Fabrication and characterization of multilayer films based on Keggin-type polyoxometalate and chitosan

Multilayer films based on Keggin-type polyoxometalate (POM) α-[SiW₁₂O₄₀]⁴⁻ (α-SiW₁₂), α-[PMo₁₂O₄₀]³⁻ (α-PMo₁₂) and cationic chitosan have been fabricated in aqueous solution via the layer-by-layer self-assembly technique (LBL). The resulting films were characterized by UV–Vis spectra, X-ray photoelectron spectra (XPS), atomic force microscopy (AFM) and cyclic voltammetry (CV) measurements. UV–Vis spectra show that the absorbance values at characteristic wavelengths of the multilayer films increase almost linearly with the number of chitosan/POM bilayers, suggesting that the deposition process is regular and highly reproducible from layer to layer. XPS spectra confirm the incorporation of chitosan and POMs into the films. AFM images indicate that the surface of the multilayer films is rather uniform and smooth. The antibacterial activities against Escherichia coli of the LBL films have also been investigated by optical density method.     

Comparative study of photoinduced wettability conversion between [PW12O40]/brookite and [SiW12O40]/brookite hybrid films

Two tungsten-based Keggin-type heteropolyacids (PW₁₂: ([PW₁₂O₄₀]³⁻) and SiW₁₂: ([SiW₁₂O₄₀]⁴⁻)) were hybridized with brookite-type TiO₂. Then photocatalytic decomposition activity, photoinduced hydrophilicity, and sustainability of the hydrophilicity in the dark were evaluated using gaseous 2-propanol (IPA) decomposition and sessile drop method. The obtained films were transparent in the visible wavelength range. Both hybrid films exhibited higher photocatalytic decomposition activity and had higher photoinduced hydrophilicizing rates than pure brookite films under UV illumination. The PW₁₂/TiO₂ film exhibited better photocatalytic performance than the SiW₁₂/TiO₂ film did. Atmosphere dependence, XPS analysis, and electrochemical experiments indicated the cause of these two films' different levels of sustainability of hydrophilicity to be differences in their electron storage capability. Results show that the electron scavenger capability and reoxidation efficiency of the heteropolyacid are key factors affecting the overall performance of wettability conversion of this hybrid film system before and after UV illumination.     

Electrochemical behaviour of self-assembly multilayer films based on iron-substituted α-Keggin polyoxotungstates

Ultrathin multilayer films containing metal-substituted polyoxometalates, [PW₁₁Fe^III(H₂O)O₃₉]⁴⁻ (PW₁₁Fe) or [SiW₁₁Fe^III(H₂O)O₃₉]⁵⁻ (SiW₁₁Fe), and poly(ethylenimine) (PEI) were prepared by the electrostatic layer-by-layer self-assembly method on a glassy carbon electrode. The multilayer films were characterized by cyclic voltammetry and scanning electron microscopy and UV-Vis absorption spectroscopy on a quartz slide was used to monitor film growth. Cyclic voltammetry indicates that the electrochemical properties of the polyoxometalates are completely maintained in the multilayer films, and the influence of scan rate on the voltammetric features showed that the first tungsten reduction process for immobilized PW₁₁Fe and SiW₁₁Fe is a surface-confined process. Studies with [Fe(CN₆)]^3−/4− as electrochemical probe showed that their permeability depends on the thickness of the multilayer films, if the outermost layer is negatively charged. Additionally, the (PEI/SiW₁₁Fe)_n multilayer films showed electrocatalytic properties towards nitrite reduction.     

Immobilization of a heteropolyacid catalyst on the aminopropyl-functionalized mesostructured cellular foam (MCF) silica

Mesostructured cellular foam (MCF) silica with high surface area (>600 m²/g) and large pore volume (≈1.16 cm³/g) was synthesized via a surfactant templating method. The MCF silica was then modified by grafting 3-aminopropyl-triethoxysilane (APTES) to create a positive charge on the surface, and thus, to provide sites for the immobilization of H₃PMo₁₂O₄₀. By taking advantage of the overall negative charge of [PMo₁₂O₄₀]³⁻, the H₃PMo₁₂O₄₀ catalyst was chemically immobilized on the aminopropyl group of the surface modified MCF silica as a charge matching component. The mesopore structure of MCF silica was maintained even after the surface modification step and the subsequent immobilization step of H₃PMo₁₂O₄₀. The H₃PMo₁₂O₄₀ species were finely and molecularly dispersed on the surface modified MCF silica via chemical immobilization.     

Confined Heteropoly Blues in Defected Zr‐MOF (Bottle Around Ship) for High‐Efficiency Oxidative Desulfurization

The Keggin‐type polyoxometalates (POMs) are effective catalysts for oxidative desulfurization (ODS) and confining these POMs in metal–organic frameworks (MOFs) is a promising strategy to improve their performances. Herein, postsynthetic modification of POMs confined in MOFs by adding thiourea creates more unsaturated metal sites as defects, promoting ODS catalytic activity. Additional modification by confining 1‐butyl‐3‐methyl imidazolium POMs in MOFs is performed to obtain higher ODS activity, owing to the affinity between electron‐rich thiophene‐based compounds and electrophilic imidazolium compounds. The ODS catalytic activities of four Zr‐MOF‐based composites (bottle around ship) including phosphomolybdate acid (PMA)/UiO‐66, [Bmim]₃PMo₁₂O₄₀/UiO‐66, PMA/Thiourea/UiO‐66, and [Bmim]₃PMo₁₂O₄₀/Thiourea/UiO‐66 are therefore investigated in detail. In order to explore the catalytic mechanism of these MOF composites, their microstructures and electronic structures are probed by various techniques such as X‐ray diffraction, thermogravimetric analysis, Fourier transform infrared, Raman, scanning electron microscope, transmission electron microscope, BET, X‐ray photoelectron spectroscopy, EPR, UV–vis, NMR spectra, and H₂‐temperature‐programmed reduction. The results reveal that phosphomolybdate blues and imidazolium phosphomolybdate blues with different Mo⁵⁺/Mo⁶⁺ ratios with the Keggin structure are confined in defected UiO‐66 for all four composites. This approach can be applied to design and synthesize other POMs/MOFs composites as efficient catalysts.     

Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors

Two series of WO _x /ZrO₂ samples are prepared by equilibrium adsorption from H₂O₂ solutions at pH 1.8 containing two different precursor anions, [W₂O₃(O₂)₄(H₂O)₂]²⁻ and [H₂W₁₂O₄₀]⁶⁻. The starting material is amorphous zirconium oxyhydroxide. The maximum W densities obtained are larger than that reported in the literature for systems synthesized by the same method using aqueous non-peroxide solutions. In the case of the metatungstate precursor, this increase is attributed to the generation of additional anchoring sites by interaction between the amorphous support and H₂O₂. The high uptake achieved when the peroxo complex is used as a precursor is a result of both the ZrO _x (OH)_4-2x –H₂O₂ interaction and low nuclearity of the adsorbing anion. The materials are characterized by XRD, DR–UV–vis, Micro-Raman and FT-IR spectroscopy. The surface acidities of samples with identical W loading prepared by equilibrium adsorption from the [H₂W₁₂O₄₀]⁶⁻–H₂O₂ system and by impregnation with aqueous solution of ammonium metatungstate are investigated by FT-IR spectroscopy of CO adsorbed at 80 K.     

Polyaniline microrods synthesized by a polyoxometalates/poly(vinyl alcohol) microfibers template

Polyaniline (PAn) microrods doped with H₄SiW₁₂O₄₀ (HPA) were obtained by a H₄SiW₁₂O₄₀/Poly(vinyl alcohol) microfibers template method. The morphology of the polyaniline microrods was observed by scanning electron microscope (SEM) photograph. The average diameter of the polyaniline microrods was between 200 and 250 nm. IR spectra and X-ray diffraction characterized the structure of the polyaniline doped with H₄SiW₁₂O₄₀. The conductivity of the polyaniline microrods was measured by a four-probe method. The highest conductivity of the polyaniline microrods was 3.9 S/cm.     

Fabrication of a 12-tungstophosphate and cadmium oxide composite film and its properties

A multilayer composite film of the 12-tungstophosphate H₃[PW₁₂O₄₀]³⁻ (PW₁₂) and cadmium oxide nanoparticles (CdO) was fabricated on quartz and silicon by the layer-by-layer (LBL) self-assembly method. The film was characterized by UV–vis spectroscopy, atomic force microscopy (AFM) and luminescence spectra. The proposed composite film exhibits higher photocatalytic activity toward methyl orange (MO) solution at pH 3.5, compared to single PW₁₂ and CdO films. The degradation rate was affected by initial concentration of PW₁₂, pH value of MO solution, inorganic ions concentration and type in MO solution. In addition, the composite film displays luminescent property and reversible electrochromic property with fast response time.     

New arsenates (V) NaKAl2O[AsO4]2 and Na2KAl3[AsO4]4

The title compounds have been synthesized by a solid state reaction route using a salt flux. Their crystal structures were determined from single crystal X-ray data. NaKAl₂O[AsO₄]₂ crystallizes with the orthorhombic K₂Fe₂O[AsO₄]₂-type, Pnma, a = 8.2368(6) Å, b = 5.5228(3) Å, c = 17.0160(13) Å and Z = 4, whereas Na₂KAl₃[AsO₄]₄ crystallizes with the orthorhombic K₃Fe₃[AsO₄]₄-type, Cmce, a = 10.5049(9), b = 20.482(2), c = 6.3574(6) Å and Z = 4. The NaKAl₂O[AsO₄]₂ structure is built up of [Al₂As₂O₉]²⁻ layers perpendicular to the c-axis which are separated by A⁺ alkali layers. The [Al₂As₂O₉]²⁻ layers consist of ribbons of edge-sharing AlO₆ octahedra, running along the a direction and which are connected through AsO₄ tetrahedra by sharing corners. The Na₂KAl₃[AsO₄]₄ structure contains [Al₃As₄O₁₆]³⁻ layers perpendicular to the b-axis separated by A⁺ alkali layers. The [Al₃As₄O₁₆]³⁻ layer consists of a layer of corner-sharing AlO₆ octahedra which are also connected to the AsO₄ tetrahedra by sharing corners.     

Physicochemical and catalytic properties of palladium supported on poly(o-methoxyaniline)

Palladium was introduced into a conjugated polymer poly(o-methoxyaniline) (POM) by reacting the powdered polymer with aqueous solution of PdCl₂ of low acidity (PdCl₂: 2.3 × 10⁻³ mol/dm³, HCl: 0.66 × 10⁻³ mol/dm³). Various Pd²⁺ complexes with Cl⁻, H₂O, OH⁻ ligands coexisted in this solution but predominated [PdCl₂(H₂O)₂] ones. Several techniques like X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron and Raman spectroscopy, extended X-ray absorption fine structure have been used to characterize the poly(o-methoxyaniline)-Pd systems. In particular, the state of Pd species in the Pd/POM of various content of palladium (2-8 wt.% Pd) and chemical changes in the polymer matrix induced by insertion of palladium were studied. The protonation and redox reactions involved on palladium incorporation resulted in palladium ions and Pd metal in the final samples. Metallic Pd produced due to spontaneous reduction of palladium ions by the polymer formed large crystalline particles 200-1000 nm in size. The Pd²⁺ species in the form of anionic complexes like [PdCl₄]²⁻ acted as the counterions at low content of palladium (2-4 wt.% Pd). At high palladium content (8 wt.% Pd), several atoms like Cl, N and/or O were identified by extended X-ray absorption fine structure (EXAFS) technique in the nearest environment of Pd atoms. The structural groups of POM (like N groups and/or OCH₃) as well as H₂O, OH⁻ molecules are, therefore, considered as probable species in the coordination sphere of palladium. The catalytic properties were studied for the as-prepared Pd/POM and the samples additionally reduced with aqueous solution of NaH₂PO₂. They were used in the hydrogenation of CC bonds in maleic acid (MAC) and CO groups in 2-ethylanthraquinone (eAQ) at 60 °C and atmospheric pressure of hydrogen using xylene-octanol-2 or water medium. The correlation between Pd/POM activities and the content of Pd metal was found. Activation of the as-prepared Pd/POM with NaH₂PO₂ improved their catalytic properties. Much higher and much stable activities were then obtained in both MAC and eAQ hydrogenation reactions.     

Thermal expansion of some garnets

Lattice constants of the garnets {Gd₃} [Gd_0.03Ga_1.97] (Ga₃)O₁₂, {Y₃} [Y_0.505Ga_1,495] (Ga₃)O₁₂, Y₃Fe₄GaO₁₂, {Y₃} [In_0.6Ga_1.4] (Ga₃)O₁₂, {Ca₂La} [Zr₂] (Ga₃O₁₂, {Y_1.05Ca_1.95}Ti_1.95Ga_3.05O₁₂, Ca₃Sn₃Ga₂O₁₂, {Ca_2.5Zr_2.5} [Zr₂] (Ga₃)O₁₂, {Na₂Ca} [Zr₂] (Ge₃)O₁₂ have been measured in the temperature range 296–1400°K. The most important results are: 1) Crystals in the system {Y₃} [Y_xGa_2−x] (Ga₃)O₁₂ are potentially good substrates for epitaxial films of the system Y₃Fe_5−xGa_xO₁₂ and 2) Our Czochralski grown crystals of GdGaG actually have the formula {Gd₃} [Gd_0.03Ga_1.97]-(Ga₃)O₁₂; the difference in the thermal expansions of the stoichiometric and Czochralski-grown materials is not significant.     

Role of bismuth and titanium in Na2O-Bi2O3-TiO2-P2O5 glasses and a model of structural units

0.60Na₂O-0.40P₂O₅ and (0.55−z)Na₂O-0.05Bi₂O₃-zTiO₂-0.40P₂O₅ glasses (0≤z≤0.15) were prepared by melting at 1000°C mixtures of Na₂CO₃, Bi₂O₃, TiO₂ and (NH₄)₂HPO₄. Differential Scanning Calorimetry (DSC) measurements give the variation of glass transition temperature (T_g) from 269°C (for 0.60Na₂O-0.40P₂O₅) to 440°C (for z=0.15). The density measurements increases from 2.25 to 3.01 g/cm³. FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO₃)_∞ chains for 0.60Na₂O-0.40P₂O₅ to P₂O₇⁴⁻ groups in the glasses containing Bi₂O₃ or both Bi₂O₃ and TiO₂. When bismuth oxide and titania are added to sodium phosphate glass, phosphate chains are depolymerized by the incorporation of distorted Bi(6) and Ti(6) units through POBi and POTi bonds. Bi₂O₃ and TiO₂ are assumed to be present as six co-ordinated octahedral [BiO_6/2]³⁻and [TiO_6/2]²⁻ units again with shared corners. This is accompanied by the simultaneous conversion of [POO_3/2] into [PO_4/2]⁺ units which achieves charge neutrality in the glasses.     

The mechanism of long phosphorescence of SrAl2−xBxO4 (0<x<0.2) and Sr4Al14−xBxO25 (0.1<x<0.4) co-doped with Eu and Dy

The role of B₂O₃ in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state MAS NMR spectra show the incorporation of boron as BO₄ in the AlO₄ framework of SrAl₂O₄ and Sr₄Al₁₄O₂₅. Phosphor, made free of glassy phases by washing with hot acetic acid+glycerol, did not show any photoconductivity under UV irradiation, indicating that the mechanism involving hole conduction in valence band is untenable for long phosphorescence. EPR studies confirm the presence of both electron and hole trap centers. Dy³⁺ forms substitutional defect complex with borate; [Dy-BO₄-V_Sr]²⁻, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO₃³⁻, i.e. [BO₃-V_O]³⁻. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO₃-V_O(e′)]⁴⁻. The holes are released from [Dy-BO₄-V_Sr(h)]¹⁻ under thermal excitation at room temperature. The recombination of electrons with holes releases energy which is expended to excite Eu²⁺ to induce long phosphorescence.     

A new organically templated monodimensional mixed valence (Fe/Fe) phosphite: (C4H12N2)[FeFe(HPO3)2F3]: Solvothermal synthesis, crystal structure, spectroscopic and magnetic properties

The organically templated (C₄H₁₂N₂)[Fe^IIFe^III(HPO₃)₂F₃] compound has been synthesized under mild solvothermal conditions. The crystal structure has been determined from X-ray single-crystal diffraction data. The compound crystallizes in the P2₁/n monoclinic space group, with the unit-cell parameters a = 12.935(1), b = 6.4476(7), c = 15.693(2) Å, β = 105.630(9)° and Z = 4. The crystal structure consists of [Fe^IIFe^III(HPO₃)₂F₃]²⁻ chains formed by a central chain built of [Fe(2)O₄F₂] edge-sharing octahedra, and two side chains formed by alternating [Fe(1)O₃F₃] octahedra and [HP(1)O₃] tetrahedra. The piperazinium cations are placed between the chains linked by ionic and hydrogen interactions. The IR and Raman spectra show the existence of two phosphite crystallographically independent. From the diffuse reflectance spectrum the D_q parameter for the iron(II) cations has been calculated (D_q = 820 cm⁻¹). The Mössbauer spectrum in the paramagnetic state shows the simultaneous presence of Fe²⁺ and Fe³⁺. The magnetic measurements indicate the existence of antiferromagnetic interactions.     

Thermal and mechanical properties of novel substrate crystal Mg0.4Al2.4O4

Mg_0.4Al_2.4O₄ single crystal was grown by the Czochralski method. The measured specific heat values are 0.804-1.06 J g^− 1 K^− 1 in the temperature range from 298.15 to 573.15 K. The calculated thermal conductivity components are 11.37, 11.47 and 10.77 W m^− 1 K^− 1 along the [111], [004] and [22?0] direction at 298.15 K. The Vickers microhardness values are 1328-1414 kg mm^− 2. These experimental results show that Mg_0.4Al_2.4O₄ crystal is a promising substrate for GaN-based LEDs.     

Novel Semiconductive Ternary Hybrid Heterostructures for Artificial Optoelectronic Synapses

Synaptic devices that mimic biological synapses are considered as promising candidates for brain-inspired devices, offering the functionalities in neuromorphic computing. However, modulation of emerging optoelectronic synaptic devices has rarely been reported. Herein, a semiconductive ternary hybrid heterostructure is prepared with a D-D’-A configuration by introducing polyoxometalate (POM) as an additional electroactive donor (D’) into a metalloviologen-based D-A framework. The obtained material features an unprecedented porous 8-connected bcu -net that accommodates nanoscale [α-SiW₁₂O₄₀]⁴⁻ counterions, displaying uncommon optoelectronic responses. Besides, the fabricated synaptic device based on this material can achieve dual-modulation of synaptic plasticity due to the synergetic effect of electron reservoir POM and photoinduced electron transfer. And it can successfully simulate learning and memory processes similar to those in biological systems. The result provides a facile and effective strategy to customize multi-modality artificial synapses in the field of crystal engineering, which opens a new direction for developing high-performance neuromorphic devices.     

2D-network of inorganic-organic hybrid material built on Keggin type polyoxometallate and amino acid: [L-C2H6NO2]3[(PO4)Mo12O36]·5H2O

A new inorganic-organic hybrid material based on polyoxometallate, [L-C₂H₆NO₂]₃[(PO₄)Mo₁₂O₃₆]·5H₂O, has been successfully synthesized and characterized by single-crystal X-ray analysis, elemental analysis, infrared and ultraviolet spectroscopy, proton nuclear magnetic resonance and differential thermal analysis techniques. The title compound crystallizes in the monoclinic space group, P2₁/c_, with a = 12.4938 (8) Å, b = 19.9326 (12) Å, c = 17.9270 (11) Å, β = 102.129 (1)°, V = 4364.8 (5) Å³, Z = 4 and R₁(wR₂) = 0.0513, 0.0877. The most remarkable structural feature of this hybrid can be described as two-dimensional inorganic infinite plane-like (2D/∞ [(PO₄)Mo₁₂O₃₆]³⁻) which forming via weak Van der Waals interactions along the z axis. The characteristic band of the Keggin anion [(PO₄)Mo₁₂O₃₆]³⁻ appears at 210 nm in the UV spectrum. Thermal analysis indicates that the Keggin anion skeleton begins to decompose at 520 °C.     

Layer-by-layer self-assembly of dye-polyoxometalate multilayer composite films and their fluorescent properties

The layer-by-layer (LbL) self-assembly technique was successfully applied to the fabrication of dye-polyoxometalate multilayer composite films consisting of two dye molecules Rhodamine B (RB) and Rhodamine 6G (R6G) and a Keggin-type polyoxometalate [α-SiW₁₂O₄₀]⁴⁻ (α-SiW₁₂). The composite films were characterized by UV-vis spectroscopy, scanning electron microscopy (SEM), and fluorescence spectroscopy. UV-vis spectra show that the characteristic absorbance values of the multilayer films increase almost linearly with the number of dye/α-SiW₁₂ bilayers, suggesting that the deposition process is regular and highly reproducible from layer to layer. SEM micrographs indicate that the film surface is a little rough with some individual granular domains. In addition, the fluorescent properties of these composite films were also investigated by fluorescence spectroscopy.     

Formation of schwertmannite and its transformation to jarosite in the presence of acidophilic iron-oxidizing microorganisms

Schwertmannite [Fe₈O₈(OH)₆SO₄] and jarosite [XFe₃(SO₄)₂(OH)₆] were synthesized from FeSO₄-mineral salts solutions that were inoculated with acidophilic iron-oxidizing bacteria (Acidithiobacillus ferrooxidans). Schwertmannite was transformed to NH₄⁺/H₃O⁺ jarosite upon increasing the aging time (19 to 40 days), temperature (36 to 45 °C), or the ammonium concentration (11.4 to 111.4 mM) in acid (about pH 2.0) media. Thus, in this phase transformation, metastable schwertmannite served as a solid phase precursor for jarosite formation. Scanning electron microscope and chemical analyses showed that the morphology, specific surface area and chemical composition of the jarosites thus produced varied depending on the experimental conditions. Because the surface areas and the molar ratios of Fe:S in the two Fe(III) hydroxysulfates are different, the transformation of schwertmannite to jarosite impacts the biogeochemistry of Fe and S as well as minor and trace metals in acid mine drainage.     

Synthesis, molecular structure, and characterization of a new 3D-layered inorganic-organic hybrid material: [d/l-C6H13O2N-H]3[(PO4)W12O36]·4.5H2O

A new 3D-layered inorganic-organic hybrid [d/l-C₆H₁₃O₂N-H]₃[(PO₄)W₁₂O₃₆]·4.5H₂O (1), as racemic material in the solid phase, has been synthesized and fully characterized by elemental microanalysis, single crystal X-ray diffraction, and infrared, Raman, and proton nuclear magnetic resonance spectroscopes. The most unique structural feature of 1 is its three-dimensional inorganic infinite tunnel-like framework that results in weak van der Waals interactions along the a-axis. A weak interlayer interaction between the titled layers provides a desirable condition to explore its potential as a host in a host-guest complex. The racemization has been observed in the crystal structure with the centric space group (P2₁/c). The latter consists of α-[(PO₄)W₁₂O₃₆]³⁻and [d/l-C₆H₁₃O₂N-H]⁺ moieties with water molecules linked together by a complex network of hydrogen bond interactions.