Ni-Al layered double hydroxides modified with citrate, malate, and tartrate: Preparation by coprecipitation and uptake of Cu from aqueous solution

We report on aqueous Cu²⁺ uptake by Ni-Al layered double hydroxides (Ni-Al LDHs) modified with citrate (C₆H₅O₇³⁻), malate (C₄H₄O₅²⁻), and tartrate (C₄H₄O₆²⁻) anions via coprecipitation. Dropwise addition of a mixed aqueous solution of Ni(NO₃)₂ and Al(NO₃)₃ to the respective organic acid solutions at a constant pH of 7.0-9.0 afforded LDHs with intercalated C₆H₅O₇³⁻ and Ni(C₆H₅O₇⁾⁻, C₄H₄O₅²⁻, and C₄H₄O₆²⁻ in their interlayers. The anions were also likely adsorbed on the LDH surface. Citrate·Ni-Al LDH could rapidly take up Cu²⁺ at a constant pH of 5.0, mainly via chelation by the intercalated and adsorbed anions, rather than coprecipitation with dissolved Al³⁺ to form Cu-Al LDH. By contrast, malate and tartrate were not active as chelating agents, probably because they formed bridges between brucite-like layers by direct coordination of the two −COO⁻ groups with Al³⁺ in those layers.     

Effects of protons and acceptor substitution on the electrical conductivity of La6WO12

Transport properties and non-stoichiometry of La_1−xCa_xW_1/6O₂ and La_1−yW_1/6O₂ (x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H⁺/D⁺ isotope exchange, and thermogravimetry in the temperature range 300-1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW_1/6O₂ with values reaching 3×10⁻³ S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.     

Intralamellar structural modifications related to the proton exchanging in K4Nb6O17 layered phase

Basic structural aspects about the layered hexaniobate of K₄Nb₆O₁₇ composition and its proton-exchanged form were investigated mainly by spectroscopic techniques. Raman spectra of hydrous K₄Nb₆O₁₇ and H₂K₂Nb₆O₁₇·H₂O show significant modifications in the 950-800 cm⁻¹ region (Nb-O stretching mode of highly distorted NbO₆ octahedra). The band at 900 cm⁻¹ shifts to 940 cm⁻¹ after the replacement of K⁺ ion by proton. Raman spectra of the original materials and the related deuterated samples are similar suggesting that no isotopic effect occurs. Major modifications were observed when H₂K₂Nb₆O₁₇ was dehydrated: the relative intensity of the band at 940 cm⁻¹ decreases and new bands seems to be present at about 860-890 cm⁻¹. The H⁺ ions should be shielded by the hydration sphere what preclude the interaction with the layers. Removing the water molecules, H⁺ ions can establish a strong interaction with oxygen atoms, decreasing the bond order of Nb-O linkage. X-ray absorption near edge structure studies performed at Nb K-edge indicate that the niobium coordination number and oxidation state remain identical after the replacement of potassium by proton. From the refinement of the fine structure, it appears that the Nb-Nb coordination shell is divided into two main contributions of about 0.33 and 0.39 nm, and interestingly the population, i.e., the number of backscattering atoms is inversed between the two hexaniobate materials.     

Reactivity and XAFS study on (1−x)CeO2-xNiO (x=0.025-0.3) system in the two-step water-splitting reaction for solar H2 production

The redox reaction of Ce⁴⁺-Ce³⁺ promoted by the catalytic function of nickel ions in a (1−x)CeO₂-xNiO solid solution was investigated for solar H₂ production by the two-step water-splitting reaction. By irradiation using an infrared imaging lamp as a solar simulator, the O₂-releasing reaction with (1−x)CeO₂-xNiO solid solution proceeded at 1673-1873 K, and its reduced form was produced. The amounts of H₂ gas evolved by the reduced form were 1.2-2.5 cm³/g and the evolved gases amounts ratio of H₂/O₂ was nearly 2, which is equal to the stoichiometric value of the water-splitting reaction (H₂O=H₂+1/2O₂). The maximum amounts of evolved H₂ and O₂ gases were obtained at the Ce:Ni mole ratio of 0.95:0.05 (x=0.05) in the (1−x)CeO₂-xNiO system. The X-ray absorption fine structure (XAFS) measurement showed that the O₂-releasing and H₂-generation reactions with (1−x)CeO₂-xNiO solid solution were repeatable with the redox system of Ce⁴⁺-Ce³⁺, which was enhanced by the catalytic function of Ni²⁺-Ni⁰.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

Structural characterization of nanocrystalline CrOOH·2H2O aerogel by X-ray diffraction

The nanostructured powder prepared by critical CO₂ extraction of the urea-assisted wet chromia gel mixture at 373 K in vacuum was studied by X-ray diffraction techniques. Thermoanalytical methods showed the presence of the lattice water molecules in the resulting phase corresponding to a chemical formula CrOOH·2H₂O. The CrOOH·2H₂O nanocrystals of 3-5 nm in diameter were observed in transmission electron microscopy and their structure was derived from the Rietveld analysis in which the disorder contribution to the X-ray scattering was implemented. The structural model shows that the hexagonal unit cell of α-CrOOH undergoes monoclinic distortion with half of the O⁻² anions and OH⁻ groups being replaced by bonded water molecules in the three-dimensional packing resulting in half of the sites in regular Cr⁺³ octahedra being vacant. Further examination of the quasi-crystalline disordered state of the CrOOH·2(H₂O) nanocrystals was performed by model independent method of Radial Distribution Function (RDF). This complementary technique is sensitive to the molecular composition and allows to assess the average atomic (or electron) density distribution and the spacings of the atomic arrangements in the nearest neighbor shells comprising the range of the crystalline order in the structure of this material.     

Synthesis of modified vermiculite by interaction with aromatic heterocyclic amines

Vermiculite of general formula [Si_6.85Al_1.15][Mg_4.68Al_0.51Fe_0.63]O₂₀(OH)₄Ca_0.128Na_0.032K_0.094 reacted with heteroaromatic amines α-, β-, and γ-picolines from aqueous solution. The products were characterized by elemental analysis, infrared spectroscopy, and X-ray diffraction. The intercalated nanocompounds maintained the crystallinity and changed the original interlayer distance of 1422 pm to 1474, 1456, and 1474 pm, for the sequence of the guest picoline molecules. Natural and intercalated vermiculite can remove copper at the solid/liquid interface; removal 0.40 mmol g⁻¹ was obtained for the original matrix, and 1.10, 0.92, and 1.33 mmol g⁻¹ for the intercalated forms. These values are near the capacity of cation exchange (CEC) of this clay mineral, which can be possibly used as source of copper removal from aqueous solution.     

Cation disorder in NaW2O6+δ·nH2−zO post-ion exchange with K, Rb, Sr, and Cs

The structure of the defect pyrochlore NaW₂O_6+δ·nH_2−zO after ion exchange with K, Rb, Sr or Cs for Na has been investigated using thermal analysis, solid-state nuclear magnetic resonance, laboratory X-ray and neutron diffraction methods. Neutron diffraction studies show that both the A-type cations (Na⁺, K⁺, Rb⁺, and/or Cs⁺) and the water molecules reside within the channels that form in the 111 direction of the W₂O₆ framework and that these strongly interact. The analytical results suggest that the water and A-type cations compete for space in the tunnels within the W₂O₆ pyrochlore framework, with the total number of water molecules and cations being approximately constant in the six samples investigated. The interplay between the cations and water explains the non-linear dependence of the a lattice parameter on the choice of cation. It appears that the ion-exchange capacity of the material will be controlled by the amount of water initially present in the sample.     

Growth and characterization of two new NLO materials from the amino acid family: l-Histidine nitrate and l-Cysteine tartrate monohydrate

Single crystals of organic nonlinear optical (NLO) materials l-Histidine nitrate (C₆H₁₀N₃O₂)⁺ · (NO₃)⁻ and l-Cysteine tartrate monohydrate (C₃H₈NO₂S)⁺ · (C₄H₅O₆)⁻ · H₂O were grown by submerged seed solution method. Characterization of the crystals was made using single crystal X-ray diffraction. Fourier transform infrared (FTIR) spectroscopic studies, optical behaviour such as UV-visible-NIR absorption spectra and second harmonic generation (SHG) conversion efficiency were investigated to explore the NLO characteristics of the above materials. Microhardness measurements and dielectric studies of the compounds were also carried out.     

Sorption study of an anionic dye - benzopurpurine 4B - on calcined and uncalcined Mg-Al layered double hydroxides

In this paper, the elimination from aqueous solution of the pollutant benzopurpurine 4B by Mg-Al-CO₃ hydrotalcite and its modified form Mg-Al-500 has been studied. Mg-Al-CO₃²⁻ layered double hydroxide with an Mg/Al molar ratio of 2.0 was synthesized by the co-precipitation method, and its modification was carried out by calcination. The affinity of these materials with a mixture of benzopurpurine 4B was studied as a function of dye-adsorbent contact time, initial pH of the solution, initial dye concentration and temperature. The results indicate that HDLs were effective in removing benzopurpurine 4B anionic dye. Their saturated adsorption capacities are very high, particularly for the calcined material Mg-Al-500 compared to the standard one Mg-Al-CO₃²⁻.The characterisation of the solid Mg-Al-500, both fresh and after removal of the dye by X-ray diffraction and infrared spectroscopy, shows that the benzopurpurine 4B adsorption on this calcined phase is enhanced by reconstruction of a matrix hydrotalcite intercalated by the dye, with basal spacing of 23.77 Å, which is larger than that of Mg-Al-CO₃²⁻ LDHs (7.57 Å).     

Effect of iron doping on the characterization and transport properties of calcium phosphate glassy semiconductors

X-ray diffraction (XRD), differential scanning calorimeter (DSC), density (d) and dc conductivity (σ) of the glasses in Fe₂O₃-CaO-P₂O₅ system were reported. The dc conductivity in the temperature range 303-453 K was measured. The overall features of these XRD curves confirm the amorphous nature of the present samples. The density of glasses increases from 2.750 to 2.892 g/cm³ with increasing Fe₂O₃ content as a result of a strengthening of cross-linking within glass network. The glass temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The glasses had conductivities ranging from 10⁻⁹ to 10⁻⁵ Sm⁻¹ at temperatures from 303 to 453 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Effect of anions on the phase stability of γ-FeOOH nanoparticles and the magnetic properties of gamma-ferric oxide derived from lepidocrocite

The effect of anions such as Cl⁻, SO₄²⁻, and HPO₄²⁻ on the phase stability of FeOOH (α or γ) during precipitation is investigated. Oxidation of Fe(OH)₂·xH₂O from FeCl₂ solution with high Cl⁻ concentration ([Cl⁻]/[Fe]=R_Cl≥8) or (NH₄)₂Fe(SO₄)₂ (FAS) with [HPO₄²⁻]/[Fe]=R_P≥0.02 yields phase-pure γ-FeOOH. In the medium ranges of R_Cl and R_P, mixed phases of α-FeOOH and γ-FeOOH are obtained. Replacement of OH⁻ by Cl⁻ with the bridging cations or strongly bonded HPO₄²⁻ ions in the matrix of the intermediate phase (Fe_x²⁺Fe_y³⁺(OH)_2x+2y−nz·xH₂O(A)_zⁿ⁻, where A is anions such as Cl⁻, SO₄²⁻, HPO₄²⁻, etc.), promoted the lower density γ-FeOOH. However, the particles are less developed and have poor crystallinity as evidenced from transmission electron microscope and thermogravimetry-differential thermal analysis of the precipitates. Whereas, monophasic, uniformly sized, nano-lath shaped particles with high aspect ratio >10 are obtained when morphology-controlling cation additives such as Pt⁴⁺, Pd²⁺ or Rh³⁺ are present in FeCl₂ (R_Cl≥8) solution. Preferential adsorption of additives on (0k0) and (h00) planes limits the growth in the perpendicular directions leading to high aspect ratios. The effect of these additives are suppressed by the phosphate ion, a strong complexing ligand, giving rise to fibrous aggregate with the length of individual particles as small as 10-30 nm. While most of the Cl⁻ ion is removed from the final precipitates on washing, phosphate remained as HPO₄²⁻ as evidenced from IR absorption spectra. Maghemite obtained by dehydroxylating γ-FeOOH contains randomly distributed micropores bringing in the relaxation effects of spins on the surface atoms as deciphered from Mössbauer spectroscopy. This leads to the low σ_s (44-48 emu/g) and H_c (120-130 Oe) for γ-Fe₂O_3−δ particles. Whereas nearly pore-free single crystalline particles obtained by reduction followed by reoxidation has high value of σ_s (73 emu/g) and H_c (320 Oe), which decreases to 30 emu/g and 75 Oe, respectively, for nanoparticles obtained from phosphate stabilized lepidocrocite. The mobility of iron ions and counter mobility of vacancies during the topotactic transformation of γ-FeOOH to magnetite to γ-Fe₂O_3−δ renders the particles pore-free.     

Photo-catalytic H2 evolution over a series of Keggin-structure heteropoly blue sensitized Pt/TiO2 under visible light irradiation

We report the visible light-induced hydrogen generation over a series of Keggin-structure heteropoly blue (HPB) anions (PW₁₂O₄₀³⁻, phosphotungstic blue (PTB), GeW₁₂O₄₀⁴⁻ (GTB), SiW₁₂O₄₀⁴⁻ (STB), BW₁₂O₄₀⁵⁻ (BTB)) sensitized Pt/TiO₂ photo-catalysts. The sensitization of TiO₂ by HPB was certified using photo-electrochemical measurements and UV-vis absorption spectra. PTB showed the most pronounced sensitization effect for TiO₂ in those HPB anions and Pt/TiO₂-PTB showed the highest hydrogen generation activity. The sensitization of TiO₂ was significantly dependent on the reduction potential of HPA, which was determined by the kind of central atom in HPA.     

Synthesis and utilization of Mg/Al hydrotalcite for removing dissolved humic acid

It has been synthesized Mg/Al layered double hydroxide anionic clay (Mg/Al hydrotalcite) through direct precipitation by adding 0.5 M NaOH solution into a mixed solution containing Mg(NO₃)₂ and Al(NO₃)₃ with molar ratio of 0.1:0.05 until the medium acidity reached pH 10.1. The synthesized Mg/Al hydrotalcite was then utilized to remove dissolved humic acid in aqueous medium. The humic acid was isolated from peat soil taken in Gambut District, South Kalimantan, Indonesia using the recommended procedure of IHSS (International Humic Substances Society). The removal of humic acid was mostly occurred through simple sorption process without accompanied by significant intercalation. The sorption was optimum at pH 9.0, with the first order rate constant, capacity and energy of sorption were 5.50 × 10⁻³ min⁻¹, 0.12 mmol g⁻¹ (69 mg g⁻¹), and 28.32 kJ mol⁻¹, respectively.     

Synthesis and characterization of Co7(OH)12(C2H4S2O6)(H2O)2—a single crystal structural study of a ferrimagnetic layered cobalt hydroxide

A novel layered hydrotalcite-like material, Co₇(H₂O)₂(OH)₁₂(C₂H₄S₂O₆), has been prepared hydrothermally and the structure determined using single crystal X-ray diffraction (a=6.2752(19) Å, b=8.361(3) Å, c=9.642(3) Å, α=96.613(5)°, β=98.230(5)°, γ=100.673(5)°, R1=0.0551). The structure consists of brucite-like sheets where 1/6 of the octahedral sites are replaced by two tetrahedrally coordinated Co(II) above and below the plane of the layer. Ethanedisulfonate anions occupy the space between layers and provide charge balance for the positively charged layers. The compound is ferrimagnetic, with a Curie temperature of 33 K, Curie-Weiss θ of −31 K, and a coercive field of 881 Oe at 5 K.     

Room temperature fabrication Oxide TFT with Y2O3 as a gate oxide and Mo contact

In this investigation, an operating voltage as low as 5 V has been achieved for Oxide TFT with Y₂O₃ as a gate oxide and a-IGZO as an active layer. The OTFT has been fabricated at room temperature using RF sputter. The mobility and threshold voltages are 11.3 cm²/V s and 3.4 V for the device with W/L = 0.8, respectively. The annealing at 400 °C in N₂ containing 5% H₂ ambient has been utilized to improve the electrical performance of TFT. The on-off current which is determined by gate dielectric has been observed to be 10⁴. It has also been observed that the dielectric properties of gate oxide deteriorate on annealing. The dielectric constant of Y₂O₃ is observed in the range between 5.1 and 5.4 measured on various devices.     

Infrared-to-red upconversion luminescence in samarium-doped antimony glasses

A new antimony-based glass system (K₂O-B₂O₃-Sb₂O₃) having low phonon energy (about 600 cm⁻¹) doped with Sm³⁺ ions has been developed. Infrared reflection spectroscopic (IRRS) studies have been employed to establish its low phonon energy. Ultraviolet-Visible-near infrared (UV-Vis-NIR) absorption and photoluminescence upconversion properties with the spectrochemistry of the 15K₂O-15B₂O₃-70Sb₂O₃ (mol%) glasses have been studied doping with different concentrations (0.1-1.0 wt%) of Sm₂O₃. UV-Vis-NIR absorption band positions have been justified with quantitative calculation of nephelauxetic parameter and covalent bonding characteristics of the host. NIR to visible upconversion has been investigated by exciting at 949 nm at room temperature. Three upconverted bands originating from the ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 transitions are found to be centered at 566 (green, weak), 602 (orange, weak) and 636 (red, remarkably strong) nm, respectively. These bands have been explained from the evaluation of the absorption, normal (downconversion) fluorescence and excitation spectra. The upconversion processes have been explained by the excited state absorption (ESA), energy transfer (ET) and cross-relaxation (CR) mechanisms involving population of the metastable (storage) energy level (⁴G_5/2) by multiphonon deexcitation effect. It is evident from the IRRS study that the upconversion phenomena are expedited by the low multiphonon relaxation rate in antimony glasses owing to their low phonon energy (602 cm⁻¹, the main and highest intensity Sb-O-Sb stretching band) which is very close to that of fluoride glasses (500-600 cm⁻¹).     

Microstructural and electrochromic properties of tungsten oxide thin films produced by surfactant mediated electrodeposition

By electrochemically controlling the structure of the surface aggregates, the grain microstructure has been optimized to yield mesoporous thin films of tungsten oxide (WO₃) at the electrode-electrolyte interface in a peroxotungstate sol in the presence of a structure-directing agent (Triton) at room temperature. Apart from the dominant ultrafine nanocrystallites and pores (5-10 nm), well-developed abutting grains (25-100 nm) and nanofibrils also constitute an integral part of the film matrix. X-ray photoemission spectra reveal the as-deposited film (WO_3−x) to be constituted by a high proportion of W⁶⁺ states with a low oxygen deficiency (x = 0.02). A relatively high W⁵⁺ content in the film, upon intercalation of 18 mC cm⁻² charge translates into a large coloring efficiency (η_VIS ∼ 70 cm² C⁻¹) and transmission modulation. At a lithium intercalation level of 22 mC cm⁻², in addition to W⁵⁺ and W⁶⁺ states, the film also comprises of W⁴⁺ states. The extremely fast color-bleach kinetics (3 and 2 s, respectively, for a 50% change in transmittance) shown by the as-deposited WO₃ film are repercussions of the mesopore morphology, the multiple nanostructures and the sixfold channels of its hexagonal modification. The film shows a high cycling stability as the switching times do not show any significant decline even after 3500 repetitive cycles. Coloration efficiency over the solar and photopic regions and current density for lithium intercalation for the as-deposited film are superior to that observed for the films annealed at 100, 250 and 500 °C. The abysmal electrochromic response of the annealed films is a consequence of surface defects like cracks and uncontrolled densification and pore shrinkage.     

Studies on the origins of the absorption spectra in PbWO4 crystal

The electronic structures and absorption spectra for both the perfect PbWO₄ (PWO) crystal and the three types of PWO crystals, containing V_Pb²⁻, V_O²⁺ and a pair of V_Pb²⁻-V_O²⁺, respectively, have been calculated using CASTEP codes with the lattice structure optimized. The calculated absorption spectra indicate that the perfect PWO crystal does not occur absorption band in the visible and near-ultraviolet region. The absorption spectra of the PWO crystal containing V_Pb²⁻ exhibit seven peaks located at 1.72 eV (720 nm), 2.16 eV (570 nm), 2.81 eV (440 nm), 3.01 eV (410 nm), 3.36 eV (365 nm), 3.70 eV (335 nm) and 4.0 eV (310 nm), respectively. The absorption spectra of the PWO crystal containing V_O²⁺ occur two peaks located at 370 nm and 420 nm. The PWO crystal containing a pair of V_Pb²⁻-V_O²⁺ does not occur absorption band in the visible and near-ultraviolet region. This leads to the conclusions that the 370 and 420 nm absorption bands are related to the existence of both V_Pb²⁻ and V_O²⁺ in the PWO crystal and the other absorption bands are related to the existence of the V_Pb²⁻ in the PWO crystal. The existence of the pair of V_Pb²⁻-V_O²⁺ has no visible effects on the optical properties. The calculated polarized optical properties are well consistent with the experimental results.     

Structure and dehydration of the pyrochlore system NaW2−yMoyO6+δ·nH2−zO between 10 and 675 K

The temperature dependence of the structure of the pyrochlore NaW_2−yMo_yO_6+δ·nH_2−zO has been investigated using a variety of diffraction and spectroscopic methods. The positions of OH⁻/H₂O molecules in the structure have been determined. Increases in temperature induce small lattice parameter changes, which are thought to result from movement of the H₂O molecules in the pyrochlore lattice.