New functionalities in abundant element oxides: ubiquitous element strategy

Recent Progress in Emerging Novel MXenes Based Materials and their Fascinating Sensing Applications

Early transition metals based 2D carbides, nitrides and carbonitrides nanomaterials are known as MXenes, a novel and extensive new class of 2D materials family. Since the first accidently synthesis based discovery of Ti₃ C₂ in 2011, more than 50 additional compositions have been experimentally reported, including at least eight distinct synthesis methods and also more than 100 stoichiometries are theoretically studied. Due to its distinctive surface chemistry, graphene like shape, metallic conductivity, high hydrophilicity, outstanding mechanical and thermal properties, redox capacity and affordable with mass-produced nature, this diverse MXenes are of tremendous scientific and technological significance. In this review, first we'll come across the MXene based nanomaterials possible synthesis methods, their advantages, limitations and future suggestions, new chemistry related to their selected properties and potential sensing applications, which will help us to explain why this family is growing very fast as compared to other 2D families. Secondly, problems that help to further improve commercialization of the MXene nanomaterials based sensors are examined, and many advances in the commercializing of the MXene nanomaterials based sensors are proposed. At the end, we'll go through the current challenges, limitations and future suggestions.

Resistance Switching in Polycrystalline C12A7 Electride

The memory (memristive) properties of an electride material based on polycrystalline mayenite (C12A7:e-) were studied. The phase composition of the material has been confirmed by such methods as XRD, TEM, Raman, and infrared spectroscopy. The electride state was confirmed by conductivity measurements and EPR using a characteristic signal from F+-like centers, but the peak at 186 cm-1, corresponding to an electride with free electrons, was not observed explicitly in the Raman spectra. The temperature dependence of current-voltage characteristics in states with low and high resistance (LRS and HRS) has been studied. In the LRS state, the temperature dependence of the current has a non-Arrhenius character and is described by the Hurd quantum tunnelling model with a Berthelot temperature of 262 K, while in the HRS state, it can be described in terms of the Arrhenius model. In the latter case, the existence of two conduction regions, "impurity" and "intrinsic", with corresponding activation energies of 25.5 and 40.6 meV, was assumed. The difference in conduction mechanisms is most likely associated with a change in the concentration of free electrons.

Molecular Electrides: An In Silico Perspective

Electrides are defined as the ionic compounds where the electron(s) serves as an anion. These electron(s) is (are) not bound to any atoms, bonds, or molecules rather than they are localized into the space, crystal voids, or interlayer between two molecular slabs. There are three major categories of electrides, known as organic electriades, inorganic electrides, and molecular electrides. The computational techniques have proven as a great tool to provide emphasis on the electride materials. In this review, we have focused on the computational methodologies and criteria that help to characterize molecular electrides. A detailed account of the computational methods and basis sets applicable for molecular electrides have been discussed along with their limitation(s) in this field. The main criterion for the identification of the electrides has also been discussed thoroughly with proper examples. The molecular electrides presented here have been justified with all the required criteria that support and proved their electride characteristics. We have also presented a few systems which have similar properties but are not considered as molecular electrides. Moreover, the applicability of the electrides in catalytic processes has also been presented.

Sc2C, a 2D Semiconducting Electride

Electrides are exotic materials that typically have electrons present in well-defined lattice sites rather than within atoms. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc₂C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc₂C and Al₂C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc₂C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.

Electrophysical Properties of Polycrystalline C12A7:e− Electride

This article demonstrates the possibility of creating memory devices based on polycrystalline mayenite. In the course of the study, structural characterization (XRD, TEM) of ceramic samples of mayenite was carried out, as well as a study of the spectral (THz range) and electrophysical characteristics. Materials obtained by calcination at high (1360–1450 °C) temperatures in an inert argon atmosphere differ in the degree of substitution of oxygen anions О2− for electrons, as indicated by the data on the unit cell parameters and dielectric constant coefficients in the range of 0.2–1.3 THz, as well as differences in the conducting properties of the samples under study by more than five orders of magnitude, from the state of the dielectric for C12A7:O2− to the conducting (metal-like) material in the state of the C12A7:e− electride. Measurements of the current–voltage characteristics of ceramic C12A7:e− showed the presence of memristive states previously detected by other authors only in the case of single crystals. The study of the stability of switching between states in terms of resistance showed that the values of currents for states with high and low resistance remain constant up to 180 switching cycles, which is two times higher than the known literature data on the stability of similar prototypes of devices. It is shown that such samples can operate in a switch mode with nonlinear resistance in the range of applied voltages from –1.3 to +1.3 V.

Mayenite Synthesis from Hydroxide Precursors: Structure Formation and Active Sites on Its Surface

We studied the formation process of a mayenite structure from hydroxide precursors in different gas media. According to X-ray diffraction data, this method allows a well-crystallized mayenite (Ca₁₂Al₁₄O₃₃ or C12A7) phase to be obtained at low (500–900 °C) temperatures with an insignificant impurity of CaO. It was shown that the lattice parameters for C12A7 obtained in an inert atmosphere (Ar) were lower when compared with similar samples in the air. These results can be explained by the different levels of oxygen nonstoichiometry in the resulting phase. We noted that sintering and crystallization of mayenite proceeds at lower temperatures in Ar than in the air medium. We found the presence of donor and acceptor active sites on the surface of mayenite, which was detected by the spin probe method. The specific (per unit surface) concentration of such sites (2.5 × 10¹⁶ m⁻² and 1.5 × 10¹⁵ m⁻² for donor and acceptor sites, respectively) is comparable to that of γ-Al₂O₃, which is traditionally used as catalyst support. This allows it to be used in adsorption and catalytic technologies, taking into account its high specific surface area (~30–50 m²/g at a low synthesis temperature).

Superatomic Chelates: The Cases of Metal Aza-Crown Ethers and Cryptands

Li, Na, and Mg⁺-coordinated hexaaza-18-crown-6 ([18]aneN₆) and 1,4,7-triazacyclononane ([9]aneN₃), Li[1.1.1]cryptand, and Na[2.2.2]cryptand species possess a diffuse electron in a quasispherical s-type orbital. They populate expanded p-, d-, f-, and g-shape orbitals in low-lying excited states and hence are identified as "superatoms". By means of quantum calculations, their superatomic shell models are revealed. The observed orbital series of M([9]aneN₃)₂ and M[18]aneN₆ (M = Li, Na, Mg⁺) are identical to the 1s, 1p, 1d, 1f, 2s, and 2p. The electronic spectra of Li[1.1.1]cryptand and Na[2.2.2]cryptand were analyzed up to the 1f¹ configuration, and their transitions were found to occur at lower energies compared to their aza-crown ethers. The introduced superatomic shell models in this work closely resemble the Aufbau principle of "solvated electrons precursors". All reported alkali metal complexes bear lower ionization potentials than any atom in the periodic table; thus, they can also be recognized as "superalkalis".

Regulating Work Function of [Ca24Al28O64]4+:4e- Electrides Via Changing Solvated Electron Characters

Since electrides behave as electron donors in catalytic reactions and device substrates due to the anionic electrons localized in periodic interstitial spaces, their work functions (Φ) could be used as key indicators in describing the high electron-donating power. Taking the [Ca₂₄Al₂₈O₆₄]⁴⁺:4e^- electride as an example, we here propose a new computational scheme of Φ and, further, characterize Φ of the bulk derivative structures of the electride for clarifying the relationship between structural characteristics and Φ. Results indicate that the external strain hardly affects Φ, but the interior heteroatom-doping and distortion bring about significant changes. All these unique variations of the bulk Φ are governed by the distribution and solvation character of anionic electrons in the cage conduction band states. The mechanism of regulating Φ revealed in this work may play a role in the rational design of electride-based catalysts and devices with a superior performance.

Recent Advances and Applications of Inorganic Electrides

Electrides, accommodating excess electrons in lattice voids as anions, have attracted considerable attention in both fundamental research and application development because of their interesting properties, such as ultralow work functions, high electronic mobility, high catalytic activity, and anisotropic electronic and optical properties. Recently, much research progress has been made in both types and applications of inorganic electrides because of the high stability. In this Perspective, we aim to summarize the recent development of inorganic electrides discovered and proposed by experiment and theoretical calculations, highlighting the main applications, including catalysis, metal-ion batteries, superconductivity, magnetism, and organic light-emitting diodes. We provide insights into the role of anionic electrons in electrides playing in the stability and properties. Finally, the problems, challenges, and opportunities are presented, which provide an outlook for future research.

Novel Mg-ion conductive oxide of μ-cordierite Mg0.6Al1.2Si1.8O6

Solid electrolytes with high Mg-ion conductivity are required to develop solid-state Mg-ion batteries. The migration energies of the Mg2+ ions of 5,576 Mg compounds tabulated from the inorganic crystal structure database (ICSD) were evaluated via high-throughput calculations. Among the computational results, we focused on the Mg2+ ion diffusion in Mg0.6Al1.2 Si1.8O6, as this material showed a relatively low migration energy for Mg2+ and was composed solely of ubiquitous elements. Furthermore, first-principles molecular dynamics calculations confirmed a single-phase Mg2+ ion conductor. The bulk material with a single Mg0.6Al1.2Si1.8O6 phase was successfully prepared using the sol-gel method. The relative density of the sample was 81%. AC impedance measurements indicated an electrical conductivity of 1.6 × 10-6 Scm-1 at 500°C. The activation energy was 1.32 eV, which is comparable to that of monoclinic-type Mg0.5Zr2(PO4)3.

Phase pure α-Mn2O3 prisms and their bifunctional electrocatalytic activity in oxygen evolution and reduction reactions

Synthesizing manganese oxide materials with exact control of the nanoparticle shape and phase is difficult, making it challenging to understand the influence of the surface structure on electrocatalysis. Here we describe an inexpensive, low-temperature method to synthesize single-crystal orthorhombic phase α-Mn₂O₃ prisms bound by the {100} facets. The synthesis is the first method to use the cation bridging effect to assist in the creation of α-Mn₂O₃ prisms. According to structural analysis using X-ray diffraction, X-ray pair-distribution function (PDF) measurements and high resolution transmission electron microscopy, the material is composed exclusively of α-Mn₂O₃ prisms, and no additional amorphous or nanocrystalline phases are present. Heating the prisms transformed the material to a more symmetrical, cubic phase α-Mn₂O₃ that exhibited strong bifunctional electrocatalytic activity for the oxygen evolution (OER) and oxygen reduction (ORR) reactions. We compared the oxygen electrode activities (OEA) and found that the α-Mn₂O₃ prisms performed 79% better than commercially-produced α-Mn₂O₃ powders, indicating that these α-Mn₂O₃ prisms perform well as inexpensive, earth-abundant materials for reversible electrodes.

Visible-Light-Sensitive Photocatalysts: Nanocluster-Grafted Titanium Dioxide for Indoor Environmental Remediation

Photocatalytic degradation of organic compounds requires photoexcited holes with strong oxidative power in the valence band (VB) of semiconductors. Although numerous types of doped semiconductors, such as nitrogen-doped TiO2, have been studied as visible-light-sensitive photocatalysts, the quantum yields of these materials were very low because of the limited oxidation power of holes in the nitrogen level above the VB. Recently, we developed visible-light-sensitive Cu(II) and Fe(III) nanocluster-grafted TiO2 using a facile impregnation method and demonstrated that visible-light absorption occurs at the interface between the nanoclusters and TiO2, as electrons in the VB of TiO2 are excited to the nanoclusters under visible-light irradiation. In addition, photogenerated holes in the VB of TiO2 efficiently oxidize organic contaminants, and the excited electrons that accumulate in nanoclusters facilitate the multielectron reduction of oxygen. Notably, Cu(II) and Fe(III) nanocluster-grafted TiO2 photocatalyst has the highest quantum yield among reported photocatalysts and has antiviral, self-cleaning, and air purification properties under illumination by indoor light fixtures equipped with white fluorescent bulbs or white light-emitting diodes.

A metal sulfide photocatalyst composed of ubiquitous elements for solar hydrogen production

A visible-light-sensitive tin sulfide photocatalyst was designed based on a ubiquitous element strategy and density functional theory (DFT) calculations.

Novel anion conductors--conductivity, thermodynamic stability and hydration of anion-substituted mayenite-type cage compounds C12A7:X (X = O, OH, Cl, F, CN, S, N)

Mayenite (Ca12Al14O33) is a highly interesting functional material not only in view of its unique crystal structure as a cage compound but also for its variety of possible applications. Its ability to incorporate foreign ions into the cage structure opens the possibility to create new types of solid electrolytes and even electrides. Therefore, the conductivity of various anion substituted mayenites was measured as a function of temperature. Due to controversial reports on the stability of mayenite under specific thermodynamic conditions (dry, wet, reducing, and high temperature), a comprehensive study on the stability was performed. Mayenite is clearly not stable under dry conditions (ppm H2O < 100) at temperatures above 1050 °C, and thus, the mayenite phase vanishes from the calcium aluminate phase diagram below a minimum humidity. Two decomposition reactions were observed and are described in detail. To get further insight into the mechanism of hydration of mayenite, the conductivity was measured as a function of water vapour pressure in a range of -5 ≤ lg[pH2O/bar] ≤ -1.6 at temperatures ranging from 1000 °C ≤ θ ≤ 1200 °C. The hydration isotherms are described with high accuracy by the underlying point defect model, which is confirmed in a wide range of water vapour pressure.

Ubiquitous element approach to plasmonic enhanced photocatalytic water splitting: the case of Ti@TiO2 core-shell nanostructure

We demonstrate a new approach to plasmonic enhanced photocatalytic water splitting by developing a novel core-shell Ti@TiO2 brush nanostructure where an elongated Ti nanorod forms a plasmonic core that concentrates light inside of a nanotubular anodic TiO2 shell. Following the ubiquitous element approach aimed at providing an enhanced device functionality without the usage of noble or rare earth elements, we utilized only inexpensive Ti to create a complex Ti@TiO2 nanostructure with an enhanced UV and Vis photocatalytic activity that emerges from the interplay between the surface plasmon resonance in the Ti core, Vis light absorption in the Ti-rich oxide layer at the Ti/TiO2 interface and UV light absorption in the nanotubular TiO2 shell.

Enhanced photoactivity with nanocluster-grafted titanium dioxide photocatalysts

Titanium dioxide (TiO2), as an excellent photocatalyst, has been intensively investigated and widely used in environmental purification. However, the wide band gap of TiO2 and rapid recombination of photogenerated charge carriers significantly limit its overall photocatalytic efficiency. Here, efficient visible-light-active photocatalysts were developed on the basis of TiO2 modified with two ubiquitous nanoclusters. In this photocatalytic system, amorphous Ti(IV) oxide nanoclusters were demonstrated to act as hole-trapping centers on the surface of TiO2 to efficiently oxidize organic contaminants, while amorphous Fe(III) or Cu(II) oxide nanoclusters mediate the reduction of oxygen molecules. Ti(IV) and Fe(III) nanoclusters-modified TiO2 exhibited the highest quantum efficiency (QE = 92.2%) and reaction rate (0.69 μmol/h) for 2-propanol decomposition among previously reported photocatalysts, even under visible-light irradiation (420-530 nm). The desirable properties of efficient photocatalytic performance with high stability under visible light with safe and ubiquitous elements composition enable these catalysts feasible for large-scale practical applications.

Solid-state source of atomic oxygen for low-temperature oxidation processes: application to pulsed laser deposition of TiO2:N films

An atomic oxygen (AO) source has been redesigned to coordinate with a pulsed laser deposition system and used to grow nitrogen-doped TiO(2) films by deposition of TiN and simultaneous irradiation of the substrate with AO. The AO source uses an incandescently heated thin tube of zirconia as an oxygen permeation media to generate pure AO of low kinetic energy. The emission flux is calibrated using a silver-coated quartz crystal microbalance. The thin shape of the probe and transverse emission geometry of this emission device allow the emission area to be positioned close to the substrate surface, enhancing the irradiation flux at the substrate. AO irradiation is crucial for formation of TiO(2) phases via oxidation of the deposited TiN laser plume, and is effective for decrease of the substrate temperature for crystallization of anatase phase to as low as around 200 °C.