Fabrication and characterization of microporous soft templated photoactive 3D materials for water disinfection in batch and continuous flow.

Water cleaning can be provided in batch mode or in continuous flow. For the latter, some kind of framework must withhold the cleaning agents from washout. Porous structures provide an ideal ratio of surface to volume for optimal access of the water to active sites and are able to facilitate rapid and efficient fluid transport to maintain a constant flow. When functionalized with suitable photoactive agents, they could be used in solar photocatalytic disinfection. In this study, we have used the sugar cube method to fabricate PDMS-based materials that contain three different classes of photosensitizers that differ in absorption wavelength and intensity, charge as well as in ability to generate singlet oxygen. The obtained sponges are characterized by scanning electron microscopy and digital microscopy. Archimede's method was used to measure porosity and density. We show that the materials can absorb visible light and generate Reactive Oxygen Species (ROS) that are required to kill bacteria. The disinfection ability was tested by examining how irradiation time and operation mode (batch vs. flow) contribute to the performance of the material. The current strategy is highly adaptable to other (medium) pressure-driven flow systems and holds promising potential for various applications, including continuous flow photoreactions.

Lead-free organic-inorganic azetidinium alternating metal cation bromide: [(CH2)3NH2]2AgBiBr6, a perovskite-related absorber.

In the last decade, organic-inorganic hybrid halide perovskite materials have developed into a very large research area in photovoltaics and optoelectronics as promising light harvesters. Lead-free double perovskites have recently been investigated as an environmentally friendly alternative to the lead-containing compositions. However, lead-free organic-inorganic hybrid halide double perovskites have so far rarely been produced due to a certain complexity in their synthesis. A number of small molecular cations have been investigated, but compositions containing azetidinium, which is a 4-membered heterocyclic molecular ring, on the A-site have hardly been considered. This study investigates the potential of [(CH2)3NH2]2AgBiBr6 as an optical absorber in photovoltaics or optoelectronics. The use of this alternative cation changes the crystal symmetry significantly. Columns of alternating metal cation form which are separated by the organic ions. While crystal symmetry is rather different from the perovskites, the overall properties as an absorber are similar. It is thus worthwhile to further investigate alternate hybrid compositions which form into other symmetries than the perovskite base structure.

Control of physical properties in BiFeO3nanoparticles via Sm3+and Co2+ion doping.

Highly crystalline BiFeO3(BFO), Bi0.97Sm0.03FeO3(Sm-BFO) and BiFe0.97Co0.03O3(Co-BFO) nanoparticles (NPs) were utilized as potential magnetic hyperthermia agents at two different frequencies in the radiofrequency (RF) range, and the effect of Sm3+and Co2+ion doping on the physical properties of the material was examined. The thermal behaviour of the as-prepared powders disclosed that the crystallization temperature of the powders is affected by the incorporation of the dopants into the BFO lattice and the Curie transition temperature is decreased upon doping. Vibrational analysis confirmed the formation of the R3c phase in all compounds through the characteristic FT-IR absorbance bands assigned to O-Fe-O bending vibration and Fe-O stretching of the octahedral FeO6group in the perovskite, as well as through Raman spectroscopy. The shift of the Raman-active phonon modes in Sm-BFO and Co-BFO NPs indicated structural distortion of the BFO lattice, which resulted in increased local polarization and enhanced visible light absorption. The aqueous dispersion of Co-BFO NPs showed the highest magnetic hyperthermia performance at 30 mT/765 kHz, entering the therapeutic temperature window for cancer treatment, whereas the heating efficiency of all samples was increased with increasing frequency from 375 to 765 kHz, making our doped nanoparticles to be suitable candidates for potential biomedical applications.

Cell Behavior Changes and Enzymatic Biodegradation of Hybrid Electrospun Poly(3-hydroxybutyrate)-Based Scaffolds with an Enhanced Piezoresponse after the Addition of Reduced Graphene Oxide.

This is the first comprehensive study of the impact of biodegradation on the structure, surface potential, mechanical and piezoelectric properties of poly(3-hydroxybutyrate) (PHB) scaffolds supplemented with reduced graphene oxide (rGO) as well as cell behavior under static and dynamic mechanical conditions. There is no effect of the rGO addition up to 1.0 wt% on the rate of enzymatic biodegradation of PHB scaffolds for 30 d. The biodegradation of scaffolds leads to the depolymerization of the amorphous phase, resulting in an increase in the degree of crystallinity. Because of more regular dipole order in the crystalline phase, surface potential of all fibers increases after the biodegradation, with a maximum (361 ± 5 mV) after the addition of 1 wt% rGO into PHB as compared to pristine PHB fibers. By contrast, PHB-0.7rGO fibers manifest the strongest effective vertical (0.59 ± 0.03 pm V-1 ) and lateral (1.06 ± 0.02 pm V-1 ) piezoresponse owing to a greater presence of electroactive ß-phase. In vitro assays involving primary human fibroblasts reveal equal biocompatibility and faster cell proliferation on PHB-0.7rGO scaffolds compared to pure PHB and nonpiezoelectric polycaprolactone scaffolds. Thus, the developed biodegradable PHB-rGO scaffolds with enhanced piezoresponse are promising for tissue-engineering applications.

Rare-earth doped BiFe0.95Mn0.05O3 nanoparticles for potential hyperthermia applications.

Ionic engineering is exploited to substitute Bi cations in BiFe0.95Mn0.05O3 NPs (BFM) with rare-earth (RE) elements (Nd, Gd, and Dy). The sol-gel synthesized RE-NPs are tested for their magnetic hyperthermia potential. RE-dopants alter the morphology of BFM NPs from elliptical to rectangular to irregular hexagonal for Nd, Gd, and Dy doping, respectively. The RE-BFM NPs are ferroelectric and show larger piezoresponse than the pristine BFO NPs. There is an increase of the maximum magnetization at 300 K of BFM up to 550% by introducing Gd. In hyperthermia tests, 3 mg/ml dispersion of NPs in water and agar could increase the temperature of the dispersion up to ∼39°C under an applied AC magnetic field of 80 mT. Although Gd doping generates the highest increment in magnetization of BFM NPs, the Dy-BFM NPs show the best hyperthermia results. These findings show that RE-doped BFO NPs are promising for hyperthermia and other biomedical applications.

Laser Ablation of NiFe2O4 and CoFe2O4 Nanoparticles.

Pulsed laser ablation in liquids was utilized to prepare NiFe2O4 (NFO) and CoFe2O4 (CFO) nanoparticles from ceramic targets. The morphology, crystallinity, composition, and particle size distribution of the colloids were investigated. We were able to identify decomposition products formed during the laser ablation process in water. Attempts to fractionate the nanoparticles using the high-gradient magnetic separation method were performed. The nanoparticles with crystallite sizes in the range of 5-100 nm possess superparamagnetic behavior and approximately 20 Am2/kg magnetization at room temperature. Their ability to absorb light in the visible range makes them potential candidates for catalysis applications in chemical reactions and in biomedicine.

High Energy Storage Density in Nanocomposites of P(VDF-TrFE-CFE) Terpolymer and BaZr0.2Ti0.8O3 Nanoparticles.

Polymer materials are actively used in dielectric capacitors, in particular for energy storage applications. An enhancement of the stored energy density can be achieved in composites of electroactive polymers and dielectric inorganic fillers with a high dielectric permittivity. In this article, we report on the energy storage characteristics of composites of relaxor terpolymer P(VDF-TrFE-CFE) and BaZr0.2Ti0.8O3 (BZT) nanoparticles. The choice of materials was dictated by their large dielectric permittivity in the vicinity of room temperature. Free-standing composite films, with BZT contents up to 5 vol.%, were prepared by solution casting. The dielectric properties of the composites were investigated over a wide range of frequencies and temperatures. It was shown that the addition of the BZT nanoparticles does not affect the relaxor behavior of the polymer matrix, but significantly increases the dielectric permittivity. The energy storage parameters were estimated from the analysis of the unipolar polarization hysteresis loops. The addition of the BZT filler resulted in the increasing discharge energy density. The best results were achieved for composites with 1.25-2.5 vol.% of BZT. In the range of electric fields to 150 MV/m, the obtained materials demonstrate a superior energy storage density compared to other P(VDF-TFE-CFE) based composites reported in the literature.

Revealing Weak Dimensional Confinement Effects in Excitonic Silver/Bismuth Double Perovskites.

Lead-free perovskites are attracting increasing interest as nontoxic materials for advanced optoelectronic applications. Here, we report on a family of silver/bismuth bromide double perovskites with lower dimensionality obtained by incorporating phenethylammonium (PEA) as an organic spacer, leading to the realization of two-dimensional double perovskites in the form of (PEA)4AgBiBr8 (n = 1) and the first reported (PEA)2CsAgBiBr7 (n = 2). In contrast to the situation prevailing in lead halide perovskites, we find a rather weak influence of electronic and dielectric confinement on the photophysics of the lead-free double perovskites, with both the 3D Cs2AgBiBr6 and the 2D n = 1 and n = 2 materials being dominated by strong excitonic effects. The large measured Stokes shift is explained by the inherent soft character of the double-perovskite lattices, rather than by the often-invoked band to band indirect recombination. We discuss the implications of these results for the use of double perovskites in light-emitting applications.

Interplay of domain structure and phase transitions: theory, experiment and functionality.

Domain walls and phase boundaries are fundamental ingredients of ferroelectrics and strongly influence their functional properties. Although both interfaces have been studied for decades, often only a phenomenological macroscopic understanding has been established. The recent developments in experiments and theory allow to address the relevant time and length scales and revisit nucleation, phase propagation and the coupling of domains and phase transitions. This review attempts to specify regularities of domain formation and evolution at ferroelectric transitions and give an overview on unusual polar topological structures that appear as transient states and at the nanoscale. We survey the benefits, validity, and limitations of experimental tools as well as simulation methods to study phase and domain interfaces. We focus on the recent success of these tools in joint scale-bridging studies to solve long lasting puzzles in the field and give an outlook on recent trends in superlattices.

Band Gap of Pb(Fe0.5Nb0.5)O3 Thin Films Prepared by Pulsed Laser Deposition.

Ferroelectric materials have gained high interest for photovoltaic applications due to their open-circuit voltage not being limited to the band gap of the material. In the past, different lead-based ferroelectric perovskite thin films such as Pb(Zr,Ti)O3 (Pb,La)(Zr,Ti)O3 and PbTiO3 were investigated with respect to their photovoltaic efficiency. Nevertheless, due to their high band gaps they only absorb photons in the UV spectral range. The well-known ferroelectric PbFe0.5Nb0.5O3 (PFN), which is in a structure similar to the other three, has not been considered as a possible candidate until now. We found that the band gap of PFN is around 2.75 eV and that the conductivity can be increased from 23 S/µm to 35 S/µm during illumination. The relatively low band gap value makes PFN a promising candidate as an absorber material.

Yttrium Oxide Freeze-Casts: Target Materials for Radioactive Ion Beams.

Highly porous yttrium oxide is fabricated as ion beam target material in order to produce radioactive ion beams via the Isotope Separation On Line (ISOL) method. Freeze casting allows the formation of an aligned pore structure in these target materials to improve the isotope release. Aqueous suspensions containing a solid loading of 10, 15, and 20 vol% were solidified with a unidirectional freeze-casting setup. The pore size and pore structure of the yttrium oxide freeze-casts are highly affected by the amount of solid loading. The porosity ranges from 72 to 84% and the crosslinking between the aligned channels increases with increasing solid loading. Thermal aging of the final target materials shows that an operation temperature of 1400 °C for 96 h has no significant effect on the microstructure. Thermo-mechanical calculation results, based on a FLUKA simulation, are compared to measured compressive strength and forecast the mechanical integrity of the target materials during operation. Even though they were developed for the particular purpose of the production of short-lived radioactive isotopes, the yttria freeze-cast scaffolds can serve multiple other purposes, such as catalyst support frameworks or high-temperature fume filters.

Effect of Composition on Polarization Hysteresis and Energy Storage Ability of P(VDF-TrFE-CFE) Relaxor Terpolymers.

The temperature dependence of the dielectric permittivity and polarization hysteresis loops of P(VDF-TrFE-CFE) polymer films with different compositions are studied. Among them, the three compositions, 51.3/48.7/6.2, 59.8/40.2/7.3, and 70/30/8.1, are characterized for the first time. Relaxor behavior is confirmed for all studied samples. Increasing the CFE content results in lowering the freezing temperature and stabilizes the ergodic relaxor state. The observed double hysteresis loops are related to the field-induced transition to a ferroelectric state. The critical field corresponding to this transition varies with the composition and temperature; it becomes larger for temperatures far from the freezing temperature. The energy storage performance is evaluated from the analysis of unipolar polarization hysteresis loops. P(VDF-TrFE-CFE) 59.8/40.2/7.3 shows the largest energy density of about 5 J·cm-3 (at the field of 200 MV·m-1) and a charge-discharge efficiency of 63%, which iscomparable with the best literature data for the neat terpolymers.

Free Molecule Studies by Perturbed γ-γ Angular Correlation: A New Path to Accurate Nuclear Quadrupole Moments.

Accurate nuclear quadrupole moment values are essential as benchmarks for nuclear structure models and for the interpretation of experimentally determined nuclear quadrupole interactions in terms of electronic and molecular structure. Here, we present a novel route to such data by combining perturbed γ-γ angular correlation measurements on free small linear molecules, realized for the first time within this work, with state-of-the-art ab initio electronic structure calculations of the electric field gradient at the probe site. This approach, also feasible for a series of other cases, is applied to Hg and Cd halides, resulting in Q(^{199}Hg,5/2^{-})=+0.674(17) b and Q(^{111}Cd,5/2^{+})=+0.664(7) b.

Role of cooperative factors in the photocatalytic activity of Ba and Mn doped BiFeO3 nanoparticles.

The escalated photocatalytic (PC) efficiency of the visible light absorber Ba-doped BiFe0.95Mn0.05O3 (BFM) nanoparticles (NPs) as compared to BiFeO3 (BFO) NPs is reported for the degradation of the organic pollutants rhodamine B and methyl orange. 1 mol% Ba-doped-BFM NPs degrade both dyes within 60 and 25 minutes under UV + visible illumination, respectively. The Ba and Mn co-doping up to 5 mol% in BFO NPs increases the specific surface area, energy of d-d transitions, and PC efficiency of the BFO NPs. The maximum PC efficiency found in 1 mol% Ba doped BFM NPs is attributed to a cooperative effect of factors like its increased light absorption ability, large surface area, active surface, reduced recombination of charge carriers, and spontaneous polarization to induce charge carrier separation. The 1 mol% Ba and 5 mol% Mn co-incorporation is found to be the optimum dopant concentration for photocatalytic applications. These properties of co-doped BFO NPs can, e.g., be exploited in the field of water splitting.

Spatial Charge Separation as the Origin of Anomalous Stark Effect in Fluorous 2D Hybrid Perovskites.

2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long-lived correlated electron-hole pairs. Such states have a marked charge-transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities.

Laser Fragmentation Synthesis of Colloidal Bismuth Ferrite Particles.

Laser fragmentation of colloidal submicron-sized bismuth ferrite particles was performed by irradiating a liquid jet to synthesize bismuth ferrite nanoparticles. This treatment achieved a size reduction from 450 nm to below 10 nm. A circular and an elliptical fluid jet were compared to control the energy distribution within the fluid jet and thereby the product size distribution and educt decomposition. The resulting colloids were analysed via UV-VIS, XRD and TEM. All methods were used to gain information on size distribution, material morphology and composition. It was found that using an elliptical liquid jet during the laser fragmentation leads to a slightly smaller and narrower size distribution of the resulting product compared to the circular jet.

Piezoelectric Response in Hybrid Micropillar Arrays of Poly(Vinylidene Fluoride) and Reduced Graphene Oxide.

This study was dedicated to the investigation of poly(vinylidene fluoride) (PVDF) micropillar arrays obtained by soft lithography followed by phase inversion at a low temperature. Reduced graphene oxide (rGO) was incorporated into the PVDF as a nucleating filler. The piezoelectric properties of the PVDF-rGO composite micropillars were explored via piezo-response force microscopy (PFM). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) showed that α, ß, and γ phases co-existed in all studied samples, with a predominance of the γ phase. The piezoresponse force microscopy (PFM) data provided the local piezoelectric response of the PVDF micropillars, which exhibited a temperature-induced downward dipole orientation in the pristine PVDF micropillars. The addition of rGO into the PVDF matrix resulted in a change in the preferred polarization direction, and the piezo-response phase angle changed from -120° to 20°-40°. The pristine PVDF and PVDF loaded with 0.1 wt % of rGO after low-temperature quenching were found to possess a piezoelectric response of 86 and 87 pm/V respectively, which are significantly higher than the |d33eff| in the case of imprinted PVDF 64 pm/V. Thus, the addition of rGO significantly affected the domain orientation (polarization) while quenching increased the piezoelectric response.

Effect of substrate orientation on local magnetoelectric coupling in bi-layered multiferroic thin films.

In this study we explore the prospect of strain-mediated magnetoelectric coupling in CoFe2O4-BaTiO3 bi-layers as a function of different interfacial boundary conditions. Pulsed laser deposition fabricated thin films on Nb:SrTiO3(100) and Nb:SrTiO3(111) single crystal substrates were characterized in terms of their peculiarities related to the structure-property relationship. Despite the homogeneous phase formation in both films, transmission electron microscopy showed that the bi-layers on Nb:SrTiO3(100) exhibit a higher number of crystallographic defects when compared to the films on Nb:SrTiO3(111). This signifies an intrinsic relationship of the defects and the substrate orientation. To analyze the consequences of these defects on the overall magnetoelectric coupling of the bi-layered films, piezoresponse force microscopy was performed in situ with an applied magnetic field. The local magnetic field dependence of the piezoresponse was obtained using principal component analysis. A detailed analysis of this dependence led to a conclusion that the bi-layers on Nb:SrTiO3(111) exhibit better strain-transfer characteristics between the magnetic and the piezoelectric layer than those which were deposited on Nb:SrTiO3(100). These strain transfer characteristics correlate well with the interface quality and the defect concentration. This study suggests that in terms of overall magnetoelectric coupling, the Nb:SrTiO3(111) grown bi-layers are expected to outperform their Nb:SrTiO3(100) grown counterparts.

Quasi-adiabatic calorimeter for direct electrocaloric measurements.

The electrocaloric effect (ECE) in ferroelectric materials is a promising candidate for small, effective, low cost, and environmentally friendly solid state cooling applications. Instead of the commonly used indirect estimates based on Maxwell's relations, direct measurements of the ECE are required to obtain reliable values. In this work, we report on a custom-made quasi-adiabatic calorimeter for direct ECE measurements. The ECE is measured for two promising lead-free materials: Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics. Adiabatic temperature changes of ΔTEC = 0.5 K at 355 K and ΔTEC = 0.3 K at 314 K were achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The quasi-adiabatic ECE measurements reliably match other direct EC measurements using a differential scanning calorimeter or an infrared camera. The data are compared to indirect EC estimations based on Maxwell's relations and show that the indirect measurements typically underestimate the effect to a certain degree.

Stress induced magnetic-domain evolution in magnetoelectric composites.

Local observation of the stress mediated magnetoelectric (ME) effect in composites has gained a great deal of interest over the last decades. However, there is an apparent lack of rigorous methods for a quantitative characterization of the ME effect at the local scale, especially in polycrystalline microstructures. In the present work, we address this issue by locally probing the surface magnetic state of barium titante-hexagonal barium ferrite (BaTiO3-BaFe12O19) ceramic composites using magnetic force microscopy (MFM). The effect of the piezoelectrically induced local stress on the magnetostrictive component (BaFe12O19, BaM) was observed in the form of the evolution of the magnetic domains. The local piezoelectric stress was induced by applying a voltage to the neighboring BaTiO3 grains, using a conductive atomic force microscopy tip. The resulting stochastic evolution of magnetic domains was studied in the context of the induced magnetoelastic anisotropy. In order to overcome the ambiguity in the domain changes observed by MFM, certain generalizations about the observed MFM contrast are put forward, followed by application of an algorithm for extracting the average micromagnetic changes. An average change in domain wall thickness of 50 nm was extracted, giving a lower limit on the corresponding induced magnetoelastic anisotropy energy. Furthermore, we demonstrate that this induced magnetomechanical energy is approximately equal to the K1 magnetocrystalline anisotropy constant of BaM, and compare it with a modeled value of applied elastic energy density. The comparison allowed us to judge the quality of the interfaces in the composite system, by roughly gauging the energy conversion ratio.