M. Thirumal

Achieving Enhanced Visible-Light-Driven Photocatalysis Using Type-II NaNbO3/CdS Core/Shell Heterostructures

Expanding the light-harvesting range and suppressing the quick recombination of photogenerated charge carriers are of paramount significance in the field of photocatalysis. One possible approach to achieve wide absorption range is to synthesize type-II core/shell heterostructures. In addition, this system also shows great promise for fast separation of charge carriers and low charge recombination rate. Herein, following the surface functionalization method using 3-mercaptopropionic acid (MPA) as a surface functionalizing agent, we report on designing NaNbO3/CdS type-II core/shell heterostructures with an absorption range extending to visible range and explore the opportunity toward degradation of methylene blue (MB) dye as a model pollutant under visible light irradiation. Characterizations including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), UV-vis diffuse reflectance spectrum (DRS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy support the growth of CdS shell onto NaNbO3 nanorods. The resulting core/shell heterostructures unveiled high surface areas, enhanced light harvesting, and appreciably increased photocatalytic activity toward MB degradation compared to individual counterparts and the photocatalytic standard, Degussa P25, under visible light irradiation. The remarkably enhanced photocatalytic activity of core/shell heterostructures could be interpreted in terms of efficient charge separation owing to core/shell morphology and resulting type-II band alignment between NaNbO3 and CdS, which creates a step-like radial potential favoring the localization of one of the carriers in the core and the other in the shell. A plausible mechanism for the degradation of MB dye over NaNbO3/CdS core/shell heterostructures is also elucidated using active species scavenger studies. Our findings imply that hydroxyl radicals (OH(•)) play a crucial role in dictating the degradation of MB under visible light. This work highlights the importance of core/shell heterostructures in leading toward new paradigms for developing highly efficient and reusable photocatalysts for the destructive oxidation of recalcitrant organic pollutants.

Synthesis and dielectric properties of magnesium niobate-magnesium tantalate solid solutions

Monophasic Oxides of the type MgNb2−xTaxO6 (0 ≤ x ≤ 2) have been synthesized by solid-state reactions using Mg(NO3)2·6H2O as the source of Mg. The oxides crystallize in the orthorhombic columbite structure till x = 1.75 and the tetragonal trirutile structure for pure MgTa2O6 (x = 2.0). Sintered pellets at 1200°C show dielectric constant varying between 16 to 28 for the entire range of composition at a frequency of 500 kHz. The dielectric loss is nearly constant with frequency, and varies between 0.004 and 0.01 for different compositions. The temperature coefficient of dielectric constant (τE) is 3 to −3 ppm/C (30 to 300°C). Scanning electron micrographs reveal a gradual decrease in grain size with increase in Ta concentration with size of 7 to 10 microns for the pure Nb phase sintered at 1200°C while the pure Ta phase shows a grain size of approximately 0.5 to 1 microns.

New double perovskites having low dielectric loss: LaBaZnTaO6, LaSrZnNbO6, and Ba2Zn0.5Ti0.5TaO6

Abstract Oxides of the type LaBaZnTaO 6 and LaSrZnNbO 6 have been synthesized by the solid-state method at 1100°C. Ba 2 Zn 0.5 Ti 0.5 TaO 6 was synthesized by the molten salt method (using NaCl-KCl as flux), the solid-state method yielding a biphasic mixture of BaTiO 3 and BaZn 1/3 Ta 2/3 O 3 . LaBaZnTaO 6 and LaSrZnNbO 6 were found to crystallize in an ordered (cubic) double perovskite structure in the Fm 3 m space group with a = 8.0867(8) and 8.001(1) A, respectively. Ba 2 Zn 0.5 Ti 0.5 TaO 6 crystallizes in the disordered cubic perovskite structure with a = 4.054(1) A. These oxides have very low dielectric losses in the region of 0.0005 to 0.003 (500 kHz). The dielectric constants of LaBaZnTaO 6 and LaSrZnNbO 6 are low (13 and 10, respectively) while Ba 2 Zn 0.5 Ti 0.5 TaO 6 has a high dielectric constant of 36 at 500 kHz.

Studies on dielectric oxide materials containing niobium and tantalum

Abstract There are now many advanced materials, which have been discovered with unique dielectric properties having been designed with an eye to specific applications. One of the areas of current interest to the material scientist and solid state chemist is to find new materials suitable for use as dielectric resonators in microwave applications and to tailor its properties. Among them the ABO3 type perovskite oxides and the AB2O6 (columbite/trirutile) type of oxides where B is either Nb or Ta have been pursued vigorously in the recent past. There is a continuous interest to optimise the dielectric properties by suitable doping at the A-site, B-site and the B′ site of the perovskite related A(B1/3B′2/3O3) structure. In addition there is an increasing demand to discover alternate routes of synthesis, which would afford these oxides at much lower temperatures. We discuss our recent results on some of the above materials.

Dominant {100} facet selectivity for enhanced photocatalytic activity of NaNbO3 in NaNbO3/CdS core/shell heterostructures

Design and engineering of crystalline advanced photocatalysts with specific facets is one of the most challenging tasks to enhance the photocatalytic performance. The surface energy of different facets is different in a crystal which leads to a corresponding change in their photocatalytic behaviour. The present study provides an experimental as well as theoretical understanding of the role of different facets of NaNbO3 in cubic and orthorhombic phases with crystals showing cubic and cuboctahedron morphologies in enhancing the photocatalytic activity of NaNbO3/CdS core/shell heterostructures. Herein, we discuss the importance of the approach of facet-selective synthesis and trace the origin of enhanced photoelectrochemical (PEC) water splitting and photocatalytic dye degradation activity for calculated surface energies of the {100} family of facets of the cubic phase and the (110) and (114) facets of the orthorhombic phase of NaNbO3. We propose that different mechanisms contribute to the enhancement of catalytic activity in these two phases. In the prepared core/shell heterostructures containing NaNbO3 as the core material, the presence of highly reactive facets of the cubic phase contributes to higher photocatalytic activity as compared to the orthorhombic phase which has a spatial charge separation assisted inter-facet charge transfer mechanism.

Phase analysis and dielectric properties of ceramics in PbO-MgO-ZnO-Nb2O5 system: A comparative study of materials obtained by ceramic and molten salt synthesis routes

Compositions of the type 3PbO-MgO/ZnO-Nb2O5 were synthesized by the ceramic route at 1000°C and sintered at 1200°C. Powder X-ray diffraction studies of the 1000°C heated products show the presence of the cubic pyrochlore and the columbite (Mg/ZnNb2O6) type phase in the ratio of 3 : 1 for all possible combinations of MgO and ZnO. Further heating at 1200°C led to a decrease in the cubic pyrochlore phase and an increase in the columbite phase by around 10%. Compacted pellets sintered further showed the appearance of the perovskite phase. Similar compositions synthesized using the KCl-NaCl molten salt method at 900°C for 6 h gave a significant amount of the cubic perovskite related phase of the type Pb(Mg/Zn)1/3Nb2/3O3 for all compositions containing MgO. The amount of the perovskite phase was nearly 55% for the Mg rich compositions and decreased with increase in Zn content, the pure Zn composition yielding mainly the cubic pyrochlore phase. On sintering these phases at 1000°C the perovskite phase content decreased. The dielectric constant of the composite materials formed by the ceramic route was in the region of 14 to 20 and varied little with frequency. The composites obtained by the molten salt method, however, showed much larger dielectric constants in the region 40–150 at 500 kHz for various compositions. The dielectric loss tangent of these composites were lower by an order (0.005–0.03 at 500 kHz) compared to the ceramic route.

Visible-Light-Driven Photoelectrochemical and Photocatalytic Performance of NaNbO3/Ag2S Core–Shell Heterostructures

Herein, we report the fabrication of visible-light-active NaNbO3 /Ag2 S staggered-gap core-shell semiconductor heterostructures with excellent photoelectrochemical activity toward water splitting, and the degradation of a model pollutant (methylene blue) was also monitored. The heterostructures show a pronounced photocurrent density of approximately 2.44 mA cm(-2) at 0.9 V versus Ag/AgCl in 0.5 m Na2 SO4 and exhibit a positive shift in onset potential by approximately 1.1 V. The high photoactivity is attributed to the efficient photoinduced interfacial charge transfer (IFCT). The core-shell design alleviates the challenges associated with the electron-hole paths across semiconductor junctions and at the electrolyte-semiconductor interface. These properties demonstrate that NaNbO3 /Ag2 S core-shell heterostructures show promising visible-light photoactivity and are also efficient, stable, and recyclable photocatalysts.

A new form of MgTa2O6 obtained by the molten salt method

Using molten salt route (with NaCl/KCl as the salt) we have been able to synthesize a new form of magnesium tantalate at 850°C. Powder X-ray diffraction data could be indexed on an orthorhombic unit cell with lattice parameters, ‘a’ = 15.36(1) Å, ‘b’ = 13.38(1) Å and ‘c’ = 12.10(1) Å. High resolution transmission electron microscopy and electron diffraction studies confirm the results obtained by X-ray studies. Energy dispersive X-ray spectroscopy helps ascertain the composition of MgTa2O6. The title compound shows a dielectric constant of ∼24 with a low dielectric loss of 0.006 at 100 kHz at room temperature. Dielectric constant is nearly unchanged with rise in temperature while the loss shows a very marginal increase (0.007 at 300°C).

Ba3ZnTa2-xNbxO9 and Ba3MgTa2-xNbxO9 : synthesis, structural and dielectric studies

Abstract Oxides of the families Ba 3 ZnTa 2−x Nb x O 9 and Ba 3 MgTa 2−x Nb x O 9 were obtained by the solid state reaction route at 1573 K and were found to crystallize in the disordered (cubic) perovskite structure. In Ba 3 ZnTa 2−x Nb x O 9 and Ba 3 MgTa 2−x Nb x O 9 the entire range (0 ≤ x ≤ 1) of solid solutions could be synthesized. The dielectric constant decreases with increase in frequency for all compositions in the range 40 Hz to 100 kHz (e r varies from 16 to 22). The dielectric loss (D) shows a broad maximum for both Ba 3 ZnTa 2−x Nb x O 9 and Ba 3 MgTa 2−x Nb x O 9. The maxima is centered around 2 kHz in the former and near 10 kHz in the latter.

Synthesis of Ba3ZnNb2O9−Sr3ZnNb2O9 solid solution and their dielectric properties

Oxides of the type, Ba3-xSrxZnNb2O9 (0 ≤x ≤3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) forx = 0 to 4.0067(2) forx = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (∼ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant atx = 1.