Rietveld Refinement in the Characterization of Crystalline Materials

This Special Issue serves as a crystallographic forum covering various aspects of material science that have in common the use of the powerful Rietveld method in the analysis of the powder XRD patterns of investigated compounds

Sol-Gel Synthesis of Ceria-Zirconia-Based High-Entropy Oxides as High-Promotion Catalysts for the Synthesis of 1,2-Diketones from Aldehyde

Efficient Lewis-acid-catalyzed direct conversion of aldehydes to 1,2-diketones in the liquid phase was enabled by using newly designed and developed ceria–zirconia-based high-entropy oxides (HEOs) as the actual catalysts. The synergistic effect of various cations incorporated in the same oxide structure (framework) was partially responsible for the efficiency of multicationic materials compared to the corresponding single-cation oxide forms. Furthermore, a clear, linear relationship between the Lewis acidity and the catalytic activity of the HEOs was observed. Due to the developed strategy, exclusively diketone-selective, recyclable, versatile heterogeneous catalytic transformation of aldehydes can be realized under mild reaction conditions

Functionalization of plasmonic metamaterials utilizing metal-organic framework thin films

We considered theoretically and experimentally a strategy to functionalize plasmonic metamaterials utilizing either a metal-organic framework (MOF) or inorganic-organic hybrids for application in adsorption-based gas sensing. MOFs are one-dimensional (1D), 2D or 3D crystalline compounds that simultaneously contain metal ions or ion clusters and organic moieties, forming thus porous networks ensuring an increased effective surface for adsorption. Metamaterials can enhance plasmonic sensor performance through metal-dielectric nanocompositing that simultaneously tailors the electromagnetic response and boosts adsorption of the targeted analyte through the use of nanopores. To perform functionalization, it is necessary to integrate one or several layers of MOF nanocrystals with the metamaterial scaffold. The simplest approach is to use dip or drop coating or the layer-by-layer technique. The scaffolds that we considered included freestanding, ultrathin membranes and sandwich structures with nanoaperture arrays. For this investigation, we used a non-aqueous sol-gel route to synthesize vanadium oxyanthracene carboxylate, a novel material with 1D crystal structure. Our results suggest that preferential concentration of analyte within the MOF pores may ensure improved adsorption and thus sensor sensitivity enhancement. Also, one may increase selectivity by introducing nanoparticle fillers or by utilizing other functionalizing materials such as catalysts or ligands

Mesoporous MgTa2O6 thin films with enhanced photocatalytic activity: On the interplay between crystallinity and mesostructure

Ordered mesoporous, crystalline MgTa2O6 thin films with a mesoscopic nanoarchitecture were synthesized by evaporation-induced self-assembly (EISA) in combination with a sol–gel procedure. Utilization of novel templates, namely the block copolymers KLE (poly(ethylene-co-butylene)-b-poly(ethylene oxide)) and PIB6000 (CH3C(CH3)2(CH2C(CH3)2)107CH2C(CH3)2C6H4O(CH2CH2O)100H), was the key to achieving a stable ordered mesoporous structure even upon crystallization of MgTa2O6 within the mesopore walls. The effect of the calcination temperature on the ability of the mesoporous films to assist the photodegradation of rhodamine B in water was studied. As a result, two maxima in the photocatalytic activity were identified in the calcination temperature range of 550–850 °C, peaking at 700 °C and 790 °C, and the origin of this was investigated by using temperature-dependent X-ray scattering. Optimal activity was obtained when the mesoporous film was heated to 790 °C; at this temperature, crystallinity was significantly high, with MgTa2O6 nanocrystals of 1.6 nm in size (averaged over all reflections), and an ordered mesoporous structure was maintained. When considering the turnover frequency of such photocatalysts, the optimized activity of the present nanoarchitectured MgTa2O6 thin film was ca. four times that of analogous anatase TiO2 films with ordered mesopores. Our study demonstrated that high crystallinity and well-developed mesoporosity have to be achieved in order to optimize the physicochemical performance of mesoporous metal-oxide films

Large-Pore Mesoporous Ho3Fe5O12Ho_{3}Fe_{5}O_{12} Thin Films with a Strong Room-Temperature Perpendicular Magnetic Anisotropy by Sol–Gel Processing

We report the evaporation-induced self-assembly synthesis of large-pore mesoporous thin films of ferrimagnetic holmium iron garnet($Ho_3Fe_5O_{12}$) from nitrate salt precursors and a polyisobutylene-blockpoly(ethylene oxide) polymer structure-directing agent. The phase composition, atomic bonding configuration, and pore structure of the top surface and the interior of the films were investigated by microscopy, scattering, and spectroscopy techniques, including synchrotron-based grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, X-ray diffraction (including Rietveld refinement), and others. The data provide evidence that the sol−gelderived material is single-phase garnet with 27 nm diameter crystallites and few defects after being heated to 850 °C in air, and the continuous network of pores averaging 23 nm in diameter is preserved to a large extent, despite a solid−solid conversion from metastable h-HoFeO$_3$ to $Ho_3Fe_5O_{12}$ during the crystallization process. Furthermore, dc magnetometry measurements show the thin films are magnetically stable with a room-temperature coercivity of ∼170 Oe and exhibit an out-of-plane easy axis with a significant perpendicular magnetic anisotropy. A strong preference for out-of-plane magnetic alignment in solution-processed mesostructured films is unique, making them attractive for application in spintronics and nanomagnetics

Triple perovskite-based triboelectric nanogenerator: a facile method of energy harvesting and self-powered information generator

A modified aqueous sol-gel reaction is used to synthesize a triple perovskite with a composition Sr3Co2WO9 (SCWO). The structural analysis conducted at room temperature reveals the presence of the cubic phase with space group Fm-3m. The microstructure of the as-synthesized SCWO particles shows a different size of particles depicting a polycrystalline nature. Triboelectrification is a trending and unique concept in energy scavenging methodologies with flexibility in choosing from a variety of materials. This paves the way to evolve eco-friendly triboelectric energy harvesters as a replacement for the limitation of batteries. The outstanding dielectric properties and low loss make triple perovskite a promising candidate for TENG. A triboelectric nanogenerator (TP-TENG) was designed, which operates in vertical contact separation mode generating an electrical response of voltage and current of 300 V and 2.2 μA, respectively, for 10 wt% polydimethylsiloxane-Sr3Co2WO9 (PDMS-SCWO) composite film. The power density of the rough surface TP-TENG was 30.5 μW/cm2, which is much higher than the power density of 5.5 μW/cm2 of plain surface TP-TENG. The positive and negative triboelectric layers of the TP-TENG were made up of aluminum and PDMS-SCWO composite film, respectively. Furthermore, the excellent flexibility and durability of TP-TENG make it suitable for sensing various gaits and information signaling (Morse code) in real-time applications, as well as for charging commercial capacitors. © 2021 Elsevier Ltd1