Aluminium substitution in Sb2S3 nanorods enhances the stability of the microstructure and high-rate capability in the alloying regime.

Alloy anodes, with twice the capacity of graphite, are promising for next-generation lithium-ion batteries (LIBs). However, poor rate-capability and cycling stability, mainly due to pulverization, have limited their application. By constraining the cutoff voltage to the alloying regime (1 V to 10 mV vs. Li/Li+), we show that Sb1.9Al0.1S3 nanorods provide excellent electrochemical performance, with an initial capacity of ∼450 mA h g-1 and excellent cycling stability with 63% retention (capacity ∼240 mA h g-1 after 1000 cycles at 5C-rate), unlike 71.4 mA h g-1 after 500 cycles observed in full-regime cycling. When conversion cycling is also involved the capacity degrades faster (<20% retention after 200 cycles) irrespective of Al doping. The contribution of alloy storage to total capacity is always larger than the conversion storage indicating the superiority of the former. The formation of crystalline Sb(Al) is noted in Sb1.9Al0.1S3, unlike amorphous Sb in Sb2S3. Retention of the nanorod microstructure in Sb1.9Al0.1S3 despite the volume expansion enhances the performance. On the contrary, the Sb2S3 nanorod electrode gets pulverized and the surface shows microcracks. Percolating Sb nanoparticles buffered by the Li2S matrix and other polysulfides enhance the performance of the electrode. These studies pave the way for high-energy and high-power density LIBs with alloy anodes.

Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3/C Composites.

Since the inception of cobalt phthalocyanine for oxygen reduction reaction (ORR), non-platinum group metals have been the central focus in the area of fuel-cell electrocatalysts. Besides Fe-Nx active sites, a large variety of species are formed during the pyrolysis, but studies related to their ORR activity have been given less importance in the literature. Fe2O3 is one among them, and this study describes the role of Fe2O3 in the ORR. The Fe2O3 is carefully synthesized on various carbon supports and characterized using X-ray photoelectron spectroscopy (XPS) spectra, high-resolution transmission electron microscopy (HRTEM) images, and surface area analysis. The characterization techniques reveal that the Fe2O3 nanoparticles are present in the pores of the carbon supports, having a particle size ranging from 4 to 15 nm. The current density of the ORR on Fe2O3/C catalysts is increased compared with bare carbon supports, as discerned from the rotating ring-disk electrode (RRDE) voltammetry experiments, demonstrating the role of size-confined Fe2O3 nanoparticles. The overall number of electrons in the ORR is increased by the introduction of Fe2O3 on the carbon support. Based on the kinetic analysis, the ORR on Fe2O3/C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to H2O2 and second 2-electron H2O2 reduction reaction (HPRR) to H2O are assigned to the graphitic carbon (carbon defects) and Fe2O3 active sites, respectively. Theoretical studies indicate that the role of Fe2O3 is to decrease the free energy of O2 adsorption and reduce the energy barrier for the reduction of *OOH to OH-. The onset potential estimated from the free energy diagram is 0.42 V, matching with the HPRR activity demonstrated using the potential-dependent rate constants plot. Fe2O3/C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.

Localized thermal spike driven morphology and electronic structure transformation in swift heavy ion irradiated TiO2 nanorods.

Irradiation of materials by high energy (∼MeV) ions causes intense electronic excitations through inelastic transfer of energy that significantly modifies physicochemical properties. We report the effect of 100 MeV Ag ion irradiation and resultant localized (∼few nm) thermal spike on vertically oriented TiO2 nanorods (∼100 nm width) towards tailoring their structural and electronic properties. Rapid quenching of the thermal spike induced molten state within ∼0.5 picosecond results in a distortion in the crystalline structure that increases with increasing fluences (ions per cm2). Microstructural investigations reveal ion track formation along with a corrugated surface of the nanorods. The thermal spike simulation validates the experimental observation of the ion track dimension (∼10 nm diameter) and melting of the nanorods. The optical absorption study shows direct bandgap values of 3.11 eV (pristine) and 3.23 eV (5 × 1012 ions per cm2) and an indirect bandgap value of 3.10 eV for the highest fluence (5 × 1013 ions per cm2). First principles electronic structure calculations corroborate the direct-to-indirect transition that is attributed to the structural distortion at the highest fluence. This work presents a unique technique to selectively tune the properties of nanorods for versatile applications.

Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-δ Thin-Film Solar Cells.

Designing solid-state perovskite oxide solar cells with large short circuit current (JSC) and open circuit voltage (VOC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large VOC (>50 V). However, they exhibit low JSC (≪µA/cm2) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (VO) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high JSC and VOC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice VO, Ca and Ti cosubstitution is used to bring out only GB-VO. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-δ/Au devices are tested for photovoltaic characteristics. Introducing VO increases the photocurrent by four orders (JSC ∼ 3 mA/cm2). On the contrary, VOC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO3. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-VO. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in JSC. The cosubstitution, following a charge compensation mechanism, decreases the lattice VO concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve VOC (3-8 V) much larger than that of BiFeO3 (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice VO concentration dominates GB-VO concentration. Notwithstanding, both lattice and GB-VO contribute to the increase in JSC; the former weakens ferroelectricity, and as a consequence, undesirably, VOC is lowered well below 0.5 V. Using optimum JSC and VOC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Enhanced high rate capability of Li intercalation in planar and edge defect-rich MoS2 nanosheets.

(i) Edge and planar defect-rich and (ii) defect-suppressed MoS2 nanosheets are fabricated by controlled annealing of wet-chemically processed precursors. Wrinkles, folds, bends, and tears lead to the introduction of severe defects in MoS2 nanosheets. These defects are suppressed and highly crystalline MoS2 nanosheets are obtained upon high-temperature annealing. The influence of defects on the electrochemical properties, particularly rate capability and cycling stability, in the Li intercalation regime (1 V to 3 V vs. Li/Li+) and conversion regime (10 mV to 3 V vs. Li/Li+) are investigated. In the intercalation regime, the initial Li intake (x in LixMoS2) for defect-rich nanosheets is larger (x ≈ 1.6) as compared to that in defect-suppressed MoS2 (x ≈ 1.2). Although the reversible initial capacity of all the anodes is nearly the same (x ≈ 0.9) at 0.05C rate, defect-rich MoS2 exhibits high rate capability (>40 mA h g-1 at 40C or 26.8 A g-1). When cycled at 10C (6.7 A g-1) for 1000 cycles, 75% capacity retention is observed. High rate capability can be attributed to the defect-rich nature of MoS2, providing faster access to lithium intercalation by a shortened diffusion length facilitated by Li adsorption at the defect sites. The defect-rich nanosheets exhibit a power density of ∼20% more than that of defect-suppressed nanosheets. For the first time, MoS2/Li cells with a high power density of 10-40 kW kg-1 in the intercalation regime have been realized. In the conversion regime, defect-rich and defect-suppressed MoS2 exhibit initial lithiation capacities of ∼1000 and ∼840 mA h g-1, respectively. Defect-rich MoS2 had a capacity of ∼800 mA h g-1 at 0.1C (67 mA g-1), whereas defect-suppressed MoS2 had a capacity of only ∼80 mA h g-1 at the same current rate. Capacity retention of 78% was observed for defect-rich MoS2 with a reversible capacity of 591 mA h g-1 when cycled at 0.1C (67 mA g-1) for 100 cycles. Despite having a lower energy density in the intercalation regime, the power density of defect-rich MoS2 in the intercalation regime is significantly larger (by three orders of magnitude) as compared to that of defect-suppressed MoS2 in the conversion regime. Defect-rich MoS2 nanosheets are promising for high-rate-capability applications when operated in the intercalation regime.

Defect stabilization in ZnO nanorods by Mg2+ doping.

Luminescent ZnO and Zn0.95Mg0.05O nanorods with length around 0.5 to 3 microm and diameter 100-150 nm were prepared by a facile solvothermal method. On hydriding at room temperature, a change of morphology from nanorods with aspect ratio 5-10 to particles of sizes 100 nm has been observed in both ZnO and Zn0.95Mg0.05O. While hydrided Zn0.95Mg0.05O showed an enhanced defect related green emission, the same got suppressed in hydrided ZnO. Even though it is observed that zinc vacancies are present in both as prepared ZnO and Zn0.95Mg0.05O, luminescence studies indicate that zinc vacancies get stabilized in Zn0.95Mg0.05O on hydrogenation.

Possible weak ferromagnetism in pure and M (Mn, Cu, Co, Fe and Tb) doped NiGa2O4 nanoparticles.

We report on the structural and magnetic properties of nanoparticles of NiGa2O4 and 5 at.% M doped (M = Mn2+, Cu2+, Co2+, Fe3+ and Tb3+) at Ga site of NiGa2O4, synthesized by gel-combustion method. The particle size, as investigated by X-ray diffraction and transmission electron microscopy, could be fine tuned by a controlled annealing process. Weak ferromagnetism becomes significant, when the particles are in the nano regime (5-7 nm). The magnetization becomes insignificant at larger particle size ( 150 nm). Cu2+ and Tb3+ doped NiGa2O4 nanoparticles showed relatively large room temperature ferromagnetism compared to other doped (Fe, Mn and Co) and undoped NiGa2O4 samples. The weak ferromagnetism observed in the nanoparticles of NiGa2O4, which is antiferromagnetic in the bulk, is due to the surface disordered states with uncompensated spins.

Improving a high-resolution fiber-optic interferometer through deposition of a TiO2 reflective coating by simple dip-coating.

Fiber-optic based interferometers are used to detect small displacements, down to the subnanometer range. Coating the end of the optical fiber with a partially reflecting thin film greatly improves the resolution of interferometers by increasing the multiple reflections between the fiber end and the measured object. In this work, we present a quick and easy thin film deposition technique to coat the end of a single optical fiber by dip-coating a metal-organic precursor, which is then decomposed in a propane flame. The coated fiber was tested for morphology and usefulness for interferometric application. We found that this coating technique is much faster and easier than conventional thin coating techniques, and yields results that are comparable or better than can be achieved with sputtering or thermal evaporation.

Magnetic-field-induced optical anisotropy in ferrofluids: a time-dependent light-scattering investigation.

We report an experimental investigation of time dependent anisotropic light scattering by an aqueous suspension of tetramethyl ammonium hydroxide coated Fe3O4 nanoparticles (approximately 6 nm) under the ON-OFF transient of an external dc magnetic field. The study employs the synchronized recording and measurement of the two magnetic-field-induced light-scattering patterns produced by two identical orthogonal He-Ne laser beams passing through the ferrofluid sample and propagating parallel and perpendicular to the applied field, respectively. From these patterns, we extract the time dependence of the induced optical anisotropy, which provides a measure of the characteristic time scale and kinematic response for field-induced structure formation in the sample. We propose that the time evolution of the scattering patterns, which is very fast at short times and significantly slower at long times, can be explained using a model based on a two-stage chain formation and coarsening processes.

Quantifying magnetic nanoparticles in non-steady flow by MRI.

OBJECTIVE: This work compares the measured R*2 of magnetic nanoparticles to their corresponding theoretical values in both gel phantoms and dynamic water flows on the basis of the static dephasing theory. MATERIALS AND METHODS: The magnetic moment of a nanoparticle solution was measured by a magnetometer. The R*2 of the nanoparticle solution doped in a gel phantom was measured at both 1.5 and 4.7 T. A total of 12 non-steady state flow experiments with different nanoparticle concentrations were conducted. The R*2 at each time point was measured. RESULTS: The theoretical R*2 on the basis of the magnetization of nanoparticles measured by the magnetometer agree within 11% of MRI measurements in the gel phantom study, a significant improvement from previous work. In dynamic flow experiments, the total R*2 calculated from each experiment agrees within 15% of the theoretical R*2 for 10 of the 12 cases. The MRI phase values are also reasonably predicted by the theory. The diffusion effect does not seem to contribute significantly. CONCLUSIONS: Under certain situations with known R*2, the static dephasing theory can be used to quantify the susceptibility or concentration of nanoparticles in either a static or dynamic flow environment at a given time point. This approach may be applied to in vivo studies.