Fabrication of Na and K based MnO2 nanocomposites for supercapacitive applications.

α-Na0.5Mn0.93O2@Na0.91MnO2 (NaMnO) and K0.48Mn1.94O5@Na0.91MnO2 (KNaMnO) nanocomposites have been synthesized using solid-state reaction method. FESEM results convey the formation of column-shaped morphology. FTIR exhibited a shift in the vibration frequency upon potassium loading. Cyclic voltammetric curves are scanned (0 V-0.8 V) at different scan rates (5 mV/s to 100 mV/s) in 1M KOH electrolyte. Galvanostatic charge-discharge characteristics, for different current densities, have shown non-linear or pseudocapacitive characteristics of the prepared electrodes. High specific capacitance of ∼361 F/g and ∼143 F/g, at a current density of 1A/g, has been achieved for KNaMnO and NaMnO samples, respectively. KNaMnO sample exhibited higher capacitive retention (116 %), up to 2000 cycles, and obeys lower series resistance, charge transfer resistance, and Warburg impedance parameters, thus, convey higher efficiency of this compound for supercapacitor applications.

Fluorescence Study of Pr3+ Doped CdS Nanoparticles and its Applications in Sensors and Detectors.

Fluorescence spectra of Pr3+ doped CdS nanoparticles, synthesized by chemical precipitation method, have been recorded at room temperature. The synthesized particles are nearly spherical shaped and the grain size is decreased with the increase in Pr3+ concentration. The chemical identity of the nanoparticles was confirmed by EDAX spectrum, the absorption peaks was confirmed by FTIR spectrum and then the recorded values were compared with the CIE diagram. The oscillator strengths of the 4f ↔ 4I transitions are parameterized in terms of three phenomenological Judd-Ofelt intensity parameters Ωλ (λ = 2, 4 and 6). Using the fluorescence data and these Ωλ parameters, theoretical and experimental study of various radiative properties viz., spontaneous emission probability (A), radiative life time , fluorescence branching ratio and stimulated emission cross-section were evaluated. The values of these parameters indicate that 3P0→ 3H4 transition can be considered to be good laser transition in the visible colour region. Also, excitation with 493 nm, leads to similar blue regions. The synthesized Pr3+ doped CdS nanomaterials could be useful for sensing and detecting devices, particularly for temperature sensing measurement and bio-sensing detection.

Structural, optical, and magnetic properties of Ag+, Mn+ and Ar+ ions implanted ZnO thin films: effect of implantation dose and stopping energy.

Herein, we systematically studied the effect of various excitation processes on the structural, optical, and magnetic properties of ZnO films implanted with 80 keV Ar+, 110 keV Mn+, and 190 keV Ag+ ions. Four different doses of 1 × 1013, 1 × 1014, 1 × 1015, and 2 × 1016 ions per cm2 were used for implantation. It was observed that the structural, optical, and magnetic properties of the implanted samples were dominantly affected at the highest dose of 2 × 1016 ions per cm2. For lower doses, insignificant changes in these properties were observed. A comparison of various processes involved in the implantation process shows that both the electronic excitation and nuclear excitation processes are responsible for the changes in the structural, optical, and magnetic properties of the implanted ZnO films.

Synchrotron radiation based X-ray techniques for analysis of cathodes in Li rechargeable batteries.

Li-ion rechargeable batteries are promising systems for large-scale energy storage solutions. Understanding the electrochemical process in the cathodes of these batteries using suitable techniques is one of the crucial steps for developing them as next-generation energy storage devices. Due to the broad energy range, synchrotron X-ray techniques provide a better option for characterizing the cathodes compared to the conventional laboratory-scale characterization instruments. This work gives an overview of various synchrotron radiation techniques for analyzing cathodes of Li-rechargeable batteries by depicting instrumental details of X-ray diffraction, X-ray absorption spectroscopy, X-ray imaging, and X-ray near-edge fine structure-imaging. Analysis and simulation procedures to get appropriate information of structural order, local electronic/atomic structure, chemical phase mapping and pores in cathodes are discussed by taking examples of various cathode materials. Applications of these synchrotron techniques are also explored to investigate oxidation state, metal-oxygen hybridization, quantitative local atomic structure, Ni oxidation phase and pore distribution in Ni-rich layered oxide cathodes.

Hierarchically Assembled Cobalt Oxynitride Nanorods and N-Doped Carbon Nanofibers for Efficient Bifunctional Oxygen Electrocatalysis with Exceptional Regenerative Efficiency.

Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath-core cobalt oxynitride (CoOx@CoNy) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoOx@CoNy nanorods, composed of metallic Co4N cores and oxidized surfaces, exhibit excellent OER activity (E = 1.69 V at 10 mA cm-2) in an alkaline medium. Although pristine NCNF or CoOx@CoNy alone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E = 0.78 V at -3 mA cm-2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoOx@CoNy nanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoOx@CoNy nanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoOx@CoNy/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn-air batteries.

Approaches to synthesize MgO nanostructures for diverse applications.

Magnesium oxide remained interesting from long time for several important phenomena like; defect induced magnetism, spin electron reflectivity, broad laser emission etc. Moreover, nanostructures of this material exhibited suitability for different kinds of applications ranging from wastewater treatment to spintronics depending upon their shape and size. In this way, researchers had grown nanostructures in the form of nanoparticles, thin films, nanotubes, nanowalls, nanobelts. Though nanoparticles and thin films are well known form of nanostructures and wide variety of synthesis approaches are available, however, limited methodology for other nanostructures are available. In order to grow these nanostructures in an optimized way an understanding of these methods is essential. Thus, this review article depicts an overview of various approaches for design of different kinds of nanostructures.

Local Electronic Structure of Calcite Investigated Using X-ray Absorption Spectroscopy at Different Span of Time.

For the present work, calcite nanoparticles was synthesized from calcium nitrate by annealing precursor at 300, 400, 500 and 600°C. Ca K-edge near edge X-ray absorption fine structure measurements revealed spectral features characteristics to the amorphous phase of calcium carbonate at 300 and 400°C. At 500 and 600°C, the spectra were analogues to the calcite phase of calcium carbonate. Simulation of extended X-ray absorption fine structure spectra envisaged that both coordination number and bond distance for Ca-O bonds decreased with annealing temperature. Both parameters attained values close to standard calcite when annealed at 600°C. The spectral features at Ca L-, O K- and C K-edge near edge X-ray absorption fine structure appeared at same positions for different ages, which envisaged the occurrence of almost same local electronic structure for different span of times.

MgO Thin Film Growth on Si(001) by Radio-Frequency Sputtering Method.

Herein, sputtering duration and annealing temperature effects on the structure and local electronic structure of MgO thin films were studied using synchrotron radiation based X-ray diffraction and X-ray absorption spectroscopic investigations. These films were grown at substrate temperature of 350 °C by varying sputtering duration from 25 min to 324 min in radio frequency (RF) sputtering method followed by post-deposition annealing at 400, 600 and 700 °C for 3 h. These films were amorphous upto certain sputtering durations, typically upto 144 min and attains crystallization thereafter. This kind of behavior was observed at all annealing temperature. The textured coefficient of crystalline films envisaged that the orientation was affected by annealing temperature. Coordination of Mg2+ ions was more distorted in amorphous films compared to crystalline films. Moreover, onset of molecular oxygen are absorbed at low annealing temperature on these films.

Soft X-ray Absorption Spectroscopic Investigation of Li(Ni0.8Co0.1Mn0.1)O2 Cathode Materials.

Herein, we report the soft X-ray absorption spectroscopic investigation for Li(Ni0.8Co0.1Mn0.1)O2 cathode material during charging and discharging. These measurements were carried out at the Mn L-, Co L-, and Ni L-edges during various stages of charging and discharging. Both the Mn and Co L-edge spectroscopic measurements reflect the invariance in the oxidation states of Mn and Co ions. The Ni L-edge measurements show the modification of the oxidation state of Ni ions during the charging and discharging process. These studies show that eg states are affected dominantly in the case of Ni ions during the charging and discharging process. The O K-edge measurements reflect modulation of metal-oxygen hybridization as envisaged from the area-ratio variation of spectral features corresponding to t2g and eg states.

Development of XANES nanoscopy on BL7C at PLS-II.

X-ray absorption near-edge structure (XANES) imaging is a powerful tool to visualize the chemical state distribution of transition-metal-based materials at synchrotron radiation facilities. In recent years, the electrochemical working rechargeable battery has been the most studied material in XANES imaging owing to the large increase of portable electronics and electric vehicles. This work acknowledges the importance of battery analysis and has developed the XANES imaging system on BL7C at Pohang Light Source-II (PLS-II). BL7C employs an undulator taper configuration to obtain an energy band >130 eV near the K-absorption edge of the target element with a minimum energy interval >0.2 eV. While measuring energy-dependent images, the zone plate translation maintains the best focus, and then various data processes such as background correction, image registration and clustering allow single XANES spectrum extraction and chemical distribution mapping. Here, the XANES imaging process is described, the XANES spectrum quality is identified and the chemical states of the partially charged cathode material used in lithium-ion batteries as an application example are examined.

Correlating the size and cation inversion factor in context of magnetic and optical behavior of CoFe2O4 nanoparticles.

Herein, the size dependent behavior of cobalt ferrite nanoparticles was investigated using synchrotron radiation based techniques. Scanning electron micrographs revealed the enhancement of particle/crystallite size with increase of annealing temperature. Moreover, the shape of these particles also changed with increase of crystallite size. Saturation magnetization increased with increase of crystallite size. The higher saturation magnetization for larger crystallite size nanoparticles was attributed to a cation distribution similar to that of bulk CoFe2O4. The optical band-gap of these nanoparticles decreased from 1.9 eV to 1.7 eV with increase of crystallite size. The enhancement of the optical band-gap for smaller crystallites was due to phenomena of optical confinement occurring in the nanoparticles. Fe L Co L-edge near edge extended X-ray absorption fine structure (NEXAFS) measurements showed that Fe and Co ions remain in the 3+ and 2+ state in these nanoparticles. The results obtained from Fe & Co K-edge X-ray absorption near edge structure (XANES)-imaging experiments further revealed that this oxidation state was possessed by even the crystallites. Extended X-ray absorption fine structure (EXAFS) measurements revealed distribution of Fe and Co ions among tetrahedral (A) and octahedral (B) sites of the spinel structure which corroborates the results obtained from Rietveld refinement of X-ray diffraction patterns (XRD). X-ray magnetic circular di-chroism (XMCD) measurements revealed negative exchange interaction among the ions situated in tetrahedral (A) and octahedral (B) sites. Theoretical and experimental calculated magnetic moments revealed the dominancy of size effects rather than the cation redistribution in the spinel lattice of CoFe2O4 nanoparticles.

Local Electronic Structure Perspectives of Nanoparticle Growth: The Case of MgO.

Herein, we reported nanoparticle growth from the point of view of the local electronic structure by taking MgO as a prototype material. These nanoparticles were obtained from the sol-gel autocombustion process. The precursor formed in this process was annealed for various temperatures ranging from 300 to 1200 °C for 0.5 and 1 h. It was observed that the amorphous phase occurred in the material synthesized at an annealing temperature of 300 °C for 1 h. This phase transformed to crystalline when the annealing temperature was increased to 350 °C. Crystallite size increased with annealing temperature; however, annealing time did not influence the crystallite size. To get deeper insights of modifications occurring at the atomic scale during crystallization growth, the local electronic structure of synthesized materials was investigated by measuring near-edge X-ray absorption fine structure at Mg, O, N, and C K-edges. These results envisaged that Mg2+ ion coordination improved with the increase of annealing temperature. It was also observed that both annealing time and annealing temperature are sensitive to the local electronic structural changes.

A manganese(ii) phthalocyanine under water-oxidation reaction: new findings.

Phthalocyanines are a promising class of ligands for manganese because of their high binding affinity. This effect is suggested to be an important factor because phthalocyanines tightly bind manganese and stabilize it under moderate conditions. The strong donor power of phthalocyanine is also suggested as a critical factor to stabilize high-valent manganese phthalocyanine. Herein, a manganese(ii) phthalocyanine, which is stable under moderate conditions, was investigated under harsh electrochemical water oxidation. By scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction, extended X-ray absorption fine structure analysis, X-ray absorption near edge structure analysis, chronoamperometry, magnetic measurements, Fourier-transform infrared spectroscopy, and electrochemical methods, it is shown that manganese phthalocyanine, a known molecular complex showing good stability under moderate conditions, could not withstand water oxidation catalysis and ultimately is altered to form catalytic oxide particles. Such nanosized Mn oxides are the true catalyst for water oxidation. Besides, we try to go a step forward to find an answer as to how Mn oxides form on the surface of the electrode.

Electrochemical water oxidation by simple manganese salts.

Recently, it has been great efforts to synthesize an efficient water-oxidizing catalyst. However, to find the true catalyst in the harsh conditions of the water-oxidation reaction is an open area in science. Herein, we showed that corrosion of some simple manganese salts, MnCO3, MnWO4, Mn3(PO4)2 · 3H2O, and Mn(VO3)2 · xH2O, under the water-electrolysis conditions at pH = 6.3, gives an amorphous manganese oxide. This conversion was studied with X-ray absorption spectroscopy (XAS), as well as, scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), spectroelectrochemistry and electrochemistry methods. When using as a water-oxidizing catalyst, such results are important to display that long-term water oxidation can change the nature of the manganese salts.

A trimetallic organometallic precursor for efficient water oxidation.

Herein, we report an iron/nickel/zinc mixed oxide as a catalyst for the electrochemical water oxidation. This catalyst was synthesized by a straightforward method for the synthesis of an iron/nickel/zinc mixed oxide through the calcination of a Fe/Ni/Zn organometallic compound. The calcined product contains Fe and Ni as crucial ions for water oxidation, accompanied by the presence of Zn ions. The removal of Zn ions from the mixed oxide provides more active sites on the surface of the catalyst. The composition of the compound was characterized by some common methods and found to be an efficient water-oxidizing catalyst. The catalyst on FTO at pH = 13 yields a current density of 12 mA/cm2 at 1.2 V (vs. Ag│AgCl). After 5 hours at 1.1 V, the electrode not only shows no decrease in performance, but also shows an increase from 4 to 7 mA/cm2 in the water oxidation activity. Tafel plot, for the electrode at pH = 13 in KOH solution (0.1 M) showed linearity for the graph of lg j vs. V with both relatively low (220.4 mV per decade) and high overpotentials (903.7 mV per decade).

Observation of Skyrmions at Room Temperature in Co2FeAl Heusler Alloy Ultrathin Film Heterostructures.

Magnetic skyrmions are topological spin-textures having immense potential for energy efficient spintronic devices. Here, we report the observation of stable skyrmions in unpatterned Ta/Co2FeAl(CFA)/MgO thin film heterostructures at room temperature in remnant state employing magnetic force microscopy. It is shown that these skyrmions consisting of ultrathin ferromagnetic CFA Heusler alloy result from strong interfacial Dzyaloshinskii-Moriya interaction (i-DMI) as evidenced by Brillouin light scattering measurements, in agreement with the results of micromagnetic simulations. We also emphasize on room temperature observation of multiple skyrmions which can be stabilized for suitable combinations of CFA layer thickness, perpendicular magnetic anisotropy, and i-DMI. These results provide a significant step towards designing of room temperature spintronic devices based on skyrmions in full Heusler alloy based thin films.

A nickel(ii) complex under water-oxidation reaction: what is the true catalyst?

In the present study, the water-oxidizing activity of nickel(ii) phthalocyanine-tetrasulfonate tetrasodium (1), which is a stable Ni(ii) complex under moderate conditions, was investigated. The role of Ni oxide in water oxidation as a true catalyst was investigated. The electrodes after water oxidation by both the complex and Ni salt were analyzed and a relation was proposed between the decomposition of the Ni complex and water oxidation. On the surface of the electrode, there are some areas without any detectable nanoparticles; thus, the detection of such Ni oxides on the surface of the electrode is not easy in the first seconds of the reaction, even using some of the usual methods such as Scanning Electron Microscopy or electrochemical analysis. Such experiments indicated that a precise analysis is necessary to reject the role of nanoparticles in the presence of Ni phthalocyanine under water oxidation. The findings also showed that under water-oxidation conditions and in the presence of the complex, Ni oxide is a good candidate for a true catalyst.

Nanosized (Ni1-x Zn x )Fe2O4 for water oxidation.

Performing water splitting for H2 production is an interesting method to store different energies. For water splitting, an efficient and stable water-oxidizing catalyst is important. Ni-Fe (hydr)oxides are among the best catalysts for water oxidation in alkaline electrolytes. An Fe amount higher than 50% in Ni-Fe (hydr)oxides increases the overpotential for water oxidation. Thus, Ni-Fe (hydr)oxides with a high ratio of Fe to Ni have rarely been focused on for water oxidation. Herein, we report water oxidation using nanosized (Ni1-x Zn x )Fe2O4. The catalyst was characterized via some methods and tested at pH values of 3, 7 and 11 in phosphate buffer. Nanosized (Ni1-x Zn x )Fe2O4 is a good catalyst for water oxidation only under alkaline conditions. In the next step, amperometry studies showed current densities of 3.50 mA cm-2 and 11.50 mA cm-2 at 1.25 V in 0.10 M and 1.0 M KOH solution, respectively. The amperometric measurements indicated high catalyst stability in both 0.10 M and 1.0 M KOH. Tafel plots were obtained in KOH solution at concentrations of both 0.10 M and 1.0 M. At pH = 13 in KOH solution (0.10 M), linearity of lg(j) vs. potential was shown, with two slopes relating to both relatively low (170.9 mV per decade) and high overpotentials (484.2 mV per decade). In 1.0 M KOH solution, the Tafel plot showed linearity of lg(j) vs. potential, with two slopes relating to both relatively low (192.5 mV per decade) and high overpotentials (545.7 mV per decade). After water oxidation, no significant change was observed in the catalyst.

Microstructure, local electronic structure and optical behaviour of zinc ferrite thin films on glass substrate.

Zinc ferrite thin films were deposited using a radio-frequency-sputtering method on glass substrates. As-deposited films were annealed at 200°C for 1, 3 and 5 h, respectively. X-ray diffraction studies revealed the amorphous nature of as-grown and annealed films. Thickness of as-deposited film is 96 nm as determined from Rutherford backscattering spectroscopy which remains almost invariant with annealing. Transmission electron microscopic investigations envisaged a low degree of crystalline order in as-deposited and annealed films. Thicknesses estimated from these measurements were almost 62 nm. Roughness values of these films were almost 1-2 nm as determined from atomic force microscopy. X-ray reflectivity measurements further support the results obtained from TEM and AFM. Near-edge X-ray absorption fine structure measurements envisaged 3+ and 2+ valence states of Fe and Zn ions in these films. UV-Vis spectra of these films were characterized by a sharp absorption in the UV region. All films exhibited almost the same value of optical band gap within experimental error, which is close to 2.86 eV.

Uptake, Distribution, and Transformation of Zerovalent Iron Nanoparticles in the Edible Plant Cucumis sativus.

Here, we investigated the fate of nanoscale zerovalent iron (nZVI) on the Cucumis sativus under both hydroponic and soil conditions. Seedlings were exposed to 0, 250, and 1000 mg/L (or mg/kg soil) nZVI during 6-9 weeks of a growth period. Ionic controls were prepared using Fe-EDTA. None of the nZVI treatments affected the plant biomass. On the basis of the total iron contents and the superparamagnetic property of nZVI-exposed roots, there was no evidence of pristine nZVI translocation from the roots to shoots. Electron microscopy revealed that the transformed iron nanoparticles are stored in the root cell membrane and the vacuoles of the leaf parenchymal cells. X-ray absorption spectroscopy identified ferric citrate (41%) and iron (oxyhydr)oxides (59%) as the main transformed products in the roots. The shoot samples indicated a larger proportion of ferric citrate (60%) compared to iron (oxyhydr)oxides (40%). The 1.8-fold higher expression of the CsHA1 gene indicated that the plant-promoted transformation of nZVI was driven by protons released from the root layers. The current data provide a basis for two potential nZVI transformation pathways in Cucumis sativus: (1) interaction with low molecular weight organic acid ligands and (2) dissolution-precipitation of the mineral products.