The pH-dependent reactions in the sonochemical synthesis of luminescent fluorides: The quest for the formation of KY3F10 crystal phases.

In this study Eu3+-doped yttrium fluorides were designed by ultrasound-assisted processes at different pH values (4.0-9.0). This novel strategy has enabled to obtain materials with intriguing morphologies and modulated crystal structures: α-KY3F10, δ-KY3F10·xH2O, and Y(OH)3-xFx. To date, the literature has primarily focused only on the α-phase of KY3F10. Yet, explaining the formation of the mostly uncharted δ-phase of KY3F10 remains a challenge. Thus, this paper offers the key to synthesizing both the α and the δ-phases of KY3F10 and also reports the first ultrasound-assisted process for the preparation of yttrium hydroxyfluorides. It is also unraveled the connection between the different pH-dependent reactions and the formation mechanisms of the compounds. In addition to this, the unique features of the Eu3+ ion have allowed to conduct a thorough study of the different materials and have endowed the compounds with photoluminescent properties. The results underscore a highly tunable optical response, with a wide gamut of color emissions (from orangish to red hues), lifetimes (from 7.9 ms to 1.1 ms) and quantum efficiencies (98-28%). The study unveils the importance of sonochemistry in obtaining luminescent fluorides with controlled crystal structures that can open up new avenues in the synthesis and design of inorganic materials.

Field-induced p-n transition in yttria-stabilized zirconia.

Oxide ion conducting yttria-stabilised zirconia ceramics show the onset of electronic conduction under a small bias voltage. Compositions with a high yttria content undergo a transition from p-type to n-type behavior at voltages in the range 2.4 to 10 V, which also depends on oxygen partial pressure. Surface reactions have a direct influence on bulk electronic conductivities, with possible implications for voltage-induced flash phenomena and resistive switching.

α-AgVO3 Decorated by Hydroxyapatite (Ca10(PO4)6(OH)2): Tuning Its Photoluminescence Emissions and Bactericidal Activity.

Defect-related luminescent materials have attracted interest because of their excellent optical properties and are considered as a less expensive and nontoxic alternative to commonly used lanthanide-based optical systems. These materials are fundamentally and technologically important for the next generation of full-color tunable light-emitting diodes as well as in the biomedical field. In this study, we report the preparation of α-silver vanadate (α-AgVO3, AV) decorated by hydroxyapatite (Ca10(PO4)6(OH)2, HA) with intense photoluminescence (PL) emissions at various HA/AV molar ratios (1:1-1:1/32) by a simple route based on chemical precipitation. The well-defined diffraction peaks observed by X-ray diffraction were all indexed to the monoclinic AV and hexagonal HA phases. Analysis of the results obtained by Fourier transform infrared spectroscopy reveals the presence of short-range structural order as deduced by the characteristic vibrational modes assigned to AV and HA systems. Characterization by scanning and transmission electron microscopies confirms the presence of AV and HA micro- and nanorods, respectively. UV-vis spectroscopy renders band gap energies of 5.80 eV for HA and in the range 2.59-2.65 eV for pure AV and HA/AV samples. The PL data reveal the presence of broad-band emission profiles, typical of defect-related optical centers in materials. Depending on the molar ratio, the emission can be completely tunable from the blue to red spectral regions; in addition, pure white color emission was obtained. On the basis of these results, we propose an order-disorder model induced by structural and interface defects to explain the PL emissions in the HA/AV system. Moreover, our results show that HA/AV composites have superior bactericidal activity against Staphylococcus aureus (methicillin-resistant and methicillin-susceptible) and can be used as a novel multifunctional material.

Tailoring the Bactericidal Activity of Ag Nanoparticles/α-Ag2WO4 Composite Induced by Electron Beam and Femtosecond Laser Irradiation: Integration of Experiment and Computational Modeling.

In nanotechnology research, significant effort is devoted to fabricating patterns of metallic nanoparticles on the surfaces of different semiconductors to find innovative materials with favorable characteristics, such as antimicrobial and photocatalytic properties, for novel applications. We present experimental and computational progress, involving a combined approach, on the antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) of as-synthesized α-Ag2WO4 samples and Ag nanoparticle composites (Ag NPs)/α-Ag2WO4. The former included two morphologies: hexagonal rod-like (α-Ag2WO4-R) and cuboid-like (α-Ag2WO4-C), and the latter included composites formed under electron beam, Ag NPs/α-Ag2WO4-RE and Ag NPs/α-Ag2WO4-CE, and femtosecond (fs) laser irradiation, Ag NPs/α-Ag2WO4-RL and Ag NPs/α-Ag2WO4-CL. Direct observations of the arrangement of Ag NPs on the Ag NPs/α-Ag2WO4 composites irradiated with an electron beam and laser, through transmission electron microscopy (TEM), high-resolution TEM, energy-dispersive X-ray spectroscopy, and field-emission scanning electron microscopy, allow us to investigate the surface morphology, chemical composition, homogeneity, and crystallinity. Therefore, these experimental factors, and in particular, the facet-dependent response of Ag NPs/α-Ag2WO4 composites were discussed and analyzed from the perspective provided by the results obtained by first-principles calculations. On this basis, α-Ag2WO4-R material proved to be a better bactericidal agent than α-Ag2WO4-C with minimum bactericidal concentration (MBC) values of 128 and 256 µg/mL, respectively. However, the Ag NPs/α-Ag2WO4-CL composite is the most efficient bactericidal agent of all tested samples (MBC = 4 µg/mL). This superior performance can be attributed to the cooperative effects of crystal facets and defect engineering. These results on the synthesis and stability of the Ag NPs/α-Ag2WO4 composites can be used for the development of highly efficient bactericidal agents for use in environmental remediation and the potential extension of methods to produce materials with catalytic applications.

Laser-induced formation of bismuth nanoparticles.

In the current communication, the synthesis of metallic Bi nanoparticles with coexisting crystallographic structures (rhombohedral, monoclinic, and cubic) obtained via direct femtosecond laser irradiation of NaBiO3 is demonstrated for the first time. By exploring the use of high laser power values, it is revealed that the promoted laser-mediated reactions lead to the synthesis of coexisting phases in metal nanoparticles, which may be a widely occurring phenomenon in other materials under femtosecond laser irradiation, and a fundamental concern for laser-based nanofabrication.

Towards the scale-up of the formation of nanoparticles on α-Ag2WO4 with bactericidal properties by femtosecond laser irradiation.

In recent years, complex nanocomposites formed by Ag nanoparticles coupled to an α-Ag2WO4 semiconductor network have emerged as promising bactericides, where the semiconductor attracts bacterial agents and Ag nanoparticles neutralize them. However, the production rate of such materials has been limited to transmission electron microscope processing, making it difficult to cross the barrier from basic research to real applications. The interaction between pulsed laser radiation and α-Ag2WO4 has revealed a new processing alternative to scale up the production of the nanocomposite resulting in a 32-fold improvement of bactericidal performance, and at the same time obtaining a new class of spherical AgxWyOz nanoparticles.

From Complex Inorganic Oxides to Ag-Bi Nanoalloy: Synthesis by Femtosecond Laser Irradiation.

Bimetallic nanoalloys with a wide variety of structures and compositions have been fabricated through many diverse techniques. Generally, various steps and chemicals are involved in their fabrication. In this study, the synthesis of Ag-Bi nanoalloys by femtosecond laser irradiation of an inorganic oxide Ag2WO4/NaBiO3 target without any chemicals like reducing agents or solvent is presented. The interaction between these materials and the ultrashort pulse of light allows the migration of Ag and Bi atoms from the crystal lattice to the particles surfaces and then to the plasma plume, where the reduction of the positively charged Ag and Bi species in their respective metallic species takes place. Subsequently, the controlled nucleation and growth of the Ag-Bi alloyed nanoparticles occurs in situ during the irradiation process in air. Although at the bulk level, these elements are highly immiscible, it was experimentally demonstrated that at nanoscale, the Ag-Bi nanoalloy can assume a randomly mixed structure with up to 6 ± 1 atom % of Bi solubilized into the face-centered cubic structure of Ag. Furthermore, the Ag-Bi binary system possesses high antibacterial activity against Staphylococcus aureus (methicillin-resistant and methicilin-susceptible), which is interesting for potential antimicrobial applications, consequently increasing their range of applicability. The present results provide potential insights into the structures formed by the Ag-Bi systems at the nanoscale and reveal a new processing method where complex inorganic oxides can be used as precursors for the controlled synthesis of alloyed bimetallic nanoparticles.

Nanofluid based on self-nanoencapsulated metal/metal alloys phase change materials with tuneable crystallisation temperature.

Nanofluids using nanoencapsulated Phase Change Materials (nePCM) allow increments in both the thermal conductivity and heat capacity of the base fluid. Incremented heat capacity is produced by the melting enthalpy of the nanoparticles core. In this work two important advances in this nanofluid type are proposed and experimentally tested. It is firstly shown that metal and metal alloy nanoparticles can be used as self-encapsulated nePCM using the metal oxide layer that forms naturally in most commercial synthesis processes as encapsulation. In line with this, Sn/SnOx nanoparticles morphology, size and thermal properties were studied by testing the suitability and performance of encapsulation at high temperatures and thermal cycling using a commercial thermal oil (Therminol 66) as the base fluid. Secondly, a mechanism to control the supercooling effect of this nePCM type based on non-eutectic alloys was developed.

Atmosphere- and Voltage-Dependent Electronic Conductivity of Oxide-Ion-Conducting Zr1-xYxO2-x/2 Ceramics.

Cubic, fluorite-structured solid solutions Zr1-xYxO2-x/2 (YSZ; x = 0.4-0.7) were prepared by sol-gel synthesis. Impedance measurements on pellets of 85% approximate density sintered at 1300 °C for 24 h showed strong evidence of oxide ion conduction with an inclined Warburg spike at low frequencies and capacitance values of ∼10-6 F cm-1 at 40 Hz. Arrhenius plots of total pellet conductivities were linear with activation energies of 1.4-1.56 eV. The conductivity decreased with x and was 2-4 orders of magnitude lower than that with optimized YSZ, x = 0.08. When the atmosphere was changed from N2 to O2 during impedance measurements, two reversible effects were seen: the Warburg spike contracted greatly, and the sample resistance decreased. These effects were more noticeable at higher x and are attributed to the introduction of p-type electronic conduction, in parallel with the preexisting oxide ion conduction. A similar reversible result was observed upon application of a direct-current (dc) bias during impedance measurements. When either pO2 is increased or a dc bias is applied, hole creation is believed to arise by the ionization of underbonded oxide ions situated near the Y3+ dopant ions. The ionized electrons are trapped at surface oxygen species, and the holes that are left on oxygen are responsible for p-type conduction. The electrolytic domain of x = 0.4-0.7 extends up to approximately 10-2 atm of O2 before p-type conduction is observed. The upper pO2 limit of the electrolytic domain of x = 0.08 is not known but is likely to be close to or slightly above 1 atm of O2.

Theoretical and Experimental Insight on Ag2CrO4 Microcrystals: Synthesis, Characterization, and Photoluminescence Properties.

Ag2CrO4 microcrystals were synthesized by means of the coprecipitation method without the use of a surfactant under three different conditions. On the basis of the theoretical and experimental results, we describe the relationship among the structural order/disorder effects, morphology, and photoluminescence of the Ag2CrO4 microcrystals. The experimental results were correlated with the theoretical findings for a deeper understanding of the relationship between the electronic structure, morphology, and photoluminescence properties. First-principles computational studies were used to calculate the geometries of bulk Ag2CrO4 and its low-index (001), (011), (110), (010), (111), and (100) facets based on a slab model. A good agreement between the experimental and the theoretical morphologies was found by varying the ratio of the superficial energy values.

Synthesis, antifungal evaluation and optical properties of silver molybdate microcrystals in different solvents: a combined experimental and theoretical study.

In this study, we investigate the structure, antifungal activity, and optical properties of ß-Ag2MoO4 using experimental and theoretical approaches. ß-Ag2MoO4 samples were prepared by a co-precipitation method using different solvents (water, ethanol and ammonia), and their antifungal activity against Candida albicans was investigated. The samples were characterized by X-ray diffraction, micro-Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy with energy dispersive spectroscopy. The optical properties were investigated by UV-Vis spectroscopy and photoluminescence measurements at room temperature. The thermodynamic equilibrium shape of the ß-Ag2MoO4 crystals was determined based on the surface energies calculated using Wulff construction. The (011) orientation was the predominant surface in the morphology. The experimental morphology was obtained by varying the surface energy ratio for each facet. A large decrease in surface energy for the (111) surface provided the experimental morphology for crystals synthesized using water and ethanol as solvents; when the surface energies for both (011) and (001) surfaces increased, the crystal morphology obtained using ammonia as a solvent was reproduced. A correlation between the exposed surfaces and antifungal activity was revealed, and an explanation to this behavior that arises from different morphologies and structural data was provided. Theoretical calculations confirm the rationality of the experimental scheme and elucidate the underlying reason for the fungistatic and fungicidal activity against Candida albicans.

Field-enhanced bulk conductivity and resistive-switching in Ca-doped BiFeO3 ceramics.

The bulk conductivity at room temperature of Ca-doped BiFeO3 ceramics is p-type and increases reversibly by up to 3 orders of magnitude under the influence of a small dc bias voltage in the range ∼3 to 20 V mm(-1). The effect occurs in both grain and grain boundary regions, is isotropic and does not involve creation of filamentary conduction pathways. It is proposed that, by means of capacitive charging and internal ionisation processes under the action of a dc bias, hole creation leads to a more conductive excited state. This gradually returns to the ground state when the dc bias is removed and the holes recombine with electrons trapped at the sample surface. The holes are believed to be created on oxygen, as O(-) ions.

Synthesis, structural characterization, and electrical properties of new oxygen-deficient tetragonal tungsten bronzes Ba2NdTi(2+x)Nb(3-x)O(15-x/2).

Oxygen-deficient tetragonal tungsten bronzes ceramics with general formula Ba(2)NdTi(2+x)Nb(3-x)O(15-x/2) (0 ≤ x ≤ 1) have been prepared by low temperature solvothermal synthesis with final firing of ceramics at 1100-1300 °C in air. Rietveld refinement of X-ray powder diffraction (XRD) and neutron powder diffraction (ND) data at room temperature of Ba(2)NdTi(3)Nb(2)O(14.5) shows that Ba and Nd are ordered on the 15-coordinate and 12-coordinate sites, respectively, Ti and Nb are disordered nonrandomly over the two octahedral sites, and oxygen vacancies locate preferentially in the coordination sphere of Nd and Ti/Nb(2) atoms. Variable frequency impedance measurements show that samples are poor electronic conductors with activation energies ∼0.8-1.7 eV, conductivities ∼1 × 10(-5) S cm(-1) at ∼725 °C and with some evidence of oxide ion conduction at high x values. Composition dependence of the dielectric properties shows a transition from classic ferroelectric behavior with Ba(2)NdTi(2)Nb(3)O(15) to a relaxor-like behavior with Ba(2)NdTi(3)Nb(2)O(14.5). At intermediate compositions, both a first-order phase transition and relaxor-like behavior are observed.

Self-assembling of Er(2)O(3)-TiO(2) mixed oxide nanoplatelets by a template-free solvothermal route.

An easy solvothermal route has been developed to synthesize the first mesoporous Er(2)O(3)-TiO(2) mixed oxide spherical particles composed of crystalline nanoplatelets, with high surface area and narrow pore size distribution. This synthetic strategy allows the preparation of materials at low temperature with interesting textural properties without the use of surfactants, as well as the control of particle size and shape. TEM and Raman analysis confirm the formation of nanocrystalline Er(2)O(3)-TiO(2) mixed oxide. Mesoscopic ordered porosity is reached through the thermal decomposition of organic moieties during the synthetic process, thus leading to a template-free methodology that can be extended to other nanostructured materials. High specific surface areas (up to 313 m(2) g(-1)) and narrow pore size distributions are achieved in comparison to the micrometric material synthesized by the traditional sol-gel route. This study opens new perspectives in the development, by solvothermal methodologies, of multifunctional materials for advanced applications by improving the classical pyrochlore properties (magnetization, heat capacity, catalysis, conductivity, etc.). In particular, since catalytic reactions take place on the surface of catalysts, the high surface area of these materials makes them promising candidates for catalysts. Furthermore, their spherical morphology makes them appropriate for advanced technologies in, for instance, ceramic inkjet printers.

Structural and spectroscopic study of a novel erbium titanate pink pigment prepared by sol-gel methodology.

Pyrochlore oxides show a large variety of physical and chemical properties depending on the ordering/disordering of the cations and oxygen vacancies. Taking account of these structural features and the luminescent properties of lanthanides, a new family of colored materials is investigated. This paper studies the structural evolution of the erbium titanate system with temperature to establish its influence on the color properties. The success on the development of color is completely related to the sol-gel preparation method, underlining its higher reactivity compared to classical solid-state synthesis. After firing at 700 degrees C, the sol-gel material develops an intense pink coloration whose intensity significantly diminishes at 800 degrees C. X-ray diffraction and Rietveld refinements indicated the presence of nanocrystals with a fluorite-like structure at 700 degrees C, responsible for the intense coloration, which suffers a gradual atomic rearrangement toward an "ideal" pyrochlore phase. These results were corroborated by infrared and Raman measurements. UV-vis spectroscopy showed the influence of the Er(3+)-O bond covalence on the spectral properties. This study opens new perspectives to the development of more ecological colored sol-gel materials based on rare earth elements. Furthermore, the combination of the optical aspects with the classical pyrochlore properties (magnetization, heat capacity, conductivity, etc.) would provide new multifunctional materials for advanced applications.