxygens (NBOs) by co-doping.

Nanoscale Effects in the Room-Temperature UV-Visible Photoluminescence from Silica Particles and Its Cancer Cell Imaging.

Silica nano/microparticles have generated significant interest for the past decades, emerging as a versatile material with a wide range of applications in photonic crystals, bioimaging, chemical sensors, and catalysis. This study focused on synthesizing silica nano/microparticles ranging from 20 nm to 1.2 μm using the Stöber and modified Stöber methods. The particles exhibited photoluminescence emission across a UV-visible range, specifically in the UV (∼290, ∼327, ∼339, and ∼377 nm), blue (∼450 nm), green (∼500 nm), yellow (∼576 nm), and red (∼634 nm) range of the electromagnetic spectrum. These emissions are due to radiative relaxation processes involving oxygen-deficient centers arising due to unrelaxed oxygen vacancies, strong interacting surface silanols, 2-fold coordinated silicon, self-trapped excitons, hydrogen-related species, strain-induced defects, and nonbridging oxygen hole centers excited via two-photon and single photon absorption. The increased PL intensity with a decreasing particle size was attributed to higher concentrations of defect sites in the case of smaller-sized particles. The MTT assay, AO/EB staining, and the DCFDA assay confirmed the biocompatible nature of silica particles in the HepG2 cell line. In addition, the cell viability assay in a normal cell line (HEK293) also showed no substantial cell death. Successful bioimaging of HepG2 cells was performed with silica nano/microparticles, which exhibited blue and green fluorescence, along with Hoechst33258 dye. Even though 20 nm-sized silica particles showed higher PL emission, particles sized above 20 nm showed better fluorescence in HepG2 cells, citing their potential in in vitro bioimaging applications.

The Effect of Boron Oxide on the Structures and Thermal Properties of Phosophosilicate Bioactive Glasses for Metallic Implants’ Coatings

To design bioactive glass compositions with optimal thermal, mechanical, and bioactive properties as coatings on Ti6Al4V metallic implants, we investigated phosphosilicate bioactive glasses based on the 6P55 composition. SiO2 was substituted with B2O3 to improve adhesion to the metallic implants and physical properties. This substitution significantly altered the glass structure and is hypothesized to improve adhesion. Computational and experimental methods revealed that boron substitution introduced BO3 and BO4 units, disrupted the Si-O network, and formed non-bridging oxygens (NBOs), resulting in a decrease in density and glass transition temperature (Tg). These changes were attributed to boron’s dual role as a network former and modifier, influencing coordination environments and connectivity. Thermal and structural analyses showed that optimal boron levels improved thermal expansion and network flexibility, which are critical for coating applications. By integrating molecular dynamics simulations and experimental techniques, this study provides valuable insights into tailoring glass compositions for enhanced performance on metallic substrates.

Reactions of Hydrogen-Passivated Silicon Vacancies in α-Quartz with Electron Holes and Hydrogen.

We used density functional theory with a hybrid functional to investigate the structure and properties of [4H]Si (hydrogarnet) defects in α-quartz as well as the reactions of these defects with electron holes and extra hydrogen atoms and ions. The results demonstrate the depassivation mechanisms of hydrogen-passivated silicon vacancies in α-quartz, providing a detailed understanding of their stability, electronic properties, and behaviour in different charge states. While fully hydrogen passivated silicon vacancies are electrically inert, the partial removal of hydrogen atoms activates these defects as hole traps, altering the defect states and influencing the electronic properties of the material. Our calculations of the hydrogen migration mechanisms predict the low energy barriers for H+, H0, and H-, with the lowest barrier of 0.28 eV for neutral hydrogen migration between parallel c-channels and a similar barrier for H+ migration along the c-channels. The reactions of electron holes and hydrogen species with [4H]Si defects lead to the breaking of O-H bonds and the formation of non-bridging oxygen hole centres (NBOHCs) within the Si vacancies. The calculated optical absorption energies of these centres are close to those attributed to individual NBOHCs in glass samples. These findings can be useful for understanding the role of [4H]Si defects in bulk and nanocrystalline quartz as well as in SiO2-based electronic devices.

A DNA phosphorothioation pathway via adenylated intermediate modulates Tdp machinery.

In prokaryotes, the non-bridging oxygen in the DNA sugar-phosphate backbone can be enzymatically replaced by a sulfur atom, resulting in phosphorothioate (PT) modification. However, the mechanism underlying the oxygen-to-sulfur substitution remains enigmatic. In this study, we discovered a hypercompact DNA phosphorothioation system, TdpABC, in extreme thermophiles. This DNA sulfuration process occurs through two sequential steps: an initial activation step by ATP to form an adenylated intermediate, followed by a substitution step where the adenyl group is replaced with a sulfur atom. Together with the TdpA-TdpB, the TdpABC system provides anti-phage defense by degrading PT-free phage DNA. Cryogenic electron microscopy structural analysis revealed that the TdpA hexamer binds one strand of encircled duplex DNA via hydrogen bonds arranged in a spiral staircase conformation. Nevertheless, the TdpAB-DNA interaction was sensitive to the hydrophobicity of the PT sulfur. PTs inhibit ATP-driven translocation and nuclease activity of TdpAB on self-DNA, thereby preventing autoimmunity.

Dielectric and spectroscopic features of Li2O/Fe2O3/In2O3/P2O5 glass systems doped with Bi2O3

A comprehensive exploration of penta glass systems’ spectroscopic and dielectric characteristics was achieved, specifically for the glass series 20Li2O–15Fe2O3–5In2O3–xBi2O3–(60–x) P2O5(x = 0, 2.5, 5, 7.5, 10, and 12.5 mol %). Both average phosphorus-phosphorus separation and crystalline volume were found to decrease depending on Bi2O3 content. The optical band gap obtained from diffuse reflectance spectra (DRS) is found to be in the range of 1.58–1.81 eV with increasing bismuth oxide content. The optical band gap increases with bismuth content, which can be ascribed to structural changes and the reduction in non-bridging oxygen (NBO). A decrease in optical parameters, viz., refractive index (2.945–2.824), molar refractivity (32.45–29.12), and molar polarizability values (12.88–11.56), occurs with Bi2O3 content. The glasses have been designated as semiconductors due to an increase in the metallization criteria values (M lie between 0.2811 and 0.3008). The obtained results exhibit contraction in the bonds of the glass matrix and an increase in rigidity of the structure at a higher content of Bi2O3.The dc and ac conductivity of the studied glasses were affected by the O/P ratio, as they attained high values at O/P > 3.0. The asymmetrical shape of the Nyquist semicircle indicated that the glasses behavior deviated from Debye relaxation. The overall behavior of the electrical properties of the studied glasses revealed their semiconducting performance. The examined glasses could be good choices for photonic applications.

First-Principles Study on Formation Mechanism of Six-Coordinated Si in Silicophosphate Glass.

Six-coordinated Si ([6]Si) structures are readily formed in silicophosphate glasses with high P2O5 contents. Although experiments and simulations have provided some information on the local configurations around [6]Si, further research on the formation mechanism of [6]Si at the atomic scale is needed. To investigate the formation mechanism of [6]Si, we performed dynamic and static analyses based on first-principles calculations. In first-principles molecular dynamics simulations with models in which all Si atoms are four-coordinated as the initial structure, we observed that the coordination number of Si increased, and the P-Q2 (P-Qn, where n represents the number of bridging oxygen atoms) changed to P-Q3. Atomic energy analysis revealed that the energy decreases with the structural change from P-Q2 to P-Q3 exceeded the energy increase with an increase in the coordination number of Si, stabilizing of the entire system. We conclude that the key factor in the formation of [6]Si is the decrease in energy associated with the change from a nonbridging oxygen in a PO4 tetrahedral structure to bridging oxygen.

Microstructure and Bioactivity of Ca- and Mg-Modified Silicon Oxycarbide-Based Amorphous Ceramics.

Silicon oxycarbide (SiOC), Ca- and Mg-modified silicon oxycarbide (SiCaOC and SiMgOC) were synthesized via sol-gel processing with subsequent pyrolysis in an inert gas atmosphere. The physicochemical structures of the materials were characterized by XRD, SEM, FTIR, and 29Si MAS NMR. Biocompatibility and in vitro bioactivity were detected by MTT, cell adhesion assay, and simulated body fluid (SBF) immersion test. Mg and Ca were successfully doped into the network structure of SiOC, and the non-bridging oxygens (NBO) were formed. The hydroxycarbonate apatite (HCA) was formed on the modified SiOC surface after soaking in simulated body fluid (SBF) for 14 days, and the HCA generation rate of SiCaOC was higher than that of SiMgOC. Accompanying the increase of bioactivity, the network connectivity (NC) of the modified SiOC decreased from 6.05 of SiOC to 5.80 of SiCaOC and 5.60 of SiMgOC. However, structural characterization and biological experiments revealed the nonlinear relationship between the biological activity and NC of the modified SiOC materials.

Grand canonical Monte Carlo and deep learning assisted enhanced sampling to characterize the distribution of Mg2+ and influence of the Drude polarizable force field on the stability of folded states of the twister ribozyme.

Molecular dynamics simulations are crucial for understanding the structural and dynamical behavior of biomolecular systems, including the impact of their environment. However, there is a gap between the time scale of these simulations and that of real-world experiments. To address this problem, various enhanced simulation methods have been developed. In addition, there has been a significant advancement of the force fields used for simulations associated with the explicit treatment of electronic polarizability. In this study, we apply oscillating chemical potential grand canonical Monte Carlo and machine learning methods to determine reaction coordinates combined with metadynamics simulations to explore the role of Mg2+ distribution and electronic polarizability in the context of the classical Drude oscillator polarizable force field on the stability of the twister ribozyme. The introduction of electronic polarizability along with the details of the distribution of Mg2+ significantly stabilizes the simulations with respect to sampling the crystallographic conformation. The introduction of electronic polarizability leads to increased stability over that obtained with the additive CHARMM36 FF reported in a previous study, allowing for a distribution of a wider range of ions to stabilize twister. Specific interactions contributing to stabilization are identified, including both those observed in the crystal structures and additional experimentally unobserved interactions. Interactions of Mg2+ with the bases are indicated to make important contributions to stabilization. Notably, the presence of specific interactions between the Mg2+ ions and bases or the non-bridging phosphate oxygens (NBPOs) leads to enhanced dipole moments of all three moieties. Mg2+-NBPO interactions led to enhanced dipoles of the phosphates but, interestingly, not in all the participating ions. The present results further indicate the importance of electronic polarizability in stabilizing RNA in molecular simulations and the complicated nature of the relationship of Mg2+-RNA interactions with the polarization response of the bases and phosphates.

Structural and Optical Properties of Alumino Lead Borate Glasses Containing Copper Oxide

The alumino lead borate glasses with small amounts of copper oxide were synthesized by melting and quenching according to the relation 50B2O3-30PbO-(20–x)Al2O3-xCuO with x = 0, 0.10, 0.25, 0.50, 0.75 and 1.00 mol%. The powder XRDs had no sharp peaks which show that the samples are amorphous. Density of the glasses increased as the content of the CuO increased. FTIR spectroscopic studies reveal the presence of BO3, BO4 –, PbO4, AlO4, pentaborate [B5O8 –], diborate [B4O7 2–] and dipentaborate B5Ø11 2– structural units. The UV-visible absorption studies showed that the refractive index, indirect energy gap, oxide ion polarizability and optical basicity had composition dependence which were related to the glass structure. As the CuO concentration increased, the refractive index decreased, indirect energy gap increased, oxide ion polarizability decreased and optical basicity decreased. Optical band gap increased with increasing CuO content as the band gap for bridging oxygens is higher than that for non-bridging oxygens.

Identifying and Quantifying Borate Environments in Borosilicate Glasses: 11B NMR-Peak Assignments Assisted by Double-Quantum Experiments.

We discuss the prospects for accurate 11B magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectral deconvolutions for reaching beyond the readily extracted borate speciations offered by the integrated resonances of the coexisting B[3] and B[4] species of the respective BO3 and BO4 network groups in borosilicate (BS) glasses. We critically review hitherto proposed 11B[3] and 11B[4] NMR-peak assignments relating to their neighboring Si, B[3] and B[4] species, as quantified by MAS NMR spectral deconvolution. Guidance to these resonance assignments was offered from double-quantum-single-quantum (2Q-1Q) 11B MAS NMR experiments that inform about the B[p]-O-B[q] network linkages. The NMR spectral deconvolutions from two BS glass series with low nonbridging oxygen (NBO) contents and fixed molar ratios nSi/nB = {1.0, 2.0} but variable network-modifying cations of alkali metals and Mg2+ revealed a dominance of B[4]-O-Si linkages, yet with a significant dependence on the BO3 population of the glass, which was rationalized by the different propensities for B[4]-O-{Si, B[3], B[4]} linkage formation. For BS glasses with comparable B and Si contents, we recommend three-peak deconvolutions of the 11B[4] spectral region, whose 11B[4](mSi) sites differ in their (average) numbers of m B[4]-O-Si and 4 - m B[4]-O-B[p] bonds, where B[p] may assume B[3] or B[4]. We also discuss the structural origin of the two rather arbitrarily classified "ring" and "non-ring" B[3] entities, where 2Q-1Q 11B NMR suggests the former to primarily constitute BO3 groups that coexist with BO4 moieties in (superstructural) ring units largely devoid of bonds to Si, whereas the "non-ring" B[3] sites involve linkages to all of B[3], B[4], and Si, with B[3]-O-Si linkages prevailing. The limitations of 11B NMR spectral deconvolutions are discussed, including the remaining challenges in analyzing NBO-rich BS glasses.

Exploration of Probing Structure, Fictive Temperature, and Homogeneity of Commercial Glass and Glass Fiber Vibrational Spectroscopic Techniques

Utilizing a set of spectroscopic techniques, i.e., Fourier transform infrared (FT-IR) in transmission (KBr method), attenuated total reflection (ATR) FT-IR, and micro-Raman, and several characterization methods relevant to the commercial production of E-glass fibers for composite reinforcement has been explored, focusing on four major areas: (i) a possible structure alignment in the fibers formed under high tension and high shear rate, (ii) the network structure difference between the skin and core of fibers, (iii) fictive temperature determination, and (iv) glass homogeneity at a nanoscale. The IR band narrowing observed from fiber samples over bulk samples was identified using the ATR method. The use of FT-IR and micro-Raman techniques enables the identification of higher non-bridging oxygen concentration near fiber skin over the core. A relationship between E-glass fictive temperature and the Raman band shift was established to predict the fictive temperature range of commercial fiberglass products. We present here for the first time a demonstration of a new FT-IR procedure for tracking commercial glass inhomogeneity with sensitivity at the nanoscale. The newly proposed glass homogeneity index (GHI) was shown to be closely related to furnace operating temperature upset and correlated with the average fiber break level during the period.

Influence of Bi2O3 on structural and optical characteristics of PbO⋅B2O3⋅Bi2O3⋅SiO2 glasses

In this study, the influence of Bi2O3 on the structural and optical characteristics of the glass samples with the molar composition of 17PbO⋅23B2O3⋅xBi2O3⋅(60-x)SiO2, prepared using the conventional melt quenching approach, is described. X-ray diffraction spectra reveal the amorphous nature of the prepared series. Density (D), molar volume (Vm), glass transition temperature (Tg), and other physical parameters have been evaluated. FTIR spectra show the presence of bismuth as [BiO3] and [BiO6] structural units and of borate as [BO3] and [BO4] structural units. It is found that the increase in non-bridging oxygens can be attributed to the increase in cut-off wavelength and decrease in bandgap energy with the rise in bismuth concentration. Urbach's energy values demonstrate that the presence of Bi2O3 in the existing glass system might affect the defect concentration. All glass samples possess a high refractive index suggesting that the current glasses may be useful for developing non-linear optical systems.

Simulation for Elucidating a Formation Mechanism of 6-Coordinated Si in Phosphate Glass

Phosphate glasses are materials that are expected to have a wide range of applications, including highly ion-conductive materials in fuel cells and biomaterials that supply therapeutic ions. Diffusion of ions in glasses plays an important role in designing glasses for these applications. The diffusion of protons is one of the most important properties of fuel cells, and the diffusion of network-modified cations such as Na+ and Ca2+ ions is closely related to glass dissolution and hydration reactions. In general, these diffusion properties are strongly dependent on the atomic-scale glass structure. We found that the difference in the morphology of PO4 tetrahedra (P-Qn : number of bridging oxygen) greatly affects the diffusion of protons and Na+ ions in phosphate glass, where these ions can diffuse much more easily near P-Q2 than near P-Q3 , and suggested that ionic conductivity and glass solubility can be designed by controlling the P-Qn distribution of phosphate glasses [1]. We also suggested that the P-Qn distribution can be controlled using 6-coordinated Si (6cSi), which is formed in phosphate glasses containing more than 40 mol% P2O5 [2]. However, the formation mechanism of 6cSi is not clear. In this study, we aim to elucidate the formation mechanism of 6cSi in phosphate glasses based on first-principles molecular dynamics (MD) simulations.The glass models for 55.0P2O5-21.3SiO2-23.7Na2O (mol%) were generated by quenching from the melt using classical MD simulations with the DL POLY [3] program. Subsequently, first-principles MD simulations using the SIESTA code [4] were performed to observe the reaction process from 4cSi to 5/6cSi. Atomic energies were calculated using orbital decomposition of the total energy by the OpenMX code [5]. For these electronic structure calculations, a localized basis with the GGA-PBE functional was used.In obtained models, atomic energies for non-bridging oxygen (NBO) are higher than those for bridging oxygen (BO). In the first-principles MD simulation, the formation of chemical reaction from "4cSi + P-Q2 " to " 5/6cSi + P-Q3 " was observed as shown in Fig. 1. The energy decreases by the change from a NBO in P-Q2 to a BO in P-Q3 is larger than the energy increase by the change for 4cSi to 5c/6cSi.It will be possible to design highly functional glasses by applying the 6cSi formation mechanism and controlling the amount of 6cSi formed.Reference:[1] K. Takada, T. Tamura, H. Maeda, and T. Kasuga, Phys. Chem. Chem. Phys. 23, 14580 (2021). [2] K. Takada, T. Tamura ,and T. Kasuga, RSC Adv. 12, 35043 (2022). [3]T. Todorov et al., J. Chem. 16, 1911 (2006). [4] J. Solar et al., J. Phys. Cond. Matt. 14, 2745 (2000). [5] T. Ozaki, Phys. Rev. B 67, 155108 (2003). Figure 1

Study on the Production Mechanism of Fluorescent Species and Its Application to White LED of NH4-Form Y-Type Zeolite

Introduction Zeolites are aluminosilicates in which all vertices of SiO4 and AlO4 tetrahedra (TO4) are shared to build a three-dimensional framework and are classified on the basis of the framework structure. Assuming that SiO4 4− in neutral SiO2 is partially replaced with AlO4 5−, the zeolite framework becomes negatively charged by the same amount of Al3+. Various cations can be introduced into the framework cages to compensate for this negative charge. The charge-compensating cation is primarily an alkali metal ion such as Na+ or K+. This cation can be easily ion-exchanged for other monovalent and polyvalent cations. Zeolites are ideal host materials because they can easily give fluorescent light by introducing luminescent centers as charge-compensating cations and are composed only of ubiquitous elements. Hayashi et al., reported that NH4-form Y-type zeolite exhibited intense fluorescence upon calcination in air atmosphere without any addition of dopants. In their study, the following fluorescence mechanism was proposed: On the halfway conversion to the H-form by heating, NH4-form zeolite was decomposed to produce oxygen vacancies. Then new excited levels appeared leading to fluorescence. This zeolite phosphor does not contain any main transition-metal nor rare-earth elements and produces bright white emission, so it will become an ecological and economical fluorescent material. In this study, the dependence of the fluorescence properties of NH4-form Y-type zeolites on calcination atmosphere and temperature was investigated and the production mechanism of fluorescent species was elucidated from detailed analyses of the calcination samples. White LEDs were fabricated from the most fluorescent samples. It is demonstrated that they can be applied practical white phosphors without rare-earth nor main transition-metal elements.Experimental The thermal properties of a commercially available NH4-form Y-type zeolite (Sigma-Aldrich, ammonium Y zeolite) underwent thermogravimetric/differential thermal analysis under air, oxygen, nitrogen, and 4% hydrogen/nitrogen mixture atmospheres at a flow rate of 150 mL/min or in vacuo at approximately 27 Pa using a rotary pump. From the TG curves, 130, 230, 280, 350, 460, 500, and 600 °C were selected as the calcination temperatures. The zeolite was calcined for 2 h at these temperatures under the above-mentioned atmospheres in a tubular furnace. The following measurements were performed on the calcined zeolite samples; X-ray diffraction (XRD), fluorescence, ultraviolet-visual (UV-Vis) absorption, electron spin resonance (ESR), Fourier transform infrared (FT-IR) absorption, and 29Si and 27Al magic-angle spin nuclear magnetic resonance (MAS-NMR) by solid state. The phosphors with the most fluorescence intensities were mixed with silicone rubber sealant in 2:1 mass ratio and were applied to a near-ultraviolet (405 nm) LED. The coated LED was then heated at 80 ºC overnight for curing. This process was repeated three times to fabricate the white LED. These LED were driven at 3.0 V, 16 mA by a stabilized power supply and illuminance and emission spectrum were measured.Results and discussion The fluorescence intensity was higher when the zeolite was calcined at 350 °C for 2 h in nitrogen atmosphere than in other atmospheres. In atmospheres containing oxygen, quenching was observed at the higher temperatures. These results indicate that calcination caused oxygen vacancies under every atmosphere which brought fluorescence properties. The spectra of ESR showed two signals corresponding to the E'-center and the non-bridging oxygen hole center (NBOHC) which were generated by the collapse of the SiO4 tetrahedron. In FT-IR spectra, the absorptions assigned to Si–O vibration decreased in the fluorescent samples. Figure shows the production mechanism of the fluorescent species. The oxygen vacancy on the SiO4 tetrahedron created SiO3 and/or SiO2 radicals and brought the zeolites to generate fluorescence. The white LED constructed with the zeolite phosphors. Driving this LED with a regulated power source could obtain illumination with color temperature of 7500 K and the tristimulus values determined xy chromaticity coordinates parameters to x = 0.272 and y = 0.341. It will be a guidelines to be able to produce a practical white phosphor that does not contain rare-earth nor transition-metal elements. Figure 1

(Invited) Improving the Stability and Transport Properties of a High Concentration of Protons in Phosphate Glass at Intermediate Temperatures

Fuel cells that operate at around 300 °C are expected as the next-generation fuel cells that can use liquid organic hydrogen carriers, such as methanol and methylcyclohexane, as fuel directly. Because of the lack of available solid electrolytes working at this temperature range, there is no fuel cell systems available at this temperature. Therefore, new electrolytes are currently being extensively investigated.Since the discovery of proton conductivity in phosphate glasses by Namikawa et al. in 1966, their proton conductivity has been investigated for applications in electrochemical devices such as fuel cells. Because conventional phosphate glasses could not hold proton carriers at temperatures above 250 °C, they were not suitable as an electrolyte for fuel cells operating at 300 °C. The reason why the glass cannot hold proton carriers at elevated temperatures is that the protons in glass exist predominantly as molecular water, which is readily released outside the glass at elevated temperatures. This limitation has recently been overcome by a proton injection method into phosphate glasses termed as alkali-proton substitution (APS), where the sodium ions in glass are electrochemically substituted with protons at high temperature (Fig. 1(a)). [1] A variety of glasses have been discovered that have high proton conductivity using APS (Fig. 1(b)). For examples, 36HO1/2–4NbO5/2–2BaO–4LaO3/2–4GeO2–1BO3/2–49PO5/2 glass exhibits high proton conductivity of 2 mScm−1 at 300 °C. [2] In order to achieve the conductivity higher than 10 mScm-1 required for practical fuel cell applications, it is necessary to further improve the stability and conductivity of proton carriers.Considering the glass formation range and the proton carrier density, the appropriate composition of the glass is around metaphosphate consisting of chain-type phosphate ions, (PO3 -)n. The deprotonation of glass fabricated using APS occurs through a dehydration-condensation reaction; thus, the migration or diffusion of PO4 tetrahedra is involved. Based on this, suppressing the viscous flow of the glass and its melt is an effective way to avoid deprotonation of the glass and its melt. Cross-linking the chain-type phosphate ions with heteroatoms that form covalent bonds with oxygen atoms is one of the ways to suppress viscous flow. Regarding proton transport, it is comprised of two fundamental processes: dissociation of the O-H bond and migration of the mobile proton; therefore, to enhance proton conductivity, controlling the strength of the O-H bond and migration of mobile protons are necessary. The strength of the O-H bond can be controlled by the bonding character of the P-O bond. The migration of mobile protons can be enhanced by enhancing the proton migration between the chain-type phosphate ions. Recently, it has been found that cross-linking between chain-type phosphate ions by heteroatoms that form a coordination polyhedron possessing non-bridging oxygen facilitates proton migration between phosphate ions. Based on these understanding, proton-conducting phosphate glass electrolytes that achieve 10 mScm-1 at 300 °C are being explored.The recent understandings mentioned above will be presented with specific examples.

Theoretical analysis of photosensitization of DNA by thionine.

In this work, we are the first to perform a theoretical analysis of photoinduced charge transfer in the intercalation complex of thionine (TH) with double-stranded DNA, which was observed in experiments. Efficient DNA binding and long-wave absorption maximum make TH an attractive photosensitizer. d(CpG)2 tetranucleotide was used as a minimal model DNA fragment. Intercalation of TH between pairs of nucleobases causes the transfer of a small negative charge (0.24 e) from the tetranucleotide to the dye. S0 → S1 photoexcitation of their complex using visible light leads to the transfer in the same direction of a significant negative charge (0.9 e). This electronic transition has a HOMO → LUMO electronic configuration, with HOMO localized on one of the two phosphate groups of the tetranucleotide, and LUMO on TH; the latter has the same shape as the LUMO of free dye. In the complex, TH, by its amino groups, forms two intermolecular H-bonds: with the deoxyribose oxygen atom of one d(CpG)2 strand and with the non-bridging oxygen atom of the phosphate group of the other strand. In this case, the H-bond TH with the phosphate group is stronger than with the sugar, but the charge transfer is carried out from another phosphate group through the sugar to the dye. Thus, charge transfer occurs along the longer of the two paths. However, the path of charge transfer depends on the parameters of the excitation since higher electronic transitions also include the second phosphate group, i.e., a short way is also used. For the calculations of the excitation of the complex, TD-DFT was used in combination with a set of ten functionals (CAM-B3LYP + D3BJ, ωB97XD, LC-ωHPBE, M052X, M062X, M06HF, M08HX, M11, MN15, and SOGGA11X), which have proven themselves well in modeling the excitation of dimers of aromatic molecules. Of these, LC-ωHPBE, which gave the best agreement with the experiment, was selected for the final calculations. It was used in combination with the 6-31 + + G(d,p) basis set and the IEFPCM solvent model. The photoinduced charge redistribution was quantitatively estimated using natural population analysis, and visually by building the frontier molecular and natural transition orbitals.

Enhancing yellow-green emission in Eu/Tb co-doped tellurite glasses controlled by Lu2O3

In this research, Lu2O3 was incorporated into Eu3+ and Tb3+ co-doped tellurite glasses to enhance structural stability and modify the yellow-green emission. A series of microstructural analyses, including Differential Scanning Calorimetry (DSC), Raman spectroscopy, and density measurements, confirmed that the addition of Lu2O3 causes the long-chain or cyclic Te–O–Te network structure to break, resulting in the formation of more [TeO3], which in turn leads to an increase in non-bridging oxygens (NBO) and lowers the phonon energy of the matrix material. Fluorescence spectral characterization revealed that both green and yellow luminescence intensities peaked when Lu2O3 concentration reached 15 %. Additionally, the Judd-Ofelt theory supports its superior laser performance. With the addition of Lu2O3,The radiative transition probability (Arad), lifetime (τrad), and branching ratio (β) of the excited states of Tb3+ and Eu3+ have all been enhanced. Under the regulation of 15 % Lu2O3, The maximum absorption cross-section of at 544 nm is 1.88 × 10−20 cm2, and the maximum emission cross-section is 2.14 × 10−20 cm2. At 611 nm, the maximum absorption cross-section is 1.05 × 10−20 cm2, and the maximum emission cross-section is 1.44 × 10−20 cm2. The decay curve indicates a fluorescence lifetime enhancement from 0.6 ms to 1 ms with Lu2O3. These results underscore the potential of Lu2O3 modulated Eu3+/Tb3+ co-doped tellurium glass as an effective yellow-green laser gain medium.

Ab Initio Molecular Dynamics Study of Trivalent Rare Earth Rich Borate Glasses: Structural Insights and Formation Mechanisms.

In this work, trivalent rare earth (RE)-rich borate glasses (30 La2O3-70 B2O3, 50 La2O3-50 B2O3, 60 La2O3-40 B2O3, and 50 Y2O3-50 B2O3) were modeled using ab initio molecular dynamics (AIMD) simulations through the melt-quenching route. It was found that the AIMD-derived structures reproduced the experimental structure factors and 11B solid-state nuclear magnetic resonance data. Isolated borate units (monomers, dimers, and trimers) terminated with nonbridging oxygen were found in the structures. Polymer units containing four or more boron atoms were identified with and without three-membered boron rings (3-rings). Increasing the proportion of La2O3 in La2O3-B2O3 glasses resulted in an increased number of isolated units, indicating that La3+ acts as a network modifier, breaking the borate glass network. The formation of these units via the melt-quenching process was detected by labeling boron species at each AIMD step from 1500 to 300 K. Representation with transition matrices clarified the specific reaction routes, leading to the formation of isolated boron units in solid glass. A key finding is the stabilization of polymer units involving 3-ring formation. The formation of isolated units is achieved through the reaction of polymers without 3-rings. The RE coordination structure was thoroughly analyzed from the perspective of shape and symmetry. Reference structures derived from the solution of the Thomson problem were compared to the AIMD-derived coordination structures and crystalline LaBO3 and YBO3. The results highlight the specificity of the Y coordination structure with 3-rings in YBO3, which is not observed in RE borate glasses. The analytical approaches and interpretations used in this study provide insights into the diverse coordination structures of glasses containing heavy elements other than REs.