Non-stoichiometric Research Articles

Оптические свойства тонких пленок SiO-=SUB=-x-=/SUB=- (x<2), полученных обработкой термического диоксида кремния в водородной плазме

Using the methods of ellipsometry, quantum chemical modeling, and photoluminescent spectroscopy, the optical properties and composition of thin films of thermal silicon oxide processed in a hydrogen electron-cyclotron resonance plasma are studied. It has been established that treatment of films in plasma leads to their depletion in oxygen and the formation of non-stoichiometric oxide SiOx <2. By comparing the experimental spectral dependence of the refractive index with the theoretically calculated from the first principles, the values ​​of the parameter x in the obtained SiOx films are determined. It was shown that an increase in the processing time of thermal SiO2 in hydrogen plasma leads to an increase in the refractive index of the film, as well as the degree of depletion of the film with oxygen. For the studied films, the dependence of the value of the parameter x on the treatment time in a hydrogen plasma is constructed.

Thermodynamic assessment of hydrogen production via solar thermochemical cycle based on MoO2/Mo by methane reduction

Inspired by the promising hydrogen production in the solar thermochemical (STC) cycle based on non-stoichiometric oxides and the operation temperature decreasing effect of methane reduction, a high-fuel-selectivity and CH4-introduced solar thermochemical cycle based on MoO2/Mo is studied. By performing HSC simulations, the energy upgradation and energy conversion potential under isothermal and non-isothermal operating conditions are compared. In the reduction step, MoO2:CH4 = 2 and 1020 K < Tred < 1600 K are found to be most favorable for syngas selectivity and methane conversion. Compared to the STC cycle without CH4, the introduction of methane yields a much higher hydrogen production, especially at the lower temperature range and atmospheric pressure. In the oxidation step, a moderately excessive water is beneficial for energy conversion whether in isothermal or non-isothermal operations, especially at H2O: Mo = 4. In the whole STC cycle, the maximum non-isothermal and isothermal efficiency can reach 0.417 and 0.391 respectively. In addition, the predicted efficiency of the second cycle is also as high as 0.454 at Tred = 1200 K and Toxi = 400 K, indicating that MoO2 could be a new and potential candidate for obtaining solar fuel by methane reduction.

Assembly of nonstoichiometric molybdenum oxide on Si as p-n junction photocathode for enhanced hydrogen evolution

We propose the utilization of a compositionally tuned MoOx catalyst on Si photocathode for the enhanced hydrogen production and improved charge separation. The efficient MoOx catalyst stoichiometry on Si photocathode was optimized by controlling the deposition gas environment and the target during RF magnetron sputtering. The optimized MoOx stoichiometry on Si photocathode exhibited one of the highest photocurrent density of ∼-30 mA cm−2 at -0.5 V vs RHE under AM 1.5 G illumination. Importantly, the deposition of MoOx on p-Si photocathode causes a significant cathodic shift in the overpotential, indicating the catalytic effect of MoOx. The valence band edge and electrochemical studies were used to analyze the band edge positions with respect to water redox potentials and the formation of p-n junction system. The results of this work demonstrate the advantage of employing a sulfur-free and oxygen-deficient MoOx catalyst on p-Si in p-n junction configuration for the efficient PEC water splitting process.

T-x-диаграмма системы Ga – Se в диапазоне составов 48.0 – 61.5 мол. % Se по данным термического анализа

Целью настоящей работы было термографическое исследование T-x диаграммы системы Ga – Se в диапазоне температур от 500 до 1100 °С и в диапазоне составов от 48.0 до 61.5 mol % Se. Методом исследования являлся дифференциальный термический анализ c компьютерной регистрацией данных. Получены свидетельства о наличии ретроградного солидуса фазы g-GaSe со стороны селена (с областью гомогенности в несколько десятых mol % при температурах выше эвтектической) и о независимом существовании близких по составу фаз e-GaSe и g-GaSe. При этом более богатая галлием фаза e-GaSe испытывает перитектический распад с образованием расплава (L2) и g-GaSe. Для темпера-туры предполагаемой перитектической реакции получено значение 921 ±2 °С. Вместе стем, на данном этапе работ не получено никаких данных в пользу существования ожидавшейся (по аналогии с системой Ga – S) высокотемпературной модификации, близкой по составу к сесквиселениду галлия (Ga2S3). Другие результаты, полученные в настоящей работе (характер и температуры плавления промежуточных фаз, температуры эвтектического и монотектического превращений, а также координата эвтектического состава), хорошо согласуются с литературными данными по исследованной системе&#x0D; &#x0D; &#x0D; &#x0D; &#x0D; ЛИТЕРАТУРА1. Kainzbauer P., Richter K. W., Ipser H. The binary Bi-Rh phase diagram: stable and metastable phases //J. Phase Equilibria and Diffusion, 2018, v. 39(1), pp. 17– 34. DOI: https://doi.org/10.1007/s11669-017-0600-52. Dolyniuk J.-A., Kaseman D. C., Sen S., Zhao J., Osterloh F. E., Kovnir K. mP-BaP3: A new phase froman old binary system // Chem. Eur. J., 2014, v. 20, pp. 10829–10837, DOI: https://doi.org/10.1002/chem.2013050783. Березин С. С., Завражнов А. Ю., Наумов А. В., Некрылов И. Н., Брежнев Н. Ю. Фазовая диаграммасистемы Ga–S в области 48.0–60.7 мол. % S // Конденсированные среды и межфазные границы, 2017,т. 19(3), с. 321–335. DOI: https://doi.org/10.17308/kcmf.2017.19/2084. Волков В. В., Сидей В. И., Наумов А. В., Некрылов И. Н., Брежнев Н. Ю., Малыгина Е. Н., Завражнов А. Ю. Высокотемпературная кубическая модификация сульфида галлия (Xs = 59 мол. %) и Т, х-диаграмма системы Ga – S // Конденсированные среды и межфазные границы, 2019, т. 21(1), с. 37–50.DOI: https://doi.org/10.17308/kcmf.2019.21/7155. Zavrazhnov A., Berezin S., Kosyakov A., Naumov A., Berezina M., Brezhnev N. J. The phase diagramof the Ga–S system in the concentration range of 48.0–60.7 mol % S // Thermal Analysis and Calorimetry,2018, v. 134(1), pp. 483–492. DOI: https://doi.org/10.1007/s10973-018-7124-z6. Okamoto H. Ga–Se (Gallium-Selenium) // J. Phase Equilibria and Diffusion, 2009, v. 30, p. 658. DOI:https://doi.org/10.1007/s11669-009-9601-37. Dieleman J., Sanders F. H. M. Phase diagram of the Ga-Se system // Phillips J. Res., 1982, v. 37(4),pp. 204 – 229.8. Zavrazhnov A. Yu. Turchen D. N., Goncharov Eu. G., Zlomanov V. P. Manometric method for thestudy of P-T-x diagrams // J. Phase Equilibria and Diffusion, 2001, v. 22(4), pp. 482–490. DOI: https://doi.org/10.1361/1054971017703330639. Shtanov V. I, Komov A. A, Tamm M. E., Atrashenko D. V., Zlomanov V. P. Gallium-selenium systemphase diagram and photoluminescence spectra of GaSe crystals // Doklady Akademii nauk SSSR, 1998, v. 361(3),pp. 357–361. (in Russ.)10. Glazov V. M., Pavlova L. M. Semiconductor and metal binary systems. Phase equilibria and chemicalthermodynamics. Springer, 1989, 327 p. DOI: https://doi.org/10.1007/978-1-4684-1680-011. Ider M. Pankajavalli R., Zhuang W. Thermochemistry of the Ga–Se System. J. Solid State Scienceand Techn., 2015, v. 4(5), Q51–Q60 DOI: https://doi.org/10.1149/2.0011507jss12. Zavrazhnov A., Naumov A., Sidey V., Pervov V. Composition control of low-volatile solids throughchemical vapor transport reactions. III. The example of gallium monoselenide: Control of the polytypicstructure, non-stoichiometry and properties // Thermochimica Acta, 2012, v. 527, pp. 118–124. DOI:https://doi.org/10.1016/j.tca.2011.10.012

Crystallization Behavior of Amorphous Si Nanoinclusions Embedded in Silicon Oxide Matrix

Herein, crystallization behavior of amorphous Si (a‐Si) nanoparticles embedded in Si oxide matrix is studied thermodynamically. The change of the Gibbs free energy of the Si nanoinclusion/Si oxide matrix system upon crystallization of the Si nanoinclusion core is calculated as a function of normalized core radius. Minimum values of the Gibbs free energy determine equilibrium crystallization states of Si nanoinclusions. The equilibrium radii of the crystallized Si parts normalized with respect to the Si particle radii, as well as the barriers for crystallization, are calculated as functions of Si size and annealing temperature. The degree of Si crystallization is shown to increase with Si nanoparticle size. The existence of minimum critical radius of Si nanoinclusions for the onset of crystallization is demonstrated. Obtained results have a good correlation with available experimental data and contribute to the understanding of the formation of crystalline Si structures in the nanosized Si/Si oxide matrix composites formed by phase separation of nonstoichiometric silicon oxide films during high‐temperature annealing.

Effect of Oxygen Interstitial Ordering on Multiple Order Parameters in Rare Earth Ferrite.

Oxygen interstitials and vacancies play a key role in modulating the microstructure and properties of nonstoichiometric oxide systems, such as those used for superconductors and multiferroics. Key to understanding the tuning mechanisms resulting from oxygen doping is a knowledge of the precise positions of these lattice defects, and of the interaction both between these defects and with many order parameters. Here, we report how such information can, for the first time, be obtained from a sample of LuFe_{2}O_{4.22} using a range of techniques including advanced electron microscopy, atomic-resolution spectroscopy, and density functional theory calculations. The results provide quantitative atomic details of the crystal unit cell, together with a description of the ferroelastic, ferroelectric, and ferromagnetic order parameters. We elucidate also the interaction between these order parameters and the positions of the oxygen interstitials in the oxygen-enriched sample. The comprehensive analysis of oxygen interstitial ordering provides insights into understanding the coupling among different degrees of freedom in rare earth ferrites and demonstrates that oxygen content regulation is a powerful tool for tuning the microstructure and properties for this class of quantum material.

The 1G4 - 3H6 electron transition process of Tm3+ promoted by nonmetallic plasmon

The coupling of rare-earth ions doped upconversion nanoparticles (UCNPs) with localized surface plasmon resonance (LSPR) of noble metal has attracted more interest because of improving luminescent efficiency of UCNPs. The nonstoichiometric semiconductor tungsten oxides can provide a possibility for the achievement of effective upconversion luminescence enhancement owning to concentrating and transferring the NIR-plasmonic energy in upconversion system. In this work, Yb3+/Tm3+-doped NaYF4 nanoparticles (NaYF4:Yb-Tm NPs)/ W18O49 nanowires (NWs) composite film which can tremendously improve the upconversion luminescence of Tm3+ as high as 140-fold has been fabricated. Investigations show that the LSPR-induced plasmonic energy transfer process produced by W18O49 NWs is the primary reason for enhancing the upconversion luminescence of NaYF4:Yb-Tm NPs, originating from much more intense excitation electric-field to generate more electrons on the excited state energy-levels of Tm3+ ions. It is anticipated that the development of non-metallic plasmon-sensitized upconversion luminescence materials may provide a platform for widely exploring application in biomedical imaging, therapeutics, and energy conversion.

Review on Recent Progress in the Development of Tungsten Oxide Based Electrodes for Electrochemical Energy Storage.

Current progress in the advancement of energy-storage devices is the most important factor that will allow the scientific community to develop resources to meet the global energy demands of the 21st century. Nanostructured materials can be used as effective electrodes for energy-storage devices because they offer various promising features, including high surface-to-volume ratios, exceptional charge-transport features, and good physicochemical properties. Until now, the successful research frontrunners have focused on the preparation of positive electrode materials for energy-storage applications; nevertheless, the electrochemical performance of negative electrodes is less frequently reported. This review mainly focuses on the current progress in the development of tungsten oxide-based electrodes for energy-storage applications, primarily supercapacitors (SCs) and batteries. Tungsten is found in various stoichiometric and nonstoichiometric oxides. Among the different tungsten oxide materials, tungsten trioxide (WO3 ) has been intensively investigated as an electrode material for different applications because of its excellent charge-transport features, unique physicochemical properties, and good resistance to corrosion. Various WO3 composites, such as WO3 /carbon, WO3 /polymers, WO3 /metal oxides, and tungsten-based binary metal oxides, have been used for application in SCs and batteries. However, pristine WO3 suffers from a relatively low specific surface area and low energy density. Therefore, it is crucial to thoroughly summarize recent progress in utilizing WO3 -based materials from various perspectives to enhance their performance. Herein, the potential- and pH-dependent behavior of tungsten in aqueous media is discussed. Recent progress in the advancement of nanostructured WO3 and tungsten oxide-based composites, along with related charge-storage mechanisms and their electrochemical performances in SCs and batteries, is systematically summarized. Finally, remarks are made on future research challenges and the prospect of using tungsten oxide-based materials to further upgrade energy-storage devices.

Magnetic Properties of Nonstoichiometric and Quasi-Binary CeFe2-Based Compounds Doped with Mn

New nonstoichiometric compounds CeFe2Mnx with a structure of the cubic Laves phase have been synthesized. The concentration range of Mn solubility has been determined. The structure and elemental composition were studied by means of X-ray powder diffraction and energy-dispersive X-ray spectroscopy. Temperature dependences of magnetization were measured. To determine Curie temperatures, magnetic susceptibility measurements were performed. The results obtained for the samples of different compositions of nonstoichiometric and quasi-binary compounds are analyzed taking into account specific features of electronic structure of Ce atoms.

Embracing the Complexity of Catalytic Structures: A Viewpoint on the Synthesis of Nonstoichiometric Mixed Metal Oxides for Catalysis

Embracing the Complexity of Catalytic Structures: A Viewpoint on the Synthesis of Nonstoichiometric Mixed Metal Oxides for Catalysis

Nonstoichiometric tungsten oxide: structure, synthesis, and applications

Stoichiometric tungsten oxide (WO3) and tungsten bronzes have received much attention due to the energy-related functional performances. In addition, as a transition metal oxide, substoichiometric tungsten oxides have manifested their unique properties and exceptional potential to lots of applications; however, no other studies have concentrated on the oxygen-deficient tungsten oxides till now. Herein, this article presents the origin of structure and synthesis of W18O49 nanomaterials chiefly, and then the general introduction of different applications, including electrochemistry, photocatalysis, near-infrared shielding and phototherapy and sensing.

Four decades of electrochemical investigation of Prussian blue

Prussian blue and its analogues are well known nowadays as promising substances for energy storage, capable of electrochemical insertion of Li+, Na+, Ca2+, and other ions. A huge amount of experimental data obtained recently is in evident contradiction with the postulates declared in the early investigations on Prussian blue electrochemistry. Nevertheless, some of these old postulates are widely in use up to now. On the basis of the data on the chemistry and the composition of the Prussian blue not previously involved in the discussion, this article examines the possible participation of hydrated ions in the redox process, the composition of the electrodeposited films of this compound, and some other important problems. Analytical data on Prussian blue, obtained in the nineteenth century and underestimated by twentieth century chemists, suggests that it is a non-stoichiometric compound. Equations describing electrochemical processes of this substance have been proposed.

Molybdenum incorporated Cu1.69ZnSnS4 kesterite photovoltaic devices with bilayer microstructure and tunable optical-electronic properties

Molybdenum (Mo) incorporated Cu1.69ZnSnS4 (CZTS) absorber has been deposited onto Mo-coated soda lime glass (SLG) by co-sputtering of Mo and non-stoichiometric quaternary compound targets. After sulfurization at 600 °C, Mo incorporation into CZTS was confirmed by X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS). From the observed shifts for the (1 1 2) and (2 2 0) peaks, both lattice parameters a and c of the CZTS unit cell were found to decrease with increasing Mo incorporation suggesting cationic substitution by Mo. The Mo incorporated CZTS has a bilayer microstructure in which the lower sub-layer adjacent to the substrate has a smaller grain size and higher porosity than the upper sub-layer. The lower sub-layer is also richer in Mo and has a graded Mo profile. Sheet resistance measurements on Mo incorporated CZTS films deposited on SLG and on quartz show resistivity that decreases with the amount of Mo in CZTS and Mo acts as an acceptor dopant. The energy band gap of CZTS on SLG increases from 1.38 eV to about 1.68 eV as a result of Mo incorporation and the absorbance of Mo incorporated CZTS is increased for wavelengths shorter than 600 nm. When Mo is co-deposited at the optimized DC sputtering power of 10 W, Mo incorporated CZTS/CdS solar cells attain a maximum power conversion efficiency (PCE) of 5.49% versus 1.63% for the reference device under 1 Sun AM 1.5 illumination. Device efficiency enhancement is due to back surface field, increased carrier concentration and reduced band tailing.

Defect-induced ferromagnetism in W18O49 nanowires

In this paper, strong room temperature ferromagnetism was observed in non-stoichiometric tungsten oxide nanostructure, sea urchin-like single crystalline W18O49 nanowires prepared by a solvothermal approach. The prepared W18O49 nanowires exhibited a 39-fold enhancement in saturation magnetic moment as compared to the transformed WO3 by annealing in air. By combining x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and electron-spin resonance (EPR) spectroscopy, a defect complex, i.e., W5+ ions accompanied by oxygen vacancies, has been identified in the prepared W18O49. The observed ferromagnetism in W18O49 is qualitatively explained through a model involving bound magnetic polarons which were induced by the coupling between the electron trapped by oxygen vacancy and the unpaired 5d electron of W5+ ion. The results provide us better insight into the defect engineering of room-temperature ferromagnetism in tungsten oxide.

Optical Properties of Nonstoichiometric Silicon Oxide SiOx (x &lt; 2)

The optical properties of amorphous nonstoichiometric silicon oxide (SiOx) films of variable composition (x = 0.62–1.92) formed by plasma-enhanced chemical vapor deposition are studied in the spectral range of 1.12–4.96 eV. Spectral ellipsometry showed that the refractive index dispersion character allows one to assign the formed SiOx films to silicon-like films, dielectrics, or intermediate-conductivity-type films depending on the content of oxygen in the gas phase during synthesis. A model of the SiOx structure for ab initio calculations is proposed and describes well the experimental optical spectra. Ab initio calculations of the dependences of the SiOx refractive index and band gap on stoichiometry parameter x are performed.

Solution-Processed Oxide Complementary Inverter via Laser Annealing and Inkjet Printing

Metal-oxide-semiconductors (MOS) have become an ideal candidate for the next-generation optoelectronic device applications. However, high processing temperature and complicate processes are still tremendous challenge in developing solution-based complementary MOS (CMOS) inverter. In this article, for the first time, femtosecond (fs) laser was used to realize controllable annealing for solution-based CMOS inverter. The achievement of nonstoichiometric p-type oxide thin films was ascribed to the photo-assisted conversion of precursor to metal-oxide (M-O) lattices along with the formation of atom vacancies in oxide lattice due to carrier excitation and relaxation using laser annealing (LA). The field effect mobility of the p-and n-type M-O thin-film transistors (TFTs) with inkjet printing (IJP) and LA was 0.91 and 7.07 cm2/V·s, respectively. Moreover, location control capacity was exploited to separately anneal the p-and n-type oxide deposited with IJP for the fabrication of TFTs, which significantly simplified the fabrication process of the inverter. CMOS inverter with high noise margin and moderate voltage gain above 10 was also obtained. Our work significantly improved the ability to selectively manipulate the functionality and properties of the irradiated materials. The results demonstrated that logic gates based on all-oxide can be large area integrated using our strategy, exhibiting attractive properties and applications of the CMOS integrated circuits in oxide electronics.

Composite Nafion Membranes with CaTiO3-δ Additive for Possible Applications in Electrochemical Devices.

A composite membrane based on a Nafion polymer matrix incorporating a non-stoichiometric calcium titanium oxide (CaTiO3−δ) additive was synthesized and characterized by means of thermal analysis, dynamic mechanical analysis, and broadband dielectric spectroscopy at different filler contents; namely two concentrations of 5 and 10 wt.% of the CaTiO3−δ additive, with respect to the dry Nafion content, were considered. The membrane with the lower amount of additive displayed the highest water affinity and the highest conductivity, indicating that a too-high dose of additive can be detrimental for these particular properties. The mechanical properties of the composite membranes are similar to those of the plain Nafion membrane and are even slightly improved by the filler addition. These findings indicate that perovskite oxides can be useful as a water-retention and reinforcing additive in low-humidity proton-exchange membranes.

Quantitative analysis of CO2 hydrate formation in porous media by proton NMR

AbstractGas hydrate is a nonstoichiometric crystal compound formed from water and gas. Most nonvisual studies on gas hydrate are unable to detect how much water is converted to hydrates, and thus, the hydrate stoichiometry calculations are inaccurate. This study investigated the CO2 hydrate formation process in porous media directly and quantitatively. The characteristics of the time‐variable consumption of hydrate formation indicated a two‐stage formation, hydrate enclathration and continuous occupancy. The enclathration stage occurred in the first 20 min of the formation when considerable heat is released. The continuous occupancy stage lasted longer than the hydrate enclathration because the empty cages in previously formed hydrates would also be occupied. The higher formation pressures can accelerate water consumption and increase cage occupancy. The compositions of completely formed CO2 hydrates at 2.7, 3.0, and 3.3 MPa and 275.15 K were determined as CO2·6.90H2O, CO2·6.70H2O, and CO2·6.49H2O, respectively.

Description of terminal substitutional solid solutions using the sublattice model

The case of the substitutional solid solutions based on elements possessing several crystallographic sites is discussed. It is concluded that, at least in some cases, especially when the solution domain is large and when strong differences of site occupancies are observed between the different sites, the sublattice model should be used as it is the case for interstitial solid solutions or non-stoichiometric intermetallic compounds. Results of experimental site occupancies are presented and compared with these obtained from the DFT calculations from which the configurational entropy can be calculated. Comparison of the enthalpy and entropy of the solid solutions is made with the available thermodynamic assessments. The β-U phase in U–Pu system is addressed but the most important cases are α-Mn and β-Mn solid solutions in different systems such as Mn–Re, Fe–Mn, Al–Mn and Al–Fe–Mn. These phases are particularly detailed and conclusions are given relative to their thermodynamic modeling emphasizing the need to use the sublattice model.

An assembled ordered W18O49 nanowire film with high SERS sensitivity and stability for the detection of RB

In this work, a novel ordered nanowire film composed of non-stoichiometric semiconductor metal oxide, W18O49, was assembled by a three-phase-interface approach. The prepared W18O49 nanowires possess relatively uniform morphologies with average diameter of about 10 nm. When employed as the SERS substrate for the detection of rhodamine B (RB), the ordered W18O49 nanowire film calcined in Ar/H2 (W18O49-H2) exhibited stronger SERS signal than W18O49 nanowire films which resulted from the existence of more oxygen vacancies due to the reduction effect of H2. The SERS detection limit of RB on W18O49-H2 nanowire films reached 10−7 M. And more importantly, the W18O49-H2 nanowire film SERS substrate possessed outstanding reproducibility for detecting RB because of the ordered as well as stable morphology, reflecting in the low relative standard deviation (RSD) (9.7%). Therefore, as a new non-noble metal SERS substrate, the W18O49-H2 nanowire film provides an important inspiration for its wide application in the detection of pollutants.