Cluster model studies on oxygen sites at the (010) surfaces of V2O5 and MoO3

Cluster model studies have been performed to examine the electronic structure and adsorption properties near structurally different oxygen sites at the (010) surfaces of vanadium pentoxide, V₂O₅, and molybdenum trioxide, MoO₃. In addition, adsorption of hydrogen at the oxygen sites and desorption of OH groups has been studied in order to find site specific differences. The electronic properties and total energies of the clusters have been obtained from ab initio density functional theory (DFT) calculations. The surface oxygen sites are found to be ionic where bridging oxygens become more negative than terminal ones. Further, hydrogen adsorbs at all oxygen sites where binding is strongest at the bridge sites on the V₂O₅(010) surface whereas on MoO₃(010) the terminal sites are preferred. The latter difference can be understood by simple geometric arguments. Surface OH groups formed by H adsorption and involving terminal oxygens are strongly bound to the surface whereas those involving bridging oxygens are mobile and become available for subsequent reactions.     

Crystal structure and multiferroic behavior of perovskite YFeO3

We present a study of multiferroic properties of YFeO₃ synthesized by means of high-energy ball milling assisted by annealing at low temperature. Fe₂O₃ and Y₂O₃ powders were mixed in a stoichiometric ratio, milled for 5?h, pressed and annealed at temperature from 773 to 1073?K. X-ray diffraction (XRD) analysis confirmed the formation of single-phase orthorhombic structure. Magnetic hysteresis loops, at room temperature, from vibrating sample magnetometry show the transition from ferromagnetic order to G-antiferromagnetic order, related to the transformation from amorphous to crystalline orthorhombic single phase. The value of Néel temperature of single phase YFeO₃ was obtained at 595?K, lower than previously reported. Dielectric behavior at room temperature of YFeO₃ single-phase sample shows a direct dependence with frequency of both dielectric constant and dielectric loss, in good agreement with Maxwell-Wagner effect. A fit made using Cole-Cole equation shows that the Low Temperature Dielectric Relaxation, LTDR, corresponds to a Debye-type relaxation. Finally, it was found that AC conductivity (σ_AC) increases linearly with frequency. All results show that YFeO₃ synthesized by high-energy ball milling assisted with annealing possess a multiferroic behavior.     

Heterostructure-based 3D-CdS/TiO2 nanotubes/Ti: Photoelectrochemical performances and interface simulation investigation

Heterojunction can effectively improve the charge separation efficiency and facilitate electron transfer, producing a strong photoelectric signal. By using 3D-TiO₂ nanotubes/Ti foil as support, CdS–TiO₂ heterojunction electrodes with different CdS proportions were fabricated as photoelectrochemical (PEC) biosensor to respond the visible irradiation and improve the PEC performance of TiO₂ nanotubes. Density functional theory (DFT) simulation was conducted to clarify the PEC process of CdS–TiO₂NTs and revealed the important role of CdS in enhancing electron–hole separation on TiO₂ nanotubes. Owing to the 3D tubular structure of the support, 2 mM CdS–TiO₂ nanotubes/Ti PEC electrode exhibited low detection limit of 0.27 μM and good sensitivity of 328.87 μA mM^?1 cm^?2 for glucose in the range of 2–9 μM under visible illumination. The fabricated CdS–TiO₂ nanotubes/Ti biosensor also showed high selectivity and good stability, which indicated a new candidate for biosensors.     

Possible origin of ferromagnetism in antiferromagnetic orthorhombic-YFeO3: A first-principles study

Rare-earth orthoferrites (RFeO₃) are well-known for the antiferromagnetic ground state. However, some of the recent experimental results suggest that the few members of RFeO₃ family possess ferromagnetism. In the present investigation we report the possible origin of ferromagnetism in antiferromagnetic YFeO₃ using density functional theory. For this purpose, we have considered pure as well as self-doping in YFeO₃ i.e. by considering the point defect at Y, Fe and O sites. Our finding suggests that the point defects in YFeO₃ results in the mixed-valence state of Fe, which may result in ferromagnetism through Zener double exchange mechanism.     

The doping concentration dependent tunable yellow luminescence of Sr5(PO4)2(SiO4):Eu

Color tunable yellow-emitting phosphors of Sr_5−5xEu_5x(PO₄)₂SiO₄ (x = 0.05-0.15) were prepared by conventional solid-state reaction method. The X-ray powder diffraction patterns, the photoluminescence excitation and emission spectra were measured. The main excitation bands of the phosphors locate at a broad band extending from 300 to 500 nm, which can match the emission of ultraviolet- and blue-emitting diode chips. The tunable luminescence color was realized by the changing Eu²⁺ doping in Sr₅(PO₄)₂SiO₄. The structure and luminescence properties were investigated. Sr_5−5x(PO₄)₂SiO₄:Eu_5x displays two typical luminescence centers, which originate from two different Sr²⁺ (Eu²⁺) sites in the host. The site-occupation, the luminescence intensity and energy transfer between the Eu²⁺ ions occupying two different crystallographic Sr²⁺ sites were discussed on the base of the luminescence spectra and crystal structure. This is helpful to improve this phosphor for a potential application as a white light emitting diode phosphor.     

Different support effect of M/ZrO2 and M/CeO2 (M = Pd and Pt) catalysts on CO adsorption: A periodic density functional study

CO adsorption over Pd₄ and Pt₄ cluster supported by c-ZrO₂(1 1 1) and CeO₂(1 1 1) catalyst systems was investigated using periodic density functional method in order to clarify the support effect on CO activation. We found that the support increases the CO activation for bridge and three-fold sites but decreases for the atop site. Moreover, it was found that the support changes the site preference for the CO adsorption. Bridge site on both the Pt₄/c-ZrO₂ and Pt₄/CeO₂ show larger CO adsorption energies than those on the other sites while the atop site is energetically preferable on isolated Pt₄ cluster. c-ZrO₂ supported Pd shows the largest CO activation with large charge transfer from the catalyst to the CO molecule. This reveals that ZrO₂ supported Pd can be a good catalyst for CO activation because of its higher probability to the three-fold site CO adsorption. We also found that positively charged M₄ clusters on the support keep their strong electron-donating properties and have enough charge density to contribute to the activation of an adsorbed CO molecule by a charge transfer.     

Grain Boundaries of Semiconducting SrTiO3 and BaTiO3 Ceramics Synthesized from Surface-Coated Powders

The chemical and electrical features of the grain boundaries in polycrystalline SrTi_0.99Nb_0.01O₃ (ST) and BaTiO₃ (BT) ceramics, which were synthesized by hot-press sintering Na- and Mn-coated semiconducting ST and BT powders, respectively, were investigated. Because of the excess negative electric charges formed near grain boundaries, electrostatic potential barriers were formed near the grain boundaries. The electrical features of the grain boundaries in ceramics are very sensitive to the amount of the coating material. When the amount of the coating material was increased from 0 to 5 wt%, the threshold voltage of the ST ceramics and the resistivity jump ratio of the BT ceramics increased from 0.7 to 81.0 V/cm and from 1.0 to 2.0 × 10³, respectively. The electrical features of the grain boundaries are related to their chemical characteristics.     

Mechanical and electrochemical properties of cubic and tetragonal LixLa0.557TiO3 perovskite oxide electrolytes

Solid oxide electrolytes with high Li ion conductivity and mechanical stability are vital for all solid-state lithium ion batteries. The perovskite material Li_xLa_0.557TiO₃ with various initial Li (0.303 ≤ x ≤ 0.370) is synthesized by traditional solid-state reaction. The cubic and tetragonal structures are prepared with fast and slow cooling, respectively. The results reveal that the Li ion conductivity of the cubic structure is higher. In fact, the bulk conductivity of 1.65 × 10^?3 S cm^?1 is obtained at room temperature for x = 0.350. The crystal structure is not affected by the Li₂O quantity. In addition, Young's modulus, hardness, and fracture toughness are determined with indentation method for both structures. The Young's modulus increases with increasing Li₂O. However, hardness and fracture toughness keep a relatively stable value independent of Li₂O quantity.     

Optical and mechanical properties of Mg-doped sialon composite with La2O3 as additive

Using 0.5 wt.% La₂O₃ as a sintering additive, Mg-doped sialon composite with the maximum infrared transmittance of 50% was fabricated by hot pressing at 1800 °C. The addition of La₂O₃ significantly promotes the densification process of Mg-doped sialon and the anisotropic growth of β-sialon grains. As a result, the sintered material exhibits high hardness (20.2 GPa), fracture toughness (4.8 MPa m^1/2) and flexural strength (664 MPa). Furthermore, the nano-sized glassy phases concentrated at triple junctions have no obviously negative impact on infrared translucency of Mg-doped sialon.     

Effects of Mn,Cl co-doping on the structure and photoluminescence properties of novel walnut-shape MAPb0.95Mn0.05I3-xClx films

The novel walnut shape MAPb_0.95Mn_0.05I_3-xCl_x film was successfully synthesized by a one-step method followed by chlorobenzene anti-solvent treatment. The bandgap energy and PL intensity of perovskite film can be effectively tuned by Mn and Cl co-doping. The enlarged bandgap energy is mainly attributed to the synergistic effect of strengthened Pb-I interaction, Cl incorporation and smaller electronegativity of Mn dopants. The stronger coupling between the Mn d- and Pb p-bands, extended carriers diffusion length and more emitting defect-associated states caused by Mn and Cl co-doping are the main reasons for the enhancing PL intensity of MAPb_0.95Mn_0.05I_3-xCl_x walnut shape film. This work not only helps to in-depth understand the correlation between co-doping elements and optical properties of nanostructured perovskite films, but also provides important strategy for future designing the lower-toxic nanostructured perovskite materials with enhanced electrical and optical properties.     

Predictions of phase stabilities,electronic structures and optical properties of potential superhard WB3

For better synthesis and use of WB₃ triborides surface coatings, structural, electronic and optical properties of hexagonal and trigonal hP4, hP8, hP16 and hR24-WB₃ triborides were predicted through the first-principles calculation. The formation enthalpies revealed that all of these WB₃ are energetically stable, and hR24-WB₃ is the best stable phase. Electronic properties showed that these WB₃ have strong B-B covalent bonds mixed with W-B covalent-ionic bonds. The calculated reflectivity that these WB₃ triborides can be used as optical shielding devices for FUV radiation. Moreover, Nd-YAG laser with wavelength of 1064?nm is available for synthetizing hP4, hP8 and hP16-WB₃ coatings, while the laser with wavelength of 532?nm is suitable for hR24-WB₃ coatings. The calculated static dielectric constants and refractive index indicated that the optical anisotropy for these WB₃ is in a sequence of hP8-WB₃ >?hP4-WB₃ >?hP16-WB₃ >?hR24-WB₃.     

Enhanced mechanical and dielectric behavior of BaTiO3/Cu composites

BaTiO₃/xCu composite ceramics with x = 0-30 wt.% were fabricated by the traditional mixing method in nitrogen gas. The mechanical properties and electric properties of the obtained composites were investigated as a function of the Cu mass fraction using a three bending test and impedance spectroscopy. The results indicated that the relative density of the sintered composites reached above 91%, the Cu-dispersed BaTiO₃ composites enhanced the mechanical properties, particularly the high fracture toughness (∼3.9 MPa m^1/2) and bending strength (∼134 MPa), compared to the monolithic BaTiO₃. Furthermore, the percolation threshold of BaTiO₃/Cu composites was x = 25 wt.%. The permittivity (?_r) markedly increased from ∼2000 for monolithic BaTiO₃ to ∼9000 with increasing Cu up to 30 wt.%. Additionally, the temperature coefficient of this system was less than 5% in the temperature range of 25-115.     

Low temperature atomic layer deposition of nickel sulfide and nickel oxide thin films using Ni(dmamb)2 as Ni precursor

Nickel(II) 1-dimethylamino-2-methyl-2-butoxide (Ni(dmamb)₂) with water and hydrogen sulfide as oxygen and sulfur sources was employed in atomic layer deposition (ALD) of nickel oxide (NiO) and nickel sulfide (NiS) thin films. Both NiO and NiS thin films demonstrate temperature-independent growth rates per cycle of 0.128?nm/cycle and 0.0765?nm/cycle, at 130–150?°C and 80–160?°C, respectively. Comparison of two nickel-based thin film materials demonstrates dissimilar deposition features depending on the reactivity of the Ni precursor, i.e., Ni(dmamb)₂ with anion sources provided by the water and hydrogen sulfide reactants. Difference in reactivity observed for NiO and NiS ALD processes is further investigated by density functional theory (DFT) simulations of surface reactions, which indicated that H₂S demonstrate higher reactivity with surface-adsorbed Ni precursor than H₂O. The material properties of ALD NiO and NiS thin films including stoichiometry, crystallinity, band structure, and electronic properties were analyzed by multiple experimental techniques, showing potential of ALD NiS as electrode or catalyst for energy-oriented devices.     

Competitive nanocrystallization of Na3ScF6 and NaYbF4 in aluminosilicate glass and optical spectroscopy of Ln3+ dopants

The precipitation of monoclinic Na₃ScF₆ nanocrystals from aluminosilicate glass with specially designed compositions of SiO₂-Al₂O₃-Na₂O-NaF-ScF₃-YbF₃ was achieved for the first time. Impressively, competitive nanocrystallization of cubic NaYbF₄ and monoclinic Na₃ScF₆ has been evidenced to be dependent on Na⁺ content and F/Na ratio in glass. Adopting Er³⁺ and Eu³⁺ dopants as structural probes, optical spectroscopic analyses verified that these emissive centers preferred to partition into NaYbF₄ nanocrystals rather than Na₃ScF₆ ones.     

Enhanced upconverted luminescence and the optical thermometry behavior of Er3+-doped BaYbF5 transparent glass ceramics

Transparent SiO₂ - Al₂O₃ - Na₂O - CaO - BaF₂ - YbF₃ glass ceramics (GC) doped with Er³⁺ ions were successfully fabricated by a melt-quenching technique with subsequent heat treatment. The formation of BaYbF₅ nano-crystalline phase was confirmed by X-ray diffraction and transmission electron microscopy. Compared to the precursor glass (PG), the clearer Stark splitting and greatly enhanced up-conversion (UC) emission in GC indicate that Er³⁺ ions mainly enter into BaYbF₅ nanocrystals with low phonon energy after crystallization. The temperature dependent on purple UC emission ratio (which is due to the Er^{3+ 4}G_11/2→⁴I_15/2 and ²H_9/2→⁴I_15/2 transitions) and common green UC emission ratio with low-power excitation in BaYbF₅ GC have been studied respectively. In addition, the UC mechanisms in PG and GC are illustrated and analyzed. The outstanding properties of Er³⁺-doped BaYbF₅ transparent GC may present potential applications in all-solid-state UC lasers and optical fiber temperature sensors.     

Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

Theoretical and experimental investigations on high temperature mechanical and thermal properties of BaZrO3

The perovskite BaZrO₃ has high phase stability from room temperature to its melting point and therefore is regarded as a promising candidate for various high-temperature applications. In this work, the mechanical and thermal properties of BaZrO₃ at high temperatures are investigated by combining first-principles calculations and experimental approaches. BaZrO₃ has moderate mechanical properties and low thermal conductivity, being comparable to other zirconium-based and silicate structural ceramics. Its remaining Young's modulus of 174.4?GPa at 1523?K is 81.6% of 213.8?GPa at room temperature. The residual flexural strength of 127.8?MPa at 1273?K is 74% of 172.4?MPa at room temperature, while the residual value at 1673?K is still 53.4?MPa. The thermal conductivity of BaZrO₃ is 5.75?W?m^?1 K^?1 at 298?K and decreases to 2.81?W?m^?1 K^?1 at 1473?K. The good high temperature mechanical and thermal properties ensure the potential high temperature applications of BaZrO₃ and our results are expected to arouse the design of BaZrO₃-based ceramics in the near future.     

Sintering behavior and mechanical properties of nano-sized Cr3C2/Al2O3 composites prepared by MOCVI process

A process using metal-organic chemical vapor infiltration (MOCVI) conducted in fluidized bed was employed for the preparation of nano-sized ceramic composites. The Cr-species was infiltrated into Al₂O₃ granules by the pyrolysis of chromium carbonyl (Cr(CO)₆) at 300–450 °C. The granulated powder was pressureless sintered or hot-pressed to achieve high density. The results showed that the dominant factors influencing the Cr-carbide phases formation, either Cr₃C₂ or Cr₇C₃, in the composite powders during the sintering process were the temperature and oxygen partial pressure in the furnace. The coated Cr-phase either in agglomerated or dispersive condition was controlled by the use of colloidal dispersion. The microstructures showed that fine (20 –600 nm) Cr_xC_y grains (≤8 vol.%) located at Al₂O₃ grain boundaries hardly retarded the densification of Al₂O₃ matrix in sintering process. The tests on hardness, strength and toughness appeared that the composites with the inclusions (Cr₃C₂) had gained the advantages over those by the rule of mixture. Even 8 vol.% ultrafine inclusions have greatly improved the mechanical properties. The strengthening and toughening mechanisms of the composites were due to grain-size reduction, homogenous dispersion of hard inclusions, and crack deflection.