Rasmita Barik

In situ synthesis of flowery-shaped α-FeOOH/Fe2O3 nanoparticles and their phase dependent supercapacitive behaviour

In situ, one-step, facile synthetic strategies for flowery-shaped iron oxide nanoparticles were developed. Herein, we report simplified controlled synthesis of 2-line ferrihydrite–goethite core–shell particles for the first time in a semi-aqueous-organic medium. The present route offered phase selectivity by controlling only the aqueous-to-organic phase ratio. The synthesised nanoparticles have high surface areas of 110 m2 g−1 and 185 m2 g−1 for 2-line ferrihydrite and core–shell goethite, respectively. Further, flowery-shaped hematite nanoparticles were obtained by annealing core–shell iron oxide nanoparticles at 400 °C. Phase purities were confirmed by XRD (X-ray diffraction), IR (infrared spectroscopy), and XPS (X-ray photo electron spectroscopy) analysis. Formation of the core–shell nanostructure for the iron oxide samples was confirmed by Mossbauer and selected area electron diffraction (SAED) studies. All the synthesized iron oxide materials were studied for their supercapacitor behaviour by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chrono-potentiometry charge–discharge measurements. Specific capacitances for core–shell α-FeOOH and α-Fe2O3 were found to be 160 F g−1 and 200 F g−1, respectively. These values were much higher as compared to the previous reported values for pure phases of iron oxides. The chrono-potentiometric charge–discharge study for all the three samples revealed their self-discharging capacities. Moreover, these iron oxide composite electrodes exhibited excellent cycling performance with >99% capacitance retention over 500 cycles. Electrochemical performance of the two-electrode system was also studied. Furthermore, the electrochemical impedance spectroscopy (EIS) demonstrated that the electrochemical resistance of α-Fe2O3 was slightly reduced with the number of cycles, indicating easier access for intercalation/deintercalation of charges in the flowery-structured materials. Thus, the present material can be used as an electrochemical supercapacitor for high-performance energy storage devices in future.

Visible light induced photo-hydroxylation of phenol to catechol over RGO–Ag3VO4 nanocomposites without the use of H2O2

RGO–Ag3VO4 nanocomposites prepared by a novel one-pot photochemical synthesis route show unusual selectivity towards catechol in the photo-hydroxylation of phenol with complete conversion.

Solvent mediated surface engineering of α-Fe2O3 nanomaterials: facet sensitive energy storage materials

The surface chemical properties of iron oxide nanomaterials are keenly studied to explore their potential for many future applications. Therefore many synthetic strategies are now being pursued to develop unique morphologies with active surfaces and unusual crystal facets for advanced uses. Here, a novel process for the formation of an α-Fe2O3 phase has been established by a facile solvent mediated precipitation route. Ethylene glycol was used as the solvent and plays an active role in controlling the surface morphology and the orientation of facets during crystal growth. The effects of various parameters on the morphology, structure of the product, and electrochemical properties were studied. Mainly high surface energy facets were stabilized by a high concentration of EG in the reaction solution. The formation of (012) or (001) facets was observed in a reaction solution with a lower concentration of EG. Hematite with a flowery morphology and having (012) plane orientation was achieved by the assembly of pseudo cubes with (012) facets and a secondary growth process. The sample obtained at an Fe : EG ratio of 1 : 2 showed an ultra-high pseudo capacitance value of 450 F g−1 related to its high surface area. The present study can be further extended for the preparation of other functional oxides with new active facets for energy storage applications.

A facile, single-step synthesis of flowery shaped, pure/lithium-doped 3D iron oxides

The shape-dependent surface properties of iron oxides are being paid increasing attention for their many advanced and synergistic applications. The present investigation deals with the preparation of pure and lithiated 3D iron oxide through a simple and single-step synthesis route. The nano-hierarchical flowers were synthesized by adopting a semi-aqueous ligated system. Here, the reagent played a double role for ligation as well as for precipitation. In the absence of lithium, goethite and ferrihydrite phases were formed, whereas formation of a mixture of hematite and ferrihydrite was observed in its presence, confirming participation of Li in phase transformation of goethite/ferrihydrite to hematite. With the progress of time, flowery shaped nanoparticles developed. Mossbauer spectroscopy revealed Li ion-induced formation of an α-Fe2O3 phase. Single-phase hematite was formed on annealing at 500 °C. The Li-doped iron oxide sample has high surface area and has a sharp distribution peak centered at 19.13 nm, showing homogeneity of the pores. On calcination of the sample at 400 °C, the surface area decreased; however, pore size distribution remained unchanged, which was an unusual trend. The annealed sample (500 °C) possessed bimodal (small and large) mesopore distribution. The fluoride adsorption behaviour and magnetic properties of the as-synthesized and annealed Li-doped samples are discussed. Magnetic properties of the samples suggest that incorporation of Li resulted in an increase of coercivity due to stabilization of the domain. The unique surface behaviour of the present samples can be further examined for other high end applications. The present synthesis strategy has the advantage of producing shape-controlled hierarchical materials with tunable surface properties, which promises the further development of other functional materials.

Metal doped mesoporous FeOOH nanorods for high performance supercapacitors

In the present study, the effect of doping of foreign atoms on the parent atoms and the application of the resultant material for energy storage are successfully investigated. A facile method is reported for successful incorporation of cobalt into the regular crystal lattice of iron oxide in ethylene glycol media. As iron oxides are reasonable, the Co doped nano-goethite is expected to be of potential use for supercapacitor application with a high specific capacitance value of 463.18 F g−1 at 0.1 A g−1 current density. It shows a cycling stability of 1000 at 1 A g−1 with 96.36% of initial capacitance. The doped goethite nanorod with a band gap of 2.82 eV and high surface area (159.74 m2 g−1) was found to be a superior electrode material for supercapacitors in terms of specific capacitance and cycling capability at a particular percentage of doping. The high discharge capacitance and its retention are attributed to high surface area and porosity of the doped iron oxide.

Solvent specific synthesis of nano corpse flowery lithiated iron oxide as an energy storage and gas sensing material

Iron oxide based materials are one of the most appealing matrices and promising futuristic materials for energy conversion/storage devices. The solvent dependent synthesis and growth of lithiated iron oxide/LiFeO2 nano flowers was established using a simple sol–gel method at low temperature. Herein, for the first time the development of flowery (corpse flower) shaped iron oxide based nano materials is reported. The effect of type of solvent on phase formation, shape and sizes of the as-synthesiszed samples was determined using charaterisation techniques such as X-ray diffraction, FTIR, Raman spectroscopy, surface area, TEM, UV, and XPS. Li was incorporated into the iron oxide matrix in ethylene glycol medium and developed a unique and uniform corpse flowery shape, depending on various reaction parameters, whereas in the presence of ethylene glycol monomethyl ether, the shape of the nano materials completely changed. The supercapacitive and gas sensing properties of some selected synthesiszed materials are evalutaed. The specific capacitance values of the materials depend on the nature of the solvent and lithium content of the as-prepared samples. The lithiated iron oxide samples exhibit a supercapacitance value of 241 F g−1 in 0.1 M Na2SO4 between −0.4 and 1 V versus Ag/AgCl. The gas sensing behaviour and optical properties are also included to open up the multidimensional applications of the samples.