Chandrani Nayak

Local structure and photocatalytic properties of sol–gel derived Mn–Li co-doped ZnO diluted magnetic semiconductor nanocrystals

The structural, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and photocatalytic properties of sol–gel derived Zn1−y−xMnyLixO (y = 0, 0.02 and x = 0, 0.01, 0.02, 0.03, 0.04, 0.06) nanoparticles (NPs) have been investigated. A small linear increase in lattice parameters ‘a’ and ‘c’ has been observed which can be attributed to the small distortion of the Zn tetrahedron. From the Mn K-edge XANES data, it can be inferred that Mn exists as Mn2+ in the 2% Mn-doped ZnO sample. Li doping oxidizes the Mn-cations in the ZnO lattice and hence Mn exists in higher oxidation states (+2 or +3) in (Li, Mn) co-doped ZnO samples. Li doping also favors Mn metal clustering whose signature can be seen in both the Mn K-edge XANES and EXAFS spectra of the (Li, Mn) co-doped samples. UV-light driven degradation of methylene blue (MB) dye aqueous solution has also been demonstrated using pure and doped ZnO and more than 90% dye degradation has been observed within only 90 min of light irradiation. The synthesized materials with visible light emission and dye degradation activity can be used effectively in future optoelectronic devices and in water purification for cleaning of dyes.

Growth of block copolymer stabilized metal nanoparticles probed simultaneously by in situ XAS and UV–Vis spectroscopy

The growth of Au and Pt nanoparticles from their respective chloride precursors using block copolymer-based reducers has been studied by simultaneous in situ measurement of XAS and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at INDUS-2 SRS at RRCAT, Indore, India. While the XANES spectra of the precursor give real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed at the intermediate stages of growth. The growth kinetics of both types of nanoparticles are found to be almost similar and are found to follow three stages, though the first stage of nucleation takes place earlier in the case of Au than in the case of Pt nanoparticles due to the difference in the reduction potential of the respective precursors. The first two stages of the growth of Au and Pt nanoparticles as obtained by in situ XAS measurements could be corroborated by simultaneous in situ measurement of UV-Vis spectroscopy also.

New thorium–bismuth oxide solid solutions with oxygen vacancy induced tunable ferromagnetism

A combined X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and magnetic and electron paramagnetic resonance (EPR) study on defect-induced magnetism in Th1−xBixO2−δ (0 ≤ x ≤ 0.3) solid solutions is presented in this paper. The solid solutions were prepared through a solid-state reaction route. The XRD patterns of the solid solutions suggest Bi dissolution up to 30 at% in the ThO2 matrix. The EXAFS study at the Bi and Th L3 edges indicates the formation of oxygen vacancies near Bi sites with an increasing trend with the Bi at% in the solid solutions. The magnetic measurements in field cooled (FC) and zero field cooled (ZFC) mode indicate an interesting observation involving ferromagnetic ordering with Curie temperature at around 70 K. The possibility of a spin glass-like phase was conclusively ruled out by a memory effect test experiment. The EPR studies revealed the presence of two different signals at g ∼ 2.05 and 2.25 due to the paramagnetic oxygen vacancy () and ferromagnetic resonance (FMR). The constant increase in intensity of the FMR signal with increasing doping level corroborates well with the increasing saturation magnetization value in magnetic measurements. By lowering the measurement temperature, this FMR signal showed an increase in its intensity along with continuous broadening and shift of the resonance field position to the lower field, which further signifies the presence of ferromagnetic ordering due to the paramagnetic oxygen vacancy in the Th1−xBixO2−δ solid solution.

Correlation of Mo dopant and photocatalytic properties of Mo incorporated TiO2: an EXAFS and photocatalytic study

The present study addresses the quantitative estimation and understanding of the nature of phases present in Mo-incorporated titania through extensive EXAFS measurements (at Mo K-edge and Ti K-edge) and to decipher their role in photodegradation of Methylene Blue (MB) dye under UV and visible irradiation. EXAFS results revealed the presence of both MoO3 nano heterophase phase and substitutional Mo dopants in the TiO2 lattice, the extent of the latter was reduced with the increasing Mo content of the sample. The presence of MoO3 in the Mo-incorporated titania was also revealed by FT-IR and TEM studies. Photocatalytic studies have shown considerable adsorption of the cationic MB-dye, perhaps due to the electronic interaction between the dye and catalytic surface. Under visible irradiation, the photocatalytic activity followed the trend: Mo-5 > Mo-2 > Mo-1 > Mo-10 ≫ TiO2, while the trend for photodegradation of MB dye under UV irradiation was as follows: Mo-5 > Mo-2 > Mo-1 > TiO2 > Mo-10. These results have been explained in the light of the structural properties of the Mo–TiO2 system obtained from EXAFS measurements. It has been observed that the relative ratio of substitutional Mo-dopant to the MoO3 phase in this tri-phasic photocatalyst plays a crucial role in augmenting its oxidative photocatalytic property.

Unusual Versatility of the Filamentous, Diazotrophic Cyanobacterium Anabaena torulosa Revealed for Its Survival during Prolonged Uranium Exposure

ABSTRACT Reports on interactions between cyanobacteria and uranyl carbonate are rare. Here, we present an interesting succession of the metabolic responses employed by a marine, filamentous, diazotrophic cyanobacterium, Anabaena torulosa for its survival following prolonged exposure to uranyl carbonate extending up to 384 h at pH 7.8 under phosphate-limited conditions. The cells sequestered uranium (U) within polyphosphates on initial exposure to 100 μM uranyl carbonate for 24 to 28 h. Further incubation until 120 h resulted in (i) significant degradation of cellular polyphosphates causing extensive chlorosis and cell lysis, (ii) akinete differentiation followed by (iii) extracellular uranyl precipitation. X-ray diffraction (XRD) analysis, fluorescence spectroscopy, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy established the identity of the bioprecipitated uranium as a U(VI) autunite-type mineral, which settled at the bottom of the vessel. Surprisingly, A. torulosa cells resurfaced as small green flakes typical of actively growing colonies on top of the test solutions within 192 to 240 h of U exposure. A consolidated investigation using kinetics, microscopy, and physiological and biochemical analyses suggested a role of inducible alkaline phosphatase activity of cell aggregates/akinetes in facilitating the germination of akinetes leading to substantial regeneration of A. torulosa by 384 h of uranyl incubation. The biomineralized uranium appeared to be stable following cell regeneration. Altogether, our results reveal novel insights into the survival mechanism adopted by A. torulosa to resist sustained uranium toxicity under phosphate-limited oxic conditions. IMPORTANCE Long-term effects of uranyl exposure in cyanobacteria under oxic phosphate-limited conditions have been inadequately explored. We conducted a comprehensive examination of the metabolic responses displayed by a marine cyanobacterium, Anabaena torulosa, to cope with prolonged exposure to uranyl carbonate at pH 7.8 under phosphate limitation. Our results highlight distinct adaptive mechanisms harbored by this cyanobacterium that enabled its natural regeneration following extensive cell lysis and uranium biomineralization under sustained uranium exposure. Such complex interactions between environmental microbes such as Anabaena torulosa and uranium over a broader time range advance our understanding on the impact of microbial processes on uranium biogeochemistry.

Extended x-ray absorption fine structure measurements on radio frequency magnetron sputtered HfO 2 thin films deposited with different oxygen partial pressures

Two sets of HfO2 thin film have been deposited by the radio frequency magnetron sputtering technique at various oxygen partial pressures, one set without any substrate bias and another set with a 50 W pulsed dc substrate bias. The films have been characterized by extended x-ray absorption fine structure (EXAFS) measurements at the Hf L3 edge, and the structural information obtained from analysis of the EXAFS data has been used to explain the macroscopic behavior of the refractive index obtained from spectroscopic ellipsometry measurements. It has been observed that the variation of refractive index with oxygen partial pressure depends on the Hf-Hf bond length for the set of films deposited without substrate bias, while for the other set of films deposited with pulsed dc substrate bias, it depends on the oxygen coordination of the nearest neighbor shell surrounding Hf sites.

High Pressure Phases and Amorphization of a Negative Thermal Expansion Compound TaVO5

Negative thermal expansion material TaVO5 is recently reported to have pressure induced structural phase transition and irreversible amorphization at 0.2 and above 8 GPa, respectively. Here, we have investigated the high pressure phase of TaVO5 using in situ neutron diffraction studies. The first high pressure phase is identified to be monoclinic P21/ c phase, same as the low temperature phase of TaVO5. On heating, amorphous TaVO5 transformed to a new crystalline phase, which showed signatures of higher coordination of vanadium indicating pressure induced amorphization (PIA). PIA observed in TaVO5 might be due to the kinetic hindrance of pressure induced decomposition (PID) into a compound with higher coordination of vanadium. Mechanism of PIA observed in TaVO5 is investigated by carrying out ex situ Raman, XRD, XPS, and XAS measurements. We have also proposed a pressure-temperature phase diagram of TaVO5 qualitatively delineating the phase boundaries between the ambient orthorhombic, monoclinic, and amorphous phases.

Selective Oxidation of Cyclohexane to Cyclohexanone Using Chromium Oxide Supported Mesoporous MCM-41 Nanospheres: Probing the Nature of Catalytically Active Chromium Sites

Highly dispersed chromium oxide supported mesoporous MCM‐41 nanosphere catalysts have been synthesized using a simple wet impregnation method. This work is devoted to a systematic study to reveal the active Cr sites in chromium oxide supported MCM‐41 nanosphere catalysts for the selective oxidation of cyclohexane to cyclohexanone. To probe the nature of the active species, we synthesized 0.5–10 wt % Cr loaded catalysts and characterized them by using XRD, UV/Vis spectroscopy, Raman spectroscopy, X‐ray photoelectron spectroscopy, extended X‐ray absorption fine structure analysis, X‐ray absorption near edge structure analysis, N2 sorption analysis, FTIR spectroscopy, 29Si NMR spectroscopy, SEM, and TEM. The liquid‐phase oxidation of cyclohexane to cyclohexanone (99 % selectivity) was performed under mild reaction conditions, and the results reveal clearly that the 5 wt % Cr loaded catalyst was optimum for the reaction. The initial composition of isolated Cr3+ species in the catalyst is the major factor that influences the enhanced activity for cyclohexane oxidation.