The contrasting effect of the Ta/Nb ratio in (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 crystals on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The effect of the Ta/Nb ratio in the (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 (0 <= x <= 4) crystals grown by a KCl flux method on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives BaNb1-xTaxO2N (0 <= x <= 1) was investigated. The Rietveld refinement of X-ray data revealed that all Ba5Nb4-xTaxO15 samples were well crystallized in the space group P (3) over bar m1 (no. 164). Phase-pure BaNb1-xTaxO2N (0 <= x <= 1) porous structures were obtained by nitridation of the flux-grown oxide crystals at 950 degrees C for 20, 25, 30, 35, and 40 h, respectively. The absorption edge of BaNb1-xTaxO2N (0 <= x <= 1) was slightly shifted from 720 to 690 nm with the increasing Ta/Nb ratio. The O-2 evolution rate gradually progressed and reached the highest value (127.24 mu mol in the first 2 h) with the Ta content up to 50 mol% but decreased at 75 and 100 mol% presumably due to the reduced specific surface area and high density of structural defects, such as grain boundaries acting as recombination centers, originated from high-temperature nitridation for prolonged periods. Transient absorption spectroscopy provided evidence for the effect of the Ta/Nb ratio on the behavior and energy states of photogenerated charge carriers, indicating a direct correlation with photocatalytic water oxidation activity of BaNb1-xTaxO2N

Hydrophobicity and polymer compatibility of POSS-modified Wyoming Na-montmorillonite for developing polymer-clay nanocomposites

The aim of the present work was to investigate the hydrophobicity and polymer compatibility of aminopropylisooctyl polyhedral oligomeric silsequioxane (POSS) - modified Na-montmorillonite (Na-MMT) towards developing polymer-clay nanocomposites. The effect of different concentrations of POSS on properties of Na-MMT was studied. The intercalation ability of the POSS molecules into the Na-MMT interlayer was analyzed by X-ray diffraction. It was found that the d(001) value was increased with increasing the POSS concentration, indicating the successful intercalation of the POSS molecules into the Na-MMT interlayer. The d(001) value was 4.12 nm at 0.4 cation exchange capacity (CEC) loading of POSS, increased at a slight rate upon further increase of CEC loading, and finally reached 4.25 nm at 1.0 CEC loading of POSS. The results of the thermogravimetric (TGA) analysis confirmed the high thermal stability of the POSS-MMT. The thermal stability was defined as a 5% mass loss (T-5) at 0.2 CEC loading of POSS was observed at 352 degrees C and slightly decreased with further increase in the POSS concentration. The porous properties, such as specific surface area (SSA), pore volume, and pore size were estimated by the adsorption of N-2 molecules on the Na-MMT surface. The SSA and pore volume were reduced with increasing the concentration of the POSS molecules due to the adsorption of the POSS molecules on the Na-MMT, while the pore size was increased upon the formation of macroporous structure. The interfacial interaction energy between water and POSS-MMT (Delta G(Clay/Water/Clay)(IF)) was used to evaluate the surface hydrophobicity, and a similar approach was also applied to assess the polymer compatibility of the developed composite. The obtained results confirm that the polymer compatibility of POSS-MMT prepared in this study is better than that of commonly used HDTMA-MMT.Türkiye Bilim, Sanayi ve Teknoloji Bakanlığı,SAN-TEZ program - 00492.STZ.2009-

Hydrothermal-induced growth of Ca10V6O25 crystals with various morphologies in a strong basic medium at different temperatures

The Ca10V6O25 crystals with various morphologies were synthesized by a facile hydrothermal method in a strong basic medium. The effects of the pH of synthesizing solution, hydrothermal reaction temperature and time on the morphology and crystallinity of Ca10V6O25 powders were investigated. The as-synthesized powders were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–vis spectrophotometer and vibrating sample magnetometer. The experimental results demonstrate that phase-pure Ca10V6O25 powders could be hydrothermally obtained at temperatures ranging from 120 °C to 180 °C for 12–48 h with the pH ≥ 12.5 of synthesizing solution. The morphology of Ca10V6O25 powders was strongly dependent on the synthesis parameters, such as pH, temperature and time. The Ca10V6O25 microspheres were obtained at 180 °C for 48 h with the pH = 12.5 of synthesizing solution. The UV–vis diffuse reflectance spectra have shown that the Ca10V6O25 powders efficiently absorb UV light with an absorption edge at about 380 nm. All the samples noticeably exhibit a superparamagnetic behavior with a nearly zero magnetic remanence (remanent magnetization). The hydrothermally synthesized Ca10V6O25 powders may be employed as a potential candidate in novel electronic and biomedical applications

Time-retrenched synthesis of BaTaO2N by localizing an NH3 delivery system for visible-light-driven photoelectrochemical water oxidation at neutral pH: Solid-state reaction or flux method?

Among 600 nm class transition-metal oxynitrides, BaTaO2N with a cubic Pm3̅m perovskite-type structure is promising for solar water oxidation due to its absorption of visible light up to 660 nm, narrower band gap (Eg = 1.9 eV), appropriate valence band edge position for oxygen evolution, good stability in concentrated alkaline solutions, and nontoxicity. However, high defect density stemmed from long high-temperature ammonolysis limits the separation and transfer efficiency of photogenerated charge carriers in BaTaO2N. Here, a NH3 delivery system is specifically localized just above the synthesis mixture to reduce the synthesis time and defect density of BaTaO2N by a fresh supply of more active nitriding species and minimizing the generation of N2 and H2. Particularly, the effects of synthesis temperature (700–950 °C), synthesis time (1–8 h), and gas composition are systematically investigated to gain insights into the formation of single-phase BaTaO2N by solid-state reaction and flux method. Time-dependent experiments conducted at 950 °C show that single-phase BaTaO2N can be synthesized ≥6 and ≥4 h by solid-state reaction and flux method, respectively, revealing the advantage of the flux method over solid-state reaction in a localized NH3 delivery system. Subsequently, the separation and transfer efficiency and kinetics of photogenerated charge carriers are studied in BaTaO2N samples. Photoelectrochemical studies made it possible to resolve trends during visible-light-induced water oxidation, evidencing the inverse relationship between recombination and charge transfer phenomena. Transient absorption spectroscopy reveals that the dynamics of the photogenerated charge carriers in both types of BaTaO2N samples are different: (i) BaTaO2N synthesized by flux method has a greater number of holes despite the similar number of deeply trapped charge carriers and (ii) solid-state reaction led to the formation of a higher number of free electrons in BaTaO2N. The findings demonstrate the advantage of reducing the transfer distance of active nitriding species to the surface of the synthesis mixture for enhancing the photoelectrochemical water oxidation of BaTaO2N at neutral pH.EC/H2020/793882/EU/Carbon-Oxynitride Coupled Artificial Photosynthesis System For Solar Water Splitting Beyond 600 nm/H2O-SPLI

Time-Retrenched Synthesis of BaTaO2N by Localizing an NH3 Delivery System for Visible-Light-Driven Photoelectrochemical Water Oxidation at Neutral pH: Solid-State Reaction or Flux Method?

Among 600 nm class transition-metal oxynitrides, BaTaO2N with a cubic Pm3¯ m perovskite-type structure is promising for solar water oxidation due to its absorption of visible light up to 660 nm, narrower band gap (Eg = 1.9 eV), appropriate valence band edge position for oxygen evolution, good stability in concentrated alkaline solutions, and nontoxicity. However, high defect density stemmed from long high-temperature ammonolysis limits the separation and transfer efficiency of photogenerated charge carriers in BaTaO2N. Here, a NH3 delivery system is specifically localized just above the synthesis mixture to reduce the synthesis time and defect density of BaTaO2N by a fresh supply of more active nitriding species and minimizing the generation of N2 and H2. Particularly, the effects of synthesis temperature (700-950 °C), synthesis time (1-8 h), and gas composition are systematically investigated to gain insights into the formation of single-phase BaTaO2N by solid-state reaction and flux method. Time-dependent experiments conducted at 950 °C show that single-phase BaTaO2N can be synthesized ≥6 and ≥4 h by solid-state reaction and flux method, respectively, revealing the advantage of the flux method over solid-state reaction in a localized NH3 delivery system. Subsequently, the separation and transfer efficiency and kinetics of photogenerated charge carriers are studied in BaTaO2N samples. Photoelectrochemical studies made it possible to resolve trends during visible-light-induced water oxidation, evidencing the inverse relationship between recombination and charge transfer phenomena. Transient absorption spectroscopy reveals that the dynamics of the photogenerated charge carriers in both types of BaTaO2N samples are different: (i) BaTaO2N synthesized by flux method has a greater number of holes despite the similar number of deeply trapped charge carriers and (ii) solid-state reaction led to the formation of a higher number of free electrons in BaTaO2N. The findings demonstrate the advantage of reducing the transfer distance of active nitriding species to the surface of the synthesis mixture for enhancing the photoelectrochemical water oxidation of BaTaO2N at neutral pH.Fil: Hojamberdiev, Mirabbos. Technishe Universitat Berlin; AlemaniaFil: Mora Hernandez, J. Manuel. Consejo Nacional de Ciencia y Tecnología; MéxicoFil: Vargas Balda, Ronald Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas; ArgentinaFil: Yamakata, Akira. Toyota Technological Institute; JapónFil: Yubuta, Kunio. Tohoku University; Japón. Kyushu University; JapónFil: Heppke, Eva Maria. Technishe Universitat Berlin; AlemaniaFil: Torres Martínez, Leticia M.. Centro de Investigacion En Materiales Avanzados; México. Universidad Autónoma de Nuevo León; MéxicoFil: Teshima, Katsuya. Shinshu University; JapónFil: Lerch, Martin. Technishe Universitat Berlin; Alemani

NH₃‑Assisted Flux Growth of Cube-like BaTaO₂N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity

As the 600 nm-class photocatalyst, BaTaO₂N is one of the promising candidates of the perovskite-type oxynitride family for photocatalytic water splitting under visible light. The oxynitrides are routinely synthesized by nitriding corresponding oxide precursors under a high-temperature NH₃ atmosphere, causing an increase in the defect density and a decrease in photocatalytic activity. To improve the photocatalytic activity by reducing the defect density and improving the crystallinity, we here demonstrate an NH₃-assisted KCl flux growth approach for the direct synthesis of BaTaO₂N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO₂N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO₂N crystals could only be grown with the solute concentrations of ≥10 mol % using the KCl flux, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO₂N crystals with an average size of about 125 nm and exposed {100} and {110} faces at 950 °C for 10 h. The time- and temperature-dependent experiments were also performed to postulate the direct growth mechanisms of cube-like BaTaO₂N submicron crystals. The BaTaO₂N crystals modified with Pt and CoO_<i>x</i> nanoparticles showed a reasonable H₂ and O₂ evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH₃-assisted KCl flux method

NH₃‑Assisted Flux-Mediated Direct Growth of LaTiO₂N Crystallites for Visible-Light-Induced Water Splitting

Photocatalytic overall water splitting on (oxy)­nitrides under visible light is one of the interesting approaches to fulfill the growing demand for clean and renewable energy. The improvement of the fabrication method is however important for reducing the defect density of (oxy)­nitride crystals. The present study aims to investigate the direct growth of the LaTiO₂N (LTON) crystallites with less defect density by an NH₃-assisted flux method and to demonstrate the visible-light-induced photocatalytic water oxidation activity in relation to their crystallite morphology. Single-phase LaTiO₂N crystallites (average size of 120 ± 39 nm) in round shape with smooth surface and high crystallinity were grown by an NH₃-assisted flux method using the KCl flux with the solute concentration of 5 mol % at 950 °C for 10 h. The photocatalytic water oxidation activity of bare and CoO_<i>x</i>-loaded LaTiO₂N crystallites grown directly by an NH₃-assisted flux method (1-step-LTON) was evaluated under visible light by comparing with the LaTiO₂N crystallites fabricated by a two-step method (2-step-LTON), converting La₂Ti₂O₇ to LaTiO₂N by high-temperature nitridation. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O₂ evolution rates of bare and CoO_<i>x</i>-loaded 1-step-LTON crystallites were 82 μmol·h^–1 and 204 μmol·h^–1, respectively, which are much higher than that of bare and CoO_<i>x</i>-loaded 2-step-LTON crystallites (37 μmol·h^–1 and 177 μmol·h^–1) due to less defect density of the LaTiO₂N crystallites achieved by a direct fabrication route using KCl flux. An NH₃-assisted flux growth is a promising route for the direct fabrication of the LaTiO₂N crystallites with less defect density that is beneficial for the enhancement of photocatalytic water oxidation half-reaction