S. Uma

Investigation of cation (Sn2+) and anion (N3−) substitution in favor of visible light photocatalytic activity in the layered perovskite K2La2Ti3O10

Noticeable lowering of the energy gaps have been achieved for the layered perovskite K(2)La(2)Ti(3)O(10) as a result of the attempts made to incorporate Sn(2+) and N(3-) ions. Incorporation of Sn(2+) ions was carried out by the ion-exchange reaction of K(2)La(2)Ti(3)O(10) with aqueous tin(II) chloride solution. Nitrogen incorporation was attempted by the solid state reaction of the parent oxide with urea around 400 °C in air. The resultant oxides have been characterized by power X-ray diffraction, UV-visible diffuse reflectance spectroscopy, and Fourier transform infrared spectroscopy. Room temperature ion-exchange was sufficient to introduce Sn(2+) ions with the resulting product of composition (Sn(0.45)K(0.2)H(0.9))La(2)Ti(3)O(10) · H(2)O. Visible light absorption was observed with the absorption edge red shift of ∼ 100 nm from that of the parent K(2)La(2)Ti(3)O(10). The lowering of the band gap was as expected by the contribution of Sn 5s orbitals to the O 2p orbitals in the formation of the valence band. Nitridation using urea resulted not only in nitrogen doping but with the additional sensitization by the presence of carbon nitride (CN) polymers, which again resulted in visible light absorption. The product oxides obtained as a result of cation and anion intended substitutional studies have been found to be useful for the visible light photocatalytic decomposition of organic dyes such as rhodamine B.

Novel Lithium-Containing Honeycomb Structures

Rock-salt-based honeycomb structures containing Te(VI) and Sb(V) with innumerable prospects of properties and applications were realized in the two new series of mixed-metal oxides of lithium, Li(8)M(2)Te(2)O(12) (M(II) = Co, Ni, Cu, Zn) and Li(8)M(2)Sb(2)O(12) (M(III) = Cr, Fe, Al, Ga). The structures of Li(8)Co(2)Te(2)O(12) and Li(8)Cu(2)Te(2)O(12) were determined by single-crystal X-ray diffraction for the first time, and mixed-occupancy Li/M was identified.

A simple one pot synthesis of cubic Cu5FeS4

Reaction of CuCl, FeCl3 and thiourea in ethylene glycol under refluxing conditions yielded micro flower shaped cubic Cu5FeS4 as confirmed from high resolution PXRD, SEM with EDX, UV-visible, Raman spectroscopy and magnetic measurements at room temperature.

Single step hydrothermal synthesis of beyerite, CaBi2O2(CO3)2 for the fabrication of UV-visible light photocatalyst BiOI/CaBi2O2(CO3)2

The mineral beyerite (CaBi2O2(CO3)2), a member of the “sillen” family possessing a layer sequence, (Bi2O2)2+–CO32−–Ca2+–CO32−–(Bi2O2)2+ has successfully been synthesized under hydrothermal conditions in a single step for the first time. Uniform rectangular plates of beyerite crystallites with a surface area of 33 m2 g−1 were obtained by heating a solution containing bismuth nitrate and calcium carbonate in ethylene glycol along with the addition of a saturated solution of sodium carbonate at 140 °C. The sample was characterized by powder X-ray diffraction combined with Rietveld refinement, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectra and diffuse reflectance measurements. The band gap estimated for beyerite was found to be 3.97 eV and was utilized subsequently to form heterostructure with BiOI (band gap of 1.76 eV), another member belonging to the sillen family. The formation of the composite BiOI/CaBi2O2(CO3)2 has been confirmed by a variety of techniques and its photocatalytic properties as a UV-visible photocatalyst has been demonstrated for the degradation of aqueous solutions of rhodamine B and phenol. Photocatalytic experiments in presence of scavengers suggested the possible mechanism occurring through holes and peroxide radicals rather than the hydroxyl radicals.

Synthesis and structural investigation of a unique columnar phase in the Bi2O3–TeO2–V2O5 System.

A new columnar phase Bi(11.65)Te(1.35)V(5)O(34-δ) (δ ∼ 1.3) containing VO(4) tetrahedra has been identified for the first time in the Bi(2)O(3)-TeO(2)-V(2)O(5) system. The phase formation and the extent of substitution of Te(4+) for Bi(3+) ions in order to stabilize V(5+) in this composition have been confirmed by the single crystal analysis, combined with the powder X-ray diffraction of the solid state synthesized bulk crystalline samples. The oxide crystallizes in a monoclinic crystal system, space group P2/c, with unit cell parameters a = 11.4616(7) Å, b = 5.7131(3) Å, c = 23.5090(18) Å, β = 101.071° (6) (Z = 2). The structure retains the basic features of the columnar oxides with the presence of [Bi(10.65)Te(1.35)O(14)](n)(9.35n+) columns along the (010) direction, surrounded by (VO(4)) tetrahedra placed in the planes parallel to (100) and (001), with an isolated bismuth atom in between the columns. The composition with a limited Te(4+) substitution, Bi(11.65)Te(1.35)V(5)O(34-δ) (δ ∼ 1.3), exists with a surprisingly high oxygen deficiency as compared to the stoichiometrically known columnar oxides such as Bi(13)Mo(4)VO(34), Bi(12)Te(1)Mo(3)V(2)O(34), and Bi(11)Te(2)Mo(2)V(3)O(34). The structure of this interesting member of the columnar family of oxides based on the single crystal X-ray diffraction and the Raman spectroscopic studies indicates the possibility of the distribution of the oxygen vacancies among the VO(4) tetrahedral units. Further confirmation for the formation of vanadium stabilized columnar structures has been provided by the successful preparation of Bi(11.65)Te(1.35)V(4)CrO(34-δ) (δ ∼ 0.83) and Bi(11.65)Te(1.35)V(4)WO(34-δ) (δ ∼ 0.83) phases. Preliminary investigation of the photocatalytic efficiencies of the oxides Bi(11.65)Te(1.35)V(5)O(34-δ), Bi(11.65)Te(1.35)V(4)CrO(34-δ), and Bi(11.65)Te(1.35)V(4)WO(34-δ) revealed moderate photocatalytic activities for the decomposition of the dyes such as Rhodamine B under UV-vis light irradiation.

Interesting cationic (Li+/Fe3+/Te6+) variations in new rocksalt ordered structures

AbstractA new series of layered oxides, Li3(Li1.5xFe3−(x+1.5x)Tex)O6, (0.1 ≤x≤ 1.0) possessing rocksalt superstructures crystallizing in monoclinic (S.G. C2/m) symmetry is reported here. Investigations based on single crystal and powder X-ray diffraction studies for the x= 1 member, Li3(Li1.5Fe0.5Te)O6, (a= 5.1834(1); b= 8.8858(2); c= 5.16840(8) Å; β= 110.660(1)∘) confirmed the stabilization of (Li1.5Fe0.5Te1.0 O6)3− honeycomb arrays with a very high amount of lithium ions. The structure for the x= 0.5 member (Li3.75Fe1.75Te0.5O6) has also been confirmed by the powder X-ray diffraction Rietveld refinements. Li3(Li1.5Fe0.5Te)O6 and Li3(Li0.75Fe1.75Te0.5)O6 oxides exhibited Curie–Weiss behaviour in the temperature range of 50–300 K with negative 𝜃 values. Their respective ionic conductivities were found to be 6.76×10−5 S cm−1 and 2.21 × 10−6 S cm−1 at 573 K. The UV-visible diffuse reflectance measurements for the different members of the series Li3(Li1.5xFe3−(x+1.5x)Tex)O6, 0.1 ≤ x ≤ 1.0) show the expected shifts in their absorption edges based on the increasing amount of Fe3+ ions starting from x= 1.0 member to x= 0.1 member. Graphical AbstractVarying the amounts of Li+, Fe3+, and Te6+ ions for appropriate charge compensation led to interesting series of oxides, Li3(Li1.5xFe3-(x+1.5x)Tex)O6, (0.1 ≤ x ≤ 1.0) with intact rocksalt superstructure ordering.

Catalytic Application of Oxygen Vacancies Induced by Bi3+ Incorporation in ThO2 Samples Obtained by Solution Combustion Synthesis

Recognizing immense advantages of solution-based combustion synthesis, its applicability to determine the extent of dissolution of Bi3+ in fluorite-structured thoria has been examined to generate high-surface-area samples with massive defects. Up to 50 mol % of thorium could be substituted with bismuth retaining fluorite structure beyond which phase separation occurred. The lattice parameters from Le-Bail refinements of their powder X-ray diffraction patterns showed marginal increase with increase in bismuth content, suggesting the competing effect between the size of the cation and the oxygen vacancy concentration. Energy-dispersive X-ray spectrometry analysis and high-resolution transmission electron microscopy measurements have also confirmed the composition and structure of the limiting composition. With progressive bismuth content, the band due to the fluorite (at 460 cm–1) diffused and a defect band in the region 570–600 cm–1 emerged in the Raman spectra. From these changes, the oxygen vacancy concentrations in these samples have been determined, which increased with increase in bismuth content. Absorbance in the visible region was noticed for bismuth-containing samples, and band gap values determined from the Kubelka–Munk function were in the range 2.34–3.24 eV. In addition to the blue emission from oxygen vacancies, 3P1 → 1S0 transition of Bi3+ was noticed in the photoluminescence spectrum. From Brunauer–Emmett–Teller measurements, the surface area of Th0.50Bi0.50O2−δ obtained by solution combustion synthesis was measured to be 265.74 m2 g–1, higher than the value (39.00 m2 g–1) for the sample prepared by solid-state synthesis. All of these factors combined with oxygen vacancies as defect centers have been found to play critical control over their use as catalyst for the reductive transformation of nitroaromatics and oxidative decolorization of organic dye molecules (methyl orange and xylenol orange). A nice correlation between oxygen vacancy concentration and pseudo first-order rate constants of these catalytic conversions has been arrived. The catalyst was found to retain its efficiency up to four cycles without undergoing any structural change during these experiments.

Novel Sb5+-containing oxide possessing unique structural features

Exploratory synthesis in spite of being a time consuming and tedious process, still remains as the best route for the identification of novel materials with interesting structures, properties and applications. Based on our success in the synthesis of new lithium based oxides [1-2], we continue to search for new phase formation in the quasi-ternary system Na2O-Fe2O3-Sb2O5 using powder and single crystal X-ray diffraction studies. Preliminary investigations (PXRD) reveal Na2FeSbO5 oxide crystallizing in orthorhombic symmetry with lattice dimensions (a = 10.87 Å; b = 15.66 Å; c = 5.32 Å) indicating a structural relation to the known brownmillerite structure [3]. However, the SXRD measurements correspond to a unique structure consisting of linked (FeO4) and (SbO6) octahedral units. Our systematic approach to solve the structure coupled with the results from other characterizations such as FTIR, Raman, DRS, magnetic measurements and ionic conductivity measurements will be presented.