Jayaraman Sundaramurthy

Electrospun α-Fe2O3 nanorods as a stable, high capacity anode material for Li-ion batteries

α-Fe2O3 nanorods are synthesized by electrospinning of polyvinylpyrrolidone (PVP)/ferric acetyl acetonate (Fe(acac)3) composite precursors and subsequent annealing at 500 °C for 5 h. X-ray diffraction and Raman spectroscopy analyses confirm the formation of a hematite structure as the predominant phase. The electron microscopy studies show that the electrospun α-Fe2O3 nanorods are composed of agglomerates of nano-sized particles and the average diameter of the nanorods is found to be 150 nm. Li-storage and cycling properties are examined by galvanostatic cycling in the voltage range 0.005–3 V vs. Li at various current densities and it is complemented by cyclic voltammetry. The electrospun α-Fe2O3 nanorods exhibit a high reversible capacity of 1095 mA h g−1 at 0.05 C, are stable up to 50 cycles and also show high rate capability, up to 2.5 C. The high rate capability and excellent cycling stability can be attributed to the unique morphology of the macroporous nanorods comprised of inter-connected nano-sized particles, thus making electrospun α-Fe2O3 a promising anode material for Li-ion batteries.

Morphologically robust NiFe2O4 nanofibers as high capacity Li-ion battery anode material.

In this work, the electrochemical performance of NiFe2O4 nanofibers synthesized by an electrospinning approach have been discussed in detail. Lithium storage properties of nanofibers are evaluated and compared with NiFe2O4 nanoparticles by galvanostatic cycling and cyclic voltammetry studies, both in half-cell configurations. Nanofibers exhibit a higher charge-storage capacity of 1000 mAh g(-1) even after 100 cycles with high Coulmbic efficiency of 100% between 10 and 100 cycles. Ex situ microscopy studies confirmed that cycled nanofiber electrodes maintained the morphology and remained intact even after 100 charge-discharge cycles. The NiFe2O4 nanofiber electrode does not experience any structural stress and eventual pulverisation during lithium cycling and hence provides an efficient electron conducting pathway. The excellent electrochemical performance of NiFe2O4 nanofibers is due to the unique porous morphology of continuous nanofibers.

Electrospun composite nanofibers and their multifaceted applications

The re-exploration of the nanostructure production technique known as electrospinning was carried out in the past decade due to its simplicity and uniqueness of producing nanostructures. As nanotechnology is one of the most promising and growing technologies today, a large amount of work is being carried out in an extensive area and shows an extremely huge potential for miraculous works in the fields of medicine and biotechnology. These nanostructures were found to be of great significance because of their inherent properties such as large surface area to volume ratio and the engineered properties such as porosity, stability and permeability. The functionality and applicability of these nanostructures were further improved by incorporating secondary phases either during electrospinning or in the post-processing resulting in the composite nanostructures. These secondary phases may include metal oxides, carbon nanotubes, precious metals, gold nanoparticles and hydroxyapatite. Nanofibrous materials that mimic the native extracellular matrix (ECM) and promote the adhesion of various cells are being developed as tissue-engineered scaffolds for the skin, bone, vasculature, heart, cornea, nervous system and other tissues. The article discusses in detail the applicability of these composite fibers in energy, sensors, filters, biotechnology and details the technological issues, research challenges and future trends.

Hierarchical electrospun nanofibers for energy harvesting, production and environmental remediation

As the demand for energy is rapidly growing worldwide ahead of energy supply, there is an impulse need to develop alternative energy-harvesting technologies to sustain economic growth. Due to their unique optical and electrical properties, one-dimensional (1D) electrospun nanostructured materials are attractive for the construction of active energy harvesting devices such as photovoltaics, photocatalysts, hydrogen energy generators, and fuel cells. 1D nanostructures produced from electrospinning possess high chemical reactivity, high surface area, low density, as well as improved light absorption and dye adsorption compared to their bulk counterparts. So, research has been focused on the synthesis of 1D nanostructured fibers made from metal oxides, composites, dopants and surface modification. Furthermore, fine tuning these NFs has facilitated fast charge transfer and efficient charge separation for improved light absorption in photocatalytic and photovoltaic properties. The recent trend in exploring these electrospun nanostructures has been promising in-terms of reducing costs and enhancing the efficiency compared to conventional materials. This review article presents the synthesis of 1D nanostructured fibers made via electrospinning and their applications in photovoltaics, photocatalysis, hydrogen energy harvesting and fuel cells. The current challenges and future perspectives for electrospun nanomaterials are also reviewed.

Unveiling TiNb2O7 as an Insertion Anode for Lithium Ion Capacitors with High Energy and Power Density

This is the first report of the utilization of TiNb2 O7 as an insertion-type anode in a lithium-ion hybrid electrochemical capacitor (Li-HEC) along with an activated carbon (AC) counter electrode derived from a coconut shell. A simple and scalable electrospinning technique is adopted to prepare one-dimensional TiNb2 O7 nanofibers that can be characterized by XRD with Rietveld refinement, SEM, and TEM. The lithium insertion properties of such electrospun TiNb2 O7 are evaluated in the half-cell configuration (Li/TiNb2 O7 ) and it is found that the reversible intercalation of lithium (≈3.45 mol) is feasible with good capacity retention characteristics. The Li-HEC is constructed with an optimized mass loading based on the electrochemical performance of both the TiNb2 O7 anode and AC counter electrode in nonaqueous media. The Li-HEC delivers very high energy and power densities of approximately 43 Wh kg(-1) and 3 kW kg(-1) , respectively. Furthermore, the AC/TiNb2 O7 Li-HEC delivers a good cyclability of 3000 cycles with about 84% of the initial value.

Enhanced super-hydrophobic and switching behavior of ZnO nanostructured surfaces prepared by simple solution – Immersion successive ionic layer adsorption and reaction process

A simple and cost-effective successive ionic layer adsorption and reaction (SILAR) method was adopted to fabricate hydrophobic ZnO nanostructured surfaces on transparent indium-tin oxide (ITO), glass and polyethylene terephthalate (PET) substrates. ZnO films deposited on different substrates show hierarchical structures like spindle, flower and spherical shape with diameters ranging from 30 to 300 nm. The photo-induced switching behaviors of ZnO film surfaces between hydrophobic and hydrophilic states were examined by water contact angle and X-ray photoelectron spectroscopy (XPS) analysis. ZnO nanostructured films had contact angles of ~140° and 160°±2 on glass and PET substrates, respectively, exhibiting hydrophobic behavior without any surface modification or treatment. Upon exposure to ultraviolet (UV) illumination, the films showed hydrophilic behavior (contact angle: 15°±2), which upon low thermal stimuli revert back to its original hydrophobic nature. Such reversible and repeatable switching behaviors were observed upon cyclical exposure to ultraviolet radiation. These biomimetic ZnO surfaces exhibit good anti-reflective properties with lower reflectance of 9% for PET substrates. Thus, the present work is significant in terms of its potential application in switching devices, solar coatings and self-cleaning smart windows.

Tunable hierarchical TiO2 nanostructures by controlled annealing of electrospun fibers: formation mechanism, morphology, crystallographic phase and photoelectrochemical performance analysis

Highly crystalline hierarchical TiO2 nanostructures of morphology ranging from one-dimensional regular fibers, hollow tubes, porous rods and spindles were achieved from electrospun TiO2/composite fibers by annealing at temperatures ranging from 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C, with a ramp rate of 5 °C min−1, and at a pressure of 1 mbar. Crystallographic structure, crystallite size, surface morphology and surface area of annealed TiO2 nanostructures were analysed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Brunauer–Emmett–Teller (BET) method. The analysis of post-annealing process on electrospun TiO2 nanofibers showed an orderly change in the crystallographic phase transformation with corresponding change in their surface morphologies. XRD and HRTEM analysis confirmed the phase transformation of highly crystalline anatase phase to rutile with crystallite size varied from 11 nm to 36 nm upon tuning the annealing temperature. Interestingly, TiO2 nanostructures annealed at 700 °C showed the formation of biphasic TiO2 hollow tubes with stoichiometry phase compositions of 45.74% anatase and 54.25% rutile. A possible formation mechanism was proposed based on series of temperature-dependent experiments. To evaluate the potential use of these TiO2 nanostructures, dye sensitized solar cell (DSSC) was fabricated using the post-annealed TiO2 nanostructures as photoanode. A higher conversion efficiency (η) of 4.56% with a short circuit current (Jsc) of 8.61 mA cm−2 was observed for highly ordered porous anatase TiO2 nanorods obtained upon annealing at 500 °C under simulated AM1.5 G (100 mW cm−2), confirming that surface area of TiO2 resulted out of porous structure played dominant role.

Free-standing electrospun carbon nanofibres—a high performance anode material for lithium-ion batteries

Free-standing carbon nanofibres (CNFs) are prepared by the carbonization of poly-acrylonitrile using a simple electro-spinning technique. The electro-spun fibres are studied as an anode material for lithium-ion batteries in half-cell configurations. The fibres showed an initial discharge capacity of 826 mAh g −1 at a current density of 200 mA g −1 and exhibited an appreciable capacity profile during cycling. The Li-storage mechanism has been explained based on the cyclic voltametric and galvanostatic cycling results. (Some figures may appear in colour only in the online journal)

Perspective of electrospun nanofibers in energy and environment

This review summarizes the recent developments of electrospun semiconducting metal oxide/polymer composite nanostructures in energy and environment related applications. Electrospinning technique has the advantage of synthesizing nanostructures with larger surface to volume ratio, higher crystallinity with phase purity and tunable morphologies like nanofibers, nanowires, nanoflowers and nanorods. The electrospun nanostructures have exhibited unique electrical, optical and catalytic properties than the bulk counter parts as well as nanomaterials synthesized through other approaches. These nanostructures have improved diffusion and interaction of molecules, transfer of electrons along the matrix and catalytic properties with further surface modification and functionalization with combination of metals and metal oxides.

Synthesis and Controlled Growth of ZnO Nanorods Based Hybrid Device Structure by Aqueous Chemical Method

In the present work, vertical ZnO nanorods (NRs) were grown onto ITO substrates by a simple two step chemical process at relatively low temperature by using successive ionic layer absorption and reaction method (SILAR) and chemical bath deposition (CBD) method. The investigated on n- ZnO/ p-Polythiophene heterojunction device have been fabricated with ZnO nanorods. Structural analysis reveals that the grown ZnO NRs exhibit (002) reflection with higher intensity, indicating that the ZnO NRs grown in c-axis orientation. FESEM image shows the surface morphology of grown ZnO nanorods was of hexagonal wurtzite structure whose diameter varies from 200 nm to 1μm. Room temperature Photoluminescence exhibited strong UV emission at ∼386 nm and a negligible green band confirms the presence of very low concentration of oxygen vacancies in the well-aligned ZnO nanorods. The current–voltage (I –V) characteristics of the heterojunctions show good rectifying diode characteristics. These results indicate that hybrid device fabricated from solution process is a promising approach for future light-emitting diodes (LEDs) devices.